A battery separator and a method for preparing the same

Abstract

The present application provides a battery separator, comprising a base film and a coating layer arranged on the base film, wherein the coating layer comprises a ceramic coating and a ceramic-PVDF mixed coating alternately arranged along a first direction parallel to the surface of the base film, wherein in the ceramic-PVDF mixed coating, from the side away from the base film to the side close to the base film, the mass content of the ceramic gradually increases, and the mass content of the PVDF gradually decreases. The battery separator can improve the bonding force between the separator and the electrode plate, reduce the thickness of the coating layer, and reduce the air permeability of the separator while ensuring sufficient high temperature resistance and electrolyte infiltration performance, thereby reducing the internal resistance of the separator and improving the electrochemical performance, rate performance, cycle performance and safety performance of the battery. The present application also provides a method for preparing a battery separator and a lithium-ion battery.

Description

Technical Field

[0001] The present application relates to the field of batteries, and in particular to a battery separator and a preparation method thereof. Background Art

[0002] Lithium-ion batteries are mainly composed of positive electrodes, negative electrodes, electrolytes and separators. The performance of the separator determines the structure of the battery and can directly affect the battery's capacity, rate performance, cycle performance and safety performance.

[0003] By adding a coating layer on the base film, the puncture resistance of the diaphragm can be increased, the thermal shrinkage rate of the diaphragm can be reduced, etc., thereby improving the performance of the diaphragm. However, the diaphragm containing a coating layer still has some problems, such as the low peeling strength between the diaphragm and the electrode plate, which is prone to faults; the diaphragm is thick, has a high air permeability, and has a large internal resistance, which affects the electrochemical properties and safety performance of the battery. These shortcomings seriously restrict the safe application of diaphragms containing coating layers in batteries. Summary of the invention

[0004] In view of this, the present application provides a battery separator and a preparation method thereof, wherein the battery separator includes a ceramic coating and a ceramic-PVDF mixed coating alternately arranged on the surface of a base film, the battery separator has a thin thickness, a low air permeability, a small internal resistance, and a high peeling strength between the separator and the electrode plate, and the electrochemical performance and safety performance of the battery using the separator are significantly improved.

[0005] In a first aspect, the present application provides a battery separator, comprising a base membrane and a coating layer arranged on the base membrane, wherein the coating layer comprises a ceramic coating and a ceramic-polyvinylidene fluoride (PVDF) mixed coating alternately arranged along a first direction parallel to the surface of the base membrane, wherein in the ceramic-PVDF mixed coating, the mass content of the ceramic gradually increases and the mass content of the PVDF gradually decreases from a side away from the base membrane to a side close to the base membrane.

[0006] In the present application, in the ceramic-PVDF mixed coating, from the side away from the base film to the side close to the base film, the mass content of the ceramic gradually increases from 15%-25% to 75-85%, and the mass content of the PVDF gradually decreases from 75%-85% to 15%-25%.

[0007] Optionally, the mass ratio of the ceramic to the PVDF is (1-9): (1-9).

[0008] In the present application, the ceramic coating comprises ceramic and a bonding component, wherein the bonding component comprises one or more of ethylene-acrylic acid copolymer, polyolefin polyacrylate, ethylene-acrylic acid fatty alcohol copolymer, fatty alcohol and acetylene alcohol.

[0009] Optionally, in the ceramic coating, the mass content of the bonding component is 0.5%-5%.

[0010] In the present application, the two ends of the coating layer in the first direction are the ceramic-PVDF hybrid coating, and the width ratio of the ceramic coating to the ceramic-PVDF hybrid coating in the first direction is (1-3):(1-3).

[0011] Optionally, the coating layer has a thickness of 2 μm-3 μm.

[0012] The battery separator provided in the first aspect of the present application has an air permeability of 80s / 100mL-100s / 100mL and has good electrical conductivity; the peel strength between the battery separator and the electrode plate is 0.006kN / m-0.009kN / m and has strong bonding force.

[0013] The second aspect of the present application provides a method for preparing a battery separator, comprising the following steps:

[0014] Add ceramic powder and the first dispersant into water and stir evenly to prepare ceramic slurry;

[0015] Add PVDF powder and a second dispersant into water and stir evenly to prepare PVDF slurry;

[0016] The ceramic slurry is mixed with the PVDF slurry, and stirred evenly to obtain a ceramic-PVDF mixed slurry;

[0017] The ceramic slurry and the ceramic-PVDF mixed slurry are alternately coated on the base membrane to obtain the battery separator, wherein the battery separator includes the base membrane and a coating layer arranged on the base membrane, wherein the coating layer includes a ceramic coating and a ceramic-PVDF mixed coating alternately arranged along a first direction parallel to the surface of the base membrane, and wherein in the ceramic-PVDF mixed coating, the mass content of the ceramic gradually increases and the mass content of the PVDF gradually decreases from the side away from the base membrane to the side close to the base membrane.

[0018] Optionally, the first dispersant includes a phosphate dispersant, and the second dispersant includes a polyacrylic acid sodium salt dispersant.

[0019] In an embodiment of the present application, a bonding material is further added to the ceramic slurry, wherein the bonding material includes one or more of ethylene-acrylic acid copolymer wax emulsion, polyolefin polyacrylate emulsion, ethylene-acrylic acid fatty alcohol copolymer emulsion, fatty alcohol emulsion and acetylene alcohol emulsion.

[0020] The ceramic slurry and the ceramic-PVDF mixed slurry are alternately coated on the base film, specifically, the ceramic slurry and the ceramic-PVDF mixed slurry are alternately sprayed on the base film.

[0021] The preparation method provided in the second aspect of the present application is simple to operate, low-cost, and environmentally friendly, and can be applied to the industrial production of battery separators.

[0022] The third aspect of the present application provides a lithium-ion battery, comprising the battery separator provided in the first aspect of the present application, or comprising the battery separator prepared by the preparation method provided in the second aspect of the present application.

[0023] In the present application, the lithium-ion battery further comprises a positive electrode, a negative electrode and an electrolyte, and the battery separator is arranged between the positive electrode and the negative electrode.

[0024] The lithium-ion battery provided in the third aspect of the present application has good electrochemical performance and safety performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A schematic diagram of the cross-sectional structure of a battery separator provided in one embodiment of the present application;

[0026] Figure 2 A schematic diagram of the cross-sectional structure of a ceramic-PVDF coating layer in a battery separator provided in one embodiment of the present application;

[0027] Figure 3 A scanning electron microscope (SEM) photograph of a ceramic-PVDF coating layer provided in one embodiment of the present application;

[0028] Figure 4 A scanning electron microscope (SEM) photograph of a ceramic-PVDF coating layer provided as a comparison of the present application. DETAILED DESCRIPTION

[0029] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0030] See also Figure 1 and Figure 2The embodiment of the present application provides a battery separator 100, which includes a base film 101 and a coating layer 102 disposed on the base film. The coating layer 102 includes a ceramic coating 10 and a ceramic-PVDF hybrid coating 20 alternately disposed along a first direction parallel to the surface of the base film. In the ceramic-PVDF hybrid coating 20, from the side B away from the base film to the side A close to the base film, the mass content of the ceramic 20a gradually increases, and the mass content of the PVDF 20b gradually decreases.

[0031] In the embodiment of the present application, the coating layer 102 includes a ceramic coating 10 and a ceramic-PVDF mixed coating 20, and the components in the coating layer that play a major role in improving the comprehensive performance of the diaphragm are ceramics and PVDF. After the ceramic is bonded to the base film, the high temperature resistance and puncture resistance of the diaphragm can be improved, thereby improving the safety of the battery. PVDF is a homopolymer or copolymer of PVDF, specifically a PVDF homopolymer or a copolymer of PVDF and other small amounts of fluorine-containing vinyl monomers. The PVDF has the characteristics of both fluororesins and general resins, has good chemical stability and electrical insulation, and is used in battery diaphragms to improve the stability and safety of the battery. Due to the presence of van der Waals forces between PVDF molecules, the diaphragm and the electrode pole piece can be bonded together to enhance the bonding force between the pole piece and the diaphragm, and improve the peel strength between the pole piece and the diaphragm, thereby improving the stability and service life of the battery. In addition, PVDF contains the element F, which can improve the electrochemical stability, solvent resistance and affinity of PVDF itself to polar electrolytes, thereby helping to improve the electrolyte absorption rate of the diaphragm. Since PVDF has good compatibility with electrolyte, PVDF can also significantly reduce the contact angle between the diaphragm and the electrolyte, so that the diaphragm maintains better wettability in the electrolyte, thereby reducing the internal resistance of the battery and increasing the discharge power of the battery.

[0032] In the embodiment of the present application, the ceramic-PVDF hybrid coating 20 includes ceramic and PVDF, and the mass content of ceramic and PVDF in the ceramic-PVDF hybrid coating 20 gradually changes from the side B far away from the base membrane to the side A close to the base membrane, the mass content of ceramic gradually increases, and the mass content of PVDF gradually decreases, that is, on the side far away from the base membrane, the mass content of PVDF is greater than the mass content of ceramic, which can make the contact area between PVDF and the electrode sheet larger, which is beneficial to improve the bonding force between the diaphragm and the electrode electrode sheet, and on the side close to the base membrane, the mass content of ceramic is greater than the mass content of PVDF, which can make the contact area between ceramic and the base membrane larger, which is helpful to improve the high temperature resistance and mechanical strength of the diaphragm. However, when the mass content of PVDF on the side far away from the base membrane accounts for too large a proportion, the effect of ceramic on the side far away from the base membrane is not obvious, and when the mass content of ceramic on the side close to the base membrane accounts for too large a proportion, the effect of PVDF on the side close to the base membrane is not obvious, therefore, controlling the mass content proportion of ceramic and PVDF on the side far away from the base membrane and the side close to the base membrane within an appropriate range can improve the comprehensive performance of the diaphragm to a greater extent. In some embodiments of the present application, from the side away from the base film to the side close to the base film, the mass content of the ceramic gradually increases from 15%-25% to 75-85%, and the mass content of PVDF gradually decreases from 75%-85% to 15%-25%. In one embodiment, for example, from the side away from the base film to the side close to the base film, the mass content of the ceramic gradually increases from 20% to 80%, and the mass content of PVDF gradually decreases from 80% to 20%. In order to further ensure the improvement effect of ceramics and PVDF on the comprehensive performance of the diaphragm, in some embodiments of the present application, the mass of the ceramic within the 30% thickness range of the side A close to the base film accounts for greater than or equal to 40%-50% of the mass of the ceramic in the entire ceramic-PVDF hybrid coating 20, that is, in the entire ceramic-PVDF hybrid coating 20, greater than or equal to 40%-50% of the mass of the ceramic is distributed in the 0%-30% thickness range of the side A close to the base film. In some embodiments of the present application, the mass of PVDF within the 30% thickness range away from the side B of the base film is greater than or equal to 40%-50% of the mass of PVDF in the entire ceramic-PVDF hybrid coating 20, that is, in the entire ceramic-PVDF hybrid coating 20, greater than or equal to 40%-50% of the mass of PVDF is distributed in the 0%-30% thickness range away from the side B of the base film.

[0033] In the implementation manner of the present application, the mass ratio of ceramic to PVDF in the ceramic-PVDF coating 20 is (1-9): (1-9). In order to make ceramic and PVDF exert a better comprehensive effect, in some embodiments of the present application, the mass ratio of ceramic to PVDF in the ceramic-PVDF coating 20 can be (1-4): 1. Specifically, the mass ratio of ceramic to PVDF can be 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1, etc.

[0034] In the embodiment of the present application, the ceramic coating 10 includes ceramic and bonding components. The bonding components can ensure that the diaphragm and the electrode pole piece show good heat bonding ability during the hot pressing process, and avoid the fault phenomenon of the battery cell after hot pressing due to insufficient bonding force, which affects the stability and safety of the battery. Specifically, the bonding component includes but is not limited to one or more of ethylene-acrylic acid copolymer, polyolefin polyacrylate, ethylene-acrylic acid fatty alcohol copolymer, fatty alcohol and acetylenic alcohol. In the ceramic coating 10, if the content of the bonding component is too low, the thermal bonding force between the pole piece and the diaphragm will be insufficient during the hot pressing process, and if the content of the bonding component is too high, the relative content of the ceramic will be reduced, which will affect the high temperature resistance of the diaphragm. In order to improve the comprehensive performance of the diaphragm, in the embodiment of the present application, the mass content of the bonding component is 0.5%-5%, and can be specifically 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, etc.

[0035] In the embodiment of the present application, the coating layer 102 can be arranged on one side or both sides of the base film 101. On the one hand, the coating layer can reduce the thermal shrinkage rate of the diaphragm and improve the puncture resistance of the diaphragm by enhancing the thickness and hardness of the diaphragm, thereby improving the safety of the battery; on the other hand, the coating material in the coating layer can maintain a higher wettability with the electrolyte, reduce the internal resistance of the battery, and increase the discharge power of the battery, thereby improving the electrochemical performance of the battery.

[0036] In the embodiment of the present application, the coating slurry is applied to the surface of the base film by spraying, so that the contact between the coating layer and the base film can be uniform and close point contact, and the proportion of the coating area in the surface area of ​​the base film can be controlled to ensure that the base film has a sufficient exposed area, so that the air permeability of the diaphragm is reduced. At the same time, the point contact form of the coating layer allows more pores to exist in the coating layer, ensuring sufficient contact between the diaphragm and the electrolyte, thereby ensuring the wettability of the diaphragm.

[0037] In the embodiment of the present application, the ceramic coating 10 and the ceramic-PVDF mixed coating 20 are alternately arranged along the first direction parallel to the surface of the base film. The alternating arrangement of the ceramic coating and the ceramic-PVDF coating can give full play to the advantages of the ceramic coating and the ceramic-PVDF coating, improve the structural stability of the diaphragm, and prevent the diaphragm from being substantially damaged due to local damage. The two ends of the coating layer 102 in the first direction are the ceramic-PVDF mixed coating 20, which can further ensure the bonding force between the diaphragm and the electrode plate, and avoid the fault phenomenon caused by insufficient bonding force between the diaphragm and the electrode plate. In some embodiments, when the total coating width of the diaphragm is fixed in the first direction, the number of alternating coatings can be controlled by changing the width of a single coating. For example, in a diaphragm with a total coating width of 20 cm along the first direction, the ceramic-PVDF coating and the ceramic coating are recorded as X and Y, respectively, and the alternating coatings can be expressed as X / Y / X, X / Y / X / Y / X, X / Y / X / Y / X / Y / X, X / Y / X / Y / X / Y / X / Y / X, etc. In some other embodiments, one or more coatings containing different components may be added to perform alternating coating of multiple coatings.

[0038] In the embodiment of the present application, the width ratio of the ceramic coating 10 and the ceramic-PVDF hybrid coating 20 in the first direction is (1-3):(1-3). In some embodiments, the width ratio can be 1:1, 1:2, 1:3, 2:1, 3:1, etc.; the coating layer thickness is 2μm-3μm. In some embodiments, the thickness can be 2μm, 2.5μm, 3μm, etc.

[0039] In the implementation mode of the present application, the finer the particle size of PVDF, the larger the specific surface area, the more contact points in the diaphragm, and the better the hot pressing effect that can be achieved. In some embodiments, the particle size D50 of PVDF is 4μm-7μm. In order to make the resulting coating smoother, the particle size can be controlled to be 4μm-5μm, specifically 4μm, 4.5μm, 5μm, etc. In some embodiments, the particle size D50 of the ceramic is 0.5μm-2μm. In order to make the resulting coating smoother, the particle size can be controlled to be 0.5μm-1μm, specifically 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, etc.

[0040] In the embodiment of the present application, the base film may be a combination of one or more of a polyethylene (PE) film, a polypropylene (PP) film and a PP / PE composite film.

[0041] The battery separator provided in the embodiment of the present application has good electrical conductivity, and its air permeability is 80s / 100mL-100s / 100mL, and can be specifically 80s / 100mL, 85s / 100mL, 90s / 100mL, 95s / 100mL, 100s / 100mL, etc. When the battery separator and the electrode plate are stacked and assembled, there is a strong bonding force between the battery separator and the electrode plate, and the peel strength between the two is 0.006kN / m-0.009kN / m, and can be specifically 0.006kN / m, 0.007kN / m, 0.008kN / m, 0.009kN / m, etc.

[0042] The present application also provides a method for preparing the above-mentioned battery separator, comprising the following steps:

[0043] (1) adding ceramic powder and a first dispersant into water and stirring evenly to prepare a ceramic slurry;

[0044] (2) adding PVDF powder and a second dispersant into water and stirring evenly to prepare PVDF slurry;

[0045] (3) mixing the ceramic slurry and the PVDF slurry and stirring them evenly to obtain a ceramic-PVDF mixed slurry;

[0046] (4) Alternately coating the ceramic slurry and the ceramic-PVDF mixed slurry on the base film to obtain a battery separator. The battery separator includes a base film and a coating layer disposed on the base film, the coating layer includes a ceramic coating layer and a ceramic-PVDF mixed coating layer alternately disposed along a first direction parallel to the surface of the base film, and in the ceramic-PVDF mixed coating layer, the mass content of the ceramic gradually increases and the mass content of the PVDF gradually decreases from the side away from the base film to the side close to the base film.

[0047] In the embodiment of the present application, the ceramic powder in the ceramic slurry can be one or more of boehmite, alumina, silicon dioxide, magnesium oxide, titanium dioxide, zirconium dioxide, and barium sulfate. The mass proportion of the ceramic powder in the ceramic slurry can be 5wt%-80wt%, and to achieve better results, it can be 25wt%-50wt%, specifically 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, etc.

[0048] In the embodiment of the present application, the first dispersant may be a phosphate dispersant. The mass proportion of the first dispersant in the ceramic slurry may be 0.5wt%-10wt%, and to achieve a better dispersion effect, may be 2wt%-8wt%, specifically 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, etc.

[0049] In the embodiment of the present application, the PVDF in the PVDF slurry can be a homopolymer of PVDF or a copolymer of PVDF and other small amounts of fluorine-containing vinyl monomers. Optionally, it can be PVDF with a weight average molecular weight greater than 1000000 g / mol. The mass proportion of PVDF in the PVDF slurry is 5wt%-50wt%. In some embodiments, it can be 5wt%-35wt%, specifically 5wt%, 10wt%, 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, etc.

[0050] In the embodiment of the present application, the second dispersant and the first dispersant are different dispersants. The second dispersant can be a sodium polyacrylate dispersant. The mass proportion of the second dispersant in the PVDF slurry is 0.5wt%-10wt%. To achieve a better dispersion effect, it can be 2wt%-8wt%, specifically 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, etc.

[0051] In the embodiment of the present application, different dispersants are added to the ceramic slurry and PVDF slurry during the preparation process, so that the components of the ceramic material and the PVDF material are independent of each other, which is conducive to the uniform dispersion of the ceramic particles and the PVDF particles, and is convenient for controlling the size of the particle size and ensuring the uniform distribution of the particle size. The ceramic slurry and the PVDF slurry can also have different hydrophilicities, specifically, the hydrophilicity of the ceramic particles is higher than that of the PVDF particles. In the ceramic-PVDF mixed slurry prepared by mixing the above-mentioned ceramic slurry and PVDF slurry, the hydrophilicity of the PVDF particles is lower, and the hydrophilicity of the ceramic particles is higher, so that the ceramic-PVDF mixed slurry can obtain the effect of PVDF being distributed toward the side away from the base film and the ceramic being distributed toward the side close to the base film during the drying process after being coated on the base film surface. That is, on the side far away from the base membrane, the mass content of PVDF is greater than the mass content of ceramics, and the contact area between PVDF and the electrode piece is larger, which is conducive to the bonding effect of PVDF on the diaphragm and the electrode piece. On the side close to the base membrane, the mass content of ceramics is greater than the mass content of PVDF, and the contact area between ceramics and the base membrane is larger, which helps the ceramics to enhance the high temperature resistance and mechanical strength of the diaphragm.

[0052] In the embodiment of the present application, the above preparation method also includes adding a bonding material to the ceramic slurry, and the bonding material can play a bonding role during the hot pressing process of the diaphragm and the electrode plate, enhance the heat-sealing ability between the diaphragm and the electrode plate, and enhance the bonding force between the diaphragm and the electrode plate after hot pressing, so as to avoid the fault phenomenon caused by insufficient bonding force between the diaphragm and the electrode plate. Optionally, the bonding material can be one or more of ethylene-acrylic acid copolymer wax emulsion, polyolefin polyacrylate emulsion, ethylene-acrylic acid fatty alcohol copolymer emulsion, fatty alcohol emulsion and acetylene alcohol emulsion, and the mass proportion of the bonding material in the ceramic slurry can be 0.5wt%-5wt%, and to achieve better results, it can be 1wt%-4wt%, and specifically can be 1wt%, 2wt%, 3wt%, 4wt%, etc.

[0053] In the embodiment of the present application, in order to improve the comprehensive performance of the diaphragm, the above-mentioned preparation method also includes adding additives during the preparation of the ceramic slurry and the PVDF slurry, and the additives include one or more of a thickener, an adhesive, a wetting agent and a defoamer. The thickener can be carboxymethyl cellulose. The adhesive can be one or more of polyacrylate, SBR (styrene-butadiene rubber) and xanthan gum in any proportion. The wetting agent can be one or more of a polyether silicone copolymer, an ethylene oxide adduct, and a fluorine polymer. The defoamer is one or more of mineral oils, silicones, and polyethers. The mass proportion of the additive in the ceramic slurry or PVDF slurry is 0.5wt%-5wt%, and to achieve better results, it can be 1wt%-4wt%, specifically 1wt%, 2wt%, 3wt%, 4wt%, etc.

[0054] In the embodiment of the present application, an adhesive is added during the preparation of the ceramic slurry and the PVDF slurry. The main function of the adhesive is to bond the coating layer to the base film, ensuring that the coating layer does not fall off the base film and does not fall off after drying. However, the adhesive is slightly weaker in enhancing the heat-sealing ability of the diaphragm and the electrode during the hot pressing process. The enhancement of the heat-sealing ability mainly depends on the PVDF component in the ceramic-PVDF coating and the bonding components in the ceramic coating and the ceramic-PVDF coating. The enhancement of the bonding force between the coating layer and the base film and the enhancement of the heat-sealing ability between the diaphragm and the electrode electrode during the hot pressing process jointly improve the peeling strength between the diaphragm and the electrode electrode.

[0055] In the embodiment of the present application, the coating is applied by spraying, and the contact between the coating layer and the base film is uniform and close point contact. The proportion of the coating area in the surface area of ​​the base film can be controlled to ensure that the base film has sufficient exposed area, so that the air permeability of the diaphragm is reduced and the internal resistance is reduced, thereby improving the cycle performance and rate performance of the battery. Since the coating layer does not cover the base film 100%, there are many pores in the coating layer, which can ensure the wettability of the diaphragm. Coating by spraying can also reduce the thickness of the coating layer, reduce the distance between the positive and negative electrode sheets, shorten the ion diffusion distance, thereby improving the rate performance of the battery, saving raw materials and reducing costs. In addition, by spraying, the nozzles at specific stations can be adjusted so that the nozzles are connected to different slurries to achieve simultaneous alternating coating of the coatings.

[0056] In the implementation manner of the present application, water is used as the solvent, which is environmentally friendly, highly safe, and has low production costs; in addition, the water-based PVDF-coated diaphragm has strong oxidation resistance and can meet the requirements of high-voltage batteries.

[0057] In the embodiment of the present application, the dispersion method of the ceramic slurry and the PVDF slurry can be sand milling, shearing, ultrasound, etc. In one embodiment, the dispersion is carried out by sand milling, and the speed of the sand mill is 500rpm-3000rpm. To achieve a better sand milling effect, it can be 500rpm-2500rpm, and specifically can be 500rpm, 1000rpm, 1500rpm, 2000rpm, 2500rpm, etc. The diameter of the zirconium ball is 0.2mm-1.8mm. To achieve a better sand milling effect, it can be 0.3mm-1.5mm, and specifically can be 0.3mm, 0.5mm, 1mm, 1.5mm, etc. The total weight is 1kg-10kg. To achieve a better sand milling effect, it can be 2kg-8kg, and specifically can be 2kg, 3kg, 4kg, 5kg, 6kg, 7kg, 8kg, etc. The sand mill speed, zirconium ball size, and sand milling time can be determined according to the quality of the raw materials and the required particle size, and sand milling is performed until the required particle size is reached.

[0058] In the embodiment of the present application, the base film can be selected from a polyethylene (PE) film, a polypropylene (PP) film, and a combination of one or more of a PP / PE composite film. Since the coating layer mainly acts on the surface of the base film to improve the performance of the diaphragm, the thickness of the base film has little effect on the effect of the coating layer and is not particularly limited. In the embodiment of the present application, the base film thickness can be various existing thicknesses, such as 6μm, 8μm, 10μm, 12μm, 14μm, etc.

[0059] In the implementation manner of the present application, the stirring line speed in the various slurry preparation processes in the above steps is 2m / s-10m / s. In order to make the obtained slurry more uniform, in the embodiment of the present application, the stirring line speed is 4m / s-6m / s, specifically 4m / s, 5m / s, 6m / s, etc. The stirring time is 10min-60min. In order to make the obtained slurry more uniform, in the embodiment of the present application, the stirring time is 30min-40min, specifically 30min, 35min, 40min, etc.

[0060] In the embodiment of the present application, during the spraying process, two or more slurries can be alternately coated simultaneously by adjusting the nozzle of a specific station to be connected to the corresponding slurry tank. The coating thickness can be 2μm-3μm, specifically 2μm, 2.5μm, 3μm, etc. The width ratio of the two coatings is (1-3):(1-3). In one embodiment, the width ratio of the two coatings is 1:1.

[0061] In the embodiment of the present application, during the spraying process, the base film travels at a speed of 25 m / s-40 m / s, specifically 25 m / s, 30 m / s, 35 m / s, 40 m / s, etc.

[0062] The diaphragm prepared by the above preparation method can reduce the coating thickness and the air permeability of the diaphragm while ensuring sufficient high temperature resistance and electrolyte wetting performance, thereby reducing the internal resistance of the diaphragm, improving the rate performance of the battery, and improving the bonding force between the diaphragm and the electrode plate. In addition, the raw materials required for the preparation method are low in price, safe and environmentally friendly.

[0063] An embodiment of the present application also provides a lithium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte and the above-mentioned battery separator or the battery separator prepared by the above-mentioned preparation method, wherein the battery separator is arranged between the positive electrode and the negative electrode.

[0064] The positive electrode includes a positive electrode current collector and a positive electrode active material, a binder and a conductive agent loaded on the positive electrode current collector, wherein the positive electrode current collector can be an aluminum foil, and the positive electrode active material can be various available positive electrode active materials, including but not limited to LiFePO4, LiMnO2, LiNiO2, LiCoO2, LiVPO4F, LiFeO2 and ternary positive electrode active materials.

[0065] The negative electrode includes a negative electrode current collector and a negative electrode active material, a binder and a conductive agent loaded on the negative electrode current collector. The negative electrode current collector can be copper foil, and the negative electrode active material can adopt all negative electrode materials that can be commercially used so far, including but not limited to one or more of natural graphite, artificial graphite, hard carbon, soft carbon, and lithium titanate.

[0066] The binder may be any binder known in the art, including but not limited to one or more of polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber and polyacrylate. The conductive agent may be any conductive agent known in the art, including but not limited to one or more of graphite, carbon fiber, carbon black, metal powder, metal oxide and fiber.

[0067] The above lithium-ion battery has excellent electrochemical performance, high stability, high safety and long service life.

[0068] The implementation scheme of this application is further explained below with multiple cases.

[0069] Example 1

[0070] A method for preparing a battery separator comprises the following steps:

[0071] (1) preparing a water-based ceramic slurry: adding 6 wt% of a phosphate dispersant, 40 wt% of boehmite powder, 3 wt% of a carboxymethyl cellulose thickener, 3 wt% of a polyacrylate adhesive, 3 wt% of an ethylene-acrylic acid copolymer wax emulsion bonding material, 3 wt% of a polyether silicone copolymer wetting agent, and 3 wt% of a mineral oil defoamer to deionized water, and obtaining a water-based ceramic slurry after sand grinding;

[0072] (2) preparing a water-based PVDF slurry: adding 6 wt % of a sodium salt of polyacrylic acid dispersant, 25 wt % of PVDF powder, 3 wt % of a carboxymethyl cellulose thickener, 3 wt % of a polyacrylate adhesive, 3 wt % of a polyether silicone copolymer wetting agent, and 3 wt % of a mineral oil defoamer to deionized water, and obtaining a water-based PVDF slurry after sand grinding;

[0073] (3) Preparing a water-based ceramic-PVDF mixed slurry: mixing the above ceramic slurry and PVDF slurry in a ratio of 1:1, stirring for 30 min at a stirring line speed of 5 m / s to obtain a ceramic-PVDF mixed slurry;

[0074] (4) Using a rotary spraying device, the ceramic-PVDF mixed slurry and the ceramic slurry are alternately coated with the same width in a first direction parallel to the surface of the base film, wherein both ends of the coating layer in the first direction are ceramic-PVDF mixed coatings, and the base film is a PE film with a thickness of 12 μm, an air permeability of 80 s / mL, a longitudinal (MD) thermal shrinkage of 4% at 105°C for 1 h, and a transverse (TD) thermal shrinkage of 2.5% at 105°C for 1 h, and the base film travel speed is 30 m / s.

[0075] Example 2

[0076] A method for preparing a battery separator, which differs from Example 1 in that: in step (1), the amount of boehmite powder added is 75wt%; in step (2), the amount of PVDF powder added is 10wt%.

[0077] Example 3

[0078] A method for preparing a battery separator, which differs from Example 1 in that: in step (1), the amount of boehmite powder added is 25wt%; in step (2), the amount of PVDF powder added is 35wt%.

[0079] Example 4

[0080] A method for preparing a battery separator, which differs from Example 1 in that: in step (1), the bonding material is a polyolefin polyacrylate emulsion.

[0081] Example 5

[0082] A method for preparing a battery separator, which differs from Example 1 in that: in step (1), the bonding material is ethylene-acrylic acid fatty alcohol copolymer emulsion.

[0083] Example 6

[0084] A method for preparing a battery separator, which differs from Example 1 in that: in step (1), the bonding material is a fatty alcohol emulsion.

[0085] Example 7

[0086] A method for preparing a battery separator, which differs from Example 1 in that: in step (1), the bonding material is an acetylene alcohol emulsion.

[0087] Comparative Example 1

[0088] A method for preparing a battery separator, which differs from Example 1 in that: in step (2), the dispersant is the same as the dispersant in step (1), both of which are phosphate dispersants.

[0089] Comparative Example 2

[0090] A method for preparing a battery separator, which differs from Example 1 in that: in step (1), no ethylene-polyolefin polyacrylate emulsion bonding material is added.

[0091] Comparative Example 3

[0092] A method for preparing a battery separator, which differs from Example 1 in that: in step (4), a roller coating device is used for gravure roller coating, and the roller coating parameters are: a feed pump speed of 80 rpm, a base film speed of 35 m / min, and a gravure roller coating speed ratio of 0.8. Wherein, the gravure roller coating speed ratio = base film speed / gravure roller speed.

[0093] The morphology of the ceramic-PVDF coating in Example 1 and Comparative Example 1 was scanned by scanning electron microscopy, and the scanning electron microscopy images obtained were as follows: Figure 3 and Figure 4 shown.

[0094] Figure 3 It is shown that in the ceramic-PVDF coating, PVDF 20b is distributed toward the side away from the base film, and ceramic 20a is distributed toward the side close to the base film. Figure 4 It shows that the ceramic and PVDF in the ceramic-PVDF coating are evenly distributed in the coating thickness direction, and there is no obvious phenomenon that PVDF20b is distributed away from the base film and ceramic 20a is distributed close to the base film. The scanning electron microscope images of Example 1 and Comparative Example 1 show that the use of different dispersants in the ceramic slurry and PVDF slurry is an important factor in causing the ceramic-PVDF coating to have PVDF20b distributed away from the base film and ceramic 20a distributed close to the base film.

[0095] In order to strongly support the beneficial effects brought by the technical solutions of the embodiments of this application, the following tests are provided:

[0096] Thickness: Tested with a thickness tester with an accuracy of ±0.1μm. Test method: Take 10 layers of diaphragm to test the total thickness, then divide it by 10, and take the average of 5-10 points in the TD direction.

[0097] Air permeability: tested using a Gurley air permeability tester, with reference to ASTM D726.

[0098] Thermal shrinkage: The test was conducted in a constant temperature forced air oven. The test method was as follows: a 20 cm × 20 cm piece of diaphragm was cut and placed in an oven at 105°C for 1 hour, then taken out and the thermal shrinkage in the transverse direction (TD) and the longitudinal direction (MD) was measured.

[0099] Peel strength: tested using a universal mechanical testing machine. Test method: hot press the diaphragm and the positive electrode at 3MPa and 80℃ for 1min, cut into 20mm width after hot pressing, peel vertically at 180 degrees, peel speed 20mm / min, national standard refers to GB / T 2792.

[0100] Diaphragm resistance: Take 1-10 layers of diaphragm and punch them into 10 sample sheets. Clamp the sample sheets between two symmetrical aluminum foil sheets to make a soft pack battery and test its impedance spectrum. The test area of ​​the diaphragm is A = 3.2 × 2.6 = 8.6 cm 2 ; Draw a curve with the number of diaphragm layers as the horizontal axis and the resistance values ​​corresponding to different numbers of layers as the vertical axis, and calculate the slope of the curve and the linear fit; when the linear fit is greater than 0.99, the resistance value of a single-layer diaphragm is equal to the slope of the curve.

[0101] The test results are shown in Table 1 below:

[0102] Table 1

[0103]

[0104] As shown in Table 1 above, the relative content of PVDF and ceramic in the ceramic-PVDF coating has an important influence on the performance of the diaphragm. It can be seen from Examples 1-3 that as the PVDF content increases and the ceramic content decreases, the high temperature resistance of the diaphragm decreases and the bonding force between the diaphragm and the electrode plate increases.

[0105] It can be seen from the various embodiments and comparative example 3 that the obvious advantage of spray coating over roller coating is that the spray coating product has a thinner thickness, a lower air permeability value, a lower resistance and a higher conductivity.

[0106] In the ceramic-PVDF mixed slurry, the dispersion of PVDF and ceramic has a significant impact on the performance of the diaphragm. It can be seen from Example 1 and Comparative Example 1 that using the same dispersant will result in a slightly larger shrinkage rate, a slightly lower adhesion, and a worse high temperature resistance.

[0107] It can be seen from the embodiments and comparative example 2 that the bonding materials including ethylene-acrylic acid copolymer wax emulsion, polyolefin polyacrylate emulsion, ethylene-acrylic acid fatty alcohol copolymer emulsion, fatty alcohol and acetylene alcohol emulsion have a significant effect on improving the bonding between the diaphragm and the electrode plate after hot pressing. Without adding bonding materials, the peel strength between the diaphragm and the electrode plate is significantly weakened.

[0108] Taking all the properties into consideration, the mass ratio of ceramic to PVDF is preferably the ratio of Example 1, but according to the needs, the ratio of Examples 2 and 3 can also be selected.

Claims

1. A battery separator, characterized in that: The battery separator includes a base film and a coating layer arranged on the base film, the coating layer includes a ceramic coating and a ceramic-PVDF mixed coating alternately arranged along a first direction parallel to the surface of the base film, in the ceramic-PVDF mixed coating, from the side away from the base film to the side close to the base film, the mass content of ceramic gradually increases, and the mass content of PVDF gradually decreases; in the ceramic-PVDF mixed coating, from the side away from the base film to the side close to the base film, the mass content of ceramic gradually increases from 15%-25% to 75%-85%, and the mass content of PVDF gradually decreases from 75%-85% to 15%-25%.

2. The battery separator according to claim 1, characterized in that The ceramic coating comprises ceramic and a bonding component, wherein the bonding component comprises one or more of ethylene-acrylic acid copolymer, polyolefin polyacrylate, ethylene-acrylic acid fatty alcohol copolymer, fatty alcohol and acetylene alcohol.

3. The battery separator according to claim 2, characterized in that In the ceramic coating, the mass content of the bonding component is 0.5%-5%.

4. The battery separator according to claim 1, characterized in that In the ceramic-PVDF mixed coating, the mass ratio of the ceramic to the PVDF is (1-9): (1-9).

5. The battery separator according to claim 1, characterized in that Both ends of the coating layer in the first direction are the ceramic-PVDF hybrid coating, and the width ratio of the ceramic coating to the ceramic-PVDF hybrid coating in the first direction is (1-3):(1-3).

6. The battery separator according to claim 1, characterized in that The coating layer has a thickness of 2 μm-3 μm.

7. The battery separator according to claim 1, characterized in that The air permeability of the battery separator is 80s / 100mL-100s / 100mL; the peel strength between the battery separator and the electrode plate is 0.006kN / m-0.009kN / m.

8. A method for preparing a battery separator according to any one of claims 1 to 7, characterized in that: The following steps are involved: Adding ceramic powder and a first dispersant into water to prepare a ceramic slurry; Adding PVDF powder and a second dispersant into water to prepare PVDF slurry; Mixing the ceramic slurry with the PVDF slurry to obtain a ceramic-PVDF mixed slurry; The ceramic slurry and the ceramic-PVDF mixed slurry are alternately coated on the base membrane to obtain the battery separator, wherein the battery separator includes the base membrane and a coating layer arranged on the base membrane, wherein the coating layer includes a ceramic coating and a ceramic-PVDF mixed coating alternately arranged along a first direction parallel to the surface of the base membrane, and wherein in the ceramic-PVDF mixed coating, the mass content of the ceramic gradually increases and the mass content of the PVDF gradually decreases from the side away from the base membrane to the side close to the base membrane.

9. The preparation method according to claim 8, characterized in that: The first dispersant includes a phosphate dispersant, and the second dispersant includes a polyacrylic acid sodium salt dispersant.

10. The preparation method according to claim 8, characterized in that: The method also includes adding a bonding material into the ceramic slurry, wherein the bonding material includes one or more of ethylene-acrylic acid copolymer wax emulsion, polyolefin polyacrylate emulsion, ethylene-acrylic acid fatty alcohol copolymer emulsion, fatty alcohol emulsion and acetylene alcohol emulsion.

11. The preparation method according to claim 8, characterized in that: The ceramic slurry and the ceramic-PVDF mixed slurry are alternately coated on the base film, specifically: The ceramic slurry and the ceramic-PVDF mixed slurry are alternately sprayed on the base film.

12. A lithium ion battery, characterized in that: The invention comprises a battery separator as described in any one of claims 1 to 7, or a battery separator prepared by the preparation method as described in claim 8.

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