A self-healing electrolyte decoupled from mechanical strength and electrical conductivity and a preparation method thereof

Abstract

The application belongs to the technical field of batteries, and discloses a self-healing electrolyte with decoupled mechanical strength and electrical conductivity and a preparation method thereof.The self-healing electrolyte with decoupled mechanical strength and electrical conductivity comprises a polymer skeleton and solvent molecules for transmitting lithium ions; the polymer skeleton is generated by copolymerization of two monomers, i.e., methyl methacrylamide providing a hydrogen bond donor and methyl methacrylate providing a hydrogen bond acceptor, in a certain proportion through free radical polymerization under the action of an initiator; and the solvent molecules are fluorinated cyclopropyl cyanide capable of competing with lithium ions on the polymer chain. Before polymerization, the polymer monomers, the initiator and the solvent molecules are mixed in a certain proportion, and a lithium salt is added, and the solvent molecules are locked in the polymer skeleton by using an in-situ polymerization method. The gel electrolyte provided by the application has high electrical conductivity, high mechanical strength and certain self-healing capacity, and can be used in lithium ion batteries and lithium metal batteries.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of batteries, and more particularly relates to a self-healing electrolyte with decoupled mechanical strength and electrical conductivity and a preparation method thereof. BACKGROUND

[0002] With the increasing demand for high energy density batteries in portable electronic devices and electric vehicles, traditional liquid electrolytes have been unable to meet the high requirements of modern battery systems for safety, stability and energy density. In particular, for lithium metal batteries, which have great potential in high energy density applications, but also face many challenges such as the growth of lithium dendrites, the compatibility of electrolytes and electrode materials, and the insufficient ion conductivity and mechanical properties of electrolytes.

[0003] In the prior art, gel electrolytes are widely used in battery systems to solve the problem of insufficient conductivity of pure solid-state batteries. However, the mechanical properties of gel electrolytes are usually weak, which limits their performance in practical applications. At present, the main methods to improve the mechanical properties of gel electrolytes include introducing cross-linking agents to generate cross-linked polymers and introducing hydrogen bond functional groups to form hydrogen bonds. However, these methods have certain limitations. On the one hand, although the increase in cross-linking degree can improve the mechanical properties of the polymer, it will cause the crystallinity of the polymer to rise, which will significantly reduce the conductivity of the gel electrolyte. On the other hand, although the introduction of hydrogen bond functional groups can enhance the mechanical properties, it will cause the preferential combination of lithium ions and hydrogen bond acceptors. This not only reduces the transmission efficiency of lithium ions, but also weakens the enhancement effect of hydrogen bonds on mechanical properties, and also reduces the self-healing ability of the material. Therefore, the existing methods to improve the mechanical properties of gel electrolytes often sacrifice the conductivity while improving the mechanical properties, and it is difficult to balance the lithium ion transmission efficiency and mechanical properties. SUMMARY

[0004] In view of the above defects or improvement needs of the prior art, the present application provides a self-healing electrolyte with decoupled mechanical strength and electrical conductivity and a preparation method thereof, which aims to develop a gel electrolyte that can provide high mechanical strength and maintain high ionic conductivity, thereby solving the technical problems of insufficient ionic conductivity and mechanical properties of electrolytes.

[0005] To achieve the above-mentioned purpose, according to one aspect of the present application, a self-healing electrolyte with decoupled mechanical strength and electrical conductivity is provided, comprising a polymer skeleton and solvent molecules for transmitting lithium ions; the polymer skeleton is generated by copolymerization of two monomers, methyl acrylamide providing hydrogen bond donors and methyl methacrylate providing hydrogen bond acceptors, in a certain proportion under the action of an initiator by free radical polymerization; the solvent molecules are fluorinated cyclopropyl cyanide, and its chemical structure is The solvent molecules have strong attraction to lithium ions, which is beneficial to desolvation of lithium ions.

[0006] Preferably, the molar ratio of the added amount of the methacrylamide monomer and the methacrylate monomer is 1:(5-20).

[0007] Preferably, the methacrylamide monomer and the methacrylate monomer are used as polymer monomers, and the added amount of the polymer monomers accounts for 10-50% of the weight percentage of the self-healing electrolyte.

[0008] Preferably, the initiator is azobisisobutyronitrile.

[0009] Preferably, the added amount of the initiator is (0.1-1) wt%, the initiation temperature is 50-80℃, and the polymerization time is 0.5-10h.

[0010] According to another aspect of the present application, a preparation method of a self-healing electrolyte with decoupling of mechanical strength and electrical conductivity is provided, comprising the following steps:

[0011] S1: preparing a precursor solution by mixing and stirring methyl methacrylate, methacrylamide, a lithium salt, solvent molecules and an initiator until the solution is clear;

[0012] S2: dropping the precursor solution in S1 on a separator of a battery, and performing drying treatment to obtain a self-healing electrolyte after molding.

[0013] Preferably, the lithium salt is one or more of lithium bisfluorosulfonylimide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluoro(oxalato)borate, lithium tetrafluoroborate and lithium hexafluorophosphate, and the added amount of the lithium salt relative to the self-healing electrolyte is 1-5 mol / L.

[0014] Preferably, the drying treatment is performed by placing the assembled battery in an oven at 50-80℃ for 0.5-10h.

[0015] According to another aspect of the present application, a lithium secondary battery is provided, comprising the self-healing electrolyte described above or prepared by the method described above.

[0016] Overall, compared with the prior art, the self-healing electrolyte with decoupling of mechanical strength and electrical conductivity and the preparation method thereof according to the present application mainly have the following beneficial effects:

[0017] 1. The self-healing electrolyte decoupled mechanical strength and electrical conductivity is proposed, which not only makes the polymer electrolyte have the self-healing ability, but also solves the trade-off problem between mechanical properties and ionic conductivity of the existing polymer solid electrolyte, realizes the decoupling of the mechanical strength and the electrical conductivity of the polymer solid electrolyte, and provides a new electrolyte solution for the development of high-performance lithium metal batteries.

[0018] 2. The application proposes a new type of polymer solid electrolyte molecular structure, which selects methyl methacrylate with carbonyl functional group and methacrylamide with amino functional group as polymer monomers, and constructs a new type of cross-linked polymer skeleton with rich hydrogen bonds, so that it has the ability of self-healing.

[0019] 3. The application selects fluorinated cyclopropyl nitrile as a solvent, and the unique structure of the solvent molecule has strong competition ability for lithium ions, which can compete with lithium ions on the polymer chain to prevent lithium ions from occupying hydrogen bond sites. This feature ensures the integrity of the hydrogen bond between the polymer segments, and avoids the decline of mechanical properties and self-healing performance caused by the combination of lithium ions and hydrogen bond acceptors. In addition, the fluorine atoms in the fluorinated cyclopropyl nitrile molecule can significantly reduce the desolvation energy barrier of lithium ions, thereby promoting the rapid transport of lithium ions. This design realizes the decoupling of the mechanical strength and the electrical conductivity of the gel electrolyte, and provides a new idea for the design of high-performance battery electrolyte. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a schematic diagram of the polymer skeleton molecular structure and the solvent molecular structure of the application;

[0021] Figure 2 is a stress-strain curve comparison diagram of tensile test of example 1 and comparative example 1 of the application.

[0022] Figure 3 is a conductivity test comparison diagram of example 1 and comparative example 1 of the application.

[0023] Figure 4 is a comparison diagram of hydrogen bond number molecular dynamics calculation results of example 1 and comparative example 1 of the application.

[0024] Figure 5 is an optical picture showing the self-healing ability of the electrolyte example 1 and comparative example 1 of the application. DETAILED DESCRIPTION

[0025] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not used to limit the present application. In addition, the technical features involved in the various embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0026] Please refer to Figure 1 The embodiment of the present application provides a self-healing electrolyte with decoupling of mechanical strength and electrical conductivity, which comprises a polymer skeleton and solvent molecules for transmitting lithium ions; the polymer skeleton is generated by copolymerization of two monomers, i.e., methyl methacrylamide providing a hydrogen bond donor and methyl methacrylate providing a hydrogen bond acceptor, in a certain proportion by free radical polymerization under the action of an initiator; and the solvent molecules are fluorinated cyclopropyl cyanide, and the chemical structure thereof is The solvent molecules have strong attraction to lithium ions, which is beneficial to desolvation of lithium ions.

[0027] The molar ratio of the added amount of the methyl methacrylamide monomer and the methyl methacrylate monomer is 1:(5-20).

[0028] The methyl methacrylamide monomer and the methyl methacrylate monomer are used as polymer monomers, and the added amount of the polymer monomers accounts for 10%-50% of the weight percentage of the self-healing electrolyte.

[0029] The initiator is azobisisobutyronitrile.

[0030] The added amount of the initiator is (0.1-1) wt%, the initiation temperature is 50-80°C, and the polymerization time is 0.5-10 h.

[0031] The embodiment of the present application further provides a preparation method of the self-healing electrolyte with decoupling of mechanical strength and electrical conductivity, which comprises the following steps:

[0032] S1: preparing a precursor solution by mixing and stirring methyl methacrylate, methyl methacrylamide, a lithium salt, solvent molecules and an initiator until the solution is clear;

[0033] S2: dropping the precursor solution in S1 on a separator of a battery, and performing drying treatment to obtain a self-healing electrolyte after molding.

[0034] The lithium salt is one or more of lithium bisfluorosulfonylimide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluoro(oxalato)borate, lithium tetrafluoroborate and lithium hexafluorophosphate, and the added amount of the lithium salt relative to the self-healing electrolyte is 1-5 mol / L.

[0035] The specific process of the drying treatment is as follows: placing the assembled battery in an oven at 50-80°C and heating for 0.5-10 h.

[0036] Example 1: Preparation of self-healing electrolyte with decoupled mechanical strength and conductivity

[0037] 0.2 g of methacrylamide, 1.8 g of methyl methacrylate, 0.05 g of azobisisobutyronitrile, and 1.5 g of lithium bisfluorosulfonylimide were weighed into 6.5 g of fluorinated cyclopropenyl cyanide solvent, stirred for 1 h to obtain a precursor solution. The precursor solution was incubated at 60°C for 2 h to obtain a self-healing electrolyte with decoupled mechanical strength and conductivity. Subsequently, the following tests were performed:

[0038] (1) Conductivity test: When assembling a stainless steel pair of symmetric batteries, the precursor solution was dropped on the separator to allow it to infiltrate the separator, and then the battery was assembled and incubated at 60°C for 2 h. The impedance of the electrolyte was measured by alternating current impedance spectroscopy, and the ionic conductivity was calculated according to the conductivity formula.

[0039] (2) Mechanical strength test: The precursor solution was coated on a polytetrafluoroethylene plate and heated at 60°C for 2 h to prepare a self-healing electrolyte with decoupled mechanical strength and conductivity. The electrolyte was tested for tensile strength using an electronic dynamic static fatigue testing machine.

[0040] (3) Calculation of the number of hydrogen bonds: Molecular dynamics software was used to calculate the number of hydrogen bonds in the gel electrolyte based on the molecular molar ratio.

[0041] (4) Self-healing test: The precursor solution was coated on a polytetrafluoroethylene plate and heated at 60°C for 2 h to prepare a self-healing electrolyte with decoupled mechanical strength and conductivity. The electrolyte was cut, and the fracture was contacted for 10 min at room temperature to observe the self-healing phenomenon.

[0042] Example 2:

[0043] 0.2 g of methacrylamide, 1.8 g of methyl methacrylate, 0.05 g of azobisisobutyronitrile, and 1.5 g of lithium bisfluorosulfonylimide were weighed into 6.5 g of fluorinated cyclopropenyl cyanide solvent, stirred for 1 h to obtain a precursor solution. The precursor solution was incubated at 60°C for 2 h to obtain a self-healing electrolyte with decoupled mechanical strength and conductivity. Subsequently, the following tests were performed:

[0044] Comparative Example 1:

[0045] 0.2 g of methacrylamide, 1.8 g of methyl methacrylate, 0.05 g of azobisisobutyronitrile, and 1.5 g of lithium bisfluorosulfonylimide were weighed into 6.5 g of fluorinated cyclopropenyl cyanide solvent, stirred for 1 h to obtain a precursor solution. The precursor solution was incubated at 60°C for 2 h to obtain a self-healing electrolyte with decoupled mechanical strength and conductivity. Subsequently, the following tests were performed:

[0046] Please refer to Figure 2 , the stress-strain curve comparison chart of the tensile test of Example 1 and Comparative Example 1 of the present application; Figure 3 is the conductivity test comparison chart of Example 1 and Comparative Example 1 of the present application;Figure 4 is a comparison chart of hydrogen bond number molecular dynamics calculation results of embodiment 1 and comparative example 1 of the present application; the performance data detection results of each embodiment are shown in Table 1.

[0047] Table 1

[0048]

[0049] From Figures 2-4 and Table 1, in comparative example 1, when using common diethyl carbonate as the solvent, the hydrogen bond sites are occupied by lithium ions, resulting in low room temperature conductivity and tensile strength of the gel electrolyte, and no self-healing ability. In embodiment 2, when using cyclopropyl cyanide with strong competition ability for lithium ions as the solvent, the lithium ions occupying the hydrogen bond sites are released, the tensile strength and conductivity of the electrolyte are improved, and it has certain self-healing ability. In embodiment 1, when using fluorocyclopropyl cyanide as the solvent, the lithium ions are further released, and the desolvation ability of the lithium ions is further enhanced, thus exhibiting high room temperature conductivity, high tensile strength, and self-healing ability.

[0050] Those skilled in the art will readily understand that the above description is only preferred embodiments of the present application and is not intended to limit the present application, and any modifications, equivalent replacements and improvements made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A self-healing electrolyte with decoupled mechanical strength and electrical conductivity, characterized in that: It includes a polymer backbone and solvent molecules for transporting lithium ions; the polymer backbone is generated by copolymerizing two monomers, methacrylamide (providing a hydrogen bond donor) and methyl methacrylate (providing a hydrogen bond acceptor), in a certain proportion via free radical polymerization under the action of an initiator; the solvent molecule is fluorocyclopropionitrile, and its chemical structure is [insert chemical structure here].

2. The self-healing electrolyte with decoupled mechanical strength and electrical conductivity as described in claim 1, characterized in that: The molar ratio of the added amounts of methacrylamide monomer and methyl methacrylate monomer is 1:(5-20).

3. The self-healing electrolyte with decoupled mechanical strength and electrical conductivity as described in claim 1, characterized in that: The methacrylamide monomer and methyl methacrylate monomer are used as polymer monomers, and the amount of polymer monomers added accounts for 10% to 50% of the weight percentage of the self-healing electrolyte.

4. The self-healing electrolyte with decoupled mechanical strength and electrical conductivity as described in claim 1, characterized in that: The initiator is azobisisobutyronitrile.

5. The self-healing electrolyte with decoupled mechanical strength and electrical conductivity as described in claim 4, characterized in that: The initiator is added at a rate of (0.1-1) wt%, the initiation temperature is 50-80℃, and the polymerization time is 0.5-10h.

6. A method for preparing a self-healing electrolyte with decoupled mechanical strength and electrical conductivity as described in any one of claims 1-5, characterized in that: Includes the following steps: S1 Preparation of precursor solution: Methyl methacrylate, methacrylamide, lithium salt, solvent molecules, and initiator are mixed and stirred until the solution is clear; S2 involves dropping the precursor solution from S1 onto the battery separator and drying it to obtain the shaped self-healing electrolyte.

7. The method for preparing a self-healing electrolyte with decoupled mechanical strength and electrical conductivity as described in claim 6, characterized in that: The lithium salt is one or more of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorooxalateborate, lithium tetrafluoroborate, and lithium hexafluorophosphate, and the amount of lithium salt added relative to the self-healing electrolyte is 1-5 mol / L.

8. The method for preparing a self-healing electrolyte with decoupled mechanical strength and electrical conductivity as described in claim 6, characterized in that: The specific process of the drying treatment is as follows: the assembled battery is placed in an oven at 50-80°C and heated for 0.5-10 hours.

9. A lithium secondary battery, characterized in that: The lithium secondary battery includes a self-healing electrolyte with decoupled mechanical strength and electrical conductivity as described in any one of claims 1-5, or a self-healing electrolyte with decoupled mechanical strength and electrical conductivity prepared by the method described in any one of claims 6-8.

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