A doPO grafted polyvinylidene fluoride-based polymer solid-state electrolyte and a preparation method thereof

Abstract

The application discloses a DOPO grafted polyvinylidene fluoride-based polymer solid-state electrolyte and a preparation method thereof. The DOPO grafted polyvinylidene fluoride-based polymer solid-state electrolyte is prepared through two-step synthesis reactions of polyvinylidene fluoride dehydrofluorination and phosphorus hydrogen addition, and a solution casting coating-drying process. The application independently synthesizes a DOPO grafted polyvinylidene fluoride polymer, which not only gives the solid-state electrolyte excellent flame retardant performance, but also significantly improves the ion conductivity, lithium ion transference number, high-voltage stability, lithium stability, charge-discharge cycle stability and other electrochemical performances of the solid-state electrolyte, and will promote the wide application of the polyvinylidene fluoride-based polymer solid-state electrolyte in high-safety and long-life solid-state lithium ion batteries.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of lithium ion battery polymer solid electrolyte preparation, and particularly relates to a DOPO grafted polyvinylidene fluoride-based polymer solid electrolyte and a preparation method thereof. BACKGROUND

[0002] Benefiting from high energy density and long cycle life, lithium ion batteries have been applied in the fields of electric vehicles, consumer electronics and large-scale energy storage. Compared with traditional liquid systems, solid-state lithium ion batteries use solid-state electrolytes to replace porous separators and flammable liquid electrolytes, which significantly improves thermal safety, eliminates leakage hazards and reduces the risk of combustion and explosion, and is therefore considered as one of the core paths for upgrading lithium battery technology. Among various solid-state electrolyte systems, polyvinylidene fluoride-based polymer solid electrolytes stand out due to their relatively excellent electrochemical stability and excellent mechanical properties. It should be noted that polyvinylidene fluoride-based polymers are not lithium ion conductors and have weak dissociation of lithium salt, and they usually rely on trace amounts of organic solvents remaining during electrolyte preparation to promote lithium salt dissociation and improve lithium ion conduction performance. However, this also to some extent aggravates the flammability of polyvinylidene fluoride-based polymer electrolytes and causes serious side reactions at the electrolyte / electrode interface, which in turn leads to lithium dendrite growth and battery cycle performance degradation.

[0003] Recent studies have shown that by introducing organic solventophilic fillers or adjusting the solvent composition during the preparation of polyvinylidene fluoride-based electrolytes, the lithium ion solvation structure in the electrolyte can be optimized, thereby effectively inhibiting the side reactions between residual solvents and electrodes, but this method still cannot solve the inherent flammability problem of polyvinylidene fluoride-based polymers. On the other hand, in order to further eliminate the safety hazards of lithium ion batteries, the flame-retardant modification of polyvinylidene fluoride-based polymer solid electrolytes by adding functional components has attracted widespread attention. However, current research in this area has mostly focused on the flammability of electrolytes, and the free flame-retardant components introduced (such as widely used organic phosphorus-based flame retardants) are also prone to uncontrollable side reactions with lithium ion battery graphite or lithium anodes, reducing the cycle performance of lithium ion batteries; how to improve the lithium ion transport performance and electrolyte / electrode interface stability of the electrolyte while improving the fire safety of the polymer solid electrolyte has rarely been reported. Therefore, it is of great theoretical significance and practical value to develop a non-flammable polyvinylidene fluoride-based polymer solid electrolyte that can simultaneously achieve high safety and excellent cycle stability of lithium ion batteries. SUMMARY

[0004] The present application aims to provide a DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) grafted polyvinylidene fluoride-based polymer solid electrolyte and a preparation method thereof, which directly solves the scientific and technical problems of intrinsic flammability, weak lithium ion conductivity, and poor electrolyte / electrode interface compatibility of polyvinylidene fluoride-based polymer solid electrolytes.

[0005] In one aspect of the present application, a preparation method of a DOPO grafted polyvinylidene fluoride-based polymer solid electrolyte is provided. According to an embodiment of the present application, the polymer matrix of the DOPO grafted polyvinylidene fluoride-based polymer solid electrolyte is a DOPO grafted modified polyvinylidene fluoride polymer, which is synthesized by two-step reactions of dehydrofluorination and DOPO phosphorus-hydrogen addition.

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

[0007] DOPO is a kind of efficient organic phosphorus flame retardant molecule, and its covalent grafting on the polyvinylidene fluoride polymer chain not only gives the electrolyte excellent overall flame retardant ability, but also strongly anchors the DOPO molecules to effectively limit their free migration, thereby greatly reducing the side reaction between DOPO and the negative electrode. At the same time, DOPO grafting can significantly reduce the crystallinity of polyvinylidene fluoride and change the polarity of the polymer, refine the size of the polyvinylidene fluoride spherulites in the electrolyte and improve the uniformity and continuity of the polymer network, realizing a fast and uniform lithium ion transmission process. In addition, the O=P-O functional groups rich in DOPO molecules can significantly improve the complexing ability of the polyvinylidene fluoride polymer chain to lithium ions, on the one hand providing an additional fast conduction path for lithium ions, and on the other hand participating in the solvation process of lithium ions, occupying the coordination space of residual solvent molecules in the electrolyte, and then inhibiting the decomposition of solvent molecules on the electrode surface, improving the stability of the electrolyte / electrode interface.

[0008] In addition, the preparation method of the DOPO grafted polyvinylidene fluoride-based polymer solid electrolyte according to the above embodiments of the present application can also have the following additional technical features:

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

[0010] In the embodiment of the present application, the dehydrofluorination reaction system comprises polyvinylidene fluoride, a catalyst and a polar organic solvent, the mass fraction of the catalyst is 0.2wt%-10wt% compared with the polyvinylidene fluoride, the temperature of the reaction is 40-100 ℃, and the reaction time is 4-48 h; under the action of the alkali catalyst, the adjacent hydrogen atom and fluorine atom in the polyvinylidene fluoride-CH2-CF2-repeating unit are removed from the chain, thereby generating an unsaturated carbon-carbon double bond on the polyvinylidene fluoride chain.

[0011] In the embodiment of the present application, the catalyst is at least one of the following alkali 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 the embodiment of the present application, the polar organic solvent is at least one of the following: N,N-dimethylformamide, N,N-dimethylacetamide, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dimethyl sulfoxide, tetrahydrofuran, ethylene glycol dimethyl ether, N-methyl pyrrolidone, acetonitrile.

[0013] In the embodiment of the present application, the phosphorus-hydrogen addition reaction system comprises the polyvinylidene fluoride after dehydrofluorination, DOPO, an initiator and a polar organic solvent, the mass fraction of the DOPO is 5wt%-20wt% compared with the polyvinylidene fluoride, the mass fraction of the initiator is 0.1wt%-2wt% compared with the original polyvinylidene fluoride, the temperature of the reaction is 40-100 ℃, and the reaction time is 4-48 h; under the action of the free radical initiator, the unsaturated carbon-carbon double bond in the polyvinylidene fluoride after dehydrofluorination is opened, and addition reaction occurs with the active P-H bond in the DOPO molecule, thereby realizing the covalent grafting process of DOPO on the polyvinylidene fluoride.

[0014] In the embodiment of the present application, the initiator is at least one of the following free radical initiators: azobisisobutyronitrile, azobisisoheptyl nitrile, dimethyl azobis isobutyrate, azobiscyclohexyl nitrile, di-tert-butyl peroxide, persulfate salt, etc.

[0015] In the embodiment of the present application, the polar organic solvent is at least one of the following: N,N-dimethylformamide, N,N-dimethylacetamide, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dimethyl sulfoxide, tetrahydrofuran, ethylene glycol dimethyl ether, N-methyl pyrrolidone, acetonitrile.

[0016] In the embodiments of the present application, the lithium salt is at least one of lithium bistrifluoromethanesulfonimide, lithium bisfluorosulfonimide, lithium perchlorate, lithium nitrate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium tetrafluoroborate, lithium hexafluorophosphate; the polar solvent is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dimethyl sulfoxide, tetrahydrofuran, ethylene glycol dimethyl ether, N-methyl pyrrolidone, acetonitrile.

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

[0018] In another aspect of the present application, the present application provides a DOPO grafted polyvinylidene fluoride-based polymer solid-state electrolyte prepared by the preparation method of the DOPO grafted polyvinylidene fluoride-based polymer solid-state electrolyte.

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

[0020] (1) Compared with polyvinylidene fluoride polymer electrolytes and simple DOPO doped polymer solid-state electrolytes, DOPO in the present application is strongly anchored to the electrolyte through covalent bonds, which not only imparts excellent flame retardant ability to the electrolyte membrane based on its free radical quenching mechanism for combustion chain reaction, but also prevents DOPO small molecules from escaping and inhibits the side reaction of DOPO with the negative electrode.

[0021] (2) DOPO grafting changes the high crystallinity and polarization properties of polyvinylidene fluoride at the molecular level, effectively promotes the dissociation of lithium salt and optimizes the polymer spherulite network in the electrolyte, and realizes the rapid and uniform migration of lithium ions in the electrochemical reaction process. More importantly, the rich oxygen-containing functional groups in the DOPO molecule also impart the ability of reversible complexation of the DOPO grafted polyvinylidene fluoride polymer chain to lithium ions, which not only promotes the rapid conduction of lithium ions along the polymer chain, but also makes the polymer chain participate in the solvation process of lithium ions, occupying the coordination space of high-activity solvent molecules and inhibiting the side reaction between them and the electrode, thereby greatly improving the stability of the electrolyte / electrode interface.

[0022] (3) DOPO, as a high-efficiency flame-retardant phosphorus-based small molecule, is chemically grafted to the polyvinylidene fluoride-based polymer solid-state electrolyte network, successfully realizing the high safety and long-period stable cycling of solid-state lithium ion batteries.

[0023] (4) The polyvinylidene fluoride-based polymer solid-state electrolyte directly solves the scientific and technological problems of intrinsic flammability, weak lithium ion conductivity and poor electrolyte / electrode interface compatibility, has good technical advancement and significant beneficial effects. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 Chemical reaction formula of polyvinylidene fluoride dehydrofluorination (a, b) and phosphorus hydrogen addition with DOPO (c) in Example 1 of the present application;

[0025] Figure 2 Picture of open flame combustion test of PVDFHF (a) prepared in Comparative Example 1 of the present application, PVDFHF+DOPO (b) prepared in Comparative Example 2 and PVDFHF-DOPO electrolyte membrane (c) prepared in Example 1 of the present application;

[0026] Figure 3 Picture of scanning electron microscope of PVDFHF (a) prepared in Comparative Example 1 of the present application, PVDFHF+DOPO (b) prepared in Comparative Example 2 and PVDFHF-DOPO electrolyte membrane (c) prepared in Example 1 of the present application (top: surface; bottom: cross section);

[0027] Figure 4 Ion conductivity of PVDFHF prepared in Comparative Example 1 of the present application, PVDFHF+DOPO prepared in Comparative Example 2 and PVDFHF-DOPO electrolyte membrane prepared in Example 1 of the present application at different temperatures;

[0028] Figure 5 Lithium ion migration number of PVDFHF (a) prepared in Comparative Example 1 of the present application, PVDFHF+DOPO (b) prepared in Comparative Example 2 and PVDFHF-DOPO electrolyte membrane (c) prepared in Example 1 of the present application at room temperature;

[0029] Figure 6 Cycle curve of stability to lithium of PVDFHF prepared in Comparative Example 1 of the present application, PVDFHF+DOPO prepared in Comparative Example 2 and PVDFHF-DOPO electrolyte membrane prepared in Example 1 of the present application at room temperature;

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

[0031] Figure 8The lithium symmetric battery assembled by the PVDFHF prepared in the present application comparative example 1, the PVDFHF+DOPO prepared in the comparative example 2 and the PVDFHF-DOPO electrolyte film prepared in the example 1 was scanned by SEM on the lithium negative electrode surface after 38 h, 50 h and 50 h of lithium cycle at room temperature respectively;

[0032] Figure 9 The XPS C 1s (a), F 1s (b), N 1s (c) and P 2p (d) spectra of the product on the lithium negative electrode surface of the lithium symmetric battery assembled by the PVDFHF prepared in the present application comparative example 1, the PVDFHF+DOPO prepared in the comparative example 2 and the PVDFHF-DOPO electrolyte film prepared in the example 1 after 38 h, 50 h and 50 h of lithium cycle at room temperature respectively;

[0033] Figure 10 The cycle performance chart of the LiFePO4(LFP) / Li battery assembled by the PVDFHF prepared in the present application comparative example 1, the PVDFHF+DOPO prepared in the comparative example 2 and the PVDFHF-DOPO electrolyte film prepared in the example 1 at room temperature;

[0034] Figure 11 The cycle performance chart of the LiNi 0.9 Co 0.05 Mn 0.05 O2 (NCM9055) / Li battery assembled by the PVDFHF prepared in the present application comparative example 1, the PVDFHF+DOPO prepared in the comparative example 2 and the PVDFHF-DOPO electrolyte film prepared in the example 1 at room temperature;

[0035] Figure 12 The cycle performance (a), voltage change curve after needling (b) and the working effect display chart of the LED screen after natural state (c), folding (d), cutting (e) and needling (f) of the LFP / Graphite (Gr) soft package battery assembled by the PVDFHF-DOPO electrolyte film prepared in the present application example 1 at room temperature. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0037] Example 1

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

[0039] Step 1, prepare 0.5 mL of a methanol (>99.7%, National Pharmaceutical Chemical Reagent Co., Ltd.) solution containing 2 wt% tetrabutylammonium hydroxide and blend it with 9.5 mL of N,N-dimethylformamide (>99.5%, National Pharmaceutical Chemical Reagent Co., Ltd.) for standby use.

[0040] Step 2, at 60 °C, fully stir and dissolve 1 g of PVDF (polyvinylidene fluoride, HSV 900, Shenzhen Keyi Zhida Technology Co., Ltd.), 0.1 g of DOPO (97%, National Pharmaceutical Chemical Reagent Co., Ltd.) and 0.8 g of LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, 99%, Arlan) in the mixed solution obtained in Step 1, and continue to stir and react for 12 h.

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

[0042] Step 4, uniformly coat the PVDFHF-DOPO polymer slurry obtained in Step 3 on a glass flat plate, and after baking at 80 °C for 24 h, a DOPO grafted PVDFHF-DOPO polymer solid-state electrolyte film is obtained.

[0043] Comparative Example 1

[0044] A method for preparing a dehydrofluorinated modified polyvinylidene fluoride polymer solid-state electrolyte film (PVDFHF) comprising the following steps:

[0045] Step 1, prepare 0.5 mL of a methanol (>99.7%, National Pharmaceutical Chemical Reagent Co., Ltd.) solution containing 2 wt% tetrabutylammonium hydroxide and blend it with 9.5 mL of N,N-dimethylformamide (>99.5%, National Pharmaceutical Chemical Reagent Co., Ltd.) for standby use.

[0046] Step 2, at 60 °C, fully stir and dissolve 1 g of PVDF (polyvinylidene fluoride, HSV 900, Shenzhen Keyi Zhida Technology Co., Ltd.) and 0.8 g of LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, 99%, Arlan) in the mixed solution obtained in Step 1 to obtain a PVDFHF slurry solution.

[0047] Step 3, PVDFHF polymer solid-state electrolyte membrane was obtained by uniformly coating the PVDFHF slurry obtained in step 2 on a glass flat plate and baking at 80 °C for 24 h.

[0048] Comparative Example 2

[0049] A method for preparing a DOPO simple blended dehydrofluorinated modified polyvinylidene fluoride polymer solid-state electrolyte membrane (PVDFHF+DOPO) includes the following steps:

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

[0051] Step 2, at 60 °C, fully stir and dissolve 1 g of PVDF (polyvinylidene fluoride, HSV 900, Shenzhen Kexin Zhida Technology Co., Ltd.), 0.1 g of DOPO (97%, National Pharmaceutical Chemical Reagent Co., Ltd.), and 0.8 g of LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, 99%, Aladdin) in the mixed solution obtained in step 1 to obtain a PVDFHF+DOPO slurry solution.

[0052] Step 3, uniformly coat the PVDFHF+DOPO polymer slurry obtained in step 2 on a glass flat plate, and bake at 80 °C for 24 h to obtain a DOPO blended PVDFHF+DOPO polymer solid-state electrolyte membrane.

[0053] Example 2

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

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

[0056] Step 2, at 60 °C, fully stir and dissolve 1 g of PVDF (polyvinylidene fluoride, HSV 900, Shenzhen Kexin Zhida Technology Co., Ltd.), 0.1 g of DOPO (97%, National Pharmaceutical Chemical Reagent Co., Ltd.), and 0.8 g of LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, 99%, Aladdin) in the mixed solution obtained in step 1, and continue to stir for 8 h.

[0057] Step 3, remove oxygen in the slurry obtained in step 2 by bubbling nitrogen gas, then add 0.01 g AIBN (azobisisobutyronitrile, 98%, Aladdin) into it, slowly stir the reaction at 80 °C for 24 h in an inert gas closed environment to obtain the PVDFHF-DOPO slurry solution.

[0058] Step 4, uniformly coat the PVDFHF-DOPO polymer slurry obtained in step 3 on a glass flat plate, and obtain the DOPO grafted PVDFHF-DOPO polymer solid electrolyte membrane after baking at 80 °C for 24 h.

[0059] Example 3

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

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

[0062] Step 2, dissolve 1 g of PVDF (polyvinylidene fluoride, HSV 900, Shenzhen Keyi Zhida Technology Co., Ltd.), 0.05 g of DOPO (97%, National Pharmaceutical Chemical Reagent Co., Ltd.) and 0.8 g of LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, 99%, Aladdin) in the mixed solution obtained in step 1 at 60 °C and continue to stir the reaction for 12 h.

[0063] Step 3, remove oxygen in the slurry obtained in step 2 by bubbling nitrogen gas, then add 0.01 g AIBN (azobisisobutyronitrile, 98%, Aladdin) into it, slowly stir the reaction at 80 °C for 24 h in an inert gas closed environment to obtain the PVDFHF-DOPO slurry solution.

[0064] Step 4, uniformly coat the PVDFHF-DOPO polymer slurry obtained in step 3 on a glass flat plate, and obtain the DOPO grafted PVDFHF-DOPO polymer solid electrolyte membrane after baking at 80 °C for 24 h.

[0065] Example 4

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

[0067] Step 1, prepare 0.5 mL of 2 wt% tetrabutylammonium hydroxide in methanol (>99.7%, National Pharmaceutical Chemical Reagent Co., Ltd.) solution and blend it with 9.5 mL of N,N-dimethylformamide (>99.5%, National Pharmaceutical Chemical Reagent Co., Ltd.) for later use.

[0068] Step 2, dissolve 1 g of PVDF (polyvinylidene fluoride, HSV 900, Shenzhen Keyi Zhida Technology Co., Ltd.), 0.15 g of DOPO (97%, National Pharmaceutical Chemical Reagent Co., Ltd.) and 0.8 g of LiTFSI (lithium bis(trifluoromethanesulfonyl)imide, 99%, Arlin) in the mixed solution obtained in Step 1 at 60 °C with sufficient stirring, and continue stirring for 12 h.

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

[0070] Step 4, uniformly coat the PVDFHF-DOPO polymer slurry obtained in Step 3 on a glass flat plate, and obtain a DOPO grafted PVDFHF-DOPO polymer solid electrolyte membrane after baking at 80 °C for 24 h.

[0071] The solid electrolyte membranes prepared in Example 1 and Comparative Examples 1-2 were tested for the following properties:

[0072] (1) The size of the polymer solid electrolyte membrane in the example and each comparative example was selected as 1 cm × 5 cm, and the combustion situation after being close to the flame of a lighted fire was observed.

[0073] Figure 2 The lighted fire resistance test pictures of the polymer solid electrolyte membranes of Comparative Example 1 PVDFHF, Comparative Example 2 PVDFHF+DOPO and Example 1 PVDFHF-DOPO, wherein the length of the ruler is 1 cm. As can be seen from the figure, the PVDFHF electrolyte is easily ignited under the lighted fire, and burns out completely within 5 s (a). While the PVDFHF+DOPO blended with DOPO and the PVDFHF-DOPO electrolyte membrane chemically anchored with DOPO both self-extinguish within 2 s after encountering the lighted fire, showing excellent flame retardancy (b, c). Figure 2 a). While the PVDFHF+DOPO blended with DOPO and the PVDFHF-DOPO electrolyte membrane chemically anchored with DOPO both self-extinguish within 2 s after encountering the lighted fire, showing excellent flame retardancy (b, c). Figure 2 b,c).

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

[0075] Figure 3Figure 3 Scanning electron microscope pictures of the surface (top) and cross-section (bottom) of the polymer solid-state electrolyte films 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 anchoring of DOPO grafting on the polymer chain, the spherulite structure of the electrolyte film is gradually refined Figure 3 b, c). In particular, the PVDFH-DOPO polymer solid-state electrolyte exhibits a continuous and integrated melt-like spherulite network structure, indicating that DOPO grafting can significantly reduce the crystallinity of polyvinylidene fluoride polymer and change the polarity of the polymer, effectively improving the uniformity and continuity of the polymer network, and realizing a fast and uniform lithium ion transmission process.

[0076] (3) Using steel sheets as positive and negative electrodes, respectively, using the polymer solid-state electrolyte films prepared in the examples and comparative examples, symmetric batteries were assembled in a glove box, and the batteries were tested by electrochemical impedance spectroscopy at different temperatures, and the ionic conductivity of the electrolyte film was calculated.

[0077] Figure 4 The ionic conductivity of the polymer solid-state electrolyte of Comparative Example 1 PVDFHF, Comparative Example 2 PVDFHF+DOPO, and Example 1 PVDFHF-DOPO at different temperatures. DOPO grafting can significantly reduce the crystallinity of polyvinylidene fluoride and change the polarity of the polymer, refine the size of polyvinylidene fluoride spherulites in the electrolyte and improve the uniformity and continuity of the polymer network, and realize a fast and uniform lithium ion transmission process; in addition, the O=P-O functional groups rich in DOPO molecules can significantly improve the reversible complexing ability of polyvinylidene fluoride polymer chains to lithium ions, effectively promoting the rapid conduction of lithium ions along the polymer chains. Therefore, the PVDFHF-DOPO electrolyte of Example 1 exhibits a significantly improved room temperature ionic conductivity compared to the comparative samples, with a value of 1.72 x 10 –4 S / cm.

[0078] (4) The polymer solid-state electrolyte films prepared in the examples and comparative examples were used as electrolytes, and lithium metal sheets were used as positive and negative electrodes to assemble lithium symmetric batteries in an argon glove box. The lithium ion transference number of the electrolyte film was determined by comprehensive testing of direct current polarization and alternating current impedance of the battery at room temperature, and the electrochemical stability of the electrolyte to the lithium metal negative electrode was evaluated by constant current charge and discharge test of the battery.

[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 lithium negative electrode surface scanning electron microscope pictures of the comparative example 1 PVDFHF, the comparative example 2 PVDFHF+DOPO and the example 1 PVDFHF-DOPO polymer solid-state electrolyte after 38 h, 50 h and 50 h of lithium cycling at room temperature, respectively. It can be seen that a large number of lithium dendrites are generated on the lithium negative electrode surface corresponding to the comparative example 1 PVDFHF and the comparative example 2 PVDFHF+DOPO electrolyte, indicating that the electrolyte / lithium negative electrode interface of the assembled lithium symmetric battery has a serious side reaction after 38 h and 50 h of cycling Figure 8 a,b). In sharp contrast, the lithium negative electrode surface of the example 1 PVDFHF-DOPO electrolyte is still smooth after 50 h of lithium cycling under the same conditions, indicating that a dense solid electrolyte interface film (SEI) is formed on the lithium negative electrode surface, which again confirms the significant improvement in the interface stability of the polymer electrolyte after DOPO grafting on the lithium metal negative electrode Figure 8 c).

[0085] Figure 9 The XPS spectra of the lithium negative electrode surface product of the comparative example 1 PVDFHF, the comparative example 2 PVDFHF+DOPO and the example 1 PVDFHF-DOPO polymer solid-state electrolyte after 38 h, 50 h and 50 h of lithium cycling at room temperature, respectively. From the C1s spectrum, it can be seen that the content of C=O and C–O on the lithium negative electrode surface corresponding to the example 1 PVDFHF-DOPO electrolyte is significantly reduced, indicating that the decomposition of DMF on the lithium negative electrode surface is effectively inhibited. The F 1s and N 1s spectra indicate that the content of LiF and Li3N on the lithium negative electrode surface corresponding to the example 1 PVDFHF-DOPO electrolyte is significantly increased, confirming that an SEI film dominated by inorganic components is formed on the lithium negative electrode surface, which can promote the uniform deposition of lithium ions on the negative electrode surface and effectively inhibit the growth of lithium dendrites. In addition, the P 2s characteristic peak intensity of the example 1 PVDFHF-DOPO electrolyte is significantly lower than that of the example 2 PVDFHF+DOPO electrolyte, indicating that the chemical anchoring of DOPO on the polyvinylidene fluoride polymer chain effectively inhibits the free migration of DOPO in the electrolyte, thereby reducing the side reaction of DOPO molecules with the lithium negative electrode and helping to maintain the interface stability.

[0086] (7) The polymer solid-state electrolyte membranes prepared in the examples and comparative examples were used as electrolytes, LFP was used as the positive electrode, and a metal lithium sheet was used as the negative electrode, a coin full battery was assembled in an argon glove box, and the battery was tested by constant current charge and discharge at room temperature and a current density of 0.5 C.

[0087] Figure 10The cycle performance of LFP / Li batteries assembled with the polymer solid-state electrolytes of Comparative Example 1 PVDFHF, Comparative Example 2 PVDFHF+DOPO, and Example 1 PVDFHF-DOPO at room temperature 0.5 C. Based on the excellent lithium ion conduction ability and the significantly improved electrolyte / electrode interface stability, the battery assembled with the Example 1 PVDFHF-DOPO electrolyte has a capacity retention rate as high as 94.27% after 850 cycles, while the batteries assembled with the Comparative Example 1 PVDFHF and Comparative Example 2 PVDFHF+DOPO electrolytes suddenly short-circuit after 167 and 420 cycles, respectively, corresponding to a capacity retention rate of only 62.55% and 76.37%.

[0088] (8) The polymer solid-state electrolyte films prepared in the examples and comparative examples were respectively used as electrolytes, NCM9055 was used as the positive electrode, and a lithium metal sheet was used as the negative electrode to assemble a coin-type full battery in an argon glove box. Constant current charge-discharge tests were performed on the battery at room temperature and a current density of 0.2 C.

[0089] Figure 11 The cycle performance of high-voltage NCM9055 / Li batteries assembled with the polymer solid-state electrolytes of Comparative Example 1 PVDFHF, Comparative Example 2 PVDFHF+DOPO, and Example 1 PVDFHF-DOPO at room temperature 0.2 C. Also based on the excellent lithium ion conduction ability and the significantly improved electrolyte / electrode interface stability, the battery assembled with the Example 1 PVDFHF-DOPO electrolyte has a capacity retention rate as high as 77.61% after 150 cycles in the high-voltage interval of 2.8-4.3 V, while the Comparative Example 1 PVDFHF electrolyte cannot match the normal cycling of the NCM9055 high-voltage positive electrode, and the battery assembled with the Comparative Example 2 PVDFHF+DOPO electrolyte also suddenly short-circuits after only 60 cycles.

[0090] (9) The LFP positive electrode sheet and the Gr negative electrode sheet were welded with tabs. The positive electrode sheet, the composite solid-state electrolyte film prepared in the example, and the negative electrode sheet were sequentially stacked in order, and the three sides were sealed after being wrapped with an aluminum plastic film. The target soft-pack battery was obtained after the above-mentioned pre-prepared soft-pack battery was vacuum heat-sealed in the glove box. Constant current charge-discharge tests were 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 needling was monitored in real time to verify its safety. In addition, after the soft-pack battery was charged to a full state, it was connected with an LED display panel to investigate the working performance of the soft-pack battery under conditions such as flat laying, folding, cutting, and needling.

[0091] Figure 12 The cycle performance of LFP / Gr soft-pack batteries assembled with the polymer solid-state electrolyte of Example 1 PVDFHF-DOPO at room temperature 0.5 C.Figure 12 a), the voltage change curve after needling (b) and the working effect of the LED screen after needling (c). Figure 12 b) and the working effect of the LED screen after needling (c). Figure 12 c), folding (d), cutting (e) and after needling (f). Figure 12 c), folding (d), cutting (e) and after needling (f). Figure 12 c), folding (d), cutting (e) and after needling (f). Figure 12 Figure 12 f). Even if applied to solid soft pack lithium ion batteries, the PVDFHF-DOPO electrolyte of Example 1 can still achieve long cycle stability of the battery, and the capacity retention rate still reaches 77.91% after 200 cycles at a current density of 0.5 C, showing a reliable practical application prospect. In addition, the assembled soft pack battery does not have abnormal conditions such as short circuit, smoking or fire after needling under full charge state, and only has a small voltage drop, and the voltage tends to be stable within 60 min thereafter; at the same time, the soft pack battery can normally light the LED lamp panel at room temperature, and can still work normally under extreme conditions such as folding, cutting and needling, indicating that the PVDFHF-DOPO electrolyte film prepared in the embodiment has excellent working safety.

[0092] The above merely describes exemplary embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

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 hydrophosphorus addition. 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. The catalyst is at least one of tetramethylammonium hydroxide, tetraethylammonium hydroxide, and tetrabutylammonium hydroxide.

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 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.

4. 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.

5. The method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte according to claim 4, characterized in that: The initiator is at least one of azobisisobutyronitrile, azobisisoheptanenitrile, dimethyl azobisisobutyrate, azobiscyclohexylformitrile, di-tert-butyl peroxide, and persulfate.

6. The method for preparing a DOPO-grafted polyvinylidene fluoride polymer solid electrolyte according to claim 4, 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.

7. 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.

8. 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.

9. A DOPO-grafted polyvinylidene fluoride polymer solid electrolyte prepared by a method according to any one of claims 1-8.

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