Reversible phase change electrolyte and method for preparing the same, secondary battery

Abstract

The application belongs to the 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, and specific structure copolymers and ionic liquids are used as main components. When the temperature is higher than the LCST temperature, the pi-pi interaction between the copolymers and the ionic liquids is broken, and phase change occurs. At the same time, the main chain of the polymer C lengthens the side chain length of the polymer A or the polymer B, and forms an obstacle to the pi-pi interaction between the copolymers and the ionic liquids. The side chain group -CX3 in the polymer C further reduces the phase change temperature of the electrolyte. The phase change electrolyte can occur phase change at a low temperature of 40-60 DEG C. When the battery temperature continues to rise to above 77 DEG C, the internal reaction of the battery completely stops, the battery heat runaway is prevented, and the battery can work efficiently in a wide temperature range.

Description

TECHNICAL FIELD

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

[0002] The rapid development of new energy vehicles puts forward higher requirements for power battery systems, and the performance requirements and safety standards of batteries continue to upgrade. However, the liquid electrolyte of mainstream power lithium ion batteries is prone to decomposition under high temperature conditions, and is accompanied by the intensification of electrode / electrolyte interface side reactions, which can easily cause thermal runaway and bring serious safety hazards. Compared with liquid electrolyte, the solid-state electrolyte of the front-line power solid-state battery has higher safety. It uses non-flammable solid electrolyte to replace flammable organic electrolyte, which can effectively alleviate the risk of high-temperature thermal runaway and combustion. However, under low temperature conditions (lower than-15℃), the sharp decline 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] 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 phase transition type; 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 polymer provides a solution to reduce the temperature influence of the battery.

[0004] At present, temperature-sensitive polymers are mainly applied to aqueous electrolyte, and phase change is achieved through the action of hydrogen bonds. For example, the invention patent with 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 liquid, and the phase transition temperature of traditional temperature-sensitive polymers in ionic liquid is relatively high (about 80-120℃), and the ionic conductivity under low temperature environment is also low.

[0005] Therefore, it is urgent to develop a phase change electrolyte that can prevent battery thermal runaway at a lower temperature in an organic solvent system and maintain a high ionic conductivity under low temperature conditions. SUMMARY

[0006] The present application aims to at least solve one of the technical problems existing in the prior art. To this end, the present application proposes a reversible phase change electrolyte and a preparation method thereof, and a secondary battery, the electrolyte can undergo phase change at a low temperature of 40-60℃, the ionic conductivity and capacity of the electrolyte material decrease, the electrochemical reaction inside the battery is blocked, and the temperature rise rate of the battery is slowed down; when the temperature is above 77℃, the reaction inside the battery is completely stopped, the thermal runaway of the battery is prevented, and the safety is high; and the ionic conductivity can still be high in a low temperature environment of-20℃.

[0007] The inventive concept of the present application is that the reversible phase change electrolyte of the present application is an organic solvent system, and specific copolymer and ionic liquid are used as main components. The copolymer undergoes phase change in the ionic liquid through the LCST phenomenon, the copolymer contains five-membered or six-membered rings and can form a large π bond; the ionic liquid is the main carrier of the electrolyte, is used for dissolving lithium salt and forming a stable solvation structure to promote the transmission of lithium ions. The cations of imidazole-based ionic liquid and pyridine-based ionic liquid contain five-membered heterocyclic ring and six-membered heterocyclic ring respectively, and can also form a large π bond. Therefore, π-π interaction occurs between the copolymer and the ionic liquid. When the temperature is lower than the LCST temperature, the polymer is dissolved in the ionic liquid in the form of a line; when the temperature is higher than the LCST temperature, the π-π interaction between the copolymer and the ionic liquid is broken, the copolymer is precipitated from the ionic liquid, and the copolymer itself agglomerates and grows into larger particles, the electrolyte changes from liquid to solid, and phase change occurs.

[0008] Meanwhile, the copolymer of the present application is copolymerized from polymer A or polymer B containing five-membered or six-membered rings and 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, and hinders the π-π interaction between the copolymer and the ionic liquid; on the other hand, the -CX3 side chain group in polymer C can assist in weakening the π-π interaction between the copolymer and the ionic liquid, and further reduce the phase change temperature of the copolymer.

[0009] In addition, a certain amount of lithium salt and specific additives (such as fluoroacetyl oxy dimethyl phosphate, [EMIM]-g-PEG, perfluoroalkyl sulfonate quaternary ammonium salt nitrate, ethyl-2,2,2-trifluoroethyl carbonate, 2-thiophene tert-butyl sulfonamide) are also added to the reversible phase change electrolyte of the present application. The lithium salt provides active lithium ions during the discharge process of the battery, and is the basis for realizing ionic conduction. The additives not only help to reduce the viscosity of the electrolyte, promote the formation of SEI film, improve the wettability of the electrode, and thus improve the interface ion conductivity; and the additives can also act as a film-forming agent to promote the formation of a stable and dense SEI film on the surface of the electrode, thereby improving the ionic conductivity of the electrolyte in a low temperature environment.

[0010] Therefore, the phase change electrolyte of the present application can change phase from liquid to solid at low temperature of 40-60℃, and the phase change process is reversible, and the phase change temperature can be controlled by adjusting the types and contents of the copolymer 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 40-60℃, 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 continuous increase of the battery temperature is slowed down; when the battery temperature continues to rise to above 77℃, the internal reaction of the battery is completely stopped, the thermal runaway of the battery is prevented, and the battery can work efficiently in a wide temperature range.

[0011] To solve the above technical problems, the first aspect of the present application provides a reversible phase change electrolyte, the raw material components of which include a copolymer, an ionic liquid and an organic solvent, and the ionic liquid is an imidazole ionic liquid or a pyridine ionic liquid.

[0012] The raw materials for preparing the copolymer include polymer A and polymer C, or include polymer B and polymer C;

[0013] The chemical structures of the polymer A, the polymer B and the polymer C are shown in formula (1), formula (2) and formula (3) respectively.

[0014] Formula (1); Formula (2);

[0015] Formula (3);

[0016] 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; R is selected from methyl, trifluoromethyl, trichloromethyl or tribromomethyl.

[0017] In some embodiments of the present application, R is selected from methyl or trifluoromethyl.

[0018] In some embodiments of the present application, x and y are integers between 200 and 800, respectively; for example, integers between 200 and 400, 300 and 500, 400 and 600, 500 and 700, 600 and 800, etc.

[0019] In some embodiments of the present application, z is an integer between 400 and 1000; for example, integers between 400 and 600, 500 and 700, 600 and 800, 700 and 900, 800 and 1000, etc.

[0020] In some embodiments of the present application, the mass ratio of the polymer A to the polymer C is (0.4-1.5):1; for example, 6:4, 5:5, 4:6, 3:7, etc.

[0021] In some embodiments of the present application, the mass ratio of the polymer B to the polymer C is (0.25-1):1; for example, 5:5, 4:6, 3:7, 2:8, etc.

[0022] In some embodiments of the present application, the imidazole-based ionic liquid is selected from at least one of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium tetrafluoroborate, 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide.

[0023] In some embodiments of the present application, the pyridine-based ionic liquid is selected from at least one of N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, N-propyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, N-methyl-N-methoxyethylpyrrolidinium bis(trifluoromethylsulfonyl)imide.

[0024] In some embodiments of the present application, the organic solvent includes a carbonate-based solvent and / or an ether-based solvent.

[0025] In some embodiments of the present application, the carbonate-based solvent is selected from at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate.

[0026] In some embodiments of the present application, the ether-based solvent is selected from at least one of tetrahydrofuran, ethylene glycol dimethyl ether, trimethyl phosphate.

[0027] In some embodiments of the present application, the raw material components include, by weight parts, the copolymer 3-15 parts, the ionic liquid 60-85 parts, and the organic solvent 1-15 parts.

[0028] In some embodiments of the present application, the raw material components include, by weight parts, the copolymer 5-12 parts, the ionic liquid 68-84 parts, and the organic solvent 2-12 parts.

[0029] In some embodiments of the present application, the raw material components further include 5-16 parts by weight of a lithium salt and 0.5-3 parts by weight of an additive.

[0030] In some embodiments of the present application, the raw material components further include 6-10 parts by weight of a lithium salt and 1-2 parts by weight of an additive.

[0031] In some embodiments of the present application, the lithium salt is selected from at least one of LiPF6, LiBF4, LiClO4, LiBOB, LiFSI, LiCF3SO3, LiTFSI, LiDFOB, LiPO2F2.

[0032] In some embodiments of the present application, the additive is selected from at least one of fluoroacetylphosphoric acid dimethyl ester, [EMIM]-g-PEG, perfluoroalkylsulfonyl quaternary ammonium salt nitrate, ethyl-2,2,2-trifluoroethyl carbonate, 2-thiophene tert-butyl sulfonamide.

[0033] A second aspect of the present application provides a method for preparing the reversible phase change electrolyte as described above, comprising the following steps:

[0034] Mixing each raw material component to prepare the reversible phase change electrolyte.

[0035] In some embodiments of the present application, the mixing is performed under an inert atmosphere.

[0036] In some embodiments of the present application, the inert atmosphere is an argon atmosphere.

[0037] In some embodiments of the present application, the method for preparing the reversible phase change electrolyte comprises the following steps:

[0038] Mixing polymer A or polymer B, polymer C, ionic liquid, organic solvent, lithium salt and additive under an argon atmosphere, and stirring uniformly to prepare the reversible phase change electrolyte.

[0039] A third aspect of the present application provides a secondary battery comprising the reversible phase change electrolyte as described above.

[0040] In some embodiments of the present application, the secondary battery is a lithium ion battery.

[0041] In some embodiments of the present application, the secondary battery further comprises a positive electrode, a negative electrode and a separator.

[0042] 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, and 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.

[0043] 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, and typical but non-limiting negative electrode active materials are, for example, graphite, silicon, metallic lithium, etc.

[0044] In some embodiments of the present application, the material of the diaphragm can be selected from conventional diaphragm materials in the art, and typical but non-limiting diaphragm materials include polyethylene, polypropylene, polyimide, polyamide, polyvinylidene fluoride, polytetrafluoroethylene, and the like.

[0045] The above technical solutions of the present application have at least the following technical effects or advantages over the prior art:

[0046] (1) The reversible phase change electrolyte of the present application is an organic solvent system, and the specific structure of the copolymer and ionic liquid is the main component. The π-π interaction between the copolymer containing five-membered or six-membered rings and the imidazole ionic liquid or pyridine ionic liquid containing five-membered or six-membered heterocyclic cations is broken when the temperature is higher than the LCST temperature. The copolymer precipitates from the ionic liquid and itself aggregates to grow into larger particles, and the electrolyte changes from liquid to solid, resulting in phase change. At the same time, the copolymer is copolymerized from polymer A or polymer B containing five-membered or six-membered rings and polymer C containing a specific backbone and a side chain group of -CX3 (X = H, F, Cl, Br). The backbone of polymer C lengthens the side chain of polymer A or polymer B, and forms an obstacle to the π-π interaction between the copolymer and the ionic liquid. The -CX3 side chain group of polymer C further weakens the π-π interaction between the copolymer and the ionic liquid, thereby reducing the phase change temperature of the electrolyte.

[0047] (2) The phase change electrolyte of the present application can undergo phase change at a low temperature of 40-60℃, and the phase change temperature can be controlled by adjusting the types and contents of the polymer and the ionic liquid, as well as the content of the lithium salt. After 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 continuous temperature rise of the battery. When the battery temperature continues to rise to above 77℃, the internal reaction of the battery completely stops, preventing the battery from overheating, and allowing the battery to work efficiently in a wide temperature range.

[0048] (3) The present application adds a certain amount of lithium salt and a specific additive to the phase change electrolyte. The additive not only helps to reduce the viscosity of the electrolyte, promote the formation of SEI film, improve the wettability of the electrode, and improve the interface ion conductivity; but also acts as a film-forming agent to promote the formation of a stable and dense SEI film on the surface of the electrode, further improve the ionic conductivity of the electrolyte in a low temperature environment, and achieve an ionic conductivity of 1.43-1.78 mS / cm for lithium ion batteries in a low temperature environment of -20℃. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 Discharge capacity graph of a lithium ion battery assembled by the reversible phase change electrolyte prepared in Example 1 of the present application;

[0050] Figure 2 Discharge capacity plot for a lithium ion battery assembled with the reversible phase change electrolyte prepared in Example 2 of the present invention;

[0051] Figure 3 Discharge capacity plot for a lithium ion battery assembled with the reversible phase change electrolyte prepared in Example 3 of the present invention;

[0052] Figure 4 Discharge capacity plot for a lithium ion battery assembled with the reversible phase change electrolyte prepared in Example 4 of the present invention;

[0053] Figure 5 Discharge capacity plot for a lithium ion battery assembled with the reversible phase change electrolyte prepared in Example 5 of the present invention. DETAILED DESCRIPTION

[0054] The present invention will be described in detail below with reference to Examples, so as to facilitate the understanding of the present invention by those skilled in the art. It is necessary to point out here that the Examples are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Non-essential improvements and adjustments to the present invention made by those skilled in the art based on the above description of the invention shall still fall within the scope of protection of the present invention. 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.

[0055] The preparation process of the copolymer in the following Examples and Comparative Examples is as follows:

[0056] Under a nitrogen atmosphere, the polymer A or polymer B and polymer C were mixed in a mass ratio, then dissolved in anhydrous dichloromethane, and then an initiator LiBF4 (added in an amount of 1 wt% of polymer C) was added. The reaction system was warmed to 60°C, at which time the LiBF4 decomposed to release BF3, which in turn initiated the ring-opening polymerization of monomer A or polymer B, and the reaction continued for 4-12 hours. After the reaction was completed, excess methanol was added to the reaction system to precipitate the polymer, which was collected by filtration or centrifugation, then washed with methanol to obtain the crude product. The crude product was dissolved in tetrahydrofuran (THF), and then the solution was poured into a poor solvent (methanol or diethyl ether) to re-precipitate the polymer. The final product was vacuum-dried at 60°C for 48 hours to obtain the purified copolymer.

[0057] The preparation process of polymer A is as follows: epichlorohydrin was slowly added to a cold mixture of 40% w / w aqueous sodium hydroxide, benzyl alcohol and tetrabutylammonium bromide (TBAB) in a stoichiometric amount corresponding to the chemical structure. After the reaction was completed, the mixture was extracted twice with diethyl ether, dried with magnesium sulfate, filtered and evaporated to dryness to obtain.

[0058] The preparation process of polymer B is as follows: epichlorohydrin is slowly added to a cold mixture of 40% w / w sodium hydroxide aqueous solution, thiophene methanol, and tetrabutylammonium bromide (TBAB) according to the corresponding amounts of the chemical structures. After the reaction is complete, the mixture is extracted twice with diethyl ether, dried with magnesium sulfate, filtered, and evaporated to dryness to obtain the polymer.

[0059] Polymer C was purchased from McLean Company.

[0060] Example 1

[0061] A reversible phase change electrolyte, the raw material components of which, by weight, include: 84 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imine salt, 2 parts of ethylene carbonate, 68 parts of LiPF6, 5 parts of copolymer, 0.2 parts of dimethyl fluoroacetoxyphosphate, and 0.8 parts of 2-thiophene tert-butylsulfonamide.

[0062] Wherein: the copolymer is formed by copolymerizing polymer A and polymer C in a mass ratio of 5:5, and the chemical structures of polymer A and polymer C are shown in formula (1) and formula (3), respectively:

[0063] Equation (1); Equation (3);

[0064] Where: x is an integer between 300 and 400, z is an integer between 700 and 800; R is a methyl group.

[0065] The preparation method of the above-mentioned reversible phase change electrolyte includes the following steps:

[0066] In an argon-filled glove box, each raw material was weighed according to its composition, mixed, and stirred at 60 rpm for 30 minutes to obtain the reversible phase change electrolyte of this embodiment.

[0067] Example 2

[0068] A reversible phase change electrolyte, the raw material components of which, by weight, include: 45 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imine salt, 33 parts of 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imine salt, 2 parts of dimethyl carbonate, 8 parts of LiTFSI, 4 parts of LiDFOB, 6 parts of copolymer, 0.4 parts of dimethyl fluoroacetoxyphosphate, 0.6 parts of [EMIM]-g-PEG, and 1 part of ethyl-2,2,2-trifluoroethyl carbonate.

[0069] Wherein: the copolymer is formed by copolymerizing polymer A and polymer C in a mass ratio of 4:6, and the chemical structures of polymer A and polymer C are shown in formula (1) and formula (3), respectively:

[0070] Equation (1); Equation (3);

[0071] Where: x is an integer between 400 and 500, z is an integer between 800 and 900; R is trifluoromethyl.

[0072] The preparation method of the reversible phase change electrolyte in Example 2 is the same as that in Example 1.

[0073] Example 3

[0074] A reversible phase change electrolyte, the raw material components of which, by weight, include: 35 parts of N-butyl-N-methylpyrrolidine bis(trifluoromethanesulfonyl)imine salt, 39 parts of N-methyl-N-methoxyethylpyrrolidine bis(trifluoromethanesulfonyl)imine 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.

[0075] Wherein: the copolymer is formed by copolymerizing polymer A and polymer C in a mass ratio of 3:7, and the chemical structures of polymer A and polymer C are shown in formula (1) and formula (3), respectively:

[0076] Equation (1); Equation (3);

[0077] Where: x is an integer between 400 and 500, z is an integer between 700 and 800; R is methyl.

[0078] The preparation method of the reversible phase change electrolyte in Example 3 is the same as that in Example 1.

[0079] Example 4

[0080] A reversible phase change electrolyte, the raw material components of which, by weight, include: 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-trifluoroethyl carbonate.

[0081] Wherein: the copolymer is formed by copolymerizing polymer B and polymer C in a mass ratio of 4:6, and the chemical structures of polymer B and polymer C are shown in formula (2) and formula (3), respectively:

[0082] Equation (2); Equation (3);

[0083] Where: y is an integer between 500 and 600, z is an integer between 500 and 600; R is methyl.

[0084] The preparation method of the reversible phase change electrolyte in Example 4 is the same as that in Example 1.

[0085] Example 5

[0086] A reversible phase change electrolyte, the raw material components of which, by weight, include: 68 parts of N-methyl-N-methoxyethylpyrrolidine bis(trifluoromethanesulfonic acid)imine salt, 12 parts of trimethyl phosphate, 10 parts of LiPO2F2, 12 parts of copolymer, 0.7 parts of dimethyl fluoroacetoxy phosphate, and 1.3 parts of ethyl-2,2,2-trifluoroethyl carbonate.

[0087] Wherein: the copolymer is formed by copolymerizing polymer B and polymer C in a mass ratio of 3:7, and the chemical structures of polymer B and polymer C are shown in formula (2) and formula (3), respectively:

[0088] Equation (2); Equation (3);

[0089] Where: y is an integer between 600 and 700, z is an integer between 600 and 700; R is a methyl group.

[0090] The preparation method of the reversible phase change electrolyte in Example 5 is the same as that in Example 1.

[0091] Comparative Example 1

[0092] The difference between Comparative Example 1 and Example 2 is that the raw material components of the reversible phase change electrolyte do not contain copolymers. The raw material components of the reversible phase change electrolyte in Comparative Example 1, by weight, include: 45 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imine salt, 33 parts of 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imine salt, 2 parts of dimethyl carbonate, 8 parts of LiTFSI, 4 parts of LiDFOB, 0.4 parts of dimethyl fluoroacetoxyphosphate, 0.6 parts of [EMIM]-g-PEG, and 1 part of ethyl-2,2,2-trifluoroethyl carbonate.

[0093] Comparative Example 2

[0094] The difference between Comparative Example 2 and Example 4 lies in the composition of the copolymer. The copolymer of Comparative Example 2 is copolymerized from polymer B and polymer C in a mass ratio of 6:4.

[0095] Comparative Example 3

[0096] The difference between Comparative Example 3 and Example 5 lies in the chemical structure of polymer C. Polymer C in Comparative Example 3 does not contain methyl groups, and its chemical structure is shown in formula (4):

[0097] Equation (4);

[0098] Where z is an integer between 600 and 700.

[0099] Comparative Example 4

[0100] 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.

[0101] Comparative Example 5

[0102] 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.

[0103] Comparative Example 6

[0104] The difference between Comparative Example 6 and Example 1 is that Comparative Example 6 uses an equal amount of copolymer D instead of the copolymer in Example 1, and polymer D is copolymerized from polymer E and polymer F, wherein polymer F is formed by adding methyl groups to the side chains of polymer A; the chemical structures of polymer E and polymer F are shown in formula (4) and formula (5), respectively:

[0105] Equation (4); Equation (5);

[0106] Where z is an integer between 600 and 700, x is an integer between 300 and 400, and R is a methyl group.

[0107] Performance testing

[0108] The reversible phase change electrolytes prepared in Examples 1-5 and Comparative Examples 1-6 were respectively added to battery casings using lithium iron phosphate as the positive electrode material, graphite as the negative electrode material, and polyvinylidene fluoride as the separator, and assembled into lithium-ion batteries. Ionic conductivity, discharge capacity, and phase change temperature were tested, wherein the temperature at which the discharge capacity reached 0 was recorded as the turn-off temperature. Specific test conditions are as follows:

[0109] 1. Ionic conductivity

[0110] With an area of ​​2cm 2 The stainless steel gasket and electrolyte are assembled into a battery. The battery is placed in a low temperature test chamber at -20℃ for 3 hours. The electrochemical workstation is connected and a sinusoidal voltage signal with an amplitude of 20mV is applied. The test frequency range is between 4-100MHz. The resistance of the electrolyte is recorded and the ionic conductivity is calculated.

[0111] 2. Discharge capacity

[0112] 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, with a discharge cutoff voltage of 2V, and 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, with a discharge cutoff voltage of 2V, and record the discharge capacity. Switch between the two temperatures.

[0113] 3. Phase transition temperature test

[0114] The phase transition temperature of an electrolyte is determined by measuring the transmittance of the electrolyte solution. A variable-temperature ultraviolet-visible spectrometer is used to measure the transmittance of the electrolyte solution. The solution is placed on the instrument's heating stage and heated at a rate of 1℃ / min. The transmittance curve is recorded, and the temperature at which the transmittance reaches 0 is defined as the phase transition temperature.

[0115] Test results are as follows Figures 1-5 As shown in Table 1.

[0116] Table 1:

[0117]

[0118] From Table 1 and Figures 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 high ionic conductivity of 1.43-1.78 mS / cm at a low temperature of -20℃; the discharge capacity at 25℃ is 154.5-159.1 mAh / g; the phase change temperature is as low as 40.2-57.6℃; and the turn-off temperature is 77℃.

[0119] Compared to Example 2, Comparative Example 1 does not contain copolymers, so the electrolyte does not undergo a phase change and there is no thermal runaway prevention effect.

[0120] Compared to Example 4, Comparative Example 2 showed a significantly higher phase transition temperature and shut-off temperature due to the excessively high mass percentage of polymer B in the copolymer, indicating that the phase transition temperature can be controlled by adjusting the polymer content.

[0121] Compared to Example 5, Comparative Example 3 shows that since polymer C does not contain methyl groups, both the phase transition temperature and the turn-off temperature are increased to a certain extent, indicating that the methyl groups in polymer C have a certain promoting effect on reducing the phase transition temperature.

[0122] Compared to Example 1, Comparative Example 4 showed an increase in phase transition temperature because an equal amount of polymer A was used to replace the copolymer in Example 1, and there was no moderating effect of polymer C. This indicates that the copolymer is more beneficial for reducing the phase transition temperature of the electrolyte than a single polymer.

[0123] Compared to Example 1, Comparative Example 5 uses an equal amount of polymer C to replace the copolymer in Example 1. Since polymer C itself does not have phase change capability, the electrolyte does not undergo phase change and there is no thermal runaway prevention effect.

[0124] Compared to Example 1, Comparative Example 6 showed poor control of phase transition temperature due to the use of an equal amount of polymer D to replace the copolymer in Example 1, resulting in a higher phase transition temperature. This indicates that the position of the substituents in the copolymer has a certain influence on the phase transition temperature of the electrolyte.

[0125] For those skilled in the art, several simple deductions or substitutions can be made without departing from the inventive concept, without requiring creative effort. Therefore, any simple improvements made to this invention by those skilled in the art based on the disclosure of this invention should be within the scope of protection of this invention. The above embodiments are preferred embodiments of this invention, and all processes similar to this invention and equivalent changes should fall within the scope of protection of this invention.

Claims

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

2. The reversible phase change electrolyte of claim 1, wherein, 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 of claim 1, wherein, The imidazole ionic liquid is selected from at least one of 1-ethyl-3-methyl imidazole bis(trifluoromethylsulfonyl) imide salt, 1-butyl-3-methyl imidazole tetrafluoroborate, 1,3-dimethyl imidazole bis(trifluoromethylsulfonyl) imide salt; The pyridine ionic liquid is selected from at least one of N-butyl-N-methyl pyrrolidine bis(trifluoromethylsulfonyl) imide salt, N-propyl-N-methyl pyrrolidine bis(trifluoromethylsulfonyl) imide salt, N-methyl-N-methoxyethyl pyrrolidine bis(trifluoromethylsulfonic acid) imide.

4. The reversible phase change electrolyte of claim 1, wherein, The organic solvent includes a carbonate solvent and / or an ether solvent.

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

6. The reversible phase change electrolyte of claim 5, wherein, The raw material components further include 5-16 parts by weight of lithium salt and 0.5-3 parts by weight of additive.

7. The reversible phase change electrolyte of claim 6, wherein, The lithium salt is selected from at least one of LiPF6, LiBF4, LiClO4, LiBOB, LiFSI, LiCF3SO3, LiTFSI, LiDFOB, LiPO2F2; The additive is selected from at least one of fluoroacetyloxy dimethyl phosphate, [EMIM]-g-PEG, perfluoroalkyl sulfonate quaternary ammonium salt nitrate, ethyl-2,2,2-trifluoroethyl carbonate, 2-thiophene tert-butyl sulfonamide.

8. A method for producing a reversible phase change electrolyte as claimed in any one of claims 1 to 7, characterized in that, The method comprises the following steps: Mixing the raw material components to prepare the reversible phase change electrolyte.

9. The method for preparing the reversible phase change electrolyte according to claim 8, characterized in that, The mixing is carried out under inert atmosphere.

10. A secondary battery characterized by comprising: The reversible phase change electrolyte of any one of claims 1-7. The reversible phase change electrolyte of any one of claims 1-7.

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