A PVDF solid-state polymer electrolyte with an interpenetrating crosslinked network structure and a preparation method and application thereof

Abstract

The application discloses a PVDF solid-state polymer electrolyte with an interpenetrating crosslinked network structure and a preparation method and application thereof, relates to the technical field of solid-state polymer electrolytes, and discloses the PVDF solid-state polymer electrolyte with the interpenetrating crosslinked network structure, which comprises a first network and a second network. The first network comprises a crosslinked framework formed by copolymerization of PVDF, a lithium salt, polyethylene glycol dimethacrylate and ethylene glycol methyl ether acrylate. The second network comprises a network structure formed by in-situ polymerization of ethylene carbonate and fluoroethylene carbonate. The second network and the first network are interpenetrated and wound. The application constructs a bicontinuous phase structure formed by interpenetration of a PVDF-based crosslinked framework and in-situ polymerization networks of ethylene carbonate and fluoroethylene carbonate, so as to reduce the crystallinity of PVDF, construct a continuous lithium ion transmission channel, improve the room temperature ionic conductivity, and improve the electrode interface compatibility.

Description

Technical Field

[0001] This invention relates to the field of solid polymer electrolyte technology, specifically to a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure, its preparation method, and its application. Background Technology

[0002] Lithium metal batteries (LMBs) are widely considered promising for applications in next-generation portable devices, transportation, and energy storage systems due to their high energy density (3860 Wh / kg). However, traditional liquid electrolytes are typically composed of volatile organic solvents (such as carbonates), posing safety hazards such as leakage, flammability, and explosion. More critically, during cycling, lithium dendrites can grow uncontrollably at the electrode interfaces, continuously consuming electrolyte and potentially puncturing the separator, leading to internal short circuits and thermal runaway.

[0003] Replacing liquid electrolytes (LE) with solid electrolytes (SSE) is one of the fundamental ways to solve the aforementioned safety issues. SSEs, with their high mechanical modulus, can physically suppress lithium dendrite growth, thereby improving battery cycle life and safety. Among various SSEs, polymer solid electrolytes (SPEs) are considered one of the most promising technologies due to their excellent flexibility, good electrode-electrolyte interface contact, and ease of film formation.

[0004] Polyvinylidene fluoride (PVDF)-based polymers, due to the presence of highly polar -CF2- groups (dielectric constant ε≈8.4) in their molecular chains, can effectively promote the dissociation of lithium salts. They also possess excellent mechanical strength (tensile strength ≥20 MPa), good solvent resistance, and thermal stability (decomposition temperature ≥470℃), making them a preferred matrix material for high-performance solid electrolytes (SPEs). However, traditional linear PVDF-based solid electrolytes suffer from significant performance bottlenecks: the tightly packed molecular chains lead to discontinuous lithium-ion transport channels, resulting in generally low room-temperature ionic conductivity (10⁻⁶ MPa). -6 ~ 10 -5 The PVDF matrix has a low efficiency (S / cm), which is insufficient to meet the requirements of high-rate charging and discharging. In addition, the interface compatibility between the single PVDF matrix and the electrode (especially the lithium metal anode) is poor, which easily forms a high-resistivity passivation layer, affecting the long-cycle stability of the battery.

[0005] To balance ionic conductivity and mechanical properties, researchers have proposed strategies for constructing cross-linked or interpenetrating network structures. For example, prior art CN102522589A (A novel gel polymer electrolyte with an interpenetrating network structure, its preparation method, and its application) discloses a method for forming an interpenetrating network gel electrolyte by mixing unsaturated ester monomers, cross-linking agents (such as polyethylene glycol di(meth)acrylate), a polymer matrix (including PVDF options), and a liquid lithium salt electrolyte (such as conventional carbonates like EC / DMC / DEC) through a one-step thermally initiated polymerization. This method improves the structural stability of the electrolyte and its adsorption capacity to liquid electrolytes.

[0006] However, this existing technology still has the following drawbacks, which limit its application in high-performance, high-safety solid-state batteries: 1) The contradiction between physical plasticization and chemical stability: Essentially, it remains a gel electrolyte with a high content of liquid electrolyte (80%-90%) that has undergone physical plasticization. The conventional carbonate solvents used (EC, DMC, DEC, etc.) are highly volatile and have low flash points, failing to fundamentally address the flammability and explosiveness risks of traditional liquid electrolytes. The presence of a large amount of free solvent also results in insufficient dimensional stability and leak resistance of the electrolyte at high temperatures.

[0007] 2) Structural limitations of one-step polymerization: The one-step polymerization method (polymerizing all components in a single step after mixing) makes it difficult to precisely control the generation sequence and microstructure of different polymer networks. This method easily leads to uneven network interweaving, which may cause local phase separation, thus affecting the uniformity of ion transport and the overall mechanical strength of the network.

[0008] 3) Lack of functional monomers: The unsaturated ester monomers used (such as MMA, BA, etc.) mainly serve to build the polymer backbone, lacking specific stabilizing functions for the electrode interface, especially for highly active lithium metal anodes or high-voltage cathodes. Side reactions between the electrolyte and the electrode are difficult to suppress effectively, and the interfacial impedance will still increase over time.

[0009] Therefore, it is necessary to design and fabricate a truly safe polymer solid electrolyte that can eliminate the dependence on large amounts of flammable liquid solvents and simultaneously achieve high ionic conductivity, excellent mechanical strength and dimensional stability, and good electrode interface compatibility through precise microstructure design, thereby meeting the application requirements of next-generation high-energy-density and high-safety lithium metal batteries. Summary of the Invention

[0010] The purpose of this invention is to provide a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure, its preparation method, and its application. By constructing a bicontinuous phase structure formed by the interpenetration of a PVDF-based cross-linked backbone with an in-situ polymerized network of ethylene carbonate and fluoroethylene carbonate, the technical problems of discontinuous ion transport channels, low room temperature ionic conductivity, and poor electrode interface compatibility in traditional PVDF-based solid electrolytes are solved.

[0011] This invention is achieved through the following technical solution: In a first aspect, embodiments of this invention provide a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure, comprising: The first network comprises a cross-linked backbone formed by copolymerization of PVDF, lithium salt, polyethylene glycol dimethacrylate and ethylene glycol methyl ether acrylate; The second network includes a network structure formed by in-situ polymerization of ethylene carbonate and fluoroethylene carbonate, wherein the second network and the first network are interpenetrating and intertwined. The second network is configured to be obtained by in-situ polymerization of a precursor solution containing ethylene carbonate, fluoroethylene carbonate and an initiator after immersing it in the first network.

[0012] In this invention, the simultaneous use of ethylene carbonate and fluoroethylene carbonate can form a stable SEI layer. Furthermore, the -COO groups on ethylene carbonate exhibit a strong dipole effect and interact with the C=O groups in the first network, which can promote the continuity of lithium-ion transport channels in the electrolyte.

[0013] As an optional implementation, the volume ratio of ethylene carbonate to fluoroethylene carbonate in the precursor solution is 1~9: 1~9.

[0014] As an optional implementation, the mass ratio of polyethylene glycol dimethacrylate to ethylene glycol methyl ether acrylate is 5~10:8~15.

[0015] This invention addresses the modification of PVDF by simultaneously adding polyethylene glycol dimethacrylate (PEG) and ethylene glycol methyl ether acrylate (EDMA), which achieve synergistic effects through structural complementarity. Specifically, PEG, as a bifunctional crosslinking agent, can construct a three-dimensional crosslinked network during polymerization, providing the electrolyte with necessary mechanical strength and structural stability, and effectively inhibiting lithium dendrite penetration. EDMA, as a monofunctional active diluent and flexible segment donor, can reduce the crosslinking density of the system, introducing a large number of amorphous polyether segments, providing continuous transport channels for lithium ions and enhancing segment mobility. Using only EDMA results in excessively high crosslinking density, excessive system rigidity, insufficient amorphous phase, hindered lithium ion transport, and decreased ionic conductivity. Using only PEG, however, fails to form an effective crosslinking network, resulting in poor electrolyte mechanical properties, difficulty in inhibiting dendrites, and susceptibility to deformation and short circuits.

[0016] As an optional implementation, the lithium salt is selected from at least one of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide ((LiFSI)), and lithium hexafluorophosphate (LiPF6).

[0017] Secondly, embodiments of the present invention provide a method for preparing a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure, comprising the following steps: S1: PVDF particles, lithium salt, polyethylene glycol dimethacrylate, ethylene glycol methyl ether acrylate, the first initiator, and an organic solvent are mixed and stirred to obtain a slurry; S2: Pour the slurry into a mold and dry it under vacuum to obtain a cross-linked network solid; S3: Prepare a mixture of in-situ polymerization precursors of ethylene carbonate (VC), fluoroethylene carbonate (FEC) and a second initiator; S4: The precursor solution is immersed in the crosslinking network and polymerized in situ to obtain a PVDF solid polymer electrolyte with an interpenetrating crosslinking network structure.

[0018] In this embodiment of the invention, both the first and second initiators are preferably AIBN. Of course, in other embodiments, ABVN, AIBI, BPO, LPO, TBPB, etc., can also be used, and no limitation is made here. In this embodiment of the invention, the addition of AIBN can make the system more gentle during the reaction, with fewer byproducts, especially for crosslinking of polyolefins, acrylates, and solid electrolytes.

[0019] As an optional implementation, the mass ratio of the PVDF particles, lithium salt, polyethylene glycol dimethacrylate, ethylene glycol methyl ether acrylate, first initiator, and in-situ polymerization precursor mixture is: (15~25):(5~10): (8~15):(0.5~1):(10.5~15.9); The amount of the second initiator added is 0.8 to 1.2 wt% of the mass of ethylene carbonate (VC).

[0020] As an optional implementation, the organic solvent is selected from at least one of N,N-dimethylformamide (DMF) and N-methylpyrrolidone (NMP).

[0021] Preferably, the organic solvent is DMF, which is suitable for preparation in multiple scenarios because it has low drying energy consumption, uniform film formation, easy induction of highly conductive β crystal phase, facilitates lithium salt dissociation, and is safer and more economical.

[0022] As an optional implementation, the stirring temperature in S1 is 40~50℃ and the stirring time is 6~12h; preferably, the stirring temperature in S1 is 45℃ and the stirring time is 8h. In this embodiment of the invention, if the stirring temperature is below 40°C, insufficient solvent kinetics will result, making it difficult for PVDF to dissolve completely. This can lead to undissolved particles, local agglomeration, and solution inhomogeneity, making subsequent film formation prone to defects and crystalline enrichment. If the stirring temperature is above 60°C, it will accelerate DMF volatilization, causing solution concentration drift. Furthermore, excessively high temperatures can cause premature local chain segment shrinkage or thermal aging of PVDF, and may also induce premature microphase separation, disrupting the uniformity of the casting solution and hindering subsequent polymerization and phase structure control. Stirring for 6–12 hours ensures complete dissolution of PVDF and stable system viscosity; too short a time will result in insufficient dissolution and solution inhomogeneity; too long a time may introduce excessive air bubbles and cause slow solvent decomposition, affecting the final mechanical and electrochemical properties of the electrolyte.

[0023] The vacuum drying temperature in S2 is 60~100℃, and the vacuum drying time is 12~24 h; preferably, the vacuum drying temperature in S2 is 80℃, and the vacuum drying time is 12 h. The in-situ polymerization temperature in S4 is 40~80℃, and the in-situ polymerization time is 2~12h; preferably, the in-situ polymerization temperature in S4 is 60℃, and the in-situ polymerization time is 6h.

[0024] As an optional implementation, the mold in S2 is a PTFE mold, which includes a disc with a diameter of 8-12 cm. Preferably, the diameter of the disc is 8 cm or 12 cm.

[0025] Thirdly, embodiments of the present invention provide an application of a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure in the preparation of lithium metal batteries. The polymer solid lithium metal battery is a lithium-lithium symmetric battery, a lithium-lithium iron phosphate (LFP) battery, etc.

[0026] Compared with the prior art, the embodiments of the present invention have the following advantages and beneficial effects: 1. The PVDF solid polymer electrolyte provided in this embodiment of the invention constructs a first network as a crosslinking backbone, which is formed by copolymerization of PVDF, lithium salt, polyethylene glycol dimethacrylate, and ethylene glycol methyl ether acrylate. A second network, which is formed by in-situ polymerization of ethylene carbonate and fluoroethylene carbonate, is interpenetrated with the first network by immersion in a mixture of in-situ polymerization precursors containing an initiator. This allows the first and second networks to interpenetrate and entangle with each other at the molecular scale to form a bicontinuous phase structure. The interpenetrating crosslinked network structure promotes the reduction of PVDF crystallinity and constructs continuous lithium-ion transport channels. At the same time, the EO (COC) segments in polyethylene glycol dimethacrylate and ethylene glycol methyl ether acrylate serve as lithium-ion transport sites to promote rapid and uniform lithium-ion migration. The in-situ polymerization method makes the electrolyte surface uniform and pore-free to reduce the risk of lithium dendrite growth and promote close contact between the solid electrolyte and the electrode interface, thereby improving ionic conductivity and interface stability.

[0027] Meanwhile, in this embodiment of the invention, a dipole interaction exists between the first network and the second network because both -COO⁻ and C=O have highly polarized covalent bonds with asymmetrical charge distribution, forming stable permanent dipoles that interact with each other through electrostatic attraction. Specifically, the oxygen in the carbonyl group (C=O) is much more electronegative than carbon, causing the electron cloud to be strongly biased towards the oxygen atom, forming a permanent dipole moment of δ⁺(C)–δ⁻(O); the carboxylate / ester group (-COO⁻) also has a highly polarized C=O and CO structure and a significant dipole moment. When these two groups approach each other, the positively charged end of one group and the negatively charged end of the other group attract each other through electrostatic attraction, thus forming a typical dipole-dipole interaction.

[0028] 2. The interpenetrating cross-linked network of this invention has a large number of EO segments. These EO segments have high electronegativity and can serve as lithium ion transport sites, promoting rapid and uniform migration of lithium ions and significantly improving the electrolyte conductivity by an order of magnitude. In addition, the EO segments can also anchor anions and a small amount of residual N,N-dimethylformamide (DMF) organic solvent, promoting lithium salt dissociation while suppressing the serious side reactions caused by DMF on the lithium metal side. The in-situ polymerization method ensures close contact between the electrolyte and the electrode interface, and the transfer of lithium ions at the interface is not adversely affected by the porosity at the interface of traditional PVDF electrolytes, thus enabling rapid and efficient transfer.

[0029] The mass ratio of polyethylene glycol dimethacrylate to ethylene glycol methyl ether acrylate is controlled within the range of 5~10:8~15. This ensures that the cross-linked backbone has sufficient mechanical strength while providing sufficient EO segment density to support efficient lithium-ion transport. This avoids the problems of excessive rigidity of the segments due to excessive cross-linking density hindering ion migration, or instability of the network structure due to excessively low cross-linking density. This achieves synergistic optimization of mechanical properties and ion conduction performance.

[0030] 3. In this embodiment of the invention, a polymer precursor mixture of in-situ ethylene carbonate (VC) and fluoroethylene carbonate (FEC) is immersed in a cross-linked PVDF polymer electrolyte, which allows the precursor mixture to penetrate more thoroughly into the PVDF electrolyte and thus promotes the formation of an interpenetrating cross-linked network.

[0031] The volume ratio of ethylene carbonate to fluoroethylene carbonate is adjustable within the range of 1 to 9:1 to 9. By controlling the ratio of the two monomers, the chemical composition and physical properties of the second network can be flexibly designed. Ethylene carbonate provides good lithium-ion solvation capability and interfacial compatibility, while fluoroethylene carbonate endows the network with excellent electrochemical stability and flame retardant properties. The synergistic effect of the two makes the electrolyte have high ionic conductivity, wide electrochemical window and enhanced safety performance.

[0032] 4. The electrolyte preparation method provided in this embodiment of the invention has low preparation cost and is simple and easy to implement; when the obtained interpenetrating network structure electrolyte is applied to a lithium-lithium button battery, it can achieve a power output of 0.1 Ah cm⁻¹. -2 0.1mAh cm -2 Cycling time exceeds 1200 hours; when used in lithium iron phosphate-lithium button batteries, it can achieve a cycle life of up to 4 mg cm⁻¹. -2 It can stably cycle 300 times at a current density of 1C under load, demonstrating excellent cycling performance.

[0033] 5. Under the process conditions of controlling the in-situ polymerization temperature at 40°C to 80°C and the time at 2h to 12h in the embodiments of the present invention, it is possible to ensure that the second network is fully polymerized and uniformly distributed inside the first network, avoiding the destruction of the first network structure or premature decomposition of the initiator due to excessively high polymerization temperature, and also avoiding monomer residue and incomplete polymerization problems caused by excessively low polymerization temperature or insufficient time, thereby ensuring the integrity of the interpenetrating network structure and the stability of electrolyte performance. Attached Figure Description

[0034] To more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be considered as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort. In the drawings: Figure 1 The images shown are cross-sectional and surface scanning electron microscope (SEM) images of the polymer solid obtained according to Example 1 and Comparative Example 1 of this invention. in Figure 1 a and Figure 1 c is a microscopic morphology image of Scale 1. Figure 1 c is Figure 1 A magnified view of part of a; Figure 1 b and Figure 1 d is a microscopic morphology image of Example 1; Figure 1 d is Figure 1 A magnified view of part b.

[0035] Figure 2 This is a comparison of the impedance tested and the calculated conductivity of the two electrolytes in the steel-steel symmetric cells of the polymer solid electrolytes prepared according to Example 1 and Comparative Example 1 of this invention. Figure 3 The polymer solid electrolyte prepared according to Example 1 and Comparative Example 1 of this invention was used in a Li||Li symmetric cell at 0.1 mA cm⁻¹. -2 0.1mAh cm -2 A schematic diagram of polarization curves under long-cycle conditions; Figure 4 According to the present invention Figure 3 SEM images of the lithium metal surface taken after the prepared battery was disassembled following cycling. in Figure 4 a represents the lithium metal microstructure of Comparative Example 1. Figure 4 b represents the lithium metal microstructure of Example 1; Figure 5The rate performance diagram shows the LFP||Li battery assembled with the polymer solid electrolyte prepared according to Example 1 and Comparative Example 1 of this invention. Figure 6 The graph shows the long-cycle performance of LFP||Li batteries assembled with polymer solid electrolytes prepared according to Example 1 and Comparative Example 1 at a current density of 1C. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0037] Therefore, the detailed description of the embodiments of the present invention provided below is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0038] Existing technology CN102522589A discloses a gel polymer electrolyte with an interpenetrating network structure, its preparation method, and its applications. This technology mixes unsaturated ester monomers, crosslinking agents, polymer matrices, and liquid lithium salt electrolytes, and forms an interpenetrating network structure gel electrolyte through one-step thermally initiated polymerization. The polymer matrix includes PVDF as an option, the crosslinking agent is polyethylene glycol di(meth)acrylate, and the liquid lithium salt electrolyte contains conventional carbonate solvents such as EC, DMC, and DEC. The principle is to physically plasticize the polymer matrix using the liquid electrolyte, and form an interpenetrating network through one-step polymerization to improve structural stability and electrolyte adsorption. However, this technology suffers from technical problems such as high volatility of liquid solvents leading to insufficient safety, difficulty in precisely controlling the uniformity of the network structure during one-step polymerization, and the lack of specific stabilizing functions at the electrode interface among the functional monomers.

[0039] To address the aforementioned issues, this invention constructs a bicontinuous phase structure formed by the interpenetration of a PVDF-based crosslinked backbone and an in-situ polymerized network of ethylene carbonate-fluoroethylene carbonate. A stepwise construction strategy is employed, first forming a first crosslinked network and then immersing it in an in-situ polymerization precursor mixture to form a second network. Furthermore, functional monomers containing EO segments and fluorinated monomers with film-forming properties are introduced to solve the technical problems of discontinuous ion transport channels, low room-temperature ionic conductivity, and poor electrode interface compatibility in traditional PVDF-based solid electrolytes.

[0040] Specifically, embodiments of the present invention provide a method for preparing a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure, comprising the following: Step 1: Mix and stir PVDF particles, lithium salt and organic solvent at 40~50℃ for 6~12h to obtain a uniform liquid with a certain viscosity. Then add polyethylene glycol dimethacrylate, ethylene glycol methyl ether acrylate and the first initiator to the above solution and continue stirring. After being fully dissolved, a slurry is obtained. Step 2: Pour the above slurry into a PTFE mold with a diameter of 12cm or 8cm, and dry it in a vacuum environment at a temperature of 60℃~100℃ for 12h~24h to obtain a cross-linked network; Step 3: Mix ethylene carbonate, fluoroethylene carbonate and the second initiator. The volume ratio of ethylene carbonate to fluoroethylene carbonate is 1:9, 3:7, 5:5, 7:3 or 9:1 to obtain an in-situ polymerization precursor mixture. Step four: Immerse the above-mentioned in-situ polymerization precursor mixture into the cross-linking network (wherein, by mass, PVDF particles are 15-25 parts, lithium salt is 5-10 parts, polyethylene glycol dimethacrylate is 5-10 parts, ethylene glycol methyl ether acrylate is 8-15 parts, the first initiator is 0.5-1 part, organic solvent is 30-50 parts, and the in-situ polymerization precursor mixture is 10.5-15.5 parts), and polymerize in situ at 40℃-80℃ for 2h-12h to obtain a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure.

[0041] The lithium salt is selected from lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, or lithium hexafluorophosphate. The organic solvent is selected from N,N-dimethylformamide or N-methylpyrrolidone. The first initiator is AIBN, the second initiator is AIBN, and the amount of the second initiator added is 0.8 to 1.2 wt% of the mass of ethylene carbonate (VC).

[0042] To better demonstrate the effects of the embodiments of the present invention, experimental verification of specific implementation methods will be carried out below.

[0043] Example 1: This embodiment of the invention provides a method for preparing a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure, comprising the following: First, PVDF powder (20 parts), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (8 parts), and 40 parts of DMF organic solvent were mixed and stirred at 45°C for 8 hours until the solution became homogeneous and had a certain viscosity. Then, polyethylene glycol dimethacrylate (8 parts), ethylene glycol methyl ether acrylate (10 parts), and AIBN (0.8 parts) were added to the above viscous solution and stirred until all substances were completely dissolved. The stirred solution was then poured into a circular PTFE plate with a diameter of 12 cm and placed in a vacuum oven. It was then vacuum dried at 80°C for 18 hours to obtain a PVDF polymer solid electrolyte membrane with a cross-linked structure. Next, FEC and VC were mixed in a volume ratio of 5:5, and then 1 wt% of AIBN by weight of VC was added. The three substances were mixed evenly to obtain an in-situ polymerization precursor mixture. The electrolyte membrane prepared above was cut into circular pieces with a diameter of 19 mm, and then 12.5 parts of the in-situ polymerization precursor mixture were added into the electrolyte membrane. The soaked electrolyte membrane was used to assemble batteries. The assembled batteries were left at room temperature for 8 hours, and then various batteries were placed at 60°C for polymerization for 6 hours to obtain a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure.

[0044] After battery assembly, allowing it to rest allows the electrodes and electrolyte to be fully wetted, ions to be evenly distributed, and a stable and uniform initial SEI film to slowly form at the interface, enabling the system to reach electrochemical equilibrium. This ensures that subsequent test data is accurate and reliable and that the battery operates stably.

[0045] Example 2: This embodiment of the invention provides a method for preparing a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure, comprising the following: First, PVDF powder (15 parts), lithium bis(fluorosulfonyl)imide (LiFSI) (5 parts), and 30 parts NMP organic solvent were mixed and stirred at 40°C for 12 hours until the solution became homogeneous and had a certain viscosity. Then, polyethylene glycol dimethacrylate (5 parts), ethylene glycol methyl ether acrylate (8 parts), and AIBN (0.5 parts) were added to the above viscous solution and stirred until all substances were completely dissolved. The stirred solution was then poured into a circular PTFE plate with a diameter of 12 cm and placed in a vacuum oven. It was then vacuum dried at 60°C for 24 hours to obtain a PVDF polymer solid electrolyte membrane with a cross-linked structure. Next, FEC and VC were mixed at a volume ratio of 1:9, and then 0.8 wt% of AIBN by weight of VC was added. The three substances were mixed uniformly to obtain an in-situ polymerization precursor mixture. The electrolyte membrane prepared above was cut into circular pieces with a diameter of 19 mm, and then 10.5 parts of the in-situ polymerization precursor mixture were added into the electrolyte membrane. The soaked electrolyte membrane was used to assemble batteries. The assembled batteries were left at room temperature for 6 hours, and then various batteries were placed at 40°C for polymerization for 12 hours to obtain a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure.

[0046] Example 3: This embodiment of the invention provides a method for preparing a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure, comprising the following: First, PVDF powder (25 parts), lithium hexafluorophosphate (LiPF6) (10 parts), and 50 parts DMF organic solvent were mixed and stirred at 50°C for 6 hours until the solution became homogeneous and had a certain viscosity. Then, polyethylene glycol dimethacrylate (10 parts), ethylene glycol methyl ether acrylate (15 parts), and AIBN (1 part) were added to the above viscous solution and stirred until all substances were completely dissolved. The stirred solution was then poured into a circular PTFE plate with a diameter of 12 cm and placed in a vacuum oven. It was then vacuum dried at 100°C for 12 hours to obtain a PVDF polymer solid electrolyte membrane with a cross-linked structure. Next, FEC and VC were mixed at a volume ratio of 9:1, and then 1.2 wt% of AIBN by mass of VC was added. The three substances were mixed uniformly to obtain an in-situ polymerization precursor mixture. The electrolyte membrane prepared above was cut into circular pieces with a diameter of 19 mm, and then 15.5 parts of the in-situ polymerization precursor mixture were added into the electrolyte membrane. The soaked electrolyte membrane was used to assemble batteries. The assembled batteries were left at room temperature for 10 h, and then various batteries were placed at 80 °C for 2 h to polymerize and obtain PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure.

[0047] Comparative Example 1 (without cross-linking structure): A method for preparing a PVDF polymer solid electrolyte membrane with a cross-linking structure is provided. The PVDF, lithium salt and DMF are mixed and stirred according to the mass parts corresponding to Example 1. Then, the stirred solution is poured into a circular PTFE plate with a diameter of 12 cm and placed in a vacuum oven. The plate is then vacuum dried at 80°C for 12 h to obtain a pure PVDF polymer solid electrolyte membrane.

[0048] Figure 1 The scanning electron microscope (SEM) images of the cross-section and surface of the electrolyte membranes prepared according to the methods described in Example 1 and Comparative Example 1 clearly show that the cross-section and surface of Comparative Example 1 have many pores, while the cross-section and surface of Example 1 have no pores. This indicates that the addition of the polymer skeleton with the interpenetrating cross-linked network promotes a more uniform and dense PVDF electrolyte membrane. Such a pore-free and dense morphology can promote the rapid and unhindered migration of lithium ions.

[0049] Figure 2 Impedance testing was conducted on the electrolyte membranes prepared according to the methods described in Example 1 and Comparative Example 1 in a steel-steel symmetric battery. The test results once again demonstrated that the transport resistance of lithium ions within the interpenetrating cross-linked network structure of the PVDF polymer electrolyte is much smaller than that of the unmodified PVDF electrolyte membrane. The conductivity of Example 1 and Comparative Example 1 was calculated using the formula σ=l / RS, where the conductivity of Example 1 reached 3.4×10⁻⁶. -4 S cm -1 The conductivity of Comparative Example 1 is only 4.28 × 10⁻⁶. -5 S cm -1 The difference in conductivity between the two electrolytes can reach an order of magnitude.

[0050] Figure 3 The electrolyte membrane prepared according to the method described in Example 1 and Comparative Example 1 was used in a Li||Li symmetric cell at a current of 0.1 mA cm⁻¹. -2 0.1mAh cm -2 The diagram shows the polarization curves under long-term cycling conditions. Comparative Example 1 exhibits an unstable and relatively large polarization voltage during cycling, while Example 1 exhibits a stable and very small polarization voltage after 1200 hours of cycling, indicating that the addition of the interpenetrating network structure can improve the compatibility between the electrolyte and lithium metal.

[0051] Figure 4 The electrolyte prepared according to the method described in Example 1 and Comparative Example 1 was used in a Li||Li symmetric cell at a current of 0.1 mA cm⁻¹. -2 0.1mAh cm -2 After long-term cycling under certain conditions, the battery was disassembled and the SEM image of the lithium metal surface was obtained by scanning electron microscopy. Figure 4a is the result of Comparative Example 1. It can be clearly seen that its surface is crisscrossed with grooves and is not uniform and flat, with many dead lithium and dendrite morphologies. Figure 4 b represents the result of Example 1, showing a uniform and flat surface without groove morphology, dead lithium, or dendrite morphology. This indicates that the PVDF polymer solid electrolyte with an interpenetrating cross-linked structure can promote uniform lithium-ion transfer and can be uniformly deposited on lithium metal.

[0052] Figure 5 and Figure 6 The figures show the rate and 1C current cycling performance of the electrolytes prepared according to the methods described in Example 1 and Comparative Example 1 in LFP||Li batteries. Comparative Example 1 exhibited a lower discharge specific capacity in the rate test: a capacity of 141 mAh g at 10 / C. -1 At C / 5, it is 137mAh g. -1 At C / 3, it is 123mAh g. -1 At C / 2, it is 115mAh g. -1 105mAh g at 1C -1 89mAh g at 2C -1 In contrast, Example 1 exhibits a higher discharge specific capacity: 168 mAh g at 10 / C. -1 At C / 5, it is 160mAh g -1 At C / 3, it is 153mAh g. -1 At C / 2, it is 146 mAh g. -1 132mAh g at 1C -1 At 2C, it is 102mAh g. -1 Furthermore, after 300 cycles at 1C, Example 1 still retained 84.85% of its capacity, while Comparative Example 1 only retained 13.56% of its capacity after 150 cycles. These results demonstrate that introducing an interpenetrating cross-linked network structure can improve the cathode compatibility of the PVDF-based electrolyte and enhance the cycle and rate performance of lithium metal batteries.

[0053] Comparative Example 2 (no interpenetrating structure, and only the first network is retained): A method for preparing a PVDF polymer solid electrolyte membrane with a cross-linked structure is provided. The PVDF, lithium salt, polyethylene glycol dimethacrylate, ethylene glycol methyl ether acrylate, first initiator and DMF are mixed and stirred according to the mass parts corresponding to Example 1. Then, the stirred solution is poured into a circular PTFE plate with a diameter of 12 cm and placed in a vacuum oven. It is then vacuum dried at 80°C for 12 h to obtain a PVDF polymer solid electrolyte membrane with a cross-linked structure.

[0054] Comparative Example 3 (One-step polymerization): A method for preparing a PVDF polymer solid electrolyte membrane with a cross-linked structure is provided. The PVDF, lithium salt, polyethylene glycol dimethacrylate, ethylene glycol methyl ether acrylate, first initiator, second initiator, FEC, VC and DMF in Example 1 are mixed and stirred. Then, the stirred solution is poured into a circular PTFE plate with a diameter of 12 cm and placed in a vacuum oven. The plate is then vacuum dried at 80°C for 12 h to obtain a PVDF-based polymer solid electrolyte membrane.

[0055] Comparative Example 4 (One-step polymerization): A method for preparing a PVDF polymer solid electrolyte membrane with a cross-linked structure is provided. The PVDF, lithium salt, polyethylene glycol dimethacrylate, ethylene glycol methyl ether acrylate, first initiator, second initiator, FEC, VC and DMF in Example 1 are mixed and stirred. The stirred solution is then poured into a circular PTFE plate with a diameter of 12 cm and placed in a vacuum oven. Prepolymerization is carried out at 60°C for 6 h, and then vacuum dried at 80°C for 12 h to obtain the PVDF-based polymer solid electrolyte membrane.

[0056] Comparative Example 5 (High Liquid Electrolyte Content): A PVDF polymer with a cross-linked structure is provided. Gel electrolyte The preparation method involves mixing and stirring the PVDF, lithium salt, polyethylene glycol dimethacrylate, ethylene glycol methyl ether acrylate, first initiator, and DMF according to the mass proportions specified in Example 1. 70 parts of liquid electrolyte containing EC / DMC / DEC are then added, and the mixture is poured into a mold and heated at 60°C for 6 hours to form a gel electrolyte.

[0057] Comparative Example 6 (EO-free monomer): A method for preparing a PVDF polymer solid electrolyte membrane with a cross-linked structure is provided. The difference from Example 1 is that methyl methacrylate (18 parts) is used instead of ethylene glycol methyl ether acrylate and polyethylene glycol dimethacrylate, while the other steps remain unchanged.

[0058] Comparative Example 7 (Fluorine-free monomer): A method for preparing a PVDF polymer solid electrolyte membrane with a cross-linked structure is provided. The difference from Example 1 is that the in-situ polymerization precursor mixture contains no fluorinated ethylene carbonate, only ethylene ethylene carbonate and the second initiator, while the other steps remain unchanged.

[0059] Comparative Example 8 (without interpenetrating structure and only retaining the second network): The difference from Example 1 is that the pure PVDF polymer solid electrolyte membrane prepared in Comparative Example 1 was subjected to in-situ polymerization in the same steps as in Example 1 to obtain a PVDF polymer solid electrolyte with a cross-linked structure.

[0060] The electrolytes prepared in Examples 1-3 and Comparative Examples 1-8 were subjected to performance testing, and the test results are shown in Table 1 below:

[0061] As shown in Table 1, the embodiments of the present invention achieve an increase in ionic conductivity and an improvement in interface stability.

[0062] As can be seen from the comparison between Example 1 and Comparative Example 2, when only the first network is retained and there is no interpenetrating structure, the lithium ion transport path is discontinuous due to the lack of a continuous lithium ion transport pathway formed by the interpenetrating network. Although the first network can partially promote the dissociation of lithium salt and increase the content of free lithium ions to a certain extent, thereby increasing the lithium ion transference number to a certain extent, its conductivity is low due to the discontinuous transport network. Furthermore, the lack of excellent film-forming materials and in-situ polymerization strategies results in poor interface compatibility. Therefore, the cycle time of Li||Li for this battery is relatively short.

[0063] A comparison of Example 1 and Comparative Example 3 shows that the PVDF-based polymer solid electrolyte membrane obtained by one-step polymerization retains some advantages of Example 1, such as a stronger ability to dissociate lithium ions, resulting in a high free lithium ion content, a relatively high lithium ion transference number, and a continuous transport network that ensures continuous lithium ion transport. However, some of the synthesized electrolyte membranes do not have the same interfacial compatibility with lithium metal as the in-situ polymerization method in Example 1. The electrolyte prepared by the one-step method exhibits solvent evaporation, leading to a rough and porous surface after film formation, which affects interfacial contact performance. Therefore, the Li||Li battery has a shorter cycle time.

[0064] As can be seen from the comparison between Example 1 and Comparative Example 4, even if the PVDF-based polymer solid electrolyte membrane is prepolymerized at 60°C for 6 hours and then vacuum dried at 80°C, the internal polymer of the electrolyte undergoes polymerization after prepolymerization, further anchoring the harmful solvent DMF that is unstable to lithium metal. This results in an excessively high residual DMF content during the subsequent vacuum process. Although the electrolyte prepared in Comparative Example 4 has higher conductivity and migration number than the previous example, the large presence of harmful DMF makes its compatibility with lithium metal worse. This is why its Li||Li battery can only cycle for 350 hours.

[0065] As can be seen from the comparison between Example 1 and Comparative Example 5, the gel electrolyte has a higher conductivity than that of Example 1, which has a liquid component content of less than 10 wt%, due to the presence of a large amount of electrolyte components. However, due to the solvation effect of the various components in the electrolyte on free lithium ions, its free lithium ion content is lower and its lithium ion transference number is also lower. In addition, the liquid content of the gel electrolyte shown in Comparative Example 5 is too high, which is far greater than the range specified for solid electrolytes.

[0066] A comparison of Example 1 and Comparative Example 6 shows that when methyl methacrylate is used instead of ethylene glycol methyl ether acrylate and polyethylene glycol dimethacrylate, methyl methacrylate, being a small molecule monomer, exhibits high crystallinity during polymerization. This high crystallinity severely hinders the dissociation of lithium salts and the transport of lithium ions, which explains the low lithium-ion transference number and conductivity of Comparative Example 6. However, the in-situ polymerization strategy still ensured the lithium metal interface contact performance of Comparative Example 6, which explains why the electrolyte prepared in Comparative Example 6 cycled for 800 hours in a Li||Li battery.

[0067] Comparing Example 1 and Comparative Example 7, it can be seen that when the in-situ polymerization precursor mixture contains only ethylene ethylene carbonate and a second initiator, without fluorinated ethylene carbonate, the electrolyte conductivity and lithium-ion transference number are somewhat reduced. This is because the regulation of lithium-ion coordination by fluorinated ethylene carbonate is lacking. Fluorinated ethylene carbonate has a high dielectric constant (~85) and strong electron-withdrawing F atoms, which can effectively weaken the ion-pair / cluster interaction between Li⁺ and TFSI⁻, converting more bound Li⁺ into free Li⁺. Therefore, the lack of fluorinated ethylene carbonate results in a relative decrease in lithium-ion transference number and conductivity. In addition, fluorinated ethylene carbonate is also a good film-forming additive. The SEI generated by the electrolyte of Comparative Example 7 without fluorinated ethylene carbonate will be unstable, thus shortening the Li||Li battery cycle time to 600h.

[0068] As can be seen from the comparison between Example 1 and Comparative Example 8, when only the second network is retained and the interpenetrating structure is absent, Comparative Example 8 exhibits low conductivity and transport number. The interpenetrating structure contains a large number of EO segments, which, due to their strong electronegativity, are often considered the main sites for lithium-ion transport. Without the interpenetrating network, the electrolyte essentially prunes these excellent lithium-ion transport sites, hindering lithium-ion transport. This is a significant factor contributing to the low lithium-ion conductivity and transport number exhibited by Comparative Example 8. However, the retention of the second network and the in-situ polymerization method ensures that Comparative Example 8 remains compatible with lithium metal, which explains why the Li||Li battery can cycle for 850 hours. Furthermore, due to the superior oxidation resistance of PVDF itself, the electrolyte maintains a high redox window (LSV) value in all examples of this invention.

[0069] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., 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 PVDF solid polymer electrolyte having an interpenetrating cross-linked network structure, characterized in that, include: The first network comprises a cross-linked backbone formed by copolymerization of PVDF, lithium salt, polyethylene glycol dimethacrylate and ethylene glycol methyl ether acrylate; The second network includes a network structure formed by in-situ polymerization of ethylene carbonate and fluoroethylene carbonate, wherein the second network and the first network are interpenetrating and intertwined. The second network is configured to be obtained by in-situ polymerization of a precursor solution containing ethylene carbonate, fluoroethylene carbonate and an initiator after immersing it in the first network.

2. The PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure according to claim 1, characterized in that, The volume ratio of ethylene carbonate to fluoroethylene carbonate in the precursor solution is 1~9:1~9.

3. The PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure according to claim 1, characterized in that, The mass ratio of polyethylene glycol dimethacrylate to ethylene glycol methyl ether acrylate is 5~10:8~15.

4. The PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure according to claim 1, characterized in that, The lithium salt is selected from at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, and lithium hexafluorophosphate.

5. A method for preparing a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure as described in any one of claims 1 to 4, characterized in that, Includes the following steps: S1: PVDF particles, lithium salt, polyethylene glycol dimethacrylate, ethylene glycol methyl ether acrylate, the first initiator, and an organic solvent are mixed and stirred to obtain a slurry; S2: Pour the slurry into a mold and dry it under vacuum to obtain a cross-linked network; S3: Prepare a mixture of in-situ polymerization precursors of ethylene carbonate, fluoroethylene carbonate and a second initiator; S4: The precursor solution is immersed in the crosslinking network and polymerized in situ to obtain a PVDF solid polymer electrolyte with an interpenetrating crosslinking network structure.

6. The method for preparing a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure according to claim 5, characterized in that, The mass ratio of the PVDF particles, lithium salt, polyethylene glycol dimethacrylate, ethylene glycol methyl ether acrylate, first initiator, and in-situ polymerization precursor mixture is: (15~25):(5~10):(8~15):(0.5~1):(10.5~15.9)； The amount of the second initiator added is 0.8 to 1.2 wt% of the mass of ethylene carbonate (VC).

7. The method for preparing a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure according to claim 5, characterized in that, The organic solvent is selected from at least one of N,N-dimethylformamide and N-methylpyrrolidone.

8. The method for preparing a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure according to claim 5, characterized in that, The stirring temperature described in S1 is 40~50℃, and the stirring time is 6~12h; The vacuum drying temperature described in S2 is 60~100℃, and the vacuum drying time is 12~24h; The in-situ polymerization temperature described in S4 is 40~80℃, and the in-situ polymerization time is 2~12h.

9. A method for preparing a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure according to claim 5, characterized in that, The mold mentioned in S2 is a PTFE mold, which includes a disc with a diameter of 8~12cm.

10. The application of a PVDF solid polymer electrolyte with an interpenetrating cross-linked network structure as described in any one of claims 1 to 4 in the preparation of lithium metal batteries.

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