Water-based PVDF (Polyvinylidene Fluoride) slurry as well as preparation method and application thereof in preparation of battery diaphragm

Abstract

The invention discloses water-based PVDF (polyvinylidene fluoride) slurry, a preparation method of the water-based PVDF slurry and application of the water-based PVDF slurry in preparation of a battery diaphragm, and relates to the technical field of new energy batteries. The preparation method of the water-based PVDF slurry comprises the following steps: S1, adding water and a demulsifier into reaction equipment, and uniformly dispersing to obtain a uniform system; s2, stirring, adding a hard monomer, an initiator and PVDF emulsion, fully stirring to a demulsification state, settling, and discharging water to obtain latex liquid with target solid content; s3, raising the temperature to 70-80 DEG C, and starting polymerization reaction for 2-5 hours to obtain the water-based PVDF slurry. The preparation method comprises the following steps: demulsifying PVDF emulsion by adopting a demulsifier, controlling the particle size, and forming a core-shell structure with PVDF colloidal particles through free radical polymerization of a hard monomer so as to reinforce the PVDF particles. Therefore, the water-based PVDF slurry with stable particles, controllable particle size and narrow distribution is obtained. And uniform and stable particles of PVDF in diaphragm coating are ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy battery, and more particularly to an aqueous PVDF slurry, a preparation method thereof, and an application thereof in the preparation of battery separator. Background Art

[0002] Polyvinylidene fluoride (PVDF) has good binding properties and electrochemical stability, and is an excellent raw material for preparing lithium battery separators. At present, PVDF is mainly applied to lithium battery separators by oil-based coating process and water-based coating process. The oil-based coating process requires the use of organic solvents such as acetone, N-methyl-2-pyrrolidone (NMP), etc., which are highly toxic and volatile during use, causing harm to the environment. To avoid the use of toxic organic solvents, the water-based coating process is usually selected. The water-based coating is to disperse PVDF resin powder in water to prepare a slurry, and then spray it on a PE film to make a separator, which can completely avoid the harm of organic solvents to the human body and the environment. However, due to the hydrophobic characteristics of PVDF powder, it is easy to agglomerate and difficult to disperse in water, resulting in large particles, uneven distribution of the PVDF coating on the separator, and thus affecting the performance of the lithium battery separator. The water-based coating needs to perform the following processes in sequence: dispersion - slurry preparation - molecular sieve filtration - ball milling - spraying, with complex processes and many steps. During the production process of the PVDF powder used, spray granulation is required, with problems such as complex production process, high production cost, and unstable particle size control during the production process. All these greatly increase the cost of PVDF during use.

[0003] In order to solve the problem of poor dispersion of existing PVDF in water, the prior art discloses an aqueous PVDF-coated separator and its preparation method and application. The preparation method of the modified aqueous PVDF powder includes the following steps: 1) Prepare emulsion A: Mix PVDF powder, the first fluoride surfactant, perfluoroalkyl acrylate, methyl methacrylate, long-chain styrene, and the first deionized water to obtain a PVDF mixed emulsion, and grind the PVDF mixed emulsion to obtain emulsion A; 2) Prepare solution B: Under a nitrogen atmosphere, mix the initiator, the second fluoride surfactant, and the second deionized water, and heat and stir to prepare solution B; 3) Prepare the modified aqueous PVDF powder: Under a nitrogen atmosphere, slowly add emulsion A to solution B, heat and stir, filter, wash, and dry to obtain the modified aqueous PVDF powder. This method utilizes the amphiphilicity of the fluoride surfactant, and the PVDF particles can be completely coated with the modified monomers, improving the dispersion of the modified PVDF particles in water. This method utilizes the cross-linking effect of perfluoroalkyl acrylate, methyl methacrylate, and long-chain styrene to improve the crystallinity of PVDF, realize the storage stability and dilution stability of the PVDF-coated slurry prepared from the modified PVDF powder, facilitate subsequent film formation, and increase the application performance of the modified PVDF powder, especially the PVDF-coated slurry. However, this method cannot control the particle size of PVDF.

[0004] The prior art also discloses a preparation method of an aqueous PVDF-coated lithium-ion battery separator, including the following steps: S1. Preparation of PVDF resin slurry: Add deionized water, vinylidene fluoride or a vinylidene fluoride mixture, an emulsifier, a chain transfer agent, and an initiator into a reactor, heat the temperature to 40-120 °C, and carry out a polymerization reaction to generate a PVDF emulsion. After demulsification and washing, a PVDF resin slurry is obtained; the vinylidene fluoride mixture is a mixture of vinylidene fluoride and a fluorinated olefin monomer; the demulsification and washing are at least one of mechanical demulsification, freezing demulsification, and salting-out demulsification; S2. Preparation of PVDF-coated slurry: Mix the PVDF resin slurry described in step S1, a binder, and deionized water evenly to obtain a PVDF-coated slurry; S3. Preparation of an aqueous PVDF-coated lithium-ion battery separator: Coating the PVDF-coated slurry described in step S2 on one or both sides of a base film to form an aqueous coating, and drying to obtain an aqueous PVDF-coated lithium-ion battery separator. However, this method also cannot control the particle size of PVDF. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the defects and shortcomings of the existing water-based PVDF slurry for battery diaphragms, such as the inability to control the particle size of PVDF, unstable particle size control, and poor dispersion stability. A preparation method of a water-based PVDF slurry is provided, in which a polymer formed by polymerization of hard monomers is demulsified and the particle size is controlled to obtain a core-shell structured water-based PVDF slurry with a specific particle size, strong particle stability, and good dispersibility, and having a polymer shell formed by polymerization of hard monomers.

[0006] Another object of the present invention is to provide a method for preparing an aqueous PVDF slurry.

[0007] Another object of the present invention is to provide an application of an aqueous PVDF slurry in the preparation of a battery separator.

[0008] Another object of the present invention is to provide a lithium battery separator.

[0009] Another object of the present invention is to provide a lithium ion battery.

[0010] The above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0011] A method for preparing an aqueous PVDF slurry comprises the following steps:

[0012] S1. Add water and demulsifier to the reaction equipment and disperse evenly to obtain a uniform system;

[0013] S2. Stirring and adding hard monomer, initiator and PVDF emulsion, stirring until demulsified, settling, draining water, and obtaining a latex liquid having a target solid content;

[0014] S3. Raise the temperature to 70-80°C and start the polymerization reaction for 2-5 hours to obtain water-based PVDF slurry.

[0015] Among them, the amount of demulsifier added in S1. is 0.8-2‰ of the water mass.

[0016] The ratio of the mass of PVDF emulsion added in S2. to the mass of water added in S1. is 1:(0.99~1.01),

[0017] S2. Medium hard monomers are monomers with a glass transition temperature Tg>0.

[0018] S2. The mass amount of medium hard monomer added is 8-20% of the solid content of PVDF emulsion, and the amount of initiator added is 3-8‰ of the mass of hard monomer.

[0019] Among them, it should be noted that:

[0020] The preparation method of the aqueous PVDF slurry of the present invention breaks the PVDF emulsion by using a demulsifier and adjusting the dosage of the demulsifier, and controls the particle size. Compared with the traditional demulsification process, the demulsification process in this invention can effectively control the particle size of PVDF colloidal particles, and the particle size control range is 1 μm to 100 μm. Then, through the free radical polymerization of the hard monomer, a core-shell structure is formed with the PVDF colloidal particles, with PVDF as the core and the hard monomer polymer as the shell, to reinforce the PVDF particles. Thus, an aqueous PVDF slurry with stable particles, controllable particle size and narrow distribution is obtained.

[0021] The present invention forms a core-shell structure through the free radical polymerization of a rigid hard monomer to reinforce the particles with the PVDF colloidal particles, making the particles stable, ensuring the stability of the particle size while ensuring the rigidity and strength of the particles, and ensuring the uniformity and stability of the PVDF particles in the diaphragm coating.

[0022] The reaction of the present invention uses water as the dispersion medium, completely avoiding the use of organic solvents, and has the advantages of small environmental pollution and low cost. And the reaction of the present invention is carried out at low temperature, with a simple process and strong production applicability.

[0023] The present invention can effectively control the demulsification particle size of the PVDF emulsion by adjusting the dosage of the demulsifier to break the PVDF emulsion. Through the swelling effect of the hard monomer, the hard monomer and the PVDF molecules penetrate each other and are fully wrapped. Then, an initiator is added to polymerize the monomer on the surface of the PVDF particles to fully wrap the PVDF molecules, achieving the effect of reinforcing the PVDF particles, making the PVDF particles stable and improving the rigidity and strength of the particles, so as to ensure the uniformity and stability of the PVDF particles in the diaphragm coating.

[0024] The addition amount of the hard monomer can control the particle size. If the amount of the hard monomer is small, the PVDF particles cannot be completely coated. If it is excessive, the particle size will be too large. At the same time, too much or too little initiator also affects the final particle size. Too much is likely to cause the particle size to be too large, and too little monomer cannot completely adhere to the PVDF particles.

[0025] The aqueous PVDF slurry prepared by the method of the present invention does not have the problem of difficult dispersion of PVDF powder compared with the traditional powder-type PVDF for diaphragms, thus solving the problem that PVDF agglomerates into large particles in the diaphragm coating, resulting in uneven diaphragm coating. The aqueous PVDF slurry can be directly used for diaphragm coating, reducing steps such as PVDF powder dispersion.

[0026] In the present invention, the addition amount of water in S1 can be the same as the mass of the PVDF emulsion.

[0027] In the present invention, the mass addition amount of the demulsifier can be 0.8‰, 0.9‰, 1.0‰, 1.1‰, 1.2‰, 1.3‰, 1.4‰, 1.5‰, 1.6‰, 1.7‰, 1.8‰, 1.9‰, 2‰ of the mass of water.

[0028] In the present invention, the mass addition amount of the hard monomer in S2 can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20% of the mass of the PVDF emulsion solids content.

[0029] In the present invention, the addition amount of the initiator in S2 can be 3‰, 4‰, 5‰, 6‰, 7‰, 8‰ of the mass of the hard monomer.

[0030] Preferably, in step S2, the hard monomer is added with stirring, the initiator is added after being uniformly dispersed, and the PVDF emulsion is added after being uniformly dispersed. Stir well until the demulsification state is reached, sediment, drain the water, and obtain a latex liquid with a target solids content and a target particle size.

[0031] It should be noted that the demulsification of the PVDF emulsion in the present invention can adopt mechanical demulsification, and the stirring speed is 400 - 500 r / min.

[0032] Preferably, the demulsifier is at least one of organic acids, inorganic acids or salts.

[0033] More preferably, the demulsifier is at least one of calcium chloride, acrylic acid or dilute hydrochloric acid. The particle size distribution of the demulsification by the preferred demulsifier is narrower, so as to greatly ensure the uniformity of the downstream diaphragm coating production.

[0034] Preferably, the hard monomer is at least one of methyl methacrylate, ethyl methacrylate, isobutyl methacrylate, butyl methacrylate, 2-hydroxyethyl methacrylate, vinyl acetate, styrene or acrylonitrile.

[0035] More preferably, the mass addition amount of the hard monomer in S2 is 10 - 15% of the PVDF emulsion solids content.

[0036] The initiator of the present invention can be an inorganic peroxide, preferably ammonium persulfate or potassium persulfate, formulated into a solution with a concentration of 8 - 12 wt%, and can be added in a one-time addition method or in multiple times. The addition of the initiator in the above S2 is preferably added in multiple times.

[0037] The PVDF emulsion of the present invention is obtained by self - preparation through conventional emulsion polymerization, wherein VDF:HFP = 90 wt%:10 wt%, the molecular weight is 380,000, and the solids content is 20 wt%.

[0038] Preferably, in S2 and S3, the oxygen content of the reaction system ≤ 30 ppm. The oxygen content in the reaction vessel can be made ≤ 30 ppm by means of vacuum pumping and nitrogen replacement.

[0039] The present invention also protects the aqueous PVDF slurry prepared by the preparation method of the aqueous PVDF slurry described in any one of the above.

[0040] The aqueous PVDF slurry of the present invention can use water as a dispersion medium, completely avoiding the use of organic solvents, and has the advantages of low environmental pollution and low cost. Moreover, the aqueous PVDF slurry of the present invention has a core-shell structure, and the polymer formed by the polymerization of hard monomers is used as the shell to reinforce the aqueous PVDF particles. The latex particles are stable, the PVDF colloidal particles have a particle size of 1-100 μm, the particle size distribution is narrow, and the dispersion stability is good.

[0041] Preferably, the particle size of the PVDF particles in the aqueous PVDF slurry is 10-30 μm.

[0042] PVDF particles with a particle size of 10-30 μm, narrow distribution, and concentrated particle size can effectively improve the uniformity of diaphragm coating.

[0043] The present invention also protects the application of the above-mentioned aqueous PVDF slurry in the preparation of battery diaphragms.

[0044] The present invention also protects a lithium battery diaphragm, which includes a substrate and a coating formed by the above-mentioned aqueous PVDF slurry coated on at least one surface of the substrate.

[0045] Compared with the traditional aqueous coating PVDF process, the coating process of the aqueous PVDF slurry of the present invention on the diaphragm is simple, greatly reducing the coating process difficulty and reducing the process steps. The steps are reduced from the original "dispersion - slurry preparation - ball milling - molecular sieve filtration - spraying" to "slurry preparation - spraying", thereby greatly reducing the process cost.

[0046] The present invention also protects a lithium ion battery, which includes a positive electrode, a negative electrode, and the above-mentioned lithium battery diaphragm.

[0047] Compared with the prior art, the beneficial effects of the present invention are:

[0048] The preparation method of the aqueous PVDF slurry of the present invention demulsifies the PVDF emulsion and controls the particle size by using a demulsifier and adjusting the dosage of the demulsifier. Compared with the traditional demulsification process, the demulsification process in this invention can effectively control the particle size of the PVDF colloidal particles, and the particle size control range is 1 μm - 100 μm. Then, through the free radical polymerization of hard monomers, a core-shell structure is formed with the PVDF colloidal particles, with PVDF as the core and the hard monomer polymer as the shell, to reinforce the PVDF particles. Thus, an aqueous PVDF slurry with stable particles, controllable particle size, and narrow distribution is obtained. Detailed Embodiments

[0049] The present invention will be further described below in conjunction with specific embodiments, but the embodiments do not limit the present invention in any form. Unless otherwise specified, the raw material reagents used in the embodiments of the present invention are conventionally purchased raw material reagents.

[0050] Example 1

[0051] A preparation method of an aqueous PVDF slurry comprises the following steps:

[0052] S1. In a 50L polymerization reactor, add 20 kg of deionized water, add 20 g of calcium chloride powder as a demulsifier, close the reactor, evacuate, and displace with nitrogen several times until the oxygen content in the reactor is less than 30 ppm. Start the stirring of the reactor and stir and disperse for 5 min to obtain a homogeneous system;

[0053] S2. At normal temperature and pressure, add 400 g of methyl methacrylate monomer (Tg = 115 °C) as a hard monomer, and stir and disperse for 10 min. Pump 0.3 g of potassium persulfate initiator (made into a 10% solution by adding water) into the polymerization kettle with a metering pump, stir and disperse for 10 min, add 20 kg of PVDF emulsion with a solid content of 20%, stir and swell for 30 min, inject deionized water for washing, sediment, and remove the excess water, so as to control and obtain a latex liquid with a target solid content.

[0054] S3. Raise the temperature in the kettle to 75 °C and keep the kettle pressure at about 3.5 MPa. After reacting for 30 min, continue to add 0.3 g of potassium persulfate initiator (made into a 10% solution by adding water) repeatedly for three times, add it once every 30 min, stop adding after continuously adding three times, continue to stir and react for 2.5 h, then keep the kettle temperature, relieve the kettle pressure to 0.2 mpa, and end the reaction while maintaining a positive pressure state, and discharge to obtain the reaction product aqueous PVDF slurry.

[0055] Among them, the mass ratio of the added amount of water to the PVDF emulsion is 1:1; the added amount of the demulsifier is 1‰ of the mass of water,

[0056] The added amount of the hard monomer is 10% of the solid content of the PVDF emulsion, and the added amount of the initiator is 3‰ of the mass of the hard monomer.

[0057] Example 2

[0058] A preparation method of an aqueous PVDF slurry, which is different from Example 1 in that:

[0059] In step S1, the mass of the calcium chloride powder is 30 g.

[0060] In step S2, the mass of the methyl methacrylate monomer is 600 g, and 0.3 g of potassium persulfate initiator (made into a 10% solution by adding water) is pumped into the polymerization kettle with a metering pump.

[0061] In step S3, after reacting for 30 min, 0.5 g of initiator potassium persulfate (made into a 10% solution by adding water) was continuously added in three repeated additions.

[0062] That is: the addition amount of the demulsifier was 1.5‰ of the water mass, the addition amount of the hard monomer was 15% of the solid content of the PVDF emulsion, and the addition amount of the initiator was 3‰ of the hard monomer mass.

[0063] The rest was the same as in Example 1 and will not be elaborated here.

[0064] Example 3

[0065] A preparation method of an aqueous PVDF slurry, different from Example 1 in that:

[0066] In step S1, the demulsifier was 20 g of acrylic acid.

[0067] The rest was the same as in Example 1 and will not be elaborated here.

[0068] Example 4

[0069] A preparation method of an aqueous PVDF slurry, different from Example 1 in that:

[0070] In step S2, the hard monomer was 400 g of ethyl methacrylate (Tg = 6°C).

[0071] The rest was the same as in Example 1 and will not be elaborated here.

[0072] Example 5

[0073] A preparation method of an aqueous PVDF slurry, different from Example 1 in that:

[0074] In step S1, the mass of calcium chloride powder was 30 g.

[0075] In step S2, the mass of methyl methacrylate monomer was 800 g, and 0.3 g of initiator potassium persulfate (made into a 10% solution by adding water) was pumped into the polymerization kettle with a metering pump.

[0076] S3. After reacting for 30 min, 0.7 g of initiator potassium persulfate (made into a 10% solution by adding water) was continuously added in three repeated additions.

[0077] That is: the addition amount of the demulsifier was 1.5‰ of the water mass,

[0078] the addition amount of the hard monomer was 20% of the solid content of the PVDF emulsion, and the addition amount of the initiator was 3‰ of the hard monomer mass.

[0079] The rest was the same as in Example 1 and will not be elaborated here.

[0080] Example 6

[0081] A preparation method of an aqueous PVDF slurry, which is different from Example 1 in that:

[0082] In step S1, 25 g of calcium chloride powder is added as a demulsifier.

[0083] The rest is the same as in Example 1 and will not be elaborated here.

[0084] That is, the addition amount of the demulsifier is 1.25‰ of the mass of water.

[0085] The rest is the same as in Example 1 and will not be elaborated here.

[0086] Comparative Example 1

[0087] Granulate the same type of PVDF emulsion powder used in the examples, then disperse the powder in water, add the glue with the same formula as in the examples, and spray it on the PE substrate to obtain a separator.

[0088] Overlap the separator and the electrode sheet, and at the same time pass through a roll press to thermally bond the separator and the electrode sheet together. The preheating temperature is 90 °C, the preheating time is 10 min, the hot pressing temperature is 90 °C, the hot pressing pressure is 0.5 Mpa, and the hot pressing time is 2 min. The side with the coating faces the electrode sheet, and then the bonded polymer-coated separator and the electrode sheet are cut into strip-shaped specimens.

[0089] Comparative Example 2

[0090] A preparation method of an aqueous PVDF slurry includes the following steps:

[0091] S1. In a 50 L polymerization reactor, add 20 kg of deionized water, close the reactor, evacuate, and displace with nitrogen several times until the oxygen content in the reactor is less than 30 ppm. Start the stirring of the reactor and stir and disperse for 5 min to obtain a homogeneous system;

[0092] S2. At normal temperature and pressure, add 400 g of methyl methacrylate monomer (Tg = 115 °C) as a hard monomer and stir and disperse for 10 min. Use a metering pump to inject 0.3 g of potassium persulfate initiator (prepared into a 10% solution with water) into the polymerization kettle, stir and disperse for 10 min, add 20 kg of PVDF emulsion with a solid content of 20%, stir and swell for 30 min, inject deionized water for washing, sediment, and remove the excess water to control the latex liquid with the target solid content.

[0093] S3. Raise the temperature in the kettle to 75 °C and keep the kettle pressure at about 3.5 MPa. After reacting for 30 min, continue to add 0.3 g of initiator potassium persulfate (prepared into a 10% solution with water), add it repeatedly three times, add it once every 30 min, stop adding after adding continuously three times, continue to stir and react until 2.5 h, then keep the kettle temperature, release the kettle pressure to 0.2 mpa, keep the positive pressure state to end the reaction, and discharge to obtain the reaction product.

[0094] Among them, the mass ratio of the added amount of water to the PVDF emulsion is 1:1;

[0095] The added amount of the hard monomer is 10% of the solid content of the PVDF emulsion, and the added amount of the initiator is 3‰ of the mass of the hard monomer.

[0096] The difference from Example 1 is that in S1, no demulsifier calcium chloride powder is added.

[0097] Comparative Example 3

[0098] A preparation method of an aqueous PVDF slurry includes the following steps:

[0099] S1. In a 50 L polymerization reactor, add 20 kg of deionized water, add 20 g of calcium chloride powder as a demulsifier, close the reactor, evacuate, and displace with nitrogen several times until the oxygen content in the reactor is less than 30 ppm. Start the reactor stirring and stir for 5 min to obtain a homogeneous system;

[0100] S2. Add 20 kg of PVDF emulsion with a solid content of 20% at normal temperature and pressure, inject deionized water for washing, sedimentation, and remove the excess water to control and obtain a latex liquid with a target solid content.

[0101] S3. Raise the temperature in the kettle to 75 °C and keep the kettle pressure at about 3.5 MPa. Continue to stir for 2.5 h, then keep the kettle temperature, release the kettle pressure to 0.2 mpa, keep the positive pressure state to end the reaction, and discharge to obtain the reaction product.

[0102] Among them, the mass ratio of the added amount of water to the PVDF emulsion is 1:1; the added amount of the demulsifier is 1‰ of the mass of water,

[0103] The added amount of the hard monomer is 10% of the solid content of the PVDF emulsion, and the added amount of the initiator is 3‰ of the mass of the hard monomer.

[0104] The difference from Example 1 is that in S2, no hard monomer methyl methacrylate monomer and initiator potassium persulfate are added.

[0105] Result detection

[0106] (1) Particle size test: Take 0.2 g of the test sample, numbered as Sample 1. Initialize the particle size tester, adjust the light obscuration of the instrument, add the emulsion dropwise to the particle size tester, and start the test to obtain the average particle size of the particles.

[0107] (2) Particle stability test under high shear: Take another 500 g of the test sample, numbered as Sample 2. Shear it at 2000 r / min under high shear dispersion for 30 min, and then use the method in (1) to test the particle size.

[0108] (3) Particle stability test during storage time: Take another 500 g of the sample, numbered as Sample 3. Place it for one month, and use the method in test method (1) to test the particle size.

[0109] Perform performance tests on the finished water-based PVDF slurries prepared in Examples 1 to 5 and Comparative Examples 1 to 3 through the above methods. The specific data are shown in Table 1 below:

[0110] Table 1

[0111]

[0112] As can be seen from the above table, for the water-based PVDF slurries prepared in Examples 1, 2, and 6 under different conditions, the dosage of the demulsifier calcium chloride is higher, and the particle size of the PVDF colloidal particles obtained by demulsification is larger. The demulsification particle size of the PVDF emulsion can be effectively controlled by adjusting the dosage of the demulsifier.

[0113] As can be seen from Example 2 and Example 5, the higher the dosage of the hard monomer, the larger the particle size of the PVDF colloidal particles obtained.

[0114] No demulsifier was added in Comparative Example 2, and a uniform system slurry could not be obtained. The particle size peak was messy, and the particle size data was not used for reference.

[0115] As can be seen from Example 1 and Comparative Example 3, without adding the hard monomer to polymerize and coat PVDF, the particle size of the formed PVDF particles changed greatly after high shear and long-term storage, indicating that the stability of the PVDF particles in the slurry was too poor.

[0116] From the particle stability test, it can be seen that compared with the comparative examples, the particles of the water-based PVDF slurry prepared in the examples of the present invention did not change under the action of high shear. After testing the particle size 30 days later, the particle size change was also very small, which proved that the latex particles of the water-based PVDF slurry had better stability. The particle size was not affected by external shear and storage time.

[0117] (4) Bonding strength test of the separator and the electrode sheet

[0118] 1) Diaphragm preparation: The aqueous PVDF slurries of Examples 1 to 6 were directly sprayed onto a PE substrate after adding glue according to the formula, and then dried to obtain Diaphragms 1 to 6. Separately, the same type of PVDF emulsion used in the example of Comparative Example 1 was spray granulated, and then the powder was dispersed in water, and the same formula glue was added and sprayed onto the PE substrate to obtain Diaphragm 7. The PVDF slurries of Comparative Example 2 and Comparative Example 3 were directly sprayed onto a PE substrate after adding glue according to the formula, and then dried to obtain Diaphragm 8 and Diaphragm 9.

[0119] 2) Adhesion test: The diaphragms with coatings of Diaphragms 1 to 9 were overlapped with the electrode sheets, and at the same time, they were passed through a roll press to thermally press and bond the diaphragms and the electrode sheets together. The preheating temperature was 90 °C, the preheating time was 10 min, the hot pressing temperature was 90 °C, the hot pressing pressure was 0.5 Mpa, and the hot pressing time was 2 min. The side with the coating faced the electrode sheet, and then the bonded polymer-coated diaphragm and the electrode sheet were cut into strip specimens. A universal tensile testing machine was used to peel the strip specimens, and the force value at the time of separation was recorded; and the peel strength between the polymer-coated diaphragm and the electrode sheet was calculated to obtain the following data.

[0120] The performance tests of the finished aqueous PVDF slurries prepared in Examples 1 to 6 and Comparative Examples 1 to 3 were carried out by the above method, and the specific data are as follows in the table:

[0121] Sample Adhesion value Peel strength Separator 1 0.36N 8.3 N / M Separator 2 0.37N 8.4 N / M Separator 3 0.36N 8.3 N / M Separator 4 0.4N 9.2 N / M Separator 5 0.36N 8.3 N / M Separator 6 0.36N 8.3 N / M Separator 7 0.20N 6.0 N / M Separator 8 - - Separator 9 0.25N 5.8 N / M

[0122] From the above data, it can be seen that compared with Diaphragm 7, Diaphragms 1 to 6 have improved adhesion values and peel strengths. The above data show that in the use of the diaphragms prepared by the aqueous PVDF slurry prepared by the method of the present invention, the particle dispersion degree of the aqueous PVDF slurry is high, and there is no particle agglomeration problem. Therefore, the adhesion value of the diaphragm and the peel strength between the diaphragm and the electrode sheet can be improved.

[0123] In the preparation process of the PVDF slurry of Comparative Example 2, no demulsifier was added, and a homogeneous slurry system could not be prepared, and there were many precipitates. Diaphragm 8 prepared was not used as a reference.

[0124] From Diaphragm 1 and Diaphragm 9, it can be seen that in Comparative Example 3, no hard monomer was added for polymerization reaction to coat PVDF, and the adhesion value and peel strength between the diaphragm 9 prepared by the obtained aqueous PVDF slurry and the electrode sheet were too low, which can show that the aqueous PVDF slurry prepared in Comparative Example 3 did not form a homogeneous dispersion system and the dispersion stability was too poor.

[0125] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limitations on the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all the implementation manners here. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the claims of the present invention.

Claims

1. A method for preparing an aqueous PVDF slurry, characterized in that: The steps include: S1. Add water and demulsifier to the reaction equipment and disperse evenly to obtain a uniform system; S2. Stirring and adding hard monomer, initiator and PVDF emulsion, stirring until demulsified, settling, draining water, and obtaining a latex liquid having a target solid content; S3. Raise the temperature to 70-80°C and start the polymerization reaction for 2-5 hours to obtain water-based PVDF slurry. Among them, the amount of demulsifier added in S1. is 0.8-2‰ of the water mass. The ratio of the mass of PVDF emulsion added in S2. to the mass of water added in S1. is 1:(0.99~1.01), S2. Medium hard monomers are monomers with a glass transition temperature Tg>0. S2. The mass amount of medium hard monomer added is 8-20% of the solid content of PVDF emulsion, and the amount of initiator added is 3-8‰ of the mass of hard monomer.

2. The method for preparing an aqueous PVDF slurry according to claim 1, characterized in that: The demulsifier is at least one of an organic acid, an inorganic acid or a salt.

3. The method for preparing an aqueous PVDF slurry according to claim 2, characterized in that: The demulsifier is at least one of calcium chloride, acrylic acid or dilute hydrochloric acid.

4. The method for preparing an aqueous PVDF slurry according to claim 1, characterized in that: The hard monomer is at least one of methyl methacrylate, ethyl methacrylate, isobutyl methacrylate, butyl methacrylate, hydroxyethyl methacrylate, vinyl acetate, styrene or acrylonitrile.

5. The method for preparing an aqueous PVDF slurry according to claim 1, characterized in that: In S2 and S3, the oxygen content of the reaction system is ≤30ppm.

6. Aqueous PVDF slurry is prepared by the method for preparing aqueous PVDF slurry according to any one of claims 1 to 5.

7. The aqueous PVDF slurry according to claim 6, characterized in that: The particle size of the PVDF particles in the aqueous PVDF slurry is 10 to 30 μm.

8. Use of the aqueous PVDF slurry according to claim 6 or 7 in the preparation of battery separators.

9. A lithium battery separator, characterized in that: The invention comprises a substrate and a coating formed by the aqueous PVDF slurry according to claim 6 or 7 and coated on at least one surface of the substrate.

10. A lithium ion battery, characterized in that: It comprises a positive electrode, a negative electrode and the lithium battery separator as claimed in claim 9.

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