WO2022163714A1

WO2022163714A1 - Separator for power storage device, and power storage device including same

Info

Publication number: WO2022163714A1
Application number: PCT/JP2022/002916
Authority: WO
Inventors: 一徳内田; 奨平森; 真也浜崎; 敏大鈴木
Original assignee: 旭化成株式会社
Priority date: 2021-01-29
Filing date: 2022-01-26
Publication date: 2022-08-04
Also published as: TWI813152B; JP7549051B2; TW202241709A; JPWO2022163714A1

Abstract

A purpose of the present invention is to provide a separator for a power storage device, said separator being provided with both high strength and high heat resistance while having high transmissivity. Provided is separator for a power storage device that has an inorganic layer that includes inorganic particles and a thermoplastic resin, at least 50 mass% of which is a polyolefin. The inorganic layer has holes and has an MFR of 0.05-5 g/10 min, inclusive.

Description

SEPARATOR FOR POWER STORAGE DEVICE AND POWER STORAGE DEVICE INCLUDING THE SAME

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

Microporous membranes, especially polyolefin-based microporous membranes, are used in many technical fields such as microfiltration membranes, battery separators, capacitor separators, and fuel cell materials, and are typified by lithium ion batteries (LIB) in particular. It is used as a secondary battery separator or a lithium ion capacitor separator. Lithium-ion batteries are being researched for various applications, such as small electronic devices such as mobile phones and laptop computers, and lithium-ion batteries or lithium-ion capacitors for hybrid vehicles and electric vehicles including plug-in hybrid vehicles. ing.

In recent years, there has been a demand for electricity storage devices typified by lithium-ion batteries that have high energy capacity, high energy density, and high output characteristics. The demand for separators that have both properties is increasing.

Patent Document 1 describes a microporous polyolefin film produced 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.).

WO2019/107119

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

Examples of embodiments of the present disclosure are listed in items [1] to [19] below.
[1]
A power storage device separator comprising an inorganic-containing layer containing inorganic particles and a thermoplastic resin containing 50% by mass or more of polyolefin,
A 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]
The separator for an electricity storage device according to item 1, wherein the inorganic-containing layer has an average pore diameter of 150 nm or more and 2000 nm or less in the MD-ND cross section.
[3]
The inorganic-containing layer has a thermoplastic resin region, an inorganic material region, and a void region in the MD-ND cross section,
Ratio (L _po /L _pi ) of the length (L _po ) of the boundary portion between the void region and the thermoplastic resin region and 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, the separator for an electricity storage device according to

item

1 or 2.
[4]
A power storage device separator comprising an inorganic-containing layer containing inorganic particles and a thermoplastic resin containing 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 the MD-ND cross section,
Ratio (L _po /L _pi ) of the length (L _po ) of the boundary portion between the void region and the thermoplastic resin region and 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 power storage device separator according to any one of items 1 to 4, wherein the inorganic particles have a particle diameter of 60 nm or more and 2000 nm or less.
[6]
6. The power storage device separator according to any one of items 1 to 5, having an air permeability of 50 seconds/100 ml or more and 300 seconds/1000 ml or less.
[7]
The electricity storage device separator includes a base material, the base material is a microporous layer containing 50% by mass or more of polypropylene,
7. The electricity storage device separator according to any one of items 1 to 6, wherein the substrate is present on one side or both sides of the inorganic-containing layer.
[8]
The electricity storage device separator includes a base material, and the thickness ratio between the inorganic-containing layer and the base material (thickness of the base material/thickness of the inorganic-containing layer) is 0.3 or more and 3.0 or less. The power storage device separator according to any one of items 1 to 7.
[9]
The power storage device separator according to any one of items 1 to 8, wherein the power storage device separator comprises a base material, and the MFR of the base material is 0.05 g/10 min or more and 0.9 g/10 min or less.
[10]
The electricity storage device separator according to any one of items 1 to 9, wherein the electricity storage device separator has a ratio of MD tensile strength to TD tensile strength (MD/TD) of 1.5 or more.
[11]
11. The power storage device separator according to any one of items 1 to 10, wherein the TD has a heat shrinkage rate of 3% or less.
[12]
12. The electricity storage device separator 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]
13. The electricity storage device separator according to any one of items 1 to 12, wherein the electricity storage device separator has a puncture strength of 100 gf/14 μm or more per 14 μm of thickness.
[14]
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% by mass or more based on the total mass of the thermoplastic resin for the entire power storage device separator. The separator for electrical storage devices according to 1.
[15]
15. The power storage device separator according to any one of items 1 to 14, wherein the polyethylene has a weight average molecular weight of 800,000 or less.
[16]
16. The electricity storage device separator 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]
17. The power storage device separator according to any one of items 1 to 16, wherein the power storage device separator has a total thickness of 5 μm or more and 30 μm or less.
[18]
18. The electricity storage device separator according to any one of items 1 to 17, wherein the inorganic-containing layer has an average pore diameter of 200 nm or more and 2000 nm or less in the MD-ND cross section.
[19]
An electricity storage device comprising the electricity storage device separator 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, high strength, and high heat resistance.

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

《Separators for power storage devices》
<Inorganic-containing layer>
The power storage device separator of the present disclosure has an inorganic-containing layer containing inorganic particles and a thermoplastic resin. The inorganic-containing layer is a microporous film that has a plurality of pores and constitutes a separator for an electricity storage device. The inorganic-containing layer may be used as a single layer, or may be used as a multilayer by laminating 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 greater than 50% by weight, 60% by weight or more, 70% by weight or more, 80% by weight or more, 90% by weight or more, 95% by weight or more, 99% by weight or more, or 100% by weight or more. % 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 an electricity storage device including the base material (hereinafter simply referred to as "the total amount of polyethylene") is It is preferably 20% by mass or more. A separator for power storage devices that melts at approximately 100°C to 140°C can be obtained because the amount of polyethylene is 20% by mass or more, based on the total standard. Therefore, the shutdown function increases the resistance between the electrodes and suppresses thermal runaway of the battery. and improve battery safety. On the other hand, if the total polyethylene content is less than 20% by mass, the melted resin may be insufficient to sufficiently increase the resistance. In addition, by containing polyethylene in an amount of 20% by mass or more based on the total amount, the crystal size of polyethylene tends to increase, and large pores are formed when lamellar pores are formed, and the pores are connected to each other, resulting in a separator for a highly permeable electricity storage device. tends to be obtained. On the other hand, if the proportion of polyethylene is less than 20% by mass based on the total amount, the proportion of large pores at the time of lamella opening becomes small, so there is a tendency that sufficient permeability cannot be obtained.

The lower limit of the amount of polyethylene on the whole standard may be more preferably 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 amount of polyethylene on an overall basis may preferably be less than 55% by weight, 50% by weight or less, 45% by weight or less, 40% by weight or less, or 15% by weight or less.

Examples of polyethylene include low-density polyethylene (LDPE), medium-density polyethylene (MDPE), and high-density polyethylene, and from the viewpoint of further increasing permeability, high-density polyethylene (HDPE) is more preferable.

The thermoplastic resin may contain thermoplastic resins other than polyethylene. Other thermoplastic resins are preferably polyolefins other than polyethylene. Polyolefins are polymers containing repeating units of monomers having carbon-carbon double bonds. Monomers constituting polyolefins 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 and the like. The polyolefin can be a homopolymer, a copolymer, a multi-stage polymer, or the like, and is preferably a homopolymer.

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

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

The weight average molecular weight of polyethylene calculated in terms of styrene is preferably 800,000 or less, more preferably 70 or less, still more preferably 650,000 or less, preferably 100,000 or more, more preferably 200,000 or more, and still 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. As a result, the inorganic particles can be dispersed in the thermoplastic resin to obtain a more uniform inorganic-containing layer, and as a result, the inter-electrode resistance suppressing effect can be enhanced. In addition, when the weight average molecular weight of polyethylene is 800,000 or less, significant variations in the concentration of inorganic particles in the inorganic-containing layer are suppressed, and the inorganic-containing layer becomes a resistance component as a whole when melted at a high temperature, so that the resistance between electrodes is maintained. easier to do. Furthermore, when the weight average molecular weight of polyethylene is 800,000 or less, the pressure during extrusion can be kept low from the viewpoint of film formation stability during extrusion, and film formation becomes easier. On the other hand, regarding the lower limit, when the weight-average molecular weight of polyethylene is 100,000 or more, the self-sustainability of the melted film increases, and winding becomes easier. Further, when the weight average molecular weight of polyethylene is 100,000 or more, the degree of orientation is improved, lamellar pores are easily formed, and the permeability is increased, which is preferable.

<Melt flow rate (MFR)>
MFR of the inorganic-containing layer containing inorganic particles and a 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. When the MFR of the inorganic-containing layer is 0.05 g/10 min or more, uniform kneading can be performed when the thermoplastic resin and the inorganic particles are kneaded under high shear conditions in a twin-screw extruder or the like, and the inorganic particles are dispersed. It is considered that this is because a uniform separator is obtained by increasing the thickness, and the effect of suppressing the inter-electrode resistance is enhanced. When the MFR of the inorganic-containing layer is 5 g/10 min or less, high MD orientation can be imparted by blowing a large amount of cold air during extrusion, and lamellar pores are easily formed, making it easy to obtain an inorganic-containing layer with high permeability.

The melt flow rate (MFR) (MFR of a single layer) of the thermoplastic resin contained in the inorganic-containing layer was measured at a load of 2.16 kg, a temperature of 190 ° C. for polyethylene, and a temperature of 230 ° C. for polypropylene. It is preferably 0.2 or more and 15 or less. It is preferably 0.25 or more and 0.30 or more, preferably 5.00 or less and 1.00 or less. The reason is not limited to theory, but when a thermoplastic resin having a high MFR and inorganic particles are kneaded, uniform kneading can be performed, and the inorganic particles are dispersed to form a uniform separator, and the inter-electrode resistance suppressing effect is enhanced. It is thought that it is from On the other hand, if the MFR is too low, the inorganic particles will not disperse, resulting in an extremely low-concentration portion of the inorganic particles in the separator, and there will be a tendency that the resistance between the electrodes cannot be maintained due to the local shortage of resistance components during high-temperature melting. In addition, from the viewpoint of film formation stability during extrusion, when the MFR is 0.2 or more, the pressure during extrusion does not rise excessively, making film formation easier. When the MFR is 15 or less, the melted film tends to be more self-sustaining, and it is easy to roll up and form a film. In addition, when the MFR is 15 or less, a decrease in the degree of orientation can be suppressed, lamellar pores are easily formed, and the permeability can be increased.

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

Examples of inorganic particles include oxide-based ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride-based ceramics such as silicon nitride, titanium nitride, and boron nitride; Silicon carbide, calcium carbonate, magnesium sulfate, aluminum sulfate, barium sulfate, aluminum hydroxide, aluminum hydroxide oxide, potassium titanate, talc, kaolinite, dikite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, Ceramics such as amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fibers. At least one selected from the group consisting of barium sulfate, titania, alumina and boehmite is preferable from the viewpoint of safety and heat resistance in the battery.

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, still more preferably 230 nm or more, and the upper limit is more preferably 1000 nm or less, still 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 gaps 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 resistance between the electrodes. be. In addition, when the particle size is 60 nm or more, the inorganic particles are easily dispersed in the inorganic-containing layer, and the inorganic particle concentration in the inorganic-containing layer is suppressed from significantly varying. It is easier to maintain resistance while When the particle diameter of the inorganic particles is 2000 nm or less, the inorganic particles are prevented from becoming a starting point of stress concentration, and the strength tends to be improved. Further, when the particle diameter of the inorganic particles is 2000 nm or less, variation in the thickness of the separator is suppressed, and the quality is improved.

The content of the 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 still more preferably 72% by mass. % 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 uniaxially stretching to form a low-pressure layer. It is easier to obtain a porous yet highly permeable separator. The reason is not limited to theory, but the amount of inorganic particles is 50% by mass or more, so that lamellar pores are perpendicular to the stretching direction and filler pores are horizontal to the stretching direction. Both occur, and the horizontal independent pores formed by the filler pores are connected by the vertical pores formed by the lamellar crystals, so even at a slight stretching ratio, the porosity is low. and high permeability can be achieved at the same time. Further, when the content ratio of the inorganic particles in the inorganic-containing layer is 95% by mass or less, the risk of film breakage is low, and thickness uniformity during film formation is improved.

<Surface treatment of inorganic particles>
In order to improve the dispersibility in the thermoplastic resin, the inorganic particles are preferably surface-treated with a surface-treating agent. Examples of surface treatment agents include saturated fatty acids and/or salts thereof (saturated fatty acid salts), unsaturated fatty acids and/or salts thereof (unsaturated fatty acid salts), aluminum-based coupling agents, polysiloxane, silane coupling agents, and the like. processing. 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, a saturated fatty acid with 8 or more carbon atoms and its salt, and a saturated fatty acid with 8 or more carbon atoms and its salt. and salts thereof are more preferred. The surface treatment agent may be applied before kneading the thermoplastic resin, or the surface treatment agent may be added after kneading. From the viewpoint of uniform coating of the surface treatment agent on the surfaces of the inorganic particles, the treatment is preferably performed 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 electrolytic solution increases, and the ion conductivity tends to improve.

Although the amount of surface treatment of the inorganic particles depends on the particle diameter of the inorganic particles, it 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.

<Basity of inorganic particles>
The basis weight of the inorganic particles is preferably 0.15 mg/cm ² or more. When the basis weight is 0.15 mg/cm ² or more, the inorganic particles are less likely to migrate into the gap between the electrodes together with the resin when the separator is melted at high temperature, and the inorganic particles remain as an inorganic particle layer that serves as a resistance component between the electrodes. , a dense inorganic particle layer tends to be formed. In addition, the inorganic particles become a resistance component and can maintain the resistance between the electrodes, which tends to improve the safety of the battery.

〈Hole〉
The inorganic-containing layer has pores. As used herein, the term "pores" means pores (also referred to as "open pores") in the microporous membrane. The inorganic-containing layer is more specifically a microporous membrane having a plurality of pores. In the MD-ND cross section (surface direction cross section), the outline of the pores (open pores) is formed from the thermoplastic resin alone or from the thermoplastic resin and the inorganic particles. Then, in the MD-ND cross section of the inorganic-containing layer, the boundary portion between the hole portion (hereinafter referred to as "open hole region") and the thermoplastic resin portion (hereinafter referred to as "thermoplastic resin region") The ratio (L _po /L _pi ) of the length (L _po ) and the interface length (L _pi ) between the thermoplastic resin region and the inorganic material portion (hereinafter referred to as the “inorganic material region”) is 0 .2 or more and 3.5 or less. That is, the ratio (L _po /L _pi ) of the length (L _po ) of the portion of the outline formed by the thermoplastic resin and 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.

L _pi increases when the amount of the thermoplastic resin coated with the inorganic particles increases. When the amount of the thermoplastic resin coated with the inorganic component increases, it becomes possible to maintain the pore structure even in a high temperature state at which the thermoplastic resin melts, and high heat resistance is likely to be obtained. On the other hand, when the amount of thermoplastic resin exposed to pores (hole spaces) increases, _Lpo increases. When the amount of the thermoplastic resin exposed to the pores increases, the thermoplastic resin comes into contact with the pores more often, so that the connectivity between the pores increases and high permeability can be easily obtained. Therefore, the preferable range of the ratio (L _po /L _pi ) is that the inorganic particle interface when viewed from the thermoplastic resin and the pore interface when viewed from the thermoplastic resin have high heat resistance and high permeability. It shows the preferred range for showing both. Since the effect as described above is exhibited 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 with the inorganic particles as cutouts and the openings with the lamellae are formed at the same time. It is thought that a connecting structure is formed between the openings by the lamellae and the openings by the lamellae, and high permeability is likely to be obtained. It is considered that the connectivity between the holes in the horizontal direction (stretching direction) derived from the filler pores and the holes in the vertical direction due to the lamellar pores is enhanced, and high permeability is likely to be obtained. A method of adjusting the ratio (L _po /L _pi ) to 0.2 or more, that is, a method of adjusting L _po to a large value with respect to L _pi is performed by melt extrusion in MD at a high drawdown ratio, and then after melt extrusion Rapid cooling by applying an extremely strong cold wind can be mentioned. When adjusted in this way, it is easier to cause not only opening due to inorganic particles but also opening due to lamellae at the same time in subsequent stretching, so the ratio (L _po /L _pi ) is adjusted to 0.2 or more. It's easy to do. On the other hand, if melt extrusion is not performed at a high drawdown ratio in the MD, or if the melt is extruded at a high drawdown ratio in the MD but the cooling efficiency is poor due to a small amount of cold air, etc., almost no lamellae are formed. It is difficult to adjust the ratio (L _po /L _pi ) to 0.2 or more.

The ratio (L _po /L _pi ) can also be adjusted by the composition of the thermoplastic resin. Regarding adjustment by composition, the ratio is adjusted to 0.2 or more by promoting lamellar opening by using a highly crystalline thermoplastic resin such as polypropylene with high stereoregularity or by adding a crystal nucleating agent. Things are mentioned.

When the ratio (L _po /L _pi ) is 3.5 or less, the pores of the separator are covered with an inorganic component composed of inorganic particles, so that the pore structure is maintained even at high temperatures, and the insulation is easily maintained and high. Easy to obtain 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 with respect to L _po , is performed by melt extrusion in MD at a high drawdown ratio, followed by MD and TD and lowering the total draw ratio of TD and lowering the draw ratio of TD. By adjusting in this way, the interface between the thermoplastic resin and the inorganic particles can be maintained without being destroyed. As a result, the length of the interface between the thermoplastic resin and the inorganic particles (L _pi ) tends to increase, and the ratio It is easy to adjust (L _po /L _pi ) to 3.5 or less. On the other hand, when melt extrusion was not performed at a high drawdown ratio in MD, or when the total draw ratio in MD and TD was high and the draw ratio in TD was not reduced, lamellar crystals were not sufficiently obtained, and filler pores were opened. Since the pores are formed by enlarging the opening starting point only, the interface between the thermoplastic resin and the inorganic particles is destroyed, and it is 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, the interface between the thermoplastic resin and the inorganic particles is separated by phase separation, and the ratio (L _po /L _pi ) is adjusted to 3.5 or less. is difficult.

Regarding adjustment by composition, the ratio is set to 3.5 or less by partially adding a thermoplastic resin modified with a polar functional group that is considered to have a high affinity with inorganic particles, such as acid-modified polyolefin. adjustment.

The lower limit of the ratio (L _po /L _pi ) is preferably 0.3 or more, more preferably 0.4 or more, still more preferably 0.5 or more, and the upper limit is preferably 3.0 or less, more preferably It is 2.5 or less, 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 power storage device separator of the present disclosure. Observation of the MD-ND cross section (10) of the inorganic-containing layer with a scanning electron microscope confirms the presence of the inorganic particles (1), the thermoplastic resin (2), and the pores (3). The portion of the inorganic particles (1) is the “inorganic material region”, the portion of the thermoplastic resin (2) is the “thermoplastic resin region”, and the portion of the pores (3) is the “void region”. The contours of the apertures (3) shown in FIG. 1 consist of a contour (4) formed from a thermoplastic resin indicated by a solid line and a contour (5) formed from inorganic particles indicated by a round dotted line. Further, the interface between the thermoplastic resin and the inorganic particles is indicated by a dashed line (6). At this time, the length of the solid line (4) corresponds to “the length (L _po ) of the boundary portion between the void region and the thermoplastic resin region”, and the length of the corner dotted line (6) corresponds to “the length of the thermoplastic resin region and The length (L _pi ) of the interface with the inorganic material region”, and their ratio (L _po /L _pi ) is 0.2 or more and 3.5 or less.

The average pore diameter of the pores in 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, still 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 for this is not limited to theory, but when the average pore size of the inorganic-containing layer is 150 nm or more, there is a tendency for the pores to form a structure in which the pores are connected to each other, resulting in high permeability. It is also believed that when the average pore diameter is 2000 nm or less, stress concentration is less likely to occur with respect to the thickness of the inorganic-containing layer, and the strength of the separator is less likely to decrease. The average pore diameter 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 Examples section.

The long pore diameters of the inorganic-containing layer are preferably arranged in one direction. In the specification of the present application, the term "slot diameter" refers to the longest line segment among the line segments connecting any two points on the outline of the opening. Although it is not limited to theory, it is preferable that the long pore diameters are aligned in one direction, because the separator is strongly oriented and the strength in the direction of the long pore diameters increases, which facilitates the battery winding operation. The arrangement of long pore 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. “Arranged in one direction” means that 90% or more of fibrils are contained within an angle range of ±20 degrees in the extending direction in the electron microscope image of the surface of the separator. That is, in the electron microscope image, when 90% or more of the long axes of the holes are included in the angle range of ±20 degrees with each other, it is determined that the arrangement of the long hole diameters is "arranged in one direction". do.

The area ratio of the openings 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 setting the area ratio of the pores to 60% or less, the ratio of the resin and the inorganic particles can be increased, and a network structure of the solid content is formed, so the elongation at the time of puncture is improved and high puncture strength is obtained. easy to get In addition, if an attempt is made to increase the opening ratio by increasing the MD draw 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 is divided, resulting in a tendency for the air permeability to deteriorate. . When the area ratio of the open pores is 20% or more, the ratio of the resin to the inorganic particles is low, so that a connection structure between the pores is easily formed, so that high permeability can be obtained.

<Elastomer>
The inorganic-containing layer may further contain an elastomer in addition to the inorganic particles. Examples of elastomers include thermoplastic elastomers and thermosetting elastomers, and thermoplastic elastomers are preferred. In the present specification, thermoplastic elastomers are included in thermoplastic resins. 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. It is possible to obtain a thin film and a high-strength separator.

<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 electricity storage device separator is improved. When the thickness is 27 µm or less, the energy density of the electricity storage device can be further increased. Considering heat resistance and energy density, the thickness is preferably 3 μm or more and 15 μm or less, more preferably 5 μm or more and 10 μm or less. In consideration of the heat resistance, ion permeability, and physical strength of the separator, the ratio of the thickness of the 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. More preferably, it is 20% or more and 60% or less.

<Base material>
The power storage device separator 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 power storage device separator may have a two-layer structure in which the base material is laminated on one side of the inorganic-containing layer, the base material is laminated on both sides of the inorganic-containing layer (as outer layers), or the inorganic-containing layer is laminated on both sides of the base material. It may have a multilayer structure of three layers in which layers are laminated, or a multilayer structure of three or more layers.

The substrate is preferably a microporous layer containing 50% by mass or more of polypropylene (hereinafter also referred to as "PP microporous layer") based on the total mass of the substrate. The amount of polypropylene in the substrate is preferably greater than 50% by weight, 60% by weight or more, 70% by weight or more, 80% by weight or more, 90% by weight or more, 95% by weight or more, 99% by weight or more, or 100% by weight. may be A substrate can be present on one or both sides of the inorganic-containing layer. That is, the power storage device separator may have a two-layer structure in which a substrate is laminated on one side of an inorganic-containing layer, or a multilayer structure of three or more layers in which a substrate is laminated on both sides of an inorganic-containing layer. good too. The MFR of the base material is preferably 0.05 g/10 min or more and 0.9 g/10 min or less regardless of whether the power storage device separator has the base material as the outer layer or the inner layer. When the MFR of the outer layer is 0.05 g/10 min or more, it is easy to form a film with a uniform thickness. When the MFR of the outer layer is 0.9 g/10 min or less, high permeability is likely to be obtained and the mechanical strength of the separator is increased.

The multilayer structure can exhibit the effect of the present invention even if it is laminated in any order, but it is a three-layer structure in which the PP microporous layer/inorganic-containing layer/PP microporous layer are laminated in this order. is particularly preferred. By having a three-layer structure of PP microporous layer/inorganic-containing layer/PP microporous layer, the inorganic-containing layer provides heat resistance, while the PP microporous layer provides mechanical strength and oxidation resistance when the electrode surface is in contact. can be maintained. The film thickness ratio between the inorganic-containing layer and the substrate (thickness of substrate/thickness of inorganic-containing layer) is preferably 0.3 or more and 3.0 or less. By setting the film thickness ratio between the inorganic layer and the base material to 0.3 or more, the resistance between the electrodes can be maintained when the separator is melted, and the safety of the battery is improved. When the film thickness ratio between the inorganic layer and the base material is 3.0 times or less, the film formation stability is improved and the film formation is facilitated.

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

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

<Puncture strength of separator>
The lower limit of the puncture strength of the power storage device separator is 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 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.

<MD/TD strength ratio of separator>
The ratio of the MD tensile strength to the TD tensile strength of the electricity storage device separator (also referred to as "MD/TD strength ratio") is preferably 1.5 or more, more preferably 6.0 or more, and still more preferably 8.0. 0 or more. When the MD/TD strength ratio is 1.5 or more, the separator is strongly oriented, so that the strength of the MD increases and the battery winding operation becomes easy. Further, when the MD/TD strength ratio is 1.5 or more, the thermal contraction rate of the TD can be reduced, so the resistance to short circuit due to thermal contraction of the TD during winding of the battery is increased. Although the upper limit of the MD/TD intensity ratio is not particularly limited, it 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, the MD is less likely to crack, and the problem of the separator splitting vertically (into the MD) during handling is less likely to occur.

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

<<Method for producing separator for power storage device>>
As a method for producing an inorganic-containing layer containing a thermoplastic resin and inorganic particles, generally, a thermoplastic resin composition and inorganic particles are mixed and dispersed, and the contents are melt-extruded to obtain a resin film. It includes a pore-forming step of opening holes in the resin sheet to make it porous, and optionally further includes a stretching step, a heat treatment step, and the like. Methods for producing an inorganic-containing layer are broadly classified into a dry method that does not use a solvent in the pore-forming step and a wet method that uses a solvent.

As a dry method, 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; For example, the material and the inorganic filler are melt-kneaded to form a sheet, and then the interface between the thermoplastic resin and the inorganic filler is exfoliated by stretching.

As a wet method, a thermoplastic resin composition and inorganic particles are mixed and dispersed, a pore-forming material is added, melted and kneaded to form a sheet, stretched as necessary, and then the pore-forming material is extracted. and a method of removing the solvent at the same time as solidifying the thermoplastic resin by immersing it in a poor solvent for the thermoplastic resin after dissolving the thermoplastic resin composition.

A single screw extruder and a twin screw extruder can be used for melt-kneading the thermoplastic resin composition. can also

The thermoplastic resin composition may optionally contain resins other than polyolefin, additives, etc., depending on the method of manufacturing the inorganic-containing layer or the physical properties of the desired inorganic-containing layer. Examples of additives include pore-forming agents, fluorine-based flow modifiers, waxes, crystal nucleating agents, antioxidants, metal soaps such as aliphatic carboxylic acid metal salts, ultraviolet absorbers, light stabilizers, and antistatic agents. , anti-fogging agents, and coloring pigments. Pore formers 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 pore-forming step, or before or after the pore-forming step. As the stretching treatment, either uniaxial stretching or biaxial stretching can be used. Uniaxial stretching is preferable, although not limited, from the viewpoint of manufacturing costs when using a dry method. By using the lamellar perforation method by uniaxial stretching, the separator for power storage devices of the present disclosure having high permeability, high strength and heat resistance can be more easily produced as described below. This will be explained below. This pore-opening method is generally a method in which a precursor (original film) having oriented lamellar crystals is obtained by melt extrusion, cold-stretched, and then hot-stretched to open pores between lamella crystals. . By mixing a high content of inorganic particles in a thermoplastic resin, melt extruding this in the MD at a high drawdown ratio, and increasing the cooling rate, a high content of inorganic particles in a resin having highly oriented lamellar crystals is obtained. It is possible to obtain a precursor (original film) before the hole forming step in which are densely arranged. Thereby, it is easier for the subsequent stretching to induce lamellar pore opening as well as pore opening due to the inorganic particles at the same time. As a result, the length (L _po ) of the interface between the thermoplastic resin and the pores tends to increase, and it is easy to adjust the ratio (L _po /L _pi ) to 0.2 or more. In addition, the formation of pores is promoted by stress concentration due to uniaxial stretching, and the total stretch ratio of MD and TD is low, and the stretch ratio of TD is low. Moreover, it is easy to adjust the MD/TD intensity ratio to 1.5 or more. Thus, the effect of the present invention can be more easily obtained by the lamellar perforation method by uniaxial stretching under specific conditions. Although the reason is not clear, the present inventors presume as follows. That is, since the inorganic particles have a larger heat capacity than the resin, the cooling is slow, and it is difficult to crystallize in the MD under general cooling conditions. . Therefore, by increasing the cooling rate of the raw film containing inorganic particles, more specifically, by increasing the air knife air volume used for cooling and quickly cooling the resin temperature of the raw film containing inorganic particles, It is preferred to orient the lamellar crystals more strongly in the MD. As a result, it is possible to obtain a raw film in which a high content of inorganic particles and a resin having lamellar crystals are densely arranged. It is speculated that a separator for power storage devices can be obtained that can simultaneously achieve high strength and heat resistance.

<Ratio of resin and inorganic material>
The content of the inorganic particles is preferably 100 parts by weight or more, more preferably 200 parts by weight or more, and still more preferably 300 parts by weight or more based on 100 parts by weight of the thermoplastic resin. If the content of inorganic particles is 100 parts by mass or more with respect 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. Then, by uniaxial stretching, the ratio (L _po /L _pi ) is adjusted to 0.2 or more, and a separator with low porosity and high permeability can be obtained. The reason is not limited to theory, but when the amount of inorganic particles is large, both lamellar pores in the direction perpendicular to the stretching direction and filler pores in the direction 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 slight stretching ratio, both low porosity and high permeability are achieved. I'm guessing. On the other hand, when the content of inorganic particles is small, the distance between the pores formed by the filler openings becomes long, making it difficult to connect the pores by the filler openings due to the lamellar openings, and high permeability cannot be obtained. presumed to be from

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

In order to suppress 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 hole forming process. The heat treatment step includes a stretching operation performed at a predetermined temperature atmosphere and a predetermined stretching rate for the purpose of adjusting physical properties, and / or a relaxation operation performed at a predetermined temperature atmosphere and a predetermined relaxation rate for the purpose of reducing stretching stress. may contain. A relaxation operation may be performed after the stretching operation. These heat treatment steps can be performed using a tenter or a roll stretching machine.

Examples of methods for producing a separator for an electricity storage device having a multilayer structure in which a plurality of microporous films containing inorganic-containing layers are laminated include a coextrusion method and a lamination method. In the co-extrusion method, the resin composition of each layer is formed into a film by co-extrusion at the same time, and the resulting multilayer raw film is stretched to open holes, thereby producing a multilayer microporous film. In the lamination method, each layer is formed separately by extrusion film formation to obtain a raw film. By laminating the obtained raw film, a multilayer raw film can be obtained, and the obtained multilayer raw film can be stretched and pore-opened to form a multilayer microporous membrane. The co-extrusion method is preferable because the inorganic-containing layer can be supported by a layer containing no inorganic particles, so that the film formation stability is improved and the inorganic particle content can be increased.

《Energy storage device》
An electricity storage device includes a positive electrode, a negative electrode, and the separator for an electricity storage device of the present disclosure described above. The power storage device separator is laminated between the positive electrode and the negative electrode.

Electricity storage devices 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, and calcium ion batteries. 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, zinc air batteries, etc. mentioned. Among these, from the viewpoint of practicality, lithium secondary batteries, lithium ion secondary batteries, lithium ion capacitors, or nickel metal hydride batteries are preferable, lithium ion secondary batteries and lithium ion capacitors are more preferable, and lithium ion capacitors are more preferable. It is a lithium ion secondary battery.

In the electric storage device, for example, the positive electrode and the negative electrode are laminated with the separator described above interposed therebetween, and are wound as necessary to form a laminated electrode body or a wound electrode body, which is then wrapped as an outer body. The positive and negative electrodes are connected to the positive and negative terminals of the exterior body via a lead body or the like, and further, 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 is exteriorized. It can be produced by sealing the exterior body after injecting it into the body.

《Measurement and evaluation method》
[Melt flow rate (MFR)]
The MFR of the polyolefin was measured in accordance with JIS K 7210 under conditions of a temperature of 230°C and a load of 2.16 kg (unit: g/10 minutes). The MFR of polypropylene was measured according to JIS K 7210 under conditions of a temperature of 230°C and a load of 2.16 kg. The MFR of polyethylene was measured according to JIS K 7210 under conditions of a temperature of 190°C and a load of 2.16 kg. The MFR of the elastomer was measured according to JIS K 7210 under conditions of a temperature of 230°C and a load of 2.16 kg.

[Ratio (L _po /L _pi )]
Preparation of Sample A separator and ruthenium tetroxide (manufactured by Rare Metallic Co., Ltd.) were allowed to coexist in a sealed container, and steam dyeing was performed for 4 hours to prepare a ruthenium-dyed separator. Main agent (Quetol 812, manufactured by Nisshin EM Co., Ltd.) 10.6 mL, curing agent (Methyl Nasic Anhydride (MNA), manufactured by Nisshin EM Co., Ltd.) 9.4 mL, and reaction accelerator (2,4,6-tris ( Dimethylaminomethyl)phenol and Nisshin EM Co., Ltd., DMP-30) (0.34 mL) were mixed and thoroughly stirred to obtain a mixed solution. The ruthenium-dyed separator was immersed in the mixture and placed in a reduced pressure environment to sufficiently impregnate the pores of the ruthenium-dyed separator with the mixture. After impregnation, the ruthenium-dyed separator was embedded in epoxy resin by curing the separator at 60° C. for 12 hours or longer. As a result, the pores in the separator were filled with the epoxy resin and cured. 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 Co., Ltd.) to prepare a smooth cross section. At this time, the cross section was processed so as to form the MD-ND plane. The resulting cross-sectional sample was fixed on a cross-sectional observation SEM sample stage with a conductive adhesive (carbon-based) and dried. Osmium coating was performed under the conditions of an adjustment knob setting of 4.5 and a discharge time of 0.5 seconds, and a microscopic sample was obtained.

Acquisition of SEM image Using a scanning electron microscope (S-4800, manufactured by Hitachi High-Tech Co., Ltd.), an acceleration voltage of 1.0 kV, an emission current of 10 μA, a probe current of High, a detector Upper + LA-BSE 100, a magnification of 10000 times, Observation was made under the conditions of 1280×960 pixels (approximately 10 nm/pixel) and a working distance of 2.0 mm. The observation field of view is determined so that the entire thickness direction of the layer containing the inorganic filler is within the observation field of view, the luminance value is not saturated, and the contrast is set as high as possible, and an 8-bit grayscale image is obtained. An electron microscope image was obtained as However, when the thickness of the layer containing the inorganic filler was larger than the observation field of view, the observation field of view was determined so that the layer containing the inorganic filler fell within the entire observation field of view.

Image processing (calculation of ratio (L _po /L _pi ))
Epoxy resin, polyolefin, and inorganic filler were observed in the obtained electron microscope image. When analyzing the interface length of each of the epoxy resin, polyolefin, and inorganic filler, if a threshold value is determined and ternarization is performed, the region with the lowest brightness (epoxy resin) and the region with the highest brightness (inorganic A part of the interface of the filler) is recognized as a medium brightness region (polyolefin), and may be analyzed as if the polyolefin exists between the epoxy resin and the inorganic filler. Therefore, the length of the interface was analyzed using the watershed algorithm described in Non-Patent Document 1 according to the procedure shown below.
(1) In the electron microscope image, select a region that includes 80% or more of the thickness of the layer containing the inorganic filler, does not include regions other than the layer containing the inorganic filler, and is as wide as possible. be the processing area. When the thickness of the layer containing the inorganic filler is larger than the observation field of view, the entire observation field of view is defined as the image processing area.
(2) In the electron microscope image, three materials, epoxy resin, polyolefin, and inorganic filler, are observed with different brightness levels. When the brightness histogram of the image processing area is calculated, it corresponds to epoxy resin, polyolefin, and inorganic filler. There are three peaks that A difference in luminance between two peaks having the maximum values of the two closest peaks among the three peaks is calculated, and this numerical value is defined as CD (FIG. 4). At this time, if the numerical value of CD cannot be determined uniquely, the value of CD is determined by fitting each of the three peaks with a Gaussian function.
(3) Perform mean shift filtering described in Non-Patent Document 2 to obtain a smoothed image. Mean Shift Filter of free software ImageJ (an open source plug-in created by Mr. Kai Uwe Barthel of FHTW-Berlin) is used for execution of mean shift filtering. Run.
(4) Two thresholds are determined using the algorithm of Non-Patent Document 3 for the obtained smoothed image, and the smaller one of the two thresholds is set as threshold A, and the larger one as threshold B. .
(5) a binary image A1 in which the luminance of pixels with luminance less than threshold A is set to 255 and the luminance of pixels with luminance greater than or equal to threshold A is set to 0; , a binary image A2 in which the brightness of pixels with brightness less than threshold A and above threshold B is set to 0; A binary image A3 is created.
(6) Erosion processing is executed using a 5×5 square kernel for each of the binary image A1, the binary image A2, and the binary image A3 (a 5×5 pixel area centered on the pixel of interest If there is at least one pixel with a brightness of 0 in the image, the brightness of the target pixel is replaced with 0 for all pixels in the image), and the binary image A1 after the contraction process, the binary image Binary image B1, binary image B2, and binary image B3 are assumed to be A2 and binary image A3, respectively.
(7) Search for watersheds in the image processing region based on the algorithm of Non-Patent Document 1; At this time, the background is the area of the binary image B1 whose luminance is 255, and the markers are the binary images B2 and B3. Specifically, in the binary image B2 and the binary image B3, a region in which pixels having a brightness of 255 are continuous is regarded as one marker, and all the markers in the binary image B2 and the binary image B3 are to perform a watershed search.
(8) For the binary image B1, the binary image B2, and the binary image B3, an area surrounded by pixels corresponding to watersheds and including at least one pixel with a luminance of 255 in the area. If so, a process is performed to replace the luminance of all pixels in that region with 255. Binary image C1, Binary image C2, and Binary image C3 are assumed to be the processed images, respectively.
(9) Binary image C1, binary image C2, and binary image C3 are dilated using a 3×3 cross-shaped kernel (one of four pixels above, below, left, and right of the pixel of interest) However, if there is a pixel with a brightness of 255, the brightness of the target pixel is replaced with 255 for all pixels in the binary image), and the binary images after dilation processing are binary images D1 and D2, respectively. A value image D2 and a binary image D3 are assumed.
(10) A numerical matrix E1 obtained by replacing the numerical values of pixels with a luminance of 255 in the binary image D1 with 1, and a numerical matrix E2 obtained by replacing the numerical values of pixels with a luminance of 255 in the binary image D2 with 10; A numerical matrix E3 is created by substituting 100 for the numerical value of a pixel having a brightness of 255 in the value image D3.
(11) A numerical matrix F is the sum of the numerical matrix E1, the numerical matrix E2, and the numerical matrix E3.
(12) Among the elements of the numerical matrix F, let G1 be the number of elements with a numerical value of 11 or 111, G2 be the number of elements with a numerical value of 101 or 111, and G3 be the number of elements with a numerical value of 110 or 111. do.
(13) Considering the pixel size of the electron microscope image, G1, G2, and G3 are converted to the length of the real space, and the length of the interface between the pore and the polyolefin is L _po , and the length of the interface between the polyolefin and the inorganic filler. Let be L _pi .

Image processing (calculation of average particle size of inorganic filler)
(1) For the smoothed image, a binary image is created in which the luminance of pixels whose luminance is less than the threshold B is set to 0, and the luminance of pixels whose luminance is equal to or higher than the threshold B is set to 255.
(2) Use the free software ImageJ to perform Thickness analysis of BoneJ (open source plug-in described in Non-Patent Document 4), read the average value of Tb.Th in the Results window, and calculate the pixel size of the electron microscope image. The average particle diameter of the inorganic filler is determined by converting the length of the actual space into consideration.

[Average pore diameter of pores]
Image processing (calculation of aperture size)
The smoothed image was binarized by setting the luminance of pixels whose luminance is less than the threshold A to 0 and the luminance of pixels whose luminance is equal to or higher than the threshold A to 255. Execute Thickness analysis of BoneJ (open source plug-in described in Non-Patent Document 2), read the value of Tb.Th mean in the Results window, and convert the value into the length of the real space did.

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

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

[Puncture strength]
A needle having a hemispherical tip with a radius of 0.5 mm was prepared, and a separator was sandwiched between two plates having an opening with a diameter (dia.) of 11 mm, and the needle, separator and plate were set. Using "MX2-50N" manufactured by Imada Co., Ltd., a puncture test was performed under the conditions of a radius of curvature 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 and the maximum puncture load (ie, puncture strength (gf)) was measured.

[MD/TD intensity ratio]
The tensile strength of the separator was measured by using a tensile tester (TG-1kN type manufactured by Minebea Co., Ltd.), setting the sample length to 35 mm before testing, and pulling the sample at a speed of 100 mm/min. The tensile strength was obtained by dividing the strength (tensile load value) when the sample yielded, or the strength (tensile load value) when the sample was cut (broken) before yielding, by the cross-sectional area of the test piece. The tensile strength was measured for each of MD and TD of the separator. The MD/TD strength ratio was obtained 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 aid, and a polyvinylidene fluoride solution as a binder were mixed in a solid state ratio of 91:5:4. They were mixed at a partial weight ratio, N-methyl-2-pyrrolidone was added as a dispersion solvent so that the solid content was 68% by weight, and the mixture was further mixed to prepare a slurry solution. After this slurry solution was applied to one side of an aluminum foil having a thickness of 15 μm, the solvent was removed by drying to obtain a positive electrode having a coating amount of 175 g/m ² . This positive electrode was rolled by a roll press to obtain a positive electrode sheet having a positive electrode mixture portion with a density of 2.4 g/cm ³ .

Preparation of Negative Electrode Sheet Artificial graphite as a negative electrode active material, styrene-butadiene rubber and an aqueous carboxymethylcellulose solution as a binder were mixed at a solid content mass ratio of 96.4:1.9:1.7, and water was used as a dispersion solvent. They were added so that the solid content was 50% by mass, and further mixed to prepare a slurry solution. After this slurry solution was applied to one side of a copper foil having a thickness of 10 μm, the solvent was removed by drying to obtain a negative electrode having a coating amount of 86 g/m ² . This negative electrode was rolled by a roll press to obtain a negative electrode sheet having a negative electrode mixture portion with a density of 1.25 g/cm ³ .

Preparation of non-aqueous electrolyte Propylene carbonate: ethylene carbonate: γ-butyl lactone = 1: 1: 2 (
LiBF ₄ was dissolved as a solute in a mixed solvent having a volume ratio of 1.0 mol/L, and trioctyl phosphate was added to a concentration of 0.5% by weight to prepare a non-aqueous electrolyte. .

Measurement of Fuse Temperature and Film Resistance (Ω·cm ² ) A sample 1 was prepared by cutting a 30 mm×30 mm square sample from the separator. In addition, the positive electrode 2A in which the positive electrode prepared by the above-described method is cut into 55 mm × 20 mm squares, the electrode active material is removed so that the applied electrode active material portion becomes 20 mm × 20 mm squares, and the current collecting foil is exposed. It was created. In addition, the negative electrode prepared by the above-described method was cut into 55 mm × 25 mm squares, and the electrode active material was removed so that the applied electrode active material portion became 25 mm × 25 mm squares, and the current collector foil was exposed. Negative electrode 2A. It was created. After that, the portions of the sample 1, the positive electrode 2A, and the negative electrode 2B coated with the electrode active material were impregnated with a non-aqueous electrolyte for 1 minute or more. Then, the negative electrode 2B, the sample 1, the positive electrode 2A, the Kapton film, and the silicon rubber having a thickness of 4 mm were laminated in this order. At this time, the sample 1 and the portion coated with the electrode active material of the positive electrode 2A were laminated so as to overlap the portion coated with the electrode active material of the negative electrode 2B. This laminate is placed on a ceramic plate in which a thermocouple is embedded, and the temperature of the heater is increased while applying a surface pressure of 2 MPa with a hydraulic press. The temperature and resistance value were continuously measured using an electrical resistance measuring device "AG-4311" (Ando Denki Co., Ltd.). The temperature was raised from a room temperature of 23° C. to 220° C. at a rate of 15° C./minute, and the resistance value was measured with an alternating current of 1 V and 1 kHz. The value calculated by multiplying the obtained resistance value (Ω) at 210° C. by the effective electrode area of 4 cm ² was taken as the F/S 210° C. membrane resistance value (Ω·cm ² ). The fuse temperature was defined as the temperature at which the impedance reached three times on the high temperature side of the minimum value.

[MD heat shrinkage and TD heat shrinkage]
The heat shrinkage rate was determined by cutting out a separator into a 5 cm square, marking 9 points at intervals of 2 cm, and wrapping it with paper. The marked sample was heat-treated at a temperature of 130° C. for 1 hour and then cooled to room temperature. Due to the accuracy of the sample measurements, or possibly due to expansion of the components within the sample, the thermal shrinkage may show negative values, but negative values were considered to be 0.0%.

<<Example 1>>
[Production of pellets containing polyethylene and inorganic particles]
Polyethylene (PE, MFR = 0.31, weight average molecular weight 610,000, surface-treated barium sulfate with an average particle diameter of 500 nm (100 parts by weight of barium sulfate, 1 part by weight of sodium stearate was used for surface treatment ) were dry-blended at a mass ratio of PE:BaSO ₄ =20:80 (% by mass), and then melt-kneaded using a twin-screw extruder HK-25D (manufactured by Parker Corporation, L/D=41). In order to minimize the decomposition and denaturation of the resin, the area from the resin input hopper to the raw material tank is completely sealed, and nitrogen is continuously flowed from the bottom of the hopper to reduce the oxygen concentration near the raw material input port to 50 ppm or less. In addition, all vents were completely sealed to eliminate air leakage into the cylinder.This oxygen concentration reduction effect greatly suppressed the decomposition and denaturation of the polymer even at high temperatures.Barium sulfate By feeding it with a twin-screw feeder, it became possible to further finely disperse barium sulfate.After melt-kneading, the strand is pulled from the die (2 holes), cooled in a water-cooled bath, and then cut using a pelletizer. As a result, pellets containing polyethylene and inorganic particles (hereinafter simply referred to as "the above pellets") were obtained.

[Preparation of microporous membrane (three layers)]
A laminated sheet was formed by a co-extrusion method. Polypropylene (PP, MFR=0.51) was melted in a 32 mmφ twin co-rotating screw extruder and fed to a circular die using a gear pump. The pellets were melted in a 32 mm diameter single screw extruder and supplied to a circular die using a gear pump. The compositions melted and kneaded by each extruder were extruded into a sheet form by a circular die capable of co-extrusion of two kinds and three layers, and the molten polymer was cooled by blowing air and wound up on a roll. The kneading temperature of the polypropylene resin was 230°C, and the extrusion rate was 2.4 kg/hr. The kneading temperature of the pellets was 230°C, the extrusion rate was 1.2 kg/hr in terms of polypropylene resin, and the pellets were extruded from the inner layer (intermediate layer) of a circular die whose temperature was set at 230°C. Immediately after extrusion, the extruded precursor (original film) was cooled by an air ring at an air volume of 3.6 m3/min per Φ300 mm width. The thickness of the original film after cooling was 16 μm. The original film was then annealed at 127° C. for 15 minutes. Next, the annealed original film is cold stretched to 10% at room temperature, then hot stretched to 100% at 115 ° C. for the film after cold stretching, and Microporosity was formed by relaxing to 92% at 125°C. After the stretching and opening, the physical properties of the obtained microporous membrane were measured. Table 1 shows the results.

<<Examples 2 to 24, Comparative Examples 1 to 3>>
A microporous membrane was obtained in the same manner as in Example 1 except that the raw material, film formation conditions, or separator physical properties were changed as shown in Tables 1 and 2, and the resulting microporous membrane was evaluated. The layer structure was adjusted by changing the extrusion ratio. 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 of the inorganic particles used in Examples 8 and 11 was to use an aluminum (Al)-based coupling agent (PRENACT AL-M) manufactured by Ajinomoto Fine-Techno Co., Inc., inorganic particles: PLENACT AL-M = 98: 2 (mass %) by dry blending.

<<Example 25>>
Operation Confirmation of Lithium Ion Secondary Battery As an electrolytic solution, an electrolytic solution containing 1 mol/L of LiPF ₆ as a lithium salt in a mixture of ethylene carbonate and ethyl methyl carbonate at a volume ratio of 1:2 was used.

Lithium-nickel-manganese-cobalt mixed oxide (LiNi _0.5 Co _0.2 Mn _0.3 O ₂ ) as a positive electrode active material, and carbon black powder (manufactured by Timcal, trade name: SuperP Li) as a conductive aid. and PVDF as a binder were mixed at a mass ratio of mixed oxide:conductive aid:binder=100:3.5:3. This mixture was applied to both sides of an aluminum foil having a thickness of 15 μm as a positive electrode current collector, dried, and then pressed with a roll press to prepare a double-sided coated positive electrode.

In a solvent, graphite powder with a particle size of 22 μm (D50) (manufactured by Hitachi Chemical Co., Ltd., trade name: MAG) as a negative electrode active material, a binder (manufactured by Zeon Corporation, trade name: BM400B), and carboxymethyl cellulose as a thickener. (manufactured by Daicel Corporation, trade name: #2200) were mixed at a mass ratio of graphite powder:binder:thickener=100:1.5:1.1. This mixture is applied to one side and both sides of a copper foil as a negative electrode current collector having a thickness of 10 μm, the solvent is removed by drying, and then the coated copper foil is pressed with a roll press to obtain a single-sided coated negative electrode. and a double-sided coated negative electrode.

The resulting positive electrode and negative electrode are placed on the opposite surfaces of the respective active materials, while sandwiching the separator of Example 1. were laminated in this order. Next, the obtained laminate was inserted into a bag (battery outer packaging) 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 terminals projecting therefrom. After that, the electrolytic solution prepared as described above was poured into a 0.8 mL bag, and the bag was vacuum-sealed to prepare a sheet-like lithium ion secondary battery.

The resulting sheet-shaped lithium ion secondary battery was housed in a constant temperature bath (manufactured by Futaba Kagaku Co., Ltd., trade name: PLM-73S) set at 25 ° C., and charged and discharged by a charging/discharging device (manufactured by Aska Denshi Co., Ltd., trade name: ACD-01) and allowed to stand for 16 hours. Then, the battery was charged at a constant current of 0.05 C and charged at a constant voltage of 4.2 V for 2 hours after the voltage reached 4.2 V, and then charged at a constant current of 0.2 C to 3.0 V. The battery was initially charged and discharged by repeating the charge-discharge cycle three times. Note that 1C indicates a current value when discharging the entire capacity of the battery in 1 hour.

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

<<Comparative Example 4>>
Operation of the lithium-ion secondary battery was checked in the same manner as in Example 25, except that the separator of Comparative Example 3 was used. As a result, the capacity retention rate was less than 90%.

<<Example 26>>
Operation confirmation 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, and 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 coated on both sides of an aluminum foil having a thickness of 15 μm, dried, and pressed using a roll press to prepare a positive electrode precursor.

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

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

The positive electrode precursor, the separator of Example 1, and the negative electrode were laminated in this order so that the positive electrode active material layer and the negative electrode active material layer faced each other with the separator sandwiched therebetween so that the outermost layer was the negative electrode, thereby obtaining an electrode laminate. The resulting 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 terminals projecting therefrom. Using a charge-discharge test device (ACD-10APS (01)) manufactured by Aska Electronics Co., Ltd., under a 45 ° C. environment, constant current charging was performed at a current value of 6 A until the voltage reached 4.5 V. Initial charging was performed by a method of continuing 0.5V constant voltage charging for 1 hour, and the negative electrode was doped with lithium.

The non-aqueous lithium storage element after doping was charged at a constant current of 10.0 A in an environment of 50° C. until the voltage reached 4.3 V. Subsequently, 4.3V constant voltage charge was performed for 5 minutes, and constant current discharge was performed until the voltage reached 2.0V at 10.0A. Then, 2.0 V constant voltage discharge was performed for 5 minutes. This work was regarded as 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 to degas, and then the aluminum laminate packaging material was sealed. A lithium ion capacitor was manufactured by the above steps.

The lithium ion capacitor subjected to the above treatment was subjected to constant current charging to a voltage of 3.8 V at a charging current of 200 C (160 A) at a temperature of 25 ° C., followed by constant current discharging to 2.2 V at a current value of 200 C. rice field. Charging and discharging were repeated with this as one cycle, and the capacity retention after 50 cycles relative to the initial capacity was measured, and the capacity retention was 90% or more.

<<Comparative Example 5>>
Operation of the lithium ion capacitor was checked in the same manner as in Example 26, except that the separator of Comparative Example 3 was used. As a result, the capacity retention rate was less than 90%.

FIG. 2 is a scanning electron microscope (SEM) image of the MD-ND cross section of the inorganic-containing layer in Example 4. 3 is a scanning electron microscope (SEM) image of the MD-ND cross section of the inorganic-containing layer in Comparative Example 3. FIG. In FIGS. 2 and 3, the brightest areas indicate inorganic particles, the darkest areas indicate pores, and the intermediate light areas indicate thermoplastic resin.

The power storage device separator of the present disclosure can achieve high strength and heat resistance at the same time while having low air permeability, and is suitable as a separator for power storage devices such as lithium ion secondary batteries and lithium ion capacitors. can be used.

1 Inorganic particles 2 Thermoplastic resin 3 Pores 4 Outline of pores formed by thermoplastic resin 5 Outline of pores formed by inorganic particles 6 Interface between thermoplastic resin and inorganic particles 10 MD of inorganic-containing layer- ND cross section

Claims

A power storage device separator comprising an inorganic-containing layer containing inorganic particles and a thermoplastic resin containing 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.
The power storage device separator according to claim 1, wherein the inorganic-containing layer has an average pore diameter of 150 nm or more and 2000 nm or less in the MD-ND cross section.
The inorganic-containing layer has a thermoplastic resin region, an inorganic material region, and a void region in the MD-ND cross section,
A ratio (L po /L pi ) of the length (L po ) of the boundary portion between the void region and the thermoplastic resin region and 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.
A power storage device separator comprising an inorganic-containing layer containing inorganic particles and a thermoplastic resin containing 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 the MD-ND cross section,
A ratio (L po /L pi ) of the length (L po ) of the boundary portion between the void region and the thermoplastic resin region and 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.
The power storage device separator according to any one of claims 1 to 4, wherein the inorganic particles have a particle diameter of 60 nm or more and 2000 nm or less.
The electricity storage device separator according to any one of claims 1 to 5, having an air permeability of 50 seconds/100 ml or more and 1000 seconds/100 ml or less.
The electricity storage device separator includes a base material, the base material is a microporous layer containing 50% by mass or more of polypropylene,
The power storage device separator according to any one of claims 1 to 6, wherein the base material is present on one side or both sides of the inorganic-containing layer.
The electricity storage device separator includes a base material, and the thickness ratio between the inorganic-containing layer and the base material (thickness of the base material/thickness of the inorganic-containing layer) is 0.3 or more and 3.0 or less. The power storage device separator according to any one of claims 1 to 7.
The electricity storage device separator according to any one of claims 1 to 8, wherein the electricity storage device separator comprises a base material, and the MFR of the base material is 0.05 g/10 min or more and 0.9 g/10 min or less.
The power storage device separator according to any one of claims 1 to 9, wherein the power storage device separator has a ratio of MD tensile strength to TD tensile strength (MD/TD) of 1.5 or more.
The power storage device separator according to any one of claims 1 to 10, wherein the TD has a heat shrinkage rate of 3% or less.
The electricity storage device separator according to any one of claims 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.
The power storage device separator according to any one of claims 1 to 12, wherein the power storage device separator has a puncture strength of 100 gf/14 µm or more per 14 µm of thickness.
Any one of claims 1 to 13, wherein the inorganic-containing layer contains polyethylene as the polyolefin, and the amount of the polyethylene is 20% by mass or more based on the total mass of the thermoplastic resin for the power storage device separator. The separator for an electricity storage device according to Item 1.
The power storage device separator according to any one of claims 1 to 14, wherein the polyethylene has a weight average molecular weight of 800,000 or less.
The power storage device separator according to any one of claims 1 to 15, wherein the inorganic-containing layer has a thickness of 1 µm or more and 27 µm or less.
The power storage device separator according to any one of claims 1 to 16, wherein the power storage device separator has a total thickness of 5 µm or more and 30 µm or less.
The power storage device separator according to any one of claims 1 to 17, wherein the inorganic-containing layer has an average pore diameter of 200 nm or more and 2000 nm or less in the MD-ND cross section.
An electricity storage device comprising the electricity storage device separator according to any one of claims 1 to 18.