A method for producing a crosslinked polyvinylidene fluoride resin powder and use thereof

Abstract

The present application relates to a kind of preparation methods of crosslinked polyvinylidene fluoride resin powder, the preparation method includes the following steps: (1) the raw material polyvinylidene fluoride resin powder with melt viscosity of 1~50kP is carried out irradiation reaction, and the irradiation dose is 50~500kGy;(2) after irradiation, heating is carried out, and the heating temperature is 120~250 ℃, keeps 1~60 min after constant temperature, and is cooled to room temperature at 5~50 ℃ / min cooling rate, and crosslinked polyvinylidene fluoride resin powder is prepared.The raw material polyvinylidene fluoride resin powder of the present application is treated after irradiation crosslinking and recrystallization, appearance does not occur agglomeration, and crosslinking product has the advantages of low melting temperature and low swelling rate simultaneously.The crosslinked polyvinylidene fluoride resin powder prepared in the present application can be applied to lithium ion battery separator as coating.

Description

Technical Field

[0001] This invention relates to the field of polymers, and more specifically to a method for preparing cross-linked polyvinylidene fluoride resin powder and its application. Background Technology

[0002] The separator in a lithium-ion battery is a crucial component for stable battery operation, typically made of polyethylene film, polypropylene film, or a composite of both. Polyolefin separators, due to internal stress during the stretching and pore-forming process, experience significant thermal shrinkage at high temperatures, potentially causing short circuits. Furthermore, their polarity differs from that of organic electrolytes, resulting in poor electrolyte wetting and affecting ion conduction. Surface modification methods, such as coating the separator with inorganic materials, heat-resistant polymers, or mixtures of both, allow the separator to maintain its shape even after reaching the polyolefin softening temperature, preventing short circuits and improving battery safety. Coating the lithium-ion battery base film with PVDF resin powder is a commonly used lithium-ion battery separator. The PVDF coating significantly improves the safety, cycle life, and cell consistency of lithium-ion batteries, while reducing battery expansion and making the battery thinner and stronger.

[0003] Lithium-ion battery separators and positive and negative electrode materials are bonded together by coated PVDF resin particles under pressure and temperature. The bonding properties of PVDF resin include dry adhesion and wet adhesion. Dry adhesion is manifested in the sufficient deformation during the casting and / or compression steps of the fluoropolymer to adhere to the electrodes or separator, and to any inorganic particles in the coating. Wet adhesion is the adhesion in the presence of electrolyte and is a key performance characteristic during normal battery use. PVDF resin swells under the influence of electrolyte, absorbing the flowing electrolyte. The higher the electrolyte temperature, the greater the swelling; severe swelling can even lead to dissolution, affecting the battery's electrical performance. Therefore, obtaining PVDF resin with high adhesion and low swelling properties is crucial for use as a lithium-ion battery binder.

[0004] Conventional PVDF resin is a semi-crystalline polymer; the lower the melting temperature, the lower the softening temperature, making it easy to deform during roll forming and exhibiting good dry adhesion. However, the lower the melting temperature, the worse the swelling performance, affecting the battery's electrical performance. To obtain PVDF resin with high adhesion and low swelling, existing technologies employ the following solutions:

[0005] Patent CN112055883A discloses a fluoropolymer coating composition comprising a low-swelling, high-crystallinity fluoropolymer phase with good wet adhesion and mechanical properties, and a softer, higher-swelling functional fluoropolymer phase with good dry adhesion. Electrodes and / or separators coated with this composition exhibit excellent wet adhesion, excellent dry adhesion, and low leaching. However, this composition cannot simultaneously improve dry adhesion and reduce swelling, and its high melting temperature necessitates roll forming at even higher temperatures during battery assembly.

[0006] Patent CN108148210A discloses a method for preparing an intramolecular cross-linked polymer, the method comprising: subjecting polyvinylidene fluoride (PVDF) to an intramolecular cross-linking reaction in a polar organic solvent under gamma-ray irradiation. The PVDF intramolecular cross-linked polymer obtained by this method has low viscosity.

[0007] Wang Guoying et al. (Melting and Recrystallization of Irradiated Polyethylene, Applied Chemistry, December 1995, Vol. 12, No. 6) disclosed the use of wide-angle X-ray diffraction (WAXD) to investigate the melting and recrystallization of irradiated polyethylene. Irradiation destroyed the crystalline structure of polyethylene, causing its melting temperature and crystallinity to decrease with increasing irradiation dose. Recrystallizing irradiated polyethylene showed a significant decrease in recrystallization degree and melting temperature with increasing irradiation dose. However, this paper did not disclose the possibility that wide-angle X-ray diffraction (WAXD) could improve adhesion and reduce swelling. Summary of the Invention

[0008] This invention aims to provide a method for preparing cross-linked polyvinylidene fluoride (PVDF) resin powder. The cross-linked PVDF resin powder obtained by this method has a low melting temperature, enabling roll forming at lower temperatures and providing good dry adhesion; it also has a low electrolyte swelling rate, reducing absorption by the fluid electrolyte; the cross-linked structure ensures that the PVDF resin powder will not dissolve in high-temperature electrolytes, ensuring adhesion at high temperatures. This cross-linked PVDF resin simultaneously improves adhesion and swelling properties.

[0009] The technical solution of the present invention is as follows:

[0010] A method for preparing cross-linked polyvinylidene fluoride resin powder, the method comprising the following steps:

[0011] (1) Irradiate the raw material polyvinylidene fluoride resin powder with a melt viscosity of 1-50 kP to a dose of 50-500 kGy.

[0012] (2) After irradiation, the product is heated to 120-250°C and kept at a constant temperature for 1-60 minutes. Then, it is cooled to room temperature at a cooling rate of 5-50°C / min to obtain cross-linked polyvinylidene fluoride resin powder.

[0013] The polyvinylidene fluoride resin powder used in this invention is selected from one of polyvinylidene fluoride homopolymer and / or copolymer of polyvinylidene fluoride and halogenated olefins; preferably, the polyvinylidene fluoride resin powder used in this invention is selected from copolymer of polyvinylidene fluoride and halogenated olefins.

[0014] The copolymer of vinylidene fluoride and halogenated olefins described in this invention can be a binary copolymer, a ternary copolymer, or a multi-component copolymer. The halogenated olefin is selected from one or more of hexafluoropropylene, trifluoroethylene, trifluorochloroethylene, tetrafluoroethylene, chlorofluoroethylene, difluorochloroethylene, fluoroethylene, vinyl chloride, vinylidene chloride, tetrachloroethylene, pentafluoropropylene, 3,3,3-trifluoropropylene, 2,3,3,3-tetrafluoropropylene, 1,3,3,3-tetrafluoropropylene, or chlorotrifluoropropylene; preferably, the halogenated olefin is selected from one or more of hexafluoropropylene, trifluoroethylene, trifluorochloroethylene, or tetrafluoroethylene; more preferably, the halogenated olefin is selected from one or more of hexafluoropropylene, trifluoroethylene, or trifluorochloroethylene; even more preferably, the halogenated olefin is selected from hexafluoropropylene.

[0015] The molar ratio of vinylidene fluoride monomer in the polyvinylidene fluoride resin powder of the present invention is 80-99.99%; more preferably, the molar ratio of vinylidene fluoride monomer is 90-99%; and even more preferably, the molar ratio of vinylidene fluoride monomer is 97.9-99%.

[0016] The polyvinylidene fluoride resin powder used in this invention requires a high melt viscosity to prevent agglomeration during high-temperature melting. The melt viscosity is tested using a capillary rheometer, according to GB / T 25278, at 230℃ and 100s. -1 The following test was conducted. The raw material, polyvinylidene fluoride resin powder, was subjected to a test at 230°C for 100 seconds. -1 The melt viscosity under the test conditions is 1–50 kPa; preferably, at 230 °C and 100 s. -1 The melt viscosity under the test conditions is 4–45 kPa; more preferably, at 230 °C and 100 s... -1 The melt viscosity under the test conditions was 10–40 kPa; more preferably, at 230 °C and 100 s... -1 The melt viscosity under the test conditions was 20–35 kPa.

[0017] The polyvinylidene fluoride resin powder used in this invention is selected from at least one of extrusion granulation crushing powder, suspension polymerization crushing powder, and emulsion polymerization coagulation powder; preferably, the polyvinylidene fluoride resin powder is selected from emulsion polymerization coagulation powder, which is resin powder obtained by emulsion polymerization coagulation and prepared according to conventional emulsion polymerization methods in the art.

[0018] The polyvinylidene fluoride (PVDF) resin powder and the cross-linked PVDF resin powder described in this invention are spherical in appearance. The cross-linked PVDF resin powder obtained by this invention does not clump or adhere. If the cross-linked PVDF resin powder clumps or adheres together, even after crushing, the resulting powder will not be spherical, thus affecting the application of lithium-ion battery separator coatings, leading to a significant reduction in porosity and severely impacting lithium-ion transport.

[0019] The polyvinylidene fluoride resin powder used in this invention is a semi-crystalline polymer, i.e., a partially crystalline polymer. When the irradiation dose is controlled at 50–500 kGy, cross-linking reactions will not occur in the crystalline regions of the polyvinylidene fluoride resin powder. At this time, the energy provided by the irradiation dose is lower than the energy required to destroy the crystalline morphology, and the cross-linking reaction only occurs in the non-crystalline regions. Preferably, the irradiation dose is 80–300 kGy; more preferably, it is 150–300 kGy.

[0020] In the heating recrystallization process of step (2) of this invention, the irradiation in step (1) causes cross-linking of the macromolecular chains, reducing the chain segment mobility and increasing crystal defects during recrystallization, thus further lowering the melting point. Through the heating recrystallization treatment in step (2), the melting point of the obtained cross-linked polyvinylidene fluoride resin powder is further reduced, and its dry adhesion is further improved. At the same time, the cross-linking structure generated in the non-crystalline region can improve the swelling performance.

[0021] In step (2) of the present invention, the heating method can be any way of generating heat, such as oven heating, infrared heating, or fluidized bed heating; preferably, the heating method is oven heating or fluidized bed heating.

[0022] In step (2) of the present invention, the heating temperature is 120-250°C depending on the melting temperature of the raw material polyvinylidene fluoride resin; preferably, the heating temperature is 140-200°C.

[0023] In step (2) of this invention, the constant temperature is maintained to allow the cross-linked polyvinylidene fluoride resin powder to recrystallize completely. The constant temperature is maintained for 1 to 60 minutes, and then cooled to room temperature at a cooling rate of 5 to 50°C / min. Preferably, the constant temperature is maintained for 6 to 30 minutes, and then cooled to room temperature at a cooling rate of 20 to 40°C / min.

[0024] Step (1) of the present invention can be carried out at room temperature or at a temperature that does not affect the morphology of the raw material polyvinylidene fluoride resin powder or the irradiation reaction of the raw material polyvinylidene fluoride resin powder; preferably, it is carried out at room temperature.

[0025] Step (1) of the present invention is carried out in an inert atmosphere, which means that it does not participate in the irradiation reaction and does not have any effect on the irradiation reaction of the raw material polyvinylidene fluoride resin powder, such as nitrogen, helium, etc.; preferably, the inert atmosphere is nitrogen.

[0026] The irradiation source in step (1) of this invention can be a radiation source that generates γ-rays, electron beams, X-rays, β-rays, etc., such as Co-60 or Cs-137 that generate γ-rays.

[0027] This invention also provides an application of cross-linked polyvinylidene fluoride (PVDF) resin powder, which is used as a coating in lithium-ion battery separators. The cross-linked PVDF resin powder and additives are mixed using an aqueous coating process to prepare an aqueous slurry, which is then coated onto one or both sides of a base membrane to obtain a lithium-ion battery separator. The aqueous coating process uses water as the dispersion medium, resulting in uniform dispersion of PVDF resin particles on the surface of the lithium-ion battery separator after coating.

[0028] The cross-linked polyvinylidene fluoride resin powder obtained by this invention is suitable for water-based coating processes.

[0029] The technical solution of the present invention has the following technical advantages compared with the prior art:

[0030] (1) The preparation method of the present invention does not damage the appearance of the raw material polyvinylidene fluoride resin powder, and the obtained cross-linked polyvinylidene fluoride resin powder does not clump or stick.

[0031] (2) The present invention performs a recrystallization process after irradiation crosslinking, which can not only further reduce the melting point and improve the adhesion performance, but also improve the swelling performance by generating a crosslinked structure in the non-crystalline region during the irradiation process. At the same time, it improves both adhesion and swelling, effectively solving the problem of neglecting one aspect in the prior art. Attached Figure Description

[0032] Figure 1 This is a scanning electron microscope image of the cross-linked polyvinylidene fluoride copolymer BII-5 prepared in Example 5 of the present invention. Detailed Implementation

[0033] The present invention will be further described below with reference to specific embodiments, but the invention is not limited to these specific embodiments. Those skilled in the art should recognize that the present invention covers all alternatives, improvements, and equivalents that may be included within the scope of the claims.

[0034] The polyvinylidene fluoride resin powder used in the embodiments and comparative examples of this invention is as follows:

[0035] (1) Polyvinylidene fluoride copolymer A (hexafluoropropylene monomer molar ratio of 1%): Poly(vinylidene fluoride-hexafluoropropylene) resin powder with a hexafluoropropylene monomer molar ratio of 1% was obtained by emulsion polymerization of vinylidene fluoride and hexafluoropropylene, and the melt viscosity was 40kP.

[0036] (2) Polyvinylidene fluoride copolymer B (hexafluoropropylene monomer molar ratio of 2.1%): Poly(vinylidene fluoride-hexafluoropropylene) resin powder with a hexafluoropropylene monomer molar ratio of 2.1% was obtained by emulsion polymerization of vinylidene fluoride and hexafluoropropylene, and the melt viscosity was 32kP.

[0037] (3) Polyvinylidene fluoride copolymer C (hexafluoropropylene monomer molar ratio of 4.4%): Poly(vinylidene fluoride-hexafluoropropylene) resin powder with a hexafluoropropylene monomer molar ratio of 4.4% was obtained by emulsion polymerization of vinylidene fluoride and hexafluoropropylene, and the melt viscosity was 25kP.

[0038] (4) Polyvinylidene fluoride copolymer D (hexafluoropropylene monomer molar ratio of 5.7%): Poly(vinylidene fluoride-hexafluoropropylene) resin powder with a hexafluoropropylene monomer molar ratio of 5.7% was obtained by emulsion polymerization of vinylidene fluoride and hexafluoropropylene, and the melt viscosity was 19kP.

[0039] (5) Polyvinylidene fluoride copolymer E (hexafluoropropylene monomer molar ratio of 10%): Poly(vinylidene fluoride-hexafluoropropylene) resin powder with a hexafluoropropylene monomer molar ratio of 10% is obtained by emulsion polymerization of vinylidene fluoride and hexafluoropropylene, and the melt viscosity is 15kP.

[0040] In the embodiments and comparative examples of this invention, the method for testing the molar ratio of hexafluoropropylene in the polyvinylidene fluoride copolymer is as follows: 20 mg of polyvinylidene fluoride resin powder is dissolved in deuterated DMSO and dissolved at 60°C for 24 h. Nuclear magnetic resonance spectroscopy is then used to analyze the solution. 19 F test, spectrum integration to obtain the peak area S1 of -CF3 and the peak area S2 of -CF2-, the molar ratio of hexafluoropropylene is calculated based on the peak area ratio 2S1 / 3S2.

[0041] Example 1

[0042] At room temperature, 1 kg of polyvinylidene fluoride copolymer B (HFP molar ratio of 2.1%) powder was placed in an aluminum foil bag, and after the air was replaced with nitrogen, it was irradiated. The irradiation source was Co-60 and the irradiation dose was 50 kGy to obtain cross-linked polyvinylidene fluoride copolymer BI-1.

[0043] Cross-linked polyvinylidene fluoride copolymer BI-1 was heated to 180℃, held at that temperature for 10 min, and then cooled to room temperature at a cooling rate of 10℃ / min to obtain cross-linked polyvinylidene fluoride copolymer BII-1.

[0044] Example 2

[0045] At room temperature, 1 kg of polyvinylidene fluoride copolymer B (HFP molar ratio of 2.1%) powder was placed in an aluminum foil bag, and after the air was replaced with nitrogen, it was irradiated. The irradiation source was Co-60 and the irradiation dose was 80 kGy to obtain cross-linked polyvinylidene fluoride copolymer BI-2.

[0046] Cross-linked polyvinylidene fluoride copolymer BI-2 was heated to 175℃, held at that temperature for 6 min, and then cooled to room temperature at a cooling rate of 20℃ / min to obtain cross-linked polyvinylidene fluoride copolymer BII-2.

[0047] Example 3

[0048] At room temperature, 1 kg of polyvinylidene fluoride copolymer B (HFP molar ratio of 2.1%) powder was placed in an aluminum foil bag, and after the air was replaced with nitrogen, it was irradiated. The irradiation source was Co-60 and the irradiation dose was 150 kGy to obtain cross-linked polyvinylidene fluoride copolymer BI-3.

[0049] Cross-linked polyvinylidene fluoride copolymer BI-3 was heated to 160℃, held at that temperature for 15 min, and then cooled to room temperature at a cooling rate of 20℃ / min to obtain cross-linked polyvinylidene fluoride copolymer BII-3.

[0050] Example 4

[0051] At room temperature, 1 kg of polyvinylidene fluoride copolymer B (HFP molar ratio of 2.1%) powder was placed in an aluminum foil bag, and after the air was replaced with nitrogen, it was irradiated. The irradiation source was Co-60 and the irradiation dose was 200 kGy to obtain cross-linked polyvinylidene fluoride copolymer BI-4.

[0052] Cross-linked polyvinylidene fluoride copolymer BI-4 was heated to 150℃, held at that temperature for 20 min, and then cooled to room temperature at a rate of 40℃ / min to obtain cross-linked polyvinylidene fluoride copolymer BII-4.

[0053] Example 5

[0054] At room temperature, 1 kg of polyvinylidene fluoride copolymer B (HFP molar ratio of 2.1%) powder was placed in an aluminum foil bag, and after the air was replaced with nitrogen, it was irradiated. The irradiation source was Co-60 and the irradiation dose was 300 kGy to obtain cross-linked polyvinylidene fluoride copolymer BI-5.

[0055] Cross-linked polyvinylidene fluoride copolymer BI-5 was heated to 150℃, held at that temperature for 15 min, and then cooled to room temperature at a cooling rate of 40℃ / min to obtain cross-linked polyvinylidene fluoride copolymer BII-5.

[0056] Example 6

[0057] At room temperature, 1 kg of polyvinylidene fluoride copolymer A (HFP molar ratio of 1%) powder was placed in an aluminum foil bag, and after the air was replaced with nitrogen, it was irradiated. The irradiation source was Co-60, and the irradiation dose was 200 kGy, to obtain cross-linked polyvinylidene fluoride copolymer AI-1.

[0058] Cross-linked polyvinylidene fluoride copolymer AI-1 was heated to 170℃, held at that temperature for 10 min, and then cooled to room temperature at a cooling rate of 40℃ / min to obtain cross-linked polyvinylidene fluoride copolymer AII-1.

[0059] Example 7

[0060] Similar to Example 6, except that the raw material polyvinylidene fluoride copolymer is polyvinylidene fluoride copolymer C (HFP molar ratio is 4.4%), to obtain cross-linked polyvinylidene fluoride copolymer CI-1.

[0061] Cross-linked polyvinylidene fluoride copolymer CI-1 was heated to 140℃, held at that temperature for 15 min, and then cooled to room temperature at a cooling rate of 40℃ / min to obtain cross-linked polyvinylidene fluoride copolymer CII-1.

[0062] Example 8

[0063] Similar to Example 6, except that the raw material is polyvinylidene fluoride copolymer D (the molar ratio of HFP is 5.7%), to obtain cross-linked polyvinylidene fluoride copolymer DI-1.

[0064] Cross-linked polyvinylidene fluoride copolymer DI-1 was heated to 125℃, held at that temperature for 7 min, and then cooled to room temperature at a cooling rate of 40℃ / min to obtain cross-linked polyvinylidene fluoride copolymer DII-1.

[0065] Example 9

[0066] Similar to Example 6, except that the raw material is polyvinylidene fluoride copolymer E (HFP molar ratio is 10%), which yields crosslinked polyvinylidene fluoride copolymer EI-1.

[0067] Cross-linked polyvinylidene fluoride copolymer EI-1 was heated to 110℃, held at that temperature for 30 min, and then cooled to room temperature at a rate of 40℃ / min to obtain cross-linked polyvinylidene fluoride copolymer EII-1.

[0068] Comparative Example 1

[0069] At room temperature, 1 kg of polyvinylidene fluoride copolymer B (HFP molar ratio of 2.1%) powder was directly heated to 160℃ without irradiation, held at the temperature for 15 min, and then cooled to room temperature at a cooling rate of 20℃ / min to obtain polyvinylidene fluoride copolymer BI.

[0070] Comparative Example 2

[0071] At room temperature, 1 kg of polyvinylidene fluoride copolymer C (HFP molar ratio of 4.4%) powder was directly heated to 140℃ without irradiation, held at the temperature for 15 min, and then cooled to room temperature at a cooling rate of 40℃ / min to obtain polyvinylidene fluoride copolymer CI.

[0072] Comparative Example 3

[0073] At room temperature, 1 kg of polyvinylidene fluoride copolymer D (HFP molar ratio of 5.7%) powder was directly heated to 125℃ without irradiation, held at the temperature for 7 min, and then cooled to room temperature at a cooling rate of 40℃ / min to obtain polyvinylidene fluoride copolymer DI.

[0074] Test case

[0075] Melting point tests were performed on copolymers BI-1, BI-2, BI-3, BI-4, BI-5, AI-1, CI-1, DI-1, and EI-1 after radiation crosslinking reaction in Examples 1-9, and copolymers BII-1, BII-2, BII-3, BII-4, BII-5, AII-1, CII-1, DII-1, and EII-1 after recrystallization reaction, as well as polyvinylidene fluoride copolymers B, C, D, BI, CI, and DI in Comparative Examples 1-3. Swelling rates were also tested at 25°C, 45°C, and 65°C for copolymers BII-1, BII-2, BII-3, BII-4, BII-5, AII-1, CII-1, DII-1, EII-1, and BI, CI, and DI. The particle appearance was also observed. The results are shown in Table 1 below.

[0076] Table 1. Results of copolymer melting point, swelling ratio and particle appearance tests

[0077]

[0078] In Table 1 above:

[0079] (1) Melting point 1 Test method: The melting point is determined by differential scanning calorimetry (DSC) according to GB / T 19466. The melting point is selected from the melting peak data of the single heating curve at a heating rate of 10℃ / min.

[0080] (2) Swelling rate 2Test method: The swelling rate of polyvinylidene fluoride (PVDF) resin powder is tested. Liquid on the surface of the particles is difficult to remove, and the swollen and softened particles cannot be filtered or centrifuged to remove the solvent between the pores. The swelling rate is tested after the powder is molded into sheets. The molding temperature is set at 250℃, preheated for 10 min, then pressurized to 10 MPa, stabilized for 5 min, and then cooled to room temperature. The cooling rate is consistent with the recrystallization cooling rate. The electrolyte is composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) in a mass ratio of 1:1:1. A 20*10*1mm PVDF resin molded sheet (m1 g) is cut and immersed in the electrolyte at different temperatures for 24 h. After removal, excess solvent is wiped off, and its mass (m2 g) is accurately weighed. The swelling rate is calculated by dividing the mass change (m2-m1) by m1.

[0081] (3) Particle appearance 3 Test method: Using a scanning electron microscope, after gold sputtering, the morphology of the primary particles of the powder was observed at a magnification of 50,000.

[0082] Table 1 shows that after irradiation crosslinking, the PVDF resin powder does not melt and agglomerate during recrystallization, maintaining its powder morphology. Increasing the cooling rate can further lower the melting point. Compared with existing technologies, the crosslinked polyvinylidene fluoride resin powder obtained by the method of this invention exhibits a lower melting point and a smaller swelling rate.

Claims

1. A method for preparing cross-linked polyvinylidene fluoride copolymer resin powder, characterized in that: The preparation method includes the following steps: (1) Irradiate the raw material polyvinylidene fluoride copolymer resin powder with a melt viscosity of 1-50 kP to a dose of 50-500 kGy. (2) After irradiation, heating is performed at a temperature of 120–250°C. The temperature is maintained for 1–60 min, and then cooled to room temperature at a rate of 5–50°C / min to obtain cross-linked polyvinylidene fluoride copolymer resin powder. The raw material, polyvinylidene fluoride copolymer, is a copolymer of vinylidene fluoride and hexafluoropropylene, and the molar ratio of vinylidene fluoride monomer in the copolymer is 90-99%.

2. The method for preparing cross-linked polyvinylidene fluoride copolymer resin powder according to claim 1, characterized in that: The melt viscosity of the raw material, polyvinylidene fluoride copolymer resin powder, is 20-35 kPa.

3. The method for preparing cross-linked polyvinylidene fluoride copolymer resin powder according to claim 1, characterized in that: The raw material, polyvinylidene fluoride copolymer resin powder, is a resin powder obtained through emulsion polymerization and coagulation.

4. The method for preparing cross-linked polyvinylidene fluoride copolymer resin powder according to claim 1, characterized in that: The cross-linked polyvinylidene fluoride copolymer resin powder has a spherical appearance.

5. The method for preparing cross-linked polyvinylidene fluoride copolymer resin powder according to claim 1, characterized in that: The irradiation dose is 150–300 kGy.

6. The method for preparing cross-linked polyvinylidene fluoride copolymer resin powder according to claim 1, characterized in that: In step (2), the heating temperature is 140-200℃, the temperature is kept constant for 6-30 minutes, and the temperature is cooled to room temperature at a rate of 20-40℃ / min.

7. An application of cross-linked polyvinylidene fluoride copolymer resin powder as a coating for lithium-ion battery separators, characterized in that: The cross-linked polyvinylidene fluoride copolymer resin powder is obtained by the preparation method of cross-linked polyvinylidene fluoride copolymer resin powder according to any one of claims 1-6.

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