Flame-retardant electrolyte and alkali metal-based battery thereof

Abstract

The present application relates to the technical field of alkali metal-based battery, and particularly relates to a flame-retardant electrolyte and an alkali metal-based battery thereof. The flame-retardant electrolyte provided by the present application comprises a solvent, the solvent comprises 10% to 100% of an ether-based phosphazene solvent in terms of volume percentage, and the ether-based phosphazene solvent has the following general structure formula (I): formula (I); wherein one or more of R1 to R6 is selected from a chain ether group which is substituted or unsubstituted by a halogen atom, a cyano group, an olefin group or an alkyne group, and R7 is selected from a cyclic ether group which is substituted or unsubstituted by a halogen atom, a cyano group, an olefin group or an alkyne group. The flame-retardant electrolyte provided by the present application adopts an ether-based phosphazene solvent with a specific structure, compared with the traditional phosphazene compound which can only be used as an electrolyte additive, the coordination effect of the introduced ether group greatly improves the solubility of the phosphazene solvent to lithium salt, so that the phosphazene solvent can be used as the main solvent of the electrolyte for the flame-retardant electrolyte of a wide-temperature and high-pressure lithium battery.

Description

Technical Field

[0001] This invention relates to the field of alkali metal-based battery technology, and more particularly to a flame-retardant electrolyte and its alkali metal-based battery. Background Technology

[0002] Lithium-ion batteries have distinguished themselves among various battery systems due to their high energy density, long effective lifespan, and environmental friendliness, successfully occupying fields such as consumer electronics, new energy vehicles, and energy storage, becoming an indispensable component in people's lives and industrial production. However, conventional lithium-ion batteries primarily use flammable organic electrolytes based on carbonates, making them prone to combustion and even explosion under conditions of abuse such as high-temperature exposure, partial short circuits, and mechanical impact. In recent years, there have been numerous incidents of spontaneous combustion and fires involving electric vehicles and mobile phones, seriously threatening people's lives and property. The low safety caused by flammable organic electrolytes has become a bottleneck factor restricting the further development of lithium-ion batteries.

[0003] Furthermore, as the applications of lithium-ion batteries become increasingly widespread, the required energy density and effective operating temperature range are also widening. However, conventional carbonate electrolytes, when reaching 4.3V (vs. Li / Li),... + At voltages of 100°C and above, and under extreme temperature conditions such as high temperatures, the electrolyte undergoes severe oxidative decomposition side reactions on the surface of the positive electrode material. This easily leads to adverse effects such as gas release, transition metal dissolution, and positive electrode material breakage, severely reducing battery stability and causing capacity decay and shortened battery life. In extreme temperature environments such as low temperatures, the significantly enhanced polarization results in a decrease in discharge voltage plateau, reduced charge / discharge capacity, rapid capacity decay, poor rate performance, and a significant reduction in energy density, severely limiting its application in aerospace, special equipment, polar expeditions, and other fields. Therefore, developing a flame-retardant or even non-flammable lithium battery electrolyte that exhibits high voltage resistance and wide temperature adaptability, while simultaneously achieving high energy density and high safety, has significant scientific and practical value.

[0004] Current publicly reported electrolyte research primarily focuses on adding functional additives to achieve flame retardancy, high voltage, or wide temperature range performance. Patent CN107834109A discloses a flame-retardant lithium-ion battery electrolyte that uses bromine-containing flame-retardant additives such as tribromoethane and dibromomethylfuran, reducing the risk of spontaneous combustion in lithium-ion batteries. However, these flame-retardant additives reduce battery capacity and energy density. Patent CN108630990A discloses a phosphine-based flame-retardant high-nickel ternary lithium-ion battery electrolyte. It utilizes high-phosphorus phosphine-based flame-retardant additives such as phenylethylene hydrogen phosphite and heptyl dipropylene glycol phosphite to achieve good flame-retardant effects. Simultaneously, the electrolyte exhibits good compatibility with the negative electrode, forming a stable solid electrolyte interface (SEI), resulting in good battery performance. However, these flame-retardant additives are mostly phenyl and phosphine-based organic compounds, which are highly toxic and easily pollute the environment, limiting their effective application. Patent CN113193231A discloses a high-voltage electrolyte for lithium-ion batteries. By adding tetramethyl thiophene-2,5-dimethylbis(methylphosphonate) (TTD) as a high-voltage additive, the lithium-ion battery can achieve stable cycling 100 times at a high voltage of 4.5V and significantly improve the cycle rate performance. However, the electrolyte cannot achieve cycling at higher cutoff voltages above 4.5V. At the same time, the electrolyte does not have flame retardancy and wide temperature range, and its function is relatively simple.

[0005] Existing flame-retardant electrolytes achieve their flame-retardant effect by adding phosphazene compounds. Phosphazene molecules contain phosphorus and nitrogen, which can play a role in flame retardancy. The two elements can work together and promote each other. Phosphazene can also be degraded endothermally to generate stable phosphates, metaphosphates, and polyphosphates containing a large amount of phosphorus, as well as non-flammable gases. These form a non-volatile, dense protective film on the surface of the burning material to prevent contact with air, thus playing a role in flame retardancy. However, most of the existing phosphazene compounds have no dissolving ability. They can only be used as electrolyte additives and cannot be used as the main solvent of the electrolyte. Their improvement on the safety and cycle performance of the electrolyte is limited, and it is difficult to meet the requirements of stable cycling and safe use under high voltage and wide temperature range at the same time.

[0006] Therefore, there is an urgent need to develop a new solvent system that can be used as the main solvent for electrolytes and also has excellent flame retardancy, high voltage stability and wide temperature adaptability, so as to promote the practical development of high-safety and high-performance lithium-ion batteries. Summary of the Invention

[0007] This invention provides a flame-retardant electrolyte and its alkali metal-based battery to address the problem that most phosphazene compounds used in existing electrolytes lack lithium salt dissolving capabilities, and can only be used as electrolyte additives rather than as the main solvent. This results in limited improvement on the safety, cycle performance, and other properties of the electrolyte, making it difficult to simultaneously meet the requirements for stable cycling and safe use under high voltage and wide temperature range.

[0008] According to a first aspect of the present invention, the present invention provides a flame-retardant electrolyte comprising a solvent, wherein the solvent comprises 10% to 100% by volume an ether-based phosphazene solvent, preferably 10%-50% or 50%-100%; more preferably 45%-55%; the ether-based phosphazene solvent having the following general structural formula (I): Equation (I); where one or two of R1 to R6 are R7 is selected from chain ether groups that are substituted or unsubstituted with halogen atoms, cyano groups, olefinic groups or alkyne groups, or cyclic ether groups that are substituted or unsubstituted with halogen atoms, cyano groups, olefinic groups or alkyne groups.

[0009] In the above-described scheme, the ether-based phosphazene solvent of the present invention significantly enhances the solubility of lithium salts through the coordination effect of the introduced ether group, making it suitable as the main solvent for electrolytes. In contrast, most existing phosphazene compounds cannot directly dissolve lithium salts and can only be used as small additives in electrolytes. Electrolytes using the ether-based phosphazene solvent of the present invention as the main solvent exhibit characteristics such as complete non-flammability, wide liquid range, good ionic conductivity, high oxidation stability, and high reduction stability. Furthermore, the ether-based phosphazene solvent can form stable positive and negative electrode passivation layers, improving the cycle stability of the electrolyte and achieving excellent safety, high-voltage cycle stability, and high-temperature and low-temperature cycle stability in lithium batteries, thus possessing high practical application value. The ether-based phosphazene solvent of the present invention can be used as the main solvent for flame-retardant electrolytes in wide-temperature, high-voltage lithium batteries.

[0010] Optionally, the volume percentage of the ether-based phosphazene solvent in the electrolyte can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100%, or other values ​​within the above range, and is not limited herein. In some specific embodiments, when the ether-based phosphazene solvent is used as a co-solvent, the volume percentage of the ether-based phosphazene solvent in the electrolyte is 10%-50%, preferably 10%-40%. In some specific embodiments, when the ether-based phosphazene solvent is used as the main solvent, the volume percentage of the ether-based phosphazene solvent in the electrolyte is 50%-100%.

[0011] The solvent in the electrolyte primarily functions to dissolve the electrolyte and conduct ions. As a crucial component of the electrolyte, the solvent dissolves electrolyte salts to form an electrolyte solution, thereby conducting ions between the positive and negative electrodes of the battery and ensuring its normal operation. The physicochemical properties of the solvent significantly impact the electrolyte's conductivity, stability, and safety. In the aforementioned scheme, by limiting the amount of ether-based phosphazene solvent added to the flame-retardant electrolyte within a reasonable range, the overall performance of the electrolyte, including safety, cycle life, conductivity, and stability, can be significantly improved.

[0012] In this invention, one or two of R1 to R6 are If R1~R6 If there are more than two, the viscosity of the ether-based phosphazene solvent will be too high, resulting in poor flowability. This may prevent the formation of a flame-retardant electrolyte with good flowability, which in turn will affect the overall performance of the flame-retardant electrolyte.

[0013] Furthermore, the chain ether group can be a straight-chain ether group or a branched-chain ether group.

[0014] Furthermore, among R1 to R6, except for those selected from... All other groups except for the one mentioned above are fluorine atoms.

[0015] In the aforementioned scheme, fluorine atoms possess a small atomic radius and high electronegativity. These characteristics result in fluorine-containing compounds exhibiting low surface energy and high thermal stability, thereby enhancing the flame-retardant properties of the material. Applying fluorine-substituted compounds to flame-retardant solvents not only significantly improves the cycle stability of batteries at high voltages but also demonstrates excellent flame-retardant properties. Therefore, this invention, by introducing fluorine atoms into ether-based phosphazene solvents through fluorine substitution, can effectively improve the flame-retardant properties of the solvent, thereby enhancing the safety and performance of the battery.

[0016] To improve the solvent's solubility for lithium salts while maintaining its flame-retardant properties, furthermore, one of the R1-R6 groups is selected from... .

[0017] To further enhance the solvent's ability to dissolve lithium salts, R7 is further defined as a chain ether group or a cyclic ether group with 1-10 oxygen atoms. Even further defined, R7 is a chain ether group or a cyclic ether group with 2-5 oxygen atoms. Even further defined, R7 is a chain ether group or a cyclic ether group with 2-3 oxygen atoms.

[0018] Further, R7 is a chain ether group or a cyclic ether group having 2-10 carbon atoms. Even further, R7 is a chain ether group or a cyclic ether group having 3-8 carbon atoms.

[0019] In some specific embodiments, the compound represented by the general formula (I) is selected from one or more of formulas I-1 to I-8:

[0021] The ether-based phosphazene solvent provided by this invention can be prepared using synthetic methods known in the art. For example, when the compound represented by general formula (I) is I-1, its preparation method may include the following steps: The first step involves dissolving ethylene glycol methyl ether and hexafluorocyclotriphosphazene in diethyl ether. After the sample is completely mixed, potassium carbonate is added, and the mixture is then sealed and stirred at room temperature for 18-30 hours. After the reaction is complete, the resulting mixture is filtered, and the solvent is removed from the filtrate under vacuum to obtain the crude product. The second step involves subjecting the crude product to vacuum distillation to obtain the pure target product.

[0022] For example, when the compound represented by general formula (I) is I-2, its preparation method may include the following steps: The first step involves dissolving diethylene glycol methyl ether and hexafluorocyclotriphosphazene in diethyl ether. After the sample is completely mixed, potassium carbonate is added, and the mixture is then sealed and stirred at room temperature for 18-30 hours. After the reaction is complete, the resulting mixture is filtered, and the solvent is removed from the filtrate under vacuum to obtain the crude product. The second step involves vacuum distillation of the crude product to obtain the pure target product.

[0023] Furthermore, the solvent also includes one or more of the following: ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate, diethyl ether, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,3-dioxolane, tetrahydrofuran, methyltetrahydrofuran, tetrahydropyran, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, methyl nonafluorobutyl ether, and bis(2,2,2-trifluoroethyl) ether.

[0024] In some specific embodiments, when the ether-based phosphazene solvent is the main solvent, the solvent further includes 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether or 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether. In some specific embodiments, when the ether-based phosphazene solvent is the main solvent, the solvent further includes 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, the amount of which is added is 0%-50% of the electrolyte volume percentage.

[0025] In some specific embodiments, when the ether-based phosphazene solvent is used as a co-solvent, the solvent further includes one or more of ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate, diethyl ether, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether. In some specific embodiments, the solvent further includes dimethyl carbonate and ethylene glycol dimethyl ether, with a volume ratio of (1-3):(1-3). When the ether-based phosphazene is used as a co-solvent, the amount of other organic solvents is 50%-100% of the electrolyte volume percentage. In some specific embodiments, the solvent also includes only ethylene glycol dimethyl ether.

[0026] In the above scheme, by further adding the specified solvents, other solvents are also included. These solvents, when used in combination with ether-based phosphazene solvents, work synergistically to better improve the overall performance of the electrolyte, such as safety, cycleability, conductivity, and stability.

[0027] Furthermore, the electrolyte provided by the present invention further includes a lithium salt electrolyte, which includes one or more of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium trifluoromethanesulfonate, lithium difluorooxalateborate, lithium tetrafluoroborate, lithium bis(oxalateborate), and lithium perchlorate. In some specific embodiments, the lithium salt electrolyte is lithium bis(fluorosulfonyl)imide.

[0028] In the above scheme, the lithium salt electrolyte in the electrolyte provides lithium ions and transports lithium ions between the positive and negative electrodes, which has a decisive influence on the physical and chemical properties of the electrolyte. By selecting a suitable type of lithium salt electrolyte, a better synergistic effect can be formed with the solvent, thereby improving the overall performance of the electrolyte, such as safety, cycle performance, conductivity, and stability.

[0029] Furthermore, the concentration of the lithium salt electrolyte is 0.01~5 mol / L.

[0030] Optionally, the concentration of the lithium salt electrolyte can be 0.01 mol / L, 0.025 mol / L, 0.05 mol / L, 0.075 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, or 5 mol / L, or other values ​​within the above range, which are not limited here.

[0031] In the above scheme, by limiting the concentration of lithium salt electrolyte in the electrolyte to a reasonable range, it is beneficial to the transport of lithium ions between the positive and negative electrodes, thereby improving battery performance.

[0032] Preferably, the flame-retardant electrolyte comprises a solvent and a lithium salt electrolyte. The solvent comprises 50%–60% by volume of the compound represented by general formula (I) and 40%–50% by volume of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. The lithium salt electrolyte is lithium difluorosulfonylimide, and the concentration of the lithium salt electrolyte is 0.5–1.5 mol / L. Preferably, the compound represented by general formula (I) is selected from the compounds represented by formulas I-2.

[0033] According to a second aspect of the present invention, the present invention also provides an alkali metal-based battery comprising the flame-retardant electrolyte described above.

[0034] The flame-retardant electrolyte of the present invention can be used alone as an electrolyte in alkali metal-based batteries, or it can be used as part of a solid-liquid composite electrolyte or a quasi-solid gel electrolyte in alkali metal-based batteries.

[0035] The alkali metal-based battery described in this invention can be a liquid alkali metal-based battery or a solid alkali metal-based battery. Correspondingly, the electrolyte is a non-aqueous liquid electrolyte or a polymer solid electrolyte.

[0036] The alkali metal-based battery of the present invention is not limited in shape and can be cylindrical, aluminum-cased, plastic-cased, or pouch-cased.

[0037] The alkali metal-based battery described in this invention can be an alkali metal battery or an alkali metal ion battery. The alkali metal can be lithium, sodium, or potassium.

[0038] Furthermore, the alkali metal-based battery also includes a positive electrode, a negative electrode, and a separator, wherein the positive electrode and the negative electrode are placed in the flame-retardant electrolyte, and the separator is placed between the positive electrode and the negative electrode.

[0039] Preferably, the positive electrode comprises one or more of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium-rich manganese, lithium manganese iron phosphate, or ternary positive electrode materials, more preferably ternary positive electrode materials, such as LiNi. x Co y Mn 1-x-y O2, where 0 < x < 1, 0 < y < 1, and x + y < 1.

[0040] Preferably, the negative electrode is a lithium metal negative electrode, a graphite negative electrode, a silicon negative electrode, a hard carbon negative electrode, or an organic negative electrode; preferably, the separator is a polypropylene film or a polyethylene film.

[0041] This invention provides an ether-based phosphazene solvent. Compared with traditional phosphazene compounds that can only be used as electrolyte additives, the ether group introduced significantly improves the solubility of phosphazene solvents for lithium salts, enabling them to be used as the main electrolyte solvent in flame-retardant electrolytes for wide-temperature, high-voltage lithium batteries.

[0042] The flame-retardant electrolyte of this invention uses ether-based phosphazene solvent as the main solvent, which makes the electrolyte exhibit characteristics such as complete non-flammability, wide liquid range, good ionic conductivity, high oxidation stability, and high reduction stability. At the same time, the ether-based phosphazene solvent can form a stable positive and negative electrode passivation layer, improve the cycle stability of the electrolyte, and achieve excellent safety, high voltage cycle stability, and high temperature and low temperature cycle stability of lithium batteries. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in this 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 some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0044] Figure 1 The diagram shows the self-extinguishing experimental test results of Comparative Example 3 and Example 1 of the present invention.

[0045] Figure 2 The graph shows the cycle performance of the NCM811 / Li batteries prepared by Comparative Example 2 and Example 2 at a cutoff voltage of 4.8 V.

[0046] Figure 3 The diagram shows the discharge voltage of the NCM811 / Li battery prepared in Example 2 of this invention at different temperature ranges. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0048] Unless otherwise specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0049] The organic solvents, lithium salt electrolytes, and additives used in the embodiments and comparative examples of this invention are all battery grade. The ether-based phosphazene solvent prepared in this invention undergoes multiple purification steps and rigorous drying.

[0050] In the following examples, the electrolyte was prepared in a glove box filled with 99.999% pure argon gas, with a moisture content of less than 0.1 ppm and a temperature of room temperature.

[0051] The compounds of formulas I-1 to I-8 used in the following examples and comparative examples were synthesized using the following methods: The preparation method of the compound of formula I-1 may include the following steps: Step 1: Dissolve 76g of ethylene glycol methyl ether and 248g of hexafluorocyclotriphosphazene in 1500ml of diethyl ether. After the sample is completely mixed, add 420g of potassium carbonate, then seal and stir at room temperature for 30h. After the reaction is complete, filter the resulting mixture, and remove the solvent from the filtrate under vacuum to obtain the crude product. Step 2: Distill the crude product under reduced pressure to obtain the target product (yield 35%). MS: [M+H] + m / z = 305.98.

[0052] The preparation method of compound I-2 may include the following steps: similar to the preparation method of compound I-1, except that ethylene glycol methyl ether is replaced with an equimolar amount of 2-(2-methoxyethoxy)ethanol. MS: [M+H] + m / z = 350.00.

[0053] The preparation method of compound I-3 may include the following steps: similar to the preparation method of compound I-1, except that ethylene glycol methyl ether is replaced with an equimolar amount of 1,3-dioxolane-4-ol. MS: [M+H] + m / z = 319.95.

[0054] The preparation method of compound I-4 may include the following steps: similar to the preparation method of compound I-1, except that ethylene glycol methyl ether is replaced with an equimolar amount of tetrahydrofuran-3-ol. MS: [M+H] + m / z = 317.98.

[0055] The preparation method of the compound of formula I-5 may include the following steps: similar to the preparation method of compound I-1, except that the ethylene glycol methyl ether is replaced with an equimolar amount of 2-(2-(2-methoxyethoxy)ethoxy)ethanol-1-ol. MS: [M+H]+ m / z = 394.03.

[0056] The preparation method of compounds of formula I-6 may include the following steps: similar to the preparation method of compound I-1, except that ethylene glycol methyl ether is replaced with an equimolar amount of 1,3-dimethoxyprop-2-ol. MS: [M+H] + m / z = 350.00.

[0057] The preparation method of compounds of formula I-7 may include the following steps: similar to the preparation method of compound I-1, except that ethylene glycol methyl ether is replaced with an equimolar amount of 2-(2-ethoxyethoxy)ethanol. MS: [M+H] + m / z = 364.02.

[0058] The preparation method of compounds of formula I-8 may include the following steps: similar to the preparation method of compound I-1, except that ethylene glycol methyl ether is replaced with two molar amounts of 2-(2-methoxyethoxy)ethanol. MS: [M+H] + m / z = 450.07.

[0059] Example 1 This embodiment provides a flame-retardant electrolyte for lithium batteries, using the compound of formula I-2 above as the main solvent, comprising 187g of lithium bis(fluorosulfonyl)imide and 1000mL of the compound of formula I-2 above.

[0060] The preparation method is as follows: In a glove box filled with argon, take 187g of lithium difluorosulfonylimide and 1000mL of the above-mentioned compound I-2, stir and let stand to obtain the product.

[0061] Example 2 This embodiment provides a flame-retardant electrolyte for lithium batteries, which uses the compound of formula I-2 above as the main solvent, including 187g of lithium difluorosulfonylimide, 500mL of the compound of formula I-2 above, and 500mL of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0062] The preparation method is as follows: In a glove box filled with argon, take 187g of lithium difluorosulfonylimide, 500mL of the compound of formula I-2 above, and 500mL of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, stir and let stand to obtain the product.

[0063] Example 3 This embodiment provides a flame-retardant electrolyte for lithium batteries, which uses the compound of formula I-4 above as a co-solvent, including 287g of lithium bis(trifluoromethanesulfonyl)imide, 600mL of ethylene glycol dimethyl ether, and 400mL of the compound of formula I-4 above.

[0064] The preparation method is as follows: In a glove box filled with argon, take 287g of lithium bis(trifluoromethanesulfonyl)imide, 600mL of ethylene glycol dimethyl ether, and 400mL of the compound of formula I-4 above, stir and let stand to obtain the product.

[0065] Example 4 This embodiment provides a flame-retardant electrolyte for lithium batteries, which uses compounds of formula I-5 as co-solvents and includes 187g of lithium difluorosulfonylimide, 400mL of dimethyl carbonate, 400mL of ethylene glycol dimethyl ether and 200mL of compounds of formula I-5.

[0066] The preparation method is as follows: In a glove box filled with argon, take 187g of lithium difluorosulfonamide, 400mL of dimethyl carbonate, 400mL of ethylene glycol dimethyl ether and 200mL of the above-mentioned compound I-5, stir and let stand to obtain the product.

[0067] Example 5 This embodiment provides a flame-retardant electrolyte for lithium batteries, which uses the compounds of formula I-6 above as the main solvent, including 187g of lithium difluorosulfonylimide, 600mL of the compounds of formula I-6 above, and 400mL of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0068] The preparation method is as follows: In a glove box filled with argon, take 187g of lithium difluorosulfonylimide, 600mL of the above-mentioned compound I-6, and 400mL of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, stir and let stand to obtain the product.

[0069] Example 6 This embodiment provides a flame-retardant electrolyte for lithium batteries, which uses the compound of formula I-2 above as the main solvent, including 187g of lithium difluorosulfonylimide, 800mL of the compound of formula I-2 above, and 200mL of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0070] The preparation method is as follows: In a glove box filled with argon, take 187g of lithium difluorosulfonylimide, 800mL of the compound of formula I-2 above, and 200mL of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, stir and let stand to obtain the product.

[0071] Example 7 This embodiment provides a flame-retardant electrolyte for lithium batteries, which uses the compounds of formula I-5 above as co-solvents, including 187g of lithium difluorosulfonylimide, 400mL of dimethyl carbonate, 500mL of ethylene glycol dimethyl ether and 100mL of the compounds of formula I-5 above.

[0072] The preparation method is as follows: Take 187g of lithium difluorosulfonylimide, 400mL of dimethyl carbonate, 500mL of ethylene glycol dimethyl ether and 100mL of the above-mentioned compound I-5, stir and let stand to obtain the product.

[0073] Example 8 This embodiment provides a flame-retardant electrolyte for lithium batteries, which differs from Embodiment 2 in that a compound of Formula I-1 is used instead of I-2. Its preparation method is the same as in Embodiment 2.

[0074] Example 9 This embodiment provides a flame-retardant electrolyte for lithium batteries, which differs from Embodiment 3 in that a compound of Formula I-3 is used instead of I-4. Its preparation method is the same as in Embodiment 3.

[0075] Example 10 This embodiment provides a flame-retardant electrolyte for lithium batteries, which differs from Embodiment 2 in that a compound of Formula I-5 is used instead of I-2. Its preparation method is the same as in Embodiment 2.

[0076] Example 11 This embodiment provides a flame-retardant electrolyte for lithium batteries, which differs from Embodiment 2 in that a compound of Formula I-7 is used instead of I-2. Its preparation method is the same as in Embodiment 2.

[0077] Example 12 This embodiment provides a flame-retardant electrolyte for lithium batteries, which differs from Embodiment 2 in that a compound of formula I-8 is used instead of I-2. Its preparation method is the same as in Embodiment 2.

[0078] Comparative Example 1 This comparative example provides an electrolyte for lithium batteries, comprising 187g of lithium bis(fluorosulfonyl)imide and 1000mL of ethylene glycol dimethyl ether.

[0079] The preparation method is as follows: In a glove box filled with argon, take 187g of lithium difluorosulfonylimide and 1000mL of ethylene glycol dimethyl ether, stir and let stand to obtain the product.

[0080] Comparative Example 2 This comparative example provides an electrolyte for lithium batteries, comprising 187g of lithium difluorosulfonylimide, 500mL of ethylene glycol dimethyl ether, and 500mL of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0081] The preparation method is as follows: In a glove box filled with argon, take 187g of lithium difluorosulfonylimide, 500mL of ethylene glycol dimethyl ether, and 500mL of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, stir and let stand to obtain the product.

[0082] Comparative Example 3 This comparative example provides an electrolyte for lithium batteries, comprising 187g of lithium bis(fluorosulfonyl)imide, 400mL of dimethyl carbonate, 400mL of diethyl carbonate, and 200mL of ethylene carbonate.

[0083] The preparation method is as follows: In a glove box filled with argon, take 187g of lithium difluorosulfonamide, 400mL of dimethyl carbonate, 400mL of diethyl carbonate and 200mL of ethylene carbonate, stir and let stand to obtain the final product.

[0084] Comparative Example 4 This comparative example provides an electrolyte for lithium batteries, comprising 187g of lithium bis(fluorosulfonyl)imide and 1000mL of ethoxypentafluorocyclotriphosphazene.

[0085] The preparation method is as follows: In a glove box filled with argon, take 187g of lithium difluorosulfonylimide and 1000mL of ethoxypentafluorocyclotriphosphazene, stir and let stand to obtain the product.

[0086] Comparative Example 5 This comparative example provides an electrolyte for lithium batteries, comprising 187g of lithium difluorosulfonylimide, 500mL of trifluoroethoxypentafluorocyclotriphosphazene, and 500mL of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0087] The preparation method is as follows: In a glove box filled with argon, take 187g of lithium difluorosulfonylimide, 500mL of trifluoroethoxypentafluorocyclotriphosphazene, and 500mL of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, stir and let stand to obtain the product.

[0088] Comparative Example 6 This embodiment provides a flame-retardant electrolyte for lithium batteries, which differs from Embodiment 1 in that it uses hexa[2-(2-methoxyethoxy)ethoxy]cyclotriphosphazene instead of I-2. Its preparation method is the same as in Embodiment 1.

[0089] Performance testing The solubility of the electrolytes prepared in the above embodiments and comparative examples was tested using the following methods: The electrolyte mixture was stirred at room temperature for one hour, and the homogeneity of the solution and whether there was any precipitation were observed. The test results are shown in Table 1.

[0090] Table 1. Comparison of electrolyte dissolution between the examples and comparative examples. Solubility Example 1 homogeneous, clear and transparent solution Example 2 homogeneous, clear and transparent solution Example 3 homogeneous, clear and transparent solution Example 4 homogeneous, clear and transparent solution Example 5 homogeneous, clear and transparent solution Example 6 homogeneous, clear and transparent solution Example 7 homogeneous, clear and transparent solution Example 8 homogeneous, clear and transparent solution Example 9 homogeneous, clear and transparent solution Example 10 homogeneous, clear and transparent solution Example 11 homogeneous, clear and transparent solution Example 12 homogeneous, clear and transparent solution Comparative Example 1 homogeneous, clear and transparent solution Comparative Example 2 homogeneous, clear and transparent solution Comparative Example 3 homogeneous, clear and transparent solution Comparative Example 4 Turbid, with obvious precipitate and insoluble matter. Comparative Example 5 Turbid, with obvious precipitate and insoluble matter. Comparative Example 6 Viscous and cloudy, with extremely poor fluidity. Comparative Examples 4, 5, and 6 were not used as electrolytes because they could not completely dissolve the electrolyte salts to form a homogeneous solution with good flowability. Therefore, they were not used in other tests. Table 1 shows that traditional phosphazene additives have poor solubility for electrolyte salts and cannot be used as electrolyte solvents.

[0091] The electrolytes prepared in the above examples, excluding Comparative Examples 4, 5, and 6, were subjected to flame retardancy tests, as follows: The igniter flame was brought close to each of the 100 μL electrolytes, held for 3 seconds, and then removed. The self-extinguishing time of the flame was measured. The experiment was repeated 3 times for each electrolyte, and the average value was calculated. The test results are shown in Table 2.

[0092] Table 2 Comparison of self-extinguishing time between the examples and comparative examples Self-extinguishing time (s / g) Example 1 0 Example 2 0 Example 3 0 Example 4 0 Example 5 0 Example 6 0 Example 7 0 Example 8 0 Example 9 0 Example 10 0 Example 11 0 Example 12 0 Comparative Example 1 110 Comparative Example 2 50 Comparative Example 3 95 Figure 1 The diagram shows the self-extinguishing test results for Comparative Example 3 and Example 1. Figure 1 As can be seen from Table 2, the ether-based phosphazene solvent used in this invention exhibits complete non-flammability and excellent flame retardancy, the electrolyte can achieve complete non-flammability, and the battery exhibits higher safety.

[0093] After assembling batteries with the electrolytes prepared in the above embodiments and comparative examples, cycle performance tests were conducted, as follows: With LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811) was used as the positive electrode, lithium foil as the negative electrode, and aluminum foil as the positive current collector. A Celgard 2325 separator was used. Button half-cells were assembled in a glove box and tested after 24 hours of rest. The cells were activated by three charge-discharge cycles at 1 / 5C between 3.0V and 4.3V at a constant temperature of 25°C. Subsequently, they were discharged at 1 / 10C at different temperatures; the discharge capacity at different temperatures is shown in Table 3. Charge-discharge cycles were performed at 1 / 3C at high cutoff voltages of 4.5V and 4.8V; the test results are shown in Table 4.

[0094] Table 3. Discharge capacity of NCM811 / Li half-cells assembled in the examples and comparative examples at different temperatures.

[0095] Table 4 shows the cycling results of the NCM811 / Li full cells assembled in the examples and comparative examples at cutoff voltages of 4.5V and 4.8V.

[0096] The capacity of Comparative Example 1 being 0 indicates that the electrolyte cannot circulate under high voltage due to its poor oxidation resistance.

[0097] Figure 2 The graph shows the cycle performance of the NCM811 / Li batteries prepared by Comparative Example 2 and Example 2 at a cutoff voltage of 4.5 V. Figure 3 The diagram shows the discharge voltage of the NCM811 / Li battery prepared in Example 2 at different temperature ranges.

[0098] From Tables 3-4 and Figures 2-3As can be seen, under high voltage charge-discharge cycles and wide temperature range discharge tests, the capacity and cycle stability of the electrolyte prepared in this embodiment are significantly better than those of the comparative example. This indicates that the ether-based phosphazene solvent provided by this invention can significantly improve the cycle stability of the battery and obtain excellent high voltage resistance and wide temperature adaptability.

[0099] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A flame-retardant electrolyte comprising a solvent, characterized in that, The solvent contains 10% to 100% by volume of an ether-based phosphazene solvent; The ether-based phosphazene solvent has the following general structural formula (I): Equation (I); Among them, one or two of R1 to R6 are R7 is selected from chain ether groups that are substituted or unsubstituted with halogen atoms, cyano groups, olefinic groups or alkyne groups, or cyclic ether groups that are substituted or unsubstituted with halogen atoms, cyano groups, olefinic groups or alkyne groups.

2. The flame-retardant electrolyte according to claim 1, characterized in that, In R1~R6, except All other groups except for the one mentioned above are fluorine atoms.

3. The flame-retardant electrolyte according to claim 1 or 2, characterized in that, R7 is a chain ether group or a cyclic ether group with 1-10 oxygen atoms.

4. The flame-retardant electrolyte according to any one of claims 1-3, characterized in that, R7 is a chain ether group or a cyclic ether group with 2-10 carbon atoms.

5. The flame-retardant electrolyte according to any one of claims 1-4, characterized in that, The compound represented by the general formula (I) is selected from one or more of formulas I-1 to I-8: 。 6. The flame-retardant electrolyte according to any one of claims 1-5, characterized in that, The solvent also includes one or more of the following: ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate, diethyl ether, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,3-dioxolane, tetrahydrofuran, methyltetrahydrofuran, tetrahydropyran, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, methyl nonafluorobutyl ether, and bis(2,2,2-trifluoroethyl) ether.

7. The flame-retardant electrolyte according to claim 6, characterized in that, When the ether-based phosphazene solvent is the main solvent, the solvent further includes 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether or 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether; when the ether-based phosphazene solvent is the co-solvent, the solvent further includes one or more of ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate, diethyl ether, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

8. The flame-retardant electrolyte according to any one of claims 1-7, characterized in that, It also includes a lithium salt electrolyte, which includes one or more of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium trifluoromethanesulfonate, lithium difluorooxalateborate, lithium tetrafluoroborate, lithium bis(oxalateborate), and lithium perchlorate; preferably, the concentration of the lithium salt electrolyte is 0.01~5 mol / L.

9. The flame-retardant electrolyte according to any one of claims 1-8, characterized in that, It includes a solvent and a lithium salt electrolyte. The solvent consists of 50% to 60% by volume of the compound represented by general formula (I) and 40% to 50% by volume of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. The lithium salt electrolyte is lithium difluorosulfonylimide, and the concentration of the lithium salt electrolyte is 0.5 to 1.5 mol / L.

10. An alkali metal-based battery, characterized in that, Includes the flame-retardant electrolyte according to any one of claims 1-9.

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