A safety flame-retardant electrolyte and its preparation method

Abstract

The present invention relates to a safe flame-retardant electrolyte and a preparation method thereof, belonging to the technical field of lithium batteries. The electrolyte includes: 11.2 - 13.8 wt% of a lithium salt, 5.5 - 7.0 wt% of a modified phosphazene flame retardant, 2.6 - 3.2 wt% of an auxiliary additive, and the balance is a composite solvent; wherein, the modified phosphazene flame retardant uses tetrafluorobutylene glycol as a raw material, is treated with sodium hydride to synthesize a fluoride with an alkoxide structure, reacts with a hexachlorocyclotriphosphazene matrix, grafts a fluorine-containing group onto the hexachlorocyclotriphosphazene, and then the end is modified with dimethylethenylchlorosilane to introduce an organosilicon and a double bond structure, endowing the electrolyte with good flame retardancy, and at the same time, a silicon-containing protective film can be formed on the surface of the electrode material, reducing the thermal decomposition of the electrode material and the solvent erosion, so that the lithium battery exhibits excellent high-temperature stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and specifically, relates to a safety flame-retardant electrolyte and a preparation method thereof. Background Art

[0002] Lithium-ion batteries have received extensive attention due to many advantages such as high energy density, long service life, low self-discharge rate, and environmental friendliness, and are widely used in mobile phones, laptops, cameras, electric vehicles, etc. Although lithium-ion batteries have great advantages as energy storage sources, there are safety and technical problems in their application process. Especially during high-rate charging and discharging, side reactions may occur simultaneously with the electrochemical reactions in the battery. These side reactions may generate a large amount of heat and gas, causing the internal temperature and pressure of the battery to continuously rise, and leading to internal self-heating reactions. When a large amount of heat and gas cannot be discharged in time, phenomena such as battery leakage and air leakage may occur. When the internal heat of the battery reaches a certain level, it will cause the electrolyte to burn, resulting in thermal runaway and even battery explosion.

[0003] Existing research shows that adding flame retardants with high flash points to the electrolyte can improve the flame resistance of flammable electrolytes, inhibit their combustion, improve the safety performance of lithium-ion batteries, and enhance the flame retardancy and thermal stability of the electrolyte. Flame retardants used in electrolytes mainly include: organophosphorus flame retardants, nitrogen-containing compound flame retardants, halogenated carbonate flame retardants, phosphazene compound flame retardants, etc. The flame retardancy mechanism of these flame retardants is to generate free radicals containing phosphorus, fluorine, nitrogen, etc., to scavenge hydrogen free radicals generated by combustion reactions, thereby blocking the combustion chain reaction. Generally, with the increase of the addition amount, the flame retardant effect is more obvious, but the impact on the performance of the electrolyte is also greater, thus affecting the performance of the battery. Therefore, based on the research on the flame retardant performance of the electrolyte and the influence of the electrolyte on the battery at high temperatures, the present application develops a safety flame-retardant electrolyte. Summary of the Invention

[0004] In order to solve the technical problems mentioned in the background art, the purpose of the present invention is to provide a safety flame-retardant electrolyte and a preparation method thereof.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A safety flame-retardant electrolyte includes the following components:

[0007] Lithium salt 11.2 - 13.8 wt%, modified phosphazene flame retardant 5.5 - 7.0 wt%, auxiliary additive 2.6 - 3.2 wt%, and the balance is a composite solvent;

[0008] The modified phosphazene flame retardant is prepared by the following method:

[0009] Step A1: Heat tetrafluorobutanediol, perfluoropentanone, and dioxane to stir and mix them. Pass a nitrogen stream for protection, keep the temperature constant at 45 - 52 °C, apply mechanical stirring at 600 - 800 rpm, intermittently add sodium hydride. After complete addition, continue to stir and react while maintaining the temperature. Control the addition reaction time of sodium hydride to be 6 - 8 h. After the reaction, reduce the pressure and rotary evaporate at a temperature not exceeding 50 °C to remove low-boiling substances including dioxane, obtaining a fluorine-modified compound;

[0010] Furthermore, the dosage ratio of tetrafluorobutanediol, sodium hydride, perfluoropentanone, and dioxane is 0.1 mol : 0.1 mol : 35 - 50 mL : 120 - 160 mL. Using dioxane as a dispersant and perfluoropentanone as a protective agent, sodium hydride reacts with the hydroxyl groups in tetrafluorobutanediol to synthesize a compound with an alkoxide structure, improving the chemical activity of the fluoride.

[0011] The specific reaction process can be expressed as follows:

[0012]

[0013] Step A2: Stir and mix the fluorine-modified compound and tetrahydrofuran, keep the temperature constant at 8 - 15 °C, apply mechanical stirring at 120 - 180 rpm, slowly add hexachlorocyclotriphosphazene. After complete addition, continue to stir and react while maintaining the temperature. Control the addition reaction time of hexachlorocyclotriphosphazene to be 3 - 5 h. After the reaction, rotary evaporate to remove tetrahydrofuran, obtaining a fluorinated phosphazene flame retardant matrix;

[0014] Furthermore, the dosage ratio of hexachlorocyclotriphosphazene, fluorine-modified compound, and tetrahydrofuran is 10 mmol : 0.08 - 0.1 mol : 460 - 540 mL. The basic alkoxide structure in the fluorine-modified compound molecule reacts with hexachlorocyclotriphosphazene to graft a fluorine-containing group onto hexachlorocyclotriphosphazene.

[0015] The specific reaction process can be expressed as follows:

[0016]

[0017] Step A3: Stir and mix the fluorinated phosphazene flame retardant matrix, dimethylethenylchlorosilane, dimethylimidazolidinone, and triethylamine, heat to 65 - 75 °C, and assist with mechanical stirring at 180 - 240 rpm for 1.5 - 2 h. After the reaction, reduce the pressure and rotary evaporate to remove excess low-boiling substances such as dimethylethenylchlorosilane and triethylamine. Repeatedly wash the rotary evaporation substrate with deionized water and remove the aqueous phase, then dry to obtain a modified phosphazene flame retardant;

[0018] Further, the dosage ratio of the phosphazene fluoride flame retardant matrix, dimethylethenylchlorosilane, dimethylimidazolidinone, and triethylamine is 10 mmol: 65 - 70 mmol: 150 - 200 mL: 5 - 8 mL. Triethylamine is used as an acid-binding agent. The chlorine-containing group in dimethylethenylchlorosilane reacts with the terminal hydroxyl group of the phosphazene fluoride flame retardant matrix, introducing an organosilicon structure and a double bond into the phosphazene fluoride flame retardant matrix.

[0019] The specific reaction process can be expressed as follows:

[0020]

[0021] Further, the lithium salt is selected from lithium hexafluorophosphate, and the composite solvent is composed of a mixture of diethyl carbonate, ethylene carbonate, propylene carbonate, and ethyl methyl carbonate.

[0022] Further, the auxiliary additive is composed of a mixture of vinylene carbonate, ethylene sulfate, 1,3 - propanesultone, and 12 - crown - 4.

[0023] A preparation method of a safety flame - retardant electrolyte specifically includes the following steps:

[0024] Step S1: Vacuum - dry the lithium salt, modified phosphazene flame retardant, auxiliary additive, and composite solvent respectively until the water content ≤ 10 ppm, and set aside.

[0025] Step S2: Weigh the dried raw materials according to the weight ratio, and mix them under the protection of dry and inert gas to obtain the safety flame - retardant electrolyte.

[0026] The beneficial effects of the present invention:

[0027] The present invention prepares a modified phosphazene flame retardant and applies it to the lithium - battery electrolyte, obtaining good flame - retardant performance and high - temperature cycling performance. The modified phosphazene flame retardant uses tetrafluorobutylene glycol as a raw material, which is treated with sodium hydride to synthesize a fluoride with an alkoxide structure, improving the chemical activity of the fluoride. Then it reacts with a hexachlorocyclotriphosphazene matrix to graft a fluorine - containing group onto the hexachlorocyclotriphosphazene, and then the end is modified by dimethylethenylchlorosilane to introduce an organosilicon and double - bond structure, which is beneficial to forming a silicon - containing protective film on the surface of the electrode material, reducing the thermal decomposition of the electrode material and the erosion of the solvent. After testing, the SET value of the prepared electrolyte is 9.74 - 11.58 s / g, and the COI value is 24.1 - 25.2%, showing excellent flame - retardant characteristics. The lithium battery made can cycle 200 times at 70 °C and 1C rate, with a charging specific capacity of more than 97 mAh / g and a Coulomb efficiency maintained above 68%, showing excellent high - temperature stability. Specific embodiments

[0028] Example 1

[0029] In this embodiment, a safe and flame-retardant electrolyte is prepared, and the specific implementation process is as follows:

[0030] 1) Prepare a modified phosphazene flame retardant

[0031] 1.1. Take perfluoromethyl hexanone and dioxane, add them to the reactor and mix well. Then add 2,2,3,3-tetrafluorobutanediol, apply mechanical stirring and heat up until a homogeneous mixture is formed. Use nitrogen to expel the air in the reactor, maintain a stable nitrogen flow for protection, keep the temperature constant at 45 °C, and apply mechanical stirring at a medium speed of 600 rpm. Add sodium hydride in 8 batches at intervals of 20 minutes each. After the complete addition of sodium hydride, keep stirring and reacting under insulation. Control the addition reaction time of sodium hydride to be 8 hours. Among them, the dosage ratio of 2,2,3,3-tetrafluorobutanediol, sodium hydride, perfluoromethyl hexanone and dioxane is 0.1 mol: 0.1 mol: 35 mL: 120 mL. After the reaction is completed, reduce the pressure to below 100 Pa, and perform rotary evaporation at 50 °C to remove dioxane, perfluoromethyl hexanone and other low-boiling substances, and obtain a fluorine-modified compound.

[0032] 1.2. Take the fluorine-modified compound and tetrahydrofuran, add them to the reactor and stir evenly. Use an ice-water bath to keep the temperature constant at 8 °C, apply mechanical stirring at 120 rpm, and slowly add hexachlorocyclotriphosphazene within 2.5 hours. After the complete addition, continue to stir and react under constant temperature. Control the addition reaction time of hexachlorocyclotriphosphazene to be 5 hours. Among them, the dosage ratio of hexachlorocyclotriphosphazene, fluorine-modified compound and tetrahydrofuran is 10 mmol: 0.08 mol: 460 mL. After the reaction is completed, perform rotary evaporation to remove tetrahydrofuran, and obtain a fluorinated phosphazene flame retardant matrix.

[0033] 1.3. Take the fluorinated phosphazene flame retardant matrix, dimethylvinylchlorosilane, dimethylimidazolidinone and triethylamine, add them to the reactor and stir to mix. Heat up to 65 °C, apply mechanical stirring at 180 rpm, and stir and react under constant temperature for 2 hours. Among them, the dosage ratio of fluorinated phosphazene flame retardant matrix, dimethylvinylchlorosilane, dimethylimidazolidinone and triethylamine is 10 mmol: 65 mmol: 150 mL: 5 mL. After the reaction is completed, perform reduced-pressure rotary evaporation to remove excessive low-boiling substances such as dimethylvinylchlorosilane and triethylamine. Add deionized water with the same mass as the rotary evaporation substrate to wash and remove the aqueous phase, repeat 2 times, and perform vacuum drying to remove water, and obtain a modified phosphazene flame retardant.

[0034] 2) Prepare a safe and flame-retardant electrolyte

[0035] 2.1. Take the following raw materials according to the weight ratio:

[0036] Lithium salt 11.2 wt%, selected from lithium hexafluorophosphate, battery-grade raw material;

[0037] Modified phosphazene flame retardant 6.0 wt%, prepared in this embodiment;

[0038] Auxiliary additive: 3.2 wt%, which is composed of vinylene carbonate, ethylene sulfate, 1,3 - propylene sultone and 12 - crown - 4, mixed in a weight ratio of 1:1.2:0.3:0.5.

[0039] Composite solvent: 79.6 wt%, which is composed of diethyl carbonate, ethylene carbonate, propylene carbonate and ethyl methyl carbonate, mixed in a weight ratio of 1:0.8:1.1:2.5.

[0040] Each raw material is vacuum - dried until the moisture content ≤ 10 ppm.

[0041] 2.2. Under the protection of high - purity argon and at a temperature of 70 °C, the dried raw materials are mixed according to the weight ratio and stirred for 3 h to obtain a safe and flame - retardant electrolyte.

[0042] Example 2

[0043] The specific implementation process for preparing the safe and flame - retardant electrolyte in this example is as follows:

[0044] 1) Preparation of modified phosphazene flame retardant

[0045] 1.1. Take perfluoropentanone and dioxane and mix them by feeding. Add 2,2,3,3 - tetrafluorobutan - 1 - ol, apply mechanical stirring and heat up until a homogeneous mixture is formed. Use nitrogen to expel the air in the reactor, maintain a stable nitrogen flow for protection, keep the temperature constant at 52 °C, apply mechanical stirring at a medium speed of 800 rpm, add sodium hydride in 5 batches at intervals of 10 min each. After the complete addition of sodium hydride, keep stirring and reacting under insulation. Control the addition reaction time of sodium hydride to be 6 h. Among them, the dosage ratio of 2,2,3,3 - tetrafluorobutan - 1 - ol, sodium hydride, perfluoropentanone and dioxane is 0.1 mol:0.1 mol:50 mL:160 mL. After the reaction ends, reduce the pressure to below 100 Pa, and perform rotary evaporation at 50 °C to remove dioxane, perfluoropentanone and other low - boiling substances to obtain a fluorine - modified compound.

[0046] 1.2. Take the fluorine - modified compound and tetrahydrofuran, stir and mix them evenly. Use an ice - water bath to keep the temperature constant at 15 °C, apply mechanical stirring at 180 rpm, and slowly add hexachlorocyclotriphosphazene within 1.5 h. After the complete addition, continue to stir and react at a constant temperature. Control the addition reaction time of hexachlorocyclotriphosphazene to be 3 h. Among them, the dosage ratio of hexachlorocyclotriphosphazene, fluorine - modified compound and tetrahydrofuran is 10 mmol:0.1 mol:540 mL. After the reaction ends, perform rotary evaporation to remove tetrahydrofuran to obtain a fluorinated phosphazene flame - retardant matrix.

[0047] 1.3. Weigh the phosphazene fluoride flame retardant matrix, dimethylvinylchlorosilane, dimethylimidazolidinone and triethylamine, add them to a reactor and stir to mix. Heat the mixture to 75 °C, apply mechanical stirring at 240 rpm, and stir at a constant temperature for 1.5 h. The dosage ratio of the phosphazene fluoride flame retardant matrix, dimethylvinylchlorosilane, dimethylimidazolidinone and triethylamine is 10 mmol: 70 mmol: 200 mL: 8 mL. After the reaction is completed, excess low-boiling substances such as dimethylvinylchlorosilane and triethylamine are removed by rotary evaporation under reduced pressure. Deionized water with the same mass as the rotary evaporation substrate is added to the rotary evaporation substrate, and the aqueous phase is removed by washing. This process is repeated 3 times, and then the product is dried under vacuum to remove water, obtaining the modified phosphazene flame retardant.

[0048] 2) Prepare a safety flame retardant electrolyte

[0049] 2.1. Weigh the following raw materials according to the weight ratio:

[0050] 12.5 wt% of lithium salt, selected from lithium hexafluorophosphate, battery-grade raw material;

[0051] 5.5 wt% of the modified phosphazene flame retardant, prepared in this example;

[0052] 2.9 wt% of auxiliary additives, which are mixed by vinylene carbonate, ethylene sulfate, 1,3 - propanesultone and 12 - crown - 4 in a weight ratio of 1:1.2:0.3:0.5.

[0053] 79.1 wt% of composite solvent, which is mixed by diethyl carbonate, ethylene carbonate, propylene carbonate and ethyl methyl carbonate in a weight ratio of 1:0.8:1.1:2.5.

[0054] Vacuum dry each raw material until the water content ≤ 10 ppm.

[0055] 2.2. Under the protection of high-purity argon and at a temperature of 70 °C, mix the dried raw materials according to the weight ratio and stir for 3 h to obtain the safety flame retardant electrolyte.

[0056] Example 3

[0057] In this example, a safety flame retardant electrolyte is prepared. The specific implementation process is as follows:

[0058] 1) Prepare the modified phosphazene flame retardant

[0059] 1.1. Take perfluorohexanone and dioxane, feed and mix them, add tetrafluorobutanediol, apply mechanical stirring and heat up until it is stirred into a homogeneous mixture. Use nitrogen to expel the air in the reactor, maintain a stable nitrogen flow for protection, keep the temperature constant at 48 °C, apply medium-speed mechanical stirring at 720 rpm, and add sodium hydride in 6 batches intermittently, with a 20-minute interval between each addition. After the complete addition of sodium hydride, keep stirring and reacting while maintaining the temperature, and control the addition reaction time of sodium hydride to be 7 h. Among them, the dosage ratio of tetrafluorobutanediol, sodium hydride, perfluorohexanone and dioxane is 0.1 mol: 0.1 mol: 50 mL: 140 mL. After the reaction is completed, reduce the pressure to below 100 Pa, and perform rotary evaporation at 50 °C to remove dioxane, perfluorohexanone and other low-boiling substances, and obtain a fluorine-modified compound.

[0060] 1.2. Take the fluorine-modified compound and tetrahydrofuran, feed and stir them evenly, keep the temperature constant using an ice-water bath, control the temperature at 12 °C, apply mechanical stirring at 180 rpm, and slowly add hexachlorocyclotriphosphazene within 2.2 h. After the complete addition, continue to stir and react while maintaining the temperature, and control the addition reaction time of hexachlorocyclotriphosphazene to be 4.5 h. Among them, the dosage ratio of hexachlorocyclotriphosphazene, fluorine-modified compound and tetrahydrofuran is 10 mmol: 0.09 mol: 520 mL. After the reaction is completed, perform rotary evaporation to remove tetrahydrofuran, and obtain a fluorinated phosphazene flame retardant matrix.

[0061] 1.3. Take the fluorinated phosphazene flame retardant matrix, dimethylvinylchlorosilane, dimethylimidazolidinone and triethylamine, feed and stir them to mix, heat up to 72 °C, apply mechanical stirring at 240 rpm, and stir and react at a constant temperature for 1.8 h. Among them, the dosage ratio of fluorinated phosphazene flame retardant matrix, dimethylvinylchlorosilane, dimethylimidazolidinone and triethylamine is 10 mmol: 70 mmol: 180 mL: 7 mL. After the reaction is completed, perform rotary evaporation under reduced pressure to remove excess low-boiling substances such as dimethylvinylchlorosilane and triethylamine. Add deionized water of equal mass to the rotary evaporation substrate, wash and remove the aqueous phase, repeat 3 times, and perform vacuum drying to remove water to obtain a modified phosphazene flame retardant.

[0062] 2) Prepare a safety flame retardant electrolyte

[0063] 2.1. Take the following raw materials according to the weight ratio:

[0064] Lithium salt 12.0 wt%, selected from lithium hexafluorophosphate, battery-grade raw material;

[0065] Modified phosphazene flame retardant 7.0 wt%, prepared in this example;

[0066] Auxiliary additive 2.7 wt%, composed of vinylene carbonate, ethylene sulfate, 1,3-propane sultone and 12-crown-4 mixed according to the weight ratio of 1:1.2:0.3:0.5.

[0067] Composite solvent: 78.3 wt%, which is a mixture of diethyl carbonate, ethylene carbonate, propylene carbonate and ethyl methyl carbonate in a weight ratio of 1:0.8:1.1:2.5.

[0068] Respectively vacuum dry each raw material until the water content ≤ 10 ppm.

[0069] 2.2. Under the protection of high-purity argon and at a temperature of 70 °C, mix the dried raw materials according to the weight ratio and stir for 3 h to obtain a safe and flame-retardant electrolyte.

[0070] Example 4

[0071] The specific implementation process for preparing the safe and flame-retardant electrolyte in this example is as follows:

[0072] 1) Prepare the modified phosphazene flame retardant

[0073] 1.1. Take perfluoropentanone and dioxane and feed them into the reactor for mixing. Add tetrafluorobutanediol, apply mechanical stirring and heat up until a homogeneous mixture is formed. Use nitrogen to expel the air in the reactor, maintain a stable nitrogen flow for protection, keep the temperature constant at 50 °C, apply mechanical stirring at a medium speed of 800 rpm, and add sodium hydride in 8 batches at intervals of 15 min each. After the complete addition of sodium hydride, keep stirring and reacting under insulation. Control the addition reaction time of sodium hydride to be 7.5 h. Among them, the dosage ratio of tetrafluorobutanediol, sodium hydride, perfluoropentanone and dioxane is 0.1 mol:0.1 mol:40 mL:150 mL. After the reaction is completed, reduce the pressure to below 100 Pa, and perform rotary evaporation at 50 °C to remove dioxane, perfluoropentanone and other low-boiling components to obtain a fluorine-modified compound.

[0074] 1.2. Take the fluorine-modified compound and tetrahydrofuran and feed them into the reactor for stirring and mixing. Keep the temperature constant using an ice-water bath at 10 °C, apply mechanical stirring at 180 rpm, and slowly add hexachlorocyclotriphosphazene within 2.5 h. After the complete addition, continue to stir and react under constant temperature. Control the addition reaction time of hexachlorocyclotriphosphazene to be 4.2 h. Among them, the dosage ratio of hexachlorocyclotriphosphazene, fluorine-modified compound and tetrahydrofuran is 10 mmol:0.1 mol:500 mL. After the reaction is completed, perform rotary evaporation to remove tetrahydrofuran to obtain a fluorinated phosphazene flame retardant matrix.

[0075] 1.3. Take the phosphazene fluoride flame retardant matrix, dimethylvinylchlorosilane, dimethylimidazolidinone and triethylamine, feed and stir them to mix, heat up to 65 °C, apply mechanical stirring at 240 rpm, and stir and react at a constant temperature for 2 h. Among them, the dosage ratio of the phosphazene fluoride flame retardant matrix, dimethylvinylchlorosilane, dimethylimidazolidinone and triethylamine is 10 mmol: 70 mmol: 180 mL: 8 mL. After the reaction, rotate and evaporate under reduced pressure to remove low-boiling substances such as excessive dimethylvinylchlorosilane and triethylamine. Add deionized water of equal mass to the rotary evaporation substrate, wash and remove the aqueous phase, repeat 3 times, and dry under vacuum to remove water to obtain the modified phosphazene flame retardant.

[0076] 2) Prepare a safety flame retardant electrolyte

[0077] 2.1. Take the following raw materials according to the weight ratio:

[0078] Lithium salt 13.8 wt%, selected from lithium hexafluorophosphate, battery-grade raw materials;

[0079] Modified phosphazene flame retardant 6.5 wt%, prepared in this example;

[0080] Auxiliary additive 2.6 wt%, composed of vinylene carbonate, ethylene sulfate, 1,3-propane sultone and 12-crown ether-4 mixed according to the weight ratio of 1:1.2:0.3:0.5.

[0081] Composite solvent 77.1 wt%, composed of diethyl carbonate, ethylene carbonate, propylene carbonate and ethyl methyl carbonate mixed according to the weight ratio of 1:0.8:1.1:2.5.

[0082] Vacuum dry each raw material until the water content ≤ 10 ppm.

[0083] 2.2. Under the protection of high-purity argon and at a temperature of 70 °C, mix the dried raw materials according to the weight ratio and stir for 3 h to obtain a safety flame retardant electrolyte.

[0084] Comparative example

[0085] The preparation process of this comparative example is the same as that of Example 3, and the modified phosphazene flame retardant is replaced by 6.2 wt% of ethoxy(pentafluoro)cyclotriphosphazene.

[0086] Take the safety flame retardant electrolytes prepared in Examples 1-4 and the comparative example as samples, and conduct the following flame retardant tests respectively:

[0087] Self-extinguishing time test: Make a cotton ball with a diameter of 8 mm from glass fiber cotton, immerse it in the sample and let it stand for 10 s, take it out and drain it, measure the liquid absorption mass of the cotton ball, ignite the cotton ball after soaking with a spray gun, record the extinguishing time of the cotton ball after removing the spray gun, and calculate the SET value.

[0088] Self-propagation rate test: Refer to the UL-94V0 standard to test the combustion propagation ability. Make a cotton swab with a length of 125 cm from glass fiber cotton, immerse the cotton swab in the sample for 10 s, take it out and drain it, ignite one end of the soaked cotton swab with a spray gun and burn for 2.5 cm, remove the spray gun, calculate the burning length and burning time of the cotton swab, and calculate the self-propagation rate.

[0089] Oxygen index test: Refer to the GB2406.2-2009 standard, and use an oxygen index combustion tester to test the oxygen index of the sample for stable combustion within 60 s.

[0090] The specific test data are shown in Table 1:

[0091] Table 1

[0092]

[0093]

[0094] It can be seen from the data in Table 1 that the SET value of the electrolyte prepared by the present invention is 9.74 - 11.58 s / g, the COI value is 24.1 - 25.2%, showing excellent flame retardant properties, and the self-propagation rate is only 12 - 18 mm / min, and the combustion is not easy to spread.

[0095] Apply the above-prepared electrolyte to a lithium iron phosphate battery for electrical performance testing, as follows:

[0096] Battery assembly: Mix battery-grade lithium iron phosphate, conductive acetylene black, and polyvinylidene fluoride in amounts of 85 wt%, 6 wt%, and 9 wt% respectively, and prepare a slurry with a solid content of 50% using N-methylpyrrolidone. According to the surface density of 6 mg / cm 2 , scrape and coat it on the surface of the copper foil, place it in a vacuum drying oven at 70 °C for 5 h to make a positive electrode plate. Use a battery-grade lithium sheet as the negative electrode, Celgard 2400 as the separator, and the electrolyte dosage is 50 μL to assemble a CR2025 type button battery;

[0097] Room temperature cycle test: Use a LAND-CT2001A battery test system to conduct constant current charge and discharge tests on the assembled battery as above. The test temperature is 25 °C, the voltage range is 2.5 - 4 V, and the rate is 0.1C. The specific test results are shown in Table 2:

[0098] Table 2

[0099]

[0100]

[0101] As can be seen from the data in Table 2, when the electrolyte prepared by the present invention is applied to a lithium iron phosphate battery, the first charge-discharge test is similar to that of the comparative example. The specific charge capacity of the first charge is slightly lower. After 2000 cycles, the charge specific capacity is higher than that of the comparative example, and the Coulomb efficiency is significantly higher than that of the comparative example.

[0102] High-temperature cycle test: The specific method is the same as that of the normal-temperature cycle test. The test temperature is 70 °C and the rate is 1C. The specific test results are shown in Table 3:

[0103] Table 3

[0104]

[0105] As can be seen from the data in Table 3, when the electrolyte prepared by the present invention is applied to a lithium iron phosphate battery, after 200 cycles at 70 °C, the charge specific capacity can still remain above 97 mAh / g, and the Coulomb efficiency remains above 68%, showing excellent high-temperature stability.

[0106] The above content is only an example and explanation of the present invention. Those skilled in the art of the present technology can make various modifications or supplements or use similar methods to replace the specific embodiments described, as long as they do not deviate from the invention or exceed the scope defined by the claims of the present invention, they should fall within the protection scope of the present invention.

Claims

1. A safety flame-retardant electrolyte, characterized in that, It includes the following components: 11.2 - 13.8 wt% of lithium salt, 5.5 - 7.0 wt% of modified phosphazene flame retardant, 2.6 - 3.2 wt% of auxiliary additive, and the balance is composite solvent; The modified phosphazene flame retardant is prepared by the following method: Step A1: Heat and mix tetrafluorobutylene glycol, perfluoromethyl isopropyl ketone and dioxane, keep the temperature constant at 45 - 52 °C under the protection of nitrogen gas flow, apply mechanical stirring at 600 - 800 rpm, intermittently add sodium hydride, control the addition reaction time of sodium hydride to be 6 - 8 h, and obtain a fluorine-modified compound after the reaction ends under reduced pressure; Step A2: Mix the fluorine-modified compound and tetrahydrofuran, keep the temperature constant at 8 - 15 °C, slowly add hexachlorocyclotriphosphazene under stirring, control the addition reaction time of hexachlorocyclotriphosphazene to be 3 - 5 h, and obtain a fluorinated phosphazene flame retardant matrix after the reaction ends by rotary evaporation; Step A3: Mix the fluorinated phosphazene flame retardant matrix, dimethylvinylchlorosilane, dimethylimidazolidinone and triethylamine, heat up to 65 - 75 °C, stir and react for 1.5 - 2 h, carry out rotary evaporation under reduced pressure after the reaction ends, then wash with deionized water and remove the aqueous phase, and dry to obtain the modified phosphazene flame retardant.

2. The safety flame-retardant electrolyte according to claim 1, wherein, The dosage ratio of tetrafluorobutylene glycol, sodium hydride, perfluoromethyl isopropyl ketone and dioxane is 0.1 mol: 0.1 mol: 35 - 50 mL: 120 - 160 mL.

3. The safety flame-retardant electrolyte according to claim 2, wherein The dosage ratio of hexachlorocyclotriphosphazene, fluorine-modified compound and tetrahydrofuran is 10 mmol: 0.08 - 0.1 mol: 460 - 540 mL.

4. The safety flame-retardant electrolyte according to claim 3, wherein, The dosage ratio of fluorinated phosphazene flame retardant matrix, dimethylvinylchlorosilane, dimethylimidazolidinone and triethylamine is 10 mmol: 65 - 70 mmol: 150 - 200 mL: 5 - 8 mL.

5. The safety flame-retardant electrolyte according to claim 1, wherein The lithium salt is lithium hexafluorophosphate, and the composite solvent is composed of diethyl carbonate, ethylene carbonate, propylene carbonate and ethyl methyl carbonate.

6. The safety flame-retardant electrolyte according to claim 1, characterized in that, The auxiliary additive is composed of vinylene carbonate, ethylene sulfate, 1,3 - propanesultone and 12 - crown - 4.

7. The preparation method of a safety flame-retardant electrolyte according to claim 1, wherein It includes the following steps: Step S1: Vacuum-dry the lithium salt, modified phosphazene flame retardant, auxiliary additive and composite solvent respectively until the water content ≤ 10 ppm, and set aside; Step S2: Weigh the dried raw materials according to the weight ratio, mix them evenly under the protection of dry inert gas to prepare a safety flame-retardant electrolyte.

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