Negative plate and lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a negative plate and a lithium ion secondary battery comprising the negative plate. Comprising a negative electrode current collector and a negative electrode active layer, the negative electrode active layer comprises a first active layer, the first active layer comprises a silicon-carbon material, and the silicon-carbon material comprises first silicon-carbon particles and second silicon-carbon particles; the sphericity degree of the first silicon-carbon particles is 0.8-1, the average particle size of the first silicon-carbon particles is 5-20 [mu] m, and the mass content of silicon is 30-60%; the second silicon carbon particles comprise secondary particles formed by a plurality of primary spherical particles; the degree of sphericity of the primary spherical particles is 0.8-1, and the average particle size of the primary spherical particles is 100 nm to 2.5 microns; the average particle size of the second silicon-carbon particles is 5-36 [mu] m, the sphericity is 0.35-0.79, and the mass content of the silicon element is 25-55%. Comprising the negative plate disclosed by the invention has relatively high energy density and also has relatively excellent cycle performance and rate capability.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a negative electrode sheet and a lithium-ion secondary battery comprising the negative electrode sheet. Background Art

[0002] Silicon materials are widely used in lithium-ion battery anode active materials due to their high specific capacity. However, silicon materials experience significant volume expansion and contraction during battery cycling, leading to material fracture during the charge and discharge cycles. As demand for battery energy density continues to increase, the specific capacity of anode active materials must also be increased, leading to an increasing proportion of silicon materials in the anode active material. This has led to increasingly significant problems with silicon materials, seriously impacting the battery's cycle and rate performance.

[0003] Therefore, it is very important to invent a battery that can achieve both high energy density and excellent cycle performance and rate performance. Summary of the Invention

[0004] The present invention aims to overcome the existing problem of silicon-doped batteries, which struggles to balance high energy density with cycle and rate performance. The present invention provides a negative electrode sheet and a lithium-ion secondary battery incorporating the same. The negative electrode sheet of the present invention exhibits superior kinetic performance and cycle stability. The lithium-ion secondary battery (hereinafter referred to as the battery) incorporating the negative electrode sheet of the present invention exhibits superior cycle and rate performance while maintaining high energy density.

[0005] Silicon-carbon particles with a large sphericity (for example, 0.8-1) have no selectivity in their expansion direction and can expand uniformly in all directions. Therefore, they can reduce many problems such as increased thickness, unstable electrode interface, and accumulation of electrode stress caused by a high proportion of silicon in the negative electrode active material. However, in actual use, it was found that the above-mentioned silicon-carbon material has poor conductivity, and the problem of material rupture still occurs during the battery cycle, which seriously affects the cycle performance and rate performance of the battery. The inventors of the present invention have found that the first silicon-carbon particles can greatly improve the volume energy density of the battery due to their large particle size (5μm-20μm), but their poor conductivity is not conducive to working at high rates. The conductivity of silicon is lower than that of graphite. Therefore, when the mass content of silicon element in the silicon-carbon material accounts for a large proportion, the overall conductivity of the silicon-carbon material will be poor. In addition, the lithium ion diffusion path inside the first silicon-carbon particles with a larger particle size is too long, and the higher sphericity will reduce the packing density of the first silicon-carbon particles, thereby reducing the effective contact points of the conductive network, thereby causing polarization to intensify.

[0006] Based on the above findings, the inventors of the present invention conducted a large number of targeted studies and found that adding a second silicon-carbon particle with a specific sphericity, including a plurality of primary spheres, to the first silicon-carbon particle and using them together as the negative electrode active material can effectively improve the overall conductivity of the silicon-carbon material and reduce the risk of its rupture during battery cycling. The reasons for this are:

[0007] First, the average particle size of the primary spherical particles in the second silicon-carbon particles is much smaller than that of the first silicon-carbon particles, which reduces the diffusion distance of lithium ions and improves the kinetic performance of the negative electrode active layer, thereby improving the charge and discharge speed of the battery.

[0008] Second, due to their high sphericity and average particle size, the first silicon-carbon particles undergo isotropic volume expansion when lithium is inserted, resulting in extremely high radial stress within the particles, ultimately causing crack propagation and active material shedding. The second silicon-carbon particles, however, are formed from smaller primary spherical particles, giving them elastic deformation capabilities. They can absorb some of this stress through compression or deformation during battery charge and discharge, mitigating the overall volume expansion of the silicon-carbon material.

[0009] Third, the heterogeneous interface between the first silicon-carbon particle and the second silicon-carbon particle can induce the volume expansion direction to change from purely radial to multi-directional dispersion, and dissipate energy through the displacement friction of adjacent particles, which is also beneficial to reduce the overall volume expansion of the silicon-carbon material.

[0010] Fourth, the surface of the second silicon-carbon particles has gaps between the primary spherical particles, which can form multi-point contact with the conductive agent in the negative electrode active layer, which is beneficial to the improvement of the overall conductive performance of the silicon-carbon material.

[0011] Fifth, due to their high sphericity, the first silicon-carbon particles form a continuous and dense SEI (Solid Electrolyte Interface) membrane on their surface during charge and discharge. However, as they expand in volume, they are prone to brittle fracture, triggering continuous decomposition of the electrolyte. The special structure of the second silicon-carbon particles, on the other hand, preferentially forms a locally stable SEI membrane, which uses pores to limit the electrolyte penetration rate, slowing the occurrence of side reactions and improving the battery's cycling stability.

[0012] Sixth, the second silicon-carbon particles with a specific sphericity have good adhesion to the surrounding materials, which is more conducive to improving the overall structural stability of the negative electrode sheet. If the sphericity of the second silicon-carbon particles is too high (for example, greater than 0.79), the second silicon-carbon particles may fall off during battery cycling, seriously affecting the battery's cycling stability.

[0013] Based on this, the inventors of the present invention proposed the following solution:

[0014] The first aspect of the present invention provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active layer located on at least one side of the negative electrode current collector, wherein the negative electrode active layer comprises a first active layer, wherein the first active layer comprises a first negative electrode active material; the first negative electrode active material comprises a silicon-carbon material, wherein the silicon-carbon material comprises first silicon-carbon particles and second silicon-carbon particles; the first silicon-carbon particles are single particles, and the sphericity of the first silicon-carbon particles is S1, 0.8≤S1≤1; the average particle size of the first silicon-carbon particles is D1, 5μm≤D1≤20μm; the first silicon-carbon particles are The mass content of silicon in the particles is C1, 30%≤C1≤60%; the second silicon-carbon particles include secondary particles formed by a plurality of primary spherical particles, the sphericity of the primary spherical particles is S2, 0.8≤S2≤1; the average particle size of the primary spherical particles is D2, 100nm≤D2≤2.5μm; the average particle size of the second silicon-carbon particles is D, 5μm≤D≤36μm, and the sphericity of the second silicon-carbon particles is 0.35-0.79; the mass content of silicon in the second silicon-carbon particles is C2, 25%≤C2≤55%.

[0015] A second aspect of the present invention provides a lithium-ion secondary battery, comprising the negative electrode sheet according to the first aspect of the present invention.

[0016] Through the above technical solution, the present invention has at least the following advantages compared with the prior art:

[0017] (1) The negative electrode sheet of the present invention has good dynamic performance and cycle stability.

[0018] (2) The lithium-ion secondary battery of the present invention has a high energy density and excellent cycle performance (including room temperature cycle performance and high temperature cycle performance) and rate performance.

[0019] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Shown is a scanning electron microscope (SEM) image of a cross section of a negative electrode sheet in an example of the present invention.

[0021] Figure 2 Shown is a SEM image of the first silicon-carbon particles in one example of the present invention.

[0022] Figure 3Shown is a cross-sectional SEM image of a negative electrode sheet in an example of the present invention.

[0023] Figure 4 FIG. 1 is a schematic diagram of the groove width in an embodiment of the present invention; wherein, Figure 4 (a)- Figure 4 (c) The two long sides of the groove are straight lines. Figure 4 In (d), the two long sides of the groove are curved.

[0024] Figure 5 The figure shows a schematic diagram of the groove spacing in an embodiment of the present invention; wherein, Figure 5 (a) is the case where two adjacent long sides are straight and parallel. Figure 5 (b) is the case where two adjacent long sides are straight lines and not parallel. Figure 5 (c) is the case where two adjacent long sides are curved lines.

[0025] Figure 6 FIG. 1 is a schematic cross-sectional view of a positive electrode sheet along the thickness direction in an embodiment of the present invention.

[0026] Figure 7 FIG. 1 is a schematic diagram of a concave region in an embodiment of the present invention. DETAILED DESCRIPTION

[0027] The following is a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0028] The first aspect of the present invention provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active layer located on at least one side of the negative electrode current collector. The negative electrode active layer comprises a first active layer, the first active layer comprises a first negative electrode active material; the first negative electrode active material comprises a silicon-carbon material, the silicon-carbon material comprises a first silicon-carbon particle and a second silicon-carbon particle. The first silicon-carbon particle is a single particle, and the sphericity of the first silicon-carbon particle is S1, 0.8≤S1≤1 (for example, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99 or 1); the average particle size of the first silicon-carbon particle is D1 , 5μm≤D1≤20μm (for example, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm or 20μm); the mass content of silicon element in the first silicon-carbon particles is C1, 30%≤C1≤60% (for example, 30%, 35%, 40%, 45%, 50%, 55% or 60%). The second silicon-carbon particles include secondary particles formed by a plurality of primary spherical particles, the sphericity of the primary spherical particles is S2, 0.8≤S2≤1 (for example, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99 or 1); the average particle size of the primary spherical particles is D2, 100 nm≤D2≤2.5 μm (for example, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900nm, 1μm, 1.5μm, 1.6μm, 2μm or 2.5μm); the average particle size of the second silicon-carbon particles is D, 5μm≤D≤36μm (for example, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm or 36μm); the sphericity of the second silicon-carbon particles is 0.35-0.79 (for example, 0.35, 0.4, 0.5, 0.6, 0.65, 0.68, 0.7, 0.75 or 0.79); the mass content of silicon element in the second silicon-carbon particles is C2, 25%≤C2≤55% (for example, 25%, 30%, 35%, 40%, 45%, 50% or 55%).

[0029] In one example, 0.9≤S1≤0.99. 0.9≤S2≤0.99.

[0030] In one example, 10 μm ≤ D1 ≤ 15 μm, 100 nm ≤ D2 ≤ 2 μm, and 15 μm ≤ D ≤ 30 μm.

[0031] In one example, 35%≤C1≤50%, and 36%≤C2≤49%.

[0032] In the present invention, "several" means that the number of the primary spherical particles in the second silicon-carbon particles is greater than or equal to 2. Figure 1 Shown is a cross-sectional scanning electron microscope (SEM) image of a negative electrode sheet in an example of the present invention. It can be seen from the figure that the silicon-carbon material includes first silicon-carbon particles and second silicon-carbon particles, wherein the first silicon-carbon particles are framed by dotted lines, and the first silicon-carbon particles are single particles; the second silicon-carbon particles are framed by solid lines, and the second silicon-carbon particles include secondary particles formed by a number of primary spherical particles. Figure 1 The scale size is 10.0 μm.

[0033] In the present invention, the average particle size D1 of the first silicon-carbon particles, the average particle size D2 of the primary spherical particles in the second silicon-carbon particles, and the average particle size D of the second silicon-carbon particles can be measured by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled and removed, and then soaked in dimethyl carbonate (DMC) solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt attached to the negative electrode sheet. The negative electrode sheet is cut using an argon ion milling instrument CP, and then observed using a SEM (using backscatter imaging mode). In this mode, the contrast of the silicon-carbon material is brighter (which can be used to distinguish the carbon-based material and the conductive agent in the negative electrode active layer). At least 10 first silicon-carbon particles, primary spherical particles, and second silicon-carbon particles are randomly selected at 5K magnification, and the particle size of each particle is measured and the average value is taken. If the number of particles is less than 10 at 5K magnification, the mirror image is taken again until 10 particles are measured. When the particles in the mirror image are regular circles, the particle size is the diameter of the regular circle; when the particles in the mirror image are not "regular circles", connect any two points on the edge of the particle to form a straight line segment inside the particle, and select the longest straight line segment inside the particle as the particle size. The term "0% SOC" can refer to the battery being discharged to 2.7V at 0.1C.

[0034] In the present invention, the sphericity S1 of the first silicon-carbon particles, the sphericity S2 of the primary spherical particles, and the sphericity of the second silicon-carbon particles can be measured by conventional methods in the art, such as discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, cutting the negative electrode sheet using an argon ion milling apparatus (CP), and observing in a SEM device using backscatter imaging mode; finding a first silicon-carbon particle and a primary spherical particle with a continuous and smooth profile, connecting any two points on the edge of the particle to form a straight line segment within the particle, selecting the longest straight line segment within the particle, and recording its length as Z1; taking the midpoint of the longest straight line segment, drawing a straight line through the midpoint to form a straight line segment with both end points at the edge of the particle, selecting the shortest straight line segment, and recording its length as Z2. The sphericity of the particle is then Z2 / Z1. At least five first silicon-carbon particles and five primary spherical particles are selected, and the sphericity is measured and the average value is taken.

[0035] In the present invention, the mass content ratio C1 of the silicon element in the first silicon-carbon particles and the mass content ratio C2 of the silicon element in the second silicon-carbon particles can be measured by conventional methods in the art, such as discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, cutting the negative electrode sheet using an argon ion milling apparatus CP, observing the silicon-carbon material using backscatter imaging mode in a SEM device to maximize magnification; using an energy dispersive spectrometer (EDS) to scan the cross-section of the first silicon-carbon particle and the second silicon-carbon particle, with the scan area being no less than 50% of the particle cross-section, and the scan range being completely within the particle cross-section, to calculate the mass content ratio of the silicon element. At least 10 particles are selected for measurement and the average value is taken.

[0036] In the present invention, the surface of the first silicon-carbon particle has a first pit. Figure 2 FIG. 1 is a SEM image of a first silicon-carbon particle in an example of the present invention. From the image, it can be seen that the first silicon-carbon particle has a first pit on its surface.

[0037] The average particle size of the first silicon-carbon particles is large, and the center of the sphere is far from the surface. The distance for lithium ions to diffuse from the surface of the particles to the inside is long and the resistance is large. Therefore, the time for lithium deintercalation / insertion is long, or even there is no time to insert or remove it. This will result in a certain loss of capacity and active lithium, causing capacity decay in the later stages of the battery cycle. Creating pits on the surface of the first silicon-carbon particles can shorten the distance from the center of the sphere to the surface, improve the transmission efficiency of lithium ions, and reduce the loss of active lithium. At the same time, the first silicon-carbon particles will expand during the cycle and squeeze the adjacent materials. The pits on the surface can reserve space for expansion, reduce the relative displacement and contact impedance caused by squeezing and shrinking between the particles, and improve the stability of the negative electrode active layer. The first pits can be manufactured by controlling the gas composition, deposition temperature and deposition rate of chemical vapor deposition. If the deposition nucleation rate is too fast or locally oversaturated, the first pits may be formed on the particle surface due to uneven deposition. It can also be manufactured by chemical corrosion and mechanical methods.

[0038] In the present invention, the number of the first pits is 1-100, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100.

[0039] In one example, the number of the first pits is 2-50.

[0040] In one example, the number of the first pits is 6-40.

[0041] In the present invention, the number of the first pits refers to the average number of first pits on a single first silicon-carbon particle; that is, the average number of first pits on the surface of each first silicon-carbon particle is 1-100, and does not mean that all first silicon-carbon particles have first pits on their surfaces. The number of the first pits can be obtained by conventional methods in the art, such as discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, or directly taking the negative electrode sheet, cutting the negative electrode sheet using an argon ion milling instrument CP, and observing it in an SEM device; selecting at least 10 first silicon-carbon particles in the electron microscope image, counting the number of first pits on half of the surface of each particle, multiplying by 2 and taking the average value, the number of first pits can be obtained.

[0042] In the present invention, the width of the first pit is 0.1 μm-3 μm, for example, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm or 3 μm.

[0043] In one example, the width of the first pit is 0.2 μm-1.5 μm.

[0044] In the present invention, the depth of the first pit is 0.01 μm-3 μm, for example, 0.01 μm, 0.05 μm, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 2 μm or 3 μm.

[0045] In one example, the depth of the first pit is 0.2 μm-1 μm.

[0046] In the present invention, the width and depth of the first pit have the conventional meanings in this field. When the pattern formed by the edge of the first pit on the surface of the first silicon-carbon particle is a "regular circle", the width of the first pit is the diameter of the regular circle; and when the pattern formed by the edge of the first pit on the surface of the first silicon-carbon particle is not a "regular circle", any two points on the edge of the pattern are connected to form a straight line segment inside the pattern, and the longest straight line segment is selected as the width of the first pit. The width of the first pit can be obtained by testing conventional methods in the field, for example, after discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, cutting the negative electrode sheet using an argon ion milling instrument CP, and observing it in an SEM device; selecting at least 20 first pits in the electron microscope image (the first pits on the same first silicon-carbon particle can be selected, or the first pits on different first silicon-carbon particles can be selected), measuring and calculating the width of each first pit, and taking the average value.

[0047] The depth of the first pit refers to the vertical distance from the lowest point in the first pit to the surface of the first silicon-carbon particle. The depth of the first pit can be measured by conventional methods in the art, such as discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, cutting the negative electrode sheet using an argon ion milling apparatus (CP), and observing it in a SEM device; selecting at least 20 first pits in the electron microscope image (the first pits can be selected from the same silicon-based material or the first pits can be selected from different first silicon-carbon particles), measuring the depth of each first pit, and taking the average value.

[0048] In the present invention, the secondary particles include N primary spherical particles, 3≤N≤1000, for example, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900 or 1000.

[0049] In one example, 150≤N≤700.

[0050] In the present invention, the primary spherical particles are formed into the secondary particles by, for example, bonding.

[0051] In the present invention, the primary spherical particles are formed into the secondary particles by, for example, agglomeration.

[0052] In the present invention, N can be obtained by testing by conventional methods in the art, such as discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, cutting the negative electrode sheet using an argon ion milling apparatus CP, and observing using backscatter imaging mode in a SEM device; selecting at least 3 secondary particles in the mirror image, counting the number of primary spherical particles in each second silicon-carbon particle, and taking the average value.

[0053] In the present invention, based on the total amount of the first silicon-carbon particles and the second silicon-carbon particles, the content of the first silicon-carbon particles is 50%-99.9%, for example, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 99.9%.

[0054] In one example, based on the total amount of the first silicon-carbon particles and the second silicon-carbon particles, the content of the first silicon-carbon particles accounts for 80%-99.9%.

[0055] In one example, based on the total amount of the first silicon-carbon particles and the second silicon-carbon particles, the content of the first silicon-carbon particles accounts for 90%-99.9%.

[0056] By regulating the number of the first and second silicon-carbon particles in the negative electrode active layer, the silicon-carbon material can be arranged more densely, which not only helps to further improve the conductivity of the silicon-carbon material itself, but also further improves the compaction of the negative electrode sheet, thereby increasing the energy density of the battery. Furthermore, mixing the first and second silicon-carbon particles in specific quantities can further reduce the risk of side reactions between the silicon-carbon material and the electrolyte, which helps to improve the cycling stability of the battery. The number of the first and second silicon-carbon particles can be measured by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled and removed. After soaking in DMC solvent for 12 hours, it is rinsed with DMC solvent to remove the lithium salt attached to the negative electrode sheet. The negative electrode sheet is cut along the thickness direction using an argon ion milling instrument CP and observed in the backscattered imaging mode of the SEM equipment to obtain a cross-section image of the negative electrode sheet along the thickness direction. At least 20 cross-section images are selected at a magnification of 1K, and the number of the first and second silicon-carbon particles in each image is counted. The total number is calculated by adding them together.

[0057] In the present invention, the second silicon-carbon particles have a coating layer on their surfaces.

[0058] In one embodiment, the coating layer includes a binder, and the binder includes at least one of polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA).

[0059] The second silicon-carbon particles are formed by bonding a large number of primary spherical particles together with a binder. The binder acts as a coating on the surface. This coating has good affinity with the conductive agent (such as carbon black, carbon nanotubes, etc.), uniformly adsorbing the conductive agent, allowing more of the conductive agent to contact the surface of the first silicon-carbon particles, thereby improving the overall electronic conductivity of the negative electrode active material.

[0060] In one example, the coating layer includes a SEI film.

[0061] In the present invention, the thickness of the coating layer is 0.05nm-2nm, for example, 0.05nm, 0.1nm, 0.2nm, 0.3nm, 0.4nm, 0.5nm, 0.6nm, 0.7nm, 0.8nm, 0.9nm, 1nm, 1.1nm, 1.2nm, 1.3nm, 1.4nm, 1.5nm, 1.6nm, 1.7nm, 1.8nm, 1.9nm or 2nm. The "thickness of the coating layer" refers to the average thickness of the coating layer on the surface of the second silicon-carbon particles. It can be obtained by conventional methods in the art, for example, discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, cutting the negative electrode sheet using an argon ion milling instrument CP, and observing in a SEM or transmission electron microscope (TEM) device using backscattered imaging mode; selecting at least 10 second silicon-carbon particles in the mirror image, selecting at least 5 sites on the surface of each second silicon-carbon particle, measuring the thickness of the coating layer at each site, taking the average value, and recording it as the thickness of the coating layer.

[0062] In the present invention, the silicon-carbon material further includes third silicon-carbon particles, the average particle size of the third silicon-carbon particles is D3, 2 μm ≤ D3 < 5 μm (for example, 2 μm, 3 μm, 4 μm or 4.9 μm); the sphericity of the third silicon-carbon particles is S3, 0.8 ≤ S3 ≤ 1 (for example, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99 or 1); the mass content of silicon element in the third silicon-carbon particles is C3, 20%≤C3≤45% (for example, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38% or 39%).

[0063] In one example, the third silicon-carbon particle is a single particle.

[0064] In one embodiment, 25%≤C3≤35%, 0.9≤S3≤0.99.

[0065] Adding the third silicon-carbon particles to the silicon-carbon material can further increase the packing density of the negative electrode active material, improve compaction, and help improve the conductivity of the negative electrode active material; thereby improving the energy density and rate performance of the battery.

[0066] Silicon-carbon materials with larger particle sizes have higher silicon content and occupy larger spaces, which can increase the surface capacity of the negative electrode active layer, thereby increasing the energy density of the battery. Smaller silicon-carbon materials fill the gaps between larger silicon-carbon materials and carbon-based materials, which can increase surface density and compaction. Smaller silicon content ratios have smaller expansions, which can reserve space for the expansion of silicon-carbon materials with larger particle sizes during battery charging, and do not occupy the expansion space of silicon-carbon materials with larger particle sizes. Therefore, when silicon-carbon materials of different particle sizes have specific silicon content ratios, they can ensure the gram capacity of the negative electrode active material and the surface density and compaction density of the negative electrode sheet, which not only improves the energy density of the battery but also alleviates the cyclic expansion of the negative electrode active layer.

[0067] In the present invention, the surface of the third silicon-carbon particle has a second pit. Similarly, providing the second pit on the surface of the third silicon-carbon particle can shorten the distance from the center of the sphere to the surface, improve the transmission efficiency of lithium ions, and reduce the loss of active lithium. At the same time, the third silicon-carbon particle will expand during the cycle, squeezing the adjacent materials. The pit on the surface can reserve space for expansion, reduce the relative displacement and contact impedance caused by squeezing and shrinking between particles, and improve the stability of the negative electrode active layer. The manufacturing method of the second pit is the same as that of the first pit and will not be repeated here.

[0068] In the present invention, the number of the second pits is 1-100, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100.

[0069] In one example, the number of the second pits is 2-20.

[0070] In the present invention, the width of the second pit is 0.05 μm-2 μm, for example, 0.05 μm, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.5 μm or 2 μm.

[0071] In one example, the width of the second pit is 0.1 μm-1.5 μm.

[0072] In the present invention, the depth of the second pit is 0.01 μm-2 μm, for example, 0.01 μm, 0.05 μm, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm or 2 μm.

[0073] In one example, the second pit has a depth of 0.1 μm-1 μm.

[0074] In the present invention, the method for testing the number, width and depth of the second pits is carried out with reference to the first pits, and will not be repeated here.

[0075] In the present invention, D1 and C1 satisfy: 6≤D1 / C1≤60, for example, 6, 7, 8, 9, 10, 20, 25, 26, 27, 28, 29, 30, 40, 50 or 60. D3 and C3 satisfy: 4≤D3 / C3≤60, for example, 4, 5, 6, 7, 8, 9, 10, 14, 15, 20, 30, 40, 50 or 60.

[0076] In one example, 20≤D1 / C1≤42.

[0077] In one example, 15≤D1 / C1≤35.

[0078] In one example, 25≤D1 / C1≤30.

[0079] In one example, 6≤D3 / C3≤20.

[0080] In one example, 8≤D3 / C3≤15.

[0081] The particle size of the first silicon-carbon particles and the third silicon-carbon particles is directly proportional to their silicon content. The larger the particle size of the first silicon-carbon particles, the higher their silicon content; conversely, the smaller the particle size of the third silicon-carbon particles, the lower their silicon content. The reason is that the first silicon-carbon particles with larger particle sizes occupy a larger space, so a higher silicon content ratio is required to ensure the gram capacity of the negative electrode active material, thereby improving the energy density of the battery. The third silicon-carbon particles with smaller particle sizes fill the gaps between the first silicon-carbon particles and the carbon-based material, which plays a role in increasing the surface density and compaction. The smaller silicon content ratio has a smaller expansion, which can reserve space for the expansion of the first silicon-carbon particles during the charging process, thereby increasing the gram capacity of the negative electrode and alleviating the cyclic expansion of the negative electrode active layer. When D1 / C1 and D3 / C3 are small, that is, the particle size is small and the silicon content is high; at this time, the third silicon-carbon particles have a large silicon content, and the first silicon-carbon particles in the negative electrode active layer are squeezed, resulting in a large displacement with other particles, the adhesion of the first silicon-carbon particles decreases, and the contact resistance increases sharply; and when D1 / C1 and D3 / C3 are large, that is, the particle size is large and the silicon content is small, the silicon content of the first silicon-carbon particles decreases, the gram capacity of the negative electrode decreases, and the energy density of the battery decreases.

[0082] In the present invention, the silicon-carbon material also includes fourth silicon-carbon particles; the average particle size of the fourth silicon-carbon particles is D4, 5μm≤D4≤35μm (for example, 5μm, 8μm, 10μm, 15μm, 20μm, 25μm, 30μm or 35μm); the sphericity of the fourth silicon-carbon particles is S4, 0.5≤S4<0.8 (for example, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75 or 0.79).

[0083] Because the highly spherical silicon-carbon material expands uniformly in all directions, it easily creates uniform gaps with other surrounding particles during repeated cycling and contraction, resulting in poor contact with other materials, increased contact resistance, and ultimately a decrease in cycling capacity. The addition of the less spherical fourth silicon-carbon particles allows their corners to fit into the gaps in the carbon-based material, providing a certain anchoring effect, preventing the silicon-carbon material from separating from the surrounding carbon-based materials. This helps improve the adhesion of the negative electrode active layer and makes the negative electrode sheet more stable during cycling.

[0084] In one example, 10 μm ≤ D4 ≤ 30 μm.

[0085] In the present invention, the negative electrode active layer further includes a second active layer. The first active layer and the second active layer are arranged along the thickness direction of the negative electrode sheet, and the second active layer is located between the negative electrode current collector and the first active layer; the second active layer includes a second negative electrode active material, the second negative electrode active material includes a first carbon-based material, and the first negative electrode active material further includes a second carbon-based material. Figure 3The figure shows a cross-sectional SEM image of a negative electrode sheet in an embodiment of the present invention. As can be seen from the figure, the negative electrode active layer includes a first active layer and a second active layer, and the second active layer is located between the negative electrode current collector and the first active layer; the first active layer includes a second carbon-based material and a silicon-carbon material, and the second active layer includes the first carbon-based material. Figure 3 The fourth silicon-carbon particle is framed by the solid line box, and the third silicon-carbon particle is framed by the solid line circle box.

[0086] In the present invention, the average particle size of the first carbon-based material is D5, 6 μm ≤ D5 ≤ 30 μm, for example, 6 μm, 10 μm, 15 μm, 20 μm, 25 μm or 30 μm. The average particle size of the second carbon-based material is D6, 2 μm ≤ D6 ≤ 10 μm, for example, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm.

[0087] In one example, 10 μm ≤ D5 ≤ 30 μm.

[0088] In one example, the average particle size of the second carbon-based material is smaller than the average particle size of the first carbon-based material.

[0089] The use of zoned coating in the thickness direction of the negative electrode sheet can further improve the negative electrode dynamics and expansion problems. The negative electrode active layer uses zoned coating, and silicon-carbon material and second carbon-based material are used in the negative electrode active layer far away from the negative electrode current collector. This coating is closer to the positive electrode sheet, and the electrolyte infiltration is more sufficient. The distance for lithium ion migration is relatively short, and it can contact the silicon-carbon material the fastest. Therefore, the number of lithium ions that can be quickly inserted into lithium is greatly increased, which is beneficial to improving the dynamic performance of the negative electrode sheet; at the same time, the particle size of the second carbon-based material in the negative electrode active layer far away from the negative electrode current collector is smaller, and the dynamics of small-particle carbon-based materials are better, which is also beneficial to the rapid deintercalation of lithium ions. The negative electrode active layer close to the negative electrode current collector uses a large-particle first carbon-based material, and no silicon-carbon material is added. The number of lithium ions migrating over long distances is reduced. In addition, the particle gaps between large-particle carbon-based materials are larger, which is beneficial to the infiltration of the electrolyte, so that the capacity attenuation of the carbon-based material in the negative electrode active layer close to the negative electrode current collector during the cycle is reduced. In addition, in terms of expansion, the silicon-carbon material in the negative electrode active layer away from the negative electrode current collector is away from the current collector, which can prevent the silicon-carbon material from directly contacting the negative electrode current collector, and avoid directly squeezing the negative electrode current collector when the silicon-carbon material expands, causing imprinting on the surface of the negative electrode current collector. At the same time, the silicon-carbon material is close to the surface of the negative electrode active layer, and has more space when expanding, which can prevent the negative electrode active layer from falling off, reduce the relative displacement of the active layer particles, and maintain stable particle contact.

[0090] In the present invention, the average particle size of the first carbon-based material is D5 and the average particle size of the second carbon-based material is D6. The average particle size can be obtained by conventional methods in the field. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled and removed, and then soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt attached to the negative electrode sheet. The negative electrode sheet is cut using an argon ion milling instrument CP, and then observed using a SEM. At least 20 first carbon-based materials and second carbon-based materials are randomly selected, and the particle size of each carbon-based material is measured to take the average value. When the particles in the mirror image are regular circles, the particle size of the particles is the diameter of the regular circle; when the particles in the mirror image are not "regular circles", any two points on the edge of the particle are connected to form a straight line segment inside the particle, and the longest straight line segment inside the particle is selected as the particle size of the particle.

[0091] In the present invention, the tensile strength of the negative electrode current collector in the width direction is σ, 300MPa≤σ≤850MPa (for example, 300MPa, 350MPa, 400MPa, 450MPa, 500MPa, 550MPa, 600MPa, 650MPa, 700MPa, 750MPa, 800MPa or 850MPa); the negative electrode current collector in the width direction is The elongation at break is A, 2% ≤ A ≤ 15% (for example, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%); the mass content of elemental silicon in the negative electrode active layer is C, 3% ≤ C ≤ 30% (for example, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 25% or 30%).

[0092] In one example, 450 MPa≤σ≤650 MPa.

[0093] In one example, 6%≤A≤10.5%.

[0094] In one example, 5%≤C≤15%.

[0095] In the present invention, the tensile strength σ and elongation at break A of the negative electrode current collector in the width direction can be tested by conventional methods in the art. For example, the battery is discharged to 0% SOC, the negative electrode sheet is disassembled and removed, and the negative electrode active layer on the surface of the negative electrode sheet is removed by washing with deionized water to obtain the negative electrode current collector. The negative electrode current collector is cut into small strips of 15 mm ± 0.2 mm along the length direction of the negative electrode sheet (the width of the small strip is 15 mm ± 0.2 mm, and the length of the small strip is the width of the negative electrode current collector). A WD-D3 electronic universal testing machine is used with a gap of 50 mm between the upper and lower clamps. The ends of the small strip along the length direction are clamped in the clamps respectively. The test is started at a speed of 50 mm / min until the equipment automatically stops the test, and the tensile strength σ and elongation at break A [A = (length at break - 50 mm) / 50 mm] are recorded. The error of the test is no more than 10% for 3 times, and the average value is taken.

[0096] In the present invention, the mass content ratio of silicon element in the negative electrode active layer can be obtained by testing conventional methods in the field, for example, after discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, soaking it in DMC solvent for 12 hours, then rinsing it with DMC solvent to remove the lithium salt attached to the negative electrode sheet, soaking the negative electrode active layer from the negative electrode current collector with deionized water, and then drying the detached negative electrode active layer, collecting the negative electrode active layer as a test sample. Using a thermogravimetric analyzer (such as a TGA 550 thermogravimetric analyzer), the test sample amount is 5mg-15mg, and in an air or oxygen atmosphere, the temperature is increased from room temperature (25℃) to 900℃ at a heating rate of 10℃ / min, and kept at 900℃ for 40min, so that the non-silicon components in the negative electrode active layer are volatilized while the silicon can be fully oxidized to silicon dioxide. The remaining substance is the ash of the negative electrode active layer. The mass content of silicon in the negative electrode active layer can be calculated based on the mass of the ash. The calculation formula is as follows: mass content of silicon in the negative electrode active layer = 7 × mass of ash / (15 × mass of test sample).

[0097] In the present invention, C / (σ×A)≤5×10 -3 , for example, 5×10 -3 , 4.5×10 -3 , 4×10 -3 , 3.5×10 -3 , 3×10 -3 , 2.5×10 -3 , 2.4×10 -3 , 2.3×10 -3 , 2.2×10 -3 , 2.1×10 -3 , 2×10 -3 , 1.5×10 -3 or 1×10 -3 .

[0098] In one example, 1.2×10 -3 ≤C / (σ×A)≤2.1×10 -3 .

[0099] The greater the silicon content in the negative electrode active layer, the greater the expansion stress of the negative electrode active material on the negative electrode current collector, and the greater the tensile strength and elongation of the negative electrode current collector are required. The tensile strength of the negative electrode current collector can resist the expansion of the negative electrode active material. When the tensile strength of the negative electrode current collector in the width direction is large, a greater expansion stress is required to stretch the negative electrode current collector; and the elongation at break of the negative electrode current collector reflects the length to which the negative electrode current collector can be stretched. The greater the elongation at break of the negative electrode current collector in the width direction, the less likely it is to be broken. Therefore, it is necessary to define the relationship between the three. When it is greater than 5×10 -3 When the silicon content is too large, or the tensile strength or elongation of the negative electrode current collector is low, the negative electrode current collector is easily stretched by stress, which can easily cause problems such as foil breakage, thickness expansion, and battery membrane shell rupture.

[0100] In one example, the negative electrode current collector includes copper foil.

[0101] In the present invention, the outer surface of the negative electrode active layer has a plurality of grooves. The depth of the grooves is 5 μm to 50 μm, for example, 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, or 50 μm. The width of the grooves is 20 μm to 150 μm, for example, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, or 150 μm. The spacing of the grooves is 500 μm to 2000 μm, for example, 500 μm, 600 μm, 700 μm, 800 μm, 900 μm, 1000 μm, 1100 μm, 1200 μm, 1300 μm, 1400 μm, 1500 μm, 1600 μm, 1700 μm, 1800 μm, 1900 μm or 2000 μm. The "several" refers to the number of grooves on the outer surface of the negative electrode active layer being greater than or equal to 2. The "outer surface" refers to the surface of the negative electrode active layer away from the negative electrode current collector.

[0102] Providing grooves on the outer surface of the negative electrode active layer can further provide buffer space for the volume expansion of the silicon-carbon material, thereby reducing the increase in the thickness of the negative electrode sheet and helping to improve the cycle stability of the battery.

[0103] In the present invention, the depth of the groove has the conventional meaning in the art, and refers to the vertical distance from the lowest point in the groove to the outer surface of the negative electrode active layer. This depth can be obtained by conventional testing methods in the art, for example, by measuring the depth of all grooves or at least five grooves on the outer surface of the negative electrode active layer using a 3D profilometer or SEM and taking the average value.

[0104] In the present invention, the projection of the groove on the negative electrode active layer includes two long sides, and the width of the groove refers to the average distance from one long side to the other long side in the length direction or width direction of the negative electrode sheet. Figure 4 FIG. 1 is a schematic diagram of the groove width in an embodiment of the present invention; wherein, Figure 4 (a)- Figure 4 (c) The two long sides of the groove are straight lines. Figure 4 The two long sides of the groove in (d) are curved. It should be noted that Figure 4 Only one groove is schematically drawn in the figure, which does not mean that there is only one groove on the surface of the negative electrode sheet of the present invention. Figure 4 (a) and Figure 4 In (b), the two long sides are arranged in parallel. Therefore, in the width direction of the negative electrode sheet, the distance from any point on one long side to the other long side is equal. In this case, the width of the groove is ( Figure 4 In the width direction), the distance d from any point on one long side to the other long side; Figure 4 In (c), the two long sides of the groove are straight lines, but they are not parallel. Therefore, the distance from any point on one long side to the other long side is not equal. In this case, the width of the groove can be averaged. That is, on one long side, based on the length of the side, 50 points are selected at equal distances (that is, the distance between each point is equal, so the selection of points can make the calculation result more accurate), and the width d corresponding to each point is measured. The average value is used to obtain the width of the groove. Figure 4 In (d), the two long sides are curved. Therefore, the distance from any point on one long side to the other long side is not equal. In this case, the width of the groove can also be averaged, that is, 50 points are randomly selected on one long side (due to Figure 4 In (d), the two long sides are curved and do not exist. Figure 4 (c) The relationship between the two long sides, therefore, 50 points can be randomly selected for measurement), and the width d corresponding to each point is measured, and the average value is taken to obtain the groove width. The groove width can be measured by conventional testing methods in the art, for example, by measuring the width of all grooves or at least 5 grooves on the outer surface of the negative electrode active layer using a 3D profilometer or SEM, and taking the average value.

[0105] In the present invention, the spacing of the grooves refers to the average distance between the two adjacent long sides of two adjacent grooves in the length direction or width direction of the negative electrode sheet. Figure 5 The figure shows a schematic diagram of the groove spacing in an embodiment of the present invention; wherein, Figure 5 (a) is the case where two adjacent long sides are straight and parallel. Figure 5 (b) is the case where two adjacent long sides are straight lines and not parallel. Figure 5 (c) is the case where two adjacent long sides are curved. Figure 5 In (a), the two adjacent long sides are straight and parallel. Therefore, in the width direction, the distance from any point on one long side to the other long side is equal. In this case, the spacing of the grooves is the distance d1 from any point on one long side to the other long side in the width direction. Figure 5 In (b), the two adjacent long sides are straight lines, but they are not parallel. Therefore, the distance from any point on one long side to the other long side is not equal. In this case, the spacing of the grooves can be averaged. That is, on one long side, based on the length of the side, 50 points are selected at equal distances (that is, the distance between each point is equal, so the selection of points can make the calculation result more accurate), and the width d1 corresponding to each point is measured. The average value is used to obtain the spacing of the grooves. Figure 5 In (c), the two adjacent long sides are curved. Therefore, the distance from any point on one long side to the other long side is not equal. In this case, the spacing of the grooves can also be averaged, that is, 50 points are randomly selected on one long side (due to Figure 5 In (c), the two long sides are curved and do not exist. Figure 5 (b) The relationship between the two long sides, therefore, 50 points can be randomly selected for measurement) and the width d1 corresponding to each point is measured. The average value is used to obtain the groove spacing. The groove spacing can be measured using conventional testing methods in the art. For example, using a 3D profilometer or SEM, the spacing of all grooves or at least five groups of adjacent grooves on the outer surface of the negative electrode active layer is measured and the average value is calculated.

[0106] In the present invention, the silicon-carbon material includes, for example, a porous carbon matrix and silicon particles located in the pores of the porous carbon matrix. The first carbon-based material and the second carbon-based material may each independently include at least one of artificial graphite, natural graphite, mesophase carbon microbeads, soft carbon, and hard carbon.

[0107] In one example, the first carbon-based material and the second carbon-based material each independently include artificial graphite.

[0108] In the present invention, the negative electrode active layer may further include a negative electrode conductive agent, a negative electrode binder, and a thickener. The negative electrode conductive agent may include, for example, at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, carbon nanotubes, and carbon fibers. The negative electrode binder may include, for example, at least one of polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, polyacrylonitrile (PAN), polyurethane, polymethacrylate, polyacrylate, and polyethylene oxide. The thickener may include, for example, at least one of carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose.

[0109] In the present invention, based on the total mass of the first negative electrode active layer, the content of the first negative electrode active material is 70%-99.7% (for example, 70%, 73%, 76%, 79%, 82%, 85%, 88%, 91%, 94%, 97%, 98.5% or 99.7%), and the content of the negative electrode conductive agent is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%). , 3%, 2%, 1%, 0.5% or 0.1%), the content of the negative electrode binder is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or 0.1%), and the content of the thickener is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or 0.1%). Based on the total mass of the second negative electrode active layer, the content of the second negative electrode active material is 70%-99.7% (for example, 70%, 73%, 76%, 79%, 82%, 85%, 88%, 91%, 94%, 97%, 98.5% or 99.7%), and the content of the negative electrode conductive agent is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%). , 2%, 1%, 0.5% or 0.1%), the content of the negative electrode binder is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or 0.1%), and the content of the thickener is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or 0.1%).

[0110] A second aspect of the present invention provides a lithium-ion secondary battery, comprising the negative electrode sheet according to the first aspect of the present invention.

[0111] In one example, the lithium-ion secondary battery further includes a positive electrode sheet and a separator, and the positive electrode sheet, the separator, and the negative electrode sheet are stacked and wound to form a winding core.

[0112] In one example, the charging cut-off voltage of the lithium-ion secondary battery is ≥4.5V.

[0113] In the present invention, the separator includes a base film and a nitrogen-containing functional layer located on at least one surface of the base film, and the nitrogen-containing functional layer is arranged facing the positive electrode sheet.

[0114] The mass content ratio of silicon elements in the negative electrode active layer and the charging cut-off voltage of the battery will affect the capacity of the battery. A higher silicon content ratio and a higher cut-off voltage can increase the capacity of the battery, but will have some negative effects on the positive electrode. For example, if the charging cut-off voltage is too high, the positive electrode active material may undergo an irreversible phase change, resulting in a capacity drop in the later stage of the cycle. At this time, the above problems can be improved by improving the diaphragm. A diaphragm containing a nitrogen-containing functional layer is provided so that the nitrogen-containing functional layer and the positive electrode sheet are arranged face to face. The nitrogen-containing groups on the nitrogen-containing functional layer can be transferred to the surface of the positive electrode active material, thereby stabilizing the metal atoms and the positive electrode crystal structure, reducing the release of active oxygen, and inhibiting the structural phase change of the positive electrode active material (such as lithium cobalt oxide) during high voltage charging and discharging, which causes the electrochemical performance of the material to decay, thereby improving the positive electrode stability and thus improving the high temperature cycle performance.

[0115] In one embodiment, the nitrogen-containing functional layer includes nitrogen-containing particles, and the nitrogen-containing particles include at least one of polyacrylonitrile, nitrile rubber, 1,3,5-triazine-2,4,6-triamine, melamine cyanurate, thiocyanate, and derivatives thereof. The derivatives are groups introduced into the above substances. As long as the derivatives contain cyano groups, the beneficial effects of the present invention can be achieved.

[0116] In one example, the nitrogen-containing functional layer includes nitrogen-containing particles, and the nitrogen-containing particles include at least one of polyacrylonitrile, nitrile rubber, 1,3,5-triazine-2,4,6-triamine, melamine cyanurate, and thiocyanate.

[0117] In the present invention, the average particle size of the nitrogen-containing particles is 100 nm-1000 nm, for example, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm or 1000 nm.

[0118] In the present invention, the average particle size of the nitrogen-containing particles can be obtained by conventional testing methods in the field. For example, after the battery is discharged to 0% SOC, the diaphragm is disassembled and removed, and after soaking in DMC solvent for 12 hours, it is rinsed with DMC solvent to remove the lithium salt attached to the diaphragm. The diaphragm is cut using an argon ion milling instrument CP, and the side of the diaphragm with a nitrogen-containing functional layer is taken as a sample. SEM observation is used, 20 nitrogen-containing particles are randomly selected, and the distance between the two longest points of the edge contour of each nitrogen-containing particle is measured. The average value is taken, which is the average particle size of the nitrogen-containing particles.

[0119] In the present invention, the thickness of the nitrogen-containing functional layer is 0.5 μm-5 μm, for example, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, or 5 μm. The thickness of the base film is 1 μm-10 μm, for example, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm.

[0120] In one example, the thickness of the nitrogen-containing functional layer is 0.5 μm-2 μm.

[0121] In the present invention, the thickness of the nitrogen-containing functional layer and the thickness of the base film can be obtained by conventional testing methods in the field. For example, after the battery is discharged to 0% SOC, the diaphragm is disassembled and removed, and after soaking in DMC solvent for 12 hours, it is rinsed with DMC to remove the lithium salt attached to the diaphragm. The area where the diaphragm exceeds the negative electrode sheet in the battery is selected, and the cross-section of the diaphragm is polished with argon ion grinding equipment. SEM observation is used, and the boundary between the base film and the nitrogen-containing functional layer can be seen in the SEM image. The thickness of the base film and the nitrogen-containing functional layer is measured separately according to the interface, and the average value is taken after measurement at 10 different locations.

[0122] In one embodiment, the separator further includes a first adhesive layer. The first adhesive layer is located on the outer surface of the nitrogen-containing functional layer. The first adhesive layer comprises at least one of PVDF, polymethyl methacrylate (PMMA), CMC, sodium carboxymethyl cellulose (CMC-Na), lithium carboxymethyl cellulose (CMC-Li), and polyvinyl pyrrolidone (PVP). The thickness of the first adhesive layer is 0.5 μm to 2 μm, for example, 0.5 μm, 1 μm, or 2 μm.

[0123] In one embodiment, the separator further includes a second adhesive layer. The second adhesive layer is located on a surface of the base film that is not provided with the nitrogen-containing functional layer. The second adhesive layer comprises at least one of PMMA, CMC, CMC-Na, CMC-Li, and PVP. The thickness of the second adhesive layer is 0.5 μm to 2 μm, for example, 0.5 μm, 1 μm, or 2 μm.

[0124] In the present invention, the thickness of the first adhesive layer and the thickness of the second adhesive layer are tested by referring to the thickness of the nitrogen-containing functional layer and the thickness of the base film, which will not be described in detail here.

[0125] In the present invention, the thickness of the nitrogen-containing functional layer, the thickness of the base film, the thickness of the first adhesive layer, and the thickness of the second adhesive layer all refer to the thickness of their respective single layers.

[0126] In one example, the diaphragm includes the substrate layer, the nitrogen-containing functional layer located on one surface of the substrate layer, the first adhesive layer located on the outer surface of the nitrogen-containing functional layer, and the second adhesive layer located on the other surface of the substrate layer.

[0127] In the present invention, the mass content of nitrogen on the surface of the diaphragm is 20%-50%, for example, 20%, 25%, 30%, 35%, 40%, 45% or 50%.

[0128] In one embodiment, the mass content of nitrogen on the surface of the diaphragm is 25%-40%.

[0129] In the present invention, the mass content ratio of nitrogen element on the surface of the diaphragm can be obtained by conventional testing methods in the field, for example, the side of the diaphragm with a nitrogen-containing functional layer is provided with a scanning electron microscope-energy dispersive X-ray spectrometer (SEM-EDS) for surface scanning testing.

[0130] In the present invention, the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer located on at least one side of the surface of the positive electrode current collector; the surface of the positive electrode active layer located near the winding center of the winding core has a convex area, and the convex area refers to an area with several convex parts; the "several" means that the number of the convex parts in the convex area is greater than or equal to 2. The surface of the positive electrode active layer located away from the winding center of the winding core has a concave area, and the concave area refers to an area with several concave parts. The "several" means that the number of the concave parts in the concave area is greater than or equal to 2. As Figure 6 The figure shows a schematic cross-sectional view of the positive electrode sheet along the thickness direction in an example of the present invention. As can be seen from the figure, the surface of the positive electrode active layer located close to the winding center of the core has several protrusions, and the surface of the positive electrode active layer located away from the winding center of the core has several concave portions.

[0131] In one example, the concave portions and the convex portions correspond to each other in positions on the surface of the positive electrode active layer.

[0132] The embossing process can produce a positive electrode sheet with concave surfaces on one side and convex surfaces on the other. This arrangement can increase the gap between the positive electrode sheet and the separator, providing a place for electrolyte infiltration and space for the volume expansion of the active material. The embossing process is to use a roller with protrusions to roll the electrode sheet. The embossing process is to use a roller with protrusions to roll the electrode sheet.

[0133] In the present invention, the depth of the recess is 10 μm-30 μm, for example, 10 μm, 15 μm, 20 μm, 25 μm or 30 μm. The spacing between the recesses is 2 mm-6 mm, for example, 2 mm, 3 mm, 4 mm, 5 mm or 6 mm. The width of the recess is 1 mm-5 mm, for example, 1 mm, 2 mm, 3 mm, 4 mm or 5 mm. The ratio of the area of ​​the positive projection of the recessed area on the surface of the positive electrode sheet to the area of ​​the positive electrode active layer away from the winding center of the core on the surface of the positive electrode sheet is 40%-90%, for example, 40%, 50%, 60%, 70%, 80% or 90%.

[0134] In the present invention, the depth of the concave portion refers to the vertical distance from the lowest point in the concave portion to the surface of the positive electrode active layer. The depth of the concave portion can be measured by conventional methods in the art, such as using a 3D profilometer to measure the depths of all or at least 20 concave portions on the surface of the positive electrode active layer and taking the average value. The spacing between the concave portions refers to the distance between the lowest points of two adjacent concave portions. It can be measured by conventional methods in the art, such as using a 3D profilometer to measure the spacing between all or at least 20 groups of concave portions on the surface of the positive electrode active layer and taking the average value.

[0135] In the present invention, the shape of the orthographic projection of the recess on the positive electrode sheet is not limited and can be a regular shape (such as a circle or a rectangle) or an irregular shape. The width of the recess refers to any two points on the orthographic projection of the recess on the positive electrode sheet, so that the line connecting the two forms a straight line segment inside the orthographic projection, and the longest straight line segment is selected as the width of the recess. The width of the recess can be obtained by conventional methods in the art, such as by using a 3D profilometer, selecting all or at least 20 recesses on the surface of the positive electrode active layer, measuring the width of each recess, and taking the average value.

[0136] In the present invention, the concave area refers to the area corresponding to the closed figure with the shortest perimeter formed by connecting the lowest points of all concave areas on the outer periphery of the positive electrode active layer away from the winding center of the core in three-dimensional space. Figure 7 The figure shows a schematic diagram of the concave region in one embodiment of the present invention. The solid black box in the figure represents a top view of the positive electrode sheet's surface away from the winding center of the core. The solid black circles represent the projections of the concave regions onto the positive electrode sheet's surface. The concave regions are hemispherical, so the center of each solid black circle is the lowest point of the concave region in three-dimensional space. The dashed black boxes in the figure represent the concave region. Similarly, the convex region refers to the area corresponding to the closed figure with the shortest circumference formed by connecting the highest points in three-dimensional space of all convex regions on the outer periphery of the positive electrode active layer near the winding center of the core.

[0137] In the present invention, the height of the protrusion is 10μm-30μm, for example, 10μm, 15μm, 20μm, 25μm or 30μm. The spacing between the protrusions is 2mm-6mm, for example, 2mm, 3mm, 4mm, 5mm or 6mm. The width of the orthographic projection of the protrusion on the surface of the positive electrode sheet is 1mm-5mm, for example, 1mm, 2mm, 3mm, 4mm or 5mm. The ratio of the area of ​​the orthographic projection of the protrusion area on the surface of the positive electrode sheet to the area of ​​the orthographic projection of the positive electrode active layer close to the winding center side of the core on the surface of the positive electrode sheet is 40%-90%, for example, 40%, 50%, 60%, 70%, 80% or 90%.

[0138] In the present invention, the height of the convex portion refers to the vertical distance from the highest point on the convex portion to the surface of the positive electrode active layer. The height of the convex portion can be obtained by conventional methods in the art, such as using a 3D profilometer to measure the heights of all convex portions or at least 20 convex portions on the surface of the positive electrode active layer and taking the average value. The spacing between the convex portions refers to the distance between the highest points on two adjacent convex portions in the length and width directions of the positive electrode sheet and can be obtained by conventional methods in the art, such as using a 3D profilometer to measure the spacing between all convex portions or at least 20 groups of convex portions in the length and width directions of the positive electrode active layer and taking the average value. The width of the orthographic projection of the convex portion on the surface of the positive electrode sheet refers to any two points on the orthographic projection of the convex portion on the positive electrode sheet, so that the line connecting the two forms a straight line segment within the orthographic projection, and the longest straight line segment is selected as the width of the convex portion. The width of the convex portion can be obtained by conventional methods in the art, such as using a 3D profilometer to select all convex portions or at least 20 convex portions on the surface of the positive electrode sheet, measure the width of each convex portion, and take the average value.

[0139] In the present invention, the positive electrode active material may include at least one of lithium cobaltate, lithium nickel cobalt manganeseate, lithium nickel cobalt aluminumate, lithium nickel cobalt manganese aluminumate, lithium manganate, lithium nickel manganeseate, lithium nickelate, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate and lithium-rich manganese-based materials.

[0140] In one example, the positive electrode active material includes lithium cobalt oxide.

[0141] In the present invention, the positive electrode active layer may further include a positive electrode conductive agent and a positive electrode binder. The positive electrode conductive agent may include, for example, at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, carbon nanotubes, and carbon fibers. The positive electrode binder may include, for example, at least one of polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, PAN, polyurethane, polymethacrylate, polyacrylate, and polyethylene oxide.

[0142] In the present invention, based on the total mass of the positive electrode active layer, the content of the positive electrode active material is 80%-99.8% (for example, 80%, 82%, 84%, 86%, 88%, 90%, 92%, 94%, 96%, 98%, 99% or 99.8%), the content of the positive electrode conductor is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or 0.1%), and the content of the positive electrode binder is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or 0.1%).

[0143] In the present invention, the lithium-ion secondary battery further comprises an electrolyte, wherein the electrolyte comprises ethyl fluoroacetate, wherein the ethyl fluoroacetate refers to a substance in which fluorine atoms are substituted at any position of ethyl acetate.

[0144] In one example, the ethyl fluoroacetate includes 2,2-difluoroethyl acetate (DFEA) and / or ethyl 2,2-difluoroacetate.

[0145] In one example, the ethyl fluoroacetate includes DFEA.

[0146] As a small molecule solvent, ethyl fluoroacetate (especially DFEA) has extremely high antioxidant properties and can exist stably for a long time at high voltage. Therefore, it can be used as a solvent for high-voltage electrolytes. When ethyl fluoroacetate (especially DFEA) is added to the electrolyte, the antioxidant stability of the electrolyte can be greatly improved, allowing the electrolyte as a whole to be stably stored for a longer period of time at high voltage, and the battery to be stably cycled for a longer period of time at high voltage.

[0147] In the present invention, based on the total mass of the electrolyte, the content of the ethyl fluoroacetate is 20%-50%, for example, 20%, 25%, 30%, 35%, 40%, 45% or 50%.

[0148] When the content of the ethyl fluoroacetate is less than 20%, the antioxidant ability of the ethyl fluoroacetate cannot be exerted due to the small amount added, and the high voltage performance of the battery cannot be improved; when the content of the ethyl fluoroacetate is greater than 50%, due to its higher viscosity, the overall viscosity of the electrolyte increases, the conductivity decreases, and the battery cycle performance decreases.

[0149] In the present invention, the mass content ratio of the ethyl fluoroacetate in the electrolyte can be measured by conventional methods in the art, such as gas chromatography (GC).

[0150] In the present invention, the electrolyte may further include lithium salts and / or additives commonly used in the art.

[0151] It should be noted that the numerical expressions such as "first" and "second" in the present invention are only used to distinguish different substances or usage methods, and do not represent a difference in order.

[0152] The present invention will be described in detail below through examples. The examples described in the present invention are only some examples of the present invention, not all examples. All other examples obtained by persons of ordinary skill in the art based on the examples of the present invention without creative work are within the scope of protection of the present invention.

[0153] In the following examples, unless otherwise specified, all materials used were commercially available analytical grade.

[0154] The following examples are used to illustrate the lithium-ion secondary battery of the present invention.

[0155] Example 1

[0156] Prepare the battery as follows:

[0157] (1) Preparation of negative electrode sheet

[0158] Artificial graphite (average particle size D5 is 13.6 μm), conductive carbon black, lithium carboxymethyl cellulose and styrene butadiene rubber are mixed in a mass ratio of 97:0.5:1.2:1.3, and deionized water is added to prepare the second negative electrode slurry; artificial graphite (average particle size D6 is 5.3 μm), silicon carbon material, carbon nanotubes, lithium carboxymethyl cellulose, styrene butadiene rubber and polyacrylic acid are mixed in a mass ratio of 58.2:38.8:0.5:0.8:0.5:1.2, and deionized water is added to prepare the second negative electrode slurry. Deionized water was added to prepare a first negative electrode slurry; the second negative electrode slurry was applied to both sides of the negative electrode current collector (copper foil, with a tensile strength σ in the width direction of 530 MPa and an elongation at break A in the width direction of 9.5%) and dried; the first negative electrode slurry was then applied to both sides of the negative electrode current collector coated with the second negative electrode slurry after drying, and after baking, roll pressing, and die cutting, grooves were etched on the outer surface of the negative electrode active layer using laser processing technology to obtain a negative electrode sheet;

[0159] Wherein, in the thickness direction of the negative electrode sheet, the size ratio of the second active layer to the first active layer is 1:1;

[0160] The silicon-carbon material comprises first silicon-carbon particles, second silicon-carbon particles, third silicon-carbon particles and fourth silicon-carbon particles mixed in a mass ratio of 85:0.1:12.4:2.5; the sphericity S1 of the first silicon-carbon particles is 0.99, the average particle size D1 is 12.7 μm, and the mass content of silicon element C1 is 42.7%; the second silicon-carbon particles include secondary particles formed by a plurality of primary spherical particles, the sphericity S2 of the primary spherical particles is 0.99, the average particle size D2 of the primary spherical particles is 1.2 μm, the average particle size D of the second silicon-carbon particles is 23.7 μm, the sphericity is 0.63, and the mass content of silicon element C2 is 42.7%; the average particle size D3 of the third silicon-carbon particles is 3.8 μm, the sphericity S3 is 0.99, and the mass content of silicon element C3 is 30.7%; the average particle size D4 of the fourth silicon-carbon particles is 25.2 μm, and the sphericity S4 is 0.76;

[0161] The first silicon-carbon particle has a first pit on its surface, the number of the first pits is 6, the width of the first pits is 0.2 μm, and the depth of the first pits is 0.2 μm; the secondary particle includes 163 primary spherical particles; based on the total number of the first silicon-carbon particles and the second silicon-carbon particles, the content of the first silicon-carbon particles accounts for 99.9%; the second silicon-carbon particle has a coating layer on its surface, the coating layer includes PAA and has a thickness of 1.1 nm; the third silicon-carbon particle has a second pit on its surface, the number of the second pits is 2, the width of the second pits is 0.1 μm, and the depth of the second pits is 0.1 μm;

[0162] D1 / C1 is 29.74, D3 / C3 is 12.38;

[0163] The mass content of elemental silicon in the negative electrode active layer is C, which is 8.25%, and C / (σ×A) is 1.64×10 -3 ;

[0164] The grooves have a depth of 25.3 μm, a width of 76 μm, and a pitch of 1217 μm.

[0165] (2) Preparation of positive electrode sheet

[0166] Lithium cobalt oxide, a positive electrode conductive agent (conductive carbon black and carbon nanotubes mixed in a mass ratio of 2:1), and polyvinylidene fluoride in a mass ratio of 97:1.5:1.5, N-methylpyrrolidone (NMP) was added, and the mixture was stirred evenly to prepare a positive electrode slurry; the positive electrode slurry was coated on both sides of an aluminum foil, dried, rolled, and slit; and then processed using a special roller with protrusions to obtain a positive electrode sheet having a concave area on one side and a convex area on the other side.

[0167] Among them, the convex area has several convex parts, and the concave area has several concave parts; the concave parts correspond to the positions of the convex parts on the surface of the positive electrode active layer one by one, the depth of the concave part is 20.3 μm, the width of the concave part is 2.5 mm, the spacing between the concave parts is 4 mm, and the ratio of the area of ​​the concave part area projected on the surface of the positive electrode sheet to the area of ​​the positive electrode active layer projected on the surface of the positive electrode sheet is 73%; the height of the convex part is 20.3 μm, the width of the convex part projected on the surface of the positive electrode sheet is 2.5 mm, the spacing between the convex parts is 4 mm, and the ratio of the area of ​​the convex part area projected on the surface of the positive electrode sheet to the area of ​​the positive electrode active coating layer projected on the surface of the positive electrode sheet is 73%.

[0168] (3) Preparation of diaphragm

[0169] The polyacrylonitrile and melamine cyanurate were ground, and then mixed with styrene-butadiene rubber and lithium polyacrylate in a mass ratio of 38.4:57.6:2:2 (polyacrylonitrile: melamine cyanurate: styrene-butadiene rubber: lithium polyacrylate), and NMP was added to obtain a nitrogen-containing functional layer slurry; the nitrogen-containing functional layer slurry was applied to one side of a polyethylene film (with a thickness of 4.5 μm) and dried (to form a nitrogen-containing functional layer with a thickness of 1.3 μm and an average particle size of nitrogen-containing particles of 498 nm); a first adhesive layer (including PVDF, PMMA and CMC, wherein the mass ratio of PVDF, PMMA and CMC was 4:4:2, forming a first adhesive layer with a thickness of 0.8 μm) was applied on the surface of the nitrogen-containing functional layer, and a second adhesive layer (including PMMA and CMC, wherein the mass ratio of PMMA and CMC was 8:2, forming a second adhesive layer with a thickness of 0.8 μm) was applied on the other side of the polyethylene film to obtain a diaphragm;

[0170] Among them, the mass content of nitrogen on the surface of the diaphragm accounts for 37.4%.

[0171] (4) Preparation of electrolyte

[0172] In a glove box (H2O <0.01ppm, O2 <0.01ppm, Ar atmosphere), EC, PC and DEC were mixed in a mass ratio of 1:3:6, and then 32.5% of DFEA and 12.5% ​​of lithium hexafluorophosphate based on the total mass of the electrolyte were added and mixed evenly. After mixing evenly, 10% of fluoroethylene carbonate, 2% of succinonitrile, 1.5% of adiponitrile and 1.5% of 1,3,6-hexanetrionitrile based on the total mass of the electrolyte were added and mixed evenly to obtain an electrolyte.

[0173] (5) Preparation of batteries

[0174] The negative electrode sheet prepared in step (1), the separator prepared in step (3) and the positive electrode sheet prepared in step (2) are wound to obtain a winding core; and a battery is obtained by packaging, baking, liquid injection, formation, secondary sealing, sorting and OCV.

[0175] Among them, the positive electrode active layer with a convex area is located close to the winding center of the core, and the positive electrode active layer with a concave area is located away from the winding center of the core; the nitrogen-containing functional layer of the diaphragm faces the positive electrode sheet.

[0176] Example 2

[0177] Prepare the battery as follows:

[0178] (1) Preparation of negative electrode sheet

[0179] Artificial graphite (average particle size D5 is 10.5 μm), conductive carbon black, lithium carboxymethyl cellulose and styrene butadiene rubber are mixed in a mass ratio of 97:0.5:1.2:1.3, and deionized water is added to prepare the second negative electrode slurry; artificial graphite (average particle size D6 is 2.4 μm), silicon carbon material, carbon nanotubes, lithium carboxymethyl cellulose, styrene butadiene rubber and polyacrylic acid are mixed in a mass ratio of 58.2:38.8:0.5:0.8:0.5:1.2, and deionized water is added to prepare the second negative electrode slurry. Deionized water is used to prepare a first negative electrode slurry; the second negative electrode slurry is applied to both sides of the negative electrode current collector (copper foil, with a tensile strength σ in the width direction of 462 MPa and an elongation at break A in the width direction of 10.4%) and dried; the first negative electrode slurry is then applied to both sides of the negative electrode current collector coated with the second negative electrode slurry after drying, and after baking, roll pressing, die cutting, and cold pressing, grooves are etched on the outer surface of the negative electrode active layer using laser processing technology to obtain a negative electrode sheet;

[0180] Wherein, in the thickness direction of the negative electrode sheet, the size ratio of the second active layer to the first active layer is 1:1;

[0181] The silicon-carbon material comprises first silicon-carbon particles, second silicon-carbon particles, third silicon-carbon particles and fourth silicon-carbon particles mixed in a mass ratio of 29:5:65:1; the sphericity S1 of the first silicon-carbon particles is 0.95, the average particle size D1 is 10.2 μm, and the mass content of silicon element C1 is 35.3%; the second silicon-carbon particles include secondary particles formed by a plurality of primary spherical particles, the sphericity S2 of the primary spherical particles is 0.95, the average particle size D2 of the primary spherical particles is 0.1 μm, the average particle size D of the second silicon-carbon particles is 15.4 μm, the sphericity is 0.42, and the mass content of silicon element C2 is 36.5%; the average particle size D3 of the third silicon-carbon particles is 2.1 μm, the sphericity S3 is 0.96, and the mass content of silicon element C3 is 25.9%; the average particle size D4 of the fourth silicon-carbon particles is 10.1 μm, and the sphericity S4 is 0.65;

[0182] The first silicon-carbon particle has a first pit on its surface, the number of the first pits is 18, the width of the first pits is 0.5 μm, and the depth of the first pits is 0.4 μm; the secondary particle includes 642 primary spherical particles; based on the total number of the first silicon-carbon particles and the second silicon-carbon particles, the content of the first silicon-carbon particles accounts for 92.5%; the second silicon-carbon particle has a coating layer on its surface, the coating layer includes PAA and has a thickness of 0.05 nm; the third silicon-carbon particle has a second pit on its surface, the number of the second pits is 8, the width of the second pits is 0.5 μm, and the depth of the second pits is 0.3 μm;

[0183] D1 / C1 is 28.9, D3 / C3 is 8.11;

[0184] The mass content of elemental silicon in the negative electrode active layer is C, which is 5.87% and C / (σ×A) is 1.22×10 -3 ;

[0185] The grooves have a depth of 5.6 μm, a width of 21.4 μm, and a pitch of 542 μm.

[0186] (2) Preparation of positive electrode sheet

[0187] Lithium cobalt oxide, a positive electrode conductive agent (conductive carbon black and carbon nanotubes mixed in a mass ratio of 2:1), and polyvinylidene fluoride in a mass ratio of 97:1.5:1.5, N-methylpyrrolidone (NMP) was added, and the mixture was stirred evenly to prepare a positive electrode slurry; the positive electrode slurry was coated on both sides of an aluminum foil, dried, rolled, and slit; and then processed using a special roller with protrusions to obtain a positive electrode sheet having a concave area on one side and a convex area on the other side.

[0188] Among them, the convex area has several convex parts, and the concave area has several concave parts; the concave parts correspond to the positions of the convex parts on the surface of the positive electrode active layer one by one, the depth of the concave part is 10.2 μm, the width of the concave part is 1.2 mm, the spacing between the concave parts is 2 mm, and the ratio of the area of ​​the concave part area projected on the surface of the positive electrode sheet to the area of ​​the positive electrode active layer projected on the surface of the positive electrode sheet is 85%; the height of the convex part is 10.2 μm, the width of the convex part projected on the surface of the positive electrode sheet is 1.2 mm, the spacing between the convex parts is 2 mm, and the ratio of the area of ​​the convex part area projected on the surface of the positive electrode sheet to the area of ​​the positive electrode active coating layer projected on the surface of the positive electrode sheet is 85%.

[0189] (3) Preparation of diaphragm

[0190] The polyacrylonitrile and 1,3,5-triazine-2,4,6-triamine were ground, and then mixed with styrene-butadiene rubber and lithium polyacrylate in a mass ratio of 40.3:55.7:2:2 (polyacrylonitrile: 1,3,5-triazine-2,4,6-triamine: styrene-butadiene rubber: lithium polyacrylate), and NMP was added to obtain a nitrogen-containing functional layer slurry; the nitrogen-containing functional layer slurry was coated on one side of a polyethylene film (with a thickness of 4 μm) and dried (to form a nitrogen-containing functional layer with a thickness of 0.5 μm and an average particle size of nitrogen-containing particles of 107 nm); a first adhesive layer (including PVDF, PMMA and CMC, wherein the mass ratio of PVDF, PMMA and CMC is 4:4:2, forming a first adhesive layer with a thickness of 2 μm) was coated on the surface of the nitrogen-containing functional layer, and a second adhesive layer (including PMMA and CMC, wherein the mass ratio of PMMA and CMC is 8:2, forming a second adhesive layer with a thickness of 2 μm) was coated on the other side of the polyethylene film to obtain a diaphragm;

[0191] Among them, the mass content of nitrogen on the surface of the diaphragm accounts for 48.2%.

[0192] (4) Preparation of electrolyte

[0193] In a glove box (H2O<0.01ppm, O2<0.01ppm, Ar atmosphere), EC, PC and DEC were mixed in a mass ratio of 1:3:6, and then 20% of DFEA and 12.5% ​​of lithium hexafluorophosphate based on the total mass of the electrolyte were added and mixed evenly. After mixing evenly, 10% of fluoroethylene carbonate, 2% of succinonitrile, 1.5% of adiponitrile and 1.5% of 1,3,6-hexanetrionitrile based on the total mass of the electrolyte were added and mixed evenly to obtain an electrolyte.

[0194] (5) Preparation of batteries

[0195] The negative electrode sheet prepared in step (1), the separator prepared in step (3) and the positive electrode sheet prepared in step (2) are wound to obtain a winding core; and a battery is obtained by packaging, baking, liquid injection, formation, secondary sealing, sorting and OCV.

[0196] Among them, the positive electrode active layer with a convex area is located close to the winding center of the core, and the positive electrode active layer with a concave area is located away from the winding center of the core; the nitrogen-containing functional layer of the diaphragm faces the positive electrode sheet.

[0197] Example 3

[0198] Prepare the battery as follows:

[0199] (1) Preparation of negative electrode sheet

[0200] Artificial graphite (average particle size D5 is 28.8 μm), conductive carbon black, lithium carboxymethyl cellulose and styrene butadiene rubber are mixed in a mass ratio of 97:0.5:1.2:1.3, and deionized water is added to prepare the second negative electrode slurry; artificial graphite (average particle size D6 is 9.6 μm), silicon carbon material, carbon nanotubes, lithium carboxymethyl cellulose, styrene butadiene rubber and polyacrylic acid are mixed in a mass ratio of 58.2:38.8:0.5:0.8:0.5:1.2, and deionized water is added to prepare the second negative electrode slurry. Deionized water is used to prepare a first negative electrode slurry; the second negative electrode slurry is applied to both sides of a negative electrode current collector (copper foil, with a tensile strength σ of 647 MPa in the width direction and an elongation at break A of 6.5% in the width direction) and dried; the first negative electrode slurry is then applied to both sides of the dried negative electrode current collector coated with the second negative electrode slurry, and after baking, roll-pressing, die-cutting, and cold-pressing, grooves are etched on the outer surface of the negative electrode active layer using laser processing technology to obtain a negative electrode sheet;

[0201] Wherein, in the thickness direction of the negative electrode sheet, the size ratio of the second active layer to the first active layer is 1:1;

[0202] The silicon-carbon material comprises first silicon-carbon particles, second silicon-carbon particles, third silicon-carbon particles, and fourth silicon-carbon particles mixed in a mass ratio of 45:10:40:5; the sphericity S1 of the first silicon-carbon particles is 0.9, the average particle size D1 is 14.8 μm, and the mass content of silicon element C1 is 49.6%; the second silicon-carbon particles include secondary particles formed by a plurality of primary spherical particles, the sphericity S2 of the primary spherical particles is 0.91, the average particle size D2 of the primary spherical particles is 1.6 μm, the average particle size D of the second silicon-carbon particles is 28.8 μm, the sphericity is 0.74, and the mass content of silicon element C2 is 48.4%; the average particle size D3 of the third silicon-carbon particles is 4.8 μm, the sphericity S3 is 0.91, and the mass content of silicon element C3 is 34.6%; the average particle size D4 of the fourth silicon-carbon particles is 29.7 μm, and the sphericity S4 is 0.52;

[0203] The first silicon-carbon particles have first pits on their surfaces, with a total number of 36, a width of 1.1 μm, and a depth of 1 μm. The secondary particles include 474 primary spherical particles. Based on the total number of the first and second silicon-carbon particles, the content of the first silicon-carbon particles accounts for 90.7%. The second silicon-carbon particles have a coating layer on their surfaces, which includes CMC and SBR and has a thickness of 1.9 nm. The third silicon-carbon particles have second pits on their surfaces, with a total number of 18, a width of 1.2 μm, and a depth of 1 μm.

[0204] D1 / C1 is 29.84, D3 / C3 is 13.87;

[0205] The mass content of elemental silicon in the negative electrode active layer is C, which is 8.65% and C / (σ×A) is 2.06×10 -3 ;

[0206] The grooves have a depth of 48.2 μm, a width of 146.6 μm, and a pitch of 1984 μm.

[0207] (2) Preparation of positive electrode sheet

[0208] Lithium cobalt oxide, a positive electrode conductive agent (conductive carbon black and carbon nanotubes mixed in a mass ratio of 2:1), and polyvinylidene fluoride in a mass ratio of 97:1.5:1.5, N-methylpyrrolidone (NMP) was added, and the mixture was stirred evenly to prepare a positive electrode slurry; the positive electrode slurry was coated on both sides of an aluminum foil, dried, rolled, and slit; and then processed using a special roller with protrusions to obtain a positive electrode sheet having a concave area on one side and a convex area on the other side.

[0209] Among them, the convex area has several convex parts, and the concave area has several concave parts; the concave parts correspond to the positions of the convex parts on the surface of the positive electrode active layer one by one, the depth of the concave part is 30 μm, the width of the concave part is 4.8 mm, the spacing between the concave parts is 6 mm, and the ratio of the area of ​​the concave part area projected on the surface of the positive electrode sheet to the area of ​​the positive electrode active layer projected on the surface of the positive electrode sheet is 42%; the height of the convex part is 30 μm, the width of the convex part projected on the surface of the positive electrode sheet is 4.8 mm, the spacing between the convex parts is 6 mm, and the ratio of the area of ​​the convex part area projected on the surface of the positive electrode sheet to the area of ​​the positive electrode active coating layer projected on the surface of the positive electrode sheet is 42%.

[0210] (3) Preparation of diaphragm

[0211] The polyacrylonitrile and melamine cyanurate were ground, and then mixed with styrene-butadiene rubber and lithium polyacrylate in a mass ratio of 75.8:20.2:2:2 (polyacrylonitrile: melamine cyanurate: styrene-butadiene rubber: lithium polyacrylate), and NMP was added to obtain a nitrogen-containing functional layer slurry; the nitrogen-containing functional layer slurry was applied to one side of a polyethylene film (with a thickness of 5 μm) and dried (to form a nitrogen-containing functional layer with a thickness of 2 μm and an average particle size of nitrogen-containing particles of 896 nm); a first adhesive layer (including PVDF, PMMA and CMC, wherein the mass ratio of PVDF, PMMA and CMC was 4:4:2, forming a first adhesive layer with a thickness of 0.5 μm) was applied on the surface of the nitrogen-containing functional layer, and a second adhesive layer (including PMMA and CMC, wherein the mass ratio of PMMA and CMC was 8:2, forming a second adhesive layer with a thickness of 0.5 μm) was applied on the other side of the polyethylene film to obtain a diaphragm;

[0212] Among them, the mass content of nitrogen on the surface of the diaphragm accounts for 29.7%.

[0213] (4) Preparation of electrolyte

[0214] In a glove box (H2O<0.01ppm, O2<0.01ppm, Ar atmosphere), EC, PC and DEC were mixed in a mass ratio of 1:3:6, and then 50% of DFEA and 12.5% ​​of lithium hexafluorophosphate based on the total mass of the electrolyte were added and mixed evenly. After mixing evenly, 10% of fluoroethylene carbonate, 2% of succinonitrile, 1.5% of adiponitrile and 1.5% of 1,3,6-hexanetrionitrile based on the total mass of the electrolyte were added and mixed evenly to obtain an electrolyte.

[0215] (5) Preparation of batteries

[0216] The negative electrode sheet prepared in step (1), the separator prepared in step (3) and the positive electrode sheet prepared in step (2) are wound to obtain a winding core; and a battery is obtained by packaging, baking, liquid injection, formation, secondary sealing, sorting and OCV.

[0217] Among them, the positive electrode active layer with a convex area is located close to the winding center of the core, and the positive electrode active layer with a concave area is located away from the winding center of the core; the nitrogen-containing functional layer of the diaphragm faces the positive electrode sheet.

[0218] Example 4

[0219] This is used to verify the impact of changes in the "sphericity S1 of the first silicon-carbon particles."

[0220] The method is carried out in accordance with Example 1, except that S1 is changed by regulating the parameters of the first pits on the surface of the first silicon-carbon particles. Specifically, the number of the first pits is 46, the width of the first pits is 0.6 μm, the depth of the first pits is 0.5 μm, and S1 is 0.8.

[0221] Example 5 Group

[0222] This set of examples is used to verify the impact of changes in the “average particle size D1 of the first silicon-carbon particles”.

[0223] This group of examples is carried out with reference to Example 1, except that D1 is changed as follows:

[0224] In Example 5a, D1 is 5.3 μm; C1 is 30.2%; the number of first pits is 2, the width of the first pit is 0.2 μm, and the depth of the first pit is 0.2 μm; D1 / C1 is 17.55; the mass content of elemental silicon in the negative electrode active layer is C, which is 6.13%, and C / (σ×A) is 1.22×10 -3 ;

[0225] In Example 5b, D1 is 19.6 μm; C1 is 57.6%; the number of first pits is 22, the width of the first pits is 0.3 μm, and the depth of the first pits is 0.2 μm; based on the total number of the first silicon-carbon particles and the second silicon-carbon particles, the content of the first silicon-carbon particles accounts for 98.2%; D1 / C1 is 34.03; the mass content of elemental silicon in the negative electrode active layer accounts for C of 10.79%, and C / (σ×A) is 2.14×10 -3 .

[0226] Example 6

[0227] Used to verify the impact of changes in the "sphericity S2 of primary spherical particles".

[0228] The same procedure is carried out with reference to Example 1, except that S2 is changed, specifically, S2 is 0.8.

[0229] Example 7

[0230] This test is used to verify the effect of changes in the average particle size D2 of the primary spherical particles.

[0231] The process is carried out with reference to Example 1, except that D2 is changed, specifically: D2 is 2.5 μm.

[0232] Example 8 Group

[0233] This set of examples is used to verify the impact of changes in the "average particle size D of the second silicon-carbon particles".

[0234] This group of examples was carried out with reference to Example 1, except that D was changed by regulating N, as follows:

[0235] In Example 8a, D is 5.5 μm; C2 is 26.1%; N is 13; based on the total number of the first silicon-carbon particles and the second silicon-carbon particles, the content of the first silicon-carbon particles accounts for 98.2%;

[0236] In Example 8b, D is 34.7 μm, C2 is 54.2%, and N is 746.

[0237] Example 9

[0238] Used to verify the impact of "whether the surface of the second silicon-carbon particles has a coating layer".

[0239] The process is carried out in accordance with Example 1, except that the second silicon-carbon particles do not have a coating layer on their surfaces.

[0240] Example 10

[0241] Used to verify the impact of changes in the "sphericity S3 of the third silicon-carbon particles".

[0242] The method is carried out in accordance with Example 1, except that S3 is changed by adjusting the parameters of the second pits on the surface of the third silicon-carbon particles. Specifically, the number of the second pits is 12, the width of the second pits is 0.3 μm, the depth of the second pits is 0.2 μm, and S3 is 0.8.

[0243] Example 11 Group

[0244] This set of examples is used to verify the impact of the change of "D1 / C1".

[0245] This group of examples were carried out with reference to Examples 2 and 3, except that D1 / C1 was changed by regulating C1, as follows:

[0246] Example 11a was carried out in accordance with Example 2, except that C1 was 49.3%; D1 / C1 was 20.69; the mass content of elemental silicon in the negative electrode active layer was C 6.68%, and C / (σ×A) was 1.33×10 -3 ;

[0247] Example 11b was carried out in accordance with Example 3, except that C1 was 35.6%; D1 / C1 was 41.57; the mass content of elemental silicon in the negative electrode active layer was 7.39%; and C / (σ×A) was 1.47×10 -3 .

[0248] Example 12 Group

[0249] This set of examples is used to verify the impact of the change of "D3 / C3".

[0250] This group of examples were carried out with reference to Examples 2 and 3, except that D3 / C3 was changed by regulating C3, as follows:

[0251] Example 12a was carried out in accordance with Example 2, except that C3 was 33.5%; D3 / C3 was 6.27; the mass content of elemental silicon in the negative electrode active layer was C 6.86%; and C / (σ×A) was 1.36×10 -3 ;

[0252] Example 12b was carried out in accordance with Example 3, except that C3 was 26.1%; D3 / C3 was 18.39; the mass content of elemental silicon in the negative electrode active layer was 7.97%; and C / (σ×A) was 1.58×10 -3 .

[0253] Example 13 Group

[0254] This group of experiments was conducted to verify the effects of changes in the "average particle size D4 of the fourth silicon-carbon particles".

[0255] This group of implementations was carried out with reference to Example 1, except that D4 was changed as follows:

[0256] Example 13a, D4 is 5.5 μm;

[0257] Example 13b, D4 is 33.6 μm.

[0258] Example 14

[0259] Used to verify the impact of changes in the "negative electrode active layer partition setting".

[0260] The process is carried out in accordance with Example 1, except that the negative electrode active layer is not partitioned in step (1), as follows:

[0261] (1) Preparation of negative electrode sheet

[0262] Artificial graphite (average particle size D6 of 5.3 μm), silicon-carbon material, carbon nanotubes, lithium carboxymethyl cellulose, styrene-butadiene rubber, and polyacrylic acid were mixed in a mass ratio of 77.6:19.4:0.5:0.8:0.5:1.2, and deionized water was added to prepare a negative electrode slurry. The negative electrode slurry was coated on both sides of a negative electrode current collector (copper foil with a tensile strength σ in the width direction of 530 MPa and an elongation at break A in the width direction of 9.5%). After baking, roll pressing, die cutting, and cold pressing, grooves were etched on the outer surface of the negative electrode active layer using laser processing technology to obtain a negative electrode sheet.

[0263] The parameters of the silicon-carbon material and the groove are the same as those in Example 1;

[0264] In addition, since the mass content of elemental silicon in the negative electrode active layer, C, has a great influence on the performance of the battery, in order to reduce the influence of C, the ratio of the negative electrode slurry was adjusted to be the same as that in Example 1.

[0265] Example 15

[0266] This is used to verify the impact of changing the positions of the second active layer and the first active layer.

[0267] The process was carried out in accordance with Example 1, except that the positions of the second active layer and the first active layer were changed. Specifically, the first negative electrode slurry was first applied to both sides of the negative electrode current collector and dried; then the second negative electrode slurry was applied to both sides of the negative electrode current collector coated with the dried first negative electrode slurry. After baking, roll pressing, die cutting, and cold pressing, a groove was etched on the outer surface of the negative electrode active layer using laser processing technology to obtain a negative electrode sheet.

[0268] Example 16

[0269] This group of examples is used to verify the impact of changes in the "mass content ratio of ethyl fluoroacetate in the electrolyte".

[0270] This group of examples was carried out with reference to Example 1, except that the mass content ratio of ethyl fluoroacetate in the electrolyte was changed, as follows:

[0271] In Example 16a, the mass content of DFEA in the electrolyte is 15%;

[0272] In Example 16b, the mass content of DFEA in the electrolyte is 55%.

[0273] Example 17

[0274] Used to verify the impact of changes in "ethyl fluoroacetate".

[0275] The same method was carried out as in Example 1, except that ethyl fluoroacetate was changed, specifically, DFEA was replaced with ethyl 2,2-difluoroacetate of the same mass.

[0276] Comparative Example 1

[0277] The process is carried out with reference to Example 1, except that the silicon-carbon material is a mixture of the second silicon-carbon particles and the fourth silicon-carbon particles in a mass ratio of 0.1:99.9.

[0278] Comparative Example 2

[0279] The process is carried out in accordance with Example 1, except that the sphericity of the second silicon-carbon particles is 0.92.

[0280] Comparative Example 3

[0281] The process is carried out with reference to Example 1, except that the silicon-carbon material only includes the first silicon-carbon particles.

[0282] Test Case

[0283] (1) Energy density test

[0284] The batteries prepared in the examples and comparative examples were subjected to energy density tests. The specific test methods are as follows:

[0285] Charge at 0.2C to an upper voltage limit of 4.5V (cut off at 0.02C), and discharge at 0.2C to a lower voltage limit of 3V, and repeat this three times; record the third discharge energy as Q; then use a 2.5D microscope to test the width L and height W of the battery, and use a PPG thickness tester to test the fully charged thickness T of the battery; the energy density is Q / (L×W×T), in units of Wh / L. The results are recorded in Table 1.

[0286] (2) Normal temperature cycle test

[0287] The battery was placed at 25°C for 1 hour, charged at 0.7C to a cut-off voltage of 4.5V, charged at constant voltage to a cut-off current of 0.05C, and discharged at 0.7C to 3V, and the above charge and discharge steps were repeated 200 times; the 200th discharge capacity was divided by the largest value among the 1st to 3rd discharge capacities, which was recorded as the capacity retention rate; the battery before the high-temperature cycle test was fully charged, and its thickness was measured and recorded as T1. After 200 cycles, the battery was fully charged and its thickness was measured and recorded as T2. The thickness expansion rate = (T2-T1) / T1; the above test results are recorded in Table 1; the battery after the cycle was dissected to observe whether the negative electrode sheet was broken. The results showed that the negative electrode sheet of the battery in the embodiment was broken after 200 cycles.

[0288] (3) 45℃ high temperature cycle test

[0289] The battery was placed at 45°C for 1 hour, charged at 0.7C to a cutoff voltage of 4.5V, charged at constant voltage to a cutoff current of 0.05C, and discharged at 0.7C to 3V. The above charge and discharge steps were repeated 200 times; the 200th discharge capacity was divided by the maximum value of the 1st to 3rd discharge capacities, which was recorded as the capacity retention rate; the battery was fully charged before the high-temperature cycle test, and its thickness was measured and recorded as T3. After 200 cycles, the battery was fully charged and its thickness was measured and recorded as T4. The thickness expansion rate = (T4-T3) / T3; the above test results are recorded in Table 1.

[0290] (4) Rate test

[0291] The batteries prepared in the examples and comparative examples were subjected to rate tests. The specific test methods are as follows:

[0292] At 25°C, discharge the battery at 0.2C to 3V, then charge it at 1C constant current to a cutoff voltage of 4.5V, record the constant current charge capacity R1, continue constant voltage charging to a cutoff current of 0.05C, record the constant voltage charge capacity R2, constant current charge ratio = R1 / (R1+R2), and record the results in Table 1.

[0293] Table 1

[0294]

[0295]

[0296] As shown in Table 1, the battery prepared using the negative electrode sheet of the present invention achieves higher energy density, room-temperature cycle performance, high-temperature cycle performance, and rate performance compared to the comparative example. Comparative Example 2 utilizes second silicon-carbon particles with higher sphericity. During the charge-discharge cycle, these second silicon-carbon particles with higher sphericity fall off, resulting in a lower cycle capacity retention rate for the battery, limiting its applicability.

[0297] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A negative electrode sheet, characterized in that: The negative electrode active layer comprises a negative electrode current collector and a negative electrode active layer located on at least one side of the negative electrode current collector, wherein the negative electrode active layer comprises a first active layer, the first active layer comprises a first negative electrode active material, the first negative electrode active material comprises a silicon-carbon material, and the silicon-carbon material comprises first silicon-carbon particles and second silicon-carbon particles; The first silicon-carbon particles are single particles, the sphericity of the first silicon-carbon particles is S1, 0.8≤S1≤1; the average particle size of the first silicon-carbon particles is D1, 5μm≤D1≤20μm; the mass content of silicon element in the first silicon-carbon particles is C1, 30%≤C1≤60%; The second silicon-carbon particles include secondary particles formed by a plurality of primary spherical particles, the sphericity of the primary spherical particles is S2, 0.8≤S2≤1; the average particle size of the primary spherical particles is D2, 100nm≤D2≤2.5μm; the average particle size of the second silicon-carbon particles is D, 5μm≤D≤36μm, and the sphericity of the second silicon-carbon particles is 0.35-0.79; the mass content of silicon element in the second silicon-carbon particles is C2, 25%≤C2≤55%.

2. The negative electrode sheet according to claim 1, wherein: 10μm≤D1≤15μm; and / or, 100 nm ≤ D2 ≤ 2 μm; and / or, 15 μm ≤ D ≤ 30 μm; and / or, 35% ≤ C1 ≤ 50%; and / or, 36% ≤ C2 ≤ 49%; And / or, the silicon-carbon material comprises a porous carbon matrix and silicon particles located in pores within the porous carbon matrix; And / or, the first silicon-carbon particles have first pits on their surfaces; Preferably, the number of the first pits is 1-100, the width of the first pits is 0.1 μm-3 μm, and the depth of the first pits is 0.01 μm-3 μm. More preferably, 0.9≤S1≤0.99; More preferably, the number of the first pits is 6-40, the width of the first pits is 0.2 μm-1.5 μm, and the depth of the first pits is 0.2 μm-1 μm.

3. The negative electrode sheet according to claim 1 or 2, wherein: Based on the total amount of the first silicon-carbon particles and the second silicon-carbon particles, the content of the first silicon-carbon particles accounts for 50%-99.9%; Preferably 80%-99.9%; More preferably, it is 90%-99.9%.

4. The negative electrode sheet according to claim 1 or 2, wherein: The silicon-carbon material further includes third silicon-carbon particles, wherein the average particle size of the third silicon-carbon particles is D3, 2 μm≤D3<5 μm; the sphericity of the third silicon-carbon particles is S3, 0.8≤S3≤1; and the mass content of silicon in the third silicon-carbon particles is C3, 20%≤C3≤45%; Preferably, 25%≤C3≤35%; Preferably, the surface of the third silicon-carbon particle has second pits; more preferably, the number of the second pits is 1-100, the width of the second pits is 0.05 μm-2 μm, and the depth of the second pits is 0.01 μm-2 μm; further preferably, the number of the second pits is 2-20, the width of the second pits is 0.1 μm-1.5 μm, and the depth of the second pits is 0.1 μm-1 μm; more preferably, 0.9≤S3≤0.99; Preferably, D1 and C1 satisfy: 6≤D1 / C1≤60; more preferably, 20≤D1 / C1≤42; Preferably, D3 and C3 satisfy: 4≤D3 / C3≤60; more preferably, 6≤D3 / C3≤20.

5. The negative electrode sheet according to claim 1 or 2, wherein: The silicon-carbon material further includes fourth silicon-carbon particles; the average particle size of the fourth silicon-carbon particles is D4, 5 μm≤D4≤35 μm; the sphericity of the fourth silicon-carbon particles is S4, 0.5≤S4<0.8; preferably, 10 μm≤D4≤30 μm; And / or, the negative electrode active layer further includes a second active layer, the second active layer and the first active layer are arranged along the thickness direction of the negative electrode sheet, and the second active layer is located between the negative electrode current collector and the first active layer; the second active layer includes a second negative electrode active material, the second negative electrode active material includes a first carbon-based material, and the first negative electrode active material further includes a second carbon-based material; Preferably, the average particle size of the second carbon-based material is smaller than the average particle size of the first carbon-based material; more preferably, the average particle size of the first carbon-based material is D5, 6 μm ≤ D5 ≤ 30 μm, and the average particle size of the second carbon-based material is D6, 2 μm ≤ D6 ≤ 10 μm; further preferably, 10 μm ≤ D5 ≤ 30 μm; Preferably, the first carbon-based material and the second carbon-based material each independently comprise artificial graphite.

6. The negative electrode sheet according to claim 1 or 2, wherein: The tensile strength of the negative electrode current collector in the width direction is σ, 300 MPa≤σ≤850 MPa; the elongation at break of the negative electrode current collector in the width direction is A, 2%≤A≤15%; the mass content of elemental silicon in the negative electrode active layer is C, 3%≤C≤30%; Preferably, 450 MPa≤σ≤650 MPa; Preferably, 6%≤A≤10.5%; Preferably, 5%≤C≤15%.

7. The negative electrode sheet according to claim 1 or 2, wherein: The outer surface of the negative electrode active layer is provided with a plurality of grooves, the depth of the grooves is 5 μm-50 μm, the width of the grooves is 20 μm-150 μm, and the spacing between the grooves is 500 μm-2000 μm.

8. A lithium ion secondary battery, characterized in that: The lithium-ion secondary battery comprises the negative electrode sheet according to any one of claims 1 to 7; Preferably, the lithium-ion secondary battery further includes a positive electrode sheet and a separator, and the positive electrode sheet, the separator and the negative electrode sheet are stacked and wound to form a winding core.

9. The lithium ion secondary battery according to claim 8, wherein The charging cut-off voltage of the lithium-ion secondary battery is ≥4.5V; The diaphragm includes a base film and a nitrogen-containing functional layer located on at least one side of the base film, wherein the nitrogen-containing functional layer is arranged facing the positive electrode sheet; Preferably, the nitrogen-containing functional layer comprises nitrogen-containing particles, and the nitrogen-containing particles comprise at least one of polyacrylonitrile, nitrile rubber, 1,3,5-triazine-2,4,6-triamine, melamine cyanurate, thiocyanate and derivatives thereof; Preferably, the thickness of the nitrogen-containing functional layer is 0.5 μm-5 μm; more preferably 0.5 μm-2 μm; Preferably, the mass content of nitrogen on the surface of the diaphragm is 20%-50%.

10. The lithium ion secondary battery according to claim 8 or 9, wherein The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer located on at least one side of the positive electrode current collector; The surface of the positive electrode active layer located near the winding center of the core has a convex area, wherein the convex area has a plurality of convex parts; the surface of the positive electrode active layer located away from the winding center of the core has a concave area, wherein the concave area has a plurality of concave parts; preferably, the depth of the concave parts is 10 μm-30 μm, the spacing between the concave parts is 2 mm-6 mm, and the width of the concave parts is 1 mm-5 mm; preferably, the ratio of the area of ​​the orthogonal projection of the concave part on the surface of the positive electrode sheet to the area of ​​the orthogonal projection of the positive electrode active layer on the side away from the winding center of the core on the surface of the positive electrode sheet is 40%-90%; And / or, the lithium-ion secondary battery further comprises an electrolyte, and the electrolyte comprises ethyl fluoroacetate; Preferably, based on the total mass of the electrolyte, the content of the ethyl fluoroacetate is 20%-50%; Preferably, the ethyl fluoroacetate includes 2,2-difluoroethyl acetate and / or ethyl 2,2-difluoroacetate.

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