Lithium ion secondary battery, preparation method of lithium ion secondary battery, power utilization device and application of lithium ion secondary battery

Abstract

The invention relates to a lithium ion secondary battery, a preparation method thereof, a power utilization device and application. The lithium ion secondary battery comprises a negative pole piece and an electrolyte, the negative pole piece comprises a negative current collector and a negative active material layer positioned on at least one side of the negative current collector; the negative electrode active material layer comprises an electronegative coating active material, and the electronegative coating active material comprises a coating layer; the coating layer comprises doped carbon, the doped carbon comprises a carbon matrix and an electronegative doping element, the carbon matrix comprises one or more of soft carbon and hard carbon, the Paulin electronegative scales of the electronegative doping element and the carbon element are respectively marked as x1 and xC, and x1-xC is more than or equal to 0.03 and less than or equal to 0.49. The negative electrode active material layer comprises a first negative electrode active layer and can also comprise a second negative electrode active layer located between the negative electrode current collector and the first negative electrode active layer. The lithium ion secondary battery has significantly improved fast charge performance.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion secondary batteries, and further to a lithium-ion secondary battery and a preparation method, an electrical device and application thereof. Background Art

[0002] The statements herein merely provide background information related to the present application and do not necessarily constitute prior art.

[0003] With the development of lithium-ion secondary battery technology, lithium-ion secondary batteries are increasingly used in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields, and are also widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations. The demand for fast charging performance of lithium-ion secondary batteries is also increasing. Summary of the invention

[0004] According to various implementations and examples of the present application, the present application provides a lithium ion secondary battery and a preparation method, an electric device and application thereof. The lithium ion secondary battery has significantly improved fast charging performance.

[0005] In a first aspect of the present application, a lithium ion secondary battery is provided.

[0006] In some embodiments, a lithium ion secondary battery is provided, comprising a negative electrode plate and an electrolyte; the negative electrode plate comprises a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector, the negative electrode active material layer comprises a first negative electrode active layer;

[0007] The first negative electrode active layer comprises a first negative electrode active material, the first negative electrode active material comprises an electronegative coated active material, the electronegative coated active material comprises a negative electrode active body and a coating layer located on at least a portion of the surface of the negative electrode active body; the coating layer comprises doped carbon, the doped carbon comprises a carbon matrix and an electronegative doping element, the carbon matrix comprises one or more of soft carbon and hard carbon, and the Pauling electronegativity scale of the electronegative doping element is denoted by x 1 , the Pauling electronegativity scale of carbon is denoted by χ C , then χ 1 With χ C The absolute value of the difference between them satisfies 0.03≤|χ 1 -x C |≤0.49.

[0008] In the lithium-ion secondary battery, an electronegative coated active material is arranged in the negative electrode active material layer in the negative electrode plate, wherein the coating layer is provided with doped carbon including electronegative doping elements, and the carbon matrix in the doped carbon may include one or more of soft carbon and hard carbon; on the one hand, the introduced electronegative doping elements have special Pauling electronegativity scale characteristics, thereby improving the polarization characteristics of the surface of the negative electrode active material, enhancing the adsorption of the negative electrode active material to foreign atoms, and weakening the binding force between the negative electrode active material and the solvated lithium ions, thereby reducing the contact angle between the surface of the negative electrode active material and the electrolyte, improving the wettability to the electrolyte, and facilitating Accelerate the rate of lithium ion embedding into the negative electrode active material; on the other hand, both the soft carbon and the hard carbon in the carbon matrix have a large degree of disorder, which is conducive to the entry of lithium ions. The carbon layers in the soft carbon are disordered and can make lithium ions transmit relatively quickly. The hard carbon can provide abundant lithium embedding sites and fast transmission channels, which is conducive to promoting faster and more lithium ion embedding into the negative electrode active material; on the other hand, the introduction of electronegative doping elements is also conducive to the formation of inorganic components in the solid electrolyte interface (SEI) film, which can enhance the lithium conductivity of the SEI film and reduce the interface impedance; based on the aforementioned multiple effects, it is conducive to improving battery dynamics and enhancing the fast charging capability of the battery.

[0009] In some embodiments, the negative electrode active material layer further includes a second negative electrode active layer, and the second negative electrode active layer is located between the negative electrode current collector and the first negative electrode active layer.

[0010] In the lithium-ion secondary battery, a first negative electrode active layer located in an upper layer and a second negative electrode active layer located in a lower layer are arranged in the negative electrode active material layer in the negative electrode plate (the direction away from the surface of the negative electrode current collector is the upper layer, and the direction toward the surface of the negative electrode current collector is the lower layer). Furthermore, an electronegative coated active material is arranged in the first negative electrode active layer located in the upper layer, wherein the coating layer is provided with doped carbon including electronegative doping elements, and the carbon matrix in the doped carbon may include one or more of soft carbon and hard carbon; on the one hand, the introduced electronegative doping elements have special Pauling electronegativity scaling characteristics, thereby improving the polarization characteristics of the surface of the negative electrode active material, enhancing the adsorption of the negative electrode active material to foreign atoms, and weakening the binding force between the negative electrode active material and the solvated lithium ions, thereby making the contact angle between the surface of the first negative electrode active material and the electrolyte On the one hand, the wettability of the electrolyte is improved, which is beneficial to accelerate the rate of lithium ion embedding into the first negative electrode active material; on the other hand, both the soft carbon and the hard carbon in the carbon matrix have a large degree of disorder, which is beneficial to the entry of lithium ions. The carbon layers in the soft carbon are disorderly stacked, which can make lithium ions transmit relatively quickly therein, and the hard carbon can provide abundant lithium embedding sites and fast transmission channels, thereby facilitating faster and more lithium ion embedding into the first negative electrode active material; on the other hand, the introduction of electronegative doping elements is also beneficial to the formation of inorganic components in the solid electrolyte interface (SEI) film, which can enhance the lithium conductivity of the SEI film and reduce the interface impedance; on the other hand, the improved wettability of the electrolyte in the first negative electrode active layer is also beneficial to improving the wettability of the electrolyte in the second negative electrode active layer; based on the aforementioned multiple effects, it is beneficial to improve the battery dynamics and enhance the fast charging capability of the battery.

[0011] In some embodiments, 0.04≤|χ 1 -x C |≤0.49.

[0012] By controlling the numerical value of the difference between the Pauling electronegativity scale of the electronegative doping element and the carbon element within the aforementioned range, the Pauling electronegativity scale of the electronegative doping element is controlled within a more appropriate range. On the one hand, it is beneficial to reduce the contact angle between the surface of the negative electrode active material and the electrolyte, and to improve the wettability of the electrolyte to the first negative electrode active material. On the other hand, by controlling the difference in electron-withdrawing ability between the electronegative doping element and the carbon atom within a more appropriate range, it is beneficial to form a stable covalent bond between the electronegative doping element and the carbon atom.

[0013] In some embodiments, the electronegative doping element is doped into the carbon matrix in a covalent bonding manner;

[0014] Optionally, at least a portion of the electronegative doping element is covalently bonded to 2 or 3 carbon atoms simultaneously;

[0015] Optionally, 80% to 100% of the electronegative doping element is covalently bonded to 2 or 3 carbon atoms at the same time;

[0016] Optionally, any one atom of the electronegative doping element is covalently bonded to 2 or 3 carbon atoms simultaneously.

[0017] By doping electronegative doping elements into the carbon matrix in a covalently bonded manner, the stability of the electronegative doping elements in the coating layer can be improved, which is beneficial to improving the stability of the electrolyte wettability and the battery cycle stability.

[0018] In some embodiments, the electronegative doping element in the doped carbon includes one or more of N, P and S.

[0019] In some embodiments, the doped carbon satisfies one or more of the following characteristics:

[0020] (ta1) the electronegative doping element comprises a bridging N atom, wherein the bridging N atom is covalently bonded to at least one carbon atom;

[0021] (ta2) the electronegative doping element comprises a bridging S atom, wherein the bridging S atom is covalently bonded to at least one carbon atom;

[0022] (ta3) The electronegative doping element includes bridging-type P atoms, and at least a portion of any covalent sites of the bridging-type P atoms are independently covalently bonded in a COP or CSP manner; optionally, 80% to 100% of any covalent sites of the bridging-type P atoms are independently covalently bonded in a COP or CSP manner.

[0023] In some embodiments, the electronegative doping element includes N element, and the doping form of the electronegative doping element in the doped carbon includes at least one of pyridinic nitrogen type and pyrrolic nitrogen type.

[0024] By controlling the doping form of the electronegative doping element in the doped carbon to include at least one of the pyridinic nitrogen type and the pyrrolic nitrogen type, the high reactivity of the doped N atoms based on the pyridinic nitrogen and pyrrolic nitrogen is conducive to better improving the adsorption of the first negative electrode active material to foreign atoms, and is conducive to better reducing the lithium ion mass transfer resistance, and is more conducive to improving the wettability of the electrolyte to the first negative electrode active layer, thereby improving the wettability of the second negative electrode active layer, and thus is conducive to better improving the fast charging performance of the battery. In addition, it is also conducive to reducing the difficulty of the doping process.

[0025] In some embodiments, the electronegative coating active material satisfies one or more of the following characteristics:

[0026] (tb1) the average thickness of the coating layer is 1 nm to 500 nm, optionally 100 nm to 500 nm, and further optionally 100 nm to 200 nm;

[0027] (tb2) the thickness of at least a portion of the coating layer is 1 nm to 1000 nm, optionally 100 nm to 1000 nm, and further optionally 100 nm to 200 nm;

[0028] (tb3) the mass percentage of the electronegative doping element in the doped carbon is 0.1% to 0.6%, optionally 0.2% to 0.6%, and further optionally 0.2% to 0.4%;

[0029] (tb4) the mass percentage of the electronegative doping element in the coating layer is 0.1% to 0.6%, optionally 0.2% to 0.6%, and further optionally 0.2% to 0.4%;

[0030] (tb5) the mass percentage of the coating layer in the electronegative coating active material is 0.2% to 5%, and can be optionally 0.5% to 3%;

[0031] (tb6) The average thickness of the coating layer is equal to the D of the first negative electrode active material. v The ratio of 50 is 0.5% to 12.5%, and can be selected as 1% to 10%;

[0032] (tb7) The mass percentage of the doped carbon in the coating layer is 80% to 100%, and can be optionally 90% to 100%;

[0033] (tb8) The sum of the mass percentages of soft carbon and hard carbon in the carbon matrix is ​​80% to 100%, and can be optionally 90% to 100%; optionally, the mass percentage of soft carbon in the carbon matrix is ​​80% to 100%, and can be further optionally 90% to 100%, or the mass percentage of hard carbon in the carbon matrix is ​​80% to 100%, and can be further optionally 90% to 100%.

[0034] The average thickness of the coating layer, the local thickness of the coating layer, the mass percentage of the electronegative doping element in the doped carbon, the mass percentage of the electronegative doping element in the coating layer, the average thickness of the coating layer and the D vWhen one or more parameters including the ratio of 1:1 to 5:50, the mass percentage of doped carbon in the coating layer and the sum of the mass percentages of soft carbon and hard carbon in the carbon matrix are controlled within the aforementioned range, it is beneficial to control the content of electronegative doping elements within a more appropriate range, which is beneficial to improving the fast charging performance of the battery based on the aforementioned multiple effects, and is also beneficial to reducing the probability of electronegative doping elements participating in side reactions, which is beneficial to achieving good battery cycle performance, but is not limited to the aforementioned theory.

[0035] In some embodiments, the electronegative coated active material accounts for 20% to 100% of the first negative electrode active material, and can be 40% to 100%. By controlling the electronegative coated active material in the first negative electrode active material within the aforementioned range, it is beneficial to better improve the battery dynamics based on the multiple effects of the electronegative coated active material and enhance the fast charging capability of the battery.

[0036] In some embodiments, the electronegative coated active material accounts for 20% to 80% of the first negative electrode active material, and can further be 40% to 80%. By controlling the electronegative coated active material in the first negative electrode active material within the aforementioned range, it is beneficial to better improve the fast charging capability of the battery and also to take into account the manufacturing cost.

[0037] In some embodiments, the electronegative coated active material includes electronegative coated secondary particles, and the negative electrode active body in the electronegative coated secondary particles is a secondary particle.

[0038] By introducing electronegative coated secondary particles into the first negative electrode active material, it is beneficial to increase the roughness of the surface of the active particles in the electronegative coated active material, increase the sites for adsorbing lithium ions, and further improve the wettability of the electrolyte to the first negative electrode active layer, and further improve the wettability of the electrolyte to the second negative electrode active layer, thereby better improving the fast charging performance of the battery.

[0039] In some embodiments, the amount of the electronegative coated secondary particles in the first negative electrode active material is 20% to 100%, and can be 40% to 100%. By controlling the amount of the electronegative coated secondary particles in the first negative electrode active material within the aforementioned range, it is beneficial to better improve the fast charging performance of the battery.

[0040] In some embodiments, the electronegative coated secondary particles account for 20% to 80% of the first negative electrode active material, and may be 40% to 80%. By controlling the electronegative coated secondary particles in the first negative electrode active material within the aforementioned range, it is beneficial to better improve the fast charging performance of the battery and also to take into account the manufacturing cost.

[0041] In some embodiments, in the electronegative coated active material, the negative electrode active body includes one or more of a carbon-based active material and a silicon-based active material.

[0042] Without limitation, the negative electrode active body in the electronegative coated active material may include one or more of a carbon-based active material and a silicon-based active material. In this case, the first negative electrode active material includes one or more of a carbon-based material and a silicon-based material. The electronegative coated active material may include electronegative coated secondary particles. When the negative electrode active body includes a carbon-based active material, it is beneficial to make the first negative electrode active material have better conductivity, which is beneficial to reduce the internal resistance and provide better fast charging capability. When the negative electrode active body includes a silicon-based active material, it is beneficial to utilize the high gram capacity of the silicon-based active material to increase the amount of lithium embedded per unit time, which is beneficial to increase the fast charging speed. In addition, it is also beneficial to increase the energy density.

[0043] In some embodiments, the first negative electrode active material satisfies one or more of the following characteristics:

[0044] (tc1) the carbon-based active material comprises one or more of artificial graphite, natural graphite, soft carbon and hard carbon;

[0045] (tc2) The first negative electrode active material includes a carbon-based material, and the mass proportion of the carbon-based material in the first negative electrode active material is 20% to 100%, optionally 20% to 80%, and further optionally 40% to 80%.

[0046] By controlling the mass proportion of the carbon-based material in the first negative electrode active material within the aforementioned range, it is beneficial to better control the volume expansion of the negative electrode during fast charging, improve the structural stability of the first negative electrode active material during the charge and discharge cycle, and also take into account the energy density of the negative electrode and the battery.

[0047] In some embodiments, the lithium-ion secondary battery satisfies one or more of the following characteristics:

[0048] (td1) the negative electrode active body in the electronegative coated active material comprises graphite;

[0049] (td2) the electronegative coated active material comprises electronegative coated secondary graphite particles, the negative electrode active body in the electronegative coated secondary graphite particles is secondary graphite particles, and the amount of the electronegative coated secondary graphite particles in the first negative electrode active material accounts for 20% to 100%, optionally 40% to 100%, and further optionally 40% to 80%;

[0050] (td3) D of the first negative electrode active material v 50 is 8μm~16μm, and can be selected as 10μm~14μm;

[0051] (td4) The porosity of the first negative electrode active layer is 22% to 32%, and can be optionally 26% to 30%;

[0052] (td5) Under at least one temperature condition between 20°C and 35°C, the ionic conductivity of the electrolyte is 13mS / cm to 18mS / cm; optionally, at 25°C, the ionic conductivity of the electrolyte is 13mS / cm to 18mS / cm.

[0053] By making the lithium-ion secondary battery satisfy one or more of the characteristics (td1), (td2), (td3), (td4) and (td5), the fast charging performance of the battery can be better improved.

[0054] By introducing graphite into the negative active body in the electronegative coated active material, it is beneficial to make the electronegative coated active material have better conductivity, which is beneficial to reduce the internal resistance and provide better fast charging capability.

[0055] By introducing electronegative coated secondary graphite particles into the electronegative coated active material, it is beneficial to take into account the advantages of both the electronegative coated secondary particles and the graphite body.

[0056] By adding a negative electrode active material D v 50 is controlled within the aforementioned range, which is beneficial to better control the degree of particle stacking in the first negative electrode active layer, is beneficial to better control the pores between particles, provides a better lithium ion transmission channel, and better improves the battery dynamics and fast charging performance.

[0057] By controlling the porosity of the first negative electrode active layer within the aforementioned range, it is beneficial to provide a better lithium ion transmission channel, so that the first negative electrode active material in the first negative electrode active layer is better infiltrated by the electrolyte, so as to better play the role of the electronegative doping element in the electronegative coated active material in adsorbing foreign atoms, better combining the solvated lithium ions, and better promoting the embedding of lithium ions into the first negative electrode active material. When the porosity of the second negative electrode active layer is controlled within the aforementioned range, the electronegative doping element has a more obvious effect on improving the fast charging performance of the battery. By controlling the ionic conductivity of the electrolyte within the aforementioned range, it is beneficial to promote the rapid transmission of lithium ions, which is beneficial to better improve the battery dynamics and the fast charging performance of the battery.

[0058] In some embodiments, the negative electrode active material layer further includes a second negative electrode active layer, and the second negative electrode active layer is located between the negative electrode current collector and the first negative electrode active layer;

[0059] The lithium-ion secondary battery meets one or more of the following characteristics:

[0060] (t1) the porosity of the first negative electrode active layer is higher than the porosity of the second negative electrode active layer;

[0061] (t2) The ratio of the difference in compaction density of the second negative electrode active layer relative to the first negative electrode active layer to the compaction density of the negative electrode sheet is recorded as R PΔ , R PΔ is 0 to 2, optionally 0 to 0.834, further optionally, 0 <R PΔ ≤0.834;

[0062] (t3) The charge rate of the first negative electrode active layer is higher than that of the second negative electrode active layer; optionally, the charge rate of the first negative electrode active layer is higher than that of the second negative electrode active layer.

[0063] By controlling the porosity of the first negative electrode active layer to be higher than that of the second negative electrode active layer, it is beneficial to take into account both the fast charging performance and the energy density of the battery. The high porosity of the first negative electrode active layer can be used to promote the rapid transmission of lithium ions, while the low porosity of the second negative electrode active layer can be used to improve the energy density of the negative electrode.

[0064] The ratio of the difference in compaction density of the second negative electrode active layer relative to the first negative electrode active layer to the compaction density of the negative electrode sheet (R PΔ ) is controlled within the aforementioned range, which is beneficial for the particle stacking degree of the first negative electrode active layer to provide a better lithium ion transmission channel, thereby better improving the battery dynamics and fast charging performance; in addition, the second negative electrode active layer can also be used to provide a higher energy density, which is beneficial for taking into account both the battery fast charging performance and energy density.

[0065] By controlling the charge rate of the first negative electrode active layer to be higher than that of the second negative electrode active layer, it is beneficial to promote the faster embedding of lithium ions into the first negative electrode active layer, and the fast charging performance of the battery can be improved. By controlling the charge rate of the first negative electrode active layer to be higher than that of the second negative electrode active layer, it is beneficial to promote the faster embedding of lithium ions into the first negative electrode active layer, and the fast charging performance of the battery can be improved.

[0066] In some embodiments, the negative electrode active material layer further includes a second negative electrode active layer, the second negative electrode active layer is located between the negative electrode current collector and the first negative electrode active layer; the second negative electrode active layer includes a second negative electrode active material, and the negative electrode plate satisfies one or more of the following characteristics:

[0067] (te1) D of the second negative electrode active material v 50 is 10μm~20μm, and can be selected as 13μm~17μm;

[0068] (te2) D of the second negative electrode active material v 50 higher than the D of the first negative electrode active material v 50;

[0069] (te3) the compaction density of the second negative electrode active layer is higher than the compaction density of the first negative electrode active layer;

[0070] (te4) The powder compaction density of the second negative electrode active layer is higher than the powder compaction density of the first negative electrode active layer.

[0071] By making the lithium-ion secondary battery meet one or more of the characteristics (te1), (te2), (te3) and (te4), it is beneficial for the lithium-ion secondary battery to have improved fast charging performance while also taking into account energy density requirements.

[0072] By adding the D v 50 is controlled within the aforementioned range, which is beneficial for the second negative electrode active layer to obtain a higher compaction density, thereby helping to improve the energy density.

[0073] By controlling the D v 50 higher than the D of the first negative electrode active material v 50, which is beneficial to making the second negative electrode active layer obtain a higher compaction density, thereby helping to improve the energy density.

[0074] By regulating the compaction density of the second negative electrode active layer in the lithium ion secondary battery to be higher than that of the first negative electrode active layer, the energy density of the lithium ion secondary battery can be improved.

[0075] By controlling the powder compaction density of the second negative electrode active layer in the lithium-ion secondary battery to be higher than the powder compaction density of the first negative electrode active layer, it is beneficial to give the second negative electrode active layer a higher compaction density during the cold pressing process of the electrode sheet, so that the second negative electrode active layer in the lithium-ion secondary battery has a higher compaction density.

[0076] In some embodiments, the negative electrode active material layer further includes a second negative electrode active layer, and the second negative electrode active layer is located between the negative electrode current collector and the first negative electrode active layer; the negative electrode plate satisfies one or more of the following characteristics:

[0077] (tf1) based on one side of the negative electrode current collector, the ratio of the surface density of the first negative electrode active layer to the surface density of the second negative electrode active layer is 2:8 to 6:4, and can be 4:6 to 5:5;

[0078] (tf2) The ratio of the thickness of the first negative electrode active layer to the sum of the thickness of the first negative electrode active layer and the second negative electrode active layer on one side of the negative electrode current collector is denoted as F H , satisfying 20%≤F H ≤65%, optionally, 40%≤F H ≤60%;

[0079] (tf3) Based on a single side of the negative electrode current collector, the thickness of the first negative electrode active layer is 10 μm to 50 μm, and can be 20 μm to 40 μm.

[0080] By controlling the ratio of the surface density of the second negative electrode active layer to the surface density of the first negative electrode active layer within the aforementioned range, it is beneficial to better balance the fast charging performance and energy density of the battery.

[0081] By controlling the thickness of the first negative electrode active layer to satisfy one or more of the above characteristics (tf2) and (tf3), it is beneficial to better utilize the electronegative doping elements in the coating layer to improve the fast charging performance of the battery while also taking into account the energy density of the battery.

[0082] In some embodiments, the surface density of the negative electrode sheet is 5 mg / cm 2 ~15mg / cm 2 .

[0083] By controlling the surface density of the negative electrode plate within the aforementioned range, it is beneficial to take into account both the fast charging performance and energy density of the battery.

[0084] In some embodiments, the lithium-ion secondary battery further includes a positive electrode plate, the positive electrode plate includes a positive electrode active layer, the positive electrode active layer includes a positive electrode active material, and the positive electrode active material includes one or more of a lithium phosphate active material and a lithium composite metal oxide active material.

[0085] By introducing lithium-containing phosphate active materials into the positive electrode active materials, it is beneficial to improve the structural stability of the positive electrode active materials during the charge and discharge cycle process, which is beneficial to extend the cycle life of the battery.

[0086] Introducing lithium composite metal oxide active materials into the positive electrode active material is beneficial to improving the positive electrode and battery energy density.

[0087] In some embodiments, the positive electrode active material includes a lithium-containing phosphate active material, and the positive electrode active material satisfies one or more of the following characteristics:

[0088] (tg1) the mass proportion of the lithium-phosphate active material in the positive electrode active layer is greater than or equal to 80%, and can be 80% to 97%;

[0089] (tg2) the lithium phosphate active material includes one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon;

[0090] (tg3) The lithium phosphate active material includes a lithium phosphate active body and a carbon coating layer located on at least a portion of the surface of the lithium phosphate active body, and the carbon coating layer includes one or more of soft carbon, hard carbon and amorphous carbon.

[0091] By controlling the mass proportion of lithium-phosphate active materials in the positive electrode active layer within the aforementioned range, it is beneficial to better extend the cycle life of the battery.

[0092] The types of lithium phosphate active materials can be flexibly selected to meet different application requirements.

[0093] By setting a carbon coating layer including one or more of soft carbon, hard carbon and amorphous carbon on the surface of the lithium phosphate active material, the conductivity of the material can also be improved, which is beneficial to improving the electrical contact network within the positive electrode plate and providing a fast and stable channel for electron transmission within the positive electrode plate, thereby helping to improve the battery's rate performance and improve the battery's fast charging capability.

[0094] In a second aspect of the present application, a method for preparing a lithium ion secondary battery is provided, which can be used to prepare the lithium ion secondary battery of the first aspect of the present application.

[0095] In some embodiments, a method for preparing a lithium-ion secondary battery is provided, comprising preparing a negative electrode sheet;

[0096] The preparation of the negative electrode sheet comprises the following steps:

[0097] A negative electrode active material layer is arranged on at least one side of the negative electrode current collector; wherein the negative electrode active material layer comprises a first negative electrode active layer, the first negative electrode active layer comprises a first negative electrode active material, the first negative electrode active material comprises an electronegative coated active material, the electronegative coated active material comprises a negative electrode active body and a coating layer located on at least a portion of the surface of the negative electrode active body; the coating layer comprises doped carbon, the doped carbon comprises a carbon matrix and an electronegative doping element, the carbon matrix comprises one or more of soft carbon and hard carbon, and the Pauling electronegativity scale of the electronegative doping element is denoted as x 1 , the Pauling electronegativity scale of carbon is denoted by χ C , then χ 1 With χ C The absolute value of the difference between them satisfies 0.03≤|χ 1 -x C |≤0.49.

[0098] In the prepared lithium-ion secondary battery, an electronegative coated active material is arranged in the negative electrode active material layer in the negative electrode plate. As described above, the electronegative doping elements and the carbon matrix in the coating layer can promote faster and more lithium ions to be embedded in the negative electrode active material through synergistic effects, which is beneficial to improve battery dynamics and enhance the battery's fast charging capability.

[0099] In some embodiments, a method for preparing a lithium-ion secondary battery is provided, comprising preparing a negative electrode sheet;

[0100] The preparation of the negative electrode sheet comprises the following steps:

[0101] A second negative electrode active layer and a first negative electrode active layer are sequentially arranged on at least one side of the negative electrode current collector; wherein the first negative electrode active layer comprises a first negative electrode active material, and the first negative electrode active material comprises an electronegative coated active material, and the definition of the electronegative coated active material is as described above.

[0102] In the prepared lithium-ion secondary battery, a first negative electrode active layer located in the upper layer and a second negative electrode active layer located in the lower layer are provided in the negative electrode active material layer of the negative electrode sheet. An electronegative coating active material is provided in the first negative electrode active layer located in the upper layer. As described above, the electronegative doping element and the carbon matrix in the coating layer can promote the faster and more lithium-ion insertion into the first negative electrode active material through synergistic effects, and are also beneficial to improving the electrolyte wettability in the second negative electrode active layer based on the improvement of the electrolyte wettability in the first negative electrode active layer; based on the above-mentioned multiple effects, it is beneficial to improve battery kinetics and enhance the fast charging ability of the battery.

[0103] In some embodiments, the carbon matrix in the doped carbon includes soft carbon, and the electronegative coating active material is prepared by a method including the following steps:

[0104] In the presence of a doping precursor, the negative electrode active body is mixed with a soft carbon precursor, and doping heat treatment and soft carbonization heat treatment are carried out to enable the soft carbon precursor and the doping precursor to jointly form doped soft carbon covering at least a part of the surface of the negative electrode active body; wherein, the doping precursor includes the electronegative doping element.

[0105] In some embodiments, the preparation method of the electronegative coating active material satisfies one or more of the following characteristics:

[0106] (th1) The doping precursor includes an N-containing precursor, and the N-containing precursor includes one or more of urea and ammonia;

[0107] (th2) The doping precursor includes an S-containing precursor, and the S-containing precursor includes one or more of sulfur and sulfur vapor;

[0108] (th3) The doping precursor includes a P-containing precursor, and the P-containing precursor includes one or more of red phosphorus and phosphorus vapor;

[0109] (th4) The doping heat treatment includes N doping treatment, and the conditions for carrying out the N doping treatment include: the heating rate is 2 °C / min to 10 °C / min, the holding temperature is 400 °C to 800 °C, and the holding time is 1 h to 6 h;

[0110] (th5) The doping heat treatment includes S doping treatment, and the conditions for carrying out the S doping treatment include: the heating rate is 2 °C / min to 10 °C / min, the holding temperature is 200 °C to 600 °C, and the holding time is 1 h to 6 h;

[0111] (th6) the doping heat treatment includes P doping treatment, and the step of performing the heat treatment includes: performing one or both of O doping treatment and S doping treatment and then performing the P doping treatment, and the conditions for performing the P doping treatment include: a heating rate of 2°C / min to 10°C / min, a holding temperature of 200°C to 600°C, and a holding time of 1h to 6h; wherein, the conditions for performing the O doping treatment include: heating at a heating rate of 2°C / min to 10°C / min in an air atmosphere, and performing the holding treatment at 400°C to 800°C in a nitrogen atmosphere, and the holding time is 1h to 6h;

[0112] (th7) In the steps of performing the doping heat treatment and the soft carbonization heat treatment, the soft carbon precursor includes asphalt;

[0113] (th8) The conditions for performing the soft carburization heat treatment include: performing a heat preservation treatment in an inert gas atmosphere at 700°C to 1300°C.

[0114] In some embodiments, the carbon matrix in the doped carbon includes hard carbon, and the electronegative coating active material is prepared by a method comprising the following steps:

[0115] Coating at least a portion of the surface of the negative electrode active body with hard carbon, introducing a doping source including an electronegative doping element, and introducing the electronegative doping element into the hard carbon by a gas replacement method to form doped hard carbon coated on at least a portion of the surface of the negative electrode active body;

[0116] Optionally, the gas replacement method uses one of ammonia gas, sulfur vapor and phosphorus vapor.

[0117] In a fourth aspect of the present application, an electrical device is provided, comprising at least one of the lithium ion secondary battery described in the first aspect of the present application and the lithium ion secondary battery prepared by the method for preparing the lithium ion secondary battery described in the second aspect of the present application.

[0118] In a fifth aspect of the present application, there is provided a use of the lithium ion secondary battery described in the first aspect of the present application in supplying electric energy and / or storing electric energy;

[0119] The application includes a process of charging the lithium-ion secondary battery at a rate greater than or equal to 2C;

[0120] Optionally, the application includes a process of charging the lithium-ion secondary battery under at least one rate condition of 2C to 6C;

[0121] Optionally, the application includes a process of charging the lithium-ion secondary battery under at least one rate condition of 2C to 4C or 4C to 6C;

[0122] Optionally, the maximum charging rate of the lithium-ion secondary battery is greater than or equal to 2C, and may be 2C to 6C, and may be further 2C to 4C or 4C to 6C.

[0123] The lithium-ion secondary battery provided in the first aspect of the present application can provide higher rate performance and good fast charging capability.

[0124] The details of one or more implementations or embodiments of the present application are set forth in the following drawings and description. Other features, objects and advantages of the present application will become apparent from the description, drawings and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0125] In order to better describe and illustrate the embodiments, examples or examples provided by the present application, reference may be made to one or more drawings. The additional details or examples used to describe the drawings should not be considered as limiting the scope of the disclosed application, the embodiments, examples or examples currently described, and any of the best modes of these applications currently understood. Moreover, the same reference numerals are used throughout the drawings to represent the same components. In the drawings:

[0126] Figure 1 It is a schematic diagram of a battery cell according to an embodiment of the present application.

[0127] Figure 2 for Figure 1 An exploded view of a battery cell according to an embodiment of the present application is shown.

[0128] Figure 3 FIG. 1 is a schematic diagram of a battery device according to an embodiment of the present application.

[0129] Figure 4 A schematic diagram of a battery pack according to an embodiment of the present application.

[0130] Figure 5 for Figure 4 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0131] Figure 6 A schematic diagram of an electrical device using a lithium-ion secondary battery according to an embodiment of the present application as a power source.

[0132] Description of reference numerals:

[0133] 1. Battery pack; 2. Upper box; 3. Lower box; 4. Battery device; 5. Battery cell; 51. Shell; 52. Electrode assembly; 53. Cover plate; 6. Electrical device. DETAILED DESCRIPTION

[0134] Below, some embodiments and examples of the lithium-ion secondary battery and its preparation method, electrical device and application of the present application are described in detail with appropriate reference to the drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0135] "Scope" disclosed in the present application can be limited in the form of lower limit and upper limit, and a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The scope limited in this way can be including end values ​​or excluding end values, and any end value can be included or not included independently, and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are also listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "ab" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation for these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to listing the parameter as, for example, integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For example, when a parameter is expressed as an integer selected from "2-10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9 and 10.

[0136] In this application, unless otherwise specified, "about" means within a reasonable range above and below the number, and the fluctuation range may vary depending on the type and value of the number. For example, it may be allowed to be within a range of ±10%, ±5%, ±2%, ±1%, etc. For example, taking "about 20°C" and its approximate value of ±1°C as an example, the approximate values ​​of 19°C, 19.5°C, etc. within the approximate range shown in "about 20°C" should also be included in the range indicated by "about 20°C".

[0137] In the present application, when "multiple", "multiple", "multiple", "several", etc. are involved, unless otherwise specified, it means that the number is greater than 2 or equal to 2. For example, "one or more" means one or ≥ (greater than or equal to) two. It is understood that when "any number" of items are involved, it means any suitable combination of multiple items, that is, the combination of "any number" of items is carried out in a way that does not conflict and can implement the present application.

[0138] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0139] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment or implementation of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments. The "implementation methods" mentioned herein have a similar understanding.

[0140] In the present application, in the open technical features or technical solutions described by the words "contain", "include", "include", etc., if there is no other explanation, additional members other than the listed members are not excluded, and it can be regarded as providing both closed features or solutions consisting of the listed members and open features or solutions including additional members in addition to the listed members. For example, a includes a1, a2 and a3. If there is no other explanation, it may also include other members or may not include additional members. It can be regarded as providing both the feature or solution of "a consists of a1, a2 and a3" or "a is selected from a1, a2 and a3", and the feature or solution of "a includes not only a1, a2 and a3, but also other members".

[0141] In the present application, unless otherwise specified, M (such as m1) means that m1 is a non-limiting example of M, and it can be understood that M is not limited to m1.

[0142] In this application, "optionally", "optional", and "optional" mean optional, that is, any one of the two parallel solutions of "yes" or "no". If there are multiple "optional" in a technical solution, unless otherwise specified and there is no contradiction or mutual restriction, each "optional" is independent. Unless otherwise specified, the descriptions of "optionally include", "optionally include", etc. in this application, taking "optionally include" as an example, mean "may include or not include".

[0143] In the present application, unless otherwise specified, the features or solutions corresponding to "and / or" include any one of two or more related listed items, and also include any and all combinations of the related listed items, wherein any and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "M and / or N" means a group consisting of M, N, and "a combination of M and N". Among them, "including M and / or N" can mean "including M, including N, and including M and N", and can also mean "including M, including N, or including M and N", which can be properly understood according to the sentence in which it is located.

[0144] Herein, the word “suitable” in “suitable combination”, “suitable method”, etc., shall be based on the technical solution that can implement the present application.

[0145] Herein, "preferred", "better", "better", "comparatively better", "preferred" and "preferred" are only used to describe implementation methods or examples with better effects, and it should be understood that they do not constitute a limitation on the scope of protection of this application. If multiple "preferred" items appear in a technical solution, unless otherwise specified and there is no contradiction or mutual restriction, each "preferred" item is independent.

[0146] In the present application, “further”, “furthermore”, “particularly”, “for example”, “such as”, “example”, etc. are used for descriptive purposes to indicate differences in content, but should not be understood as limiting the scope of protection of the present application.

[0147] In the present application, the terms "first", "second", "third", "fourth", etc. in "the first aspect", "the second aspect", "the third aspect", "the fourth aspect", etc. are used only for descriptive purposes and cannot be understood as indicating or implying relative importance or quantity, nor can they be understood as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first", "second", "third", "fourth", etc. only serve the purpose of non-exhaustive enumeration and description, and it should be understood that they do not constitute a closed limitation on quantity.

[0148] In this application, unless otherwise clearly specified and limited, the term "connection" and other terms should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection. For ordinary technicians in this field, the appropriate meanings of the above terms in this application can be understood according to the circumstances.

[0149] In the present application, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. In the present application, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean a relative positional relationship in terms of horizontal height, or may mean that there is an attachment relationship without limiting the relative positional relationship in terms of horizontal height.

[0150] In the present application, the term "room temperature" generally refers to 4°C to 35°C, and may refer to 20°C±5°C. In some embodiments or examples of the present application, room temperature refers to 20°C to 30°C.

[0151] In this application, when the units of a data range are mentioned, if there is a unit only after the right endpoint, it means that the units of the left endpoint and the right endpoint are the same. For example, 3-5μm or 3-5μm both mean that the units of the left endpoint "3" and the right endpoint "5" are both μm (micrometers), which have the same meaning as 3μm-5μm. In addition, similar descriptions of other parameters such as temperature and size are also understood in the same way.

[0152] In this application, "greater than or equal to" and "≥" have the same meaning and can be used interchangeably; "less than or equal to" and "≤" have the same meaning and can be used interchangeably; "greater than" can be equivalently expressed as ">", and "less than" can be equivalently expressed as "<". In this application, unless otherwise specified, "greater than or equal to" and "≥" can be regarded as providing two solutions of "greater than" and "equal to". In this application, unless otherwise specified, "less than or equal to" and "≤" can be regarded as providing two solutions of "less than" and "equal to".

[0153] In the present application, exemplary descriptions such as "in some implementation modes (or examples)" and "in one implementation mode (or example)" may include but are not limited to the following meanings: these solutions may be combined with other solutions in a suitable manner to form new technical solutions.

[0154] The improved effects described in this application, unless otherwise stated, are not intended to be limited to any theory.

[0155] In the present application, for a certain parameter, when two or more test methods are provided, as long as the test result of at least one test method is within the described range, they are all included in the protection scope of the present application.

[0156] At present, the demand for fast charging performance of lithium-ion secondary batteries is getting higher and higher, which requires lithium ions to be able to be quickly transmitted in the negative electrode plate. However, in traditional lithium-ion secondary batteries, the part of the negative electrode plate away from the surface of the negative electrode plate is not well wetted by the electrolyte, which affects the further improvement of the battery's fast charging performance.

[0157] According to various embodiments and examples of the present application, the present application provides a lithium ion secondary battery and a preparation method thereof, an electric device and an application thereof. The lithium ion secondary battery comprises a negative electrode plate and an electrolyte, the negative electrode plate comprises a negative electrode active material layer, and the negative electrode active material layer comprises an electronegative coated active material. The lithium ion secondary battery has significantly improved fast charging performance.

[0158] It can be understood that the negative electrode active material in the negative electrode active material layer includes an electronegative coating active material.

[0159] In some embodiments, the lithium ion secondary battery comprises a negative electrode plate and an electrolyte; the negative electrode plate comprises a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector; the negative electrode active material layer comprises an electronegative coated active material, the electronegative coated active material comprises a coating layer; the coating layer comprises doped carbon, the doped carbon comprises a carbon matrix and an electronegative doping element, the carbon matrix comprises one or more of soft carbon and hard carbon, and the Pauling electronegativity scales of the electronegative doping element and the carbon element are respectively denoted as x 1 , χ C , satisfying 0.03≤|χ 1 -x C |≤0.49. The negative electrode active material layer includes a first negative electrode active layer, and may further include a second negative electrode active layer located between the negative electrode current collector and the first negative electrode active layer. The lithium ion secondary battery has significantly improved fast charging performance.

[0160] It is understood that the first negative electrode active layer includes an electronegative coating active material.

[0161] In some embodiments, the negative electrode active material layer includes a first negative electrode active layer, the first negative electrode active layer includes a first negative electrode active material, and the first negative electrode active material includes an electronegative coating active material.

[0162] In some embodiments, the electronegative coated active material includes a negative electrode active body and a coating layer located on at least a portion of the surface of the negative electrode active body, the coating layer includes doped carbon, the doped carbon includes a carbon matrix and electronegative doping elements, and the carbon matrix includes one or more of soft carbon and hard carbon.

[0163] In this application, unless otherwise specified, the term "lithium-ion secondary battery" refers to a secondary battery whose active ions include lithium ions, and "lithium-ion battery cell" refers to a battery cell whose active ions include lithium ions. Typically, a lithium-ion secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. During the battery charge and discharge process, active ions are embedded and removed back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, which mainly prevents the positive and negative electrodes from short-circuiting, while allowing ions to pass through.

[0164] In this application, unless otherwise specified, the "electrode active material layer" includes at least one of the positive electrode active material layer of the positive electrode sheet and the negative electrode active material layer of the negative electrode sheet. Depending on the specific circumstances, the electrode active material layer may refer to the positive electrode active material layer or the negative electrode active material layer. In this application, the "positive electrode active material layer" may also be recorded as the "positive electrode active layer", and the "negative electrode active material layer" may also be recorded as the "negative electrode active layer".

[0165] In the present application, the term "negative electrode sheet" includes a negative electrode active material layer, and the negative electrode active material layer includes a negative electrode active material. The term "negative electrode active material" refers to a material used in a negative electrode sheet that can reversibly insert and extract active ions.

[0166] In this application, unless otherwise specified, "negative electrode sheet" includes a negative electrode current collector. "Negative electrode current collector" refers to a structure responsible for collecting and conducting electrons at the negative electrode. In the negative electrode sheet, the negative electrode active layer is located on at least one side of the negative electrode current collector, and can be located on one side or both sides of the negative electrode current collector.

[0167] In the present application, the term "positive electrode sheet" includes a positive electrode active material layer, and the positive electrode active material layer includes a positive electrode active material. The term "positive electrode active material" refers to a material used in a positive electrode sheet that can reversibly extract and insert active ions.

[0168] In this application, unless otherwise specified, "positive electrode sheet" includes positive electrode current collector. "Positive electrode current collector" refers to the structure responsible for collecting and conducting electrons at the positive electrode. In the positive electrode sheet, the positive electrode active layer is located on at least one side of the positive electrode current collector, and can be located on one side or both sides of the positive electrode current collector.

[0169] In the present application, unless otherwise specified, "separation membrane" and "diaphragm" have the same meaning and can be used interchangeably.

[0170] In a first aspect of the present application, a lithium ion secondary battery is provided.

[0171] In some embodiments, a lithium ion secondary battery is provided, comprising a negative electrode plate and an electrolyte; the negative electrode plate comprises a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector, the negative electrode active material layer comprising a first negative electrode active layer;

[0172] The first negative electrode active layer includes a first negative electrode active material, the first negative electrode active material includes an electronegative coating active material, the electronegative coating active material includes a negative electrode active body (which may be referred to as a first active body) and a coating layer located on at least a portion of a surface of the negative electrode active body;

[0173] The coating layer includes doped carbon, the doped carbon includes a carbon matrix and an electronegative doping element, the carbon matrix may include one or more of soft carbon and hard carbon, and the Pauling electronegativity scale of the electronegative doping element is denoted by x 1 , the Pauling electronegativity scale of carbon is denoted by χ C , then χ 1 With χ C The absolute value of the difference between them satisfies 0.03≤|χ 1 -x C |≤0.49.

[0174] In some embodiments, the negative electrode active material layer also includes a second negative electrode active layer, and the second negative electrode active layer is located between the negative electrode collector and the first negative electrode active layer; in this case, a lithium-ion secondary battery is provided, which includes a negative electrode plate and an electrolyte; the negative electrode plate includes a negative electrode collector and a second negative electrode active layer and a first negative electrode active layer sequentially arranged on at least one side of the negative electrode collector.

[0175] In the present application, the "negative electrode active material layer" in the negative electrode sheet may include one or more negative electrode active layers, that is, the negative electrode active material layer may be a single-layer structure or a multi-layer structure. Unless otherwise specified, the negative electrode active material layer includes at least a first negative electrode active layer. In some embodiments, the negative electrode active material layer is a single-layer structure, including only one negative electrode active layer. In this case, the negative electrode active material layer is the first negative electrode active layer.

[0176] In the present application, unless otherwise specified, "electronegative coated active material" belongs to the category of negative electrode active material, which includes a negative electrode active body and a coating layer located on at least a portion of the surface of the negative electrode active body; the "negative electrode active body" can be recorded as a "first active body" and has the ability to reversibly embed and release active ions; the coating layer includes doped carbon.

[0177] In the present application, unless otherwise specified, "doped carbon" includes a carbon matrix and an electronegative doping element. Generally, in electronegative coated active materials, the carbon matrix of the doped carbon in the coating layer is a different substance from the negative electrode active body. In some embodiments, the carbon matrix may include one or more of soft carbon and hard carbon. Doped carbon whose carbon matrix is ​​soft carbon may be recorded as "doped soft carbon", and it can be understood that the doped soft carbon includes soft carbon and electronegative doping elements; doped carbon whose carbon matrix is ​​hard carbon may be recorded as "doped hard carbon", and it can be understood that the doped hard carbon includes hard carbon and electronegative doping elements. Doped carbon may include one or more of doped soft carbon and doped hard carbon.

[0178] In this application, "soft carbon" and "hard carbon" have well-known meanings in the art. Soft carbon can be graphitized after further high-temperature treatment, while hard carbon is difficult to graphitize even after further high-temperature treatment. The carbon layers in soft carbon are stacked disorderly, so that lithium ions can be transported relatively quickly. The crystals inside hard carbon are arranged in a disordered manner and have many pores, which can provide abundant lithium insertion sites and fast transmission channels.

[0179] In this application, unless otherwise specified, 1 -x C A pair of “|” in | is used to indicate an absolute value, which is a positive number.

[0180] In this application, unless otherwise specified, "Pauling electronegativity scale" has a well-known meaning in the art. It is a parameter proposed by American chemist Linus Pauling, which can be used to reflect the level of the ability of atoms of a certain type of element to attract electrons, and can be used to characterize the electronegativity of the element. The larger the Pauling electronegativity scale, the stronger the ability of the corresponding atom to attract electrons and the stronger the electronegativity. The Pauling electronegativity scale of each element can be consulted in a chemical handbook or known literature. In the Pauling electronegativity scale, the electronegativity of fluorine is defined as 4.0, which is the highest electronegativity of all elements. In addition, illustratively, the Pauling electronegativity scales of carbon (C), nitrogen (N), sulfur (S) and phosphorus (P) are 2.55, 3.04, 2.58 and 2.19, respectively.

[0181] In some embodiments of the lithium-ion secondary battery, an electronegative coated active material is provided in the negative electrode active material layer in the negative electrode plate, wherein the coating layer is provided with doped carbon including electronegative doping elements, and the carbon matrix in the doped carbon may include one or more of soft carbon and hard carbon; on the one hand, the introduced electronegative doping elements have special Pauling electronegativity scaling characteristics, thereby improving the polarization characteristics of the surface of the negative electrode active material, enhancing the adsorption of the negative electrode active material to foreign atoms, and weakening the binding force between the negative electrode active material and the solvated lithium ions, thereby reducing the contact angle between the surface of the negative electrode active material and the electrolyte, and improving the wettability to the electrolyte, It is beneficial to accelerate the rate of lithium ion embedding into the negative electrode active material; on the other hand, both the soft carbon and the hard carbon in the carbon matrix have a large degree of disorder, which is beneficial to the entry of lithium ions. The carbon layers in the soft carbon are disordered and can make lithium ions transmit relatively quickly. The hard carbon can provide abundant lithium embedding sites and fast transmission channels, which is beneficial to promote faster and more lithium ion embedding into the negative electrode active material; on the other hand, the introduction of electronegative doping elements is also beneficial to the formation of inorganic components in the solid electrolyte interface (SEI) film, which can enhance the lithium conductivity of the SEI film and reduce the interface impedance; based on the aforementioned multiple effects, it is beneficial to improve battery dynamics and enhance the fast charging capability of the battery.

[0182] In some embodiments of the lithium-ion secondary battery, a first negative electrode active layer located in the upper layer and a second negative electrode active layer located in the lower layer are provided in the negative electrode active material layer in the negative electrode plate (the direction away from the surface of the negative electrode current collector is the upper layer, and the direction toward the surface of the negative electrode current collector is the lower layer). Furthermore, an electronegative coated active material is provided in the first negative electrode active layer located in the upper layer, wherein the coating layer is provided with doped carbon including electronegative doping elements, and the carbon matrix in the doped carbon may include one or more of soft carbon and hard carbon. On the one hand, the introduced electronegative doping elements have special Pauling electronegativity scaling characteristics, thereby improving the polarization characteristics of the surface of the negative electrode active material, enhancing the adsorption of the negative electrode active material to foreign atoms, and weakening the binding force between the negative electrode active material and the solvated lithium ions, thereby making the surface of the first negative electrode active material and the electrolyte between the electrolyte and the negative electrode active material. The contact angle is reduced, and the wettability to the electrolyte is improved, which is beneficial to accelerating the rate of lithium ion embedding into the first negative electrode active material; on the other hand, both the soft carbon and the hard carbon in the carbon matrix have a large degree of disorder, which is beneficial to the entry of lithium ions. The carbon layers in the soft carbon are disordered, which can make lithium ions transmit relatively quickly therein, and the hard carbon can provide abundant lithium embedding sites and fast transmission channels, thereby facilitating faster and more lithium ion embedding into the first negative electrode active material; on the other hand, the introduction of electronegative doping elements is also beneficial to the formation of inorganic components in the solid electrolyte interface (SEI) film, which can enhance the lithium conductivity of the SEI film and reduce the interface impedance; on the other hand, the improved wettability of the electrolyte in the first negative electrode active layer is also beneficial to improving the wettability of the electrolyte in the second negative electrode active layer; based on the aforementioned multiple effects, it is beneficial to improve battery dynamics and enhance the fast charging capability of the battery.

[0183] In this application, unless otherwise specified, in the thickness direction of the negative electrode sheet, for the negative electrode active material layer, the direction close to the surface of the negative electrode sheet is recorded as "upper", and the direction away from the surface of the negative electrode sheet is recorded as "lower". Taking the negative electrode sheet including the negative electrode current collector as an example, the direction away from the surface of the negative electrode current collector is the upper direction, and the direction toward the surface of the negative electrode current collector is the lower direction.

[0184] In the present application, the cross section of the negative electrode sheet can be subjected to microscopic morphology observation to observe the microscopic morphology of each negative electrode active layer in the negative electrode active material layer in the negative electrode sheet and the boundary between different negative electrode active layers, and then the thickness of different negative electrode active layers can be determined. "Cross section of the negative electrode sheet" refers to a cross section perpendicular to the thickness of the negative electrode sheet. Further, the cross section of the negative electrode sheet can also be subjected to microscopic morphology observation and combined with component analysis (such as energy dispersive spectrometer (EDS) etc.) to identify the element type to confirm the composition of different negative electrode active layers in the negative electrode active material layer. Non-limitingly, instruments or equipment including but not limited to focused electron beam (FIB) electron microscope (non-limiting examples such as FEI Scios 2HiVac equipment, etc.), ion cross section polisher (non-limiting examples such as IB-09010CP argon ion cross section polisher, IB-19500CP ion cross section polisher, etc. of JEOL, Japan) can be used to obtain the cross section of the negative electrode sheet, and the cross section of the negative electrode sheet can also be obtained by plasma quenching method. The microscopic morphology observation method can use instruments or equipment including but not limited to scanning electron microscopy (SEM) technology. Without limitation, a high-resolution field emission scanning electron microscope can be used; non-limiting examples of SEM instruments include the Sigma 300 scanning electron microscope and Apreo 2SEM field emission scanning electron microscope of ZEISS Company of Germany.

[0185] Those skilled in the art can identify the components in the negative electrode active material layer, the first negative electrode active layer, and the second negative electrode active layer by one or more of the following detection methods known in the art, including but not limited to: Fourier transform infrared (FT-IR) spectroscopy, ultraviolet spectroscopy, nuclear magnetic resonance spectroscopy (HNMR), 1 H NMR) method, gel permeation chromatography (GPC) method, high performance liquid chromatography (HPLC) method, mass spectrometry, X-ray photoelectron spectroscopy (XPS) method, X-ray diffraction (XRD) method, Raman spectroscopy (Raman) method, single crystal X-ray diffraction (SCXRD) method, inductively coupled plasma spectroscopy (ICP) method, energy dispersive spectrometer (EDS) analysis, etc. The sample preparation methods and test methods of these test methods are known to those skilled in the art, and the test parameters can be appropriately adjusted according to the structure of the material or substance and the sample characteristics.

[0186] As a non-limiting example, EDS can be used to distinguish carbon from silicon, and then to distinguish carbon-based materials from silicon-based materials. For another example, EDS can be used to detect the type and content of the conductive agent, but is not limited thereto.

[0187] Taking the negative electrode active materials including natural graphite and artificial graphite as non-limiting examples, natural graphite and artificial graphite can be distinguished by the appearance morphology of the particles; X-ray diffraction (XRD) analysis test can be further performed. In the XRD spectrum, if the characteristic peak near 2θ26.5° is very sharp and has a high intensity, it is natural graphite; if the characteristic peak near 2θ26.5° is relatively wide and has a weak intensity, it is artificial graphite.

[0188] Taking the negative electrode active materials including graphite and soft carbon as an example, Raman spectroscopy can be used to distinguish graphite and soft carbon; in detail, the characteristic peak information of the carbon component in the spectrum (such as the intensity ratio of D peak / G peak, I D / G ) to analyze soft carbon. Both D peak and G peak are Raman characteristic peaks of carbon atom crystal. D peak represents the defects of carbon atom crystal. The more defects there are, the greater the intensity of D peak is. The intensity of D peak can reflect the content of amorphous (turbostratified stacking) area. G peak represents the in-plane stretching vibration of sp2 hybridization of carbon atoms. The intensity of G peak can reflect the content of graphitized (layered structure) area. As the degree of disorder of carbon atoms increases, the intensity ratio of D peak to G peak also increases. You can also compare Raman spectra I D / G Standard Raman spectrum of graphite I D / G The difference between the D peak and the G peak of the Raman spectrum can be used to distinguish between graphite and hard carbon. Similarly, the difference between the D peak and the G peak of the Raman spectrum can be used to distinguish between natural graphite and artificial graphite.

[0189] In this application, unless otherwise specified, the constituent materials of the negative electrode active material layer are recorded as negative electrode materials, the constituent materials of the first negative electrode active layer are recorded as first negative electrode materials, and the constituent materials of the second negative electrode active layer are recorded as second negative electrode materials. The first negative electrode material includes the first negative electrode active material, and the second negative electrode material includes the second negative electrode active material. The "first" and "second" in "first negative electrode material", "second negative electrode material", "first negative electrode active material", and "second negative electrode active material" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or quantity, nor can they be understood as implicitly indicating the importance or quantity of the indicated technical features.

[0190] The test sample of the first negative electrode active material can be obtained by the following method to detect whether it includes an electronegative coated active material: disassemble the battery cell to obtain the negative electrode plate, soak and clean it with a solvent (such as dimethyl carbonate (DMC) etc.) to remove the residual electrolyte; scrape the powder from the first negative electrode active layer of the negative electrode plate to obtain the first negative electrode material; for example, a certain amount of powder sample can be scraped from a position close to the surface of the negative electrode plate, and a sample with a thickness of 10 μm can be scraped. Further, a solvent (such as N-methylpyrrolidone (NMP) etc.) can be used to fully soak the obtained first negative electrode material to dissolve the organic components therein such as binders, thickeners, etc., and ultrasonic dispersion can be optionally combined to promote dissolution, and then washed and filtered, and the collected solid phase is used as the powder to be tested for the first negative electrode active material. It should be noted that the small amount of conductive agent that may be contained in the test sample has little effect on the test results. Similarly, a certain amount of powder sample in the negative electrode active material layer can be scraped from a position close to the negative electrode current collector to obtain the second negative electrode active material (such as a sample with a thickness of 10 μm), and further subjected to similar treatment to obtain a sample to be tested of the second negative electrode active sample.

[0191] The following method can be used to detect and analyze the coating structure of the electronegative coated active material; after cutting the powder using FIB (focused ion beam), the particle cross-sectional morphology is observed under TEM (transmission electron microscope), and a clear boundary can be observed at the coating interface. The thickness and average thickness of the coating layer can be analyzed and calculated based on the TEM image; further combined with one or more methods such as energy dispersive spectroscopy (EDS) analysis, Raman spectroscopy, X-ray diffraction (XRD) method, the material types in the coating layer and the negative electrode active body can be confirmed respectively.

[0192] The elemental composition and component types of the coating layer in the electronegative coated active material can be analyzed by or with reference to the aforementioned method. As a non-limiting example, the detection of the type of electronegative doping elements can be carried out by methods including but not limited to X-ray photoelectron spectroscopy (XPS). The following method can be used: the negative electrode active material sample is converted into an aerosol, introduced into a high-temperature plasma, and the type and content of the electronegative doping element are quantitatively analyzed using the characteristic spectrum generated after the element is ionized. Non-limiting examples of electronegative doping elements include nitrogen (N), phosphorus (P) and sulfur (S).

[0193] In some embodiments, 0.03≤|χ 1 -x C |≤0.49, optionally, 0.04≤|χ 1 -x C |≤0.49. Without limitation, |χ 1 -x C| It can also be any of the following values ​​or a range consisting of any two of the following values: 0.03, 0.04, 0.05, 0.06, 0.08, 0.1, 0.12, 0.14, 0.15, 0.16, 0.18, 0.2, 0.22, 0.23, 0.24, 0.25, 0.26, 0.28, 0.3, 0.32, 0.34, 0.35, 0.36, 0.38, 0.4, 0.42, 0.44, 0.45, 0.46, 0.48, 0.49, etc.

[0194] By controlling the numerical value of the difference between the Pauling electronegativity scale of the electronegative doping element and the carbon element within the aforementioned range, the Pauling electronegativity scale of the electronegative doping element is controlled within a more appropriate range. On the one hand, it is beneficial to reduce the contact angle between the surface of the negative electrode active material and the electrolyte, and to improve the wettability of the electrolyte to the first negative electrode active material. On the other hand, by controlling the difference in electron-withdrawing ability between the electronegative doping element and the carbon atom within a more appropriate range, it is beneficial to form a stable covalent bond between the electronegative doping element and the carbon atom.

[0195] In some embodiments, the electronegative doping element is covalently doped into the carbon matrix. Unless otherwise specified, "the electronegative doping element is covalently doped into the carbon matrix" means that the electronegative doping element is covalently doped into the carbon skeleton of the carbon matrix.

[0196] In some embodiments, at least a portion of the electronegative doping element is covalently bonded to multiple (eg, 2 or 3) carbon atoms simultaneously.

[0197] Optionally, the carbon matrix can be one or more of soft carbon and hard carbon. In some embodiments, the electronegative doping element is covalently doped into the soft carbon. In some embodiments, the electronegative doping element is covalently doped into the hard carbon.

[0198] In some embodiments, 80% to 100% of the electronegative doping elements are covalently bonded to multiple (such as 2 or 3) carbon atoms at the same time. In a non-limiting manner, the proportion of "electronegative doping elements covalently bonded to 2 or 3 carbon atoms at the same time" in the "electronegative doping elements in the electronegative coated active material" can be 80% to 100%, optionally 90% to 100%, or any of the following percentages or a range selected from any two of the following percentages: 80%, 82%, 84%, 85%, 86%, 88%, 90%, 92%, 94%, 95%, 96%, 98%, 99%, 100%, etc.

[0199] By doping electronegative doping elements into the carbon matrix in a covalently bonded manner, the stability of the electronegative doping elements in the coating layer can be improved, which is beneficial to improving the stability of the electrolyte wettability and the battery cycle stability.

[0200] In some embodiments, the electronegative doping element in the doped carbon may include one or more of N, P, and S.

[0201] N, P, S, etc. all have different electronegativity relative to C atoms and can bond with C atoms.

[0202] Nitrogen (N) atoms have a larger electronegativity difference relative to C atoms, and are easier to bond with C atoms. At the same time, more defects are formed due to the adsorption of foreign atoms. Taking the combination of N and carbon matrix as an example, there may be but are not limited to the following three bonding methods: pyridinic nitrogen, pyrrolic nitrogen and graphitic nitrogen. Among them, the higher reactivity of pyridinic nitrogen and pyrrolic nitrogen is conducive to better improving the adsorption of foreign atoms by the first negative electrode active material, which is conducive to better reducing the resistance to lithium ion mass transfer, and is more conducive to improving the wettability of the electrolyte to the first negative electrode active layer, thereby improving the wettability of the second negative electrode active layer, which is conducive to better improving the fast charging performance of the battery.

[0203] In the present application, “foreign atoms” adsorbable by the electronegative doping element refer to atoms from outside the negative electrode active material, and generally may include atoms from the electrolyte, for example, may include electrolyte anions from the electrolyte.

[0204] The surface polarization caused by sulfur (S) element doping is lower than that of N element. The process difficulty of introducing S element doping is higher than that of N element doping. The doping amount of N element can be controlled to be higher than that of S element.

[0205] Phosphorus (P) element is not easy to bond with C element, and usually at least one element of O and S needs to be introduced to bond with the element to form COP or CSP form.

[0206] In some embodiments, among the electronegative doping elements of the doped carbon, the doping amount of N is higher than the doping amount of P, and is also higher than the doping amount of S.

[0207] In some embodiments, the doped carbon satisfies one or more of the following characteristics:

[0208] (ta1) the electronegative doping element includes a bridging N atom, the bridging N atom being covalently bonded to at least one carbon atom;

[0209] (ta2) the electronegative doping element includes a bridging S atom, wherein the bridging S atom is covalently bonded to at least one carbon atom;

[0210] (ta3) The electronegative doping elements include bridging-type P atoms, and any covalent sites of at least a portion of the bridging-type P atoms are independently covalently bonded in a COP or CSP manner; optionally, any covalent sites of 80% to 100% of the bridging-type P atoms are independently covalently bonded in a COP or CSP manner.

[0211] In the present application, unless otherwise specified, a “bridging atom” refers to an atom that is covalently bonded to at least two adjacent atoms, such that the bridging atom exhibits a covalent bond valence state of two or more.

[0212] In some embodiments, in the doped carbon, the electronegative doping element includes a bridging N atom covalently bonded to at least one carbon atom.

[0213] In some embodiments, in the doped carbon, the electronegative doping element includes a bridging S atom covalently bonded to at least one carbon atom.

[0214] In some embodiments, in the doped carbon, the electronegative doping element includes a bridging P atom, and any covalent site of at least a portion of the bridging P atoms is independently covalently bonded in the form of COP or CSP. In some embodiments, 80% to 100% of the number of bridging P atoms are independently covalently bonded in the form of COP or CSP. In a non-limiting manner, the proportion of "bridging P atoms covalently bonded in the form of COP or CSP at any covalent site" in the "bridged P atoms in the electronegative coated active material" can be 80% to 100%, optionally 90% to 100%, and can also be any of the following percentages or a range selected from any two of the following percentages: 80%, 82%, 84%, 85%, 86%, 88%, 90%, 92%, 94%, 95%, 96%, 98%, 99%, 100%, etc.

[0215] In some embodiments, the electronegative doping element includes N element, and the doping form of the electronegative doping element in the doped carbon includes at least one of pyridinic nitrogen type and pyrrolic nitrogen type.

[0216] By controlling the doping form of the electronegative doping element in the doped carbon to include at least one of the pyridinic nitrogen type and the pyrrolic nitrogen type, the high reactivity of the doped N atoms based on the pyridinic nitrogen and pyrrolic nitrogen is conducive to better improving the adsorption of the first negative electrode active material to foreign atoms, and is conducive to better reducing the lithium ion mass transfer resistance, and is more conducive to improving the wettability of the electrolyte to the first negative electrode active layer, thereby improving the wettability of the second negative electrode active layer, and thus is conducive to better improving the fast charging performance of the battery. In addition, it is also conducive to reducing the difficulty of the doping process.

[0217] In some embodiments, the electronegative coating active material satisfies one or more of the following characteristics:

[0218] (tb1) The average thickness of the coating layer is 1 nm to 500 nm, and may be 100 nm to 500 nm, and may be further 100 nm to 200 nm;

[0219] (tb2) the thickness of at least a portion of the coating layer is 1 nm to 1000 nm, optionally 100 nm to 1000 nm, and further optionally 100 nm to 200 nm;

[0220] (tb3) the mass percentage of the electronegative doping element in the doped carbon is 0.1% to 0.6%, optionally 0.2% to 0.6%, and further optionally 0.2% to 0.4%;

[0221] (tb4) the mass percentage of the electronegative doping element in the coating layer is 0.1% to 0.6%, optionally 0.2% to 0.6%, and further optionally 0.2% to 0.4%;

[0222] (tb5) The mass percentage of the coating layer in the electronegative coating active material is 0.2% to 5%, and can be optionally 0.5% to 3%;

[0223] (tb6) The average thickness of the coating layer and the D of the first negative electrode active material v The ratio of 50 is 0.5% to 12.5%, and can be selected as 1% to 10%;

[0224] (tb7) The mass percentage of the doped carbon in the coating layer is 80% to 100%, and can be optionally 90% to 100%;

[0225] (tb8) The sum of the mass percentages of soft carbon and hard carbon in the carbon matrix is ​​80% to 100%, and can be optionally 90% to 100%; optionally, the mass percentage of soft carbon in the carbon matrix is ​​80% to 100%, and can be further optionally 90% to 100%, or the mass percentage of hard carbon in the carbon matrix is ​​80% to 100%, and can be further optionally 90% to 100%.

[0226] Without limitation, the average thickness of the coating layer in the electronegative coated active material can be 1nm to 500nm, optionally 100nm to 500nm, further optionally 100nm to 200nm, and can also be any of the following thicknesses or a range consisting of any two of the following thicknesses: 5nm, 10nm, 20nm, 40nm, 50nm, 60nm, 80nm, 90nm, 100nm, 120nm, 140nm, 150nm, 160nm, 180nm, 200nm, 250nm, 300nm, 400nm, 500nm, etc.

[0227] Without limitation, in the electronegative coated active material, the thickness of at least a portion of the coating layer can be 1nm to 1000nm, optionally 100nm to 1000nm, further optionally 100nm to 200nm, and can also be any of the following thicknesses or a range consisting of any two of the following thicknesses: 5nm, 10nm, 20nm, 40nm, 50nm, 60nm, 80nm, 90nm, 100nm, 120nm, 140nm, 150nm, 160nm, 180nm, 200nm, 250nm, 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, etc.

[0228] Without limitation, in the electronegative coated active material, the mass percentage of the electronegative doping element in the doped carbon can be 0.1% to 0.6%, optionally 0.2% to 0.6%, further optionally 0.2% to 0.4%, and can also be any of the following percentages or a range consisting of any two of the following percentages: 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, etc.

[0229] Without limitation, in the electronegative coated active material, the mass percentage of the electronegative doping element in the coating layer can be 0.1% to 0.6%, optionally 0.2% to 0.6%, further optionally 0.2% to 0.4%, and can also be any of the following percentages or a range consisting of any two of the following percentages: 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, etc.

[0230] Without limitation, the mass percentage of the coating layer in the electronegative coated active material can be 0.2% to 5%, optionally 0.5% to 3%, or any of the following percentages or a range consisting of any two of the following percentages: 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.25%, 1.5%, 1.6%, 1.8%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, etc.

[0231] In a non-limiting manner, the average thickness of the coating layer in the electronegative coating active material is about the D v The ratio of 50 can be 0.5% to 12.5%, can be optionally 1% to 10%, can be further optionally 2% to 8%, can also be any of the following percentages or a range consisting of any two of the following percentages: 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.25%, 1.5%, 1.6%, 1.8%, 2%, 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 12.5%, etc.

[0232] Without limitation, the mass percentage of doped carbon in the coating layer can be 80% to 100%, optionally 90% to 100%, or any of the following percentages or a range consisting of any two of the following percentages: 80%, 82%, 84%, 85%, 86%, 88%, 90%, 94%, 95%, 96%, 98%, 100%, etc.

[0233] Without limitation, the sum of the mass percentages of soft carbon and hard carbon in the carbon matrix can be 80% to 100%, optionally 90% to 100%, or any of the following percentages or a range consisting of any two of the following percentages: 80%, 82%, 84%, 85%, 86%, 88%, 90%, 94%, 95%, 96%, 98%, 100%, etc.

[0234] In some embodiments, the mass percentage of soft carbon in the carbon matrix can be 80% to 100%, optionally 90% to 100%, or any of the following percentages or a range consisting of any two of the following percentages: 80%, 82%, 84%, 85%, 86%, 88%, 90%, 94%, 95%, 96%, 98%, 100%, etc.

[0235] In other embodiments, the mass percentage of hard carbon in the carbon matrix can be 80% to 100%, optionally 90% to 100%, or any of the following percentages or a range consisting of any two of the following percentages: 80%, 82%, 84%, 85%, 86%, 88%, 90%, 94%, 95%, 96%, 98%, 100%, etc.

[0236] Without limitation, the sum of the mass percentages of doped soft carbon and doped hard carbon in the doped carbon or coating layer can be 80% to 100%, optionally 90% to 100%, or any of the following percentages or a range consisting of any two of the following percentages: 80%, 82%, 84%, 85%, 86%, 88%, 90%, 94%, 95%, 96%, 98%, 100%, etc.

[0237] In some embodiments, the mass percentage of doped soft carbon in the doped carbon or coating layer can be 80% to 100%, optionally 90% to 100%, or any of the following percentages or a range consisting of any two of the following percentages: 80%, 82%, 84%, 85%, 86%, 88%, 90%, 94%, 95%, 96%, 98%, 100%, etc.

[0238] In other embodiments, the mass percentage of doped hard carbon in the doped carbon or coating layer can be 80% to 100%, optionally 90% to 100%, or any of the following percentages or a range consisting of any two of the following percentages: 80%, 82%, 84%, 85%, 86%, 88%, 90%, 94%, 95%, 96%, 98%, 100%, etc.

[0239] The average thickness of the coating layer, the local thickness of the coating layer, the mass percentage of the electronegative doping element in the doped carbon, the mass percentage of the electronegative doping element in the coating layer, the average thickness of the coating layer and the D v 50, the sum of the mass percentage of doped carbon in the coating layer and the mass percentage of soft carbon and hard carbon in the carbon matrix, and one or more of the mass percentages of doped soft carbon and doped hard carbon in the doped carbon or coating layer, the mass percentage of doped soft carbon in the doped carbon or coating layer, and the mass percentage of doped hard carbon in the doped carbon or coating layer are controlled within the aforementioned range, which is beneficial for controlling the content of electronegative doping elements within a more appropriate range, which is beneficial for improving the fast charging performance of the battery based on the aforementioned multiple effects, and is also beneficial for reducing the probability of electronegative doping elements participating in side reactions, and is beneficial for achieving good battery cycle performance, but is not limited to the aforementioned theory.

[0240] In some embodiments, the amount of the electronegative coated active material in the first negative electrode active material is 20% to 100%, optionally 40% to 100%, or any of the following percentages or a range consisting of any two of the following percentages: 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, etc. By controlling the amount of the electronegative coated active material in the first negative electrode active material within the aforementioned range, it is beneficial to better improve the battery dynamics based on the multiple effects of the electronegative coated active material and enhance the fast charging capability of the battery.

[0241] Detection and analysis methods such as scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and X-ray photoelectron spectroscopy (XPS) can be used in combination to identify the electrochemically negative coated active material, and then the proportion of the electrochemically negative coated active material in the first negative electrode active material can be statistically obtained.

[0242] In some embodiments, the proportion of the electrochemically negative coated active material in the first negative electrode active material is 20% to 80%, further optionally 40% to 80%, and can also be any one of the following percentages or a range composed of any two of the following percentages: 20%, 30%, 40%, 50%, 60%, 70%, 80%, etc. By controlling the proportion of the electrochemically negative coated active material in the first negative electrode active material within the foregoing range, it is beneficial to better improve the fast charging ability of the battery and also beneficial to take into account the manufacturing cost.

[0243] In some embodiments, the first negative electrode active material includes secondary particles; further, the first negative electrode active material may optionally further include non-agglomerated primary particles. In some of these embodiments, the proportion of the secondary particles in the first negative electrode active material is greater than or equal to 20%, optionally 30% to 80%, further optionally 30% to 60%, and can also be any one of the following percentages or a range composed of any two of the following percentages: 20%, 30%, 40%, 50%, 60%, 70%, 80%, etc.

[0244] By introducing secondary particles into the first negative electrode active material, it is beneficial to increase the surface roughness of the active particles in the first negative electrode active material, increase the sites for adsorbing lithium ions, and further beneficial to improve the wettability of the electrolyte, improve the wettability of the electrolyte to the first negative electrode active layer, and can also improve the wettability to the second negative electrode active layer, thereby better improving the fast charging performance of the battery.

[0245] In this application, unless otherwise specified, the "primary particle" in the electrode active material is the basic unit of the particles in the electrode active material. It can be understood that there are primary particles in the electrode active material. Taking the negative electrode as an example, in the negative electrode active material, the primary particles can be in a non-agglomerated state or form aggregates. The non-agglomerated primary particles can be called "non-agglomerated primary particles", and the aggregates of primary particles can be called "secondary particles". For those skilled in the art, non-agglomerated primary particles and secondary particles can be identified based on the particle morphology diagram of the electrode active material. Taking the negative electrode active material as an example, the particle morphology diagram of the electrode active material can be obtained by using the test results of a scanning electron microscope (such as ZEISS Sigma 300), and the sample to be tested can be obtained by laying and adhering the electrode active material on a conductive adhesive.

[0246] In some embodiments, the electronegative coated active material includes electronegative coated secondary particles, and the negative active body in the electronegative coated secondary particles is a secondary particle.

[0247] In the present application, "electronegative coated secondary particles" refer to electronegative coated active materials whose negative electrode active body is a secondary particle. It can be understood that the electronegative coated secondary particles include a negative electrode active body and a coating layer located on at least a portion of the surface of the negative electrode active body, and the negative electrode active body is a secondary particle. The definition of the coating layer can refer to the coating layer in the electronegative coated active material. Electronegative coated secondary particles also belong to the category of secondary particles.

[0248] By introducing electronegative coated secondary particles into the first negative electrode active material, it is beneficial to increase the roughness of the surface of the active particles in the electronegative coated active material, increase the sites for adsorbing lithium ions, and further improve the wettability of the electrolyte to the first negative electrode active layer, and further improve the wettability of the electrolyte to the second negative electrode active layer, thereby better improving the fast charging performance of the battery.

[0249] In a non-limiting manner, the amount of electronegative coated secondary particles in the first negative electrode active material may be greater than or equal to 20%, optionally 20% to 100%, optionally 40% to 100%, or any of the following percentages or a range selected from any two of the following percentages: 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, etc. By controlling the amount of electronegative coated secondary particles in the first negative electrode active material within the aforementioned range, it is beneficial to better improve the fast charging performance of the battery.

[0250] In some embodiments, the amount of electronegative coated secondary particles in the first negative electrode active material may be 20% to 80%, optionally 40% to 80%, or any of the following percentages or a range consisting of any two of the following percentages: 20%, 30%, 40%, 50%, 60%, 70%, 80%, etc. By controlling the amount of electronegative coated secondary particles in the first negative electrode active material within the aforementioned range, it is beneficial to better improve the fast charging performance of the battery and also beneficial to take into account the manufacturing cost.

[0251] In some embodiments, in the electronegative coated active material, the negative electrode active body includes one or more of a carbon-based active material and a silicon-based active material.

[0252] It can be understood that the electronegative coated active material whose negative electrode active body includes carbon-based active material belongs to carbon-based material, and the electronegative coated active material whose negative electrode active body includes silicon-based active material belongs to silicon-based material.

[0253] Without limitation, the carbon-based active material may include one or more of artificial graphite, natural graphite, soft carbon, and hard carbon.

[0254] Without limitation, the carbon-based active material may include graphite, and further, the graphite may include one or more of artificial graphite and natural graphite.

[0255] Without limitation, the silicon-based active material may include one or more of elemental silicon, silicon-oxygen compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys.

[0256] In some embodiments, in the electronegative coated active material, the negative electrode active body includes a carbon-based active material. Further, the negative electrode active body includes one or more of graphite, soft carbon, and hard carbon, and may further include one or more of artificial graphite, natural graphite, soft carbon, and hard carbon. Optionally, the electronegative coated active material includes electronegative coated secondary particles.

[0257] Without limitation, the negative electrode active body in the electronegative coated active material may include one or more of a carbon-based active material and a silicon-based active material. In this case, the first negative electrode active material includes one or more of a carbon-based material and a silicon-based material. The electronegative coated active material may include electronegative coated secondary particles. When the negative electrode active body includes a carbon-based active material, it is beneficial to make the first negative electrode active material have better conductivity, which is beneficial to reduce the internal resistance and provide better fast charging capability. When the negative electrode active body includes a silicon-based active material, it is beneficial to utilize the high gram capacity of the silicon-based active material to increase the amount of lithium embedded per unit time, which is beneficial to increase the fast charging speed. In addition, it is also beneficial to increase the energy density.

[0258] In some embodiments, the first negative electrode active material includes one or more of a carbon-based material and a silicon-based material.

[0259] In some embodiments, the first negative electrode active material includes a carbon-based material. In a non-limiting manner, the mass proportion of the carbon-based material in the first negative electrode active material may be 20% to 100%, optionally 20% to 80%, further optionally 40% to 80%, or any of the following percentages or a range selected from any two of the following percentages: 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, etc.

[0260] By controlling the mass proportion of the carbon-based material in the first negative electrode active material within the aforementioned range, it is beneficial to better control the volume expansion of the negative electrode during fast charging, improve the structural stability of the first negative electrode active material during the charge and discharge cycle, and also take into account the energy density of the negative electrode and the battery.

[0261] In some embodiments, the negative electrode active body in the electronegative coated active material includes graphite. Further, the graphite may include one or more of artificial graphite and natural graphite.

[0262] In some embodiments, the negative electrode active body in the electronegative coated active material includes graphite and soft carbon, and further, the graphite may include artificial graphite, in which case the negative electrode active body includes artificial graphite and soft carbon. Non-limitingly, the mass percentage of soft carbon relative to graphite may be 1% to 10%, optionally 1% to 5%, or any of the following percentages or a range selected from any two of the following percentages: 1%, 2%, 3%, 4%, 5%, 6%, 8%, 10%, etc.

[0263] In some embodiments, the negative electrode active body in the electronegative coated active material includes graphite and hard carbon, and further, the graphite may include artificial graphite, in which case the negative electrode active body includes artificial graphite and hard carbon. Non-limitingly, the mass percentage of hard carbon relative to graphite may be 1% to 10%, optionally 1% to 5%, or any of the following percentages or a range selected from any two of the following percentages: 1%, 2%, 3%, 4%, 5%, 6%, 8%, 10%, etc.

[0264] In some embodiments, the mass proportion of the electronegative coated active material of the negative electrode active body including graphite in the first negative electrode active material can be 20% to 100%, optionally 20% to 80%, further optionally 40% to 80%, and can also be any of the following percentages or a range selected from any two of the following percentages: 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, etc.

[0265] In some embodiments, the mass proportion of the negative electrode active body including the electronegative coated active material of soft carbon in the first negative electrode active material can be 0-10%, optionally 1%-10%, further optionally 1%-5%, and can also be any of the following percentages or a range selected from any two of the following percentages: 0%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 8%, 10%, etc.

[0266] In some embodiments, the mass proportion of the negative electrode active body including the electronegative coated active material of hard carbon in the first negative electrode active material can be 0-10%, optionally 1%-10%, further optionally 1%-5%, and can also be any of the following percentages or a range composed of any two of the following percentages: 0%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 8%, 10%, etc.

[0267] In some embodiments, the lithium-ion secondary battery satisfies one or more of the following characteristics (any numerical parameter in the following characteristics can also be selected from any suitable numerical value or range in the context):

[0268] (td1) the negative electrode active body in the electronegative coated active material includes graphite;

[0269] (td2) the electronegative coated active material includes electronegative coated secondary graphite particles, the negative electrode active body in the electronegative coated secondary graphite particles includes secondary graphite particles, and the amount of the electronegative coated secondary graphite particles in the first negative electrode active material accounts for 20% to 100%, optionally 40% to 100%, and further optionally 40% to 80%;

[0270] (td3) D of the first negative electrode active material v 50 is 8μm~16μm, and can be selected as 10μm~14μm;

[0271] (td4) the porosity of the first negative electrode active layer is 22% to 32%, and can be optionally 26% to 30%;

[0272] (td5) Under at least one temperature condition between 20°C and 35°C, the ionic conductivity of the electrolyte is 13mS / cm to 18mS / cm; optionally, at 25°C, the ionic conductivity of the electrolyte is 13mS / cm to 18mS / cm.

[0273] By making the lithium-ion secondary battery satisfy one or more of the characteristics (td1), (td2), (td3), (td4) and (td5), the fast charging performance of the battery can be better improved.

[0274] In some embodiments, the negative active body in the electronegative coated active material includes graphite. In this case, the electronegative coated active material includes electronegative coated graphite. In some embodiments, the negative active body in the electronegative coated active material is graphite, that is, a graphite body.

[0275] In the present application, a "graphite body" consists of graphite.

[0276] In the present application, the electronegative coated active material whose negative electrode active body is graphite can be recorded as "electronegative coated graphite"; the electronegative coated graphite includes graphite and a coating layer located at least on a portion of the graphite surface. The definition of the coating layer in the electronegative coated graphite can refer to the coating layer in the electronegative coated active material.

[0277] Without limitation, the amount of electronegative coated graphite in the first negative electrode active material can be 20% to 100%, optionally 40% to 100%, further optionally 40% to 80%, and can also be any of the following percentages or a range consisting of any two of the following percentages: 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, etc.

[0278] By introducing graphite into the negative active body in the electronegative coated active material, it is beneficial to make the electronegative coated active material have better conductivity, which is beneficial to reduce the internal resistance and provide better fast charging capability.

[0279] In some embodiments, the electronegative coated active material includes electronegative coated secondary graphite particles. In this case, the negative active body in the electronegative coated active material includes secondary graphite particles. In some embodiments, the electronegative coated active material is electronegative coated secondary graphite particles.

[0280] In the present application, "secondary graphite particles" refer to aggregates of primary graphite particles, and "primary graphite particles" are basic graphite grain units.

[0281] In the present application, the electronegative coated active material whose negative electrode active body is secondary graphite particles can be recorded as "electronegative coated secondary graphite particles"; the electronegative coated secondary graphite particles include secondary graphite particles and a coating layer located at least partially on the surface of the secondary graphite particles. The definition of the coating layer in the electronegative coated secondary graphite particles can refer to the coating layer in the electronegative coated active material.

[0282] Without limitation, the amount of electronegative coated secondary graphite particles in the first negative electrode active material can be 20% to 100%, optionally 40% to 100%, further optionally 40% to 80%, and can also be any of the following percentages or a range consisting of any two of the following percentages: 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, etc.

[0283] By introducing electronegative coated secondary graphite particles into the electronegative coated active material, it is beneficial to take into account the advantages of both the electronegative coated secondary particles and the graphite body.

[0284] Without limitation, the first negative electrode active material may include a soft carbon coated secondary particle graphite material (at this time, the electronegative coated active material includes a soft carbon coated secondary particle graphite material); the soft carbon coated secondary particle graphite material includes secondary particle graphite and a coating layer located at least a portion of the surface of the secondary particle graphite, and the coating layer includes doped soft carbon. Doped soft carbon is doped carbon whose carbon matrix is ​​soft carbon. Without limitation, the amount of soft carbon coated secondary particle graphite material in the first negative electrode active material can be 20% to 100%, optionally 40% to 100%, further optionally 40% to 80%, and can also be any of the following percentages or a range selected from any two of the following percentages: 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, etc.

[0285] Without limitation, the first negative electrode active material may include a hard carbon coated secondary particle graphite material (at this time, the electronegative coated active material includes a hard carbon coated secondary particle graphite material); the hard carbon coated secondary particle graphite material includes secondary particle graphite and a coating layer located at least a portion of the surface of the secondary particle graphite, and the coating layer includes doped hard carbon. Doped hard carbon is doped carbon whose carbon matrix is ​​hard carbon. Without limitation, the amount of hard carbon coated secondary particle graphite material in the first negative electrode active material can be 20% to 100%, optionally 40% to 100%, further optionally 40% to 80%, and can also be any of the following percentages or a range selected from any two of the following percentages: 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, etc.

[0286] In some embodiments, D of the first negative electrode active material v 50 is 8μm~16μm, and can be optionally 10μm~14μm. It can also be any of the following values ​​or a range consisting of any two of the following values: 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, etc.

[0287] In this application, unless otherwise specified, D v 50 refers to the particle size corresponding to when the cumulative volume distribution percentage of the material reaches 50%. This parameter indicates that the particle size of 50% of the material volume is less than or equal to D v 50, and 50% of the particles by volume are larger than D v 50. Those skilled in the art will appreciate that v50, and can be measured by instruments and methods known in the art. For example, it can be conveniently measured by a laser particle size analyzer, such as the Mastersizer 2000E laser particle size analyzer of Malvern Instruments Ltd., UK, and the LS-909 laser particle size analyzer (OMEC). The D of the material can be determined by reference to the GB / T19077-2016 / ISO 13320:2009 standard process. v 50 for testing. The detailed test process includes: taking an appropriate amount of the sample to be tested, adding a solvent (the solvent can be deionized water, and the sample concentration can be controlled at 8% to 12% shading), ultrasonic treatment for 5 minutes (53KHz / 120W) to fully disperse the sample, and then measuring the sample in accordance with GB / T19077-2016 / ISO 13320:2009 standard. After the sample is poured into the injection tower, it circulates with the solution to the test optical path system. When the particles are irradiated by the laser beam, the particle size distribution characteristics of the particles can be obtained by receiving and measuring the energy distribution of the scattered light. The particle size volume distribution diagram is drawn according to the test data, and D is obtained from the distribution diagram. v 50. In order to avoid the influence of agglomeration during the drying process on the particle size test, the wet sample after washing was taken for dispersion test. D of the first negative electrode active material and the second negative electrode active material v 50 can be tested using the above method.

[0288] By adding a negative electrode active material D v 50 is controlled within the aforementioned range, which is beneficial to better control the degree of particle stacking in the first negative electrode active layer, is beneficial to better control the pores between particles, provides a better lithium ion transmission channel, and better improves the battery dynamics and fast charging performance.

[0289] In some embodiments, the porosity of the negative electrode active material layer is 20% to 32%, optionally 22% to 32%, further optionally 26% to 30%, and can also be any of the following percentages or a range consisting of any two of the following percentages: 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 32%, etc.

[0290] By controlling the porosity of the negative electrode active material layer within the aforementioned range, it is beneficial to provide a better lithium ion transmission channel, so that the negative electrode active material is better infiltrated by the electrolyte, thereby better exerting the role of the electronegative doping elements in the electronegative coated active material in adsorbing foreign atoms, and better promoting the embedding of lithium ions into the negative electrode active material.

[0291] In some embodiments, in the negative electrode sheet of a lithium-ion secondary battery or in the negative electrode sheet obtained after cold pressing, the porosity of the first negative electrode active layer is 22% to 32%, optionally 26% to 30%, or any of the following percentages or a range selected from any two of the following percentages: 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 32%, etc.

[0292] In the present application, "porosity" refers to the ratio of the pore volume in the material to the total volume, which can be expressed as a percentage. The "porosity" of the second negative electrode active layer refers to the ratio of the pore volume in the second negative electrode active layer to the volume of the second negative electrode active layer. The "porosity" of the first negative electrode active layer refers to the ratio of the pore volume in the first negative electrode active layer to the volume of the first negative electrode active layer. The "porosity" of the negative active material layer of the negative electrode sheet refers to the ratio of the pore volume in the negative active material layer to the volume of the entire negative active material layer, which can also be recorded as the "porosity of the negative electrode sheet".

[0293] By controlling the porosity of the first negative electrode active layer within the aforementioned range, it is beneficial to provide a better lithium ion transmission channel, so that the first negative electrode active material in the first negative electrode active layer is better infiltrated by the electrolyte, so as to better play the role of the electronegative doping element in the electronegative coating active material in adsorbing foreign atoms, better combining the solvated lithium ions, and better promoting the embedding of lithium ions into the first negative electrode active material. When the porosity of the second negative electrode active layer is controlled within the aforementioned range, the electronegative doping element has a more obvious effect on improving the fast charging performance of the battery.

[0294] In the present application, the porosity (δ N ) can be measured using instruments and methods known in the art. For example, GB / T 24586-2009 can be referred to and the gas displacement method can be used for measurement. The porosity (δ N ):

[0295] Porosity δ N =(V N2 -V N1 ) / V N2 × 100%, where V N2 =S N ×D N ×n.

[0296] The test sample is n negative electrode discs; S N is the area of ​​the negative electrode active material layer in a single negative electrode disc, in cm 2 ;D N , the thickness of the negative electrode active material layer in a single negative electrode disc, in cm; VN1 , true volume of the test sample, cm 3 ; V N2 , the apparent volume of the test sample, in cm 3 .

[0297] Pretreatment: Punch the negative electrode discs. In a drying room, use tweezers to select ≥20 discs with good appearance and no powder falling off the edges and put them into the sample cup. The test sample is n negative electrode discs. Record the number of discs n and calculate the apparent volume V of the test sample. N2 .

[0298] True volume V N1 Test: Place the sample cup containing the test sample in the true density tester, close the test system, and introduce helium according to the program. By detecting the pressure of the gas in the sample chamber and the expansion chamber, the real volume is calculated according to Bohr's law (PV=nRT), thereby obtaining the porosity of the test sample, which can be recorded as the porosity of the negative electrode active material layer (δ N ).

[0299] The above method can not only test the negative electrode sheets taken out after disassembling the battery cell, but also test the negative electrode sheets obtained after cold pressing.

[0300] For example, the porosity (δ N ) and the porosity of the second negative electrode active layer (δ 2 ), when δ N Greater than δ 2 When the porosity of the first negative electrode active material layer is greater than that of the second negative electrode active layer, it can be considered that “the porosity of the first negative electrode active material layer is higher than that of the second negative electrode active layer”. N and the thickness D of the second negative electrode active layer 2 , the thickness D of the first negative electrode active layer 1 , and then according to formula D N ×δ N =D 1 ×δ 1 +D 2 ×δ 2 , the porosity of the first negative electrode active layer (δ 1 ).

[0301] For example, the porosity (δ 2 ) can be obtained by testing as follows:

[0302] (1) Obtain a sample to be tested including at least a portion of the second negative electrode active layer and excluding the first negative electrode active layer: disassemble the battery cell, take the negative electrode plate, and use a micrometer to test the plate thickness L; take another negative electrode plate, wipe off the negative electrode active material layer on both sides of the surface to leave the remaining empty collector foil, and test the thickness L0, then the total thickness of the negative electrode active material layer on both sides of the negative electrode collector is L-L0; scrape the negative electrode active material layer on one surface of the negative electrode plate to expose the negative electrode collector surface, scrape the negative electrode material on the other surface until the plate thickness is L1=L0+Δd, and collect the powder of the Δd thickness portion of the negative electrode active material layer under the thickness of L1. The thickness of the second negative electrode active layer can be determined in advance in combination with the observation results of the cross-sectional morphology of the plate, and then the first negative electrode active layer is completely removed when the thickness is controlled to be L1, and the remaining negative electrode active material layer portion corresponds to part or all of the second negative electrode active layer. For example, Δd can be exemplarily 15μm, 20μm, etc. In the sample to be tested, only the second negative electrode active layer on one side of the negative electrode current collector is retained.

[0303] (2) The porosity of the second negative electrode active layer (δ 2 ), refer to the method for testing the porosity of the negative electrode sheet to test the porosity of the sample to be tested, and record it as the porosity of the second negative electrode active layer.

[0304] Exemplarily, the following method can also be used to test the porosity of the negative electrode active material layer in the negative electrode sheet, and the porosity of the first negative electrode active layer and the second negative electrode active layer can also be tested and / or compared: disassemble the battery cell and take the negative electrode sheet; punch the negative electrode sheet into small discs, and use the nano-space dynamic resolution and layer-by-layer cutting technology of FIB-SEM (focused electron beam electron microscope-scanning electron microscope) to reconstruct the three-dimensional structure of the sample, and use an energy dispersive spectrometer (EDS) to analyze the distribution and proportion of each element, and obtain the porosity of the first negative electrode active layer, the second negative electrode active layer and the overall negative electrode active material layer through software quantitative analysis. The FEI Scios 2HiVac device can be used for testing.

[0305] In some embodiments, under at least one temperature condition of 20° C. to 35° C., the ionic conductivity of the electrolyte may be 13 mS / cm to 18 mS / cm, or any of the following values ​​or a range consisting of any two of the following values: 13 mS / cm, 14 mS / cm, 15 mS / cm, 16 mS / cm, 17 mS / cm, 18 mS / cm, etc. The test temperature may be 25° C. Without limitation, at 25° C., the ionic conductivity of the electrolyte may be 13 mS / cm to 18 mS / cm, or any of the following values ​​or a range consisting of any two of the following values: 13 mS / cm, 14 mS / cm, 15 mS / cm, 16 mS / cm, 17 mS / cm, 18 mS / cm, etc.

[0306] In this application, unless otherwise specified, the "ionic conductivity" of the electrolyte has a well-known meaning in the art, and can be tested and analyzed using existing methods in the art. The ionic conductivity can be obtained by testing with a conductivity tester, such as a DDSJ-318 conductivity meter. The test can be performed with reference to the method of HG-T 4067-2015. The test temperature can be 25±0.1°C. Without limitation, the test can be performed using a method comprising the following steps:

[0307] Pretreatment: Take the standard liquid and keep it at a constant temperature of 25℃ (deviation ±0.1℃), and take the test liquid and keep it at a constant temperature of the test temperature (deviation ±0.1℃);

[0308] Test: Test the instrument with two standard solutions at 25°C. After calibration, clean the electrode and place the test sample electrode vertically into the liquid to be tested. Click to start the test and wait for the data to stabilize for more than 10 seconds to record the test results.

[0309] By controlling the ionic conductivity of the electrolyte within the aforementioned range, it is beneficial to promote the rapid transmission of lithium ions, which is beneficial to better improve battery dynamics and battery fast charging performance.

[0310] The ionic conductivity of the electrolyte of some embodiments may refer to the context of this application. An electrolyte with higher conductivity may be obtained by selecting a solvent with low viscosity, but is not limited thereto.

[0311] Without limitation, the electrolyte includes a non-aqueous solvent, the non-aqueous solvent includes a low-viscosity solvent, the low-viscosity solvent may include ethyl acetate, methyl acetate, dimethyl carbonate, ethyl methyl carbonate, and the like, as well as one or more solvents having a viscosity less than or equal to that of at least one of the aforementioned reagents at 25°C.

[0312] In this application, unless otherwise specified, the viscosity of the solvent or electrolyte can be tested by conventional methods in the art, and can be measured by instruments and methods known in the art, for example, the national standard GB / T10247-2008 "Viscosity Measurement Method" can be referred to, and the test can be performed based on the rotational viscometer in Appendix D of the national standard GB / T10247-2008. Non-limitingly, the viscosity of the solvent or electrolyte can be tested by the following method: a certain mass of the sample to be tested is placed in a sample container, and the rotational viscometer with the instrument model DV2TLV produced by Brookfield is used for testing.

[0313] In some embodiments, the negative electrode active material layer further includes a second negative electrode active layer, and the second negative electrode active layer is located between the negative electrode current collector and the first negative electrode active layer;

[0314] Lithium-ion secondary batteries meet one or more of the following characteristics:

[0315] (t1) the porosity of the first negative electrode active layer is higher than the porosity of the second negative electrode active layer;

[0316] (t2) The ratio of the difference in compaction density of the second negative electrode active layer relative to the first negative electrode active layer relative to the compaction density of the negative electrode sheet is recorded as R PΔ , R PΔ is 0 to 2, optionally 0 to 0.834, further optionally, 0 <R PΔ ≤0.834;

[0317] (t3) The charge rate of the first negative electrode active layer is higher than that of the second negative electrode active layer; optionally, the charge rate of the first negative electrode active layer is higher than that of the second negative electrode active layer.

[0318] In some embodiments, the porosity of the first negative electrode active layer is higher than the porosity of the second negative electrode active layer.

[0319] By controlling the porosity of the first negative electrode active layer to be higher than that of the second negative electrode active layer, it is beneficial to take into account both the fast charging performance and the energy density of the battery. The high porosity of the first negative electrode active layer can be used to promote the rapid transmission of lithium ions, while the low porosity of the second negative electrode active layer can be used to improve the energy density of the negative electrode.

[0320] In the present application, in a lithium-ion secondary battery, the ratio of the difference in compaction density of the second negative electrode active layer relative to the first negative electrode active layer relative to the compaction density of the negative electrode sheet can be recorded as R PΔ .

[0321] In some embodiments, R PΔ is 0 to 2, optionally 0 to 0.834, further optionally, 0 <R PΔ≤0.834, and can also be any of the following values ​​or a range consisting of any two of the following values: 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.075, 0.08, 0.085, 0.09, 0.095, 0.1, 0.11, 0.12, 0.125, 0.13, 0.133, 0.134, 0.135, 0.14, 0.1 45, 0.15, 0.155, 0.16, 0.17, 0.175, 0.18, 0.19, 0.2, 0.21, 0.22, 0.225, 0.23, 0.235, 0.24, 0.245, 0.25, 0.26, 0.265, 0.266, 0.27, 0.275, 0.28, 0.285, 0.286, 0.29, 0.3, 0.33, 0.34, 1 / 3、0.35、0.36、0.38、0.4、0.417、0.42、0.45、0.48、0.49、0.5、0.55、0.6、0.61、0.62、0.625、0.66、2 / 3、0.67、0.672、0.673、0.674、0.675、0.68、0.7、0.72、0.74、0.75、0.76、0.78、0 .80, 0.82, 0.83, 0.833, 5 / 6, 0.834, 0.85, 0.86, 0.87, 0.88, 0.90, 0.95, 1, 1.05, 1.08, 1.09, 1.1, 1.12, 1.15, 1.2, 1.3, 1.4, 1.5, 1.6, 5 / 3, 1.67, 1.8, 1.9, 1.95, 1.96, 1.98, 1.99, 2, etc.

[0322] In some embodiments, R PΔ Greater than 0.

[0323] Without limitation, R PΔ You can also select from any of the following ranges: 0 <R PΔ ≤2,0 <R PΔ ≤1.67, 0 <R PΔ ≤5 / 3, 0 <R PΔ ≤1.2, 0 <R PΔ ≤1.1,0 <R PΔ ≤1,0 <R PΔ ≤0.834, 0 <R PΔ ≤5 / 6, 0 <R PΔ ≤0.625, 0 <R PΔ ≤0.42, 0~1.67, 0 to 5 / 3, 0~1.2, 0~1.1, 0~1, 0 to 5 / 6, 0~0.625, 0~0.42, etc.

[0324] The ratio of the difference in compaction density of the second negative electrode active layer relative to the first negative electrode active layer to the compaction density of the negative electrode sheet (R PΔ ) is controlled within the aforementioned range, which is beneficial for the particle stacking degree of the first negative electrode active layer to provide a better lithium ion transmission channel, thereby better improving the battery dynamics and fast charging performance; in addition, the second negative electrode active layer can also be used to provide a higher energy density, which is beneficial for taking into account both the battery fast charging performance and energy density.

[0325] Compared with the negative electrode sheet obtained after cold pressing, the volume of the negative electrode sheet in the lithium-ion secondary battery will have a certain rebound, so that the compaction density of the negative electrode sheet in the lithium-ion secondary battery is lower than that of the negative electrode sheet obtained after cold pressing.

[0326] In the present application, in the negative electrode sheet obtained after cold pressing, the ratio of the difference in compaction density of the second negative electrode active layer relative to the first negative electrode active layer relative to the compaction density of the negative electrode sheet can be recorded as R PΔ0 . R PΔ0 With R PΔ When the volume expansion rate of the first negative electrode active layer and the second negative electrode active layer is equal, R PΔ0 With R PΔ The values ​​are theoretically equal.

[0327] In the present application, unless otherwise specified, "the negative electrode sheet obtained after cold pressing" refers to the state of the negative electrode sheet obtained after cold pressing just after the cold pressing. It can be considered that in this state, no or almost no rebound of the volume of the sheet has occurred.

[0328] In some embodiments, in the negative electrode sheet obtained after cold pressing, the compaction density of the first negative electrode active layer is 1.40 g / cm 3 ~1.60g / cm 3 The compaction density of the second negative electrode active layer is 1.65g / cm 3 ~1.90g / cm 3 .

[0329] In some embodiments, in a lithium-ion secondary battery, the difference between the compaction density of the second negative electrode active layer and the compaction density of the first negative electrode active layer (which can be denoted as P Δ ) is 0g / cm 3 ~0.5g / cm 3 , can be selected as 0g / cm 3 ~0.48g / cm 3 , further optional to 0.04g / cm 3 ~0.48g / cm 3 , can also be any of the following values ​​or a range consisting of any two of the following values: 0g / cm3 , 0.01g / cm 3 , 0.02g / cm 3 , 0.04g / cm 3 , 0.05g / cm 3 , 0.06g / cm 3 , 0.08g / cm 3 , 0.1g / cm 3 , 0.12g / cm 3 , 0.15g / cm 3 , 0.16g / cm 3 , 0.18g / cm 3 , 0.2g / cm 3 , 0.22g / cm 3 , 0.24g / cm 3 , 0.25g / cm 3 , 0.3g / cm 3 , 0.35g / cm 3 , 0.4g / cm 3 , 0.45g / cm 3 , 0.48g / cm 3 , 0.5g / cm 3 wait.

[0330] In some embodiments, the charge rate of the first negative electrode active layer is higher than that of the second negative electrode active layer; optionally, the charge rate of the first negative electrode active layer is higher than that of the second negative electrode active layer.

[0331] In the present application, the "rate" of the first negative electrode active layer and the second negative electrode active layer is a parameter reflecting the charge and discharge capabilities, and the "charging rate" is a parameter reflecting the charging capability. The higher the charging rate, the better the fast charging performance.

[0332] The following method can be used to prepare the first active layer electrode sheet and the second active layer electrode sheet: based on the negative electrode sheet obtained by cold pressing or the negative electrode sheet obtained by disassembling the battery cell, the first negative electrode material is extracted from the first negative electrode active layer, and the second negative electrode material is extracted from the second negative electrode active layer, and they are resuspended into uniform slurries with deionized water, respectively, and recorded as the first resuspended slurry and the second resuspended slurry. The second resuspended slurry is applied to one side surface of the negative electrode collector copper foil, dried, and cold pressed to obtain the second active layer electrode sheet. The first resuspended slurry is applied to one side surface of the negative electrode collector copper foil, dried, and cold pressed to obtain the first active layer electrode sheet. The difference between the first resuspended slurry and the second resuspended slurry is only the difference between the first negative electrode material and the second negative electrode material, and the coating weight, drying and cold pressing parameters are the same. The first active layer electrode sheet and the second active layer electrode sheet can be respectively combined with lithium sheets to form a buckle battery for rate testing. If the charge-withdrawing ratio of the first active layer electrode sheet is higher than that of the second active layer electrode sheet, it is considered that "the charge-withdrawing ratio of the first negative electrode active layer is higher than that of the second negative electrode active layer". The charge-withdrawing ratio test method available in the art can be used for testing.

[0333] In this application, unless otherwise specified, the rate or charge rate of the first negative electrode active layer and the second negative electrode active layer may be compared using the following method:

[0334] The first negative electrode active material, conductive carbon black (Super P), stabilizer sodium carboxymethyl cellulose and binder styrene-butadiene rubber (SBR) are dispersed in deionized water in a certain mass ratio (the mass ratio can be 97.3:0.7:1.2:0.8) to form a first slurry.

[0335] The second negative electrode active material, conductive carbon black (Super P), stabilizer sodium carboxymethyl cellulose and binder SBR are dispersed in deionized water according to a certain mass ratio (the mass ratio can be 96.4:1.0:1.2:1.4) to form a second slurry.

[0336] The first slurry and the second slurry are respectively coated on a single side surface of a copper foil current collector and dried in an oven for later use.

[0337] A metal lithium plate is used as a counter electrode; a polypropylene (PP) film is used as a separator; ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain a mixed solvent, and then lithium hexafluorophosphate (LiPF6) is dissolved in the above mixed solvent to obtain an electrolyte, wherein the concentration of LiPF6 is 1 mol / L.

[0338] All components were assembled into a CR2430 button cell in an argon-protected glove box. "CR" represents the international IEC number of button-type lithium manganese batteries, with a diameter of 24 mm and a thickness of 30 mm. After standing for 12 hours, the obtained button cell was discharged at a constant current of 0.05C to 0.005V, and then discharged at a constant current of 10μA to 0.005V. Then stand for 5 minutes, charge the obtained button cell at a constant current of 0.1C to 2V, stand for 5 minutes, and record the charging capacity C0. The battery was placed at a constant temperature of 25°C for 2h, and the charge and discharge tests were carried out at 1C0, 2C0, 3C0, 4C0, and 5C0 rates to obtain the capacity retention rate. Under the same cycle conditions and number of cycles, the higher the capacity retention rate, the better the rate performance, and it can be considered that the "rate" is higher.

[0339] In addition, using the same charging conditions and charging at the same rate to the same SOC (such as 80% SOC, the cut-off current can be 0.01C), the shorter the time used, the better the charging rate performance, and it can be considered that the "charging rate" is higher.

[0340] By controlling the charge rate of the first negative electrode active layer to be higher than that of the second negative electrode active layer, it is beneficial to promote the faster embedding of lithium ions into the first negative electrode active layer, and the fast charging performance of the battery can be improved. By controlling the charge rate of the first negative electrode active layer to be higher than that of the second negative electrode active layer, it is beneficial to promote the faster embedding of lithium ions into the first negative electrode active layer, and the fast charging performance of the battery can be improved.

[0341] In some embodiments, in the negative electrode sheet of the lithium ion secondary battery or in the negative electrode sheet obtained after cold pressing, the negative electrode active material layer further includes a second negative electrode active layer, and the second negative electrode active layer is located between the negative electrode current collector and the first negative electrode active layer; the second negative electrode active layer includes a second negative electrode active material, and the negative electrode sheet satisfies one or more of the following characteristics (any numerical parameter in the following characteristics can also be selected from any suitable numerical value or range in the context):

[0342] (te1) D of the second negative electrode active material v 50 is 10μm~20μm, and can be selected as 13μm~17μm;

[0343] (te2) D of the second negative electrode active material v 50 higher than the D of the first negative electrode active material v 50;

[0344] (te3) the compaction density of the second negative electrode active layer is higher than the compaction density of the first negative electrode active layer;

[0345] (te4) The powder compaction density of the second negative electrode active layer is higher than the powder compaction density of the first negative electrode active layer.

[0346] By making the lithium-ion secondary battery meet one or more of the characteristics (te1), (te2), (te3) and (te4), it is beneficial for the lithium-ion secondary battery to have improved fast charging performance while also taking into account energy density requirements.

[0347] In some embodiments, D of the second negative electrode active material v 50 is 10μm~20μm, and can be optionally 13μm~17μm. It can also be any of the following values ​​or a range consisting of any two of the following values: 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, etc.

[0348] By adding the D v 50 is controlled within the aforementioned range, which is beneficial for the second negative electrode active layer to obtain a higher compaction density, thereby helping to improve the energy density.

[0349] In some embodiments, D of the second negative electrode active material v 50 higher than the D of the first negative electrode active material v 50.

[0350] By controlling the D v 50 higher than the D of the first negative electrode active material v 50, which is beneficial to making the second negative electrode active layer obtain a higher compaction density, thereby helping to improve the energy density.

[0351] In some embodiments, the compaction density of the negative electrode sheet in the lithium ion secondary battery is greater than or equal to 1.40 g / cm 3 , optional 1.40g / cm 3 ~1.75g / cm 3 , further optional to 1.50g / cm 3 ~1.75g / cm 3 , can also be any of the following compaction densities or a range consisting of any two of the following compaction densities: 1.40 g / cm 3 , 1.45g / cm 3 , 1.50g / cm 3 , 1.55g / cm 3 , 1.60g / cm 3 , 1.65g / cm 3 , 1.70g / cm 3 , 1.75g / cm 3 wait.

[0352] In this application, unless otherwise specified, the "compacted density" of a negative electrode sheet refers to the ratio of the mass to the volume of the negative electrode active material layer in the negative electrode sheet.

[0353] In a non-limiting manner, the compaction density of the negative electrode sheet in the lithium-ion secondary battery can be tested by the following method: dismantling the battery to obtain the negative electrode sheet, punching the obtained negative electrode sheet into a size S 0 (such as 1540.25mm 2 ) and measure the mass M of the small disc. B and thickness L B Take the negative electrode sheet from another area, wipe off the negative electrode active material layer on the surface and remove the remaining negative electrode current collector foil (which can be recorded as empty negative electrode current collector foil), and also punch out into an area S 0 Weigh the mass M of the empty negative electrode current collector foil 0 and thickness L 0 , then the compaction density PD of the negative electrode sheet B =(M B -M 0 ) / [S 0 ×(L B -L 0 )]. Test a plurality of small discs (such as at least 10, further such as at least 20) and take an average value.

[0354] By controlling the compaction density of the negative electrode plate in the lithium-ion secondary battery within the aforementioned range, the liquid phase impedance accounts for a higher proportion of the impedance inside the negative electrode plate, and the quaternary ammonium salt compound improves the liquid phase impedance by guiding the lithium ions in the electrolyte. Therefore, by controlling the compaction density of the negative electrode plate within the aforementioned range, it is beneficial to better play the role of the quaternary ammonium salt compound in improving the fast charging performance of the battery.

[0355] In some embodiments, the compaction density of the second negative electrode active layer is higher than the compaction density of the first negative electrode active layer.

[0356] In the present application, unless otherwise specified, the "compacted density" of the second negative electrode active layer refers to the ratio of the mass to the volume of the second negative electrode active layer, and the "compacted density of the first negative electrode active layer" refers to the ratio of the mass to the volume of the first negative electrode active layer.

[0357] For example, the compaction density of the second negative electrode active layer in the negative electrode sheet of a lithium-ion secondary battery can be tested by the following method: dismantling the battery to obtain the negative electrode sheet, erasing part of the negative electrode active material layer, obtaining a sheet sample including at least a part of the second negative electrode active layer and excluding the first negative electrode active layer, and punching out the sheet sample into an area S 0 (such as 1540.25mm 2 ) small disc. The mass M of the pole piece samples were measured respectively. 2and thickness L 2 , the same area S 0 The mass of the empty negative electrode current collector foil M 0 and thickness L 0 , then the compaction density PD of the pole piece sample is 2 =(M 2 -M 0 ) / [S 0 ×(L 2 -L 0 )], which can be used as the compaction density of the second negative electrode active layer.

[0358] When the compaction density of the negative electrode sheet PD B Lower than the compaction density PD of the second negative electrode active layer 2 , it can be considered that "the compaction density of the second negative electrode active layer is higher than the compaction density of the first negative electrode active layer".

[0359] By regulating the compaction density of the second negative electrode active layer in the lithium ion secondary battery to be higher than that of the first negative electrode active layer, the energy density of the lithium ion secondary battery can be improved.

[0360] In some embodiments, the powder compaction density of the second negative electrode active layer is higher than the powder compaction density of the first negative electrode active layer.

[0361] In the present application, unless otherwise specified, the "powder compaction density" of the second negative electrode active layer refers to the powder compaction density of the powder material constituting the second negative electrode active layer, and the "powder compaction density" of the first negative electrode active layer refers to the powder compaction density of the powder material constituting the first negative electrode active layer.

[0362] In this application, "powder compaction density" has a well-known meaning in the art, and refers to the ratio of mass to volume of the powder material after compaction under a certain pressure. The "powder compaction density" of the negative electrode material can be measured using instruments and methods known in the art. For example, it can be measured with an electronic pressure testing machine (such as UTM7305) with reference to standard GB / T24533-2009. An exemplary test method is as follows: weigh a material to be tested with a mass of M (such as 1 g), add a bottom area of ​​A (such as 1.327 cm 2 ) in the mold, pressurized to a certain pressure P 0 (such as 3 to 5 tons (3T to 5T), such as 3T, 4T, 5T), maintain the pressure for a certain time (such as 5T for 30s), then release the pressure, maintain it for a period of time (such as 10s), and then record and calculate the negative electrode material at the pressure P 0 The above method can be used to compare or obtain the first negative electrode material powder in the first negative electrode active layer and the second negative electrode material powder in the second negative electrode active layer.

[0363] By controlling the powder compaction density of the second negative electrode active layer in the lithium-ion secondary battery to be higher than the powder compaction density of the first negative electrode active layer, it is beneficial to give the second negative electrode active layer a higher compaction density during the cold pressing process of the electrode sheet, so that the second negative electrode active layer in the lithium-ion secondary battery has a higher compaction density.

[0364] In some embodiments, the negative electrode active material layer further includes a second negative electrode active layer, and the second negative electrode active layer is located between the negative electrode current collector and the first negative electrode active layer; the negative electrode plate satisfies one or more of the following characteristics (any numerical parameter in the following characteristics can also be selected from any suitable value or range in the context):

[0365] (tf1) based on a single side of the negative electrode current collector, the ratio of the surface density of the first negative electrode active layer to the surface density of the second negative electrode active layer is 2:8 to 6:4, and can be 4:6 to 5:5;

[0366] (tf2) The ratio of the thickness of the first negative electrode active layer to the sum of the thickness of the first negative electrode active layer and the second negative electrode active layer on one side of the negative electrode current collector is denoted as F H , satisfying 20%≤F H ≤65%, optionally, 40%≤F H ≤60%;

[0367] (tf3) The thickness of the first negative electrode active layer on one side of the negative electrode current collector is 10 μm to 50 μm, and can be 20 μm to 40 μm.

[0368] In some embodiments, based on a single side of the negative electrode current collector, the ratio of the surface density of the second negative electrode active layer to the surface density of the first negative electrode active layer is 2:8 to 6:4, optionally 4:6 to 5:5, or any of the following ratios or a range consisting of any two of the following ratios: 2:8 (corresponding to 1:4, also corresponding to 0.25), 0.3, 1:3, 0.4, 0.45, 0.5, 0.55, 0.6, 2:3 (corresponding to 4:6), 0.67, 0.7, 0.75, 0.8, 0.75, 0.9, 0.95, 1, 1.05, 1.1, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5 (corresponding to 6:4), etc.

[0369] In the present application, unless otherwise specified, "the surface density of the second negative electrode active layer measured on a single side of the negative electrode current collector" refers to the ratio of the mass of the second negative electrode active layer on a single side of the negative electrode current collector to the area of ​​the second negative electrode active layer on that side, and "the surface density of the first negative electrode active layer" refers to the ratio of the mass of the first negative electrode active layer on a single side of the negative electrode current collector to the area of ​​the first negative electrode active layer on that side, and the corresponding "area" is equal to the positive projection area of ​​the second negative electrode active layer and the first negative electrode active layer along the thickness direction of the electrode sheet. Numerically, the surface density of the second negative electrode active layer measured on a single side of the negative electrode current collector is equal to the product of the compaction density of the second negative electrode active layer and the thickness of the second negative electrode active layer. The mass and area data can be obtained by referring to the test method for the compaction density of the second negative electrode active layer. Numerically, the surface density of the first negative electrode active layer measured on a single side of the negative electrode current collector is equal to the product of the compaction density of the first negative electrode active layer and the thickness of the second negative electrode active layer. The mass and area data can be obtained by referring to the test method for the compaction density of the first negative electrode active layer.

[0370] By controlling the ratio of the surface density of the second negative electrode active layer to the surface density of the first negative electrode active layer within the aforementioned range, it is beneficial to better balance the fast charging performance and energy density of the battery.

[0371] In the present application, “the ratio of the thickness of the first negative electrode active layer to the sum of the thickness of the first negative electrode active layer and the second negative electrode active layer” is denoted as F H .

[0372] In some embodiments, based on a single side of the negative electrode current collector, 20%≤F H ≤65%, optionally, 40%≤F H ≤60%. Without limitation, F H It can also be any of the following percentages or a range consisting of any two of the following percentages: 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, etc.

[0373] In some embodiments, in the negative electrode sheet of a lithium ion secondary battery or in the negative electrode sheet obtained after cold pressing, the thickness of the first negative electrode active layer can be 10 μm to 50 μm, optionally 20 μm to 40 μm, or any of the following values ​​or a range consisting of any two of the following values: 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, etc., measured on a single side of the negative electrode current collector.

[0374] By controlling the thickness of the first negative electrode active layer to satisfy one or more of the above characteristics (tf2) and (tf3), it is beneficial to better exert the effect of the electronegative doping elements in the coating layer on improving the fast charging performance of the battery while taking into account the energy density of the battery. As a non-limiting theory, by controlling the thickness of the first negative electrode active layer to satisfy one or more of the above characteristics (tf2) and (tf3), it is beneficial to better exert the effect of the electronegative doping elements in the coating layer on improving the wettability of the electrolyte, and it is beneficial to better improve the wettability of the electrolyte to the first negative electrode active layer, and then improve the wettability of the second negative electrode active layer, so as to better improve the fast charging performance of the battery.

[0375] In some embodiments, the surface density of the negative electrode sheet is 5 mg / cm 2 ~15mg / cm 2 , can also be any of the following values ​​or a range consisting of any two of the following values: 5 mg / cm 2 , 6mg / cm 2 , 7mg / cm 2 , 8mg / cm 2 , 9mg / cm 2 、10mg / cm 2 、11mg / cm 2 , 12mg / cm 2 、13mg / cm 2 、14mg / cm 2 、15mg / cm 2 wait.

[0376] In this application, unless otherwise specified, "the surface density of the negative electrode sheet measured on one side of the negative electrode current collector" is equal to the ratio of the mass of the negative electrode active material layer on one side of the negative electrode current collector to the area of ​​the negative electrode active material layer, and the corresponding "area" is equal to the positive projection area of ​​the negative electrode active material layer along the thickness direction of the electrode sheet. Reference can be made to the test method for the compaction density of the negative electrode sheet, according to (M B -M 0 ) / S 0 Calculated.

[0377] By controlling the surface density of the negative electrode plate within the aforementioned range, it is beneficial to take into account both the fast charging performance and energy density of the battery.

[0378] In some embodiments, the lithium-ion secondary battery further includes a positive electrode plate, the positive electrode plate includes a positive electrode active layer, the positive electrode active layer includes a positive electrode active material, and the positive electrode active material includes one or more of a lithium phosphate active material and a lithium composite metal oxide active material.

[0379] In some embodiments, the positive electrode active material includes a lithium-containing phosphate-based active material.

[0380] In the present application, unless otherwise specified, the lithium-containing phosphate active material may include at least one of lithium-containing phosphate and its modified product. The lithium-containing phosphate active material may have an olivine structure. Unless otherwise specified, "lithium-containing phosphate" refers to a material including lithium, transition metal elements and phosphate ions (PO 4 3- ) positive electrode active material. Non-limiting examples of olivine-structured lithium-containing phosphates may include, but are not limited to, one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0381] By introducing lithium-containing phosphate active materials into the positive electrode active materials, it is beneficial to improve the structural stability of the positive electrode active materials during the charge and discharge cycle process, which is beneficial to extend the cycle life of the battery.

[0382] In some embodiments, the positive electrode active material includes a lithium composite metal oxide-based active material.

[0383] In the present application, unless otherwise specified, the lithium composite metal oxide type active material includes at least one of lithium composite metal oxide and its modified product. Unless otherwise specified, "lithium composite metal oxide" refers to a positive electrode active material including lithium element, non-lithium metal element and oxygen element. Generally, the non-lithium metal elements of lithium composite metal oxide include transition metal elements, and therefore, lithium composite metal oxide can also be referred to as "lithium transition metal oxide". The lithium composite metal oxide type active material can have a crystal structure such as a layered structure and a spinel structure suitable for positive electrode active materials. In some embodiments, the lithium composite metal oxide type active material comprises a layered structure. In some embodiments, the lithium composite metal oxide type active material is a layered structure.

[0384] Introducing lithium composite metal oxide active materials into the positive electrode active material is beneficial to improving the positive electrode and battery energy density.

[0385] In this application, unless otherwise specified, "a modified substance of a positive electrode active material" includes the positive electrode active material itself and the modified element. Furthermore, the modified element may exist in the form of a doping element, a coating element, or a combination of a doping element and a coating element. Unless otherwise specified, "a modified substance of a positive electrode active material" still falls within the scope of positive electrode active material.

[0386] In this application, unless otherwise specified, the "doping element" involved in the positive electrode active material refers to the modifying element doped in the positive electrode active material; unless otherwise specified, the "coating element" involved in the positive electrode active material means that the positive electrode active material includes the positive electrode active particle body and a coating layer covering at least a part of the surface of the positive electrode active particle body, wherein the coating element is the modifying element in the coating layer. As a non-limiting example, in the positive electrode active material, "the modifying element exists in the form of a combination of a doping element and a coating element" means that the positive electrode active material includes the positive electrode active particle body and a coating layer covering at least a part of the surface of the positive electrode active particle body, at least a part of the modifying element is doped in the positive electrode active material body, and at least a part of the modifying element is also included in the coating layer. Both the doping modification method for introducing the doping element and the coating modification method for introducing the coating element can adopt or refer to the existing modification methods in the art, including but not limited to the selection of element types, doping amounts, and coating amounts. In some embodiments, the positive electrode active particle body can be the positive electrode active material itself or its doping modified product. In some embodiments, the doping element can include one or more of Na, K, Ca, Ba, Sb, Ti, Zr, W, Sr, Nb, Mo, Si, Mg, B, Cr, Ta, etc. In some embodiments, the coating element can include one or more of Ti, Mg, Nb, C, etc.

[0387] In some embodiments, the positive electrode active material includes a lithium-containing phosphate-based active material, and the positive electrode active material satisfies one or more of the following characteristics (any numerical parameter in the following characteristics can also be selected from any suitable value or range in the context):

[0388] (tg1) The mass ratio of the lithium-containing phosphate-based active material in the positive electrode active layer is greater than or equal to 80%, and can be optionally 80% - 97%;

[0389] (tg2) The lithium-containing phosphate-based active material includes one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, and a composite material of lithium manganese iron phosphate and carbon;

[0390] (tg3) The lithium-containing phosphate-based active material includes a lithium-containing phosphate-based active body and a carbon coating layer covering at least a part of the surface of the lithium-containing phosphate-based active body, and the carbon coating layer includes one or more of soft carbon, hard carbon, and amorphous carbon.

[0391] In some embodiments, the mass proportion of lithium-phosphate active materials in the positive electrode active layer may be greater than or equal to 80%, optionally 80% to 97%, and further optionally 95% to 97%. Without limitation, the mass proportion of lithium-phosphate active materials in the positive electrode active layer may also be any of the following percentages or a range selected from any two of the following percentages: 80%, 82%, 84%, 85%, 86%, 88%, 90%, 92%, 93%, 94%, 95%, 95.2%, 95.4%, 95.5%, 96%, 96.5%, 96.6%, 96.8%, 96.9%, 97%, etc.

[0392] By controlling the mass proportion of lithium-phosphate active materials in the positive electrode active layer within the aforementioned range, it is beneficial to better extend the cycle life of the battery.

[0393] In some embodiments, the lithium phosphate-based active material includes one or more of lithium iron phosphate (LFP), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon. Examples of lithium iron phosphate include LiFePO 4 Examples of lithium manganese phosphate include LiMnPO 4 .

[0394] The types of lithium phosphate active materials can be flexibly selected to meet different application requirements.

[0395] In some embodiments, the lithium phosphate-containing active material includes a lithium phosphate-containing active body and a carbon coating layer located on at least a portion of the surface of the lithium phosphate-containing active body. Optionally, the carbon coating layer includes one or more of soft carbon, hard carbon and amorphous carbon.

[0396] The definitions of soft carbon and hard carbon can be found in the previous article.

[0397] In some embodiments, the carbon coating layer includes soft carbon.

[0398] In some embodiments, the carbon coating layer includes hard carbon.

[0399] In some embodiments, the carbon coating includes amorphous carbon.

[0400] In this application, unless otherwise specified, "amorphous carbon" refers to a transitional carbon material with a very low degree of graphitization crystallization and a nearly amorphous form (or a structure without a fixed shape and periodicity). In this application, "amorphous carbon" may refer to a product of carbonization of an organic carbon source. Similarly, amorphous carbon can be identified based on the difference in intensity between the D peak and the G peak of the Raman spectrum.

[0401] By providing a carbon coating layer including one or more of soft carbon, hard carbon, and amorphous carbon on the surface of the lithium-containing phosphate active material, the electrical conductivity of the material can also be improved, which is beneficial to improving the electrical contact network in the positive electrode sheet and providing a fast and stable channel for electron transport in the positive electrode sheet. Therefore, it is beneficial to enhance the rate performance of the battery and improve the fast charging ability of the battery.

[0402] In some embodiments, the lithium-containing phosphate active body includes at least one of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, etc.

[0403] In some embodiments, the positive electrode active material includes a lithium iron phosphate-based positive electrode active material. The "lithium iron phosphate-based positive electrode active material" includes at least a lithium iron phosphate body, and further may include a carbon coating layer on at least a part of the surface of the lithium iron phosphate body. Optionally, the carbon coating layer in the lithium iron phosphate-based positive electrode active material may include one or more of soft carbon, hard carbon, and amorphous carbon. Without limitation, the lithium iron phosphate-based positive electrode active material may include carbon-coated lithium iron phosphate.

[0404] In some embodiments, the positive electrode active material includes carbon-coated lithium iron phosphate. The carbon-coated lithium iron phosphate includes a lithium iron phosphate body and a carbon coating layer on at least a part of the surface of the lithium iron phosphate body. Further, the carbon coating layer may include one or more of soft carbon, hard carbon, and amorphous carbon. At this time, the positive electrode active material includes a lithium-containing phosphate active material, and the lithium-containing phosphate active material includes carbon-coated lithium iron phosphate.

[0405] Those skilled in the art can use conventional techniques to select and adjust the mass ratio of the carbon coating layer in the lithium iron phosphate-based positive electrode active material (an example of the lithium iron phosphate-based positive electrode active material is carbon-coated lithium iron phosphate) and the thickness or average thickness of the carbon coating layer. Without limitation, the mass ratio of the carbon coating layer in the lithium iron phosphate-based positive electrode active material (such as carbon-coated lithium iron phosphate) can be 0.2% - 2%, but is not limited thereto. Without limitation, in the lithium iron phosphate-based positive electrode active material, the average thickness of the carbon coating layer can be 10 nm - 20 nm, but is not limited thereto. Without limitation, in the lithium iron phosphate-based positive electrode active material, the thickness of the carbon coating layer in a part of the region can be 10 nm - 20 nm, but is not limited thereto.

[0406] In some embodiments, the positive electrode active material includes soft-carbon-coated lithium iron phosphate. Further, the soft-carbon-coated lithium iron phosphate includes a lithium iron phosphate body and soft carbon on at least a part of the surface of the lithium iron phosphate body.

[0407] The positive electrode active material in the positive electrode active layer can be detected by fully discharging the battery, disassembling the battery cell, taking out the positive electrode plate, scraping the material of the positive electrode active layer, and using elemental analysis methods such as inductively coupled plasma (ICP) spectroscopy to test and analyze the type and proportion of elements, thereby confirming the elemental composition and chemical formula of the positive electrode active material.

[0408] In some embodiments, the positive electrode active material includes a positive electrode active body and a coating layer located on the positive electrode active body.

[0409] For positive electrode active materials including a coating layer (an example of a coating layer is a carbon coating layer), the cross-section can be cut using FIB (focused ion beam) and the particle cross-sectional morphology can be observed under TEM (transmission electron microscope). A clear boundary can be observed at the coating interface, and the thickness and average thickness of the coating layer can be analyzed and calculated based on the TEM image. Further combined with one or more methods such as energy dispersive spectroscopy (EDS) analysis and Raman spectroscopy, the types of substances in the coating layer and the positive electrode active body can be confirmed respectively.

[0410] The following are some additional descriptions about the positive electrode.

[0411] The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active layer includes a positive electrode active material.

[0412] The definitions of the positive electrode sheet, positive electrode active layer, and positive electrode active material in some embodiments may refer to the context of this application.

[0413] Without limitation, the mass percentage of the positive electrode active material in the positive electrode active layer may be greater than or equal to 80%, and further may be greater than or equal to 90%.

[0414] As a non-limiting example, the positive electrode current collector has two surfaces that are opposite to each other in its thickness direction, and the positive electrode active layer is disposed on any one or both of the two surfaces of the positive electrode current collector that are opposite to each other.

[0415] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. In the positive electrode current collector, the composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. In the positive electrode current collector, the composite current collector may be obtained by forming a metal material on a polymer material base layer. In the positive electrode current collector, non-limiting examples of the metal material may include at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. In a non-limiting manner, in the positive electrode current collector, the polymer material base layer may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0416] The types of positive electrode active materials can be found in the above text. The positive electrode active materials may also include other types of positive electrode active materials known in the art for lithium ion secondary batteries. The positive electrode active materials may be used alone or in combination of two or more.

[0417] As non-limiting examples, the positive electrode active material may include, but is not limited to, one or more of the following materials: olivine-structured lithium-containing phosphates, lithium transition metal oxides, and modifications thereof.

[0418] In some embodiments, the positive electrode active material includes a lithium phosphate containing an olivine structure. Non-limiting examples of lithium phosphate containing an olivine structure may include, but are not limited to, one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon. Examples of lithium iron phosphate include LiFePO 4 Examples of lithium manganese phosphate include LiMnPO 4 .

[0419] In some embodiments, the positive electrode active material includes a lithium transition metal oxide. Examples of lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and modified compounds thereof. Non-limiting examples of lithium cobalt oxides may include LiCoO 2 Non-limiting examples of lithium nickel oxides may include LiNiO 2 Non-limiting examples of lithium manganese oxides may include LiMnO 2 、LiMn 2 O 4 etc.; Non-limiting examples of lithium nickel cobalt manganese oxides may include LiNi 1 / 3 Co 1 / 3 Mn1 / 3 O 2 (Also referred to as NCM 333 )、LiNi 0.5 Co 0.2 Mn 0.3 O 2 (Also referred to as NCM 523 )、LiNi 0.5 Co 0.25 Mn 0.25 O 2 (Also referred to as NCM 211 )、LiNi 0.6 Co 0.2 Mn 0.2 O 2 (Also referred to as NCM 622 )、LiNi 0.8 Co 0.1 Mn 0.1 O 2 (Also referred to as NCM 811 ) etc. Non-limiting examples of lithium nickel cobalt aluminum oxide may include LiNi 0.80 Co 0.15 Al 0.05 O 2 .

[0420] It is understandable that the battery will be accompanied by lithium (Li) deintercalation and consumption during the charging and discharging process, and the content of Li in the positive electrode sheet is different when the battery is discharged to different states. In the exemplary description of the positive electrode active material in this application, unless otherwise specified, the content of Li can be the initial state of the material or the non-initial state after the charge and discharge cycle. The positive electrode active material is applied to the positive electrode in the battery system, and after the charge and discharge cycle, the content of Li in the positive electrode active material at the positive electrode usually changes. Among them, the content of Li can be measured by atomic molar content, but is not limited to this. Regarding "the content of Li is the initial state of the material", the initial state of the material refers to the state before being made into a positive electrode active layer. It can be understood that the new material or new substance obtained by appropriately modifying the listed positive electrode active materials is also within the scope of the positive electrode active material. The aforementioned appropriate modification refers to the acceptable modification method for the positive electrode active material, and non-limiting examples include one or more of coating modification and doping modification.

[0421] In the exemplary description of the positive electrode active material in this application, the content of oxygen (O) is only a theoretical state value. The release of oxygen from the lattice will cause the atomic molar content of oxygen to change, and the actual content of O will fluctuate. Among them, the content of O can be measured by atomic molar content, but is not limited to this.

[0422] In some embodiments, the positive electrode active layer optionally includes a binder. As a non-limiting example, the binder may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin. Generally, the mass percentage of the binder in the positive electrode active layer can be 0 to 10%, further 0 to 8%, and further 1% to 5%, based on the total mass of the positive electrode active layer.

[0423] In some embodiments, the positive electrode active layer optionally includes a conductive agent. As a non-limiting example, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. Typically, the mass percentage of the conductive agent in the positive electrode active layer may be 0 to 10%, further 0 to 8%, and further 0 to 5%, based on the total mass of the positive electrode active layer.

[0424] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive active material, the conductive agent, the binder and any other components, are dispersed in a solvent to form a positive electrode slurry. Further, the positive electrode slurry is coated on at least one side of the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained. Cold pressing can be performed by a cold rolling mill. The type of solvent in the positive electrode slurry can include but is not limited to any of the aforementioned embodiments, for example, it can include N-methylpyrrolidone (NMP), and further can be NMP. The surface of the positive electrode collector coated with the positive electrode slurry can be on a single surface of the positive electrode collector, or on both surfaces of the positive electrode collector. The solid content of the positive electrode slurry can be 40wt% to 80wt%. The viscosity of the positive electrode slurry at room temperature can be adjusted to 5000mPa·s to 25000mPa·s.

[0425] In some embodiments, the positive electrode active material includes a lithium-containing phosphate active material, and further, the mass proportion of the lithium-containing phosphate active material in the positive electrode active material can be greater than or equal to 50%, but is not limited thereto. When applying the positive electrode slurry, the coating surface density (based on the coating surface density on both sides) on a dry weight basis (excluding the solvent) can also be (0.1-0.6) g / 1540.25 mm 2 , but not limited to this. The compaction density of the positive electrode sheet can be 1.9g / cm 3 ~2.7g / cm 3 .

[0426] In some embodiments, the positive electrode active material includes a lithium composite metal oxide active material, and further, the mass proportion of the lithium composite metal oxide active material in the positive electrode active material may be greater than or equal to 50%, but is not limited thereto. When the positive electrode slurry is coated, the coating surface density on a dry weight basis (excluding the solvent) may be 15 mg / cm 2 ~35mg / cm 2 , measured by the double-sided coating surface density. The compacted density of the positive electrode sheet can be 3.0g / cm 3 ~3.6g / cm 3 , optional 3.3g / cm 3 ~3.5g / cm 3 .

[0427] In the present application, unless otherwise specified, the compaction density of the positive electrode sheet refers to the ratio of the mass of the positive electrode active layer to its volume.

[0428] The following are some additional descriptions about the negative electrode.

[0429] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material. The negative electrode active material layer includes at least a first negative electrode active layer. The first negative electrode active layer includes a first negative electrode active material.

[0430] It can be understood that the negative electrode active material includes a first negative electrode active material.

[0431] In some embodiments, the negative electrode active material layer further includes a second negative electrode active layer, and the second negative electrode active layer is located between the first negative electrode active layer and the negative electrode current collector. The second negative electrode active layer includes a second negative electrode active material. The types of the first negative electrode active material and the second negative electrode active material can be the same or different, and the description in the context can be referred to. In this case, the negative electrode active material includes a first negative electrode active material and a second negative electrode active material.

[0432] In some embodiments, the negative electrode active material layer on at least one side includes a first negative electrode active layer and a second negative electrode active layer, and the second negative electrode active layer is located between the first negative electrode active layer and the negative electrode current collector.

[0433] The definitions of the negative electrode sheet, the negative electrode active material layer, the first negative electrode active layer and the second negative electrode active layer, the negative electrode active material, the first negative electrode active material and the second negative electrode active material in some embodiments can be referred to in the context of this application.

[0434] As a non-limiting example, the negative electrode sheet includes a negative electrode collector and a second negative electrode active layer and a first negative electrode active layer sequentially disposed on at least one side of the negative electrode collector.

[0435] Without limitation, the mass percentage of the negative electrode active material in the negative electrode active material layer may be greater than or equal to 80%, and further may be greater than or equal to 90%.

[0436] Without limitation, the mass percentage of the first negative electrode active material in the first negative electrode active layer may be greater than or equal to 80%, and further may be greater than or equal to 90%.

[0437] Without limitation, the mass percentage of the second negative electrode active material in the second negative electrode active layer may be greater than or equal to 80%, and further may be greater than or equal to 90%.

[0438] As a non-limiting example, the negative electrode current collector has two surfaces facing each other in its thickness direction, and the negative electrode active material layer is disposed on any one or both of the two surfaces facing each other of the negative electrode current collector.

[0439] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as a metal foil, a copper foil may be used. In the negative electrode current collector, the composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. In the negative electrode current collector, the composite current collector may be formed by forming a metal material on a polymer material base layer. In the negative electrode current collector, non-limiting examples of the metal material may include one or more of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. In a non-limiting manner, in the negative electrode current collector, the polymer material base layer may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0440] In addition to the types of negative electrode active materials involved in the context implementation methods and examples, the negative electrode active materials in the negative electrode active material layer may also include other types of negative electrode active materials suitable for lithium-ion secondary batteries that are well known in the art. The negative electrode active material may be used alone or in combination of two or more. In some embodiments, the negative electrode active material also includes one or more of tin-based materials and lithium titanate. In the negative electrode active material layer, these negative electrode active materials may be used alone or in combination of two or more. In the first negative electrode active layer and the second negative electrode active layer, each independently, these negative electrode active materials may be used alone or in combination of two or more.

[0441] Without limitation, the first negative electrode active material may also be other types of negative electrode active materials known in the art for lithium-ion secondary batteries.

[0442] Without limitation, the second negative electrode active material may adopt other types of negative electrode active materials for lithium ion secondary batteries known in the art. The second negative electrode active material may include one or more of carbon-based materials, silicon-based materials, tin-based materials, and lithium titanate. The carbon-based material may include one or more of artificial graphite, natural graphite, soft carbon, and hard carbon. The silicon-based material may include one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may include one or more of elemental tin, tin oxide compounds, and tin alloys. As a non-limiting example, the second negative electrode active material may include one or more of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. However, the second negative electrode active material is not limited to these materials or substances, and other traditional materials that can be used as negative electrode active materials for batteries can also be used. In the second negative electrode active layer, these negative electrode active materials can be used alone or in combination of two or more. In some embodiments, the second negative electrode active material includes one or more of carbon-based materials and silicon-based materials. In some embodiments, the second negative electrode active material includes a carbon-based material. In some embodiments, the mass percentage of the carbon-based material in the second negative electrode active material may be 60% to 100%, may be 80% to 100%, may further be 90% to 100%, and may further be 97% to 100%.

[0443] In some embodiments, the negative active material layer may optionally include a binder.

[0444] In some embodiments, the first negative electrode active layer and the second negative electrode active layer each independently optionally include a binder.

[0445] Without limitation, in the negative electrode sheet, the binder may include one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS). The binder types in the first negative electrode active layer and the second negative electrode active layer may be the same or different.

[0446] Without limitation, the mass percentage of the binder in the negative electrode active material layer may be 0% to 20%, further 0% to 10%, further 0 to 5%, further 1% to 5%, further optionally 1% to 3%.

[0447] Without limitation, the mass percentage of the binder in the first negative electrode active layer may be 0% to 20%, further 0% to 10%, further 0 to 5%, further 1% to 5%, further optionally 1% to 3%.

[0448] Without limitation, the mass percentage of the binder in the second negative electrode active layer may be 0% to 20%, further 0% to 10%, further 0 to 5%, further 1% to 5%, further optionally 1% to 3%.

[0449] In some embodiments, the negative active material layer may optionally include a conductive agent.

[0450] In some embodiments, the first negative electrode active layer and the second negative electrode active layer each independently optionally include a conductive agent.

[0451] Without limitation, in the negative electrode sheet, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers. The types of conductive agents in the first negative electrode active layer and the second negative electrode active layer may be the same or different.

[0452] Without limitation, the mass percentage of the conductive agent in the negative electrode active material layer may be 0% to 15%, further optionally 0% to 10%, and further optionally 0% to 5%.

[0453] In a non-limiting manner, the mass percentage of the conductive agent in the first negative electrode active layer may be 0% to 15%, further optionally 0% to 10%, and further optionally 0% to 5%.

[0454] In a non-limiting manner, the mass percentage of the conductive agent in the second negative electrode active layer may be 0% to 15%, further optionally 0% to 10%, and further optionally 0% to 5%.

[0455] In some embodiments, the negative electrode active material layer may optionally include other additives, such as a thickener (e.g., sodium carboxymethyl cellulose (CMC-Na)), etc. In a non-limiting manner, the mass percentage of other additives in the negative electrode active material layer may be 0% to 15%, further optionally 0% to 10%, further optionally 0% to 5%, further optionally 0% to 3%, further optionally 0% to 2%.

[0456] In some embodiments, the first negative electrode active layer and the second negative electrode active layer may each independently optionally include other additives, such as a thickener (such as sodium carboxymethyl cellulose (CMC-Na)), etc. Non-limitingly, the mass percentage of other additives in the first negative electrode active layer or the second negative electrode active layer may independently be 0% to 15%, further optionally 0% to 10%, further optionally 0% to 5%, further optionally 0% to 3%, further optionally 0% to 2%.

[0457] In some embodiments, a negative electrode sheet is prepared by a method comprising the following steps, which can be used to prepare a negative electrode sheet with a single-layer structure of a negative electrode active material layer:

[0458] S110: preparing negative electrode slurry. The above components for preparing the negative electrode active material layer, such as negative electrode active material, conductive agent, binder and any other components are dispersed in a solvent (a non-limiting example of the solvent is deionized water) to form a negative electrode slurry. It is understood that the negative electrode active material includes an electronegative coated active material.

[0459] S120: Prepare negative electrode sheets. The negative electrode slurry is coated on at least one side of the negative electrode current collector. After drying, cold pressing and other processes, a negative electrode active material layer can be formed accordingly to obtain a negative electrode sheet. Cold pressing can be performed using a cold rolling mill. The surface of the negative electrode current collector coated with the negative electrode slurry can be on a single surface of the negative electrode current collector or on both surfaces of the negative electrode current collector. The solid content of the negative electrode slurry can be 30wt% to 70wt%, and can be optionally 40wt% to 60wt%. The viscosity of the negative electrode slurry at room temperature can be adjusted to 2000mPa·s to 10000mPa·s, and can be optionally 3000mPa·s to 10000mPa·s.

[0460] In some embodiments, a negative electrode sheet is prepared by a method comprising the following steps: S110' and S120'; which can be used to prepare a negative electrode sheet having a negative electrode active material layer including a first negative electrode active layer and a second negative electrode active layer.

[0461] S110': Prepare a first negative electrode slurry and a second negative electrode slurry. The components for preparing the first negative electrode active layer, such as the first negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (a non-limiting example of the solvent is deionized water) to form a first negative electrode slurry. The components for preparing the second negative electrode active layer, such as the second negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (a non-limiting example of the solvent is deionized water) to form a second negative electrode slurry.

[0462] S120': Prepare the negative electrode sheet. Use a double-layer coating machine to coat the second negative electrode slurry and the first negative electrode slurry on at least one side of the negative electrode current collector. Apply the second negative electrode slurry first and then the first negative electrode slurry. After drying, cold pressing and other processes, the second negative electrode active layer and the first negative electrode active layer can be formed accordingly. The second negative electrode active layer is located between the first negative electrode active layer and the negative electrode current collector; wherein the non-solvent component of the first negative electrode slurry forms the first negative electrode active layer, and the non-solvent component of the second negative electrode slurry forms the second negative electrode active layer. Cold pressing can be performed using a cold rolling mill. The surface of the negative electrode current collector coated with the first negative electrode slurry and the second negative electrode slurry can be on a single surface of the negative electrode current collector or on both surfaces of the negative electrode current collector. The solid content of the first negative electrode slurry and the second negative electrode slurry can be independently 30wt% to 70wt%, and can be independently 40wt% to 60wt%. The viscosity of the first negative electrode slurry and the second negative electrode slurry at room temperature can be independently adjusted to 2000 mPa·s to 10000 mPa·s, and can be independently selected to be 3000 mPa·s to 10000 mPa·s.

[0463] The coating surface density of the negative electrode sheet and the compaction density of the negative electrode sheet can be found in the context of this application.

[0464] Compared with the negative electrode sheet obtained after cold pressing, the volume of the negative electrode sheet in the lithium-ion secondary battery will have a certain rebound, so that the compaction density of the negative electrode sheet in the lithium-ion secondary battery is lower than that of the negative electrode sheet obtained after cold pressing.

[0465] The electrolyte is exemplarily described below.

[0466] The electrolyte has the function of conducting ions between the positive electrode and the negative electrode. The appropriate electrolyte can be selected according to the needs.

[0467] The electrolyte solution includes an electrolyte salt and a solvent. The solvent includes a non-aqueous solvent.

[0468] In some embodiments, the electrolyte is a non-aqueous electrolyte. The non-aqueous electrolyte may include an electrolyte salt and a solvent.

[0469] The concentration of the electrolyte salt in the electrolyte solution may generally be 0.5 mol / L to 5 mol / L.

[0470] In some embodiments, the electrolyte salt comprises a lithium electrolyte salt.

[0471] In the present invention, the solvent in the non-aqueous electrolyte may include ethylene carbonate (EC, ), propylene carbonate (PC, ), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC, ), one or more of fluoroethylene carbonate (FEC), methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0472] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high or low temperature performance, etc.

[0473] In some embodiments, the additives in the electrolyte may include, but are not limited to, one or more of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethylethylene carbonate (TFPC), and the like.

[0474] The separator is exemplarily described below.

[0475] The definition of the isolation membrane of some embodiments may be found in the context of this application.

[0476] The present application has no particular limitation on the type of diaphragm membrane, and any known diaphragm membrane with a porous structure having good chemical stability and mechanical stability may be selected.

[0477] In some embodiments, the material of the diaphragm membrane may include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The diaphragm membrane may be a single-layer film or a multi-layer composite film, without particular limitation. When the diaphragm membrane is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0478] In some embodiments, the thickness of the separator film is 4 μm to 40 μm, and optionally 7 μm to 20 μm.

[0479] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

[0480] In some embodiments, the lithium-ion secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0481] In some embodiments, the outer packaging of the lithium ion secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the lithium ion secondary battery can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, and further, non-limiting examples of plastic can include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0482] A lithium-ion secondary battery includes at least one battery cell. A lithium-ion secondary battery may include one or more battery cells.

[0483] The present application has no particular limitation on the shape of the battery cell, which may be cylindrical, square or any other shape. Figure 1 The battery cell 5 is a square structure as an example.

[0484] In some embodiments, reference Figure 2 , the outer packaging may include a shell 51 and a cover plate 53. Among them, the shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, and those skilled in the art can select according to actual needs.

[0485] The lithium-ion secondary battery may be a battery device 4 or a battery pack 1 .

[0486] The battery device includes at least one battery cell. The number of battery cells contained in the battery device can be one or more, and those skilled in the art can select a suitable number according to the application and capacity of the battery device.

[0487] Figure 3 4 is an example of a battery device. Figure 3 In the battery device 4, the plurality of battery cells 5 may be arranged in sequence along the length direction of the battery device 4. Of course, they may also be arranged in any other manner. Further, the plurality of battery cells 5 may be fixed by fasteners.

[0488] Optionally, the battery device 4 may further include a housing having a receiving space, and the plurality of battery cells 5 are received in the receiving space.

[0489] In some embodiments, the battery devices described above may also be assembled into a battery pack. The number of battery devices contained in the battery pack may be one or more. Those skilled in the art may select a suitable number according to the application and capacity of the battery pack.

[0490] Figure 4 and Figure 5 1 is a battery pack 1 as an example. Figure 4 and Figure 5 The battery pack 1 may include a battery box and a plurality of battery devices 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery devices 4. The plurality of battery devices 4 can be arranged in the battery box in any manner.

[0491] In another aspect of the present application, a negative electrode plate is provided, which may have the same characteristics as the negative electrode plate described in the first aspect of the present application, or may be in the state after cold pressing and before being soaked in electrolyte corresponding to the negative electrode plate described in the first aspect of the present application.

[0492] In some embodiments, a negative electrode sheet is provided, comprising a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector, wherein the negative electrode active material layer comprises a first negative electrode active layer;

[0493] The first negative electrode active layer includes a first negative electrode active material, the first negative electrode active material includes an electronegative coated active material, the electronegative coated active material includes a negative electrode active body and a coating layer located on at least a portion of the surface of the negative electrode active body; the coating layer includes doped carbon, the doped carbon includes a carbon matrix and an electronegative doping element, the carbon matrix includes one or more of soft carbon and hard carbon, and the Pauling electronegativity scale of the electronegative doping element is denoted by x 1 , the Pauling electronegativity scale of carbon is denoted by χ C , then χ 1 With χ C The absolute value of the difference between them satisfies 0.03≤|χ 1 -x C |≤0.49.

[0494] As mentioned above, by arranging an electronegative coated active material in the negative electrode active material layer of the negative electrode plate, as described above, the electronegative doping elements and the carbon matrix in the coating layer can promote the faster and more embedding of lithium ions into the negative electrode active material through a synergistic effect, thereby improving the lithium conductivity of the SEI film, reducing the interface impedance, and helping to improve the fast charging capability of the battery.

[0495] In some embodiments, the negative electrode active material layer also includes a second negative electrode active layer, and the second negative electrode active layer is located between the negative electrode collector and the first negative electrode active layer. In this case, the negative electrode sheet includes a negative electrode collector and the second negative electrode active layer and the first negative electrode active layer arranged in sequence on at least one side of the negative electrode collector.

[0496] In some embodiments, a negative electrode sheet is provided, which includes a negative electrode collector and a second negative electrode active layer and a first negative electrode active layer sequentially arranged on at least one side of the negative electrode collector; the first negative electrode active layer includes a first negative electrode active material, and the first negative electrode active material includes an electronegative coated active material.

[0497] The definition of electronegative coating active material can be found in the first aspect of the present application.

[0498] By arranging a first negative electrode active layer located at the upper layer and a second negative electrode active layer located at the lower layer on the negative electrode plate, and arranging an electronegative coated active material in the first negative electrode active layer, as described above, the electronegative doping elements and the carbon matrix in the coating layer can promote the faster and more lithium ion embedding into the first negative electrode active material through synergistic effect, thereby improving the lithium conductivity of the SEI film, reducing the interface impedance, and improving the wettability of the electrolyte in the first negative electrode active layer. Based on the improvement of the wettability of the electrolyte in the first negative electrode active layer, the wettability of the electrolyte in the second negative electrode active layer can be further improved. Based on the multiple effects, it is beneficial to improve the fast charging capability of the battery.

[0499] In the present application, the effect of the electronegative coated active material on the electrolyte wettability of the negative electrode active material layer or the first negative electrode active layer can be evaluated by the change in the liquid absorption rate. The greater the liquid absorption rate, the better the electrolyte wettability can be considered. The following method can be used for testing and analysis: fix the negative electrode plate to be tested on the sample table, drip the electrolyte on the surface of the negative electrode plate, and use a stopwatch to time; record the weight increase and time; calculate the liquid absorption rate on the surface of the negative electrode plate by the change in weight over time, which can be regarded as the liquid absorption rate of the first negative electrode active layer. Non-limiting examples of electrolytes include the electrolyte formula of Example 1, and the electrolyte composition of the same electrolyte as that in the lithium-ion secondary battery can also be used to test the liquid absorption rate, and commercially available electrolytes such as electrolyte E30 can also be used. In some examples, the electrolyte is composed of a solvent and 1 mol / L lithium hexafluorophosphate (LiPF6), and the solvent composition is ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1.

[0500] In some embodiments, the negative electrode plate is the negative electrode plate defined in the first aspect of the present application.

[0501] In a second aspect of the present application, a method for preparing a lithium ion secondary battery is provided, which can be used to prepare the lithium ion secondary battery of the first aspect of the present application.

[0502] In one embodiment, a method for preparing a lithium-ion secondary battery is provided, which includes preparing a negative electrode plate.

[0503] In yet another aspect of the present application, a method for preparing a negative electrode plate is also provided.

[0504] In some embodiments, preparing the negative electrode sheet comprises the following steps:

[0505] S100: a negative electrode active material layer is provided on at least one side of the negative electrode current collector; wherein the negative electrode active material layer comprises a first negative electrode active layer, the first negative electrode active layer comprises a first negative electrode active material, the first negative electrode active material comprises an electronegative coated active material, the electronegative coated active material comprises a negative electrode active body and a coating layer located at least in part on the surface of the negative electrode active body; the coating layer comprises doped carbon, the doped carbon comprises a carbon matrix and an electronegative doping element, the carbon matrix comprises one or more of soft carbon and hard carbon, and the Pauling electronegativity scale of the electronegative doping element is denoted as x 1 , the Pauling electronegativity scale of carbon is denoted by χ C , then χ 1 With χ C The absolute value of the difference between them satisfies 0.03≤|χ 1 -x C |≤0.49.

[0506] In the prepared lithium-ion secondary battery, an electronegative coated active material is arranged in the negative electrode active material layer in the negative electrode plate. As described above, the electronegative doping elements and the carbon matrix in the coating layer can promote faster and more lithium ions to be embedded in the negative electrode active material through synergistic effects, which is beneficial to improve battery dynamics and enhance the battery's fast charging capability.

[0507] In some embodiments, a method for preparing a lithium-ion secondary battery is provided, comprising preparing a negative electrode sheet;

[0508] The preparation of the negative electrode sheet includes the following steps:

[0509] A second negative electrode active layer and a first negative electrode active layer are sequentially arranged on at least one side of the negative electrode current collector; wherein the first negative electrode active layer includes a first negative electrode active material, and the first negative electrode active material includes an electronegative coated active material, and the definition of the electronegative coated active material can be as described in the first aspect of the present application.

[0510] In the prepared lithium-ion secondary battery, a first negative electrode active layer located at the upper layer and a second negative electrode active layer located at the lower layer are arranged in the negative electrode active material layer in the negative electrode plate, and an electronegative coated active material is arranged in the first negative electrode active layer located at the upper layer. As described above, the electronegative doping elements and the carbon matrix in the coating layer can promote faster and more lithium ions to be embedded in the first negative electrode active material through a synergistic effect, and it is also beneficial to improve the wettability of the electrolyte in the second negative electrode active layer based on the improvement of the wettability of the electrolyte in the first negative electrode active layer; based on the aforementioned multiple effects, it is beneficial to improve the battery dynamics and enhance the fast charging capability of the battery.

[0511] In one embodiment, preparing the negative electrode sheet includes the following steps:

[0512] S100’: Sequentially dispose a second negative electrode active layer and a first negative electrode active layer on at least one side of the negative electrode current collector; wherein, the first negative electrode active layer includes a first negative electrode active material, the first negative electrode active material includes an electro-negativity coated active material, the electro-negativity coated active material includes a negative electrode active body and at least a part of a coating layer located on the surface of the negative electrode active body; the coating layer includes doped carbon, the doped carbon includes a carbon matrix and an electro-negativity doping element, the carbon matrix includes one or more of soft carbon and hard carbon, and the Pauling electronegativity scale of the electro-negativity doping element is denoted as χ 1 , denote the Pauling electronegativity scale of carbon element as χ C , then χ 1 and χ C The absolute value of the difference between them satisfies 0.03 ≤ |χ 1 - χ C | ≤ 0.49.

[0513] In some embodiments, the carbon matrix in the doped carbon includes soft carbon. At this time, the coating layer in the electro-negativity coated active material includes doped soft carbon, and the electro-negativity coated active material can be prepared by a method including the following steps:

[0514] S10: In the presence of a doping precursor, mix the negative electrode active body with a soft carbon precursor, perform doping heat treatment and soft carbonization heat treatment, so that the soft carbon precursor and the doping precursor jointly form doped soft carbon covering at least a part of the surface of the negative electrode active body; wherein, the doping precursor includes an electro-negativity doping element. A non-limiting example of the soft carbon precursor is pitch.

[0515] By controlling the amount of the doping precursor relative to the soft carbon precursor, the mass percentage of the electro-negativity doping element in the doped carbon and in the coating layer can be adjusted. In some embodiments, the mass percentage of the doping precursor relative to the soft carbon precursor is greater than or equal to 0.5%, and the prepared electro-negativity coated active material satisfies one or both of the following characteristics: (i) the mass percentage of the electro-negativity doping element in the doped carbon is greater than or equal to 0.1%, (ii) the mass percentage of the electro-negativity doping element in the coating layer is greater than or equal to 0.1%. The mass percentage of the electro-negativity doping element in the doped carbon and the mass percentage of the electro-negativity doping element in the coating layer can also refer to the first aspect of this application.

[0516] The doping conditions for introducing the electro-negativity doping element can be appropriately adjusted according to the difficulty of introducing this doping element. For example, different heat preservation temperatures can be selected, and one or both of the heating rate and the heat preservation time can also be adjusted.

[0517] In some embodiments, in step S10, the doping precursor and the soft carbon precursor asphalt are first mixed, and then the precursor mixture is used to coat the surface of the negative electrode active body (corresponding to the first active body), and the doping heat treatment and the soft carbonization heat treatment can be performed in the presence of the precursor mixture. Non-limiting examples of negative electrode active bodies include graphite. When the electronegative doping element includes the P element, one or both of the O-doping treatment and the S-doping treatment can be performed before P doping. This method is beneficial to improving the uniformity of the coating. It can be understood that the doping heat treatment and the soft carbonization heat treatment can be carried out simultaneously. During the soft carbonization heat treatment, the electronegative doping element in the doping precursor can combine with carbon (C) to form a new chemical structure, and at the same time has high defects, can effectively adsorb foreign atoms, and can improve the wettability of the negative electrode sheet and the fast charging capability of the battery.

[0518] In some embodiments, the doping precursor includes a N-containing precursor. Considerations for selecting the N-containing precursor may include compatibility with asphalt. The N-containing precursor may include one or more of urea and ammonia. Without limitation, the N-containing precursor may also include other organic nitrogen compounds, such as one or more of amine compounds, amide compounds, nitro compounds, and nitrogen heterocyclic compounds. In some of these embodiments, the N-containing precursor may also include one or more of aliphatic amines, aromatic amines, polyamines, aliphatic amides, aromatic amides, nitrobenzene, nitrotoluene, pyridine, quinoline, and carbazole.

[0519] In some embodiments, the doping precursor includes a S-containing precursor. The S-containing precursor may include one or more of sulfur and sulfur vapor. Without limitation, the S-containing precursor may also include other types of S-containing compounds, such as one or more of sulfurized petroleum resin, sulfurized rubber, thiosulfate, mercaptan, and thioamidine.

[0520] In some embodiments, the doping precursor includes a P-containing precursor, and the P-containing precursor includes one or more of red phosphorus and phosphorus vapor. Without limitation, the P-containing precursor may also include other P-containing compounds, such as one or more of phosphate esters, polyphosphate esters, phosphides, and phosphorus heterocyclic compounds.

[0521] In some embodiments, in step S10, the doping heat treatment includes N doping treatment, and the conditions for performing the N doping treatment include: a heating rate of 2°C / min to 10°C / min, a holding temperature of 400°C to 800°C, and a holding time of 1h to 6h. In the step of performing the N doping treatment, the heating rate may be any rate of 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, or may be selected from a range consisting of any two of the following rates. In the step of performing the N doping treatment, the holding temperature may be 400°C to 800°C, or may be any of the following temperatures or a range consisting of any two of the following temperatures: 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, or the like. In the step of N-doping treatment, the insulation time can be 1h to 6h, or any of the following time lengths or a range consisting of any two of the following time lengths: 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 6h, etc.

[0522] By controlling appropriate process conditions, the presence of S and P elements in the form of single-substance impurities can be reduced or avoided, which is beneficial to suppress the probability of lithium insertion reaction of S and P single-substance impurities, suppress the aggravation of the volume expansion of the negative electrode plate, and reduce the risk of damage to the coating layer, which is beneficial to achieve good long-term battery performance. By controlling the conditions for S doping and P doping under more appropriate conditions (for example, the doping temperature can be slightly increased when S doping is performed, for example, at least one of O and S can be introduced before introducing P), it is beneficial to make the battery have better long-term cycle performance. In some embodiments, when S doping is performed, one or more of the following conditions can also be introduced: reduce the heating rate, extend the insulation time, and reduce the sulfur (S) vapor flow rate.

[0523] In some embodiments, in step S10, the doping heat treatment includes S doping treatment, and the conditions for performing the S doping treatment include: a heating rate of 2°C / min to 10°C / min, a holding temperature of 200°C to 600°C, and a holding time of 1h to 6h. In the step of performing the S doping treatment, the heating rate may be any rate of 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, or may be selected from a range consisting of any two of the following rates. In the step of performing the S doping treatment, the holding temperature may be 200°C to 600°C, or may be any of the following temperatures or may be selected from a range consisting of any two of the following temperatures: 200°C, 250°C, 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, or the like. In the step of S doping treatment, the insulation time can be 1h to 6h, or any of the following time lengths or a range consisting of any two of the following time lengths: 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 6h, etc.

[0524] In some embodiments, in step S10, the doping heat treatment includes P doping treatment, and the step of performing heat treatment includes: performing one or both of O doping treatment and S doping treatment and then performing P doping treatment, and the conditions for performing P doping treatment include: a heating rate of 2°C / min to 10°C / min, a holding temperature of 200°C to 600°C, and a holding time of 1h to 6h; wherein, the conditions for performing O doping treatment include: heating at a heating rate of 2°C / min to 10°C / min in an air atmosphere, and performing holding treatment at 400°C to 800°C in a nitrogen atmosphere, and a holding time of 1h to 6h. In the step of performing P doping treatment, the heating rate can be any rate of 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, etc., and can also be selected from the range of any two of the above rates, such as 2°C / min to 10°C / min, and can be selected from 2°C / min to 6°C / min. In a non-limiting manner, in the step of performing the P doping treatment, the holding temperature may be 200°C to 600°C, or any one of the following temperatures or a range consisting of any two of the following temperatures: 200°C, 250°C, 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, etc. In a non-limiting manner, in the step of performing the P doping treatment, the holding temperature may be 300°C to 700°C, or any one of the following temperatures or a range consisting of any two of the following temperatures: 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, etc. In the step of P doping treatment, the insulation time can be 1h to 6h, or any of the following time lengths or a range consisting of any two of the following time lengths: 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 6h, etc.

[0525] In some embodiments, in step S10 , in the steps of performing the doping heat treatment and the soft carbonization heat treatment, the soft carbon precursor includes a liquid precursor.

[0526] In some embodiments, the conditions for the soft carburization heat treatment include: heat preservation treatment under an inert atmosphere and 700°C to 1300°C. A non-limiting example of an inert atmosphere is a nitrogen atmosphere. Non-limitingly, the conditions for the heat preservation treatment can be any of the following temperatures or a range consisting of any two of the following temperatures: 700°C, 800°C, 900°C, 1000°C, 1020°C, 1040°C, 1050°C, 1060°C, 1080°C, 1100°C, 1120°C, 1140°C, 1150°C, 1160°C, 1180°C, 1200°C, 1250°C, 1300°C, etc.

[0527] In some embodiments, the method for preparing the electronegative coated active material satisfies one or more of the following characteristics (any numerical parameter in the following characteristics can also be selected from any suitable numerical value or range in the context):

[0528] (th1) the doping precursor includes a N-containing precursor, and the N-containing precursor may include one or more of urea and ammonia;

[0529] (th2) the doping precursor includes a S-containing precursor, and the S-containing precursor may include one or more of sulfur and sulfur vapor;

[0530] (th3) the doping precursor includes a P-containing precursor, and the P-containing precursor includes one or more of red phosphorus and phosphorus vapor;

[0531] (th4) The doping heat treatment includes N doping treatment. The conditions for performing the N doping treatment include: a heating rate of 2°C / min to 10°C / min,

[0532] The holding temperature is 400℃~800℃, and the holding time is 1h~6h;

[0533] (th5) the doping heat treatment includes S doping treatment, and the conditions for the S doping treatment include: a heating rate of 2°C / min to 10°C / min, a holding temperature of 200°C to 600°C, and a holding time of 1h to 6h;

[0534] (th6) The doping heat treatment includes P doping treatment, and the step of performing the heat treatment includes: performing one or both of O doping treatment and S doping treatment and then performing P doping treatment, and the conditions for performing the P doping treatment include: a heating rate of 2°C / min to 10°C / min, a holding temperature of 200°C to 600°C, and a holding time of 1h to 6h; wherein, the conditions for performing the O doping treatment include: heating at a heating rate of 2°C / min to 10°C / min in an air atmosphere, and performing the holding treatment in a nitrogen atmosphere at 400°C to 800°C, and the holding time is 1h to 6h; further, the conditions for the S doping treatment can refer to feature (th5), but are not limited thereto;

[0535] (th7) in the steps of performing doping heat treatment and performing soft carbonization heat treatment, the soft carbon precursor includes a liquid precursor;

[0536] (th8) The conditions for soft carburization heat treatment include: heat preservation treatment in an inert atmosphere at 700°C to 1300°C.

[0537] In some embodiments, the soft carbon precursor includes asphalt; further, the temperature for the soft carbonization treatment may be 1150°C, but is not limited thereto, and may be heat-insulated at 700°C to 1300°C, or heat-insulated at 1000°C to 1200°C, or may be any of the following temperatures or a range selected from any two of the following temperatures: 700°C, 800°C, 900°C, 1000°C, 1020°C, 1040°C, 1050°C, 1060°C, 1080°C, 1100°C, 1120°C, 1140°C, 1150°C, 1160°C, 1180°C, 1200°C, 1250°C, 1300°C, etc. By more reasonably controlling the amount of asphalt used, it is beneficial for the asphalt to better maintain the effect of the fast ion ring, to improve the uniformity of the coating of the asphalt, and to form more carbon end faces to make the lithium insertion more efficient. The fast charging capacity of the battery can be improved through the aforementioned multiple effects.

[0538] By introducing pitch into the soft carbon precursor, one or more of the following advantages can be achieved: (1) Rich carbon source: It can be converted into a carbon-based substance with a high carbon content by high-temperature treatment, for example, it can be converted into soft carbon; (2) Controllable structure: Pitch can form complex microporous and mesoporous structures during pyrolysis, which helps to increase the specific surface area and porosity of the formed carbon-based substance, thereby improving the electrochemical performance of lithium-ion batteries; (3) Cost-effectiveness: Pitch is an industrial by-product with a wide source, relatively low price, and relatively high cost-effectiveness; (4) Promote graphite (5) Improved interface performance: Asphalt-derived carbon-based materials can improve the interface performance between soft carbon and electrolyte, reduce the occurrence of side reactions, and improve the cycle stability and safety of batteries; (6) Easy to modify: Asphalt can be chemically modified to introduce heteroatoms (such as N, P, S, etc.), which can further optimize the electrochemical properties of carbon-based materials.

[0539] At the battery level, the introduction of electronegative doping elements such as N, P, and S may have one or more of the following effects:

[0540] (1) Structural stability: The incorporation of N, P, and S elements may change the interlayer distance and structure of the carbon-based residue. Controlling the appropriate doping amount is conducive to controlling the interlayer distance change of the carbon-based material within a more appropriate range, which is beneficial to improving the structural stability and cycle performance of the battery during the insertion and deinsertion of lithium ions.

[0541] (2) Electrochemical performance: When doping, by controlling the probability of N, P, and S doping elements reacting with the electrolyte, the formation of additional by-products can be reduced or avoided. For example, the thickening of the SEI film (solid electrolyte interface film) and the increase in the internal resistance of the battery can be inhibited, which is conducive to achieving better battery dynamics and cycle stability. In addition, by controlling the doping elements within a more appropriate content range, it is also helpful to inhibit or avoid the doping elements from participating in additional electrochemical reactions, reduce the probability of consuming active lithium ions, and help achieve better battery capacity.

[0542] (3) Thermal stability: The introduction of N, P, and S elements may affect the thermal stability of the battery. For example, by introducing the S element and controlling it within a more appropriate range, it is beneficial to improve the stability of the battery at high temperatures and better control the thermal runaway risk of the battery.

[0543] (4) Capacity and actual energy density: By controlling the doping amount of N, P, and S elements within a more appropriate range, the conductivity of graphite and the diffusion rate of lithium ions can be improved by doping with N, P, and S elements, thereby increasing the battery capacity and actual energy density. At the same time, it can also effectively reduce the risk of damage to the carbon-based material structure and effectively improve the overall performance of the battery.

[0544] In some embodiments, during N-doping and P-doping, the heating rate is 2°C / min to 6°C / min, and the precursor containing N and P is mixed with asphalt and a negative electrode active body (such as graphite) at 1150°C, and the mixture is kept warm for 9h to 12h.

[0545] In some embodiments, during the S doping process, the heating rate is 6°C / min to 10°C / min, and the S-containing precursor is mixed with asphalt and a negative electrode active body (such as graphite) at 1150°C, and the mixture is kept warm for 9h to 12h.

[0546] In some embodiments, in the electronegative coated active material, the coating layer includes doped soft carbon, and in this case, the carbon matrix in the coating layer includes soft carbon.

[0547] In some embodiments, the electronegative coating active material includes a negative electrode active body and doped soft carbon located on at least a portion of the surface of the negative electrode active body.

[0548] Without limitation, the negative electrode active body in the electronegative coated active material can be any suitable negative electrode active material, and reference can also be made to the first aspect of the present application. In some embodiments, the negative electrode active body in the electronegative coated active material includes a carbon-based active material, further, the carbon-based active material can include graphite, and further, the carbon-based active material can be graphite.

[0549] In some embodiments, the negative electrode active body in the electronegative coated active material includes graphite, and further, the graphite may include one or more of artificial graphite and natural graphite.

[0550] In some embodiments, the soft carbon precursor is asphalt, the doping precursor is one or more of sulfur, urea and red phosphorus, and further, the doping precursor is one of sulfur, urea and red phosphorus. Furthermore, the negative electrode active body is secondary particle graphite.

[0551] In some embodiments, one or more of the following methods ① to ⑤ are used to perform electronegative doping and coating treatment on the negative electrode active body without a coating layer to introduce doped soft carbon including electronegative doping elements into the coating layer.

[0552] In some embodiments, the electronegative coated negative electrode active material (:

[0553] Method ①: first physically mix the asphalt and the electronegative doping precursor to obtain a precursor mixture, and then carbonize the precursor mixture and the secondary graphite particles to form a coating layer including soft carbon on the surface of the secondary graphite particles. When S doping is introduced, the heating rate can be reduced and the insulation time can be extended; when P is introduced, after introducing O / S (where O / S can represent one or both of O and S), O / S can be bonded with P to form one or both of COP bonds and CSP bonds.

[0554] An "electronegative doping precursor" is a precursor that provides an electronegative doping element.

[0555] In some embodiments, the electronegative coated active material is prepared by a method (which may be referred to as method ②) comprising the following steps:

[0556] Method ②: Use urea as the N-containing precursor.

[0557] In some embodiments, the electronegative coated active material is prepared by a method (which may be referred to as method ③) comprising the following steps:

[0558] Method ③: Sulfur is used as the S-containing precursor. Sulfur can be directly added to asphalt to form sulfided asphalt.

[0559] In some embodiments, the electronegative coated active material is prepared by a method (which may be referred to as method ④) comprising the following steps:

[0560] Method ④: Using red phosphorus as the P-containing precursor.

[0561] In some embodiments, the electronegative coated active material is prepared by a method (which may be referred to as method ⑤) comprising the following steps:

[0562] Method ⑤: The doping heat treatment and soft carbonization heat treatment process includes the regulation of the heating rate, holding temperature, and holding time; in a protective atmosphere, a heating rate of 2°C / min to 10°C / min is adopted, and the temperature is kept at 700°C to 1300°C for 6h to 12h to prepare an electronegative coated active material in which the carbon matrix in the coating layer includes soft carbon.

[0563] In some embodiments, one or both of the following methods (i) to (ii) are used to obtain a new electronegative coated active material by element replacement based on the electronegative coated active material to change the element composition of the coating layer. In this case, the coating temperature may not be affected by the asphalt.

[0564] In some embodiments, the electronegative coated active material is prepared by a method (which may be referred to as method (i)) comprising the following steps:

[0565] Method (i): N-doping and / or P-doping of the carbon matrix in the coating layer: 3 In the atmosphere, the electronegative coated active material is heated at a rate of 2°C / min to 10°C / min and kept at 400°C to 800°C for 1h to 6h to obtain a new electronegative coated active material, whose coating layer includes the electronegative doping element N. When P doping is performed, the ammonia atmosphere can be changed to a phosphorus vapor atmosphere.

[0566] In some embodiments, the electronegative coated active material is prepared by a method (which may be referred to as method (ii)) comprising the following steps:

[0567] Method (ii): S-doping the carbon matrix in the coating layer: mixing the electronegative coating active material with asphalt and sulfur, and heating it at 200°C to 600°C in an atmosphere of S vapor at a heating rate of 2°C / min to 10°C / min for 1h to 6h to obtain a new electronegative coating active material, whose coating layer includes the electronegative doping element S.

[0568] It is understood that the electronegative coated active material can be prepared by using asphalt as a soft carbon precursor, but is not limited thereto.

[0569] In some embodiments, the electronegative coated active material is prepared by one or both of the methods (a) and (b) comprising the following steps: introducing two electronegative doping elements, when a P doping element is included, introducing them by O or S grafting (i.e., first introducing O or S to make the subsequent P introduction easier).

[0570] In some embodiments, the electronegative coated active material is prepared using a method (a) comprising the following steps:

[0571] Method (a) S-doping (S-ization) the carbon matrix in the coating layer: mixing the negative electrode active body with asphalt and sulfur, and in an atmosphere of S vapor, heating at a rate of 2°C / min to 10°C / min, and keeping the temperature at 200°C to 600°C for 1h to 6h to obtain an S-doped product (an electronegative coated active material, the electronegative doping elements in the doped carbon include S).

[0572] Oxidation pretreatment (oxidation): The S-doped product is heated at a rate of 2°C / min to 10°C / min and kept at 400°C to 800°C for 1h to 6h to obtain an O-doped product; wherein, the atmosphere during the heating process is air and the atmosphere during the keeping process is nitrogen, which is beneficial to reduce or prevent the oxidation weight loss of asphalt.

[0573] In some embodiments, the electronegative coated active material is prepared using a method (b) comprising the following steps:

[0574] Method (b) adopts a step-by-step P-treatment method: the S and / or O-doped product obtained by S-treatment and / or oxidation is placed in an atmosphere of P vapor, heated at a rate of 2°C / min to 10°C / min, and kept at 200°C to 600°C for 1h to 6h.

[0575] In some embodiments, the carbon matrix in the doped carbon includes hard carbon. In this case, the coating layer in the electronegative coated active material includes doped hard carbon. The electronegative coated active material can be prepared by a method comprising the following steps:

[0576] At least a portion of the surface of the negative electrode active body is coated with hard carbon, a doping source including an electronegative doping element is introduced, and the electronegative doping element is introduced into the hard carbon to form a doped hard carbon coated on at least a portion of the surface of the negative electrode active body. The electronegative doping element can be introduced into the hard carbon by a gas replacement method.

[0577] Those skilled in the art can use existing methods to prepare hard carbon coated on at least a portion of the surface of the negative electrode active body.

[0578] In some embodiments, the gas replacement method uses one of ammonia gas, sulfur vapor, and phosphorus vapor.

[0579] In some embodiments, taking the negative electrode active body as graphite and hard carbon coating of graphite as an example, a method including the following steps can be used to coat the surface of the first negative electrode body with hard carbon to prepare a hard carbon-coated negative electrode active material: a hard carbon precursor and graphite are mixed in liquid phase and dispersed by stirring; the hard carbon precursor is solidified on the graphite surface by drying at 100°C to 300°C, and then carbonized at 700°C to 1300°C for 1h to 6h, and the heating rate of the carbonization treatment can be 2°C / min to 10°C / min; during the carbonization process, the hard carbon precursor is surface polymerized on the graphite surface to achieve hard carbon coating on at least a portion of the surface of the negative electrode active body, and the prepared hard carbon-coated negative electrode active material includes a graphite body (the corresponding negative electrode active body is graphite) and a hard carbon coating layer located on at least a portion of the surface of the graphite body.

[0580] A hard carbon precursor containing a relatively high content of electronegative doping elements may be selected, and non-limiting examples of the hard carbon precursor may include one or more of polyimide, melamine, polyacrylonitrile (PAN), and the like.

[0581] In some embodiments, the electronegative doping elements may be introduced into the hard carbon in the coating layer based on the hard carbon coated negative electrode active material by the following gas replacement method, but are not limited to these doping methods:

[0582] N-doping and / or S-doping: placing the hard carbon-coated negative electrode active material in ammonia NH 3 Or in an atmosphere of S vapor, at a heating rate of 2°C / min to 10°C / min, keep at 400°C to 700°C for 1h to 6h;

[0583] Oxidation treatment: the hard carbon coated negative electrode active material is heated at a rate of 2°C / min to 10°C / min and kept at 400°C to 700°C for 1h to 6h; the atmosphere during the heating process is air and the atmosphere during the insulation process is nitrogen, which is beneficial to reduce or prevent oxidation weight loss of the coating layer;

[0584] P doping: place the S-doped or oxidized hard carbon-coated negative electrode active material in an atmosphere of P vapor, and heat it at 400°C to 700°C for 1h to 6h at a heating rate of 2°C / min to 10°C / min.

[0585] In the third aspect of the present application, an electrical device is provided, which includes the lithium-ion secondary battery described in the first aspect of the present application, the aforementioned negative electrode plate, and at least one of the lithium-ion secondary batteries prepared by the preparation method of the lithium-ion secondary battery described in the second aspect of the present application.

[0586] The electrical device including the aforementioned lithium-ion secondary battery can have the advantageous effects of the aforementioned lithium-ion secondary battery, including but not limited to having improved fast charging performance.

[0587] The electrical device including the aforementioned negative electrode sheet can have the advantageous effects of the aforementioned negative electrode sheet, including but not limited to having improved fast charging performance.

[0588] In some embodiments, the electrical device includes the lithium-ion secondary battery according to any of the embodiments provided in the present application.

[0589] The lithium-ion secondary battery can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device can include, but is not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc. Among them, the mobile device can be, for example, a mobile phone, a laptop computer, etc.; the electric vehicle can be, for example, a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, an electric motorcycle, an electric tool, etc., but is not limited to this. The electrical device can also be applied to military equipment, aerospace and other fields, and can also be applied to energy storage power systems such as hydraulic, thermal, wind and solar power stations.

[0590] As the electrical device, the lithium-ion secondary battery can be selected according to its usage requirements.

[0591] Figure 6 This is the electrical device 6 as an example. The electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the electrical device for the lithium-ion secondary battery, a battery device or a battery pack can be used.

[0592] Another example of the device can be a mobile phone, a tablet computer, a laptop computer, etc. This device usually requires thinning, and a lithium-ion secondary battery can be used as the power source.

[0593] In the fourth aspect of the present application, there is provided an application of the lithium-ion secondary battery described in the first aspect of the present application in supplying electrical energy and / or storing electrical energy;

[0594] This application includes the process of charging the lithium-ion secondary battery under a rate condition of higher than or equal to 2C, that is, the lithium-ion secondary battery can provide a charging rate greater than or equal to 2C.

[0595] In this application, unless otherwise specified, the "rate" of a battery has a well-known meaning in the art, which refers to the current value required for the battery to charge to its rated capacity or discharge its rated capacity within a specified time, and the unit is expressed in C. Unless otherwise specified, the "specified time" is 1 hour (h), 1C means that charging and discharging are completed in 1 hour, and 1 / 3C means that charging and discharging are completed in 3 hours. The battery rate reflects the charging and discharging capabilities of the battery under different currents. A high rate means that the battery can charge and discharge quickly in a short time. The higher the rate, the better the fast charging performance.

[0596] In this application, unless otherwise specified, for the rate, "1C" means the current value required to charge the battery from zero to full or from full to empty within 1 hour. For the charging rate, when charging from zero at a rate of 1C, the battery will be fully charged in 1 hour.

[0597] In some embodiments, the application includes a process of charging a lithium ion secondary battery under at least one rate condition of 2C to 6C, that is, the lithium ion secondary battery can provide a charging rate of 2C to 6C.

[0598] In some embodiments, the application includes a process of charging a lithium-ion secondary battery under at least one rate condition of 2C to 4C.

[0599] In some embodiments, the application includes a process of charging a lithium-ion secondary battery under at least one rate condition of 4C to 6C.

[0600] Without limitation, the lithium-ion secondary battery can be charged at any of the following rates, or can be charged at a rate greater than or equal to any of the following rates, or can be charged at a rate within a range consisting of any two of the following rates: 2C, 3C, 4C, 5C, 6C, etc.

[0601] In some embodiments, the lithium-ion secondary battery can provide a charge rate greater than or equal to 2C.

[0602] In some embodiments, the lithium-ion secondary battery can provide a charge rate of 2C to 6C.

[0603] In the present application, “capable of providing a charging rate Cx” means that the battery cell can be charged under the condition of the charging rate Cx; for example, it can be charged to 97% SOC, but is not limited to this SOC state.

[0604] In some embodiments, the maximum charge rate of the lithium ion secondary battery may be greater than or equal to 2C, and may be 2C to 6C, and may be 2C to 4C or 4C to 6C.

[0605] In some embodiments, the maximum charge rate of the lithium-ion secondary battery can be greater than 2C (C max >2C), can be greater than 2C and less than or equal to 6C (2 <C max ≤6C), and can further be greater than 2C and less than or equal to 4C (2 <C max ≤4C) or can be greater than or equal to 4C and less than or equal to 6C (4C≤C max ≤6C).

[0606] Without limitation, the maximum charge rate of the lithium-ion secondary battery may also be any one of the following charge rates or a range consisting of any two of the following charge rates: 2C, 3C, 4C, 5C, 6C, etc.

[0607] In this application, the maximum charge rate (which can be recorded as C) of a lithium-ion secondary battery is max )” has a well-known meaning in the industry and can be obtained by testing using conventional methods in the art. For example, the lithium precipitation window curve can be obtained by testing at different charging rates, and the critical charging rate at which lithium precipitation occurs is used as the test value of the maximum charging rate of the battery. The test parameters can be as follows: the battery to be tested is charged to a voltage of 3.8V at different rates (such as 1C, 2C, 2.5C, 3C, 3.5C, 4C, ...), then charged to a current of ≤0.05C at constant voltage, left to stand for 5 minutes, and then charged to 3.8V at a constant current of 0.33C, left to stand for 5 minutes, and the battery is disassembled to observe the lithium precipitation of the negative electrode. Exemplarily, a series of parallel samples can be prepared, starting from 1C, and tested at intervals of 0.1C until lithium precipitation occurs at the negative electrode. In order to reduce the amount of samples, a large interval can be selected to determine the range of the maximum charging rate, and then a small interval can be selected to more accurately determine the maximum charging rate. The intervals of the rate can be selected from 1C, 0.5C, 0.2C, and 0.1C, respectively.

[0608] Below, some embodiments of the present application are described. The described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The embodiments described below are exemplary and are only used to explain the present application, and should not be understood as limitations on the present application and its applications. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

[0609] If the techniques or conditions are not specified in the examples, the above description, or the techniques or conditions described in the literature in the field, or the product instructions are used. If the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be obtained commercially, or can be synthesized in a conventional manner from commercially available products.

[0610] In the following specific embodiments, the measured parameters of raw material components may have slight deviations within the range of weighing accuracy unless otherwise specified. For temperature and time parameters, acceptable deviations caused by instrument test accuracy or operation accuracy are allowed.

[0611] In the following examples, room temperature refers to 20°C to 30°C.

[0612] In the following examples, unless otherwise specified, the parameters involved can be confirmed by referring to the test methods described above. For example, the percentage of secondary graphite particles in the first negative electrode active material and the percentage of secondary graphite particles in the negative electrode active material can be confirmed based on SEM (such as Sigma 300 scanning electron microscope of ZEISS, Germany) test results; the D ratio of the first negative electrode active material and the second negative electrode active material can be confirmed based on SEM test results. v 50 Malvern 2000 (MasterSizer 2000) laser particle size analyzer can be used for testing; for coating layers in electronegative coated active materials and carbon coating layers in lithium iron phosphate-based positive electrode active materials, transmission electron microscope JEM-F200 combined with EDS (energy spectrometer) can be used for testing and analysis; for the doping form of electronegative doping elements in electronegative coated active materials, X-ray photoelectron spectrometer (XPS, Thermo Scientific ESCALAB Xi+) can also be used for testing and analysis. For example, the compaction density of the negative electrode sheet, the comparison of the compaction density of the first negative electrode active layer and the second negative electrode active layer, the comparison of the surface density of the first negative electrode active layer and the second negative electrode active layer (measured on a single side of the negative electrode current collector), the porosity of the first negative electrode active layer and the porosity of the second negative electrode active layer, the porosity of the negative electrode active material layer, the difference in the compaction density of the second negative electrode active layer relative to the first negative electrode active layer (P Δ and P Δ0 ), the ratio of the difference in compaction density of the second negative electrode active layer relative to the first negative electrode active layer to the compaction density of the negative electrode sheet (R PΔ and R PΔ0 ), rate comparison of the first negative electrode active layer and the second negative electrode active layer, charge rate comparison of the first negative electrode active layer and the second negative electrode active layer, etc. For example, the ion conductivity test of the electrolyte is also involved, using a DDSJ-318 conductivity meter and referring to the HG-T 4067-2015 detection method.

[0613] 1. Method for preparing electronegative coated active material for use as first negative electrode active material

[0614] The negative electrode active body used for coating is artificial graphite and includes secondary graphite particles, and the amount of the secondary graphite particles in the negative electrode active body raw material accounts for 20% to 100% (optionally 40% to 100%, further optionally 40% to 80), Dv 50 is 8μm to 16μm (optionally 10μm to 14μm). The negative electrode active body corresponds to the negative electrode active body in the electronegative coated active material. The prepared electronegative coated active material includes electronegative coated secondary graphite particles, and accordingly, the amount of electronegative coated secondary graphite particles in the first negative electrode active material accounts for 20% to 100% (optionally 40% to 100%, further optionally 40% to 80).

[0615] (a) The carbon matrix in the coating layer is soft carbon.

[0616] Electronegative coated active materials are prepared by one or more of methods ① to ⑤, (i) and (ii), (a) and (b), wherein the "electronegative doping precursor" is a precursor that provides electronegative doping elements. The electronegative coated active material is prepared by using asphalt as a soft carbon precursor. The doping heat treatment can be performed first, followed by the soft carbonization heat treatment, or the doping heat treatment and the soft carbonization heat treatment can be performed simultaneously. The heating rate for the soft carbonization heat treatment is 2°C / min to 10°C / min, the insulation temperature is 700°C to 1300°C, and the insulation time is 1h to 6h.

[0617] Method ① Physically mix the asphalt and the electronegative doping precursor to obtain a precursor mixture, and carbonize the precursor mixture and the negative electrode active body to form a coating layer including soft carbon on the surface of the negative electrode active body. When S doping is introduced, the heating rate can be reduced and the insulation time can be extended; when P is introduced, after introducing O / S (where O / S can represent one or both of O and S), O / S can be bonded with P to form one or both of COP bonds and CSP bonds.

[0618] Method ② uses urea as the N-containing precursor.

[0619] Method ③ uses sulfur as the S-containing precursor. Sulfur can be directly added to asphalt to form sulfided asphalt.

[0620] Method ④ uses red phosphorus as the P-containing precursor.

[0621] Method ⑤ The doping heat treatment and soft carbonization heat treatment process includes the regulation of the heating rate, the holding temperature, and the holding time; in a protective atmosphere, a heating rate of 2°C / min to 10°C / min is adopted, and the temperature is kept at 700°C to 1300°C for 6h to 12h to prepare an electronegative coated active material; in this method, the doping heat treatment and the soft carbonization heat treatment process are carried out simultaneously.

[0622] Method (i) N-doping and / or P-doping the carbon matrix in the coating layer: 3In the atmosphere, the electronegative coated active material is heated at a rate of 2°C / min to 10°C / min and kept at 400°C to 800°C for 1h to 6h to obtain a new electronegative coated active material, whose coating layer includes the electronegative doping element N. When P doping is performed, the ammonia atmosphere is changed to a phosphorus vapor atmosphere.

[0623] Method (ii) S-doping the carbon matrix in the coating layer: mixing the electronegative coating active material with asphalt and sulfur, and heating it at 200°C to 600°C in an atmosphere of S vapor at a heating rate of 2°C / min to 10°C / min for 1h to 6h to obtain a new electronegative coating active material, whose coating layer includes the electronegative doping element S.

[0624] Two electronegative doping elements S and P are introduced by methods (a) and (b) comprising the following steps, wherein the step of introducing the P doping element is introduced by O or S grafting (i.e., O or S is introduced first to make the subsequent P introduction easier).

[0625] Method (a) S-doping (S-ization) of the carbon matrix in the coating layer: mixing the negative electrode active body with asphalt and sulfur, and keeping it at 200°C to 600°C in an atmosphere of S vapor at a heating rate of 2°C / min to 10°C / min for 1h to 6h.

[0626] Oxidation pretreatment (oxidation): The electronegative coated active material is heated at a rate of 2°C / min to 10°C / min and kept at 400°C to 800°C for 1h to 6h; the atmosphere during the heating process is air and during the insulation process is nitrogen to reduce or prevent the oxidation weight loss of the asphalt.

[0627] Method (b) adopts a step-by-step P-treatment method: the electronegative coated active material obtained by S-treatment and / or oxidation is placed in an atmosphere of P vapor, heated at a rate of 2°C / min to 10°C / min, and kept at 200°C to 600°C for 1h to 6h.

[0628] (ii) The carbon matrix in the coating layer is hard carbon.

[0629] The method for preparing an electronegative coating active material whose carbon substrate is hard carbon comprises the following steps S210 and S220.

[0630] S210: preparing hard carbon coated negative electrode active material.

[0631] A hard carbon precursor with a high nitrogen content (polyacrylonitrile (PAN)) is mixed with graphite in liquid phase and dispersed by stirring; the hard carbon precursor is dried at 100°C to 300°C to solidify the hard carbon precursor on the graphite surface, and then carbonized at 700°C to 1300°C for 1h to 6h, and the heating rate of the carbonization treatment can be 2°C / min to 10°C / min; during the carbonization process, the hard carbon precursor is surface polymerized on the graphite surface and converted into hard carbon, and the prepared hard carbon-coated negative electrode active material includes a graphite body and a hard carbon coating layer located at least a portion of the surface of the graphite body, and the hard carbon coating layer has a high nitrogen content and can be recorded as a high nitrogen hard carbon coating layer.

[0632] S220: introducing electronegative doping elements into the hard carbon coating layer of the hard carbon coated negative electrode active material by a gas replacement method.

[0633] N-doping and / or P-doping of the hard carbon coating: When N-doping is performed, the NH 3 In the atmosphere, the hard carbon coated negative electrode active material is heated at a rate of 2°C / min to 10°C / min, and kept at 400°C to 800°C for 1h to 6h to obtain an electronegative coated active material, wherein the coating layer includes an electronegative doping element N. When P doping is performed, the ammonia atmosphere is changed to a phosphorus vapor atmosphere.

[0634] The hard carbon coating layer is doped with S: the hard carbon coated negative electrode active material is placed in an atmosphere of S vapor, heated at a rate of 2°C / min to 10°C / min, and kept at 200°C to 600°C for 1h to 6h to obtain an electronegative coated active material, wherein the coating layer includes the electronegative doping element S.

[0635] Two electronegative doping elements S and P can also be introduced by methods (a2) and (b2) comprising the following steps, wherein the step of introducing the P doping element is introduced by O or S grafting (i.e., O or S is introduced first to make the subsequent P introduction easier).

[0636] Method (a2) S-doping (S-ization) of the carbon matrix in the coating layer: mixing the hard carbon-coated negative electrode active material with sulfur, and keeping it at 200°C to 600°C for 1h to 6h at a heating rate of 2°C / min to 10°C / min in an atmosphere of S vapor.

[0637] Oxidation pretreatment (oxidation): The hard carbon-coated negative electrode active material is heated at a rate of 2°C / min to 10°C / min and kept at 400°C to 800°C for 1h to 6h; the atmosphere during the heating process is air and the atmosphere during the insulation process is nitrogen to reduce or prevent the oxidation weight loss of the asphalt.

[0638] Method (b2) adopts a step-by-step P treatment method: the electronegative coated active material obtained by S treatment and / or oxidation is placed in an atmosphere of P vapor, heated at a rate of 2°C / min to 10°C / min, and kept at 200°C to 600°C for 1h to 6h.

[0639] In some embodiments, based on the hard carbon coated negative electrode active material, the following methods can be used to introduce electronegative doping elements into the hard carbon in the coating layer, but are not limited to these doping methods:

[0640] 1. Perform N-doping or S-doping: place the hard carbon-coated negative electrode active material in ammonia NH 3 Or in an atmosphere of S vapor, at a heating rate of 2°C / min to 10°C / min, keep at 400°C to 700°C for 1h to 6h;

[0641] 2. Oxidation treatment: the hard carbon coated negative electrode active material is heated at a rate of 2°C / min to 10°C / min and kept at 400°C to 700°C for 1h to 6h; the atmosphere during the heating process is air and the atmosphere during the insulation process is nitrogen, which is beneficial to reduce or prevent oxidation weight loss of the coating layer;

[0642] 3. P doping: place the S-doped or oxidized hard carbon-coated negative electrode active material in an atmosphere of P vapor, and heat it at 400°C to 700°C for 1h to 6h at a heating rate of 2°C / min to 10°C / min.

[0643] 2. Preparation of negative electrode sheet and lithium-ion secondary battery

[0644] Example 1.

[0645] (1) Preparation of positive electrode sheet:

[0646] The positive electrode active material lithium iron phosphate (LFP, surface coated with soft carbon), conductive agent (Super P), and binder (PVDF) were mixed in a ratio of 96:2:2, and solvent N-methylpyrrolidone (NMP) was added. The mixture was stirred in a vacuum mixer until the system became uniform and transparent to obtain a positive electrode slurry with a solid content of 60wt%; the coating surface density (single side) was 0.18g / 1540.25m 2 (about 11.7mg / cm 2 ) The positive electrode slurry is evenly coated on both sides of the positive electrode current collector aluminum foil, and the coating surface density on both sides is basically the same; the positive electrode current collector coated with the positive electrode slurry is dried at room temperature and then transferred to an oven for drying, and then cold pressed and cut to obtain the positive electrode sheet. The compacted density of the positive electrode sheet is 2.50g / cm 3 .

[0647] The lithium iron phosphate-based positive electrode active material is lithium iron phosphate coated with soft carbon, and the mass proportion of soft carbon in the positive electrode active material is about 1%.

[0648] (2) Preparation of negative electrode sheet:

[0649] The powder of the first negative electrode active material (corresponding to the first negative electrode active layer in the upper layer) is mixed with a thickener (sodium carboxymethyl cellulose), a binder (SBR), and a conductive agent (Super P) in a ratio of 97.0:0.8:1.0:1.2, and mixed evenly with a solvent (deionized water) under the action of a vacuum mixer to prepare a first negative electrode slurry with a solid content of 48wt%.

[0650] The powder of the second negative electrode active material (corresponding to the second negative electrode active layer in the lower layer) is mixed with a thickener (sodium carboxymethyl cellulose), a binder (SBR), and a conductive agent (Super P) in a ratio of 96.4:1.0:1.2:1.4, and mixed evenly with a solvent (deionized water) under the action of a vacuum mixer to prepare a second negative electrode slurry with a solid content of 48wt%.

[0651] The second negative electrode slurry and the first negative electrode slurry are uniformly coated on both sides of the negative electrode current collector copper foil by an extrusion coater. The second negative electrode slurry (lower layer) is coated first, and then the first negative electrode slurry (upper layer) is coated. The single-side coating surface density of the negative electrode current collector is controlled to be 0.13g / 1540.25mm 2 (corresponding to about 8.44mg / cm 2 ), the upper and lower coating weights (one-side) ratio is controlled to be about 4:6, the coating surface densities of the upper and lower layers on both sides of the negative electrode current collector are basically the same, after oven drying, using a cold press to compact, the compaction density of the negative electrode sheet is controlled to be 1.65g / cm 3 The cold pressed electrode sheet is subjected to striping and cutting processes to obtain a negative electrode sheet. The negative electrode sheet is composed of a negative electrode current collector and a negative electrode active material layer located on both sides of the negative electrode current collector. The negative electrode active material layer is composed of a first negative electrode active layer and a second negative electrode active layer. The second negative electrode active layer is located between the negative electrode current collector and the first negative electrode active layer.

[0652] First negative electrode active layer (upper layer): the first negative electrode active material is electronegative coated graphite, the negative electrode active body is artificial graphite and secondary particle graphite (in this case, the first negative electrode active material is electronegative coated secondary particle graphite, which is both electronegative coated graphite and electronegative coated secondary particle), the coating layer is doped soft carbon, the carbon matrix is ​​soft carbon, and the doped soft carbon includes electronegative doping element N. D of the first negative electrode active material v50 is 12μm. Artificial graphite is used as the negative electrode active body, and the negative electrode active body is secondary particle graphite. Method ⑤ is adopted, and the doping heat treatment and soft carbonization heat treatment are carried out simultaneously. In a nitrogen atmosphere, asphalt is used as a soft carbon precursor and urea is used as a N-containing precursor. The negative electrode active body is subjected to N doping treatment. The heating rate during doping is 5℃ / min, the insulation temperature is 1000℃, and the insulation time is 4h. The mass percentage of urea relative to asphalt is about 0.8%.

[0653] Second negative electrode active layer (lower layer): The second negative electrode active material is artificial graphite and secondary graphite particles. v 50 is 15μm.

[0654] In Example 1, the ratio of the surface density of the first negative electrode active layer to the surface density of the second negative electrode active layer is about 4:6. The powder compaction density of the second negative electrode active material is higher than the powder compaction density of the first negative electrode active material. The ratio of the thickness of the first negative electrode active layer (upper layer) to the sum of the thickness of the first negative electrode active layer and the second negative electrode active layer (F H ) is in the range of 20% to 65%, and further in the range of 40% to 60%, F H At this time, the compaction density of the second negative electrode active layer (about 1.75g / cm 3 ) is higher than the compaction density of the first negative electrode active layer (about 1.5 g / cm 3 The thickness of the first negative electrode active layer on one side is in the range of 20 μm to 40 μm. The difference between the compaction density of the second negative electrode active layer and the compaction density of the first negative electrode active layer (which can be recorded as P Δ0 ) is about 0.25g / cm 3 , P Δ0 The ratio of the compaction density of the negative electrode (which can be recorded as R PΔ0 ) is about 0.152, R PΔ0 It is in the range of 0 to 2, and further in the range of 0 to 0.834.

[0655] In Example 1, the porosity of the first negative electrode active layer is higher than that of the second negative electrode active layer, wherein the porosity of the first negative electrode active layer located on the upper layer is about 26.14%.

[0656] (3) Isolation film: A polyethylene film with a thickness of 12 microns (μm) is selected.

[0657] (4) Preparation of electrolyte: Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed in a mass ratio of 30:70 to obtain an organic solvent, and then fully dried lithium salt lithium hexafluorophosphate (LiPF6) is dissolved in the mixed organic solvent, and vinylene carbonate (VC), fluoroethylene carbonate (FEC) and vinyl sulfate (DTD) are added to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1 mol / L. The electrolyte contains VC with a mass concentration of 2.5wt%, FEC with a mass concentration of 1wt%, and DTD with a mass concentration of 0.5wt%.

[0658] The ionic conductivity of the electrolyte at 25°C is 15 mS / cm.

[0659] (5) Assembling secondary batteries: stacking and winding the positive electrode sheet, the separator, and the negative electrode sheet in order to obtain an electrode assembly; placing the electrode assembly in an outer package, injecting electrolyte after drying, and undergoing vacuum packaging, standing, forming, shaping and other processes to obtain a lithium-ion secondary battery.

[0660] Example 2. A first negative electrode active material, a negative electrode plate and a lithium ion secondary battery are prepared by a method substantially the same as that of Example 1, except that: in the step of preparing the first negative electrode active material, the N doping treatment is changed to firstly performing the O doping treatment and then performing the P doping treatment, and red phosphorus is used as the P-containing precursor; a different first negative electrode active material is used to prepare the negative electrode plate; and a different negative electrode plate is used to prepare the lithium ion secondary battery. The remaining steps are the same as those of Example 1.

[0661] The negative electrode active body is firstly subjected to O doping treatment and then subjected to P doping treatment.

[0662] O-doping treatment was performed: the temperature was increased at a heating rate of 6°C / min in an air atmosphere, and the temperature was kept at 600°C in a nitrogen atmosphere for 4 hours to obtain an O-doped product; the atmosphere during the heating process was air, and the atmosphere during the keeping process was nitrogen.

[0663] The O-doped product was subjected to P-doping treatment: in a nitrogen atmosphere, asphalt was used as a soft carbon precursor, and red phosphorus was used as a P-containing precursor. The heating rate during doping was 5°C / min, the holding temperature was 1000°C, and the holding time was 6h.

[0664] Example 3. A first negative electrode active material, a negative electrode plate and a lithium ion secondary battery are prepared by a method substantially the same as that of Example 1, except that: in the step of preparing the first negative electrode active material, the N doping treatment is replaced by the S doping treatment, and sulfur is used as the S-containing precursor; a different first negative electrode active material is used to prepare the negative electrode plate; and a different negative electrode plate is used to prepare the lithium ion secondary battery. The remaining steps are the same as those of Example 1.

[0665] S doping treatment: In a nitrogen atmosphere, asphalt is used as a soft carbon precursor, sulfur is used as a S-containing precursor, the heating rate is 5°C / min, the holding temperature is 1000°C, and the holding time is 5h.

[0666] Example 4. A first negative electrode active material, a negative electrode sheet and a lithium ion secondary battery are prepared by a method substantially the same as that of Example 1, except that: in the step of preparing the first negative electrode active material, the N doping treatment is changed to first S doping treatment and then P doping treatment; a different first negative electrode active material is used to prepare the negative electrode sheet; a different negative electrode sheet is used to prepare the lithium ion secondary battery. The remaining steps are the same as those of Example 1.

[0667] S doping treatment: In a nitrogen atmosphere, asphalt was used as a soft carbon precursor, sulfur was used as a S-containing precursor, the heating rate was 6°C / min, the holding temperature was 400°C, and the holding time was 4h to obtain an S-doped product.

[0668] The S-doped product was subjected to P-doping treatment: in a nitrogen atmosphere, red phosphorus was used as a P-containing precursor, the heating rate during doping was 5°C / min, the holding temperature was 1000°C, and the holding time was 6h.

[0669] Example 5. A first negative electrode active material, a negative electrode plate and a lithium-ion secondary battery are prepared by a method substantially the same as that of Example 1, except that: in the step of preparing the first negative electrode active material, the mass percentage of urea relative to asphalt is about 1.2%; different first negative electrode active materials are used to prepare negative electrode plates; different negative electrode plates are used to prepare lithium-ion secondary batteries. The remaining operation steps are the same as those of Example 1. Compared with Example 1, the mass proportion of N in the doped carbon and the coating layer in Example 5 is increased.

[0670] Example 6. A first negative electrode active material, a negative electrode sheet and a lithium ion secondary battery were prepared by a method substantially the same as that of Example 1, except that in the step of preparing the electrolyte, the amount ratio of the solvent EC and EMC was changed so that the ion conductivity of the electrolyte at 25° C. was about 13.7 mS / cm, and a lithium ion secondary battery was prepared using a different electrolyte. The remaining steps were the same as those of Example 1.

[0671] Example 7. A negative electrode sheet and a lithium-ion secondary battery are prepared by using a method substantially the same as that of Example 1, except that in the step of preparing the electrolyte, the composition of the electrolyte is changed, and a lithium-ion secondary battery is prepared using a different electrolyte. The remaining steps are the same as those of Example 1.

[0672] In Example 7, the type of solvent is changed, and the solvent is ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a volume ratio of 1:2:1; the type and amount of the electrolyte lithium salt, the type and amount of the additive are the same as in Example 1.

[0673] The electrolyte of Example 7 has an ionic conductivity of about 17.8 mS / cm at 25°C.

[0674] Embodiment 8-9. The first negative electrode active material, negative electrode sheet and lithium ion secondary battery are prepared by the method basically the same as that of embodiment 1, except that: in the step of preparing the negative electrode sheet, the coating weight ratio of the first negative electrode slurry and the second negative electrode slurry is changed, that is, the ratio of the surface density of the first negative electrode active layer to the surface density of the second negative electrode active layer is changed, and the sum of the coating surface densities of both sides and the two layers of the negative electrode sheet remains basically unchanged; different negative electrode sheets are used to prepare lithium ion secondary batteries. The remaining operation steps are the same as those of embodiment 1.

[0675] In Example 8, the ratio of the surface density of the first negative electrode active layer to the surface density of the second negative electrode active layer is 2:8.

[0676] In Example 9, the ratio of the surface density of the first negative electrode active layer to the surface density of the second negative electrode active layer is 6:4.

[0677] In Examples 10-11, the first negative electrode active material, the negative electrode sheet and the lithium ion secondary battery are prepared by the same method as in Example 1, except that: in the step of preparing the negative electrode sheet, the type of electronegative coating active material in the first negative electrode active layer is changed, and the type of negative electrode active body is changed; different negative electrode sheets are used to prepare lithium ion secondary batteries. The remaining steps are the same as in Example 1.

[0678] The negative electrode active body of Example 10 adopts a combination of artificial graphite and soft carbon in a mass ratio of 1:2%.

[0679] The negative electrode active body of Example 11 adopts a combination of artificial graphite and hard carbon in a mass ratio of 1:2%.

[0680] Example 12. A first negative electrode active material, a negative electrode plate and a lithium-ion secondary battery are prepared by a method substantially the same as that of Example 1, except that in the step of preparing the positive electrode plate, the type of positive electrode active material is changed to a ternary positive electrode material NCM811 (LiNi 0.8 Co 0.1 Mn 0.1 O 2 ); different positive electrode sheets were used to prepare lithium-ion secondary batteries. The remaining steps were the same as those in Example 1.

[0681] Example 13. A first negative electrode active material, a negative electrode plate and a lithium ion secondary battery are prepared by a method substantially the same as that of Example 1, except that: in the electronegative coated active material used as the first negative electrode active material, the carbon matrix of the doped carbon in the coating layer is hard carbon, and the electronegative doping element is N; different first negative electrode active materials are used to prepare negative electrode plates; different negative electrode plates are used to prepare lithium ion secondary batteries. The remaining steps are the same as those of Example 1.

[0682] (1) Preparation of hard carbon-coated negative electrode active materials

[0683] A hard carbon precursor (polyacrylonitrile (PAN)) with a high nitrogen content is mixed with graphite in liquid phase and dispersed by stirring; the hard carbon precursor is solidified on the graphite surface by drying at 200°C, and then carbonized at 1000°C for 3 hours with a heating rate of 5°C / min to prepare a hard carbon-coated negative electrode active material with a hard carbon coating layer on its surface having a high nitrogen content.

[0684] (2) The electronegative doping element N is introduced into the hard carbon coating layer of the hard carbon coated negative electrode active material by using the gas replacement method.

[0685] N-doping of the hard carbon coating: 3 In the atmosphere, the hard carbon coated negative electrode active material is heated at a rate of 5°C / min and kept at 600°C for 4 hours to obtain an electronegative coated active material. The coating layer includes a carbon matrix and an electronegative doping element N, and the carbon matrix is ​​hard carbon.

[0686] Example 14. A negative electrode sheet and a lithium-ion secondary battery were prepared by using a method substantially the same as that of Example 1, except that the negative electrode active material layer of the negative electrode sheet adopted a single-layer structure, the first negative electrode active material of Example 1 was used as the negative electrode active material of Example 14, and a lithium-ion secondary battery was prepared using a different negative electrode sheet. The negative electrode sheet was prepared by the following method:

[0687] The powder of the negative electrode active material (electronegative coated graphite) is mixed with a thickener (sodium carboxymethyl cellulose), a binder (SBR), and a conductive agent (SuperP) in a ratio of 96:0.8:2.0:1.2, and mixed evenly with a solvent (deionized water) under the action of a vacuum mixer to prepare a negative electrode slurry with a solid content of 48wt%.

[0688] The negative electrode slurry is evenly coated on both sides of the negative electrode current collector copper foil by an extrusion coater (the coating parameters on both sides of the negative electrode current collector are basically the same), and the coating weight on one side of the negative electrode current collector is 0.13g / 1540.25mm 2 After drying in an oven, the negative electrode is compacted using a cold press to control the compaction density of the negative electrode to 1.60g / cm 3The cold pressed electrode sheet is subjected to striping and cutting processes to obtain the negative electrode sheet. Based on the single side of the negative electrode current collector, the surface density of the negative electrode sheet is about 8.44 mg / cm 2 .

[0689] The negative electrode active material is electronegative coated graphite, the negative electrode active body is artificial graphite and secondary graphite particles (in this case, the negative electrode active material is electronegative coated secondary graphite particles, which is both electronegative coated graphite and electronegative coated secondary particles), the coating layer is doped soft carbon, the carbon matrix is ​​soft carbon, and the doped soft carbon includes electronegative doping element N. v 50 is 12 μm. Artificial graphite is used as the negative electrode active body, and the negative electrode active body is secondary particle graphite.

[0690] In this example, the porosity of the negative electrode active material layer is about 28.91%.

[0691] Comparative Examples 1-4. A first negative electrode active material, a negative electrode sheet and a lithium ion secondary battery were prepared by substantially the same method as in Example 1, except that the doping precursor in the coating step was omitted when preparing the first negative electrode active material, and no electronegative doping was performed.

[0692] Comparative Examples 1-4 respectively adopt the same method as Examples 1 and 10-12 to prepare negative electrode sheets and lithium-ion secondary batteries, except that: when the first negative electrode active material is soft-carbon coated, no doping precursor is introduced, and only asphalt is used for coating, the mass ratio of the first negative electrode active material, the conductive agent, the stabilizer and the binder is the same as that in Examples 1 and 10-12, different first negative electrode active materials are used to prepare negative electrode sheets; different negative electrode sheets are used to prepare lithium-ion secondary batteries; the remaining operation steps are the same as those of the corresponding Examples 1 and 10-12. The sum of the double-sided coating surface density of the negative electrode sheet is the same as that of the corresponding Examples 1 and 10-12.

[0693] Comparative Example 5. A first negative electrode active material, a negative electrode sheet, and a lithium ion secondary battery were prepared by a method substantially the same as that of Example 13, except that: no electronegative doping element was introduced when preparing the first negative electrode active material; a different first negative electrode active material was used to prepare the negative electrode sheet; and a different negative electrode sheet was used to prepare the lithium ion secondary battery. The first negative electrode active material of Comparative Example 5 was a hard carbon-coated negative electrode active material, and the hard carbon was not doped with an electronegative doping element.

[0694] As a non-limiting example, in the electronegative coated active materials of Examples 1-13, the average thickness of the coating layer is all in the range of 1nm to 500nm, further in the range of 100nm to 500nm, and further in the range of 100nm to 200nm; respectively satisfying that the thickness of at least a portion of the coating layer is in the range of 1nm to 1000nm; respectively satisfying that the thickness of at least a portion of the coating layer is in the range of 100nm to 1000nm; respectively satisfying that the thickness of at least a portion of the coating layer is in the range of 100nm to 200nm.

[0695] As a non-limiting example, in the electronegative coated active materials of Examples 1-13, the mass percentage of the electronegative doping element in the doped carbon and the coating layer is in the range of 0.1% to 1%, and in some examples in the range of 0.2% to 0.6%.

[0696] As a non-limiting example, in the electronegative coated active materials of Examples 1-13, the mass percentage of the coating layer in the electronegative coated active material is all within a range of 0.2% to 5%, and further within a range of 0.5% to 3%.

[0697] As a non-limiting example, in the electronegative coated active materials of Examples 1-13, the average thickness of the coating layer is about the same as D of the first negative electrode active material. v 50 are all within the range of 0.5% to 12.5%, and further within the range of 1% to 10%.

[0698] As a non-limiting example, in Examples 1-13, the ratio of the thickness of the first negative electrode active layer (upper layer) to the sum of the thickness of the first negative electrode active layer and the second negative electrode active layer (F H ) are all within the range of 20% to 65%, and further within the range of 40% to 60%. The single-side thickness of the first negative electrode active layer is all within the range of 10 μm to 50 μm, and further within the range of 20 μm to 40 μm.

[0699] The porosity of the first negative electrode active layer in Examples 1-13 is higher than that of the second negative electrode active layer, and the porosity of the first negative electrode active layer is in the range of 24% to 28%; the ratio of the difference in compaction density of the second negative electrode active layer relative to the first negative electrode active layer in the negative electrode sheet obtained after cold pressing to the compaction density of the negative electrode sheet (R PΔ0 ) are all in the range of -0.834 to 2, and most of them are in the range of 0 to 2.

[0700] Test Methods and Analysis

[0701] 1. Negative electrode sheet and negative electrode active material layer

[0702] The first negative electrode active layer is referred to as an upper layer, and the second negative electrode active layer is referred to as a lower layer.

[0703] 1. X-ray photoelectron spectroscopy (XPS) test.

[0704] XPS instrument: Thermo Scientific ESCALAB Xi+.

[0705] The clean and uncontaminated sample is placed in an ultra-high vacuum environment to prevent molecules in the air from interfering with the test results. Then, high-energy X-rays (1486.6eV aluminum Kα radiation) are irradiated onto the surface of the sample to excite the electrons on the sample surface, and the detector is used to capture and record their kinetic energy. By analyzing the kinetic energy distribution of these photoelectrons, the types of electronegative doping elements on the surface of the sample and their bonding mode with carbon (C) are analyzed.

[0706] 2. Liquid absorption rate of negative electrode

[0707] Sample to be tested: The negative electrode sheet obtained by cold pressing is used as the negative electrode sheet sample to be tested; or the battery cell is disassembled, the negative electrode sheet is taken out, and it is cleaned with solvent dimethyl carbonate, and dried to obtain the negative electrode sheet sample to be tested.

[0708] The dried negative electrode sheet was cut into 20mm×10mm sheets, the thickness of the negative electrode sheet was tested and recorded, the negative electrode sheet was fixed on the sample table, and E30 electrolyte was dripped and timed with a stopwatch; the weight increase and time were recorded; the liquid absorption rate of the negative electrode sheet was calculated by the change in weight over time.

[0709] 3. Porosity test

[0710] In each embodiment and each comparative example, the negative electrode sheet obtained after cold pressing is used as the sample to be tested.

[0711] Test of porosity of the sample to be tested: refer to GB / T 24586-2009. The porosity of the sample to be tested is tested by true density porosity analysis, and the porosity is calculated by measuring the true density and apparent density of the sample to be tested.

[0712] The sample to be tested is processed into a disc with no powder falling off the edge in a drying room, and 20 or more discs with good appearance are selected with tweezers and placed in the sample cup. The number of discs is recorded, and the apparent volume is calculated. Then the sample cup containing the sample is placed in the true density tester, the test system is sealed, and helium is introduced according to the program. By detecting the pressure of the gas in the sample chamber and the expansion chamber, the real volume is calculated according to Bohr's law (PV=nRT), thereby obtaining the porosity of the sample to be tested.

[0713] (1) Regarding the porosity of the negative electrode sheet, the negative electrode sheet is used as the sample to be tested and its porosity is measured using the above method.

[0714] (2) For a negative electrode sheet having a first negative electrode active layer and a second negative electrode active layer, the following test method can be used to confirm the porosity (δ 1 ):

[0715] The porosity of the following samples A and B were tested using the above method, and then the porosity of the following samples A and B was calculated according to formula D N ×δ N =D 1 ×δ 1 +D 2 ×δ 2 , the porosity of the first negative electrode active layer (δ 1 ), D N is the total thickness of the negative electrode active material layer in the negative electrode sheet to be tested, D 1 is the thickness of the first negative electrode active layer, D 2 is the thickness of the second negative electrode active layer:

[0716] Sample A: Negative electrode sheet, used to measure the porosity of the negative electrode sheet (δ N );

[0717] Sample B: For the negative electrode sheet provided with the first negative electrode active layer and the second negative electrode active layer, the upper layer of powder was scraped off to obtain the electrode sheet sample after removing the first negative electrode active layer, which was used to measure the porosity (δ 2 ).

[0718] The thickness of the second negative electrode active layer can be roughly determined by referring to the observation results of the cross-sectional morphology of the electrode piece, so as to control the remaining thickness on the negative electrode current collector of sample B to come from the second negative electrode active layer.

[0719] 2. Battery Performance [0720...

Claims

1. A lithium ion secondary battery, characterized in that: It includes a negative electrode plate and an electrolyte; the negative electrode plate includes a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector, and the negative electrode active material layer includes a first negative electrode active layer; The first negative electrode active layer comprises a first negative electrode active material, the first negative electrode active material comprises an electronegative coated active material, the electronegative coated active material comprises a negative electrode active body and a coating layer located on at least a portion of the surface of the negative electrode active body; the coating layer comprises doped carbon, the doped carbon comprises a carbon matrix and an electronegative doping element, the carbon matrix comprises one or more of soft carbon and hard carbon, the Pauling electronegativity scale of the electronegative doping element is denoted as χ1, and the Pauling electronegativity scale of the carbon element is denoted as χ C , then χ1 and χ C The absolute value of the difference between them satisfies 0.03≤|χ1-χ C |≤0.

49.

2. The lithium ion secondary battery according to claim 1, characterized in that: The negative electrode active material layer further includes a second negative electrode active layer, and the second negative electrode active layer is located between the negative electrode collector and the first negative electrode active layer.

3. The lithium ion secondary battery according to claim 1 or 2, characterized in that: 0.04≤|χ1-χ C |≤0.49。 4. The lithium ion secondary battery according to any one of claims 1 to 2, characterized in that The electronegative doping element is doped into the carbon matrix in a covalent bonding manner; Optionally, at least a portion of the electronegative doping elements are covalently bonded to 2 or 3 carbon atoms simultaneously; Optionally, 80% to 100% of the electronegative doping element is covalently bonded to 2 or 3 carbon atoms at the same time; Optionally, any one atom of the electronegative doping element is covalently bonded to 2 or 3 carbon atoms simultaneously.

5. The lithium ion secondary battery according to any one of claims 1, 2 and 4, characterized in that: The electronegative doping elements in the doped carbon include one or more of N, P and S.

6. The lithium ion secondary battery according to claim 5, characterized in that: The doped carbon satisfies one or more of the following characteristics: (ta1) the electronegative doping element comprises a bridging N atom, wherein the bridging N atom is covalently bonded to at least one carbon atom; (ta2) the electronegative doping element comprises a bridging S atom, wherein the bridging S atom is covalently bonded to at least one carbon atom; (ta3) The electronegative doping element includes bridging-type P atoms, and at least a portion of any covalent sites of the bridging-type P atoms are independently covalently bonded in a COP or CSP manner; optionally, 80% to 100% of any covalent sites of the bridging-type P atoms are independently covalently bonded in a COP or CSP manner.

7. The lithium ion secondary battery according to claim 5 or 6, characterized in that: The electronegative doping element includes N element, and the doping form of the electronegative doping element in the doped carbon includes at least one of pyridine nitrogen type and pyrrole nitrogen type.

8. The lithium ion secondary battery according to any one of claims 1 to 7, characterized in that The electronegative coated active material satisfies one or more of the following characteristics: (tb1) the average thickness of the coating layer is 1 nm to 500 nm, optionally 100 nm to 500 nm, and further optionally 100 nm to 200 nm; (tb2) the thickness of at least a portion of the coating layer is 1 nm to 1000 nm, optionally 100 nm to 1000 nm, and further optionally 100 nm to 200 nm; (tb3) the mass percentage of the electronegative doping element in the doped carbon is 0.1% to 0.6%, optionally 0.2% to 0.6%, and further optionally 0.2% to 0.4%; (tb4) the mass percentage of the electronegative doping element in the coating layer is 0.1% to 0.6%, optionally 0.2% to 0.6%, and further optionally 0.2% to 0.4%; (tb5) the mass percentage of the coating layer in the electronegative coating active material is 0.2% to 5%, and can be optionally 0.5% to 3%; (tb6) The average thickness of the coating layer is proportional to the D of the first negative electrode active material. v The ratio of 50 is 0.5% to 12.5%, and can be selected as 1% to 10%; (tb7) The mass percentage of the doped carbon in the coating layer is 80% to 100%, and can be optionally 90% to 100%; (tb8) The mass percentage of soft carbon and hard carbon in the carbon matrix is ​​80% to 100%, and can be optionally 90% to 100%; optionally, the mass percentage of soft carbon in the carbon matrix is ​​80% to 100%, and can be further optionally 90% to 100%, or the mass percentage of hard carbon in the carbon matrix is ​​80% to 100%, and can be further optionally 90% to 100%.

9. The lithium ion secondary battery according to any one of claims 1 to 8, characterized in that The electronegative coating active material accounts for 20% to 100% of the first negative electrode active material, and may be 40% to 100%; Alternatively, the electronegative coated active material accounts for 20% to 80% of the first negative electrode active material, and further optionally 40% to 80%.

10. The lithium ion secondary battery according to claim 9, characterized in that: The electronegative coated active material comprises electronegative coated secondary particles, wherein the negative electrode active body in the electronegative coated secondary particles is a secondary particle; Optionally, the electronegative coated secondary particles account for 20% to 100% of the first negative electrode active material, and further optionally 40% to 100%; Alternatively, the electronegative coated secondary particles account for 20% to 80% of the first negative electrode active material, and may further be 40% to 80%.

11. The lithium ion secondary battery according to any one of claims 1 to 10, characterized in that: The negative electrode active body includes one or more of a carbon-based active material and a silicon-based active material; Optionally, the first negative electrode active material satisfies one or more of the following characteristics: (tc1) the carbon-based active material comprises one or more of artificial graphite, natural graphite, soft carbon and hard carbon; (tc2) The first negative electrode active material includes a carbon-based material, and the mass proportion of the carbon-based material in the first negative electrode active material is 20% to 100%, optionally 20% to 80%, and further optionally 40% to 80%.

12. The lithium ion secondary battery according to any one of claims 1 to 11, characterized in that: The lithium-ion secondary battery meets one or more of the following characteristics: (td1) the negative electrode active body in the electronegative coated active material comprises graphite; (td2) the electronegative coated active material comprises electronegative coated secondary graphite particles, the negative electrode active body in the electronegative coated secondary graphite particles is secondary graphite particles, and the amount of the electronegative coated secondary graphite particles in the first negative electrode active material accounts for 20% to 100%, optionally 40% to 100%, and further optionally 40% to 80%; (td3) D of the first negative electrode active material v 50 is 8μm~16μm, and can be selected as 10μm~14μm; (td4) The porosity of the first negative electrode active layer is 22% to 32%, and can be optionally 26% to 30%; (td5) Under at least one temperature condition between 20°C and 35°C, the ionic conductivity of the electrolyte is 13mS / cm to 18mS / cm; optionally, at 25°C, the ionic conductivity of the electrolyte is 13mS / cm to 18mS / cm.

13. The lithium ion secondary battery according to any one of claims 1, 3 to 12, characterized in that: The negative electrode active material layer further includes a second negative electrode active layer, wherein the second negative electrode active layer is located between the negative electrode current collector and the first negative electrode active layer; The lithium-ion secondary battery meets one or more of the following characteristics: (t1) the porosity of the first negative electrode active layer is higher than the porosity of the second negative electrode active layer; (t2) The ratio of the difference in compaction density of the second negative electrode active layer relative to the first negative electrode active layer to the compaction density of the negative electrode sheet is recorded as R PΔ , R PΔ is 0 to 2, optionally 0 to 0.834, further optionally, 0 <R PΔ ≤0.834; (t3) The charge rate of the first negative electrode active layer is higher than that of the second negative electrode active layer; optionally, the charge rate of the first negative electrode active layer is higher than that of the second negative electrode active layer.

14. The lithium ion secondary battery according to any one of claims 1, 3 to 13, characterized in that: The negative electrode active material layer further includes a second negative electrode active layer, and the second negative electrode active layer is located between the negative electrode current collector and the first negative electrode active layer; the second negative electrode active layer includes a second negative electrode active material, and the negative electrode plate meets one or more of the following characteristics: (te1) D of the second negative electrode active material v 50 is 10μm~20μm, and can be selected as 13μm~17μm; (te2) D of the second negative electrode active material v 50 higher than the D of the first negative electrode active material v 50; (te3) the compaction density of the second negative electrode active layer is higher than the compaction density of the first negative electrode active layer; (te4) The powder compaction density of the second negative electrode active layer is higher than the powder compaction density of the first negative electrode active layer.

15. The lithium ion secondary battery according to any one of claims 1, 3 to 14, characterized in that: The negative electrode active material layer further includes a second negative electrode active layer, and the second negative electrode active layer is located between the negative electrode current collector and the first negative electrode active layer; the negative electrode plate meets one or more of the following characteristics: (tf1) based on one side of the negative electrode current collector, the ratio of the surface density of the first negative electrode active layer to the surface density of the second negative electrode active layer is 2:8 to 6:4, and can be 4:6 to 5:5; (tf2) The ratio of the thickness of the first negative electrode active layer to the sum of the thickness of the first negative electrode active layer and the second negative electrode active layer on one side of the negative electrode current collector is denoted as F H , satisfying 20%≤F H ≤65%, optionally, 40%≤F H ≤60%; (tf3) Based on a single side of the negative electrode current collector, the thickness of the first negative electrode active layer is 10 μm to 50 μm, and can be 20 μm to 40 μm.

16. The lithium ion secondary battery according to any one of claims 1 to 15, characterized in that: The surface density of the negative electrode sheet is 5 mg / cm 2 ~15mg / cm 2 .

17. The lithium ion secondary battery according to any one of claims 1 to 16, characterized in that: The lithium-ion secondary battery further comprises a positive electrode sheet, the positive electrode sheet comprises a positive electrode active layer, the positive electrode active layer comprises a positive electrode active material, and the positive electrode active material comprises one or more of a lithium-containing phosphate active material and a lithium composite metal oxide active material.

18. The lithium ion secondary battery according to claim 17, characterized in that: The positive electrode active material includes a lithium-containing phosphate active material, and the positive electrode active material satisfies one or more of the following characteristics: (tg1) the mass proportion of the lithium-phosphate active material in the positive electrode active layer is greater than or equal to 80%, and can be 80% to 97%; (tg2) the lithium phosphate active material includes one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon; (tg3) The lithium phosphate active material includes a lithium phosphate active body and a carbon coating layer located on at least a portion of the surface of the lithium phosphate active body, and the carbon coating layer includes one or more of soft carbon, hard carbon and amorphous carbon.

19. A method for preparing a lithium ion secondary battery, characterized in that: Including preparing a negative electrode plate; The preparation of the negative electrode sheet comprises the following steps: A negative electrode active material layer is arranged on at least one side of the negative electrode current collector; wherein the negative electrode active material layer comprises a first negative electrode active layer, the first negative electrode active layer comprises a first negative electrode active material, the first negative electrode active material comprises an electronegative coated active material, the electronegative coated active material comprises a negative electrode active body and a coating layer located at least in part on the surface of the negative electrode active body; the coating layer comprises doped carbon, the doped carbon comprises a carbon matrix and an electronegative doping element, the carbon matrix comprises one or more of soft carbon and hard carbon, the Pauling electronegativity scale of the electronegative doping element is denoted as χ1, and the Pauling electronegativity scale of the carbon element is denoted as χ C , then χ1 and χ C The absolute value of the difference between them satisfies 0.03≤|χ1-χ C |≤0.

49.

20. The method for preparing a lithium ion secondary battery according to claim 19, characterized in that: The carbon matrix in the doped carbon includes soft carbon, and the electronegative coating active material is prepared by a method comprising the following steps: In the presence of a doping precursor, the negative electrode active body is mixed with a soft carbon precursor, and a doping heat treatment and a soft carbonization heat treatment are performed, so that the soft carbon precursor and the doping precursor together form a doped soft carbon coated on at least a portion of the surface of the negative electrode active body; wherein the doping precursor includes the electronegative doping element.

21. The method for preparing a lithium-ion secondary battery according to claim 20, characterized in that: The method for preparing the electronegative coated active material satisfies one or more of the following characteristics: (th1) the doping precursor includes a N-containing precursor, and the N-containing precursor includes one or more of urea and ammonia; (th2) the doping precursor includes a S-containing precursor, and the S-containing precursor includes one or more of sulfur and sulfur vapor; (th3) the doping precursor includes a P-containing precursor, and the P-containing precursor includes one or more of red phosphorus and phosphorus vapor; (th4) the doping heat treatment includes N doping treatment, and the conditions for performing the N doping treatment include: a heating rate of 2°C / min to 10°C / min, a holding temperature of 400°C to 800°C, and a holding time of 1h to 6h; (th5) the doping heat treatment includes S doping treatment, and the conditions for performing the S doping treatment include: a heating rate of 2°C / min to 10°C / min, a holding temperature of 200°C to 600°C, and a holding time of 1h to 6h; (th6) the doping heat treatment includes P doping treatment, and the step of performing the heat treatment includes: performing one or both of O doping treatment and S doping treatment and then performing the P doping treatment, and the conditions for performing the P doping treatment include: a heating rate of 2°C / min to 10°C / min, a holding temperature of 200°C to 600°C, and a holding time of 1h to 6h; wherein, the conditions for performing the O doping treatment include: heating at a heating rate of 2°C / min to 10°C / min in an air atmosphere, and performing the holding treatment at 400°C to 800°C in a nitrogen atmosphere, and the holding time is 1h to 6h; (th7) In the steps of performing the doping heat treatment and the soft carbonization heat treatment, the soft carbon precursor includes asphalt; (th8) The conditions for performing the soft carburization heat treatment include: performing a heat preservation treatment in an inert gas atmosphere at 700°C to 1300°C.

22. The method for preparing a lithium ion secondary battery according to claim 19, characterized in that: The carbon matrix in the doped carbon includes hard carbon, and the electronegative coating active material is prepared by a method comprising the following steps: Coating at least a portion of the surface of the negative electrode active body with hard carbon, introducing a doping source including an electronegative doping element, and introducing the electronegative doping element into the hard carbon by a gas replacement method to form doped hard carbon coated on at least a portion of the surface of the negative electrode active body; Optionally, the gas replacement method uses one of ammonia gas, sulfur vapor and phosphorus vapor.

23. An electrical device, characterized in that: The invention comprises at least one of the lithium ion secondary battery described in any one of claims 1 to 18 and the lithium ion secondary battery prepared by the method for preparing a lithium ion secondary battery described in any one of claims 19 to 22.

24. Use of the lithium ion secondary battery according to any one of claims 1 to 18 in supplying and / or storing electric energy; The application includes a process of charging the lithium-ion secondary battery at a rate greater than or equal to 2C; Optionally, the application includes a process of charging the lithium-ion secondary battery under at least one rate condition of 2C to 6C; Optionally, the application includes a process of charging the lithium-ion secondary battery under at least one rate condition of 2C to 4C or 4C to 6C; Optionally, the maximum charging rate of the lithium-ion secondary battery is greater than or equal to 2C, and may be 2C to 6C, and may be further 2C to 4C or 4C to 6C.

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