A polyvinylidene fluoride resin, a method for preparing the same, and use thereof

Abstract

This invention discloses a polyvinylidene fluoride (PVDF) resin, its preparation method, and its application. The preparation method includes the following steps: (1) preparing PVDF resin emulsion particles by emulsion polymerization of PVDF monomer; (2) adding a reducing agent and a fluorinated solvent to the obtained PVDF resin emulsion particles; (3) adding PVDF monomer, a dispersant, and an oil-soluble initiator to the emulsion obtained in step (2), and then heating to carry out a suspension polymerization reaction; (4) separating the product after the suspension polymerization is completed to obtain the PVDF resin. The PVDF resin of this invention has high flexibility, high product purity, and high adhesion.

Description

Technical Field

[0001] This invention belongs to the field of adhesives, specifically relating to a polyvinylidene fluoride resin, its preparation method, and its application. Background Technology

[0002] Polyvinylidene fluoride (PVDF) resin refers to homopolymers of vinylidene fluoride (VDF) or copolymers of VDF and vinyl monomers. Due to the high bond energy and short bond length of the carbon-fluorine bonds in PVDF resin, it possesses excellent chemical resistance, corrosion resistance, high-temperature resistance, oxidation resistance, weather resistance, UV resistance, and radiation resistance. PVDF resin is widely used in photovoltaics, aerospace, lithium batteries, semiconductors, water treatment, chemicals, and construction due to its outstanding properties.

[0003] In recent years, with the rapid development of new energy vehicles, the annual output of lithium batteries has continued to climb, and the demand for polyvinylidene fluoride (PVDF) resin, a lithium battery binder, has risen accordingly. In addition to the increased demand, higher requirements are being placed on the performance of PVDF resin. On the one hand, to improve battery safety, the purity of PVDF resin is required to be higher; on the other hand, to meet the requirements for increased energy density, the flexibility and adhesion of PVDF resin are required to be higher.

[0004] Existing polyvinylidene fluoride resins are mainly produced by two processes: emulsion polymerization and suspension polymerization. Polyvinylidene fluoride resins prepared by emulsion polymerization have lower crystallinity and higher product flexibility. When used as a lithium battery binder, it can improve the cracking problem during the winding of lithium electrode sheets. However, because the original particle size of emulsion polymerized products is in the nanometer range, they cannot be used directly and must be coagulated by adding a coagulant. This operation introduces impurities into the product, which are not easy to wash off, posing a potential safety hazard to lithium batteries.

[0005] The polyvinylidene fluoride resin prepared by suspension polymerization has a narrow molecular weight distribution, strong adhesion, but low purity. However, its high crystallinity results in insufficient flexibility.

[0006] Solvay patent US10428211 discloses a physically blended polyvinylidene fluoride resin, which physically mixes polyvinylidene fluoride resin prepared by suspension homopolymerization with polyvinylidene fluoride resin prepared by suspension copolymerization to improve its performance in lithium battery applications.

[0007] Arkema patent CN 107787348 discloses a process for preparing polyvinylidene fluoride resin by blending three components A, B, and C. A comprises a polyvinylidene fluoride (PVDF) homopolymer containing a portion of ultra-high molecular weight chains; B is a PVDF elastomer copolymer and at least one other fluorinated comonomer that can copolymerize with VDF; and C is a VDF homopolymer with a lower viscosity than component A.

[0008] The aforementioned patent aims to improve performance by mixing polyvinylidene fluoride resins with different properties. However, physical blending methods cannot improve the poor flexibility of products prepared by suspension processes and the low purity of products prepared by emulsion processes. At the same time, the performance uniformity of physically blended products is insufficient.

[0009] In conclusion, it is crucial to develop a polyvinylidene fluoride resin with high purity, strong adhesion, and comprehensive performance, as well as its preparation method and applications. Summary of the Invention

[0010] The purpose of this invention is to provide a polyvinylidene fluoride resin with high product purity, strong adhesion and low crystallinity, its preparation method and its application.

[0011] To achieve the above objectives, the present invention provides a method for preparing polyvinylidene fluoride resin, comprising the following steps:

[0012] (1) Polyvinylidene fluoride monomer was used to prepare polyvinylidene fluoride resin emulsion particles by emulsion polymerization.

[0013] (2) Stir the obtained polyvinylidene fluoride resin emulsion particles at low temperature, and then add a reducing agent and a fluorinated solvent to the emulsion.

[0014] (3) Add vinylidene fluoride monomer to the emulsion obtained in step (2), and add dispersant and oil-soluble initiator. Optionally, other comonomers can be added to the reactor, and then the temperature is raised to carry out suspension polymerization reaction.

[0015] (4) After suspension polymerization, the product is separated and post-processed to obtain polyvinylidene fluoride resin.

[0016] In some specific embodiments of the present invention, the method for preparing polyvinylidene fluoride resin emulsion particles in step (1) is as follows: adding polyvinylidene fluoride monomer, emulsifier, initiator, and deionized water into a reaction vessel, and carrying out emulsion polymerization reaction in the reaction vessel.

[0017] In some specific embodiments of the present invention, in the preparation method of polyvinylidene fluoride resin emulsion particles in step (1), the polymerization reaction temperature is 75-120℃.

[0018] In some specific embodiments of the present invention, in the preparation method of polyvinylidene fluoride resin emulsion particles in step (1), the reaction is carried out under stirring conditions, and the stirring speed is 30-90 rpm.

[0019] In some specific embodiments of the present invention, in the method for preparing polyvinylidene fluoride resin emulsion particles, the emulsifier is a water-soluble emulsifier selected from anionic or cationic emulsifiers, including but not limited to sodium perfluoropolyether carboxylate, ammonium perfluoropolyether carboxylate, potassium perfluorooctanoate, ammonium perfluorooctanoate, etc., and the amount of the emulsifier is 0.3-1.4 wt% of the mass of the added polyvinylidene fluoride monomer.

[0020] In some specific embodiments of the present invention, the initiator in step (1) is a persulfate, including but not limited to potassium persulfate, ammonium persulfate, etc., and the amount of the initiator is 0.08-0.6 wt% of the initial vinylidene fluoride monomer mass.

[0021] In some specific embodiments of the present invention, the particle size of the polyvinylidene fluoride resin emulsion in step (1) is 30-200 nm, preferably 30-120 nm.

[0022] In some specific embodiments of the present invention, the temperature of the low-temperature stirring in step (2) is 0-9°C, and the stirring time at low temperature is 20-50 min.

[0023] In some specific embodiments of the present invention, the reducing agent in step (2) is a sulfite or a bisulfite, selected from one or more of sodium metabisulfite, sodium sulfite, sodium bisulfite, ferrous sulfate, potassium sulfite, potassium bisulfite, ammonium bisulfite, and sodium dithionite, and the amount of the reducing agent added is 50-80 wt% of the mass of the initiator added in step (1).

[0024] In some specific embodiments of the present invention, the fluorinated solvent in step (2) is a perfluoroalkane, preferably one or more of perfluoropentane, perfluorohexane, perfluoroheptane, and perfluorooctane, and the amount of the fluorinated solvent added is 12-25% of the mass of vinylidene fluoride monomer.

[0025] In some specific embodiments of the present invention, the amount of vinylidene fluoride monomer added in step (3) is 2.1-3.1 times the mass of vinylidene fluoride added in step (1).

[0026] In some specific embodiments of the present invention, the dispersant in step (3) is one or more of hydroxyethyl cellulose, hydroxypropyl cellulose, sodium carboxymethyl cellulose, and polyvinyl alcohol, and the amount of the dispersant added is 0.08-0.7 wt% of the mass of the added vinylidene fluoride.

[0027] In some specific embodiments of the present invention, the oil-soluble initiator in step (3) is a peroxide initiator or an azo initiator, such as one or more of diethyl percarbonate, dicyclohexyl percarbonate, diisopropyl percarbonate, 2,2′-azobis(4-methoxy-2,4-dimethylpentanonitrile), dilauroyl peroxide, 2,2′-azobis(2,4-dimethylpentanonitrile), azobisisobutyronitrile, and benzoyl peroxide; the amount of oil-soluble initiator added is 0.11-0.85 wt% of the mass of the added vinylidene fluoride.

[0028] In some specific embodiments of the present invention, the other comonomers in step (3) are one or more of methacrylic acid, methyl methacrylate, acrylic acid, hydroxyethyl acrylate, styrene, acrylonitrile, tetrafluoroethylene, trifluorochloroethylene, and hexafluoropropylene, and the amount of the comonomer added is 0.4-5.5% of the mass of the added vinylidene fluoride.

[0029] In some specific embodiments of the present invention, the suspension polymerization reaction temperature in step (3) is 40-62°C.

[0030] In some specific embodiments of the present invention, the stirring speed of the suspension polymerization process in step (3) is 370-700 rpm.

[0031] In some specific embodiments of the present invention, step (4) can be performed by pressure filtration to separate the product, and after separation, the product is washed and dried to obtain polyvinylidene fluoride resin.

[0032] The present invention also provides a polyvinylidene fluoride resin prepared by the preparation method described above.

[0033] Another aspect of the present invention provides the application of the polyvinylidene fluoride resin prepared by the aforementioned preparation method, which can be used as a lithium battery adhesive.

[0034] The beneficial effects of this invention are as follows:

[0035] The polyvinylidene fluoride resin of this invention has low crystallinity and high product flexibility, which can effectively improve the cracking problem during the winding of lithium electrode sheets. Moreover, this invention can agglomerate the latex particles in the suspended particles without adding a coagulant after the latex particles are obtained by emulsion polymerization. This method avoids the introduction of impurities caused by the addition of coagulants and solves the problem of insufficient purity due to many impurities in emulsion polymerization.

[0036] This invention adds a reducing agent to the emulsion after emulsion polymerization to deactivate the residual water-soluble initiator, thus preventing the water-soluble initiator from reacting in the aqueous phase during suspension polymerization and producing low molecular weight resin that affects product performance. At the same time, by adding a fluorinated solvent to the emulsion, the surface of the latex particles swells, allowing the latex particles to exist more stably inside the suspension droplets, thus preventing the latex particles from becoming free and settling.

[0037] This invention prepares highly flexible polyvinylidene fluoride (PVDF) resin emulsion particles through emulsion polymerization, and then performs suspension polymerization on the PVDF resin emulsion particles. The resulting resin has high adhesion and high purity, making it suitable for lithium battery adhesives. Using the PVDF resin of this invention as an adhesive results in high adhesion, making the battery electrodes less prone to cracking, improving safety performance, and giving it high value for industrial applications. Detailed Implementation

[0038] The method of the present invention will be further illustrated below through specific embodiments, but the present invention is not limited to the listed embodiments, and should also include any other known modifications within the scope of the claims of the present invention.

[0039] The main testing methods are as follows:

[0040] Particle size

[0041] The particle size of polyvinylidene fluoride resin particles was tested using a Dandong Bettersize 3000PLUS laser particle size analyzer.

[0042] Bond strength

[0043] The adhesive strength of the polymer after being prepared as an electrode was determined according to ISO 4624 standard (adhesion pull-off test). The electrode preparation process is as follows: At room temperature, 1g of the resin produced in the examples or comparative examples was dissolved in 50g of NMP under mechanical stirring. 2g of conductive carbon black and 30g of lithium cobalt oxide (LiCoO2) were added while stirring, and the mixture was thoroughly mixed to ensure homogeneity. The mixture was then degassed under vacuum and spread onto aluminum foil using a spatula. Finally, the aluminum foil with the electrode material mixture spread on it was placed in a vacuum oven and dried at 60°C for 12 hours to obtain the electrode sheet. The adhesive strength of the electrode material on the electrode sheet was determined using an INSTRON tensile testing machine.

[0044] Crystallinity

[0045] The test was performed using a METTLER DSC-1 differential scanning calorimeter.

[0046] impurity content

[0047] The impurity content of the product was tested by HNMR, and the nuclear magnetic resonance analysis was performed on a Bruker 400 nuclear magnetic resonance spectrometer.

[0048] The main raw material information is as follows:

[0049] Vinylidene fluoride: Wanhua Chemical Group Co., Ltd., industrial product;

[0050] Sodium perfluoropolyether carboxylate: Sigma, analytical grade;

[0051] Ammonium perfluorooctanoate: Sigma, analytical grade;

[0052] Potassium persulfate: Aladdin, analytical grade;

[0053] Ammonium persulfate: Aladdin, analytical grade;

[0054] Sodium metabisulfite: Jinan Jiaxu Chemical Co., Ltd., industrial product;

[0055] Sodium sulfite: Jinan Jiaxu Chemical Co., Ltd., industrial product;

[0056] Perfluoroheptane: Aladdin, analytical grade;

[0057] Perfluorooctane: Aladdin, analytical grade;

[0058] Acrylic acid: Wanhua Chemical Group Co., Ltd., industrial product;

[0059] Hexafluoropropylene: Beijing Innocare, analytical grade;

[0060] Hydroxyethyl cellulose: Merck Chemicals, analytical grade;

[0061] Polyvinyl alcohol: Sigma, analytical grade;

[0062] Azobisisobutyronitrile: Aladdin, analytical grade;

[0063] Diisopropyl peroxide: Aladdin, analytical grade.

[0064] Example 1

[0065] The 5L high-pressure reactor was cleaned and purged. After evacuating the reactor, nitrogen was added to bring it to positive pressure. 1800g of deionized water, 4.08g of sodium perfluoropolyether carboxylate emulsifier, 1.92g of potassium persulfate water-soluble initiator, and 600g of vinylidene fluoride monomer were added to the reactor. The temperature was then raised to 78℃, and the stirring speed was maintained at 34 rpm. After 6 hours of emulsion polymerization, a polyvinylidene fluoride resin emulsion with an average particle size of 75nm was obtained. The temperature inside the reactor was lowered to 2°C and stirred at this temperature for 34 minutes. Then, 1.02 g of sodium metabisulfite and 144 g of perfluoroheptane were added to the reactor. After the temperature inside the reactor was raised to room temperature, 5.47 g of hydroxyethyl cellulose dispersant, 7.63 g of diisopropyl peroxide dicarbonate oil-soluble initiator, 1440 g of vinylidene fluoride monomer and 33.12 g of acrylic acid monomer were added. The temperature was raised to the reaction temperature of 55°C and the stirring speed was maintained at 520 rpm for suspension polymerization. After 5 hours, the temperature was lowered, and then the product was subjected to pressure filtration, washing and drying to obtain polyvinylidene fluoride resin.

[0066] Example 2

[0067] The 5L high-pressure reactor was cleaned and purged thoroughly. After evacuating the reactor, nitrogen was added to bring it to positive pressure. 1800g of deionized water, 1.86g of ammonium perfluorooctanoate emulsifier, 3.42g of ammonium persulfate water-soluble initiator, and 600g of polyvinylidene fluoride monomer were added to the reactor. The temperature was then raised to 120℃, and the stirring speed was maintained at 86 rpm. After 6 hours of emulsion polymerization, a polyvinylidene fluoride resin emulsion with an average particle size of 114nm was obtained. The temperature inside the reactor was lowered to 9°C and stirred at this temperature for 20 minutes. Then, 2.57g of sodium sulfite and 102g of perfluorooctane were added to the reactor as reducing agents. After the temperature inside the reactor was raised to room temperature, 1.62g of polyvinyl alcohol as a dispersant, 14.76g of azobisisobutyronitrile as an oil-soluble initiator, 1800g of vinylidene fluoride monomer and 9.9g of hexafluoropropylene monomer were added. The temperature was raised to the reaction temperature of 43°C and the stirring speed was maintained at 390 rpm for suspension polymerization. After 5 hours, the temperature was lowered, and then the product was subjected to pressure filtration, washing and drying to obtain polyvinylidene fluoride resin.

[0068] Example 3

[0069] The 5L high-pressure reactor was cleaned and purged. After evacuating the reactor, nitrogen was added to bring it to positive pressure. 1800g of deionized water, 7.8g of sodium perfluoropolyether carboxylate emulsifier, 0.48g of potassium persulfate water-soluble initiator, and 600g of vinylidene fluoride monomer were added to the reactor. The temperature was then raised to the reaction temperature of 98℃, and the stirring speed was maintained at 70 rpm. After 6 hours of emulsion polymerization, a polyvinylidene fluoride resin emulsion with an average particle size of 38nm was obtained. The temperature inside the reactor was lowered to 6°C and stirred at this temperature for 48 minutes. Then, 0.31 g of sodium metabisulfite and 78 g of perfluorooctane were added to the reactor as reducing agents. After the temperature inside the reactor was raised to room temperature, 11.59 g of hydroxyethyl cellulose dispersant, 2.02 g of diisopropyl peroxide dicarbonate as an oil-soluble initiator, 1690 g of vinylidene fluoride monomer and 85.68 g of hexafluoropropylene monomer were added. The temperature was raised to the reaction temperature of 62°C and the stirring speed was maintained at 685 rpm for suspension polymerization. After 5 hours, the temperature was lowered, and then the product was subjected to pressure filtration, washing and drying to obtain polyvinylidene fluoride resin.

[0070] Example 4

[0071] The 5L high-pressure reactor was thoroughly cleaned and purged. After evacuating the reactor, nitrogen was added to bring it to positive pressure. 1800g of deionized water, 3.66g of ammonium perfluorooctanoate (PFOA) emulsifier, 0.84g of potassium persulfate (KPS) water-soluble initiator, and 600g of vinylidene fluoride monomer were added to the reactor. The temperature was then raised to 85°C, and the stirring speed was maintained at 55 rpm. After 6 hours of emulsion polymerization, a polyvinylidene fluoride resin emulsion with an average particle size of 68nm was obtained. The temperature inside the reactor was lowered to 5°C and stirred at this temperature for 35 minutes. Then, 0.5g of sodium sulfite and 102g of perfluoroheptane were added to the reactor. After the temperature inside the reactor was raised to room temperature, 2.81g of hydroxyethyl cellulose dispersant, 5.0g of diisopropyl peroxide dicarbonate oil-soluble initiator, 1560g of vinylidene fluoride monomer and 28.08g of acrylic acid monomer were added. The temperature was raised to the reaction temperature of 48°C and the stirring speed was maintained at 440 rpm for suspension polymerization. After 5 hours, the temperature was lowered, and then the product was subjected to pressure filtration, washing and drying to obtain polyvinylidene fluoride resin.

[0072] Comparative Example 1

[0073] The 5L high-pressure reactor was thoroughly cleaned and purged. After evacuating the reactor, nitrogen was added to bring it to positive pressure. 1800g of deionized water, 4.08g of sodium perfluoropolyether carboxylate emulsifier, 1.92g of potassium persulfate water-soluble initiator, and 600g of vinylidene fluoride monomer were added to the reactor. The temperature was then raised to 78℃, and the stirring speed was maintained at 34 rpm for 6 hours to carry out emulsion polymerization, resulting in a polyvinylidene fluoride resin emulsion. 94g of a 5% calcium chloride aqueous solution was added to the emulsion for demulsification and coagulation. The emulsion was then subjected to pressure filtration, washing, and drying to obtain the polyvinylidene fluoride resin product.

[0074] Comparative Example 2

[0075] The 5L high-pressure reactor was thoroughly cleaned and purged. After evacuating the reactor, nitrogen was added to bring it to positive pressure. 1800g of deionized water, 5.47g of hydroxyethyl cellulose dispersant, 7.63g of diisopropyl peroxide dicarbonate oil-soluble initiator, 1440g of vinylidene fluoride monomer, and 33.12g of acrylic acid monomer were added to the reactor. The temperature was raised to 55℃, and the stirring speed was maintained at 520 rpm for suspension polymerization. After 5 hours, the mixture was cooled, and then subjected to pressure filtration, washing, and drying to obtain the polyvinylidene fluoride resin product.

[0076] Comparative Example 3

[0077] The 5L high-pressure reactor was cleaned and purged. After evacuating the reactor, nitrogen was added to bring it to positive pressure. 1800g of deionized water, 4.08g of sodium perfluoropolyether carboxylate emulsifier, 1.92g of potassium persulfate water-soluble initiator, and 600g of vinylidene fluoride monomer were added to the reactor. The temperature was then raised to 78℃, and the stirring speed was maintained at 34 rpm. After 6 hours of emulsion polymerization, a polyvinylidene fluoride resin emulsion with an average particle size of 75nm was obtained. The temperature inside the reactor was lowered to 2°C and stirred at this temperature for 34 minutes. Then, 144g of perfluoroheptane was added to the reactor. After the temperature inside the reactor was raised to room temperature, 5.47g of hydroxyethyl cellulose dispersant, 7.63g of diisopropyl peroxide dicarbonate oil-soluble initiator, 1440g of vinylidene fluoride monomer and 33.12g of acrylic acid monomer were added. The temperature was raised to the reaction temperature of 55°C and the stirring speed was maintained at 520 rpm for suspension polymerization. After 5 hours, the temperature was lowered, and then the product was subjected to pressure filtration, washing and drying to obtain polyvinylidene fluoride resin.

[0078] Comparative Example 4

[0079] The 5L high-pressure reactor was cleaned and purged. After evacuating the reactor, nitrogen was added to bring it to positive pressure. 1800g of deionized water, 4.08g of sodium perfluoropolyether carboxylate emulsifier, 1.92g of potassium persulfate water-soluble initiator, and 600g of vinylidene fluoride monomer were added to the reactor. The temperature was then raised to 78℃, and the stirring speed was maintained at 34 rpm. After 6 hours of emulsion polymerization, a polyvinylidene fluoride resin emulsion with an average particle size of 75nm was obtained. The temperature inside the reactor was lowered to 2°C and stirred at this temperature for 34 minutes. Then, 1.02 g of sodium metabisulfite, a reducing agent, was added to the reactor. After the temperature inside the reactor was raised to room temperature, 5.47 g of hydroxyethyl cellulose, a dispersant, 7.63 g of diisopropyl peroxide, an oil-soluble initiator, 1440 g of vinylidene fluoride monomer, and 33.12 g of acrylic acid monomer were added. The temperature was raised to the reaction temperature of 55°C, and the stirring speed was maintained at 520 rpm for suspension polymerization. After 5 hours, the temperature was lowered, and then the product was subjected to pressure filtration, washing, and drying to obtain the polyvinylidene fluoride resin product.

[0080] The properties of the polyvinylidene fluoride resins obtained in the above embodiments and comparative examples are shown in Table 1 below:

[0081] Table 1 Performance parameters of the examples and comparative examples

[0082]

[0083]

[0084] As can be seen from the table, the polyvinylidene fluoride resin prepared by the method of the present invention has high adhesion, low impurity content, and good overall product performance.

[0085] Those skilled in the art will understand that modifications or adjustments can be made to the present invention based on the teachings of this specification. These modifications or adjustments should also be within the scope defined by the claims of the present invention.

Claims

1. A method for preparing polyvinylidene fluoride resin, characterized in that, Includes the following steps: (1) Polyvinylidene fluoride monomer was used to prepare polyvinylidene fluoride resin emulsion particles by emulsion polymerization; (2) Add a reducing agent and a fluorinated solvent to the obtained polyvinylidene fluoride resin emulsion particles; (3) Add vinylidene fluoride monomer to the emulsion obtained in step (2), and add dispersant and oil-soluble initiator, and then heat up to carry out suspension polymerization reaction; (4) After suspension polymerization, the product is separated to obtain polyvinylidene fluoride resin; The amount of vinylidene fluoride monomer added in step (3) is 2.1-3.1 times the mass of vinylidene fluoride added in step (1).

2. The preparation method according to claim 1, characterized in that, The preparation method of polyvinylidene fluoride resin emulsion particles in step (1) is as follows: add polyvinylidene fluoride monomer, emulsifier, initiator and deionized water into a reaction vessel, and carry out emulsion polymerization reaction in the reaction vessel.

3. The preparation method according to claim 2, characterized in that, In the preparation method of polyvinylidene fluoride resin emulsion particles in step (1), the polymerization reaction temperature is 75-120℃.

4. The preparation method according to claim 2, characterized in that, In the preparation method of polyvinylidene fluoride resin emulsion particles in step (1), the reaction is carried out under stirring conditions, and the stirring speed is 30-90 rpm.

5. The preparation method according to claim 2, characterized in that, The emulsifier is a water-soluble emulsifier, selected from anionic or cationic emulsifiers, and selected from one or more of sodium perfluoropolyether carboxylate, ammonium perfluoropolyether carboxylate, potassium perfluorooctanoate, and ammonium perfluorooctanoate. The amount of the emulsifier is 0.3-1.4 wt% of the mass of the added vinylidene fluoride monomer.

6. The preparation method according to claim 2, characterized in that, The initiator is a persulfate, selected from potassium persulfate and ammonium persulfate, and the amount of the initiator is 0.08-0.6 wt% of the initial vinylidene fluoride monomer mass.

7. The preparation method according to claim 1, characterized in that, The obtained polyvinylidene fluoride resin emulsion has a particle size of 30-200 nm.

8. The preparation method according to claim 7, characterized in that, The obtained polyvinylidene fluoride resin emulsion has a particle size of 30-120 nm.

9. The preparation method according to claim 1, characterized in that, In step (2), the obtained polyvinylidene fluoride resin emulsion particles are stirred at low temperature, and then a reducing agent and a fluorinated solvent are added; the temperature of the low-temperature stirring is 0-9℃, and the stirring time at low temperature is 20-50min.

10. The preparation method according to claim 1, characterized in that, In step (2), the reducing agent is a sulfite or a bisulfite, selected from one or more of sodium metabisulfite, sodium sulfite, sodium bisulfite, ferrous sulfate, potassium sulfite, potassium bisulfite, ammonium bisulfite, and sodium dithionite.

11. The preparation method according to claim 1, characterized in that, The amount of reducing agent added is 50-80 wt% of the mass of the initiator added in step (1).

12. The preparation method according to claim 1, characterized in that, The fluorinated solvent in step (2) is a perfluoroalkane.

13. The preparation method according to claim 12, characterized in that, The fluorinated solvent in step (2) is one or more of perfluoropentane, perfluorohexane, perfluoroheptane, and perfluorooctane.

14. The preparation method according to claim 12, characterized in that, In step (2), the amount of fluorinated solvent added is 12-25% of the mass of vinylidene fluoride monomer.

15. The preparation method according to claim 1, characterized in that, In step (3), the dispersant is one or more of hydroxyethyl cellulose, hydroxypropyl cellulose, sodium carboxymethyl cellulose, and polyvinyl alcohol, and the amount of the dispersant added is 0.08-0.7 wt% of the mass of the added vinylidene fluoride.

16. The preparation method according to claim 1, characterized in that, In step (3), the oil-soluble initiator is a peroxide initiator or an azo initiator.

17. The preparation method according to claim 16, characterized in that, In step (3), the oil-soluble initiator is one or more of the following: diethyl peroxide, dicyclohexyl peroxide, diisopropyl peroxide, 2,2′-azobis(4-methoxy-2,4-dimethylpentanonitrile), dilauroyl peroxide, 2,2′-azobis(2,4-dimethylpentanonitrile), azobisisobutyronitrile, and benzoyl peroxide.

18. The preparation method according to claim 1, characterized in that, The amount of oil-soluble initiator added is 0.11-0.85 wt% of the added vinylidene fluoride.

19. The preparation method according to claim 1, characterized in that, In step (3), optionally, other comonomers are added to the reactor. The other comonomers are one or more of the following: methacrylic acid, methyl methacrylate, acrylic acid, hydroxyethyl acrylate, styrene, acrylonitrile, tetrafluoroethylene, trifluorochloroethylene, and hexafluoropropylene.

20. The preparation method according to claim 19, characterized in that, The amount of comonomer added is 0.4-5.5% of the mass of the added vinylidene fluoride.

21. The preparation method according to claim 1, characterized in that, The suspension polymerization reaction temperature in step (3) is 40-62℃.

22. The preparation method according to claim 1, characterized in that, The stirring speed during the suspension polymerization process in step (3) is 370-700 rpm.

23. The preparation method according to claim 1, characterized in that, Step (4) The product is separated by pressure filtration. After separation, it is washed and dried to obtain polyvinylidene fluoride resin.

24. Polyvinylidene fluoride resin prepared by the preparation method according to any one of claims 1-23.

25. The application of the polyvinylidene fluoride resin prepared by the preparation method according to any one of claims 1-23, wherein it is used as a lithium battery adhesive.

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