Battery diaphragm suitable for low-temperature hot pressing process and preparation method thereof

Abstract

The invention provides a battery diaphragm and a preparation method thereof, and the battery diaphragm comprises a base film and a bonding layer, the bonding layer is formed on at least one side of the base film, the bonding layer comprises a PVDF polymer, and the PVDF polymer comprises a core region containing VDF and a shell region covering the core region and containing HFP. Therefore, the bonding layer can exert high bonding force with the pole piece under a certain pressure at the hot pressing temperature from 25 DEG C to 120 DEG C.

Description

Technical Field

[0001] The present invention relates to the technical field of battery separators, and particularly to a battery separator suitable for low-temperature hot pressing process and a preparation method thereof. Background Art

[0002] A lithium-ion battery is a device that converts chemical energy into electrical energy and has received extensive attention due to its environmental friendliness and great potential in high-efficiency energy storage. Currently, lithium batteries on the market can be simply divided into square batteries, soft-pack batteries, and cylindrical batteries according to the packaging type. The composition of a lithium-ion battery mainly uses lithium metal oxide as the positive electrode, graphite as the negative electrode, a mixed solvent package as the electrolyte, and a polymer porous membrane as the separator. The separator separates the positive and negative electrodes to prevent short circuits.

[0003] Currently, the bare battery cells are first cold-pressed and shaped by the battery cell client to prevent jitter on the conveyor belt. This requires a certain adhesion force between the separator and the electrode plate during cold pressing. Subsequently, the battery cells enter the tunnel furnace for preheating. However, to meet the requirements of the client for improving the process efficiency and reducing energy consumption, the preheating time will be reduced, which may lead to insufficient preheating inside the battery cells and a soft phenomenon inside the battery cells after hot pressing. Another way to reduce energy consumption is to reduce the original hot pressing temperature from 90°C to about 60°C. Based on these two methods, higher requirements are put forward for the separator, that is, the separator and the electrode plate have a high adhesion force at a relatively low hot pressing temperature.

[0004] Currently, in the market, to meet the requirement of high adhesion force between the separator and the electrode plate, PVDF or PMMA adhesive layers are usually coated on the base film layer or the ceramic layer; however, the softening point temperatures of conventional PVDF and PMMA are relatively high, and the adhesion force between the separator and the electrode plate will only be generated at a relatively high hot pressing temperature (90°C). Summary of the Invention

[0005] In view of the requirements of the battery cell client for improving the process efficiency and reducing energy consumption, the present invention develops a separator that has a high adhesion force under different hot pressing temperature conditions to solve the problems of a narrow hot pressing process window and a high hot pressing temperature between the existing separator and the electrode plate.

[0006] Therefore, the first aspect of the present invention is implemented as follows:

[0007] A battery separator, comprising: a base film and an adhesive layer; the adhesive layer is formed on at least one side of the base film, and the adhesive layer comprises a PVDF polymer, and the PVDF polymer comprises a core region containing VDF and a shell region covering the core region and containing HFP.

[0008] In a preferred embodiment, the adhesive layer further comprises: polymer microspheres, and the polymer microspheres comprise a core and a shell covering the core and containing cellulose fibrils.

[0009] In a preferred embodiment, the mass ratio between the PVDF polymer and the polymer microspheres is 1:(0.1 to 10).

[0010] In a preferred embodiment, the mass ratio between the HFP and the VDF is 1:(8 to 10).

[0011] In a preferred embodiment, the core of the polymer microspheres comprises one or more of polyvinyl acetate, polyacrylamide, polymethyl methacrylate, and polystyrene.

[0012] In a preferred embodiment, the core of the polymer microspheres sequentially comprises, from the inside to the outside, a first core containing the polyvinyl acetate, a second core coating the first core and containing the polyacrylamide, a third core coating the second core and containing the polymethyl methacrylate, and a fourth core coating the third core and containing the polystyrene.

[0013] In a preferred embodiment, the ratio of the mass of the cellulose fibrils to the total mass of the polyvinyl acetate, the polyacrylamide, the polymethyl methacrylate, and the polystyrene is 1:(50 to 200).

[0014] In a preferred embodiment, the mass ratio between the polyvinyl acetate, the polyacrylamide, the polymethyl methacrylate, and the polystyrene is 1:(0.5 to 5):(1 to 10):(1 to 10).

[0015] In a preferred embodiment, the adhesive layer further comprises one or more of a polyacrylonitrile adhesive and a polyacrylate adhesive.

[0016] The second aspect of the present invention is implemented as follows:

[0017] A method for preparing a battery separator, which comprises: preparing a PVDF polymer and preparing a slurry and coating and drying; preparing the PVDF polymer comprises: mixing and stirring VDF and an emulsifier to obtain a polymer precursor; after mixing and reacting the polymer precursor with an initiator, then adding HFP and reacting further to obtain a PVDF emulsion; and spray-drying the PVDF emulsion to obtain the PVDF polymer; preparing the slurry and coating and drying comprises: preparing the PVDF polymer into a slurry; and coating the slurry on at least one side of a base film and drying.

[0018] In a preferred embodiment, the emulsifier comprises one or more of sodium stearate, sodium laurate, sodium oleate, sodium dodecyl sulfate, sodium hexadecyl benzene sulfonate, and dodecyl benzene sulfonic acid.

[0019] In a preferred embodiment, the initiator includes one or more of ammonium persulfate, potassium persulfate, and metal salts of peroxides.

[0020] In a preferred embodiment, the method for preparing the battery separator further includes: preparing polymer microspheres; preparing polymer microspheres includes: mixing one or more of vinyl acetate, acrylamide, methyl methacrylate, and styrene with a cellulose fibril suspension and stirring evenly to obtain an oil-water mixture, followed by ultrasonic fragmentation to obtain a Pickering emulsion; and mixing and reacting the Pickering emulsion with a water-soluble initiator to obtain polymer microspheres; preparing the slurry and coating and drying, and preparing the PVDF polymer into a slurry further includes: mixing the PVDF polymer with the polymer microspheres to obtain the slurry.

[0021] In a preferred embodiment, the water-soluble initiator includes one or more of ammonium persulfate, potassium persulfate, and metal salts of peroxides.

[0022] In a preferred embodiment, the cellulose fibrils include one or more of absorbent cotton, pulp, wood pulp, and hemp.

[0023] In a preferred embodiment, the mass ratio between the emulsifier and the VDF is 1:(5 to 10).

[0024] In a preferred embodiment, the mass ratio between the initiator, the HFP, and the polymer precursor is 1:(30 to 70):(300 to 700).

[0025] In a preferred embodiment, the oil-water mass ratio of the oil-water mixture is 1:(2 to 8).

[0026] In a preferred embodiment, the mass ratio between the water-soluble initiator and the Pickering emulsion is 1:(200 to 400).

[0027] In a preferred embodiment, the mass ratio between the PVDF polymer and the polymer microspheres is 1:(0.1 to 10).

[0028] The present invention has the following advantages and beneficial effects compared with the prior art:

[0029] (1) During the polymerization of PVDF, the reaction rate is controlled by regulating the content of HFP monomer and the addition time of HFP monomer, so that the prepared PVDF has a core-shell-like structure and a relatively low softening point, where the core is the crystalline region of VDF and the shell is the non-crystalline region of HFP. Since the shell is a non-crystalline region and has a lower Tg temperature, a higher adhesion force can be generated at a lower hot pressing temperature.

[0030] (2) Using methyl methacrylate, styrene, acrylamide, or vinyl acetate as monomers, cellulose fibrils as stabilizers, and performing Pickering emulsion polymerization with an initiator (such as ammonium persulfate). During the polymerization process, by regulating the monomer ratio and using cellulose fibrils as the shell, the adhesion of the prepared microspheres can be prevented, and at the same time, the prepared polymer has a lower Tg temperature, and a lower hot pressing temperature can also generate a higher adhesive force.

[0031] (3) Physically blend the prepared PVDF and polymer microspheres to establish different softening point temperature gradients, so that the prepared separator can be adapted to the hot pressing processes of different battery manufacturers. Especially under low-temperature conditions, both the separator and the electrode have a strong adhesive force.

[0032] (4) Appropriately add polyacrylonitrile and polyacrylic acid to the slurry formulation to improve the adhesion between PVDF and polymer microspheres on the surface of the base film and the cohesion between the coatings.

[0033] (5) The adhesive layer can exert a high adhesive force with the electrode under a certain pressure, and the hot pressing temperature can range from 25°C to 120°C.

[0034] (6) It can not only ensure that the separator has excellent heat resistance but also has high adhesiveness. Only one coating is required. Compared with the currently commonly used two-coating method, the final energy consumption and cost are greatly reduced. Moreover, the prepared polymer emulsion has high stability, reducing the risks during the production process. Description of the Drawings

[0035] Figure 1 Schematically present the process of forming PVDF aggregates by spray drying PVDF emulsion;

[0036] Figure 2 Schematically present the structure of polymer microspheres. Detailed Description of the Invention

[0037] The following details the specific embodiments of the present invention in conjunction with the drawings. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not used to limit the present invention.

[0038] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values and individual point values of each range, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0039] Emulsion polymerization is a common synthetic method in polymer chemistry. It refers to the process in which monomers form an emulsion in water under the action of an emulsifier and undergo a polymerization reaction under the action of an initiator. The core of the process is to form a stable emulsion system. The polymerization process includes three stages: chain initiation, chain growth, and chain termination. Because of its environmental friendliness, high production efficiency, and flexible control of monomer ratios, it is currently widely used in the polymer field.

[0040] Pickering emulsion refers to an emulsion formed by a class of solid particles replacing surfactants as stabilizers. With its excellent stability and biocompatibility, it is widely used in coatings, biomedicine, cosmetics and other fields, showing good application scenarios.

[0041] The preferred embodiment of the present invention uses VDF and HFP as monomers, potassium persulfate as initiator, and sodium dodecyl sulfate as emulsifier to carry out emulsion polymerization. During the polymerization process, the reaction rate is controlled by regulating the HFP monomer content and the HFP monomer addition time, so that the prepared PVDF has a relatively low softening point and this PVDF has a core-shell structure, wherein the core is the crystalline region VDF and the shell is the non-crystalline region HFP. Since the shell is the non-crystalline region with a lower Tg temperature, a lower hot pressing temperature can produce a higher adhesion. On the other hand, Pickering emulsion polymerization is carried out with methyl methacrylate, styrene, acrylamide, and vinyl acetate as monomers, ammonium persulfate as initiator, and cellulose fibrils as stabilizers. During the polymerization process, the monomer ratio is regulated and the outer shell is used as cellulose fibrils to prevent the prepared microspheres from sticking. At the same time, this polymer has a lower Tg temperature, and a lower hot pressing temperature can also produce a higher adhesion. The prepared PVDF and polymer microspheres are physically blended respectively to establish different softening point temperature gradients, so that the coated layer diaphragm can be adapted to the hot pressing process of different battery manufacturers. Especially at low temperatures, the diaphragm and the electrode have strong adhesion.

[0042] Here, the first embodiment of the present invention mentions a battery separator, comprising: a base film and an adhesive layer; the adhesive layer is formed on at least one side of the base film, and the adhesive layer comprises a PVDF (polyvinylidene fluoride) polymer, and may further comprise polymer microspheres; the PVDF polymer comprises a core region comprising VDF (vinylidene fluoride) and a shell region coating the core region and comprising HFP (hexafluoropropylene); the polymer microspheres comprise an inner core and an outer shell coating the inner core and comprising cellulose fibrils.

[0043] The base film can be a polyolefin base film, preferably one or more of a polyethylene base film and a polypropylene base film. The adhesive layer can be formed on one or both sides of the base film. Under the condition of including polymer microspheres, the mass ratio between the PVDF polymer and the polymer microspheres can be 1:(0.1 to 10), preferably 1:(any value among 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values). In addition, the adhesive layer can further include: one or more of a polyacrylonitrile adhesive and a polyacrylate adhesive; the polyacrylate adhesive can be one or more of a polymethacrylate, an ethyl acrylate, a n-butyl acrylate, a tert-butyl acrylate, and an octadecyl acrylate. Under the condition of including a polyacrylonitrile adhesive and a polyacrylate adhesive, the mass ratio between the polyacrylate adhesive, the polyacrylonitrile adhesive, the PVDF polymer and the polymer microspheres can be 1:(1 to 5):(5 to 20):(5 to 20), preferably 1:(any value among 1, 2, 3, 4, 5 or the range between any two values):(any value among 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or the range between any two values):(any value among 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or the range between any two values).

[0044] The mass ratio between HFP and VDF can be 1:(8 to 10), preferably 1:(any value among 8, 8.2, 8.4, 8.6, 8.8, 9, 9.2, 9.4, 9.6, 9.8, 10 or the range between any two values). As Figure 1 shown, the PVDF polymer can be formed by spray-drying a PVDF emulsion; the particles of the PVDF emulsion belong to primary particles, and their particle size can be 200 nm to 300 nm; the PVDF polymer is in powder form and belongs to agglomerated particles, and its D50 particle size can be 6 μm to 10 μm.

[0045] The core of the polymer microspheres can include one or more of polyvinyl acetate, polyacrylamide, polymethyl methacrylate, and polystyrene. The examples of the core of the polymer microspheres are illustrated as follows:

[0046] The polymer microspheres may comprise a core containing polyvinyl acetate and a shell coating the core and containing cellulose fibrils. Under such conditions, the mass ratio between the cellulose fibrils and the polyvinyl acetate may be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values).

[0047] The polymer microspheres may comprise a core containing polyacrylamide and a shell coating the core and containing cellulose fibrils. Under such conditions, the mass ratio between the cellulose fibrils and the polyacrylamide may be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values).

[0048] The polymer microspheres may comprise a core containing polymethyl methacrylate and a shell coating the core and containing cellulose fibrils. Under such conditions, the mass ratio between the cellulose fibrils and the polymethyl methacrylate may be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values).

[0049] The polymer microspheres may comprise a core containing polystyrene and a shell coating the core and containing cellulose fibrils. Under such conditions, the mass ratio between the cellulose fibrils and the polystyrene may be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values).

[0050] The polymer microspheres may include a core containing polyvinyl acetate and polyacrylamide, and a shell coating the core and containing cellulose fibrils. Under these conditions, the ratio between the mass of the cellulose fibrils and the total mass of the polyvinyl acetate and polyacrylamide may be 1:(50 to 200), preferably 1:(any value from 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The core of the polymer microspheres may sequentially include a first core containing polyvinyl acetate from the inside to the outside, and a second core coating the first core and containing polyacrylamide. The mass ratio between the polyvinyl acetate and the polyacrylamide may be 1:(0.5 to 5), preferably 1:(any value from 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 or the range between any two values).

[0051] The polymer microspheres may include a core containing polyvinyl acetate and polymethyl methacrylate, and a shell coating the core and containing cellulose fibrils. Under these conditions, the ratio between the mass of the cellulose fibrils and the total mass of the polyvinyl acetate and polymethyl methacrylate may be 1:(50 to 200), preferably 1:(any value from 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The core of the polymer microspheres may sequentially include a first core containing polyvinyl acetate from the inside to the outside, and a second core coating the first core and containing polymethyl methacrylate. The mass ratio between the polyvinyl acetate and the polymethyl methacrylate may be 1:(1 to 10), preferably 1:(any value from 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values).

[0052] The polymer microspheres may include a core containing polyvinyl acetate and polystyrene, and a shell coating the core and containing cellulose fibrils. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of polyvinyl acetate and polystyrene may be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The core of the polymer microspheres may sequentially include a first core containing polyvinyl acetate from the inside to the outside, and a second core coating the first core and containing polystyrene. The mass ratio between polyvinyl acetate and polystyrene may be 1:(1 to 10), preferably 1:(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values).

[0053] The polymer microspheres may include a core containing polyacrylamide and polymethyl methacrylate, and a shell coating the core and containing cellulose fibrils. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of polyacrylamide and polymethyl methacrylate may be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The core of the polymer microspheres may sequentially include a first core containing polyacrylamide from the inside to the outside, and a second core coating the first core and containing polymethyl methacrylate. The mass ratio between polyacrylamide and polymethyl methacrylate may be 1:(0.2 to 20), preferably 1:(any value among 0.2, 0.4, 0.6, 0.8, 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 or the range between any two values).

[0054] The polymer microspheres may comprise a core containing polyacrylamide and polystyrene, and a shell coating the core and containing cellulose fibrils. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of polyacrylamide and polystyrene may be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The core of the polymer microspheres may sequentially include a first core containing polyacrylamide from the inside to the outside, and a second core coating the first core and containing polystyrene. The mass ratio between polyacrylamide and polystyrene may be 1:(0.2 to 20), preferably 1:(any value among 0.2, 0.4, 0.6, 0.8, 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 or the range between any two values).

[0055] The polymer microspheres may comprise a core containing polymethyl methacrylate and polystyrene, and a shell coating the core and containing cellulose fibrils. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of polymethyl methacrylate and polystyrene may be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The core of the polymer microspheres may sequentially include a first core containing polymethyl methacrylate from the inside to the outside, and a second core coating the first core and containing polystyrene. The mass ratio between polymethyl methacrylate and polystyrene may be 1:(0.1 to 10), preferably 1:(any value among 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values).

[0056] The polymer microspheres may include a core containing polyvinyl acetate, polyacrylamide, and polymethyl methacrylate, and a shell coating the core and containing cellulose fibrils. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of polyvinyl acetate, polyacrylamide, and polymethyl methacrylate may be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The core of the polymer microspheres may sequentially include a first core containing polyvinyl acetate, a second core coating the first core and containing polyacrylamide, and a third core coating the second core and containing polymethyl methacrylate from the inside to the outside. The mass ratio between polyvinyl acetate, polyacrylamide, and polymethyl methacrylate may be 1:(0.5 to 5):(1 to 10), preferably 1:(any value among 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 or the range between any two values):(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values).

[0057] The polymer microspheres may include a core containing polyvinyl acetate, polyacrylamide, and polystyrene, and a shell coating the core and containing cellulose fibrils. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of polyvinyl acetate, polyacrylamide, and polystyrene may be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The core of the polymer microspheres may sequentially include a first core containing polyvinyl acetate, a second core coating the first core and containing polyacrylamide, and a third core coating the second core and containing polystyrene from the inside to the outside. The mass ratio between polyvinyl acetate, polyacrylamide, and polystyrene may be 1:(0.5 to 5):(1 to 10), preferably 1:(any value among 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 or the range between any two values):(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values).

[0058] The polymer microspheres may include a core containing polyvinyl acetate, polymethyl methacrylate, and polystyrene, and a shell coating the core and containing cellulose fibrils. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of polyvinyl acetate, polymethyl methacrylate, and polystyrene may be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The core of the polymer microspheres may sequentially include a first core containing polyvinyl acetate, a second core coating the first core and containing polymethyl methacrylate, and a third core coating the second core and containing polystyrene from the inside to the outside. The mass ratio between polyvinyl acetate, polymethyl methacrylate, and polystyrene may be 1:(1 to 10):(1 to 10), preferably 1:(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values):(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values).

[0059] The polymer microspheres may include a core containing polyacrylamide, polymethyl methacrylate, and polystyrene, and a shell coating the core and containing cellulose fibrils. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of polyacrylamide, polymethyl methacrylate, and polystyrene may be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The core of the polymer microspheres may sequentially include a first core containing polyacrylamide, a second core coating the first core and containing polymethyl methacrylate, and a third core coating the second core and containing polystyrene from the inside to the outside. The mass ratio between polyacrylamide, polymethyl methacrylate, and polystyrene may be 1:(0.2 to 20):(0.2 to 20), preferably 1:(any value among 0.2, 0.4, 0.6, 0.8, 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 or the range between any two values):(any value among 0.2, 0.4, 0.6, 0.8, 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 or the range between any two values).

[0060] The polymer microspheres may include a core containing polyvinyl acetate, polyacrylamide, polymethyl methacrylate, and polystyrene, and a shell covering the core and containing cellulose fibrils. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of polyvinyl acetate, polyacrylamide, polymethyl methacrylate, and polystyrene may be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). As Figure 2 shown, the core of the polymer microspheres may sequentially include a first core containing polyvinyl acetate, a second core covering the first core and containing polyacrylamide, a third core covering the second core and containing polymethyl methacrylate, and a fourth core covering the third core and containing polystyrene from the inside to the outside. The mass ratio between polyvinyl acetate, polyacrylamide, polymethyl methacrylate, and polystyrene may be 1:(0.5 to 5):(1 to 10):(1 to 10), preferably 1:(any value among 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 or the range between any two values):(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values):(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values).

[0061] Here, a second embodiment of the present invention mentions a method for preparing a battery separator. This method can be used to prepare the battery separator of the first embodiment, but should not be limited thereto. This method includes the following steps: preparing a PVDF polymer, preparing a slurry, coating and drying, and may further include preparing polymer microspheres.

[0062] First, during the process of preparing the PVDF polymer, VDF and an emulsifier are first mixed and stirred to obtain a polymer precursor; then, after the polymer precursor is mixed and reacted with an initiator, HFP is added and reacted again to obtain a PVDF emulsion; finally, the PVDF emulsion is spray-dried to obtain the PVDF polymer.

[0063] The emulsifier may include one or more of sodium stearate, sodium laurate, sodium oleate, sodium dodecyl sulfate, sodium hexadecyl benzene sulfonate, and dodecyl benzene sulfonic acid. The mass ratio between the emulsifier and VDF may be 1:(5 to 10), preferably 1:(any value among 5, 5.25, 5.5, 5.75, 6, 6.25, 6.5, 6.75, 7, 7.25, 7.5, 7.75, 8, 8.25, 8.5, 8.75, 9, 9.25, 9.5, 9.75, 10 or the range between any two values). The initiator may include one or more of ammonium persulfate, potassium persulfate, and metal salts of peroxides. The mass ratio among the initiator, HFP, and the polymer precursor may be 1:(30 to 70):(300 to 700), preferably 1:(any value among 30, 35, 40, 45, 50, 55, 60, 65, 70 or the range between any two values):(any value among 300, 350, 400, 450, 500, 550, 600, 650, 700 or the range between any two values).

[0064] Before mixing and stirring VDF with the emulsifier, it can be purged with an inert gas first; preferably purged with an inert gas at 80°C to 120°C (more preferably 100°C), and then cooled to 40°C to 50°C (more preferably 45°C). The inert gas can be one or more of nitrogen and argon. When mixing and stirring VDF with the emulsifier, it can be mechanically stirred at a sealed 60°C to 70°C (preferably 65°C) for 1 h to 3 h (preferably 2 h), and then cooled to obtain the polymer precursor.

[0065] When mixing and reacting the polymer precursor with the initiator, it can be mechanically stirred at a sealed 70°C to 80°C (preferably 75°C) for 1 h to 3 h (preferably 2 h). The reaction time for adding HFP and then reacting can be 3 h to 5 h, preferably any value among 3 h, 4 h, 5 h or the range between any two values. Before obtaining the PVDF emulsion, the product obtained by adding HFP and then reacting can be filtered, preferably filtered through a 100-mesh to 200-mesh filter screen, more preferably filtered through a 150-mesh filter screen.

[0066] The processing time for spray-drying the PVDF emulsion can be 10 min to 60 min, preferably any value among 10 min, 20 min, 30 min, 40 min, 50 min, 60 min or the range between any two values.

[0067] Secondly, during the preparation of polymer microspheres, after mixing one or more of vinyl acetate, acrylamide, methyl methacrylate, and styrene with a cellulose fibril suspension and stirring evenly to obtain an oil-water mixed liquid, ultrasonic fragmentation treatment is performed to obtain a Pickering emulsion; finally, the Pickering emulsion is mixed and reacted with a water-soluble initiator to obtain polymer microspheres.

[0068] The cellulose fibrils may include one or more of cotton linter, pulp, wood pulp, and hemp. The concentration of the cellulose fibril suspension may be 0.1 wt% to 0.4 wt%, preferably any value or the range between any two values among 0.1 wt%, 0.2 wt%, 0.3 wt%, and 0.4 wt%. The water-soluble initiator may include one or more of ammonium persulfate, potassium persulfate, and metal salts of peroxides. The mass ratio between the water-soluble initiator and the Pickering emulsion may be 1:(200 to 400), preferably 1:(any value or the range between any two values among 200, 250, 300, 350, and 400). The oil-water mass ratio of the oil-water mixed liquid may be 1:(2 to 8), preferably 1:(any value or the range between any two values among 2, 3, 4, 5, 6, 7, and 8).

[0069] When obtaining the oil-water mixed liquid, vinyl acetate may be mixed and stirred evenly with the cellulose fibril suspension. Under this condition, the mass ratio between the cellulose fibrils and vinyl acetate may be 1:(50 to 200), preferably 1:(any value or the range between any two values among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, and 200). The mass ratio between vinyl acetate and the cellulose fibril suspension may be 1:(4 to 30), preferably 1:(any value or the range between any two values among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, and 30).

[0070] When obtaining the oil-water mixed liquid, acrylamide can be mixed and stirred evenly with the cellulose fibril suspension. Under this condition, the mass ratio between the cellulose fibrils and acrylamide can be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The mass ratio between acrylamide and the cellulose fibril suspension can be 1:(4 to 30), preferably 1:(any value among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or the range between any two values).

[0071] When obtaining the oil-water mixed liquid, methyl methacrylate can be mixed and stirred evenly with the cellulose fibril suspension. Under this condition, the mass ratio between the cellulose fibrils and methyl methacrylate can be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The mass ratio between methyl methacrylate and the cellulose fibril suspension can be 1:(4 to 30), preferably 1:(any value among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or the range between any two values).

[0072] When obtaining the oil-water mixed liquid, styrene can be mixed and stirred evenly with the cellulose fibril suspension. Under such conditions, the mass ratio between the cellulose fibrils and styrene can be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The mass ratio between styrene and the cellulose fibril suspension can be 1:(4 to 30), preferably 1:(any value among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or the range between any two values).

[0073] When obtaining the oil-water mixed liquid, vinyl acetate, acrylamide and the cellulose fibril suspension can be mixed and stirred evenly. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of vinyl acetate and acrylamide can be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The ratio between the total mass of vinyl acetate and acrylamide and the mass of the cellulose fibril suspension can be 1:(4 to 30), preferably 1:(any value among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or the range between any two values). The mass ratio between vinyl acetate and acrylamide can be 1:(0.5 to 5), preferably 1:(any value among 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 or the range between any two values).

[0074] When obtaining the oil-water mixed liquid, vinyl acetate, methyl methacrylate and the cellulose fibril suspension can be mixed and stirred evenly. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of vinyl acetate and methyl methacrylate can be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The ratio between the total mass of vinyl acetate and methyl methacrylate and the mass of the cellulose fibril suspension can be 1:(4 to 30), preferably 1:(any value among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or the range between any two values). The mass ratio between vinyl acetate and methyl methacrylate can be 1:(1 to 10), preferably 1:(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values).

[0075] When obtaining the oil-water mixed liquid, vinyl acetate, styrene and the cellulose fibril suspension can be mixed and stirred evenly. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of vinyl acetate and styrene can be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The ratio between the total mass of vinyl acetate and styrene and the mass of the cellulose fibril suspension can be 1:(4 to 30), preferably 1:(any value among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or the range between any two values). The mass ratio between vinyl acetate and styrene can be 1:(1 to 10), preferably 1:(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values).

[0076] When obtaining the oil-water mixed liquid, acrylamide, methyl methacrylate and the cellulose fibril suspension can be mixed and stirred evenly. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of acrylamide and methyl methacrylate can be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The ratio between the total mass of acrylamide and methyl methacrylate and the mass of the cellulose fibril suspension can be 1:(4 to 30), preferably 1:(any value among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or the range between any two values). The mass ratio between acrylamide and methyl methacrylate can be 1:(0.2 to 20), preferably 1:(any value among 0.2, 0.4, 0.6, 0.8, 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 or the range between any two values).

[0077] When obtaining the oil-water mixed liquid, acrylamide, styrene and the cellulose fibril suspension can be mixed and stirred evenly. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of acrylamide and styrene can be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The ratio between the total mass of acrylamide and styrene and the mass of the cellulose fibril suspension can be 1:(4 to 30), preferably 1:(any value among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or the range between any two values). The mass ratio between acrylamide and styrene can be 1:(0.2 to 20), preferably 1:(any value among 0.2, 0.4, 0.6, 0.8, 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 or the range between any two values).

[0078] When obtaining the oil-water mixed liquid, methyl methacrylate, styrene and the cellulose fibril suspension can be mixed and stirred evenly. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of methyl methacrylate and styrene can be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The ratio between the total mass of methyl methacrylate and styrene and the mass of the cellulose fibril suspension can be 1:(4 to 30), preferably 1:(any value among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or the range between any two values). The mass ratio between methyl methacrylate and styrene can be 1:(0.1 to 10), preferably 1:(any value among 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values).

[0079] When obtaining the oil-water mixed liquid, vinyl acetate, acrylamide, methyl methacrylate and cellulose fibril suspension can be mixed and stirred evenly. Under this condition, the ratio between the mass of cellulose fibrils and the total mass of vinyl acetate, acrylamide and methyl methacrylate can be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The ratio between the total mass of vinyl acetate, acrylamide and methyl methacrylate and the mass of cellulose fibril suspension can be 1:(4 to 30), preferably 1:(any value among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or the range between any two values). The mass ratio among vinyl acetate, acrylamide and methyl methacrylate can be 1:(0.5 to 5):(1 to 10), preferably 1:(any value among 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 or the range between any two values):(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values).

[0080] When obtaining the oil-water mixed liquid, vinyl acetate, acrylamide, styrene and the cellulose fibril suspension can be mixed and stirred evenly. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of vinyl acetate, acrylamide and styrene can be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The ratio between the total mass of vinyl acetate, acrylamide and styrene and the mass of the cellulose fibril suspension can be 1:(4 to 30), preferably 1:(any value among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or the range between any two values). The mass ratio among vinyl acetate, acrylamide and styrene can be 1:(0.5 to 5):(1 to 10), preferably 1:(any value among 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 or the range between any two values):(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values).

[0081] When obtaining the oil-water mixed liquid, vinyl acetate, methyl methacrylate, styrene and the cellulose fibril suspension can be mixed and stirred evenly. Under such conditions, the ratio between the mass of the cellulose fibrils and the total mass of vinyl acetate, methyl methacrylate and styrene can be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The ratio between the total mass of vinyl acetate, methyl methacrylate and styrene and the mass of the cellulose fibril suspension can be 1:(4 to 30), preferably 1:(any value among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or the range between any two values). The mass ratio among vinyl acetate, methyl methacrylate and styrene can be 1:(1 to 10):(1 to 10), preferably 1:(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values):(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values).

[0082] When obtaining the oil-water mixed liquid, acrylamide, methyl methacrylate, styrene and cellulose fibril suspension can be mixed and stirred evenly. Under these conditions, the ratio between the mass of cellulose fibrils and the total mass of acrylamide, methyl methacrylate and styrene can be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The ratio between the total mass of acrylamide, methyl methacrylate and styrene and the mass of cellulose fibril suspension can be 1:(4 to 30), preferably 1:(any value among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or the range between any two values). The mass ratio among acrylamide, methyl methacrylate and styrene can be 1:(0.2 to 20):(0.2 to 20), preferably 1:(any value among 0.2, 0.4, 0.6, 0.8, 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 or the range between any two values):(any value among 0.2, 0.4, 0.6, 0.8, 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 or the range between any two values).

[0083] When obtaining the oil-water mixed liquid, vinyl acetate, acrylamide, methyl methacrylate, styrene and the cellulose fibril suspension can be mixed and stirred evenly. Under this condition, the ratio between the mass of the cellulose fibrils and the total mass of vinyl acetate, acrylamide, methyl methacrylate and styrene can be 1:(50 to 200), preferably 1:(any value among 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 or the range between any two values). The ratio between the total mass of vinyl acetate, acrylamide, methyl methacrylate and styrene and the mass of the cellulose fibril suspension can be 1:(4 to 30), preferably 1:(any value among 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or the range between any two values). The mass ratio between vinyl acetate, acrylamide, methyl methacrylate and styrene can be 1:(0.5 to 5):(1 to 10):(1 to 10), preferably 1:(any value among 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 or the range between any two values):(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values):(any value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values).

[0084] Before mixing and reacting the Pickering emulsion with the water-soluble initiator, the Pickering emulsion can be first passed through an inert gas for deoxygenation; preferably, an inert gas at 65°C to 75°C (more preferably 70°C) is passed through for 20 min to 60 min (more preferably 30 min) for deoxygenation.

[0085] When mixing and reacting the Pickering emulsion with the water-soluble initiator, the Pickering emulsion and the water-soluble initiator can be first mixed and reacted at 65°C to 75°C (preferably 70°C) for 1.5 h to 3 h (preferably 2 h), then the temperature is raised to 85°C to 95°C (preferably 90°C) and reacted for 15 min to 30 min (preferably 20 min), and finally cooled to room temperature to obtain polymer microspheres. Before obtaining the polymer microspheres, the product obtained by mixing and reacting the Pickering emulsion with the water-soluble initiator can be filtered, preferably filtered with a 100-mesh to 200-mesh filter screen, and more preferably filtered with a 150-mesh filter screen.

[0086] Finally, during the preparation of the slurry and the coating and drying process, first prepare the PVDF polymer into a slurry; then, coat the slurry on at least one side of the base film and dry it.

[0087] When coating the slurry on at least one side of the base film, the slurry can be coated on one side or both sides of the base film. In addition, when coating the slurry on at least one side of the base film, one or more of spraying, brushing, roll coating, knife coating, dip coating, screen printing coating, slot coating, spin coating, curtain coating can be used. The drying time can be from 0.5 min to 5 min, preferably any value among 0.5 min, 1 min, 1.5 min, 2 min, 2.5 min, 3 min, 3.5 min, 4 min, 4.5 min, 5 min or the range between any two values; the drying temperature can be from 50 °C to 100 °C, preferably any value among 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, 100 °C or the range between any two values.

[0088] The power of the ultrasonic crushing treatment can be from 500 W to 1000 W, preferably any value among 500 W, 600 W, 700 W, 800 W, 900 W, 1000 W or the range between any two values; the time can be from 5 min to 15 min, preferably any value among 5 min, 6 min, 7 min, 8 min, 9 min, 10 min, 11 min, 12 min, 13 min, 14 min, 15 min or the range between any two values.

[0089] Under the conditions including the preparation of polymer microspheres, during the preparation of the slurry and the coating and drying process, first mix the PVDF polymer and the polymer microspheres to obtain a slurry; then, coat the slurry on at least one side of the base film and dry it.

[0090] The mass ratio between the PVDF polymer and the polymer microspheres can be 1:(0.1 to 10), preferably 1:(any value among 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or the range between any two values). The base film can be a polyolefin base film, preferably one or more of a polyethylene base film and a polypropylene base film.

[0091] When mixing the PVDF polymer with polymer microspheres, one or more of polyacrylonitrile binder and polyacrylate binder can be further added to obtain a slurry; the polyacrylate binder can be one or more of polymethacrylate, polyethyl acrylate, n-butyl acrylate, tert-butyl acrylate, and octadecyl acrylate. Under the conditions including polyacrylonitrile binder and polyacrylate binder, the mass ratio among the polyacrylate binder, polyacrylonitrile binder, PVDF polymer, and polymer microspheres can be 1:(1 to 5):(5 to 20):(5 to 20), preferably 1:(1, 2, 3, 4, 5):(any value among 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or the range between any two values):(any value among 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or the range between any two values). When mixing the PVDF polymer with polymer microspheres, water can be further added to obtain a slurry.

[0092] The present invention is exemplarily illustrated by the following examples:

[0093] Example 1

[0094] The preparation method of powdered PVDF specifically includes the following steps:

[0095] (1) Weigh 50 parts by mass of VDF monomer, purge with nitrogen at 100 °C for 20 min, add 8 parts by mass of sodium dodecyl sulfate after cooling to 45 °C, mechanically stir at 65 °C for 2 h after sealing, and then cool down to obtain a polymer precursor;

[0096] (2) Mix 50 parts by mass of the obtained polymer precursor and 0.1 part by mass of potassium persulfate, seal and heat to 75 °C for reaction for 2 h, add 5 parts by mass of HFP monomer, continue to react for 4 h, then cool to room temperature and filter through a 150-mesh filter screen to obtain a PVDF emulsion;

[0097] (3) Add 50 parts by mass of the obtained PVDF emulsion to a spray drying device and mechanically process for 30 min to obtain PVDF powder.

[0098] The preparation method of polymer microspheres specifically includes the following steps:

[0099] (1) Weigh 10 parts by mass of styrene monomer, 10 parts by mass of methyl methacrylate monomer, 5 parts by mass of acrylamide monomer, and 5 parts by mass of vinyl acetate monomer, mix them and drop them into 120 parts by mass of a 0.2 wt% cellulose fibril suspension, disperse evenly, and then process in an ultrasonic crusher with a power of 600 W for 10 min to obtain a Pickering emulsion;

[0100] (2) Weigh 50 parts by mass of the obtained Pickering emulsion, place it in a constant-temperature device at 70 °C, purge with nitrogen for 30 min, then add 0.2 parts by mass of the water-soluble initiator ammonium persulfate and react for 2 h. Then, raise the temperature to 90 °C and react for 20 min, and then cool to room temperature. Filter through a 150-mesh sieve to remove the filter residue, and polymer microspheres can be obtained.

[0101] The preparation method of the lithium battery separator specifically includes the following steps:

[0102] (1) Weigh 10 parts by mass of the obtained PVDF powder, 10 parts by mass of the obtained polymer microspheres, 3 parts by mass of polyacrylonitrile, and 1 part by mass of polymethacrylate, and add them successively to 200 parts by mass of deionized water. Disperse them evenly to obtain a slurry.

[0103] (2) Take 3 parts by mass of the obtained slurry, use a laboratory small sprayer to evenly spray the slurry on the polyethylene film, and then bake it in an oven at 60 °C for 1 min to obtain the lithium battery separator.

[0104] Comparative Example 1

[0105] The preparation method of the powdered PVDF specifically includes the following steps:

[0106] (1) Weigh 50 parts by mass of VDF monomer, purge with nitrogen at 100 °C for 20 min, cool to 45 °C, then add 8 parts by mass of sodium dodecyl sulfate, seal and stir mechanically at 65 °C for 2 h, and then cool down to obtain a polymer precursor.

[0107] (2) Mix 50 parts by mass of the obtained polymer precursor, 5 parts by mass of HFP monomer, and 0.1 part by mass of potassium persulfate, seal and heat to 75 °C and react for 2 h. Then, cool to room temperature and filter through a 150-mesh filter to obtain a PVDF emulsion.

[0108] (3) Add 50 parts by mass of the obtained PVDF emulsion to a spray drying device and mechanically process it for 30 min to obtain PVDF powder.

[0109] The preparation method of the polymer microspheres specifically includes the following steps:

[0110] (1) Weigh 10 parts by mass of styrene monomer, 10 parts by mass of methyl methacrylate monomer, 5 parts by mass of acrylamide monomer, and 5 parts by mass of vinyl acetate monomer, mix them, and drop them into 240 parts by mass of a 0.2 wt% cellulose fibril suspension. After dispersing them evenly, process them in an ultrasonic crusher with a power of 600 W for 10 min to obtain a Pickering emulsion.

[0111] (2) Weigh 50 parts by mass of the obtained Pickering emulsion, place it in a constant-temperature device at 70 °C, purge with nitrogen for 30 min, then add 0.2 parts by mass of the water-soluble initiator ammonium persulfate and react for 2 h. Then, raise the temperature to 90 °C and react for 20 min. After cooling to room temperature, filter through a 150-mesh sieve to remove the filter residue, and polymer microspheres can be obtained.

[0112] The preparation method of a lithium battery separator specifically includes the following steps:

[0113] (1) Weigh 10 parts by mass of the obtained PVDF powder, 10 parts by mass of the obtained polymer microspheres, 3 parts by mass of polyacrylonitrile, and 1 part by mass of polymethacrylate, and sequentially add them to 200 parts by mass of deionized water. Disperse them evenly to obtain a slurry.

[0114] (2) Take 3 parts by mass of the obtained slurry, use a laboratory small sprayer to evenly spray the slurry on a polyethylene film, and then dry it in an oven at 60 °C for 1 min to obtain a lithium battery separator.

[0115] Comparative Example 2

[0116] The preparation method of powdered PVDF specifically includes the following steps:

[0117] (1) Weigh 50 parts by mass of VDF monomer, purge with nitrogen at 100 °C for 20 min, cool to 45 °C, add 8 parts by mass of sodium dodecyl sulfate, seal, and mechanically stir at 65 °C for 2 h. Then, cool down to obtain a polymer precursor.

[0118] (2) Mix 50 parts by mass of the obtained polymer precursor and 0.1 part by mass of potassium persulfate, seal, and heat to 75 °C and react for 2 h. Then, add 5 parts by mass of HFP monomer, continue to react for 2 h, cool to room temperature, and filter through a 150-mesh filter to obtain a PVDF emulsion.

[0119] (3) Add 50 parts by mass of the obtained PVDF emulsion to a spray drying device and mechanically process for 30 min to obtain PVDF powder.

[0120] The preparation method of polymer microspheres specifically includes the following steps:

[0121] (1) Weigh 10 parts by mass of styrene monomer, 10 parts by mass of methyl methacrylate monomer, 5 parts by mass of acrylamide monomer, and 5 parts by mass of vinyl acetate monomer, mix them, and drop them into 120 parts by mass of a 0.5 wt% cellulose fibril suspension. After dispersing them evenly, process them in an ultrasonic crusher with a power of 600 W for 10 min to obtain a Pickering emulsion.

[0122] (2) Weigh 50 parts by mass of the obtained Pickering emulsion, place it in a constant temperature device at 70 °C, purge with nitrogen for 30 min, then add 0.2 parts by mass of the water-soluble initiator ammonium persulfate and react for 2 h. Then, raise the temperature to 90 °C and react for 20 min. After cooling to room temperature, pass it through a 150-mesh sieve to remove the filter residue, and polymer microspheres can be obtained.

[0123] The preparation method of the lithium battery separator specifically includes the following steps:

[0124] (1) Weigh 10 parts by mass of the obtained PVDF powder, 10 parts by mass of the obtained polymer microspheres, 3 parts by mass of polyacrylonitrile, and 1 part by mass of polymethacrylate, and sequentially add them to 200 parts by mass of deionized water, and disperse them evenly to obtain a slurry;

[0125] (2) Take 3 parts by mass of the obtained slurry, use a laboratory small sprayer to evenly spray the slurry on a polyethylene film, and then dry it in an oven at 60 °C for 1 min to obtain a lithium battery separator.

[0126] Perform performance tests on the Pickering emulsions prepared in Example 1, Comparative Example 1, and Comparative Example 2, and count their stability, particle size, viscosity and other parameters. The test data are shown in Table 1.

[0127] It can be seen from Table 1 that the Pickering emulsions of Example 1 and Comparative Example 2 are relatively stable, and the viscosity of the Pickering emulsion of Example 1 is lower than that of Comparative Example 2, indicating that the Pickering emulsion of Example 1 is relatively easy to apply.

[0128] Table 1

[0129]

[0130] Perform performance tests on the separators prepared in Example 1, Comparative Example 1, and Comparative Example 2, and count their air permeability, internal resistance, powder falling rate, adhesion strength with the positive electrode at different temperatures and other parameters. The test data are shown in Table 2.

[0131] It can be seen from Table 2 that the separator of Example 1 is superior to Comparative Examples 1 and 2 in terms of air permeability, self-adhesion, and adhesion with the positive electrode at different temperatures. Moreover, the internal resistance of the separator of Example 1 is also lower than that of Comparative Examples 1 and 2. Generally speaking, the separator of Example 1 can provide excellent air permeability, self-adhesion, and low internal resistance under the condition of meeting the requirement of high adhesion force with the electrode sheet at a relatively low hot pressing temperature.

[0132] Table 2

[0133]

[0134]

[0135] The above content related to well-known common knowledge will not be described in detail, and those skilled in the art can understand it.

[0136] The above are only some specific embodiments of the present invention and are not intended to limit the present invention. 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 present invention. The technical scope of this invention is not limited to the content in the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A battery separator, characterized in that, Comprising: A base film and an adhesive layer; the adhesive layer is formed on at least one side of the base film, and the adhesive layer comprises a PVDF polymer, and the PVDF polymer comprises a core region containing VDF and a shell region covering the core region and containing HFP.

2. The battery separator according to claim 1, wherein The adhesive layer further comprises: polymer microspheres, and the polymer microspheres comprise a core and a shell covering the core and containing cellulose fibrils.

3. The battery separator according to claim 2, wherein, The mass ratio between the PVDF polymer and the polymer microspheres is 1:(0.1 to 10).

4. The battery separator according to claim 1, wherein The mass ratio between the HFP and the VDF is 1:(8 to 10).

5. The battery separator according to claim 2, wherein The core of the polymer microspheres comprises one or more of polyvinyl acetate, polyacrylamide, polymethyl methacrylate, and polystyrene.

6. The battery separator according to claim 5, characterized in that, The core of the polymer microspheres sequentially comprises a first core containing the polyvinyl acetate, a second core covering the first core and containing the polyacrylamide, a third core covering the second core and containing the polymethyl methacrylate, and a fourth core covering the third core and containing the polystyrene from the inside to the outside.

7. The battery separator according to claim 6, wherein The ratio of the mass of the cellulose fibrils to the total mass of the polyvinyl acetate, the polyacrylamide, the polymethyl methacrylate, and the polystyrene is 1:(50 to 200).

8. The battery separator according to claim 6, characterized in that, The mass ratio between the polyvinyl acetate, the polyacrylamide, the polymethyl methacrylate, and the polystyrene is 1:(0.5 to 5):(1 to 10):(1 to 10).

9. The battery separator according to claim 1, wherein, The adhesive layer further comprises: one or more of polyacrylonitrile adhesives and polyacrylate adhesives.

10. A method for preparing a battery separator, characterized in that, Comprising: Preparing the PVDF polymer and preparing the slurry and coating and drying; Preparing the PVDF polymer comprises: Mixing and stirring VDF and an emulsifier to obtain a polymer precursor; After mixing and reacting the polymer precursor with an initiator, then adding HFP and reacting again to obtain a PVDF emulsion; and Spray-drying the PVDF emulsion to obtain the PVDF polymer; Preparing the slurry and coating and drying comprises: Preparing the PVDF polymer into a slurry; and Coating the slurry on at least one side of the base film and drying.

11. The method for preparing a battery separator according to claim 10, characterized in that, The emulsifier comprises one or more of sodium stearate, sodium laurate, sodium oleate, sodium dodecyl sulfate, sodium hexadecyl benzene sulfonate, and dodecyl benzene sulfonic acid.

12. The preparation method of the battery separator according to claim 10, wherein, The initiator comprises one or more of ammonium persulfate, potassium persulfate, and metal salts of peroxides.

13. The preparation method of the battery separator according to claim 10, wherein, Further comprising: preparing polymer microspheres; Preparing polymer microspheres comprises: Mixing and stirring one or more of vinyl acetate, acrylamide, methyl methacrylate, and styrene with a cellulose fibril suspension to obtain an oil-water mixed liquid, and then performing ultrasonic fragmentation treatment to obtain a Pickering emulsion; and Mixing and reacting the Pickering emulsion with a water-soluble initiator to obtain polymer microspheres; Preparing the PVDF polymer into a slurry in preparing the slurry and coating and drying further comprises: Mixing the PVDF polymer and the polymer microspheres to obtain the slurry.

14. The method for preparing a battery separator according to claim 13, wherein The water-soluble initiator comprises one or more of ammonium persulfate, potassium persulfate, and metal salts of peroxides.

15. The method for preparing a battery separator according to claim 13, characterized in that, The cellulose fibrils include one or more of absorbent cotton, pulp, wood pulp, and hemp.

16. The method for preparing a battery separator according to claim 10, wherein, The mass ratio between the emulsifier and the VDF is 1:(5 to 10).

17. The method for preparing a battery separator according to claim 10, characterized in that, The mass ratio among the initiator, the HFP, and the polymer precursor is 1:(30 to 70):(300 to 700).

18. The preparation method of the battery separator according to claim 13, wherein, The mass ratio of oil to water in the oil-water mixed liquid is 1:(2 to 8).

19. The method for preparing a battery separator according to claim 13, characterized in that, The mass ratio between the water-soluble initiator and the Pickering emulsion is 1:(200 to 400).

20. The method for preparing a battery separator according to claim 13, wherein The mass ratio between the PVDF polymer and the polymer microspheres is 1:(0.1 to 10).

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