A preparation method of an interface-enhanced positive electrode for a sulfide all-solid-state battery

Abstract

This invention relates to a method for preparing an interface-enhanced cathode for sulfide all-solid-state batteries, comprising the following steps: reacting a PVDF prepolymer containing active hydrogen end groups with a crosslinking agent containing dynamic covalent bonds by heating to obtain a modified PVDF binder; then mixing this binder with cathode material, solid electrolyte, conductive agent, and coupling agent to obtain a composite cathode slurry; coating this slurry onto a current collector; simultaneously drying it using microwave and infrared irradiation; and finally hot-pressing it to obtain the interface-enhanced cathode. This method first significantly improves the solubility of the end-functionalized PVDF in non-polar solvents and endows the binder with self-healing ability by dynamically covalently crosslinking it; subsequently, introducing a coupling agent into the composite cathode slurry greatly enhances the solid-solid interface bonding strength through chemical bonding; finally, a synergistic drying process using microwave and infrared radiation is employed to achieve rapid and uniform drying of the electrode.

Description

Technical Field

[0001] This invention belongs to the field of all-solid-state lithium battery technology, specifically relating to a method for preparing an interface-enhanced cathode for sulfide all-solid-state batteries, and more particularly to a method for preparing a high-performance sulfide all-solid-state battery composite cathode through dynamic covalent bond modification of binders, interface coupling agent enhancement, and efficient synergistic drying process. Background Technology

[0002] In the process of moving from the laboratory to large-scale manufacturing of sulfide all-solid-state batteries, there are three interconnected and critical technical bottlenecks in the preparation of their composite cathodes that urgently need to be addressed:

[0003] (1) The compatibility problem between traditional binders and solvent systems. The preparation of composite cathodes requires the uniform mixing of active materials, solid electrolytes, conductive agents and binders. To avoid irreversible chemical corrosion of sulfide electrolytes by polar solvents (such as N-methylpyrrolidone, NMP), non-polar solvents such as decane and toluene must be used. However, the most commonly used and stable polyvinylidene fluoride (PVDF) binder in the industry has extremely low solubility in such solvents and cannot form a stable and uniform slurry. Although non-polar solvent-soluble alternative binders such as polyisobutylene and polystyrene can be used, these materials generally have problems such as narrow electrochemical windows (easily decomposed at high potentials), weak adhesion or poor thermal stability, making it difficult to meet the long-term cycling requirements of high-voltage ternary cathode materials.

[0004] (2) Risk of mechanical failure due to insufficient solid-solid interface bonding strength. During long-term charge-discharge cycles, electrode materials (especially high-nickel ternary materials) undergo continuous lattice volume changes. The resulting periodic stress easily leads to microcracks, contact loss, or even complete peeling at the fragile solid-solid interface, thereby interrupting the electron and ion transport network, causing a sharp increase in battery internal resistance and rapid capacity decay. Existing technologies mostly focus on increasing the compaction density of the electrode to increase contact, but this sacrifices ion transport channels and cannot fundamentally solve the problem of chemical strength of interfacial bonding.

[0005] (3) Low efficiency and difficulty in uniformity control of electrode drying process. Unlike traditional liquid battery slurry, sulfide cathode slurry based on non-polar solvents has the characteristics of high solid content, high viscosity and slow solvent evaporation rate. Existing hot air or vacuum drying processes have the disadvantages of high energy consumption, long drying time and slow heat conduction from the surface to the inside. This not only leads to low production efficiency and increased production cost, but more seriously, the slow and uneven drying process will cause the migration and redistribution of binder and fine particles due to the movement of the solvent evaporation front, resulting in component segregation, density gradient and stress concentration inside the electrode. The final electrode is often dense on the surface and loose on the inside, with tortuous and uneven ion and electron transport paths, which seriously restricts the overall performance of the battery.

[0006] In summary, existing technologies often attempt to solve one of the aforementioned problems in isolation, lacking a systematic and synergistic solution. The industrialization of sulfide all-solid-state batteries urgently requires a full-chain innovation that can simultaneously overcome the three major bottlenecks of compatibility, stability, and manufacturing efficiency, from binder molecule design and interface enhancement to efficient molding processes. Summary of the Invention

[0007] The purpose of this invention is to address the problems existing in the prior art by providing a method for preparing an interface-enhanced cathode for sulfide all-solid-state batteries, which is a technical solution that systematically solves the bottleneck in the preparation of cathodes for sulfide all-solid-state batteries.

[0008] This invention significantly improves the solubility of PVDF in non-polar solvents and endows the binder with self-healing capabilities by dynamically modifying PVDF through covalent crosslinking (such as disulfide bonds). Secondly, by adding interfacial coupling agents such as silanes to the composite cathode slurry, strong chemical bonds are formed between the active material, electrolyte, and binder, significantly enhancing interfacial adhesion strength. Finally, a combined microwave and infrared radiation drying process is employed, utilizing the complementary advantages of microwave bulk heating and infrared surface heating to achieve rapid and uniform drying of the electrode. The synergistic application of these three innovative technologies fundamentally solves the three core problems of poor solvent compatibility, easy interface peeling, and low drying efficiency in traditional processes.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] In a first aspect, the present invention provides a method for preparing an interface-enhanced cathode for a sulfide all-solid-state battery, comprising the following steps:

[0011] S1. Dissolve PVDF prepolymer and crosslinking agent in a solvent and heat to react to obtain modified PVDF binder. The end groups of PVDF prepolymer are end groups containing active hydrogen, and the crosslinking agent is a crosslinking agent containing dynamic covalent bonds.

[0012] S2. Add the positive electrode material, sulfide solid electrolyte, conductive agent, and modified PVDF binder to a non-polar solvent. After initial dispersion, a mixed slurry is obtained. Add a coupling agent and disperse evenly again to obtain a composite positive electrode slurry.

[0013] S3. The composite positive electrode slurry is coated onto the current collector and dried by simultaneous microwave and infrared irradiation to obtain the electrode precursor. Then, it is hot-pressed to obtain the composite positive electrode, i.e., the interface-enhanced positive electrode.

[0014] In one embodiment of the present invention, in step S1, the terminal group containing active hydrogen is one or more of thiol, amino, carboxyl, and hydroxyl groups. Preferably, the PVDF prepolymer includes one or more of the following: thiol-terminated PVDF (-SH-terminated), amino-terminated PVDF (-NH2-terminated), carboxyl-terminated PVDF (-COOH-terminated), PVDF-HFP copolymer (polyvinylidene fluoride-hexafluoropropylene, -SH-terminated), and PVDF-TrFE copolymer (polyvinylidene fluoride-trifluoroethylene, -OH-terminated).

[0015] In one embodiment of the present invention, in step S1, the dynamic covalent bond is one or more of disulfide bonds, borate ester bonds, and Diels-Alder bonds. Preferably, the crosslinking agent includes one or more of dithiodiethylene glycol, 1,2-dithioethane, dithiodipropionic acid, bismaleimides, and 1,4-phenylenediobionic acid.

[0016] Conventional crosslinking agents form irreversible, permanent chemical crosslinks. While this may improve mechanical strength, it renders the polymer completely insoluble or even unable to swell in nonpolar solvents, resulting in a complete loss of solution processability and rendering it unusable for slurry preparation. Furthermore, the permanent network lacks self-healing capabilities.

[0017] Active hydrogen end groups (-SH, -NH2, -COOH) are key sites for specific and efficient chemical reactions with dynamic crosslinking agents. They can form covalent bonds with specific functional groups on the crosslinking agent (such as disulfide bonds, epoxy groups, borate groups, etc.), thereby "stitching" the crosslinking agent to the end of the PVDF molecular chain and constructing a three-dimensional network with the PVDF chain as the backbone and dynamic covalent bonds as the connection points. Without these active end groups, the crosslinking reaction cannot be effectively initiated.

[0018] In one embodiment of the present invention, in step S1, the solvent is N,N-dimethylformamide.

[0019] In one embodiment of the present invention, in step S1, the mass ratio of PVDF prepolymer to crosslinking agent is 100:(5-30). The amount ratio of PVDF prepolymer to solvent is 100g:800-1500ml.

[0020] In one embodiment of the present invention, in step S1, the heating reaction temperature is 60-80°C and the time is 6-12 hours.

[0021] In one embodiment of the present invention, in step S1, the PVDF prepolymer is terminally aminated polyvinylidene fluoride, the crosslinking agent is dithiodipropionic acid, and a catalyst (N-hydroxysuccinimide) is also used when modifying the PVDF binder. The terminally aminated polyvinylidene fluoride is first dissolved in a solvent, and under nitrogen protection and ice-water bath cooling, an activation solution containing the crosslinking agent and catalyst is added dropwise, followed by a heating reaction. The activation solution is prepared by dissolving the crosslinking agent and catalyst in a solvent and activating them for 30-40 minutes before adding them dropwise.

[0022] As one embodiment of the present invention, in step S1, the PVDF prepolymer is a carboxyl-terminated polyvinylidene fluoride-hexafluoropropylene copolymer, the crosslinking agent is bismaleimide, and a catalyst (4-dimethylaminopyridine) is also used when modifying the PVDF binder. First, the carboxyl-terminated polyvinylidene fluoride-hexafluoropropylene copolymer and the catalyst are dissolved in a solvent, stirred and activated for 1-3 hours at room temperature under nitrogen protection, and then the PVDF solution is added dropwise under ice-water bath cooling, and then heated for reaction.

[0023] As one embodiment of the present invention, in step S1, the PVDF prepolymer is terminally aminated polyvinylidene fluoride, the crosslinking agent is dithiodipropionic acid, and a catalyst is also used when modifying the PVDF binder. The terminally aminated polyvinylidene fluoride is first dissolved in a solvent, and under nitrogen protection and ice-water bath cooling, an activation liquid containing crosslinking agent and catalyst is dripped in, and then heated to react.

[0024] In one embodiment of the present invention, in step S1, the product of the heated reaction is precipitated, washed, and dried to obtain a modified PVDF binder, which is a white powdery dynamically cross-linked PVDF binder. Precipitation is achieved by introducing the reaction solution into methanol to precipitate the precipitate. Washing is performed by washing the precipitate with methanol. The drying temperature is 80°C, and the drying time is 12 hours.

[0025] In one embodiment of the present invention, in step S2, the cathode material is a ternary cathode material. The ternary cathode material includes one or more of NCM111, NCM523, NCM622, and NCM811.

[0026] In one embodiment of the present invention, in step S2, the sulfide solid electrolyte includes Li6PS5Cl (LPSCl) and Li 10 GeP2S 12 (LGPS), Li7P3S 11 One or more of Li3PS4 and Li2S-P2S5 glass ceramics.

[0027] As one embodiment of the present invention, in step S2, the conductive agent includes one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, carbon nanofibers, Super P, and acetylene black.

[0028] As one embodiment of the present invention, in step S2, the nonpolar solvent includes one or more of n-hexane, decane, dodecane, mesitylene, toluene, and p-xylene.

[0029] As one embodiment of the present invention, in step S2, the mass ratio of the positive electrode material, the sulfide solid electrolyte, the conductive agent and the cross-linked PVDF binder is (70-85):(10-25):(2-5):(3-6).

[0030] In one embodiment of the present invention, in step S2, the mass ratio of the positive electrode material to the non-polar solvent is 70-85:200.

[0031] In one embodiment of the present invention, in step S2, the initial dispersion speed is 1500-4000 rpm, the time is 5-8 hours, and the temperature is 20-30℃.

[0032] As one embodiment of the present invention, in step S2, the coupling agent includes one or more of γ-aminopropyltriethoxysilane (KH-550), γ-glycidoxypropyltrimethoxysilane (KH-560), γ-mercaptopropyltrimethoxysilane, isopropyltris(dioctylphosphoyloxy)titanate, triethyl phosphate, and bis(2-ethylhexyl) phosphate.

[0033] As one embodiment of the present invention, in step S2, the amount of coupling agent added is 0.1-1.0 wt% of the total solid mass in the mixed slurry; the solids include positive electrode material, sulfide solid electrolyte, conductive agent, and modified PVDF binder.

[0034] As one embodiment of the present invention, in step S2, the rotation speed range for re-dispersion is 1500-4000 rpm; the time is 1-2 hours; and the temperature is 20-30℃.

[0035] In one embodiment of the present invention, in step S3, the microwave power density is 3-8 kW / m²; the infrared wavelength range is 2.0-4.0 μm; the infrared power density is 5-15 kW / m²; and the synchronous irradiation drying time is 1-5 min.

[0036] In one embodiment of the present invention, in step S3, the thickness of the electrode precursor is 100-150 μm.

[0037] In one embodiment of the present invention, in step S3, the temperature of the hot pressing treatment is 80-100℃, the pressure is 10-20 MPa, and the time is 5-15 min.

[0038] In one embodiment of the present invention, in step S3, the thickness of the composite positive electrode sheet is 80-100 μm.

[0039] Secondly, the present invention provides a modified PVDF binder prepared by the above method step S1.

[0040] Thirdly, the present invention provides a composite positive electrode sheet prepared by the above method.

[0041] Fourthly, the present invention provides a sulfide all-solid-state battery comprising the above-mentioned composite positive electrode sheet, wherein the negative electrode active material is one or more of micron-sized silicon, nano-sized silicon, graphite, and lithium-silicon alloy; the sulfide electrolyte is a Li6PS5Cl type solid electrolyte; and the positive electrode is the positive electrode sheet prepared by the above method.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] (1) Overcoming the contradiction between solvent compatibility and mechanical properties of the binder: By dynamically covalently crosslinking PVDF, its solubility in non-polar solvents was successfully improved, avoiding the corrosion of sulfide electrolytes by the solvent. At the same time, the dynamic network endows the binder with self-healing ability, which can repair microcracks caused by volume changes during electrode cycling, significantly improving the cycle life of the electrode.

[0044] (2) Molecular-level reinforcement of the composite electrode interface was achieved: By adding a trace amount of coupling agent, a strong "chemical bridge" was constructed at the three-phase interface of the positive electrode active material, sulfide electrolyte and binder, upgrading the traditional physical point contact to a strong chemical bond. This makes the interface less prone to peeling when the electrode is subjected to huge cyclic stress and effectively suppresses capacity decay caused by contact failure.

[0045] (3) The electrode drying process has been innovated, which greatly improves manufacturing efficiency and consistency: The microwave-infrared synergistic drying technology utilizes the complementary effect of microwave bulk heating and infrared surface heating to significantly shorten the drying time. This process avoids component segregation and stress cracks caused by uneven drying, makes the electrode pore distribution more uniform, optimizes the ion / electron transport path, and significantly improves the batch consistency of electrodes.

[0046] (4) Comprehensive improvement of the electrochemical performance of all-solid-state batteries: The synergistic application of the above three technologies enables the prepared composite cathode to retain up to 99.4% capacity after 20 cycles at 0.5C rate, providing a practical and feasible full-chain solution for the high-performance, low-cost and large-scale manufacturing of sulfide all-solid-state batteries. Detailed Implementation

[0047] The present invention will now be described in detail with reference to specific embodiments. The following examples are implemented under the premise of the technical solution of the present invention, providing detailed implementation methods and specific operating procedures, which will help those skilled in the art to further understand the present invention. It should be noted that the scope of protection of the present invention is not limited to the following embodiments; any adjustments and improvements made under the concept of the present invention are all within the scope of protection of the present invention.

[0048] This invention provides a method for preparing an interface-enhanced cathode for sulfide all-solid-state batteries, comprising the following steps:

[0049] (1) Preparation of dynamically crosslinked modified PVDF adhesive:

[0050] A PVDF prepolymer with active hydrogen end groups (such as thiol or amino groups) is dissolved in N,N-dimethylformamide with a crosslinking agent containing dynamic covalent bonds (preferably disulfide bonds), and the reaction is carried out at a certain temperature. After precipitation, washing and drying, a white powdery dynamic crosslinked PVDF binder is obtained.

[0051] (2) Preparation of composite cathode slurry:

[0052] The ternary positive electrode active material, sulfide solid electrolyte, conductive agent and the dynamically cross-linked PVDF binder obtained in step (1) are added to a non-polar solvent in a certain mass ratio, and a uniform slurry is obtained by high-speed dispersion. Then a certain amount of silane coupling agent is added and high-speed dispersion is carried out again to obtain a composite positive electrode slurry.

[0053] (3) Coating and co-drying:

[0054] The slurry is coated onto an aluminum current collector and dried by simultaneous irradiation for a certain period of time under appropriate microwave power and infrared wavelength conditions using a microwave-infrared synergistic drying device to form an electrode precursor with a thickness of 100-150 μm.

[0055] (4) Hot pressing:

[0056] After drying, the electrode sheet is placed in a hot press and hot-pressed under certain temperature and pressure conditions. After cooling, a composite positive electrode sheet with a thickness of 80-100 μm is obtained.

[0057] The polyvinylidene fluoride (PVDF) prepolymers terminated with thiol groups (-SH), amino groups (-NH2), carboxyl groups (-COOH), or hydroxyl groups (-OH) used in this invention are known polymer materials with defined terminal functional group structures. Their preparation methods are known in the art and can be achieved through living / controlled polymerization techniques such as reversible addition-fragmentation chain transfer (RAFT) polymerization and atom transfer radical polymerization (ATRP), using chain transfer agents or initiators with corresponding protecting functional groups to carry out efficient nucleophilic substitution reactions to achieve end-group conversion. This method is well known to those skilled in the art. This invention does not limit the specific synthetic route of the prepolymers.

[0058] Terminal hydroxylation (-OH): Introduced by reacting the active terminal group with a hydroxylating agent (such as sodium hydroquinone or by acidification after reaction with ethylene oxide).

[0059] Terminal carboxylation (-COOH): Introduced by reacting the active terminal group with a nucleophilic reagent containing a carboxyl group (such as acidic hydrolysis after reaction with sodium cyanide, or hydrolysis decarboxylation after direct reaction with sodium malonate).

[0060] Terminal amination (-NH2): Introduced by reacting the active terminal group with an amino-containing nucleophile (such as potassium phthalimide followed by hydrazinolysis, or sodium azide followed by reduction).

[0061] Terminal thiolization (-SH): Introduced by reacting the active terminal group with a nucleophilic reagent containing a thiol group (such as potassium thioacetate followed by hydrolysis, or direct reaction with thiourea followed by alkaline hydrolysis).

[0062] Example 1

[0063] A method for preparing a high-performance composite cathode for sulfide all-solid-state batteries, comprising the following steps:

[0064] (1) Preparation of dynamically crosslinked modified PVDF adhesive:

[0065] The PVDF prepolymer used was a thiol-terminated PVDF-HFP copolymer with a 70% end-capping rate and a number-average molecular weight (Mn) of approximately 45,000 g / mol. The crosslinking agent was dithiodiethylene glycol (analytical grade), and the solvent was N,N-dimethylformamide (DMF, anhydrous grade). 10.0 g of the PVDF prepolymer and 1.2 g of the crosslinking agent were completely dissolved in 120 mL of DMF (mass ratio PVDF:crosslinking agent = 100:12) and placed in a three-necked flask equipped with a magnetic stirrer, a condenser, and a nitrogen inlet. After purging with nitrogen, the oil bath was heated to 70°C, and the reaction was continuously stirred at this temperature for 8 hours. The reaction solution was then slowly poured into 500 mL of vigorously stirred methanol, and a white fibrous product immediately precipitated. The precipitate was filtered through a Buchner funnel and washed three times with fresh methanol. The resulting white solid was placed in a vacuum drying oven and dried at 80°C for 12 hours to obtain a dynamically crosslinked PVDF binder powder.

[0066] (2) Preparation of composite cathode slurry

[0067] 75 g of NCM811, 18 g of Li6PS5Cl, 3 g of conductive multi-walled carbon nanotubes, and 4 g of cross-linked PVDF binder were added to 200 g of decane solvent. The mixture was continuously dispersed at 3000 rpm for 6 hours using a high-speed planetary mixer (with a jacketed water bath temperature control), while maintaining the water bath temperature at 25±5℃, to obtain a homogeneous primary slurry. Then, 0.5 g of γ-aminopropyltriethoxysilane (KH-550) coupling agent was added to the primary slurry, and the mixing speed was adjusted to 2500 rpm, with dispersion continuing for 1.5 hours to ensure thorough mixing and reaction of the coupling agent, resulting in the final composite cathode slurry. The slurry exhibited a uniform dark gray color with no visible particle agglomeration.

[0068] (3) Coating and co-drying

[0069] The above slurry was uniformly coated onto a 15μm thick aluminum foil current collector using an automatic coating machine, with the wet film thickness controlled at 130μm. The wet film was then placed in a microwave-infrared synergistic drying chamber (nitrogen atmosphere). The microwave power density was set to 5 kW / m². 2 The infrared radiation wavelength is 3.0 μm and the power density is 10 kW / m². 2 The electrode passes through the conveyor belt at a constant speed and is subjected to 2.5 minutes of synchrotron irradiation drying. After drying, the wet film transforms into a structurally complete electrode precursor with a thickness of approximately 105 μm.

[0070] (4) Hot pressing

[0071] The dried electrode precursor was cut into sheets and placed in a flatbed hot press; it was hot-pressed at 90℃ and 15 MPa for 10 minutes. After hot pressing, it was naturally cooled to room temperature to obtain the final composite positive electrode sheet. The electrode sheet had a smooth and dense surface, no cracks, and a total thickness of 90±5μm.

[0072] Example 2

[0073] A method for preparing a high-performance composite cathode for sulfide all-solid-state batteries, comprising the following steps:

[0074] (1) Preparation of dynamically crosslinked modified PVDF adhesive:

[0075] The PVDF prepolymer used was a hydroxylated PVDF-TrFE copolymer (-OH capped, TrFE content 15%) with a number-average molecular weight Mn≈60,000 g / mol. The crosslinking agent was 1,4-phenylenediboric acid (analytical grade), and the solvent was N,N-dimethylformamide (DMF, anhydrous grade). 15.0 g of the PVDF prepolymer, 0.9 g of the crosslinking agent, and 0.009 g of the catalyst p-toluenesulfonic acid were completely dissolved in 180 mL of DMF (mass ratio PVDF:crosslinking agent = 100:6). The reaction apparatus was the same as in Example 1. Under nitrogen protection, the oil bath was heated to 65°C, and the reaction was continuously stirred at this temperature for 10 hours. This reaction constructs a dynamic network by forming reversible borate ester bonds. The reaction solution was then slowly poured into 500 mL of vigorously stirred methanol, and a white fibrous product immediately precipitated. The precipitate was filtered through a Buchner funnel and washed three times with fresh methanol. The resulting pale yellow solid was placed in a vacuum drying oven and dried at 75°C for 15 hours to obtain dynamically cross-linked PVDF binder powder.

[0076] (2) Preparation of composite cathode slurry

[0077] 78 g of NCM811, 16 g of Li6PS5Cl, 3 g of conductive multi-walled carbon nanotubes, and 3 g of cross-linked PVDF binder were added to 220 g of decane solvent. The mixture was continuously dispersed for 7 hours at 3500 rpm using a high-speed planetary mixer (with a jacketed water bath temperature control), while maintaining the water bath temperature at 25±5℃, to obtain a homogeneous primary slurry. Then, 0.3 g of coupling agent γ-glycidoxypropyltrimethoxysilane was added to the primary slurry, and the mixing speed was adjusted to 2000 rpm, with dispersion continuing for 2 hours to ensure thorough mixing and reaction of the coupling agent, resulting in the final composite cathode slurry. The slurry exhibited a uniform dark gray color with no visible particle agglomeration.

[0078] (3) Coating and co-drying

[0079] The above slurry was uniformly coated onto a 15μm thick aluminum foil current collector using an automatic coating machine, with the wet film thickness controlled at 140μm. The wet film was then placed in a microwave-infrared synergistic drying chamber (nitrogen atmosphere). The microwave power density was set to 4 kW / m². 2 The infrared radiation wavelength is 2.8 μm and the power density is 12 kW / m². 2 The electrode passes through the conveyor belt at a constant speed and is subjected to synchrotron irradiation drying for 3 minutes. After drying, the wet film transforms into a structurally complete electrode precursor with a thickness of approximately 110 μm.

[0080] (4) Hot pressing

[0081] The dried electrode precursor was cut into sheets and placed in a flatbed hot press; it was hot-pressed at 85℃ and 18MPa for 12 minutes. After hot pressing, it was naturally cooled to room temperature to obtain the final composite positive electrode sheet. The electrode sheet had a smooth and dense surface, no cracks, and a total thickness of 95±5μm.

[0082] Example 3

[0083] A method for preparing a high-performance composite cathode for sulfide all-solid-state batteries, comprising the following steps:

[0084] (1) Preparation of dynamically crosslinked modified PVDF binder: The PVDF prepolymer is end-aminated polyvinylidene fluoride (-NH2-terminated), with a weight-average molecular weight (Mw) of about 80,000 g / mol, and the amount used is 10.0 g. The crosslinking agent is dithiodipropionic acid, and the amount used is 1.5 g (that is, the mass ratio of PVDF prepolymer to crosslinking agent is 100:15).

[0085] The specific procedure is as follows: 10.0 g of terminally aminated PVDF prepolymer was completely dissolved in 120 mL of DMF to form a homogeneous and transparent solution A. In another dry container, 1.5 g of dithiodipropionic acid and 1.8 g of the catalyst N-hydroxysuccinimide were dissolved together in 20 mL of DMF and activated by stirring at room temperature for 30 minutes to obtain activated solution B. Under nitrogen protection and ice-water bath cooling, activated solution B was slowly added dropwise to the vigorously stirred solution A. After the addition was complete, the ice bath was removed, the temperature of the reaction system was raised to 60°C, and the reaction was continuously stirred at this temperature for 12 hours. The entire reaction was carried out under a nitrogen atmosphere. After the reaction was completed, the reaction solution was slowly poured into a vigorously stirred methanol / water mixed solvent (volume ratio 4:1). A white fibrous product immediately precipitated. The precipitate was collected by filtration and washed four times with the above methanol / water mixed solvent to completely remove unreacted reagents, catalysts, and byproducts. Finally, the product was dried in a vacuum oven at 60°C for 24 hours to obtain a white to light yellow elastic solid.

[0086] The subsequent preparation, coating and co-drying of the composite cathode slurry, and hot pressing molding operations are similar to those in Example 1.

[0087] Example 4

[0088] A method for preparing a high-performance composite cathode for sulfide all-solid-state batteries, comprising the following steps:

[0089] (1) Preparation of dynamically crosslinked modified PVDF binder: The PVDF prepolymer is a carboxyl-terminated polyvinylidene fluoride-hexafluoropropylene copolymer (-COOH end-capped, HFP content ~8%) with a weight average molecular weight (Mw) of about 100,000 g / mol and a dosage of 10.0 g; the crosslinking agent is bismaleimide with a dosage of 0.54 g (the mass ratio of PVDF prepolymer to crosslinking agent is about 100:5.4).

[0090] The specific procedure was as follows: 10.0 g of carboxyl-terminated PVDF-HFP was completely dissolved in 100 mL of DMF. 1.50 g of 4-dimethylaminopyridine catalyst was added, and the mixture was stirred and activated for 2 hours at room temperature under nitrogen protection. 0.54 g of bismaleimide was dissolved in 20 mL of DMF, and this solution was slowly added dropwise to the PVDF solution while cooling in an ice bath. After the addition was complete, the ice bath was removed, the temperature was raised to 65 °C, and the reaction was continuously stirred for 10 hours. The reaction solution was poured into 1.2 L of acetone to precipitate the solid. After filtration, the solid was washed five times with acetone and then dried under vacuum at 60 °C for 3 hours to obtain a light brown solid.

[0091] The subsequent preparation, coating and co-drying of the composite cathode slurry, and hot pressing molding operations are similar to those in Example 1.

[0092] Comparative Example 1

[0093] 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.

[0094] Comparative Example 2

[0095] In this comparative example, the addition of coupling agent in step (2) of Example 1 was omitted, while other conditions and parameters remained unchanged. In this comparative example, the slurry viscosity was low, at 1260 mPa·s, and overflow occurred during the coating process. The electrode test impedance was 10.85Ω, the ionic conductivity was 1.32mS / cm, the porosity of the positive electrode was 10.5%, the compaction density was 2.8 g / cm3, the peel strength to the substrate aluminum foil was 3.7N / m, the battery performance was 196.7 mAh / g at 0.1C, the first-cycle coulombic efficiency was 84.2%, the discharge specific capacity at 0.5C was 185.8 mAh / g, the discharge specific capacity after 20 cycles was 179.6 mAh / g, and the capacity retention rate was 96.7%.

[0096] In Example 1, the slurry viscosity was 1740 mPa·s, the coating and film surface were normal, the electrode impedance was 7.32 Ω, the ionic conductivity was 2.84 mS / cm, the porosity of the positive electrode was 6.7%, and the compaction density was 3.2 g / cm³. 3 The peel strength to the aluminum foil substrate is 6.5 N / m. The battery performance is as follows: 0.1C discharge specific capacity is 218.5 mAh / g, first-cycle coulombic efficiency is 88.9%, 0.5C discharge specific capacity is 197.6 mAh / g, 20-cycle discharge specific capacity is 196.4 mAh / g, and capacity retention is 99.4%.

[0097] The absence of coupling agent in this comparative example leads to severely insufficient solid-solid interface adhesion, poor electrode mechanical integrity, and low electrode peel strength; high and continuously increasing interfacial contact resistance, large battery polarization, inability to effectively buffer and transfer stress, and exacerbation of interfacial side reactions; accelerated irreversible consumption of active lithium and electrolyte, which not only increases impedance but also directly leads to irreversible capacity decay.

[0098] Comparative Example 3

[0099] In this comparative example, the addition of coupling agent in step (2) of Example 1 was omitted, and the microwave-infrared synergistic drying treatment in step (3) was omitted. General vacuum oven drying technology was used, with a drying temperature of 80°C and a drying time of 12 hours; the remaining conditions and parameters remained unchanged.

[0100] In this comparative example, the slurry viscosity was relatively low, at 1230 mPa·s, resulting in edge overflow during coating. The drying time in the oven was 12 hours. The electrode impedance was 11.62 Ω, the ionic conductivity was 1.04 mS / cm, the porosity of the positive electrode was 11.4%, and the compacted density was 2.7 g / cm³. 3The peel strength to the aluminum foil substrate is 3.5 N / m. The battery performance is as follows: 0.1C discharge specific capacity is 190.5 mAh / g, first-cycle coulombic efficiency is 82.1%, 0.5C discharge specific capacity is 178.4 mAh / g, 20-cycle discharge specific capacity is 170.5 mAh / g, and capacity retention is 95.6%.

[0101] In this comparative example, the absence of coupling agent and microwave-infrared synergistic drying results in internal stress (such as shrinkage stress) generated during the drying process acting directly on the fragile interface, inducing more microcracks. Weak areas in the non-uniform structure fail first during cycling, becoming the "breakthrough point" for performance degradation and accelerating the collapse of the overall electrode. The high-impedance interface and tortuous transmission path work together to exacerbate battery polarization, resulting in a complete loss of rate performance.

[0102] Comparative Example 4

[0103] In this comparative example, only the microwave-infrared synergistic drying process in step (3) of Example 1 was omitted, and a general vacuum oven drying technique was used. The drying temperature was 80°C and the drying time was 12 hours. All other conditions and parameters remained unchanged.

[0104] In this comparative example, the slurry viscosity was normal at 1730 mPa·s, but the drying time in the oven was 12 hours; the electrode impedance was 9.21 Ω, the ionic conductivity was 1.63 mS / cm, the porosity of the positive electrode was 8.2%, and the compacted density was 3.0 g / cm³. 3 The peel strength to the aluminum foil substrate is 5.1 N / m. The battery performance is as follows: discharge specific capacity at 0.1C is 198.3 mAh / g, first-cycle coulombic efficiency is 85.3%, discharge specific capacity at 0.5C is 187.4 mAh / g, discharge specific capacity after 20 cycles is 181.4 mAh / g, and capacity retention is 96.8%.

[0105] In this comparative example, microwave-infrared synergistic drying was not used. Traditional hot air or vacuum conduction heating required the solvent to evaporate layer by layer from the surface inward, resulting in extremely low drying efficiency, long drying time, and uneven drying leading to microstructural defects. Heat was conducted from the outside inward, and the surface solvent evaporated first, forming a dense hard shell that hindered the escape of the internal solvent. After the internal solvent was heated and vaporized, pressure accumulated, which easily led to defects such as blistering and cracking of the electrode. The slow drying process would exacerbate the migration of binder and small particles caused by capillary forces, forming a density and composition gradient from the surface to the inside. The surface layer may be blocked by ions due to binder enrichment, while the inner layer may have weak bonding due to insufficient binder.

[0106] Comparative Example 5

[0107] In this comparative example, only the crosslinking modification treatment in step (1) of Example 1 was removed, and only the terminal thiolized PVDF-HFP copolymer was used for the preparation of the composite positive electrode slurry, while the other conditions and parameters remained unchanged.

[0108] In this comparative example, the slurry viscosity was relatively low, at 1470 mPa·s, and the drying time was 2.5 min. The electrode impedance was 9.43 Ω, the ionic conductivity was 1.54 mS / cm, the porosity of the positive electrode was 8.7%, and the compacted density was 2.9 g / cm³. 3 The peel strength to the aluminum foil substrate is 4.6 N / m. The battery performance is as follows: discharge specific capacity at 0.1C is 202.3 mAh / g, first-cycle coulombic efficiency is 88.7%, discharge specific capacity at 0.5C is 189.5 mAh / g, discharge specific capacity after 20 cycles is 184.3 mAh / g, and capacity retention is 97.2%.

[0109] In this comparative example, the PVDF-HFP with end-thiol groups was not crosslinked, which resulted in the inability to form a stable network. After the solvent evaporated, the linear polymer formed a film only through chain entanglement and physical action, resulting in limited adhesion. Under the continuous stress of battery cycling, the polymer chains will undergo creep and irreversible plastic deformation, which will cause the physical contact between the binder and the active particles and electrolyte particles to gradually loosen or even detach, ultimately leading to a rapid decrease in usable capacity.

[0110] Comparative Example 6

[0111] In this comparative example, only the infrared processing in step (3) of Example 1 is omitted, and only microwave processing is used, with a microwave power density of 15 kW / m². 2 All other conditions and parameters remain unchanged.

[0112] In this comparative example, the slurry viscosity was normal at 1750 mPa·s, but the drying time using microwave treatment alone was 30 min; the electrode impedance was 7.95 Ω, the ionic conductivity was 2.53 mS / cm, the porosity of the positive electrode was 6.9%, and the compacted density was 3.1 g / cm³. 3 The peel strength to the aluminum foil substrate is 6.1 N / m. The battery performance is as follows: 0.1C discharge specific capacity is 212.4 mAh / g, first-cycle coulombic efficiency is 88.1%, 0.5C discharge specific capacity is 192.4 mAh / g, 20-cycle discharge specific capacity is 187.8 mAh / g, and capacity retention is 97.6%.

[0113] In this comparative example, using only microwave drying will first increase the drying time, and secondly, when the solvent vapor generated inside escapes outward, it will condense on the surface when it encounters cold, or cause the surface polymer (binder) to solidify prematurely. This results in insufficient wetting and bonding of the interface between the active material, electrolyte particles and the binder, especially in the surface and bottom layers. Consequently, the electron and ion transport network becomes discontinuous, local interfacial impedance increases, and the battery capacity is affected.

[0114] Comparative Example 7

[0115] In this comparative example, only the microwave processing in step (3) of Example 1 is omitted, and only infrared processing is used, with an infrared power density of 15 kW / m². 2 All other conditions and parameters remain unchanged.

[0116] In this comparative example, the slurry viscosity was normal at 1740 mPa·s, but the drying time using only infrared treatment was 60 min; the electrode impedance was 7.98 Ω, the ionic conductivity was 2.50 mS / cm, the porosity of the positive electrode was 7.2%, and the compaction density was 3.0 g / cm³. 3 The peel strength to the aluminum foil substrate is 5.8 N / m. The battery performance is as follows: 0.1C discharge specific capacity is 210.3 mAh / g, first-cycle coulombic efficiency is 87.7%, 0.5C discharge specific capacity is 190.3 mAh / g, 20-cycle discharge specific capacity is 184.6 mAh / g, and capacity retention is 97.0%.

[0117] In this comparative example, using only infrared drying results in a slow drying rate and low production efficiency. Infrared is a surface heating technology, where heat is conducted from the electrode surface to the interior, resulting in slow heat transfer. Heat and solvent evaporation begin from the surface, causing the "drying front" to gradually advance inward. This leads to severe binder migration and damages electrode uniformity. This directly increases the battery's internal resistance, deteriorates rate performance, and makes it prone to peeling off from the current collector during cycling.

[0118] Comparative Example 8

[0119] In this comparative example, conventional (non-terminated functionalized) PVDF-HFP was used instead of the thiolized PVDF-HFP copolymer in Example 1, and reacted with the same dithiodiethylene glycol under exactly the same conditions.

[0120] The product in this comparative example is almost insoluble in nonpolar solvents, making it impossible to prepare adhesives or slurries. This is because the active hydrogen end groups (-SH, -NH2, -COOH) are key sites for specific and efficient chemical reactions with the dynamic crosslinking agent. They can form covalent bonds with specific functional groups on the crosslinking agent (such as disulfide bonds, epoxy groups, borate groups, etc.), thereby "stitching" the crosslinking agent to the end of the PVDF molecular chain, constructing a three-dimensional network with the PVDF chain as the backbone and dynamic covalent bonds as the connection points. Without these active end groups, the crosslinking reaction cannot be effectively initiated.

[0121] Comparative Example 9

[0122] In this comparative example, the conventional crosslinking agent 1,4-butanediol diglycidyl ether (BDDE) was used to replace the dithiodiethylene glycol in Example 1 for crosslinking modification treatment, while the other conditions and parameters remained unchanged.

[0123] The modified PVDF binder cannot swell and dissolve in non-polar solvents, remaining as a solid powder and settling at the bottom of the bottle, making it impossible to prepare adhesives or slurries. This is because the epoxy groups in the BDDE structure undergo a ring-opening addition reaction with the terminal thiol groups of PVDF, forming non-hydrolyzable CSC and COC bonds, which are typical permanent chemical crosslinks. The permanent crosslink network cannot be destroyed or swollen in solvents, and the material completely loses its solution processability, making it impossible to use wet coating processes.

[0124] The examples and comparative examples were tested using the following methods:

[0125] (1) Testing of slurry state and coating film state

[0126] The slurry state and coating film state of the examples and comparative examples were compared, and the results are shown in Table 1.

[0127] Table 1

[0128]

[0129] (2) Testing of ionic conductivity:

[0130] 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 top and bottom 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.

[0131] Table 2

[0132]

[0133] (3) Porosity test of the positive electrode sheet:

[0134] The experiment used a high-performance, fully automated mercury porosimeter, model Micromeritics AutoPore V 9620. Pressures were applied from approximately 0.6 to 50 PSI in the low-pressure station (LP) and from 20 to 60,000 PSI in the high-pressure station (HP). A glass cone probe was used as the sample container to apply pressure to the sample, which had a size of 1 cm x 1 cm. The porosity of the positive electrode was calculated based on the volume of mercury porosimeter entering the pores.

[0135] 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 applied to the center of the steel plate, smoothing it firmly to ensure a tight fit. The double-sided tape was then peeled off, and the electrode was attached to the tape, ensuring a perfect match between the electrode and the tape. 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 fixing the test sample, the peel speed was 0.2mm / s, and the test began.

[0136] The test results of porosity, peel strength and compaction density of the positive electrode sheet are shown in Table 3.

[0137] Table 3

[0138]

[0139] (4) Full battery performance test:

[0140] 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.

[0141] Table 4

[0142]

[0143] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that these are merely illustrative examples, and any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing an interface-enhanced cathode for sulfide all-solid-state batteries, characterized in that, Includes the following steps: S1. Dissolve PVDF prepolymer and crosslinking agent in a solvent and heat to react to obtain modified PVDF binder. The end groups of PVDF prepolymer are end groups containing active hydrogen, and the crosslinking agent is a crosslinking agent containing dynamic covalent bonds. S2. Add the positive electrode material, sulfide solid electrolyte, conductive agent, and modified PVDF binder to a non-polar solvent. After initial dispersion, a mixed slurry is obtained. Add a coupling agent and disperse evenly again to obtain a composite positive electrode slurry. S3. The composite positive electrode slurry is coated on the current collector and dried by simultaneous microwave and infrared irradiation to obtain the electrode precursor. Then, it is hot-pressed to obtain the composite positive electrode, i.e., the interface-enhanced positive electrode. In step S1, the PVDF prepolymer includes one or more of the following: terminally thiolized PVDF, terminally aminated PVDF, terminally carboxylated PVDF, PVDF-HFP copolymer, and PVDF-TrFE copolymer; Crosslinking agents include one or more of dithiodiethylene glycol, 1,2-dithioethane, dithiodipropionic acid, bismaleimide, and 1,4-phenylenediboronic acid; The power density of the microwaves in step S3 is 3-8 kW / m 2 ; the power density of the infrared radiation is 5-15 kW / m 2 ; and the time of simultaneous irradiation for drying is 1-5 min.

2. The preparation method according to claim 1, characterized in that, In step S1, the mass ratio of PVDF prepolymer to crosslinking agent is 100:5-30; And / or, in step S1, the ratio of PVDF prepolymer to solvent is 100g: 800-1500ml; And / or, in step S1, the heating reaction is carried out at a temperature of 60-80°C for 6-12 hours.

3. The preparation method according to claim 1, characterized in that, In step S2, the cathode material is a ternary cathode material; the ternary cathode material includes one or more of NCM111, NCM523, NCM622, and NCM811; And / or, in step S2, the sulfide solid electrolyte includes Li6PS5Cl, Li 10 GeP2S 12 Li7P3S 11 One or more of the following: Li3PS4, Li2S-P2S5 glass-ceramics; And / or, in step S2, the conductive agent includes one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, carbon nanofibers, Super P, and acetylene black; And / or, in step S2, the nonpolar solvent includes one or more of n-hexane, decane, dodecane, mesitylene, toluene, and p-xylene.

4. The preparation method according to claim 1, characterized in that, In step S2, the coupling agent includes one or more of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane, isopropyltris(dioctylphosphoyloxy)titanate, triethyl phosphate, and bis(2-ethylhexyl) phosphate. And / or, in step S2, the amount of coupling agent added is 0.1-1.0 wt% of the total solid mass in the mixed slurry; the solids include positive electrode material, sulfide solid electrolyte, conductive agent, and modified PVDF binder.

5. The preparation method according to claim 1, characterized in that, In step S3, the thickness of the electrode precursor is 100-150 μm; And / or, in step S3, the hot pressing temperature is 80-100℃, the pressure is 10-20 MPa, and the time is 5-15 min; And / or, in step S3, the thickness of the composite positive electrode sheet is 80-100 μm.

6. A composite positive electrode sheet obtained by the preparation method as described in claim 1.

7. A sulfide all-solid-state battery, characterized in that, The sulfide all-solid-state battery includes a negative electrode active material, a sulfide electrolyte, and a composite positive electrode as described in claim 6.

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