Negative plate and battery

Abstract

The invention relates to a negative plate and a battery. The negative plate comprises a negative current collector and a negative active material layer arranged on at least one side surface of the negative current collector; a negative electrode active material in the negative electrode active material layer comprises a silicon material and a carbon material; the particle sizes of the silicon material and the carbon material meet the following relational expressions: 2 < D10Si < 0.5 * D10C formula 1, and D90Si < 0.5 * D90C formula 2, in the relational expressions 1 and 2, D10Si is the particle size D10 of the silicon material, and the unit is [mu] m; d10C is the particle size D10 of the carbon material, and the unit is [mu] m; d90Si is the particle size D90 of the silicon material, and the unit is [mu] m; and D90C is the particle size D90 of the carbon material, and the unit is [mu] m. According to the scheme provided by the invention, the battery can present excellent normal-temperature cycle performance and high-temperature cycle performance.

Description

Technical Field

[0001] The present application relates to the technical field of batteries, and particularly to a negative electrode sheet and a battery. Background Art

[0002] Lithium-ion batteries are widely used in fields such as 3C digital, power tools, aerospace, energy storage, and electric vehicles due to their advantages of high specific energy, no memory effect, and long cycle life. The rapid development of electronic information technology and consumer products has put forward higher requirements for the high voltage and high energy density of lithium-ion batteries.

[0003] In related technologies, silicon materials have received extensive attention and research due to their advantages of high theoretical capacity, wide raw material sources, non-toxicity, and environmental friendliness, and are expected to become the next-generation high-energy density negative electrode materials. However, during the charge and discharge process, the volume change of silicon materials is as high as 300%. The large volume change is likely to cause electrode particle pulverization and active material shedding, resulting in a decrease in the conductivity of the negative electrode active material layer, reducing the cycle performance of the battery, and the deterioration of the cycle performance of the battery is more obvious under high-temperature conditions.

[0004] Therefore, there is an urgent need to develop a negative electrode sheet that can enable the battery to exhibit excellent room-temperature cycle performance and high-temperature cycle performance. Summary of the Invention

[0005] To solve or partially solve the problems existing in related technologies, the present application provides a negative electrode sheet and a battery, which can enable the battery to exhibit excellent room-temperature cycle performance and high-temperature cycle performance.

[0006] The first aspect of the present application provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector; the negative electrode active material in the negative electrode active material layer includes a silicon material and a carbon material; The particle sizes of the silicon material and the carbon material satisfy the following relationship: 2 < D10 Si < 0.5×D10 C Formula 1 D90 Si < 0.5×D90 C Formula 2 In the above-mentioned relationship 1 and relationship 2, D10 Si is the D10 particle size of the silicon material, with the unit of μm; D10 C is the D10 particle size of the carbon material, with the unit of μm; D90 Si is the D90 particle size of the silicon material, with the unit of μm; D90 C is the D90 particle size of the carbon material, with the unit of μm.

[0007] For the negative electrode sheet as described above, wherein, D90C / D10 C <2.5.

[0008] The negative electrode sheet as described above, wherein in the XRD diffraction pattern of the negative electrode active material layer, there is no diffraction peak at a diffraction angle of 2θ = 26° to 30°.

[0009] The negative electrode sheet as described above, wherein 2.7 ≤ D10 Si ≤ 5.7; and / or, 6 ≤ D50 Si ≤ 9, wherein D50 Si is the particle size D50 of the silicon material, with the unit of μm; and / or, 9 ≤ D90 Si ≤ 12.

[0010] The negative electrode sheet as described above, wherein 6 ≤ D10 C ≤ 9; and / or, 11 ≤ D50 C ≤ 14, wherein D50 C is the particle size D50 of the carbon material, with the unit of μm; and / or, 15 ≤ D90 C ≤ 24.

[0011] The negative electrode sheet as described above, wherein the cross-section of the silicon material is a quasi-polygon, the number of sides of the quasi-polygon is not less than 5, and the included angle formed between two adjacent side strips in the quasi-polygon is not less than 60°.

[0012] The negative electrode sheet as described above, wherein the mass percentage content of the silicon material in the negative electrode active material is less than 20%.

[0013] The negative electrode sheet as described above, wherein the silicon material includes one or more of silicon monoxide, lithiated silicon monoxide, silicon-carbon composite materials; and / or, the carbon material includes one or more of natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon.

[0014] The negative electrode sheet as described above, wherein the negative electrode active material layer further includes a conductive agent, and the conductive agent includes one or more of single-walled carbon nanotubes and multi-walled carbon nanotubes; preferably, the mass percentage content of the conductive agent in the negative electrode active material layer is 0.02% to 1.5%; and / or, the negative electrode active material layer further includes a binder, and the binder includes one or more of polyacrylic acid, polyacrylonitrile, polyacrylamide, styrene-butadiene rubber; preferably, the mass percentage content of the binder in the negative electrode active material layer is 1% to 4%; and / or, The negative electrode active material layer further includes a dispersant, and the dispersant includes one or more of lithium carboxymethyl cellulose and sodium carboxymethyl cellulose; preferably, the mass percentage content of the dispersant in the negative electrode active material layer is not higher than 1%.

[0015] In a second aspect of the present application, a battery is provided, which includes the negative electrode sheet as described above.

[0016] The technical solution provided by the present application may include the following beneficial effects: When the particle size relationship between the silicon material and the carbon material is within the range of the above relational expression, the silicon material and the carbon material are combined. The carbon material is mainly large particles, and the silicon particles are mainly small particles. The silicon particles are distributed in the voids of the carbon material. On the one hand, it can improve the particle packing density, thereby improving the compaction density of the negative electrode active layer, making the battery exhibit a higher energy density. On the other hand, it can inhibit the volume expansion of the silicon material, avoid the breakage or pulverization and shedding of the negative electrode active material layer caused by the volume expansion of the silicon material, improve the conductivity of the negative electrode sheet, and thereby improve the cycle performance of the battery.

[0017] It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and cannot limit the present application. Detailed Description of the Invention

[0018] To make the present application easy to understand, the present application will be described in detail below. However, before describing the present application in detail, it should be understood that the present application is not limited to the described specific embodiments. It should also be understood that the terms used herein are only for describing specific embodiments and do not represent restrictive.

[0019] When a numerical range is provided, it should be understood that each intermediate value between the upper and lower limits of the range and any other specified or intermediate value in the specified range is covered by the present application. The upper and lower limits of these smaller ranges can be independently included in the smaller ranges and are also covered by the present application, subject to any explicitly excluded limits in the specified range. When the specified range includes one or both of the limits, the range excluding either or both of the included limits is also included in the present application.

[0020] Unless otherwise defined, all terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which this application belongs. Although any methods and materials equivalent to those described herein can also be used in the implementation or testing of the present application, the preferred methods and materials are now described.

[0021] In the related art, silicon materials have received extensive attention and research due to their advantages such as high theoretical capacity, wide raw material sources, non-toxicity, and environmental friendliness, and are expected to become the next-generation anode materials with high energy density. However, during the charge and discharge process, the volume change of silicon materials is as high as 300%, and the large volume change is likely to cause electrode particle pulverization and active material shedding, resulting in a decrease in the conductivity of the anode active material layer, reducing the cycle performance of the battery, and the deterioration of the cycle performance of the battery is more obvious under high-temperature conditions.

[0022] In view of the above problems, an embodiment of the present application provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector; the negative electrode active material in the negative electrode active material layer includes a silicon material and a carbon material; The particle sizes of the silicon material and the carbon material satisfy the following relational expressions: 2 < D10 Si < 0.5 × D10 C Equation 1 D90 Si < 0.5 × D90 C Equation 2 In Relational Expression 1 and Relational Expression 2, D10 Si is the D10 of the silicon material, with the unit of μm; D10 C is the D10 of the carbon material, with the unit of μm; D90 Si is the D90 of the silicon material, with the unit of μm; D90 C is the D90 of the carbon material, with the unit of μm.

[0023] The present application does not limit the selection of the negative electrode current collector, which can be selected according to actual needs. For example, copper foil can be selected. The negative electrode active material layer of the present application includes a negative electrode active material. The negative electrode active material of the present application includes a carbon material and a silicon material. The present application does not limit the selection of the carbon material, which can be selected according to actual needs. For example, graphite, carbon black, hard carbon, soft carbon, etc. can be selected; the present application does not limit the selection of the silicon material, which can be selected according to actual needs. For example, silicon oxide, pre-lithiated silicon oxide, pre-magnesiated silicon oxide material, silicon-carbon composite material, elemental silicon, etc. can be selected.

[0024] D10 in the present application refers to the particle size D10, that is, in the particle size distribution, the particles smaller than this particle size account for 10% of the total number of particles. Similarly, D90 refers to the particle size D90, that is, in the particle size distribution, the particles smaller than this particle size account for 90% of the total number of particles.

[0025] According to the above solution provided by the present application, after applying the negative electrode sheet to a battery, the battery has excellent room temperature and high temperature cycling performance. The applicant analyzed this principle and believes that the reason is that when the particle size relationship between the silicon material and the carbon material is within the range of the above relational formula, the silicon material and the carbon material are combined. The carbon material mainly consists of large particles, and the silicon particles mainly consist of small particles. The silicon particles are distributed in the voids of the carbon material. On the one hand, it can improve the particle packing density, thereby improving the compaction density of the negative electrode active layer, making the battery exhibit a higher energy density. On the other hand, it can inhibit the volume expansion of the silicon material, avoid the damage or pulverization and shedding of the negative electrode active material layer caused by the volume expansion of the silicon material, improve the conductivity of the negative electrode sheet, and thus improve the cycling performance of the battery.

[0026] Specifically, the present application tests the negative electrode active material layer, including the following steps: cutting the electrode sheet by cross-section polishing to obtain an electrode sheet cross-section; performing SEM testing on the electrode sheet cross-section at a magnification of 5000 times to observe the silicon material particles and carbon material particles, randomly selecting 100 silicon material particles and 100 carbon material particles, respectively calibrating the particle size of each particle, and then statistically analyzing the particle sizes of the 100 tested silicon material particles or carbon material particles to obtain D10 and D90 of the silicon material and D10 and D90 of the carbon material in the negative electrode active material layer.

[0027] In a specific embodiment, D90 C / D10 C <2.5, for example D90 C / D10 C can be 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3 or 2.4, etc. When the ratio of D90 to D10 of the carbon material is within the above range, the particle size distribution of the carbon material is concentrated, the particle size of the carbon material is uniform, the carbon material can be distributed more closely and uniformly on the surface of the silicon material, inhibit the expansion of the silicon material to a greater extent, improve the cycling performance of the battery, and at the same time, the carbon material with uniform particle size in the negative electrode active material layer can construct a more uniform and efficient electron transport channel, making the insertion and extraction and diffusion of lithium ions in the negative electrode active material layer smoother, improving the conductivity of the negative electrode active material layer, and thus improving the cycling performance of the battery.

[0028] In a specific embodiment, in the XRD diffraction pattern of the negative electrode active material layer, there is no diffraction peak at a diffraction angle 2θ of 26° to 30°. In this application, the negative electrode active material layer is subjected to XRD testing to obtain the XRD diffraction pattern of the negative electrode active material layer. In this XRD diffraction pattern, there is no diffraction peak at a diffraction angle 2θ of 26° to 30°, that is, no silicon diffraction peak appears, indicating that the silicon material in the negative electrode active material layer is low-crystalline silicon. When there is no diffraction peak in the negative electrode active material layer, the volume expansion of the low-crystalline silicon is low, resulting in a low volume expansion of the negative electrode active material layer, and further improving problems such as pulverization and shedding of the negative electrode active material layer to a greater extent, and further improving the cycle performance of the battery.

[0029] Specifically, in this application, the negative electrode active material layer is subjected to XRD testing to obtain the XRD diffraction pattern of the negative electrode active material layer, and by analyzing the XRD diffraction pattern, it is determined whether there is a diffraction peak at a diffraction angle 2θ of 26° to 30°.

[0030] In a specific embodiment, 2.7 ≤ D10 Si ≤ 5.7. For example, D10 Si can be 2.7, 3, 3.5, 4, 4.5, 5, 5.5, or 5.7, etc.

[0031] In a specific embodiment, 6 ≤ D50 Si ≤ 9, where D50 Si is the particle size D50 of the silicon material, with the unit of μm. For example, D50 Si can be 6, 6.5, 7, 7.5, 8, 8.5, or 9, etc.

[0032] In a specific embodiment, 9 ≤ D90 Si ≤ 12. For example, D90 Si can be 9, 9.5, 10, 10.5, 11, 11.5, or 12, etc.

[0033] When the D10, D50, and D90 of the silicon material are within the above ranges, the particle size of the silicon material is reasonable, and the silicon material can be better distributed in the voids of the carbon material, inhibiting the expansion of the silicon material, avoiding pulverization and shedding of the negative electrode active material layer, and at the same time avoiding a large specific surface area of the silicon material caused by too small a particle size of the silicon material, thereby avoiding an increase in side reactions between the silicon material and the electrolyte and reducing the consumption of the electrolyte, thus improving the cycle performance of the battery.

[0034] In a specific embodiment, 6 ≤ D10 C ≤ 9. For example, D10 C can be 6, 6.5, 7, 7.5, 8, 8.5, or 9, etc.

[0035] In a specific embodiment, 11 ≤ D50C ≤14, where D50 C is the particle size D50 of the carbon material, with the unit of μm. For example, D50 C can be 11, 11.5, 12, 12.5, 13, 13.5 or 14, etc.

[0036] In a specific embodiment, 15 ≤ D90 C ≤ 24. For example, D90 C can be 15, 16, 17, 18, 19, 20, 21, 22, 23 or 24, etc.

[0037] When the D10, D50, and D90 of the carbon material are within the above ranges, the particle size of the carbon material is reasonable, and the carbon material can be fully and evenly distributed on the surface of the silicon material, suppressing the expansion of the silicon material to a greater extent, reducing the probability of pulverization and shedding of the negative electrode active material layer. At the same time, the carbon material can form a more efficient and stable conductive network, further improving the conductivity of the negative electrode active material layer, thereby making the cycle performance of the battery better.

[0038] In a specific embodiment, the cross-section of the silicon material is a quasi-polygon, and the number of sides of the quasi-polygon is not less than 5, and the angle formed between two adjacent side strips in the quasi-polygon is not less than 60°. The shape of the cross-section of the silicon material in this application is a quasi-polygon similar to a polygon with sides and angles. The number of sides of this quasi-polygon is not less than 5, for example, the number of sides can be 5, 6, 7, 8, 9 or 10, etc. The angle of this quasi-polygon is not less than 60°. The angles in this quasi-polygon can be the same, not completely the same, or completely different. For example, the angles in the quasi-polygon can be 60°, 70°, 80°, 90°, 100°, 110° or 120°, etc. When the cross-sectional shape parameters of the silicon material are within the above ranges, the number of sides and the angle of the silicon material are reasonable, which can avoid the appearance of acute angles on the surface of the silicon material, avoid the rupture of the silicon material during the rolling process, and at the same time avoid the deterioration of the battery performance caused by the hard silicon material damaging the surface of the carbon material. In addition, it can also avoid the silicon material being sharp and piercing the separator, avoiding an increase in battery self-discharge, and can avoid the uneven expansion of the silicon material caused by the increase in the anisotropy of the silicon material, thereby ensuring better cycle performance of the battery.

[0039] Specifically, this application conducts SEM tests on the negative electrode active material layer, including the following steps: performing SEM tests on the negative electrode sheet, maintaining a magnification of 5000 times, selecting a preset number, for example, 10 silicon particle materials, and observing the number of side strips and the angle size of each silicon particle; calculating the average value of the number of side strips of the 10 silicon particle materials obtained from the test, and at the same time recording the degree of the minimum angle to obtain the number of side strips and the angle of the silicon material.

[0040] In a specific embodiment, the mass percentage of the silicon material in the negative electrode active material is less than 20%, that is, the ratio of the mass of the silicon material to the total mass of the silicon material and the carbon material is less than 20%. For example, the mass percentage of the silicon material in the negative electrode active material can be 5%, 7%, 10%, 12%, 15% or 18%, etc. When the mass percentage of the silicon material in the negative electrode active material is within the above range, the content of the silicon material in the negative electrode active material is relatively low, and the carbon material can effectively inhibit the volume expansion of the silicon material, thereby reducing the volume expansion of the negative electrode active material layer, and thus improving the cycle performance of the battery. At the same time, the addition of the silicon material can ensure the high energy density of the battery.

[0041] In a specific embodiment, the silicon material includes one or more of silicon monoxide, lithiated silicon monoxide, and silicon-carbon composite materials.

[0042] In a specific embodiment, the carbon material includes one or more of natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, and soft carbon.

[0043] In a specific embodiment, the negative electrode active material layer further includes a conductive agent, and the conductive agent includes one or more of single-walled carbon nanotubes and multi-walled carbon nanotubes; the mass percentage of the conductive agent in the negative electrode active material layer is 0.02% - 1.5%. For example, the mass percentage of the conductive agent in the negative electrode active material layer can be 0.02%, 0.05%, 0.1%, 0.5%, 1% or 1.5%, etc. Adding a conductive agent to the negative electrode active material layer can improve the conductivity of the negative electrode active material layer, facilitate the rapid migration of lithium ions in the negative electrode active material layer, and contribute to improving the cycle performance of the battery.

[0044] In a specific embodiment, the negative electrode active material layer further includes a binder, and the binder includes one or more of polyacrylic acid, polyacrylonitrile, polyacrylamide, and styrene-butadiene rubber; the mass percentage of the binder in the negative electrode active material layer is 1% - 4%. For example, the mass percentage of the binder in the negative electrode active material layer can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5% or 4%, etc. The addition of the binder in the negative electrode active material layer can attach the negative electrode active material and the conductive agent to the surface of the negative electrode current collector, prevent the pulverization of the negative electrode active material layer, improve the structural stability of the negative electrode active material layer, and also can limit the volume expansion of the silicon material to a certain extent, thereby improving the cycle performance of the battery.

[0045] In a specific embodiment, the negative electrode active material layer further includes a dispersant, and the dispersant includes one or more of lithium carboxymethyl cellulose and sodium carboxymethyl cellulose; the mass percentage content of the dispersant in the negative electrode active material layer is not higher than 1%, for example, the mass percentage content of the dispersant in the negative electrode active material layer is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1%, etc. When the mass percentage content of the dispersant is within the above range, the dispersant can be dispersed on the surfaces of the silicon material and the carbon material, avoid the agglomeration of the silicon material and the carbon material, ensure the uniform distribution of the silicon material and the carbon material, and at the same time can enhance the stability of the negative electrode active material layer, reduce the sedimentation and agglomeration of the active substances during the cyclic charge and discharge process, and improve the cyclic performance of the battery.

[0046] The second aspect of the present application provides a battery, including the above-mentioned negative electrode sheet.

[0047] In a specific embodiment, the battery of the present application further includes a positive electrode sheet, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer coated on the positive electrode current collector, the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes at least one of lithium cobaltate, lithium nickel cobalt manganate, and lithium iron phosphate. When the above compounds are selected as the positive electrode active material, the positive electrode active material can fully exert its performance and improve the electrochemical performance of the lithium ion battery.

[0048] In the embodiments of the present application, there is no particular limitation on the type of the positive electrode current collector, and it can be any known material suitable for use as the positive electrode current collector. In one of the embodiments, the positive electrode current collector includes metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, etc., and carbon materials such as carbon cloth and carbon paper. Preferably, the positive electrode current collector is a metal material.

[0049] In a specific embodiment, the positive electrode active material layer further includes a conductive agent and a binder. The conductive agent can include, for example, carbon materials such as natural graphite, artificial graphite, acetylene black, needle coke, carbon nanotubes, and graphene. The binder can include, for example, polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose.

[0050] In a specific embodiment, the battery of the present application further includes an electrolyte, and the electrolyte is an electrolyte well-known in the art that can be used in a battery and makes the battery have excellent electrochemical performance, including a lithium salt and an organic solvent, and can be specifically set according to needs.

[0051] In a specific embodiment, the battery further includes a separator. The embodiments of the present application do not particularly limit the material and shape of the separator, as long as the effects of the present application are not significantly damaged. It may include substances in the form of porous sheets or non-woven fabrics with excellent liquid retention properties, etc. The materials of the resin or glass fiber separator include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, polyethersulfone, etc., and can be specifically set according to needs.

[0052] In a specific embodiment, the battery may include an outer package, which can be used to encapsulate the above-mentioned electrode assembly and electrolyte.

[0053] In a specific embodiment, the outer package of the battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic. As plastics, polypropylene, polybutylene terephthalate, and polybutylene succinate, etc. can be listed.

[0054] The present application does not particularly limit the shape of the secondary battery, and it can be cylindrical, square, or any other shape.

[0055] Hereinafter, the present application will be further described in detail through specific examples.

[0056] Example 1 1. Preparation of the negative electrode sheet Silicon-carbon particles, graphite, conductive carbon nanotubes, polyacrylic acid (PAA), styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC-Na) were proportioned according to a solid mass ratio of 5:90.4:0.1:2:2:0.5 with reference to the following batching process. Among them, the D10 of the silicon-carbon particles was 3.2 μm, the D50 was 7.8 μm, and the D90 was 8.9 μm; the D10 of the graphite was 7.6 μm, the D50 was 12.5 μm, and the D90 was 18.6 μm; the cross-section of the silicon-carbon particles was a quasi-polygon with 7 side lines, and the minimum included angle formed between two adjacent side lines was 78°: Step 1: Dry-mix the silicon-carbon particles and conductive carbon nanotubes at a stirring speed of 25 r / min for 30 min to obtain a premix; Step 2: Add 50% of the PAA and CMC-Na glue solution to the premix for the first stirring treatment, with a revolution speed of 25 r / min, a rotation speed of 200 r / min, and a stirring time of 60 min to obtain a first mixture; Step 3: Add graphite, 50% of the CMC-Na glue solution, and deionized water accounting for 10% of the total solid weight to the first mixture for the second stirring treatment, with a revolution speed of 30 r / min, a rotation speed of 2000 r / min, and a stirring time of 120 min to obtain a second mixture; Step 4: Add SBR latex to the second mixture and perform the third stirring process with a revolution speed of 25 r / min, a rotation speed of 500 r / min, and a stirring time of 45 min. Then, perform vacuum degassing to obtain the negative electrode slurry. Step 5: Uniformly coat the negative electrode slurry on one surface of a negative electrode current collector copper foil with a thickness of 10 μm, and dry it at 110 °C to obtain a negative electrode sheet with a negative electrode material layer coated on one side and a coating thickness of 150 μm. Repeat the above steps on the other surface of the negative electrode sheet to obtain a negative electrode sheet with negative electrode active material layers coated on both sides. Then, cut the negative electrode sheet into a specification of 74 mm × 867 mm for standby.

[0057] Perform XRD testing on the prepared negative electrode sheet, analyze the XRD diffraction pattern, and no diffraction peak appears at 2θ of 26° - 30° in the XRD diffraction pattern.

[0058] 2. Preparation of the positive electrode sheet Mix the positive electrode active material lithium cobaltate, conductive carbon black, and polyvinylidene fluoride (PVDF) binder evenly in a mass ratio of 95:2.5:2.5, fully stir and mix in N-methylpyrrolidone solvent, formulate a slurry with a solid content of 75%, and stir evenly to obtain the positive electrode slurry. Uniformly coat the positive electrode slurry on one surface of an aluminum foil with a thickness of 12 μm, dry it at 90 °C, and obtain a positive electrode sheet with a positive electrode active material layer thickness of 110 μm after cold pressing. Then, repeat the above steps on the other surface of the positive electrode sheet to obtain a positive electrode sheet with positive electrode active material layers coated on both sides. Cut the positive electrode sheet into a specification of 76 mm × 851 mm, weld the tab, and set it aside for use.

[0059] 3. Preparation of the electrolyte In an environment with a water content of less than 10 ppm, mix propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) in a mass ratio of 1:3:6, and add 8% butyl butyrate based on the total mass of the electrolyte; then add lithium hexafluorophosphate (LiPF 6 6) to the solvent to dissolve and mix evenly, and then add fluoroethylene carbonate (FEC) to obtain the electrolyte. Among them, the molar concentration of LiPF 6 6 in the electrolyte is 1.15 mol / L, and the mass concentration of FEC in the electrolyte is 10.1%.

[0060] 4. Fabrication of the lithium-ion battery Use a polyethylene porous polymer film as the separator. Stack the positive electrode sheet, the separator, and the negative electrode sheet in sequence, with the separator in the middle of the positive and negative electrodes to play a separating role. Then wind the stacked electrode sheets and the separator to obtain an electrode assembly. Place the electrode assembly in a pre-formed aluminum-plastic film shell, remove moisture at 80°C, inject the prepared electrolyte, and obtain a lithium-ion battery through processes such as vacuum packaging, standing, forming, and shaping.

[0061] The main differences between Examples 2 to 6 and Comparative Examples 1 to 7 and Example 1 are the different parameters of the silicon material and the carbon material. Refer to Table 1.

[0062] Table 1

[0063] The main differences between Examples 7 to 11 and Comparative Examples 8 to 13 and Example 1 are the different mass ratios of silicon-carbon particles, graphite, conductive carbon nanotubes, polyacrylic acid, styrene-butadiene rubber, and sodium carboxymethyl cellulose. Refer to Table 2.

[0064] Table 2

[0065] Test the batteries prepared in the above examples and comparative examples for the following items.

[0066] Test for the ultimate compaction density of the negative electrode active material layer: Perform a pair-rolling treatment on the negative electrode, gradually increase the pair-rolling pressure until the surface of the electrode sheet shines, and use the compaction density corresponding to the maximum pressure at which the electrode sheet does not shine as the ultimate compaction density.

[0067] Battery performance test: 1. Cycle performance test In a constant temperature oven at (45 ± 2)°C and (25 ± 2)°C respectively, charge the lithium-ion battery at a constant current and constant voltage of 0.7C to 4.53V, charge at a constant voltage to 0.05C, stand for 5 minutes, and then discharge at 0.5C to 3V. The capacity obtained by this step is the initial capacity. Perform a cycle test using 0.7C charging / 0.5C discharging, and use the ratio of the capacity at 600 cycles to the initial capacity to obtain the capacity retention rate.

[0068] For each group of 5 batteries, calculate the average value of the room temperature cycle capacity retention rate and the high temperature cycle capacity retention rate and record it in Table 3.

[0069] 2. Swelling rate test Use a micrometer to measure the thickness of the lithium-ion battery when it is half-charged, i.e., at a 50% state of charge (SOC), as the initial thickness. After 600 cycles of full charge, i.e., at a 100% SOC state, use a micrometer to measure the thickness of the lithium-ion battery at this time. By comparing it with the thickness of the lithium-ion battery when it was initially half-charged, the expansion rate of the fully charged lithium-ion battery at this time can be obtained.

[0070] For each group of 5 batteries, the average values of the room-temperature cycle expansion rate and the high-temperature cycle expansion rate calculated are recorded in Table 3.

[0071] Table 3

[0072] The following conclusions can be drawn from the results of the examples and comparative examples: In Examples 1 to 11, different anode materials and different anode formulations were used under the ingredient parameter conditions of this application, and the battery cells had good cycle retention and thickness expansion. In Comparative Example 1, the D10 of the silicon particles was small, the specific surface area of the anode increased, the SEI film formation consumed more lithium ions, side reactions increased during the cycle, the electrolyte was consumed quickly, the cycle retention of the battery cell decreased, and the thickness expansion increased. In Comparative Example 2, the particle size ratio of the silicon particles and graphite was unreasonable. The mismatch in particle size led to a decrease in the compaction density of the anode, overpressure and material structure damage occurred during the rolling of the electrode sheet. On the one hand, the overpressure led to poor kinetics, and on the other hand, the destruction of the material surface structure increased the side reaction with the electrolyte, so the cycle performance deteriorated. In Comparative Example 3, silicon diffraction peaks appeared in the XRD pattern of the silicon anode. The crystalline silicon material had a larger volume expansion. Microcracks appeared inside the silicon-carbon material during the cycle, and the material failed rapidly, corresponding to the deterioration of the cycle performance and expansion. In Comparative Example 4, the silicon anode material had fewer sides and sharper side angles. The silicon particles were easily fractured during the rolling process, and the sharp corners of the silicon particles also damaged the surface of the graphite, resulting in an increase in side reactions. Moreover, the anisotropy of the particles with fewer sides increased, and the expansion was uneven, resulting in an increase in expansion. In Comparative Examples 5 to 7, the particle size ratio of the silicon particles and graphite was unreasonable, the compaction of the anode electrode sheet decreased significantly, the energy density was lost, and the cycle expansion also increased to a certain extent. In Comparative Example 8, the silicon content was too high, and the cycle retention and thickness expansion deteriorated. In Comparative Example 9, the high content of the conductive agent led to poor slurry fluidity, and severe fish-scale patterns appeared during coating, making it impossible to fabricate a battery cell. In Comparative Example 10, the content of the conductive agent was too low, and a stable conductive network could not be formed inside the electrode sheet, resulting in the deterioration of the cycle performance. In Comparative Example 11, the content of the binder was too low, unable to limit the expansion of the silicon anode, and the expansion of the battery cell increased. In Comparative Examples 12 to 13, the contents of the binder and the dispersant were too high, resulting in difficult lithium ion migration inside the electrode sheet, obvious lithium deposition at room temperature during cycling, and large swelling of the battery cell.

Claims

1. A negative electrode sheet, characterized in that: It comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one side of the negative electrode current collector; the negative electrode active material in the negative electrode active material layer comprises silicon material and carbon material; The particle sizes of the silicon material and the carbon material satisfy the following relationship: 2<D10 Si <0.5×D10 C Formula 1 D90 Si <0.5×D90 C Formula 2 In the above equations 1 and 2, D10 Si is the particle size D10 of the silicon material, in μm; D10 C is the particle size D10 of the carbon material, in μm; D90 Si is the particle size D90 of the silicon material, in μm; D90 C is the particle size D90 of the carbon material, in μm.

2. The negative electrode sheet according to claim 1, characterized in that: D90 C / D10 C <2.5。 3. The negative electrode sheet according to claim 1, characterized in that: In the XRD diffraction spectrum of the negative electrode active material layer, there is no diffraction peak at a diffraction angle of 2θ=26°-30°.

4. The negative electrode sheet according to claim 1, characterized in that: 2.7≤D10 Si ≤5.7； and / or, 6 ≤ D50 Si ≤9, where D50 Si is the particle size D50 of the silicon material, in μm; and / or, 9≤D90 Si ≤12.

5. The negative electrode sheet according to claim 1, characterized in that: 6≤D10 C ≤9； and / or, 11≤D50 C ≤14, where D50 C is the particle size D50 of the carbon material, in μm; and / or, 15 ≤ D90 C ≤24.

6. The negative electrode sheet according to claim 1, characterized in that: The cross section of the silicon material is a quasi-polygon, the number of side strips of the quasi-polygon is not less than 5, and the angle generated between two adjacent side strips in the quasi-polygon is not less than 60°.

7. The negative electrode sheet according to claim 1, characterized in that: The mass percentage of the silicon material in the negative electrode active material is less than 20%.

8. The negative electrode sheet according to claim 1, characterized in that: The silicon material includes one or more of silicon oxide, lithiated silicon oxide, and silicon-carbon composite materials; and / or, The carbon material includes one or more of natural graphite, artificial graphite, mesophase carbon microbeads, hard carbon, and soft carbon.

9. The negative electrode sheet according to claim 1, characterized in that: The negative electrode active material layer further includes a conductive agent, and the conductive agent includes one or more of single-walled carbon nanotubes and multi-walled carbon nanotubes; preferably, the mass percentage of the conductive agent in the negative electrode active material layer is 0.02% to 1.5%; and / or, The negative electrode active material layer further includes a binder, and the binder includes one or more of polyacrylic acid, polyacrylonitrile, polyacrylamide, and styrene-butadiene rubber; preferably, the mass percentage of the binder in the negative electrode active material layer is 1% to 4%; and / or, The negative electrode active material layer further includes a dispersant, and the dispersant includes one or more of lithium carboxymethyl cellulose and sodium carboxymethyl cellulose; preferably, the mass percentage of the dispersant in the negative electrode active material layer is not higher than 1%.

10. A battery, characterized in that: A negative electrode sheet comprising any one of claims 1 to 9.

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