DOPO-grafted polyvinylidene fluoride-based polymer solid electrolyte and preparation method thereof

Abstract

The invention discloses a DOPO (9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide)-grafted polyvinylidene fluoride-based polymer solid electrolyte and a preparation method thereof, and the DOPO-grafted polyvinylidene fluoride-based polymer solid electrolyte is prepared by two-step synthesis reaction of polyvinylidene fluoride dehydrofluorination and phosphorus-hydrogen addition in combination with a solution pouring coating-drying process. The DOPO grafted polyvinylidene fluoride polymer is autonomously synthesized, so that the solid electrolyte is endowed with excellent flame retardant property, and the electrochemical properties of the solid electrolyte, such as ionic conductivity, lithium ion transference number, high-voltage resistance stability, lithium stability, charge-discharge cycle stability and the like, are also remarkably improved; and wide application of the polyvinylidene fluoride-based polymer solid electrolyte in high-safety and long-life solid-state lithium ion batteries is promoted.

Description

Technical Field

[0001] This invention belongs to the field of preparation of polymer solid electrolytes for lithium-ion batteries, specifically relating to a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte and its preparation method. Background Technology

[0002] Benefiting from their high energy density and long cycle life, lithium-ion batteries have been widely adopted in electric vehicles, consumer electronics, and large-scale energy storage. Compared to traditional liquid systems, solid-state lithium-ion batteries use solid electrolytes instead of porous membranes and flammable liquid electrolytes, significantly improving thermal safety while eliminating leakage hazards and reducing the risk of combustion and explosion. Therefore, they are considered one of the core pathways for lithium battery technology upgrades. Among various solid-state electrolyte systems, polyvinylidene fluoride (PVDF) solid-state electrolytes stand out due to their relatively excellent electrochemical stability and superior mechanical properties. It is important to note that PVDF itself is not a lithium-ion conductor and has a weak ability to dissociate lithium salts. It typically relies on trace amounts of organic solvents remaining from the electrolyte preparation process to promote lithium salt dissociation and improve lithium-ion conductivity. However, this also exacerbates the flammability of PVDF electrolytes to some extent and triggers severe side reactions at the electrolyte / electrode interface, leading to lithium dendrite growth and battery cycle performance degradation.

[0003] Recent studies have shown that introducing organic solvent-loving fillers or adjusting the solvent composition during the preparation of polyvinylidene fluoride (PVDF) electrolytes can optimize the lithium-ion solvation structure within the electrolyte, thereby effectively suppressing side reactions between residual solvents and electrodes. However, this method still fails to solve the inherent flammability problem of PVDF polymers. On the other hand, to further eliminate the safety hazards of lithium-ion batteries, flame-retardant modification of PVDF polymer solid electrolytes by adding functional components has attracted widespread attention. However, current research in this area mostly focuses on the flammability of the electrolyte, and the introduced free flame-retardant components (such as widely used organophosphorus flame retardants) are also prone to uncontrollable side reactions with the graphite or lithium anode of lithium-ion batteries, reducing the cycle performance of lithium-ion batteries. There are few reports on how to improve the lithium-ion transport performance and electrolyte / electrode interface stability of polymer solid electrolytes while simultaneously enhancing their fire resistance and safety. Therefore, developing a non-flammable PVDF polymer solid electrolyte that can simultaneously achieve high safety and excellent cycle stability in lithium-ion batteries has significant theoretical and practical value. Summary of the Invention

[0004] The purpose of this invention is to provide a DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) grafted polyvinylidene fluoride polymer solid electrolyte and its preparation method, which directly solves the scientific and technological problems faced by polyvinylidene fluoride polymer solid electrolytes, such as intrinsic flammability, weak lithium-ion conductivity and poor electrolyte / electrode interface compatibility.

[0005] In one aspect of the present invention, a method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte is provided. According to an embodiment of the present invention, the polymer matrix of the DOPO-grafted polyvinylidene fluoride polymer solid electrolyte is a DOPO-grafted modified polyvinylidene fluoride polymer, which is synthesized via a two-step reaction involving dehydrofluorination and phosphohydroaddition with DOPO.

[0006] It should be noted that in the above-mentioned technical solution of DOPO-grafted polyvinylidene fluoride polymer solid electrolyte, lithium salt can be added to the solution system in any step, and the order of addition of lithium salt will not affect the reaction effect of the present invention.

[0007] DOPO, as a highly efficient organophosphorus flame retardant molecule, covalently grafted onto polyvinylidene fluoride (PVDF) polymer chains, not only endows the electrolyte with excellent overall flame retardancy but also strongly anchors DOPO molecules to effectively restrict their free migration, thereby significantly reducing side reactions between DOPO and the negative electrode. Simultaneously, DOPO grafting can significantly reduce the crystallinity of PVDF and alter polymer polarity, refine the PVDF spherulite size within the electrolyte, and improve the uniformity and continuity of the polymer network, achieving a rapid and uniform lithium-ion transport process. Furthermore, the abundant O=P–O functional groups in the DOPO molecule significantly enhance the complexation ability of the PVDF polymer chain for lithium ions. On the one hand, this provides an additional rapid conduction pathway for lithium ions; on the other hand, it can participate in the lithium-ion solvation process, crowding out the coordination space of residual solvent molecules within the electrolyte, thereby inhibiting the decomposition of solvent molecules on the electrode surface and improving the stability of the electrolyte / electrode interface.

[0008] In addition, the method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte according to the above embodiments of the present invention may also have the following additional technical features:

[0009] In an embodiment of the present invention, the DOPO-grafted polyvinylidene fluoride polymer solid electrolyte is prepared by a solution casting, coating-drying process using a DOPO-grafted modified polyvinylidene fluoride polymer and a polar solvent solution of lithium salt.

[0010] In an embodiment of the present invention, the defluorination reaction system includes polyvinylidene fluoride, a catalyst, and a polar organic solvent. The catalyst has a mass fraction of 0.2wt%-10wt% relative to polyvinylidene fluoride. The reaction temperature is 40-100 °C, and the reaction time is 4-48 h. Under the action of an alkaline catalyst, adjacent hydrogen and fluorine atoms in the polyvinylidene fluoride –CH2–CF2– repeating unit are removed from the chain, thereby generating unsaturated carbon-carbon double bonds on the polyvinylidene fluoride chain.

[0011] In embodiments of the present invention, the catalyst is at least one of the following alkaline catalysts: tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrabutylammonium hydroxide, hexadecyltrimethylammonium hydroxide, 1,8-diazabicyclo[5.4.0]undec-7-ene, triethylamine, pyridine, sodium hydroxide, potassium hydroxide, etc.

[0012] In embodiments of the present invention, the polar organic solvent is at least one selected from N,N-dimethylformamide, N,N-dimethylacetamide, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dimethyl ethyl carbonate, dimethyl sulfoxide, tetrahydrofuran, ethylene glycol dimethyl ether, N-methylpyrrolidone, and acetonitrile.

[0013] In an embodiment of the present invention, the phosphohydroaddition reaction system includes dehydrofluorinated polyvinylidene fluoride (PVDF), DOPO, an initiator, and a polar organic solvent. The DOPO has a mass fraction of 5 wt%-20 wt% relative to PVDF, and the initiator has a mass fraction of 0.1 wt%-2 wt% relative to the original PVDF. The reaction temperature is 40-100 °C, and the reaction time is 4-48 h. Under the action of a free radical initiator, the unsaturated carbon-carbon double bonds in the dehydrofluorinated PVDF are opened, and an addition reaction occurs with the active PH bonds in the DOPO molecule, thereby realizing the covalent grafting process of DOPO onto PVDF.

[0014] In embodiments of the present invention, the initiator is at least one of free radical initiators such as azobisisobutyronitrile, azobisisoheptanenitrile, dimethyl azobisisobutyrate, azobiscyclohexylformitrile, di-tert-butyl peroxide, and persulfate.

[0015] In embodiments of the present invention, the polar organic solvent is at least one selected from N,N-dimethylformamide, N,N-dimethylacetamide, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dimethyl ethyl carbonate, dimethyl sulfoxide, tetrahydrofuran, ethylene glycol dimethyl ether, N-methylpyrrolidone, and acetonitrile.

[0016] In embodiments of the present invention, the lithium salt is at least one selected from lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium nitrate, lithium dioxoborate, lithium difluorooxalate borate, lithium tetrafluoroborate, and lithium hexafluorophosphate; the polar solvent is at least one selected from N,N-dimethylformamide, N,N-dimethylacetamide, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, diethyl carbonate, dimethyl ethyl carbonate, dimethyl sulfoxide, tetrahydrofuran, ethylene glycol dimethyl ether, N-methylpyrrolidone, and acetonitrile.

[0017] In an embodiment of the present invention, the mass ratio of the lithium salt to polyvinylidene fluoride is 0.2:1-1.5:1; the drying process conditions are drying at 40-100 °C for 12-48 h.

[0018] In another aspect of the present invention, the present invention provides a method for preparing the DOPO-grafted polyvinylidene fluoride polymer solid electrolyte as described above.

[0019] Compared with the prior art, the beneficial effects of the present invention are reflected in:

[0020] (1) Compared with polyvinylidene fluoride polymer electrolytes and polymer solid electrolytes simply doped with DOPO, in this invention, DOPO is strongly anchored to the electrolyte through covalent bonds. On the one hand, based on its free radical quenching mechanism for combustion chain reactions, it endows the electrolyte membrane with excellent flame retardant ability. On the other hand, it can prevent the escape of small DOPO molecules and inhibit their side reactions with the negative electrode.

[0021] (2) DOPO grafting alters the high crystallinity and polarization properties of polyvinylidene fluoride at the molecular level, effectively promoting lithium salt dissociation and optimizing the polymer spherulite network within the electrolyte, thus enabling rapid and uniform lithium ion migration during electrochemical reactions. More importantly, the abundant oxygen-containing functional groups within the DOPO molecule endow the DOPO-grafted modified polyvinylidene fluoride polymer chain with the ability to reversibly coordinate with lithium ions. This not only promotes the rapid conduction of lithium ions along the polymer chain but also allows the polymer chain to participate in the solvation process of lithium ions, crowding out the coordination space of highly active solvent molecules and suppressing their side reactions with the electrode, thereby significantly improving the stability of the electrolyte / electrode interface.

[0022] (3) DOPO, a highly efficient flame-retardant phosphorus-based small molecule, was chemically grafted onto the polyvinylidene fluoride polymer solid electrolyte network, successfully achieving high safety and long-term stable cycling of solid lithium-ion batteries.

[0023] (4) This invention directly addresses the scientific and technological challenges faced by polyvinylidene fluoride polymer solid electrolytes, such as intrinsic flammability, weak lithium-ion conductivity, and poor compatibility at the electrolyte / electrode interface. It has good technological advancement and significant beneficial effects. Attached Figure Description

[0024] Figure 1 The chemical reaction formulas for the defluorination of polyvinylidene fluoride (a, b) and the hydrophosphoric addition of DOPO to polyvinylidene fluoride (c) in Example 1 of the present invention are shown.

[0025] Figure 2 Images of open flame combustion tests of PVDFHF (a) prepared in Comparative Example 1, PVDFHF+DOPO (b) prepared in Comparative Example 2, and PVDFHF-DOPO electrolyte membrane (c) prepared in Example 1 of this invention;

[0026] Figure 3 Scanning electron microscope (SEM) images (top: surface; bottom: cross section) of PVDFHF (a) prepared in Comparative Example 1, PVDFHF+DOPO (b) prepared in Comparative Example 2, and PVDFHF-DOPO electrolyte membrane (c) prepared in Example 1 of this invention.

[0027] Figure 4 The ionic conductivity of the PVDFHF prepared in Comparative Example 1, the PVDFHF+DOPO prepared in Comparative Example 2, and the PVDFHF-DOPO prepared in Example 1 at different temperatures.

[0028] Figure 5 The lithium-ion transport number at room temperature of the PVDFHF (a) prepared in Comparative Example 1, the PVDFHF+DOPO (b) prepared in Comparative Example 2, and the PVDFHF-DOPO electrolyte membrane (c) prepared in Example 1 of this invention;

[0029] Figure 6 The lithium stability cycling curves of the PVDFHF prepared in Comparative Example 1, the PVDFHF+DOPO prepared in Comparative Example 2, and the PVDFHF-DOPO prepared in Example 1 at room temperature are shown.

[0030] Figure 7 Raman spectra (TFSI) of PVDFHF (a) prepared in Comparative Example 1, PVDFHF+DOPO (b) prepared in Comparative Example 2, and PVDFHF-DOPO electrolyte membrane (c) prepared in Example 1 at room temperature. – (ion coordination state)

[0031] Figure 8SEM images of the lithium anode surface after lithium cycling at room temperature for 38 h, 50 h and 50 h, respectively, assembled from the PVDFHF (a) prepared in Comparative Example 1, the PVDFHF+DOPO (b) prepared in Comparative Example 2 and the PVDFHF-DOPO electrolyte membrane prepared in Example 1.

[0032] Figure 9 The X-ray photoelectron spectroscopy (XPS) spectra of lithium anode surface products after lithium cycling for 38 h, 50 h and 50 h at room temperature are shown for lithium symmetric batteries assembled with PVDFHF prepared in Comparative Example 1, PVDFHF+DOPO prepared in Comparative Example 2 and PVDFHF-DOPO prepared in Example 1.

[0033] Figure 10 The graph shows the cycling performance of LiFePO4(LFP) / Li batteries assembled with the PVDFHF prepared in Comparative Example 1, the PVDFHF+DOPO prepared in Comparative Example 2, and the PVDFHF-DOPO electrolyte membrane prepared in Example 1 at room temperature.

[0034] Figure 11 The LiNi assembled from the PVDFHF prepared in Comparative Example 1, the PVDFHF+DOPO prepared in Comparative Example 2, and the PVDFHF-DOPO electrolyte membrane prepared in Example 1 of this invention. 0.9 Co 0.05 Mn 0.05 Cycling performance of O2 (NCM9055) / Li battery at room temperature;

[0035] Figure 12 The cycling performance (a) of the LFP / graphite (Gr) pouch cell assembled with the PVDFHF-DOPO electrolyte membrane prepared in Example 1 of this invention at room temperature, the voltage change curve after needle puncture (b), and the working effect of lighting an LED screen in the natural state (c), folded (d), cut (e), and after needle puncture (f) are shown. Detailed Implementation

[0036] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0037] Example 1

[0038] A method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte (PVDFHF-DOPO) includes the following steps:

[0039] Step 1: Prepare a 0.5 mL methanol (>99.7%, Sinopharm Chemical Reagent Co., Ltd.) solution containing 2 wt% tetrabutylammonium hydroxide, and mix it with 9.5 mL N,N-dimethylformamide (>99.5%, Sinopharm Chemical Reagent Co., Ltd.) for later use.

[0040] Step 2: At 60 °C, 1 g PVDF (polyvinylidene fluoride, HSV 900, Shenzhen Kejing Zhida Technology Co., Ltd.), 0.1 g DOPO (97%, Sinopharm Chemical Reagent Co., Ltd.) and 0.8 g LiTFSI (lithium bis(trifluoromethanesulfonylimide, 99%, Aladdin) were thoroughly stirred and dissolved in the mixed solution obtained in Step 1, and the reaction was continued to be stirred for 12 h.

[0041] Step 3: Remove oxygen from the slurry obtained in Step 2 by blowing nitrogen gas, and then add 0.01 g of AIBN (azobisisobutyronitrile, 98%, Aladdin). In an inert gas-sealed environment, slowly stir and react at 80 °C for 24 h to obtain PVDFHF-DOPO slurry solution.

[0042] Step 4: Coat the PVDFHF-DOPO polymer slurry obtained in Step 3 evenly on a glass plate, and bake it at 80 °C for 24 h to obtain the DOPO-grafted PVDFHF-DOPO polymer solid electrolyte membrane.

[0043] Comparative Example 1

[0044] A method for preparing a defluorinated hydrogen-modified polyvinylidene fluoride polymer solid electrolyte membrane (PVDFHF) includes the following steps:

[0045] Step 1: Prepare a 0.5 mL methanol (>99.7%, Sinopharm Chemical Reagent Co., Ltd.) solution containing 2 wt% tetrabutylammonium hydroxide, and mix it with 9.5 mL N,N-dimethylformamide (>99.5%, Sinopharm Chemical Reagent Co., Ltd.) for later use.

[0046] Step 2: At 60 °C, 1 g of PVDF (polyvinylidene fluoride, HSV 900, Shenzhen Kejing Zhida Technology Co., Ltd.) and 0.8 g of LiTFSI (lithium bis(trifluoromethanesulfonylimide, 99%), are thoroughly stirred and dissolved in the mixed solution obtained in Step 1 to obtain PVDFHF slurry solution.

[0047] Step 3: The PVDFHF slurry obtained in Step 2 is evenly coated on a glass plate and baked at 80 °C for 24 h to obtain a hydrogen-defluorinated PVDFHF polymer solid electrolyte membrane.

[0048] Comparative Example 2

[0049] A method for preparing a defluorinated hydrogen fluoride modified polyvinylidene fluoride polymer solid electrolyte membrane (PVDFHF+DOPO) with simple blending of DOPO includes the following steps:

[0050] Step 1: Prepare a 0.5 mL methanol (>99.7%, Sinopharm Chemical Reagent Co., Ltd.) solution containing 2 wt% tetrabutylammonium hydroxide, and mix it with 9.5 mL N,N-dimethylformamide (>99.5%, Sinopharm Chemical Reagent Co., Ltd.) for later use.

[0051] Step 2: At 60 °C, 1 g PVDF (polyvinylidene fluoride, HSV 900, Shenzhen Kejing Zhida Technology Co., Ltd.), 0.1 g DOPO (97%, Sinopharm Chemical Reagent Co., Ltd.) and 0.8 g LiTFSI (lithium bis(trifluoromethanesulfonylimide), 99%, Aladdin) are thoroughly stirred and dissolved in the mixed solution obtained in Step 1 to obtain a PVDFHF+DOPO slurry solution.

[0052] Step 3: Coat the PVDFHF+DOPO polymer slurry obtained in Step 2 evenly on a glass plate, and bake it at 80 ℃ for 24 h to obtain the DOPO blended PVDFHF+DOPO polymer solid electrolyte membrane.

[0053] Example 2

[0054] A method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte includes the following steps:

[0055] Step 1: Prepare a 0.5 mL methanol (>99.7%, Sinopharm Chemical Reagent Co., Ltd.) solution containing 10 wt% tetrabutylammonium hydroxide, and mix it with 9.5 mL N,N-dimethylformamide (>99.5%, Sinopharm Chemical Reagent Co., Ltd.) for later use.

[0056] Step 2: At 60 °C, 1 g PVDF (polyvinylidene fluoride, HSV 900, Shenzhen Kejing Zhida Technology Co., Ltd.), 0.1 g DOPO (97%, Sinopharm Chemical Reagent Co., Ltd.) and 0.8 g LiTFSI (lithium bis(trifluoromethanesulfonylimide), 99%, Aladdin) were thoroughly stirred and dissolved in the mixed solution obtained in Step 1, and the reaction was continued to be stirred for 8 h.

[0057] Step 3: Remove oxygen from the slurry obtained in Step 2 by blowing nitrogen gas, and then add 0.01 g of AIBN (azobisisobutyronitrile, 98%, Aladdin). In an inert gas-sealed environment, slowly stir and react at 80 °C for 24 h to obtain PVDFHF-DOPO slurry solution.

[0058] Step 4: Coat the PVDFHF-DOPO polymer slurry obtained in Step 3 evenly on a glass plate, and bake it at 80 °C for 24 h to obtain the DOPO-grafted PVDFHF-DOPO polymer solid electrolyte membrane.

[0059] Example 3

[0060] A method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte includes the following steps:

[0061] Step 1: Prepare a 0.5 mL methanol (>99.7%, Sinopharm Chemical Reagent Co., Ltd.) solution containing 2 wt% tetrabutylammonium hydroxide, and mix it with 9.5 mL N,N-dimethylformamide (>99.5%, Sinopharm Chemical Reagent Co., Ltd.) for later use.

[0062] Step 2: At 60 °C, 1 g PVDF (polyvinylidene fluoride, HSV 900, Shenzhen Kejing Zhida Technology Co., Ltd.), 0.05 g DOPO (97%, Sinopharm Chemical Reagent Co., Ltd.) and 0.8 g LiTFSI (lithium bis(trifluoromethanesulfonylimide), 99%, Aladdin) were thoroughly stirred and dissolved in the mixed solution obtained in Step 1, and the reaction was continued to be stirred for 12 h.

[0063] Step 3: Remove oxygen from the slurry obtained in Step 2 by blowing nitrogen gas, and then add 0.01 g of AIBN (azobisisobutyronitrile, 98%, Aladdin). In an inert gas-sealed environment, slowly stir and react at 80 °C for 24 h to obtain PVDFHF-DOPO slurry solution.

[0064] Step 4: Coat the PVDFHF-DOPO polymer slurry obtained in Step 3 evenly on a glass plate, and bake it at 80 °C for 24 h to obtain the DOPO-grafted PVDFHF-DOPO polymer solid electrolyte membrane.

[0065] Example 4

[0066] A method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte includes the following steps:

[0067] Step 1: Prepare a 0.5 mL methanol (>99.7%, Sinopharm Chemical Reagent Co., Ltd.) solution containing 2 wt% tetrabutylammonium hydroxide, and mix it with 9.5 mL N,N-dimethylformamide (>99.5%, Sinopharm Chemical Reagent Co., Ltd.) for later use.

[0068] Step 2: At 60 °C, 1 g PVDF (polyvinylidene fluoride, HSV 900, Shenzhen Kejing Zhida Technology Co., Ltd.), 0.15 g DOPO (97%, Sinopharm Chemical Reagent Co., Ltd.) and 0.8 g LiTFSI (lithium bis(trifluoromethanesulfonylimide), 99%, Aladdin) were thoroughly stirred and dissolved in the mixed solution obtained in Step 1, and the reaction was continued to be stirred for 12 h.

[0069] Step 3: Remove oxygen from the slurry obtained in Step 2 by blowing nitrogen gas, and then add 0.01 g of AIBN (azobisisobutyronitrile, 98%, Aladdin). In an inert gas-sealed environment, slowly stir and react at 80 °C for 36 h to obtain PVDFHF-DOPO slurry solution.

[0070] Step 4: Coat the PVDFHF-DOPO polymer slurry obtained in Step 3 evenly on a glass plate, and bake it at 80 °C for 24 h to obtain the DOPO-grafted PVDFHF-DOPO polymer solid electrolyte membrane.

[0071] The solid electrolyte membranes prepared in Example 1 and Comparative Examples 1-2 were subjected to the following performance tests:

[0072] (1) The polymer solid electrolyte membrane in the examples and comparative examples was selected to be 1 cm × 5 cm in size, and its combustion was observed when it was brought close to an open flame.

[0073] Figure 2 Images show the open flame flammability test results of the polymer solid electrolyte membranes of Comparative Example 1 (PVDFHF), Comparative Example 2 (PVDFHF+DOPO), and Example 1 (PVDFHF-DOPO). The scale bar is 1 cm long. The images show that the PVDFHF electrolyte is easily ignited by an open flame and burns completely within 5 seconds. Figure 2 a). Both PVDFHF+DOPO electrolyte membranes blended with DOPO and PVDFHF-DOPO electrolyte membranes with chemically anchored DOPO exhibit self-extinguishing properties within 2 seconds of exposure to an open flame, demonstrating excellent flame retardancy. Figure 2 b,c).

[0074] (2) The surface and cross-sectional microstructure morphology of the polymer solid electrolyte in the examples and comparative examples were observed by scanning electron microscopy.

[0075] Figure 3 The images show scanning electron microscope (SEM) images of the surface (top) and cross-section (bottom) of the polymer solid electrolyte membranes of Comparative Example 1 PVDFHF, Comparative Example 2 PVDFHF+DOPO, and Example 1 PVDFHF-DOPO. Figure 3 a indicates that the PVDFHF electrolyte has a typical large-size polymer spherulite structure, while with the introduction of DOPO blending and the grafting and anchoring of DOPO onto the polymer chain, the spherulite structure of the electrolyte membrane gradually becomes finer. Figure 3 (b, c). In particular, the PVDFH-DOPO polymer solid electrolyte exhibits a continuous, dense, and integrated molten spherulite network structure, indicating that DOPO grafting can significantly reduce the crystallinity of polyvinylidene fluoride polymer and change the polymer polarity, effectively improving the uniformity and continuity of the polymer network and achieving a rapid and uniform lithium-ion transport process.

[0076] (3) Using steel sheets as positive and negative electrodes, symmetrical batteries were assembled in a glove box using polymer solid electrolyte membranes prepared in the examples and comparative examples, respectively. Electrochemical impedance spectroscopy tests were performed on the batteries at different temperatures, and the ionic conductivity of the electrolyte membrane was calculated.

[0077] Figure 4 The ionic conductivity of the polymer solid electrolytes in Comparative Example 1 (PVDFHF), Comparative Example 2 (PVDFHF+DOPO), and Example 1 (PVDFHF-DOPO) at different temperatures was measured. DOPO grafting significantly reduces the crystallinity of polyvinylidene fluoride (PVDF) and alters polymer polarity, refining the size of PVDF spherulites within the electrolyte and improving the uniformity and continuity of the polymer network, thus achieving a rapid and uniform lithium-ion transport process. Furthermore, the abundant O=P–O functional groups in the DOPO molecule significantly enhance the reversible complexation ability of the PVDF polymer chain for lithium ions, effectively promoting rapid lithium-ion conduction along the polymer chain. Therefore, the PVDFHF-DOPO electrolyte in Example 1 exhibits a significantly improved room-temperature ionic conductivity compared to the comparative samples, reaching 1.72 × 10⁻⁶. –4 S / cm.

[0078] (4) The polymer solid electrolyte membranes prepared in the examples and comparative examples were used as electrolytes, and lithium metal sheets were used as positive and negative electrodes. They were assembled into lithium symmetric batteries in an argon glove box. The lithium ion transference number of the electrolyte membrane was determined by a comprehensive test of DC polarization and AC impedance at room temperature. At the same time, the constant current charge and discharge test was performed on the battery to evaluate the electrochemical stability of the electrolyte to the lithium metal negative electrode.

[0079] Figure 5The lithium-ion transference number (LTL) of the polymer solid electrolytes in Comparative Example 1 (PVDFHF), Comparative Example 2 (PVDFHF+DOPO), and Example 1 (PVDFHF-DOPO) at room temperature is shown. Also benefiting from the optimized polyvinylidene fluoride spherulite network structure within the electrolyte after DOPO grafting and the enhanced lithium-conductivity of the polymer chains, the PVDFHF-DOPO electrolyte in Example 1 exhibited a significantly higher LTL compared to the comparative samples at room temperature, reaching 0.48.

[0080] Figure 6 Comparative Example 1 (PVDFHF), Comparative Example 2 (PVDFHF+DOPO), and Example 1 (PVDFHF-DOPO) polymer solid electrolyte were tested at room temperature at 0.1 mA / cm². 2 Lithium stability at current density. Comparative Example 1 (PVDFHF) and Comparative Example 2 (PVDFHF+DOPO) electrolytes experienced short circuits after 38 h and 107 h of lithium cycling, respectively, while the PVDFHF-DOPO electrolyte of Example 1 maintained stable operation after 2500 h of cycling, confirming that the DOPO-grafted PVDFHF-DOPO electrolyte has significantly improved lithium interface stability.

[0081] (5) Raman spectroscopy was used to analyze the TFSI in the polymer solid electrolyte membranes prepared in the examples and comparative examples. – The coordination state of ions indirectly assesses the solvation structure of lithium ions within the electrolyte.

[0082] Figure 7 The Raman spectra (TFSI) of Comparative Example 1 PVDFHF, Comparative Example 2 PVDFHF+DOPO, and Example 1 PVDFHF-DOPO polymer solid electrolyte at room temperature are shown. – (Ion coordination state). It can be seen that the TFSI anion in the PVDFHF-DOPO electrolyte of Example 1... – The significantly increased content of DOPO indicates that grafting altered the polymer's polarity, effectively promoting lithium salt dissociation. Simultaneously, the content of ionic aggregates "AGGs" also increased significantly, suggesting that TFSI... – Enhanced coordination with lithium ions facilitates the formation of TFSI. – The dominant lithium-ion solvation shell inhibits the decomposition of DMF on the electrode surface, thereby improving the stability of the electrolyte / electrode interface.

[0083] (6) The microstructure morphology of the lithium anode surface of the polymer solid electrolyte assembled in the examples and comparative examples after a certain number of cycles was observed by scanning electron microscopy, and the chemical composition of the surface products was analyzed by XPS characterization.

[0084] Figure 8 The images show scanning electron microscope (SEM) images of the lithium anode surfaces of Comparative Example 1 (PVDFHF), Comparative Example 2 (PVDFHF+DOPO), and Example 1 (PVDFHF-DOPO) polymer solid electrolytes after lithium cycling for 38 h, 50 h, and 50 h at room temperature. It can be seen that a large number of lithium dendrites were formed on the lithium anode surfaces of Comparative Example 1 (PVDFHF) and Comparative Example 2 (PVDFHF+DOPO) electrolytes, indicating that severe side reactions occurred at the electrolyte / lithium anode interface of the assembled lithium symmetric batteries after cycling for 38 h and 50 h. Figure 8 a, b). In stark contrast, under the same lithium cycling conditions, the lithium anode surface of the PVDFHF-DOPO electrolyte in Example 1 remained smooth after 50 h, indicating the formation of a dense solid electrolyte interphase (SEI) film on the lithium anode surface. This further confirms the significantly improved interfacial stability of the lithium metal anode by the DOPO-grafted polymer electrolyte. Figure 8 c).

[0085] Figure 9 XPS spectra of lithium anode surface products after lithium cycling for 38 h, 50 h, and 50 h at room temperature for Comparative Example 1 PVDFHF, Comparative Example 2 PVDFHF+DOPO, and Example 1 PVDFHF-DOPO polymer solid electrolytes, respectively. The C1s spectrum shows a significant reduction in the C=O and C–O content on the lithium anode surface corresponding to Example 1 PVDFHF-DOPO electrolyte, indicating that DMF decomposition on the lithium anode surface was effectively suppressed. The F 1s and N 1s spectra show a significant increase in the LiF and Li3N content on the lithium anode surface corresponding to Example 1 PVDFHF-DOPO electrolyte, confirming the formation of an inorganic-dominated SEI film on the lithium anode surface, which promotes uniform lithium ion deposition and effectively inhibits lithium dendrite growth. Furthermore, the intensity of the P 2s characteristic peak of the PVDFHF-DOPO electrolyte in Example 1 was significantly lower than that of the PVDFHF+DOPO electrolyte in Example 2, indicating that the chemical anchoring of DOPO to the polyvinylidene fluoride polymer chain effectively suppressed its free migration within the electrolyte, thereby reducing the side reactions between DOPO molecules and the lithium anode and helping to maintain interface stability.

[0086] (7) The polymer solid electrolyte membranes prepared in the examples and comparative examples were used as electrolytes, LFP was used as positive electrode and lithium metal sheet was used as negative electrode. Coin cells were assembled in an argon glove box and constant current charge-discharge test was performed on the cells at room temperature and 0.5 C current density.

[0087] Figure 10The cycling performance of LFP / Li batteries assembled with Comparative Example 1 (PVDFHF), Comparative Example 2 (PVDFHF+DOPO), and Example 1 (PVDFHF-DOPO) polymer solid electrolytes at room temperature (0.5 C) is shown. Based on excellent lithium-ion conductivity and significantly improved electrolyte / electrode interface stability, the battery assembled with Example 1 (PVDFHF-DOPO) electrolyte maintained a capacity retention of 94.27% after 850 cycles. In contrast, the batteries assembled with Comparative Example 1 (PVDFHF) and Comparative Example 2 (PVDFHF+DOPO) electrolytes experienced sudden short circuits after 167 and 420 cycles, respectively, with corresponding capacity retentions of only 62.55% and 76.37%.

[0088] (8) The polymer solid electrolyte membranes prepared in the examples and comparative examples were used as electrolytes, NCM9055 was used as the positive electrode and lithium metal sheet was used as the negative electrode. The coin cells were assembled in an argon glove box and constant current charge-discharge tests were performed on the cells at room temperature and 0.2 C current density.

[0089] Figure 11 This section describes the cycling performance of high-voltage NCM9055 / Li batteries assembled with Comparative Example 1 (PVDFHF), Comparative Example 2 (PVDFHF+DOPO), and Example 1 (PVDFHF-DOPO) polymer solid electrolyte at room temperature (0.2 C). Based on excellent lithium-ion conductivity and significantly improved electrolyte / electrode interface stability, the battery assembled with Example 1 (PVDFHF-DOPO electrolyte) maintained a capacity retention of 77.61% after 150 cycles in the 2.8-4.3 V high-voltage range. In contrast, Comparative Example 1 (PVDFHF electrolyte) could barely keep up with the normal cycling of the high-voltage NCM9055 cathode, and the battery assembled with Comparative Example 2 (PVDFHF+DOPO electrolyte) suddenly short-circuited after only 60 cycles.

[0090] (9) Weld tabs to the LFP positive electrode and Gr negative electrode, stack the positive electrode, the composite solid electrolyte membrane prepared in the example, and the negative electrode in sequence, and seal three sides after wrapping them with aluminum-plastic film. The prefabricated soft-pack battery is then vacuum-heat-sealed in a glove box to obtain the target soft-pack battery. A constant current charge-discharge test is performed on the battery at room temperature and a current density of 0.5 C, and the voltage change trend of the soft-pack battery after needle penetration is monitored in real time to verify its safety. Furthermore, after fully charging the soft-pack battery, it is connected to an LED display panel to examine its performance under flat, folded, cut, and needle-penetrated conditions.

[0091] Figure 12 The cycling performance of the LFP / Gr soft-pack battery assembled with the PVDFHF-DOPO polymer solid electrolyte in Example 1 at room temperature and 0.5 C ( Figure 12 a), Voltage change curve after acupuncture ( Figure 12 b) and in the natural state ( Figure 12 c) Folding Figure 12 d) Cutting ( Figure 12 e) and after acupuncture ( Figure 12 f) Demonstration of the LED screen's working effect. Even when applied to solid-state pouch lithium-ion batteries, the PVDFHF-DOPO electrolyte of Example 1 still achieves stable long-term battery cycling. After 200 cycles at a current density of 0.5 C, its capacity retention rate still reaches 77.91%, demonstrating reliable practical application prospects. Furthermore, the assembled pouch battery, when fully charged, did not exhibit any abnormalities such as short circuits, smoke, or fire after being punctured; it only experienced a slight voltage drop, which stabilized within 60 minutes. Simultaneously, the pouch battery could normally illuminate the LED panel at room temperature and continued to function normally even under extreme conditions such as folding, cutting, and puncture, indicating that the PVDFHF-DOPO electrolyte membrane prepared in this example possesses excellent operational safety.

[0092] The above description is merely an exemplary embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte, characterized in that: The polymer matrix of the DOPO-grafted polyvinylidene fluoride polymer solid electrolyte is a DOPO-grafted modified polyvinylidene fluoride polymer, which is synthesized through a two-step reaction of dehydrofluorination and DOPO phosphohydroaddition.

2. The method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte according to claim 1, characterized in that: The DOPO-grafted polyvinylidene fluoride polymer solid electrolyte is prepared by a solution casting, coating, and drying process using a DOPO-grafted modified polyvinylidene fluoride polymer and a polar solvent solution of lithium salt.

3. The method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte according to claim 1, characterized in that: The defluorination reaction system includes polyvinylidene fluoride, a catalyst, and a polar organic solvent. The catalyst has a mass fraction of 0.2wt%-10wt% relative to polyvinylidene fluoride. The reaction temperature is 40-100 °C, and the reaction time is 4-48 h.

4. The method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte according to claim 3, characterized in that: The catalyst is at least one of tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrabutylammonium hydroxide, hexadecyltrimethylammonium hydroxide, 1,8-diazabicyclo[5.4.0]undec-7-ene, triethylamine, pyridine, sodium hydroxide, and potassium hydroxide; the polar organic solvent is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dimethyl sulfoxide, tetrahydrofuran, ethylene glycol dimethyl ether, N-methylpyrrolidone, and acetonitrile.

5. The method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte according to claim 1, characterized in that: The phosphohydroaddition reaction system includes dehydrofluorinated polyvinylidene fluoride (PVDF), DOPO, an initiator, and a polar organic solvent. The DOPO has a mass fraction of 5 wt%-20 wt% relative to PVDF, the initiator has a mass fraction of 0.1 wt%-2 wt% relative to PVDF, the reaction temperature is 40-100 °C, and the reaction time is 4-48 h.

6. The method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte according to claim 5, characterized in that: The initiator is at least one of azobisisobutyronitrile, azobisisoheptanenitrile, dimethyl azobisisobutyrate, azobiscyclohexylformitrile, di-tert-butyl peroxide, and persulfate.

7. The method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte according to claim 5, characterized in that: The polar organic solvent is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dimethyl sulfoxide, tetrahydrofuran, ethylene glycol dimethyl ether, N-methylpyrrolidone, and acetonitrile.

8. The method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte according to claim 2, characterized in that: The lithium salt is at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium nitrate, lithium dioxoborate, lithium difluorooxalate borate, lithium tetrafluoroborate, and lithium hexafluorophosphate; the polar solvent is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dimethyl ethyl carbonate, dimethyl sulfoxide, tetrahydrofuran, ethylene glycol dimethyl ether, N-methylpyrrolidone, and acetonitrile.

9. The method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte according to claim 2, characterized in that: The mass ratio of lithium salt to polyvinylidene fluoride is 0.2:1-1.5:1; the drying conditions in the solution casting coating-drying process are 40-100 °C for 12-48 h.

10. A DOPO-grafted polyvinylidene fluoride polymer solid electrolyte prepared by a method according to any one of claims 1-9.

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