Negative electrode plate, and battery

Abstract

A negative electrode plate, and a battery comprising the negative electrode plate. The negative electrode plate comprises a negative current collector and coatings located on one side surface or two side surfaces of the negative current collector; the coatings include a first coating and a second coating; the first coating is located between the negative current collector and the second coating, and the first coating is a non-continuous coating; the first coating includes several image blocks, and if the sum of the projection areas of the several image blocks in the first coating on the negative current collector is S1, and the projection areas of the coatings on the negative current collector is S, then S1 < S. The electrical impedance and DCIR of the negative electrode plate are low; a battery having the negative electrode plate has a small low-temperature expansion rate, a long cycle life, ameliorated lithium precipitation, and improved quick charging capability.

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. Background Art

[0002] With the large-scale application of lithium-ion batteries in electronic products and electric vehicles, the demand for improving the energy density of lithium-ion batteries is increasing. Using silicon-based anodes to completely or partially replace graphite anodes is the simplest and most effective way to increase battery energy density. However, the application of silicon-based materials still faces a series of challenges. This is mainly because the silicon anode can embed and release more lithium ions during the charge and discharge process, causing 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, while also continuously consuming electrolyte, ultimately causing capacity decay and cycle failure.

[0003] Since the pure silicon system has problems such as excessive expansion and low initial efficiency, the current method of graphite doping with silicon is generally used to construct the negative electrode plate. However, the problem of large expansion still exists. The graphite-doped silicon negative electrode system has the following problems at the plate level: 1) Too little binder distribution on the surface of the silicon negative electrode: In the graphite-doped silicon system, the binder distribution on the surface of the silicon particles is insufficient, and the bonding network is easily destroyed; 2) The pores between silicon and graphite particles continue to increase: The expansion rates of silicon and graphite after de-lithiation are quite different, and the particles are prone to movement and rearrangement, resulting in the continuous increase of particle pores; 3) The binder needs to withstand more cyclic stretching: Due to the difference in the de-lithiation potential of silicon and graphite, silicon first intercalates lithium and graphite later, resulting in the binder undergoing two stretching cycles in each cycle and needing to withstand more fatigue strength. Therefore, the current graphite-doped silicon negative electrode has not been able to effectively solve the problem of large expansion, especially in the later stages of the cycle, the thickness of the battery cell seriously exceeds the standard, and even causes safety accidents.

[0004] Summary of the Invention

[0005] In order to effectively solve the problem of large expansion of graphite-doped silicon negative electrode, an elastic coating layer can be set on the surface of silicon particles. However, due to the poor electronic conductivity of silicon particles and elastic coating layer, the impedance and DCIR of the battery will be too large, resulting in insufficient rate performance of the battery.

[0006] The present disclosure aims to overcome the aforementioned problems of the prior art by providing a negative electrode sheet and a battery including the same. The negative electrode sheet disclosed herein has low impedance and DCIR. Batteries including the disclosed negative electrode sheet have a low low-temperature expansion rate, a long cycle life, improved lithium plating, and enhanced fast-charging capabilities.

[0007] Research has found that by reducing the impedance and DCIR of the negative electrode sheet, the fast charging performance of the battery can be improved. In order to reduce the impedance and DCIR of the negative electrode sheet, the structure of the negative electrode sheet can be changed.

[0008] To achieve the above-mentioned objectives, the present disclosure provides a negative electrode sheet in a first aspect, comprising a negative electrode current collector and a coating located on one or both surfaces of the negative electrode current collector, wherein the coating comprises a first coating and a second coating, wherein the first coating is located between the negative electrode current collector and the second coating, and the first coating comprises a plurality of blocks, wherein the sum of the areas of the projections of the plurality of blocks in the first coating on the negative electrode current collector is S1, and the area of ​​the projection of the coating on the negative electrode current collector is S, then S1<S.

[0009] A second aspect of the present disclosure provides a battery, comprising the negative electrode sheet described in the first aspect of the present disclosure.

[0010] Through the above technical solution, the present disclosure has at least the following advantages compared with the prior art:

[0011] (1) The negative electrode sheet disclosed herein has low impedance;

[0012] (2) The negative electrode sheet disclosed herein has a low DCIR;

[0013] (3) The battery disclosed herein has low internal resistance and good fast charging capability;

[0014] (4) The battery disclosed herein can improve the lithium plating problem;

[0015] (5) The battery disclosed herein has a low expansion rate in low-temperature environments;

[0016] (6) The battery disclosed herein has a long cycle life.

[0017] Other features and advantages of the present disclosure will be described in detail in the following detailed description.

[0018] 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

[0019] FIG1 is a schematic structural diagram of a negative electrode sheet of the present disclosure (wherein 1 is a negative electrode current collector, 21 is a first coating layer, and 22 is a second coating layer).

[0020] FIG2 is a schematic diagram showing the projection of the first coating layer on the negative electrode current collector according to the present disclosure.

[0021] FIG3 is a schematic diagram showing that the projection of the first coating layer on the negative electrode current collector of the present disclosure is a continuous area.

[0022] FIG4 is a schematic diagram showing a discontinuous area where the projection of the first coating layer on the negative electrode current collector of the present disclosure is formed.

[0023] FIG5 is a SEM image of the negative electrode sheet of the present disclosure (wherein 211 is a first silicon particle and 212 is a coating layer).

[0024] FIG6 is a SEM image of the first coating layer in the negative electrode sheet of the present disclosure.

[0025] FIG7 is a schematic diagram showing the center distance between adjacent first silicon particles in a cross section of a negative electrode sheet.

[0026] FIG8 is a schematic diagram showing the center distance between adjacent first silicon particles in another cross section of a negative electrode sheet.

[0027] FIG9 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. DETAILED DESCRIPTION

[0028] The technical solutions of the present application will be described in further detail below in conjunction with specific embodiments. It should be understood that the following embodiments are merely illustrative and explain the present application and should not be construed as limiting the scope of protection of the present application. All technologies implemented based on the above content of the present application are encompassed within the scope that the present application is intended to protect. Herein, unless otherwise specified, data ranges include endpoints.

[0029] It should be noted that the numerical expressions such as "first" and "second" in the present disclosure are only used to distinguish different substances or usage methods, and do not represent a difference in order.

[0030] In a first aspect of the present disclosure, a negative electrode sheet is provided, comprising a negative electrode current collector and a coating located on one or both surfaces of the negative electrode current collector, wherein the coating comprises a first coating and a second coating, wherein the first coating is located between the negative electrode current collector and the second coating, and wherein the first coating comprises a plurality of blocks, wherein the sum of the areas of projections of the plurality of blocks in the first coating on the negative electrode current collector is S1, and the area of ​​the projection of the coating on the negative electrode current collector is S, then S1<S.

[0031] As shown in FIG1 , the negative electrode sheet includes a negative electrode current collector 1 and a coating 2 located on one or both sides of the negative electrode current collector 1 . The coating includes a first coating 21 and a second coating 22 . The first coating 21 is located between the negative electrode current collector 1 and the second coating 22 .

[0032] To reduce the impedance and DCIR of the negative electrode sheet, the projected area of ​​the first coating on the negative electrode current collector can be set to S1 < the projected area of ​​the coating on the negative electrode current collector, which is S. For negative electrode sheets that meet the above specific structure, a portion of the second coating can be in direct contact with the negative electrode current collector, thereby effectively reducing the impedance and DCIR of the negative electrode sheet and improving the fast charging capability of the battery.

[0033] In the present disclosure, by designing the negative electrode sheet with the above-mentioned specific structure, the negative electrode sheet has achieved lower impedance and lower DCIR than the existing technology, thereby enabling the battery to have better fast charging performance and longer cycle life. To further improve the effect, one or more of the technical features can be further optimized.

[0034] In one example, the ratio of the projected area S1 of the first coating on the negative electrode current collector to the projected area S of the coating on the negative electrode current collector is (0.2-0.99):1 (for example, 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 0.99:1). Limiting the ratio of the projected area S1 of the first coating on the negative electrode current collector to the projected area S of the coating on the negative electrode current collector to the above-mentioned specific range can further reduce the impedance and DCIR of the negative electrode sheet. When the ratio of the projected area S1 of the first coating on the negative electrode current collector to the projected area S of the coating on the negative electrode current collector is lower than 0.2, the surface density of the first coating is too high and the thickness of the local coating is too thick, resulting in uneven thickness of the overall battery and easy battery deformation during the cycle process.

[0035] In one example, a ratio of an area S1 of a projection of the first coating layer on the negative electrode current collector to an area S of a projection of the coating layer on the negative electrode current collector is (0.6-0.9):1.

[0036] In one example, a ratio of an area S1 of a projection of the first coating layer on the negative electrode current collector to an area S of a projection of the coating layer on the negative electrode current collector is (0.6-0.8):1;

[0037] The first coating layer may include a plurality of blocks (wherein, "several" means ≥1), and the projection of the blocks in the first coating layer on the negative electrode current collector may be circular, square, strip, polygonal, patterned or irregular, as shown in FIG2 . FIG2 is only for showing the shape of the projection of the first coating layer on the negative electrode current collector. In one example, the projection of the first coating layer on the negative electrode current collector has multiple shapes. In another example, the projection of the first coating layer on the negative electrode current collector has only one shape. Regardless of the shape of the projection of the first coating layer on the negative electrode current collector, any shape that can meet the definition of the specific relationship between the area S1 of the projection of the first coating layer on the negative electrode current collector and the area S of the projection of the coating layer on the negative electrode current collector in the present disclosure can achieve the effect of reducing the negative electrode sheet impedance and DCIR, and improving the fast charging performance of the battery.

[0038] The projection of the first coating layer on the negative electrode current collector may be a continuous block (as shown in FIG3 ) or may include multiple non-continuous blocks (as shown in FIG2 ). When the projection of the first coating layer on the negative electrode current collector is composed of multiple non-continuous blocks, the area S1 of the projection of the first coating layer on the negative electrode current collector is the sum of the areas of the multiple non-continuous blocks.

[0039] In one example, the first coating is applied discontinuously.

[0040] In one example, the projection of the first coating layer on the negative electrode current collector is a discontinuous stripe (eg, a zebra shape).

[0041] In one example, in the projection of the coating on the negative electrode current collector, the sum of the areas not coated with the first coating is S2, and then S1=S-S2.

[0042] In one example, the first coating layer includes first silicon particles and a coating layer located on the surface of the first silicon particles, and the second coating layer includes a second active material and an optional second silicon particle, wherein the second active material includes one or more of graphite particles and hard carbon particles. By arranging the silicon particles and the graphite particles and / or hard carbon particles in two different coating layers (the silicon particles are located in the first coating layer, and the graphite particles and / or hard carbon particles are located in the second coating layer), the problem of easy movement and rearrangement between the particles due to the large difference in the expansion coefficients of the silicon particles and the carbon material, thereby causing the porosity between the particles to increase, can be reduced or even overcome. It is also possible to reduce the number of stretching times of the binder per cycle, reduce the demand for fatigue strength of the binder, and thus improve the cycle life of the battery. 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, and prevent demolding during charging and discharging. In addition, the polymer material also has the function of a binder, realizing the function of bonding the second coating layer and the current collector, thereby giving full play to the comprehensive performance of the silicon particles and graphite.

[0043] 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).

[0044] As can be seen in Figure 6, the entire surface of the first silicon particles (light gray) is covered by the coating layer (dark), 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 non-destructible bonding network, further reducing the expansion rate of the negative electrode sheet and improving the safety of the negative electrode sheet. It also improves the adhesion between the coating layer and the second coating layer.

[0045] In one embodiment, the coating layer includes 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 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 silicon particles, but also has conductive properties, improving the conductivity of the first coating layer. The presence of the coating layer can also reduce thermal failure of silicon particles caused by short circuits and high temperatures, thereby improving the safety of the negative electrode sheet.

[0046] 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 (for example, 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 buffer space provided by the coating layer for the volume change of the silicon particles is not large enough, and the expansion rate of the negative electrode sheet is still large; when R is greater than 100, although the buffer space provided by the coating layer for the volume change of the silicon particles is large enough to reduce the expansion rate of the negative electrode sheet, it will increase the internal resistance of the battery. When the ratio R of the center distance between adjacent first silicon particles to the median particle size Dv50 of the first silicon particles is limited to the above-mentioned specific range, the coating layer can provide a large buffer space for the volume change of the silicon particles while ensuring a low internal resistance of the battery, thereby improving the overall performance of the battery.

[0047] In one example, a ratio R of a center distance between adjacent first silicon particles to a median particle size of the first silicon particles is (0.1-10):1.

[0048] 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.

[0049] 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).

[0050] In one example, the center distance between adjacent first silicon particles is 0.1 μm-500 μm.

[0051] 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. This is the most important function of carbon nanotubes as a conductive agent. At the same time, since S1<S, the internal resistance of the battery can be further reduced. 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 conductivity, 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 large-scale conductive network 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 and reducing the low-temperature expansion rate of the battery.

[0052] 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.

[0053] 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.

[0054] 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).

[0055] According to a specific embodiment, the median particle size Dv50 of the first silicon particles is 5 μm-20 μm (e.g., 5 μm, 7 μm, 10 μm, 12 μm, 15 μm, 17 μm, 20 μm). In the present disclosure, the median particle size Dv50 can be measured using a laser particle size analyzer.

[0056] In one example, the median particle size Dv50 of the first silicon particles is 6 μm-15 μm.

[0057] In one example, the first silicon particles include one or more of silicon-carbon particles, silicon-oxygen particles, and nano-silicon particles.

[0058] In one example, the silicon-carbon particles include particles formed by mixing silicon or partially oxidized silicon with amorphous or crystalline carbon.

[0059] 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.

[0060] In one embodiment, the silicon-oxygen particles comprise 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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 partially enter 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.

[0067] 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、5000m 2When the specific surface area of ​​the first conductive agent is limited to the above specific range, sufficient space can be provided for the polymer material to enter the pores of the first conductive agent.

[0068] In one embodiment, the specific surface area of ​​the first conductive agent is 5m 2 / g-1000m 2 / g.

[0069] In one example, the first conductive agent includes one or more of carbon black, conductive graphite, carbon fiber, and graphene.

[0070] In one embodiment, the first conductive agent includes one or more of carbon black, conductive graphite, and carbon fiber. Selecting the above-mentioned specific first conductive agent can provide more space for the extruded polymer material.

[0071] In one example, the coating layer further includes a first dispersant.

[0072] 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.

[0073] 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%).

[0074] 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%.

[0075] In one example, the second coating layer further includes a second conductive agent, a binder, and a second dispersant.

[0076] In one example, the graphite particles include natural graphite and / or artificial graphite.

[0077] 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.

[0078] In one example, the median particle size Dv50 of the second active material is 4 μm-25 μm.

[0079] In one example, the second active material includes graphite and / or hard carbon.

[0080] 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).

[0081] In one example, the second silicon particles include one or more of silicon-carbon particles, silicon-oxygen particles, and nano-silicon particles.

[0082] In one example, the silicon-carbon particles include particles formed by mixing silicon or partially oxidized silicon with amorphous or crystalline carbon.

[0083] 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.

[0084] In one embodiment, the silicon-oxygen particles include a mixture of amorphous silicon and oxygen.

[0085] 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.

[0086] Here, M includes one or more of Li and Mg.

[0087] 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.

[0088] In one example, the silicon oxide particles include one or more of a mixture of amorphous silicon and ytterbium, Li2SiO3 (crystalline), and Li2Si2O5 (crystalline).

[0089] In one embodiment, the surface of the silicon oxide particles comprises an amorphous carbon coating layer.

[0090] In one example, the second conductive agent includes one or more of carbon black, conductive graphite, carbon fiber, carbon nanotube, and graphene.

[0091] In one example, the binder includes one or more of butadiene-styrene polymer, polystyrene-acrylate polymer, acrylate polymer, polydimethylsiloxane, and polyurethane material.

[0092] 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.

[0093] 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%).

[0094] 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.

[0095] When the weight content of the second conductive agent is 0 wt %, it means that the conductive agent does not exist in the second coating layer.

[0096] 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 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%.

[0097] In one embodiment, the surface of the first coating layer away from the negative electrode current collector has a protrusion (as shown in FIG9 ). 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 excessive local lithium embedding of the first silicon particles, and improving the cycle structural stability of the negative electrode sheet.

[0098] 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 8 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.

[0099] In one example, the flatness of the first coating layer is 8 μm-20 μm.

[0100] In the present disclosure, flatness can be obtained by the following method: 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 9, 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.

[0101] 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.

[0102] According to a specific embodiment, the thickness d1 of the first coating layer is 0.5 μm-50 μm (for example, 0.5 μm, 1 μm, 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm).

[0103] 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.

[0104] In one example, the thickness d1 of the first coating layer is 1 μm-40 μm.

[0105] According to a specific embodiment, the thickness d2 of the second coating layer is 5 μm-100 μm (for example, 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm).

[0106] In one example, the thickness d2 of the second coating layer is 10 μm-80 μm.

[0107] 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.

[0108] As shown in Figure 9, 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.

[0109] In one example, the negative electrode current collector includes one or more of copper foil, composite copper foil, and porous copper foil.

[0110] 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 limited to the above-mentioned specific range, adhesion can be ensured, the negative electrode sheet does not fall off powder, the internal resistance is low, and the dynamic requirements of the battery can be met.

[0111] 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.

[0112] In one embodiment, the surface density of the negative electrode sheet is 4 mg / cm 2 -11 mg / cm 2 .

[0113] 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 3When the compaction density of the negative electrode sheet is limited to the above-mentioned specific 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.

[0114] In one embodiment, the compaction density of the negative electrode sheet is 1.6 g / cm 3 -1.8g / cm 3 .

[0115] A second aspect of the present disclosure provides a battery, comprising the negative electrode sheet described in the first aspect of the present disclosure.

[0116] The materials of the battery except the negative electrode sheet can be made according to the methods in the art, and can achieve the effects of low expansion rate, long cycle life and good fast charging performance.

[0117] The battery may be a lithium-ion battery.

[0118] Since the battery disclosed herein includes the negative electrode sheet disclosed herein, the battery expansion rate is reduced, the cycle life is extended, and the fast charging performance is improved.

[0119] 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.

[0120] The following examples are used to illustrate the negative electrode sheet of the present disclosure.

[0121] Example 1

[0122] (1) Preparation of ingredients

[0123] Negative electrode current collector: copper foil;

[0124] 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);

[0125] Second coating: 98.2 parts by weight of graphite particles (artificial graphite), 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).

[0126] (2) Preparation of negative electrode sheet

[0127] The first silicon particles, the first conductive agent (specific surface area of ​​800m 2 / g), a polymer material and a first dispersant were dispersed in deionized water and stirred to obtain a uniformly dispersed first slurry. The solid content of the first slurry was 16%, and the viscosity was 3500mPa·s-6500mPa·s. The first slurry was passed through a 150-mesh gauze and evenly coated on the surfaces of both sides of the negative electrode current collector to form a first coating. The projection of the first coating on the negative electrode current collector was discontinuous strips (see Figure 4), and the coating surface density was 0.94mg / 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;

[0128] The second conductive agent, 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, the negative electrode sheet was obtained, wherein the total surface density was: 5.80 mg / cm 2 , compacted density is 1.72g / cm 3 For details, see Table 1-1 and Table 1-2.

[0129] Example 2 group

[0130] 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.

[0131] Example 3 group

[0132] 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.

[0133] Example 4 Group

[0134] 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.

[0135] Example 5 group

[0136] 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.

[0137] Example 6

[0138] The same process as in Example 1 was performed, except that second silicon particles were added to the second coating layer. Example 6a: 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 6b: 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).

[0139] Example 7 Group

[0140] 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.

[0141] Example 8

[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 9

[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 10

[0146] 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.

[0147] Example 11 Group

[0148] The method was carried out in accordance with Example 1, except that the ratio of the sum of the projected areas S1 of the blocks in the first coating layer on the negative electrode current collector to the projected area S of the coating layer on the negative electrode current collector was changed. For details, see Table 1-1 and Table 1-2.

[0149] Example 12 Group

[0150] The same procedure was followed as in Example 1, except that the shape of the blocks in the first coating layer was changed. For details, see Table 1-1 and Table 1-2.

[0151] Example 13 Group

[0152] This example group was carried out in accordance with Example 1, except that the flatness of the first coating layer was changed by rolling. In Example 13a, before applying the second coating layer, the negative electrode sheet including the first coating layer was rolled to make the compaction of the first coating layer 1.0 g / cm 3 In Example 13b, the negative electrode sheet including the first coating was rolled to a compaction of 0.61 g / cm 3 In Example 13c, 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.

[0153] Table 1-1

[0154] Table 1-2 * indicates the same as Example 1; - indicates absence.

[0155] Comparative Example 1

[0156] (1) Preparation of ingredients

[0157] Negative electrode current collector: copper foil;

[0158] 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.

[0159] (2) Preparation of negative electrode sheet

[0160] 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 .

[0161] Comparative Example 2

[0162] The same procedure is carried out as in Example 1, except that the projected area S1 of the first coating layer on the negative electrode current collector is equal to the projected area S of the coating layer on the negative electrode current collector.

[0163] Preparation Example

[0164] Batteries were prepared using the negative electrode sheets obtained in the examples and comparative examples in the following manners.

[0165] (1) Positive electrode

[0166] 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 .

[0167] (2) Negative electrode

[0168] The negative electrode sheets obtained in the above embodiments and comparative examples were used respectively.

[0169] (3) Electrolyte

[0170] 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.

[0171] (4) Isolation film

[0172] A coated polyethylene isolation film with a thickness of 8 μm was selected.

[0173] (5) Preparation of lithium-ion batteries

[0174] 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.

[0175] Test Case

[0176] The batteries obtained in the examples and comparative examples were tested as follows:

[0177] (1) DC internal resistance (DCIR) test

[0178] 1) Stand at 0℃±2℃ for 1 hour;

[0179] 2) 0.1C discharge to the lower limit voltage (the lower limit voltage is 3.0V);

[0180] 3) Let it stand for 10 minutes;

[0181] 4) Charge at 0.1C to the upper voltage limit (upper voltage is 4.53V), cut off current 0.05C, and rest for 10 minutes;

[0182] 5) 0.1C discharge to the lower limit voltage (for initial capacity);

[0183] 6) Rest for 10 minutes, charge at 0.1C to the upper voltage limit, and cut off current 0.05C;

[0184] 7) Stand still for 10 minutes, release at 0.1C for 10 seconds (200ms sampling), and release at 1C for 360 seconds (200ms sampling)

[0185] 8) Let it rest for 15 minutes, and repeat steps 7 to 8 until the voltage drops to the lower limit voltage;

[0186] The discharge current in steps 7) to 8) is calculated using the initial capacity (Note: The 0.1C current and the 1C current are both calculated based on the same capacity. The end voltage of a 0.1C discharge for 10 seconds is V1, and the corresponding voltage of a 1C discharge for 5 seconds is V2).

[0187] The DICR at 100% SOC was collected. The DCIR calculation method is: (V1-V2) / (1C-0.1C). The test results are shown in Table 2.

[0188] (2) 0℃ cycle test

[0189] Take the battery with 50% SOC after OCV measurement, test its voltage, internal resistance and thickness T1, then place the battery in a (0±3)℃ environment, let it stand for 3 hours, and when the battery body reaches (0±3)℃, charge it to 4.2V at a constant current of 1.5C, then charge it to 4.5V at a constant current and constant voltage of 0.7C, 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 of 100 / 200 / 300 / 400 / 500 weeks is recorded respectively. The discharge capacity of the cycles of 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 of 100 / 200 / 300 / 400 / 500 weeks. For example, the discharge capacity of the first cycle is Q1, and the discharge capacity after 500 cycles is Q100. The capacity retention rate after 500 cycles is Q500 / Q1 × 100%. Compare this to the capacity retention rate of a 500T cell. Discharge at 0.2C every 100 cycles for low-rate capacity recovery. After cycling for 100 / 200 / 300 / 400 / 500 cycles, remove the fully charged battery from the 0°C incubator and immediately measure its thickness 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 after 500 cycles is T500, the thickness expansion rate (%) is (T500 - T1) / T1 × 100%. Compare this to the expansion rate of a 500T cell. After cycling, dissect the fully charged cell 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". When a large area of ​​the negative electrode sheet turns gray or black, the result is expressed as "severe lithium deposition".

[0190] (3) 132°C furnace temperature test

[0191] At 25°C±5°C, discharge at 0.2C to the lower voltage limit (3.0V) and allow to rest for 10 minutes. Charge at 0.7C to the upper voltage limit (4.53V) with a cutoff current of 0.05C. At 25°C±5°C, fully charged batteries were tested for voltage, internal resistance, and thickness, and pre-test photos were taken. Fully charged batteries were placed in an oven and heated at a rate of 5±2°C / min. When the oven temperature reached 132°C±2°C, the temperature was maintained constant for 60 minutes. Three batteries were tested per group. The batteries were observed from the beginning to the end of the test to see if they caught fire or exploded. This served as the oven temperature acceptance criteria. No fire or explosion was considered a pass; a fire failure was considered a fail. Results are expressed as "number of passes / number of tests." "3 / 3" indicates a pass in all three tests, and "1 / 3" indicates a pass in all three tests.

[0192] The obtained results are recorded in Table 2.

[0193] Table 2

[0194] It can be seen from Table 2 that, through the comparative examples and the embodiments, it can be seen that the DC resistance of the battery made from the negative electrode sheet of the embodiment is significantly reduced, the low-temperature expansion rate is significantly reduced, the cycle capacity retention rate is significantly improved, the lithium plating situation is improved, and the furnace temperature test results are improved, indicating that the negative electrode sheet of the present invention and the battery including the negative electrode sheet, by setting the sum of the areas of the projections of several blocks in the first coating on the negative electrode current collector S1 < the area of ​​the projection of the coating on the negative electrode current collector to S, reduce the impedance of the battery, improve the fast charging performance of the battery, reduce the expansion rate of the battery in a low temperature environment, extend the cycle life of the battery, and improve the lithium plating situation of the battery.

[0195] 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 collector and a coating located on one side or both sides of the negative electrode collector, the coating includes a first coating and a second coating, the first coating is located between the negative electrode collector and the second coating, the first coating includes a plurality of blocks, the sum of the areas of projections of the plurality of blocks in the first coating on the negative electrode collector is S1, the area of ​​the projection of the coating on the negative electrode collector is S, then S1<S.

2. The negative electrode sheet according to claim 1, wherein: The ratio of the sum of the areas S1 of the projections of several blocks in the first coating on the negative electrode current collector to the area S of the projection of the coating on the negative electrode current collector is (0.2-0.99):1, preferably (0.6-0.9):1; more preferably (0.6-0.8):

1.

3. The negative electrode sheet according to claim 1 or 2, wherein: The first coating is a discontinuous coating; And / or, the projection of the blocks in the first coating on the negative electrode current collector is a circle, a square, a strip, a polygon, a pattern or an irregular shape; And / or, in the projection of the coating on the negative electrode current collector, the sum of the areas not coated with the first coating is S2, then S1=S-S2.

4. The negative electrode sheet according to any one of claims 1 to 3, wherein: The first coating layer 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 a ratio R of a center distance between adjacent first silicon particles to a median particle size Dv50 of the first silicon particles is (0.01-100):1; And / or, 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.

5. The negative electrode sheet according to any one of claims 1 to 4, 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.

6. The negative electrode sheet according to claim 4 or 5, wherein: 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; And / or, the first silicon particles include one or more of silicon-carbon particles, silicon-oxygen particles and nano-silicon particles.

7. The negative electrode sheet according to claim 6, wherein: The silicon oxide particles include a chemical formula of M a Si b O x of substances, where 0≤a<5, 0≤b<3, 0≤x≤2; 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.

8. The negative electrode sheet according to any one of claims 4 to 7, 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 carbon black, conductive graphite, carbon fiber and graphene.

9. The negative electrode sheet according to any one of claims 4 to 8, 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%.

10. The negative electrode sheet according to any one of claims 4 to 9, 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.

11. The negative electrode sheet according to any one of claims 4 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 conductive graphite, acetylene black, conductive carbon black Super-P, Ketjen black and / or hard carbon.

13. The negative electrode sheet according to any one of claims 1 to 12, wherein: The thickness d1 of the first coating is 0.5 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.

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

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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