Negative electrode material and preparation method thereof, secondary battery and electronic equipment

Abstract

The invention provides a negative electrode material and a preparation method thereof, a secondary battery and an electronic device, the negative electrode material comprises silicon-carbon particles, the oxygen content of the silicon-carbon particles is X%, 9 < = X < = 20, the silicon-carbon particles comprise internal areas and surface areas, the surface areas refer to areas less than 1 [mu] m away from the surfaces of the silicon-carbon particles, and the surface areas refer to areas less than 1 [mu] m away from the surfaces of the silicon-carbon particles. The internal region refers to other regions except the surface region in the silicon-carbon particles, the oxygen content of the internal region is X1%, and 9 < = X1 < = 21. According to the negative electrode material provided by the invention, the first coulombic efficiency of a cold-pressed anode can be improved, and the thermal runaway temperature of a secondary battery is improved.

Description

Technical Field

[0001] The present application relates to the field of electrochemical energy storage, and in particular to a negative electrode material, a preparation method of the negative electrode material, a secondary battery using the negative electrode material, and an electronic device using the secondary battery. Background Art

[0002] Lithium-ion batteries have the advantages of high energy density, high working voltage, good safety, and environmental friendliness, and are widely used in consumer electronics fields such as mobile phones and laptop computers. However, the negative electrodes of traditional commercial lithium-ion batteries are basically limited to graphite materials, and their low capacity (LiC 6 , 372 mAh g -1 ) has greatly hindered the improvement of energy density. In recent years, silicon-based materials have gradually entered the industrialization track of lithium-ion battery negative electrodes with their high specific capacity (Li 15 Si 4 , 3579 mAh g -1 ), rich resource reserves, and a reasonable lithiation platform (<0.4V vs. Li / Li + ), and are considered to be the most promising negative electrode materials for the next generation.

[0003] Despite the above advantages, silicon-based materials will undergo large volume changes during the lithiation / delithiation process, resulting in a decline in the cycling performance of lithium-ion batteries, and causing the negative electrode active material particles in the electrode to pulverize and fail, which hinders the full play of the electrochemical performance of silicon-based materials. To address this problem, in the prior art, silicon-based materials are usually nano-sized and compounded with porous carbon. Although it can prevent the negative electrode active material particles from breaking to a certain extent, in high-temperature application conditions, the thermal runaway safety of secondary batteries still needs to be improved. Summary of the Invention

[0004] The present application provides a negative electrode material, a preparation method thereof, a secondary battery, and an electronic device.

[0005] In the first aspect of the present application, a negative electrode material is provided. The negative electrode material includes silicon-carbon particles, the oxygen content of the silicon-carbon particles is X%, 9 ≤ X ≤ 20. The silicon-carbon particles include an internal region and a surface region. The surface region refers to the region less than 1 μm away from the surface of the silicon-carbon particles, and the internal region refers to the other regions of the silicon-carbon particles except the surface region. The oxygen content of the internal region is X1%, 9 ≤ X1 ≤ 21.

[0006] In this application, both the overall and internal regions of the silicon-carbon particles have a relatively high oxygen content. The oxygen content of the silicon-carbon particles and the oxygen content of the internal region are within the above-mentioned suitable ranges, indicating that the oxygen distribution within the silicon-carbon particles is relatively uniform. When the silicon-carbon particles are cold-pressed at the anode, the high pressure will cause the silicon-carbon particles to break, and the internal Si component will react with water and oxygen in the air, resulting in a decrease in the specific capacity and the first efficiency. However, the appropriate amount of oxygen in the above-mentioned silicon-carbon particles and the internal region can convert the nanosilicon into SiOx in the internal region, reducing the reactivity of some silicon, thereby reducing the degree of reaction of silicon with water / oxygen in the air, reducing the massive consumption of active lithium, reducing the loss of irreversible capacity, reducing the loss of the first Coulombic efficiency of the silicon-carbon particles after cold pressing, and increasing the first Coulombic efficiency of the anode after cold pressing. At the same time, under the condition of high oxygen content in the silicon-carbon particles, a large amount of LiSixOy will be formed in the negative electrode material when the secondary battery is in a fully charged state. The large amount of LiSixOy will disperse the distribution of the silicon-lithium alloy. LiSixOy plays a role in isolating between the electrolyte and the silicon-lithium alloy, reducing the contact probability between the electrolyte and the silicon-lithium alloy, and increasing the thermal runaway temperature of the secondary battery.

[0007] Based on the first aspect, in some possible implementation manners, the silicon-carbon particles include porous carbon and silicon particles, and at least part of the silicon particles are located in the pores of the porous carbon. The porous carbon serves as the framework of the silicon-carbon particles, enabling the silicon-carbon particles to have good electrical conductivity and cycle stability. Moreover, the porous carbon as the substrate for silicon deposition can inhibit the volume expansion of silicon during the lithium insertion process, thereby reducing the expansion stress and improving the cycle performance and expansion performance of the silicon-carbon particles.

[0008] Based on the first aspect, in some possible implementation manners, the oxygen content of the surface region is X2%, and 7 ≤ X2 ≤ 17. During the cold pressing of the silicon-carbon particles at the anode, the oxygen in the surface region can convert the nanosilicon into SiOx, reducing the reactivity of some silicon, thereby reducing the degree of reaction of silicon with water / oxygen in the air, reducing the loss of the first Coulombic efficiency of the silicon-carbon particles after cold pressing, and further increasing the first Coulombic efficiency of the anode after cold pressing. And it is also beneficial to increase the thermal runaway temperature of the secondary battery.

[0009] Based on the first aspect, in some possible implementation manners, the oxygen content of the silicon-carbon particles satisfies: 10 ≤ X ≤ 15 and 10 ≤ X1 ≤ 15. This is beneficial to further increase the first Coulombic efficiency of the anode after cold pressing and increase the thermal runaway temperature of the secondary battery.

[0010] Based on the first aspect, in some possible embodiments, the differential capacity curve of the negative electrode material during de-lithiation has a first peak and a second peak at 200 mV to 320 mV and 600 mV to 900 mV respectively, and the peak intensity ratio of the first peak to the second peak is I1, where 1.2 ≤ I1 ≤ 3.2. This is beneficial for further forming a large amount of LiSixOy, reducing the polarization during the first charge and discharge, reducing the loss of the first Coulombic efficiency of the silicon-carbon particles after cold pressing, improving the first Coulombic efficiency and cyclic expansion performance of the anode after cold pressing, and increasing the thermal runaway temperature of the secondary battery.

[0011] Based on the first aspect, in some possible embodiments, in the X-ray photoelectron spectrum of the silicon-carbon particles, a Si-O peak and a Si-Si peak are respectively present at 101 eV to 104 eV and 96 eV to 99 eV, and the peak intensity ratio of the Si-O peak to the Si-Si peak is I2, where 0.6 ≤ I2 ≤ 1.4. This is beneficial for reducing the reactivity of some silicon, thereby reducing the degree of reaction between silicon and water / oxygen in the air, and is beneficial for improving the first Coulombic efficiency and cyclic expansion performance of the anode after cold pressing; moreover, it is also beneficial for forming a large amount of LiSixOy, reducing the contact probability between the electrolyte and the silicon-lithium alloy, and increasing the thermal runaway temperature of the secondary battery.

[0012] The second aspect of the present application provides a method for preparing silicon-carbon particles in the negative electrode material, comprising the following steps: S1) providing a porous carbon framework, introducing a mixed gas into the porous carbon framework, with a deposition time of 1 h to 20 h and a reaction temperature of 400 °C to 500 °C to obtain a core body, wherein the mixed gas includes silane gas, carbon dioxide, and an inert gas, the mass percentage content of carbon dioxide in the mixed gas is 2% to 20%, and the flow rate of the mixed gas is 100 sccm to 500 sccm; S2) introducing an alkane gas into the core body to form a carbon layer on the core body to obtain the silicon-carbon particles. During the above preparation process, it is possible to increase the oxygen content in the internal region and the uniformity of the entire silicon-carbon particles, and enable the silicon-carbon particles to obtain an appropriate content of oxygen, thereby facilitating the silicon-carbon particles prepared to improve the first Coulombic efficiency of the anode after cold pressing and increasing the thermal runaway temperature of the secondary battery.

[0013] The third aspect of the present application provides a secondary battery, comprising a negative electrode sheet, a positive electrode sheet, and an electrolyte. The negative electrode sheet further includes the negative electrode material described above or the negative electrode material obtained by the preparation method described above, and the negative electrode material further includes graphite. Graphite has a certain flexibility, and its combination with the silicon-carbon particles can alleviate the overall volume expansion of the negative electrode material layer. At the same time, using both graphite and the silicon-carbon particles as the negative electrode material can also make full use of the advantages of both the silicon-carbon particles and graphite to achieve better electrochemical performance.

[0014] Based on the third aspect, in some possible embodiments, in the fully discharged state, the mass content of lithium in the anode material is Y%, where 0.1 ≤ Y ≤ 0.9. This is conducive to the formation of a large amount of LiSixOy and improves the ionic conductivity of the silicon-carbon particles, thereby improving the first Coulombic efficiency of the anode material after cold pressing.

[0015] Based on the third aspect, in some possible embodiments, the average circularity of the silicon-carbon particles is C1, and the average circularity of the graphite is C2, where |C1 - C2| ≤ 0.2. The average circularity of the silicon-carbon particles and the graphite is similar, and the morphological difference between the silicon-carbon particles and the graphite is small. The proportion of tips and irregular regions in the anode material is small. In the cold-pressed anode sheet, it can reduce the impact caused by the crushing generated by squeezing the irregular regions during the cold pressing process, reduce the loss of the first Coulombic efficiency and specific capacity of the silicon-carbon particles, thereby improving the first Coulombic efficiency and specific capacity of the anode material and reducing the thermal runaway temperature of the secondary battery.

[0016] Based on the third aspect, in some possible embodiments, the electrolyte includes a first lithium salt, and the first lithium salt is selected from at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, and lithium trifluoromethanesulfonate. Based on the mass of the electrolyte, the mass percentage content of the first lithium salt is S1%, where 0.01 ≤ S1 ≤ 2. This is conducive to further improving the first Coulombic efficiency of the anode after cold pressing and increasing the thermal runaway temperature of the secondary battery.

[0017] Based on the third aspect, in some possible embodiments, 0.5 ≤ S1 ≤ 1, which is conducive to further improving the first Coulombic efficiency of the anode after cold pressing and increasing the thermal runaway temperature of the secondary battery.

[0018] Based on the third aspect, in some possible embodiments, the electrolyte includes a second lithium salt, and the second lithium salt is selected from lithium tetrafluoroborate and lithium difluorophosphate. Based on the mass of the electrolyte, the mass percentage content of the second lithium salt is S2%, where 0.5 ≤ S2 ≤ 2. This is conducive to further improving the first Coulombic efficiency of the anode after cold pressing and increasing the thermal runaway temperature of the secondary battery.

[0019] Based on the third aspect, in some possible embodiments, 1 ≤ S2 / S1 ≤ 5. This is conducive to further improving the first Coulombic efficiency of the anode after cold pressing and increasing the thermal runaway temperature of the secondary battery.

[0020] The fourth aspect of the present application provides an electronic device, including a secondary battery. The secondary battery supplies power to the load in the electronic device. The secondary battery including the anode sheet has an excellent thermal runaway temperature, which is beneficial to improving the service life and safety of the electronic device. Specific Embodiments

[0021] The technical solutions in the embodiments of the present application will be clearly and detailedly described below. Apparently, the described embodiments are some, but not all, of the embodiments of the present application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present application belongs. The terms used in the specification of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application.

[0022] During the cold pressing process of the anode, a relatively large roller pressure will cause the silicon-carbon particles to break and the surface coating layer to fall off. After the silicon-carbon particles break or the surface protective layer of the silicon-carbon particles is damaged, the nano-silicon inside the silicon-carbon particles is prone to react with water / oxygen in the air to produce SiO 2 , which affects the specific capacity and the first Coulomb efficiency. Moreover, in a lithium-ion battery, the exposed interface of the silicon-carbon particles will also exacerbate the consumption of the electrolyte and reduce the cycle life of the lithium-ion battery.

[0023] Lithium-ion batteries will encounter high-temperature working conditions during application. Under high-temperature working conditions, the side reaction between the Si-Li alloy and the electrolyte (such as LiPF 6 / ethylene carbonate EC-diethyl carbonate DEC) will be exacerbated. The reaction formula of the side reaction: LixSi + EC → LiF + CO 2 ↑ + other decomposition products, which will lead to a sharp increase in the risk of thermal runaway of the lithium-ion battery. Therefore, suppressing the energy density loss caused by anode cold pressing and increasing the thermal runaway temperature are crucial for the application of silicon-based battery cells.

[0024] Therefore, an embodiment of the present application provides a secondary battery, which includes a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located inside the housing.

[0025] The housing can be a packaging bag obtained by encapsulating with a packaging film (such as an aluminum-plastic film). For example, the secondary battery is a soft-pack battery. In some other embodiments, the secondary battery can also be a steel-shell battery, an aluminum-shell battery, etc.

[0026] The electrode assembly includes a positive electrode plate, a negative electrode plate, and a separator. The separator is disposed between the positive electrode plate and the negative electrode plate. The electrode assembly can be a stacked structure, which is formed by alternately laminating the positive electrode plate, the separator, and the negative electrode plate. In some other embodiments, the electrode assembly can also be a wound structure, which is formed by laminating and winding the positive electrode plate, the separator, and the negative electrode plate.

[0027] Negative electrode plate

[0028] The negative electrode plate includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The negative electrode current collector can use at least one of copper foil, nickel foil, stainless steel foil, titanium foil, composite current collector, or carbon-based current collector, etc. The negative electrode material layer includes a negative electrode material.

[0029] One embodiment of the present application provides a negative electrode material, which includes silicon-carbon particles. The oxygen content of the silicon-carbon particles is X%, where 9 ≤ X ≤ 20. The silicon-carbon particles include an inner region and a surface region. The surface region refers to the region less than 1 μm away from the surface of the silicon-carbon particles, and the inner region refers to the other regions of the silicon-carbon particles except the surface region. The oxygen content of the inner region is X1%, where 9 ≤ X1 ≤ 21.

[0030] In the present application, both the overall silicon-carbon particles and the inner region have a relatively high oxygen content. The oxygen content of the silicon-carbon particles and the oxygen content of the inner region are within the above suitable ranges, indicating that the oxygen distribution in the silicon-carbon particles is relatively uniform. When the silicon-carbon particles are cold-pressed at the anode, high pressure will cause the silicon-carbon particles to break, and the internal Si component will react with water and oxygen in the air, resulting in a decrease in the specific capacity and the first efficiency. However, the appropriate amount of oxygen in the above-mentioned silicon-carbon particles and the inner region can convert the nano-silicon into SiOx in the inner region, reducing the reactivity of some silicon, thereby reducing the degree of reaction between silicon and water / oxygen in the air, reducing the large consumption of active lithium, reducing the loss of irreversible capacity, reducing the loss of the first Coulombic efficiency of the silicon-carbon particles after cold pressing, and improving the first Coulombic efficiency of the anode after cold pressing. At the same time, under the condition of high oxygen content in the silicon-carbon particles, a large amount of LiSixOy will be formed in the negative electrode material when the secondary battery is fully charged. The large amount of LiSixOy will disperse the distribution of the silicon-lithium alloy. LiSixOy plays a role in isolating between the electrolyte and the silicon-lithium alloy, reducing the contact probability between the electrolyte and the silicon-lithium alloy, and increasing the thermal runaway temperature of the secondary battery.

[0031] If the oxygen content in the silicon-carbon particles is low, or the oxygen content in the inner region is low, such as less than 9, the oxygen content is insufficient, increasing the proportion of exposed nano-silicon. When cold-pressing the anode, the surface of the broken silicon particles reacts quickly with H 2 O / O 2 to generate a SiO 2 passivation layer, and its reaction formula is: Si + O 2 → SiO 2 , Si + H 2 O → SiO 2 + H2↑), which will consume a large amount of active lithium and cause irreversible capacity loss, reducing the first Coulombic efficiency of the negative electrode material after cold-pressing the anode. When the oxygen content is insufficient, at high temperature, it also exacerbates the side reaction between the Si-Li alloy and the electrolyte (such as LiPF 6 / EC-DEC). The reaction formula of the side reaction is: Li 15 Si 4 + 6EC → LiF + CO 2↑ + other decomposition products), reducing the formation of LiSixOy. The deficiency of LiSixOy may cause the exothermic peak temperature of the secondary battery to decrease by 20°C - 30°C, increasing the risk of thermal runaway.

[0032] If the oxygen content in the silicon-carbon particles is relatively high, such as greater than 20%, or the oxygen content in the internal region is relatively high, such as greater than 21%, the excessive oxygen content will generate a large amount of SiO 2 , a large amount of SiO 2 will form an insulating interface layer, blocking the electron conduction path, increasing the contact resistance between the silicon-carbon particles, thereby increasing the resistivity after cold-pressing the anode sheet (increasing by about 50% to 100%), increasing the polarization during the first charge and discharge, reducing the initial Coulomb efficiency after cold-pressing the anode, and reducing the capacity contribution of the active silicon, reducing the reversible capacity of the negative electrode material. Moreover, the large amount of SiO 2 will also form a dense SiO 2 layer, which is not conducive to improving the lithium-ion insertion / extraction kinetics and reducing the initial Coulomb efficiency of the negative electrode material. The excessive oxygen content is also not conducive to increasing the thermal runaway temperature of the secondary battery.

[0033] In some embodiments, the oxygen content of the silicon-carbon particles is X%, and X can be 9, 10, 12, 14, 15, 16, 17, 18, 20, or any value within the range composed of any two of the above values.

[0034] In some embodiments, the oxygen content of the internal region in the silicon-carbon particles is X1%, and X1 can be 9, 10, 12, 14, 15, 16, 17, 18, 21, or any value within the range composed of any two of the above values.

[0035] In some embodiments, the silicon-carbon particles include porous carbon and silicon particles, and at least part of the silicon particles are located in the pores of the porous carbon. The porous carbon serves as the skeleton of the silicon-carbon particles, enabling the silicon-carbon particles to have good electrical conductivity and cycle stability. Moreover, the porous carbon as the substrate for silicon deposition can inhibit the volume expansion of silicon during the lithium insertion process, thereby reducing the expansion stress and improving the cycle performance and expansion performance of the silicon-carbon particles. In some embodiments, part of the silicon particles can be located in the pores of the porous carbon, and another part of the silicon particles are located on the surface of the porous carbon.

[0036] In some embodiments, the oxygen content of the surface area is X2%, 7≤X2≤17. The oxygen content in the surface area is within the above range, the oxygen content of the surface area and the internal area of ​​the entire silicon-carbon particle is relatively high, and the oxygen content of the entire silicon-carbon particle is evenly distributed. During the cold pressing of the silicon-carbon particles at the anode, the oxygen in the surface area can convert nano-silicon into SiOx, reduce the reactivity of part of the silicon, thereby reducing the degree of reaction between silicon and water / oxygen in the air, reducing the loss of the first coulomb efficiency of the silicon-carbon particles after cold pressing, and further improving the first coulomb efficiency of the anode after cold pressing. And the surface area contains a high amount of oxygen, which is conducive to the formation of a large amount of LiSixOy. A large amount of LiSixOy will disperse the distribution of silicon-lithium alloys, play an isolating role between the electrolyte and the silicon-lithium alloy, and reduce the contact probability of the electrolyte and the silicon-lithium alloy, and increase the thermal runaway temperature of the secondary battery. In some embodiments, the oxygen content of the surface area is X2%, and X2 can be 7, 8, 10, 14, 15, 16, 17 or any value within the range composed of any two of the above values. In addition, since the silicon-carbon particles in the present application are subjected to an outer carbon coating and a pre-lithiation treatment, the oxygen content on the surface of the silicon-carbon particles is slightly lower than the oxygen content inside the silicon-carbon particles.

[0037] In some embodiments, the silicon-carbon particles satisfy: 10≤X≤15 and 10≤X1≤15; this is beneficial to further improve the first coulombic efficiency of the anode after cold pressing and to increase the thermal runaway temperature of the secondary battery.

[0038] In some embodiments, the differential capacity curve of the negative electrode material delithiation has a first peak and a second peak at 200mV to 320mV and 600mV to 900mV, respectively, and the peak intensity ratio of the first peak to the second peak is I1, 1.2≤I1≤3.2. The differential capacity-voltage curve of the negative electrode material delithiation can be understood as taking the charge and discharge specific capacity of the negative electrode material as the horizontal coordinate and the voltage as the vertical coordinate, and obtaining the charge and discharge curve of the negative electrode material, and then taking the first-order derivative of the delithiation specific capacity of the negative electrode material with respect to the voltage as the vertical coordinate and the voltage as the horizontal coordinate, and then obtaining the differential capacity-voltage curve of the negative electrode material delithiation. The differential capacity-voltage curve can reflect the capacity contained in the negative electrode material within the unit voltage range. If the capacity at a certain voltage platform is higher, it means that a lot of capacity will be contributed within a very small voltage fluctuation range, and a characteristic peak will be shown on the curve. Each characteristic peak represents an electrochemical reaction, and the peak height of each characteristic peak also indicates the contribution of the corresponding electrochemical reaction to the capacity.

[0039] The first peak of the negative electrode material is the de-lithiation characteristic peak of the transformation of LixSi (x is about 3.5) to LiySi (y is about 2); the second peak is the partially reversible de-lithiation characteristic peak of the transformation of LiSixOy to SiOx. Among them, the ratio of the first peak to the second peak can reflect the influence of the oxygen content in the negative electrode material on the de-lithiation reaction of the negative electrode material. When I1 is within the above range, it indicates that the silicon-carbon particles contain an appropriate oxygen content, which is conducive to further forming a large amount of LiSixOy, reducing the polarization of the first charge and discharge, reducing the loss of the first Coulombic efficiency of the silicon-carbon particles after cold pressing, improving the first Coulombic efficiency and cycle expansion performance of the anode after cold pressing, and increasing the thermal runaway temperature of the secondary battery. In some embodiments, I1 can be 1.2, 1.3, 1.5, 1.8, 2.0, 2.3, 2.5, 2.7, 3.0, 3.2 or any value within the range composed of any two of the above values.

[0040] In some embodiments, in the X-ray photoelectron spectrum of the silicon-carbon particles, there are Si-O peaks and Si-Si peaks at 101 eV to 104 eV and 96 eV to 99 eV respectively, and the peak intensity ratio of the Si-O peak to the Si-Si peak is I2, and 0.6 ≤ I2 ≤ 1.4. In the X-ray photoelectron spectrum, the intensity of the Si-O peak at 101 eV to 104 eV can reflect the proportion of the content of SiOx in the surface region of the silicon-carbon particles, and the intensity of the Si-Si peak at 96 eV to 99 eV can reflect the proportion of the content of Si in the surface region of the silicon-carbon particles. When the peak intensity ratio I2 is within the above appropriate range, the appropriate content of SiOx is conducive to reducing the reaction activity of some silicon, thereby reducing the degree of reaction between silicon and water / oxygen in the air, and is conducive to improving the first Coulombic efficiency and cycle expansion performance of the anode after cold pressing; and it is also conducive to forming a large amount of LiSixOy, reducing the contact probability between the electrolyte and the silicon-lithium alloy, and increasing the thermal runaway temperature of the secondary battery. In some embodiments, I2 can be 0.6, 0.7, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4 or any value within the range composed of any two of the above values.

[0041] In some embodiments, the negative electrode material further includes graphite. The average circularity of the silicon-carbon particles is C1, and the average circularity of the graphite is C2, where |C1 - C2| ≤ 0.2. The graphite includes at least one of natural graphite or artificial graphite. Since graphite has a certain flexibility, its combination with the silicon-carbon particles can alleviate the overall volume expansion of the negative electrode material layer. At the same time, using both the silicon-carbon particles and graphite as the negative electrode material can fully utilize the advantages of both to achieve better electrochemical performance. The inventors found that the smaller |C1 - C2| is, the closer the average circularity of the silicon-carbon particles and graphite is, the smaller the morphological difference between the silicon-carbon particles and graphite is, and the smaller the proportion of tips and irregular regions in the negative electrode material. In the cold-pressed anode sheet, it can reduce the impact caused by the crushing of the irregular regions during the cold pressing process, reduce the loss of the initial Coulombic efficiency and specific capacity of the silicon-carbon particles, thereby improving the initial Coulombic efficiency and specific capacity of the negative electrode material and increasing the thermal runaway temperature of the secondary battery. In some embodiments, the value of |C1 - C2| can be 0, 0.05, 0.1, 0.15, 0.2 or any value within the range formed by any two of the above numerical values. In some embodiments, |C1 - C2| ≤ 0.1.

[0042] In some embodiments, in the fully discharged state, the mass content of lithium in the negative electrode material is Y%, where 0.1 ≤ Y ≤ 0.9. The lithium content in the silicon-carbon particles within the above range is beneficial for the formation of a large amount of LiSixOy and can improve the ionic conductivity of the silicon-carbon particles, thereby improving the initial Coulombic efficiency of the negative electrode material after cold pressing. In some embodiments, Y can be 0.1, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or any value within the range formed by any two of the above numerical values. The negative electrode material also contains graphite. After cleaning the negative electrode material to remove the binder and electrolyte, the graphite in the negative electrode material contains almost no lithium, and lithium exists in the silicon-carbon particle material. Therefore, the lithium content described here can be understood as the lithium content in the silicon-carbon material.

[0043] In some embodiments, the mass content of lithium in the silicon-carbon material is 0.5% to 4.2%.

[0044] In some embodiments, the electrolyte includes a first lithium salt selected from at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, and lithium trifluoromethanesulfonate. Based on the mass of the electrolyte, the mass percentage of the first lithium salt is S1%, where 0.01 ≤ S1 ≤ 2. This is beneficial for improving the initial Coulombic efficiency of the anode after cold pressing and increasing the thermal runaway temperature of the secondary battery. When the mass percentage of the first lithium salt is within the above range, it is conducive to further improving the initial Coulombic efficiency of the anode after cold pressing and increasing the thermal runaway temperature of the secondary battery. In some embodiments, the mass percentage S1 of the first lithium salt can be 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, or any value within the range formed by any two of the above values. Preferably, 0.5 ≤ S1 ≤ 1.

[0045] In some embodiments, the electrolyte includes a second lithium salt selected from lithium tetrafluoroborate and lithium difluorophosphate. Based on the mass of the electrolyte, the mass percentage of the second lithium salt is S2%, where 0.5 ≤ S2 ≤ 2. This is beneficial for improving the initial Coulombic efficiency of the anode after cold pressing and increasing the thermal runaway temperature of the secondary battery. When the mass percentage of the second lithium salt is within the above range, it is conducive to further improving the initial Coulombic efficiency of the anode after cold pressing and increasing the thermal runaway temperature of the secondary battery. In some embodiments, the mass percentage S2 of the second lithium salt can be 0.5, 0.75, 0.9, 1, 1.3, 1.45, 1.6, 1.7, 1.8, 1.9, 2, or any value within the range formed by any two of the above values.

[0046] In some embodiments, 1 ≤ S2 / S1 ≤ 5. When the ratio of the mass percentages of the first lithium salt and the second lithium salt in the electrolyte is within the above range, it is conducive to further improving the initial Coulombic efficiency of the anode after cold pressing and increasing the thermal runaway temperature of the secondary battery. In some embodiments, the ratio of S2 / S1 can be 1, 2, 3, 4, 5, or any value within the range formed by any two of the above values.

[0047] This application also provides a method for preparing silicon-carbon particles in a negative electrode material, including the following steps:

[0048] S1) Provide a porous carbon framework, introduce a mixed gas into the porous carbon framework, with a deposition time of 1 h to 20 h and a reaction temperature of 400 °C to 500 °C, to obtain a core body. The mixed gas includes silane gas, carbon dioxide, and an inert gas, where the mass percentage of carbon dioxide in the mixed gas is 2% to 20%, and the flow rate of the mixed gas is 100 sccm to 500 sccm.

[0049] The mass percentage of carbon dioxide in the above-mentioned mixed gas, the flow rate of the mixed gas, the deposition time, the reaction temperature, and the time for coating carbon on the core to form a carbon layer in the subsequent step S2) will all affect the oxygen content in the silicon-carbon particles. The greater the mass percentage of carbon dioxide, or the greater the flow rate of the mixed gas, or the longer the deposition time and the higher the reaction temperature, the more conducive it is to increasing the oxygen content in the silicon-carbon particles. The longer the carbon coating time, the more conducive it is to reducing the oxygen content in the silicon-carbon particles, and more conducive to reducing the oxygen content in the surface area.

[0050] In some embodiments, the reaction temperature can be 400 °C, 430 °C, 450 °C, 470 °C, 500 °C or any value within the range formed by any two of the above values. The time for depositing silicon can be 1 h, 2 h, 5 h, 7 h, 8 h, 10 h, 12 h, 14 h, 15 h, 17 h, 19 h, 20 h or any value within the range formed by any two of the above values.

[0051] In some embodiments, the mass percentage of carbon dioxide in the mixed gas can be 2%, 5%, 8%, 10%, 12%, 15%, 17%, 19%, 20% or any value within the range formed by any two of the above values. The flow rate of the mixed gas can be 100 sccm, 150 sccm, 200 sccm, 300 sccm, 400 sccm, 500 sccm or any value within the range formed by any two of the above values.

[0052] Among them, the heating rate for slow heating can be but not limited to 0.5 °C / min to 5 °C / min, and the mixed gas contains 2% to 20% of silane, 2% to 20% of carbon dioxide and 60% to 92% of inert gas (such as but not limited to nitrogen) by mass percentage.

[0053] S2) Introduce an alkane gas into the core to form a carbon layer on the core to obtain silicon-carbon particles.

[0054] Specifically, at a temperature of 500°C to 1000°C, an alkane mixture gas is introduced into the core body and maintained for 2 h to 20 h to form a carbon layer of a carbon-coated layer covering the core body. Then, the atmosphere of the alkane mixture gas is switched to an inert atmosphere (such as but not limited to a nitrogen atmosphere) and cooled to room temperature to obtain the silicon-carbon particles. Among them, the alkane mixture gas contains 5% to 100% by mass of alkane (such as but not limited to acetylene) and 0% to 95% of inert gas (such as but not limited to argon). The flow rate of the introduced alkane mixture gas is 100 sccm to 500 sccm. In some embodiments, the flow rate of the introduced alkane mixture gas can be 100 sccm, 150 sccm, 200 sccm, 300 sccm, 400 sccm, 500 sccm, or any value within the range composed of any two of the above values. In some embodiments, the reaction temperature of the alkane mixture gas can be 500°C, 600°C, 700°C, 800°C, 900°C, 1000°C, or any value within the range composed of any two of the above values. The reaction time can be 2 h, 5 h, 7 h, 8 h, 10 h, 12 h, 14 h, 15 h, 17 h, 19 h, 20 h, or any value within the range composed of any two of the above values.

[0055] S3) The silicon-carbon particles are also pre-lithiated to obtain pre-lithiated silicon-carbon particles.

[0056] Specifically, in an oxygen-free / water-free environment, the silicon-carbon particles, dimethylbiphenyl, methyltetrahydrofuran, and lithium sheets are stirred evenly at room temperature for 1 h to 10 h. Among them, the mass ratio of the silicon-carbon particles to the lithium sheets is (10:1) to (100:1). The lithium sheets are fully dissolved in dimethylbiphenyl and methyltetrahydrofuran. After removing the excess dimethylbiphenyl and methyltetrahydrofuran, it is heated to 500°C to 700°C under nitrogen, and the heating time is 1 h to 5 h. After natural cooling, pre-lithiated silicon-carbon-oxygen particles are obtained.

[0057] In this step, through the pre-lithiation treatment, the silicon-carbon particles contain an appropriate amount of lithium to improve the ionic conductivity of the silicon-carbon particles and the first Coulomb efficiency of the negative electrode material.

[0058] In some embodiments, the mass ratio of the silicon-carbon particles to the lithium sheets can be 100:1, 90:1, 80:1, 70:1, 60:1, 50:1, 40:1, 30:1, 20:1, 10:1, or any ratio within the range composed of any two of the above values.

[0059] In some embodiments, the pre-lithiation heating time can be 500 °C, 550 °C, 600 °C, 650 °C, 700 °C, or any value within the range formed by any two of the above values. The heating time can be 1 h, 2 h, 3 h, 4 h, 5 h, or any value within the range formed by any two of the above values.

[0060] The negative electrode material layer further includes a binder and a conductive agent. The binder is used to bond the negative electrode material particles to facilitate the formation of a film layer, and at the same time, it can also improve the bonding force between the negative electrode material layer and the negative electrode current collector. In some embodiments, the binder may include, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon, etc.

[0061] The conductive agent in the negative electrode material layer includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based materials may include, but are not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based materials may include, but are not limited to, metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0062] Separator

[0063] There are no particular limitations on the material and shape of the separator used in the secondary battery of the present application, and it can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or an inorganic substance formed of a material stable to the electrolyte of the present application.

[0064] For example, the separator may include a base material layer and a surface treatment layer. The base material layer is a non-woven fabric, a film, or a composite film having a porous structure, and the material of the base material layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite film can be selected.

[0065] A surface treatment layer is provided on at least one surface of the base material layer. The surface treatment layer can be a polymer layer, an inorganic layer, or a layer formed by mixing polymers and inorganic substances. The inorganic layer includes inorganic particles and a binder. The inorganic particles are selected from at least one of alumina, silica, magnesia, titania, hafnium dioxide, stannic oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, copolymer of vinylidene fluoride - hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene alkoxide, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer contains a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene alkoxide, polyvinylidene fluoride, and poly(vinylidene fluoride - hexafluoropropylene).

[0066] Electrolyte

[0067] According to some embodiments of the present application, the electrolyte includes an organic solvent, a lithium salt, and an optional additive. The organic solvent in the electrolyte of the present application can be any organic solvent known in the prior art that can be used as a solvent for the electrolyte. There is no limitation on the electrolyte used in the electrolyte according to the present application, and it can be any electrolyte known in the prior art. The additive of the electrolyte according to the present application can be any additive known in the prior art that can be used as an electrolyte additive.

[0068] In some embodiments, the organic solvent includes, but is not limited to: ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate, or ethyl propionate. In some embodiments, the organic solvent includes an ether solvent, such as including at least one of 1,3 - dioxolane (DOL) and ethylene glycol dimethyl ether (DME). In some embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments, the lithium salt includes, but is not limited to: lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), lithium difluorophosphate (LiPO 2 F 2 ), lithium bis(trifluoromethanesulfonyl)imide LiN(CF 3 SO 2 ) 2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO 2 F) 2 )(LiFSI), lithium bis(oxalato)borate LiB(C 2 O 4) 2 (LiBOB) or lithium difluoro(oxalato)borate LiBF 2 (C 2 O 4 )(LiDFOB). In some embodiments, the additive includes at least one of fluoroethylene carbonate and adiponitrile.

[0069] Positive electrode sheet

[0070] The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer provided on the positive electrode current collector. The positive electrode current collector can be made of aluminum foil, nickel foil, etc. The positive electrode active layer contains a positive electrode active material, and the positive electrode active material includes a compound that reversibly intercalates and deintercalates lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive electrode active material may include a lithium transition metal composite oxide. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel. In some embodiments, the positive electrode active material may include, but is not limited to, at least one of lithium cobaltate, lithium nickel manganese cobaltate, lithium nickel manganese aluminate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel-type lithium manganate, spinel-type lithium nickel manganate, and lithium titanate.

[0071] The positive electrode active layer further contains a binder for binding the positive electrode active material particles to facilitate the formation of a film layer and at the same time improve the binding force between the positive electrode active layer and the positive electrode current collector. In some embodiments, the binder may include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0072] The positive electrode active layer may further contain a conductive material, and the conductive material includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based materials may include, but are not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0073] The above secondary battery is applied to an electronic device to supply power to a load in the electronic device. Moreover, the above secondary battery including the negative electrode sheet has excellent initial Coulomb efficiency and thermal runaway temperature, which is beneficial to improving the service life and safety of the electronic device. Among them, the electronic device may include, but is not limited to, a laptop computer, a pen input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, a liquid crystal TV, a portable cleaner, a portable CD player, a minidisc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flash, a camera, a large household battery, and a lithium-ion capacitor, etc.

[0074] The present application will be described below through specific examples and comparative examples. Those skilled in the art should understand that the preparation methods described in the present application are only examples, and any other suitable preparation methods are within the scope of the present application.

[0075] Example 1-1

[0076] (1) Preparation of silicon-carbon particles:

[0077] Under an argon atmosphere, 100 g of a porous carbon framework (the particle size Dv50 of the porous carbon is 6 μm to 10 μm) is heated to 420 °C at a rate of 2 °C / min, and then the argon atmosphere is switched to a silane-carbon dioxide-nitrogen mixed gas (by mass percentage, 20% silane, 4% carbon dioxide, and 76% nitrogen), and deposited at 420 °C / 200 sccm for 10 h to obtain a core body.

[0078] The mixed gas atmosphere is switched to an acetylene mixed gas (by mass percentage, 20% acetylene and 80% argon), the acetylene mixed gas is introduced into the core body, deposited at 500 °C for 3 h, then the acetylene mixed gas is switched to nitrogen, and finally cooled to room temperature of 25 °C to obtain silicon-carbon particles.

[0079] Under an oxygen-free / anhydrous environment, 100 g of silicon-carbon particles, 3.64 g of dimethylbiphenyl, 200 ml of methyltetrahydrofuran, and 1.09 g of lithium pieces are stirred evenly at room temperature for 1 h to 10 h, the excess methyltetrahydrofuran is removed, and then heated to 550 °C under nitrogen, and naturally cooled to obtain pre-lithiated silicon-carbon particles.

[0080] (2) Preparation of the negative electrode sheet: Mix the negative electrode materials (graphite and the above-mentioned pre-lithiated silicon-carbon particles mixed at a weight ratio of 80:20), carbon nanotubes, polymethyl acrylate (SBP), and sodium carboxymethyl cellulose at a weight ratio of 97:1:1:1, add deionized water as a solvent, and formulate a negative electrode slurry with a solid content of 40 wt%. After stirring evenly with a vacuum mixer, the negative electrode slurry is obtained. The negative electrode slurry is uniformly coated on one surface of a negative electrode current collector copper foil with a thickness of 12 μm, and dried at 85°C to obtain a negative electrode sheet with a single-sided coated negative electrode material layer. When coating, the coating weight of the negative electrode material layer is 10 mg / cm 2 . Then repeat the above steps on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coated negative electrode material layer. Then, through cold pressing, slicing, and slitting, it is dried under vacuum conditions at 120°C for 12 h to obtain a negative electrode sheet with a specification of 78 mm × 875 mm. After welding the tab, it is ready for use. Among them, the compaction density of the negative electrode material layer after cold pressing is 1.7 g / cm 3 ; The negative electrode materials include artificial graphite and the silicon-carbon material prepared above, and the mass ratio of artificial graphite to silicon-carbon particles is 80:20.

[0081] (3) Preparation of the positive electrode sheet: Mix the positive electrode active material lithium cobalt oxide, conductive carbon black Super P, and positive electrode binder polyvinylidene fluoride at a mass ratio of 97:1.4:1.6, add N-methylpyrrolidone as a solvent, and formulate a positive electrode slurry with a solid content of 72 wt%. After stirring evenly with a vacuum mixer, the positive electrode slurry is obtained. The positive electrode slurry is uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 12 μm, and dried at 85°C to obtain a positive electrode sheet with a single-sided coated positive electrode material layer. When coating, the coating weight of the positive electrode material layer is 19 mg / cm 2 . Then repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coated positive electrode material layer. Then, through cold pressing, slicing, and slitting, it is dried under vacuum conditions at 85°C for 4 h to obtain a positive electrode sheet with a specification of 74 mm × 867 mm. After welding the tab, it is ready for use. Among them, the compaction density of the positive electrode material layer after cold pressing is 4.2 g / cm 3 .

[0082] (4) Preparation of the electrolyte: In a dry argon environment, add LiPF 6 to a mixed solution of 1,3-propane sultone (1,3-PS), propyl propionate (PP), ethyl propionate (EP), fluoroethylene carbonate (FEC), propylene carbonate (PC), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium difluorophosphate. Among them, based on the mass of the electrolyte, LiPF 6The mass fraction is 12.5% for, 3% for 1,3-PS, 8% for PC, 10% for EC, 10% for FEC, 0.01% for LiFSI, 0.5% for lithium difluorophosphate, and the balance is EP and PP, with the mass ratio of EP to PP being 1:3.

[0083] (5) Preparation of the separator: A porous polyethylene film with a thickness of 7 μm (provided by Celgard) was used as the separator.

[0084] (6) Preparation of the lithium-ion full cell: The positive electrode sheet, separator, negative electrode sheet, and separator were stacked in sequence, with the separator placed between the positive electrode sheet and the negative electrode sheet to play a separating role, and then wound to obtain an electrode assembly. The electrode assembly was placed in an aluminum-plastic film packaging bag, dried at 80 °C, and then electrolyte was injected. After vacuum packaging, standing, forming, shaping, and capacity testing processes, a soft-pack lithium-ion battery was obtained.

[0085] Examples 1-2 to 1-11

[0086] The differences between Examples 1-2 to 1-11 and Example 1-1 are the mass fraction of carbon dioxide in the mixed gas and the carbon coating time. For specific preparation parameters, please refer to Tables 1 and 2, and the rest are the same as Example 1-1.

[0087] Comparative Examples 1 to 2

[0088] The differences between Comparative Examples 1 to 2 and Example 1-1 are the mass fraction of carbon dioxide in the mixed gas and the carbon coating time. For specific preparation parameters, please refer to Tables 1 and 2, and the rest are the same as Example 1-1.

[0089] Examples 2-1 to 2-12

[0090] The differences between Examples 2-1 to 2-12 and Example 1-1 are the mass fraction of carbon dioxide in the mixed gas, the carbon coating time, and the mass ratio of the silicon-carbon material particles to the lithium foil. For specific preparation parameters, please refer to Tables 3 and 4, and the rest are the same as Example 1-1.

[0091] Examples 2-13 to 2-16

[0092] The differences between Examples 2-13 to 2-16 and Example 2-7 are that by controlling the roundness of the porous carbon, the average roundness of the silicon-carbon particles in the prepared negative electrode material is changed. For specific preparation parameters, please refer to Table 5, and the rest are the same as Example 1-1.

[0093] Examples 3-1 to 3-11

[0094] Examples 3-1 to 3-11 are different from Examples 1-5 in that the components of the electrolyte in the lithium-ion battery are adjusted. Except for adjusting the parameters of the relevant electrolyte components according to Table 6, the rest are the same as in Examples 1-5.

[0095] Relevant test methods for the negative electrode material

[0096] (1) Test method for the oxygen content of silicon-carbon particles:

[0097] Use a nitrogen-oxygen-hydrogen analyzer to test the oxygen content. Weigh 0.2 g of the silicon-carbon material particle sample, mix it evenly with the flux (graphite), put it into a graphite crucible, and place it in the instrument sampler. Heat the high-temperature furnace to 2500 °C. The silicon-carbon material particle sample melts in an argon gas stream, and oxygen is released in the form of CO or CO 2 form, and the carrier gas transports it to the detection system. CO / CO 2 passes through an infrared detector, and the absorbance is proportional to the oxygen content. The oxygen content can be obtained through the data system.

[0098] (2) Test method for the oxygen content of the surface area and internal area of silicon-carbon particles:

[0099] Under an argon gas atmosphere, use focused ion beam cutting (FIB) to cut the cross-section of the silicon-carbon material particles to obtain a thin sample for analysis. Observe and calibrate the internal area and surface area of the silicon-carbon material particles through TEM. Use TEM EDX to take multiple regions (5 times, take the average value) to obtain the oxygen content of the internal area and surface area of the silicon-carbon material particles.

[0100] (3) Test method for the differential capacity curve of lithium deintercalation of the negative electrode material:

[0101] After the lithium-ion coin half-cell is discharged at a small current of 0.05C / 50μA / 10μA in three stages to 0.005V, record the first discharge capacity of the lithium-ion coin half-cell; then charge it at a constant current of 0.1C to 1.5V, and record the first charge capacity of the lithium-ion coin half-cell. The first reversible specific capacity of the negative electrode material from 0.005V to 1.5V = the first charge capacity of the button battery / the mass of the negative electrode material. Then perform slope function processing on the two groups of specific capacity-voltage data in the charging stage (obtain DQ / DV corresponding to each voltage), take one data every 15 points, and use voltage as the abscissa and DQ / DV as the ordinate to obtain the differential capacity curve of lithium deintercalation of the negative electrode material.

[0102] (4) Test method for X-ray photoelectron spectroscopy:

[0103] Use XPS to analyze the components of the surface Si layer of the silicon-carbon material particles. Stick the silicon-carbon material particles on the double-sided tape of the sample stage and put them into the sample chamber for testing.

[0104] Peak deconvolution calculation: a. Charge calibration of the original data is performed based on the standard value of 284.8 eV for foreign contaminated carbon. b. Energy spectrum peak deconvolution fitting is carried out through XPS peaks. Peak deconvolution rule: For the split orbits of the same element, their full width at half maximum should be as close as possible to be consistent. The Gaussian-Lorentz ratio of each data should be kept consistent (generally directly adopt the default value of 80). c. The integral intensity ratio calculation of the spectrum after peak deconvolution fitting is carried out through scientific drawing software (Origin).

[0105] (5) Test method for the average circularity of silicon-carbon material particles and graphite:

[0106] Use a ZEISS-SEM (sigma-02-33) scanning electron microscope to observe the particle samples of the negative electrode active material. Randomly select 20 silicon-carbon material particles (or graphite), and calculate their perimeter equivalent diameter and area equivalent diameter respectively. The sphericity of each silicon-carbon material particle (or graphite) = perimeter equivalent diameter / area equivalent diameter. Calculate the arithmetic mean of the sphericity of 20 silicon-carbon material particles (or graphite), which is used as the sphericity of the silicon-carbon material particles (or graphite).

[0107] (6) Test method for lithium content:

[0108] After the lithium-ion full battery is fully discharged, disassemble the lithium-ion full battery, and wash the negative electrode material with DMC. Take 2 g of silicon-carbon material and place it in a PTFE beaker. Add 10 mL of concentrated nitric acid and 2 mL of hydrofluoric acid, place it on a 220-degree hot plate heater, heat and digest until almost dry, and then slowly add 10 mL of nitric acid and continue heating and dissolving. Then filter and separate with deionized water, collect the filtrate, and then perform constant volume ICP element test to obtain the mass percentage of Li.

[0109] Performance test of lithium-ion battery:

[0110] (1) Test of anode specific capacity / first Coulombic efficiency / first efficiency loss after pulping - coating - cold pressing:

[0111] Preparation of the negative electrode sheet of the coin-type half cell: Disperse pre-lithiated silicon-carbon particles, polyacrylic acid lithium (PAALi), conductive carbon (SP), and single-walled carbon nanotube dispersion in an appropriate amount of deionized water according to the weight ratio of 84:10:5:1 and stir well to form a uniform negative electrode slurry, where the solid content of the negative electrode slurry is 40 wt%. Coat this slurry on the negative electrode current collector copper foil, dry it in a vacuum at 120 °C for 12 hours, and then divide the dried electrode sheet into two groups. One group is reserved after cutting and slitting, marked as the non-cold-pressed silicon-carbon negative electrode. One group is cold-pressed (the pressure of the roller press is 15 t - three times of cold pressing), cut, and slit, and then marked as the cold-pressed silicon-carbon negative electrode.

[0112] Using the electrolytes prepared in each of the examples or comparative examples as the electrolytes of the coin half-cells, a 7-μm polypropylene / polyethylene composite separator was used, and a lithium sheet and a silicon-carbon electrode sheet were used as counter electrodes respectively, and coin half-cells were assembled in a glove box respectively.

[0113] First, the two groups of coin half-cells were discharged at a small current in three stages of 0.05C / 50μA / 10μA to 0.005V respectively, and the first discharge capacity of the coin cell was recorded; then they were charged at a constant current of 0.1C to 0.8V, and the first charge capacity of the coin cell was recorded. The first reversible specific capacity of the negative electrode active material from 0.005V to 0.8V = the first charge capacity of the coin cell / the mass of the negative electrode active material. The initial coulombic efficiency (ICE) = the first reversible specific capacity / the first discharge specific capacity. The ICE loss of the first efficiency: the first efficiency loss = the first efficiency of the non-cold-pressed group - the first efficiency of the cold-pressed group.

[0114] (2) Test of thermal runaway temperature:

[0115] The thermal runaway temperature of the fully charged lithium-ion battery was tested using an explosion-proof high-temperature test chamber. The thermocouple wire was attached to the center of the surface of the lithium-ion battery, and the lithium-ion battery was placed vertically in the chamber and heated at a rate of 5 ± 1°C to 128 ± 1°C and held for 60 minutes. The criterion for non-runaway was that the battery cell did not catch fire or explode. If the group of lithium-ion batteries did not experience thermal runaway, a new batch of lithium-ion batteries was selected and placed vertically in the chamber and heated at a rate of 5 ± 1°C to 130 ± 1°C and held for 60 minutes. This was repeated, increasing the heating temperature step by step to 128, 130, 132, 134, 136°C until the thermal runaway temperature of each group of lithium-ion batteries was measured.

[0116] Among them, the preparation conditions and test results of each example and comparative example are recorded in Tables 1 to 7.

[0117] Table 1

[0118]

[0119] Table 2

[0120]

[0121] Combining Tables 1 and 2 above, compared with the comparative examples, in Examples 1-1 to 1-11, during the preparation of the silicon-carbon particle material, when the mass ratio of carbon dioxide in the mixed gas, the carbon coating time, the oxygen content in the silicon-carbon particles, and the oxygen content in the internal region all have a relatively high oxygen content and meet a specific range, the initial coulombic efficiency of the anode after cold pressing of the negative electrode material can be improved, and the thermal runaway temperature of the secondary battery can be increased.

[0122] In addition, when the oxygen content in the surface area also meets a specific range, the first Coulombic efficiency of the anode after cold pressing of the negative electrode material can be further improved, and the thermal runaway temperature of the secondary battery can be increased.

[0123] In addition, it can be seen that by increasing the mass ratio of CO within the scope of this application 2 the oxygen content of the silicon-carbon particles, the oxygen content in the internal area is X1, and the oxygen content in the surface area is X2 can be increased.

[0124] Table 3

[0125]

[0126]

[0127] Table 4

[0128]

[0129]

[0130] Combining Table 3 and Table 4, in the above embodiments, during the preparation of the silicon-carbon particle material, the mass ratio of carbon dioxide in the mixed gas, the carbon coating time, and the mass ratio of the silicon-carbon material particles to the lithium foil, when the X, X1 in the silicon-carbon particles, the differential capacity curve of the negative electrode material, the ratio of the Si-O peak to the Si-Si peak, and the mass content of lithium in the negative electrode material are respectively within specific ranges, can all further improve the first Coulombic efficiency of the anode after cold pressing of the negative electrode material and increase the thermal runaway temperature of the secondary battery.

[0131] Table 5

[0132]

[0133] In Table 5, during the negative electrode material process, when the difference in the average circularity of graphite and silicon-carbon particles meets a specific range, the first Coulombic efficiency of the anode after cold pressing of the negative electrode material can be further improved, and the thermal runaway temperature of the secondary battery can be increased.

[0134] Table 6

[0135]

[0136] Table 7

[0137]

[0138]

[0139] In Tables 6 and 7, when the electrolyte contains either the first lithium salt or the second lithium salt, and the mass percentage ratios S1 and S2 satisfy an appropriate range, and when the electrolyte contains both the first lithium salt and the second lithium salt, and the mass ratio S2 / S1 satisfies an appropriate range, it is possible to further improve the first Coulombic efficiency of the anode after cold pressing of the negative electrode material while taking into account excellent specific capacity, and to increase the thermal runaway temperature of the secondary battery.

[0140] The above disclosure is only for the preferred embodiments of the present application. Of course, the present application cannot be limited by this. Therefore, equivalent changes made according to the present application still fall within the scope covered by the present application.

Claims

1. A negative electrode material, characterized in that: The negative electrode material includes silicon-carbon particles, the oxygen content of the silicon-carbon particles is X%, 9≤X≤20, the silicon-carbon particles include an internal area and a surface area, the surface area refers to an area less than 1 μm away from the surface of the silicon-carbon particles, the internal area refers to other areas of the silicon-carbon particles except the surface area, and the oxygen content of the internal area is X1%, 9≤X 1≤21.

2. The negative electrode material according to claim 1, characterized in that The silicon-carbon particles include porous carbon and silicon particles, wherein the silicon particles are at least partially located in the pores of the porous carbon.

3. The negative electrode material according to claim 1, characterized in that The oxygen content of the surface area is X2%, 7≤X2≤17.

4. The negative electrode material according to any one of claims 1 to 3, characterized in that The negative electrode material satisfies at least one of the following conditions: (1) The oxygen content of the silicon-carbon particles satisfies: 10≤X≤15 and 10≤X 1≤15; (2) The differential capacity curve of the negative electrode material after lithium removal has a first peak and a second peak at 200 mV to 320 mV and 600 mV to 900 mV, respectively, and the peak intensity ratio of the first peak to the second peak is I1, 1.2≤I1≤3.2; (3) In the X-ray photoelectron energy spectrum of the silicon-carbon particles, there are Si-O peaks and Si-Si peaks at 101eV to 104eV and 96eV to 99eV, respectively, and the peak intensity ratio of the Si-O peak to the Si-Si peak is I2, 0.6≤I2≤1.

4.

5. A method for preparing silicon-carbon particles in a negative electrode material according to any one of claims 1 to 4, characterized in that: The following steps are involved: S1) providing a porous carbon skeleton, introducing a mixed gas into the porous carbon skeleton, the deposition time is 1h to 20h, the reaction temperature is 400°C to 500°C, to obtain a core body, wherein the mixed gas includes silane gas, carbon dioxide and an inert gas, the mass percentage of the carbon dioxide in the mixed gas is 2% to 20%, and the flow rate of the mixed gas is 100sccm to 500sccm; S2) introducing an alkane gas into the core body to form a carbon layer on the core body to obtain the silicon-carbon particles.

6. A secondary battery comprising a negative electrode sheet, a positive electrode sheet and an electrolyte, characterized in that: The negative electrode sheet further comprises the negative electrode material according to any one of claims 1 to 4 or the negative electrode material obtained by the preparation method according to claim 5, and the negative electrode material further comprises graphite.

7. The secondary battery according to claim 6, characterized in that: In a fully discharged state, the mass content of lithium in the negative electrode material is Y%, and 0.1≤Y≤0.

9.

8. The secondary battery according to claim 6, wherein: The average circularity of the silicon-carbon particles is C1, the average circularity of the graphite is C2, and |C 1-C2|≤0.

2.

9. The secondary battery according to claim 6, characterized in that: The electrolyte satisfies one of the following conditions: (1) The electrolyte includes a first lithium salt, wherein the first lithium salt is selected from at least one of lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide and lithium trifluoromethylsulfonate, and based on the mass of the electrolyte, the mass percentage of the first lithium salt is S1%, 0.01≤S1≤2, preferably, 0.5≤S1≤1; (2) The electrolyte includes a second lithium salt, the second lithium salt is selected from lithium tetrafluoroborate and lithium difluorophosphate, and the mass percentage of the second lithium salt is S2% based on the mass of the electrolyte, 0.5≤S2≤2; (3) 1≤S2 / S1≤5.

10. An electronic device, characterized in that: The invention comprises the secondary battery according to any one of claims 7 to 9.

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