Separator for power storage devices, and power storage device

Abstract

Provided is a separator for power storage devices, the separator comprising a separator base material constituted by a microporous film containing a polyolefin as a main component, the separator including a microporous layer (A) having: a trunk height of 500 nm-1000 nm inclusive, calculated by analysis of a scanning electron microscope (SEM) image of an MD-ND cross section of the separator base material; and an MD length of 1000 nm-1900 nm inclusive, calculated by analysis of the SEM image of the MD-ND cross section of the separator base material.

Description

Separator for power storage device and power storage device

[0001] The present disclosure relates to a separator for an electricity storage device.

[0002] Microporous membranes, particularly polyolefin-based microporous membranes, are used in many technical fields, such as microfiltration membranes, battery separators, capacitor separators, and fuel cell materials, and are particularly used as separators for power storage devices, such as lithium secondary batteries and lithium ion secondary batteries. Lithium ion batteries are used in a variety of applications, including small electronic devices such as mobile phones and notebook personal computers, as well as electric vehicles, including hybrid vehicles and plug-in hybrid vehicles.

[0003] In recent years, there has been a demand for lithium-ion batteries with high energy capacity, high energy density, and high output characteristics, and this has led to an increasing demand for thin-film separators that offer excellent battery performance, reliability, and safety.

[0004] For example, US Patent No. 6,299,999 describes a multilayer microporous thin film or membrane that can improve properties including dielectric breakdown and strength. A preferred multilayer microporous membrane comprises a microlayer and one or more stacked barriers.

[0005] Patent Document 2 describes a separator for an electricity storage device that has high strength and can be made thin, and discloses a microporous membrane that contains a polyolefin as a main component and has a melt tension of 30 mN or less when measured at a temperature of 230°C, and a melt flow rate (MFR) of 0.9 g / 10 min or less when measured under a load of 2.16 kg at a temperature of 230°C.

[0006] Patent Document 3 describes a separator for an electricity storage device that is excellent in product safety, and discloses a microporous film that contains a polypropylene resin and a thermoplastic elastomer and has a specific MFR and morphology.

[0007] Patent Document 4 describes a separator for a lithium-ion lithium battery that has excellent gas permeability and shutdown properties, and discloses a porous film that has a mixed resin layer containing a polypropylene-based resin, a polyethylene-based resin, and a thermoplastic resin whose crystalline melting peak temperature or glass transition temperature is equal to or lower than the crystalline melting peak temperature of the polyethylene-based resin, and that has β activity.

[0008] Patent Document 5 describes a battery separator including a microporous membrane that is excellent in TD tensile strength, puncture strength, and air permeability.

[0009] Patent Document 6 describes a porous membrane that combines high mechanical stability and low air resistance and a separator using the same, and discloses a polypropylene porous membrane that exhibits a specific mesopentad fraction and TREF (temperature rising elution fractionation) measurement value.

[0010] International Publication No. 2018 / 089748 International Publication No. 2020 / 196120 International Publication No. 2019 / 103947 JP 2010-111832 A International Publication No. 2018 / 217990 JP 2014-133839 A

[0011] In the methods described in Patent Documents 1 to 6, a thin film, high strength, excellent air permeability, and the like are achieved by using specific membrane properties or resin mixtures. However, separators that combine high strength and low air permeability are required for electricity storage devices, and from this perspective, there is room for further improvement.

[0012] Therefore, the problem to be solved by the first embodiment of the present invention is to provide a separator for an electricity storage device that combines high puncture strength and low air permeability, and the problem to be solved by the second embodiment of the present invention is to provide a separator for an electricity storage device that combines high puncture strength, high TD tensile strength, and low air permeability.

[0013] After extensive investigations, the present inventors discovered that the above-mentioned problems can be solved by using a separator substrate containing a polyolefin as a main component and having a specific pore structure, thereby completing a first embodiment of the present invention. Furthermore, after extensive investigations, the present inventors discovered that the above-mentioned problems can be solved by using a separator substrate containing polypropylene and having a specific composition and porosity, thereby completing a second embodiment of the present invention. Specifically, the present invention is as follows: (1) A separator for an electricity storage device, comprising a microporous membrane containing a polyolefin as a main component as a separator substrate, the separator comprising a microporous layer (A) having a trunk height of 500 nm or more and 1000 nm or less, calculated from analysis of a scanning electron microscope (SEM) image of an MD-ND cross section of the separator substrate, and an MD length of 1000 nm or more and 1900 nm or less, calculated from analysis of a SEM image of an MD-ND cross section of the separator substrate. (2) The separator for an electric storage device according to item 1, wherein the microporous layer (A) comprises a thermoplastic elastomer containing one or more repeating units selected from the group consisting of a polyolefin different from the main component, a polyolefin copolymer, and a copolymer of polystyrene and a polyolefin. (3) The separator for an electric storage device according to item 1 or 2, wherein the microporous layer (A) comprises a thermoplastic elastomer containing, as a repeating unit, one or more repeating units selected from the group consisting of ethylene, propylene, and 1-butene. (4) The separator for an electric storage device according to any one of items 1 to 3, wherein the polyolefin contains 80.0% by mass or more and 99.5% by mass or less of polypropylene and 0.5% by mass or more and 20.0% by mass or less of a thermoplastic elastomer, based on the total mass of the microporous layer (A). (5) In a composition image at a magnification of 30,000 times obtained by a scanning electron microscope (SEM) of an MD-ND cross section of the microporous layer (A) stained with a ruthenium compound, a polymer matrix and fibrils are present, and the polymer matrix and the fibrils each have an area of ​​100 nm ２ 0.1 μm or more ２ The following bright dyed areas are present, and the area of ​​the entire image showing only the microporous layer (A) is 100 nm ２ 0.1 μm or more２ The total area ratio of the following bright dyed parts is S １ %, 0.5≦S １ (6) The separator for an electric storage device according to any one of items 1 to 5, wherein the integrated elution amount of the microporous layer (A) at 100°C or higher and 130°C or lower, as measured by cross fractionation chromatography (CFC), is 80.0% by mass or higher and 99.5% by mass or lower of the total elution amount, and the integrated elution amount of the microporous layer (A) at 20°C or higher and lower than 100°C is 0.5% by mass or higher and 20.0% by mass or lower of the total elution amount. (7) The separator for an electric storage device according to any one of items 1 to 6, wherein the area average long pore diameter calculated by analysis of an SEM image of an MD-ND cross section of the microporous layer (A) is 50 nm or higher and 500 nm or lower. (8) The electricity storage device separator according to any one of items 1 to 7, wherein the microporous layer (A) has a melt flow rate (MFR) of 1.00 g / 10 min or less when measured under a load of 2.16 kg at a temperature of 230°C. (9) The electricity storage device separator according to any one of items 1 to 8, wherein the microporous layer (A) has a melt tension Mt at 240°C of 10 mN or more and 35 mN or less. (10) The electricity storage device separator according to any one of items 1 to 9, wherein the separator substrate has a thickness of 3 μm or more and 20 μm or less. (11) The electricity storage device separator according to any one of items 1 to 10, wherein the separator substrate has a porosity of 30% or more and 60% or less. (12) The separator for an electric storage device according to any one of items 1 to 11, wherein the separator substrate has a TD heat shrinkage of 5% or less at 105°C for 1 hour. (13) The separator substrate has an MD tensile strength of 1800 kgf / cm ２(14) The separator for an electricity storage device according to any one of items 1 to 13, wherein the microporous layer (A) has a melt flow rate (MFR) of 0.20 g / 10 min or more when measured under a load of 2.16 kg at a temperature of 230°C. (15) The separator for an electricity storage device according to any one of items 1 to 14, wherein the polyolefin is polypropylene, and the weight average molecular weight (Mw) of the polypropylene is 300,000 or more and 1,300,000 or less. (16) The separator for an electricity storage device according to item 15, wherein the molecular weight distribution (Mw / Mn) of the polypropylene, which is the value obtained by dividing the weight average molecular weight (Mw) by the number average molecular weight (Mn), is 3 or more and 30 or less. (17) The separator for an electricity storage device according to item 15, wherein the polypropylene １３ (18) The separator for a storage battery device according to any one of items 1 to 17, which contains polyethylene in an amount of 0.5% by mass or more and 20.0% by mass or less, based on the total mass of the microporous layer (A). (19) The separator substrate has a melt tension Mt of the microporous layer (A) at 240°C. Ａ and a melt tension Mt Ｂ(20) An electricity storage device separator according to any one of items 1 to 19, comprising a positive electrode containing lithium iron phosphate as a positive electrode active material, a negative electrode, and the electricity storage device separator according to any one of items 1 to 19, disposed between the positive electrode and the negative electrode. (21) An electricity storage device separator comprising a microporous layer (A) containing polypropylene as a separator substrate, wherein the integrated elution amount of the microporous layer (A) at 100°C or higher and 130°C or lower, as measured by cross fractionation chromatography (CFC), is 80.0% to 99.5% by mass of the total elution amount, and the integrated elution amount of the microporous layer (A) at 20°C or higher and lower than 100°C is 0.5% to 20.0% by mass of the total elution amount, and the porosity of the separator substrate is 30.0% to 45.0%. (22) The separator for a power storage device according to item 21, wherein an elution temperature-elution amount curve of the microporous layer (A) measured by cross fractionation chromatography (CFC) has at least two peaks, the peak temperature of the higher-temperature peak being 105°C or higher and 125°C or lower, and the peak temperature of the lower-temperature peak being 35°C or higher and 100°C or lower. (23) The weight-average molecular weight (Mw Ｈ ) is 300,000 or more and 1,300,000 or less, and the weight average molecular weight (Mw Ｌ (24) The separator for an electric storage device according to any one of items 21 to 23, wherein the microporous layer (A) has a melt flow rate (MFR) of 1.0 g / 10 min or less when measured under a load of 2.16 kg at a temperature of 230°C. (25) The separator for an electric storage device according to item 22, wherein the melt tension Mt Ａ(26) The separator for an electricity storage device according to any one of items 21 to 25, wherein the weight average molecular weight (Mw) of the microporous layer (A) is 250,000 or more and 1,500,000 or less. (27) The separator for an electricity storage device according to item 26, wherein the molecular weight distribution (Mw / Mn) of the microporous layer (A), which is the value obtained by dividing the weight average molecular weight (Mw) by the number average molecular weight (Mn), is 3 or more and 30 or less. (28) １３(29) The separator for an electric storage device according to any one of items 21 to 28, wherein the pentad fraction of the polypropylene measured by C-NMR (nuclear magnetic resonance) is 94.0% or more. (30) The separator for an electric storage device according to item 29, wherein the thermoplastic elastomer is 0.5% by mass or more and 20.0% by mass or less, based on the total mass of the microporous layer (A). (31) The separator for an electric storage device according to item 29, wherein the thermoplastic elastomer contains, as a repeating unit, one or more selected from the group consisting of ethylene, propylene, and 1-butene. (32) The separator for an electric storage device according to any one of items 21 to 31, wherein the thickness of the separator substrate is 3 μm or more and 20 μm or less. (33) The separator for a power storage device according to any one of items 21 to 32, wherein the stem height calculated by analysis of a scanning electron microscope (SEM) image of an MD-ND cross section of the separator substrate is 500 nm or more and 1,000 nm or less. (34) The separator for a power storage device according to any one of items 21 to 33, wherein the area average long pore diameter calculated by analysis of a scanning electron microscope (SEM) image of an MD-ND cross section of the microporous layer (A) is 50 nm or more and 500 nm or less. (35) The separator for a power storage device according to any one of items 21 to 34, wherein the separator substrate has a TD heat shrinkage of 5% or less after 1 hour at a temperature of 105°C. (36) The separator for a power storage device according to any one of items 21 to 35, wherein the separator substrate has an MD heat shrinkage of 20% or less after 1 hour at a temperature of 105°C. (37) The separator for an electric storage device according to any one of items 21 to 36, wherein the separator substrate has an MD tensile elongation of 20% or more and 60% or less. (38) The separator substrate has a melt tension Mt Ａ and a melt tension Mt Ｂ(39) An electricity storage device comprising: a positive electrode containing lithium iron phosphate as a positive electrode active material, a negative electrode, and the electricity storage device separator according to any one of items 21 to 38, disposed between the positive electrode and the negative electrode.

[0014] According to a first embodiment of the present invention, it is possible to provide a separator for an electricity storage device that has both high puncture strength and low air permeability. According to a second embodiment of the present invention, it is possible to provide a separator for an electricity storage device that has both high puncture strength and low air permeability and is also resistant to tearing due to its high TD tensile strength.

[0015] FIG. 1 is an example of a schematic diagram illustrating the relationship between the polymer matrix, connecting domains, fibrils, and pores in the MD-ND (thickness direction) cross section of a microporous layer (A) produced by uniaxial stretching in the film-forming direction (MD) according to the present disclosure. FIG. 2 is an example of a schematic diagram illustrating the relationship between the polymer matrix, fibrils, domains that become brightly dyed areas, and pores in the cross section of a microporous layer (A) produced by uniaxial stretching in the film-forming direction (MD) according to the present disclosure. FIG. 3 is a cutout diagram illustrating the detection points used to calculate the trunk height in a scanning electron microscope (SEM) image of the MD-ND cross section of a separator substrate, where fibrils have been removed by image analysis and the image has been binarized into resin and pore portions. FIG. 4 is a cutout diagram illustrating the detection points used to calculate the MD length in a scanning electron microscope (SEM) image of the MD-ND cross section of a microporous layer, where the image has been binarized into resin and pore portions by image analysis. FIG. 5 is a graph showing the elution temperature-elution amount curve in cross fractionation chromatography (CFC) measurement.

[0016] In this specification, various measurements are carried out based on the methods described in the Examples unless otherwise specified. In this specification, the upper or lower limit of a numerical range described in stages may be replaced by the upper or lower limit of a corresponding numerical range described in another stage, and may also be replaced by the corresponding value described in the Examples. In this specification, the term "step" includes not only an independent step but also a step that cannot be clearly distinguished from other steps, as long as the function of that step is achieved.

[0017] First embodiment

[0018] <<Electricity Storage Device Separator>> The electricity storage device separator of the first embodiment of the present disclosure has, as a separator substrate, a microporous layer containing a polyolefin as a primary component. The microporous layer in the present disclosure corresponds to the microporous layer (A) below. When the microporous layer (A) is a single layer, it may constitute a single-layer microporous membrane, and when it is a plurality of microporous layers (A) or when the microporous layer (A) is combined with another layer, it may constitute a multilayer microporous membrane. If desired, the separator substrate may include a microporous layer (B) containing a polyolefin as a primary component, in addition to the microporous layer (A). Furthermore, the separator substrate may further have a coating layer (also referred to as a "surface layer," "coating layer," etc.; hereinafter simply referred to as a "coating layer") on the microporous layer (A) and / or the microporous layer (B). In the present disclosure, the term "microporous layer" refers to each microporous layer constituting the substrate of a separator, the term "separator substrate" refers to the substrate of a separator excluding any coating layers, and the term "separator" refers to the entire separator including any coating layers.

[0019] <Microporous Layer (A)> The separator for an electricity storage device according to the first embodiment of the present disclosure has a microporous layer (A). The separator for an electricity storage device may have only one microporous layer (A), or two or more microporous layers (A). The microporous layer (A) is primarily composed of polyolefin, more preferably polypropylene and / or polyethylene. This allows for good pore openability and good battery performance. The microporous layer (A) is even more preferably composed of polypropylene. This allows for good battery performance to be maintained even after storage at high temperatures (e.g., 130°C). In this specification, "primarily composed of polypropylene" means that the microporous layer (A) contains 50% by mass or more of polypropylene, based on the total mass of the microporous layer (A). The lower limit of the polypropylene content in the microporous layer (A) is 50% by mass or more, and preferably 55% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more, from the viewpoints of the wettability, thinning, and shutdown characteristics of the separator, etc. The upper limit of the polypropylene content in the microporous layer (A) is not limited, and may be, for example, 97.5% by mass or less, 98% by mass or less, 98.5% by mass or less, or 99% by mass or less, or may be 100% by mass.

[0020] <MD length of microporous layer (A)> An example of the morphology of the microporous layer (A) of the present disclosure produced by uniaxial stretching in the film formation direction (MD) is shown schematically in Figure 1. It is preferred that a plurality of fibrils (3) extend along the MD of the microporous layer (A) between a plurality of polymer matrices (1), that linking domains (2-1) are oriented parallel to the MD of the microporous layer (A) inside or on the surface of the polymer matrices (1) and / or fibrils (3), and that the portions excluding the polymer matrices (1), fibrils (3), and linking domains (2-1) are voids (4). The presence of linking domains inside or on the surface of the polymer matrix and / or fibrils reinforces the polymer matrix and fibrils without excessively crushing the voids, and that the fibrils and / or linking domains connect as many polymer matrices as possible. This promotes stress propagation between polymer matrices while maintaining an appropriate amount of pores, and alleviates localized stress on the polymer matrix in the pin puncture strength failure mode, thereby suppressing failure due to cracking of the polymer matrix. It is presumed that this makes it possible to form a microporous layer (A) that combines low air permeability and high pin puncture strength. When fibrils connect more polymer matrices, the morphology preferably has a structure in which fibrils are arranged in the MD, as shown schematically in the connecting domain (2-1) in Figure 1. The polymer matrix preferably contains at least polypropylene, and when produced by uniaxial stretching, the structure is one in which lamellar crystals are arranged. The fibrils preferably contain at least polypropylene, and when produced by uniaxial stretching, the fibrils are formed by stretching the polymer chains of the polymer matrix during pore opening, and the connecting domains preferably contain polyethylene and / or a thermoplastic elastomer.

[0021] The MD length of the microporous layer (A) of the first embodiment of the present disclosure is 1000 nm or more and 1900 nm or less. The MD length is correlated with the structure in which the fibrils connecting the polymer matrices are arranged side by side in the MD and / or with the connecting domains. As the MD length increases, the fibrils and / or connecting domains connect more polymer matrices, relieving local stress and thereby achieving good pin puncture strength. From the viewpoint of obtaining a microporous layer (A) with high pin puncture strength, the lower limit of the MD length is preferably 1050 nm or more, more preferably 1100 nm or more, even more preferably 1150 nm or more, still more preferably 1200 nm or more, particularly preferably 1250 nm or more, and most preferably 1300 nm or more. From the viewpoint of ensuring sufficient pores and obtaining good air permeability, the upper limit of the MD length is preferably 1,850 nm or less, more preferably 1,800 nm or less, even more preferably 1,780 nm or less, still more preferably 1,750 nm or less, particularly preferably 1,720 nm or less, and most preferably 1,700 nm or less.

[0022] The MD length in the present disclosure is calculated by analyzing a scanning electron microscope (SEM) image of the MD-ND cross section of the separator substrate, and the analysis and calculation methods are described in detail in the Examples.

[0023] To achieve a good MD length of the microporous layer (A), the microporous layer (A) may contain polyethylene and / or a thermoplastic elastomer as an additive. When a thermoplastic elastomer is contained, from the viewpoints of increasing the MD length and achieving film-formability, thinning, low air permeability, and high pin puncture strength, the lower limit of the content of the thermoplastic elastomer in the microporous layer (A) is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, even more preferably 2.0% by mass or more, and still more preferably 3.0% by mass or more, based on the total mass of the microporous layer (A). The upper limit of the thermoplastic elastomer content in the microporous layer (A), from the viewpoint of ensuring sufficient pores to obtain good air permeability and maintaining pore openness, is preferably 20.0% by mass or less, more preferably 15.0% by mass or less, even more preferably 10.0% by mass or less, still more preferably 8.0% by mass or less, particularly preferably 7.5% by mass or less, extremely preferably 6.0% by mass or less, and most preferably 5.0% by mass or less, based on the total mass of the microporous layer (A). When polyethylene is contained, the lower limit of the polyethylene content, based on the total mass of the microporous layer (A), from the viewpoint of increasing the MD length and achieving both high pin puncture strength and low air permeability, is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, even more preferably 2.0% by mass or more, and even more preferably 3.0% by mass or more. The upper limit of the polyethylene content is preferably 20.0% by mass or less, more preferably 15.0% by mass or less, even more preferably 12.5% ​​by mass or less, even more preferably 10.0% by mass or less, and particularly preferably 7.5% by mass or less, based on the total mass of the microporous layer (A), from the viewpoint of maintaining openness. When the microporous layer of the present disclosure contains polypropylene as a polyolefin and a thermoplastic elastomer as an additive, it preferably contains 80.0% by mass or more and 99.5% by mass or less of polypropylene and 0.5% by mass or more and 20.0% by mass or less of the thermoplastic elastomer, based on the total mass of the microporous layer.

[0024] <Material of Microporous Layer (A)> The microporous layer (A) of the first embodiment of the present disclosure is primarily composed of a polyolefin. By using a polyolefin as the primary component, the substrate or separator has good pore opening and can achieve good battery performance. When the microporous layer (A) contains polypropylene, the polypropylene of the microporous layer (A) may be the same material as the polypropylene of the microporous layer (B) described below, or may be a polypropylene with a different chemical structure, more specifically, a polypropylene differing in at least one of monomer composition, stereoregularity, molecular weight, and crystalline structure. The stereoregularity of the polypropylene is not limited, and examples thereof include atactic, isotactic, or syndiotactic homopolymers. The polypropylene according to the present disclosure is preferably a highly crystalline isotactic or syndiotactic homopolymer.

[0025] The polypropylene of the microporous layer (A) is preferably a homopolymer, but may also be a copolymer, such as a block polymer, in which a small amount of a comonomer other than propylene, such as an α-olefin comonomer, is copolymerized. The amount of propylene structures contained as repeating units in the polypropylene is not limited, but may be, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The amount of repeating units derived from comonomers other than the propylene structure contained in the polypropylene is not limited, but may be, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. The polypropylene can be used alone or in combination of two or more types.

[0026] The weight-average molecular weight (Mw) of the polypropylene of the microporous layer (A) is preferably 300,000 or more from the viewpoint of high pin puncture strength of the microporous layer, and is preferably 1,300,000 or less from the viewpoints of good film-forming properties, productivity, thinning, and ensuring low air permeability. The Mw of the polypropylene is more preferably 500,000 or more and 1,200,000 or less, even more preferably 650,000 or more and 1,100,000 or less, still more preferably 750,000 or more and 1,000,000 or less, and particularly preferably 800,000 or more and 1,000,000 or less.

[0027] The upper limit of the value (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) of the polypropylene of the microporous layer (A) by the number average molecular weight (Mn) is preferably 30 or less, more preferably 20 or less, 18 or less, 16 or less, 14 or less, or 12 or less. Setting the Mw / Mn of the polypropylene to 30 or less tends to ensure good film-forming properties, productivity, and thinning. The lower limit of the Mw / Mn of the polypropylene is preferably 3 or more, more preferably 4 or more, 4.5 or more, or 5.0 or more. Setting the Mw / Mn of the polypropylene to 3 or more maintains appropriate molecular entanglement, resulting in good film-forming stability. The weight average molecular weight, number average molecular weight, and Mw / Mn of the polypropylene of the present disclosure are polystyrene-equivalent molecular weights obtained by GPC (gel permeation chromatography) measurement.

[0028] The density of the polypropylene of the microporous layer (A) is preferably 0.85 g / cm ３ or more, for example, 0.88 g / cm ３ Above, 0.89g / cm ３ or more, or 0.90 g / cm ３ The density of polypropylene is preferably 1.1 g / cm or more. ３ Below, for example, 1.0 g / cm ３ Below, 0.98g / cm ３ Below, 0.97g / cm ３ Below, 0.96g / cm ３ Below, 0.95g / cm ３ Below, 0.94g / cm ３Below, 0.93g / cm ３ or less, or 0.92 g / cm ３ The density of polypropylene is related to the crystallinity of polypropylene, and the density of polypropylene may be 0.85 g / cm or less. ３ By setting the above, the productivity of the microporous layer is improved, which is particularly advantageous in the dry method.

[0029] From the viewpoint of obtaining a microporous layer with low air permeability, the lower limit of the pentad fraction of the polypropylene in the microporous layer (A) is preferably 94.0% or more, 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 the polypropylene is not limited, but may be 99.9% or less, 99.8% or less, or 99.5% or less. The pentad fraction of the polypropylene is １３ Measured by C-NMR (nuclear magnetic resonance).

[0030] A pentad fraction of 94.0% or more of the polypropylene in the microporous layer (A) indicates high crystallinity of the polypropylene. In separators obtained by the stretching method, particularly the dry method, pores are formed by stretching the amorphous portions between crystalline portions. Therefore, when the polypropylene has high crystallinity, the pores become good and the air permeability can be kept low, enabling the battery to have high input / output.

[0031] Examples of polyethylene for the microporous layer (A) include ultra-high molecular weight polyethylene, high-density polyethylene, low-density polyethylene, and linear low-density polyethylene. These may be used alone or in combination of two or more. Also suitable are polyethylene with a narrow molecular weight distribution produced using a metallocene catalyst, high-density polyethylene, and polyethylene produced by multistage polymerization. Among these, high-density polyethylene is preferred from the viewpoints of film-forming properties and high pin puncture strength.

[0032] The polyethylene of the microporous layer (A) is preferably a homopolymer, but may also be a copolymer, such as a block polymer, in which a small amount of a comonomer other than ethylene, such as an α-olefin comonomer, is copolymerized. The amount of ethylene structures contained as repeating units in the polyethylene is not limited, but may be, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The amount of repeating units derived from comonomers other than ethylene structures contained in the polyethylene is not limited, but may be, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. The polyethylene may be used alone or in combination of two or more types.

[0033] The weight-average molecular weight (Mw) of the polyethylene in the microporous layer (A) is preferably 100,000 or more from the viewpoint of high pin puncture strength due to the formation of connected domains in the microporous layer, and is preferably 1,800,000 or less from the viewpoint of ensuring good film-formability, thinning, and productivity. The Mw of the polyethylene is more preferably 150,000 or more and 1,500,000 or less, even more preferably 200,000 or more and 1,200,000 or less, still more preferably 250,000 or more and 900,000 or less, and particularly preferably 300,000 or more and 600,000 or less.

[0034] The upper limit of the value (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) by the number average molecular weight (Mn) of the polyethylene of the microporous layer (A) is preferably 24 or less, more preferably 22 or less, 20 or less, or 18 or less. Setting the Mw / Mn of the polyethylene to 24 or less tends to ensure good film-forming properties, thinning, and productivity. The lower limit of the Mw / Mn of the polyethylene is preferably 3 or more, more preferably 4 or more, 4.5 or more, or 5.0 or more. Setting the Mw / Mn of the polyethylene to 3 or more maintains appropriate molecular entanglement, resulting in good film-forming stability. The weight average molecular weight, number average molecular weight, and Mw / Mn of the polyethylene of the present disclosure are polystyrene-equivalent molecular weights obtained by GPC (gel permeation chromatography) measurement.

[0035] The density of the polyethylene of the microporous layer (A) is preferably 0.85 g / cm ３ or more, for example, 0.88 g / cm ３ Above, 0.89g / cm ３ or more, or 0.90 g / cm ３ The density of polyethylene is preferably 1.1 g / cm or more. ３ Below, for example, 1.0 g / cm ３ Below, 0.98g / cm ３ Below, 0.97g / cm ３ Below, 0.96g / cm ３ Below, 0.95g / cm ３ Below, 0.94g / cm ３ Below, 0.93g / cm ３ or less, or 0.92 g / cm ３ The density of polyethylene is related to the crystallinity of the polyethylene, and the density of polyethylene may be 0.85 g / cm or less. ３ By doing so, a microporous layer with high pin puncture strength can be obtained due to the formation of linked domains.

[0036] The microporous layer (A) is primarily composed of polyolefin, but may contain polyolefins other than polypropylene and polyethylene. Polyolefins are polymers containing, as repeating units, monomers having a carbon-carbon double bond. Monomers constituting polyolefins other than polypropylene and polyethylene include, but are not limited to, monomers having 4 to 10 carbon atoms and having a carbon-carbon double bond, such as 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Examples of polyolefins include homopolymers, copolymers, and multistage polymers.

[0037] The microporous layer (A) of the first embodiment of the present disclosure contains a polyolefin as a main component and may also contain a thermoplastic elastomer. Examples of thermoplastic elastomers include a polyolefin different from the main component, a polyolefin copolymer, and a copolymer of polystyrene and a polyolefin. A polyolefin is a polymer containing a monomer having a carbon-carbon double bond as a repeating unit. Monomers constituting the polyolefin include, but are not limited to, monomers having 2 to 10 carbon atoms (C2 to C10) and a carbon-carbon double bond, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Examples of polyolefins include low-crystalline polypropylenes having low stereoregularity regions. Monomers constituting the polyolefin copolymer may be used alone or in combination of two or more. The polyolefin copolymer may be a random copolymer or a block copolymer. Examples of copolymers of polystyrene and polyolefin include styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer (SEBS), styrene-ethylene-styrene copolymer, styrene-(ethylene-butene)-olefin copolymer (SEBC), and olefin-(ethylene-butene)-styrene copolymer (CEBS), which may be hydrogenated polymers. These copolymers may be random copolymers or block copolymers, and are preferably block copolymers.

[0038] When the microporous layer (A) of the first embodiment of the present disclosure contains only a thermoplastic elastomer as an additive, the thermoplastic elastomer contained therein is preferably a copolymer containing one or more repeating units selected from the group consisting of propylene, 1-butene, 1-hexene, and 1-octene that are compatible with polypropylene when the main component is polypropylene, from the viewpoint of obtaining a microporous layer (A) that combines high pin puncture strength and low air permeability by playing a role in connecting the polymer matrix and strengthening the connection between domains. Furthermore, when the main component is polypropylene, the copolymer preferably contains ethylene as a repeating unit that is incompatible with polypropylene, from the viewpoint of alleviating local stress applied to the polymer matrix in the pin puncture failure mode and suppressing failure due to cracking of the polymer matrix. Among these, ethylene / propylene (C2C3) copolymer, ethylene / 1-butene (C2C4) copolymer, ethylene / 1-hexene (C2C6) copolymer, ethylene / 1-octene (C2C8) copolymer, olefin-(ethylene-butene)-olefin copolymer (CEBC), and olefin-(ethylene-butene)-styrene copolymer (CEBS) are more preferred, and ethylene / propylene (C2C3) copolymer, ethylene-terminated olefin-(ethylene-butene)-olefin copolymer (CEBC), and ethylene-terminated olefin-(ethylene-butene)-styrene copolymer (CEBS) are even more preferred. Here, the ethylene-butene structure is structurally similar to propylene and therefore exhibits high affinity with polypropylene. The thermoplastic elastomers can be used alone or in combination of two or more.

[0039] When the microporous layer (A) of the first embodiment of the present disclosure contains polyethylene and a thermoplastic elastomer as additives, the thermoplastic elastomer contained therein is preferably a copolymer that is compatible with polypropylene and / or polyethylene and contains one or more repeating units selected from the group consisting of ethylene, propylene, and 1-butene, from the viewpoint of obtaining a microporous layer (A) that combines high pin puncture strength and low air permeability by fulfilling the role of connecting the polymer matrix and strengthening the connection between domains. Among these, ethylene / propylene (C2C3) copolymer, ethylene / 1-hexene (C2C6) copolymer, ethylene / 1-octene (C2C8) copolymer, olefin-(ethylene-butene)-olefin copolymer (CEBC), and olefin-(ethylene-butene)-styrene copolymer (CEBS) are more preferred, and ethylene / propylene (C2C3) copolymer, ethylene-terminated olefin-(ethylene-butene)-olefin copolymer (CEBC), and ethylene-terminated olefin-(ethylene-butene)-styrene copolymer (CEBS) are even more preferred. Here, the ethylene-butene structure is structurally similar to propylene and therefore exhibits high affinity with polypropylene. The thermoplastic elastomers can be used alone or in combination of two or more.

[0040] <Measurement values ​​of microporous layer (A) by cross fractionation chromatography (CFC)> The preferred range of the measurement values ​​of the microporous layer (A) in the first embodiment of the present disclosure by cross fractionation chromatography (CFC) is the same as that in the second embodiment described below.

[0041] <Melt flow rate (MFR) of microporous layer (A)> The melt flow rate (MFR) of the microporous layer (A) of the present disclosure is preferably 1.00 g / 10 min or less. From the viewpoint of obtaining a microporous layer (A) with higher pin puncture strength, the upper limit of the melt flow rate (MFR) of the microporous layer (A) (MFR of a single layer) is preferably 1.00 g / 10 min or less, more preferably 0.90 g / 10 min or less, even more preferably 0.80 g / 10 min or less, still more preferably 0.70 g / 10 min or less, and most preferably 0.60 g / 10 min or less. The lower limit of the MFR of the microporous layer (A) (single layer MFR) is not limited, but from the viewpoint of obtaining a microporous layer (A) with lower air permeability, film-forming properties, and a thinner film, it may be, for example, 0.20 g / 10 min or more, 0.25 g / 10 min or more, 0.30 g / 10 min or more, 0.35 g / 10 min or more, 0.40 g / 10 min or more, or 0.45 g / 10 min or more. The MFR of the microporous layer (A) is measured under conditions of a load of 2.16 kg and a temperature of 230°C. An MFR of the microporous layer (A) of 1.00 g / 10 min or less means that the molecular weight of the polyolefin contained in the microporous layer (A) is significantly high. A high molecular weight of the polyolefin increases the number of tie molecules that bond crystalline phases together, and therefore tends to result in a microporous layer (A) with high pin puncture strength. Furthermore, when the microporous layer (A) has an MFR of 0.20 g / 10 min or more, the melt tension of the microporous layer (A) does not become too high, making it possible to ensure good film-forming properties, thin film thickness, and productivity.

[0042] From the viewpoint of obtaining a microporous layer (A) that has high pin puncture strength, low air permeability, and a thin film, the MFR of the polypropylene of the microporous layer (A) is preferably 0.2 to 0.9 g / 10 min when measured under conditions of a load of 2.16 kg and a temperature of 230° C. From the viewpoint of obtaining a microporous layer (A) with even higher pin puncture strength, the upper limit of the MFR of the polypropylene may be, for example, 0.8 g / 10 min or less, 0.7 g / 10 min or less, 0.65 g / 10 min or less, 0.6 g / 10 min or less, or 0.55 g / 10 min or less. The lower limit of the MFR of the polypropylene is not limited, but from the viewpoint of obtaining a microporous layer (A) with lower air permeability, film-formability, and a thinner film, it may be, for example, 0.2 g / 10 min or more, 0.25 g / 10 min or more, 0.3 g / 10 min or more, 0.35 g / 10 min or more, 0.4 g / 10 min or more, or 0.45 g / 10 min or more.

[0043] From the viewpoint of obtaining a microporous layer (A) that has high pin puncture strength, low air permeability, and a thin film, the polyethylene of the microporous layer (A) preferably has an MFR of 0.005 to 10.0 g / 10 min when measured under conditions of a load of 2.16 kg and a temperature of 190°C. From the viewpoint of obtaining a microporous layer (A) with higher pin puncture strength and good film-forming stability, the upper limit of the MFR of the polyethylene may be, for example, 8.0 g / 10 min or less, 6.0 g / 10 min or less, 4.0 g / 10 min or less, 3.0 g / 10 min or less, 2.0 g / 10 min or less, 1.5 g / 10 min or less, 1.0 g / 10 min or less, 0.8 g / 10 min or less, 0.6 g / 10 min or less, 0.5 g / 10 min or less, 0.4 g / 10 min or less, 0.35 g / 10 min or less, 0.3 g / 10 min or less, 0.25 g / 10 min or less, 0.2 g / 10 min or less, 0.15 g / 10 min or less, 0.1 g / 10 min or less, or 0.05 g / 10 min or less. The lower limit of the MFR of the polyethylene is not limited, but from the viewpoint of obtaining a microporous layer (A) with lower air permeability, film-formability, and a thinner film, it may be, for example, 0.008 g / 10 min or more, 0.01 g / 10 min or more, 0.012 g / 10 min or more, 0.015 g / 10 min or more, 0.018 g / 10 min or more, or 0.02 g / 10 min or more.

[0044] From the viewpoint of obtaining a microporous layer (A) having high pin puncture strength, low air permeability, a thin film, and good film-forming stability, the MFR of the thermoplastic elastomer of the microporous layer (A) is preferably 0.1 to 100.0 g / 10 min when measured under conditions of a load of 2.16 kg and a temperature of 230° C. The upper limit of the MFR of the thermoplastic elastomer may be, for example, 80.0 g / 10 min or less, 60.0 g / 10 min or less, 40.0 g / 10 min or less, 30.0 g / 10 min or less, 20.0 g / 10 min or less, 15.0 g / 10 min or less, 10.0 g / 10 min or less, 8.0 g / 10 min or less, 6.0 g / 10 min or less, or 5.0 g / 10 min or less, from the viewpoint of being uniformly kneaded with the polyolefin and obtaining a microporous layer (A) having high pin puncture strength and good film-forming stability. The lower limit of the MFR of the thermoplastic elastomer is not limited, but from the viewpoint of obtaining a microporous layer (A) with lower air permeability, film-formability, and a thinner film, it may be, for example, 0.5 g / 10 min or more, 1.0 g / 10 min or more, 1.5 g / 10 min or more, 2.0 g / 10 min or more, 2.5 g / 10 min or more, or 3.0 g / 10 min or more.

[0045] <Mw and Mw / Mn of Microporous Layer (A)> The weight-average molecular weight (Mw) of the microporous layer (A) is preferably 250,000 or more from the viewpoint of obtaining a microporous layer (A) with higher pin puncture strength, and is preferably 1,500,000 or less from the viewpoints of good film-formability, productivity, thinning, and ensuring low air permeability. The Mw of the microporous layer (A) is more preferably 400,000 or more and 1,300,000 or less, even more preferably 500,000 or more and 1,200,000 or less, still more preferably 600,000 or more and 1,100,000 or less, and particularly preferably 700,000 or more and 1,000,000 or less.

[0046] The upper limit of the value (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) of the microporous layer (A) by the number average molecular weight (Mn) is preferably 30 or less, more preferably 25 or less, 20 or less, 18 or less, 16 or less, 14 or less, 12 or less, or 10 or less. Setting the Mw / Mn of the microporous layer (A) to 30 or less tends to ensure good film-formability, productivity, and thinning. The lower limit of the Mw / Mn of the microporous layer (A) is preferably 3 or more, more preferably 4 or more, 4.5 or more, or 5.0 or more. Setting the Mw / Mn of the microporous layer (A) to 3 or more maintains appropriate molecular entanglement, resulting in good film-forming stability. The weight average molecular weight, number average molecular weight, and Mw / Mn of the microporous layer (A) of the present disclosure are polystyrene-equivalent molecular weights obtained by GPC (gel permeation chromatography) measurement. Furthermore, since the microporous layer (A) of the present disclosure contains polyolefins including polypropylene and polyethylene as main components and also contains a thermoplastic elastomer, the Mw and Mw / Mn of the microporous layer (A) described above are values ​​that reflect the influence of the combination of these constituent materials.

[0047] <Melt tension of microporous layer (A)> Melt tension Mt of microporous layer (A) at 240°C Ａ (Single layer Mt Ａ The upper limit of the melt tension Mt of the microporous layer (A) is preferably 35 mN or less, more preferably 32 mN or less, even more preferably 30 mN or less, still more preferably 28 mN or less, and particularly preferably 26 mN or less, from the viewpoints of good film-forming properties, productivity, thinning, and obtaining a microporous layer (A) with low air permeability. Ａ (Single layer Mt Ａ ) is preferably 10 mN or more, more preferably 13 mN or more, even more preferably 16 mN or more, still more preferably 18 mN or more, and particularly preferably 19 mN or more, from the viewpoint of obtaining a microporous layer (A) with higher pin puncture strength.

[0048] <DSC parameters of microporous layer (A)> In the microporous layer (A), it is preferable that, in a DSC curve (where the vertical axis represents heat flow and the horizontal axis represents temperature) during the temperature rise process in differential scanning calorimetry (DSC), there is an endothermic peak A having a peak value in the range of 100°C to 145°C and an endothermic peak B having a peak value in the range of 155°C to 175°C. In addition, in the DSC curve, the microporous layer (A) has an area S of endothermic peak A. Ａ and the area S of endothermic peak B Ｂ Ratio S Ｂ / S Ａ is preferably 2 or more and 200 or less. It is believed that endothermic peak A is derived from at least polyethylene among the components contained in the microporous layer (A), and endothermic peak B is derived from at least polypropylene among the components contained in the microporous layer (A). From the viewpoint of obtaining a microporous layer (A) with high pin puncture strength, the peak value range of endothermic peak A of the microporous layer (A) is preferably 100°C or more and 145°C or less, more preferably 110°C or more and 142°C or less, even more preferably 120°C or more and 140°C or less, and even more preferably 125°C or more and 135°C or less. From the viewpoint of obtaining a microporous layer (A) with high pin puncture strength and low air permeability, the peak value range of endothermic peak B of the microporous layer (A) is preferably 155°C or more and 175°C or less, more preferably 158°C or more and 173°C or less, even more preferably 160°C or more and 171°C or less, and even more preferably 163°C or more and 169°C or less. In addition, S of the microporous layer (A) Ｂ / S Ａ is considered to reflect the ratio between at least the polypropylene content and at least the polyethylene content, and from the viewpoint of maintaining good battery performance even after storage at high temperatures (e.g., 130°C) and maintaining good film-forming properties, puncture strength, and air permeability, it is preferably 2 or more and 200 or less, more preferably 4 or more and 100 or less, even more preferably 6 or more and 50 or less, even more preferably 8 or more and 30 or less, and particularly preferably 10 or more and 25 or less.

[0049] <Area-average long pore diameter of microporous layer (A)> The area-average long pore diameter (hereinafter also simply referred to as "area-average long pore diameter") in the MD-ND cross section of the microporous layer (A) is preferably 50 nm or more and 500 nm or less. In the present disclosure, "ND" refers to the thickness direction of the microporous layer, and "MD" refers to the film-forming direction of the microporous layer. For example, the MD of a separator having a microporous layer is the longitudinal direction in the case of a roll. "Long pore diameter" means the pore diameter in the MD. Furthermore, when there are two or more microporous layers (A) and / or microporous layers (B), the area-average long pore diameters of the microporous layer (A) and the microporous layer (B) are compared based on the average area-average long pore diameter value of each layer. The lower limit of the area-average pore diameter of the microporous layer (A) is preferably 50 nm or more, more preferably 80 nm or more, even more preferably 100 nm or more, still more preferably 120 nm or more, and particularly preferably 130 nm or more, from the viewpoint of ensuring high input / output in the electricity storage device and obtaining a microporous layer (A) with low air permeability. The upper limit of the area-average pore diameter of the microporous layer (A) is preferably 500 nm or less, more preferably 400 nm or less, even more preferably 350 nm or less, still more preferably 300 nm or less, particularly preferably 250 nm or less, and most preferably 210 nm or less, from the viewpoint of obtaining a microporous layer (A) with high pin puncture strength.

[0050] The area-average pore diameter can be measured by observing a cross-sectional SEM (scanning electron microscope) of the separator's MD-ND cross section and analyzing the obtained image. Detailed conditions are shown in the Examples. When measuring the average pore diameter from a cross-sectional SEM image, the number-average pore diameter and the area-average pore diameter can be calculated, but in the present disclosure, the area-average pore diameter is used as the average pore diameter to better correlate with the separator's physical properties.

[0051] <Lightly stained portion of microporous layer (A)> In a composition image at a magnification of 30,000 times obtained by a scanning electron microscope (SEM) of an MD-ND cross section of the microporous layer (A) stained with a ruthenium compound, a polymer matrix and fibrils are present, and the polymer matrix and the fibrils each have an area of ​​100 nm ２ 0.1 μm or more ２It is preferable that the following bright staining portions exist. Here, the bright staining portions in the present disclosure refer to domain portions that are observed as bright regions in the composition image due to differences in the degree of staining by the ruthenium compound caused by differences in the constituent material and / or molecular structure from polypropylene, which is the main component of the polymer matrix and fibrils. In addition, the amorphous portions of polypropylene, which is the main component of the polymer matrix and fibrils, can also become bright staining portions, but the areas per unit area are 100 nm ２ By setting the above-mentioned light dyed portions and lower area limits, it is intended to exclude amorphous portions of polypropylene. In addition, from the viewpoint of obtaining a microporous layer (A) having high pin puncture strength, the lower limit of the area per light dyed portion is set to 150 nm ２ More than 200 nm is preferable. ２ More preferably, 250 nm or more ２ More preferably, 300 nm or more ２ The upper limit of the area per bright dyed portion is 0.08 μm from the viewpoint of obtaining a microporous layer (A) having good pore opening properties and high puncture strength. ２ Preferably, 0.06 μm or less ２ More preferably, 0.05 μm or less ２ More preferably, 0.04 μm or less ２ More preferably, 0.03 μm or less ２ Particularly preferred is 0.02 μm or less ２ The following are most preferred:

[0052] The area of ​​the entire image showing only the microporous layer (A) is 100 nm ２ 0.1 μm or more ２ The total area ratio of the following bright stained areas is S １ %, 0.5≦S １ It is preferable that S is ≦15.0. １From the viewpoint of obtaining a microporous layer (A) that has both low air permeability and high pin puncture strength by reinforcing the polymer matrix and fibrils without excessively crushing the pores, the lower limit of S is more preferably 0.75 or more, even more preferably 1.0 or more, even more preferably 1.25 or more, particularly preferably 1.5 or more, extremely preferably 1.75 or more, and most preferably 2.0 or more. １ From the viewpoint of maintaining pore openness and obtaining a microporous layer (A) having low air permeability and high pin puncture strength, the upper limit is more preferably 12.0 or less, even more preferably 10.0 or less, still more preferably 8.0 or less, particularly preferably 6.0 or less, extremely preferably 5.0 or less, and most preferably 4.0 or less.

[0053] Area of ​​stained bright area and total area ratio S １ The total area ratio S can be measured by observing the MD-ND cross section of a separator dyed with a ruthenium compound with a cross-sectional SEM (scanning electron microscope) and analyzing the composition image obtained at a magnification of 30,000 times. Detailed conditions are shown in the examples. １ The denominator for calculating the area is the entire image showing only the microporous layer (A), and the area of ​​the entire image including not only the polymer matrix and fibrils but also pores is used.

[0054] As described above, the microporous layer (A) of the present disclosure preferably contains polypropylene and a thermoplastic elastomer as main components and has a structure including a polymer matrix and fibrils, and the light-dyed areas in the composition image are preferably present in the polymer matrix and fibrils. The morphology of the microporous layer (A) of the present disclosure is not limited, but an example of the morphology when produced by uniaxial stretching in the film formation direction (MD) is shown schematically in Figure 2. Preferably, a plurality of fibrils (3) extend along the MD of the microporous layer (A) between a plurality of polymer matrices (1), and domains (2-2) that become the light-dyed areas are oriented parallel to the MD of the microporous layer (A) within or on the surfaces of the polymer matrices (1) and fibrils (3). The areas excluding the polymer matrix (1), fibrils (3), and domains (2-2) that become the light-dyed areas are voids (4). It is preferred that the polymer matrix contains at least polypropylene, and when produced by uniaxial stretching, the structure will be one in which lamellar crystals are arranged, the fibrils contain at least polypropylene, and when produced by uniaxial stretching, the fibrils are formed by stretching the polymer chains of the polymer matrix during perforation by stretching, and the domains that become the bright dyed areas contain a thermoplastic elastomer.

[0055] Without being limited by theory, it is believed that it is important for the microporous layer (A) of the present disclosure to form domains that become light-dyed areas, and that reinforcing the polymer matrix and fibrils without excessively crushing the pores makes it possible to form a microporous layer (A) that combines low air permeability and high pin puncture strength. Examples of failure modes for pin puncture strength in the microporous layer (A) include cracking of the polymer matrix. The presence of connecting domains inside or on the surface of the polymer matrix is ​​thought to alleviate the stress applied to the polymer matrix at the time of fracture, thereby achieving high pin puncture strength. The microporous layer (A) of the present disclosure preferably contains or adds a thermoplastic elastomer to polypropylene, and the thermoplastic elastomer preferably has a site that is incompatible with polypropylene. It is believed that some or all of the thermoplastic elastomer that is not mixed with the polymer matrix containing polypropylene as the main component forms domains that are different from the polymer matrix (that become light-dyed areas).

[0056] Furthermore, when the microporous layer (A) of the present disclosure is primarily composed of polypropylene, polyethylene is preferred from the viewpoint of obtaining a microporous layer (A) that combines low air permeability and high pin puncture strength by forming domains that are dyed brightly in the polymer matrix and fibrils primarily composed of polypropylene, which are generally immiscible with polypropylene when added together with a thermoplastic elastomer. When polyethylene is contained in or added to polypropylene, polypropylene and polyethylene generally do not mix, so some or all of the polyethylene that does not mix with the polymer matrix containing polypropylene as the primary component forms domains that are distinct from the polymer matrix. When a thermoplastic elastomer is added to polyethylene, it is presumed that the addition of a thermoplastic elastomer improves the dispersibility of polypropylene and polyethylene or plays a role in connecting polypropylene and polyethylene to strengthen the connection between the domains, thereby promoting the above-mentioned effects as a connecting domain. From the viewpoint of combining low air permeability and high pin puncture strength, it is preferable that the connecting domain containing polyethylene extend uninterrupted along the MD, as shown in FIG. 1.

[0057] <Porosity of Microporous Layer (A)> The porosity of the microporous layer (A) in the first embodiment of the present disclosure is preferably 30% or more from the viewpoint of avoiding clogging in the electricity storage device and obtaining a microporous layer (A) with low air permeability, and is preferably 60% or less from the viewpoint of obtaining a microporous layer (A) with high pin puncture strength. The lower limit of the porosity of the microporous layer (A) may be, for example, 35% or more, 40% or more, 43% or more, 45% or more, or 47% or more. The upper limit of the porosity of the microporous layer (A) may be, for example, 57% or less, 54% or less, or 53% or less.

[0058] <Thickness of Microporous Layer (A)> When the substrate of the separator for an electricity storage device has a single-layer structure having only one microporous layer (A), the upper limit of the thickness of the microporous layer (A) is preferably 20 μm or less, for example, 18 μm or less, 16 μm or less, 14 μm or less, 13 μm or less, 12 μm or less, 11.5 μm or less, 11 μm or less, 10.5 μm or less, or 10 μm or less, from the viewpoint of increasing the energy density of the electricity storage device and reducing the air permeability of the microporous layer (A). When the substrate has a single-layer structure, the lower limit of the thickness of the microporous layer (A) is preferably 3 μm or more, for example, 4 μm or more, 5 μm or more, 6 μm or more, 7 μm or more, 7.5 μm or more, 8 μm or more, 8.5 μm or more, or 9 μm or more, from the viewpoint of obtaining a microporous layer (A) with high pin puncture strength.

[0059] When the substrate of the separator for an electricity storage device has a multilayer structure including one or more microporous layers (A), the upper limit of the thickness of the microporous layer (A) may be preferably 10 μm or less, 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, from the viewpoint of increasing the energy density of the electricity storage device and reducing the air permeability of the separator substrate. When the substrate has a multilayer structure, the lower limit of the thickness of the microporous layer (A) may be preferably 1 μm or more, for example, 2 μm or more, 3 μm or more, or 3.5 μm or more, from the viewpoint of obtaining a separator substrate with high pin puncture strength.

[0060] <Additives for Microporous Layer (A)> The microporous layer (A) containing a polyolefin as a main component may further contain, as necessary, additives such as an elastomer, a crystal nucleating agent, an antioxidant, a filler, etc. The amount of the additive is not particularly limited, and may be, for example, 0.01% by mass or more, 0.1% by mass or more, or 1% by mass or more, and may be, for example, 20% by mass or less, 10% by mass or less, or 7% by mass or less, based on the total mass of the microporous layer (A).

[0061] <Microporous Layer (B)> The separator for an electricity storage device of the present disclosure optionally has a microporous layer (B) in addition to the microporous layer (A). The separator for an electricity storage device may have only one microporous layer (B) or two or more microporous layers (B). The microporous layer (B) is also primarily composed of polyolefin, more preferably polypropylene and / or polyethylene. This allows for good pore openability and good battery performance. The microporous layer (B) is more preferably primarily composed of polypropylene, which allows for good battery performance to be maintained even after storage at high temperatures (e.g., 130°C). In the present disclosure, the microporous layer (B) being "primarily composed of" polypropylene means that it contains 50% by mass or more of polypropylene, based on the total mass of the microporous layer (B). The lower limit of the polypropylene content in the microporous layer (B) may be preferably 55% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more from the viewpoints of the wettability of the separator, thinning of the film, etc. The upper limit of the polypropylene content in the microporous layer (B) is not limited, but may be, for example, 98% by mass or less, 99% by mass or less, or 100% by mass.

[0062] <Material of Microporous Layer (B)> The polypropylene of the microporous layer (B) may be the same material as the polypropylene of the microporous layer (A) described above, or may be a polypropylene with a different chemical structure, more specifically, a polypropylene with a different at least one of monomer composition, stereoregularity, molecular weight, and crystalline structure. The stereoregularity of the polypropylene of the microporous layer (B) is not limited, and examples thereof include atactic, isotactic, or syndiotactic homopolymers. The polypropylene according to the present disclosure is preferably an isotactic or syndiotactic highly crystalline homopolymer.

[0063] The polypropylene of the microporous layer (B) is preferably a homopolymer, but may also be a copolymer, such as a block polymer, in which a small amount of a comonomer other than propylene, such as an α-olefin comonomer, is copolymerized. The amount of propylene structures contained as repeating units in the polypropylene is not limited, but may be, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The amount of repeating units derived from comonomers other than the propylene structure contained in the polypropylene is not limited, but may be, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. The polypropylene may be used alone or in combination of two or more types.

[0064] The weight-average molecular weight (Mw) of the polypropylene of the microporous layer (B) is preferably 250,000 or more from the viewpoint of the strength of the microporous layer, etc., and is preferably 1,000,000 or less from the viewpoint of increasing the pore size of the microporous layer and achieving good air permeability. The Mw of the polypropylene is more preferably 400,000 or more and 950,000 or less, even more preferably 550,000 or more and 900,000 or less, still more preferably 600,000 or more and 900,000 or less, and particularly preferably 700,000 or more and 900,000 or less.

[0065] The upper limit of the value (Mw / Mn) obtained by dividing the weight-average molecular weight (Mw) of the polypropylene of the microporous layer (B) 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. The smaller the Mw / Mn value of the polypropylene, the lower the melt tension of the resulting microporous layer tends to be. Therefore, it is preferable that the Mw / Mn value of the polypropylene is 7 or less in order to control the melt tension of the microporous layer (B) to a low level. Furthermore, Mw / Mn may be preferably 1.1 or more, for example, 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. By having an Mw / Mn of 1.1 or more, appropriate molecular entanglement is maintained, which may improve stability during film formation. The weight average molecular weight, number average molecular weight, and Mw / Mn of the polyolefin of the microporous layer (B) of the present disclosure are polystyrene-equivalent molecular weights obtained by GPC (gel permeation chromatography) measurement, as in the case of the polypropylene of the microporous layer (A).

[0066] The density of the polypropylene of the microporous layer (B) is preferably 0.85 g / cm ３ or more, for example, 0.88 g / cm ３ Above, 0.89g / cm ３ or more, or 0.90 g / cm ３ The density of polypropylene is preferably 1.1 g / cm or more. ３ Below, for example, 1.0 g / cm ３ Below, 0.98g / cm ３ Below, 0.97g / cm ３ Below, 0.96g / cm ３ Below, 0.95g / cm ３ Below, 0.94g / cm ３ Below, 0.93g / cm ３ or less, or 0.92 g / cm ３ The density of polyolefins is related to the crystallinity of polypropylene, and the density of polypropylene can be 0.85 g / cm or less. ３ By setting the above, the productivity of the microporous layer is improved, which is particularly advantageous in the dry method.

[0067] The microporous layer (B) may contain other resins in addition to polypropylene. Examples of other resins include polyolefins other than polypropylene (also referred to as "other polyolefins"). Polyolefins are polymers containing a monomer having a carbon-carbon double bond as a repeating unit. Monomers constituting polyolefins other than polypropylene include, but are not limited to, monomers having 2 or 4 to 10 carbon atoms and a carbon-carbon double bond, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene.

[0068] The microporous layer (B) may contain a thermoplastic elastomer in addition to polypropylene. Examples of thermoplastic elastomers include, but are not limited to, polypropylenes other than the main component, polyolefins other than polypropylene (also referred to as "other polyolefins"), and copolymers of polystyrene and polyolefins. Examples of polypropylene include low-crystalline polypropylenes having low stereoregularity regions. Polyolefins are polymers containing a monomer having a carbon-carbon double bond as a repeating unit. Examples of monomers constituting polyolefins other than polypropylene include, but are not limited to, monomers having 2 or 4 to 10 carbon atoms and a carbon-carbon double bond, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Polyolefins may be, for example, homopolymers, copolymers, or multi-stage polymers. For example, polyethylene may be included. Preferred examples of copolymers of polystyrene and polyolefin include styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer (SEBS), hydrogenated styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), styrene-ethylene-styrene copolymer, olefin crystalline-(ethylene-butene)-olefin crystalline copolymer (CEBC), hydrogenated olefin crystalline-(ethylene-butene)-olefin crystalline copolymer (hydrogenated CEBC), styrene-(ethylene-butene)-olefin crystalline copolymer (SEBC), hydrogenated styrene-(ethylene-butene)-olefin crystalline copolymer (hydrogenated SEBC), etc. Particularly preferred are hydrogenated styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), hydrogenated styrene-(ethylene-butene)-olefin crystalline copolymer (hydrogenated SEBC), and styrene-(ethylene-propylene)-styrene copolymer (SEPS).

[0069] The thermoplastic elastomer contained in the microporous layer (B) is preferably an elastomer incompatible with polypropylene from the viewpoints of pore opening and increasing the pore size. The elastomer incompatible with polypropylene is not particularly limited, but preferred examples include copolymers of polyethylene and other polyolefins and copolymers of polystyrene and polyolefins. Preferred examples of the copolymer of polystyrene and polyolefin include styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer (SEBS), hydrogenated styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), styrene-ethylene-styrene copolymer, olefin crystalline-(ethylene-butene)-olefin crystalline copolymer (CEBC), hydrogenated olefin crystalline-(ethylene-butene)-olefin crystalline copolymer (hydrogenated CEBC), styrene-(ethylene-butene)-olefin crystalline copolymer (SEBC), and hydrogenated styrene-(ethylene-butene)-olefin crystalline copolymer (hydrogenated SEBC). Particularly preferred are hydrogenated styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), hydrogenated styrene-(ethylene-butene)-olefin crystalline copolymer (hydrogenated SEBC), and styrene-(ethylene-propylene)-styrene copolymer (SEPS).

[0070] <Melt flow rate (MFR) of microporous layer (B)> From the viewpoint of obtaining a microporous layer (B) with higher strength, the upper limit of the melt flow rate (MFR) of the microporous layer (B) (MFR of a single layer) is preferably 8.00 g / 10 min or less, and may be, for example, 6.00 g / 10 min or less, 4.00 g / 10 min or less, 3.00 g / 10 min or less, 2.00 g / 10 min or less, or 1.10 g / 10 min or less. From the viewpoint of exhibiting good air permeability, the lower limit of the MFR of the microporous layer (B) (MFR of a single layer) is not limited, and may be, for example, 0.30 g / 10 min or more, 0.35 g / 10 min or more, 0.40 g / 10 min or more, 0.45 g / 10 min or more, or 0.50 g / 10 min or more. The MFR of the microporous layer (B) is measured under conditions of a load of 2.16 kg and a temperature of 230°C. An MFR of the microporous layer (B) of 8.00 g / 10 min or less means that the molecular weight of the polyolefin contained in the microporous layer (B) is significantly high. A high molecular weight polyolefin increases the number of tie molecules that bond crystalline substances together, tending to result in a microporous layer (B) with high strength. An MFR of the microporous layer (B) of 0.30 g / 10 min or more prevents the melt tension of the microporous layer (B) from becoming too high, making it easier to obtain a separator that exhibits good air permeability.

[0071] The MFR of the polypropylene of the microporous layer (B) is preferably 8.0 g / 10 min or less, and may be, for example, 6.0 g / 10 min or less, 4.0 g / 10 min or less, 3.0 g / 10 min or less, 2.0 g / 10 min or less, or 1.1 g / 10 min or less, from the viewpoint of obtaining a microporous layer (B) with higher strength. The lower limit of the MFR of the microporous layer (B) (single-layer MFR) is not limited, from the viewpoint of exhibiting good air permeability, and may be, for example, 0.3 g / 10 min or more, 0.35 g / 10 min or more, 0.4 g / 10 min or more, 0.45 g / 10 min or more, or 0.5 g / 10 min or more.

[0072] The MFR of the microporous layer (B) is preferably higher than the MFR of the microporous layer (A). By making the MFR of the microporous layer (B) higher than the MFR of the microporous layer (A), the pore size of the microporous layer (B) in the obtained separator can be controlled to be larger than the pore size of the microporous layer (A).

[0073] <Pentad fraction of microporous layer (B)> From the viewpoint of obtaining a microporous layer with low air permeability, the lower limit of the pentad fraction of the polypropylene of the microporous layer (B) is preferably 94.0% or more, 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 the polypropylene is not limited, but may be 99.9% or less, 99.8% or less, or 99.5% or less. The pentad fraction of the polypropylene of the microporous layer (B) is, as in the case of the polypropylene of the microporous layer (A), １３ Measured by C-NMR (nuclear magnetic resonance).

[0074] A pentad fraction of 94.0% or more of the polypropylene in the microporous layer (B) indicates high crystallinity of the polypropylene. In separators obtained by the stretching perforation method, particularly the dry method, pores are formed by stretching the amorphous portions between crystalline portions. Therefore, when the polypropylene has high crystallinity, the perforation property is good and the air permeability can be kept low, thereby enabling a high output battery.

[0075] <Melt tension of microporous layer (B)> Melt tension Mt of microporous layer (B) at 240°C Ｂ The melt tension Mt is preferably 4 mN or more and 30 mN or less. Ｂ From the viewpoint of good film-forming properties and productivity of the separator substrate having the microporous layer (A) and the microporous layer (B), the lower limit of the melt tension Mt is preferably 4 mN or more, more preferably 7 mN or more, even more preferably 10 mN or more, particularly preferably 13 mN or more, and most preferably 16 mN or more. ＢThe upper limit of the melt tension Mt of the microporous layers (A) and (B) at a temperature of 240°C is preferably 30 mN or less, more preferably 28 mN or less, even more preferably 26 mN or less, and most preferably 24 mN or less, from the viewpoint of achieving a large pore diameter and exhibiting good air permeability. Ａ and Mt. Ｂ may be different from each other in terms of the area average long pore diameter of the microporous membrane.

[0076] <Area-average pore diameter of microporous layer (B)> The area-average pore diameter (hereinafter also simply referred to as "area-average pore diameter") in the MD-ND cross section of the microporous layer (B) is preferably larger than the area-average pore diameter of the microporous layer (A). For details about the relationship with the area-average pore diameter of the microporous layer (A), see the section <Area-average pore diameter of microporous layer (A)>.

[0077] The area average pore diameter of the microporous layer (B) in the MD-ND cross section is preferably 100 nm to 600 nm, more preferably 120 nm to 500 nm, even more preferably 140 nm to 400 nm, and still more preferably 160 nm to 350 nm. When the area average pore diameter of the microporous layer (B) is within this range, good pin puncture strength and air permeability can be obtained.

[0078] <Porosity of Microporous Layer (B)> The porosity of the microporous layer (B) is preferably 20% or more from the viewpoint of avoiding clogging in the electricity storage device and obtaining good air permeability of the separator, and is preferably 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, even more preferably 30% or more and 60% or less, and particularly preferably 35% or more and 60% or less.

[0079] <Thickness of Microporous Layer (B)> The thickness of the microporous layer (B) may be preferably 10 μm or less, 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, from the viewpoint of increasing the energy density of the electricity storage device, etc. The lower limit of the thickness of the microporous layer (B) may be preferably 1 μm or more, for example, 2 μm or more, 3 μm or more, or 3.5 μm or more, from the viewpoint of strength, etc.

[0080] <Additives for Microporous Layer (B)> In addition to polyolefin, the microporous layer (B) may further contain additives such as elastomers, crystal nucleating agents, antioxidants, fillers, etc. The amount of the additives is not particularly limited, and may be, for example, 0.01% by mass or more, 0.1% by mass or more, or 1% by mass or more, and may be, for example, 10% by mass or less, 7% by mass or less, or 5% by mass or less, based on the total mass of the microporous layer (B).

[0081] <Relationship between microporous layer (A) and microporous layer (B)> Melt tension Mt of microporous layer (A) at 240°C Ａ and the melt tension Mt of the microporous layer (B) at 240°C. Ｂ Relative to Mt Ａ / Mt Ｂ is preferably 1.05 or more and 4.0 or less. Ａ / Mt Ｂ By making Mt 1.05 or more, the pore size of the microporous layer (A) of the obtained separator can be controlled to be sufficiently small and the pore size of the microporous layer (B) can be controlled to be sufficiently large, thereby achieving both good voltage resistance and good air permeability. Ａ / Mt Ｂ By making Mt 4.0 or less, it is possible to obtain a separator having good pore opening and air permeability. Ａ / Mt Ｂ is more preferably 1.1 or more and 3.5 or less, even more preferably 1.15 or more and 3.3 or less, still more preferably 1.2 or more and 3.0 or less, and particularly preferably 1.25 or more and 2.5 or less.

[0082] MFR of the microporous layer (B) (MFR Ｂ ) and the MFR of the microporous layer (A) (MFR Ａ ) ratio, MFR Ｂ / MFR Ａ The MFR is preferably 1.02 or more and 10.0 or less. Ｂ / MFR Ａ By making the MFR equal to or greater than 1.02, the pore size of the microporous layer (A) of the obtained separator can be controlled to be sufficiently small, and the pore size of the microporous layer (B) can be controlled to be sufficiently large, thereby achieving both good voltage resistance and good air permeability. Ｂ / MFR ＡBy making the MFR equal to or less than 10.0, it is possible to obtain a separator having stable film-forming properties, productivity, and good pore opening and air permeability. Ｂ / MFR Ａ is more preferably 1.05 or more and 6.0 or less, even more preferably 1.1 or more and 5.0 or less, still more preferably 1.1 or more and 4.0 or less, and particularly preferably 1.1 or more and 3.0 or less.

[0083] Weight average molecular weight Mw of polypropylene of microporous layer (A) Ａ and the weight average molecular weight Mw of the polypropylene of the microporous layer (B). Ｂ The ratio of Mw Ａ / Mw Ｂ is preferably 1.02 or more and 2.0 or less. Ａ / Mw Ｂ By making the melt tension ratio Mt of the microporous layer (A) to the microporous layer (B) 1.02 or more, Ａ / Mt Ｂ As a result, it becomes possible to effectively suppress dendrite formation in the electricity storage device. Ａ / Mt Ｂ By making the Mw 2.0 or less, it is possible to obtain a separator having stable film-forming properties, productivity, and good pore opening properties and air permeability. Ａ / Mw Ｂ is preferably 1.02 or more and 1.8 or less, more preferably 1.03 or more and 1.6 or less, and most preferably 1.05 or more and 1.4 or less.

[0084] <Layer Structure of Separator Substrate> The substrate of a separator for an electricity storage device (also simply referred to as "separator substrate" in the present disclosure) may be any microporous membrane used in a separator for an electricity storage device. It may have a single-layer structure containing only a microporous layer (A), or a multilayer structure containing at least one microporous layer (A). Alternatively, it may have a multilayer structure containing at least one microporous layer (A) and at least one microporous layer (B). Furthermore, the separator substrate may have a multilayer structure of three or more layers containing two or more layers of at least one microporous layer (A) and / or microporous layer (B). Examples of such a 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). The separator substrate may also have layers other than the microporous layer (A) and the microporous layer (B). For example, examples of layers other than the microporous layer (A) and the microporous layer (B) include a microporous layer mainly composed of a polyolefin other than (A) and (B), a layer containing an inorganic substance, and a layer containing a heat-resistant resin. The separator substrate may also have a multilayer structure of four or more layers, such as microporous layer (A) / microporous layer (B) / microporous layer (C) / microporous layer (A). From the viewpoints of ease of production and suppression of curling of the separator, a symmetrical laminate structure is preferred.

[0085] <Stem Height of Separator Substrate> The stem height in the MD-ND cross section of the separator substrate is 500 nm or more and 1000 nm or less. The stem height is correlated with the tortuosity of the pores and is a value that indicates the pore structure different from the pore diameter. As the stem height increases, the tortuosity of the pores decreases, and as the stem height decreases, the tortuosity of the pores increases, resulting in good voltage resistance. Furthermore, increased tortuosity forms a dense network, promoting stress dispersion and resulting in good pin puncture strength. From the viewpoint of pin puncture strength, the upper limit of the stem height is preferably 950 nm or less, more preferably 920 nm or less, even more preferably 900 nm or less, even more preferably 850 nm or less, particularly preferably 820 nm or less, extremely preferably 800 nm or less, and most preferably 750 nm or less. From the viewpoint of obtaining good air permeability, the lower limit of the trunk height is preferably 520 nm or more, more preferably 540 nm or more, even more preferably 560 nm or more, still more preferably 580 nm or more, and particularly preferably 600 nm or more.

[0086] The stem height can be calculated by performing cross-sectional SEM observation of the MD-ND cross section of the separator substrate, removing fibrils by image analysis, and measuring the ND length of the remaining lamellae by image analysis. Detailed conditions are shown in the Examples. When measuring the stem height from a cross-sectional SEM image, the number-average stem height and the length-average stem height can be calculated. However, in this specification, the length-average stem height is used as the stem height to better correlate with the physical properties of the separator.

[0087] <Thickness of Separator Substrate> From the viewpoint of increasing the energy density and increasing the input / output of the power storage device, the upper limit of the thickness of the separator substrate is preferably 20 μm or less, and may be, for example, 18 μm or less, 16 μm or less, 14 μm or less, 13 μm or less, 12 μm or less, 11.5 μm or less, 11 μm or less, 10.5 μm or less, or 10 μm or less. From the viewpoint of obtaining a microporous layer (A) or separator substrate with high pin puncture strength, the lower limit of the thickness of the separator substrate is preferably 3 μm or more, and may be, for example, 4 μm or more, 5 μm or more, 6 μm or more, 7 μm or more, 7.5 μm or more, 8 μm or more, 8.5 μm or more, or 9 μm or more.

[0088] <Air Permeability (Air Permeability Resistance) of Separator Substrate> The upper limit of the air permeability of the separator substrate according to the first embodiment of the present disclosure is preferably 400 sec / 100 cm from the viewpoint of ensuring good output in the power storage device. ３ Less than 350 seconds / 100 cm, more preferably ３ More preferably, 300 seconds / 100 cm or less ３ More preferably, 250 seconds / 100 cm or less ３ Particularly preferably 220 seconds / 100 cm or less ３ Below 200 seconds / 100 cm, most preferably ３ The lower limit of the air permeability of the separator substrate is not limited, but is, for example, 10 seconds / 100 cm ３ Above, 20 seconds / 100cm ３ or more, or 30 seconds / 100 cm ３ It may be more than that.

[0089] <Porosity of Separator Substrate> The porosity of the separator substrate in the first embodiment of the present disclosure is preferably 30% or more from the viewpoint of avoiding clogging in the power storage device and obtaining good air permeability of the separator, and is preferably 60% or less from the viewpoint of maintaining the separator's pin puncture strength. The lower limit of the porosity of the separator substrate may be, for example, 35% or more, 40% or more, 43% or more, 45% or more, or 47% or more. The upper limit of the porosity of the separator substrate may be, for example, 57% or less, 54% or less, or 53% or less.

[0090] <Puncture Strength of Separator Substrate> The lower limit of the puncture strength of the separator substrate according to the first embodiment of the present disclosure is, when converted to a thickness of 10 μm, preferably 150 gf or more (approximately 1.47 N or more), more preferably 180 gf or more, 200 gf or more, 220 gf or more, or 240 gf or more, and particularly preferably 260 gf or more, 270 gf or more, or 280 gf or more. The upper limit of the puncture strength of the separator substrate is not limited, but may be preferably 500 gf or less, for example 450 gf or less, or 400 gf or less, when converted to a thickness of 10 μm.

[0091] <Withstand voltage of separator substrate> From the viewpoint of suppressing short circuits in the electricity storage device, the withstand voltage of the separator substrate, when converted to a thickness of 10 μm of the separator substrate, is preferably 0.80 kV or more, more preferably 0.90 kV or more, even more preferably 0.95 kV or more, particularly preferably 1.00 kV or more, and most preferably 1.05 kV or more.

[0092] <Heat Shrinkage of Separator Substrate> The separator substrate preferably has a transverse direction (TD) heat shrinkage of 5% or less after heat treatment at 105°C for 1 hour, and more preferably a value between -1.0% and 3.0%. That is, a separator substrate with a TD heat shrinkage of 5% or less at 105°C exhibits very little TD heat shrinkage even at high temperatures. Having a heat shrinkage of 5% or less or 3.0% or less can effectively prevent short circuits at high temperatures. The reason for the heat shrinkage of -1.0% or more is that during measurement of the heat shrinkage, the substrate may expand in the TD, resulting in a negative value below 0%. The heat shrinkage may be 0% or more, or greater than 0%. A method for producing a separator substrate with a heat shrinkage of 5% or less or between -1.0% and 3.0% can be, for example, a separator production method using uniaxial MD stretching, preferably a dry uniaxial stretching method. In a separator manufacturing method using biaxial stretching in MD and TD, as typified by a wet separator, the thermal shrinkage in TD is generally very large, whereas in a uniaxially stretched dry separator, it is easy to obtain a separator substrate having a thermal shrinkage rate of 5% or less, or -1.0% or more and 3.0% or less.

[0093] The separator substrate has a heat shrinkage rate in the film formation direction (MD) after heat treatment at 105°C for 1 hour, from the viewpoints of productivity of the electricity storage device and suppression of short circuits at high temperatures, of preferably 20% or less, more preferably 15% or less, even more preferably 10% or less, even more preferably 8% or less, particularly preferably 6% or less, and most preferably 5.5% or less. The lower limit of the heat shrinkage rate is not limited, but may be preferably 0.1% or more, for example, 0.3% or more, or 0.5% or more. As described above, when the separator substrate of the present disclosure is produced by uniaxial stretching in the MD and the microporous layer (A) has connecting domains arranged in parallel in the MD, it is believed that stress applied to the polymer matrix can be alleviated even during heat shrinkage in the MD, and a low MD heat shrinkage rate can be achieved.

[0094] <Tensile Strength of Separator Substrate> The separator substrate of the first embodiment of the present disclosure preferably has a tensile strength in the MD of 1500 kgf / cm from the viewpoint of operability during battery winding and high puncture strength. ２ or more (approximately 14.7 kN / cm ２ or more), more preferably 1600 kgf / cm ２ More preferably, 1700 kgf / cm ２ More preferably, 1800 kgf / cm ２ More than 1900 kgf / cm, most preferably ２ The upper limit of the tensile strength in the MD of the separator substrate is not limited, but is preferably 4000 kgf / cm ２ For example, 3800 kgf / cm ２ Below, 3500kgf / cm ２ Below, 3200kgf / cm ２ or less, or 3000 kgf / cm ２ It may be the following:

[0095] <Tensile elongation of separator substrate> From the viewpoints of productivity and high pin puncture strength of the power storage device, the MD tensile elongation of the separator substrate is preferably 20% or more, more preferably 24% or more, even more preferably 26.5% or more, still more preferably 28% or more, and particularly preferably 30% or more. From the viewpoint of processability, the upper limit of the MD tensile elongation of the separator substrate is preferably 60% or less, more preferably 55% or less, even more preferably 50% or less, still more preferably 45% or less, and particularly preferably 40% or less. In the separator substrate of the present disclosure, as described above, when the separator substrate is produced by uniaxial stretching in the MD and the microporous layer (A) has connecting domains parallel to the MD, it is possible to alleviate the stress applied to the polymer matrix even during tension in the MD, and it is thought that a high MD tensile elongation can be achieved.

[0096] Second embodiment

[0097] <<Separator for Electricity Storage Device>> The separator for an electricity storage device according to a second embodiment of the present disclosure has a microporous layer (A) containing polypropylene as a separator substrate. If desired, the separator substrate may include a microporous layer (B) containing a polyolefin as a main component, in addition to the microporous layer (A). The separator substrate may further include a coating layer (also referred to as a "surface layer," "coating layer," etc.; hereinafter simply referred to as a "coating layer") on the microporous layer (A) and / or the microporous layer (B). In the present disclosure, the term "microporous layer" refers to each microporous layer constituting the separator substrate, the term "separator substrate" refers to the separator substrate excluding any coating layers, and the term "separator" refers to the entire separator, including any coating layers.

[0098] <Microporous layer (A)> The separator for an electricity storage device according to the second embodiment of the present disclosure has a microporous layer (A). The separator for an electricity storage device may have only one microporous layer (A), or two or more microporous layers (A). The microporous layer (A) contains polypropylene. In the present disclosure, the microporous layer (A) may be in the form of a membrane, and the integrated elution amount of the microporous membrane (A) at 100°C to 130°C, as measured by cross fractionation chromatography (CFC), is 80.0% to 99.5% by mass of the total elution amount, and the integrated elution amount at 20°C to less than 100°C is 0.5% to 20.0% by mass of the total elution amount.

[0099] <Material of Microporous Layer (A)> The microporous layer (A) of the second embodiment of the present disclosure contains polypropylene, which allows the battery to maintain good battery performance even after storage at high temperatures (e.g., 130°C). From the viewpoints of separator wettability, thinning, and shutdown characteristics, the lower limit of the polypropylene content in the microporous layer (A) is preferably 50% by mass or more, more preferably 60% by mass or more, 70% by mass or more, 80% by mass or more, 85% by mass or more, 90% by mass or more, 92.5% by mass or more, or 95% by mass or more, based on the total mass of the microporous layer (A). From the viewpoint of maintaining good pin puncture strength and air permeability, the upper limit of the polypropylene content in the microporous layer (A) is, for example, 97.5% by mass or less, 98% by mass or less, 98.5% by mass or less, 99% by mass or less, or 99.5% by mass or less, based on the total mass of the microporous layer (A).

[0100] The microporous layer (A) of the second embodiment of the present disclosure contains polypropylene, but the polypropylene described in the first embodiment may be used.

[0101] The microporous layer (A) of the second embodiment of the present disclosure contains polypropylene but may also contain a thermoplastic elastomer. Examples of thermoplastic elastomers include polyolefins other than the main component polypropylene, polyolefin copolymers, and copolymers of polystyrene and polyolefins. Polyolefins are polymers containing a monomer having a carbon-carbon double bond as a repeating unit. Examples of monomers constituting polyolefins include, but are not limited to, monomers having 2 to 10 carbon atoms (C2 to C10) and a carbon-carbon double bond, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Examples of polyolefins include low-crystalline polypropylenes having low stereoregularity regions. Monomers constituting polyolefin copolymers may be used alone or in combination of two or more. Examples of copolymers of polystyrene and polyolefin include styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer (SEBS), styrene-ethylene-styrene copolymer, styrene-(ethylene-butene)-olefin copolymer (SEBC), and olefin-(ethylene-butene)-styrene copolymer (CEBS), which may be hydrogenated polymers. These copolymers may be random copolymers or block copolymers, and are preferably block copolymers.

[0102] When the microporous layer (A) of the second embodiment of the present disclosure contains a thermoplastic elastomer, the thermoplastic elastomer preferably has a portion that is incompatible with polypropylene, from the viewpoint of obtaining a microporous layer (A) that combines low air permeability and high pin puncture strength by forming connecting domains in a polymer matrix and / or fibrils mainly composed of polypropylene, as described below. Furthermore, in addition to having a portion that is incompatible with polypropylene, the thermoplastic elastomer preferably also has a portion that is compatible with polypropylene, from the viewpoint of enhancing the dispersibility and connection between the polypropylene and the thermoplastic elastomer. From these viewpoints, the thermoplastic elastomer is preferably a copolymer containing, as a repeating unit, one or more repeating units selected from the group consisting of ethylene, propylene, and 1-butene. Among these, ethylene / propylene (C2C3) copolymer, ethylene / 1-butene (C2C4) copolymer, ethylene / 1-hexene (C2C6) copolymer, ethylene / 1-octene (C2C8) copolymer, styrene-(ethylene-butene)-styrene copolymer (SEBS), olefin-(ethylene-butene)-olefin copolymer (CEBC), and olefin-(ethylene-butene)-styrene copolymer (CEBS) are more preferred. Here, the ethylene-butene structure is structurally similar to propylene and therefore exhibits high affinity with polypropylene. The thermoplastic elastomers can be used alone or in combination of two or more.

[0103] When the microporous layer (A) of the second embodiment of the present disclosure contains a thermoplastic elastomer, the lower limit of the content of the thermoplastic elastomer in the microporous layer (A) is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, even more preferably 2.0% by mass or more, and even more preferably 3.0% by mass or more, based on the total mass of the microporous layer (A), from the viewpoints of film-formability, thinning, low air permeability, and high pin puncture strength. The upper limit of the content of the thermoplastic elastomer in the microporous layer (A) is preferably 20.0% by mass or less, more preferably 15.0% by mass or less, even more preferably 10.0% by mass or less, even more preferably 7.5% by mass or less, and particularly preferably 5.0% by mass or less, based on the total mass of the microporous layer (A), from the viewpoints of maintaining pore openness.

[0104] The microporous layer (A) of the second embodiment of the present disclosure contains polypropylene, but may contain a polyolefin other than polypropylene as the main component. Polyolefins are polymers containing, as repeating units, monomers having a carbon-carbon double bond. Monomers constituting polyolefins other than polypropylene include, but are not limited to, monomers having 2 or 4 to 10 carbon atoms and a carbon-carbon double bond, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Examples of polyolefins include homopolymers, copolymers, and multi-stage polymers. Among these, polyethylene is preferred from the viewpoint of obtaining a microporous layer (A) that combines low air permeability and high pin puncture strength by forming connecting domains in a polymer matrix and / or fibrils mainly composed of polypropylene, as described below, when added together with a thermoplastic elastomer.

[0105] When the microporous layer (A) of the second embodiment of the present disclosure contains polyethylene, the lower limit of the polyethylene content, based on the total mass of the microporous layer (A), is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, even more preferably 2.0% by mass or more, and even more preferably 3.0% by mass or more, from the viewpoint of achieving both high pin puncture strength and low air permeability. The upper limit of the polyethylene content, based on the total mass of the microporous layer (A), is preferably 20.0% by mass or less, more preferably 15.0% by mass or less, even more preferably 12.5% ​​by mass or less, even more preferably 10.0% by mass or less, and particularly preferably 7.5% by mass or less, from the viewpoint of maintaining pore openness.

[0106] When the microporous layer (A) of the second embodiment of the present disclosure contains polyethylene, the polyethylene described in the first embodiment may be used as the polyethylene.

[0107] <Measured values ​​of microporous layer (A) by cross fractionation chromatography (CFC)> The integrated elution amount at 100°C or higher and 130°C or lower of the microporous membrane (A) of the second embodiment of the present disclosure, measured by cross fractionation chromatography (CFC), is from 80.0% to 99.5% by mass of the total elution amount, and the integrated elution amount at 20°C or higher and lower than 100°C is from 0.5% to 20.0% by mass of the total elution amount.

[0108] Here, "cross fractionation chromatography (CFC)" is a measurement technique that combines a temperature rising elution fractionation (TREF) section that performs crystallinity fractionation with a GPC section that performs molecular weight fractionation, and by directly connecting the TREF section and the GPC section, it is possible to analyze the correlation between the composition distribution and molecular weight distribution of components with different crystallinity. Note that measurement in the TREF section may be referred to as measurement by CFC.

[0109] Measurements using CFC are performed as follows: The sample to be measured is dissolved in an eluent. Thereafter, when cooled at a constant temperature, highly crystalline components crystallize first, followed by less crystalline components as the temperature decreases. Next, when the temperature is increased in stages, components with low to high crystallinity are eluted, and the concentrations of the eluted components at each temperature are detected. Then, an elution temperature-elution volume curve is measured using the elution temperature (°C) of the sample and the elution volume (mass%) at that time, with the elution temperature on the vertical axis and the elution volume on the horizontal axis, and the elution volume and integrated elution volume at each temperature can be obtained. At the same time, the molecular weight at each elution temperature is measured in the GPC section, and the weight average molecular weight (Mw, described below) at each temperature is calculated. Ｈ or Mw Ｌ 5 is a graph showing the elution temperature-elution amount curve in the CFC measurement.

[0110] The elution components of the microporous membrane (A) at 100°C to 130°C as measured by CFC preferably include at least polypropylene, and may also include polyethylene. From the viewpoint of high pin puncture strength and good shutdown properties, the lower limit of the integrated elution amount of the microporous membrane (A) at 100°C to 130°C as measured by CFC is preferably 85% by mass or more of the total elution amount, more preferably 90% by mass or more, even more preferably 92.5% by mass or more, and even more preferably 95% by mass or more. From the viewpoint of maintaining good pin puncture strength and air permeability, the upper limit of the integrated elution amount of the microporous membrane (A) at 100°C to 130°C as measured by CFC is, for example, preferably 99% by mass or less of the total elution amount, more preferably 98.5% by mass or less, even more preferably 98% by mass or less, and even more preferably 97.5% by mass or less.

[0111] The elution components of the microporous membrane (A) at 20°C or higher but lower than 100°C as measured by CFC are not limited, but preferably include a thermoplastic elastomer and / or polyethylene, and may also include a low-crystalline component of a polyolefin. The lower limit of the integrated elution amount of the microporous membrane (A) at 20°C or higher but lower than 100°C as measured by CFC is preferably 1.0% by mass or higher of the total elution amount, more preferably 2.0% by mass or higher, even more preferably 2.5% by mass or higher, and even more preferably 3.0% by mass or higher, from the viewpoints of film-formability, thinning, low air permeability, and high pin puncture strength. The upper limit of the integrated elution amount of the microporous membrane (A) at 20°C or higher but lower than 100°C as measured by CFC is preferably 15.0% by mass or lower of the total elution amount, more preferably 12.5% ​​by mass or lower, even more preferably 10.0% by mass or lower, even more preferably 7.5% by mass or lower, and particularly preferably 5.0% by mass or lower, from the viewpoint of maintaining pore openability.

[0112] In the elution temperature-elution amount curve of the microporous membrane (A) of the present disclosure measured by cross fractionation chromatography (CFC), it is preferable that there are at least two peaks, a high-temperature peak and a low-temperature peak. As described above, the elution temperature is an indicator of the crystallinity of the contained components, with higher temperatures indicating higher crystallinity and lower temperatures indicating lower crystallinity. The high-temperature peak preferably contains at least polypropylene. From the viewpoint of obtaining a microporous layer (A) with high pin puncture strength, high TD tensile strength, and high heat resistance, the lower limit of the peak temperature showing the maximum elution amount in the high-temperature peak is preferably 105°C or higher, more preferably 108°C or higher, even more preferably 111°C or higher, even more preferably 113°C or higher, and particularly preferably 115°C or higher. The upper limit of the peak temperature showing the maximum elution amount in the higher temperature peak is preferably 125°C or lower, more preferably 124°C or lower, even more preferably 123°C or lower, even more preferably 122.5°C or lower, and particularly preferably 122°C or lower, from the viewpoint of ensuring good film-formability, productivity, thin film thickness, and low air permeability. On the other hand, the lower temperature peak is preferably, but not limited to, a thermoplastic elastomer and / or polyethylene. The lower limit of the peak temperature showing the maximum elution amount in the lower temperature peak is preferably 35°C or higher, more preferably 37°C or higher, even more preferably 39°C or higher, even more preferably 41°C or higher, and particularly preferably 43°C or higher, from the viewpoint of obtaining a microporous layer (A) having high pin puncture strength and good film-formability stability. The upper limit of the peak temperature showing the maximum elution amount in the low-temperature peak is preferably 100°C or lower, more preferably 90°C or lower, even more preferably 80°C or lower, still more preferably 70°C or lower, particularly preferably 60°C or lower, and most preferably 50°C or lower, from the viewpoint of obtaining a microporous layer (A) with low air permeability, film-formability, and thinness.

[0113] In the elution temperature-elution amount curve of the microporous membrane (A) of the present disclosure measured by cross fractionation chromatography (CFC), the weight average molecular weight (Mw ＨThe lower limit of the weight average molecular weight (Mw) at the temperature of the high-temperature peak is preferably 300,000 or more, more preferably 500,000 or more, even more preferably 650,000 or more, even more preferably 750,000 or more, and particularly preferably 800,000 or more, from the viewpoint of obtaining a microporous layer (A) having high pin puncture strength and high TD tensile strength. Ｈ The upper limit of the weight average molecular weight (Mw) at the temperature of the low-temperature peak is preferably 1,300,000 or less, more preferably 1,200,000 or less, even more preferably 1,100,000 or less, still more preferably 1,050,000 or less, and particularly preferably 1,000,000 or less, from the viewpoint of obtaining a microporous layer (A) with good film formation stability, thinning, and low air permeability. Ｌ The lower limit of the weight average molecular weight (Mw) at the temperature of the low-temperature peak is preferably 50,000 or more, more preferably 80,000 or more, even more preferably 100,000 or more, still more preferably 125,000 or more, and particularly preferably 150,000 or more, from the viewpoint of obtaining a microporous layer (A) with high pin puncture strength. Ｌ From the viewpoint of obtaining a microporous layer (A) with good film-forming properties and low air permeability, the upper limit of the weight average molecular weight (Mw) of the microporous membrane (A) at each elution temperature measured by CFC of the present disclosure is preferably 1.8 million or less, more preferably 1.4 million or less, even more preferably 1 million or less, even more preferably 750,000 or less, particularly preferably 500,000 or less, and most preferably 250,000 or less. Ｈ and Mw Ｌ ) is the molecular weight in polystyrene equivalent.

[0114] <Melt flow rate (MFR) of microporous layer (A)> The preferred melt flow rates (MFR) of the microporous layer (A) of the second embodiment of the present disclosure, the MFR of the polypropylene, the MFR of the thermoplastic elastomer, and the MFR of the polyethylene are as described for the first embodiment.

[0115] <Mw and Mw / Mn of Microporous Layer (A)> The preferred weight-average molecular weight (Mw) and the value obtained by dividing the weight-average molecular weight (Mw) by the number-average molecular weight (Mn) (Mw / Mn) of the microporous layer (A) in the second embodiment of the present disclosure are as described for the first embodiment.

[0116] <Melt tension of microporous layer (A)> The preferred melt tension Mt at 240°C of the microporous layer (A) of the second embodiment of the present disclosure Ａ (Single layer Mt Ａ ) are as described for the first embodiment.

[0117] <Morphology of Microporous Layer (A)> The preferred morphology of the microporous layer (A) in the second embodiment of the present disclosure is as described above in the first embodiment.

[0118] <Area Average Pore Diameter of Microporous Layer (A)> The preferred area average pore diameter of the microporous layer (A) of the second embodiment of the present disclosure is as described for the first embodiment.

[0119] <Porosity of microporous layer (A)> The porosity of the microporous layer (A) in the second embodiment of the present disclosure is preferably 30% or more from the viewpoint of avoiding clogging in an electricity storage device and obtaining a microporous layer (A) with low air permeability, and is preferably 45% or less from the viewpoint of obtaining a microporous layer (A) with high pin puncture strength, high TD strength, and tear resistance. The porosity of the microporous layer (A) is more preferably 31% or more and 42% or less, even more preferably 32% or more and 40% or less, and particularly preferably 33% or more and 38% or less.

[0120] <Thickness of Microporous Layer (A)> The preferred thickness of the microporous layer (A) in the second embodiment of the present disclosure is as described for the first embodiment.

[0121] <Additives for Microporous Layer (A)> The microporous layer (A) of the second embodiment of the present disclosure containing polypropylene may further contain, in addition to polypropylene, additives such as an elastomer, a crystal nucleating agent, an antioxidant, a filler, etc. The amount of the additive is not particularly limited, and may be, for example, 0.01% by mass or more, 0.1% by mass or more, or 1% by mass or more, and may be, for example, 20% by mass or less, 10% by mass or less, or 7% by mass or less, based on the total mass of the microporous layer (A).

[0122] <Microporous Layer (B)> The separator for an electricity storage device according to the second embodiment of the present disclosure optionally includes a microporous layer (B). The separator for an electricity storage device may include only one microporous layer (B), or two or more microporous layers (B). The microporous layer (B) is preferably primarily composed of polyolefin, more preferably polypropylene and / or polyethylene. This allows for good pore openability and good battery performance. The microporous layer (B) is even more preferably primarily composed of polypropylene, which allows for good battery performance to be maintained even after storage at high temperatures (e.g., 130°C). In the present disclosure, the expression "primarily composed of" polypropylene means that the microporous layer (B) contains 50% by mass or more of polypropylene, based on the total mass of the microporous layer (B). The lower limit of the polypropylene content in the microporous layer (B) may be preferably 55% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more from the viewpoints of the wettability of the separator, thinning of the film, etc. The upper limit of the polypropylene content in the microporous layer (B) is not limited, but may be, for example, 98% by mass or less, 99% by mass or less, or 100% by mass.

[0123] The preferred materials, MFR, pentad fraction, melt tension, area average pore diameter, porosity, thickness, and additives of the microporous layer (B) of the second embodiment of the present disclosure are the same as those described for the first embodiment.

[0124] The preferred relationship between microporous layer (A) and microporous layer (B), the layer structure of the separator substrate, the stem height of the separator substrate, and the thickness of the separator substrate in the second embodiment of the present disclosure are as described for the first embodiment.

[0125] <Air Permeability (Air Permeability Resistance) of Separator Substrate> The upper limit of the air permeability of the separator substrate according to the second embodiment of the present disclosure is preferably 400 sec / 100 cm from the viewpoint of ensuring good output in the power storage device. ３ Less than 350 seconds / 100 cm, more preferably ３ More preferably, 300 seconds / 100 cm or less ３ More preferably, 280 seconds / 100 cm or less ３or less than 250 seconds / 100 cm ３ Particularly preferably 220 seconds / 100 cm or less ３ Below 200 seconds / 100 cm, most preferably ３ The lower limit of the air permeability of the separator substrate is not limited, but is, for example, 10 seconds / 100 cm ３ Above, 20 seconds / 100cm ３ or more, or 30 seconds / 100 cm ３ It may be more than that.

[0126] <Porosity of Separator Substrate> The porosity of the separator substrate of the second embodiment of the present disclosure is 30.0% or more and 45.0% or less. Without being limited by theory, for example, when manufactured by uniaxial stretching in the film formation direction (MD), the morphology is as shown in Figure 1 . However, because it is oriented in the MD, the separator is prone to tearing in the transverse direction (TD). When the porosity of the separator substrate is low, the separator has fewer and / or smaller pores and a larger basis weight (weight per unit area), indicating relatively fewer fibrils and a larger polymer matrix. It is presumed that, because there are many tie molecules that can be extracted from the polymer matrix when stretched in the TD, a high TD tensile strength is obtained, resulting in a separator that is less prone to tearing. The lower limit of the porosity of the separator substrate is preferably 31% or more, more preferably 32% or more, even more preferably 33% or more, even more preferably 34% or more, and particularly preferably 35% or more, from the viewpoint of avoiding clogging in the electricity storage device and obtaining good separator air permeability. The upper limit of the porosity of the separator substrate is preferably 42% or less, more preferably 41% or less, even more preferably 40% or less, even more preferably 39% or less, and particularly preferably 38% or less, from the viewpoint of obtaining a microporous layer (A) that has high pin puncture strength and high TD strength and is tear-resistant.

[0127] <Puncture Strength of Separator Substrate> The lower limit of the puncture strength of the separator substrate according to the second embodiment of the present disclosure is preferably 150 gf or more (approximately 1.47 N or more), more preferably 180 gf or more, 200 gf or more, 220 gf or more, or 240 gf or more, and particularly preferably 250 gf or more, 260 gf or more, 270 gf or more, or 280 gf or more, when converted to a thickness of 10 μm. The upper limit of the puncture strength of the separator substrate is not limited, but may be preferably 500 gf or less, for example 450 gf or less, or 400 gf or less, when converted to a thickness of 10 μm.

[0128] <Thermal shrinkage rate of separator substrate> The preferred thermal shrinkage rate of the separator substrate of the second embodiment of the present disclosure is as described for the first embodiment.

[0129] <Tensile Strength and Tensile Elongation of Separator Substrate> The separator substrate of the second embodiment of the present disclosure preferably has a tensile strength in the MD of 1500 kgf / cm from the viewpoint of operability during battery winding and high puncture strength. ２ or more (approximately 14.7 kN / cm ２ or more), more preferably 1600 kgf / cm ２ More preferably, 1700 kgf / cm ２ More preferably, 1800 kgf / cm ２ More than 1900 kgf / cm, most preferably ２ The upper limit of the tensile strength in the MD of the separator substrate is not limited, but is preferably 4000 kgf / cm ２ For example, 3800 kgf / cm ２ Below, 3500kgf / cm ２ Below, 3200kgf / cm ２ or less, or 3000 kgf / cm ２ It may be the following:

[0130] The separator substrate according to the second embodiment of the present disclosure preferably has a TD tensile strength of 100 kgf / cm from the viewpoint of ensuring productivity and yield of the electricity storage device due to the resistance of the separator to tearing. ２ More preferably, 120 kgf / cm ２More preferably, 140 kgf / cm ２ More preferably, 160 kgf / cm ２ More preferably, 180 kgf / cm ２ More than 200 kgf / cm, most preferably ２ The upper limit of the TD tensile strength of the separator substrate is not limited, but is preferably 4000 kgf / cm ２ For example, 3000 kgf / cm ２ Below, 2000kgf / cm ２ Below, 1000kgf / cm ２ Below, 500kgf / cm ２ or less, or 300 kgf / cm ２ It may be the following:

[0131] The separator substrate of the second embodiment of the present disclosure has an MD tensile elongation of preferably 20% or more, more preferably 24% or more, even more preferably 26.5% or more, still more preferably 28% or more, and particularly preferably 30% or more, from the viewpoints of productivity and high pin puncture strength of the electricity storage device. The upper limit of the MD tensile elongation of the separator substrate is preferably 60% or less, more preferably 55% or less, even more preferably 50% or less, still more preferably 45% or less, and particularly preferably 40% or less, from the viewpoint of processability. As described above, when the separator substrate of the present disclosure is produced by uniaxial stretching in the MD and the microporous layer (A) has connecting domains parallel to the MD, it is believed that stress applied to the polymer matrix can be relaxed even during tension in the MD, and high MD tensile elongation can be achieved.

[0132] <<Method for manufacturing separator for power storage device>> The method for manufacturing the separator for power storage device according to the first and second embodiments includes a melt-extrusion step of melt-extruding a resin composition containing polypropylene as a main component (hereinafter also referred to as a "polypropylene-based resin composition") to obtain a resin sheet (precursor sheet), and a hole-forming step of opening holes in the obtained precursor sheet to make it porous. Methods for manufacturing the microporous layer are broadly divided into dry methods in which no solvent is used in the hole-forming step, and wet methods in which a solvent is used.

[0133] Examples of dry methods include a method in which a polypropylene-based resin composition is melt-kneaded and extruded, and then the polypropylene crystal interface is peeled off by heat treatment and stretching; and a method in which a polypropylene-based resin composition and an inorganic filler are melt-kneaded and formed into a film, and then the interface between the polypropylene and the inorganic filler is peeled off by stretching.

[0134] Examples of the wet method include a method in which a polypropylene resin composition and a pore-forming material are melt-kneaded to form a film, which is stretched as necessary, and then the pore-forming material is extracted; and a method in which a polypropylene resin composition is dissolved, and then the film is immersed in a poor solvent for polypropylene to solidify the polypropylene and simultaneously remove the solvent.

[0135] A single-screw extruder or a twin-screw extruder can be used for melt-kneading the polypropylene-based resin composition. In addition to these, for example, a kneader, a Laboplastomill, a kneading roll, a Banbury mixer, etc. can also be used.

[0136] The polypropylene resin composition may optionally contain resins other than polypropylene, additives, etc., depending on the method for producing the microporous layer or the desired physical properties of the microporous layer. Examples of additives include pore-forming materials, fluorine-based flow modifiers, waxes, crystal nucleating materials, antioxidants, metal soaps such as metal salts of aliphatic carboxylic acids, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments. Examples of pore-forming materials include plasticizers, inorganic fillers, and combinations thereof.

[0137] Examples of the plasticizer include hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; and higher alcohols such as oleyl alcohol and stearyl alcohol.

[0138] Examples of inorganic fillers include oxide ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fiber.

[0139] The method for producing the separator substrate according to the first and second embodiments is preferably a dry lamellar crystal opening process in which the polypropylene crystal interface is peeled off by heat treatment and stretching. Here, the method for producing a separator substrate having a microporous layer (A) and a microporous layer (B) is preferably at least one of the following methods (i) and (ii): (i) a method for producing a separator substrate by coextrusion, in which the microporous layer (A) and the microporous layer (B) are coextruded and then subjected to annealing, cold stretching, hot stretching, and heat-relaxing; and (ii) a method for producing a separator substrate by lamination, in which the microporous layer (A) and the microporous layer (B) are separately extruded and then laminated together, and then subjected to annealing, cold stretching, hot stretching, and heat-relaxing.

[0140] Of the co-extrusion process (i) and lamination process (ii), the co-extrusion process (i) is preferred from the viewpoint of production costs, etc. In the co-extrusion process (i), the extrusion film formation conditions for the microporous layers (A) and (B) are preferably such that the resin is discharged at as low a temperature as possible and effectively quenched by blowing low-temperature air. After film formation, it is preferable to quench with air, and the temperature of the blown air is preferably 20°C or less, more preferably 15°C or less. By blowing cold air controlled at such a low temperature, the resin after film formation is uniformly oriented in the MD.

[0141] Manufacturing Method According to the First Embodiment In both the co-extrusion process (i) and the lamination process (ii), the manufacturing method of the separator substrate according to the first embodiment may include an annealing step after extrusion film formation. The annealing step tends to grow the crystal structure of the microporous layers (A) and (B), improving pore openness. Annealing at a specific temperature for a predetermined time tends to enable both the microporous layers (A) and (B) to have a good area-average long pore diameter, high porosity, low air permeability, and high pin puncture strength. This is thought to be because the crystals grow without disrupting the crystal structure, resulting in high pore openness. In the annealing step, the lower limit of the temperature range is preferably 115°C or higher, more preferably 120°C or higher, even more preferably 125°C or higher, still more preferably 130°C or higher, and particularly preferably 135°C or higher, and the upper limit of the temperature range is preferably 160°C or lower, more preferably 155°C or lower, even more preferably 150°C or lower, and the annealing treatment is preferably carried out for 20 minutes or longer, more preferably 60 minutes or longer. This results in highly oriented crystals of the polypropylene, which is the main component, and results in high pore opening during the subsequent stretching step, good area average long pore diameter, high porosity, low air permeability, and high pin puncture strength, which is preferable from the viewpoint of realizing high input / output and high energy density of the electricity storage device.

[0142] The method for producing the separator substrate of the first embodiment may include a stretching step after the annealing step. Either uniaxial stretching or biaxial stretching can be used as the stretching treatment. Although not limited thereto, uniaxial stretching is preferred from the viewpoints of production costs when using a dry method and reducing thermal shrinkage in the TD. The MD stretch ratio of cold stretching ((dimension after stretching - dimension before stretching) / dimension before stretching x 100 (%)) is preferably in the range of 5% to 50%, more preferably 20% to 45%, and even more preferably 25% to 45%. If it is above the lower limit, the amount of cracking during cold stretching increases, making it easier to obtain small pore diameters during hot stretching, and a moderately good stem height can be obtained, rather than being excessively large. Furthermore, the small pore diameters allow domains (resulting in brightly dyed areas) as shown in Figures 1 and 2 to be connected, increasing the MD length. Due to stress relaxation of the connected domains at the time of fracture, fracture is less likely to progress, enabling elongation of the polyolefin, which is the main component. As a result, a highly oriented structure of lamellae is achieved in the high strain range, resulting in a separator with high pin puncture strength. On the other hand, if it is below the upper limit, the probability of membrane rupture during stretching due to excessive cold stretching is reduced, ensuring separator productivity and yield. The temperature of the cold stretching is preferably 10° C. or higher and 50° C. or lower, more preferably 20° C. or higher and 30° C. or lower, and from the viewpoint of production costs, the cold stretching may be performed at room temperature (23±2° C.) From the viewpoints of improving both the pin puncture strength and low air permeability of the resulting separator substrate, production costs, and reduction of TD thermal shrinkage, uniaxial stretching is preferred.

[0143] In order to suppress thermal shrinkage of the separator substrate, a heat treatment step may be performed for the purpose of heat setting after the stretching step or the hole-forming step. The heat treatment step may include a hot stretching operation performed at a predetermined temperature and a predetermined stretch ratio for the purpose of adjusting physical properties, and / or a heat-relaxing operation performed at a predetermined temperature and a predetermined relaxation ratio for the purpose of reducing shrinkage stress imparted during film formation and stretching. The heat-relaxing operation may be performed after the hot stretching operation.

[0144] In the hot stretching of the first embodiment, the lower limit of the stretch ratio is preferably 140% or more, more preferably 160% or more, even more preferably 170% or more, even more preferably 180% or more, particularly preferably 190% or more, and most preferably 200% or more, based on the MD dimension before stretching (100%). The upper limit of the stretch ratio is preferably 280% or less, more preferably 260% or less, even more preferably 250% or less, and even more preferably 240% or less, based on the MD dimension before stretching (100%). If the stretch ratio is greater than the lower limit, extended chains are formed, resulting in strong fibrils. The connecting domains are also elongated in the MD, and the fibrils and / or connecting domains can connect more polymer matrices. As a result, the MD length is increased, and a separator with high pin puncture strength is obtained. Furthermore, in the heat treatment step, performing the stretching operation at a specific stretch ratio or greater tends to enable a good trunk height to be obtained. The reason for this is thought to be that below a certain stretching ratio, pore opening takes precedence, and structural changes accompanied by changes in trunk height occur after the pore opening process. On the other hand, if the stretching ratio is below the upper limit, pore collapse due to necking in the TD and thickness directions caused by excessive hot stretching can be suppressed, thereby obtaining good air permeability. In the heat-relaxing after hot stretching, the MD is preferably relaxed by 10% to 50%, more preferably 20% to 45%. These heat treatment steps can be performed using a tenter or roll stretching machine. The temperature of the heat treatment step is preferably 120°C to 160°C, more preferably 130°C to 155°C. When a thermoplastic elastomer is contained, the heat treatment is performed at or above its melting point to relieve the stress applied to the skeleton of the polypropylene, the main component, during hot stretching, and uniformly extend the fibrils in the MD, thereby obtaining a separator with improved rigidity and toughness and high pin puncture strength.

[0145] From the viewpoint of achieving both high pin puncture strength and low air permeability in the thin-film separator, and from the viewpoint of obtaining a morphology such as that shown in Figure 1 and / or Figure 2, it is preferable to carry out the above-mentioned annealing treatment and hot stretching operation in the separator manufacturing method. From these viewpoints, another aspect of the present disclosure provides a method for manufacturing a separator for an electricity storage device, including the following steps: an annealing step of annealing a resin sheet containing polypropylene, polyethylene, and a thermoplastic elastomer at a temperature of 135°C or higher and 160°C or lower; and a heat treatment step of stretching the annealed resin sheet in the MD at a temperature of 130°C or higher and 155°C or lower. The resin sheet, annealing conditions, and heat treatment conditions involving the hot stretching operation may be as described above.

[0146] Manufacturing Method According to a Second Embodiment In both the co-extrusion process (i) and the lamination process (ii), the manufacturing method of the separator substrate according to the second embodiment may include an annealing step after extrusion film formation. The annealing step tends to grow the crystalline structure of the microporous layers (A) and (B), improving pore openness. Annealing at a high temperature and / or for a long time that does not exceed the melting point of the components contained therein allows crystals to grow without disrupting the crystalline structure, resulting in high pore openness. It is believed that both the microporous layers (A) and (B) can achieve good area-average long pore diameter, high porosity, low air permeability, and high pin puncture strength. On the other hand, excessively high pore openness can reduce TD tensile strength and result in a separator substrate that is prone to tearing. Therefore, it is preferable to control the balance of physical properties of the separator substrate by annealing at a specific temperature for a predetermined time. In the annealing step, the lower limit of the temperature range is preferably 115°C or higher, more preferably 120°C or higher, and the upper limit of the temperature range is preferably 150°C or lower, more preferably 145°C or lower, even more preferably 140°C or lower, still more preferably 135°C or lower, and particularly preferably 130°C or lower, and the annealing treatment is preferably carried out for 20 minutes or longer, more preferably 60 minutes or longer. This allows the polypropylene crystals contained therein to be moderately highly oriented, resulting in moderately high pore openness during subsequent stretching. In addition, in the case of uniaxial stretching in the MD, excessive pulling of tie molecules from the polypropylene crystals in the MD is suppressed, and tie molecules that can be pulled against TD tensile fracture are ensured, thereby realizing a separator with low air permeability and high pin puncture strength and high TD tensile strength due to a high elastic modulus. This allows for high input / output and high energy density in an electricity storage device, and is preferable from the viewpoint of ensuring productivity and yield of electricity storage devices due to the separator's resistance to tearing.

[0147] The method for producing a separator substrate according to the second embodiment may include a stretching step after the annealing step. Either uniaxial stretching or biaxial stretching can be used as the stretching treatment. Although not limited thereto, uniaxial stretching is preferred from the viewpoints of production costs when using a dry method and reducing thermal shrinkage in the TD. The MD stretch ratio of cold stretching ((dimension after stretching - dimension before stretching) / dimension before stretching x 100 (%)) is preferably in the range of 5% to 50%, more preferably 10% to 45%, and even more preferably 20% to 40%. By increasing the amount of cracking during cold stretching, small pore sizes are more likely to be obtained during hot stretching, resulting in a moderately good, not excessive, trunk height. Furthermore, the small pore sizes result in connection of domains as shown in Figures 1 and 2. The stress relaxation of the connected domains at the time of fracture makes fracture less likely to progress, allowing the polypropylene contained therein to be elongated. As a result, a highly oriented structure of lamellae is achieved in the high strain range, resulting in a separator with high pin puncture strength. The cold stretching temperature is preferably 10°C to 50°C, more preferably 20°C to 30°C. From the perspective of production costs, cold stretching may be performed at room temperature (23±2°C). From the viewpoints of improving both the pin puncture strength and low air permeability of the resulting separator substrate, production costs, and reduction of thermal shrinkage in the TD, uniaxial stretching is preferred.

[0148] In order to suppress thermal shrinkage of the separator substrate, a heat treatment step may be performed for the purpose of heat setting after the stretching step or the hole-forming step. The heat treatment step may include a hot stretching operation performed at a predetermined temperature and a predetermined stretch ratio for the purpose of adjusting physical properties, and / or a heat-relaxing operation performed at a predetermined temperature and a predetermined relaxation ratio for the purpose of reducing shrinkage stress imparted during film formation and stretching. A heat-relaxing operation may be performed after the hot stretching operation. In the second embodiment, the MD dimension before stretching is taken as 100%, and the stretching is preferably performed to 120% to 220% or less, more preferably 125% to 200%, even more preferably 130% to 180%, and particularly preferably 130% to 160%. This allows the formation of extended chains, resulting in strong fibrils. Furthermore, the tie molecules are not excessively pulled out from the polypropylene crystals in the MD, but are secured to be able to be pulled out against TD tensile failure, resulting in a separator with high pin puncture strength and high TD tensile strength. Furthermore, in the heat treatment step, stretching at a specific draw ratio or higher tends to result in a good trunk height. This is thought to be because, below a certain draw ratio, pore opening is prioritized, and structural changes accompanied by changes in trunk height occur after the pore opening process. In the heat relaxation step after hot stretching, the MD relaxation is preferably 10% to 50%, more preferably 20% to 45%. These heat treatment steps can be performed using a tenter or roll stretching machine. The temperature in the heat treatment step is preferably 120°C to 160°C, more preferably 130°C to 155°C. When the low-crystalline component contains, for example, polyethylene and / or a thermoplastic elastomer, the heat treatment is carried out at a temperature equal to or higher than the melting point of the polyethylene and / or thermoplastic elastomer, thereby alleviating the stress applied to the polypropylene skeleton during hot stretching and causing the fibrils to extend uniformly in the MD, thereby obtaining a separator with improved rigidity and toughness, high puncture strength, and a low air permeability with a good area average long pore diameter.

[0149] The obtained separator substrates of the first and second embodiments can be used as they are as separators for power storage devices. Optionally, one or both surfaces of the separator substrate may be provided with an additional layer such as a coating layer, and the separator substrate may be subjected to a surface treatment such as a corona treatment, if necessary.

[0150] The power storage device of the present disclosure includes the power storage device separator according to the first or second embodiment of the present disclosure. The power storage device of the present disclosure preferably has a positive electrode and a negative electrode, and the power storage device separator according to the present disclosure is preferably disposed between the positive electrode and the negative electrode.

[0151] Examples of the power storage device according to the first and second embodiments include, but are not limited to, lithium secondary batteries (including all-solid-state lithium batteries, lithium-sulfur batteries, and lithium-air batteries), lithium ion secondary batteries, sodium secondary batteries, sodium ion secondary batteries, magnesium secondary batteries, magnesium ion secondary batteries, calcium secondary batteries, calcium ion secondary batteries, aluminum secondary batteries, aluminum ion secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries, electric double layer capacitors, lithium ion capacitors, redox flow batteries, and zinc-air batteries. Among these, from the viewpoints of high energy density, low cost, and durability, lithium secondary batteries, lithium ion secondary batteries, and lithium ion capacitors are preferred, and lithium ion secondary batteries are more preferred.

[0152] The electricity storage device can be produced, for example, by stacking a positive electrode and a negative electrode with the separator described above interposed therebetween and winding them as necessary to form a stacked electrode body or a wound electrode body, which is then loaded into an outer casing, connecting the positive and negative electrodes to the positive and negative electrode terminals of the outer casing via lead bodies or the like, and further injecting a non-aqueous electrolyte solution containing a non-aqueous solvent such as a chain or cyclic carbonate and an electrolyte such as a lithium salt into the outer casing, and then sealing the outer casing.

[0153] The present electricity storage device is more preferably a lithium ion secondary battery, and preferred embodiments of the lithium ion secondary battery will now be described.

[0154] The positive electrode is not particularly limited as long as it functions as a positive electrode of a lithium ion secondary battery, and known positive electrodes can be used. The positive electrode preferably contains, as a positive electrode active material, one or more materials selected from the group consisting of materials capable of absorbing and releasing lithium ions. From the viewpoints of battery capacity and safety, the positive electrode is preferably LiCoO ２ Lithium cobalt oxide, represented by Li ２ Mn ２ O ４ Spinel-based lithium manganese oxides, such as Li ２ Mn １．５ Ni ０．５ O ４ Spinel-based lithium nickel manganese oxides, such as LiNiO ２ Lithium nickel oxide, LiMO ２ (M represents two or more elements selected from the group consisting of Ni, Mn, Co, Al, and Mg), a lithium-containing composite metal oxide represented by LiFePO ４ Among these, from the viewpoint of high safety and long-term stability, LiCoO ２ Lithium cobalt oxide, represented by LiNiO ２ Lithium nickel oxide, LiMO ２ (M represents two or more elements selected from the group consisting of Ni, Mn, Co, Al, and Mg), a lithium-containing composite metal oxide represented by LiFePO ４ Particularly preferred is a lithium iron phosphate compound represented by LiFePO ４ It is a lithium iron phosphate compound represented by the formula:

[0155] The negative electrode is not particularly limited as long as it functions as a negative electrode for a lithium-ion secondary battery, and may be any known material. The negative electrode preferably contains, as the negative electrode active material, one or more materials selected from the group consisting of materials capable of absorbing and releasing lithium ions and metallic lithium. That is, the negative electrode preferably contains, as the negative electrode active material, one or more materials selected from the group consisting of metallic lithium, carbon materials, materials containing elements capable of forming an alloy with lithium, and lithium-containing compounds. Examples of such materials include, in addition to metallic lithium, carbon materials such as hard carbon, soft carbon, artificial graphite, natural graphite, graphite, pyrolytic carbon, coke, glassy carbon, fired bodies of organic polymer compounds, mesocarbon microbeads, carbon fibers, activated carbon, graphite, carbon colloids, and carbon black.

[0156] The measurement methods and evaluation methods employed in this example are described below. In this example, the "separator substrate" corresponds to the "separator," so in the following description, the "separator substrate" may be read as the "separator."

[0157] <<Measurement and Evaluation Methods>> The measurement and evaluation methods according to the first and second embodiments are as follows. [Measurement of Melt Flow Rate (MFR)] The melt flow rate (MFR) of a microporous layer such as the microporous layer (A) and the microporous layer (B) was measured in accordance with JIS K 7210 at a temperature of 230°C and a load of 2.16 kg (unit: g / 10 min). The MFR of polypropylene or elastomer was measured in accordance with JIS K 7210 at a temperature of 230°C and a load of 2.16 kg. The melt flow rate (MFR) of polyethylene was measured in accordance with JIS K 7210 at a temperature of 190°C and a load of 2.16 kg.

[0158] [Measurement of Mw and Mn by GPC (Gel Permeation Chromatography)] Using an Agilent PL-GPC220, standard polystyrene was measured under the following conditions to create a calibration curve. Chromatography was also performed on the sample polymer or microporous layer under the same conditions, and based on the calibration curve, the polystyrene-equivalent weight average molecular weight (Mw), number average molecular weight (Mn), and MWD (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) by the number average molecular weight (Mn) of the polymer or microporous layer were calculated under the following conditions. Column: TSKgel GMHHR-H(20) HT (7.8 mm I.D. x 30 cm) (2 columns) Mobile phase: 1,2,4-trichlorobenzene Detector: RI Column temperature: 160°C Sample concentration: 1 mg / ml Calibration curve: polystyrene

[0159] [Measurement of Melt Tension] The melt tension (mN) of the microporous layer (A) and the microporous layer (B) was measured using a Capillograph manufactured by Toyo Seiki Seisakusho, Ltd. under the following conditions: Capillary: diameter 1.0 mm, length 20 mm Cylinder extrusion speed: 2 mm / min Take-up speed: 60 m / min Temperature: 240°C

[0160] [Measurement of Pentad Fraction] The pentad fraction of polypropylene was assigned based on the description in the Polymer Analysis Handbook (edited by the Japan Society for Analytical Chemistry). １３ Calculation was performed from the C-NMR spectrum using the peak height method. １３ The C-NMR spectrum was measured using a JEOL-ECZ500 by dissolving polypropylene pellets in o-dichlorobenzene-d at a measurement temperature of 145°C and accumulating 25,000 times.

[0161] [DSC Measurement] DSC measurement of the microporous layer (A) was carried out using a Shimadzu DSC-60 under the following conditions: - Sample amount: Approximately 5 mg - Cell used: Aluminum crimp cell (diameter (dia.) 5.8 mm) - Atmosphere used: Nitrogen (flow rate 50 mL / min) - Temperature program: 1st step: Heat from room temperature to 230°C at 10°C / min and hold for 5 minutes 2nd step: Heat down to 20°C at 10°C / min and hold for 5 minutes 3rd step: Heat up to 200°C at 10°C / min and hold for 5 minutes From the DSC curve (vertical axis is heat flow, horizontal axis is temperature) during the temperature rise in the 3rd step, the peak temperatures of endothermic peaks A and B were read using the analysis software TA-60 attached to the device, and the area S of endothermic peak A was calculated. Ａ and the area S of endothermic peak B Ｂ Calculate S Ｂ / S Ａ asked for.

[0162] [Measurement of Thickness (μm)] The thickness (μm) of the separator substrate was measured using a Mitutoyo Digimatic Indicator IDC112 at room temperature of 23±2° C. The thickness of each microporous layer was calculated from cross-sectional SEM image data obtained by the method for evaluating the area average long pore diameter described below.

[0163] [Measurement of Porosity (%)] A sample having a size of 10 cm x 10 cm was cut from the separator or the microporous layer, and its volume (cm ３ ) and mass (g), and calculate the density (g / cm ３ ) and the porosity was calculated using the following formula: Porosity (%) = (volume - mass / density) / volume x 100

[0164] [Air permeability (sec / 100cm) ３ The air permeability (sec / 100 cm) of the separator substrate was measured using a Gurley air permeability meter conforming to JIS P-8117. ３ The air permeability (sec / 100 cm) was measured. ３ ) by the separator substrate thickness (μm) and multiplying by 10 μm to obtain the air permeability (sec / 100 cm) when the thickness of the separator substrate is converted to 10 μm. ３ ・10 μm) was also determined.

[0165] [MD Heat Shrinkage (%), TD Heat Shrinkage (%)] The separator substrate was cut into a 50 mm square in each of the MD and TD directions, and the resulting sample was placed on copy paper in a hot air dryer (manufactured by Yamato Scientific Co., Ltd., DF1032) and heat-treated at 105°C for 1 hour in air at normal pressure. The sample was removed from the hot air dryer and allowed to cool at 25°C for 10 minutes, after which the dimensional shrinkage was determined. Heat shrinkage (%): (dimension before heating (mm) - dimension after heating (mm)) / (dimension before heating (mm)) x 100

[0166] [Puncture Strength] A needle with a hemispherical tip and a radius of 0.5 mm was prepared, and a separator was sandwiched between two plates with openings of 11 mm in diameter (dia.), and the needle, separator, and plates were set in place. A puncture test was performed using an "MX2-50N" made by Imada Co., Ltd., under the conditions of a needle tip curvature radius of 0.5 mm, an opening diameter of 11 mm in the separator holding plate, and a puncture speed of 25 mm / min. The needle and separator were brought into contact, and the maximum puncture load (i.e., puncture strength (gf)) was measured. The puncture strength (gf) was divided by the separator substrate thickness (μm) and multiplied by 10 μm to determine the puncture strength (gf / 10 μm) when the separator substrate thickness was converted to 10 μm.

[0167] [MD Tensile Strength, MD Tensile Elongation, TD Tensile Strength] The tensile strength of the separator was measured using a tensile tester (Model TG-1kN manufactured by Minebea Co., Ltd.) by setting the sample length before the test to 35 mm and pulling the sample at a speed of 100 mm / min. The strength (tensile load value) at which the sample broke (fractured) was divided by the cross-sectional area of ​​the test piece to determine the tensile strength (kgf / cm ２ The elongation (percentage of elongation from before the test) at which the sample broke was taken as the tensile elongation (%). The tensile strength and tensile elongation in the MD of the separator, and the tensile strength in the TD were measured.

[0168] [Measurement of withstand voltage (kV)] The separator substrate sandwiched between aluminum foils was set on the sample stage of a withstand voltage tester (grade) manufactured by Kikusui Electronics Co., Ltd., and an electrode indenter was placed thereon. The voltage was increased at a starting voltage of 0 kV and a voltage increase rate of 0.025 kV / sec, and the voltage value when a current of 0.2 mA flowed between the electrodes was taken as the withstand voltage value. The withstand voltage value (kV) was also divided by the separator thickness (μm) and multiplied by 10 μm to determine the withstand voltage (kV / 10 μm) when the thickness of the separator substrate was converted to 10 μm.

[0169] [Area-average pore diameter (nm)] The area-average pore diameter was measured by image analysis of cross-sectional SEM observation. As a pretreatment, the separator was stained with ruthenium, and a cross-sectional sample was prepared by freeze-fracturing. The cross-section was the MD-ND plane. The cross-sectional sample was fixed to a SEM sample stage for cross-sectional observation with a conductive adhesive (carbon-based) and dried. After that, an osmium coating was performed as a conductive treatment using an osmium coater (HPC-30W, manufactured by Vacuum Device Co., Ltd.) under conditions of an applied voltage adjustment knob setting of 4.5 and a discharge time of 0.5 seconds, and a microscopic specimen was prepared. Next, using a scanning electron microscope (S-4800 manufactured by Hitachi High-Technologies Corporation), eight random points on the cross section of each microporous layer of the microporous membrane were observed under conditions of an acceleration voltage of 1 kV, a detection signal LA10, a working distance of 5 mm, and a magnification of 30,000x.

[0170] The observed image was trimmed using a function of OpenCV, an image analysis library, in a programming language Python (registered trademark) environment so that only the cross section of one arbitrary microporous layer was included, and the surface, outer layer, and other microporous layers were removed. The image was then binarized using the Otsu method to separate the resin portion from the pore portion, and the average major axis of the pore portion was calculated. At this time, the area of ​​the pores existing across the photographed range and outside the photographed range was 0.001 nm ２ The following pores were excluded from the measurement: The average diameter was calculated by averaging the area of ​​each pore.

[0171] [Trunk height (nm)] The trunk height was measured by image analysis of cross-sectional SEM observation. As in the calculation of the area average pore diameter, cross-sectional samples and microscopic samples were prepared, and then three arbitrary points on the cross section of the microporous membrane were observed under conditions of an accelerating voltage of 1 kV, a detection signal LA10, a working distance of 5 mm, and a magnification of 5000x.

[0172] The observed image was trimmed using functions in the OpenCV image analysis library in the Python (registered trademark) programming language environment to include only the cross section of one arbitrary microporous layer. After removing the surface and exterior, fibril removal processing was performed by repeatedly blurring only the ND, and binarization processing was performed using the Otsu method to separate the resin portion from the pore portion. The fibril removal processing was performed 100 times using a Gaussian filter in a rod-shaped processing range of 3 pixels in the ND and 1 pixel in the MD. After binarization processing, noise was removed by sequentially performing opening and closing processing in an elliptical processing range of 7 pixels in the major axis in the ND and 3 pixels in the minor axis in the MD, and a fibril-removed image was obtained.

[0173] The stem height was calculated from the fibril-removed image. The fibril-removed image was cut out at 1 pixel in MD, and the entire length of the resin part of ND was detected. This was repeated to include all of MD, and the length of the resin part of ND over the entire range of the fibril-removed image was detected. Figure 3 is a schematic diagram of a cut-out portion of the fibril-removed image. The bright areas in the figure are resin parts (5), and the dark areas are hole parts (6), and an example of a location cut out at 1 pixel in MD and detected is the part indicated by the double arrow. A weighted average was calculated for the obtained numerical values ​​of the resin part length of ND, with the length of the resin part of ND as the weight, and the obtained value was used as the stem height. If the length of the resin part of ND is L, the stem height H can be calculated using the following formula: By using a weighted average using n lengths L, the trunk height H becomes a value that is more highly correlated with the puncture strength and withstand voltage.

[0174] [MD length (nm)] The MD length was measured by image analysis of cross-sectional SEM observation. As in the calculation of the area average pore diameter, cross-sectional samples and microscopic samples were prepared, and then three arbitrary points on the cross section of the microporous membrane were observed under the conditions of an accelerating voltage of 1 kV, a detection signal LA10, a working distance of 5 mm, and a magnification of 5000x.

[0175] The observed image was trimmed using functions of OpenCV, an image analysis library, in an environment using the programming language Python (registered trademark) so that only the cross section of the separator substrate was included, and after removing the surface and exterior, the image was binarized using the Otsu method to obtain a binarized image in which the resin portion and the pore portion were separated.

[0176] The MD length was calculated from the above binarized image. The fibril-removed image was cut out in ND1 pixel units, and the entire length of the resin part in the MD was detected. This was repeated to include all ND, and the length of the resin part in the MD over the entire range of the binarized image was detected. Figure 4 is a schematic diagram of a portion cut out of the binarized image. The bright areas in the figure are resin parts (5), and the dark areas are hole parts (6), and an example of a location cut out and detected in ND1 pixel units is the part indicated by the double arrow. A weighted average was calculated for the obtained numerical values ​​of the resin part in the MD, with the length of the resin part in the MD as the weight, and the obtained value was taken as the MD length. If the length of the resin part in the MD is l, then the MD length L ＭＤ is the following formula: The MD length L is calculated by using a weighted average of n lengths l. ＭＤ is a value that is more highly correlated with puncture strength.

[0177] [Analysis of Stained Bright Areas] Stained bright areas were measured by image analysis of cross-sectional SEM observation. As in the calculation of the area-average pore diameter, cross-sectional samples and microscopic samples were prepared, and then three random points on the microporous membrane surface were observed using a scanning electron microscope (Hitachi High-Tech SU8220) under conditions of an acceleration voltage of 2 kV, a detection signal of LA100, a working distance of 5 mm, and a magnification of 30,000 times. The brightness was set to avoid saturation and to maximize contrast, and a composition image was obtained as an 8-bit grayscale image.

[0178] The acquired composition image was processed using the image processing software ImageJ according to the following procedure. Bright stained areas were extracted, and the area and total area ratio were analyzed. (1) In the acquired composition image, an area showing only the microporous membrane or microporous layer was selected as the image processing area. (2) Using the Filters function of ImageJ, an averaging process was performed using Mean with the argument Radius set to 2 pixels. (3) For the obtained averaged image, the Threshold function was used to check the image and pixel value histogram (the horizontal axis is pixel brightness value, and the vertical axis is the number of pixels) and set the maximum and minimum threshold values ​​for distinguishing bright stained areas. The maximum threshold value was set to 255. The minimum threshold value is set so that the light areas of the image can be selected. However, if it is difficult to distinguish the light areas, the peak height of the highest brightness peak in the histogram is set to 100, and the minimum threshold value is set to the brightness value (horizontal axis) at a height of 30 on the high brightness side. (4) The Analyze Particles function is used to select particles with an area of ​​100 nm ２ 0.1 μm or more ２ For the following bright stained areas, the maximum area per area and the total area ratio S １ Calculate the percentage.

[0179] [Measurement by cross fractionation chromatography (CFC)] Measurement by cross fractionation chromatography (CFC) was performed as follows. First, a sample of the separator substrate or microporous layer was dissolved in an eluent and introduced into the device. The sample solution was held at 145°C for 30 minutes and 135°C for 30 minutes. Next, the temperature was lowered to -17°C at a rate of 1.0°C / min and then held for 60 minutes. Thereafter, the temperature was sequentially raised at a rate of 40°C / min. First, the temperature was raised from -17°C to -10°C, then from -10°C to 0°C, then from 0°C to 80°C at 5°C intervals, then from 80°C to 104°C at 3°C ​​intervals, then from 104°C to 126°C at 2°C intervals, then from 126°C to 130°C, and finally from 130°C to 140°C at 5°C intervals. At each temperature, the temperature was maintained for 3 minutes for elution time plus 19 minutes for analysis time, after which the temperature was raised and the concentration of the components eluted at each temperature was detected. Then, an elution temperature-elution amount curve was measured using the elution temperature (°C) of the component and the elution amount (mass%) at that time, with the elution temperature on the vertical axis and the elution amount on the horizontal axis, and the elution amount and integrated elution amount at each temperature were determined. Simultaneously, the molecular weight at each elution temperature was measured in the GPC section, and the weight average molecular weight at each temperature was determined. Data analysis was performed using the data processing software "CFC calc." Apparatus: CFC2 type cross-fractionation chromatograph manufactured by Polymer Char Detector: IR4 type infrared spectrophotometer manufactured by Polymer Char Detection wavelength: 3.42 μm GPC column: Shodex HT-806M x 3 manufactured by Showa Denko Column temperature: 135°C Column calibration: monodisperse polystyrene manufactured by Tosoh Corporation Molecular weight calibration method: standard calibration method (polystyrene equivalent) Eluent: o-dichlorobenzene (ODCB), BHT added Flow rate: 1.0 mL / min Sample concentration: 90 mg / 30 mL Injection volume: 0.5 mL

[0180] Examples and Comparative Examples According to the First Embodiment

[0181] Example 1-1 Preparation of Polypropylene Resin Composition Pellets of a high molecular weight polypropylene resin (PP1, MFR = 0.51) and an ethylene / 1-butene copolymer (C2C4, MFR = 6.7) shown in Table 1 were dry blended in a mass ratio of PP1:C2C4 = 50.0:50.0 (mass%), and then melt-kneaded using a TEM26SS (manufactured by Toshiba Machine Co., Ltd., L / D = 48.5). After melt-kneading, a strand was pulled from a die (3 holes), cooled in a water-cooled bath, and then cut using a pelletizer to obtain polypropylene resin composition pellets.

[0182] [Preparation of Microporous Layer] The resin for the microporous layer (A) was a dry blend of the high molecular weight polypropylene resin (PP1, MFR = 0.51) shown in Table 1 and the polypropylene resin composition pellets at a mass ratio of PP1:polypropylene resin composition pellets = 92.0:8.0 (mass%), and then melted in a 2.5-inch extruder and fed to a single-layer inflation die using a gear pump. The temperature of the inflation die was set to 240°C, and the molten polymer was extruded from the inflation die. The extruded resin was cooled by blown air and then wound onto a roll to obtain a single-layer precursor sheet consisting of a microporous layer (A) with a thickness of approximately 12 μm. Here, the lip distance (lip clearance) of the inflation die was set to 1.8 mm, and extrusion was performed at a discharge rate of 9 kg / h.

[0183] The resulting precursor sheet was then placed in a dryer and annealed at 150°C for 180 minutes. The annealed precursor sheet was then cold stretched 30% in the MD at room temperature, and the stretched film was placed in a 135°C oven without shrinkage. The pre-stretched dimensions were set to 100%, and the sheet was hot stretched 200% in the MD. Then, the sheet was heat-relaxed 44% in the MD to obtain a separator substrate having a single-layer structure consisting of a microporous layer (A). The structure and physical properties of the resulting separator substrate are shown in Table 1.

[0184] Example 1-2 A separator substrate was obtained in the same manner as in Example 1-1, except that the substrate was cold stretched by 40% and hot stretched by 220% in an oven at 135° C. The evaluation results of the obtained separator substrate are shown in Table 1.

[0185] Example 1-3 A separator substrate was obtained in the same manner as in Example 1-2, except that hot stretching of 240% was performed in an oven at 135° C. The evaluation results of the obtained separator substrate are shown in Table 1.

[0186] Example 1-4 A separator substrate was obtained in the same manner as in Example 1-2, except that, when preparing the polypropylene resin composition, pellets of a high molecular weight polypropylene resin (PP1, MFR = 0.51), a high molecular weight polyethylene resin (PE1, MFR = 0.02), and an ethylene / propylene copolymer (C2C3, MFR = 9.5) shown in Table 1 were dry-blended and melt-kneaded in a mass ratio of PP1:PE1:C2C3 = 74.2:13.8:12.0 (% by mass), and the high molecular weight polypropylene resin (PP1, MFR = 0.51) and polypropylene resin composition pellets shown in Table 1 were dry-blended in a mass ratio of PP1:polypropylene resin composition pellets = 73.0:27.0 (% by mass) as the resin for the microporous layer, and the die and extrusion temperature were finely adjusted to stabilize the extrusion. The evaluation results of the obtained separator substrate are shown in Table 1.

[0187] Example 1-5 A separator substrate was obtained in the same manner as in Example 1-4, except that hot stretching of 240% was performed in an oven at 135° C. The evaluation results of the obtained separator substrate are shown in Table 1.

[0188] Examples 1-6 and 1-9 Separator substrates were obtained in the same manner as in Example 1-2, except that the raw materials were changed as shown in Table 1. The evaluation results of the obtained separator substrates are shown in Table 1.

[0189] Examples 1-7, 1-8, 11, and 16 Separator substrates were obtained in the same manner as in Example 1-1, except that the raw materials were changed as shown in Table 1. The evaluation results of the obtained separator substrates are shown in Table 1.

[0190] Example 1-10 A separator substrate was obtained in the same manner as in Example 1-1, except that the precursor sheet was obtained using PP1:polypropylene resin composition pellets in a mass ratio of 84.0:16.0 (mass%). The evaluation results of the obtained separator substrate are shown in Table 1.

[0191] Example 1-12 A separator substrate was obtained in the same manner as in Example 1-1, except that the precursor sheet was obtained using PP1:polypropylene resin composition pellets in a mass ratio of 96.0:4.0 (mass%). The evaluation results of the obtained separator substrate are shown in Table 1.

[0192] Example 1-13 A separator substrate was obtained in the same manner as in Example 1-2, except that when preparing the polypropylene resin composition, pellets of a high molecular weight polypropylene resin (PP1, MFR = 0.51) and an ethylene / propylene copolymer (C2C3, MFR = 9.5) shown in Table 1 were dry-blended and melt-kneaded in a mass ratio of PP1:C2C3 = 91.2:8.8 (mass%), and that the resin for the microporous layer (A) was a mixture of a high molecular weight polypropylene resin (PP1, MFR = 0.51) and polypropylene resin composition pellets shown in Table 1 in a mass ratio of PP1:polypropylene resin composition pellets = 61.6:38.4 (mass%). The evaluation results of the obtained separator substrate are shown in Table 1.

[0193] Example 1-14 A separator substrate was obtained in the same manner as in Example 1-1, except that, when preparing the polypropylene resin composition, pellets of a high molecular weight polypropylene resin (PP1, MFR = 0.51), a polyethylene resin (PE2, MFR = 1.0), and an ethylene / propylene copolymer (C2C3, MFR = 9.5) shown in Table 1 were dry-blended and melt-kneaded in a mass ratio of PP1:PE2:C2C3 = 82.8:9.2:8.0 (% by mass), and that, as the resin for the microporous layer (A), a high molecular weight polypropylene resin (PP1, MFR = 0.51) shown in Table 1 and polypropylene resin composition pellets were dry-blended in a mass ratio of PP1:polypropylene resin composition pellets = 56.5:43.5 (% by mass), and the die and extrusion temperature were finely adjusted to stabilize the extrusion. The evaluation results of the obtained separator substrate are shown in Table 1.

[0194] Example 1-15 For the resin of the microporous layer (A), pellets of a high molecular weight polypropylene resin (PP1, MFR = 0.51) and an ethylene / 1-butene copolymer (C2C4, MFR = 6.7) shown in Table 1 were dry-blended in a mass ratio of PP1:C2C4 = 96.0:4.0 (mass%), and then melted in a 2.5-inch extruder and supplied to both outer layers of a two-kind, three-layer co-extrusion blown film die using a gear pump. For the resin of the microporous layer (B), 100% by mass of a high molecular weight polypropylene resin (PP4, MFR = 0.31) was melted in a 2.5-inch extruder and supplied to the inner layer of the two-kind, three-layer co-extrusion blown film die using a gear pump. The temperature of the inflation die was set to 235°C, and the molten polymer was extruded from the inflation die. The extruded resin was cooled by blown air while being wound around a roll to obtain a precursor sheet with an A / B / A layer structure approximately 12 μm thick. The lip clearance of the inflation die was set to 1.8 mm, and extrusion was performed at a discharge rate of 9 kg / h. Next, annealing, cold stretching, hot stretching, and heat relaxation were performed in the same manner as in Example 1-1 to obtain a separator substrate having a three-layer structure of microporous layer (A) / microporous layer (B) / microporous layer (A). The resulting separator was evaluated. The evaluation results are shown in Table 1.

[0195] Example 1-17 A separator substrate was obtained in the same manner as in Example 1-15, except that the raw materials and composition were changed as shown in Table 1. The evaluation results of the obtained separator substrate are shown in Table 1.

[0196] Comparative Example 1-1 A separator substrate was obtained in the same manner as in Example 1-1, except that, when obtaining a precursor sheet, 100.0% (by mass) of a high molecular weight polypropylene resin (PP3, MFR = 0.91) shown in Table 1 was used, and the precursor sheet was hot-stretched by 180% in an oven at 135°C and heat-relaxed by 50%. The evaluation results of the obtained separator substrate are shown in Table 1.

[0197] Comparative Example 1-2 A separator substrate was obtained in the same manner as in Example 1-1, except that the substrate was cold stretched by 10%, hot stretched by 160% in an oven at 135°C, and heat-relaxed by 39%. The evaluation results of the obtained separator substrate are shown in Table 1.

[0198] Comparative Example 1-3 A separator substrate was obtained in the same manner as in Example 1-4, except that the substrate was cold stretched by 40% and hot stretched by 260% in an oven at 135° C. The evaluation results of the obtained separator substrate are shown in Table 1.

[0199]

[0200]

[0201] Examples and Comparative Examples According to the Second Embodiment

[0202] Example 2-1 Preparation of Microporous Layer For the resin of the microporous layer (A), pellets of a high molecular weight polypropylene resin (PP1, MFR = 0.51) and an ethylene / 1-butene copolymer (C2C4, MFR = 6.7) shown in Table 2 were dry-blended in a mass ratio of PP1:C2C4 = 96.0:4.0 (mass%), then melted in a 2.5-inch extruder and fed to a single-layer inflation die using a gear pump. The temperature of the inflation die was set to 240°C, and the molten polymer was extruded from the inflation die. The extruded resin was cooled by blown air and taken up on a roll to obtain a single-layer precursor sheet consisting of a microporous layer (A) with a thickness of approximately 12 μm. The lip distance (lip clearance) of the inflation die was set to 1.8 mm, and extrusion was performed at a discharge rate of 9 kg / h.

[0203] The resulting precursor sheet was then placed in a dryer and annealed at 120°C for 180 minutes. The annealed precursor sheet was then cold stretched 30% in the MD at room temperature, and the stretched film was placed in a 135°C oven without shrinkage. The pre-stretched dimension was set to 100%, and the sheet was hot stretched 130% in the MD. Then, the sheet was heat-relaxed 44% in the MD to obtain a separator substrate having a single-layer structure composed of a microporous layer (A). The structure and physical property evaluation results of the resulting separator substrate are shown in Table 2. The elution temperature-elution amount curve for the CFC measurement of Example 2-1 is shown in Figure 5.

[0204] Examples 2-2 to 2-8, 2-10 to 2-11 and Comparative Examples 2-2 to 2-5 Separator substrates having a single-layer structure were obtained in the same manner as in Example 2-1, except that the raw materials were changed as shown in Table 2, the mass ratios of the polypropylene resin and elastomer pellets were adjusted according to the compositions shown in Table 2, and the production conditions were adjusted as shown in Table 2. The evaluation results of the obtained separator substrates are shown in Table 2.

[0205] Example 2-9 Preparation of Polypropylene Resin Composition Pellets of a high molecular weight polypropylene resin (PP1, MFR = 0.51), a polyethylene resin (PE1, MFR = 0.02), and an olefin-(ethylene-butene)-olefin copolymer (CEBC, MFR = 2.5) shown in Table 2 were dry blended in a mass ratio of PP1:PE1:CEBC = 82.8:9.2:8.0 (mass%), and then melt-kneaded using a TEM26SS (manufactured by Toshiba Machine Co., Ltd., L / D = 48.5). After melt-kneading, a strand was pulled from a die (3 holes), cooled in a water-cooled bath, and then cut using a pelletizer to obtain polypropylene resin composition pellets. [Preparation of microporous layer] A high molecular weight polypropylene resin (PP1, MFR = 0.51) shown in Table 2 was dry-blended with the above polypropylene resin composition pellets in a mass ratio of PP1:polypropylene resin composition pellets = 56.5:43.5 (mass%) as the resin for the microporous layer (A), and a separator substrate having a single-layer structure was obtained in the same manner as in Example 2-1, except that the die and the extrusion temperature at which extrusion was stable were finely adjusted. The evaluation results of the obtained separator substrate are shown in Table 2.

[0206] Comparative Examples 2-1 and 2-7 to 2-9 Separator substrates having a single-layer structure were obtained in the same manner as in Example 2-9, except that no elastomer was used, and 100% by mass of a high-molecular-weight polypropylene resin shown in Table 2 was melted in a 2.5-inch extruder as the resin for the microporous layer (A), and the production conditions were adjusted as shown in Table 2. The evaluation results of the obtained separator substrates are shown in Table 2.

[0207] Comparative Example 2-6 A separator substrate having a single-layer structure was obtained in the same manner as in Example 2-1, except that the mass ratios of the polypropylene resin, polyethylene resin, and elastomer pellets were adjusted according to the composition shown in Table 2, and the production conditions were adjusted according to Table 2. The evaluation results of the obtained separator substrate are shown in Table 2.

[0208] Example 2-12 Preparation of microporous layer As the resin for the microporous layer (A), pellets of a high molecular weight polypropylene resin (PP2, MFR = 0.60) and an ethylene / 1-butene copolymer (C2C4, MFR = 6.7) shown in Table 2 were dry-blended in a mass ratio of PP1:C2C4 = 96.0:4.0 (mass%), and then melted in a 2.5-inch extruder and supplied to both outer layers of a two-kind, three-layer co-extrusion blown film die using a gear pump. As the resin for the microporous layer (B), 100% by mass of a high molecular weight polypropylene resin (PP4, MFR = 0.30) was melted in a 2.5-inch extruder and supplied to the inner layer of the two-kind, three-layer co-extrusion blown film die using a gear pump. The temperature of the inflation die was set to 250°C, and the molten polymer was extruded from the inflation die. The extruded resin was cooled by blown air while being wound around a roll to obtain a precursor sheet with an A / B / A layer structure approximately 12 μm thick. The lip distance (lip clearance) of the inflation die was set to 1.8 mm, and extrusion was performed at a discharge rate of 9 kg / h. Next, a separator substrate having a three-layer structure of microporous layer (A) / microporous layer (B) / microporous layer (A) was obtained in the same manner as in Example 2-1, except that the production conditions were adjusted as shown in Table 2. The resulting separator was evaluated. The evaluation results are shown in Table 2.

[0209] Comparative Example 2-10 A separator substrate having a three-layer structure of microporous layer (A) / microporous layer (B) / microporous layer (A) was obtained in the same manner as in Example 2-12, except that no elastomer was used, and 100% by mass of the high-molecular-weight polypropylene resin shown in Table 2 was melted in a 2.5-inch extruder as the resin for the microporous layer (A), and the extrusion temperature at which the die and discharge were stable was finely adjusted. The evaluation results of the obtained separator substrate are shown in Table 2.

[0210]

[0211]

[0212]

[0213]

[0214] The separator for an electricity storage device according to the present disclosure can be suitably used as a separator for an electricity storage device, such as a lithium ion secondary battery.

[0215] 1 Polymer matrix 2-1 Connected domain (thick line) 2-2 Domain that becomes light stained area 3 Fibril (thin line) 4 Hole 5 Resin area (light area) 6 Hole area (dark area)

Claims

1. A separator for an electricity storage device, comprising a microporous membrane mainly composed of polyolefin as a separator substrate, the separator substrate comprising a microporous layer (A) having a stem height of 500 nm or more and 1000 nm or less, calculated from analysis of a scanning electron microscope (SEM) image of an MD-ND cross section of the separator substrate, and a MD length of 1000 nm or more and 1900 nm or less, calculated from analysis of a SEM image of the MD-ND cross section of the separator substrate.

2. The separator for an electric storage device according to claim 1, wherein the microporous layer (A) contains a thermoplastic elastomer containing at least one member selected from the group consisting of a polyolefin different from the main component, a polyolefin copolymer, and a copolymer of polystyrene and a polyolefin.

3. The separator for an electricity storage device according to claim 1 or 2, wherein the microporous layer (A) contains a thermoplastic elastomer containing, as a repeating unit, one or more members selected from the group consisting of ethylene, propylene, and 1-butene.

4. A separator for an electricity storage device as described in claim 1 or 2, comprising, based on the total mass of the microporous layer (A), 80.0 mass% or more and 99.5 mass% or less of polypropylene as the polyolefin and 0.5 mass% or more and 20.0 mass% or less of a thermoplastic elastomer.

5. In a composition image at a magnification of 30,000 times obtained by a scanning electron microscope (SEM) of an MD-ND cross section of the microporous layer (A) stained with a ruthenium compound, a polymer matrix and fibrils are present, and the polymer matrix and the fibrils each have an area of ​​100 nm ２ 0.1 μm or more ２ The following stained bright areas are present, and the area of ​​the entire image showing only the microporous layer (A) is 100 nm ２ 0.1 μm or more ２ The total area ratio of the dyed bright parts is S １ %, 0.5≦S １ The separator for an electricity storage device according to claim 1 or 2, wherein the average molecular weight is ≦15.

0.

6. A separator for a storage device as described in claim 1 or 2, wherein the integrated elution amount of the microporous layer (A) at 100°C or higher and 130°C or lower, as measured by cross fractionation chromatography (CFC), is 80.0% by mass or higher and 99.5% by mass or lower of the total elution amount, and the integrated elution amount of the microporous layer (A) at 20°C or higher and lower than 100°C is 0.5% by mass or higher and 20.0% by mass or lower of the total elution amount.

7. The separator for an electricity storage device according to claim 1 or 2, wherein the area average long pore diameter calculated by analysis of a SEM image of an MD-ND cross section of the microporous layer (A) is 50 nm or more and 500 nm or less.

8. The separator for an electricity storage device according to claim 1 or 2, wherein the microporous layer (A) has a melt flow rate (MFR) of 1.00 g / 10 min or less when measured under a load of 2.16 kg at a temperature of 230° C.

9. The separator for an electricity storage device according to claim 1 or 2, wherein the melt tension Mt of the microporous layer (A) at 240°C is 10 mN or more and 35 mN or less.

10. The separator for an electricity storage device according to claim 1 or 2, wherein the thickness of the separator substrate is 3 μm or more and 20 μm or less.

11. The separator for an electricity storage device according to claim 1 or 2, wherein the porosity of the separator substrate is 30% or more and 60% or less.

12. The separator for an electricity storage device according to claim 1 or 2, wherein the separator substrate has a TD heat shrinkage rate of 5% or less at 105° C. for 1 hour.

13. The separator substrate has an MD tensile strength of 1800 kgf / cm ２ The separator for an electricity storage device according to claim 1 or 2.

14. The separator for an electricity storage device according to claim 1 or 2, wherein the melt flow rate (MFR) of the microporous layer (A) measured under a load of 2.16 kg at a temperature of 230° C. is 0.20 g / 10 min or more.

15. The separator for an electricity storage device according to claim 1 or 2, wherein the polyolefin is polypropylene, and the weight average molecular weight (Mw) of the polypropylene is 300,000 or more and 1,300,000 or less.

16. The separator for use in an electricity storage device according to claim 15, wherein the polypropylene has a molecular weight distribution (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) by the number average molecular weight (Mn) of 3 or more and 30 or less.

17. The polypropylene １３ The separator for an electricity storage device according to claim 15, which has a pentad fraction of 94.0% or more as measured by C-NMR (nuclear magnetic resonance).

18. The separator for an electricity storage device according to claim 1 or 2, comprising 0.5 mass % or more and 20.0 mass % or less of polyethylene, based on the total mass of the microporous layer (A).

19. The separator substrate has a melt tension Mt of the microporous layer (A) at 240°C. Ａ The melt tension Mt Ｂ The separator for an electricity storage device according to claim 1 or 2, comprising a microporous layer (B) which is 20. An electricity storage device comprising a positive electrode containing lithium iron phosphate as a positive electrode active material, a negative electrode, and the separator for an electricity storage device according to claim 1 or 2 disposed between the positive electrode and the negative electrode.

21. A separator for an electricity storage device comprising a microporous layer (A) containing polypropylene as a separator substrate, wherein the integrated elution amount of the microporous layer (A) at 100°C or higher and 130°C or lower, as measured by cross fractionation chromatography (CFC), is 80.0% to 99.5% by mass of the total elution amount, and the integrated elution amount of the microporous layer (A) at 20°C or higher and lower than 100°C is 0.5% to 20.0% by mass of the total elution amount, and the porosity of the separator substrate is 30.0% to 45.0%.

22. The electricity storage device separator according to claim 21, wherein an elution temperature-elution amount curve of the microporous layer (A) measured by cross fractionation chromatography (CFC) has at least two peaks, the peak temperature of the higher peak being 105°C or higher and 125°C or lower, and the peak temperature of the lower peak being 35°C or higher and 100°C or lower.

23. In the elution temperature-elution amount curve of the microporous layer (A) measured by cross fractionation chromatography (CFC), the weight average molecular weight (Mw Ｈ ) is 300,000 or more and 1,300,000 or less, and the weight average molecular weight (Mw Ｌ 23. The separator for use in an electric storage device according to claim 22, wherein the molecular weight per unit area (Mn) is 50,000 or more and 1,800,000 or less.

24. The separator for an electricity storage device according to claim 21 or 22, wherein the microporous layer (A) has a melt flow rate (MFR) of 1.0 g / 10 min or less when measured under a load of 2.16 kg at a temperature of 230° C.

25. Melt tension Mt of the microporous layer (A) at 240°C Ａ The separator for an electricity storage device according to claim 21 or 22, wherein the compressive strength is 10 mN or more and 35 mN or less.

26. The separator for an electricity storage device according to claim 21 or 22, wherein the weight average molecular weight (Mw) of the microporous layer (A) is 250,000 or more and 1,500,000 or less.

27. The separator for an electricity storage device according to claim 26, wherein the molecular weight distribution (Mw / Mn), which is the value obtained by dividing the weight average molecular weight (Mw) of the microporous layer (A) by the number average molecular weight (Mn), is 3 or more and 30 or less.

28. １３ The separator for an electric storage device according to claim 21 or 22, wherein the polypropylene has a pentad fraction of 94.0% or more as measured by C-NMR (nuclear magnetic resonance).

29. The separator for an electricity storage device according to claim 21 or 22, comprising a thermoplastic elastomer in an amount of 0.5 mass % or more and 20.0 mass % or less, based on the total mass of the microporous layer (A).

30. The separator for use in an electricity storage device according to claim 29, comprising 0.5 mass % or more and 20.0 mass % or less of polyethylene, based on the total mass of the microporous layer (A).

31. The electricity storage device separator according to claim 29, wherein the thermoplastic elastomer contains, as a repeating unit, at least one type selected from the group consisting of ethylene, propylene, and 1-butene.

32. The separator for an electricity storage device according to claim 21 or 22, wherein the thickness of the separator substrate is 3 μm or more and 20 μm or less.

33. The separator for use in an electricity storage device according to claim 21 or 22, wherein the stem height calculated by analysis of a scanning electron microscope (SEM) image of an MD-ND cross section of the separator substrate is 500 nm or more and 1000 nm or less.

34. The electricity storage device separator according to claim 21 or 22, wherein the area average long pore diameter calculated by analysis of a scanning electron microscope (SEM) image of an MD-ND cross section of the microporous layer (A) is 50 nm or more and 500 nm or less.

35. The separator for an electricity storage device according to claim 21 or 22, wherein the separator substrate has a TD heat shrinkage rate of 5% or less for 1 hour at a temperature of 105°C.

36. The separator for an electricity storage device according to claim 21 or 22, wherein the separator substrate has an MD heat shrinkage rate of 20% or less for 1 hour at a temperature of 105°C.

37. The separator for an electricity storage device according to claim 21 or 22, wherein the separator substrate has an MD tensile elongation of 20% or more and 60% or less.

38. The separator substrate has a melt tension Mt of the microporous layer (A) at 240°C. Ａ The melt tension Mt Ｂ The separator for an electricity storage device according to claim 21 or 22, comprising a microporous layer (B) which is 39. An electricity storage device comprising: a positive electrode containing lithium iron phosphate as a positive electrode active material; a negative electrode; and the electricity storage device separator according to claim 21 or 22 disposed between the positive electrode and the negative electrode.

Images