Composite electrolyte, preparation method and application thereof

Abstract

The application discloses a composite electrolyte and a preparation method and application thereof. The composite electrolyte comprises a lithium salt, a solvent, a Lewis acid precursor, a Lewis acid capturing agent and a thermal response monomer, the mass content of the Lewis acid precursor is 0.1-5%, the mass content of the Lewis acid capturing agent is 2-5%, and the mass content of the thermal response monomer is 5-30%. The composite electrolyte can rapidly polymerize at high temperature, so that the electrolyte thermal stability and the ion transmission path are cut off, meanwhile, the electrolyte can stably keep liquid state in a non-high-temperature environment, and the electrochemical performance is not affected, and the electrolyte is compatible with various storage and use environments. When the electrolyte is applied to a lithium ion battery, the electrolyte can significantly inhibit gas production and fire in a battery thermal runaway process, improves the safety of the battery, and does not affect the electrical performance of the battery in a normal operation state.

Description

Technical Field

[0001] This invention relates to a composite electrolyte, its preparation method, and its application in lithium-ion batteries, belonging to the field of lithium battery technology. Background Technology

[0002] Lithium-ion batteries, as a crucial component of electrochemical energy storage systems, play a vital role in the new energy ecosystem. However, they still suffer from safety issues, being susceptible to thermal runaway under abusive conditions such as thermal, electrical, and mechanical stresses, posing a significant challenge to the stable operation of the new energy system.

[0003] Commercial lithium-ion battery electrolytes are mainly composed of carbonate solvents, which are prone to combustion during battery thermal runaway, generating large amounts of flammable and explosive gases. Therefore, using electrolytes with higher thermal stability has become an important research direction to improve the safety of lithium-ion batteries.

[0004] Gelation can effectively improve the thermal stability of electrolytes, but current technologies typically achieve electrolyte gelation directly during battery fabrication through heating, radiation, or other methods. This leads to a decrease in electrolyte ionic conductivity and poor contact between the electrolyte and the positive and negative electrodes, severely impacting battery electrical performance. Furthermore, since current technologies often use free radical initiators to achieve electrolyte gelation, the storage and usage environment of the electrolyte requires strict control.

[0005] Patent publication number CN111430780B discloses a raw material composition for a gel electrolyte in lithium-ion batteries. This composition contains a non-aqueous organic solvent, an electrolyte lithium salt, additives such as inorganic acids, organic esters, and / or nitrs and acrylates. By mixing the composition with a thermal initiator, in-situ gelation of the electrolyte is achieved, significantly improving battery safety. Patent publication number CN117497843B discloses a gel electrolyte for lithium batteries and its preparation method, as well as a method for preparing a gel electrolyte lithium battery. This relates to the field of lithium battery technology. The method for preparing a gel electrolyte for lithium batteries involves pre-preparing a precursor polymer M by polymerizing monomers A, B, and C and an initiator, and then mixing M with the electrolyte to prepare a gel electrolyte. The stepwise polymerization method reduces the influence of the monomers and initiator on the electrolyte performance, improving the low-temperature rate performance of the lithium battery. However, both of the above technologies use free radical initiators to achieve gelation, which places strict requirements on the storage and usage environment of the electrolyte. In addition, all of the above technologies involve electrolyte gelation during battery manufacturing, which reduces the ionic conductivity of the electrolyte and affects the battery's electrical performance.

[0006] Therefore, it is crucial to develop an electrolyte with high thermal stability and high ionic conductivity to improve the safety and electrochemical performance of lithium batteries. Summary of the Invention

[0007] To address the problems existing in the prior art, the first objective of this invention is to provide a composite electrolyte. This electrolyte exhibits strong thermal stability and high ionic conductivity, effectively improving the overall performance of the battery.

[0008] The second objective of this invention is to provide a method for preparing a composite electrolyte. This method is simple, inexpensive, and suitable for industrial production.

[0009] A third objective of this invention is to provide an application of a composite electrolyte. This electrolyte, when used in lithium-ion batteries, can significantly improve battery safety while ensuring excellent electrochemical performance.

[0010] To achieve the above-mentioned technical objectives, the present invention provides a composite electrolyte comprising lithium salt, solvent, Lewis acid precursor, Lewis acid scavenger and thermally responsive monomer, wherein the mass content of the Lewis acid precursor is 0.1-5%, the mass content of the Lewis acid scavenger is 2-5%, and the mass content of the thermally responsive monomer is 5-30%.

[0011] This invention constructs a Lewis acid sustained-release mechanism by introducing a Lewis acid precursor and a Lewis acid trapping agent. Under normal non-high-temperature conditions, the Lewis acid precursor decomposes slowly, releasing only a small amount of Lewis acid during long-term storage. The Lewis acid trapping agent, such as isothiocyanate, silazane, or silamine, can coordinate with the electron-deficient Lewis acid through nitrogen, sulfur, and oxygen atoms containing lone pairs of electrons, thereby neutralizing its acidity and blocking the initiation of cationic polymerization of monomers in the electrolyte. Nevertheless, due to significant steric hindrance, the coordination compound formed by the trapping agent and the Lewis acid in this invention has limited stability. Under high-temperature conditions, it will reversibly decompose and release the Lewis acid again. Simultaneously, the decomposition rate of the Lewis acid in the electrolyte will significantly increase at high temperatures, far exceeding the binding capacity of the trapping agent. This further rapidly initiates the cationic polymerization of thermally responsive monomers, achieving rapid thermal response solidification of the electrolyte, thereby improving the electrolyte's thermal stability and cutting off internal ion pathways, significantly enhancing battery safety. In summary, this invention achieves long-term stable storage of the electrolyte at room temperature and rapid high-temperature polymerization by introducing Lewis acid precursors and scavengers, ensuring the high ionic conductivity of the electrolyte under normal conditions, so that the electrochemical performance of the electrolyte is not affected, and also improving the safety and stability of the electrolyte.

[0012] In addition, controlling the contents of Lewis acid, Lewis acid scavenger, and thermally responsive monomer within appropriate ranges can ensure excellent overall performance of the electrolyte. Excessive Lewis acid precursor content, insufficient Lewis acid scavenger content, or excessive thermally responsive monomer content will affect the electrochemical performance of the electrolyte under normal conditions. Conversely, insufficient Lewis acid precursor content, excessive Lewis acid scavenger content, or insufficient thermally responsive monomer content will affect the thermal response curing effect of the electrolyte under high-temperature conditions, thus limiting the high-temperature safety performance of the electrolyte.

[0013] As a preferred embodiment, the Lewis acid precursor includes at least one of lithium hexafluorophosphate, lithium difluorooxalate boronate, tris(pentafluorophenyl)borane, tris(pentafluorophenyl)phosphine, lithium tetrafluoroborate, tin tetrachloride, titanium tetrachloride, titanium tetrabromide, and aluminum trifluoromethanesulfonate. This type of Lewis acid precursor exhibits good stability in non-high-temperature environments but readily decomposes to produce Lewis acids under high-temperature conditions.

[0014] As a preferred embodiment, the Lewis acid scavenger comprises at least one selected from trimethylsilyl isocyanate, (trimethylsilyl) isothiocyanate, 4-(trifluoromethyl)phenyl isothiocyanate, hexamethyldisilazane, and heptamethyldisilazane. This type of Lewis acid scavenger exhibits moderate binding affinity to Lewis acids, making it suitable for the system of this invention.

[0015] As a preferred embodiment, the thermally responsive monomer comprises at least one of 2-fluorostyrene, 3-fluorostyrene, 4-fluorostyrene, 4-(trifluoromethyl)styrene, 2,3,4,5,6-pentafluorostyrene, 4-fluoro-α-methylstyrene, acrylamidomethylpropanesulfonic acid, and perfluorooctyl acrylate.

[0016] As a preferred embodiment, the composite electrolyte also contains film-forming additives.

[0017] As a preferred embodiment, the content of the film-forming additive is 1 to 7 wt% of the composite electrolyte.

[0018] As a preferred embodiment, the film-forming additive includes at least one of vinylene carbonate, vinyl sulfate, fluorovinyl carbonate, methylene disulfonate, 1,3-propanesulfonate lactone, and vinyl vinyl carbonate.

[0019] As a preferred embodiment, the lithium salt includes at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, and lithium nitrate.

[0020] As a preferred embodiment, the lithium salt has a mass content of 10-15.5%.

[0021] As a preferred embodiment, the solvent includes at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene carbonate, γ-butyrolactone, methyl acetate, propyl acetate, and butyl acetate.

[0022] The present invention also provides a method for preparing a composite electrolyte, which involves mixing raw materials comprising lithium salt, solvent, Lewis acid precursor, Lewis acid scavenger and thermally responsive monomer to obtain the electrolyte.

[0023] The present invention also provides an application of a composite electrolyte, which is used as an electrolyte in lithium-ion batteries.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] (1) The composite electrolyte provided by the present invention can rapidly respond to polymerization and solidification in high temperature environment, and stably maintain the liquid form of electrolyte in non-high temperature conventional environment, ensuring the electrochemical performance of electrolyte, significantly improving the overall stability of electrolyte, and having good compatibility with storage and use environment;

[0026] (2) The composite electrolyte does not need to be gelled in advance during the battery preparation process. The electrolyte has high ionic conductivity and good contact and wetting with the positive and negative electrodes, which does not affect the battery's electrical performance.

[0027] (3) The composite electrolyte provided by the present invention can be rapidly solidified at high temperature. While improving the thermal stability of the electrolyte, it can also cut off the internal ion pathway of the battery and act as a supporting material to isolate the positive and negative electrodes, prevent large-area short circuits inside the battery, and greatly improve the battery safety performance. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0029] Figure 1 This is a thermal response curing effect diagram of the composite electrolyte of Example 1 of the present invention at 120°C.

[0030] Figure 2 This is a comparison chart of the cycle performance of lithium-ion batteries corresponding to Example 1 and Comparative Example 2 of the present invention. Detailed Implementation

[0031] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] In the following examples, unless otherwise specified, all raw materials and equipment used are commercially available, and all solvents used are of analytical grade.

[0033] Lithium hexafluorophosphate: CAS number 21324-40-3, purchased from Suzhou Duoduo Reagent Co., Ltd.

[0034] Lithium difluorooxalate borate: CAS No. 409071-16-5, purchased from Suzhou Duoduo Reagent Co., Ltd.

[0035] Lithium tetrafluoroborate: CAS number 14283-07-9, purchased from Suzhou Duoduo Reagent Co., Ltd.

[0036] Trimethylsilyl isocyanate: CAS No. 1118-02-1, purchased from Shanghai McLean Biochemical Technology Co., Ltd.

[0037] Hexamethyldisilazane: CAS No. 999-97-3, purchased from Shanghai Maclean Biochemical Technology Co., Ltd.

[0038] 4-Fluoro-styrene: CAS No. 405-99-2, purchased from Shanghai Maclean Biochemical Technology Co., Ltd.

[0039] Acrylamidomethylpropanesulfonic acid: CAS No. 15214-89-8, purchased from Shanghai Maclean Biochemical Technology Co., Ltd.

[0040] Lithium bis(fluorosulfonyl)imide: CAS No. 9002-81-7, purchased from Suzhou Duoduo Reagent Co., Ltd.

[0041] Lithium perchlorate: CAS number 7791-03-9, purchased from Suzhou Duoduo Reagent Co., Ltd.

[0042] Ethylene carbonate: CAS No. 96-49-1, purchased from Suzhou Duoduo Reagent Co., Ltd.

[0043] Dimethyl carbonate: CAS number 616-38-6, purchased from Suzhou Duoduo Reagent Co., Ltd.

[0044] Ethyl methyl carbonate: CAS No. 623-53-0, purchased from Suzhou Duoduo Reagent Co., Ltd.

[0045] Ethylene carbonate: CAS No. 872-36-6, purchased from Suzhou Duoduo Reagent Co., Ltd.

[0046] Vinyl sulfate: CAS number 1072-53-3, purchased from Suzhou Duoduo Reagent Co., Ltd.

[0047] Fluoroethylene carbonate: CAS number 114435-02-8, purchased from Suzhou Duoduo Reagent Co., Ltd.

[0048] Polyolefin membrane: Celgard 2400 membrane, 25μm thick, single-layer PP material, 41% porosity, purchased from Celgard, USA.

[0049] Commercial lithium iron phosphate cathode: areal density 13.5 mg / cm³ 2 The active substance content was 95.3 wt%, purchased from Shenzhen Kejing Zhida Technology Co., Ltd.

[0050] Commercial graphite anode: areal density 6.3 mg / cm³ 2 The active substance content was 95.3 wt%, purchased from Shenzhen Kejing Zhida Technology Co., Ltd.

[0051] In the following examples, room temperature refers to 23±2℃.

[0052] Example 1

[0053] In an argon-protected glove box (water and oxygen content both less than 0.1 ppm), weigh the following substances in sequence:

[0054] Lewis acid precursor: 0.20 g lithium boron difluorooxalate;

[0055] Lewis acid scavenger: 0.40 g trimethylsilyl isocyanate;

[0056] Thermally responsive monomer: 1.5g 4-fluorostyrene;

[0057] Lithium salt: 1.2g lithium difluorosulfonylimide;

[0058] Solvents: 2.00g dimethyl carbonate, 2.00g ethylene carbonate, and 2.50g ethyl methyl carbonate;

[0059] Film-forming additives: 0.10g vinylene carbonate, 0.05g fluoroethylene carbonate, and 0.05g vinyl sulfate;

[0060] All the above substances were mixed at room temperature and stirred for 6 hours until the lithium salt was completely dissolved, thus preparing the safe electrolyte S1.

[0061] Example 2

[0062] The electrolyte was prepared using the method of Example 1, except that lithium difluorooxalate boronate was replaced with an equal mass of lithium tetrafluoroborate to obtain safe electrolyte S2.

[0063] Example 3

[0064] The electrolyte was prepared using the method of Example 1, except that trimethylsilyl isocyanate was replaced with an equal mass of hexamethyldisilazane to obtain safe electrolyte S3.

[0065] Example 4

[0066] The electrolyte was prepared using the method of Example 1, except that 4-fluorostyrene was replaced with an equal mass of acrylamide methylpropanesulfonic acid to obtain safe electrolyte S4.

[0067] Example 5

[0068] The electrolyte was prepared using the method of Example 1, except that lithium bis(fluorosulfonyl)imide was replaced with an equal mass of lithium perchlorate to obtain safe electrolyte S5.

[0069] Example 6

[0070] The electrolyte was prepared using the method described in Example 1, except that the raw material ratio was as follows:

[0071] Lewis acid precursor: 0.10 g lithium difluoroborate;

[0072] Lewis acid scavenger: 0.20 g trimethylsilyl isocyanate;

[0073] Thermally responsive monomer: 2.0g 4-fluorostyrene;

[0074] Lithium salt: 1.2g lithium difluorosulfonylimide;

[0075] Solvents: 1.80g dimethyl carbonate, 2.00g ethylene carbonate, 2.50g methyl ethyl carbonate;

[0076] Film-forming additives: 0.10 g vinylene carbonate, 0.05 g fluoroethylene carbonate, 0.05 g vinyl sulfate. This example yields safe electrolyte S6.

[0077] Example 7

[0078] The electrolyte was prepared using the method of Example 2, except that lithium bis(fluorosulfonyl)imide was replaced with an equal mass of lithium perchlorate to obtain safe electrolyte S7.

[0079] Example 8

[0080] The electrolyte was prepared using the method of Example 1, except that no film-forming additives were used.

[0081] Comparative Example 1

[0082] The commercial lithium-ion battery electrolyte has a specific composition of 1 M LiPF6 EC / EMC / DMC = 1 / 1 / 1 (wt%), that is, the solvent is a mixture of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate in a mass ratio of 1:1:1, and the lithium hexafluorophosphate concentration is 1 mol / L.

[0083] Comparative Example 2

[0084] The conventional gelled electrolyte has the following composition: 1 M LiPF6 EC / EMC / DMC = 1 / 1 / 1 (wt%) + 0.1 wt% azobisisobutyronitrile + 10 wt% methyl methacrylate + 10 wt% ethylene glycol dimethacrylate. That is, the solvent is a mixture of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate in a mass ratio of 1:1:1, and the concentration of lithium hexafluorophosphate in the electrolyte is 1 mol / L. Then, 0.1 wt% azobisisobutyronitrile, 10 wt% methyl methacrylate and 10 wt% ethylene glycol dimethacrylate are added according to the mass of the lithium-containing mixture.

[0085] Comparative Example 3

[0086] The electrolyte was prepared using the method of Example 1, except that Lewis acid precursors were not added.

[0087] Comparative Example 4

[0088] The electrolyte was prepared using the method of Example 1, except that Lewis acid scavengers were not added.

[0089] Comparative Example 5

[0090] The electrolyte was prepared using the method of Example 1, except that no thermally responsive monomer was added.

[0091] Comparative Example 6

[0092] The electrolyte was prepared using the method of Example 1, except that the Lewis acid scavenger content was increased to 1.0 g.

[0093] Comparative Example 7

[0094] The electrolyte was prepared using the method of Example 1, except that the content of Lewis acid precursor was increased to 0.6 g and the content of Lewis acid scavenger was decreased to 0.1 g.

[0095] Commercial lithium iron phosphate cathodes, commercial graphite anodes, commercial separators (polyolefin separators), and electrolytes prepared in each example and comparative example were assembled into lithium-ion batteries. Specifically, for Comparative Example 2, the commercial lithium iron phosphate cathode, commercial graphite anode, commercial separator (polyolefin separator), and electrolyte prepared in Comparative Example 2 were assembled into a battery and heated at 60°C for 48 hours to obtain the corresponding lithium-ion battery.

[0096] Performance tests were conducted on the aforementioned lithium-ion batteries, including electrolyte high-temperature response test, electrolyte long-term storage stability test, battery cycle performance test, and battery nail penetration test. The results are shown in Table 1. The electrolyte high-temperature response test was conducted as follows: 2.0 g of electrolyte was weighed into a transparent glass bottle in an argon-protected glove box (water and oxygen content both less than 0.1 ppm), sealed, and then transferred to a 120°C forced-air oven for heating. The electrolyte thermal response time and degree of solidification were recorded.

[0097]

[0098] As shown in Table 1, the composite electrolyte prepared in the embodiments of the present invention can rapidly achieve complete solidification at high temperatures, and its thermal response speed is faster and its degree of solidification is higher than that of Comparative Example 2, which represents a conventional gelled electrolyte. Comparative Example 6 shows that an excessively high content of scavenging agent in the electrolyte will affect the thermal response of the electrolyte, making it impossible for the electrolyte to solidify quickly, thereby reducing the high-temperature safety performance of the electrolyte.

[0099] Figure 1 The image shows the thermal response curing of the composite electrolyte S1 prepared in Example 1 of this invention at 120°C. As can be seen from the image, the electrolyte can be completely cured at high temperature and appears as a white solid, indicating a high degree of polymerization.

[0100] Long-term storage stability test of each electrolyte: 2.0g of electrolyte was weighed into a transparent glass bottle in an argon-protected glove box (water and oxygen content were both less than 0.1ppm), sealed, and then transferred to room temperature and normal humidity environment for storage. The spontaneous gelation time and solidification degree of the electrolyte were recorded. The test results are shown in Table 2.

[0101]

[0102] As shown in Table 2, the composite electrolyte prepared in this invention can maintain a liquid state at room temperature for a long time without the need for special low-temperature and light-protected storage. Its stability is significantly better than that of Comparative Example 2, which represents a conventional gelled electrolyte, and also better than that of Comparative Example 4 without a trapping agent. Comparative Example 7 shows that when the Lewis acid content in the electrolyte is too high, the electrolyte will solidify in non-high-temperature environments, thereby affecting the electrochemical performance of the battery.

[0103] Battery cycle performance tests were conducted on batteries assembled with various electrolytes: Cyclic tests were conducted using a Ruineng charge-discharge tester, specifically: (1) The batteries were fully activated by charging and discharging 3 times at a rate of 0.1C within a voltage range of 2.5-3.65V; (2) The batteries were charged and discharged at a rate of 0.5C within a voltage range of 2.5-3.65V, and the discharge capacity retention rate and energy efficiency were recorded after 100 cycles. The discharge capacity retention rate was calculated as: discharge capacity of the 100th cycle / discharge capacity of the first cycle × 100%. The test results are shown in Table 3.

[0104]

[0105] As shown in Table 3, the lithium-ion battery using the composite electrolyte prepared according to the present invention exhibits good discharge capacity retention and energy efficiency, indicating that the composite electrolyte of the present invention has no negative impact on the battery's electrical performance and its performance is significantly better than that of conventional gelled electrolytes (Comparative Example 2). Meanwhile, the data from Comparative Examples 6 and 7 show that excessively high levels of Lewis acid precursors or scavengers in the electrolyte will affect the battery's charge-discharge performance. In particular, excessively high Lewis acid content will significantly reduce the battery's discharge capacity retention and energy efficiency.

[0106] Figure 2 The graph shows the cycle performance test results of lithium-ion batteries prepared using the electrolytes of Example 1 and Comparative Example 2 of the present invention. As can be seen from the graph, the cycle performance of the lithium-ion battery corresponding to Example 1 of the present invention is significantly better than that of the lithium-ion battery using a conventional gelled electrolyte (Comparative Example 2).

[0107] Battery needle penetration tests were performed on batteries using the electrolytes of each embodiment and comparative example: the batteries were charged to 3.65V on a Ruineng charge-discharge tester, and then needle penetration and heating tests were performed. The steel needle used for the needle penetration test had a diameter of 6mm and an insertion speed of 25mm / s. The state of the battery after needle penetration was recorded, and the test results are shown in Table 4.

[0108]

[0109] As shown in Table 4, the lithium-ion batteries using the electrolyte in the embodiments of the present invention do not smoke or catch fire after being punctured, demonstrating superior safety compared to those using commercially available electrolytes (Comparative Example 1) and conventional gelled electrolytes (Comparative Example 2). This indicates that the composite electrolyte of the present invention can effectively improve battery safety. Meanwhile, the data from Comparative Example 6 shows that excessively high levels of scavenging agents in the electrolyte significantly reduce the fire safety performance of lithium batteries.

[0110] Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A composite electrolyte, characterized by: The lithium salt, the solvent, the Lewis acid precursor, the Lewis acid capturing agent and the thermal responsive monomer, the mass content of the Lewis acid precursor is 0.1-5%, the mass content of the Lewis acid capturing agent is 2-5%, and the mass content of the thermal responsive monomer is 5-30%; The Lewis acid capturing agent includes at least one of trimethylsilyl isocyanate, (trimethylsilyl) isothiocyanate, 4-(trifluoromethyl) phenyl isothiocyanate, hexamethyldisilazane and heptamethyldisilazane. The thermal responsive monomer includes at least one of 2-fluorostyrene, 3-fluorostyrene, 4-fluorostyrene, 4-(trifluoromethyl) styrene, 2,3,4,5,6-pentafluorostyrene, 4-fluoro-alpha-methylstyrene, acrylamidomethylpropane sulfonic acid and perfluorooctyl acrylate.

2. The composite electrolyte according to claim 1, characterized in that: The Lewis acid precursor includes at least one of lithium hexafluorophosphate, lithium difluoro(oxalato)borate, tris(pentafluorophenyl)borane, tris(pentafluorophenyl)phosphine, lithium tetrafluoroborate, tin tetrachloride, titanium tetrachloride, titanium tetrabromide and aluminum triflate.

3. The composite electrolyte according to claim 1 or 2, characterized in that: The composite electrolyte further contains a film-forming additive; The content of the film-forming additive is 1-7 wt% of the composite electrolyte.

4. The composite electrolyte according to claim 3, characterized in that: The film-forming additive includes at least one of vinylene carbonate, vinyl sulfate, fluorinated vinyl carbonate, methane disulfonic acid methylene, 1,3-propane sultone and vinyl vinyl carbonate.

5. The composite electrolyte according to claim 1, characterized in that: The lithium salt includes at least one of lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate and lithium nitrate; The mass content of the lithium salt is 10-15.5%.

6. The composite electrolyte of claim 1 wherein: The solvent includes at least one of vinyl carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene carbonate, gamma-butyrolactone, methyl acetate, propyl acetate and butyl acetate.

7. A method of preparing a composite electrolyte according to any one of claims 1 to 6, characterised in that: Mixing raw materials including lithium salt, solvent, Lewis acid precursor, Lewis acid capturing agent and thermal responsive monomer, and then the composite electrolyte is obtained.

8. Use of a composite electrolyte according to any one of claims 1 to 6, characterized in that: The composite electrolyte is used as an electrolyte for lithium ion batteries.

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