Modified PVDF (Polyvinylidene Fluoride) binder for sulfide all-solid-state battery as well as preparation method and application of modified PVDF binder

Abstract

The invention discloses a modified PVDF (Polyvinylidene Fluoride) binder for a sulfide all-solid-state battery as well as a preparation method and application of the modified PVDF binder. The preparation method of the modified PVDF binder comprises the following steps: dissolving polyvinylidene fluoride, a polymerizable monomer and a chain transfer agent in N, N-dimethylformamide, and carrying out a modification reaction to obtain a block copolymer; precipitating and separating out in methanol, washing, and drying in vacuum to obtain the white powdery modified PVDF binder. The invention also discloses the modified PVDF binder prepared by the method, a composite positive electrode with a gradient structure, and an all-solid-state battery comprising the composite positive electrode. The prepared modified PVDF has good solubility in a non-polar solvent, and erosion of the polar solvent to sulfide electrolyte is avoided. A continuous gradient structure from an electron enrichment area to an ion enrichment area is constructed in the pole piece, the prepared composite positive pole piece has excellent electron and ion conduction at the same time, and the problems of interface transmission bottleneck and solvent compatibility in a sulfide system are effectively solved.

Description

Technical Field

[0001] This invention relates to the field of all-solid-state lithium battery technology, and more specifically, to a modified PVDF binder for sulfide all-solid-state batteries, its preparation method, and its application. Background Technology

[0002] In the industrialization process of sulfide all-solid-state batteries, the preparation of composite cathodes faces a series of key technical bottlenecks that urgently need to be addressed: (1) Chemical incompatibility between solvent systems and sulfide electrolytes. Traditional lithium-ion battery cathode slurries generally use strong polar solvents such as N-methylpyrrolidone (NMP) to dissolve polyvinylidene fluoride (PVDF) binders. However, these polar solvents will undergo violent chemical reactions with sulfide solid electrolytes, leading to electrolyte decomposition, the generation of hydrogen sulfide (H2S) gas, and irreversible decay of ionic conductivity, which seriously damages battery performance and safety. (2) Solubility barriers of traditional binders in non-polar solvents. To avoid solvent corrosion, non-polar solvents such as decane and toluene are required. However, commercial PVDF has extremely low solubility in these solvents and cannot form a uniform and stable slurry. The non-polar solvent-soluble binders such as polyisobutylene and polystyrene used in existing studies have solved the processability problem, but they generally have problems such as insufficient bonding strength and poor electrochemical stability (easy to oxidize and decompose under high voltage), which cannot meet the long-term cycling requirements of high-nickel ternary cathode materials. (3) The internal charge transport dynamics of composite cathodes are limited. Traditional slurry preparation process mixes active materials, solid electrolytes, conductive agents and binders at one time, resulting in random distribution of each component and tortuous ion and electron transport paths. The solid-solid contact between active materials and electrolytes is poor and the interface impedance is high; at the same time, the electronic conductive network and the ion conductive network interfere with each other, forming a transport bottleneck, resulting in severe battery polarization, poor rate performance and low utilization of active materials. (4) Insufficient electrode structure design and process optimization. Existing electrode forming processes focus more on macroscopic densification and lack active design and control of microscopic transport channels. During the drying process, the capillary force generated by solvent evaporation can easily cause binder migration and particle agglomeration, forming uneven pores and interfaces, which further deteriorates the electrochemical performance.

[0003] In summary, developing a specialized binder that combines non-polar solvent solubility, high bonding strength, and a wide electrochemical window, along with innovative electrode structure design and fabrication processes, to construct an efficient ion / electron dual continuous transport network, is key to overcoming the performance bottleneck of sulfide all-solid-state batteries and promoting their industrial application. Summary of the Invention

[0004] To address the aforementioned technical problems, the present invention aims to provide a modified PVDF binder for sulfide all-solid-state batteries, its preparation method, and its application. This comprehensive solution systematically addresses core challenges in the preparation of sulfide all-solid-state battery cathodes, including solvent corrosion, interfacial transport, and structural optimization. The solution includes: First, synthesizing a PVDF-b-polyolefin block copolymer as a novel binder using reversible addition-fragmentation chain transfer polymerization technology, enabling it to dissolve well in non-polar solvents and thus completely avoiding chemical corrosion of the sulfide electrolyte. Second, innovatively employing functional partitioning and sequential coating processes, a current collector-side slurry targeting high electronic conductivity and an electrolyte-side slurry targeting high ionic conductivity are prepared separately. By precisely controlling the coating sequence, a composite cathode structure with a component gradient is formed. This invention, through breakthroughs in both molecular design and process innovation, achieves optimization and decoupling of the charge transport path within the electrode, significantly improving the rate performance and cycle stability of the battery.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] In a first aspect, the present invention provides a method for preparing a modified PVDF binder for sulfide all-solid-state batteries, comprising the following steps: dissolving polyvinylidene fluoride (PVDF), polymerizable monomers and chain transfer agents in N,N-dimethylformamide (NMP) to carry out a modification reaction to obtain a block copolymer; precipitating the reaction product in methanol, and after washing and vacuum drying, obtaining a white powdery modified PVDF binder.

[0007] As some specific embodiments of the present invention, the polyvinylidene fluoride is selected from PVDF homopolymer or PVDF-HFP copolymer.

[0008] As some specific embodiments of the present invention, the weight-average molecular weight of the polyvinylidene fluoride is >600,000 g / mol or between 300,000 and 600,000 g / mol; And / or, the PVDF is selected from any one of non-functionalized PVDF, carboxyl-terminated (-COOH)-functionalized PVDF, and hydroxyl-terminated (-OH)-functionalized PVDF.

[0009] As some specific embodiments of the present invention, the polymerizable monomer is selected from at least one of styrene and its derivatives, propylene, 1-butene, butadiene, isoprene, methyl methacrylate, n-butyl acrylate, and trifluoroethyl methacrylate.

[0010] As some specific embodiments of the present invention, the chain transfer agent is selected from at least one of 2-cyano-2-propylbenzotrithiocarbonate, 4-cyano-4-(thiobenzoylthio)valerate, and dithiocarbamates. The chain transfer agent is a RAFT (reversible addition-fragmentation chain transfer) reagent.

[0011] As some specific embodiments of the present invention, the temperature of the modification reaction is 60-80°C; the time is 6-12 hours.

[0012] As some specific embodiments of the present invention, the mass ratio of polyvinylidene fluoride to polymerizable monomer is 1:0.2-1.5; the amount of chain transfer agent is 0.5%-5% of the mass of polymerizable monomer.

[0013] As some specific embodiments of the present invention, the detergent used for washing is methanol, and the number of washing cycles is 3-5. And / or, the vacuum drying temperature is 80-100℃ and the time is 12-18 hours.

[0014] Secondly, the present invention provides a modified PVDF binder for sulfide all-solid-state batteries, prepared by the preparation method described in any of the preceding claims. The modified PVDF binder is a block copolymer with an amphiphilic structure, comprising solubilizing blocks composed of polyvinylidene fluoride segments and polar functional blocks composed of polymer segments of the polymerizable monomers.

[0015] As some specific embodiments of the present invention, the modified PVDF binder has a solubility of more than 6 wt% in a non-polar solvent.

[0016] Thirdly, the present invention provides a method for preparing a composite cathode with a gradient structure, comprising the following steps: S1. Preparation of gradient functional slurry, including: Preparation of slurry A: The ternary cathode material, carbon nanotube conductive agent, sulfide electrolyte and the modified PVDF binder as described above are added to a non-polar solvent at a mass ratio of 70-80:10-15:5-10:3-5 and dispersed at high speed to obtain a uniform electron conduction layer slurry A. Preparation of slurry B: The ternary cathode material, sulfide electrolyte, conductive carbon black and the modified PVDF binder as described above are added to a non-polar solvent in a mass ratio of 60-70:25-35:2-5:3-5 and dispersed at high speed to obtain a uniform ion-conducting layer slurry B. S2. Sequential coating and gradient structure construction: Slurry A is evenly coated onto the aluminum foil current collector and preliminarily dried to form the bottom layer; slurry B is then coated onto the bottom layer and dried to form the top layer. S3. Electrode forming and post-processing: The product obtained in step S2 is subjected to hot pressing, followed by vacuum drying to obtain a composite cathode with a gradient structure.

[0017] As some specific embodiments of the present invention, in step S1, the ternary cathode material is selected from one or more of NCM111, NCM523, NCM622 and NCM811.

[0018] As some specific embodiments of the present invention, in step S1, the sulfide electrolyte is selected from Li6PS5Cl (LPSCl), Li 10 GeP2S 12 (LGPS), Li7P3S 11 One or more of Li3PS4 and Li2S-P2S5 glass ceramics.

[0019] As some specific embodiments of the present invention, in step S1, the nonpolar solvent is selected from one or more of n-hexane, decane, dodecane, mesitylene, toluene, and p-xylene.

[0020] As some specific embodiments of the present invention, in step S1, the high-speed dispersion speed is 1500-4000 rpm; the time is 1-3 hours.

[0021] As some specific embodiments of the present invention, in step S2, after coating slurry A, the initial drying temperature is 40-60°C; after coating slurry B, the drying temperature is 60-80°C.

[0022] As some specific embodiments of the present invention, in step S2, the thickness of the bottom layer is 40-50 μm; the thickness of the top layer is 40-50 μm.

[0023] The main difference between slurry A and B is the ratio control of conductive agent and ion-conducting agent in the composite cathode system, which has a synergistic effect on electron and ion transport near the cathode current collector and near the electrolyte layer.

[0024] The core function of slurry A (bottom layer, electron conduction layer) is to establish an efficient "electronic highway". This layer is closely attached to the aluminum foil current collector. Its formula (high proportion of carbon nanotube conductive agent and ternary cathode material) is designed to solve the electronic conduction bottleneck, build a high-speed electron network at the bottom layer, and reduce the electrode / current collector interface impedance. The core function of slurry B (surface layer, ion-conducting layer) is to construct a robust "ion transport hub." This layer is located on the outer side of the electrode and is in direct contact with the sulfide electrolyte membrane. Its formulation (high proportion of sulfide electrolyte) is designed to optimize ion transport and interfacial stability. It establishes high-speed ion channels in the surface layer, buffers and matches the interface, and alleviates cyclic stress.

[0025] As some specific embodiments of the present invention, in step S3, the temperature of the hot pressing treatment is 100-120℃; the pressure is 10-15MPa; and the time is 5-10 minutes.

[0026] As some specific embodiments of the present invention, in step S3, the temperature of the vacuum drying is 100-120°C; and the time is 4-8 hours.

[0027] As some specific embodiments of the present invention, in step S3, the thickness of the composite positive electrode sheet is 80-100 μm.

[0028] Fourthly, the present invention provides a composite cathode with a gradient structure, which is prepared by the preparation method described in any of the preceding claims.

[0029] Fifthly, the present invention provides a sulfide all-solid-state battery comprising a composite cathode with a gradient structure as described above.

[0030] As some specific embodiments of the present invention, the negative electrode active material in the sulfide all-solid-state battery is selected from one or more of micron-sized silicon, nano-sized silicon, graphite, and lithium-silicon alloy; the sulfide electrolyte is Li6PS5Cl.

[0031] Compared with the prior art, the present invention has the following beneficial effects: 1. Solved the contradiction between solvent corrosion and processability: By synthesizing PVDF-b-PS block copolymer binder, its solubility in non-polar solvents is increased to more than 6wt%, which avoids the chemical corrosion of sulfide electrolytes by polar solvents and ensures good processability and film-forming properties of the slurry.

[0032] 2. A highly efficient charge transport network was constructed, significantly optimizing the electrochemical kinetics within the electrode. Through functionally partitioned slurry design and sequential coating processes, a clear functional gradient was created within the electrode: a high-speed electron transport layer, primarily composed of a three-dimensional carbon nanotube network, was formed near the current collector, ensuring low-resistance and rapid electron transport from the current collector to the active material; a highly efficient ion conduction layer, primarily composed of a sulfide electrolyte continuous phase, was formed near the electrolyte, providing a smooth path for lithium ion migration between the active material and the solid electrolyte interface. A smooth transition between the two layers is achieved through a continuous gradient of components, effectively avoiding the congestion caused by the intersection and competition of electron / ion transport paths in traditional uniform electrodes.

[0033] 3. Significantly improves the overall performance of the battery: The sulfide all-solid-state battery assembled with the gradient structure cathode prepared by this invention retains more than 99.9% of its capacity after 20 cycles at 0.5C rate. At the same time, the electrode still maintains a suitable porosity of 6% after hot pressing, which balances compactness and ion transport efficiency.

[0034] 4. Possesses promising industrialization prospects: The preparation method is compatible with traditional coating equipment, requiring only adjustments to the slurry formulation and coating program, without the need to modify core production equipment; the non-polar solvent used is recyclable, and the synthesis route of the modified PVDF binder is mature and cost-controllable, providing a practical and feasible technical solution for the large-scale manufacturing of sulfide all-solid-state batteries.

[0035] 5. This invention is the first to propose the synthesis of PVDF-based binders with well-defined amphiphilic block structures via active / controlled free radical polymerization (Reversible Addition-Fragmentation Chain Transfer RAFT). PVDF serves as the macromolecular chain transfer agent, acting as the "seed" and first segment of the entire polymerization reaction. Under the action of the RAFT reagent, polymerizable monomers are initiated, and their polymer chains grow linearly from the ends of the PVDF chains. The final product is a diblock structure consisting of PVDF and PS segments linearly connected in series. This solves the problem of unstable solid-solid interfaces in the three phases (high-nickel ternary cathode material / sulfide electrolyte / binder) of sulfide-based all-solid-state batteries.

[0036] (1) Innovations were made in structural design: from "random grafting / blending" to "controllable block".

[0037] Existing technologies typically employ traditional free radical copolymerization to modify PVDF, forming structures with other side chains grafted onto the PVDF backbone. This method produces products with a wide molecular weight distribution and random chain segment sequences, aiming to improve the solubility or general adhesion of PVDF in common polar solvents (such as NMP). This invention explicitly uses RAFT polymerization to synthesize a defined block copolymer such as PVDF-HFP-b-PS (polyvinylidene fluoride-hexafluoropropylene)-block-polystyrene. The PVDF-HFP segment acts as the anchoring segment, utilizing the chemical inertness and certain electrochemical oxidation resistance of the fluorinated segment to achieve compatibility with non-polar solvents and provide the basic framework. The PS segment acts as the functional segment; its benzene ring structure can generate strong π-π conjugation or dipole-dipole interactions with sulfur and phosphorus atoms on the surface of sulfide electrolyte particles and transition metal oxides on the surface of ternary materials, thereby achieving specific and strong adsorption. By directionally constructing block structures with defined interfacial adsorption functions, specific interfacial problems are addressed.

[0038] (2) This invention explicitly uses the modified PVDF binder for the cathode of a sulfide all-solid-state battery. The extreme demands of this scenario are: a) the binder must be processed in a non-polar solvent; b) it must simultaneously and firmly bond the high surface energy ternary material and the highly reactive sulfide electrolyte; c) it must maintain interfacial stability during long-term electrochemical cycling. The block structure design of this invention can act like a "molecular rivet," with one end anchored in the binder substrate / solvent and the other end chemically "nailed" to the surface of the active particles. Attached Figure Description

[0039] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 The charge-discharge curves of the sulfide all-solid-state battery prepared in Example 1 at a rate of 0.1C are shown. Figure 2 Charge-discharge curves of the sulfide all-solid-state battery prepared for Comparative Example 2 at a rate of 0.1C; Figure 3 Charge-discharge curves of the sulfide all-solid-state battery prepared for Comparative Example 3 at a rate of 0.1C; Figure 4 The charge-discharge curves of the sulfide all-solid-state battery prepared for Comparative Example 6 at a rate of 0.1C. Detailed Implementation

[0040] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0041] Example 1 A method for preparing a modified PVDF binder and gradient cathode for sulfide all-solid-state batteries, comprising the following steps: (1) Synthesis of modified PVDF binder The selected copolymer is a carboxyl-terminated PVDF-HFP copolymer with a weight-average molecular weight (Mw) of approximately 450,000 g / mol, used in an amount of 10.0 g. The polymerizable monomer is styrene, used in an amount of 7.5 g (PVDF to polymerizable monomer mass ratio of 1:0.75). The chain transfer agent is 2-cyano-2-propylbenzotrithiocarbonate, used in an amount of 0.15 g (2% of the styrene monomer mass). The solvent is N,N-dimethylformamide (DMF), used in an amount of 200 g (approximately 11.4 times the total solid mass). The reaction process involved adding the above-mentioned raw materials to a three-necked flask equipped with a stirrer and reflux condenser. Under a nitrogen atmosphere, the mixture was heated to 75°C and stirred continuously at this temperature for 10 hours. After the reaction was complete, the reaction solution was slowly poured into a large amount of methanol to precipitate the product, which was then collected as a white solid by filtration. The product was subsequently washed four times with methanol to thoroughly remove unreacted monomers, chain transfer agents, and solvents. Finally, the product was dried in a vacuum oven at 90°C for 15 hours to obtain a white powdery modified PVDF binder (denoted as PVDF-HFP-b-PS).

[0042] (2) Preparation of graded functional slurry Preparation of slurry A (electron conduction layer): The formula mass ratio is NCM811 cathode material: carbon nanotubes (CNT): Li6PS5Cl electrolyte: modified PVDF binder = 75:12:8:5; The specific operation is to take 75g of NCM811 powder, 12g of CNT, 8g of Li6PS5Cl powder, and 5g of the modified PVDF binder obtained in step (1) and add them to 200g of decane solvent. Using a high-speed planetary mixer, premix at 2000 rpm for 15 minutes, and then disperse at 3200 rpm for 60 minutes. During the dispersion process, the slurry temperature is controlled at 30±5℃ by water bath, and finally a uniform and viscous slurry A is obtained.

[0043] Preparation of Slurry B (ion-conducting layer): The formula mass ratio is NCM811 cathode material: Li6PS5Cl electrolyte: conductive carbon black (Super P): modified PVDF binder = 65:28:3:4; Specifically, take 65g of NCM811 powder, 28g of Li6PS5Cl powder, 3g of Super P, and 4g of modified PVDF binder, and add them to 200g of decane solvent. Disperse using the same equipment and procedure as slurry A to obtain slurry B.

[0044] (3) Sequential coating and gradient structure construction Underlayer coating (slurry A): Using a precision coating machine, slurry A is uniformly coated onto an aluminum foil current collector with a thickness of 15 μm, and the wet film thickness is controlled to be 110 μm. The coated electrode is then transferred to a 50°C forced-air drying oven for initial drying for 30 minutes to allow most of the solvent to evaporate, forming an underlayer with a thickness of approximately 45 μm. Topcoat (Slurry B): On the dried substrate surface, slurry B is applied again using the same coating machine, with the wet film thickness controlled at 110 μm. The electrode is then transferred to a 70°C forced-air drying oven and dried for 40 minutes to form a topcoat with a thickness of approximately 45 μm.

[0045] (4) Electrode forming and post-processing Hot pressing: The dried double-layer electrode sheets are cut to the specified size and placed in a flatbed hot press. They are then hot-pressed for 8 minutes at 110℃ and 12MPa. Vacuum drying: The hot-pressed electrode sheet is immediately transferred to a vacuum oven and dried at 110℃ and a vacuum degree below -0.095MPa for 6 hours to completely remove any trace amounts of solvent and moisture that may remain; a gradient structure composite positive electrode sheet with a total thickness of about 90±5μm, a smooth surface, and a dense structure is obtained.

[0046] Example 2 A method for preparing a modified PVDF binder and gradient cathode for sulfide all-solid-state batteries, comprising the following steps: (1) Synthesis of modified PVDF binder A low-to-medium degree PVDF homopolymer with a weight-average molecular weight (Mw) of approximately 350,000 g / mol was selected, and the dosage was 10.0 g. The polymerizable monomer was n-butyl acrylate (BA), and the dosage was 5.0 g (i.e., the mass ratio of PVDF to monomer was 1:0.5). The chain transfer agent was 4-cyano-4-(thiobenzoylthio)valerate, and the dosage was 0.10 g (accounting for 2% of the BA monomer mass). The solvent was N,N-dimethylformamide (DMF), and the dosage was 180 g (approximately 12 times the total solid mass). Reaction process: The above raw materials were added to the reactor, heated to 68°C under nitrogen protection, and stirred continuously at this temperature for 8 hours. After the reaction was completed, the reaction solution was poured into excess methanol to precipitate, and filtered to obtain a solid product. The product was washed three times with methanol and then dried in a vacuum oven at 85°C for 14 hours to obtain a white powdery modified PVDF binder (denoted as PVDF-b-PBA).

[0047] (2) Preparation of graded functional slurry Preparation of slurry A (electron conduction layer): The formula mass ratio is NCM622. Cathode material: Carbon nanotubes (CNTs): Li7P3S 11 Electrolyte: Modified PVDF binder = 78:11:6:5; Take 78 g of NCM622 powder, 11 g of CNT, and Li7P3S. 11 6g of powder and 5g of modified PVDF binder were added to 200g of octane solvent; the mixture was continuously dispersed at 2500 rpm for 90 minutes using a high-shear dispersing emulsifier, while controlling the slurry temperature to ≤ 35℃ during the dispersion process, to obtain a uniform slurry A. Preparation of slurry B (ion-conducting layer): The formula mass ratio is NCM622 cathode material: Li7P3S 11 Electrolyte: Conductive carbon black (Super P): Modified PVDF binder = 65:28:3:4; Take 65 g of NCM622 powder, Li7P3S 11 28 g of powder, 3 g of SuperP, and 4 g of modified PVDF binder were added to 200 g of octane solvent; the mixture was dispersed at 2800 rpm for 75 minutes using the same equipment to obtain slurry B.

[0048] (3) Sequential coating and gradient structure construction Underlayer coating (slurry A): Slurry A is coated onto the aluminum foil current collector, with the wet film thickness controlled at 105 μm. It is then continuously dried in a drying tunnel at 45°C for 25 minutes to form an underlayer with a thickness of approximately 42 μm. Topcoat (Slurry B): Slurry B is applied to the substrate surface, with a wet film thickness controlled at 105 μm. It is then dried in a drying tunnel at 65°C for 35 minutes to form a topcoat with a thickness of approximately 43 μm.

[0049] (4) Electrode forming and post-processing Hot pressing treatment: Place the dried double-layer electrode sheet in a hot press and hot press it for 6 minutes at 105℃ and 11 MPa. Vacuum drying: The hot-pressed electrode sheet is placed in a vacuum oven and dried at 105℃ for 5 hours; a gradient structure composite positive electrode sheet with a total thickness of about 88±4 μm is obtained.

[0050] Comparative Example 1 In this comparative example, step (1) in Example 1 was omitted, and the PVDF binder was not modified and was directly used in the slurry system. However, since the unmodified PVDF cannot be completely dissolved in non-polar solvents such as decane and octane, the preparation of the slurry system in step (2) could not be completed, and the positive electrode sheet could not be prepared.

[0051] Comparative Example 2 In this comparative example, the preparation of electron conduction layer slurry A in step (2) of Example 1 was omitted. Only slurry B was prepared and used for coating. The coating thickness was controlled at about 90 μm. All other parameters remained unchanged and were prepared according to the steps of Example 1.

[0052] Comparative Example 3 In this comparative example, the preparation of ion-conducting layer slurry B in step (2) of Example 1 is omitted. Only slurry A is prepared and used for coating. The coating thickness is controlled at about 90 μm. All other parameters remain unchanged and are prepared according to the steps of Example 1.

[0053] Comparative Example 4 In this comparative example, the preparation of ion-conducting layer slurry B in step (2) of Example 1 is omitted. When preparing electron-conducting layer slurry A, 12% of carbon nanotubes are replaced with 6% carbon nanotubes and 6% conductive carbon black, while the remaining parameters remain unchanged.

[0054] Comparative Example 5 In this comparative example, step (1) of Example 2 was omitted, the PVDF binder was not modified, and polybutyl acrylate was physically blended with unmodified PVDF, which was then used in subsequent experiments. This was to demonstrate the irreplaceable nature of chemically bonded block structures relative to physical mixing. The results showed that the blend was prone to phase separation in the solvent, the slurry was unstable, and normal electrode sheets could not be prepared.

[0055] Comparative Example 6 In this comparative example, the use of the chain transfer agent 2-cyano-2-propylbenzotrithiocarbonate RAFT reagent in step (1) of Example 1 was omitted, and PVDF-HFP-r-PS (random copolymer) was synthesized by conventional free radical polymerization (using conventional chain transfer agent 2-mercaptoethanol). The remaining steps remained unchanged and were prepared according to the steps of Example 1.

[0056] Effect Example The following tests were performed on each embodiment and comparative example: 1. The slurry state and coating film state of the examples and comparative examples were compared, and the results are shown in Table 1.

[0057] Table 1. Slurry State and Film Formation State

[0058] 2. Ionic conductivity test: The prepared positive electrode sheet was cut into small circular pieces with a diameter of 10 mm, and then loaded into a pressure battery mold. It was pressed under a pressure of 1 ton and held for 1 minute. Then, 100 mg of sulfide electrolyte powder was assembled on the upper and lower surfaces of the positive electrode sheet, and pressed under a pressure of 1 ton and held for 1 minute. The AC impedance was tested using an electrochemical workstation of model CHI660E at room temperature and in a normal atmospheric atmosphere, and the ionic conductivity was calculated. The results are shown in Table 2.

[0059] Table 2 Ionic Conductivity Test Table

[0060] 3. Porosity, peeling and compaction density tests (1) Porosity test of the positive electrode: A high-performance fully automatic mercury porosimeter, model MicromeriticsAutoPore V 9620, was used in the experiment. The pressure applied in the low-pressure station (LP) was about 0.6 to 50 PSI, and the pressure applied in the high-pressure station (HP) was 20 to 60,000 PSI. A cone probe made of glass was used as the sample container to apply pressure to the sample. The sample size was 1 cm × 1 cm. The porosity of the positive electrode was calculated based on the volume of mercury entering the pores.

[0061] (2) 180° peel test of positive electrode: A universal tensile testing machine was used. The sample size was 1cm × 10cm. A flat, thin steel plate was taken, and a strip of double-sided tape was first attached to the center of the steel plate. The tape was then smoothed to ensure that the double-sided tape was tightly attached to the center of the steel plate. The double-sided tape was peeled off, and the electrode was attached to the tape. It was necessary to ensure that the electrode and the tape were properly matched and attached. The steel plate with the attached electrode was inserted into the lower clamp of the tensile testing machine and fixed vertically. The electrode without tape was inserted into the upper clamp and fixed so that the electrode attached to the tape was at a 180° angle to the electrode fixed in the upper clamp. After the test sample was fixed, the peeling speed was 0.2mm / s, and the test began.

[0062] (3) Compacted density test: All operations must be performed in a glove box filled with inert gas (such as argon) (H2O, O2 < 0.1 ppm). The electrode sheet is compacted at 100 MPa and 25°C. The prepared positive electrode sheet is cut into small circular pieces with a diameter of 10 mm. The mass (m) of the sample is weighed using an electronic balance, and the data is recorded. The thickness is measured at at least 5 different points randomly selected on the sample using calipers or a micrometer, and the average thickness is calculated. The compaction density (g / cm³) is calculated by dividing the mass by the volume. 3 ).

[0063] The test results of porosity, peel strength and compaction density of the positive electrode sheet are shown in Table 3.

[0064] Table 3. Test results of porosity, peel strength, and compaction density.

[0065] 4. Full battery performance test: 100 mg of Li6PS5Cl sulfide electrolyte was weighed and placed in a pressure mold, and a pressure of 1 ton was applied and held for 1 minute. Then, a small circular positive electrode sheet with a diameter of 10 mm was placed on one side of the electrolyte. After assembly, a pressure of 1 ton was applied and held for 1 minute. Next, 30 mg of negative electrode active material (micron-sized silicon D50 = 5 μm) was weighed and spread evenly on the other side of the electrolyte. Copper foil was used as the negative electrode current collector. After assembly, a pressure of 1 ton was applied and held for 1 minute. Finally, a sulfide all-solid-state lithium-ion battery was obtained for testing. The Xinwei Battery Testing System (model CT-4000) was used to conduct charge-discharge tests on the all-solid-state battery. The charge-discharge rate was 0.2C-0.5C, the voltage range was 4.3-2.5V, and the temperature was 28℃. The charge-discharge capacity of the sulfide all-solid-state battery was tested, and the test results are shown in Table 4.

[0066] Table 4. Results of Charge / Discharge Capacity Test

[0067] Figure 1-4 The figures show the charge-discharge curves of the sulfide all-solid-state batteries prepared in Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 6 at a rate of 0.1C.

[0068] As can be seen from the above test results, the test impedance in Example 1 was 6.76Ω, the ionic conductivity was 3.41mS / cm, the porosity of the positive electrode sheet was 6.5%, and the compaction density was 3.2g / cm³. 3 The peel strength to the aluminum foil substrate is 3.5 N / m. The battery performance is as follows: discharge specific capacity at 0.1C is 213.5 mAh / g, first-cycle coulombic efficiency is 88.7%, discharge specific capacity at 0.5C is 193.5 mAh / g, discharge specific capacity after 20 cycles is 193.3 mAh / g, and capacity retention is 99.9%.

[0069] Comparative Example 2 showed normal slurry and film formation conditions, with a test impedance of 6.20 Ω, ionic conductivity of 3.64 mS / cm, a porosity of 7.4% for the positive electrode, and a compaction density of 2.8 g / cm³. 3 The peel strength to the aluminum foil substrate is 3.0 N / m. The battery performance is as follows: discharge specific capacity at 0.1C is 197.6 mAh / g, first-cycle coulombic efficiency is 83.2%, discharge specific capacity at 0.5C is 175.3 mAh / g, discharge specific capacity after 20 cycles is 167.5 mAh / g, and capacity retention is 95.5%.

[0070] Because Comparative Example 2 only has an ion-conducting layer B, the overall content of sulfide electrolyte is relatively high, so its ion conductivity is slightly higher than that of Example 1. However, it lacks an electronic conductive layer, which means that electrons cannot be effectively transported to the interior or deep within the active material particles encapsulated by the solid electrolyte. This results in extremely low utilization of the active material, severely insufficient capacity, severe polarization, extremely poor rate performance, uneven reaction, accelerated local degradation and failure, continuous increase in interfacial impedance, and shortened cycle life.

[0071] In Comparative Example 3, the slurry and film formation were normal, the test impedance was 9.37 Ω, the ionic conductivity was 1.46 mS / cm, the porosity of the positive electrode was 7.8%, and the compaction density was 2.6 g / cm³. 3 The peel strength from the aluminum foil substrate is 2.9 N / m. The battery performance is as follows: discharge specific capacity at 0.1C is 187.5 mAh / g, first-cycle coulombic efficiency is 81.4%, discharge specific capacity at 0.5C is 170.4 mAh / g, discharge specific capacity after 20 cycles is 161.6 mAh / g, and capacity retention is 94.8%. Because only the electronically conductive layer A exists in Comparative Example 3, the ionic conductivity of the electrode is significantly lower. This leads to discontinuities or excessive resistance in the lithium-ion transport network, causing lithium-ion diffusion in the solid medium to be much slower than that of electrons, which is a rate-controlling step. Impaired ion channels can directly "suffocate" the reaction, resulting in sluggish reaction kinetics, severely limited rate performance, low bulk utilization of the active material, inability to fully utilize capacity, uneven current distribution, and ultimately, localized overload and failure.

[0072] In Comparative Example 4, the slurry and film formation were normal, the test impedance was 9.45Ω, the ionic conductivity was 1.42 mS / cm, the porosity of the positive electrode was 7.7%, and the compaction density was 2.6 g / cm³. 3 The peel strength from the aluminum foil substrate was 2.9 N / m. The battery performance was as follows: discharge specific capacity at 0.1C was 186.9 mAh / g, first-cycle coulombic efficiency was 81.6%, discharge specific capacity at 0.5C was 170.2 mAh / g, discharge specific capacity after 20 cycles was 161.3 mAh / g, and capacity retention was 94.7%. In Comparative Example 4, the total content of conductive agent was maintained at the same level as in Comparative Example 3, and two types of conductive agents were used. The overall performance was similar to that of Comparative Example 3, with no significant difference. However, it had the same problem as Comparative Example 3, resulting in discontinuity or excessive resistance in the lithium-ion transport network, and the overall performance was worse than that of Example 1.

[0073] In Comparative Example 6, the slurry had low fluidity and could form a film, but the overall quality of the film was lower than that of Example 1. The test impedance was 10.32 Ω, the ionic conductivity was 1.13 mS / cm, the porosity of the positive electrode was 7.9%, and the compaction density was 2.5 g / cm³. 3 The peel strength to the aluminum foil substrate is 2.6 N / m. The battery performance is as follows: discharge specific capacity at 0.1C is 184.6 mAh / g, first-cycle coulombic efficiency is 80.5%, discharge specific capacity at 0.5C is 168.7 mAh / g, discharge specific capacity after 20 cycles is 156.8 mAh / g, and capacity retention is 92.9%. In Comparative Example 6, due to the random distribution of styrene units, long-range, coherent PS functional blocks could not be formed, resulting in weak interface-specific adsorption capacity. Although solubility could be improved, the peel strength and full-cell cycle life of the prepared electrode were significantly lower than those in Example 1.

[0074] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing a modified PVDF binder for sulfide all-solid-state batteries, characterized in that, The process includes the following steps: dissolving polyvinylidene fluoride, polymerizable monomers, and chain transfer agents in N,N-dimethylformamide to carry out a modification reaction to obtain a block copolymer; precipitating the reaction product in methanol, washing it, and vacuum drying it to obtain a modified PVDF binder; The polyvinylidene fluoride is selected from PVDF homopolymer or PVDF-HFP copolymer; The polymerizable monomer is selected from at least one of styrene and its derivatives, propylene, 1-butene, butadiene, isoprene, methyl methacrylate, n-butyl acrylate, and trifluoroethyl methacrylate. The chain transfer agent is selected from at least one of 2-cyano-2-propylbenzotrithiocarbonate, 4-cyano-4-(thiobenzoylthio)valerate, and dithiocarbamates.

2. The preparation method according to claim 1, characterized in that, The modification reaction is carried out at a temperature of 60-80℃ for 6-12 hours. And / or, the mass ratio of the polyvinylidene fluoride to the polymerizable monomer is 1:0.2-1.5; the amount of the chain transfer agent is 0.5%-5% of the mass of the polymerizable monomer; And / or, the detergent used for the washing is methanol, and the washing is performed 3-5 times; And / or, the vacuum drying temperature is 80-100℃ and the time is 12-18 hours.

3. A modified PVDF binder for sulfide all-solid-state batteries, characterized in that, The modified PVDF binder is prepared by the preparation method described in claim 1 or 2. It is a block copolymer with an amphiphilic structure, comprising solubilizing blocks composed of polyvinylidene fluoride segments and polar functional blocks composed of polymer segments of the polymerizable monomer.

4. A method for preparing a composite cathode with a gradient structure, characterized in that, Includes the following steps: S1. Preparation of gradient functional slurry, including: Preparation of slurry A: The ternary cathode material, carbon nanotube conductive agent, sulfide electrolyte and the modified PVDF binder as described in claim 3 are added to a non-polar solvent at a mass ratio of 70-80:10-15:5-10:3-5 and dispersed at high speed to obtain a uniform electron conduction layer slurry A; Preparation of slurry B: The ternary cathode material, sulfide electrolyte, conductive carbon black and the modified PVDF binder as described in claim 3 are added to a non-polar solvent in a mass ratio of 60-70:25-35:2-5:3-5 and dispersed at high speed to obtain a uniform ion-conducting layer slurry B. S2. Sequential coating and gradient structure construction: Slurry A is evenly coated onto the aluminum foil current collector and preliminarily dried to form the bottom layer; slurry B is then coated onto the bottom layer and dried to form the top layer. S3. Electrode forming and post-processing: The product obtained in step S2 is subjected to hot pressing, followed by vacuum drying to obtain a composite cathode with a gradient structure.

5. The preparation method according to claim 4, characterized in that, In step S1, the ternary cathode material is selected from one or more of NCM111, NCM523, NCM622 and NCM811; And / or, the sulfide electrolyte is selected from Li6PS5Cl, Li 10 GeP2S 12 Li7P3S 11 One or more of the following: Li3PS4, Li2S-P2S5 glass-ceramics; And / or, the nonpolar solvent is selected from one or more of n-hexane, decane, dodecane, mesitylene, toluene, and p-xylene; And / or, the high-speed dispersion is carried out at a rotation speed of 1500-4000 rpm for 1-3 hours.

6. The preparation method according to claim 4, characterized in that, In step S2, after applying slurry A, the initial drying temperature is 40-60℃; after applying slurry B, the drying temperature is 60-80℃. And / or, the thickness of the bottom layer is 40-50 μm; the thickness of the top layer is 40-50 μm.

7. The preparation method according to claim 4, characterized in that, In step S3, the hot pressing treatment is performed at a temperature of 100-120℃, a pressure of 10-15MPa, and a time of 5-10 minutes. And / or, the vacuum drying temperature is 100-120°C; the time is 4-8 hours; And / or, the thickness of the composite positive electrode is 80-100 μm.

8. A composite positive electrode with a gradient structure, characterized in that, It is prepared by the preparation method described in any one of claims 4-7.

9. A sulfide all-solid-state battery, characterized in that, It includes a composite cathode with a gradient structure as described in claim 8.

10. The sulfide all-solid-state battery according to claim 9, characterized in that, The negative electrode active material in the sulfide all-solid-state battery is selected from one or more of micron-sized silicon, nano-sized silicon, graphite, and lithium-silicon alloy; the sulfide electrolyte is Li6PS5Cl.

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