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 coating arranged on at least one side surface of the negative current collector; the negative electrode coating comprises an inert layer, a first active layer and a second active layer which are stacked in sequence, and the second active layer is arranged on the surface of the negative electrode current collector; wherein the inert layer comprises a first binder; the first active layer comprises a first active material, and the first active material comprises a silicon material and a first carbon material; the second active layer includes a second active material including a second carbon material. According to the scheme provided by the invention, the battery can present excellent cycle performance.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a negative electrode sheet and a battery. Background Art

[0002] Lithium-ion batteries are widely used in 3C digital, power tools, aerospace, energy storage, power vehicles and other fields due to their advantages such as 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 the related technology, the current commercial negative electrode material is mainly graphite negative electrode, but graphite is far from meeting the current energy density requirements. Negative electrodes containing silicon materials have received extensive attention and research due to their high theoretical capacity, low lithium insertion potential, abundant raw materials, non-toxicity and environmental protection. However, silicon materials will expand and shrink to a large extent during the charging and discharging process, which can easily cause the electrode particles to pulverize and the active material to fall off, reducing the conductivity of the negative electrode coating, and ultimately causing a significant deterioration in the battery's cycle performance.

[0004] Therefore, it is urgent to develop a battery that combines high energy density and high cycle performance. Summary of the invention

[0005] In order to solve or partially solve the problems existing in the related art, the present application provides a negative electrode sheet and a battery, which can make the battery exhibit excellent cycle performance.

[0006] In a first aspect, the present application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode coating disposed on at least one side of the negative electrode current collector; the negative electrode coating comprises an inert layer, a first active layer and a second active layer stacked in sequence, and the second active layer is disposed on the surface of the negative electrode current collector; wherein: The inert layer includes a first binder; the first active layer includes a first active material, the first active material includes a silicon material and a first carbon material; the second active layer includes a second active material, the second active material includes a second carbon material.

[0007] In the negative electrode sheet as described above, the thickness of the inert layer is 0.2 μm to 2 μm.

[0008] The negative electrode sheet as described above, wherein the porosity of the inert layer is 20% to 60%.

[0009] The negative electrode sheet as described above, wherein the tortuosity of the inert layer is 1-4.

[0010] The negative electrode sheet as described above, wherein the first binder comprises at least one of polyurethane, polymethyl methacrylate, polyacrylonitrile, polyvinylidene fluoride, and polyacrylamide.

[0011] The negative electrode sheet as described above, wherein the negative electrode sheet satisfies the following relationship: 0<H 1 / (H 1 +H 2 )≤50%; Among them, H 1 is the thickness of the first active layer; H 2 is the thickness of the second active layer.

[0012] The negative electrode sheet as described above, wherein the particle size D50 of the silicon material in the first active layer is 5 μm to 20 μm.

[0013] The negative electrode sheet as described above, wherein the particle size D50 of the silicon material in the first active layer is 6 μm to 12 μm.

[0014] The negative electrode sheet as described above, wherein the silicon material includes at least one of silicon element, silicon-oxygen material, and silicon-carbon material.

[0015] The negative electrode sheet as described above, wherein the first carbon material includes at least one of graphite, graphene, and mesophase carbon microbeads.

[0016] In the negative electrode sheet as described above, the mass percentage of the first active material in the first active layer is 91% to 98.9%.

[0017] In the negative electrode sheet as described above, the mass percentage of the silicon material in the first active material is 1% to 50%.

[0018] The negative electrode sheet as described above, wherein the second carbon material comprises at least one of artificial graphite, natural graphite, and mesophase carbon microbeads.

[0019] The negative electrode sheet as described above, wherein the resistance of the negative electrode sheet is 5mΩ~500mΩ.

[0020] The negative electrode sheet as described above, wherein the first active layer and the second active layer further include a conductive agent, and the conductive agent includes at least one of carbon nanotubes, conductive carbon black, graphene, vapor-grown carbon fibers, and lamellar graphite.

[0021] The negative electrode sheet as described above, wherein the first active layer and the second active layer further include a second binder, and the second binder includes at least one of styrene-butadiene rubber, polyacrylic acid, polyacrylamide, polyacrylonitrile, polyurethane, polyethylene oxide, polypropylene alcohol, polyimide, sodium carboxymethyl cellulose, and polyvinylidene fluoride.

[0022] A second aspect of the present application provides a method for preparing a negative electrode sheet, which comprises the following steps: A first slurry, a second slurry and a third slurry are prepared respectively; wherein the first slurry includes the first binder; the second slurry includes the silicon material and the first carbon material; and the third slurry includes the second carbon material; The third slurry is coated on at least one side of the surface of the negative electrode current collector to form the second active layer; the second slurry is coated on the side of the second active layer away from the negative electrode current collector to form the first active layer; the first slurry is coated on the side of the first active layer away from the negative electrode current collector to form the inert layer.

[0023] A third aspect of the present application provides a battery, wherein the battery comprises a positive electrode sheet, and a negative electrode sheet as described above or a negative electrode sheet prepared according to the above method for preparing a negative electrode sheet.

[0024] In the battery as described above, the positive electrode sheet comprises a positive electrode current collector and a positive electrode coating disposed on at least one side of the positive electrode current collector; the positive electrode coating and the first active layer are separated by the inert layer.

[0025] The technical solution provided by the present application may include the following beneficial effects: on the one hand, the first active layer including silicon material in the negative electrode sheet is arranged between the inert layer including the binder and the second active layer including the carbon material. During the charging and discharging process of the battery, the expansion stress generated by the silicon material can be absorbed by the flexible characteristics of the binder and can also be absorbed by the carbon material in the first active layer and the second active layer, thereby achieving the effect of inhibiting the expansion of the silicon material, avoiding the pulverization and shedding of the negative electrode coating, improving the deintercalation rate and transmission rate of lithium ions in the negative electrode coating, and improving the cycle performance of the battery; on the other hand, the highly swellable binder in the inert layer has a high lithium ion affinity and can be fully infiltrated by the electrolyte, thereby improving the lithium conductivity of the negative electrode sheet and the cycle performance of the battery; in addition, the silicon material is concentrated in the first active layer. Under the condition of the same amount of silicon material, the thickness of the active layer containing the silicon material can be reduced. Even if the silicon material expands greatly, due to the small base thickness of the active layer, the increase in its absolute expansion thickness is limited, thereby effectively inhibiting the expansion of the negative electrode coating, thereby improving the cycle performance of the battery.

[0026] It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of the structure of the negative electrode.

[0028] Description of reference numerals: 1- inert layer; 2- first active layer; 3- second active layer; 4-Negative electrode current collector. DETAILED DESCRIPTION

[0029] 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 specific embodiments described. It should also be understood that the terms used herein are only for describing specific embodiments and are not intended to be limiting.

[0030] Where a numerical range is provided, it is understood that each intervening value between the upper and lower limits of the range and any other specified or intervening values ​​in the specified range is encompassed within the present application. The upper and lower limits of these smaller ranges may be independently included in the smaller ranges and are also encompassed within the present application, subject to any explicitly excluded limits in the specified ranges. Where a specified range includes one or two limits, ranges excluding either or both of those included limits are also encompassed within the present application.

[0031] Unless otherwise defined, all terms used herein have the same meaning as those commonly understood by those of ordinary skill in the art to which the present application belongs. Although any methods and materials or equivalent methods and materials described herein can also be used in the practice or testing of the present application, preferred methods and materials are now described.

[0032] Among the related technologies, silicon negative electrodes have received extensive attention and research due to their advantages such as high theoretical capacity, low lithium insertion potential, abundant raw materials, non-toxicity and environmental protection. However, silicon materials will expand and contract to a large extent during the charging and discharging process, which can easily cause the electrode particles to pulverize and the active materials to fall off, reducing the conductivity of the negative electrode coating, and ultimately causing a significant deterioration in the battery's cycle performance.

[0033] In view of the above problems, the present application provides a negative electrode sheet, such as Figure 1 As shown, the negative electrode sheet includes a negative electrode current collector 4 and a negative electrode coating disposed on at least one side of the negative electrode current collector; the negative electrode coating includes an inert layer 1, a first active layer 2 and a second active layer 3 stacked in sequence, and the second active layer 3 is disposed on the surface of the negative electrode current collector 4; The inert layer 1 includes a first binder; the first active layer 2 includes a first active material including a silicon material and a first carbon material; the second active layer 3 includes a second active material including a second carbon material.

[0034] 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 coating of the present application includes three layers, namely an inert layer, a first active layer and a second active layer, which are stacked in sequence, the second active layer is arranged on the surface of the negative electrode current collector, the first active layer is arranged on the surface of the second active layer away from the current collector, and the inert layer is arranged on the surface of the first active layer away from the current collector.

[0035] The inert layer of the present application includes a first binder, which is a highly adhesive and highly swellable binder. The high swelling property of the binder means that the weight swelling rate of the binder is greater than 50% after the binder is immersed in an electrolyte at 45°C for 7 days. The binder can be selected according to actual needs. For example, the first binder may include polyurethane (PU), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyacrylamide (PAM), etc.

[0036] The first active layer of the present application includes a first active material, and the first active material includes a silicon material and a first carbon material. The present application does not limit the selection of silicon materials, and the selection can be made according to actual needs, for example, silicon oxide, silicon-carbon composite materials, elemental silicon, pre-lithiation silicon oxide, pre-magnesiation silicon oxide materials, etc. The present application does not limit the selection of the first carbon material, and the selection can be made according to actual needs, for example, graphite, graphene, mesophase carbon microspheres, carbon black, hard carbon, soft carbon, etc.

[0037] The second active layer of the present application includes a second active material, and the second active material includes a second carbon material. The present application does not limit the selection of the second carbon material, and the second carbon material can be selected according to actual needs, for example, graphite, graphene, mesophase carbon microspheres, carbon black, hard carbon, soft carbon, etc. can be selected.

[0038] According to the above solution provided in the present application, after the negative electrode sheet is applied to a battery, the battery has excellent cycle performance. The applicant analyzed this principle and believed that the reason is that, on the one hand, the first active layer including silicon material in the negative electrode sheet is arranged between the inert layer including the binder and the second active layer including the carbon material. The expansion stress generated by the silicon material during the battery charging and discharging process can be absorbed by the flexible characteristics of the binder and the carbon material in the first active layer and the second active layer, thereby achieving the effect of inhibiting the expansion of the silicon material, avoiding the pulverization and shedding of the negative electrode coating, and improving the deintercalation rate and transmission rate of lithium ions in the negative electrode coating, and improving the cycle performance of the battery; on the other hand, the highly swellable binder in the inert layer has a high lithium ion affinity and can be fully infiltrated by the electrolyte, thereby improving the lithium conductivity of the negative electrode sheet and the cycle performance of the battery; in addition, the silicon material is concentrated in the first active layer. Under the condition of the same amount of silicon material, the thickness of the active layer containing the silicon material can be reduced. Even if the silicon material expands more, due to the small thickness base of the active layer, the increase in its absolute expansion thickness is limited, thereby effectively inhibiting the expansion of the negative electrode coating, thereby improving the cycle performance of the battery.

[0039] In a specific embodiment, the thickness of the inert layer is 0.2μm~2μm. For example, the thickness of the inert layer can be 0.2μm, 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, 1.6μm, 1.7μm, 1.8μm, 1.9μm or 2μm, etc. When the thickness of the inert layer is within the above range, the inert layer can absorb the expansion stress of the silicon material in the first active layer, reduce the probability of powdering and falling off of the negative electrode coating, and the inert layer can be better infiltrated by the electrolyte, thereby greatly improving the deintercalation and transmission rate of lithium ions in the negative electrode coating, thereby improving the cycle performance of the battery. At the same time, the diaphragm in the battery can be eliminated and replaced by the inert layer. Since the thickness of the inert layer is smaller than that of the diaphragm, the overall thickness of the battery can be reduced, which is beneficial to improving the energy density of the battery.

[0040] In a specific embodiment, the porosity of the inert layer is 20% to 60%, for example, the porosity may be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55% or 60%, etc. When the porosity of the inert layer is within the above range, the electrolyte can better infiltrate the inert layer, so that lithium ions can better migrate and diffuse in the negative electrode coating, thereby improving the cycle performance of the battery, and also enabling the inert layer to replace the separator, thereby improving the energy density of the battery.

[0041] In a specific embodiment, the tortuosity of the inert layer is 1 to 4, for example, the tortuosity can be 1, 1.5, 2, 2.5, 3, 3.5 or 4. The tortuosity of the present application refers to the ratio of the actual pore depth of the inert layer to the thickness of the inert layer. When the tortuosity of the inert layer is within the above range, the pores in the inert layer can ensure the efficient migration and diffusion of lithium ions, thereby improving the lithium conductivity of the negative electrode coating, thereby making the cycle performance of the battery better.

[0042] Specifically, the porosity and tortuosity of the inert layer of the present application can be achieved by adjusting the selection of the first binder and the thickness of the inert layer.

[0043] In a specific embodiment, the first binder includes at least one of polyurethane (PU), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), and polyacrylamide (PAM). When the first binder is selected from the above polymers, the first binder can form an inert layer with suitable porosity and tortuosity and high stability, which can effectively absorb the expansion stress of the silicon material and can be fully infiltrated by the electrolyte, thereby improving the lithium conductivity of the negative electrode sheet and thus improving the cycle performance of the battery.

[0044] In a specific embodiment, the negative electrode sheet satisfies the following relationship: 0<H 1 / (H 1 +H 2 )≤50%; where H 1 is the thickness of the first active layer, in μm; H 2 is the thickness of the second active layer, in μm. For example, H 1 / (H 1 +H 2 ) can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, etc. When the thickness relationship between the first active layer and the second active layer is within the above range, the thickness of the first active layer can be ensured to be relatively low, so that the expansion stress of the silicon material in the first active layer can be better absorbed by the carbon material in the second active layer, reducing the risk of pulverization and shedding of the negative electrode coating. At the same time, under the condition of the same amount of silicon material, the reduction in the thickness of the first active layer can increase the porosity of the first active layer and improve the ion mobility of the negative electrode coating, thereby further improving the cycle performance of the battery.

[0045] In a specific embodiment, the particle size D50 of the silicon material in the first active layer is 5 μm to 20 μm, for example, the particle size D50 of the silicon material can be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or 20 μm, etc. When the particle size D50 of the silicon material in the first active layer is within the above range, the silicon material can be evenly dispersed in the first active layer to form a uniform and dense first active layer, which helps to improve the energy density of the battery, and at the same time, the silicon material can expand evenly everywhere to avoid problems such as breakage of the silicon material, and the inert layer and the second active layer can absorb the expansion stress generated by the silicon material to a greater extent, ensuring the stability of the negative electrode coating, thereby further improving the cycle performance of the battery. Preferably, the particle size D50 of the silicon material in the first active layer is 6 μm to 12 μm.

[0046] Specifically, the particle size D50 of the silicon material in the first active layer of the present application can be tested by a Malvern 3000 laser particle size analyzer, including the following steps: cutting the pole piece, polishing the cross section after exposing the cross section, placing the cross section in a SEM to identify the first active layer area, and then performing big data precise measurement of the radius of the silicon particles in the layer area to obtain D50, that is, the particle size D50 of the silicon material in the first active layer.

[0047] In a specific embodiment, the silicon material includes at least one of silicon, silicon-oxygen material, and silicon-carbon material. The present application selects the above silicon material to enable the silicon material to fully exert its electrochemical properties and ensure the energy density of the battery.

[0048] In a specific embodiment, the first carbon material includes at least one of graphite, graphene, and mesophase carbon microspheres. The present application adds the first carbon material to the first active material, so that the first carbon material is fully mixed with the silicon material, further absorbing the expansion stress of the silicon material, avoiding the drastic expansion of the first active layer, thereby improving the stability of the negative electrode coating to a greater extent, thereby making the battery cycle performance better.

[0049] In a specific embodiment, the mass percentage of the first active material in the first active layer is 70% to 98%, for example, the mass percentage of the first active material in the first active layer is 70%, 75%, 80%, 85%, 90%, 95%, 98%, etc. When the mass percentage of the first active material is within the above range, a sufficient amount of active material can improve the conductivity of the negative electrode coating and ensure the energy of the negative electrode coating, so that the battery has both high cycle performance and high energy density.

[0050] In a specific embodiment, the mass percentage of the silicon material in the first active material is 1% to 50%, for example, the mass percentage of the silicon material in the first active material can be 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or 50%, etc. When the mass percentage of the silicon material is within the above range, the silicon material can give full play to its electrochemical properties to ensure the energy density of the battery, and the expansion of the silicon material can be suppressed by the inert layer and the second active layer, avoiding excessive expansion of the silicon material to cause the negative electrode coating to pulverize or fall off, thereby improving the cycle performance of the battery.

[0051] In a specific embodiment, the second carbon material includes at least one of artificial graphite, natural graphite, and mesophase carbon microspheres. The present application selects the above carbon material as the second active layer, which can improve the conductivity of the negative electrode coating and effectively inhibit the expansion of the first active layer, thereby helping to improve the cycle performance of the battery.

[0052] In a specific embodiment, the resistance of the negative electrode sheet is 5mΩ~500mΩ, for example, the resistance of the negative electrode sheet can be 5mΩ, 10mΩ, 50mΩ, 100mΩ, 150mΩ, 200mΩ, 250mΩ, 300mΩ, 350mΩ, 400mΩ, 450mΩ or 500mΩ, etc. The negative electrode current collector in the negative electrode sheet of the present application can select a copper foil with a thickness of 6μm. When the resistance of the negative electrode sheet is within the above range, the conductivity of the negative electrode sheet is high, which is conducive to the transmission and deintercalation of lithium ions, thereby further improving the cycle performance of the battery.

[0053] Specifically, the resistance of the negative electrode sheet of the present application can be achieved by adjusting the composition and thickness of the negative electrode coating.

[0054] Specifically, the resistance of the negative electrode sheet can be tested by a film resistance meter, including the following steps: cutting the negative electrode sheet into small discs with regular shapes and required sizes, and then placing the small discs under the probe of the film resistance meter for testing to obtain the resistance of the negative electrode sheet.

[0055] In a specific embodiment, the first active layer and the second active layer further include a conductive agent, and the conductive agent includes at least one of carbon nanotubes (CNT), conductive carbon black (SP), graphene, vapor grown carbon fiber (VGCF), and lamellar graphite.

[0056] In a specific embodiment, the first active layer and the second active layer further include a second binder, and the second binder includes at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polyacrylamide (PAM), polyacrylonitrile (PAN), polyurethane (PU), polyethylene oxide (PEO), polypropylene alcohol (PVA), polyimide (PI), sodium carboxymethyl cellulose (CMC), and polyvinylidene fluoride (PVDF).

[0057] A second aspect of the present application provides a method for preparing a negative electrode sheet, comprising the following steps: S1. Prepare a first slurry, a second slurry and a third slurry respectively; wherein the first slurry includes a first binder; the second slurry includes a silicon material and a first carbon material; and the third slurry includes a second carbon material; S2. Coating a third slurry on at least one side of the negative electrode current collector to form a second active layer; coating a second slurry on a side of the second active layer away from the negative electrode current collector to form a first active layer; coating the first slurry on a side of the first active layer away from the negative electrode current collector to form an inert layer.

[0058] Specifically, in step S1, while providing the negative electrode current collector, a first slurry, a second slurry and a third slurry are prepared respectively: the first slurry is prepared using a first binder, the second slurry is prepared using a silicon material and a first carbon material, and the third slurry is prepared using a second carbon material.

[0059] In a preferred embodiment, the first slurry uses a first adhesive in a colloidal state, and the first slurry does not contain other components.

[0060] In a preferred embodiment, the second slurry is prepared by uniformly mixing a silicon material, a first carbon material, a conductive agent, and a second binder in a solvent; wherein the silicon material includes at least one of silicon element, silicon oxide material, and silicon carbon material, the first carbon material includes at least one of graphite, graphene, and mesophase carbon microspheres, the conductive agent includes at least one of carbon nanotubes, conductive carbon black, graphene, vapor-grown carbon fiber, and lamellar graphite, and the second binder includes at least one of styrene-butadiene rubber, polyacrylic acid, polyacrylamide, polyacrylonitrile, polyurethane, polyethylene oxide, polypropylene alcohol, polyimide, sodium carboxymethyl cellulose, and polyvinylidene fluoride.

[0061] In a preferred embodiment, the mass ratio of the total mass of the silicon material and the first carbon material, the conductive agent, and the second binder is (91-98.9): (0.1-2): (1-7); the mass of the silicon material accounts for 1%-50% of the total mass of the silicon material and the first carbon material.

[0062] In a preferred embodiment, the third slurry is prepared by uniformly mixing a second carbon material, a conductive agent, and a second binder in a solvent; wherein the second carbon material includes at least one of artificial graphite, natural graphite, and mesophase carbon microspheres, the conductive agent includes at least one of carbon nanotubes, conductive carbon black, graphene, vapor-grown carbon fiber, and lamellar graphite, and the second binder includes at least one of styrene-butadiene rubber, polyacrylic acid, polyacrylamide, polyacrylonitrile, polyurethane, polyethylene oxide, polypropylene alcohol, polyimide, sodium carboxymethyl cellulose, and polyvinylidene fluoride.

[0063] In a preferred embodiment, the mass ratio of the second carbon material, the conductive agent, and the second binder is (95-99.2): (0-2): (0.8-3).

[0064] In step S2, the third slurry is coated on the surface of the negative electrode current collector to form a second active layer, and then the second slurry is coated on the surface of the second active layer to form a first active layer, and then the first slurry is coated on the surface of the first active layer to form an inert layer. The inert layer, the first active layer, the second active layer, and the negative electrode current collector are stacked in sequence.

[0065] In a preferred embodiment, the coating of the third slurry and the second slurry is carried out simultaneously to form the second active layer and the first active layer at the same time; the coating of the first slurry can be carried out simultaneously with the coating of the third slurry and the second slurry, or can be carried out after the coating of the third slurry and the second slurry, which can be selected according to the actual coating equipment.

[0066] The present application can prepare a negative electrode sheet with a specific structure through a three-layer coating method, so that the expansion stress of the silicon material in the first active layer can be absorbed by the inert layer and the second active layer, thereby achieving the effect of inhibiting expansion, improving the stability of the negative electrode coating, thereby improving the deintercalation and transmission rate of lithium ions in the negative electrode coating, improving the cycle performance of the battery, and can improve the lithium ion affinity of the negative electrode coating, further improve the lithium ion transmission performance of the negative electrode coating, thereby improving the cycle performance of the battery. In addition, the preparation method is simple in process and has low equipment requirements, which is conducive to the industrial application of the preparation method.

[0067] The third aspect of the present application provides a battery, comprising the above-mentioned negative electrode sheet or a negative electrode sheet prepared according to the above-mentioned method for preparing the negative electrode sheet. The battery exhibits excellent cycle performance.

[0068] 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 coating coated on the positive electrode current collector, the positive electrode coating includes a positive electrode active material, and the positive electrode active material includes at least one of lithium manganese oxide, nickel cobalt lithium manganese oxide ternary material, nickel manganese oxide, lithium-rich manganese-based material, and nickel cobalt aluminum ternary material. When the positive electrode active material is selected from the above compounds, the positive electrode active material can give full play to its performance and improve the electrochemical performance of the lithium-ion battery.

[0069] In the embodiments of the present application, there is no particular restriction on the type of the positive electrode current collector, which can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, and carbon materials such as carbon cloth and carbon paper. Preferably, the positive electrode current collector is a metal material.

[0070] In a specific embodiment, the positive electrode coating further includes a conductive agent and a binder. The conductive agent includes at least one of carbon materials such as natural graphite, artificial graphite, acetylene black, needle coke, carbon nanotubes, graphene, vapor-grown carbon fiber (VGCF), etc. The binder includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, polyvinylidene fluoride, and polytetrafluoroethylene.

[0071] In a specific embodiment, the battery of the present application further includes an electrolyte, which is an electrolyte known in the art that can be used in batteries and has excellent electrochemical properties of the battery, including lithium salts and organic solvents, and can be specifically configured as needed.

[0072] In a specific embodiment, the lithium-ion battery also includes a diaphragm. The embodiment of the present application has no particular restrictions on the material and shape of the diaphragm, as long as it does not significantly damage the effect of the present application. It can include porous sheet-like or non-woven fabric-like materials with excellent liquid retention, etc. The materials of the resin or glass fiber diaphragm include but are not limited to polyolefins, aromatic polyamides, polytetrafluoroethylene, polyether sulfone, etc., and can be specifically set according to needs.

[0073] In a preferred embodiment, the positive electrode coating and the first active layer are separated by an inert layer, that is, the battery does not include a separator, and the positive electrode coating and the negative electrode coating are in direct contact. The inert layer in the negative electrode coating replaces the separator, avoiding direct contact between the positive electrode coating and the first active layer, avoiding battery short circuit, and also making the overall thickness of the battery smaller, reducing the volume of the battery, and improving the energy density of the battery.

[0074] In a specific embodiment, the battery may include an outer packaging, which can be used to encapsulate the electrode assembly and the electrolyte.

[0075] In a specific embodiment, the outer packaging of the battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, etc. can be listed.

[0076] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape.

[0077] The present application has no particular restrictions on the application fields of lithium-ion batteries, and they can be used in consumer batteries, new energy vehicle power batteries, energy storage batteries and other fields.

[0078] Hereinafter, the present application will be further described in detail through specific embodiments.

[0079] Example 1 1. Preparation of negative electrode Step 1: using polyurethane (PU) glue as the first slurry; The active material (25% silicon-carbon material + 75% graphite), dispersant (CMC), and binder (PAA) were stirred at a mass ratio of 98:1:1 for 15 minutes, and then water was added to adjust the solid content to 55%, and then a secondary dispersion was performed for 60 minutes to obtain a second slurry; the particle size D50 of the silicon-carbon material was 8 μm; The active material (graphite), dispersant (CMC), and binder (SBR) were stirred at a ratio of 98:1:1 for 15 minutes, and then water was added to adjust the solid content to 55%, and then a secondary dispersion was performed for 60 minutes to obtain a third slurry.

[0080] Step 2: uniformly coat the third slurry on one surface of the negative electrode current collector copper foil with a thickness of 6 μm to obtain a second active layer, then coat the second slurry on the surface of the second active layer to obtain a first active layer, and finally coat the first slurry on the surface of the first active layer to obtain an inert layer, and repeat the above operation on the other surface of the negative electrode current collector copper foil to obtain a negative electrode sheet.

[0081] The thickness of the inert layer is 1.5 μm, and the ratio of the thickness of the first active layer to the total thickness of the first active layer and the second active layer is H. 1 / (H 1 +H 2 ) is 20%.

[0082] A mercury intrusion test was performed on the negative electrode sheet, and the porosity of the inert layer was found to be 40%. An ionic impedance test was performed on the negative electrode sheet, and the tortuosity of the inert layer was found to be 2.

[0083] The film resistance test of the negative electrode sheet showed that the resistance of the negative electrode sheet was 30 mΩ.

[0084] 2. Preparation of positive electrode The positive electrode active material lithium cobalt oxide, conductive carbon black (SP) and polyvinylidene fluoride (PVDF) binder are mixed evenly at a mass ratio of 97.5:1.5:1, fully stirred and mixed in N-methylpyrrolidone solvent, and prepared into a slurry with a solid content of 75%, and stirred evenly to obtain a positive electrode slurry. The positive electrode slurry is evenly coated on one surface of an aluminum foil with a thickness of 12μm, dried at 90°C, and cold pressed to obtain a positive electrode sheet with a positive electrode coating thickness of 110μm, and then the above steps are repeated on the other surface of the positive electrode sheet to obtain a positive electrode sheet coated with a positive electrode coating on both sides. The positive electrode sheet is cut into a specification of 76mm×851mm and welded to the pole ear for standby use.

[0085] 3. Preparation of electrolyte In an environment with a water content of less than 10 ppm, propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) were mixed in a mass ratio of 1:3:6, and 8% butyl butyrate was added based on the total mass of the electrolyte; then lithium hexafluorophosphate (LiPF 6 ), dissolve it and mix it evenly, then add fluoroethylene carbonate (FEC) to obtain an electrolyte. 6 The molar concentration in the electrolyte is 1.15 mol / L, and the mass concentration of FEC in the electrolyte is 10.1%.

[0086] 4. Production of lithium-ion batteries The positive electrode sheet and the negative electrode sheet are stacked in order, and the stacked electrode sheets and the isolation film are wound to obtain an electrode assembly. The electrode assembly is placed in the aluminum-plastic film that has been punched and formed, and the moisture is removed at 80°C, and the prepared electrolyte is injected. After vacuum packaging, standing, forming, shaping and other processes, a lithium-ion battery is obtained.

[0087] The main difference between Examples 2 to 26 and Comparative Examples 3 to 12 and Example 1 is that the parameters of the inert layer, the first active layer, and the second active layer are different, see Table 1 and Table 2.

[0088] Comparative Example 1 The difference between this comparative example and Example 1 is the preparation of the negative electrode sheet: the active material (graphite): dispersant (CMC): binder (PAA) is stirred at a ratio of 98:1:1 for 15 minutes, and then water is added to adjust the solid content to 55%, and then secondary dispersion is performed for 60 minutes to obtain a negative electrode slurry. The negative electrode slurry is coated on the surface of the negative electrode current collector and dried to obtain a negative electrode sheet.

[0089] Comparative Example 2 The difference between this comparative example and Example 1 is the preparation of the negative electrode sheet: the active material (25% silicon-carbon material + 75% graphite, the D50 of the silicon-carbon material is 8 μm), dispersant (CMC), and binder (PAA) are stirred for 15 minutes at a mass ratio of 98:1:1, and then water is added to adjust the solid content to 55%, and then secondary dispersion is performed for 60 minutes to obtain a negative electrode slurry. The negative electrode slurry is coated on the surface of the negative electrode current collector and dried to obtain a negative electrode sheet.

[0090] Table 1

[0091] Table 2

[0092] Test example The batteries prepared in the examples and comparative examples were tested for the following properties: 1. Cycle performance test In a constant temperature box at (25±2)℃, the lithium-ion battery was charged to 4.45V at 0.2C constant current and constant voltage, and then charged to 0.05C at constant voltage. After standing for 5 minutes, it was discharged to 3V at 0.2C. The capacity obtained in this step was taken as the initial capacity, and a cycle test was performed using 1C charge / 1C discharge. When the capacity decayed to 80% of the initial capacity, the cycle life was obtained.

[0093] There are 5 batteries in each group, and the average cycle life is recorded in Table 3.

[0094] 2. Expansion rate test The thickness of the lithium-ion battery when half-charged, i.e., at 50% state of charge (SOC), is measured with a screw micrometer as the initial thickness. After 200 cycles, when the battery is fully charged, i.e., at 100% SOC, the thickness of the lithium-ion battery at this time is measured with a screw micrometer again. By comparing the thickness with the initial thickness of the lithium-ion battery when half-charged, the expansion rate of the fully charged lithium-ion battery can be obtained.

[0095] There are 5 batteries in each group, and the average value of the calculated cycle expansion rate is recorded in Table 3.

[0096] Table 3

[0097] Table 3

[0098] From Tables 1 to 3, we can see that: According to the comparison of Examples 1 to 5, when the inert layer is made of a binder such as polyurethane, polymethyl methacrylate, polyacrylonitrile, polyvinylidene fluoride, polyacrylamide, etc., the cycle performance of the battery is improved.

[0099] According to the comparison between Examples 6 to 9 and Comparative Examples 3 and 4, it can be seen that when the thickness of the inert layer is in the range of 0.2 μm to 2 μm, the cycle performance of the battery is better.

[0100] According to the comparison between Example 8 and Example 10 and the comparison between Examples 4, 11, 12 and Comparative Examples 5 and 6, it can be seen that when the porosity of the inert layer is 20% to 60% and the tortuosity of the inert layer is 1 to 4, it is beneficial to improve the cycle performance of the battery.

[0101] According to the comparison between Examples 12 and 13 and between Examples 4, 14 to 16 and Comparative Example 7, when 0<H 1 / (H 1 +H 2 )≤50%, the cycle performance of the battery can be further improved.

[0102] According to the comparison of Examples 4, 17 and 18, it can be seen that when the silicon material is selected from silicon alone, silicon-oxygen material and silicon-carbon material, the cycle performance of the battery can be improved.

[0103] According to the comparison of Examples 4, 19 to 22 and Comparative Examples 8 and 9, it can be seen that when the particle size D50 of the silicon material in the first active layer is 5 μm to 20 μm, it is beneficial to improve the cycle performance of the battery; when the particle size D50 of the silicon material is 6 μm to 12 μm, the first coulombic efficiency, rate performance and cycle performance of the battery are better.

[0104] According to the comparison between Examples 4, 23, 24 and Comparative Example 10, when the mass percentage of the silicon material in the first active material is 1% to 50%, the cycle performance of the battery can be improved.

[0105] According to the comparison between Examples 4, 25, 26 and Comparative Examples 11, 12, when the mass percentage of the first active material in the first active layer is 91%-98.9%, the electrochemical performance of the battery is improved.

[0106] The embodiments of the present application have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The selection of terms used herein is intended to best explain the principles of the embodiments, practical applications, or improvements to the technology in the market, or to enable other persons of ordinary skill in the art to understand the embodiments disclosed herein.

Claims

1. A negative electrode sheet, characterized in that: It comprises a negative electrode current collector and a negative electrode coating disposed on at least one side of the negative electrode current collector; the negative electrode coating comprises an inert layer, a first active layer and a second active layer stacked in sequence, and the second active layer is disposed on the surface of the negative electrode current collector; wherein: The inert layer includes a first binder; the first active layer includes a first active material, the first active material includes a silicon material and a first carbon material; the second active layer includes a second active material, the second active material includes a second carbon material.

2. The negative electrode sheet according to claim 1, characterized in that: The thickness of the inert layer is 0.2 μm to 2 μm.

3. The negative electrode sheet according to claim 1, characterized in that: The porosity of the inert layer is 20% to 60%.

4. The negative electrode sheet according to claim 1, characterized in that: The tortuosity of the inert layer is 1-4.

5. The negative electrode sheet according to claim 1, characterized in that: The first binder includes at least one of polyurethane, polymethyl methacrylate, polyacrylonitrile, polyvinylidene fluoride, and polyacrylamide.

6. The negative electrode sheet according to claim 1, characterized in that: The negative electrode sheet satisfies the following relationship: 0<H1 / (H1+H2)≤50%; Wherein, H1 is the thickness of the first active layer; H2 is the thickness of the second active layer.

7. The negative electrode sheet according to claim 1, characterized in that: The particle size D50 of the silicon material in the first active layer is 5 μm to 20 μm.

8. The negative electrode sheet according to claim 7, characterized in that: The particle size D50 of the silicon material in the first active layer is 6 μm to 12 μm.

9. The negative electrode sheet according to claim 1, characterized in that: The silicon material includes at least one of silicon element, silicon-oxygen material, and silicon-carbon material.

10. The negative electrode sheet according to claim 1, characterized in that: The first carbon material includes at least one of graphite, graphene, and mesophase carbon microbeads.

11. The negative electrode sheet according to claim 7, characterized in that: The mass percentage of the first active material in the first active layer is 91% to 98.9%.

12. The negative electrode sheet according to claim 7, characterized in that: The mass percentage of the silicon material in the first active material is 1% to 50%.

13. The negative electrode sheet according to claim 1, characterized in that: The second carbon material includes at least one of artificial graphite, natural graphite, and mesophase carbon microbeads.

14. The negative electrode sheet according to claim 1, characterized in that: The resistance of the negative electrode sheet is 5mΩ~500mΩ.

15. The negative electrode sheet according to claim 1, characterized in that: The first active layer and the second active layer further include a conductive agent, and the conductive agent includes at least one of carbon nanotubes, conductive carbon black, graphene, vapor-grown carbon fibers, and lamellar graphite.

16. The negative electrode sheet according to claim 1, characterized in that: The first active layer and the second active layer further include a second binder, and the second binder includes at least one of styrene-butadiene rubber, polyacrylic acid, polyacrylamide, polyacrylonitrile, polyurethane, polyethylene oxide, polyacryl alcohol, polyimide, sodium carboxymethyl cellulose, and polyvinylidene fluoride.

17. A method for preparing a negative electrode sheet according to any one of claims 1 to 16, characterized in that: The following steps are involved: A first slurry, a second slurry and a third slurry are prepared respectively; wherein the first slurry includes the first binder; the second slurry includes the silicon material and the first carbon material; and the third slurry includes the second carbon material; The third slurry is coated on at least one side of the surface of the negative electrode current collector to form the second active layer; the second slurry is coated on the side of the second active layer away from the negative electrode current collector to form the first active layer; the first slurry is coated on the side of the first active layer away from the negative electrode current collector to form the inert layer.

18. A battery, characterized in that: The battery comprises a positive electrode sheet, and a negative electrode sheet according to any one of claims 1 to 16 or a negative electrode sheet prepared by the method for preparing a negative electrode sheet according to claim 17.

19. The battery according to claim 18, characterized in that The positive electrode sheet comprises a positive electrode current collector and a positive electrode coating disposed on at least one side of the positive electrode current collector; the positive electrode coating and the first active layer are separated by the inert layer.

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