Preparation method and application of safe electrolyte

Abstract

The invention discloses a preparation method and application of a safe electrolyte, and belongs to the technical field of electrolytes.The preparation method comprises the steps that a carbonic ester solvent, imidazolyl ionic liquid, a lithium salt and a composite additive are dried and dewatered, the carbonic ester solvent and the imidazolyl ionic liquid are prepared into an electrolyte organic solvent in a water-free and oxygen-free glove box, and the electrolyte organic solvent is added into a lithium ion battery; adding the lithium salt and the composite additive into the electrolyte organic solvent, and uniformly mixing to obtain the safe electrolyte; the safe electrolyte keeps the liquid-state high ionic conductivity characteristic in a normal working temperature interval, liquid-solid phase change is rapidly generated when the temperature of the battery rises to a critical threshold value, a three-dimensional net-shaped solid-state barrier layer is formed, a lithium ion transmission path is cut off so as to restrain the hot chain reaction, meanwhile, lithium fluoride salt with dynamic coordination capacity is introduced into the safe electrolyte system, and therefore the safety performance of the lithium ion battery is improved. A stable oxidation-reduction potential window is maintained in the phase change process, the integrity of electrical performance after high-temperature phase change is ensured, the electrolyte reversibly recovers to be in a liquid state after cooling, and the performance of the battery is basically not attenuated.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrolytes, and specifically relates to a preparation method and application of a safe electrolyte. Background Art

[0002] With the continuous improvement of the energy density of lithium-ion batteries, their thermal safety issues have become a key bottleneck restricting the development of high-safety application scenarios (such as electric vehicles and energy storage systems). Traditional carbonate-based electrolytes are prone to violent decomposition and gasification at high temperatures, and uncontrollable side reactions occur with electrode materials, leading to a sharp increase in the risk of thermal runaway. Existing technologies mainly improve thermal stability by adding flame retardants, developing solid electrolytes, or using high-boiling-point solvents, but there are still significant defects: flame retardants easily deteriorate the electrochemical performance of electrolytes; solid electrolytes face problems such as high interfacial impedance and poor cycle stability; while high-boiling-point solvents can delay thermal failure but cannot actively cut off the ion transport path during the heat accumulation stage.

[0003] In recent years, thermal management strategies based on phase change materials have received attention, but their integration into the electrolyte system still faces two major challenges: one is the poor compatibility of traditional phase change materials (such as paraffin) with lithium salts / solvents, which easily leads to phase separation; the other is that existing phase change electrolytes mostly adopt irreversible curing mechanisms, which can block thermal runaway but sacrifice the repairability of the battery. In addition, the stability of the redox potential of the electrolyte at high temperatures directly affects the controllability of the positive electrode interface reaction. The traditional system is prone to potential instability during the phase change process, exacerbating metal dissolution and structural degradation. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method and application of a safe electrolyte. The present invention proposes a reversible liquid-solid phase change electrolyte system with temperature-responsive characteristics, that is, a safe electrolyte.

[0005] The safe electrolyte maintains high ionic conductivity in the liquid state within the normal operating temperature range (-20~60°C). When the battery temperature rises to the critical threshold (80~120°C), it quickly undergoes a liquid-solid phase change to form a three-dimensional network solid barrier layer, cutting off the lithium-ion transport path to inhibit the thermal chain reaction. At the same time, this safe electrolyte system introduces a lithium fluoride salt with dynamic coordination ability, maintaining a stable redox potential window (>4.5V vs. Li+ / Li) during the phase change process to ensure the integrity of the electrical performance after high-temperature phase change. After cooling, the electrolyte reversibly returns to the liquid state, and the battery performance basically does not decay, solving the problems that the traditional electrolyte system is prone to potential instability and exacerbating metal dissolution and structural degradation during the phase change process.

[0006] The purpose of the present invention can be achieved through the following technical solutions: A preparation method of a safe electrolyte, comprising the following steps: Step 1: Dry and remove water from the carbonate solvent, imidazolium ionic liquid, lithium salt, and composite additive, and control their water content < 20 ppm; Step 2: Prepare an electrolyte organic solvent by mixing the carbonate solvent and imidazolium ionic liquid in an anhydrous and anaerobic glove box; Step 3: Add the lithium salt and composite additive to the electrolyte organic solvent and mix evenly to obtain a safe electrolyte.

[0007] Preferably, by mass percentage, the safe electrolyte comprises the following components: Electrolyte organic solvent 70 - 90%wt, lithium salt 5 - 20%wt, composite additive 0.1 - 10%wt.

[0008] Preferably, the content of the imidazolium ionic liquid in the electrolyte organic solvent is 10 - 50%wt.

[0009] Preferably, the carbonate solvent is composed of ethylene carbonate and diethyl carbonate mixed in a mass ratio of 1:1.

[0010] Preferably, the imidazolium ionic liquid is any one of 1,3 - dimethylimidazolium bis(trifluoromethanesulfonyl)imide, 1 - ethyl - 3 - methylimidazolium hexafluorophosphate, 1 - butyl - 3 - methylimidazolium bis(trifluoromethanesulfonyl)imide, 1 - octyl - 3 - methylimidazolium tetrafluoroborate, and 1 - hydroxyethyl - 3 - methylimidazolium chloride.

[0011] Preferably, the lithium salt is any one or a combination of any mass ratios of lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluoro(oxalato)borate, and lithium bis(fluorosulfonyl)imide.

[0012] Preferably, the composite additive is any one or a combination of any mass ratios of poly(N - isopropylacrylamide), poly(benzyl methacrylate), polyetheramine, and poly(N - isopropylacrylamide - glycidyl methacrylate).

[0013] The present invention also provides an application of the safe electrolyte in a lithium - ion battery, and the lithium - ion battery further comprises a positive electrode, a negative electrode, and a separator.

[0014] Preferably, the active material in the positive electrode is LiCoO2, LiMn2O4, LiFePO4, LiNi x Co y Mn z O2, LiNi x Co y Mn z M 1-x-y-z O and LiNi x Co y Al z N1-x-y-z One or more of O2; wherein, M and N are each independently selected from one of Mg, Al, Mo, Zn, B, Zr, La, Ga, Cr, V and Ti, 1≥x≥0.5, 0.5≥y≥0, 0.5≥z≥0, and x + y + z ≤ 1, and the values of x, y, and z make the general formula satisfy the valence balance.

[0015] Preferably, the active material in the negative electrode includes one or more of natural graphite, artificial graphite, and silicon-carbon composite materials. The separator is a polyolefin porous membrane.

[0016] Advantages of the present invention: 1. The safety electrolyte prepared by the preparation method of the present invention has the function of active thermal safety regulation. The safety electrolyte undergoes rapid reversible liquid-solid phase change at high temperature (80~120°C) to form a three-dimensional solid barrier layer, actively cutting off the ion transport path, effectively inhibiting the thermal runaway chain reaction, enabling the battery to achieve "self-power-off" protection under extreme temperature rise conditions, which is significantly higher than the traditional electrolyte system (usually <130°C).

[0017] 2. The safety electrolyte prepared by the preparation method of the present invention has reversible repair characteristics. Different from the irreversible solidified electrolyte, the safety electrolyte system can return to the liquid state and reconstruct a uniform ion-conducting network after the temperature drops, and the reversible recovery rates of battery capacity and impedance are >98%. It solves the pain point of "one-time failure" of traditional thermal protection electrolytes and extends the safety of the entire battery life cycle.

[0018] 3. The preparation method of the present invention has compatibility and cost advantages. It adopts a solution blending process, adapts to existing electrolyte injection equipment, and does not require production line transformation; compared with solid electrolytes, the safety electrolyte has a smaller addition amount of phase change materials, and the cost is much lower than that of solid electrolyte technology. Description of the drawings

[0019] Figure 1 It is a comparison chart of the room temperature performance and performance at 80°C of the electrolytes prepared in Example 1 and Comparative Example 1 of the present invention in coin cells; Figure 2 It is a comparison chart of the cycling performance at high temperature of the electrolytes prepared in Example 1 and Comparative Example 1 of the present invention in coin cells. Detailed implementation manners

[0020] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0021] In the examples, the imidazolium-based ionic liquids include but are not limited to 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-octyl-3-methylimidazolium tetrafluoroborate, and 1-hydroxyethyl-3-methylimidazolium chloride.

[0022] The lithium salts include but are not limited to lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluoro(oxalato)borate, and lithium bis(fluorosulfonyl)imide.

[0023] The composite additives include but are not limited to poly(N-isopropylacrylamide), poly(benzyl methacrylate), polyetheramine, and poly(N-isopropylacrylamide-glycidyl methacrylate).

[0024] Example 1: This example provides a safe electrolyte. By mass percentage, the safe electrolyte includes the following components: 85%wt of electrolyte organic solvent, 10%wt of lithium salt, and 5%wt of composite additive.

[0025] The specific preparation method of the safe electrolyte is as follows: Step 1: Mix ethylene carbonate and diethyl carbonate in a mass ratio of 1:1 to obtain a carbonate solvent. Dry and remove water from the carbonate solvent, imidazolium-based ionic liquid, lithium salt, and composite additive, and control its water content < 20 ppm.

[0026] Step 2: Prepare the electrolyte organic solvent from the carbonate solvent and imidazolium-based ionic liquid in an anhydrous and anaerobic glove box. The mass ratio of the carbonate solvent to the imidazolium-based ionic liquid is 2:1, and the imidazolium-based ionic liquid selected is 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide.

[0027] Step 3: Add the lithium salt and composite additive to the electrolyte organic solvent prepared in Step 2 and mix evenly to obtain the safe electrolyte. Among them, the lithium salt selected is lithium bis(trifluoromethanesulfonyl)imide, and the composite additive selected is poly(benzyl methacrylate).

[0028] Example 2: This example provides a safe electrolyte. The difference between Example 2 and Example 1 is that the mass ratio of the carbonate solvent to the imidazolium-based ionic liquid is 9:1.

[0029] By mass percentage, the safe electrolyte includes the following components: 85%wt of electrolyte organic solvent, 10%wt of lithium salt, and 5%wt of composite additive.

[0030] The specific preparation method of the safe electrolyte is as follows: Step 1: Mix ethylene carbonate and diethyl carbonate in a mass ratio of 1:1 to obtain a carbonate solvent. Dry and remove water from the carbonate solvent, imidazolium ionic liquid, lithium salt, and composite additive, and control the water content to be <20 ppm. Step 2: Prepare an electrolyte organic solvent from the carbonate solvent and imidazolium ionic liquid in Step 1 in an anhydrous and oxygen-free glove box. The mass ratio of the carbonate solvent to the imidazolium ionic liquid is 9:1, and the imidazolium ionic liquid is 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.

[0031] Step 3: Add the lithium salt and composite additive to the electrolyte organic solvent in Step 2 and mix evenly to obtain a safe electrolyte. The lithium salt is lithium bis(trifluoromethylsulfonyl)imide, and the composite additive is poly(benzyl methacrylate).

[0032] Example 3: This example provides a safe electrolyte, which is different from Example 1 in that the mass ratio of the carbonate solvent to the imidazolium ionic liquid is 1:1.

[0033] By mass percentage, the safe electrolyte includes the following components: 85%wt of electrolyte organic solvent, 10%wt of lithium salt, and 5%wt of composite additive.

[0034] The specific preparation method of the safe electrolyte is as follows: Step 1: Mix ethylene carbonate and diethyl carbonate in a mass ratio of 1:1 to obtain a carbonate solvent. Dry and remove water from the carbonate solvent, imidazolium ionic liquid, lithium salt, and composite additive, and control the water content to be <20 ppm.

[0035] Step 2: Prepare an electrolyte organic solvent from the carbonate solvent and imidazolium ionic liquid in Step 1 in an anhydrous and oxygen-free glove box. The mass ratio of the carbonate solvent to the imidazolium ionic liquid is 1:1, and the imidazolium ionic liquid is 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.

[0036] Step 3: Add the lithium salt and composite additive to the electrolyte organic solvent in Step 2 and mix evenly to obtain a safe electrolyte. The lithium salt is lithium bis(trifluoromethylsulfonyl)imide, and the composite additive is poly(benzyl methacrylate).

[0037] Example 4: This example provides a safe electrolyte, which is different from Example 1 in that the concentration of the composite additive in the safety is adjusted to 0.1%.

[0038] By mass percentage, the safe electrolyte includes the following components: 89.9%wt of electrolyte organic solvent, 10%wt of lithium salt, and 0.1%wt of composite additive.

[0039] The specific preparation method of the safe electrolyte is as follows: Step 1: Mix ethylene carbonate and diethyl carbonate in a mass ratio of 1:1 to obtain a carbonate solvent. Dry and remove water from the carbonate solvent, imidazolium ionic liquid, lithium salt, and composite additive, and control the water content to be <20 ppm.

[0040] Step 2: Prepare an electrolyte organic solvent from the carbonate solvent and imidazolium ionic liquid in Step 1 in an anhydrous and anaerobic glove box. The mass ratio of the carbonate solvent to the imidazolium ionic liquid is 2:1, and the imidazolium ionic liquid is 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.

[0041] Step 3: Add the lithium salt and composite additive to the electrolyte organic solvent in Step 2 and mix evenly to obtain a safe electrolyte. The lithium salt is lithium bis(trifluoromethylsulfonyl)imide, and the composite additive is poly(benzyl methacrylate).

[0042] Example 5: This example provides a safe electrolyte, which is different from Example 1 in that the concentration of the composite additive in the safety is adjusted to 10%.

[0043] By mass percentage, the safe electrolyte includes the following components: 80%wt of electrolyte organic solvent, 10%wt of lithium salt, and 10%wt of composite additive.

[0044] The specific preparation method of the safe electrolyte is as follows: Step 1: Mix ethylene carbonate and diethyl carbonate in a mass ratio of 1:1 to obtain a carbonate solvent. Dry and remove water from the carbonate solvent, imidazolium ionic liquid, lithium salt, and composite additive, and control the water content to be <20 ppm.

[0045] Step 2: Prepare an electrolyte organic solvent from the carbonate solvent and imidazolium ionic liquid in Step 1 in an anhydrous and anaerobic glove box. The mass ratio of the carbonate solvent to the imidazolium ionic liquid is 2:1, and the imidazolium ionic liquid is 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.

[0046] Step 3: Add the lithium salt and composite additive to the electrolyte organic solvent in Step 2 and mix evenly to obtain a safe electrolyte. The lithium salt is lithium bis(trifluoromethylsulfonyl)imide, and the composite additive is poly(benzyl methacrylate).

[0047] Example 6: This example provides a safe electrolyte, which is different from Example 1 in that the imidazolium ionic liquid is 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide.

[0048] The safe electrolyte includes the following components: 85%wt of electrolyte organic solvent, 10%wt of lithium salt, and 5%wt of composite additive.

[0049] The specific preparation method of the safe electrolyte is as follows: Step 1: Mix ethylene carbonate and diethyl carbonate in a mass ratio of 1:1 to obtain a carbonate solvent. Dry and remove water from the carbonate solvent, imidazolium ionic liquid, lithium salt, and composite additive, and control its water content < 20 ppm.

[0050] Step 2: Prepare an electrolyte organic solvent from the carbonate solvent and imidazolium ionic liquid in Step 1 in an anhydrous and oxygen-free glove box. The mass ratio of the carbonate solvent to the imidazolium ionic liquid is 2:1, and the imidazolium ionic liquid is selected as 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide.

[0051] Step 3: Add the lithium salt and composite additive to the electrolyte organic solvent in Step 2 and mix evenly to obtain a safe electrolyte. The lithium salt is selected as lithium bis(trifluoromethanesulfonyl)imide, and the composite additive is selected as poly(benzyl methacrylate).

[0052] Example 7: This example provides a safe electrolyte. The difference from Example 1 is that the imidazolium ionic liquid is selected as 1-ethyl-3-methylimidazolium hexafluorophosphate, and the lithium salt is selected as lithium hexafluorophosphate.

[0053] The safe electrolyte includes the following components: 85%wt of electrolyte organic solvent, 10%wt of lithium salt, and 5%wt of composite additive.

[0054] The specific preparation method of the safe electrolyte is as follows: Step 1: Mix ethylene carbonate and diethyl carbonate in a mass ratio of 1:1 to obtain a carbonate solvent. Dry and remove water from the carbonate solvent, imidazolium ionic liquid, lithium salt, and composite additive, and control its water content < 20 ppm.

[0055] Step 2: Prepare an electrolyte organic solvent from the carbonate solvent and imidazolium ionic liquid in Step 1 in an anhydrous and oxygen-free glove box. The mass ratio of the carbonate solvent to the imidazolium ionic liquid is 2:1, and the imidazolium ionic liquid is selected as 1-ethyl-3-methylimidazolium hexafluorophosphate.

[0056] Step 3: Add the lithium salt and composite additive to the electrolyte organic solvent in Step 2 and mix evenly to obtain a safe electrolyte. The lithium salt is selected as lithium hexafluorophosphate, and the composite additive is selected as poly(benzyl methacrylate).

[0057] Example 8: This example provides a safe electrolyte. The difference from Example 1 is that the composite additive is selected as poly(N-isopropylacrylamide). By mass percentage, the safe electrolyte includes the following components: 85%wt of electrolyte organic solvent, 10%wt of lithium salt, and 5%wt of composite additive.

[0058] The specific preparation method of the safety electrolyte is as follows: Step 1: Mix ethylene carbonate and diethyl carbonate in a mass ratio of 1:1 to obtain a carbonate solvent. Dry and remove water from the carbonate solvent, imidazolium ionic liquid, lithium salt, and composite additive, and control its water content < 20 ppm.

[0059] Step 2: Prepare an electrolyte organic solvent from the carbonate solvent and imidazolium ionic liquid in Step 1 in an anhydrous and oxygen-free glove box. The mass ratio of the carbonate solvent to the imidazolium ionic liquid is 2:1, and the imidazolium ionic liquid is 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.

[0060] Step 3: Add a lithium salt and a composite additive to the electrolyte organic solvent in Step 2 and mix evenly to obtain a safety electrolyte. The lithium salt is lithium bis(trifluoromethylsulfonyl)imide, and the composite additive is poly(N-isopropylacrylamide).

[0061] Comparative Example 1: Based on Example 1, no imidazolium ionic liquid is added to the electrolyte organic solvent, and no composite additive is added in Step 3. The remaining steps remain unchanged to prepare a safety electrolyte.

[0062] Comparative Example 2: Based on Example 1, no imidazolium ionic liquid is added to the electrolyte organic solvent, and the remaining steps remain unchanged to prepare a safety electrolyte.

[0063] Comparative Example 3: Based on Example 1, no composite additive is added in Step 3, and the remaining steps remain unchanged to prepare a safety electrolyte.

[0064] Application Example: The application of a safety electrolyte in a lithium-ion battery, which also includes a positive electrode, a negative electrode, and a separator.

[0065] The active material in the positive electrode can be one or more of LiCoO2, LiMn2O4, LiFePO4, LiNi x Co y Mn z O2, LiNi x Co y Mn z M 1-x-y-z O or LiNi x Co y Al z N 1-x-y-z O2; wherein, M and N are each independently selected from one or more of Mg, Al, Mo, Zn, B, Zr, La, Ga, Cr, V, and Ti, 1≥x≥0.5, 0.5≥y≥0, 0.5≥z≥0, and x + y + z ≤ 1, and the values of x, y, and z make the general formula satisfy valence balance.

[0066] The active material in the negative electrode includes one or more of natural graphite, artificial graphite, and silicon-carbon composite materials. The separator is a polyolefin porous membrane.

[0067] In this application example, NCM811 is selected as the positive electrode active material. The positive electrode active material, conductive carbon black, and binder PVDF are mixed in a mass ratio of 8:1:1, and then dispersed in the solvent NMP to obtain a positive electrode slurry with a solid content of 50%. The positive electrode slurry is uniformly coated on the positive electrode current collector aluminum foil, dried, rolled, and die-cut by a wooden mold to obtain the positive electrode.

[0068] The environmentally friendly electrolytes in Examples 1-8 and Comparative Examples 1-3 and the positive electrode in the application example are assembled into a coin-type half-cell for electrochemical testing. A lithium sheet is selected as the negative electrode, and the test voltage is 2.8V-4.3V. The results are shown in Table 1: Table 1 As can be seen from Table 1, by using the phase change safety electrolysis of Examples 1-8 of the present invention, the impedance can be effectively increased at high temperatures, and the battery capacity can be reduced to less than 1000 mAh. And it has good comprehensive electrochemical performance. The capacity retention rate after 100 cycles at room temperature reaches more than 95%, and the capacity retention rate after 100 cycles at high temperature is more than 90%. While the capacity retention rate after 100 cycles at room temperature in Comparative Examples 1, 2, and 3 is less than 93%, and the capacity retention rate after 100 cycles at high temperature is less than 90%. Compared with the examples, the irreversible capacity loss at high temperatures is larger and the cycle performance is poorer.

[0069] Among them, for the comparison of the room temperature performance and the performance at 80°C of the electrolytes prepared in Example 1 and Comparative Example 1 in a coin-type battery, please refer to Figure 1 , and for the comparison of the cycle performance at high temperatures, please refer to Figure 2 . It can be clearly seen that the cycle performance of Example 1 is better than that of Comparative Example 1, indicating that Example 1 can improve the cycle temperature performance at high temperatures.

[0070] It should be noted that in this article, terms such as "including", "comprising", or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article, or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or elements inherent to such a process, method, article, or device.

[0071] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention.

Claims

1. A method for preparing a safe electrolyte, characterized in that, It includes the following steps: Step 1: Dry and remove water from the carbonate solvent, imidazolium ionic liquid, lithium salt and composite additive, and control its water content < 20 ppm; Step 2: Prepare an electrolyte organic solvent by mixing the carbonate solvent and the imidazolium ionic liquid in an anhydrous and oxygen-free glove box; Step 3: Add the lithium salt and the composite additive to the electrolyte organic solvent and mix evenly to obtain a safe electrolyte.

2. The preparation method of a safe electrolyte according to claim 1, wherein By mass percentage, the safe electrolyte includes the following components: 70 - 90%wt of electrolyte organic solvent, 5 - 20%wt of lithium salt, 0.1 - 10%wt of composite additive.

3. The preparation method of a safe electrolyte according to claim 1, wherein In Step 1, the carbonate solvent is composed of ethylene carbonate and diethyl carbonate mixed in a mass ratio of 1:

1.

4. The preparation method of a safe electrolyte according to claim 1, characterized in that In Step 2, the content of the imidazolium ionic liquid in the electrolyte organic solvent is 10 - 50%wt.

5. The preparation method of a safe electrolyte according to claim 1, wherein, The imidazolium ionic liquid is any one of 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-octyl-3-methylimidazolium tetrafluoroborate and 1-hydroxyethyl-3-methylimidazolium chloride.

6. The preparation method of a safe electrolyte according to claim 1, characterized in that, The lithium salt is any one or a combination of any mass ratios of lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluoro(oxalato)borate, lithium bis(fluorosulfonyl)imide.

7. The preparation method of a safe electrolyte according to claim 1, characterized in that, The composite additive is any one or a combination of any mass ratios of poly(N-isopropylacrylamide), poly(benzyl methacrylate), polyetheramine, poly(N-isopropylacrylamide-glycidyl methacrylate).

8. Application of a safe electrolyte in a lithium-ion battery, characterized in that, The safe electrolyte is prepared by the preparation method described in any one of claims 1 - 7.

9. The application of a safety electrolyte in a lithium-ion battery according to claim 8, characterized in that, The lithium-ion battery further includes a positive electrode, and the active material in the positive electrode is one or more of LiCoO2, LiMn2O4, LiFePO4, LiNi x Co y Mn z O2, LiNi x Co y Mn z M 1-x-y-z O and LiNi x Co y Al z N 1-x-y-z O2, combined in any mass ratio.

10. The application of a safety electrolyte in a lithium-ion battery according to claim 9, characterized in that, General formula LiNi x Co y Mn z O2, LiNi x Co y Mn z M 1-x-y-z O and LiNi x Co y Al z N 1-x-y-z O2, where 1 ≥ x ≥ 0.5, 0.5 ≥ y ≥ 0, 0.5 ≥ z ≥ 0, and x + y + z ≤ 1, and the values of x, y, and z are such that the general formula satisfies valence balance; M and N are each independently selected from one of Mg, Al, Mo, Zn, B, Zr, La, Ga, Cr, V, and Ti.

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