A crosslinked fluoropolymer electrolyte and its preparation method

Abstract

The present invention provides a crosslinked fluoropolymer electrolyte and a preparation method thereof, aiming to solve the problems of low electrolyte conductivity and mechanical strength of polyethylene oxide in the prior art. The present invention develops a novel crosslinked fluorinated polyethylene glycol-imide polymer electrolyte matrix material, which has multiple functional improvements. Among them, the polyethylene glycol structure endows the electrolyte with the functions of lithium ion conduction and wetting the electrode sheet, the imide structure endows the polymer electrolyte membrane with high mechanical strength and thermal stability functions, and the fluorinated structure endows the polymer electrolyte with better electrochemical stability functions. The three can act simultaneously, giving the polymer electrolyte higher ionic conductivity, mechanical strength, thermal stability and electrochemical stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of solid-state lithium batteries, and particularly relates to a cross-linked fluoropolymer electrolyte and a preparation method thereof. Background Art

[0002] Lithium-ion batteries have been widely used due to their advantages such as high specific energy, high working voltage, long cycle life, low self-discharge rate, and environmental friendliness. However, traditional liquid lithium-ion batteries contain organic carbonate-based liquid electrolytes, which have poor thermal stability and are flammable. When the battery is abused and generates heat, there are potential safety hazards such as leakage, fire, and explosion. Compared with traditional liquid lithium-ion batteries, solid-state lithium batteries use solid electrolytes that are non-flammable and have good thermal stability to replace organic electrolytes, and can have higher energy density, longer cycle life, and better safety, thus becoming a research hotspot in lithium-ion batteries in recent years.

[0003] Inorganic solid electrolytes have been studied due to their advantages such as high mechanical stability, thermal stability, chemical stability, and good electrochemical stability. However, the interface problem is the biggest application obstacle for inorganic solid electrolytes. In addition to inorganic solid electrolytes, polymer solid electrolytes have also received great attention due to their strong mechanical strength, high elasticity, and low cost. Polymer electrolytes are composed of a polymer matrix and a lithium salt. In solid polymers, ion transport mainly depends on the amorphous phase in the polymer matrix. As the content of the amorphous region increases, the ionic conductivity of the polymer electrolyte also increases. Therefore, researchers have done a lot of research on reducing the glass transition temperature of the polymer, stabilizing the amorphous region, and increasing the content of the amorphous region. So far, among pure solid polymer electrolytes, the most studied are polymer electrolytes based on polyethylene oxide. However, due to the relatively high crystallinity of polyethylene oxide, the migration of lithium ions is severely inhibited, resulting in low ionic conductivity at room temperature. At the same time, its electrochemical window is relatively narrow. These disadvantages have greatly hindered their further application in lithium batteries. Although researchers have carried out a lot of modification on the polyethylene oxide chain to reduce its crystallinity, the improvement of room temperature ionic conductivity is limited, and still cannot meet the requirements of high mechanical strength, high ionic conductivity, and wide electrochemical window of the electrolyte. In order to improve its conductivity, mechanical properties, thermal properties, and electrochemical properties, and at the same time have reliable cycle performance and good electrode compatibility, it is urgent to develop a new matrix material to fundamentally improve it. Summary of the Invention

[0004] The purpose of the present invention is to solve the problems described in the background art, and provide a cross-linked fluoropolymer electrolyte and a preparation method thereof.

[0005] A crosslinked fluoropolymer electrolyte according to the present invention, the crosslinked fluoropolymer electrolyte comprising a crosslinked fluorinated polyethylene glycol-imide and a lithium salt, wherein the chemical structural formula of the crosslinked fluorinated polyethylene glycol-imide is as follows:

[0006]

[0007] wherein q and p are both integers, and q = 10 - 50, p = 1 - 20;

[0008] R is wherein n and m are both integers, and n = 10 - 100, m = 10 - 50.

[0009] Further, the is selected from any one of ;

[0010] The X is selected from -CH 2 , -CH 2 (CH 2 ) in any one;

[0011] The Y is selected from -CH 2 , -OCH 2 , -OCH 2 (CH 2 ) in any one.

[0012] A method for preparing the crosslinked fluoropolymer electrolyte of the present invention comprises the following specific steps:

[0013] S1. Add the dianhydride to a reactor filled with inert gas, add N,N - methylpyrrolidone, stir at 0 - 5 °C until the dianhydride is completely dissolved, then add the diamine polydiethanol, stir and react for 5 - 20 h, add maleic anhydride, continue to react for 2 - 8 h, add triethylamine and acetic anhydride, continue to stir and react for 5 - 24 h, and obtain the polyethylene glycol-imide powder through precipitation, filtration, and drying;

[0014] S2. Completely dissolve the polyethylene glycol-imide powder prepared in S1 in N,N - methylpyrrolidone, add the fluorinated acrylate, stir well, add azobisisobutyronitrile, and continue to react at 40 - 80 °C for 3 - 10 h to obtain the fluorinated polyethylene glycol-imide solution;

[0015] S3. Add the lithium salt to the fluorinated polyethylene glycol-imide solution prepared in S2, stir for 2 - 5 h, pour it into a polytetrafluoroethylene mold, and dry to obtain the crosslinked fluoropolymer electrolyte.

[0016] Further, the dianhydride monomer is selected from any one or more of pyromellitic dianhydride, 2,3,3',4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-biphenylether dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, hexafluorodiacid dianhydride, and bisphenol A type diether dianhydride.

[0017] Further, the lithium salt is selected from any one or more of lithium hexafluorophosphate, lithium bis(trifluoromethylsulfonyl)imide, lithium difluoromethylsulfonylimide, lithium perchlorate, lithium tetrafluoroborate, and lithium bis(oxalato)borate.

[0018] Further, the molar ratio of the dianhydride, diaminopolyethylene glycol, and maleic anhydride is 1:(1 to 1.1):(0.1 to 0.5); the addition amounts of the triethylamine and the acetic anhydride are 0.1% to 0.5% of the total mass of the dianhydride, diaminopolyethylene glycol, and maleic anhydride.

[0019] Further, the molar ratio of the polyethylene glycol-imide to the fluorinated acrylate is 1:(1 to 1.2); the addition amount of the azobisisobutyronitrile is 0.1% to 0.5% of the total mass of the polyethylene glycol-imide and the fluorinated acrylate.

[0020] Further, the weight ratio of the crosslinked fluorinated polyethylene glycol-imide to the lithium salt is (70 to 90):(10 to 30).

[0021] Advantages of the present invention:

[0022] 1. The advantage of the present invention lies in the development of a novel crosslinked fluorinated polyethylene glycol-imide polymer electrolyte matrix material, which has multiple functional improvements. Among them, the polyethylene glycol structure endows the electrolyte with the function of conducting lithium ions and wetting the electrode sheet, the imide structure endows the polymer electrolyte membrane with high mechanical strength and thermal stability functions, and the fluorinated structure endows the polymer electrolyte with better electrochemical stability functions. The three can act simultaneously, giving the polymer electrolyte higher ionic conductivity, mechanical strength, thermal stability, and electrochemical stability.

[0023] 2. The advantage of the present invention lies in that the prepared polyethylene glycol-imide not only introduces the polyethylene glycol structure and the imide structure, but also uses maleic anhydride as a capping agent to give double bond end groups to the molecular chain, and undergoes free radical polymerization with the fluorinated acrylate under the action of an initiator to further crosslink, improving the mechanical strength of the polymer electrolyte membrane, reducing the polymer crystallinity, increasing the ionic conductivity of the electrolyte, and simultaneously introducing a fluorinated structure, which has a very strong electronegativity and can improve the dissociation degree of the electrolyte lithium salt and the processing performance to a certain extent. Specific embodiments

[0024] The present invention will be further described below in conjunction with specific embodiments, in order that those skilled in the art can more clearly understand the content of the technical solution. In addition, the description of this part is only exemplary and explanatory, and should not have any restrictive effect on the protection scope of the present invention.

[0025] Example 1:

[0026] A preparation method of a crosslinked fluorinated polymer electrolyte in this example:

[0027] S1. Add 1 mol of 4,4'-biphenyl ether dianhydride into a reactor filled with nitrogen, and add 500 mL of N,N-methylpyrrolidone. Stir at 0 °C until 4,4'-biphenyl ether dianhydride is completely dissolved, then add 1 mol of diamine polydiethanol, stir and react for 15 h, add 0.2 mol of maleic anhydride, continue to react for 6 h, add triethylamine and acetic anhydride, and continue to stir and react for 24 h. Obtain polyethylene glycol-imide powder through precipitation, filtration, and drying;

[0028] S2. Completely dissolve 1 mol of the polyethylene glycol-imide powder prepared in S1 in N,N-methylpyrrolidone, add 1 mol of fluorinated acrylate, stir well, add azobisisobutyronitrile, and continue to react at 60 °C for 6 h. Filter and dry to obtain crosslinked fluorinated polyethylene glycol-imide;

[0029] S3. Take 80 parts by weight of the crosslinked fluorinated polyethylene glycol-imide prepared in S2 and dissolve it in N,N-methylpyrrolidone. Add 20 parts by weight of lithium bis(trifluoromethanesulfonyl)imide to the solution, stir for 2 h, pour it into a polytetrafluoroethylene mold, and dry to obtain a crosslinked fluorinated polymer electrolyte.

[0030] Example 2:

[0031] A preparation method of a crosslinked fluorinated polymer electrolyte in this example:

[0032] S1. Add 1 mol of 4,4'-biphenyl ether dianhydride into a reactor filled with nitrogen, and add 500 mL of N,N-methylpyrrolidone. Stir at 0 °C until 4,4'-biphenyl ether dianhydride is completely dissolved, then add 1.1 mol of diamine polydiethanol, stir and react for 15 h, add 0.4 mol of maleic anhydride, continue to react for 6 h, add triethylamine and acetic anhydride, and continue to stir and react for 24 h. Obtain polyethylene glycol-imide powder through precipitation, filtration, and drying;

[0033] S2. Completely dissolve 1 mol of the polyethylene glycol - imide powder prepared in S1 in N,N - methylpyrrolidone, add 1 mol of fluorinated acrylate, stir well, add azobisisobutyronitrile, and continue to react at 60 °C for 6 h. Filter and dry to obtain cross - linked fluorinated polyethylene glycol - imide.

[0034] S3. Take 80 parts by weight of the cross - linked fluorinated polyethylene glycol - imide prepared in S2 and dissolve it in N,N - methylpyrrolidone. Add 20 parts by weight of lithium bis(trifluoromethanesulfonyl)imide to the solution, stir for 2 h, pour it into a polytetrafluoroethylene mold, and dry to obtain a cross - linked fluorinated polymer electrolyte.

[0035] Example 3:

[0036] A preparation method of a cross - linked fluorinated polymer electrolyte in this example:

[0037] S1. Add 1 mol of 4,4'-biphenyl ether dianhydride to a reactor filled with nitrogen, and add 500 mL of N,N - methylpyrrolidone. Stir at 0 °C until 4,4'-biphenyl ether dianhydride is completely dissolved. Then add 1 mol of diaminopolyethylene glycol, stir and react for 15 h, add 0.2 mol of maleic anhydride, continue to react for 6 h, add triethylamine and acetic anhydride, and continue to stir and react for 24 h. Obtain polyethylene glycol - imide powder through precipitation, filtration, and drying.

[0038] S2. Completely dissolve 1 mol of the polyethylene glycol - imide powder prepared in S1 in N,N - methylpyrrolidone, add 1.2 mol of fluorinated acrylate, stir well, add azobisisobutyronitrile, and continue to react at 60 °C for 6 h. Filter and dry to obtain cross - linked fluorinated polyethylene glycol - imide.

[0039] S3. Take 80 parts of the cross - linked fluorinated polyethylene glycol - imide prepared in S2 and dissolve it in N,N - methylpyrrolidone. Add 20 parts of lithium bis(trifluoromethanesulfonyl)imide to the solution, stir for 2 h, pour it into a polytetrafluoroethylene mold, and dry to obtain a cross - linked fluorinated polymer electrolyte.

[0040] Example 4:

[0041] A preparation method of a cross - linked fluorinated polymer electrolyte in this example:

[0042] S1. Add 1 mol of 4,4'-biphenyl ether dianhydride into a reactor filled with nitrogen, and add 500 mL of N,N-methylpyrrolidone. Stir at 0 °C until 4,4'-biphenyl ether dianhydride is completely dissolved. Then add 1 mol of diamine poly(ethylene glycol), stir and react for 15 h, add 0.2 mol of maleic anhydride, continue to react for 6 h, add triethylamine and acetic anhydride, and continue to stir and react for 24 h. Obtain poly(ethylene glycol)-imide powder through precipitation, filtration, and drying;

[0043] S2. Completely dissolve 1 mol of the poly(ethylene glycol)-imide powder prepared in S1 in N,N-methylpyrrolidone, add 1.2 mol of fluorinated acrylate, stir well, add azobisisobutyronitrile, and continue to react at 60 °C for 6 h. Filter and dry to obtain crosslinked fluorinated poly(ethylene glycol)-imide;

[0044] S3. Take 90 parts of the crosslinked fluorinated poly(ethylene glycol)-imide prepared in S2 and dissolve it in N,N-methylpyrrolidone. Add 10 parts of lithium bis(trifluoromethylsulfonyl)imide to the solution, stir for 2 h, pour it into a polytetrafluoroethylene mold, and dry to obtain a crosslinked fluorinated polymer electrolyte.

[0045] Example 5:

[0046] A preparation method of a crosslinked fluorinated polymer electrolyte in this example:

[0047] S1. Add 1 mol of 4,4'-biphenyl ether dianhydride into a reactor filled with nitrogen, and add 500 mL of N,N-methylpyrrolidone. Stir at 0 °C until 4,4'-biphenyl ether dianhydride is completely dissolved. Then add 1 mol of diamine poly(ethylene glycol), stir and react for 15 h, add 0.2 mol of maleic anhydride, continue to react for 6 h, add triethylamine and acetic anhydride, and continue to stir and react for 24 h. Obtain poly(ethylene glycol)-imide powder through precipitation, filtration, and drying;

[0048] S2. Completely dissolve 1 mol of the poly(ethylene glycol)-imide powder prepared in S1 in N,N-methylpyrrolidone, add 1.2 mol of fluorinated acrylate, stir well, add azobisisobutyronitrile, and continue to react at 60 °C for 6 h. Filter and dry to obtain crosslinked fluorinated poly(ethylene glycol)-imide;

[0049] S3. Take 70 parts of the crosslinked fluorinated poly(ethylene glycol)-imide prepared in S2 and dissolve it in N,N-methylpyrrolidone. Add 30 parts of lithium bis(trifluoromethylsulfonyl)imide to the solution, stir for 2 h, pour it into a polytetrafluoroethylene mold, and dry to obtain a crosslinked fluorinated polymer electrolyte.

[0050] Example 6:

[0051] A method for preparing a cross-linked fluorinated polymer electrolyte in this embodiment:

[0052] S1. Add 1 mol of hexafluorodianhydride to a reactor filled with nitrogen, and add 500 mL of N,N-methylpyrrolidone, stir at 0°C until 4,4'-biphenyl ether dianhydride is completely dissolved, then add 1 mol of diamine polyglycol, stir and react for 15 hours, add 0.2 mol of maleic anhydride, continue to react for 6 hours, add triethylamine and acetic anhydride, continue to stir and react for 24 hours, and obtain polyethylene glycol-imide powder by precipitation, filtration and drying;

[0053] S2, completely dissolve 1 mol of the polyethylene glycol-imide powder prepared in S1 into N,N-methylpyrrolidone, add 1.2 mol of fluorine-containing acrylate, stir well, add azobisisobutyronitrile, continue to react at 60°C for 6h, filter and dry to obtain cross-linked fluorine-containing polyethylene glycol-imide;

[0054] S3. Take 80 parts of the cross-linked fluorinated polyethylene glycol-imide prepared in S2 and dissolve it in N,N-methylpyrrolidone, add 20 parts of lithium bis(trifluoromethylsulfonyl)imide to the solution, stir for 2 hours, pour into a polytetrafluoroethylene mold, and dry to obtain a cross-linked fluorinated polymer electrolyte.

[0055] It should be noted that the lithium salt used in the above embodiments is selected from one or more of lithium hexafluorophosphate, lithium bis(trifluoromethylsulfonyl)imide, lithium difluoromethylsulfonylimide, lithium perchlorate, lithium tetrafluoroborate, and lithium dioxalate borate; the dianhydride monomer used is selected from one or more of pyromellitic anhydride, 2,3,3',4'-biphenyltetracarboxylic anhydride, 3,3',4,4'-biphenyltetracarboxylic anhydride, 3,3',4,4'-benzophenonetetracarboxylic anhydride, 4,4'-biphenyl ether anhydride, 2,3,6,7-naphthalenetetracarboxylic anhydride, hexafluoro dianhydride, and bisphenol A type diether anhydride.

[0056] Comparative Example 1

[0057] 80 parts of polyethylene oxide were dissolved in N,N-methylpyrrolidone, 20 parts of lithium bis(trifluoromethylsulfonyl)imide were added to the solution, stirred for 2 hours, poured into a polytetrafluoroethylene mold, and dried to obtain a polyethylene oxide polymer electrolyte.

[0058] Test method:

[0059] Tensile strength test method: Referring to GB / T36363-2018 standard, the tensile strength of the composite solid electrolyte membrane was tested in a universal tensile test at a rate of 10 mm / min.

[0060] Ionic conductivity test: The ionic conductivity of the electrolyte is measured by assembling a symmetrical cell with steel sheets as blocking electrodes on an electrochemical workstation in the frequency range of 0.1 Hz to 10 6 Hz. The ionic conductivity is calculated according to the following formula: σ = L / (R*S), where σ is the ionic conductivity of the composite solid electrolyte membrane, L is the thickness of the composite solid electrolyte membrane, S is the area of the composite solid electrolyte membrane, and R is the bulk resistance of the composite solid electrolyte membrane.

[0061] Electrochemical window test: Cyclic voltammetry is used for the test.

[0062]

[0063] Through comparison, it can be found that a cross-linked fluoropolymer electrolyte prepared by the method of the present invention has an ionic conductivity and mechanical strength much higher than those of the polyethylene oxide polymer electrolyte in the prior art.

Claims

1. A crosslinked fluoropolymer electrolyte, characterized in that, the crosslinked fluoropolymer electrolyte comprises a crosslinked fluoropolyethylene glycol-imide and a lithium salt, wherein the chemical structural formula of the crosslinked fluoropolyethylene glycol-imide is as follows: wherein, q and p are both integers, and q = 10 - 50, p = 1 - 20; R is wherein n and m are both integers, and n = 10 to 100, m = 10 to 50; Selected from Any one of them, wherein X is selected from -CH 2 , -CH 2 (CH 2 ) Any one of them, Y is selected from -CH 2 , -OCH 2 , -OCH 2 (CH 2 ) Any one of them.

2. A preparation method of the crosslinked fluoropolymer electrolyte according to claim 1, characterized in that, it comprises the following specific steps: S1. Add the dianhydride into a reactor filled with inert gas, add N,N-methylpyrrolidone, stir at 0 - 5 °C until the dianhydride is completely dissolved, then add the diamine polydiethanol, stir and react for 5 - 20 h, add maleic anhydride, continue to react for 2 - 8 h, add triethylamine and acetic anhydride, continue to stir and react for 5 - 24 h, and obtain the polyethylene glycol-imide powder through precipitation, filtration, and drying; S2. Completely dissolve the polyethylene glycol-imide powder prepared in S1 into N,N-methylpyrrolidone, add the fluorinated acrylate, stir well, add azobisisobutyronitrile, and continue to react for 3 - 10 h under the condition of 40 - 80 °C to obtain the fluorinated polyethylene glycol-imide solution; S3. Add the lithium salt into the fluorinated polyethylene glycol-imide solution prepared in S2, stir for 2 - 5 h, pour it into a polytetrafluoroethylene mold, and dry it to obtain the crosslinked fluoropolymer electrolyte.

3. A preparation method of a crosslinked fluoropolymer electrolyte according to claim 2, characterized in that, the dianhydride is selected from any one or more of pyromellitic dianhydride, 2,3,3',4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-biphenylether dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, hexafluorodiacid anhydride, bisphenol A type diether dianhydride.

4. A preparation method of a crosslinked fluoropolymer electrolyte according to claim 2, characterized in that, the lithium salt is selected from any one or more of lithium hexafluorophosphate, lithium bis(trifluoromethylsulfonyl)imide, lithium difluoromethylsulfonylimide, lithium perchlorate, lithium tetrafluoroborate, lithium bis(oxalato)borate.

5. A preparation method of a crosslinked fluoropolymer electrolyte according to claim 2, characterized in that, the molar ratio of the dianhydride, diamine polydiethanol, and maleic anhydride is 1:(1 - 1.1):(0.1 - 0.5); the addition amounts of the triethylamine and the acetic anhydride are 0.1% - 0.5% of the total mass of the dianhydride, diamine polydiethanol, and maleic anhydride.

6. A preparation method of a crosslinked fluoropolymer electrolyte according to claim 2, characterized in that, the molar ratio of the polyethylene glycol-imide to the fluorinated acrylate is 1:(1 - 1.2); the addition amount of the azobisisobutyronitrile is 0.1% - 0.5% of the total mass of the polyethylene glycol-imide and the fluorinated acrylate.

7. A preparation method of a crosslinked fluoropolymer electrolyte according to claim 2, characterized in that, The weight ratio of the crosslinked fluorinated polyethylene glycol-imide to the lithium salt is (70-90):(10-30).

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