Separator for power storage device and power storage device

Abstract

Provided is a separator for a power storage device, said separator comprising a separator substrate having a microporous layer (A) containing polypropylene. The separator substrate has a melt flow rate (MFR) of 0.20 g / 10 min - 0.90 g / 10 min as measured under a load of 2.16 kg and at a temperature of 230°C. A main fiber width calculated through analysis of a scanning electron microscope (SEM) image of an MD-ND cross section of the separator substrate is 150 nm - 500 nm.

Description

Separators for energy storage devices and energy storage devices

[0001] This disclosure relates to separators for energy storage devices, etc.

[0002] Microporous membranes, particularly polyolefin-based microporous membranes, are used in many technological fields, including precision filtration membranes, battery separators, capacitor separators, and fuel cell materials. They are especially used as separators for energy 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 computers, as well as electric vehicles, including hybrid vehicles and plug-in hybrid vehicles.

[0003] In recent years, there has been a growing demand for lithium-ion batteries with high energy capacity, high energy density, and high output characteristics. Consequently, there is a growing demand for separators that are thin films and offer excellent battery performance, reliability, and safety.

[0004] For example, Patent Document 1 describes a multilayer microporous thin film or film that can improve properties including dielectric breakdown and strength. The preferred multilayer microporous film described in Patent Document 1 includes a microlayer and one or more stacked barriers.

[0005] Patent Document 2 describes a separator for energy storage devices that is high-strength and can be made into a thin film. This microporous film is mainly composed of polyolefin, 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 at a load of 2.16 kg and a temperature of 230°C.

[0006] Patent Document 3 describes a separator for energy storage devices that is excellent in terms of product safety, and discloses a microporous membrane comprising a polypropylene resin and a thermoplastic elastomer, having a specific melt flow rate (MFR) and morphology.

[0007] Patent Document 4 describes a battery separator that includes a microporous membrane with excellent TD tensile strength, puncture strength, and air permeability.

[0008] International Publication No. 2018 / 089748, International Publication No. 2020 / 196120, International Publication No. 2019 / 103947, International Publication No. 2018 / 217990

[0009] The methods described in Patent Documents 1 to 4 achieve thin films, high strength, and excellent air permeability, respectively, through specific film properties and resin mixtures. However, energy storage devices require separators that are thin, high-strength, and have low air permeability, and there was room for further improvement in this regard. In particular, there was a problem with the separator being prone to tearing during thinning, resulting in poor tear resistance (MD tensile strength, TD tensile strength, TD tensile elongation, and first puncture strength at fracture).

[0010] Therefore, the problem that the present invention aims to solve is to provide a separator for energy storage devices that combines excellent tear resistance (high MD tensile strength, high TD tensile strength, and high TD tensile elongation), low air permeability, and high process suitability, and that can be made into a thin film.

[0011] The present inventors, after diligent research, have found that the above problems can be solved by using a separator substrate that contains polypropylene as the main component and has a specific composition and structure, and have completed the present invention. That is, the present invention is as follows: (1) A separator for an energy storage device comprising a separator substrate having a microporous layer (A) containing polypropylene, wherein the melt flow rate (MFR) of the separator substrate measured at a load of 2.16 kg and a temperature of 230°C is 0.20 g / 10 min or more and 0.90 g / 10 min or less, and the stem width calculated from the analysis of scanning electron microscope (SEM) images of the MD-ND cross section of the separator substrate is 150 nm or more and 500 nm or less. (2) The separator for an energy storage device according to item 1, wherein the separator substrate contains more than 0.0 mol% and 3.0 mol% or less of an olefin structure other than a propylene structure as repeating units, based on the total resin of the separator substrate. (3) A separator for energy storage devices according to item 1 or 2, wherein the stem width calculated from the analysis of scanning electron microscope (SEM) images of the MD-ND cross-section of the separator substrate is 250 nm or more and 500 nm or less. (4) A separator for energy storage devices according to any one of items 1 to 3, wherein the stem height calculated from the analysis of scanning electron microscope (SEM) images of the MD-ND cross-section of the separator substrate is 500 nm or more and 1000 nm or less. (5) A separator for energy storage devices according to any one of items 1 to 4, wherein the melt flow rate (MFR) of the separator substrate measured at a load of 2.16 kg and a temperature of 230 °C is 0.20 g / 10 min or more and 0.6 g / 10 min or less. (6) A separator for energy storage devices according to any one of items 1 to 5, wherein the area-average pore diameter calculated from the analysis of scanning electron microscope (SEM) images of the MD-ND cross section of the separator substrate is 100 nm or more and 300 nm or less. (7) A separator for energy storage devices according to item 2, wherein the olefin structure other than the propylene structure contained in the separator substrate contains one or more selected from the group consisting of ethylene, 1-butene and 1-octene as repeating units.(8) The separator substrate comprises a propylene / olefin random copolymer, wherein the propylene / olefin random copolymer contains 60 mol% or more of propylene structures as repeating units, as described in any one of items 1 to 7, as a separator for energy storage devices. (9) The separator substrate comprises a c-axis crystal orientation of 0.915 or more and 0.940 or less, as described in any one of items 1 to 8, as a separator for energy storage devices. (10) The separator substrate comprises a c-axis oriented crystallite size of 17.0 nm or more and 19.5 nm or less, as described in any one of items 1 to 9, as a separator for energy storage devices. (11) The separator substrate comprises 50.0% by mass or more and 90.0% by mass or less of homopolypropylene, and 10.0% by mass or more and 50.0% by mass or less of thermoplastic resin other than homopolypropylene, based on the total mass of the separator substrate, as described in any one of items 1 to 10, as a separator for energy storage devices. (12) The separator substrate is a separator for energy storage devices according to any one of items 1 to 11, wherein the residue after extraction with boiling n-hexane for 6 hours is 90% by weight or more. (13) The separator for energy storage devices according to any one of items 1 to 12, wherein the temperature on the lower end of the range of half the crystal melting peak obtained from differential scanning calorimetry of the separator substrate at a heating rate of 10°C / min is 155.0°C or higher and 164.0°C or lower. (14) The separator substrate is a separator for energy storage devices according to any one of items 1 to 13, wherein the thickness of the separator substrate is 7.0 μm or more and 12.0 μm or less. (15) The separator substrate is a separator for energy storage devices according to any one of items 1 to 14, wherein the porosity of the separator substrate is 28% or more and 52% or less. (16) A separator for an energy storage device according to any one of items 1 to 15, wherein the TD thermal shrinkage rate of the separator substrate in one hour at a temperature of 105°C is 5% or less, and the MD thermal shrinkage rate in one hour at a temperature of 105°C is 20% or less. (17) A separator for an energy storage device according to any one of items 1 to 16, wherein the first puncture strength of the separator substrate is 250 gf or more when the thickness of the separator substrate is converted to 10 μm.(18) A storage device comprising a positive electrode, a negative electrode, and a separator for a storage device according to any one of items 1 to 17, disposed between the positive electrode and the negative electrode. (19) The storage device according to item 18, wherein the positive electrode contains lithium iron phosphate as a positive electrode active material.

[0012] Other aspects of the present disclosure are listed below. (20) A separator for an energy storage device comprising a separator substrate having a microporous layer mainly composed of polypropylene, wherein the microporous layer contains, with respect to the total resin of the separator substrate, an olefin structure other than the propylene structure as repeating units in an amount greater than 0.0 mol% and less than or equal to 8.0 mol%, the stem height calculated from the analysis of scanning electron microscope (SEM) images of the MD-ND cross section of the separator substrate being 500 nm or more and 1000 nm or less, and the stem width calculated from the analysis of scanning electron microscope (SEM) images of the MD-ND cross section of the separator substrate being 150 nm or more and 500 nm or less. (21) A separator for an energy storage device comprising a separator substrate mainly composed of polypropylene, wherein the c-axis crystal orientation of the separator substrate is 0.915 or more and 0.940 or less. (22) A separator for an energy storage device comprising a separator substrate mainly composed of polypropylene, wherein the c-axis oriented crystallite size of the separator substrate is 17.0 nm or more and 19.5 nm or less.

[0013] According to the present invention, it is possible to provide a separator for energy storage devices that combines excellent tear resistance (high MD tensile strength, high TD tensile strength, high TD tensile elongation, and first puncture strength), low air permeability, and high process suitability, and that can be made into a thin film.

[0014] Figure 1 is a diagram showing a portion of a scanning electron microscope (SEM) image of an 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 areas. The diagram is cropped to illustrate the detection points used for calculating stem height. Figure 2 is a diagram showing a portion of a scanning electron microscope (SEM) image of an 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 areas. The diagram is cropped to illustrate the detection points used for calculating stem width. This is a schematic diagram to explain the puncture test. This is a schematic diagram to explain the relationship between the needle and the separator in the puncture test. This is a graph showing the relationship between displacement and stress in the puncture test. Figure 6 is a schematic diagram explaining the melting peak and peak temperature obtained from differential scanning calorimetry (DSC) of the separator substrate, and the temperature at which the half value of the melting peak and the temperature at which the cumulative heat of fusion reaches 25% can be calculated from there.

[0015] In this specification, all measurements are performed according to the methods described in the Examples unless otherwise specified. In this specification, upper or lower limits in numerical ranges described in steps may be replaced with upper or lower limits in other numerical ranges described in steps, and further, with the corresponding values ​​described in the Examples. In this specification, "process" is included not only when it is an independent process, but also when it is not clearly distinguishable from other processes, as long as the function of the process is achieved.

[0016] 《Separator for Energy Storage Devices》 The separator for energy storage devices of this disclosure comprises a separator substrate having a microporous layer containing polypropylene. The separator substrate refers to a single separator substrate constituting the separator for energy storage devices, and may be used as a single layer or as a multilayer by laminating two or more layers. In the case of two layers, in addition to the microporous layer (A), a microporous layer (B) mainly composed of polypropylene may also be included. Furthermore, the separator substrate may have a coating layer (also called a "surface layer," "coating layer," etc.; hereinafter simply referred to as a "coating layer"). In this disclosure, "microporous layer" means each microporous layer constituting the separator substrate, "separator substrate" means the separator substrate excluding any coating layer, and "separator" means the entire separator including any coating layer.

[0017] <Materials for Separator Substrate> The separator substrate of this disclosure contains polypropylene. It is preferable that the separator substrate contains polypropylene as the main agent or main component. The lower limit of the polypropylene content in the separator substrate 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 separator substrate, from the viewpoint of wettability, thinning, and shutdown characteristics of the separator. The upper limit of the polypropylene content in the separator substrate 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 separator substrate, from the viewpoint of maintaining good battery performance even after storage at high temperatures (e.g., 130°C).

[0018] The main component of the separator substrate is preferably homopolypropylene, which is a homopolymer of propylene or a copolymer with a small amount of ethylene that does not impair its crystallinity. Polypropylene can be used alone or in mixtures of two or more types.

[0019] The weight-average molecular weight (Mw) of the homopolypropylene separator substrate is preferably 300,000 or more from the viewpoint of high tear resistance of the microporous layer, and preferably 1,300,000 or less from the viewpoint of ensuring good film formation, productivity, thin film formation, and low air permeability. The Mw of the homopolypropylene 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, even 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.

[0020] The upper limit of the value (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) of the homopolypropylene in the separator base material by the number average molecular weight (Mn) is preferably 20 or less, more preferably 18 or less, 16 or less, 14 or less, or 12 or less. By setting Mw / Mn of the homopolypropylene to 20 or less, there is a tendency that good film-forming properties, productivity, and thinning can be ensured. Further, the lower limit of Mw / Mn of the homopolypropylene is preferably 3 or more, more preferably 4 or more, 4.5 or more, or 5.0 or more. By setting Mw / Mn of the homopolypropylene to 3 or more, appropriate molecular entanglement is maintained and good film-forming stability is obtained. The weight average molecular weight, number average molecular weight, and Mw / Mn of the polypropylene of the present disclosure are molecular weights in terms of polystyrene obtained by GPC (gel permeation chromatography) measurement.

[0021] The density of the homopolypropylene in the separator base material is preferably 0.85 g / cm ３ or more, for example 0.88 g / cm ３ or more, 0.89 g / cm ３ or more, or 0.90 g / cm ３ or more. The density of the homopolypropylene is preferably 1.1 g / cm ３ or less, for example 1.0 g / cm ３ or less, 0.98 g / cm ３ or less, 0.97 g / cm ３ or less, 0.96 g / cm ３ or less, 0.95 g / cm ３ or less, 0.94 g / cm ３ or less, 0.93 g / cm ３ or less, or 0.92 g / cm ３ or less. The density of the homopolypropylene is related to the crystallinity of the homopolypropylene, and by setting the density of the homopolypropylene to 0.85 g / cm ３ or more, the productivity of the microporous layer is improved, which is particularly advantageous in the dry process.

[0022] The lower limit of the pentad fraction of homopolypropylene in the separator substrate is preferably 94.0% or higher, for example, 95.0% or higher, 96.0% or higher, 96.5% or higher, 97.0% or higher, 97.5% or higher, 98.0% or higher, 98.5% or higher, or 99.0% or higher, from the viewpoint of obtaining a microporous layer with low air permeability. The upper limit of the pentad fraction of homopolypropylene is not limited, but may be 99.9% or lower, 99.8% or lower, or 99.5% or lower. The pentad fraction of homopolypropylene is １３ It is measured using C-NMR (nuclear magnetic resonance).

[0023] A pentad fraction of 94.0% or higher in homopolypropylene indicates high crystallinity of the polypropylene. Separators obtained by the stretch-opening method, especially the dry method, open by stretching the amorphous portions between crystalline particles. Therefore, high crystallinity of the polypropylene results in good porosity and allows for low air permeability, enabling higher input / output capabilities for batteries.

[0024] Melt tension Mt of homopolypropylene separator substrate at 240°C ＡPP (Single layer Mt ＡPP As for the upper limit of ), from the viewpoint of obtaining a microporous layer (A) with good film formation properties, productivity, thinning, and low air permeability, it is preferably 35 mN or less, more preferably 32 mN or less, even more preferably 30 mN or less, even more preferably 28 mN or less, and particularly preferably 26 mN or less. The melt tension Mt of homopolypropylene in the microporous layer (A) ＡPP (Single layer Mt ＡPP As for the lower limit of ), from the viewpoint of obtaining a microporous layer (A) with higher tear resistance, it is preferably 10 mN or more, more preferably 13 mN or more, even more preferably 16 mN or more, even more preferably 18 mN or more, and particularly preferably 19 mN or more.

[0025] The lower limit of the homopolypropylene content in the separator substrate is preferably 50.0% by mass or more, more preferably 55.0% by mass or more, even more preferably 60% by mass or more, and even more preferably 65% ​​by mass or more, based on the total mass of the separator substrate, from the viewpoint of ensuring the porosity necessary for high MD tensile strength and low air permeability. The upper limit of the homopolypropylene content in the separator substrate is preferably 99% by mass or less, more preferably 95% by mass or less, even more preferably 90.0% by mass or less, even more preferably 85% by mass or less, particularly preferably 80% by mass or less, and most preferably 75% by mass or less, based on the total mass of the separator substrate, from the viewpoint of obtaining good battery performance. This allows for an appropriate orientation of the polypropylene crystals, neither too high nor too low, and enables optimal design of the stem height and stem width after stretching, making it easier to obtain a separator with excellent permeability and strength. Furthermore, good battery performance can be maintained even after storage at high temperatures (e.g., 130°C).

[0026] The separator substrate of this disclosure preferably contains 0.0 mol% to 8.0 mol% of olefin structures other than propylene structures as repeating units, based on the total resin, and more preferably contains more than 0.0 mol% and 8.0 mol% or less.

[0027] In this disclosure, in order to obtain a separator substrate that is highly resistant to tearing and has low air permeability, possessing high MD tensile strength, high TD tensile strength, and high TD tensile elongation, it is important to give the polypropylene crystals, which are the main component, an appropriate orientation that is neither too high nor too low, when manufacturing the separator substrate by uniaxial stretching in the film formation direction (MD). When the polypropylene crystals are highly oriented, the porosity in the stretching process improves, resulting in low air permeability, and tie molecules are efficiently drawn out in the MD direction, forming strong fibrils and thus obtaining high MD tensile strength. However, if the orientation is too high, the tie molecules are localized in the MD direction, and tie molecules that connect the crystals in the TD direction are not secured, resulting in a separator that is prone to tearing, with reduced TD tensile strength and TD tensile elongation. Therefore, by controlling the orientation of the polypyrropylene crystals to an appropriate degree, tie molecules that can be drawn out in response to TD tensile fracture are secured, thereby improving TD tensile strength and TD tensile elongation, and obtaining a separator with high tear resistance. If the material has excellent tear resistance, when stress is applied to the separator substrate through a puncture test or similar method, the first fracture depth increases, and the first fracture puncture strength tends to improve.

[0028] The lower limit of the amount of olefin structures other than propylene structures included as repeating units in the separator substrate is more preferably 0.2 mol% or more, even more preferably 0.4 mol% or more, even more preferably 0.6 mol% or more, even more preferably 0.8 mol% or more, particularly preferably 1.0 mol% or more, and most preferably 1.5 mol% or more, based on the total resin of the separator substrate. If the amount of olefin structures other than propylene structures is 0.2 mol% or more, components other than propylene are more likely to be present in the amorphous region of the main component polypropylene, and the orientation of the amorphous region can be reduced. As a result, the number of tie molecules connecting the TD crystals increases, so high TD tensile strength and high TD tensile elongation can be achieved, and a separator with good tear resistance can be obtained. On the other hand, the upper limit of the olefin structure other than the propylene structure included as a repeating unit in the separator substrate is preferably 6.0 mol% or less, more preferably 5.0 mol% or less, even more preferably 4.0 mol% or less, even more preferably 3.0 mol% or less, and particularly preferably 2.0 mol% or less, based on the total resin of the separator substrate. If the olefin structure other than the propylene structure is 8.0 mol% or less, porosity and MD fibril formation are ensured without excessively reducing the crystal orientation of the main component, polypropylene, resulting in a separator with low air permeability and high MD tensile strength. In addition, when using a circular die, which has high production efficiency in separator manufacturing, adhesion of the two layers due to components other than propylene can be avoided during folding after bubble extrusion, thus avoiding film breakage problems in the subsequent peeling process and achieving high process suitability. From the same viewpoint as above, it is preferable that the olefin structure other than the propylene structure includes one or more selected from the group consisting of ethylene, 1-butene, and 1-octene as a repeating unit.

[0029] The separator substrate of this disclosure preferably contains additives other than homopolypropylene. Preferred additives are thermoplastic resins other than homopolypropylene, and more preferably thermoplastic resins with high compatibility with homopolypropylene. Examples of thermoplastic resins include polyolefins and copolymers of polyolefins. The copolymer of polyolefin may be a copolymer obtained by copolymerizing a small amount of comonomer other than propylene, such as an α-olefin comonomer, for example, a random or block polymer. The amount of propylene structure contained as a repeating unit in the polyolefin is not limited, but may be, for example, 60 mol% or more, 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 polyolefin is not limited, but may be, for example, 40% or less, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. Polyolefins can be used individually or in mixtures of two or more types. Polyolefins are polymers that contain monomers having carbon-carbon double bonds as repeating units. Examples of monomers constituting polyolefins are not limited to those having carbon-carbon double bonds and 2 to 10 carbon atoms (C2 to C10), such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Examples of polyolefins include low-crystallinity polypropylene having a low-steric-regularity region. Monomers constituting polyolefin copolymers may be used individually or as a mixture of two or more. These copolymers may be random copolymers or block copolymers, and are preferably random copolymers. From the same viewpoint as above, the separator substrate preferably contains a propylene / olefin random copolymer, and the propylene / olefin random copolymer preferably contains 60 mol% or more of the propylene structure as a repeating unit.

[0030] As the thermoplastic resin included in the separator substrate of this disclosure, a random copolymer of random polypropylene composed of polypropylene and polyethylene is preferred, from the viewpoint of obtaining a microporous layer that combines excellent tear resistance (high MD tensile strength, high TD tensile strength, and high TD tensile elongation) with low air permeability and high process suitability, while also enabling thin film formation. The randomness makes it easier for amorphous regions to exist, allowing for lower orientation of the amorphous regions, which is preferable for suppressing tearing.

[0031] These thermoplastic resins have the advantage of being highly compatible with polypropylene and having low crystallinity, which makes it easier to mitigate the orientation of the amorphous region. In other words, these copolymer components are high molecular weight components that are less likely to be oriented to the MD of the amorphous region of the separator substrate.

[0032] As described above, the separator substrate preferably contains additives other than homopolypropylene, and thermoplastic resins other than homopolypropylene are preferred as additives. The lower limit of the content of additives other than homopolypropylene is preferably 10.0% by mass or more, more preferably 15.0% by mass or more, and still preferably 20.0% by mass or more, based on the total mass of the separator substrate, from the viewpoint of film formation, thin film formation, low air permeability and excellent tear resistance. 25.0% by mass or more is even more preferred. The upper limit of the content of additives other than homopolypropylene is preferably 50.0% by mass or less, more preferably 45.0% by mass or less, even more preferably 40.0% by mass or less, and still more preferably 35.0% by mass or less, based on the total mass of the separator substrate, from the viewpoint of maintaining porosity and avoiding adhesion problems of the film due to leaching of components other than propylene, and high process suitability.

[0033] <Melt Flow Rate (MFR) of Separator Substrate> The melt flow rate (MFR) of the separator substrate of this disclosure is preferably 1.0 g / 10 min or less. From the viewpoint of obtaining a microporous layer (A) with better tear resistance, the upper limit of the melt flow rate (MFR) of the separator substrate (single layer MFR) is preferably 0.90 g / 10 min or less, more preferably 0.8 g / 10 min or less, even more preferably 0.7 g / 10 min or less, and even more preferably 0.6 g / 10 min or less. The lower limit of the MFR (MFR of a single layer) of the separator substrate is not limited, but from the viewpoint of obtaining a microporous layer (A) with lower air permeability, film formation properties, and thinning capabilities, it may be, for example, 0.20 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. The MFR of the separator substrate is measured under conditions of a load of 2.16 kg and a temperature of 230 °C. An MFR of 1.0 g / 10 min or less of the separator substrate means that the molecular weight of the polyolefin contained in the separator substrate is relatively high. A high molecular weight of polyolefin results in more tie molecules that bind crystalline particles together, which tends to yield a microporous layer (A) with excellent tear resistance. Furthermore, by having an MFR of 0.2 g / 10 min or higher for the separator substrate, the melt tension of the microporous layer (A) does not become too high, making it possible to ensure good film formation, thinning, and productivity.

[0034] The MFR of the polypropylene separator substrate is preferably 0.2 to 0.9 g / 10 min when measured under a load of 2.16 kg and a temperature of 230°C, from the viewpoint of obtaining excellent tear resistance, low air permeability, and a thin microporous layer (A). The upper limit of the polypropylene MFR 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, from the viewpoint of obtaining a microporous layer (A) with even better tear resistance. The lower limit of the MFR of polypropylene is not limited, but from the viewpoint of obtaining a microporous layer (A) with lower air permeability, film-forming properties, and thinning capabilities, 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.

[0035] When a thermoplastic resin is included as an additive other than the main component of the separator substrate, the MFR of the thermoplastic resin is preferably 0.1 to 100.0 g / 10 min when measured under a load of 2.16 kg and a temperature of 230°C, from the viewpoint of obtaining a microporous layer (A) with excellent tear resistance, low air permeability, thin film, and good film formation stability. The upper limit of the MFR of the thermoplastic resin 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 obtaining a microporous layer (A) that is uniformly kneaded with polyolefin and has high tear resistance and good film-forming stability. The lower limit of the MFR of the thermoplastic resin is not limited, but from the viewpoint of obtaining a microporous layer (A) with lower air permeability, film-forming properties, and thinning capabilities, 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.

[0036] <Mw and Mw / Mn of the separator substrate> The weight-average molecular weight (Mw) of the separator substrate is preferably 250,000 or more from the viewpoint of obtaining a microporous layer (A) with superior tear resistance, and preferably 1,500,000 or less from the viewpoint of ensuring good film formation, productivity, thinning, and 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, even 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.

[0037] The upper limit of the value obtained by dividing the weight-average molecular weight (Mw) of the separator substrate by the number-average molecular weight (Mn) (Mw / Mn) is preferably 30 or less, and 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 separator substrate to 30 or less tends to ensure good film formation, productivity, and thin film formation. The lower limit of the Mw / Mn of the separator substrate 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 separator substrate to 3 or more maintains appropriate molecular entanglement and provides good film formation stability. Note that the weight-average molecular weight, number-average molecular weight, and Mw / Mn of the separator substrate in this disclosure are polystyrene-convertible molecular weights obtained by GPC (gel permeation chromatography) measurement. Furthermore, if the separator substrate of this disclosure is mainly composed of polypropylene and also contains additives such as thermoplastic resin, the Mw and Mw / Mn values ​​of the separator substrate described above will reflect the influence of the combination of these constituent materials.

[0038] <Melting Tension of Separator Substrate> Melting tension of separator substrate at 240°C Mt Ａ (Single layer Mt ＡAs for the upper limit of ), from the viewpoint of obtaining a microporous layer (A) with good film formation properties, productivity, thinning, and low air permeability, it is preferably 35 mN or less, more preferably 32 mN or less, even more preferably 30 mN or less, even more preferably 28 mN or less, and particularly preferably 26 mN or less. Melt tension Mt of separator substrate Ａ (Single layer Mt Ａ As for the lower limit of ), from the viewpoint of obtaining a microporous layer (A) with higher tear resistance, it is preferably 10 mN or more, more preferably 13 mN or more, even more preferably 16 mN or more, even more preferably 18 mN or more, and particularly preferably 19 mN or more.

[0039] <C3 / C2 Molar Unit Ratio of Separator Substrate> The molar unit ratio of carbon-3 olefin (C3) to carbon-2 olefin (C2) of the separator substrate is preferably 10 to 300, more preferably 20 to 250, even more preferably 40 to 200, significantly preferably 50 to 150, and particularly preferably 60 to 120, based on the total amount of olefin units. A molar unit ratio of 10 or more results in a resin composition with high compatibility with polypropylene, which is preferable because it prevents adhesion between the two layers during folding after bubble extrusion when using a circular die, which has high production efficiency in separator manufacturing, thus preventing film breakage problems in the subsequent peeling process. A molar unit ratio of 300 or less makes it easier for components other than polypropylene to be present in the amorphous region, which reduces the orientation of the amorphous region, thus improving tear resistance and making it easier to increase the first puncture strength.

[0040] <Area-average elongated pore diameter of separator substrate> The area-average elongated pore diameter (hereinafter also simply referred to as "area-average elongated pore diameter") of the separator substrate in the MD-ND cross-section is preferably 50 nm or more and 300 nm or less. In this disclosure, "ND" indicates the thickness direction of the substrate or microporous layer, and "MD" indicates the film formation direction of the substrate or microporous layer. For example, the MD of a separator substrate having a microporous layer is the longitudinal direction if it is a roll. "Elongated pore diameter" means the pore diameter of the MD. Furthermore, if there are two or more microporous layers (A) and / or microporous layers (B), the area-average elongated pore diameters of microporous layer (A) and microporous layer (B) are compared based on the average area-average elongated pore diameter value of each layer. The lower limit of the area-average elongated pore diameter of the separator substrate is preferably 50 nm or more, more preferably 80 nm or more, even more preferably 100 nm or more, even more preferably 120 nm or more, and particularly preferably 130 nm or more, from the viewpoint of ensuring high input / output in the energy storage device and obtaining a substrate or microporous layer (A) with low air permeability. The upper limit of the area-average elongated pore diameter of the separator substrate is preferably 300 nm or less, more preferably 250 nm or less, even more preferably 200 nm or less, and even more preferably 180 nm or less, from the viewpoint of obtaining a substrate or microporous layer (A) with high tear resistance.

[0041] The area-average pore diameter can be measured by performing cross-sectional SEM observation of the MD-ND cross-section of the separator and analyzing the resulting image. Detailed conditions are shown in the examples. When measuring the average pore diameter from the cross-sectional SEM image, both the number-average pore diameter and the area-average pore diameter can be calculated, but in this disclosure, the area-average pore diameter is used as the average pore diameter to obtain a better correlation with the physical properties of the separator. In the case of a multilayer structure, the average value for each layer is calculated and divided by the layer ratio to obtain the average pore diameter for the entire separator substrate.

[0042] <Thickness of Separator Substrate> When the separator substrate of a separator for an energy storage device is composed of a single-layer structure having only one microporous layer, the upper limit of the thickness of the separator substrate is preferably 12.0 μm or less, for example, 11.5 μm or less, 11.0 μm or less, 10.5 μm or less, or 10.0 μm or less, from the viewpoint of increasing the energy density of the energy storage device and reducing the air permeability of the microporous layer. The lower limit of the thickness of the separator substrate when composed of a single-layer structure is preferably 7.0 μm or more, 7.5 μm or more, 8.0 μm or more, 8.5 μm or more, or 9.0 μm or more, from the viewpoint of obtaining a separator substrate with high puncture strength.

[0043] <Low Half-Maximum Temperature of Melting Peak for Separator Substrate> In this disclosure, the low half-maximum temperature of the melting peak obtained from differential scanning calorimetry (DSC) refers to the lower temperature among the temperatures at which the half-maximum of the melting peak occurs. When polypropylene is the main component, the low half-maximum temperature of the melting peak correlates with the degree of orientation of the crystalline and amorphous parts. The lower limit of the low half-maximum temperature of the melting peak of the separator substrate is preferably 155.0°C or higher, more preferably 155.5°C or higher, even more preferably 156.0°C or higher, and most preferably 156.5°C or higher. By designing the low half-maximum temperature of the melting peak to be 155.0°C or higher, the degree of crystalline orientation does not become too low, and low air permeability can be obtained. The upper limit of the low half-maximum temperature of the melting peak is preferably 164.0°C or lower, more preferably 163.5°C or lower, even more preferably 163.0°C or lower, and most preferably 162.5°C or lower. By setting the melting peak half-maximum low temperature to 164.0°C or lower, the degree of orientation of the crystalline and amorphous regions can be designed to be low. It is thought that lowering the degree of orientation of the amorphous regions increases the number of tie molecules connecting the crystals of the TD, which is presumed to improve the TD strength, suppress cracking, and improve the first fracture puncture strength. The heating rate of the DSC can be set to, for example, 10°C / min.

[0044] <Additives for Separator Substrate> The separator substrate, which has polypropylene as its main component, may further contain additives such as elastomers, nucleating agents, antioxidants, and fillers as needed, in addition to polypropylene. The amount of additives is not particularly limited, but may be, for example, 0.01% by mass or more, 0.1% by mass or more, or 1% by mass or more, 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).

[0045] <Relationship between microporous layer (A) and microporous layer (B)> When the separator substrate has a two-layer configuration or a configuration with more than two layers, the MFR of microporous layer (B) Ｂ ) and microporous layer (A) MFR (MFR Ａ ) ratio, MFR Ｂ / MFR Ａ The MFR is preferably between 1.02 and 10.0. Ｂ / MFR Ａ By setting this to 1.02 or higher, the pore size of the microporous layer (A) of the resulting 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 dielectric strength and air permeability. MFR Ｂ / MFR Ａ By setting the value to 10.0 or less, it becomes possible to obtain a separator with stable film formation properties, productivity, and good porosity and air permeability. Ｂ / MFR Ａ More preferably, it is 1.05 to 6.0, even more preferably 1.1 to 5.0, even more preferably 1.1 to 4.0, and particularly preferably 1.1 to 3.0.

[0046] Weight-average molecular weight Mw of polypropylene in the microporous layer (A) Ａ and the weight-average molecular weight Mw of the polypropylene in the microporous layer (B) Ｂ The ratio of Mw Ａ / Mw Ｂ It is preferably 1.02 or more and 2.0 or less. Mw Ａ / Mw Ｂ By setting this to 1.02 or higher, the melt tension ratio Mt of the microporous layer (A) and the microporous layer (B) Ａ / Mt ＢThis makes it possible to control the dendrite suppression effect in energy storage devices effectively. Ａ / Mt Ｂ By setting the ratio to 2.0 or less, it becomes possible to obtain a separator with stable film formation properties, productivity, and good porosity and air permeability. Ａ / Mw Ｂ Preferably, it is 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.

[0047] <Layer Structure of Separator Substrate> The substrate for a separator for energy storage devices (hereinafter also simply referred to as "separator substrate") has at least one microporous layer (A), or at least one microporous layer (A) and at least one microporous layer (B). The separator substrate may also have a multilayer structure of three or more layers, having at least two layers of microporous layer (A) and / or microporous layer (B). For example, a two-layer structure of microporous layer (A) / microporous layer (B), a three-layer structure of microporous layer (A) / microporous layer (B) / microporous layer (A), etc. The separator substrate may also have layers other than microporous layer (A) and microporous layer (B). For example, layers other than microporous layer (A) and microporous layer (B) include, for instance, a microporous layer mainly composed of polyolefins other than (A) and (B), a layer containing inorganic materials, and a layer containing heat-resistant resin. For example, the separator substrate may have a multilayer structure of four or more layers, such as microporous layer (A) / microporous layer (B) / microporous layer (C) / microporous layer (A). A symmetrical laminated structure is preferred from the viewpoint of ease of manufacturing and suppression of separator curling.

[0048] <Stem height of separator substrate> The stem height of the separator substrate in the MD-ND cross-section of the separator substrate of this disclosure is 500 nm or more and 1000 nm or less. The stem height is correlated with the curvature of the pores and is a numerical value that indicates a pore structure different from the pore diameter. When the stem height is high, the curvature of the pores decreases, and when the stem height is low, the curvature of the pores increases, and good dielectric strength can be obtained. 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, particularly preferably 850 nm or less, and most preferably 820 nm or less, from the viewpoint of obtaining good dielectric strength, high MD tensile strength and high puncture strength. The lower limit of the stem height is preferably 520 nm or more, more preferably 540 nm or more, even more preferably 560 nm or more, particularly preferably 580 nm or more, and most preferably 600 nm or more, from the viewpoint of obtaining good air permeability, high TD tensile strength and high TD tensile elongation.

[0049] The stem height can be calculated by performing cross-sectional SEM observation of the MD-ND cross-section of the separator substrate, removing the fibrils by image analysis, and measuring the length of the remaining ND lamellae by image analysis. Detailed conditions are shown in the examples. When measuring the stem height from the cross-sectional SEM image, the number-average stem height and the length-average stem height can be calculated, but in this specification, the length-average stem height is used as the stem height to obtain a better correlation with the physical properties of the separator.

[0050] <Stem width of separator substrate> The stem width of the separator substrate in the MD-ND cross-section of the separator substrate of this disclosure is 150 nm or more and 500 nm or less. The lower limit of the stem width is preferably 170 nm or more, more preferably 190 nm or more, even more preferably 205 nm or more, even more preferably 220 nm or more, even more preferably 230 nm or more, particularly preferably 240 nm or more, and most preferably 250 nm or more. The upper limit of the stem width is preferably 450 nm or less, more preferably 400 nm or less, even more preferably 380 nm or less, even more preferably 360 nm or less, particularly preferably 340 nm or less, and most preferably 320 nm or less. The stem width correlates with the toughness of the polymer matrix. If the stem width is 150 nm or more, sufficient tie molecular weights are secured to connect the TD crystals inside or on the surface of the polymer matrix and / or between the polymer matrix, making it possible to resist the stress applied to the polymer matrix when the TD breaks, resulting in high TD tensile strength and high TD tensile elongation, and excellent tear resistance, thus obtaining high first fracture puncture strength. Furthermore, if the stem width is 500 nm or less, porosity is maintained, low air permeability is obtained, and strong MD fibrils are secured, resulting in a separator with high MD tensile strength. In the case of a separator substrate of the present invention, for example, a separator substrate having a laminated structure including three microporous layers, the stem width of each layer is determined by dividing it into layers in the thickness direction (divided by the number of layers), and the separator substrate of the present invention is defined as having a stem width of 150 nm or more and 500 nm or less for all layers. Furthermore, the lower limit of the trunk width is preferably 170 nm or more, more preferably 190 nm or more, even more preferably 205 nm or more, even more preferably 220 nm or more, even more preferably 230 nm or more, most preferably 240 nm or more, and most preferably 250 nm or more. The upper limit of the trunk width is preferably 450 nm or less, more preferably 400 nm or less, even more preferably 380 nm or less, even more preferably 360 nm or less, especially preferably 340 nm or less, and most preferably 320 nm or less.

[0051] The stem width can be calculated by performing a cross-sectional SEM observation of the MD-ND cross-section of the separator substrate, removing the fibrils by image analysis, and measuring the length of the remaining MD lamellae by image analysis, similar to the stem height described above. Detailed conditions are shown in the examples. When measuring the stem width from the cross-sectional SEM image, the number-average stem width and the length-average stem width can be calculated, but in this specification, the length-average stem width is used as the stem width to obtain a better correlation with the physical properties of the separator.

[0052] <Thickness of Separator Substrate> The upper limit of the thickness of the separator substrate is preferably 12.0 μm or less, for example, 11.5 μm or less, 11.0 μm or less, 10.5 μm or less, or 10.0 μm or less, from the viewpoint of increasing the energy density of the energy storage device and reducing the air permeability of the substrate or microporous layer. The lower limit of the thickness of the separator substrate is preferably 7.0 μm or more, 7.5 μm or more, 8.0 μm or more, 8.5 μm or more, or 9.0 μm or more, from the viewpoint of obtaining a substrate or microporous layer with high puncture strength.

[0053] <Air permeability (air resistance) of separator substrate> From the viewpoint of ensuring good output in the energy storage device, the upper limit of the air permeability of the separator substrate is preferably 400 seconds / 100 cm. ３ More preferably, 350 seconds / 100 cm ３ More preferably, 300 seconds / 100 cm ３ More preferably, 250 seconds / 100 cm ３ The following applies: The lower limit of the air permeability of the separator substrate is not limited, but for example, 50 seconds / 100 cm ３ Above, 100 seconds / 100cm ３ Above, or 150 seconds / 100 cm ３ Above, or 200 seconds / 100 cm ３ That's all.

[0054] <Porosity of Separator Substrate> The porosity of the separator substrate is preferably 28% or more from the viewpoint of avoiding clogging in the energy storage device and obtaining good air permeability of the separator, and preferably 52% or less from the viewpoint of obtaining a substrate with high puncture strength or a microporous layer. The porosity of the separator substrate is more preferably 32% to 48%, and even more preferably 36% to 44%.

[0055] <Puncture Strength of Separator Substrate> The lower limit of the puncture strength (first breaking puncture strength or maximum breaking puncture strength) of the separator substrate 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, when the thickness of the separator substrate is converted to 10 μm, and particularly preferably 250 gf or more, 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 when the thickness of the separator substrate is converted to 10 μm, it is preferably 500 gf or less, for example, 450 gf or less, or 400 gf or less.

[0056] <Thermal Shrinkage Rate of Separator Substrate> The thermal shrinkage rate of the separator substrate in the width direction (TD) after heat treatment at 105°C for 1 hour is preferably 5% or less, and more preferably -1.0% to 3.0%. In other words, a separator substrate with a TD thermal shrinkage rate of 5% or less at 105°C means that the thermal shrinkage in the TD is very small even at high temperatures. By having a thermal shrinkage rate of 5% or less or 3.0% or less, short circuits at high temperatures can be effectively suppressed. The reason why the thermal shrinkage rate should be -1.0% or more is that when measuring the thermal shrinkage rate, the substrate expands in the TD, and the thermal shrinkage rate may become a negative value less than 0%. The thermal shrinkage rate may be 0% or more, or greater than 0%. As a method for manufacturing a separator substrate with a thermal shrinkage rate of 5% or less, or -1.0% to 3.0%, for example, a method for manufacturing a separator by uniaxial stretching of the medium-distance (MD) is given, and preferably a method for manufacturing by a dry uniaxial stretching method is given. In the manufacturing method of separators by biaxial stretching of MD and TD, as exemplified by wet separators, the thermal shrinkage of TD is generally very large, whereas in dry separators produced by uniaxial stretching, it is easier to obtain separator substrates with a thermal shrinkage rate of 5% or less, or between -1.0% and 3.0%.

[0057] The thermal shrinkage rate in the film formation direction (MD) after heat treatment at 105°C for 1 hour of the separator substrate is 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, from the viewpoint of productivity of energy storage devices and suppression of short circuits at high temperatures. The lower limit of the thermal shrinkage rate is not limited, but is preferably 0.1% or more, for example, 0.3% or more, or 0.5% or more.

[0058] <Tensile strength, tensile elongation, and breaking elongation of the separator substrate> The tensile strength of the MD of the separator substrate is preferably 1500 kgf / cm² from the viewpoint of operability during battery winding and excellent tear resistance. ２ or more (approximately 14.7 kN / cm ２ (More than above), more preferably 1700 kgf / cm² ２ More preferably, 1800 kgf / cm² ２More preferably, 1900 kgf / cm² ２ In particular, 1960 kgf / cm² is preferred. ２ In summary, the most preferred load is 2000 kgf / cm². ２ That concludes the explanation. The upper limit of the tensile strength of the MD of the separator substrate is not limited, but is preferably 4000 kgf / cm². ２ For example, 3800 kgf / cm² ２ Below, 3500kgf / cm ２ Below, 3200kgf / cm ２ Below, 3000kgf / cm ２ Below, 2800kgf / cm ２ The following, or 2500 kgf / cm² ２ The following is acceptable:

[0059] The tensile elongation of the MD of the separator substrate is preferably 30% or more, more preferably 35% or more, and even more preferably 40% or more, from the viewpoint of productivity of energy storage devices and excellent tear resistance. The upper limit of the tensile elongation of the MD of the separator substrate is preferably 60% or less, more preferably 55% or less, even more preferably 50% or less, and even more preferably 45% or less, from the viewpoint of processability.

[0060] The TD breaking strength of the separator substrate is preferably 200 kgf / cm², from the viewpoint of productivity of energy storage devices and excellent tear resistance. ２ Above, a comfortable temperature of 220 kgf / cm² ２ More preferably 230 kgf / cm² ２ More preferably 240 kgf / cm² ２ In particular, 250 kgf / cm² is preferred. ２ That concludes the explanation. From the viewpoint of processability, the upper limit of the TD breaking strength of the separator substrate is preferably 350 kgf / cm. ２ More preferably, 330 kgf / cm² ２ More preferably, 310 kgf / cm² ２ More preferably, 290 kgf / cm² ２The following applies to the separator substrate of this disclosure. As described above, when manufacturing by uniaxial stretching of the microporous layer (A), if the crystal orientation of the microporous layer (A) is relaxed in the manufacturing process and composition design, the number of tie molecules between crystals of the TD can be increased, and the stress applied to the polymer matrix during tensile stress on the TD can be increased, thereby enabling the realization of high TD fracture strength.

[0061] The TD elongation at break of the separator substrate is preferably 800% or more, more preferably 850% or more, and even more preferably 900% or more, from the viewpoint of productivity of energy storage devices and excellent tear resistance. The upper limit of the MD tensile elongation of the separator substrate is preferably 1500% or less, more preferably 1300% or less, even more preferably 1100% or less, and even more preferably 1000% or less, from the viewpoint of processability. In the separator substrate of this disclosure, as described above, when manufacturing by uniaxial stretching of the MD, if the crystal orientation of the MD of the microporous layer (A) is relaxed in the manufacturing process and composition design, the number of tie molecules between crystals of the TD can be increased, and the stress on the polymer matrix during TD tension can be relaxed, making it possible to achieve a high TD elongation at break.

[0062] <Extraction Residue of Separator Substrate> The residue of the separator substrate after extraction with boiling n-hexane for 6 hours is preferably 90% by weight or more, more preferably 95% by weight or more, even more preferably 98% by weight or more, and particularly preferably 99.5% by weight or more. When the above residue is 90% by weight or more, even thin film substrates tend to have excellent tear resistance, low air permeability, high process suitability and low thermal shrinkage, and also have good peelability. Additives with an MFR of 5.0 or higher tend to be almost completely extracted under these conditions, and the residue can be determined from the MFR and amount of the additive.

[0063] The c-axis crystal orientation of the separator substrate of this disclosure is preferably 0.915 or more and 0.940 or less, with a lower limit of preferably 0.915 or more, more preferably 0.920 or more, even more preferably 0.923 or more, and particularly preferably 0.925 or more. The upper limit is preferably 0.940 or less, more preferably 0.937 or less, even more preferably 0.934 or less, particularly 0.932 or less, and most preferably 0.930 or less. When the c-axis crystal orientation of the separator substrate is 0.915 or more, orientational crystallization progresses, improving porosity in the stretching process and making it easier to achieve low air permeability. On the other hand, when the c-axis crystal orientation is 0.940 or less, when stress is applied to the separator substrate by puncture testing or the like, stress concentration on the MD is mitigated, suppressing cracking, increasing the first fracture elongation, and making it easier to improve the first fracture puncture strength. In particular, when the substrate thickness is 12 μm or less, the drawdown ratio (precursor thickness / extruded thickness) tends to increase due to the constraints of the lip clearance during extrusion, and the degree of c-axis crystal orientation tends to increase. As a result, separators obtained by extruding with a thickness of 12 μm or less become prone to tearing, making it difficult to achieve high first puncture strength. Therefore, it is necessary to achieve a more suitable degree of c-axis orientation through composition design including compatible polymers and process design of high-temperature extrusion.

[0064] While not limited to theory, the degree of c-axis crystal orientation of the separator substrate in this disclosure is important, and a decrease in the degree of c-axis crystal orientation tends to correlate with a decrease in the degree of amorphous orientation. This is thought to increase the number of tie molecules connecting the TD crystals, which is presumed to improve TD strength, suppress cracking, and improve the first fracture puncture strength. One fracture mode for puncture strength is cracking of the polymer matrix. It is thought that an increase in the tie molecular weight of TD inside or on the surface of the polymer matrix and / or between the polymer matrix makes it possible to resist the stress applied to the polymer matrix at the time of fracture, thus enabling the manifestation of high puncture strength.

[0065] From the same viewpoint as above, the c-axis oriented crystallite size of the separator substrate according to this disclosure is preferably 17.0 nm or more and 19.5 nm or less, more preferably 17.5 nm or more and 19.2 nm or less, even more preferably 18.0 nm or more and 19.0 nm or less, and particularly preferably 18.4 nm or more and 18.8 nm or less. When the crystallite size is 17.0 nm or more, the degree of crystal orientation tends to be low, and excessive pore formation is suppressed, so the porosity can be adjusted to a low level, making it easier to secure the basis weight of the separator substrate and presumably improving the first fracture puncture strength. When the crystallite size is 19.5 nm or less, the tie molecules present in the amorphous region become sufficiently long relative to the crystallite size, so the number of tie molecules connecting the TD crystals increases, as a result the TD strength improves, cracking is suppressed, and furthermore the formed pore diameter becomes smaller, resulting in a dense pore structure and presumably improving the first fracture puncture strength. Furthermore, when the crystallite size is 19.5 nm or less, the amount of microcrystals increases, which improves the connectivity of the pore structure and makes it easier to achieve low air permeability.

[0066] 《Method for Manufacturing Separators for Energy Storage Devices》 The method for manufacturing separators for energy storage devices includes a melt extrusion step to obtain a resin sheet (precursor sheet) by melt extruding a resin composition mainly composed of polypropylene (hereinafter also referred to as "polypropylene-based resin composition"), and a pore formation step to create pores in the obtained precursor sheet. Methods for manufacturing microporous layers are broadly classified into a dry method that does not use solvents in the pore formation step and a wet method that uses solvents.

[0067] Examples of dry methods include a method in which a polypropylene 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 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.

[0068] Wet methods include a method in which a polypropylene resin composition and a pore-forming agent are melt-kneaded together to form a film, which is then stretched as needed, and the pore-forming agent is extracted; and a method in which the polypropylene resin composition is dissolved, and then immersed in a poor solvent for polypropylene to solidify the polypropylene while simultaneously removing the solvent.

[0069] For melt-mixing polypropylene resin compositions, single-screw extruders and twin-screw extruders can be used. In addition to these, kneaders, laboplast mills, kneading rolls, and Banbury mixers can also be used.

[0070] The polypropylene resin composition may optionally contain resins other than polypropylene and additives, depending on the method for manufacturing the microporous layer or the physical properties of the desired microporous layer. Examples of additives include pore-forming agents, fluorine-based fluid modifiers, waxes, crystal nucleating agents, antioxidants, metal soaps such as aliphatic carboxylic acid metal salts, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments. Examples of pore-forming agents include plasticizers, inorganic fillers, or combinations thereof. In the method for manufacturing separators for energy storage devices, from the viewpoint of appropriately controlling the stem width based on raw materials, it is preferable to add a compatible olefin polymer other than polypropylene to polypropylene, and / or adjust the homopolypropylene content in the polypropylene resin composition to 50% to 80% by mass.

[0071] Examples of plasticizers include hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; and higher alcohols such as oleyl alcohol and stearyl alcohol.

[0072] 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, amethyst, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fibers.

[0073] As a method for manufacturing a separator substrate, a dry lamellar crystal opening process is preferred, in which the polypropylene crystal interface is peeled off by heat treatment and stretching. Here, as a method for manufacturing a separator substrate having a microporous layer (A) and a microporous layer (B), it is preferable to use at least one of the following methods (i) and (ii): (i) A method for manufacturing a separator substrate by co-extrusion, in which the microporous layer (A) and the microporous layer (B) are co-extruded and subjected to annealing, cold stretching, hot stretching, and thermal relaxation steps; and (ii) A method for manufacturing a separator substrate by lamination, in which the microporous layer (A) and the microporous layer (B) are extruded separately, bonded together by lamination, and then subjected to annealing, cold stretching, hot stretching, and thermal relaxation steps.

[0074] Of the above co-extrusion process (i) and lamination process (ii), the co-extrusion process (i) is preferred from the viewpoint of manufacturing cost, etc. In the co-extrusion process (i), the extrusion film formation conditions for the microporous layers (A) and (B) are as follows: the upper limit of thickness is preferably 16 μm or less, for example 14 μm or less, 13 μm or less, 12 μm or less, or 11 μm or less; the lower limit of thickness is preferably 6.0 μm or more, for example 7.0 μm or more, 8.0 μm or more, or 9.0 μm; the lower limit of preferred temperature is preferably 245°C or higher, 250°C or higher, or 255°C or higher; and the upper limit of preferred temperature is preferably 280°C or lower, 270°C or lower, or 265°C or lower. It is preferable to form the film at a high temperature of 245°C or higher as described above. By using high-temperature film-forming conditions, the orientation of polypropylene is relaxed (reduced), and the crystallite size tends to become smaller, resulting in a moderately good trunk width that is not too small, thus suppressing cracking and improving the first puncture strength. On the other hand, by using a film-forming temperature of 280°C or lower, resin decomposition can be suppressed, and the orientation does not become excessively low, porosity can be ensured, and a moderately good trunk width that is not too large can be obtained, resulting in low air permeability, high TD tensile strength and high TD tensile elongation, and excellent crack resistance.

[0075] In both the co-extrusion process (i) and the lamination process (ii) described above, the method for manufacturing the separator substrate may include an annealing step after extrusion film formation. By performing the annealing step, the crystal structure of the microporous layers (A) and (B) tends to grow, and the porosity tends to improve. By applying annealing at a specific temperature for a predetermined time, it tends to be possible to obtain good area-average pore diameter, high porosity, low air permeability, high puncture strength, and high tear resistance in both the microporous layers (A) and (B). This is thought to be because the crystals grow without disrupting the crystal structure, and high porosity is obtained. In the annealing step, the annealing treatment is preferably performed at a temperature range of 115°C to 160°C, more preferably 135°C to 150°C, for 20 minutes or more, more preferably 60 minutes or more. This ensures that the polypropylene crystals, the main component, are sufficiently oriented to create pores, resulting in appropriate porosity during subsequent stretching processes, good stem width and height, and a separator with low air permeability, high modulus of elasticity, high puncture strength, and high tear resistance. This is preferable from the viewpoint of achieving high input / output and high energy density in energy storage devices.

[0076] The method for manufacturing the separator substrate may include a stretching step after the annealing step. Either uniaxial stretching or biaxial stretching can be used for the stretching process. While not limited to this, uniaxial stretching is preferred from the viewpoint of manufacturing costs when using the dry method and reducing thermal shrinkage of the TD. The stretching ratio of the MD in cold stretching ((dimensions after stretching - dimensions before stretching) / dimensions before stretching × 100 (%)) is preferably in the range of 5% to 50%, more preferably 20% to 45%, and even more preferably 25% to 45%. This increases the amount of crack initiation during cold stretching, making it easier to obtain small pore diameters and crystallite sizes during hot stretching. A moderately good trunk height is obtained without being excessive, and furthermore, the smaller pore diameter makes it difficult for fracture to progress due to stress relaxation, thus enabling the production of a separator with high puncture strength and high tear resistance. By applying film formation conditions that make crystal orientation difficult to achieve with these cold stretching conditions, it is easier to obtain a smaller crystallite size. The cold drawing temperature is preferably 10°C to 50°C, more preferably 20°C to 30°C, and may be carried out at room temperature (23±2°C) from the viewpoint of manufacturing cost. Uniaxial drawing is preferred from the viewpoint of improving both the high tear resistance and low air permeability of the resulting separator substrate, as well as from the viewpoint of manufacturing cost and reduction of thermal shrinkage of TD.

[0077] To suppress thermal shrinkage of the separator substrate, a heat treatment step may be performed after the stretching step or the pore formation step for the purpose of thermal fixation. The heat treatment step may include a hot stretching operation performed at a predetermined temperature and stretching ratio for the purpose of adjusting the physical properties, and / or a thermal relaxation operation performed at a predetermined temperature and relaxation ratio for the purpose of reducing the shrinkage stress applied during film formation and stretching. The thermal relaxation operation may be performed after the hot stretching operation. In hot stretching and thermal relaxation, the MD dimension before stretching is set to 100%, and is preferably stretched to 140% to 280%, more preferably to 150% to 260%, and even more preferably to 160% to 240%. This forms a stretched chain and a strong fibril, thereby obtaining a separator with high puncture strength and high MD tensile strength. Furthermore, in the above heat treatment step, it tends to be possible to obtain a good stem height by performing the stretching operation at a stretching ratio above a certain level. The reason for this is thought to be that below a certain stretching ratio, pore formation takes precedence, and after the pore formation process, a structural change accompanied by a change in stem height occurs. In the thermal relaxation after hot stretching, the MD is preferably relaxed by 10% to 50%, more preferably by 20% to 45%. These heat treatment steps can be carried out using a tenter or a roll stretcher. The temperature of the heat treatment step is preferably 120°C to 160°C, and more preferably 130°C to 155°C. By heat treating at a moderately high temperature within a range that does not exceed the melting point of the main component polypropylene, sufficient fibrils are drawn out, a moderately good stem height that is not excessive is obtained, high MD tensile strength and high TD tensile strength are obtained, and a separator with excellent tear resistance is obtained.

[0078] The resulting separator substrate can be used as is as a separator for energy storage devices. Optionally, a further layer, such as a coating layer, may be provided on one or both sides of the separator substrate, and surface treatments such as corona treatment may be applied as needed.

[0079] Energy Storage Device The energy storage device of the present disclosure comprises a separator for energy storage devices of the present disclosure. The energy storage device of the present disclosure has a positive electrode and a negative electrode, and it is preferable that the separator for energy storage devices of the present disclosure is placed between the positive electrode and the negative electrode.

[0080] Examples of energy storage devices, though not limited to them, include 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, lithium secondary batteries, lithium-ion secondary batteries, or lithium-ion capacitors are preferred from the viewpoint of high energy density, low cost, and durability, and lithium-ion secondary batteries are more preferred.

[0081] An energy storage device can be manufactured, for example, by stacking a positive electrode and a negative electrode via the separator described above, winding them as necessary to form a laminated electrode body or a wound electrode body, loading this into an outer casing, connecting the positive and negative electrodes to the positive and negative terminals of the outer casing via lead members or the like, and then injecting a non-aqueous electrolyte containing a non-aqueous solvent such as a chain-like or cyclic carbonate and an electrolyte such as a lithium salt into the outer casing before sealing the outer casing.

[0082] The energy storage device is more preferably a lithium-ion secondary battery, and a preferred embodiment of the lithium-ion secondary battery is described below.

[0083] The positive electrode is not particularly limited as long as it acts as the positive electrode of a lithium-ion secondary battery, and known ones can be used. Preferably, the positive electrode contains one or more materials selected from the group consisting of materials capable of intercalating and releasing lithium ions as the positive electrode active material. From the viewpoint of battery capacity and safety, LiCoO2 is preferred as the positive electrode. ２ Lithium cobalt oxide, Li ２ Mn ２ O ４ Spinel-type lithium manganese oxides, such as Li ２ Mn １．５ Ni ０．５ O ４The spinel-type lithium nickel manganese oxide represented by ２ The lithium nickel oxide represented by ２ The lithium-containing composite metal oxide represented by LiMO (M represents two or more elements selected from the group consisting of Ni, Mn, Co, Al, and Mg), and the lithium iron phosphate compound represented by LiFePO ４ are exemplified. Among these, from the viewpoints of high safety and long-term stability, more preferably, the lithium cobalt oxide represented by LiCoO ２ The lithium nickel oxide represented by ２ The lithium nickel oxide represented by ２ The lithium-containing composite metal oxide represented by LiMO (M represents two or more elements selected from the group consisting of Ni, Mn, Co, Al, and Mg), and the lithium iron phosphate compound represented by LiFePO ４ are exemplified, and particularly preferably, the lithium iron phosphate compound represented by LiFePO ４ is the lithium iron phosphate compound represented by

[0084] The negative electrode is not particularly limited as long as it functions as the negative electrode of a lithium-ion secondary battery, and a known one may be used. It is preferable that the negative electrode contains one or more materials selected from the group consisting of a material capable of occluding and releasing lithium ions as a negative electrode active material and metallic lithium. That is, it is preferable that the negative electrode contains, as the negative electrode active material, one or more materials selected from the group consisting of metallic lithium, a carbon material, a material containing an element capable of forming an alloy with lithium, and a lithium-containing compound. Such materials include, in addition to metallic lithium, for example, carbon materials represented by hard carbon, soft carbon, artificial graphite, natural graphite, graphite, pyrolytic carbon, coke, vitreous carbon, a fired body of an organic polymer compound, mesocarbon microbeads, carbon fiber, activated carbon, graphite, carbon colloids, and carbon black.

[0085] Hereinafter, the measurement method and evaluation method adopted in this example will be described. In this example, since the "separator substrate" corresponds to the "separator", in the following description, the "separator substrate" may be read as the "separator".

[0086] 《Measurement and Evaluation Methods》 [Measurement of Melt Flow Rate (MFR)] The melt flow rate (MFR) of microporous layer (A) and microporous layer (B) was measured in accordance with JIS K 7210 under conditions of a temperature of 230°C and a load of 2.16 kg (unit is g / 10 min). The MFR of polypropylene and some additives was measured in accordance with JIS K 7210 under conditions of a temperature of 230°C and a load of 2.16 kg. The melt flow rate (MFR) of other additives was measured in accordance with JIS K 7210 under conditions of a temperature of 190°C and a load of 2.16 kg (shown in parentheses in Table 1).

[0087] [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. The sample polymer was also chromatographed under similar conditions, and based on the calibration curve, the weight-average molecular weight (Mw), number-average molecular weight (Mn), and MWD (Mw / Mn) (weight-average molecular weight (Mw) divided by the number-average molecular weight (Mn)) of the polymer in polystyrene equivalent were calculated under the following conditions. Column: TSKgel GMHHR-H(20) HT (7.8 mm I.D. × 30 cm) x 2 Mobile phase: 1,2,4-trichlorobenzene Detector: RI Column temperature: 160°C Sample concentration: 1 mg / ml Calibration curve: Polystyrene

[0088] [Measurement of Melt Tension] The melt tension (mN) of microporous layer (A) and microporous layer (B) was measured using a capillary tube manufactured by Toyo Seiki Seisakusho under the following conditions: • Capillary: 1.0 mm diameter, 20 mm length • Cylinder extrusion speed: 2 mm / min • Take-up speed: 60 m / min • Temperature: 240°C

[0089] [Measurement of Pentad Fraction] The pentad fraction of polypropylene was attributed based on the description in the Polymer Analysis Handbook (edited by the Japan Society for Analytical Chemistry). １３ The peak height was calculated from the C-NMR spectrum using the peak height method. １３The measurement of the 13C-NMR spectrum was carried out using a JEOL-ECZ500. Polypropylene pellets were dissolved in o-dichlorobenzene-d, and the measurement was performed at a measurement temperature of 145°C and an integration number of 25,000 times.

[0090] [Measurement of the molar unit ratio of C3 to C2] The molar unit ratio of C3 to C2 (C2 content of the copolymer consisting only of C3 and C2 units) can be determined from the integration intensity obtained by 13C-NMR measurement. The measurement of the 13C-NMR spectrum was carried out using an AVANCE NEO 600 (manufactured by Bruker). The sample was dissolved in o-dichlorobenzene-d, and the measurement was performed at a measurement temperature of 130°C and an integration number of 8,000 times. The sample concentration at this time was 10 wt%. Regarding the peak assignment method, CH ３ was set to 12 ppm - 23 ppm, and CH ２ +CH was set to 23 ppm - 54 ppm. Let the C2 fraction of the C3C2 copolymer be m (0 < m < 1). Then, the ratio of the number of CH ３ and CH, CH ２ is CH ３ :(CH ２ +CH)=1 - m:2. Therefore, from the following formula: m = 1 - 2×CH ３ / (CH ２ +CH), the molar unit ratio of C3 / C2 is (1 - m) / m.

[0091] [Measurement of thickness (μm)] Using a digital indicator IDC112 manufactured by Mitutoyo, the thickness (μm) of the separator substrate was measured at room temperature of 23 ± 2°C. The thickness of each microporous layer was calculated from the image data by cross-sectional SEM obtained by the evaluation method of the area-average pore size described below.

[0092] [Measurement of porosity (%)] A sample having a size of 10 cm × 10 cm square was cut from the separator or the microporous layer, and its volume (cm ３ ) and mass (g) were determined. From these and the density (g / cm ３ ), the porosity was calculated using the following formula. Porosity (%)=(Volume - Mass / Density) / Volume×100

[0093] [Air permeability (seconds / 100 cm ３Using a Gurley type air permeability meter conforming to JIS P-8117, the air permeability of the separator substrate (seconds / 100 cm) was measured. ３ The following was measured: Air permeability (seconds / 100cm). ３ Dividing this by the separator substrate thickness (μm) and multiplying by 10 μm gives the air permeability (seconds / 100 cm) when the separator substrate thickness is converted to 10 μm. ３ ) was sought.

[0094] [MD Heat Shrinkage Rate (%), TD Heat Shrinkage Rate (%)] Samples obtained by cutting separator substrate into 50 mm squares for both MD and TD were placed on copy paper and heat-treated in a hot air dryer (Yamato Scientific Co., Ltd., DF1032) at atmospheric pressure and 105°C for 1 hour. The samples were removed from the hot air dryer, allowed to cool at 25°C for 10 minutes, and then the dimensional shrinkage rate was determined. Heat shrinkage rate (%): (Dimensions before heating (mm) - Dimensions after heating (mm)) / (Dimensions before heating (mm)) × 100

[0095] [Puncture Strength and Puncture Depth] The measurement of puncture strength and puncture depth in this disclosure will be described below with reference to Figures 3 to 5. As illustrated in Figure 3, a needle (4) with a hemispherical tip with a radius of 0.5 mm was prepared, and a separator (3) was placed between two plates (5, 5) having an opening with a diameter (dia.) of 11 mm, and the needle (4), separator (3), and plates (5, 5) were set up. Using an MX2-50N manufactured by Imada Co., Ltd., a puncture test was performed under the conditions of a needle tip radius of curvature of 0.5 mm, an opening diameter of 11 mm for the separator holding plate, and a puncture speed of 25 mm / min. The needle (4) and the separator (3) were brought into contact, and the maximum puncture load (i.e., puncture strength (gf)) was measured as illustrated in Figure 5. The puncture strength (gf) was divided by the separator substrate thickness (μm) and multiplied by 10 μm to determine the puncture strength (gf) when the separator substrate thickness was converted to 10 μm. As shown in Figure 4, the load at which the separator (3) first ruptured longitudinally in the MD direction, either on one or both sides of the needle (4), and the load dropped sharply was defined as the first fracture strength. In Figure 3(c), the depth from when the needle (4) touched the separator (3) to the first fracture point was defined as the first fracture depth (D). The puncture was continued, and as illustrated in Figure 4(c), the load at which the separator (3) finally fractured completely in the TD direction was measured as the final fracture strength. As illustrated in Figures 4 and 5, the displacement (mm) of the needle from when the needle (4) touched the separator (3) until the final fracture was reached was measured as the final fracture depth.

[0096] [MD Tensile Strength, MD Tensile Elongation, TD Tensile Strength, TD Tensile Elongation] The tensile strength of the separator was measured using a tensile testing machine (TG-1kN type, manufactured by Minebea Co., Ltd.) with a sample length of 35 mm before testing, by pulling the sample at a speed of 100 mm / min. The tensile strength (kgf / cm²) was calculated by dividing the strength (tensile load value) at which the sample broke (fractured) by the cross-sectional area of ​​the test piece. ２ The tensile strength and tensile elongation were measured for the MD and TD sections of the separator.

[0097] [Area-average pore diameter (nm)] The area-average pore diameter was measured by image analysis from cross-sectional SEM observation. As a pretreatment, a separator was stained with ruthenium, and a cross-sectional sample was prepared by freeze-fracturing. The cross-section was an MD-ND plane. The above cross-sectional sample was fixed to the SEM sample stage for cross-sectional observation with a conductive adhesive (carbon-based), dried, and then, as a conductive treatment, an osmium coating was performed using an osmium coater (HPC-30W, manufactured by Vacuum Device Co., Ltd.) under the conditions of applied voltage adjustment knob setting 4.5 and discharge time 0.5 seconds, and the sample was prepared for microscopic observation. Next, using a scanning electron microscope (Hitachi High-Technologies S-4800), eight arbitrary points on each microporous layer cross-section of the microporous film were observed under the conditions of acceleration voltage 1kV, detection signal LA10, working distance 5mm, and magnification 30000x.

[0098] The observed images were cropped using the OpenCV image analysis library in a Python® programming environment to include only the cross-section of a single microporous layer. After removing the surface, exterior, and other microporous layers, the images were 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. In this process, the pore area that spanned both the imaging range and the area outside the imaging range was 0.001 nm. ２ The following holes were excluded from the measurement. The average diameter was calculated by area averaging from the area of ​​each hole.

[0099] [Stem height (nm), stem width (nm)] Stem height and stem width were measured by image analysis from cross-sectional SEM observation. Similar to the calculation of area-average pore diameter, after preparing cross-sectional samples and microscopic samples, three arbitrary points on the cross-section of the microporous membrane were observed under the conditions of acceleration voltage of 1 kV, detection signal LA10, working distance of 5 mm, and magnification of 5000x. For the stem width of each layer of the three-layer laminated structure, the center of each layer, which was divided into three sections in the thickness direction, was measured under the same conditions except that the magnification was 30,000x.

[0100] The observed images were trimmed using the OpenCV image analysis library in a Python programming environment to include only the cross-section of a single microporous layer. After removing the surface and external areas, fibril removal was performed by repeatedly blurring only the ND (non-distributed) area. Binarization was then performed using the Otsu method to separate the resin and pore areas. The fibril removal process was performed by repeating a Gaussian filter 100 times in a rod-shaped processing area of ​​3 pixels for ND and 1 pixel for MD (medium-distributed) areas. After binarization, noise was removed by sequentially performing opening and closing processes in an elliptical processing area of ​​7 pixels in the major axis for ND and 3 pixels in the minor axis for MD, obtaining a fibril-removed image.

[0101] The trunk height and trunk width were calculated from the fibril-removed image described above. For trunk height, the fibril-removed image was cropped using MD1 pixels, the length of all ND resin portions was detected, and this process was repeated until all MD portions were included, thereby detecting the length of the ND resin portions across the entire range of the fibril-removed image. For trunk width, the fibril-removed image was cropped using ND1 pixels, the length of all MD resin portions was detected, and this process was repeated until all ND portions were included, thereby detecting the length of the MD resin portions across the entire range of the fibril-removed image.

[0102] Figures 1 and 2 are schematic diagrams showing excisions of parts of fibril-removed images. In the figures, the light areas represent resin parts (1), and the dark areas represent pores (2). The double-headed arrows indicate an example of an area to be extracted and detected, with a trunk height of MD1 pixel and a trunk width of ND1 pixel. A weighted average was calculated for the obtained ND or MD resin part lengths, using the length of the ND or MD resin part as the weight, and the resulting values ​​were defined as trunk height and trunk width, respectively. If the length of the ND or MD resin part is L, the trunk height or trunk width H is given by the following formula: It is calculated as follows. By using a weighted average of n lengths L, the trunk height or trunk width H becomes a value that correlates more highly with puncture strength, crack resistance, and voltage resistance.

[0103] [Amount of olefin structure other than propylene (mol%)] The amount of olefin structure other than propylene included as a repeating unit is １３It can be determined from the integrated intensity obtained by C-NMR measurement. １３ ¹³C-NMR spectra were measured using a Bruker AVANCE NEO600, with the sample dissolved in o-dichlorobenzene-d, at a measurement temperature of 130°C, and with 8000 cumulative measurements. The sample concentration at this time was 10 wt%. For peak assignment, the method described in the Polymer Analysis Handbook (edited by the Japan Society for Analytical Chemistry) was used as a reference. For example, if the sample consists only of a 3-carbon olefin (C3) which is propylene, and a 2-carbon olefin (C2) other than propylene, then CH ３ 12-23 ppm, CH ２ +CH was set to 23-54 ppm. In the case of copolymers consisting only of C3 and C2, if the amount of olefin (C2) structure other than propylene (C3) is m mole%, then CH ３ and CH ２ The ratio of CH units is CH ３ : (CH ２ (+CH) = (1 - m / 100): 2, therefore, m = {1 - 2 × CH} ３ / (CH ２ The result is (+CH)) × 100.

[0104] [Peelability] Separator substrates were attached to a plastic film (Toray Industries PET film, Lumirror S10) using double-sided adhesive tape (Nichiban N

[0105] [DSC measurement]

[0106] (Measurement of Microporous Layer (A)) DSC measurements of the microporous layer (A) or separator substrate were performed 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: Nitrogen (flow rate 50 mL / min) • Temperature program: 1st step: Heat from room temperature to 230°C at 10°C / min, held for 5 min 2nd step: Cool down to 20°C at 10°C / min, held for 5 min 3rd step: Heat up to 200°C at 10°C / min, held for 5 min From the DSC curve (vertical axis: heat flow, horizontal axis: temperature) during the heating process in the 3rd step, the peak temperatures of endothermic peaks A and B were read using the analysis software TA-60 attached to the instrument, and the area S of endothermic peak A was calculated. Ａ and the area S of the endothermic peak B Ｂ Calculate S Ｂ / S Ａ They sought it.

[0107] (Measurement of Separator Substrate) DSC measurement of the separator substrate was performed 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: Nitrogen (flow rate 50 mL / min) • Temperature program: Step 1: Heat increased from 30°C to 230°C at a rate of 10°C / min, held for 5 min. Step 2: Cooled down to 30°C at a rate of 10°C / min, held for 5 min. Step 3: Heat increased to 230°C at a rate of 10°C / min, held for 5 min. A DSC curve (with heat flow on the vertical axis and temperature on the horizontal axis) was obtained from the heating process in Step 1.

[0108] [Melting Peak Half-Maximum Low Temperature, 25% Melting Heat Temperature] In the DSC measurement of the above microporous layer (A) or separator substrate, the melting peak half-maximum low temperature and the 25% melting heat temperature were calculated from the endothermic peak data from 100°C to 200°C in the DSC curve during the heating process of the first step. The baseline g(x) is defined as a line that satisfies the following two conditions. The DSC curve f(x) and baseline g(x) have two intersection points within the range of 100°C to 200°C, and always satisfy g(x) ≥ f(x) in the figure. The region enclosed by the DSC curve f(x) and baseline g(x) was defined as the endothermic peak. Figure 6 is a schematic diagram of the endothermic peak. The lower temperature (7) and the higher temperature (8) of the half-maximum temperature of the melting peak temperature (6) of the endothermic peak were calculated, respectively. Furthermore, the area of ​​the melting peak indicates the amount of heat absorbed during melting, and the temperature at which the cumulative amount of heat of fusion reaches 25% (9) was also calculated, starting from the low-temperature side.

[0109] [Measurement of crystallite size and crystallinity] [Measurement] Transmission-type WAXS measurements were performed using a Rigaku Nanopix-SP X-ray diffractometer. Cu-Kα rays were incident on the sample, and the diffracted light was detected by a HyPix-6000 (two-dimensional semiconductor detector). The sample was positioned so that the MD was in the vertical direction, and the X-rays were incident perpendicularly to the sample. The measurement was performed under conditions of a sample-detector distance of 96 mm, excitation voltage of 40 kV, and current of 30 mA. The optical system was collimated (1st slit 1.2 mmφ, guard slit 0.35 mmφ).

[0110] Measurement of crystallite size and crystallinity [Data processing] 1. When the sector-averaged structure is oriented, the X-ray scattering pattern becomes anisotropic. Therefore, instead of using a circular average which averages over all azimuthal angles φ, we mainly used a one-dimensional sector-averaged scattering profile Iφs < φ < φe (2θ) which averages only the scattering at a specific azimuthal angle. Iφs < φ < φe (2θ) means sector-averaging over the azimuthal angle range φs < φ < φe, and is calculated by Equation 2. The azimuthal angle φ is defined as 0° at the 12 o'clock position in the two-dimensional scattering pattern, with clockwise being the positive direction.

[0111] 2. The one-dimensional profile calculated using scattering intensity correction formula 2 includes scattering from sources other than the sample, such as window material and air scattering, in addition to scattering originating from the sample. Furthermore, the scattering intensity depends on the instrument and sample thickness. To correct for these factors, empty cell scattering and absolute intensity correction were performed (Equation 3).

[0112] 3. To evaluate the degree of orientation distribution of the azimuthal component, the range in which the scattering of interest exists (2θ) ｃ < 2θ < 2θ ｅ The azimuthal distribution of scattered light intensity in ) was calculated using Equation 4. Furthermore, to facilitate data comparison, the intensity of I(φ) was normalized using Equation 5.

[0113] [Parameter Analysis] 1. PP(110) crystallite size (calculated from sector-averaged profile) A one-dimensional sector-averaged profile corrected for scattering intensity was obtained in the azimuthal angle range of 70° < φ < 110° (azimuthal angle φ is defined as 0° at 12 o'clock in the two-dimensional scattering pattern, with clockwise being the positive direction). For this sector-averaged profile, the Multi-peak Fit function of Wave Metrics' Igor Pro 8.00 software was used to approximate both crystalline and amorphous peaks with a Gaussian function as shown below and perform peak separation. Specifically, a straight line was drawn connecting the points at 2θ = 6° and 2θ = 20° in the profile, and this was used as the baseline. Three peaks were considered as crystalline peaks, and the initial values ​​of the three peak positions were set to 2θ = 14.0°, 16.9°, and 18.5°. The initial values ​​of the peak full width at half maximum were set to 0.4°, 0.4°, and 0.5° in order from the smallest angle peak. The initial value of the peak height was the scattering intensity at the peak position of each peak. Two amorphous peaks were considered, with their peak positions fixed at 2θ = 14.9° and 19.18°, and their full width at half maximum (FMAX) fixed at 4.69° and 7.03° from the smaller angle side. Similar to the crystalline peaks, the initial values ​​for the amorphous peak heights were set to the scattering intensities at 2θ = 14.9° and 19.18°. Peak separation was performed on the profile with the amorphous peaks fixed to the above values, without any constraints on peak height, and with the crystalline peaks also unconstrained. If, as a result, there were peaks whose peak positions differed significantly from the initial values, or peaks whose magnitudes were negative, the peak positions of the crystalline peaks were fixed to the above values, and peak separation was performed. The PP(110) crystallite size was calculated by substituting the FMAX of the peaks near 2θ = 14.0° obtained from the peak separation results into Scherrer's formula below.

[0114] 2. Calculation of Orientation An azimuth angle distribution was obtained for the crystal peaks in the range where scattering originating from the PP(110) plane exists (13° < 2θ < 15.3°) according to equations 3 and 4. Using the Multi-peak Fit function of Wave Metrics' Igor Pro 8.00 software, peak separation was performed by approximating the three peaks with a Gaussian function as shown below. Specifically, a straight line was drawn connecting the points φ = 0° and 359° on the profile, and this was used as the baseline. Considering the two peaks, the initial values ​​of the two peak positions were set to φ = 90° and 270°. The initial value of the peak full width at half maximum was set to 10° for both. The initial value of the peak height was the normalized scattering intensity at the peak position of each peak. Peak separation was performed on the azimuth angle distribution without imposing any constraints on the two peaks. As a result, if there were peaks whose peak positions differed significantly from the initial values, or if there were peaks whose magnitudes were negative, the peak positions of the two peaks were fixed to the above values, and peak separation was performed (an example of the results of peak separation in a normalized azimuth distribution is shown below).

[0115] The PP(110) crystal orientation degree f was calculated using the following formula based on the full width at half maximum (FWHM1 and FWHM2) of the two peaks in the azimuthal angle distribution obtained from the peak separation.

[0116] Experiment I

[0117] Example 1 [Preparation of Polypropylene Resin Composition] Pellets of high molecular weight polypropylene resin (PP1, MFR = 0.50) and propylene / ethylene random copolymer (rC3C2-1, MFR = 0.75) shown in Table 1 were dry blended in a mass ratio of PP1:rC3C2-1 = 50.0:50.0 (mass%), and then melt-kneaded using TEM26SS (manufactured by Toshiba Machine Co., Ltd., L / D = 48.5). After melt-kneading, the strands were pulled from a die (3 holes), cooled in a water-cooled bath, and then cut using a pelletizer to obtain polypropylene resin composition pellets.

[0118] [Preparation of Microporous Layer and Separator Substrate] As the resin for the microporous layer (A), the high molecular weight polypropylene resin (PP1, MFR = 0.50) shown in Table 1 and the above polypropylene resin composition pellets were dry blended in a mass ratio of PP1:polypropylene resin composition pellets = 50.0:50.0 (mass%), then melted in a 2.5-inch extruder and supplied to a single-layer inflation die using a gear pump. The inflation die temperature was set to 255°C, and after the molten polymer was discharged from the inflation die, the discharged resin was cooled by blown air and 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 clearance of the inflation die was set to 1.8 mm, and discharge was performed under a discharge rate condition of 9 kg / h.

[0119] Next, the obtained precursor sheet was placed in a dryer and annealed at 130°C for 180 minutes. After that, the annealed precursor sheet was cold-stretched to 30% MD at room temperature, and without shrinking the stretched film, it was placed in an oven at 135°C and hot-stretched to 160% MD, with the pre-stretch dimensions being 100%, and then thermally relaxed to 46% MD to obtain a separator substrate having a single-layer structure consisting of a microporous layer (A). The structure and physical properties of the obtained separator substrate are shown in Table 1.

[0120] Examples 2-8 and 12-13: Separator substrates having a single-layer structure were obtained by following the same method as in Example 1, except that the raw materials were changed as shown in Table 1, the mass ratio of polypropylene resin and additive pellets was adjusted as shown in Table 1, and the manufacturing conditions were adjusted as shown in Table 1. The evaluation results of the obtained separator substrates are shown in Table 1.

[0121] Example 9 [Preparation of Microporous Layer and Separator Substrate] As the resin for the microporous layer (A), the high molecular weight polypropylene resin (PP1, MFR = 0.50) shown in Table 1 and the same polypropylene resin composition pellets as in Example 1 were dry blended in a mass ratio of PP1:polypropylene resin composition pellets = 60.0:40.0 (mass%), then melted in a 2.5-inch extruder and supplied to both outer layers of a two-type, three-layer co-extrusion inflation die using a gear pump. Also, as the resin for the microporous layer (B), 100% by mass of high molecular weight polypropylene resin (PP1, MFR = 0.50) was melted in a 2.5-inch extruder and supplied to the inner layer of the above two-type, three-layer co-extrusion inflation die using a gear pump. The inflation die temperature was set to 255°C, and after the molten polymer was extruded from the inflation die, the extruded resin was cooled by blown air and wound onto a roll to obtain a precursor sheet with an A / B / A layer structure and a thickness of approximately 12 μm. Here, the lip clearance of the inflation die was set to 1.8 mm, and extrusion was performed under an extrusion rate of 9 kg / h. Next, a separator substrate having a three-layer structure with a thickness ratio of microporous layer (A) / microporous layer (B) / microporous layer (A) = 1 / 2 / 1 was obtained by following the same method as in Example 1, except that the manufacturing conditions were adjusted as shown in Table 1. The evaluation results of the obtained separator substrate are shown in Table 1.

[0122] Examples 10-11 and 14: Separator substrates having a three-layer structure were obtained by following the same method as in Example 9, except that the raw materials were changed as shown in Table 1, the mass ratio of polypropylene resin and additive pellets was adjusted as shown in Table 1, and the manufacturing conditions were adjusted as shown in Table 1. The evaluation results of the obtained separator substrates are shown in Table 1.

[0123] <Comparative Example 1> A separator substrate having a single-layer structure was obtained by following the same method as in Example 1, except that a polypropylene resin composition was not prepared, and 100% by mass of the high molecular weight polypropylene resin shown in Table 1 was melted in a 2.5-inch extruder as the resin for the microporous layer (A), and the extrusion temperature was finely adjusted to stabilize the die and discharge. The evaluation results of the obtained separator substrate are shown in Table 1.

[0124] Comparative Examples 2-3: Without preparing a polypropylene resin composition, 100% by mass of the high molecular weight polypropylene resin shown in Table 1 was melted in a 2.5-inch extruder as the resin for the microporous layer (A), and a separator substrate having a three-layer structure was obtained by the same method as in Example 9, except that the extrusion temperature was finely adjusted to stabilize the die and discharge. The evaluation results of the obtained separator substrates are shown in Table 1.

[0125]

[0126]

[0127]

[0128] Experiment II

[0129] Example II-1 [Preparation of Polypropylene Resin Composition] Pellets of high molecular weight polypropylene resin (PP1, MFR = 0.51) and ethylene / propylene random copolymer (r-C2C3, MFR = 0.75) shown in Table 2 were dry blended in a mass ratio of PP1:r-C2C3 = 50.0:50.0 (mass%), and then melt-kneaded using TEM26SS (manufactured by Toshiba Machine Co., Ltd., L / D = 48.5). After melt-kneading, the strands were pulled from a die (3 holes), cooled in a water-cooled bath, and then cut using a pelletizer to obtain polypropylene resin composition pellets.

[0130] [Preparation of Microporous Layer and Separator (Substrate)] As shown in Table 2, the above pellets were co-extruded or laminated in a co-extruder or lamination die for the microporous layer (A), and high molecular weight polypropylene resin (PP1, MFR = 0.51) was co-extruded or laminated for the microporous layer (B). If necessary, the materials were further subjected to treatments such as annealing (e.g., 150°C, 180 mins), cold stretching (e.g., room temperature, MD 30%), hot stretching (e.g., 135°C, MD 200% relative to 100% of the pre-stretch dimensions), stretch opening (e.g., annealing at 150°C for 180 mins, cold stretching to 30% at room temperature, hot stretching to 200% at 135°C, and relaxing by 44%), and thinning (e.g., stretching, heat fixing, peeling or slicing) to produce a separator substrate with a layer (B) / layer (A) / layer (B) thickness ratio of 1 / 2 / 1. The evaluation results of the obtained separator substrates are also shown in Table 2.

[0131] Examples II-3 to 5, 8, 10, and Comparative Example II-2: Separator substrates having a single-layer or laminated structure were obtained according to the same method as in Example II-1, except that the raw materials, layer configuration, substrate configuration, process conditions, and (layer) (thickness) ratio were changed as shown in Table 2. The evaluation results of the obtained separator substrates are shown in Table 2.

[0132] Examples II-2 and II-7: Without preparing a polypropylene resin composition, the resin and additives for the microporous layer (A) shown in Table 2 were melted in a 2.5-inch extruder, and a separator substrate having a single-layer structure was obtained according to the same method as in Example II-1, except that the extrusion temperature was finely adjusted to stabilize the die and discharge. The evaluation results of the obtained separator substrate are shown in Table 2.

[0133] Examples II-6, II-9, and Comparative Example II-1: Separator substrates having a single-layer structure were obtained by following the same method as in Example II-1, except that the resin of the microporous layer (A) shown in Table 2 was melted in a 2.5-inch extruder, and the extrusion temperature was finely adjusted to stabilize the die and discharge, without using a co-extruder or lamination die, and without using a single-layer die. The evaluation results of the obtained separator substrates are shown in Table 2.

[0134]

[0135]

[0136] Experiment III

[0137] Example III-1 [Preparation of Polypropylene Resin Composition] Pellets of high molecular weight polypropylene resin (PP1, MFR = 0.51) and ethylene / propylene random copolymer (r-C2C3 Maru1, MFR = 0.5) shown in Table 3 were dry blended in a mass ratio of PP1:r-C2C3 Maru1 = 50.0:50.0 (mass%), and then melt-kneaded using TEM26SS (manufactured by Toshiba Machine Co., Ltd., L / D = 48.5). After melt-kneading, the strands were pulled from a die (3 holes), cooled in a water-cooled bath, and then cut using a pelletizer to obtain polypropylene resin composition pellets.

[0138] [Preparation of Microporous Layer and Separator (Substrate)] As shown in Table 3, the above polypropylene resin composition pellets were dry blended in a mass ratio of PP1:polypropylene resin composition pellets = 20.0:80.0 (mass%) so that the mass ratio was PP1:r-C2C3 1 = 60.0:40.0 (mass%). The mixture was then melted in a 2.5-inch extruder and supplied to a single-layer inflation die using a gear pump. The inflation die temperature was set to 255°C, and after the molten polymer was discharged from the inflation die, the discharged resin was cooled by blown air and wound onto a roll to obtain a single-layer precursor sheet consisting of a microporous layer with a thickness of approximately 12 μm. Here, the lip clearance of the inflation die was set to 1.8 mm, and discharge was performed under a discharge rate of 9 kg / h.

[0139] Next, the obtained precursor sheet was placed in a dryer and annealed at 150°C for 180 minutes. After that, the annealed precursor sheet was cold-stretched to 30% MD at room temperature, and without shrinking the stretched film, it was placed in an oven at 135°C, and hot-stretched to 160% MD, with the pre-stretch dimensions set to 100%, and then thermally relaxed to 46% MD, thereby obtaining a separator substrate having a single-layer structure consisting of a microporous layer. The evaluation results of the structure and physical properties of the obtained separator substrate are shown in Table 3.

[0140] Example III-2: Polypropylene resin composition pellets were prepared in the same manner as in Example III-1. Separator substrates were obtained by dry blending the above polypropylene resin composition pellets in a mass ratio of PP1:polypropylene resin composition pellets = 50.0:50.0 (mass%) so that the mass ratio of PP1:r-C2C3 pellets = 75.0:25.0 (mass%), and by performing annealing treatment at 130°C. The evaluation results of the obtained separator substrates are shown in Table 3.

[0141] Example III-3: A separator substrate was obtained according to the same method as in Example III-2, except that a polypropylene resin composition pellet was prepared using PP1 and ethylene / propylene random copolymer (r-C2C3 round 2, MFR = 0.75) pellets. The evaluation results of the obtained separator substrate are shown in Table 3.

[0142] Example III-4: A separator substrate was obtained by following the same method as in Example III-3, except that the medium-density material (MD) was hot-stretched to 200% and then thermally relaxed to 45%. The evaluation results of the obtained separator substrate are shown in Table 3.

[0143] Example III-5: Polypropylene resin composition pellets were prepared in the same manner as in Example III-3, melted in a 2.5-inch extruder, and supplied to both outer layers of a two-type, three-layer co-extrusion inflation die using a gear pump. In addition, 100% by mass of high molecular weight polypropylene resin (PP1, MFR = 0.51) was melted in a 2.5-inch extruder as the resin for the microporous layer (B) and supplied to the inner layer of the two-type, three-layer co-extrusion inflation die using a gear pump. Except for this, a separator substrate having a three-layer structure with a thickness ratio of microporous layer (A) / microporous layer (B) / microporous layer (A) = 1 / 2 / 1 was obtained by following the same method as in Example III-3. The evaluation results of the obtained separator substrate are shown in Table 3.

[0144] Example III-6: A separator substrate was obtained according to the same method as in Example III-5, except that the medium-density material (MD) was hot-stretched to 200% and then thermally relaxed to 45%. The evaluation results of the obtained separator substrate are shown in Table 3.

[0145] Example III-7: As shown in Table 3, a separator substrate was obtained according to the same method as in Example III-2, except that a polypropylene resin composition pellet was prepared using PP1 and ethylene / propylene random copolymer (r-C2C3 round 3, MFR = 2.4) pellets. The evaluation results of the obtained separator substrate are shown in Table 3.

[0146] Example III-8: As shown in Table 3, a separator substrate was obtained according to the same method as in Example III-2, except that a polypropylene resin composition pellet was prepared using PP1 and ethylene / propylene random copolymer (r-C2C3 round 4, MFR = 0.5) pellets. The evaluation results of the obtained separator substrate are shown in Table 3.

[0147] Example III-9: A separator substrate was obtained according to the same method as in Example III-2, except that annealing treatment was performed at 150°C. The evaluation results of the obtained separator substrate are shown in Table 3.

[0148] Example III-10: A separator substrate was obtained according to the same method as in Example III-1, except that annealing treatment was performed at 130°C. The evaluation results of the obtained separator substrate are shown in Table 3.

[0149] Example III-11: A separator substrate was obtained using the same method as in Example III-2, except that polypropylene resin composition pellets were not prepared and only PP1 was used. The evaluation results of the obtained separator substrate are shown in Table 3.

[0150] Example III-12: A separator substrate was obtained according to the same method as in Example III-5, except that polypropylene resin composition pellets were prepared with a mass ratio of PP1:r-C2C3 round 2 = 30.0:70.0 (mass%). The evaluation results of the obtained separator substrate are shown in Table 3.

[0151] Comparative Example III-1: A separator substrate was obtained according to the same method as in Example III-11, except that the inflation die temperature was set to 240°C to obtain a precursor sheet, annealing was performed at 150°C, 40% cold stretching was performed on the MD, 220% hot stretching was performed on the MD, and 44% thermal relaxation was performed on the MD. The evaluation results of the obtained separator substrate are shown in Table 3.

[0152] 《Comparative Example III-2》A separator substrate was obtained in the same manner as in Example III-5, except that pellets of PP1 and ethylene / octene copolymer (C2C8 elastomer, MFR=1) were used, the polypropylene resin composition pellets were dry-blended in a mass ratio of PP1:polypropylene resin composition pellets = 60.0:40.0 (mass%) so that the mass ratio of PP1:C2C8 elastomer = 80.0:20.0 (mass%) was used, and the inflation die temperature was set to 240°C to obtain a precursor sheet. The evaluation results of the obtained separator substrate are shown in Table 3.

[0153]

[0154]

[0155]

[0156] The separator for energy storage devices of this disclosure can be suitably used as a separator for energy storage devices, such as lithium-ion secondary batteries.

[0157] 1. Resin part (light area) 2. Hole part (dark area) 3. Separator 4. Needle 5. Plate 6. Melting peak temperature 7. The lower of the two temperatures that represent half of the melting peak 8. The lower of the two temperatures that represent half of the melting peak 9. Temperature at which the cumulative heat of fusion reaches 25% D. First fracture depth dia. Diameter

Claims

1. A separator for an energy storage device comprising a separator substrate having a microporous layer (A) containing polypropylene, wherein the separator substrate has a melt flow rate (MFR) of 0.20 g / 10 min or more and 0.90 g / 10 min or less when measured at a load of 2.16 kg and a temperature of 230 °C, and the stem width calculated from the analysis of scanning electron microscope (SEM) images of the MD-ND cross-section of the separator substrate is 150 nm or more and 500 nm or less.

2. The separator for energy storage devices according to claim 1, wherein the separator substrate contains, based on the total resin, an olefin structure other than the propylene structure as a repeating unit, in an amount greater than 0.0 mol% and less than or equal to 3.0 mol%.

3. The separator for energy storage devices according to claim 1, wherein the stem width calculated from the analysis of scanning electron microscope (SEM) images of the MD-ND cross-section of the separator substrate is 250 nm or more and 500 nm or less.

4. The separator for an energy storage device according to claim 1, wherein the stem height calculated from the analysis of scanning electron microscope (SEM) images of the MD-ND cross-section of the separator substrate is 500 nm or more and 1000 nm or less.

5. The separator substrate has a melt flow rate (MFR) of 0.20 g / 10 min or more and 0.6 g / 10 min or less when measured at a load of 2.16 kg and a temperature of 230 °C, as described in claim 1, for use as a separator for an energy storage device.

6. The separator for an energy storage device according to claim 1, wherein the area-average pore diameter calculated from the analysis of scanning electron microscope (SEM) images of the MD-ND cross-section of the separator substrate is 100 nm or more and 300 nm or less.

7. The separator for an energy storage device according to claim 2, wherein the olefin structure other than the propylene structure contained in the separator substrate comprises one or more repeating units selected from the group consisting of ethylene, 1-butene, and 1-octene.

8. The separator substrate comprises a propylene / olefin random copolymer, wherein the propylene / olefin random copolymer contains 60 mol% or more of a propylene structure as a repeating unit, as described in claim 1, for use as a separator for an energy storage device.

9. The separator for an energy storage device according to claim 1, wherein the c-axis crystal orientation of the separator substrate is 0.915 or more and 0.940 or less.

10. The separator for an energy storage device according to claim 1, wherein the c-axis oriented crystallite size of the separator substrate is 17.0 nm or more and 19.5 nm or less.

11. The separator substrate according to claim 1, wherein the separator substrate contains 50.0% by mass or more and 90.0% by mass or less of homopolypropylene, and 10.0% by mass or more and 50.0% by mass or less of thermoplastic resin other than homopolypropylene, based on the total mass of the separator substrate.

12. The separator substrate has a residue of 90% by weight or more after extraction with boiling n-hexane for 6 hours, as described in claim 1, for a power storage device.

13. The separator for an energy storage device according to claim 1, wherein the temperature on the lower end of the range of temperatures at which the crystal melting peak obtained from differential scanning calorimetry of the separator substrate at a heating rate of 10°C / min is half the peak is between 155.0°C and 164.0°C.

14. The separator for an energy storage device according to claim 1, wherein the thickness of the separator substrate is 7.0 μm or more and 12.0 μm or less.

15. The separator for energy storage device according to claim 1, wherein the porosity of the separator substrate is 28% or more and 52% or less.

16. The separator for an energy storage device according to claim 1, wherein the TD thermal shrinkage rate of the separator substrate in one hour at a temperature of 105°C is 5% or less, and the MD thermal shrinkage rate in one hour at a temperature of 105°C is 20% or less.

17. The separator for an energy storage device according to claim 1, wherein the first puncture strength of the separator substrate is 250 gf or more when the thickness of the separator substrate is converted to 10 μm.

18. An energy storage device comprising a positive electrode, a negative electrode, and a separator for an energy storage device according to any one of claims 1 to 17, disposed between the positive electrode and the negative electrode.

19. The energy storage device according to claim 18, wherein the positive electrode contains lithium iron phosphate as the positive electrode active material.

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