Negative electrode plate and battery

Abstract

The present disclosure relates to the technical field of batteries, in particular to a negative electrode plate and a battery comprising the negative electrode plate. The negative electrode plate comprises a negative current collector, a first coating located on the surface of the negative current collector and a second coating located on the surface of the first coating; the first coating comprises first silicon particles and a coating layer located on the surface of the first silicon particles; the coating layer comprises a polymer material, carbon nanotubes, and a first conductive agent, and the ratio R of the center distance between adjacent particles to the median particle size Dv50 of the first silicon particles is (0.01-100):1. The negative electrode plate in the present disclosure has a low expansion rate and high safety performance. A battery comprising the negative electrode plate of the present disclosure has a low expansion rate, high cycling stability, and high safety performance.

Description

Negative electrode sheet and battery Technical Field

[0001] The present disclosure relates to the technical field of batteries, and in particular to a negative electrode sheet and a battery including the negative electrode sheet.

[0002] Background of the Invention

[0003] Lithium-ion batteries have high energy density and cycle life and have been widely used in consumer electronics and electric vehicles. Due to the continuous improvement of lithium-ion battery energy density, the average cruising range of new energy vehicles has increased from about 100 kilometers in 2013 to more than 600 kilometers in 2022. However, the existing lithium-ion battery chemical system has basically reached its limit and it is difficult to make a major breakthrough. Silicon-based negative electrodes have extremely high theoretical capacity (Si is 4200mAh / g, SiO x The capacity of the silicon-based negative electrode is 2680 mAh / g, which is much greater than the capacity of the current graphite negative electrode (370 mAh / g). Therefore, using a silicon-based negative electrode to completely or partially replace the graphite negative electrode is the simplest and most effective way to increase the energy density of the battery.

[0004] However, silicon-based materials still face a series of challenges. The most significant technical bottleneck is how to effectively suppress the volume expansion of the silicon anode. Since the silicon anode can embed and release more lithium ions during the charge and discharge process, it triggers a huge volume expansion and contraction effect, which in turn causes cracks and even pulverization. This leads to the continuous rupture, repeated formation, and continuous thickening of the SEI film. This also continuously consumes electrolyte, ultimately causing capacity decay and cycle failure. Especially in the late stages of the cycle, the battery cell thickness seriously exceeds the standard, and even causes safety accidents. Limited by the large expansion of the silicon anode and poor cycle stability, high-energy-density lithium-ion batteries based on silicon-based anodes have not yet been widely used.

[0005] Therefore, it is very important to invent a battery with low expansion rate, high cycle stability and high safety performance.

[0006] Summary of the Invention

[0007] Because pure silicon systems suffer from excessive expansion and low initial efficiency, graphite-doped silicon is currently commonly used to construct negative electrode plates. As shown in Figures 1 and 2, graphite and silicon are mixed and coated on the surface of the negative electrode current collector to form a graphite-silicon blended coating. However, this still poses the problem of high expansion. Graphite-doped silicon anode systems present several issues at the electrode level: 1) Too little binder is distributed on the silicon anode surface: In graphite-doped silicon systems, insufficient binder is distributed on the surface of silicon particles, easily disrupting the bonding network. 2) The pores between silicon and graphite particles continue to increase: The expansion differences between silicon and graphite are significant. In graphite-doped silicon systems, silicon and graphite particles with different expansion rates easily move and rearrange in the graphite-doped silicon system, increasing the porosity between the particles. 3) The binder needs to withstand more cyclic stretching: The difference in potential between graphite and silicon for lithium intercalation and deintercalation causes silicon to intercalate lithium first, followed by graphite. This results in the binder undergoing two stretching cycles per cycle, requiring it to withstand increased fatigue strength. Therefore, current graphite-doped silicon anodes have been unable to effectively address the problem of large silicon volume expansion.

[0008] The present invention aims to overcome the aforementioned problems of the prior art by providing a negative electrode sheet and a battery including the negative electrode sheet. The negative electrode sheet disclosed herein has a low expansion rate and high safety performance; a battery including the negative electrode sheet disclosed herein has a low expansion rate, high cycle stability, and high safety performance.

[0009] The inventors of the present disclosure have discovered that by improving the uniformity of the binder distribution on the surface of silicon particles and reducing the increase in porosity between particles, the expansion rate of the negative electrode sheet can be reduced and the safety of the negative electrode sheet can be improved, thereby reducing the expansion rate of the battery and improving the cycle stability and safety performance of the battery.

[0010] The inventors of this disclosure have further researched and discovered that, in order to improve the uniformity of the binder distribution on the silicon particle surface and reduce the increase in porosity between particles, the structure of the negative electrode sheet can be modified to overcome the adverse effects of volume changes in silicon particles in a graphite-doped silicon system. After extensive research, the inventors of this disclosure have discovered a specific structure that can improve the uniformity of the binder distribution on the silicon particle surface.

[0011] In order to achieve the above-mentioned objectives, the first aspect of the present disclosure provides a negative electrode sheet, which includes a negative electrode current collector, a first coating located on one side or both sides of the negative electrode current collector, and a second coating located on the surface of the first coating, the first coating including first silicon particles and a coating layer located on the surface of the first silicon particles, the coating layer including a polymer material, carbon nanotubes and a first conductive agent, and the ratio R of the center distance between adjacent first silicon particles to the median particle size Dv50 of the first silicon particles is (0.01-100):1.

[0012] A second aspect of the present disclosure provides a battery, comprising the negative electrode sheet described in the first aspect of the present disclosure.

[0013] Through the above technical solution, the present disclosure has at least the following advantages compared with the prior art:

[0014] (1) The negative electrode sheet disclosed herein has a low expansion rate;

[0015] (2) The negative electrode sheet disclosed herein has high safety performance;

[0016] (3) The battery expansion rate of the present disclosure is low;

[0017] (4) The battery disclosed herein has high cycle stability;

[0018] (5) The battery disclosed herein has high safety performance.

[0019] Other features and advantages of the present disclosure will be described in detail in the following detailed description.

[0020] 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

[0021] FIG1 is a schematic diagram showing the structure of a graphite-doped silicon negative electrode sheet in the prior art (wherein 3 represents a negative electrode current collector, 4 represents silicon, and 5 represents graphite).

[0022] FIG. 2 shows an SEM image of a graphite-doped silicon negative electrode sheet in the prior art.

[0023] FIG3 is a schematic structural diagram of the negative electrode sheet of the present disclosure.

[0024] FIG. 4 is a SEM image of a negative electrode sheet according to an embodiment of the present disclosure.

[0025] FIG5 is a SEM image of a negative electrode sheet according to another embodiment of the present disclosure.

[0026] FIG6 is a schematic diagram showing an uneven surface where the first coating layer and the second coating layer of the negative electrode sheet of the present disclosure are in contact.

[0027] FIG7 is a schematic diagram showing the center distance between adjacent first silicon particles in a cross section of a negative electrode sheet.

[0028] FIG8 is a schematic diagram showing the center distance between adjacent first silicon particles in another cross section of a negative electrode sheet. DETAILED DESCRIPTION

[0029] The following describes the specific embodiments of the present disclosure in detail. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present disclosure and are not intended to limit the present disclosure.

[0030] It should be noted that the numerical expressions such as "first" and "second" in this disclosure are only used to distinguish different substances or usage methods, and do not represent a difference in order.

[0031] In a first aspect, the present disclosure provides a negative electrode sheet, comprising a negative electrode current collector, a first coating located on one or both sides of the negative electrode current collector, and a second coating located on the surface of the first coating, wherein the first coating comprises first silicon particles and a coating layer located on the surface of the first silicon particles, the coating layer comprises a polymer material, carbon nanotubes, and a first conductive agent, and the ratio R of the center distance between adjacent first silicon particles to the median particle size Dv50 of the first silicon particles is (0.01-100):1.

[0032] As shown in Figures 3 and 4, the negative electrode sheet includes a negative electrode current collector 3, a first coating 1 located on one or both sides of the negative electrode current collector 3, and a second coating 2 located on the surface of the first coating. The first coating 1 includes first silicon particles 11 and a coating 12 located on the surface of the first silicon particles 11. The coating 12 includes a polymer material, carbon nanotubes, and a first conductive agent. By placing the first silicon particles and the second active material (including one or more of graphite particles and hard carbon particles) in two different coatings (the first silicon particles are located in the first coating, and the second active material is located in the second coating), the problem of easy movement and rearrangement between the particles due to the large difference in expansion coefficient between the first silicon particles and the second active material, which causes increased porosity between the particles, can be reduced or even overcome. It can also reduce the number of times the binder is stretched per cycle, reducing the requirement for binder fatigue strength, and thus improving the battery's cycling stability. At the same time, the polymer material is an elastic material, which can effectively inhibit the expansion of silicon, strengthen the bonding of the negative electrode active layer, improve the stability of the bonding network, and prevent demolding during charging and discharging. The stable existence of the bonding network is beneficial to the stability of the conductive network in the electrode, reduces the risk of conductive network failure, and improves the cycle capacity retention rate. In addition, the polymer material also acts as a binder, achieving the function of bonding the second coating and the current collector, thereby giving full play to the comprehensive performance of silicon particles and graphite.

[0033] The coating layer may include a polymer material, carbon nanotubes, and a first conductive agent. The polymer material in the coating layer can provide a buffer space for volume changes of the silicon particles, and the first conductive agent in the coating layer is conductive, providing an electron channel for the coating layer. Therefore, the coating layer not only provides a buffer space for volume changes of the silicon particles, but also has conductive properties, thereby improving the conductivity of the first coating layer. Moreover, the presence of the coating layer can reduce thermal failure of the silicon particles caused by short circuits and high temperatures, thereby improving the safety performance of the negative electrode sheet.

[0034] The ratio R of the center-to-center distance between adjacent first silicon particles to the median particle size Dv50 of the first silicon particles can be (0.01-100):1 (e.g., 0.01:1, 0.05:1, 0.1:1, 0.5:1, 1:1, 5:1, 10:1, 50:1, 60:1, 70:1, 80:1, 90:1, 100:1). When R is less than 0.01, the coating layer does not provide sufficient buffer space for volume changes of the silicon particles, resulting in a high expansion rate of the negative electrode sheet. When R is greater than 100, although the coating layer provides sufficient buffer space for volume changes of the silicon particles, reducing the expansion rate of the negative electrode sheet, the internal resistance of the battery may increase. When the ratio R of the thickness of the coating layer to the median particle size Dv50 of the first silicon particles is controlled within the above range, the coating layer can partially or completely coat the first silicon particles, ensuring that the coating layer exists around the first silicon particles. In this way, the coating layer provides a larger buffer space for the volume change of the silicon particles while ensuring that the internal resistance of the battery is low, thereby improving the overall performance of the battery.

[0035] As shown in Figures 7 and 8, in a cross-sectional view at any position on the negative electrode sheet, the distance between the two most distant points in the cross-section of each first silicon particle is d, with the midpoint of d being denoted as O. Taking the two first silicon particles in Figure 7 as an example, the distance between the two most distant points in the cross-section of the lower first silicon particle is d1, with the midpoint of d1 being O1. The distance between the two most distant points in the cross-section of the upper first silicon particle is d2, with the midpoint of d2 being O2. The center-to-center distance between adjacent first silicon particles is represented by the most distant distance between the O's of any two adjacent first silicon particles. Two adjacent first silicon particles represent two first silicon particles that are directly adjacent, with no other silicon particles between them. In Figure 8, first silicon particles A and B, as well as first silicon particles A and C, are both considered adjacent first silicon particles, while first silicon particles B and C are not, because first silicon particles A and D are between first silicon particles B and C. In a cross-sectional view of a negative electrode sheet, there may be multiple adjacent first silicon particles, such as shown in Figure 8. In this case, the center-to-center distance between adjacent first silicon particles is the greatest distance between the O's of any two adjacent first silicon particles. Specifically, the greatest distance among O1O2, O1O3, O1O4, O3O4, and O2O4 is the center-to-center distance between adjacent first silicon particles. Ten first silicon particles in the cross-sectional view are selected, and the average of the center-to-center distances between adjacent first silicon particles is calculated and recorded as the center-to-center distance between adjacent first silicon particles.

[0036] In the present disclosure, by designing the negative electrode sheet to have the above-mentioned specific structure, the negative electrode sheet has been able to achieve a lower expansion rate and higher safety than the prior art. To further improve the effect, one or more of the technical features can be further optimized.

[0037] The first silicon particles are dispersed in the coating layer, and the coating layer adheres to the surface of the negative electrode current collector. Therefore, the thickness of the coating layer is equal to the thickness of the first coating layer. The surface of the first silicon particles can be completely covered by the coating layer, or partially covered by the coating layer (for example, the first silicon particles are in contact with the negative electrode current collector, or the first silicon particles are partially exposed on the surface of the first coating layer so that the first silicon particles are partially covered by the coating layer).

[0038] As shown in Figure 5, the entire surface of the first silicon particles (light gray) is covered by the coating layer (dark color), and the second coating layer is located on the surface of the coating layer (first coating layer). This ensures that during cycling, the coating layer on the surface of the first silicon particles forms a durable bonding network, further reducing the expansion rate of the negative electrode sheet and improving its safety. It also improves the adhesion between the coating layer and the second coating layer.

[0039] In one example, the ratio R of the center distance between adjacent particles to the median particle size Dv50 of the first silicon particle is (0.1-10):1.

[0040] According to a specific embodiment, the center distance between adjacent first silicon particles is 0.05μm-2000μm (for example, 0.05μm, 0.1μm, 1μm, 10μm, 20μm, 50μm, 100μm, 200μm, 300μm, 400μm, 500μm, 600μm, 700μm, 800μm, 900μm, 1000μm, 1300μm, 1500μm, 1700μm, 2000μm).

[0041] In one example, the center distance between adjacent first silicon particles is 0.1 μm-500 μm.

[0042] In one example, the carbon nanotubes have a diameter of 0.5 nm to 40 nm (e.g., 0.5 nm, 1 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm), and / or a length of 1 μm to 40 μm (e.g., 1 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm), and / or an aspect ratio of 25 to 80,000 (e.g., 25, 100, 250, 300, 400, 500, 600, 700, 800, 900, 1000, 3000, 10,000, 50,000, 80,000). The carbon nanotubes in the coating layer have the following functions: 1) They can reduce the internal resistance of lithium batteries, which is the most important function of carbon nanotubes as a conductive agent; 2) They can improve the bonding strength of lithium battery pole pieces and reduce the amount of adhesive used. The connection strength between zero-dimensional conductive carbon black and the adhesive is relatively low, while carbon nanotubes are a material with a high aspect ratio. They not only have the function of conducting electricity, but also have a certain function of enhancing the bonding strength with the adhesive. 3) They can extend the cycle life of the battery. As a conductive additive with a high aspect ratio, carbon nanotubes can construct a conductive network on a large scale inside the battery, thereby alleviating problems such as cracking caused by the expansion and contraction of materials such as electrodes. By limiting the above-mentioned specific characteristics of carbon nanotubes, the conductive properties of carbon nanotubes can be further improved, and the strength of the conductive network formed by carbon nanotubes can be increased, thereby improving the cycle capacity retention rate of the battery.

[0043] In one example, the diameter of the carbon nanotube is 1.2 nm-10 nm, and / or the length of the carbon nanotube is 2 μm-30 μm, and / or the aspect ratio of the carbon nanotube is 1000-25000.

[0044] According to a specific embodiment, the median particle size Dv50 of the first silicon particles is 5 μm-20 μm (e.g., 5 μm, 8 μm, 10 μm, 13 μm, 15 μm, 18 μm, 20 μm). In the present disclosure, the median particle size Dv50 can be measured using a laser particle size analyzer.

[0045] In one example, the median particle size Dv50 of the first silicon particles is 6 μm-15 μm.

[0046] In one example, the first silicon particles include one or more of silicon-carbon particles, silicon-oxygen particles, and nano-silicon particles.

[0047] In one example, the silicon-carbon particles include particles formed by mixing silicon or partially oxidized silicon with amorphous or crystalline carbon.

[0048] In one embodiment, the silicon-carbon particles include particles formed by silicon or partially oxidized silicon filling (including partial filling or complete filling) in the pores of porous amorphous carbon or porous crystalline carbon. The porous amorphous carbon or porous crystalline carbon is mainly used as a structural support and may also have a certain reversible lithium storage capacity. Silicon or partially oxidized silicon has a high reversible lithium storage capacity. Filling them in the pores of porous amorphous carbon or porous crystalline carbon can prevent silicon or partially oxidized silicon from agglomerating and breaking, thereby improving the structural stability of the silicon-based particles.

[0049] In one embodiment, the silicon-oxygen particles include a mixture of amorphous silicon and oxygen. Due to the oxygen element, the silicon-oxygen particles form inert or low-reactivity components such as silicates after initial lithium insertion. These components can act as a buffer matrix for silicon expansion, thereby improving the structural stability of the silicon-based particles.

[0050] In one embodiment, the silicon oxide particles include a chemical formula of M a Si b O x A substance where 0≤a<5, 0≤b<3, and 0≤x≤2.

[0051] Wherein, M includes one or more of Li and Mg. The M element, including a specific element, can combine with O and Si to form an inert or low-activity silicate, thereby reducing the amount of side reactions during the initial lithium insertion of the silicon-based particles and improving the initial coulombic efficiency of the silicon-based particles.

[0052] In one example, the silicon oxide particles include a mixture of amorphous silicon and oxygen, or one or more of Si, SiO2, Li2SiO3, Li2Si2O5, Li4SiO4, MgSiO3, Mg2SiO4, and Li2O.

[0053] In one embodiment, the silicon-oxygen particles include one or more of a mixture of amorphous silicon and oxygen, Li2SiO3 (crystalline), and Li2Si2O5 (crystalline). The Li2SiO3 and Li2Si2O5 are crystalline phases formed when the silicon-oxygen particles are doped with lithium. The formation of Li2SiO3 (crystalline) and Li2Si2O5 (crystalline) can improve the first coulombic efficiency of the silicon-based particles.

[0054] In one embodiment, the surface of the silicon oxide particles comprises an amorphous carbon coating layer, which can isolate the electrolyte, thereby inhibiting side reactions on the surface of the silicon oxide particles and improving the structural stability of the silicon oxide particles.

[0055] In one embodiment, the polymer material is an elastic material. The elasticity of the polymer material can provide a buffer space for the coating layer to accommodate the volume change of the silicon particles, and can also adapt to the volume change of the silicon particles, ensuring that its adhesiveness does not fail during the cycle.

[0056] In one example, the polymer material includes one or more of butadiene-styrene polymer, polystyrene-acrylate polymer, acrylate polymer, polydimethylsiloxane, polyurethane, and polyacrylic acid-acrylonitrile copolymer (PAA-PAN copolymer).

[0057] In one embodiment, the first conductive agent has a porous structure. When the first conductive agent has a porous structure, the polymer material can occupy the space of the porous structure in the first conductive agent, thereby providing more buffer space for the volume change of the silicon particles. For example, when the volume of the silicon particles expands, the squeezed polymer material can be partially compressed and occupy the pores of the first conductive agent, thereby providing more buffer space for the silicon particles. At the same time, the porous carbon is conducive to the infiltration of the electrolyte, improving the conductivity of lithium ions, thereby improving the overall electrochemical performance of the negative electrode.

[0058] According to a specific embodiment, the specific surface area of ​​the first conductive agent is 1m 2 / g-5000m 2 / g (for example, 1m 2 / g、5m 2 / g、10m 2 / g, 50m 2 / g、100m 2 / g、500m 2 / g、1000m 2 / g、2000m 2 / g、3000m 2 / g、4000m 2 / g、5000m2 When the specific surface area of ​​the first conductive agent is controlled within the above range, sufficient space can be provided for the polymer material to enter the pores of the first conductive agent.

[0059] In one embodiment, the specific surface area of ​​the first conductive agent is 5m 2 / g-1000m 2 / g.

[0060] In one example, the first conductive agent includes one or more of conductive carbon black, conductive graphite, carbon fiber, and graphene.

[0061] In one embodiment, the first conductive agent includes one or more of conductive carbon black, conductive graphite, and carbon fiber. The selection of the first conductive agent can provide more space for the extruded polymer material.

[0062] In one example, the coating layer further includes a first dispersant.

[0063] In one example, the first dispersant includes one or more of sodium carboxymethyl cellulose, sodium polyacrylate, sodium alginate, lithium carboxymethyl cellulose, polyacrylic acid, polyacrylamide, polyvinyl alcohol, polyacrylic acid-acrylamide copolymer, polyvinyl alcohol-acrylic acid copolymer, polyacrylic acid-acrylonitrile copolymer and polyacrylic acid-acrylonitrile-acrylamide copolymer.

[0064] According to a specific embodiment, based on the total weight of the first coating, the weight content of the first silicon particles is 10wt%-90wt% (for example, 10wt%, 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, 55wt%, 60wt%, 65wt%, 70wt%, 75wt%, 80wt%, 85wt%, 90wt%), the weight content of the polymer material is 2wt%-60wt% (for example, 2wt%, 5wt%, 10wt%, 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, 55wt%, 60wt%), and the carbon nanotubes are preferably quartz and sintered. The weight content of the rice tube is 0.05wt%-20wt% (for example, 0.1wt%, 0.2wt%, 0.5wt%, 0.8wt%, 1wt%, 5wt%, 10wt%), the weight content of the first conductive agent is 0.5wt%-50wt% (for example, 0.5wt%, 1wt%, 5wt%, 10wt%, 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%), and the weight content of the first dispersant is 0.5wt%-45wt% (for example, 0.5wt%, 1wt%, 5wt%, 10wt%, 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%).

[0065] In one example, based on the total weight of the first coating, the weight content of the first silicon particles is 30wt%-80wt%, the weight content of the polymer material is 4wt%-50wt%, the weight content of the carbon nanoparticles is 0.1wt%-10wt%, the weight content of the first conductive agent is 4wt%-40wt%, and the weight content of the dispersant is 2wt%-20wt%.

[0066] In one example, the second coating layer includes a second active material and optionally (“optionally” means it may or may not be present) second silicon particles, and the second active material includes one or more of graphite particles and hard carbon particles.

[0067] In one example, the second coating layer includes a second active material.

[0068] In one example, the graphite particles include natural graphite and / or artificial graphite.

[0069] In one example, the second coating layer further includes a second conductive agent, a binder, and a second dispersant.

[0070] According to a specific embodiment, the median particle size Dv50 of the second active material is 1 μm-30 μm (e.g., 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm). Within this range, the particle size of the second active material is moderate, surface side reactions are reduced, lithium insertion kinetics is good, and cycle stability is high.

[0071] In one example, the median particle size Dv50 of the second active material is 4 μm-25 μm.

[0072] In one example, the median particle size Dv50 of the second silicon particles is 5 μm-20 μm (eg, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm).

[0073] In one example, the second silicon particles include one or more of silicon-carbon particles, silicon-oxygen particles, and nano-silicon particles.

[0074] In one example, the silicon-carbon particles include particles formed by mixing silicon or partially oxidized silicon with amorphous or crystalline carbon.

[0075] In one example, the silicon carbon particles include particles formed by silicon or partially oxidized silicon filling (including partially filling or completely filling) pores in porous amorphous carbon or porous crystalline carbon.

[0076] In one embodiment, the silicon-oxygen particles include a mixture of amorphous silicon and oxygen.

[0077] In one embodiment, the silicon oxide particles include a chemical formula of M a Si b O x A substance where 0≤a<5, 0≤b<3, and 0≤x≤2.

[0078] Here, M includes one or more of Li and Mg.

[0079] In one example, the silicon oxide particles include a mixture of amorphous silicon and oxygen, or one or more of Si, SiO, SiO2, Li2SiO3, Li2Si2O5, Li4SiO4, MgSiO3, Mg2SiO4, and Li2O.

[0080] In one example, the silicon oxide particles include one or more of a mixture of amorphous silicon and ytterbium, Li2SiO3 (crystalline), and Li2Si2O5 (crystalline).

[0081] In one embodiment, the surface of the silicon oxide particles comprises an amorphous carbon coating layer.

[0082] In one example, the second conductive agent includes one or more of conductive carbon black, conductive graphite, carbon fiber, carbon nanotube, and graphene.

[0083] In one example, the binder includes one or more of butadiene-styrene polymer, polystyrene-acrylate polymer, acrylate polymer, polydimethylsiloxane, and polyurethane material.

[0084] In one example, the second dispersant includes one or more of sodium carboxymethyl cellulose, sodium polyacrylate, sodium alginate, lithium carboxymethyl cellulose, polyacrylic acid, polyacrylamide, polyvinyl alcohol, polyacrylic acid-acrylamide copolymer, polyvinyl alcohol-acrylic acid copolymer, polyacrylic acid-acrylonitrile copolymer and polyacrylic acid-acrylonitrile-acrylamide copolymer.

[0085] According to a specific embodiment, based on the total weight of the second coating layer, the weight content of the second active material is 70wt%-99wt% (for example, 70wt%, 75wt%, 80wt%, 85wt%, 90wt%, 95wt%, 98wt%), the weight content of the second silicon particles is 0wt%-5wt% (for example, 0wt%, 0.1wt%, 0.5wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%), and the weight content of the second conductive agent is 0wt%-20wt% (for example, 0wt%, 0.1wt%, 0.5wt%). , 1wt%, 5wt%, 10wt%, 15wt%, 20wt%), the weight content of the binder is 0.5wt%-20wt% (for example, 0.5wt%, 1wt%, 3wt%, 5wt%, 8wt%, 10wt%, 12wt%, 15wt%, 18wt%, 20wt%), and the weight content of the second dispersant is 0.2wt%-20wt% (for example, 0.2wt%, 0.3wt%, 0.6wt%, 1wt%, 3wt%, 5wt%, 8wt%, 10wt%, 12wt%, 15wt%, 18wt%, 20wt%).

[0086] When the weight content of the second silicon particles is 0 wt %, it means that the second silicon particles do not exist in the second coating layer, and in this case, the expansion rate of the negative electrode sheet can be further reduced.

[0087] When the weight content of the second conductive agent is 0 wt %, it means that the second conductive agent does not exist in the second coating layer.

[0088] In one example, based on the total weight of the second coating, the weight content of the second active material is 80wt%-98.2wt%, the weight content of the second silicon particles is 0wt%-3wt%, the weight content of the second conductive agent is 0wt%-5wt%, the weight content of the binder is 0.5wt%-10wt%, and the weight content of the second dispersant is 0.5wt%-10wt%.

[0089] In one embodiment, the surface of the first coating layer away from the negative electrode current collector has a protrusion (as shown in FIG6 ). Based on the shape and stacking method of the first silicon particles, the first coating layer forms a protrusion close to the surface of the second coating layer, that is, the surface of the first coating layer in contact with the second coating layer is uneven. This uneven surface can increase the contact area between the first silicon particles and the second coating layer, thereby improving the uniformity of lithium ion embedding into the silicon-based particles, effectively suppressing the problem of excessive expansion caused by localized excessive lithium embedding of the first silicon particles, and improving the cycle structural stability of the negative electrode sheet.

[0090] In one embodiment, the flatness β of the first coating layer is 4 μm-30 μm (e.g., 4 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm). When β is less than 4 μm, the first coating layer away from the current collector surface is relatively flat, the contact area between the first silicon particles and the second coating layer is small, and the lithium insertion uniformity of the silicon-based particles is poor. When β is greater than 30 μm, the distribution of the first silicon particles in the first coating layer becomes significantly non-uniform, which can cause excessive expansion in local areas and affect the battery cycle performance.

[0091] In one example, the flatness of the first coating layer is 8 μm-20 μm.

[0092] In the present disclosure, flatness can be obtained in the following manner: cutting in a direction perpendicular to the surface of the negative electrode sheet to obtain a cross section of the negative electrode sheet, and using SEM to observe any area of ​​the negative electrode sheet with a length of not less than 80 μm. As shown in Figure 6, along the direction perpendicular to the surface of the negative electrode sheet, draw a vertical straight line toward the negative electrode coating at any point A on the surface of the negative electrode current collector. The straight line passes through the first coating and the second coating in sequence. When the straight line has more than or equal to one intersection with the edge of the cross section of the first silicon particle in the first coating, the intersection farthest from A is taken as B, and the length of the AB line segment is taken as the distribution width f of the first silicon particle at point A; when the straight line has no intersection with the edge of the cross section of the first silicon particle, the distribution width of the silicon-based particles at point A is defined as 0. The maximum value of f in the observation area is f max , the minimum value is f min , with β=f max -f min is the flatness of the first coating.

[0093] Methods for controlling the flatness may include selecting first silicon particles with an appropriate particle size, controlling the content of the first silicon particles, applying the second coating layer without rolling the first coating layer, and the like.

[0094] According to a specific embodiment, the thickness d1 of the first coating layer is 0.1 μm-50 μm (e.g., 0.1 μm, 0.5 μm, 1 μm, 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm). Within this range, the problem of coating scraping caused by a too small thickness of the first coating layer can be avoided; the phenomenon of silicon-based particles being too widely distributed along the thickness direction due to a too large thickness of the first coating layer can also be prevented, thereby improving the uniformity of lithium insertion in the silicon-based particles.

[0095] In the present disclosure, the thickness of a coating refers to the thickness of the coating on one side. For example, the thickness of the first coating refers to the thickness of the first coating on one side (the first coating on the negative electrode current collector side) when the first coating is located on one side of the negative electrode current collector. When the first coating is located on both sides of the negative electrode current collector, the thickness of the first coating on both sides of the negative electrode current collector is the same, and the thickness of the first coating refers to the thickness of the first coating on one side of the negative electrode current collector. The thickness of the second coating also refers to the thickness of the coating on one side.

[0096] In one example, the thickness d1 of the first coating layer is 1 μm-40 μm.

[0097] According to a specific embodiment, the thickness d2 of the second coating layer is 5 μm-100 μm (e.g., 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm). Within this range, the problem of coating scraping caused by a too small thickness of the second coating layer can be avoided; and the problem of excessive lithium polarization and uneven lithium insertion caused by a too large thickness of the second coating layer can also be prevented.

[0098] In one example, the thickness d2 of the second coating layer is 10 μm-80 μm.

[0099] In one embodiment, the thickness d1 of the second coating layer is greater than the thickness d2 of the first coating layer. When the thickness d1 of the second coating layer is greater than the thickness d2 of the first coating layer, the second coating layer is relatively thick, and the expansion of the electrode during the cycle is small, which is conducive to reducing the overall expansion rate of the electrode.

[0100] As shown in Figure 6, the thickness of the first coating layer and the thickness of the second coating layer can be obtained by cutting the negative electrode sheet perpendicular to the surface to obtain a cross-section of the negative electrode sheet, and observing any area of ​​the negative electrode sheet with a length of not less than 80 μm using an SEM. A vertical straight line is drawn perpendicular to the surface of the negative electrode sheet from any point A on the surface of the negative electrode current collector toward the negative electrode coating layer. This straight line passes through the first coating layer and the second coating layer in sequence. This straight line has at least one intersection with the edge of the cross-section of the first silicon particle in the first coating layer. The intersection point farthest from A is taken as B, and the intersection point of this straight line with the side of the second coating layer away from the negative electrode current collector surface is taken as C. The maximum length of the line segment AB in the observation area is taken as the thickness a of the first coating layer, and the length BC is taken as the thickness b of the second coating layer.

[0101] In one example, the negative electrode current collector includes one or more of copper foil, composite copper foil, and porous copper foil.

[0102] According to a specific embodiment, the surface density of the negative electrode sheet is 1 mg / cm 2 -15mg / cm 2 (e.g., 1 mg / cm 2 , 2mg / cm 2 , 3mg / cm 2 , 4mg / cm 2 , 5mg / cm 2 , 6mg / cm 2 , 7mg / cm 2 , 8mg / cm 2 , 9mg / cm 2 , 10mg / cm 2 , 15mg / cm 2 When the surface density of the negative electrode sheet is controlled within the above range, adhesion can be ensured, the negative electrode sheet does not fall off, the internal resistance is low, and the dynamic requirements of the battery can be met.

[0103] The surface density of the negative electrode is the sum of the surface density of the first coating layer and the surface density of the second coating layer. In this disclosure, surface density refers to the surface density of a single surface. Taking the surface density of the first coating layer as an example, when the first coating layer is applied to one side of the negative electrode current collector, the surface density of the first coating layer is the surface density of that side (i.e., the side with the first coating layer); when the first coating layer is applied to both sides of the negative electrode current collector, the surface density of the first coating layer on both sides is the same, and the surface density of the first coating layer is the surface density of either side.

[0104] In one embodiment, the surface density of the negative electrode sheet is 4 mg / cm 2 -11mg / cm 2 .

[0105] According to a specific embodiment, the compaction density of the negative electrode sheet is 1g / cm 3 -2g / cm 3 (For example, 1g / cm 3 , 1.3g / cm 3 , 1.5g / cm 3 , 1.55g / cm 3 , 1.6g / cm 3 , 1.65g / cm 3 , 1.7g / cm 3 , 1.75g / cm 3 , 1.8g / cm 3 , 1.85g / cm 3 , 1.9g / cm 3 , 2g / cm 3 When the compaction density of the negative electrode sheet is controlled within the above range, it can be ensured that the negative electrode sheet is not excessively rolled, and the porosity is appropriate, which is conducive to the infiltration of the electrolyte.

[0106] In one embodiment, the compaction density of the negative electrode sheet is 1.6 g / cm 3 -1.8g / cm 3 .

[0107] A second aspect of the present disclosure provides a battery, comprising the negative electrode sheet described in the first aspect of the present disclosure.

[0108] The materials of the battery except the negative electrode sheet can be prepared according to the methods in the art, and can achieve the effects of low expansion rate, high cycle stability and high safety performance.

[0109] The battery may be a lithium-ion battery.

[0110] Since the battery disclosed herein includes the negative electrode sheet disclosed herein, the battery expansion rate is reduced, the cycle stability is improved, and the safety performance is enhanced.

[0111] The present disclosure will be described in detail below through examples. The examples described in this disclosure are only a portion of the examples of the present disclosure, not all of the examples. Based on the examples in this disclosure, all other examples obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this disclosure.

[0112] The following examples are used to illustrate the negative electrode sheet of the present disclosure.

[0113] Example 1

[0114] (1) Preparation of ingredients

[0115] Negative electrode current collector: copper foil;

[0116] First coating layer: first silicon particles (silicon carbon particles (silicon carbon particles are particles formed by silicon in the pores of porous amorphous carbon), with a median particle size Dv50 of 9.01 μm) 50 parts by weight, coating layer (first conductive agent (conductive carbon black) 10 parts by weight and (carbon nanotube conductive agent) 1.6 parts by weight, polymer material (polybutadiene-styrene (SBR)) 20 parts by weight, first dispersant (sodium carboxymethyl cellulose (CMC-Na)) 18.4 parts by weight);

[0117] Second coating: 97.7 parts by weight of the second active material (artificial graphite), 0.5 parts by weight of the second conductive agent (conductive carbon black), 1.2 parts by weight of the binder (polybutadiene-styrene), and 0.6 parts by weight of the second dispersant (lithium carboxymethyl cellulose).

[0118] (2) Preparation of negative electrode sheet

[0119] The first silicon particles, the first conductive agent (specific surface area of ​​800m 2 / g), a polymer material and a first dispersant are dispersed in deionized water, and stirred to obtain a uniformly dispersed first slurry. The first slurry has a solid content of 16% and a viscosity of 3500mPa·s-6500mPa·s. After passing the first slurry through a 150-mesh gauze, it is evenly coated on the surface of both sides of the negative electrode current collector to form a first coating. The first coating has an uneven surface formed based on the surface of the first silicon particles, and the coating surface density is 0.75mg / cm 2 After drying at 80℃ for 10h, the first coating was rolled to a compaction density of 0.8g / cm 3 , obtaining a negative electrode sheet including a first coating layer;

[0120] The second active material, conductive agent, binder and second dispersant were dispersed in deionized water and stirred to obtain a uniformly dispersed second slurry. The second slurry had a solid content of 48% and a viscosity of 3500mPa·s-6500mPa·s. The second slurry was passed through a 150-mesh gauze and evenly coated on the surface of the first coating to form a second coating. The coating surface density was 5.18mg / cm 2 After drying at 80°C for 10 hours, a negative electrode sheet was obtained, wherein the surface density was: 5.80 mg / cm 2 , compacted density is 1.72g / cm 3 .

[0121] Example 2 group

[0122] This embodiment group is carried out with reference to embodiment 1, except that the ratio R of the center distance between adjacent first silicon particles to the median particle size Dv50 of the first silicon particles is adjusted by changing the median particle size Dv50 of the first silicon particles and / or the center distance between adjacent first silicon particles. See Table 1-1 and Table 1-2 for details.

[0123] Example 3 group

[0124] This embodiment group was carried out in accordance with the embodiment 1, except that the specific surface area of ​​the first conductive agent was adjusted by changing the specific selection of the first conductive agent, as shown in Table 1-1 and Table 1-2. Inevitably, the center-to-center distance between adjacent first silicon particles would change.

[0125] Example 4 Group

[0126] This example group was carried out with reference to Example 1, except that the weight content of the first silicon particles in the first coating layer was changed. For details, see Table 1-1 and Table 1-2.

[0127] Example 5 Group

[0128] This example group was carried out in accordance with Example 1, except that the weight content of the first conductive agent in the first coating layer was changed, as shown in Table 1-1 and Table 1-2. Inevitably, the center-to-center distance between adjacent first silicon particles was changed.

[0129] Example 6

[0130] This embodiment group was carried out with reference to the embodiment 1, except that the specific selection of polymer materials was changed. For details, see Table 1-1 and Table 1-2.

[0131] Example 7 Group

[0132] This embodiment group was carried out with reference to the embodiment 1, except that the specific selection of the first silicon particles was changed. For details, see Table 1-1 and Table 1-2.

[0133] Example 8 Group

[0134] The process was carried out in the same manner as in Example 1, except that second silicon particles were added to the second coating layer. Example 8a: The second coating layer contained: 97.7 parts by weight of (95% graphite particles + 5% second silicon particles), 0.5 parts by weight of a second conductive agent (conductive carbon black), 1.2 parts by weight of a binder (polybutadiene-styrene), and 0.6 parts by weight of a second dispersant (lithium carboxymethyl cellulose). Example 8b: The second coating layer contained: 97.7 parts by weight of (95% graphite particles + 2.5% second silicon particles), 0.5 parts by weight of a second conductive agent (conductive carbon black), 1.2 parts by weight of a binder (polybutadiene-styrene), and 0.6 parts by weight of a second dispersant (lithium carboxymethyl cellulose).

[0135] Example 9 Group

[0136] This example group was carried out with reference to Example 1, except that one or more of the diameter of the carbon nanotubes, the length of the carbon nanotubes, and the aspect ratio of the carbon nanotubes were changed. For details, see Table 1-1 and Table 1-2.

[0137] Example 10 Group

[0138] The process was carried out in accordance with Example 1, except that the content of carbon nanotubes in the first coating layer was changed. For details, see Table 1-1 and Table 1-2.

[0139] Example 11

[0140] The same procedure was followed as in Example 1, except that the composition of the first coating layer was changed. For details, see Table 1-1 and Table 1-2.

[0141] Example 12

[0142] The same procedure was followed as in Example 1, except that the composition of the first coating layer was changed. For details, see Table 1-1 and Table 1-2.

[0143] Example 13

[0144] The same procedure was followed as in Example 1, except that the composition of the first coating layer was changed. For details, see Table 1-1 and Table 1-2.

[0145] Example 14 Group

[0146] This example group was carried out in accordance with Example 1, except that the flatness of the first coating layer was changed by roller pressing. In Example 14a, before applying the second coating layer, the negative electrode sheet including the first coating layer was roller pressed to make the compaction of the first coating layer 1.0 g / cm 3 In Example 14b, the negative electrode sheet including the first coating was rolled to a compaction of 0.61 g / cm 3 In Example 14c, the negative electrode sheet including the first coating was rolled to a compaction of 0.48 g / cm 3 For details, see Table 1-1 and Table 1-2.

[0147] Table 1-1

[0148] *Same as Example 1.

[0149] Table 1-2

[0150] * indicates the same as Example 1;

[0151] - means it does not exist.

[0152] Comparative Example 1

[0153] (1) Preparation of ingredients

[0154] Negative electrode current collector: copper foil;

[0155] The graphite / silicon composite negative electrode material (mass ratio is 93:7): 96.6wt%, conductive agent (conductive carbon black) 0.8wt% weight part and carbon nanotubes 0.2wt% weight part, binder (SBR) 1.4wt% weight part, dispersant (lithium carboxymethyl cellulose) 1wt% weight part.

[0156] (2) Preparation of negative electrode sheet

[0157] Graphite, silicon, conductive agent and binder were dispersed in deionized water and stirred to obtain a uniformly dispersed negative electrode slurry with a solid content of 48% and a viscosity of 3500mPa·s-6500mPa·s. The negative electrode slurry was passed through a 150-mesh gauze and evenly coated on the surface of both sides of the negative electrode current collector with a coating surface density of 5.84mg / cm 2 After drying at 80℃ for 10h, the negative electrode sheet was obtained, and its compaction density was 1.72g / cm 2 .

[0158] Comparative Example 2

[0159] The process is carried out in accordance with Example 1, except that the average value of the ratio R of the center distance between adjacent first silicon particles to the median particle size Dv50 of the first silicon particles is 0.006.

[0160] Comparative Example 3

[0161] The process was carried out in accordance with Example 1, except that the average value of the ratio R of the center distance between adjacent first silicon particles to the median particle size Dv50 of the first silicon particles was 102.4.

[0162] Comparative Example 4

[0163] The process was carried out in accordance with Example 1, except that no carbon nanotubes were added.

[0164] Comparative Example 5

[0165] The same process as in Example 1 was carried out except that no polymer material was added.

[0166] Preparation Example

[0167] Batteries were prepared using the negative electrode sheets obtained in the examples and comparative examples in the following manners.

[0168] (1) Positive electrode

[0169] The positive electrode active material lithium cobalt oxide (LCO), the binder polyvinylidene fluoride (PVDF), and the conductive agent acetylene black are mixed in a weight ratio of 97.6:1.2:1.2, and N-methylpyrrolidone (NMP) is added. The mixture is stirred in a vacuum mixer until the mixed system becomes a uniform and fluid positive electrode slurry; the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil; the coated aluminum foil is baked in an oven with 5 different temperature gradients, and then dried in an oven at 120°C for 8 hours, and then rolled and cut to obtain the desired positive electrode sheet. The single-side density of the positive electrode sheet is: 13.7 mg / cm 2 , compaction: 4.15g / cm 2 .

[0170] (2) Negative electrode

[0171] The negative electrode sheets obtained in the above embodiments and comparative examples were used respectively.

[0172] (3) Electrolyte

[0173] In a glove box filled with inert gas (argon) (H2O <0.1ppm, O2 <0.1ppm), carbonate solvents (ethylene carbonate, propylene carbonate, diethyl carbonate in a mass ratio of 1:1:1) and carboxylate solvents (propyl propionate, ethyl propionate in a mass ratio of 1:1) were mixed evenly, and then 1.25mol / L of fully dried lithium hexafluorophosphate (LiPF6) was quickly added thereto, dissolved in a non-aqueous organic solvent, stirred evenly, and after passing the moisture and free acid tests, an electrolyte was obtained.

[0174] (4) Isolation film

[0175] A coated polyethylene isolation film with a thickness of 8 μm was selected.

[0176] (5) Preparation of lithium-ion batteries

[0177] The positive electrode sheet of step (1), the isolation film of step (4), and the negative electrode sheet of step (2) are stacked in order, ensuring that the isolation film is located between the positive and negative electrode sheets to play an isolating role, and then a bare cell without liquid injection is obtained by winding; the bare cell is placed in an outer packaging foil, and the electrolyte of step (3) is injected into the dried bare cell. After vacuum packaging, standing, forming, shaping, sorting and other processes, a lithium-ion battery is obtained.

[0178] Test Case

[0179] The batteries obtained in the examples and comparative examples were tested as follows:

[0180] (1) 25℃ cycle test

[0181] Take the battery with 50% SOC after OCV measurement, test its voltage, internal resistance and thickness T1, then place the battery in a (25±3)℃ environment and let it stand for 3 hours. When the battery body reaches (25±3)℃, charge it to 4.2V at 3C constant current, then charge it to 4.5V at 0.7C constant current and constant voltage, with a cut-off current of 0.05C, and then discharge it to 3V at 0.5C. The charge and discharge cycles are 100 / 200 / 300 / 400 / 500 weeks, and the discharge capacity of the cycles 100 / 200 / 300 / 400 / 500 weeks is recorded respectively. The discharge capacity of the cycles 100 / 200 / 300 / 400 / 500 weeks is divided by the discharge capacity of the first cycle to obtain the capacity retention rate of the cycles 100 / 200 / 300 / 400 / 500 weeks. For example, the discharge capacity of the first cycle is Q1, and the discharge capacity of the 500th cycle is Q500. Then the capacity retention rate of the 100th cycle = Q500 / Q1×100%. Record the capacity retention rate of the 500T battery.

[0182] Discharge at 0.2C every 100 cycles, performing low-rate capacity recovery. After cycling for 100 / 200 / 300 / 400 / 500 cycles, remove the fully charged battery from the 25°C incubator and immediately measure its thickness at full charge after 100 / 200 / 300 / 400 / 500 cycles. Calculate the thickness expansion rate of the battery at 100 / 200 / 300 / 400 / 500 cycles, respectively. For example, if the thickness of the fully charged battery after 500 cycles is T500, then the thickness expansion rate (%) = (T500 - T1) / T1 × 100%. Record the expansion rate of the 500T battery. After the cycle is completed, the battery is fully charged and dissected to observe the lithium deposition. When there is no lithium deposition, the surface of the negative electrode sheet is golden yellow after lithium insertion. When the battery impedance is large and the kinetics are poor, lithium deposition will occur. At this time, the edge of the negative electrode sheet or a small local area will turn gray or black. This is caused by the oxidation of the deposited lithium. When lithium deposition is severe, a large area of ​​the negative electrode sheet turns gray or black. Therefore, when the surface of the negative electrode sheet is observed to be golden yellow, the result is expressed as "no lithium deposition", when the edge of the negative electrode sheet or a small local area turns gray or black, the result is expressed as "lithium deposition", and when a large area of ​​the negative electrode sheet turns gray or black, the result is expressed as "severe lithium deposition".

[0183] (2) 45℃ cycle test

[0184] Take the battery with 50% SOC after OCV measurement, test its voltage, internal resistance and thickness M1, then place the battery in a (45±3)℃ environment and let it stand for 3 hours. When the battery body reaches (45±3)℃, charge it to 4.2V at 3C constant current, then charge it to 4.5V at 0.7C constant current and constant voltage, with a cut-off current of 0.05C, and then discharge it to 3V at 0.5C. The charge and discharge cycles are 100 / 200 / 300 / 400 / 500 weeks, and the discharge capacity of the cycles 100 / 200 / 300 / 400 / 500 weeks is recorded respectively. The discharge capacity of the cycles 100 / 200 / 300 / 400 / 500 weeks is divided by the discharge capacity of the first cycle to obtain the capacity retention rate of the cycles 100 / 200 / 300 / 400 / 500 weeks. For example, the discharge capacity of the first cycle is P1, and the discharge capacity of the 500th cycle is P500. Then the capacity retention rate of the 100th cycle = P500 / P1*100%. Record the capacity retention rate of the 500T battery. After the cycle is completed, the battery is fully charged and dissected to observe the lithium deposition. Among them, when there is no lithium deposition, the surface of the negative electrode sheet is golden yellow after lithium insertion. When the battery impedance is large and the kinetics are poor, lithium deposition will occur. At this time, the edge of the negative electrode sheet or a small local area will turn gray or black. This is caused by the oxidation of the deposited lithium. When lithium deposition is severe, a large area of ​​the negative electrode sheet turns gray or black. Therefore, when the surface of the negative electrode sheet is observed to be golden yellow, the result is expressed as "no lithium deposition", when the edge of the negative electrode sheet or a small local area turns gray or black, the result is expressed as "lithium deposition", and when a large area of ​​the negative electrode sheet turns gray or black, the result is expressed as "severe lithium deposition".

[0185] Discharge at 0.2C every 100 cycles, performing low-rate capacity recovery. After cycling for 100 / 200 / 300 / 400 / 500 cycles, remove the fully charged battery from the 45°C incubator and immediately measure its thickness at full charge after 100 / 200 / 300 / 400 / 500 cycles. Calculate the thickness expansion rate of the battery at 100 / 200 / 300 / 400 / 500 cycles, respectively. For example, if the fully charged thickness after 500 cycles is M500, then the thickness expansion rate (%) = (M500 - M1) / M1 × 100%. Record the expansion rate of the 500T battery.

[0186] (3) 132°C furnace temperature test

[0187] At 25°C ± 5°C, discharge at 0.2C to the lower voltage limit and allow to rest for 10 minutes. Charge at 0.7C to the upper voltage limit with a cutoff current of 0.05C. At 25°C ± 5°C, test the fully charged voltage, internal resistance, and thickness, and take photos before testing. Fully charged batteries are placed in an oven and heated at a rate of 5±2°C / min. When the oven temperature reaches 132°C ± 2°C, maintain the temperature for 60 minutes. Three batteries are tested per group. Observe from the start to the end of the test for fire or explosion, which serves as the oven temperature acceptance criteria. No fire or explosion is considered a pass; a fire is considered a fail. Results are expressed as "number of passes / number of tests." "3 / 3" indicates three passes out of three tests, and "1 / 3" indicates one pass out of three tests.

[0188] The obtained results are recorded in Table 2.

[0189] Table 2

[0190] As can be seen from Table 2, it can be seen from the comparative examples and the embodiments that the room temperature and high temperature cycle capacity retention rate of the battery made from the negative electrode sheet of the embodiment is significantly improved, the room temperature and high temperature cycle battery cell expansion rate is significantly reduced, the room temperature and high temperature cycle lithium precipitation situation is improved, and the furnace temperature test result is improved, indicating that the negative electrode sheet of the present disclosure and the battery including the negative electrode sheet overcome the adverse effects of the volume change of silicon particles in the graphite-doped silicon system by changing the structure of the negative electrode sheet, reduce the expansion rate of the battery, and improve the cycle stability and safety performance of the battery.

[0191] The preferred embodiments of the present disclosure are described in detail above, but the present disclosure is not limited thereto. Within the technical concept of the present disclosure, various simple variations of the technical solution of the present disclosure 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 by the present disclosure and fall within the scope of protection of the present disclosure.

Claims

1. A negative electrode sheet, characterized in that: The negative electrode sheet includes a negative electrode current collector, a first coating located on one side or both sides of the negative electrode current collector, and a second coating located on the surface of the first coating, the first coating includes first silicon particles and a coating layer located on the surface of the first silicon particles, the coating layer includes a polymer material, carbon nanotubes and a first conductive agent, and the ratio R of the center distance between adjacent first silicon particles to the median particle size Dv50 of the first silicon particles is (0.01-100):

1.

2. The negative electrode sheet according to claim 1, wherein: The ratio R of the center distance between adjacent first silicon particles to the median particle size Dv50 of the first silicon particles is (0.1-10): 1; and / or, the center distance between adjacent first silicon particles is 0.05 μm-2000 μm; And / or, the median particle size Dv50 of the first silicon particles is 5 μm-20 μm.

3. The negative electrode sheet according to claim 1 or 2, wherein: The diameter of the carbon nanotube is 0.5 nm-40 nm, and / or the length of the carbon nanotube is 1 μm-40 μm, and / or the aspect ratio of the carbon nanotube is 25-80000; And / or, the polymer material includes one or more of butadiene-styrene polymer, polystyrene-acrylate polymer, acrylate polymer, polydimethylsiloxane, polyurethane and polyacrylic acid-acrylonitrile copolymer.

4. The negative electrode sheet according to any one of claims 1 to 3, wherein: The first silicon particles include one or more of silicon-carbon particles, silicon-oxygen particles and nano-silicon particles.

5. The negative electrode sheet according to any one of claims 1 to 4, wherein: The first conductive agent has a porous structure; And / or, the specific surface area of ​​the first conductive agent is 1m 2 / g-5000m 2 / g, preferably 5m 2 / g-1000m 2 / g; And / or, the first conductive agent includes one or more of conductive carbon black, conductive graphite, carbon fiber and graphene.

6. The negative electrode sheet according to any one of claims 1 to 5, wherein: The coating layer also includes a first dispersant. Based on the total weight of the first coating, the weight content of the first silicon particles is 10wt%-90wt%, the weight content of the polymer material is 2wt%-60wt%, the weight content of the carbon nanotubes is 0.05wt%-20wt%, the weight content of the first conductive agent is 0.5wt%-50wt%, and the weight content of the first dispersant is 0.5wt%-45wt%.

7. The negative electrode sheet according to claim 6, wherein: The first dispersant includes one or more of sodium carboxymethyl cellulose, sodium polyacrylate, sodium alginate, lithium carboxymethyl cellulose, polyacrylic acid, polyacrylamide, polyvinyl alcohol, polyacrylic acid-acrylamide copolymer, polyvinyl alcohol-acrylic acid copolymer, polyacrylic acid-acrylonitrile copolymer and polyacrylic acid-acrylonitrile-acrylamide copolymer.

8. The negative electrode sheet according to any one of claims 1 to 7, wherein: The second coating layer includes a second active material and optionally second silicon particles, the second active material including one or more of graphite particles and hard carbon particles.

9. The negative electrode sheet according to claim 8, wherein: The median particle size Dv50 of the second active substance is 1 μm-30 μm, preferably 4 μm-25 μm; And / or, the median particle size Dv50 of the second silicon particles is 5 μm-20 μm; And / or, the second silicon particles include one or more of silicon-carbon particles, silicon-oxygen particles and nano-silicon particles.

10. The negative electrode sheet according to claim 4 or 9, wherein: The silicon oxide particles include a chemical formula of M a Si b O x A substance, wherein 0≤a<5, 0≤b<3, 0≤x≤2, and M includes one or more of Li and Mg; And / or, the surface of the silicon oxide particles contains an amorphous carbon coating layer; and / or, the silicon-carbon particles include particles formed by a mixture of silicon or partially oxidized silicon and amorphous or crystalline carbon; And / or, the silicon carbon particles include particles formed by silicon or partially oxidized silicon filling pores in porous amorphous carbon or porous crystalline carbon.

11. The negative electrode sheet according to any one of claims 8 to 10, wherein: The second coating layer also includes a second conductive agent, a binder and a second dispersant. Based on the total weight of the second coating layer, the weight content of the second active substance is 70wt%-98wt%, the weight content of the second silicon particles is 0wt%-5wt%, the weight content of the second conductive agent is 0wt%-20wt%, the weight content of the binder is 0.5wt%-20wt%, and the weight content of the second dispersant is 0.2wt%-20wt%.

12. The negative electrode sheet according to claim 11, wherein: The second conductive agent includes one or more of conductive graphite, carbon fiber, conductive carbon black, carbon nanotube and graphene.

13. The negative electrode sheet according to any one of claims 1 to 12, wherein: The surface density of the negative electrode sheet is 1-15 mg / cm 2 , preferably 4-11 mg / cm 2 ; And / or, the compaction density of the negative electrode sheet is 1-2 g / cm 3 , preferably 1.6-1.8 g / cm 3 .

14. The negative electrode sheet according to any one of claims 1 to 13, wherein: The thickness d1 of the first coating is 0.1 μm-50 μm, preferably 1 μm-40 μm; and / or, the thickness d2 of the second coating is 5 μm-100 μm, preferably 10 μm-80 μm; and / or, the thickness d1 of the second coating layer is greater than the thickness d2 of the first coating layer; And / or, the flatness β of the first coating layer is 4 μm-30 μm.

15. A battery, characterized in that: The battery comprises the negative electrode sheet according to any one of claims 1 to 14.

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