A safe low-temperature lithium ion battery electrolyte and a preparation and application method thereof

Abstract

The application belongs to the technical field of lithium ion batteries, and particularly relates to a safe low-temperature lithium ion battery electrolyte and a preparation and application method thereof; the electrolyte comprises a lithium salt, a composite organic solvent and an additive; the molar concentration of lithium ions in the lithium salt in the composite organic solvent is 3-12 mol / L; the composite organic solvent comprises a eutectic solvent and a low-freezing-point solvent, the eutectic solvent is prepared by mixing ethylene carbonate and dimethylacetamide; the low-freezing-point solvent is a mixed solvent of 1,3 dioxolane, dichloromethane and tetrafluoroethane; the additive comprises a cyclic sulfonic acid ester and an imidazolium-based ionic liquid, and the additive is dissolved in advance and then gradually added into the electrolyte. The formation of lithium dendrites is further relieved, and the compatibility between the cycle stability of the metal lithium battery and the non-flammability of the electrolyte is realized. The lithium ion battery constructed by using the electrolyte has excellent rate performance and cycle stability, and can work safely and stably at-60 DEG C to 100 DEG C.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a safe low-temperature lithium-ion battery electrolyte and its preparation and application methods. Background Technology

[0002] Lithium-ion batteries are among the most commercially available and widely used electrochemical energy storage devices. However, their capacity typically decreases rapidly below 0°C, severely limiting their use in extremely cold regions. The electrolyte is one of the main factors affecting the low-temperature performance of lithium-ion batteries; therefore, rationally controlling the electrolyte composition and constructing a low-temperature electrolyte structure is beneficial for improving the low-temperature performance of lithium-ion batteries.

[0003] From the perspective of electrolyte physicochemical properties, low-temperature environments inevitably affect properties such as ionic conductivity and viscosity. Slow molecular thermal motion and enhanced intermolecular interactions at low temperatures severely hinder lithium-ion migration within the bulk electrolyte. The interface between the electrolyte and the electrode is a crucial site for charge transfer and electrode reactions. The slow charge transfer rate and severely impaired reaction kinetics at low temperatures limit the charge transfer and electrochemical reaction rates of lithium ions at the electrode-electrolyte interface. Currently, improving the low-temperature performance of electrolytes by using solvents with low freezing points and low polarity is the main approach to optimizing the low-temperature performance of lithium-ion batteries. However, most methods struggle to achieve both excellent low-temperature performance and good cycle stability. Therefore, designing and constructing a safe, stable, and high-performance low-temperature lithium-ion battery electrolyte is an urgent problem to be solved.

[0004] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the present invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0005] The first objective of this invention is to provide a safe low-temperature lithium-ion battery electrolyte, which enables lithium-ion batteries to be constructed with excellent rate performance and cycle stability, while also allowing them to operate safely at temperatures ranging from -60°C to 100°C.

[0006] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0007] A safe low-temperature lithium-ion battery electrolyte includes a lithium salt, a composite organic solvent, and additives;

[0008] The molar concentration of lithium ions in the lithium salt within the composite organic solvent is 3–12 mol / L. High-concentration lithium salts form very stable "solvated ion clusters" in the composite organic solvent. This protects the composite organic solvent from side reactions with metallic lithium, thus allowing the electrolyte to exhibit interfacial stability similar to the high-concentration system. A preferred molar concentration is 3–5 mol / L, which ensures a wide voltage window while minimizing the use of lithium salts and reducing costs.

[0009] The composite organic solvent comprises a eutectic solvent and a low-freezing-point solvent, with a volume ratio of 1:1. The eutectic solvent is prepared by mixing ethylene carbonate (EC) and dimethylacetamide (DMAC). DMAC acts as a hydrogen bond donor, and EC as a hydrogen bond acceptor. Upon mixing, hydrogen bonds are formed, and the strong hydrogen bonding lowers the freezing point of the resulting mixture, thus creating the eutectic system. This also helps prevent the precipitation of EC solids at low temperatures. After mixing, EC dissolves completely and its solubility is not affected by the introduction of other solvents. The low-freezing-point solvent is a mixture of 1,3-dioxane (DIOX), dichloromethane, and tetrafluoroethane; this lowers the overall freezing point of the electrolyte, ensuring good fluidity even at low temperatures.

[0010] Preferably, the volume ratio of 1,3-dioxolane, dichloromethane, tetrafluoroethane, and dimethylacetamide is 8–10:2–4:1–2:4–7. This ratio design allows for multifunctional complementarity, balancing low-temperature conductivity and high-temperature stability, ensuring good electrolyte performance over a wide temperature range. Simultaneously, it improves conductivity, reduces viscosity, optimizes lithium salt solubility, and maintains fluidity at low temperatures.

[0011] In the composite organic solvent designed in this invention, the low-freezing-point solvent exhibits excellent antioxidant properties. Dimethylacetamide acts as a hydrogen bond donor solvent and, when mixed with the low-freezing-point solvent, forms a low-viscosity, electrochemically inert diluent. This ensures a wide voltage window for the high-concentration electrolyte and solves the high viscosity problem caused by high salt concentration. While maintaining high solvent stability, the freezing point of the electrolyte is lowered, allowing it to remain liquid at low temperatures, enabling rapid lithium-ion migration and ensuring stable operation of the lithium-ion battery at low temperatures. By designing a high-concentration lithium salt electrolyte, this invention can adjust the solvation configuration of lithium ions, reduce the number of solvated lithium ions, decrease the desolvation energy barrier, ensure rapid lithium intercalation in the lithium-ion battery, and improve the rate capability and low-temperature performance of the lithium-ion battery.

[0012] Unlike traditional high-concentration electrolytes, this invention incorporates a large amount of "inactive diluents." These diluents (such as tetrafluoroethane and dimethylacetamide) do not directly form solvates with lithium ions. They primarily reduce the overall viscosity and melting point of the system, while also not significantly affecting the properties of the solvated ion clusters formed by high-concentration lithium salts. This allows the electrolyte to maintain good fluidity even at low temperatures, thereby improving lithium-ion transport efficiency.

[0013] The additives include cyclic sulfonate esters and imidazolium-based ionic liquids. These additives are pre-dissolved in a mixture of dichloromethane and dimethylacetamide in a 1:1 volume ratio, and then gradually added to the electrolyte. Dichloromethane is a low-polarity, low-viscosity solvent with excellent dissolving power, effectively dissolving the cyclic sulfonate esters and making them easier to disperse in the mixed solution. Dimethylacetamide is a highly polar solvent, effectively dissolving the imidazolium-based ionic liquid, and has a low viscosity, helping to reduce the overall viscosity of the mixture, facilitating subsequent mixing and handling. This invention, by pre-dissolving the cyclic sulfonate esters and imidazolium-based ionic liquids in a mixture of dichloromethane and dimethylacetamide, ensures that these additives are thoroughly and uniformly mixed before being added to the main electrolyte, avoiding problems caused by uneven dispersion of the additives in the electrolyte.

[0014] Preferably, the cyclic sulfonate is methylene methane disulfonate and 1,1-cyclopropanediethanol sulfinate, and the imidazolium-based ionic liquid is 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide.

[0015] Cyclic sulfonates, as battery additives, exhibit excellent high-temperature cycling performance and can reduce electrolyte polarization, thereby improving the battery's discharge platform. They also enhance the electrolyte's antioxidant properties, maintaining its stability and effectively mitigating the poor high-temperature performance and stability caused by the use of large amounts of low-temperature solvents. Furthermore, after decomposition, sulfonates can form a sulfonate-containing SEI layer with good lithium-ion conductivity. Working in conjunction with EC solvents, this forms a thin and stable SEI layer, effectively reducing the battery's internal resistance, thus improving cycle performance and extending battery life. The fluorinated groups and imidazole ring structure in 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide can also form a dense SEI layer on the lithium metal anode surface. This invention, through the design of three mixed additives, further mitigates lithium dendrite formation, achieving compatibility between battery cycle stability and electrolyte non-flammability.

[0016] Meanwhile, in the composite organic solvent, the hydrogen bond network formed by EC and DMAC results in a melting point of the mixed system that is much lower than that of EC and DMAC individually. 1-Butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide can act as both a hydrogen bond donor and acceptor. When added as an additive, it not only alleviates dendrite growth problems and improves battery life, but also acts as a bridging agent for hydrogen bonds, making the hydrogen bond network of the entire system more compact and stable. This further enhances the eutectic solvent system of EC and DMAC, lowers the electrolyte freezing point, and improves the lithium-ion solvation structure. Furthermore, due to the protective effect of 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, DMAC can be better and more uniformly distributed on the negative electrode surface, thus effectively improving its interfacial compatibility.

[0017] Preferably, the molar concentrations of methylene disulfonate, 1,1-cyclopropanediethanol sulfinate, and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide are all 0.05–0.2 mol / L, with a volume ratio of 1–3:1–3:2–4. The three additives work synergistically to maximize their respective functions and achieve multi-dimensional control of the electrolyte. 1-Butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide has very low volatility and non-flammability, and its large volume proportion can significantly improve the overall safety of the electrolyte, reducing the risk of combustion and thermal runaway.

[0018] Preferably, the lithium salt is a mixture of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide; the molar ratio of lithium bis(trifluoromethanesulfonyl)imide to lithium bis(fluorosulfonyl)imide is 1 to 3:1. These two lithium salts have advantages such as high stability (no decomposition below 200℃), excellent low-temperature performance, good hydrolytic stability, and greater environmental friendliness. They can effectively reduce the high and low temperature resistance of the SEI layer formed on the electrode surface at low temperatures, reduce capacity loss of the lithium battery during storage, and thus provide high battery capacity and electrochemical performance.

[0019] The second objective of this invention is to provide a method for preparing a safe low-temperature lithium-ion battery electrolyte, which fully solves the problems of conductivity, stability and safety of traditional electrolytes under low-temperature conditions.

[0020] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0021] A method for preparing a safe low-temperature lithium-ion battery electrolyte includes the following steps;

[0022] S1: Heat EC to a liquid state, and mix it with dimethylacetamide at 50-80°C in a certain proportion and stir for 30 minutes, then let it cool naturally to room temperature to obtain mixed solvent 1;

[0023] In this invention, EC is first dissolved in DMAC by heating, which forms a relatively homogeneous solution. Since DMAC is highly compatible with EC, this mixing order helps to prevent the precipitation of EC solids under low-temperature conditions. After mixing, EC is completely dissolved and its solubility is not affected by the introduction of other solvents.

[0024] S2: Mixing a low freezing point solvent and mixed solvent 1 to obtain mixed solvent 2;

[0025] This invention mixes EC and DMAC with a low freezing point solvent to ensure that the composite solvent system remains liquid under extremely low temperature conditions, thereby enhancing the low-temperature performance of the electrolyte.

[0026] S3: Add the two lithium salts to the mixed solvent 2 in proportion and stir thoroughly to dissolve them to obtain a mixed solution;

[0027] S4: Mix dichloromethane and dimethylacetamide to prepare mixed solvent 3;

[0028] S5: Slowly add the additives to the mixed solvent 3 in sequence until fully dissolved, add them to the mixed solution, and stir at -10℃ for 30 to 40 minutes to obtain a low-temperature lithium-ion battery electrolyte.

[0029] This invention pre-dissolves the additives in the mixed solvent 3, which helps ensure their uniform dissolution and distribution, avoiding the problem of excessively high local concentrations that may occur when directly added to the mixed solution. The slow addition process to the mixed solution allows for control of the addition rate and stirring conditions, preventing excessive concentration gradients between different components that could affect the uniformity and stability of the electrolyte. Stirring at a low temperature of -10°C simulates the state of the electrolyte in a low-temperature environment, ensuring that each component maintains good solubility and stability at low temperatures.

[0030] The third objective of this invention is to provide a method for applying a safe low-temperature lithium-ion battery electrolyte, wherein the lithium-ion battery constructed with this electrolyte has excellent rate performance and cycle stability, and can operate safely at temperatures ranging from -60°C to 100°C.

[0031] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0032] The above-mentioned safe low-temperature lithium-ion battery electrolyte is applied to a low-temperature lithium-ion battery. The lithium-ion battery is composed of the above-mentioned low-temperature electrolyte, lithium iron phosphate positive electrode, porous graphite negative electrode and separator; the lithium iron phosphate positive electrode and the porous graphite negative electrode are respectively disposed on both sides of the separator; the lithium-ion battery can operate at -60℃.

[0033] By employing the above technical solution, the present invention has at least the following beneficial effects:

[0034] 1. The dual-salt electrolyte system constructed in this invention, with a high salt concentration of 3–12 mol / L, enables Li… + The amount of external solvation is reduced, while the two lithium salts effectively enhance the solvation structure of the anion, accelerate the reaction kinetics at the interface, improve the migration rate of lithium ions at the electrode interface at low temperatures, and enhance the low-temperature performance of the battery. Furthermore, through optimized design of the co-solvent electrolyte system, the solvation layer structure of the lithium salt and dimethylacetamide-based electrolyte is not affected by the diluents 1,3-dioxolane, dichloromethane, and tetrafluoroethane. This ensures that the electrolyte has a wide voltage stability window of 0–4.85 V, maintains a viscosity of 0.35 Pa·s at -60℃, and achieves an improved ionic conductivity of 1.2 mS / cm. -1 It exhibits good wetting ability and demonstrates chemical and electrochemical stability to metallic lithium.

[0035] 2. This invention uses three electrolyte additives. On the one hand, the synergistic effect of 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide and DMAC enhances the interfacial stability of lithium metal, further improving the solvation structure of the electrolyte containing a large number of aggregates, which helps the formation of anion-derived SEI, effectively alleviating the formation of lithium dendrites, and improving coulombic efficiency, thereby achieving compatibility between the cycle stability of lithium metal batteries and the non-flammability of electrolytes. On the other hand, the two cyclic sulfonate-based electrolyte additives, methanedisulfonate methylene and 1,1-cyclopropanediethanol sulfinate, can reduce the polarization of the electrolyte, thereby improving the discharge platform of the battery; and further improve the antioxidant properties of the electrolyte, maintain the stable state of the electrolyte, effectively reduce the internal resistance of the battery, thereby improving the safe cycle performance of the battery and extending the battery's service life.

[0036] 3. The safe low-temperature electrolyte prepared by the above-mentioned preparation method is applied to lithium-ion batteries. In the assembled full cell, the capacity retention rate is 86% after 1000 cycles at -60°C. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 The charging and discharging test curves of the lithium-ion battery at different temperatures in Example 1 are shown.

[0039] Figure 2 This is a diagram showing the cycle life of the lithium-ion battery at low temperatures in Example 1. Detailed Implementation

[0040] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0042] In embodiments of the present invention, the specific sources of the various materials are as follows:

[0043]

[0044] Example 1

[0045] S1: Heat EC to a liquid state, and mix it with dimethylacetamide at a volume ratio of 8:6 at 60°C for 30 minutes. Then let it cool naturally to room temperature to obtain mixed solvent 1.

[0046] S2: Mixed solvent 2 is prepared by mixing 1,3-dioxolane, dichloromethane, and tetrafluoroethane with mixed solvent 1; the volume ratio of the four solvents, 1,3-dioxolane, dichloromethane, tetrafluoroethane, and dimethylacetamide, is 10:3:1:6.

[0047] S3: Lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide are added to mixed solvent 2 in a molar ratio of 1:1 and stirred thoroughly to dissolve and obtain a mixed solution; the molar concentration of lithium ions in the two salts in the low freezing point solvent is 5 mol / L.

[0048] S4: Mix dichloromethane and dimethylacetamide in a volume ratio of 1:1 to prepare mixed solvent 3;

[0049] S5: Methylene methane disulfonate, 1,1-cyclopropanediethanol sulfinate, and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide are slowly added to mixed solvent 3 in a volume ratio of 1:1:2 and dissolved completely. The molar concentration of methylene methane disulfonate is 0.05 mol / L. Then, the solution is added to the mixed solution and stirred at -10℃ for 30-40 min to obtain a low-temperature lithium-ion battery electrolyte.

[0050] The prepared low-temperature electrolyte was used as the electrolyte for lithium-ion batteries. It was assembled into lithium-ion batteries together with lithium iron phosphate cathode, porous graphite anode and PP separator, and its electrochemical performance was tested.

[0051] Example 2

[0052] S1: Heat EC to a liquid state, and mix it with dimethylacetamide at a volume ratio of 1:1 at 60°C for 30 minutes. Then let it cool naturally to room temperature to obtain mixed solvent 1.

[0053] S2: Mixed solvent 2 is prepared by mixing 1,3-dioxolane, dichloromethane, and tetrafluoroethane with mixed solvent 1; the volume ratio of the four solvents, 1,3-dioxolane, dichloromethane, tetrafluoroethane, and dimethylacetamide, is 9:4:1:7.

[0054] S3: Lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide are added to mixed solvent 2 in a molar ratio of 1:1 and stirred thoroughly to dissolve and obtain a mixed solution; the molar concentration of lithium ions in the two salts in the low freezing point solvent is 8 mol / L.

[0055] S4: Mix dichloromethane and dimethylacetamide in a volume ratio of 1:1 to prepare mixed solvent 3;

[0056] S5: Methylene methane disulfonate, 1,1-cyclopropanediethanol sulfinate, and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide are slowly added to mixed solvent 3 in a volume ratio of 1:1:2 and dissolved completely. The molar concentration of methylene methane disulfonate is 0.05 mol / L. Then, the solution is added to the mixed solution and stirred at -10℃ for 30-40 min to obtain a low-temperature lithium-ion battery electrolyte.

[0057] The prepared low-temperature electrolyte was used as the electrolyte for lithium-ion batteries. It was assembled into lithium-ion batteries together with lithium iron phosphate cathode, porous graphite anode and PP separator, and its electrochemical performance was tested.

[0058] Comparative Example 1

[0059] A commercial electrolyte was used: 1M LiPF6 dissolved in EC, DEC, and EMC (volume ratio 1:1:1) as the electrolyte for lithium-ion batteries. The lithium-ion battery was assembled with a lithium iron phosphate cathode, a porous graphite anode, and a PP separator, and its electrochemical performance was tested.

[0060] Comparative Example 2

[0061] Two lithium salts, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide, were added to 1,3-dioxane (DIOX) in a molar ratio of 1:1 and stirred thoroughly to dissolve, resulting in a mixed solution. The molar concentration of lithium ions in the two salts in the low freezing point solvent was 5 mol / L.

[0062] The prepared low-temperature electrolyte was used as the electrolyte for lithium-ion batteries. It was assembled into lithium-ion batteries together with lithium iron phosphate cathode, porous graphite anode and PP separator, and its electrochemical performance was tested.

[0063] Comparative Example 3

[0064] S1: Heat EC to a liquid state, and mix it with dimethylacetamide at a volume ratio of 8:6 at 60°C for 30 minutes. Then let it cool naturally to room temperature to obtain mixed solvent 1.

[0065] S2: Mixed solvent 2 is prepared by mixing 1,3-dioxolane, dichloromethane, and tetrafluoroethane with mixed solvent 1; the volume ratio of the four solvents, 1,3-dioxolane, dichloromethane, tetrafluoroethane, and dimethylacetamide, is 10:3:1:6.

[0066] S3. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide are added to the low freezing point solvent in S2 in a molar ratio of 1:1 and stirred thoroughly to dissolve, so as to obtain an electrolyte solution. The molar concentration of lithium ions in the two salts in the low freezing point solvent is 5 mol / L.

[0067] The prepared low-temperature electrolyte was used as the electrolyte for lithium-ion batteries. It was assembled into lithium-ion batteries together with lithium iron phosphate cathode, porous graphite anode and PP separator, and its electrochemical performance was tested.

[0068] The lithium-ion battery prepared in Example 1 was subjected to charge-discharge tests at different temperatures, and the test results are as follows: Figure 1 As shown; the lithium-ion battery prepared in Example 1 was subjected to cycle life tests at room temperature and low temperature, and the test results are as follows. Figure 2 As shown.

[0069] Table 1 lists the electrochemical performance test parameters and results of lithium-ion batteries for some examples and comparative examples.

[0070] Table 1. Electrochemical performance test results of lithium-ion batteries

[0071]

[0072]

[0073] The performance test results in the table show that the lithium-ion battery constructed using the low-temperature electrolyte prepared by the method of this invention exhibits superior low-temperature performance and safety stability. In Comparative Example 2, only the low-freezing-point solvent 1,3-dioxane (DIOX) and lithium salt were used to prepare the electrolyte. Although the freezing point of the electrolyte was reduced, the battery stability was poor, and the capacity retention rate at low temperatures was low. In Comparative Example 3, both a eutectic solvent and a low-freezing-point solvent were used, but the lack of stabilizing additives resulted in poor battery stability.

[0074] Based on the above examples and experimental data, the low-temperature electrolyte prepared according to the process parameters of Example 1 exhibits excellent low-temperature performance when used to construct lithium-ion batteries, maintaining more than 75% of the properties of room temperature at -60°C.

[0075] Those skilled in the art should understand that this invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to this invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A safe low-temperature lithium-ion battery electrolyte, characterized in that, The electrolyte comprises a lithium salt, a composite organic solvent, and additives. The lithium salt has a lithium ion molar concentration of 3-12 mol / L in the composite organic solvent. The composite organic solvent includes a eutectic solvent and a low-freezing-point solvent. The eutectic solvent is prepared by mixing ethylene carbonate and dimethylacetamide. The low-freezing-point solvent is a mixture of 1,3-dioxolane, dichloromethane, and tetrafluoroethane. The additives include cyclic sulfonates and imidazolium-based ionic liquids. The additives are pre-dissolved in a mixed solution of dichloromethane and dimethylacetamide and then gradually added to the electrolyte. In a mixed solution of dichloromethane and dimethylacetamide, the volume ratio of dichloromethane to dimethylacetamide is 1:

1. The lithium salt is a mixed salt of lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide; the molar ratio of lithium bis(trifluoromethanesulfonyl)imide to lithium bis(fluorosulfonyl)imide is 1~3:1; The cyclic sulfonate is methylene disulfonate and 1,1-cyclopropanediethanol sulfinate, and the imidazolium-based ionic liquid is 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide.

2. The safe low-temperature lithium-ion battery electrolyte according to claim 1, characterized in that, The molar concentrations of methane disulfonate, 1,1-cyclopropanediethanol sulfinate, and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide are all 0.05-0.2 mol / L, and the volume ratio is 1-3:1-3:2-4.

3. The safe low-temperature lithium-ion battery electrolyte according to claim 1, characterized in that, The volume ratio of 1,3-dioxolane, dichloromethane, tetrafluoroethane, and dimethylacetamide is 8~10:2~4:1~2:4~7.

4. The safe low-temperature lithium-ion battery electrolyte according to claim 1, characterized in that, The electrolyte has a voltage stability window of 0~4.85V and can maintain a viscosity of 0.35Pa·s at 60℃.

5. A method for preparing a safe low-temperature lithium-ion battery electrolyte according to any one of claims 1 to 4, characterized in that, The following steps are included; S1: Heat EC until it becomes liquid, mix it with dimethylacetamide and stir until homogeneous, then let it cool naturally to room temperature to obtain mixed solvent 1; S2: Mixing a low freezing point solvent and mixed solvent 1 to obtain mixed solvent 2; S3: Add lithium salt to mixed solvent 2 in proportion and stir thoroughly to dissolve and obtain a mixed solution; S4: Mix dichloromethane and dimethylacetamide to prepare mixed solvent 3; S5: Slowly add the additive to the mixed solvent 3 until fully dissolved, add it to the mixed solution, and stir at 10°C for 30~40 minutes to obtain a low-temperature lithium-ion battery electrolyte.

6. The method for preparing a safe low-temperature lithium-ion battery electrolyte according to claim 5, characterized in that, In step S1, EC is heated to a liquid state and mixed with dimethylacetamide at a temperature of 50~80℃ for 30 minutes.

7. Applying the safe low-temperature lithium-ion battery electrolyte according to any one of claims 1 to 4 to a low-temperature lithium-ion battery, characterized in that: The lithium-ion battery is composed of the aforementioned low-temperature electrolyte, a lithium iron phosphate positive electrode, a porous graphite negative electrode, and a separator; the lithium iron phosphate positive electrode and the porous graphite negative electrode are respectively disposed on both sides of the separator; the lithium-ion battery can operate at 60°C.

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