Laminated porous membrane and method for producing laminated porous membrane

Abstract

To provide a laminated porous membrane and a method for producing a laminated porous membrane that are superior in heat resistance and resistance.SOLUTION: A laminated porous membrane has a porous layer containing inorganic particles on at least one surface of a polyolefin microporous membrane, wherein an area ratio of the inorganic particles in a cross-section of the porous layer is 55% or more and 70% or less, and a void area deviation is 0.05 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a laminated porous membrane having a porous layer on at least one side of a polyolefin microporous membrane, and a method for producing the laminated porous membrane. [Background technology]

[0002] Thermoplastic resin porous 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, polyolefin microporous membranes are preferably used as separators for lithium ion secondary batteries because they have ion permeability due to electrolyte impregnation, excellent electrical insulation, electrolyte resistance, and oxidation resistance, and also have a pore-blocking effect that interrupts 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 pore closure for some reason, the polyolefin microporous membrane may rupture. This phenomenon is not limited to polyolefins, and cannot be avoided above the melting point of the resin that constitutes the porous membrane.

[0005] In response to this, laminated porous membranes have been adopted in which a heat-resistant porous layer mainly composed of inorganic particles and a binder resin is coated on a polyolefin microporous membrane. By using this laminated porous membrane as a separator, the heat-resistant porous layer suppresses shrinkage of the polyolefin microporous membrane due to temperature rise. However, since the inclusion of inorganic particles in the porous layer increases electrical resistance, attempts have been made in recent years to achieve both high heat resistance and low resistance.

[0006] For example, Patent Document 1 describes a laminated porous membrane in which a porous layer primarily composed of inorganic particles and a binder resin is coated on a polyolefin microporous membrane, and by specifying the valley area ratio of the inorganic particle-containing porous layer surface and the aspect ratio of the inorganic particles, a separator with low resistance and high heat resistance can be provided. Patent Document 2 describes a separator with high heat resistance and low resistance can be provided by specifying the proportion of voids within a predetermined size range in the cross section of the inorganic particle-containing porous layer. Patent Document 3 describes a separator with high heat resistance can be provided by including inorganic particles, a water-insoluble binder, a water-soluble binder, and a polyacrylic acid-based dispersant as components constituting the porous layer. Patent Document 4 describes a separator with high heat resistance and low resistance can be provided by partially sintering or recrystallizing and bonding primary particles of inorganic particles to form secondary particles, and setting the porosity in the cross section of the porous layer to 50% or more. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2023-135588 [Patent Document 2] Patent No. 7058803 [Patent Document 3] Patent No. 7305895 [Patent Document 4] Japanese Patent Application Laid-Open No. 2006-147569 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the techniques described in the above Patent Documents 1 to 4 are still insufficient in terms of achieving both high heat resistance and low resistance, and therefore, there is a demand for a better laminated porous membrane.

[0009] An object of the present invention is to provide a laminated porous membrane having excellent heat resistance and low electrical resistance, and a method for producing the laminated porous membrane. [Means for solving the problem]

[0010] In order to solve the above problems, the present invention has the following configuration. [I] A laminated porous membrane having a porous layer containing inorganic particles and a binder resin on at least one side of a polyolefin microporous membrane, wherein the area ratio of the inorganic particles in the cross section of the porous layer is 55% or more and 70% or less, and the void area deviation is 0.05 or less. [II] The laminated porous membrane according to [I], wherein the binder resin contains a polyacrylamide resin. [III] The laminated porous membrane according to [I] or [II], wherein the inorganic particles have an average circularity of 0.50 or more and 0.70 or less, and an average area envelopment ratio of 0.85 or more and 0.95 or less. [IV] The laminated porous film according to any one of [I] to [III], wherein the inorganic particles contain at least one kind selected from alumina, barium sulfate, and boehmite. [V] A slurry for coating a porous layer, which contains inorganic particles and a binder resin and has a pH of 3.5 or more and 5.5 or less. [VI] A method for producing a laminated porous membrane having a porous layer containing inorganic particles and a binder resin on at least one surface of a polyolefin microporous membrane, the method comprising the steps of applying a slurry containing inorganic particles and a binder resin to at least one surface of the polyolefin microporous membrane and then drying the applied slurry, wherein the pH of the slurry is 3.5 or more and 5.5 or less. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a laminated porous membrane having excellent heat resistance and low electrical resistance, and a method for producing the laminated porous membrane. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the embodiments described below.

[0013] One embodiment of the present invention is a laminated porous membrane having a porous layer containing inorganic particles on at least one surface of a polyolefin microporous membrane.

[0014] <Polyolefin microporous membrane> In an embodiment of the present invention, the thickness of the polyolefin microporous membrane is not particularly limited as long as it functions as a laminated porous membrane, but is preferably 25 μm or less. It is more preferably 3 μm or more and 20 μm or less, and even more preferably 5 μm or more and 14 μm or less. A polyolefin microporous membrane having a thickness of 25 μm or less can achieve both practical membrane strength and pore-blocking function, and is suitable for increasing the capacity of the battery without restricting the area per unit volume of the battery case.

[0015] The air resistance of the polyolefin microporous membrane is preferably 30 sec / 100 cc air or more and 300 sec / 100 cc air or less. It is more preferably 40 sec / 100 cc air or more and 250 sec / 100 cc air or less, and even more preferably 50 sec / 100 cc air or more and 200 sec / 100 cc air or less. An air resistance of 30 sec / 100 cc air or more provides sufficient mechanical strength and insulation, reducing the possibility of short circuits occurring during battery charge and discharge. An air resistance of 300 sec / 100 cc air or less provides sufficient battery charge and discharge characteristics, particularly ion permeability (charge and discharge operating voltage) and battery life (closely related to the amount of electrolyte retained), allowing the battery to fully function.

[0016] The porosity of the polyolefin microporous membrane is preferably 20% or more and 70% or less, more preferably 30% or more and 60% or less, and even more preferably 55% or less. A porosity of 20% 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.

[0017] The average pore size of the polyolefin microporous membrane has a significant effect on the pore-blocking function, and is therefore preferably 0.01 μm or more and 1.0 μm or less. It is more preferably 0.02 μm or more and 0.5 μm or less, and even more preferably 0.03 μm or more and 0.3 μm or less. When the average pore size of the polyolefin microporous membrane is 0.01 μm or more, clogging of the pores by the porous layer composition when the porous layer is laminated is suppressed, and the air resistance and electrical resistance become favorable. When the average pore size is 1.0 μm or less, deterioration of the air resistance and electrical resistance due to clogging of the pores by the porous layer composition and deterioration of the safety of the battery due to the occurrence of micro-short circuits are suppressed, and this is favorable.

[0018] Furthermore, when the average pore size of the polyolefin microporous membrane is 0.01 μm or more and 1.0 μm or less, the anchor effect of the porous layer composition provides sufficient adhesive strength of the porous layer to the polyolefin microporous membrane, and when the porous layer is laminated, the air resistance and electrical resistance do not deteriorate significantly, the response of the pore blocking phenomenon to temperature does not become slow, and the pore blocking temperature does not shift to a higher temperature due to the heating rate. The above-mentioned average pore size is measured by the bubble point method specified in JIS K 3832:1990.

[0019] The polyolefin resin constituting the polyolefin microporous 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.

[0020] Among these, polyethylene is particularly preferred from the viewpoint of excellent pore-blocking performance. Hereinafter, the polyolefin resin used in the present invention will be described in detail using polyethylene as an example, but the embodiment of the present invention is not limited thereto.

[0021] Examples of polyethylene include ultra-high molecular weight polyethylene, high-density polyethylene, medium-density polyethylene, and low-density polyethylene. The polymerization catalyst is not particularly limited, 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. While polyethylene may be a single substance, a mixture of two or more polyethylenes is preferred. The polyethylene mixture may be a mixture of two or more ultra-high molecular weight polyethylenes with 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 polyethylenes selected from the group consisting of ultra-high molecular weight polyethylenes, high-density polyethylenes, medium-density polyethylenes, and low-density polyethylenes.

[0022] The polyolefin microporous membrane preferably has a 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. When the constituent resin has a melting point of 70°C or higher and 150°C or lower, the pore-blocking function is exerted during normal use, preventing the battery from becoming unusable, and the pore-blocking function is exerted during an abnormal reaction, ensuring safety.

[0023] <Porous layer> The laminated porous membrane according to an embodiment of the present invention comprises a porous layer provided on at least one surface of the polyolefin microporous membrane, and the porous layer contains inorganic particles and a binder resin. The porous layer may be provided on only one side of the polyolefin microporous membrane or on both sides. When provided on only one side, the number of steps for forming the porous layer is reduced, thereby further reducing production costs. When provided on both sides, the thermal shrinkage of the polyolefin microporous membrane is suppressed from both sides, thereby more effectively reducing the thermal shrinkage rate of the laminated porous membrane.

[0024] The cross section of the porous layer of the laminated porous membrane of the present invention must have an area ratio of inorganic particles of 55% or more and 70% or less, as determined by the measurement method described below. When the area ratio of inorganic particles in the cross section of the porous layer is within a specified range, heat resistance is improved and electrical resistance can be reduced. It is more preferably 58% or more and 66% or less. If the area ratio of inorganic particles is less than 55%, the voids in the porous layer may become too large, resulting in insufficient contact between the inorganic particles and the heat resistance may be reduced. Furthermore, if the area ratio of inorganic particles exceeds 70%, the voids in the porous layer may become too small, making it difficult for ions to move, resulting in reduced electrical resistance. The area ratio of inorganic particles in the cross section of the porous layer can be determined by binarizing the inorganic particles and voids in an image of the porous layer cross section obtained with a scanning electron microscope (SEM) under the measurement conditions described below. The binder resin contained in the porous layer of the laminated porous membrane of the present invention is present around the inorganic particles, and the inorganic particles and the binder resin become the same phase during binarization processing. Therefore, the area ratio of the inorganic particles in the cross section of the porous layer, the average circularity of the inorganic particles, and the average area envelopment ratio of the inorganic particles, which are obtained by binarization processing of the cross section image of the porous layer obtained by a scanning electron microscope (SEM) in the present invention, are obtained as values ​​including the inorganic particles and the binder resin.

[0025] The porous layer cross section of the present invention must have a void area deviation of 0.05 or less, as determined by the measurement method described below. The void area deviation in the porous layer cross section can be determined by binarizing the porous layer cross-sectional image obtained by SEM, as described above, into inorganic particles and voids. When the void area deviation in the porous layer cross section is 0.05 or less, uniform voids are formed between the inorganic particles, which is favorable for heat resistance and electrical resistance. When the void area deviation exceeds 0.05, the voids in the porous layer become too small or too large in some places. In the former case, ions become less mobile, resulting in poor electrical resistance, and in the latter case, the contact points between the inorganic particles become insufficient, resulting in poor heat resistance.

[0026] The inorganic particles in the cross section of the porous layer of the present invention preferably have an average circularity of 0.50 or more and 0.70 or less, as determined by the measurement method described below. The average circularity of the inorganic particles in the cross section of the porous layer can be determined by binarizing the inorganic particles and voids in a cross section image of the porous layer obtained by SEM, as described above. The average circularity is a numerical representation of a perfectly circular shape when it is 1, and an elongated shape as it approaches 0. In the present invention, an average circularity of 0.50 or more and 0.70 or less facilitates the formation of the voids described in the present invention during porous layer formation, which is favorable for heat resistance and electrical resistance. By setting the average circularity to 0.50 or more, the inorganic particles have an elongated shape, and the inorganic particles pile up due to distortion during porous layer formation, which can prevent insufficient contact between the inorganic particles and a deterioration in heat resistance. Furthermore, by setting the average circularity to 0.70 or less, the inorganic particles have a shape close to a perfect circle, and the inorganic particles are densely stacked during the formation of the porous layer, which makes it difficult for ions to move and prevents deterioration of electrical resistance. The average circularity is more preferably 0.55 to 0.69, and even more preferably 0.61 to 0.68.

[0027] The inorganic particles in the cross section of the porous layer of the present invention preferably have an average area envelopment ratio of 0.85 or more and 0.95 or less, as determined by the measurement method described below. The average area envelopment ratio of the inorganic particles in the cross section of the porous layer can be determined by binarizing the inorganic particles and voids in a cross section image of the porous layer obtained by SEM, as described above. The average area envelopment ratio relates to the contour of the inorganic particle (the ratio of the area of ​​the inorganic particle wrapped around the actual perimeter to the area of ​​the inorganic particle wrapped around the perimeter including the dents). A value of 1 indicates no dents, and as the ratio approaches 0, the area of ​​the dents increases. In the present invention, when the average area envelopment ratio of the inorganic particles in the cross section of the porous layer is 0.85 or more and 0.95 or less, the voids described in the present invention are easily formed during the formation of the porous layer, which is favorable for heat resistance and electrical resistance. By maintaining an average area envelopment ratio of 0.85 or more, it is possible to prevent small particles generated by dents from penetrating the voids between the inorganic particles that form the porous layer, thereby preventing an increase in electrical resistance. Furthermore, by setting the average area envelopment ratio to 0.95 or less, it is possible to prevent the inclusion of inorganic particles that are not disintegrated during the formation of the porous layer, resulting in insufficient contact between the inorganic particles and a deterioration in heat resistance.The average area envelopment ratio is more preferably 0.86 or more and 0.94 or less.

[0028] In the present invention, the inorganic particles in the cross section of the porous layer preferably have a low average circularity and a high average area envelopment ratio relative to a perfect circle, that is, a shape with few depressions on the surface rather than a perfect circle.

[0029] <Inorganic particles> The inorganic particles contained in the porous layer of the present invention are not particularly limited in material as long as they are electrochemically stable. Specific examples include anions such as sodium oxide, potassium oxide, magnesium oxide, calcium oxide, barium oxide, lanthanum oxide, cerium oxide, strontium oxide, vanadium oxide, SiO2-MgO (magnesium silicate), SiO2-CaO (calcium silicate), hydrotalcite, sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, barium carbonate, lanthanum carbonate, cerium carbonate, basic titanate, basic silicic titanate, basic copper acetate, basic lead sulfate, layered double hydroxides (Mg-Al type, Mg-Fe type, Ni-Fe type, Li-Al type), layered double hydroxide-alumina silica gel composite, boehmite, alumina, zinc oxide, lead oxide, iron oxide, iron oxyhydroxide, hematite, bismuth oxide, tin oxide, titanium oxide, and zirconium oxide. The adsorbents may be selected from the group consisting of cation adsorbents such as zirconium phosphate, titanium phosphate, apatite, non-basic titanates, niobates, and niobium titanates; oxide ceramics such as zeolites, calcium sulfate, magnesium sulfate, aluminum sulfate, gypsum, barium sulfate, alumina trihydrate (ATH), fumed silica, precipitated silica, zirconia, and yttria; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; silicon carbide; layered silicates such as kaolinite, talc, dickite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, amesite, and bendonite; asbestos, diatomaceous earth, glass fiber; synthetic layered silicates such as mica or fluoromica; and zinc borate. These may be used alone or in combination. Among these, alumina, barium sulfate, and boehmite are particularly preferred, with boehmite being more preferred. Specifically, by using angular boehmite, it is possible to reduce the deviation in void area in the cross section of the porous layer, thereby improving the heat resistance.

[0030] The inorganic particles preferably have an average particle diameter D50 of 0.1 μm or more and 2.5 μm or less when the volume-based cumulative ratio is 50%. It is more preferably 0.3 μm or more and 2.0 μm or less, and even more preferably 0.5 μm or more and 1.0 μm or less. By setting the average particle diameter D50 of the inorganic particles to be equal to or greater than the aforementioned range, it is possible to prevent the gaps between the inorganic particles in the porous layer from becoming too small, which makes it difficult for ions to move and results in a deterioration in electrical resistance. Furthermore, by setting the average particle diameter D50 of the inorganic particles to be equal to or less than the aforementioned range, it is possible to prevent the gaps between the inorganic particles in the porous layer from becoming too wide, which results in insufficient contact between the inorganic particles and a deterioration in heat resistance.

[0031] <Binder resin> The binder resin contained in the porous layer of the present invention is not particularly limited, but is preferably one that has both the effect of binding inorganic particles constituting the porous layer together and the effect of adhering the porous layer to the polyolefin microporous membrane. Specifically, one or more resins selected from the group consisting of (meth)acrylic acid copolymer resins, polyacrylamide resins, polyvinylidene fluoride resins, polyvinyl alcohol resins, polyimide resins, polyamideimide resins, polyamide resins, and poly(meth)aramid resins can be used, and commercially available aqueous solutions or aqueous dispersions can be used. Specific examples of acrylic resins include the "Polysol" series manufactured by Showa Denko K.K., the "BM" series manufactured by Zeon Corporation, "Jurymer" (registered trademark) AT-210, ET-410, "Aron" (registered trademark) A-104, AS-2000, and NW-7060 manufactured by Toagosei Co., Ltd., the "LIOACCUM" (registered trademark) series manufactured by Toyochem Co., Ltd., TRD202A and TRD102A manufactured by JSR Corporation, "Polystron" (registered trademark) 117, 705, and 1280 manufactured by Arakawa Chemical Industries, Ltd., the "Kogam" (registered trademark) series manufactured by Showa Denko K.K., and WEM-200U and WEM-3000 manufactured by Taisei Fine Chemical Co., Ltd. Specific examples of polyvinyl alcohol include Kuraray Poval (registered trademark) 3-98 and 3-88 manufactured by Kuraray Co., Ltd., and Gohsenol (registered trademark) N-300 and GH-20 manufactured by Mitsubishi Chemical Corporation. Among these, polyacrylamide resins are preferred because they are highly versatile and easily bond inorganic particles together. There is a method in which the condensation degree of a hydrolysis condensate of a silane compound is adjusted and used as a binder resin, but since it is difficult to adjust the condensation degree, it is preferable not to use such a condensate as a binder resin in the present invention.

[0032] <Additives> The porous layer may appropriately contain a dispersant for improving the dispersion stability of inorganic particles, a thickener and a wetting agent for improving coatability, a thermosetting agent and a crosslinking agent for improving heat resistance, etc.

[0033] <Weight composition ratio of porous layer> In an embodiment of the present invention, the content of inorganic particles contained in the porous layer is 92% by mass or more and 99% by mass or less, with the total composition forming the heat-resistant porous layer being 100% by mass. It is more preferably 93% by mass or more and 98% by mass or less. When the content of inorganic particles is 92% by mass or more, an increase in air permeation resistance can be suppressed, which is preferable.

[0034] <Average thickness of porous layer> In an embodiment of the present invention, the average thickness of the porous layer is preferably 1.0 μm or more and 5.0 μm or less. It is more preferably 1.0 μm or more and 4.5 μm or less, even more preferably 1.0 μm or more and 4.0 μm or less, and particularly preferably 1.0 μm or more and 3.0 μm or less. By setting the thickness of the porous layer to 1.0 μm or more, the amount of electrolyte solution permeating into the porous layer is reduced, thereby preventing penetration into the polyolefin microporous membrane from being poorly promoted. Furthermore, by setting the thickness of the porous layer to 5.0 μm or less, the volume of the porous layer increases, preventing the electrolyte solution from accumulating inside the porous layer and preventing penetration into the polyolefin microporous membrane from being poorly promoted.

[0035] <Laminated porous membrane> The laminated porous membrane of the present invention preferably has a heat shrinkage rate of 10% or less in both the longitudinal direction (MD) and the transverse direction (TD) as determined by the measurement method described below. By achieving such a heat shrinkage rate, when used as a separator, the separating function can be maintained even at high temperatures, and the heat resistance is excellent, which is preferable. More preferably, the heat shrinkage rate is 4% or less in both MD and TD.

[0036] <Method for forming porous layer and laminated porous membrane> The method for forming the porous layer and laminated porous membrane in the present invention is not particularly limited, but they can be obtained, for example, by the following steps. (a) Preparation of a porous layer coating slurry containing inorganic particles and a binder resin. (b) adjusting the pH of the slurry. (c) applying the pH-adjusted slurry to at least one or both surfaces of a microporous polyolefin membrane; (d) After the application, the solvent is dried with a dryer to form a porous layer. In the step (a), water is preferably used as the dispersion medium. A mixture of water and a hydrophilic solvent such as methanol, ethanol, or N-methylpyrrolidone may be used as the dispersion medium, provided that the dispersion stability of the porous layer coating slurry is not impaired. A known method can be used to prepare a porous layer coating slurry containing at least inorganic particles and a binder resin. Examples of such methods include ball mills, bead mills, planetary ball mills, vibrating ball mills, sand mills, colloid mills, roll mills, high-speed impeller dispersion, dispersers, homogenizers, planetary mixers and planetary kneaders, ultrasonic dispersion, and mechanical stirring using a stirring blade or the like.

[0037] In order to achieve a preferred embodiment of the present invention in which the average circularity of the inorganic particles in the cross section of the porous layer is 0.50 to 0.70 and the average area envelopment ratio is 0.85 to 0.95, it is desirable to disintegrate the inorganic particle agglomerates in this process and to achieve a dispersion state in which the inorganic particles are free of chips or dents. If the inorganic particles are too dispersed, small particles resulting from chips or dents may penetrate into the voids between the inorganic particles that form the porous layer, increasing electrical resistance. On the other hand, if the inorganic particles are too dispersed, the inorganic particle agglomerates cannot be disintegrated, and the presence of the agglomerates during porous layer formation may result in insufficient contact between the inorganic particles, resulting in poor heat resistance.

[0038] The above-mentioned dispersed state is a state in which, in the step of dispersing inorganic particles in an agglomerated state in a dispersion medium, the agglomerates can be broken down without applying excessive energy to the particles, and the inorganic particles are dispersed while maintaining the size, shape, crystalline structure, surface state, etc. of the primary particles. Specifically, for example, when a bead mill dispersing device is used, this can be obtained by using ceramic beads with a smaller bead diameter or beads with a smaller bead specific gravity, or by adjusting the rotor peripheral speed of the bead mill dispersing device.

[0039] To achieve the above-mentioned dispersed state, 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.

[0040] By setting the bead particle size to 0.3 mm or more, the mass per bead is small and the shear stress generated between the beads is small, so that aggregates of inorganic particles cannot be sufficiently broken down, and the inclusion of the aggregates during porous layer formation results in insufficient contact between the inorganic particles, which can prevent deterioration of heat resistance.By setting the bead particle size to 1.0 mm or less, the impact force per bead increases, which causes already broken down individual inorganic particles to be crushed even finer, and the small particles enter the gaps between the inorganic particles that form the porous layer, preventing ion movement and preventing increased electrical resistance. The ceramic beads may be made of at least one material selected from the group consisting of alumina, zirconia, and silicon nitride. The method for adjusting the dispersion state by changing the rotor peripheral speed of the bead mill disperser is not particularly limited, but examples include a method of adjusting the conditions depending on the average circularity and average area envelopment of the inorganic particles in the cross section of the porous layer. As mentioned above, the average circularity is a numerical representation of the fact that a value of 1 indicates a perfect circle, and as the value approaches 0, the shape becomes more elongated. The average area envelopment relates to the contour of the inorganic particles (the ratio of the area obtained by wrapping the actual perimeter around the inorganic particle to the area obtained by wrapping the perimeter including the dents), and is a numerical representation of the fact that a value of 1 indicates no dents, and as the value approaches 0, the area of ​​the dents increases. Specifically, for example, when angular inorganic particles are used, in order to break down agglomerates and achieve a dispersion state in which the inorganic particles are free of chips or dents, it is preferable to achieve a dispersion state in which the average circularity is small and the average area envelopment is large.

[0041] To achieve the above-mentioned dispersion state, the rotor peripheral speed is preferably 7 m / sec or more and 12 m / sec or less. If the rotor peripheral speed is below the preferred range, the shear stress generated between the beads is small, so that the aggregates of inorganic particles cannot be sufficiently disintegrated, and the presence of the aggregates during porous layer formation may result in insufficient contact between the inorganic particles, resulting in poor heat resistance. If the rotor peripheral speed exceeds the preferred range, the shear stress generated between the beads becomes too large, causing chipping and denting of the inorganic particles. The resulting small particles may enter the voids between the inorganic particles that form the porous layer, hindering ion migration and increasing electrical resistance.

[0042] Although the conditions using a bead mill dispersing device have been described above as an example of a method for dispersing inorganic particles, the present invention is not limited to such an example. Even when other dispersing devices or stirring devices are used, inorganic particles suitable for the present invention can be prepared by controlling the shear stress of the dispersion and stirring conditions, breaking down aggregates of inorganic particles, and suppressing chipping or denting of the inorganic particles.

[0043] In the present invention, it is preferable to include a step of adjusting the pH of the slurry obtained in the step (a). If the step (b) of adjusting the pH of the slurry is included, the additional steps will be more time-consuming and costly, and therefore it is not generally carried out. However, the present inventors have found that the area ratio occupied by inorganic particles in the cross section of the porous layer can be controlled by adjusting the pH of the slurry.

[0044] The method for adjusting the slurry pH is not particularly limited, but for example, a lowered pH of the slurry can be achieved by adding an acidic solvent such as acetic acid to the slurry and mixing it, whereas a higher pH of the slurry can be achieved by adding an alkaline solvent such as aqueous ammonia to the slurry and mixing it.

[0045] The purpose of adjusting the pH of the slurry is to control the area ratio occupied by inorganic particles in the cross section of the porous layer. Specifically, when forming a porous layer using a slurry containing boehmite as the inorganic particles and polyacrylamide resin as the binder resin, the area ratio occupied by the inorganic particles can be reduced by making the pH acidic. On the other hand, the area ratio occupied by the inorganic particles can be increased by making the pH alkaline. Although the mechanism is unclear, it is assumed that the pH changes the ease of spreading of the binder molecular chains contained in the slurry, which in turn changes the spreading of the inorganic particles bound to the binder.

[0046] The pH of the slurry is preferably 3.5 or more and 5.5 or less. It is more preferably 4.0 or more and 5.0 or less. If the pH of the slurry is less than 3.5, the area ratio of the inorganic particles in the cross section of the porous layer is too small, resulting in insufficient contact between the inorganic particles and possibly a deterioration in heat resistance. If the pH of the slurry exceeds 5.5, the area ratio of the inorganic particles in the cross section of the porous layer is too large, hindering ion migration and possibly resulting in a deterioration in electrical resistance.

[0047] In the step (c), the pH-adjusted slurry can be applied to at least one or both surfaces of the polyolefin microporous membrane by any known method, such as direct gravure coating, reverse gravure coating, kiss reverse gravure coating, direct bar coating, air knife coating, Mayer bar coating, pipe doctor coating, blade coating, and die coating, which can be used alone or in combination.

[0048] In step (d), the solvent is dried with a dryer after application to form a porous membrane, and known methods can be used. Examples include drying with a heated roll, drying with air, and drying in a drying oven. Although not particularly limited, the drying temperature is preferably 25°C or higher and 100°C or lower. If the drying temperature is lower than 25°C, it may take a long time to dry the solvent, which may result in a decrease in productivity. If the drying temperature is higher than 100°C, the polyolefin microporous membrane may shrink excessively, which may result in a decrease in productivity.

[0049] One embodiment of the present invention is a slurry for coating a porous layer, which contains inorganic particles and a binder resin and has a pH of 3.5 to 5.5. By applying this slurry to at least one surface of a polyolefin microporous membrane, a laminated porous membrane with excellent heat resistance and low resistance can be obtained.

[0050] Another embodiment of the present invention is a method for producing a laminated porous membrane having a porous layer containing inorganic particles and a binder resin on at least one surface of a polyolefin microporous membrane, the method comprising the steps of applying a slurry containing inorganic particles and a binder resin to at least one surface of the polyolefin microporous membrane and then drying the applied slurry, wherein the pH of the slurry is 3.5 to 5.5. By this production method, a laminated porous membrane having excellent heat resistance and low resistance can be obtained. [Example]

[0051] 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.

[0052] <Thickness [μm]> The thickness of the polyolefin microporous membrane and the laminated porous membrane was determined by averaging five measurements 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 spherical ultrahard probe under a load of 0.15 N. Furthermore, the thickness [μm] of the porous layer was determined by washing the laminated porous membrane with the same solvent as that contained in the slurry, removing the porous layer, measuring the polyolefin microporous membrane with the contact-type film thickness meter, and calculating the thickness using the following formula: Porous layer thickness [μm] = laminated porous membrane thickness [μm] - polyolefin microporous membrane thickness [μm] <Particle diameter of inorganic particles [μm]> The particle diameter D50 of the inorganic particles used to prepare the slurry was measured at a volume-based accumulation rate of 50% using a laser diffraction particle size distribution analyzer (Microtrac, MT3300-EXII) in accordance with JIS Z 8825 (2013), and is shown in Table 1. The measurement conditions are as described below. Solvent conditions Solvent type: Water Solvent refractive index: 1.33 Particle Conditions Transparency: Transparent Shape: non-spherical Particle Refractive Index: Alumina: 1.77, barium sulfate: 1.65, boehmite: 1.62 <Area ratio of inorganic particles in the cross section of the porous layer, void area deviation, average circularity and average area envelopment of inorganic particles> The laminated porous membrane was treated using an ion milling device (IM4000 manufactured by Hitachi High-Technologies Corporation) so as to obtain a cross section in the thickness direction. Next, an LEI image of the cross section of the obtained laminated porous membrane was taken at a magnification of 10,000 times (accelerating voltage: 2.0 kV) using a scanning electron microscope (JEOL Ltd. JSM6701F). <Calculation of the area ratio occupied by inorganic particles, as well as the average circularity and average area envelopment ratio of inorganic particles> The area ratio of inorganic particles in the cross section of the porous layer, as well as the average circularity and average area envelopment of inorganic particles, are calculated using the image processing software Fiji (Fuji Is Just ImageJ) according to the following methods (1)-(10). (1) First, open the cross-sectional SEM image of the target laminated porous membrane by selecting [File] → [Open]. Next, measure the known distance in the image using the line selection tool [Straight]. Then, select [Analyze] → [Set Scale] and enter the known distance and measurement unit to set the scale. (2) Next, select an evaluation area for binarization using the region selection tool [Rectangle]. The desired region is selected by removing the area from the interface between the polyolefin microporous membrane and the porous layer to the porous layer side by 0.3 μm and the area from the outermost layer of the porous layer to a thickness of 0.3 μm in the thickness direction of the laminated porous membrane, and selecting the remaining area. Then, select the entire image in the direction perpendicular to the thickness direction of the laminated porous membrane. (3) Next, select [Image] → [Crop] to display only the selected area. At this time, the field of view area of ​​the selected range is calculated. (4) Next, perform contrast enhancement processing on the cross-sectional SEM image. Go to [Process] → [Enhance Contrast], set [Saturated Pixels] to 0.3%, check [Equalize histogram], and click [OK]. This processing enhances the contrast of the image, making bright areas (inorganic particles) brighter and dark areas (voids) darker. (5) Next, go to [Process] → [Filters] → [Median], enter 3.0 in [Radius pixels], and click [OK]. By performing this process, you can remove noise while preserving the outline of the inorganic particles. (6) Next, perform binarization by selecting [Process] → [Binary] → [Make Binary]. This process allows you to obtain a binary image in which inorganic particles are black and voids are white. (7) Next, select [Edit] → [Invert] to invert the black and white, and obtain an image in which the inorganic particles are white and the voids are black. (8) Next, select [Process] → [Binary] → [Watershed] to separate connected inorganic particles through binarization. (9) Next, click [Analyze] → [Analyze Particles], enter [0-Infinity] in the [Size (μm^2)] field, enter [0.00-1.00] in the [Circularity] field, and then check [Display results], [Clear results], [Add to Manager], and [Include holes]. Click [OK] to obtain the area, circularity, and area envelopment ratio of each inorganic particle relative to the field of view. Here, circularity and area envelopment ratio are uniquely determined by the software mentioned above, and are calculated using the following formulas:

[0053] Circularity = 4π x area of ​​inorganic particle / square of circumference of inorganic particle Area envelopment = area wrapped around actual perimeter / area wrapped around envelope perimeter (10) The area ratio of the inorganic particles in the cross section of the porous layer, as well as the average circularity and average area envelopment of the inorganic particles are calculated using the following formulas.

[0054] Area ratio of inorganic particles = total area of ​​each inorganic particle / field of view area Average circularity of inorganic particles = total circularity of each inorganic particle / total number of inorganic particles Average area envelopment of inorganic particles = total area envelopment of each inorganic particle / total number of inorganic particles <Void area deviation> The void area deviation in the cross section of the porous layer is calculated by the methods (1)-(6) above and (11)-(15) below. (11) Next, select [Process] → [Binary] → [Watershed] to separate connected voids using binarization processing. (12) Next, click [Analyze] → [Analyze Particle], enter [0-Infinity] in the [Size (μm^2)] field, and then check [Display results], [Clear results], [Add to Manager], and [Include holes]. Click [OK] to obtain the area of ​​each void relative to the field of view. (13) Calculate the void area deviation in the porous layer cross section using the following formula.

[0055] Void area deviation = √(Σ(Each void area - average void area) 2 ) / total number of voids <Thermal shrinkage rate (%) of laminated porous membrane> The heat shrinkage rates of the laminated porous membrane in the MD direction (length direction) and TD direction (width direction) were measured by the following method. (1) Three pieces of the laminated porous membrane were cut out, each measuring 100 mm in the MD direction and 100 mm in the TD direction. Two points were marked at two locations that were 25 mm from the center of gravity of the square in the MD direction. The distance between these two points, approximately 50 mm, was defined as the initial dimension in the MD direction. Furthermore, two points were marked at two locations that were 25 mm from the center of gravity of the square in the TD direction. The distance between these two points, approximately 50 mm, was defined as the initial dimension in the TD direction. These initial dimensions were measured in 0.1 mm increments as the initial dimensions in each direction using a transparent glass scale (measurement accuracy 0.1 mm). (2) The laminated porous membrane was sandwiched between two sheets of A3 size paper, placed in an oven at 150°C, and left for 1 hour. After that, the laminated porous membrane was taken out of the oven and left at room temperature for 30 minutes. (3) Using the glass scale, measurements were taken between two points in the MD direction of the laminated porous membrane to determine the dimension after shrinkage, and measurements were taken between two points in the TD direction to determine the dimension after shrinkage. These measurements were taken in 0.1 mm increments.

[0056] The heat shrinkage rates (%) in the MD and TD directions were calculated from the initial dimensions and the dimensions after shrinkage using the following formula. The heat shrinkage rates in the MD and TD directions of the three sheets obtained were averaged, and the average value was used as the heat shrinkage rate of the laminated porous membrane in the MD and TD directions. Furthermore, a heat shrinkage rate of the laminated porous membrane in the MD and TD directions of less than 4% was rated as particularly good, a heat shrinkage rate of 10% or less was rated as good, and a heat shrinkage rate of more than 10% was rated as unsatisfactory.

[0057] <Electrical resistance> The electrical resistance of the laminated porous membrane was measured by the following method. A CR2032-type coin cell was prepared so that the number of laminated porous membranes was one. Specifically, the cut-out laminated porous membrane was impregnated with an electrolyte (1M-LiPF6 / EC:EMC (4:6 vol%)). This was sealed under reduced pressure in a coin-shaped case to prepare a total of two cells. The cells were placed in a thermostatic bath at 25°C, and the resistance of the cells was measured by an AC impedance method at an amplitude of 20 mV and a frequency of 200 kHz. The average value of the measured resistance values ​​of the cells was taken as the electrical resistance (Ω) of the laminated porous membrane. A resistance of 0.70Ω or less was considered particularly good and was indicated as A, a resistance of more than 0.70Ω but not more than 0.80Ω was considered good and was indicated as B, and a resistance of more than 0.80Ω was considered insufficient and was indicated as C.

[0058] Example 1 As shown in Table 1, 96.3 parts by weight of boehmite (particle diameter D50 = 0.5 μm) inorganic particles were used, and 0.4 parts by weight (active ingredient) of a polyacrylic acid dispersant (Aron (registered trademark), manufactured by Toa Gosei Co., Ltd.) was prepared and added to water. Next, while stirring at 800 rpm using a mixer (Three-One Motor, manufactured by Toki Sangyo Co., Ltd.) equipped with a disperser-type blade, 1.4 parts by weight (active ingredient) of polyacrylamide resin (Polystron 117, manufactured by Arakawa Chemical Industries Co., Ltd.) serving as a binder resin and the entire amount of boehmite were added. Next, the mixture was stirred at 1200 rpm for 1 hour, yielding a mixed solution with a solids content of 60% by weight.

[0059] The resulting mixture was dispersed three times using a bead mill disperser (Picomir PCM-LR, manufactured by Asada Iron Works Co., Ltd.) and zirconia beads with a bead particle size of 0.5 mm (Toray Industries, Inc., Treceram φ0.5 mm) under conditions of a bead filling rate of 70% by volume, a peripheral speed of 10 m / sec, and a flow rate of 15 kg / h to obtain a masterbatch liquid.

[0060] While stirring the obtained masterbatch liquid at 800 rpm with the stirrer, 1.4 parts by weight (active ingredient) of acrylic resin (Polysol, manufactured by Showa Denko K.K.), 0.6 parts by weight (active ingredient) of wetting agent (SN Wet 366, manufactured by San Nopco Ltd.), and water were added. The mixture was then stirred at 800 rpm with the stirrer for 30 minutes to obtain a slurry with a solid content of 55% by weight.

[0061] Acetic acid solvent was added to the obtained slurry while stirring it at 800 rpm with the stirrer, to obtain a slurry for coating a porous layer having a solid content of 50% by weight and a pH adjusted to 3.5.

[0062] The pH-adjusted slurry was applied to one side of a polyolefin microporous membrane (8 μm thick) by reverse gravure coating and dried to prepare a multilayer porous membrane having a porous layer of 2 μm thick. The evaluation results of the prepared multilayer porous membrane are shown in Table 1.

[0063] <Examples 2 to 4, Comparative Examples 1 to 4> Except for changing the pH of the porous layer coating slurry, a laminated porous membrane was produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0064] <Examples 5 to 7, Comparative Examples 5 to 6> Except for changing the pH of the slurry for applying the porous layer and the peripheral speed during dispersion of the bead mill disperser, a laminated porous membrane was produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0065] <Examples 8 to 9, Comparative Examples 7 to 8> Except for changing the pH of the slurry for applying the porous layer, and the type and particle size of the inorganic particles, a laminated porous membrane was produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0066]

Table 1

Claims

1. A laminated porous membrane having a porous layer containing inorganic particles and a binder resin on at least one surface of a polyolefin microporous membrane, wherein the area ratio of the inorganic particles in a cross section of the porous layer is 55% or more and 70% or less, and the void area deviation is 0.05 or less.

2. The laminated porous membrane according to claim 1 , wherein the binder resin comprises a polyacrylamide resin.

3. 3. The laminated porous membrane according to claim 1, wherein the inorganic particles have an average circularity of 0.50 or more and 0.70 or less, and an average area envelopment ratio of 0.85 or more and 0.95 or less.

4. 3. The laminated porous membrane according to claim 1, wherein the inorganic particles contain at least one kind selected from the group consisting of alumina, barium sulfate, and boehmite.

5. A slurry for coating a porous layer, comprising inorganic particles and a binder resin, and having a pH of 3.5 or more and 5.5 or less.

6. A method for producing a laminated porous membrane having a porous layer containing inorganic particles and a binder resin on at least one surface of a polyolefin microporous membrane, the method comprising the steps of applying a slurry containing inorganic particles and a binder resin to at least one surface of the polyolefin microporous membrane and then drying the applied slurry, wherein the pH of the slurry is 3.5 or more and 5.5 or less.

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