Battery separators

Abstract

[Problem] The purpose of the present invention is to provide a separator which has excellent heat resistance, while exhibiting improved electrolyte solution pourability during the production of a battery. [Solution] The present invention is a separator for batteries, said separator having a polyolefin porous film and a heat-resistant porous layer that is provided on at least one surface of the porous film, and said separator being characterized in that: the heat-resistant porous layer contains inorganic particles and an organic synthetic resin component; and among the inorganic particles, the number of particles (B) having a particle diameter of 100 nm or less and adhering to the surface of an inorganic particle (A) is 5.0 or less, said inorganic particle (A) having a particle diameter of 0.3 μm or more.

Description

[Technical Field]

[0001] The present invention relates to a battery separator having a polyolefin porous membrane and a heat-resistant porous layer on at least one side of the porous membrane. The battery separator according to the embodiment of the present invention can be useful as a separator for a lithium-ion secondary battery. [Background technology]

[0002] Porous thermoplastic resin membranes are widely used as materials for separating substances, selectively permeating, and isolating them, etc. Examples of such membranes include battery separators used in lithium ion secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries, polymer batteries, etc., separators for electric double layer capacitors, various filters such as reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes, breathable waterproof clothing, and medical materials.

[0003] In particular, as a separator for a lithium ion secondary battery, a polyolefin porous membrane is preferably used, which has ion permeability due to electrolyte impregnation, excellent electrical insulation, electrolyte resistance, and oxidation resistance, and also has a pore-blocking effect that cuts off current at temperatures of about 120 to 150°C during abnormal battery temperature rise and suppresses excessive temperature rise.

[0004] However, if the temperature continues to rise after the pores are blocked for some reason, the polyolefin porous membrane may break. This phenomenon is not limited to the use of polyolefin, and cannot be avoided above the melting point of the resin that constitutes the porous membrane.

[0005] In response to this, a heat-resistant separator is used in which a heat-resistant porous layer composed mainly of inorganic particles and a binder resin is coated on the polyolefin porous membrane. By using this heat-resistant separator, the shrinkage of the polyolefin porous membrane due to temperature rise is suppressed by the heat-resistant porous layer. In such a separator, in order to improve the electrolyte injection property in the injection step in the battery production, for example, A separator having a heat-resistant porous layer formed using inorganic particles and a resin such as a specific polyamide, polyimide, or polyamideimide as a resin binder (Patent Document 1); a separator having a heat-resistant porous layer formed from a porous membrane composition containing a specific water-soluble thickener, a carbodiimide compound crosslinking agent having a monomer unit derived from a dihydric or higher alcohol, and a particulate polymer (paragraph

[0034] of Patent Document 2); a separator that satisfies a specific formula in the relationship between the critical surface tension of the outermost surface of a heat-resistant porous layer and the critical surface tension of the porous film when the heat-resistant porous layer is peeled off at the interface with the porous film (Patent Document 3); However, it is still not sufficient, and there is a demand for separators with better electrolyte injectability. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-87562 [Patent Document 2] WO2014 / 024991 publication [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-49774 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a separator that has excellent heat resistance and improved electrolyte injection properties during battery production. [Means for solving the problem]

[0008] The present inventors have conducted extensive research in light of the prior art, A battery separator having a polyolefin porous membrane and a heat-resistant porous layer provided on at least one surface of the porous membrane, the heat-resistant porous layer contains inorganic particles and an organic synthetic resin component, It has been found that this problem can be solved by providing a battery separator characterized in that the average number of particles (B) having a particle diameter of 100 nm or less attached to the surface of inorganic particles (A) having a particle diameter of 0.3 μm or more is 5.0 or less. A more preferred embodiment is (1) The inorganic particles are precipitated barium sulfate; (2) The precipitated barium sulfate is a particle synthesized by the Glauber's salt method. (3) When the total amount of the inorganic particle component and the organic synthetic resin component contained in the heat-resistant porous layer is taken as 100% by weight, The inorganic particles are contained in an amount of 50% by weight to 99% by weight. (4) The organic synthetic resin component is containing one or more resins selected from the group consisting of (meth)acrylic acid copolymer resin, polyacrylamide resin, polyvinylidene fluoride resin, polyvinyl alcohol resin, polyimide resin, polyamideimide resin, polyamide resin, and poly(meth)aramid resin; is. [Effects of the Invention]

[0009] According to an embodiment of the present invention, a separator having excellent heat resistance and excellent electrolyte injection properties in battery production can be provided. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a scanning electron microscope image of the surface of a heat-resistant porous layer of a separator produced in Example 1 of the present invention. [Figure 2] 1 is a scanning electron microscope image of the surface of a heat-resistant porous layer of a separator produced in Comparative Example 1 of the present invention. [Figure 3] 1 is a scanning electron microscope image of the surface of a heat-resistant porous layer of a separator produced in Comparative Example 2 of the present invention. [Figure 4] 1 is a photograph showing an evaluation of the wetting and spreading property of an electrolyte solution of a separator produced in Example 1 of the present invention. [Figure 5]1 is a photograph showing an evaluation of the wetting and spreading property of an electrolyte solution of a separator produced in Comparative Example 1 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the embodiments described below.

[0012] A battery separator according to an embodiment of the present invention has a polyolefin porous membrane and a heat-resistant porous layer provided on at least one surface of the porous membrane.

[0013] [Porous polyolefin membrane] The thickness of the polyolefin porous membrane in the embodiment of the present invention is not particularly limited as long as it functions as a battery separator, but is preferably 25 μm or less. More preferably, it is 7 μm or more and 20 μm or less, and even more preferably, it is 9 μm or more and 16 μm or less. When the thickness of the polyolefin porous membrane is 25 μm or less, it is possible to achieve both practical membrane strength and pore-blocking function, and the area per unit volume of the battery case is not restricted, which is suitable for increasing the capacity of the battery.

[0014] The air resistance of the polyolefin porous membrane is 30 sec / 100 cm 3 Air or above, 200sec / 100cm 3 Air or less is preferable, and 40 sec / 100 cm is more preferable. 3 Air or above, 150sec / 100cm 3 Air or less, more preferably 50 sec / 100 cm 3 Air or above, 100sec / 100cm 3 Air resistance is 30sec / 100cm or less. 3 If the resistance is above 200 sec / 100 cm, sufficient mechanical strength and insulation are obtained, reducing the possibility of short circuits occurring during battery charging and discharging. 3When the voltage is equal to or lower than air, the battery has sufficient charge / discharge characteristics, particularly ion permeability (charge / discharge operating voltage) and battery life (closely related to the amount of electrolyte retained), and can fully function as a battery.

[0015] The porosity of the polyolefin porous membrane is preferably 20% or more and 70% or less, more preferably 30% or more and 60% or less, and even more preferably 55% or less. A porosity of 30% or more and 70% or less provides sufficient battery charge / discharge characteristics, particularly ion permeability (charge / discharge operating voltage) and battery life (closely related to the amount of electrolyte retained), allowing the battery to fully function, and providing sufficient mechanical strength and insulation, reducing the possibility of short circuits occurring during charge / discharge.

[0016] The average pore size of the polyolefin porous membrane has a significant effect on pore blocking performance, and is therefore preferably 0.01 μm or more and 1.0 μm or less. It is more preferably 0.02 μm or more and 0.5 μm or less, and even more preferably 0.03 μm or more and 0.3 μm or less. If the average pore size of the polyolefin porous membrane is less than 0.01 μm, clogging of the pores by the organic synthesis component may occur when the heat-resistant porous layer is deposited, resulting in a deterioration in air resistance and electrical resistance. If the average pore size is 1 μm or more, clogging of the pores by the heat-resistant porous layer composition may occur, resulting in a deterioration in air resistance and electrical resistance, or a micro-short circuit may occur, reducing the safety of the battery. When the average pore size of polyolefin porous membrane is more than 0.01 μ m and less than 1.0 μ m, the anchoring effect of binder can obtain sufficient adhesion strength of heat-resistant porous layer to polyolefin porous membrane, and when heat-resistant porous layer is laminated, the air resistance and electrical resistance do not deteriorate significantly, and the response of pore blocking phenomenon to temperature does not become slow, and the pore blocking temperature does not shift to higher temperature side due to the change of temperature rise rate.The average pore size referred to in the present invention is the measurement value obtained by the bubble point method specified in JIS K 3832:1990.

[0017] Although the polyolefin resin constituting the polyolefin porous membrane is not particularly limited, polyethylene and polypropylene are preferred because, in addition to basic properties such as electrical insulation and ion permeability, they have a pore-blocking effect that cuts off current and suppresses excessive temperature rise when the battery temperature rises abnormally. The polyolefin resin may be a single material or a mixture of two or more different polyolefin resins, such as a mixture of polyethylene and polypropylene, or a copolymer of different olefins.

[0018] Among these, polyethylene is particularly preferred from the viewpoint of excellent pore-blocking performance. Hereinafter, the polyolefin resin used in the present invention will be described in detail using polyethylene as an example, but the embodiment of the present invention is not limited thereto.

[0019] Examples of polyethylene include ultra-high molecular weight polyethylene, high-density polyethylene, medium-density polyethylene, and low-density polyethylene. There are no particular limitations on the polymerization catalyst, and examples include Ziegler-Natta catalysts, Phillips catalysts, and metallocene catalysts. These polyethylenes may be ethylene homopolymers or copolymers containing small amounts of other α-olefins. Suitable α-olefins other than ethylene include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, (meth)acrylic acid, esters of (meth)acrylic acid, and styrene.

[0020] The polyethylene may be a single substance, but is preferably a mixture of two or more types of polyethylene. The polyethylene mixture may be a mixture of two or more types of ultra-high molecular weight polyethylenes having different weight average molecular weights (Mw), a mixture of similar high-density polyethylenes, medium-density polyethylenes, and low-density polyethylenes, or a mixture of two or more types of polyethylenes selected from the group consisting of ultra-high molecular weight polyethylenes, high-density polyethylenes, medium-density polyethylenes, and low-density polyethylenes.

[0021] The polyolefin porous membrane must have the function of blocking pores when an abnormality occurs in the charge / discharge reaction. Therefore, the melting point (softening point) of the constituent resin is preferably 70°C or higher and 150°C or lower, more preferably 80°C or higher and 140°C or lower, and even more preferably 100°C or higher and 130°C or lower. If the constituent resin has a melting point of 70°C or higher and 150°C or lower, the pore blocking function will not be exerted during normal use, preventing the battery from becoming unusable, and the pore blocking function will be exerted during an abnormal reaction, ensuring safety.

[0022] [Heat-resistant porous layer] A battery separator according to an embodiment of the present invention has a heat-resistant porous layer provided on at least one surface of the polyolefin porous membrane, and contains inorganic particles and an organic synthetic resin component.

[0023] The heat-resistant porous layer may be provided on only one side of the polyolefin porous membrane, or on both sides.When provided on only one side, the process of forming the heat-resistant porous layer is reduced, and production costs can be further reduced.When provided on both sides, the heat-induced shrinkage of the polyolefin porous membrane is suppressed from both sides, and the heat-induced shrinkage rate of the battery separator can be more effectively reduced.

[0024] [Inorganic particles] The inorganic particles in the present invention have a specific relationship between inorganic particles (A) and particles (B) in the heat-resistant layer, as defined below. The number of particles (B) with a particle diameter of 100 nm or less attached to the surface of inorganic particles (A) with a particle diameter of 0.3 μm or more is 5.0 or less. Preferably, it is 3.0 or less, and more preferably, it is 1.0 or less. Here, the inorganic particles (A) of the present invention are particles with a minor axis diameter of 0.3 μm or more in a backscattered electron image (BEI) obtained by observing the inorganic particles in a heat-resistant porous layer provided on the surface of a battery separator at 30,000 times magnification using a scanning electron microscope (hereinafter referred to as SEM). Particles (B) are particles with a minor axis diameter of 100 nm or less attached to the surface of the inorganic particles (A).

[0025] The number of particles (B) attached to inorganic particles (A) is calculated by counting the number of particles (B) attached to 20 randomly selected inorganic particles (A), and is the average number of particles attached per inorganic particle (A). If the number of particles (B) attached to the surface of inorganic particles (A) exceeds 5.0, electrolyte injection performance during battery manufacturing may be impaired. This is thought to be due to the fact that the surfaces of lotus leaves and rose petals are dotted with special fine particles, which form a microscopic nano-level uneven structure that creates super-water-repellent properties (the so-called lotus effect). Similarly, the microscopic nano-level uneven structure formed by the attachment of particles (B) to the surface of inorganic particles (A) may impair the wetting of the electrolyte to the surface of inorganic particles (A). When the average number of particles (B) attached to the surface of the inorganic particles (A) is 5.0 or less, a fine nano-level uneven structure is not formed on the surface of the inorganic particles (A), and the wettability of the electrolyte to the surface of the inorganic particles (A) is good, resulting in good electrolyte injection properties in battery production.

[0026] In order to limit the number of particles (B) with a particle diameter of 100 nm or less adhering to the surface of inorganic particles (A) with a particle diameter of 0.3 μm or more, which is one of the constituent elements of the present invention, to 5.0 or less, the volume-based average particle diameter of the inorganic particles in the heat-resistant layer determined by a particle size distribution analyzer using a laser diffraction scattering method is preferably 0.4 μm or more, and more preferably 0.5 μm or more. Although a volume-based average particle diameter determined by a particle size distribution analyzer using a laser diffraction scattering method is not sufficient, it is possible to control the number of particles (B) with a particle diameter of 100 nm or less adhering to the surface of inorganic particles (A) with a particle diameter of 0.3 μm or more to 5.0 or less. The upper limit of the average particle diameter of the inorganic particles is 2.0 μm or less. If the average particle diameter of the inorganic particles is greater than 2.0 μm, the number of contact points between individual inorganic particles in the heat-resistant porous layer will be reduced, resulting in a decrease in the number of particles (B) adhering to the surface of inorganic particles (A) with a particle diameter of 0.3 μm or more. pore layer The structure of the polyolefin porous membrane becomes brittle, making it difficult to suppress shrinkage of the polyolefin porous membrane at high temperatures, or the number of coarse particles increases, causing unevenness in the surface shape of the heat-resistant porous layer and resulting in the generation of streaks and the like in the manufacturing method of the heat-resistant porous layer described below.

[0027] The inorganic particles of the present invention are not particularly limited in material as long as they are electrochemically stable. Specific examples include sodium oxide, potassium oxide, magnesium oxide, calcium oxide, barium oxide, lanthanum oxide, cerium oxide, strontium oxide, vanadium oxide, SiO2-MgO (magnesium silicate), SiO2-CaO (calcium silicate), hydrotalcite, sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, barium carbonate, lanthanum carbonate, cerium carbonate, basic titanate, basic silicic titanate, basic copper acetate, basic lead sulfate, layered double hydroxides (Mg-Al type, Mg-Fe type, Ni-Fe type, Li-Al type), layered double hydroxide-alumina silica gel composite, boehmite, alumina, zinc oxide, lead oxide, iron oxide, iron oxyhydroxide, hematite, bismuth oxide, tin oxide, titanium oxide, and zirconium oxide. The adsorbent may be selected from the group consisting of anion adsorbents, cation adsorbents such as zirconium phosphate, titanium phosphate, apatite, non-basic titanates, niobates, and niobium titanates, oxide ceramics such as zeolites, calcium sulfate, magnesium sulfate, aluminum sulfate, gypsum, barium sulfate, alumina trihydrate (ATH), fumed silica, precipitated silica, zirconia, and yttria, nitride ceramics such as silicon nitride, titanium nitride, and boron nitride, silicon carbide, layered silicates such as kaolinite, talc, dickite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, amesite, and bentonite, asbestos, diatomaceous earth, glass fiber, synthetic layered silicates such as mica or fluoromica, and zinc borate. These may be used alone or in combination. Among these, barium sulfate is particularly preferred, and precipitated barium sulfate is more preferred. Specifically, barium sulfate particles are obtained by a method of obtaining barium sulfate by adding sulfuric acid to barium carbonate or barium sulfide (sulfuric acid method), or by a method of obtaining barium sulfate by adding sodium sulfate to barium chloride (Miura's salt method).By using barium sulfate particles produced by a synthetic method, the particle size of the inorganic particles can be controlled with high precision, and the number of particles (B) with a particle size of 100 nm or less attached to the surface of inorganic particles (A) with a particle size of 0.3 μm or more, which is one of the components of the present invention, can be controlled to 5.0 or less.

[0028] The particles (B) of the present invention are not particularly limited and may be organic particles or inorganic particles. Preferably, they are inorganic particles, and more preferably they are made of the same material as the inorganic particles (A).

[0029] The barium sulfate particles used in the present invention are preferably precipitated barium sulfate particles obtained by a synthetic method, particularly barium sulfate particles synthesized by the Glauber's salt method, which uses barium chloride as a starting material and reacts it with sodium sulfate (Glauber's salt).The reason for this is that barium sulfate particles synthesized by the Glauber's salt method generate very little hydrogen sulfide during the course of research into barium sulfate particles, and can suppress the generation of corrosive gases.

[0030] [Organic synthetic resin component] The organic synthetic resin component in the embodiment of the present invention has both the effect of binding together the inorganic particles that constitute the heat-resistant porous layer and the effect of adhering the heat-resistant porous layer to the polyolefin porous membrane. Specifically, one or more resins selected from the group consisting of (meth)acrylic acid copolymer resin, polyacrylamide resin, polyvinylidene fluoride resin, polyvinyl alcohol resin, polyimide resin, polyamideimide resin, polyamide resin, and poly(meth)aramid resin can be used, and commercially available aqueous solutions or dispersions can be used. Specific examples of acrylic resins include the "Polysol" series manufactured by Showa Denko K.K., the "BM" series manufactured by Zeon Corporation, "Jurymer" (registered trademark) AT-210, ET-410, "Aron" (registered trademark) A-104, AS-2000, and NW-7060 manufactured by Toagosei Co., Ltd., the "LIOACCUM" (registered trademark) series manufactured by Toyochem Co., Ltd., TRD202A and TRD102A manufactured by JSR Corporation, "Polystron" (registered trademark) 117, 705, and 1280 manufactured by Arakawa Chemical Industries, Ltd., the "Kogam" (registered trademark) series manufactured by Showa Denko K.K., and WEM-200U and WEM-3000 manufactured by Taisei Fine Chemical Co., Ltd. Specific examples of polyvinyl alcohol include Kuraray Poval (registered trademark) 3-98 and 3-88 manufactured by Kuraray Co., Ltd., and Gohsenol (registered trademark) N-300 and GH-20 manufactured by Mitsubishi Chemical Corporation. Among these, acrylic resins are preferred because of their versatility and the ease with which barium sulfate particles bond together. When a particulate dispersion of an organic synthetic resin component is used to form a heat-resistant porous layer by coating and drying, the average primary particle diameter of the particles of the organic synthetic resin component after the formation of the heat-resistant porous layer is preferably 100 nm or greater if the particle shape is not maintained after the formation of the heat-resistant porous layer, or if the particle shape is maintained after the formation of the heat-resistant porous layer. Having an average primary particle diameter of 100 nm or greater allows the number of particles (B) with a particle diameter of 100 nm or less attached to the surface of inorganic particles (A) with a particle diameter of 0.3 μm or greater, which is one of the components of the present invention, to be controlled to 5.0 or less, thereby improving the electrolyte injectability during battery production.

[0031] The heat-resistant porous layer may appropriately contain a dispersant for improving the dispersion stability of inorganic particles, a thickener and a wetting agent for improving coatability, a thermosetting resin and a crosslinking agent for improving heat resistance, etc.

[0032] [Weight composition ratio of heat-resistant porous layer] The content of inorganic particles contained in the heat-resistant porous layer in an embodiment of the present invention is 50% by mass or more and 99% by mass or less, with the total of the inorganic particles and the organic synthetic resin component being 100% by mass. It is more preferably 77% by mass or more and 95% by mass or less, and even more preferably 80% by mass or more and 93% by mass or less.

[0033] If the content of inorganic particles is less than 50% by mass, the gaps between the individual inorganic particles in the heat-resistant porous layer will become clogged with the organic synthetic resin component, narrowing or lengthening the ion migration path, resulting in increased electrical resistance and air resistance.

[0034] If the content of inorganic particles is more than 99% by mass, the organic synthetic resin component that binds the individual inorganic particles together becomes insufficient, and the structure of the heat-resistant porous layer cannot be maintained.

[0035] When the content of inorganic particles is 50% by mass or more and 99% by mass or less, the gaps between the individual inorganic particles in the heat-resistant porous layer are less likely to be clogged with the organic synthetic resin component, thereby achieving good electrical resistivity and air resistance, and there is no shortage of the binder that binds the inorganic particles together, thereby suppressing shrinkage of the polyolefin porous membrane due to heat.

[0036] [Average thickness of heat-resistant porous layer] In an embodiment of the present invention, the average thickness of the heat-resistant porous layer is preferably 2.0 μm or more and 10 μm or less. More preferably, it is 2.5 μm or more and 6 μm or less, and even more preferably, it is 3.0 μm or more and 4.0 μm or less. If the thickness of the heat-resistant porous layer is less than 2.0 μm, it may not be possible to suppress shrinkage of the polyolefin porous membrane due to heat. If the average thickness of the heat-resistant porous layer is more than 10 μm, the ion migration path becomes longer, which may increase the air permeation resistance, or the distance between the positive and negative electrodes of the battery cell may increase, which may increase the proportion of the battery separator in the battery cell capacity and increase the electrical resistance. pore layerWhen the average thickness is 2.0 μm or more and 10 μm or less, the air resistance and the electrical resistance do not increase.

[0037] [Method for forming heat-resistant porous layer] The heat-resistant porous layer for obtaining the present invention can be obtained by the following steps. (a) Preparation of coating dispersion for heat-resistant porous layer. (b) A step of coating at least one or both surfaces of a polyolefin porous membrane with the slurry. (c) After the coating, the solvent is dried with a dryer to form a heat-resistant porous layer.

[0038] In step (a), water is preferably used as the dispersion medium. A mixture of water and a hydrophilic solvent such as methanol, ethanol, or N-methylpyrrolidone may be used as the dispersion medium, provided that the dispersion stability of the heat-resistant porous layer coating dispersion is not impaired. Known methods can be used to prepare a heat-resistant porous layer coating dispersion containing at least inorganic particles and an organic synthetic resin. Examples include ball mills, bead mills, planetary ball mills, vibrating ball mills, sand mills, colloid mills, roll mills, high-speed impeller dispersion, dispersers, homogenizers, planetary mixers and planetary kneaders, ultrasonic dispersion, and mechanical stirring using a stirring blade. To limit the number of particles (B) with a particle diameter of 100 nm or less attached to the surface of inorganic particles (A) with a particle diameter of 0.3 μm or more, which is one of the components of the present invention, to 5.0 or less, a mild dispersion that minimizes cracking and chipping of the inorganic particles is desirable in this step. If the dispersion is excessive, the inorganic particles (A) may crack, exposing many new active surfaces, or many new particles (B) of 100 nm or less may be generated, and the particles (B) may be attracted to and adhere to the active surfaces of the inorganic particles (A) generated by the cracks. Mild dispersion can reduce the generation of new active surfaces generated by the cracks of the inorganic particles (A) and the number of particles (B) newly generated by the cracks, and can reduce the number of particles (B) of 100 nm or less in diameter attached to the surface of inorganic particles (A) with a particle diameter of 0.3 μm or more to 5.0 or less. The term "mild dispersion" as used herein refers to a state in which inorganic particles in an aggregated state are dispersed in a dispersion medium without applying excessive energy to the particles, while maintaining the size, shape, crystalline structure, surface state, etc. of the primary particles of the inorganic particles. Specifically, for example, when using a bead mill dispersion device, a mild dispersion state can be obtained by using beads with a smaller bead diameter or a smaller bead specific gravity.

[0039] In the step (b), the coating dispersion for the heat-resistant porous layer can be coated on at least one or both sides of the polyolefin porous membrane by a known method, such as reverse roll coating, gravure coating, small-diameter gravure coater, kiss coating, roll brush, air knife coating, Mayer bar coating, pipe doctor coating, blade coating, and die coating, which can be used alone or in combination.

[0040] The battery separator according to the embodiment of the present invention can be used as a battery separator for secondary batteries such as nickel-metal hydride batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, and lithium-sulfur batteries, etc. In particular, it is preferably used as a separator for lithium-ion secondary batteries. [Example]

[0041] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. The values ​​measured in the examples were obtained by the following methods.

[0042] 1. Inorganic particle size (μm) The particle size of the inorganic particles was measured using a laser diffraction particle size distribution analyzer (LA-960V2, manufactured by Horiba, Ltd.) in accordance with JIS Z8825 (2013), and the following physical properties were measured. 1) Particle diameter D50 (μm) when the volumetric accumulation rate is 50% 2. Number of particles (B) adhering to the surface of inorganic particles (A) Using a scanning electron microscope (manufactured by JEOL Ltd., model JSM-6700F, hereinafter referred to as SEM), inorganic particles in a heat-resistant porous layer provided on the surface of a battery separator were observed at 30,000x magnification at an acceleration voltage of 2.0 kV, and the resulting backscattered electron images (BEI) were taken. Particles with a minor axis diameter of 0.3 μm or more were designated as inorganic particles (A), and particles with a minor axis diameter of 100 nm or less attached to the surface of the inorganic particles (A) were designated as particles (B). Twenty inorganic particles (A) were randomly selected, and the particles (B) attached to their surfaces were counted. The average value was rounded to one decimal place.

[0043] 3. Thickness (μm) The thickness of the polyolefin porous membrane and the battery separator was determined by averaging five measured values ​​using a contact-type film thickness meter ("Litematic" (registered trademark) series 318, manufactured by Mitutoyo Corporation). Measurements were performed using a 9.5 mm diameter ultrahard spherical probe under a load of 0.01 N. Furthermore, the thickness (μm) of the heat-resistant porous layer was determined by washing the battery separator with the same solvent as that contained in the slurry, removing the heat-resistant porous layer, measuring the polyolefin porous membrane with the contact-type film thickness meter, and calculating the thickness using the following formula:

[0044] Thickness of heat-resistant porous layer (μm) = thickness of battery separator (μm) - thickness of polyolefin porous membrane (μm).

[0045] 4. Heat shrinkage rate (%) of battery separator (heat resistance) The heat resistance of the battery separator was measured in the MD (longitudinal direction) and TD (transverse direction) directions of the battery separator by the following method, the detailed procedure of which is explained below.

[0046] 1) Cut out three pieces of battery separator measuring 100mm x 100mm, place a transparent glass scale (measurement accuracy 0.1mm) on them, and measure the distance between the midpoints of the two opposing sides of the battery separator as the length in the MD direction and the length in the TD direction, respectively, to determine the initial dimensions (mm).

[0047] 2) The battery separator was sandwiched between two sheets of A3 size paper and placed in an oven at 130°C for 1 hour. After that, the battery separator was removed and left to cool for 30 minutes.

[0048] 3) The distance between the midpoints of the two opposing sides of the battery separator was measured again using the glass scale, and this was recorded as the dimension after shrinkage (mm). The measurement position was the same as the position where the initial dimension was measured, and if the edge of the battery separator was curled, it was unfolded before measurement. Using the obtained initial dimension and the dimension after shrinkage, the length in the MD direction, the length in the TD direction, and the thermal shrinkage rate (%) for each were calculated using the following formula.

[0049] Heat shrinkage rate (%) = {initial dimension (mm) - dimension after shrinkage (mm)} / initial dimension (mm) x 100.

[0050] 5. Wetting and spreading of electrolyte on battery separators The wetting and spreading properties of the battery separator with the electrolyte were measured by the following method, the detailed procedure of which is explained below. 1) A battery separator was cut into a size of MD 100 mm x 100 mm, and this was used as a measurement sample. 2) The sample was left in a dry room at a temperature of 23°C and a dew point of -50°C for 24 hours. 3) In the dry room, the separator was placed with the heat-resistant porous membrane facing up, and 5 mm from each end in the MD direction of the separator was held horizontally with clips so as not to cause wrinkles. 4) Polycarbonate liquid was used as the measurement liquid, and 0.5 μL of the measurement liquid was collected using a microsyringe and gently dropped onto the measurement sample. 5) Photograph the measurement sample under the conditions 8 minutes after the drop of the measurement liquid has been dropped, and measure the area (cm) of the droplet that has spread with the sample liquid from the photographed image. 2 ) was measured.

[0051] Example 1 [Production of battery separators] 100 parts by weight of particle A (barium sulfate (Miura salt method), D50 = 1.2 μm) shown in Table 1, 0.5 parts by weight (active ingredient) of polyacrylic acid dispersant (SA / HAPS = 85 / 15 mol% copolymer, Mw = 6000), and water were added, and the mixture was mixed with 0.1 mm diameter alumina beads (TB-1, bead specific gravity 3.9 g / cm). 3 The mixture was dispersed in a bead mill using a bead mill (manufactured by Taimei Chemical Industry Co., Ltd.) to obtain a dispersion with an active ingredient ratio of 60% by weight.

[0052] To the resulting dispersion, 1.5 parts by weight of a partially sodium-neutralized polyacrylic acid having a degree of neutralization of 50% (Viscomate NP-700, manufactured by Showa Denko K.K.) was added as a thickener, 3.0 parts by weight (active ingredient) of an acrylic emulsion (Polysol AP-4735, manufactured by Showa Denko K.K.) as a binder, 0.5 parts by weight (active ingredient) of a wetting agent (SN Wet 366, manufactured by San Nopco Ltd.), and water were added and stirred to prepare a coating solution with a solids content of 50% by weight.

[0053] The obtained coating liquid was applied to one side (one side) of a polyethylene porous membrane a (thickness: 10 μm, "SETELA" (registered trademark) manufactured by Toray Industries, Inc.) shown in Table 2 by a microgravure method, and then dried to prepare a battery separator having a heat-resistant porous layer with a thickness of 4 μm. The battery separators thus prepared were evaluated for the thickness of the heat-resistant porous layer, the number of particles (B) attached to the surface of the inorganic particles (A), the thermal shrinkage rate (%) of the separator, and the wetting and spreading property of the electrolyte. The results are shown in Table 3.

[0054] (Examples 2 and 3, Comparative Examples 1 to 4) Battery separators were produced and evaluated in the same manner as in Example 1, except that particle A in Example 1 was changed to particles B to G shown in Table 1. The results are shown in Table 3.

[0055] Example 4 A battery separator was produced and evaluated in the same manner as in Example 1, except that the bead mill dispersion in Example 1 was replaced by dispersion using an ultrasonic homogenizer (24 kHz, horn diameter 14 mmΦ). The results are shown in Table 3.

[0056] (Comparative Example 5) A battery separator was prepared in the same manner as in Example 4, except that particles A in Example 1 were replaced with particles D shown in Table 1, and evaluation was carried out. The results of beriberi are shown in Table 3.

[0057] (Comparative Example 6) The 0.1 mm diameter alumina beads of Example 1 were replaced with 1.0 mm diameter zirconia beads (Toraceram® beads, bead specific gravity 6.0 g / cm 3 A battery separator was produced in the same manner as in Example 1, except that the resin used was replaced with a polyester resin (manufactured by Toray Industries, Inc.), and evaluation was carried out. The results are shown in Table 3.

[0058] Examples 5 to 7 Battery separators were produced and evaluated in the same manner as in Example 1, except that the porous polyethylene membrane a in Example 1 was replaced with the porous polyethylene membranes b to d shown in Table 2. The results are shown in Table 3.

[0059] ( reference Example 8) 100 parts by weight of particles H shown in Table 1, 1.5 parts by weight (active ingredient) of polyvinyl alcohol (K-17C, manufactured by Denka Co., Ltd.), and water were added, and the mixture was dispersed using a bead mill in the same manner as in Example 1 to obtain a dispersion with an active ingredient ratio of 40% by weight. Next, battery separators were produced and evaluated in the same manner as in Example 1, and the results are shown in Table 3. As is clear from Table 3, the battery separators of Examples 1 to 8 had good separator heat shrinkage rates (%) and good electrolyte wetting and spreading properties.

[0060] [Table 1]

[0061] [Table 2]

[0062] [Table 3] [Industrial Applicability]

[0063] The separator of the present invention can be suitably used as a battery separator preferably used in non-aqueous electrolyte batteries such as lithium ion batteries. [Explanation of symbols]

[0064] 1 Inorganic particles (A) 2 particles (B)

Claims

1. A battery separator having a polyolefin porous membrane and a heat-resistant porous layer provided on at least one surface of the porous membrane, the heat-resistant porous layer contains inorganic particles and an organic synthetic resin component, the inorganic particles are precipitated barium sulfate, The precipitated barium sulfate is a particle synthesized by a Glauber's salt method, A battery separator, wherein among the inorganic particles, inorganic particles (A) having a particle diameter of 0.3 μm or more have 5.0 or less particles (B) having a particle diameter of 100 nm or less attached to their surfaces. Here, the number of particles (B) adhering to inorganic particles (A) is the average number of particles (B) adhering to each inorganic particle (A), which is calculated by counting the number of particles (B) adhering to 20 inorganic particles (A) arbitrarily selected from a range of 2 μm in length and 2 μm in width in an image obtained by observing the inorganic particles in the heat-resistant porous layer provided on the surface of a battery separator with a scanning electron microscope at 30,000 magnifications.

2. The organic synthetic resin component is 2. The battery separator according to claim 1, comprising at least one resin selected from the group consisting of (meth)acrylic acid copolymer resin, polyacrylamide resin, polyvinylidene fluoride resin, polyvinyl alcohol resin, polyimide resin, polyamideimide resin, polyamide resin, and poly(meth)aramid resin.

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