Microporous dielectric separator and method of making and using same

Abstract

The application discloses a microporous dielectric diaphragm and a preparation method and application thereof, and belongs to the technical field of supercapacitors. The preparation method comprises the following steps: sequentially performing enzyme treatment and silanization treatment on a fiber substrate, and then performing vacuum drying to obtain a pretreated microfiber substrate; mixing polyvinylidene fluoride and a pore-forming agent, stirring and dissolving the mixture in a solvent, and then standing and defoaming to obtain a spraying liquid; spraying the spraying liquid on the pretreated microfiber substrate, and then sequentially performing soaking, drying and hot roller pressing to obtain the microporous dielectric diaphragm. The application further discloses the microporous dielectric diaphragm prepared by the method and application thereof. The application can effectively solve the problems that the existing diaphragm is not suitable for high-voltage supercapacitors and that there is strong polarization effect at the interface under high voltage, and has a wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of supercapacitor technology, specifically relating to a microporous dielectric membrane, its preparation method, and its application. Background Technology

[0002] Supercapacitors, characterized by high response speed, high charge / discharge efficiency, and high power density, also possess advantages such as long cycle life and high safety, making them promising for high-frequency, long-term energy storage and regulation. With the development of modern technology, high-voltage supercapacitors (≥3V) retain the characteristics of traditional supercapacitors while also offering advantages such as increased energy density (approximately 23.5% higher than 2.7V low-voltage products) and optimized power density (approximately 3.7 times higher than 2.7V low-voltage products). Most importantly, space-constrained applications urgently require the development of high-voltage supercapacitor cells to reduce the size of supercapacitor modules. Therefore, developing supercapacitors capable of high-response, long-cycle charge / discharge under harsh high-voltage conditions is crucial. However, the strong polarization interface failure at the electrode material-electrolyte interface in high-voltage supercapacitors is the core bottleneck restricting the long-cycle stability of these devices.

[0003] To date, researchers have conducted extensive and in-depth studies on interface modification, electrolyte additives, and electrode design to address these issues. Strategies such as chemical enhancement and in-situ solidification of the electrode interface, adding low-viscosity non-solvents to the electrolyte, and constructing mechanically interlocking and biomimetic structures have been employed to improve electrode performance. While these methods effectively suppress the strong polarization effect in high-voltage supercapacitor systems, they often involve cumbersome modification processes, expensive additives, and invariably increase the cost of the supercapacitor.

[0004] The separator is one of the most important components of a supercapacitor, crucial for preventing physical contact between the positive and negative electrodes and ensuring good interfacial contact. Currently, commonly used separators for supercapacitors are polyolefin separators and cellulose separators. Polyolefin separators, due to their poor thermal stability, shrink significantly above 125°C, thus failing to meet the operating requirements of high-voltage supercapacitors. Cellulose separators, on the other hand, possess excellent thermal stability, exhibiting no significant shrinkage below 200°C, making them the first choice for high-voltage supercapacitor separators. However, regardless of whether it's a polyolefin or cellulose separator, the oxygen-containing functional groups on their surface readily undergo strong polarization reactions with the organic electrolyte under high voltage, leading to electrolyte failure and significantly impacting the device's cycle life.

[0005] Therefore, existing technologies lack a membrane solution that can actively regulate the interfacial electric field while possessing a compacted structure to ensure stable ion transport and improve device cycle life to meet the needs of high-voltage supercapacitors. Summary of the Invention

[0006] The problem this invention aims to solve is to provide a microporous dielectric membrane, its preparation method, and its application, in order to address the issues that existing membranes are not suitable for high-voltage supercapacitors and exhibit strong polarization effects at the interface under high voltage.

[0007] The technical solution adopted to solve the technical problem is to provide a method for preparing a microporous dielectric membrane, including the following steps: (1) The fiber substrate was sequentially enzymatically treated and silanized, and then vacuum dried to obtain a pretreated microfiber substrate. (2) Mix polyvinylidene fluoride and pore-forming agent, stir to dissolve in solvent, let stand to remove bubbles, and obtain spray liquid; (3) Spray the coating liquid onto the pretreated microfiber substrate, and then soak, dry and hot roll press in sequence to obtain the product.

[0008] The beneficial effects of the above-mentioned technical solution of the present invention are as follows: In the method of the present invention, the fiber substrate is first enzymatically treated to gently etch the fiber surface, making it rough and generating more microstructures; then, a silanization treatment is performed to introduce a chemically bonded silane coupling agent layer, resulting in a pretreated microfiber substrate with a rough surface and containing low hydroxyl functional groups; then, polyvinylidene fluoride is used as the main body of the spraying liquid, which can significantly improve the dielectric constant of the diaphragm, so that a more stable electric field distribution can be formed at the interface between the microporous dielectric diaphragm and the electrode, reducing excessive charge accumulation. At the same time, the mixed pore-forming agent can create a large number of uniformly sized and evenly distributed nano / micro-sized pores; further, the spraying process can more precisely control the thickness and uniformity of the coating, avoiding the coating from being too thick and clogging the pores or too thin and causing defects; after soaking to wash off the pore-forming agent, a through-hole, open microporous structure is formed, followed by drying and hot rolling, realizing the formation of high mechanical strength and dense microporous structure of the diaphragm.

[0009] Preferably, in step (1), the fiber substrate is a ramie fiber bundle with a diameter of 0.1~3 μm and a thickness of 20~45 μm; the enzyme treatment is carried out in a cellulase solution with a mass concentration of 2~5% at a temperature of 40~50℃ for 2~4 h; the silanization treatment is carried out in a silane coupling agent with a mass concentration of 1~5% for 2~6 h; and the vacuum drying temperature is 80~100℃ for 2~4 h.

[0010] More preferably, in step (1), the fiber substrate is a ramie fiber bundle with a diameter of 2 μm and a thickness of 35~45 μm; the enzyme treatment is carried out in a cellulase solution with a mass concentration of 2.5% at a temperature of 40°C for 2~4 h; the silanization treatment is carried out in a silane coupling agent with a mass concentration of 2% for 3~4 h; and the vacuum drying temperature is 85°C for 3 h.

[0011] More preferably, the silane coupling agent is at least one of γ-aminopropyltriethoxysilane, γ-(2,3-epoxypropoxy)propyltrimethoxysilane and γ-(methacryloyloxy)propyltrimethoxysilane.

[0012] More preferably, the silane coupling agent is γ-aminopropyltriethoxysilane.

[0013] Preferably, in step (2), the pore-forming agent is polyvinyl alcohol; the solvent is at least one of N-methylpyrrolidone, dimethylformamide, dimethylacetamide, dimethyl sulfoxide and triethyl phosphate; and the ratio of polyvinylidene fluoride, pore-forming agent and solvent is 1 g: 0.2~0.8 g: 8~24 mL.

[0014] More preferably, the solvent is N-methylpyrrolidone; the ratio of polyvinylidene fluoride, pore-forming agent and solvent is 1 g: 0.2~0.6 g: 10~16 mL.

[0015] Preferably, in step (2), the stirring and dissolving temperature is 40~60℃ and the time is 2.5~6 h; the standing and degassing temperature is 35~45℃ and the time is 2.5~6 h.

[0016] More preferably, in step (2), the stirring and dissolving temperature is 45°C and the time is 4~5 h; the standing and degassing temperature is 40°C and the time is 3 h.

[0017] Preferably, step (3) includes the following steps: spraying the coating liquid onto one or both sides of the pretreated microfiber substrate, then immersing the pretreated microfiber substrate in an ethanol aqueous solution for 5-8 min, then drying it at 40-50℃ for 24-36 h, and then hot rolling it to obtain the final product.

[0018] More preferably, step (3) includes the following steps: spraying the coating liquid onto one or both sides of the pretreated microfiber substrate, then immersing the pretreated microfiber substrate in an ethanol aqueous solution for 5 min, then drying it at 40°C for 24 h and then hot rolling it to obtain the final product.

[0019] More preferably, the spray gun used for spraying has a nozzle diameter of 0.2~1 mm and a spraying air pressure of 0.4~0.8 MPa; the volume ratio of water to anhydrous ethanol in the ethanol-water solution is 0.5:0.5~2.

[0020] More preferably, the spray gun used for spraying has a nozzle diameter of 0.2~0.3 mm and a spraying air pressure of 0.4 MPa; the volume ratio of water to anhydrous ethanol in the ethanol-water solution is 0.5:0.5~1.

[0021] More preferably, the hot rolling speed is 0.8~1 m / min, the rolling pressure is 5~9 MPa, the rolling temperature is 55~65℃, and the number of rolling cycles is 2~4.

[0022] More preferably, the hot rolling speed is 0.8 m / min, the rolling pressure is 5~6 MPa, the rolling temperature is 60℃, and the number of rolling cycles is 3.

[0023] The present invention also provides a microporous dielectric membrane prepared by the above preparation method.

[0024] The present invention also provides the application of the above-mentioned microporous dielectric membrane in supercapacitors.

[0025] The present invention also provides the application of the above-mentioned microporous dielectric membrane in high-voltage supercapacitors.

[0026] The present invention has the following beneficial effects: The microporous dielectric membrane of this invention features high dielectric constant, dense microporous structure, high mechanical strength, and strong moisture resistance. It can effectively suppress interfacial polarization under high-voltage operating conditions, improve ion transport efficiency and interfacial stability, thereby significantly improving the cycle life of high-voltage supercapacitors. Compared with the commonly used NKK membrane, it is more suitable for high-voltage supercapacitors, suitable for large-scale industrial production, and has the advantages of suppressing interfacial polarization and improving cycle life, thus solving the problem of strong interfacial polarization in high-voltage supercapacitors. Attached Figure Description

[0027] Figure 1 The images show scanning electron microscope (SEM) images of the morphology of the microporous dielectric membranes; where (a) is the PVDF-CMF of Example 1; and (b) is the PVDF-C of Comparative Example 1. Figure 2 The infrared spectra of the microporous dielectric membranes are shown below; where (a) is the PVDF-CMF of Example 1 and the commercial membrane NKK; and (b) is the PVDF-C of Comparative Example 1. Figure 3 The figures show a comparison of the tensile strength of microporous dielectric membranes; (a) is a comparison of the transverse tensile strength of PVDF-CMF from Example 1 and commercial membrane NKK; (b) is a comparison of the longitudinal tensile strength of PVDF-CMF from Example 1 and commercial membrane NKK; and (c) is a comparison of the longitudinal tensile strength of PVDF-C, PVDF-C1, PVDF-C2 and PVDF-C3 in the comparative examples. Figure 4 The graph shows a comparison of dielectric constants; where (a) is PVDF-CMF from Example 1 and commercial diaphragm NKK; and (b) is PVDF-C, PVDF-C1, PVDF-C2 and PVDF-C3 from the comparative examples. Figure 5 The diagram shows the porosity and liquid absorption rate of PVDF-CMF and commercial NKK membranes in Example 1; Figure 6 The diagram shows the ionic conductivity of PVDF-CMF and the commercial membrane NKK from Example 1. Figure 7 The CV curves of PVDF-CMF and commercial NKK separators in Example 1 are shown at 3 V. Figure 8 The diagram shows the long-cycle operation of the microporous dielectric membrane at 3 V; where (a) is the PVDF-CMF of Example 1 and the commercial membrane NKK; and (b) is the PVDF-C, PVDF-C1, PVDF-C2 and PVDF-C3 of the comparative examples. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention; that is, the described embodiments are only a part of the embodiments of this invention, and not all of them.

[0029] Therefore, the following detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0030] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0031] Example 1 A method for preparing a microporous dielectric membrane includes the following steps: (1) After cutting ramie fiber bundles with a diameter of 2 μm and a thickness of 35 μm into 10*10 cm, they were immersed in a 2.5% cellulase solution and enzymatically treated at 40°C for 3 h. After enzyme treatment, they were washed with water until neutral and then immersed in a 2% γ-aminopropyltriethoxysilane solution for silanization treatment for 3 h. Finally, they were placed in a vacuum drying oven at 85°C and vacuum dried for 3 h to obtain a pretreated microfiber substrate. (2) Mix 1 g of polyvinylidene fluoride and 0.2 g of polyvinyl alcohol and dissolve them in 10 mL of N-methylpyrrolidone. Heat the mixture to 45°C and stir for 4 h until completely dissolved. Place the mixture in a vacuum drying oven at 40°C and let it stand for 3 h to remove bubbles, thus obtaining the spraying liquid. (3) The spraying liquid was sprayed onto both sides of the pretreated microfiber substrate using a spray gun with a nozzle diameter of 0.2 mm. The spraying air pressure was 0.4 MPa during the spraying process, and the distance between the spray nozzle and the pretreated microfiber substrate was 10 cm. After the spraying was completed, the substrate was soaked in an ethanol aqueous solution for 5 min and then taken out. It was then dried in a constant temperature drying oven at 40℃ for 24 h. The substrate was then placed on a roller press, and the roller pressing speed was set to 0.8 m / min, the roller pressing pressure was 5 MPa, and the roller pressing temperature was 60℃. The roller pressing was repeated 3 times to obtain the final product. The volume ratio of water to anhydrous ethanol in the ethanol aqueous solution was 0.5:1.

[0032] Example 2 A method for preparing a microporous dielectric membrane includes the following steps: (1) After cutting ramie fiber bundles with a diameter of 2 μm and a thickness of 35 μm into 10*10 cm, they were immersed in a 2.5% cellulase solution and enzymatically treated at 40℃ for 4 h. After enzyme treatment, they were washed with water until neutral and then immersed in a 2% γ-aminopropyltriethoxysilane solution for silanization treatment for 3 h. After that, they were placed in a vacuum drying oven at 85℃ and vacuum dried for 3 h to obtain the pretreated microfiber substrate. (2) Mix 1 g of polyvinylidene fluoride and 0.4 g of polyvinyl alcohol and dissolve them in 12 mL of N-methylpyrrolidone. Heat the mixture to 45°C and stir for 4 h until completely dissolved. Place the mixture in a vacuum drying oven at 40°C and let it stand for 3 h to remove bubbles, thus obtaining the spraying liquid. (3) The spraying liquid was sprayed onto both sides of the pretreated microfiber substrate using a spray gun with a nozzle diameter of 0.2 mm. The spraying air pressure was 0.4 MPa during the spraying process, and the distance between the spray nozzle and the pretreated microfiber substrate was 10 cm. After the spraying was completed, the substrate was soaked in an ethanol aqueous solution for 5 min and then taken out. It was then dried in a constant temperature drying oven at 40℃ for 24 h. The substrate was then placed on a roller press, and the roller pressing speed was set to 0.8 m / min, the roller pressing pressure was 5 MPa, and the roller pressing temperature was 60℃. The roller pressing was repeated 3 times to obtain the final product. The volume ratio of water to anhydrous ethanol in the ethanol aqueous solution was 0.5:1.

[0033] Example 3 A method for preparing a microporous dielectric membrane includes the following steps: (1) After cutting ramie fiber bundles with a diameter of 2 μm and a thickness of 40 μm into 10*10 cm, they were immersed in a 2.5% cellulase solution and enzymatically treated at 40℃ for 2 h. After enzyme treatment, they were washed with water until neutral and then immersed in a 2% γ-aminopropyltriethoxysilane solution for silanization treatment for 4 h. Finally, they were placed in a vacuum drying oven at 85℃ and vacuum dried for 3 h to obtain a pretreated microfiber substrate. (2) Mix 1 g of polyvinylidene fluoride and 0.6 g of polyvinyl alcohol and dissolve them in 16 mL of N-methylpyrrolidone. Heat the mixture to 45°C and stir for 5 h until completely dissolved. Place the mixture in a vacuum drying oven at 40°C and let it stand for 3 h to remove bubbles, thus obtaining the spraying liquid. (3) The spraying liquid was sprayed onto both sides of the pretreated microfiber substrate using a spray gun with a nozzle diameter of 0.3 mm. The spraying air pressure was 0.4 MPa during the spraying process, and the distance between the spray nozzle and the pretreated microfiber substrate was 10 cm. After the spraying was completed, the substrate was soaked in an ethanol aqueous solution for 5 min and then taken out. It was then dried in a constant temperature drying oven at 40℃ for 24 h. The substrate was then placed on a roller press, and the roller pressing speed was set to 0.8 m / min, the roller pressing pressure was 5 MPa, and the roller pressing temperature was 60℃. The roller pressing was repeated 3 times to obtain the final product. The volume ratio of water to anhydrous ethanol in the ethanol aqueous solution was 0.5:0.5.

[0034] Example 4 A method for preparing a microporous dielectric membrane includes the following steps: (1) After cutting ramie fiber bundles with a diameter of 2 μm and a thickness of 45 μm into 10*10 cm, they were immersed in a 2.5% cellulase solution and enzymatically treated at 40°C for 2 h. After enzyme treatment, they were washed with water until neutral and then immersed in a 2% γ-aminopropyltriethoxysilane solution for silanization treatment for 4 h. Finally, they were placed in a vacuum drying oven at 85°C and vacuum dried for 3 h to obtain a pretreated microfiber substrate. (2) Mix 1 g of polyvinylidene fluoride and 0.6 g of polyvinyl alcohol and dissolve them in 16 mL of N-methylpyrrolidone. Heat the mixture to 45°C and stir for 5 h until completely dissolved. Place the mixture in a vacuum drying oven at 40°C and let it stand for 3 h to remove bubbles, thus obtaining the spraying liquid. (3) The spraying liquid was sprayed onto both sides of the pretreated microfiber substrate using a spray gun with a nozzle diameter of 0.3 mm. The spraying air pressure was 0.4 MPa during the spraying process, and the distance between the spray nozzle and the pretreated microfiber substrate was 10 cm. After the spraying was completed, the substrate was soaked in an ethanol aqueous solution for 5 min and then taken out and dried in a constant temperature drying oven at 40℃ for 24 h. The substrate was then placed on a roller press, and the roller pressing speed was set to 0.8 m / min, the roller pressing pressure was 6 MPa, and the roller pressing temperature was 60℃. The roller pressing was repeated 3 times to obtain the final product. The volume ratio of water to anhydrous ethanol in the ethanol aqueous solution was 0.5:0.5.

[0035] Comparative Example 1 A method for preparing a microporous dielectric membrane, compared with Example 1, eliminates the enzyme treatment and silanization treatment in step (1), and directly uses ramie fiber bundles with a diameter of 2 μm and a thickness of 35 μm as a pretreated microfiber substrate after being cut into 10*10 cm sizes. The remaining steps and parameters are the same as in Example 1.

[0036] Comparative Example 2 A method for preparing a microporous dielectric membrane, compared with Example 1, step (1) is as follows: ramie fiber bundles with a diameter of 2 μm and a thickness of 35 μm are cut into 10*10 cm sizes, immersed in a cellulase solution with a mass concentration of 2.5%, and enzymatically treated at 40°C for 3 h; after enzyme treatment, the membrane is washed with water until neutral, and then placed in a vacuum drying oven at 85°C for vacuum drying for 3 h to obtain a pretreated microfiber substrate; the remaining steps and parameters are the same as in Example 1.

[0037] Comparative Example 3 A method for preparing a microporous dielectric membrane, compared with Example 1, step (1) is as follows: ramie fiber bundles with a diameter of 2 μm and a thickness of 35 μm are cut into 10*10 cm sizes, immersed in a 2% γ-aminopropyltriethoxysilane solution for silanization treatment for 3 h, and then placed in a vacuum drying oven at 85°C for vacuum drying for 3 h to obtain a pretreated microfiber substrate; the remaining steps and parameters are the same as in Example 1.

[0038] Comparative Example 4 A method for preparing a microporous dielectric membrane, compared with Example 1, step (2) does not involve the addition of polyvinyl alcohol; the remaining steps and parameters are the same as in Example 1.

[0039] Experimental Example The microporous dielectric membrane prepared in Example 1 is designated as PVDF-CMF, the microporous dielectric membrane prepared in Comparative Example 1 is designated as PVDF-C, the microporous dielectric membrane prepared in Comparative Example 2 is designated as PVDF-C1, the microporous dielectric membrane prepared in Comparative Example 3 is designated as PVDF-C2, and the microporous dielectric membrane prepared in Comparative Example 4 is designated as PVDF-C3.

[0040] 1. Morphological and structural characterization 1.1 Scanning electron microscopy analysis Scanning electron microscopy was performed on the surfaces of PVDF-CMF and PVDF-C, and the results are as follows: Figure 1 As shown. From Figure 1 As can be seen, the surface of PVDF-CMF forms a uniform and dense pore structure, and the uniform pores can further homogenize the flux of ions and charges. In contrast, the PVDF-C fiber substrate is used directly as a membrane without any treatment, and its appearance is dense with almost no visible pores.

[0041] 1.2 Infrared Spectroscopy Analysis Fourier transform infrared spectroscopy was performed on PVDF-CMF, commercially available NKK diaphragm, and PVDF-C. The results are shown in [Figure number missing]. Figure 2 .

[0042] from Figure 2 As can be seen from this, PVDF-CMF at 850 cm -1 The characteristic peak at 3334 cm⁻¹ corresponds to the bending vibration peak of the CF₂ group, which is a typical characteristic peak of PVDF, indicating that polyvinylidene fluoride was successfully introduced into the membrane. -1 The peak intensity of the hydroxyl functional group at the coating was weakened, and the band showed a significant red shift and broadening, which is significantly different from that of commercial NKK membranes. This indicates that a large number of hydrogen bonds were formed between the CF bonds of the sprayed polyvinylidene fluoride and the -OH bonds of cellulose. The presence of this interaction further enhances the mechanical properties of PVDF-CMF. In contrast, the CF2 characteristic peak of PVDF-C is sharp, and the hydroxyl peak is extremely strong, indicating that PVDF failed to effectively bind to the fiber.

[0043] 2. Mechanical property analysis The mechanical properties of PVDF-CMF, commercial diaphragm NKK, and PVDF-C, PVDF-C1, PVDF-C2, and PVDF-C3 were tested. The mechanical property tests were conducted using a Sansi Zongheng UTM4000 electronic universal testing machine, referring to the GB / T 36363-2018 standard. The results are as follows: Figure 3 As shown.

[0044] from Figure 3 As can be seen, the tensile strength of the PVDF-CMF membrane along the Y-axis is enhanced to 123.62 MPa, and the tensile strength along the X-axis is 62.85 MPa, demonstrating excellent mechanical properties. In contrast, the PVDF-C membrane exhibits extremely poor adhesion to the PVDF layer, only 65.3 MPa, resulting in large-area coating peeling during rolling. While enzyme treatment of PVDF-C1 increases surface roughness and hydroxyl exposure, the lack of silanization sealing results in a still highly polar fiber surface. Its adhesion to PVDF is better than PVDF-C but worse than PVDF-CMF. The PVDF-C2 membrane, due to lack of enzyme treatment, has a smooth fiber surface, and the silane coupling agent grafting is insufficient and uneven, resulting in poor mechanical properties, with a tensile strength only 67% of that of PVDF-CMF. The tensile strength of PVDF-C3 is similar to that of the PVDF-C membrane, but its mechanical properties are poor.

[0045] 3. Analysis of the electrochemical performance of the diaphragm 3.1 Comparison of Dielectric Constants The dielectric constants of PVDF-CMF, commercial separator NKK, and PVDF-C, PVDF-C1, PVDF-C2, and PVDF-C3 were determined. The specific steps were as follows: First, the materials to be tested were prepared into circular, flat thin sheets with a diameter of 10 mm. Then, silver was deposited on both sides using ion sputtering to obtain the test samples. Next, the dielectric constants of the test samples were measured using a Novocontrol concept 80 testing instrument. The results are shown below. Figure 4 .

[0046] from Figure 4 As can be seen from the diagram, the dielectric constant of the PVDF-CMF prepared in Example 1 of this invention is 9, which is much higher than the 3.2 of the commercially available NKK separator. This high dielectric property effectively reduces the direct electrical impact and corrosion of the electrodes caused by the high electric field under high voltage. In contrast, the dielectric constant of PVDF-C is only 4.2, that of PVDF-C1 is only 5.5, and that of PVDF-C2 is only 6.9, all significantly inferior to that of this application.

[0047] 3.2 Analysis of liquid absorption rate, porosity and ionic conductivity Liquid absorbance, porosity, and ionic conductivity of PVDF-CMF and commercially available NKK separators were determined. The specific steps were as follows: Liquid absorbance was measured using a gravimetric method, assessing its absorbency by measuring the mass change of the sample after immersion in the electrolyte for 2 hours; porosity was measured using a liquid saturation method, where the test sample was immersed in n-butanol solution for 1 hour, then removed, blotted dry with lint-free paper, and weighed. The porosity was calculated by determining the mass change of the adsorbed n-butanol solution; ionic conductivity was tested by assembling a blocking electrode, and the impedance was measured using a Chenhua 660E device before calculating the ionic conductivity. The results are as follows: Figures 5-6 As shown.

[0048] from Figures 5-6 As can be seen from the figure, the PVDF-CMF prepared in Example 1 of the present invention has a high content of 7.82 × 10⁻⁶. -3 The conductivity of S / cm is due to the synergistic effect of high porosity (37.80%) and high liquid absorption (81.35%) brought about by the densified microporous structure.

[0049] 4. Electrochemical performance analysis of supercapacitors Supercapacitors were assembled using PVDF-CMF, commercial NKK separators, and PVDF-C, PVDF-C1, PVDF-C2, and PVDF-C3 separators, respectively, with YP-50 activated carbon as the electrode material and 1M TEATFB electrolyte. Performance testing was conducted on button-sized supercapacitors assembled using a Xinwei CT-4008T testing equipment. A constant current charge-discharge test method was employed, with a test current density of 1 A / g, a voltage range of 0–3 V, and 20,000 cycles, conducted at a constant temperature of 25°C. Results are as follows: Figures 7-8 As shown.

[0050] from Figure 7 The CV curves show that the PVDF-CMF prepared by spraying polyvinylidene fluoride onto a ramie fiber substrate has a polarization current density of only 2.61 mA cm⁻¹ at 3 V. -2 This is significantly lower than NKK's 3.65 mA cm⁻¹. -2 This directly proves that the interface polarization phenomenon has been effectively suppressed.

[0051] At the same time, from Figure 8 As can be seen, the PVDF-CMF of this invention achieves an ultra-long cycle life of 20,000 cycles, far exceeding that of the NKK membrane. The PVDF-CMF of this invention has an initial specific capacitance of 22.66 F / g, and after 20,000 cycles, it still retains a specific charge of 12.42 F / g, with a capacity retention of 54.88%. In contrast, the specific capacitance of the NKK membrane rapidly decreases to 3.9 F / g after 10,500 cycles, and by 14,000 cycles, the capacity retention is only 18.45%. The significant improvement in cycle stability of this invention is due to the combined effect of enzyme-silane synergistic pretreatment and the densified dielectric layer, which fundamentally solves the problems of interfacial side reactions and internal resistance growth. The dielectric properties and densified structure of polyvinylidene fluoride effectively suppress the uneven migration and concentration polarization of anions and cations in the electrolyte at the membrane-electrode interface under high-voltage operating conditions, thereby improving the cycle life of the capacitor under high voltage.

[0052] The comparative examples show that the capacity of PVDF-C rapidly decays to less than 60% of its initial value after 5000 cycles. This indicates that the untreated fiber surface has excessively high polarity, poor compatibility with the dielectric polymer, and cannot form a stable and effective dielectric functional layer, thus failing to meet the requirements for high-voltage applications. The capacity retention of PVDF-C1 drops below 50% after approximately 8000 cycles, indicating that simply increasing roughness without reducing surface polarity still leads to severe interfacial side reactions. The capacity retention of PVDF-C2 also decreases significantly. The results for PVDF-C1 and PVDF-C2 demonstrate that both enzyme treatment and silanization are indispensable. Enzyme treatment provides a sufficient and robust anchoring basis for silanization, while silanization transforms the high specific surface area resulting from enzyme treatment into a stable, low-polarity interface. The results from PVDF-C3 show that although it has high mechanical strength, the assembled capacitor has extremely low specific capacitance due to the obstruction of ion transport caused by low porosity, and severe polarization during high current charging and discharging. This indicates that the pore-forming agent is crucial in forming a dense microporous structure in this invention, which is the physical basis for achieving high ionic conductivity and low polarization.

[0053] The enzyme-silane synergistic pretreatment of the fiber substrate and the introduction of a pore-forming agent in the coating solution are key to achieving a balance of high dielectric constant, high strength, and high ionic conductivity. Comparative examples show that the absence of any pretreatment step or key component results in significant defects in the performance of the prepared membrane, failing to meet the requirements for long-term stable operation of high-voltage supercapacitors.

[0054] The present invention has been described according to the above embodiments. It should be understood that the above embodiments do not limit the present invention in any way. All technical solutions obtained by equivalent substitution or equivalent transformation fall within the scope of the present invention.

Claims

1. A method for preparing a microporous dielectric membrane for a supercapacitor, characterized in that, Includes the following steps: (1) The fiber substrate was sequentially enzymatically treated and silanized, and then vacuum dried to obtain a pretreated microfiber substrate. (2) Mix polyvinylidene fluoride and pore-forming agent, stir to dissolve in solvent, let stand to remove bubbles, and obtain spray liquid; (3) Spray the coating liquid onto the pretreated microfiber substrate, and then soak, dry and hot roll press in sequence to obtain the product; The fiber substrate in step (1) is a ramie fiber bundle with a diameter of 0.1~3 μm and a thickness of 20~45 μm; the enzyme treatment is carried out in a cellulase solution with a mass concentration of 2~5% at a temperature of 40~50℃ for 2~4 h; the silanization treatment is carried out in a silane coupling agent with a mass concentration of 1~5% for 2~6 h; the vacuum drying temperature is 80~100℃ for 2~4 h. In step (2), the pore-forming agent is polyvinyl alcohol; the solvent is at least one of N-methylpyrrolidone, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, and triethyl phosphate; the ratio of polyvinylidene fluoride, pore-forming agent, and solvent is 1 g: 0.2~0.8 g: 8~24 mL; Step (3) includes the following steps: spraying the coating liquid onto one or both sides of the pretreated microfiber substrate, then immersing the pretreated microfiber substrate in an ethanol aqueous solution for 5-8 minutes, then drying it at 40-50°C for 24-36 hours, and then hot rolling it to obtain the final product.

2. The method for preparing the microporous dielectric membrane as described in claim 1, characterized in that, The silane coupling agent is at least one of γ-aminopropyltriethoxysilane, γ-(2,3-epoxypropoxy)propyltrimethoxysilane, and γ-(methacryloyloxy)propyltrimethoxysilane.

3. The method for preparing the microporous dielectric membrane as described in claim 1, characterized in that, In step (2), the stirring and dissolving temperature is 40~60℃ and the time is 2.5~6 h; the standing and degassing temperature is 35~45℃ and the time is 2.5~6 h.

4. The method for preparing the microporous dielectric membrane as described in claim 1, characterized in that, The spray gun used for spraying has a nozzle diameter of 0.2~1 mm and a spraying air pressure of 0.4~0.8 MPa; the volume ratio of water to anhydrous ethanol in the ethanol-water solution is 0.5:0.5~2.

5. The method for preparing the microporous dielectric membrane as described in claim 1, characterized in that, The hot roller pressing has a pressing speed of 0.8~1 m / min, a pressing pressure of 5~9 MPa, a pressing temperature of 55~65℃, and a pressing cycle of 2~4 times.

6. A microporous dielectric membrane prepared by any one of claims 1 to 5.

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