Battery separator and manufacturing method thereof

Abstract

The present invention relates to a separator for batteries comprising a porous polyolefin membrane and a heat-resistant porous layer arranged on at least one surface of the porous polyolefin membrane, wherein: the heat-resistant porous layer contains barium sulfate particles and an organic synthetic resin component; and the content of the barium sulfate particles, the increase in the air resistance per a thickness of 1 μm of the heat-resistant porous layer, the shrinkage after being left to stand for 1 hour in an atmosphere at 130°C, and the hydrogen sulfide concentration are respectively within specific ranges.

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 surface 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.

[0007] Furthermore, as lithium-ion secondary batteries have become increasingly important as highly efficient energy devices, efforts are being made to further increase their energy density, and studies are being conducted to increase the capacity of the components inside the container in order to improve battery capacity. In this trend, the separator located between the positive and negative electrodes has a smaller margin compared to the increase in electrode area, and it is predicted that the positioning accuracy of the separator relative to the electrodes will continue to improve.

[0008] 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.

[0009] Patent Document 2 discloses a separator that contains 2 to 20 parts by weight of barium sulfate per 100 parts by weight of a microporous membrane, thereby enabling the relative positions of the electrodes and the separator to be detected in an X-ray examination.

[0010] Example 1 of Patent Document 3 discloses a separator in which polyvinylidene fluoride resin (VDF-HFP copolymer, VDF:HFP (molar ratio) = 97.6:2.4, weight-average molecular weight 1,130,000) is dissolved in a mixed solvent of dimethylacetamide (DMAc) and tripropylene glycol (TPG) (DMAc:TPG = 80:20 [mass ratio]) to give a resin concentration of 4 mass%, and barium sulfate particles (average primary particle size 0.10 μm) are further stirred and mixed, and the resulting coating liquid is applied to a polyethylene microporous membrane. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent No. 6337512 [Patent Document 2] Japanese Patent No. 5898405 [Patent Document 3] Japanese Patent No. 6526359 Summary of the Invention [Problem to be solved by the invention]

[0012] Battery separators are placed between the positive and negative electrodes of lithium secondary batteries. Because battery separators are components that insulate the flow of electrons between the positive and negative electrodes, it is important to position the battery separator appropriately. Recently, there has been a constant demand for higher capacity in lithium-ion secondary batteries, and research is underway to maximize the area of ​​electrodes in order to maximize the amount that can be packed into the container.

[0013] On the other hand, the margin available for battery separators is becoming narrower, requiring ever-improved accuracy in their position relative to electrodes. Generally, electrode current collectors are made of metals such as copper or aluminum, which shield against X-rays. Therefore, misalignment can be detected by X-ray imaging during the X-ray inspection process used to check the state of the electrodes inside the battery. Battery separators also become misaligned, just like positive and negative electrodes, but managing the misalignment of battery separators requires the ability to be observed simultaneously with the electrodes.

[0014] In addition, to improve the safety of batteries, the heat-resistant porous layer is required to have higher heat resistance. To improve heat resistance, inorganic particles may be blended in addition to heat-resistant resins. Furthermore, to suppress separator shrinkage due to heat, particles with small diameters may be blended in to improve the particle packing density.

[0015] However, when the packing density is improved, the gap between particles becomes narrower, which causes a problem of a large increase in the air permeation resistance.The heat-resistant porous layer referred to in the present invention refers to a modified porous layer specialized for heat resistance.The increase in the air permeation resistance means the difference between the air permeation resistance of the polyolefin porous membrane as a substrate and the air permeation resistance of the battery separator laminated with the heat-resistant porous layer.

[0016] Furthermore, lithium-ion secondary batteries contain electrolytes, which are important for battery reactions, and are highly sensitive to water, potentially generating gases such as hydrogen fluoride and consuming the electrolyte, resulting in a decrease in battery performance. Taking boehmite, which is contained as inorganic particles in the modified porous layer of a typical battery separator, as an example, boehmite contains water molecules within its structure and has numerous hydroxyl groups on its particle surface, which form hydrogen bonds with moisture in the air and thereby adsorb a large amount of water. The modified porous layer also contains water, and when it comes into contact with the electrolyte in the battery, it reacts with the electrolyte, generating gases such as hydrofluoric acid and causing a decrease in battery performance.

[0017] An object of the present invention is to provide a battery separator that allows for observation of misalignment with electrodes in an X-ray inspection step in the manufacturing process of a secondary battery, suppresses thermal shrinkage, and yet exhibits a small increase in air permeation resistance and a low moisture content. [Means for solving the problem]

[0018] In order to solve the above problems, a battery separator and a method for producing the same according to an embodiment of the present invention have the following configurations. (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 70% by volume or more and 96% by volume or less, with the total of the barium sulfate particles and the organic synthetic resin component being 100% by volume, and the amount of the barium sulfate particles is 1.8 g / m 2 Above, 19.8g / m 2 Included below are: A battery separator characterized in that the heat-resistant porous layer has an increase in air permeation resistance of 10.0 sec / 100 cc Air or less per 1 μm of thickness, a shrinkage rate of 8.0% or less when left in an atmosphere at 130°C for 1 hour, and a hydrogen sulfide concentration of 0.3 ppm by volume or less. (The hydrogen sulfide concentration is measured using 5 ml of battery separator. 2 The mixture was placed in a 1L sealed container and left at 60°C for 24 hours. The gas in the container was measured using a gas detector tube method specified in JIS K 0804:2014. The measured value was measured using a 1m battery separator. 2 This is the value converted to per (2) The battery separator according to (1), wherein the barium sulfate particles are precipitated barium sulfate. (3) The battery separator according to (2), wherein the precipitated barium sulfate is produced by a Glauber's salt process. (4) The battery separator according to any one of (1) to (3) above, wherein the barium sulfate particles have an average particle size of 0.3 μm or more and 2.0 μm or less. (5) The battery separator according to any one of (1) to (4) above, wherein the heat-resistant porous layer has a thickness of 1 μm or more and 8 μm or less. (6) The battery separator according to any one of (1) to (5) above, wherein the organic synthetic resin component contains a dispersant and a binder. (7) The battery separator according to (6) above, wherein the dispersant is a cellulose-based resin. (8) The battery separator according to (6) or (7), wherein the binder is an acrylic resin. (9) The battery separator according to any one of (1) to (8) above, which has a moisture content of 500 ppm or less. (10) A method for producing a battery separator according to any one of (1) to (9), comprising the steps of: mixing barium sulfate particles, a dispersant, and a solvent to obtain a mixed solution; and then 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 65 vol. % or more and 85 vol. % or less to prepare a masterbatch solution. [Effects of the Invention]

[0019] An embodiment of the present invention can provide a battery separator that enables observation of misalignment with electrodes in an X-ray inspection step during a secondary battery manufacturing process, suppresses thermal shrinkage while minimizing the increase in air permeation resistance, suppresses hydrogen sulfide that can accelerate battery degradation, and has low moisture content. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the embodiments described below.

[0021] 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.

[0022] [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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] [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.

[0032] The content of the barium sulfate particles per unit area in the heat-resistant porous layer is 1.8 g / m 2 Above, 19.8g / m 2 Preferably, it is 4.3 g / m or less. 2 More than 16.8g / m 2 Less than 6.9 g / m, more preferably 2 More than 13.8g / m 2 The following is the result.

[0033] The content of barium sulfate particles is 1.8 g / m 2If the content of barium sulfate particles is less than 19.8 g / m, the image of the battery separator taken by X-ray may appear faint, making it difficult to identify the position of the separator, and shrinkage of the polyolefin porous membrane due to heat may not be suppressed. 2 If the thickness is larger, the image of the battery separator taken with an X-ray may appear darker, making it difficult to distinguish the boundary line between the electrodes, or the distance between the positive and negative electrodes of the battery cell may become larger, increasing the proportion of the battery separator in the battery cell capacity and reducing the battery capacity density.

[0034] The content of barium sulfate particles is 1.8 g / m 2 Above, 19.8g / m 2 If the thickness is less than this, it becomes possible to distinguish the boundary line between the image of the battery separator and the image of the electrode taken by X-ray, and it is possible to suppress shrinkage of the polyolefin porous membrane due to heat, and further, it does not reduce the capacity density of the battery.

[0035] The thickness of the heat-resistant porous layer is preferably 1 μm or more and 8 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.

[0036] If the thickness of the heat-resistant porous layer is less than 1 μm, the battery separator will appear faint in an X-ray image, making it difficult to identify the position of the separator and preventing the polyolefin porous membrane from shrinking due to heat.If the thickness of the heat-resistant porous layer is more than 8 μm, the battery separator will appear dark in an X-ray image, making it difficult to identify the boundary line between the electrodes.Also, the distance between the positive and negative electrodes of the battery cell will increase, increasing the proportion of the battery separator in the battery cell capacity and resulting in a decrease in the battery capacity density.

[0037] When the thickness of the heat-resistant porous layer is 1 μm or more and 8 μm or less, it becomes possible to distinguish the boundary line between the image of the battery separator and the image of the electrode taken by X-ray, and it is possible to suppress shrinkage of the polyolefin porous membrane due to heat, and further, the capacity density of the battery is not reduced.

[0038] 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.

[0039] In the present invention, the X-ray imaging method is not particularly limited, but in order to clearly identify the misalignment between the electrode and the battery separator, it is preferable to use a scintillator with higher sensitivity. International Publication No. 2017-187818 issue" The use of the cell-type scintillator described in paragraph

[0112] of the preceding paragraph is preferable because it allows for clear identification of misalignment between the electrode and the battery separator. Specifically, a laminate of a negative electrode using copper foil as a current collector and a battery separator is prepared, and then an X-ray image of the test laminate can be taken using an X-ray detector (VAREX 1515DX, pixel pitch 127 μm) and the cell-type scintillator (pixel pitch 127 μm) under the following conditions:

[0040] X-ray source tube voltage: 80 kV, X-ray source focal diameter: 20 μm, Source-FPD distance: 247mm, Source-object distance: 82 mm (magnification 3x), Shooting time: 1 sec (5 shots taken at 0.2 sec shooting time and averaged) From the obtained image, it is possible to determine whether or not there is a boundary line between the battery separator and the electrode.

[0041] The increase in air permeation resistance per 1 μm of thickness of the heat-resistant porous layer is 10.0 sec / 100 cc Air or less, preferably 7.0 sec / 100 cc Air or less, and more preferably 4.0 sec / 100 cc Air or less.

[0042] If the increase in air permeation resistance per 1 μm of heat-resistant porous layer thickness is greater than 10.0 sec / 100 ccAir, the resistance to the diffusion and migration of lithium ions inside the battery may increase, or by-products generated by the battery reaction may clog the porous layer, resulting in a significant decrease in battery performance.If the increase in air permeation resistance per 1 μm of heat-resistant porous layer thickness is 10.0 sec / 100 ccAir or less, the resistance to the diffusion and migration of lithium ions inside the battery will not increase, and battery performance will not be significantly reduced.

[0043] In order to make the increase in air permeation resistance per 1 μm of thickness of the heat-resistant porous layer 10.0 sec / 100 ccAir or less, for example, the manufacturing method of the battery separator according to the embodiment of the present invention described below can be used.

[0044] The battery separator has a shrinkage rate of 8.0% or less when left in a 130°C atmosphere for 1 hour. It is preferably 6.0% or less, and more preferably 4.0% or less. If the battery separator has a shrinkage rate of more than 8.0% when left in a 130°C atmosphere for 1 hour, the battery separator may shrink when the battery is exposed to heat, making it impossible to maintain insulation between the positive and negative electrodes. If the battery separator has a shrinkage rate of 8.0% or less when left in a 130°C atmosphere for 1 hour, the battery separator can maintain insulation between the positive and negative electrodes when exposed to heat.

[0045] [Porosity of heat-resistant porous layer] The porosity of the heat-resistant porous layer is preferably 30% or more and 65% or less, more preferably 35% or more and 60% or less, and even more preferably 40% or more and 55% or less.

[0046] If the porosity of the heat-resistant porous layer is less than 30%, the gaps between the individual barium sulfate particles in the heat-resistant porous layer will be narrow, and the increase in air permeation resistance per 1 μm of thickness of the heat-resistant porous layer may be greater than 10.0 sec / 100 ccAir.If the porosity of the heat-resistant porous layer is greater than 65%, the gaps between the individual barium sulfate particles in the heat-resistant porous layer will be wide, and the structure of the heat-resistant porous layer will become brittle, and it may be difficult to suppress shrinkage of the polyolefin porous membrane due to heat.

[0047] When the porosity of the heat-resistant porous layer is 30% or more and 65% or less, the increase in air permeation resistance per 1 μm of thickness of the heat-resistant porous layer is 10.0 sec / 100 ccAir or less, and shrinkage of the polyolefin porous membrane due to heat can be suppressed.

[0048] Next, the composition constituting the heat-resistant porous layer will be described in detail. The heat-resistant porous layer in the embodiment of the present invention contains barium sulfate particles and an organic synthetic resin component.

[0049] [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 preventing smaller particles from entering the gaps between the barium sulfate particles, making it easier to maintain the voids in the heat-resistant porous layer, thereby suppressing the increase in air resistance.

[0050] 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).

[0051] Although the barium sulfate particles used in the present invention are expensive, it is preferable to use 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, during the course of our research into barium sulfate particles, we discovered that barium sulfate particles synthesized by the Glauber's salt method generate very little hydrogen sulfide and can suppress the generation of corrosive gases.

[0052] The average particle size of the barium sulfate particles is preferably 0.3 μm or more and 2.0 μm or less, more preferably 0.4 μm or more and 1.5 μm or less, and even more preferably 0.5 μm or more and 1.0 μm or less.

[0053] 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 will be narrow, and the increase in air permeability resistance per μm of thickness of the heat-resistant porous layer may be greater than 10.0 sec / 100 ccAir.If the average particle size of the barium sulfate particles is more than 2.0 μm, the gaps between the individual barium sulfate particles in the heat-resistant porous layer will be wide, and the structure of the heat-resistant porous layer will become brittle, and it may be difficult to suppress shrinkage of the polyolefin porous membrane due to heat.

[0054] When the average particle size of the barium sulfate particles is 0.3 μm or more and 2.0 μm or less, the increase in air permeation resistance per μm of thickness of the heat-resistant porous layer is 10.0 sec / 100 ccAir or less, and shrinkage of the polyolefin porous membrane due to heat can be suppressed.

[0055] 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.). The major axes of 100 randomly selected barium sulfate particles were then measured, and the average value was taken as the average particle size.

[0056] 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.

[0057] [Volume 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 70% by volume or more and 96% by volume or less, more preferably 77% by volume or more and 93% by volume or less, and even more preferably 85% by volume or more and 90% by volume or less, based on 100% by volume of the total of the barium sulfate particles and the organic synthetic resin component.

[0058] If the content of barium sulfate particles is less than 70% by volume, the gaps between the individual barium sulfate particles in the heat-resistant porous layer become clogged with the organic synthetic resin component, which may prevent the increase in air permeability resistance per μm of thickness of the heat-resistant porous layer from being 10.0 sec / 100 ccAir or less, or may cause the organic synthetic resin component present in the gaps between the barium sulfate particles to shrink when the battery separator is exposed to heat, making it difficult to suppress the shrinkage of the polyolefin porous membrane due to heat.

[0059] If the content of barium sulfate particles is greater than 96% by volume, 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, and it may become difficult to suppress shrinkage of the polyolefin porous membrane due to heat.

[0060] When the content of barium sulfate particles is 70% by volume or more and 96% by volume 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, and the increase in air permeation resistance per 1 μm of thickness of the heat-resistant porous layer is reduced to 10.0 sec / 100 In addition, there is no shortage of the binder that binds the barium sulfate particles together, which makes it possible to suppress shrinkage of the polyolefin porous membrane due to heat.

[0061] [Hydrogen sulfide concentration] Battery separator 1m according to an embodiment of the present invention 2 The concentration of hydrogen sulfide generated from the vicinity is 0.3 ppm by volume or less, preferably 0.2 ppm by volume or less, and more preferably 0.1 ppm by volume or less.

[0062] Battery separator 1m 2 If the concentration of hydrogen sulfide generated from the battery is greater than 0.3 ppm by volume, gas may be generated inside the battery cell, or the electrode current collector may deteriorate due to an oxidation reaction between the current collector and hydrogen sulfide, resulting in a shortened battery life. 2 When the concentration of hydrogen sulfide generated around the battery cell is 0.3 ppm by volume or less, gas generation inside the battery cell can be suppressed, and deterioration of the electrode current collector can be suppressed.

[0063] The hydrogen sulfide concentration here is 5 m 2 The mixture was placed in a 1L sealed container and left at 60°C for 24 hours. The gas in the container was measured using a gas detector tube method specified in JIS K 0804:2014. The measured value was measured using a 1m battery separator. 2 This is the value converted to per

[0064] Battery separator 1m 2 The method for reducing the concentration of hydrogen sulfide generated from around the separator to 0.3 volume ppm or less is not particularly limited, but may be, for example, a method of heat-treating barium sulfate produced by a sulfuric acid method among precipitated barium sulfate, or a method of washing the barium sulfate with sufficient water and then drying the water. 2 Even if the concentration of hydrogen sulfide generated around the separator is greater than 0.3 ppm by volume, the separator can be obtained by subjecting the separator to an appropriate heat treatment.

[0065] [Specific surface area of ​​barium sulfate particles] Barium sulfate particles do not have hydroxyl groups on their surface, so they are less affected by water molecules adsorbed to the surface, and this can prevent the generation of gases such as hydrogen fluoride that are generated by the reaction between water and the electrolyte, as well as the deterioration of battery performance due to the consumption of the electrolyte. The specific surface area of ​​barium sulfate particles is 1.0 m 2 / g or more, 18.0m 2 / g or less is preferable, and 2.0m 2 / g or more, 12.0m 2 / g or less, more preferably 3.0m 2 / g or more, 6.0m 2 / g or less.

[0066] The specific surface area of ​​barium sulfate particles is 1.0m 2 If the average particle size is less than 1 / g, the particle size of each barium sulfate particle in the heat-resistant porous layer will be larger than the thickness of the heat-resistant porous layer, which may result in the barium sulfate particles falling off the battery separator or the increased distance between the positive and negative electrodes of the battery cell, increasing the separator's share of the battery cell capacity and reducing the capacity density of the battery.

[0067] The specific surface area of ​​barium sulfate particles is 18.0 m 2 If it is greater than 1 / g, the amount of water adsorbed on the surface of the barium sulfate particles increases, and the moisture content of the battery separator may become high.

[0068] The specific surface area of ​​barium sulfate particles is 1.0m 2 / g or more, 18.0m 2 / g or less is preferable because it prevents the barium sulfate particles from falling off, the capacity density of the battery from decreasing, and the moisture content of the battery separator from increasing.

[0069] The moisture content referred to here is a measurement value obtained by leaving 1 g of battery separator in an atmosphere with a dew point of -60°C for 24 hours, and then heating it for 10 minutes at 150°C under a nitrogen atmosphere using a Karl Fischer moisture content meter (MKC-610, Kyoto Electronics Manufacturing Co., Ltd.) placed in an atmosphere with a dew point of -60°C.

[0070] [Organic synthetic resin component] The organic synthetic resin component in an embodiment of the present invention includes a binder and a dispersant.

[0071] [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, an acrylic resin, polyvinyl alcohol, poly-N-vinylacetamide, etc. can be used, and a commercially available aqueous solution or aqueous dispersion can be used.

[0072] 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.

[0073] Specific examples of polyvinyl alcohol include "Kuraray Poval" (registered trademark) 3-98 and 3-88 manufactured by Kuraray Co., Ltd., and "Gosenol" (registered trademark) N-300 and GH-20 manufactured by Mitsubishi Chemical Corporation.

[0074] A specific example of poly-N-vinylacetamide is GE191-104 manufactured by Showa Denko KK Among them, acrylic resins are preferred because they are highly versatile and easily bond barium sulfate particles together.

[0075] The content of the binder in the heat-resistant porous layer is not particularly limited, but is preferably 3.2% by volume or more and 24.0% by volume or less, more preferably 5.6% by volume or more and 18.5% by volume or less, and even more preferably 8.0% by volume or more and 12.1% by volume or less, based on 100% by volume of the total of the barium sulfate particles and the organic synthetic resin component.

[0076] If the binder content is less than 3.2% by volume, there will be insufficient binder to bind the individual barium sulfate particles together, and the structure of the heat-resistant porous layer will not be maintained, which may make it difficult to suppress shrinkage of the polyolefin porous membrane due to heat.

[0077] If the binder content is more than 24.0% by volume, the gaps between individual barium sulfate particles in the heat-resistant porous layer will be clogged with the binder, making it impossible to keep the increase in air permeability resistance per μm of heat-resistant porous layer thickness below 10.0 sec / 100 ccAir. Also, when the battery separator is exposed to heat, the binder present in the gaps between the barium sulfate particles will shrink, making it difficult to suppress heat-induced shrinkage of the polyolefin porous membrane.

[0078] When the binder content is 3.2% by volume or more and 24.0% by volume or less, there is no shortage of the binder that binds the barium sulfate particles together, which prevents the polyolefin porous membrane from shrinking due to heat. Furthermore, the gaps between the individual barium sulfate particles in the heat-resistant porous layer are not clogged with the binder, which prevents the increase in air permeation resistance per 1 μm of thickness of the heat-resistant porous layer from becoming 10.0 sec / 100 ccAir or less.

[0079] [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.

[0080] 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.

[0081] [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 1.0 mm or less to obtain a master batch liquid. (c) A step of adding a binder to the masterbatch liquid and further adding other additives to obtain a slurry.

[0082] [solvent] The solvent used in step (a) can be any solvent capable of dissolving the dispersant and dissolving or dispersing the binder, with water being preferred. The solvent may contain a small amount of alcohol to improve the formability of the heat-resistant porous layer.

[0083] [Dispersant] In the embodiment of the present invention, for example, a cellulose-based resin, an anionic surfactant, a cationic surfactant, a nonionic surfactant, a silicone surfactant, or the like can be used as the dispersant.

[0084] 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.

[0085] Specific examples of anionic surfactants 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.

[0086] 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.

[0087] Specific examples of nonionic surfactants include Emulgen 103 and Emulgen 705 manufactured by Kao Chemical Corporation.

[0088] Specific examples of silicone surfactants include SN Wet 125 manufactured by San Nopco Ltd. Among them, water-soluble polymers are preferred for allowing the dispersant to efficiently act on the barium sulfate particles. Of these, carboxymethyl cellulose derivatives are more preferred because they have excellent dispersibility of barium sulfate particles, oxidation resistance, are easily available, and also contribute to improving heat resistance.

[0089] The content of the dispersant in the heat-resistant porous layer is not particularly limited, but is preferably 0.8% by volume or more and 5.9% by volume or less, more preferably 1.4% by volume or more and 4.5% by volume or less, and even more preferably 2.0% by volume or more and 3.0% by volume or less, based on 100% by volume of the total of the barium sulfate particles and the organic synthetic resin component.

[0090] If the content of the dispersant is less than 0.8% by volume, 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. As a result, gaps larger than the particle size of the barium sulfate particles are likely to occur in the structure of the heat-resistant porous layer, and heat-induced shrinkage of the polyolefin porous membrane may not be suppressed.

[0091] If the amount of dispersant added is more than 5.9% by volume, the individual barium sulfate particles once disintegrated in step (b) will re-aggregate due to the dispersant, and undispersed aggregates will remain in the heat-resistant porous layer. As a result, gaps larger than the particle size of the barium sulfate particles will easily form in the structure of the heat-resistant porous layer, and heat-induced shrinkage of the polyolefin porous membrane may not be suppressed.

[0092] When the amount of dispersant added is 0.8 volume % or more and 5.9 volume % or less, the individual barium sulfate particles remain dispersed without re-aggregating, making it less likely that gaps larger than the barium sulfate particles will form in the heat-resistant porous layer, and shrinkage of the polyolefin porous membrane due to heat can be suppressed.

[0093] 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.

[0094] Step (b) uses a bead mill disperser to collide ceramic beads with the barium sulfate particle aggregates 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 appropriately disperse fragile particles.

[0095] [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.

[0096] When 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 disintegrated, and agglomerates larger than the thickness of the heat-resistant porous layer remain in the slurry. This can lead to the formation of gaps larger than the diameter of the barium sulfate particles in the structure of the heat-resistant porous layer, making it impossible to suppress the shrinkage of the polyolefin porous membrane due to heat. Furthermore, the distance between the positive and negative electrodes of the battery cell increases, increasing the proportion of the separator in the battery cell capacity and resulting in a decrease in the battery capacity density.

[0097] If the bead particle size 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 particle size of the barium sulfate particles to form in the structure of the heat-resistant porous layer, making it impossible to prevent the polyolefin porous film from shrinking due to heat, and increasing the distance between the positive and negative electrodes of the battery cell, which will increase the proportion of the separator in the battery cell capacity and reduce the battery's capacity density.

[0098] When the bead diameter is 0.3 mm or more and 1.0 mm or less, a sufficient disintegration effect is obtained for the agglomerates of barium sulfate particles, so that no particles larger than the thickness of the heat-resistant porous layer remain, which suppresses the shrinkage of the polyolefin porous membrane due to heat and prevents a decrease in the capacity density of the battery.

[0099] The ceramic beads may be made of at least one material selected from alumina, zirconia, and silicon nitride.

[0100] [Bead filling rate] The bead filling rate of the ceramic beads is preferably 65% ​​by volume or more and 85% by volume or less, and more preferably 70% by volume or more and 80% by volume or less. Here, the bead filling rate is defined as 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.

[0101] If the bead filling rate is less than 65% 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 residual agglomerates of barium sulfate particles, which may cause the agglomerates to fall off the heat-resistant porous layer or increase the distance between the positive and negative electrodes of the battery cell, increasing the proportion of the separator in the battery cell capacity and reducing the capacity density of the battery.

[0102] If the bead filling rate is greater than 85% by volume, the number of contact points between the ceramic beads will be excessive, which may cause the individual barium sulfate particles that have already been crushed to smaller particles. Therefore, the fine particles may enter the gaps between the barium sulfate particles that form the heat-resistant porous layer, resulting in the heat-resistant porous The increase in air resistance per 1 μm of layer thickness may be greater than 10.0 sec / 100 cc Air.

[0103] When the filling rate of the ceramic beads is 65% by volume or more and 85% by volume or less, a sufficient effect of breaking down the agglomerates of barium sulfate particles can be obtained. layer This prevents particles larger than the thickness of the battery separator from falling off, preventing agglomerates of barium sulfate particles from falling off and reducing the battery's capacity density. Furthermore, by crushing individual barium sulfate particles, the generation of fine particles can be suppressed, so the increase in air permeability resistance per 1 μm of heat-resistant porous layer thickness can be kept to 10.0 sec / 100 ccAir or less.

[0104] 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).

[0105] 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).

[0106] [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.

[0107] 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.

[0108] [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.

[0109] 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.

[0110] In the step (e), the drying temperature of the dryer is not particularly limited, but is preferably 40°C or higher and 90°C or lower, more preferably 45°C or higher and 80°C or lower, and even more preferably 50°C or higher and 70°C or lower.

[0111] If the drying temperature is lower than 40°C, the solvent cannot be sufficiently dried, so that the solvent remains in the heat-resistant porous layer, and the moisture content of the battery separator may become high, especially when the solvent is water.If the drying temperature is higher than 90°C, the polyolefin porous membrane may shrink due to heat before the heat-resistant porous layer is formed.

[0112] A drying temperature of 40° C. or higher and 90° C. or lower is preferred because it allows the solvent to be removed efficiently without causing the polyolefin porous membrane to shrink.

[0113] 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]

[0114] 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.

[0115] 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: 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.

[0116] 2.Air resistance (sec / 100ccAir) Using an Oken air permeability resistance meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.), samples of the polyolefin porous membrane and battery separator were fixed to prevent wrinkles and measured according to JIS P8117. The samples were 100 mm square, and measurements were taken at five points, the center and four corners of the sample, and the average value was used as the air permeability resistance (sec / 100 cc Air). If the length of one side of the sample is less than 100 mm, values ​​measured at five points 50 mm apart may be used. The increase in air permeability resistance of the heat-resistant porous layer (sec / 100 cc Air) was determined by washing the battery separator with the same solvent as the slurry, removing the heat-resistant porous layer, measuring the resulting polyolefin porous membrane with the Oken air permeability resistance meter, and calculating it using the following formula: This value was further divided by the thickness of the heat-resistant porous layer to obtain the increase in air permeability resistance (sec / 100 cc Air) per 1 μm of heat-resistant porous layer thickness. Increase in air resistance (sec / 100ccAir) = Air resistance of porous membrane (sec / 100cc Air) - Air resistance of battery separator (sec / 100cc Air)

[0117] 3.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.). The major axes of 100 randomly selected barium sulfate particles were then measured, and the average value was taken as the average particle size.

[0118] 4. Hydrogen sulfide concentration (ppm by volume) The hydrogen sulfide concentration is measured by measuring the volume of the battery separator 5m. 2 The mixture was placed in a 1 L glass container (Aibottle NEO GL-45, manufactured by AS ONE Corporation) and left in an oven at 60°C for 24 hours. The gas in the container was then measured three times using a Kitagawa detector (AP-20, manufactured by Komyo Rikagaku Kogyo Co., Ltd.) and a hydrogen sulfide detector tube (120U, manufactured by Komyo Rikagaku Kogyo Co., Ltd.) in accordance with the gas detection tube method specified in JIS K 0804:2014. The average value was calculated as the concentration of hydrogen sulfide in 1 m of battery separator. 2 Converted to per.

[0119] 5. X-ray imaging method The X-ray imaging method is International Publication No. 2017-187818 issue" The cell-type scintillator described in paragraph

[0112] was fabricated and the test was carried out. Specifically, first, a test laminate was fabricated to identify and evaluate the winding position of the electrode and the battery separator. Four negative electrodes (30 mm × 30 mm) using copper foil as a current collector and three battery separators (34 mm × 34 mm) were prepared, and the battery separators were alternately stacked while being shifted by 1 mm in any direction to obtain a test laminate. Next, an X-ray image of the test laminate was taken using an X-ray detector (1515DX manufactured by VAREX, pixel pitch 127 μm) and the cell-type scintillator (pixel pitch 127 μm) under the following conditions. X-ray source tube voltage: 80 kV, X-ray source focal diameter: 20 μm, Source-FPD distance: 247mm, Source-object distance: 82 mm (magnification 3x), Shooting time: 1 sec (5 shots taken at 0.2 sec shooting time and averaged) The presence or absence of a boundary line between the battery separator and the electrode was judged from the obtained images. The judgment criteria are shown below. ○: Misalignment between separator and electrode can be identified ×: The misalignment between the separator and the electrode cannot be identified.

[0120] 6.Heat shrinkage rate (%) The heat resistance of the polyolefin porous membrane and the battery separator was measured in the MD direction (longitudinal direction) and TD direction (transverse direction) of the battery separator by the following method. The detailed procedure is explained below. 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). 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. 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. Heat shrinkage rate (%) = {initial dimension (mm) - dimension after shrinkage (mm)} / initial dimension (mm) x 100

[0121] 7.Moisture content (ppm) The moisture content was measured by leaving 1 g of battery separator in an atmosphere with a dew point of -60°C for 24 hours, and then heating it for 10 minutes at 150°C in a nitrogen atmosphere using a Karl Fischer moisture content meter (Kyoto Electronics Manufacturing Co., Ltd. MKC-610) in an atmosphere with a dew point of -60°C.

[0122] In the examples and comparative examples of the present invention, the following barium sulfate particles were used. Barium sulfate A: Glauber's salt method, nominal particle size: 0.6 μm Barium sulfate B: Sulfuric acid method (derived from barium carbonate), nominal particle size: 0.6 μm, heat treated Barium sulfate C: sulfuric acid method, (derived from barium carbonate), nominal particle size: 0.6 μm, Barium sulfate D: Glauber's salt method, nominal particle size: 0.35 μm, Barium sulfate E: Glauber's salt method, nominal particle size: 2.0 μm, Barium sulfate F: Glauber's salt method, nominal particle size: 0.1 μm, Barium sulfate G: Glauber's salt method, nominal particle size: 2.3 μm, The heat treatment referred to here means leaving the barium sulfate particles in a constant temperature incubator DKN302 (manufactured by Yamato Scientific Co., Ltd.) at 200°C for 24 hours with the vent open to 25%, and then slowly cooling for 2 hours.

[0123] 8. 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.

[0124] [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.

[0125] [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 bag 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.

[0126] [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. 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.

[0127] Example 1 [Production of battery separators] One part by weight of sodium carboxymethylcellulose was added to 100 parts by weight of water, and the mixture was stirred for 120 minutes at 600 rpm using a Three-One Motor (manufactured by Toki Sangyo Co., Ltd.) equipped with a disper-type blade. Further, 1.14 parts by weight of the barium sulfate A was added to 100 parts by weight of water while stirring, and the mixture was further stirred for 60 minutes to obtain a mixed solution. The resulting mixture was milled twice using a bead mill disperser (Picomill PCM-LR, manufactured by Asada Iron Works Co., Ltd.) and zirconia beads with a bead diameter of 0.5 mm (Toray Ceram φ0.5 mm, manufactured by Toray Industries, Inc.) under conditions of a bead filling rate of 75% by volume, a peripheral speed of 10 m / sec, and a flow rate of 16 kg / hr, to obtain a masterbatch liquid. To 100 parts by weight of the obtained masterbatch liquid, 3.5 parts by weight of an acrylic resin water dispersion with a solid content of 40% by weight and 6.5 parts by weight of water were added while stirring at 500 rpm using a Three-One Motor (manufactured by Toki Sangyo Co., Ltd.) equipped with a dispersing blade, and further stirring was continued for 10 minutes. Next, to 100 parts by weight of the masterbatch liquid, 0.1 parts by weight of a fluorine-based surfactant with a solid content of 100% by weight was added while stirring, and further stirring was continued for 10 minutes to obtain a slurry. The obtained slurry was coated on one side of a 12 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. The obtained battery separator contained 88.8% by volume of barium sulfate particles and 11.2% by volume of the organic synthetic resin component (dispersant: 2.2% by volume, binder: 9.0% by volume), where the total of the barium sulfate particles and the organic synthetic resin component in the heat-resistant porous layer was 100% by volume. The barium sulfate particles contained in the heat-resistant porous layer were 9.2 g / m 2 It was.

[0128] Example 2 The total of the barium sulfate particles and the organic synthetic resin component in the heat-resistant porous layer was taken as 100% by volume, and the barium sulfate particles were 71.0% by volume, the organic synthetic resin component was 29.0% by volume (dispersant: 5.8% by volume, binder: 23.2% by volume), and the barium sulfate particles contained in the heat-resistant porous layer were 7.4 g / m 2A battery separator was obtained in the same manner as in Example 1, except that:

[0129] Example 3 The total of the barium sulfate particles and the organic synthetic resin component in the heat-resistant porous layer is taken as 100% by volume, and the barium sulfate particles are 80.0% by volume, the organic synthetic resin component is 20.0% by volume (dispersant: 4.0% by volume, binder: 16.0% by volume), and the barium sulfate particles contained in the heat-resistant porous layer are 8.3 g / m 2 A battery separator was obtained in the same manner as in Example 1, except that:

[0130] Example 4 The total of the barium sulfate particles and the organic synthetic resin component in the heat-resistant porous layer is taken as 100% by volume, and the barium sulfate particles are 95.0% by volume, the organic synthetic resin component is 5.0% by volume (dispersant: 1.0% by volume, binder: 4.0% by volume), and the barium sulfate particles contained in the heat-resistant porous layer are 9.8 g / m 2 A battery separator was obtained in the same manner as in Example 1, except that:

[0131] Example 5 The total of the barium sulfate particles and the organic synthetic resin component in the heat-resistant porous layer is taken as 100% by volume, and the barium sulfate particles are 92.0% by volume, the organic synthetic resin component is 8.0% by volume (dispersant: 1.6% by volume, binder: 6.4% by volume), and the barium sulfate particles contained in the heat-resistant porous layer are 9.5 g / m 2 A battery separator was obtained in the same manner as in Example 1, except that:

[0132] Example 6 A heat-resistant porous layer was provided on one side of the polyolefin porous membrane with a thickness of 1.1 μm, and the barium sulfate particles contained in the heat-resistant porous layer were 2.5 g / m 2 A battery separator was obtained in the same manner as in Example 1, except that:

[0133] Example 7 A heat-resistant porous layer was provided on one side of the polyolefin porous membrane with a thickness of 2.1 μm, and the barium sulfate particles contained in the heat-resistant porous layer were 4.8 g / m2 A battery separator was obtained in the same manner as in Example 1, except that:

[0134] Example 8 A heat-resistant porous layer was provided on one side of the polyolefin porous membrane with a thickness of 3.0 μm, and the barium sulfate particles contained in the heat-resistant porous layer were 6.9 g / m 2 A battery separator was obtained in the same manner as in Example 1, except that:

[0135] Example 9 A heat-resistant porous layer with a thickness of 3.9 μm was provided on one side of the polyolefin porous membrane, and a heat-resistant porous layer with a thickness of 4.0 μm was provided on the other side. The barium sulfate particles contained in the heat-resistant porous layer were 18.1 g / m 2 A battery separator was obtained in the same manner as in Example 1, except that:

[0136] Example 10 A heat-resistant porous layer with a thickness of 3.0 μm was provided on one side of the polyolefin porous membrane, and another 3.0 μm was provided on the other side. The barium sulfate particles contained in the heat-resistant porous layer were 13.8 g / m 2 A battery separator was obtained in the same manner as in Example 1, except that:

[0137] Example 11 A heat-resistant porous layer with a thickness of 2.5 μm was provided on one side of the polyolefin porous membrane, and another 2.5 μm thick layer was provided on the other side. The barium sulfate particles contained in the heat-resistant porous layer were 11.5 g / m 2 A battery separator was obtained in the same manner as in Example 1, except that:

[0138] Example 12 A battery separator was obtained in the same manner as in Example 1, except that barium sulfate particles were changed to barium sulfate B.

[0139] Example 13 A battery separator was obtained in the same manner as in Example 1, except that barium sulfate particles were changed to barium sulfate D.

[0140] Example 14 A battery separator was obtained in the same manner as in Example 1, except that barium sulfate particles were changed to barium sulfate E.

[0141] Example 15 A battery separator was obtained in the same manner as in Example 1, except that the dispersant in the organic synthetic resin component was changed to hydroxymethyl cellulose.

[0142] Example 16 A battery separator was obtained in the same manner as in Example 1, except that the dispersant in the organic synthetic resin component was changed to sodium polyacrylate.

[0143] Example 17 A battery separator was obtained in the same manner as in Example 1, except that the binder in the organic synthetic resin component was changed to polyvinyl alcohol.

[0144] Example 18 A battery separator was obtained in the same manner as in Example 1, except that the ceramic beads were changed to zirconia beads with a particle size of 0.3 mm (Toray Ceram φ0.3 mm).

[0145] Example 19 A battery separator was obtained in the same manner as in Example 1, except that the ceramic beads were changed to zirconia beads with a particle size of 0.8 mm (Toray Toraceram φ0.8 mm).

[0146] Example 20 A battery separator was obtained in the same manner as in Example 1, except that the ceramic bead filling rate was changed to 70% by volume.

[0147] Example 21 A battery separator was obtained in the same manner as in Example 1, except that the ceramic bead filling rate was changed to 80% by volume.

[0148] The results of Examples 1 to 21 are shown in Table 1. In all of Examples 1 to 21, the misalignment between the separator and the electrodes was identifiable by X-ray imaging, the thermal shrinkage rate was kept low, the increase in air permeation resistance was small, and the moisture regain was also kept low.

[0149] (Comparative Example 1) The total of the barium sulfate particles and the organic synthetic resin component in the heat-resistant porous layer was taken as 100% by volume, and the barium sulfate particles were 69.0% by volume, the organic synthetic resin component was 31.0% by volume (dispersant: 6.1% by volume, binder: 24.9% by volume), and the barium sulfate particles contained in the heat-resistant porous layer were 7.2 g / m 2 Except for this, a battery separator was obtained in the same manner as in Example 1. The results of Comparative Example 1 are shown in Table 1. In Comparative Example 1, since the content of barium sulfate particles was less than 70% by volume, the increase in air permeation resistance per μm of thickness of the heat-resistant porous layer was large, and the thermal shrinkage rate was also high.

[0150] (Comparative Example 2) The total of the barium sulfate particles and the organic synthetic resin component in the heat-resistant porous layer is taken as 100% by volume, and the barium sulfate particles are 97.0% by volume, the organic synthetic resin component is 3.0% by volume (dispersant: 0.6% by volume, binder: 2.4% by volume), and the barium sulfate particles contained in the heat-resistant porous layer are 10.0 g / m 2 An attempt was made to produce a battery separator. However, significant detachment of the heat-resistant porous layer from the polyolefin porous membrane occurred, and a battery separator could not be obtained. In Table 1, "-" indicates that measurement was not possible because a battery separator could not be obtained.

[0151] (Comparative Example 3) A heat-resistant porous layer with a thickness of 0.7 μm was provided on one side of the polyolefin porous membrane, and the barium sulfate particles contained in the heat-resistant porous layer were 1.6 g / m 2 The results of Comparative Example 3 are shown in Table 1. In Comparative Example 3, the content of barium sulfate particles was 1.8 g / m 2Because the difference was less than this, the misalignment between the separator and the electrodes could not be identified by X-ray imaging, and the thermal shrinkage rate was also high.

[0152] Comparative Example 4 A heat-resistant porous layer with a thickness of 4.5 μm was provided on one side of the polyolefin porous membrane, and another 4.5 μm was provided on the other side. The barium sulfate particles contained in the heat-resistant porous layer were 20.7 g / m 2 The results of Comparative Example 4 are shown in Table 1. In Comparative Example 4, the content of barium sulfate particles was 19.8 g / m 2 Because it was larger, the misalignment of the separator and electrode could not be identified by X-ray imaging.

[0153] (Comparative Example 5) A battery separator was obtained in the same manner as in Example 1, except that the barium sulfate particles were changed to barium sulfate C. The results of Comparative Example 5 are shown in Table 1. Comparative Example 5 showed a high hydrogen sulfide concentration.

[0154] (Comparative Example 6) A battery separator was obtained in the same manner as in Example 1, except that the barium sulfate particles were changed to barium sulfate F. The results of Comparative Example 6 are shown in Table 1. In Comparative Example 6, the average particle size of the barium sulfate was small, and the increase in air permeation resistance per 1 μm of thickness of the heat-resistant porous layer was large.

[0155] (Comparative Example 7) A battery separator was obtained in the same manner as in Example 1, except that the barium sulfate particles were changed to barium sulfate G. The results of Comparative Example 7 are shown in Table 1. Comparative Example 7 resulted in a large thermal shrinkage rate.

[0156] (Comparative Example 8) A battery separator was obtained in the same manner as in Example 1, except that the ceramic beads were changed to zirconia beads with a particle size of 0.2 mm (Toray Toraceram φ0.2 mm). The results of Comparative Example 8 are shown in Table 1. Comparative Example 8 showed a large thermal shrinkage rate.

[0157] (Comparative Example 9) A battery separator was obtained in the same manner as in Example 1, except that the ceramic beads were changed to zirconia beads with a particle size of 1.5 mm (Toray Ceram φ1.5 mm, manufactured by Toray Industries, Inc.). The results of Comparative Example 9 are shown in Table 1. Comparative Example 9 showed a large thermal shrinkage rate.

[0158] (Comparative Example 10) A battery separator was obtained in the same manner as in Example 1, except that the filling rate of the ceramic beads was changed to 90% by volume. The results of Comparative Example 10 are shown in Table 1. In Comparative Example 10, the average particle size of the barium sulfate was small, and the increase in air permeation resistance per 1 μm of thickness of the heat-resistant porous layer was large.

[0159] (Comparative Example 11) A battery separator was obtained in the same manner as in Example 1, except that the barium sulfate particles were replaced with aluminum oxide particles having an average particle size of 0.5 μm. The results of Comparative Example 11 are shown in Table 1. Since Comparative Example 11 did not contain barium sulfate, the misalignment between the separator and the electrodes could not be identified by X-ray imaging, and the moisture content was also high.

[0160] (Comparative Example 12) A battery separator was obtained in the same manner as in Example 1, except that the barium sulfate particles were replaced with boehmite particles having an average particle size of 0.4 μm. The results of Comparative Example 12 are shown in Table 1. Since Comparative Example 12 did not contain barium sulfate, the misalignment between the separator and the electrodes could not be identified by X-ray imaging, and the moisture content was also high.

[0161] [Table 1]

[0162] Although various embodiments have been described above with reference to the drawings, it goes without saying that the present invention is not limited to such examples. It is clear that a person skilled in the art can conceive of various modifications or alterations within the scope of the claims, and it is understood that these also naturally fall within the technical scope of the present invention. Furthermore, the components of the above-described embodiments may be combined in any manner without departing from the spirit of the invention.

[0163] This application is based on a Japanese patent application (Patent Application No. 2019-149148) filed on August 15, 2019, the contents of which are incorporated herein by reference. [Industrial Applicability]

[0164] 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 70% by volume or more and 96% by volume or less, with the total of the barium sulfate particles and the organic synthetic resin component being 100% by volume, and the barium sulfate particles are contained in the heat-resistant porous layer in an amount of 1.8 g / m ２ Above, 19.8g / m ２ Included below are: A battery separator characterized in that the heat-resistant porous layer has an increase in air permeation resistance per 1 μm of thickness of 10.0 sec / 100 cc Air or less, a shrinkage rate when left in an atmosphere at 130° C. for 1 hour of 8.0% or less in both MD (longitudinal direction) and TD (transverse direction), and a hydrogen sulfide concentration of 0.1 ppm by volume or less. (The hydrogen sulfide concentration is measured after 5 m of battery separator. ２ The mixture was sealed in a sealed container with a capacity of 1 L and left to stand in an atmosphere at 60°C for 24 hours. After that, the gas in the container was measured by the gas detector tube method specified in JIS K 0804:2014. ２ This is a value converted to per

2. 2. The battery separator according to claim 1, wherein the barium sulfate particles have an average particle size of 0.3 μm or more and 2.0 μm or less.

3. 3. The battery separator according to claim 1, wherein the heat-resistant porous layer has a thickness of 1 μm or more and 8 μm or less.

4. The battery separator according to any one of claims 1 to 3, wherein the organic synthetic resin component comprises a dispersant and a binder.

5. The battery separator according to claim 4 , wherein the dispersant is a cellulose-based resin.

6. 6. The battery separator according to claim 4, wherein the binder is an acrylic resin.

7. The battery separator according to any one of claims 1 to 6, which has a moisture content of 500 ppm or less.

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