Separator for energy storage device and energy storage device

Abstract

The present invention provides a separator for an energy storage device, comprising a microporous membrane with polyolefin as the main component as the separator substrate, wherein the trunk height calculated from the analysis of the scanning electron microscope (SEM) image of the MD-ND cross section of the separator substrate is 500 nm to 1000 nm, and includes a microporous layer (A) with an MD length calculated from the analysis of the SEM image of the MD-ND cross section of the separator substrate being 1000 nm to 1900 nm.

Description

Technical Field

[0001] This invention relates to a separator for an energy storage device, etc. Prior Technology

[0002] Microporous membranes, especially polyolefin-based microporous membranes, are used in numerous technical fields such as microfiltration membranes, battery separators, capacitor separators, and fuel cell materials. They are particularly used as separators in energy storage devices, such as lithium-ion batteries. Lithium-ion batteries are used not only in small electronic devices such as mobile phones and laptops, but also in various applications including electric vehicles such as hybrid and plug-in hybrid vehicles.

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

[0004] For example, Patent Document 1 describes a multilayer microporous thin film or membrane that can improve properties including insulation failure and strength. A preferred multilayer microporous membrane comprises microlayers and one or more stacked barrier layers.

[0005] Patent document 2 describes a high-strength and thin-film-compatible separator for an energy storage device, which discloses a microporous membrane with polyolefin as the main component, a melt tension of less than 30 mN when measured at 230°C, and a melt flow rate (MFR) of less than 0.9 g / 10 min when measured at a load of 2.16 kg and a temperature of 230°C.

[0006] Patent document 3 describes a separator for an energy storage device with excellent product safety, and discloses a microporous membrane containing polypropylene resin and thermoplastic elastomer, having a specific MFR and morphology.

[0007] Patent document 4 describes a separator for lithium-ion batteries with excellent air permeability and shutdown characteristics. It discloses a porous membrane having a mixed resin layer containing polypropylene resin, polyethylene resin, and thermoplastic resin whose crystallization melting peak temperature or glass transition temperature is below the crystallization melting peak temperature of polyethylene resin, and having β-activity.

[0008] Patent document 5 describes a battery separator comprising a microporous membrane with excellent TD tensile strength, puncture strength, and air permeability.

[0009] Patent document 6 describes a porous membrane that balances high mechanical stability and low air permeability resistance, and discloses a polypropylene porous membrane that displays a specific internal racemic pentagonal component ratio and TREF (temperature-induced dissolution fractionation) measurement value using its separator. [Previous Technical Documents] [Patent Literature]

[0010] [Patent Document 1] International Publication No. 2018 / 089748 [Patent Document 2] International Publication No. 2020 / 196120 [Patent Document 3] International Publication No. 2019 / 103947 [Patent Document 4] Japanese Patent Application Publication No. 2010-111832 [Patent Document 5] International Publication No. 2018 / 217990 [Patent Document 6] Japanese Patent Application Publication No. 2014-133839 Summary of the Invention

[0011] [The problem the invention aims to solve] In the methods described in Patent Documents 1 to 6, specific membrane properties or resin mixtures are used to exhibit thin film, high strength, and excellent air permeability, respectively. However, energy storage devices require a separator that balances high strength and low air permeability. From this point of view, there is room for further improvement.

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

[0013] Through repeated and dedicated research, the inventors discovered that by using a separator substrate containing polyolefin as the main component and having a specific pore structure, the above-mentioned problems can be solved, thereby completing the first embodiment of the present invention. Furthermore, through repeated and dedicated research, it was discovered that by using a separator substrate containing polypropylene and having a specific composition and porosity, the aforementioned problems could be solved, thereby completing the second embodiment of the present invention. That is, the present invention is as described below. (1) A separator for an energy storage device, comprising a microporous membrane with polyolefin as the main component as the separator substrate, and Based on the analysis of the scanning electron microscope (SEM) image of the MD-ND cross-section of the aforementioned separator substrate, the trunk height calculated is between 500 nm and 1000 nm. It includes a microporous layer (A) with an MD length of 1000 nm to 1900 nm calculated from the SEM image of the MD-ND cross section of the aforementioned separator substrate. (2) The separator for the energy storage device of Item 1, wherein the microporous layer (A) comprises one or more thermoplastic elastomers selected from the group consisting of polyolefins, copolymers of polyolefins, and copolymers of polystyrene and polyolefins that are different from the main component. (3) The separator for the energy storage device as described in item 1 or 2, wherein the microporous layer (A) comprises a thermoplastic elastomer containing one or more of the group consisting of ethylene, propylene and 1-butene as repeating units. (4) The separator for the energy storage device of any one of items 1 to 3, wherein, based on the total mass of the microporous layer (A) mentioned above, it contains 80.0% to 99.5% by mass of polypropylene as the polyolefin mentioned above, and contains 0.5% to 20.0% by mass of thermoplastic elastomer. (5) For any of the energy storage devices in items 1 to 4, in the composition image obtained by scanning electron microscopy (SEM) at a magnification of 30,000 times using the MD-ND cross section of the microporous layer (A) dyed with ruthenium compound, there are polymer matrix and fibrils, and each of the polymer matrix and fibrils has a dyed bright area with an area of ​​100 nm 2 or more and 0.1 μm 2 or less, and when the total area ratio of the dyed bright areas with an area of ​​100 nm 2 or more and 0.1 μm 2 or less relative to the total area ratio of the image of only the microporous layer (A) is taken, 0.5 ≤ S 1 ≤ 15.0. (6) For any of the energy storage devices in items 1 to 5, the integral dissolution of the microporous layer (A) at temperatures between 100°C and 130°C, as determined by cross-gradient chromatography (CFC), is between 80.0% and 99.5% of the total dissolution, and the integral dissolution at temperatures between 20°C and 100°C is between 0.5% and 20.0% of the total dissolution. (7) For any of the energy storage devices in items 1 to 6, the area average long pore diameter calculated from the analysis of the SEM image of the MD-ND cross section of the microporous layer (A) is 50 nm or more and 500 nm or less. (8) For any of the energy storage devices in items 1 to 7, the melt flow rate (MFR) of the microporous layer (A) is less than 1.00 g / 10 min when measured under a load of 2.16 kg and a temperature of 230°C. (9) For any of the energy storage devices in items 1 to 8, the melt tension Mt of the microporous layer (A) at 240°C is 10 mN or more and 35 mN or less. (10) A separator for an energy storage device, as described in any of items 1 to 9, wherein the thickness of the substrate of the separator is more than 3 μm and less than 20 μm. (11) A separator for an energy storage device, as described in any of items 1 to 10, wherein the porosity of the substrate of the separator is more than 30% and less than 60%. (12) For any of the energy storage devices in items 1 to 11, the TD heat shrinkage rate of the substrate of the aforementioned separator at 105°C for 1 hour is less than 5%. (13) A separator for an energy storage device, such as any one of items 1 to 12, wherein the MD tensile strength of the substrate of the separator is 1800 kgf / cm2 or higher. (14) For any of the energy storage devices in items 1 to 13, the melt flow rate (MFR) of the microporous layer (A) is 0.20 g / 10 min or more when measured under a load of 2.16 kg and a temperature of 230°C. (15) The separator for the energy storage device of any of items 1 to 14, wherein the polyolefin is polypropylene and the weight average molecular weight (Mw) of the polypropylene is more than 300,000 and less than 1,300,000. (16) For example, the separator for the energy storage device of item 15, wherein the weight average molecular weight (Mw) of the polypropylene is divided by the number average molecular weight (Mn), i.e., the molecular weight distribution (Mw / Mn) is 3 or more and 30 or less. (17) For the separator of the energy storage device such as Item 15 or 16, the five-component ratio of the polypropylene mentioned above, as determined by 13C-NMR (nuclear magnetic resonance method), is 94.0% or more. (18) A separator for an energy storage device, as described in any of items 1 to 17, wherein the separator contains 0.5% to 20.0% polyethylene by mass based on the total mass of the microporous layer (A). (19) A separator for an energy storage device as described in any of items 1 to 18, wherein the substrate of the separator includes a microporous layer (B) with a melt tension Mt B that is different from the melt tension Mt A at 240°C of the microporous layer (A). (20) An energy storage device comprising a positive electrode containing lithium iron phosphate as the positive electrode active material, a negative electrode, and a separator for the energy storage device disposed between the positive electrode and the negative electrode as described in any one of items 1 to 19. (21) A separator for an energy storage device, comprising a microporous layer (A) containing polypropylene as a separator substrate, and The integrated dissolution amount of the aforementioned microporous layer (A) determined by cross-fractional chromatography (CFC) at temperatures between 100°C and 130°C is 80.0% to 99.5% of the total dissolution amount; the integrated dissolution amount at temperatures between 20°C and 100°C is 0.5% to 20.0% of the total dissolution amount. The porosity of the above-mentioned separator substrate is 30.0% to 45.0%. (22) For example, the separator for the energy storage device of item 21, wherein in the dissolution temperature-dissolution amount curve of the microporous layer (A) measured by cross-gradient chromatography (CFC), there are at least two peaks, the peak temperature of the high temperature side peak is above 105°C and below 125°C, and the peak temperature of the low temperature side peak is above 35°C and below 100°C. (23) For the separator of the energy storage device as described in Item 22, in the dissolution temperature-dissolution amount curve of the microporous layer (A) determined by cross-gradient chromatography (CFC), the weight average molecular weight (MwH) at the temperature of the high-temperature side peak is more than 300,000 and less than 1,300,000, and the weight average molecular weight (MwL) at the temperature of the low-temperature side peak is more than 50,000 and less than 1,800,000. (24) For any of the energy storage devices in items 21 to 23, the melt flow rate (MFR) of the microporous layer (A) is less than 1.0 g / 10 min when measured at a load of 2.16 kg and a temperature of 230°C. (25) For any of the energy storage devices in items 21 to 24, the melt tension Mt A of the microporous layer (A) at 240°C is more than 10 mN and less than 35 mN. (26) For any of the energy storage devices in items 21 to 25, the weight average molecular weight (Mw) of the microporous layer (A) is more than 250,000 and less than 1,500,000. (27) For example, the separator for the energy storage device of item 26, wherein the value obtained by dividing the weight average molecular weight (Mw) of the microporous layer (A) by the number average molecular weight (Mn), i.e., the molecular weight distribution (Mw / Mn), is 3 or more and 30 or less. (28) For any of the energy storage devices in items 21 to 27, the five-component ratio of the polypropylene described above, as determined by 13C-NMR (nuclear magnetic resonance method), is 94.0% or more. (29) A separator for an energy storage device, as described in any of items 21 to 28, wherein the separator contains 0.5% to 20.0% by mass of thermoplastic elastomer based on the total mass of the microporous layer (A). (30) The separator for the energy storage device, as in item 29, contains 0.5% to 20.0% polyethylene by mass based on the total mass of the microporous layer (A) mentioned above. (31) The separator for the energy storage device, as in item 29, wherein the thermoplastic elastomer contains one or more of the group consisting of ethylene, propylene and 1-butene as repeating units. (32) A separator for an energy storage device, as described in any of items 21 to 31, wherein the thickness of the substrate of the separator is more than 3 μm and less than 20 μm. (33) The separator for an energy storage device, as described in any of items 21 to 32, wherein the trunk height calculated from the analysis of the scanning electron microscope (SEM) image of the MD-ND cross section of the separator substrate is more than 500 nm and less than 1000 nm. (34) The separator for an energy storage device, as described in any of items 21 to 33, wherein the area-average long pore diameter calculated from the analysis of the scanning electron microscope (SEM) image of the MD-ND cross section of the microporous layer (A) is 50 nm or more and 500 nm or less. (35) A separator for an energy storage device, as described in any of items 21 to 34, wherein the TD heat shrinkage rate of the substrate of the separator is less than 5% after 1 hour at a temperature of 105°C. (36) A separator for an energy storage device, as described in any of items 21 to 35, wherein the MD heat shrinkage rate of the substrate of the separator at a temperature of 105°C for 1 hour is less than 20%. (37) A separator for an energy storage device, as described in any of items 21 to 36, wherein the MD tensile elongation of the substrate of the separator is more than 20% and less than 60%. (38) A separator for an energy storage device, as described in any of items 21 to 37, wherein the substrate of the separator includes a microporous layer (B) with a melt tension Mt B that is different from the melt tension Mt A of the microporous layer (A) at 240°C. (39) An energy storage device comprising a positive electrode containing lithium iron phosphate as the positive electrode active material, a negative electrode, and a separator for an energy storage device disposed between the positive electrode and the negative electrode as described in any one of items 21 to 38. [Effects of the Invention]

[0014] According to a first embodiment of the present invention, a separator for an energy storage device that combines high puncture strength and low air permeability can be provided. According to a second embodiment of the present invention, a separator for an energy storage device can be provided that combines high puncture strength and low air permeability, and is therefore not easily broken due to its high TD tensile strength. Simple Explanation of the Diagram

[0015] Figure 1 is a schematic diagram illustrating the relationship between the polymer matrix, connecting regions, fibrils, and pores in the MD-ND (thickness direction) cross section of the microporous layer (A) when it is manufactured by uniaxial extension in the film-forming direction (MD) according to the present invention. Figure 2 is a schematic diagram illustrating the relationship between the polymer matrix, fibrils, dyed bright areas, and pores in a cross-section of the microporous layer (A) when the microporous layer (A) is manufactured by uniaxial extension in the film-forming direction (MD) according to the present invention. Figure 3 illustrates the detection area for calculating the trunk height in a scanning electron microscope (SEM) image of the MD-ND cross-section of the separator substrate, obtained by removing the fibrils through image analysis and binarizing the resin part and the pore part. Figure 4 illustrates the detection area for calculating the MD length in a scanning electron microscope (SEM) image of the MD-ND cross-section of a microporous layer, obtained by image analysis and binarization of the resin portion and pore portion. Figure 5 is a graph showing the dissolution temperature-dissolution amount curve determined by cross-fractional chromatography (CFC). Implementation

[0016] Unless otherwise specified, all measurements in this specification are performed based on the methods described in the embodiments. In this specification, the upper or lower limit values ​​within the numerical ranges described in stages can be replaced with the corresponding upper or lower limit values ​​within the numerical ranges described in other stages, and further, with the corresponding values ​​described in the embodiments. In this specification, the term "step" not only refers to an independent step, but also includes steps that cannot be clearly distinguished from other steps, as long as the function of the step is achieved.

[0017] First Implementation Method

[0018] Separators for energy storage devices The separator for an energy storage device according to the first embodiment of the present invention has a microporous layer containing polyolefin as the main component as the separator substrate. The microporous layer in the present invention corresponds to the following microporous layer (A). When the microporous layer (A) is a single layer, a single-layer microporous membrane can be formed; and when there are multiple layers of microporous layer (A) or when the microporous layer (A) is combined with other layers, a multilayer microporous membrane can be formed. The separator substrate may, as needed, include a microporous layer (B) with polyolefin as the main component in addition to the microporous layer (A). Furthermore, the separator substrate may have a coating layer (also referred to as a "surface layer," "coating layer," etc.; hereinafter simply referred to as "coating layer") on the microporous layer (A) and / or the microporous layer (B). In this invention, the term "microporous layer" refers to each layer of the microporous material constituting the substrate of the separator, the term "separator substrate" refers to the substrate of the separator excluding any coating layer, and the term "separator" refers to the separator as a whole, which also includes any coating layer.

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

[0020] <MD length of microporous layer (A)> FIG1 provides an example of the configuration of the microporous layer (A) of the present invention when it is manufactured by uniaxial extension in the film-forming direction (MD). Preferably, between a plurality of polymer matrices (1), a plurality of fibrils (3) extend along the MD of the microporous layer (A). In the interior or surface of the polymer matrices (1) and / or fibrils (3), the connecting regions (2-1) are aligned side by side with the MD of the microporous layer (A). The portion other than the polymer matrices (1), fibrils (3) and connecting regions (2-1) is a pore (4). Preferably, by having connecting regions in the interior or surface of the polymer matrices and / or fibrils without excessively damaging the pores, the polymer matrices and fibrils are reinforced, thereby allowing the fibrils and / or connecting regions to connect more polymer matrices. It is speculated that this can adequately ensure porosity and promote stress propagation between polymer matrices, mitigating the localized stress applied to the polymer matrix during the puncture strength failure mode, thereby suppressing damage caused by polymer matrix rupture, and forming a microporous layer (A) with both low permeability and high puncture strength. When more polymer matrices are connected by fibrils, the morphology is preferably, in a pattern as shown in Figure 1, the connecting region (2-1), forming a structure parallel to the MD. Preferably, the polymer matrix contains at least polypropylene, and in the case of uniaxial extension, it forms a layered crystalline structure; the fibrils contain at least polypropylene, and in the case of uniaxial extension, the polymer chains of the polymer matrix extend during the extension opening; the connecting region contains polyethylene and / or thermoplastic elastomer.

[0021] The microporous layer (A) of the first embodiment of the present invention has a microporous layer length (MD) of 1000 nm or more and 1900 nm or less. The MD length is related to the structure and / or the bonding region where the fibrils connecting the polymer matrix are arranged side by side with the MD. If the MD length is increased, more polymer matrix is ​​connected to the fibrils and / or the bonding region, which alleviates local stress and thereby obtains good puncture strength. From the viewpoint of obtaining a microporous layer (A) with high puncture strength, the lower limit of the MD length is preferably 1050 nm or more, more preferably 1100 nm or more, more preferably 1150 nm or more, more preferably 1200 nm or more, particularly preferably 1250 nm or more, and most preferably 1300 nm or more. From the perspective of ensuring sufficient porosity and obtaining good air permeability, the upper limit of the MD length is preferably below 1850 nm, more preferably below 1800 nm, further preferably below 1780 nm, further preferably below 1750 nm, especially preferably below 1720 nm, and most preferably below 1700 nm.

[0022] The MD length of this invention is calculated based on the analysis of the scanning electron microscope (SEM) image of the MD-ND cross section of the separator substrate, and the analysis and calculation method is detailed in the embodiments.

[0023] To obtain a good MD length in the microporous layer (A), the microporous layer (A) may contain polyethylene and / or thermoplastic elastomers as additives. When thermoplastic elastomers are included, from the viewpoints of increasing MD length, film-forming properties, thin-film properties, low air permeability, and high puncture strength, the lower limit of the thermoplastic elastomer content in the microporous layer (A), based on the total mass of the microporous layer (A), is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, further preferably 2.0% by mass or more, and further preferably 3.0% by mass or more. As an upper limit for the content of thermoplastic elastomer in the microporous layer (A), from the viewpoint of ensuring sufficient porosity, obtaining good air permeability, and maintaining open-cell properties, based on the total mass of the microporous layer (A), it is preferably 20.0% by mass or less, more preferably 15.0% by mass or less, further preferably 10.0% by mass or less, further preferably 8.0% by mass or less, particularly preferably 7.5% by mass or less, significantly preferably 6.0% by mass or less, and most preferably 5.0% by mass or less. When polyethylene is present, as a lower limit for the polyethylene content, based on the total mass of the microporous layer (A), from the viewpoint of increasing MD length and balancing high puncture strength and low air permeability, it is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, further preferably 2.0% by mass or more, and further preferably 3.0% by mass or more. As an upper limit for the polyethylene content, based on the total mass of the microporous layer (A), from the viewpoint of maintaining open-cell properties, it is preferably 20.0% by mass or less, more preferably 15.0% by mass or less, further preferably 12.5% ​​by mass or less, further preferably 10.0% by mass or less, and particularly preferably 7.5% by mass or less. When the microporous layer of the present invention contains polypropylene as a polyolefin and thermoplastic elastomer as an additive, based on the total mass of the microporous layer, it is preferably containing 80.0% by mass or more and 99.5% by mass of polypropylene, and 0.5% by mass or more and 20.0% by mass of thermoplastic elastomer.

[0024] <Materials of Microporous Layers (A)> The microporous layer (A) of the first embodiment of the present invention is mainly composed of polyolefin. By using polyolefin as the main component, the substrate or separator has good porosity, and good battery performance can be obtained. When the microporous layer (A) contains polypropylene, the polypropylene of the microporous layer (A) can be the same material as the polypropylene of the microporous layer (B) described below, or it can be a polypropylene with a different chemical structure. More specifically, it can be a polypropylene with at least one different monomer composition, stereoregularity, molecular weight, and crystal structure. The stereoregularity of the polypropylene is not limited; examples include heteropolymers, homopolymers, and parallel homopolymers. The polypropylene of the present invention is preferably a homopolymer or parallel homopolymer with high crystallinity.

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

[0026] The weight-average molecular weight (Mw) of the polypropylene in the microporous layer (A) is preferably 300,000 or higher from the viewpoint of high puncture strength of the microporous layer, and preferably 1,300,000 or lower from the viewpoint of ensuring good film-forming properties, manufacturability, thin-film formation, and low air permeability. The Mw of the polypropylene is more preferably 500,000 or higher and 1,200,000 or lower, more preferably 650,000 or higher and 1,100,000 or lower, more preferably 750,000 or higher and 1,000,000 or lower, and especially preferably 800,000 or higher and 1,000,000 or lower.

[0027] The upper limit of the weight-average molecular weight (Mw) of the polypropylene in the microporous layer (A) divided by the number-average molecular weight (Mn) (Mw / Mn) is preferably 30 or less, more preferably 20 or less, 18 or less, 16 or less, 14 or less, or 12 or less. By keeping the Mw / Mn of the polypropylene below 30, there is a tendency to ensure good film-forming properties, productivity, and thin film formation. Furthermore, the lower limit of the Mw / Mn of the polypropylene is preferably 3 or more, more preferably 4 or more, 4.5 or more, or 5.0 or more. By keeping the Mw / Mn of the polypropylene above 3, appropriate molecular entanglement can be maintained, resulting in good film-forming stability. Moreover, the weight-average molecular weight, number-average molecular weight, and Mw / Mn of the polypropylene in this invention are molecular weights converted from polystyrene obtained by GPC (gel permeation chromatography).

[0028] The density of polypropylene in the microporous layer (A) is preferably 0.85 g / cm³ or higher, for example, 0.88 g / cm³ or higher, 0.89 g / cm³ or higher, or 0.90 g / cm³ or higher. The density of polypropylene is preferably 1.1 g / cm³ or lower, for example, 1.0 g / cm³ or lower, 0.98 g / cm³ or lower, 0.97 g / cm³ or lower, 0.96 g / cm³ or lower, 0.95 g / cm³ or lower, 0.94 g / cm³ or lower, 0.93 g / cm³ or lower, or 0.92 g / cm³ or lower. The density of polypropylene is related to its crystallinity. By achieving a polypropylene density of 0.85 g / cm³ or higher, the productivity of the microporous layer is improved, especially in dry processes.

[0029] As a lower limit for the five-component ratio of polypropylene in the microporous layer (A), from the viewpoint of obtaining a microporous layer with low air permeability, it 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. There is no upper limit for the five-component ratio of polypropylene, and it can be 99.9% or lower, 99.8% or lower, or 99.5% or lower. The five-component ratio of polypropylene is determined by 13C-NMR (nuclear magnetic resonance method).

[0030] The polypropylene in the so-called microporous layer (A) has a five-component ratio of over 94.0%, indicating that the polypropylene has a high crystallinity. The separator obtained by the extended opening method, especially the dry method, opens the pores by extending the amorphous parts between the crystalline materials. Therefore, if the polypropylene has a high crystallinity, the opening property becomes good, and the air permeability can be suppressed to a low level, thus enabling the battery to achieve high input and output.

[0031] Examples of polyethylene as the microporous layer (A) include ultra-high molecular weight polyethylene, high-density polyethylene, low-density polyethylene, and linear low-density polyethylene. One or more of these can be used alone. Furthermore, polyethylene with a narrow molecular weight distribution obtained using a metallocene catalyst, or even high-density polyethylene obtained through multi-stage polymerization, can also be used. Of the above, high-density polyethylene is preferred from the viewpoint of film-forming properties and high puncture strength.

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

[0033] The weight-average molecular weight (Mw) of polyethylene as the microporous layer (A) is preferably 100,000 or more from the viewpoint of achieving high puncture strength due to the formation of connecting regions in the microporous layer, and preferably 1,800,000 or less from the viewpoint of ensuring good film-forming properties, thin-film properties, and manufacturability. The Mw of polyethylene is more preferably 150,000 or more and 1,500,000 or less, more preferably 200,000 or more and 1,200,000 or less, more preferably 250,000 or more and 900,000 or less, and particularly preferably 300,000 or more and 600,000 or less.

[0034] The upper limit of the value (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) of polyethylene in the microporous layer (A) by the number average molecular weight (Mn) is preferably 24 or less, more preferably 22 or less, 20 or less, or 18 or less. By making the Mw / Mn of polyethylene 24 or less, there is a tendency to ensure good film-forming properties, thin film formation, and productivity. Furthermore, the lower limit of the Mw / Mn of polyethylene is preferably 3 or more, more preferably 4 or more, 4.5 or more, or 5.0 or more. By making the Mw / Mn of polyethylene 3 or more, appropriate molecular entanglement can be maintained, and good film-forming stability can be obtained. Moreover, the weight average molecular weight, number average molecular weight, and Mw / Mn of polyethylene in this invention are molecular weights converted from polystyrene obtained by GPC (gel permeation chromatography).

[0035] The density of polyethylene in the microporous layer (A) is preferably 0.85 g / cm³ or higher, for example, 0.88 g / cm³ or higher, 0.89 g / cm³ or higher, or 0.90 g / cm³ or higher. The density of polyethylene is preferably 1.1 g / cm³ or lower, for example, 1.0 g / cm³ or lower, 0.98 g / cm³ or lower, 0.97 g / cm³ or lower, 0.96 g / cm³ or lower, 0.95 g / cm³ or lower, 0.94 g / cm³ or lower, 0.93 g / cm³ or lower, or 0.92 g / cm³ or lower. The density of polyethylene is related to its crystallinity; by making the polyethylene density 0.85 g / cm³ or higher, a microporous layer with high puncture strength achieved by forming interconnected regions can be obtained.

[0036] The microporous layer (A) is mainly composed of polyolefins, and may contain polyolefins other than polypropylene and polyethylene. Polyolefins refer to polymers containing monomers with carbon-carbon double bonds as repeating units. There are no limitations on the monomers constituting polyolefins other than polypropylene and polyethylene; examples include monomers with 4 to 10 carbon atoms having carbon-carbon double bonds, such as 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Examples of polyolefins include homopolymers, copolymers, and multi-stage polymers.

[0037] The microporous layer (A) of the first embodiment of the present invention contains a polyolefin as the main component and may also contain a thermoplastic elastomer. Examples of thermoplastic elastomers include: polyolefins different from the main component, copolymers of polyolefins, and copolymers of polystyrene and polyolefins. A polyolefin refers to a polymer containing monomers having carbon-carbon double bonds as repeating units. There is no limitation on the monomers constituting the polyolefin; examples include monomers with 2 to 10 carbon atoms (C2 to C10) having carbon-carbon double bonds, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Examples of polyolefins include low-crystallinity polypropylene with low stereoregularity regions. One monomer may be used alone, or two or more monomers may be used in combination as monomers constituting the copolymer of the polyolefin. The copolymer of the polyolefin may be a random copolymer or a block copolymer. Examples of copolymers of polystyrene and polyolefins include: styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer (SEBS), styrene-ethylene-styrene copolymer, styrene-(ethylene-butene)-olefin copolymer (SEBC), and olefin-(ethylene-butene)-styrene copolymer (CEBS), etc., which may be hydrogenated polymers. These copolymers may be random copolymers or block copolymers, preferably block copolymers.

[0038] In the case where the microporous layer (A) of the first embodiment of the present invention contains only a thermoplastic elastomer as an additive, from the viewpoint that the thermoplastic elastomer contains a microporous layer (A) that balances high puncture strength and low air permeability by playing a role in connecting the polymer matrix and strengthening the connection of regions, when the main component is polypropylene, it is preferable to contain a copolymer containing one or more repeating units selected from the group consisting of propylene, 1-butene, 1-hexene, and 1-octene as compatible with polypropylene. Furthermore, from the viewpoint that the copolymer mitigates the localized stress applied to the polymer matrix during the failure mode of puncture strength and suppresses the damage caused by the rupture of the polymer matrix, when the main component is polypropylene, it is preferable to contain ethylene as a repeating unit that is incompatible with polypropylene. More preferably, these are ethylene / propylene (C2C3) copolymers, ethylene / 1-butene (C2C4) copolymers, ethylene / 1-hexene (C2C6) copolymers, ethylene / 1-octene (C2C8) copolymers, olefin-(ethylene-butene)-olefin copolymers (CEBC), and olefin-(ethylene-butene)-styrene copolymers (CEBS), and even more preferably, ethylene / propylene (C2C3) copolymers, ethylene-terminated olefin-(ethylene-butene)-olefin copolymers (CEBC), and ethylene-terminated olefin-(ethylene-butene)-styrene copolymers (CEBS). Here, the ethylene-butene structure is structurally similar to propylene, thus exhibiting a higher affinity for polypropylene. The thermoplastic elastomer can be used alone or in combination of two or more.

[0039] In the case where the microporous layer (A) of the first embodiment of the present invention contains polyethylene and thermoplastic elastomer as additives, the thermoplastic elastomer, by playing a role in connecting the polymer matrix and strengthening the connection of regions, can achieve a microporous layer (A) that balances high puncture strength and low air permeability. It is preferable that the thermoplastic elastomer contains one or more copolymers selected from the group consisting of ethylene, propylene, and 1-butene as repeating units compatible with polypropylene and / or polyethylene. More preferably, these are ethylene / propylene (C2C3) copolymers, ethylene / 1-hexene (C2C6) copolymers, ethylene / 1-octene (C2C8) copolymers, olefin-(ethylene-butene)-olefin copolymers (CEBC), and olefin-(ethylene-butene)-styrene copolymers (CEBS). Even more preferably, these are ethylene / propylene (C2C3) copolymers, ethylene-terminated olefin-(ethylene-butene)-olefin copolymers (CEBC), and ethylene-terminated olefin-(ethylene-butene)-styrene copolymers (CEBS). Here, the ethylene-butene structure is structurally similar to propylene, thus exhibiting a high affinity for polypropylene. Thermoplastic elastomers can be used alone or in combination of two or more.

[0040] <Measurement values ​​of microporous layer (A) by cross-fractional chromatography (CFC)> The preferred range of the measured values ​​of the microporous layer (A) of the first embodiment of the present invention by cross-grading chromatography (CFC) is the same as that of the second embodiment described below.

[0041] <Mel Flow Rate (MFR) of Microporous Layer (A)> The melt flow rate (MFR) of the microporous layer (A) of the present invention is preferably 1.00 g / 10 min or less. As an upper limit of the melt flow rate (MFR) of the microporous layer (A) (MFR of a single layer), from the viewpoint of obtaining a microporous layer (A) with higher puncture strength, it is preferably 1.00 g / 10 min or less, more preferably 0.90 g / 10 min or less, further preferably 0.80 g / 10 min or less, further preferably 0.70 g / 10 min or less, and most preferably 0.60 g / 10 min or less. There is no lower limit to the MFR (MFR of a single layer) of the microporous layer (A). From the viewpoint of obtaining microporous layers (A) with lower permeability, film-forming properties, and thin film formation, it can be, for example, 0.20 g / 10 min or more, 0.25 g / 10 min or more, 0.30 g / 10 min or more, 0.35 g / 10 min or more, 0.40 g / 10 min or more, or 0.45 g / 10 min or more. The MFR of the microporous layer (A) was measured under a load of 2.16 kg and a temperature of 230°C. An MFR of 1.00 g / 10 min or less for the microporous layer (A) indicates that the molecular weight of the polyolefin contained in the microporous layer (A) is significantly higher. By increasing the molecular weight of the polyolefin, more molecules are bonded together to bind the crystalline substances, thus tending to obtain microporous layers (A) with high puncture strength. Furthermore, by ensuring that the MFR of the microporous layer (A) is 0.20 g / 10 min or higher, the melt tension of the microporous layer (A) will not be too high, thus ensuring good film-forming properties, thin film formation, and productivity.

[0042] For the microporous layer (A) of polypropylene, from the viewpoint of obtaining a microporous layer (A) with high puncture strength, low air permeability, and a thin film, the MFR is preferably 0.2 to 0.9 g / 10 min when measured under conditions of a load of 2.16 kg and a temperature of 230°C. As an upper limit for the MFR of polypropylene, from the viewpoint of obtaining a microporous layer (A) with even higher puncture strength, it can 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. There is no lower limit for the MFR of polypropylene. From the viewpoint of obtaining a microporous layer (A) with lower air permeability, film-forming properties and thin film formation, it can be, for example, 0.2 g / 10 min or more, 0.25 g / 10 min or more, 0.3 g / 10 min or more, 0.35 g / 10 min or more, 0.4 g / 10 min or more, or 0.45 g / 10 min or more.

[0043] For polyethylene MFR as a microporous layer (A), from the viewpoint of obtaining a microporous layer (A) with high puncture strength, low air permeability and a thin film, the preferred value when measured under a load of 2.16 kg and a temperature of 190°C is 0.005~10.0 g / 10 min. From the viewpoint of obtaining a microporous layer (A) with higher puncture strength and good film-forming stability, the upper limit of the MFR of polyethylene can be, for example, 8.0 g / 10 min or less, 6.0 g / 10 min or less, 4.0 g / 10 min or less, 3.0 g / 10 min or less, 2.0 g / 10 min or less, 1.5 g / 10 min or less, 1.0 g / 10 min or less, 0.8 g / 10 min or less, 0.6 g / 10 min or less, 0.5 g / 10 min or less, 0.4 g / 10 min or less, 0.35 g / 10 min or less, 0.3 g / 10 min or less, 0.25 g / 10 min or less, 0.2 g / 10 min or less, 0.15 g / 10 min or less, 0.1 g / 10 min or less, or 0.05 g / 10 min or less. There is no limit to the lower limit of MFR for polyethylene. From the viewpoint of obtaining a microporous layer (A) with lower air permeability, film-forming properties and thin film formation, it can be, for example, 0.008 g / 10 min or more, 0.01 g / 10 min or more, 0.012 g / 10 min or more, 0.015 g / 10 min or more, 0.018 g / 10 min or more, or 0.02 g / 10 min or more.

[0044] For the thermoplastic elastomer used as the microporous layer (A), from the viewpoint of obtaining a microporous layer (A) with high puncture strength, low air permeability, thin film, and good film-forming stability, the preferred MFR is 0.1 to 100.0 g / 10 min when measured under a load of 2.16 kg and a temperature of 230°C. As for the upper limit of the MFR of the thermoplastic elastomer, from the viewpoint of obtaining a microporous layer (A) with high puncture strength and good film-forming stability through uniform mixing with polyolefins, for example, it can be 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. There is no limit to the lower limit of the MFR of thermoplastic elastomers. From the point of view of obtaining microporous layers (A) with lower air permeability, film-forming properties and thin film formation, it can be, for example, 0.5 g / 10 min or more, 1.0 g / 10 min or more, 1.5 g / 10 min or more, 2.0 g / 10 min or more, 2.5 g / 10 min or more, or 3.0 g / 10 min or more.

[0045] <Mw and Mw / Mn of the microporous layer (A)> The weight-average molecular weight (Mw) of the microporous layer (A) is preferably 250,000 or more from the viewpoint of obtaining a microporous layer (A) with higher puncture strength, and preferably 1,500,000 or less from the viewpoint of ensuring good film-forming properties, manufacturability, thin-film formation and low air permeability. The Mw of the microporous layer (A) is more preferably 400,000 or more and 1,300,000 or less, more preferably 500,000 or more and 1,200,000 or less, more preferably 600,000 or more and 1,100,000 or less, and even more preferably 700,000 or more and 1,000,000 or less.

[0046] The upper limit of the value (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) of the microporous layer (A) by the number average molecular weight (Mn) is preferably 30 or less, more preferably 25 or less, 20 or less, 18 or less, 16 or less, 14 or less, 12 or less, or 10 or less. By making the Mw / Mn of the microporous layer (A) 30 or less, there is a tendency to ensure good film-forming properties, productivity, and thin film formation. Furthermore, the lower limit of the Mw / Mn of the microporous layer (A) is preferably 3 or more, more preferably 4 or more, 4.5 or more, or 5.0 or more. By making the Mw / Mn of the microporous layer (A) 3 or more, appropriate molecular entanglement can be maintained, and good film-forming stability can be obtained. Moreover, the weight average molecular weight, number average molecular weight, and Mw / Mn of the microporous layer (A) of the present invention are molecular weights converted from polystyrene obtained by GPC (gel permeation chromatography). Furthermore, the microporous layer (A) of the present invention is mainly composed of polyolefin containing polypropylene and polyethylene, and also contains thermoplastic elastomer. Therefore, the Mw and Mw / Mn of the microporous layer (A) are values ​​that reflect the influence of the composition of these constituent materials.

[0047] <Melting Tension of Microporous Layer (A)> As an upper limit value for the melt tension Mt A (Mt A of a single layer) of the microporous layer (A) at 240°C, from the viewpoint of obtaining a microporous layer (A) with good film-forming properties, productivity, thin film formation and low air permeability, it is preferably 35 mN or less, more preferably 32 mN or less, further preferably 30 mN or less, further preferably 28 mN or less, and especially preferably 26 mN or less. As a lower limit value for the melt tension Mt A (Mt A of a single layer) of the microporous layer (A), from the viewpoint of obtaining a microporous layer (A) with higher puncture strength, it is preferably 10 mN or more, more preferably 13 mN or more, further preferably 16 mN or more, further preferably 18 mN or more, and especially preferably 19 mN or more.

[0048] <DSC parameters of microporous layer (A)> The microporous layer (A) preferably exhibits an endothermic peak A with a peak value in the range of 100°C to 145°C and an endothermic peak B with a peak value in the range of 155°C to 175°C in the DSC curve (vertical axis: heat flux, horizontal axis: temperature) of the heating process in differential scanning calorimetry (DSC). Furthermore, the ratio of the area SA of the endothermic peak A to the area SB of the endothermic peak B in the DSC curve of the microporous layer (A) is preferably 2 to 200. It is assumed that the endothermic peak A in the microporous layer (A) is at least derived from polyethylene, and the endothermic peak B in the microporous layer (A) is at least derived from polypropylene. From the viewpoint of obtaining a microporous layer (A) with high puncture strength, the range of the peak value of the endothermic peak A of the microporous layer (A) is preferably 100°C to 145°C, more preferably 110°C to 142°C, further preferably 120°C to 140°C, and even more preferably 125°C to 135°C. From the viewpoint of obtaining a microporous layer (A) with high puncture strength and low air permeability, the range of the peak value of the endothermic peak B of the microporous layer (A) is preferably 155°C to 175°C, more preferably 158°C to 173°C, further preferably 160°C to 171°C, and even more preferably 163°C to 169°C. Furthermore, it is believed that the SB / SA ratio of the microporous layer (A) reflects the ratio of at least polypropylene content to at least polyethylene content. From the perspective of maintaining good battery performance after storage at high temperature (e.g., 130°C), and maintaining good film-forming properties, puncture strength and air permeability, it is preferred to be 2 or more and 200 or less, more preferably 4 or more and 100 or less, further preferably 6 or more and 50 or less, further preferably 8 or more and 30 or less, and especially preferably 10 or more and 25 or less.

[0049] <Area-average long pore diameter of microporous layer (A)> The area-average long pore diameter (hereinafter also referred to as "area-average long pore diameter") of the MD-ND cross section of the microporous layer (A) is preferably 50 nm to 500 nm. In this invention, "ND" refers to the thickness direction of the microporous layer, and "MD" refers to the film formation direction of the microporous layer. For example, if the MD of the separator having the microporous layer is a roll, then it is the length direction. The term "long pore diameter" refers to the pore diameter of the MD. Furthermore, when there are two or more microporous layers (A) and / or microporous layers (B), the area-average long pore diameter of microporous layer (A) and microporous layer (B) is compared based on the average area-average long pore diameter value of each layer. As a lower limit for the area-average long pore diameter of the microporous layer (A), from the viewpoint of ensuring high input / output in energy storage devices and obtaining a microporous layer (A) with low permeability, it is preferably 50 nm or more, more preferably 80 nm or more, further preferably 100 nm or more, further preferably 120 nm or more, and especially preferably 130 nm or more. Furthermore, as an upper limit for the area-average long pore diameter of the microporous layer (A), from the viewpoint of obtaining a microporous layer (A) with high puncture strength, it is preferably 500 nm or less, more preferably 400 nm or less, further preferably 350 nm or less, further preferably 300 nm or less, especially preferably 250 nm or less, and most preferably 210 nm or less.

[0050] The area-average long aperture can be observed by cross-sectional SEM (scanning electron microscope) of the MD-ND section of the separator, and determined by image analysis based on the obtained image. Detailed conditions are shown in the embodiments. Furthermore, when determining the average aperture from the cross-sectional SEM image, the number-average aperture and area-average aperture can be calculated. However, in order to obtain a greater correlation with the physical properties of the separator, the area-average aperture is used as the average aperture in this invention.

[0051] <Dyeing of the bright part of the microporous layer (A)> Preferably, in a compositional image obtained by scanning electron microscopy (SEM) at a magnification of 30,000x using the MD-ND cross-section of the microporous layer (A) stained with ruthenium compound, a polymer matrix and fibrils are present, and each of the polymer matrix and fibrils contains a brightly stained region with an area of ​​100 nm² to 0.1 μm². Here, the brightly stained region in this invention refers to an area observed as a bright region in the compositional image due to a difference in the constituent material and / or molecular structure of polypropylene, which is the main component of the polymer matrix and fibrils, resulting in a difference in the degree of ruthenium compound staining. Furthermore, the amorphous portion of polypropylene, the main component of the polymer matrix and fibrils, can also be a brightly stained region, but by setting a lower area limit for the brightly stained region, i.e., each with an area of ​​100 nm² or more, it is intended that the amorphous portion of polypropylene is excluded. Furthermore, from the viewpoint of obtaining a microporous layer (A) with high puncture strength, the lower limit of the area of ​​each stained bright area is preferably 150 nm² or more, more preferably 200 nm² or more, further preferably 250 nm² or more, and further preferably 300 nm² or more. From the viewpoint of obtaining a microporous layer (A) with good porosity and high puncture strength, the upper limit of the area of ​​each stained bright area is preferably 0.08 μm² or less, more preferably 0.06 μm² or less, further preferably 0.05 μm² or less, further preferably 0.04 μm² or less, particularly preferably 0.03 μm² or less, and most preferably 0.02 μm² or less.

[0052] When S1% is defined as the total area ratio of the dyed bright areas with an area of ​​100 nm² to 0.1 μm² relative to the total area of ​​the image consisting only of the microporous layer (A), it is preferably 0.5 ≤ S1 ≤ 15.0. As a lower limit for S1, from the viewpoint of obtaining a microporous layer (A) with both low air permeability and high puncture strength by reinforcing the polymer matrix and fibrils without excessively damaging the pores, it is more preferably 0.75 or higher, more preferably 1.0 or higher, more preferably 1.25 or higher, especially preferably 1.5 or higher, significantly more preferably 1.75 or higher, and most preferably 2.0 or higher. As the upper limit of S1, from the viewpoint of maintaining porosity and obtaining a microporous layer (A) with low air permeability and high puncture strength, it is preferably 12.0 or less, even more preferably 10.0 or less, even more preferably 8.0 or less, especially preferably 6.0 or less, significantly more preferably 5.0 or less, and most preferably 4.0 or less.

[0053] The area of ​​the stained bright area and the total area ratio S1 can be determined by cross-sectional SEM (scanning electron microscope) observation of the MD-ND cross-section of the separator stained with ruthenium compound, and image analysis of the obtained compositional image at a magnification of 30,000. Detailed conditions are shown in the examples. Furthermore, the denominator when calculating the total area ratio S1 is the entire image of the microporous layer (A) taken only, using the area of ​​the entire image that includes not only the polymer matrix and fibrils but also the pores.

[0054] Preferably, the microporous layer (A) of the present invention contains polypropylene and thermoplastic elastomer as the main components as the constituent materials, as described above, and has a polymer matrix and fibrils as the structure. In the above composition image, the dyed bright area exists in the polymer matrix and fibrils. The morphology of the microporous layer (A) of the present invention is not limited. For example, an example of the morphology when it is manufactured by uniaxial extension in the film-forming direction (MD) is shown in FIG2. Preferably, between a plurality of polymer matrices (1), a plurality of fibrils (3) extend along the MD of the microporous layer (A). In the interior or on the surface of the polymer matrix (1) and the fibrils (3), the regions (2-2) that become dyed bright areas are aligned side by side with the MD of the microporous layer (A). The portion other than the polymer matrix (1), the fibrils (3) and the regions (2-2) that become dyed bright areas are pores (4). Preferably, the polymer matrix contains at least polypropylene, and when manufactured by uniaxial extension, it forms a structure with layered crystal arrangement. The fibrils contain at least polypropylene, and when manufactured by uniaxial extension, the polymer chains of the polymer matrix extend during the extension and opening process, forming a thermoplastic elastomer in the dyed bright area.

[0055] Theoretically, there are no limitations. The key aspect of the microporous layer (A) of this invention is that it preferably forms the area that becomes the visible part of the dyed area. It is speculated that a microporous layer (A) with both low air permeability and high puncture strength can be formed by reinforcing the polymer matrix and fibrils without excessively damaging the pores. In the microporous layer (A), the failure mode of puncture strength can be exemplified by the rupture of the polymer matrix. It is believed that the presence of connecting regions inside or on the surface of the polymer matrix can mitigate the stress applied to the polymer matrix during failure, thus exhibiting high puncture strength. In the microporous layer (A) of this invention, it is preferable that polypropylene contains or has added thermoplastic elastomers. The thermoplastic elastomers preferably have portions that are immiscible with polypropylene. It is believed that a portion or all of the thermoplastic elastomer that does not mix with the polymer matrix containing the main component, polypropylene, forms an area different from the polymer matrix (becoming the visible part of the dyed area).

[0056] Furthermore, when polypropylene is the main component in the microporous layer (A) of the present invention, polyethylene is preferred from the viewpoint that it is formed in the polymer matrix and fibrils containing polypropylene, and is not usually mixed with polypropylene, but is added together with thermoplastic elastomer to form a region that is dyed bright, thereby obtaining a microporous layer (A) that balances low permeability and high puncture strength. When polypropylene contains or has added polyethylene, polypropylene and polyethylene are usually not mixed, so a portion or all of the polyethylene that is not mixed with the polymer matrix containing polypropylene forms a region different from the polymer matrix. When thermoplastic elastomer is added to polyethylene, it is presumed that by improving the dispersibility of polypropylene and polyethylene, or by connecting polypropylene and polyethylene, and strengthening the connection of regions, the above-mentioned effect as a connecting region is promoted. As a connecting region containing polyethylene, from the viewpoint of balancing low permeability and high puncture strength, it is preferred that it continue uninterruptedly along MD as shown in FIG1.

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

[0058] <Thickness of the microporous layer (A)> When the substrate of the separator for an energy storage device is constructed with a single-layer structure having only one microporous layer (A), the upper limit of the thickness of the microporous layer (A) is preferably 20 μm or less, from the viewpoint of achieving high energy density in the energy storage device and low permeability of the microporous layer (A). For example, it can be 18 μm or less, 16 μm or less, 14 μm or less, 13 μm or less, 12 μm or less, 11.5 μm or less, 11 μm or less, 10.5 μm or less, or 10 μm or less. As for the lower limit of the thickness of the microporous layer (A) when it is constructed with a single-layer structure, from the viewpoint of obtaining a microporous layer (A) with high puncture strength, it is preferably 3 μm or more. For example, it can be 4 μm or more, 5 μm or more, 6 μm or more, 7 μm or more, 7.5 μm or more, 8 μm or more, 8.5 μm or more, or 9 μm or more.

[0059] When the substrate of the separator for an energy storage device is constructed with a multilayer structure comprising one or more microporous layers (A), the upper limit of the thickness of the microporous layer (A) is preferably 10 μm or less, from the viewpoint of achieving high energy density in the energy storage device and low permeability of the separator substrate. For example, it can be 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm or less, or 4 μm or less. As for the lower limit of the thickness of the microporous layer (A) in the case of a multilayer structure, from the viewpoint of obtaining a separator substrate with high puncture strength, it is preferably 1 μm or more, for example, it can be 2 μm or more, 3 μm or more, or 3.5 μm or more.

[0060] <Additives for Microporous Layer (A)> The microporous layer (A), mainly composed of polyolefins, may contain elastomers, nucleating agents, antioxidants, fillers, and other additives as needed, in addition to polypropylene, polyethylene, and thermoplastic elastomers. The amount of additives is not particularly limited, but is based on the total mass of the microporous layer (A), and may be 0.01% or more, 0.1% or more, or 1% or more, or less than 20% or less, 10% or less, or less than 7% or less.

[0061] <Microporous layer (B)> The separator for the energy storage device of the present invention may, as needed, have a microporous layer (B) in addition to the microporous layer (A). The separator for the energy storage device may have only one microporous layer (B), or it may have two or more layers. The microporous layer (B) is mainly composed of polyolefin, more preferably polypropylene and / or polyethylene. This provides good porosity and good battery performance. The microporous layer (B) is further preferably mainly composed of polypropylene, thereby maintaining good battery performance even after storage at high temperatures (e.g., 130°C). In the present invention, the term "mainly composed of polypropylene" in the microporous layer (B) means that, based on the total mass of the microporous layer (B), it contains 50% by mass or more of polypropylene. As a lower limit for the polypropylene content in the microporous layer (B), from the viewpoint of wettability and film formation of the separator, it is preferably 55% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more. There is no upper limit for the polypropylene content in the microporous layer (B), for example, it can be 98% by mass or less, 99% by mass or less, or even 100% by mass.

[0062] <Materials for Microporous Layers (B)> The polypropylene of the microporous layer (B) can be the same material as the polypropylene of the microporous layer (A) described above, or it can be a polypropylene with a different chemical structure. More specifically, it can be a polypropylene with at least one different monomer composition, stereoregularity, molecular weight, and crystal structure. The stereoregularity of the polypropylene of the microporous layer (B) is not limited; examples include heteropolymers, homopolymers, and parallel homopolymers. Preferably, the polypropylene of the present invention is a homopolymer or parallel homopolymer with high crystallinity.

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

[0064] The weight-average molecular weight (Mw) of the polypropylene in the microporous layer (B) is preferably 250,000 or higher from the viewpoint of the strength of the microporous layer, and preferably 1,000,000 or lower from the viewpoint of increasing the pore size of the microporous layer and exhibiting good air permeability. The Mw of the polypropylene is more preferably 400,000 or higher and 950,000 or lower, more preferably 550,000 or higher and 900,000 or lower, more preferably 600,000 or higher and 900,000 or lower, and especially preferably 700,000 or higher and 900,000 or lower.

[0065] The upper limit of the Mw / Mn ratio (the value obtained by dividing the weight average molecular weight (Mw) of polypropylene in the microporous layer (B) by the number average molecular weight (Mn) is preferably 7 or less, more preferably 6.5 or less, 6 or less, 5.5 or less, or 5 or less. The smaller the Mw / Mn value of polypropylene, the lower the melt tension of the resulting microporous layer tends to be. Therefore, when the Mw / Mn value of polypropylene is 7 or less, it is preferable to control the melt tension of the microporous layer (B) to be relatively low. Furthermore, Mw / Mn is preferably 1.1 or more, for example, 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. By making Mw / Mn 1.1 or more, it is possible to maintain appropriate molecular entanglement, resulting in good stability during film formation. Furthermore, the weight-average molecular weight, number-average molecular weight, and Mw / Mn of the polyolefin in the microporous layer (B) of the present invention are similar to those of the polypropylene in the microporous layer (A), and are obtained by measuring the molecular weight of polystyrene using GPC (gel permeation chromatography).

[0066] The density of the polypropylene in the microporous layer (B) is preferably 0.85 g / cm³ or higher, for example, 0.88 g / cm³ or higher, 0.89 g / cm³ or higher, or 0.90 g / cm³ or higher. The density of the polypropylene is preferably 1.1 g / cm³ or lower, for example, 1.0 g / cm³ or lower, 0.98 g / cm³ or lower, 0.97 g / cm³ or lower, 0.96 g / cm³ or lower, 0.95 g / cm³ or lower, 0.94 g / cm³ or lower, 0.93 g / cm³ or lower, or 0.92 g / cm³ or lower. The density of the polyolefin is related to the crystallinity of the polypropylene. By making the density of the polypropylene 0.85 g / cm³ or higher, the productivity of the microporous layer is improved, especially in dry processes.

[0067] The microporous layer (B) may contain resins other than polypropylene. Examples of other resins include polyolefins other than polypropylene (also called "other polyolefins"). Polyolefins refer to polymers containing monomers with carbon-carbon double bonds as repeating units. There are no limitations on the monomers constituting polyolefins other than polypropylene, but examples include monomers with 2 or 4 to 10 carbon atoms having carbon-carbon double bonds, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene.

[0068] The microporous layer (B) may contain thermoplastic elastomers other than polypropylene. There are no particular limitations on the thermoplastic elastomer; examples include: polypropylene different from the main component, polyolefins other than polypropylene (also called "other polyolefins"), and copolymers of polystyrene and polyolefins. Examples of polypropylene include: low-crystallinity polypropylene with low stereoregularity regions. Polyolefins refer to polymers containing monomers with carbon-carbon double bonds as repeating units. There are no limitations on the monomers constituting polyolefins other than polypropylene; examples include: monomers with 2 or 4 to 10 carbon atoms having carbon-carbon double bonds, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Polyolefins may be homopolymers, copolymers, or multi-stage polymers; polyethylene may also be included as an example. Examples of copolymers of polystyrene and polyolefins include: styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer (SEBS), hydrogenated styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), styrene-ethylene-styrene copolymer, olefin crystallization-(ethylene-butene)-olefin crystallization copolymer (CEBC), hydrogenated olefin crystallization-(ethylene-butene)-olefin crystallization copolymer (hydrogenated CEBC), styrene-(ethylene-butene)-olefin crystallization copolymer (SEBC), and hydrogenated styrene-(ethylene-butene)-olefin crystallization copolymer (hydrogenated SEBC). Particularly preferred are hydrogenated styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), hydrogenated styrene-(ethylene-butene)-olefin crystallization copolymer (hydrogenated SEBC), and styrene-(ethylene-propylene)-styrene copolymer (SEPS).

[0069] As a thermoplastic elastomer contained in the microporous layer (B), from the viewpoint of open-pore properties and enlarging pore size, it is preferable to be an elastomer that is incompatible with polypropylene. There are no particular limitations on the elastomer that is incompatible with polypropylene, but preferred examples include: copolymers of polyethylene with other polyolefins, and copolymers of polystyrene with polyolefins. Preferred examples of copolymers of polystyrene with polyolefins include: styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer (SEBS), hydrogenated styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), styrene-ethylene-styrene copolymer, olefin crystallization-(ethylene-butene)-olefin crystallization copolymer (CEBC), hydrogenated olefin crystallization-(ethylene-butene)-olefin crystallization copolymer (hydrogenated CEBC), styrene-(ethylene-butene)-olefin crystallization copolymer (SEBC), and hydrogenated styrene-(ethylene-butene)-olefin crystallization copolymer (hydrogenated SEBC), etc. The preferred options are hydrogenated styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), hydrogenated styrene-(ethylene-butene)-olefin crystalline copolymer (hydrogenated SEBC), and styrene-(ethylene-propylene)-styrene copolymer (SEPS).

[0070] Melt Flow Rate (MFR) of Microporous Layer (B) The upper limit of the melt flow rate (MFR) of the microporous layer (B) (for monolayers) is preferably 8.00 g / 10 min or less, from the viewpoint of obtaining a microporous layer (B) with higher strength. For example, it can be 6.00 g / 10 min or less, 4.00 g / 10 min or less, 3.00 g / 10 min or less, 2.00 g / 10 min or less, or 1.10 g / 10 min or less. The lower limit of the MFR of the microporous layer (B) (for monolayers) is not limited from the viewpoint of exhibiting good air permeability. For example, it can be 0.30 g / 10 min or more, 0.35 g / 10 min or more, 0.40 g / 10 min or more, 0.45 g / 10 min or more, or 0.50 g / 10 min or more. The MFR of the microporous layer (B) was measured under a load of 2.16 kg and a temperature of 230°C. A melt flow rate (MFR) of 8.00 g / 10 min or less for the microporous layer (B) indicates a significantly higher molecular weight of the polyolefin contained in the microporous layer (B). By increasing the molecular weight of the polyolefin, more molecules are bonded together to the crystalline components, thus tending to obtain a microporous layer (B) with high strength. Furthermore, by making the MFR of the microporous layer (B) 0.30 g / 10 min or more, the melt tension of the microporous layer (B) will not be too high, making it easier to obtain a separator with good air permeability.

[0071] From the viewpoint of obtaining a microporous layer (B) with higher strength, the MFR of the polypropylene in the microporous layer (B) is preferably 8.0 g / 10 min or less, for example, it can be 6.0 g / 10 min or less, 4.0 g / 10 min or less, 3.0 g / 10 min or less, 2.0 g / 10 min or less, or 1.1 g / 10 min or less. As for the lower limit value of the MFR of the microporous layer (B) (the MFR of a single layer), from the viewpoint of performing good air permeability, there is no limitation, for example, it can be 0.3 g / 10 min or more, 0.35 g / 10 min or more, 0.4 g / 10 min or more, 0.45 g / 10 min or more, or 0.5 g / 10 min or more.

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

[0073] <Five-component ratio of microporous layer (B)> As a lower limit for the five-component ratio of polypropylene in the microporous layer (B), from the viewpoint of obtaining a microporous layer with low air permeability, it 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. There is no upper limit for the five-component ratio of polypropylene, and it can be 99.9% or lower, 99.8% or lower, or 99.5% or lower. The five-component ratio of polypropylene in the microporous layer (B) is determined by 13C-NMR (nuclear magnetic resonance) in the same manner as in the microporous layer (A).

[0074] The so-called microporous layer (B) of polypropylene has a five-component ratio of over 94.0%, which indicates that the polypropylene has a high crystallinity. The separator obtained by the extended opening method, especially the dry method, opens the pores by extending the amorphous parts between the crystalline materials. Therefore, if the polypropylene has a high crystallinity, the opening property becomes good, and the air permeability can be suppressed to a low level, thus enabling the battery to achieve high output.

[0075] <Melting Tension of Microporous Layer (B)> The melt tension MtB of the microporous layer (B) at 240°C is preferably 4 mN or more and 30 mN or less. As the lower limit of the melt tension MtB, from the viewpoint of good film-forming properties and manufacturability of the substrate separating the microporous layer (A) and microporous layer (B), it is preferably 4 mN or more, more preferably 7 mN or more, further preferably 10 mN or more, particularly preferably 13 mN or more, and most preferably 16 mN or more. As the upper limit of the melt tension MtB, from the viewpoint of achieving large pore size and good air permeability, it is preferably 30 mN or less, more preferably 28 mN or less, further preferably 26 mN or less, and most preferably 24 mN or less. The melt tensions MtA and MtB of the microporous layers (A) and (B) at 240°C may differ from each other from the viewpoint of the area-average long pore diameter of the microporous membrane.

[0076] <Area-average long pore diameter of microporous layer (B)> The area-average long pore diameter (hereinafter referred to as "area-average long pore diameter") of the MD-ND cross section of the microporous layer (B) is preferably larger than that of the microporous layer (A). For details on the relationship with the area-average long pore diameter of the microporous layer (A), please refer to the section on "Area-average long pore diameter of microporous layer (A)".

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

[0078] <Porosity of Microporous Layer (B)> The porosity of the microporous layer (B) is preferably 20% or more from the viewpoint of avoiding clogging in the energy storage device and obtaining good air permeability of the separator, and preferably 70% or less from the viewpoint of maintaining the strength of the separator. The porosity of the microporous layer (B) is more preferably 25% or more and 65% or less, more preferably 30% or more and 60% or less, and even more preferably 35% or more and 60% or less.

[0079] <Thickness of the microporous layer (B)> From the viewpoint of achieving high energy density in energy storage devices, the thickness of the microporous layer (B) is preferably 10 μm or less, for example, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm or less, or 4 μm or less. From the viewpoint of strength, the lower limit of the thickness of the microporous layer (B) is preferably 1 μm or more, for example, 2 μm or more, 3 μm or more, or 3.5 μm or more.

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

[0081] <Relationship between microporous layer (A) and microporous layer (B)> The ratio of the melt tension MtA of the microporous layer (A) at 240°C to the melt tension MtB of the microporous layer (B) at 240°C, MtA / MtB, is preferably 1.05 or higher and 4.0 or lower. Here, by making MtA / MtB 1.05 or higher, the pore size of the microporous layer (A) of the obtained separator can be sufficiently reduced, and by controlling the pore size of the microporous layer (B) to be sufficiently large, good withstand voltage and air permeability can be achieved. By making MtA / MtB 4.0 or lower, a separator with good porosity and air permeability can be obtained. Furthermore, MtA / MtB is more preferably 1.1 or higher and 3.5 or lower, more preferably 1.15 or higher and 3.3 or lower, more preferably 1.2 or higher and 3.0 or lower, and particularly preferably 1.25 or higher and 2.5 or lower.

[0082] The MFR (MFR B) ratio of the microporous layer (B) to the MFR (MFR A) of the microporous layer (A), MFR B / MFR A, is preferably 1.02 or higher and 10.0 or lower. By making MFR B / MFR A 1.02 or higher, the pore size of the microporous layer (A) of the obtained separator can be sufficiently reduced, and the pore size of the microporous layer (B) can be controlled to be sufficiently large, thus achieving both good voltage withstand capability and air permeability. By making MFR B / MFR A 10.0 or lower, a separator with stable film-forming properties, manufacturability, and good porosity and air permeability can be obtained. Furthermore, MFR B / MFR A is more preferably 1.05 or higher and 6.0 or lower, more preferably 1.1 or higher and 5.0 or lower, more preferably 1.1 or higher and 4.0 or lower, and especially preferably 1.1 or higher and 3.0 or lower.

[0083] The weight-average molecular weight (MwA) of polypropylene in the microporous layer (A) and the weight-average molecular weight (MwB) of polypropylene in the microporous layer (B), MwA / MwB, is preferably 1.02 or higher and 2.0 or lower. By making MwA / MwB 1.02 or higher, the melt tension ratio (MtA / MtB) of the microporous layer (A) and microporous layer (B) can be controlled to be relatively high, resulting in a good suppression effect on dendritic crystallization in the energy storage device. By making MtA / MtB 2.0 or lower, a separator with stable film-forming properties, manufacturability, and good porosity and permeability can be obtained. Furthermore, MwA / MwB is preferably 1.02 or higher and 1.8 or lower, more preferably 1.03 or higher and 1.6 or lower, and most preferably 1.05 or higher and 1.4 or lower.

[0084] <Layer Structure of Separator Substrate> The substrate for the separator in an energy storage device (hereinafter referred to as the "separator substrate") can be any microporous membrane used for a separator in an energy storage device. It can be a single-layer structure with only a microporous layer (A), or a multi-layer structure having at least one microporous layer (A). Alternatively, it can be a multi-layer structure having at least one microporous layer (A) and one microporous layer (B). Furthermore, the separator substrate can also be a multi-layer structure with three or more layers, having at least one of two or more microporous layers (A) and / or at least one microporous layer (B). Examples include a two-layer structure of microporous layer (A) / microporous layer (B), and a three-layer structure of microporous layer (A) / microporous layer (B) / microporous layer (A). Additionally, the separator substrate can have layers other than microporous layers (A) and (B). For example, layers other than microporous layer (A) and microporous layer (B) can include: microporous layers mainly composed of polyolefins, layers containing inorganic materials, and layers containing heat-resistant resins, etc. Furthermore, the separator substrate can also be a multilayer structure with four or more layers, such as microporous layer (A) / microporous layer (B) / microporous layer (C) / microporous layer (A). From the viewpoints of ease of manufacturing and suppression of curling of the separator, a symmetrical laminated structure is preferred.

[0085] <Main height of the substrate of the separator> The trunk height of the MD-ND cross-section of the separator substrate is between 500 nm and 1000 nm. The trunk height is related to the tortuosity of the hole and represents a numerical value indicating a hole structure different from the hole diameter. A higher trunk height results in lower tortuosity, while a lower trunk height results in higher tortuosity, leading to better withstand voltage. Furthermore, by increasing tortuosity, a dense mesh structure can be formed, promoting stress dispersion and thus achieving good puncture strength. From the viewpoint of puncture strength, the upper limit of the trunk height is preferably 950 nm or less, more preferably 920 nm or less, further preferably 900 nm or less, further preferably 850 nm or less, especially preferably 820 nm or less, significantly preferably 800 nm or less, and most preferably 750 nm or less. From the perspective of achieving good air permeability, the lower limit of the main trunk height is preferably 520 nm or higher, more preferably 540 nm or higher, even more preferably 560 nm or higher, even more preferably 580 nm or higher, and especially preferably 600 nm or higher.

[0086] The trunk height can be calculated as follows: A cross-sectional SEM image of the MD-ND section of the separator substrate is performed. The fibrils are removed by image analysis, and the length of the remaining thin ND layer is determined by image analysis. Detailed conditions are shown in the embodiments. Furthermore, when determining the trunk height based on the cross-sectional SEM image, the number-average trunk height and length-average trunk height can be calculated. However, to obtain a greater correlation with the physical properties of the separator, the length-average trunk height is used as the trunk height in this specification.

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

[0088] <Air permeability (air tightness) of the substrate of the separator> From the viewpoint of ensuring good output in an energy storage device, the upper limit of the air permeability of the separator substrate according to the first embodiment of the present invention is preferably 400 seconds / 100 cm³ or less, more preferably 350 seconds / 100 cm³ or less, further preferably 300 seconds / 100 cm³ or less, further preferably 250 seconds / 100 cm³ or less, particularly preferably 220 seconds / 100 cm³ or less, and most preferably 200 seconds / 100 cm³ or less. The lower limit of the air permeability of the separator substrate is not limited, and for example, it can be 10 seconds / 100 cm³ or more, 20 seconds / 100 cm³ or more, or 30 seconds / 100 cm³ or more.

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

[0090] <Puncture Strength of Separator Substrate> As a lower limit value for the puncture strength of the separator substrate in the first embodiment of the present invention, when the thickness of the separator substrate is converted to 10 μm, it is preferably 150 gf or more (about 1.47 N or more), more preferably 180 gf or more, 200 gf or more, 220 gf or more, or 240 gf or more, and even more preferably 260 gf or more, 270 gf or more, or 280 gf or more. The upper limit value for 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.

[0091] <Voltage Withstandability of Separator Substrate> From the viewpoint of suppressing short circuits in the energy storage device, the withstand voltage of the separator substrate is preferably 0.80 kV or higher, more preferably 0.90 kV or higher, and even more preferably 0.95 kV or higher, especially preferably 1.00 kV or higher, and most preferably 1.05 kV or higher, when the thickness of the separator substrate is converted to 10 μm.

[0092] <Thermal Shrinkage Rate of Separator Substrate> The thermal shrinkage rate in the width direction (TD) of the separator substrate after heat treatment at 105°C for 1 hour is preferably 5% or less, and more preferably -1.0% to 3.0% or less. That is, a separator substrate with a TD thermal shrinkage rate of 5% or less at 105°C indicates that even at high temperatures, the TD thermal shrinkage is very small. By achieving a thermal shrinkage rate of 5% or less or 3.0% or less, short circuits at high temperatures can be effectively suppressed. The reason for a thermal shrinkage rate of -1.0% or more is as follows: when measuring the thermal shrinkage rate, the substrate expands in the TD, and sometimes the thermal shrinkage rate becomes a negative value less than 0%. This thermal shrinkage rate can 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% or less, for example, a method for manufacturing a separator by uniaxial extension of the MD (Medium-Density Matrix), and more preferably, a method for manufacturing by a dry method of uniaxial extension. In manufacturing methods of separators that extend in the biaxial direction by MD and TD, such as wet separators, the thermal shrinkage of TD is usually very large. In contrast, in dry separators that extend in the uniaxial direction, it is easy to obtain separator substrates with a thermal shrinkage rate of less than 5% or more than -1.0% and less than 3.0%.

[0093] The thermal shrinkage rate of the separator substrate in the film-forming direction (MD) after heat treatment at 105°C for 1 hour is preferably 20% or less, more preferably 15% or less, further preferably 10% or less, further preferably 8% or less, particularly preferably 6% or less, and most preferably 5.5% or less, from the viewpoint of the productivity of the energy storage device and the suppression of short circuits at high temperatures. This lower limit of thermal shrinkage rate is not limited, but is preferably 0.1% or more, for example, 0.3% or more or 0.5% or more. In the separator substrate of the present invention, as described above, when it is manufactured by uniaxial extension of MD, and when there is a connecting region in the microporous layer (A) parallel to MD, it is believed that even during the thermal shrinkage of MD, the stress applied to the polymer matrix can be mitigated, and a lower MD thermal shrinkage rate can be achieved.

[0094] <Tensile Strength of Separator Substrate> The tensile strength of the separator substrate MD, as described in the first embodiment of the present invention, is preferably 1500 kgf / cm² or higher (approximately 14.7 kN / cm² or higher) from the viewpoint of battery winding operability and high puncture resistance, more preferably 1600 kgf / cm² or higher, further preferably 1700 kgf / cm² or higher, even more preferably 1800 kgf / cm² or higher, and most preferably 1900 kgf / cm² or higher. There is no upper limit to the tensile strength of the separator substrate MD, but it is preferably 4000 kgf / cm² or lower, for example, 3800 kgf / cm² or lower, 3500 kgf / cm² or lower, 3200 kgf / cm² or lower, or 3000 kgf / cm² or lower.

[0095] <Tension elongation of the substrate of the separator> From the viewpoint of manufacturability and high puncture strength, the tensile elongation of the MD substrate as the separator is preferably 20% or more, more preferably 24% or more, further preferably 26.5% or more, further preferably 28% or more, and even more preferably 30% or more. From the viewpoint of processability, the upper limit of the tensile elongation of the MD substrate as the separator is preferably 60% or less, more preferably 55% or less, further preferably 50% or less, further preferably 45% or less, and even more preferably 40% or less. In the separator substrate of the present invention, as described above, when it is manufactured by uniaxial extension of the MD, and when a connecting region exists parallel to the MD in the microporous layer (A), it is believed that even during the stretching of the MD, the stress applied to the polymer matrix can be mitigated, and a higher tensile elongation of the MD can be achieved.

[0096] Second Implementation Method

[0097] Separators for energy storage devices The separator for an energy storage device according to the second embodiment of the present invention has a microporous layer (A) containing polypropylene as the separator substrate. The separator substrate may, as needed, include a microporous layer (B) mainly composed of polyolefin, in addition to the microporous layer (A). Furthermore, the separator substrate may have a coating layer (also referred to as a "surface layer," "coating layer," etc.; hereinafter simply referred to as a "coating layer") on the microporous layer (A) and / or the microporous layer (B). In the present invention, "microporous layer" refers to each layer of the microporous material constituting the separator substrate, "separator substrate" refers to the separator substrate excluding any coating layer, and "separator" refers to the separator as a whole, also including any coating layer.

[0098] <Microporous layer (A)> The separator for the energy storage device according to the second embodiment of the present invention has a microporous layer (A). The separator for the energy storage device may have only one microporous layer (A) or two or more layers. The microporous layer (A) contains polypropylene. In the present invention, the microporous layer (A) may be in the form of a membrane, wherein the integral dissolution amount of the microporous membrane (A) at 100°C to 130°C, as determined by cross-fractional chromatography (CFC), is 80.0% to 99.5% by mass of the total dissolution amount, and the integral dissolution amount at 20°C to 100°C is 0.5% to 20.0% by mass of the total dissolution amount.

[0099] <Materials of Microporous Layers (A)> The microporous layer (A) of the second embodiment of the present invention contains polypropylene, thereby maintaining good battery performance even after storage at high temperatures (e.g., 130°C). As a lower limit for the polypropylene content in the microporous layer (A), from the viewpoints of wettability, thin-film formation, and shutdown characteristics of the separator, it is preferably 50% by mass or more, more preferably 60% by mass or more, 70% by mass or more, 80% by mass or more, 85% by mass or more, 90% by mass or more, 92.5% by mass or more, or 95% by mass or more, based on the total mass of the microporous layer (A). As an upper limit for the polypropylene content in the microporous layer (A), from the viewpoint of maintaining good puncture strength and air permeability, it is, for example, 97.5% by mass or less, 98% by mass or less, 98.5% by mass or less, 99% by mass or less, or 99.5% by mass or less, based on the total mass of the microporous layer (A).

[0100] The microporous layer (A) of the second embodiment of the present invention contains polypropylene, and the polypropylene described in the first embodiment can be used.

[0101] The microporous layer (A) of the second embodiment of the present invention contains polypropylene, but may contain a thermoplastic elastomer. Examples of thermoplastic elastomers include: polyolefins different from the main component polypropylene, copolymers of polyolefins, and copolymers of polystyrene and polyolefins. Polyolefins refer to polymers containing monomers having carbon-carbon double bonds as repeating units. There are no limitations on the monomers constituting polyolefins; examples include monomers with 2 to 10 carbon atoms (C2 to C10) having carbon-carbon double bonds, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Examples of polyolefins include low-crystallinity polypropylene with low stereoregularity regions. One monomer may be used alone, or two or more monomers may be used in combination as monomers constituting the copolymer of the polyolefin. Examples of copolymers of polystyrene and polyolefins include: styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer (SEBS), styrene-ethylene-styrene copolymer, styrene-(ethylene-butene)-olefin copolymer (SEBC), and olefin-(ethylene-butene)-styrene copolymer (CEBS), etc., which may be hydrogenated polymers. These copolymers may be random copolymers or block copolymers, preferably block copolymers.

[0102] In the case where the microporous layer (A) of the second embodiment of the present invention contains a thermoplastic elastomer, from the viewpoint of obtaining a microporous layer (A) that balances low air permeability and high puncture strength by forming connecting regions in the polymer matrix and / or fibrils with polypropylene as the main component, it is preferable to have regions that are immiscible with polypropylene. Furthermore, in addition to having regions that are immiscible with polypropylene, from the viewpoint of strengthening the dispersibility or bonding between polypropylene and the thermoplastic elastomer, it is preferable to also have regions that are miscible. From these viewpoints, the thermoplastic elastomer is preferably a copolymer containing one or more repeating units selected from the group consisting of ethylene, propylene, and 1-butene. Preferred copolymers include ethylene / propylene (C2C3) copolymers, ethylene / 1-butene (C2C4) copolymers, ethylene / 1-hexene (C2C6) copolymers, ethylene / 1-octene (C2C8) copolymers, styrene-(ethylene-butene)-styrene copolymers (SEBS), olefin-(ethylene-butene)-olefin copolymers (CEBC), and olefin-(ethylene-butene)-styrene copolymers (CEBS). Here, ethylene-butene has a structurally similar structure to propylene, thus exhibiting a higher affinity for polypropylene. The thermoplastic elastomer can be used alone or in combination with two or more.

[0103] In the case where the microporous layer (A) of the second embodiment of the present invention contains a thermoplastic elastomer, the lower limit of the content of thermoplastic elastomer in the microporous layer (A) is, based on the total mass of the microporous layer (A), preferably 0.5% by mass or more, more preferably 1.0% by mass or more, further preferably 2.0% by mass or more, and further preferably 3.0% by mass or more, from the viewpoint of maintaining open pores. The upper limit of the content of thermoplastic elastomer in the microporous layer (A) is, based on the total mass of the microporous layer (A), preferably 20.0% by mass or less, more preferably 15.0% by mass or less, further preferably 10.0% by mass or less, further preferably 7.5% by mass or less, and particularly preferably 5.0% by mass or less.

[0104] The microporous layer (A) of the second embodiment of the present invention contains polypropylene, but may contain polyolefins other than polypropylene as the main component. Polyolefins refer to polymers containing monomers having carbon-carbon double bonds as repeating units. There is no limitation on the monomers constituting polyolefins other than polypropylene; examples include monomers with 2 or 4 to 10 carbon atoms having carbon-carbon double bonds, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Examples of polyolefins include homopolymers, copolymers, or multi-stage polymers. From the viewpoint of obtaining a microporous layer (A) that balances low air permeability and high puncture strength by adding a thermoplastic elastomer together with polypropylene, typically without mixing with polypropylene, to form a connecting region in the polymer matrix and / or fibrils mainly composed of polypropylene, polyethylene is preferred.

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

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

[0107] <Measurement values ​​of microporous layer (A) by cross-fractional chromatography (CFC)> According to the second embodiment of the present invention, the microporous membrane (A) has an integral dissolution rate of 80.0% to 99.5% of the total dissolution rate at temperatures between 100°C and 130°C as determined by cross-fractional chromatography (CFC), and an integral dissolution rate of 0.5% to 20.0% of the total dissolution rate at temperatures between 20°C and 100°C.

[0108] Here, "cross-fractional chromatography (CFC)" refers to a method that combines the temperature stripping fraction (TREF) for crystallinity fractionation with the gas chromatography-mass spectrometry (GPC) for molecular weight fractionation. By directly connecting the TREF and GPC sections, the relationship between the compositional distribution and molecular weight distribution of components with different crystallinities can be analyzed. Furthermore, sometimes the determination of the TREF section is referred to as the determination of CFC.

[0109] The determination of CFCs is performed as follows: The sample to be tested is dissolved in a precipitate. Then, upon cooling at a certain temperature, the more crystalline components crystallize first, followed by the less crystalline components as the temperature decreases. Next, if the temperature is increased in stages, the components with lower crystalline properties dissolve sequentially from the less crystalline to the more crystalline, and the concentration of the dissolved components at each temperature is measured. Furthermore, a dissolution temperature-dissolution amount curve is plotted with the dissolution temperature as the vertical axis and the dissolution amount as the horizontal axis, based on the sample's dissolution temperature (°C) and the amount dissolved (mass %) at that point. This yields the dissolution amount and integral dissolution amount at each temperature. Simultaneously, the molecular weight of the GPC section at each dissolution temperature is measured, thus obtaining the weight-average molecular weight (MwH or MwL) at each temperature. Figure 5 shows the dissolution temperature-dissolution amount curve for CFC determination.

[0110] As for the dissolution components of the microporous membrane (A) at 100°C to 130°C when measured by CFC, it is preferable to contain at least polypropylene, and polyethylene may also be present. As for the lower limit of the integral dissolution amount of the microporous membrane (A) at 100°C to 130°C when measured by CFC, from the viewpoint of high puncture strength and good shut-off characteristics, it is preferably 85% by mass or more, more preferably 90% by mass or more, and even more preferably 92.5% by mass or more, and even more preferably 95% by mass or more. As for the upper limit of the integral dissolution amount of the microporous membrane (A) at 100°C to 130°C when measured by CFC, from the viewpoint of maintaining good puncture strength and permeability, it is, for example, preferably 99% by mass or less, more preferably 98.5% by mass or less, even more preferably 98% by mass or less, and even more preferably 97.5% by mass or less.

[0111] The components that dissolve in the microporous membrane (A) at temperatures above 20°C but below 100°C when measured by CFCs are not limited, but preferably contain thermoplastic elastomers and / or polyethylene, and may also contain low-crystallinity components of polyolefins. As a lower limit for the integral dissolution amount of the microporous membrane (A) at temperatures above 20°C but below 100°C when measured by CFCs, from the viewpoints of film-forming properties, thin-film properties, low permeability, and high puncture strength, it is preferably 1.0% by mass or more, more preferably 2.0% by mass or more, further preferably 2.5% by mass or more, and further preferably 3.0% by mass or more. As the upper limit of the integral dissolution amount of the microporous membrane (A) at 20°C or higher but not reaching 100°C as determined by CFC, from the point of view of maintaining porosity, it is preferable to be 15.0% by mass or less of the total dissolution amount, more preferably 12.5% ​​by mass or less, further preferably 10.0% by mass or less, further preferably 7.5% by mass or less, and especially preferably 5.0% by mass or less.

[0112] Preferably, in the dissolution temperature-dissolution amount curve of the microporous membrane (A) of the present invention determined by cross-fractional chromatography (CFC), there are at least two peaks, namely a high-temperature side peak and a low-temperature side peak. As mentioned above, the dissolution temperature is an indicator of the crystallinity of the contained components; the higher the temperature, the higher the crystallinity, and the lower the temperature, the lower the crystallinity. The high-temperature side peak preferably contains at least polypropylene. As the lower limit of the peak temperature representing the maximum dissolution amount of the high-temperature side peak, from the viewpoint of obtaining a microporous layer (A) with high puncture strength, high TD tensile strength, and high heat resistance, it is preferably 105°C or higher, more preferably 108°C or higher, further preferably 111°C or higher, further preferably 113°C or higher, and especially preferably 115°C or higher. The upper limit of the peak temperature representing the maximum dissolution amount for the high-temperature side peak is preferably below 125°C, more preferably below 124°C, further preferably below 123°C, more preferably below 122.5°C, and especially preferably below 122°C, from the viewpoint of ensuring good film-forming properties, productivity, thin-film formation, and low permeability. On the other hand, the low-temperature side peak is not limited, but it is preferably containing thermoplastic elastomers and / or polyethylene. The lower limit of the peak temperature representing the maximum dissolution amount for the low-temperature side peak is preferably above 35°C, more preferably above 37°C, further preferably above 39°C, more preferably above 41°C, and especially preferably above 43°C, from the viewpoint of obtaining a microporous layer (A) with high puncture strength and good film-forming stability. From the viewpoint of obtaining a microporous layer (A) with low permeability, film-forming properties and thin film formation, the upper limit of the peak temperature representing the maximum dissolution of the low-temperature side peak is preferably below 100°C, more preferably below 90°C, further preferably below 80°C, further preferably below 70°C, especially preferably below 60°C, and most preferably below 50°C.

[0113] In the dissolution temperature-dissolution amount curve of the microporous membrane (A) of the present invention determined by cross-fractional chromatography (CFC), the lower limit of the weight average molecular weight (MwH) at the aforementioned high-temperature side peak temperature is preferably 300,000 or more, more preferably 500,000 or more, further preferably 650,000 or more, further preferably 750,000 or more, and even more preferably 800,000 or more, as far as the viewpoint of obtaining a microporous layer (A) with high puncture strength and high TD tensile strength. The upper limit of the weight average molecular weight (MwH) at the high-temperature side peak temperature is preferably 1,300,000 or less, more preferably 1,200,000 or less, further preferably 1,100,000 or less, further preferably 1,050,000 or less, and even more preferably 1,000,000 or less, as far as the viewpoint of obtaining a microporous layer (A) with good film-forming stability, thin-film formation, and low permeability. As a lower limit for the weight-average molecular weight (MwL) at the low-temperature side-wave peak temperature, from the viewpoint of obtaining a microporous layer (A) with high puncture strength, it is preferably 50,000 or more, more preferably 80,000 or more, further preferably 100,000 or more, further preferably 125,000 or more, and even more preferably 150,000 or more. As an upper limit for the weight-average molecular weight (MwL) at the low-temperature side-wave peak temperature, from the viewpoint of obtaining a microporous layer (A) with good film-forming properties and low air permeability, it is preferably 1,800,000 or less, more preferably 1,400,000 or less, further preferably 1,000,000 or less, further preferably 750,000 or less, even more preferably 500,000 or less, and most preferably 250,000 or less. Furthermore, the weight-average molecular weight (MwH and MwL) of the microporous membrane (A) measured by CFC in this invention at each dissolution temperature is the molecular weight converted from polystyrene.

[0114] <Mel Flow Rate (MFR) of Microporous Layer (A)> The preferred melt flow rate (MFR) of the microporous layer (A) in the second embodiment of the present invention, the MFR of polypropylene, the MFR of thermoplastic elastomer, and the MFR of polyethylene are as described in the first embodiment.

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

[0116] <Melting Tension of Microporous Layer (A)> The preferred melt tension Mt A (Mt A of a single layer) of the microporous layer (A) in the second embodiment of the present invention at 240°C is as described in the first embodiment.

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

[0118] <Area-average long pore diameter of microporous layer (A)> The preferred area-average long pore diameter of the microporous layer (A) in the second embodiment of the present invention is as described in the first embodiment.

[0119] <Porosity of Microporous Layer (A)> The porosity of the microporous layer (A) in the second embodiment of the present invention is preferably 30% or more from the viewpoint of avoiding clogging in the energy storage device and obtaining a microporous layer (A) with low air permeability. From the viewpoint of obtaining a microporous layer (A) with high puncture strength and high TD strength, and which is not easily broken, it is preferably 45% or less. The porosity of the microporous layer (A) is more preferably 31% or more and 42% or less, more preferably 32% or more and 40% or less, and even more preferably 33% or more and 38% or less.

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

[0121] <Additives for Microporous Layer (A)> In addition to polypropylene, the microporous layer (A) of the second embodiment of the present invention may further contain additives such as elastomers, nucleating agents, antioxidants, and fillers, as needed. The amount of additives is not particularly limited, and can be based on the total mass of the microporous layer (A), for example, 0.01% by mass or more, 0.1% by mass or more, or 1% by mass or more, and for example, 20% by mass or less, 10% by mass or less, or 7% by mass or less.

[0122] <Microporous layer (B)> The separator for the energy storage device according to the second embodiment of the present invention has a microporous layer (B) as required. The separator for the energy storage device may have only one microporous layer (B) or two or more layers. The microporous layer (B) is preferably mainly composed of polyolefin, more preferably mainly composed of polypropylene and / or polyethylene. This provides good porosity and good battery performance. The microporous layer (B) is further preferably mainly composed of polypropylene, thereby maintaining good battery performance even after storage at high temperatures (e.g., 130°C). In the present invention, the term "mainly composed of polypropylene" in the microporous layer (B) means that, based on the total mass of the microporous layer (B), it contains 50% by mass or more of polypropylene. As a lower limit for the polypropylene content in the microporous layer (B), from the viewpoint of wettability and film formation of the separator, it is preferably 55% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more. There is no upper limit for the polypropylene content in the microporous layer (B), for example, it can be 98% by mass or less, 99% by mass or less, or even 100% by mass.

[0123] The preferred materials, MFR, five-component ratio, melt tension, area-average long pore diameter, porosity, thickness, and additives of the microporous layer (B) of the second embodiment of the present invention are as described in the first embodiment.

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

[0125] <Air permeability (air tightness) of the substrate of the separator> From the viewpoint of ensuring good output in an energy storage device, the upper limit of the air permeability of the separator substrate in the second embodiment of the present invention is preferably 400 seconds / 100 cm³ or less, more preferably 350 seconds / 100 cm³ or less, further preferably 300 seconds / 100 cm³ or less, further preferably 280 seconds / 100 cm³ or less or 250 seconds / 100 cm³ or less, particularly preferably 220 seconds / 100 cm³ or less, and most preferably 200 seconds / 100 cm³ or less. The lower limit of the air permeability of the separator substrate is not limited, and for example, it can be 10 seconds / 100 cm³ or more, 20 seconds / 100 cm³ or more, or 30 seconds / 100 cm³ or more.

[0126] <Porosity of the substrate for separators> The porosity of the separator substrate in the second embodiment of the present invention is 30.0% to 45.0%. Theoretically, there is no limitation; for example, the morphology of a separator manufactured by uniaxial extension in the film-forming direction (MD) is shown in Figure 1. Due to the MD alignment, it becomes a separator that is prone to breakage in the width direction (TD). A lower porosity in the separator substrate indicates fewer and / or smaller pores in the separator, resulting in a higher weight per area. It also indicates relatively fewer fibrils and more polymer matrix. It is presumed that due to stretching towards the TD, more binding molecules can be drawn from the polymer matrix, thus achieving high TD tensile strength and becoming a separator that is not easily broken. As a lower limit value for the porosity of the separator substrate, from the viewpoint of avoiding clogging in the energy storage device and obtaining good air permeability of the separator, it is preferably 31% or more, more preferably 32% or more, further preferably 33% or more, further preferably 34% or more, and most preferably 35% or more. From the viewpoint of obtaining a microporous layer (A) with high puncture strength and high TD strength that is not easily broken, the upper limit of the porosity of the separator substrate is preferably 42% or less, more preferably 41% or less, further preferably 40% or less, further preferably 39% or less, and especially preferably 38% or less.

[0127] <Puncture Strength of Separator Substrate> As a lower limit value for the puncture strength of the separator substrate in the second embodiment of the present invention, when the thickness of the separator substrate is converted to 10 μm, it is preferably 150 gf or more (approximately 1.47 N or more), more preferably 180 gf or more, 200 gf or more, 220 gf or more, or 240 gf or more, and even more preferably 250 gf or more, 260 gf or more, 270 gf or more, or 280 gf or more. The upper limit value for 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.

[0128] <Thermal Shrinkage Rate of Separator Substrate> The preferred thermal shrinkage rate of the separator substrate in the second embodiment of the present invention is as described in the first embodiment.

[0129] <Tensile strength and elongation of the substrate of the separator> The tensile strength of the separator substrate MD, as described in the second embodiment of the present invention, is preferably 1500 kgf / cm² or higher (approximately 14.7 kN / cm² or higher) from the viewpoint of battery winding operability and high puncture strength. More preferably, it is 1600 kgf / cm² or higher, even more preferably 1700 kgf / cm² or higher, particularly preferably 1800 kgf / cm² or higher, and most preferably 1900 kgf / cm² or higher. There is no upper limit to the tensile strength of the separator substrate MD, but it is preferably 4000 kgf / cm² or lower, for example, 3800 kgf / cm² or lower, 3500 kgf / cm² or lower, 3200 kgf / cm² or lower, or 3000 kgf / cm² or lower.

[0130] From the viewpoint of ensuring the productivity and yield of the energy storage device by preventing the separator from easily breaking, the tensile strength of the separator substrate TD in the second embodiment of the present invention is preferably 100 kgf / cm² or more, more preferably 120 kgf / cm² or more, further preferably 140 kgf / cm² or more, further preferably 160 kgf / cm² or more, particularly preferably 180 kgf / cm² or more, and most preferably 200 kgf / cm² or more. The upper limit of the tensile strength of the separator substrate TD is not limited, but it is preferably 4000 kgf / cm² or less, for example, it can be 3000 kgf / cm² or less, 2000 kgf / cm² or less, 1000 kgf / cm² or less, 500 kgf / cm² or less, or 300 kgf / cm² or less.

[0131] The tensile elongation of the MD substrate of the separator substrate according to the second embodiment of the present invention is preferably 20% or more, more preferably 24% or more, further preferably 26.5% or more, further preferably 28% or more, and even more preferably 30% or more, from the viewpoint of manufacturability and high puncture strength. As the upper limit of the tensile elongation of the MD substrate of the separator substrate, from the viewpoint of processability, it is preferably 60% or less, more preferably 55% or less, further preferably 50% or less, further preferably 45% or less, and even more preferably 40% or less. In the separator substrate of the present invention, as described above, when it is manufactured by uniaxial extension of the MD, and when a connecting region exists parallel to the MD in the microporous layer (A), it is believed that even when the MD is stretched, the stress applied to the polymer matrix can be mitigated, and a higher tensile elongation of the MD can be achieved.

[0132] Manufacturing method of separators for energy storage devices The manufacturing methods for the separator of the energy storage device according to the first and second embodiments include: a melt extrusion step of obtaining a resin sheet (precursor sheet) by melt extruding a resin composition with polypropylene as the main component (hereinafter also referred to as "polypropylene-based resin composition"); and a pore-forming step of opening and porousening the obtained precursor sheet. The manufacturing methods of the microporous layer are generally divided into dry methods that do not use solvents in the pore-forming step and wet methods that use solvents.

[0133] Examples of dry methods include: a method in which a polypropylene resin composition is melt-blended and extruded, and then heat-treated and stretched to separate the polypropylene crystal interface; and a method in which a polypropylene resin composition is melt-blended with an inorganic filler and formed into a film, and then stretched to separate the polypropylene and inorganic filler interface.

[0134] Examples of wet methods include: melting and mixing a polypropylene resin composition with a pore-forming material and forming it into a film, then stretching it as needed and extracting the pore-forming material; or dissolving a polypropylene resin composition and immersing it in a solvent that is unsuitable for polypropylene to allow the polypropylene to solidify, while simultaneously removing the solvent.

[0135] In the melt mixing of polypropylene resin compositions, single-shaft extruders and twin-shaft extruders can be used, as well as kneaders, Laboplastomills, mixing rollers and Bamboo mixers.

[0136] The polypropylene-based resin composition may contain resins other than polypropylene and additives, depending on the manufacturing method of the microporous layer or the physical properties of the target microporous layer. Examples of additives include: pore-forming materials, fluorinated flow modifiers, waxes, 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 materials include: plasticizers, inorganic fillers, or combinations thereof.

[0137] Examples of plasticizers include: liquid paraffin, paraffin and other hydrocarbons; dioctyl phthalate, dibutyl phthalate and other esters; oleyl alcohol, stearyl alcohol and other higher alcohols.

[0138] Examples of inorganic fillers include: oxide ceramics such as alumina, silicon oxide (silicon oxide), titanium oxide, zirconium oxide, magnesium oxide, cerium oxide, yttrium oxide, 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, hydrous kaolin, pyrophyllite, montmorillonite, sericite, mica, magnesia chlorite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomite, and quartz sand; and glass fiber.

[0139] The preferred method for manufacturing the separator substrate according to the first and second embodiments is a dry layered crystallization opening process that involves heat treatment and stretching to peel off the polypropylene crystallization interface. Here, the preferred method for manufacturing the separator substrate having a microporous layer (A) and a microporous layer (B) is to use at least one of the following methods (i) and (ii): (i) A method for manufacturing a separator substrate by co-extrusion of a microporous layer (A) and a microporous layer (B) into a film, which is then subjected to annealing, cold stretching, hot stretching, and heat tempering steps; and (ii) A method for manufacturing a spacer substrate by extruding microporous layer (A) and microporous layer (B) separately into films, laminating them together, and then feeding them into annealing, cold stretching, hot stretching and heat easing steps.

[0140] Of the co-extrusion process (i) and lamination process (ii) described above, co-extrusion process (i) is preferred from the viewpoint of manufacturing cost. In co-extrusion process (i), the extrusion film-forming conditions for the microporous layers (A) and (B) are preferably achieved by spraying the resin at the lowest possible temperature and effectively cooling it by blowing low-temperature air. It is also preferable to rapidly cool the film with air after film formation; the temperature of the blowing air is preferably below 20°C, and more preferably below 15°C. By blowing this controlled low-temperature air, the resin in the film is uniformly aligned in the microstructure (MD).

[0141] Manufacturing method of the first embodiment As a method for manufacturing the separator substrate in the first embodiment, both the co-extrusion process (i) and the lamination process (ii) described above can include an annealing step after extrusion film formation. By performing the annealing step, the crystalline structure of the microporous layers (A) and (B) grows, tending to improve porosity. Through annealing at a specific temperature for a specified time, both microporous layers (A) and (B) tend to obtain good area-average pore size, high porosity, low air permeability, and high puncture strength. The reason for this is that crystal growth is achieved without disrupting the crystalline structure, resulting in higher porosity. In the annealing step, the lower limit of the temperature range is preferably above 115°C, more preferably above 120°C, further preferably above 125°C, further preferably above 130°C, and especially preferably above 135°C. The upper limit of the temperature range is preferably below 160°C, more preferably below 155°C, and further preferably below 150°C. The annealing process is preferably performed for at least 20 minutes, and more preferably at least 60 minutes. This allows the polypropylene, as the main component, to achieve high crystal orientation, resulting in higher porosity during subsequent steps. This leads to good area-average pore size, high porosity, low air permeability, and high puncture strength, exhibiting high input / output and high energy density in the energy storage device, which is preferable from the above perspective.

[0142] The manufacturing method of the separator substrate according to the first embodiment may include a stretching step after the annealing step. As a stretching treatment, either uniaxial stretching or biaxial stretching can be used. From the viewpoints of reducing manufacturing costs when using the dry method and reducing thermal shrinkage of TD, uniaxial stretching is preferred, but there is no limitation. The elongation rate of the MD during cold stretching ((length after stretching - length before stretching) / length before stretching × 100 (%)) is preferably 5% to 50%, more preferably 20% to 45%, and even more preferably 25% to 45%. If it is above the lower limit, the amount of cracking during cold stretching increases, which makes it easier to obtain a smaller pore size during hot stretching. This results in a moderately good trunk height that is not too large. Furthermore, by achieving a small pore size, the regions (becoming the dyed bright areas) shown in Figures 1 and 2 are connected, and the MD length increases. By easing the stress in the connected regions during failure, failure is less likely to occur. The polyolefin, as the main component, can elongate, resulting in a highly aligned layered crystal structure in the high-strain region. Therefore, a separator with high puncture strength can be obtained. On the other hand, if it is below the upper limit, the cold stretching is too high, thereby suppressing the probability of film breakage during stretching and ensuring the productivity and yield of the separator. The preferred temperature for cold stretching is 10°C to 50°C, and more preferably 20°C to 30°C. From the perspective of manufacturing cost, it can be carried out at room temperature (23±2°C). From the perspectives of improving both the puncture strength and low permeability of the obtained separator substrate, manufacturing cost, and reducing the thermal shrinkage of TD, uniaxial stretching is preferred.

[0143] To suppress thermal shrinkage of the separator substrate, a heat treatment step can be performed after the stretching step or the hole forming step for the purpose of heat fixation. The heat treatment step may include: a heat stretching operation performed at a specified temperature and a specified stretching ratio for the purpose of adjusting physical properties, and / or a heat mitigation operation performed at a specified temperature and a specified mitigation ratio for the purpose of reducing shrinkage stress imparted during film formation and stretching. The heat mitigation operation can be performed after the heat stretching operation.

[0144] In the heat stretching of the first embodiment, the lower limit of the elongation rate is set to 100%, preferably 140% or more, more preferably 160% or more, even more preferably 170% or more, even more preferably 180% or more, particularly preferably 190% or more, and most preferably 200% or more. The upper limit of the elongation rate is set to 100%, and the stretching is preferably 280% or less, more preferably 260% or less, even more preferably 250% or less, and even more preferably 240% or less. If the value is above the lower limit, an extended chain is formed, becoming a strong fibril. The connecting region is also extended in the MD, and more polymer matrix can be connected to the fibril and / or connecting region. As a result, the length of the MD also increases, and a separator with high puncture strength is obtained. Furthermore, in the above heat treatment step, by performing the stretching operation at a specific elongation ratio or higher, there is a tendency to obtain a good trunk height. The rationale is as follows: Below a specific elongation ratio, pores are preferentially formed, and after the pore-forming process, structural changes occur accompanied by variations in the trunk height. On the other hand, below the upper limit, porosity damage caused by excessive thermal stretching leading to TD and necking in the film thickness direction can be suppressed, thus achieving good air permeability. During the thermal tempering after thermal stretching, the tempering rate in MD is preferably 10% to 50%, more preferably 20% to 45%. These heat treatment steps can be performed using a tenter frame or a roll stretcher. The temperature of the heat treatment step is preferably 120°C to 160°C, and even more preferably 130°C to 155°C. In cases containing thermoplastic elastomers, by performing the above heat treatment above their melting point, the stress exerted on the polypropylene skeleton of the main component during thermal stretching can be mitigated, and the fibrils are uniformly stretched in MD, thereby obtaining a high-puncture-strength separator with improved rigidity and toughness.

[0145] From the viewpoint of balancing high puncture strength and low air permeability of the membrane separator, and from the viewpoint of obtaining a shape as shown in, for example, Figures 1 and / or 2, it is preferable to perform the above-mentioned annealing and thermal stretching operations in the manufacturing method of the separator. From these viewpoints, in other embodiments of the present invention, a method for manufacturing a separator for a storage device including the following steps is provided: An annealing step involving annealing resin sheets containing polypropylene, polyethylene, and thermoplastic elastomers at a temperature between 135°C and 160°C; and The annealed resin sheet is subjected to a heat treatment step of MD stretching at a temperature of 130°C to 155°C. The resin sheet, annealing conditions, and heat treatment conditions for the accompanying heat stretching operation can be described as above.

[0146] Manufacturing method of the second embodiment As a method for manufacturing the separator substrate according to the second embodiment, both the co-extrusion process (i) and the lamination process (ii) described above can include an annealing step after extrusion film formation. By performing the annealing step, the crystalline structure of the microporous layers (A) and (B) grows, and there is a tendency to improve the porosity. It is believed that by subjecting annealing at a high temperature and / or a long time not exceeding the melting point of the contained components, crystal growth can be achieved without disrupting the crystalline structure, resulting in higher porosity. Both the microporous layers (A) and (B) can obtain good area-average long pore diameter, high porosity, low air permeability, and high puncture strength. On the other hand, if excessively high porosity is obtained, the TD tensile strength decreases, and it can become a separator substrate that is prone to breakage. Therefore, it is preferable to control the physical property balance of the separator substrate by subjecting it to annealing at a specific temperature for a specified time. In the annealing step, the lower limit of the temperature range is preferably above 115°C, more preferably above 120°C, and the upper limit of the temperature range is preferably below 150°C, more preferably below 145°C, further preferably below 140°C, further preferably below 135°C, and especially preferably below 130°C. The annealing process is preferably performed for at least 20 minutes, more preferably at least 60 minutes. This results in a moderately high orientation of the polypropylene crystals, achieving moderately high porosity during subsequent extension steps. Furthermore, during uniaxial extension towards the molecular matrix (MD), excessive extraction of bound molecules from the polypropylene crystals into the MD is suppressed, ensuring that bound molecules can be extracted against tensile stress (TD). This achieves a separator with low permeability, high elastic modulus resulting in high puncture strength and high TD tensile strength, exhibiting high input / output and high energy density in the energy storage device. Moreover, the separator is less prone to breakage, ensuring the productivity and yield of the energy storage device. From the above perspectives, this is preferable.

[0147] The manufacturing method of the separator substrate in the second embodiment may include a stretching step after the annealing step. As a stretching treatment, either uniaxial stretching or biaxial stretching can be used. From the viewpoints of reducing manufacturing costs when using the dry method and reducing thermal shrinkage of TD, uniaxial stretching is preferred, but there is no limitation. The elongation rate of the MD during cold stretching ((length after stretching - length before stretching) / length before stretching × 100 (%)) is preferably 5% to 50%, more preferably 10% to 45%, and even more preferably 20% to 40%. By increasing the amount of cracking during cold stretching, it is easier to obtain a smaller pore size during hot stretching, resulting in a moderately good trunk height that is not too large. Furthermore, by achieving a small pore size, the regions shown in Figures 1 and 2 are connected. The stress in the connected regions is relieved during failure, making failure less likely. The polypropylene contained therein can elongate, resulting in a highly oriented, layered crystalline structure in high-strain regions, thus obtaining a separator with high puncture strength. The temperature for cold stretching is preferably 10°C to 50°C, more preferably 20°C to 30°C. From a manufacturing cost perspective, it can be carried out at room temperature (23±2°C). From the perspectives of improving both the puncture strength and low permeability of the obtained separator substrate, manufacturing cost, and reducing the heat shrinkage of TD, uniaxial extension is preferable.

[0148] To suppress thermal shrinkage of the separator substrate, a heat treatment step can be performed after the stretching step or the hole forming step for the purpose of heat fixation. The heat treatment step may include: a heat stretching operation performed at a specified temperature and a specified stretching ratio for the purpose of adjusting physical properties, and / or a heat mitigation operation performed at a specified temperature and a specified mitigation ratio for the purpose of reducing shrinkage stress imparted during film formation and stretching. The heat mitigation operation can be performed after the heat stretching operation. In the heat stretching of the second embodiment, the MD size before stretching is set to 100%, and the stretching is preferably 120% to 220%, more preferably 125% to 200%, further preferably 130% to 180%, and even more preferably 130% to 160%. This forms an extended chain, becoming a strong fibril, and the bound molecules are not excessively drawn out from the polypropylene crystallization into the MD, ensuring that the bound molecules can be drawn out against TD tensile damage, thereby obtaining a separator with high puncture strength and high TD tensile strength. Furthermore, in the above-mentioned heat treatment step, by performing the stretching operation at a specific elongation ratio or higher, there is a tendency to obtain a good trunk height. The reason for this is as follows: at a specific elongation ratio below a certain elongation ratio, pores are preferentially opened, and after the pore opening process, structural changes occur accompanied by changes in trunk height. During the heat treatment after heat stretching, the MD is preferably softened by 10% to 50%, more preferably by 20% to 45%. These heat treatment steps can be performed using a tenter frame 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. For example, when polyethylene and / or thermoplastic elastomers are used as low-crystallinity components, the above-mentioned heat treatment, which is performed above their melting point, can alleviate the stress on the polypropylene skeleton during heat stretching, and the fibrils are stretched uniformly in the MD, thereby obtaining a separator with high puncture strength, improved rigidity and toughness, and good area average long pore size and low air permeability.

[0149] The separator substrates obtained in the first and second embodiments can be used directly as separators for energy storage devices. Further layers such as coatings can be provided on one or both sides of the separator substrate, and surface treatments such as corona treatment can be performed as needed.

[0150] Energy Storage Devices The energy storage device of the present invention includes a separator for energy storage devices according to the first or second embodiment of the present invention. The energy storage device of the present invention has a positive electrode and a negative electrode, and preferably the separator for energy storage devices of the present invention is disposed between the positive electrode and the negative electrode.

[0151] The energy storage device used in the first and second embodiments is not limited, and examples 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, double-layer capacitors, lithium-ion capacitors, redox flow batteries, and zinc-air batteries, etc. Among these, from the viewpoint of high energy density, low cost, and durability, lithium secondary batteries, lithium-ion secondary batteries, or lithium-ion capacitors are preferred, and lithium-ion secondary batteries are even more preferred.

[0152] The energy storage device can be manufactured, for example, by overlapping the positive and negative electrodes with the separators described above, and winding them as needed to form a multilayer electrode body or a wound electrode body. After that, it is filled into an outer casing. The positive and negative electrodes are connected to the positive and negative terminals of the outer casing via leads or the like. Then, a non-aqueous electrolyte containing a non-aqueous solvent such as chain or cyclic carbonates and an electrolyte such as lithium salts is injected into the outer casing, and the outer casing is sealed.

[0153] This energy storage device is preferably a lithium-ion secondary battery; here, a preferred configuration of the lithium-ion secondary battery is described.

[0154] The cathode is not particularly limited as long as it functions as the cathode of a lithium-ion secondary battery, and known materials can be used. Preferably, the cathode contains one or more materials selected from the group that can absorb and release lithium ions as the cathode active material. From the viewpoint of battery capacity and safety, the cathode is preferably a lithium cobalt oxide represented by LiCoO₂, a spinel-based lithium manganese oxide represented by Li₂Mn₂O₄, a spinel-based lithium nickel manganese oxide represented by Li₂Mn₁.₅Ni₀.₅O₄, a lithium nickel oxide represented by LiNiO₂, a lithium-containing composite metal oxide represented by LiMO₂ (where M represents two or more elements selected from the group consisting of Ni, Mn, Co, Al, and Mg), or a lithium iron phosphate compound represented by LiFePO₄. From the perspective of high safety and long-term stability, lithium cobalt oxide represented by LiCoO2, lithium nickel oxide represented by LiNiO2, lithium-containing composite metal oxide represented by LiMO2 (M represents two or more elements selected from the group composed of Ni, Mn, Co, Al and Mg), and lithium iron phosphate compounds represented by LiFePO4 are preferred, especially lithium iron phosphate compounds represented by LiFePO4.

[0155] The negative electrode is not particularly limited as long as it functions as the negative electrode in a lithium-ion secondary battery, and is generally known. Preferably, the negative electrode contains one or more materials selected from the group consisting of materials capable of absorbing and releasing lithium ions and metallic lithium as the negative electrode active material. That is, the negative electrode preferably contains one or more materials selected from the group consisting of metallic lithium, carbon materials, materials containing elements capable of forming alloys with lithium, and lithium-containing compounds as the negative electrode active material. Examples of such materials, besides metallic lithium, include, for example, hard carbon, soft carbon, artificial graphite, natural graphite, graphite, pyrolytic carbon, coke, glassy carbon, calcined organic polymer compounds, mesophase carbon microspheres, carbon fibers, activated carbon, graphite, carbon colloids, and carbon black. [Example]

[0156] The measurement and evaluation methods used in this embodiment will be described below. In this embodiment, "separator substrate" is equivalent to "separator", therefore, in the following description, "separator substrate" can be referred to as "separator".

[0157] Measurement and Evaluation Methods The measurement and evaluation methods for the first and second embodiments are described below. [Determination of Melt Flow Rate (MFR)] The melt flow rate (MFR) of microporous layers (A) and (B) was determined according to JIS K 7210 at a temperature of 230°C and a load of 2.16 kg (unit: g / 10 min). The MFR of polypropylene or elastomer was determined according to JIS K 7210 at a temperature of 230°C and a load of 2.16 kg. The melt flow rate (MFR) of polyethylene was determined according to JIS K 7210 at a temperature of 190°C and a load of 2.16 kg.

[0158] [Determination 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 polymer or microporous layer of the sample was also measured under the same conditions. Based on the calibration curve, the converted weight-average molecular weight (Mw), number-average molecular weight (Mn), and MWD (Mw / Mn) obtained by dividing the weight-average molecular weight (Mw) by the number-average molecular weight (Mn) for the polymer or microporous layer of polystyrene were calculated according to the following conditions. Column: 2 TSKgel GMHHR-H(20) HT (7.8 mm I.D. × 30 cm) Mobile phase: 1,2,4-trichlorobenzene Detector: RI Column temperature: 160℃ Sample concentration: 1 mg / ml Calibration curve: Polystyrene

[0159] [Determination of Melt Tension] The melt tension (mN) of microporous layer (A) and microporous layer (B) was measured using a Capillograph manufactured by Toyo Seiki Co., Ltd. under the following conditions. • Capillary tube: 1.0 mm in diameter, 20 mm in length • Extrusion speed: 2 mm / min • Winding speed: 60 m / min Temperature: 240℃

[0160] [Determination of the proportions of the five-element components] The proportions of the five components of polypropylene were calculated using the peak height method based on the 13C-NMR spectrum recorded in the Handbook of Polymer Analysis (edited by the Japan Society for Analytical Chemistry). The 13C-NMR spectroscopy was performed using a JEOL-ECZ500, dissolving polypropylene particles in o-dichlorobenzene-d, at a measurement temperature of 145°C and a cumulative measurement of 25,000 times.

[0161] [DSC Measurement] The DSC measurement of the microporous layer (A) was performed using a DSC-60 manufactured by Shimadzu Corporation under the following conditions. • Sample size: Approximately 5 mg • Pool used: Aluminum rolled edge pool (diameter 5.8 mm) • Operating environment: Nitrogen gas (flow rate 50 mL / min) Temperature program: Step 1: Increase the temperature from room temperature to 230°C at a rate of 10°C / min, and hold for 5 min. Step 2: Cool down to 20℃ at a rate of 10℃ / min and hold for 5 min. Step 3: Increase the temperature to 200℃ at a rate of 10℃ / min and hold for 5 min. From the DSC curve (with heat flow on the vertical axis and temperature on the horizontal axis) obtained during the heating process in step 3 above, use the analysis software TA-60 provided with the device to read the peak temperatures of endothermic peaks A and B, calculate the area SA of endothermic peak A and the area SB of endothermic peak B, and then calculate SB / SA.

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

[0163] [Determination of Porosity (%)] Take a sample with dimensions of 10 cm × 10 cm square from the separator or microporous layer, calculate its volume (cm 3) and mass (g), and then calculate its porosity using the following formula based on its density (g / cm 3). Porosity (%) = (Volume - Mass / Density) / Volume × 100

[0164] [Breathability (seconds / 100 cm³)] The air permeability (seconds / 100 cm³) of the separator substrate was measured using a Gurley air permeability meter according to JIS P-8117. The air permeability (seconds / 100 cm³) was divided by the thickness of the separator substrate (μm) and multiplied by 10 μm to obtain the air permeability (seconds / 100 cm³·10 μm) when the thickness of the separator substrate is converted to 10 μm.

[0165] [MD heat shrinkage rate (%), TD heat shrinkage rate (%)] Samples were obtained by cutting squares with MD / TD ratios of 50 mm from the substrate of the separator. The samples were placed on copy paper and then placed in a hot air dryer (manufactured by Yamato Scientific, DF1032) and heat-treated at 105°C for 1 hour under normal atmospheric pressure. The samples were then removed from the hot air dryer and cooled at 25°C for 10 minutes. The dimensional shrinkage rate was then calculated. Heat shrinkage rate (%): (Dimensions before heating (mm) - Dimensions after heating (mm)) / (Dimensions before heating (mm)) × 100

[0166] [Puncture strength] Prepare a hemispherical needle with a tip radius of 0.5 mm. Position the separator between two plates with openings of 11 mm in diameter. Using an IMADA Corporation "MX2-50N" puncture tester, conduct a puncture test with a needle tip radius of curvature of 0.5 mm, a separator holding plate opening diameter of 11 mm, and a puncture speed of 25 mm / min. With the needle in contact with the separator, measure the maximum puncture load (i.e., puncture strength (gf)). Divide the puncture strength (gf) by the separator substrate thickness (μm) and multiply by 10 μm to calculate the puncture strength (gf / 10 μm) when the separator substrate thickness is converted to 10 μm.

[0167] [MD tensile strength, MD tensile elongation, TD tensile strength] The tensile strength of the separator was determined as follows: using a tensile testing machine (Minebea TG-1kN type), the specimen length was set to 35 mm before the test, and the specimen was stretched at a speed of 100 mm / min. The strength at fracture (tensile load value) was divided by the cross-sectional area of ​​the specimen, and the resulting value was taken as the tensile strength (kgf / cm²). The elongation at fracture (the elongation ratio from before the test) was taken as the tensile elongation (%). The tensile strength and tensile elongation were measured for the MD (Medium Degree) of the separator, and the tensile strength was measured for the TD (Dielectric Tensile ...

[0168] [Withstand Voltage Measurement (kV)] A withstand voltage tester (grade) manufactured by Kikusui Electronics Industry Co., Ltd. was used. A separator substrate sandwiched between aluminum foils was placed on the sample stage. Electrode indenters were installed, and the voltage was increased from an initial voltage of 0 kV at a rate of 0.025 kV / second. The voltage value when a current of 0.2 mA flows between the electrodes was taken as the withstand voltage value. Furthermore, the withstand voltage value (kV) was divided by the separator thickness (μm) and multiplied by 10 μm to calculate the withstand voltage (kV / 10 μm) when the thickness of the separator substrate was converted to 10 μm.

[0169] [Average area long aperture (nm)] The area-average long pore diameter was determined by image analysis from cross-sectional SEM observations. As a pretreatment, the separators were stained with ruthenium and cryogenically cut to prepare cross-sectional specimens. The cross-section was set as the MD-ND plane. The cross-sectional specimens were fixed to the SEM specimen stage for cross-sectional observation using a conductive adhesive (carbon-based), dried, and then subjected to conductivity treatment. Osmium coating was performed using an osmium coating machine (HPC-30W, manufactured by VACUUM DEVICE Co., Ltd.) with the applied voltage adjustment handle set to 4.5 and the discharge time 0.5 seconds, serving as the microscopic examination specimen. Next, using a scanning electron microscope (Hitachi High-Technologies S-4800), eight arbitrary points on each microporous layer cross-section of the microporous membrane were observed under conditions of accelerating voltage 1 kV, detection signal LA10, actuation distance 5 mm, and magnification 30000x.

[0170] For the observed images, using functions from OpenCV (a library for image parsing) in the Python programming language environment, trimming was performed to include only a cross-section of any single microporous layer. After removing the surface, external, and other microporous layers, binarization was performed using the Otsu method to distinguish between the resin portion and the pore portion, and the average major diameter of the pore portion was calculated. At this point, pores with an area of ​​less than 0.001 nm² existing both outside and across the imaging range were excluded from the measurement. The average diameter was calculated by averaging the areas of each pore.

[0171] [Truncation Height (nm)] The trunk height was determined by analyzing images observed using cross-sectional SEM. Similar to the calculation of the area-average long pore diameter, after preparing cross-sectional and microscopic specimens, any three points on the cross-section of the microporous membrane were observed under the conditions of accelerating voltage 1 kV, detection signal LA10, actuation distance 5 mm, and magnification 5000.

[0172] For the observed image, using functions from OpenCV (a library for image parsing) in the Python programming language environment, trimming was performed by including only a cross-section of any one microporous layer. After removing the surface and external parts, fibrillation was performed by repeatedly blurring only the ND layer. Otsu's method was used for binarization to distinguish between the resin layer and the pore layer. The above fibrillation removal process was performed by repeatedly applying a Gaussian filter 100 times within a 3-pixel bar-shaped processing area on the ND and 1 pixel on the MD layer. After binarization, opening and closing processing were sequentially performed within an elliptical processing area with a major axis of 7 pixels on the ND and a minor axis of 3 pixels on the MD layer to remove noise, thereby obtaining the fibrillation-removed image.

[0173] From the above fiber removal image, the trunk height is calculated. The fiber removal image is cropped to a size of 1 pixel of MD, and the length of the resin part of all ND is detected. This is repeated in a way that includes all MD, and the length of the resin part of ND in the entire range of the fiber removal image is detected. Figure 3 is a schematic diagram of a portion of the cropped fiber removal image. In the figure, the bright part is the resin part (5), the dark part is the hole part (6), and an example of the part cropped and detected to a size of 1 pixel of MD is the part indicated by two arrows. For the obtained length of the resin part of ND, a weighted average is calculated with the length of the resin part of ND as the weight, and the obtained value is used as the trunk height. If the length of the resin part of ND is set as L, the trunk height H is calculated by the following formula: [Number 1] . By using a weighted average of n lengths L, the trunk height H becomes a value that is more correlated with puncture intensity and withstand voltage.

[0174] [MD length (nm)] The MD length was determined by image analysis from cross-sectional SEM observation. Similar to the calculation of the area-average long pore diameter, after preparing cross-sectional and microscopic specimens, any three points on the cross-section of the microporous membrane were observed under the conditions of accelerating voltage 1 kV, detection signal LA10, actuation distance 5 mm, and magnification 5000.

[0175] For the observed image, in the Python programming language (registered trademark) environment, OpenCV functions, which are used as image parsing libraries, were used to trim the image by including only the cross-section of the substrate of the separator. After removing the surface and the exterior, the Otsu method was used for binarization to obtain a binarized image that distinguishes the resin part and the hole part.

[0176] From the above binarized image, the MD length is calculated. The fiber removal image is cropped out at a size of 1 pixel of ND, and the length of the resin part of all MDs is detected. This process is repeated to include all NDs, and the length of the resin part of the MDs in the entire range of the binarized image is detected. Figure 4 is a schematic diagram of a portion of the cropped binarized image. In the figure, the bright part is the resin part (5), the dark part is the hole part (6), and an example of the part cropped and detected at a size of 1 pixel of ND is the part indicated by two arrows. For the obtained length of the resin part of MD, a weighted average is calculated with the length of the resin part of MD as the weight, and the obtained value is taken as the MD length. If the length of the resin part of MD is set to l, then the MD length LMD is calculated by the following formula: [Number 2] . By using a weighted average of n lengths l, the MD length LMD becomes a value that is more correlated with puncture intensity.

[0177] [Analysis of the coloring of the bright areas] The staining of the bright area was determined by image analysis from cross-sectional SEM observations. Similar to the calculation of the area-average long pore diameter, after preparing cross-sectional and microscopic specimens, a scanning electron microscope (Hitachi High-Technologies SU8220) was used to observe any three points on the surface of the microporous membrane under conditions of 2 kV accelerating voltage, LA100 detection signal, 5 mm actuation distance, and 30,000x magnification. The microscope was set to achieve unsaturated brightness and the highest possible contrast to acquire a composite image as an 8-bit grayscale image.

[0178] For the acquired composite image, use the image processing software ImageJ, following the order shown below, to extract the stained bright areas and analyze the area and total area ratio. (1) In the acquired composite image, select the area where only the microporous membrane or microporous layer is photographed as the image processing area. (2) Using the Filters function of Image J, the Mean is used to set the Radius of the argument to 2 piexels to perform averaging. (3) For the obtained averaged image, use the Threshold function to confirm the image and pixel value histogram (horizontal axis is the pixel brightness value, vertical axis is the number of pixels), and set the maximum and minimum values ​​of the threshold used to distinguish the bright areas of the color. The maximum value of the threshold is set to 255. Regarding the minimum value of the threshold, it is set in a way that allows selection of the white bright areas in the image. However, when it is difficult to distinguish the bright areas, the peak height of the highest brightness peak in the above histogram is set to 100, and the minimum value of the threshold is set at the brightness value (horizontal axis) at a height of 30 on the high brightness side. (4) Using the Analyze Particles function, calculate the maximum area of ​​each stained bright part with an area of ​​more than 100 nm 2 and less than 0.1 μm 2 and the total area ratio S1 relative to the whole image.

[0179] [Determination by cross-fractional chromatography (CFC)] The determination was performed using cross-fractional chromatography (CFC) as follows: First, the sample of the separator substrate or microporous layer was dissolved in the elution solution and introduced into the apparatus. The sample solution was held at 145°C for 30 minutes followed by 135°C for 30 minutes. Next, the temperature was lowered to -17°C at a rate of 1.0°C / min and held for 60 minutes. Subsequently, the temperature was sequentially increased at a rate of 40°C / min. The temperature was first increased from -17°C to -10°C, then from -10°C to 0°C, then increased from 0°C to 80°C in 5°C intervals, then from 80°C to 104°C in 3°C intervals, then from 104°C to 126°C in 2°C intervals, then from 126°C to 130°C, and finally from 130°C to 140°C in 5°C intervals. Then, at each temperature, the temperature was held for a dissolution time of 3 minutes followed by an analysis time of 19 minutes before further heating, and the concentration of the dissolved components at each temperature was measured. Furthermore, based on the dissolution temperature (°C) and the amount dissolved (mass%) at that point, a dissolution temperature-dissolution amount curve was constructed with dissolution temperature as the vertical axis and dissolution amount as the horizontal axis, to determine the dissolution amount and integral dissolution amount at each temperature. Simultaneously, the molecular weight of the GPC component at each dissolution temperature was measured, and the weight-average molecular weight at each temperature was calculated. For data processing, the data analysis software "CFC calc" was used. • Apparatus: CFC2 cross-grading chromatography system manufactured by Polymer Char. • Detector: IR4 infrared spectrophotometer manufactured by Polymer Char. • Detection wavelength: 3.42 μm • GPC tubing: 3 Shodex HT-806M tubes manufactured by Showa Denko Co., Ltd. • Column temperature: 135℃ • Column calibration: Monodisperse polystyrene manufactured by Tosoh Corporation • Molecular weight calibration method: Standard calibration method (polystyrene conversion) • Solution: o-dichlorobenzene (ODCB), with added BHT • Flow rate: 1.0 mL / min • Sample concentration: 90 mg / 30 mL Injection volume: 0.5 mL

[0180] Examples and Comparative Examples of the First Embodiment

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

[0182] [Fabrication of Microporous Layers] The high molecular weight polypropylene resin (PP1, MFR=0.51) shown in Table 1, which serves as the resin for the microporous layer (A), was dry-blended with the aforementioned polypropylene resin composition particles at a mass ratio of PP1:polypropylene resin composition particles = 92.0:8.0 (mass %). The mixture was then melted using a 2.5-inch extruder and supplied to a single-layer blow molding die using a gear pump. The temperature of the blow molding die was set to 240°C. After the molten polymer was ejected from the blow molding die, it was cooled by blowing air while being wound into a roll, thereby obtaining a precursor sheet with a single-layer structure containing the microporous layer (A) approximately 12 μm thick. Here, the die lip distance (die lip gap) of the blow molding die was set to 1.8 mm, and the ejection rate was 9 kg / h.

[0183] Subsequently, the obtained precursor sheet was placed in a dryer and annealed at 150°C for 180 minutes. Afterward, the annealed precursor sheet was cold-stretched by 30% in a microstructure (MD) at room temperature to prevent shrinkage, and then placed in an oven at 135°C. The initial dimension was set to 100%, and a 200% hot stretch was performed in the MD. Following this, a 44% heat tempering was applied in the MD, thereby obtaining a separator substrate with a single-layer structure including a microporous layer (A). The evaluation results of the structure and properties of the obtained separator substrate are shown in Table 1.

[0184] Examples 1-2 Except for a 40% cold stretch and a 220% hot stretch in an oven at 135°C, the separator substrate was obtained using the same method as in Examples 1-1. The evaluation results of the obtained separator substrate are shown in Table 1.

[0185] Examples 1-3 Except for undergoing a 240% thermal stretch in an oven at 135°C, the separator substrate was obtained using the same method as in Examples 1-2. The evaluation results of the obtained separator substrate are shown in Table 1.

[0186] Examples 1-4 Except for the following steps in preparing the polypropylene resin composition: High molecular weight polypropylene resin (PP1, MFR=0.51), high molecular weight polyethylene resin (PE1, MFR=0.02), and ethylene / propylene copolymer (C2C3, MFR=9.5) particles as shown in Table 1 were dry-blended at a mass ratio of PP1:PE1:C2C3=74.2:13.8:12.0 (mass%), and the high molecular weight polypropylene resin (PP1, MFR=0.51) as the microporous layer resin as shown in Table 1 was dry-blended with polypropylene resin composition particles at a mass ratio of PP1:polypropylene resin composition particles=73.0:27.0 (mass%), and fine-tuning was performed on the die and the extrusion temperature for stable ejection, the separator substrate was obtained according to the same method as in Examples 1-2. The evaluation results of the obtained separator substrate are shown in Table 1.

[0187] Examples 1-5 Except for undergoing a 240% thermal stretch in an oven at 135°C, the separator substrate was obtained using the same method as in Examples 1-4. The evaluation results of the obtained separator substrate are shown in Table 1.

[0188] Examples 1-6, 9 Except for the changes in raw materials as shown in Table 1, the separator substrate was obtained using the same method as in Examples 1-2. The evaluation results of the obtained separator substrate are shown in Table 1.

[0189] Examples 1-7, 8, 11, and 16 Except for the changes in raw materials shown in Table 1, the separator substrate was obtained using the same method as in Example 1-1. The evaluation results of the obtained separator substrate are shown in Table 1.

[0190] Examples 1-10 Except that when obtaining the precursor sheet, a mass ratio of PP1:polypropylene resin composition particles = 84.0:16.0 (mass %) was used, the separator substrate was obtained according to the same method as in Examples 1-1. The evaluation results of the obtained separator substrate are shown in Table 1.

[0191] Examples 1-12 Except that when obtaining the precursor sheet, the PP1:polypropylene resin composition particles were used at a mass ratio of 96.0:4.0 (mass%), the separator substrate was obtained according to the same method as in Examples 1-1. The evaluation results of the obtained separator substrate are shown in Table 1.

[0192] Examples 1-13 Except for the preparation of the polypropylene resin composition, in which high molecular weight polypropylene resin (PP1, MFR=0.51) and ethylene / propylene copolymer (C2C3, MFR=9.5) particles as shown in Table 1 are dry-blended and melt-mixed at a mass ratio of PP1:C2C3=91.2:8.8 (mass%), and in which high molecular weight polypropylene resin (PP1, MFR=0.51) as the resin of the microporous layer (A) is used with polypropylene resin composition particles at a mass ratio of PP1:polypropylene resin composition particles=61.6:38.4 (mass%), the separator substrate was obtained according to the same method as in Examples 1-2. The evaluation results of the obtained separator substrate are shown in Table 1.

[0193] Examples 1-14 Except for the preparation of the polypropylene resin composition, in which the high molecular weight polypropylene resin (PP1, MFR=0.51), polyethylene resin (PE2, MFR=1.0), and ethylene / propylene copolymer (C2C3, MFR=9.5) particles shown in Table 1 were dry-blended and melt-mixed at a mass ratio of PP1:PE2:C2C3=82.8:9.2:8.0 (mass%), and the high molecular weight polypropylene resin (PP1, MFR=0.51) shown in Table 1 (as the resin of the microporous layer (A)) was dry-blended with the polypropylene resin composition particles at a mass ratio of PP1:polypropylene resin composition particles=56.5:43.5 (mass%), and the extrusion temperature for stable ejection was finely adjusted, the separator substrate was obtained according to the same method as in Examples 1-1. The evaluation results of the obtained separator substrate are shown in Table 1.

[0194] Examples 1-15 Particles of high molecular weight polypropylene resin (PP1, MFR=0.51) and ethylene / 1-butene copolymer (C2C4, MFR=6.7), as the resin for the microporous layer (A) shown in Table 1, were dry-blended at a mass ratio of PP1:C2C4=96.0:4.0 (mass%) and melted using a 2.5-inch extruder. The melt was then supplied to the two outer layers of a three-layer co-extrusion blow-blown die using a gear pump. 100% by mass of high molecular weight polypropylene resin (PP4, MFR=0.31), as the resin for the microporous layer (B), was melted using a 2.5-inch extruder and supplied to the inner layer of the same three-layer co-extrusion blow-blown die using a gear pump. The blow-blown die temperature was set to 235°C. After the molten polymer was ejected from the blow-blown die, it was cooled by blowing air while being wound into a roll, thereby obtaining a precursor sheet with an A / B / A layer structure approximately 12 μm thick. Here, the distance between the mold lips of the blow molding die (mold lip gap) is set to 1.8 mm, and the blow molding is carried out at a blow rate of 9 kg / h. Subsequently, following the same method as in Example 1-1, annealing, cold stretching, hot stretching, and heat tempering are performed to obtain a separator substrate with a three-layer structure of microporous layer (A) / microporous layer (B) / microporous layer (A). The obtained separator is evaluated. The evaluation results are shown in Table 1.

[0195] Examples 1-17 Except for the changes in raw materials and composition as shown in Table 1, the separator substrate was obtained using the same method as in Examples 1-15. The evaluation results of the obtained separator substrate are shown in Table 1.

[0196] Comparative Example 1-1 Except that when obtaining the precursor sheet, 100.0% (by mass%) of the high molecular weight polypropylene resin (PP3, MFR=0.91) shown in Table 1 was used for 180% thermal stretching in an oven at 135°C and 50% thermal quenching, the separator substrate was obtained according to the same method as in Examples 1-1. The evaluation results of the obtained separator substrate are shown in Table 1.

[0197] Comparative Examples 1-2 Except for a 10% cold stretch, a 160% hot stretch in an oven at 135°C, and a 39% heat quench, the separator substrate was obtained using the same method as in Examples 1-1. The evaluation results of the obtained separator substrate are shown in Table 1.

[0198] Comparative Examples 1-3 Except for a 40% cold stretch and a 260% hot stretch in an oven at 135°C, the separator substrate was obtained using the same method as in Examples 1-4. The evaluation results of the obtained separator substrate are shown in Table 1.

[0199] [Table 1-1]

[0200] [Table 1-2]

[0201] Examples and Comparative Examples of the Second Embodiment

[0202] Example 2-1 [Fabrication of Microporous Layers] Particles of high molecular weight polypropylene resin (PP1, MFR=0.51) and ethylene / 1-butene copolymer (C2C4, MFR=6.7), as shown in Table 2, were dry-blended at a mass ratio of PP1:C2C4=96.0:4.0 (mass%). The mixture was then melted using a 2.5-inch extruder and fed to a single-layer blow molding die using a gear pump. The blow molding die temperature was set to 240°C. After the molten polymer was extruded from the blow molding die, it was cooled by blowing air while being wound into a roll, thereby obtaining a precursor sheet with a single-layer structure containing the microporous layer (A) of approximately 12 μm thickness. Here, the die lip distance (die lip gap) of the blow molding die was set to 1.8 mm, and extrusion was carried out at an extrusion rate of 9 kg / h.

[0203] Subsequently, the obtained precursor sheet was placed in a dryer and annealed at 120°C for 180 minutes. Afterward, the annealed precursor sheet was cold-stretched by 30% in a microstructure (MD) at room temperature to prevent shrinkage, and then placed in an oven at 135°C. The initial dimension was set to 100%, and the sheet was hot-stretched to 130% in the MD. Following this, it was heat-retarded by 44% in the MD, thereby obtaining a separator substrate with a single-layer structure including a microporous layer (A). The evaluation results of the structure and properties of the obtained separator substrate are shown in Table 2. The dissolution temperature-dissolution rate curves for CFC determination in Example 2-1 are shown in Figure 5.

[0204] Examples 2-2~8, 2-10~11 and Comparative Examples 2-2~5 Except for the changes in raw materials as shown in Table 2, the adjustment of the mass ratio of polypropylene resin and elastomer particles as shown in Table 2, and the adjustment of manufacturing conditions as shown in Table 2, a separator substrate with a single-layer structure was obtained according to the same method as in Example 2-1. The evaluation results of the obtained separator substrate are shown in Table 2.

[0205] Examples 2-9 [Preparation of Polypropylene Resin Composition] The high molecular weight polypropylene resin (PP1, MFR=0.51), polyethylene resin (PE1, MFR=0.02), and olefin-(ethylene-butene)-olefin copolymer (CEBC, MFR=2.5) particles shown in Table 2 were dry-blended at a mass ratio of PP1:PE1:CEBC=82.8:9.2:8.0 (mass%), and then melt-blended using TEM26SS (manufactured by Toshiba Machine Co., Ltd., L / D=48.5). After melt-blending, the material was drawn from the die (3 holes) and cooled in a water-cooled bath, then cut using a granulator to obtain polypropylene resin composition particles. [Fabrication of Microporous Layers] Except for dry mixing of the high molecular weight polypropylene resin (PP1, MFR=0.51) shown in Table 2 (as the resin for the microporous layer (A)) with the above-mentioned polypropylene resin composition particles at a mass ratio of PP1:polypropylene resin composition particles = 56.5:43.5 (mass%), and fine-tuning the extrusion temperature for stable nozzle and ejection, a separator substrate with a single-layer structure was obtained according to the same method as in Example 2-1. The evaluation results of the obtained separator substrate are shown in Table 2.

[0206] Comparative Examples 2-1 and 2-7~9 Except for the absence of an elastomer, 100% by mass of the high molecular weight polypropylene resin shown in Table 2, which serves as the microporous layer (A), was melted using a 2.5-inch extruder. The manufacturing conditions were adjusted as shown in Table 2. Following the same method as in Examples 2-9, a separator substrate with a single-layer structure was obtained. The evaluation results of the obtained separator substrate are shown in Table 2.

[0207] Comparative Examples 2-6 Except for adjusting the mass ratio of polypropylene resin, polyethylene resin, and elastomer particles as shown in Table 2, and adjusting the manufacturing conditions as shown in Table 2, a separator substrate with a single-layer structure was obtained according to the same method as in Example 2-1. The evaluation results of the obtained separator substrate are shown in Table 2.

[0208] Example 2-12 [Fabrication of Microporous Layers] Particles of high molecular weight polypropylene resin (PP2, MFR=0.60) and ethylene / 1-butene copolymer (C2C4, MFR=6.7), as shown in Table 2, were dry-blended at a mass ratio of PP1:C2C4=96.0:4.0 (mass%) and melted using a 2.5-inch extruder. The melt was then supplied to the two outer layers of a three-layer co-extrusion blow-blown die using a gear pump. 100% by mass of high molecular weight polypropylene resin (PP4, MFR=0.30), as the microporous layer (B), was melted using a 2.5-inch extruder and supplied to the inner layer of the same three-layer co-extrusion blow-blown die using a gear pump. The blow-blown die temperature was set to 250°C. After the molten polymer was ejected from the blow-blown die, it was cooled by blowing air while being wound into a roll, thereby obtaining a precursor sheet with an A / B / A layer structure approximately 12 μm thick. Here, the distance between the mold lips of the blow molding die (mold lip clearance) is set to 1.8 mm, and the blow molding is carried out at an ejection rate of 9 kg / h. Then, except for adjusting the manufacturing conditions as shown in Table 2, a separator substrate with a three-layer structure of microporous layer (A) / microporous layer (B) / microporous layer (A) is obtained according to the same method as in Example 2-1. The obtained separator is evaluated. The evaluation results are shown in Table 2.

[0209] Comparative Example 2-10 Except for the absence of an elastomer, 100% by weight of the high molecular weight polypropylene resin shown in Table 2, which serves as the microporous layer (A), was melted using a 2.5-inch extruder, and the die and extrusion temperature were finely adjusted for stability. Following the same method as in Examples 2-12, a separator substrate with a three-layer structure of microporous layer (A) / microporous layer (B) / microporous layer (A) was obtained. The evaluation results of the obtained separator substrate are shown below.

[0210] [Table 2-1] Table 2.

[0211] [Table 2-2] Table 2.

[0212] [Table 2-3] Table 2.

[0213] [Table 2-4] Table 2. [Industrial Applicability]

[0214] The separator for energy storage devices of the present invention can be well used as a separator for energy storage devices, such as lithium-ion secondary batteries.

[0215] 1: Polymer matrix 2-1: Connecting regions (thick lines) 2-2: The area that becomes the bright part of the dyed area 3: Fibrous fibers (fine yarns) 4: Porosity 5: Resin section (visible part) 6: Hole (dark part)

Claims

1. A separator for an energy storage device, comprising a microporous membrane with polyolefin as the main component as the separator substrate, wherein the trunk height calculated from the analysis of a scanning electron microscope (SEM) image of the MD (film formation direction)-ND (thickness direction) cross section of the separator substrate is 500 nm to 1000 nm, and includes a microporous layer (A) with an MD length calculated from the analysis of the SEM image of the MD-ND cross section of the separator substrate being 1000 nm to 1900 nm.

2. The separator for the energy storage device as claimed in claim 1, wherein the microporous layer (A) comprises one or more thermoplastic elastomers selected from the group consisting of polyolefins, copolymers of polyolefins, and copolymers of polystyrene and polyolefins that are different from the main component.

3. The separator for an energy storage device as claimed in claim 1 or 2, wherein the microporous layer (A) comprises a thermoplastic elastomer containing one or more of the group consisting of ethylene, propylene, and 1-butene as repeating units.

4. The separator for the energy storage device as claimed in claim 1 or 2, wherein, based on the total mass of the aforementioned microporous layer (A), it contains 80.0% to 99.5% by mass of polypropylene as the aforementioned polyolefin, and contains 0.5% to 20.0% by mass of thermoplastic elastomer.

5. The separator for the energy storage device as claimed in claim 1 or 2, wherein in the compositional image obtained by scanning electron microscopy (SEM) at a magnification of 30,000x on the MD-ND cross section of the microporous layer (A) dyed with ruthenium compound, there are polymer matrix and fibrils, and each of the polymer matrix and the fibrils has a dyed bright area with an area of ​​100 nm2 to 0.1 μm2, and when the total area ratio of the dyed bright areas with an area of ​​100 nm2 to 0.1 μm2 to the total area of ​​the image of only the microporous layer (A) is taken is set as S1%, 0.5 ≤ S1 ≤ 15.

0.

6. For the separator of the energy storage device as requested in item 1 or 2, wherein the integral dissolution amount of the microporous layer (A) at 100°C to 130°C, as determined by cross-grading chromatography (CFC), is 80.0% to 99.5% of the total dissolution amount, and the integral dissolution amount at 20°C to 100°C is 0.5% to 20.0% of the total dissolution amount.

7. The separator for the energy storage device as requested in item 1 or 2, wherein the area-average long pore diameter calculated from the analysis of the SEM image of the MD-ND cross section of the microporous layer (A) is 50 nm or more and 500 nm or less.

8. For the energy storage device separator as requested in item 1 or 2, wherein the melt flow rate (MFR) of the microporous layer (A) is less than 1.00 g / 10 min when measured under a load of 2.16 kg and a temperature of 230°C.

9. The separator for the energy storage device as requested in item 1 or 2, wherein the melt tension Mt of the microporous layer (A) at 240°C is 10 mN or more and 35 mN or less.

10. The separator for an energy storage device as claimed in claim 1 or 2, wherein the thickness of the separator substrate is 3 μm or more and 20 μm or less.

11. The separator for an energy storage device as claimed in claim 1 or 2, wherein the porosity of the separator substrate is 30% or more and 60% or less.

12. The separator for the energy storage device as claimed in item 1 or 2, wherein the TD heat shrinkage rate of the substrate of the separator at 105°C for 1 hour is less than 5%.

13. The separator for an energy storage device as claimed in claim 1 or 2, wherein the MD tensile strength of the substrate of the separator is 1800 kgf / cm2 or higher.

14. For the energy storage device separator as requested in item 1 or 2, wherein the melt flow rate (MFR) of the microporous layer (A) is 0.20 g / 10 min or more when measured under a load of 2.16 kg and a temperature of 230°C.

15. The separator for the energy storage device as claimed in claim 1 or 2, wherein the polyolefin is polypropylene and the weight average molecular weight (Mw) of the polypropylene is more than 300,000 and less than 1,300,000.

16. The separator for the energy storage device as claimed in claim 15, wherein the weight average molecular weight (Mw) of the polypropylene divided by the number average molecular weight (Mn), i.e., the molecular weight distribution (Mw / Mn), is 3 or more and 30 or less.

17. The separator for the energy storage device as claimed in claim 15, wherein the polypropylene described above has a five-component composition of 94.0% or more as determined by 13C-NMR (nuclear magnetic resonance method).

18. The separator for the energy storage device as claimed in claim 1 or 2, wherein, based on the total mass of the aforementioned microporous layer (A), it contains 0.5% by mass and 20.0% by mass of polyethylene.

19. A separator for an energy storage device as claimed in claim 1 or 2, wherein the separator substrate comprises a microporous layer (B) having a melt tension MtB different from that of the microporous layer (A) at 240°C.

20. An energy storage device comprising a positive electrode containing lithium iron phosphate as the positive electrode active material, a negative electrode, and a separator for an energy storage device disposed between the positive electrode and the negative electrode as claimed in claim 1 or 2.

21. A separator for an energy storage device, comprising a microporous layer (A) containing polypropylene as a separator substrate, wherein the integral dissolution amount of the microporous layer (A) at 100°C to 130°C, as determined by cross-fractional chromatography (CFC), is 80.0% to 99.5% by mass of the total dissolution amount, and the integral dissolution amount at 20°C to 100°C is 0.5% to 20.0% by mass of the total dissolution amount, and the porosity of the separator substrate is 30.0% to 45.0%.

22. The separator for the energy storage device as claimed in claim 21, wherein in the dissolution temperature-dissolution amount curve of the microporous layer (A) determined by cross-gradient chromatography (CFC), there are at least two peaks, the peak temperature of the high-temperature side peak is above 105°C and below 125°C, and the peak temperature of the low-temperature side peak is above 35°C and below 100°C.

23. The separator for the energy storage device as claimed in claim 22, wherein in the dissolution temperature-dissolution amount curve of the microporous layer (A) determined by cross-fractional chromatography (CFC), the weight average molecular weight (MwH) at the high-temperature side peak temperature is 300,000 or more and 1,300,000 or less, and the weight average molecular weight (MwL) at the low-temperature side peak temperature is 50,000 or more and 1,800,000 or less.

24. The separator for the energy storage device as requested in item 21 or 22, wherein the melt flow rate (MFR) of the microporous layer (A) is less than 1.0 g / 10 min when measured under a load of 2.16 kg and a temperature of 230°C.

25. The separator for the energy storage device as claimed in claim 21 or 22, wherein the melt tension MtA of the microporous layer (A) at 240°C is 10 mN or more and 35 mN or less.

26. The separator for an energy storage device as claimed in claim 21 or 22, wherein the weight average molecular weight (Mw) of the aforementioned microporous layer (A) is more than 250,000 and less than 1,500,000.

27. The separator for the energy storage device as claimed in claim 26, wherein the value obtained by dividing the weight average molecular weight (Mw) of the microporous layer (A) by the number average molecular weight (Mn), i.e., the molecular weight distribution (Mw / Mn), is 3 or more and 30 or less.

28. The separator for the energy storage device as requested in item 21 or 22, wherein the five-component component percentage of the aforementioned polypropylene, as determined by 13C-NMR (nuclear magnetic resonance method), is 94.0% or more.

29. The separator for an energy storage device as claimed in claim 21 or 22, wherein, based on the total mass of the aforementioned microporous layer (A), it contains 0.5% by mass and 20.0% by mass of thermoplastic elastomer.

30. The separator for an energy storage device as claimed in claim 29, wherein, based on the total mass of the aforementioned microporous layer (A), it contains 0.5% by mass and 20.0% by mass of polyethylene.

31. The separator for an energy storage device as claimed in claim 29, wherein the thermoplastic elastomer contains one or more of the group consisting of ethylene, propylene, and 1-butene as repeating units.

32. The separator for an energy storage device as claimed in claim 21 or 22, wherein the thickness of the substrate of the separator is more than 3 μm and less than 20 μm.

33. The separator for an energy storage device as claimed in claim 21 or 22, wherein the trunk height calculated from the analysis of a scanning electron microscope (SEM) image of the MD-ND cross section of the separator substrate is 500 nm or more and 1000 nm or less.

34. The separator for the energy storage device as claimed in claim 21 or 22, wherein the area-average long pore diameter calculated from the analysis of the scanning electron microscope (SEM) image of the MD-ND cross section of the microporous layer (A) is 50 nm or more and 500 nm or less.

35. The separator for an energy storage device as claimed in claim 21 or 22, wherein the TD heat shrinkage rate of the substrate of the separator at a temperature of 105°C for 1 hour is less than 5%.

36. The separator for an energy storage device as claimed in claim 21 or 22, wherein the MD heat shrinkage rate of the substrate of the separator at a temperature of 105°C for 1 hour is less than 20%.

37. The separator for an energy storage device as claimed in claim 21 or 22, wherein the MD tensile elongation of the separator substrate is more than 20% and less than 60%.

38. The separator for an energy storage device as claimed in claim 21 or 22, wherein the separator substrate comprises a microporous layer (B) having a melt tension MtB different from the melt tension MtA of the microporous layer (A) at 240°C.

39. An energy storage device comprising a positive electrode containing lithium iron phosphate as the positive electrode active material, a negative electrode, and a separator for the energy storage device disposed between the positive electrode and the negative electrode as claimed in claim 21 or 22.

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