Negative electrode and battery

Abstract

This application relates to a negative electrode sheet and a battery. The negative electrode sheet includes a negative electrode current collector and a negative electrode coating disposed on at least one surface 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, with the second active layer disposed on the surface of the negative electrode current collector. The inert layer includes a first binder; the first active layer includes a first active material, which includes a silicon material and a first carbon material; and the second active layer includes a second active material, which includes a second carbon material. The solution provided by this application can enable the battery to exhibit 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 relevant technology, the current commercial negative electrode material is mainly graphite, but graphite is far from meeting today's energy density requirements. Anodes containing silicon materials have attracted widespread attention and research due to their high theoretical capacity, low lithium insertion potential, abundant raw materials, non-toxicity, and environmental friendliness. However, silicon materials undergo significant expansion and contraction during the charge and discharge process, which can easily cause electrode particle pulverization and active material shedding, reducing the conductivity of the negative electrode coating and ultimately leading to significant degradation of 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 enable the battery to 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, the second active layer being disposed on the surface of the negative electrode current collector; wherein:

[0007] The inert layer includes a first binder; the first active layer includes a first active material including a silicon material and a first carbon material; and the second active layer includes a second active material including a second carbon material.

[0008] In the negative electrode sheet as described above, the thickness of the inert layer is 0.2 μm to 2 μm.

[0009] The negative electrode sheet as described above, wherein the porosity of the inert layer is 20% to 60%.

[0010] The negative electrode sheet as described above, wherein the tortuosity of the inert layer is 1-4.

[0011] The negative electrode sheet as described above, wherein the first binder includes at least one of polyurethane, polymethyl methacrylate, polyacrylonitrile, polyvinylidene fluoride, and polyacrylamide.

[0012] The negative electrode sheet as described above, wherein the negative electrode sheet satisfies the following relationship: 0

[0013] Wherein, H1 is the thickness of the first active layer; H2 is the thickness of the second active layer.

[0014] 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.

[0015] 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.

[0016] The negative electrode sheet as described above, wherein the silicon material includes at least one of silicon element, silicon oxide material, and silicon carbon material.

[0017] In the negative electrode sheet as described above, the first carbon material includes at least one of graphite, graphene, and mesocarbon microbeads.

[0018] 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%.

[0019] In the negative electrode sheet as described above, the mass percentage of the silicon material in the first active material is 1% to 50%.

[0020] In the negative electrode sheet as described above, the second carbon material comprises at least one of artificial graphite, natural graphite, and mesophase carbon microbeads.

[0021] The negative electrode sheet as described above, wherein the resistance of the negative electrode sheet is 5mΩ~500mΩ.

[0022] 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.

[0023] 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.

[0024] ​A second aspect of the present application provides a method for preparing a negative electrode sheet, comprising the following steps:

[0025] 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;

[0026] The third slurry is coated on at least one side 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; and 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.

[0027] A third aspect of the present application provides a battery, wherein the battery includes a positive electrode sheet, and a negative electrode sheet as described above or a negative electrode sheet prepared according to the above-mentioned method for preparing a negative electrode sheet.

[0028] In the battery as described above, the positive electrode sheet includes a positive electrode current collector and a positive electrode coating provided 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.

[0029] The technical solution provided by the present application can include the following beneficial effects: on the one hand, the first active layer comprising silicon material in the negative electrode sheet is arranged between the inert layer comprising a binder and the second active layer comprising a carbon material. During the charge and discharge process of the battery, the expansion stress generated by the silicon material can be absorbed by the flexible properties of the binder and the carbon material in the first and second active layers, thereby achieving the effect of suppressing the expansion of the silicon material, avoiding the pulverization and shedding of the negative electrode coating, and improving the deintercalation and transmission rate of lithium ions in the negative electrode coating, thereby improving the cycle performance of the battery. On the other hand, the highly swellable binder in the inert layer has a high affinity for lithium ions 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 comprising silicon material can be reduced. Even if the silicon material expands significantly, the increase in its absolute expansion thickness is limited due to the small thickness base of the active layer, thereby effectively suppressing the expansion of the negative electrode coating and improving the cycle performance of the battery.

[0030] 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

[0031] Figure 1 Schematic diagram of the structure of the negative electrode.

[0032] Description of reference numerals:

[0033] 1- inert layer;

[0034] 2-first active layer;

[0035] 3- second active layer;

[0036] 4-Negative electrode current collector. DETAILED DESCRIPTION

[0037] To facilitate understanding of the present application, 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 for the purpose of describing specific embodiments only and are not intended to be limiting.

[0038] Where a range of values is provided, it is understood that each intervening value between the upper and lower limits of the stated range and any other stated or intervening values in the stated range is encompassed herein. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed herein, subject to any express exclusions in the stated ranges. Where a stated range includes one or both limits, ranges excluding either or both of those included limits are also encompassed herein.

[0039] 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 practice or testing of this application, preferred methods and materials are now described.

[0040] Among related technologies, silicon anodes have garnered widespread attention and research due to their high theoretical capacity, low lithium insertion potential, abundant raw materials, non-toxicity, and environmental friendliness. However, silicon materials undergo significant expansion and contraction during the charge and discharge process, which can easily lead to pulverization of electrode particles and shedding of active materials, reducing the conductivity of the anode coating and ultimately significantly degrading the battery's cycling performance.

[0041] In order to solve the above problems, the present invention provides a negative electrode sheet. Figure 1 As shown, the negative electrode sheet includes a negative electrode current collector 4 and a negative electrode coating provided 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 provided on the surface of the negative electrode current collector 4;

[0042] 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; and the second active layer 3 includes a second active material including a second carbon material.

[0043] This application does not limit the choice of negative electrode current collector, which can be selected according to actual needs, for example, copper foil can be selected. The negative electrode coating of this application includes three layers, namely an inert layer, a first active layer, and a second active layer. The inert layer, the first active layer, and the second active layer 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.

[0044] The inert layer of the present application includes a first adhesive, which is a highly adhesive and highly swellable adhesive. The high swellability of the adhesive means that the weight swelling rate of the adhesive is greater than 50% after the adhesive is immersed in an electrolyte at 45°C for 7 days. The adhesive can be selected according to actual needs. For example, the first adhesive may include polyurethane (PU), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyacrylamide (PAM), etc.

[0045] 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 choice of silicon material, and the selection can be made according to actual needs, for example, silicon oxide, silicon-carbon composite material, elemental silicon, pre-lithiated silicon oxide, pre-magnesium silicon oxide material, etc. The present application does not limit the choice of the first carbon material, and the selection can be made according to actual needs, for example, graphite, graphene, mesophase carbon microbeads, carbon black, hard carbon, soft carbon, etc.

[0046] The second active layer of the present application includes a second active material, which includes a second carbon material. The present application does not limit the choice of the second carbon material, and the second carbon material can be selected according to actual needs, for example, graphite, graphene, mesocarbon microbeads, carbon black, hard carbon, soft carbon, etc.

[0047] 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 comprising silicon material in the negative electrode sheet is arranged between the inert layer comprising a binder and the second active layer comprising a carbon material. During the charge and discharge process of the battery, the expansion stress generated by the silicon material can be absorbed by the flexible properties of the binder and the carbon material in the first and second active layers, thereby achieving the effect of suppressing the expansion of the silicon material, preventing the pulverization and shedding of the negative electrode coating, and improving the deintercalation and transmission rate of lithium ions in the negative electrode coating, thereby improving the cycle performance of the battery. On the other hand, the highly swellable binder in the inert layer has a high affinity for lithium ions 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 comprising silicon material can be reduced. Even if the silicon material expands significantly, due to the small base thickness of the active layer, the increase in its absolute expansion thickness is limited, thereby effectively suppressing the expansion of the negative electrode coating and improving the cycle performance of the battery.

[0048] In a specific embodiment, the thickness of the inert layer is 0.2 μm to 2 μm. For example, the thickness of the inert layer can be 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm or 2 μm, 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 pulverization 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.

[0049] In one specific embodiment, the porosity of the inert layer is 20% to 60%, for example, the porosity can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, or 60%. When the porosity of the inert layer is within the above range, the electrolyte can better infiltrate the inert layer, allowing lithium ions to better migrate and diffuse in the negative electrode coating, thereby improving the cycle performance of the battery. At the same time, the inert layer can also replace the separator, thereby increasing the energy density of the battery.

[0050] In one 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 herein 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 efficient migration and diffusion of lithium ions, thereby improving the lithium conductivity of the negative electrode coating and, in turn, improving the cycle performance of the battery.

[0051] Specifically, the porosity and tortuosity of the inert layer of the present application can be achieved by regulating the selection of the first binder and the thickness of the inert layer.

[0052] In one specific embodiment, the first binder includes at least one of polyurethane (PU), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), and polyacrylamide (PAM). When these polymers are used as the first binder, the first binder can form an inert layer with suitable porosity and tortuosity, as well as high stability. This layer can effectively absorb the expansion stress of the silicon material while being fully wetted by the electrolyte, thereby improving the lithium conductivity of the negative electrode sheet and thus enhancing the battery's cycling performance.

[0053] In one specific embodiment, the negative electrode sheet satisfies the following relationship: 0

[0054] ​In one 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. 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, forming a uniform and dense first active layer, which helps to improve the energy density of the battery. At the same time, it can ensure uniform expansion of the silicon material, avoiding problems such as silicon material breakage. In addition, 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.

[0055] 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 electrode, polishing the cross-section after exposing the cross-section, placing the cross-section in an SEM to identify the first active layer area, and then performing big data precise measurement of the radius of the silicon particles in this layer area to obtain D50, that is, the particle size D50 of the silicon material in the first active layer.

[0056] 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 materials to enable the silicon materials to fully exert their electrochemical properties and ensure the energy density of the battery.

[0057] In one embodiment, the first carbon material comprises at least one of graphite, graphene, and mesocarbon microbeads. The present application adds the first carbon material to the first active material to fully mix the first carbon material with the silicon material, further absorbing the expansion stress of the silicon material and preventing drastic expansion of the first active layer. This significantly improves the stability of the negative electrode coating and further enhances the battery's cycling performance.

[0058] In one 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 the active material can improve the conductivity of the negative electrode coating and ensure the energy utilization of the negative electrode coating, thereby achieving both high cycle performance and high energy density in the battery.

[0059] 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%. When the mass percentage of the silicon material is within the above range, the silicon material can fully exert its electrochemical properties, ensuring the energy density of the battery. At the same time, the expansion of the silicon material can be suppressed by the inert layer and the second active layer, preventing excessive expansion of the silicon material from causing pulverization or shedding of the negative electrode coating, thereby improving the cycle performance of the battery.

[0060] In one embodiment, the second carbon material comprises at least one of artificial graphite, natural graphite, and mesocarbon microbeads. The use of such carbon materials as the second active layer in this application can enhance the conductivity of the negative electrode coating and effectively inhibit the expansion of the first active layer, thereby improving the battery's cycling performance.

[0061] In one specific embodiment, the resistance of the negative electrode sheet is 5mΩ to 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. The negative electrode current collector in the negative electrode sheet of the present application can be made of copper foil with a thickness of 6μm. When the resistance of the negative electrode sheet is within the above range, the negative electrode sheet has high conductivity, which is conducive to the transmission and deintercalation of lithium ions, thereby further improving the cycle performance of the battery.

[0062] Specifically, the resistance of the negative electrode sheet of the present application can be achieved by regulating the composition and thickness of the negative electrode coating.

[0063] Specifically, the resistance of the negative electrode sheet can be tested by a sheet resistance meter, which includes the following steps: cutting the negative electrode sheet into small discs with regular shapes and sizes that meet the requirements, and then placing the above small discs under the probes of the sheet resistance meter for testing to obtain the resistance of the negative electrode sheet.

[0064] 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 (CNTs), conductive carbon black (SP), graphene, vapor grown carbon fiber (VGCF), and lamellar graphite.

[0065] 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).

[0066] A second aspect of the present application provides a method for preparing a negative electrode sheet, comprising the following steps:

[0067] 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;

[0068] 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 the side of the second active layer away from the negative electrode current collector to form a first active layer; coating the first slurry on the side of the first active layer away from the negative electrode current collector to form an inert layer.

[0069] 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.

[0070] In a preferred embodiment, the first slurry is a first adhesive in a colloidal state, and the first slurry does not contain other components.

[0071] In a preferred embodiment, the second slurry is prepared by uniformly mixing 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.

[0072] 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.

[0073] 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.

[0074] 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).

[0075] In step S2, the third slurry is applied to the surface of the negative electrode current collector to form a second active layer. Subsequently, the second slurry is applied to the surface of the second active layer to form a first active layer. The first slurry is then applied to the surface of the first active layer to form an inert layer. The inert layer, first active layer, second active layer, and negative electrode current collector are stacked in this order.

[0076] 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.

[0077] This application utilizes a three-layer coating method to produce a negative electrode sheet with a specific structure. This allows the expansion stress of the silicon material in the first active layer to be absorbed by the inert layer and the second active layer, thereby suppressing expansion and improving the stability of the negative electrode coating. This, in turn, increases the rate of lithium ion insertion, extraction, and transfer within the negative electrode coating, thereby improving the battery's cycling performance. Furthermore, the negative electrode coating's lithium ion affinity is enhanced, further improving its lithium ion transfer performance, thereby improving the battery's cycling performance. Furthermore, this preparation method is simple and requires minimal equipment, facilitating its industrial application.

[0078] A 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.

[0079] In one specific embodiment, the battery of the present application further includes a positive electrode sheet, the positive electrode sheet including a positive electrode current collector and a positive electrode coating coated on the positive electrode current collector, the positive electrode coating including a positive electrode active material, and the positive electrode active material including at least one of lithium manganese oxide, a ternary material of nickel cobalt lithium manganese oxide, lithium nickel manganese oxide, a lithium-rich manganese-based material, and a ternary material of nickel cobalt aluminum. When the positive electrode active material is selected from the above compounds, the positive electrode active material can fully demonstrate its performance and enhance the electrochemical performance of the lithium-ion battery.

[0080] In the embodiments of the present application, there are no particular restrictions on the type of positive electrode current collector; it can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes a metal material such as aluminum, stainless steel, nickel plating, titanium, tantalum, or a carbon material such as carbon cloth or carbon paper. Preferably, the positive electrode current collector is a metal material.

[0081] In one specific embodiment, the positive electrode coating further comprises a conductive agent and a binder. The conductive agent comprises at least one of carbon materials such as natural graphite, artificial graphite, acetylene black, needle coke, carbon nanotubes, graphene, and vapor-grown carbon fiber (VGCF). The binder comprises at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, polyvinylidene fluoride, and polytetrafluoroethylene.

[0082] In a specific embodiment, the battery of the present application further includes an electrolyte, which is a well-known electrolyte in the art that can be used in batteries and has excellent electrochemical performance of the battery, including lithium salts and organic solvents, and can be specifically configured as needed.

[0083] 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, polyethersulfone, etc., and can be specifically set according to needs.

[0084] 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, preventing direct contact between the positive electrode coating and the first active layer, thus preventing battery short circuits. It also reduces the overall thickness of the battery, reducing the battery volume and increasing the battery's energy density.

[0085] In a specific embodiment, the battery may include an outer packaging, which can be used to encapsulate the electrode assembly and the electrolyte.

[0086] In one embodiment, the outer packaging of the battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0087] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape.

[0088] This application has no particular restrictions on the application areas of lithium-ion batteries, and they can be used in consumer batteries, new energy vehicle power batteries, energy storage batteries and other fields.

[0089] Hereinafter, the present application will be further described in detail through specific embodiments.

[0090] Example 1

[0091] 1. Preparation of negative electrode sheet

[0092] Step 1: using polyurethane (PU) glue as the first slurry;

[0093] 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%. 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.

[0094] 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%. A secondary dispersion was performed for 60 minutes to obtain a third slurry.

[0095] Step 2: Evenly 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. Repeat the above operation on the other surface of the negative electrode current collector copper foil to obtain a negative electrode sheet.

[0096] The thickness of the inert layer is 1.5 μm, and the ratio H1 / (H1+H2) of the thickness of the first active layer to the total thickness of the first active layer and the second active layer is 20%.

[0097] 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.

[0098] The negative electrode sheet was tested for sheet resistance, and the resistance of the negative electrode sheet was found to be 30 mΩ.

[0099] 2. Preparation of positive electrode

[0100] The positive electrode active material, lithium cobalt oxide, conductive carbon black (SP), and polyvinylidene fluoride (PVDF) binder were mixed uniformly in a mass ratio of 97.5:1.5:1. The mixture was thoroughly stirred in N-methylpyrrolidone solvent to form a slurry with a solid content of 75%. This mixture was then stirred to obtain a positive electrode slurry. The positive electrode slurry was evenly coated on one surface of a 12μm-thick aluminum foil, dried at 90°C, and cold-pressed to obtain a positive electrode sheet with a positive electrode coating thickness of 110μm. The above steps were repeated on the other surface of the positive electrode sheet to obtain a double-sided positive electrode sheet. The positive electrode sheet was cut into 76mm×851mm dimensions and the tabs were welded before use.

[0101] 3. Preparation of electrolyte

[0102] 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. 8% butyl butyrate was added based on the total mass of the electrolyte. Lithium hexafluorophosphate (LiPF6) was then added to the solvent, dissolved and mixed thoroughly, and fluoroethylene carbonate (FEC) was added to obtain an electrolyte. The molar concentration of LiPF6 in the electrolyte was 1.15 mol / L, and the mass concentration of FEC in the electrolyte was 10.1%.

[0103] 4. Production of lithium-ion batteries

[0104] The positive and negative electrodes are stacked in sequence, and the stacked electrodes are wound with a separator to form an electrode assembly. The electrode assembly is placed in a pre-molded aluminum-plastic film, dehydrated at 80°C, and injected with the prepared electrolyte. After vacuum packaging, standing, formation, and shaping, the lithium-ion battery is obtained.

[0105] 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. Please refer to Table 1 and Table 2.

[0106] Comparative Example 1

[0107] This comparative example differs from Example 1 in the preparation of the negative electrode sheet: The active material (graphite): dispersant (CMC): binder (PAA) were stirred at a ratio of 98:1:1 for 15 minutes. Water was then added to adjust the solids content to 55%, and a secondary dispersion process was performed for 60 minutes to obtain a negative electrode slurry. The negative electrode slurry was applied to the surface of the negative electrode current collector and dried to obtain a negative electrode sheet.

[0108] Comparative Example 2

[0109] This comparative example differs from Example 1 in the preparation of the negative electrode sheet: The active material (25% silicon-carbon material + 75% graphite, with a D50 of 8 μm for the silicon-carbon material), dispersant (CMC), and binder (PAA) were stirred at a mass ratio of 98:1:1 for 15 minutes. Water was then added to adjust the solids content to 55%, and a secondary dispersion process was performed for 60 minutes to obtain a negative electrode slurry. The negative electrode slurry was applied to the surface of the negative electrode current collector and dried to obtain a negative electrode sheet.

[0110] Table 1

[0111]

[0112] Table 2

[0113]

[0114] Test example

[0115] The batteries prepared in the examples and comparative examples were tested for the following properties:

[0116] 1. Cycle performance test

[0117] In a constant temperature chamber at (25±2)°C, charge the lithium-ion battery at a constant current and constant voltage of 0.2C to 4.45V, then charge it at a constant voltage of 0.05C. After standing for 5 minutes, discharge it at 0.2C to 3V. The capacity obtained in this step is regarded as the initial capacity. Cycle testing is performed using 1C charge / 1C discharge. When the capacity decays to 80% of the initial capacity, the cycle life is obtained.

[0118] There are 5 batteries in each group, and the average cycle life is recorded in Table 3.

[0119] 2. Expansion rate test

[0120] Use a screw micrometer to measure the thickness of the lithium-ion battery when it is half-charged, that is, at 50% state of charge (SOC) as the initial thickness. After 200 cycles, fully charge it, that is, at 100% SOC. Use a screw micrometer to measure the thickness of the lithium-ion battery at this time. Compare it with the thickness of the lithium-ion battery at the initial half-charge state to obtain the expansion rate of the fully charged lithium-ion battery at this time.

[0121] There are 5 batteries in each group, and the average value of the calculated cycle expansion rate is recorded in Table 3.

[0122] Table 3

[0123]

[0124] Table 3

[0125]

[0126] From Tables 1 to 3, we can see that:

[0127] According to the comparison of Examples 1 to 5, when the inert layer uses a binder such as polyurethane, polymethyl methacrylate, polyacrylonitrile, polyvinylidene fluoride, or polyacrylamide, the cycle performance of the battery is improved.

[0128] 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.

[0129] 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.

[0130] According to the comparison between Examples 12 and 13 and between Examples 4, 14 to 16 and Comparative Example 7, it can be seen that when 0

[0131] According to the comparison of Examples 4, 17 and 18, it can be seen that when the silicon material is selected from silicon element, silicon oxide material and silicon carbon material, the cycle performance of the battery can be improved.

[0132] 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.

[0133] According to the comparison between Examples 4, 23, and 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.

[0134] According to the comparison between Examples 4, 25, and 26 and Comparative Examples 11 and 12, when the mass percentage of the first active material in the first active layer is 91% to 98.9%, the electrochemical performance of the battery is improved.

[0135] ​The embodiments of the present application have been described above. The above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is selected to best explain the principles of the embodiments, their practical applications, or improvements to the technology in the market, or to enable other persons skilled in the art to understand the embodiments disclosed herein.

Claims

1. A negative electrode sheet, characterized in that: The invention comprises a negative electrode current collector and a negative electrode coating provided 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 provided on the surface of the negative electrode current collector; wherein: The inert layer includes a first binder, and the thickness of the inert layer is 0.2 μm to 2 μm; the first active layer includes a first active material, the first active material includes a silicon material and a first carbon material, the silicon material is a silicon-carbon material, and the mass percentage of the silicon material in the first active material is 1% to 50%, the mass percentage of the first active material in the first active layer is 91% to 98.9%, and the particle size D50 of the silicon material in the first active layer is 5 μm to 20 μm; the second active layer includes a second active material, and the second active material includes a second carbon material; 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.

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

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

4. 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.

5. 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 6 μm to 12 μm.

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

7. 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.

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

9. 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.

10. 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, polypropylene alcohol, polyimide, sodium carboxymethyl cellulose, and polyvinylidene fluoride.

11. A method for preparing a negative electrode sheet according to any one of claims 1 to 10, 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 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; and 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.

12. 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 10 or a negative electrode sheet prepared according to the method for preparing a negative electrode sheet according to claim 11.

13. The battery according to claim 12, characterized in that The positive electrode sheet includes 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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