Separator for power storage device and power storage device using same

Abstract

Provided are: a separator for an electric power storage device, the separator having high liquid absorption and high strength; and an electric power storage device in which the separator is used. The separator for an electric power storage device includes a microporous membrane (A) containing an inorganic filler and a polyolefin resin. The microporous membrane (A) has a MFR of 0.05-5 inclusive, includes not less than 20 mass% but less than 100 mass% of the organic filler, has an average pore diameter of 100-1500 nm inclusive for the pores in a cross-section ND-MD, and has an air permeability of 340 sec / 100 mL or less.

Description

[Technical Field]

[0001] The present disclosure relates to a separator for an electricity storage device and an electricity storage device using the same. [Background technology]

[0002] It is generally known that the use of a nonwoven fabric such as cellulose as a microporous membrane for a separator for an electricity storage device (hereinafter simply referred to as "separator") can provide a microporous membrane with high electrolyte absorption. However, the large fiber diameter of nonwoven fabric makes it difficult to form a thin membrane.

[0003] A separator using a microporous membrane containing an inorganic filler inside is known as a method for achieving a thin separator with high liquid absorption. For example, Patent Document 1 describes a separator for a nonaqueous electrolyte secondary battery composed of a polymer porous membrane containing an inorganic filler. However, a microporous membrane containing an inorganic filler inside is prone to poor pore formation, such as the formation of isolated pores at the pore origin.

[0004] A method of biaxially stretching during the production of a microporous membrane is known to form continuous pores to achieve high liquid absorption, but biaxial stretching tends to increase the amount of thermal shrinkage, making it difficult to achieve high TD dimensional precision.

[0005] Microporous membranes are also known in which a layer containing no inorganic filler is provided as the outermost layer, but if the perforation method differs from that of the inner layer, it is difficult to find optimal perforation conditions, such as stretching. Furthermore, a method is known in which a coating material containing an inorganic filler and a resin binder is laminated on the surface of a microporous membrane. For example, Patent Document 2 describes a laminated porous film in which a coating layer containing a filler and a resin binder is laminated on at least one side of a polyolefin resin porous film. However, with this method, there is a concern that the binder component contained in the coating material may clog the pores of the microporous membrane. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-71979 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-20437 [Non-patent literature]

[0007] [Non-Patent Document 1] PS. Liao, TS. Chen, and PC. Chung, “A Fast Algorithm for Multilevel Thresholding”, Journal of Information Science and Engineering, vol. 17, 2001, pp.713-727 [Non-patent document 2] Michael Doube, Michal M Klosowski, Ignacio Arganda-Carreras, Fabrice P Cordelieres, Robert P Dougherty, Jonathan S Jackson, Benjamin Schmid, John R Hutchinson, and Sandra J Shefelbinea, “BoneJ: Free and extensible bone image analysis in ImageJ” Bone Volume 47, Issue 6, December 2010, pp.1076-1079 Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present disclosure is to provide a separator for an electricity storage device that has high liquid absorption and high strength, and an electricity storage device using the same. [Means for solving the problem]

[0009] [1] A separator for an electricity storage device, comprising a microporous membrane (A) containing an inorganic filler and a polyolefin resin, The microporous membrane (A) has an MFR of 0.05 or more and 5 or less, The microporous membrane (A) contains the inorganic filler in an amount of 20% by mass or more and less than 100% by mass, and the average pore size of the pores in the ND-MD cross section is 100 nm or more and 1500 nm or less, A separator for an electricity storage device having an air permeability of 340 seconds / 100 ml or less. [2] 2. The separator for an electricity storage device according to item 1, wherein the microporous membrane (A) contains 30% by mass or more and 90% by mass or less of the inorganic filler. [3] 3. The separator for an electricity storage device according to item 1 or 2, wherein the microporous membrane (A) contains 60% by mass or more and 90% by mass or less of the inorganic filler. [4] 4. The separator for an electricity storage device according to any one of items 1 to 3, wherein the microporous membrane (A) has an average pore size of 200 nm or more and 840 nm or less in the ND-MD cross section. [5] 5. The separator for an electricity storage device according to any one of items 1 to 4, having an air permeability of 50 seconds / 100 ml or more and 250 seconds / 100 ml or less. [6] 6. The separator for an electricity storage device according to any one of items 1 to 5, wherein the inorganic filler has a particle size of 60 nm or more and 2000 nm or less. [7] 7. The separator for an electricity storage device according to any one of items 1 to 6, having a tortuosity of 1.2 or more and 3.0 or less. [8] 8. The separator for a power storage device according to any one of items 1 to 7, wherein the separator for a power storage device has a TD heat shrinkage rate at 130° C. of 4% or less. [9] MD tensile strength is 500kg / cm 2 More than 2000kg / cm 2 9. The separator for an electricity storage device according to any one of items 1 to 8, wherein:

[10] 10. The separator for an electricity storage device according to any one of items 1 to 9, wherein the ratio of tensile strength in MD to tensile strength in TD is 1.5 or more and 30 or less.

[11] 11. The separator for an electricity storage device according to any one of items 1 to 10, wherein the thickness of the microporous membrane (A) is 1 μm or more and 27 μm or less.

[12] 12. The separator for an electricity storage device according to any one of items 1 to 11, having a total thickness of 5 μm or more and 30 μm or less.

[13] 13. The separator for an electricity storage device according to any one of items 1 to 12, having a puncture strength per 14 μm of thickness of 50 gf / 14 μm or more and 550 gf / 14 μm or less.

[14] 14. The separator for an electricity storage device according to any one of items 1 to 13, wherein the inorganic filler has a Mohs hardness of 5 or less.

[15] 15. The separator for an electricity storage device according to any one of items 1 to 14, comprising microporous membranes (B) containing a polyolefin resin as a main component, disposed on both sides of the microporous membrane (A). [Effects of the Invention]

[0010] The present disclosure can provide a separator for an electricity storage device having high liquid absorption and high strength, and an electricity storage device using the same. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is an example of a luminance histogram (horizontal axis: luminance, vertical axis: frequency) of an image processing area. DETAILED DESCRIPTION OF THE INVENTION

[0012] <Separator for power storage device> The separator for an electricity storage device of the present disclosure (hereinafter also simply referred to as "separator") has an inorganic filler-containing layer (microporous membrane (A)) containing an inorganic filler and a polyolefin. The MFR of the microporous membrane (A) is 0.05 or more and less than 5. The microporous membrane (A) contains 20% by mass or more and less than 100% by mass of an inorganic filler, has an average pore size of 100 nm or more and 1500 nm or less in the ND-MD cross section, and has an air permeability of 340 sec / 100 ml or less. Each of the characteristics will be described in detail below.

[0013] <Microporous membrane (A)> In the present disclosure, the inorganic filler-containing layer containing an inorganic filler and a polyolefin is also referred to as a "microporous membrane (A)." The microporous membrane (A) contains an inorganic filler and a polyolefin resin. The inclusion of an inorganic filler improves the wettability of the separator to the electrolyte solution, thereby improving the electrolyte solution absorption. Furthermore, the inclusion of an inorganic filler facilitates the formation of large pores, with the interface between the inorganic filler and the polyolefin resin acting as the pore opening origin during lamella perforation, and facilitates the formation of a pore interconnection structure, thereby enabling a microporous membrane with high liquid absorption. Melt extrusion at a high drawdown ratio in the MD during membrane formation provides lamellae with high crystal orientation, which facilitates the formation of interconnected pores in the thickness direction during perforation. Furthermore, the lamellae with high crystal orientation facilitate the formation of interconnected pores, enabling a microporous membrane with high liquid absorption.

[0014] The content of the inorganic filler in the microporous membrane (A) is from 20 to less than 100% by mass, preferably from 20 to 90% by mass, from 30 to 90% by mass, more preferably from 25 to 85% by mass, even more preferably from 30 to 80% by mass, still more preferably from 35 to 75% by mass, and particularly preferably from 40 to 70% by mass, based on the total mass of the microporous membrane (A). The content of the inorganic filler may be preferably from 40 to 90% by mass, more preferably from 50 to 90% by mass, and even more preferably from 60 to 90% by mass. When the inorganic filler content in the microporous membrane (A) is 20% by mass or more, it is believed, without being limited by theory, that a balanced lamellar pore opening and filler pore opening occur, and the vertical pores formed by the lamellar crystals connect the horizontally independent pores formed by the filler pore openings, thereby achieving high permeability and high liquid absorption. There is no upper limit to the inorganic filler content, but for example, if it is 90% by mass or less, a layer containing inorganic filler and lamellar crystals can be formed by melt extrusion at a high drawdown ratio, and then uniaxial stretching can produce a separator with high permeability. Furthermore, there is a low risk of membrane rupture, and thickness uniformity during membrane formation tends to be improved. Furthermore, if it is 70% by mass or less, the inorganic filler forms a continuous resin network structure, maintaining high MD strength and facilitating the production of a wound body in which electrodes and separators are wound.

[0015] Examples of inorganic fillers include oxide ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, magnesium sulfate, aluminum sulfate, barium sulfate, aluminum hydroxide, aluminum oxide hydroxide, potassium titanate, talc, kaolinite, dickite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fibers. From the viewpoint of imparting safety and heat resistance within the battery, at least one selected from the group consisting of barium sulfate, titania, alumina, and boehmite is preferred.

[0016] The particle diameter of the inorganic filler is preferably 60 nm to 2000 nm, preferably 100 nm to 1800 nm, more preferably 200 nm to 1500 nm, and even more preferably 350 nm to 1000 nm. From the viewpoint of film formation stability during extrusion, a particle diameter of 60 nm or greater facilitates film formation without excessive increases in extrusion pressure. Furthermore, a particle diameter of 60 nm or greater allows the inorganic filler to be more dispersed, making it less likely that a significantly low inorganic filler concentration will occur in the separator, and the filler will become a resistive component during high-temperature melting, making it easier to maintain interelectrode resistance. Furthermore, the inorganic filler is less likely to wear the piping and reduce the amount of metallic foreign matter contained in the separator. An inorganic filler diameter of 2000 nm or less reduces the risk of film rupture when the lamellae open, is less likely to become a starting point for stress concentration relative to the layer thickness, and improves strength. Furthermore, the separator thickness is less likely to vary, improving quality. A particle diameter of 2000 nm or less provides a large inorganic filler surface area, making it easier to achieve high liquid absorption.

[0017] The Mohs hardness of the inorganic filler is preferably 5 or less. When the inorganic filler comes into contact with the piping flow path of the film formation equipment, abrasion with the metal surface is suppressed. For example, abrasion of the SUS material used for the piping is reduced, and the risk of contamination inside the separator can be reduced. When the separator is incorporated into a battery, the contaminated metal foreign matter will leach out during charge / discharge cycles, forming dendrites, particularly on the negative electrode side, which can cause a short circuit in the battery, which is undesirable.

[0018] The inorganic filler can be surface-treated with a surface treatment agent to improve dispersibility in the polyolefin resin. Examples of such surface treatment agents include treatment with saturated fatty acids and / or their salts (saturated fatty acid salts), unsaturated fatty acids and / or their salts (unsaturated fatty acid salts), polysiloxanes, and silane coupling agents. From the perspective of dispersibility in the polyolefin resin, treatment agents include saturated fatty acids and their salts, unsaturated fatty acids and their salts, saturated fatty acids having 8 or more carbon atoms and their salts, and unsaturated fatty acids having 8 or more carbon atoms and their salts. The inorganic filler is highly dispersed, and a large amount of the inorganic filler surface is exposed inside the separator, improving liquid absorption.

[0019] The surface hydrophilicity of the inorganic filler is preferably 0.1 or more and 0.8 or less. When the surface hydrophilicity is 0.8 or less, the inorganic filler is easily dispersed and aggregation can be suppressed. When the surface hydrophilicity is 0.1 or more, affinity to the electrolyte solution increases, and ionic conductivity tends to improve.

[0020] The amount of surface treatment of the inorganic filler depends on the particle size of the inorganic filler, but is preferably 0.1% by mass or more and 10% by mass or less based on the total mass of the surface-treated inorganic filler. When the amount of surface treatment is 10% by mass or less, excess surface treatment agent can be reduced, and when the amount of surface treatment is 0.1% by mass or more, good dispersibility can be obtained.

[0021] The inorganic filler weight is 0.15 mg / cm 2 The weight per unit area is preferably 0.15 mg / cm or more. 2In this case, the inorganic filler is less likely to migrate into the inter-electrode gap together with the resin when the separator is melted at high temperature, and the inorganic filler remains as an inorganic filler layer that serves as a resistance component between the electrodes, which tends to form a dense inorganic filler layer.The inorganic filler then serves as a resistance component, making it possible to maintain the inter-electrode resistance, and the safety of the battery tends to improve.

[0022] The microporous membrane (A) contains a polyolefin resin. The polyolefin resin is a polymer containing a monomer having a carbon-carbon double bond as a repeating unit. Monomers constituting the polyolefin resin are not limited, but include monomers having 3 to 10 carbon atoms and a carbon-carbon double bond, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. The polyolefin may be a homopolymer, copolymer, or multi-stage polymer, and is preferably a homopolymer.

[0023] The content of the polyolefin resin in the microporous membrane (A) is preferably 10 to 80% by mass, more preferably 20 to 70% by mass, even more preferably 20 to 60% by mass, still more preferably 20 to 50% by mass, and particularly preferably 20 to 40% by mass. When the amount of the polyolefin resin based on the total mass of the microporous membrane (A) is within this range, a separator having high permeability and high strength is easily obtained.

[0024] The polyolefin resin is preferably at least one selected from the group consisting of polyethylene, polypropylene, and a copolymer of polyethylene and polypropylene from the viewpoint of shutdown characteristics, etc. Examples of polyethylene include low-density polyethylene (LDPE), medium-density polyethylene (MDPE), and high-density polyethylene, and high-density polyethylene (HDPE) is more preferred from the viewpoint of easily obtaining large pore sizes and further enhancing permeability and liquid absorption.

[0025] The MFR of the polyolefin resin contained in the microporous membrane (A) is preferably 0.2 or more and 15 or less when measured under a load of 2.16 kg at 190°C if the main component is polyethylene, or at 230°C if the main component is other polyethylene, particularly polypropylene. It is preferably 0.25 or more, 0.30 or more, and preferably 5.00 or less, 1.00 or less. The reason for this, without being limited by theory, is thought to be that kneading a thermoplastic resin with a high MFR polyolefin resin with an inorganic filler results in uniform kneading, dispersing the inorganic filler to form a uniform separator, and when the separator is incorporated into a battery, a uniform insulating layer of the inorganic filler can be formed when the separator is melted at high temperature, thereby enhancing the effect of maintaining resistance between electrodes. On the other hand, if the MFR of the polyolefin resin is too low, the inorganic filler will not disperse, resulting in areas of extremely low inorganic filler concentration within the separator. When the separator is incorporated into a battery, the insulating layer formed by the inorganic filler during high-temperature melting will be locally insufficient, resulting in an uneven inorganic filler layer and the tendency to be unable to maintain inter-electrode resistance. Furthermore, from the perspective of film formation stability during extrusion, if the MFR of the polyolefin resin is 0.2 or higher, the risk of film rupture at high drawdown ratios tends to increase due to the crystal orientation during melt extrusion. If the MFR of the polyolefin resin is 15 or lower, the crystal orientation is facilitated, suppressing the decrease in orientation degree, facilitating lamellar opening, and improving permeability.

[0026] The MFR of the microporous membrane (A) is 0.05 or more and 5 or less. The MFR of the microporous membrane (A) is measured under a load of 2.16 kg and at a temperature of 230°C. The MFR of the microporous membrane (A) is almost the same as the MFR of the raw material composition forming the microporous membrane (A). When the MFR of the microporous membrane (A) is 0.05 or more, the pressure during extrusion does not increase excessively, facilitating membrane formation, thereby improving membrane formation stability during extrusion. Furthermore, when the MFR of the microporous membrane (A) is 0.05 or more, the microporous membrane (A) without the microporous membrane (B) laminated thereon flows through piping during melt extrusion, reducing wear on the inorganic filler and the surface of the equipment piping, and reducing the content of metal components in the separator, thereby improving the short-circuit resistance of the battery. When the MFR of the microporous membrane (A) is 5 or less, the decrease in the degree of orientation can be suppressed, lamellae can be easily opened, and liquid absorption can be improved. The MFR is preferably 0.1 or more and 3 or less, and more preferably 0.2 or more and 2 or less.

[0027] The microporous membrane (A) has pores, the outline of which is formed by the polyolefin resin alone or by the inorganic filler and the polyolefin resin. The pores in the microporous membrane (A) preferably include pores formed by the inorganic filler and the polyolefin resin. The average pore size (average pore diameter) of the pores in the ND-MD cross section may be 100 nm to 2000 nm, preferably 150 nm to 1500 nm, more preferably 200 nm to 840 nm, even more preferably 250 nm to 800 nm, and even more preferably 300 nm to 750 nm. When the average pore diameter is 100 nm or more, the pores tend to be connected to each other, resulting in high permeability. Furthermore, when the average pore diameter is 1500 nm or less, the thickness of the microporous membrane (A) is less likely to become a starting point for stress concentration, and therefore the strength of the separator is less likely to decrease. Furthermore, an average pore diameter of 1500 nm or less increases the withstand voltage and reduces the likelihood of short-circuiting in the battery, improving the yield during battery production. The average pore diameter of the microporous membrane (A) can be measured by observing the ND-MD cross section of the microporous membrane (A) with a scanning electron microscope (SEM), as described later in the Examples section.

[0028] The pore diameters of the microporous membrane (A) are preferably aligned in one direction. In this specification, the term "pore diameter" refers to the longest line segment connecting any two points on the contour of an aperture. Without being limited by theory, unidirectional alignment of the pore diameters is preferred because it strongly orients the crystals of the resin constituting the separator, increasing its strength in the pore diameter direction and facilitating the production of a wound body in which electrodes and a separator are wound together when fabricating a cylindrical battery. The alignment of the pore diameters can be confirmed by observing the cross section of the microporous membrane (A) with a scanning electron microscope (SEM), as described later in the Examples section. "Aligned in one direction" means that, in the electron microscope image of the separator surface, 90% or more of the fibrils fall within an angular range of ±20 degrees relative to their extension direction. That is, the pore diameters are determined to be "aligned in one direction" when, in the electron microscope image, the major axes of 90% or more of the pores fall within an angular range of ±20 degrees relative to each other.

[0029] The area ratio of open pores to the cross-sectional area, which is an index of the porosity of the microporous membrane (A), is 20% or more and 60% or less. It is preferably 30% or more, more preferably 35% or more, preferably 55% or less, and more preferably 50% or less. By setting the area ratio of open pores to 60% or less, the proportion of resin and inorganic filler can be increased, and a network structure of solids is formed. This reduces stress concentration during piercing, improves breaking elongation, and makes it easier to obtain high pierce strength. Furthermore, when attempting to increase the MD stretch ratio to increase the open pore ratio, the pores tend to be crushed in the thickness direction, disrupting the interconnected structure between pores and reducing air permeability. When the area ratio of open pores is 20% or more, the low proportion of resin and inorganic filler makes it easier to form an interconnected structure between pores, resulting in high permeability.

[0030] The microporous membrane (A) may further contain an elastomer in addition to the polyolefin resin and inorganic filler. Examples of elastomers include thermoplastic elastomers and thermosetting elastomers, and preferably thermoplastic elastomers. In this specification, thermoplastic elastomers are included in the term "thermoplastic resin." When the microporous membrane (A) contains a thermoplastic elastomer, the melt tension can be reduced without compromising the balance between strength and air permeability. As a result, when a high MFR thermoplastic resin that is expected to have high strength is used, the inclusion of the thermoplastic resin makes it possible to reduce the thickness even when the thermoplastic resin has a high melt tension, and a thin, high-strength separator can be obtained.

[0031] The thickness of the microporous membrane (A) is preferably 1 μm or more and 27 μm or less, more preferably 1 μm or more and 20 μm or less. A thickness of 1 μm or more improves the heat resistance of the separator for an electricity storage device. A thickness of 27 μm or less can further increase the energy density of the electricity storage device. In consideration of heat resistance and energy density, the thickness is preferably 3 μm or more and 15 μm or less, more preferably 5 μm or more and 10 μm or less. In consideration of the heat resistance, ion permeability, and physical strength of the separator, the ratio of the thickness of the microporous membrane (A) to the total thickness of the separator is preferably 15% or more and 90% or less, more preferably 20% or more and 80% or less, and even more preferably 20% or more and 60% or less.

[0032] <Microporous membrane (B)> The separator for an electricity storage device of the present disclosure may include, in addition to the microporous membrane (A), a microporous membrane (B) composed primarily of a polyolefin resin, disposed on one or both sides of the microporous membrane (A). In this specification, "major component" refers to a component contained in an amount of more than 50% by mass, and may be 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, 99% by mass or more, or 100% by mass. It is preferable that the microporous membrane (B) does not contain an inorganic filler.

[0033] The microporous membrane (B) is preferably a microporous layer primarily composed of a polyolefin resin. Having a polyolefin outer layer improves film-forming properties, allows melt extrusion at a high MD drawdown ratio during film formation, and provides high crystal orientation, making it easier to achieve high permeability. A three-layer structure of polyolefin microporous layer / inorganic-containing layer / polyolefin microporous layer provides high liquid absorption due to the inorganic-containing layer while maintaining mechanical strength due to the polyolefin microporous layer and oxidation resistance when in contact with an electrode surface. From the viewpoint of abrasion resistance, the polyolefin microporous layer is preferably the outermost layer, and a five-layer or nine-layer structure of polyolefin microporous layer / inorganic-containing layer / polyolefin microporous layer / inorganic-containing layer / polyolefin microporous layer / inorganic-containing layer / polyolefin microporous layer may also be used. From the viewpoint of thin film properties, a three-layer structure is preferred.

[0034] As the polyolefin resin for the microporous film (B), polyethylene and polypropylene are preferred because they are easy to open lamellae, and polypropylene is preferred from the viewpoint of puncture resistance (hereinafter also referred to as "PP microporous layer"). The thickness of the microporous film (B) is preferably 10% to 90% of the total thickness of the separator. When it is 10% or more, the proportion of layers having a network structure in which resins are continuously connected increases, resulting in high strength. When it is 90% or less, the necessary thickness of the inorganic layer suitable for high liquid absorption can be ensured, thereby achieving high liquid absorption.

[0035] The stereoregularity of the polypropylene of the microporous layer (B) is not limited, but examples thereof include atactic, isotactic, or syndiotactic homopolymers, etc. The polypropylene according to the present disclosure is preferably an isotactic or syndiotactic highly crystalline homopolymer.

[0036] The polypropylene of the microporous layer (B) is preferably a homopolymer, but may also be a copolymer, such as a block polymer, in which a small amount of a comonomer other than propylene, such as an α-olefin comonomer, is copolymerized. The amount of propylene structures contained as repeating units in the polypropylene is not limited, but may be, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The amount of repeating units derived from comonomers other than the propylene structure contained in the polypropylene is not limited, but may be, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. The polypropylene can be used alone or in combination of two or more types.

[0037] The MFR of the polypropylene of the microporous layer (B) is preferably 8.0 g / 10 min or less, and may be, for example, 6.0 g / 10 min or less, 4.0 g / 10 min or less, 3.0 g / 10 min or less, 2.0 g / 10 min or less, or 1.1 g / 10 min or less, from the viewpoint of obtaining a microporous layer (B) with higher strength. The lower limit of the MFR of the microporous layer (B) is not limited, from the viewpoint of achieving good film-forming properties and film thickness uniformity, and may be, for example, 0.3 g / 10 min or more, 0.35 g / 10 min or more, 0.4 g / 10 min or more, 0.45 g / 10 min or more, or 0.5 g / 10 min or more.

[0038] The thickness of the microporous membrane (B) is preferably 1 μm or more and 10 μm or less, more preferably 3 μm or more and 6 μm or less. A thickness of 1 μm or more allows the microporous membrane (A) to be covered without layer defects (missing parts), thereby reducing wear of the SUS material used in the piping and reducing the risk of substances from the piping getting into the separator. A thickness of 10 μm or less makes it easier to achieve high liquid absorption due to the microporous layer (A) containing an inorganic filler.

[0039] <Separator> The separator for an electricity storage device may be a single-layer separator consisting of only the microporous membrane (A), or a two-layer separator in which the microporous membrane (B) is laminated on the microporous membrane (A). The separator for an electricity storage device may be a three-layer separator having a microporous membrane (B) / microporous membrane (A) / microporous membrane (B) structure in which the microporous membrane (A) is the inner layer and the microporous membrane (B) is the outer layer. The separator for an electricity storage device may have a three-layer or more structure. The three-layer structure of a PO microporous layer / inorganic-containing layer / PO microporous layer is formed by disposing a microporous membrane (B) containing a polyolefin resin as the main component (hereinafter also referred to as a "PO microporous layer") on both sides of a microporous membrane (A) containing an inorganic filler (hereinafter also referred to as an "inorganic-containing layer"), thereby reducing the chance of the inorganic filler coming into contact with the piping flow paths of the film formation equipment and suppressing wear of the metal surface. In particular, contact of the inorganic filler with the lip portion, which has a small cross-sectional area during extrusion, a high flow rate, and is considered to be at the highest risk of wear due to corrosion when the trace additive comes into contact with air, can be avoided. In this way, the contact opportunity between the inorganic filler and the surface of the equipment piping can be reduced or eliminated, and the possibility of metal components such as Fe being contained in the separator can be reduced, thereby improving the short-circuit resistance of the battery.

[0040] The TD heat shrinkage rate of the separator for an electricity storage device at 130°C is 4% or less, preferably 3% or less, and more preferably 1% or less. When the TD heat shrinkage rate at 130°C is 4% or less, the degree of crystal orientation increases, making lamellar pores more likely to open, and vertical cracks are more likely to occur during pore opening. As a result, the pores expand during subsequent thermal stretching, resulting in a highly straight pore structure and tending to improve liquid absorption. Furthermore, when the TD heat shrinkage rate is 4% or less, an insulating film can be formed that maintains the separator area, maintaining the insulation between electrodes and increasing resistance to short circuits due to TD heat shrinkage in a laminate cell short circuit test. The lower limit of the heat shrinkage rate is not particularly limited, but is, for example, -4% or more, preferably -3% or more, more preferably -2% or more, and even more preferably -1% or more. When the TD heat shrinkage rate is -4% or more, the separator is less likely to wrinkle when the battery temperature rises, and uniform stress is applied to the separator, making it less likely to tear. The reason why the TD thermal shrinkage is a negative value is that the components contained inside the separator may expand due to heating.

[0041] The air permeability (also referred to as "air permeability resistance") of the separator for an electricity storage device may be 400 sec / 100 ml or less, preferably 340 sec / 100 ml or less, more preferably 50 sec / 100 ml or more and 340 sec / 100 ml or less, even more preferably 50 sec / 100 ml or more and 300 sec / 100 ml or less, still more preferably 50 sec / 100 ml or more and 250 sec / 100 ml or less, particularly preferably 100 sec / 100 ml or more and 230 sec / 100 ml or less, and particularly preferably 120 sec / 100 ml or more and 210 sec / 100 ml or less. The air permeability may be preferably 100 sec / 100 ml or more and 340 sec / 100 ml or less, more preferably 120 sec / 100 ml or more and 340 sec / 100 ml or less. An air permeability of 50 sec / 100 ml or more provides a network structure of continuous pores, thereby providing a separator with high strength. If the absorption rate is 300 seconds / 100 ml or less, a structure in which the pores are interconnected is obtained, resulting in high liquid absorption.

[0042] The total thickness of the separator for an electricity storage device is preferably 5 μm or more and 30 μm or less, more preferably 5 μm or more and 27 μm or less. When the thickness is 5 μm or more, the heat resistance of the separator for an electricity storage device is improved. When the thickness is 30 μm or less, the energy density of the electricity storage device can be increased.

[0043] The lower limit of the puncture strength of the separator for an electricity storage device may be 50 gf / 14 μm or more, and preferably 100 gf / 14 μm or more, for example, 130 gf / 14 μm or more, or 160 gf / 14 μm or more, when the thickness of the separator is converted to 14 μm. The upper limit of the puncture strength of the multilayer structure is not limited, but may be preferably 550 gf / 14 μm or less, for example, 500 gf / 14 μm or less, or 480 gf / 14 μm or less, when the thickness of the entire multilayer structure is converted to 14 μm.

[0044] The separator for an electricity storage device preferably has a MD tensile strength of 500 kg / cm 2 More than 2000kg / cm 2 Less than 800 kg / cm, more preferably 2 More than 1800kg / cm 2 More preferably, 1000 kg / cm or less 2 More than 1600kg / cm 2 MD tensile strength is 500 kg / cm or less. 2 If the thickness is more than this, the separator is strongly oriented, increasing the MD strength, and it becomes easier to manufacture a wound body in which the electrode and the separator are wound. 2 It is preferable that the wavelength is 2000 cm or less. 2 If it is less than this, cracks are less likely to occur in the MD, and the separator is less likely to tear vertically (in the MD) during handling.

[0045] The ratio of the MD tensile strength to the TD tensile strength of the separator for an electricity storage device (also referred to as the "MD / TD strength ratio") is preferably 1.5 or more and 30 or less, more preferably 6.0 or more and 20 or less, and even more preferably 8.0 or more and 15 or less. When the MD / TD strength ratio is 1.5 or more, the crystals of the resin component constituting the separator are strongly oriented, thereby increasing the MD strength. This makes it easier to produce a wound body in which electrodes and a separator are wound. Furthermore, when the MD / TD strength ratio is 1.5 or more, the TD thermal shrinkage rate can be relatively reduced, thereby increasing resistance to short circuits due to TD thermal shrinkage during battery winding. The upper limit of the MD / TD strength ratio is not particularly limited, but is preferably 30 or less, more preferably 20 or less, and more preferably 15 or less. When the MD / TD strength ratio is 30 or less, cracks are less likely to occur in the MD, and the separator is less likely to tear longitudinally (in the MD) during unwinding, winding, and other web handling of the separator in the battery production process.

[0046] The tortuosity of the separator for an electricity storage device is preferably 1.2 to 3.0, more preferably 1.4 to 2.5, and even more preferably 1.5 to 2.0. The liquid absorption of a microporous membrane is thought to be mainly determined by the wettability of the inorganic filler surface to the electrolyte and the tortuosity. A tortuosity of 3.0 or less shortens the path that the electrolyte takes from one surface of the microporous membrane to the opposite surface, resulting in high liquid absorption. A tortuosity of 1.2 or more can maintain high withstand voltage and high pin puncture strength, improving battery safety.

[0047] The water content of the separator for an electricity storage device is preferably 1000 ppm or less. When the water content of the separator for an electricity storage device is 1000 ppm or less, adverse effects on the battery characteristics are reduced.

[0048] <<Method for manufacturing separator for electricity storage device>> A method for producing a microporous membrane (A) containing an inorganic filler and a polyolefin resin generally includes a melt-extrusion step in which a polyolefin resin and an inorganic filler are mixed and dispersed, and melt-extruded to obtain a resin film, and a pore-forming step in which the obtained resin sheet is perforated to make it porous, and optionally further includes a stretching step, a heat treatment step, etc. The method for producing a microporous membrane (A) is roughly divided into a dry method in which no solvent is used in the pore-forming step, and a wet method in which a solvent is used.

[0049] Dry methods include a method in which a polyolefin resin and an inorganic filler are mixed and dispersed in a dry state, melt-kneaded, and extruded, and then heat-treated and stretched to peel off the polyolefin resin crystal interface; and a method in which a polyolefin resin and an inorganic filler are melt-kneaded and molded into a sheet, and then stretched to peel off the interface between the polyolefin resin and the inorganic filler.

[0050] Examples of the wet method include a method in which a polyolefin resin composition and an inorganic filler are mixed and dispersed, a liquid pore-forming material is added, and the mixture is melt-kneaded to form a sheet, which is stretched as necessary, and then the pore-forming material is extracted; and a method in which a polyolefin resin composition is dissolved in an organic solvent, and then the mixture is immersed in a poor solvent that has low solubility in the polyolefin resin, thereby solidifying the polyolefin resin and simultaneously removing the organic solvent.

[0051] A single-screw extruder or a twin-screw extruder can be used for melt-kneading the polyolefin resin composition, and other devices such as a kneader, a Laboplastomill, a kneading roll, and a Banbury mixer can also be used.

[0052] The polyolefin resin composition may optionally contain resins other than polyolefin resins, additives, etc., depending on the production method for the microporous membrane (A) or the physical properties of the target microporous membrane (A). Examples of additives include pore-forming materials, fluorine-based flow modifiers, waxes, crystal nucleating materials, antioxidants, metal soaps such as metal salts of aliphatic carboxylic acids, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments. Examples of pore-forming materials include plasticizers, inorganic fillers, and combinations thereof.

[0053] Examples of the plasticizer include hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; and higher alcohols such as oleyl alcohol and stearyl alcohol.

[0054] A stretching step may be performed during the hole formation step, or before or after the hole formation step. As the stretching process, either uniaxial stretching or biaxial stretching can be used. Although not limited thereto, uniaxial stretching is preferred from the viewpoint of production costs when using a dry method. By using the lamellar hole-opening method by uniaxial stretching, it is possible to more easily produce the separator for an electricity storage device of the present disclosure having high liquid absorption and high strength (more specifically, for example, high pin puncture strength), as described below. This will be explained below. This hole-opening method generally involves obtaining a precursor (raw film) in which lamellar crystals are oriented by melt extrusion, cold stretching this, and then hot stretching it to open pores between the lamellar crystals. The present inventors conducted extensive research to achieve both high liquid absorption and high strength using the lamellar perforation method, and found that, preferably, by lowering the extruder temperature of the microporous membrane (A) containing an inorganic filler to form the inner layer and increasing the air flow rate immediately after extrusion to efficiently cool the membrane, the degree of crystal orientation can be increased, despite the generally high heat capacity of the inorganic filler. This allows the microporous membrane (A) to have a highly open pore structure and high liquid absorption. Furthermore, by perforating a highly oriented raw film at a low stretch ratio, high resin density can be maintained while achieving high pin puncture strength. This allows even a single-layer separator made of the microporous membrane (A) to have high liquid absorption while maintaining strength.

[0055] To suppress shrinkage of the microporous film, a heat treatment step may be performed after the stretching step or the hole-forming step for the purpose of heat setting. The heat treatment step may include a stretching operation performed in a predetermined temperature atmosphere and at a predetermined stretch ratio for the purpose of adjusting physical properties, and / or a relaxation operation performed in a predetermined temperature atmosphere and at a predetermined relaxation ratio for the purpose of reducing stretching stress. The relaxation operation may be performed after the stretching operation. These heat treatment steps may be performed using a tenter or roll stretching machine.

[0056] The microporous membrane (B) may be laminated on one or both sides of the microporous membrane (A). Examples of methods for laminating the microporous membrane (B) include coextrusion and lamination. In the coextrusion method, the resin compositions for each layer are simultaneously coextruded to form a film, and the resulting multilayer raw film is stretched and perforated to produce a multilayer microporous membrane. In the lamination method, each layer is separately extrusion-formed to obtain a raw film. The resulting raw films are laminated to obtain a multilayer raw film, and the resulting multilayer raw film is stretched and perforated to produce a multilayer microporous membrane. The coextrusion method is preferable because it improves film formation stability and allows for a high inorganic filler content, since the microporous membrane (A) can be supported by layers that do not contain inorganic filler.

[0057] <Energy storage device> The power storage device includes a positive electrode, a negative electrode, and the above-described power storage device separator of the present disclosure. The power storage device separator is laminated between the positive electrode and the negative electrode.

[0058] Examples of the power storage device include, but are not limited to, lithium secondary batteries, lithium ion secondary batteries, sodium secondary batteries, sodium ion secondary batteries, magnesium secondary batteries, magnesium ion secondary batteries, calcium secondary batteries, calcium ion secondary batteries, aluminum secondary batteries, aluminum ion secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries, electric double layer capacitors, lithium ion capacitors, redox flow batteries, lithium-sulfur batteries, lithium-air batteries, and zinc-air batteries. Among these, from the viewpoint of practicality, lithium secondary batteries, lithium ion secondary batteries, nickel-metal hydride batteries, and lithium ion capacitors are preferred, and lithium ion secondary batteries are more preferred.

[0059] The electricity storage device can be produced, for example, by stacking a positive electrode and a negative electrode with the separator described above interposed therebetween and winding them as necessary to form a stacked electrode body or a wound electrode body, which is then loaded into an outer casing, connecting the positive and negative electrodes to the positive and negative electrode terminals of the outer casing via lead bodies or the like, and further injecting a non-aqueous electrolyte solution containing a non-aqueous solvent such as a chain or cyclic carbonate and an electrolyte such as a lithium salt into the outer casing, and then sealing the outer casing. [Example]

[0060] <<Measurement and Evaluation Methods>> Melt Flow Rate (MFR) The MFR of thermoplastic resins was measured in accordance with JIS K 7210 at a temperature of 230°C and a load of 2.16 kg (unit: g / 10 min). The MFR of polypropylene was measured in accordance with JIS K 7210 at a temperature of 230°C and a load of 2.16 kg. The MFR of polyethylene was measured in accordance with JIS K 7210 at a temperature of 190°C and a load of 2.16 kg. The MFR of elastomers was measured in accordance with JIS K 7210 at a temperature of 230°C and a load of 2.16 kg.

[0061] When the MFR of the microporous membrane (A) was measured after removal from the laminate sheet, it was measured as follows: An adhesive tape was attached to the outer layer of the microporous membrane (B) on the surface of the laminated microporous sheet, and then peeled off to remove the microporous membrane (A), and the MFR was measured in accordance with JIS K 7210 under conditions of a temperature of 230°C and a load of 2.16 kg.

[0062] [Area ratio of inorganic filler, pores and resin] Electron microscopy preparation: A separator and ruthenium tetroxide (Rare Metallic Co., Ltd.) were placed in a sealed container and dyed with steam for 4 hours to produce a ruthenium-dyed separator. 10.6 mL of epoxy resin (Quetol 812, Nissin EM Co., Ltd.), 9.4 mL of curing agent (methyl nadic anhydride (MNA), Nissin EM Co., Ltd.), and 0.34 mL of reaction accelerator (2,4,6-Tris(dimethyl amino methyl)phenol, Nissin EM Co., Ltd., DMP-30) were mixed and thoroughly stirred. The ruthenium-dyed separator was immersed in the mixture and placed under reduced pressure to fully impregnate the pores of the ruthenium-dyed separator. The ruthenium-dyed separator was then embedded in the epoxy resin by curing at 60°C for at least 12 hours. After embedding in epoxy resin, the cross-section was roughly processed with a razor blade, followed by cross-sectional milling using an ion milling machine (E3500 Plus, Hitachi High-Tech Corporation) to prepare a cross-sectional sample. The cross-section was in the ND-MD plane. The cross-sectional sample was fixed to a SEM sample stage for cross-sectional observation using a conductive adhesive (carbon-based). After drying, an osmium coating was applied as a conductive treatment using an osmium coater (HPC-30W, Vacuum Device Co., Ltd.) with the applied voltage adjustment knob set to 4.5 and a discharge time of 0.5 seconds. This prepared the sample for microscopic examination.

[0063] Electron microscope image acquisition: The above-mentioned microscopic sample was observed in an SEM (S-4800, manufactured by Hitachi High-Tech Corporation) under the conditions of an acceleration voltage of 1.0 kV, an emission current of 10 μA, a probe current of High, a detector Upper+LA-BSE100, a pixel resolution of approximately 10 nm / pix, and a working distance of 2.0 mm. The observation field was determined so that the entire thickness direction of the layer containing the inorganic filler was included within the observation field, and the settings were made so that the brightness did not saturate and the contrast was as high as possible, and an electron microscope image was obtained. When the thickness of the layer containing the inorganic filler was larger than the observation field, the observation field was determined so that the layer containing the inorganic filler was included in the entire observation field, and the settings were made so that the brightness did not saturate and the contrast was as high as possible, and an electron microscope image was obtained.

[0064] Image processing (calculation of pore ratio and inorganic filler ratio): In the electron microscope image, a region as large as possible, with a horizontal width of at least 20 μm, was selected as the image processing region, encompassing at least 80% of the thickness of the inorganic filler-containing layer, excluding any areas other than the inorganic filler-containing layer. When the thickness of the inorganic filler-containing layer was larger than the observation field, the entire observation field was used as the image processing region. Image processing was performed using the free software ImageJ. A brightness histogram of the image processing region was displayed, and the difference in brightness between the two maximum values ​​of the two closest peaks among the three peaks (indicated by arrows in Figure 1) was calculated, as shown in Figure 1. This value was used as the CD. A smoothed image was created using a Mean Shift Filter (an open-source plugin created by Kai Uwe Barthel of FHTW-Berlin). The spatial radius, an argument of the Mean Shift Filter, was set to 3, and the color distance was set to the CD value. Multi-Otsu Threshold (an open-source plugin created using the algorithm described in Non-Patent Document 1) was used to determine two non-zero thresholds for the smoothed image. The smaller of the two thresholds was designated as threshold A, and the larger was designated as threshold B. In the smoothed image, pixels with a brightness less than threshold A were deemed to be voids, pixels with a brightness equal to or greater than threshold A but less than threshold B were deemed to be polyolefin, and pixels with a brightness equal to or greater than threshold B were deemed to be inorganic filler, with the number of void pixels relative to the total number of pixels being taken as the void area ratio. Similarly, the polyolefin area ratio and the inorganic filler area ratio were calculated. The void area ratio, polyolefin area ratio, and inorganic filler area ratio were calculated using electron microscope images of five or more fields of view, and the average values ​​were taken as the void ratio, polyolefin ratio, and inorganic filler ratio, respectively.

[0065] Image processing (calculating the average particle size of inorganic fillers): (1) A binary image is created from the smoothed image by setting the brightness of pixels whose brightness is less than threshold B to 0 and the brightness of pixels whose brightness is greater than or equal to threshold B to 255. (2) Using the free software ImageJ, a thickness analysis is performed using BoneJ (an open source plug-in described in Non-Patent Document 2), the average Tb and Th values ​​in the Results window are read, and the value converted into a length in real space taking into account the pixel size of the electron microscope image is used as the average particle diameter of the inorganic filler.

[0066] [Average pore size] Image processing (pore size calculation): The smoothed image was binarized by setting the brightness of pixels with brightness less than threshold A to 0 and the brightness of pixels with brightness equal to or greater than threshold A to 255. Thickness analysis was performed using BoneJ (an open source plug-in described in Non-Patent Document 2), and the Tb. Th mean value in the Results window was read and converted to a length in real space, which was used as the average pore diameter.

[0067] [Thickness (μm)] The thickness (μm) of the separator was measured using a Mitutoyo Digimatic Indicator IDC112 at room temperature of 23±2°C. The thickness of the inorganic-containing layer was measured by drawing a center line between the inorganic-containing layer and the interface or another layer obtained from the SEM image and measuring the length between the lines.

[0068] [Air permeability resistance (sec / 100ml)] The air resistance (seconds / 100 ml) of the separator was measured using a Gurley air permeability meter in accordance with JIS P-8117.

[0069] [Piercing strength] A needle with a hemispherical tip and a radius of 0.5 mm was prepared, and a separator was sandwiched between two plates with openings of 11 mm diameter. The needle, separator, and plates were then set. A puncture test was performed using an Imada MX2-50N needle with a 0.5 mm radius of curvature at the needle tip, an 11 mm diameter opening in the separator holding plate, and a puncture speed of 25 mm / min. The needle was brought into contact with the separator, and the maximum puncture load (i.e., puncture strength (gf)) was measured.

[0070] [MD tensile strength, TD tensile strength, and MD / TD strength ratio] The tensile strength of the separator was measured using a tensile testing machine (Minebea Co., Ltd., TG-1kN type) by setting the sample length before testing to 35 mm and pulling the sample at a speed of 100 mm / min. The tensile strength was determined by dividing the strength (tensile load value) when the sample yielded, or the strength (tensile load value) when it broke (fractured) before yielding, by the cross-sectional area of ​​the test piece. The tensile strength was measured in both the MD and TD of the separator. The MD / TD strength ratio was calculated by dividing the MD tensile strength by the TD tensile strength.

[0071] [MD heat shrinkage and TD heat shrinkage] To determine the thermal shrinkage rate, a separator was cut into a 5cm square, marked at 9 points at 2cm intervals, and wrapped in paper. The marked sample was heat-treated at 130°C for 1 hour, then cooled to room temperature. The MD and TD lengths were measured at 3 points each to determine the shrinkage rate.

[0072] [curvature rate] The average pore size of a separator for an electricity storage device can be measured by the gas-liquid method. Specifically, it is known that the fluid inside a capillary follows Knudsen flow when the mean free path of the fluid is larger than the pore diameter of the capillary, and follows Poiseuille flow when the mean free path is smaller. Therefore, it is assumed that the air flow follows Knudsen flow when measuring the air permeability of a separator, and the water flow follows Poiseuille flow when measuring the water permeability. In this case, the average pore diameter d (μm) and tortuosity τa (dimensionless) of the porous membrane are related to the air permeation rate constant R gas (m 3 / (m 2 ·sec·Pa)), water permeation rate constant R liq (m 3 / (m 2 The following equation was used to calculate the thickness of the film from the air pressure (m / s), the molecular velocity of air (ν), the viscosity of water (η), the standard pressure (Ps) (=101325 Pa), the porosity (ε) (%), and the film thickness (L) (μm). d=2ν×(R liq / R gas )×(16η / 3Ps)×10 6 τa=(d×(ε / 100)×ν / (3L×Ps×R gas )) 1 / 2 where R gas was calculated from the air permeability (sec) using the following formula: R gas =0.0001 / (air permeability × (6.424 × 10 -4 )×(0.01276×101325)) Also, R liq is the permeability (cm 3 / (cm 2 The following formula was used to calculate the pressure (MPa) from the pressure (MPa) (sec·Pa). R liq =water permeability / 100 The water permeability was determined as follows: A separator that had been immersed in ethanol beforehand was set in a stainless steel liquid permeation cell with a diameter of 41 mm, and after washing the ethanol off the separator with water, water was passed through the separator at a differential pressure of approximately 50,000 Pa. The amount of water permeated (cm 3 ) was used to calculate the amount of water permeable per unit time, unit pressure, and unit area, which was taken as the water permeability. ν is the gas constant R (= 8.314 J / (K·mol)), absolute temperature T (K), pi, and the average molecular weight of air M (= 2.896 × 10 -2 kg / mol) using the following formula: ν=((8R×T) / (π×M)) 1 / 2

[0073] [Moisture content] The separator was cut into pieces ranging from 0.15 g to 0.20 g and pretreated (left to stand) for 12 hours in an environment of 23°C and 40% relative humidity. The weight was then measured and used as the sample weight (g). The moisture content (μg) of the pretreated sample was measured using a Karl Fischer apparatus. The heating and vaporization conditions for the measurement were 150°C and 10 minutes. Hydranal Coulomat CG-K (manufactured by SIGMA-ALDRICH) was used as the cathode reagent, and Hydranal Coulomat AK (manufactured by SIGMA-ALDRICH) was used as the anode reagent. Moisture content (ppm) = water weight (μg) / sample weight (g)

[0074] [Method for evaluating drop absorbency (s)] The drop absorbency was measured by cutting a separator into a 5 cm square and placing it on a smooth plate inside an argon box. 5 μl of DMC was measured into a microsyringe and dropped onto the separator surface. The time it took for the droplet to disappear was measured to determine the drop absorbency.

[0075] [Fe (ppm) (wear resistance) evaluation method] Approximately 0.2 g of separator was weighed into a sealed decomposition vessel made of fluororesin, and 5 mL of high-purity nitric acid was added thereto. The mixture was heated at 200 °C for 20 minutes using a microwave decomposition device (Milestone General Co., Ltd., product name "ETHOS TC", model number 125571), and then the volume was adjusted to 50 mL with ultrapure water. Then, measurements were performed using an ICP mass spectrometer (Thermo Fisher Scientific Co., Ltd., product name "X Series X7 ICP-MS", model number X0126).

[0076] Example 1 [Preparation of a composition containing polyethylene resin and inorganic filler] Polyethylene resin (PE, MFR = 0.31) and surface-treated barium sulfate with an average particle size of 500 nm (100 parts barium sulfate per 1 part sodium stearate) were dry-blended at a mass ratio of PE:BaSO4 = 20:80 (mass%) and then melt-kneaded using a twin-screw extruder HK-25D (Parker Corporation, L / D = 41). To minimize resin decomposition and denaturation, the resin inlet hopper and raw material tank were completely sealed, and nitrogen was continuously flowed from the bottom of the hopper to maintain the oxygen concentration near the raw material inlet at 50 ppm or less. All vents were also completely sealed to prevent air leakage into the cylinder. This oxygen concentration reduction significantly suppressed polymer decomposition and denaturation, even at high temperatures. Barium sulfate was further finely dispersed by adding it using a twin-screw feeder. After melt-kneading, a strand was pulled from a die (two holes), cooled in a water-cooled bath, and then cut using a pelletizer to obtain pellets.

[0077] [Preparation of microporous membrane (three layers)] A laminated sheet was formed by coextrusion. Polypropylene resin (PP, MFR = 0.51) was melted in a 32 mm diameter twin-screw co-rotating extruder and fed to a circular die using a gear pump. The barium sulfate-containing pellets were melted in a 32 mm diameter single-screw extruder and fed to a circular die using a gear pump. The melted and mixed compositions from each extruder were extruded into a sheet through a two-component, three-layer circular die. The molten polymer was cooled with blown air and then wound on a roll. The polypropylene resin was extruded from the outer layer (the outer two layers) of the circular die set at 230 °C at a rate of 2.4 kg / hr. The barium sulfate-containing pellets were extruded from the inner layer (the middle layer) of the circular die set at 220 °C at a rate of 1.2 kg / hr (equivalent to polyethylene resin) at a temperature of 230 °C. The extruded precursor (raw film) is then air-rolled immediately after extrusion to form a film of 3.6 m per Φ300 mm width. 3The raw film was cooled with an air flow rate of 1 / min. After cooling, the thickness of the raw film was 16 μm. The raw film was then annealed at 127°C for 15 minutes. The annealed raw film was then cold-stretched to 10% at room temperature, and the cold-stretched film was then hot-stretched to 120% at 115°C, and the hot-stretched film was relaxed to 92% at 125°C to form micropores. After the above stretching and perforation, the physical properties of the resulting microporous membrane were measured. The results are shown in Table 1. Example 9 [Preparation of microporous membrane (single layer)] The barium sulfate-containing pellets were melted in a 32 mmφ single-screw extruder and fed to a circular die using a gear pump. The composition melted and kneaded in the extruder was extruded into a sheet through the circular die, and the molten polymer was cooled by blown air and then wound up on a roll. The kneading temperature of the barium sulfate-containing pellets was 230°C, and the extrusion rate was 1.2 kg / hr in terms of polypropylene resin, extruded from the inner layer (intermediate layer) of the circular die set at 230°C. The extruded precursor (raw film) was then air-circulated immediately after extrusion to form a sheet of 3.6 m per Φ300 mm width. 3 The film was cooled with an air flow rate of 1 / min. The thickness of the raw film after cooling was 16 μm. The raw film was then annealed at 127°C for 15 minutes. The annealed raw film was then cold stretched to 10% at room temperature, and the cold-stretched film was then hot stretched to 110% at 115°C, and the hot-stretched film was relaxed to 92% at 125°C to form micropores. After the above stretching and perforation, the physical properties of the resulting microporous film were measured. The results are shown in Table 1.

[0078] Examples 2 to 8, 10 to 15, and Comparative Examples 1 to 8 The raw materials, membrane-forming conditions, or separator properties were changed as shown in Tables 1 and 2, and a microporous membrane was obtained in the same manner as in Example 1 for a three-layer membrane or in the same manner as in Example 9 for a single-layer membrane, and the resulting microporous membrane was evaluated. The layer configuration was adjusted by changing the ratio of the extrusion rates.

[0079] [Table 1]

[0080] [Table 2] [Industrial Applicability]

[0081] The separator for an electricity storage device according to the present disclosure has high liquid absorption, high strength and abrasion resistance, and can be suitably used as a separator for an electricity storage device such as a lithium ion secondary battery.

Claims

1. A separator for an electricity storage device, comprising a microporous membrane (A) containing an inorganic filler and a polyolefin resin, the microporous membrane (A) has an MFR of 0.05 g / 10 min or more and 5 g / 10 min or less, measured under conditions of a temperature of 230°C and a load of 2.16 kg; The microporous membrane (A) contains the inorganic filler in an amount of 20% by mass or more and less than 100% by mass, and the pores have an average pore size in an ND-MD cross section of 100 nm or more and 1500 nm or less, The air permeability is 340 seconds / 100 ml or less, the tensile strength in the MD is 1000 kg / cm 2 or more and 2000 kg / cm 2 or less; The separator for an electricity storage device, wherein the ratio of the tensile strength in the MD to the tensile strength in the TD is 1.5 or more and 30 or less.

2. The separator for an electricity storage device according to claim 1 , wherein the microporous film (A) contains the inorganic filler in an amount of 30% by mass or more and 90% by mass or less.

3. The separator for an electricity storage device according to claim 2 , wherein the microporous film (A) contains the inorganic filler in an amount of 60% by mass or more and 90% by mass or less.

4. 4. The electricity storage device separator according to claim 3, wherein the microporous membrane (A) has pores with an average diameter of 200 nm or more and 840 nm or less in an ND-MD cross section.

5. The separator for an electricity storage device according to claim 4, which has an air permeability of 50 seconds / 100 ml or more and 250 seconds / 100 ml or less.

6. The separator for an electricity storage device according to claim 5 , wherein the inorganic filler has a particle size of 60 nm or more and 2000 nm or less.

7. The separator for an electricity storage device according to claim 5 , wherein the tortuosity is 1.2 or more and 3.0 or less.

8. 6. The separator for a power storage device according to claim 5, wherein the separator for a power storage device has a thermal shrinkage rate of 4% or less in the TD after being heat-treated at a temperature of 130°C for 1 hour and then cooled to room temperature.

9. The separator for an electricity storage device according to claim 5 , wherein the microporous membrane (A) has a thickness of 1 μm or more and 27 μm or less.

10. The separator for an electricity storage device according to claim 9 , which has a total thickness of 5 μm or more and 30 μm or less.

11. The separator for an electricity storage device according to claim 5 , having a pin puncture strength per 14 μm of thickness of 50 gf / 14 μm or more and 550 gf / 14 μm or less.

12. The separator for an electricity storage device according to claim 5 , wherein the inorganic filler has a Mohs hardness of 5 or less.

13. The separator for an electricity storage device according to claim 5 , further comprising: a microporous membrane (B) mainly composed of a polyolefin resin, the microporous membrane (B) being disposed on both sides of the microporous membrane (A).

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