Electrolyte additive for enhancing thermal stability of lithium battery and preparation method thereof

Abstract

The application relates to the technical field of lithium ion batteries, in particular to an electrolyte additive for enhancing the thermal stability of lithium batteries and a preparation method thereof. The method comprises the following steps: reacting cyclotriphosphazene containing an allyloxy group and a triazole group with 1-vinylimidazole under the action of an acid-binding agent, washing and drying the reaction product by beating and vacuum drying to obtain the electrolyte additive. The cyclotriphosphazene containing an allyloxy group and a triazole group is prepared by etherification reaction of cyclotriphosphazene containing a triazole group and a hydroxyl group and an allyl halide, and the cyclotriphosphazene containing a triazole group and a hydroxyl group is obtained by azide substitution and azide-alkyne click cycloaddition reaction of hexachlorocyclotriphosphazene. The obtained additive contains a cyclotriphosphazene skeleton, a triazole group and a vinyl imidazole group, and after being applied to lithium battery electrolyte, the additive can significantly improve the thermal runaway starting temperature and the cycle capacity retention rate of the battery, shorten the self-extinguishing time of the electrolyte, and effectively enhance the thermal stability and safety of the lithium battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to an electrolyte additive that enhances the thermal stability of lithium batteries and its preparation method. Background Technology

[0002] Lithium-ion batteries dominate portable electronic devices, electric vehicles, and energy storage systems due to their high energy density and long cycle life. However, with the continuous improvement of energy density and the widespread application of high-voltage cathode materials, battery thermal safety issues are becoming increasingly prominent. The electrolyte, composed of low-flash-point organic carbonate solvents, is highly susceptible to decomposition and the release of large amounts of heat under conditions such as high temperature, overcharging, or internal short circuits, potentially leading to battery thermal runaway. Adding functional additives to the electrolyte is one of the effective ways to improve battery thermal stability and safety, and it is currently a hot research topic.

[0003] To address the issues of insufficient thermal stability of electrolytes and poor compatibility with electrode interfaces, researchers have developed various functional additives. Among existing technologies, cyclotriphosphazene compounds have attracted widespread attention due to their unique alternating phosphorus and nitrogen cyclic structure and excellent thermal stability. Chinese patent application CN121307200A discloses an electrolyte additive containing ethoxypentafluorocyclotriphosphazene, which improves battery interface stability and safety performance through its synergistic effect with lithium difluorodioxarate phosphate and fluoroethylene carbonate. Chinese patent application CN115579521A discloses an electrolyte additive, an electrolyte, and a lithium-ion battery; by introducing an electrolyte additive with an ethoxypentafluorocyclotriphosphazene structure, the electrolyte exhibits excellent flame retardancy. Chinese patent application CN121769248A discloses an electrolyte containing a triazole compound. The electrolyte includes a lithium salt, a non-aqueous solvent, and additives. The additives include 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, a triazole compound, and 2-mercapto-4-(4-pyridyl)thiazole. Adding these additives to the electrolyte can construct a dense, uniform, and stable composite CEI film on the positive electrode surface. However, these additives typically focus on only a single function, and their effectiveness in simultaneously achieving flame retardancy and long-term cycle stability is still not ideal.

[0004] Despite some progress in current research, electrolyte additives that combine flame retardancy and good cycle stability remain scarce. Currently reported cyclotriphosphazene additives typically focus on flame retardancy, while there is still room for improvement in stabilizing the electrode interface and enhancing battery cycle life. Therefore, developing multifunctional additives that combine flame retardancy and cycle stability is of significant practical importance for improving the thermal stability and safety of lithium-ion batteries. Summary of the Invention

[0005] This invention provides an electrolyte additive for enhancing the thermal stability of lithium batteries and its preparation method. The obtained additive contains a cyclotriphosphazene skeleton, a triazole group, and a vinylimidazolium group. When applied to lithium battery electrolytes, it can significantly improve the thermal runaway initiation temperature and cycle capacity retention of the battery, and shorten the self-extinguishing time of the electrolyte, thereby effectively enhancing the thermal stability and safety of lithium batteries.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing an electrolyte additive to enhance the thermal stability of lithium batteries includes the following steps: S1. Under a nitrogen atmosphere, cyclotriphosphazene containing allyloxy and triazole groups is dissolved in an anhydrous solvent, 1-vinylimidazolium and a binding agent are added, and the temperature is raised to 50~80℃ and reacted for 8~16h. After the reaction is completed, it is naturally cooled to 25℃, and then placed in an ice-water bath to cool to 0~10℃. Anhydrous lower alcohol is added, and the temperature is controlled at 0~10℃ and stirred for 0.5~1h. Then the ice-water bath is removed, and the temperature is naturally raised to 25℃ and stirred for another 0.5~1h. S2. Filtration to remove insoluble matter and collect the filtrate. The solvent in the filtrate is recovered by vacuum distillation. The residue is washed with ethyl acetate, and after solid-liquid separation, it is vacuum dried at 40~60℃ for 6~8h to obtain an electrolyte additive that enhances the thermal stability of lithium batteries.

[0007] Furthermore, the cyclotriphosphazene containing allyloxy and triazole groups is prepared by the following method: cyclotriphosphazene containing triazole and hydroxyl groups is fed to an allyl halide at a molar ratio of 1:(1.5~3.0), potassium carbonate and a solvent are added, wherein the molar ratio of potassium carbonate to cyclotriphosphazene containing triazole and hydroxyl groups is (1.0~2.0):1, and the amount of solvent used is 5~20 times the mass of cyclotriphosphazene containing triazole and hydroxyl groups. The reaction is carried out at 20~60℃ for 4~12h, and after the reaction, the mixture is washed with water and dried to obtain cyclotriphosphazene containing allyloxy and triazole groups.

[0008] Furthermore, the allyl halide is selected from allyl bromide and allyl chloride; the solvent is selected from N,N-dimethylformamide and acetone.

[0009] Furthermore, the cyclotriphosphazene containing triazole and hydroxyl groups is prepared by the following method: Step 1: In a nitrogen atmosphere, hexachlorocyclotriphosphazene and sodium azide are dissolved in a polar solvent at a molar ratio of 1:(1.0~2.0) and reacted at 0~10℃ for 2~6h. After the reaction is completed, the intermediate containing azide group is obtained by extraction and drying. Step 2: Dissolve the cyclotriphosphazene intermediate containing an azide group and propargyl alcohol in an organic solvent, and react at 25-50°C for 6-24 hours in the presence of a monovalent copper catalyst and an inorganic base. After the reaction is completed, filter, concentrate, and wash to obtain cyclotriphosphazene containing triazole and hydroxyl groups.

[0010] Furthermore, in step one, the polar solvent is selected from any one of anhydrous acetonitrile, anhydrous N,N-dimethylformamide, and anhydrous dimethyl sulfoxide, and the amount of polar solvent used is 5 to 20 times the mass of hexachlorocyclotriphosphazene.

[0011] Furthermore, in step two, the monovalent copper catalyst is selected from either cuprous iodide or cuprous bromide, and the molar ratio of the monovalent copper catalyst to the cyclotriphosphazene intermediate containing an azide group is (0.05~0.20):1; the inorganic base is selected from either potassium carbonate or sodium bicarbonate, and the molar ratio of the inorganic base to the cyclotriphosphazene intermediate containing an azide group is (1.0~3.0):1; the organic solvent is selected from either tetrahydrofuran or acetonitrile, and the amount of organic solvent used is 3~15 times the mass of the cyclotriphosphazene intermediate containing an azide group.

[0012] Furthermore, in step two, the molar ratio of the azido-containing cyclotriphosphazene intermediate to propargyl alcohol is 1:(1.0~2.5).

[0013] Furthermore, in step S1, the molar ratio of 1-vinylimidazole to the cyclotriphosphazene containing allyloxy and triazole groups is (1~4):1; the acid-binding agent is selected from any one of triethylamine, potassium carbonate, and cesium carbonate, and the molar ratio of the acid-binding agent to 1-vinylimidazole is (1~2):1.

[0014] Furthermore, in step S1, the anhydrous solvent is selected from any one of anhydrous acetonitrile, anhydrous tetrahydrofuran, and anhydrous N,N-dimethylformamide, and the amount of the anhydrous solvent is 5 to 20 times the mass of the cyclotriphosphazene containing allyloxy and triazole groups; the anhydrous lower alcohol is selected from any one of anhydrous methanol and anhydrous ethanol, and the amount of the anhydrous lower alcohol is 2 to 10 times the mass of the cyclotriphosphazene containing allyloxy and triazole groups.

[0015] This invention also provides an electrolyte additive that enhances the thermal stability of lithium batteries, prepared by the method described above.

[0016] Compared with the prior art, the beneficial effects of the present invention are: (1) This invention uses hexachlorocyclotriphosphazene as the starting material. First, it utilizes the nucleophilic substitution reaction of the active chlorine atom on the cyclotriphosphazene by the azide anion to gradually replace the chlorine atom with the azide group, thereby introducing the azide group onto the ring. Subsequently, the azide group undergoes a cyclization reaction with the carbon-carbon triple bond in propargyl alcohol to construct a five-membered triazole ring and simultaneously introduce a hydroxyl group, obtaining an intermediate containing a triazole group and a hydroxyl group. The triazole group contains multiple nitrogen atoms, which can form stable coordination complexes with the transition metal ions dissolved in the cathode material, thereby inhibiting the effect of these metal ions on the electrolyte. The catalyst in the process reduces side reactions. Then, under alkaline conditions, the hydroxyl group of the intermediate is converted to allyloxy, which not only eliminates the hygroscopic side reaction that the hydroxyl group might initiate in the electrolyte but also provides an active site for further reactions. Finally, the allyloxy reacts with 1-vinylimidazole to introduce a vinylimidazole group onto the cyclotriphosphazene. This group can form a stable interfacial film on the electrode surface, effectively inhibiting the continuous decomposition of the electrolyte at the electrode interface. Simultaneously, the nitrogen atom on its imidazole ring can synergistically cooperate with the triazole group to enhance the complexation effect on metal ions. The cyclotriphosphazene skeleton itself has excellent thermal stability and chemical inertness, maintaining structural integrity at high temperatures and providing a stable molecular carrier for the aforementioned functional groups.

[0017] (2) The additives in this invention are applied to lithium battery electrolytes, which can effectively improve the cycle stability and thermal runaway suppression capability of the battery at high temperatures, while shortening the self-extinguishing time of the electrolyte, thus ensuring the safety performance of high energy density lithium batteries. Detailed Implementation

[0018] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0019] Example 1

[0020] A method for preparing an electrolyte additive to enhance the thermal stability of lithium batteries includes the following steps: S1. Under a nitrogen atmosphere, cyclotriphosphazene containing allyloxy and triazole groups is dissolved in anhydrous acetonitrile in 5 times its mass. 1-Vinylimidazole and triethylamine, an acid-binding agent, are added. The molar ratio of 1-vinylimidazole to cyclotriphosphazene containing allyloxy and triazole groups is 1:1, and the molar ratio of triethylamine to 1-vinylimidazole is 1:1. The mixture is heated to 50°C and reacted for 8 hours. After the reaction is completed, it is naturally cooled to 25°C and then placed in an ice-water bath to cool to 0°C. Anhydrous ethanol containing 2 times its mass of cyclotriphosphazene containing allyloxy and triazole groups is added. The mixture is stirred at 0°C for 0.5 hours. Then the ice-water bath is removed, and the mixture is naturally heated to 25°C and stirred for another 0.5 hours. S2. Filtration to remove insoluble matter and collect the filtrate. The solvent in the filtrate is recovered by vacuum distillation. The residue is washed with ethyl acetate, and after solid-liquid separation, it is dried under vacuum at 40°C for 6 hours to obtain an electrolyte additive that enhances the thermal stability of lithium batteries.

[0021] In this embodiment, the cyclotriphosphazene containing allyloxy and triazole groups was prepared by the following method: cyclotriphosphazene containing triazole and hydroxyl groups was fed with allyl bromide at a molar ratio of 1:1.5, potassium carbonate and N,N-dimethylformamide were added, wherein the molar ratio of potassium carbonate to cyclotriphosphazene containing triazole and hydroxyl groups was 1.0:1, and the amount of N,N-dimethylformamide was 5 times the mass of cyclotriphosphazene containing triazole and hydroxyl groups. The reaction was carried out at 20°C for 4 hours, and after the reaction, the mixture was washed with water and dried to obtain the cyclotriphosphazene containing allyloxy and triazole groups.

[0022] In this embodiment, the cyclotriphosphazene containing triazole and hydroxyl groups was prepared by the following method: Step 1: In a nitrogen atmosphere, hexachlorocyclotriphosphazene and sodium azide are dissolved in anhydrous acetonitrile at a molar ratio of 1:1.0. The amount of anhydrous acetonitrile is 5 times the mass of hexachlorocyclotriphosphazene. The reaction is carried out at 0°C for 2 hours. After the reaction is completed, the mixture is extracted and dried to obtain an azide-containing cyclotriphosphazene intermediate. Step 2: Dissolve the cyclotriphosphazene intermediate containing an azide group and propargyl alcohol in tetrahydrofuran at a molar ratio of 1:1.0. React at 25°C for 6 hours in the presence of cuprous iodide and potassium carbonate. After the reaction is complete, filter, concentrate, and wash to obtain cyclotriphosphazene containing triazole and hydroxyl groups. The molar ratio of cuprous iodide to the cyclotriphosphazene intermediate containing an azide group is 0.05:1, the molar ratio of potassium carbonate to the cyclotriphosphazene intermediate containing an azide group is 1.0:1, and the amount of tetrahydrofuran used is 3 times the mass of the cyclotriphosphazene intermediate containing an azide group.

[0023] Example 2

[0024] A method for preparing an electrolyte additive to enhance the thermal stability of lithium batteries includes the following steps: S1. Under a nitrogen atmosphere, cyclotriphosphazene containing allyloxy and triazole groups is dissolved in 12 times its mass of anhydrous acetonitrile. 1-Vinylimidazole and triethylamine, an acid-binding agent, are added. The molar ratio of 1-vinylimidazole to cyclotriphosphazene containing allyloxy and triazole groups is 2.5:1, and the molar ratio of triethylamine to 1-vinylimidazole is 1.5:1. The mixture is heated to 65°C and reacted for 12 hours. After the reaction is completed, it is naturally cooled to 25°C and then placed in an ice-water bath to cool to 5°C. Anhydrous ethanol containing 6 times its mass of cyclotriphosphazene containing allyloxy and triazole groups is added. The mixture is stirred at 5°C for 0.75 hours. Then the ice-water bath is removed, and the mixture is naturally heated to 25°C and stirred for another 0.75 hours. S2. Filtration to remove insoluble matter and collect the filtrate. The solvent in the filtrate is recovered by vacuum distillation. The residue is washed with ethyl acetate, and after solid-liquid separation, it is dried under vacuum at 50°C for 7 hours to obtain an electrolyte additive that enhances the thermal stability of lithium batteries.

[0025] In this embodiment, the cyclotriphosphazene containing allyloxy and triazole groups was prepared by the following method: cyclotriphosphazene containing triazole and hydroxyl groups was fed with allyl bromide at a molar ratio of 1:2.2, potassium carbonate and N,N-dimethylformamide were added, wherein the molar ratio of potassium carbonate to cyclotriphosphazene containing triazole and hydroxyl groups was 1.5:1, and the amount of N,N-dimethylformamide was 12 times the mass of cyclotriphosphazene containing triazole and hydroxyl groups. The reaction was carried out at 40°C for 8 hours, and after the reaction, the mixture was washed with water and dried to obtain the cyclotriphosphazene containing allyloxy and triazole groups.

[0026] In this embodiment, the cyclotriphosphazene containing triazole and hydroxyl groups was prepared by the following method: Step 1: In a nitrogen atmosphere, hexachlorocyclotriphosphazene and sodium azide are dissolved in anhydrous acetonitrile at a molar ratio of 1:1.5. The amount of anhydrous acetonitrile is 12 times the mass of hexachlorocyclotriphosphazene. The reaction is carried out at 5°C for 4 hours. After the reaction is completed, the intermediate containing azide group is obtained by extraction and drying. Step 2: The cyclotriphosphazene intermediate containing an azide group and propargyl alcohol were dissolved in tetrahydrofuran at a molar ratio of 1:1.8. The mixture was reacted at 38°C for 15 hours in the presence of cuprous iodide and potassium carbonate. After the reaction was completed, the mixture was filtered, concentrated, and washed to obtain cyclotriphosphazene containing triazole and hydroxyl groups. The molar ratio of cuprous iodide to the cyclotriphosphazene intermediate containing an azide group was 0.12:1, the molar ratio of potassium carbonate to the cyclotriphosphazene intermediate containing an azide group was 2.0:1, and the amount of tetrahydrofuran used was 9 times the mass of the cyclotriphosphazene intermediate containing an azide group.

[0027] Example 3

[0028] A method for preparing an electrolyte additive to enhance the thermal stability of lithium batteries includes the following steps: S1. Under a nitrogen atmosphere, cyclotriphosphazene containing allyloxy and triazole groups is dissolved in anhydrous acetonitrile in 20 times its mass. 1-vinylimidazole and triethylamine, an acid-binding agent, are added. The molar ratio of 1-vinylimidazole to cyclotriphosphazene containing allyloxy and triazole groups is 4:1, and the molar ratio of triethylamine to 1-vinylimidazole is 2:1. The mixture is heated to 80°C and reacted for 16 hours. After the reaction is completed, it is naturally cooled to 25°C and then placed in an ice-water bath to cool to 10°C. Anhydrous ethanol containing 10 times its mass of cyclotriphosphazene containing allyloxy and triazole groups is added. The mixture is stirred at 10°C for 1 hour. Then the ice-water bath is removed, and the mixture is naturally heated to 25°C and stirred for another hour. S2. Filtration to remove insoluble matter and collect the filtrate. The solvent in the filtrate is recovered by vacuum distillation. The residue is washed with ethyl acetate, and after solid-liquid separation, it is dried under vacuum at 60°C for 8 hours to obtain an electrolyte additive that enhances the thermal stability of lithium batteries.

[0029] In this embodiment, the cyclotriphosphazene containing allyloxy and triazole groups was prepared by the following method: cyclotriphosphazene containing triazole and hydroxyl groups was fed with allyl bromide at a molar ratio of 1:3.0, potassium carbonate and N,N-dimethylformamide were added, wherein the molar ratio of potassium carbonate to cyclotriphosphazene containing triazole and hydroxyl groups was 2.0:1, and the amount of N,N-dimethylformamide was 20 times the mass of cyclotriphosphazene containing triazole and hydroxyl groups. The reaction was carried out at 60°C for 12 hours, and after the reaction, the mixture was washed with water and dried to obtain the cyclotriphosphazene containing allyloxy and triazole groups.

[0030] In this embodiment, the cyclotriphosphazene containing triazole and hydroxyl groups was prepared by the following method: Step 1: In a nitrogen atmosphere, hexachlorocyclotriphosphazene and sodium azide are dissolved in anhydrous acetonitrile at a molar ratio of 1:2.0. The amount of anhydrous acetonitrile is 20 times the mass of hexachlorocyclotriphosphazene. The reaction is carried out at 10°C for 6 hours. After the reaction is completed, the intermediate containing azide group is obtained by extraction and drying. Step 2: The cyclotriphosphazene intermediate containing an azide group and propargyl alcohol are dissolved in tetrahydrofuran at a molar ratio of 1:2.5. The reaction is carried out at 50°C for 24 hours in the presence of cuprous iodide and potassium carbonate. After the reaction is completed, the mixture is filtered, concentrated, and washed to obtain cyclotriphosphazene containing triazole and hydroxyl groups. The molar ratio of cuprous iodide to the cyclotriphosphazene intermediate containing an azide group is 0.20:1, the molar ratio of potassium carbonate to the cyclotriphosphazene intermediate containing an azide group is 3.0:1, and the amount of tetrahydrofuran used is 15 times the mass of the cyclotriphosphazene intermediate containing an azide group.

[0031] Comparative Example 1 The difference from Example 1 is that in this comparative example, hexachlorocyclotriphosphazene is directly used as an electrolyte additive.

[0032] Comparative Example 2 The difference from Example 1 is that in this comparative example, only cyclotriphosphazenes containing triazole and hydroxyl groups were synthesized and used as electrolyte additives.

[0033] Comparative Example 3 The difference from Example 1 is that in this comparative example, only cyclotriphosphazenes containing allyloxy and triazole groups were synthesized and used as electrolyte additives.

[0034] Performance testing: The electrolyte additives obtained in Examples 1-3 and Comparative Examples 1-3 were added to the base electrolyte (1M LiPF6 dissolved in a 1:1 volume ratio mixture of ethylene carbonate EC and dimethyl carbonate DMC) at a dosage of 3wt%, respectively. After stirring until completely dissolved, the electrolytes were assembled into batteries, and the following performance tests were performed. The test results are shown in Table 1. 1. Thermal runaway initiation temperature: The temperature at which the battery self-heating rate continuously reaches or exceeds 0.02℃ / min is recorded as the thermal runaway initiation temperature. 2. Cycle capacity retention rate: Referring to the cycle life test method in GB / T 36276-2023 "Lithium-ion Batteries for Power Storage", under the condition of 25℃±2℃, charge at a constant current and constant voltage of 1C to the upper limit cutoff voltage and the constant voltage cutoff current is 0.05C, and then discharge at a constant current of 1C to the lower limit cutoff voltage. Repeat the charge and discharge cycle 500 times, and calculate the capacity retention rate as the percentage of the capacity of the 500th discharge cycle to the capacity of the first discharge cycle. 3. Self-extinguishing time: The electrolyte self-extinguishing time SET method was used. Glass fiber filter paper was taken, and 0.5g of the electrolyte to be tested was added to fully wet it. After fixing it horizontally, the flame was applied to the electrolyte for 30s. After removing the flame, the self-extinguishing time of the electrolyte flame was recorded. The test was repeated 3 times and the average value was taken.

[0035] Table 1

[0036] As can be seen from the test results in Table 1, the electrolyte additives prepared in Examples 1-3 of this invention are significantly superior to the comparative examples in terms of thermal runaway initiation temperature, cycle capacity retention, and self-extinguishing time. Comparative Example 1 contains only a hexachlorocyclotriphosphazene skeleton, which cannot effectively inhibit electrolyte decomposition and stabilize the electrode interface, resulting in poor performance in all aspects. The residual hydroxyl groups in Comparative Example 2 are prone to side reactions in the electrolyte, leading to a low cycle retention and limited improvement in thermal stability. Although Comparative Example 3 contains allyloxy and double bonds, it lacks vinylimidazolium groups and cannot form a synergistic coordination effect with triazole. Its interface stability and flame retardant effect are inferior to those of the examples of this invention. This indicates that by sequentially introducing triazole and vinylimidazolium groups into the cyclotriphosphazene skeleton, the additives obtained in this invention can effectively improve the thermal stability and flame retardant performance of lithium batteries, while also improving long-term cycle stability.

[0037] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0038] Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the present invention.

Claims

1. A method for preparing an electrolyte additive to enhance the thermal stability of lithium batteries, characterized in that, Includes the following steps: S1. Under a nitrogen atmosphere, cyclotriphosphazene containing allyloxy and triazole groups is dissolved in an anhydrous solvent, 1-vinylimidazolium and a binding agent are added, and the temperature is raised to 50~80℃ and reacted for 8~16h. After the reaction is completed, it is naturally cooled to 25℃, and then placed in an ice-water bath to cool to 0~10℃. Anhydrous lower alcohol is added, and the temperature is controlled at 0~10℃ and stirred for 0.5~1h. Then the ice-water bath is removed, and the temperature is naturally raised to 25℃ and stirred for another 0.5~1h. S2. Filtration to remove insoluble matter and collect the filtrate. The solvent in the filtrate is recovered by vacuum distillation. The residue is washed with ethyl acetate, and after solid-liquid separation, it is vacuum dried at 40~60℃ for 6~8h to obtain an electrolyte additive that enhances the thermal stability of lithium batteries.

2. The method for preparing the electrolyte additive for enhancing the thermal stability of lithium batteries according to claim 1, characterized in that, The cyclotriphosphazene containing allyloxy and triazole groups is prepared by the following method: cyclotriphosphazene containing triazole and hydroxyl groups is fed to an allyl halide at a molar ratio of 1:(1.5~3.0), potassium carbonate and solvent are added, wherein the molar ratio of potassium carbonate to cyclotriphosphazene containing triazole and hydroxyl groups is (1.0~2.0):1, and the amount of solvent is 5~20 times the mass of cyclotriphosphazene containing triazole and hydroxyl groups. The reaction is carried out at 20~60℃ for 4~12h, and after the reaction, the mixture is washed with water and dried to obtain cyclotriphosphazene containing allyloxy and triazole groups.

3. The method for preparing the electrolyte additive for enhancing the thermal stability of lithium batteries according to claim 2, characterized in that, The allyl halide is selected from allyl bromide and allyl chloride; the solvent is selected from N,N-dimethylformamide and acetone.

4. The method for preparing the electrolyte additive for enhancing the thermal stability of lithium batteries according to claim 2, characterized in that, The cyclotriphosphazene containing triazole and hydroxyl groups is prepared by the following method: Step 1: In a nitrogen atmosphere, hexachlorocyclotriphosphazene and sodium azide are dissolved in a polar solvent at a molar ratio of 1:(1.0~2.0) and reacted at 0~10℃ for 2~6h. After the reaction is completed, the intermediate containing azide group is obtained by extraction and drying. Step 2: Dissolve the cyclotriphosphazene intermediate containing an azide group and propargyl alcohol in an organic solvent, and react at 25-50°C for 6-24 hours in the presence of a monovalent copper catalyst and an inorganic base. After the reaction is completed, filter, concentrate, and wash to obtain cyclotriphosphazene containing triazole and hydroxyl groups.

5. The method for preparing the electrolyte additive for enhancing the thermal stability of lithium batteries according to claim 4, characterized in that, In step one, the polar solvent is selected from any one of anhydrous acetonitrile, anhydrous N,N-dimethylformamide, and anhydrous dimethyl sulfoxide, and the amount of polar solvent used is 5 to 20 times the mass of hexachlorocyclotriphosphazene.

6. The method for preparing the electrolyte additive for enhancing the thermal stability of lithium batteries according to claim 4, characterized in that, In step two, the monovalent copper catalyst is selected from either cuprous iodide or cuprous bromide, and the molar ratio of the monovalent copper catalyst to the cyclotriphosphazene intermediate containing an azide group is (0.05~0.20):1; the inorganic base is selected from either potassium carbonate or sodium bicarbonate, and the molar ratio of the inorganic base to the cyclotriphosphazene intermediate containing an azide group is (1.0~3.0):1; the organic solvent is selected from either tetrahydrofuran or acetonitrile, and the amount of organic solvent used is 3~15 times the mass of the cyclotriphosphazene intermediate containing an azide group.

7. The method for preparing the electrolyte additive for enhancing the thermal stability of lithium batteries according to claim 4, characterized in that, In step two, the molar ratio of the azido-containing cyclotriphosphazene intermediate to propargyl alcohol is 1:(1.0~2.5).

8. The method for preparing the electrolyte additive for enhancing the thermal stability of lithium batteries according to claim 1, characterized in that, In step S1, the molar ratio of 1-vinylimidazole to the cyclotriphosphazene containing allyloxy and triazole groups is (1~4):1; the acid-binding agent is selected from any one of triethylamine, potassium carbonate, and cesium carbonate, and the molar ratio of the acid-binding agent to 1-vinylimidazole is (1~2):

1.

9. The method for preparing the electrolyte additive for enhancing the thermal stability of lithium batteries according to claim 1, characterized in that, In step S1, the anhydrous solvent is selected from any one of anhydrous acetonitrile, anhydrous tetrahydrofuran, and anhydrous N,N-dimethylformamide, and the amount of anhydrous solvent used is 5 to 20 times the mass of the cyclotriphosphazene containing allyloxy and triazole groups; the anhydrous lower alcohol is selected from any one of anhydrous methanol and anhydrous ethanol, and the amount of anhydrous lower alcohol used is 2 to 10 times the mass of the cyclotriphosphazene containing allyloxy and triazole groups.

10. An electrolyte additive for enhancing the thermal stability of lithium batteries, characterized in that, Prepared by the preparation method according to any one of claims 1-9.

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