JP7549051B2 - Separator for power storage device and power storage device including same - Google Patents

Description

The present disclosure relates to a separator for an electricity storage device and an electricity storage device including the same.

Microporous membranes, particularly polyolefin-based microporous membranes, are used in many technical fields such as microfiltration membranes, battery separators, condenser separators, and fuel cell materials, and are particularly used as separators for secondary batteries such as lithium ion batteries (LIBs) or separators for lithium ion capacitors. Lithium ion batteries are being researched for use in small electronic devices such as mobile phones and notebook personal computers, and lithium ion batteries or lithium ion capacitors are being researched for use in hybrid automobiles and electric automobiles including plug-in hybrid automobiles.

In recent years, there has been a demand for energy storage devices, such as lithium-ion batteries, that have high energy capacity, high energy density, and high output characteristics. This has led to an increased demand for separators that combine high strength (e.g., high puncture strength), low air permeability, and high heat resistance.

Patent document 1 describes a microporous polyolefin film made from a mixture of a polyolefin having a viscosity average molecular weight of 600,000 or more and an inorganic filler (claim 11, examples, etc.).

International Publication No. 2019/107119

L. Vincent, and P. Soille, “Watersheds in Digital Spaces: An Efficient Algorithm Based on Immersion Simulations”, IEEE Transaction on Pattern Analysis and Machine Intelligence, vol. 13, No. 6, June 1991, pp.583- 598 D. Comaniciu, and P. Meer, “Mean Shift: A Robust Approach toward Feature Space Analysis”, IEEE Transaction on Pattern Analysis and Machine Intelligence, vol. 24, 2002, pp.603-619. 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 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

It is known that a separator with excellent permeability can be obtained by using an inorganic-containing layer containing a polyolefin resin and inorganic particles. However, when inorganic particles are generally used, the pores at the interface peeling portion are greatly enlarged by stretching, making it difficult to provide a separator with high strength and high heat resistance. On the other hand, if the pores are adjusted to be small, it is difficult to maintain high permeability. One of the objects of the present disclosure is to provide a separator for an electricity storage device that has high permeability while also having high strength and high heat resistance.

Examples of embodiments of the present disclosure are listed in the following items [1] to [19].
[1]
A separator for an electricity storage device, comprising an inorganic-containing layer including inorganic particles and a thermoplastic resin including 50% by mass or more of polyolefin,
The separator for an electricity storage device, wherein the inorganic-containing layer has pores, and the MFR of the inorganic-containing layer is 0.05 g/10 min or more and 5 g/10 min or less.
[2]
2. The separator for a power storage device according to item 1, wherein the inorganic-containing layer has pores with an average pore size of 150 nm or more and 2000 nm or less in an MD-ND cross section.
[3]
The inorganic-containing layer has a thermoplastic resin region, an inorganic material region, and a void region in an MD-ND cross section,
3. The separator for an electric storage device according to item 1 or 2, wherein a ratio (L _po /L _pi ) of a length (L _po ) of a boundary portion between the void region and the thermoplastic resin region to a length (L _pi ) of an interface between the thermoplastic resin region and the inorganic material region is 0.2 or more and 3.5 or less.
[4]
A separator for an electricity storage device, comprising an inorganic-containing layer including inorganic particles and a thermoplastic resin including 50% by mass or more of polyolefin,
The inorganic-containing layer has a thermoplastic resin region, an inorganic material region, and a void region in an MD-ND cross section,
A separator for an electricity storage device, wherein the ratio (L _po /L pi ) of the length (L _po ) of the boundary portion between the void region and the thermoplastic resin region to the length (L _pi ) of the interface between the thermoplastic _resin region and the inorganic material region is 0.2 or more and 3.5 or less.
[5]
5. The separator for an electricity storage device according to any one of items 1 to 4, wherein the inorganic particles have a particle size of 60 nm or more and 2000 nm or less.
[6]
6. The separator for an electricity storage device according to any one of items 1 to 5, having an air permeability of 50 sec/100 ml or more and 300 sec/1000 ml or less.
[7]
the electricity storage device separator includes a substrate, the substrate being a microporous layer containing 50% by mass or more of polypropylene;
7. The separator for a storage battery device according to any one of items 1 to 6, wherein the substrate is present on one or both sides of the inorganic-containing layer.
[8]
8. The separator for an electricity storage device according to any one of items 1 to 7, wherein the electricity storage device separator includes a substrate, and a thickness ratio of the inorganic-containing layer to the substrate (thickness of the substrate/thickness of the inorganic-containing layer) is 0.3 or more and 3.0 or less.
[9]
9. The separator for an electricity storage device according to any one of items 1 to 8, wherein the electricity storage device separator comprises a substrate, and the substrate has an MFR of 0.05 g/10 min or more and 0.9 g/10 min or less.
[10]
10. The separator for use in an electricity storage device according to any one of items 1 to 9, wherein the ratio of the tensile strength in MD to the tensile strength in TD (MD/TD) of the separator for use in an electricity storage device is 1.5 or more.
[11]
11. The separator for use in an electricity storage device according to any one of items 1 to 10, having a TD heat shrinkage rate of 3% or less.
[12]
Item 12. The separator for a power storage device according to any one of items 1 to 11, wherein the inorganic-containing layer contains the inorganic particles in an amount of 50% by mass or more and 95% by mass or less, based on the total mass of the inorganic-containing layer.
[13]
Item 13. The separator for an electricity storage device according to any one of items 1 to 12, wherein the separator for an electricity storage device has a puncture strength of 100 gf/14 μm or more per 14 μm of thickness.
[14]
Item 14. The separator for a storage battery device according to any one of items 1 to 13, wherein the inorganic-containing layer contains polyethylene as the polyolefin, and the amount of the polyethylene is 20 mass% or more based on the total mass of the thermoplastic resin of the entire separator for the storage battery device.
[15]
15. The separator for use in an electricity storage device according to any one of items 1 to 14, wherein the polyethylene has a weight average molecular weight of 800,000 or less.
[16]
Item 16. The separator for an electrical storage device according to any one of items 1 to 15, wherein the inorganic-containing layer has a thickness of 1 μm or more and 27 μm or less.
[17]
Item 17. The separator for an electricity storage device according to any one of items 1 to 16, wherein the separator for an electricity storage device has a total thickness of 5 μm or more and 30 μm or less.
[18]
Item 18. The separator for a power storage device according to any one of items 1 to 17, wherein the inorganic-containing layer has pores with an average pore size of 200 nm or more and 2000 nm or less in an MD-ND cross section.
[19]
Item 19. An electricity storage device comprising the separator for an electricity storage device according to any one of items 1 to 18.

The present disclosure can provide a separator for an electricity storage device that has high permeability while also having high strength and high heat resistance.

FIG. 1 is a schematic diagram showing an MD-ND cross section of an inorganic-containing layer in an electricity storage device separator according to the present disclosure. FIG. 2 is a scanning electron microscope (SEM) image of an MD-ND cross section of the inorganic-containing layer in Example 4. FIG. 3 is a scanning electron microscope (SEM) image of an MD-ND cross section of an inorganic-containing layer in Comparative Example 3. FIG. 4 is an example of a luminance histogram (horizontal axis: luminance, vertical axis: frequency) of an image processing area.

<Separator for power storage device>
<Inorganic-containing layer>
The separator for an electric storage device according to the present disclosure includes an inorganic-containing layer that includes inorganic particles and a thermoplastic resin. The inorganic-containing layer is a microporous film that has a plurality of pores and constitutes the separator for an electric storage device. The inorganic-containing layer may be used as a single layer or as a multilayer formed by stacking two or more layers.

<Thermoplastic resin>
The thermoplastic resin contained in the inorganic-containing layer contains 50% by mass or more of polyolefin based on the total mass of the thermoplastic resin. The amount of polyolefin resin in the thermoplastic resin is preferably 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.

The polyolefin is preferably polyethylene. The amount of polyethylene contained in the inorganic-containing layer based on the total mass of the thermoplastic resin contained in the entire separator for a power storage device including the substrate (hereinafter simply referred to as the "total polyethylene amount") is preferably 20% by mass or more. When the total polyethylene amount is 20% by mass or more, a separator for a power storage device that melts at about 100°C to 140°C is obtained, so that the shutdown function can increase the resistance between the electrodes to suppress thermal runaway of the battery, improving the safety of the battery. On the other hand, if the total polyethylene amount is less than 20% by mass, there may be a shortage of melting resin and sufficient resistance increase may not be possible. In addition, by containing 20% by mass or more of polyethylene on a total basis, the crystal size of the polyethylene tends to become large, and large openings are formed when the lamellar opening is opened, and the openings are connected to each other, so that a separator for a power storage device with high permeability tends to be obtained. On the other hand, if the proportion of polyethylene is less than 20% by mass on a total basis, the proportion of large openings when the lamellar opening is opened is small, so sufficient permeability tends not to be obtained.

The lower limit of the total polyethylene content may more preferably be 20% by mass or more, 25% by mass or more, 30% by mass or more, or 35% by mass or more. The upper limit of the total polyethylene content may preferably be less than 55% by mass, 50% by mass or less, 45% by mass or less, 40% by mass or less, or 15% by mass or less.

Examples of polyethylene include low density polyethylene (LDPE), medium density polyethylene (MDPE), and high density polyethylene, with high density polyethylene (HDPE) being more preferred from the viewpoint of increasing permeability.

The thermoplastic resin may contain a thermoplastic resin other than polyethylene. The other thermoplastic resin is preferably a polyolefin other than polyethylene. A polyolefin is a polymer containing a monomer having a carbon-carbon double bond as a repeating unit. Monomers constituting a polyolefin include, but are not limited to, monomers having 3 to 10 carbon atoms and having a carbon-carbon double bond, such as 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.

As polyolefins other than polyethylene, specifically, polypropylene and copolymers of polyethylene and polypropylene are preferred from the viewpoint of shutdown characteristics, etc.

The amount of thermoplastic resin contained in the inorganic-containing layer is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, based on the total mass of the inorganic-containing layer, and is preferably 50% by mass or less, more preferably 30% by mass or less, even more preferably 25% by mass or less. When the amount of thermoplastic resin based on the total mass of the inorganic-containing layer is within this range, a separator with high strength while having high permeability is easily obtained.

The weight average molecular weight of polyethylene calculated in terms of styrene is preferably 800,000 or less, more preferably 70 or less, and even more preferably 650,000 or less, and is preferably 100,000 or more, more preferably 200,000 or more, and even more preferably 300,000 or more. When the weight average molecular weight of polyethylene is 800,000 or less, the inorganic particles can be kneaded more uniformly. This allows the inorganic particles to be dispersed in the thermoplastic resin to obtain a more uniform inorganic-containing layer, resulting in a higher interelectrode resistance suppression effect. In addition, when the weight average molecular weight of polyethylene is 800,000 or less, the significant variation in inorganic particle concentration in the inorganic-containing layer is suppressed, and the inorganic-containing layer becomes a resistive component as a whole during high-temperature melting, making it easier to maintain the interelectrode resistance. Furthermore, when the weight average molecular weight of polyethylene is 800,000 or less, the pressure during extrusion is kept low from the viewpoint of film formation stability during extrusion, making film formation easier. On the other hand, as for the lower limit, when the weight average molecular weight of polyethylene is 100,000 or more, the self-supporting property of the molten film is increased, making it easier to wind it up. In addition, when the weight average molecular weight of the polyethylene is 100,000 or more, the degree of orientation is improved, lamellar pores are easily formed, and permeability is increased, which is preferable.

<Melt Flow Rate (MFR)>
The MFR of the inorganic-containing layer containing inorganic particles and thermoplastic resin is 0.05 g/10 min or more and 5 g/10 min or less, preferably 0.1 g/10 min or more and 2.5 g/10 min or less, more preferably 0.2 g/10 min or more and 1.0 g/10 min or less. It is believed that the MFR of the inorganic-containing layer is 0.05 g/10 min or more, and when the thermoplastic resin and the inorganic particles are kneaded under high shear conditions using a twin-screw extruder or the like, they can be kneaded uniformly, and the inorganic particles are dispersed to form a uniform separator, thereby enhancing the effect of suppressing the resistance between electrodes. When the MFR of the inorganic-containing layer is 5 g/10 min or less, a high MD orientation can be imparted by blowing a large amount of cold air during extrusion, which makes it easier to open lamellae and easily obtain an inorganic-containing layer with high permeability.

The melt flow rate (MFR) (single layer MFR) of the thermoplastic resin contained in the inorganic-containing layer is preferably 0.2 or more and 15 or less when measured under a load of 2.16 kg and at a temperature of 190°C for polyethylene and at a temperature of 230°C for polypropylene. It is preferably 0.25 or more, 0.30 or more, and preferably 5.00 or less, 1.00 or less. The reason is not limited to theory, but it is thought that when a thermoplastic resin with a high MFR is kneaded with inorganic particles, uniform kneading is possible, the inorganic particles are dispersed to form a uniform separator, and the effect of suppressing the resistance between electrodes is high. On the other hand, if the MFR is too low, the inorganic particles do not disperse, and a significantly low concentration part of the inorganic particles occurs in the separator, and the resistance component is locally insufficient during high-temperature melting, so that the resistance between electrodes tends to be unable to be maintained. In addition, from the viewpoint of film formation stability during extrusion, if the MFR is 0.2 or more, the pressure during extrusion does not increase extremely, making it easier to form a film. When the MFR is 15 or less, the molten membrane tends to have high self-supporting property, and is easy to wind up and form into a membrane. Also, when the MFR is 15 or less, the decrease in the degree of orientation can be suppressed, lamellar pores can be easily opened, and the permeability can be increased.

<Inorganic particles>
The inorganic-containing layer contains inorganic particles (also called "inorganic filler.") When the inorganic-containing layer contains inorganic particles, a separator having high heat resistance can be obtained.

Examples of inorganic particles 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 safety and heat resistance in the battery, at least one selected from the group consisting of barium sulfate, titania, alumina, and boehmite is preferable.

The particle diameter of the inorganic particles is preferably 60 nm or more and 2000 nm or less, the lower limit is more preferably 150 nm or more, and even more preferably 230 nm or more, and the upper limit is more preferably 1000 nm or less, and even more preferably 800 nm or less. When the particle diameter of the inorganic particles is 60 nm or more, the particles are less likely to migrate into the gap between the electrodes during high-temperature melting, and the inorganic particles become a resistance component by remaining between the electrodes, making it easier to maintain the interelectrode resistance. In addition, when the particle diameter is 60 nm or more, the inorganic particles are easily dispersed in the inorganic-containing layer, and the significant variation in the inorganic particle concentration in the inorganic-containing layer is suppressed, and the inorganic-containing layer becomes a resistance component overall during high-temperature melting, making it easier to maintain the interelectrode resistance. When the particle diameter of the inorganic particles is 2000 nm or less, the inorganic particles are suppressed from becoming the starting point of stress concentration, and the strength tends to improve. In addition, when the particle diameter of the inorganic particles is 2000 nm or less, the variation in the thickness of the separator is suppressed, and the quality is improved.

The content of inorganic particles in the inorganic-containing layer is preferably 50% by mass or more and 95% by mass or less, based on the total mass of the inorganic-containing layer, and the lower limit is more preferably 65% by mass or more, and even more preferably 72% by mass or more. When the content of inorganic particles in the inorganic-containing layer is 50% by mass or more, a layer having inorganic particles and lamellar crystals can be formed by melt extrusion at a high drawdown ratio, and then uniaxial stretching is performed, making it easier to obtain a separator with low porosity and high permeability. The reason for this is not limited to theory, but when the amount of inorganic particles is 50% by mass or more, both lamellar openings perpendicular to the stretching direction and filler openings horizontal to the stretching direction occur, and the vertical openings formed by the lamellar crystals connect the independent horizontal openings formed by the filler openings, so that it is thought that low porosity and high permeability can be achieved even at a small stretching ratio. Furthermore, if the content of inorganic particles in the inorganic-containing layer is 95 mass % or less, the risk of film breakage is low and the thickness uniformity during film formation is good.

Surface Treatment of Inorganic Particles
In order to improve the dispersibility in the thermoplastic resin, it is preferable to use inorganic particles that have been surface-treated with a surface treatment agent. Examples of the surface treatment agent include treatment with saturated fatty acid and/or its salt (saturated fatty acid salt), unsaturated fatty acid and/or its salt (unsaturated fatty acid salt), aluminum-based coupling agent, polysiloxane, silane coupling agent, etc. From the viewpoint of dispersibility in the thermoplastic resin, the surface treatment agent is preferably a saturated fatty acid and its salt, an unsaturated fatty acid and its salt, and more preferably a saturated fatty acid and its salt having 8 or more carbon atoms, an unsaturated fatty acid and its salt having 8 or more carbon atoms. The surface treatment agent may be applied before kneading the thermoplastic resin, or may be added after kneading. From the viewpoint of uniform coating of the surface treatment agent on the inorganic particle surface, it is preferable to treat before kneading. By treating with these surface treatment agents, the inorganic particles are highly dispersed in the resin, and higher heat resistance can be achieved.

The surface hydrophilicity of the inorganic particles is preferably 0.1 or more and 0.8 or less. When the surface hydrophilicity is 0.8 or less, the inorganic particles are better dispersed in the thermoplastic resin and aggregation can be suppressed. When the surface hydrophilicity of the inorganic particles is 0.1 or more, the affinity for the electrolyte is increased and the ionic conductivity tends to improve.

The amount of surface treatment of inorganic particles depends on the particle size of the inorganic particles, 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 particles. 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.

<Weight of inorganic particles>
The basis weight of the inorganic particles is preferably 0.15 mg/ ^cm2 or more. When the basis weight is 0.15 mg/ ^cm2 or more, the inorganic particles are less likely to migrate to the interelectrode gap together with the resin when the separator is melted at high temperature, and the inorganic particles remain as an inorganic particle layer that becomes a resistance component between the electrodes, so that a dense inorganic particle layer tends to be formed. The inorganic particles become a resistance component, which can maintain the interelectrode resistance, and the safety of the battery tends to be improved.

Hole
The inorganic-containing layer has holes. In the present specification, "hole" means a hole (also called "open hole") in a microporous film. More specifically, the inorganic-containing layer is a microporous film having a plurality of open holes. In the MD-ND cross section (plane direction cross section), the outline of the hole (open hole) is formed only from the thermoplastic resin, or from the thermoplastic resin and inorganic particles. In the MD-ND cross section of the inorganic-containing layer, the ratio (L po /L pi ) of the length (L po ) of the boundary part between the hole part (hereinafter referred to as the "open hole region") and the thermoplastic resin part (hereinafter referred to as the "thermoplastic resin region") to the length (L _pi ) of the interface between the thermoplastic _resin region and the inorganic material part (hereinafter referred to as _the "inorganic material region") is preferably 0.2 or more _and 3.5 or less. That is, it is preferable that the ratio (L _po /L _pi ) of the length (L _po ) of the portion of the contour formed by the thermoplastic resin to the length (L _pi ) of the interface between the thermoplastic resin and the inorganic particles is 0.2 or more and 3.5 or less.

When the amount of the thermoplastic resin covered by the inorganic particles increases, L _pi increases. When the amount of the thermoplastic resin covered by the inorganic component increases, it is possible to maintain the pore structure even at high temperatures when the thermoplastic resin melts, and high heat resistance is easily obtained. On the other hand, when the amount of the thermoplastic resin exposed to the pores (pore space) increases, L _po increases. When the amount of the thermoplastic resin exposed to the pores increases, the thermoplastic resin comes into contact with the pores more, so that the connectivity between the pores increases and high permeability is easily obtained. Therefore, the preferred range of the ratio (L _po /L _pi ) shows the preferred range for showing both high heat resistance and high permeability of the inorganic particle interface when viewed from the thermoplastic resin, and the pore interface when viewed from the thermoplastic resin. Since the above-mentioned effects are shown regardless of the pore size and particle diameter, it is appropriate to express it as the ratio (L _po /L _pi ).

When the ratio (L _po /L _pi ) is 0.2 or more, the openings made by the inorganic particles and the openings made by the lamellae are simultaneously formed, which makes the pores larger and forms a connection structure between the openings made by the inorganic particles and the openings made by the lamellae, which is considered to be easy to obtain high permeability. It is considered that the connection between the horizontal (stretching direction) holes derived from the filler openings and the vertical holes made by the lamellae openings is increased, which makes it easy to obtain high permeability. A method of adjusting the ratio (L _po /L _pi ) to 0.2 or more, that is, a method of adjusting L _po to be large relative to L _pi , is to melt extrude at a high drawdown ratio in MD, and then apply extremely strong cold air after melt extrusion to rapidly cool. When adjusted in this way, it is easier to simultaneously cause openings by the inorganic particles as well as openings by the lamellae in the later stretching, so that the ratio (L _po /L _pi ) is easily adjusted to 0.2 or more. On the other hand, if melt extrusion is not performed at a high drawdown ratio in the MD, or if melt extrusion is performed at a high drawdown ratio in the MD but the cooling efficiency is poor due to a small amount of cold air, etc., there will be almost no lamellae opening, and it will be difficult to adjust the ratio (L _po /L _pi ) to 0.2 or more.

The ratio ( _Lpo / _Lpi ) can also be adjusted by the composition of the thermoplastic resin, for example, by using a highly crystalline thermoplastic resin such as polypropylene having high stereoregularity, or by adding a crystal nucleating agent to promote lamellar opening and adjust the ratio to 0.2 or more.

When the ratio (L _po /L _pi ) is 3.5 or less, the pores of the separator are covered with inorganic components made of inorganic particles, so that the pore structure is maintained even at high temperatures, making it easy to maintain insulation and obtain high heat resistance. A method of adjusting the ratio (L _po /L _pi ) to 3.5 or less, that is, a method of adjusting L _pi to be large relative to L _po , includes melt extrusion at a high drawdown ratio in MD, followed by lowering the total stretch ratio in MD and TD and lowering the stretch ratio in TD. By adjusting in this way, the interface between the thermoplastic resin and the inorganic particles can be maintained without being destroyed, and as a result, the length (L _pi ) of the interface between the thermoplastic resin and the inorganic particles tends to be large, and it is easy to adjust the ratio (L _po /L _pi ) to 3.5 or less. On the other hand, if melt extrusion is not performed with a high drawdown ratio in MD, or if the total stretch ratio in MD and TD is not high and the stretch ratio in TD is not small, lamellar crystals are not obtained sufficiently, and the opening origin of the filler alone is enlarged to form holes, destroying the interface between the thermoplastic resin and the inorganic particles, making it difficult to adjust the ratio (L _po /L _pi ) to 3.5 or less. Even when liquid paraffin is introduced and a wet method is used for preparation, the interface between the thermoplastic resin and the inorganic particles is divided by phase separation, making it difficult to adjust the ratio (L _po /L _pi ) to 3.5 or less.

Regarding adjustments to the composition, one option is to adjust the ratio to 3.5 or less by adding a portion of a thermoplastic resin modified with a polar functional group that is thought to have a high affinity with inorganic particles, such as an acid-modified polyolefin.

The lower limit of the ratio (L _po /L _pi ) is preferably 0.3 or more, more preferably 0.4 or more, and even more preferably 0.5 or more, and the upper limit is preferably 3.0 or less, more preferably 2.5 or less, and even more preferably 2.0 or less.

FIG. 1 is a schematic diagram showing an MD-ND cross section of an inorganic-containing layer in a separator for an electric storage device according to the present disclosure. When the MD-ND cross section (10) of the inorganic-containing layer is observed with a scanning electron microscope, the presence of inorganic particles (1), thermoplastic resin (2) and pores (3) can be confirmed. The inorganic particles (1) are the "inorganic material region", the thermoplastic resin (2) is the "thermoplastic resin region", and the pores (3) are the "void region". The contour of the pores (3) shown in FIG. 1 is composed of a contour (4) formed from the thermoplastic resin shown by a solid line, and a contour (5) formed from the inorganic particles shown by a dotted circle line. The interface between the thermoplastic resin and the inorganic particles is shown by a dotted square line (6). In this case, the length of the solid line (4) corresponds to the "length of the boundary between the void region and the thermoplastic resin region (L _po )," and the length of the dotted line (6) corresponds to the "length of the interface between the thermoplastic resin region and the inorganic material region (L _pi )," and the ratio thereof (L _po /L _pi ) is 0.2 or more and 3.5 or less.

The average pore size of the inorganic-containing layer is preferably 150 nm or more and 2000 nm or less, more preferably 200 nm or more and 2000 nm or less, even more preferably 200 nm or more and 1500 nm or less, and even more preferably 250 nm or more and 1000 nm or less, in the MD-ND cross section. The reason is not limited to theory, but when the average pore size of the inorganic-containing layer is 150 nm or more, the open pores tend to be connected to each other and have high permeability. In addition, when the average pore size is 2000 nm or less, it is unlikely to become a starting point of stress concentration with respect to the thickness of the inorganic-containing layer, and it is thought that the strength of the separator is unlikely to decrease. The average pore size of the inorganic-containing layer can be measured by observing the MD-ND cross section of the inorganic-containing layer with a scanning electron microscope (SEM), as described later in the example section.

It is preferable that the long hole diameters of the inorganic-containing layer are arranged in one direction. In this specification, the "long hole diameter" refers to the longest line segment among the line segments connecting any two points on the contour of the opening. Although not limited by theory, if the long hole diameters are arranged in one direction, the separator is strongly oriented, the strength in the long hole diameter direction is increased, and the battery winding operation is easier, which is preferable. The arrangement of the long hole diameters can be confirmed by observing the cross section of the inorganic-containing layer with a scanning electron microscope (SEM), as described later in the Examples section. "Aligned in one direction" means that in the above electron microscope image of the separator surface, 90% or more of the fibrils are included within an angle range of ±20 degrees in the extension direction. In other words, if the long axes of 90% or more of the holes are included within an angle range of ±20 degrees from each other in the above electron microscope image, the arrangement of the long hole diameters is determined to be "aligned in one direction".

The area ratio of the open pores to the MD-ND cross-sectional area of the inorganic-containing layer 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 making the area ratio of the open pores 60% or less, the ratio of resin and inorganic particles can be increased, and a network structure of the solid content is formed, so that the elongation at the time of piercing is improved and high piercing strength is easily obtained. In addition, if an attempt is made to increase the open pore ratio by increasing the MD stretching ratio in order to increase the pore ratio, the pores tend to be crushed in the thickness direction, and the connection structure between the pores tends to be broken, resulting in a deterioration in air permeability. When the area ratio of the open pores is 20% or more, the low ratio of resin and inorganic particles makes it easy to form a connection structure between the pores, so high permeability is obtained.

<Elastomer>
The inorganic-containing layer may further contain an elastomer in addition to the inorganic particles. Examples of the elastomer include thermoplastic elastomers and thermosetting elastomers, and preferably thermoplastic elastomers. In the present specification, the thermoplastic elastomer is included in the thermoplastic resin. By containing a thermoplastic elastomer in the inorganic-containing layer, the melt tension can be reduced without impairing the balance between strength and air permeability, so that even a thermoplastic resin with a low MFR and a high melt tension can be thinned, and a thin, high-strength separator can be obtained.

<Thickness of inorganic-containing layer>
The thickness of the inorganic-containing layer is preferably 1 μm or more and 27 μm or less, more preferably 1 μm or more and 20 μm or less. When the thickness is 1 μm or more, the heat resistance of the separator for the electric storage device is improved. When the thickness is 27 μm or less, the energy density of the electric storage device can be further increased. In consideration of the heat resistance and the 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 addition, in consideration of the heat resistance, ion permeability, and physical strength of the separator, the ratio of the thickness of the inorganic-containing layer to the thickness of the entire 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.

<Substrate>
The separator for an electric storage device of the present disclosure may include a substrate in addition to the inorganic-containing layer. The substrate may be present on one or both sides of the inorganic-containing layer. That is, the separator for an electric storage device may have a two-layer structure in which a substrate is laminated on one side of the inorganic-containing layer, a multilayer structure in which substrates are laminated (as outer layers) on both sides of the inorganic-containing layer, or a multilayer structure of three or more layers in which inorganic-containing layers are laminated on both sides of the substrate.

The substrate is preferably a microporous layer (hereinafter also referred to as "PP microporous layer") containing 50% by mass or more of polypropylene based on the total mass of the substrate. The amount of polypropylene in the substrate is preferably 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. The substrate can be present on one or both sides of the inorganic-containing layer. That is, the separator for the electric storage device may have a two-layer structure in which the substrate is laminated on one side of the inorganic-containing layer, or may have a multi-layer structure of three or more layers in which the substrate is laminated on both sides of the inorganic-containing layer. Whether the separator for the electric storage device has a substrate as an outer layer or a substrate as an inner layer, the MFR of the substrate is preferably 0.05 g/10 min or more and 0.9 g/10 min or less. When the MFR of the outer layer is 0.05 g/10 min or more, it is easy to form a membrane with a uniform thickness. When the MFR of the outer layer is 0.9 g/10 min or less, high permeability is easily obtained, and the mechanical strength of the separator is increased.

The multilayer structure can exhibit the effects of the present invention regardless of the order in which the layers are laminated, but a three-layer structure in which the layers are laminated in the order of PP microporous layer/inorganic-containing layer/PP microporous layer is particularly preferable. By having a three-layer structure of PP microporous layer/inorganic-containing layer/PP microporous layer, the heat resistance properties of the inorganic-containing layer can be provided, while the mechanical strength and oxidation resistance at the time of electrode surface contact can be maintained by the PP microporous layer. The thickness ratio of the inorganic-containing layer to the substrate (substrate thickness/inorganic-containing layer thickness) is preferably 0.3 to 3.0. By making the thickness ratio of the inorganic layer to the substrate 0.3 or more, the interelectrode resistance can be maintained when the separator is melted, and the safety of the battery is improved. When the thickness ratio of the inorganic layer to the substrate is 3.0 times or less, the film formation stability is improved and film formation is easy.

<Separator thickness>
The total thickness of the separator for the electric 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 the electric storage device is improved. When the thickness is 30 μm or less, the energy density of the electric storage device can be increased.

<Air permeability of separator>
The upper limit of the air permeability (also referred to as "air permeability resistance") of the electricity storage device separator is preferably 1000 sec/100 ml or less, 800 sec/100 ml or less, 600 sec/100 ml or less, 500 sec/100 ml or less, 400 sec/100 ml or less, 300 sec/100 ml or less, 250 sec/100 ml or less, or 200 sec/100 ml or less, and the lower limit may be preferably 50 sec/100 ml or more, 80 sec/100 ml or more, or 100 sec/100 ml or more.

<Separator puncture strength>
The lower limit of the puncture strength of the separator for an electric storage device may be 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.

<MD/TD strength ratio of separator>
The ratio of the tensile strength in the MD to the tensile strength in the TD of the separator for an electric storage device (also referred to as the "MD/TD strength ratio") is preferably 1.5 or more, more preferably 6.0 or more, and even more preferably 8.0 or more. When the MD/TD strength ratio is 1.5 or more, the separator is strongly oriented, so that the strength in the MD is increased, and the battery winding operation is easy. In addition, when the MD/TD strength ratio is 1.5 or more, the thermal shrinkage rate of the TD can be reduced, so that the resistance to short circuits caused by thermal shrinkage of the TD during battery winding is increased. 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 even more preferably 15 or less. When the MD/TD strength ratio is 30 or less, cracks are unlikely to occur in the MD, and the separator is unlikely to tear vertically (in the MD) during handling.

<Thermal shrinkage rate of separator>
The heat shrinkage rate of the TD of the separator for an electricity storage device is preferably 3% or less, more preferably 1% or less. When the heat shrinkage rate of the TD is 1% or less, resistance to short circuit caused by heat shrinkage of the TD during winding of the battery is increased.

<<Method for producing separator for power storage device>>
A method for producing an inorganic-containing layer containing a thermoplastic resin and inorganic particles generally includes a melt extrusion step of mixing and dispersing a thermoplastic resin composition and inorganic particles, and melt extruding the mixture to obtain a resin film, and a hole forming step of opening holes in the obtained resin sheet to make it porous, and optionally further includes a stretching step, a heat treatment step, etc. The method for producing an inorganic-containing layer is roughly divided into a dry method in which no solvent is used in the hole forming step, and a wet method in which a solvent is used.

Dry methods include a method in which a thermoplastic resin composition and inorganic particles are mixed and dispersed in a dry state, melt-kneaded, and extruded, and then the thermoplastic resin crystal interface is peeled off by heat treatment and stretching; and a method in which a thermoplastic resin composition and inorganic filler are melt-kneaded and formed into a sheet, and then the interface between the thermoplastic resin and the inorganic filler is peeled off by stretching.

Examples of wet methods include a method in which a thermoplastic resin composition is mixed and dispersed with inorganic particles, a pore-forming material is added, the mixture is melt-kneaded and formed into a sheet, which is stretched as necessary, and then the pore-forming material is extracted; and a method in which the thermoplastic resin composition is dissolved and then immersed in a poor solvent for the thermoplastic resin to solidify the thermoplastic resin and simultaneously remove the solvent.

A single-screw extruder or a twin-screw extruder can be used for melt-kneading the thermoplastic resin composition. In addition to these, other devices such as a kneader, a lab plast mill, a kneading roll, and a Banbury mixer can also be used.

The thermoplastic resin composition may optionally contain resins other than polyolefins and additives, etc., depending on the manufacturing method of the inorganic-containing layer or the physical properties of the intended inorganic-containing layer. 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 color pigments. Examples of pore-forming materials include plasticizers, inorganic fillers, or combinations thereof.

Examples of plasticizers 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.

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, uniaxial stretching is preferred from the viewpoint of manufacturing costs when using a dry method. By using the lamellar hole opening method by uniaxial stretching, the separator for the power storage device of the present disclosure having high permeability, high strength and heat resistance can be more easily manufactured as described below. This will be described below. This hole opening method is generally a method in which a precursor (original film) in which lamellar crystals are oriented by melt extrusion is obtained, and this is cold stretched and then hot stretched to open holes between the lamellar crystals. By mixing a high content of inorganic particles into a thermoplastic resin, melt extruding this in the MD at a high drawdown ratio, and increasing the cooling rate, it is possible to obtain a precursor (original film) before the hole formation step in which a high content of inorganic particles is densely arranged in a resin having highly oriented lamellar crystals. As a result, it is easier to simultaneously cause not only the opening of holes by inorganic particles but also the opening of holes by lamellae in the subsequent stretching. As a result, the length (L _po ) of the interface between the thermoplastic resin and the pores tends to be large, and it is easy to adjust the ratio (L _po /L _pi ) to 0.2 or more. In addition, the stress concentration caused by the uniaxial stretching promotes the formation of holes, and even if the total stretching ratio in MD and TD is low and the stretching ratio in TD is low, the film has high permeability and the MD/TD strength ratio can be easily adjusted to 1.5 or more. In this way, the effect of the present invention can be more easily obtained by the lamellar opening method using uniaxial stretching under specific conditions. The reason is unclear, but the inventors have speculated as follows. That is, inorganic particles have a larger heat capacity than resin, so they are cooled slowly, and under general cooling conditions, they are difficult to crystallize in MD, and hole formation is inhibited in the subsequent uniaxial stretching, and high permeability tends not to be obtained. Therefore, it is preferable to increase the cooling speed of the raw film containing inorganic particles, more specifically, to increase the air knife volume used for cooling and to quickly cool the resin temperature of the raw film containing inorganic particles, thereby strongly orienting the lamellar crystals in the MD. This makes it possible to obtain a raw film in which a high content of inorganic particles and a resin having lamellar crystals are densely arranged, and it is presumed that a separator for an electricity storage device can be obtained that can simultaneously achieve high permeability, high strength, and heat resistance by promoting hole formation even at a small stretch ratio.

<Ratio of resin and inorganic materials>
The content of inorganic particles is preferably 100 parts by weight or more, more preferably 200 parts by weight or more, and even more preferably 300 parts by weight or more, based on 100 parts by weight of thermoplastic resin. If the content of inorganic particles is 100 parts by weight or more relative to a crystalline resin such as polyolefin, in one embodiment, a layer having inorganic particles and lamellar crystals can be formed by melt extrusion at a high drawdown ratio, and then uniaxial stretching is performed to adjust the ratio (L _po /L _pi ) to 0.2 or more, thereby obtaining a separator with low porosity but high permeability. The reason for this is not limited to theory, but when the amount of inorganic particles is large, both lamellar openings perpendicular to the stretching direction and filler openings horizontal to the stretching direction occur, and the vertical pores formed by the lamellar crystals connect the horizontal independent pores formed by the filler openings, so that even at a small stretching ratio, it is presumed that low porosity and high permeability are compatible. On the other hand, when the inorganic particle content is low, the distance between the pores formed by the filler pores becomes large, making it difficult for the pores to be connected by the filler pores through the lamellar pores, and it is presumed that high permeability cannot be obtained.

The content of inorganic particles in the inorganic-containing layer is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, based on the total mass of the inorganic-containing layer, and is preferably 95% by mass or less, and more preferably 90% by mass or less. If the content of inorganic particles is 50% by mass or more based on the total mass of the inorganic-containing layer, a separator with low porosity and high permeability can be obtained for the same reasons as above, which is preferable.

In order to suppress shrinkage of the microporous film, a heat treatment step may be performed for the purpose of heat setting after the stretching step or the hole formation step. 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. A relaxation operation may be performed after the stretching operation. These heat treatment steps may be performed using a tenter or roll stretching machine.

Examples of methods for producing a separator for a storage device having a multilayer structure in which multiple microporous films including inorganic-containing layers are laminated include a co-extrusion method and a lamination method. In the co-extrusion method, the resin compositions of each layer are simultaneously co-extruded to form a film, and the resulting multilayer raw film is stretched and perforated to produce a multilayer microporous film. In the lamination method, each layer is separately extrusion-formed to obtain a raw film. The resulting raw film is laminated to obtain a multilayer raw film, and the resulting multilayer raw film is stretched and perforated to produce a multilayer microporous film. The co-extrusion method is preferable in that the inorganic-containing layer can be supported by a layer that does not contain inorganic particles, improving film formation stability and allowing a large inorganic particle content.

<Electricity 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.

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-hydrogen 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, lithium ion capacitors, and nickel-hydrogen batteries are preferred, lithium ion secondary batteries and lithium ion capacitors are more preferred, and lithium ion secondary batteries are even more preferred.

The energy storage device can be produced, for example, by stacking positive and negative electrodes with the separator described above between them and winding them as necessary to form a laminated 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 a lead body 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.

Measurement and evaluation methods
[Melt flow rate (MFR)]
The MFR of polyolefin was measured at a temperature of 230° C. and a load of 2.16 kg (unit: g/10 min) in accordance with JIS K 7210. The MFR of polypropylene was measured at a temperature of 230° C. and a load of 2.16 kg in accordance with JIS K 7210. The MFR of polyethylene was measured at a temperature of 190° C. and a load of 2.16 kg in accordance with JIS K 7210. The MFR of elastomer was measured at a temperature of 230° C. and a load of 2.16 kg in accordance with JIS K 7210.

[Ratio ( _Lpo / _Lpi )]
Preparation of Samples A separator and ruthenium tetroxide (manufactured by Rare Metallic Co., Ltd.) were placed in a sealed container and dyed with steam for 4 hours to prepare a separator dyed with ruthenium. (manufactured by Nisshin EM Co., Ltd.) 10.6 mL, a curing agent (methylnadic anhydride (MNA), manufactured by Nisshin EM Co., Ltd.) 9.4 mL, and a reaction accelerator (2,4,6-tris(dimethylaminomethyl)phenol, manufactured by Nisshin EM Co., Ltd.) The separator dyed with ruthenium was immersed in the mixture and placed under reduced pressure to separate the ruthenium from the separator. The pores of the dyed separator were thoroughly impregnated with the mixture. After impregnation, the separator was cured at 60° C. for 12 hours or more, so that the ruthenium-dyed separator was embedded in the epoxy resin. The internal voids were filled with epoxy resin and allowed to harden. After embedding in epoxy resin, the cross section was roughly processed with a razor or the like, and then the cross section was milled using an ion milling device (E-3500 Plus, manufactured by Hitachi High-Tech Corporation) to prepare a smooth cross section. The cross section was processed to have an MD-ND plane. The obtained cross section sample was fixed to a SEM sample stage for cross section observation with a conductive adhesive (carbon-based), dried, and then subjected to conductive treatment using an osmium coater (HPC-30W Using a microscope (manufactured by Vacuum Devices Co., Ltd.), osmium coating was carried out under the conditions of an applied voltage adjustment knob setting of 4.5 and a discharge time of 0.5 seconds, to prepare a specimen for microscopic examination.

Acquisition of SEM images The above-mentioned microscopic sample was observed with a scanning electron microscope (S-4800, manufactured by Hitachi High-Tech Corporation) under the following conditions: acceleration voltage 1.0 kV, emission current 10 μA, probe current High, detector Upper + LA-BSE100, magnification 10,000 times, pixel number 1280 × 960 (about 10 nm / pixel), and working distance 2.0 mm. The observation field was determined so that the entire thickness direction of the layer containing the inorganic filler was within the observation field, and the brightness value was set so that it was not saturated and the contrast was as high as possible, and an electron microscope image was obtained as an 8-bit grayscale image. However, 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 contained in the entire observation field.

Image Processing (Calculation of Ratio ( _Lpo / _Lpi ))
In the obtained electron microscope image, epoxy resin, polyolefin, and inorganic filler were observed. When analyzing the interface length of each of these epoxy resin, polyolefin, and inorganic filler, if a threshold value is set and ternary is performed, a part of the interface between the lowest brightness area (epoxy resin) and the highest brightness area (inorganic filler) may be recognized as an area of medium brightness (polyolefin), and the analysis may be performed as if polyolefin exists between the epoxy resin and the inorganic filler. Therefore, the interface length was analyzed using the watershed algorithm described in Non-Patent Document 1 according to the procedure shown below.
(1) In an electron microscope image, a region that includes 80% or more of the thickness of the layer containing the inorganic filler, does not include any region other than the layer containing the inorganic filler, and is as large as possible is selected as the image processing region. When the thickness of the layer containing the inorganic filler is larger than the observation field, the entire observation field is used as the image processing region.
(2) In the electron microscope image, three materials, epoxy resin, polyolefin, and inorganic filler, are observed with different brightnesses, and when the brightness histogram of the image processing area is calculated, three peaks corresponding to epoxy resin, polyolefin, and inorganic filler are present. The difference in brightness between the two points that have the maximum values of the two closest peaks among these three peaks is calculated, and this value is taken as CD (Figure 4). At this time, if the value of CD cannot be uniquely determined, the value of CD is determined by fitting each of the three peaks with a Gaussian function.
(3) Execute the mean shift filtering described in Non-Patent Document 2 to obtain a smoothed image. To execute the mean shift filtering, use the Mean Shift Filter (an open source plug-in created by Kai Uwe Barthel of FHTW-Berlin) of the free software ImageJ, and execute the filtering with the arguments Spatial radius set to 3 and Color distance set to CD.
(4) For the smoothed image thus obtained, two thresholds are determined using the algorithm of Non-Patent Document 3, and the smaller of the two thresholds is designated as threshold A and the larger of the two thresholds is designated as threshold B.
(5) Create a binary image A1 in which the brightness of pixels with a brightness less than threshold A is set to 255 and the brightness of pixels with a brightness equal to or greater than threshold A is set to 0; a binary image A2 in which the brightness of pixels with a brightness equal to or greater than threshold A and less than threshold B is set to 255 and the brightness of pixels with a brightness less than threshold A and equal to or greater than threshold B is set to 0; and a binary image A3 in which the brightness of pixels with a brightness equal to or greater than threshold B is set to 255 and the brightness of pixels with a brightness less than threshold B is set to 0.
(6) A 5 x 5 square kernel is used to perform a shrinkage process on each of binary images A1, A2, and A3 (if there is even one pixel with a brightness of 0 within a 5 pixel x 5 pixel area centered on the pixel of interest, the brightness of the pixel of interest is replaced with 0, and this is performed on all pixels in the image), and the binary images A1, A2, and A3 after the shrinkage process are designated as binary images B1, B2, and B3, respectively.
(7) Based on the algorithm of Non-Patent Document 1, watersheds are searched for in the image processing area. In this case, the background is the area in binary image B1 with a brightness of 255, and binary images B2 and B3 are used as markers. Specifically, an area in binary image B2 and binary image B3 where pixels with a brightness of 255 are consecutive is regarded as one marker, and the search for watersheds is performed using all markers in binary image B2 and binary image B3.
(8) For binary images B1, B2, and B3, if a region is surrounded by pixels corresponding to a watershed and contains at least one pixel with a brightness of 255, a process is performed to replace the brightness of all pixels in that region with 255. The images after the process are designated as binary images C1, C2, and C3, respectively.
(9) An expansion process is performed on binary images C1, C2, and C3 using a 3 x 3 cross kernel (if there is even one pixel with a brightness of 255 among the four pixels above, below, left, and right of the pixel of interest, the brightness of the pixel of interest is replaced with 255, and this process is performed on all pixels in the binary image), and the binary images after the expansion process are designated as binary images D1, D2, and D3, respectively.
(10) Create a numerical matrix E1 by replacing the numerical values of pixels in the binary image D1 with a brightness of 255 with 1, a numerical matrix E2 by replacing the numerical values of pixels in the binary image D2 with a brightness of 255 with 10, and a numerical matrix E3 by replacing the numerical values of pixels in the binary image D3 with a brightness of 255 with 100.
(11) Let F be the sum of the numerical matrix E1, the numerical matrix E2, and the numerical matrix E3.
(12) Among the components of the numerical matrix F, the number of components whose numerical values are 11 or 111 is defined as G1, the number of components whose numerical values are 101 or 111 is defined as G2, and the number of components whose numerical values are 110 or 111 is defined as G3.
(13) Taking into account the pixel size of the electron microscope image, G1, G2, and G3 are converted into lengths in real space and regarded as the interface length _Lpo between the pores and the polyolefin, and the interface length _Lpi between the polyolefin and the inorganic filler, respectively.

Image processing (calculation of average particle size of inorganic filler)
(1) A binary image is created from the smoothed image in which the brightness of pixels whose brightness is less than a threshold value B is set to 0 and the brightness of pixels whose brightness is equal to or greater than the threshold value B is set to 255.
(2) Using the free software ImageJ, perform thickness analysis of BoneJ (an open source plug-in described in Non-Patent Document 4), read the average value of Tb. Th in the Results window, and convert the value into a length in real space taking into account the pixel size of the electron microscope image to determine the average particle diameter of the inorganic filler.

[Average pore size]
Image processing (calculation of aperture size)
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 value of Tb.Th mean in the Results window was read and the value converted to the length in real space was used as the average pore size of the pores.

[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 in the SEM image and measuring the length between the lines.

[Air permeability resistance (sec/100ml)]
The air permeability resistance (sec/100 ml) of the separator was measured using a Gurley air permeability meter conforming to JIS P-8117.

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

[MD/TD intensity ratio]
The tensile strength of the separator was measured using a tensile tester (TG-1kN type manufactured by Minebea Co., Ltd.) by setting the length of the sample before the test to 35 mm and pulling the sample at a speed of 100 mm/min. The strength (tensile load value) at the time of breaking (fracture) was measured, or if the specimen broke before yielding, the strength (tensile load value) at the time of breaking was divided by the cross-sectional area of the specimen to determine the tensile strength. The tensile strength was measured in each of the MD and TD. The MD/TD strength ratio was calculated by dividing the MD tensile strength by the TD tensile strength.

[F/S fuse temperature and 210°C resistance]
Preparation of positive electrode sheet LiNi _1/3 Mn _1/3 Co _1/3 O ₂ as a positive electrode active material, carbon black as a conductive assistant, and polyvinylidene fluoride solution as a binder were mixed in a solid content mass ratio of 91:5:4, and N-methyl-2-pyrrolidone was added as a dispersion solvent so that the solid content was 68 mass%, and further mixed to prepare a slurry solution. This slurry solution was applied to one side of an aluminum foil with a thickness of 15 μm, and then the solvent was dried and removed to obtain a positive electrode with a coating amount of 175 g / m ^2. This positive electrode was rolled with a roll press to obtain a positive electrode sheet with a density of 2.4 g / cm ³ in the positive electrode mixture portion.

Preparation of negative electrode sheet Artificial graphite as a negative electrode active material, styrene butadiene rubber as a binder, and a carboxymethyl cellulose aqueous solution were mixed in a solid content mass ratio of 96.4: 1.9: 1.7, and water was added as a dispersion solvent so that the solid content was 50 mass%, and further mixed to prepare a slurry solution. This slurry solution was applied to one side of a copper foil having a thickness of 10 μm, and then the solvent was dried and removed to obtain a negative electrode with a coating amount of 86 g / m ^2. This negative electrode was rolled with a roll press to obtain a negative electrode sheet with a density of 1.25 g / cm ³ in the negative electrode mixture part.

Preparation of non-aqueous electrolyte: Propylene carbonate: Ethylene carbonate: γ-butyl lactone = 1: 1: 2 (
_LiBF4 as a solute was dissolved in a mixed solvent of 0.05 wt. % and 0.05 wt. % of trioctyl phosphate was further added thereto to prepare a nonaqueous electrolyte solution.

Measurement of fuse temperature and film resistance (Ω·cm ² ) A 30 mm×30 mm square sample was cut from the separator to prepare sample 1. The positive electrode prepared by the above-mentioned method was cut into a 55 mm×20 mm square, and the electrode active material was removed so that the applied electrode active material portion was 20 mm×20 mm square to prepare a positive electrode 2A in which the current collector foil was exposed. The negative electrode prepared by the above-mentioned method was cut into a 55 mm×25 mm square, and the electrode active material was removed so that the applied electrode active material portion was 25 mm×25 mm square to prepare a negative electrode 2A in which the current collector foil was exposed. After that, the nonaqueous electrolyte was impregnated for 1 minute or more in the portions of sample 1, positive electrode 2A, and negative electrode 2B in which the electrode active material was applied. Then, negative electrode 2B, sample 1, positive electrode 2A, Kapton film, and 4 mm thick silicone rubber were laminated in this order. At this time, the sample 1 and the positive electrode 2A were laminated so that the portions coated with the electrode active material of the negative electrode 2B were overlapped. The laminate was placed on a ceramic plate in which a thermocouple was embedded, and the heater was heated while applying a surface pressure of 2 MPa with a hydraulic press, and the temperature and resistance value were continuously measured using an AC electrical resistance measuring device "AG-4311" (Ando Electric Co., Ltd.) to which the collector portions of the positive electrode 2A and the negative electrode 2B were connected. The temperature was raised from room temperature 23°C to 220°C at a rate of 15°C/min, and the resistance value was measured with an AC of 1V and 1kHz. The value calculated by multiplying the obtained resistance value (Ω) at 210°C by the effective electrode area of ⁴ cm2 was taken as the F/S 210°C film resistance value (Ω· ^cm2 ). The fuse temperature was taken as the temperature at which the impedance reached three times the minimum value on the high temperature side.

[MD heat shrinkage and TD heat shrinkage]
The heat shrinkage was measured by cutting a separator into a 5 cm square, marking 9 points at 2 cm intervals, and wrapping it in paper. The marked sample was heat-treated at 130°C for 1 hour, and then cooled to room temperature. The lengths of the MD and TD were measured at 3 points each to determine the shrinkage. Due to the accuracy of the sample measurement, or possibly due to the expansion of the components in the sample, the heat shrinkage may show a negative value, but when a negative value was shown, it was considered to be 0.0%.

Example 1
[Preparation of pellets containing polyethylene and inorganic particles]
Polyethylene (PE, MFR = 0.31, weight average molecular weight 610,000 and surface-treated barium sulfate with an average particle diameter of 500 nm (1 part by mass of sodium stearate was used for surface treatment for 100 parts by mass of barium sulfate) were used as PE:BaSO ₄ After dry blending at a mass ratio of 20:80 (mass%), melt-kneading was performed using a twin-screw extruder HK-25D (Parker Corporation, L/D = 41). In order to minimize decomposition and denaturation of the resin, the resin inlet hopper was completely sealed from the raw material tank, and nitrogen was continuously flowed from the bottom of the hopper to control the oxygen concentration near the raw material inlet to 50 ppm or less. In addition, all vents were completely sealed to eliminate air leakage into the cylinder. This oxygen concentration reduction effect significantly suppressed decomposition and denaturation of the polymer even at high temperatures. Barium sulfate was further finely dispersed by feeding it with a twin-screw feeder. After melt-kneading, the strand was pulled from the die (two holes) and cooled in a water-cooled bath, and then cut using a pelletizer to obtain pellets containing polyethylene and inorganic particles (hereinafter simply referred to as "the pellets").

[Preparation of microporous membrane (three layers)]
A laminated sheet was formed by co-extrusion. Polypropylene (PP, MFR = 0.51) was melted in a 32 mmφ twin-screw extruder and fed to a circular die using a gear pump. The pellets were melted in a 32 mmφ single-screw extruder and fed to a circular die using a gear pump. The composition melted and mixed by each extruder was extruded into a sheet shape by a two-kind, three-layer co-extrusion-capable circular die, and the molten polymer was cooled by blown air and then wound on a roll. The polypropylene resin was extruded from the outer layer (two outer layers) of a circular die set at a temperature of 230 ° C. at a mixing temperature of 230 ° C. and an extrusion rate of 2.4 kg / hr. The pellets were extruded from the inner layer (middle layer) of a circular die set at a temperature of 230 ° C. at a mixing temperature of 230 ° C. and an extrusion rate of 1.2 kg / hr in terms of polypropylene resin. The extruded precursor (raw film) was cooled immediately after extrusion with an air flow rate of 3.6 m3/min per Φ300 mm width by air ring. 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 then the cold stretched film was hot stretched to 100% 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 obtained microporous film were measured. The results are shown in Table 1.

Examples 2 to 24 and Comparative Examples 1 to 3
A microporous membrane was obtained in the same manner as in Example 1, except that the raw materials, membrane forming conditions, or separator properties were changed as shown in Tables 1 and 2, and the obtained microporous membrane was evaluated. The layer structure was adjusted by changing the ratio of the extrusion amount. The polyethylene used in Example 9 had an MFR of 0.35 and a weight average molecular weight of 510,000. The surface treatment method for the inorganic particles used in Examples 8 and 11 was a treatment method in which an aluminum (Al)-based coupling agent (Plenact AL-M) manufactured by Ajinomoto Fine-Techno Co., Inc. was dry-blended at a mass ratio of inorganic particles:Plenact AL-M of 98:2 (mass%).

Example 25
Confirmation of operation of lithium ion secondary battery As an electrolyte, an electrolyte in which ethylene carbonate and ethyl methyl carbonate were mixed in a volume ratio of 1:2 and LiPF ₆ was added as a lithium salt at 1 mol/L was used.

A lithium nickel _{manganese cobalt} mixed oxide ( _{LiNi0.5Co0.2Mn0.3O2} ) was used as the positive _electrode active material, carbon black powder (manufactured by Timcal, product name: SuperP Li) was used as the conductive additive, and PVDF was used as the binder, and the mass ratio of the mixed oxide:conductive additive:binder was 100:3.5:3. This mixture was applied to both sides of an aluminum foil serving as a positive electrode current collector with a thickness of 15 μm, dried, and then pressed with a roll press to prepare a double-sided coated positive electrode.

Graphite powder with a particle size of 22 μm (D50) (Hitachi Chemical Co., Ltd., product name: MAG) as the negative electrode active material, binder (Zeon Corporation, product name: BM400B), and carboxymethylcellulose (Daicel Corporation, product name: #2200) as the thickener were mixed in a solvent at a mass ratio of graphite powder:binder:thickener = 100:1.5:1.1. This mixture was applied to one side and both sides of copper foil as a negative electrode current collector with a thickness of 10 μm, the solvent was dried and removed, and then the applied copper foil was pressed with a roll press to prepare a single-sided coated negative electrode and a double-sided coated negative electrode, respectively.

The obtained positive and negative electrodes were laminated in the order of one-sided coated negative electrode/both-sided coated positive electrode/both-sided coated negative electrode/both-sided coated positive electrode/one-sided coated negative electrode, with the separator of Example 1 sandwiched between the opposing surfaces of the active materials. Next, the obtained laminate was inserted into a bag (battery exterior) made of a laminate film in which both sides of aluminum foil (thickness 40 μm) were coated with a resin layer, with the positive and negative terminals protruding. After that, 0.8 mL of the electrolyte prepared as described above was poured into the bag, and the bag was vacuum sealed to prepare a sheet-shaped lithium ion secondary battery.

The obtained sheet-shaped lithium ion secondary battery was placed in a thermostatic chamber (manufactured by Futaba Scientific Co., Ltd., product name: PLM-73S) set at 25°C, connected to a charge/discharge device (manufactured by Aska Electronics Co., Ltd., product name: ACD-01), and left to stand for 16 hours. The battery was then charged at a constant current of 0.05 C, and after the voltage reached 4.2 V, it was charged at a constant voltage of 4.2 V for 2 hours, and then discharged at a constant current of 0.2 C to 3.0 V. This charge/discharge cycle was repeated three times to perform initial charging and discharging of the battery. Note that 1 C refers to the current value when the entire capacity of the battery is discharged in 1 hour.

The lithium secondary battery that had been treated as described above was discharged at a discharge current of 1 C at a temperature of 25° C. to a discharge end voltage of 3 V, and then charged at a charge current of 1 C to a charge end voltage of 4.1 V. This constitutes one cycle. Charging and discharging were repeated, and the capacity retention rate after 50 cycles relative to the initial capacity was measured, and the capacity retention rate was 90% or more.

Comparative Example 4
The operation of the lithium ion secondary battery was confirmed in the same manner as in Example 25 except that the separator was changed to that of Comparative Example 3. The capacity retention rate was found to be less than 90%.

Example 26
Confirmation of operation of lithium ion capacitor As a positive electrode precursor, 58.0 parts by mass of activated carbon with an average particle size of 5.5 μm, 32.0 parts by mass of lithium carbonate, 4.0 parts by mass of acetylene black, 3.5 parts by mass of acrylic latex, 1.5 parts by mass of CMC (carboxymethyl cellulose), and 1.0 parts by mass of PVP (polyvinylpyrrolidone) were mixed. This mixture was applied to both sides of an aluminum foil with a thickness of 15 μm, dried, and pressed using a roll press machine to prepare a positive electrode precursor.

As the negative electrode active material, 83 parts by mass of artificial graphite with an average particle size of 4.5 μm, 4 parts by mass of composite carbon material, 9 parts by mass of acetylene black, 2 parts by mass of styrene-butadiene copolymer, and an aqueous CMC (carboxymethylcellulose) solution were added so that the CMC amount was 2 parts by mass. This mixture was applied as the negative electrode coating liquid to both sides of electrolytic copper foil with a thickness of 10 μm, dried at a drying temperature of 60°C, and pressed using a roll press machine to create a negative electrode.

As the organic solvent, a mixed solvent of ethylene carbonate (EC): dimethyl carbonate (DMC): methyl ethyl carbonate (EMC) = 34: 44: 22 (volume ratio) was used, and each electrolyte salt was dissolved so that the concentration ratio of LiN(SO ₂ F) ₂ and LiPF ₆ was 25: 75 (molar ratio) relative to the total nonaqueous electrolyte solution, and the sum of the concentrations of LiN(SO ₂ F) ₂ and LiPF ₆ was 1.2 mol/L.

The positive electrode precursor, the separator of Example 1, and the negative electrode were laminated in this order so that the outermost layer was the negative electrode, with the positive electrode active material layer and the negative electrode active material layer facing each other with the separator sandwiched therebetween, to obtain an electrode laminate. The obtained laminate was inserted into a bag (battery exterior) made of a laminate film in which both sides of an aluminum foil (thickness 40 μm) were coated with a resin layer, with the positive and negative electrode terminals protruding. Using a charge/discharge tester (ACD-10APS (01)) manufactured by Asuka Electronics Co., Ltd., constant current charging was performed at a current value of 6 A in an environment of 45°C until the voltage reached 4.5 V, and then initial charging was performed by a method of continuing constant voltage charging at 4.5 V for 1 hour, and lithium was doped into the negative electrode.

After doping, the non-aqueous lithium storage element was charged at a constant current of 10.0 A in a 50°C environment until a voltage of 4.3 V was reached. Next, a constant voltage charge of 4.3 V was performed for 5 minutes, and a constant voltage discharge of 10.0 A was performed until a voltage of 2.0 V was reached. Then, a constant voltage discharge of 2.0 V was performed for 5 minutes. This operation constitutes one cycle, and a total of 5 cycles were performed. After aging at 60°C, a portion of the aluminum laminate packaging material was opened and degassed, and the aluminum laminate packaging material was sealed. A lithium ion capacitor was manufactured by the above process.

The lithium ion capacitor that had been treated as described above was charged at a constant current of 200 C (160 A) to a voltage of 3.8 V at a temperature of 25° C., and then discharged at a constant current of 200 C to 2.2 V. This constitutes one cycle, and charging and discharging were repeated. The capacity retention rate after 50 cycles relative to the initial capacity was measured, and the capacity retention rate was 90% or more.

Comparative Example 5
The operation of the lithium ion capacitor was confirmed in the same manner as in Example 26 except that the separator was changed to that of Comparative Example 3. The capacity retention rate was less than 90%.

Figure 2 is a scanning electron microscope (SEM) image of the MD-ND cross section of the inorganic-containing layer in Example 4. Figure 3 is a scanning electron microscope (SEM) image of the MD-ND cross section of the inorganic-containing layer in Comparative Example 3. In Figures 2 and 3, the lightest areas represent inorganic particles, the darkest areas represent open pores, and the areas of intermediate brightness represent thermoplastic resin.

The separator for an electricity storage device disclosed herein has low air permeability while simultaneously achieving high strength and heat resistance, and can be suitably used as a separator for electricity storage devices, such as lithium ion secondary batteries and lithium ion capacitors.

REFERENCE SIGNS LIST 1 inorganic particle 2 thermoplastic resin 3 aperture 4 outline of aperture formed by thermoplastic resin 5 outline of aperture formed by inorganic particle 6 interface between thermoplastic resin and inorganic particle 10 MD-ND cross section of inorganic-containing layer

Claims (15)

A separator for an electricity storage device, comprising an inorganic-containing layer including inorganic particles and a thermoplastic resin including 50% by mass or more of polyolefin,
The inorganic-containing layer has a thermoplastic resin region, an inorganic material region, and a void region in an MD-ND cross section,
a ratio (L _po / L pi ) of a length (L _po ) of a boundary portion between the void region and the thermoplastic resin region to a length (L _pi ) of an interface between the thermoplastic resin region and the inorganic material region _is 0.2 or more and 3.0 or less;
The thickness of the inorganic-containing layer is 3 μm or more and 27 μm or less,
The separator for an electricity storage device has a ratio of tensile strength in MD to tensile strength in TD (MD/TD) of 8.0 or more. The electricity storage device separator according to claim 1 , wherein the inorganic-containing layer has pores having an average pore diameter of 150 nm or more and 2000 nm or less in an MD-ND cross section. The separator for an electricity storage device according to claim 1 or 2 , wherein the inorganic particles have a particle diameter of 60 nm or more and 2000 nm or less. The separator for an electricity storage device according to any one of claims 1 to 3 , having an air permeability of 50 sec/100 ml or more and 1000 sec/100 ml or less. The separator for the electricity storage device includes a substrate, the substrate being a microporous layer containing 50% by mass or more of polypropylene,
The separator for an electricity storage device according to claim 1 , wherein the substrate is present on one or both sides of the inorganic-containing layer. The separator for an electricity storage device according to any one of claims 1 to 5, wherein the separator for an electricity storage device includes a substrate, and a thickness ratio of the inorganic-containing layer to the substrate (thickness of the substrate/thickness of the inorganic-containing layer) is 0.3 or more and 3.0 or less. The separator for an electricity storage device according to any one of claims 1 to 6 , wherein the separator for an electricity storage device comprises a substrate, and the MFR of the substrate is 0.05 g/10 min or more and 0.9 g/10 min or less. The separator for an electricity storage device according to any one of claims 1 to 7 , which has a TD heat shrinkage rate of 3% or less. The separator for an electric storage device according to claim 1 , wherein the inorganic-containing layer contains the inorganic particles in an amount of 50% by mass or more and 95% by mass or less, based on the total mass of the inorganic-containing layer. The separator for use in an electricity storage device according to any one of claims 1 to 9 , wherein the separator for use in an electricity storage device has a puncture strength per 14 µm of thickness of 100 gf/14 µm or more. The separator for a storage device according to any one of claims 1 to 10 , wherein the inorganic-containing layer contains polyethylene as the polyolefin, and the amount of the polyethylene is 20 mass% or more based on the total mass of the thermoplastic resin of the entire separator for a storage device. The separator for use in an electricity storage device according to claim 11 , wherein the polyethylene has a weight average molecular weight of 800,000 or less. The separator for an electricity storage device according to any one of claims 1 to 12 , wherein the separator for an electricity storage device has a total thickness of 5 µm or more and 30 µm or less. The separator for an electricity storage device according to claim 1 , wherein the inorganic-containing layer has pores having an average pore diameter of 200 nm or more and 2000 nm or less in an MD-ND cross section. An electricity storage device comprising the separator for an electricity storage device according to any one of claims 1 to 14 .