Low-temperature phase change electrolyte, preparation method thereof and secondary battery

Abstract

The invention belongs to the related technical field of batteries, and discloses a low-temperature phase change electrolyte, a preparation method thereof and a secondary battery. The low-temperature phase change electrolyte is an organic solvent system, Pi-Pi interaction between a polymer containing a five-membered ring or a six-membered ring and imidazole ionic liquid with cations containing five-membered heterocyclic rings is utilized, when the temperature is higher than the LCST temperature, the Pi-Pi interaction between the polymer and the imidazole ionic liquid breaks, and phase change occurs. Meanwhile, benzene rings and thiophene of the polymer contain electron withdrawing groups or C1-C3 alkyl groups, and ethyoxyl / ether chain segments on the basis of a methyl acrylate structure, so that the phase change temperature of the polymer can be effectively reduced, and the phase change temperature of the electrolyte is further reduced. The phase-change electrolyte disclosed by the invention can generate phase change at a low temperature of 35-60 DEG C, and when the temperature of the battery is increased to 75 DEG C or above, the internal reaction of the battery is completely stopped, so that thermal runaway of the battery is prevented in the whole heating process.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of batteries, and particularly relates to a low-temperature phase change electrolyte and a preparation method thereof and a secondary battery. BACKGROUND

[0002] Lithium ion batteries have the advantages of high specific energy, light weight, long cycle life, etc., and dominate the application fields of consumer electronics, new energy vehicles, energy storage, etc. However, most of the current lithium ion batteries use organic solvents as electrolytes, which have a series of safety problems in high-temperature environments, such as electrolyte decomposition, gasification, etc. In addition, these electrolytes are flammable, which makes liquid lithium ion batteries face serious safety challenges in high-temperature environments.

[0003] Solid-state batteries use non-flammable solid electrolytes to replace flammable organic solvents, which can greatly reduce the safety hazards related to high-temperature thermal runaway and electrolyte combustion, and is a solution to improve battery safety. Moreover, solid-state batteries can also resist lithium dendrites, so they can match metal lithium with ultra-high theoretical specific capacity (3860 mAh / g) as the negative electrode material. However, solid-state batteries face the challenge of a significant decrease in ionic conductivity in low-temperature environments (below -15℃), which is one of the key bottlenecks restricting the application of solid-state batteries in low-temperature scenarios. For example, the ionic conductivity of sulfide solid-state electrolytes at -20℃ is only 0.1-0.2 mS / cm.

[0004] Temperature-sensitive polymers are a class of high molecular materials that can undergo reversible phase transition in response to temperature changes. They can be divided into lowest critical solution temperature (LCST) type and upper critical solution temperature (UCST) type according to the type of phase transition. When the temperature is higher than LCST, the polymer changes from hydrophilic dissolution to hydrophobic precipitation. When the temperature is lower than UCST, the polymer precipitates from the solution. Therefore, LCST type temperature-sensitive polymers provide a solution to the high-temperature thermal runaway problem of electrolytes.

[0005] Currently, temperature-sensitive polymers are mainly applied to aqueous electrolytes, and phase transition is achieved through the action of hydrogen bonds. For example, the invention patent with the publication number CN110938170A discloses a reversible overheat protection aqueous electrolyte based on methyl cellulose grafted isopropyl acrylamide. However, in order to adapt to lithium ion battery systems with higher voltage, the polymer needs to undergo phase transition in ionic liquids. However, the phase transition temperature of traditional temperature-sensitive polymers in ionic liquids is relatively high (about 80-120℃), and the ionic conductivity in low-temperature environments is also relatively low.

[0006] Therefore, it is urgent to develop a phase change electrolyte that has a lower phase transition temperature in an organic solvent system and can maintain a high ionic conductivity in a low-temperature environment. SUMMARY

[0007] The present application aims to at least solve one of the technical problems existing in the prior art. To this end, the present application provides a low-temperature phase change electrolyte and a preparation method thereof, and a secondary battery, which can undergo phase change at a low temperature of 35-60℃ and still maintain high ionic conductivity in a low-temperature environment of -20℃.

[0008] The inventive concept of the present application is that the low-temperature phase change electrolyte of the present application is an organic solvent system, and a specific polymer and an imidazole-based ionic liquid are used as main raw materials. The polymer undergoes phase change in the ionic liquid through the LCST phenomenon, and the polymer contains a five-membered ring (thiophene) or a six-membered ring (benzene ring), which can form a large π bond. The cation of the imidazole-based ionic liquid contains a five-membered heterocyclic ring, which can also form a large π bond. Therefore, π-π interaction occurs between the polymer and the imidazole-based ionic liquid. When the temperature is lower than the LCST temperature, the polymer is dissolved in the ionic liquid in the form of a thread; when the temperature is higher than the LCST temperature, the π-π interaction between the polymer and the imidazole-based ionic liquid is broken, the polymer precipitates from the ionic liquid, and itself undergoes agglomeration to grow into larger particles, and the electrolyte changes from a liquid state to a solid state, thereby undergoing phase change.

[0009] Meanwhile, the benzene ring and the thiophene of the polymer of the present application both contain a substituent group, which can be an electron-withdrawing group such as a trifluoromethyl group, a trichloromethyl group, a cyano group, or a nitro group. These electron-withdrawing groups can reduce the electron cloud density of the benzene ring and the thiophene, and weaken the π-π interaction between the polymer and the ionic liquid. The weaker the π-π interaction, the lower the phase change temperature of the polymer. The substituent group can also be a C1-C3 alkyl group, which increases the steric hindrance of the benzene ring and the thiophene, and also weakens the π-π interaction between the polymer and the ionic liquid, thereby reducing the phase change temperature of the polymer. In addition, by adding an ethoxy (EO) / ether group segment to the structure of methyl acrylate, the molecular flexibility can be improved, and the phase change temperature of the polymer can also be reduced.

[0010] In addition, a certain amount of lithium salt and specific additives (such as 2-thiouracil, 4-aminophenol, 3-sulfobenzoic acid, dimethyl sulfate, crown ether compounds, cryptand compounds) are added to the low-temperature phase change electrolyte of the present application. The additives are used to reduce the viscosity of the ionic liquid on the one hand, and to form complexes with lithium ions on the other hand, thereby promoting the dissociation of the lithium salt and improving the ionic conductivity of the electrolyte in a low-temperature environment.

[0011] Therefore, the phase change electrolyte of the present application can change phase from liquid to solid at low temperature of 35-60℃, and the phase change process is reversible, and the phase change temperature can be controlled by adjusting the types and contents of the polymer and the ionic liquid, and the content of the lithium salt. At the normal working temperature of the lithium ion battery (20-30℃), the electrolyte is in liquid state; when the battery temperature rises to 35-60℃, the electrolyte changes phase, and the decomposition temperature (about 80℃) of the SEI film has not been reached; after the phase change, the ionic conductivity of the electrolyte decreases, the capacity of the battery decreases, the electrochemical reaction in the battery is gradually blocked, and the temperature of the battery continues to rise; when the temperature of the battery rises to above 75℃, the internal reaction of the battery completely stops, and the heat runaway of the battery during the whole temperature rising process is effectively prevented.

[0012] To solve the above technical problems, the first aspect of the present application provides a low-temperature phase change electrolyte, the raw material components of which include a polymer, an ionic liquid and an organic solvent, the ionic liquid is an imidazole-based ionic liquid, and the polymer is selected from one of the structures shown in formula (1) to formula (3): Formula (1), Formula (2), Formula (3); wherein R1, R2 and R3 are independently selected from C1-C3 alkyl, trifluoromethyl, trichloromethyl, cyano or nitro, and n is an integer between 100 and 1500.

[0013] In some embodiments of the present application, R1, R2 and R3 are independently selected from methyl or trifluoromethyl.

[0014] In some embodiments of the present application, n is an integer between 200 and 1200; for example, an integer between 200 and 600, 500 and 800, 700 and 1000, 900 and 1100, 1000 and 1200, etc.

[0015] In some embodiments of the present application, the cation of the ionic liquid is selected from one of the structures shown in formula (4) to formula (6): Formula (4), Formula (5), Formula (6); wherein m is an integer between 1 and 16; the anion of the ionic liquid is selected from I - , Cl - , F - , [PF6] - , [TFSI] - , [FSI]- , [TFSM] - , [FTFSI] - , [CTFSI] - , [SbF6] - , [NTf2] - .

[0016] In some embodiments of the present application, the anion is selected from any one of F - , [FSI] - , [TFSM] - , [NTf2] - .

[0017] In some embodiments of the present application, the m is an integer between 1-10, for example, 1, 2, 5, 6, 8, 10, etc., including but not limited to the listed values.

[0018] In some embodiments of the present application, the organic solvent is selected from at least one of vinyl carbonate, propylene carbonate, ethanol, ethylene glycol, glycerol, methyl formate, methyl acetate, methyl butyrate, ethyl propionate, trifluorinated propylene carbonate, fluorinated vinyl carbonate, nonafluoromethyl butyl ether, dimethyl sulfoxide, acetonitrile.

[0019] In some embodiments of the present application, the organic solvent is selected from at least one of vinyl carbonate, trifluorinated propylene carbonate, fluorinated vinyl carbonate.

[0020] In some embodiments of the present application, the raw material components include, by weight parts: polymer 2-13 parts, ionic liquid 30-70 parts, and organic solvent 10-30 parts.

[0021] In some embodiments of the present application, the raw material components include, by weight parts: polymer 3-10 parts, ionic liquid 40-70 parts, and organic solvent 15-30 parts.

[0022] In some embodiments of the present application, the raw material components include, by weight parts: polymer 4-7 parts, ionic liquid 45-65 parts, and organic solvent 20-27 parts.

[0023] In some embodiments of the present application, the raw material components further include 5-25 parts by weight of lithium salt and 0.1-5 parts by weight of additive.

[0024] In some embodiments of the present application, the raw material components further include 7-22 parts by weight of lithium salt and 1-2 parts by weight of additive.

[0025] In some embodiments of the present application, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(oxalato)borate, lithium bis(trifluoromethylsulfonylimide), lithium bis(fluorosulfonyl)imide.

[0026] In some embodiments of the present application, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium bis(trifluoromethylsulfonylimide), lithium bis(fluorosulfonyl)imide.

[0027] In some embodiments of the present application, the additive is selected from at least one of 2-thiouracil, 4-aminophenol, 3-sulfobenzoic acid, dimethyl sulfate, crown ether compounds, cryptand compounds.

[0028] In some embodiments of the present application, the crown ether compound is selected from at least one of 18-crown-6 ether, 15-crown-5 ether.

[0029] In some embodiments of the present application, the cryptand compound is selected from at least one of cryptand 2,2,1, cryptand 2,3,1.

[0030] In some embodiments of the present application, the additive is selected from at least one of 2-thiouracil, 4-aminophenol, dimethyl sulfate, 18-crown-6 ether.

[0031] A second aspect of the present application provides a preparation method of the low-temperature phase change electrolyte described above, comprising the following steps: Mixing each raw material component to prepare the low-temperature phase change electrolyte.

[0032] In some embodiments of the present application, the mixing is carried out under an inert atmosphere.

[0033] In some embodiments of the present application, the inert atmosphere is an argon atmosphere.

[0034] In some embodiments of the present application, the preparation method of the low-temperature phase change electrolyte comprises the following steps: Mixing the polymer, the ionic liquid, the organic solvent, the lithium salt and the additive under an argon atmosphere, and stirring uniformly to prepare the low-temperature phase change electrolyte.

[0035] A third aspect of the present application provides a secondary battery comprising the low-temperature phase change electrolyte described above.

[0036] In some embodiments of the present application, the secondary battery is a lithium ion battery.

[0037] In some embodiments of the present application, the secondary battery further comprises a positive electrode, a negative electrode and a separator.

[0038] In some embodiments of the present application, the active material of the positive electrode can be selected from conventional positive electrode active materials in the art, typical but non-limiting positive electrode active materials are, for example, lithium iron phosphate (LiFePO4), lithium cobaltate (LiCoO2), lithium manganese oxide, lithium nickel oxide, ternary material, etc.

[0039] In some embodiments of the present application, the active material of the negative electrode can be selected from conventional negative electrode active materials in the art, typical but non-limiting negative electrode active materials are, for example, graphite, silicon, metallic lithium, etc.

[0040] In some embodiments of the present application, the material of the separator can be selected from conventional separator materials in the art, typical but non-limiting separator materials are, for example, polyethylene, polypropylene, polyimide, polyamide, polyvinylidene fluoride, polytetrafluoroethylene, etc.

[0041] The above technical solutions of the present application have at least the following technical effects or advantages relative to the prior art: (1) The low-temperature phase change electrolyte of the present application is an organic solvent system, using a specific structure of polymer and imidazole ionic liquid as the main raw material. The π-π interaction occurs between the polymer containing five-membered ring (thiophene) or six-membered ring (benzene ring) and the imidazole ionic liquid with five-membered heterocyclic cation. When the temperature is higher than the LCST temperature, the π-π interaction between the polymer and the imidazole ionic liquid breaks, the polymer precipitates from the ionic liquid, itself agglomerates and grows into larger particles, the electrolyte changes from liquid to solid, and phase transition occurs. At the same time, the benzene ring and thiophene of the polymer both contain electron-withdrawing groups or C1-C3 alkyl groups, as well as ethoxyl (EO) / ether group segments based on the structure of methyl acrylate, which can effectively reduce the phase transition temperature of the polymer, and further reduce the phase transition temperature of the electrolyte.

[0042] (2) The phase change electrolyte of the present application can undergo phase transition at a low temperature of 35-60℃, and the phase transition temperature can be controlled by adjusting the types and contents of the polymer and the ionic liquid, as well as the content of lithium salt. After phase transition occurs, the ionic conductivity of the electrolyte decreases, the capacity of the battery decreases, the electrochemical reaction inside the battery is gradually blocked, and the continuous temperature rise of the battery is slowed down; when the temperature of the battery rises to above 75℃, the internal reaction of the battery completely stops, effectively preventing the battery from experiencing thermal runaway during the whole process of temperature rise.

[0043] (3) By adding a certain amount of lithium salt and specific additives to the phase change electrolyte, the additives can not only reduce the viscosity of the ionic liquid, but also form complexes with lithium ions, promoting the dissociation of lithium salt, thereby improving the ionic conductivity of the electrolyte in a low temperature environment, and realizing that the ionic conductivity of the lithium ion battery can still reach 1.35-1.84 mS / cm in a low temperature environment of-20℃. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 Discharge capacity graph of a lithium ion battery assembled with the low-temperature phase change electrolyte prepared in Example 1 of the present application; Figure 2 Discharge capacity graph of a lithium ion battery assembled with the low-temperature phase change electrolyte prepared in Example 2 of the present application; Figure 3 Discharge capacity graph of a lithium ion battery assembled with the low-temperature phase change electrolyte prepared in Example 3 of the present application; Figure 4 Discharge capacity graph of a lithium ion battery assembled with the low-temperature phase change electrolyte prepared in Example 4 of the present application; Figure 5 Discharge capacity graph of a lithium ion battery assembled with the low-temperature phase change electrolyte prepared in Example 5 of the present application. DETAILED DESCRIPTION

[0045] The present application will be described in detail below with reference to Examples, so as to facilitate the understanding of the present application by those skilled in the art. It is necessary to point out here that the Examples are only used to further illustrate the present application and cannot be understood as limiting the protection scope of the present application. Non-essential improvements and adjustments of the present application made by those skilled in the art according to the above description should still fall within the protection scope of the present application. Meanwhile, the raw materials mentioned below which are not described in detail are all commercially available products; the process steps or preparation methods which are not mentioned in detail are all known to those skilled in the art.

[0046] Example 1 A low-temperature phase change electrolyte, whose raw material components include, by weight fraction: 6 parts of polymer, 54 parts of ionic liquid, 12 parts of lithium hexafluorophosphate, 5 parts of lithium bis-trifluoromethylsulfonylimide, 21 parts of ethylene carbonate and 2 parts of 4-aminophenol.

[0047] The chemical structure of the polymer is shown in formula (1): Formula (1); Wherein: R1 is methyl, and n is an integer between 1000 and 1200.

[0048] The anion of the ionic liquid is F - The chemical structure of the cation is shown in formula (6): Formula (6); Wherein: m is 5.

[0049] The preparation method of the above low-temperature phase change electrolyte includes the following steps: In an argon-filled glove box, the raw materials were weighed according to the raw material components, mixed, and stirred at a speed of 60 revolutions per minute for 30 minutes to prepare the low-temperature phase change electrolyte of the example.

[0050] Example 2 A low-temperature phase change electrolyte, whose raw material components include, by weight: 4 parts of a polymer, 45 parts of an ionic liquid, 17 parts of lithium bisfluorosulfonylimide, 5 parts of lithium bis-trifluoromethylsulfonylimide, 27 parts of fluoroethylene carbonate, 0.5 parts of dimethyl sulfate, and 1.5 parts of 18-crown-6 ether.

[0051] The chemical structure of the polymer is shown in formula (2): Formula (2); Wherein: R2 is a methyl group, and n is an integer between 900 and 1100.

[0052] The anion of the ionic liquid is [NTf2] - , and the chemical structure of the cation is shown in formula (4): Formula (4); Wherein: m is 2.

[0053] The preparation method of the low-temperature phase change electrolyte of Example 2 is the same as that of Example 1.

[0054] Example 3 A low-temperature phase change electrolyte, whose raw material components include, by weight: 7 parts of a polymer, 65 parts of an ionic liquid, 7 parts of lithium bisfluorosulfonylimide, 20 parts of trifluoroethylene carbonate, and 1 part of 2-thiouracil.

[0055] The chemical structure of the polymer is shown in formula (2): Formula (2); Wherein: R2 is a trifluoromethyl group, and n is an integer between 500 and 800.

[0056] The anion of the ionic liquid is [FSI] - , and the chemical structure of the cation is shown in formula (5): Formula (5); Wherein: m is 2.

[0057] The preparation method of the low-temperature phase change electrolyte of Example 3 is the same as that of Example 1.

[0058] Example 4 A low-temperature phase change electrolyte, whose raw material components include, by weight: 5 parts of a polymer, 64 parts of an ionic liquid, 8 parts of lithium bis-trifluoromethylsulfonylimide, 2 parts of lithium hexafluorophosphate, 20 parts of fluoroethylene carbonate, and 1 part of dimethyl sulfate.

[0059] wherein the chemical structure of the polymer is shown in formula (1): formula (1); wherein R1 is trifluoromethyl, and n is an integer between 200 and 600.

[0060] The anion of the ionic liquid is [TFSM] - and the chemical structure of the cation is shown in formula (5): formula (5); wherein m is 1.

[0061] The preparation method of the low-temperature phase change electrolyte of Example 4 is the same as that of Example 1.

[0062] Example 5 A low-temperature phase change electrolyte, the raw material components of which include, by weight fraction: 6 parts of polymer, 55 parts of ionic liquid, 14 parts of lithium hexafluorophosphate, 2 parts of lithium bisfluorosulfonylimide, 21 parts of fluoroethylene carbonate, and 2 parts of 4-aminophenol.

[0063] wherein the chemical structure of the polymer is shown in formula (3): formula (3); wherein R3 is methyl, and n is an integer between 100 and 300.

[0064] The anion of the ionic liquid is [CTFSI] - and the chemical structure of the cation is shown in formula (4): formula (4); wherein m is 1.

[0065] The preparation method of the low-temperature phase change electrolyte of Example 5 is the same as that of Example 1.

[0066] Comparative Example 1 Comparative Example 1 differs from Example 1 in that the raw material components of the low-temperature phase change electrolyte do not include a polymer. The raw material components of the low-temperature phase change electrolyte of Comparative Example 1 include, by weight fraction: 54 parts of ionic liquid, 12 parts of lithium hexafluorophosphate, 5 parts of lithium bis-trifluoromethylsulfonylimide, 21 parts of ethylene carbonate, and 2 parts of 4-aminophenol.

[0067] wherein the anion of the ionic liquid is F - and the chemical structure of the cation is shown in formula (6): formula (6); wherein m is 5.

[0068] Comparative Example 2 The difference between Comparative Example 2 and Example 3 is only in the chemical structure of the polymer, the polymer of Comparative Example 2 does not contain substituent on the five-membered ring, and the chemical structure is shown in formula (7): Formula (7); In which: n is an integer between 500-800.

[0069] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the raw material components of the low-temperature phase change electrolyte do not contain the additive 4-aminophenol, and the raw material components of the low-temperature phase change electrolyte of Comparative Example 3 include: 6 parts of polymer, 54 parts of ionic liquid, 12 parts of lithium hexafluorophosphate, 5 parts of lithium bis-trifluoromethylsulfonylimide, and 21 parts of ethylene carbonate.

[0070] In which: the chemical structure of the polymer is shown in formula (1): Formula (1); In which: R1 is methyl, and n is an integer between 1000-1200.

[0071] The anion of the ionic liquid is F - , and the chemical structure of the cation is shown in formula (6): Formula (6); In which: m is 5.

[0072] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is only in the chemical structure of the polymer, the polymer of Comparative Example 4 is PMMA, which does not contain a benzene ring, and the chemical structure is shown in formula (8): Formula (8); In which: n is an integer between 500-800.

[0073] Comparative Example 5 The difference between Comparative Example 5 and Example 1 is only in the chemical structure of the polymer, the benzene ring of the polymer of Comparative Example 5 contains other substituents, and the chemical structure is shown in formula (9): Formula (9); In which: n is an integer between 500-800.

[0074] Comparative Example 6 The difference between Comparative Example 6 and Example 1 is only in the chemical structure of the ionic liquid, the cation of the ionic liquid of Comparative Example 6 does not contain a five-membered ring, and the chemical structure is shown in formula (10), and the anion is F - .

[0075] Formula (10).

[0076] Performance test The low temperature phase change electrolyte prepared in the above examples 1-5 and comparative examples 1-6 was respectively added into a battery shell with lithium iron phosphate as the positive material, lithium metal as the negative electrode and polyethylene as the separator, and a lithium ion battery was assembled. The ion conductivity, discharge capacity and phase change temperature were tested, wherein the temperature at which the discharge capacity was 0 was recorded as the shutdown temperature. The specific test conditions were as follows: 1. Ion conductivity A stainless steel gasket with an area of 2cm 2 was assembled with the low temperature phase change electrolyte prepared in the above examples 1-5 and comparative examples 1-6 to form a battery, and the battery was placed in a low temperature test box at-20℃ for 3 hours. An electrochemical workstation was connected, and the electrochemical workstation applied a sinusoidal voltage signal with an amplitude of 20mV. The frequency range of the test was between 4-100MHz, the resistance of the electrolyte was recorded, and the ion conductivity was calculated.

[0077] 2. Discharge capacity The battery was placed in a high and low temperature test box, the temperature was set to 25℃, the battery was placed for 1 hour, a charge-discharge instrument was connected, and the battery was discharged at a rate of 0.2C, the discharge cutoff voltage was 2V, the capacity of the first discharge at 25℃ was recorded, the temperature of the test box was adjusted to 75℃, the battery was placed for 1 hour, and the battery was discharged at a rate of 0.2C, the discharge cutoff voltage was 2V, the discharge capacity was recorded, and the two temperatures were switched back and forth.

[0078] 3. Phase change temperature test The phase change temperature of the electrolyte was determined by measuring the transmittance of the electrolyte solution. A variable temperature ultraviolet-visible spectrometer was used to determine the transmittance of the electrolyte solution. The solution was placed on the heating table of the instrument, and the solution was heated at a heating rate of 1℃ / min. The transmittance curve was recorded, wherein the temperature at which the transmittance reached 0 was the phase change temperature.

[0079] The test results are shown in Figures 1-5 and Table 1.

[0080] Table 1:

[0081] As can be seen from Table 1 and Figures 1-5 , the lithium ion battery assembled from the low temperature phase change electrolyte of the examples 1-5 of the present application can maintain a high ion conductivity of 1.35-1.84mS / cm in a low temperature environment of-20℃, a discharge capacity of 154.7-160.1mAh / g at 25℃, a phase change temperature as low as 35.5-58.5℃, and a shutdown temperature of 75℃.

[0082] Comparative Example 1 relative to Example 1, since no polymer, the electrolyte does not phase change, no thermal runaway prevention effect.

[0083] Comparative Example 2 relative to Example 3, since the polymer does not contain substituents on the five-membered ring, the phase transition temperature of the electrolyte is 74.5°C higher than that of Example 1, indicating that the substituents have a significant effect on reducing the phase transition temperature.

[0084] Comparative Example 3 relative to Example 1, since no additive 4-aminophenol, the ionic conductivity of the electrolyte is decreased by 33.5% compared with Example 1, indicating that the additive 4-aminophenol can effectively improve the ionic conductivity of the electrolyte at low temperature.

[0085] Comparative Example 4 relative to Example 1, since the traditional polymer PMMA is used, the electrolyte does not phase change, no thermal runaway prevention effect.

[0086] Comparative Example 5 relative to Example 1, respectively, since the benzene ring of the polymer contains other substituents, it cannot effectively regulate the π-π interaction, the electrolyte does not phase change, no thermal runaway prevention effect.

[0087] Comparative Example 6 relative to Example 1, since the ionic liquid does not contain a five-membered ring, it fails to form π-π interaction, the electrolyte also does not phase change, no thermal runaway prevention effect. From the detection results of Comparative Examples 4-6, only a polymer with a specific structure can phase change and have a thermal runaway prevention effect.

[0088] For those skilled in the art to which the present application belongs, without departing from the concept of the present application, a number of simple deductions or substitutions can be made without the need for creative labor. Therefore, the simple improvements made by those skilled in the art to the present application based on the disclosure of the present application should be within the scope of protection of the present application. The above examples are preferred embodiments of the present application, and any equivalent changes made to the process and the equivalent changes made to the present application should be within the scope of protection of the present application.

Claims

1. A low-temperature phase change electrolyte, characterized in that: The raw material components include polymer, ionic liquid and organic solvent, the ionic liquid is an imidazole ionic liquid, and the polymer is selected from one of the structures shown in formula (1) to formula (3): Formula (1) Formula (2) Formula (3); Wherein: R1, R2, R3 are independently selected from C1-C3 alkyl, trifluoromethyl, trichloromethyl, cyano or nitro, and n is an integer between 100-1500.

2. The low-temperature phase change electrolyte according to claim 1, characterized in that The cation of the ionic liquid is selected from one of the structures shown in formula (4) to formula (6): Formula (4) Formula (5) Formula (6); Wherein: m is an integer between 1 and 16; The anion of the ionic liquid is selected from I - 、Cl - 、F - PF6 - , [TFSI] - , [FSI] - , [TFSM] - , [FTFSI] - , [CTFSI] - , [SbF6] - , [NTf2] - Any one of .

3. The low-temperature phase change electrolyte according to claim 1 or 2, characterized in that: The organic solvent is selected from at least one of ethylene carbonate, propylene carbonate, ethanol, ethylene glycol, glycerol, methyl formate, methyl acetate, methyl butyrate, ethyl propionate, trifluoropropylene carbonate, fluoroethylene carbonate, nonafluoromethyl butyl ether, dimethyl sulfoxide, and acetonitrile.

4. The low-temperature phase change electrolyte according to claim 1, characterized in that The raw material components include, by weight, 2-13 parts of polymer, 30-70 parts of ionic liquid and 10-30 parts of organic solvent.

5. The low-temperature phase change electrolyte according to claim 4, characterized in that: The raw material components further include 5-25 parts by weight of lithium salt and 0.1-5 parts by weight of additives.

6. The low-temperature phase change electrolyte according to claim 5, characterized in that The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(oxalatoborate), lithium bis(trifluoromethylsulfonylimide), lithium bis(fluorooxalatoborate), and lithium bis(fluorosulfonylimide).

7. The low-temperature phase change electrolyte according to claim 5, characterized in that The additive is selected from at least one of 2-thiouracil, 4-aminophenol, 3-sulfobenzoic acid, dimethyl sulfate, crown ether compounds, and cryptand compounds.

8. A method for preparing a low-temperature phase-change electrolyte according to any one of claims 1 to 7, characterized in that: The following steps are involved: The raw material components are mixed to prepare the low-temperature phase-change electrolyte.

9. The method for preparing a low-temperature phase-change electrolyte according to claim 8, characterized in that: The mixing is performed under an inert atmosphere.

10. A secondary battery, characterized in that: The invention comprises the low-temperature phase-change electrolyte according to any one of claims 1 to 7.

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