Polyimide composite nanofiber membrane for lithium ion battery diaphragm and preparation method of polyimide composite nanofiber membrane

Abstract

The invention discloses a polyimide composite nanofiber membrane for a lithium ion battery diaphragm and a preparation method of the polyimide composite nanofiber membrane, polyimide is modified by selecting ionic liquid, orthosilicate is added for hydrolysis to form a cross-linked structure, and the polyimide composite nanofiber membrane is prepared by adopting a high-voltage electrostatic spinning method. The obtained polyimide composite nanofiber membrane has high porosity and liquid absorption rate and excellent heat resistance, and the ionic conductivity is not lower than 1.2 * 10 <-3 > S / cm.

Description

Technical Field

[0001] The present invention relates to the technical field of thin films, and particularly relates to a polyimide composite nanofiber membrane for a lithium-ion battery separator and a preparation method thereof. Background Art

[0002] The replacement of combustion engine vehicles with electric vehicles has received extensive attention and expectation. New energy vehicles and new energy power generation have also become projects actively developed and promoted by countries around the world. Among many new energy sources, lithium-ion batteries have become the most popular energy storage systems due to their high energy density, no memory effect, long cycle life, low self-discharge rate, etc., and are also one of the most promising candidates for large power sources of electric vehicles and smart grids.

[0003] As an important component of the battery, the battery separator plays a role in separating the positive and negative electrodes and providing a channel for lithium-ion transmission. Currently, the commercially widely used lithium-ion battery separators are mostly polypropylene (PP), polyethylene (PE) and their composite membranes. These membrane materials are inexpensive and have stable performance, but their thermal stability is poor. At high temperatures, they will affect the isolation of the positive and negative electrodes, resulting in internal short circuits of the battery and even triggering safety accidents, which limits the development of lithium-ion batteries. For example, the melting point of the polyethylene (PE) membrane is 130°C. When the temperature exceeds 130°C, the battery separator will melt and form closed pores; the melting point of polypropylene (PP) is 163°C. When the temperature is about 150°C, the shrinkage rate of the separator will reach 30% or even higher. The high shrinkage rate makes it easy to occur safety accidents when using this separator. In addition, due to the chemical structure of polyolefins themselves, their wettability, porosity and electrolyte absorption rate are relatively low, resulting in the need to improve the comprehensive performance of the battery.

[0004] In view of the deficiencies of polyolefin materials themselves, in addition to modifying traditional polyolefin materials, researchers have also actively developed new separator materials. Polyimide (PI) is an organic polymer membrane material that has been studied more at present. Compared with polyolefin materials, polyimide (PI) has outstanding high-temperature resistance due to the conjugated effect of the aromatic heterocycles it contains, up to 400°C. In addition, polyimide has excellent mechanical properties, excellent electrical insulation, high dielectric strength, and also has outstanding creep resistance and dimensional stability, and can be used for a long time at a temperature of 250°C to 300°C. These outstanding properties make it possible for polyimide to be applied in the field of battery separators. Summary of the Invention

[0005] The object of the present invention is to provide a polyimide composite nanofiber film for a lithium-ion battery separator and a preparation method thereof, so as to solve some problems existing in the prior art. The polyimide is modified with an alkylimidazolium bis(trifluoromethanesulfonyl)imide salt, and then through the hydrolysis of an orthosilicate ester, a polyimide composite nanofiber film with a cross-linked structure, high porosity and liquid absorption rate is prepared, and it has good thermal stability.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] One of the technical solutions of the present invention: A preparation method of a polyimide composite nanofiber membrane for a lithium-ion battery separator, comprising the following steps:

[0008] S1: Dissolve the dianhydride monomer in a solvent in an ice-water bath, then add the diamine monomer in batches and multiple times. After mixing, stir and react under an inert gas. When the climbing rod phenomenon appears, remove the ice-water bath, add the solvent in batches, and stir to obtain a polyamic acid solution;

[0009] S2: Add an alkylimidazolium bis(trifluoromethanesulfonyl)imide to the polyamic acid solution obtained in S1, and stir for 12 - 20 h to obtain an ionic liquid / polyamic acid mixed solution;

[0010] S3: Add an orthosilicate ester to the ionic liquid / polyamic acid mixed solution obtained in S2 in batches and multiple times. After stirring for 2 - 5 h, place the obtained mixed solution in a syringe, connect the spinneret to a high-voltage generating device, adjust the receiving distance, and perform electrospinning. Control the thickness of the obtained fiber membrane by controlling the spinning time;

[0011] S4: Place the fiber membrane obtained in S3 in a high-temperature forced-air drying oven, raise the temperature under program control, keep it at 100 - 350 °C for 1 - 2 h respectively, then place the fiber membrane under tension in a sealed container pre-filled with a mixed hydrolysis solution, and the fiber membrane does not contact the hydrolysis solution. Keep it at 60 - 80 °C for 12 - 36 h, simultaneously perform hydrolysis and acid-induced cross-linking. Finally, keep the hydrolyzed fiber membrane at 300 - 350 °C for 2 - 3 h to obtain the polyimide composite nanofiber membrane.

[0012] In some embodiments of the present invention, the dianhydride monomer in S1 is pyromellitic dianhydride; the solvent is at least one of N,N'-dimethylformamide, N,N'-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0013] In some embodiments of the present invention, the diamine monomer in S1 is any one of 4,4'-diaminodiphenyl ether, 4,4'-diaminobenzanilide, 4,4'-diaminodiphenylmethane, and 2,4,6-trimethyl-1,3-phenylenediamine.

[0014] In some embodiments of the present invention, the molar ratio of the dianhydride monomer to the diamine monomer in S1 is 1 - 1.3:1.

[0015] In some embodiments of the present invention, the alkylimidazole bis(trifluoromethanesulfonyl)imide in S2 is any one of 1-methyl-3-methylimidazole bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazole bis(trifluoromethanesulfonyl)imide, and 1-butyl-3-methylimidazole bis(trifluoromethanesulfonyl)imide.

[0016] In some embodiments of the present invention, the added mass of the alkylimidazole bis(trifluoromethanesulfonyl)imide accounts for 3.5 - 12% of the mass of the polyamic acid solution.

[0017] The addition of the ionic liquid in the present invention increases the polarity of the spinning solution system, resulting in a greater electric field force during spinning, more sufficient stretching, and finally a more dense pore structure and more uniform pore size in the obtained fiber membrane. In addition, due to the presence of a relatively large volume -CF3 in the ionic liquid, the close packing of molecular chains is prevented, resulting in a larger chain spacing, a larger free volume in the fiber membrane, and multiple available hydrogen bond sites. Moreover, a cross-linked network formed by the hydrolysis of tetraalkyl orthosilicate is utilized to achieve a stable and free spatial structure inside the fiber membrane. Additionally, due to the good affinity between the polar imidazole ring in the ionic liquid and the electrolyte, the fiber membrane can retain more electrolyte, further improving the liquid absorption rate.

[0018] However, the presence of too much ionic liquid will affect the subsequent hydrolysis of tetraalkyl orthosilicate to form a cross-linked structure, thus affecting the thermal stability of the obtained fiber membrane. After several experiments, the inventor preferably selects that the added mass of the ionic liquid accounts for 3.5 - 12% of the mass of the polyamic acid solution.

[0019] In some embodiments of the present invention, the tetraalkyl orthosilicate in S3 is at least one of tetramethyl orthosilicate, tetraethyl orthosilicate, propyl orthosilicate, and tetrabutyl orthosilicate, and its added mass accounts for 20 - 30% of the mass of the ionic liquid / polyamic acid mixed solution.

[0020] In some embodiments of the present invention, in S3, the conditions for electrospinning are: the receiving distance is 20 - 25 cm, the spinning voltage is 10 - 18 kV, the rotation speed of the receiving roller is 300 - 400 rpm, the feeding speed of the syringe pump is 0.003 - 0.005 ml / min, the relative humidity is 30 - 50%, and the ambient temperature is 25°C.

[0021] In some embodiments of the present invention, the hydrolysis solution in S4 is an acidic solution prepared by mixing 37% hydrochloric acid, ethanol, and deionized water in a volume ratio of 0.042 - 0.045:2:1, and the volume used is 30 - 40 ml.

[0022] The second technical solution of the present invention: A polyimide composite nanofiber membrane prepared by the above preparation method.

[0023] The third technical solution of the present invention: An application of the above polyimide composite nanofiber membrane in a lithium-ion battery.

[0024] Beneficial effects: Compared with the prior art, the polyimide composite nanofiber membrane prepared by the present invention has relatively uniform pore sizes, high porosity and liquid absorption rate, excellent ionic conductivity, and good thermal stability. Detailed implementation manners

[0025] The various exemplary implementation manners of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, characteristics, and implementation schemes of the present invention.

[0026] It should be understood that the terms used in the present invention are only for describing specific implementation manners and are not used to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0027] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0028] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific implementation manners of the present invention specification, which are obvious to those skilled in the art. Other implementation manners obtained from the specification of the present invention are obvious to those skilled in the art. The specification and examples of this application are only exemplary.

[0029] Example 1

[0030] S1: Dissolve 5.45 g (0.025 mol) of pyromellitic dianhydride in 45 g of N,N'-dimethylformamide in an ice-water bath. Then add 5.5 g (0.025 mol) of 4,4'-diaminodiphenyl ether in two batches. After mixing, stir the reaction under a nitrogen atmosphere. When the "climbing rod" phenomenon appears, remove the ice-water bath and add a total of 10 g of N,N'-dimethylformamide in three batches (to homogenize the molecular chain length), and stir for 6 h to obtain a polyamic acid solution;

[0031] S2: Add 8.154 g of 1-methyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide to the viscous liquid obtained in S1 and stir for 12 h to obtain an ionic liquid / polyamic acid mixed solution;

[0032] S3: Add a total of 15.22 g of tetramethyl orthosilicate to the ionic liquid / polyamic acid mixed solution obtained in S2 in three batches. After stirring for 2 h, place the obtained mixed solution in a 20 ml syringe. Select a needle with a diameter of 1.2 mm as the spinneret. Connect the spinneret to a high-voltage generating device, adjust the receiving distance to 20 cm, the spinning voltage range is 10 - 18 kV, the receiving roller rotation speed is 400 rpm, and perform electrospinning at 25 °C and 30% humidity with a feeding speed of 0.005 ml / min, and control the spinning time to be 12 h;

[0033] S4: Place the fiber membrane obtained in S3 in a high-temperature forced-air drying oven and heat it up under program control. Keep it at 100 °C for 1 h, 150 °C for 1 h, 200 °C for 1 h, 250 °C for 1 h, 300 °C for 1 h, and 350 °C for 1 h respectively. Then place the fiber membrane under tension in a sealed container pre-filled with 30 ml of a mixed hydrolysis solution (37% hydrochloric acid: ethanol: deionized water = 0.042:2:1), and ensure that the fiber membrane does not contact the hydrolysis solution. Keep it at 60 °C for 36 h to simultaneously perform hydrolysis and acid-induced crosslinking. Finally, keep the hydrolyzed fiber membrane at 300 °C for 3 h to obtain the polyimide composite nanofiber membrane.

[0034] Example 2

[0035] S1: Dissolve 10.9 g (0.05 mol) of pyromellitic dianhydride in 60 g of N,N'-dimethylacetamide in an ice-water bath. Then add 9.5 g (0.042 mol) of 4,4'-diaminobenzanilide in three batches. After mixing, stir the reaction under a nitrogen atmosphere. When the "climbing rod" phenomenon appears, remove the ice-water bath and add a total of 15 g of N,N'-dimethylacetamide in three batches (to homogenize the molecular chain length), and stir for 6 h to obtain a polyamic acid solution;

[0036] S2: Add 7.632 g of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide to the viscous liquid obtained in S1, and stir for 12 h to obtain an ionic liquid / polyamic acid mixed solution;

[0037] S3: Add a total of 25.758 g of tetraethyl orthosilicate to the ionic liquid / polyamic acid mixed solution obtained in S2 in three portions. After stirring for 3 h, place the obtained mixed solution in a 20 ml syringe. Select a needle with a diameter of 1.2 mm as the spinneret. Connect the spinneret to a high-voltage generating device. Adjust the receiving distance to 22 cm, the spinning voltage range to 10 - 18 kV, the receiving roller rotation speed to 350 rpm, and perform electrospinning at 25 °C and 40% humidity at a feeding rate of 0.004 ml / min, and control the spinning time to 13 h;

[0038] S4: Place the fiber membrane obtained in S3 in a high-temperature forced-air drying oven, and increase the temperature by program control. Keep it at 100 °C for 1 h, 150 °C for 1 h, 200 °C for 1 h, 250 °C for 1 h, 300 °C for 1 h, 350 °C for 1 h respectively. Then place the fiber membrane under tension in a sealed container pre-filled with 35 ml of a mixed hydrolysis solution (37% hydrochloric acid: ethanol: deionized water = 0.043:2:1), and ensure that the fiber membrane does not contact the hydrolysis solution. Keep it at 70 °C for 24 h to simultaneously perform hydrolysis and acid-induced crosslinking. Finally, keep the hydrolyzed fiber membrane at 320 °C for 2.5 h to obtain the polyimide composite nanofiber membrane.

[0039] Example 3

[0040] S1: Dissolve 21.8 g (0.1 mol) of pyromellitic dianhydride in 80 g of N-methylpyrrolidone in an ice-water bath, and then add 15.27 g (0.077 mol) of 4,4'-diaminodiphenylmethane in four batches. After mixing, stir and react under a nitrogen atmosphere. When the climbing rod phenomenon appears, remove the ice-water bath, and add a total of 20 g of N-methylpyrrolidone (to homogenize the molecular chain length) in three batches, and stir for 6 h to obtain a polyamic acid solution;

[0041] S2: Add 17.65 g of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide to the viscous liquid obtained in S1, and stir for 12 h to obtain an ionic liquid / polyamic acid mixed solution;

[0042] S3: Add a total of 49.72 g of tetrabutyl orthosilicate to the ionic liquid / polyamic acid mixed solution obtained in S2 in three portions. After stirring for 4 h, place the resulting mixed solution in a 20 ml syringe. Select a needle with a diameter of 1.2 mm as the spinneret, connect the spinneret to a high-voltage generating device, adjust the receiving distance to 25 cm, the spinning voltage range to 10 - 18 kV, the receiving roller speed to 300 rpm, and perform electrospinning at 25°C and 50% humidity at a feeding rate of 0.003 ml / min, controlling the spinning time to 13 h;

[0043] S4: Place the fiber membrane obtained in S3 in a high-temperature forced-air drying oven, and heat it up through program control. Keep it at 100°C for 1 h, 150°C for 1 h, 200°C for 1 h, 250°C for 1 h, 300°C for 1 h, and 350°C for 1 h respectively. Then place the fiber membrane under tension in a sealed container pre-filled with 40 ml of a mixed hydrolysis solution (37% hydrochloric acid: ethanol: deionized water = 0.045:2:1), and ensure that the fiber membrane does not contact the hydrolysis solution. Keep it at 80°C for 12 h to simultaneously perform hydrolysis and acid-induced crosslinking. Finally, keep the hydrolyzed fiber membrane at 350°C for 2 h to obtain the polyimide composite nanofiber membrane.

[0044] Example 4

[0045] The process is similar to that of Example 3, except that the added mass of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide is 20.56 g (accounting for 15% of the mass of the polyamic acid solution)

[0046] Comparative Example 1

[0047] The process is similar to that of Example 3, except that 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide is not added.

[0048] Comparative Example 2

[0049] The process is similar to that of Example 3, except that tetrabutyl orthosilicate is not added.

[0050] Comparative Example 3

[0051] The process is similar to that of Example 3, except that neither 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide nor tetrabutyl orthosilicate is added.

[0052] Performance Test

[0053] Thermal stability: Use a thermogravimetric analyzer (TGA-1000A, Shanghai Yingnuo Precision Instrument Co., Ltd.) to test the heat resistance of the fiber membrane. The test conditions are 30 - 800°C, the heating rate is 25°C / min, and N2 protection;

[0054] Porosity: Cut the fiber membrane into small squares of 20 mm × 20 mm. Use a high-precision thickness gauge to measure the thickness of the fiber membrane and calculate the volume of the fiber membrane. Weigh it and immerse it in n-butanol. After soaking at room temperature for 2 h, take it out, clamp it between two pieces of filter paper, place a 100 g weight on the filter paper and press for 10 s to remove the excess n-butanol, and weigh the soaked fiber membrane. Calculate the porosity of the fiber membrane according to the following formula: where M1 is the mass (g) of the fiber membrane before soaking, M2 is the mass (g) of the fiber membrane after soaking; ρ b is the density of n-butanol (g / cm 3 ); V is the volume (cm 3 ) of the fiber membrane before soaking;

[0055] Liquid absorption rate: The process is the same as above, and the calculation formula is:

[0056] Ionic conductivity: Use an electrochemical workstation (BMP3, Biologic Company, Germany) to test the ionic conductivity of the fiber membrane. Assemble a stainless steel sheet / diaphragm / stainless steel sheet system with a CR 2032 type battery component, and use the alternating current impedance method to measure the bulk resistance of the fiber membrane. The test frequency is 1 - 105 Hz. Calculate the ionic conductivity of the fiber membrane according to the following formula: where η is the ionic conductivity (S / cm), d is the thickness (cm) of the fiber membrane; R is the bulk resistance (Ω) of the fiber membrane, S is the area (cm 2 ) of the fiber membrane.

[0057] The test results are shown in Table 1:

[0058] Table 1 Performance test results of the fiber membranes obtained in Examples 1 - 4 and Comparative Examples 1 - 3

[0059]

[0060] As can be seen from the data in Table 1, the polyimide nanofiber membrane provided by the present invention is far superior to the unmodified polyimide membrane in terms of heat resistance, and also has a high porosity and high liquid absorption rate that can improve the capacity of lithium-ion batteries. The excellent ionic conductivity can achieve smoother lithium-ion transmission, broadening the application of lithium-ion batteries in fields such as high-power mechanical equipment, aerospace, and energy storage systems.

Claims

1. A preparation method of a polyimide composite nanofiber membrane for a lithium-ion battery separator, characterized in that, It includes the following steps: S1: Dissolve the dianhydride monomer in a solvent in an ice-water bath, then add the diamine monomer in batches multiple times. After mixing, stir and react under an inert gas. When the "climbing rod" phenomenon appears, remove the ice-water bath, add the solvent in batches, and stir to obtain a polyamic acid solution; S2: Add alkylimidazole bis(trifluoromethanesulfonyl)imide to the polyamic acid solution obtained in S1, and stir for 12 - 20 h to obtain an ionic liquid / polyamic acid mixed solution; S3: Add tetraalkyl orthosilicate to the ionic liquid / polyamic acid mixed solution obtained in S2 in batches multiple times. After stirring for 2 - 5 h, place the obtained mixed solution in a syringe, connect the spinneret to a high-voltage generating device, adjust the receiving distance, and perform electrospinning. Control the thickness of the obtained fiber membrane by controlling the spinning time; S4: Place the fiber membrane obtained in S3 in a high-temperature air blast drying oven, heat it up under program control, keep it at 100 - 350 °C for 1 - 2 h respectively, then place the fiber membrane under tension in a sealed container pre-filled with a mixed hydrolysis solution, and ensure that the fiber membrane does not contact the hydrolysis solution. Keep it at 60 - 80 °C for 12 - 36 h to simultaneously carry out hydrolysis and acid-induced crosslinking. Finally, keep the hydrolyzed fiber membrane at 300 - 350 °C for 2 - 3 h to obtain the polyimide composite nanofiber membrane.

2. The preparation method of the polyimide composite nanofiber membrane for a lithium-ion battery separator according to claim 1, wherein The dianhydride monomer in S1 is pyromellitic dianhydride; the solvent is at least one of N,N'-dimethylformamide, N,N'-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

3. The preparation method of the polyimide composite nanofiber membrane for a lithium-ion battery separator according to claim 1, characterized in that, The diamine monomer in S1 is any one of 4,4'-diaminodiphenyl ether, 4,4'-diaminobenzanilide, 4,4'-diaminodiphenylmethane, and 2,4,6-trimethyl-1,3-phenylenediamine.

4. The preparation method of the polyimide composite nanofiber membrane for a lithium-ion battery separator according to claim 1, characterized in that, The molar ratio of the dianhydride monomer to the diamine monomer in S1 is 1 - 1.3:

1.

5. The preparation method of the polyimide composite nanofiber membrane for a lithium-ion battery separator according to claim 1, characterized in that The alkylimidazole bis(trifluoromethanesulfonyl)imide in S2 is any one of 1-methyl-3-methylimidazole bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazole bis(trifluoromethanesulfonyl)imide, and 1-butyl-3-methylimidazole bis(trifluoromethanesulfonyl)imide.

6. The preparation method of the polyimide composite nanofiber membrane for a lithium ion battery separator according to claim 5, characterized in that, The added mass of the alkylimidazole bis(trifluoromethanesulfonyl)imide accounts for 3.5 - 12% of the mass of the viscous liquid.

7. The preparation method of the polyimide composite nanofiber membrane for a lithium-ion battery separator according to claim 1, characterized in that, The tetraalkyl orthosilicate in S3 is at least one of tetramethyl orthosilicate, tetraethyl orthosilicate, propyl orthosilicate, and tetrabutyl orthosilicate, and its added mass accounts for 20 - 30% of the mass of the ionic liquid / polyamic acid mixed solution.

8. The preparation method of the polyimide composite nanofiber membrane for a lithium-ion battery separator according to claim 1, characterized in that, In S3, the conditions for electrospinning are: the receiving distance is 20 - 25 cm, the spinning voltage is 10 - 18 kV, the rotation speed of the receiving roller is 300 - 400 rpm, the feeding speed of the syringe pump is 0.003 - 0.005 ml / min, the relative humidity is 30 - 50%, and the ambient temperature is 25 °C.

9. The preparation method of the polyimide composite nanofiber membrane for a lithium-ion battery separator according to claim 1, characterized in that, The hydrolysis solution in S4 is an acidic solution prepared by mixing 37% hydrochloric acid, ethanol, and deionized water according to a volume ratio of 0.042 - 0.045:2:1, and the volume dosage is 30 - 40 ml.

10. The polyimide composite nanofiber membrane prepared by the preparation method according to any one of claims 1 - 9.

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