Preparation method of three-dimensional porous polyvinylidene fluoride / MOF composite gel electrolyte

Abstract

The present invention discloses a method for preparing a three-dimensional porous polyvinylidene fluoride / MOF composite gel electrolyte, which belongs to the technical field of functional high molecular polymer films. The present invention aims to solve the technical problems of low ionic conductivity and low mechanical properties of gel electrolytes at room temperature. The method of the present invention is as follows: first dissolve PVDF in DMF, then add lithium salt, blend ethylene carbonate and MIL‑125 (Ti) nanoparticles, evenly disperse, transfer to a film-forming container and dry, and finally soak in a liquid electrolyte. The present invention adopts a simple and easy-to-operate casting film-forming method to prepare a composite gel electrolyte with high ionic conductivity and strong mechanical properties, with a conductivity of 1.5×10 ‑ 3 Scm ‑1 The high ionic conductivity and maximum strain degree of 86.8% at room temperature have greatly improved the ionic conductivity and mechanical properties compared with the traditional gel electrolyte based on polymer film.

Description

Technical Field

[0001] The invention belongs to the technical field of functional high molecular polymer films, and in particular relates to a three-dimensional porous polyvinylidene fluoride / MOF composite gel electrolyte and a preparation method thereof. Background Art

[0002] Liquid electrolytes have advantages such as high ionic conductivity and good interfacial compatibility, but their flammability and instability to lithium metal limit their application in high energy density batteries. Solid electrolytes can solve the problems of flammability and instability of lithium metal in liquid electrolytes, and inorganic solid electrolytes have high ionic conductivity, and solid polymer electrolytes have advantages such as high flexibility, easy film formation, low cost and safety. However, inorganic solid electrolytes still have the problem of poor interfacial compatibility, and polymer electrolytes still exhibit poor ionic conductivity at room temperature, which seriously hinders practical applications.

[0003] As an intermediate state between liquid electrolyte and solid electrolyte, gel electrolyte is expected to have high ionic conductivity at room temperature, good interfacial compatibility and high mechanical properties. However, the high crystallinity of the polymer matrix itself limits the segment motion at room temperature, resulting in inevitable low-speed lithium ion transmission. At the same time, the presence of liquid components in the gel electrolyte sacrifices the mechanical properties of the polymer itself. Adding organic fillers can reduce the crystallinity of the polymer, but the reduction in crystallinity is often accompanied by weakened mechanical properties. Adding inorganic fillers can improve the mechanical properties of the polymer, but the uneven dispersion in the matrix can easily cause slow and uneven lithium ion transmission, inducing the growth of lithium dendrites. Therefore, it is urgent to prepare a composite gel electrolyte with both high ionic conductivity and high mechanical properties at room temperature. Summary of the invention

[0004] The present invention aims to solve the technical problems of low ionic conductivity and low mechanical properties of gel electrolytes at room temperature, and provides a toughened high ionic conductivity composite gel electrolyte with polyvinylidene fluoride / MOF polymer film as a matrix and a preparation method thereof.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] The object of the present invention is to provide a method for preparing a three-dimensional porous polyvinylidene fluoride / MOF composite gel electrolyte, which is carried out according to the following steps:

[0007] Step 1, dispersing polyvinylidene fluoride (PVDF) in N,N-dimethylformamide (DMF), then adding lithium salt, heating under magnetic stirring after uniform dispersion, and then standing;

[0008] Step 2: adding MIL-125 (Ti) nanoparticles and ethylene carbonate (EC), magnetically stirring at room temperature until uniform (taking 10h-12h), casting in a mold, standing to remove bubbles, drying, and then immersing in a liquid electrolyte to obtain the composite gel electrolyte.

[0009] It is further defined that the MIL-125 (Ti) nanoparticles are prepared by the following steps: adding terephthalic acid to an organic solvent, mechanically stirring until uniform, adding butyl titanate dropwise while stirring, and continuing to stir for a certain period of time after the addition is completed; then hydrothermal reaction, differential centrifugation after the reaction is completed, collecting the precipitate and drying it to obtain MIL-125 (Ti) nanoparticles.

[0010] It is further defined that the organic solvent used to prepare the MIL-125 (Ti) nanoparticles is composed of DMF and methanol in a volume ratio of (8-10):1.

[0011] It is further defined that during the preparation of the MIL-125 (Ti) nanoparticles, the stirring speed is 60 rpm to 100 rpm.

[0012] It is further defined that, during the preparation of the MIL-125 (Ti) nanoparticles, the hydrothermal reaction is carried out at 145° C.-160° C. for at least 24 hours.

[0013] It is further defined that, during the preparation of the MIL-125 (Ti) nanoparticles, the drying is forced air drying at 60° C. to 70° C. for 12 hours.

[0014] It is further defined that, in the process of preparing MIL-125 (Ti) nanoparticles, the mass of each drop of butyl titanate is 0.02±0.005 g.

[0015] It is further defined that in the process of preparing MIL-125 (Ti) nanoparticles, the amount ratio of terephthalic acid, organic solvent and butyl titanate is (1-5) g: (40-80) mL: (1.36-1.76) g.

[0016] It is further defined that in the process of preparing MIL-125 (Ti) nanoparticles, the differential centrifugation treatment process is: differential centrifugation treatment at 4000 rpm to 6000 rpm at room temperature for 5 min to 10 min, filtering, adding DMF to the precipitate, and repeating differential centrifugation 2 to 5 times.

[0017] It is further defined that in step 1, the lithium salt is one of LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) and LiPF6 (lithium hexafluorophosphate).

[0018] It is further defined that in step 1, the mass ratio of polyvinylidene fluoride to lithium salt is 1:(0.05-0.5); the mass ratio of polyvinylidene fluoride to DMF is 1:(10-30).

[0019] It is further defined that in step 1, heating is performed at 40°C to 60°C for 10h to 12h under magnetic stirring at a speed of 300rpm to 400rpm.

[0020] It is further defined that in step one, the standing time is 3h-4h.

[0021] It is further defined that in step 2, the mass ratio of polyvinylidene fluoride, MIL-125 (Ti) nanoparticles and vinyl carbonate is 1: (0.01-0.2): (0.3-0.5).

[0022] It is further defined that in step 2, the speed of magnetic stirring is 400 rpm-500 rpm.

[0023] It is further defined that in step 2, the standing time is 3h-4h.

[0024] It is further defined that in step 2, the liquid electrolyte is a secondary electrolyte (LB-008), and the membrane with a diameter of 19 mm is soaked in a ratio of 0.01 g to 0.5 g of the liquid electrolyte, and the soaking time is 1 h to 2 h.

[0025] A three-dimensional porous polyvinylidene fluoride / MOF composite gel electrolyte prepared by any of the above methods.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The composite gel electrolyte with polyvinylidene fluoride / MOF polymer film as the matrix prepared by the present invention is suitable for composite gel electrolytes with high ionic conductivity and high mechanical properties. The organic filler ethylene carbonate (EC) and the inorganic MOF filler MIL-125 (Ti) are introduced into the polyvinylidene fluoride polymer matrix together, wherein MIL-125 (Ti) is embedded in the three-dimensional porous structure of the polymer. Ethylene carbonate reduces the crystallinity of the polymer matrix, increases the absorption of the liquid electrolyte, and promotes the segment movement of the high molecular polymer, thereby strengthening the movement of lithium ions in the liquid component. The MIL-125 (Ti) nanoparticles synergistically promote the transfer and uniform transmission of lithium ions in the electrolyte, and improve the mechanical properties of the polyvinylidene fluoride / MOF polymer film and the composite gel electrolyte.

[0028] The present invention adopts a simple and easy-to-operate casting film-forming method to prepare a composite gel electrolyte with high ionic conductivity and strong mechanical properties. -3 S cm -1The high ionic conductivity and maximum strain degree of 86.8% at room temperature have greatly improved the ionic conductivity and mechanical properties compared with the traditional gel electrolyte based on polymer film.

[0029] In order to further understand the features and technical contents of the present invention, please refer to the following detailed description and drawings of the present invention. However, the attached drawings are only provided for reference and illustration and are not intended to limit the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the SEM image of polyvinylidene fluoride / MOF polymer film (MIL-125(Ti)-PVDF / LiTFSI / EC);

[0031] Figure 2 The stress-strain curves are those of polyvinylidene fluoride / MOF polymer film (MIL-125(Ti)-PVDF / LiTFSI / EC), polyvinylidene fluoride polymer film without MOF filler (PVDF / LiTFSI / EC), and polyvinylidene fluoride film without filler and lithium salt (PVDF).

[0032] Figure 3 Impedance curves of the composite gel electrolyte (MIL-125(Ti)-PVDF / LiTFSI / EC GPE) with polyvinylidene fluoride / MOF polymer film (MIL-125(Ti)-PVDF / LiTFSI / EC) as the matrix, the gel electrolyte (PVDF / LiTFSI / EC GPE) with polyvinylidene fluoride polymer film without MOF filler (PVDF / LiTFSI / EC) as the matrix, and the gel electrolyte (PVDF / LiTFSI GPE) with polyvinylidene fluoride film without filler (PVDF / LiTFSI) as the matrix. DETAILED DESCRIPTION

[0033] The present invention will be further described below in conjunction with specific examples. It should be understood that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0034] In order to make the results, preparation process and effects of the present invention more clear, the present invention is further described in detail below in conjunction with the 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.

[0035] The experimental methods used in the following examples are conventional methods unless otherwise specified. The materials, reagents, methods and instruments used are conventional materials, reagents, methods and instruments in the art unless otherwise specified, and can be obtained through commercial channels by those skilled in the art.

[0036] The MIL-125 (Ti) nanoparticles used below are prepared by the following steps: 3 g of terephthalic acid is added to 60 mL of an organic solvent, and mechanically stirred at a rate of 100 rpm until uniform, and 1.56 g of butyl titanate is added dropwise while stirring, and the mass of each drop of butyl titanate is 0.02±0.005 g, and stirring is continued for a certain time after the addition is completed; then a hydrothermal reaction is performed at 150° C. for 24 h, and after the reaction is completed, differential centrifugation is performed, and the precipitate is collected and air-dried at 60° C. to 70° C. for 12 h to obtain MIL-125 (Ti) nanoparticles;

[0037] The organic solvent used to prepare MIL-125 (Ti) nanoparticles was prepared by DMF and methanol in a volume ratio of 9:1;

[0038] The differential centrifugation treatment process is as follows: differential centrifugation treatment at 5000 rpm for 10 min at room temperature, filtration, adding DMF to the precipitate, and repeating differential centrifugation 4 times.

[0039] Embodiment 1:

[0040] The operation process of preparing the polyvinylidene fluoride film (PVDF film) of this embodiment is as follows:

[0041] Step 1: Disperse 10g of polyvinylidene fluoride in 300g of N,N-dimethylformamide solvent, and stir magnetically at 400rpm for 10h at 60℃. After stirring, let it stand at room temperature for 12h to fully dissolve.

[0042] The fully dissolved solution in step 2 and step 1 is transferred to a film-forming container, and after being allowed to stand and stabilize, it is transferred to a forced air drying oven and dried at 65° C. for 12 hours to obtain a polyvinylidene fluoride polymer film (labeled as PVDF film). The prepared PVDF film contains no filler and no lithium salt.

[0043] The operation process of preparing the gel electrolyte (PVDF / LiTFSI GPE) in this embodiment is as follows:

[0044] Step 1: Disperse 10g of polyvinylidene fluoride in 300g of N,N-dimethylformamide solvent, and stir magnetically at 400rpm for 10h at 60℃. After stirring, let it stand at room temperature for 12h to fully dissolve.

[0045] Step 2: Add 4g of lithium bis(trifluoromethanesulfonyl)imide to the fully dissolved solution in step 1, stir magnetically at 500rpm for 12h at room temperature to fully dissolve the solute, and let the uniform solution stand at room temperature to remove bubbles. Transfer the uniform solution with bubbles removed to a film-forming container with a diameter of 80mm, let it stand until the solution is stable, and then transfer it to a blast drying oven and dry it at 65℃ for 12h to obtain a filler-free polyvinylidene fluoride film (PVDF / LiTFSI).

[0046] Finally, the unfilled polyvinylidene fluoride film cut into a circular shape with a diameter of 19 mm was immersed in 0.06 g of secondary electrolyte (LB-008) for 1 h to obtain a gel electrolyte (PVDF / LiTFSIGPE) with the unfilled polyvinylidene fluoride film as a matrix.

[0047] The operation process of preparing the composite gel electrolyte (PVDF / LiTFSI / EC GPE) in this embodiment is as follows:

[0048] Step 1: Disperse 10g of polyvinylidene fluoride in 300g of N,N-dimethylformamide solvent, and stir magnetically at 400rpm for 10h at 60℃. After stirring, let it stand at room temperature for 12h to fully dissolve.

[0049] Step 2: Add 4 g of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 4 g of ethylene carbonate filler to the fully dissolved solution in step 1.

[0050] The solution was stirred magnetically at 500 rpm for 12 h at room temperature to fully dissolve the solute, and the homogeneous solution was allowed to stand at room temperature to remove bubbles.

[0051] The solution was then transferred to a film-forming container with a diameter of 80 mm, and after being allowed to stand for stabilization, it was transferred to a forced air drying oven and dried at 65° C. for 12 h to obtain a polyvinylidene fluoride polymer film (PVDF / LiTFSI / EC) without MOF filler.

[0052] Step 3: Cut the MOF-free polyvinylidene fluoride polymer film (PVDF / LiTFSI / EC) obtained in step 2 into circular pieces with a diameter of 19 mm and immerse them in 0.06 g of secondary electrolyte (LB-008) for 1 hour to obtain a composite gel electrolyte (PVDF / LiTFSI / EC GPE) with the MOF-free polyvinylidene fluoride polymer film as the matrix.

[0053] The operation process of preparing the composite gel electrolyte (MIL-125(Ti)-PVDF / LiTFSI / EC GPE) in this embodiment is as follows:

[0054] Step 1: Disperse 10g of polyvinylidene fluoride in 300g of N,N-dimethylformamide solvent, and stir magnetically at 400rpm for 10h at 60℃. After stirring, let it stand at room temperature for 12h to fully dissolve.

[0055] Step 2: Add 4g of lithium bis(trifluoromethanesulfonyl)imide, 4g of ethylene carbonate filler and 2g of MIL-125(Ti) filler to the fully dissolved solution in step 1, stir magnetically at 500rpm for 12h at room temperature to fully dissolve the solute, and let the uniform solution stand at room temperature to remove bubbles. Then transfer the solution to a film-forming container with a diameter of 80mm, and after standing and stabilizing, transfer it to a forced air drying oven and dry it at 65°C for 12h to obtain a polyvinylidene fluoride / MOF polymer film (MIL-125(Ti)-PVDF / LiTFSI / EC).

[0056] Step 3: Cut the polyvinylidene fluoride / MOF polymer film (MIL-125(Ti)-PVDF / LiTFSI / EC) obtained in step 2 into circular polyvinylidene fluoride / MOF polymer film with a diameter of 19 mm and immerse it in 0.06 g of secondary electrolyte (LB-008) for 1 hour to obtain a composite gel electrolyte (MIL-125(Ti)-PVDF / LiTFSI / EC GPE) with polyvinylidene fluoride / MOF polymer film as the matrix.

[0057] The product obtained by the above preparation process was structurally characterized.

[0058] ① The microstructure of the polyvinylidene fluoride / MOF polymer film (MIL-125(Ti)-PVDF / LiTFSI / EC) prepared in Example 1 was characterized, and the SEM images were as follows: Figure 1 As shown, the SEM image shows that the polyvinylidene fluoride / MOF polymer film has a three-dimensional porous structure.

[0059] ② The polyvinylidene fluoride / MOF polymer film (MIL-125(Ti)-PVDF / LiTFSI / EC) prepared in Example 1, the polyvinylidene fluoride polymer film without MOF filler (PVDF / LiTFSI / EC) and the polyvinylidene fluoride film without filler and lithium salt (PVDF) were subjected to tensile tests. The results are as follows: Figure 2 As shown by Figure 2 It can be seen that the maximum strain of the polyvinylidene fluoride / MOF polymer film is 86.8%, the maximum strain of the polyvinylidene fluoride polymer film without MOF filler is 20.7%, and the maximum strain of the polyvinylidene fluoride film without filler and lithium salt is 7.0%. The above results show that the introduction of inorganic MOF filler MIL-125 (Ti) reduces the crystallinity of the polyvinylidene fluoride polymer without MOF filler, which helps to increase the maximum strain and improve the mechanical properties.

[0060] ③ The impedance curve impedance test of the composite gel electrolyte with polyvinylidene fluoride / MOF polymer film (MIL-125(Ti)-PVDF / LiTFSI / EC) as the matrix prepared in Example 1, the gel electrolyte with polyvinylidene fluoride polymer film without MOF filler (PVDF / LiTFSI / EC) as the matrix and the gel electrolyte with polyvinylidene fluoride film without filler (PVDF / LiTFSI) as the matrix, the results are as follows Figure 3 As shown by Figure 3 It can be seen that the ionic conductivity of the composite gel electrolyte with (MIL-125(Ti)-PVDF / LiTFSI / EC) as the matrix is ​​calculated to be about 1.50×10 -3 S cm -1 The ionic conductivity of the gel electrolyte based on (PVDF / LiTFSI / EC) is calculated to be about 6.98×10 -4 S cm -1 The ionic conductivity of the gel electrolyte based on (PVDF / LiTFSI) is calculated to be about 4.21×10 -4 S cm -1 The above results indicate that the addition of ethylene carbonate filler reduces the crystallinity of the polymer system, which is conducive to the transfer of more lithium ions, and the MIL-125(Ti) nanoparticles synergistically promote the transfer of lithium ions in the gel electrolyte and improve the ionic conductivity.

[0061] Example 2: In this example, the preparation of the composite gel electrolyte with three-dimensional porous polyvinylidene fluoride / MOF polymer film as the matrix is ​​carried out according to the following steps:

[0062] 1. Weighing:

[0063] Polyvinylidene fluoride 10g, N,N-dimethylformamide 300g, lithium bis(trifluoromethanesulfonyl)imide 4g, ethylene carbonate 4g, MIL-125(Ti) 1g, liquid electrolyte 0.06g.

[0064] 2. The operation process of preparing composite gel electrolyte is as follows:

[0065] 10 g of polyvinylidene fluoride was dispersed in 300 g of N,N-dimethylformamide solvent and magnetically stirred at 400 rpm for 10 h at 60 °C. After stirring, it was allowed to stand at room temperature for 12 h to fully dissolve.

[0066] After the dissolution was completed, 4 g of lithium bis(trifluoromethanesulfonyl)imide, 4 g of ethylene carbonate filler and 1 g of MIL-125 (Ti) filler were added to the solution, and magnetic stirring was performed at 500 rpm for 12 h at room temperature to fully dissolve the solute. The uniform solution was allowed to stand at room temperature to remove bubbles.

[0067] The solution was then transferred to a film-forming container with a diameter of 80 mm, and after being allowed to stand and stabilize, it was transferred to a forced air drying oven and dried at 65° C. for 12 h to obtain a polyvinylidene fluoride / MOF polymer film (MIL-125(Ti)-PVDF / LiTFSI / EC).

[0068] The obtained polyvinylidene fluoride / MOF polymer film (MIL-125(Ti)-PVDF / LiTFSI / EC) was cut into a circular shape with a diameter of 19 mm and immersed in 0.06 g of secondary electrolyte (LB-008) for 1 hour to obtain a composite gel electrolyte (MIL-125(Ti)-PVDF / LiTFSI / ECGPE) with polyvinylidene fluoride / MOF polymer film as the matrix.

[0069] Example 3: In this example, the preparation of the composite gel electrolyte with three-dimensional porous polyvinylidene fluoride / MOF polymer film as the matrix is ​​carried out according to the following steps:

[0070] 1. Weighing:

[0071] Polyvinylidene fluoride 10g, N,N-dimethylformamide 300g, lithium bis(trifluoromethanesulfonyl)imide 4g, ethylene carbonate 4g, MIL-125(Ti) 0.1g, secondary electrolyte (LB-008) 0.06g.

[0072] 2. The operation process of preparing composite gel electrolyte is as follows:

[0073] 10 g of polyvinylidene fluoride was dispersed in 300 g of N,N-dimethylformamide solvent and magnetically stirred at 400 rpm for 10 h at 60 °C. After stirring, it was allowed to stand at room temperature for 12 h to fully dissolve.

[0074] After the dissolution was completed, 4 g of lithium bis(trifluoromethanesulfonyl)imide, 4 g of ethylene carbonate and 0.1 g of MIL-125 (Ti) filler were added to the solution, and magnetic stirring was performed at 500 rpm for 12 h at room temperature to fully dissolve the solute. The uniform solution was allowed to stand at room temperature to remove bubbles.

[0075] The solution was then transferred to a film-forming container with a diameter of 80 mm, and after being allowed to stand and stabilize, it was transferred to a forced air drying oven and dried at 65° C. for 12 h to obtain a polyvinylidene fluoride / MOF polymer film (MIL-125(Ti)-PVDF / LiTFSI / EC).

[0076] The obtained polyvinylidene fluoride / MOF polymer film (MIL-125(Ti)-PVDF / LiTFSI / EC) was cut into a circle with a diameter of 19 mm and immersed in 0.06 g of secondary electrolyte (LB-008) for 1 hour to obtain a composite gel electrolyte (MIL-125(Ti)-PVDF / LiTFSI / ECGPE) with polyvinylidene fluoride / MOF polymer film as the matrix.

[0077] The above description is only a preferred embodiment of the present invention. In view of the fact that technicians in the field of the present invention can make appropriate changes and modifications to the above implementation, the present invention is not limited to the specific implementation described above. Some modifications and changes to the present invention should also fall within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a three-dimensional porous polyvinylidene fluoride / MOF composite gel electrolyte, characterized in that: The preparation method is carried out according to the following steps: Step 1: Disperse 10 g of polyvinylidene fluoride in 300 g of N,N-dimethylformamide solvent, stir magnetically at 400 rpm for 10 h at 60 ° C, and stand at room temperature for 12 h to fully dissolve; Step 2: Add 4 g of lithium bis(trifluoromethanesulfonyl)imide, 4 g of ethylene carbonate filler and 2 g of MIL-125 (Ti) filler to the fully dissolved solution, stir magnetically at 500 rpm for 12 h at room temperature to fully dissolve the solute, and let the uniform solution stand at room temperature to remove bubbles; The solution was then transferred to a film-forming container with a diameter of 80 mm, and after being allowed to stand for stabilization, it was transferred to a forced air drying oven and dried at 65°C for 12 h. The polyvinylidene fluoride / MOF polymer film was then cut into a circular shape with a diameter of 19 mm and immersed in 0.06 g of secondary electrolyte LB-008 for 1 h to obtain a three-dimensional porous polyvinylidene fluoride / MOF composite gel electrolyte. MIL-125 (Ti) nanoparticles are prepared by the following steps: 3 g of terephthalic acid is added to 60 mL of an organic solvent, and the mixture is mechanically stirred at a rate of 100 rpm until uniform, and 1.56 g of butyl titanate is added dropwise while stirring, and the mass of each drop of butyl titanate is 0.02±0.005 g. After the addition is completed, stirring is continued for a certain period of time; then, a hydrothermal reaction is carried out at 150° C. for 24 h, and after the reaction is completed, differential centrifugation is carried out, and the precipitate is collected and dried at 60° C. to 70° C. for 12 h to obtain MIL-125 (Ti) nanoparticles; The organic solvent used to prepare MIL-125 (Ti) nanoparticles was prepared by DMF and methanol in a volume ratio of 9:1; The differential centrifugation treatment process is as follows: differential centrifugation treatment at 5000 rpm for 10 min at room temperature, filtration, adding DMF to the precipitate, and repeating differential centrifugation 4 times.

2. A composite gel electrolyte prepared by the method of claim 1.

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