A network-structured solid polymer electrolyte based on a phenyl cross-linking agent, a preparation method thereof, and a solid-state lithium secondary battery

Abstract

The present invention provides a network-structured solid polymer electrolyte based on a phenyl cross-linking agent, a preparation method thereof, and a solid-state lithium secondary battery. The specific preparation method is as follows: ① Dissolve the phenyl cross-linking agent, amino-terminated polyethylene glycol, and lithium salt in an organic solvent to obtain a mixed solution; ② Coat the mixed solution into a film and then heat it in a vacuum drying oven, and cool it to obtain a network-structured solid polymer electrolyte based on the phenyl cross-linking agent. By constructing a network structure in the solid polymer electrolyte, the present invention can inhibit the crystallization of PEO, improve the ionic conductivity of the solid polymer electrolyte at room temperature, and at the same time, the benzene ring structure in the phenyl cross-linking agent can enhance the mechanical strength and mechanical stability of the solid polymer electrolyte, so that the electrolyte has excellent lithium dendrite inhibition performance. In addition, the present invention also provides a solid-state lithium secondary battery containing the solid polymer electrolyte, which has excellent long-term cycle stability.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium secondary batteries, and particularly relates to a network-structured solid polymer electrolyte based on a phenyl cross-linking agent, a preparation method thereof, and a solid-state lithium secondary battery. Background Art

[0002] With the rapid development of emerging fields such as new energy vehicles, aerospace, and intelligent electronics, people have increasingly higher requirements for the safety, reliability, and energy density of energy storage devices. Lithium metal anodes have broad application prospects in the next-generation battery system due to their extremely high theoretical specific capacity (3860 mAh g -1 ). However, lithium metal has high reactivity, resulting in an unstable solid electrolyte interface on its surface and being prone to the generation of lithium dendrites during cycling, seriously affecting the cycle life and safety of the battery. At the same time, the currently widely used organic electrolytes have safety problems such as easy leakage, flammability, and explosiveness. Using a solid polymer electrolyte to replace the organic electrolyte can not only solve the safety problems of the battery, but also improve the interface stability, inhibit the growth of lithium dendrites, and at the same time has good flexibility and processing performance, good contact with the electrode interface, can reduce the interface impedance of the solid-state battery, and improve the energy density of the battery, and has received more and more attention in all-solid-state lithium secondary batteries.

[0003] In the solid polymer electrolyte system, polyethylene oxide (PEO) has become the main matrix material because of its high dielectric constant, excellent lithium salt dissociation ability, and stability to lithium. However, it has a high crystallinity at room temperature, resulting in the inability of lithium ions to rapidly transport within the PEO crystal structure, and poor ionic conductivity at room temperature (~10 -6 S cm -1 ). When the temperature rises above the melting point, the crystalline regions of PEO melt, the ionic conductivity increases, but it will lose dimensional stability and the mechanical properties will decrease significantly, easily leading to battery short circuits.

[0004] To enhance the room-temperature ionic conductivity of solid polymer electrolytes, researchers have proposed methods such as introducing plasticizers, inorganic fillers, polymer blending, and copolymerization. Introducing plasticizers can effectively improve the ionic conductivity of the electrolyte, but it will lead to a decrease in mechanical properties, while other methods have limited improvement in the ionic conductivity of the electrolyte. Forming a network structure through cross-linking can reduce the polymer crystallinity, while improving the ionic transport ability of the electrolyte and maintaining good mechanical properties. For example, researchers cross-linked trimethylolpropane triglycidyl ether with polyethylene glycol diamine, and the ionic conductivity of the prepared cross-linked network polymer electrolyte can reach up to 10 -4 S cm -1(Journal of Power Sources, 2016, 331, 322 - 331). However, the mechanical properties of the electrolyte membrane still need to be improved.

[0005] Therefore, there is an urgent need to develop a solid polymer electrolyte with high mechanical properties, high ionic conductivity, and the ability to inhibit lithium dendrites, so that the prepared lithium secondary battery has a high discharge specific capacity and good cycling performance. Summary of the Invention

[0006] Aiming at the problems of low ionic conductivity at room temperature and poor mechanical properties at high temperature existing in current solid polymer electrolytes, the present invention proposes a network - structured solid polymer electrolyte based on a phenyl cross - linker and its preparation method. By constructing a network structure in the solid polymer electrolyte, the crystallization of PEO can be inhibited, the ionic conductivity of the solid polymer electrolyte at room temperature can be improved. At the same time, the benzene ring structure in the phenyl cross - linker can enhance the mechanical strength and mechanical stability of the solid polymer electrolyte, making the electrolyte have excellent lithium dendrite inhibition performance. In addition, the present invention also provides a solid - state lithium secondary battery containing this solid polymer electrolyte, which has excellent long - term cycling stability.

[0007] The present invention realizes the above technical objectives through the following technical means.

[0008] A preparation method of a network - structured solid polymer electrolyte based on a phenyl cross - linker, comprising the following steps:

[0009] S1: Dissolve the phenyl cross - linker, amino - terminated polyethylene glycol, and lithium salt in an organic solvent to obtain a mixed solution;

[0010] S2: Coat the mixed solution into a film and place it in a vacuum drying oven. Under vacuum heating conditions, a chemical cross - linking reaction occurs, and then it is gradually cooled to room temperature to obtain a network - structured solid polymer electrolyte based on the phenyl cross - linker.

[0011] Further, the phenyl cross - linker is tris - glycidyl - p - aminophenol, as shown in Formula I:

[0012]

[0013] Or, it is N,N,N,N - tetra - glycidyl - 4,4 - diamino - diphenylmethane, as shown in Formula II:

[0014]

[0015] Further, the amino - terminated polyethylene glycol is polyethylene glycol diamine or polyethylene glycol capped with bis(3 - aminopropyl), and the molecular weight of the amino - terminated polyethylene glycol is 400 - 10000.

[0016] Further, the lithium salt is LiY, where Y is (FSO2)2N - or (CF3SO2)2N - ; the organic solvent is N-N dimethylformamide.

[0017] Further, the molar ratio of the phenyl crosslinker to the amino-terminated polyethylene glycol is 1:0.75 - 2; the molar ratio of the monomer unit EO of the amino-terminated polyethylene glycol to the lithium salt is 8 - 20:1.

[0018] Further, the specific steps of the chemical crosslinking reaction under the vacuum heating condition in step S2 are as follows: first react in a vacuum environment at 90°C for 10 h, then raise the temperature to 110°C and keep it warm for 12 h; the cooling method is to cool to room temperature with the vacuum drying oven.

[0019] The network-structured solid polymer electrolyte based on the phenyl crosslinker prepared by the preparation method according to any one of the above has a network structure formed by crosslinking of epoxy groups in the phenyl crosslinker and amino groups in the amino-terminated polyethylene glycol, and the lithium salt is dispersed in the network structure.

[0020] Further, the Young's modulus of the solid polymer electrolyte is 0.31 Mpa, the elongation at break is 38%, and the conductivity at 90°C is 4.45*10 -4 S·cm -1 .

[0021] A solid-state lithium secondary battery made of the above solid polymer electrolyte.

[0022] Further, the capacity retention rate of the solid-state lithium secondary battery is 92.8% after 100 cycles at a charge-discharge rate of 0.2C at 90°C.

[0023] The beneficial effects of the present invention are as follows:

[0024] 1. The network structure of the solid polymer electrolyte provided by the present invention can fully inhibit the crystallization of the polymer, which is beneficial to promoting the transport of lithium ions in the electrolyte and improving the ionic conductivity of the solid polymer electrolyte at room temperature. At the same time, the crosslinked network containing a benzene ring structure further enhances the mechanical properties of the electrolyte, making the electrolyte have excellent lithium dendrite inhibition performance. Its symmetric lithium battery can stably cycle for more than 2500 hours at a current density of 0.5 mA cm -2 The lithium secondary battery containing the network-structured solid polymer electrolyte has excellent cycling performance at a charge-discharge rate of 0.2C at 90°C. After 100 cycles, the capacity retention rate is 92.8%, which is significantly better than the conventional PEO-based polymer electrolyte.

[0025] 2. The present invention provides a simple and efficient method for preparing a solid polymer electrolyte. The film-forming process is simple, the film is formed quickly, the efficiency is high, and the conventional equipment used is easy to control and operate, so that the solid polymer electrolyte can be prepared in large quantities. Description of the Drawings

[0026] Figure 1 It is the infrared spectrum of the solid polymer electrolyte described in the embodiment of the present invention;

[0027] Figure 2 It is the ionic conductivity diagram of the solid polymer electrolyte described in the embodiment of the present invention;

[0028] Figure 3 It is the tensile property diagram of the solid polymer electrolyte described in the embodiment of the present invention;

[0029] Figure 4 It is the electrochemical cycle diagram of the solid-state lithium secondary battery described in the embodiment of the present invention at a charge-discharge rate of 90 °C and 0.2C;

[0030] Figure 5 It is for the solid-state lithium symmetric battery described in the embodiment of the present invention at 90 °C and 0.5 mA·cm -2 Symmetric battery cycle diagram at the current density. Detailed Embodiments

[0031] The present invention will be further described below in conjunction with the drawings and specific embodiments, but the protection scope of the present invention is not limited thereto.

[0032] The preparation method of the network-structured solid polymer electrolyte based on a phenyl crosslinking agent described in this embodiment includes the following steps:

[0033] S1: Dissolve the phenyl crosslinking agent, amino-terminated polyethylene glycol, and lithium salt in an organic solvent to obtain a mixed solution;

[0034] S2: Coat the mixed solution into a film and place it in a vacuum drying oven. A chemical crosslinking reaction occurs under vacuum heating conditions, and then it is gradually cooled to room temperature to obtain a network-structured solid polymer electrolyte based on the phenyl crosslinking agent.

[0035] Among them, the phenyl crosslinking agent is tris(2,3-epoxypropyl)-p-aminophenol or N,N,N,N-tetraglycidyl-4,4'-diaminodiphenylmethane; the amino-terminated polyethylene glycol is polyethylene glycol diamine or polyethylene glycol capped with bis(3-aminopropyl); the lithium salt is lithium bis(fluorosulfonyl)imide or lithium bis(trifluoromethanesulfonyl)imide; the organic solvent is N,N-dimethylformamide.

[0036] Example 1:

[0037] The preparation method of the network-structured solid polymer electrolyte based on phenyl crosslinker in this embodiment is as follows:

[0038] S1: Dissolve 24.5 mg of N,N,N,N-tetraglycidyl-4,4-diaminodiphenylmethane, 231.9 mg of poly(ethylene glycol) bis(3-aminopropyl) terminated, and 61.6 mg of lithium bis(fluorosulfonyl)imide in 1 mL of N,N-dimethylformamide solvent, and stir for 3 hours to completely dissolve to obtain a mixed solution. The molar ratio of N,N,N,N-tetraglycidyl-4,4-diaminodiphenylmethane to poly(ethylene glycol) bis(3-aminopropyl) terminated is 1:2, the molar ratio of the monomer unit EO (ethylene oxide) of poly(ethylene glycol) bis(3-aminopropyl) terminated to lithium bis(fluorosulfonyl)imide is 16:1, and the structural formulas of N,N,N,N-tetraglycidyl-4,4-diaminodiphenylmethane, poly(ethylene glycol) bis(3-aminopropyl) terminated, and lithium bis(fluorosulfonyl)imide are shown in Table 1 below;

[0039] Table 1: Summary table of the structural formulas of phenyl crosslinker, amino-terminated polyethylene glycol, and lithium salt

[0040]

[0041] S2: Coat the mixed solution on a 7.5 cm * 2.5 cm glass slide, then transfer it to a vacuum drying oven, and heat and react in a vacuum environment at 90 °C for 10 h to cause a chemical crosslinking reaction between the epoxy groups in N,N,N,N-tetraglycidyl-4,4-diaminodiphenylmethane and the amino groups in poly(ethylene glycol) bis(3-aminopropyl) terminated. Then raise the temperature to 110 °C and keep it warm for 12 h to complete the reaction and completely remove N,N-dimethylformamide. After the reaction is complete, cool it to room temperature with the vacuum drying oven to obtain a network-structured solid polymer electrolyte based on phenyl crosslinker.

[0042] Figure 1 is the infrared spectrum of the solid polymer electrolyte prepared in this embodiment. It can be seen from the figure that the peak at 910 cm -1 (this peak is the wave peak of the epoxy group) disappears, indicating that a chemical crosslinking reaction occurs between the epoxy group and the amino group. At the same time, it can also be found that the wave peak near 1500 cm -1 is the benzene ring structure.

[0043] Measurement of ionic conductivity: In this embodiment, the alternating current impedance method is used to test the solid polymer electrolyte. The alternating current impedance test frequency range is 0.1 Hz to 100 kHz, and the amplitude is 5 mV. Before the test, in the range of 30 to 90 °C, with a temperature measurement interval of 10 °C, let the solid polymer electrolyte stand for about 1 hour in each temperature measurement interval, and test three times in each temperature measurement interval and take the average value to obtain the bulk impedance of the electrolyte in this temperature measurement interval. The calculation formula for the ionic conductivity σ of the electrolyte is as follows:

[0044]

[0045] Among them, R b , L, and S are the bulk impedance (Ω), thickness (cm), and effective area (cm 2 ) of the electrolyte, respectively.

[0046] Figure 2 is the ionic conductivity graph of the solid polymer electrolyte prepared in this example. It can be seen from the figure that as the temperature increases, the conductivity of the electrolyte also increases. The conductivity measured at 30 °C is 2.16×10 -5 S·cm -1 , while the conductivity measured at 90 °C is 4.45×10 -4 S·cm -1 .

[0047] Figure 3 is the tensile property graph of the solid polymer electrolyte prepared in this example. It can be seen from the figure that the Young's modulus of the solid polymer electrolyte prepared in this example is as high as 0.31 Mpa, and the elongation at break is 38%.

[0048] This example also relates to a solid-state lithium secondary battery, which includes a positive electrode cover, a positive electrode sheet, the solid polymer electrolyte prepared by the above preparation method, a lithium sheet negative electrode, and a negative electrode cover. The solid polymer electrolyte is sandwiched between the positive electrode sheet and the lithium sheet negative electrode. The preparation method of the above solid-state lithium secondary battery is as follows:

[0049] K1: Dissolve polyethylene oxide and lithium salt in acetonitrile, and heat and stir to completely dissolve them;

[0050] K2: Continuously add lithium iron phosphate, conductive carbon black, and polyvinylidene fluoride to the solution, and add 1-methyl-2-pyrrolidone. After mixing evenly, ball mill for 40 min to obtain a well-ground slurry, where the molar ratio of each component is: lithium iron phosphate: (polyethylene oxide + lithium salt): conductive carbon black: polyvinylidene fluoride = 6:2:1:1;

[0051] K3: Coat the slurry evenly on the surface of the aluminum foil, and then put it into a vacuum drying oven together, and dry it in a vacuum environment at 60 °C for 10 h;

[0052] K4: Cut the dried membrane into circular thin slices with a diameter of 10 mm and a thickness of 20 μm, which are the positive electrode sheets of the solid-state lithium secondary battery;

[0053] K5: Stack the positive electrode case cover, positive electrode sheet, solid polymer electrolyte, lithium metal negative electrode, and negative electrode case cover in order from bottom to top in an argon - protected glove box, press them tightly, and then place them on a stamping machine for stamping to obtain the assembled solid - state lithium secondary battery.

[0054] Figure 4 This is the electrochemical cycling graph of the solid - state lithium secondary battery assembled in this example at a charge - discharge rate of 0.2C and a temperature of 90°C. As can be seen from the figure, the solid polymer electrolyte has excellent cycling performance at a charge - discharge rate of 0.2C and a temperature of 90°C. After 100 cycles, the capacity retention rate is 92.8% and the Coulombic efficiency is 99.8%.

[0055] Charge - discharge test of symmetric lithium battery: In this example, a Neware CT - 4008Tn type battery test system is used, the test current density is 0.5 mA / cm -2 , the test temperature is 90°C, and the battery is charged for 3 hours and discharged for 3 hours in each charge - discharge cycle. The test results are as Figure 5 shown. The prepared solid - state lithium symmetric battery can stably cycle for more than 2500 hours at a current density of 0.5 mA·cm -2 .

[0056] The above - described embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above - described embodiments. Without departing from the essence of the present invention, any obvious improvements, substitutions, or modifications that those skilled in the art can make all fall within the protection scope of the present invention.

Claims

1. A preparation method of a network-structured solid polymer electrolyte based on a phenyl crosslinking agent, characterized in that, It includes the following steps: S1: Co - dissolve a phenyl cross - linker, amino - terminated polyethylene glycol, and a lithium salt in an organic solvent to obtain a mixed solution; the phenyl cross - linker is triglycidyl - p - aminophenol, as shown in Formula I: Or, it is N,N,N,N - tetra - glycidyl - 4,4 - diaminodiphenylmethane, as shown in Formula II: S2: After coating the mixed solution into a film, place it in a vacuum drying oven. Under vacuum heating conditions, a chemical cross - linking reaction occurs, and then it is gradually cooled to room temperature to obtain a network - structured solid polymer electrolyte based on the phenyl cross - linker.

2. The preparation method according to claim 1, characterized in that, The amino - terminated polyethylene glycol is polyethylene glycol diamine or polyethylene glycol capped with bis(3 - aminopropyl). The molecular weight of the amino - terminated polyethylene glycol is 400 - 10000.

3. The preparation method according to claim 1, characterized in that, The lithium salt is LiY, where Y is (FSO2)2N - or (CF3SO2)2N - ; The organic solvent is N,N-dimethylformamide.

4. The preparation method according to claim 1, characterized in that, The molar ratio of the phenyl cross - linker to the amino - terminated polyethylene glycol is 1:0.75 - 2; the molar ratio of the monomer unit EO of the amino - terminated polyethylene glycol to the lithium salt is 8 - 20:

1.

5. The preparation method according to claim 1, characterized in that, The specific steps of the chemical cross - linking reaction under the vacuum heating conditions in Step S2 are: First, react in a vacuum environment at 90 °C for 10 h, then raise the temperature to 110 °C and keep it for 12 h; the cooling method is to cool to room temperature with the vacuum drying oven.

6. A network-structured solid polymer electrolyte based on a phenyl crosslinking agent prepared by the preparation method according to any one of claims 1 to 5, characterized in that, It has a network structure formed by cross - linking of epoxy groups in the phenyl cross - linker and amino groups in the amino - terminated polyethylene glycol, and the lithium salt is dispersed in the network structure.

7. The solid polymer electrolyte according to claim 6, characterized in that, The Young's modulus of the solid polymer electrolyte is 0.31 Mpa, the elongation at break is 38%, and the conductivity at 90 °C is 4.45*10 -4 S·cm -1 .

8. A solid-state lithium secondary battery made of the solid polymer electrolyte according to claim 6.

9. The solid-state lithium secondary battery according to claim 8, characterized in that, The capacity retention rate of the solid - state lithium secondary battery is 92.8% after 100 cycles at a charge - discharge rate of 0.2C at 90 °C.

Images