Lithium-ion secondary battery, electric apparatus, and use

Abstract

A lithium-ion secondary battery, an electric apparatus, and a use. The lithium-ion secondary battery comprises a negative electrode sheet. The negative electrode sheet comprises a negative electrode current collector, and a second negative electrode active layer and a first negative electrode active layer which are sequentially arranged on at least one side of the negative electrode current collector. The second negative electrode active layer is located between the negative electrode current collector and the first negative electrode active layer; and the second negative electrode active layer comprises a second negative electrode active material and quaternary ammonium salt compounds.

Description

Lithium-ion secondary batteries, electrical devices and applications

[0001] Related applications

[0002] This application claims priority to Chinese patent application No. CN2025100803681, filed on January 17, 2025, entitled "Lithium-ion secondary battery, power supply device and application", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of lithium-ion secondary battery technology, and further to a lithium-ion secondary battery, an electrical device, and an application. Background Technology

[0004] The statements herein are provided only as background information in connection with this application and do not necessarily constitute prior art.

[0005] With the technological advancements in lithium-ion rechargeable batteries, they are increasingly being used in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and many other fields. They are also widely used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants. The demand for fast-charging performance in lithium-ion rechargeable batteries is also increasing. Summary of the Invention

[0006] According to various embodiments and examples of this application, this application provides a lithium-ion secondary battery, a power-consuming device, and an application. This lithium-ion secondary battery exhibits significantly improved fast-charging performance.

[0007] In a first aspect of this application, a lithium-ion secondary battery is provided, wherein a quaternary ammonium salt compound is disposed in the negative electrode active material layer of the negative electrode sheet.

[0008] In some embodiments, a lithium-ion secondary battery is provided, which includes a negative electrode sheet and an electrolyte; the negative electrode sheet includes a negative current collector and a second negative active layer and a first negative active layer sequentially disposed on at least one side of the negative current collector, the second negative active layer being located between the negative current collector and the first negative active layer;

[0009] The second negative electrode active layer includes a second negative electrode active material and a quaternary ammonium salt compound. The second negative electrode active material includes a carbon-based material, and the quaternary ammonium salt compound includes quaternary ammonium cations.

[0010] In this lithium-ion secondary battery, a first negative electrode active layer and a second negative electrode active layer (with the direction away from the surface of the negative electrode current collector being considered as the upper layer and the direction towards the surface of the negative electrode current collector being considered as the lower layer) can be disposed in the negative electrode active material layer of the negative electrode sheet. Furthermore, a quaternary ammonium salt compound is disposed in the second negative electrode active layer. The quaternary ammonium salt compound includes hydrophilic, positively charged quaternary ammonium ions. For the negative electrode sheet immersed in the electrolyte, the electrostatic effect based on the quaternary ammonium ions can be used to attract electrolyte anions in the electrolyte, promote the rapid dissociation of electrolyte lithium salt in the electrolyte, significantly improve the electrolyte wettability of the second negative electrode active layer and the kinetics of the negative electrode sheet, and thus significantly improve the fast charging performance of the battery.

[0011] In some embodiments, the electrolyte includes a non-aqueous solvent.

[0012] In some embodiments, the electrolyte comprises an electrolyte lithium salt and a non-aqueous solvent.

[0013] In some embodiments, the quaternary ammonium salt compound includes a quaternary ammonium cation and a hydrocarbon chain covalently bonded to the quaternary ammonium cation.

[0014] In some embodiments, the hydrocarbon chain in the quaternary ammonium salt compound includes an alkyl chain.

[0015] By incorporating hydrocarbon chains into quaternary ammonium salt compounds, such as alkyl chains, the chain structure of the hydrocarbon chains facilitates better and more stable loading and encapsulation of the quaternary ammonium salt compounds on the surface of the second negative electrode active material. The carbon-carbon skeleton provided by the hydrocarbon chains facilitates better and more stable adsorption of the quaternary ammonium salt compounds on the surface of carbon-based materials (which have a certain degree of oleophilicity). Furthermore, combined with the electrostatic attraction of the quaternary ammonium ions in the quaternary ammonium salt compounds, it is beneficial to guide lithium ions in the electrolyte to be guided to the surface of the second negative electrode active material more quickly, which is beneficial to further improve the kinetics of the negative electrode sheet and the fast charging performance of the battery.

[0016] In some embodiments, the quaternary ammonium salt compound satisfies one or more of the following characteristics:

[0017] (ta1) The hydrocarbon chain is an alkyl chain;

[0018] (ta2) The hydrocarbon chain contains 12 to 18 carbon atoms;

[0019] (ta3) The molecular weight of the quaternary ammonium salt compound is less than or equal to 600 Da.

[0020] In some embodiments, the hydrocarbon chain in the quaternary ammonium salt compound is C. 12-18 Alkyl chain.

[0021] By controlling the number of carbon atoms in the hydrocarbon chain of the quaternary ammonium salt compound and / or the molecular weight of the quaternary ammonium salt compound within the aforementioned range, the length of the hydrocarbon chain can be adjusted to a more suitable range. On the one hand, this facilitates better and more stable encapsulation of the quaternary ammonium salt compound on the surface of the second negative electrode active material, and helps to suppress the detachment of the quaternary ammonium salt compound from the surface of the second negative electrode active material during charging and discharging. Furthermore, by combining with the quaternary ammonium ions exposed to the electrolyte, it can promote the faster and more stable guidance of lithium ions in the electrolyte to the surface of the second negative electrode active material, thereby increasing the transport rate of lithium ions inside the negative electrode sheet. On the other hand, it can also better control the degree of encapsulation of the hydrocarbon chain on the surface of the second negative electrode active material, so that the active sites on the surface of the second negative electrode active material can better contact the electrolyte. This is beneficial to better improve the dynamics of the negative electrode sheet and the fast charging performance of the battery.

[0022] In some embodiments, the quaternary ammonium salt compound satisfies one or more of the following characteristics:

[0023] (tb1) The structure of the quaternary ammonium cation is -N + (R1R2R3), where R1 and R2 are each independently C 1-3 Alkyl group, R3 is C 1- 3. Alkyl or hydroxyethyl; optionally, R1 and R2 are each independently methyl, and R3 is methyl or hydroxyethyl;

[0024] (tb2) The quaternary ammonium salt type compound includes anion, which includes one or more of nitrate, carbonate, bicarbonate and phosphate.

[0025] In some embodiments, the quaternary ammonium salt compound includes one or more of the following: octadecyl dimethyl hydroxyethyl quaternary ammonium nitrate, N,N-dimethyl-N-(2-hydroxyethyl)hexadecyl quaternary ammonium phosphate, dodecyl trimethyl quaternary ammonium nitrate, octadecyl trimethyl quaternary ammonium phosphate, dodecyl trimethyl quaternary ammonium phosphate, dodecyl trimethyl quaternary ammonium carbonate, and dodecyl trimethyl quaternary ammonium bicarbonate.

[0026] By introducing one or more of nitrate, carbonate, bicarbonate, and phosphate into the anions of quaternary ammonium salt compounds, it is beneficial to better control the binding ability between anions and quaternary ammonium ions in the quaternary ammonium salt compounds, making it easier for quaternary ammonium ions to dissociate. These anions can have lower electronegativity than electrolyte anions, which is more conducive to promoting the formation of quaternary ammonium ion-electrolyte anion structure. This is more conducive to the role of the quaternary ammonium ion-electrolyte anion structure in guiding the aggregation of lithium ions in the electrolyte to the second negative electrode active material, and is beneficial to improving the kinetics of the negative electrode sheet and the fast charging performance of the battery.

[0027] In some embodiments, the quaternary ammonium salt compound accounts for 0.2% to 2% of the mass of the second negative electrode active layer.

[0028] In some embodiments, the quaternary ammonium salt compound accounts for 0.2% to 1.5% of the mass of the second negative electrode active layer.

[0029] By controlling the mass proportion of quaternary ammonium salt compounds in the second negative electrode active layer within the aforementioned range, it is beneficial to better leverage the role of quaternary ammonium salt compounds in improving the battery's fast-charging performance. It also helps to better control the decrease in electronic conductivity on the surface of the second negative electrode active material caused by the encapsulation of quaternary ammonium salt compounds, thus further improving the battery's fast-charging performance. Furthermore, it also helps to maintain a high lithium storage capacity in the second negative electrode active layer, which is beneficial for balancing the energy density of both the negative electrode and the battery.

[0030] In some embodiments, the carbon-based material accounts for 80% to 100% of the mass of the second negative electrode active material.

[0031] In some embodiments, the negative electrode sheet satisfies one or more of the following characteristics:

[0032] (tc1) The carbon-based material includes one or more of artificial graphite, natural graphite, soft carbon, and hard carbon;

[0033] (tc2) The carbon-based material accounts for 94.5% to 97.5% of the mass of the second negative electrode active layer.

[0034] In some embodiments, the carbon-based material includes one or more of artificial graphite, natural graphite, graphite, soft carbon, and hard carbon.

[0035] Natural graphite is prone to lateral deformation when subjected to cold pressing. Introducing natural graphite into the second negative electrode active material can help improve the compaction density and areal capacity of the second negative electrode active layer, thereby improving the energy density of the negative electrode sheet.

[0036] Compared to natural graphite, artificial graphite has a more stable structure and fewer internal defects, which results in a slower decay of available storage sites for lithium ions during cycling and more stable cycling performance.

[0037] Both hard carbon and soft carbon exhibit high levels of disorder, which facilitates lithium-ion entry. Hard carbon provides abundant lithium intercalation sites and rapid transport channels; the disordered stacking of carbon layers in soft carbon allows for relatively rapid lithium-ion transport. The introduction of at least one of hard carbon and soft carbon is beneficial for improving the negative electrode and battery kinetics.

[0038] In some embodiments, the carbon-based material accounts for 94.5% to 97.5% of the mass of the second negative electrode active layer, and optionally 95.0% to 97.0%.

[0039] By controlling the mass ratio of carbon-based materials in the second negative electrode active layer within the aforementioned range, it is beneficial to better encapsulate quaternary ammonium salt compounds on the surface of the second negative electrode active material. In addition, the energy density of both the negative electrode and the battery can be taken into account.

[0040] In some embodiments, the second negative electrode active layer includes an adhesive, said adhesive being styrene-butadiene rubber.

[0041] By introducing styrene-butadiene rubber (SBR) into the binder of the second negative electrode active layer, SBR can have a non-chain structure. In terms of occupying the encapsulation sites on the surface of the second negative electrode active material, SBR is less likely to compete with quaternary ammonium salt compounds. This is conducive to better simultaneous encapsulation of the binder and quaternary ammonium salt compounds on the surface of the second negative electrode active material. This can not only achieve a good electrical contact network, but also give full play to the role of quaternary ammonium salt compounds in guiding lithium ions in the electrolyte, which is beneficial to improving the fast charging performance of the battery.

[0042] In some embodiments, the second negative electrode active layer comprises a carbon-based material and a binder, wherein the binder comprises styrene-butadiene rubber.

[0043] By introducing carbon-based materials into the second negative electrode active material of the second negative electrode active layer and introducing styrene-butadiene rubber into the binder, styrene-butadiene rubber, with its suitable oleophilicity, can better synergistically enhance the binding effect between the binder, carbon-based materials, and quaternary ammonium salt compounds. Compared to the strong binding effect of oily binders (such as polyvinylidene fluoride (PVDF)) on quaternary ammonium salt compounds, styrene-butadiene rubber is more conducive to better leveraging the binding effect between quaternary ammonium salt compounds and carbon-based materials, and better utilizing the role of quaternary ammonium salt compounds in improving the fast charging performance of the battery.

[0044] In some embodiments, the glass transition temperature of the styrene-butadiene rubber is 5°C to 70°C.

[0045] In some embodiments, the glass transition temperature of the styrene-butadiene rubber is 30°C to 50°C.

[0046] By controlling the glass transition temperature of styrene-butadiene rubber within the aforementioned range, it is beneficial to enable the second negative electrode active layer to have a better pore structure after rolling, which is beneficial to promote better wetting of the second negative electrode active layer by the electrolyte.

[0047] In some embodiments, the first negative electrode active layer includes a first negative electrode active material, the first negative electrode active material including 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 including one or more of soft carbon, hard carbon and amorphous carbon.

[0048] By setting a coating layer on the surface of the first negative electrode active material in the first negative electrode active layer, and setting one or more of soft carbon, hard carbon and amorphous carbon in the coating layer, the lithium-ion transport channels on the surface of the first negative electrode active material can be optimized, lithium-ion transport can be promoted, and battery kinetics and battery fast charging performance can be further improved.

[0049] In some embodiments, the first negative electrode active layer includes a first negative electrode active material;

[0050] The lithium-ion secondary battery satisfies one or more of the following characteristics:

[0051] (td1) The first negative electrode active material includes coated graphite, the coated graphite including a graphite body and a coating layer located on at least a portion of the surface of the graphite body, the coating layer including one or more of soft carbon, hard carbon and amorphous carbon;

[0052] (td2) The first negative electrode active material includes secondary particulate graphite, the secondary particulate graphite includes a secondary particulate graphite body, and the proportion of the secondary particulate graphite in the first negative electrode active material is greater than or equal to 20%, optionally 30% to 80%; optionally, the secondary particulate graphite includes carbon-coated secondary particulate graphite, the carbon-coated secondary particulate graphite includes the secondary particulate graphite body and a carbon coating layer located on at least a portion of the surface of the secondary particulate graphite body, the carbon coating layer in the carbon-coated secondary particulate graphite includes one or more of soft carbon, hard carbon and amorphous carbon;

[0053] (td3) The first negative electrode active material includes graphite material, wherein the OI value of the graphite material is 2 to 15, and can be selected as 2 to 10;

[0054] (td4) D of the first negative electrode active material v 50 is 10μm to 18μm, and can be selected as 12μm to 16μm;

[0055] (td5) The porosity of the first negative electrode active layer is higher than that of the second negative electrode active layer;

[0056] (td6) The compacted density of the powder of the first negative electrode active material or the compacted density of the powder of the first negative electrode active layer is 1.60 g / cm³. 3 ~1.80g / cm 3 ;

[0057] (td7) The rate of charge of the first negative electrode active layer is higher than that of the second negative electrode active layer; optionally, the charging rate of the first negative electrode active layer is higher than that of the second negative electrode active layer.

[0058] (td8) At at least one temperature condition from 20°C to 35°C, the ionic conductivity of the electrolyte is 13 mS / cm to 18 mS / cm; optionally, at 25°C, the ionic conductivity of the electrolyte is 13 mS / cm to 18 mS / cm.

[0059] The fast-charging performance of a lithium-ion secondary battery can be improved by making it satisfy one or more of the following characteristics: (td1), (td2), (td3), (td4), (td5), (td6), (td7), and (td8).

[0060] By introducing coated graphite into the first negative electrode active layer, and setting one or more of soft carbon, hard carbon and amorphous carbon in the coating layer, the lithium-ion transport channels on the surface of the first negative electrode active material can be optimized, promoting lithium-ion transport and further improving battery kinetics and fast-charging performance.

[0061] By introducing secondary particulate graphite into the first negative electrode active layer, and taking advantage of the characteristic that secondary particles are formed by the agglomeration of primary particles, and based on the disordered orientation of each primary particle, it is beneficial to improve the isotropic characteristics when lithium ions are inserted into the first negative electrode active material, increase the lithium ion insertion sites on the surface of the first negative electrode active material, improve the lithium ion insertion rate, and improve the fast charging performance of the battery.

[0062] By controlling the OI value of the graphite material in the first negative electrode active material within the aforementioned range, it is beneficial to enhance the isotropic characteristics of the first negative electrode active material, increase the lithium ion insertion channels, and make the lithium ion diffusion dynamics in the first negative electrode active layer better, thereby improving the fast charging performance of the battery.

[0063] By using the D of the first negative electrode active material v Controlling the concentration of 50% within the aforementioned range helps to better control the degree of particle accumulation in the first negative electrode active layer, better control the porosity between particles, provide better lithium-ion transport channels, and better improve battery dynamics and fast charging performance.

[0064] 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 balance the fast charging performance and energy density of the battery. The relatively high porosity of the first negative electrode active layer can be used to promote the rapid transport of lithium ions, while the relatively low porosity of the second negative electrode active layer can be used to improve the energy density of the negative electrode.

[0065] By controlling the powder compaction density of the first negative electrode active material or the powder compaction density of the first negative electrode active layer within the aforementioned range, it is beneficial to provide a better lithium-ion transport channel by increasing the particle packing degree of the first negative electrode active layer, thereby improving battery dynamics and fast charging performance. In addition, the second negative electrode active layer can be used to provide a higher energy density, which is beneficial to balance the fast charging performance and energy density of lithium-ion secondary batteries.

[0066] By controlling the charging 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 insertion of lithium ions into the first negative electrode active layer, which is more conducive to improving the fast charging performance of the battery.

[0067] By controlling the ionic conductivity of the electrolyte within the aforementioned range, it is beneficial to promote the rapid transport of lithium ions, which in turn helps to improve battery dynamics and fast-charging performance.

[0068] In some embodiments, the negative electrode in the lithium-ion secondary battery satisfies one or more of the following characteristics:

[0069] (te1) D of the second negative electrode active material v 50 is 12μm to 21μm, and can be selected as 14μm to 20μm;

[0070] (te2) The first negative electrode active layer includes a first negative electrode active material, and the second negative electrode active material D v 50 is higher than the D of the first negative electrode active material v 50;

[0071] (te3) The compaction density of the second negative electrode active layer is higher than that of the first negative electrode active layer;

[0072] (te4) The powder compaction density of the second negative electrode active material is higher than that of the first negative electrode active material, or the powder compaction density of the second negative electrode active layer is higher than that of the first negative electrode active layer; optionally, the ratio of the powder compaction density of the second negative electrode active material to that of the first negative electrode active material is 1.05 to 1.35, further optionally 1.10 to 1.30, even more preferably 1.10 to 1.28 or 1.15 to 1.30; optionally, the ratio of the powder compaction density of the second negative electrode active layer to that of the first negative electrode active layer is 1.05 to 1.35, further preferably 1.10 to 1.30, even more preferably 1.10 to 1.28 or 1.15 to 1.30;

[0073] (te5) The compacted density of the powder of the second negative electrode active material or the compacted density of the powder of the second negative electrode active layer is 1.85 g / cm³. 3 ~2.05g / cm 3 .

[0074] By enabling lithium-ion secondary batteries to meet one or more of the characteristics (te1), (te2), (te3), (te4), and (te5), it is beneficial to enable lithium-ion secondary batteries to have improved fast-charging performance while also meeting energy density requirements.

[0075] By using the D of the second negative electrode active material v Keeping 50 within the aforementioned range is beneficial for achieving a higher compaction density in the second negative electrode active layer, which in turn helps to improve the energy density.

[0076] By controlling the D of the second negative electrode active material v 50 higher than the D of the first negative electrode active material v 50 is beneficial for achieving a higher compaction density in the second negative electrode active layer, which in turn helps to improve the energy density.

[0077] By adjusting the compaction density of the second negative electrode active layer in a lithium-ion secondary battery to be higher than that of the first negative electrode active layer, it is beneficial to improve the energy density of the lithium-ion secondary battery.

[0078] By controlling the powder compaction density of the second negative electrode active material in the lithium-ion secondary battery to be higher than that of the first negative electrode active material, or by controlling the powder compaction density of the second negative electrode active layer to be higher than that of the first negative electrode active layer, it is beneficial to impart a higher compaction density to the second negative electrode active layer during the cold pressing process of the electrode sheet, thus giving the second negative electrode active layer in the lithium-ion secondary battery a higher compaction density. Furthermore, by controlling the ratio of the powder compaction density of the second negative electrode active material to that of the first negative electrode active material, or by controlling the ratio of the powder compaction density of the second negative electrode active layer to that 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 lithium-ion secondary battery.

[0079] By controlling the powder compaction density of the second negative electrode active material or the powder compaction density of the second negative electrode active layer within the aforementioned range, it is beneficial to achieve a higher energy density in the secondary battery. Furthermore, the particle packing can be relatively dense; in this case, the quaternary ammonium salt compounds introduced into the second negative electrode active layer may have a more significant effect on improving the battery's fast-charging performance.

[0080] In some embodiments, the negative electrode in the lithium-ion secondary battery satisfies one or more of the following characteristics:

[0081] (tf1) On one side of the negative electrode current collector, the ratio of the areal density of the second negative electrode active layer to the areal density of the first negative electrode active layer is 3:2 to 2:3.

[0082] (tf2) Taking the thickness ratio of the first negative electrode active layer to the second negative electrode active layer on one side of the negative electrode current collector as denoted as f H , satisfying f H ≤1.6, optionally, 1.1≤f H ≤1.6, and further optionally, 1.1≤f H ≤1.3;

[0083] (tf3) Measured on one side of the negative electrode current collector, the thickness of the first negative electrode active layer is less than or equal to 50 μm, optionally 20 μm to 50 μm, and further optionally 30 μm to 40 μm.

[0084] By controlling the ratio of the areal density of the second negative electrode active layer to the areal density of the first negative electrode active layer within the aforementioned range, it is beneficial to balance the battery's fast charging performance and energy density.

[0085] By controlling the thickness of the first negative electrode active layer to satisfy one or more of the above features (tf2) and (tf3), the distance between the quaternary ammonium salt compound and the surface of the negative electrode sheet can be adjusted, which is beneficial to better promote the transport of lithium ions to the second negative electrode active layer located below, thereby improving the fast charging performance of the battery.

[0086] In a first aspect of this application, a lithium-ion secondary battery is also provided, which includes a negative electrode sheet and an electrolyte; the negative electrode sheet includes a negative current collector and a negative active material layer located on at least one side of the negative current collector;

[0087] The negative electrode active material layer includes a negative electrode active material and a quaternary ammonium salt compound. The negative electrode 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 one or more of soft carbon, hard carbon, and amorphous carbon. The quaternary ammonium salt compound includes quaternary ammonium cations.

[0088] By setting a coating layer on the surface of the negative electrode active material layer, and setting one or more of soft carbon, hard carbon, and amorphous carbon in the coating layer, the lithium-ion transport channels on the surface of the negative electrode active material can be optimized, promoting lithium-ion transport and improving battery kinetics and fast-charging performance. Furthermore, by setting quaternary ammonium salt compounds in the negative electrode active material layer, the electrostatic interaction based on quaternary ammonium ions can attract electrolyte anions in the electrolyte, promoting the rapid dissociation of electrolyte lithium salts in the electrolyte. The formed quaternary ammonium ion-electrolyte anion structure can also guide the rapid transport of lithium ions in the electrolyte to the surface of the negative electrode active material. Based on the aforementioned multiple effects, the fast-charging performance of the battery can be further improved.

[0089] In some embodiments, the negative electrode sheet satisfies one or more of the following characteristics:

[0090] (tg1) The quaternary ammonium salt type compounds are as defined above;

[0091] (tg2) The quaternary ammonium salt compound has a mass percentage of 0.2% to 2% in the negative electrode active material layer, and can be selected as 0.2% to 1.5%;

[0092] (tg3) The negative electrode active material includes a carbon-based material; optionally, the carbon-based material includes one or more of artificial graphite, natural graphite, soft carbon and hard carbon;

[0093] (tg4) The negative electrode active material includes a carbon-based material, wherein the carbon-based material accounts for 94.5% to 97.5% of the mass of the negative electrode active material layer, and is optionally 95.0% to 97.0%;

[0094] (tg5) The negative electrode active material layer includes a binder, which includes styrene-butadiene rubber; optionally, the glass transition temperature of the styrene-butadiene rubber is 5℃~70℃, and more preferably 30℃~50℃;

[0095] (tg6) The negative electrode active material includes coated graphite, which includes a graphite body and a coating layer located on at least a portion of the surface of the graphite body. The coating layer includes one or more of soft carbon, hard carbon and amorphous carbon.

[0096] (tg7) The negative electrode active material includes secondary particulate graphite, which includes a secondary particulate graphite body. The proportion of secondary particulate graphite in the negative electrode active material is greater than or equal to 20%, and optionally 30% to 60%. Optionally, the secondary particulate graphite includes carbon-coated secondary particulate graphite, which includes the secondary particulate graphite body and a carbon coating layer located on at least a portion of the surface of the secondary particulate graphite body. The carbon coating layer in the carbon-coated secondary particulate graphite includes one or more of soft carbon, hard carbon, and amorphous carbon.

[0097] (tg8) The negative electrode active material includes graphite material, and the OI value of the graphite material is 2 to 15, which can be selected as 2 to 10;

[0098] (tg9) D of the negative electrode active material v 50 is 11μm to 20μm, and can be selected as 13μm to 18μm;

[0099] (tg10) The porosity of the negative electrode active material layer is 15% to 35%, and can be selected as 25% to 30%;

[0100] (tg11) At at least one temperature condition from 20°C to 35°C, the ionic conductivity of the electrolyte is 13 mS / cm to 18 mS / cm; optionally, at 25°C, the ionic conductivity of the electrolyte is 13 mS / cm to 18 mS / cm.

[0101] (tg12) The negative electrode active material layer includes a second negative electrode active layer, which is defined as in the first aspect of this application.

[0102] By controlling the mass percentage of quaternary ammonium salt compounds in the negative electrode active material layer within the aforementioned range, the role of quaternary ammonium salt compounds in improving the fast-charging performance of the battery can be better utilized. Furthermore, the decrease in electronic conductivity on the surface of the negative electrode active material caused by the encapsulation of quaternary ammonium salt compounds can be better controlled, which is beneficial for improving the battery's fast-charging performance. Moreover, it also helps maintain a high lithium storage capacity in the negative electrode active material layer, which is beneficial for balancing the energy density of both the negative electrode and the battery.

[0103] Carbon-based materials are beneficial for better adsorption of hydrocarbon chains in quaternary ammonium salt compounds, which in turn promotes better encapsulation of quaternary ammonium salt compounds on the surface of the negative electrode active material. This, in turn, helps to better guide lithium ions in the electrolyte to the surface of the negative electrode active material more quickly. Furthermore, carbon-based materials also have good electronic conductivity. In addition, carbon-based materials help to provide better stability of the negative electrode active material during fast charging, making it less prone to pulverization. Through the aforementioned multiple effects, it is beneficial to improve the dynamics of the negative electrode sheet and the fast charging performance of the battery.

[0104] Introducing natural graphite into the negative electrode active material can improve the compaction density and areal capacity of the negative electrode active material layer, thereby increasing the energy density of the negative electrode sheet. Introducing artificial graphite into the negative electrode active material can improve cycle performance. Introducing at least one of hard carbon and soft carbon into the negative electrode active material can improve the negative electrode sheet and battery kinetics.

[0105] By controlling the mass ratio of carbon-based materials in the negative electrode active material layer within the aforementioned range, it is beneficial to better coat the quaternary ammonium salt compound on the surface of the negative electrode active material. In addition, the energy density of the negative electrode and the battery can also be taken into account.

[0106] By introducing carbon-based materials into the negative electrode active material layer and styrene-butadiene rubber into the binder, it is beneficial to better simultaneously encapsulate the binder and quaternary ammonium salt compounds on the surface of the negative electrode active material. This not only achieves a good electrical contact network but also fully leverages the role of quaternary ammonium salt compounds in guiding lithium ions in the electrolyte, which is conducive to better improving the fast charging performance of the battery.

[0107] By controlling the glass transition temperature of styrene-butadiene rubber within the aforementioned range, it is beneficial to enable the negative electrode active material layer to have a better pore structure after rolling, which is beneficial to promote better wetting of the negative electrode active material layer by the electrolyte.

[0108] By setting a coating layer on the surface of the negative electrode active material layer, and setting one or more of soft carbon, hard carbon and amorphous carbon in the coating layer, the lithium-ion transport channels on the surface of the negative electrode active material can be optimized, lithium-ion transport can be promoted, and battery kinetics and fast charging performance can be further improved.

[0109] By introducing coated graphite into the negative electrode active material layer, and setting one or more of soft carbon, hard carbon and amorphous carbon in the coating layer, the lithium-ion transport channels on the surface of the negative electrode active material can be optimized, promoting lithium-ion transport and further improving battery kinetics and fast-charging performance.

[0110] By introducing secondary particulate graphite into the negative electrode active material layer, and taking advantage of the characteristic that secondary particles are formed by the agglomeration of primary particles, and based on the disordered orientation of each primary particle, it is beneficial to improve the isotropic characteristics when lithium ions are intercalated into the negative electrode active material, increase the lithium ion intercalation sites on the surface of the negative electrode active material, improve the lithium ion intercalation rate, and improve the fast charging performance of the battery.

[0111] By controlling the OI value of graphite in the negative electrode active material within the aforementioned range, it is beneficial to enhance the isotropic characteristics of the negative electrode active material, increase the lithium ion insertion channels, and make the lithium ion diffusion dynamics in the negative electrode active material layer better, thereby improving the fast charging performance of the battery.

[0112] By using the D of the negative electrode active material v Controlling the concentration of 50% within the aforementioned range helps to better control the degree of particle accumulation in the negative electrode active material layer, better control the porosity between particles, provide better lithium-ion transport channels, and better improve battery dynamics and fast charging performance.

[0113] When the porosity of the negative electrode active material layer is controlled within the aforementioned range, the particle packing is relatively dense. At this time, the quaternary ammonium salt compound introduced into the negative electrode active material layer has a more significant effect on improving the battery's fast charging performance.

[0114] By controlling the ionic conductivity of the electrolyte within the aforementioned range, it is beneficial to promote the rapid transport of lithium ions, which in turn helps to improve battery dynamics and fast-charging performance.

[0115] In some embodiments, the areal density of the negative electrode sheet is 5 mg / cm³, calculated on one side of the negative electrode current collector. 2 ~15mg / cm 2 .

[0116] By controlling the areal density of the negative electrode sheet within the aforementioned range, it is beneficial to balance the battery's fast charging performance and energy density.

[0117] In some embodiments, the electrolyte comprises an electrolyte salt, which comprises an electrolyte anion;

[0118] The electrolyte anions include one or more of tetrafluoroborate, hexafluoroarsenate, hexafluorophosphate, trifluoromethanesulfonate, difluorophosphate, difluorooxalateborate, tetrafluorooxalate phosphate, difluorodioxalate phosphate, difluorosulfonylimide, and ditrifluoromethanesulfonylimide.

[0119] By selecting the aforementioned types of electrolyte anions, it is beneficial to enhance the binding capacity of electrolyte anions to quaternary ammonium ions, better promote the dissociation of quaternary ammonium ions and anions in quaternary ammonium salt compounds, promote the attraction of quaternary ammonium salt compounds to electrolyte anions and the guidance of lithium ions in the electrolyte, improve the transport rate of lithium ions to the second negative electrode active material, and better improve the dynamics of the negative electrode sheet and the fast charging performance of the battery.

[0120] The aforementioned anions have stronger electronegativity than one or more of nitrate, carbonate, bicarbonate and phosphate, and have a stronger affinity for quaternary ammonium ions.

[0121] In some embodiments, the electrolyte salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0122] In some embodiments, the lithium-ion secondary battery further includes a positive electrode sheet, the positive electrode sheet includes a positive active layer, the positive active layer includes a positive active material, and the positive active material includes one or more of lithium phosphate active materials and lithium composite metal oxide active materials.

[0123] Introducing lithium phosphate-containing active materials into the positive electrode active material can improve the structural stability of the positive electrode active material during charge-discharge cycles and extend the cycle life of the battery.

[0124] Introducing lithium-composite metal oxide active materials into the positive electrode active material is beneficial to improving the energy density of the positive electrode and the battery.

[0125] In some embodiments, the positive electrode active material includes a lithium phosphate-based active material, and the positive electrode active material satisfies one or more of the following characteristics:

[0126] (th1) The mass percentage of the lithium phosphate-containing active material in the positive electrode active layer is greater than or equal to 80%, and can be selected as 80% to 97%;

[0127] (th2) The lithium-containing phosphate active materials include one or more of lithium iron phosphate, lithium iron phosphate and carbon composite materials, lithium manganese phosphate, lithium manganese phosphate and carbon composite materials, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composite materials.

[0128] (th3) 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. The carbon coating layer in the lithium phosphate active material includes one or more of soft carbon, hard carbon and amorphous carbon.

[0129] By controlling the mass ratio of lithium phosphate active materials in the positive electrode active layer within the aforementioned range, it is beneficial to extend the cycle life of the battery.

[0130] The types of lithium phosphate active materials can be flexibly selected to meet different application needs.

[0131] By setting one or more carbon coating layers, including soft carbon, hard carbon, and amorphous carbon, on the surface of lithium phosphate active materials, the conductivity of the material can be improved. This is beneficial for improving the electrical contact network within the positive electrode, providing a fast and stable channel for electron transport within the positive electrode, thereby improving the rate performance and fast charging capability of the battery.

[0132] In a second aspect of this application, an electrical device is provided, which includes the lithium-ion secondary battery described in the first aspect of this application.

[0133] In a third aspect of this application, the application of the lithium-ion secondary battery described in the first aspect of this application in supplying and / or storing electrical energy is provided;

[0134] The application includes the process of charging the lithium-ion secondary battery at a rate of 2C or higher.

[0135] In some embodiments, the application includes the process of charging the lithium-ion secondary battery at at least one rate of 2C to 6C or 4C to 6C.

[0136] Optionally, the application includes the process of charging the lithium-ion secondary battery at at least one rate condition from 2C to 4C;

[0137] Optionally, the maximum charging rate of the lithium-ion secondary battery is greater than or equal to 2C, and can be selected as 2C to 6C, or further selected as 2C to 4C or 4C to 6C.

[0138] The lithium-ion secondary battery provided in the first aspect of this application can provide high rate performance and good fast charging capability.

[0139] Details of one or more embodiments or examples of this application are set forth in the following drawings and description. Other features, objects, and advantages of this application will become apparent from the specification, drawings, and claims. Attached Figure Description

[0140] To better describe and illustrate the embodiments, examples, or models provided in this application, reference may be made to one or more accompanying drawings. Additional details or examples used to describe the drawings should not be considered as limiting the scope of any of the disclosed applications, the currently described embodiments, examples, or models, or the best mode of these applications as currently understood. Furthermore, the same reference numerals denote the same parts throughout the drawings. It should also be noted that the drawings are drawn in a simplified form and are only intended to facilitate and clarify the illustration of this application. The various dimensions of each part shown in the drawings are arbitrarily shown and may be precise or not drawn to scale. For example, the dimensions of parts are appropriately exaggerated in some places in the drawings to make the illustration clearer. Unless otherwise specified, the parts in the drawings are not drawn to scale. This application does not limit every dimension of every part.

[0141] In the attached diagram:

[0142] Figure 1 is a schematic diagram of the negative electrode sheet in one embodiment of this application.

[0143] Figure 2 is a schematic diagram of the negative electrode sheet in another embodiment of this application.

[0144] Figure 3 is a schematic diagram of the negative electrode sheet in another embodiment of this application.

[0145] Figure 4 is a schematic diagram of a battery cell according to one embodiment of this application.

[0146] Figure 5 is an exploded view of a battery cell according to an embodiment of this application shown in Figure 4.

[0147] Figure 6 is a schematic diagram of a battery device according to an embodiment of this application.

[0148] Figure 7 is a schematic diagram of a battery pack according to one embodiment of this application.

[0149] Figure 8 is an exploded view of the battery pack of one embodiment of this application shown in Figure 7.

[0150] Figure 9 is a schematic diagram of an electrical device using a lithium-ion secondary battery as a power source according to an embodiment of this application.

[0151] Explanation of reference numerals in the attached figures: 202, negative electrode active particle; 208, quaternary ammonium salt compound; 200, negative electrode sheet; 210, negative electrode current collector; 220, negative electrode active material layer; 222, second negative electrode active layer; 224, first negative electrode active layer; 1, battery pack; 2, upper casing; 3, lower casing; 4, battery assembly; 5, battery cell; 51, casing; 52, electrode assembly; 53, cover plate; 6, electrical device.

[0152] It should be noted that the shape and size of the negative electrode active particles 202, quaternary ammonium salt compounds 208, etc. shown in Figures 1-3 do not represent or are not used to limit the shape and size of the actual substances, and the quantities shown do not represent or are not used to limit the actual quantities and proportions. Detailed Implementation

[0153] The following describes in detail some embodiments and examples of the lithium-ion secondary battery, power supply device, and application of this application with appropriate reference to the accompanying drawings. However, some unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0154] The "range" disclosed in this application can be defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints. Any endpoint can be included or excluded independently and can be combined arbitrarily; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​1 and 2 are listed, and maximum range values ​​3, 4, and 5 are also listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0" and "5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when describing a parameter as an integer ≥ 2, it is equivalent to listing integers such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12 for that parameter. For instance, when describing a parameter as an integer selected from "2-10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0155] In this application, unless otherwise specified, "about" means within a reasonable range above and below the stated number, and the range of fluctuation may vary depending on the type and value of the stated number. For example, a range of ±10%, ±5%, ±2%, ±1%, etc., may be allowed. For example, taking "about 20°C" and its approximation as ±1°C, approximate values ​​such as 19°C, 19.5°C, etc., within the approximation range indicated by "about 20°C" should also be included in the range indicated by "about 20°C".

[0156] In this application, the terms "multiple," "various," "multiple items," "several," etc., unless otherwise specified, refer to a quantity greater than or equal to 2. For example, "one or more" means one or more (greater than or equal to) two. It can be understood that when "any number of" items are involved, it refers to any suitable combination of multiple items, that is, a combination of "any number of" items in a manner that does not conflict and enables the implementation of this application.

[0157] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0158] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments. The term "implementation" as used herein has a similar understanding.

[0159] In this application, open-ended technical features or solutions described using terms such as "containing," "comprising," or "including" do not exclude additional members beyond those listed unless otherwise specified. They can be considered as providing both closed-ended features or solutions comprised of the listed members and open-ended features or solutions that include additional members beyond the listed members. For example, if 'a' includes a1, a2, and a3, it may also include other members or exclude additional members unless otherwise specified. This can be considered as providing both features or solutions where "a consists of a1, a2, and a3" or "a is selected from a1, a2, and a3," and features or solutions where "a includes not only a1, a2, and a3, but also other members."

[0160] In this application, unless otherwise specified, M (e.g., m1) means that m1 is a non-limiting example of M, and it is understood that M is not limited to m1.

[0161] In this application, "optionally," "optionally," and "optional" mean that something is optional, that is, it is selected from either "with" or "without." If multiple "options" appear in a technical solution, unless otherwise specified and there are no contradictions or mutual constraints, each "option" is independent. Unless otherwise specified, the descriptions such as "optionally include" and "optionally contain" in this application, taking "optionally include" as an example, mean "may include or not include."

[0162] In this application, unless otherwise specified, the features or solutions corresponding to "and / or" include any one of two or more of the related listed items, as well as any and all combinations of the related listed items. 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" represents the group consisting of M, N, and "a combination of M and N". "Containing M and / or N" can mean "containing M, containing N, and containing both M and N", or "containing M, containing N, or containing both M and N", and can be appropriately understood according to the context.

[0163] In this document, the word "suitable" in "suitable combination" or "suitable method" refers to the technical solution that can implement this application.

[0164] In this document, terms such as "preferred," "better," "more effective," "gooder," and "superior" are merely descriptions of more effective implementation methods or embodiments and should be understood not to limit the scope of protection of this application. If multiple "preferred" terms appear in a technical solution, unless otherwise specified and there are no contradictions or mutual constraints, each "preferred" term shall be independent.

[0165] In this application, terms such as "further," "even more," "especially," "for example," "as," and "example" are used for descriptive purposes to indicate differences in content, but should not be construed as limiting the scope of protection of this application.

[0166] In this application, the terms "first aspect," "second aspect," "third aspect," "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," "fourth," etc., serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on quantity.

[0167] In this application, unless otherwise expressly specified and limited, the term "connection" and other such terms should be interpreted broadly, for example, as a fixed connection, a detachable connection, or an integral part. Those skilled in the art will understand the appropriate meaning of the above terms in this application based on the context.

[0168] In this application, unless otherwise expressly specified and limited, the phrase "above" or "below" the second feature can mean that the first and second features are in direct contact, or that the first and second features are in indirect contact through an intermediate medium. In this application, unless otherwise expressly specified and limited, the phrase "above" or "below" the second feature can indicate a horizontal positional relationship, or it can simply indicate the existence of an attachment relationship without specifying a horizontal positional relationship.

[0169] In this application, the term "room temperature" generally refers to 4℃ to 35℃, and may refer to 20℃ ± 5℃. In some embodiments or examples of this application, room temperature refers to 20℃ to 30℃.

[0170] In this application, when a unit is specified for a data range, if it is only followed by the right endpoint, it indicates that the units for the left and right endpoints are the same. For example, 3~5μm or 3-5μm both mean that the units for the left endpoint "3" and the right endpoint "5" are both μm (micrometers), and have the same meaning as 3μm~5μm. Furthermore, similar descriptions of other parameters such as temperature and size are interpreted in the same way.

[0171] 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 represented as ">", and "less than" can be equivalently represented as "<". In this application, unless otherwise specified, "greater than or equal to" and "≥" can be considered as providing two additional solutions: "greater than" and "equal to". In this application, unless otherwise specified, "less than or equal to" and "≤" can be considered as providing two additional solutions: "less than" and "equal to".

[0172] In this application, the exemplary descriptions such as "in some implementations (or embodiments)" and "in one implementation (or embodiment)" may cover, but are not limited to, the following meanings: these solutions can be combined with other solutions in a suitable manner to form new technical solutions.

[0173] In this application, when two or more test methods are provided for a certain parameter, all test results of at least one test method that are within the described range are included in the protection scope of this application.

[0174] Unless otherwise stated, the improvements described in this application are not intended to be limited to any theoretical constraints.

[0175] According to various embodiments and examples of this application, this application provides a lithium-ion secondary battery, a power-consuming device, and an application. This lithium-ion secondary battery exhibits significantly improved fast-charging performance.

[0176] In this application, unless otherwise specified, the term "lithium-ion secondary battery" refers to a secondary battery in which the active ions include lithium ions, and "lithium-ion battery cell" refers to a battery cell in which the active ions include lithium ions. Typically, a lithium-ion secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0177] In this application, unless otherwise specified, "electrode active material layer" includes at least one of the positive active material layer of the positive electrode sheet and the negative active material layer of the negative electrode sheet. Depending on the specific circumstances, the electrode active material layer may refer to either the positive active material layer or the negative active material layer. In this application, "positive active material layer" may also be referred to as "positive active layer" and "negative active material layer" may also be referred to as "negative active layer".

[0178] In this application, the term "negative electrode sheet" includes a negative electrode active material layer, which includes a negative electrode active material. The term "negative electrode active material" refers to a material used in a negative electrode sheet that is capable of reversibly inserting and de-inserting active ions.

[0179] In this application, unless otherwise specified, "negative electrode sheet" includes a negative electrode current collector. A "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 may be located on one or both sides of the negative electrode current collector.

[0180] In this application, the term "positive electrode sheet" includes a positive electrode active material layer, which includes a positive electrode active material. The term "positive electrode active material" refers to a material used in a positive electrode sheet that is capable of reversibly extracting and inserting active ions.

[0181] In this application, unless otherwise specified, "positive electrode sheet" includes a positive current collector. A "positive current collector" refers to a structure responsible for collecting and conducting electrons at the positive electrode. In the positive electrode sheet, the positive active layer is located on at least one side of the positive current collector, and may be located on one or both sides of the positive current collector.

[0182] In this application, unless otherwise specified, "separation membrane" and "diaphragm" have the same meaning and can be used interchangeably.

[0183] In a first aspect of this application, a lithium-ion secondary battery is provided, which includes a negative electrode sheet, the negative electrode sheet including a negative electrode active material layer, wherein a quaternary ammonium salt compound is disposed in the negative electrode active material layer.

[0184] In some embodiments, a lithium-ion secondary battery is provided, comprising a negative electrode and an electrolyte, wherein the negative electrode includes a first negative active layer and a second negative active layer, the second negative active layer being located on the side of the first negative active layer away from the surface of the negative electrode. That is, the second negative active layer is further away from the surface of the negative electrode relative to the first negative active layer.

[0185] In some embodiments, a lithium-ion secondary battery is provided, which includes a negative electrode sheet and an electrolyte. The negative electrode sheet includes a negative electrode active material layer, which includes a first negative electrode active layer and a second negative electrode active layer. The second negative electrode active layer is located on the side of the first negative electrode active layer away from the surface of the negative electrode sheet.

[0186] In some embodiments, a lithium-ion secondary battery is provided, comprising a negative electrode and an electrolyte; the negative electrode includes a negative current collector and a second negative active layer and a first negative active layer sequentially disposed on at least one side of the negative current collector, the second negative active layer being located between the negative current collector and the first negative active layer; the second negative active layer includes a second negative active material and a quaternary ammonium salt compound, the quaternary ammonium salt compound including quaternary ammonium cations. The second negative active material may include a carbon-based material.

[0187] In this application, in the thickness direction of the negative electrode sheet, for the negative electrode active material layer, the direction closer to the surface of the negative electrode sheet can be denoted as "upper" and the direction farther away from the surface of the negative electrode sheet can be denoted as "lower". Taking a negative electrode sheet including a negative electrode current collector as an example, the direction farther away from the surface of the negative electrode current collector is called upper, and the direction toward the surface of the negative electrode current collector is called lower.

[0188] In this application, unless otherwise specified, "a second negative electrode active layer and a first negative electrode active layer are sequentially disposed on at least one side of the negative electrode current collector" means that the second negative electrode active layer is located between the negative electrode current collector and the first negative electrode active layer.

[0189] In this application, unless otherwise specified, a "quaternary ammonium salt compound" is a compound comprising a quaternary ammonium ion, including at least a quaternary ammonium cation, and having the ability to bind electrolyte anions in an electrolyte solution through ion exchange. Unless otherwise specified, the nitrogen atom of the quaternary ammonium cation in the quaternary ammonium salt compound is covalently bonded to a carbon atom; that is, the quaternary ammonium salt compound comprises a quaternary ammonium cation and a carbon atom covalently bonded to the quaternary ammonium cation. Non-limitingly, the quaternary ammonium salt compound may optionally include an antistatic agent comprising a quaternary ammonium ion. Non-limitingly, the quaternary ammonium salt compound is a small molecule compound; unless otherwise specified, the molecular weight of the quaternary ammonium salt compound is less than or equal to 600 Da. Unless otherwise specified, the "molecular weight" of the quaternary ammonium salt compound refers to the molecular mass measured in Daltons (Da), where 1 Dalton equals... 12 One-twelfth of the atomic mass of carbon. Non-limitingly, the molecular weight of the quaternary ammonium salt compound is 150 Da to 600 Da, optionally 160 Da to 500 Da, further optionally 200 Da to 500 Da, and may also be any of the following molecular weights or a range selected from any two of the following molecular weights: 180 Da, 190 Da, 200 Da, 220 Da, 250 Da, 260 Da, 280 Da, 300 Da, 350 Da, 400 Da, 450 Da, 500 Da, 600 Da, etc.

[0190] In some embodiments, the quaternary ammonium salt compound has low solubility in the electrolyte or is insoluble in the electrolyte, which makes the adsorption of the quaternary ammonium salt compound on the surface of the negative electrode active material more stable.

[0191] In this application, unless otherwise specified, "electrolyte anion" refers to the anion carried by the electrolyte lithium salt in the electrolyte.

[0192] In this lithium-ion secondary battery, a first negative electrode active layer and a second negative electrode active layer (with the direction away from the surface of the negative electrode current collector being considered as the upper layer and the direction towards the surface of the negative electrode current collector being considered as the lower layer) can be disposed in the negative electrode active material layer of the negative electrode sheet. Furthermore, a quaternary ammonium salt compound is disposed in the second negative electrode active layer. The quaternary ammonium salt compound includes hydrophilic, positively charged quaternary ammonium ions. For the negative electrode sheet immersed in the electrolyte, the electrostatic effect based on the quaternary ammonium ions can be used to attract electrolyte anions in the electrolyte, promote the rapid dissociation of electrolyte lithium salt in the electrolyte, significantly improve the electrolyte wettability of the second negative electrode active layer and the kinetics of the negative electrode sheet, and thus significantly improve the fast charging performance of the battery.

[0193] In this application, the cross-section of the negative electrode sheet can be observed for microscopic morphology to examine the microscopic morphology of each negative electrode active layer in the negative electrode active material layer and the boundaries between different negative electrode active layers, thereby determining the thickness of different negative electrode active layers. "Cross-section of the negative electrode sheet" refers to a section perpendicular to the thickness of the negative electrode sheet. Furthermore, the cross-section of the negative electrode sheet can be observed for microscopic morphology and combined with compositional analysis (such as energy dispersive spectroscopy (EDS)) to identify elemental types, thus confirming the composition of different negative electrode active layers in the negative electrode active material layer. Non-limitingly, the cross-section of the negative electrode sheet can be obtained using instruments or equipment including but not limited to focused electron beam (FIB) microscopes (non-limiting examples such as the FEI Scios 2HiVac device), ion cross-section polishers (non-limiting examples such as the IB-09010CP argon ion cross-section polisher and IB-19500CP ion cross-section polisher from JEOL Corporation of Japan), or by plasma quenching. Microscopic morphology observation methods can employ instruments or equipment including but not limited to scanning electron microscopy (SEM) technology. In particular, high-resolution field emission scanning electron microscopes can be used. Examples of non-limiting SEM instruments include the Sigma 300 scanning electron microscope and the Apreo 2SEM field emission scanning electron microscope from ZEISS GmbH, Germany.

[0194] 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 using 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, and proton nuclear magnetic resonance (NMR) spectroscopy. 1 Methods include 1H NMR, gel permeation chromatography (GPC), high performance liquid chromatography (HPLC), mass spectrometry, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, single crystal X-ray diffraction (SCXRD), inductively coupled plasma optical emission spectrometry (ICP), and energy dispersive spectroscopy (EDS). The sample preparation and testing methods for these 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 characteristics of the sample.

[0195] Unless otherwise specified in this application, the following method can be used to extract samples from the negative electrode active material layer of the negative electrode sheet to further detect whether it contains quaternary ammonium salt compounds and the types of quaternary ammonium salt compounds. The detailed steps are as follows: disassemble the battery cell, scrape a certain amount of powder (e.g., a certain mass M0) from the second negative electrode active layer of the negative electrode sheet, use acetone as the extractant, and continuously extract using a Soxhlet extractor to obtain the extract (mass denoted as M1), which is used as the sample to be tested. Non-limitingly, a sample thickness of 10 μm can be scraped. Non-limitingly, an Agilent liquid chromatography-mass spectrometry instrument can be used to detect the components of the quaternary ammonium salt compounds.

[0196] Non-limitingly, the content of identified quaternary ammonium salt compounds can be determined by the following method: A Waters ACQUITY ARC high-performance liquid chromatograph with a PDA detector can be used, referring to the method in GB / T 32268-2015, to confirm the content of quaternary ammonium salt compounds by comparing the retention time and peak area in the high-performance liquid chromatogram. The content of quaternary ammonium salt compounds in the second negative electrode active layer can be further calculated by combining the mass of the scraped powder M0, the mass of the extract M1, and the amount of test sample M2, such as the mass percentage of quaternary ammonium salt compounds in the second negative electrode active layer.

[0197] In this application, unless otherwise specified, the constituent materials of the negative electrode active material layer are referred to as negative electrode material, the constituent materials of the first negative electrode active layer are referred to as first negative electrode material, and the constituent materials of the second negative electrode active layer are referred to as second negative electrode material. 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 terms "first" and "second" in "first negative electrode material," "second negative electrode material," "first negative electrode active material," and "second negative electrode active material" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features.

[0198] In this application, the content of quaternary ammonium salt compounds in the second negative electrode active layer can be detected and analyzed using the following method. A similar method can also be used to detect and analyze the content of quaternary ammonium salt compounds in the entire negative electrode active material layer of the negative electrode sheet:

[0199] (S1) Powder scraping: Disassemble the battery cell, take the negative electrode sheet, and use a micrometer to measure the thickness L of the electrode sheet; take another negative electrode sheet, wipe off the negative active material layer on both sides of the remaining empty current collector foil, and measure the thickness L0. The total thickness of the negative active material layer on both sides of the negative current collector is L-L0; scrape off the negative active material layer on one surface of the negative electrode sheet until the bare negative current collector surface is exposed, and scrape off the negative material on the surface of the other negative electrode sheet until the electrode thickness is L1 = L0 + Δd. Collect the powder of the Δd thickness portion of the negative active material layer below the L1 thickness for testing the quaternary ammonium salt compound content. The thickness of the second negative active layer can be determined in advance based on the observation results of the electrode cross-sectional morphology, thereby controlling that when the thickness is L1, the first negative active layer has been completely removed, and the remaining negative active material layer corresponds to part or all of the second negative active layer. For example, Δd can be 20 μm.

[0200] (S2) Separation: Weigh the powder (corresponding to the second negative electrode material) collected in the Δd thickness part in step (S1) by mass (denoted as M0), mix the powder with organic solvent (the mass of organic solvent is denoted as M1) at a ratio of 1g powder to 100g organic solvent, perform ultrasonication, vortexing, centrifugation, and filter membrane filtration, and the mass of the filtrate obtained is M2.

[0201] Those skilled in the art can choose a suitable organic solvent to collect the sample of the quaternary ammonium salt compound. Taking Example 1 as an example, the ammonium salt compound is dodecyltrimethyl quaternary ammonium phosphate, which is soluble in aqueous solutions of alcohol reagents such as methanol and ethanol. In step (S2), the powder can be ultrasonicated, vortexed, centrifuged, and filtered through a 0.22 μm organic filter membrane using an aqueous methanol solution.

[0202] (S3) Detection: High-performance liquid chromatography (HPLC) using an Acquit H class UPLC-mass spectrometry system coupled with a Waters Xevo G2-XS Qtof instrument was employed. HPLC detection followed GB / T 16631-2008, and mass spectrometry detection followed GB / T-6041-2002. A reversed-phase C18 column was used for separation. The mobile phase was selected from solvents capable of dissolving quaternary ammonium compounds (for example, in Example 1, 0.5% (v / v) methanol-water solution was used as the mobile phase for dodecyltrimethylammonium phosphate).

[0203] Using selected quaternary ammonium salts at different concentrations as standard samples, a standard curve of detection signal versus quaternary ammonium salt concentration was constructed. The selected quaternary ammonium salt can be dodecyltrimethylammonium phosphate. The concentration gradient can be selected as 0.02, 0.05, 0.08, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, or 2.0 times (M2-M1) / M1.

[0204] The filtrate in step (S2) is tested, and the retention time and peak area are compared with those of the standard sample. The concentration of the quaternary ammonium salt compound separated in step (S2) is calculated as C. The mass of the quaternary ammonium salt compound is calculated according to the formula M3 = M2 × C. Then, the mass percentage of the quaternary ammonium salt compound in the second negative electrode active layer can be calculated as M3 / M0 × 100%.

[0205] It should be noted that in step (S2), the conductive agent and other components in the second negative electrode active layer are insoluble in the organic solvent. Some binder components (such as styrene-butadiene rubber) may be soluble in the organic solvent; however, the molecular weight of the binder components is usually high, for example, above 10 kDa (1 kDa = 1000 Da), and their polarity is usually weaker than that of quaternary ammonium salt compounds. In step (S3), when performing high-performance liquid chromatography (HPLC) detection, a chromatographic column (such as a C18 column) can be used to separate the binder components from the quaternary ammonium salt compounds.

[0206] In some embodiments, the electrolyte includes an electrolyte lithium salt and a non-aqueous solvent.

[0207] In some embodiments, the second negative electrode active material includes a carbon-based material. The carbon atoms in the quaternary ammonium salt compound can enable the quaternary ammonium salt compound to be better adsorbed on the surface of the carbon-based material.

[0208] In some embodiments, the surface of the particles of the second negative electrode active material is provided with carbonaceous material. The carbonaceous material may include one or more of artificial graphite, natural graphite, soft carbon, and hard carbon.

[0209] In some embodiments, the quaternary ammonium salt compound includes a quaternary ammonium cation, a hydrocarbon chain covalently bonded to the quaternary ammonium cation, and an anion.

[0210] In this application, unless otherwise specified, "hydrocarbon chain" refers to a chain-like structure composed of carbon and hydrogen atoms; the hydrocarbon chain in quaternary ammonium salt compounds can provide a carbon-carbon skeleton, and the hydrocarbon chain is weakly polar or nonpolar. In this application, unless otherwise specified, "carbon-carbon skeleton" and "carbon skeleton" have the same meaning and can be used interchangeably; unless otherwise specified, "carbon-carbon skeleton" refers to a skeleton composed of carbon atoms.

[0211] By incorporating hydrocarbon chains into quaternary ammonium salt compounds, such as alkyl chains, the chain structure of the hydrocarbon chains facilitates better and more stable loading and encapsulation of the quaternary ammonium salt compounds on the surface of the second negative electrode active material. The carbon-carbon skeleton provided by the hydrocarbon chains facilitates better and more stable adsorption of the quaternary ammonium salt compounds on the surface of carbon-based materials (which have a certain degree of oleophilicity). Furthermore, combined with the electrostatic attraction of the quaternary ammonium ions in the quaternary ammonium salt compounds, it is beneficial to guide lithium ions in the electrolyte to be guided to the surface of the second negative electrode active material more quickly, which is beneficial to further improve the kinetics of the negative electrode sheet and the fast charging performance of the battery.

[0212] In some embodiments, the hydrocarbon chain in the quaternary ammonium salt compound includes an alkyl chain. In some of these embodiments, the hydrocarbon chain in the quaternary ammonium salt compound is an alkyl chain.

[0213] In this application, unless otherwise specified, "alkyl chain" refers to a saturated chain structure composed of carbon atoms and hydrogen atoms, and the alkyl chain in quaternary ammonium salt compounds can provide a saturated carbon-carbon skeleton.

[0214] In this application, the term "hydrocarbon group" refers to a monovalent residue formed by the loss of a hydrogen atom from a hydrocarbon compound comprising a primary (normal) carbon atom, a secondary carbon atom, a tertiary carbon atom, a quaternary carbon atom, or a combination thereof. Phrases containing this term, such as "C", are also included. 12-18 "Hydrocarbon group" refers to a hydrocarbon group containing 12 to 18 carbon atoms, which can be independently C12 and C23 each time they appear. 12 hydrocarbon group, C 13 hydrocarbon group, C 14 hydrocarbon group, C 15 hydrocarbon group, C 16 hydrocarbon group, C 17 hydrocarbon group or C 18 Hydrocarbon group.

[0215] In this application, the term "alkyl" refers to a monovalent residue formed by the loss of a hydrogen atom from a saturated hydrocarbon comprising a primary (normal) carbon atom, or a secondary carbon atom, or a tertiary carbon atom, or a quaternary carbon atom, or a combination thereof. Phrases containing this term, such as "C", are also included. 12-18 "Alkyl" refers to an alkyl group containing 12 to 18 carbon atoms, and each time it appears, it can independently be C16. 12 Alkyl, C 13 Alkyl, C 14 Alkyl, C 15 Alkyl, C 16 Alkyl, C 17 Alkyl or C 18 alkyl.

[0216] In this application, unless otherwise specified, the number of carbon atoms in a compound or group may be described using the subscript "C". For example, "C12-18 "C" indicates that it has 12 to 18 carbon atoms. 12-18 "Each time it appears, it can be independently classified as C." 12 C 13 C 14 C 15 C 16 C 17 Or C 18 .

[0217] In some implementations, the hydrocarbon chain is a linear chain.

[0218] In some embodiments, the alkyl chain is a linear chain.

[0219] Unless otherwise specified, a "linear chain" refers to a chain-like structure formed by the sequential connection of main chain atoms. In this case, there are no ring structures and no branches.

[0220] In some embodiments, the quaternary ammonium salt compound satisfies one or more of the following characteristics (any numerical parameter of the following characteristics may also be selected from any suitable value or range in the context):

[0221] (ta1) The hydrocarbon chain is an alkyl chain;

[0222] (ta2) The number of carbon atoms in the hydrocarbon chain is 12 to 18 (selected as 12, 13, 14, 15, 16, 17, 18 or selected from any two of the aforementioned integers);

[0223] (ta3) Quaternary ammonium salt compounds have a molecular weight of less than or equal to 600 Da;

[0224] (ta4) The hydrocarbon chain is a linear chain;

[0225] The hydrocarbon chain length in (ta5) ammonium salt compounds is 12 to 18 main chain carbon atoms.

[0226] In some embodiments, the hydrocarbon chain in the quaternary ammonium salt compound is C. 12-18 Alkyl chain. C 12-18 The number of carbon atoms in the alkyl chain can be 12 to 18, or it can be 12, 13, 14, 15, 16, 17, 18 or any two of the aforementioned integers.

[0227] In some embodiments, the hydrocarbon chain length in the ammonium salt compound is 12 to 18 main chain carbon atoms. Optionally, the hydrocarbon chain length may also be a range of 12, 13, 14, 15, 16, 17, 18, or any two of the aforementioned integers, in terms of the number of main chain carbon atoms. Further optionally, the hydrocarbon chain may be an alkyl chain.

[0228] By controlling the number of carbon atoms in the hydrocarbon chain of the quaternary ammonium salt compound and / or the molecular weight of the quaternary ammonium salt compound within the aforementioned range, the length of the hydrocarbon chain can be adjusted to a more suitable range. On the one hand, this facilitates better and more stable encapsulation of the quaternary ammonium salt compound on the surface of the second negative electrode active material during slurry stirring, and helps to suppress the detachment of the quaternary ammonium salt compound from the surface of the second negative electrode active material during charging and discharging. Furthermore, by combining with the quaternary ammonium ions exposed to the electrolyte, it can promote the faster and more stable guidance of lithium ions in the electrolyte to the surface of the second negative electrode active material, thereby increasing the transport rate of lithium ions inside the negative electrode sheet. On the other hand, it can also better control the degree of encapsulation of the hydrocarbon chain on the surface of the second negative electrode active material, so that the active sites on the surface of the second negative electrode active material can better contact the electrolyte. This is beneficial to better improve the dynamics of the negative electrode sheet and the fast charging performance of the battery.

[0229] In some embodiments, the quaternary ammonium salt compound satisfies one or more of the following characteristics:

[0230] (tb1) The structure of the quaternary ammonium cation is -N + (R1R2R3), where R1 and R2 are each independently C 1-3 Alkyl group, R3 is C 1-3 Alkyl or hydroxyethyl; optionally, R1 and R2 are each independently methyl, and R3 is methyl or hydroxyethyl;

[0231] (tb2) Quaternary ammonium salt compounds include anions, which include one or more of nitrate, carbonate, bicarbonate and phosphate.

[0232] In this application, "C" is involved. 1-3 "alkyl" can be methyl, ethyl, or propyl.

[0233] In some embodiments, the anions in the quaternary ammonium salt compounds include one or more of nitrate, carbonate, bicarbonate, and phosphate.

[0234] According to Pauling's electronegativity scaling theory, the Pauling electronegativity scales for nitrate, carbonate, bicarbonate, and phosphate are generally considered to be as follows: nitrate 3.0 > carbonate 2.5 > bicarbonate 2.5 > phosphate 2.0. Based on the same theory, the Pauling electronegativity scales for some common electrolyte anions are generally considered to have the following values: tetrafluoroborate 4.0 ≈ hexafluoroarsenate 4.0 ≈ hexafluorophosphate 4.0 ≈ bis(fluorosulfonyl)imide 4.0 ≈ bis(trifluoromethanesulfonyl)imide 4.0 ≈ trifluoromethanesulfonate 4.0 ≈ lithium difluorophosphate 4.0 ≈ difluorooxalateborate 4.0 ≈ lithium tetrafluorooxalateborate 4.0 ≈ lithium difluorodioxalateborate 4.0 > perchlorate 3.5 ≈ lithium dioxalateborate 3.5. The Pauling electronegativity scale for anions can be determined based on the Pauling electronegativity scale value of the most electronegative element among the anions. Pauling electronegativity scales for different elements can be found in handbooks or existing literature.

[0235] In some embodiments, the quaternary ammonium salt compound includes one or more of the following: octadecyl dimethyl hydroxyethyl quaternary ammonium nitrate, N,N-dimethyl-N-(2-hydroxyethyl)hexadecyl quaternary ammonium phosphate, dodecyl trimethyl quaternary ammonium nitrate, octadecyl trimethyl quaternary ammonium phosphate, dodecyl trimethyl quaternary ammonium phosphate, dodecyl trimethyl quaternary ammonium carbonate, and dodecyl trimethyl quaternary ammonium bicarbonate.

[0236] By introducing one or more of nitrate, carbonate, bicarbonate, and phosphate into the anions of quaternary ammonium salt compounds, it is beneficial to better control the binding ability between anions and quaternary ammonium ions in the quaternary ammonium salt compounds, making it easier for quaternary ammonium ions to dissociate. These anions can have lower electronegativity than electrolyte anions, which is more conducive to promoting the formation of quaternary ammonium ion-electrolyte anion structure. This is more conducive to the role of the quaternary ammonium ion-electrolyte anion structure in guiding the aggregation of lithium ions in the electrolyte to the second negative electrode active material, and is beneficial to improving the kinetics of the negative electrode sheet and the fast charging performance of the battery.

[0237] In some embodiments, the quaternary ammonium salt compound may account for 0.2% to 2% of the mass of the second negative electrode active layer, and may be selected as 0.2% to 1.5%.

[0238] In some embodiments, the quaternary ammonium salt compound accounts for 0.2% to 1.5% of the mass of the second negative electrode active layer.

[0239] Non-limitingly, the mass percentage of the quaternary ammonium salt compound in the second negative electrode active layer can also be any of the following percentages or a range selected from any two of the following percentages: 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2%, etc.

[0240] By controlling the mass proportion of quaternary ammonium salt compounds in the second negative electrode active layer within the aforementioned range, it is beneficial to better leverage the role of quaternary ammonium salt compounds in improving the battery's fast-charging performance. It also helps to better control the decrease in electronic conductivity on the surface of the second negative electrode active material caused by the encapsulation of quaternary ammonium salt compounds, thus further improving the battery's fast-charging performance. Furthermore, it also helps to maintain a high lithium storage capacity in the second negative electrode active layer, which is beneficial for balancing the energy density of both the negative electrode and the battery.

[0241] In some embodiments, the mass percentage of carbon-based material in the second negative electrode 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%, 97%, 98%, 99%, 100%, etc.

[0242] In some embodiments, the second negative electrode active material includes a graphite material. The graphite material may include one or more of synthetic graphite and natural graphite.

[0243] In some embodiments, the graphite-based material in the second negative electrode active material can be 80% to 100% by mass, 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%, 97%, 98%, 99%, 100%, etc.

[0244] In some embodiments, the surface of the particles of the second negative electrode active material is provided with graphite. The graphite may include one or more of artificial graphite and natural graphite.

[0245] In some implementations, the negative electrode sheet satisfies one or more of the following characteristics (any numerical parameter of the following characteristics may also be selected from any suitable value or range in the context):

[0246] (tc1) Carbon-based materials include one or more of artificial graphite, natural graphite, soft carbon, and hard carbon;

[0247] (tc2) The mass percentage of carbon-based materials in the second negative electrode active layer is 94.5% to 97.5%, and can be selected as 95.0% to 97.0%.

[0248] In some embodiments, the carbon-based material includes one or more of artificial graphite, natural graphite, graphite, soft carbon, and hard carbon.

[0249] In some embodiments, the carbon-based material includes graphite, and further, the graphite may include one or more of artificial graphite and natural graphite.

[0250] Natural graphite is prone to lateral deformation when subjected to cold pressing. Introducing natural graphite into the second negative electrode active material can help improve the compaction density and areal capacity of the second negative electrode active layer, thereby improving the energy density of the negative electrode sheet.

[0251] Compared to natural graphite, artificial graphite has a more stable structure and fewer internal defects, which results in a slower decay of available storage sites for lithium ions during cycling and more stable cycling performance.

[0252] Both hard carbon and soft carbon exhibit high levels of disorder, which facilitates lithium-ion entry. Hard carbon provides abundant lithium intercalation sites and rapid transport channels; the disordered stacking of carbon layers in soft carbon allows for relatively rapid lithium-ion transport. The introduction of at least one of hard carbon and soft carbon is beneficial for improving the negative electrode and battery kinetics.

[0253] In some embodiments, the carbon-based material in the second negative electrode active layer accounts for 94.5% to 97.5% by mass, optionally 95.0% to 97.0%, and may also be any of the following percentages or a range selected from any two of the following percentages: 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, etc.

[0254] By controlling the mass ratio of carbon-based materials in the second negative electrode active layer within the aforementioned range, it is beneficial to better encapsulate quaternary ammonium salt compounds on the surface of the second negative electrode active material. In addition, the energy density of both the negative electrode and the battery can be taken into account.

[0255] In some embodiments, the second negative electrode active layer includes an adhesive, which includes styrene-butadiene rubber (SBR).

[0256] By introducing styrene-butadiene rubber (SBR) into the binder of the second negative electrode active layer, SBR can have a non-chain structure. In terms of occupying the encapsulation sites on the surface of the second negative electrode active material, SBR is less likely to compete with quaternary ammonium salt compounds. This is conducive to better simultaneous encapsulation of the binder and quaternary ammonium salt compounds on the surface of the second negative electrode active material. This can not only achieve a good electrical contact network, but also give full play to the role of quaternary ammonium salt compounds in guiding lithium ions in the electrolyte, which is beneficial to improving the fast charging performance of the battery.

[0257] In some embodiments, the second negative electrode active layer comprises a carbon-based material and a binder, the binder being styrene-butadiene rubber.

[0258] By introducing carbon-based materials into the second negative electrode active material of the second negative electrode active layer and introducing styrene-butadiene rubber into the binder, styrene-butadiene rubber, with its suitable oleophilicity, can better synergize the binding effect between the binder, carbon-based materials, and quaternary ammonium salt compounds. Compared with the strong binding effect of oily binders (such as polyvinylidene fluoride (PVDF)) on quaternary ammonium salt compounds, styrene-butadiene rubber is beneficial to better exert the binding effect between quaternary ammonium salt compounds and carbon-based materials, and better exert the role of quaternary ammonium salt compounds in improving the fast charging performance of the battery.

[0259] In some embodiments, the glass transition temperature of styrene-butadiene rubber is 5°C to 70°C, optionally 30°C to 50°C, and may further be any of the following temperatures or selected from any two of the following temperature ranges: 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, etc.

[0260] In some embodiments, the glass transition temperature of styrene-butadiene rubber is 30°C to 50°C.

[0261] In this application, "glass transition temperature (Tg)" has a well-known meaning in the art, referring to the temperature at which a polymer transitions from a glassy state to a rubbery state, or the temperature range within which it transitions from a rubbery state to a glassy state. Generally, the higher the glass transition temperature, the larger the molecular weight. Polymers can undergo molecular chain motion above their Tg. The glass transition temperature can be determined using conventional methods in the art, including but not limited to differential scanning calorimetry (DSC).

[0262] By controlling the glass transition temperature of styrene-butadiene rubber within the aforementioned range, it is beneficial to enable the second negative electrode active layer to have a better pore structure after rolling, which is beneficial to promote better wetting of the second negative electrode active layer by the electrolyte.

[0263] It is understandable that the first negative electrode active layer includes the first negative electrode active material.

[0264] In some embodiments, the first negative electrode active layer includes a first 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 coating layer includes one or more of soft carbon, hard carbon, and amorphous carbon. Non-limitingly, the mass percentage of the coating layer in the first negative electrode active material can be 0.1% to 4.0%, optionally 1.0% to 2.0%. Non-limitingly, the sum of the mass percentages of soft carbon, hard carbon, and amorphous carbon in the coating layer in the first negative electrode active material can be 0.1% to 3.0%, optionally 1.0% to 2.0%.

[0265] "Soft carbon" and "hard carbon" have well-known meanings in the art. Soft carbon can be graphitized through further high-temperature treatment, while hard carbon is difficult to graphitize even with further high-temperature treatment. The disordered stacking of carbon layers in soft carbon allows for relatively rapid lithium-ion transport. Hard carbon has a disordered internal crystal arrangement and numerous pores, providing abundant lithium intercalation sites and rapid transport channels.

[0266] In this application, unless otherwise specified, "amorphous carbon" refers to transitional carbon materials with a very low degree of graphitization and crystallization, exhibiting an approximately amorphous morphology (or lacking a fixed shape and periodic structural regularity). In this application, "amorphous carbon" may also refer to the product of carbonization treatment of an organic carbon source.

[0267] By setting a coating layer on the surface of the first negative electrode active material in the first negative electrode active layer, and setting one or more of soft carbon, hard carbon and amorphous carbon in the coating layer, the lithium-ion transport channels on the surface of the first negative electrode active material can be optimized, lithium-ion transport can be promoted, and battery kinetics and battery fast charging performance can be further improved.

[0268] In some implementations, the lithium-ion secondary battery satisfies one or more of the following characteristics (any numerical parameter of the following characteristics may also be selected from any suitable value or range in the context):

[0269] (td1) The first negative electrode active material includes coated graphite, which includes a graphite body and a coating layer located on at least a portion of the surface of the graphite body. The coating layer in the coated graphite includes one or more of soft carbon, hard carbon and amorphous carbon.

[0270] (td2) The first negative electrode active material includes secondary particulate graphite, which includes a secondary particulate graphite body. The proportion of secondary particulate graphite in the first negative electrode active material is greater than or equal to 20%, and can be selected as 30% to 80%. Optionally, the secondary particulate graphite includes carbon-coated secondary particulate graphite. The carbon-coated secondary particulate graphite includes a secondary particulate graphite body and a carbon coating layer (which can be referred to as the first carbon coating layer) located on at least a portion of the surface of the secondary particulate graphite body. The carbon coating layer (i.e., the first carbon coating layer) in the carbon-coated secondary particulate graphite includes one or more of soft carbon, hard carbon, and amorphous carbon.

[0271] (td3) The first negative electrode active material includes graphite material, and the OI value of the graphite material can be 2 to 15, or can be 2 to 10;

[0272] (td4) D of the first negative electrode active material v 50 is 10μm to 18μm, and can be selected as 12μm to 16μm;

[0273] (td5) The porosity of the first negative electrode active layer is higher than that of the second negative electrode active layer;

[0274] (td6) The compacted powder density of the first negative electrode active material or the compacted powder density of the first negative electrode active layer is 1.60 g / cm³. 3 ~1.80g / cm 3 ;

[0275] (td7) The rate of charge of the first negative electrode active layer is higher than that of the second negative electrode active layer; optionally, the charging rate of the first negative electrode active layer is higher than that of the second negative electrode active layer.

[0276] (td8) The electrolyte has an ionic conductivity of 13 mS / cm to 18 mS / cm at at least one temperature condition between 20°C and 35°C; optionally, the electrolyte has an ionic conductivity of 13 mS / cm to 18 mS / cm at 25°C.

[0277] The fast-charging performance of a lithium-ion secondary battery can be improved by making it satisfy one or more of the following characteristics: (td1), (td2), (td3), (td4), (td5), (td6), (td7), and (td8).

[0278] In some embodiments, the first negative electrode active material includes coated graphite, which includes a graphite body and a coating layer located on at least a portion of the surface of the graphite body. The coating layer may include one or more of soft carbon, hard carbon, and amorphous carbon.

[0279] In this application, the “graphite body” is composed of graphite.

[0280] By introducing coated graphite into the first negative electrode active layer, and setting one or more of soft carbon, hard carbon and amorphous carbon in the coating layer, the lithium-ion transport channels on the surface of the first negative electrode active material can be optimized, promoting lithium-ion transport and further improving battery kinetics and fast-charging performance.

[0281] Test samples of coated graphite can be tested using the following method: The negative electrode sheet obtained from the disassembled battery cell is soaked and cleaned with a solvent (such as dimethyl carbonate (DMC)) to remove residual electrolyte; powder is scraped from different thickness positions of the negative electrode sheet to obtain powder samples of the first and second negative electrode materials, respectively; the powder is cross-sectionally cut using FIB (Focused Ion Beam) and the cross-sectional morphology of the particles is observed under TEM (Transmission Electron Microscopy). A clear boundary can be observed at the coating interface. Based on the TEM image, the thickness and average thickness of the coating layer can be analyzed and calculated; further, by combining one or more methods such as energy dispersive spectroscopy (EDS), Raman spectroscopy, and X-ray diffraction (XRD), the material types of the coating layer and the graphite body can be confirmed.

[0282] The elemental composition and composition of the coating layer in coated graphite can be analyzed using or by referring to the methods described above.

[0283] Using the distinction between natural graphite and artificial graphite as a non-limiting example, natural graphite and artificial graphite can be distinguished by X-ray diffraction (XRD) analysis. In the XRD pattern, if the characteristic peak near 2θ26.5° is very sharp and has high intensity, it is natural graphite; if the characteristic peak near 2θ26.5° is relatively broad and has weak intensity, it is artificial graphite.

[0284] Taking the distinction between graphite and amorphous carbon as an example, Raman spectroscopy can be used for testing and analysis. The analysis can be based on the characteristic peak information of carbon components in the spectrum (such as the intensity ratio of the D peak to the G peak, I...). D / G Analysis of amorphous carbon was performed. Both the D and G peaks are Raman characteristic peaks of carbon atom crystals. The D peak represents defects in the carbon atom crystal; the more defects, the greater the intensity of the D peak. The intensity of the D peak reflects the content of amorphous (e.g., disordered layer stacking) regions. The G peak represents the in-plane stretching vibrations of sp2 hybridization of carbon atoms; the intensity of the G peak reflects the content of graphitized (layered structure) regions. As the degree of disorder of carbon atoms increases, the intensity ratio of the D peak to the G peak also increases. Raman spectra can also be compared. D / G Standard Raman spectrum of graphite I D / G The difference between the two peaks can be used to determine whether the material being tested contains amorphous carbon. Similarly, the difference in intensity between the D and G peaks in Raman spectra can be used to distinguish between natural and artificial graphite.

[0285] In some embodiments, the first negative electrode active material includes secondary particulate graphite. Non-limitingly, the amount of secondary particulate graphite in the first negative electrode active material can be greater than or equal to 20%, optionally 20% to 80%, further optionally 30% to 60%, and can also be any of the following percentages or a range selected from any two of the following percentages: 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, etc.

[0286] In this application, "secondary particulate graphite" is a graphite-based material that includes a secondary particulate graphite body, and optionally includes a coating layer located on at least a portion of the surface of the secondary particulate graphite body. The secondary particulate graphite can be uncoated graphite, coated graphite, or a combination of uncoated and coated graphite.

[0287] In this application, "non-coated graphite" means graphite without a coating layer.

[0288] In this application, unless otherwise specified, "primary particle" refers to the basic unit of particles in a material. Primary particles can be in a non-agglomerated state, referred to as "non-agglomerated primary particles"; multiple primary particles can aggregate into a single aggregated particle, referred to as a "secondary particle," that is, a "secondary particle" refers to an aggregate of primary particles. Secondary particles are formed by the aggregation of primary particles, and secondary particles can have a relatively random or relatively disordered orientation as a whole; therefore, secondary particles exhibit stronger isotropy.

[0289] In this application, "secondary particle graphite body" or "secondary particle graphite" refers to an aggregate of primary particle graphite, where "primary particle graphite" is the basic graphite grain unit. It can be understood that secondary particle graphite belongs to the secondary particle type of graphite.

[0290] Non-limitingly, the "proportion of secondary particulate graphite in the first negative electrode active material" can be obtained through statistical analysis of the SEM scan images of the first negative electrode active material. The following method can be used: the first negative electrode active material is laid and adhered to conductive adhesive, and the particle morphology is tested using a scanning electron microscope (e.g., ZEISS Sigma 300). The number of particles in the obtained SEM images, whether secondary or non-agglomerated primary particles, is counted. Multiple regions are randomly selected for scanning tests, and the number of secondary particles and non-agglomerated primary particles in each region is counted. The proportion of secondary particles in each region is calculated, and the average of the proportions of secondary particles in multiple test regions is taken as the test value of the "proportion of secondary particles in the first negative electrode active material". Taking graphite as an example, this corresponds to the "proportion of secondary particulate graphite in the first negative electrode active material"; taking graphite as an example, this corresponds to the "proportion of secondary particulate graphite in the first negative electrode active material". Similar methods can be used to test and obtain parameters such as "the proportion of secondary particles in the second negative electrode active material", "the proportion of secondary particulate graphite in the second negative electrode active material", and "the proportion of secondary particulate graphite in the first negative electrode active material". Furthermore, similar methods can be used to obtain parameters such as "the proportion of secondary particulate graphite in the negative electrode active material" and "the proportion of secondary particles in the negative electrode active material".

[0291] In this application, "graphite material" refers to a negative electrode active material containing a graphite matrix, which includes at least a graphite matrix and optionally a coating layer located on at least a portion of the surface of the graphite matrix. The graphite material can be uncoated graphite, coated graphite, or a combination of uncoated and coated graphite. The graphite material can be secondary particles, non-agglomerated primary particles, or a combination of secondary and non-agglomerated primary particles. When the graphite material can be secondary particles, it corresponds to secondary particle type graphite.

[0292] By introducing secondary particulate graphite into the first negative electrode active layer, and taking advantage of the characteristic that secondary particles are formed by the agglomeration of primary particles, and based on the disordered orientation of each primary particle, it is beneficial to improve the isotropic characteristics when lithium ions are inserted into the first negative electrode active material, increase the lithium ion insertion sites on the surface of the first negative electrode active material, improve the lithium ion insertion rate, and improve the fast charging performance of the battery.

[0293] In some embodiments, the first negative electrode active material includes graphite material, and the OI value of the graphite material can be 2 to 15, optionally 2 to 10, or any of the following values ​​or a range selected from any two of the following values: 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, etc.

[0294] In this application, unless otherwise specified, the “OI value” of graphite material refers to the intensity ratio of the (004) crystal plane to the (110) crystal plane in the X-ray diffraction pattern of graphite material after roll forming. The level of the OI value can be used to characterize the orientation degree of graphite particle packing and can reflect the degree of isotropy of graphite particle packing.

[0295] The OI value (GOI) of graphite materials can be determined by X-ray diffraction (XRD) testing, referring to Appendix F of the national standard GB / T 24533-2019 "Graphite Anode Materials for Lithium-ion Batteries". The powder sample to be tested can be rolled into sheets, and XRD testing can be performed on the sheet samples to obtain X-ray diffraction patterns. After rolling the sample, the orientation (OI value) of artificial or natural hexagonal graphite on the electrode sheet is analyzed using X-ray polycrystalline diffraction. The peak area C of the (004) crystal plane diffraction peak is obtained after analysis and calculation using XRD pattern analysis software such as Highscore Plus or Jade. 004 The peak area C of the diffraction peak of the (110) crystal plane 110 OI value = C 004 / C 110 X-ray diffractometers (such as Bruker-D8 advance) can be used for testing.

[0296] Non-limitingly, samples for OI value testing can be obtained using the following method: disassemble the battery cell, remove the negative electrode sheet, and soak and clean it with a solvent such as dimethyl carbonate; scrape powder from the first negative electrode active layer, and then thoroughly soak the powder material extracted from the first negative electrode active layer with a solvent (such as N-methylpyrrolidone (NMP)) to dissolve the organic components such as binders and thickeners in the solvent (ultrasonic dispersion can also be used to promote dissolution). After washing and filtering, the collected solid phase is used as the test powder of the first negative electrode active material. After rolling the test powder sample into a sheet, XRD testing is performed on the sheet sample. The powder compaction density of the first negative electrode active material can also be tested.

[0297] By controlling the OI value of the graphite material in the first negative electrode active material within the aforementioned range, it is beneficial to enhance the isotropic characteristics of the first negative electrode active material, increase the lithium ion insertion channels, and make the lithium ion diffusion dynamics in the first negative electrode active layer better, thereby improving the fast charging performance of the battery.

[0298] The aforementioned features (td1) to (td6) can be combined in any suitable manner. For example, the first negative electrode active material can simultaneously satisfy features (td1) and (td2), in which case the coated graphite can also be secondary particle graphite. Exemplarily, the first negative electrode active material includes secondary particles, and the secondary particle graphite includes carbon-coated secondary particle graphite. Non-limitingly, the carbon-coated secondary particle graphite includes a carbon coating layer (i.e., a first carbon coating layer) located on the secondary particle graphite body and located on at least a portion of the surface of the secondary particle graphite body. Further, the carbon coating layer (i.e., the first carbon coating layer) in the carbon-coated secondary particle graphite can include one or more of soft carbon, hard carbon, and amorphous carbon. For example, the first negative electrode active material can simultaneously satisfy characteristics (td2) and (td3). For instance, the first negative electrode active material includes graphite material, which includes secondary particulate graphite. The OI value of the graphite material can be 2 to 15, optionally 2 to 10, or any of the following values ​​or a range selected from any two of the following values: 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, etc.

[0299] In this application, the terms "first," "third," "fourth," and "fifth" in "first carbon coating layer," "third carbon coating layer," "fourth carbon coating layer," and "fifth carbon coating layer" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features.

[0300] In some embodiments, the D of the first negative electrode active material v 50 can be 10μm to 18μm, can be selected from 12μm to 16μm, or can be any of the following values ​​or a range composed of any two of the following values: 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, etc.

[0301] Unless otherwise stated in this application, D v 50 refers to the particle size corresponding to a cumulative volume distribution percentage of 50% in the material. This parameter indicates that the particle size of 50% of the material's volume is less than or equal to D. v 50, and particles accounting for 50% of the material volume have a particle size greater than D. v 50. Those skilled in the art will understand D vThe meaning of 50 can be determined using instruments and methods known in the art. For example, it can be conveniently determined using a laser particle size analyzer, such as the Mastersizer 2000E laser particle size analyzer from Malvern Instruments Ltd., UK, or the LS-909 laser particle size analyzer (Omega). The standard procedure GB / T19077-2016 / ISO 13320:2009 can be referenced for the determination of D in the first negative electrode active material. v 50 is tested. The detailed test procedure includes: taking an appropriate amount of the sample to be tested, adding solvent (the solvent can be deionized water, and the sample concentration can be controlled at 8%~12% opacity), sonicating for 5 minutes (53KHz / 120W) to fully disperse the sample, and then measuring the sample according to 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. Under the irradiation of the laser beam, the particle size distribution characteristics can be obtained by receiving and measuring the energy distribution of the scattered light. Based on the test data, a particle size volume distribution map is plotted, and D is obtained from the distribution map. v 50. To avoid agglomeration during the drying process affecting particle size testing, a dispersion test was performed on the washed and moistened sample.

[0302] By using the D of the first negative electrode active material v Controlling the concentration of 50% within the aforementioned range helps to better control the degree of particle accumulation in the first negative electrode active layer, better control the porosity between particles, provide better lithium-ion transport channels, and better improve battery dynamics and fast charging performance.

[0303] In some embodiments, the porosity of the first negative electrode active layer is higher than that of the second negative electrode active layer.

[0304] In this application, the "porosity" of a membrane layer refers to the percentage of pore volume to total volume, which can be expressed as a percentage. The "porosity" of the second negative electrode active layer refers to the percentage of pore volume in the second negative electrode active layer relative to the total volume of the second negative electrode active layer. The "porosity" of the first negative electrode active layer refers to the percentage of pore volume in the first negative electrode active layer relative to the total volume of the first negative electrode active layer. The "porosity" of the negative electrode active material layer of the negative electrode sheet refers to the percentage of pore volume in the negative electrode active material layer relative to the total volume of the negative electrode active material layer, which can also be denoted as "porosity of the negative electrode sheet".

[0305] 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 balance the fast charging performance and energy density of the battery. The relatively high porosity of the first negative electrode active layer can be used to promote the rapid transport of lithium ions, while the relatively low porosity of the second negative electrode active layer can be used to improve the energy density of the negative electrode.

[0306] In this application, for the overall negative electrode sheet, the porosity (δ) of the negative electrode active material layer is... N The measurement can be performed using instruments and methods known in the art. For example, GB / T 24586-2009 can be referenced, using the gas displacement method. This method can test not only the negative electrode removed after disassembling the battery cell, but also the negative electrode obtained after cold pressing.

[0307] Pretreatment: The negative electrode sheets are punched. In a drying room, ≥20 discs with good appearance and no powder shedding from the edges are selected using tweezers and placed into a sample cup. The test sample consists of n negative electrode disc discs. Record the number of discs n and calculate the apparent volume V of the test sample. N2 V N2 =S N ×D N ×n.

[0308] Test Analysis: The sample cup containing the test sample is placed in a true density analyzer. The test system is sealed, and helium gas is introduced according to the procedure. The pressure of the gas in the sample chamber and the expansion chamber is detected, and the true volume V of the test sample is calculated according to Bohr's Law (PV = nRT). N1 Thus, the porosity δ of the test sample can be calculated. N =(V N2 -V N1 ) / V N2 ×100% can be recorded as the porosity of the negative electrode active material layer.

[0309] The test sample consists of n negative electrode discs; S N The area of ​​the negative electrode active material layer in a single negative electrode disc, in cm². 2 ;D N The thickness of the negative electrode active material layer in a single negative electrode disc, in cm; V N1 The true volume of the test sample, in cm 3 V N2 The apparent volume of the test sample, in cm³. 3 .

[0310] For example, the porosity of the second negative electrode active layer can be obtained by testing using the following method:

[0311] (1) Obtain a test sample 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 sheet, and use a micrometer to measure the electrode sheet thickness L; take another negative electrode sheet, wipe off the negative electrode active material layer on both sides of the remaining empty current collector foil, and measure the thickness L0. The total thickness of the negative electrode active material layer on both sides of the negative electrode current collector is L-L0; scrape off the negative electrode active material layer on one surface of the negative electrode sheet to expose the negative electrode current collector surface, and scrape off the negative electrode material on the other surface until the electrode sheet thickness is L1 = L0 + Δd. Collect the powder of the Δd thickness portion of the negative electrode active material layer below the L1 thickness. The thickness of the second negative electrode active layer can be determined in advance based on the observation results of the electrode sheet cross-sectional morphology, thereby controlling that when the thickness is L1, the first negative electrode active layer has been completely removed, and the remaining negative electrode active material layer corresponds to part or all of the second negative electrode active layer. For example, Δd can be exemplarily 15μm, 20μm, etc. The sample to be tested retains only the second negative electrode active layer on one side of the negative electrode current collector.

[0312] (2) Test the porosity (δ2) of the second negative electrode active layer: Test the porosity of the sample to be tested by referring to the method for testing the porosity of the negative electrode sheet, and record it as the porosity of the second negative electrode active layer.

[0313] When δ N When the porosity is greater than δ2, it can be considered that "the porosity of the first negative electrode active material layer is higher than the porosity of the second negative electrode active layer". Furthermore, for example, the porosity (δ1) of the first negative electrode active layer can be determined according to formula D. N ×δ N =D1×δ1+D2×δ2 is calculated; where δ N δ1 represents the porosity of the entire negative electrode active material layer, δ2 represents the porosity of the second negative electrode active layer, and D represents the porosity of the second negative electrode active layer. N D1 is the total thickness of the negative electrode active material layer, D2 is the thickness of the second negative electrode active layer, and D1 is the thickness of the first negative electrode active layer.

[0314] In addition, by way of example, the porosity of the negative electrode active material layer in the negative electrode sheet can also be tested, 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 remove the negative electrode sheet; punch the negative electrode sheet into small discs, and use the nanoscale spatial dynamic resolution and layer-by-layer cutting technique of FIB-SEM (Focused Electron Beam Electron Microscopy-Scanning Electron Microscopy) to reconstruct the three-dimensional structure of the sample; use energy dispersive spectroscopy (EDS) to analyze the distribution and proportion of each element; and use software to quantitatively analyze the porosity of the first negative electrode active layer, the second negative electrode active layer, and the overall negative electrode active material layer. The FEI Scios 2HiVac instrument can be used for testing.

[0315] In some embodiments, the powder compaction density of the first negative electrode active material or the powder compaction density of the first negative electrode active layer is 1.60 g / cm³. 3 ~1.80g / cm 3 It can also be any of the following values ​​or a range selected from any two of the following values: 1.60 g / cm³ 3 1.62g / cm 3 1.64 g / cm 3 1.65g / cm 3 1.66 g / cm 3 1.68g / cm 3 1.70g / cm 3 1.72g / cm 3 1.74 g / cm 3 1.75g / cm 3 1.76 g / cm 3 1.78g / cm 3 1.80g / cm 3 wait.

[0316] In this application, unless otherwise specified, 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, and 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.

[0317] In this application, the term "powder compaction density" has a well-known meaning in the art, referring to the ratio of mass to volume of a powder material after compaction under a certain pressure. In this application, the powder compaction density of the negative electrode active layer in the negative electrode active material layer refers to the powder compaction density of the negative electrode material constituting that negative electrode active layer. "Powder material" and "powder material" have the same meaning and can be used interchangeably.

[0318] The "powder compaction density" of powder materials can be determined using instruments and methods known in the art. For example, it can be determined using an electronic pressure testing machine (e.g., UTM7305 model) according to standard GB / T24533-2009. An exemplary test method is as follows: Weigh out a mass M (e.g., 1g) of the material to be tested, and add a bottom area A (e.g., 1.327cm²). 2 In a mold, pressure is applied to a certain pressure P0 (e.g., 3-5 tons (3T-5T), such as 3T, 4T, 5T), and held for a certain time (e.g., 5T for 30s). Then the pressure is released, and the pressure is maintained for a period of time (e.g., 10s). The compacted density of the powder under pressure P0 is then recorded and calculated. Unless otherwise specified, the pressure for testing the compacted density of the powder is 5T.

[0319] The compacted powder density of the first negative electrode material in the first negative electrode active layer and the second negative electrode material in the second negative electrode active layer can be tested to compare or obtain the compacted powder density of the first and second negative electrode active layers. The first and second negative electrode materials can be extracted from the negative electrode sheet after disassembling the battery cell, or they can be extracted from the negative electrode sheet obtained after cold pressing. Before testing, the first and second negative electrode materials can be ultrasonically dispersed in a solvent and then dried.

[0320] The powder raw materials of the first or second negative electrode active material can also be tested to obtain the powder compaction density of the first and second negative electrode active materials. Alternatively, the battery can be disassembled to obtain the test powders of the first and / or second negative electrode active materials, and then the powder compaction density can be tested. The preparation method of the test powders can be found in the OI testing section.

[0321] In this application, unless otherwise stated, "negative electrode sheet obtained after cold pressing" refers to the state of the negative electrode sheet obtained after cold pressing immediately after the cold pressing is completed. It can be considered that in this state, no or almost no rebound of the electrode sheet volume has occurred.

[0322] By controlling the powder compaction density of the first negative electrode active material or the powder compaction density of the first negative electrode active layer within the aforementioned range, it is beneficial to provide a better lithium-ion transport channel by increasing the particle packing degree of the first negative electrode active layer, thereby improving battery dynamics and fast charging performance. In addition, the second negative electrode active layer can be used to provide a higher energy density, which is beneficial to balance the fast charging performance and energy density of lithium-ion secondary batteries.

[0323] Typically, in lithium-ion secondary batteries, the measured powder compaction density of the first negative electrode active material is close to the measured powder compaction density of the first negative electrode active layer, and the measured powder compaction density of the second negative electrode active material is close to the measured powder compaction density of the second negative electrode active layer.

[0324] In some implementations, the rate capability of the first negative electrode active layer is higher than that of the second negative electrode active layer.

[0325] In some implementations, the charging rate of the first negative electrode active layer is higher than that of the second negative electrode active layer.

[0326] In this application, the "rate" of the first negative electrode active layer and the second negative electrode active layer is a parameter reflecting the charging and discharging capability, and the "charging rate" is a parameter reflecting the charging capability. The higher the charging rate, the better the fast charging performance.

[0327] The first and second active layer electrodes can be prepared using the following method: Based on a negative electrode obtained through cold pressing or dismantling a battery cell, a first negative electrode material is extracted from the first negative electrode active layer, and a second negative electrode material is extracted from the second negative electrode active layer. These are then resuspended in deionized water to form uniform slurries, denoted as the first resuspension slurry and the second resuspension slurry, respectively. The second resuspension slurry is coated onto one side of the copper foil of the negative electrode current collector, dried, and cold-pressed to obtain the second active layer electrode. Conversely, the first resuspension slurry is coated onto one side of the copper foil of the negative electrode current collector, dried, and cold-pressed to obtain the first active layer electrode. The difference between the first and second resuspension slurries lies in the difference between the first and second negative electrode materials; the coating weight, drying, and cold-pressing parameters are the same. Furthermore, the first and second active layer electrodes are respectively combined with lithium foil to form coin cells, and rate testing is performed. If the charge rate of the coin cell prepared from the first active layer electrode is higher than that prepared from the second active layer electrode, then it is considered that "the charge rate of the first negative active layer is higher than that of the second negative active layer". Existing methods in the art for testing the coin cell charge rate can be used for this test. Similarly, if the charge rate of the coin cell prepared from the first active layer electrode is higher than that prepared from the second active layer electrode, then it is considered that "the charge rate of the first negative active layer is higher than that of the second negative active layer". Existing methods in the art for testing the coin cell charge rate can be used for this test.

[0328] Unless otherwise specified in this application, the rate capability or charging rate of the first negative electrode active layer and the second negative electrode active layer can be compared using the following method:

[0329] The first negative electrode active material, conductive agent conductive carbon black (Super P), stabilizer sodium carboxymethyl cellulose, and binder styrene-butadiene rubber (SBR) are dispersed in deionized water at a mass ratio of 97.3:0.7:1.2:0.8 to form the first slurry.

[0330] The second negative electrode active material, conductive agent conductive carbon black (Super P), stabilizer sodium carboxymethyl cellulose and binder SBR are dispersed in deionized water at a mass ratio of 97.3:0.7:1.2:0.8 to form a second slurry.

[0331] The first and second slurries are respectively coated onto one side of the copper foil current collector and dried in an oven for later use.

[0332] A lithium metal plate is used as the counter electrode; a polypropylene (PP) membrane is used as the 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.

[0333] All components were assembled into a CR2430 button cell in an argon-protected glove box. "CR" represents the international IEC designation for coin-type lithium manganese batteries, with a diameter of 24 mm and a thickness of 30 mm. After resting for 12 hours, the resulting button cell was discharged at a constant current of 0.05C to 0.005V, and then discharged again at a constant current of 10μA to 0.005V. After resting for 5 minutes, the resulting button cell was charged at a constant current of 0.1C to 2V, and then rested for 5 minutes; the charging capacity C0 was recorded. The battery was placed at a constant temperature of 25℃ for 2 hours, and charge-discharge tests were performed at rates of 1C0, 2C0, 3C0, 4C0, and 5C0 to determine the capacity retention rate. Under the same cycling conditions and number of cycles, a higher capacity retention rate indicates better rate performance, and the "rate" can be considered higher.

[0334] Furthermore, under the same charging conditions, charging to the same SOC (e.g., 80% SOC, with a cutoff current of 0.01C) at the same rate, the shorter the time taken, the better the charging rate performance, and the higher the "charging rate" can be considered.

[0335] By controlling the 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 insertion of lithium ions into the first negative electrode active layer, which is more conducive to improving the fast charging performance of the battery.

[0336] By controlling the charging 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 insertion of lithium ions into the first negative electrode active layer, which is more conducive to improving the fast charging performance of the battery.

[0337] In some embodiments, at at least one temperature condition from 20°C to 35°C, the ionic conductivity of the electrolyte can be from 13 mS / cm to 18 mS / cm, or it can be any of the following values ​​or a range selected from 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 can be 25°C. Non-limitingly, at 25°C, the ionic conductivity of the electrolyte can be from 13 mS / cm to 18 mS / cm, or it can be any of the following values ​​or a range selected from 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.

[0338] In this application, unless otherwise specified, the term "ionic conductivity" of the electrolyte has a well-known meaning in the art and can be tested and analyzed using existing methods in the field. Ionic conductivity can be obtained using a conductivity meter, such as the DDSJ-318 conductivity meter. The test temperature can be 25±0.1℃. The test can be performed according to the method in HG-T 4067-2015. Non-limitingly, the test can be performed using a method including the following steps:

[0339] 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℃);

[0340] Test: The instrument was calibrated using two standard solutions at 25℃. After calibration and cleaning the electrode, the test sample electrode was vertically placed into the liquid to be tested. Click "Start Test" and record the test results after the data stabilized for more than 10 seconds.

[0341] By controlling the ionic conductivity of the electrolyte within the aforementioned range, it is beneficial to better improve battery dynamics and fast-charging performance.

[0342] The ionic conductivity of the electrolyte in some embodiments can be found in the context of this application. Electrolytes with high conductivity can be obtained by selecting solvents with low viscosity characteristics, but are not limited thereto.

[0343] In a non-limiting sense, the electrolyte includes non-aqueous solvents, which include low-viscosity solvents. Low-viscosity solvents may include one or more of ethyl acetate, methyl acetate, dimethyl carbonate, ethyl methyl carbonate, etc., and solvents whose viscosity at 25°C is less than or equal to that of at least one of the aforementioned reagents.

[0344] Unless otherwise specified in this application, the viscosity of solvents or electrolytes can be tested using conventional methods in the art, and can be determined using instruments and methods known in the art. For example, it can be measured according to the national standard GB / T10247-2008 "Methods for Viscosity Measurement", and can be performed based on the rotational viscometer in Appendix D of the national standard GB / T10247-2008. Non-limitingly, the viscosity of solvents or electrolytes can be tested by placing a certain mass of the sample to be tested in a sample container and using a Brookfield DV2TLV rotational viscometer.

[0345] In some embodiments, the negative electrode in a lithium-ion secondary battery satisfies one or more of the following characteristics (any numerical parameter of the following characteristics may also be selected from any suitable value or range in the context):

[0346] (te1) D of the second negative electrode active material v 50 is 12μm to 21μm, can be selected as 14μm to 20μm, and can be further selected as 14μm to 19μm;

[0347] (te2) D of the second negative electrode active material v 50 higher than the D of the first negative electrode active material v 50;

[0348] (te3) The compaction density of the second negative electrode active layer is higher than that of the first negative electrode active layer;

[0349] (te4) The powder compaction density of the second negative electrode active material is higher than that of the first negative electrode active material, or the powder compaction density of the second negative electrode active layer is higher than that of the first negative electrode active layer; optionally, the ratio of the powder compaction density of the second negative electrode active material to that of the first negative electrode active material is 1.05 to 1.35, further optionally 1.10 to 1.30, even more preferably 1.10 to 1.28 or 1.15 to 1.30, even more preferably 1.15 to 1.28; optionally, the ratio of the powder compaction density of the second negative electrode active layer to that of the first negative electrode active layer is 1.05 to 1.35, further preferably 1.10 to 1.30, even more preferably 1.10 to 1.28 or 1.15 to 1.30;

[0350] (te5) The compacted density of the powder in the second negative electrode active layer is 1.85 g / cm³. 3 ~2.05g / cm 3 .

[0351] By enabling lithium-ion secondary batteries to meet one or more of the characteristics (te1), (te2), (te3), (te4), and (te5), it is beneficial to enable lithium-ion secondary batteries to have improved fast-charging performance while also meeting energy density requirements.

[0352] In some embodiments, the second negative electrode active material includes secondary particulate graphite. Non-limitingly, the percentage of secondary particulate graphite in the second negative electrode active material is not particularly limited, and can be 0% to 100%, optionally 20% to 100%, further optionally 50% to 100%, and even further optionally 80% to 100%, or any of the following percentages or a range selected from any two of the following percentages: 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, etc.

[0353] In some embodiments, the D of the second negative electrode active material v 50 can be 12μm to 21μm, can be selected from 14μm to 20μm, can be further selected from 14μm to 19μm, and can also be any of the following values ​​or a range selected from any two of the following values: 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, etc.

[0354] By using the D of the second negative electrode active material vKeeping 50 within the aforementioned range is beneficial for achieving a higher compaction density in the second negative electrode active layer, which in turn helps to improve the energy density.

[0355] In some embodiments, the D of the second negative electrode active material v 50 higher than the D of the first negative electrode active material v 50.

[0356] By controlling the D of the second negative electrode active material v 50 higher than the D of the first negative electrode active material v 50 is beneficial for achieving a higher compaction density in the second negative electrode active layer, which in turn helps to improve the energy density.

[0357] 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 / cm³. 3 1.50g / cm³ is an optional value. 3 ~1.75g / cm 3 It can also be any of the following compaction densities or a range selected from any two of the following compaction densities: 1.40 g / cm³ 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.

[0358] In this application, unless otherwise specified, the “compacted density” of the 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.

[0359] Non-limitingly, the compaction density of the negative electrode sheet in a lithium-ion secondary battery can be tested using the following method: disassemble the battery to obtain the negative electrode sheet, and punch the obtained negative electrode sheet into a piece with an area S0 (e.g., 1540.25 mm²). 2 The small circular disc is used to measure its mass M. B and thickness L B Take another negative electrode sheet from a different region, wipe off the surface negative electrode active material layer, and cut the remaining negative electrode current collector foil (which can be denoted as empty negative electrode current collector foil) into small circular pieces with an area of ​​S0. Weigh the empty negative electrode current collector foil and record the mass as M0 and the thickness as L0. Then, the compaction density PD of the negative electrode sheet is... B =(M B -M0) / [S0×(L B -L0)]. Test multiple small discs and take the average value.

[0360] By controlling the compaction density of the negative electrode sheet in the lithium-ion secondary battery within the aforementioned range, the liquid phase impedance occupies a higher proportion of the impedance inside the negative electrode sheet. Quaternary ammonium salt compounds improve the liquid phase impedance by guiding lithium ions in the electrolyte. Therefore, controlling the compaction density of the negative electrode sheet within the aforementioned range is beneficial to better leveraging the role of quaternary ammonium salt compounds in improving the fast charging performance of the battery.

[0361] Compared to the negative electrode sheet obtained after cold pressing, the volume of the negative electrode sheet in a lithium-ion secondary battery will have a certain rebound, resulting in a lower compaction density of the negative electrode sheet in a lithium-ion secondary battery compared to the negative electrode sheet obtained after cold pressing.

[0362] In some embodiments, the compaction density of the second negative electrode active layer is higher than that of the first negative electrode active layer.

[0363] In this 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.

[0364] 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 using the following method: disassemble the battery to obtain the negative electrode sheet, wipe off part of the negative electrode active material layer to obtain an electrode sample including at least a portion of the second negative electrode active layer but excluding the first negative electrode active layer, and punch it into an area S0 (e.g., 1540.25 mm²). 2 Small circular pieces of material. The mass M2 and thickness L2 of the electrode sample are measured respectively. The mass M0 and thickness L0 of the empty negative electrode current collector foil with the same area S0 are measured respectively. Then the compaction density PD2 of the electrode sample = (M2-M0) / [S0×(L2-L0)] can be used as the compaction density of the second negative electrode active layer.

[0365] When the compaction density PD of the negative electrode sheet B The compaction density PD2 of the second negative electrode active layer is lower than that of the first negative electrode active layer, so it can be considered that "the compaction density of the second negative electrode active layer is higher than that of the first negative electrode active layer".

[0366] By adjusting the compaction density of the second negative electrode active layer in a lithium-ion secondary battery to be higher than that of the first negative electrode active layer, it is beneficial to improve the energy density of the lithium-ion secondary battery.

[0367] In some embodiments, the powder compaction density of the second negative electrode active material is higher than that of the first negative electrode active material.

[0368] In some embodiments, the ratio of the compacted density of the second negative electrode active material to the compacted density of the first negative electrode active material can be 1.05 to 1.35, optionally 1.10 to 1.30, further optionally 1.10 to 1.28 or 1.15 to 1.30, or any of the following ratios or a range selected from any two of the following ratios: 1.05, 1.06, 1.08, 1.10, 1.11, 1.12, 1.14, 1.15, 1.16, 1.18, 1.20, 1.22, 1.24, 1.25, 1.26, 1.28, 1.30, etc., or selected from the following ranges: 1.05 to 1.28, 1.05 to 1.30, 1.1 to 1.3, 1.10 to 1.35, 1.15 to 1.28, etc.

[0369] In some embodiments, the powder compaction density of the second negative electrode active layer is higher than that of the first negative electrode active layer.

[0370] In some embodiments, the ratio of the compacted density of the powder in the second negative electrode active layer to the compacted density of the powder in the first negative electrode active layer can be 1.05 to 1.35, optionally 1.10 to 1.30, further optionally 1.10 to 1.28 or 1.15 to 1.30, or any of the following ratios or a range selected from any two of the following ratios: 1.05, 1.06, 1.08, 1.10, 1.11, 1.12, 1.14, 1.15, 1.16, 1.18, 1.20, 1.22, 1.24, 1.25, 1.26, 1.28, 1.30, etc., or selected from the following ranges: 1.05 to 1.30, 1.05 to 1.28, 1.1 to 1.3, 1.10 to 1.35, 1.15 to 1.28, etc.

[0371] By controlling the powder compaction density of the second negative electrode active material in the lithium-ion secondary battery to be higher than that of the first negative electrode active material, or by controlling the powder compaction density of the second negative electrode active layer to be higher than that of the first negative electrode active layer, it is beneficial to impart a higher compaction density to the second negative electrode active layer during the cold pressing process of the electrode sheet, thus giving the second negative electrode active layer in the lithium-ion secondary battery a higher compaction density. Furthermore, by controlling the ratio of the powder compaction density of the second negative electrode active material to that of the first negative electrode active material, or by controlling the ratio of the powder compaction density of the second negative electrode active layer to that 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 lithium-ion secondary battery.

[0372] In some embodiments, the powder compaction density of the second negative electrode active material or the powder compaction density of the second negative electrode active layer is 1.85 g / cm³. 3 ~2.05g / cm3 It can also be any of the following values ​​or a range consisting of any two of the following values: 1.85 g / cm³ 3 1.86 g / cm 3 1.88g / cm 3 1.90g / cm 3 1.92g / cm 3 1.94 g / cm 3 1.95g / cm 3 1.96g / cm 3 1.98g / cm 3 2.00g / cm 3 2.02 g / cm 3 2.04 g / cm 3 2.05g / cm 3 wait.

[0373] By controlling the powder compaction density of the second negative electrode active material or the powder compaction density of the second negative electrode active layer within the aforementioned range, it is beneficial to achieve a higher energy density in the secondary battery. Furthermore, the particle packing can be relatively dense; in this case, the quaternary ammonium salt compounds introduced into the second negative electrode active layer may have a more significant effect on improving the battery's fast-charging performance.

[0374] In some implementations, the negative electrode sheet satisfies one or more of the following characteristics (any numerical parameter of the following characteristics may also be selected from any suitable value or range in the context):

[0375] (tf1) On one side of the negative electrode current collector, the ratio of the areal density of the second negative electrode active layer to the areal density of the first negative electrode active layer is 3:2 to 2:3.

[0376] (tf2) Taking the thickness ratio of the first negative electrode active layer to the second negative electrode active layer on one side of the negative electrode current collector as denoted as f H , satisfying f H ≤1.6, optionally, 1.1≤f H ≤1.6, and further optionally, 1.1≤f H ≤1.3;

[0377] (tf3) On one side of the negative electrode current collector, the thickness of the first negative electrode active layer is less than or equal to 50 μm, and can be selected as 30 μm to 40 μm.

[0378] In this application, unless otherwise stated, "on one side of the negative electrode current collector" means that the described features are located on the same side of the negative electrode current collector.

[0379] In some embodiments, the ratio of the areal density of the second negative electrode active layer to the areal density of the first negative electrode active layer, measured on one side of the negative electrode current collector, is 3:2 to 2:3. It can also be any of the following ratios or a range selected from any two of the following ratios: 2:3, 0.67, 0.7, 0.75, 0.8, 0.75, 0.9, 0.95, 1 (corresponding to 1:1), 1.05, 1.1, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5 (i.e., 3:2), etc.

[0380] In this application, unless otherwise specified, "the areal density of the second negative electrode active layer, measured on one side of the negative electrode current collector," refers to the ratio of the mass of the second negative electrode active layer on one side of the negative electrode current collector to the area of ​​the second negative electrode active layer on that side. "The areal density of the first negative electrode active layer," refers to the ratio of the mass of the first negative electrode active layer on one side of the negative electrode current collector to the area of ​​the first negative electrode active layer on that side. The corresponding "area" is equal to the projected area of ​​each of the second and first negative electrode active layers along the electrode thickness direction. Numerically, measured on one side of the negative electrode current collector, the areal density of the second negative electrode active layer is equal to the product of the compacted density of the second negative electrode active layer and the thickness of the second negative electrode active layer. Mass and area data can be obtained by referring to the test method for the compacted density of the second negative electrode active layer. Numerically, measured on one side of the negative electrode current collector, the areal density of the first negative electrode active layer is equal to the product of the compacted density of the first negative electrode active layer and the thickness of the second negative electrode active layer. Mass and area data can be obtained by referring to the test method for the compacted density of the first negative electrode active layer.

[0381] By controlling the ratio of the areal density of the second negative electrode active layer to the areal density of the first negative electrode active layer within the aforementioned range, it is beneficial to balance the battery's fast charging performance and energy density.

[0382] In some embodiments, the thickness ratio of the first negative electrode active layer to the second negative electrode active layer, measured on one side of the negative electrode current collector, is denoted as f. H , satisfying f H ≤1.6, optionally, 1.1≤f H ≤1.6, and further optionally, 1.1≤f H ≤1.3, f H It can also be any of the following values ​​or a range consisting of any two of the following values: 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, etc.

[0383] In some embodiments, the thickness of the first negative electrode active layer is less than or equal to 50 μm on one side of the negative electrode current collector, and can be selected as 20 μm to 50 μm, further selected as 30 μm to 50 μm, and even further selected as 30 μm to 40 μm. It can also be any of the following values ​​or a range selected from any two of the following values: 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, etc.

[0384] In this application, unless otherwise specified, "the thickness ratio of the first negative electrode active layer to the second negative electrode active layer, measured on one side of the negative electrode current collector" refers to the thickness ratio of the first negative electrode active layer to the second negative electrode active layer on the same side of the negative electrode current collector. In this application, unless otherwise specified, "the first negative electrode active layer, measured on one side of the negative electrode current collector" refers to the thickness of the first negative electrode active layer on one side of the negative electrode current collector.

[0385] By controlling the thickness of the first negative electrode active layer to satisfy one or more of the above features (tf2) and (tf3), the distance between the quaternary ammonium salt compound and the surface of the negative electrode sheet can be adjusted, which is beneficial to better promote the transport of lithium ions to the second negative electrode active layer located below, thereby improving the fast charging performance of the battery.

[0386] In a first aspect of this application, a lithium-ion secondary battery is also provided, comprising a negative electrode and an electrolyte; the negative electrode includes a negative active material layer, the negative active material layer comprising a negative active material and a quaternary ammonium salt compound. As previously defined, the quaternary ammonium salt compound comprises a quaternary ammonium cation.

[0387] By setting quaternary ammonium salt compounds in the negative electrode active material layer, the electrostatic effect based on quaternary ammonium ions can be used to attract electrolyte anions in the electrolyte, promote the rapid dissociation of electrolyte lithium salt in the electrolyte, and the formed quaternary ammonium ion-electrolyte anion structure can also guide lithium ions in the electrolyte to be rapidly transported to the surface of the negative electrode active material, which can significantly improve the dynamics of the negative electrode sheet, and thus significantly improve the fast charging performance of the battery.

[0388] In some embodiments, the negative electrode includes a negative current collector and a layer of negative active material located on at least one side of the negative current collector.

[0389] In some embodiments, a lithium-ion secondary battery is also provided, which includes a negative electrode sheet and an electrolyte; the negative electrode sheet includes a negative current collector and a negative active material layer located on at least one side of the negative current collector;

[0390] The negative electrode active material layer includes a negative electrode active material and a quaternary ammonium salt compound. The negative electrode 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 one or more of soft carbon, hard carbon, and amorphous carbon. The quaternary ammonium salt compound includes quaternary ammonium cations.

[0391] By setting a coating layer on the surface of the negative electrode active material layer, and setting one or more of soft carbon, hard carbon, and amorphous carbon in the coating layer, the lithium-ion transport channels on the surface of the negative electrode active material can be optimized, promoting lithium-ion transport and improving battery kinetics and fast-charging performance. Furthermore, by setting quaternary ammonium salt compounds in the negative electrode active material layer, the electrostatic interaction based on quaternary ammonium ions can attract electrolyte anions in the electrolyte, promoting the rapid dissociation of electrolyte lithium salts in the electrolyte. The formed quaternary ammonium ion-electrolyte anion structure can also guide the rapid transport of lithium ions in the electrolyte to the surface of the negative electrode active material. Based on the aforementioned multiple effects, the fast-charging performance of the battery can be further improved.

[0392] The negative electrode active material layer 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.

[0393] In some implementations, the negative electrode active material layer is a single-layer structure.

[0394] In some implementations, the negative electrode active material includes a carbon-based material.

[0395] In some embodiments, the mass percentage of carbon-based materials in the negative electrode 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%, 97%, 98%, 99%, 100%, etc.

[0396] In some embodiments, the surface of the negative electrode active material particles is provided with carbonaceous material. The carbonaceous material may include one or more of artificial graphite, natural graphite, soft carbon, and hard carbon.

[0397] In some embodiments, the negative electrode active material includes a graphite material. The graphite material may include one or more of synthetic graphite and natural graphite.

[0398] In some embodiments, the mass percentage of graphite-based material in the negative electrode 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%, 97%, 98%, 99%, 100%, etc.

[0399] In some embodiments, the surface of the negative electrode active material particles is provided with graphite. Graphite may include one or more of artificial graphite and natural graphite.

[0400] In some implementations, the negative electrode sheet satisfies one or more of the following characteristics:

[0401] (tg1) Quaternary ammonium salt compounds are as defined above;

[0402] (tg2) The mass percentage of quaternary ammonium salt compounds in the negative electrode active material layer is 0.2% to 2%, which can be selected from 0.2% to 1.5%, or it can be any of the following percentages or a range selected from any two of the following percentages: 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2%, etc.

[0403] (tg3) The negative electrode active material includes carbon-based materials; optionally, the carbon-based material includes one or more of artificial graphite, natural graphite, soft carbon and hard carbon;

[0404] (tg4) The negative electrode active material includes carbon-based materials, and the mass percentage of carbon-based materials in the negative electrode active material layer is 94.5% to 97.5%, which can be selected as 95.0% to 97.0%, or any of the following percentages or a range selected from any two of the following percentages: 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, etc.

[0405] (tg5) The negative electrode active material layer includes a binder, which includes styrene-butadiene rubber; optionally, the glass transition temperature of the styrene-butadiene rubber is 5℃~70℃, further optionally 30℃~50℃, and may also be any of the following temperatures or selected from any two of the following temperature ranges: 5℃, 10℃, 15℃, 20℃, 25℃, 30℃, 35℃, 40℃, 45℃, 50℃, etc.

[0406] (tg6) The negative electrode active material includes coated graphite, which includes a graphite body and a coating layer located on at least a portion of the surface of the graphite body. The coating layer includes one or more of soft carbon, hard carbon and amorphous carbon.

[0407] (tg7) The negative electrode active material includes secondary particulate graphite, which includes a secondary particulate graphite body. The proportion of secondary particulate graphite in the negative electrode active material is greater than or equal to 20%, and can be selected from 30% to 60%. It can also be any of the following percentages or a range selected from any two of the following percentages: 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, etc. Optionally, the secondary particulate graphite includes carbon-coated secondary particulate graphite. The carbon-coated secondary particulate graphite includes a secondary particulate graphite body and a carbon coating layer located on at least a portion of the surface of the secondary particulate graphite body. The carbon coating layer in the carbon-coated secondary particulate graphite includes one or more of soft carbon, hard carbon, and amorphous carbon.

[0408] (tg8) The negative electrode active material includes graphite material, and the OI value of the graphite material is 2 to 15, which can be selected as 2 to 10, or any of the following values ​​or a range composed of any two of the following values: 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, etc.

[0409] (tg9) D of the negative electrode active material v 50 is 11μm to 20μm, can be selected from 13μm to 18μm, and can also be any of the following values ​​or a range composed of any two of the following values: 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, etc.

[0410] (tg10) The porosity of the negative electrode active material layer is 15% to 35%, which can be selected as 25% to 30%, or any of the following porosities or a range selected from any two of the following porosities: 15%, 16%, 18%, 20%, 22%, 24%, 25%, 26%, 8%, 30%, 32%, 34%, 35%, etc.

[0411] (tg11) At at least one temperature condition from 20°C to 35°C, the ionic conductivity of the electrolyte is 13 mS / cm to 18 mS / cm; optionally, at 25°C, the ionic conductivity of the electrolyte is 13 mS / cm to 18 mS / cm, and may also be any of the following values ​​or a range selected from 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.

[0412] (tg12) The negative electrode active material layer includes a second negative electrode active layer, the definition of which can be found in the first aspect of this application.

[0413] For the testing methods of the relevant parameters in features (tg1) to (tg11), please refer to the relevant testing methods in the context.

[0414] By controlling the mass percentage of quaternary ammonium salt compounds in the negative electrode active material layer within the aforementioned range, the role of quaternary ammonium salt compounds in improving the fast-charging performance of the battery can be better utilized. Furthermore, the decrease in electronic conductivity on the surface of the negative electrode active material caused by the encapsulation of quaternary ammonium salt compounds can be better controlled, which is beneficial for improving the battery's fast-charging performance. Moreover, it also helps maintain a high lithium storage capacity in the negative electrode active material layer, which is beneficial for balancing the energy density of both the negative electrode and the battery.

[0415] Carbon-based materials are beneficial for better adsorption of hydrocarbon chains in quaternary ammonium salt compounds, which in turn promotes better encapsulation of quaternary ammonium salt compounds on the surface of the negative electrode active material. This, in turn, helps to better guide lithium ions in the electrolyte to the surface of the negative electrode active material more quickly. Furthermore, carbon-based materials also have good electronic conductivity. In addition, carbon-based materials help to provide better stability of the negative electrode active material during fast charging, making it less prone to pulverization. Through the aforementioned multiple effects, it is beneficial to improve the dynamics of the negative electrode sheet and the fast charging performance of the battery.

[0416] Introducing natural graphite into the negative electrode active material can improve the compaction density and areal capacity of the negative electrode active material layer, thereby increasing the energy density of the negative electrode sheet. Introducing artificial graphite into the negative electrode active material can improve cycle performance. Introducing at least one of hard carbon and soft carbon into the negative electrode active material can improve the negative electrode sheet and battery kinetics.

[0417] By controlling the mass ratio of carbon-based materials in the negative electrode active material layer within the aforementioned range, it is beneficial to better coat the quaternary ammonium salt compound on the surface of the negative electrode active material. In addition, the energy density of the negative electrode and the battery can also be taken into account.

[0418] By introducing carbon-based materials into the negative electrode active material layer and styrene-butadiene rubber into the binder, it is beneficial to better simultaneously encapsulate the binder and quaternary ammonium salt compounds on the surface of the negative electrode active material. This not only achieves a good electrical contact network but also fully leverages the role of quaternary ammonium salt compounds in guiding lithium ions in the electrolyte, which is conducive to better improving the fast charging performance of the battery.

[0419] By controlling the glass transition temperature of styrene-butadiene rubber within the aforementioned range, it is beneficial to enable the negative electrode active material layer to have a better pore structure after rolling, which is beneficial to promote better wetting of the negative electrode active material layer by the electrolyte.

[0420] By setting a coating layer on the surface of the negative electrode active material layer, and setting one or more of soft carbon, hard carbon and amorphous carbon in the coating layer, the lithium-ion transport channels on the surface of the negative electrode active material can be optimized, lithium-ion transport can be promoted, and battery kinetics and fast charging performance can be further improved.

[0421] By introducing coated graphite into the negative electrode active material layer, and setting one or more of soft carbon, hard carbon and amorphous carbon in the coating layer, the lithium-ion transport channels on the surface of the negative electrode active material can be optimized, promoting lithium-ion transport and further improving battery kinetics and fast-charging performance.

[0422] By introducing secondary particulate graphite into the negative electrode active material layer, and taking advantage of the characteristic that secondary particles are formed by the agglomeration of primary particles, and based on the disordered orientation of each primary particle, it is beneficial to improve the isotropic characteristics when lithium ions are intercalated into the negative electrode active material, increase the lithium ion intercalation sites on the surface of the negative electrode active material, improve the lithium ion intercalation rate, and improve the fast charging performance of the battery.

[0423] By controlling the OI value of graphite in the negative electrode active material within the aforementioned range, it is beneficial to enhance the isotropic characteristics of the negative electrode active material, increase the lithium ion insertion channels, and make the lithium ion diffusion dynamics in the negative electrode active material layer better, thereby improving the fast charging performance of the battery.

[0424] By using the D of the negative electrode active material v Controlling the concentration of 50% within the aforementioned range helps to better control the degree of particle accumulation in the negative electrode active material layer, better control the porosity between particles, provide better lithium-ion transport channels, and better improve battery dynamics and fast charging performance.

[0425] When the porosity of the negative electrode active material layer is controlled within the aforementioned range, the particle packing is relatively dense. At this time, the quaternary ammonium salt compound introduced into the negative electrode active material layer has a more significant effect on improving the battery's fast charging performance.

[0426] By controlling the ionic conductivity of the electrolyte within the aforementioned range, it is beneficial to promote the rapid transport of lithium ions, which in turn helps to improve battery dynamics and fast-charging performance.

[0427] In some embodiments, the areal density of the negative electrode sheet is 5 mg / cm³, calculated on one side of the negative electrode current collector. 2 ~15mg / cm 2It can also be any of the following values ​​or a range consisting of any two of the following values: 5 mg / cm³ 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.

[0428] In this application, unless otherwise specified, "the areal 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. The corresponding "area" is equal to the orthogonal projected area of ​​the negative electrode active material layer along the thickness direction of the electrode sheet. Refer to the test method for the compaction density of the negative electrode sheet, based on (M... B -M0) / S0 is calculated.

[0429] By controlling the areal density of the negative electrode sheet within the aforementioned range, it is beneficial to balance the battery's fast charging performance and energy density.

[0430] In some embodiments, the electrolyte comprises an electrolyte salt, which comprises an electrolyte anion. Without limitation, the electrolyte anion may comprise one or more of tetrafluoroborate, hexafluoroarsenate, hexafluorophosphate, trifluoromethanesulfonate, difluorophosphate, difluorooxalateborate, tetrafluorooxalate phosphate, difluorodioxalate phosphate, bis(fluorosulfonyl)imide, and bis(trifluoromethanesulfonyl)imide.

[0431] By selecting the aforementioned types of electrolyte anions, it is beneficial to enhance the binding capacity of electrolyte anions to quaternary ammonium ions, better promote the dissociation of quaternary ammonium ions and anions in quaternary ammonium salt compounds, promote the attraction of quaternary ammonium salt compounds to electrolyte anions and the guidance of lithium ions in the electrolyte, improve the transport rate of lithium ions to the second negative electrode active material, and better improve the dynamics of the negative electrode sheet and the fast charging performance of the battery.

[0432] The aforementioned anions have stronger electronegativity than one or more of nitrate, carbonate, bicarbonate and phosphate, and have a stronger affinity for quaternary ammonium ions.

[0433] In some embodiments, the electrolyte salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0434] In some embodiments, the lithium-ion secondary battery further includes a positive electrode sheet, which includes a positive active layer, and the positive active layer includes a positive active material, which includes one or more of lithium phosphate active materials and lithium composite metal oxide active materials.

[0435] In some embodiments, the positive electrode active material includes lithium phosphate-based active materials.

[0436] In this application, unless otherwise specified, lithium phosphate-containing active materials may include at least one of lithium phosphates and their modifications. Lithium phosphate-containing active materials may have an olivine structure. Unless otherwise specified, "lithium phosphate" refers to materials containing lithium, transition metal elements, and phosphate ions (PO4). 3- The positive electrode active material is olivine-structured lithium phosphate. Non-limiting examples of lithium phosphates may include, but are not limited to, one or more of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium iron manganese phosphate, and lithium iron manganese phosphate and carbon composites.

[0437] Introducing lithium phosphate-containing active materials into the positive electrode active material can improve the structural stability of the positive electrode active material during charge-discharge cycles and extend the cycle life of the battery.

[0438] In some embodiments, the positive electrode active material includes lithium-ion composite metal oxide active materials.

[0439] In this application, unless otherwise specified, lithium composite metal oxide active materials include at least one of lithium composite metal oxides and their modified forms. Unless otherwise specified, "lithium composite metal oxide" refers to a positive electrode active material comprising lithium, non-lithium metal elements, and oxygen. Typically, the non-lithium metal elements in lithium composite metal oxides include transition metal elements; therefore, lithium composite metal oxides can also be called "lithium transition metal oxides." Lithium composite metal oxide active materials can have crystal structures suitable for positive electrode active materials, such as layered structures or spinel structures. In some embodiments, the lithium composite metal oxide active material comprises a layered structure. In some embodiments, the lithium composite metal oxide active material has a layered structure.

[0440] Introducing lithium-composite metal oxide active materials into the positive electrode active material is beneficial to improving the energy density of the positive electrode and the battery.

[0441] In this application, unless otherwise specified, "a modified positive electrode active material" includes the positive electrode active material itself and the modifying element. Furthermore, the modifying element may exist as a dopant element, a coating element, or a combination of a dopant element and a coating element. Unless otherwise specified, "a modified positive electrode active material" still falls within the scope of positive electrode active materials.

[0442] In this application, unless otherwise specified, "doping element" in positive electrode active material refers to a modifying element doped into the positive electrode active material; unless otherwise specified, "coating element" in positive electrode active material refers to a positive electrode active material comprising a positive electrode active particle body and a coating layer located on at least a portion of the surface of the positive electrode active particle body, wherein the coating element is a modifying element located in the coating layer. As a non-limiting example, in positive electrode active material, "the modifying element exists in a combination of doping element and coating element" means that the positive electrode active material comprises a positive electrode active particle body and a coating layer located on at least a portion of the surface of the positive electrode active particle body, at least a portion of the modifying element is doped into the positive electrode active particle body, and at least a portion of the modifying element is also contained in the coating layer. Both the doping modification method of introducing doping elements and the coating modification method of introducing coating elements can adopt or refer to existing modification methods in the art, including but not limited to the selection of element type, doping amount, and coating amount. In some embodiments, the positive electrode active particle body can be the positive electrode active material itself or its doped modified form. In some embodiments, the doping element may 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 may include one or more of Ti, Mg, Nb, C, etc.

[0443] In some embodiments, the positive electrode active material includes a lithium phosphate-based active material, which satisfies one or more of the following characteristics (any numerical parameter of the following characteristics may also be selected from any suitable value or range in the context):

[0444] (th1) The mass percentage of lithium phosphate active material in the positive electrode active layer is greater than or equal to 80%, and can be selected as 80% to 97%;

[0445] (th2) Lithium-containing phosphate active materials include one or more of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0446] (th3) Lithium phosphate active materials include a lithium phosphate active body and a carbon coating layer (which may be referred to as the third carbon coating layer) located on at least a portion of the surface of the lithium phosphate active body. The carbon coating layer (third carbon coating layer) in the lithium phosphate active material includes one or more of soft carbon, hard carbon and amorphous carbon.

[0447] In some embodiments, the mass percentage of lithium phosphate-containing active material in the positive electrode active layer can be greater than or equal to 80%, optionally 80% to 97%, and further optionally 95% to 97%. Non-limitingly, the mass percentage of lithium phosphate-containing active material in the positive electrode active layer 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%, 93%, 94%, 95%, 95.2%, 95.4%, 95.5%, 96%, 96.5%, 96.6%, 96.8%, 96.9%, 97%, etc.

[0448] By controlling the mass ratio of lithium phosphate active materials in the positive electrode active layer within the aforementioned range, it is beneficial to extend the cycle life of the battery.

[0449] In some embodiments, the lithium phosphate-containing active material includes one or more of lithium iron phosphate (LFP), lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium iron manganese phosphate, and lithium iron manganese phosphate and carbon composites. Examples of lithium iron phosphate include LiFePO4. Examples of lithium manganese phosphate include LiMnPO4.

[0450] The types of lithium phosphate active materials can be flexibly selected to meet different application needs.

[0451] In some embodiments, the lithium phosphate active material includes a lithium phosphate active body and a carbon coating layer (i.e., a third carbon coating layer) located on at least a portion of the surface of the lithium phosphate active body. Optionally, the carbon coating layer (i.e., the third carbon coating layer) in the lithium phosphate active material includes one or more of soft carbon, hard carbon, and amorphous carbon.

[0452] By setting one or more carbon coating layers, including soft carbon, hard carbon, and amorphous carbon, on the surface of lithium phosphate active materials, the conductivity of the material can be improved. This is beneficial for improving the electrical contact network within the positive electrode, providing a fast and stable channel for electron transport within the positive electrode, thereby improving the rate performance and fast charging capability of the battery.

[0453] In some embodiments, the lithium phosphate active substrate includes at least one of lithium iron phosphate, lithium manganese phosphate, and lithium manganese iron phosphate.

[0454] In some embodiments, the positive electrode active material includes a lithium iron phosphate (LFP)-based positive electrode active material. The "lithium iron phosphate-based positive electrode active material" at least includes a lithium iron phosphate body, and may further include a carbon coating layer (referred to as a fourth carbon coating layer) located on at least a portion of the surface of the lithium iron phosphate body. Optionally, the carbon coating layer (i.e., the fourth 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. Non-limitingly, the lithium iron phosphate-based positive electrode active material may include carbon-coated lithium iron phosphate.

[0455] In some embodiments, the positive electrode active material includes carbon-coated lithium iron phosphate, which comprises a lithium iron phosphate body and a carbon coating layer (referred to as the fifth carbon coating layer) located on at least a portion of the surface of the lithium iron phosphate body. Further, the carbon coating layer (i.e., the fifth carbon coating layer) in the carbon-coated lithium iron phosphate may include one or more of soft carbon, hard carbon, and amorphous carbon. In some embodiments, the carbon coating layer comprises soft carbon. In this case, the positive electrode active material includes a lithium phosphate active material, which includes carbon-coated lithium iron phosphate.

[0456] Those skilled in the art can use conventional techniques to select and control the mass percentage of the carbon coating layer in lithium iron phosphate-based cathode active materials (examples of which include carbon-coated lithium iron phosphate) and the thickness or average thickness of the carbon coating layer. Non-limitingly, the mass percentage of the carbon coating layer in the lithium iron phosphate-based cathode active material (e.g., carbon-coated lithium iron phosphate) can be 0.2% to 2%, but is not limited thereto. Non-limitingly, in lithium iron phosphate-based cathode active materials, the average thickness of the carbon coating layer can be 10 nm to 20 nm, but is not limited thereto.

[0457] In some embodiments, the positive electrode active material includes soft carbon-coated lithium iron phosphate. Further, soft carbon-coated lithium iron phosphate includes a lithium iron phosphate body and soft carbon located on at least a portion of the surface of the lithium iron phosphate body.

[0458] The detection of positive electrode active material in the positive electrode active layer can be carried out by disassembling the battery after it is fully discharged, removing the positive electrode plate, scraping off 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 types and proportions of elements, thereby confirming the elemental composition and chemical formula of the positive electrode active material.

[0459] 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.

[0460] For cathode active materials that include a coating layer (e.g., a carbon coating layer), a cross-section can be obtained using FIB (Focused Ion Beam) and the particle cross-sectional morphology can be observed under TEM (Transmission Electron Microscopy). A clear boundary can be observed at the coating interface, and the thickness and average thickness of the coating layer can be calculated based on the TEM image. Further analysis using one or more methods such as energy-dispersive spectroscopy (EDS) and Raman spectroscopy can identify the types of substances in both the coating layer and the cathode active material.

[0461] The following are some other descriptions of the positive electrode sheet.

[0462] The positive electrode includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector, the positive active layer including a positive active material.

[0463] The definitions of the positive electrode sheet, positive electrode active layer, and positive electrode active material in some embodiments can be found in the context of this application.

[0464] Without limitation, the mass fraction of the positive electrode active material in the positive electrode active layer can be greater than or equal to 80%, and more particularly greater than or equal to 90%.

[0465] As a non-limiting example, the positive current collector has two surfaces that are opposite to each other in its own thickness direction, and the positive active layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0466] 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 substrate and a metal layer formed on at least one surface of the polymer material substrate. In the positive electrode current collector, the composite current collector may be obtained by forming a metal material on the polymer material substrate. Non-limiting examples of the metal material in the positive electrode current collector may include at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. Non-limitingly, in the positive electrode current collector, the polymer material substrate may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0467] The types of positive electrode active materials can be found above. Positive electrode active materials may also include other types of positive electrode active materials known in the art for use in lithium-ion secondary batteries. A single positive electrode active material may be used alone, or two or more may be used in combination.

[0468] As a non-limiting example, the positive electrode active material may include, but is not limited to, one or more of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modifications.

[0469] In some embodiments, the positive electrode active material comprises a lithium-containing phosphate with an olivine structure. Non-limiting examples of lithium-containing phosphates with an olivine structure include, but are not limited to, one or more of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium iron manganese phosphate, and lithium iron manganese phosphate and carbon composites. Examples of lithium iron phosphate include LiFePO4. Examples of lithium manganese phosphate include LiMnPO4.

[0470] In some embodiments, the positive electrode active material includes a lithium transition metal oxide. Examples of lithium transition metal oxides 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 their modified compounds. Non-limiting examples of lithium cobalt oxide may include LiCoO2; non-limiting examples of lithium nickel oxide may include LiNiO2; non-limiting examples of lithium manganese oxide may include LiMnO2, LiMn2O4, etc.; non-limiting examples of lithium nickel cobalt manganese oxide may include LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 Examples of lithium nickel cobalt aluminum oxides include LiNi, etc. 0.80 Co 0.15 Al 0.05 O2.

[0471] Understandably, lithium (Li) is intercalated and deintercalated during the charging and discharging process of a battery, and the Li content in the positive electrode varies depending on the state of discharge. In the exemplary description of the positive electrode active material in this application, unless otherwise specified, the Li content can be the initial state of the material or a non-initial state after charge-discharge cycles. When the positive electrode active material is applied to the positive electrode in a battery system, the Li content in the positive electrode active material at the positive electrode will usually change after charge-discharge cycles. The Li content can be measured in atomic molar content, but is not limited to this. Regarding "Li content is the initial state of the material," the initial state of the material refers to the state before it is formed into the positive electrode active layer. It is understood that new materials or substances obtained by appropriately modifying the listed positive electrode active materials are also within the scope of positive electrode active materials. The aforementioned appropriate modification refers to an acceptable modification method for the positive electrode active material, and non-limiting examples include one or more of coating modification and doping modification.

[0472] In the exemplary description of the positive electrode active material in this application, the oxygen (O) content is only a theoretical value. Lattice oxygen release will cause changes in the atomic molar content of oxygen, and the actual O content will fluctuate. The O content can be measured in atomic molar content, but is not limited to this.

[0473] 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), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins. Typically, the mass fraction of the binder in the positive electrode active layer can be 0–10%, more further 0–8%, and even more further 1%–5%, based on the total mass of the positive electrode active layer.

[0474] 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 fraction of the conductive agent in the positive electrode active layer can be 0–10%, more further 0–8%, and even further 0–5%, based on the total mass of the positive electrode active layer.

[0475] In some embodiments, the positive electrode sheet can be prepared by dispersing the components used to prepare the positive electrode sheet, such as the positive active material, conductive agent, binder, and any other components, in a solvent to form a positive electrode slurry. Further, the positive electrode slurry is coated onto at least one surface of the positive current collector, and after drying, cold pressing, and other processes, the positive electrode sheet is obtained. Cold pressing can be performed using a cold rolling mill. The solvent in the positive electrode slurry can be, but is not limited to, any of the solvents described in the foregoing embodiments, for example, it can include N-methylpyrrolidone (NMP), and more specifically, NMP. The surface of the positive current collector coated with the positive electrode slurry can be a single surface of the positive current collector or both surfaces of the positive current collector. The solid content of the positive electrode slurry can be 40 wt% to 80 wt%. The viscosity of the positive electrode slurry at room temperature can be adjusted to 5000 mPa·s to 25000 mPa·s.

[0476] In some embodiments, the positive electrode active material includes a lithium phosphate-containing active material. Further, the mass percentage of the lithium phosphate-containing active material in the positive electrode active material can be greater than or equal to 50%, but is not limited thereto. When coating the positive electrode slurry, the coating areal density (based on dry weight, minus solvent) (based on the coating areal density of both sides) can also be (0.1–0.6) g / 1540.25 mm². 2 However, this is not the only possibility. The compaction density of the positive electrode sheet can be 1.9 g / cm³. 3 ~3.0g / cm 3 .

[0477] In some embodiments, the positive electrode active material includes a lithium composite metal oxide active material. Further, the mass percentage of the lithium composite metal oxide active material in the positive electrode active material can be greater than or equal to 50%, but is not limited thereto. When coating the positive electrode slurry, the coating surface density, based on dry weight (excluding solvent), can be 15 mg / cm³. 2 ~35mg / cm 2 The compaction density of the positive electrode sheet can be measured by the surface density of the coating on both sides. 3 ~3.6g / cm 3 3.3g / cm³ is an option. 3 ~3.5g / cm 3 .

[0478] In this 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.

[0479] The following are some other descriptions of the negative electrode plate.

[0480] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, wherein the negative active material layer includes a negative active material.

[0481] In this 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.

[0482] In some embodiments, the negative electrode active material layer includes a second negative electrode active layer, the definition of which can be found in the context of this application.

[0483] In some embodiments, at least one side of the negative electrode active material layer includes a first negative electrode active layer and a second negative electrode active layer, with the second negative electrode active layer located between the first negative electrode active layer and the negative electrode current collector.

[0484] The first negative electrode active layer includes a first negative electrode active material. The second negative electrode active layer includes a second negative electrode active material. The first and second negative electrode active materials may be the same or different, as described in the context.

[0485] The definitions of the negative electrode sheet, negative electrode active material layer, first negative electrode active layer and second negative electrode active layer, negative electrode active material, first negative electrode active material and second negative electrode active material in some embodiments can be found in the context of this application.

[0486] As a non-limiting example, the negative electrode sheet includes a negative current collector and a second negative active layer and a first negative active layer disposed sequentially on at least one side of the negative current collector, the second negative active layer being located between the negative current collector and the first negative active layer.

[0487] 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 may further be greater than or equal to 90%.

[0488] Without limitation, the mass fraction of the first negative electrode active material in the first negative electrode active layer may be greater than or equal to 80%, and may further be greater than or equal to 90%.

[0489] Without limitation, the mass fraction of the second negative electrode active material in the second negative electrode active layer may be greater than or equal to 80%, and more particularly greater than or equal to 90%.

[0490] As a non-limiting example, the negative electrode current collector has two surfaces that are opposite to each other in its own thickness direction, and the negative electrode active material layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0491] Figure 1 is a schematic diagram of the negative electrode sheet structure in one embodiment of this application. The negative electrode sheet 200 includes a negative current collector 210 and a second negative active layer 222 and a first negative active layer 224 sequentially disposed on one side of the negative current collector 210. The second negative active layer 222 includes negative active particles 202 and a quaternary ammonium salt compound 208. In this case, the second negative active material in the second negative active layer includes the negative active particles 202. The negative active material layer includes both the first and second negative active layers. The negative active material layer may also be located on both sides of the negative current collector.

[0492] Figure 2 is a schematic diagram of the negative electrode sheet in another embodiment of this application. The negative electrode sheet 200 includes a negative current collector 210 and a second negative active layer 222 and a first negative active layer 224 sequentially disposed on both sides of the negative current collector 210. The second negative active layer 222 includes negative active particles 202 and a quaternary ammonium salt compound 208. In this case, the second negative active material in the second negative active layer includes the negative active particles 202. In this case, the negative active material layer includes the first negative active layer and the second negative active layer. The negative active material layer may also be located on both sides of the negative current collector.

[0493] Figure 3 is a schematic diagram of the negative electrode sheet in one embodiment of this application. The negative electrode sheet 200 includes a negative current collector 210 and a negative active material layer 220 located on one side of the negative current collector 210. The negative active material layer 220 includes negative active particles 202 and quaternary ammonium salt compounds 208. In this case, the negative active material in the negative active material layer includes negative active particles 202. The negative active material layer may also be located on both sides of the negative current collector.

[0494] In this application, unless otherwise specified, negative electrode active particles are particulate matter in negative electrode active materials.

[0495] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. In the negative electrode current collector, the composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. In the negative electrode current collector, the composite current collector may be formed by forming a metal material on the polymer material substrate. Non-limiting examples of the metal material in the negative electrode current collector may include one or more of copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. Non-limitingly, in the negative electrode current collector, the polymer material substrate may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0496] In addition to the types of negative electrode active materials mentioned in the context and examples, the negative electrode active materials in the negative electrode active material layer, the first negative electrode active layer, and the second negative electrode active layer may also independently include other types of negative electrode active materials known in the art and suitable for lithium-ion secondary batteries. 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, these negative electrode active materials 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.

[0497] In some embodiments, the negative electrode active material layer may optionally include a binder.

[0498] In some embodiments, the first negative electrode active layer and the second negative electrode active layer may each optionally include a binder independently.

[0499] Non-limitingly, 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 types of binders in the first negative electrode active layer and the second negative electrode active layer may be the same or different.

[0500] Non-limitingly, the mass percentage of the binder in the negative electrode active material layer can be 0% to 20%, further can be 0% to 10%, even further can be 0% to 5%, even further can be 1% to 5%, and even further can be 1% to 3%.

[0501] Non-limiting, the mass fraction of the binder in the first negative electrode active layer can be 0% to 20%, further can be 0% to 10%, even further can be 0% to 5%, even further can be 1% to 5%, and even further can be 1% to 3%.

[0502] Non-limitingly, the mass fraction of the binder in the second negative electrode active layer can be 0% to 20%, further can be 0% to 10%, even further can be 0% to 5%, even further can be 1% to 5%, and even more preferably 1% to 3%.

[0503] The definition of the adhesive in some implementations can be found in the context of this application.

[0504] In some embodiments, the negative electrode active material layer may optionally include a conductive agent.

[0505] In some embodiments, the first negative electrode active layer and the second negative electrode active layer may each optionally include a conductive agent independently.

[0506] In a non-limiting sense, the conductive agent in the negative electrode 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.

[0507] Non-limitingly, the mass percentage of the conductive agent in the negative electrode active material layer can be 0% to 15%, more preferably 0% to 10%, and even more preferably 0% to 5%.

[0508] Non-limitingly, the mass fraction of the conductive agent in the first negative electrode active layer can be 0% to 15%, more preferably 0% to 10%, and even more preferably 0% to 5%.

[0509] Non-limitingly, the mass fraction of the conductive agent in the second negative electrode active layer can be 0% to 15%, more preferably 0% to 10%, and even more preferably 0% to 5%.

[0510] In some embodiments, the negative electrode active material layer may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)). Non-limitingly, the mass percentage of these other additives in the negative electrode active material layer may be 0% to 15%, more preferably 0% to 10%, even more preferably 0% to 5%, even more preferably 0% to 3%, and even more preferably 0% to 2%.

[0511] In some embodiments, the first and second negative electrode active layers may each optionally and independently include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)). Non-limitingly, the mass fraction of these other additives in the first or second negative electrode active layer may independently be 0% to 15%, more preferably 0% to 10%, even more preferably 0% to 5%, even more preferably 0% to 3%, and even more preferably 0% to 2%.

[0512] In some embodiments, a method comprising the following steps is used to prepare a negative electrode sheet, which can be used to prepare a negative electrode sheet in which the negative electrode active material layer has a single-layer structure:

[0513] S110: Preparation of negative electrode slurry. The components used to prepare the negative electrode active material layer, such as negative electrode active material, quaternary ammonium salt compound, conductive agent, binder and any other components, are dispersed in a solvent (a non-limiting example of a solvent is deionized water) to form a negative electrode slurry.

[0514] S120: Preparation of the negative electrode sheet. A negative electrode slurry is coated onto at least one surface of the negative electrode current collector. After drying and cold pressing, a negative electrode active material layer is formed, obtaining the 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 a single surface or both surfaces of the negative electrode current collector. The solid content of the negative electrode slurry can be 30wt%–70wt%, optionally 40wt%–60wt%. The viscosity of the negative electrode slurry at room temperature can be adjusted to 2000mPa·s–10000mPa·s, optionally 3000mPa·s–10000mPa·s.

[0515] In some embodiments, a method comprising the following steps is used to prepare the negative electrode sheet: S100' and S200'; this method can be used to prepare a negative electrode sheet in which the negative electrode active material layer includes a first negative electrode active layer and a second negative electrode active layer.

[0516] S100': Preparation of a first negative electrode slurry and a second negative electrode slurry. The components described above for preparing the first negative electrode active layer, such as a first negative electrode active material, a conductive agent, a binder, and any other components, are dispersed in a solvent (a non-limiting example of a solvent is deionized water) to form a first negative electrode slurry. The components described above for preparing the second negative electrode active layer, such as a second negative electrode active material, a quaternary ammonium salt compound, a conductive agent, a binder, and any other components, are dispersed in a solvent (a non-limiting example of a solvent is deionized water) to form a second negative electrode slurry.

[0517] S200': Preparation of the negative electrode sheet. A double-layer coating machine is used to coat a second negative electrode slurry and a first negative electrode slurry onto at least one surface of the negative electrode current collector. The second negative electrode slurry is coated first, followed by the first negative electrode slurry. After drying and cold pressing, a second negative electrode active layer and a first negative electrode active layer are formed accordingly. The second negative electrode active layer is located between the first negative electrode active layer and the negative electrode current collector. The non-solvent components of the first negative electrode slurry form the first negative electrode active layer, and the non-solvent components of the second negative electrode slurry form the second negative electrode active layer. Cold pressing can be performed using a cold rolling mill. The surfaces of the negative electrode current collector coated with the first and second negative electrode slurries can be a single surface or both surfaces of the negative electrode current collector. The solid content of the first and second negative electrode slurries can each be independently 30wt% to 70wt%, or independently selectable as 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.

[0518] The coating density and compaction density of the negative electrode sheet can be found in the context of this application.

[0519] The electrolyte is described below as an example.

[0520] The electrolyte serves to conduct ions between the positive and negative electrodes. A suitable electrolyte can be selected based on specific requirements.

[0521] Electrolytes consist of electrolyte salts and solvents. Solvents include non-aqueous solvents.

[0522] In some embodiments, the electrolyte is a non-aqueous electrolyte. A non-aqueous electrolyte may include an electrolyte salt and a solvent.

[0523] The concentration of electrolyte salts in the electrolyte solution is typically 0.5 mol / L to 5 mol / L.

[0524] In some embodiments, the electrolyte salt includes an electrolyte lithium salt. The definition of the electrolyte salt in some embodiments can be found in the context of this application.

[0525] In some embodiments, the solvent in the non-aqueous electrolyte may include ethylene carbonate (EC). ), propylene carbonate (PC, propylene carbonate) ), 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 the following: fluoroethylene carbonate (FEC), methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0526] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0527] 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), trifluoromethyl ethylene carbonate (TFPC), etc.

[0528] The following is an exemplary description of the separator membrane.

[0529] The definition of the isolation membrane in some implementations can be found in the context of this application.

[0530] This application does not impose any particular restrictions on the type of diaphragm; any well-known porous diaphragm with good chemical and mechanical stability can be selected.

[0531] In some embodiments, the diaphragm material may include one or more of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The diaphragm may be a single-layer film or a multi-layer composite film, without particular limitation. When the diaphragm is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0532] In some embodiments, the thickness of the diaphragm is 4 μm to 40 μm, and optionally 7 μm to 15 μm.

[0533] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0534] In some embodiments, the lithium-ion secondary battery may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0535] In some embodiments, the outer packaging of the lithium-ion secondary battery can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the lithium-ion secondary battery can also be a soft pack, such as a pouch. The soft pack can be made of plastic; further, non-limiting examples of plastic may include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0536] A lithium-ion secondary battery includes at least one battery cell. A lithium-ion secondary battery may include one or more battery cells.

[0537] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 4 shows a square battery cell 5 as an example.

[0538] In some embodiments, referring to FIG5, the outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. The electrode assembly 52 is immersed in an electrolyte. The number of electrode assemblies 52 contained in the battery cell 5 may be one or more, which can be selected by those skilled in the art according to actual needs.

[0539] The lithium-ion secondary battery can be a battery device 4 or a battery pack 1.

[0540] The battery device includes at least one battery cell. The number of battery cells in the battery device can be one or more, and those skilled in the art can select an appropriate number according to the application and capacity of the battery device.

[0541] Figure 6 shows a battery device 4 as an example. Referring to Figure 6, in the battery device 4, multiple battery cells 5 can be arranged sequentially along the length of the battery device 4. Of course, they can also be arranged in any other arbitrary way. Furthermore, the multiple battery cells 5 can be fixed in place by fasteners.

[0542] Optionally, the battery device 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0543] In some embodiments, the battery devices described above can also be assembled into a battery pack, and the number of battery devices contained in the battery pack can be one or more. Those skilled in the art can select an appropriate number according to the application and capacity of the battery pack.

[0544] Figures 7 and 8 show a battery pack 1 as an example. Referring to Figures 7 and 8, the battery pack 1 may include a battery compartment and multiple battery devices 4 disposed within the battery compartment. The battery compartment includes an upper compartment 2 and a lower compartment 3, the upper compartment 2 covering the lower compartment 3 to form a closed space for accommodating the battery devices 4. The multiple battery devices 4 can be arranged in any manner within the battery compartment.

[0545] In another aspect of this application, a negative electrode sheet is provided, which may have the same features as the negative electrode sheet described in the first aspect of this application, or may be in the state after cold pressing and before immersion in electrolyte corresponding to the negative electrode sheet described in the first aspect of this application.

[0546] In some embodiments, a negative electrode sheet is provided, which includes a negative current collector and a negative active material layer located on at least one side of the negative current collector. The negative active material layer includes a negative active material and a quaternary ammonium salt compound, wherein the quaternary ammonium salt compound includes a quaternary ammonium cation.

[0547] In some embodiments, a negative electrode sheet is provided, which includes a negative electrode current 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 current collector, wherein the second negative electrode active layer is located between the negative electrode current collector and the first negative electrode active layer.

[0548] The second negative electrode active layer includes a second negative electrode active material and a quaternary ammonium salt compound.

[0549] Introducing a quaternary ammonium salt compound into the second negative electrode active layer significantly improves its electrolyte wettability. The effect of introducing the quaternary ammonium salt compound on the electrolyte wettability of the second negative electrode active layer can be compared by observing changes in the liquid absorption rate. A higher liquid absorption rate indicates better electrolyte wettability. The following methods can be used for testing and analysis.

[0550] Based on the negative electrode sheet obtained by cold pressing or dismantling of a battery cell, the second negative electrode material can be extracted from the second negative electrode active layer using the method of the first aspect of the application. This material is then resuspended in deionized water to form a uniform slurry, denoted as the second resuspension slurry. The second resuspension slurry is coated onto one side of the copper foil of the negative electrode current collector, dried, and cold-pressed to obtain the second active layer electrode sheet. The non-solvent components in the second resuspension slurry are substantially the same as those in the second negative electrode slurry. Alternatively, the second negative electrode slurry used in preparing the negative electrode sheet can be used instead of the second resuspension slurry to prepare the second active layer electrode sheet.

[0551] The liquid absorption rate can be tested using the following method: fix the electrode to be tested on the sample stage, add electrolyte, and time with a stopwatch; record the weight increase and time; calculate the liquid absorption rate of the electrode by the change in weight over time. Non-limiting examples of electrolytes include the electrolyte formulation in Example 1. Electrolytes with the same composition as those used in lithium-ion secondary batteries can also be used to test the liquid absorption rate. Commercially available electrolytes such as electrolyte E30 can also be used. In some examples, the electrolyte composition consists of a solvent and 1 mol / L lithium hexafluorophosphate (LiPF6), with the solvent consisting of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1.

[0552] In some embodiments, a negative electrode sheet is provided, which includes a negative current collector and a negative active material layer located on at least one side of the negative current collector; the negative active material layer includes a negative active material and a quaternary ammonium salt compound, the negative active material includes a negative active body and a coating layer located on at least a portion of the surface of the negative active body, the coating layer includes one or more of soft carbon, hard carbon and amorphous carbon, and the quaternary ammonium salt compound includes quaternary ammonium cations.

[0553] By setting a coating layer on the surface of the negative electrode active material layer, and incorporating one or more of soft carbon, hard carbon, and amorphous carbon in the coating layer, the lithium-ion transport channels on the surface of the negative electrode active material can be optimized, promoting lithium-ion transport and improving battery kinetics and fast-charging performance. Furthermore, by incorporating quaternary ammonium salt compounds in the negative electrode active material layer, the electrostatic interaction based on quaternary ammonium ions can attract electrolyte anions in the electrolyte, promoting rapid dissociation of the electrolyte lithium salt in the electrolyte. The formed quaternary ammonium ion-electrolyte anion structure can also guide the rapid transport of lithium ions in the electrolyte to the surface of the negative electrode active material. Based on the aforementioned multiple effects, the fast-charging performance of the battery can be further improved.

[0554] In some embodiments, the negative electrode is the negative electrode as defined in the first aspect of this application.

[0555] In a second aspect of this application, an electrical device is provided, which includes the lithium-ion secondary battery described in the first aspect of this application.

[0556] Electrical devices that include the aforementioned lithium-ion secondary batteries may have the advantages of the aforementioned lithium-ion secondary batteries, including but not limited to improved fast charging performance.

[0557] Electrical devices including the aforementioned negative electrode may have the advantages of the aforementioned negative electrode, including but not limited to improved fast charging performance.

[0558] In some embodiments, the electrical device includes a lithium-ion secondary battery according to any of the embodiments provided in this application.

[0559] Lithium-ion secondary batteries can be used as a power source for electrical devices or as an energy storage unit for electrical devices. Electrical devices can include, but are not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc. Mobile devices can include, for example, mobile phones and laptops; electric vehicles can include, for example, pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, electric motorcycles, power tools, etc., but are not limited to these. This type of electrical device can also be applied to military equipment, aerospace, and other fields, as well as to energy storage power systems such as hydroelectric, thermal, wind, and solar power plants.

[0560] As an electrical device, lithium-ion rechargeable batteries can be selected according to its usage requirements.

[0561] Figure 9 shows an example of an electrical device 6. This electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the lithium-ion secondary battery for this electrical device, a battery device or battery pack can be used.

[0562] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a lithium-ion rechargeable battery as their power source.

[0563] In a third aspect of this application, the application of the lithium-ion secondary battery described in the first aspect of this application in supplying and / or storing electrical energy is provided;

[0564] The application includes the process of charging a lithium-ion secondary battery at a rate of 2C or higher, that is, the lithium-ion secondary battery can provide a charging rate of 2C or higher.

[0565] In this application, unless otherwise specified, the term "rate" for a battery has a well-known meaning in the art, referring to the current required to charge or discharge a battery to its rated capacity within a specified time, expressed in C. Unless otherwise specified, "specified time" is 1 hour (h), 1C means completing charge / discharge in 1 hour, and 1 / 3C means completing charge / discharge in 3 hours. The battery rate reflects the battery's charging and discharging capabilities at different currents. A higher rate means the battery can charge and discharge quickly in a short time. The higher the rate, the better the fast-charging performance.

[0566] In this application, unless otherwise specified, for the charging rate, "1C" refers to the current required to fully charge the battery from zero charge to full charge or from full charge to complete discharge within one hour. For the charging rate, when charging from zero charge at a 1C rate, the battery will be fully charged in one hour.

[0567] In some embodiments, the application includes a process of charging a lithium-ion secondary battery at at least one rate from 2C to 6C, that is, the lithium-ion secondary battery is capable of providing a charging rate from 2C to 6C.

[0568] In some embodiments, the application includes a process of charging a lithium-ion secondary battery at at least one rate from 2C to 4C.

[0569] In some embodiments, the application includes a process of charging a lithium-ion secondary battery at at least one rate from 4C to 6C.

[0570] In a non-limiting manner, the lithium-ion secondary battery can be charged at any of the following rates, or at a rate greater than or equal to any of the following rates, or at a rate selected from any two of the following rates: 2C, 3C, 4C, 5C, 6C, etc.

[0571] In some implementations, lithium-ion secondary batteries can provide a charging rate of 2C or higher.

[0572] In some implementations, lithium-ion secondary batteries can provide charging rates of 2C to 6C.

[0573] In this application, "able to provide a charging rate Cx" means that the battery cell can be charged under the condition of charging rate Cx; for example, it can be charged to 97% SOC, but it is not limited to this SOC state.

[0574] In some embodiments, the maximum charging rate of the lithium-ion secondary battery can be greater than or equal to 2C, and can be selected as 2C to 6C, or further selected as 2C to 4C or 4C to 6C.

[0575] In some implementations, the maximum charging rate of the lithium-ion secondary battery can be greater than 2C (C max >2C), can be selected as greater than 2C and less than or equal to 6C (2 <C max ≤6C), further optionally greater than 2C and less than or equal to 4C (2 <C max ≤4C) or optionally greater than or equal to 4C and less than or equal to 6C (4C≤C) max ≤6C).

[0576] Without limitation, the maximum charging rate of the lithium-ion secondary battery may also be any of the following charging rates or a range selected from any two of the following charging rates: 2C, 3C, 4C, 5C, 6C, etc.

[0577] In this application, the "maximum charging rate (which can be denoted as C)" of the lithium-ion secondary battery is defined as... max The term "lithium plating" has a well-known meaning within the industry and can be obtained through testing using conventional methods within the field. For example, tests can be conducted at different charging rates to obtain a lithium plating window curve, and the critical charging rate at which lithium plating occurs can be used as the test value of the battery's maximum charging rate. Test parameters can be as follows: The battery under test is charged at a constant current to 4.4V at different rates (e.g., 1C, 2C, 2.5C, 3C, 3.5C, 4C, ...), then charged at a constant voltage until the current is ≤0.05C, left to stand for 5 minutes, and then charged at a constant current of 0.33C to 4.4V, left to stand for 5 minutes, and then disassembled to observe the lithium plating at the negative electrode. For example, a series of parallel samples can be prepared, starting from 1C and tested at 0.1C intervals until lithium plating appears at the negative electrode. To reduce the sample size, a larger interval can be selected first to determine the range of the maximum charging rate, and then a smaller interval can be selected to more accurately determine the maximum charging rate. The intervals can be 1C, 0.5C, 0.2C, and 0.1C, respectively.

[0578] The following describes some embodiments of this application. The described embodiments are only a part of the embodiments of this application, and not all of them. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application and its applications. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0579] Unless otherwise specified in the examples, the procedures described above, or those described in the literature in this field, or those described in the product instructions, shall be followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products, or products that can be synthesized using conventional methods from commercially available products.

[0580] In the specific embodiments described below, the measurement parameters involving raw material components may have slight deviations within the weighing accuracy range unless otherwise specified. Temperature and time parameters are subject to acceptable deviations due to instrument testing accuracy or operational precision.

[0581] In the following examples, room temperature refers to 20°C to 30°C.

[0582] The glass transition temperature of the styrene-butadiene rubber used in the following examples is approximately 40°C, within the range of 40°C to 50°C.

[0583] 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 particulate graphite in the first negative electrode active material, and the percentage of secondary particulate graphite in the negative electrode active material, can be confirmed by SEM (Sigma 300 scanning electron microscope from ZEISS, Germany) test results; the D of the first and second negative electrode active materials... vThe OI value of 50 can be tested using a Malvern 2000 (MasterSizer 2000) laser particle size analyzer; the OI values ​​of the first and second negative electrode active materials can be obtained using a Bruker-D8 advance X-ray diffractometer (XRD). The carbon coating layer can be tested using a JEM-F200 transmission electron microscope combined with an EDS (Energy Dispersive Spectrometer). Examples include the compaction density of the negative electrode sheet, a comparison of the compaction densities of the first and second negative electrode active layers, a comparison of the areal density of the first and second negative electrode active layers (based on one side of the negative electrode current collector), a comparison of the porosity of the first and second negative electrode active layers, the porosity of the negative electrode active material layer, the powder compaction density of the second and first negative electrode active materials and the ratio of the two powder compaction densities, the powder compaction density of the second and first negative electrode active layers and the ratio of the two powder compaction densities, a comparison of the charging rates of the first and second negative electrode active layers, and a comparison of the charging rates of the first and second negative electrode active layers. Another example involves the testing of the ionic conductivity of the electrolyte, using a DDSJ-318 conductivity meter, referring to the testing method in HG-T 4067-2015.

[0584] In the following examples, unless otherwise specified, the alkyl chain is a linear chain. For example, the octadecyl group in octadecyltrimethyl quaternary aminophosphate is n-octadecyl.

[0585] Example 1.

[0586] 1. Preparation of the positive electrode sheet:

[0587] The positive electrode sheet is composed of powder containing lithium iron phosphate-based positive electrode active material and a positive electrode current collector.

[0588] The positive electrode active material LFP (lithium iron phosphate, surface-coated with soft carbon), conductive agent carbon black (Super P), and binder polyvinylidene fluoride (PVDF) were mixed evenly in an appropriate amount of solvent N-methylpyrrolidone (NMP) at a mass ratio of 97.9:0.3:1.8 to obtain a positive electrode slurry with a solid content of 60wt%. The coating weight (both sides) was 0.360g / 1540.25mm. 2 (approximately 23.4 mg / cm²) 2 The positive electrode slurry is coated onto both sides of the positive electrode current collector aluminum foil. Through processes such as drying, cold pressing, slitting, and cutting, the positive electrode sheet is obtained. The compacted density of the positive electrode sheet is 2.60 g / cm³. 3 .

[0589] The lithium iron phosphate-based cathode active material is lithium iron phosphate coated with soft carbon, and the mass percentage of soft carbon in the cathode active material is about 1%.

[0590] 2. Preparation of the negative electrode sheet:

[0591] The second negative electrode active material, quaternary ammonium salt compound, conductive carbon black (Super P), stabilizer sodium carboxymethyl cellulose (CMC-Na), and binder styrene-butadiene rubber (SBR) were dispersed in deionized water at a mass ratio of 96.0:0.5:0.4:1.1:2.0 to form the second negative electrode slurry (corresponding to the lower second negative electrode active layer), with a solid content of 50 wt%.

[0592] The first negative electrode active material, conductive agent conductive carbon black (Super P), stabilizer sodium carboxymethyl cellulose and binder SBR are dispersed in deionized water at a mass ratio of 97.3:0.7:1.2:0.8 to form the first negative electrode slurry (corresponding to the upper first negative electrode active layer), which does not include quaternary ammonium salt compounds and has a solid content of 50 wt%.

[0593] The second negative electrode slurry and the first negative electrode slurry were uniformly coated on both sides of the copper foil of the negative electrode current collector using an extrusion coating machine (the coating parameters on both sides of the negative electrode current collector were basically the same). The second negative electrode slurry (corresponding to the second negative electrode active layer) was coated first, followed by the first negative electrode slurry (corresponding to the first negative electrode active layer). The coating weights of the upper and lower layers (on a single side) were controlled to be 0.08g / 1540.25mm. 2 and 0.08g / 1540.25mm 2 After being dried in an oven, the negative electrode sheet is compacted using a cold press to control its compaction density at 1.65 g / cm³. 3 The cold-pressed electrode sheet is then slitting and cutting to obtain the negative electrode sheet. Based on one side of the negative current collector, the areal density of the negative electrode sheet is approximately 10.4 mg / cm³. 2 .

[0594] First negative electrode active layer (upper layer): The first negative electrode active material is coated graphite, further comprising artificial graphite coated with soft carbon (the mass percentage of soft carbon in the coating layer in the first negative electrode active material is approximately 2%). v The value of 50 is 14 μm. The first negative electrode active material includes secondary particle graphite, and the secondary particle graphite accounts for about 50% of the first negative electrode active material. The OI value of the first negative electrode active material is 5.

[0595] Second negative electrode active layer (lower layer): The second negative electrode active material is artificial graphite (without a coating layer), D v The 50 is 18μm, and the second negative electrode active material includes secondary particle graphite, which accounts for about 100% of the total amount in the second negative electrode active material; the OI value of the second negative electrode active material is 12.

[0596] The thickness ratio (f) of the first negative electrode active layer (upper layer) to the second negative electrode active layer (lower layer) in the negative electrode sheet. H If f is greater than 1, H Within the range of 1.1 to 1.3, f H The thickness is approximately 1.2, and the thickness of one side of the first negative electrode active layer is in the range of 30 μm to 40 μm. At this time, the compaction density of the second negative electrode active layer is higher than that of the first negative electrode active layer.

[0597] 3. Separating membrane: A 12μm thick polypropylene membrane is used as the separating membrane.

[0598] 4. Electrolyte preparation: Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a mass ratio of 30:70 to obtain an organic solvent. Thoroughly dried LiPF6 was dissolved in this organic solvent, and vinylene carbonate (VC), fluoroethylene carbonate (FEC), and ethylene sulfate (DTD) were added to prepare an electrolyte with a LiPF6 concentration of 1 mol / L. The electrolyte contained 2.5 wt% VC, 1 wt% FEC, and 0.5 wt% DTD.

[0599] The electrolyte has an ionic conductivity of 15 mS / cm at 25°C.

[0600] 5. Preparation of secondary battery: The positive electrode, separator and negative electrode are stacked and wound in sequence to obtain the electrode assembly; the electrode assembly is placed in the outer packaging, dried and injected with electrolyte, and then subjected to vacuum sealing, standing, formation and shaping processes to obtain the lithium-ion secondary battery.

[0601] Examples 2-4. Negative electrode sheets and lithium-ion secondary batteries were prepared using essentially the same method as in Example 1, except that the mass percentage of the quaternary ammonium salt compound in the second negative electrode active layer was changed during the negative electrode sheet preparation step, resulting in the use of different negative electrode sheets to prepare lithium-ion secondary batteries. See Table 1 for details.

[0602] Examples 5-8. Negative electrode sheets and lithium-ion secondary batteries were prepared using essentially the same method as in Example 1, except that the type of quaternary ammonium salt compound was changed in the negative electrode sheet preparation step, and different negative electrode sheets were used to prepare lithium-ion secondary batteries. See Table 1. The quaternary ammonium salt compounds in Examples 5-8 were octadecyltrimethyl quaternary ammonium phosphate, dodecyltrimethyl quaternary ammonium nitrate, dodecyltrimethyl quaternary ammonium carbonate, and dodecyltrimethyl quaternary ammonium bicarbonate, respectively.

[0603] Examples 9-10. Negative electrode sheets and lithium-ion secondary batteries were prepared using essentially the same method as in Example 1, except that the composition of the electrolyte was changed in the electrolyte preparation step, and different electrolytes were used to prepare the lithium-ion secondary batteries. See Table 1 for details.

[0604] Example 9 uses lithium hexafluorophosphate (LiPF6) and lithium bisfluorosulfonyl imide (LiFSI) in a molar ratio of 1:0.1, with molar volume concentrations of 1 mol / L and 0.1 mol / L in the electrolyte, respectively; the solvent type, additive type and amount are the same as in Example 1.

[0605] Example 10 changed the solvent to ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:2:1; the type and amount of electrolyte lithium salt, and the type and amount of additives were the same as in Example 1.

[0606] The electrolytes of Examples 9-10 all had ionic conductivity in the range of 16 mS / cm to 18 mS / cm at 25°C.

[0607] Examples 11-12. A negative electrode sheet and a lithium-ion secondary battery were prepared using essentially the same method as in Example 1, except that the thickness ratio of the first negative electrode active layer to the second negative electrode active layer was changed in the negative electrode sheet preparation step; that is, the thickness ratio of the first negative electrode active layer to the second negative electrode active layer (f) was changed. H The sum of the coating surface densities on both sides of the negative electrode sheet is basically the same as in Example 1; lithium-ion secondary batteries are prepared using different negative electrode sheets. See Table 1 for details.

[0608] Examples 13-14. Negative electrode sheets and lithium-ion secondary batteries were prepared using essentially the same method as in Example 1, except that: in the step of preparing the negative electrode sheet, the type of the second negative electrode active material was changed, and the coating weight of the first and second negative electrode slurries was adjusted to change f. H The sum of the coating surface densities on both sides of the negative electrode sheet is basically the same as in Example 1; lithium-ion secondary batteries are prepared using different negative electrode sheets. See Table 1 for details.

[0609] In Example 13, the second negative electrode active material is a combination of artificial graphite and soft carbon with a mass ratio of 1:5%, maintaining the same mass ratio of the second negative electrode active material in the second negative electrode active layer as in Example 1.

[0610] In Example 14, the second negative electrode active material is a combination of artificial graphite and hard carbon with a mass ratio of 1:5%, maintaining the same mass ratio of the second negative electrode active material in the second negative electrode active layer as in Example 1.

[0611] Example 15. A negative electrode sheet and a lithium-ion secondary battery were prepared using a method essentially the same as in Example 1, except that: in the step of preparing the negative electrode sheet, the D of the first negative electrode active material... v 50 is 12μm, the D of the second negative electrode active material v50 represents 20 μm; the coating surface density ratio was adjusted so that the thickness ratio of the first negative electrode active layer to the second negative electrode active layer was basically the same as in Example 1; lithium-ion secondary batteries were prepared using different negative electrode sheets. See Table 1 for details.

[0612] Example 16. A negative electrode and a lithium-ion secondary battery were prepared using a method essentially the same as in Example 1, except that the type of positive electrode active material was changed to ternary positive electrode material NCM811 (LiNi) in the step of preparing the positive electrode. 0.8 Co 0.1 Mn 0.1 (O2), lithium-ion secondary batteries were prepared using different positive electrode sheets. See Table 1 for details.

[0613] Example 17. A negative electrode sheet was prepared using essentially the same method as in Example 1, except that the negative electrode active material layer of the negative electrode sheet adopted a single-layer structure. A lithium-ion secondary battery was prepared using different negative electrode sheets. In this example, the negative electrode sheet was prepared using the following method:

[0614] The negative electrode active material, quaternary ammonium salt compound, conductive carbon black (Super P), stabilizer sodium carboxymethyl cellulose (CMC-Na), and binder styrene-butadiene rubber (SBR) were dispersed in deionized water at a mass ratio of 96.0:0.5:0.4:1.1:2.0 to form a negative electrode slurry with a solid content of 50 wt%.

[0615] The negative electrode slurry was uniformly coated onto both sides of the copper foil of the negative electrode current collector using an extrusion coating machine (the coating parameters on both sides of the negative electrode current collector were basically the same). The coating weight on one side of the negative electrode current collector was 0.16g / 1540.25mm. 2 After being dried in an oven, the negative electrode sheet is compacted using a cold press to control its compaction density to 1.65 g / cm³. 3 The cold-pressed electrode sheet is then slitting and cutting to obtain the negative electrode sheet. Based on one side of the negative current collector, the areal density of the negative electrode sheet is approximately 10.4 mg / cm³. 2 .

[0616] The negative electrode active material is artificial graphite coated with soft carbon (the mass percentage of soft carbon in the coating layer in the first negative electrode active material is 2%), D v The value of 50 is 14 μm. The first negative electrode active material includes secondary particle graphite, and the secondary particle graphite accounts for about 50% of the first negative electrode active material. The OI value of the first negative electrode active material is 5.

[0617] Comparative Example 1. No quaternary ammonium salt compound is set in the second negative electrode active layer.

[0618] The negative electrode sheet and lithium-ion secondary battery were prepared using essentially the same method as in Example 1, except that: in the step of preparing the negative electrode sheet, the quaternary ammonium salt compound was omitted from the second negative electrode slurry; the mass ratio of the second negative electrode active material, conductive agent, stabilizer, and binder was the same as in Example 1; and the remaining operating steps were the same as in Example 1. Different negative electrode sheets were used to prepare the lithium-ion secondary battery. The sum of the surface densities of the coatings on both sides of the negative electrode sheet was the same as in Example 1.

[0619] Comparative Example 2. No quaternary ammonium salt compounds were used in the second negative electrode active layer.

[0620] The negative electrode sheet and lithium-ion secondary battery were prepared using essentially the same method as in Example 1, except that the second negative electrode slurry was replaced with the first negative electrode slurry in the negative electrode sheet preparation step (i.e., the second negative electrode active layer did not contain a quaternary ammonium salt compound), while the remaining operation steps were the same as in Example 1; different negative electrode sheets were used to prepare the lithium-ion secondary battery. The sum of the surface densities of the coatings on both sides of the negative electrode sheet was the same as in Example 1.

[0621] Comparative Example 3. No quaternary ammonium salt compounds were used in the second negative electrode active layer.

[0622] The negative electrode sheet and lithium-ion secondary battery were prepared using essentially the same method as in Example 16, except that: in the step of preparing the negative electrode sheet, the quaternary ammonium salt compound was omitted from the second negative electrode slurry; the mass ratio of the second negative electrode active material, conductive agent, stabilizer, and binder was the same as in Example 16; and the remaining operating steps were the same as in Example 16. Different negative electrode sheets were used to prepare the lithium-ion secondary battery. The sum of the surface densities of the coatings on both sides of the negative electrode sheet was the same as in Example 16.

[0623] Comparative Example 4. No quaternary ammonium salt compounds were used in the second negative electrode active layer.

[0624] The negative electrode sheet and lithium-ion secondary battery were prepared using essentially the same method as in Example 16, except that the second negative electrode slurry was replaced with the first negative electrode slurry in the negative electrode sheet preparation step (i.e., the second negative electrode active layer did not contain a quaternary ammonium salt compound), while the remaining operation steps were the same as in Example 16; different negative electrode sheets were used to prepare the lithium-ion secondary battery. The sum of the surface densities of the coatings on both sides of the negative electrode sheet was the same as in Example 16.

[0625] Comparative Example 5. A negative electrode sheet and a lithium-ion secondary battery were prepared using essentially the same method as in Example 17, except that: in the step of preparing the negative electrode sheet, the quaternary ammonium salt compound was omitted from the negative electrode slurry; the mass ratio of the negative electrode active material, conductive agent, stabilizer, and binder was the same as in Example 17; and the remaining operational steps were the same as in Example 17; a different negative electrode sheet was used to prepare the lithium-ion secondary battery. The sum of the surface densities of the coatings on both sides of the negative electrode sheet was the same as in Example 17.

[0626] In Examples 1-16, the porosity of the first negative electrode active layer is higher than that of the second negative electrode active layer. The porosity of the negative electrode sheets in Examples 1-17 is in the range of 15% to 35%. Taking Examples 1 and 17 as examples, the porosity of the negative electrode sheet in Example 1 is approximately 25%, and the porosity of the negative electrode sheet in Example 2 is approximately 22%.

[0627] In Examples 1-16, the powder compaction density of the second negative electrode active material was higher than that of the first negative electrode active material, and the powder compaction density of the second negative electrode active material was all around 1.85 g / cm³. 3 ~2.05g / cm 3 Within this range, the ratio of the compacted powder density of the second negative electrode active material to that of the first negative electrode active material is consistently between 1.05 and 1.35. Taking Example 1 as an example, the compacted powder density of the second negative electrode active material is approximately 1.9 g / cm³. 3 Within the specified range, the compacted density of the first negative electrode active material powder is approximately 1.7 g / cm³. 3 Within the range.

[0628] Table 1.

[0629] In Table 1, "same as Example 1" under "Types of Quaternary Ammonium Salt Compounds" indicates dodecyltrimethylquaternary ammonium phosphate.

[0630] Test Methods and Analysis

[0631] (I) Negative Electrode Performance Testing

[0632] Liquid absorption rate test

[0633] Disassemble the battery cell, remove the negative electrode sheet, and soak and clean it with dimethyl carbonate (DMC). Scrape powder from the second negative electrode active layer near the negative electrode current collector to obtain a powder sample of the second negative electrode material. Resuspend the second negative electrode material in deionized water to form a uniform slurry, denoted as the second resuspension slurry. Coat the second resuspension slurry onto one side of the copper foil of the negative electrode current collector, dry, and cold press to obtain the second active layer electrode sheet. The liquid absorption rate is tested using the following method: fix the electrode sheet to be tested on the sample stage, add electrolyte E30 (commercially available), and time with a stopwatch; record the weight increase and time; calculate the liquid absorption rate of the electrode sheet by the change in weight over time. The measured liquid absorption rate can be recorded as the "liquid absorption rate of the second negative electrode active layer".

[0634] (II) Battery Performance Testing

[0635] 1. Battery energy density

[0636] At 25℃, the battery under test was charged to 3.8V at a constant current and constant voltage of 0.33C, with a cutoff current of 0.05C. After standing for 30 minutes, it was discharged to the designed 2.0V at 0.33C, and the discharge energy P was recorded. X (Wh), the cell volume is recorded as V. X (Unit: Liters (L)), then the energy density of the battery (Wh / L) = P X / V X .

[0637] 2. Fast charging capability test

[0638] The lithium-ion secondary battery under test was placed at room temperature (25℃) and charged to 3.8V using a constant current rate of 0.33C. Then, it was charged to 0.05C using a constant voltage rate, allowed to rest for 5 minutes, and then discharged to 2.0V using a constant current rate of 0.33C. The constant current discharge capacity was recorded as the initial capacity C0. The battery was then sequentially charged to 3.8V (full cell potential) or 0mV (negative electrode cutoff potential) using constant current rates of 0.5C0, 1C0, 1.5C0, 2C0, 2.0C0, 3C0, and 3.5C0 (either condition indicates completion of charging). After each charge, it was discharged to 2.0V using a constant current rate of 0.33C0. The negative electrode potential was recorded at 10% SOC intervals. Rate-negative electrode potential curves were plotted for different SOCs. Linear fitting was performed to obtain the charging rate corresponding to a negative electrode potential of 0mV at different SOCs, denoted as Cx (x = 2–8). The charging time t for a lithium-ion secondary battery to charge from 10% SOC to 80% SOC is calculated using the formula (1 / C2+1 / C3+1 / C4+1 / C5+1 / C6+1 / C7+1 / C8)×0.1×60. c (min).

[0639] Test results can be found in "Charging time t from 10% SOC to 80% SOC". c ".

[0640] The shorter the time t0, the better the battery's fast charging performance.

[0641] SOC (State of Charge) indicates the state of charge. When "SOC = 0", it means that the battery is fully discharged, and when "SOC = 100%", it means that the battery is fully charged.

[0642] 3. Fast charging cycle performance test

[0643] The battery cell is placed in a three-piece steel plate clamp. The clamp is equipped with a pressure sensor, and the initial clamping force is 3000N.

[0644] At 25℃, the battery under test was charged at a constant current of 0.33C to the charging cutoff voltage of 3.8V, then charged at a constant voltage to a current of 0.05C, allowed to stand for 5 minutes, and then discharged at a constant current of 0.33C to the discharge cutoff voltage of 2.0V. Its initial capacity was recorded as C0. The cell was then charged at 0.33C0 to 10% SOC. From 10% SOC to 80% SOC, the fast charging strategy described above for "Fast Charging Capability Test" was applied. Then, the cell was charged at 0.33C0 to the cutoff voltage of 4.25V, then charged at a constant voltage to a current of 0.05C, allowed to stand for 5 minutes, and then discharged at 0.33C0. The discharge capacity Cn of each cycle was recorded until 1000 cycles were completed. The cycle capacity retention rate (i.e., C1000 / C1×100%) was then calculated.

[0645] The test results can be found in "Capacity retention rate after 1000 fast charging cycles at 25℃".

[0646] A higher cycle retention rate indicates a better fast charging cycle life.

[0647] 4. Ratio performance

[0648] The first negative electrode slurry and the second negative electrode slurry, corresponding to the embodiments and comparative examples, were respectively coated onto one side of the copper foil current collector and dried in an oven for later use.

[0649] A lithium metal plate was used as the counter electrode, and a polypropylene (PP) membrane was used as the separator. Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain a mixed solvent. LiPF6 was then uniformly dissolved in the mixed solvent to obtain an electrolyte with a LiPF6 concentration of 1 mol / L.

[0650] All components were assembled into a CR2430 button cell in an argon-protected glove box. "CR" represents the international IEC designation for button-type lithium manganese batteries, which are 24 mm in diameter and 30 mm thick.

[0651] After resting for 12 hours, the obtained button cell battery was discharged at a constant current of 0.05C to 0.005V, and then discharged again at a constant current of 10μA to 0.005V. After resting for 5 minutes, the obtained button cell battery was charged at a constant current of 0.1C to 2V, and then rested for 5 minutes. The charging capacity C0 was recorded. The battery was placed at a constant temperature of 25℃ for 2 hours, and charge-discharge tests were performed at rates of 1C0, 2C0, 3C0, 4C0, and 5C0 to obtain the capacity retention rate. At the same rate, the higher the capacity retention rate, the better the rate performance.

[0652] 5. Maximum charging rate of lithium-ion secondary batteries

[0653] A series of parallel samples of the test batteries were tested at different charging rates to obtain lithium plating window curves. The critical charging rate at which lithium plating occurred was taken as the test value of the battery's maximum charging rate. The test parameters are as follows: The test battery was charged at a constant current to 3.8V at the target rate (e.g., 1C, 2C, 2.5C, 3C, 3.5C, 4C, ...), then charged at a constant voltage until the current ≤ 0.05C, allowed to stand for 5 minutes, and then charged at a constant current of 0.33C to 3.8V, allowed to stand for 5 minutes, and then disassembled to observe the lithium plating on the negative electrode. To reduce the sample size, a large interval of charging rate conditions was first selected to determine the range of the maximum charging rate, and then a small interval of charging rate conditions was selected to more accurately determine the maximum charging rate. The interval of the charging rate can be reduced sequentially according to the accuracy requirements. For example, one or more intervals of 1C, 0.5C, 0.2C, and 0.1C can be selected.

[0654] In the tests of Examples 1-17 and Comparative Examples 1-4, the selected magnification intervals were 1C, 0.5C, 0.2C, and 0.1C, respectively.

[0655] For some of the test results regarding battery performance, please refer to Table 2-3.

[0656] Test Results and Analysis

[0657] Based on the test results of the liquid absorption rate, after introducing a quaternary ammonium salt compound into the second negative electrode active layer, the liquid absorption rate of the second negative electrode active layer in Examples 1-17 was significantly improved compared with that in Comparative Examples 1-4, and the electrolyte wettability of the second negative electrode active layer was significantly improved.

[0658] The lithium-ion secondary batteries in Examples 1-17 all had quaternary ammonium salt compounds deposited in the second negative electrode active layer. The fast-charging performance of Examples 1-15 was significantly improved compared to Comparative Examples 1-2, Example 16 compared to Comparative Examples 3-4, and Example 17 compared to Comparative Example 5. The charging time t from 10% SOC to 80% SOC was significantly reduced. c It is significantly shortened.

[0659] The maximum charge rate of the lithium-ion secondary batteries in Examples 1-17 is all higher than 2C. Taking Example 1 as an example, the maximum charge rate of the lithium-ion secondary battery in Example 1 is 2.8C. The maximum charge rates of Examples 1-17 are in the range of 2.4C to 3.3C. The maximum charge rates of Comparative Examples 1-5 are in the range of 1.7C to 2.0C.

[0660] According to the "rate performance" test, the CR2430 coin cells assembled with the first negative electrode slurry in Examples 1-17 all exhibited higher rate performance than the CR2430 coin cells assembled with the first negative electrode slurry in Comparative Examples 1-4. The introduction of a quaternary ammonium salt compound into the second negative electrode active layer improved its rate performance.

[0661] Furthermore, in Examples 1-16, the rate of increase of the first negative electrode active layer is higher than that of the second negative electrode active layer.

[0662] The cycle performance of Examples 1-15 was improved compared to Comparative Examples 1-2, Example 16 compared to Comparative Examples 3-4, and Example 17 compared to Comparative Example 5.

[0663] The lithium-ion secondary batteries in Examples 1-17 also have high energy density; some test results can be found in Table 3.

[0664] Table 2.

[0665] Table 3.

[0666] The descriptions of the various implementation methods and embodiments above tend to emphasize the differences between them. Similarities or resemblances can be referenced interchangeably, and for the sake of brevity, they will not be repeated here. The technical features of the implementation methods and embodiments described above can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments have been described. However, as long as the combinations of these technical features do not contradict each other, they should be considered within the scope of this specification.

[0667] It should be noted that this application is not limited to the above-described embodiments and examples. The above-described embodiments and examples are merely examples, and any embodiments and examples that have the same structure and achieve the same effect as the technical concept within the scope of this application are included in the technical scope of this application. The embodiments and examples described above only illustrate several embodiments and examples of this application, and although the descriptions are relatively detailed, they should not be construed as limiting the scope of the patent. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments or examples, and other ways of constructing embodiments or examples by combining some of the constituent elements of the embodiments or examples, are also included in the scope of this application without departing from the spirit of this application.

Claims

1. A lithium-ion secondary battery, comprising a negative electrode sheet and an electrolyte; the negative electrode sheet comprising a negative current collector and a second negative active layer and a first negative active layer sequentially disposed on at least one side of the negative current collector, the second negative active layer being located between the negative current collector and the first negative active layer; The second negative electrode active layer includes a second negative electrode active material and a quaternary ammonium salt compound. The second negative electrode active material includes a carbon-based material, and the quaternary ammonium salt compound includes quaternary ammonium cations.

2. The lithium-ion secondary battery according to claim 1, wherein, The quaternary ammonium salt compound also includes a hydrocarbon chain covalently bonded to the quaternary ammonium cation.

3. The lithium-ion secondary battery according to claim 2, wherein, The hydrocarbon chains in the quaternary ammonium salt type compounds include alkyl chains.

4. The lithium-ion secondary battery according to claim 2 or 3, wherein, The quaternary ammonium salt compound satisfies one or more of the following characteristics: (ta1) The hydrocarbon chain is an alkyl chain; (ta2) The hydrocarbon chain contains 12 to 18 carbon atoms; (ta3) The molecular weight of the quaternary ammonium salt compound is less than or equal to 600 Da.

5. The lithium-ion secondary battery according to any one of claims 2 to 4, wherein, The quaternary ammonium salt compound satisfies one or more of the following characteristics: (tb1) The structure of the quaternary ammonium cation is -N + (R1R2R3), where R1 and R2 are each independently C 1-3 Alkyl group, R3 is C 1- 3. Alkyl or hydroxyethyl; optionally, R1 and R2 are each independently methyl, and R3 is methyl or hydroxyethyl; (tb2) The quaternary ammonium salt type compound also includes anions, which include one or more of nitrate, carbonate, bicarbonate and phosphate.

6. The lithium-ion secondary battery according to claim 1, wherein, The quaternary ammonium salt compounds include one or more of the following: octadecyl dimethyl hydroxyethyl quaternary ammonium nitrate, N,N-dimethyl-N-(2-hydroxyethyl)hexadecyl quaternary ammonium phosphate, octadecyl trimethyl quaternary ammonium phosphate, dodecyl trimethyl quaternary ammonium phosphate, dodecyl trimethyl quaternary ammonium nitrate, dodecyl trimethyl quaternary ammonium carbonate, and dodecyl trimethyl quaternary ammonium bicarbonate.

7. The lithium-ion secondary battery according to any one of claims 1 to 6, wherein, The quaternary ammonium salt compound accounts for 0.2% to 2% of the mass of the second negative electrode active layer.

8. The lithium-ion secondary battery according to claim 7, wherein, The quaternary ammonium salt compound accounts for 0.2% to 1.5% of the mass of the second negative electrode active layer.

9. The lithium-ion secondary battery according to any one of claims 1 to 8, wherein, The carbon-based material accounts for 80% to 100% of the mass of the second negative electrode active material.

10. The lithium-ion secondary battery according to any one of claims 1 to 9, wherein, The negative electrode sheet satisfies one or more of the following characteristics: (tc1) The carbon-based material includes one or more of artificial graphite, natural graphite, soft carbon, and hard carbon; (tc2) The carbon-based material accounts for 94.5% to 97.5% of the mass of the second negative electrode active layer.

11. The lithium-ion secondary battery according to claim 9 or 10, wherein, The carbon-based material accounts for 95.0% to 97.0% of the mass of the second negative electrode active layer.

12. The lithium-ion secondary battery according to any one of claims 1 to 11, wherein, The second negative electrode active layer includes a binder, which includes styrene-butadiene rubber.

13. The lithium-ion secondary battery according to claim 12, wherein, The glass transition temperature of the styrene-butadiene rubber is 5℃~70℃, and can be selected as 30℃~50℃.

14. The lithium-ion secondary battery according to any one of claims 1 to 13, wherein, The first negative electrode active layer includes a first 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 coating layer includes one or more of soft carbon, hard carbon, and amorphous carbon.

15. The lithium-ion secondary battery according to any one of claims 1 to 14, wherein, The first negative electrode active layer includes a first negative electrode active material; The lithium-ion secondary battery satisfies one or more of the following characteristics: (td1) The first negative electrode active material includes coated graphite, the coated graphite including a graphite body and a coating layer located on at least a portion of the surface of the graphite body, the coating layer including one or more of soft carbon, hard carbon and amorphous carbon; (td2) The first negative electrode active material includes secondary particulate graphite, the secondary particulate graphite includes a secondary particulate graphite body, and the proportion of the secondary particulate graphite in the first negative electrode active material is greater than or equal to 20%, optionally 30% to 80%; optionally, the secondary particulate graphite includes carbon-coated secondary particulate graphite, the carbon-coated secondary particulate graphite includes the secondary particulate graphite body and a carbon coating layer located on at least a portion of the surface of the secondary particulate graphite body, the carbon coating layer in the carbon-coated secondary particulate graphite includes one or more of soft carbon, hard carbon and amorphous carbon; (td3) The first negative electrode active material includes graphite material, wherein the OI value of the graphite material is 2 to 15, and can be selected as 2 to 10; (td4) D of the first negative electrode active material v 50 is 10μm to 18μm, and can be selected as 12μm to 16μm; (td5) The porosity of the first negative electrode active layer is higher than that of the second negative electrode active layer; (td6) The compacted density of the powder of the first negative electrode active material or the compacted density of the powder of the first negative electrode active layer is 1.60 g / cm³. 3 ~1.80g / cm 3 ; (td7) The rate of charge of the first negative electrode active layer is higher than that of the second negative electrode active layer; Optionally, the charging rate of the first negative electrode active layer is higher than that of the second negative electrode active layer. (td8) At at least one temperature condition from 20°C to 35°C, the ionic conductivity of the electrolyte is 13 mS / cm to 18 mS / cm; optionally, at 25°C, the ionic conductivity of the electrolyte is 13 mS / cm to 18 mS / cm.

16. The lithium-ion secondary battery according to any one of claims 1 to 15, wherein, The negative electrode sheet satisfies one or more of the following characteristics: (te1) D of the second negative electrode active material v 50 is 12μm to 21μm, and can be selected as 14μm to 20μm; (te2) The first negative electrode active layer includes a first negative electrode active material, and the second negative electrode active material D v 50 is 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 that of the first negative electrode active layer; (te4) The powder compaction density of the second negative electrode active material is higher than that of the first negative electrode active material, or the powder compaction density of the second negative electrode active layer is higher than that of the first negative electrode active layer; optionally, the ratio of the powder compaction density of the second negative electrode active material to that of the first negative electrode active material is 1.05 to 1.35, further optionally 1.10 to 1.30, and even more preferably 1.10 to 1.28; optionally, the ratio of the powder compaction density of the second negative electrode active layer to that of the first negative electrode active layer is 1.05 to 1.35, further preferably 1.10 to 1.30, and even more preferably 1.10 to 1.28; (te5) The compacted density of the powder of the second negative electrode active material or the compacted density of the powder of the second negative electrode active layer is 1.85 g / cm³. 3 ~2.05g / cm 3 .

17. The lithium-ion secondary battery according to any one of claims 1 to 16, wherein, The negative electrode sheet satisfies one or more of the following characteristics: (tf1) On one side of the negative electrode current collector, the ratio of the areal density of the second negative electrode active layer to the areal density of the first negative electrode active layer is 3:2 to 2:

3. (tf2) Taking the thickness ratio of the first negative electrode active layer to the second negative electrode active layer on one side of the negative electrode current collector as denoted as f H , satisfying f H ≤1.6, optionally, 1.1≤f H ≤1.6, and further optionally, 1.1≤f H ≤1.3; (tf3) Measured on one side of the negative electrode current collector, the thickness of the first negative electrode active layer is less than or equal to 50 μm, optionally 20 μm to 50 μm, and further optionally 30 μm to 40 μm.

18. A lithium-ion secondary battery, comprising a negative electrode and an electrolyte; the negative electrode comprises a negative current collector and a negative active material layer located on at least one side of the negative current collector; The negative electrode active material layer includes a negative electrode active material and a quaternary ammonium salt compound. The negative electrode 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 one or more of soft carbon, hard carbon, and amorphous carbon. The quaternary ammonium salt compound includes quaternary ammonium cations.

19. The lithium-ion secondary battery according to claim 18, wherein, The negative electrode sheet satisfies one or more of the following characteristics: (tg1) The quaternary ammonium salt compound as defined in any one of claims 2 to 6; (tg2) The quaternary ammonium salt compound has a mass percentage of 0.2% to 2% in the negative electrode active material layer, and can be selected as 0.2% to 1.5%; (tg3) The negative electrode active material includes a carbon-based material; optionally, the carbon-based material includes one or more of artificial graphite, natural graphite, soft carbon and hard carbon; (tg4) The negative electrode active material includes a carbon-based material, wherein the mass percentage of the carbon-based material in the negative electrode active material layer is 94.5% to 97.5%, and optionally 95.0% to 97.0%; (tg5) The negative electrode active material layer includes a binder, which includes styrene-butadiene rubber; optionally, the glass transition temperature of the styrene-butadiene rubber is 5℃~70℃, and more preferably 30℃~50℃; (tg6) The negative electrode active material includes coated graphite, which includes a graphite body and a coating layer located on at least a portion of the surface of the graphite body. The coating layer includes one or more of soft carbon, hard carbon and amorphous carbon. (tg7) The negative electrode active material includes secondary particulate graphite, which includes a secondary particulate graphite body. The proportion of secondary particulate graphite in the negative electrode active material is greater than or equal to 20%, and optionally 30% to 60%. Optionally, the secondary particulate graphite includes carbon-coated secondary particulate graphite, which includes the secondary particulate graphite body and a carbon coating layer located on at least a portion of the surface of the secondary particulate graphite body. The carbon coating layer in the carbon-coated secondary particulate graphite includes one or more of soft carbon, hard carbon, and amorphous carbon. (tg8) The negative electrode active material includes graphite material, and the OI value of the graphite material is 2 to 15, which can be selected as 2 to 10; (tg9) D of the negative electrode active material v 50 is 11μm to 20μm, and can be selected as 13μm to 18μm; (tg10) The porosity of the negative electrode active material layer is 15% to 35%, and can be selected as 25% to 30%; (tg11) At at least one temperature condition from 20°C to 35°C, the ionic conductivity of the electrolyte is 13 mS / cm to 18 mS / cm; optionally, at 25°C, the ionic conductivity of the electrolyte is 13 mS / cm to 18 mS / cm. (tg12) The negative electrode active material layer includes a second negative electrode active layer, which is defined as in any one of claims 1 to 13 and 16.

20. The lithium-ion secondary battery according to any one of claims 1 to 19, wherein, The areal density of the negative electrode sheet is 5 mg / cm³, calculated on one side of the negative electrode current collector. 2 ~15mg / cm 2 .

21. The lithium-ion secondary battery according to any one of claims 1 to 20, wherein, The electrolyte includes an electrolyte salt, which includes electrolyte anions; The electrolyte anions include one or more of tetrafluoroborate, hexafluoroarsenate, hexafluorophosphate, trifluoromethanesulfonate, difluorophosphate, difluorooxalateborate, tetrafluorooxalate phosphate, difluorodioxalate phosphate, difluorosulfonylimide, and ditrifluoromethanesulfonylimide.

22. The lithium-ion secondary battery according to claim 21, wherein, The electrolyte salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

23. The lithium-ion secondary battery according to any one of claims 1 to 22, wherein, The lithium-ion secondary battery further includes a positive electrode sheet, the positive electrode sheet includes a positive active layer, the positive active layer includes a positive active material, and the positive active material includes one or more of lithium phosphate active materials and lithium composite metal oxide active materials.

24. The lithium-ion secondary battery according to claim 23, wherein, The positive electrode active material includes a lithium phosphate-containing active material, and the positive electrode active material satisfies one or more of the following characteristics: (th1) The mass percentage of the lithium phosphate-containing active material in the positive electrode active layer is greater than or equal to 80%, and can be selected as 80% to 97%; (th2) The lithium-containing phosphate active materials include one or more of lithium iron phosphate, lithium iron phosphate and carbon composite materials, lithium manganese phosphate, lithium manganese phosphate and carbon composite materials, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composite materials. (th3) 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. The carbon coating layer in the lithium phosphate active material includes one or more of soft carbon, hard carbon and amorphous carbon.

25. An electrical device comprising a lithium-ion secondary battery as described in any one of claims 1 to 24.

26. The use of the lithium-ion secondary battery according to any one of claims 1 to 24 in supplying and / or storing electrical energy; The application includes the process of charging the lithium-ion secondary battery at a rate of 2C or higher.

27. The application according to claim 26, wherein, The application includes the process of charging the lithium-ion secondary battery at at least one rate from 2C to 6C. Optionally, the application includes the process of charging the lithium-ion secondary battery at at least one rate 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 can be selected as 2C to 6C, or further selected as 2C to 4C or 4C to 6C.

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