CN118318346A - Separator for power storage device and power storage device using the same - Google Patents

Abstract

Provided are a separator for a power storage device having high liquid absorption and high strength and a power storage device using the separator. The separator for the power storage device comprises a microporous membrane (A) containing an inorganic filler and a polyolefin resin. The MFR of the microporous membrane (A) is greater than or equal to 0.05 and less than or equal to 5, the microporous membrane (A) comprises more than 20% by mass and less than 100% by mass of the inorganic filler, the average pore size of the pores in the ND-MD cross section is greater than or equal to 100 nm and less than or equal to 1500 nm, and the air permeability is less than or equal to 340 seconds/100 ml.

Description

Separator for power storage device and power storage device using the same

Technical Field

The present disclosure relates to a separator for an electric storage device and an electric storage device using the separator.

Background technique

Generally, a microporous membrane with high electrolyte absorption can be provided by using a nonwoven fabric such as cellulose as a microporous membrane used in a separator for a power storage device (hereinafter also referred to as a "separator"). However, since the fiber diameter of the nonwoven fabric is large, it is difficult to make a thin film.

As a method for realizing a separator with a thin film and high liquid absorption, a separator using a microporous membrane containing an inorganic filler is known. For example, Patent Document 1 records a separator for a non-aqueous 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 opening such as the opening starting point being formed as an independent hole.

From the viewpoint of obtaining high liquid absorption, a method of biaxially stretching a microporous membrane during production to form interconnected pores is known. However, biaxial stretching tends to increase the amount of heat shrinkage, making it difficult to achieve high dimensional accuracy in TD.

In addition, it is also known that the layer without inorganic filler is arranged on the outermost microporous film, but if it is different from the opening mode of the inner layer, it is difficult to find the opening conditions such as the best stretching. And then, it is also known that there is a method for the coating containing inorganic filler and resin binder on the surface of the microporous film. For example, patent documentation 2 records the laminated porous film formed by the coating layer with filler and resin binder on at least one side of the polyolefin resin porous film. However, in this method, the binder component contained in the coating is likely to block the hole of the microporous film.

Prior art literature

Patent Literature

Patent Document 1: Japanese Patent Application Publication No. 2005-71979

Patent Document 2: Japanese Patent Application Publication No. 2012-20437

Non-patent literature

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 Klowski, 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 that the invention aims to solve

An object of the present disclosure is to provide a separator for an electricity storage device having high liquid absorption and high strength, and an electricity storage device using the separator.

Solutions for solving problems

[1]

A separator for an electrical storage device, comprising a microporous membrane (A) containing an inorganic filler and a polyolefin resin,

The MFR of the microporous membrane (A) is 0.05 or more and 5 or less,

The microporous membrane (A) contains 20 mass % or more and less than 100 mass % of the inorganic filler, and the average pore diameter of the pores in the ND-MD cross section is 100 nm or more and 1500 nm or less,

The separator for a power storage device has an air permeability of 340 sec/100 ml or less.

[2]

The separator for a power storage device according to item 1, wherein the microporous film (A) contains 30% by mass or more and 90% by mass or less of the inorganic filler.

[3]

The separator for a power storage device according to item 1 or 2, wherein the microporous film (A) contains 60% by mass or more and 90% by mass or less of the inorganic filler.

[4]

The separator for a power storage device according to any one of items 1 to 3, wherein an average pore diameter of the pores in the ND-MD cross section of the microporous membrane (A) is 200 nm or more and 840 nm or less.

[5]

The separator for a power storage device according to any one of items 1 to 4, wherein the air permeability is 50 sec/100 ml or more and 250 sec/100 ml or less.

[6]

The separator for a power storage device according to any one of items 1 to 5, wherein a particle diameter of the inorganic filler is 60 nm or more and 2000 nm or less.

[7]

The separator for a power storage device according to any one of items 1 to 6, wherein the degree of curvature is 1.2 or more and 3.0 or less.

[8]

The separator for an electric storage device according to any one of items 1 to 7, wherein the heat shrinkage rate of the separator in TD at 130° C. is 4% or less.

[9]

The separator for a power storage device according to any one of items 1 to 8, wherein the tensile strength in MD is 500 kg/cm ² or more and 2000 kg/cm ² or less.

[10]

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

[11]

The separator for a power storage device according to any one of items 1 to 10, wherein the thickness of the microporous film (A) is 1 μm or more and 27 μm or less.

[12]

The separator for a power storage device according to any one of items 1 to 11, wherein the total thickness is 5 μm or more and 30 μm or less.

[13]

The separator for a power storage device according to any one of items 1 to 12, wherein the puncture strength per 14 μm thickness is 50 gf/14 μm or more and 550 gf/14 μm or less.

[14]

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

[15]

The separator for a power storage device according to any one of items 1 to 14, comprising a microporous membrane (B) containing a polyolefin resin as a main component and disposed on both surfaces of the microporous membrane (A).

Effects of the Invention

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 separator.

BRIEF DESCRIPTION OF THE DRAWINGS

[Fig. 1] Fig. 1 is an example of a brightness histogram (horizontal axis: brightness, vertical axis: frequency) of an image processing area.

Detailed ways

《Separators for power storage devices》

The separator for a power storage device disclosed in the present invention (hereinafter also 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 5 or less. The microporous membrane (A) contains 20% by mass or more and less than 100% by mass of an inorganic filler, and the average pore size of the pores in the ND-MD cross section is 100 nm or more and 1500 nm or less, and the air permeability is 340 seconds / 100 ml or less. The following is a detailed description of each feature.

＜Microporous membrane (A)＞

In the present disclosure, the inorganic filler-containing layer including the inorganic filler and the polyolefin is also referred to as a "microporous membrane (A)". The microporous membrane (A) contains an inorganic filler and a polyolefin resin. By containing the inorganic filler, the wettability of the separator to the electrolyte is improved, and the liquid absorption of the electrolyte can be improved. In addition, by containing the inorganic filler, the interface between the inorganic filler and the polyolefin resin becomes the starting point of the hole opening when the layer is opened, and it is easy to form large pores and a connecting structure of the pores, so that a microporous membrane with high liquid absorption can be obtained. By melt extrusion at a high draw ratio in MD during film formation, layered crystals with high crystal orientation can be obtained, and interconnected pores can be easily formed in the thickness direction when the pores are opened. In addition, since it is easy to form interconnected pores of the pores through the layered crystals with high crystal orientation, a microporous membrane with high liquid absorption can be obtained.

Based on the total mass of the microporous membrane (A), the content of the inorganic filler in the microporous membrane (A) is 20% by mass or more and less than 100% by mass, preferably 20% by mass or more and 90% by mass or less, 30% by mass or more and 90% by mass or less, more preferably 25% by mass or more and 85% by mass or less, further preferably 30% by mass or more and 80% by mass or less, further preferably 35% by mass or more and 75% by mass or less, particularly preferably 40% by mass or more and 70% by mass or less. The content of the inorganic filler is preferably 40% by mass or more and 90% by mass or less, more preferably 50% by mass or more and 90% by mass or less, and further preferably 60% by mass or more and 90% by mass or less. If the content ratio of the inorganic filler in the microporous membrane (A) is 20% by mass or more, it is not limited theoretically, but it can be considered that: the layer opening and the filler opening occur in a balanced manner, and the pores in the vertical direction formed by the layered crystals are connected to each other by the independent pores in the horizontal direction formed by the filler openings, so that high permeability and high liquid absorption can be achieved. The upper limit of the content ratio of the inorganic filler is not limited. For example, when it is less than 90% by mass, a layer with an inorganic filler and a layered crystal can be formed by melt extrusion with a high draw ratio, and then uniaxial stretching is performed, thereby obtaining a highly permeable separator. In addition, there is a tendency that the risk of film breakage is small and the thickness uniformity during film formation becomes good. In addition, if it is less than 70% by mass, the network structure of the resin can be formed without being interrupted by the inorganic filler, so that a high MD strength can be maintained, and the winding body of the winding electrode and the separator can be easily manufactured.

Examples of the inorganic filler include oxide ceramics such as aluminum oxide, silicon dioxide, titanium dioxide, zirconium oxide, magnesium oxide, cerium oxide, yttrium oxide, 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 hydroxide, potassium titanate, talc, kaolinite, dickite, pearl clay, halloysite, pyrophyllite, montmorillonite, sericite, mica, magnesia chlorite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fiber, etc. From the viewpoint of providing safety and heat resistance in the battery, at least one selected from the group consisting of barium sulfate, titanium dioxide, aluminum oxide, and boehmite is preferred.

The particle size of the inorganic filler is preferably 60 nm or more and 2000 nm or less, preferably 100 nm or more and 1800 nm or less, more preferably 200 nm or more and 1500 nm or less, and further preferably 350 nm or more and 1000 nm or less. From the viewpoint of film forming stability during extrusion, when the particle size is 60 nm or more, the pressure during extrusion will not rise extremely, and film forming is easy. In addition, if it is 60 nm or more, the inorganic filler is further dispersed, and it is not easy to produce a significant low concentration portion of the inorganic filler in the separator, and it becomes a resistance component when melted at high temperature, and it is easy to maintain the resistance between electrodes. In addition, it is not easy to wear the piping, and the metal foreign matter contained in the separator is reduced. When the inorganic filler is 2000 nm or less, the risk of film breakage during layer opening is reduced, and it is not easy to become the starting point of stress concentration relative to the layer thickness, and the strength is improved. In addition, the thickness of the separator is not easy to produce unevenness, and the quality is improved. When it is 2000 nm or less, the inorganic filler surface area is large, so it is easy to obtain high liquid absorption.

The Mohs hardness of the inorganic filler is preferably 5 or less. When the Mohs hardness is 5 or less, the inorganic filler is inhibited from wearing the metal surface when in contact with the piping flow path of the film forming equipment. For example, the wear of the SUS material used for piping is reduced, and the risk of mixing into the separator can be reduced. When the separator is installed in the battery, the mixed metal foreign matter is sometimes dissolved during the charge and discharge cycle, especially forming dendrites on the negative electrode side, which may cause a short circuit of the battery, so it is not preferred.

In order to improve the dispersibility of polyolefin resin, inorganic fillers can use substances that have been surface treated by a surface treatment agent. As the surface treatment agent, treatments using saturated fatty acids and/or their salts (saturated fatty acid salts), unsaturated fatty acids and/or their salts (unsaturated fatty acid salts), polysiloxanes, silane coupling agents, etc. can be cited. From the perspective of the dispersibility of polyolefin resin, the treatment agent is a saturated fatty acid and its salt, an unsaturated fatty acid and its salt, a saturated fatty acid and its salt with a carbon number of 8 or more, and an unsaturated fatty acid and its salt with a carbon number of 8 or more. Since the inorganic filler is highly dispersed, the surface of the inorganic filler is exposed more inside the separator, so the liquid absorption is improved.

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. If the surface hydrophilicity is 0.1 or more, the affinity for the electrolyte is improved, and there is a tendency for the ion conductivity to be improved.

The surface treatment amount of the inorganic filler also depends on the particle size of the inorganic filler, but based on the total mass of the inorganic filler after the surface treatment, it is preferably 0.1% by mass or more and 10% by mass or less. When the surface treatment amount is 10% by mass or less, the remaining surface treatment agent can be reduced, and when the surface treatment amount is 0.1% by mass or more, good dispersibility can be obtained.

The weight per unit area of the inorganic filler is preferably 0.15 mg/ ^cm2 or more. If the weight per unit area is 0.15 mg/ ^cm2 or more, the inorganic filler is not easily transferred to the inter-electrode gap along with the resin during the high-temperature melting of the separator, and the inorganic filler remains in the form of an inorganic filler layer that becomes a resistance component between the electrodes, so there is a tendency to form a dense inorganic filler layer. In addition, the inorganic filler becomes a resistance component and can maintain the resistance between the electrodes, and there is a tendency to improve the safety of the battery.

The microporous membrane (A) comprises a polyolefin resin. A polyolefin resin is a polymer containing a monomer having a carbon-carbon double bond in the form of a repeating unit. The monomer constituting the polyolefin resin is not limited, and examples thereof include monomers having 3 to 10 carbon atoms having 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, a copolymer, or a multi-stage polymer, and is preferably a homopolymer.

The content of the polyolefin resin in the microporous membrane (A) is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more and 70% by mass or less, further preferably 20% by mass or more and 60% by mass or less, further preferably 20% by mass or more and 50% by mass or less, and particularly preferably 20% by mass or more and 40% by mass or less. 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 can be easily obtained.

As the polyolefin resin, at least one selected from the group consisting of polyethylene, polypropylene, and copolymers of polyethylene and polypropylene is preferred from the viewpoint of shutdown characteristics, etc. As polyethylene, low-density polyethylene (LDPE), medium-density polyethylene (MDPE), and high-density polyethylene can be cited, and high-density polyethylene (HDPE) is more preferred from the viewpoint of easily obtaining a large pore size and further improving permeability and liquid absorption.

The MFR of the polyolefin resin contained in the microporous membrane (A) is measured under a load of 2.16 kg, at a temperature of 190°C when polyethylene is the main component, and at a temperature of 230°C when other than this, especially when polypropylene is the main component. At this time, it is preferably 0.2 or more and 15 or less. It is preferably 0.25 or more and 0.30 or more, and preferably 5.00 or less and 1.00 or less. It is not limited theoretically, but the reason can be considered to be that if a thermoplastic resin with a high MFR of a polyolefin resin is kneaded with an inorganic filler, uniform kneading can be performed, and the inorganic filler is dispersed to form a uniform separator. When the separator is loaded into a battery, the separator can form an insulating layer of a uniform inorganic filler layer when it is melted at a high temperature, so the effect of maintaining the resistance between the electrodes becomes higher. On the other hand, when the MFR of the polyolefin resin is too low, the inorganic filler is not dispersed and a significant low-concentration portion of the inorganic filler is generated in the separator. Therefore, when the separator is loaded into the battery, the inorganic filler is locally insufficient in the insulating layer formed when it is melted at high temperature, so it becomes an uneven inorganic filler layer and there is a tendency that the resistance between the electrodes cannot be maintained. In addition, from the perspective of film forming stability during extrusion, when the MFR of the polyolefin resin is 0.2 or more, there is a tendency that the risk of film breakage becomes higher when the draw ratio is high in order to carry out crystal orientation during melt extrusion. When the MFR of the polyolefin resin is 15 or less, the crystal becomes easy to orient, the decrease in the degree of orientation can be suppressed, and it becomes easy to open the layer, and the permeability is also improved.

The MFR of the microporous membrane (A) is greater than 0.05 and less than 5. The MFR of the microporous membrane (A) is measured at a load of 2.16 kg and a temperature of 230°C. The MFR of the microporous membrane (A) is almost unchanged from the MFR of the raw material composition forming the microporous membrane (A). When the MFR of the microporous membrane (A) is greater than 0.05, the pressure during extrusion will not rise extremely, and it is easy to form a film, so the film forming stability during extrusion is improved. In addition, when the MFR of the microporous membrane (A) is greater than 0.05, the flow in the piping during the melt extrusion of the microporous membrane (A) without the microporous membrane (B) can reduce the wear of the inorganic filler and the surface of the equipment piping, and reduce the content of the metal component of the separator, so that the short circuit resistance of the battery can be improved. When the MFR of the microporous membrane (A) is less than 5, the decrease in orientation can be suppressed, the layer opening can be easily performed, and the 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.

The microporous membrane (A) has an opening, and the profile of the opening is formed only by a polyolefin resin, or is formed by an inorganic filler and a polyolefin resin. The opening of the microporous membrane (A) preferably includes a hole composed of an inorganic filler and a polyolefin resin. The average pore size (average pore size) of the hole in the ND-MD cross section can be more than 100nm and less than 2000nm, preferably more than 150nm and less than 1500nm, more preferably more than 200nm and less than 840nm, more preferably more than 250nm and less than 800nm, and more preferably more than 300nm and less than 750nm. If the average pore size is more than 100nm, it becomes a structure in which pores are connected to each other, and there is a tendency to obtain high permeability. In addition, when the average pore size is less than 1500nm, it is difficult to become the starting point of stress concentration relative to the thickness of the microporous membrane (A), so the strength of the separator is not easily reduced. Furthermore, when the average pore size is 1500 nm or less, the withstand voltage becomes higher and the battery is less likely to short-circuit, thereby improving the yield rate during battery manufacturing. The average pore size of the microporous membrane (A) can be measured by observing the ND-MD cross section of the microporous membrane (A) using a scanning electron microscope (SEM) as described later in the Examples column.

The long aperture of the microporous membrane (A) is preferably arranged in one direction. In the present specification, "long aperture" refers to the longest line segment in the line segment connecting any two points on the contour of the opening. It is not limited theoretically. If the long aperture is arranged in one direction, the crystallization of the resin constituting the separator is strongly oriented, and the strength in the direction of the long aperture becomes larger, and it is easy to manufacture the winding body of the wound electrode and the separator when making a cylindrical battery, so it is preferred. As described later in the embodiment column, the arrangement of the long aperture can be confirmed by observing the cross section of the microporous membrane (A) with a scanning electron microscope (SEM). "Arranged in one direction" means that in the above-mentioned electron microscope image of the surface of the separator, more than 90% of the original fibers are contained in the angle range of ±20 degrees in the extension direction. That is, in the above-mentioned electron microscope image, when the major axis of more than 90% of the pores is contained in the angle range of ±20 degrees from each other, it is judged that the arrangement of the aperture is "arranged in one direction".

The area ratio of the openings in the cross-sectional area, which is an index of the porosity of the microporous membrane (A), is 20% or more and 60% or less. Preferably, it is 30% or more, more preferably 35% or more, preferably 55% or less, and more preferably 50% or less. By setting the area ratio of the openings to 60% or less, the ratio of the resin to the inorganic filler can be increased to form a network structure of the solid component. Therefore, the stress concentration during puncture is relaxed, the elongation at break is improved, and it is easy to obtain a high puncture strength. In addition, if the stretching ratio of MD is increased in order to increase the opening ratio, the hole is easily crushed in the thickness direction, and there is a tendency that the connection structure between the holes is divided and the air permeability is deteriorated. When the area ratio of the openings is 20% or more, the ratio of the resin to the inorganic filler is low, so that it is easy to form a connection structure between the holes, so that high permeability can be obtained.

Microporous membrane (A) may also contain an elastomer in addition to containing a polyolefin resin and an inorganic filler. As an elastomer, thermoplastic elastomers and thermosetting elastomers etc. can be listed, preferably thermoplastic elastomers. In the present specification, thermoplastic elastomers are contained in thermoplastic resins. By making microporous membrane (A) contain thermoplastic elastomers, melt tension can be reduced without damaging the balance of strength and air permeability. Thus, when using a thermoplastic resin with a high MFR expected to be high in strength, even if it is a high melt tension, it is possible to filmize by containing a thermoplastic resin, and a separator as a thin film and high strength can be obtained.

The thickness of the microporous membrane (A) is preferably from 1 μm to 27 μm, more preferably from 1 μm to 20 μm. When the thickness is 1 μm or more, the heat resistance of the separator for the storage device is improved. When the thickness is 27 μm or less, the energy density of the storage device can be further improved. Considering the heat resistance and energy density, it is preferably from 3 μm to 15 μm, and more preferably from 5 μm to 10 μm. In addition, considering the heat resistance, ion permeability and physical strength of the separator, the proportion of the thickness of the microporous membrane (A) in the overall thickness of the separator is preferably from 15% to 90%, more preferably from 20% to 80%, and more preferably from 20% to 60%.

＜Microporous membrane (B)＞

The separator for the storage device disclosed in the present invention may include a microporous film (B) with a polyolefin resin as the main component, which is arranged on one or both sides of the microporous film (A). In the present application specification, "main component" refers to a component with a content of more than 50% by mass, which 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. The microporous film (B) preferably does not contain an inorganic filler.

The microporous membrane (B) is preferably a microporous layer with a polyolefin resin as the main component. By having an outer layer of polyolefin, the film-forming property becomes good, and it can be melt-extruded at a high draw ratio in MD during film formation, and a high crystal orientation can be obtained, so it is easy to obtain high permeability. In the case of a three-layer structure with a polyolefin microporous layer/containing inorganic layer/polyolefin microporous layer, it is possible to have a high liquid absorption brought by the inorganic layer, and maintain the mechanical strength brought by the polyolefin microporous layer and the oxidation resistance when the electrode surface is in contact. From the viewpoint of wear resistance, the polyolefin microporous layer is preferably the outermost layer, and can be a 5-layer structure of polyolefin microporous layer/containing inorganic layer/polyolefin microporous layer/containing inorganic layer/polyolefin microporous layer/containing inorganic layer/polyolefin microporous layer, 9-layer structure. From the viewpoint of thin film properties, it is preferably a 3-layer structure.

From the perspective of easy layer opening, the polyolefin resin used as the microporous membrane (B) is preferably polyethylene or polypropylene. From the perspective of strength such as puncture, polypropylene (hereinafter also referred to as "PP microporous layer") is preferred. The thickness of the microporous membrane (B) is preferably 10% or more and 90% or less relative to the total thickness of the separator. If it is 10% or more, the proportion of the layer having a mesh structure formed by continuously connecting the resin increases, so that it can be made high-strength. If it is 90% or less, the necessary thickness of the inorganic layer suitable for high liquid absorption can be ensured, so high liquid absorption can be achieved.

The stereoregularity of the polypropylene of the microporous layer (B) is not limited, and examples thereof include atactic, isotactic, or syndiotactic homopolymers, etc. The polypropylene of the present invention is preferably an isotactic or syndiotactic highly crystalline homopolymer.

The polypropylene of the microporous layer (B) is preferably a homopolymer, and may also be a copolymer such as a block polymer formed by copolymerizing a small amount of comonomers other than propylene, such as α-olefin comonomers. The amount of propylene structure contained as a repeating unit in the polypropylene is not limited, for example, it may be 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 from comonomers other than the propylene structure contained in the polypropylene is not limited, for example, it may be 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. Polypropylene may be used alone or in combination of two or more.

From the viewpoint of obtaining a microporous layer (B) with higher strength, the MFR of the polypropylene of the microporous layer (B) is preferably 8.0 g/10 minutes or less, for example, 6.0 g/10 minutes or less, 4.0 g/10 minutes or less, 3.0 g/10 minutes or less, 2.0 g/10 minutes or less, or 1.1 g/10 minutes or less. From the viewpoint of exhibiting good film-forming properties and uniform film thickness, the lower limit of the MFR of the microporous layer (B) is not limited, for example, 0.3 g/10 minutes or more, 0.35 g/10 minutes or more, 0.4 g/10 minutes or more, 0.45 g/10 minutes or more, or 0.5 g/10 minutes or more.

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. If the thickness is 1 μm or more, the microporous membrane (A) can be coated without generating defects (defective portions) of the layer, so that the wear of the SUS material used for piping is reduced, and the risk of substances from the piping being mixed into the separator is easily reduced. When the thickness is 10 μm or less, it is easy to obtain high liquid absorption based on the microporous layer (A) containing inorganic fillers.

〈Separator〉

The separator for the storage device can be a separator of a single-layer structure consisting only of a microporous film (A), or a separator of a double-layer structure in which a microporous film (B) is stacked on the microporous film (A). The separator for the storage device can be a separator of a three-layer structure of a microporous film (B)/microporous film (A)/microporous film (B) with a microporous film (A) as an inner layer and a microporous film (B) as an outer layer. The separator for the storage device can have a structure of more than three layers. By having a three-layer structure of a microporous film (B) (hereinafter also referred to as "PO microporous layer") having a polyolefin resin as the main component on both sides of a microporous film (A) (hereinafter also referred to as "inorganic layer"), the inorganic filler is less likely to contact the piping flow path of the film-forming equipment, and the wear of the metal surface is suppressed. In particular, the lip portion, which is considered to have the highest risk of wear due to the small cross-sectional area during extrusion, the high flow rate, and the corrosion when the trace additive contacts the air, can avoid contact with the inorganic filler. As a result, the contact between the inorganic filler and the surface of the equipment piping can be reduced or eliminated, and the possibility of containing metal components such as Fe in the separator can be reduced, thereby improving the short-circuit resistance of the battery.

The heat shrinkage of TD at 130°C of the separator for the storage device is 4% or less, preferably 3% or less, and more preferably 1% or less. If the heat shrinkage of TD at 130°C is 4% or less, the degree of crystal orientation becomes high, which makes it easy for the layer to open and cracks in the vertical direction when the opening occurs. As a result, the pores are enlarged by subsequent heat stretching, and a pore structure with high straightness can be obtained, and the liquid absorption tends to be higher. In addition, if the heat shrinkage of TD is 4% or less, an insulating film that maintains the area of the separator can be formed, and the insulation between the electrodes can be maintained, so the resistance to short circuit caused by the heat shrinkage of TD in the laminated unit short circuit test becomes higher. The lower limit of the heat shrinkage is not particularly limited, for example, it is -4% or more, preferably -3% or more, more preferably -2% or more, and further preferably -1% or more. If the heat shrinkage of TD is -4% or more, wrinkles are not easy to form when the temperature of the battery rises, and the problem of cracking is not easy to cause due to the equal stress applied to the separator. The reason why the thermal shrinkage of TD takes a negative value is that the components contained in the separator may expand due to heating.

The air permeability (also referred to as "air permeability resistance") of the separator for the power storage device may be 400 seconds/100ml or less, preferably 340 seconds/100ml or less, more preferably 50 seconds/100ml or more and 340 seconds/100ml or less, further preferably 50 seconds/100ml or more and 300 seconds/100ml or less, further preferably 50 seconds/100ml or more and 250 seconds/100ml or less, particularly preferably 100 seconds/100ml or more and 230 seconds/100ml or less, particularly preferably 120 seconds/100ml or more and 210 seconds/100ml or less. The air permeability is preferably 100 seconds/100ml or more and 340 seconds/100ml or less, and may also be more preferably 120 seconds/100ml or more and 340 seconds/100ml or less. When the air permeability is 50 sec/100 ml or more, a network structure of continuous pores can be obtained, thereby obtaining a high-strength separator. When it is 300 sec/100 ml or less, a structure in which the pores are connected to each other can be obtained, thereby obtaining high liquid absorption.

The total thickness of the separator for the power storage device is preferably 5 μm or more and 30 μm or less, and 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 the power storage device is improved. When the thickness is 30 μm or less, the energy density of the power storage device can be improved.

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

The tensile strength of the MD of the separator for the storage device is preferably 500kg/ ^cm2 or more and 2000kg/cm2 or ^less , more preferably 800kg/ ^cm2 or more and 1800kg/ ^cm2 or less, and further preferably 1000kg/ ^cm2 or more and 1600kg/ ^cm2 or less. If the MD tensile strength is 500kg/ ^cm2 or more, the strength of the MD becomes large by applying strong orientation to the separator, and it is easy to manufacture a wound body of a wound electrode and a separator. There is no particular limitation, but it is preferably 2000kg/ ^cm2 or less. If it is 2000kg/ ^cm2 or less, cracks are not easy to occur on the MD, and the problem of the separator cracking in the longitudinal direction (MD) is not easy to occur during use.

The ratio of the tensile strength of the MD to the tensile strength of the TD of the separator for the 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 further preferably 8.0 or more and 15 or less. If the MD/TD strength ratio is 1.5 or more, the strength of the MD becomes greater by applying strong orientation to the crystals of the resin component constituting the separator. Thus, it is easy to manufacture a wound body of wound electrodes and separators. In addition, if the MD/TD strength ratio is 1.5 or more, the thermal shrinkage rate of the TD can be relatively reduced, so the resistance to short circuits caused by the thermal shrinkage of the TD during battery winding becomes higher. The upper limit of the MD/TD strength ratio is not particularly limited, preferably 30 or less, preferably 20 or less, and preferably 15 or less. If the MD/TD strength ratio is 30 or less, cracks are not likely to occur in the MD, and the problem of cracking of the separator in the longitudinal direction (MD) is not likely to occur during the unwinding, winding, and other web handling of the separator in the battery production process.

The curvature of the separator for the storage device is preferably 1.2 or more and 3.0 or less, more preferably 1.4 or more and 2.5 or less, and more preferably 1.5 or more and 2.0 or less. For the liquid absorption of the microporous membrane, the wettability and curvature of the surface of the inorganic filler to the electrolyte are considered to be the main controlling factors. If the curvature is 3.0 or less, the path of the electrolyte from the surface of the microporous membrane to the surface on the opposite side becomes shorter, so it is believed that high liquid absorption can be obtained. When the curvature is 1.2 or more, the withstand voltage can be maintained high, the puncture strength can be maintained high, and the safety of the battery can be improved.

The water content of the separator for an electrical storage device is preferably 1000 ppm or less. By setting the water content of the separator for an electrical storage device to 1000 ppm or less, adverse effects on battery characteristics can be reduced.

《Method for manufacturing separator for power storage device》

The method for producing a microporous membrane (A) containing an inorganic filler and a polyolefin resin generally includes: a melt extrusion step, in which the polyolefin resin and the inorganic filler are mixed, dispersed, and melt extruded to obtain a resin film; and a pore forming step, in which the obtained resin sheet is opened to make it porous; and optionally a stretching step and 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.

As dry methods, there are: 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 the interface of the polyolefin resin crystal is peeled off by heat treatment and stretching; and a method in which a polyolefin resin and an inorganic filler are melt-kneaded and formed on a sheet, and then the interface between the polyolefin resin and the inorganic filler is peeled off by stretching.

As wet methods, there are: a method in which a polyolefin resin composition is dispersed with an inorganic filler, a liquid pore-forming material is added and melt-kneaded to form a sheet, and the sheet is stretched as required to extract the pore-forming material; and a method in which a polyolefin resin composition is dissolved in an organic solvent and then immersed in a poor solvent having low solubility in the polyolefin resin to solidify the polyolefin resin and remove the organic solvent at the same time.

The polyolefin resin composition may be melt-kneaded using a single-screw extruder or a twin-screw extruder. In addition, for example, a kneader, a Labo Plastomill, a kneading roll, a Banbury mixer, etc. may be used.

The polyolefin resin composition may optionally contain resins other than polyolefin resins and additives according to the manufacturing method of the microporous film (A) or the physical properties of the target microporous film (A). As additives, for example, pore-forming materials, fluorine-based flow modifiers, waxes, crystal nucleating agents, antioxidants, metal soaps such as aliphatic carboxylic acid metal salts, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments, etc. As pore-forming materials, plasticizers, inorganic fillers, or combinations thereof may be cited.

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.

The stretching process can also be performed in the hole forming process, or before or after the hole forming process. As a stretching treatment, any one of uniaxial stretching or biaxial stretching can be used. Although not limited, from the perspective of manufacturing cost when using a dry method, uniaxial stretching is preferred. By using a layer opening method based on uniaxial stretching, it is possible to more easily manufacture a separator for a storage device of the present disclosure having high liquid absorption and high strength (more specifically, for example, high puncture strength) as described below. Hereinafter, this situation will be described. The opening method is generally a method in which a precursor (blank film) in which layered crystals are oriented is obtained by melt extrusion, and it is cold stretched and then hot stretched to open pores between layered crystals. The inventors of the present invention have conducted in-depth research in order to take into account high liquid absorption and high strength by the layer opening method, and found that it is preferred that: by lowering the extruder temperature of the microporous membrane (A) containing an inorganic filler as an inner layer, and increasing the air volume of the air ring (air ring) just after extrusion, the degree of crystal orientation can be increased despite the generally high heat capacity of the inorganic filler. Thus, a high open-pore structure of the microporous membrane (A) can be achieved, and high liquid absorption can be achieved. Furthermore, by using a highly oriented raw film while opening holes at a low stretch ratio, a high resin density can be maintained and a high puncture strength can be achieved. Thus, even a single-layer separator of the microporous membrane (A) can have high liquid absorption while maintaining strength.

In order to suppress the shrinkage of the microporous film, a heat treatment process may be performed for the purpose of heat setting after the stretching process or after the pore forming process. The heat treatment process may include: a stretching operation performed in a specified temperature atmosphere and a specified stretching rate for the purpose of adjusting physical properties, and/or a relaxation operation performed in a specified temperature atmosphere and a specified relaxation rate for the purpose of reducing tensile stress. A relaxation operation may also be performed after the stretching operation. These heat treatment processes may be performed using a tenter or a roller stretching machine.

It is also possible to laminate the microporous film (B) on one or both sides of the microporous film (A). As a method for laminating the microporous film (B), for example, coextrusion and lamination can be cited. In the coextrusion method, the resin composition of each layer can be coextruded simultaneously to form a film, and the obtained multi-layer blank film is stretched to form a hole to make a multi-layer microporous film. In the lamination method, each layer is formed into a film by extrusion film formation to obtain a blank film. By laminating the blank film obtained, a multi-layer blank film can be obtained, and the obtained multi-layer blank film is stretched to form a hole, thereby making a multi-layer microporous film. In the coextrusion method, since the microporous film (A) can be supported by a layer without inorganic filler, the film forming stability is improved, which is preferred in terms of being able to increase the content of inorganic filler.

《Electricity Storage Device》

The power storage device includes a positive electrode, a negative electrode, and the above-described separator for a power storage device of the present disclosure. The separator for a power storage device is stacked between the positive electrode and the negative electrode.

As the power storage device, there is no particular limitation, and examples thereof include 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-hydrogen batteries, nickel-cadmium batteries, double-layer capacitors, lithium ion capacitors, redox flow batteries, lithium-sulfur batteries, lithium-air batteries, zinc-air batteries, etc. Among these, from the viewpoint of practicality, preferably lithium secondary batteries, lithium ion secondary batteries, nickel-hydrogen batteries, or lithium ion capacitors, more preferably lithium ion secondary batteries.

The power storage device can be manufactured, for example, as follows: the positive electrode and the negative electrode are overlapped with each other via the separator described above, and wound as needed to form a stacked electrode body or a wound electrode body, which is then loaded into an outer casing, the positive and negative electrodes are connected to the positive and negative terminals of the outer casing by means of a lead body, etc., and a non-aqueous electrolyte containing a non-aqueous solvent such as a chain or cyclic carbonate and an electrolyte such as a lithium salt is injected into the outer casing, and the outer casing is sealed, thereby manufacturing the power storage device.

Example

《Measurement and evaluation methods》

[Melt flow rate (MFR)]

The MFR of thermoplastic resins is measured according to JIS K 7210 at a temperature of 230°C and a load of 2.16 kg (unit: g/10 minutes). The MFR of polypropylene is measured according to JIS K 7210 at a temperature of 230°C and a load of 2.16 kg. The MFR of polyethylene is measured according to JIS K 7210 at a temperature of 190°C and a load of 2.16 kg. The MFR of elastomers is measured according to JIS K 7210 at a temperature of 230°C and a load of 2.16 kg.

The MFR of the microporous film (A) was measured when it was taken out from the laminated sheet. That is, the microporous film (B) located on the surface of the microporous sheet of the laminated structure was bonded to the outer layer with an adhesive tape and peeled off, and the microporous film (A) was taken out, and then measured at a temperature of 230°C and a load of 2.16 kg according to JISK 7210.

[Area ratio of inorganic filler, pores and resin]

Pre-treatment for electron microscope observation:

A separator and ruthenium tetroxide (manufactured by Rare Metallic Co., Ltd.) are coexisted 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, manufactured by Nissin EM Co., Ltd.), 9.4 mL of curing agent (methylnadic anhydride (MNA), manufactured by Nissin EM Co., Ltd.) and 0.34 mL of reaction accelerator (2,4,6-tris (dimethylaminomethyl) phenol) (2,4,6-Tris (dimethylaminomethyl) phenol), manufactured by Nissin EM Co., Ltd., DMP-30) are mixed, and the separator dyed with ruthenium is immersed in the fully stirred mixed solution, and the mixture is placed under a reduced pressure environment to fully penetrate the pores of the separator dyed with ruthenium. Then, the separator dyed with ruthenium is embedded in the epoxy resin by curing at 60°C for more than 12 hours. After embedding with epoxy resin, the cross section is roughly processed with a blade, etc., and then, the cross section is milled using an ion milling device (E3500Plus, manufactured by Hitachi High-Tech Co., Ltd.) to produce a cross section sample. The cross section is the ND-MD surface. The cross section sample is fixed to the SEM sample stage for cross section observation using a conductive adhesive (carbon-based) and dried. As a conductive treatment, an osmium coater (HPC-30W, manufactured by Vacuum Device Co., Ltd.) is used to apply osmium under the conditions of setting the applied voltage adjustment knob to 4.5 and the discharge time to 0.5 seconds, thereby obtaining a microscopic sample.

Acquisition of electron microscope images:

The above-mentioned microscopic sample was examined with a SEM (S-4800, manufactured by Hitachi High-Tech Co., Ltd.) under conditions of an acceleration voltage of 1.0 kV, an emission current of 10 μA, a probe current of High, a detector of Upper+LA-BSE 100, a pixel resolution of about 10 nm/pix, and a working distance of 2.0 mm, so that the entire area in the thickness direction of the layer containing the inorganic filler was contained within the observation field of view, the brightness was set to be unsaturated, and the contrast was set to be as high as possible, and an electron microscope image was obtained. In the case where the thickness of the layer containing the inorganic filler is greater than the observation field of view, the observation field of view was determined in such a way that the layer containing the inorganic filler was contained in the entire observation field of view, the brightness was set to be unsaturated, and the contrast was set to be as high as possible, and an electron microscope image was obtained.

Image processing (calculation of pore ratio and inorganic filler ratio):

In the above electron microscope image, the following area is selected as the image processing area: an area as large as possible that contains more than 80% of the thickness of the layer containing inorganic fillers and does not contain areas other than the layer containing inorganic fillers, and has a width of more than 20 μm in the horizontal direction. In the case where the thickness of the layer containing inorganic fillers is greater than the observation field of view, the entire observation field of view is used as the image processing area. The free software ImageJ was used for image processing. The brightness histogram of the above image processing area is shown, and the brightness difference between the two points that become the maximum value of the two closest peaks of the three peaks shown in Figure 1 is calculated (indicated by arrows in Figure 1), and the value is used as CD. Use Mean Shift Filter (an open source plug-in made by Kai UweBarthel of FHTW-Berlin) to make a smoothed image. At this time, the independent variable Spatial radius of Mean Shift Filter is set to 3, and Color Resistance is set to the value of the above CD. For the above smoothed image, use Multi Otsu Threshold (an open source plug-in made using the algorithm of non-patent document 1) to determine two thresholds that are not 0. The smaller of the two thresholds is set as threshold A, and the larger one is set as threshold B. In the above-mentioned smoothed image, pixels with brightness less than threshold A are regarded as pores, pixels with brightness greater than threshold A and less than threshold B are regarded as polyolefins, pixels with brightness greater than threshold B are regarded as inorganic fillers, and the number of pixels of pores relative to the total number of pixels is taken as the area occupancy of pores. The area occupancy of polyolefins and the area occupancy of inorganic fillers are calculated in the same way. Using electron microscope images of more than 5 fields of view, the area occupancy of the above-mentioned pores, the area occupancy of the above-mentioned polyolefins, and the area occupancy of the above-mentioned inorganic fillers are calculated, and the respective average values are taken as the pore ratio, polyolefin ratio, and inorganic filler ratio.

Image processing (calculation of average particle size of inorganic filler):

(1) For the image after the smoothing process, a binary image is created in which the brightness of pixels with brightness less than a threshold value B is set to 0 and the brightness of pixels with brightness greater than or equal to the threshold value B is set to 255.

(2) Use the free software ImageJ to perform Thickness analysis of BoneJ (an open source plug-in described in Non-Patent Document 2), read the average value of Tb.Th in the Results window, and convert it into the length of the actual space taking into account the pixel size of the electron microscope image. The obtained value is used as the average particle size of the inorganic filler.

[Average pore size]

Image processing (calculation of pore size):

For the above-mentioned smoothed image, the brightness of pixels with brightness less than threshold A is set to 0, and the brightness of pixels with brightness greater than threshold A is set to 255, thereby binarizing the image. The Thickness analysis of BoneJ (an open source plug-in described in non-patent document 2) is performed, and the value of Tb.Th mean in the Results window is read, and the value converted into the length of the actual space is used as the average pore diameter of the pores.

[Thickness (μm)]

The thickness (μm) of the separator was measured at room temperature 23±2° C. using a digital indicator (IDC 112) manufactured by Mitutoyo. The thickness of the inorganic layer was measured by drawing a center line between the inorganic layer and the interface or other layers obtained from the SEM image and measuring the length between the lines.

[Breathability resistance (seconds/100ml)]

The air permeability resistance (seconds/100 ml) of the separator was measured using a Gurley type air permeometer in accordance with JISP-8117.

[Puncture Strength]

A hemispherical needle with a tip radius of 0.5 mm was prepared, and a separator was clamped between two plates with an opening of 11 mm in diameter (dia.), and the needle, separator, and plate were set. Using "MX2-50N" manufactured by IMADA Co., Ltd., a puncture test was performed under the conditions that the curvature radius of the tip of the needle was 0.5 mm, the opening diameter of the plate holding the separator was 11 mm, and the puncture speed was 25 mm/min. The needle was brought into contact with the separator, and the maximum puncture load (i.e., puncture strength (gf)) was measured.

[MD tensile strength and TD tensile strength, and MD/TD strength ratio]

The tensile strength of the separator is measured by using a tensile tester (TG-1kN type manufactured by Minebea Co., Ltd.) with the sample length before the test set to 35 mm and the sample stretched at a speed of 100 mm/min. The strength (tensile load value) when the sample yields or the strength (tensile load value) when it is cut (broken) before yielding is divided by the cross-sectional area of the test piece as the tensile strength. The tensile strength is measured for the MD and TD of the separator respectively. The MD/TD strength ratio is obtained by dividing the MD tensile strength by the TD tensile strength.

[MD heat shrinkage and TD heat shrinkage]

Regarding the heat shrinkage, the separator was cut into 5 cm squares, marked at 9 locations at 2 cm intervals, and wrapped in paper. The marked sample was heat treated at 130°C for 1 hour, then cooled to room temperature, and the MD and TD lengths were measured at 3 locations to determine the shrinkage.

[Curvature]

The average pore size of the separator for the power storage device can be measured by the gas-liquid method. Specifically, it is known that for the fluid inside the capillary, when the mean free path of the fluid is greater than the pore size of the capillary, it follows the Knudzen flow, and when it is less than the pore size of the capillary, it follows the Poiseuille flow. Therefore, it is assumed that in the air permeability measurement of the separator, the flow of air follows the Knudzen flow, and in the water permeability measurement, the flow of water follows the Poiseuille flow.

In this case, the average pore size d (μm) and tortuosity τa (dimensionless) of the porous membrane are determined using the following formula from the air permeation rate constant R _gas (m ³ /(m ² ·sec·Pa)), the water permeation rate constant R _liq (m ³ /(m ² ·sec·Pa)), the air molecular velocity v (m/sec), the water viscosity η (Pa·sec), the standard pressure Ps (=101325 Pa), the porosity ε (%), and the membrane thickness L (μm).

d＝2ν×(R _liq /R _gas )×(16η/3Ps)×10 ⁶

τa＝(d×(ε/100)×ν/(3L×Ps×R _gas )) ^1/2

Here, R _gas is calculated from the air permeability (seconds) using the following formula.

R _gas = 0.0001/(air permeability × (6.424 × 10 ^-4 ) × (0.01276 × 101325))

In addition, R _liq was determined from the water permeability (cm ³ /(cm ² ·sec·Pa)) using the following formula.

R _liq = water permeability/100

It should be noted that the water permeability was determined as follows. A separator pre-immersed in ethanol was installed in a stainless steel permeation tank with a diameter of 41 mm. After the ethanol in the separator was washed with water, water was allowed to pass through at a pressure difference of about 50,000 Pa. The water permeability per unit time, unit pressure, and unit area was calculated from the water permeability (cm ³ ) in 120 seconds, and the result was taken as the water permeability. In addition, ν was determined from the gas constant R (=8.314 J/(K·mol)), absolute temperature T (K), pi, and the average molecular weight M of air (=2.896×10 ^{- 2} kg/mol) using the following formula.

ν＝((8R×T)/(π×M)) ^1/2

[Moisture content]

The separator was cut into pieces in the range of 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. Then, its weight was measured as the sample weight (g). The water weight (μg) of the pretreated sample was measured using a Karl Fischer apparatus. It should be noted that the heating and vaporization conditions during the measurement were set to 150°C and 10 minutes. In addition, HYDRANAL-Coulomat CG-K (manufactured by SIGMA-ALDRICH) was used as a cathode reagent, and HYDRANAL-Coulomat AK (manufactured by SIGMA-ALDRICH) was used as an anode reagent.

Moisture content (ppm) = water weight (μg) / sample weight (g)

[Evaluation method of droplet absorption (s)]

For the droplet absorbency, the separator was cut into 5 cm squares and placed on a smooth plate in an argon box. 5 μl of DMC was measured with a microsyringe and dropped onto the separator surface. The time until the droplet disappeared was measured to determine the droplet absorbency.

[Evaluation method of Fe (ppm) (wear resistance)]

About 0.2 g of the separator was weighed into a sealed decomposition container made of fluororesin, 5 mL of high-purity nitric acid was added thereto, and the mixture was heated at 200°C for 20 minutes using a microwave decomposition device (manufactured by Milestone General K.K., trade name "ETHOS TC", machine number 125571), and then the volume was fixed to 50 mL with ultrapure water. Then, the measurement was performed using an ICP mass spectrometer (manufactured by Thermo Fisher Scientific, trade name "X Series X7 ICP-MS", machine number X0126).

《Example 1》

[Preparation of a composition containing a polyethylene resin and an inorganic filler]

Polyethylene resin (PE, MFR = 0.31) and surface-treated barium sulfate (1 part by mass of sodium stearate is used for surface treatment relative to 100 parts by mass of barium sulfate) with an average particle size of 500 nm were dry-mixed at a mass ratio of PE:BaSO ₄ = 20:80 (mass%), and then melt-kneaded using a twin-screw extruder HK-25D (manufactured by Parker Corporation, L/D = 41). In order to suppress the decomposition and modification of the resin as much as possible, the resin feeding hopper port to the raw material tank is completely sealed, and nitrogen is continuously flowed from the bottom of the hopper to control the oxygen concentration near the raw material feeding port to less than 50 ppm. In addition, the exhaust part is completely sealed to eliminate the part where air leaks into the barrel. The effect of reducing the oxygen concentration greatly suppresses the decomposition and modification of the polymer even at high temperatures. The barium sulfate is fed using a twin-screw feeder, thereby further finely dispersing the barium sulfate. After melt kneading, strands were drawn out from a die (two holes) and cooled in a water cooling bath, and then cut using a pelletizer to obtain pellets.

[Production of microporous membrane (three layers)]

A laminated sheet is formed by a co-extrusion method. A polypropylene resin (PP, MFR=0.51) is melted using a 32mmφ twin-screw co-rotating screw extruder and supplied to a circular die using a gear pump. The above-mentioned pellets containing barium sulfate are melted using a 32mmφ single-screw extruder and supplied to a circular die using a gear pump. The compositions obtained by melt-kneading through each extruder are extruded into sheets through two types of three-layer co-extruded circular dies, and the molten polymer is cooled by blowing air and then wound into a roll. The polypropylene resin is extruded from the outer layer (the second layer on the surface) of the circular die set at a temperature of 230°C at a mixing temperature of 230°C and an extrusion rate of 2.4kg/hr. The pellets containing barium sulfate are extruded from the inner layer (middle layer) of the circular die set at a temperature of 220°C at a mixing temperature of 230°C and an extrusion rate of 1.2kg/hr in terms of polyethylene resin. The extruded precursor (blank film) was cooled by an air ring at an air volume of 3.6 m ³ /min per Φ300 mm width immediately after extrusion. The thickness of the cooled blank film was 16 μm. Next, the blank film was annealed at 127°C for 15 minutes. Next, the annealed blank film was cold-stretched to 10% at room temperature, and then the cold-stretched film was hot-stretched to 120% at 115°C, and the hot-stretched film was relaxed to 92% at 125°C, thereby forming microporous. After the above-mentioned stretching and opening, the physical properties of the obtained microporous film were measured. The results are shown in Table 1.

Example 9

[Production of microporous membrane (single layer)]

The pellets containing barium sulfate are melted by a 32mmφ single-screw extruder and supplied to a circular die using a gear pump. The composition melt-kneaded by the extruder is extruded into a sheet through a circular die, and the molten polymer is cooled by blowing air and then wound into a roll. At this time, the mixing temperature of the pellets containing barium sulfate is 230°C, and the extrusion amount is 1.2kg/hr in terms of polypropylene resin, and it is extruded from the inner layer (middle layer) of a circular die set at a temperature of 230°C. The extruded precursor (blank film) is cooled by an air ring at an air volume of ^3.6m3 /min per Φ300mm width just after extrusion. The thickness of the cooled blank film is 16μm. Next, the blank film is annealed at 127°C for 15 minutes. Next, the annealed blank film was cold stretched to 10% at room temperature, then the cold stretched film was hot stretched to 110% at 115°C, and the hot stretched film was relaxed to 92% at 125°C, thereby forming micropores. After the above stretching and pore opening, the physical properties of the obtained microporous film were measured. The results are shown in Table 1.

《Examples 2 to 8, Examples 10 to 15 and Comparative Examples 1 to 8》

As shown in Tables 1 and 2, the raw materials, film forming conditions or separator properties were changed, and in the case of three layers, a microporous membrane was obtained by the same method as in Example 1, and in the case of a single layer, a microporous membrane was obtained by the same method as in Example 9, and the obtained microporous membrane was evaluated. The layer structure was adjusted by changing the ratio of the extrusion amount.

[Table 1]

[Table 2]

Industrial Applicability

The separator for an electric storage device disclosed by the present invention has high liquid absorption, high strength and wear resistance, and can be suitably used as a separator for an electric storage device, for example, a lithium ion secondary battery.

Claims (15)

1. A separator for an electric storage device comprising a microporous film (A) containing an inorganic filler and a polyolefin resin,

The MFR of the microporous membrane (A) is 0.05 to 5,

The microporous membrane (A) comprises 20 mass% or more and less than 100 mass% of the inorganic filler, and the average pore diameter of the pores in ND-MD section is 100nm or more and 1500nm or less,

The separator for an electrical storage device has an air permeability of 340 seconds/100 ml or less.

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

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

4. The separator for an electric storage device according to claim 3, wherein an average pore diameter of the pores in an ND-MD section of the microporous film (a) is 200nm or more and 840nm or less.

5. The separator for an electric storage device according to claim 4, wherein the air permeability is 50 seconds/100 ml or more and 250 seconds/100 ml or less.

6. The separator for an electric storage device according to claim 5, wherein the particle diameter of the inorganic filler is 60nm or more and 2000nm or less.

7. The separator for an electric storage device according to claim 5, wherein the degree of bending is 1.2 or more and 3.0 or less.

8. The separator for an electric storage device according to claim 5, wherein the heat shrinkage rate of TD at 130 ℃ of the separator for an electric storage device is 4% or less.

9. The separator for an electric storage device according to claim 5, wherein the tensile strength in the MD is 500kg/cm ² or more and 2000kg/cm ² or less.

10. The separator for an electric storage device according to claim 9, wherein a ratio of tensile strength in MD to tensile strength in TD is 1.5 or more and 30 or less.

11. The separator for an electrical storage device according to claim 5, wherein the thickness of the microporous film (a) is 1 μm or more and 27 μm or less.

12. The separator for an electric storage device according to claim 11, having a total thickness of 5 μm or more and 30 μm or less.

13. The separator for an electric storage device according to claim 5, wherein the puncture strength per 14 μm thickness is 50gf/14 μm or more and 550gf/14 μm or less.

14. The separator for an electrical storage device according to claim 5, wherein the inorganic filler has a mohs hardness of 5 or less.

15. The separator for an electric storage device according to claim 5, comprising a microporous film (B) containing a polyolefin resin as a main component, disposed on both sides of the microporous film (a).