Lithium ion battery and method of manufacturing the same

Abstract

This invention provides a lithium-ion battery and its preparation method. The lithium-ion battery includes a positive electrode, a negative electrode, and a separator. The negative electrode includes a current collector. The current collector has a first surface and a second surface disposed opposite to each other. A base coating, a negative electrode active layer, and a top coating are independently and sequentially stacked on the first surface and the second surface. The base coating includes a first conductive agent and a first binder. The first conductive agent includes an amorphous carbon conductive material and a first carbon-based two-dimensional conductive material. The native particle size of the amorphous carbon conductive material is ≤80 nm. The elastic modulus of the first binder is ≥0.5 GPa. The top coating includes a second carbon-based two-dimensional conductive material. The surface friction coefficient of the separator is ≤0.45. The lithium-ion battery of this invention can reduce the compressive and tensile forces on the current collector caused by volume expansion during charging and discharging, thereby reducing the risk of electrode breakage.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a lithium-ion battery and its preparation method. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, long lifespan, and excellent charge / discharge performance, have rapidly become the preferred power source for portable electronic devices, electric vehicles, and energy storage systems. However, the volume expansion of lithium-ion batteries during charge and discharge exerts compressive and tensile forces on the current collector copper and aluminum foils. After long-term cycling, the electrode foils may break, affecting the battery's capacity, internal resistance, and rate performance. In severe cases, this can cause an open circuit, rendering the battery unusable, a situation particularly common in cylindrical lithium-ion batteries.

[0003] Currently, most improvement methods primarily focus on enhancing the material and thickness of the foil itself, and optimizing the internal structural layout of the battery to distribute stress. For example, using higher-strength alloy foil, increasing foil thickness, or introducing additional space in the battery design to buffer the effects of volume expansion. However, these methods often come at the cost of battery energy density and fail to fundamentally solve the problem. Therefore, there is an urgent need to develop a method to improve cell breakage. Summary of the Invention

[0004] The main objective of this invention is to provide a lithium-ion battery and its preparation method to solve the problem that the volume expansion of lithium-ion batteries during charging and discharging causes the electrodes to break easily.

[0005] To achieve the above objectives, according to one aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode, a negative electrode, and a separator. The negative electrode includes a current collector; the current collector has a first surface and a second surface disposed opposite to each other; a base coating, a negative electrode active layer, and a top coating are independently and sequentially stacked on the first surface and the second surface, respectively; wherein the base coating includes a first conductive agent and a first binder; the first conductive agent includes an amorphous carbon conductive material and a first carbon-based two-dimensional conductive material; the native particle size of the amorphous carbon conductive material is ≤80nm; the elastic modulus of the first binder is ≥0.5GPa; the top coating includes a second carbon-based two-dimensional conductive material; and the surface friction coefficient of the separator is ≤0.45.

[0006] Furthermore, the mass ratio of the first conductive agent to the first binder in the base coating is 4~6:4~6; and / or, the thickness of the base coating is 1~3μm.

[0007] Further, the mass ratio of the amorphous carbon conductive material to the first carbon-based two-dimensional conductive material is 1~2:2~3; and / or, the amorphous carbon conductive material is selected from any one or more of carbon black, acetylene black, and Ketjen black; the original particle size of the amorphous carbon conductive material is 20~80 nm; and / or, the first carbon-based two-dimensional conductive material is selected from any one or more of graphene, Mxenes, and graphite sheets; and / or, the first binder is selected from any one or more of polyurethane, polyacrylic acid, and polyaniline; wherein, the molecular weight of polyurethane is 100,000~1,000,000 g / mol, and the elastic modulus of polyurethane is 0.5~2 GPa; the molecular weight of polyacrylic acid is 200,000~500,000 g / mol, and the elastic modulus of polyacrylic acid is 2.5~6 GPa; the molecular weight of polyaniline is 150,000~2,000,000 g / mol, and the elastic modulus of polyaniline is 0.5~2 GPa.

[0008] Furthermore, the thickness of the top coating is 0.5~2μm; and / or, the second carbon-based two-dimensional conductive material is selected from any one or more of graphene, MXenes, and graphite sheets.

[0009] Furthermore, the negative electrode active layer comprises a negative electrode active material, a second conductive agent, and a second binder; the mass ratio of the negative electrode active material, the second conductive agent, and the second binder is 90~98:1~5:1~5; the negative electrode active material is selected from any one or more of graphite, silicon-based materials, and elemental phosphorus; the second conductive agent is selected from any one or more of conductive carbon black, graphene, carbon nanotubes, and graphite sheets; the second binder is selected from any one or more of sodium carboxymethyl cellulose, styrene-butadiene rubber latex, and polyacrylic acid; and / or, the thickness of the negative electrode active layer is 50~120 μm.

[0010] Furthermore, the current collector is selected from any one or more of copper foil, composite current collector, and porous current collector; and / or, the thickness of the current collector is 4~10μm.

[0011] Furthermore, the thickness ratio of the current collector, the base coating, the negative electrode active layer, and the top coating is 6~8:1.5~2.5:60~100:1~1.5.

[0012] Furthermore, the surface friction coefficient of the diaphragm is 0.15~0.45; the diaphragm includes a base membrane and a composite coating disposed on both sides of the base membrane; the composite coating includes polyvinylidene fluoride and aluminum oxide; the mass ratio of polyvinylidene fluoride to aluminum oxide is 20~30:70~80; the thickness of the composite coating is 2~5μm; and / or, the outer surface of the diaphragm has a lubricating layer, the lubricating layer including a lubricant; the lubricant is selected from any one or more of polytetrafluoroethylene microspheres, molybdenum disulfide and boron nitride; the thickness of the lubricating layer is 0.4~1.5μm.

[0013] According to another aspect of the present invention, a method for preparing the above-mentioned lithium-ion battery is provided, the method comprising: step S1, mixing raw materials including a first conductive agent, a first binder and a first solvent to obtain a bottom coating slurry; coating the bottom coating slurry on opposite surfaces of a current collector and drying it to form a bottom coating layer; step S2, coating a negative electrode slurry on the surface of the bottom coating layer away from the current collector and drying it to form a negative electrode active layer; step S3, mixing raw materials including a second carbon-based two-dimensional conductive material and a second solvent to obtain a top coating slurry; coating the top coating slurry on the surface of the negative electrode active layer away from the bottom coating layer and drying it to form a top coating layer, thereby obtaining a negative electrode sheet; step S4, assembling the negative electrode sheet, the positive electrode sheet and the separator and injecting an electrolyte to obtain a lithium-ion battery.

[0014] Further, in step S1 above, the raw materials including the first adhesive and the first solvent are mixed to obtain an adhesive solution; the raw materials including the adhesive solution and the first conductive agent are mixed to obtain a base coat slurry; wherein, the first solvent is water; the solid content of the adhesive solution is 4~10%; and / or, the solid content of the base coat slurry is 40~50%; the viscosity of the base coat slurry is 100~500mps; and / or, in step S3 above, the second solvent is water; the solid content of the top coat slurry is 30~40%.

[0015] Applying the technical solution of this invention, the volume expansion of a lithium-ion battery during charging and discharging exerts extrusion and tension on the copper and aluminum foil current collectors. The extrusion force is limited by space (group margin), while the tension is mainly due to radial force, primarily influenced by the pressure of the extrusion, the contact area, and the friction method. Therefore, the lithium-ion battery of this application, employing the aforementioned negative electrode structure and a separator with a low coefficient of friction, can reduce the extrusion and tension on the positive and negative current collectors caused by volume expansion during charging and discharging, thereby reducing the risk of current collector foil breakage. Specifically, 1) for the negative electrode, this application provides a base coating on both sides of the current collector. Adding small-particle-size amorphous carbon conductive material to the base coating not only improves conductivity and reduces the battery's internal resistance but also reduces friction on the foil caused by the expansion of the active material. Adding a first carbon-based two-dimensional conductive material improves conductivity, and due to its unique two-dimensional morphology, it also provides additional mechanical support and stress buffering, thereby improving the structural stability of the negative electrode. Adding a high-elasticity modulus first binder improves the adhesion between the negative electrode active layer and the current collector, reducing slippage between them. This further reduces the direct friction between the active material expansion and the current collector. Simultaneously, the high-elasticity modulus polyurethane binder enhances the stress dispersion capability of the negative electrode, thereby improving its structural stability. A top coating layer, applied to the surface of the negative electrode active layer away from the bottom coating layer, with the addition of a second carbon-based two-dimensional conductive material, reduces radial friction, allowing the electrode to stretch more easily within the battery, thus reducing the risk of electrode breakage and lowering the tensile strength requirements for electrode extension. 2) Using a low-friction coefficient separator reduces resistance during winding and prevents slippage, effectively reducing friction between electrodes during charge-discharge cycles. In summary, the lithium-ion battery of this application reduces the risk of electrode breakage after long-term cycling, thereby improving cycle performance and cycle life. Detailed Implementation

[0016] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0017] As analyzed in the background section of this application, existing lithium-ion batteries suffer from volume expansion during charging and discharging, which leads to easy breakage of the electrode sheets. In order to solve the above problems, this application provides a lithium-ion battery and its preparation method.

[0018] In a typical embodiment of this application, a lithium-ion battery is provided, including a positive electrode, a negative electrode, and a separator. The negative electrode includes a current collector. The current collector has a first surface and a second surface disposed opposite to each other. A base coating, a negative electrode active layer, and a top coating are independently and sequentially stacked on the first surface and the second surface. The base coating includes a first conductive agent and a first binder. The first conductive agent includes an amorphous carbon conductive material and a first carbon-based two-dimensional conductive material. The primary particle size (first particle size) of the amorphous carbon conductive material is ≤80nm. The elastic modulus of the first binder is ≥0.5GPa. The top coating includes a second carbon-based two-dimensional conductive material. The surface friction coefficient of the separator is ≤0.45.

[0019] The volume expansion of lithium-ion batteries during charging and discharging exerts compressive and tensile forces on the copper and aluminum foil current collectors. The compressive force is limited by space (group margin), while the tensile force is mainly due to radial force, influenced primarily by the pressure of the compression, contact area, and friction mode. Therefore, the lithium-ion battery of this application, employing the aforementioned negative electrode structure and a low-friction separator, can reduce the compressive and tensile forces on the positive and negative current collectors caused by volume expansion during charging and discharging, thereby reducing the risk of current collector foil breakage. Specifically, 1) for the negative electrode, this application provides a base coating on both sides of the current collector. Adding small-particle-size amorphous carbon conductive material to the base coating not only improves conductivity and reduces the battery's internal resistance but also reduces friction on the foil caused by the expansion of the active material. Adding a first carbon-based two-dimensional conductive material improves conductivity and, due to its unique two-dimensional morphology, provides additional mechanical support and stress buffering, thereby improving the structural stability of the negative electrode. Adding a high-elasticity modulus first binder improves the adhesion between the negative electrode active layer and the current collector, reducing slippage between them. This further reduces the direct friction between the active material expansion and the current collector. Simultaneously, the high-elasticity modulus polyurethane binder enhances the stress dispersion capability of the negative electrode, thereby improving its structural stability. A top coating layer, applied to the surface of the negative electrode active layer away from the bottom coating layer, with the addition of a second carbon-based two-dimensional conductive material, reduces radial friction, allowing the electrode to stretch more easily within the battery, thus reducing the risk of electrode breakage and lowering the tensile strength requirements for electrode extension. 2) Using a low-friction coefficient separator reduces resistance during winding and prevents slippage, effectively reducing friction between electrodes during charge-discharge cycles. In summary, the lithium-ion battery of this application reduces the risk of electrode breakage after long-term cycling, thereby improving cycle performance and cycle life.

[0020] In one embodiment of this application, the mass ratio of the first conductive agent to the first binder in the base coating is 4~6:4~6; and / or, the thickness of the base coating is 1~3μm.

[0021] Preferably controlling the mass ratio of the first conductive agent to the first binder in the base coating within the aforementioned range helps to ensure that the base coating has good conductivity and sufficient adhesion, thereby enhancing the bonding strength between the negative electrode active coating and the current collector, reducing interfacial impedance, and thus improving the electrochemical performance of the battery. Preferably, the thickness of the base coating within the aforementioned range helps to provide sufficient adhesion and conductivity while reducing the additional cost and weight increase caused by an excessively thick coating, thereby optimizing the energy density of the battery. Furthermore, the thickness of the base coating can be 1 μm, 1.5 μm, 2 μm, 2.5 μm, or 3 μm, etc.

[0022] In one embodiment of this application, the mass ratio of the amorphous carbon conductive material, the first carbon-based two-dimensional conductive material, and the first binder is 1~2:2~3; and / or, the amorphous carbon conductive material is selected from any one or more of carbon black, acetylene black, and Ketjen black; the native particle size of the amorphous carbon conductive material is 20~80 nm; and / or, the first carbon-based two-dimensional conductive material is selected from any one or more of graphene, MXenes, and graphite sheets; and / or, the first binder is selected from any one or more of polyurethane, polyacrylic acid, and polyaniline; wherein, the molecular weight of polyurethane is 100,000~1,000,000 g / mol, and the elastic modulus of polyurethane is 0.5~2 GPa; the molecular weight of polyacrylic acid is 200,000~500,000 g / mol, and the elastic modulus of polyacrylic acid is 2.5~6 GPa; the molecular weight of polyaniline is 150,000~2,000,000 g / mol, and the elastic modulus of polyaniline is 0.5~2 GPa.

[0023] Preferably controlling the mass ratio of amorphous carbon conductive material to the first carbon-based two-dimensional conductive material within the aforementioned range helps improve the conductivity of the base coating and the stress dispersion capability of the negative electrode. Preferably, the type of amorphous carbon conductive material being within the aforementioned range also helps improve the conductivity of the base coating. Preferably, controlling the particle size of the amorphous carbon conductive material being within the aforementioned range helps reduce the friction of the current collector caused by the expansion of the active material. Furthermore, the native particle size of the amorphous carbon conductive material can be 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, or 80 nm, etc.

[0024] The preferred type of the first carbon-based two-dimensional conductive material is within the aforementioned range, which not only helps to further improve the conductivity of the base coating, but also helps to enhance the mechanical support and stress buffering of the base coating, thereby improving the structural stability of the negative electrode sheet. The preferred type of the first binder is within the aforementioned range, which not only helps to improve the adhesion between the negative electrode active layer and the current collector, but also helps to improve the stress dispersion capability of the negative electrode sheet, thereby further improving the structural stability of the negative electrode sheet.

[0025] Furthermore, the molecular weight of polyurethane can be 100,000 g / mol, 200,000 g / mol, 400,000 g / mol, 600,000 g / mol, 800,000 g / mol, or 1,000,000 g / mol, etc. The elastic modulus of polyurethane can be 0.5 GPa, 0.8 GPa, 1 GPa, 1.5 GPa, 1.8 GPa, or 2 GPa, etc. The molecular weight of polyacrylic acid can be 200,000 g / mol, 300,000 g / mol, 400,000 g / mol, or 500,000 g / mol, etc. The elastic modulus of polyacrylic acid can be 2.5 GPa, 3 GPa, 4 GPa, 5 GPa, or 6 GPa, etc. The molecular weight of polyaniline can be 150,000, 500,000 g / mol, 700,000 g / mol, 1,000,000 g / mol, 1,200,000 g / mol, 1,500,000 g / mol, 1,700,000 g / mol, or 2,000,000 g / mol, etc. The elastic modulus of polyaniline can be 0.5 GPa, 0.8 GPa, 1 GPa, 1.5 GPa, 1.8 GPa or 2 GPa, etc.

[0026] In one embodiment of this application, the thickness of the top coating is 0.5~2μm; and / or, the second carbon-based two-dimensional conductive material is selected from any one or more of graphene, MXenes and graphite sheets.

[0027] The preferred thickness of the top coating within the aforementioned range helps to further reduce radial friction on the current collector, thereby further reducing the risk of electrode breakage. Furthermore, the thickness of the top coating can be 0.5 μm, 0.8 μm, 1 μm, 1.5 μm, 1.8 μm, or 2 μm, etc. The preferred type of the second carbon-based two-dimensional conductive material within the aforementioned range helps to further reduce radial friction, allowing the electrode to expand more easily within the battery, thereby further reducing the risk of electrode breakage.

[0028] The above MXenes are a class of two-dimensional transition metal carbides, nitrides, or carbonitrides.

[0029] In one embodiment of this application, the negative electrode active layer includes a negative electrode active material, a second conductive agent, and a second binder; the mass ratio of the negative electrode active material, the second conductive agent, and the second binder is 90~98:1~5:1~5; the negative electrode active material is selected from any one or more of graphite, silicon-based materials, and elemental phosphorus; the second conductive agent is selected from any one or more of conductive carbon black, graphene, carbon nanotubes, and graphite sheets; the second binder is selected from any one or more of polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene-butadiene rubber latex, and polyacrylic acid; and / or, the thickness of the negative electrode active layer is 50~120 μm.

[0030] Preferably controlling the types and mass ratios of the negative electrode active material, the second conductive agent, and the second binder in the negative electrode active layer within the aforementioned range helps to improve the energy density and cycle stability of the negative electrode sheet. Preferably, the thickness of the negative electrode active layer within the aforementioned range helps to maintain a thin electrode design, improve the overall energy density of the battery, and reduce internal resistance, thereby improving the battery's cycle performance. Furthermore, the thickness of the negative electrode active layer can be 50 μm, 60 μm, 70 μm, 90 μm, 100 μm, or 120 μm, etc.

[0031] In one embodiment of this application, the current collector is selected from any one or more of copper foil, composite current collector, and porous current collector; and / or, the thickness of the current collector is 4~10μm.

[0032] Preferring a current collector type within the above-mentioned range helps improve the stability and conductivity of the negative electrode. The composite current collector can be a polymer-based film composite with a double-sided metal layer (such as PET / CPP / PI+Cu). Preferring a current collector thickness within the above-mentioned range helps to further optimize the battery's weight and volume while maintaining mechanical strength, thereby further improving the battery's electrochemical performance. Furthermore, the current collector thickness can be 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, or 10μm, etc.

[0033] In one embodiment of this application, the thickness ratio of the current collector, the base coating, the negative electrode active layer, and the top coating is 6~8:1.5~2.5:60~100:1~1.5.

[0034] Preferentially selecting a thickness ratio of current collector, bottom coating, negative electrode active layer, and top coating within the above range helps to optimize the internal structure of the battery, improve the electrochemical performance of the negative electrode, and reduce the compressive and tensile forces on the current collector caused by the volume expansion of the active material during lithium-ion charging and discharging, thereby reducing the risk of electrode breakage.

[0035] In one embodiment of this application, the surface friction coefficient of the diaphragm is 0.15~0.45; the diaphragm includes a base membrane and a composite coating disposed on both sides of the base membrane; the composite coating includes polyvinylidene fluoride and aluminum oxide; the mass ratio of polyvinylidene fluoride to aluminum oxide is 20~30:70~80; the thickness of the composite coating is 2~5μm; and / or, the outer surface of the diaphragm has a lubricating layer, the lubricating layer includes a lubricant; the lubricant is selected from any one or more of polytetrafluoroethylene microspheres, molybdenum disulfide and boron nitride; the thickness of the lubricating layer is 0.4~1.5μm.

[0036] A separator with a composite coating is preferred. Controlling the thickness of the composite coating and the mass ratio of polyvinylidene fluoride (PVDF) to aluminum oxide (ANO3) within the aforementioned range helps improve the thermal stability and mechanical strength of the separator. A separator with a lubricating layer of the aforementioned thickness is preferred, as it helps further reduce the friction between the separator and the electrode, thereby reducing material damage caused by excessive friction during winding and improving the stability and consistency of the battery's internal structure. Simultaneously, using the aforementioned type of lubricant helps further reduce the separator's coefficient of friction, facilitating smooth cell winding, reducing slippage, and thus improving battery production efficiency. Furthermore, the surface coefficient of friction of the separator can be 0.15, 0.2, 0.25, 0.3, 0.25, 0.4, or 0.45, etc. The thickness of the lubricating layer can be 0.4 μm, 0.6 μm, 0.8 μm, 1 μm, 1.2 μm, or 1.5 μm, etc.

[0037] In another typical embodiment of this application, a method for preparing the above-mentioned lithium-ion battery is provided. The method includes: step S1, mixing a first binder and a first solvent to obtain a slurry; preparing a base coating slurry by mixing raw materials including a first conductive agent, a first binder, and a first solvent; coating the base coating slurry onto the opposite surfaces of both sides of a current collector and drying it to form a base coating layer; step S2, coating a negative electrode slurry onto the surface of the base coating layer away from the current collector and drying it to form a negative electrode active layer; step S3, mixing raw materials including a second carbon-based two-dimensional conductive material and a second solvent to obtain a top coating slurry; coating the top coating slurry onto the surface of the negative electrode active layer away from the base coating layer and drying it to form a top coating layer, thus obtaining a negative electrode sheet; and step S4, assembling the negative electrode sheet, the positive electrode sheet, and the separator, and then injecting an electrolyte to obtain a lithium-ion battery.

[0038] The lithium-ion battery prepared by the method of this application can significantly reduce the risk of electrode foil breakage after long-term cycling, thereby improving the cycle performance and cycle life of the lithium-ion battery. Specifically, in step S1, the bottom coating slurry obtained by mixing the above raw materials is coated on both sides of the current collector to form a bottom coating layer, which not only enhances the adhesion between the negative electrode active coating and the current collector, but also reduces the frictional force on the current collector caused by the volume expansion of the active material. In step S2, a negative electrode active layer is formed on the two sides of the bottom coating layer away from the current collector, which can improve the energy density and cycle performance of the negative electrode. In step S3, a top coating layer is formed on the two sides of the negative electrode active layer away from the bottom coating layer, which can reduce radial friction, thereby helping the electrode to stretch during the winding process, further reducing the risk of electrode breakage, and also reducing the requirements for the current collector's extensibility, thereby improving the battery's reliability and cycle life. In step S5, the separator with a low surface friction coefficient is assembled with the positive and negative electrodes to form a cell, which can reduce the resistance during the winding process and prevent slippage, thereby effectively reducing the friction between the electrodes during the battery's charge and discharge cycles. Furthermore, the preparation method of this application is easy to mass-produce and has good compatibility with production processes.

[0039] In one embodiment of this application, in step S1 above, raw materials including a first adhesive and a first solvent are mixed to obtain an adhesive solution; raw materials including the adhesive solution and a first conductive agent are mixed to obtain a base coat slurry; the first solvent is water; the solid content of the adhesive solution is 4~10%; and / or, the solid content of the base coat slurry is 40~50%; the viscosity of the base coat slurry is 100~500mps; and / or, in step S3 above, the second solvent is water; the solid content of the top coat slurry is 30~40%.

[0040] In step S1, it is preferable that the type of the first solvent is within the aforementioned range, which helps the first adhesive to be fully dispersed in the first solvent, forming an adhesive solution with the aforementioned solid content. It is also preferable that the solid content and viscosity of the primer slurry are controlled within the aforementioned range, which helps to ensure uniformity and consistency of the coating.

[0041] In step S3, it is preferable that the type of the second solvent is within the above range, which helps the second carbon-based two-dimensional conductive material to be fully dispersed in the second solvent, thereby improving the uniformity and consistency of the coating.

[0042] The positive electrode includes, but is not limited to, lithium manganese iron phosphate positive electrode and lithium nickel manganese cobalt oxide positive electrode. The electrolyte can be any commonly used formulation in this field.

[0043] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0044] Example 1

[0045] Preparation of the negative electrode: A first binder, polyurethane, with a molecular weight of 500,000 g / mol and an elastic modulus of 1 GPa, and a first solvent, deionized water, are mixed to obtain a slurry with a solid content of 6%. The slurry, a first conductive agent, and a first carbon-based two-dimensional conductive material, graphene, are mixed to obtain a base coating slurry with a solid content of 45% and a viscosity of 300 mps. The first conductive agent consists of amorphous carbon black with a native particle size of 50 nm and the first carbon-based two-dimensional conductive material, graphene, with a mass ratio of 1.5:2.5. The mass ratio of the first conductive agent to the first binder is 1:1. The base coating slurry is coated onto both sides of an 8 μm thick copper current collector foil using a gravure coating method and then dried. A first base coating and a second base coating, each with a thickness of 2 μm, are sequentially formed on the first and second surfaces of the current collector, respectively, where they are positioned opposite each other.

[0046] By weight, 97 parts of the negative electrode active materials silicon-carbon and graphite, 1.5 parts of the second conductive agent (conductive carbon black and single-walled carbon nanotubes, mass ratio 1:1), and 1.5 parts of the second binder (styrene-butadiene rubber and polyacrylic acid, mass ratio 1:1) were mixed with deionized water to obtain a negative electrode active slurry with a solid content of 20%. The negative electrode active slurry was coated on the surface of the base layer away from the current collector using a double-sided coating method and then dried. A first negative electrode active layer and a second negative electrode active layer, each with a thickness of 85 μm, were sequentially formed on the surfaces of the first and second base layers, respectively.

[0047] A top coating slurry with a solid content of 35% is obtained by mixing graphene, a second carbon-based two-dimensional conductive material, and deionized water, a second solvent. The top coating slurry is then coated onto the surface of the negative electrode active layer away from the base coating layer using a double-sided coating method and dried. A first top coating layer and a second top coating layer, each with a thickness of 1 μm, are sequentially formed on the surfaces of the first and second negative electrode active layers, respectively, to obtain the negative electrode sheet. The structure of the negative electrode sheet consists of a first top coating layer, a first negative electrode active layer, a first base coating layer, a current collector, a second base coating layer, a second negative electrode active layer, and a second top coating layer, stacked sequentially.

[0048] The diaphragm consists of a polyethylene base membrane and two composite coatings on its two surfaces, each 2 μm thick. These composite coatings contain polyvinylidene fluoride (PVDF) and aluminum oxide (ANO3) in a 25:75 mass ratio. Each of the two composite coatings, on the surface furthest from the base membrane, contains a 1 μm thick lubricating layer composed of polytetrafluoroethylene (PTFE) microspheres as a lubricant. The diaphragm has a surface friction coefficient of 0.3.

[0049] Preparation of the positive electrode sheet: By weight, 98 parts of high-nickel ternary positive electrode material (NCM811), 1 part of conductive carbon black, and 1 part of polyvinylidene fluoride binder were mixed with NMP solvent to obtain a positive electrode active slurry with a solid content of 70%. The positive electrode active slurry was coated on the surface of the current collector aluminum foil using a double-sided coating method and then dried to form a positive electrode active layer with a thickness of 120 μm.

[0050] Electrolyte: 1.0 mol / L LiPF6, organic solvent (ethylene carbonate EC, dimethyl carbonate EMC and ethyl methyl carbonate DMC in a volume ratio of 1:1:1) and 3% mixed additives (ethylene carbonate VC and fluoroethylene carbonate FEC) are mixed to obtain the electrolyte.

[0051] A separator is sandwiched between the positive and negative electrode plates, and the cells are wound together to form a battery cell. The battery cell is then placed in a battery casing and electrolyte is injected to obtain a lithium-ion battery.

[0052] Example 2

[0053] The difference from Example 1 lies in the preparation of the negative electrode: Polyacrylic acid (500,000 g / mol molecular weight, 6 GPa elastic modulus) and deionized water (1 solvent) are mixed to obtain a 4% solid content adhesive. The adhesive, a first conductive agent, and graphene (a first carbon-based two-dimensional conductive material) are mixed to obtain a 40% solid content base coating slurry with a viscosity of 100 mps. The first conductive agent consists of acetylene black (an amorphous carbon conductive material with a native particle size of 20 nm) and Mxenes (a first carbon-based two-dimensional conductive material), with a mass ratio of 1:2. The mass ratio of the first conductive agent to the first binder is 4:6. The base coating slurry is applied to both sides of a 10 μm thick copper current collector foil using a gravure coating method and then dried. A first base coating and a second base coating, each 1 μm thick, are then sequentially formed on the first and second surfaces of the current collector, respectively.

[0054] By weight, 95 parts of the negative electrode active materials silicon-carbon and graphite, 3 parts of the second conductive agent (conductive carbon black and single-walled carbon nanotubes, mass ratio 1:1), and 2 parts of the second binder (styrene-butadiene rubber and polyacrylic acid, mass ratio 1:1) were mixed with deionized water to obtain a negative electrode active slurry with a solid content of 30%. The negative electrode active slurry was coated on the surface of the base layer away from the current collector using a double-sided coating method and then dried. A first negative electrode active layer and a second negative electrode active layer, each with a thickness of 120 μm, were sequentially formed on the surfaces of the first and second base layers, respectively.

[0055] A top coating slurry with a solid content of 30% is obtained by mixing graphene, a second carbon-based two-dimensional conductive material, and deionized water, a second solvent. The top coating slurry is then coated onto the surface of the negative electrode active layer away from the base coating layer using a double-sided coating method and dried. A first top coating layer and a second top coating layer, each with a thickness of 0.5 μm, are sequentially formed on the surfaces of the first and second negative electrode active layers, respectively, to obtain the negative electrode sheet. The structure of the negative electrode sheet consists of a first top coating layer, a first negative electrode active layer, a first base coating layer, a current collector, a second base coating layer, a second negative electrode active layer, and a second top coating layer, stacked sequentially.

[0056] Separator: The base membrane is a polyethylene membrane, and both surfaces of the base membrane contain a 2μm thick composite coating containing polyvinylidene fluoride and aluminum oxide in a mass ratio of 20:80. The surfaces of the two composite coatings furthest from the base membrane each contain a 0.4μm thick lubricating layer containing molybdenum disulfide as a lubricant. The surface friction coefficient of the separator is 0.45, ultimately resulting in a lithium-ion battery.

[0057] Example 3

[0058] The difference from Example 1 is that the thickness of the lubricating layer in the separator is 1.5 μm, the lubricating layer contains boron nitride, the surface friction coefficient of the separator is 0.15, and a lithium-ion battery is finally obtained.

[0059] Example 4

[0060] The difference from Example 1 is that the thickness of the lubricating layer in the separator is 0.2 μm, the lubricating layer contains boron nitride, the surface friction coefficient of the separator is 0.15, and a lithium-ion battery is finally obtained.

[0061] Example 5

[0062] The difference from Example 1 is in the preparation of the negative electrode: the mass ratio of amorphous carbon conductive material and first carbon-based two-dimensional conductive material in the bottom coating is 2:3, the original particle size of the amorphous carbon conductive material is 80nm, and finally a lithium-ion battery is obtained.

[0063] Example 6

[0064] The difference from Example 1 is in the preparation of the negative electrode: the mass ratio of amorphous carbon conductive material and first carbon-based two-dimensional conductive material in the bottom coating is 3:2, the original particle size of the amorphous carbon conductive material is 10nm, and finally a lithium-ion battery is obtained.

[0065] Example 7

[0066] The difference from Example 1 is in the preparation of the negative electrode: the total mass ratio of the first conductive agent to the first binder in the bottom coating is 6:4, the molecular weight of the first binder polyaniline is 2 million g / mol, and the elastic modulus is 2 GPa, finally obtaining a lithium-ion battery.

[0067] Example 8

[0068] The difference from Example 1 is in the preparation of the negative electrode: the total mass ratio of the first conductive agent to the first binder in the bottom coating is 3:7, the molecular weight of the first binder polyaniline is 150,000 g / mol, and the elastic modulus is 0.5 GPa, and finally a lithium-ion battery is obtained.

[0069] Example 9

[0070] The difference from Example 1 lies in the preparation of the negative electrode: the thickness of the current collector is 6 μm, the thickness of the bottom coating (single side) is 1.5 μm, the thickness of the negative electrode active layer (single side) is 100 μm, the thickness of the top coating (single side) is 1.5 μm, and the thickness ratio of the current collector, bottom coating, negative electrode active layer and top coating is 6:1.5:100:1.5, finally obtaining a lithium-ion battery.

[0071] Example 10

[0072] The difference from Example 1 is in the preparation of the negative electrode sheet: the thickness of the current collector is 6 μm, the thickness of the bottom coating (single side) is 3 μm, the thickness of the negative electrode active layer (single side) is 100 μm, the thickness of the top coating (single side) is 0.5 μm, and the thickness ratio of the current collector, bottom coating, negative electrode active layer and top coating is 6:3:100:0.5, finally obtaining a lithium-ion battery.

[0073] Comparative Example 1

[0074] The difference from Example 1 is in the preparation of the negative electrode: the two sides of the current collector have only the negative electrode active layer, the total thickness of the negative electrode is the same as in Example 1, the separator is a polyethylene film, and finally a lithium-ion battery is obtained.

[0075] Comparative Example 2

[0076] The difference from Example 1 is in the preparation of the negative electrode: the elastic modulus of the first binder in the bottom coating is 0.2 GPa; the original particle size of the amorphous carbon conductive material is 100 nm; and the second carbon-based two-dimensional conductive material in the top coating is replaced with conductive carbon black, thus obtaining a lithium-ion battery.

[0077] Comparative Example 3

[0078] The difference from Example 1 is that the negative electrode does not contain a base coating, and the total thickness of the negative electrode is the same as in Example 1, ultimately resulting in a lithium-ion battery.

[0079] Comparative Example 4

[0080] The difference from Example 1 is that the negative electrode does not contain a top coating, and the total thickness of the negative electrode is the same as in Example 1, ultimately resulting in a lithium-ion battery.

[0081] Comparative Example 5

[0082] The difference from Example 1 is that the surface friction coefficient of the separator is 0.6, resulting in a lithium-ion battery.

[0083] Test method:

[0084] Outer ring fracture condition: After initialization and volume control, the ring was disassembled and observed, and the fracture condition was observed by computed tomography (CT) imaging.

[0085] The test results are shown in Table 1.

[0086] Table 1

[0087]

[0088] As can be seen from the above, Comparative Example 1 lacks both a top coating and a bottom coating. During the charging and discharging process, the current collector cannot withstand the compressive and tensile forces exerted by the volume expansion, leading to electrode breakage. In Comparative Example 2, the first binder has a low elastic modulus, causing easy slippage between the negative electrode active layer and the current collector, and failing to effectively disperse the stress of the negative electrode, resulting in electrode breakage. The material used in the top coating cannot effectively reduce radial friction, also increasing the risk of electrode breakage. Comparative Example 3 does not contain a bottom coating, resulting in greater friction on the foil due to the expansion of the active material, leading to electrode breakage. Comparative Example 4 lacks a top coating, resulting in greater radial friction, leading to electrode breakage. In Comparative Example 5, the separator has a high surface friction coefficient, resulting in greater resistance during winding and greater friction between the electrodes, increasing the risk of electrode breakage. Comparative Examples 1, 3, and 4, by increasing the thickness of the current collector to achieve a uniform total thickness of the negative electrode, enhance the mechanical strength of the electrode to some extent, but this leads to a decrease in battery capacity.

[0089] Although the negative electrode plates in Examples 4, 6, 8 and 10 developed cracks, they did not break and could still be recycled.

[0090] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0091] The volume expansion of lithium-ion batteries during charging and discharging exerts compressive and tensile forces on the copper and aluminum foil current collectors. The compressive force is limited by space (group margin), while the tensile force is mainly due to radial force, influenced primarily by the pressure of the compression, contact area, and friction mode. Therefore, the lithium-ion battery of this application, employing the aforementioned negative electrode structure and a low-friction separator, can reduce the compressive and tensile forces on the positive and negative current collectors caused by volume expansion during charging and discharging, thereby reducing the risk of current collector foil breakage. Specifically, 1) for the negative electrode, this application provides a base coating on both sides of the current collector. Adding small-particle-size amorphous carbon conductive material to the base coating not only improves conductivity and reduces the battery's internal resistance but also reduces friction on the foil caused by the expansion of the active material. Adding a first carbon-based two-dimensional conductive material improves conductivity and, due to its unique two-dimensional morphology, provides additional mechanical support and stress buffering, thereby improving the structural stability of the negative electrode. Adding a high-elasticity modulus first binder improves the adhesion between the negative electrode active layer and the current collector, reducing slippage between them. This further reduces the direct friction between the active material expansion and the current collector. Simultaneously, the high-elasticity modulus polyurethane binder enhances the stress dispersion capability of the negative electrode, thereby improving its structural stability. A top coating layer, applied to the surface of the negative electrode active layer away from the bottom coating layer, with the addition of a second carbon-based two-dimensional conductive material, reduces radial friction, allowing the electrode to stretch more easily within the battery, thus reducing the risk of electrode breakage and lowering the tensile strength requirements for electrode extension. 2) Using a low-friction coefficient separator reduces resistance during winding and prevents slippage, effectively reducing friction between electrodes during charge-discharge cycles. In summary, the lithium-ion battery of this application reduces the risk of electrode breakage after long-term cycling, thereby improving cycle performance and cycle life.

[0092] The above are merely embodiments of the present invention and are not intended to limit the invention. Those skilled in the art will recognize that the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A lithium-ion battery, comprising a positive electrode, a negative electrode, and a separator, characterized in that, The negative electrode sheet includes a current collector; the current collector has a first surface and a second surface disposed opposite to each other; a base coating, a negative electrode active layer and a top coating are independently and sequentially stacked on the first surface and the second surface respectively; The base coating comprises a first conductive agent and a first binder; the first conductive agent comprises an amorphous carbon conductive material and a first carbon-based two-dimensional conductive material; the native particle size of the amorphous carbon conductive material is ≤80nm; the elastic modulus of the first binder is ≥0.5GPa. The top coating comprises a second carbon-based two-dimensional conductive material; The surface friction coefficient of the diaphragm is ≤0.

45.

2. The lithium-ion battery according to claim 1, characterized in that, The mass ratio of the first conductive agent to the first binder in the base coating is 4~6:4~6; and / or the thickness of the base coating is 1~3μm.

3. The lithium-ion battery according to claim 1 or 2, characterized in that, The mass ratio of the amorphous carbon conductive material to the first carbon-based two-dimensional conductive material is 1~2:2~3; And / or, the amorphous carbon conductive material is selected from any one or more of carbon black, acetylene black and Ketjen black; the original particle size of the amorphous carbon conductive material is 20~80nm; And / or, the first carbon-based two-dimensional conductive material is selected from any one or more of graphene, Mxenes, and graphite sheets; And / or, the first adhesive is selected from any one or more of polyurethane, polyacrylic acid, and polyaniline; wherein the molecular weight of the polyurethane is 100,000 to 1,000,000 g / mol, and the elastic modulus of the polyurethane is 0.5 to 2 GPa; the molecular weight of the polyacrylic acid is 200,000 to 500,000 g / mol, and the elastic modulus of the polyacrylic acid is 2.5 to 6 GPa; the molecular weight of the polyaniline is 150,000 to 2,000,000 g / mol, and the elastic modulus of the polyaniline is 0.5 to 2 GPa.

4. The lithium-ion battery according to any one of claims 1 to 3, characterized in that, The thickness of the top coating is 0.5~2μm; and / or, the second carbon-based two-dimensional conductive material is selected from any one or more of graphene, MXenes and graphite sheets.

5. The lithium-ion battery according to any one of claims 1 to 4, characterized in that, The negative electrode active layer comprises a negative electrode active material, a second conductive agent, and a second binder; the mass ratio of the negative electrode active material, the second conductive agent, and the second binder is 90~98:1~5:1~5; The negative electrode active material is selected from any one or more of graphite, silicon-based materials, and elemental phosphorus; the second conductive agent is selected from any one or more of conductive carbon black, graphene, carbon nanotubes, and graphite sheets; the second binder is selected from any one or more of sodium carboxymethyl cellulose, styrene-butadiene rubber latex, and polyacrylic acid. And / or, the thickness of the negative electrode active layer is 50~120μm.

6. The lithium-ion battery according to any one of claims 1 to 5, characterized in that, The current collector is selected from any one or more of copper foil, composite current collector, and porous current collector; and / or, the thickness of the current collector is 4~10μm.

7. The lithium-ion battery according to any one of claims 1 to 6, characterized in that, The thickness ratio of the current collector, the base coating, the negative electrode active layer, and the top coating is 6~8:1.5~2.5:60~100:1~1.

5.

8. The lithium-ion battery according to any one of claims 1 to 7, characterized in that, The surface friction coefficient of the diaphragm is 0.15~0.45; the diaphragm includes a base membrane and a composite coating disposed on both sides of the base membrane; the composite coating includes polyvinylidene fluoride and aluminum oxide; the mass ratio of polyvinylidene fluoride to aluminum oxide is 20~30:70~80; the thickness of the composite coating is 2~5μm; And / or, the outer surface of the diaphragm has a lubricating layer, the lubricating layer comprising a lubricant; the lubricant is selected from any one or more of polytetrafluoroethylene microspheres, molybdenum disulfide and boron nitride; the thickness of the lubricating layer is 0.4~1.5μm.

9. A method for preparing a lithium-ion battery according to any one of claims 1 to 8, characterized in that, The preparation method includes: Step S1: Mix raw materials including a first conductive agent, a first binder and a first solvent to obtain a base coating slurry; apply the base coating slurry to the opposite surfaces on both sides of the current collector and then dry it to form a base coating layer; Step S2: After coating the negative electrode slurry onto the surface of the base coating away from the current collector, dry it to form the negative electrode active layer; Step S3: Mix the raw materials including the second carbon-based two-dimensional conductive material and the second solvent to obtain a top coating slurry; coat the top coating slurry onto the surface of the negative electrode active layer away from the bottom coating layer and then dry it to form a top coating layer, thereby obtaining a negative electrode sheet; Step S4: After assembling the negative electrode, positive electrode and separator, inject the electrolyte to obtain the lithium-ion battery.

10. The preparation method according to claim 9, characterized in that, In step S1, raw materials including the first adhesive and the first solvent are mixed to obtain an adhesive solution; raw materials including the adhesive solution and the first conductive agent are mixed to obtain the base coating slurry; wherein, the first solvent is water; the solid content of the adhesive solution is 4~10%; and / or, the solid content of the base coating slurry is 40~50%; the viscosity of the base coating slurry is 100~500mps; And / or, in step S3, the second solvent is water; the solid content of the top coating slurry is 30-40%.

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