Polyvinylidene fluoride-based composite solid electrolyte membrane and preparation method thereof

Abstract

The present invention discloses a polyvinylidene fluoride-based composite solid electrolyte membrane and a preparation method thereof. The present invention controls the relevant parameters of steam-induced phase separation to influence the degree of phase separation, thereby forming a uniformly connected high-pore structure and a suitable pore size to adapt to the particle size of the inorganic solid electrolyte powder, thereby enhancing the interfacial bonding force between the inorganic solid electrolyte powder and the polyvinylidene fluoride organic matrix. In addition, in order to further enhance the effect of steam-induced phase separation, the present invention also optimizes the types and contents of auxiliary agents and modifiers, adds annealing post-treatment to further enhance the performance of the polyvinylidene fluoride-based composite solid electrolyte membrane, enhances the composite effect of the electrolyte, and can improve interface defects, thereby facilitating the formation of a stable composite solid electrolyte membrane, broadening the path of lithium ion transmission, and enhancing the conductivity and cycle stability of the battery.

Description

Technical Field

[0001] The present invention belongs to the field of solid electrolyte materials, and in particular relates to a polyvinylidene fluoride-based composite solid electrolyte membrane and a preparation method thereof. Background Art

[0002] In today's rapidly developing energy sector, lithium-ion batteries, as the mainstream energy storage device, have performance improvements and innovations that are directly related to the future development of electric vehicles, portable electronic devices, and large-scale energy storage systems. However, despite significant success in commercial applications, the organic liquid electrolyte system used in traditional lithium-ion batteries has gradually exposed its inherent shortcomings, becoming a bottleneck restricting its further development. Liquid electrolytes are prone to side reactions under high temperature, high pressure, or mechanical stress, resulting in a decrease in the internal stability of the battery. At the same time, the volatilization and leakage of the electrolyte not only reduce the safety performance of the battery, but also accelerate the decay of capacity and shorten the cycle life. This makes it difficult to meet the urgent demand for high energy density, long cycle life, and high safety performance lithium batteries in emerging fields such as new energy vehicles, aerospace, and wearable devices.

[0003] To address these issues, both the scientific research community and the industrial community have turned their attention to the research and development of solid-state electrolytes. Solid-state electrolytes are considered one of the key materials for the next generation of lithium batteries due to their non-flammability, no leakage, and good high-temperature stability. Currently, solid-state electrolytes are primarily classified into two categories: polymer electrolytes and inorganic electrolytes. Polymer electrolytes have the advantages of good flexibility and ease of processing and molding, but their relatively low ionic conductivity makes it difficult to meet the demands of high power output. Inorganic electrolytes, on the other hand, have high ionic conductivity and good chemical stability, but their brittleness and poor interfacial compatibility limit their application in complex battery structures.

[0004] Given the limitations of single electrolytes, the concept of composite electrolytes has emerged. By combining polymer electrolytes with inorganic electrolytes, the advantages of both can be fully utilized, while their respective deficiencies can be overcome, resulting in the preparation of solid-state electrolyte materials with superior performance. Among the various polymer electrolytes, polyvinylidene fluoride-based electrolytes have become a hot topic in composite electrolyte research due to their excellent chemical and thermal stability and high ion transport capacity.

[0005] How to balance the compatibility and compounding between composite electrolytes to obtain electrolyte materials with higher ionic conductivity is a key issue that needs to be urgently solved in the current field of solid-state electrolyte research. Summary of the Invention

[0006] The present invention addresses the problems in the prior art and discloses a polyvinylidene fluoride-based composite solid electrolyte membrane and a method for preparing the same. The present invention regulates the relevant parameters of steam-induced phase separation to influence the degree of phase separation, forming a uniformly connected high-porosity structure and an appropriate pore size to match the particle size of the inorganic solid electrolyte powder. To further enhance the effect of steam-induced phase separation and form a uniform and stable porous structure, the present invention also optimizes the types and contents of additives and modifiers, and adds post-annealing treatment to further enhance the performance of the polyvinylidene fluoride-based composite solid electrolyte membrane. This results in a stable composite solid electrolyte membrane, improving the battery's electrical conductivity and cycling stability.

[0007] In a first aspect, the present invention provides a method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane, comprising the following steps:

[0008] S1: mixing the inorganic solid electrolyte powder, additives and solvent to obtain a slurry;

[0009] S2: adding a polyvinyl fluoride-based organic matrix and a modifier to the slurry described in S1, and stirring uniformly to obtain a polyvinyl fluoride-based polymer solution;

[0010] S3: The polyvinylidene fluoride-based polymer solution described in S2 is coated with a thin film obtained by a tabletop coating machine, and a porous polyvinylidene fluoride film is prepared by a steam-induced phase separation method;

[0011] S4: Immersing the polyvinylidene fluoride-based membrane described in S3 in a solid electrolyte slurry, and subjecting it to ultrasonic and drying post-treatment to finally obtain a polyvinylidene fluoride-based composite solid electrolyte membrane.

[0012] The steam-induced phase separation technique involves placing the film described in S3 in a constant temperature and humidity environment and adjusting the relevant parameters of the steam-induced phase separation technique to allow non-solvent vapor to flow into the film, thereby promoting phase separation of the film and thereby preparing a polyvinylidene fluoride-based film having a porous structure. In one embodiment of the present invention, the non-solvent vapor used for the steam-induced phase separation is water vapor.

[0013] When polyvinylidene fluoride-based polymers undergo steam-induced phase separation, their high crystallinity may make it difficult for steam to penetrate into the interior of the polymer, thereby limiting the phase separation process; and the strong molecular chain rigidity may also limit the mobility of the molecular chain under steam induction, thereby affecting the formation of a porous structure and being unable to effectively combine with inorganic solid electrolyte powders, thereby affecting the formation of a composite solid electrolyte membrane, which is not conducive to the improvement of ionic conductivity and electrochemical performance.

[0014] Therefore, on the one hand, we need to continuously study and adjust the relevant parameters of the steam-induced phase separation technology to affect the phase separation degree and porous structure of the polyvinyl fluoride-based polymer, form a suitable porosity to adapt to the particle size of the inorganic solid electrolyte powder, and form a stable and uniform composite solid electrolyte membrane; at the same time, by adjusting the parameters, its porous structure will not be damaged during the steam-induced phase separation process. On the other hand, it is necessary to optimize the constituent materials of the polyvinyl fluoride-based polymer solution to improve the physical and chemical properties of the polyvinyl fluoride-based polymer, thereby enhancing its adaptability in the phase separation process and the overall performance of the final product. In addition, in order to further improve the performance of the polyvinyl fluoride-based membrane obtained by the steam-induced phase separation technology, it is also necessary to optimize the post-processing of the polyvinyl fluoride-based membrane obtained by the steam-induced phase separation technology to obtain a more uniform and stable porous structure and improve the overall performance of the composite solid electrolyte membrane.

[0015] As a further solution, the S3 mid-table coating machine has a temperature range of 30°C-90°C, a blade thickness range of 50μm-200μm, and a coating speed range of 0.1m / min-5m / min. Appropriate coating machine temperature, blade thickness, and coating speed facilitate the production of a film material that is favorable for steam permeation, promotes the subsequent steam-induced phase separation process, and ensures uniformity and a regular pore structure in the polyvinylidene fluoride-based film.

[0016] As a further solution, the particle size D50 of the inorganic solid electrolyte powder in S1 can be selected from 80nm≤D50≤300nm. Selecting an appropriate particle size range (80nm≤D50≤300nm) can ensure that the inorganic solid electrolyte powder has good dispersion in the polyvinylidene fluoride polymer solution and is evenly filled into the pore structure during the steam-induced phase separation process. It also improves the interfacial bonding between the inorganic solid electrolyte powder and the polyvinylidene fluoride organic matrix, forming a stable composite solid electrolyte membrane, and improving the battery's electrical conductivity and cycle stability.

[0017] After extensive research by the applicant, the parameters that have a key impact on the steam-induced phase separation technology have been selected, including the steam temperature, steam humidity and steam induction time of steam-induced phase separation; the steam temperature and steam humidity mentioned in the present invention represent the required steam temperature and humidity set by the constant temperature and humidity chamber used, where the humidity represents the water vapor content in the air in the constant temperature and humidity chamber.

[0018] As a further solution, based on experience and experimental summary, it is concluded that when the steam temperature for steam-induced phase separation in S3 is selected from 50-100°C, the steam humidity is selected from 40-90%, and the steam induction time is selected from 40-300s, a stable porous structure can be maintained and the inorganic solid electrolyte powder can be ensured to be uniformly dispersed in the pores; a polyvinylidene fluoride-based composite solid electrolyte membrane with high ionic conductivity and cycle performance is formed.

[0019] As a further solution, the inorganic solid electrolyte powder in S1 includes one or more of garnet-type solid electrolyte powder, perovskite-type solid electrolyte powder, or NASICON-type solid electrolyte powder. As some specific examples, the garnet-type solid electrolyte powder includes lithium lanthanum zirconium oxide (LLZO) or a powder obtained by element replacement, doping, or modification of LLZO; the perovskite-type solid electrolyte includes lithium lanthanum titanate (LLTO) or a powder obtained by element replacement, doping, or modification of LLTO; and the NASICON-type solid electrolyte includes LATP or a powder obtained by element replacement, doping, or modification of LATP.

[0020] As a further embodiment, the solvent in S1 includes one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, triethyl phosphate, methyl acetate, ethyl acetate and acetone.

[0021] As a further embodiment, the polyvinylidene fluoride-based organic matrix in S2 includes one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trifluoroethylene, polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene, or polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene;

[0022] As a further solution, the content W of the inorganic solid electrolyte powder in S1 can be selected from 5%≤W≤30%, where W is the ratio of the mass of the inorganic solid electrolyte powder to the total mass of the S1 slurry; the content X of the polyvinyl fluoride-based organic matrix in S2 can be selected from 10%≤X≤30%, where X is the ratio of the mass of the polyvinyl fluoride-based organic matrix to the total mass of the slurry in S1.

[0023] Furthermore, the weight average molecular weight of the polyvinylidene fluoride-based organic matrix is ​​100,000-1,200,000.

[0024] In order to further improve the comprehensive performance of the polyvinyl fluoride-based composite solid electrolyte membrane, the present invention also considers the preparation of a porous polyvinyl fluoride-based composite solid electrolyte membrane by optimizing additives and modifiers and adding an annealing post-treatment process during the preparation stage of the polyvinyl fluoride polymer solution.

[0025] As a further solution, the auxiliary agent in S1 can be selected from one or more of polyolefin auxiliary agents with cyclic branches, polyol auxiliary agents, and alkyl salt auxiliary agents; as some specific examples: the polyolefin auxiliary agent with cyclic branches can be polyvinyl pyrrolidone, polystyrene, polyvinyl pyrazole, etc.; the polyol can be polyethylene glycol, polyvinyl alcohol, polyethylene glycol octylphenyl ether, etc.; the alkyl salt auxiliary agents include lauryl magnesium sulfate, sodium lauryl sulfate, sodium dodecyl sulfonate, sodium dodecylbenzenesulfonate, ammonium lauryl sulfate, lithium lauryl sulfate, etc.

[0026] As a further solution, the modifier in S2 can be selected from one or more of a silane coupling agent or a fluorocarbon surfactant; as some specific examples: the silane coupling agent is γ-aminopropyltriethoxysilane (KH-550), diethylenetriaminepropyltrimethoxysilane (NQ-62), N-(β-aminoethyl)-γ-aminopropyltriethoxysilane (KH-791), N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane (KH-792), N-aminoethyl-3-aminopropylmethyldimethoxysilane (WD-53 ), 3-isocyanatepropyltriethoxysilane, 3-aminopropyltrimethoxysilane (APTMS), γ-aminopropylmethyldiethoxysilane, NN diethylaminopropyltrimethoxysilane (KH-703), bis(3-trimethoxysilylpropyl)amine (BTMSPA) or more; the fluorocarbon surfactant is FC-4430, FS-65, TF-310, perfluoro-p-ethylhexanesulfonic acid, perfluoroalkyl poly(ethylene oxide) glycol, polyethylene glycol perfluorobutyrate, and N-(diethylene glycol) perfluoroamide or more. FC-4430 is a surfactant available from 3M, FS-65 is a surfactant available from Chemours, and TF-310 is a commercially available environmentally friendly fluorocarbon surfactant.

[0027] As a further solution, the auxiliary agent is preferably a polyolefin auxiliary agent with a cyclic branch: a polyol auxiliary agent with a mass ratio of 0.5-2; the modifier is preferably a silane coupling agent: a fluorocarbon surfactant with a mass ratio of 0.5-2; the proportion of the total mass of the auxiliary agent to the total mass of the slurry in S1 can be selected from 5-15%; the proportion of the total mass of the modifier to the total mass of the slurry in S1 can be selected from 0.1-5%.

[0028] In order to ensure the uniform dispersion and stable performance of the additives, the present invention mixes polyolefin additives and polyol additives in a certain proportion; the synergistic use of the two makes it easier to form a uniform and stable pore structure, reduce interface defects, and enable lithium ions to be transmitted not only in the inorganic solid electrolyte filled in the pore structure, but also at the interface between the organic polymer and the inorganic solid electrolyte, effectively broadening the lithium ion transmission path and reducing the transmission energy barrier of the composite solid electrolyte at the interface.

[0029] The combination of fluorocarbon surfactants and silane coupling agents can further optimize the performance of composite solid electrolyte membranes. During the steam induction process, a stable inorganic solid electrolyte powder-polyvinylidene fluoride organic matrix bonding layer is formed, creating a uniform lithium ion transmission channel and filling structural defects to form a denser and more complete interface, thereby improving the ionic conductivity of the composite solid electrolyte membrane.

[0030] Furthermore, the auxiliary agent is preferably a polyolefin auxiliary agent with cyclic branches: a polyol auxiliary agent in a mass ratio of 1:1; the modifier is preferably a silane coupling agent: a fluorocarbon surfactant in a mass ratio of 1:1.

[0031] Furthermore, the auxiliary agent is preferably selected from polyvinyl pyrrolidone and polyethylene glycol.

[0032] Furthermore, the molecular weight of polyvinyl pyrrolidone is selected from 30,000-70,000, and the content of polyvinyl pyrrolidone is selected from 3%-8%; wherein the content of polyvinyl pyrrolidone is the mass of polyvinyl pyrrolidone to the total mass of the slurry in S1.

[0033] The molecular weight and content of the additives work together to make the steam-induced phase separation process more efficient and controllable.

[0034] In order to further improve the performance of the polyvinylidene fluoride-based composite solid electrolyte membrane, the present invention also optimizes the post-annealing treatment of the polyvinylidene fluoride membrane obtained by steam-induced phase separation.

[0035] As a further solution, the annealing treatment in S4 is to adjust the annealing temperature to 100-150° C. and the annealing time to 1-4 hours. Selecting a suitable annealing temperature range (100-150° C.) and screening the optimal annealing time (1-4 hours) ensures that the performance and structure of the membrane are optimally balanced.

[0036] As a further embodiment, the solid electrolyte slurry in S4 is preferably an aqueous solid electrolyte slurry; the aqueous solid electrolyte slurry is an aqueous mixture containing solid electrolyte particles. Specifically, the slurry may be selected from LLZO aqueous solid electrolyte slurry, LLTO aqueous solid electrolyte slurry, or LATP aqueous solid electrolyte slurry. The primary solvent of the aqueous solid electrolyte is deionized water, which helps reduce production costs, minimize wastewater discharge, and alleviate environmental pollution.

[0037] Furthermore, in the aqueous solid electrolyte slurry, the solid electrolyte particle content accounts for 10%-40% of the total content of the aqueous solid electrolyte slurry, and the solid electrolyte particle size D50 is selected from 100nm-500nm.

[0038] As a further solution, in the ultrasonic treatment in S4, the ultrasonic temperature can be selected from 20-50°C, and the ultrasonic time can be selected from 0.5h-12h.

[0039] Ultrasonic temperature and time within the controllable range are beneficial to the polyvinylidene fluoride-based composite solid electrolyte membrane base membrane and internal pores being mixed with inorganic solid electrolyte powder, thereby improving the transmission efficiency of lithium ions.

[0040] As a further solution, the drying method in S4 can be selected from freeze drying, oven drying, radiation drying or vacuum drying.

[0041] In a second aspect, the present invention provides a polyvinylidene fluoride-based composite solid electrolyte membrane, which is obtained by the preparation method described in the first aspect. The polyvinylidene fluoride-based composite solid electrolyte membrane has a thickness of 20 μm to 150 μm.

[0042] As a further solution, the porosity of the polyvinylidene fluoride-based composite solid electrolyte membrane is preferably 80-95%; and the most probable pore diameter is preferably 2 μm-5 μm.

[0043] The polyvinylidene fluoride-based composite solid electrolyte membrane has a continuously interconnected pore structure and can effectively accommodate more inorganic solid electrolyte powders; the appropriate thickness range improves the lithium ion transmission efficiency and the electrical conductivity of the solid electrolyte.

[0044] In a third aspect, the present invention provides a battery comprising a positive electrode, a negative electrode, and the polyvinylidene fluoride-based composite solid electrolyte membrane according to the second aspect.

[0045] The characteristics and beneficial effects of the present invention are:

[0046] (1) The present invention affects the degree of phase separation by regulating the relevant parameters of steam-induced phase separation to form a high-porous structure with uniform connectivity and a suitable pore size to adapt to the particle size of the inorganic solid electrolyte powder, thereby enhancing the interfacial bonding force between the inorganic solid electrolyte powder and the polyvinylidene fluoride organic matrix, which is conducive to forming a stable composite solid electrolyte membrane and improving the conductivity and cycle stability of the battery.

[0047] (2) In order to further enhance the effect of steam-induced phase separation, the present invention also optimizes the types and contents of additives and modifiers, which is beneficial to the uniform diffusion and filling of inorganic solid electrolytes in polyvinylidene fluoride organic matrices, thereby enhancing the composite effect of the electrolyte; and can improve interface defects, broaden the path of lithium ion transmission, and improve ionic conductivity.

[0048] (3) In order to further improve the performance of the polyvinyl fluoride based composite solid electrolyte membrane, the present invention also optimizes the annealing post-treatment of the polyvinyl fluoride based membrane obtained by steam-induced phase separation; eliminates the stress that may accumulate during the steam induction process, further improves the stability of the membrane structure, improves the membrane dielectric constant, and obtains a polyvinyl fluoride based composite solid electrolyte membrane with excellent ionic conductivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments or descriptions of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0050] Figure 1 This is a SEM image of the polyvinylidene fluoride-based porous base membrane obtained in Example 6 of the present invention;

[0051] Figure 2 This is a SEM image of the polyvinylidene fluoride-based composite solid electrolyte membrane obtained in Example 6 of the present invention. DETAILED DESCRIPTION

[0052] In order to facilitate understanding of the present invention, the present invention will be described in more detail below, and embodiments of the present invention are given, but the scope of the present invention is not limited thereby.

[0053] To further enhance the performance of polyvinylidene fluoride-based composite solid-state electrolytes, this invention employs vapor-induced phase separation technology to create a porous electrolyte membrane. This porous structure not only provides more channels for lithium ion transport, improving the electrolyte's ionic conductivity, but also helps mitigate the effects of volume changes on the electrolyte membrane during charge and discharge, thereby enhancing the battery's cycling stability.

[0054] However, the present invention found that the steam-induced phase separation technology faces some technical difficulties and challenges in the process of preparing polyvinyl fluoride-based composite solid electrolyte membranes, which is not conducive to the formation of a uniform and stable porous structure, and the inorganic solid electrolyte powder is unevenly distributed, resulting in the inability to effectively improve the conductivity. Therefore, the present invention has considered how to optimize the composition of polyvinyl fluoride-based materials to obtain an ideal steam-induced phase separation effect and a suitable pore structure, and conducted the following experimental research.

[0055] In a first aspect, the present invention provides a method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane, comprising the following steps:

[0056] S1: mixing the inorganic solid electrolyte powder, additives and solvent to obtain a slurry;

[0057] S2: adding a polyvinyl fluoride-based organic matrix and a modifier to the slurry described in S1, and stirring uniformly to obtain a polyvinyl fluoride-based polymer solution;

[0058] S3: After scraping the polyvinylidene fluoride vinyl-based polymer solution described in S2 using a desktop coating machine, a porous polyvinylidene fluoride film is prepared by a steam-induced phase separation method;

[0059] S4: Immersing the polyvinylidene fluoride-based membrane described in S3 in a solid electrolyte slurry, and subjecting it to ultrasonic and drying post-treatment to finally obtain a polyvinylidene fluoride-based composite solid electrolyte membrane.

[0060] Polyvinylidene fluoride (PVDF) materials have high crystallinity and molecular chain rigidity, which gives them good chemical stability and excellent mechanical properties when used in solid electrolyte materials, resulting in higher ionic conductivity, enhanced battery energy efficiency and cycle performance. However, when steam-induced phase separation is performed on a PVDF-based polymer solution after scraping, its high crystallinity may make it difficult for steam to penetrate the polymer interior, thereby limiting the phase separation process. In addition, the strong molecular chain rigidity may also limit the mobility of the molecular chain under steam induction, thereby affecting the formation of a porous structure, resulting in pores formed during the phase separation process that are too large or too small, and unevenly distributed. It is also impossible to effectively combine with inorganic solid electrolyte powders, thereby affecting the formation of a composite solid electrolyte membrane and hindering the improvement of ionic conductivity and electrochemical performance.

[0061] Therefore, on the one hand, we need to continuously study and adjust the relevant parameters of the steam-induced phase separation technology to affect the phase separation degree and porous structure of the polyvinyl fluoride-based polymer, form a suitable porosity to adapt to the particle size of the inorganic solid electrolyte powder, and form a stable and uniform composite solid electrolyte membrane; at the same time, by adjusting the parameters, its porous structure will not be damaged during the steam-induced phase separation process. On the other hand, it is necessary to optimize the constituent materials of the polyvinyl fluoride-based polymer solution to improve the physical and chemical properties of the polyvinyl fluoride-based polymer, thereby enhancing its adaptability in the phase separation process and the overall performance of the final product. In addition, in order to further improve the performance of the polyvinyl fluoride-based membrane obtained by the steam-induced phase separation technology, it is also necessary to optimize the post-processing of the polyvinyl fluoride-based membrane obtained by the steam-induced phase separation technology to obtain a more uniform and stable porous structure and improve the overall performance of the composite solid electrolyte membrane.

[0062] As a further solution, the S3 mid-table coating machine has a temperature range of 30°C-90°C, a blade thickness range of 50μm-200μm, and a coating speed range of 0.1m / min-5m / min. Appropriate coating machine temperature, blade thickness, and coating speed facilitate the production of a film material that is favorable for steam permeation, promotes the subsequent steam-induced phase separation process, and ensures uniformity and a regular pore structure in the polyvinylidene fluoride-based film.

[0063] As a further solution, the particle size D50 of the inorganic solid electrolyte powder in S1 can be selected from 80nm≤D50≤300nm. Selecting an appropriate particle size range (80nm≤D50≤300nm) can ensure that the inorganic solid electrolyte powder has good dispersion in the polyvinylidene fluoride polymer solution and is evenly filled into the pore structure during the steam-induced phase separation process. It also improves the interfacial bonding between the inorganic solid electrolyte powder and the polyvinylidene fluoride organic matrix, forming a stable composite solid electrolyte membrane, and improving the battery's electrical conductivity and cycle stability.

[0064] After extensive research by the applicant, the parameters that have a key impact on the steam-induced phase separation technology have been selected, including the steam temperature, steam humidity and steam induction time of steam-induced phase separation; the steam temperature and steam humidity mentioned in the present invention represent the required steam temperature and humidity set by the constant temperature and humidity chamber used, where the humidity represents the water vapor content in the air in the constant temperature and humidity chamber.

[0065] As a further solution, based on experience and experimental summary, it is concluded that when the steam temperature for steam-induced phase separation in S3 is selected from 50-100°C, the steam humidity is selected from 40-90%, and the steam induction time is selected from 40-300s, a stable porous structure can be maintained and the inorganic solid electrolyte powder can be ensured to be uniformly dispersed in the pores; a polyvinylidene fluoride-based composite solid electrolyte membrane with high ionic conductivity and cycle performance is formed.

[0066] When performing steam-induced phase separation on a polyvinyl fluoride-based polymer solution, selecting a higher steam temperature (50-100°C) is beneficial to increasing the phase separation rate, promoting the steam-induced phase separation process, forming a more uniform network porous structure, and facilitating the uniform embedding of the inorganic solid electrolyte powder. The steam temperature within the optimized range can not only increase the thermal motion of the molecular chain, reduce crystallinity, and increase the rate of phase separation; it also helps to reduce the rigidity of the molecular chain, making the molecular chain softer, increasing the mobility of the chain segments, forming a uniform and stable pore structure, and better enhancing the interfacial bonding ability between the inorganic solid electrolyte and the polyvinyl fluoride-based organic matrix, thereby improving the uniformity and stability of the composite solid electrolyte. However, excessively high temperatures may lead to the decomposition of the polyvinyl fluoride material, thereby affecting the formation of its porous structure and its bonding with the inorganic solid electrolyte. It may also cause the phase separation rate to be too fast, and the porous structure cannot be fully induced, which is not conducive to the improvement of electrochemical performance.

[0067] When a higher steam temperature is selected, it is necessary to match the appropriate steam humidity (40-90%). This optimization range can maintain a high heat transfer efficiency; and when the humidity is high, the interaction between steam and polyvinylidene fluoride-based materials is enhanced, the polyvinylidene fluoride chain segments are prone to relaxation, and the rigidity of the molecular chain is reduced, which helps to form more amorphous regions and increase the phase separation process. Higher steam temperature and humidity are conducive to the violent movement of molecules, forming a more open and connected pore structure, which is conducive to the uniform filling of inorganic solid electrolyte powder in the pores, broadening the transmission path of lithium ions and improving ionic conductivity. However, lower steam humidity is not enough to induce steam phase separation, thereby affecting the phase separation process and is not conducive to the formation of a high-porosity structure.

[0068] Under optimized steam temperature and humidity conditions, in order to more fully integrate the inorganic solid electrolyte powder, it is also necessary to match the steam induction time (40-300s) to ensure that the phase separation process proceeds fully without excessive induction, which may lead to membrane structure damage. A shorter steam induction time may not fully promote solvent evaporation and phase separation, resulting in incomplete formation of the porous structure, which in turn affects the formation of the composite solid electrolyte. On the other hand, an excessively long induction time may cause the membrane structure to over-relax, resulting in excessive pore size or structural collapse, which is not conducive to the stable existence of the inorganic solid electrolyte powder in its pores, affecting the composite effect and even reducing its electrochemical performance.

[0069] Therefore, it is necessary to comprehensively regulate the steam induction parameters and select the appropriate inorganic solid electrolyte particle size to prepare a stable polyvinylidene fluoride-based composite solid electrolyte membrane.

[0070] As a further solution, the inorganic solid electrolyte powder in S1 includes one or more of garnet-type solid electrolyte powder, perovskite-type solid electrolyte powder, or NASICON-type solid electrolyte powder. As some specific examples, the garnet-type solid electrolyte powder includes lithium lanthanum zirconium oxide (LLZO) or a powder obtained by element replacement, doping, or modification of LLZO; the perovskite-type solid electrolyte includes lithium lanthanum titanate (LLTO) or a powder obtained by element replacement, doping, or modification of LLTO; and the NASICON-type solid electrolyte includes LATP or a powder obtained by element replacement, doping, or modification of LATP.

[0071] As a further embodiment, the solvent in S1 includes one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, triethyl phosphate, methyl acetate, ethyl acetate and acetone.

[0072] As a further embodiment, the polyvinylidene fluoride-based organic matrix in S2 includes one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trifluoroethylene, polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene, or polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene;

[0073] As a further solution, the content W of the inorganic solid electrolyte powder in S1 can be selected from 5%≤W≤30%, where W is the ratio of the mass of the inorganic solid electrolyte powder to the total mass of the S1 slurry; the content X of the polyvinyl fluoride-based organic matrix in S2 can be selected from 10%≤X≤30%, where X is the ratio of the mass of the polyvinyl fluoride-based organic matrix to the total mass of the slurry in S1.

[0074] By optimizing the content of inorganic solid electrolyte powder and polyvinylidene fluoride-based organic matrix, the porosity and pore size distribution of the porous membrane can be further regulated to ensure that the inorganic solid electrolyte powder is evenly embedded in the pores and exists stably; thereby achieving fine control of the electrolyte membrane performance.

[0075] Furthermore, the weight average molecular weight of the polyvinylidene fluoride-based organic matrix is ​​100,000-1,200,000.

[0076] In order to further improve the comprehensive performance of the polyvinyl fluoride-based composite solid electrolyte membrane, the present invention also considers the preparation of a porous polyvinyl fluoride-based composite solid electrolyte membrane by optimizing additives and modifiers and adding an annealing post-treatment process during the preparation stage of the polyvinyl fluoride polymer solution.

[0077] As a further solution, the auxiliary agent in S1 can be selected from one or more of polyolefin auxiliary agents with cyclic branches, polyol auxiliary agents, and alkyl salt auxiliary agents; as some specific examples: the polyolefin auxiliary agent with cyclic branches can be polyvinyl pyrrolidone, polystyrene, polyvinyl pyrazole, etc.; the polyol can be polyethylene glycol, polyvinyl alcohol, polyethylene glycol octylphenyl ether, etc.; the alkyl salt auxiliary agents include lauryl magnesium sulfate, sodium lauryl sulfate, sodium dodecyl sulfonate, sodium dodecylbenzenesulfonate, ammonium lauryl sulfate, lithium lauryl sulfate, etc.

[0078] On the one hand, the addition of additives can promote the uniform diffusion of inorganic solid electrolyte powder in the pores of polyvinylidene fluoride-based organic matrices, enhance the surface activity of inorganic solid electrolytes, and help improve the binding ability of inorganic solid electrolytes and polyvinylidene fluoride polymer matrices during steam-induced phase separation. On the other hand, the addition of additives can also promote the dispersion and dissolution of organic polymer chains, thereby accelerating the speed of steam flowing into the polymer solution during steam-induced phase separation, accelerating the thermodynamic and kinetic driving forces of phase separation, and forming a pore structure with continuous interconnection.

[0079] As a further solution, the modifier in S2 can be selected from one or more of a silane coupling agent or a fluorocarbon surfactant; as some specific examples: the silane coupling agent is γ-aminopropyltriethoxysilane (KH-550), diethylenetriaminepropyltrimethoxysilane (NQ-62), N-(β-aminoethyl)-γ-aminopropyltriethoxysilane (KH-791), N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane (KH-792), N-aminoethyl-3-aminopropylmethyldimethoxysilane (WD-53 ), 3-isocyanatepropyltriethoxysilane, 3-aminopropyltrimethoxysilane (APTMS), γ-aminopropylmethyldiethoxysilane, NN diethylaminopropyltrimethoxysilane (KH-703), bis(3-trimethoxysilylpropyl)amine (BTMSPA) one or more; the fluorocarbon surfactant is FC-4430, FS-65, TF-310, perfluoro-p-ethylhexanesulfonic acid, perfluoroalkyl poly(oxyethylene) glycol, polyethylene glycol perfluorobutyrate and N-(diethylene glycol) perfluoroamide one or more.

[0080] During the steam induction process, excessive steam humidity may reduce the activity of the polymer material, affecting the phase separation effect. To overcome this problem, the introduction of modifiers can effectively increase the activity of the polymer solution and enhance its responsiveness to steam. Silane coupling agents crosslink with polymer molecular chains, improving their thermal stability and mechanical properties, thereby maintaining good structural stability during the steam induction process. Fluorocarbon surfactants reduce surface tension, improving the wettability and fluidity of the polymer solution, and promoting more uniform steam penetration and diffusion, thereby forming a more uniform pore structure.

[0081] As a further solution, the auxiliary agent is preferably a polyolefin auxiliary agent with a cyclic branch: a polyol auxiliary agent with a mass ratio of 0.5-2; the modifier is preferably a silane coupling agent: a fluorocarbon surfactant with a mass ratio of 0.5-2; the proportion of the total mass of the auxiliary agent to the total mass of the slurry in S1 can be selected from 5-15%; the proportion of the total mass of the modifier to the total mass of the slurry in S1 can be selected from 0.1-5%.

[0082] In order to ensure the uniform dispersion and stable performance of the additives, the present invention mixes polyolefin additives and polyol additives in a certain proportion; the cyclic branched polyolefin additives can form a stable complex with the inorganic solid electrolyte powder, thereby being uniformly dispersed in the polymer matrix during the phase separation process; the polyol additives further increase the viscosity of the polymer solution, so that the inorganic solid electrolyte powder and the polymer matrix are more stably composited during the steam-induced phase separation process; the two additives are used in conjunction with each other to more easily form a uniform and stable pore structure, reduce interface defects, and enable lithium ions to be transmitted not only in the inorganic solid electrolyte filled in the pore structure, but also at the interface between the organic polymer and the inorganic solid electrolyte, effectively broadening the lithium ion transmission path and reducing the transmission energy barrier of the composite solid electrolyte at the interface.

[0083] The combination of fluorocarbon surfactants and silane coupling agents can further optimize the performance of composite solid electrolyte membranes. The addition of silane coupling agents enhances the reactivity between the polyvinylidene fluoride-based organic matrix and the inorganic solid electrolyte powder through a coupling reaction, improving its responsiveness to steam and facilitating the formation of a stable inorganic solid electrolyte powder-polyvinylidene fluoride-based organic matrix bonding layer during the steam-induced phase separation process, creating a uniform lithium ion transmission channel. Fluorocarbon surfactants and polyvinylidene fluoride-based organic matrices can be uniformly mixed and dispersed evenly on the membrane surface and within the internal pore structure during the steam-induced phase separation process, filling structural defects and forming a denser and more complete interface, thereby improving the ionic conductivity of the composite solid electrolyte membrane.

[0084] Furthermore, the auxiliary agent is preferably a polyolefin auxiliary agent with cyclic branches: a polyol auxiliary agent in a mass ratio of 1:1; the modifier is preferably a silane coupling agent: a fluorocarbon surfactant in a mass ratio of 1:1.

[0085] Furthermore, the auxiliary agent is preferably selected from polyvinyl pyrrolidone and polyethylene glycol.

[0086] Furthermore, the molecular weight of polyvinyl pyrrolidone is selected from 30,000-70,000, and the content of polyvinyl pyrrolidone is selected from 3%-8%; wherein the content of polyvinyl pyrrolidone is the mass of polyvinyl pyrrolidone to the total mass of the slurry in S1.

[0087] Preferred additives for the present invention are polyvinyl pyrrolidone and polyethylene glycol. Polyethylene glycol can increase the viscosity of the polymer solution, further strengthening the interfacial bonding between the inorganic solid electrolyte powder and the organic polymer during steam induction. Polyvinyl pyrrolidone, containing pyrrolidone rings, has a stronger hydrophilicity, thereby accelerating steam penetration and diffusion, improving the efficiency of phase separation. The two synergistically form a more stable composite solid electrolyte structure. Furthermore, polyvinyl pyrrolidone has high thermal and chemical stability, which helps maintain the integrity and stability of the porous structure during subsequent steam-induced phase separation and post-annealing treatment.

[0088] In addition, optimizing the molecular weight and content of polyvinyl pyrrolidone can further regulate the formation of polyvinylidene fluoride composite solid electrolyte membranes. Selecting polyvinyl pyrrolidone with a high molecular weight (30,000-70,000) can entangle with polyvinylidene fluoride chain segments to form a uniform network structure. This structure can effectively reduce the formation of crystalline regions during steam-induced phase separation, while promoting the influx of non-solvent vapor, increasing the phase separation rate, and obtaining a more uniform porous structure with a reasonable pore size distribution, which is conducive to the uniform and stable distribution of inorganic solid electrolyte powders in the pores. However, when the molecular weight is too high or too low, it may affect the stability of the polymer solution and the uniformity of the phase separation process. The pores formed are not conducive to the embedding of inorganic solid electrolyte powders, reducing the composite effect.

[0089] The molecular weight and content of the additive need to work in synergy. When the content is too low, the additive may not fully function, affecting the formation of the pore structure and the uniform embedding of the inorganic solid electrolyte powder. When the content is too high, the solution fluidity may deteriorate, affecting the phase separation process, hindering the formation of a uniform porous structure with appropriate pore size, and reducing the performance of the composite solid electrolyte membrane. Therefore, an appropriate amount of polyvinyl pyrrolidone (3%-8%) can make the steam-induced phase separation process more efficient and controllable.

[0090] In order to further improve the performance of the polyvinylidene fluoride-based composite solid electrolyte membrane, the present invention also optimizes the post-annealing treatment of the polyvinylidene fluoride membrane obtained by steam-induced phase separation.

[0091] As a further solution, the annealing treatment in S4 is to adjust the annealing temperature to 100-150° C. and the annealing time to 1-4 hours.

[0092] After steam induction, stress may accumulate within the membrane due to processes such as solvent volatilization and phase separation. Appropriate annealing temperature and time can help eliminate residual stress in the porous structure, reduce the formation of microcracks, and prevent membrane rupture from affecting the porous structure. However, too short an annealing time may not fully eliminate stress and defects within the membrane, while too long an annealing time or too high an annealing temperature may lead to excessive shrinkage of the membrane and destruction of the pore structure, reducing ionic conductivity. Therefore, we selected a suitable annealing temperature range (100-150°C) and screened for the optimal annealing time (1-4 hours) to ensure an optimal balance between membrane performance and structure.

[0093] As a further embodiment, the solid electrolyte slurry in S4 is preferably an aqueous solid electrolyte slurry; the aqueous solid electrolyte slurry is an aqueous mixture containing solid electrolyte particles. Specifically, the slurry may be selected from LLZO aqueous solid electrolyte slurry, LLTO aqueous solid electrolyte slurry, or LATP aqueous solid electrolyte slurry. The primary solvent of the aqueous solid electrolyte is deionized water, which helps reduce production costs, minimize wastewater discharge, and alleviate environmental pollution.

[0094] Furthermore, in the aqueous solid electrolyte slurry, the solid electrolyte particle content accounts for 10%-40% of the total content of the aqueous solid electrolyte slurry, and the solid electrolyte particle size D150 is selected from 100nm-500nm.

[0095] As a further solution, in the ultrasonic treatment in S4, the ultrasonic temperature can be selected from 20-50°C, and the ultrasonic time can be selected from 0.5h-12h.

[0096] Ultrasonic temperature and time within the controllable range are beneficial to the polyvinylidene fluoride-based composite solid electrolyte membrane base membrane and internal pores being mixed with inorganic solid electrolyte powder, thereby improving the transmission efficiency of lithium ions.

[0097] As a further solution, the drying method in S4 can be selected from freeze drying, oven drying, radiation drying or vacuum drying.

[0098] In a second aspect, the present invention provides a polyvinylidene fluoride-based composite solid electrolyte membrane, which is obtained by the preparation method described in the first aspect. The polyvinylidene fluoride-based composite solid electrolyte membrane has a thickness of 20 μm to 150 μm.

[0099] As a further solution, the porosity of the polyvinylidene fluoride-based composite solid electrolyte membrane is preferably 80-95%; and the most probable pore diameter is preferably 2 μm-5 μm.

[0100] This polyvinylidene fluoride-based composite solid electrolyte membrane features a continuously interconnected pore structure, which facilitates the rapid transport of lithium ions. Due to its high porosity and appropriate pore size distribution, the membrane can effectively accommodate a larger amount of inorganic solid electrolyte powder, thereby improving overall ionic conductivity. Furthermore, the appropriate thickness range of the polyvinylidene fluoride membrane avoids the problem of excessively thick membrane layers leading to an extended ion transport path, thereby improving lithium ion transport efficiency and enhancing the conductivity of the solid electrolyte.

[0101] In a third aspect, the present invention provides a battery comprising a positive electrode, a negative electrode, and the polyvinylidene fluoride-based composite solid electrolyte membrane according to the second aspect.

[0102] As a specific example of the implementation of the present invention, a detailed case is provided as follows:

[0103] Example 1

[0104] 1) Weigh 2.5 g of LLTO powder (particle size D50: 100 nm) and 9% polyvinyl pyrrolidone (weight-average molecular weight: 60,000) in 43.5 g of N-methylpyrrolidone, seal the mixture, and ultrasonically disperse the mixture at 60° C. for 12 h to obtain a uniformly dispersed slurry. The polyvinyl pyrrolidone content is the percentage of the total mass of the slurry.

[0105] 2) Add 10.0g of polyvinylidene fluoride (weight average molecular weight 600,000) and 1g of 3-isocyanatepropyltriethoxysilane to the above slurry, stir at 600 rpm for 12h at 60°C to mix evenly to form a homogeneous solution, place in a 60°C oven, and let stand for degassing for 12h. Pour the degassed solution onto a clean glass plate, use a 100μm scraper to scrape the film at a speed of 1m / min on a 60°C desktop coating machine, place it in a constant temperature and humidity chamber (temperature 55°C, humidity 65%), steam-induced for 80s, phase separation and solidification to form a film, peel off the solidified film and soak it in deionized water, take it out and freeze-dry it to obtain a porous base film;

[0106] 3) The porous base membrane was fixed on a square metal clamp frame and immersed in an aqueous LLTO solid electrolyte slurry (solid content of 20%, particle size D150 of 150 nm). After ultrasonic treatment at 30°C for 8 hours, the membrane was removed, vacuum dried at 60°C, and then hot-pressed on a flat plate (temperature of 60°C) to obtain a composite solid electrolyte membrane with a membrane thickness of 75 μm.

[0107] Example 2

[0108] 1) Weigh 2.5 g of LLZO powder (particle size D50: 100 nm) and 9% polyethylene glycol (weight-average molecular weight: 400) in 43.5 g of N-methylpyrrolidone, seal the mixture, and ultrasonically disperse it at 60° C. for 12 h to obtain a uniformly dispersed slurry; wherein the polyethylene glycol content is the mass of the polyethylene glycol relative to the total mass of the slurry.

[0109] 2) Add 10.0g of polyvinylidene fluoride (weight average molecular weight 600,000) and 1g of FC-4430 to the above slurry, stir at 600 rpm for 12 hours at 60°C to mix evenly to form a homogeneous solution, place in a 60°C oven, and let it stand for 12 hours to degas. Pour the degassed solution onto a clean glass plate, use a 100μm scraper to scrape the film at a speed of 1m / min on a 60°C desktop coating machine, place it in a constant temperature and humidity chamber (temperature 70°C, humidity 85%), steam-induce for 70 seconds, and then phase separate and solidify to form a film. Peel off the solidified film and soak it in deionized water. After removal, freeze-dry to obtain a porous base film.

[0110] 3) The porous base membrane was fixed on a square metal clamp frame and immersed in an aqueous LLZO solid electrolyte slurry (solid content of 20%, particle size D150 of 150 nm). After ultrasonic treatment at 30°C for 8 hours, the membrane was taken out, vacuum dried at 60°C, and then hot-pressed on a flat plate (temperature of 60°C) to obtain a composite solid electrolyte membrane with a membrane thickness of 78 μm.

[0111] Example 3

[0112] 1) Weigh 2.5 g of LATP powder (particle size D50, 100 nm), 4% polyethylene glycol (weight-average molecular weight, 400), and 4% polyvinylpyrrolidone (weight-average molecular weight, 40,000) in 43.5 g of N-methylpyrrolidone, seal the mixture, and ultrasonically disperse at 60°C for 12 h to obtain a uniformly dispersed slurry.

[0113] 2) Add 10.0g of polyvinylidene fluoride (weight average molecular weight 600,000), 0.5g of 3-isocyanatepropyltriethoxysilane and 0.5g of FC-4430 to the above slurry, stir at 600 rpm for 12h at 60°C to mix evenly to form a homogeneous solution, place in a 60°C oven, and let stand for degassing for 12h. Pour the degassed solution onto a clean glass plate, use a 100μm scraper to scrape the film at a speed of 1m / min on a 60°C desktop coating machine, place it in a constant temperature and humidity chamber (temperature 60°C, humidity 80%), steam-induced for 60s, phase separation and solidification to form a film, peel off the solidified film and soak it in deionized water, take it out and freeze-dry it to obtain a porous base film;

[0114] 3) The porous base membrane was fixed on a square metal clamp frame and immersed in an aqueous LATP solid electrolyte slurry (solid content of 20%, particle size D150 of 150 nm). After ultrasonic treatment at 30°C for 8 h, the membrane was removed, vacuum dried at 60°C, and then hot pressed on a flat plate (temperature of 60°C) to obtain a composite solid electrolyte membrane with a thickness of 70 μm.

[0115] Example 4

[0116] The specific process is the same as that of Example 3, except that: 3% polyethylene glycol (weight-average molecular weight of 400) and 6% polyvinyl pyrrolidone (weight-average molecular weight of 50,000); 0.3g of 3-isocyanatepropyltriethoxysilane and 0.6g of FC-4430; and the thickness of the composite solid electrolyte membrane is 74μm.

[0117] Example 5

[0118] The specific process is the same as that of Example 3, except that: 6% polyethylene glycol (weight-average molecular weight of 400) and 3% polyvinyl pyrrolidone (weight-average molecular weight of 35,000); 0.6g of 3-isocyanatepropyltriethoxysilane and 0.3g of FC-4430; and the thickness of the composite solid electrolyte membrane is 77μm.

[0119] Example 6

[0120] The specific process is the same as that of Example 3, except that after obtaining the composite solid electrolyte membrane, the composite solid electrolyte membrane is sandwiched between two glass sheets, placed in a programmable temperature controlled oven, heated to 120°C at a rate of 1°C / min, annealed for 2 hours, and then cooled to 25°C at a rate of 1°C / min. The SEM image of the polyvinylidene fluoride porous base membrane is shown in FIG. Figure 1 As shown; the SEM image of the polyvinylidene fluoride composite solid electrolyte membrane is as shown Figure 2 As shown, the thickness of the polyvinylidene fluoride-based composite solid electrolyte membrane is 50 μm.

[0121] Example 7

[0122] The specific process is the same as that of Example 6, except that the annealing temperature is 150° C., the annealing time is 1 h, and the thickness of the polyvinylidene fluoride-based composite solid electrolyte membrane is 55 μm.

[0123] Comparative Example 1

[0124] The specific process is the same as that of Example 3, except that no LATP powder is added.

[0125] Comparative Example 2

[0126] The specific process is the same as that of Example 3, except that the particle size D50 of the LATP powder is 500 nm.

[0127] Comparative Example 3

[0128] The specific process is the same as that of Example 3, except that polyethylene glycol and polyvinyl pyrrolidone are not added.

[0129] Comparative Example 4

[0130] The specific process is the same as that of Example 3, except that 3-isocyanatepropyltriethoxysilane and FC-4430 are not added.

[0131] Comparative Example 5

[0132] The specific process is the same as that of Example 3, except that the steam induction temperature in the constant temperature and humidity chamber is 120°C.

[0133] Comparative Example 6

[0134] The specific process is the same as that of Example 3, except that the steam-induced humidity in the constant temperature and humidity chamber is 30%.

[0135] Comparative Example 7

[0136] The specific process is the same as that of Example 3, except that the steam induction time in the constant temperature and humidity chamber is 40 seconds.

[0137] Comparative Example 8

[0138] The specific process is the same as that of Example 3, except that the content of polyvinyl pyrrolidone is 20% and the weight average molecular weight is 90,000.

[0139] Comparative Example 9

[0140] The specific process is the same as that of Example 3, except that the added amount of 3-isocyanatepropyltriethoxysilane and FC-4430 is 2.0 g.

[0141] Comparative Example 10

[0142] The specific process is the same as that of Example 6, except that the annealing temperature is 160° C. and the annealing time is 1 h.

[0143] Specific test conditions and methods:

[0144] The surface morphology of the porous base membrane and the composite solid electrolyte membrane was tested and analyzed by scanning electron microscopy, and gold spraying treatment was performed with a gold spraying time of 30 s and an acceleration voltage of 3 kV.

[0145] The porosity of the porous base membrane was tested by a mercury intrusion instrument (test pressure: low pressure 50 psi, high pressure 30000 psi).

[0146] The most probable pore size of porous base membranes is determined using the gas-liquid displacement method. Using isopropyl alcohol as the wetting agent and nitrogen as the working gas, pressure is applied on one side to expel the wetting agent from the membrane. The most probable pore size is calculated by measuring the gas permeation flux at different inlet pressures.

[0147] The slurry wetting time of the porous base membrane was measured using a contact angle meter. A syringe was used to drop the aqueous electrolyte slurry onto the membrane surface, and the time until the contact angle reached 0° was recorded. This was the slurry wetting time.

[0148] The dielectric constant of the composite solid electrolyte membrane was tested by an automatic component analyzer. The test frequency range was 20~3×10 5 Hz.

[0149] The ionic conductivity of the composite solid electrolyte membrane was measured by electrochemical impedance spectroscopy. Stainless steel sheets were used as positive and negative electrodes, and button cells were assembled for electrochemical impedance spectroscopy. The test frequency range was 0.1-10 6 Hz, amplitude 10mV.

[0150] Lithium iron phosphate was used as the positive electrode and lithium metal as the negative electrode to assemble button batteries. The prepared button batteries were subjected to constant current charge and discharge tests using a battery testing system. After three cycles of activation at a rate of 0.1C, they were subsequently cycled at a rate of 0.5C.

[0151] Table 1

[0152]

[0153]

[0154] It can be seen from Examples 1-7 and Comparative Examples 1-10 that by selecting a suitable range of steam induction parameters and an appropriate inorganic solid electrolyte particle size range, the electrical conductivity can be effectively improved, a higher porosity and a larger pore size can be obtained, and the wettability of the solid electrolyte slurry is good. This shows that the steam induction parameters within the control range can achieve a good phase separation effect and form a uniformly connected network porous structure; and can adapt to the intercalation of inorganic solid electrolyte powders, broaden the transmission path of lithium ions, thereby improving the ionic conductivity. Moreover, by adding additives and modifiers and optimizing their types and contents, the formation of a high-porosity structure can be further promoted, and the ionic conductivity improvement effect is obvious; this shows that the optimization of additives and modifiers can improve the steam-induced phase separation effect; in addition, the polyvinyl fluoride-based composite solid electrolyte membrane is annealed and its annealing parameters are optimized to obtain a higher membrane dielectric constant, which shows that adding annealing can further improve the performance of the composite solid electrolyte membrane, greatly improving its electrical conductivity and the cycle performance of the battery.

[0155] Depend on Figure 1It can be seen that a high-porosity polyvinylidene fluoride porous base membrane with uniform connectivity is formed. Figure 2 It can be seen that the inorganic solid electrolyte powder is evenly filled in the pores of the polyvinylidene fluoride-based porous base membrane, forming a uniform and tightly connected polyvinyllidene fluoride-based composite solid electrolyte membrane.

[0156] It can be seen from Example 3 and Comparative Example 1 that adding inorganic solid electrolyte powder to form a composite solid electrolyte membrane can effectively improve the ionic conductivity of the membrane. This is because the inorganic solid electrolyte powder can be filled in the porous structure of the organic polymer membrane layer, broadening the transmission path of lithium ions and improving the ionic conductivity; it can be seen from Example 3 and Comparative Example 2 that when the particle size of the inorganic solid electrolyte powder is too large, it is not conducive to its embedding in the pore structure, and thus it cannot be effectively composited with the polyvinylidene fluoride-based organic matrix, and an effective improvement in conductivity and cycle efficiency cannot be achieved.

[0157] It can be seen from Example 3 and Comparative Examples 5, 6, and 7 that through the mutual regulation of steam induction temperature, humidity, and time, a higher porosity and pore size are obtained, and the slurry wettability is good, indicating that a higher phase separation effect can be obtained within the optimized parameter range, forming a stable porous structure, which is conducive to the uniform dispersion of inorganic solid electrolyte powder in the pores; forming a polyvinylidene fluoride-based composite solid electrolyte membrane with high ionic conductivity and cycle performance. When the steam induction temperature, humidity or induction time are inappropriate, the porosity, pore size, and slurry infiltration effect are all low. This is because when the steam temperature is too high, the phase separation rate is too fast, the steam induction is insufficient, and it is not conducive to the formation of a porous structure with uniform high porosity; when the steam humidity is too low, the steam volume is insufficient to complete the phase separation, resulting in low porosity and pore size, and poor wettability of the membrane in the slurry; when the steam induction time is short, it is not conducive to the full progress of the phase separation process; membranes with low porosity and pore size are not conducive to the combination of inorganic solid electrolyte particles, and cannot effectively broaden the lithium ion transmission channel, thereby reducing the conductivity and battery cycle performance.

[0158] It can be seen from Example 3 and Comparative Examples 3, 4, 8, and 9 that the addition of an appropriate amount of additives and modifiers can promote steam-induced phase separation, achieve the preparation of a high-porosity structure, significantly improve the wettability of the porous base membrane to the slurry, obtain higher ionic conductivity, and better battery cycle performance; this is because the addition of additives and modifiers can increase the reactivity of the polyvinylidene fluoride-based solution, promote the formation of a uniformly connected high-porosity structure, and facilitate the uniform diffusion of the inorganic solid electrolyte powder in the pores of the polyvinylidene fluoride polymer matrix. At the same time, the presence of additives and modifiers can further enhance the binding ability of the inorganic solid electrolyte and the polyvinylidene fluoride polymer matrix during the steam induction process, fill structural defects, and form a denser and more complete interface, so that lithium ions can not only be transmitted in the inorganic solid electrolyte filled in the pore structure, but also at the interface between the organic polymer and the inorganic solid electrolyte, effectively broadening the path of lithium ion transmission, thereby improving conductivity; however, the addition of excessive additives or modifiers will be adsorbed on the polymer interface due to its own low ion transmission capacity, hindering ion transmission and reducing conductivity. Therefore, appropriate amounts of additives or modifiers are compounded and used in appropriate proportions to better exert synergistic effects, maintain good structural stability during the steam induction process, help steam penetrate and diffuse more evenly, and form a more uniform and continuously interconnected pore structure.

[0159] It can be seen from Examples 3 and 6 that adding annealing treatment can effectively improve the dielectric constant of the membrane and obtain higher ionic conductivity, which shows that annealing treatment can further improve the performance of the composite solid electrolyte membrane.

[0160] As shown in Example 6 and Comparative Example 10, when the annealing temperature is high, the conductivity is low, and both the porosity and pore size are significantly reduced. This indicates that the appropriate annealing temperature and time further enhance the stability of the membrane's porous structure, resulting in higher conductivity performance. However, excessively high annealing temperatures can also damage the porous structure of the composite solid electrolyte membrane. The solid electrolyte embedded in its pores can be coated with polyvinylidene fluoride polymer, hindering lithium ion transport and leading to reduced conductivity and cycling performance.

[0161] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane, comprising the following steps: S1: mixing the inorganic solid electrolyte powder, additives and solvent to obtain a slurry; S2: adding a polyvinyl fluoride-based organic matrix and a modifier to the slurry described in S1, and stirring uniformly to obtain a polyvinyl fluoride-based polymer solution; S3: The film obtained by coating the polyvinylidene fluoride vinyl polymer solution described in S2 with a desktop coating machine is subjected to a steam-induced phase separation method to prepare a porous polyvinylidene fluoride film; the steam-induced phase separation, wherein the steam temperature is selected from 50-100 o C, steam humidity is selected from 40-90%, and steam induction time is selected from 40-300s; S4: Immersing the polyvinylidene fluoride-based membrane described in S3 in a solid electrolyte slurry, and subjecting it to ultrasonic and drying post-treatment to finally obtain a polyvinylidene fluoride-based composite solid electrolyte membrane.

2. The method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 1, wherein: The inorganic solid electrolyte powder in S1 has a particle size D50 selected from 80nm≤D50≤300nm.

3. The method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 1, wherein: The temperature of the S3 medium-sized table coating machine is selected from 30° C. to 90° C., the scraper thickness is selected from 50 μm to 200 μm, and the scraping speed is selected from 0.1 m / min to 5 m / min.

4. The method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 1, wherein: The inorganic solid electrolyte powder in S1 includes one or more of garnet-type solid electrolyte powder, perovskite-type solid electrolyte powder or NASICON-type solid electrolyte powder; the polyvinylidene fluoride-based organic matrix in S2 includes one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trifluoroethylene, polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene or polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene.

5. The method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 1, wherein: The content W of the inorganic solid electrolyte powder in S1 is selected from 5%≤W≤30%, where W is the ratio of the mass of the inorganic solid electrolyte powder to the total mass of the S1 slurry.

6. The method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 1, wherein: The content X of the polyvinyl fluoride-based organic matrix in S2 is selected from 10%≤X≤30%, where X is the ratio of the mass of the polyvinyl fluoride-based organic matrix to the total mass of the slurry in S1.

7. The method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 1, wherein: The weight average molecular weight of the polyvinylidene fluoride-based organic matrix is ​​100,000-1,200,000.

8. The method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 1, wherein: The auxiliary agent in S1 is selected from one or more of polyolefin auxiliary agents with cyclic branches, polyol auxiliary agents, and alkyl salt auxiliary agents; the solvent includes one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, triethyl phosphate, methyl acetate, ethyl acetate and acetone; the modifier in S2 is selected from one or more of silane coupling agents or fluorocarbon surfactants.

9. The method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 1, wherein: The auxiliary agent is selected from a composite of a polyolefin auxiliary agent with a cyclic branch and a polyol auxiliary agent, and the mass ratio of the two is 0.5-2; the modifier is selected from a composite of a silane coupling agent and a fluorocarbon surfactant, and the mass ratio of the two is 0.5-2; the proportion of the total mass of the auxiliary agent to the total mass of the slurry in S1 is selected from 5-15%; the proportion of the total mass of the modifier to the total mass of the slurry in S1 is selected from 0.1-5%.

10. The method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 9, characterized in that: The auxiliary agent is selected from a composite of a polyolefin auxiliary agent with a cyclic branch and a polyol auxiliary agent, and the mass ratio of the two is 1:1; the modifier is selected from a composite of a silane coupling agent and a fluorocarbon surfactant, and the mass ratio of the two is 1:

1.

11. The method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 9, characterized in that: The auxiliary agent is selected from polyvinyl pyrrolidone and polyethylene glycol.

12. The method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 11, characterized in that: The molecular weight of polyvinyl pyrrolidone is selected from 30,000 to 70,000, and the content of polyvinyl pyrrolidone is selected from 3% to 8%; wherein the content of polyvinyl pyrrolidone is the mass of polyvinyl pyrrolidone to the total mass of the slurry in S1.

13. The method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 1, wherein: The annealing treatment in S4 is to adjust the annealing temperature to 100-150 o C, annealing time is 1-4h.

14. The method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 1, wherein: The solid electrolyte slurry in S4 is an aqueous solid electrolyte slurry, and the aqueous solid electrolyte slurry is an aqueous mixture containing solid electrolyte particles; the ultrasonic treatment, the ultrasonic temperature is selected from 20-50°C, and the ultrasonic time is selected from 0.5h-12h; the drying method is selected from one of freeze drying, oven drying, radiation drying or vacuum drying.

15. The method for preparing a polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 14, characterized in that: The solid electrolyte particle content of the aqueous solid electrolyte slurry accounts for 10%-40% of the total content of the aqueous solid electrolyte slurry, and the solid electrolyte particle size D150 is selected from 100nm-500nm.

16. A polyvinylidene fluoride-based composite solid electrolyte membrane, wherein the polyvinylidene fluoride-based composite solid electrolyte membrane is obtained by the preparation method according to any one of claims 1 to 15; the polyvinylidene fluoride-based composite solid electrolyte membrane has a thickness of 20 μm to 150 μm. 17 . A battery comprising a positive electrode, a negative electrode, and the polyvinylidene fluoride-based composite solid electrolyte membrane according to claim 16 .

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