Separator for battery

Abstract

[Problem] The purpose of the present invention is to provide a separator for batteries, said separator having low air resistance, low electrical resistance, good high-output characteristics and low moisture content ratio, while being suppressed in gas generation. [Solution] The present invention relates to a separator for batteries, said separator comprising a polyolefin porous membrane and a heat-resistant porous layer that is provided on at least one surface of the porous membrane. This separator for batteries is characterized in that: the heat-resistant porous layer contains barium sulfate particles and an organic synthetic resin component; the amount of particles having a particle diameter of 0.5 μm or less is 20% by volume or less; the amount of particles having a particle diameter of 3.0 μm or more is 10% by volume or less; the barium sulfate particles are contained in the heat-resistant porous layer in an amount of from 70% by volume to 98% by volume if the total of the barium sulfate particles and the organic synthetic resin component is taken as 100% by volume; the average thickness of the heat-resistant porous layer is from 2 μm to 10 μm; the moisture ratio of the separator is 400 ppm or less; and the content of hydrogen sulfide is 0.2 × 10-3 mg / m2 or less.

Description

Technical Field

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

Background Art

[0002] Thermoplastic resin porous membranes are widely used as material separation, selective permeation, and separation materials. For example, separators for batteries used in lithium-ion secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, and polymer batteries, separators for electric double-layer capacitors, reverse osmosis filtration membranes, ultrafiltration membranes, and various filters such as microfiltration membranes, moisture-permeable and waterproof clothing, and medical materials.

[0003] Particularly, as a separator for a lithium-ion secondary battery, a polyolefin porous membrane having ion permeability by impregnation with an electrolyte solution, excellent electrical insulation, electrolyte resistance, and oxidation resistance, and having a pore-blocking effect of interrupting current at a temperature of about 120 to 150°C when the battery abnormally overheats and suppressing excessive temperature rise is preferably used.

[0004] However, when the temperature continues to rise even after pore blockage for some reason, the polyolefin porous membrane may rupture. This phenomenon is not limited to the case where polyolefin is used, and it cannot be avoided at a temperature equal to or higher than the melting point of the resin constituting the porous membrane.

[0005] Particularly, the separator for lithium-ion secondary batteries is deeply related to battery characteristics, battery productivity, and battery safety, and excellent mechanical properties, heat resistance, permeability, dimensional stability, pore blocking characteristics (shutdown characteristics), and melt film rupture characteristics (melt down characteristics) are required. In recent years, especially when used in in-vehicle lithium-ion batteries, it is necessary to shorten the charging time of the battery and improve the acceleration performance, and rapid charging (high-current charging) and increased power consumption (high-current discharging) are required as the required characteristics of the battery. Along with this, the requirements for the separator have become even higher in terms of improving output characteristics. For this purpose, various modified porous layers have been studied for laminating on porous membranes.

[0006] As the modified porous layer, polyamideimide resin, polyimide resin, polyamide resin, and / or fluororesin having excellent heat resistance and electrolyte permeability and excellent electrode adhesiveness are preferably used. In addition, water-soluble or water-dispersible binders that can laminate the modified porous layer using a relatively simple water washing process and drying process are also widely used. The modified porous layer refers to a layer containing a resin that imparts or improves at least one or more functions such as heat resistance, adhesiveness to the electrode material, high ion permeability, and high output characteristics.

[0007] In Example 1 of Patent Document 1, a separator is disclosed in which heat resistance and battery stability are improved by coating a slurry containing barium sulfate particles and poly(meth)acrylamide on a polyethylene separator with a thickness of 12 μm by gravure coating.

[0008] Patent Document 2 discloses a separator that can detect the relative positions of the electrodes and the separator in X-ray inspection by containing 2 to 20 parts by weight of barium sulfate per 100 parts by weight of the microporous membrane.

[0009] In Example 1 of Patent Document 3, polyvinylidene fluoride-based resin (VDF-HFP copolymer, VDF:HFP (molar ratio) = 97.6:2.4, weight average molecular weight 1,130,000) was dissolved in a mixed solvent of dimethylacetamide (DMAc) and tripropylene glycol (TPG) (DMAc:TPG = 80:20 [mass ratio]) so that the resin concentration became 4% by mass. Further, barium sulfate particles (average primary particle diameter 0.10 μm) were stirred and mixed, and a separator in which the obtained coating liquid was applied to a polyethylene microporous membrane is disclosed.

[0010] In Patent Document 4, by making the average particle diameter D20 of inorganic particles larger than the average pore diameter of pores opening on the surface of the microporous membrane, the inorganic particles do not enter the pores on the surface of the microporous membrane, making the pores less likely to collapse, and a separator that achieves both high ion permeability and pressure resistance is disclosed.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0012] A separator for a battery is a member for insulating the flow of electrons when the temperature of a lithium secondary battery abnormally rises. To improve the safety of the battery, higher heat resistance is required for the heat-resistant porous layer. In addition to heat-resistant resins, inorganic particles may be blended to improve heat resistance, and furthermore, shrinkage of the separator due to heat can be suppressed. However, as the proportion of inorganic particles in the heat-resistant porous layer increases, the gaps between the particles become narrower, the air permeability resistance and the electrical resistivity increase, and there is a problem that the output characteristics deteriorate.

[0013] In addition, a lithium-ion secondary battery contains an electrolyte important for the battery reaction, which reacts very sensitively to water and may cause gas generation such as hydrogen fluoride and deterioration of battery performance due to consumption of the electrolyte. Taking boehmite contained as inorganic particles in a modified porous layer of a general battery separator as an example, boehmite contains water molecules in its structure and has a large number of hydroxyl groups on the particle surface, and has the property of adsorbing a large amount of moisture by forming hydrogen bonds with moisture in the air. Similarly, it contains moisture in the modified porous layer and reacts with the electrolyte when it comes into contact with the electrolyte in the battery, causing gas generation such as hydrofluoric acid and deterioration of battery performance. In Patent Document 1, it is proposed to use a mixture of a specific barium sulfate and a specific synthetic resin, but gas generation may occur and it is not sufficient.

[0014] An object of the present invention is to provide a battery separator having low air permeability resistance and electrical resistivity, excellent high output characteristics, low moisture content rate, and suppressing gas generation.

Means for Solving the Problems

[0015] The inventors of the present invention have conducted intensive studies and a battery separator having a polyolefin porous membrane and a heat-resistant porous layer provided on at least one side of the porous membrane, the heat-resistant porous layer contains barium sulfate particles and an organic synthetic resin component, the barium sulfate particles are Particles with a particle diameter of 0.5 μm or less are 20% by volume or less, and particles with a particle diameter of 3.0 μm or more are 10% by volume or less. The barium sulfate particles are contained in an amount of 70% by volume or more and 98% by volume or less, with the total of the barium sulfate particles and the organic synthetic resin component being 100% by volume. The average thickness of the heat-resistant porous layer is 2 μm or more and 10 μm or less. The water content of the separator is 400 ppm or less. The content of hydrogen sulfide is 0.2 × 10 -3 mg / m 2 or less. It has been found that the problem is solved by a battery separator characterized by the above.

[0016] A more preferred embodiment is (1) The barium sulfate particles are precipitated barium sulfate. (2) The precipitated barium sulfate is produced by the glauber's salt method using barium chloride as a raw material. (3) The BET specific surface area of the barium sulfate particles is 2.0 m 2 / g or more and less than 3.0 m 2 / g. (4) The organic synthetic component contains one or more selected from the group consisting of (meth)acrylic acid copolymer resin, polyacrylamide resin, polyvinylidene fluoride resin, polyvinyl alcohol resin, polyimide resin, polyamideimide resin, polyamide resin, and poly(meth)aramid resin. (5) The air permeability resistance of the polyolefin porous membrane is 30 seconds / 100 cm 3 Air or more and 200 seconds / 100 cm 3 or less. That's it.

Effects of the Invention

[0017] According to the embodiment of the present invention, it is possible to provide a battery separator having low air permeability resistance and electrical resistance, good high output characteristics, low water content, and suppressing hydrogen sulfide generation.

Brief Description of the Drawings

[0018]

Figure 1

Mode for Carrying Out the Invention

[0019] Hereinafter, embodiments of the present invention will be described in detail. Note that the present invention is not limited to the embodiments described below.

[0020] The separator for a battery according to an embodiment of the present invention has a polyolefin porous membrane and a heat-resistant porous layer provided on at least one side of the porous membrane.

[0021] [Polyolefin Porous Membrane] The thickness of the polyolefin porous membrane in the embodiment of the present invention is not particularly limited as long as it has the function of 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 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, the area per unit volume of the battery case is not restricted, and it is suitable for increasing the capacity of the battery.

[0022] The air permeability resistance of the polyolefin porous membrane is 30 sec / 100 cm 3 Air or more and 200 sec / 100 cm 3 Air or less is preferable. More preferably, it is 40 sec / 100 cm 3 Air or more and 150 sec / 100 cm 3 Air or less, and even more preferably 50 sec / 100 cm 3 Air or more and 100 sec / 100 cm 3 Air or less. When the air permeability resistance is 30 sec / 100 cm 3 Air or more, sufficient mechanical strength and insulation can be obtained, and the possibility of short circuit during charging and discharging of the battery is reduced. 200 sec / 100 cm 3When it is below Air, it is sufficient in terms of sufficient charge-discharge characteristics of the battery, particularly ion permeability (charge-discharge operating voltage) and battery life (which is closely related to the amount of electrolyte retained), and can fully exhibit the function as a battery.

[0023] The porosity of the polyolefin porous membrane is preferably 20% or more and 70% or less. More preferably, it is 30% or more and 60% or less, and still more preferably 55% or less. When the porosity is 30% or more and 70% or less, it is sufficient in terms of sufficient charge-discharge characteristics of the battery, particularly ion permeability (charge-discharge operating voltage) and battery life (which is closely related to the amount of electrolyte retained), can fully exhibit the function as a battery, and the possibility of short circuit during charge-discharge is reduced due to sufficient mechanical strength and insulation being obtained.

[0024] Since the average pore diameter of the polyolefin porous membrane greatly affects the pore blocking performance, it is preferably 0.01 μm or more and 1.0 μm or less. More preferably, it is 0.02 μm or more and 0.5 μm or less, and still more preferably 0.03 μm or more and 0.3 μm or less. If the average pore diameter of the polyolefin porous membrane is less than 0.01 μm, pore clogging may occur due to organic synthetic components when the heat-resistant porous layer is deposited, and the air permeability resistance and electrical resistance may deteriorate. If it is 1 μm or more, pore clogging may occur due to the heat-resistant porous layer composition, the air permeability resistance and electrical resistance may deteriorate, or the safety of the battery may decrease due to the occurrence of micro short circuits. When the average pore diameter of the polyolefin porous membrane is 0.01 μm or more and 1.0 μm or less, due to the anchor effect of the binder, sufficient adhesion strength of the heat-resistant porous layer to the polyolefin porous membrane can be obtained. When the heat-resistant porous layer is laminated, the air permeability resistance and electrical resistance do not deteriorate significantly, and the response of the pore blocking phenomenon to temperature does not become slow, and the pore blocking temperature due to the change in the heating rate rarely shifts to the higher temperature side. The average pore diameter referred to in the present invention is the measured value obtained by the bubble point method defined in JIS K 3832:1990.

[0025] The polyolefin resin constituting the polyolefin porous membrane is not particularly limited, but polyethylene and polypropylene are preferred. Also, it may be a single substance or a mixture of two or more different polyolefin resins, for example, 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 can have a pore-blocking effect that shuts off the current and suppresses excessive temperature rise when the battery abnormally heats up. Among them, polyethylene is particularly preferred from the viewpoint of excellent pore-blocking performance. Hereinafter, polyethylene will be described in detail as an example of the polyolefin resin used in the present invention, but the embodiments of the present invention are not limited thereto.

[0026] Examples of polyethylene include ultra-high molecular weight polyethylene, high density polyethylene, medium density polyethylene, and low density polyethylene. Also, there is no particular limitation on the polymerization catalyst, and examples include Ziegler-Natta catalysts, Phillips catalysts, metallocene catalysts, and the like. These polyethylenes may be not only homopolymers of ethylene but also copolymers containing a small amount of other α-olefins. Preferred 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, styrene, and the like.

[0027] Polyethylene may be a single substance, but is preferably a mixture of two or more polyethylenes. As the polyethylene mixture, a mixture of two or more ultra-high molecular weight polyethylenes having different weight average molecular weights (Mw), a similar mixture of high density polyethylene, medium density polyethylene, and low density polyethylene, or a mixture of two or more polyethylenes selected from the group consisting of ultra-high molecular weight polyethylene, high density polyethylene, medium density polyethylene, and low density polyethylene may be used. The polyolefin porous membrane has a function of closing pores during abnormal charge and discharge reactions. Therefore, the melting point (softening point) of the resin constituting the membrane is preferably 70°C or higher and 150°C or lower. More preferably, it is 80°C or higher and 140°C or lower, and even more preferably, it is 100°C or higher and 130°C or lower. When the melting point of the resin constituting the membrane is 70°C or higher and 150°C or lower, the pore closing function will not be manifested during normal use, and the battery will not become unusable. In addition, the safety can be ensured by the manifestation of the pore closing function during abnormal reactions.

[0028] [Heat-resistant porous layer] The separator for a battery according to an embodiment of the present invention is provided with a heat-resistant porous layer on at least one side of the above polyolefin porous membrane, and contains barium sulfate particles and an organic synthetic resin component.

[0029] 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 the production cost can be further suppressed. When provided on both sides, the shrinkage of the polyolefin porous membrane due to heat can be suppressed from both sides, and the shrinkage rate of the separator for a battery due to heat can be more effectively reduced.

[0030] [Barium sulfate particles] The barium sulfate particles have 20% by volume or less of particles with a particle diameter of 0.5 μm or less and 10% by volume or less of particles with a particle diameter of 3.0 μm or more. Preferably, the particles with a particle diameter of 0.5 μm or less are 15% by volume or less, and the particles with a particle diameter of 3.0 μm or more are 8% by volume or less. More preferably, the particles with a particle diameter of 0.5 μm or less are 10% by volume or less, and the particles with a particle diameter of 3.0 μm or more are 6% by volume or less. When the particles with a particle diameter of 0.5 μm or less are more than 20% by volume, the gaps between the inorganic particles in the heat-resistant porous layer are filled up, the migration path of lithium ions inside the battery becomes narrow or long, resulting in an increase in membrane resistance. In addition, when the particles clog the pores of the polyolefin porous membrane, the performance of the battery may be significantly reduced. When the particles with a particle size of 3.0 μm or more exceed 10% by volume, the number of contacts between individual inorganic particles in the heat-resistant porous layer decreases, making the structure of the heat-resistant porous layer brittle. As a result, it becomes difficult to suppress the shrinkage of the polyolefin porous membrane at high temperatures, or the number of coarse particles increases, causing unevenness in the surface shape of the heat-resistant porous layer, and streaks may occur in the manufacturing method of the heat-resistant porous layer described below. When the particles with a particle size of 0.5 μm or less are 20% by volume or less and the particles with a particle size of 3.0 μm or more are 10% by volume or less, the gaps between the inorganic particles in the heat-resistant porous layer are not completely filled, and the particles are less likely to clog the pores of the polyolefin porous membrane, so the membrane resistance can be reduced.

[0031] The particle size referred to here means the particle size when a cumulative curve is obtained with the total volume as 100% when measured using a laser diffraction particle size distribution measuring device. The average particle size of barium sulfate particles was measured using a laser diffraction particle size distribution measuring device (LA-960V2, manufactured by Horiba, Ltd.) in accordance with JIS Z8825 (2013), and the volume average particle size (μm) was defined as the particle size when the volume-based cumulative ratio was 50%. The barium sulfate particles of the present invention are produced by a synthetic method. Specifically, they are barium sulfate particles obtained by a method of adding sulfuric acid to barium carbonate or barium sulfide to obtain barium sulfate (sulfuric acid method), or a method of adding sodium sulfate to barium chloride to obtain barium sulfate (sodium sulfate method).

[0032] The barium sulfate particles used in the present invention are expensive, but it is preferable to use sedimentary barium sulfate particles obtained by a synthetic method, especially barium sulfate particles synthesized by the sodium sulfate method in which barium chloride is used as a starting material and reacted with sodium sulfate (sodium sulfate). The reason for this is that during the study of barium sulfate particles, the barium sulfate particles synthesized by the sodium sulfate method generate extremely little hydrogen sulfide and can suppress the generation of corrosive gases.

[0033] In the embodiments of the present invention, the shape of the barium sulfate particles is not particularly defined, and barium sulfate particles of various shapes can be used. Specifically, spherical, substantially spherical, plate-like, needle-like, polyhedral shapes, etc. can be mentioned, and any of them is acceptable.

[0034] [Organic synthetic resin component] In the embodiments of the present invention, the organic synthetic resin component has the effects of binding barium sulfate particles constituting the heat-resistant porous layer to each other and adhering the heat-resistant porous layer to the polyolefin porous membrane. Specifically, one or more selected from the group of (meth)acrylic acid copolymer resins, polyacrylamide resins, polyvinylidene fluoride resins, polyvinyl alcohol resins, polyimide resins, polyamideimide resins, polyamide resins, poly(meth)aramid resins can be used, and commercially available aqueous solutions or aqueous dispersions can be used. As acrylic resins, specifically, "Polyzole" series manufactured by Showa Denko K.K., "BM" series manufactured by Nippon Zeon Co., Ltd., "Jurimer" (registered trademark) AT-210, ET-410, "Aron" (registered trademark) A-104, AS-2000, NW-7060 manufactured by Toagosei Co., Ltd., "LIOACCUM" (registered trademark) series manufactured by Toyochem Co., Ltd., TRD202A, TRD102A manufactured by JSR Corporation, "Polystron" (registered trademark) 117, 705, 1280 manufactured by Arakawa Chemical Industries, Ltd., "Cogum" (registered trademark) series manufactured by Showa Denko K.K., WEM-200U, and WEM-3000 manufactured by Daisheng Fine Chemical Co., Ltd. can be mentioned. As polyvinyl alcohol, specifically, "Kuraray Poval" (registered trademark) 3-98, 3-88 manufactured by Kuraray Co., Ltd., "Gosenol" (registered trademark) N-300, GH-20 manufactured by Mitsubishi Chemical Corporation can be mentioned. Among them, acrylic resins with high versatility and easy binding of barium sulfate particles to each other are preferred.

[0035] The heat-resistant porous layer may appropriately contain a thickener, a wetting agent, etc. for the purpose of improving coatability, and a thermosetting resin, a crosslinking agent, etc. for the purpose of improving heat resistance.

[0036] [Volume composition ratio of heat-resistant porous layer] In the heat-resistant porous layer in the embodiment of the present invention, the content of barium sulfate particles is 70% by volume or more and 98% by volume or less, with the total of barium sulfate particles and the organic synthetic resin component being 100% by volume. More preferably, it is 77% by volume or more and 93% by volume or less, and still more preferably, it is 85% by volume or more and 90% by volume or less.

[0037] When the content of barium sulfate particles is less than 70% by volume, the gaps between individual barium sulfate particles in the heat-resistant porous layer are clogged with the organic synthetic resin component, so that the ion migration path becomes narrow or long, increasing the electrical resistivity and air permeability resistance.

[0038] When the content of barium sulfate particles is more than 98% by volume, the organic synthetic resin component that holds individual barium sulfate particles together is insufficient, and the structure as a heat-resistant porous layer cannot be maintained.

[0039] When the content of barium sulfate particles is 70% by volume or more and 98% by volume or less, the gaps between individual barium sulfate particles in the heat-resistant porous layer are less likely to be clogged with the organic synthetic resin component, enabling good electrical resistivity and air permeability resistance to be obtained, and since there is no shortage of the binder that holds barium sulfate particles together, shrinkage of the polyolefin porous membrane due to heat can be suppressed.

[0040] [Average thickness of the heat-resistant porous layer] In the embodiment of the present invention, the average thickness of the heat-resistant porous layer is preferably 2.0 μm or more and 10 μm or less. More preferably, it is 2.5 μm or more and 6 μm or less, and even more preferably, it is 3.0 μm or more and 4.0 μm or less. If the thickness of the heat-resistant porous layer is less than 2.0 μm, it may not be possible to suppress the shrinkage of the polyolefin porous membrane due to heat. If the average thickness of the heat-resistant porous layer is greater than 10 μm, the ion migration path becomes longer, so the air permeability resistance may increase, or the proportion of the battery separator in the battery cell capacity may increase due to the increase in the inter-pole distance between the positive and negative electrodes of the battery cell, and the electrical resistivity may increase. When the average thickness of the heat-resistant porous layer is 2.0 μm or more and 10 μm or less, it is almost impossible for the air permeability resistance or the electrical resistivity to increase.

[0041] [Water content of barium sulfate] Since barium sulfate particles do not have hydroxyl groups on their particle surfaces, the influence of water molecules adsorbed on the surface is small, and it is possible to suppress gas generation such as hydrofluoric acid generated by the reaction of water and the electrolyte, and the deterioration of battery characteristics due to the consumption of the electrolyte. The specific surface area of barium sulfate particles is 1.0 m 2 / g or more and 18.0 m 2 / g or less is preferable. More preferably, it is 2.0 m 2 / g or more and 12.0 m 2 / g or less, and even more preferably, it is 2.0 m 2 / g or more and 3.0 m 2 / g or less.

[0042] If the specific surface area of barium sulfate particles is less than 1.0 m 2 / g, the particle size of individual barium sulfate particles in the heat-resistant porous layer may be larger than the thickness of the heat-resistant porous layer. Therefore, the barium sulfate particles may fall off from the battery separator, or the proportion of the separator in the battery cell capacity may increase due to the increase in the inter-pole distance between the positive and negative electrodes of the battery cell, and the capacity density of the battery may decrease. If the specific surface area of barium sulfate particles is 18.0 m 2If it is greater than / g, the amount of water adsorbed on the surface of barium sulfate particles increases, and the water content of the battery separator may increase. When the specific surface area of the barium sulfate particles is 1.0 m 2 / g or more and 18.0 m 2 / g or less, it is preferable because the barium sulfate particles do not fall off, the capacity density of the battery does not decrease, and the water content of the battery separator does not increase. The water content referred to here is measured by connecting a Karl Fischer moisture meter (Kyoto Electronics Industry Co., Ltd. MKC-610) and a moisture vaporization device (ADP-611 manufactured by Kyoto Electronics Industry Co., Ltd.) placed in a dry box under a dew point of -60°C with a gas outlet pipe. After leaving 1 g of the battery separator in a dew point of -60°C atmosphere for 24 hours, it is heated at 150°C for 10 minutes under a nitrogen atmosphere using the moisture vaporization device, and the moisture contained in the gas flowing out from the gas outlet pipe into the Karl Fischer moisture meter is measured.

[0043] [Hydrogen sulfide content] The hydrogen sulfide contained in the battery separator according to the embodiment of the present invention is 0.2×10 -3 mg / m 2 or less. Preferably, it is 0.15×10 -3 mg / m 2 or less, and more preferably 0.1×10 -3 mg / m 2 or less. If it is greater than 0.2×10 -3 mg / m 2 , gas may be generated inside the battery cell, or the current collector of the electrode may deteriorate due to the oxidation reaction of hydrogen sulfide with the current collector, and the life of the battery may decrease. If it is 0.2×10 -3 mg / m 2 or less, the generation of gas inside the battery cell can be suppressed, and the deterioration of the current collector of the electrode can be suppressed. The hydrogen sulfide content referred to here is obtained by enclosing 5 m 2 of the battery separator in a sealed container with a volume of 1 L, leaving it for 24 hours in an atmosphere of 60°C, and then obtaining a measured value X [volume ppm] of the gas in the container by the gas detector tube method defined in JIS K 0804:2014, and calculating the amount of hydrogen sulfide contained per unit area of the separator [mg / m 2 by the following calculation.

[0044] [Number]

[0045] Here, the gas density of hydrogen sulfide was 1.5392 [g / L] (1 atom, 0 °C; "Revised Seventh Edition Chemical Engineering Handbook (Maruzen Publishing)") was used.

[0046] Separator for battery per 1 m 2 Hydrogen sulfide contained in the separator for battery generated per is 0.2 × 10 -3 mg / m 2 The method below is not particularly limited. For example, it may be a method of heat-treating barium sulfate produced by the sulfuric acid method among precipitated barium sulfates, or a method of drying the moisture after washing with sufficient water. Further, for the separator for battery according to the embodiment of the present invention, even when the hydrogen sulfide contained in the separator for battery is larger than 0.2 × 10 -3 mg / m 2 it can be obtained by appropriately performing heat treatment on the separator for battery.

[0047] Next, the manufacturing method of the separator in the embodiment of the present invention will be specifically described.

[0048] [Method for forming heat-resistant porous layer] The heat-resistant porous layer for obtaining the present invention can be obtained by the following steps. (a) Preparation of coating dispersion for heat-resistant porous layer. (b) A step of coating the slurry on at least one side or both sides of the polyolefin porous membrane. (c) After the coating, a step of drying the solvent with a dryer to form a heat-resistant porous layer.

[0049] In the step (b), a known method can be used to coat at least one side or both sides of the polyolefin porous membrane with a coating dispersion for a heat-resistant porous layer. For example, the reverse roll coating method, the gravure coating method, the small-diameter gravure coater method, the kiss coating method, the roll brush method, the air knife coating method, the Meyer bar coating method, the pipe doctor method, the blade coating method, the die coating method, etc. can be mentioned. These methods can be carried out alone or in combination. It can be carried out in combination.

[0050] The separator for a battery according to an embodiment of the present invention can be used as a separator for a battery such as a secondary battery such as a nickel-hydrogen battery, a nickel-cadmium battery, a nickel-zinc battery, a silver-zinc battery, a lithium-ion secondary battery, a lithium polymer secondary battery, and a lithium-sulfur battery. In particular, it is preferably used as a separator for a lithium-ion secondary battery.

Examples

[0051] Hereinafter, examples will be shown and specifically described, but the present invention is not limited by these examples. The measured values in the examples are the values obtained by the following methods.

[0052] 1. Air permeability resistance (sec / 100cm 3 Air) Using a Wangyan type air permeability resistance meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T), each sample of the polyolefin porous membrane and the battery separator was fixed so as not to have wrinkles, and measured according to JIS P8117. The sample was 100 mm square, and the measurement points were 5 points at the center and 4 corners of the sample. The average value was used as the air permeability resistance (sec / 100cm 3 Air). When the length of one side of the sample is less than 100 mm, the values measured at 5 points at 50 mm intervals may be used.

[0053] 2. Thickness (μm) The thickness of the polyolefin porous membrane and the separator for battery was determined by averaging five measurement values using a contact thickness gauge ("Lightmatic" (registered trademark) series 318, manufactured by Mitutoyo Corporation). The measurement was carried out using a super-hard spherical probe of φ9.5 mm under the condition of a load of 0.01 N. Further, the thickness (μm) of the heat-resistant porous layer was measured with the above-mentioned contact thickness gauge for a polyolefin porous membrane obtained by washing the battery separator with the same liquid as the solvent contained in the slurry and removing the heat-resistant porous layer, and was obtained by the following calculation formula.

[0054] Thickness of heat-resistant porous layer (μm) = Thickness of battery separator (μm) - Thickness of polyolefin porous membrane (μm) 3. Particle size (μm) The particle size of barium sulfate particles was measured for the following physical property values using a laser diffraction particle size distribution analyzer (LA-960V2, manufactured by Horiba, Ltd.) in accordance with JIS Z8825 (2013). 1) Volume average particle size (μm) = Particle size when the volume-based cumulative ratio is 50% 2) Content of particles of 0.5 μm or less (%) = (Volume-based cumulative ratio of 0.5 μm or less) × 100 3) Content of particles of 3.0 μm or more (%) = {1 - (Volume-based cumulative ratio of 3.0 μm or less)} × 100.

[0055] 4. Hydrogen sulfide content In accordance with the measurement method of the hydrogen sulfide content, three measurements were carried out using a Kitagawa detector (AP-20, manufactured by Koito Rikagaku Kogyo Co., Ltd.) and a hydrogen sulfide detection tube (120U, manufactured by Koito Rikagaku Kogyo Co., Ltd.), and the average value was calculated as the hydrogen sulfide content per 1 m of the battery separator. 2 Here, when the measured value using the gas detection tube was below the detection limit, the detection limit value was used to calculate the average value and the hydrogen sulfide content.

[0056] 5. Moisture content (weight ppm) The moisture content was measured by connecting a Karl Fischer moisture meter (MKC-610, manufactured by Kyoto Electronics Industry Co., Ltd.) and a moisture vaporization device (ADP-611, manufactured by Kyoto Electronics Industry Co., Ltd.) placed in a dry box under a dew point of -60°C with a gas outlet pipe. After leaving 1 g of the battery separator in the atmosphere with a dew point of -60°C for 24 hours, it was heated at 150°C for 10 minutes under a nitrogen atmosphere using the moisture vaporization device, and the moisture contained in the gas flowing out from the gas outlet pipe into the Karl Fischer moisture meter was measured.

[0057] 6. Electrical Resistivity The electrical resistivity of the battery separator was measured by the following method. Coin cells of type CR2032 were prepared with 3, 4, and 5 sheets of the battery separator, respectively. Specifically, the cut-out battery separator was impregnated with an electrolytic solution (1 M-LiPF6 / EC:EMC (4:6 vol%)). This was vacuum-sealed in a coin-shaped case to prepare a cell. The cell was placed in a constant temperature bath at 25°C, and the resistance of the cell was measured by the AC impedance method at an amplitude of 20 mV and a frequency of 200 kHz. The measured resistance values of the cells were plotted against the number of battery separators, and this plot was linearly approximated to obtain the slope. This slope was multiplied by the measurement area to obtain the electrical resistivity (ohm·cm 2 ) per sheet of the battery separator.

[0058] 7. Thermal Shrinkage Rate (%) The heat resistance of 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 will be described below.

[0059] 1) Cut out three pieces of the battery separator with a size of 100 mm × 100 mm, place a transparent glass scale (measurement accuracy 0.1 mm) on it, and measure the distance between the midpoints of the two opposite sides of the battery separator as the length in the MD direction and the length in the TD direction, respectively, as the initial dimensions (mm).

[0060] 2) The battery separator was sandwiched between two sheets of A3-sized paper and placed in an oven at 130°C for 1 hour. After that, the battery separator was taken out and allowed to cool for 30 minutes.

[0061] 3) The distance between the midpoints of the two opposite sides of the battery separator was measured again using the glass scale, and the dimension after shrinkage (mm) was obtained. The measurement position at this time was the same as the position where the initial dimension was measured. When the end of the battery separator was curled, it was flattened for measurement. Using the obtained initial dimension and the dimension after shrinkage, the length in the MD direction and the length in the TD direction, and the thermal shrinkage rate (%) of each were obtained using the following calculation formula.

[0062] Thermal shrinkage rate (%) = {Initial dimension (mm) - Dimension after shrinkage (mm)} / Initial dimension (mm) × 100.

[0063] 8. Physical properties of the battery cell [Fabrication of the 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 cobaltate as an active material and 1.8 parts by mass of carbon black and mixed to obtain a positive electrode mixture-containing slurry. This positive electrode mixture-containing slurry was uniformly coated on 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. Then, it was compression-molded by a roll press to make the density of the positive electrode layer excluding the current collector 3.6 g / cm 3 to fabricate a positive electrode.

[0064] [Fabrication of the negative electrode] An aqueous solution containing 1.0 part by mass of sodium carboxymethyl cellulose was added to 98 parts by mass of artificial graphite as an active material and mixed. Further, a styrene-butadiene latex containing 1.0 part by mass as a solid content as a binder was added and mixed to obtain a negative electrode mixture-containing slurry. This negative electrode mixture-containing slurry was uniformly coated on 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. Then, it was compression-molded by a roll press to make the density of the negative electrode layer excluding the current collector 1.45 g / cm 3 to fabricate a negative electrode.

[0065] [Fabrication of Test Battery] Using the above positive and negative electrodes with tabs and each microporous membrane, a wound body was fabricated. Next, the wound body was placed in an aluminum laminate bag, and an electrolytic solution (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 dropped in and sealed with a vacuum laminator. Then, it was charged at 0.2C (C represents the current value at which the battery can be fully charged in 1 hour, which is 300 mA in the case of this battery) to 10% of the total capacity. After that, one side of the laminate was opened for gas venting and immediately sealed again with a vacuum sealer. Next, it was charged at a constant current and constant voltage of 0.1C, 4.35V, and a cut-off current of 0.05C, and then discharged at a constant current of 0.1C to 3V. Thereafter, it was charged at a constant current and constant voltage of 0.2C, 4.35V, and a cut-off current of 0.05C, and then discharged at a constant current of 0.2C to 3V. This charge and discharge at 0.2C were repeated 3 times. This was used as a 300 mAh-class test battery.

[0066] [High Load Test] Using the above test battery, an output characteristic test was carried out. After charging at a constant current and constant voltage of 0.2C, 4.35V, and a cut-off current of 0.05C, it was discharged at a constant current of 0.2C to 3V, and this capacity was recorded as the 0.2C discharge capacity. Next, it was charged at a constant current and constant voltage of 0.2C, 4.35V, and a cut-off current of 0.05C, and then discharged at a constant current of 5C to 3V, and this capacity was recorded as the 5C discharge capacity.

[0067] The 5C discharge capacity retention rate was calculated by the following formula.

[0068] 5C discharge capacity retention rate = [5C discharge capacity] / [0.2C discharge capacity] The same process was performed on a total of 3 test batteries, and the average value of the 5C discharge capacity retention rate was used as the output characteristic.

[0069] [Cycle Characteristic Test] After finishing the output characteristic test, the test battery was charged at a constant current and constant voltage of 0.5C, 4.35V, and a cut-off current of 0.05C. Then, it was discharged at a constant current of 0.2C until 3V, and this capacity was recorded as the first discharge capacity. The battery in this state was charged and discharged under the following conditions.

[0070] Charge: Constant current and constant voltage charge at 1C, 4.35V, cut-off current 0.05C; Discharge: Constant current discharge at 1C, 3V; Measurement temperature: 25°C. This was carried out on a total of 3 test batteries, and the average value of the ratio of the 2000th discharge capacity to the first discharge capacity, that is, the capacity retention rate, was calculated and used as an index of the cycle characteristics.

[0071] (Example 1) [Fabrication of battery separator] 100 parts by weight of particle A (barium sulfate (Glauber's salt method), D50 = 1.2μm) shown in Table 1, 0.5 part by weight (active ingredient) of a polyacrylic acid-based dispersant ("Aron" (registered trademark) A-6114 manufactured by Toagosei Co., Ltd.), and water were added, and dispersed with a bead mill to obtain a dispersion with an active ingredient rate of 60% by weight.

[0072] To the obtained dispersion, 1.5 parts by weight of a sodium partial neutralization product of polyacrylic acid with a neutralization degree of 50% (Viscomeet NP-700 manufactured by Showa Denko KK) as a thickener, 5.0 parts by weight (active ingredient) of an acrylic emulsion (Polyzol AP-4735 manufactured by Showa Denko KK) as a binder, 0.5 part by weight (active ingredient) of a wetting agent (product name "SN Wet 366" manufactured by San Nopco Ltd.), and water were added and stirred to prepare a coating liquid with a solid content rate of 50% by weight.

[0073] The obtained coating liquid was applied to one side (one surface) of the polyethylene porous membrane a (thickness 10μm, "SETELA" (registered trademark) manufactured by Toray Industries, Inc.) shown in Table 2 by the microgravure method and dried to fabricate a battery separator having a heat-resistant porous layer with a thickness of 4μm. Regarding the fabricated battery separator, evaluations of the heat-resistant porous layer thickness, hydrogen sulfide content, moisture rate, high-load test and cycle test of the battery, and heat shrinkage rate were carried out, and the results are shown in Table 3.

[0074] (Example 2, Comparative Examples 1 - 5) A separator for a battery was produced and evaluated in the same manner as in Example 1, except that the particle A in Example 1 was changed to particles B - G shown in Table 1, and the results are shown in Table 3.

[0075] (Examples 3 - 5, Comparative Examples 6 - 7) A separator for a battery was produced and evaluated in the same manner as in Example 1, except that the single - side coating of the heat - resistant porous layer in Example 1 with a film thickness of 4 μm was changed to the coating surface and film thickness described in Table 3, and the results are shown in Table 3.

[0076] (Examples 6 - 7, Comparative Examples 8 - 9) A separator for a battery was produced and evaluated in the same manner as in Example 1, except that the weight ratio of the active ingredient in the coating liquid of Example 1 was changed as described in Table 3, and the results are shown in Table 3. A separator for a battery was produced and evaluated in the same manner as in Example 1, except that the polyolefin porous membrane a in Example 1 was changed to the polyolefin porous membranes described in Table 2, and the results are shown in Table 3. (Examples 8 - 10) On each one - side of the polyethylene microporous film in Example 1, which was changed to the polyethylene porous membrane b (thickness 15.8 μm, "SETELA" (registered trademark) manufactured by Toray Industries, Inc.), polyethylene microporous film c (thickness 5.2 μm, "SETELA" (registered trademark) manufactured by Toray Industries, Inc.), and polyethylene porous membrane d (thickness 14.5 μm, "SETELA" (registered trademark) manufactured by Toray Industries, Inc.) shown in Table 2, a heat - resistant porous layer with a thickness of 4 μm was applied by the micro - gravure method and dried to produce a separator for a battery. As is clear from Table 3, the separators for batteries of Examples 1 to 10 showed good discharge characteristics with 65% or more in the high - load test, good heat resistance with a heat shrinkage rate of 5% or less at 130 °C, and good battery capacity retention rate of 70% or more after 2000 cycles.

[0077]

Table 1

[0078]

Table 2

[0079]

Table 3

Industrial Applicability

[0080] The separator of the present invention can be suitably used as a battery separator preferably used for non-aqueous electrolyte batteries such as lithium ion batteries.

Claims

1. A separator for a battery having a polyolefin porous membrane and a heat-resistant porous layer provided on at least one side of the polyolefin porous membrane, wherein the heat-resistant porous layer contains barium sulfate particles and an organic synthetic resin component, and the barium sulfate particles have 20% by volume or less of particles having a particle diameter of 0.5 μm or less and 10% by volume or less of particles having a particle diameter of 3.0 μm or more. The barium sulfate particles are contained in the heat-resistant porous layer at 70% by volume or more and 98% by volume or less based on the total of the barium sulfate particles and the organic synthetic resin component being 100% by volume. The average thickness of the heat-resistant porous layer is 2 μm or more and 10 μm or less. The moisture content of the separator is 400 ppm or less. The hydrogen sulfide content is 0.2 × 10 －３ mg / m ２ or less, and the separator for a battery is characterized by this.

2. The battery separator according to claim 1, wherein the barium sulfate particles are precipitated barium sulfate.

3. The battery separator according to claim 2, wherein the precipitated barium sulfate is produced by the glauber's salt method using barium chloride as a raw material.

4. The BET specific surface area of the barium sulfate particles is 2.0 m ２ / g or more and less than 3.0 m ２ / g, and the separator for a battery according to any one of claims 1 to 3, characterized in that.

5. The battery separator according to any one of claims 1 to 4, wherein the organic synthetic resin component contains one or more selected from the group consisting of (meth)acrylic acid copolymer resin, polyacrylamide resin, polyvinylidene fluoride resin, polyvinyl alcohol resin, polyimide resin, polyamideimide resin, polyamide resin, and poly(meth)aramid resin.

6. The air permeability resistance of the polyolefin porous membrane is 30 seconds / 100 cm ３ or more and 200 seconds / 100 cm ３ The separator for a battery according to any one of claims 1 to 5, characterized in that it is as described above.

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