A PVDF coated diaphragm and preparation method thereof

Abstract

The present invention discloses a PVDF coated diaphragm and a preparation method thereof, which belongs to the field of diaphragm technology. A polyimide membrane is used as a base membrane, and doped and modified PVDF nanofibers are mixed with inorganic nanoparticles and coated on the surface of the base membrane to form a coating. Tourmaline-doped PVDF is used to prepare modified PVDF nanofibers through electrospinning. The PVDF polymer and tourmaline work synergistically to promote the formation of β-phase PVDF, giving it excellent electrochemical and mechanical properties. Inorganic nanoparticles are added to the mixed slurry to further improve the heat resistance and electrolyte affinity of the diaphragm. The diaphragm is prepared by coating the surface of the mixed slurry of modified PVD nanofibers and inorganic nanoparticles with a polyimide base membrane. The diaphragm has good thermal stability, electrolyte affinity, excellent electrochemical and mechanical properties, giving it a clear competitive advantage in diaphragm applications.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and in particular relates to a PVDF coated diaphragm and a preparation method thereof. Background Art

[0002] The separator is a key component in electrochemical cells. Its primary function is to separate the positive and negative electrodes, allowing ions to pass through while hindering electron transport. It is a thin film with a microporous structure. Currently, large-scale commercial separator production is primarily based on polyolefins. However, polyolefins have a low melting point and poor thermal stability, posing safety risks when used at high temperatures. Furthermore, polyolefin separators have low polarity and poor affinity with the electrolyte.

[0003] Compared with polyolefin diaphragms, polyimide has higher heat resistance and stability, better comprehensive mechanical properties, and higher puncture strength. These excellent properties provide a wide range of applications for polyimide materials in the diaphragm field, making it an ideal diaphragm material.

[0004] Surface coating modification involves depositing or coating a functional layer on the surface of a base membrane. Modifying the membrane by coating it with organic polymers and inorganic nanomaterials can improve the membrane's mechanical, electrochemical, and thermal stability. Polyvinylidene fluoride (PVDF) is often used as a binder and can also be used as a modified coating material on the membrane surface. This coating can enhance the bond between the membrane and the electrode, and improve the thermal stability of the base membrane.

[0005] Polyvinylidene fluoride has multiple crystalline phases, and PVDF in each crystalline phase has different physical and chemical properties. Among them, β-phase PVDF exhibits excellent electrical properties. A larger β-phase content will lead to higher polarity, which will help lithium ions migrate faster in the separator and improve cycle performance. Doping tourmaline can effectively promote the formation of high-polarity crystalline β-phase of PVDF, giving it better electrochemical properties. Summary of the Invention

[0006] The object of the present invention is to provide a PVDF coated diaphragm and a preparation method thereof, so as to improve the electrochemical performance and electrolyte wettability of the lithium battery diaphragm.

[0007] The purpose of the present invention can be achieved through the following technical solutions:

[0008] The present invention provides a PVDF coated diaphragm, which comprises a base membrane, wherein the base membrane is a polyimide microporous membrane, and a mixed slurry is coated on the upper and lower surfaces of the base membrane to form a coating; the mixed slurry comprises modified PVDF nanofibers, inorganic nanoparticles, a dispersant and a thickener; the modified PVDF nanofibers are PVDF-doped tourmaline powder electrospun nanofibers.

[0009] Furthermore, the polyimide microporous membrane has a thickness of 15-20 μm.

[0010] Furthermore, the coating has a thickness of 2-6 μm.

[0011] Furthermore, the inorganic nanoparticles are one or more of aluminum oxide nanoparticles, silicon dioxide nanoparticles, titanium dioxide nanoparticles, zinc oxide, and magnesium oxide, and the particle size of the inorganic nanoparticles is 10-500 nm;

[0012] The dispersant is one of polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene glycol, and paraffin, or a mixture of any proportion;

[0013] The thickener is sodium carboxymethyl cellulose, which can adjust the viscosity of the slurry and make the coating more uniform.

[0014] Furthermore, the particle size of the tourmaline powder is 300-400 nm.

[0015] Furthermore, in the mixed slurry, the mass fraction of modified PVDF nanofibers is 20%-40%, the mass fraction of inorganic nanoparticles is 0.2%-3%, the mass fraction of dispersant is 0.05%-1%, and the mass fraction of thickener is 0.01%-1%.

[0016] The present invention also provides a method for preparing a PVDF coated diaphragm, comprising the following steps:

[0017] Step 1: Prepare modified PVDF solution:

[0018] PVDF powder is added to an organic solvent, heated to 60-80°C, and stirred continuously for 2-4 hours to dissolve to obtain a PVDF solution. Tourmaline powder is added to the PVDF solution and ultrasonically dispersed for 6-10 hours to obtain a uniformly mixed modified PVDF spinning solution. If the dispersion time is too short, the tourmaline powder cannot be evenly dispersed and cannot fully contact with the PVDF.

[0019] Step 2: Preparation of modified PVDF nanofibers:

[0020] The modified PVDF spinning solution is drawn into a syringe, placed on a single-channel syringe pump, and modified PVDF nanofibers are obtained by electrospinning;

[0021] Step 3: Prepare mixed slurry:

[0022] Add the modified PVDF nanofibers to the same organic solvent as in step (1), add inorganic nanoparticles and a dispersant, heat to 40-60°C, mix and stir for 6-12 hours, add a thickener after uniform dispersion, and stir again for 3-4 hours to obtain a mixed slurry;

[0023] Step 4: Prepare the diaphragm:

[0024] The mixed slurry is evenly coated on the upper and lower surfaces of the polyimide base film, and dried at a constant temperature of 80-100°C for 6-10 hours. Heating and drying helps to remove the solvent. After drying, a coating is formed to prepare a PVDF coated diaphragm.

[0025] Furthermore, the organic solvent is one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and N-methylpyrrolidone.

[0026] Furthermore, the mass fraction of PVDF powder in the PVDF solution is 10%-20%, and the added amount of tourmaline powder is 5%-15% of the mass of the PVDF powder.

[0027] Furthermore, the electrospinning parameters are: voltage 15-20 kV, liquid pushing speed 1-10 μm / min, and PVDF can promote the formation of β phase under the action of electrospinning high voltage power supply.

[0028] Beneficial effects of the present invention:

[0029] (1) The present invention prepares modified PVDF nanofibers by first doping tourmaline powder into PVDF and then obtaining the modified PVDF nanofibers through an electrospinning process. Due to the spontaneous electrical polarity of the tourmaline powder, the PVDF polarization can be induced to form β-phase PVDF. The β-crystal content is further optimized by the action of a high-voltage electrospinning power supply, thereby effectively improving the hydrophilicity and mechanical properties of the fiber and the ionic conductivity and puncture strength of the diaphragm.

[0030] (2) The present invention prepares a mixed slurry for coating, and the addition of inorganic nanoparticles to the slurry can further enhance the temperature resistance of the diaphragm. The surface adsorption of tourmaline is utilized to prevent the inorganic nanoparticles from falling off, while avoiding the pore blocking effect caused by the agglomeration of the inorganic nanoparticles, thereby reducing the resistance to lithium ion migration and improving the ionic conductivity.

[0031] (4) The present invention adopts a polyimide diaphragm as the base membrane. Compared with traditional polyolefin diaphragms, polyimide has better temperature resistance and mechanical properties. A mixed slurry prepared by modified PVDF nanofibers and inorganic nanoparticles is coated on the surface of the polyimide membrane, which overcomes the defect of poor hydrophilicity of the polyimide membrane and enhances the adhesion between the diaphragm and the electrode. The obtained PVDF coated diaphragm has strong puncture resistance, high temperature resistance, high ionic conductivity and good electrolyte affinity. DETAILED DESCRIPTION

[0032] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0033] Example 1

[0034] Dissolve PVDF powder in N,N-dimethylformamide, heat to 60°C, and stir continuously for 4 hours to obtain a PVDF solution with a PVDF mass fraction of 10%. Add tourmaline powder with a particle size of 300 nm to the PVDF solution at a mass fraction of 0.5%, and ultrasonically disperse for 10 hours to obtain a uniformly mixed modified PVDF spinning solution.

[0035] The modified PVDF spinning solution was pumped into a syringe, placed on a single-channel syringe pump, and modified PVDF nanofibers were obtained by electrospinning. The electrospinning parameters were set as follows: voltage 15 kV, liquid pushing speed 10 μm / min;

[0036] Modified PVDF nanofibers were added to N,N-dimethylformamide, with a mass fraction of the modified PVDF nanofibers being 20%. Alumina nanoparticles with a particle size of 280 nm and polyvinyl alcohol were also added, with a mass fraction of the alumina nanoparticles being 0.5% and a mass fraction of the polyvinyl alcohol being 0.1%. The mixture was heated to 60° C. and stirred for 6 h. After uniform dispersion, 0.05% sodium carboxymethyl cellulose was added, and the mixture was stirred for another 4 h to obtain a mixed slurry.

[0037] The mixed slurry is evenly coated on the upper and lower surfaces of the polyimide base film, dried at a constant temperature of 80°C for 8 hours, and a coating is formed after drying to obtain a PVDF coated membrane. The thickness of the polyimide base membrane is 15 μm, the coating thickness is 6 μm, and the PVDF coated membrane thickness is 21 μm.

[0038] Example 2

[0039] Dissolve PVDF powder in N,N-dimethylformamide, heat to 60°C, and stir continuously for 4 hours to obtain a PVDF solution with a PVDF mass fraction of 10%. Add tourmaline powder with a particle size of 300 nm to the PVDF solution at a mass fraction of 1%, and ultrasonically disperse for 10 hours to obtain a uniformly mixed modified PVDF spinning solution.

[0040] The modified PVDF spinning solution was pumped into a syringe, placed on a single-channel syringe pump, and modified PVDF nanofibers were obtained by electrospinning. The electrospinning parameters were set as follows: voltage 15 kV, liquid pushing speed 10 μm / min;

[0041] Modified PVDF nanofibers were added to N,N-dimethylformamide, with a mass fraction of the modified PVDF nanofibers being 20%. Alumina nanoparticles with a particle size of 280 nm and polyvinyl alcohol were also added, with a mass fraction of the alumina nanoparticles being 0.5% and a mass fraction of the polyvinyl alcohol being 0.1%. The mixture was heated to 60° C. and stirred for 6 h. After uniform dispersion, 0.05% sodium carboxymethyl cellulose was added, and the mixture was stirred for another 4 h to obtain a mixed slurry.

[0042] The mixed slurry is evenly coated on the upper and lower surfaces of the polyimide base film, dried at a constant temperature of 80°C for 8 hours, and a coating is formed after drying to prepare a PVDF coated membrane. The thickness of the polyimide base membrane is 15 μm, the thickness of the top coating is 6 μm, and the thickness of the PVDF coated membrane is 21 μm.

[0043] Example 3

[0044] Dissolve PVDF powder in N,N-dimethylformamide, heat to 60°C, and stir continuously for 4 hours to obtain a PVDF solution with a PVDF mass fraction of 10%. Add tourmaline powder with a particle size of 300 nm to the PVDF solution at a mass fraction of 1.5%, and ultrasonically disperse for 10 hours to obtain a uniformly mixed modified PVDF spinning solution.

[0045] The modified PVDF spinning solution was pumped into a syringe, placed on a single-channel syringe pump, and modified PVDF nanofibers were obtained by electrospinning. The electrospinning parameters were set as follows: voltage 15 kV, liquid pushing speed 10 μm / min;

[0046] Modified PVDF nanofibers were added to N,N-dimethylformamide, with a mass fraction of the modified PVDF nanofibers being 20%. Alumina nanoparticles with a particle size of 280 nm and polyvinyl alcohol were also added, with a mass fraction of the alumina nanoparticles being 0.5% and a mass fraction of the polyvinyl alcohol being 0.1%. The mixture was heated to 60° C. and stirred for 6 h. After uniform dispersion, 0.05% sodium carboxymethyl cellulose was added, and the mixture was stirred for another 4 h to obtain a mixed slurry.

[0047] The mixed slurry is evenly coated on the upper and lower surfaces of the polyimide base film, dried at a constant temperature of 80°C for 8 hours, and a coating is formed after drying to obtain a PVDF coated membrane. The thickness of the polyimide base membrane is 15 μm, the coating thickness is 6 μm, and the PVDF coated membrane thickness is 21 μm.

[0048] Comparative Example 1

[0049] This comparative example is compared with Example 3, except that tourmaline powder is not added, and the specific steps are as follows:

[0050] PVDF powder was dissolved in N,N-dimethylformamide, heated to 60°C, and stirred continuously for 4 hours to obtain a PVDF solution with a PVDF mass fraction of 10%. A uniformly mixed PVDF spinning solution was obtained. The PVDF spinning solution was pumped into a syringe, placed on a single-channel syringe pump, and PVDF nanofibers were obtained by electrospinning. The electrospinning parameters were set as follows: voltage 15 kV, liquid pushing speed 10 μm / min;

[0051] PVDF nanofibers were added to N,N-dimethylformamide at a PVDF nanofiber mass fraction of 20%. Alumina nanoparticles with a particle size of 280 nm and polyvinyl alcohol were also added at a mass fraction of 0.5% and 0.1%, respectively. The mixture was heated to 60° C. and stirred for 6 hours. After uniform dispersion, 0.05% sodium carboxymethyl cellulose was added, and the mixture was stirred for another 4 hours to obtain a mixed slurry.

[0052] The mixed slurry is evenly coated on the upper and lower surfaces of the polyimide base film, dried at a constant temperature of 80°C for 8 hours, and a coating is formed after drying to obtain a PVDF coated membrane. The thickness of the polyimide base membrane is 15 μm, the coating thickness is 6 μm, and the PVDF coated membrane thickness is 21 μm.

[0053] Comparative Example 2

[0054] Compared with Example 3, this comparative example is different in that the tourmaline powder in PVDF is replaced with nano-titanium dioxide particles. The specific steps are as follows:

[0055] PVDF powder was dissolved in N,N-dimethylformamide, heated to 60°C, and stirred continuously for 4 hours to obtain a PVDF solution with a PVDF mass fraction of 10%. Nano-titanium dioxide particles with a particle size of 300 nm were added to the PVDF solution with a mass fraction of 1.5%. Ultrasonic dispersion was performed for 10 hours to obtain a uniformly mixed modified PVDF spinning solution.

[0056] The modified PVDF spinning solution was pumped into a syringe, placed on a single-channel syringe pump, and modified PVDF nanofibers were obtained by electrospinning. The electrospinning parameters were set as follows: voltage 15 kV, liquid pushing speed 10 μm / min;

[0057] Modified PVDF nanofibers were added to N,N-dimethylformamide, with a mass fraction of the modified PVDF nanofibers being 20%. Alumina nanoparticles with a particle size of 280 nm and polyvinyl alcohol were also added, with a mass fraction of the alumina nanoparticles being 0.5% and a mass fraction of the polyvinyl alcohol being 0.1%. The mixture was heated to 60° C. and stirred for 6 h. After uniform dispersion, 0.05% sodium carboxymethyl cellulose was added, and the mixture was stirred for another 4 h to obtain a mixed slurry.

[0058] The mixed slurry is evenly coated on the upper and lower surfaces of the polyimide base film, dried at a constant temperature of 80°C for 8 hours, and a coating is formed after drying to obtain a PVDF coated membrane. The thickness of the polyimide base membrane is 15 μm, the coating thickness is 6 μm, and the PVDF coated membrane thickness is 21 μm.

[0059] Comparative Example 3

[0060] Compared with Example 3, this comparative example does not add aluminum oxide nanoparticles and polyvinyl alcohol. The specific steps are as follows:

[0061] Dissolve PVDF powder in N,N-dimethylformamide, heat to 60°C, and stir continuously for 4 hours to obtain a PVDF solution with a PVDF mass fraction of 10%. Add tourmaline powder with a particle size of 300 nm to the PVDF solution at a mass fraction of 1%, and ultrasonically disperse for 10 hours to obtain a uniformly mixed modified PVDF spinning solution.

[0062] The modified PVDF spinning solution was pumped into a syringe, placed on a single-channel syringe pump, and modified PVDF nanofibers were obtained by electrospinning. The electrospinning parameters were set as follows: voltage 15 kV, liquid pushing speed 10 μm / min;

[0063] The modified PVDF nanofibers were added to N,N-dimethylformamide with a mass fraction of 20%; the mixture was heated to 60°C and stirred for 6 hours; after uniform dispersion, 0.05% sodium carboxymethyl cellulose was added and stirred for another 4 hours to obtain a mixed slurry;

[0064] The mixed slurry is evenly coated on the upper and lower surfaces of the polyimide base film, dried at a constant temperature of 80°C for 8 hours, and a coating is formed after drying to obtain a PVDF coated membrane. The thickness of the polyimide base membrane is 15 μm, the coating thickness is 6 μm, and the PVDF coated membrane thickness is 21 μm.

[0065] Comparative Example 4

[0066] Compared with Example 3, this comparative example differs in that the base film adopts a polypropylene microporous film, and the specific steps are as follows:

[0067] Dissolve PVDF powder in N,N-dimethylformamide, heat to 60°C, and stir continuously for 4 hours to obtain a PVDF solution with a PVDF mass fraction of 10%. Add tourmaline powder with a particle size of 300 nm to the PVDF solution at a mass fraction of 1.5%, and ultrasonically disperse for 10 hours to obtain a uniformly mixed modified PVDF spinning solution.

[0068] The modified PVDF spinning solution was pumped into a syringe, placed on a single-channel syringe pump, and modified PVDF nanofibers were obtained by electrospinning. The electrospinning parameters were set as follows: voltage 15 kV, liquid pushing speed 10 μm / min;

[0069] Modified PVDF nanofibers were added to N,N-dimethylformamide, with a mass fraction of the modified PVDF nanofibers being 20%. Alumina nanoparticles with a particle size of 280 nm and polyvinyl alcohol were also added, with a mass fraction of the alumina nanoparticles being 0.5% and a mass fraction of the polyvinyl alcohol being 0.1%. The mixture was heated to 60° C. and stirred for 6 h. After uniform dispersion, 0.05% sodium carboxymethyl cellulose was added, and the mixture was stirred for another 4 h to obtain a mixed slurry.

[0070] The mixed slurry was evenly coated on the upper and lower surfaces of the polypropylene microporous membrane, dried at a constant temperature of 80°C for 8 hours, and a coating was formed after drying to prepare a PVDF coated membrane. The thickness of the polypropylene microporous membrane was 15 μm, the coating thickness was 6 μm, and the PVDF coated membrane thickness was 21 μm.

[0071] Performance tests were performed on Examples 1 to 3 and Comparative Examples 1 to 4 according to the battery separator performance test standard. The results are shown in Table 1.

[0072] Performance test refers to GB / T 36363-2018 "Lithium-ion battery separator performance test"

[0073] The thermal shrinkage test method is: place the sample in a 150°C oven for heat treatment for 60 minutes, and then measure the change in the diaphragm area before and after the heat treatment.

[0074] Puncture strength: Puncture strength test is carried out using CMT series universal puncture testing machine. Five samples are tested and the average value is calculated. The reference standard is used for testing.

[0075] Table 1

[0076]

[0077] It can be seen from Table 1 that in Examples 1 to 3, as the content of tourmaline powder increases, the ionic conductivity of the prepared membranes gradually increases, the contact angle gradually decreases, and the mechanical properties of the membranes are further enhanced.

[0078] In Comparative Example 1, tourmaline powder is not added, and the ionic conductivity of the diaphragm is greatly reduced, and the puncture strength is reduced; in Comparative Example 2, the tourmaline powder in Example 3 is replaced with nano-titanium dioxide particles. The ionic conductivity and electrolyte wettability of the diaphragm are better than those in Comparative Example 1, but much lower than those in Example 3. This is because the physical modification caused by the hydrophilicity of nano-titanium dioxide itself cannot increase the β-phase content in PVDF, and the inorganic nanoparticles in the coating are easy to fall off, and its modification effect is far inferior to that of tourmaline powder.

[0079] Comparative Example 3, which omitted the addition of inorganic nanoparticles and dispersant, exhibited a significantly greater thermal shrinkage and slightly larger contact angle than Example 3, resulting in decreased performance. Comparative Example 4, which replaced the base film material with a polypropylene base film based on Example 3, exhibited inferior performance across all aspects of the prepared membrane, with significantly reduced thermal stability and potential safety risks. Comprehensive analysis indicates that the PVDF-coated membrane prepared by mixing modified PVDF nanofibers with inorganic nanoparticles exhibits superior performance across the board, demonstrating outstanding competitiveness.

[0080] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0081] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A PVDF coated diaphragm, characterized in that: include: A base film, wherein the base film is a polyimide microporous film, and the upper and lower surfaces of the base film are coated with a mixed slurry to form a coating; The mixed slurry comprises modified PVDF nanofibers, inorganic nanoparticles, a dispersant and a thickener; the modified PVDF nanofibers are PVDF-doped tourmaline powder electrostatically spun nanofibers.

2. A PVDF coated diaphragm according to claim 1, characterized in that: The thickness of the polyimide microporous membrane is 15-20 μm.

3. A PVDF coated diaphragm according to claim 1, characterized in that: The coating thickness is 2-6 μm.

4. A PVDF coated diaphragm according to claim 3, characterized in that: The inorganic nanoparticles are one or more of aluminum oxide nanoparticles, silicon dioxide nanoparticles, titanium dioxide nanoparticles, zinc oxide and magnesium oxide, and the particle size of the inorganic nanoparticles is 10-500 nm; The dispersant is one of polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene glycol and paraffin or a mixture of any proportion; The thickener is sodium carboxymethyl cellulose.

5. The PVDF coated diaphragm according to claim 1, characterized in that: The particle size of the tourmaline powder is 300-400 nm.

6. A PVDF coated diaphragm according to claim 1, characterized in that: The mass fraction of the modified PVDF nanofiber in the mixed slurry is 20%-40%, the mass fraction of the inorganic nanoparticles is 0.2%-3%, the mass fraction of the dispersant is 0.05%-1%, and the mass fraction of the thickener is 0.01%-1%.

7. A method for preparing a PVDF coated diaphragm according to any one of claims 1 to 6, characterized in that: The steps are as follows: (1) Preparation of modified PVDF solution: Add PVDF powder to an organic solvent, heat to 60-80°C, and stir continuously for 2-4 hours to dissolve to obtain a PVDF solution. Add tourmaline powder to the PVDF solution and ultrasonically disperse for 6-10 hours to obtain a uniformly mixed modified PVDF spinning solution. (2) Preparation of modified PVDF nanofibers: The modified PVDF spinning solution is drawn into a syringe, placed on a single-channel syringe pump, and modified PVDF nanofibers are obtained by electrospinning; (3) Preparation of mixed slurry: Add the modified PVDF nanofibers to the same organic solvent as in step (1), add inorganic nanoparticles and a dispersant, heat to 40-60°C, mix and stir for 6-12 hours, add a thickener after uniform dispersion, and stir again for 3-4 hours to obtain a mixed slurry; (4) Preparation of diaphragm: The mixed slurry is evenly coated on the upper and lower surfaces of the polyimide base film, and dried at a constant temperature of 80-100° C. for 6-10 hours. After drying, a coating is formed to prepare a PVDF coated diaphragm.

8. The method for preparing a PVDF coated diaphragm according to claim 7, characterized in that: The organic solvent is one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide and N-methylpyrrolidone.

9. The method for preparing a PVDF coated diaphragm according to claim 7, characterized in that: The mass fraction of the PVDF powder in the PVDF solution is 10%-20%, and the added amount of the tourmaline powder is 5%-15% of the mass of the PVDF powder.

10. The method for preparing a PVDF coated diaphragm according to claim 7, characterized in that: The electrospinning parameters are: voltage 15-20 kV, liquid pushing speed 1-10 μm / min.

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