Safe low-temperature lithium ion battery electrolyte and preparation and application methods thereof

Abstract

The invention 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. Comprising 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, and the eutectic solvent is prepared by mixing ethylene carbonate and dimethylacetamide; the low freezing point solvent is a mixed solvent of 1, 3-dioxolame, dichloromethane and tetrafluoroethane; the additive comprises cyclic sulfonate and imidazolium-based ionic liquid, and the additive is dissolved in advance and then is gradually added into the electrolyte. The formation of lithium dendrites is further relieved, and the compatibility between the cycling stability of the metal lithium battery and the incombustibility of the electrolyte is realized. The lithium ion battery constructed by the electrolyte has excellent rate capability and cycling stability, and can work safely and stably at the temperature of-60 DEG C to 100 DEG C.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a safe low-temperature lithium ion battery electrolyte and a preparation and application method thereof. Background Art

[0002] Lithium-ion batteries are one of the most commercially available and widely used electrochemical energy storage devices. However, the capacity of lithium-ion batteries usually decreases rapidly below 0°C, which severely limits their use in cold regions. The electrolyte is one of the main reasons affecting the low-temperature performance of lithium-ion batteries. Therefore, it is beneficial to improve the low-temperature performance of lithium-ion batteries by rationally regulating the electrolyte composition and constructing a low-temperature electrolyte structure.

[0003] From the perspective of the physical and chemical properties of the electrolyte, the low temperature environment will inevitably affect the properties of the electrolyte such as ionic conductivity and viscosity. The slow molecular thermal motion and enhanced intermolecular interactions at low temperatures cause the migration of lithium ions in the main electrolyte to be severely hindered. The interface between the electrolyte and the electrode is the key site for charge transfer and electrode reaction. The slow charge transfer rate and severely hindered reaction kinetics at low temperatures limit the charge transfer and electrochemical reaction rate of lithium ions at the electrode-electrolyte interface. At present, improving the low-temperature performance of the electrolyte by using solvents with low freezing points and low polarity is the main means to optimize the low-temperature performance of lithium-ion batteries, but most of them are difficult to reconcile excellent low-temperature performance and good cycle stability. Therefore, designing and constructing a safe, stable, and low-temperature performance lithium-ion battery electrolyte is an urgent problem to be solved.

[0004] The information disclosed in this background technology section is only intended to deepen the understanding of the overall background technology of the present invention, and should not be regarded as acknowledging or suggesting in any form that the information constitutes the prior art known to those skilled in the art. Summary of the invention

[0005] The first object of the present invention is to provide a safe low-temperature lithium-ion battery electrolyte. The lithium-ion battery constructed with this electrolyte has excellent rate performance and cycle stability, and can work safely at -60°C to 100°C.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions:

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

[0008] The molar concentration of lithium ions in lithium salt in the composite organic solvent is 3-12 mol / L; high-concentration lithium salt forms a very stable "solvated ion cluster" in the composite organic solvent. This protects the composite organic solvent from participating in the side reaction with metallic lithium, so that the electrolyte exhibits similar interface stability to the high-concentration system. The preferred molar concentration is 3-5 mol / L, which can minimize the use of lithium salt and reduce costs while ensuring a wide voltage window.

[0009] The composite organic solvent includes a low eutectic solvent and a low freezing point solvent. The volume ratio between the low eutectic solvent and the low freezing point solvent is 1:1. The low eutectic solvent is prepared by mixing ethylene carbonate (EC) and dimethylacetamide (DMAC); DMAC acts as a hydrogen bond donor and EC acts as a hydrogen bond acceptor. After the two are mixed, hydrogen bonds are formed. The strong hydrogen bond effect reduces the freezing point of the formed mixed liquid, which is a low eutectic system. At the same time, it helps to avoid the precipitation of EC solid under low temperature conditions. After mixing, EC is completely dissolved, and its dissolution state will not be affected by the introduction of other solvents. The low freezing point solvent is a mixed solvent of 1,3 dioxolane (DIOX), dichloromethane and tetrafluoroethane; it can reduce the overall freezing point of the electrolyte and ensure that it still has good fluidity under low temperature conditions.

[0010] Preferably, the volume ratio of 1,3-dioxolane, dichloromethane, tetrafluoroethane, and dimethylacetamide is 8-10:2-4:1-2:4-7. The above ratio design makes the multi-function complementary, can take into account both low-temperature conductivity and high-temperature stability, and ensure that the electrolyte maintains good performance in a wide temperature range. At the same time, it improves conductivity, reduces viscosity, optimizes the solubility of lithium salts, and maintains fluidity at low temperatures.

[0011] In the composite organic solvent designed by the present invention, the low freezing point solvent has excellent antioxidant properties. While dimethylacetamide acts as a hydrogen bond donor solvent, it is mixed with the low freezing point solvent to form an electrochemically inert diluent with low viscosity, which fully guarantees the wide voltage window of the high-concentration electrolyte and solves the high viscosity problem caused by high salt concentration. While ensuring the high stability of the solvent, the freezing point of the electrolyte can be lowered, so that the electrolyte can still remain liquid at low temperatures, and lithium ions can still achieve rapid migration, ensuring the stable operation of lithium-ion batteries at low temperatures. By designing a high-concentration lithium salt electrolyte, the present invention can adjust the solvation configuration of lithium ions, reduce the number of lithium ion solvations, reduce the desolvation energy barrier, ensure a rapid lithium insertion process in lithium-ion batteries, and improve the rate and low-temperature performance of lithium-ion batteries.

[0012] Unlike traditional high-concentration electrolytes, the present invention adds a large amount of "inactive diluents". These diluents (such as tetrafluoroethane and dimethylacetamide) will not directly form solvates with lithium ions, and mainly play a role in reducing the overall viscosity and melting point of the entire system. At the same time, they will not significantly affect the properties of the solvated ion clusters formed by high-concentration lithium salts, allowing the electrolyte to maintain good fluidity under low temperature conditions, thereby improving the lithium ion transmission efficiency.

[0013] The additives include cyclic sulfonates and imidazolium-based ionic liquids, which are pre-dissolved in a mixed solution of dichloromethane and dimethylacetamide, wherein the volume ratio of dichloromethane to dimethylacetamide is 1:1, and then gradually added to the electrolyte. Dichloromethane is a low-polarity, low-viscosity solvent with good dissolving power, which can effectively dissolve cyclic sulfonates and make them easier to disperse in the mixed solution; dimethylacetamide is a highly polar solvent that can effectively dissolve imidazolium-based ionic liquids and has a lower viscosity, which can help reduce the viscosity of the overall mixed solution and facilitate subsequent mixing and operation. The present invention can ensure that these additives are fully mixed and uniform before being added to the main electrolyte by pre-dissolving the cyclic sulfonates and imidazolium-based ionic liquids in a mixed solution of dichloromethane and dimethylacetamide. Problems caused by uneven dispersion of additives in the electrolyte are avoided.

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

[0015] As a battery additive, cyclic sulfonate has good high-temperature cycle performance and can reduce the polarization of the electrolyte, thereby improving the discharge platform of the battery. At the same time, it can also improve the antioxidant performance of the electrolyte, maintain the stable state of the electrolyte, and effectively improve the problems of poor high-temperature performance and poor stability caused by the use of a large amount of low-temperature solvents. In addition, after decomposition, the sulfonate can form a sulfonate-containing SEI layer with good lithium ion conductivity. Working together with the EC solvent, the formed SEI layer is thin and has stable composition, which can effectively reduce the internal resistance of the battery, thereby improving the battery's cycle performance and extending the battery's service life. The fluorinated groups and imidazole ring structures contained in 1-butyl-2,3-dimethylimidazole bis(trifluoromethanesulfonyl)imide can also form a dense SEI layer on the surface of the lithium metal negative electrode. The present invention further alleviates the formation of lithium dendrites by designing three mixed additives, thereby achieving compatibility between battery cycle stability and electrolyte non-flammability.

[0016] At the same time, in the composite organic solvent, the hydrogen bond network formed by EC and DMAC makes the melting point of the mixed system much lower than the melting points of EC and DMAC respectively. 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide can be used as both a hydrogen bond donor and a hydrogen bond acceptor. When added as an additive, it can not only alleviate the problem of dendrite growth and improve the battery life, but also act as a bridging hydrogen bond, making the hydrogen bond network of the entire system more compact and stable, thereby further enhancing the low eutectic solvent system of EC and DMAC, reducing the freezing point of the electrolyte, and improving the lithium ion solvation structure. In addition, due to the protective effect of 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, DMAC can be better and more evenly distributed on the negative electrode surface, thereby effectively improving its interfacial compatibility.

[0017] Preferably, the molar concentrations of methanedisulfonic acid methylene ester, 1,1-cyclopropane dimethanol sulfinic acid ester, and 1-butyl-2,3-dimethylimidazole bis(trifluoromethanesulfonyl)imide are all 0.05-0.2 mol / L, and the volume ratio is 1-3:1-3:2-4. The three additives work together to maximize their respective functions and achieve multi-dimensional regulation of the electrolyte. 1-Butyl-2,3-dimethylimidazole bis(trifluoromethanesulfonyl)imide has very low volatility and non-flammability, and a large volume ratio, which can significantly improve the overall safety of the electrolyte and reduce the risk of combustion and thermal runaway.

[0018] Preferably, the lithium salt is a mixed salt of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide; the molar ratio of lithium bis(trifluoromethylsulfonyl)imide to lithium bis(fluorosulfonyl)imide is 1 to 3:1. These two lithium salts have the advantages of high stability (no decomposition below 200°C), excellent low temperature performance, good hydrolysis stability and more environmentally friendly, and can effectively reduce the high and low temperature resistance of the SEI layer formed on the surface of the electrode plate at low temperatures, reduce the capacity loss of the lithium battery during storage, and thus provide high battery capacity and electrochemical performance of the battery.

[0019] The second object of the present 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 technical objectives of the present invention are achieved through the following technical solutions:

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

[0022] S1: EC is heated to a liquid state, and mixed with dimethylacetamide in a certain proportion at a temperature of 50-80°C and stirred for 30 min, and then naturally cooled to room temperature to obtain a mixed solvent 1;

[0023] The present invention heats and dissolves EC into DMAC first, so as to form a relatively uniform solution. Since DMAC is well compatible with EC, this first mixing sequence helps to avoid precipitation of EC solid under low temperature conditions. After mixing, EC is completely dissolved, and its dissolution state is not affected by the introduction of other solvents.

[0024] S2: mixing the low freezing point solvent and the mixed solvent 1 to prepare the mixed solvent 2;

[0025] In the present invention, EC and DMAC are mixed with a low freezing point solvent to ensure that the composite solvent system remains in a liquid state under extremely low temperature conditions, thereby enhancing the low temperature performance of the electrolyte.

[0026] S3: adding two lithium salts into the mixed solvent 2 in proportion, stirring and dissolving them to obtain a mixed solution;

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

[0028] S5: slowly add the additives into the mixed solvent 3 in sequence to fully dissolve, add the additives into the mixed solution, place at -10°C and stir for 30-40 minutes to obtain a low-temperature lithium-ion battery electrolyte.

[0029] The present invention pre-dissolves the additives in the mixed solvent 3, which helps to ensure that they are evenly dissolved and distributed, avoiding the problem of excessive local concentration that may be caused by direct addition to the mixed solution. The process of slowly adding the additives to the mixed solution can control the addition speed and stirring conditions to avoid excessive concentration gradients between different components, thereby affecting the uniformity and stability of the electrolyte. Stirring at a low temperature of -10°C can simulate the state of the electrolyte in a low temperature environment, ensuring that each component still has good solubility and stability at low temperatures.

[0030] The third object of the present invention is to provide an application method of a safe low-temperature lithium-ion battery electrolyte. The lithium-ion battery constructed with this electrolyte has excellent rate performance and cycle stability, and can work safely at -60°C to 100°C.

[0031] The above technical objectives of the present invention are achieved through the following technical solutions:

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

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

[0034] 1. The dual salt electrolyte system constructed by the present invention, the high concentration salt system of 3 to 12 mol / L makes Li + The number of solvations outside the electrolyte is reduced, and the two lithium salts effectively enhance the solvation structure of the anion, accelerate the reaction kinetics at the interface, increase the migration rate of lithium ions at the electrode interface at low temperatures, and improve the low-temperature performance of the battery. At the same time, by optimizing the design of the co-solvent electrolyte system, the solvation layer structure of the lithium salt and the dimethylacetamide-based electrolyte is not affected by the diluent 1,3-dioxolane, dichloromethane and tetrafluoroethane, which ensures that the electrolyte has a wide voltage stability window of 0-4.85V, maintains a viscosity of 0.35Pa·s at -60℃, and improves the ionic conductivity to 1.2mS cm -1 and wetting ability, and exhibits chemical and electrochemical stability toward metallic lithium.

[0035] 2. The present invention uses three electrolyte additives. On the one hand, the interfacial stability of metallic lithium is enhanced through the synergistic effect of 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide and DMAC, and the solvation structure containing a large number of aggregates in the electrolyte is further improved, which is conducive to the generation of anion-derived SEI, effectively alleviates the formation of lithium dendrites, and improves the coulombic efficiency, thereby achieving compatibility between the cycle stability of metallic lithium batteries and the non-flammability of electrolytes; on the other hand, two cyclic sulfonate-based electrolyte additives, methanedisulfonic acid methylene ester and 1,1-cyclopropane dimethanol sulfinic acid ester, can reduce the polarization degree of the electrolyte, thereby improving the discharge platform of the battery; and further improve the antioxidant performance 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 service life of the battery.

[0036] 3. The safe low-temperature electrolyte prepared by the above-mentioned preparation method of the present invention is applied to lithium-ion batteries. In the assembled full battery, the capacity retention rate after 1000 cycles at -60°C is 86%. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0038] Figure 1 The charge and discharge test curves of the lithium ion battery at different temperatures in Example 1;

[0039] Figure 2 This is a diagram of the cycle life of the lithium-ion battery in Example 1 at low temperature. DETAILED DESCRIPTION

[0040] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

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

[0043]

[0044] Example 1

[0045] S1: EC was heated to a liquid state, and mixed with dimethylacetamide at a volume ratio of 8:6 at 60°C and stirred for 30 min, and then naturally cooled to room temperature to obtain a mixed solvent 1;

[0046] S2: 1,3 dioxolane, dichloromethane, tetrafluoroethane and mixed solvent 1 are mixed to prepare mixed solvent 2; the volume ratio of the four solvents of 1,3 dioxolane, dichloromethane, tetrafluoroethane and dimethylacetamide is 10:3:1:6;

[0047] S3: adding two lithium salts, lithium bis(trifluoromethylsulfonyl imide) and lithium bis(fluorosulfonyl imide), into the mixed solvent 2 in a molar ratio of 1:1, and stirring and dissolving them to 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: mixing dichloromethane and dimethylacetamide in a volume ratio of 1:1 to prepare a mixed solvent 3;

[0049] S5: methylene disulfonate, 1,1-cyclopropane dimethanol sulfinic acid ester and 1-butyl-2,3-dimethylimidazole bis(trifluoromethanesulfonyl)imide are slowly added to the mixed solvent 3 in a volume ratio of 1:1:2 to fully dissolve, wherein the molar concentration of methylene disulfonate is 0.05 mol / L, and then added to the mixed solution, placed at -10°C and stirred for 30 to 40 minutes to obtain a low-temperature lithium-ion battery electrolyte.

[0050] The prepared low-temperature electrolyte was used as a lithium-ion battery electrolyte, assembled into a lithium-ion battery together with a lithium iron phosphate positive electrode, a porous graphite negative electrode and a PP separator, and its electrochemical properties were tested.

[0051] Example 2

[0052] S1: EC was heated to a liquid state, and mixed with dimethylacetamide at a volume ratio of 1:1 at 60°C and stirred for 30 min, and then naturally cooled to room temperature to obtain a mixed solvent 1;

[0053] S2: 1,3 dioxolane, dichloromethane, tetrafluoroethane and mixed solvent 1 are mixed to prepare mixed solvent 2; the volume ratio of the four solvents of 1,3 dioxolane, dichloromethane, tetrafluoroethane and dimethylacetamide is 9:4:1:7;

[0054] S3: adding two lithium salts, lithium bis(trifluoromethylsulfonyl imide) and lithium bis(fluorosulfonyl imide), into the mixed solvent 2 in a molar ratio of 1:1, and stirring and dissolving them to 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: mixing dichloromethane and dimethylacetamide in a volume ratio of 1:1 to prepare a mixed solvent 3;

[0056] S5: methylene disulfonate, 1,1-cyclopropane dimethanol sulfinic acid ester and 1-butyl-2,3-dimethylimidazole bis(trifluoromethanesulfonyl)imide are slowly added to the mixed solvent 3 in a volume ratio of 1:1:2 to fully dissolve, wherein the molar concentration of methylene disulfonate is 0.05 mol / L, and then added to the mixed solution, placed at -10°C and stirred for 30 to 40 minutes to obtain a low-temperature lithium-ion battery electrolyte.

[0057] The prepared low-temperature electrolyte was used as a lithium-ion battery electrolyte, assembled into a lithium-ion battery together with a lithium iron phosphate positive electrode, a porous graphite negative electrode and a PP separator, and its electrochemical properties were tested.

[0058] Comparative Example 1

[0059] Commercial electrolyte was used: 1M LiPF6 was dissolved in EC, DEC, and EMC (volume ratio of 1:1:1) as the lithium-ion battery electrolyte, and assembled into a lithium-ion battery together with a lithium iron phosphate positive electrode, a porous graphite negative electrode, and a pp separator, and its electrochemical performance was tested.

[0060] Comparative Example 2

[0061] Two lithium salts, lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide, are added to 1,3-dioxolane (DIOX) in a molar ratio of 1:1, and stirred and dissolved to obtain a mixed solution, wherein the molar concentration of lithium ions in the two salts in the low freezing point solvent is 5 mol / L;

[0062] The prepared low-temperature electrolyte was used as a lithium-ion battery electrolyte, assembled into a lithium-ion battery together with a lithium iron phosphate positive electrode, a porous graphite negative electrode and a PP separator, and its electrochemical properties were tested.

[0063] Comparative Example 3

[0064] S1: EC was heated to a liquid state, and mixed with dimethylacetamide at a volume ratio of 8:6 at 60°C and stirred for 30 min, and then naturally cooled to room temperature to obtain a mixed solvent 1;

[0065] S2: 1,3 dioxolane, dichloromethane, tetrafluoroethane and mixed solvent 1 are mixed to prepare mixed solvent 2; the volume ratio of the four solvents of 1,3 dioxolane, dichloromethane, tetrafluoroethane and dimethylacetamide is 10:3:1:6;

[0066] S3. Add two lithium salts, lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide, in a molar ratio of 1:1 to the low freezing point solvent in S2, and stir and dissolve them to obtain an electrolyte solution, wherein 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 a lithium-ion battery electrolyte, assembled into a lithium-ion battery together with a lithium iron phosphate positive electrode, a porous graphite negative electrode and a PP separator, and its electrochemical properties were tested.

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

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

[0070] Table 1 Lithium-ion battery electrochemical performance test results

[0071]

[0072]

[0073] It can be seen from the performance test results in the table that the lithium-ion battery constructed by the low-temperature electrolyte prepared by the preparation method of the present invention has more excellent low-temperature performance and safety stability. In Comparative Example 2, only the low-freezing point solvent 1,3-dioxolane (DIOX) and lithium salt are used to prepare the electrolyte. Although the freezing point of the electrolyte is reduced, the battery stability is poor and the capacity retention rate is low at low temperatures. In Comparative Example 3, a low eutectic solvent and a low freezing point solvent are used, but there is a lack of stabilizing additives, and the battery stability is poor.

[0074] In summary, the low-temperature electrolyte prepared based on the process parameters of Example 1 has excellent low-temperature performance when used to construct a lithium-ion battery, and can maintain more than 75% of the room temperature performance at -60°C.

[0075] Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A safe low-temperature lithium-ion battery electrolyte, characterized in that: Including lithium salt, composite organic solvent and additive; The molar concentration of lithium ions in the lithium salt in the composite organic solvent is 3 to 12 mol / L; The composite organic solvent includes a low eutectic solvent and a low freezing point solvent, wherein the low 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 cyclic sulfonic acid ester and imidazolium-based ionic liquid, and the additive is pre-dissolved in a mixed solution of dichloromethane and dimethylacetamide, and then gradually added into the electrolyte.

2. A safe low-temperature lithium-ion battery electrolyte according to claim 1, characterized in that: In the mixed solution of dichloromethane and dimethylacetamide, the volume ratio of dichloromethane to dimethylacetamide is 1:

1.

3. A safe low-temperature lithium-ion battery electrolyte according to claim 1, characterized in that: The lithium salt is a mixed salt of lithium bis(trifluoromethylsulfonyl imide) and lithium bis(fluorosulfonyl imide) salt; the molar ratio of lithium bis(trifluoromethylsulfonyl imide) and lithium bis(fluorosulfonyl imide) salt is 1-3:

1.

4. A safe low-temperature lithium-ion battery electrolyte according to claim 1, characterized in that: The cyclic sulfonate is methylene disulfonate and 1,1-cyclopropane dimethanol sulfinic acid ester, and the imidazolium-based ionic liquid is 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide.

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

6. A 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.

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

8. The method for preparing a safe low-temperature lithium-ion battery electrolyte according to any one of claims 1 to 7, characterized in that: The following steps are included: S1: Heat EC to liquid state and mix with dimethylacetamide and stir evenly, then cool naturally to room temperature to obtain mixed solvent 1; S2: mixing the low freezing point solvent and the mixed solvent 1 to prepare the mixed solvent 2; S3: adding lithium salt into the mixed solvent 2 in proportion, stirring and dissolving to obtain a mixed solution; S4: mixing dichloromethane and dimethylacetamide to prepare a mixed solvent 3; S5: slowly add the additive into the mixed solvent 3 to fully dissolve it, add it into the mixed solution, place it at -10°C and stir for 30-40 min to obtain a low-temperature lithium-ion battery electrolyte.

9. The method for preparing a safe low-temperature lithium-ion battery electrolyte according to claim 8, characterized in that: In step S1, EC is heated to a liquid state, and mixed with dimethylacetamide in a certain proportion at a temperature of 50-80°C and stirred for 30 minutes.

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

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