Fluorine-containing polymer resin powder for diaphragm coating as well as preparation method and application of fluorine-containing polymer resin powder

Abstract

The invention discloses fluorine-containing polymer resin powder for a diaphragm coating, the fluorine-containing polymer resin powder comprises polyvinylidene fluoride and a water-based binder, the mass percentage of the water-based binder relative to the mass percentage of the polyvinylidene fluoride is 1-8%, the average particle size D50 of the fluorine-containing polymer resin powder is 3-15 [mu] m, the average particle size D5 is greater than 1.5 [mu] m, and the average particle size D5 is greater than 1.5 [mu] m. When the fluorine-containing polymer resin powder is used for preparing the diaphragm coating aqueous slurry, the aqueous binder is not dissolved in water. The fluorine-containing polymer resin powder disclosed by the invention is controllable in powder particle size and low in content of small-particle-size aggregates, and is particularly suitable for preparing aqueous slurry for coating a lithium battery diaphragm.

Description

Technical Field

[0001] This invention relates to the field of polymers, and more specifically to a fluoropolymer resin powder for diaphragm coating, its preparation method, and its application. Background Technology

[0002] Coating lithium-ion battery separators with PVDF resin powder can significantly improve the safety, cycle life, and cell consistency of lithium-ion batteries, while reducing battery expansion rate and making the batteries thinner and stronger. PVDF coating of lithium-ion battery separators commonly employs both oil-based and water-based coating processes. Water-based coating processes use water-based slurries prepared by mixing PVDF powder with additives and water.

[0003] PVDF powder is typically obtained through spray drying. Patent document CN115353750A discloses a method for preparing polymer powder, spray drying equipment, battery separators, and secondary batteries, where the PVDF powder is prepared by direct spray drying of an emulsion. Patent document CN112055883A describes, in its application examples, the spray drying of PVDF-based latex into fine powder with an average particle size of 1–30 μm. PVDF emulsions contain various organic additives; during spray drying, some of these additives volatilize, forming waste gas, while others remain in the PVDF powder. The fluorinated additives are highly toxic and bioaccumulative, and cannot be effectively recovered or degraded as waste gas.

[0004] Demulsifying and washing PVDF emulsions removes most of the additives, reducing impurities and contributing to battery stability. However, after demulsification and washing, the PVDF polymer floats on the water as a slurry, exhibiting a paste-like consistency and unable to flow. Patent document CN102453166A utilizes mechanical stirring and aeration, along with hot water washing, to remove reaction additives such as surfactants, co-emulsifiers, and molecular weight regulators adhering to the polymer particles, resulting in a high-quality PVDF polymer.

[0005] After post-processing steps such as demulsification and washing, PVDF emulsions result in a slurry that separates from water, exhibiting poor flowability. During spray drying granulation, this slurry layering can clog the pipes, preventing the formation of large-diameter particles and resulting in a wide particle size distribution. Furthermore, the demulsification of PVDF emulsions creates numerous small-diameter agglomerates, which, after spray drying, form small particles that can clog membrane pores, increasing permeability without contributing to the peel strength of the lithium-ion battery separator. Summary of the Invention

[0006] To address the issue of wide particle size distribution and high content of small-diameter powder in PVDF powder obtained by direct drying after demulsification and washing of PVDF emulsion, this invention provides a fluoropolymer resin powder for diaphragm coating. The fluoropolymer resin powder comprises polyvinylidene fluoride (PVDF) and an aqueous binder. The aqueous binder stabilizes the PVDF slurry after demulsification and washing, preventing stratification. The resulting fluoropolymer resin powder, after drying, has a narrow particle size distribution and low content of small-diameter powder.

[0007] During the demulsification and washing process of polyvinylidene fluoride (PVDF) emulsion, latex particles agglomerate to form secondary particles. Agitation introduces a large amount of air into these secondary particles, making them lighter than water. After agitation, these PVDF secondary particles float on the surface, forming a poorly flowing paste. This is why the PVDF slurry after demulsification and washing is prone to stratification. Although the drying process exerts some shearing force on the slurry, it cannot eliminate all large-diameter particles.

[0008] When the polyvinylidene fluoride slurry after demulsification and washing is mixed with an aqueous binder, the aqueous binder adsorbs onto the secondary particles during high-speed dispersion, playing a role in dispersion and thickening, improving the stability of the slurry, resulting in a narrow particle size distribution and a low content of small-diameter particles; at the same time, it can also reduce the risk of sticking to the pipe walls during pipeline transportation.

[0009] First, the present invention provides a fluoropolymer resin powder for diaphragm coating, wherein the fluoropolymer resin powder comprises polyvinylidene fluoride and an aqueous binder.

[0010] The water-based adhesive has a mass percentage of 1-8% relative to the mass of polyvinylidene fluoride.

[0011] The average particle size D50 of the fluoropolymer resin powder is 3–15 μm, where D5 > 1.5 μm.

[0012] In the process of preparing the aqueous slurry for the diaphragm coating, the aqueous binder of the fluoropolymer resin powder is insoluble in water.

[0013] The mass percentage of the water-based binder relative to the mass of polyvinylidene fluoride is 1-8%. If the amount of water-based binder added is greater than 8%, the particle size of the dried powder will be large, resulting in a thick coating on the diaphragm and low adhesion. If the amount of water-based binder added is less than 1%, the slurry stability will be poor, which is not conducive to drying. In addition, the water-based binder has a poor bonding effect on PVDF secondary particles, and there are a large number of small-diameter particles in the dried powder.

[0014] The water-based adhesive is a water-based adhesive available in the art. Preferably, the water-based adhesive is selected from at least one of polyvinyl alcohol, carboxymethyl cellulose, acrylonitrile copolymer, styrene-butadiene rubber, and acrylic copolymer. More preferably, the water-based adhesive is selected from at least one of polyvinyl alcohol and acrylonitrile copolymer.

[0015] In one embodiment, the water-based adhesive is polyvinyl alcohol, and the degree of hydrolysis of the polyvinyl alcohol is greater than or equal to 90%, preferably, the degree of hydrolysis of the polyvinyl alcohol is greater than or equal to 98%.

[0016] In another embodiment, the water-based binder is an acrylonitrile copolymer having crosslinking groups, which forms a crosslinked polymer after drying.

[0017] The polyvinylidene fluoride (PVDF) contains 70-98% PVDF monomer units and 2-30% comonomer units; preferably, the PVDF contains 80-97% PVDF monomer units; more preferably, the PVDF contains 85-96% PVDF monomer units.

[0018] The mass ratio of the polyvinylidene fluoride comonomer units is 2-30%; preferably, the mass ratio of the polyvinylidene fluoride comonomer units is 3-20%; more preferably, the mass ratio of the polyvinylidene fluoride comonomer units is 4-15%.

[0019] The comonomer is selected from at least one 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, and chlorotrifluoropropylene; preferably, the comonomer is selected from at least one of hexafluoropropylene, tetrafluoroethylene, trifluoroethylene, and trifluorochloroethylene; more preferably, the comonomer is selected from at least one of hexafluoropropylene and tetrafluoroethylene.

[0020] In one embodiment, the polyvinylidene fluoride is a mixture of polyvinylidene fluoride copolymers. The mixture can be obtained by emulsion mixing or by mixing materials after demulsification; preferably, the mixture is obtained by emulsion mixing.

[0021] Second, the present invention provides a method for preparing fluoropolymer resin powder for diaphragm coating, comprising the following steps:

[0022] S1: After demulsification and washing, polyvinylidene fluoride emulsion is diluted with water to obtain a first slurry with a polyvinylidene fluoride mass percentage of 5-15%.

[0023] S2: The first slurry is mixed with an aqueous binder to obtain a second slurry, wherein the mass percentage of the aqueous binder relative to the mass percentage of polyvinylidene fluoride is 1-8%;

[0024] S3: Disperse the second slurry to obtain a third slurry with an average particle size D50 of 2-10 μm;

[0025] S4: Dry the third slurry to obtain a fluoropolymer resin powder with an average particle size D50 of 3-15 μm and D5 > 1.5 μm.

[0026] Furthermore, in step S1, the conductivity of the first slurry is less than 5 μS / cm, preferably less than 2 μS / cm.

[0027] Furthermore, in step S2, the aqueous adhesive is an aqueous solution or an emulsion.

[0028] The water-based adhesive is a water-based adhesive available in the art. Preferably, the water-based adhesive is selected from at least one of polyvinyl alcohol, carboxymethyl cellulose, acrylonitrile copolymer, styrene-butadiene rubber, and acrylic copolymer. More preferably, the water-based adhesive is selected from at least one of polyvinyl alcohol and acrylonitrile copolymer.

[0029] In one embodiment, the water-based adhesive is polyvinyl alcohol, and the degree of hydrolysis of the polyvinyl alcohol is greater than or equal to 90%, preferably, the degree of hydrolysis of the polyvinyl alcohol is greater than or equal to 98%.

[0030] In another embodiment, the water-based binder is an acrylonitrile copolymer having crosslinking groups, which forms a crosslinked polymer after drying.

[0031] Furthermore, in step S3, the dispersion is carried out using a high-speed disperser, and the dispersion disc is a serrated disc.

[0032] The purpose of step S3 is to disperse the large-diameter particles that aggregate secondary particles, and to prevent the formation of large-diameter particles during the drying step.

[0033] The average particle size D50 of the third slurry is 2 to 10 μm, preferably 3 to 8 μm.

[0034] Further, in step S4, the drying is performed using spray drying or airflow drying; preferably, the drying is performed using spray drying. The spray drying can be performed using centrifugal spray drying, airflow spray drying, or pressure spray drying, preferably using centrifugal spray drying or airflow spray drying.

[0035] The average particle size D50 of the fluoropolymer resin powder is 3-15 μm, and D5 > 1.5 μm; preferably, the average particle size D50 is 4-9 μm, and D5 > 2 μm. D5 indicates that 5% of the powder particles have a diameter smaller than this size. For example, D5 > 1.5 μm means that 5% of the powder particles have a diameter smaller than 1.5 μm.

[0036] In one implementation, the order of steps S1 and S2 can be reversed, that is, step S2 is placed before step S1.

[0037] Third, the present invention provides an application of fluoropolymer resin powder for separator coating, wherein the fluoropolymer resin powder is used to prepare an aqueous slurry for lithium battery separator coating.

[0038] The fluoropolymer resin powder contains a hydrophilic binder, which can shorten the preparation time of the water-based slurry.

[0039] Fourth, the present invention provides a method for preparing an aqueous slurry of fluoropolymer resin powder for diaphragm coating, comprising the step of mixing the fluoropolymer resin powder, dispersant and water at high speed to obtain an aqueous slurry with an average particle size D50 of 5 to 15 μm.

[0040] The fluoropolymer resin powder includes polyvinylidene fluoride and an aqueous binder.

[0041] Furthermore, during high-speed stirring, the aqueous binder in the fluoropolymer resin powder is insoluble in water at 30°C; preferably, it is insoluble in water at 50°C; more preferably, it is insoluble in water at 60°C.

[0042] Compared with the prior art, the beneficial effects of the present invention are as follows: the fluoropolymer resin powder for diaphragm coating provided by the present invention has a low content of small particle size powder; there is no waste gas emission during the drying process; the slurry is stable and does not separate during the preparation process, so the particle size of the powder can be controlled. Attached Figure Description

[0043] Figure 1 This is a scanning electron microscope image of the fluoropolymer resin powder obtained in Example 1 of the present invention.

[0044] Figure 2 This is a particle size distribution diagram of the fluoropolymer resin powder obtained in Example 1 of the present invention.

[0045] Figure 3 This is a scanning electron microscope image of the fluoropolymer resin powder obtained in Comparative Example 1 of the present invention.

[0046] Figure 4 This is a particle size distribution diagram of the fluoropolymer resin powder obtained in Comparative Example 1 of the present invention. Detailed Implementation

[0047] 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.

[0048] Appearance test method for fluoropolymer resin powder

[0049] Scanning electron microscopy (SEM) was used to examine the powder morphology and particle size at magnifications of 500–1000x after gold sputtering.

[0050] Fluoropolymer Resin Particle Size Testing Method

[0051] Second slurry particle size test method: Take 1g of the second slurry and add it to 20g of ethanol. After sonication for 5min, use a laser particle size analyzer to test it.

[0052] Powder particle size testing method: Add 0.1g of powder to 20g of ethanol, sonicate for 5min, and then test with a laser particle size analyzer.

[0053] Example 1

[0054] A vinylidene fluoride-hexafluoropropylene copolymer (hexafluoropropylene mass percentage 9%) emulsion was demulsified by mechanical stirring, washed with deionized water until the conductivity was 5 μS / cm, and diluted with pure water to a first slurry with a solid content of 6%. 10 kg of the first slurry was taken and 420 g of polyvinyl alcohol aqueous solution (solid content 10%) was added, and the mixture was stirred to obtain a stable second slurry. The second slurry was dispersed in a high-speed disperser at 3000 rpm for 20 min to obtain a third slurry with an average particle size D50 of 5.1 μm. The third slurry was spray-dried at an inlet temperature of 140℃ and an outlet temperature of 80℃ to obtain a fluoropolymer resin powder. The fluoropolymer resin powder was spherical with an average particle size D50 of 8.3 μm and a D5 of 2.1 μm.

[0055] Example 2

[0056] A vinylidene fluoride-hexafluoropropylene copolymer emulsion (hexafluoropropylene mass percentage 9%) was demulsified by mechanical stirring, washed with deionized water until the conductivity was 2 μS / cm, and diluted with pure water to a first slurry with a solid content of 14%. 10 kg of the first slurry was taken, and 300 g of acrylonitrile multi-component copolymer emulsion (solid content 15%) was added and stirred to obtain a stable second slurry. The second slurry was dispersed in a high-speed disperser at 1000 rpm for 60 min to obtain a third slurry with an average particle size D50 of 9.8 μm. The third slurry was spray-dried at an inlet temperature of 140℃ and an outlet temperature of 80℃ to obtain a fluoropolymer resin powder. The fluoropolymer resin powder was spherical with an average particle size D50 of 12.8 μm and a D5 of 2.7 μm.

[0057] Example 3

[0058] A vinylidene fluoride-hexafluoropropylene copolymer emulsion (hexafluoropropylene mass percentage 4%) was demulsified by mechanical stirring, washed with deionized water until the conductivity was 2 μS / cm, and diluted with pure water to a first slurry with a solid content of 14%. 10 kg of the first slurry was taken, and 200 g of acrylonitrile multi-component copolymer emulsion (solid content 15%) was added and stirred to obtain a stable second slurry. The second slurry was dispersed in a high-speed disperser at 3000 rpm for 60 min to obtain a third slurry with an average particle size D50 of 6.8 μm. The third slurry was spray-dried at an inlet temperature of 150℃ and an outlet temperature of 80℃ to obtain a fluoropolymer resin powder. The fluoropolymer resin powder was spherical with an average particle size D50 of 7.9 μm and a D5 of 2.0 μm.

[0059] Example 4

[0060] A vinylidene fluoride-hexafluoropropylene copolymer (hexafluoropropylene mass percentage 13%) emulsion was demulsified by mechanical stirring, washed with deionized water until the conductivity was 4 μS / cm, and diluted with pure water to a first slurry with a solid content of 10%. 10 kg of the first slurry was taken and 200 g of polyvinyl alcohol aqueous solution (solid content 10%) was added, and the mixture was stirred to obtain a stable second slurry. The second slurry was dispersed at 3000 rpm for 40 min using a high-speed disperser to obtain a third slurry with an average particle size D50 of 11.1 μm. The third slurry was spray-dried at an inlet temperature of 110℃ and an outlet temperature of 60℃ to obtain a fluoropolymer resin powder. The fluoropolymer resin powder was spherical with an average particle size D50 of 13.6 μm and a D5 of 1.7 μm.

[0061] Comparative Example 1

[0062] The vinylidene fluoride-hexafluoropropylene copolymer (hexafluoropropylene mass percentage 9%) emulsion was demulsified by mechanical stirring, washed with deionized water until the conductivity was 2 μS / cm, and diluted with pure water to a first slurry with a solid content of 6%. The first slurry was dispersed in a high-speed disperser at 3000 rpm for 30 min to obtain a second slurry with an average particle size D50 of 10.1 μm. After standing for 5 min, the slurry separated into layers. The second slurry was spray-dried at an inlet temperature of 140℃ and an outlet temperature of 80℃ to obtain polyvinylidene fluoride powder. The polyvinylidene fluoride powder has a wide particle size distribution and irregular shape, with an average particle size D50 of 10.8 μm and a D5 of 0.91 μm.

[0063] Comparative Example 2

[0064] The vinylidene fluoride-hexafluoropropylene copolymer (hexafluoropropylene mass percentage 9%) emulsion was demulsified by mechanical stirring, washed with deionized water until the conductivity was 5 μS / cm, and diluted with pure water to a solid content of 6% as a first slurry. 10 kg of the first slurry was taken and 48 g of polyvinyl alcohol aqueous solution (solid content 10%) was added, stirred and mixed, and allowed to stand for 5 min to separate into layers. The second slurry was dispersed in a high-speed disperser at 3000 rpm for 20 min to obtain a third slurry with an average particle size D50 of 9.2 μm. The third slurry was spray-dried at an inlet temperature of 140℃ and an outlet temperature of 80℃ to obtain a fluoropolymer resin powder. The fluoropolymer resin powder was spherical with an average particle size D50 of 9.7 μm and a D5 of 0.98 μm.

[0065] Comparative Example 3

[0066] A vinylidene fluoride-hexafluoropropylene copolymer (hexafluoropropylene mass percentage 9%) emulsion was demulsified by mechanical stirring, washed with deionized water until the conductivity was 5 μS / cm, and diluted with pure water to a solid content of 6% to form a first slurry. 10 kg of the first slurry was taken and 600 g of polyvinyl alcohol aqueous solution (solid content 10%) was added, and the mixture was stirred to obtain a second slurry, which was allowed to stand for 5 min without stratification. The second slurry was dispersed in a high-speed disperser at 3000 rpm for 20 min to obtain a third slurry with an average particle size D50 of 7.1 μm. The third slurry was spray-dried at an inlet temperature of 140℃ and an outlet temperature of 80℃ to obtain a fluoropolymer resin powder. The fluoropolymer resin powder was spherical with an average particle size D50 of 21.5 μm and a D5 of 4.17 μm.

[0067] Comparative Example 4

[0068] Polyvinylidene fluoride homopolymer emulsion was demulsified by mechanical stirring, washed with deionized water until the conductivity was 2 μS / cm, and diluted with pure water to a first slurry with a solid content of 10%. The first slurry was dispersed in a high-speed disperser at 4000 rpm for 60 min to obtain a second slurry with an average particle size D50 of 5.6 μm. After standing for 5 min, the slurry separated into layers. The second slurry was spray-dried at an inlet temperature of 160℃ and an outlet temperature of 90℃ to obtain polyvinylidene fluoride powder. The polyvinylidene fluoride powder has a wide particle size distribution and irregular shape, with an average particle size D50 of 17.5 μm and a D5 of 0.82 μm.

[0069] Application Examples

[0070] The fluoropolymer resin powders of Examples 1-4 and Comparative Examples 1-4 were used to prepare diaphragm coating slurries: 1218g of pure water and 240g of dispersant were weighed and poured into a 5L plastic bucket, and stirred for 15min at 1000rpm using a high-speed disperser; 324g of polyvinylidene fluoride powder was added and stirred for 60min at 2000rpm; 1218g of pure water was added and stirred for 60min at 2500rpm; the slurry was then filtered through a 150-mesh sieve to obtain the diaphragm coating slurry.

[0071] The above-mentioned diaphragm coating slurry was coated once with a 20μm wire rod and dried at 70℃ for 10 minutes to obtain the coated diaphragm.

[0072] The coated separator and the positive electrode sheet are rolled together at 80°C to obtain a composite electrode sheet.

[0073] The air permeability of the coated diaphragm was tested, and the bonding strength of the composite electrode was tested. The data are detailed in Table 1.

[0074] Table 1 Test Results

[0075]

[0076]

[0077] As can be seen from Table 1, the average particle size D50 of the polyvinylidene fluoride resin powder in Examples 1 to 4 is 3 to 15 μm, with D5 > 1.5 μm, maintaining low air permeability and excellent adhesion to the positive electrode sheet.

[0078] As can be seen from Example 1 and Comparative Example 1, there are fewer small-sized agglomerates and a narrower particle size distribution, resulting in a lower air permeability value for the coated diaphragm and improved bonding strength of the composite electrode.

[0079] As can be seen from Examples 1, 2, and 3, when the amount of water-based binder added is less than 1%, there are more small-particle agglomerates, which increases the air permeability and reduces the bonding strength; when the amount of water-based binder added is greater than 8%, the coating thickness of the diaphragm is uneven, it is easy to shed powder, and the bonding strength is low.

Claims

1. A fluoropolymer resin powder for diaphragm coating, characterized in that: The fluoropolymer resin powder includes polyvinylidene fluoride and an aqueous binder. The water-based adhesive has a mass percentage of 1-8% relative to the mass of polyvinylidene fluoride. The average particle size D50 of the fluoropolymer resin powder is 3–15 μm, where D5 > 1.5 μm. In the process of preparing the aqueous slurry for the diaphragm coating, the aqueous binder of the fluoropolymer resin powder is insoluble in water.

2. The fluoropolymer resin powder for diaphragm coating according to claim 1, characterized in that: The water-based adhesive is selected from at least one of polyvinyl alcohol, carboxymethyl cellulose, acrylonitrile copolymer, styrene-butadiene rubber, and acrylic acid copolymer.

3. The fluoropolymer resin powder for diaphragm coating according to claim 2, characterized in that: The water-based adhesive is polyvinyl alcohol, and the degree of alcoholysis of the polyvinyl alcohol is greater than or equal to 90%.

4. The fluoropolymer resin powder for diaphragm coating according to claim 2, characterized in that: The water-based binder is an acrylonitrile copolymer, which has crosslinking groups and can form a crosslinked structure after drying.

5. The fluoropolymer resin powder for diaphragm coating according to claim 1, characterized in that: The polyvinylidene fluoride (PVDF) contains 70-98% PVDF monomer units and 2-30% comonomer units. The comonomer is selected from at least one of hexafluoropropylene, trifluoroethylene, trifluorochloroethylene, tetrafluoroethylene, chlorofluoroethylene, difluorochloroethylene, fluoroethylene, vinyl chloride, vinyl chloride, PVDF, tetrachloroethylene, pentafluoropropylene, 3,3,3-trifluoropropylene, 2,3,3,3-tetrafluoropropylene, 1,3,3,3-tetrafluoropropylene, and chlorotrifluoropropylene.

6. A method for preparing a fluoropolymer resin powder for a diaphragm coating according to any one of claims 1-5, characterized in that: Includes the following steps: S1: After demulsification and washing, polyvinylidene fluoride emulsion is diluted with water to obtain a first slurry with a polyvinylidene fluoride mass percentage of 5-15%. S2: The first slurry is mixed with an aqueous binder to obtain a second slurry, wherein the mass percentage of the aqueous binder relative to the mass percentage of polyvinylidene fluoride is 1-8%; S3: Disperse the second slurry to obtain a third slurry with an average particle size D50 of 2-10 μm; S4: Dry the third slurry to obtain a fluoropolymer resin powder with an average particle size D50 of 3-15 μm and D5 > 1.5 μm.

7. The method for preparing fluoropolymer resin powder for diaphragm coating according to claim 6, characterized in that: In step S4, the drying process employs spray drying or airflow drying.

8. The method for preparing fluoropolymer resin powder for diaphragm coating according to claim 6, characterized in that: In step S3, the dispersion is carried out using a high-speed disperser, and the dispersion disc is a serrated disc.

9. The application of a fluoropolymer resin powder for a diaphragm coating according to any one of claims 1-5, characterized in that: The fluoropolymer resin powder is used to prepare an aqueous slurry for coating lithium battery separators.

10. A method for preparing an aqueous slurry comprising the fluoropolymer resin powder for diaphragm coating according to any one of claims 1-5, characterized in that: The process includes the step of mixing the fluoropolymer resin powder, dispersant and water at high speed to obtain an aqueous slurry with an average particle size D50 of 5 to 15 μm.

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