Negative plate, preparation method thereof and solid-state battery

Abstract

The invention discloses a negative plate, a preparation method thereof and a solid-state battery, and relates to the technical field of battery materials. The disclosed negative plate comprises a current collector, and a bottom layer with the thickness of 10-50 [mu] m, a middle layer with the thickness of 10-50 [mu] m and a surface layer with the thickness of 10-50 [mu] m which are sequentially arranged on the surface of the current collector, the bottom layer comprises the following components in percentage by mass: 1-30% of small-particle-size solid electrolyte and 0.5-3 microns of D50; the component of the middle layer comprises medium-particle-size solid electrolyte, the mass ratio of the medium-particle-size solid electrolyte is 5-30%, and D50 is 3-5 [mu] m; the component of the surface layer comprises a large-particle-size solid electrolyte, the mass ratio of the large-particle-size solid electrolyte is 5-30%, and D50 is 5-10 [mu] m. Compared with a common negative plate, the negative plate provided by the invention optimizes the ion transmission path, relieves the interface stress and improves the electrochemical performance.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and in particular to a negative electrode sheet, a preparation method thereof, and a solid-state battery. Background Art

[0002] With the development of battery technology, all-solid-state batteries have gradually become the focus of the industry. In all-solid-state batteries, sulfide solid electrolytes have high ionic conductivity, comparable to liquid electrolytes. In addition, the good ductility of sulfide solid electrolytes is conducive to the large-scale production of all-solid-state batteries. However, due to the large volume expansion of the silicon-carbon negative electrode and the growth of lithium dendrites, the application of this type of all-solid-state battery is limited. At present, the mainstream solutions to this problem are as follows: 1. Silicon-carbon anode surface coating: A carbon layer or elastic polymer (such as polyacrylonitrile) is coated on the silicon surface to mitigate volume expansion. This method has the advantages of reducing direct contact between silicon and the electrolyte, inhibiting side reactions, and partially mitigating volume expansion. However, its disadvantages are that the coating hinders lithium ion diffusion, the process is complex (requiring CVD or solution coating), and the cost is high.

[0003] 2. Interface doping (e.g., LiF layer): Introducing an interfacial layer such as LiF or Li3N at the anode / electrolyte interface improves interfacial stability. Advantages of this method include suppressing interfacial side reactions and improving interfacial lithium ion mobility. Disadvantages include the need for an additional deposition step (e.g., ALD), poor compatibility with sulfide electrolytes, and the tendency for the interfacial layer to break after long-term cycling.

[0004] 3. Uniformly mixed sulfide electrolyte: The sulfide electrolyte is uniformly mixed with the silicon-carbon material (e.g., by ball milling) to prepare a composite anode. This method offers advantages such as a simple process and a continuous ion transport path. However, it also offers disadvantages such as an inability to alleviate volume expansion stress. Silicon expansion leads to contact failure between particles, resulting in increased impedance.

[0005] 4. High-pressure pressing: High pressure (>50 MPa) is applied to the composite anode to improve density and ionic contact. This process has the advantages of increasing electrode density and reducing porosity. However, high pressure can cause sulfide particles to break up, reducing ionic conductivity and concentrating silicon expansion stress, which can exacerbate interfacial cracking.

[0006] 5. Dry electrode process: This process uses a dry film formation (solvent-free) process to prepare composite anodes, reducing environmental pollution. Advantages include environmental friendliness, low cost, and suitability for large-scale production. Disadvantages include poor dispersibility of the conductive agent / binder, low electronic conductivity, and weak inter-electrode bonding.

[0007] Therefore, the current method has certain defects. In view of this, the present invention is proposed. Summary of the Invention

[0008] The object of the present invention is to provide a negative electrode sheet, a preparation method thereof and a solid-state battery, aiming to improve at least one problem mentioned in the background art.

[0009] The present invention is achieved in that: In a first aspect, the present invention provides a solid-state battery negative electrode sheet, comprising a current collector and a bottom layer with a thickness of 10 to 50 μm, an intermediate layer with a thickness of 10 to 50 μm, and a surface layer with a thickness of 10 to 50 μm, which are sequentially arranged on the surface of the current collector; The bottom layer consists of small-particle solid electrolyte, silicon-carbon composite material, adhesive and conductive agent, of which the small-particle solid electrolyte accounts for 1-30% by mass and the D50 of the small-particle solid electrolyte is 0.5-3 μm. The components of the intermediate layer include medium-sized solid electrolyte, silicon-carbon composite material, adhesive and conductive agent, of which the medium-sized solid electrolyte accounts for 5-30% by mass and the D50 of the medium-sized solid electrolyte is 3-5 μm; The components of the surface layer include large-particle solid electrolyte, silicon-carbon composite material, adhesive and conductive agent, among which the mass proportion of large-particle solid electrolyte is 5~30%, and the D50 of large-particle solid electrolyte is 5~10μm.

[0010] In an optional embodiment, the small-particle solid electrolyte is a sulfide solid electrolyte; and / or, the medium-sized solid electrolyte is a sulfide solid electrolyte; And / or, the large-particle solid electrolyte is a sulfide solid electrolyte.

[0011] In an optional embodiment, the sulfide solid electrolyte is selected from LPS, L3PS 4-a X a 、Li6PS5X、Li 5.5 PS 4.5 X 1.5 , LiSiPSX and Li 11-a M 2-a P 1+a S 12 At least one of the following, wherein X is selected from at least one of F, Cl, Br, and I, M is selected from at least one of Ge, Si, and Sn, and 0.01≤a≤1.

[0012] In an optional embodiment, the solid electrolyte in the bottom layer accounts for 1-30% by mass, the silicon-carbon composite material accounts for 60-95% by mass, the adhesive accounts for 0.1-5% by mass, and the conductive agent accounts for 0.1-5% by mass; And / or, in the intermediate layer, the solid electrolyte accounts for 5-30% by mass, the silicon-carbon composite material accounts for 60-90% by mass, the adhesive accounts for 0.1-5% by mass, and the conductive agent accounts for 0.1-5% by mass; And / or, in the surface layer, the solid electrolyte accounts for 5-30% by mass, the silicon-carbon composite material accounts for 60-90% by mass, the adhesive accounts for 0.1-5% by mass, and the conductive agent accounts for 0.1-5% by mass.

[0013] In an optional embodiment, at least one of the following features (1) to (4) is included: (1) The conductive agent is selected from at least one of acetylene black, Ketjen black, carbon nanofibers, single-walled carbon nanotubes, and multi-walled carbon nanotubes; (2) The adhesive is selected from at least one of polytetrafluoroethylene, styrene-butadiene rubber, nitrile rubber, hydrogenated nitrile rubber, butadiene rubber, polyisobutylene and polyimide; (3) Silicon-carbon composite material is a mixture of silicon nanopowder and graphite.

[0014] (4) The slurry solvent is selected from at least one or more of toluene, xylene, anisole, butyl acetate, ethanol, deionized water, n-hexane and decane.

[0015] In an alternative embodiment, the current collector is copper foil.

[0016] In a second aspect, the present invention provides a method for preparing a negative electrode sheet according to any one of the aforementioned embodiments, comprising: Applying a first slurry for forming a small-particle solid electrolyte layer on the surface of a current collector, and then performing a first drying and a first pressing to obtain a first intermediate material; Applying a second slurry for forming a medium-sized solid electrolyte layer on the surface of the first intermediate material, followed by a second drying and a second pressing to obtain a second intermediate material; Applying a third slurry for forming a large-particle solid electrolyte layer on the surface of the second intermediate material, followed by a third drying and a third pressing to obtain a third intermediate material; The third intermediate material is placed in a protective atmosphere and heat-treated to cross-link and solidify the adhesive.

[0017] In an optional embodiment, at least one of the following features (1) to (3) is included: (1) The methods of first drying and / or second drying and / or third drying include: Pre-dry at 25-50°C for 10-100 minutes, then increase the temperature to 40-80°C and vacuum dry for 60-600 minutes; (2) The adhesive is hydrogenated nitrile rubber, the heat treatment temperature is 80~200℃, and the heat treatment time is 0.5~2h; (3) The protective atmosphere is argon atmosphere.

[0018] In an optional embodiment, the first hot pressing pressure is 1-5 MPa, the pressing time is 5-20 min, and the pressing temperature is 60-200° C.; and / or, the second pressing pressure is 3-10 MPa, the pressing time is 5-20 min, and the pressing temperature is 60-200° C.; And / or, the third pressing pressure is 5-15 MPa, the pressing time is 5-20 min, and the pressing temperature is 60-200°C.

[0019] In a third aspect, the present invention provides a solid-state battery comprising a negative electrode sheet according to any one of the aforementioned embodiments.

[0020] The present invention has the following beneficial effects: The negative electrode sheet provided by the embodiment of the present invention has a surface layer on the outermost side (close to the electrolyte), in which the solid electrolyte particles contained are relatively large, forming a porous skeleton structure to alleviate the volume expansion of silicon and provide ion transmission channels; the middle intermediate layer contains a moderate amount of solid electrolyte to balance porosity and ion contact surface; and a bottom layer is provided on the innermost side (close to the current collector), in which the solid electrolyte particles contained are the smallest, used to fill the pores and improve electronic conductivity. Due to the specific configuration of the three-layer solid electrolyte layer, the negative electrode sheet provided by the embodiment of the present invention can avoid excessive porosity caused by large particles or interfacial side reactions caused by small particles, optimize the ion transmission path, alleviate interfacial stress, and improve the electrochemical performance of the negative electrode sheet. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0022] Figure 1 A schematic structural diagram of a negative electrode sheet provided in an embodiment of the present invention; Figure 2 This is a SEM image of the negative electrode sheet of Example 1; Figure 3 This is the SEM image of the negative electrode sheet of Comparative Example 1; Figure 4 This is a graph showing the test results of the ionic conductivity of the negative electrode sheets of Example 1 and Comparative Example 1 tested by EIS. DETAILED DESCRIPTION

[0023] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are used. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be purchased commercially.

[0024] The features and performance of the present invention are further described in detail below with reference to the embodiments.

[0025] An embodiment of the present invention provides a negative electrode sheet, comprising a current collector and a bottom layer with a thickness of 10 to 50 μm, an intermediate layer with a thickness of 10 to 50 μm, and a surface layer with a thickness of 10 to 50 μm, which are sequentially arranged on the surface of the current collector; The bottom layer consists of small-particle solid electrolyte, silicon-carbon composite material, adhesive and conductive agent, of which the small-particle solid electrolyte accounts for 1-30% by mass and the D50 of the small-particle solid electrolyte is 0.5-1 μm. The components of the intermediate layer include medium-sized solid electrolyte, silicon-carbon composite material, adhesive and conductive agent, of which the medium-sized solid electrolyte accounts for 5-30% by mass and the D50 of the medium-sized solid electrolyte is 3-5 μm; The components of the surface layer include large-particle solid electrolyte, silicon-carbon composite material, adhesive and conductive agent, among which the mass proportion of large-particle solid electrolyte is 5~30%, and the D50 of large-particle solid electrolyte is 5~10μm.

[0026] The negative electrode sheet provided by the embodiment of the present invention has a surface layer on the outermost side (close to the electrolyte), in which the solid electrolyte particles contained are relatively large, forming a porous skeleton structure to alleviate the volume expansion of silicon and provide ion transmission channels; the middle intermediate layer contains a moderate amount of solid electrolyte to balance porosity and ion contact surface; and a bottom layer is provided on the innermost side (close to the current collector), in which the solid electrolyte particles contained are the smallest, used to fill the pores and improve electronic conductivity. Due to the specific configuration of the three-layer solid electrolyte layer, the negative electrode sheet provided by the embodiment of the present invention can avoid excessive porosity caused by large particles or interfacial side reactions caused by small particles, optimize the ion transmission path, alleviate interfacial stress, and improve the electrochemical performance of the negative electrode sheet.

[0027] Preferably, the small-particle solid electrolyte is a sulfide solid electrolyte; and / or, the medium-particle solid electrolyte is a sulfide solid electrolyte; and / or, the large-particle solid electrolyte is a sulfide solid electrolyte.

[0028] Compared with other types of electrolytes, sulfide solid electrolytes have advantages such as better ionic conductivity, better mechanical properties, high thermal stability, and low interfacial impedance. Therefore, sulfide solid electrolytes are preferably used in this application.

[0029] Optionally, the sulfide solid electrolyte is selected from LPS, L3PS 4-a X a 、Li6PS5X、Li 5.5 PS 4.5 X 1.5 , LiSiPSX and Li 11-a M 2-a P 1+a S 12 At least one of the following, wherein X is selected from at least one of F, Cl, Br, and I, M is selected from at least one of Ge, Si, and Sn, and 0.01≤a≤1.

[0030] Optionally, in order to make the negative electrode sheet have better electrochemical performance, the solid electrolyte in the bottom layer accounts for 1~30% by mass, the silicon-carbon composite material accounts for 60~95% by mass, the adhesive accounts for 0.1~5% by mass, and the conductive agent accounts for 0.1~5% by mass; the solid electrolyte in the middle layer accounts for 5~30% by mass, the silicon-carbon composite material accounts for 60~90% by mass, the adhesive accounts for 0.1~5% by mass, and the conductive agent accounts for 0.1~5% by mass; the solid electrolyte in the surface layer accounts for 5~30% by mass, the silicon-carbon composite material accounts for 60~90% by mass, the adhesive accounts for 0.1~5% by mass, and the conductive agent accounts for 0.1~5% by mass.

[0031] Optionally, the conductive agent is selected from at least one of acetylene black, Ketjen black (ECP), carbon nanofiber (VGCF), single-walled carbon nanotube and multi-walled carbon nanotube.

[0032] Preferably, a mixture of a point-shaped conductive agent and a linear conductive agent can further increase the electronic conductivity. The point-shaped conductive agent includes acetylene black, Ketjen black, etc., and the linear conductive agent includes carbon nanofiber, carbon nanotube, etc.

[0033] Optionally, the adhesive is selected from at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), nitrile rubber (NBR), hydrogenated nitrile rubber (H-NBR), butadiene rubber (BR), polyisobutylene (PIB) and polyimide (PI).

[0034] Optionally, the silicon-carbon composite material is a mixture of silicon nanopowder and graphite. Preferably, the mass ratio of silicon to carbon is 5 to 10:90.

[0035] The method for preparing the negative electrode sheet provided in an embodiment of the present invention includes: Applying a first slurry for forming a small-particle solid electrolyte layer on the surface of a current collector, and then performing a first drying and a first pressing to obtain a first intermediate material; Applying a second slurry for forming a medium-sized solid electrolyte layer on the surface of the first intermediate material, followed by a second drying and a second pressing to obtain a second intermediate material; Applying a third slurry for forming a large-particle solid electrolyte layer on the surface of the second intermediate material, followed by a third drying and a third pressing to obtain a third intermediate material; The third intermediate material is placed in a protective atmosphere and heat-treated to cross-link and solidify the adhesive.

[0036] The above-mentioned preparation method provided in the embodiment of the present invention can produce a negative electrode sheet having an active layer with three layers of particle size gradient distribution. It should be noted that in addition to the negative electrode sheet provided in the embodiment of the present invention that can be produced by the above-mentioned preparation method, a multi-layer structure can also be formed on the surface of the current collector by spin coating, dipping, or electrostatic spraying, which can also be used to prepare the negative electrode sheet provided in the embodiment of the present invention.

[0037] Specifically: S1. Material handling The solid electrolyte is graded. Taking Li6PS5Cl as an example, Li6PS5Cl is crushed into three levels of particle size using a ball mill: Large particle size layer: D50 = 5~10 μm (used for the surface layer to buffer stress); Medium particle size layer: D50 = 3~5 μm (used for the middle layer, balancing porosity); Small particle size layer: D50 = 0.5~3 μm (used for the bottom layer to improve density).

[0038] Silicon-carbon composite material: Silicon nanoparticles (50~100nm) are mixed with graphite and made by ball milling.

[0039] Prepare appropriate amounts of conductive agent and adhesive. Nitrile rubber is preferred as the adhesive, as it has good bonding properties and good interface contact, effectively preventing excessive interface impedance from affecting rate performance.

[0040] S2. Preparation of slurry The raw materials are mixed with a solvent according to the proportion of the components in the small-particle solid electrolyte layer to obtain a first slurry with a solid content of 30-60%; According to the proportion of each component in the medium-sized solid electrolyte layer, these raw materials are mixed with a solvent to obtain a second slurry with a solid content of 30-65%; The raw materials are mixed with a solvent according to the proportion of each component in the large-particle solid electrolyte layer to obtain a third slurry with a solid content of 30-70%.

[0041] Alternatively, the solvent may be a mixture of one or more of toluene, xylene, butyl acetate, and anisole. Preferably, the solvent is anisole combined with butyl acetate, wherein the volume proportion of butyl acetate is 1-10%, which can better improve the electrochemical performance.

[0042] S3, coating, drying and pressing Using a multi-stage slot coater, the first slurry is applied to the current collector surface. The slurry is then pre-dried at 25-50°C for 10-100 minutes, then vacuum-dried at 40-80°C for 60-600 minutes. The coating is then pressed under a pressure of 10-20 MPa to obtain the first intermediate material. Using a multi-stage slot coater, the second slurry is applied to the surface of the first intermediate material. The material is then pre-dried at 25-50°C for 10-100 minutes, then vacuum-dried at 40-80°C for 60-600 minutes. The second intermediate material is then pressed under a pressure of 15-25 MPa to form a coating. The third slurry is applied to the surface of the second intermediate material using a multi-stage slot coater. The material is then pre-dried at 25-50°C for 10-100 minutes, then vacuum-dried at 40-80°C for 60-600 minutes. The third intermediate material is then pressed under a pressure of 20-30 MPa to form a coating.

[0043] In this step, the pressing process is carried out under heating conditions, and the pressing temperature should not exceed the thermal stability temperature of the binder.

[0044] S4. Heat treatment The third intermediate material is placed in a protective atmosphere for heat treatment to crosslink and solidify the adhesive, thereby improving the mechanical strength of the electrode.

[0045] Preferably, the adhesive is nitrile rubber, which is heat-resistant and has a high elastic modulus. Correspondingly, the heat treatment temperature is 80-200° C., and the heat treatment time is 0.5-2 hours.

[0046] It should be noted that if other materials are selected as adhesives, a corresponding heat treatment temperature must be set to allow them to crosslink and cure.

[0047] Optionally, the protective atmosphere is an argon atmosphere.

[0048] The negative electrode sheet and its preparation method provided by the present invention, compared with the process of silicon-carbon negative electrode surface coating, do not require a coating layer, directly buffer stress through gradient electrolyte distribution, and have a better ion transmission path; compared with the interface doping process, the interface is naturally optimized by the gradient structure, no additional interface layer is required, and the process complexity is reduced; compared with the uniform mixed sulfide electrolyte process, the solid electrolyte particle size gradient distribution design, the surface large particles buffer stress, the bottom small particles are densified, and contact failure is suppressed; compared with the high-pressure pressing process, the gradient pressing pressure is used to optimize the compaction density layer by layer, avoid particle crushing, and take into account mechanical strength and conductivity; compared with the dry electrode process, the wet layered coating collaborative process is adopted to achieve high-precision gradient coating, layered pressing optimizes interlayer bonding, and takes into account performance and environmental protection. Therefore, the solid-state battery negative electrode sheet and its preparation method provided by the present invention have better technical effects than the prior art.

[0049] The present invention also provides a solid-state battery including the negative electrode provided in the present invention. The solid-state battery has better electrochemical performance due to the inclusion of the negative electrode provided in the present invention.

[0050] Optionally, the positive electrode material of the solid-state battery can be lithium iron phosphate, lithium cobalt oxide, NCM622, NCM90, etc.

[0051] Example 1 Providing silicon-carbon composite materials: Silicon nanoparticles (50-100nm) and graphite are mixed and ball-milled in a mass ratio of 1:9; Provide conductive agent and adhesive: Ketjen Black, carbon nanofiber (VGCF) as composite conductive agent, nitrile rubber (NBR) as adhesive; Provide solid electrolytes: D50 of large, medium and small particle size solid electrolytes are 8μm, 4μm and 0.8μm respectively; The above materials were mixed with anisole composite solvent in a ratio of 80:15:4:1, to obtain a first slurry with a solid content of 70%. The above materials were mixed with anisole composite solvent in a ratio of silicon-carbon composite material: medium-sized Li6PS5Cl: composite conductive agent: binder = 80:15:4:1 to obtain a second slurry with a solid content of 65%; The above materials were evenly mixed with anisole composite solvent according to the ratio of silicon-carbon composite material: large-particle Li6PS5Cl: composite conductive agent: binder = 75:20:4:1 to obtain a third slurry with a solid content of 60%.

[0052] The first slurry was coated on a copper foil, and then pre-dried at 45°C for 20 minutes, and then vacuum-dried at 70°C for 2 hours, and finally pressed at 3 MPa for 10 minutes to obtain a first intermediate material; The second slurry was coated on the first intermediate material, and then pre-dried at 45°C for 20 minutes, and then vacuum-dried at 70°C for 2 hours, and finally pressed at 5 MPa for 10 minutes to obtain a second intermediate material; The third slurry was coated on the second intermediate material, and then pre-dried at 45° C. for 20 min, then the temperature was increased to 70° C. and vacuum dried for 8 h, and finally pressed at 10 MPa for 10 min to obtain a third intermediate material.

[0053] The third intermediate material was heat-treated at 180° C. for 1 h in an argon atmosphere to obtain a negative electrode sheet. The thicknesses of the three composite negative electrode active layers from the bottom layer to the surface layer were 20 μm, 30 μm, and 30 μm, respectively.

[0054] Example 2 Providing silicon-carbon composite materials: Silicon nanoparticles (50-100nm) and graphite are mixed and ball-milled in a mass ratio of 1:9; Provide conductive agent and adhesive: Ketjen Black, carbon nanofiber (VGCF) as composite conductive agent, nitrile rubber (NBR) as adhesive; Provide solid electrolytes: D50 of large, medium and small particle size solid electrolytes are 10μm, 5μm and 3μm respectively; The above materials were mixed with anisole composite solvent in a ratio of 60:30:5:5 according to the ratio of silicon-carbon composite material: small-particle Li6PS5Cl: composite conductive agent: binder to obtain a first slurry with a solid content of 70%; The above materials were mixed with anisole composite solvent according to the ratio of silicon-carbon composite material: medium-sized Li6PS5Cl: composite conductive agent: binder = 60:30:5:5 to obtain a second slurry with a solid content of 65%; The above materials were evenly mixed with anisole composite solvent according to the ratio of silicon-carbon composite material: large-particle Li6PS5Cl: composite conductive agent: binder = 60:30:5:5 to obtain a third slurry with a solid content of 60%.

[0055] The first slurry was coated on a copper foil, and then pre-dried at 50°C for 10 minutes, and then vacuum-dried at 80°C for 60 minutes, and finally pressed at 1 MPa for 5 minutes to obtain a first intermediate material; The second slurry was coated on the first intermediate material, and then pre-dried at 50°C for 10 minutes, and then vacuum-dried at 80°C for 60 minutes, and finally pressed at 3 MPa for 5 minutes to obtain a second intermediate material; The third slurry was coated on the second intermediate material, and then pre-dried at 50° C. for 10 min, then the temperature was increased to 80° C. and vacuum dried for 60 min, and finally pressed at 5 MPa for 5 min to obtain a third intermediate material.

[0056] The third intermediate material was heat-treated in an argon atmosphere at 200° C. for 0.5 h to obtain a negative electrode sheet. The thickness of the composite negative electrode active layer from the bottom layer to the surface layer was 50 μm, 10 μm, and 10 μm.

[0057] Example 3 Providing silicon-carbon composite materials: Silicon nanoparticles (50-100nm) and graphite are mixed and ball-milled in a mass ratio of 1:9; Provide conductive agent and adhesive: Ketjen Black, carbon nanofiber (VGCF) as composite conductive agent, nitrile rubber (NBR) as adhesive; Provide solid electrolytes: D50 of large, medium and small particle size solid electrolytes are 5μm, 3μm and 0.5μm respectively; The above materials were mixed with anisole composite solvent in a ratio of silicon-carbon composite material: small-particle Li6PS5Cl: composite conductive agent: binder = 95:3:0.1:1.9 to obtain a first slurry with a solid content of 70%; The above materials were mixed with anisole composite solvent according to the ratio of silicon-carbon composite material: medium-sized Li6PS5Cl: composite conductive agent: binder = 90: 5: 4: 1 to obtain a second slurry with a solid content of 65%; The above materials were evenly mixed with anisole composite solvent according to the ratio of silicon-carbon composite material: large-particle Li6PS5Cl: composite conductive agent: binder = 90: 5: 4: 1 to obtain a third slurry with a solid content of 60%.

[0058] The first slurry was coated on a copper foil, and then pre-dried at 25° C. for 100 minutes, and then vacuum-dried at 40° C. for 600 minutes, and finally pressed at 5 MPa for 20 minutes to obtain a first intermediate material; The second slurry was coated on the first intermediate material, and then pre-dried at 25°C for 100 minutes, and then vacuum-dried at 40°C for 600 minutes, and finally pressed at 10 MPa for 20 minutes to obtain a second intermediate material; The third slurry was coated on the second intermediate material, and then pre-dried at 25° C. for 100 min, then vacuum-dried at 40° C. for 600 min, and finally pressed at 15 MPa for 20 min to obtain a third intermediate material.

[0059] The third intermediate material was heat-treated at 80° C. for 2 h in an argon atmosphere to obtain a negative electrode sheet. The thickness of the composite negative electrode active layer from the bottom layer to the surface layer was 10 μm, 50 μm, and 50 μm.

[0060] Example 4 This embodiment is basically the same as embodiment 1, except that the conductive agent is replaced by Ketjen black and carbon nanofibers in a mass ratio of 1:1 in equal amounts.

[0061] Comparative Example 1 This comparative example provides a method for preparing a negative electrode sheet, wherein the prepared negative electrode sheet has only one layer of coating. The specific preparation method is as follows: Providing silicon-carbon composite materials: Silicon nanoparticles (50-100nm) and graphite are mixed and ball-milled in a mass ratio of 1:9; Provide conductive agents and adhesives: Ketjen black, carbon nanofiber as composite conductive agents, nitrile rubber (NBR) as adhesive; The D50 of the particles in the solid electrolyte is 4 μm; The above materials were mixed with anisole composite solvent according to the ratio of silicon-carbon composite material: medium-sized Li6PS5Cl: composite conductive agent: binder = 90:5:4:1 to obtain a second slurry with a solid content of 65%; The slurry was coated onto the current collector, and then pre-dried at 45°C for 20 minutes, then vacuum-dried at 70°C for 2 hours, and finally pressed at 5 MPa for 10 minutes to obtain an intermediate material. The intermediate material was heat treated at 180° C. for 1 h in an argon atmosphere to obtain a negative electrode sheet. The thickness of the composite negative electrode sheet was 80 μm.

[0062] Comparative Example 2 This comparative example is basically the same as Example 2, except that: on the basis of the constant thickness of the total active layer, the thickness of the surface layer is 60 μm, the thickness of the middle layer is 5 μm, and the thickness of the bottom layer is 5 μm.

[0063] Comparative Example 3 This comparative example is basically the same as Example 3, except that the thickness of the surface layer is 5 μm, the thickness of the middle layer is 55 μm, and the thickness of the bottom layer is 50 μm, while the thickness of the total active layer remains unchanged.

[0064] Comparative Example 4 This comparative example is basically the same as Example 1, except that the total component content of the bottom layer and the middle layer remains unchanged, and the ratio of the substances in the two layers is changed, specifically: In the bottom layer, silicon-carbon composite material: small-particle Li6PS5Cl: composite conductive agent: binder = 50:38:6:6; In the middle layer, silicon-carbon composite material: medium-sized Li6PS5Cl: composite conductive agent: binder = 50:36:7:7.

[0065] Experimental example The negative electrode sheets prepared in various embodiments and comparative examples were assembled into full batteries for testing. The positive electrode active material was NCM811 and the solid electrolyte layer was Li6PS5Cl.

[0066] The electrochemical performance of the batteries assembled in each example and comparative example was tested using the following method: activation at 0.1C for five cycles, followed by rate performance testing at 1 / 3C, 0.5C, and 1C. The batteries were then cycled at 0.5C for 500 cycles, and the capacity retention was recorded. The test results are recorded in Table 1.

[0067] Table 1 Electrochemical performance test of negative electrode sheets of various embodiments and comparative examples

[0068] Table 1 shows that the negative electrode sheets prepared in various embodiments of the present invention have good electrochemical performance after being assembled into solid-state batteries. Comparing Comparative Example 1 with each embodiment, the cycle capacity retention rate and rate performance of Comparative Example 1 are significantly worse, which shows that the gradient distribution of the solid electrolyte particle size in the active layer provided by the present invention can significantly improve the electrochemical performance of the negative electrode sheet; The thickness of the surface layer of Comparative Examples 2 and 3 is not within the range required by the present invention. Comparative Examples 2 and 3 are compared with the corresponding embodiments. Comparative Examples 2 and 3 are significantly worse than the corresponding embodiments. This shows that the thickness of the surface layer should be within the range required by the present invention to achieve a significant improvement in electrochemical performance. If the thickness of the surface layer is too small, the lithium ion deposition is uneven, the structure is unstable, and lithium dendrites are easily generated, resulting in poor cycle performance. If the thickness of the surface layer is too large, the lithium ion diffusion path is extended, the proportion of active material is reduced, resulting in poor rate performance, and affecting energy density. The content of small- and medium-sized solid electrolytes in Comparative Example 4 is not within the range required by the present invention. Comparative Example 4 is compared with the corresponding embodiments. Comparative Example 4 is significantly worse than the corresponding embodiments. This shows that the solid electrolyte content in the bottom layer and the middle layer should be within the range required by the present invention to achieve a significant improvement in electrochemical performance. If the content of small- and medium-sized solid electrolytes is too little, the ion diffusion path is discontinuous, resulting in a decrease in capacity retention. If the content of small- and medium-sized solid electrolytes is too much, the active material content is reduced, the electronic conductivity is also reduced, resulting in polarization, and poor cycle performance. Experimental Example 2 (1) After 500 cycles, the batteries of Example 1 and Comparative Example 1 were disassembled and the surface cracks of the negative electrode sheets were observed. The SEM images of the negative electrode sheets of Example 1 and Comparative Example 1 are as follows: Figure 2 and Figure 3 shown.

[0069] As can be seen from the figure, the surface cracks of the negative electrode sheet of Example 1 of the present invention are very few, while the cracks of the negative electrode sheet of Comparative Example 1 are significantly more. According to calculations, the crack reduction rate of the negative electrode sheet of the present invention can reach >50% compared with the negative electrode sheet of the comparative example. This is mainly due to the large particles on the surface forming a porous skeleton, which can effectively buffer the expansion stress of silicon.

[0070] (2) After 500 cycles, the batteries of Example 1 and Comparative Example 1 were disassembled to observe whether there was lithium dendrite precipitation on the negative electrode surface. Example 1 showed no precipitation, while Comparative Example 1 showed lithium dendrite precipitation; (3) EIS tests the ionic conductivity of the negative electrode sheets of Example 1 and Comparative Example 1. The ionic conductivity of Example 1 and Comparative Example 1 were 2.1×10 -3 S / cm, 8.5×10 -4 S / cm, the comparison between the two shows that the embodiment of the present invention can make Li + The migration path is shortened. Test data such as Figure 4 shown.

[0071] (4) The electronic conductivity of Example 1 and Example 4 was measured by four-probe method. The test result of Example 1 was 1.6×10 - 3 S / cm, the electronic conductivity of Example 4 is >2×10 -2 S / cm, indicating that the mixed use of point-shaped conductive agents and linear conductive agents can increase the electronic conductivity of the negative electrode sheet.

[0072] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A negative electrode sheet, characterized in that: The invention comprises a current collector and a bottom layer with a thickness of 10 to 50 μm, an intermediate layer with a thickness of 10 to 50 μm, and a surface layer with a thickness of 10 to 50 μm, which are sequentially arranged on the surface of the current collector; The bottom layer comprises a small-particle solid electrolyte, a silicon-carbon composite material, an adhesive, and a conductive agent, wherein the small-particle solid electrolyte accounts for 1 to 30% by mass, and the D50 of the small-particle solid electrolyte is 0.5 to 3 μm; The components of the intermediate layer include a medium-sized solid electrolyte, a silicon-carbon composite material, a binder, and a conductive agent, wherein the medium-sized solid electrolyte accounts for 5 to 30% by weight, and the D50 of the medium-sized solid electrolyte is 3 to 5 μm; The components of the surface layer include large-particle solid electrolyte, silicon-carbon composite material, adhesive and conductive agent, wherein the mass proportion of the large-particle solid electrolyte is 5-30%, and the D50 of the large-particle solid electrolyte is 5-10 μm.

2. The negative electrode sheet according to claim 1, characterized in that: The small-particle solid electrolyte is a sulfide solid electrolyte; And / or, the medium-sized solid electrolyte is a sulfide solid electrolyte; And / or, the large-particle solid electrolyte is a sulfide solid electrolyte.

3. The negative electrode sheet according to claim 2, characterized in that: The sulfide solid electrolyte is selected from LPS, L3PS 4- a X a 、Li6PS5X、Li 5.5 PS 4.5 X 1.5 , LiSiPSX and Li 11-a M 2-a P 1+a S 12 At least one of the following, wherein X is selected from at least one of F, Cl, Br, and I, M is selected from at least one of Ge, Si, and Sn, and 0.01≤a≤1.

4. The negative electrode sheet according to claim 1, characterized in that: In the bottom layer, the solid electrolyte accounts for 1-30% by mass, the silicon-carbon composite material accounts for 60-95% by mass, the adhesive accounts for 0.1-5% by mass, and the conductive agent accounts for 0.1-5% by mass; And / or, in the intermediate layer, the solid electrolyte accounts for 5-30% by mass, the silicon-carbon composite material accounts for 60-90% by mass, the adhesive accounts for 0.1-5% by mass, and the conductive agent accounts for 0.1-5% by mass; And / or, in the surface layer, the solid electrolyte accounts for 5-30% by mass, the silicon-carbon composite material accounts for 60-90% by mass, the adhesive accounts for 0.1-5% by mass, and the conductive agent accounts for 0.1-5% by mass.

5. The negative electrode sheet according to claim 1, characterized in that: Include at least one of the following features (1) to (4): (1) The conductive agent is selected from at least one of acetylene black, Ketjen black, carbon nanofibers, single-walled carbon nanotubes, and multi-walled carbon nanotubes; (2) The adhesive is selected from at least one of polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, nitrile rubber, hydrogenated nitrile rubber, styrene-butadiene-styrene block copolymer, butadiene rubber, polyisobutylene and polyimide; (3) The silicon-carbon composite material is a mixed powder of silicon nanopowder and carbon powder; (4) The slurry uses a solvent selected from at least one of toluene, xylene, anisole, ethyl acetate, n-hexane and butyl acetate.

6. The negative electrode sheet according to claim 1, characterized in that: The current collector is nickel-plated copper foil.

7. A method for preparing a negative electrode sheet according to any one of claims 1 to 6, characterized in that: include: Applying the first slurry for forming the small-particle solid electrolyte layer on the surface of the current collector, and then performing a first drying and a first pressing to obtain a first intermediate material; Applying a second slurry for forming the medium-sized solid electrolyte layer on the surface of the first intermediate material, followed by a second drying and a second pressing to obtain a second intermediate material; The third slurry for forming the large-particle solid electrolyte layer is coated on the surface of the second intermediate material, and then the third intermediate material is obtained after a third drying and a third pressing.

8. The preparation method according to claim 7, characterized in that Include at least one of the following features (1) to (3): (1) The first drying and / or the second drying and / or the third drying include: Pre-dry at 25-50°C for 10-100 minutes, then increase the temperature to 40-80°C and vacuum dry for 60-600 minutes; (2) The adhesive is nitrile rubber, the heat treatment temperature is 80~200℃, and the heat treatment time is 0.5~2h; (3) The protective atmosphere is argon atmosphere.

9. The preparation method according to claim 7, characterized in that The first hot pressing pressure is 1-5 MPa, the pressing time is 5-20 min, and the pressing temperature is 60-200°C; And / or, the second pressing pressure is 3-10 MPa, the pressing time is 5-20 min, and the pressing temperature is 60-200° C.; And / or, the third pressing pressure is 5-15 MPa, the pressing time is 5-20 min, and the pressing temperature is 60-200°C.

10. A solid-state battery, characterized in that: The invention comprises the negative electrode sheet according to any one of claims 1 to 6 or the negative electrode sheet prepared by the preparation method according to any one of claims 7 to 9.

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