A solid polymer electrolyte, its preparation method and application

Abstract

This invention discloses a solid polymer electrolyte, its preparation method, and its applications. The preparation method of this invention includes two main steps: the preparation of a mixed solution and the preparation of an electrolyte membrane. Based on the high polarization intensity of poly(vinylidene fluoride-trifluoroethylene), this invention utilizes its ability to promote the dissociation of lithium salts and improve the ionic conductivity of solid polymer electrolytes to prepare a high-performance solid polymer electrolyte. This solid polymer electrolyte exhibits high room temperature ionic conductivity and lithium-ion transference number. Furthermore, lithium metal batteries based on the solid polymer electrolyte prepared by this invention have high discharge specific capacity and good cycle stability, showing broad development prospects.

Description

Technical Field

[0001] This invention belongs to the field of lithium metal battery technology, specifically relating to a solid polymer electrolyte, its preparation method, and its application. Background Technology

[0002] In existing lithium-ion batteries, the medium for transporting lithium ions is the electrolyte. The solvent in the electrolyte is highly polar, promoting efficient dissociation of lithium salts, thus giving the electrolyte extremely high room-temperature ionic conductivity. However, electrolytes are flammable and prone to leakage, posing safety hazards such as combustion and even explosion to lithium-ion batteries. To improve safety and stability, an effective method is to replace the flammable and leaky electrolyte with a non-flammable solid electrolyte. Among various solid electrolytes, solid polymer electrolytes have good ductility and flexibility, and are easy to process, thus attracting increasing attention from researchers. However, due to the low polarity of the polymer matrix itself, it cannot promote significant dissociation of lithium salts, resulting in extremely low room-temperature ionic conductivity for solid polymer electrolytes.

[0003] Inspired by the strong polarity of electrolytes, increasing the polarity of the polymer matrix is ​​an effective method to enhance the dissociation ability of lithium salts. Compared to other polymers, polyvinylidene fluoride (PVDF) has relatively high polarity. Its polarity is closely related to its molecular chain conformation. Typical melt and solution processing easily leads to an alternating cis-trans (TGTG) conformation of the molecular chains, corresponding to the α phase. Melt crystallization under high temperature or shear field, or solution crystallization in dimethylformamide at low temperature, easily forms the TTTG conformation, corresponding to the γ phase. Melt crystallization under high pressure, high electric field polarization treatment, or stretching treatment easily forms the TTTT conformation of PVDF, corresponding to the β phase. In the TGTG conformation, adjacent molecular chain dipoles cancel each other out, resulting in zero macroscopic polarization; in the TTTG conformation, molecular chain dipoles partially cancel each other out, exhibiting weak polarity; in the TTTT conformation, all dipoles are aligned in the same direction, exhibiting strong polarity. Therefore, to enhance the polarization intensity of polyvinylidene fluoride and improve its ability to dissociate lithium salts, it is necessary to induce as much TTTT conformation as possible.

[0004] Unlike polyvinylidene fluoride (PVDF), in poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], only the molar content of trifluoroethylene monomer needs to be controlled at 20-50% to ensure the material consistently exhibits a strongly polar TTTT conformation, unaffected by external processing conditions. Therefore, PVDF-trifluoroethylene is an ideal solid-state polymer electrolyte matrix. According to research, no preparation methods for PVDF-trifluoroethylene solid-state polymer electrolytes have been reported, nor have any solid-state lithium metal batteries based on this electrolyte been reported. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a solid polymer electrolyte, its preparation method, and its application. In the preparation process, poly(vinylidene fluoride-trifluoroethylene) is innovatively used as a raw material to obtain a solid electrolyte with high ionic conductivity and lithium-ion transference number. Moreover, the lithium metal battery prepared based on the solid electrolyte has good battery cycle performance and stability.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for preparing a solid polymer electrolyte includes the following steps:

[0008] (1) Preparation of mixed solution: Poly(vinylidene fluoride-trifluoroethylene) copolymer and lithium salt are added to solvent, mixed and stirred evenly to obtain mixed solution;

[0009] (2) Preparation of electrolyte membrane: The mixed solution obtained in step (1) is placed in a glass petri dish and evaporated at high temperature to obtain the solid polymer electrolyte.

[0010] Preferably, in step (1), the weight-average relative molecular mass of the poly(vinylidene fluoride-trifluoroethylene) copolymer is 450,000 g / mol, and the molar content of trifluoroethylene in the poly(vinylidene fluoride-trifluoroethylene) copolymer is 20-45%.

[0011] Preferably, in step (1), the lithium salt is one of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI).

[0012] More preferably, the purity of the lithium salt is greater than 99.9%.

[0013] Preferably, in step (1), the mass ratio of poly(vinylidene fluoride-trifluoroethylene) to lithium salt is 2:1 to 4.

[0014] Preferably, in step (1), the solvent is one of N,N-dimethylformamide, N-methylpyrrolidone, and tetrahydrofuran.

[0015] Preferably, in step (2), the evaporation process includes blower drying and vacuum drying in sequence, wherein the temperature and time of blower drying are 45-65℃ and 18-22h, respectively, and the temperature and time of vacuum drying are 45-65℃ and 1-3h, respectively.

[0016] Meanwhile, the present invention claims protection for solid polymer electrolytes prepared by any of the methods described above.

[0017] Meanwhile, this invention claims protection for the application of the prepared solid polymer electrolyte in lithium metal batteries.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] 1. The poly(vinylidene fluoride-trifluoroethylene) selected in this invention has high polarization intensity, which can promote the dissociation of lithium salt and improve the ionic conductivity of solid polymer electrolyte. At the same time, lithium metal batteries containing this electrolyte also have high discharge specific capacity and good cycle stability.

[0020] 2. The poly(vinylidene fluoride-trifluoroethylene) used in this invention has a weight-average relative molecular mass of 450,000 g / mol, wherein the molar content of trifluoroethylene is 20-45%. When the molar content of trifluoroethylene is 20-45%, poly(vinylidene fluoride-trifluoroethylene) always exhibits a highly polar TTTT conformation, which can promote the dissociation of lithium salt to generate more free lithium ions, thereby promoting the improvement of lithium ion conductivity and lithium ion transference number. In addition, the cycle performance of lithium metal batteries prepared based on this solid polymer electrolyte is also greatly improved.

[0021] 3. The mass ratio of poly(vinylidene fluoride-trifluoroethylene) to lithium salt selected in this invention is 2:1 to 4, which ensures a high ionic conductivity. Although poly(vinylidene fluoride-trifluoroethylene) has strong polarity and can promote the dissociation of salt, the appropriate proportion of lithium salt can also greatly affect the ionic conductivity. This is because when the salt content is low, the number of free lithium ions that can be dissociated is limited; when the salt content is too high, the electrolyte viscosity is too high, which hinders the transport of lithium ions. This invention rationally uses the mass ratio of poly(vinylidene fluoride-trifluoroethylene) to lithium salt to maintain the ionic conductivity at a high level.

[0022] 4. In this invention, the excess solvent in the mixed solution can be removed by blowing air drying, and vacuum drying can further remove the water (water in the air) in the mixed solution, so as to avoid the presence of water in the solid polymer electrolyte, which would affect the application of lithium-ion batteries and further enhance its stability in the application of lithium metal batteries.

[0023] 5. The present invention obtains a solid polymer electrolyte by dissolving poly(vinylidene fluoride-trifluoroethylene) and lithium salt in a solvent and then drying them. This method is simple to operate, low in cost, and suitable for large-scale production. Attached Figure Description

[0024] Figure 1 The Li / Li symmetric cells of Example 1 and Comparative Example 3 were tested at 25°C and 0.05 mA / cm². 2 Cyclic performance at current density.

[0025] Figure 2 LiNi for Example 1 and Comparative Example 3 0.8Co 0.1 Mn 0.1 Cyclic performance of O2(NCM811) / Li battery at 25℃ and 1C charge / discharge rate. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. Of course, the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0027] Although the steps in this invention are arranged by reference numerals, this is not intended to limit the order of the steps. Unless the order of the steps is explicitly stated or the execution of a step requires other steps as a basis, the relative order of the steps can be adjusted. It is understood that the term "and / or" as used herein refers to and covers any and all possible combinations of one or more of the associated listed items.

[0028] Unless otherwise specified, all chemical reagents and materials used in this invention were purchased through commercial channels.

[0029] Example 1

[0030] A method for preparing a solid poly(vinylidene fluoride-trifluoroethylene) electrolyte includes the following steps:

[0031] (1) Preparation of mixed solution: Poly(vinylidene fluoride-trifluoroethylene) with a trifluoroethylene content of 20 mol% and lithium bis(trifluoromethanesulfonylimide) were mixed at a mass ratio of 1:1 and then added to N,N-dimethylformamide solvent. The mixture was stirred at room temperature for 10 hours to form a homogeneous solution. The weight-average relative molecular mass of the poly(vinylidene fluoride-trifluoroethylene) was 450,000 g / mol.

[0032] (2) Preparation of electrolyte membrane: Pour the mixed solution from (1) into a glass petri dish, place it in a 55°C oven for 19 hours, then place it in a 55°C vacuum oven for 1 hour, and then peel the formed membrane off the glass petri dish to obtain poly(vinylidene fluoride-trifluoroethylene) / bis(trifluoromethanesulfonyl)imide lithium solid polymer electrolyte.

[0033] Example 2

[0034] A method for preparing a solid poly(vinylidene fluoride-trifluoroethylene) electrolyte includes the following steps:

[0035] (1) Preparation of mixed solution: Poly(vinylidene fluoride-trifluoroethylene) with a trifluoroethylene content of 20 mol% and lithium difluorosulfonyl imide were mixed at a mass ratio of 1:1 and then added to N,N-dimethylformamide solvent. The mixture was stirred at room temperature for 10 hours to form a homogeneous solution. The weight-average relative molecular mass of the poly(vinylidene fluoride-trifluoroethylene) was 450,000 g / mol.

[0036] (2) Preparation of electrolyte membrane: Pour the mixed solution from (1) into a glass petri dish, place it in a forced-air oven at 55°C for 19h, then place it in a vacuum oven at 55°C for 1h, and then peel the formed membrane off the glass petri dish to obtain the poly(vinylidene fluoride-trifluoroethylene) / bis(fluorosulfonylimide) lithium solid polymer electrolyte.

[0037] The difference between Example 2 and Example 1 is that the lithium salt used in this example is lithium bis(fluorosulfonyl)imide.

[0038] Example 3

[0039] A method for preparing a solid poly(vinylidene fluoride-trifluoroethylene) electrolyte includes the following steps:

[0040] (1) Preparation of mixed solution: Poly(vinylidene fluoride-trifluoroethylene) with a trifluoroethylene content of 25 mol% and lithium bis(trifluoromethanesulfonyl imide) were mixed at a mass ratio of 1:1 and then added to N,N-dimethylformamide solvent. The mixture was stirred at room temperature for 10 hours to form a homogeneous solution. The weight-average relative molecular mass of the poly(vinylidene fluoride-trifluoroethylene) was 450,000 g / mol.

[0041] (2) Preparation of electrolyte membrane: Pour the mixed solution from (1) into a glass petri dish, place it in a 55°C oven for 19 hours, then place it in a 55°C vacuum oven for 1 hour, and then peel the formed membrane off the glass petri dish to obtain poly(vinylidene fluoride-trifluoroethylene) / bis(trifluoromethanesulfonyl)imide lithium solid polymer electrolyte.

[0042] The difference between Example 3 and Example 1 is that the poly(vinylidene fluoride-trifluoroethylene) used in this example contains 25 mol% trifluoroethylene.

[0043] Example 4

[0044] A method for preparing a solid poly(vinylidene fluoride-trifluoroethylene) electrolyte includes the following steps:

[0045] (1) Preparation of mixed solution: Poly(vinylidene fluoride-trifluoroethylene) with a trifluoroethylene content of 30 mol% and lithium bis(trifluoromethanesulfonylimide) were mixed at a mass ratio of 1:1 and then added to N,N-dimethylformamide solvent. The mixture was stirred at room temperature for 10 hours to form a homogeneous solution. The weight-average relative molecular mass of the poly(vinylidene fluoride-trifluoroethylene) was 450,000 g / mol.

[0046] (2) Preparation of electrolyte membrane: Pour the mixed solution from (1) into a glass petri dish, place it in a 55°C oven for 19 hours, then place it in a 55°C vacuum oven for 1 hour, and then peel the formed membrane off the glass petri dish to obtain poly(vinylidene fluoride-trifluoroethylene) / bis(trifluoromethanesulfonyl)imide lithium solid polymer electrolyte.

[0047] The difference between Example 4 and Example 1 is that the poly(vinylidene fluoride-trifluoroethylene) used in this example contains 30 mol% trifluoroethylene.

[0048] Example 5

[0049] A method for preparing a solid poly(vinylidene fluoride-trifluoroethylene) electrolyte includes the following steps:

[0050] (1) Preparation of mixed solution: Poly(vinylidene fluoride-trifluoroethylene) with a trifluoroethylene content of 45 mol% and lithium bis(trifluoromethanesulfonylimide) were mixed at a mass ratio of 1:1 and then added to N,N-dimethylformamide solvent. The mixture was stirred at room temperature for 10 hours to form a homogeneous solution. The weight-average relative molecular mass of the poly(vinylidene fluoride-trifluoroethylene) was 450,000 g / mol.

[0051] (2) Preparation of electrolyte membrane: Pour the mixed solution from (1) into a glass petri dish, place it in a 55°C oven for 19 hours, and then place it in a 55°C vacuum oven for 2 hours. After that, peel the formed membrane off the glass petri dish to obtain the poly(vinylidene fluoride-trifluoroethylene) / bis(trifluoromethanesulfonyl)imide lithium solid polymer electrolyte.

[0052] The difference between Example 5 and Example 1 is that the poly(vinylidene fluoride-trifluoroethylene) used in this example has a trifluoroethylene content of 45 mol% and a vacuum drying time of 2 h.

[0053] Comparative Example 1

[0054] A method for preparing a solid poly(vinylidene fluoride-trifluoroethylene) electrolyte includes the following steps:

[0055] (1) Preparation of mixed solution: Poly(vinylidene fluoride-trifluoroethylene) with a trifluoroethylene content of 20 mol% and lithium bis(trifluoromethanesulfonylimide) were mixed at a mass ratio of 3:1 and then added to N,N-dimethylformamide solvent. The mixture was stirred at room temperature for 10 hours to form a homogeneous solution. The weight-average relative molecular mass of the poly(vinylidene fluoride-trifluoroethylene) was 450,000 g / mol.

[0056] (2) Preparation of electrolyte membrane: Pour the mixed solution from (1) into a glass petri dish, place it in a forced-air oven at 55°C for 19 hours, and then place it in a vacuum oven at 55°C for 1 hour. After that, peel the formed membrane off the glass petri dish to obtain the poly(vinylidene fluoride-trifluoroethylene) / bis(trifluoromethanesulfonylimide) lithium solid polymer electrolyte.

[0057] The difference between Comparative Example 1 and Example 1 is that the mass ratio of poly(vinylidene fluoride-trifluoroethylene) and lithium bis(trifluoromethanesulfonylimide) used in this comparative example is 3:1.

[0058] Comparative Example 2

[0059] A method for preparing a solid poly(vinylidene fluoride-trifluoroethylene) electrolyte includes the following steps:

[0060] (1) Preparation of mixed solution: Poly(vinylidene fluoride-trifluoroethylene) with a trifluoroethylene content of 20 mol% and lithium bis(trifluoromethanesulfonylimide) were mixed at a mass ratio of 1:3 and then added to N,N-dimethylformamide solvent. The mixture was stirred at room temperature for 10 hours to form a homogeneous solution. The weight-average relative molecular mass of the poly(vinylidene fluoride-trifluoroethylene) was 450,000 g / mol.

[0061] (2) Preparation of electrolyte membrane: Pour the mixed solution from (1) into a glass petri dish, place it in a forced-air oven at 55°C for 19 hours, and then place it in a vacuum oven at 55°C for 1 hour. After that, peel the formed membrane off the glass petri dish to obtain the poly(vinylidene fluoride-trifluoroethylene) / bis(trifluoromethanesulfonylimide) lithium solid polymer electrolyte.

[0062] The difference between Comparative Example 2 and Example 1 is that the mass ratio of poly(vinylidene fluoride-trifluoroethylene) and lithium bis(trifluoromethanesulfonylimide) used in this comparative example is 1:3.

[0063] Comparative Example 3

[0064] A method for preparing a solid polyvinylidene fluoride electrolyte includes the following steps:

[0065] (1) Preparation of mixed solution: Polyvinylidene fluoride and lithium bis(trifluoromethanesulfonyl)imide were mixed at a mass ratio of 1:1 and then added to N,N-dimethylformamide solvent. The mixture was stirred at room temperature for 10 hours to form a homogeneous solution.

[0066] (2) Preparation of electrolyte membrane: Pour the mixed solution from (1) into a glass petri dish, place it in a forced-air oven at 55°C for 19 hours, and then place it in a vacuum oven at 55°C for 3 hours. After that, peel the formed membrane off the glass petri dish to obtain the polyvinylidene fluoride / bis(trifluoromethanesulfonyl)imide lithium solid polymer electrolyte.

[0067] The difference between Comparative Example 3 and Example 1 is that the polymer matrix used in this comparative example is polyvinylidene fluoride, and the vacuum drying time is 3 hours.

[0068] Comparative Example 4

[0069] A method for preparing a solid polyvinylidene fluoride electrolyte includes the following steps:

[0070] (1) Preparation of mixed solution: Polyvinylidene fluoride and lithium difluorosulfonyl imide were mixed at a mass ratio of 1:1 and then added to N,N-dimethylformamide solvent. The mixture was stirred at room temperature for 10 hours to form a homogeneous solution.

[0071] (2) Preparation of electrolyte membrane: Pour the mixed solution from (1) into a glass petri dish, place it in a forced-air oven at 55°C for 19 hours, and then place it in a vacuum oven at 55°C for 3 hours. After that, peel the formed membrane off the glass petri dish to obtain the polyvinylidene fluoride / bis(fluorosulfonyl)imide lithium solid polymer electrolyte.

[0072] The difference between Comparative Example 4 and Example 1 is that the polymer matrix used in this comparative example is polyvinylidene fluoride, the lithium salt used is lithium bis(fluorosulfonyl)imide, and the vacuum drying time is 3 hours.

[0073] The products prepared in Examples 1-5 and Comparative Examples 1-4 of this application were tested for ionic conductivity, activation energy, lithium-ion transference number, and battery cycle performance. The specific test methods are as follows:

[0074] (1) Ionic conductivity test: The solid polymer electrolyte is assembled into a steel sheet-electrolyte-steel sheet (SS / SPEs / SS) battery in a glove box filled with argon gas. The AC impedance is tested using an electrochemical workstation, and the ionic conductivity is calculated using the following formula 1.

[0075]

[0076] Where L is the thickness of the electrolyte membrane, R is the EIS impedance of the SS / SPEs / SS battery, and S is the area of ​​the steel sheet.

[0077] (2) Activation energy calculation: After obtaining the ionic conductivity at different temperatures, the data is plotted and fitted, and the activation energy of lithium ion migration is calculated by combining it with Formula 2.

[0078]

[0079] Where σ0 is the pre-exponential factor, E a It is the activation energy.

[0080] (3) Ion transport number calculation: After assembling the electrolyte into a lithium metal-electrolyte-lithium metal (Li / SPEs / Li) battery, EIS testing was performed at room temperature with a frequency of 7MHz to 1Hz and an amplitude of 10mV. The impedance R0 was recorded at this time. Then, a polarization voltage of 10mV was continuously applied, and the initial response current I0 was recorded. When the steady-state current (I0) was reached... s After that, stop applying voltage and perform the EIS test again, recording the impedance R at this point. s The calculation formula is shown in Figure 3:

[0081]

[0082] Where I0 is the initial response current, R0 is the initial impedance, and I s For steady-state current, R s The impedance at which steady-state current is achieved.

[0083] (4) Battery cycle performance test: The above electrolytes were assembled into NCM811 / lithium metal full cells and tested at 25°C and 1C rate.

[0084] The test results for the product performance in Examples 1-5 and Comparative Examples 1-4 are shown in Table 1.

[0085] Table 1

[0086]

[0087] Results analysis:

[0088] (1) Under the same preparation conditions, compared with Comparative Example 3, Examples 1-5 use highly polar poly(vinylidene fluoride-trifluoroethylene) as the matrix to prepare solid polymer electrolytes, resulting in higher ionic conductivity, lower activation energy, and better battery cycle performance. This indicates that highly polar poly(vinylidene fluoride-trifluoroethylene) can promote efficient dissociation of lithium salts, thereby significantly improving the electrolyte ionic conductivity and improving battery cycle stability.

[0089] (2) Compared with Comparative Examples 1 and 2, Example 1 exhibits higher ionic conductivity, lower activation energy, and better battery cycle performance. This indicates that the ratio of poly(vinylidene fluoride-trifluoroethylene) to lithium salt needs to be controlled at a specific level. Too low a lithium salt content will result in a lower number of lithium ions, affecting the construction of efficient ion transport channels; too high a lithium salt content may lead to ion aggregation, increasing electrolyte viscosity and also affecting efficient lithium ion transport. Therefore, a 1:1 ratio of poly(vinylidene fluoride-trifluoroethylene) to lithium salt will achieve the highest ionic conductivity.

[0090] (3) In Examples 1 and 2-5, the ionic conductivity decreased with the increase of trifluoroethylene monomer content. This is because the polarity of the polymer decreases with the increase of trifluoroethylene monomer content, and thus the ability to dissociate lithium salts decreases.

[0091] (4) To further test the ability of the prepared solid polymer electrolyte to suppress lithium dendrite growth, this application assembled the above Example 1 into a lithium-lithium symmetric battery for testing. The experimental results are as follows: Figure 1 As shown, the lithium-lithium symmetric battery containing the solid polymer electrolyte of Example 1 has a current density of 0.05 mA / cm² at room temperature. 2 At the same test conditions, the lithium-ion battery containing the solid polymer electrolyte in Comparative Example 3 could cycle stably for 1980 hours with a relatively low polarization voltage of 40mV. However, under the same test conditions, the lithium-ion battery containing the solid polymer electrolyte in Comparative Example 3 only cycled for 715 hours before short-circuiting, and its polarization voltage was much higher (55mV). This indicates that the solid poly(vinylidene fluoride-trifluoroethylene) electrolyte has a good ability to suppress lithium dendrites because its system contains more free lithium ions, providing more opportunities for the uniform deposition of lithium ions on the lithium anode.

[0092] (5) To illustrate the matching capability between the prepared solid polymer electrolyte and the high-voltage NCM811 cathode, this application assembled the above Example 1 into an NCM811-lithium full cell for long-cycle performance testing. Figure 2 As shown, the NCM811-lithium full cell containing the solid polymer electrolyte of Example 1 can stably cycle for more than 600 cycles at room temperature and 1C rate, and retains 92.1% of its capacity after 300 cycles; while the NCM811-lithium full cell containing the solid polymer electrolyte of Comparative Example 3 exhibits drastic capacity decay at room temperature and 1C rate, with a capacity retention of only 20.5% after 300 cycles. This indicates that solid poly(vinylidene fluoride-trifluoroethylene) electrolyte can be better matched with high-voltage cathodes, which is helpful for the preparation of high-energy-density lithium metal batteries.

[0093] In summary, this invention utilizes the high polarization intensity of poly(vinylidene fluoride-trifluoroethylene) to promote the dissociation of lithium salts and enhance the ionic conductivity of solid polymer electrolytes, thereby preparing a high-performance solid polymer electrolyte. This solid polymer electrolyte exhibits high room temperature ionic conductivity and lithium-ion transference number. Furthermore, lithium metal batteries constructed using the solid polymer electrolyte prepared in this invention as raw material demonstrate high discharge specific capacity and good cycle stability, showing promising development prospects.

Claims

1. A method for preparing a solid polymer electrolyte, characterized in that, Includes the following steps: (1) Preparation of mixed solution: Poly(vinylidene fluoride-trifluoroethylene) copolymer and lithium salt are added to solvent, mixed and stirred evenly to obtain mixed solution; (2) Preparation of electrolyte membrane: The mixed solution obtained in step (1) is placed in a glass petri dish and evaporated at high temperature to obtain the solid polymer electrolyte. In step (1), the weight-average relative molecular mass of the poly(vinylidene fluoride-trifluoroethylene) copolymer is 450,000 g / mol, and the molar content of trifluoroethylene in the poly(vinylidene fluoride-trifluoroethylene) copolymer is 20-45%; the solvent is one of N,N-dimethylformamide, N-methylpyrrolidone, and tetrahydrofuran.

2. The method for preparing a solid polymer electrolyte according to claim 1, characterized in that, In step (1), the lithium salt is one of lithium bis(trifluoromethanesulfonyl)imide or lithium bis(fluorosulfonyl)imide.

3. The method for preparing a solid polymer electrolyte according to claim 1, characterized in that, In step (1), the purity of the lithium salt is greater than 99.9%.

4. The method for preparing a solid polymer electrolyte according to claim 1, characterized in that, In step (1), the mass ratio of the poly(vinylidene fluoride-trifluoroethylene) copolymer to the lithium salt is 2:1~4.

5. The method for preparing a solid polymer electrolyte according to claim 1, characterized in that, In step (2), the evaporation process includes, in sequence, forced-air evaporation and vacuum evaporation, wherein the temperature and time of forced-air drying are 45~65℃. o C. Vacuum drying for 18-22 hours, with a temperature and time of 45-65°C. o C, 1~3 h.

6. A solid polymer electrolyte prepared by the method according to any one of claims 1 to 5.

7. The application of the solid polymer electrolyte of claim 6 in a lithium metal battery.

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