Preparation method of high-safety metal lithium battery diaphragm

Abstract

The application discloses a preparation method of a high-safety metal lithium battery diaphragm and relates to the technical field of lithium battery diaphragms, and comprises the following preparation steps: step one, modified PAA microspheres, polyvinylpyrrolidone and sodium carboxymethyl cellulose are added into a mixed solvent, ultrasonic dispersion is carried out for 20-30 minutes, a magnetic stirrer is used to stir at a rotating speed of 1200-1500 r / min for 3.5-4 hours, and microsphere slurry is obtained; step two, the microsphere slurry is coated on both sides of the diaphragm with a gap of 80-100 mm on an automatic doctor blade coater, and then placed in a blast drying oven for drying at 45-50 DEG C for 6-8 hours, so that the high-safety metal lithium battery diaphragm is obtained. According to the preparation method of the high-safety metal lithium battery diaphragm, the heat-resistant temperature of the diaphragm is effectively improved through the combined action of polyvinylidene fluoride modified boehmite, the diaphragm is prevented from shrinking or being damaged under high-temperature conditions, and therefore the safety and service life of the lithium battery are improved; and the prepared diaphragm has good chemical stability and is not easy to be corroded by chemical substances such as electrolyte.

Description

Technical Field

[0001] This invention relates to the field of lithium battery separator technology, specifically to a method for preparing a high-safety metallic lithium battery separator. Background Technology

[0002] Chinese Patent Publication No. CN104051694A discloses a lithium polyoxometalate composite separator for lithium batteries and its preparation method. The composite separator includes a PVDF-HFP copolymer, SiO2, and a lithium polyoxometalate. The lithium polyoxometalate has a three-dimensional framework structure, and lithium ions combine with the lithium polyoxometalate anions. During battery charging and discharging, the lithium ions of the lithium polyoxometalate can continuously exchange with the lithium ions in the electrolyte, realizing the combination of the separator material and the lithium ions in the electrolyte, reducing the battery's internal resistance and improving the battery's rate performance. However, the battery separator prepared by the above-mentioned prior art cannot guarantee safety under high temperature conditions. Therefore, this invention provides a method for preparing a high-safety lithium metal battery separator to solve the above problems. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention provides a method for preparing a high-safety lithium metal battery separator, thus solving the problems mentioned in the background section.

[0004] To achieve the above objectives, the present invention provides a method for preparing a high-safety lithium metal battery separator, comprising the following preparation steps: Step 1: Add modified PAA microspheres, polyvinylpyrrolidone, and sodium carboxymethyl cellulose to a mixed solvent, ultrasonically disperse for 20-30 minutes, and then stir with a magnetic stirrer at a speed of 1200-1500 r / min for 3.5-4 hours to obtain microsphere slurry. Step 2: On an automatic blade coating machine, the microsphere slurry is coated on both sides of the separator with a gap of 80-100µm. Then, it is placed in a forced-air drying oven at 45-50℃ for 6-8 hours to obtain a high-safety metal lithium battery separator.

[0005] Preferably, the weight proportions of each component added to the lithium metal battery separator are: 8-15 parts of modified PAA microspheres, 0.5-2 parts of polyvinylpyrrolidone, 0.5-2 parts of sodium carboxymethyl cellulose, and 25-45 parts of mixed solvent.

[0006] Preferably, in step one, the mixed solvent is prepared by mixing deionized water and anhydrous ethanol, and the volume ratio of deionized water to anhydrous ethanol is 1:(0.8-1.5).

[0007] Preferably, the method for preparing the diaphragm is as follows: (1) Place 15-20g of polyvinylidene fluoride and 3.5-6g of polymethyl methacrylate in a vacuum drying oven at 60-70℃ and dry for 10-12h. (2) Pour 60-70 mL of N,N-dimethylformamide into a water bath and heat it to 35-45℃. After stirring mechanically for 8-10 h, add 0.8-1.2 g of modified boehmite and continue stirring. After mixing evenly, let it stand for 40-60 min to obtain the spinning solution. (3) Turn on the heat lamp of the spinning machine, use a syringe to draw out the spinning solution, and start spinning when the humidity in the spinning machine chamber drops to 20%RH and the temperature is 40-45℃. After spinning is completed, place it in a vacuum drying oven at 50-60℃ for 10-14 hours to dry, and the diaphragm is obtained.

[0008] Preferably, the spinning voltage is 15-25kV, the liquid pushing speed is 0.002-0.004mm / s, and the receiving distance is 16-18cm.

[0009] Preferably, the modified boehmite is prepared by: placing 25-30g of boehmite in a drying oven for drying, then pouring it into 50-65g of KH550, ultrasonically treating it for 20-30min, then heating it to 130-150℃, and stirring and refluxing it at this temperature for 1.5-2h. The product is then washed with deionized water until the supernatant is neutral, and then dried in a drying oven at 50-60℃ to obtain the modified boehmite.

[0010] Preferably, the modified PAA microspheres are prepared by: S1. After heating the PAA microspheres in a forced-air oven at 260-300℃ for 1.5-2.5h, immerse them in a sodium hydroxide solution with a concentration of 0.2mol / L for 10-20min. S2. After washing with deionized water 3-5 times, immerse in 3w% acetic acid solution for 20-30 minutes. S3. Remove and wash with deionized water 3-5 times to obtain modified PAA microspheres.

[0011] Preferably, the method for preparing the PAA microspheres is as follows: Under ice bath conditions of S11 and 0-2℃, 35-50 mL of N,N-dimethylformamide is poured into a mixed solution of 10-12.5 g of pyromellitic anhydride and 8.5-10 g of 4,4′-diaminodiphenyl ether. S12. After heating to 50-60℃, react at this temperature for 40-48 hours. S13. Electrostatic spraying is performed under the parameters of 15-20kV positive pressure and 4-6kV negative pressure to obtain PAA microspheres.

[0012] Beneficial effects This invention provides a method for preparing a highly safe lithium metal battery separator. Compared with existing technologies, it has the following advantages: (1) The preparation method of this high-safety lithium metal battery separator effectively improves the heat resistance temperature of the separator through the combined action of polyvinylidene fluoride modified boehmite, preventing the separator from shrinking or being damaged under high temperature conditions, thereby improving the safety and service life of the lithium battery; and the prepared separator has good chemical stability and is not easily corroded by electrolytes and other chemicals, ensuring that the performance of the separator does not decline significantly during long-term use; the addition of modified boehmite can enhance the puncture resistance of the separator, preventing the separator from being punctured due to the expansion of positive and negative electrode materials or other reasons inside the battery, thereby avoiding safety problems such as short circuits caused by direct contact between positive and negative electrodes, and improving the safety of the separator during use; polyvinylidene fluoride and polymethyl methacrylate have good affinity with electrolyte, and can effectively absorb and retain electrolyte; N,N-dimethylformamide as a solvent can make the components fully mixed, which is conducive to forming a stable electrolyte system, thereby improving ionic conductivity, reducing the internal resistance of the battery, and improving the charge-discharge performance and rate performance of the battery.

[0013] (2) The preparation method of this high-safety lithium metal battery separator involves modifying PAA microspheres to deprotonate the carboxyl groups on the surface of the PAA microspheres, increasing their hydrophilicity and making them easier to absorb and retain electrolyte, thereby improving ionic conductivity, reducing battery internal resistance, and enhancing the battery's charge-discharge performance and rate performance. At the same time, the surface properties of the PAA microspheres can be adjusted to further optimize their affinity with the electrolyte, ensuring that the separator can stably retain the electrolyte during charge-discharge, providing a good medium environment for lithium-ion transport. The modified PAA microspheres can better interact with other components in the separator material to form a stable network structure, thereby improving the tensile strength and tear resistance of the separator, enabling it to withstand greater external forces during battery assembly and use, making it less prone to breakage and extending the battery's service life. Attached Figure Description

[0014] Figure 1 Here are the SEM images of the PAA microspheres before and after modification. Figure 2 Infrared spectra of boehmite before and after modification provided by this invention; Figure 3 Comparison chart of thermal dimensional stability test of lithium battery separator provided by the present invention; Figure 4 A comparison chart of the high-temperature safety test results of the assembled battery provided for this invention, showing Fact 1 and Comparative Example 1. Detailed Implementation

[0015] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] Example 1

[0017] A method for preparing a high-safety lithium metal battery separator includes the following preparation steps: Step 1: Add 8g of modified PAA microspheres, 2g of polyvinylpyrrolidone, and 2g of sodium carboxymethyl cellulose to 45g of a mixed solution of deionized water and anhydrous ethanol in a volume ratio of 1:0.8. After ultrasonic dispersion for 20min, stir with a magnetic stirrer at a speed of 1200r / min for 3.5h to obtain microsphere slurry. The preparation method of the diaphragm is as follows: 15g of polyvinylidene fluoride and 3.5g of polymethyl methacrylate are placed in a vacuum drying oven at 60℃ and dried for 10h; then 60mL of N,N-dimethylformamide is poured in, the water bath temperature is raised to 35℃, and mechanical stirring is carried out for 8h. 0.8g of modified boehmite is added, and stirring is continued. After the mixture is evenly mixed, it is allowed to stand for 40min to obtain the spinning solution; the heating lamp of the spinning machine is turned on, and the spinning solution is drawn out with a syringe. When the humidity in the spinning machine chamber drops to 20%RH and the temperature is 40℃, spinning is started. The spinning voltage is 15kV, the liquid pushing speed is 0.002mm / s, and the receiving distance is 16cm. After spinning is completed, it is placed in a vacuum drying oven at 50℃ and dried for 10h to obtain the diaphragm. The preparation method of modified boehmite is as follows: 25g of boehmite is placed in a drying oven and dried, then poured into 50g of KH550, ultrasonically treated for 20min, heated to 130℃, and stirred and refluxed for 1.5h at this temperature. The product is washed with deionized water until the supernatant is neutral, and then placed in a drying oven at 50℃ to dry, thus obtaining modified boehmite. The modified PAA microspheres were prepared as follows: Under ice bath conditions at 0°C, 35 mL of N,N-dimethylformamide was added to a mixed solution of 10 g of pyromellitic anhydride and 8.5 g of 4,4′-diaminodiphenyl ether; the temperature was raised to 50°C, and the reaction was carried out for 40 h at this temperature; electrostatic spraying was performed under parameters of 15 kV positive pressure and 4 kV negative pressure to obtain PAA microspheres; the PAA microspheres were heated in a 260°C forced-air oven for 1.5 h, then immersed in a 0.2 mol / L sodium hydroxide solution for 10 min; after washing three times with deionized water, they were immersed in a 3 w% acetic acid solution for 20 min; after removal, they were washed three more times with deionized water to obtain the modified PAA microspheres. Step 2: On an automatic blade coating machine, the microsphere slurry is coated on both sides of the separator with a gap of 80µm. Then, it is placed in a forced-air drying oven at 45℃ for 6 hours to obtain a high-safety metal lithium battery separator.

[0018] Example 2

[0019] A method for preparing a high-safety lithium metal battery separator includes the following preparation steps: Step 1: Add 12g of modified PAA microspheres, 1g of polyvinylpyrrolidone, and 1.5g of sodium carboxymethyl cellulose to 35g of a 1:1 volume ratio of deionized water and anhydrous ethanol mixture. After ultrasonic dispersion for 25min, stir with a magnetic stirrer at 1300r / min for 3.8h to obtain microsphere slurry. The preparation method of the diaphragm is as follows: 17g of polyvinylidene fluoride and 4.5g of polymethyl methacrylate are placed in a vacuum drying oven at 65℃ and dried for 11h; then 65mL of N,N-dimethylformamide is poured in, the water bath temperature is raised to 40℃, and mechanical stirring is carried out for 9h. 1g of modified boehmite is added, and stirring is continued. After the mixture is evenly mixed, it is allowed to stand for 50min to obtain the spinning solution; the heating lamp of the spinning machine is turned on, and the spinning solution is drawn out with a syringe. When the humidity in the spinning machine chamber drops to 20%RH and the temperature is 42℃, spinning is started. The spinning voltage is 20kV, the liquid pushing speed is 0.003mm / s, and the receiving distance is 17cm. After spinning is completed, it is placed in a vacuum drying oven at 55℃ and dried for 12h to obtain the diaphragm. The preparation method of modified boehmite is as follows: 28g of boehmite is placed in a drying oven and dried, then poured into 60g of KH550, ultrasonically treated for 25min, heated to 140℃, and stirred and refluxed for 1.8h at this temperature. The product is washed with deionized water until the supernatant is neutral, and then placed in a drying oven at 55℃ to dry, thus obtaining modified boehmite. The modified PAA microspheres were prepared as follows: Under ice bath conditions at 1°C, 40 mL of N,N-dimethylformamide was added to a mixed solution of 11.5 g of pyromellitic anhydride and 9 g of 4,4′-diaminodiphenyl ether; the temperature was raised to 55°C, and the reaction was carried out for 44 h at this temperature; electrostatic spraying was performed under parameters of 18 kV positive pressure and 5 kV negative pressure to obtain PAA microspheres; the PAA microspheres were heated in a 280°C oven for 2 h, then immersed in a 0.2 mol / L sodium hydroxide solution for 15 min; after washing four times with deionized water, they were immersed in a 3 w% acetic acid solution for 25 min; after removal, they were washed four more times with deionized water to obtain the modified PAA microspheres. Step 2: On an automatic blade coating machine, the microsphere slurry is coated on both sides of the separator with a gap of 90µm. Then, it is placed in a forced-air drying oven at 48℃ for 7 hours to obtain a high-safety metal lithium battery separator.

[0020] Example 3

[0021] A method for preparing a high-safety lithium metal battery separator includes the following preparation steps: Step 1: Add 15g of modified PAA microspheres, 0.5g of polyvinylpyrrolidone, and 0.5g of sodium carboxymethyl cellulose to 25g of a mixed solution of deionized water and anhydrous ethanol in a volume ratio of 1:0.8. After ultrasonic dispersion for 30min, stir with a magnetic stirrer at a speed of 1500r / min for 4h to obtain microsphere slurry. The preparation method of the diaphragm is as follows: 20g of polyvinylidene fluoride and 6g of polymethyl methacrylate are placed in a vacuum drying oven at 70℃ and dried for 12h; then 70mL of N,N-dimethylformamide is poured in, the water bath temperature is raised to 45℃, and mechanical stirring is carried out for 10h. 1.2g of modified boehmite is added, and stirring is continued. After the mixture is evenly mixed, it is allowed to stand for 60min to obtain the spinning solution; the heating lamp of the spinning machine is turned on, and the spinning solution is drawn out with a syringe. When the humidity in the spinning machine chamber drops to 20%RH and the temperature is 45℃, spinning is started. The spinning voltage is 25kV, the liquid pushing speed is 0.004mm / s, and the receiving distance is 18cm. After spinning is completed, it is placed in a vacuum drying oven at 60℃ and dried for 14h to obtain the diaphragm. The preparation method of modified boehmite is as follows: 30g of boehmite is placed in a drying oven and dried, then poured into 65g of KH550, ultrasonically treated for 30min, heated to 150℃, and stirred and refluxed for 2h at this temperature. The product is washed with deionized water until the supernatant is neutral, and then placed in a drying oven at 60℃ to dry, thus obtaining modified boehmite. The modified PAA microspheres were prepared as follows: 50 mL of N,N-dimethylformamide was added to a mixed solution of 12.5 g of pyromellitic anhydride and 10 g of 4,4′-diaminodiphenyl ether under ice bath conditions at 2°C. The temperature was raised to 60°C, and the reaction was carried out for 48 h at this temperature. Electrostatic spraying was performed under positive pressure of 20 kV and negative pressure of 6 kV to obtain PAA microspheres. The PAA microspheres were then heated in a 300°C oven for 2.5 h, and then immersed in a 0.2 mol / L sodium hydroxide solution for 20 min. After washing five times with deionized water, they were immersed in a 3 w% acetic acid solution for 30 min. After removal, they were washed five more times with deionized water to obtain the modified PAA microspheres. Step 2: On an automatic blade coating machine, the microsphere slurry is coated on both sides of the separator with a gap of 100µm. Then, it is placed in a forced-air drying oven at 50℃ for 8 hours to obtain a high-safety metal lithium battery separator.

[0022] Comparative Example 1 Compared with Example 1, the difference is that the modified PAA microspheres in Example 1 are replaced with PAA microspheres; the rest remain the same.

[0023] Comparative Example 2 Compared with Example 1, the difference is that the modified boehmite in Example 1 is replaced with boehmite; the rest remains the same.

[0024] like Figure 1 As shown, the surface of the unmodified PAA microspheres is smooth, and the spherical structure is complete, round, and uniform in size distribution. The surface of the modified PAA microspheres is no longer smooth and becomes extremely rough, with many cracks and spots. This is because the etching of the alkaline solution destroys the amide ring in the molecule, resulting in many rough spots on the surface of the microspheres. However, it does not destroy the spherical structure of the microspheres, but only causes ring opening on the surface of the microspheres, thereby introducing the -COOH structure.

[0025] like Figure 2 As shown, the absorption peaks around 1384 cm⁻¹ in boehmite are the overtone bands of its surface hydroxyl groups; while around 3500 cm⁻¹ is a strong hydroxyl absorption peak on the surface of boehmite; the stretching vibration peaks of Al-O₆ are located at 502 cm⁻¹ and 615 cm⁻¹, and the OH vibration peaks are located at 739 cm⁻¹, 1070 cm⁻¹, 3097 cm⁻¹ and 3308 cm⁻¹, respectively; in the modified boehmite, the stretching vibration peaks of CH in CH at 2927 cm⁻¹ and 2850 cm⁻¹ are the peaks, and the CH₂ group originates from the silane coupling agent, indicating that the aluminum hydroxyl groups of boehmite and the alkoxy groups of the silane coupling agent underwent a condensation reaction, and the coupling agent was successfully grafted onto the boehmite.

[0026] Electrochemical impedance spectroscopy (EIS) testing: Lithium-ion battery separators with a diameter of 16 mm were cut using a separator punching machine, and their thickness was measured using a thickness gauge. The separators were then dried in a vacuum oven at 60°C for 10 hours. In a glove box, CR2023 button cells were assembled with stainless steel sheets as positive and negative electrodes and LPF6 / (EC, DEC, DMC = 1:1:1) electrolyte. After assembly, the cells were allowed to stand for 12 hours. In an electrochemical workstation, AC impedance mode was selected, with an amplitude of 5 mV, a frequency range of 1-10 kHz, and an equilibration time of 2 seconds. The results are shown in Table 1. The ionic conductivity formula is:

[0027] In the formula, σ is the ionic conductivity, mS / cm; d is the thickness of the diaphragm, cm; Rb is the internal resistance of the diaphragm, Ω; and AS is the contact area between the diaphragm and the stainless steel electrode, cm2.

[0028] Mechanical performance testing: The lithium battery separator was cut into 20mm x 150mm membrane strips and clamped in a mold. The tensile strength and elongation at break of the membrane were determined using an XLW intelligent electronic tensile testing machine. The test conditions were: tensile rate 25mm / min, test temperature 23±2℃, and test humidity 50±5%. The results are shown in Table 1.

[0029] Table 1

[0030] Thermal dimensional stability test: First, the lithium battery separator was cut into a circle with a diameter of 15mm and placed in an oven at 160℃. The state was observed after 1 hour and 2 hours. The results are as follows: Figure 3 As shown, compared to Comparative Examples 1-2, the lithium battery separators prepared in Examples 1-3 only exhibit slight shrinkage.

[0031] High-temperature safety test: After the assembled button half-cells were left to stand for 12 hours, they were charged at a rate of 0.1C to a voltage of 4.3V, and then placed in an oven at a constant temperature of 140℃ to discharge at a rate of 0.1C; Figure 4 As shown: The battery assembled in Comparative Example 1 exhibited a significant voltage drop at around 22 minutes, and the voltage dropped to 0 at 27 minutes, indicating that a short circuit occurred inside the battery at this time; The battery assembled in Example 1 did not exhibit a significant voltage drop in the first 150 minutes, but the voltage dropped rapidly at around 160 minutes, and the voltage dropped to 0 at 167 minutes. This indicates that the battery assembled with the lithium battery separator in Example 1 has a much longer survival time at a high temperature of 140°C than the battery assembled with the lithium battery separator in Comparative Example 1, indicating that the battery assembled with the lithium battery separator in Example 1 has excellent safety performance at high temperatures.

[0032] Furthermore, any content not described in detail in this specification is existing technology known to those skilled in the art.

[0033] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0034] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a high-safety lithium metal battery separator, characterized in that, The preparation steps include the following: Step 1: Add modified PAA microspheres, polyvinylpyrrolidone, and sodium carboxymethyl cellulose to a mixed solvent, ultrasonically disperse for 20-30 minutes, and then stir with a magnetic stirrer at a speed of 1200-1500 r / min for 3.5-4 hours to obtain microsphere slurry. Step 2: On an automatic blade coating machine, the microsphere slurry is coated on both sides of the separator with a gap of 80-100µm. Then, it is placed in a forced-air drying oven at 45-50℃ for 6-8 hours to obtain a high-safety metal lithium battery separator. The method for preparing the diaphragm is as follows: (1) Place 15-20g of polyvinylidene fluoride and 3.5-6g of polymethyl methacrylate in a vacuum drying oven at 60-70℃ and dry for 10-12h. (2) Pour 60-70 mL of N,N-dimethylformamide into a water bath and heat it to 35-45℃. After stirring mechanically for 8-10 h, add 0.8-1.2 g of modified boehmite and continue stirring. After mixing evenly, let it stand for 40-60 min to obtain the spinning solution. (3) Turn on the heating lamp of the spinning machine, use a syringe to draw the spinning solution, and start spinning when the humidity in the spinning machine chamber drops to 20%RH and the temperature is 40-45℃. After spinning is completed, place it in a vacuum drying oven at 50-60℃ for 10-14 hours to dry, and the diaphragm will be obtained. The preparation method of modified boehmite is as follows: 25-30g of boehmite is placed in a drying oven and dried, then poured into 50-65g of KH550, ultrasonically treated for 20-30min, heated to 130-150℃, and stirred and refluxed at this temperature for 1.5-2h. The product is washed with deionized water until the supernatant is neutral, and then dried in a drying oven at 50-60℃ to obtain modified boehmite. The method for preparing the modified PAA microspheres is as follows: S1. After heating the PAA microspheres in a forced-air oven at 260-300℃ for 1.5-2.5h, immerse them in a sodium hydroxide solution with a concentration of 0.2mol / L for 10-20min. S2. After washing with deionized water 3-5 times, immerse in 3w% acetic acid solution for 20-30 minutes. S3. Remove and wash with deionized water 3-5 times to obtain modified PAA microspheres.

2. The method for preparing a high-safety lithium metal battery separator according to claim 1, characterized in that: The weight proportions of each component added to the lithium metal battery separator are as follows: 8-15 parts modified PAA microspheres, 0.5-2 parts polyvinylpyrrolidone, 0.5-2 parts sodium carboxymethyl cellulose, and 25-45 parts mixed solvent.

3. The method for preparing a high-safety lithium metal battery separator according to claim 1, characterized in that: In step one, the mixed solvent is prepared by mixing deionized water and anhydrous ethanol, and the volume ratio of deionized water to anhydrous ethanol is 1:(0.8-1.5).

4. The method for preparing a high-safety lithium metal battery separator according to claim 1, characterized in that: in, The spinning voltage is 15-25kV, the liquid pushing speed is 0.002-0.004mm / s, and the receiving distance is 16-18cm.

5. The method for preparing a high-safety lithium metal battery separator according to claim 1, characterized in that: The method for preparing the PAA microspheres is as follows: Under ice bath conditions of S11 and 0-2℃, 35-50 mL of N,N-dimethylformamide is poured into a mixed solution of 10-12.5 g of pyromellitic anhydride and 8.5-10 g of 4,4′-diaminodiphenyl ether. S12. After heating to 50-60℃, react at this temperature for 40-48 hours. S13. Electrostatic spraying is performed under the parameters of 15-20kV positive pressure and 4-6kV negative pressure to obtain PAA microspheres.

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