A positive electrode sheet, a method for manufacturing the same, and a solid-state battery

Abstract

The present application relates to the technical field of solid-state batteries, and provides a positive electrode sheet, a preparation method thereof and a solid-state battery. The positive electrode sheet comprises a positive electrode current collector, a first positive electrode active material layer arranged on at least one side surface of the positive electrode current collector, and a second positive electrode active material layer located on the side surface of the first positive electrode active material layer away from the positive electrode current collector; the first positive electrode active material layer comprises a positive electrode active material, a solid-state electrolyte, a conductive agent and a first binder; the first binder is a fluorine-containing high molecular material, and the first binder comprises binder A and binder B; the binder A is a fluorine-containing polymer fiber with a molecular weight of 3 million to 6 million; the binder B has a molecular weight of 0.8 million to 1.2 million; the second positive electrode active material layer comprises a positive electrode active material, a solid-state electrolyte, a conductive agent and a second binder; and the second binder comprises a polymer fiber containing a carboxyl functional group. The present application solves the problems of insufficient toughness of the positive electrode sheet, easy breakage and interface impedance growth.

Description

Technical Field

[0001] This invention relates to the technical field of solid-state batteries, specifically to a positive electrode sheet, its preparation method, and a solid-state battery. Background Technology

[0002] In the field of solid-state batteries, dry electrode technology has attracted much attention due to its advantages such as environmental friendliness and simple process. However, current dry cathode sheets for solid-state batteries face significant technical bottlenecks: on the one hand, the cathode sheets have low toughness and are prone to breakage during subsequent processing such as winding and cutting, which seriously affects production feasibility; on the other hand, it is difficult to suppress the increase in interfacial impedance between high-nickel materials and solid electrolytes, leading to a decline in battery cycle performance and restricting the practical application of solid-state batteries. Summary of the Invention

[0003] The purpose of this invention is to overcome the aforementioned problems in the prior art and provide a positive electrode sheet, its preparation method, and a solid-state battery. The positive electrode sheet is constructed by layering a first positive electrode active material layer and a second positive electrode active material layer. A flexible network is formed using fluorine-containing materials of different molecular weights in the first binder, enhancing the bonding strength. Combined with the chemical bonding between the carboxyl-containing polymer fibers in the second binder and the positive electrode active material, the problems of insufficient toughness and easy breakage of the positive electrode sheet, as well as the increase in interfacial impedance, are solved. This achieves synergistic optimization of the mechanical properties and interfacial stability of the positive electrode sheet.

[0004] To achieve the above objectives, a first aspect of the present invention provides a positive electrode sheet, the positive electrode sheet comprising a positive current collector, a first positive active material layer disposed on at least one surface of the positive current collector, and a second positive active material layer disposed on the surface of the first positive active material layer away from the positive current collector; the first positive active material layer comprises a positive active material, a solid electrolyte, a conductive agent, and a first binder; the first binder is a fluorinated polymer material, and the first binder comprises binder A and binder B; binder A is a fluorinated polymer fiber with a molecular weight of 3 million to 6 million; the molecular weight of binder B is 800,000 to 1.2 million; the second positive active material layer comprises a positive active material, a solid electrolyte, a conductive agent, and a second binder; the second binder comprises a polymer fiber containing carboxyl functional groups.

[0005] A second aspect of the present invention provides a method for preparing the positive electrode sheet described in the first aspect, comprising the following steps:

[0006] Step S1: Dry mix the positive electrode active material and the solid electrolyte to obtain a mixture.

[0007] Step S2: Divide the mixture obtained in step S1 into two parts, add a conductive agent and a first binder to one part of the mixture, and perform dry mixing to obtain the first mixture.

[0008] A conductive agent and a second binder are added to another mixture, and the mixture is then dry-mixed to obtain a second mixture.

[0009] Step S3: The first mixture and the second mixture obtained in step S2 are discharged sequentially and subjected to the first pressing process to obtain a composite film;

[0010] Step S4: The first mixture side of the composite membrane obtained in step S3 is attached to the surface of the positive electrode current collector, and then the second pressing process and the third pressing process are performed in sequence.

[0011] A third aspect of the present invention provides a solid-state battery, comprising a positive electrode sheet provided in the first aspect, or a positive electrode sheet obtained by the preparation method provided in the second aspect.

[0012] The present invention, by adopting the above technical solution, has the following beneficial effects:

[0013] The positive electrode sheet provided by the present invention solves the problems of insufficient toughness and easy breakage of the positive electrode sheet and the increase of interfacial impedance by layering a first positive electrode active material layer and a second positive electrode active material layer, utilizing fluorine-containing materials of different molecular weights in the first binder to form a flexible network and enhance the bonding strength, and combining the chemical bonding effect of carboxyl polymer fibers in the second binder with the positive electrode active material. This achieves synergistic optimization of the mechanical properties and interfacial stability of the positive electrode sheet.

[0014] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and should be understood to include values ​​close to those ranges or values. For numerical ranges, endpoint values ​​of various ranges, endpoint values ​​of various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. In this document, unless otherwise specified, data ranges include endpoints. Detailed Implementation

[0015] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0016] Unless otherwise defined, all scientific and technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art.

[0017] The first aspect of the present invention provides a positive electrode sheet, the positive electrode sheet comprising a positive current collector, a first positive active material layer disposed on at least one side surface of the positive current collector, and a second positive active material layer disposed on the side surface of the first positive active material layer away from the positive current collector; the first positive active material layer comprises a positive active material, a solid electrolyte, a conductive agent, and a first binder; the first binder is a fluorinated polymer material, and the first binder comprises binder A and binder B; binder A is a fluorinated polymer fiber with a molecular weight of 3 million to 6 million; the molecular weight of binder B is 800,000 to 1.2 million; the second positive active material layer comprises a positive active material, a solid electrolyte, a conductive agent, and a second binder; the second binder comprises a polymer fiber containing carboxyl functional groups.

[0018] The positive electrode sheet provided by this invention features a gradient distribution of two active material layers. The inner layer (first active material layer) focuses on improving the mechanical properties of the positive electrode sheet. High-molecular-weight binder A undergoes shearing and fiberization during electrode pressing, forming a continuous flexible network framework. The extensibility of its molecular chains endows the positive electrode sheet with bending resistance and inhibits breakage. Medium-molecular-weight binder B forms hydrogen bonds with the hydroxyl groups on the surface of the active material through the strong electronegativity of fluorine atoms on its molecular chains. Simultaneously, its molecular chain entanglement enhances the bonding strength between positive electrode active material particles and between the positive electrode active material particles and the current collector, improving the overall mechanical stability of the positive electrode sheet. The outer layer (second active material layer) strengthens interfacial stability. The second binder (polymer fiber containing carboxyl functional groups) utilizes the interaction between carboxyl groups and hydroxyl groups on the surface of the positive electrode active material to form a chemical anchoring structure, enhancing the interfacial bonding force of the outer active material layer, inhibiting interfacial side reactions, and reducing impedance growth. This invention, through the design of the layered structure and gradient binder system of the positive electrode sheet, achieves synergistic optimization of improved positive electrode sheet toughness and stable interfacial impedance, meeting the requirements of subsequent processing (winding, cutting) and electrochemical performance.

[0019] For example, the molecular weight of the binder A is 3 million to 6 million, such as 3 million, 3.5 million, 4 million, 5 million, 5.5 million, 6 million, or any value within the range of any pair of values ​​mentioned above. This invention limits the molecular weight of binder A to the above range to ensure that binder A is fully fibrous under compression and shear force, forming a continuous flexible network skeleton, thus giving the positive electrode sheet resistance to bending and stretching. It avoids situations where the molecular weight is less than 3 million, resulting in excessively short molecular chains that are difficult to stretch into a fibrous structure under shear force during compression, leading to a sparse fibrous network and easy breakage of the positive electrode sheet when bent; it also avoids situations where the molecular weight is greater than 6 million, resulting in excessively long molecular chains that cause severe agglomeration, making it difficult to disperse evenly in the active material and easily forming large clumps during mixing; simultaneously, excessively high molecular weight reduces fluidity during compression, making fiber network formation difficult and weakening stretching.

[0020] For example, the molecular weight of the binder B is 800,000 to 1,200,000, such as 800,000, 900,000, 1,000,000, 1,100,000, 1,200,000, or any value within the range of any two of the above values. This invention limits the molecular weight of the binder B to the above range. The binder B enhances the bonding strength between particles and with the current collector through molecular chain entanglement and hydrogen bonding between fluorine atoms and the hydroxyl groups on the surface of the active material. It avoids situations where the molecular weight is less than 800,000, resulting in too short molecular chains, weak entanglement, and inability to effectively connect the active material particles, leading to reduced peel strength between the positive electrode and the current collector, and easy shedding of the active material during processing. It also avoids situations where the molecular weight is greater than 1,200,000, resulting in excessively high viscosity, which can easily coat the conductive agent or solid electrolyte during dry mixing, blocking active sites and affecting ion conduction. Furthermore, excessively high molecular weight reduces the flexibility of the positive electrode, weakens the synergistic effect with the fibrous network formed by the binder A, and leads to a decrease in the overall mechanical properties of the positive electrode.

[0021] In this invention, the molecular weights of adhesive A and adhesive B can be directly provided by the manufacturer as required.

[0022] In some embodiments, the adhesive A comprises polytetrafluoroethylene fibers.

[0023] In some embodiments, the adhesive B comprises polyvinylidene fluoride.

[0024] In some embodiments, the mass percentage of the first binder, based on the total mass of the first positive electrode active material layer, is 2%-3%, for example, it can be 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, or any value within the range of the above pairs. This invention limits the mass percentage of the first binder to the above range to ensure that binder A and binder B form a sufficient flexible fiber network framework in the first active material layer and guarantee bonding strength, while avoiding excessive first binder that would reduce the proportion of positive electrode active material and affect battery capacity and ion conduction.

[0025] In some embodiments, the mass ratio of binder A to binder B is (1-5):1, for example, it can be 1:1, 2:1, 3:1, 4:1, 5:1, or any ratio within the range defined in this invention. In this invention, binder A has a greater impact on the electrode peel strength than binder B. By further limiting the mass ratio of binder A to binder B to the above range, it can be avoided that when the mass ratio is less than 1:1, there is too little binder A and too much binder B, resulting in a sparse fibrous network, a significant decrease in the toughness of the positive electrode, and a relative decrease in peel strength. It can also be avoided that when the mass ratio is greater than 5:1, there is too much binder A and too little binder B. Although the peel strength of the electrode is improved, the excessive binder A will easily agglomerate and cannot extend to form an effective fibrous network structure, which will reduce the flexibility of the positive electrode and make it easy to break.

[0026] In some embodiments, the Dv50 of the binder A is 100μm-400μm, for example, it can be 100μm, 200μm, 300μm, 400μm, or any value within the range of these values. A suitable particle size ensures that the binder A is uniformly dispersed during the positive electrode preparation process, and is fully fiberized by shear force during pressing. When the particle size is less than 100μm, the specific surface area of ​​the binder A is large, making it prone to agglomeration during positive electrode preparation and unable to be uniformly distributed in the active material, resulting in a discontinuous fiber network and localized weakness in the positive electrode toughness. When the particle size is greater than 400μm, the binder A is difficult to fully fiberize by shear force, resulting in a fiber network with excessively large pore sizes that cannot effectively connect the active material particles, leading to a decrease in the ductility of the positive electrode.

[0027] In some embodiments, the Dv50 of the adhesive B is 80μm-120μm, for example, it can be 80μm, 90μm, 100μm, 110μm, 120μm, or any value within the range of any two of the above values. The particle size of the adhesive B within this range allows it to uniformly fill the gaps in the fiber network formed by the adhesive A, enhancing the bonding strength through molecular chain entanglement and hydrogen bonding, while avoiding uneven dispersion due to excessively large particle size or decreased conductivity due to excessively small particle size.

[0028] In this invention, the Dv50 of adhesive A and the Dv50 of adhesive B can be tested by particle size analysis—laser particle size distribution instrument method.

[0029] In some embodiments, the mass percentage of the second binder, based on the total mass of the second positive electrode active material layer, is 0.3%-1%, for example, it can be 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or any value within the range of the above pairs. The mass percentage of the second binder within the above range ensures that the carboxyl-containing polymer fibers form sufficient interfacial chemical bonding sites in the second active material layer. This enhances the interfacial bonding force through the bonding between carboxyl groups and hydroxyl groups on the surface of the positive electrode active material, while avoiding excessive addition that could cover the active sites of the solid electrolyte.

[0030] It should be noted that the mass percentage of the first binder (including binder A and binder B) and the second binder in the positive electrode active material layer refers to the percentage of the mass of the first binder and the mass of the second binder relative to the total mass of each component (such as positive electrode active material, solid electrolyte, conductive agent, etc.) in the positive electrode active material layer when preparing the positive electrode sheet.

[0031] In some embodiments, the carboxyl-containing polymer fiber includes one or both of polyacrylic acid fiber and carboxymethyl cellulose nanofiber.

[0032] In some embodiments, the diameter of the carboxyl-functionalized polymer fiber is 50nm-150nm, for example, it can be 50nm, 80nm, 100nm, 120nm, 150nm, or any value within the range of any two of the above values; the length of the polymer fiber is 200nm-500nm, for example, it can be 200nm, 300nm, 400nm, 500nm, or any value within the range of any two of the above values. Nanoscale diameter carboxyl-functionalized polymer fibers can uniformly fill the gaps between positive electrode active material particles, forming a flexible transition layer at the interface, buffering the volume expansion stress during cycling, and suppressing interfacial delamination. A diameter less than 50nm is avoided, as it easily leads to fiber agglomeration, ineffective dispersion, uneven interfacial bonding, and abnormally high local impedance; a diameter greater than 200nm is also avoided, as it leads to poor interfacial contact, increased ion conduction resistance, and decreased conductivity. A suitable length allows polymer fibers containing carboxyl functional groups to bridging multiple active material particles, forming a three-dimensional network bonded structure. This enhances the interfacial mechanical strength and ion conduction continuity. A length less than 200 nm is avoided, as the polymer fibers containing carboxyl functional groups cannot effectively connect particles, resulting in weak interfacial bonding and a tendency for microcracks to form during cycling. A length greater than 500 nm is also avoided, as the polymer fibers containing carboxyl functional groups are prone to tangling and clumping during cathode preparation, leading to uneven dispersion and localized binder enrichment areas on the cathode, affecting the consistency of electrochemical performance.

[0033] In some embodiments, the positive electrode active material includes positive electrode active material particles and an interface buffer layer located on the surface of the positive electrode active material particles; the interface buffer layer includes lithium phosphate (Li3PO4). The interface buffer layer can isolate the positive electrode active material from direct contact with the solid electrolyte, suppress the formation of impedance layers such as Li2CO3 by side reactions, and work synergistically with the second binder to further suppress the growth of interface impedance of the cycled positive electrode sheet.

[0034] In some embodiments, the mass ratio of the positive electrode active material to the solid electrolyte, based on the total mass of the first and second positive electrode active material layers, is (7-10):1. For example, it can be 7:1, 8:1, 9:1, 10:1, or any ratio within the range defined by this invention. By controlling the mass ratio of the positive electrode active material to the solid electrolyte within the above range, this invention enables the formation of a complete Li3PO4 interface buffer layer on the surface of the positive electrode active material particles through plasma activation. Furthermore, while ensuring sufficient capacity from the positive electrode active material, the solid electrolyte forms a continuous ion conduction network, balancing energy density and ion conduction. This avoids insufficient solid electrolyte, which reduces ion conductivity and leads to a decrease in battery capacity retention.

[0035] In some embodiments, the thickness ratio of the first positive electrode active material layer to the second positive electrode active material layer is 1:(1-5), for example, it can be 1:1, 1:2, 1:3, 1:4, 1:5, or any ratio within the range defined by this invention. This invention controls the thickness ratio of the first positive electrode active material layer to the second positive electrode active material layer within the above range to avoid the first positive electrode active material layer being too thin, resulting in insufficient fiber network content, decreased membrane extensibility, and susceptibility to cracking; it also avoids the second positive electrode active material layer being too thin, leading to weak interfacial bonding between the carboxyl-containing polymer fibers and the -OH groups on the surface of the positive electrode active material, which would fail to effectively suppress interfacial side reactions and solve the problem of interfacial impedance growth during cycling.

[0036] In some embodiments, the total thickness of the first positive electrode active material layer and the second positive electrode active material layer is 300μm-350μm, for example, it can be 300μm, 310μm, 320μm, 330μm, 340μm, 350μm or any point value in the range of the above two point values.

[0037] In some embodiments, the positive electrode active material includes a ternary positive electrode active material, wherein the chemical formula of the ternary material is Li. a Ni x Co y Mn z O2, where 0.9≤a≤1.1, 0.7≤x<1, 0≤y≤1, 0≤z≤1.

[0038] In some embodiments, the solid electrolyte includes at least one of a sulfide solid electrolyte, an oxide solid electrolyte, and a polymer-based solid electrolyte.

[0039] In some embodiments, the solid electrolyte is selected from sulfide solid electrolytes.

[0040] In some embodiments, the sulfide solid electrolyte includes glassy 20Li2S-80P2S5 and glass-ceramic Li7P3S. 11 , Crystalline Li6PS6Cl, Li 10 GeP2S 12 At least one of them.

[0041] In this invention, the conductive agent in the first positive electrode active material layer and the second positive electrode active material layer is not particularly limited. Conventional conductive agents used in solid-state batteries can be selected, such as conductive carbon black or one or more mixed conductive agents from carbon nanotubes, graphene, and carbon fibers.

[0042] A second aspect of the present invention provides a method for preparing the positive electrode sheet of the first aspect, comprising the following steps:

[0043] Step S1: Dry mix the positive electrode active material and the solid electrolyte to obtain a mixture.

[0044] Step S2: Divide the mixture obtained in step S1 into two parts, add a conductive agent and a first binder to one part of the mixture, and perform dry mixing to obtain the first mixture.

[0045] A conductive agent and a second binder are added to another mixture, and the mixture is then dry-mixed to obtain a second mixture.

[0046] Step S3: The first mixture and the second mixture obtained in step S2 are discharged sequentially and subjected to the first pressing process to obtain a composite film;

[0047] Step S4: The first mixture side of the composite membrane obtained in step S3 is attached to the surface of the positive electrode current collector, and then the second pressing process and the third pressing process are performed in sequence.

[0048] In some embodiments, step S1 further includes a plasma activation treatment step after dry mixing; the mixture obtained in step S1 is placed in an inert atmosphere and irradiated with plasma energy to generate an interface buffer layer on the surface of the positive electrode active material particles.

[0049] In some embodiments, the inert gas in the inert atmosphere may be selected from at least one of argon (Ar), helium (He), and nitrogen (N2).

[0050] In some embodiments, the inert gas in the inert atmosphere is selected from argon.

[0051] In some embodiments, the power of plasma activation treatment is 150W-250W, for example, it can be 150W, 170W, 190W, 210W, 230W, 250W, or any value within the range of any two of the above values; the irradiation time is 3min-10min, for example, it can be 3min, 4min, 5min, 6min, 7min, 8min, 9min, 10min, or any value within the range of any two of the above values. If the power is too low or the time is too short, the activation energy is insufficient, the Li3PO4 layer is too thin, and it cannot effectively isolate the positive electrode active material from the solid electrolyte, resulting in an increase in impedance due to side reactions; if the power is too high or the time is too long, the lattice distortion on the surface of the positive electrode active material particles will occur, the crystallinity of the Li3PO4 layer will decrease, the ion conduction resistance will increase, and the rate performance will degrade.

[0052] In some embodiments, the conditions for the first pressing process in step S3 include: a pressure of 5MPa-15MPa, for example, 5MPa, 7MPa, 9MPa, 11MPa, 13MPa, 15MPa, or any value within the range of any two of the above values; and a temperature of 20℃-40℃, for example, 20℃, 25℃, 30℃, 35℃, 40℃, or any value within the range of any two of the above values. During the first pressing process, preliminary molding is achieved under low temperature and low pressure, causing the first and second mixtures to bond into a film. If the pressure is too low (below 5MPa) or the temperature is too low (below 20℃), the interparticle bonding force is insufficient, the film is easily broken, and it cannot be peeled off; while if the pressure is too high (above 15MPa) or the temperature is too high (above 40℃), the binder softens and agglomerates prematurely, resulting in uneven film density, making subsequent optimization of the positive electrode structure impossible.

[0053] In some embodiments, the conditions for the second pressing process in step S4 include: a pressure of 15MPa-25MPa, for example, 15MPa, 17MPa, 19MPa, 21MPa, 23MPa, 25MPa, or any value within the range of any two of the above values; and a temperature of 60℃-80℃, for example, 60℃, 65℃, 70℃, 75℃, 80℃, or any value within the range of any two of the above values. During the second pressing process, the medium-temperature and high-pressure conditions activate the fluidity of binder A and binder B, filling the pores retained in the first pressing stage. Binder A fully fibrillates to form a continuous network, and binder B strengthens the bonding between particles through hydrogen bonds. If the temperature is too low (below 60℃) or the pressure is too low (below 15MPa), binder A does not soften, the formed fiber network is discontinuous, and the membrane's extensibility decreases. If the temperature is too high (above 80℃) or the pressure is too high (above 25MPa), binder A overheats and decomposes, binder B's crystallinity decreases, and the membrane hardens and cracks easily.

[0054] In some embodiments, the conditions for the third pressing process in step S4 include: a pressure of 20MPa-30MPa, for example, 20MPa, 22MPa, 24MPa, 26MPa, 28MPa, 30MPa, or any value within the range of any two of the above values; and a temperature of 15℃-30℃, for example, 15℃, 18℃, 20℃, 22℃, 24℃, 26℃, 28℃, 30℃, or any value within the range of any two of the above values. During the third pressing process, the fiber orientation of binder A and the second binder can be further adjusted to form an isotropic structure through molecular chain segment rearrangement, making the fiber structure more stable and uniform. If the temperature is too high (above 30℃) or the pressure is too low (below 20MPa): the fiber orientation loosens, the membrane becomes significantly anisotropic, and cracks along the orientation direction when bent; if the pressure is too high (above 30MPa) or the temperature is too low (below 15℃): the membrane density is too high, the ion conduction path narrows, and the impedance increase rebounds.

[0055] A third aspect of the present invention provides a solid-state battery, comprising a positive electrode sheet provided in the first aspect, or a positive electrode sheet obtained by the preparation method provided in the second aspect.

[0056] This invention utilizes plasma-activated positive electrode active material and a layered mixing process to form an interfacial buffer layer that synergistically suppresses side reactions with carboxyl fibers, significantly reducing the rate of increase in battery interfacial impedance. Through a three-stage pressing process—pre-pressing, hot-pressing for mesh formation, and cold-pressing for shaping—the tensile strength and toughness of the positive electrode sheet are significantly improved. The solid-state battery obtained by this invention can be applied to all-solid-state power batteries (EVs / drones), flexible wearable device batteries, and high-energy-density energy storage systems.

[0057] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0058] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0059] The present invention will now be described in detail with reference to specific embodiments, which are intended to understand rather than limit the invention.

[0060] Example 1

[0061] 1. Preparation of the positive electrode:

[0062] (1) The positive electrode active material (NCM9235; chemical formula LiNi) 0.92 Co 0.03 Mn 0.05 O2 (Dv50 is 6μm) and solid electrolyte (Li6PS5Cl) were dry-mixed and then plasma-activated (Ar atmosphere, power 200W, 5min) to obtain the mixture.

[0063] (2) Divide the mixture obtained in (1) into two parts, add a conductive agent (vapor-grown carbon fiber, abbreviated as VGCF) and a first binder to one part of the mixture, and then perform dry mixing to obtain the first mixture.

[0064] VGCF and a second binder (polyacrylic acid nanofibers, 100 nm in diameter and 350 nm in length) were added to another mixture, and the mixture was dry-mixed to obtain the second mixture.

[0065] (3) The first mixture and the second mixture obtained in step (2) are discharged sequentially and subjected to the first pressing treatment (10MPa, 30℃, 15s) to obtain a composite film.

[0066] (4) The first mixture of the composite membrane obtained in step (3) is attached to the surface of the positive electrode current collector, and a second pressing treatment (20MPa, 70℃, 15s) and a third pressing treatment (25MPa, 25℃, 15s) are performed in sequence to obtain a membrane with a total thickness of 320μm for the first positive electrode active material layer and the second positive electrode active material layer. The thickness ratio of the first positive electrode active material layer to the second positive electrode active material layer is 1:3.

[0067] The mass ratio of positive electrode active material: solid electrolyte: first binder: second binder: conductive agent is 85.5:10:2:0.5:2; the first binder is polytetrafluoroethylene (binder A) and polyvinylidene fluoride (binder B), and the mass ratio of the two is 3:1.

[0068] 2. Preparation of the negative electrode:

[0069] Nano Ag powder, nano carbon black, and polyvinylidene fluoride (PVDF) binder are dispersed and slurried in a double planetary mixer at a mass ratio of 72:24:4, with N-methylpyrrolidone (NMP) as the solvent. The slurry is then sprayed onto the surface of copper foil to form a sheet, which is then rolled and slit to obtain the negative electrode sheet of the target specification.

[0070] 3. Solid electrolyte film: Solid electrolyte Li6PS5Cl powder and PTFE powder are mixed at a mass ratio of 3:1, dispersed and mixed evenly at high speed, and then film-forming and rolled to obtain a solid electrolyte film with a thickness of 40μm.

[0071] 4. Battery assembly

[0072] After the negative electrode (welded tab), solid electrolyte film, and positive electrode (welded tab) are stacked in sequence, they are encapsulated with aluminum-plastic film and then compacted by an isostatic pressing device to achieve close contact between the electrode and the electrolyte film; low-current formation is performed, and constant current and constant voltage charging is carried out at 0.1C to 40% SOC.

[0073] Performance testing

[0074] i) Physical performance testing

[0075] (1) Tensile strength test:

[0076] 1. Sampling: Avoid the coating edge (>10mm from the edge) and take samples along the coating direction (MD) and transverse direction (TD) respectively (to assess anisotropy); the total length of the sample is ≥100mm, the gauge length is 25mm (width) × 80mm (length), and the clamping end is ≥15mm (widened for anti-slip).

[0077] 2. Sample loading: Set the fixture spacing to 50mm; the coating surface should face inwards, and the current collector should face outwards (to reduce fixture damage to the coating); pretension 0.1N (to eliminate wrinkles);

[0078] 3. Test conditions: Tensile speed: 10 mm / min (simulating actual winding speed); Termination condition: specimen fracture or strain > 100%;

[0079] 4. Calculation formula: Electrode tensile strength σ = F max / (W×T); Electrode tensile strength (F) max : The maximum tensile force that the specimen can withstand during the tensile process, i.e., the maximum load before fracture; W: width; T: thickness).

[0080] (2) Flexibility test: Cut the positive electrode sheet to 15mm in length and 10mm in width, fold the electrode sheet 180° in half, and fold it back and forth 3 times. If the result is that it is not transparent and does not break, it passes the flexibility test; otherwise, it fails.

[0081] (3) Peel strength test: The peel strength between the tape sample and the stainless steel plate was tested according to the method standard GB / T 2792-2014 "Test method for peel strength of adhesive tape".

[0082] ① Place the adhesive tape sample in an environment of 23±1℃ and 50±5%RH for 24 hours to condition it.

[0083] ② Wipe the surface of the stainless steel plate with acetone 4 times and let it air dry for 10 minutes.

[0084] ③ Remove the four layers of adhesive tape from the surface of the sample, and use a cutter to cut a sample with a width of 24 mm and a length of 300 mm from the remaining sample.

[0085] ④ Fold one end of the cut sample together with adhesive tape to form a folded layer approximately 12 mm long. Attach the other end of the sample to one end of the steel plate and roll it twice with an adhesive tape roller at a speed of 600 mm / min.

[0086] ⑤ Peel off about 25mm of adhesive tape from the steel plate at the folded end of the sample, and clamp the steel plate at this end and the free end of the sample in the upper and lower fixtures of the equipment, respectively.

[0087] ⑥ Set the test speed (300mm / min), sample width (24mm) and other parameters, click the test option, the test will start, the equipment will automatically record the force value during the peeling process, and report the peel strength of the sample accordingly.

[0088] ⑦ Repeat the procedures of sample cutting, preparation, and testing in ③-⑥, and test the peel strength values ​​of the three samples.

[0089] ii) Electrical performance test: At 25°C, with the voltage range set to 2.5V-4.25V, the prepared full cell was charged at a constant current rate of 0.33C to 4.25V, and then charged at a constant voltage rate until the current cutoff was 0.05C. After standing for 30 minutes, it was discharged at a constant current rate of 0.33C to 2.5V, and then discharged at a constant voltage rate until the current cutoff was 0.05C. After standing for 30 minutes, the above charging process was repeated. This process was used to test the capacity retention rate by performing 300 full charge and discharge cycles.

[0090] iii) Interface impedance test: Before and after cycling (steps are the same as ii) Electrical performance test steps) Place the positive electrode on the film resistance tester and test the film resistance (standard test method for film resistance).

[0091] The single-probe method uses a resistor meter fixed at one end and a moving terminal at the other end to test the sample resistance. The controllable-pressure single-probe device fixes one end to a controllable-pressure device and places the other end at the edge of the sample. At the same time, parameters such as test pressure and holding time are set on the MRMS software, and the test is started. The software automatically reads data such as electrode resistance.

[0092] iv) Particle size Dv50 test

[0093] The Dv50 of binder A and the particle size analysis of the Dv50 of binder A were tested using a laser particle size analyzer method.

[0094] The specific steps include:

[0095] I. Sample Pretreatment

[0096] Dispersion media: Water-soluble samples → ultrapure water; Oil-soluble samples → anhydrous ethanol / isopropanol;

[0097] Dispersant addition: Add 0.1% sodium hexametaphosphate (to prevent agglomeration);

[0098] Concentration control: transmittance 10%-20% (liquid appears slightly turbid upon visual inspection).

[0099] II. Instrument Preparation

[0100] Background calibration: Start the instrument → Place the pure dispersion medium → Perform background measurement (automatic background subtraction);

[0101] Parameter settings: Refractive index: Input sample / medium value (e.g., SiO2: 1.46, water: 1.33); Opacity range: 8%-15% (too low requires concentration, too high requires dilution).

[0102] III. Test Operation

[0103] Ultrasonic treatment for 30 seconds → Inject into the sample cell → Start the test → Repeat 3 times.

[0104] IV. Data Analysis

[0105] Output results: Dv10, Dv50, Dv90, distribution curves;

[0106] Validation: Three tests; Dv50 deviation <3%; distribution curve overlap >95%.

[0107] Examples 2, 3, and Comparative Examples 1-4 were performed in accordance with Example 1, with the main differences shown in Table 1. Specifically, in Example 2, the molecular weights of adhesive A and adhesive B were changed; in Example 3, the diameter and length of the second adhesive were changed.

[0108] Table 1

[0109]

[0110]

[0111] Comparative Examples 5-7 were carried out in accordance with Example 1, with the main differences shown in Table 2. The specific differences from Example 1 are as follows: Comparative Example 5 replaced PTFE in the original Example 1 with an equal amount of PVDF; Comparative Example 6 replaced PVDF in the original Example 1 with an equal amount of PI (polyimide); and Comparative Example 7 replaced PTFE in the original Example 1 with an equal amount of PVDF, and simultaneously replaced PVDF in the original Example 1 with an equal amount of PI.

[0112] Table 2

[0113] Comparative Example 5 Not approved 92.1％ 0.36 Comparative Example 6 Not approved 91.3％ 0.31 Comparative Example 7 Not approved 88.7％ 0.62

[0114] Examples 4 through 7 were performed in accordance with Example 1, with the main differences shown in Table 3. Specifically, Example 4 mainly changed the mass ratio of binder A to binder B; Example 5 mainly changed the mass ratio of the positive electrode active material to the solid electrolyte; Example 6 mainly changed the proportion of the first binder; and Example 7 mainly changed the proportion of the second binder.

[0115] Table 3

[0116]

[0117]

[0118] Examples 8-11 were performed in accordance with Example 1 to verify the compression processing parameters. The main differences are shown in Table 4.

[0119] Table 4

[0120]

[0121]

[0122] Example 12 was performed in accordance with Example 1 to verify the Dv50 particle size of adhesive A and adhesive B. The main differences are shown in Table 5.

[0123] Table 5

[0124]

[0125] As shown in Tables 1-5, the positive electrode sheet provided by the present invention, by layering a first positive electrode active material layer and a second positive electrode active material layer, utilizes a first binder (binder A and binder B) to form a flexible network and enhance the bonding strength, and combines the chemical bonding effect of the carboxyl polymer fiber in the second binder with the positive electrode active material, solves the problems of insufficient toughness and easy breakage of the positive electrode sheet and the increase of interfacial impedance. It achieves synergistic optimization of the mechanical properties and interfacial stability of the positive electrode sheet, and meets the requirements of subsequent processing (winding, cutting) and electrochemical performance.

[0126] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element. Furthermore, it should be noted that the scope of the methods and apparatuses in the embodiments of the present invention is not limited to performing functions in the order shown or discussed, but may also include performing functions substantially simultaneously or in the reverse order, depending on the functions involved. For example, the described methods may be performed in a different order than described, and various steps may be added, omitted, or combined. Additionally, features described with reference to certain examples may be combined in other examples.

[0127] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A positive electrode plate, characterized in that, The positive electrode sheet includes a positive current collector, a first positive active material layer disposed on at least one side surface of the positive current collector, and a second positive active material layer located on the side surface of the first positive active material layer away from the positive current collector. The first positive electrode active material layer includes a positive electrode active material, a solid electrolyte, a conductive agent, and a first binder; the first binder is a fluorinated polymer material, and the first binder includes binder A and binder B; binder A is a fluorinated polymer fiber with a molecular weight of 3 million to 6 million; the molecular weight of binder B is 800,000 to 1.2 million; The second positive electrode active material layer includes a positive electrode active material, a solid electrolyte, a conductive agent, and a second binder; the second binder includes polymer fibers containing carboxyl functional groups; based on the total mass of the first positive electrode active material layer, the mass percentage of the first binder is 2%-3%; based on the total mass of the second positive electrode active material layer, the mass percentage of the second binder is 0.3%-1%. The mass ratio of adhesive A to adhesive B is (1-5):1; The Dv50 of adhesive A is 100 μm-400 μm; the Dv50 of adhesive B is 80 μm-120 μm. The adhesive A comprises polytetrafluoroethylene; the adhesive B comprises polyvinylidene fluoride.

2. The positive electrode sheet according to claim 1, characterized in that, The polymer fiber containing carboxyl functional groups has a diameter of 50 nm-150 nm and a length of 200 nm-500 nm.

3. The positive electrode sheet according to claim 1, characterized in that, The positive electrode active material includes positive electrode active material particles and an interface buffer layer located on the surface of the positive electrode active material particles; the interface buffer layer includes lithium phosphate.

4. The positive electrode sheet according to any one of claims 1-3, characterized in that, Based on the total mass of the first positive electrode active material layer and the second positive electrode active material layer, the mass ratio of the positive electrode active material to the solid electrolyte is (7-10):

1.

5. The positive electrode sheet according to any one of claims 1-3, characterized in that, The thickness ratio of the first positive electrode active material layer to the second positive electrode active material layer is 1:(1-5). And / or, the total thickness of the first positive electrode active material layer and the second positive electrode active material layer is 300 μm-350 μm.

6. A method for preparing the positive electrode sheet according to any one of claims 1-5, characterized in that, Includes the following steps: Step S1: Dry mix the positive electrode active material and the solid electrolyte to obtain a mixture. Step S2: Divide the mixture obtained in step S1 into two parts, add a conductive agent and a first binder to one part of the mixture, and perform dry mixing to obtain the first mixture. A conductive agent and a second binder are added to another mixture, and the mixture is then dry-mixed to obtain a second mixture. Step S3: The first mixture and the second mixture obtained in step S2 are discharged sequentially and subjected to the first pressing process to obtain a composite film; Step S4: The first mixture side of the composite membrane obtained in step S3 is attached to the surface of the positive electrode current collector, and then the second pressing process and the third pressing process are performed in sequence.

7. The method according to claim 6, characterized in that, In step S1, after dry mixing, a plasma activation treatment step is also included; the mixture obtained in step S1 is placed in an inert atmosphere and irradiated with plasma energy to generate an interface buffer layer on the surface of the positive electrode active material particles.

8. The method according to claim 7, characterized in that, The power of plasma activation treatment is 150W-250W, and the irradiation time is 3 min-10 min.

9. The method according to claim 6, characterized in that, In step S3, the conditions for the first pressing process include: pressure of 5 MPa-15 MPa and temperature of 20℃-40℃.

10. The method according to claim 6, characterized in that, In step S4, the conditions for the second pressing process include: pressure of 15 MPa-25 MPa and temperature of 60℃-80℃.

11. The method according to claim 6, characterized in that, In step S4, the conditions for the third pressing process include: pressure of 20 MPa-30 MPa and temperature of 15℃-30℃.

12. A solid-state battery, characterized in that, This includes the positive electrode sheet according to any one of claims 1-5, or the positive electrode sheet obtained by the preparation method according to any one of claims 6-11.

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