A fluorinated polyimide-based composite solid electrolyte, a preparation method thereof, and a lithium battery

Abstract

The invention discloses a kind of fluorinated polyimide-based composite solid electrolyte and its preparation method and lithium battery;The preparation method includes:Fluorinated polyimide is prepared using chemical imidization, cross-linking is made to occur inside fluorinated polyimide by adding a cross-linking agent, then SN and LITFSI are dissolved in N, N-dimethylformamide, in the cross-linked fluorinated polyimide solution after imidization is added, then film-forming is prepared, the obtained film is formed into a porous membrane by suction filtration in a sand core funnel using a nanofiltration membrane, then put into anhydrous ethanol and soak, then remove the solvent and perform thermal cross-linking, obtain a fluorinated polyimide-based composite solid electrolyte. The preparation method of the present invention allows FPI and SN to be prepared into a uniform electrolyte membrane after mixing, significantly improves the rate performance of lithium ion batteries, while enhancing the mechanical properties of electrolyte membrane.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a fluorine-containing polyimide-based composite solid electrolyte, a preparation method thereof, and a lithium battery. Background Art

[0002] Lithium-ion batteries, due to their high energy density, light weight, flexibility, and long life, have been widely used in new energy vehicles, energy storage power stations, and various electronic devices such as mobile phones and computers. However, spontaneous combustion and explosion of lithium batteries are now a frequent occurrence, attributed to the decomposition of the liquid electrolyte in lithium batteries at high temperatures and high voltages, which can cause explosions. Consequently, attention has turned to solid polymer electrolytes, which offer excellent safety, thermal stability, electrochemical stability, flammability, and mechanical strength. Furthermore, these electrolytes offer good compatibility with electrodes and can replace the role of separators. PEO-based electrolytes have emerged as promising candidates due to their high lithium ion conductivity, good processability, and high energy density, but they still require improvement in mechanical properties, high-temperature resistance, and safety. Polyimide materials, on the other hand, have gained popularity due to their high-temperature resistance, excellent mechanical properties, high electrochemical activity, and high theoretical capacity. Fluorinated polyimide (FPI) further improves processability, increases the dielectric constant, enhances the membrane's surface polarity, improves interfacial compatibility with electrodes, reduces polymer crystallinity, promotes Li+ dissociation, and reduces bulk resistance. However, they have very low ionic conductivity (10 at room temperature) compared to liquid electrolytes. -11 to 10 -5 S / cm), which has led to the search for a new composite electrolyte design to enhance the ionic conductivity of FPI-based SPE.

[0003] Succinonitrile (SN)-based electrolytes have great potential for practical applications in all-solid-state lithium metal batteries due to their high room-temperature ionic conductivity, wide electrochemical window, and good thermal stability. However, poor mechanical strength, low stability to lithium metal, and poor interfacial compatibility with electrodes have hindered the further application of SN-based electrolytes in all-solid-state lithium metal batteries. FPI polymer electrolytes can just make up for the disadvantages of SN electrolytes, improving mechanical properties, enhancing interfacial compatibility, and high stability to lithium metal. The FPI-SN-based electrolyte formed by the composite of the two is expected to become one of the directions for the design of a new generation of electrolytes, while ensuring electrical properties while also having mechanical and thermal properties. However, there are still problems in the preparation of FPI-SN-based electrolytes. The two cannot be blended because FPI and SN are not compatible. SN cannot enter the interior of FPI, and the mixture of the two cannot be prepared into a uniform electrolyte membrane. In addition, SN also acts as a plasticizer, which will reduce the mechanical properties of the polymer, making it impossible to achieve the ideal mechanical properties. Summary of the Invention

[0004] In order to overcome the above-mentioned shortcomings and deficiencies of the prior art, the purpose of the present invention is to provide a method for preparing a fluorinated polyimide-based composite solid electrolyte, so that FPI and SN can be mixed to form a uniform electrolyte membrane, which significantly improves the rate performance of lithium-ion batteries and enhances the mechanical properties of the electrolyte membrane.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] The present invention provides a method for preparing a fluorinated polyimide-based composite solid electrolyte, comprising the following steps:

[0007] (1) synthesizing a fluorinated polyamic acid precursor solution by reacting a fluorinated aromatic diamine and an excess of aromatic dianhydride in an organic solvent; the molar ratio of the fluorinated aromatic diamine to the aromatic dianhydride is 1:(1.03-1.1);

[0008] (2) adding a silane coupling agent to the fluorinated polyamic acid precursor solution prepared in step (1) to obtain a chemically cross-linked fluorinated polyamic acid solution;

[0009] (3) adding acetic anhydride and pyridine to the chemically cross-linked fluorinated polyamic acid solution prepared in step (2) to perform chemical imidization to obtain a cross-linked fluorinated polyimide solution; adding succinonitrile and lithium bis(trifluoromethanesulfonyl)imide to the cross-linked fluorinated polyimide solution to obtain a composite cross-linked fluorinated polyimide solution;

[0010] (4) forming a film from the composite cross-linked fluorinated polyimide solution obtained in step (3) to obtain a composite cross-linked fluorinated polyimide electrolyte membrane; the mass percentage of the organic solvent in the composite cross-linked fluorinated polyimide electrolyte membrane is 84 to 85%;

[0011] (5) placing a nanofiltration membrane on a sand core filter plate of a sand core funnel, placing the composite cross-linked fluorinated polyimide electrolyte membrane prepared in step (4) on the nanofiltration membrane, performing suction filtration to extract the organic solvent in the composite cross-linked fluorinated polyimide electrolyte membrane, and then soaking it in anhydrous ethanol to obtain a porous composite cross-linked fluorinated polyimide electrolyte membrane;

[0012] (6) removing the solvent from the porous composite cross-linked fluorinated polyimide electrolyte membrane obtained in step (5) and subjecting it to thermal cross-linking under vacuum conditions to obtain a fluorinated polyimide-based composite solid electrolyte.

[0013] Preferably, the fluorine-containing aromatic diamine in step (1) has one of the following structures:

[0014]

[0015]

[0016] The fluorine-containing aromatic dianhydride has one of the following structural formulas:

[0017]

[0018] Preferably, the organic solvent in step (1) is N,N-dimethylformamide.

[0019] Preferably, the silane coupling agent in step (2) is one of bis(3-trimethoxysilylpropyl)amine, N-[3-(trimethoxysilyl)propyl]ethylenediamine and (3-aminopropyl)triethoxysilane.

[0020] Preferably, in step (3), the molar ratio of succinonitrile: lithium bis(trifluoromethanesulfonyl)imide: fluorinated polyimide is 1:(0.5-1):(1.5-2).

[0021] Preferably, the pore size of the nanofiltration membrane in step (5) is 2 to 10 nm.

[0022] Preferably, the soaking in anhydrous ethanol in step (5) is specifically: soaking in anhydrous ethanol for 24 to 72 hours.

[0023] Preferably, in step (6), the porous composite cross-linked fluorinated polyimide electrolyte membrane obtained in step (5) is subjected to thermal cross-linking under vacuum conditions after removing the solvent, specifically:

[0024] First, the porous composite cross-linked fluorinated polyimide electrolyte membrane obtained in step (5) is placed in a blast oven to remove the solvent, and then placed in a vacuum oven, and the temperature is raised from room temperature to 50-60°C and kept warm for 1-4 hours. After the temperature is raised to 70-80°C and kept warm for 4-8 hours, then raised to 90-100°C and kept warm for 4-8 hours, and finally raised to 110-120°C and kept warm for 1-4 hours. After thermal cross-linking, a fluorinated polyimide-based composite solid electrolyte is obtained.

[0025] The present invention also provides a fluorine-containing polyimide-based composite solid electrolyte, which is prepared by the preparation method of the fluorine-containing polyimide-based composite solid electrolyte.

[0026] The present invention also provides a lithium ion battery comprising the fluorine-containing polyimide-based composite solid electrolyte.

[0027] Preferably, the amount of bis(3-trimethoxysilylpropyl)amine (BTMSPA) added in step (2) is 5-10% of the mass of the fluorinated polyamic acid precursor.

[0028] Preferably, the film preparation in step (4) is specifically: coating with a film applicator (scraper), film casting or spraying with an electrostatic spinning method to obtain a composite cross-linked fluorinated polyimide electrolyte membrane containing a small amount of impurities.

[0029] Preferably, the amount of acetic anhydride added in step (3) is 16.5-17.5 wt% of the composite cross-linked fluorinated polyimide electrolyte membrane; the amount of pyridine added in step (3) is 12.5-13.5 wt% of the composite cross-linked fluorinated polyimide electrolyte membrane.

[0030] Preferably, when a film applicator (scraper) is used for coating in step (4), the size of the scraper is 200 to 500 μm, and the thickness of the film is between 50 and 150 μm.

[0031] Preferably, the nanofiltration membrane in step (5) is a polycarbonate nanofiltration membrane.

[0032] The principle of the present invention is:

[0033] The present invention utilizes a chemical imidization method to prepare a fluorinated polyimide. By adding a crosslinking agent, crosslinking is generated within the fluorinated polyimide. SN and LITFSI are then dissolved in an organic solvent and added to the imidized crosslinked polyimide solution. The membrane is then formed into a undried membrane containing 84-85 wt% of organic solvent. The resulting membrane is filtered through a nanofiltration membrane with precise and uniformly distributed pores in a sand core funnel to remove most of the organic solvent, making the composite crosslinked fluorinated polyimide membrane uniformly porous. Succinonitrile and lithium salt are then separated from the solvent and uniformly stored within the membrane. The membrane is then placed in anhydrous ethanol to remove impurities while further storing the succinonitrile and lithium salt within the membrane using a phase separation method. Furthermore, while heating to remove the solvent, the membrane undergoes thermal crosslinking, ultimately producing a composite porous crosslinked fluorinated polyimide membrane. The composite porous cross-linked fluorinated polyimide membrane prepared by the present invention incorporates SN. Although SN as a plasticizer reduces the mechanical properties of the system, the introduction of cross-linked FPI significantly improves the mechanical properties. Furthermore, SN alone is very unstable as an ion-conducting medium and easily undergoes side reactions with lithium sheets or electrodes, leading to instability in the electrolyte system and a significant reduction in the battery's cycle life. However, the introduction of cross-linked FPI as a three-dimensional porous framework allows the SN to be evenly dispersed in the cross-linked FPI, significantly improving the cycle life. Finally, because FPI contains carbonyl groups, it can also serve as a positive electrode material in the electrolyte system, providing the battery with a certain amount of additional capacity.

[0034] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0035] (1) The preparation method of the fluorinated polyimide-based composite solid electrolyte of the present invention is to place the composite cross-linked fluorinated polyimide electrolyte membrane in a sand core funnel and use a nanofiltration membrane for filtration, so that the composite cross-linked fluorinated polyimide electrolyte membrane becomes uniformly porous, and SN and lithium salt can be smoothly stored in the membrane, solving the problem that SN and FPI cannot be mixed and prepared.

[0036] (2) The preparation method of the fluorinated polyimide-based composite solid electrolyte of the present invention abandons the traditional method of dissolving FPI in DMAC and then blending FPI and SN because the cross-linked FPI and SN are not compatible and cannot be blended. Instead, SN is added and mixed directly during the synthesis of the cross-linked FPI. After the imide cyclization of the FPI, SN is added to the porous cross-linked fluorinated polyimide solution and mixed, so that the SN is evenly mixed in the porous cross-linked polyimide. Subsequently, the SN and the cross-linked FPI are filtered through a sand core funnel to form a three-dimensional porous skeleton electrolyte membrane, thereby solving the problem of incompatibility between the cross-linked FPI and SN.

[0037] (3) The preparation method of the fluorinated polyimide-based composite solid electrolyte of the present invention uses porous cross-linked FPI instead of ordinary FPI. Firstly, it is convenient for SN to enter the FPI membrane from the pores. Secondly, the addition of SN reduces the mechanical properties of the system, making the three-dimensional skeleton mechanically weak and unable to effectively inhibit the growth of lithium dendrites. Therefore, cross-linked FPI is used to greatly improve the mechanical properties to meet the needs of the electrolyte system.

[0038] (4) The preparation method of the fluorinated polyimide-based composite solid electrolyte of the present invention cross-links the fluorinated polyimide through a cross-linking agent, thereby greatly improving the mechanical strength and reducing the effect of SN as a plasticizer on the mechanical strength.

[0039] (5) The preparation method of the fluorinated polyimide-based composite solid electrolyte of the present invention introduces fluorine groups into polyimide, improves processing performance, increases the dielectric constant of the material, enhances the surface polarity and interfacial compatibility of the membrane, and promotes Li + dissociation and transport.

[0040] (6) The introduction of SN into the fluorinated polyimide-based composite solid electrolyte of the present invention greatly improves the ionic conductivity at room temperature and improves the electrochemical window, thereby greatly improving the electrical performance; the introduction of cross-linked FPI improves the mechanical properties, enhances the interfacial compatibility, improves the thermal stability, and promotes the Li + In addition, because FPI has a carbonyl group, it can also be used as a positive electrode material during battery operation, providing a certain amount of additional capacity to the battery, making the battery equipped with this composite electrolyte have better electrochemical performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 Surface SEM image (a) of the finished composite porous cross-linked fluorinated polyimide electrolyte membrane in Example 1 of the present invention and cross-sectional SEM image (b) of the finished porous cross-linked fluorinated polyimide electrolyte membrane in Comparative Example 1 of the present invention.

[0042] Figure 2 Graph showing the long cycle performance of the electrolyte membranes of Example 1 (FPI-SN) of the present invention and Comparative Example 2 (PEO-SN) at 25°C and 0.2C.

[0043] Figure 3 The rate cycle diagram of the electrolyte membrane of Example 1 (FPI-SN) of the present invention and Comparative Example 2 (PEO-SN) at 25 degrees is 0.1C-0.2C-0.5C-1C-2-1C-0.5C-0.2C-0.1C.

[0044] Figure 4 1 and 2 are LSV diagrams of the electrolyte membranes of Example 1 (FPI-SN) of the present invention and Comparative Example 2 (PEO-SN) at 25°C.

[0045] Figure 5 Graphs showing the tensile properties of the electrolyte membranes of Example 1(a) and Comparative Example 2(b) of the present invention.

[0046] Figure 6 These are the test results of thermal dimensional stability of the electrolyte membranes of Example 1 and Comparative Example 2 of the present invention at 30°C-60°C-90°C.

[0047] Figure 7 Ignition test results of the electrolyte membranes of Example 1 of the present invention and Comparative Example 2 under flame. DETAILED DESCRIPTION

[0048] The present invention will be further described in detail below with reference to the examples, but the embodiments of the present invention are not limited thereto.

[0049] Example 1

[0050] Step 1: Dissolve 0.8569 g of fluorinated diamine (TFDB) and 1.2243 g of fluorinated dianhydride (molar ratio 1:1.03, ensuring a slight excess of dianhydride to obtain a completely anhydride-terminated polyamic acid oligomer) in 12 mL of N,N-dimethylformamide (DMF). Stir in an ice-water bath under a nitrogen atmosphere for 12 hours, wait for the viscosity of the solution in the container to increase, and obtain a completely reacted anhydride-terminated fluorinated polyamic acid precursor solution.

[0051] Step 2: Add the crosslinking agent bis(3-trimethoxysilylpropyl) (BTMSPA) into the container

[0052] 0.1486 g of fluorinated polyimide was added, and after vigorous stirring for 1 h, 2.2654 g of acetic anhydride and 1.7555 g of pyridine were added dropwise to the solution, and chemical imidization was carried out for 1.5 h to form a cross-linked fluorinated polyimide solution (C-12FPI).

[0053] Step 3: 1.0406 g of succinonitrile (SN) and 1.0406 g of lithium bis(trifluoromethylsulfonyl)imide (LITFSI) were dissolved in DMF at 50 ° C., then added to C-12FPI, stirred for 20 minutes, and then immediately poured onto a polytetrafluoroethylene plate. The film was coated with a surgical spatula with a size of 250 μm. After shaping, a membrane with a DMF content of 84.6 wt% was obtained. After tearing it off, it was immediately placed in a sand core funnel equipped with a polycarbonate nanofiltration membrane for filtration, wherein the polycarbonate nanofiltration membrane was placed on the sand core filter plate of the sand core funnel, and the membrane was placed on the polycarbonate nanofiltration membrane; the filtered membrane was then soaked in anhydrous ethanol until it was washed to obtain a porous composite cross-linked fluorinated polyimide electrolyte membrane (C-12FPI-SN) free of impurities.

[0054] Step 4: First, place the membrane in a blast oven at 80°C to remove the remaining solvent, and then place it in a vacuum oven for thermal crosslinking. Heat it from room temperature to 55°C and keep it warm for 4 hours. After that, heat it to 75°C and keep it warm for 8 hours. Then heat it to 95°C and keep it warm for 8 hours. Finally, heat it to 110-120°C and keep it warm for 4 hours to obtain a finished composite porous cross-linked fluorinated polyimide electrolyte membrane, that is, a fluorinated polyimide-based composite solid electrolyte.

[0055] Example 2

[0056] In this embodiment, the fluorine-containing diamine is

[0057]

[0058] In this embodiment, the fluorine-containing dianhydride is

[0059]

[0060] The steps of this embodiment are similar to those of embodiment 1, except that the diamine monomer is replaced by 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene instead of 2,2'-bis(trifluoromethyl)diaminobiphenyl to prepare a fluorinated polyimide-based composite solid electrolyte.

[0061] Example 3

[0062] In this embodiment, the fluorine-containing diamine is

[0063]

[0064] In this embodiment, the fluorine-containing dianhydride is

[0065]

[0066] The steps of this embodiment are similar to those of embodiment 1, except that the dianhydride monomer is replaced by 9,9-bis(trifluoromethyl)-2,3,6,7-xanthenetetracarboxylic dianhydride instead of 4,4'-(hexafluoroisopropylene)diphthalic anhydride; and a fluorinated polyimide-based composite solid electrolyte is prepared.

[0067] Example 4

[0068] The steps of this embodiment are similar to those of Example 1, except that the cross-linking agent is replaced by (3-aminopropyl)triethoxysilane (KH550) instead of bis(3-trimethoxysilylpropyl)amine (BTMSPA) to prepare a fluorinated polyimide-based composite solid electrolyte.

[0069] Comparative Example 1

[0070] The steps of Comparative Example 1 are similar to those of Example 1, except that SN is not added.

[0071] Comparative Example 2

[0072] The polyethylene oxide PEO (M=300000) used in this comparative example 2 is

[0073]

[0074] Step 1: Dissolve 2.0812 g of polyethylene oxide (PEO) in 15 mL of N,N-dimethylformamide and stir at 50°C under a nitrogen atmosphere until the PEO is completely dissolved and the viscosity of the solution in the container increases, thereby obtaining a PEO solution.

[0075] Step 2: Dissolve 1.0406 g of succinonitrile (SN) and 1.0406 g of lithium bis(trifluoromethylsulfonyl)imide (LITFSI) in DMF at 50°C, then add them to the PEO solution and stir for 2 hours. After mixing is complete, cast the mixture onto a polytetrafluoroethylene plate to control the film thickness to be between 75 and 150 μm.

[0076] Step 3: First, place the membrane in a forced air oven at 50°C to remove the solvent, and then place it in a vacuum oven at 50°C to remove moisture, and finally obtain a composite polyethylene oxide electrolyte membrane (PEO-SN).

[0077] SEM test of electrolyte membrane:

[0078] The SEM test results of the fluorinated polyimide-based composite electrolyte membrane of Example 1 and the electrolyte membrane of Comparative Example 1 are as follows: Figure 1As shown, it can be clearly seen that both materials form a uniform porous structure, and in the example in which SN is added, SN is better distributed in the pores of the membrane, proving that the preparation method of Example 1 can effectively mix FPI with SN and LITFSI.

[0079] Battery cycle performance test of lithium-ion battery at 0.2C:

[0080] Comparing the lithium ion batteries using the fluorinated polyimide-based composite solid electrolyte membrane of Example 1 and the composite polyethylene oxide electrolyte membrane of Comparative Example 2, the battery cycle performance is as follows: Figure 2 As shown in the figure, compared with the traditional PEO-based composite electrolyte, the carbonyl-containing fluorinated polyimide-based composite electrolyte system has a higher specific capacity and can act as a part of the positive electrode in the battery; the cycle life is long, and it still has a certain specific capacity after 1500 cycles, the cycle remains stable, and the lithium dendrite growth phenomenon is suppressed to a certain extent; Figure 3 The PEO-SN curve in the figure begins to become disordered after 300 cycles, and the battery cycle is unstable.

[0081] Rate performance test of lithium-ion batteries:

[0082] Comparing the lithium ion batteries using the fluorinated polyimide-based composite solid electrolyte membrane of Example 1 and the polyethylene oxide-based composite electrolyte membrane of Comparative Example 2, the rate cycling performance is as follows: Figure 3 As shown, the results show that at different rates, the fluorinated polyimide-based composite solid electrolyte prepared in Example 1 of the present invention has a specific capacity far exceeding that of the system in Comparative Example 2.

[0083] LSV test of lithium-ion batteries:

[0084] Comparing the lithium ion battery of the fluorinated polyimide-based composite solid electrolyte membrane of Example 1 and the polyethylene oxide composite electrolyte of Comparative Example 2, the LSV test results are as follows: Figure 4 As shown, it can be seen that the lithium battery assembled with the fluorinated polyimide-based composite electrolyte prepared by the embodiment of the present invention has a higher decomposition voltage than that of the polyethylene oxide composite electrolyte. The decomposition voltage in the embodiment reaches about 5.5V, while the decomposition voltage of Comparative Example 2 is only about 4.5V, which proves that the embodiment system has better high voltage resistance, a wider range of operating voltage, and improved safety.

[0085] Tensile test of electrolyte membrane:

[0086] Comparing the fluorinated polyimide-based composite solid electrolyte membrane of Example 1 and the polyethylene oxide composite electrolyte membrane of Comparative Example 2, the tensile test results are as follows: Figure 5As shown, the tensile strength of the embodiment system is 40 MPa and the elongation is 196%, which are much greater than the tensile strength of 0.2 MPa and the elongation of 25% of the comparative example 2 system, indicating that the fluorinated polyimide-based composite electrolyte membrane has better mechanical strength, can effectively inhibit dendrite growth and prevent puncture, and has better processability and is tougher, and can be used to meet the requirements of different shapes.

[0087] Thermal dimensional stability test of electrolyte membrane:

[0088] The thermal dimensional stability test of the fluorinated polyimide-based composite solid electrolyte membrane of Example 1 and the polyethylene oxide composite electrolyte membrane of Comparative Example 2 is as follows: Figure 6 As shown in the figure, it can be seen that when the temperature reaches 60°C, the dimensions of the polyethylene oxide composite electrolyte membrane have become unstable, and when the temperature reaches 90°C, it completely loses its shape. However, the fluorinated polyimide-based composite electrolyte membrane can still maintain dimensional stability at 90°C, indicating that the fluorinated polyimide-based composite electrolyte membrane is more thermally stable than the polyethylene oxide composite electrolyte membrane, and its safety is improved.

[0089] Ignition test of electrolyte membrane

[0090] Comparing the fluorinated polyimide-based composite solid electrolyte membrane of Example 1 and the polyethylene oxide composite electrolyte membrane of Comparative Example 2, the ignition test results are as follows: Figure 7 As shown in the figure, compared to the polyethylene oxide composite electrolyte membrane, the fluorinated polyimide-based composite electrolyte membrane ignites more slowly and maintains a certain shape after 40 seconds, turning into a carbonized solid after complete combustion. In contrast, the polyethylene oxide composite electrolyte membrane burns rapidly upon contact with the flame, completing combustion in a very short time and turning into a black liquid. These results effectively demonstrate that the fluorinated polyimide-based composite electrolyte membrane has better flame retardancy and safety performance, and is less prone to spontaneous combustion and explosion.

[0091] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing a fluorinated polyimide-based composite solid electrolyte, characterized in that: The following steps are involved: (1) synthesizing a fluorinated polyamic acid precursor solution by reacting a fluorinated aromatic diamine and an excess of aromatic dianhydride in an organic solvent; the molar ratio of the fluorinated aromatic diamine to the aromatic dianhydride is 1:(1.03-1.1); The fluorine-containing aromatic diamine has one of the following structures: The fluorine-containing aromatic dianhydride has one of the following structural formulas: (2) adding a silane coupling agent to the fluorinated polyamic acid precursor solution prepared in step (1) to obtain a chemically cross-linked fluorinated polyamic acid solution; (3) adding acetic anhydride and pyridine to the chemically cross-linked fluorinated polyamic acid solution prepared in step (2) to perform chemical imidization to obtain a cross-linked fluorinated polyimide solution; adding succinonitrile and lithium bis(trifluoromethanesulfonyl)imide to a cross-linked fluorinated polyimide solution to obtain a composite cross-linked fluorinated polyimide solution; (4) forming a film from the composite cross-linked fluorinated polyimide solution obtained in step (3) to obtain a composite cross-linked fluorinated polyimide electrolyte membrane; the mass percentage of the organic solvent in the composite cross-linked fluorinated polyimide electrolyte membrane is 84 to 85%; (5) placing a nanofiltration membrane on a sand core filter plate of a sand core funnel, placing the composite cross-linked fluorinated polyimide electrolyte membrane prepared in step (4) on the nanofiltration membrane, performing suction filtration to extract the organic solvent in the composite cross-linked fluorinated polyimide electrolyte membrane, and then soaking it in anhydrous ethanol to obtain a porous composite cross-linked fluorinated polyimide electrolyte membrane; (6) removing the solvent from the porous composite cross-linked fluorinated polyimide electrolyte membrane obtained in step (5), and then thermally cross-linking the membrane under vacuum conditions to obtain a fluorinated polyimide-based composite solid electrolyte.

2. The method for preparing a fluorinated polyimide-based composite solid electrolyte according to claim 1, wherein: The organic solvent in step (1) is N,N-dimethylformamide.

3. The method for preparing a fluorinated polyimide-based composite solid electrolyte according to claim 1, wherein: The silane coupling agent in step (2) is one of bis(3-trimethoxysilylpropyl)amine, N-[3-(trimethoxysilyl)propyl]ethylenediamine and (3-aminopropyl)triethoxysilane.

4. The method for preparing a fluorinated polyimide-based composite solid electrolyte according to claim 1, wherein: In step (3), the molar ratio of succinonitrile: lithium bis(trifluoromethanesulfonyl)imide: fluorinated polyimide is 1:(0.5-1):(1.5-2).

5. The method for preparing the fluorinated polyimide-based composite solid electrolyte according to claim 1, wherein: The pore size of the nanofiltration membrane in step (5) is 2 to 10 nm.

6. The method for preparing a fluorinated polyimide-based composite solid electrolyte according to claim 1, wherein: The soaking in anhydrous ethanol described in step (5) is specifically: soaking in anhydrous ethanol for 24 to 72 hours.

7. The method for preparing the fluorinated polyimide-based composite solid electrolyte according to claim 1, characterized in that: Step (6) is to remove the solvent from the porous composite cross-linked fluorinated polyimide electrolyte membrane obtained in step (5) and then perform thermal cross-linking under vacuum conditions, specifically: First, the porous composite cross-linked fluorinated polyimide electrolyte membrane obtained in step (5) is placed in a blast oven to remove the solvent, and then placed in a vacuum oven, and the temperature is raised from room temperature to 50-60°C and kept warm for 1-4 hours. After the temperature is raised to 70-80°C and kept warm for 4-8 hours, then raised to 90-100°C and kept warm for 4-8 hours, and finally raised to 110-120°C and kept warm for 1-4 hours. After thermal cross-linking, a fluorinated polyimide-based composite solid electrolyte is obtained.

8. Fluorinated polyimide-based composite solid electrolyte, characterized in that: The fluorinated polyimide-based composite solid electrolyte is prepared by the preparation method of any one of claims 1 to 7.

9. A lithium-ion battery, characterized in that: It includes the fluorinated polyimide-based composite solid electrolyte according to claim 8.

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