Reversible phase change electrolyte, preparation method thereof and secondary battery

Abstract

The invention belongs to the related technical field of batteries, and discloses a reversible phase change electrolyte, a preparation method thereof and a secondary battery. The reversible phase change electrolyte is an organic solvent system, a copolymer with a specific structure and an ionic liquid are used as main components, the pi-pi interaction between the copolymer and the ionic liquid is utilized, when the temperature is higher than the LCST temperature, the pi-pi interaction between the copolymer and the ionic liquid breaks, and phase change occurs. Meanwhile, the main chain of the polymer C enables the length of the side chain of the polymer A or the polymer B to be increased, and hindering is formed for the pi-pi interaction between the copolymer and the ionic liquid; and a-CX3 side chain group in the polymer C further promotes the reduction of the phase change temperature of the electrolyte. The phase-change electrolyte disclosed by the invention can generate phase change at a low temperature of 40-60 DEG C, and when the temperature of the battery continues to rise to 77 DEG C or above, the internal reaction of the battery is completely stopped, so that thermal runaway of the battery is prevented, and the battery can efficiently work in a wide temperature range.

Description

Technical Field

[0001] The present invention belongs to the technical field related to batteries, and in particular relates to a reversible phase change electrolyte, a preparation method thereof, and a secondary battery. Background Art

[0002] The rapid development of new energy vehicles has placed higher demands on power battery systems, and battery performance requirements and safety standards continue to upgrade. However, the liquid electrolytes of mainstream power lithium-ion batteries are easily decomposed under high temperature conditions, and with the intensification of side reactions at the electrode / electrolyte interface, they can easily cause thermal runaway and pose serious safety hazards. Compared with liquid electrolytes, the solid electrolytes of cutting-edge power solid-state batteries have higher safety. They use non-flammable solid electrolytes instead of flammable organic electrolytes, which can effectively alleviate the risks of high-temperature thermal runaway and combustion. However, in low-temperature environments (below -15°C), the sharp drop in ionic conductivity is a shortcoming of solid-state batteries, which restricts their application in low-temperature scenarios. Therefore, temperature is an important factor affecting the performance of lithium-ion batteries and solid-state batteries.

[0003] Thermosensitive polymers are a class of polymer materials that undergo reversible phase transitions in response to temperature. These phase transitions can be categorized as either lower critical solution temperature (LCST) or upper critical solution temperature (UCST). When the temperature is above the LCST, the polymer undergoes a hydrophilic phase transition and precipitates out of solution. When the temperature is below the UCST, the polymer precipitates out of solution. Therefore, LCST-type thermosensitive polymers offer a solution for mitigating the temperature effects of batteries.

[0004] Currently, thermosensitive polymers are primarily used in aqueous electrolytes, achieving phase transitions through hydrogen bonding. For example, patent publication CN110938170A discloses a reversible overheat-protected aqueous electrolyte based on methylcellulose grafted with isopropyl acrylamide. However, to adapt to higher-voltage lithium-ion battery systems, the polymer needs to undergo a phase transition in an ionic liquid. Conventional thermosensitive polymers in ionic liquids have relatively high phase transition temperatures (approximately 80-120°C) and low ionic conductivity at low temperatures.

[0005] Therefore, there is an urgent need to develop a phase change electrolyte that can prevent battery thermal runaway at lower temperatures in an organic solvent system and maintain high ionic conductivity in low temperature environments. Summary of the Invention

[0006] The present invention aims to solve at least one of the technical problems existing in the above-mentioned prior art. To this end, the present invention provides a reversible phase-change electrolyte, a preparation method thereof, and a secondary battery. The electrolyte undergoes a phase change at low temperatures of 40-60°C, reducing the ionic conductivity and capacity of the electrolyte material, blocking the electrochemical reactions within the battery and slowing the rate of temperature increase. At temperatures above 77°C, the internal reactions in the battery completely cease, preventing thermal runaway and providing high safety. Furthermore, the electrolyte can maintain high ionic conductivity even at low temperatures of -20°C.

[0007] The inventive concept of the present invention is as follows: The reversible phase-change electrolyte of the present invention is an organic solvent system, mainly composed of a specific copolymer and an ionic liquid. The copolymer undergoes a phase change in the ionic liquid via the LCST phenomenon. The copolymer contains five-membered or six-membered rings that can form large π bonds. The ionic liquid is the primary carrier of the electrolyte, used to dissolve lithium salts and form a stable solvation structure to facilitate lithium ion transport. The cations of imidazole and pyridine ionic liquids contain five-membered and six-membered heterocyclic rings, respectively, which can also form large π bonds. Therefore, π-π interactions occur between the copolymer and the ionic liquid. When the temperature is below the LCST temperature, the polymer dissolves in the ionic liquid in a linear form. When the temperature is above the LCST temperature, the π-π interactions between the copolymer and the ionic liquid break, and the copolymer precipitates from the ionic liquid, agglomerates, and grows into larger particles. The electrolyte transforms from a liquid to a solid state, undergoing a phase change.

[0008] At the same time, the copolymer of the present invention is formed by copolymerizing a polymer A or polymer B containing a five-membered ring or a six-membered ring and a polymer C containing a specific main chain and a side chain group of -CX3 (X=H, F, Cl, Br). On the one hand, the main chain of polymer C lengthens the side chain length of polymer A or polymer B, thereby hindering the π-π interaction between the copolymer and the ionic liquid; on the other hand, the -CX3 side chain group in polymer C can help weaken the π-π interaction between the copolymer and the ionic liquid, further reducing the phase transition temperature of the copolymer.

[0009] In addition, the reversible phase-change electrolyte of the present invention also contains a certain amount of lithium salt and specific additives (such as fluoroacetoxy dimethyl phosphate, [EMIM]-g-PEG, perfluoroalkylsulfonyl quaternary ammonium nitrate, ethyl-2,2,2-trifluoroethane carbonate, and 2-thiophene tert-butylsulfonamide). The lithium salt provides active lithium ions during battery discharge and is the basis for ionic conductivity. The additive not only helps reduce the viscosity of the electrolyte, promotes the formation of the SEI film, and enhances the wettability of the electrode, thereby improving interfacial ionic conductivity; it also acts as a film-forming agent, promoting the formation of a stable and dense SEI film on the electrode surface, thereby improving the electrolyte's ionic conductivity in low-temperature environments.

[0010] Therefore, the phase-change electrolyte of the present invention can undergo a phase transition at low temperatures of 40-60°C, converting from a liquid to a solid state. This phase transition process is reversible, and the phase transition temperature can be controlled by adjusting the types and contents of the copolymer and ionic liquid, as well as the content of the lithium salt. At the normal operating temperature of a lithium-ion battery (20-30°C), the electrolyte is liquid. When the battery temperature rises to 40-60°C, the electrolyte undergoes a phase transition, which does not yet reach the decomposition temperature of the SEI membrane (approximately 80°C). After the phase transition occurs, the electrolyte's ionic conductivity decreases, reducing the battery's capacity, gradually blocking the electrochemical reactions within the battery and slowing the further increase in battery temperature. When the battery temperature continues to rise to above 77°C, the internal reactions completely cease, preventing thermal runaway and enabling the battery to operate efficiently over a wide temperature range.

[0011] To solve the above technical problems, the first aspect of the present invention provides a reversible phase change electrolyte, the raw material components of which include a copolymer, an ionic liquid and an organic solvent, wherein the ionic liquid is an imidazole ionic liquid or a pyridine ionic liquid; The raw materials for preparing the copolymer include polymer A and polymer C, or include polymer B and polymer C; The chemical structures of polymer A, polymer B and polymer C are shown in formula (1), formula (2) and formula (3), respectively: Formula (1); Formula (2); Formula (3); wherein x is an integer between 100 and 1000, y is an integer between 100 and 1000, and z is an integer between 300 and 1200; and R is selected from methyl, trifluoromethyl, trichloromethyl or tribromomethyl.

[0012] In some embodiments of the present invention, the R is selected from methyl or trifluoromethyl.

[0013] In some embodiments of the present invention, x and y are integers between 200-800, for example, integers between 200-400, 300-500, 400-600, 500-700, 600-800, etc.

[0014] In some embodiments of the present invention, z is an integer between 400-1000; for example, an integer between 400-600, 500-700, 600-800, 700-900, 800-1000, etc.

[0015] In some embodiments of the present invention, the mass ratio of polymer A to polymer C is (0.4-1.5):1; for example, 6:4, 5:5, 4:6, 3:7, etc.

[0016] In some embodiments of the present invention, the mass ratio of polymer B to polymer C is (0.25-1):1; for example, 5:5, 4:6, 3:7, 2:8, etc.

[0017] In some embodiments of the present invention, the imidazolium ionic liquid is selected from at least one of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium tetrafluoroborate, and 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide.

[0018] In some embodiments of the present invention, the pyridine ionic liquid is selected from at least one of N-butyl-N-methylpyrrolidine bis(trifluoromethylsulfonyl)imide, N-propyl-N-methylpyrrolidine bis(trifluoromethylsulfonyl)imide, and N-methyl-N-methoxyethylpyrrolidine bis(trifluoromethylsulfonyl)imide.

[0019] In some embodiments of the present invention, the organic solvent includes a carbonate solvent and / or an ether solvent.

[0020] In some embodiments of the present invention, the carbonate solvent is selected from at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0021] In some embodiments of the present invention, the ether solvent is selected from at least one of tetrahydrofuran, ethylene glycol dimethyl ether, and trimethyl phosphate.

[0022] In some embodiments of the present invention, the raw material components include, by weight: 3-15 parts of copolymer, 60-85 parts of ionic liquid and 1-15 parts of organic solvent.

[0023] In some embodiments of the present invention, the raw material components include, by weight, 5-12 parts of copolymer, 68-84 parts of ionic liquid, and 2-12 parts of organic solvent.

[0024] In some embodiments of the present invention, the raw material components further include 5-16 parts by weight of lithium salt and 0.5-3 parts by weight of additives.

[0025] In some embodiments of the present invention, the raw material components further include 6-10 parts by weight of lithium salt and 1-2 parts by weight of additives.

[0026] In some embodiments of the present invention, the lithium salt is selected from at least one of LiPF6, LiBF4, LiClO4, LiBOB, LiFSI, LiCF3SO3, LiTFSI, LiDFOB, and LiPO2F2.

[0027] In some embodiments of the present invention, the additive is selected from at least one of fluoroacetoxy dimethyl phosphate, [EMIM]-g-PEG, perfluoroalkylsulfonyl quaternary ammonium nitrate, ethyl-2,2,2-trifluoroethane carbonate, and 2-thiophene tert-butyl sulfonamide.

[0028] A second aspect of the present invention provides a method for preparing the above-mentioned reversible phase change electrolyte, comprising the following steps: The raw material components are mixed to prepare the reversible phase change electrolyte.

[0029] In some embodiments of the present invention, the mixing is performed under an inert atmosphere.

[0030] In some embodiments of the present invention, the inert atmosphere is an argon atmosphere.

[0031] In some embodiments of the present invention, the method for preparing the reversible phase change electrolyte comprises the following steps: Under an argon atmosphere, polymer A or polymer B, polymer C, ionic liquid, organic solvent, lithium salt and additives are mixed and stirred evenly to prepare the reversible phase change electrolyte.

[0032] A third aspect of the present invention provides a secondary battery comprising the above-mentioned reversible phase-change electrolyte.

[0033] In some embodiments of the present invention, the secondary battery is a lithium-ion battery.

[0034] In some embodiments of the present invention, the secondary battery further includes a positive electrode, a negative electrode, and a separator.

[0035] In some embodiments of the present invention, 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 include lithium iron phosphate (LiFePO4), lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium nickel oxide, ternary materials, etc.

[0036] In some embodiments of the present invention, 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 include graphite, silicon, metallic lithium, and the like.

[0037] In some embodiments of the present invention, the material of the diaphragm can be selected from conventional diaphragm materials in the art. Typical but non-limiting diaphragm materials include polyethylene, polypropylene, polyimide, polyamide, polyvinylidene fluoride, polytetrafluoroethylene, etc.

[0038] Compared with the prior art, the above technical solution of the present invention has at least the following technical effects or advantages: (1) The reversible phase change electrolyte of the present invention is an organic solvent system with a copolymer of a specific structure and an ionic liquid as the main components. It utilizes the π-π interaction between a copolymer containing a five-membered ring or a six-membered ring and an imidazole ionic liquid or a pyridine ionic liquid whose cation contains a five-membered heterocyclic ring or a six-membered heterocyclic ring. When the temperature is higher than the LCST temperature, the π-π interaction between the copolymer and the ionic liquid breaks, the copolymer precipitates from the ionic liquid, agglomerates itself, grows into larger particles, and the electrolyte transforms from a liquid to a solid, undergoing a phase change. At the same time, the copolymer is copolymerized by polymer A or polymer B containing a five-membered ring or a six-membered ring and polymer C containing a specific main chain and a side chain group of -CX3 (X=H, F, Cl, Br). The main chain of polymer C lengthens the side chain of polymer A or polymer B, thereby hindering the π-π interaction between the copolymer and the ionic liquid. The -CX3 side chain group in polymer C further weakens the π-π interaction between the copolymer and the ionic liquid, thereby reducing the phase change temperature of the electrolyte.

[0039] (2) The phase-change electrolyte of the present invention can undergo phase change at low temperatures of 40-60°C. The phase change temperature can be controlled by adjusting the type and content of the polymer and ionic liquid, as well as the content of the lithium salt. After the phase change occurs, the ionic conductivity of the electrolyte decreases, the capacity of the battery decreases, and the electrochemical reaction inside the battery is gradually blocked, slowing down the further increase in the battery temperature. When the battery temperature continues to rise to above 77°C, the internal reaction of the battery completely stops, preventing the battery from thermal runaway, allowing the battery to operate efficiently in a wide temperature range.

[0040] (3) The present invention adds a certain amount of lithium salt and specific additives to the phase change electrolyte. The additives not only help to reduce the viscosity of the electrolyte, promote the formation of the SEI film, improve the wettability of the electrode, and improve the interfacial ion conductivity; but also serve as a film-forming agent to promote the formation of a stable and dense SEI film on the electrode surface, further improving the ionic conductivity of the electrolyte in a low temperature environment, and achieving an ionic conductivity of the lithium ion battery of 1.43-1.78mS / cm in a low temperature environment of -20°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 Graph showing the discharge capacity of a lithium-ion battery assembled with the reversible phase-change electrolyte prepared in Example 1 of the present invention; Figure 2 Graph showing the discharge capacity of a lithium-ion battery assembled with the reversible phase-change electrolyte prepared in Example 2 of the present invention; Figure 3 Graph showing the discharge capacity of a lithium-ion battery assembled with the reversible phase-change electrolyte prepared in Example 3 of the present invention; Figure 4Graph showing the discharge capacity of a lithium-ion battery assembled with the reversible phase-change electrolyte prepared in Example 4 of the present invention; Figure 5 This is a diagram of the discharge capacity of a lithium-ion battery assembled with the reversible phase-change electrolyte prepared in Example 5 of the present invention. DETAILED DESCRIPTION

[0042] The present invention is described in detail below with reference to the examples to facilitate understanding of the present invention by those skilled in the art. It is necessary to point out that the examples are only used to further illustrate the present invention and are not to be construed as limiting the scope of protection of the present invention. Non-essential improvements and adjustments made to the present invention by those skilled in the art based on the above-mentioned invention should still fall within the scope of protection of the present invention. At the same time, the raw materials mentioned below that are not described in detail are all commercially available products; the process steps or preparation methods that are not mentioned in detail are all process steps or preparation methods known to those skilled in the art.

[0043] The preparation process of the copolymer in the following examples and comparative examples is as follows: Under a nitrogen atmosphere, polymers A or B and C are mixed in a specific mass ratio and dissolved in anhydrous dichloromethane. Then, the initiator LiBF4 (1 wt% of polymer C) is added. The reaction system is heated to 60°C, where LiBF4 decomposes to release BF3, which in turn initiates a ring-opening polymerization of monomers A or B. The reaction lasts for 4-12 hours. After the reaction is complete, excess methanol is added to the reaction system to precipitate the polymer, which is collected by filtration or centrifugation and then washed with methanol to obtain a crude product. The crude product is dissolved in tetrahydrofuran (THF), and the solution is poured into a poor solvent (methanol or diethyl ether) to reprecipitate the polymer. The final product is vacuum-dried at 60°C for 48 hours to obtain a purified copolymer.

[0044] Polymer A was prepared as follows: Epichlorohydrin was slowly added to a cold mixture of 40% w / w aqueous sodium hydroxide solution, benzyl alcohol, and tetrabutylammonium bromide (TBAB) in the amounts corresponding to the chemical structures. After the reaction was complete, the mixture was extracted twice with diethyl ether, dried over magnesium sulfate, filtered, and evaporated to dryness.

[0045] Polymer B was prepared as follows: Epichlorohydrin was slowly added to a cold mixture of 40% w / w aqueous sodium hydroxide solution, thiophene methanol, and tetrabutylammonium bromide (TBAB) in the amounts corresponding to the chemical structures. After the reaction was complete, the mixture was extracted twice with diethyl ether, dried over magnesium sulfate, filtered, and evaporated to dryness.

[0046] Polymer C was purchased from Maclean.

[0047] Example 1 A reversible phase-change electrolyte, whose raw material components, measured by weight, include: 84 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide salt, 2 parts of ethylene carbonate, 68 parts of LiPF, 5 parts of copolymer, 0.2 parts of fluoroacetoxy dimethyl phosphate, and 0.8 parts of 2-thiophene tert-butyl sulfonamide.

[0048] Wherein: the copolymer is formed by copolymerizing polymer A and polymer C in a mass ratio of 5:5. The chemical structures of polymer A and polymer C are shown in formula (1) and formula (3) respectively: Formula (1); Formula (3); Wherein: x is an integer between 300-400, z is an integer between 700-800; and R is a methyl group.

[0049] The method for preparing the reversible phase change electrolyte comprises the following steps: In a glove box filled with argon, raw materials were weighed according to their composition, mixed, and stirred at a speed of 60 rpm for 30 minutes to prepare the reversible phase change electrolyte of this embodiment.

[0050] Example 2 A reversible phase change electrolyte, whose raw material components include, by weight: 45 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide salt, 33 parts of 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide salt, 2 parts of dimethyl carbonate, 8 parts of LiTFSI, 4 parts of LiDFOB, 6 parts of copolymer, 0.4 parts of fluoroacetoxy dimethyl phosphate, 0.6 parts of [EMIM]-g-PEG, and 1 part of ethyl 2,2,2-trifluoroethane carbonate.

[0051] Wherein: the copolymer is formed by copolymerizing polymer A and polymer C in a mass ratio of 4:6. The chemical structures of polymer A and polymer C are shown in formula (1) and formula (3) respectively: Formula (1); Formula (3); Wherein: x is an integer between 400-500, z is an integer between 800-900; and R is a trifluoromethyl group.

[0052] The preparation method of the reversible phase change electrolyte of Example 2 is the same as that of Example 1.

[0053] Example 3 A reversible phase change electrolyte, whose raw material components, measured by weight, include: 35 parts of N-butyl-N-methylpyrrolidine bis(trifluoromethylsulfonyl)imide salt, 39 parts of N-methyl-N-methoxyethylpyrrolidine bis(trifluoromethylsulfonyl)imide salt, 6 parts of dimethyltetrahydrofuran carbonate, 10 parts of LiDFOB, 8 parts of copolymer, 0.8 parts of perfluoroalkylsulfonyl quaternary ammonium salt nitrate, and 1.2 parts of dimethyl fluoroacetoxyphosphate.

[0054] Wherein: the copolymer is formed by copolymerization of polymer A and polymer C in a mass ratio of 3:7. The chemical structures of polymer A and polymer C are shown in formula (1) and formula (3), respectively: Formula (1); Formula (3); Wherein: x is an integer between 400-500, z is an integer between 700-800; and R is a methyl group.

[0055] The preparation method of the reversible phase change electrolyte of Example 3 is the same as that of Example 1.

[0056] Example 4 A reversible phase change electrolyte, whose raw material components include, by weight: 72 parts of 1-butyl-3-methylimidazolium tetrafluoroborate, 4 parts of propylene carbonate, 2 parts of dimethyl carbonate, 10 parts of LiClO4, 10 parts of copolymer, and 2 parts of ethyl-2,2,2-trifluoroethane carbonate.

[0057] Wherein: the copolymer is formed by copolymerization of polymer B and polymer C in a mass ratio of 4:6. The chemical structures of polymer B and polymer C are shown in formula (2) and formula (3) respectively: Formula (2); Formula (3); Wherein: y is an integer between 500-600, z is an integer between 500-600; and R is a methyl group.

[0058] The preparation method of the reversible phase change electrolyte of Example 4 is the same as that of Example 1.

[0059] Example 5 A reversible phase-change electrolyte, whose raw material components, measured in parts by weight, include: 68 parts of N-methyl-N-methoxyethyl pyrrolidine bis(trifluoromethylsulfonate)imide salt, 12 parts of trimethyl phosphate, 10 parts of LiPO2F2, 12 parts of copolymer, 0.7 parts of fluoroacetoxy dimethyl phosphate, and 1.3 parts of ethyl-2,2,2-trifluoroethane carbonate.

[0060] Wherein: the copolymer is formed by copolymerization of polymer B and polymer C in a mass ratio of 3:7. The chemical structures of polymer B and polymer C are shown in formula (2) and formula (3) respectively: Formula (2); Formula (3); Wherein: y is an integer between 600-700, z is an integer between 600-700; and R is a methyl group.

[0061] The preparation method of the reversible phase change electrolyte of Example 5 is the same as that of Example 1.

[0062] Comparative Example 1 The difference between Comparative Example 1 and Example 2 is that the raw material components of the reversible phase change electrolyte do not contain a copolymer. The raw material components of the reversible phase change electrolyte of Comparative Example 1 include, by weight: 45 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 33 parts of 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, 2 parts of dimethyl carbonate, 8 parts of LiTFSI, 4 parts of LiDFOB, 0.4 parts of dimethyl fluoroacetyl phosphate, 0.6 parts of [EMIM]-g-PEG, and 1 part of ethyl-2,2,2-trifluoroethane carbonate.

[0063] Comparative Example 2 The difference between Comparative Example 2 and Example 4 is that the composition of the copolymer is different. The copolymer of Comparative Example 2 is copolymerized by polymer B and polymer C in a mass ratio of 6:4.

[0064] Comparative Example 3 The difference between Comparative Example 3 and Example 5 is that the chemical structure of polymer C is different. The polymer C of Comparative Example 3 does not contain a methyl group, and its chemical structure is shown in formula (4): Formula (4); Wherein: z is an integer between 600 and 700.

[0065] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that Comparative Example 4 uses an equal amount of polymer A to replace the copolymer in Example 1.

[0066] Comparative Example 5 The difference between Comparative Example 5 and Example 1 is that Comparative Example 5 uses an equal amount of polymer C to replace the copolymer in Example 1.

[0067] Comparative Example 6 The difference between Comparative Example 6 and Example 1 is that Comparative Example 6 uses an equal amount of copolymer D to replace the copolymer of Example 1, and polymer D is copolymerized by polymer E and polymer F, wherein polymer F is obtained by adding a methyl group to the side chain of polymer A; the chemical structures of polymer E and polymer F are shown in formula (4) and formula (5), respectively: Formula (4); Formula (5); Wherein: z is an integer between 600-700, x is an integer between 300-400, and R is a methyl group.

[0068] Performance Testing The reversible phase-change electrolytes prepared in Examples 1-5 and Comparative Examples 1-6 were added to battery housings containing lithium iron phosphate as the positive electrode material, graphite as the negative electrode material, and polyvinylidene fluoride as the separator to form lithium-ion batteries. Ionic conductivity, discharge capacity, and phase transition temperature were tested, with the temperature at which the discharge capacity reached zero being designated the shutdown temperature. Specific test conditions are as follows: 1. Ionic conductivity The area is 2cm 2 The stainless steel gasket and electrolyte were assembled into a battery, and the battery was placed in a low-temperature test box at -20°C for 3 hours. The electrochemical workstation was connected to apply a sinusoidal voltage signal with an amplitude of 20mV. The test frequency range was between 4-100MHz, the resistance of the electrolyte was recorded, and the ionic conductivity was calculated.

[0069] 2. Discharge capacity Place the battery in a high and low temperature test chamber, set the temperature to 25℃, let the battery stand for 1 hour, connect the charge and discharge instrument, discharge at a rate of 0.2C, and the discharge cut-off voltage is 2V. Record the capacity of the first discharge at 25℃, adjust the test chamber temperature to 75℃, let the battery stand for 1 hour, discharge at a rate of 0.2C, and the discharge cut-off voltage is 2V, record the discharge capacity, and switch back and forth between the two temperatures.

[0070] 3. Phase change temperature test The electrolyte phase transition temperature is determined by measuring the transmittance of the electrolyte solution. This is measured using a variable temperature UV-Vis spectrometer. The solution is placed on the instrument's built-in heating stage and heated at a rate of 1°C / min. The transmittance curve is recorded, with the phase transition temperature being the temperature at which the transmittance reaches 0.

[0071] The test results are as follows Figure 1-5 As shown in Table 1.

[0072] Table 1:

[0073] From Table 1 and Figure 1-5 It can be seen that the batteries assembled with the reversible phase change electrolytes of Examples 1-5 of the present invention maintain a relatively high ionic conductivity of 1.43-1.78 mS / cm in a low temperature environment of -20°C; the discharge capacity at 25°C is 154.5-159.1 mAh / g, the phase change temperature is as low as 40.2-57.6°C, and the shutdown temperature is 77°C.

[0074] Compared with Example 2, Comparative Example 1 does not contain a copolymer, so the electrolyte does not undergo phase change and has no thermal runaway prevention effect.

[0075] In Comparative Example 2 relative to Example 4, due to the excessively high mass proportion of polymer B in the copolymer, the phase transition temperature and the shutdown temperature are significantly higher than those of Example 4, indicating that the phase transition temperature can be controlled by adjusting the polymer content.

[0076] In Comparative Example 3, compared with Example 5, since the structure of polymer C does not contain methyl groups, the phase transition temperature and the shutdown temperature are both increased to a certain extent compared with Example 5, indicating that the methyl groups in polymer C have a certain promoting effect on lowering the phase transition temperature.

[0077] In Comparative Example 4, compared with Example 1, an equal amount of polymer A is used to replace the copolymer of Example 1, and there is no regulating effect of polymer C, so the phase transition temperature increases, indicating that the copolymer is more conducive to lowering the phase transition temperature of the electrolyte than a single polymer.

[0078] In Comparative Example 5, as compared to Example 1, an equal amount of polymer C is used to replace the copolymer of Example 1. Polymer C itself does not have phase change capability, the electrolyte does not undergo phase change, and there is no thermal runaway prevention effect.

[0079] Compared with Example 1, in Comparative Example 6, since an equal amount of polymer D is used to replace the copolymer of Example 1, the control effect of the phase transition temperature is poor, and the phase transition temperature is higher, indicating that the position of the copolymer substituent has a certain influence on the phase transition temperature of the electrolyte.

[0080] For those skilled in the art to which the present invention belongs, a number of simple deductions or substitutions can be made without departing from the concept of the present invention, without having to resort to creative work. Therefore, based on the disclosure of the present invention, simple improvements made by those skilled in the art to the present invention should be within the scope of protection of the present invention. The above embodiments are preferred embodiments of the present invention, and all processes similar to the present invention and equivalent changes made should fall within the scope of protection of the present invention.

Claims

1. A reversible phase change electrolyte, characterized in that: The raw material components include copolymer, ionic liquid and organic solvent, wherein the ionic liquid is imidazole ionic liquid or pyridine ionic liquid; The raw materials for preparing the copolymer include polymer A and polymer C, or include polymer B and polymer C; The chemical structures of polymer A, polymer B and polymer C are shown in formula (1), formula (2) and formula (3), respectively: Formula (1); Formula (2); Formula (3); wherein x is an integer between 100 and 1000, y is an integer between 100 and 1000, and z is an integer between 300 and 1200; and R is selected from methyl, trifluoromethyl, trichloromethyl or tribromomethyl.

2. The reversible phase change electrolyte according to claim 1, characterized in that The mass ratio of the polymer A to the polymer C is (0.4-1.5):1, and the mass ratio of the polymer B to the polymer C is (0.25-1):

1.

3. The reversible phase change electrolyte according to claim 1, characterized in that The imidazole ionic liquid is selected from at least one of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium tetrafluoroborate, and 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide; The pyridine ionic liquid is selected from at least one of N-butyl-N-methylpyrrolidine bis(trifluoromethylsulfonyl)imide, N-propyl-N-methylpyrrolidine bis(trifluoromethylsulfonyl)imide, and N-methyl-N-methoxyethylpyrrolidine bis(trifluoromethylsulfonyl)imide.

4. The reversible phase change electrolyte according to claim 1, characterized in that The organic solvent includes a carbonate solvent and / or an ether solvent.

5. The reversible phase change electrolyte according to claim 1, characterized in that The raw material components include, by weight, 3-15 parts of copolymer, 60-85 parts of ionic liquid and 1-15 parts of organic solvent.

6. The reversible phase change electrolyte according to claim 5, characterized in that The raw material components further include 5-16 parts by weight of lithium salt and 0.5-3 parts by weight of additives.

7. The reversible phase change electrolyte according to claim 6, characterized in that The lithium salt is selected from at least one of LiPF6, LiBF4, LiClO4, LiBOB, LiFSI, LiCF3SO3, LiTFSI, LiDFOB, and LiPO2F2; The additive is selected from at least one of fluoroacetoxy dimethyl phosphate, [EMIM]-g-PEG, perfluoroalkylsulfonyl quaternary ammonium salt nitrate, ethyl-2,2,2-trifluoroethane carbonate, and 2-thiophene tert-butyl sulfonamide.

8. A method for preparing a reversible 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 reversible phase change electrolyte.

9. The method for preparing a reversible 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 reversible phase change electrolyte according to any one of claims 1 to 7.

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