Gel electrolyte based on in-situ polymerization and preparation method of lithium ion battery of gel electrolyte

Abstract

The invention belongs to the technical field of lithium ion batteries, and relates to a gel electrolyte based on in-situ polymerization and a preparation method of a lithium ion battery. The preparation method comprises the following steps: adding a cross-linking monomer with a structure with more than three arms and a polyfluorinated acrylate monomer into an electrolyte, and adding an initiator to obtain a monomer solution; then injecting into the lithium ion battery, placing and aging to enable the monomer solution to fully infiltrate a positive plate, a negative plate and a diaphragm; and performing thermal polymerization on the aged battery cell in situ to obtain the gel electrolyte lithium ion battery based on in-situ polymerization. According to the invention, a polymethylacrylic acid based gel electrolyte is prepared by using a method of in-situ polymerization of a polyfluorine monomer with interface film-forming characteristics and polymerization characteristics and a multi-arm cross-linking monomer with high polymerization activity; wherein the monomer with double characteristics can participate in polymerization, and the residual monomer can serve as an interface film-forming additive, so that more LiF is formed in a solid electrolyte membrane of a negative electrode and a positive electrode-electrolyte phase, and the interface stability is enhanced.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a gel electrolyte based on in-situ polymerization and a preparation method thereof for lithium-ion batteries. Background Art

[0002] Safety problems such as leakage, inflammability, and explosiveness of liquid electrolytes affect the application of lithium batteries. Introducing solid electrolytes, such as polymer electrolytes, can improve such problems and thus promote the practical application of lithium metal batteries. However, the polymer electrolytes prepared by traditional methods are generally thick, reducing the energy density of the battery, increasing the transmission distance of lithium ions, and at the same time, it is difficult to fill the electrode voids and achieve good contact between the electrolyte and the electrode.

[0003] In-situ polymerization is a simple preparation method for polymer electrolytes compatible with existing commercial battery production, and in-situ polymerization gel electrolytes can reduce the risk of solvent leakage. Compared with non-in-situ gel electrolytes, it can make the electrode / electrolyte interface closer and greatly reduce the interfacial impedance. Existing Chinese patent CN118630309A constructs a new cross-linked gel electrolyte by regulating the contents of nitrile compounds containing two carbon-carbon double bonds and acrylate esters in the prepolymer solution, enhancing the lithium-ion transmission ability of the gel electrolyte; another Chinese patent CN118336103A uses fluorinated acrylate monomers, and the synthesized fluorinated polymer can encapsulate the electrolyte solution, reducing the outflow of the solvent. At the same time, the fluorinated backbone is not flammable, so the risk of the battery catching fire under heating conditions is reduced; there is also Chinese patent CN118099516A that uses two monomers for cross-linking polymerization. The first monomer is an olefin monomer containing a sulfonic acid group, and the second monomer is an acrylate monomer. This gel electrolyte not only has a good solid-solid contact interface and compatibility with the positive and negative electrodes, but also has high tensile strength and high ionic conductivity.

[0004] Currently, most patents related to gel electrolytes focus on improving the ionic conductivity, flame retardancy, antioxidant properties, etc. of the electrolytes. In particular, the review article "Great challenges and New Paradigm of The In situ Polymerization Technology Inside Lithium Batteries" published in ADVANCED FUNCTIONAL MATERIALS points out that one of the greatest challenges in current in-situ polymerization is the high chemical / electrochemical reactivity and high-voltage fragility of residual monomers. Residual monomers can lead to problems such as decreased electrochemical stability, reduced ionic conductivity, and increased interfacial resistance, thus affecting the performance and safety of lithium-ion batteries. At the same time, most in-situ polymerization methods use free-radical-initiated polymerization of acrylate monomers. To improve the ionic conductivity, linear structure monomers with an excessive number of ether bonds are often used. The linear polymers formed by such monomers have poor mechanical and oxidation stability and are difficult to adapt to long-term cycling and high-voltage environments. Summary of the Invention

[0005] To solve the deficiencies described in the above prior art, especially the problems that residual monomers can lead to decreased electrochemical stability, reduced ionic conductivity, increased interfacial resistance, etc., and the mechanical and oxidation stability of polymers with multiple ether bonds is poor and difficult to adapt to long-term cycling and high-voltage environments, the present invention provides a preparation method of a gel electrolyte based on in-situ polymerization and its lithium-ion battery.

[0006] The technical solution adopted by the present invention is as follows: A preparation method of a gel electrolyte lithium-ion battery based on in-situ polymerization, comprising the following steps: Step 1: Add a first monomer and a second monomer to an electrolyte solution, stir evenly, then add an initiator, and continue stirring to obtain a monomer solution; the electrolyte solution includes an organic solvent and an electrolyte salt; the first monomer contains a chain polymerization functional group; the second monomer participates in forming a solid electrolyte film at the negative electrode and copolymerizes with the first monomer; Step 2: Inject the monomer solution obtained in Step 1 into the lithium-ion battery, let it stand and age for a period of time so that the monomer solution can fully infiltrate the positive and negative electrode plates and the separator; Step 3: Thermally polymerize the aged battery cell in-situ. The pressure of the thermal polymerization is 0.4 MPa to 0.8 MPa, and the temperature is 50 °C to 80 °C, to obtain a gel electrolyte lithium-ion battery based on in-situ polymerization.

[0007] Further, the first monomer is a cross-linked monomer with a molecular structure having three or more arms.

[0008] Preferably, the first monomer is one of 1,3,5-tris(2-methyl-2-propen-1-yl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, triallyl isocyanurate, tris(2-acryloyloxyethyl) isocyanurate, trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, ditrimethylolpropane tetraacrylate, trimethylolpropane-tris[3-(2-methylaziridinyl)propionate], and trimethylolpropane tris(3-aziridinylpropionate).

[0009] Furthermore, the second monomer is a fluorinated acrylate monomer.

[0010] Preferably, the second monomer is one of hexafluorobutyl methacrylate, hexafluoroisopropyl methacrylate, 2,2,3,3,4,4,4-heptafluorobutyl methacrylate, dodecafluoroheptyl methacrylate, tert-butyl 2-trifluoromethylacrylate, 2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadecafluorononyl methacrylate, 3-(perfluorobutyl)-2-hydroxypropyl methacrylate, and 1H,1H,2H,2H-nonafluorohexyl acrylate.

[0011] Furthermore, the molar ratio of the first monomer to the second monomer is 1:0.5 - 3.

[0012] Preferably, the molar ratio of the first monomer to the second monomer is 1:1 - 2.5.

[0013] A gel electrolyte based on in-situ polymerization is a polymethyl methacrylate (PMMA)-based gel electrolyte formed by in-situ polymerization of a crosslinking monomer with a molecular structure of three or more arms and a fluorinated acrylate monomer.

[0014] The beneficial effects of the present invention are as follows: 1. The present invention uses a method of in-situ polymerization of a fluorinated monomer with dual properties of interfacial film-forming and polymerization properties and a multi-arm crosslinking monomer with high polymerization activity (i.e., a crosslinking monomer with a molecular structure of three or more arms) to prepare a polymethyl methacrylate (PMMA)-based gel electrolyte; the fluorinated monomer with dual properties can not only participate in polymerization, but also its residual monomer can act as an interfacial film-forming additive to form more LiF in the solid electrolyte interface (SEI) of the negative electrode and the cathode-electrolyte interface (CEI); LiF has a lower diffusion barrier and stronger mechanical strength, enhancing the interfacial stability, thereby improving the cycle stability of the battery cell, and this monomer can solve the adverse effects of residual monomers on lithium-ion batteries; both monomers contain fewer ether bonds and the multi-arm crosslinking monomer contains an electron-donating group, increasing the electron density of the chain segments, thereby jointly improving the oxidation stability of the gel electrolyte.

[0015] 2. The second monomer of the present invention is a polyfluorinated monomer with interfacial film-forming properties and polymerization properties. Since fluorine has a strong electron-withdrawing effect and antioxidant ability, participating in the polymerization of the gel electrolyte can protect the polymer framework from oxidation. At the same time, F in the polymer chain reduces the nucleophilicity of O, resulting in a decrease in the binding strength between O and Li+. Polymer fluorination weakens the coordination between Li+ and O, reducing the migration energy barrier of polymer segments. In addition to polymerization properties, this monomer also has interfacial film-forming properties. Fluorine groups can promote the formation of an SEI and CEI rich in LiF, which has a low diffusion energy barrier and strong mechanical strength, enhancing interfacial stability. Therefore, this monomer can not only optimize the electrochemical properties of the gel electrolyte but also solve the adverse effects of residual monomers on lithium-ion batteries.

[0016] 3. The multi-arm crosslinking monomer with high polymerization activity in the present invention can not only reduce the negative impact of residual monomers but also form a three-dimensional (3D) crosslinked polymer network, enabling the PMMA crosslinked gel electrolyte to have a high solution absorption capacity, reducing the direct contact between the electrolyte and the active material, and further reducing the decomposition of the electrolyte. The multi-arm crosslinking monomer contains electron-donating groups, increasing the electron density of the chain segments and further improving the oxidation stability of the polymer electrolyte. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 are the dQ / dV curves of Comparative Example 3 and Comparative Example 2; Figure 2 are the DCR curves of Comparative Example 3 and Comparative Example 2; Figure 3 are the 0.5C / 1C normal temperature cycle curves of Comparative Example 3 and Comparative Example 2; Figure 4 are the 0.5C / 1C 45°C cycle curves of Comparative Example 3 and Comparative Example 2; Figure 5 are the DCR curves of Example 1 and Comparative Example 10; Figure 6 are the 0.5C / 1C normal temperature cycle curves of Example 1 and Comparative Example 10; Figure 7 are the 0.5C / 1C 45°C cycle curves of Example 1 and Comparative Example 10. DETAILED DESCRIPTION OF THE INVENTION

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0019] A preparation method of a gel electrolyte lithium-ion battery based on in-situ polymerization, comprising the following steps: Step 1: Add the first monomer and the second monomer into the electrolyte solution. After stirring evenly, add the initiator and continue stirring to obtain a monomer solution; the electrolyte solution includes an organic solvent and an electrolyte salt; the first monomer contains a chain polymerization functional group; the second monomer can not only participate in the formation of a solid electrolyte membrane (SEI) at the negative electrode, but also copolymerize with the first monomer; Step 2: Inject the monomer solution obtained in Step 1 into the lithium-ion battery. After standing for aging for a period of time, the monomer solution can fully infiltrate the positive and negative electrode plates and the separator; Step 3: Thermally polymerize the cell completed in aging in Step 2 in-situ. The pressure of the thermal polymerization is 0.4 MPa to 0.8 MPa, and the temperature is 50 °C to 80 °C, to obtain a gel electrolyte lithium-ion battery based on in-situ polymerization.

[0020] Furthermore, the first monomer is a cross-linked monomer with a molecular structure of more than three arms, simply referred to as a multi-arm cross-linked monomer. Specifically, it can be selected from 1,3,5-tris(2-methyl-2-propen-1-yl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, triallyl isocyanurate, tris(2-acryloyloxyethyl) isocyanurate, trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, ditrimethylolpropane tetraacrylate, trimethylolpropane-tris[3-(2-methylaziridinyl)propionate], trimethylolpropane tris(3-aziridinyl propionate).

[0021] Furthermore, the second monomer is a polyfluoroacrylate monomer, simply referred to as a polyfluoro monomer. Specifically, it can be selected from hexafluorobutyl methacrylate, hexafluoroisopropyl methacrylate, 2,2,3,3,4,4,4-heptafluorobutyl methacrylate, dodecafluoroheptyl methacrylate, tert-butyl 2-trifluoromethylacrylate, 2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadecafluorononyl methacrylate, 3-(perfluorobutyl)-2-hydroxypropyl methacrylate, 1H,1H,2H,2H-nonafluorohexyl acrylate.

[0022] Furthermore, the molar ratio of the first monomer to the second monomer is 1:0.5 - 3, preferably 1:(1 - 2.5).

[0023] Furthermore, the initiator is one of azobisisoheptonitrile, azobisisobutyronitrile, benzoyl peroxide, 2,4-dimethylvaleronitrile, dimethyl 2,2'-azobis(2-methylpropionate), methyl ethyl ketone peroxide, di-tert-butyl peroxide, diisopropylbenzene peroxide. Preferably, the initiator accounts for 1% - 5% of the total mass of the two monomers.

[0024] Preferably, the electrolyte is EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC and 1 M LiPF6.

[0025] Example 1 A method for preparing a gel electrolyte lithium-ion battery based on in-situ polymerization includes the following steps: S1, Preparation of the gel polymer electrolyte prepolymer solution: Add 2.4 g of the first monomer 1,3,5-tris(2-methyl-2-propen-1-yl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione and the second monomer hexafluorobutyl methacrylate in a molar ratio of 1:1 to 98 g of the electrolyte (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC and 1 M LiPF6); Stir at room temperature for 2 hours, and after stirring evenly, add 0.5 g of the initiator azodiisobutyronitrile, and continue to stir for 1 hour to obtain the gel polymer electrolyte prepolymer solution.

[0026] S2, Aging: Inject the gel polymer electrolyte prepolymer solution in step S1 into the battery interior according to an injection coefficient of 1.7 by mass, and age at 25°C for 36 hours to fully wet the electrode sheets and the separator with the electrolyte.

[0027] S3, Curing: Place the battery cell completed in aging in step S2 at 70°C for 2 hours and apply a pressure of 0.5 Mpa to complete in-situ thermal polymerization.

[0028] S4, Testing: Subject the battery cell completed in curing in step S3 to formation and grading, and test the internal resistance of the battery cell and the charge-discharge cycle test.

[0029] Example 2 The method for preparing a gel electrolyte lithium-ion battery based on in-situ polymerization in this example is different from that in Example 1 in that 2.4 g of the first monomer 1,3,5-tris(2-methyl-2-propen-1-yl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione and the second monomer hexafluorobutyl methacrylate are added to 98 g of the electrolyte (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC and 1 M LiPF6) in a molar ratio of 2:1; the rest is the same as in Example 1.

[0030] Example 3 The preparation method of a gel electrolyte lithium-ion battery based on in-situ polymerization in this example is different from that in Example 1 in that the first monomer 1,3,5-tris(2-methylprop-2-en-1-yl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione and the second monomer hexafluorobutyl methacrylate with a total mass of 2.4 g are added to 98 g of electrolyte solution (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC, and 1 M LiPF6) at a molar ratio of 1:2; the rest is the same as in Example 1.

[0031] Example 4 The preparation method of a gel electrolyte lithium-ion battery based on in-situ polymerization in this example is different from that in Example 1 in that the first monomer 1,3,5-tris(2-methylprop-2-en-1-yl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione and the second monomer hexafluorobutyl methacrylate with a total mass of 2.4 g are added to 98 g of electrolyte solution (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC, and 1 M LiPF6) at a molar ratio of 1:3; the rest is the same as in Example 1.

[0032] Example 5 The preparation method of a gel electrolyte lithium-ion battery based on in-situ polymerization in this example is different from that in Example 1 in that the first monomer triallyl isocyanurate and the second monomer hexafluoroisopropyl methacrylate with a total mass of 2.4 g are added to 98 g of electrolyte solution (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC, and 1 M LiPF6) at a molar ratio of 1:1; the rest is the same as in Example 1.

[0033] Example 6 The preparation method of a gel electrolyte lithium-ion battery based on in-situ polymerization in this example is different from that in Example 1 in that the first monomer tris(2-acryloyloxyethyl) isocyanurate and the second monomer 2,2,3,3,4,4,4-heptafluorobutyl methacrylate with a total mass of 2.4 g are added to 98 g of electrolyte solution (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC, and 1 M LiPF6) at a molar ratio of 1:1; the rest is the same as in Example 1.

[0034] Example 7 A preparation method of a gel electrolyte lithium-ion battery based on in-situ polymerization in this example, which is different from Example 1 in that the first monomer trimethylolpropane triacrylate and the second monomer dodecafluoroheptyl methacrylate with a total mass of 2.4 g are added to 98 g of electrolyte (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC and 1 M LiPF6) in a molar ratio of 1:1; the rest is the same as in Example 1.

[0035] Example 8 A preparation method of a gel electrolyte lithium-ion battery based on in-situ polymerization in this example, which is different from Example 1 in that the first monomer ethoxylated trimethylolpropane triacrylate and the second monomer tert-butyl 2-(trifluoromethyl)acrylate with a total mass of 2.4 g are added to 98 g of electrolyte (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC and 1 M LiPF6) in a molar ratio of 1:1; the rest is the same as in Example 1.

[0036] Example 9 A preparation method of a gel electrolyte lithium-ion battery based on in-situ polymerization in this example, which is different from Example 1 in that the first monomer trimethylolpropane trimethacrylate and the second monomer 2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadecafluorononyl methacrylate with a total mass of 2.4 g are added to 98 g of electrolyte (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC and 1 M LiPF6) in a molar ratio of 1:1; the rest is the same as in Example 1.

[0037] Example 10 A preparation method of a gel electrolyte lithium-ion battery based on in-situ polymerization in this example, which is different from Example 1 in that the first monomer ditrimethylolpropane tetraacrylate and the second monomer 3-(perfluorobutyl)-2-hydroxypropyl methacrylate with a total mass of 2.4 g are added to 98 g of electrolyte (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC and 1 M LiPF6) in a molar ratio of 1:1; the rest is the same as in Example 1.

[0038] Comparative Example 1 A preparation method of a gel electrolyte lithium-ion battery includes the following steps: S1. Preparation of Gel Polymer Electrolyte Prepolymer Solution: Add 2.4 g of 1,3,5-tris(2-methyl-2-propen-1-yl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione to 98 g of electrolyte solution (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC, and 1 M LiPF6); stir at room temperature for 2 hours. After stirring evenly, add 0.5 g of initiator azobisisoheptonitrile and continue stirring for 1 hour to obtain the gel polymer electrolyte prepolymer solution.

[0039] S2. Aging: Inject the gel polymer electrolyte prepolymer solution in Step S1 into the battery interior according to a filling coefficient of 1.7 by mass, and age at 25 °C for 36 hours to fully wet the electrode sheets and separator with the electrolyte solution.

[0040] S3. Curing: Place the battery cell completed in aging in Step S2 on a specific device, apply a pressure of 0.5 Mpa to the battery cell, and place it at 70 °C for 2 hours to complete in-situ thermal polymerization.

[0041] S4. Subject the battery cell completed in curing in Step S3 to formation and grading, and test the internal resistance of the battery cell and charge-discharge cycle test.

[0042] Comparative Example 2 A method for preparing a lithium-ion battery with an electrolyte solution containing additives, comprising the following steps: S1. Preparation of Electrolyte Solution Containing Additives: Add 0.05 g of hexafluorobutyl methacrylate to 100 g of electrolyte solution (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC, and 1 M LiPF6); stir at room temperature for 2 hours. After stirring evenly, obtain the electrolyte solution containing additives.

[0043] S2. Inject the electrolyte solution in Step 1 into the battery interior according to a filling coefficient of 1.7 by mass, and age at 25 °C for 36 hours to fully wet the electrode sheets and separator with the electrolyte solution.

[0044] S3. Subject the battery cell completed in aging in Step 2 to formation and grading, and test the internal resistance of the battery cell and charge-discharge cycle test.

[0045] Comparative Example 3 A method for preparing a lithium-ion battery with a liquid electrolyte, comprising the following steps: S1. Inject a conventional electrolyte solution (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC, and 1 M LiPF6) into the battery interior according to a filling coefficient of 1.7 by mass, and age at 25 °C for 36 hours to fully wet the electrode sheets and separator with the electrolyte solution.

[0046] S2. Subject the cells that have completed aging in step S1 to formation and grading, and test the internal resistance of the cells and the charge-discharge cycle test, and test the internal resistance of the cells and the charge-discharge cycle test.

[0047] Comparative Example 4 The difference from Example 1 is that the second monomer is replaced with 1,6-hexanediol diacrylate without fluorine element. That is, a total of 2.4 g of the first monomer 1,3,5-tris(2-methyl-2-propen-1-yl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione and the second monomer 1,6-hexanediol diacrylate are added to 98 g of the electrolyte in a molar ratio of 1:1, and the rest is the same as in Example 1.

[0048] Comparative Example 5 The difference from Example 1 is that the second monomer is replaced with trifluoroethyl methacrylate with less fluorine element. That is, a total of 2.4 g of the first monomer 1,3,5-tris(2-methyl-2-propen-1-yl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione and the second monomer trifluoroethyl methacrylate are added to 98 g of the electrolyte in a molar ratio of 1:1, and the rest is the same as in Example 1.

[0049] Comparative Example 6 The difference from Comparative Example 2 is that 0.05 g of 1,6-hexanediol diacrylate is added to 100 g of the electrolyte (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC and 1 M LiPF6); the rest is the same as in Comparative Example 2.

[0050] Comparative Example 7 The difference from Comparative Example 2 is that 0.05 g of trifluoroethyl methacrylate is added to 100 g of the electrolyte (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC and 1 M LiPF6); the rest is the same as in Comparative Example 2.

[0051] Comparative Example 8 The difference from Example 1 is that the first monomer is replaced with linear (ethylene glycol) diacrylate; that is, a total of 2.4 g of (ethylene glycol) diacrylate and the second monomer hexafluorobutyl methacrylate are added to 98 g of the electrolyte (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC and 1 M LiPF6) in a molar ratio of 1:1, and the rest is the same as in Example 1.

[0052] Comparative Example 9 The difference from Example 1 is that the first monomer is replaced with the linear substance pentaerythritol triacrylate; that is, 2.4 g of pentaerythritol triacrylate and the second monomer hexafluorobutyl methacrylate are added to 98 g of electrolyte (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC, and 1 M LiPF6) in a molar ratio of 1:1, and the rest is the same as in Example 1.

[0053] Comparative Example 10 The difference from Example 1 is that the first monomer is replaced with the linear substance pentaerythritol triacrylate and the second monomer is replaced with the fluorine-free 1,6-hexanediol diacrylate; that is, 2.4 g of pentaerythritol triacrylate and the second monomer 1,6-hexanediol diacrylate are added to 98 g of electrolyte (EC / EMC / DME = 2 / 1 / 5, 0.5% VC, 0.5% FEC, and 1 M LiPF6) in a molar ratio of 1:1, and the rest is the same as in Example 1.

[0054] The battery cells used in all of the above examples and comparative examples are soft-pack batteries, and the separator is a polypropylene separator. The formation conditions are as follows: charge at a current of 0.2C to 4.2V at 25°C, and then stand at 45°C for 24h - 72h. The cycling mode is: cycle at a constant temperature of 45°C, charge at 0.5C and discharge at 1C, the constant voltage cut-off current rate is 0.02C, and the cut-off voltage range is 2.5V - 4.2V. The test results are shown in the following table.

[0055] According to the test results recorded in the above table, comparing Examples 1, 5 - 10, Comparative Examples 1, 4, 5, 10, the gel electrolyte in which fluorinated acrylate participates in polymerization has a wider electrochemical stability window of the electrolyte compared to acrylate containing only ether oxygen bonds. Moreover, the higher the fluorine content in the acrylate, the wider its electrochemical window. This is because the fluorinated gel electrolyte can form a hard and stable fluoride-rich (such as LiF) protective layer on the anode and cathode surfaces, thereby improving the stability of the electrode and electrolyte interface. The strong electrostatic attraction between Li⁺ ions and F⁻ ions in LiF forms an unusually stable ionic lattice. At the same time, due to the strong polarity of fluorine atoms, most anions and solvent molecules participate in the solvation of Li⁺ ions, reducing the redox reactions of free solvent molecules and anions at the positive and negative electrode interfaces, and thus establishing a wider electrochemical window. Comparing Comparative Examples 2, 3, 6, 7, the fluorinated acrylate used as an electrolyte additive can also improve the electrochemical stability window of the electrolyte. The fluoride-rich protective layer induced by the fluorinated acrylate can also stabilize the deposition and stripping of Li⁺ ions, improving the cycle stability. Comparing Example 1, Comparative Examples 8, 9, the ionic conductivity of the cross-linking agent with multiple arms has an advantage compared to the linear cross-linking agent. At the same time, the addition of the three-arm cross-linking agent can form a three-dimensional cross-linked structure, effectively binding the liquid electrolyte, reducing its contact with the electrode and thus reducing the side reactions of the liquid electrolyte. Comparing Examples 1 - 4, the ratio of the multi-arm cross-linking monomer to the fluorinated acrylate will affect its electrochemical performance. If the proportion of the fluorinated acrylate is too high, the ionic conductivity will be greatly affected. The high degree of fluorination will weaken the interaction between Li⁺ and dipoles, resulting in the aggregation of a large number of ions and restricting the ionic conductivity of the electrolyte. Comparative Example 1 shows that for the simple polymerization framework of the multi-arm cross-linking monomer, its ionic conductivity and electrochemical window will be greatly affected.

[0056] In addition, Comparative Example 2 using the second monomer as an additive was compared with Comparative Example 3 using a conventional electrolyte. As Figure 1 shown by the dQ / dV curve, it indicates that hexafluorobutyl methacrylate can form a film by oxidation on the negative electrode side. And compared with the curve without the addition of hexafluorobutyl methacrylate, the solvent film-forming peak shifts to the right, proving that hexafluorobutyl methacrylate can form a good film on the negative electrode, reducing the interfacial impedance and polarization; Figure 2 The DCR curve of Figure 3 further verifies this argument. The DCR of the battery cell with hexafluorobutyl methacrylate added to the electrolyte is significantly lower than that of the battery cell without addition. From Figure 4 and Figures 5 to 7The gel electrolyte containing 1,1,1,3,3,3-hexafluorobutyl methacrylate participating in the polymerization is beneficial to improving the interface, reducing the impedance, and enhancing the high-temperature and room-temperature cycling performance.

Claims

1. A method for preparing a gel electrolyte lithium ion battery based on in-situ polymerization, characterized in that: It includes the following steps: Step 1: Add the first monomer and the second monomer into the electrolyte, stir evenly and then add the initiator, and continue to stir to obtain a monomer solution; the electrolyte includes an organic solvent and an electrolyte salt; the first monomer contains a chain polymerization functional group; the second monomer participates in forming a solid electrolyte film at the negative electrode and copolymerizes with the first monomer; Step 2: Inject the monomer solution obtained in Step 1 into the lithium-ion battery, and place it for aging for a period of time so that the monomer solution can fully infiltrate the positive and negative electrode plates and the separator; Step 3: Thermally polymerize the aged battery cell in situ in Step 2, the pressure of the thermal polymerization is 0.4 MPa to 0.8 MPa, and the temperature is 50 °C to 80 °C, to obtain a gel electrolyte lithium-ion battery based on in-situ polymerization.

2. The method for preparing a gel electrolyte lithium ion battery based on in-situ polymerization according to claim 1, characterized in that: The first monomer is a crosslinked monomer with a molecular structure of more than three arms.

3. The preparation method of a gel electrolyte lithium-ion battery based on in-situ polymerization according to claim 2, characterized in that, The first monomer is one of 1,3,5-tris(2-methyl-2-propen-1-yl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, triallyl isocyanurate, tris(2-acryloyloxyethyl) isocyanurate, trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, ditrimethylolpropane tetraacrylate, trimethylolpropane tris[3-(2-methylaziridinyl)propionate], trimethylolpropane tris(3-aziridinylpropionate).

4. The preparation method of a gel electrolyte lithium-ion battery based on in-situ polymerization according to claim 1, wherein The second monomer is a polyfluorinated acrylate monomer.

5. The preparation method of a gel electrolyte lithium-ion battery based on in-situ polymerization according to claim 4, characterized in that, The second monomer is one of hexylfluorobutyl methacrylate, hexafluoroisopropyl methacrylate, 2,2,3,3,4,4,4-heptafluorobutyl methacrylate, dodecafluoroheptyl methacrylate, tert-butyl 2-trifluoromethylacrylate, 2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadecafluorononyl methacrylate, 3-(perfluorobutyl)-2-hydroxypropyl methacrylate, 1H,1H,2H,2H-nonafluorohexyl acrylate.

6. The method for preparing a gel electrolyte lithium ion battery based on in-situ polymerization according to claim 1, characterized in that: The molar ratio of the first monomer to the second monomer is 1:0.5 to 3.

7. The preparation method of a gel electrolyte lithium-ion battery based on in-situ polymerization according to claim 6, characterized in that, The molar ratio of the first monomer to the second monomer is 1:1 to 2.

5.

8. A gel electrolyte based on in-situ polymerization, characterized in that, It is a polymethyl methacrylate (PMMA)-based gel electrolyte formed by in-situ polymerization of a crosslinked monomer with a molecular structure of more than three arms and a polyfluorinated acrylate monomer.

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