Battery separator and manufacturing method thereof

Abstract

To provide a separator for a battery, with which high output characteristics of a battery and battery characteristics such as cycle characteristics can be improved.SOLUTION: A separator for a battery includes a polyolefin porous membrane and a heat-resistant porous layer provided at least one side of the polyolefin porous membrane. The heat-resistant porous layer contains barium sulfate particles and an organic synthetic resin component. The content of the barium sulfate particles in the heat-resistant porous layer is 77 wt.% or more and 99 wt.% or less based on 100 wt.% of the total of the barium sulfate particles and the organic synthetic resin component. A gross value at an incident angle 60° into the heat-resistant porous layer is 9% or more and less than 33%.SELECTED DRAWING: None

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 polyolefin porous membrane, and a method for producing the same. 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 viscosity of the polyolefin constituting the membrane will decrease and the membrane will shrink, which may cause the polyolefin porous membrane to break. This phenomenon is not limited to the case of using polyolefin, and even when using other thermoplastic resins, it cannot be avoided above the melting point of the resin constituting the porous membrane.

[0005] In particular, separators for lithium-ion secondary batteries are deeply related to battery characteristics, battery productivity, and battery safety, and are required to have excellent mechanical properties, heat resistance, permeability, dimensional stability, pore-blocking properties (shutdown properties), and meltdown properties. Furthermore, improved adhesion to electrode materials is required to improve battery cycle characteristics, and improved electrolyte permeability is required to improve productivity. To achieve this, various modified porous layers have been layered on porous membranes.

[0006] The modified porous layer is preferably made of a polyamideimide resin, a polyimide resin, a polyamide resin, and / or a fluorine-based resin, which has excellent heat resistance and electrolyte permeability, and / or an electrode-adhesive property. Water-soluble or water-dispersible binders are also widely used, allowing the modified porous layer to be laminated using relatively simple water washing and drying processes. The modified porous layer refers to a layer containing a resin that imparts or improves at least one of the following functions: heat resistance, adhesion to the electrode material, and electrolyte permeability. Furthermore, as lithium-ion secondary batteries have become increasingly important as highly efficient energy devices, efforts are being made to further increase power output and energy density, and efforts are being made to reduce resistance during output and improve battery capacity through thinner components. Amid these trends, separators are also required to be highly durable, and it is predicted that the electrical resistance of the modified porous layer will continue to be further reduced.

[0007] Example 1 of Patent Document 1 discloses a separator that has improved heat resistance and battery stability by applying a slurry containing barium sulfate particles and poly(meth)acrylamide onto a 12 μm-thick polyethylene separator by gravure coating.

[0008] Patent Document 2 discloses a method in which a metal oxide filler is dispersed by a media dispersion method such as a bead mill, and a binder whose main component is a water-soluble resin is used to improve heat resistance, resistance to powder shedding, and battery characteristics. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 6337512 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-208780 Summary of the Invention [Problem to be solved by the invention]

[0010] To improve the safety of batteries, the heat-resistant porous layer is required to have higher heat resistance. In order to improve heat resistance, boehmite, aluminum oxide, magnesium oxide, magnesium hydroxide, etc. are preferably used as inorganic particles in addition to heat-resistant resins. When forming the modified porous layer, the inorganic particles are preferably dispersed in a solvent such as water to improve processability. The heat-resistant porous layer referred to in the present invention refers to a modified porous layer specialized for heat resistance. Furthermore, soft inorganic particles may be preferably used in terms of reducing wear on manufacturing equipment, etc. In addition, particles produced by a synthetic method have a regular particle surface shape, and when a modified porous layer is formed, a structure with voids is likely to be formed, which may improve battery characteristics. However, if the conditions for the dispersion treatment are not optimal, new particles generated by cracking or chipping of inorganic particles may enter between the unbroken inorganic particles, filling the voids and degrading the battery characteristics.

[0011] An object of the present invention is to provide a battery separator that can improve battery characteristics such as high power output characteristics and cycle characteristics of the battery. [Means for solving the problem]

[0012] In order to solve the above problems, the present inventors have conducted extensive research and have come to the conclusion that the above problems can be solved by the following means. (1) A battery separator having a polyolefin porous membrane and a heat-resistant porous layer provided on at least one surface of the polyolefin porous membrane, the heat-resistant porous layer contains barium sulfate particles and an organic synthetic resin component, The barium sulfate particles are contained in the heat-resistant porous layer in an amount of 77% by weight or more and 99% by weight or less, assuming that the total weight of the barium sulfate particles and the organic synthetic resin component in the heat-resistant porous layer is 100% by weight, and the gloss value of the heat-resistant porous layer at an incident angle of 60° is 9% or more and less than 33%. (2) The battery separator according to (1), wherein the barium sulfate particles are precipitated barium sulfate. (3) The battery separator according to either (1) or (2), wherein the barium sulfate particles have an average particle size of 0.3 μm or more and 3.0 μm or less. (4) The battery separator according to any one of (1) to (3), wherein the organic synthetic resin component contains a dispersant and a binder. (5) The battery separator according to (4), characterized in that the organic synthetic resin component contains at least one selected from the group consisting of (meth)acrylic acid copolymer resin, polyvinylidene fluoride resin, polyacrylamide copolymer resin, polyimide resin, polyamideimide resin, and poly(meth)aramid resin.

[0013] (6) The battery separator according to (4) or (5), wherein the dispersant contains at least one selected from the group consisting of polyacrylic acid copolymer resins and carboxymethyl cellulose resins. (7) A method for producing a mixed solution by mixing barium sulfate particles, a dispersant, and a solvent, The method for producing a battery separator according to any one of (1) to (6), further comprising the step of dispersing the mixed solution in a bead mill disperser using ceramic beads having a particle size of 0.3 mm or more and 1.0 mm or less at a filling rate of 40 vol.% or more and 75 vol.% or less, to prepare a masterbatch solution. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a battery separator that can improve the high power output characteristics and cycle characteristics of a battery. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the embodiments described below.

[0016] A battery separator according to an embodiment of the present invention comprises a polyolefin porous membrane and a heat-resistant porous layer provided on at least one surface of the polyolefin porous membrane.

[0017] [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.

[0018] The air resistance of the polyolefin porous membrane is preferably 300 sec / 100 cc air or less. More preferably, it is 200 sec / 100 cc air or less, and even more preferably, it is 150 sec / 100 cc air or less. There is no particular limitation on the preferable lower limit. When the air resistance is 300 sec / 100 cc air or less, 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. Furthermore, sufficient mechanical strength and insulation properties are obtained, reducing the possibility of short circuits occurring during charge / discharge.

[0019] The porosity of the polyolefin porous membrane is preferably 30% or more and 70% or less, more preferably 35% or more and 60% or less, and even more preferably 40% or more and 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.

[0020] The average pore size of polyolefin porous membrane has a great influence on pore-blocking function, so it is preferably 0.01 μm or more and 1.0 μm or less.More preferably 0.05 μm or more and 0.5 μm or less, and even more preferably 0.1 μm or more and 0.3 μm or less.When the average pore size of polyolefin porous membrane is 0.01 μm or more and 1.0 μm or less, the anchoring effect of binder can obtain sufficient adhesion strength of the heat-resistant porous layer to the polyolefin porous membrane, and when the heat-resistant porous layer is laminated, the air resistance does not deteriorate significantly, and the response of the pore-blocking phenomenon to the temperature does not become slow, and the pore-blocking temperature does not shift to a higher temperature side due to the heating rate.

[0021] The average pore size of polyolefin porous membranes was measured using the following method. A sample was fixed onto a measurement cell using double-sided tape, platinum or gold was vacuum-deposited for several minutes, and the membrane surface was measured using an SEM at an appropriate magnification. Ten locations were randomly selected from the image obtained by SEM measurement, and the number-average value of the pore sizes at those 10 locations was used as the average pore size of the sample.

[0022] The polyolefin resin constituting the polyolefin porous membrane is not particularly limited, but polyethylene or polypropylene is preferred. It may be a single polyolefin resin or a mixture of two or more different polyolefin resins, such as a mixture of polyethylene and polypropylene, or a copolymer of different olefins. This is because, in addition to basic properties such as electrical insulation and ion permeability, it has a pore-blocking effect that cuts off current and suppresses excessive temperature rise during abnormal temperature rise in the battery.

[0023] Among these, polyethylene is particularly preferred from the viewpoint of its excellent pore-blocking function. 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.

[0024] 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.

[0025] 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.

[0026] The polyethylene porous membrane must have the function of blocking pores in the event of an abnormal 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.

[0027] [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 barium sulfate particles and an organic synthetic resin component.

[0028] 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.

[0029] [Barium sulfate particles] Barium sulfate particles are widely used because they are relatively inexpensive and can be obtained by crushing the natural material barite. Other examples include precipitated barium sulfate particles with uniform particle size obtained by synthetic methods. When barium sulfate particles have a uniform particle size, they form a heat-resistant porous layer while minimizing the intrusion of smaller particles into the gaps between the barium sulfate particles, making it easier to maintain the voids in the heat-resistant porous layer, thereby improving battery performance.

[0030] Specifically, precipitated barium sulfate particles are barium sulfate particles 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).

[0031] The average particle size of the barium sulfate particles is preferably 0.3 μm or more and 3.0 μm or less, more preferably 0.6 μm or more and 2.2 μm or less, and even more preferably 0.9 μm or more and 1.5 μm or less.

[0032] If the average particle size of the barium sulfate particles is less than 0.3 μm, the gaps between the individual barium sulfate particles in the heat-resistant porous layer may become narrow, which may result in a deterioration in battery performance.If the average particle size of the barium sulfate particles is more than 3.0 μm, the individual barium sulfate particles in the heat-resistant porous layer may become large, which may result in an increase in the distance between the electrodes, which may result in a deterioration in battery performance.

[0033] When the average particle size of the barium sulfate particles is 0.3 μm or more and 3.0 μm or less, the battery characteristics are not deteriorated.

[0034] The average particle size of the barium sulfate particles was determined by taking an LEI image of the barium sulfate particles on the surface of the heat-resistant porous layer at a magnification of 10,000 times (accelerating voltage 2.0 kV) using a scanning electron microscope (JSM6701F manufactured by JEOL Ltd.). Next, the major axes of 100 randomly selected barium sulfate particles were measured, and the number average value was taken as the average particle size.

[0035] The shape of the barium sulfate particles in the embodiment of the present invention is not particularly limited, and barium sulfate particles of various shapes can be used. Specifically, any of spherical, nearly spherical, plate-like, acicular, and polyhedral shapes can be used.

[0036] [Organic synthetic resin component] The organic synthetic resin component in an embodiment of the present invention includes a binder and a dispersant.

[0037] [binder] The binder in the embodiment of the present invention has both the effect of binding barium sulfate particles constituting the heat-resistant porous layer together and the effect of adhering the heat-resistant porous layer to the polyolefin porous membrane. Specifically, at least one selected from the group consisting of (meth)acrylic acid copolymer resin, polyvinylidene fluoride resin, polyacrylamide copolymer resin, polyamide-imide resin, and poly(meth)aramid resin can be used, and a commercially available aqueous solution or aqueous dispersion can be used. The binder used in the present invention is preferably a water dispersion, because the water-insoluble organic synthetic resin component is less likely to clog the pores on the surface of the polyolefin porous membrane during the formation of the heat-resistant porous layer, and can suppress an increase in the resistance of the battery separator.

[0038] Specific examples of acrylic resins include "Jurymer" (registered trademark) AT-210, ET-410, "Aron" (registered trademark) A-104, AS-2000, and NW-7060 manufactured by Toa Gosei 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.

[0039] [Other additives] The heat-resistant porous layer may contain a thermosetting agent, a crosslinking agent, etc., as appropriate, for the purpose of improving heat resistance.

[0040] [Weight composition ratio of heat-resistant porous layer] The content of barium sulfate particles in the heat-resistant porous layer in an embodiment of the present invention is 77% by weight or more and 99% by weight or less, more preferably 85% by weight or more and 98% by weight or less, and even more preferably 93% by weight or more and 97% by weight or less, based on 100% by weight of the total of the barium sulfate particles and the organic synthetic resin component.

[0041] If the content of barium sulfate particles is less than 77% by weight, the gaps between the individual barium sulfate particles in the heat-resistant porous layer will be clogged with the organic synthetic resin component, which may result in a decrease in battery performance.

[0042] If the content of barium sulfate particles is greater than 99% by weight, there will be a shortage of binder that binds the individual barium sulfate particles together, making it impossible to maintain the structure of the heat-resistant porous layer, which may impair the function of the battery.

[0043] When the content of barium sulfate particles is 77% by weight or more and 99% by weight or less, the gaps between the individual barium sulfate particles in the heat-resistant porous layer are not clogged with the organic synthetic resin component, which prevents the battery performance from deteriorating, and there is no shortage of the binder that binds the barium sulfate particles together, which prevents the battery's function from being impaired.

[0044] [Average thickness of heat-resistant porous layer] The thickness of the heat-resistant porous layer is preferably 1 μm or more and 10 μm or less, more preferably 2 μm or more and 7 μm or less, and even more preferably 3 μm or more and 6 μm or less.

[0045] If the thickness of the heat-resistant porous layer is less than 1 μm, it may not be possible to suppress the shrinkage of the polyolefin porous membrane due to heat. If the thickness of the heat-resistant porous layer is more than 10 μm, the distance between the positive and negative electrodes of the battery cell may increase, which may increase the electrical resistance. If the thickness of the heat-resistant porous layer is 1 μm or more and 10 μm or less, it is possible to suppress the shrinkage of the polyolefin porous membrane and to suppress an increase in electrical resistance.

[0046] [Gross value of heat-resistant porous layer] According to an embodiment of the present invention, the heat-resistant porous layer can easily determine the battery characteristics of a battery separator when made into a battery cell based on the optical characteristics of its surface. Specifically, when dispersing barium sulfate particles in a slurry in the process of forming a heat-resistant porous layer, if the dispersion load is too low, particle aggregates may remain on the surface, lengthening the ion migration distance inside the battery and potentially degrading the battery characteristics. On the other hand, if the dispersion load is too high, cracked or chipped particles may occur, filling the gaps between particles that are not cracked or chipped, thereby inhibiting ion permeation inside the battery and potentially degrading the battery characteristics. Particle aggregates have a lower surface smoothness, making them less likely to reflect light, while cracked or chipped particles have their interfaces (cleavage planes) exposed on the surface, making them more likely to reflect light.Therefore, the condition of the particles in the heat-resistant porous layer can be determined, for example, by gloss value or whiteness.

[0047] The gloss value referred to here is a value measured according to JIS Z 8741:1997, and is an index showing the amount of reflected light (glossiness) when light is irradiated onto the heat-resistant porous layer.

[0048] The gloss value of the heat-resistant porous layer at an incident angle of 60° is preferably 9% or more and less than 33%, more preferably 13% or more and less than 28%, and even more preferably 18% or more and less than 23%. If the gloss value of the heat-resistant porous layer is less than 9%, aggregates of barium sulfate particles will remain on the surface of the heat-resistant porous layer, widening the distance between the negative and positive electrodes of the battery cell, which may increase the distance that ions must travel inside the battery and degrade battery performance.If the gloss value is 33% or more, cracked or chipped barium sulfate particles may get between the cracked or chipped particles, inhibiting ion transmission inside the battery and degrading battery performance. When the gloss value of the heat-resistant porous layer is 9% or more and less than 33%, aggregates of barium sulfate particles do not remain on the surface of the heat-resistant porous layer, preventing the distance between the negative and positive electrodes of the battery cell from increasing, thereby improving battery performance. Also, broken or chipped barium sulfate particles do not get between particles that are not cracked or chipped, improving battery performance.

[0049] Next, a method for manufacturing a battery separator according to an embodiment of the present invention will be described. The method for manufacturing a battery separator is carried out in the order of producing a slurry and then forming a heat-resistant porous layer. Here, the slurry is a liquid that is coated on the polyolefin porous membrane when forming the heat-resistant porous layer, and contains barium sulfate particles, an organic synthetic resin component, and a solvent. In addition, additives may be blended as needed.

[0050] [Slurry manufacturing method] A method for producing a slurry for obtaining a heat-resistant porous layer according to an embodiment of the present invention includes the following steps: (a) A process in which a dispersant is added to a solvent containing water as the main component, and then barium sulfate particles are added and stirred to obtain a mixed liquid. (b) A step of dispersing the mixture in a bead mill disperser using ceramic beads with a particle size of 0.3 to 1.0 mm to obtain a masterbatch liquid. (c) A step of adding a binder to the masterbatch liquid and further adding other additives to obtain a slurry.

[0051] [solvent] The solvent used in step (a) can be any solvent capable of dissolving the dispersant and dissolving or dispersing the binder, and among these, water is preferred. The solvent may contain a small amount of alcohol in order to improve the formability of the heat-resistant porous layer.

[0052] [Dispersant] The dispersant in the embodiment of the present invention can be, for example, at least one selected from the group consisting of polyacrylic acid copolymer resin and carboxymethyl cellulose resin, and a commercially available aqueous solution of a water-soluble resin can be used. In addition, anionic surfactants, cationic surfactants, nonionic surfactants, silicone surfactants, etc. can be used. Representative examples of polyacrylic acid copolymer resins include DL-40 and TL-37 manufactured by Nippon Shokubai Co., Ltd., and Aron (registered trademark) A-6012 and A-6114 manufactured by Toagosei Co., Ltd.

[0053] Representative examples of cellulose-based resins include carboxymethyl cellulose and its derivatives, such as 1120, 1220, SP200, SE400, and DN-100L manufactured by Daicel FineChem Ltd., Sunrose (registered trademark) FJ08HC and A04SH manufactured by Nippon Paper Industries Co., Ltd., and Cellogen (registered trademark) 7A and WS-C manufactured by Daiichi Kogyo Seiyaku Co., Ltd.

[0054] Specific examples of cationic surfactants include SN Dispersant 4215 and Nopcosperse 092, both manufactured by San Nopco Ltd. Specific examples of amphoteric surfactants include Amphitol 20BS and Amphitol 20N, both manufactured by Kao Chemical Corporation.

[0055] Specific examples of nonionic surfactants include Emulgen 103 and Emulgen 705 manufactured by Kao Chemical Corporation.

[0056] Specific examples of silicone surfactants include SN Wet 125 manufactured by San Nopco Ltd.

[0057] The content of the dispersant in the heat-resistant porous layer is not particularly limited, but is preferably 0.1% by weight or more and 5.3% by weight or less, more preferably 0.2% by volume or more and 3.0% by volume or less, and even more preferably 0.4% by weight or more and 1.0% by weight or less, based on 100% by weight of the total of the barium sulfate particles and the organic synthetic resin component.

[0058] If the content of the dispersant is less than 0.1 wt%, the aggregates of individual barium sulfate particles cannot be sufficiently dispersed in the step (b), and the aggregated particles remain in the heat-resistant porous layer, which may increase the distance between the positive and negative electrodes during battery cell fabrication and result in a deterioration in battery performance.

[0059] If the amount of dispersant added is more than 5.3 wt %, the individual barium sulfate particles once crushed in step (b) will re-aggregate due to the dispersant, and undispersed aggregates will remain in the heat-resistant porous layer. As a result, the distance between the positive and negative electrodes will increase during battery cell fabrication, which may result in a deterioration in battery performance.

[0060] When the amount of dispersant added is 0.1% by weight or more and 5.3% by weight or less, the individual barium sulfate particles remain dispersed without re-aggregating, which prevents the distance between the positive and negative electrodes from increasing during battery cell fabrication and a decrease in battery performance.

[0061] In the step (a), the stirring method is not particularly limited, but stirring with a dispersing blade, a planetary mixer, a paint shaker, a ball mill, an ultrasonic disperser, a homogenizer, a planetary mixer, etc. may be used. Furthermore, in order for the dispersant to effectively act on the barium sulfate particles in the solvent, it is important to add the barium sulfate particles in a state in which the dispersant is sufficiently dissolved in the solvent. Therefore, it is preferable to add the dispersant to the solvent, followed by the barium sulfate particles, in that order.

[0062] Step (b) uses a bead mill disperser to collide ceramic beads with aggregates of barium sulfate particles contained in the mixed solution, thereby disintegrating them into individual barium sulfate particles. Generally, in processes for disintegrating fragile particles such as barium sulfate, media-less dispersers that do not use ceramic beads are considered suitable because they cause less damage to the particles. In the present invention, a bead mill disperser is used, and by adjusting the bead particle size and bead packing rate to suitable conditions, it is possible to suitably disperse fragile particles.

[0063] [Bead particle size] The particle size of the ceramic beads is preferably 0.3 mm or more and 1.0 mm or less, more preferably 0.4 mm or more and 0.8 mm or less, and even more preferably 0.5 mm or more and 0.7 mm or less.

[0064] If the bead diameter is less than 0.3 mm, the mass of each ceramic bead is small, resulting in less shear stress between the ceramic beads. As a result, the barium sulfate particle agglomerates cannot be sufficiently broken down, and agglomerates larger than the thickness of the heat-resistant porous layer remain in the slurry. This can lead to the formation of gaps in the heat-resistant porous layer that are larger than the diameter of the barium sulfate particles, increasing the distance between the positive and negative electrodes of the battery cell and potentially degrading electrical characteristics.

[0065] If the bead diameter is larger than 1.0 mm, there will be fewer contact points between the ceramic beads, and agglomerates larger than the thickness of the heat-resistant porous layer will remain in the slurry. This will make it easier for gaps larger than the diameter of the barium sulfate particles to form in the structure of the heat-resistant porous layer, which may increase the distance between the positive and negative electrodes of the battery cell and reduce battery performance.

[0066] When the bead diameter is 0.3 mm or more and 1.0 mm or less, a sufficient effect of breaking down agglomerates of barium sulfate particles can be obtained, so that no particles larger than the thickness of the heat-resistant porous layer remain, and the battery performance is not deteriorated.

[0067] The ceramic beads may be made of at least one material selected from alumina, zirconia, and silicon nitride.

[0068] [Bead filling rate] The bead filling rate of the ceramic beads is preferably 40% by volume or more and 75% by volume or less, more preferably 45% by volume or more and 65% by volume or less, and even more preferably 50% by volume or more and 55% by volume or less. Here, the bead filling rate is the ratio of the weight (g) of the ceramic beads used to the packing density (g / cm 3 ) and the resulting volume (cm 3 ) and then the vessel capacity (cm 3 ) is the volume fraction of the ceramic beads divided by the volume fraction of the ceramic beads.

[0069] If the bead filling rate is less than 40% by volume, the amount of ceramic beads present in the vessel is small, resulting in fewer contact points between the ceramic beads and more likely to result in agglomerates of barium sulfate particles remaining. This can lead to the agglomerates of barium sulfate particles falling off the heat-resistant porous layer or an increase in the distance between the positive and negative electrodes of the battery cell, which can degrade the battery performance.

[0070] If the bead filling rate is greater than 75% by volume, the number of contact points between the ceramic beads will be excessive, which may result in the individual barium sulfate particles that have already been crushed being crushed into smaller particles. As a result, cracked or chipped particles may get into the gaps between the barium sulfate particles that form the heat-resistant porous layer, which may degrade the battery performance.

[0071] When the filling rate of the ceramic beads is 40% by volume or more and 75% by volume or less, a sufficient disintegration effect is achieved for the barium sulfate particle aggregates. As a result, no particles larger than the thickness of the heat-resistant porous membrane remain, and the battery performance is not impaired. Furthermore, the occurrence of cracked or chipped barium sulfate particles can be suppressed, so the battery performance is not impaired.

[0072] In the step (c), the stirring method is not particularly limited, and may be stirring with a dispersing blade, a planetary mixer, a paint shaker, a ball mill, an ultrasonic disperser, a homogenizer, a planetary mixer, etc. It is important not to add the binder to the mixed solution before the bead mill dispersion treatment in the step (b).

[0073] That is, the mixture is subjected to heat and high shear force generated by the bead mill dispersion treatment, which may cause the binder to gel or aggregate. This makes it difficult for the binder to bind the individual barium sulfate particles together, making it impossible to maintain the structure of the heat-resistant porous layer. Therefore, it may not be possible to suppress the shrinkage of the polyolefin porous membrane due to heat. Therefore, it is preferable to add the binder in step (c).

[0074] [Surfactants] The slurry obtained in step (c) may contain a surfactant as needed to form a heat-resistant porous layer with a more uniform thickness on the polyolefin porous membrane. Surfactants include wetting agents, leveling agents, and antifoaming agents. To maintain the dispersion of the barium sulfate particles, the surfactant is preferably added last after the binder is thoroughly mixed.

[0075] Next, a method for forming a heat-resistant porous layer on a polyolefin porous membrane in an embodiment of the present invention will be described.

[0076] [Method for forming heat-resistant porous layer] The method for forming a heat-resistant porous layer according to the present invention has the following features: (d) A step of coating at least one surface of a polyolefin porous membrane with the slurry. (e) After the coating, the solvent is dried with a dryer to form a heat-resistant porous layer. is.

[0077] In the step (d), the slurry (heat-resistant porous layer) can be coated on at least one side of the polyolefin porous membrane by a known method. Examples include 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. These methods can be used alone or in combination.

[0078] 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]

[0079] 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.

[0080] 1. 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:

[0081] Thickness of heat-resistant porous layer (μm) = Thickness of battery separator (μm) - Thickness of polyolefin porous membrane (μm) In Table 1, the thickness of the heat-resistant porous layer on side A means the thickness of the heat-resistant porous layer provided on one side of the polyolefin porous membrane, and the thickness of the heat-resistant porous layer on side B means the thickness of the heat-resistant porous layer provided on the other side.

[0082] 2.Average particle size (μm) The average particle size of the barium sulfate particles was determined by taking an LEI image of the barium sulfate particles on the surface of the heat-resistant porous layer at a magnification of 10,000 times (accelerating voltage 2.0 kV) using a scanning electron microscope (JSM6701F manufactured by JEOL Ltd.). Next, the major axes of 100 randomly selected barium sulfate particles were measured, and the number average value was taken as the average particle size.

[0083] 3. Gloss value Measurements were taken in accordance with JIS Z-8741 (1997) using a digital variable angle glossmeter UGV-5D manufactured by Suga Test Instruments Co., Ltd., at an incident angle of 60° and a receiving angle of 60°.

[0084] The surface of the heat-resistant porous layer was measured at three points so that the incident and received light of the glossmeter were parallel to the MD direction of the film sample, and the average of the three gloss values ​​measured at each surface was calculated. The value of the surface with the highest average value was taken as the MD gloss value of the polyolefin microporous membrane.

[0085] In addition, measurements were taken at three points on the surface of the heat-resistant porous layer so that the incident and receiving light of the glossmeter were parallel to the TD direction of the film sample, and the average value of the three gloss values ​​measured on each surface was calculated. The value of the surface with the highest average value was taken as the gloss value in the TD direction of the polyolefin microporous membrane.

[0086] The average of the MD gloss value and TD gloss value calculated by the above method was calculated as the average gloss value.

[0087] 4. Battery cycle characteristic test [Preparation of positive electrode] An NMP solution containing 1.2 parts by mass of PVDF as a binder was added to 97 parts by mass of lithium cobalt oxide and 1.8 parts by mass of carbon black as active materials and mixed to prepare a positive electrode mixture slurry. This positive electrode mixture slurry was uniformly applied to both sides of a positive electrode current collector made of aluminum foil with a thickness of 20 μm and dried to form a positive electrode layer. The positive electrode layer was then compression-molded using a roll press to reduce the density of the positive electrode layer excluding the current collector to 3.6 g / cm. 3 A positive electrode was prepared by the above procedure.

[0088] [Preparation of negative electrode] An aqueous solution containing 1.0 part by mass of sodium carboxymethylcellulose was added to and mixed with 98 parts by mass of artificial graphite as an active material, and then styrene butadiene latex containing 1.0 part by mass of solids as a binder was added and mixed to prepare a negative electrode mixture-containing slurry. This negative electrode mixture-containing slurry was uniformly applied to both sides of a negative electrode current collector made of copper foil with a thickness of 10 μm and dried to form a negative electrode layer. The negative electrode layer was then compression-molded using a roll press to reduce the density of the negative electrode layer excluding the current collector to 1.45 g / cm. 3 A negative electrode was prepared.

[0089] [Preparation of test battery] A wound battery was fabricated using the tabbed cathode and anode electrodes and the respective microporous membranes. The wound battery was then placed in an aluminum laminate bag, and electrolyte (1.1 mol / L LiPF6, ethylene carbonate / ethyl methyl carbonate / diethylene carbonate = 3 / 5 / 2 (volume ratio) with 0.5 wt% vinylene carbonate and 2 wt% fluoroethylene carbonate added) was dripped into it. The battery was then sealed with a vacuum laminator. The battery was then charged to 10% of its full capacity at 0.2 C (C represents the current value at which the battery can be fully charged in 1 hour, and in this case, 300 mA). One side of the laminate was opened to allow gas to escape, and the battery was immediately resealed with a vacuum sealer. The battery was then charged at a constant current of 0.1 C, 4.35 V, with a cutoff current of 0.05 C, and then discharged at a constant current of 0.1 C to 3 V. The battery was then charged at a constant current and voltage of 0.2 C, 4.35 V, with a cutoff current of 0.05 C, and then discharged at a constant current of 0.2 C, 3 V. This 0.2 C charge and discharge cycle was repeated three times to produce a 300 mAh-class test battery.

[0090] [High load test] An output characteristic test was conducted using the test battery. After charging at a constant current and constant voltage of 0.2 C to 4.35 V with a cutoff current of 0.05 C, the battery was discharged at a constant current of 0.2 C to 3 V, and this capacity was recorded as the 0.2 C discharge capacity. Next, the battery was charged at a constant current and constant voltage of 0.2 C to 4.35 V with a cutoff current of 0.05 C, and then discharged at a constant current of 5 C to 3 V, and this capacity was recorded as the 5 C discharge capacity.

[0091] The 5C discharge capacity retention rate was calculated using the following formula.

[0092] 5C discharge capacity maintenance rate = [5C discharge capacity] / [0.2C discharge capacity] This same treatment was carried out on a total of three test batteries, and the average value of the 5C discharge capacity retention rate was taken as the output characteristic.

[0093] [Cycle characteristic test] After the output characteristics test, the test battery was charged at a constant current and constant voltage of 0.5C, 4.35V, and a cutoff current of 0.05C, and then discharged at a constant current of 0.2C to 3V, and this capacity was recorded as the first discharge capacity. The battery in this state was charged and discharged under the following conditions.

[0094] Charging: 1C, 4.35V constant current constant voltage charging, cut-off current 0.05C Discharge: 1C, 3V constant current discharge Measurement temperature: 25℃ The test was carried out on a total of three test batteries, and the ratio of the 1500th discharge capacity to the first discharge capacity, i.e., the average capacity retention rate, was calculated and used as an index of cycle characteristics.

[0095] In the examples and comparative examples of the present invention, the following barium sulfate particles were used.

[0096] Particle A: barium sulfate (Mitrasodium sulfate method), average particle size: 1.2 μm, Particle B: barium sulfate (Mitrasodium sulfate method), average particle size: 0.6 μm, Particle C: barium sulfate (Mitrasodium sulfate method), average particle size: 2.1 μm, Particle D: barium sulfate (Mitrasodium sulfate method), average particle size: 0.2 μm, Particle E: barium sulfate (Mitrasodium sulfate method), average particle size: 3.3 μm, Particle F: barium sulfate (sulfuric acid method, derived from barium carbonate), average particle size: 1.2 μm.

[0097] Example 1 [Production of battery separators] To 100 parts by weight of particles A (barium sulfate (Miuranium salt method), average particle size = 1.2 μm) shown in Table 1, 0.5 parts by weight (active ingredient) of a polyacrylic acid dispersant ("Aron" (registered trademark) A-6114, manufactured by Toa Gosei Co., Ltd.) was prepared and added to water. Next, the entire amount of particles A was added while stirring with a Three-One Motor (manufactured by Toki Sangyo Co., Ltd.) equipped with a disperser-type blade. Next, stirring was carried out at 1000 rpm for 60 minutes, yielding a mixed solution with a solid content of 60% by weight.

[0098] The resulting mixture was milled once using a bead mill disperser (Picomill PCM-LR, manufactured by Asada Iron Works Co., Ltd.) and zirconia beads with a bead particle size of 0.5 mm (Toray Ceram φ0.5 mm, manufactured by Toray Industries, Inc.) under conditions of a bead filling rate of 55% by volume, a peripheral speed of 6 m / sec, and a flow rate of 16 kg / hr, to obtain a masterbatch liquid.

[0099] To the resulting masterbatch liquid, 3.5 parts by weight (active ingredient) of an acrylic emulsion (Polysol AP-4735, manufactured by Showa Denko K.K.) was added as a binder, and water was then added. Next, 0.2 parts by weight of a wetting agent (SN Wet 366, manufactured by San Nopco Ltd.) was added, and the mixture was stirred at 500 rpm for 10 minutes using a Three-One Motor (manufactured by Toki Sangyo Co., Ltd.) equipped with a disperser blade, yielding a slurry with a solid content of 55% by weight. The obtained slurry was coated on one side of a 10 μm thick polyolefin porous membrane by microgravure coating at a drying temperature of 50°C and a conveying speed of 4 m / min to obtain a battery separator with a heat-resistant porous layer having a thickness of 4 μm.

[0100] The obtained battery separator was evaluated for the gloss value of the heat-resistant porous layer, and subjected to a high-load battery test and a cycle test. The results are shown in Table 2.

[0101] (Examples 2 and 3, Comparative Example 1) Battery separators were obtained and evaluated in the same manner as in Example 1, except that the content of barium sulfate particles in Example 1 was changed as shown in Table 2. The results are shown in Table 2.

[0102] (Comparative Example 2) An attempt was made to obtain a battery separator in the same manner as in Example 1, except that the content of barium sulfate particles in Example 1 was changed as shown in Table 2. However, the heat-resistant porous layer fell off after coating, and evaluation was not possible.

[0103] (Examples 4 to 6, Comparative Examples 3 and 4) Battery separators were obtained in the same manner as in Example 1, except that particle A in Example 1 was changed to particles B to F shown in Table 1, and evaluations were carried out. The results are shown in Table 2.

[0104] (Examples 7 to 9, Comparative Examples 5 and 6) Battery separators were obtained and evaluated in the same manner as in Example 1, except that the bead particle size and bead packing ratio in Example 1 were changed as shown in Table 2. The results are shown in Table 2.

[0105] [Table 1]

[0106] [Table 2] [Industrial Applicability]

[0107] 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.

Claims

1. A battery separator having a polyolefin porous membrane and a heat-resistant porous layer provided on at least one surface of the polyolefin porous membrane, the heat-resistant porous layer contains barium sulfate particles and an organic synthetic resin component, The barium sulfate particles are contained in the heat-resistant porous layer in an amount of 77% by weight or more and 99% by weight or less, with the total weight of the barium sulfate particles and the organic synthetic resin component being 100% by weight, and the heat-resistant porous layer has a gloss value of 9% or more and less than 33% at an incident angle of 60°.

2. 10. The battery separator of claim 1, wherein said barium sulfate particles are precipitated barium sulfate.

3. 3. The battery separator according to claim 1, wherein the barium sulfate particles have an average particle size of 0.3 μm or more and 3.0 μm or less.

4. 4. The battery separator according to claim 1, wherein the organic synthetic resin component comprises a dispersant and a binder.

5. A battery separator as described in claim 4, wherein the content of the dispersant in the heat-resistant porous layer is 0.1% by weight or more and 5.3% by weight or less, with the total of the barium sulfate particles and the organic synthetic resin component being 100% by weight.

6. 6. The battery separator according to claim 4, wherein the organic synthetic resin component comprises at least one selected from the group consisting of (meth)acrylic acid copolymer resin, polyvinylidene fluoride resin, polyacrylamide copolymer resin, polyamide-imide resin, and poly(meth)aramid resin.

7. 7. The battery separator according to claim 4, wherein the dispersant contains at least one selected from the group consisting of polyacrylic acid copolymer resin and carboxymethyl cellulose resin.

8. The method includes a step of mixing barium sulfate particles, a dispersant, and a solvent to obtain a mixed solution, the mixed solution is subjected to a dispersion treatment in a bead mill disperser using ceramic beads having a particle size of 0.3 mm or more and 1.0 mm or less at a filling rate of 40 vol% or more and 75 vol% or less, thereby preparing a masterbatch solution.

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