CN117242635A

CN117242635A - Separator for power storage device and power storage device

Info

Publication number: CN117242635A
Application number: CN202280032775.XA
Authority: CN
Inventors: 浜崎真也; 浅见阳子; 高桥真生; 吉田大贵; 武田久
Original assignee: Asahi Kasei Corp; Celgard LLC
Current assignee: Asahi Kasei Corp; Celgard LLC
Priority date: 2021-03-16
Filing date: 2022-03-14
Publication date: 2023-12-15
Also published as: JP2022142699A

Abstract

The present disclosure provides a separator for an electric storage device capable of reducing clogging and having excellent thermal stability, and an electric storage device using the same. The separator for an electric storage device described above contains a microporous layer (a) and a microporous layer (B) that contain 70wt% or more of polypropylene, and the area average main pore diameter of the microporous layer (B) in the ND-MD section is 0.95 times or less the area average main pore diameter of the microporous layer (a) in the ND-MD section. Alternatively, a separator for an electric storage device contains 70wt% or more of polyolefin, and the area average main pore diameter of the first porous surface (X) of the separator is 1.05 times or more and 10 times or less the area average main pore diameter of the second porous surface (Y) on the opposite side thereof.

Description

Separator for power storage device and power storage device

FIELD

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

Background

Microporous membranes, particularly polyolefin-based microporous membranes, are used in a wide variety of technical fields such as microfiltration membranes, battery separators, capacitor separators, materials for fuel cells, and the like, and particularly as separators for secondary batteries typified by lithium ion batteries. Lithium ion batteries are used in various types of applications, for example, in applications for small electronic devices such as mobile phones and notebook computers, and electric vehicles including hybrid vehicles and plug-in hybrid vehicles.

In recent years, lithium ion batteries having high energy capacity, high energy density, and high output characteristics have been demanded. With this trend, the demand for separators having reduced thickness, excellent battery performance, and excellent battery reliability and safety is increasing.

For example, patent document 1 discloses a thin multilayer microporous film or membrane capable of exhibiting improved characteristics, including improved dielectric breakdown and strength, compared to a conventional single-layer or three-layer microporous membrane having the same thickness. Preferred multilayer microporous films include one microlayer and one or more layer barriers.

Patent document 2 discloses a separator for an electrical storage device, which includes a microporous membrane containing polyolefin as a main component, wherein the microporous membrane has a melt tension of 30mN or less measured at a temperature of 230 ℃, and wherein the microporous membrane has a Melt Flow Rate (MFR) of 0.9g/10min or less measured at a load of 2.16kg and a temperature of 230 ℃.

[ quotation list ]

[ patent literature ]

[ patent document 1] WO 2018/089748

[ patent document 2] WO 2020/196120

SUMMARY

[ technical problem ]

There is a problem in that the separator is clogged by deposits generated due to cycle degradation, which shortens the cycle life. Further, as the size of the battery increases, a separator that exhibits excellent gas permeability and dimensional stability even after exposure to high temperatures is required.

Accordingly, it is an object of the present disclosure to provide a separator for an electric storage device capable of reducing clogging and having excellent thermal stability, and an electric storage device using the same.

Technical scheme

Examples of embodiments of the present disclosure will be listed in items [1] to [22] below.

[1]

A separator for an electrical storage device, comprising a substrate including:

a microporous layer (A) containing 70wt% or more of polypropylene; and

a microporous layer (B) containing 70wt% or more of polypropylene,

wherein the area average main pore diameter (area average major pore diameter) of pores contained in the microporous layer (B) in the ND-MD section is 0.95 times or less the area average main pore diameter of pores contained in the microporous layer (A) in the ND-MD section.

[2]

The separator for an electric storage device according to item 1, wherein the area average main pore diameter of pores contained in the microporous layer (B) in the ND-MD section is 0.30 times or more and 0.90 times or less the area average main pore diameter of pores contained in the microporous layer (a) in the ND-MD section.

[3]

The separator for an electric storage device according to item 1 or 2, wherein the substrate has a change rate of air permeability of 100% or less when the substrate with the end portion fixed is heated in the atmosphere at 140 ℃ for 30 minutes.

[4]

The separator for an electric storage device according to any one of items 1 to 3, wherein the area average main pore diameter of pores contained in the microporous layer (a) in an ND-MD section is 100nm or more and 600nm or less.

[5]

The separator for an electric storage device according to any one of items 1 to 4, wherein the microporous layer (a) constitutes an outermost layer on both sides of the substrate.

[6]

The separator for an electric storage device according to any one of items 1 to 5, wherein the substrate further comprises a microporous layer (C) containing 50wt% or more of polyolefin.

[7]

The separator for an electric storage device according to item 6, wherein the area average main pore diameter of pores contained in the microporous layer (C) in the ND-MD section is 0.20 times or more and 0.90 times or less the area average main pore diameter of pores contained in the microporous layer (B) in the ND-MD section.

[8]

The separator for an electric storage device according to item 6 or 7, wherein the substrate comprises a structure in which a microporous layer (a), a microporous layer (B), and a microporous layer (C) are sequentially laminated.

[9]

The separator for an electric storage device according to any one of items 1 to 8, wherein the substrate comprises a structure in which a microporous layer (a), a microporous layer (B), and a microporous layer (a) are laminated in this order.

[10]

The separator for an electric storage device according to any one of items 1 to 8, wherein, when a substrate surface on a side of the microporous layer (a) is defined as a first porous surface (X) and a substrate surface on a side opposite to the first porous surface (X) is defined as a second porous surface (Y), the area-average main pore diameter (S) of pores contained in the first porous surface (X) _X ) Is the area-average main pore diameter (S) of pores contained in the second porous surface (Y) _Y ) 1.05 times or more and 10 times or less of (a) the total length of the container.

[11]

The separator for an electric storage device according to item 10, wherein the average main pore diameter (S _X ) 80nm or more and 600nm or less.

[12]

The separator for an electric storage device according to any one of claims 1 to 11, wherein the substrate has a heat shrinkage of-1.0% or more and 3.0 or less in the width direction measured after heating at 150 ℃ for 1 hour.

[13]

An electric storage device comprising a positive electrode, a negative electrode, and the separator for an electric storage device according to any one of claims 1 to 12.

[14]

The power storage device according to item 13, wherein the microporous layer (a) is arranged to face the anode side.

[15]

The power storage device according to item 13 or 14, wherein the positive electrode contains lithium iron phosphate as a positive electrode active material.

[16]

A separator for an electric storage device includes a substrate containing 70wt% or more of polyolefin and having a first porous surface (X) and a second porous surface (Y) on the opposite side to the first porous surface,

wherein the pores contained in the first porous surface (X) have an area-average main pore diameter (S _X ) Is the area-average primary pore diameter (S) of the pores contained in the second porous surface (Y) _Y ) 1.05 times or more and 10 times or less of (a) the total length of the container.

[17]

The separator for an electric storage device according to item 16, wherein the average main pore diameter (S _X ) 80nm or more and 600nm or moreLower.

[18]

The separator for an electrical storage device according to item 16 or 17, wherein the polyolefin is polypropylene.

[19]

The separator for an electric storage device according to any one of claims 16 to 18, wherein the substrate has a heat shrinkage in a width direction of-1.0% or more and 3.0 or less, measured after heating at 150 ℃ for 1 hour.

[20]

An electric storage device comprising a positive electrode, a negative electrode, and the separator for an electric storage device according to any one of claims 16 to 19.

[21]

The power storage device according to item 20, wherein the first porous surface (X) is configured to face the anode side.

[22]

The power storage device according to item 20 or item 21, wherein the positive electrode contains lithium iron phosphate as a positive electrode active material.

[23]

A microporous membrane comprising a substrate comprising:

a microporous layer (A) containing 70wt% or more of polypropylene; and

a microporous layer (B) containing 70wt% or more of polypropylene,

wherein the average main pore diameter of the pores contained in the microporous layer (B) in the ND-MD section is 0.95 times or less the average main pore diameter of the pores contained in the microporous layer (A) in the ND-MD section.

[ advantageous effects of the invention ]

The present disclosure provides a separator for an electric storage device capable of reducing clogging and having excellent thermal stability, and an electric storage device using the same.

Description of the embodiments

Separator for electric storage device

The separator for an electrical storage device according to the present disclosure includes a substrate including a microporous layer containing 70wt% or more of polyolefin. The polyolefin is preferably polypropylene. The substrate may be composed of a single (one layer) microporous layer containing 70wt% or more of polypropylene, or may also contain a microporous layer (a) containing 70wt% or more of polypropylene and a microporous layer (B) containing 70wt% or more of polypropylene. The substrate may further have a coating layer (also referred to as a "surface layer", "cover layer" or the like) on one or both surfaces thereof. In the specification of the present application, the term "microporous layer" refers to each microporous layer constituting a substrate of a separator, the term "substrate" refers to a substrate of a separator excluding any disposed coating layer, and the term "separator" refers to the entire separator including any disposed coating layer. It is preferred that the substrate does not include a layer containing 50wt% or more of polyethylene.

< microporous layer (A) >

The separator for an electrical storage device according to the present disclosure preferably contains a microporous layer (a) containing 70wt% or more of polypropylene. The separator for the power storage device may include only one microporous layer (a), or may include two or more microporous layers (a). At least one of the microporous layers (A) constitutes an outermost layer on at least one side of the substrate. In the case where the separator for an electric storage device includes two or more microporous layers (a), the microporous layers (a) may constitute the outermost layers on both sides of the substrate. The microporous layer (A) contains 70wt% or more of polypropylene, which makes it possible to maintain good battery performance after storage at high temperature (140 ℃). The lower limit of the polypropylene content in the microporous layer (a) is 70wt% or more, preferably 75wt% or more, 80wt% or more, 85wt% or more, or 90wt% or more from the viewpoint of wettability, thinning, shutdown characteristics, etc. of the separator. The upper limit of the polypropylene content in the microporous layer (a) is not so limited, and may be combined with any of these lower limits, and may be, for example, 80wt% or less, 90wt% or less, 95wt% or less, 98wt% or less, or 99wt% or less, or may be 100wt%.

< Material of microporous layer (A) >)

The microporous layer (A) contains 70wt% or more of polypropylene. The polypropylene contained in the microporous layer (a) may be the same material as the polypropylene contained in the microporous layer (B) and the microporous layer (C) which will be described later, or may also be polypropylene different in chemical structure, more specifically polypropylene different in at least one of monomer composition, stereoregularity, molecular weight, crystal structure, and the like from the polypropylene in the microporous layer (B) and the microporous layer (C).

There is no limitation on the stereoregularity of polypropylene, but polypropylene may be, for example, atactic (atactic), isotactic (isotic) or syndiotactic (syndiotatic) homopolymer. The polypropylene according to the present disclosure is preferably a highly crystalline isotactic or syndiotactic homopolymer.

The polypropylene contained in the microporous layer (a) is preferably a homopolymer, but may also be a copolymer in which a small amount of a comonomer other than propylene (such as an α -olefin comonomer) is copolymerized, for example, as a block polymer. The amount of propylene structure contained as a repeating unit in the polypropylene may be, but is not limited to, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The amount of repeating units of the comonomer may be, but is not limited to, for example, 30 mole% or less, 20 mole% or less, 10 mole% or less, 5 mole% or less, or 1 mole% or less. One polypropylene may be used alone, or two or more polypropylenes may be used as a mixture.

The weight average molecular weight (Mw) of the polypropylene contained in the microporous layer (A) is preferably 300,000 or more from the viewpoint of improving the strength of the microporous layer or the like, and is preferably 1,500,000 or less from the viewpoint of increasing the pore diameter of the microporous layer and avoiding clogging. The Mw of the polypropylene is more preferably 500,000 or more and 1,300,000 or less, still more preferably 600,000 or more and 1,100,000 or less, still more preferably 700,000 or more and 1,050,000 or less, particularly preferably 800,000 or more and 1,000,000 or less.

The upper limit value of the value (Mw/Mn), which is obtained by dividing the weight average molecular weight (Mw) of the polypropylene contained in the microporous layer (A) by the number average molecular weight (Mn) thereof, is preferably 7 or less, more preferably 6.5 or less, 6 or less, 5.5 or less or 5 or less. The lower the Mw/Mn value of the polypropylene compared to the microporous layer (B), the lower the melt tension and the larger the pore size of the resulting microporous layer (A). The lower limit of Mw/Mn, which may be combined with any of these upper limits, is preferably 1 or more, and may be, for example, 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. When Mw/Mn is 1 or more, there are cases where: proper molecular entanglement can be maintained, and stability during film formation can be improved. It is noted that the weight average molecular weight, the number average molecular weight, and the Mw/Mn of the polyolefin according to the present disclosure are molecular weights in terms of polystyrene as determined by GPC (gel permeation chromatography) measurement.

The polypropylene contained in the microporous layer (A) preferably has a density of 0.85g/cm ³ Or higher, and may be, for example, 0.88g/cm ³ Or higher, 0.89g/cm ³ Or higher, 0.90g/cm ³ Or higher. The upper limit of the polypropylene density which can be combined with any of these lower limits is preferably 1.1g/cm ³ Or lower, and may be, for example, 1.0g/cm ³ Or lower, 0.98g/cm ³ Or lower, 0.97g/cm ³ Or lower, 0.96g/cm ³ Or lower, 0.95g/cm ³ Or lower, 0.94g/cm ³ Or lower, 0.93g/cm ³ Or lower or 0.92g/cm ³ Or lower. The density of the polyolefin is related to the crystallinity of the polypropylene by adjusting the density of the polypropylene to 0.85g/cm ³ Or higher, the yield of the microporous layer can be increased, which makes it particularly advantageous in the case of using a dry process.

The microporous layer (A) may contain another resin as long as it contains 70% by weight or more of polypropylene. This other resin may be, for example, a polyolefin other than polypropylene (also referred to as "other polyolefin") or a copolymer of polystyrene and polyolefin. Polyolefins are polymers which contain monomers having carbon-carbon double bonds as repeating units. Examples of monomers constituting the polyolefin other than polypropylene include, but are not limited to, monomers having a carbon-carbon double bond and having 2 or 4 to 10 carbon atoms, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene. The polyolefin is, for example, a homopolymer, copolymer, multistage polymer or the like, and is preferably a homopolymer. Specifically, the polyolefin is preferably polyethylene in view of shutdown characteristics and the like. Preferred examples of the copolymer of polystyrene and polyolefin include styrene- (ethylene-propylene) -styrene copolymer (SEPS), styrene- (ethylene-butylene) -styrene copolymer and styrene-ethylene-styrene copolymer. Styrene- (ethylene-propylene) -styrene copolymers (SEPS) are particularly preferred.

The weight average molecular weight (Mw) of the other polyolefin is preferably 300,000 or more from the viewpoint of improving the strength of the microporous layer and the like, and is preferably 1,500,000 or less from the viewpoint of increasing the pore diameter of the microporous layer and avoiding clogging to obtain a high output. The Mw of the polyolefin is more preferably 500,000 or more and 1,300,000 or less, still more preferably 600,000 or more and 1,100,000 or less, still more preferably 700,000 or more and 1,000,000 or less, particularly preferably 800,000 or more and 960,000 or less.

The upper limit value of the value (Mw/Mn), which is obtained by dividing the weight average molecular weight (Mw) of the other polyolefin by the number average molecular weight (Mn), is preferably 7 or less, more preferably 6.5 or less, 6 or less, 5.5 or less or 5 or less. Further, the lower limit of the Mw/Mn of the polyolefin contained in the microporous layer (a) which can be combined with any of these upper limits is preferably 1 or more, and may be, for example, 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. When Mw/Mn is 1 or more, there are cases where: proper molecular entanglement can be maintained, and stability during film formation can be improved.

< Melt Flow Rate (MFR) of microporous layer (A) >)

From the viewpoint of obtaining a microporous layer (a) having higher strength, the upper limit value of the Melt Flow Rate (MFR) of the microporous layer (a) (MFR of the individual layers) is preferably 4.0g/10min or less, and may be, for example, 3.0g/10min or less, 2.0g/10min or less, 1.5g/10min or less, or 1.1g/10min or less. The lower limit value of the MFR (MFR of the individual layer) of the microporous layer (a) that can be combined with any one of these upper limits is not limited, and may be, for example, 0.3g/10min or more, 0.35g/10min or more, 0.4g/10min or more, 0.45g/10min or more, 0.5g/10min or more from the viewpoint of formability and the like of the microporous layer (a). The MFR of the microporous layer (A) was measured under a load of 2.16kg and a temperature of 230 ℃.

The fact that the microporous layer (A) has an MFR of 4.0g/10min or less means that the polyolefin contained in the microporous layer (A) has a molecular weight as high as a certain degree. Furthermore, the fact that the polyolefin has a high molecular weight indicates that there are a large number of linking molecules linking the crystalline material, and this makes it easier to obtain a microporous layer (a) having high strength. When the MFR of the microporous layer (a) is 0.3g/10min or more, the melt tension during formation of the microporous layer (a) can be reduced, and the pore diameter of the microporous layer (a) can be increased (compared with that of the microporous layer (B)), and thus is preferable.

From the viewpoint of obtaining a microporous layer (A) having high strength, the MFR of polypropylene contained in the microporous layer (A) is preferably 0.3 to 4.0g/10min when measured under a load of 2.16kg and a temperature of 230 ℃. From the viewpoint of obtaining a microporous layer having high strength, the upper limit value of the MFR of polypropylene may be, for example, 3.0g/10min or less, 2.0g/10min or less, 1.5g/10min or less, or 1.1g/10min or less. The lower limit value of the MFR of the polypropylene that can be combined with any one of these upper limits is not limited from the viewpoint of the formability and the like of the microporous layer (a), and may be, for example, 0.3g/10min or more, 0.35g/10min or more, 0.4g/10min or more, or 0.45g/10min or more.

< pentad fraction (pentad fraction) of microporous layer (A) >)

The lower limit value of the pentad fraction of the polypropylene contained in the microporous layer (a) is preferably 94.0% or more from the viewpoint of obtaining a microporous layer having low air permeability, and may be, for example, 95.0% or more, 96.0% or more, 96.5% or more, 97.0% or more, 97.5% or more, 98.0% or more, 98.5% or more, or 99.0% or more. The upper limit of the pentad fraction of polypropylene that can be combined with any of these lower limits can be, but is not limited to, 99.9% or less, 99.8% or less, or 99.5% or less. The pentad fraction of polypropylene is obtained by ¹³ Measured by C-NMR (nuclear magnetic resonance).

The fact that the pentad fraction of polypropylene is 94.0% or more indicates that polypropylene has high crystallinity. In the separator obtained by the stretching hole forming process, particularly by the dry method, amorphous portions between crystalline materials are stretched to form holes. Therefore, when polypropylene has high crystallinity, good pore forming property can be obtained, and also the air permeability can be reduced to a low level, thereby enabling high battery output.

< area average Main pore diameter of microporous layer (A) >)

The area average main pore diameter (hereinafter also simply referred to as "area average main pore diameter") in the ND-MD section of the pores contained in the microporous layer (a) is preferably larger than the area average main pore diameter of the microporous layer (B). The area average main pore diameter of the microporous layer (B) is preferably not more than 0.99 times the area average main pore diameter of the microporous layer (a). In the present specification, the term "ND" refers to the thickness direction of the microporous layer, and the term "MD" refers to the direction in which the microporous layer is formed. For example, if a separator including one or more microporous layers is in a roll form, the MD direction of the separator is the machine direction. The term "main pore size" refers to pore size in the MD. Further, when the substrate includes two or more microporous layers (a) and/or microporous layers (B), the area average main pore diameters of the microporous layers (a) and (B) are compared based on the average value of the area average main pore diameters of the respective layers.

The fact that the area average main pore diameter of the microporous layer (B) is 0.99 times or less the area average main pore diameter of the microporous layer (a) means that the microporous layer (a) as the outermost layer of the substrate is a microporous layer having a pore diameter larger than that of the microporous layer (B). When the area average main pore diameter of the microporous layer (B) is 0.99 times or more than that of the microporous layer (a), both reduction of clogging and prevention of short circuit can be achieved in a film with balanced properties. More specifically, it is considered that when the microporous layer (a) as the outermost layer has a large pore diameter, clogging of the separator due to deposition can be reduced in battery evaluation (cycle test), and when the microporous layer (B) located at the inner layer has a small pore diameter, occurrence of short circuits in the thus-produced power storage device can be prevented. The area average main pore diameter of the microporous layer (B) is preferably 0.95 times or less, more preferably 0.90 times or less, the area average main pore diameter of the microporous layer (a). The lower limit of the area-average main pore diameter of the microporous layer (B) which can be combined with any one of these upper limits is preferably 0.30 times or more, more preferably 0.40 times or more the area-average main pore diameter of the microporous layer (a). It is considered that by adjusting the area average main pore diameter of the microporous layer (B) to 0.30 times or more the area average main pore diameter of the microporous layer (a), a sufficient strength of the separator can be ensured.

The area-average main pore diameter of the microporous layer (A) is preferably 100nm or more and 600nm or less. When the area average main pore diameter of the microporous layer (a) is 100nm or more, clogging of the separator due to deposits in the power storage device can be more effectively reduced; on the other hand, when the area average main pore diameter of the microporous layer (A) is 600nm or less, the strength of the separator can be further improved. In the preferred embodiment, it is more important to reduce clogging due to deposits in the power storage device, and for this reason, it is more important that the microporous layer (a) has an area average main pore diameter of 100nm or more. The area-average main pore diameter of the microporous layer (A) is more preferably 150nm or more and 550nm or less, still more preferably 180nm or more and 500nm or less, still more preferably 200nm or more and 450nm or less.

The area average primary pore size can be measured by the following procedure: the MD-ND section of the separator was observed by cross-sectional SEM, and an image analysis was performed on a region of 20 μm×3 μm (MD direction×ND direction) in the obtained image. Detailed conditions will be described in examples. In the case of measuring the average pore diameter from the cross-sectional SEM image, the number average pore diameter and the area average pore diameter can be calculated. However, in the calculation of the number average pore diameter, even very small pores are counted as 1 pore, which makes it difficult to obtain a sufficient correlation with the physical properties of the separator. Therefore, the area average pore diameter is used as the average pore diameter in the specification of the present application so that the correlation with the physical properties of the separator can be obtained.

< porosity of microporous layer (A) >)

The microporous layer (a) preferably has a porosity of 20% or more from the viewpoint of avoiding clogging in the resulting power storage device and improving the air permeability of the resulting separator, and preferably has a porosity of 70% or less from the viewpoint of maintaining the strength of the separator. The porosity of the microporous layer (a) is more preferably 25% or more and 65% or less, still more preferably 30% or more and 60% or less, particularly preferably 35% or more and 55% or less.

< thickness of microporous layer (A) >)

The thickness of the microporous layer (a) is preferably, for example, 10 μm or less from the viewpoint of achieving high energy density of the resulting power storage device, and may be, for example, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm or less, or 4 μm or less. The lower limit value of the thickness of the microporous layer (a) that can be combined with any one of these upper limits is preferably 1 μm or more from the viewpoint of improving strength and the like, and may be, for example, 2 μm or more, 3 μm or more, or 3.5 μm or more.

< additive for microporous layer (A) >)

In addition to polypropylene, if desired, the microporous layer (A) containing 70% by weight or more of polypropylene may further contain additives such as an elastomer, a crystallization nucleating agent, an antioxidant, a filler, and the like. The amount of the additive is not particularly limited, and is, for example, 0.01wt% or more, 0.1wt% or more, or 1wt% or more based on the total mass of the microporous layer (a). The upper limit of the amount of additive that can be combined with any of these lower limits can be 20wt% or less, 10wt% or less, or 7wt% or less.

< microporous layer (B) >)

The separator for an electrical storage device according to the present disclosure includes a microporous layer (B). The separator for the electric storage device may contain only one microporous layer (B), or may contain two or more microporous layers (B). The microporous layer (B) also contains 70wt% or more of polypropylene, which enables good battery performance to be maintained after storage at high temperature (140 ℃). The lower limit of the polypropylene content in the microporous layer (B) may be preferably 75wt% or more, 80wt% or more, 85wt% or more, 90wt% or more or 95wt% or more from the viewpoint of wettability, thinning, shutdown characteristics, etc. of the separator. The upper limit of the polypropylene content in the microporous layer (B) which may be combined with any one of these lower limits is not limited, and may be, for example, 80wt% or less, 90wt% or less, 95wt% or less, 98wt% or less or 99wt% or less, or may be 100wt%.

< Material of microporous layer (B) >)

The microporous layer (B) contains 70wt% or more of polypropylene. The polypropylene contained in the microporous layer (B) may be the same material as the polypropylene contained in the microporous layer (a) and the microporous layer (C) to be described later, or may also be polypropylene different in chemical structure, more specifically polypropylene different in at least one of monomer composition, stereoregularity, molecular weight, crystal structure, and the like from the polypropylene in the microporous layer (a) and the microporous layer (C).

There is no limitation on the stereoregularity of the polypropylene contained in the microporous layer (B), but the polypropylene may be, for example, an atactic, isotactic or syndiotactic homopolymer. The polypropylene according to the present disclosure is preferably a highly crystalline isotactic or syndiotactic homopolymer.

The polypropylene contained in the microporous layer (B) is preferably a homopolymer, but may also be a copolymer in which a small amount of a comonomer other than propylene (such as an α -olefin comonomer) is copolymerized, for example, as a block polymer. The amount of propylene structure contained as a repeating unit in the polypropylene may be, but is not limited to, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The upper limit of the amount of the repeating unit of the comonomer may be, but is not limited to, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. One polypropylene may be used alone, or two or more polypropylenes may be used as a mixture.

The weight average molecular weight (Mw) of the polypropylene contained in the microporous layer (B) is preferably 300,000 or more from the viewpoint of improving the strength of the microporous layer or the like, and is preferably 1,500,000 or less from the viewpoint of increasing the pore diameter of the microporous layer and avoiding clogging. The Mw of the polypropylene is more preferably 500,000 or more and 1,300,000 or less, still more preferably 600,000 or more and 1,100,000 or less, still more preferably 700,000 or more and 1,050,000 or less, particularly preferably 800,000 or more and 1,000,000 or less.

The upper limit value of the value (Mw/Mn), which is obtained by dividing the weight average molecular weight (Mw) of the polypropylene contained in the microporous layer (B) by the number average molecular weight (Mn) thereof, is preferably 20 or less, more preferably 15 or less. The lower limit of Mw/Mn, which may be combined with any of these upper limits, is preferably 4 or more, and may be, for example, 4.5 or more, 5.0 or more, or 5.5 or more. The higher the Mw/Mn value of the polypropylene, the higher the melt tension of the resulting microporous layer tends to be compared with the microporous layer (B). Therefore, the fact that the Mw/Mn value of polypropylene is 4 or more means that the melt tension of the microporous layer (B) can be controlled to be higher than that of the microporous layer (A), and also means that as a result, the pore size of the microporous layer (B) can be controlled to be smaller than that of the microporous layer (A), and is thus preferable. It is noted that the weight average molecular weight, the number average molecular weight, and the Mw/Mn of the polyolefin according to the present disclosure are molecular weights in terms of polystyrene as measured by GPC (gel permeation chromatography) measurement.

The polypropylene contained in the microporous layer (B) preferably has a density of 0.85g/cm ³ Or higher, and may be, for example, 0.88g/cm ³ Or higher, 0.89g/cm ³ Or higher or 0.90g/cm ³ Or higher. The upper limit of the polypropylene density which can be combined with any of these lower limits is preferably 1.1g/cm ³ Or lower, and may be, for example, 1.0g/cm ³ Or lower, 0.98g/cm ³ Or lower, 0.97g/cm ³ Or lower, 0.96g/cm ³ Or lower, 0.95g/cm ³ Or lower, 0.94g/cm ³ Or lower, 0.93g/cm ³ Or lower or 0.92g/cm ³ Or lower. The density of the polyolefin is related to the crystallinity of the polypropylene by adjusting the density of the polypropylene to 0.85g/cm ³ Or higher, the yield of the microporous layer can be increased, which makes it particularly advantageous in the case of using a dry process.

The microporous layer (B) may further contain another resin as long as the microporous layer (B) contains 70wt% or more of polypropylene. The other resin may be, for example, a polyolefin other than polypropylene (also referred to as "other polyolefin") or a copolymer of polystyrene and polyolefin. Polyolefins are polymers which contain monomers having carbon-carbon double bonds as repeating units. Examples of polyolefin monomers constituting the outside of polypropylene include, but are not limited to, monomers having a carbon-carbon double bond and having 2 or 4 to 10 carbon atoms, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene. The polyolefin is, for example, a homopolymer, copolymer, multistage polymer or the like, and is preferably a homopolymer. Specifically, the polyolefin is preferably polyethylene in view of shutdown characteristics and the like.

< Melt Flow Rate (MFR) of microporous layer (B) >)

From the viewpoint of obtaining a microporous layer (B) having higher strength, the upper limit value of the Melt Flow Rate (MFR) of the microporous layer (B) (MFR of the individual layers) is preferably 1.5g/10min or less, and may be, for example, 1.4g/10min or less, 1.3g/10min or less, 1.2g/10min or less, or 1.1g/10min or less. The lower limit value of the MFR (MFR of the individual layer) of the microporous layer (B) that can be combined with any one of these upper limits is not limited, and may be, for example, 0.2g/10min or more, 0.25g/10min or more, 0.3g/10min or more, 0.35g/10min or more, or 0.4g/10min or more from the viewpoint of formability and the like of the microporous layer (B). The MFR of the microporous layer (B) was measured under a load of 2.16kg and a temperature of 230 ℃.

The fact that the microporous layer (B) has an MFR of 1.5g/10min or less means that the polyolefin contained in the microporous layer (B) has a molecular weight as high as a certain degree. Furthermore, the fact that the polyolefin has a high molecular weight indicates that there are a large number of linking molecules linking the crystalline material, and this makes it easier to obtain a microporous layer (B) having high strength. In addition, the melt tension can be kept at a high level, which is advantageous in control to achieve a small pore size. When the MFR of the microporous layer (B) is 0.2g/10min or more, the melt tension of the microporous layer (B) can be prevented from being excessively low, and a microporous layer having high strength and reduced thickness can be more easily obtained.

From the viewpoint of obtaining a microporous layer (B) having high strength, the MFR of polypropylene contained in the microporous layer (B) is preferably 0.2 to 1.5g/10min when measured under a load of 2.16kg and a temperature of 230 ℃. From the viewpoint of obtaining a microporous layer having higher strength, the upper limit value of the MFR of polypropylene may be, for example, 1.4g/10min or less, 1.3g/10min or less, 1.2g/10min or less, or 1.1g/10min or less. The lower limit value of the MFR of the polypropylene that can be combined with any one of these upper limits is not limited from the viewpoint of the formability and the like of the microporous layer (B), and may be, for example, 0.25g/10min or more, 0.3g/10min or more, 0.35g/10min or more, or 0.4g/10min or more.

The MFR of microporous layer (B) is preferably lower than that of microporous layer (a). By adjusting the MFR of the microporous layer (B) to be lower than that of the microporous layer (a), the pore size of the microporous layer (a) of the resulting separator can be controlled to be larger than that of the microporous layer (B) of the separator. The ratio of the MFR of the microporous layer (B) to the MFR of the microporous layer (a) is preferably 0.95 or less, more preferably 0.90 or less, still more preferably 0.85 or less. The lower limit of the above ratio, which may be combined with any one of these upper limits, is preferably 0.2 or more, more preferably 0.3 or more, still more preferably 0.4 or more, from the viewpoint of film formation stability.

< five-membered fraction of microporous layer (B)

The lower limit value of the pentad fraction of the polypropylene contained in the microporous layer (B) is preferably 94.0% or more from the viewpoint of obtaining a microporous layer having low air permeability, and may be, for example, 95.0% or more, 96.0% or more, 96.5% or more, 97.0% or more, 97.5% or more, 98.0% or more, 98.5% or more, or 99.0% or more. The upper limit of the pentad fraction of polypropylene that can be combined with any of these lower limits can be, but is not limited to, 99.9% or less, 99.8% or less, or 99.5% or less. The pentad fraction of polypropylene is obtained by ¹³ Measured by C-NMR (nuclear magnetic resonance).

< area average Main pore diameter of microporous layer (B) >)

The area average main pore diameter (hereinafter also simply referred to as "area average main pore diameter") in the ND-MD section of the pores contained in the microporous layer (B) is smaller than that of the microporous layer (a). For details of the relationship with the area average main pore diameter of the microporous layer (a), see the section of < area average main pore diameter of microporous layer (a >).

The area-average main pore diameter of the microporous layer (B) is preferably 40nm or more and 500nm or less, more preferably 60nm or more and 450nm or less, still more preferably 80nm or more and 400nm or less, still more preferably 100nm or more and 350nm or less. When the area average main pore diameter of the microporous layer (B) is within the above range, clogging of the resulting separator can be reduced more effectively and short-circuiting can be prevented.

< porosity of microporous layer (B) >)

The microporous layer (B) preferably has a porosity of 20% or more from the viewpoint of avoiding clogging in the resulting power storage device and improving the air permeability of the resulting separator, and preferably has a porosity of 70% or less from the viewpoint of maintaining the strength of the separator. The porosity of the microporous layer (B) is more preferably 25% or more and 65% or less, still more preferably 30% or more and 60% or less, particularly preferably 35% or more and 55% or less.

< thickness of microporous layer (B) >)

The thickness of the microporous layer (B) according to the present disclosure is preferably, for example, 10 μm or less from the viewpoint of achieving high energy density of the resulting power storage device, and may be, for example, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm or less, or 4 μm or less. The lower limit value of the thickness of the microporous layer (B) that can be combined with any one of these upper limits is preferably 1 μm or more from the viewpoint of improving strength and the like, and may be, for example, 2 μm or more, 3 μm or more, or 3.5 μm or more.

< additive for microporous layer (B) >)

In addition to polypropylene, if desired, the microporous layer (B) containing 70% by weight or more of polypropylene may further contain additives such as an elastomer, a crystallization nucleating agent, an antioxidant, a filler, and the like. The amount of the additive is not particularly limited, and is, for example, 0.01wt% or more, 0.1wt% or more, or 1wt% or more based on the total mass of the microporous layer (B). The upper limit of the amount of additive that can be combined with any of these lower limits can be 10wt% or less, 7wt% or less, or 5wt% or less.

< microporous layer (C) >)

In addition to the microporous layer (a) and the microporous layer (B), the separator for an electric storage device according to the present disclosure may further include a microporous layer (C) containing 50wt% or more of polyolefin. In this case, the separator for the power storage device may contain only one microporous layer (C), or may contain two or more microporous layers (C). The microporous layer (C) preferably contains 50wt% or more of polypropylene. This enables good battery performance to be maintained after storage at high temperature (140 ℃). The lower limit of the polypropylene content in the microporous layer (C) may be preferably 55wt% or more, 60wt% or more, 70wt% or more, 80wt% or more, 90wt% or more or 95wt% or more from the viewpoint of wettability, thinning, shutdown characteristics, etc. of the separator. The upper limit of the polypropylene content in the microporous layer (C) that can be combined with any one of these lower limits is not limited, and may be, for example, 60wt% or less, 70wt% or less, 80wt% or less, 90wt% or less, 95wt% or less, 98wt% or less, or 99wt% or less, or may be 100wt%.

< Material of microporous layer (C) >)

The polypropylene contained in the microporous layer (C) may be the same material as the polypropylene contained in the microporous layer (a) and the microporous layer (B), or may be polypropylene different in chemical structure, more specifically, polypropylene different in at least one of monomer composition, stereoregularity, molecular weight, crystal structure, and the like from the polypropylene contained in the microporous layer (a) and the microporous layer (B).

There is no limitation on the stereoregularity of the polypropylene contained in the microporous layer (C), but the polypropylene may be, for example, an atactic, isotactic or syndiotactic homopolymer. The polypropylene according to the present disclosure is preferably a highly crystalline isotactic or syndiotactic homopolymer.

The polypropylene contained in the microporous layer (C) is preferably a homopolymer, but may also be a copolymer in which a small amount of a comonomer other than propylene (such as an alpha-olefin comonomer) is copolymerized, for example, as a block polymer. The amount of the propylene structure contained as the repeating unit in the polypropylene may be, but is not limited to, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The upper limit of the amount of the repeating unit of the comonomer may be, but is not limited to, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. One polypropylene may be used alone, or two or more polypropylenes may be used as a mixture.

The weight average molecular weight (Mw) of the polypropylene contained in the microporous layer (C) is preferably 300,000 or more from the viewpoint of improving the strength of the microporous layer and the like, and is preferably 1,500,000 or less from the viewpoint of increasing the pore diameter of the microporous layer and avoiding clogging. The Mw of the polypropylene is more preferably 500,000 or more and 1,300,000 or less, still more preferably 600,000 or more and 1,100,000 or less, still more preferably 700,000 or more and 1,050,000 or less, and particularly preferably 800,000 or more and 1,000,000 or less.

The upper limit value of the value (Mw/Mn) obtained by dividing the weight average molecular weight (Mw) of the polypropylene contained in the microporous layer (C) by the number average molecular weight (Mn) thereof is preferably 20 or less, more preferably 15 or less. The lower limit value of Mw/Mn, which can be combined with any of these upper limits, is preferably 4 or more, and may be, for example, 4.5 or more, 5.0 or more, or 5.5 or more. The higher the Mw/Mn value of the polypropylene, the higher the melt tension of the resulting microporous layer tends to be. Therefore, a value of Mw/Mn of polypropylene of 4 or more means that the melt tension of the microporous layer (C) can be controlled to be higher than that of the microporous layer (B), and also means that as a result, the pore size of the microporous layer (C) can be controlled to be smaller than that of the microporous layer (B), and thus is preferable. It is noted that the weight average molecular weight, the number average molecular weight, and the Mw/Mn of the polyolefin according to the present disclosure are molecular weights in terms of polystyrene as determined by GPC (gel permeation chromatography) measurement.

The density of the polypropylene contained in the microporous layer (C) is preferably0.85g/cm ³ Or higher, and may be, for example, 0.88g/cm ³ Or higher, 0.89g/cm ³ Or higher or 0.90g/cm ³ . The upper limit of the polypropylene density which may be combined with any of these lower limits is preferably 1.1g/cm ³ Or lower and may be, for example, 1.0g/cm ³ Or lower, 0.98g/cm ³ Or lower, 0.97g/cm ³ Or lower, 0.96g/cm ³ Or lower, 0.95g/cm ³ Or lower, 0.94g/cm ³ Or lower, 0.93g/cm ³ Or lower or 0.92g/cm ³ Or lower. The density of the polyolefin is related to the crystallinity of the polypropylene by adjusting the density of the polypropylene to 0.85g/cm ³ Or higher, the productivity of the microporous layer can be improved, which makes it particularly advantageous in the case of using a dry method.

The microporous layer (C) may contain another resin in addition to polypropylene. This other resin may be, for example, a polyolefin other than polypropylene (also referred to as "other polyolefin") or a copolymer of polystyrene and polyolefin. Polyolefins are polymers which contain monomers having carbon-carbon double bonds as repeating units. Examples of polyolefin monomers constituting the outside of polypropylene include, but are not limited to, monomers having a carbon-carbon double bond and having 2 or 4 to 10 carbon atoms, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene. The polyolefin is, for example, a homopolymer, copolymer, multistage polymer or the like, and is preferably a homopolymer. Specifically, the polyolefin is preferably polyethylene in view of shutdown characteristics and the like.

< Melt Flow Rate (MFR) of microporous layer (C) >)

From the viewpoint of obtaining a microporous layer (C) having higher strength, the upper limit value of the Melt Flow Rate (MFR) of the microporous layer (C) (MFR of the individual layers) is preferably 1.5g/10min or less, and may be, for example, 1.4g/10min or less, 1.3g/10min or less, 1.2g/10min or less, or 1.1g/10min or less. The lower limit value of the MFR (MFR of the individual layer) of the microporous layer (C) that can be combined with any one of these upper limits is not limited, and may be, for example, 0.2g/10min or more, 0.25g/10min or more, 0.3g/10min or more, 0.35g/10min or more, or 0.4g/10min or more from the viewpoint of formability and the like of the microporous layer (C). The MFR of the microporous layer (C) was measured under a load of 2.16kg and a temperature of 230 ℃.

The fact that the microporous layer (C) has an MFR of 1.5g/10min or less means that the polyolefin contained in the microporous layer (C) has a molecular weight as high as a certain degree. Furthermore, the fact that the polyolefin has a high molecular weight indicates that there are a large number of linking molecules linking the crystalline material, and this makes it easier to obtain a microporous layer (C) having high strength. When the MFR of the microporous layer (C) is 0.2g/10min or more, the melt tension of the microporous layer (C) can be prevented from being excessively low, and a microporous layer having high strength and reduced thickness can be more easily obtained.

From the viewpoint of obtaining a microporous layer (C) having high strength, the MFR of polypropylene contained in the microporous layer (C) is preferably 0.2 to 1.5g/10min when measured under a load of 2.16kg and a temperature of 230 ℃. From the viewpoint of obtaining a microporous layer having higher strength, the upper limit value of the MFR of polypropylene may be, for example, 1.4g/10min or less, 1.3g/10min or less, 1.2g/10min or less, or 1.1g/10min or less. The lower limit value of the MFR of the polypropylene that can be combined with any one of these upper limits is not limited from the viewpoint of the formability and the like of the microporous layer (C), and may be, for example, 0.25g/10min or more, 0.3g/10min or more, 0.35g/10min or more, or 0.4g/10min or more.

< five-tuple fraction of microporous layer (C) >)

The lower limit value of the pentad fraction of the polypropylene contained in the microporous layer (C) is preferably 94.0% or more from the viewpoint of obtaining a microporous layer having low air permeability, and may be, for example, 95.0% or more, 96.0% or more, 96.5% or more, 97.0% or more, 97.5% or more, 98.0% or more, 98.5% or more, or 99.0% or more. The upper limit of the pentad fraction of polypropylene that can be combined with any of these lower limits can be, but is not limited to, 99.9% or less, 99.8% or less, or 99.5% or less. The pentad fraction of polypropylene is obtained by ¹³ Measured by C-NMR (nuclear magnetic resonance).

< area average Main pore diameter of microporous layer (C) >)

The area average main pore diameter (hereinafter also simply referred to as "area average main pore diameter") in the ND-MD section of the pores contained in the microporous layer (C) is preferably smaller than the area average main pore diameter of the microporous layer (B). Specifically, the area average main pore diameter of the microporous layer (C) is preferably 0.20 times or more and 0.90 times or less, more preferably 0.50 times or more and 0.90 times or less the area average main pore diameter of the microporous layer (B). This makes it possible to more effectively reduce clogging of the resulting separator and prevent short circuits. When the substrate includes two or more microporous layers (C) and/or microporous layers (B), the area average main pore diameters of the microporous layers (C) and (B) are compared based on the average value of the area average main pore diameters of the layers of each category.

The area-average main pore diameter of the microporous layer (C) is preferably 20nm or more and 450nm or less, more preferably 40nm or more and 400nm or less, still more preferably 60nm or more and 350nm or less, still more preferably 80nm or more and 300nm or less. When the area average main pore diameter of the microporous layer (C) is within the above range, clogging of the separator can be more effectively reduced and short circuit can be prevented.

< porosity of microporous layer (C) >)

The microporous layer (C) preferably has a porosity of 20% or more from the viewpoint of avoiding clogging in the resulting power storage device and improving the air permeability of the resulting separator, and preferably has a porosity of 70% or less from the viewpoint of maintaining the strength of the separator. The porosity of the microporous layer (C) is more preferably 25% or more and 65% or less, still more preferably 30% or more and 60% or less, particularly preferably 35% or more and 55% or less.

< thickness of microporous layer (C) >)

The thickness of the microporous layer (C) according to the present disclosure is preferably, for example, 10 μm or less from the viewpoint of achieving high energy density of the resulting power storage device, and may be, for example, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm or less, or 4 μm or less. The lower limit value of the thickness of the microporous layer (C) that can be combined with any one of these upper limits is preferably 1 μm or more from the viewpoint of improving strength and the like, and may be, for example, 2 μm or more, 3 μm or more, or 3.5 μm or more.

< additive for microporous layer (C) >)

In addition to polypropylene, the microporous layer (C) may further contain additives such as an elastomer, a crystallization nucleating agent, an antioxidant, a filler, etc., if necessary. The amount of the additive is not particularly limited, and is, for example, 0.01wt% or more, 0.1wt% or more, or 1wt% or more based on the total mass of the microporous layer (C). The upper limit of the amount of additive that can be combined with any of these lower limits can be 10wt% or less, 7wt% or less, or 5wt% or less.

< area average principal pore diameter of substrate surface >

The substrate preferably contains polyolefin as a main component, and has a first porous surface (X) and a second porous surface (Y) on the opposite side to the first porous surface (X). The area average main pore diameter (SX) of the pores contained in the first porous surface (X) is preferably 1.05 times or more and 10 times or less, more preferably 1.1 times or more and 5 times or less, still more preferably 1.2 times or more and 3 times or less the area average main pore diameter (SY) of the pores contained in the second porous surface (Y). The surface (X) and the surface (Y) may be constituted by the surfaces of a single (one layer) microporous layer; alternatively, the surface (X) may be constituted by the surface of one microporous layer or the surface of a laminated microporous layer in which two or more layers are laminated, and the surface (Y) may be constituted by the surface of another microporous layer or the laminated microporous layer. When the substrate has a layered structure including at least one of each of the microporous layer (a) and the microporous layer (B), the surface on the microporous layer (a) side corresponds to the first porous surface (X), and the surface of the microporous film on the opposite side from the microporous layer (a) corresponds to the second porous surface (Y).

The area average main pore diameter of each surface can be measured by observing the surface of the separator with SEM and performing image analysis on the obtained image. The term "main pore size" refers to pore size in the MD. The term "MD" refers to the direction in which a microporous layer is formed. For example, if a separator comprising one or more microporous layers is in a roll form, the MD of the separator is the machine direction. In the case of measuring the average pore diameter from the surface SEM image, the number average pore diameter (number average pore diameter) and the area average pore diameter (area average pore diameter) can be calculated. However, in the calculation of the number average pore diameter, even very small pores are counted as 1 pore, and thus it is difficult to obtain a sufficient correlation with the physical properties of the separator. Therefore, the area average pore diameter is used as the average pore diameter in the specification of the present application, so that the correlation with the physical properties of the separator can be obtained.

The fact that the area average main pore diameter (SX) of the surface (X) is not less than 1.05 times the area average main pore diameter (SY) of the surface (Y) means that the surface (X) has a pore diameter larger than that of the surface (Y). When the area average main pore diameter (SX) of the surface (X) is not less than 1.05 times the area average main pore diameter (SY) of the surface (Y), clogging of the substrate in battery evaluation (cycle test) can be reduced, and occurrence of short-circuiting due to foreign matter mixing can be prevented. When the area average main pore diameter (SX) of the surface (X) is not more than 10 times the area average main pore diameter (SY) of the surface (Y), it is considered that the strength of the separator can be sufficiently ensured.

The area average main pore diameter (SX) of the surface (X) is preferably 80nm or more and 600nm or less, more preferably 120nm or more and 550nm or less, still more preferably 130nm or more and 500nm or less, still more preferably 140nm or more and 450nm or less. When the area average main pore diameter of the surface (X) is 80nm or more, clogging of the separator due to deposits in the power storage device can be more effectively reduced; when the area average main pore diameter is 600nm or less, the strength of the separator can be further improved. It is preferable to reduce clogging due to deposits in the power storage device, and for this reason, the area average main pore diameter of the surface (X) is preferably 80nm or more.

The area average main pore diameter (SY) of the surface (Y) is preferably 20nm or more and 500nm or less, more preferably 30nm or more and 450nm or less, still more preferably 40nm or more and 400nm or less, still more preferably 50nm or more and 350nm or less. When the area average main pore diameter of the microporous layer (B) is within the above range, clogging of the separator can be more effectively reduced and short circuit can be prevented.

< layer Structure of substrate >

A substrate (also simply referred to as "substrate" in the specification of the present application) of a separator for an electric storage device is constituted of a single (one-layer) microporous layer, or contains at least one of each of the microporous layer (a) and the microporous layer (B). The substrate may have a multi-layer structure of three or more layers, including two or more microporous layers (a) and/or microporous layers (B). Examples of the multilayer structure include a two-layer structure of microporous layer (a)/microporous layer (B) and a three-layer structure of microporous layer (a)/microporous layer (B)/microporous layer (a). Further, the substrate may include a microporous layer (a) and a layer other than the microporous layer (B). Examples of the layers other than the microporous layer (a) and the microporous layer (B) include the microporous layer (C), the inorganic substance-containing layer, and the heat-resistant resin-containing layer. For example, the substrate may have a multilayer structure of four or more layers, such as a microporous layer (a)/microporous layer (B)/microporous layer (C)/microporous layer (a) structure. A symmetrical laminated structure is preferable from the viewpoints of ease of manufacture, prevention of curling of the separator, and the like.

In the case where the substrate includes the microporous layer (C), the substrate preferably has a three-layer structure of microporous layer (a)/microporous layer (B)/microporous layer (C). When the substrate has such a layer structure, clogging of the separator can be reduced more effectively and short circuits can be prevented.

< substrate thickness >

The upper limit value of the thickness of the substrate is preferably, for example, 25 μm or less from the viewpoint of achieving high energy density of the resulting power storage device, and may be, for example, 22 μm or less, 20 μm or less, 18 μm or less, 16 μm or less, 14 μm or less, or 12 μm or less. The lower limit value of the thickness of the substrate that can be combined with any one of these upper limit values is preferably 6 μm or more, and may be, for example, 7 μm or more, from the viewpoint of improving strength and the like. 8 μm or more, 9 μm or more, or 10 μm or more.

< air permeability (air resistance) of substrate >

When the thickness of the substrate is converted to 16. Mu.m, the upper limit value of the air permeability of the substrate is preferably 290 seconds/100 cm ³ Or lower and may be, for example, 280 seconds/100 cm ³ Or lower, 270 seconds/100 cm ³ Or lower, 260 seconds/100 cm ³ Or less or 250 seconds/100 cm ³ Or lower. The lower limit value of the air permeability of the substrate that can be combined with any one of these upper limits is not limited, and may be, for example, 50 seconds/100 cm when the thickness of the substrate is converted to 16 μm ³ Or higher, 60 seconds/100 cm ³ Or higher or 70 seconds/100 cm ³ Or higher.

< air permeability (air resistance) of substrate after high temperature treatment >

When the end portion of the substrate is fixed and heated in an air atmosphere at 140 ℃ for 30 minutes (hereinafter also simply referred to as "after high temperature treatment"), the substrate according to the present disclosure preferably has a change rate of air permeability of 100% or less. The air permeability change rate can be determined by the following formula:

air permeability change rate (%) = { air permeability after heating (seconds/100 cm ³ ) Air permeability before heating (seconds/100 cm) ³ ) Air permeability after heating (seconds/100 cm) ³ )×100

The expression "fixing the end portion of the substrate" refers to a case where the substrate is subjected to heat treatment in a state where the end portion is fixed, in order to assume that the separator is fixed in the production of the power storage device.

In the case of a conventional separator comprising a substrate comprising a layer based on polyethylene, there is a gas permeability exceeding 5000 seconds/100 cm after high temperature treatment ³ Is the case for (a). In contrast, a change rate of 100% or less means that the change in air permeability is extremely small even after exposure to high temperature due to a drying process or the like. When the change rate of the air permeability is 100% or less, in the manufacture of the power storage device, good battery performance can be ensured after drying at high temperature. The air permeability change rate is preferably 80% or less, 60% or less, 40% or less, 20% or less, 10% or less or 5% or less, and may be in combination with any of these upper limits The lower limit of the combination is preferably, but not limited to, -5% or higher, -3% or higher, -2% or higher, -1% or higher, 0% or higher or greater than 0%.

When the thickness of the substrate is converted to 16 μm, the upper limit value of the air permeability of the substrate after high temperature treatment according to the present disclosure is preferably 580 seconds/100 cm ³ Or lower and may be, for example, 500 seconds/100 cm ³ Or lower, 450 seconds/100 cm ³ Or lower, 400 seconds/100 cm ³ Or less or 350 seconds/100 cm ³ Or lower. When the thickness of the substrate is converted to 16 μm, there is no limitation on the lower limit value of the air permeability of the substrate after high temperature treatment, which may be combined with any one of these upper limits, and it may be, for example, 50 seconds/100 cm ³ Or higher, 60 seconds/100 cm ³ Or higher or 70 seconds/100 cm ³ Or higher.

< porosity of substrate >

The substrate preferably has a porosity of 20% or more from the viewpoint of avoiding clogging in the resulting power storage device and improving the air permeability of the resulting separator; from the viewpoint of maintaining the strength of the separator, it is preferable to have a porosity of 70% or less. The porosity of the substrate is more preferably 25% or more and 65% or less, still more preferably 30% or more and 60% or less, particularly preferably 35% or more and 55% or less.

< puncture Strength of substrate >

When the thickness of the substrate is converted to 16 μm, the lower limit value of the puncture strength of the substrate is preferably 230gf or more, 240gf or more, 250gf or more, 260gf or more, 280gf or more, 300gf or more, or 320gf or more. The upper limit value of the puncture strength of the substrate that can be combined with any one of these lower limits is not limited, and is preferably 550gf or less when the thickness of the substrate is converted to 16 μm, and may be 500gf or less or 480gf or less, for example.

< thermal shrinkage of substrate >

The substrate preferably has a heat shrinkage of-1.0% or more and 3.0% or less in the width direction (TD) when measured after heat treatment at 150 ℃ for 1 hour. That is, the fact that the above heat shrinkage rate is within the above range means that the substrate has an extremely low heat shrinkage rate in the width direction even at a high temperature. When the above heat shrinkage is 3.0% or less, the occurrence of short circuit at high temperature can be effectively prevented. The reason why the above-mentioned heat shrinkage is-1.0% or more is that there is a case where the substrate expands slightly in the width direction at the time of measuring the heat shrinkage, resulting in a heat shrinkage of less than 0%, that is, a negative value. The heat shrinkage may be 0% or more, or may exceed 0%. The above-described substrate having a heat shrinkage of-1.0% or more and 3.0% or less can be manufactured, for example, by a method such as dry uniaxial stretching. In general, the wet separator has extremely high heat shrinkage in the width direction. In contrast, in the case of a uniaxially stretched dry separator, a substrate in which the above-mentioned heat shrinkage is-1.0% or more and 3.0% or less can be more easily obtained regardless of the ratio of pore diameters of the inner layer and the outer layer.

Method for producing separator for electric storage device

A method of manufacturing a separator for an electrical storage device, comprising: a melt extrusion step of melt-extruding a resin composition (hereinafter also referred to as "polypropylene-based resin composition") containing polypropylene as a main component to obtain a resin film; and a pore-forming step of forming pores in the obtained resin film to make the film porous. Methods of manufacturing the microporous layer can be roughly classified into a dry method (no solvent is used in the pore-forming step) and a wet method (solvent is used in the pore-forming step).

Examples of the dry method include the following: melt blending and extruding a polypropylene-based resin composition, and then heat treating and stretching the extrudate to produce delamination at the interfaces between polypropylene crystals; and the method as follows: the polypropylene-based resin composition and the inorganic filler are melt blended to form a film, and then the film is stretched, resulting in delamination at the interface between the polypropylene and the inorganic filler.

Examples of the wet method include the following: melt blending the polypropylene-based resin composition and the pore-forming material to form a film, stretching the film as required, and then extracting the pore-forming material; and the method as follows: the polypropylene-based resin composition is melted and then immersed in a poor solvent for polypropylene to solidify the polypropylene, while removing the solvent.

For melt blending of the polypropylene-based resin composition, a single screw extruder and a twin screw extruder may be used. In addition to these extruders, for example, kneaders, labo Plasto mills, mixing rolls, banbury mixers (Banbury mixer), or the like may also be used.

Depending on the method of making the microporous layer, or depending on the physical properties of the microporous layer of interest, the polypropylene-based resin composition may optionally contain resins other than polypropylene, additives, and the like. Examples of additives include pore forming materials, fluorine-based flow modifiers, waxes, crystal nucleating agents, antioxidants, metal fatty acid salts such as aliphatic metal carboxylates, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and color pigments. Examples of pore forming materials include plasticizers, inorganic fillers, and combinations thereof.

Examples of plasticizers include: hydrocarbons such as liquid paraffin and paraffin; esters such as dioctyl phthalate and dibutyl phthalate; higher alcohols such as oleyl alcohol and stearyl alcohol.

Examples of the inorganic filler include oxide-based ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride-based ceramics such as silicon nitride, titanium nitride, boron nitride, and the like; ceramics such as silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, magnesium stone, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and (3) glass fibers.

A process of forming layered crystal pores by a dry method is preferable as a manufacturing method of a substrate in which heat treatment and stretching are performed to generate delamination at an interface between polypropylene crystals. As a method of manufacturing the substrate including the microporous layer (a) and the microporous layer (B), it is preferable to use at least one of the following processes (i) and (ii):

(i) A process for producing a substrate by forming a coextruded film, wherein a microporous layer (A) and a microporous layer (B) are formed by coextruding respective resin compositions, and the coextruded film is subjected to annealing, cold stretching, hot stretching and thermal relaxation steps; and

(ii) A process for producing a substrate by lamination, in which a microporous layer (a) and a microporous layer (B) are formed by extruding respective resin compositions separately, the extruded films are laminated and stuck to each other, and then the resulting laminate is subjected to annealing, cold stretching, hot stretching, and thermal relaxation steps.

Also, in the case where the substrate further includes the microporous layer (C), examples of the manufacturing method of the substrate include the following processes:

(i) A process for producing a substrate by forming a coextruded film, wherein a microporous layer (A) and a microporous layer (B) and a microporous layer (C) are formed by coextruding respective resin compositions, and the coextruded film is subjected to annealing, cold stretching, hot stretching and thermal relaxation steps; and

(ii) A process for producing a substrate by lamination, wherein a microporous layer (a) and a microporous layer (B) and a microporous layer (C) are formed by extruding at least one resin composition separately from the other resin composition, the extruded films are laminated and adhered to each other, and then the resulting laminate is subjected to annealing, cold stretching, hot stretching and thermal relaxation steps.

The absolute values of the area average main pore diameters of the microporous layer (a) and the microporous layer (B) and optionally the microporous layer (C) in the ND-MD section and their ratios may be adjusted to be within the preferred range of the present disclosure, for example, by a method of changing the molecular weight of polypropylene contained in each layer, a method of adding an additive, or the like. The inventors have found that the pore size of one microporous layer can be controlled to be smaller than that of the other layer by using polypropylene in the one microporous layer having a molecular weight higher than that of polypropylene used in the other layer. The present inventors have also found that the pore size of one microporous layer can be controlled to be larger than that of the other layer by adding an additive having a specific structure represented by a styrene-olefin copolymer to the one microporous layer. Further, the present inventors have found that by strictly controlling the pore diameters of the respective layers, good battery performance and heat resistance can be obtained while preventing short circuits.

When the substrate has a layered structure composed of two or more layers, the absolute values of the area average main pore diameter (SX) of the surface (X) and the area average main pore diameter (SY) of the surface (Y) and their ratio can be adjusted to be within the preferred range of the present disclosure, for example, by a method of changing the molecular weight of polypropylene contained in each layer having each surface, a method of adding an additive, or the like. The pore size of one microporous layer having a surface on one side can be controlled to be smaller than that of another layer having a surface on the other side by using polypropylene in the one microporous layer having a molecular weight higher than that of polypropylene used in the other layer. Further, the pore diameter of one microporous layer having a surface on one side can be controlled to be larger than that of another layer having a surface on the other side by adding an additive having a specific structure represented by a styrene-olefin copolymer to the one microporous layer. Further, the present inventors have found that by strictly controlling the pore diameters of the respective layers, good battery performance and heat resistance can be obtained while preventing short circuits.

When the substrate is composed of a single (one layer) microporous layer, the absolute values of the area average main pore diameter (SX) of the surface (X) and the area average main pore diameter (SY) of the surface (Y) and their ratio can be adjusted to be within the preferred scope of the present disclosure, for example, by a method of forming a molecular weight gradient in a single layer during film formation, a method of allowing a larger amount of additive to be contained on one side of the layer. Such a method enables to control the pore size on the surface of one side having a lower molecular weight or a higher content of additives to be larger than the pore size on the other side.

A multilayer separator composed of a layer containing polypropylene as a main component and having a small pore diameter and a layer containing polyethylene and having a large pore diameter has been conventionally known. Without being limited by theory, by using polyethylene having a high crystallinity and a crystal size larger than that of polypropylene, the pore size of the polyethylene-containing layer can be controlled to be larger than that of a layer having polypropylene as a main component. However, when the substrate includes a layer having polyethylene as a main component, there are problems as follows: the heat treatment at a high temperature equal to or higher than the melting point (128 ℃) of polyethylene causes clogging of the pores, with the result that it cannot function as a separator. On the other hand, in a substrate having a multilayer structure (which does not include a layer mainly composed of polyethylene and is composed of a layer mainly composed of polypropylene), it is extremely difficult to individually control the pore diameters of the respective layers and form a multilayer structure in which the pore diameters of the respective layers are different.

Among the coextrusion process (i) and the lamination process (ii), the coextrusion process (i) is preferable from the viewpoint of manufacturing cost and the like. In the coextrusion process (i), as conditions for extrusion film formation of the microporous layers (a) to (C), it is preferable to extrude the resin at a temperature as low as possible and to efficiently cool the coextruded film by blowing low-temperature air thereto. After film formation, the coextruded film is preferably cooled rapidly by blowing, the temperature of which is preferably 20 ℃ or less, more preferably 15 ℃ or less. By blowing the cold air controlled at such a low temperature, the resin after film formation is uniformly oriented in the MD direction while being rapidly cooled.

The method of manufacturing the substrate may include an annealing step after the extrusion film is formed. The annealing step is performed to facilitate the growth of the crystal structure of the microporous layers (a) to (C) and to improve the pore-forming property. By performing the annealing step at a specific temperature for a long period of time, a favorable area average main pore diameter tends to be obtained in all of the microporous layers (a) to (C). The reason for this is considered to be that since crystals can be grown without disturbance of the crystal structure, high pore-forming properties can be obtained. Further, by performing the annealing step at a specific temperature for a long period of time, a good area average main pore diameter tends to be obtained in both the microporous layer (a) and the microporous layer (B). The reason for this is considered to be that since crystals can be grown without disturbance of the crystal structure, high pore-forming properties can be obtained. In the annealing step, it is preferable to perform the annealing treatment in a temperature range of 115 ℃ or more and 160 ℃ or less, preferably 20 minutes or more, more preferably 60 minutes or more, from the viewpoint of obtaining a good area-average main pore diameter, and obtaining a good area-average main pore diameter to prevent clogging of the resulting power storage device.

After the annealing step, the manufacturing method of the substrate may include a stretching step in the pore-forming step or before or after the pore-forming step. For the stretching treatment, either a uniaxial stretching method or a biaxial stretching method may be used. Although not limited thereto, uniaxial stretching is preferable in the case of using a dry method, for example, from the viewpoint of production cost. Biaxial stretching is preferable, for example, from the viewpoint of improving the strength of the resulting substrate. Examples of the biaxial stretching include methods of these, such as simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple stretching, and the like. Simultaneous biaxial stretching is preferable from the viewpoints of improving puncture strength, stretching uniformity, and closing characteristics. Further, sequential biaxial stretching is preferable from the viewpoint of easy control of plane orientation. When a sheet-like molded article is biaxially stretched at a high magnification, molecules are oriented in the planar direction, and a substrate which is not easily torn and has high puncture strength is easily obtained.

In order to reduce the heat shrinkage of the substrate, a heat treatment step for heat-setting may be performed after the stretching step or after the hole forming step. The heat treatment step may include: stretching operation, which is performed under a predetermined temperature atmosphere and a predetermined stretching ratio, with the aim of adjusting physical properties; and/or a relaxation operation, which is carried out under a predetermined temperature atmosphere and a predetermined relaxation rate, with the aim of reducing the tensile stress. The relaxing operation may be performed after the stretching operation is performed. Such a heat treatment step may be performed using a tenter frame or a roll stretcher.

The resulting substrate itself may be used as a separator of an electrical storage device. Optionally, a coating may be further provided on one or both surfaces of the substrate.

< electric storage device >

The power storage device according to the present disclosure includes a separator for the power storage device according to the present disclosure. The power storage device according to the present disclosure includes a positive electrode and a negative electrode, and the separator for the power storage device is preferably laminated between the positive electrode and the negative electrode. The microporous layer (a) constituting the outermost layer of the substrate is preferably disposed so as to face the negative electrode side. Since clogging of the separator in the electrical storage device is mainly caused by deposition on the surface of the anode, clogging of the separator can be effectively reduced by disposing the microporous layer (a) having a relatively large pore diameter to face the anode side. The surface (X) of the substrate is preferably arranged to face the negative side. Since clogging of the separator in the power storage device is mainly caused by deposits on the surface of the anode, clogging of the separator can be effectively reduced by disposing the surface (X) having a relatively large pore diameter to face the anode side.

Examples of the electric storage device include, but are not limited to, lithium secondary batteries, lithium ion secondary batteries, sodium ion secondary batteries, magnesium ion secondary batteries, calcium ion 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. Among them, a lithium secondary battery, a lithium ion secondary battery, a nickel hydrogen battery or a lithium ion capacitor is preferable from the viewpoint of practical use, and a lithium ion secondary battery is more preferable.

The power storage device can be manufactured, for example, by stacking a positive electrode and a negative electrode with the separator interposed therebetween. The obtained laminate is wound as necessary to form a laminated electrode body or a wound electrode body. Then, the laminated electrode body or the wound electrode body is housed outside. Connecting the anode and the cathode with external anode and cathode terminals through leads and the like; further, a nonaqueous electrolyte solution containing a nonaqueous solvent such as a acyclic or cyclic carbonate and an electrolyte such as a lithium salt is injected to the outside. The outside is then sealed.

The power storage device is more preferably a lithium ion secondary battery. Preferred embodiments of the lithium ion secondary battery will now be described. However, the power storage device according to the present disclosure is not limited to the lithium ion secondary battery.

The positive electrode is not particularly limited as long as it functions as a positive electrode of a lithium ion secondary battery, and a known positive electrode may be used. The positive electrode preferably contains one selected from materials capable of occluding and releasing lithium ionsOr a plurality of materials as the positive electrode active material. Preferred examples of the positive electrode include, from the viewpoint of battery capacity and safety: by LiCoO ₂ Lithium cobalt oxide as a representative; by Li ₂ Mn ₂ O ₄ A representative spinel-based lithium manganese oxide; by Li ₂ Mn _1.5 Ni _0.5 O ₄ Spinel-based lithium nickel manganese oxide as a representative; by LiNiO ₂ Lithium nickel oxide as a representative; in LiMO form ₂ (wherein M represents two or more elements selected from Ni, mn, co, al and Mg); in LiFePO form ₄ A lithium iron phosphate compound is shown. Among them, from the viewpoints of high safety and long-term stability, more preferable is: by LiCoO ₂ Lithium cobalt oxide as a representative; by LiNiO ₂ Lithium nickel oxide as a representative; in LiMO form ₂ (wherein M represents two or more elements selected from Ni, mn, co, al and Mg); in LiFePO form ₄ Lithium iron phosphate compounds represented, particularly preferably in the form of LiFePO ₄ A lithium iron phosphate compound is shown.

The anode is not particularly limited as long as it functions as an anode of a lithium ion secondary battery, and may be a known anode. The negative electrode preferably contains, as a negative electrode active material, one or more materials selected from lithium metal and materials capable of occluding and releasing lithium ions. That is, the anode preferably contains, as an anode active material, one or more materials selected from lithium metal, a carbon material, a material containing an element capable of forming an alloy with lithium, and a lithium-containing compound. Examples of such materials include, in addition to lithium metal, carbon materials typified by hard carbon, soft carbon, artificial graphite, natural graphite, pyrolytic carbon, coke, vitreous carbon, burned products of organic high molecular compounds, mesophase carbon microbeads, carbon fibers, activated carbon, graphite, carbon colloid, and carbon black.

Examples

< measurement and evaluation method >

[ measurement of Melt Flow Rate (MFR) ]

The Melt Flow Rate (MFR) (unit: g/10 min) of each microporous layer was measured in accordance with JIS K7210 under the conditions of a temperature of 230℃and a load of 2.16 kg. The MFR of polypropylene was measured in accordance with JIS K7210 under the conditions of a temperature of 230℃and a load of 2.16 kg. However, according to JIS K7210, the Melt Flow Rate (MFR) of polyethylene and the Melt Flow Rate (MFR) of a microporous layer containing 50% by weight or more of polyethylene were measured at a temperature of 190℃and a load of 2.16 kg.

Mw and Mn were measured by GPC (gel permeation chromatography)

Calibration curves were prepared using an Agilent PL-GPC220 under the following conditions to measure standard polystyrene. The respective sample polymers were also measured by chromatography under the same conditions, and based on the calibration curve, the weight average molecular weight (Mw), the number average molecular weight (Mn) and the value (Mw/Mn) obtained by dividing the weight average molecular weight (Mw) by the number average molecular weight (Mn) in terms of polystyrene were calculated for each polymer under the following conditions.

Chromatographic column: two TSK gel GMHHR-H (20) HT (7.8 mm I.D..times.30 cm) columns

Mobile phase: 1,2, 4-trichlorobenzene

The detector comprises: RI (RI)

Column temperature: 160 DEG C

Sample concentration: 1mg/ml

Calibration curve: polystyrene

[ measurement of melt tension ]

Melt tension (mN) of each microporous membrane was measured using a capillograph manufactured by Toyo Seiki co., ltd under the following conditions.

Capillary: diameter 1.0mm and length 20mm

Cylinder extrusion speed: 2 mm/min

Take-up speed: 60 m/min

Temperature: 230 DEG C

[ measurement of five-tuple fraction ]

According to the peak height method described in the handbook of Polymer analysis (Japanese society of analytical chemistry) ¹³ C-NMR spectrum to calculate the pentad fraction of polypropylene. By melting polypropylene pellets in o-dichlorobenzene-d, under the conditions of a measurement temperature of 145 ℃ and a cumulative number of 25,000 times, using JEOL-ECZ 500 ¹³ C-NMR spectrum.

[ measurement of thickness (. Mu.m) ]

The thickness (μm) of the substrate was measured at room temperature of 23.+ -. 2 ℃ using a Digimatic indicator IDC 112 manufactured by Mitutoyo corporation. The thickness of each microporous layer was calculated from image data obtained by cross-sectional SEM, and an evaluation method of the area average main pore diameter will be described later.

[ measurement of porosity (%) ]

Samples having square dimensions of 10cm by 10cm were cut from the separator or each microporous layer and the volume (cm) of the samples was measured ³ ) And mass (g), and from these measurements and density (g/cm) using the following formula ³ ) Porosity was calculated.

Porosity (%) = (volume-mass/density)/volume×100

[ measurement of air permeability (second/100 cm) ³ )]

The air resistance (seconds/100 cm) of the substrate was measured according to JIS P-8117 using a Gurley air permeability tester ³ ) And the measured air resistance was divided by the thickness and multiplied by 16 to calculate the air permeability in terms of 16 μm thickness.

[ air permeability after high-temperature treatment (seconds/100 cm) ³ ) Is measured by (a)]

The substrate was cut into squares of 100mm×100mm in MD and TD directions to obtain samples, the samples were placed in a hot air dryer (DF 1032, manufactured by Yamato Science co., ltd.) and the ends of the four sides of the squares were fixed on a metal frame, and heat-treated in the atmosphere at 140 ℃ for 30 minutes under normal pressure. After the heat treatment, the sample was taken out of the hot air dryer, cooled at room temperature for 10 minutes, and the substrate was removed from the metal frame. Then, the air resistance (seconds/100 cm) of the substrate was measured using a Gurley air permeability tester according to JIS P-8117 ³ ) And the measured air resistance was divided by the thickness and multiplied by 16 to calculate the air permeability (in 16 μm thickness) after the high temperature treatment. The rate of change of air permeability was determined according to the following equation.

[ measurement of TD Heat shrinkage (%) ]

The substrate was cut into squares of 50mm×50mm in MD and TD directions to obtain a sample, and the sample was placed in a hot air dryer (DF 1032, manufactured by Yamato Science co., ltd.) and subjected to heat treatment at 150 ℃ for 1 hour in the atmosphere at normal pressure. After the heat treatment, the sample was taken out of the hot air dryer, cooled at room temperature for 10 minutes, and then the dimensional shrinkage was measured. The respective samples are placed on a material such as copy paper so as not to adhere to the inner wall of the dryer or the like, and so that the samples are not fused with each other.

Heat shrinkage (%): (dimension before heating (mm) -dimension after heating (mm))/(dimension before heating (mm)) ×100

[ measurement of area average Main pore diameter ]

(1) Area average main pore diameter in ND-MD section

The area average main pore diameter in the ND-MD section was measured by image analysis in the section SEM observation. As a pretreatment, the separator was ruthenium stained, and an ND-MD section sample was prepared by freeze fracture. The thus prepared cross-section sample was fixed on an SEM stage with a conductive adhesive (carbon-based) for cross-section observation, dried, and then as a conductive treatment, osmium-coated with an osmium coater (HPC-30W, manufactured by Vacuum Device inc.) under the conditions that the voltage application adjustment knob was set to 4.5 and the discharge time was 0.5 seconds, to prepare a microscopic sample. Thereafter, any three points in the ND-MD section of the microporous membrane were observed using a scanning electron microscope (S-4800, manufactured by Hitachi High Technologies Inc.) under conditions of an acceleration voltage of 1kV, a LA10 detection signal, an operating distance of 5mm, and a magnification of 5000 times.

Each observation Image was binarized into a resin portion and a hole portion using Image processing software Image J and Otsu method, and the average main diameter of the hole portion was calculated. At this time, a microporous portion existing across each imaging region and a region other than the imaging region was excluded from the object to be measured, and had a thickness of 0.001 μm ² Or lower pore area. The average diameter was calculated from the area of each hole based on the area average value. To avoid overestimating the contributions of very small holes, the average is calculated based on the area average, which isInstead of a number average value based on dividing by the number of wells, a weighted average of the individual well areas.

(2) Area average primary aperture on substrate surface

The area average main pore diameter on the substrate surface was measured by image analysis in surface SEM observation. When the substrate has one or more optional coatings on one or more surfaces thereof, the one or more coatings are removed by stripping the one or more coatings by hand as a pretreatment after immersing the substrate in acetone for 3 minutes. Thereafter, the substrate was washed with water and then dried at room temperature overnight. The thus prepared sample was fixed on an SEM stage with a conductive adhesive (carbon-based) for surface observation, dried, and then as a conductive treatment, osmium-coated with an osmium coater (HPC-30W, manufactured by Vacuum Device inc.) under the conditions that the voltage application adjustment knob was set to 4.5 and the discharge time was 0.5 seconds, to prepare a microscopic sample. Thereafter, any three points on the surface of the corresponding microporous membrane were observed using a scanning electron microscope (S-4800, manufactured by Hitachi High Technologies inc.) under conditions of an acceleration voltage of 1kV, a LA10 detection signal, an operating distance of 5mm, and a magnification of 5000 times. An area of 20 μm (in the MD direction). Times.3 μm (in the ND direction) in the obtained image was taken as an observation image. Each observation Image was binarized into a resin portion and a hole portion using Image processing software Image J and Otsu method, and the average main diameter of the hole portion was calculated. At this time, a microporous portion existing in a region spanning each imaging region and outside the imaging region was excluded from the object to be measured, and had a thickness of 0.001 μm ² Or lower pore area. The average diameter was calculated from the area of each hole based on the area average value.

[ cycle Capacity Retention Rate and occlusion assessment ]

LiPF as lithium salt was used in a volume ratio of 1:2 of 1mol/L ₆ Adding the electrolyte into a mixture of ethylene carbonate and methyl ethyl carbonate.

Lithium/nickel/manganese/cobalt mixed oxide (LiNi) as positive electrode active material _0.5 Co _0.2 Mn _0.3 O ₂ ) Carbon black powder as a conductive aid and PVDF as a binder to mixOxide: conductive auxiliary agent: binder = 100:3.5:3 mass ratio. The resulting mixture was coated on both surfaces of an aluminum foil as a positive electrode current collector, each having a thickness of 15 μm, dried, and then pressed with a roll press to prepare a double-sided coated positive electrode.

Graphite powder (artificial graphite) having a particle diameter of 22 μm (D50) as a negative electrode active material, a binder (polystyrene-butadiene latex), and carboxymethyl cellulose as a thickener were mixed as graphite powder: and (2) an adhesive: thickener = 100:1.5:1.1 by mass. The resulting mixture was coated on one surface or both surfaces of a copper foil as a negative electrode current collector, each having a thickness of 10 μm, the solvent was removed by drying, and then the coated copper foil was pressed with a roll press to prepare a single-sided coated negative electrode or a double-sided coated negative electrode.

The positive electrode and the negative electrode thus obtained were laminated in the order of single-sided coated negative electrode/double-sided coated positive electrode/double-sided coated negative electrode/single-sided coated positive electrode, with a separator manufactured as described below interposed between the electrodes so as to face each active material, and the MD edge of the separator was fixed with a seal. At this time, the microporous layer (a) is disposed so as to face the anode. Subsequently, in a state where the positive electrode terminal and the negative electrode terminal were mounted thereto in a protruding manner, the resulting laminated film was inserted into the inside of a pouch (battery exterior) constituted by a laminated film obtained by coating both surfaces of an aluminum foil (thickness: 40 μm) with a resin layer. After the resultant was dried in the atmosphere at 80 ℃ for 12 hours, 0.8mL of the electrolyte prepared as described above was injected into the pouch, and the pouch was vacuum-sealed, to prepare a sheet-shaped lithium ion secondary battery.

The obtained sheet-shaped lithium ion secondary battery was placed in a constant temperature bin controlled at 25 ℃ and connected to a charging and discharging device, and left for 16 hours. Subsequently, the battery was subjected to three charge and discharge cycles, each consisting of: charging at a constant current of 0.05C; after the voltage reached 4.35V, charging for 2 hours at a constant voltage of 4.35V; then discharge to 3.0V at 0.2C constant current; the initial charge and discharge of the battery are performed. "1C" means a current value in the case where the entire capacity of the battery is discharged within one hour.

After the initial charge and discharge described above, the battery was placed in a constant temperature bin controlled at 25 ℃. Thereafter, the battery was subjected to 100 charge and discharge cycles, each consisting of: charging at a constant current of 1C; after the voltage reached 4.35V, charging for 1 hour at a constant voltage of 4.35V; then discharging to 3.0V with a constant current of 1C; battery cycle testing is performed.

The value (percentage) obtained by dividing the discharge capacity (mAh) at the 100 th cycle by the discharge capacity (mAh) at the 1 st cycle was used as the cycle capacity retention rate. Further, the sheet-shaped lithium ion secondary battery after the completion of the 100 th cycle was disassembled in an argon atmosphere, and the separator was taken out and washed 3 times by immersing in ethyl methyl carbonate. Then, a 1mm square area of the negative electrode side surface of the separator was observed with a microscope to confirm the presence or absence of clogging on the separator surface. When 50% or more of the pores of the separator surface are covered with the deposit, the surface is evaluated as "clogged"; and when 50% or more of the pores of the separator surface were not covered with the deposit, the surface was evaluated as "unblocked". The presence or absence of the above-described clogging was confirmed at 10 positions, and the proportion of the positions evaluated as "clogging" was calculated.

[ short-circuit evaluation ]

5 sheet-shaped lithium ion secondary batteries were prepared by the method described in the section "cycle capacity retention rate evaluation".

Each of the obtained sheet-shaped lithium ion secondary batteries was placed in a constant temperature bin controlled at 25 ℃ and connected to a charge and discharge device, and left for 16 hours. Subsequently, each cell was subjected to three charge and discharge cycles, each cycle consisting of: charging at a constant current of 0.05C; after the voltage reached 4.35V, charging for 2 hours at a constant voltage of 4.35V; then discharged to 3.0V at a constant current of 0.2C; initial charge and discharge of the battery are performed. "1C" means a current value in the case where the entire capacity of the battery is discharged within one hour.

After the initial charge and discharge, each of the sheet-shaped lithium ion secondary batteries was charged with a constant current of 0.5C, and after the voltage reached 4.5V, was charged with a constant voltage of 4.5V for 1 hour in a state of being pressurized to 0.5MPa at 25 ℃. Thereafter, each cell was left in an open state for one hour. A battery whose voltage did not reach 4.5V even after constant current charging was performed for 1 hour, and a battery whose voltage was reduced to 4.3V or less in 1 hour while remaining in an open circuit state were evaluated as "short", and a proportion evaluated as "short" was calculated.

[ evaluation of high temperature drying resistance ]

Lithium/nickel/manganese/cobalt mixed oxide (LiNi) as positive electrode active material _0.5 Co _0.2 Mn _0.3 O ₂ ) Carbon black powder as a conductive aid and PVDF as a binder to mix oxides: conductive auxiliary agent: adhesive = 100:3.5:3 mass ratio. The resulting mixture was coated on both surfaces of an aluminum foil as a positive electrode current collector, each having a thickness of 15 μm, dried, and then pressed with a roll press to prepare a double-sided coated positive electrode. .

The positive electrode and the negative electrode thus obtained were laminated in the order of single-sided coated negative electrode/double-sided coated positive electrode/double-sided coated negative electrode/single-sided coated positive electrode, with a separator manufactured as described below interposed between the electrodes so as to face each active material, and the MD edge of the separator was fixed with a seal. At this time, the microporous layer (a) is disposed so as to face the anode. Subsequently, in a state where the positive electrode terminal and the negative electrode terminal were mounted thereto in a protruding manner, the resulting laminated film was inserted into the inside of a pouch (battery exterior) composed of laminated films obtained by coating both surfaces of an aluminum foil (thickness: 40 μm) with a resin layer. After the resultant was dried in the atmosphere at 140 ℃ for 30 minutes, 0.8mL of the electrolyte prepared as described above was injected into the pouch, and the pouch was vacuum-sealed, to prepare a sheet-shaped lithium ion secondary battery that had been subjected to a high-temperature drying treatment. The obtained sheet-shaped lithium ion secondary battery was placed in a constant temperature bin controlled at 25 ℃ and connected to a charging and discharging device, and left for 16 hours. Subsequently, the battery was charged at a constant current of 0.5C to confirm whether the battery was chargeable. In a normal secondary battery, a target voltage of 4.35V is reached within 3 hours; in a cell that loses ionic conductivity, the voltage rapidly rises to 5V or higher within a few minutes, thereby activating an emergency stop. If the emergency stop has been activated, the battery is assessed as not being chargeable.

< example 1>

[ preparation of microporous layer ]

As the resin of the microporous layer (a), 95wt% of a polypropylene resin having a high molecular weight (indicated as "PP1" in table 1; MFR (230 ℃) =1.0 g/10 min, density=0.91 g/cm ³ ) And 5wt% of a random copolymer type elastomer of ethylene and butene (represented as "C2C4" in Table 1) were dry blended to obtain a resin material. The resulting resin material was melted in a 2.5 inch extruder and fed to the two outer layers of the two three layer coextrusion T die using a gear pump. Further, as the resin of the microporous layer (B), a polypropylene resin having a high molecular weight (represented as "PP1" in table 1; MFR (230 ℃) =1.0 g/10 min, density=0.91 g/cm) was used in a 2.5-inch extruder ³ ) The inner layers of the above two three-layer coextrusion T dies were melted and fed using a gear pump. The temperature of the T die was set to 220 ℃, the molten polymer was extruded from the T die, and then the resin extrudate was wound on a roll while being cooled by blowing air, to obtain a precursor sheet having an a/B/a layer structure with a thickness of about 17 μm. At this time, the die lip width in the T die TD direction was set to 500mm, the inter-die lip distance (die lip gap) of the T die was set to 2.4mm, and extrusion was performed at an extrusion rate of 6 kg/h.

Subsequently, the resulting precursor was put into a dryer, and an annealing treatment was performed at 120 ℃ for 20 minutes. Thereafter, the annealed precursor was cold-stretched by 20% at room temperature, and the stretched film was put into an oven controlled at 125 ℃ without shrinking it, hot-stretched by 140%, and then relaxed by 15%, to obtain a substrate having a three-layer structure consisting of layers a/B/a.

Examples 2 to 4, examples 7 to 16 and comparative examples 1 to 8>

Respective microporous films were obtained in the same manner as in example 1 except that the raw materials and stretching conditions shown in tables 1 to 3 were used, and the obtained separators were evaluated. In tables 1 to 3, "PP1", "PP2", "PP3", "PP4" and "PP5" represent polypropylene resins shown in table 4. In tables 1 to 3, "SEPS" represents a styrene-ethylene/propylene-styrene block copolymer. In tables 1 to 3, "PE" represents polyethylene (MFR (190 ℃) =0.4 g/10 min).

< example 5>

Two-layer coextrusion T dies were installed instead of two three-layer coextrusion T dies, and film formation was performed under the same conditions as in example 1 using the raw materials shown in table 1, to obtain a precursor sheet having an a/B layer structure and a thickness of about 17 μm.

Subsequently, the resulting precursor was put into a dryer, and an annealing treatment was performed at 120 ℃ for 20 minutes. Thereafter, the annealed precursor was cold-stretched by 20% at room temperature, and the stretched film was put into an oven controlled at 125 ℃ without shrinking it, hot-stretched by 140%, and then relaxed by 15%, to obtain a substrate having a two-layer structure composed of layers a/B.

< example 6>

Three-layer coextrusion T dies were installed instead of two three-layer coextrusion T dies, and film formation was performed under the same conditions as in example 1 using the raw materials shown in table 1, to obtain a precursor sheet having an a/B/C layer structure and a thickness of about 17 μm.

Subsequently, the resulting precursor was put into a dryer, and an annealing treatment was performed at 120 ℃ for 20 minutes. Thereafter, the annealed precursor was cold-stretched by 20% at room temperature, and the stretched film was put into an oven controlled at 125 ℃ without shrinking it, hot-stretched by 140%, and then relaxed by 15%, to obtain a substrate having a three-layer structure consisting of layers a/B/C.

< example 17>

To be used as positive electrode active material in LiFePO ₄ Represented are lithium iron phosphate compound, carbon black powder as a conductive aid and PVDF as a binder to mix oxides: conductive auxiliary agent: adhesive = 92:5:3 mass ratio. The resulting mixture was coated on both surfaces of an aluminum foil as a positive electrode current collector, each having a thickness of 15 μm, dried, and then pressed with a roll press to prepare a double-sided coated positive electrode.

The positive electrode and the negative electrode thus obtained were laminated in the order of single-sided coated negative electrode/double-sided coated positive electrode/double-sided coated negative electrode/single-sided coated positive electrode, with the separator manufactured in example 10 interposed between the electrodes so as to face each active material, and the MD edge of the separator was fixed with a seal. At this time, the microporous layer (a) is disposed so as to face the anode. Subsequently, in a state where the positive electrode terminal and the negative electrode terminal were mounted thereto in a protruding manner, the resulting laminate was inserted into the inside of a pouch (battery exterior) composed of laminated films obtained by coating both surfaces of an aluminum foil (thickness: 40 μm) with a resin layer. After the resultant was dried in the atmosphere at 80 ℃ for 12 hours, 0.8mL of the electrolyte prepared as described above was injected into the pouch, and the pouch was vacuum-sealed, to prepare a sheet-shaped lithium ion secondary battery whose positive electrode contained a lithium iron phosphate compound.

The obtained sheet-shaped lithium ion secondary battery (the positive electrode of which contains a lithium iron phosphate compound) was placed in a constant temperature bin controlled at 25 ℃ and connected to a charge and discharge device, and left for 16 hours. Subsequently, the battery was subjected to three charge and discharge cycles, each consisting of: charging at a constant current of 0.05C; after the voltage reached 3.65V, charging for 2 hours at a constant voltage of 3.65V; then discharged to 2.4V at a constant current of 0.2C; initial charge and discharge of the battery are performed. "1C" means a current value in the case where the entire capacity of the battery is discharged within one hour.

After the initial charge and discharge described above, the battery was placed in a constant temperature bin controlled at 25 ℃. Thereafter, the battery was subjected to 10 charge and discharge cycles, each consisting of: charging at a constant current of 1C; after the voltage reached 3.65V, charging for 1 hour at a constant voltage of 3.65V; then discharged to 2.4V at a constant current of 1C. It has been demonstrated that the resulting battery is capable of being charged and discharged in a desired manner.

TABLE 1-1

TABLE 1-2

TABLE 2-1

TABLE 2-2

TABLE 3-1

TABLE 3-2

TABLE 4 Table 4

Example 18

[ preparation of microporous layer ]

As the resin of the microporous layer (a), 95wt% of a polypropylene resin having a high molecular weight (indicated as "PP1" in table 5; MFR (230 ℃) =1.0 g/10 min, density=0.91 g/cm ³ ) And 5wt% of a styrene-ethylene/propylene-styrene block copolymer (denoted as "SEPS" in Table 5) to give a resin material. The resulting resin material was melted in a 2.5 inch extruder and fed to the outer layer on the side of the two-layer coextrusion T die using a gear pump. Further, as the resin of the microporous layer (B), a polypropylene resin having a high molecular weight (represented as "PP2" in table 5; MFR (230 ℃) =0.5 g/10 min, density=0.91 g/cm) was used in a 2.5-inch extruder ³ ) Melted and fed to the outer layer on the other side of the two-layer coextrusion T die using a gear pump. The temperature of the T die was set to 220 ℃, the molten polymer was extruded from the T die, and then the resin extrudate was wound on a roll while being cooled by blowing air, to obtain a precursor sheet having an a/B/layer structure with a thickness of about 17 μm. At this time, the die lip width in the T die TD direction was set to 500mm, the inter-die lip distance (die lip gap) of the T die was set to 2.4mm, and extrusion was performed at an extrusion rate of 6 kg/h.

Subsequently, the resulting precursor was put into a dryer, and an annealing treatment was performed at 120 ℃ for 20 minutes. Thereafter, the annealed precursor was cold-stretched by 20% at room temperature, and the stretched film was put into an oven controlled at 125 ℃ without shrinking it, hot-stretched by 140%, and then relaxed by 15%, to obtain a substrate having a two-layer structure composed of layers a/B. In the resulting substrate, the surface on the microporous layer (a) side (constituting surface (X)) has a larger pore diameter, and the surface on the microporous layer (B) side (constituting surface (Y)) has a smaller pore diameter.

Example 19

Each microporous membrane was obtained in the same manner as in example 18 except that the raw materials shown in table 5 were used, and the obtained separator was evaluated. In table 5, "C2C4" represents a random copolymer type elastomer of ethylene and butene.

Example 20

Three-layer coextrusion T-die were installed instead of two-layer coextrusion T-die, and film formation was performed under the same conditions as in example 18 using the raw materials shown in table 5, to obtain a precursor sheet having an a/C/B layer structure with a thickness of about 17 μm.

Subsequently, the resulting precursor was put into a dryer, and an annealing treatment was performed at 120 ℃ for 20 minutes. Thereafter, the annealed precursor was cold-stretched by 20% at room temperature, and the stretched film was put into an oven controlled at 125 ℃ without shrinking it, hot-stretched by 140%, and then relaxed by 15%, to obtain a separator substrate having a three-layer structure composed of layers a/C/B. In the obtained substrate, the surface (constituting surface (X)) on the microporous layer (a) side has a large pore diameter, and the surface (constituting surface (Y)) on the microporous layer (B) side has a small pore diameter.

Comparative examples 9 and 10

Two three-layer coextrusion T-die were mounted instead of two-layer coextrusion T-die, and film formation was performed under the same conditions as in example 18 using the raw materials shown in table 5. A precursor sheet having an A/C/B layer structure was obtained with a thickness of about 17. Mu.m. In table 5, "PE" means polyethylene (MFR (230 ℃) =0.4 g/10 min).

Subsequently, the resulting precursor was put into a dryer, and an annealing treatment was performed at 120 ℃ for 20 minutes. Thereafter, the annealed precursor was cold-stretched by 20% at room temperature, and the stretched film was put into an oven controlled at 125 ℃ without shrinking it, hot-stretched by 140%, and then relaxed by 15%, to obtain a separator substrate having a three-layer structure composed of layers a/C/B.

TABLE 5

Industrial application

The separator for an electrical storage device according to the present disclosure may be suitably used as a separator for an electrical storage device (e.g., a lithium ion secondary battery).

Claims

1. A separator for an electrical storage device, comprising a substrate including:

a microporous layer (A) containing 70wt% or more of polypropylene; and

a microporous layer (B) containing 70wt% or more of polypropylene,

wherein the average main pore diameter of the pores contained in the microporous layer (B) in the ND-MD section is not more than 0.95 times the average main pore diameter of the pores contained in the microporous layer (A) in the ND-MD section.

2. The separator for an electric storage device according to claim 1, wherein an area average main pore diameter of pores contained in the microporous layer (B) in an ND-MD section is not less than 0.30 times and not more than 0.90 times an area average main pore diameter of pores contained in the microporous layer (a) in an ND-MD section.

3. The separator for an electric storage device according to claim 1 or 2, wherein the substrate has a change rate of air permeability of 100% or less when heated in the atmosphere at 140 ℃ for 30 minutes in a state where an end portion of the substrate is fixed.

4. The separator for an electric storage device according to any one of claims 1 to 3, wherein the pores contained in the microporous layer (a) have an area average main pore diameter of 100nm or more and 600nm or less in an ND-MD section.

5. The separator for an electric storage device according to any one of claims 1 to 4, wherein the microporous layer (a) constitutes an outermost layer on both sides of the substrate.

6. The separator for an electric storage device according to any one of claims 1 to 5, wherein the substrate further comprises a microporous layer (C) containing 50wt% or more of polyolefin.

7. The separator for an electric storage device according to claim 6, wherein an area average main pore diameter of pores contained in the microporous layer (C) in an ND-MD section is 0.20 times or more and 0.90 times or less of an area average main pore diameter of pores contained in the microporous layer (B) in an ND-MD section.

8. The separator for an electric storage device according to claim 6 or 7, wherein the substrate includes a structure of a microporous layer (a), a microporous layer (B), and a microporous layer (C) laminated in this order.

9. The separator for an electric storage device according to any one of claims 1 to 8, wherein the substrate has a structure in which a microporous layer (a), a microporous layer (B), and a microporous layer (a) are laminated in this order.

10. The separator for an electric storage device according to any one of claims 1 to 8, wherein when a surface of the substrate on the microporous layer (a) side is defined as a first porous surface (X) and a surface of the substrate on the opposite side from the first porous surface (X) is defined as a second porous surface (Y), the first porous surface (X) contains pores having an area average main pore diameter (S _X ) Is the area-average main pore diameter (S) of pores contained in the second porous surface (Y) _Y ) 1.05 times or more and 10 times or less of (a) the total length of the container.

11. The separator for an electrical storage device according to claim 10, wherein an average main pore diameter (S _X ) 80nm or more and 600nm or less.

12. The separator for an electric storage device according to any one of claims 1 to 11, wherein the substrate has a heat shrinkage of-1.0% or more and 3.0% or less in the width direction, when measured after heating at 150 ℃ for 1 hour.

13. An electrical storage device comprising a positive electrode, a negative electrode, and the separator for an electrical storage device according to any one of claims 1 to 12.

14. The power storage device according to claim 13, wherein the microporous layer (a) is provided so as to face the negative electrode side.

15. The power storage device according to claim 13 or 14, wherein the positive electrode contains lithium iron phosphate as a positive electrode active material.

16. A separator for an electric storage device includes a substrate containing 70wt% or more of polyolefin and having a first porous surface (X) and a second porous surface (Y) on the opposite side to the first porous surface (X),

17. The separator for an electrical storage device according to claim 16, wherein an average main pore diameter (S _X ) 80nm or more and 600nm or less.

18. The separator for an electric storage device according to claim 16 or 17, wherein the polyolefin is polypropylene.

19. The separator for an electric storage device according to any one of claims 16 to 18, wherein the substrate has a heat shrinkage ratio of-1.0% or more and 3.0% or less in a width direction, measured after heating at 150 ℃ for 1 hour.

20. An electric storage device comprising a positive electrode, a negative electrode, and the separator for an electric storage device according to any one of claims 16 to 19.

21. The power storage device according to claim 20, wherein the first porous surface (X) is provided so as to face the negative electrode side.

22. The power storage device according to claim 20 or 21, wherein the positive electrode contains lithium iron phosphate as a positive electrode active material.

23. A microporous membrane comprising a substrate comprising:

a microporous layer (A) containing 70wt% or more of polypropylene; and

a microporous layer (B) containing 70wt% or more of polypropylene,