A Gel Electrolyte for Lithium-Ion Batteries, Its Preparation Method and Application

Abstract

The present invention relates to the technical field of lithium-ion batteries, and particularly relates to a gel electrolyte for lithium-ion batteries, a preparation method thereof, and an application thereof. The present invention provides a gel electrolyte for lithium-ion batteries, comprising: a polymer matrix having a crosslinked network structure, a plasticizer, a filler, and a lithium salt; the matrix polymer is a crosslinked product of methyl methacrylate, acrylonitrile, and vinyltrimethoxysilane, the plasticizer is a mixed solution of at least one of dimethyl carbonate, ethylene carbonate, or ethyl methyl carbonate and N,N-dimethylformamide, and the filler is zinc oxide. This gel electrolyte can be obtained only through simple radical polymerization, coating, soaking, etc., and the preparation method is simple and feasible, and it is easy to realize large-scale production. The room temperature conductivity of this gel electrolyte can reach 10.3×10 ‑3 S / cm, the tensile strength can reach 2.3 MPa, and the liquid absorption rate is greater than 310%, and it has great application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly to a gel electrolyte for lithium-ion batteries, a preparation method thereof, and an application thereof. Background Art

[0002] Among the electrolytes used in lithium-ion batteries, liquid electrolytes, as electrolytes with advantages such as high ionic conductivity, good electrode compatibility, wide temperature window, and no heavy metal pollution, are widely used in various electronic products and electrical appliances. However, liquid electrolytes have problems such as low safety, easy leakage of the electrolyte solution, and difficulty in inhibiting the growth of lithium dendrites, which seriously affect the development and application of liquid electrolytes.

[0003] Gel polymer electrolyte, also known as gel electrolyte, is a semi-solid electrolyte obtained by mixing a liquid and a solid. It is obtained by filling a liquid electrolyte into the network structure of polymer molecules. Among them, the polymer molecules are a cross-linked three-dimensional network structure, and the voids in the network structure are filled with a liquid plasticizer, and a lithium salt is dissolved in the polymer and the plasticizer. Compared with liquid electrolytes, gel electrolytes have advantages such as lower leakage risk, good elasticity and flexibility, and high safety.

[0004] However, the room temperature conductivity and mechanical strength of gel electrolytes still need to be further improved. At present, although there are methods to improve the ionic conductivity of electrolytes by adjusting the ratio of high molecular weight polymers to low molecular weight polymers, or to enhance the mechanical strength and electrochemical stability of electrolytes by adding nano-fillers, these methods all have the contradiction that the conductivity and mechanical strength cannot be satisfied at the same time, and an ideal effect cannot be obtained. Summary of the Invention

[0005] In view of this, the present invention provides a gel electrolyte for lithium-ion batteries, a preparation method thereof, and an application thereof. The gel electrolyte has high room temperature conductivity, excellent mechanical strength and liquid absorption rate, and the preparation method is simple and feasible.

[0006] To solve the above technical problems, the first aspect of the present invention provides a gel electrolyte for lithium-ion batteries, including: a polymer matrix with a cross-linked network structure, a plasticizer, a filler, and a lithium salt; the matrix polymer is a cross-linked product of methyl methacrylate, acrylonitrile, and vinyltrimethoxysilane, the plasticizer is a mixed solution of at least one of dimethyl carbonate, ethylene carbonate, or ethyl methyl carbonate and N,N-dimethylformamide, and the filler is zinc oxide.

[0007] The present invention prepares a cross-linked polymer using methyl methacrylate, acrylonitrile, and vinyltrimethoxysilane as reaction monomers. Taking the silicon-oxygen bond in vinyltrimethoxysilane as the cross-linking point, the molecular chains of polymethyl methacrylate, polyacrylonitrile, and their copolymers are cross-linked to form a network structure. In addition to the strong interaction between the carbon-oxygen bond in polymethyl methacrylate and the plasticizer, the carbon-oxygen bond itself can also act as a plasticizer, reducing the high crystallinity brought by the rigid acrylonitrile in the polymer, thereby ensuring that the resulting gel electrolyte has a high room-temperature conductivity. In addition to cross-linking polymethyl methacrylate with polyacrylonitrile or its copolymer to obtain a gel electrolyte with a network structure, the oxygen atom in the uncross-linked silicon-oxygen bond of vinyltrimethoxysilane can also undergo a grafting reaction with the zinc atom of the filler, thereby indirectly "connecting in series" the filler and the polymer, disrupting the ordered arrangement of the polyacrylonitrile molecular chain, further reducing the polymer crystallinity, and improving the conductivity. In addition, the polymer matrix provided by the present invention has a cross-linked network structure, providing a more stable "framework" structure to ensure that the gel electrolyte has excellent mechanical strength.

[0008] In addition, during the experiment, the inventors accidentally found that when a mixed solution of a certain amount of N,N-dimethylformamide and a traditional plasticizer was used as the electrolyte, the room-temperature conductivity of the resulting gel electrolyte was significantly improved.

[0009] Combined with the first aspect, the mass ratio of methyl methacrylate to acrylonitrile is 1:5 to 10, and the mass ratio of methyl methacrylate to vinyltrimethoxysilane is 1:1 to 2. This range of mass ratios can ensure that the gel electrolyte has good conductive performance and mechanical properties at the same time.

[0010] Preferably, the mass ratio of methyl methacrylate to acrylonitrile is 1:8, and the mass ratio of vinyltrimethoxysilane to acrylonitrile is 1:14.

[0011] Combined with the first aspect, the mass ratio of the filler zinc oxide to vinyltrimethoxysilane is 1:3 to 5.

[0012] Combined with the first aspect, the lithium salt is selected from lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium difluorooxalate borate, or lithium trifluoromethanesulfonate. The above lithium salts have good solubility in the plasticizer and are more likely to "penetrate" into the pores between the molecular chains of the polymer matrix together with the plasticizer.

[0013] Combined with the first aspect, the mass fraction of N,N-dimethylformamide in the plasticizer is 15% to 30%. This proportion range of N,N-dimethylformamide can ensure that the resulting gel electrolyte has both a high room-temperature conductivity and good mechanical strength.

[0014] The second aspect of the present invention provides a preparation method for a gel electrolyte for a lithium-ion battery, and the steps include:

[0015] S1. Add an initiator to the mixed solution of methyl methacrylate, acrylonitrile and vinyltrimethoxysilane, stir and dissolve to obtain a first solution;

[0016] S2. Under inert conditions, drop the first solution into an aqueous solution at 65-80 °C containing a dispersant, and at the same time stir for a polymerization reaction. After the reaction is completed, adjust the pH of the solution to 8-9, add a filler zinc oxide, stir at the same temperature for 2-5 h, and filter to obtain an electrolyte precursor;

[0017] S3. Under inert conditions, dissolve the electrolyte precursor in an organic solvent, perform flat film coating, and after the film is cured, soak it in a plasticizer containing a lithium salt for 15-30 min to obtain the gel electrolyte.

[0018] Through a simple free radical polymerization reaction, the present invention makes methyl methacrylate, acrylonitrile and vinyltrimethoxysilane react to form a cross-linked polymer with a network structure, and then prepares a gel electrolyte by means of dissolution and film casting. The steps are simple and easy to implement, and it is easy to realize batch production.

[0019] Combined with the second aspect, the concentration of the lithium salt in the plasticizer is 0.8-1 mol / L.

[0020] Combined with the second aspect, the mass ratio of the electrolyte precursor in the organic solvent is 10%-30%.

[0021] Preferably, the initiator is benzoyl peroxide.

[0022] The third aspect of the present invention provides a lithium-ion battery prepared by using the above gel electrolyte.

[0023] The beneficial effects of the present invention: The present invention provides a gel electrolyte with a cross-linked network structure. Through ingenious improvement of the electrolyte components and preparation method, a gel electrolyte with high room temperature conductivity (up to 10.3×10 -3 S / cm), excellent mechanical strength (the highest tensile strength can reach 2.3 MPa), and a liquid absorption rate > 310% is successfully obtained. Specific Embodiments

[0024] In order to make the purpose, technical solutions and advantages of the present invention clearer, the following further elaborates on the present invention in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0025] Polymethyl methacrylate is a chemically stable, colorless, transparent amorphous polymer with a high liquid electrolyte absorption capacity and excellent interfacial compatibility with lithium metal. However, its film-forming ability is poor, resulting in a poor mechanical strength of the obtained film. Some studies have prepared polymer films by copolymerizing and crosslinking methyl methacrylate and acrylonitrile, and then obtained gel electrolytes by absorbing electrolytes containing lithium salts. However, the room temperature conductivity of the obtained gel electrolytes is still at a low level.

[0026] In view of this, the present invention provides a gel electrolyte. By improving the components and preparation method of the electrolyte, a gel electrolyte with significantly improved room temperature conductivity and still having good mechanical strength is obtained, which has great practical application value.

[0027] The purity of each monomer used in the following examples is at least 99.98%.

[0028] Example 1

[0029] This example provides a gel electrolyte for lithium-ion batteries, which is prepared by the following method:

[0030] S1. Accurately weigh 5.0 g of methyl methacrylate, 35.0 g of acrylonitrile and 7.5 g of vinyltrimethoxysilane. After mixing evenly, add 2.2 g of benzoyl peroxide and stir to dissolve to obtain a first solution.

[0031] S2. Under the protection of an inert gas, slowly dropwise add the first solution into a 100 mL aqueous solution containing 4.5 g of gelatin at 75 °C, and at the same time stir at a speed of 400 r / min for a polymerization reaction for 6 h. After the reaction is completed, adjust the pH of the solution to 8-9 with a sodium hydroxide solution, add 2 g of zinc oxide, stir at the same temperature for 3.5 h, filter, and dry to obtain an electrolyte precursor with a cross-linked network structure.

[0032] S3. Under the protection of an inert gas, dissolve 30 g of the electrolyte precursor in 100 g of N,N-dimethylformamide, and then apply it to the surface of a glass plate or a polytetrafluoroethylene plate. After film formation, dry it at 50 °C for 6 h. Then immerse the obtained film in 300 g of a plasticizer (including 25 g of N,N-dimethylformamide and 225 g of dimethyl carbonate) with a concentration of 0.9 mol / L of lithium bis(trifluoromethanesulfonyl)imide for 20 min, take it out to obtain a gel electrolyte with a thickness of 32 μm.

[0033] Example 2

[0034] This example provides a gel electrolyte for lithium-ion batteries, which is prepared by the following method:

[0035] S1. Accurately weigh 5.0 g of methyl methacrylate, 25.0 g of acrylonitrile, and 5.0 g of vinyltrimethoxysilane. After mixing evenly, add 1.4 g of benzoyl peroxide and stir to dissolve to obtain the first solution;

[0036] S2. Under the protection of inert gas, slowly dropwise add the first solution into 35 mL of an aqueous solution containing 3.2 g of gelatin at 65 °C, and simultaneously stir at a speed of 400 r / min for a polymerization reaction for 7 h. After the reaction is completed, adjust the pH of the solution to 8 - 9 with sodium hydroxide solution, add 1.3 g of zinc oxide, stir at the same temperature for 5 h, filter, and dry to obtain an electrolyte precursor with a cross-linked network structure;

[0037] S3. Under the protection of inert gas, dissolve 20 g of the electrolyte precursor in 75 g of N,N-dimethylformamide, and then apply it to the surface of a glass plate or a polytetrafluoroethylene plate. After film formation, dry it at 50 °C for 6 h. Then immerse the obtained film in 70 g of a plasticizer (including 8 g of N,N-dimethylformamide and 62 g of ethylene carbonate) with a concentration of 0.8 mol / L of lithium difluorooxalate borate for 15 min, take it out to obtain a gel electrolyte with a thickness of 29 μm.

[0038] Example 3

[0039] This example provides a gel electrolyte for a lithium-ion battery, which is prepared according to the following method:

[0040] S1. Accurately weigh 5.0 g of methyl methacrylate, 50.0 g of acrylonitrile, and 10.0 g of vinyltrimethoxysilane. After mixing evenly, add 3.4 g of benzoyl peroxide and stir to dissolve to obtain the first solution;

[0041] S2. Under the protection of inert gas, slowly dropwise add the first solution into 160 mL of an aqueous solution containing 5.9 g of gelatin at 80 °C, and simultaneously stir at a speed of 400 r / min for a polymerization reaction for 5 h. After the reaction is completed, adjust the pH of the solution to 8 - 9 with sodium hydroxide solution, add 3 g of zinc oxide, stir at the same temperature for 2 h, filter, and dry to obtain an electrolyte precursor with a cross-linked network structure;

[0042] S3. Under the protection of inert gas, dissolve 45 g of the electrolyte precursor in 300 g of N,N-dimethylformamide, and then apply it to the surface of a glass plate or a polytetrafluoroethylene plate. After film formation, dry it at 50 °C for 6 h. Then immerse the obtained film in 205 g of a plasticizer (including 20 g of N,N-dimethylformamide and 185 g of ethyl methyl carbonate) with a concentration of 1 mol / L of lithium bis(fluorosulfonyl)imide for 30 min, take it out to obtain a gel electrolyte with a thickness of 38 μm.

[0043] Comparative Example 1

[0044] This comparative example provides a gel electrolyte for a lithium-ion battery. Its composition and preparation method are basically similar to those of Example 1, except that vinyltrimethoxysilane is replaced by an equimolar amount of polyethylene glycol dimethacrylate, and the rest of the preparation method is the same as that of Example 1. The thickness of the obtained gel electrolyte is 35 μm.

[0045] Comparative Example 2

[0046] This comparative example provides a gel electrolyte for a lithium-ion battery. Its composition and preparation method are basically similar to those of Example 1, except that in step S2, after the polymerization reaction is completed, pH adjustment and addition of zinc oxide are not carried out, and the filtration and drying steps are directly carried out to obtain an electrolyte precursor with no zinc oxide grafted on the polymer chain. Steps S1 and S3 are the same as those of Example 1. The thickness of the obtained gel electrolyte is 29 μm.

[0047] Comparative Example 3

[0048] This comparative example provides a gel electrolyte for a lithium-ion battery. Its composition and preparation method are basically similar to those of Example 1, except that the plasticizer used in step S3 does not contain N,N-dimethylformamide, and the rest of the steps are the same as those of Example 1. The thickness of the obtained gel electrolyte is 33 μm.

[0049] Examination Example 1

[0050] Tensile strength tests were respectively carried out on the gel electrolytes obtained in Examples 1 to 3 and Comparative Examples 1 to 3 (the length and width of the test samples were 20 mm and 12.5 mm respectively, and the tensile rate was 12 mm / min). The results are shown in Table 1. It can be seen that the tensile strengths of the gel electrolytes obtained in Examples 1 to 3 are all above 2.0 MPa; and from the results of Comparative Example 1, it can be known that when the crosslinking agent used is a polymer with a certain length, the tensile strength of the obtained gel electrolyte decreases.

[0051] Test Example 2

[0052] The gel electrolytes obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were respectively subjected to an alternating current impedance experiment at room temperature to test their conductivity. The results are shown in Table 1.

[0053] Table 1

[0054]

[0055] The above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution of the present invention and its inventive concept, making equivalent substitutions or changes, shall be covered by the protection scope of the present invention.

Claims

1. A gel electrolyte for a lithium-ion battery, characterized in that, Comprising: A polymer matrix with a crosslinked network structure, a plasticizer, a filler, and a lithium salt; the matrix polymer is a crosslinked product of methyl methacrylate, acrylonitrile, and vinyltrimethoxysilane, the plasticizer is a mixed solution of at least one of dimethyl carbonate, ethylene carbonate, or ethyl methyl carbonate and N,N-dimethylformamide, the filler is zinc oxide; the mass ratio of methyl methacrylate to acrylonitrile is 1:5 to 10, and the mass ratio of methyl methacrylate to vinyltrimethoxysilane is 1:1 to 2; The gel electrolyte is prepared according to the following method: S1. Add an initiator to the mixed solution of methyl methacrylate, acrylonitrile, and vinyltrimethoxysilane, stir and dissolve to obtain a first solution; S2. Under inert conditions, drop the first solution into an aqueous solution at 65 - 80 °C containing a dispersant, while stirring for a polymerization reaction. After the reaction is completed, adjust the pH of the solution to 8 - 9, add the filler zinc oxide, stir at the same temperature for 2 - 5 h, filter to obtain an electrolyte precursor with a crosslinked network structure; S3. Under inert conditions, dissolve the electrolyte precursor in an organic solvent, perform a flat film coating, and after the film is cured, immerse it in a plasticizer containing a lithium salt for 15 - 30 min to obtain the gel electrolyte.

2. The gel electrolyte for a lithium-ion battery according to claim 1, characterized in that, The mass ratio of the filler zinc oxide to vinyltrimethoxysilane is 1:3 to 5.

3. The gel electrolyte for a lithium-ion battery according to claim 1, characterized in that, The lithium salt is selected from lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium difluorooxalate borate, or lithium trifluoromethanesulfonate.

4. The gel electrolyte for a lithium-ion battery according to claim 1, wherein The mass proportion of N,N-dimethylformamide in the plasticizer is 15% - 30%.

5. The gel electrolyte for a lithium-ion battery according to claim 1, wherein, The concentration of the lithium salt in the plasticizer is 0.8 - 1 mol / L.

6. The gel electrolyte for a lithium-ion battery according to claim 1, characterized in that, The mass proportion of the electrolyte precursor in the organic solvent is 10% - 30%.

7. A lithium-ion battery, characterized in that, Prepared by using the gel electrolyte according to any one of claims 1 - 6.

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