Separators for power storage devices

Abstract

To provide a separator for a power storage device which has high strength and suppresses clogging due to long-term use.SOLUTION: In a separator for a power storage device including a microporous layer (X) whose main component is polyolefin (A), the melt flow rate (MFR) of the microporous layer (X) at a load of 2.16 kg and a temperature of 230°C is 0.90 g / 10 min or less, and in the MD-TD surface observation or the ND-MD cross-sectional observation of the microporous layer (X) by a scanning electron microscope (SEM), the average long pore diameter of the pore existing in the microporous layer (X) is 100 nm or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a separator for an electricity storage device. [Background technology]

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

[0003] In recent years, there has been a demand for lithium-ion batteries with high energy capacity, high energy density, and high output characteristics, and this has led to an increasing demand for separators that are thin and have high strength (for example, high puncture strength).

[0004] Patent Document 1 describes a polypropylene resin composition for microporous films, which contains 5 to 30% by weight of a polypropylene resin (X) and 95 to 70% by weight of a polypropylene resin (Y) from the viewpoint of improving the rigidity of the microporous film and reducing the product defect rate at a certain thickness. The polypropylene resin (X) described in Patent Document 1 has a specific melt flow rate (MFR), molecular weight distribution (Mw / Mn), and long chain branching structure. The polypropylene resin (Y) described in Patent Document 1 has a specific MFR and excludes the polypropylene resin (X).

[0005] Patent Document 2 describes a microporous membrane having the following three layers (a) to (c) from the viewpoint of an improved balance of electrochemical stability, high meltdown temperature, high electrolyte affinity, and low water retention of the microporous membrane: (a) 6.0 × 10 5 a first layer containing 40.0 to 85.0% by weight of isotactic polypropylene having a weight average molecular weight (Mw) of 10.0 to 15.0% by weight; (b) a second layer containing a polyolefin; and (c) 6.0 × 10 5 A third layer containing 40.0 to 85.0% by weight of isotactic polypropylene having a Mw of at least 100%.

[0006] Patent Document 3 describes a melting point T mA a first microporous film comprising a first resin composition having a melting point T mA Lower melting point T mB and a second microporous film containing a second resin composition having the formula: wherein the extensional viscosity is 18,000 to 40,000 Pa·s and the shear viscosity is 5,000 to 10,000 Pa·s.

[0007] Patent Document 4 describes a propylene-based resin microporous film made of a propylene-based resin having 25 to 60% by weight of components with a molecular weight of 50,000 or less, 19 to 30% by weight of components with a molecular weight of 700,000 or more, a weight-average molecular weight of 350,000 to 500,000, and a melt tension of 1.1 to 3.2 g.

[0008] Patent Document 5 describes a polypropylene resin composition for forming a microporous film, which contains as an essential component a propylene homopolymer, and which has an intrinsic viscosity [η] of 1 dl / g or more and less than 7 dl / g, a mesopentad fraction in the range of 94.0 to 99.5%, an elution integral up to 100°C during heating of 10% or less, a melting point of 153 to 167°C, and in an elution temperature-elution amount curve, the peak-top temperature of the maximum peak is between 105 and 130°C, and the half-width of the peak is 7.0°C or less.

[0009] Patent Document 6 describes a polyolefin microporous membrane containing a polypropylene-based resin, which has a meltdown temperature of 195°C or more and 230°C or less, and describes that the polypropylene-based resin may have a weight-average molecular weight of 500,000 or more and 800,000 or less and a molecular weight distribution of 7.5 or more and 16 or less.

[0010] Patent Document 7 describes a laminated microporous film comprising a first microporous film made of a first resin composition and a second microporous film made of a second resin composition, wherein the MFR of both the first resin composition and the second resin composition is 1.0 g / 10 min or less, and the molecular weight distribution (Mw / Mn) of the resin contained in the first resin composition is 10 or more. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 2018-030992 [Patent Document 2] Special Publication No. 2013-517152 [Patent Document 3] Japanese Patent Application Publication No. 2016-022676 [Patent Document 4] Japanese Patent Application Laid-Open No. 2012-092286 [Patent Document 5] International Publication No. 2010 / 079784 [Patent Document 6] International Publication No. 2017 / 138512 [Patent Document 7] Japanese Patent Application Laid-Open No. 2016-22679 Summary of the Invention [Problem to be solved by the invention]

[0012] It is known that the use of high molecular weight polyolefins in microporous membranes can provide microporous membranes with high strength.

[0013] However, when a high-molecular-weight polyolefin is used to produce a microporous layer, the film-forming properties and pore opening during production tend to be insufficient, resulting in a small pore size in the microporous membrane and a deterioration in air permeability (i.e., air resistance). In addition, the high viscosity when melted tends to make it difficult to produce a thin separator (e.g., a thickness of 20 μm or less).

[0014] Therefore, an object of the present invention is to provide a thin-film separator for an electricity storage device that has high strength and is less susceptible to clogging over long-term use. Furthermore, the present invention aims to provide a separator for an electricity storage device that has high strength (particularly high puncture strength) and high dimensional stability at high temperatures, which contributes to high safety of the electricity storage device, while also being high-power and capable of being thinned. [Means for solving the problem]

[0015] The present inventors have conducted extensive research to solve the above problems and have found that using a microporous layer containing a polyolefin having a specific melt flow rate (MFR) as a main component and having a specific average long pore size is advantageous in solving the above problems, thereby completing the present invention. Examples of embodiments of the present invention are listed below. [1] A microporous layer (X) whose main component is polyolefin (A), the microporous layer (X) has a melt flow rate (MFR) of 0.90 g / 10 min or less under a load of 2.16 kg and at a temperature of 230°C, and, in MD-TD surface observation or ND-MD cross-sectional observation of the microporous layer (X) with a scanning electron microscope (SEM), the average long pore diameter of pores present in the microporous layer (X) is 100 nm or more; Separators for power storage devices. [2] In MD-TD surface observation or ND-MD cross-sectional observation of the microporous layer (X) by SEM, the maximum longitudinal pore diameter of the pores present in the microporous layer (X) is 100 nm or more and 400 nm or less. The separator for an electricity storage device according to the first aspect. [3] The melt tension of the microporous layer (X) measured at a temperature of 230°C is 16 mN or more. The separator for an electricity storage device according to the first or second aspect. [4] The melt tension of the microporous layer (X) measured at a temperature of 230°C is 16 mN or more and 40 mN or less. The separator for an electricity storage device according to the first or second aspect. [5] The ratio (SMD / STD) of the tensile strength in the machine direction (SMD) to the tensile strength in the width direction (STD) of the separator for an electricity storage device is SMD / STD>5. The separator for an electricity storage device according to any one of the above first to fourth aspects. [6] The separator for an electric storage device has a heat shrinkage rate of 1% or less in TD and 4% or less in MD after heat treatment at 105°C for 1 hour, The electricity storage device separator according to any one of aspects 1 to 5, wherein the electricity storage device separator has a thermal shrinkage of 1% or less in TD and 10% or less in MD after heat treatment at 120° C. for 1 hour. [7] The air permeability of the separator for an electricity storage device when converted to a thickness of 14 μm is 250 sec / 100 cm 3 Below is the The separator for an electricity storage device according to any one of the above first to sixth aspects. [8] The separator for an electricity storage device according to any one of aspects 1 to 7, having a thickness of 8 μm or more and 18 μm or less, and a puncture strength of 230 gf or more when converted to a thickness of 14 μm. [9] The polyolefin (A) comprises polypropylene. The separator for an electricity storage device according to any one of the above first to eighth aspects.

[10] The ratio of the polypropylene to the polyolefin (A) is 50 to 100% by mass. The separator for an electricity storage device according to aspect 9.

[11] The pentad fraction of the polypropylene is 95.0% or more. The separator for an electricity storage device according to aspect 9 or 10.

[12] The weight average molecular weight (Mw) of the microporous layer (X) is 500,000 or more and 1,500,000 or less. The separator for an electricity storage device according to any one of the above first to eleventh aspects.

[13] The value (Mw / Mn) obtained by dividing Mw by number average molecular weight (Mn) of the microporous layer (X) is 6 or less. The separator for an electricity storage device according to any one of the above first to twelfth aspects.

[14] The separator for an electricity storage device according to any one of the above embodiments 1 to 13, further comprising a microporous layer (Y) containing polyolefin (B) as a main component.

[15] The separator for an electricity storage device according to aspect 14, wherein the polyolefin (B) is polyethylene.

[16] An electricity storage device comprising the separator for an electricity storage device according to any one of the first to fifteenth aspects. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a separator for an electricity storage device that has high strength and can suppress clogging in an electricity storage device such as a lithium ion secondary battery. Furthermore, according to the present invention, it is possible to provide a separator for an electricity storage device that has high strength (particularly high puncture strength) and high dimensional stability at high temperatures, which contributes to high safety of the electricity storage device, while also having high output and being able to be thinned. Note that the above description should not be construed as disclosing all embodiments of the present invention and all advantages related to the present invention. Further embodiments and advantages of the present invention will become apparent from the following description. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail for the purpose of illustrating the present invention, but the present invention is not limited to this embodiment. Note that the weight average molecular weight (Mw), number average molecular weight (Mn), and Mw / Mn of the polyolefin of the present embodiment are polystyrene-equivalent molecular weights obtained by GPC (gel permeation chromatography) measurement.

[0018] <Separator for power storage device> <Microporous layer> The separator for an electricity storage device of this embodiment includes a microporous layer primarily composed of a polyolefin. In this embodiment, the microporous layer refers to one microporous layer constituting the separator for an electricity storage device, and may be a multilayer structure formed by stacking two or more layers. In one aspect, the separator for an electricity storage device includes a microporous layer (X) primarily composed of a polyolefin (A). The separator for an electricity storage device may further include a microporous layer (Y) primarily composed of a polyolefin (B), if desired.

[0019] Even when the separator is composed of multiple microporous layers, each microporous layer can be easily peeled off and collected by attaching adhesive tape to the edge of the separator and pulling it, and physical properties such as melt tension, melt flow rate (MFR), molecular weight, long pore diameter, porosity, and thickness, which will be described later, can be measured.

[0020] <Microporous layer (X)> The separator for an electricity storage device of this embodiment has a microporous layer (X). The separator for an electricity storage device may have only one microporous layer (X) or two or more microporous layers. In this embodiment, "containing polyolefin (A) as a main component" means that the microporous layer (X) contains 50 mass% or more of polyolefin (A) based on the total mass of the microporous layer (X). The lower limit of the content of polyolefin (A) in the microporous layer (X) is 50 mass% or more, and preferably 55 mass% or more, 60 mass% or more, 70 mass% or more, 80 mass% or more, 90 mass% or more, or 95 mass% or more, from the viewpoints of the wettability of the separator to the electrolyte, thinning of the film, and shutdown properties. The upper limit of the content of polyolefin (A) in the microporous layer (X) may be, for example, 60% by mass or less, 70% by mass or less, 80% by mass or less, 90% by mass or less, 95% by mass or less, 98% by mass or less, or 99% by mass or less, or may be 100% by mass.

[0021] <Polyolefin (A)> The polyolefin (A) in this embodiment is a polymer containing a monomer having a carbon-carbon double bond as a repeating unit. Examples of monomers constituting the polyolefin (A) include, but are not limited to, monomers having 1 or more but less than 10 carbon atoms and having a carbon-carbon double bond, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. The polyolefin (A) may be, for example, a homopolymer, a copolymer, or a multi-stage polymer, and is preferably a homopolymer.

[0022] Specifically, from the viewpoint of shutdown characteristics, etc., the polyolefin (A) is preferably polyethylene, polypropylene, a copolymer of ethylene and propylene, or a mixture thereof. The polyolefin (A) is more preferably polypropylene. From the same viewpoint, the lower limit of the ratio of polypropylene to polyolefin (A), the main component of the microporous layer (X), is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The lower limit of the ratio of polypropylene to polyolefin (A), the main component of the microporous layer (X), is preferably 100% by mass or less.

[0023] The stereoregularity of the polypropylene is not limited, and examples thereof include atactic, isotactic, or syndiotactic polypropylene. The polypropylene according to this embodiment is preferably a highly crystalline isotactic or syndiotactic homopolymer.

[0024] The polypropylene usable as the polyolefin (A) is preferably a homopolymer, but may also be a copolymer, such as a block polymer, copolymerized with a comonomer other than propylene, such as an α-olefin comonomer. The lower limit of the amount of propylene structures contained as repeating units in the polypropylene may be, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The polypropylene may contain repeating units other than the propylene structure. In such cases, the upper limit of the amount of repeating units derived from the comonomer (excluding the propylene structure) may be, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. The polypropylene may be used singly or in combination of two or more types.

[0025] The lower limit of the weight average molecular weight (Mw) of the polyolefin (A) is preferably 300,000 or more, more preferably 500,000 or more, even more preferably 600,000 or more, and particularly preferably 800,000 or more, from the viewpoint of the strength of the microporous layer (X), etc. The upper limit of the weight average molecular weight (Mw) of the polyolefin (A) is preferably 1,500,000 or less, more preferably 1,300,000 or less, even more preferably 1,100,000 or less, still more preferably 1,000,000 or less, and particularly preferably 960,000 or less, from the viewpoint of increasing the pore size of the microporous layer (X), suppressing clogging, and obtaining high output.

[0026] The upper limit of the value (Mw / Mn) obtained by dividing the weight-average molecular weight (Mw) of the polyolefin (A) by the number-average molecular weight (Mn) is preferably 7 or less, 6.5 or less, 6 or less, 5.5 or less, or 5 or less. The smaller the Mw / Mn value, the less entanglement of molecules there is, and the melt tension of the resulting microporous layer (X) tends to be lower. Therefore, it is preferable that the Mw / Mn value of the polyolefin (A) is 7 or less, because the melt tension of the microporous layer (X) can be controlled low and the microporous layer (X) can be produced more stably as a thinner film. The lower limit of the Mw / Mn value of the polyolefin (A) is preferably 1 or more, 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. An Mw / Mn of 1 or more maintains appropriate molecular entanglement, tending to improve stability during film formation.

[0027] Conventional separators often have a thickness of 20 μm to 40 μm, and it has been difficult to produce separators thinner than this, especially those with a multilayer structure. This is thought to be because the use of high-molecular-weight polyolefins increases the melt tension during film formation, making it difficult to stably produce a thin microporous layer. Controlling the molecular weight and molecular weight distribution (Mw / Mn) of polyolefin (A) within the above-described numerical ranges is advantageous from the perspective of producing thinner separators.

[0028] When the polyolefin (A) is polypropylene, the lower limit of the weight average molecular weight (Mw) of the polypropylene is preferably 300,000 or more, more preferably 500,000 or more, even more preferably 600,000 or more, still more preferably 700,000 or more, and particularly preferably 800,000 or more, from the viewpoint of the strength of the microporous layer (X). The upper limit of the Mw of the polypropylene is preferably 1,500,000 or less, more preferably 1,300,000 or less, even more preferably 1,100,000 or less, still more preferably 1,000,000 or less, and particularly preferably 960,000 or less, from the viewpoint of increasing the pore size of the microporous layer and suppressing clogging.

[0029] When the polyolefin (A) is polypropylene, the upper limit of the value (Mw / Mn) obtained by dividing the weight-average molecular weight (Mw) of the polypropylene by the number-average molecular weight (Mn) is preferably 7 or less, 6.5 or less, 6 or less, 5.5 or less, or 5 or less. The smaller the Mw / Mn value of the polypropylene, the smaller the melt tension of the resulting microporous layer (X) tends to be. Therefore, a Mw / Mn value of 7 or less for the polypropylene is preferred for controlling the melt tension of the microporous layer (X) to 30 mN or less. Furthermore, the Mw / Mn of the polypropylene is preferably 1 or more, 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. When the Mw / Mn of the polypropylene is 1 or more, appropriate molecular entanglement is maintained, and stability during film formation tends to be good.

[0030] The lower limit of the density of the polyolefin (A) is preferably 0.85 g / cm 3 More than 0.88g / cm 3 More than 0.89g / cm 3 or more, or 0.90 g / cm 3 The upper limit of the density of the polyolefin (A) is preferably 1.1 g / cm 3 Below 1.0g / cm 3 Below, 0.98g / cm 3 Below, 0.97g / cm 3 Below, 0.96g / cm 3 Below, 0.95g / cm 3 Below, 0.94g / cm 3 Below, 0.93g / cm 3 or less than 0.92 g / cm 3 The density of the polyolefin (A) is related to the crystallinity of the polyolefin (A), and the density of the polyolefin (A) is 0.85 g / cm or less. 3 By doing so, the pore opening of the microporous layer (X) is improved, which is particularly advantageous in a method for making a resin raw sheet porous by a dry method (hereinafter referred to as a dry method).

[0031] <Melt tension of the microporous layer (X)> The upper limit of the melt tension of the microporous layer (X) (melt tension of a single layer) measured at 230°C is preferably 40 mN or less, more preferably 38 mN or less, even more preferably 35 mN or less, and most preferably 30 mN or less, from the viewpoint of formability of the microporous layer (X). The lower limit of the melt tension of the microporous layer (X) (melt tension of a single layer) is preferably 16 mN or more, more preferably 17 mN or more, even more preferably 20 mN or more, and most preferably 24 mN or more, from the viewpoint of the strength of the microporous layer (X).

[0032] <Melt flow rate (MFR)> The upper limit of the melt flow rate (MFR) of the microporous layer (X) (i.e., the MFR of a single layer) measured under a load of 2.16 kg at a temperature of 230°C is 0.9 g / 10 min or less, and may be, for example, 0.90 g / 10 min or less, 0.85 g / 10 min or less, 0.7 g / 10 min or less, 0.65 g / 10 min or less, 0.6 g / 10 min or less, or 0.55 g / 10 min or less, from the viewpoint of obtaining a high-strength separator. In view of the formability of the microporous layer (X), the lower limit of the MFR of the microporous layer (X) (single layer MFR) may be, for example, 0.15 g / 10 min or more, 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, 0.38 g / 10 min or more, 0.4 g / 10 min or more, 0.45 g / 10 min or more, 0.50 g / 10 min or more, 0.55 g / 10 min or more, 0.60 g / 10 min or more, 0.65 g / 10 min or more, or 0.70 g / 10 min or more.

[0033] When the molecular weight of the polyolefin (A) contained in the microporous layer (X) is high, the MFR of the microporous layer (X) tends to be 0.9 g / 10 min or less. A high molecular weight polyolefin increases the number of tie molecules that bond crystalline substances together, tending to produce a high-strength microporous layer. In one aspect, a polyolefin (A) with a molecular weight distribution expressed as Mw / Mn of 7 or less and an MFR of 0.9 g / 10 min or less, particularly 0.6 g / 10 min or less, is advantageous from the perspective of producing a high-strength, thin-film separator for an electricity storage device. In one aspect, when the MFR of the microporous layer (X) is 0.15 g / 10 min or more, the melt tension of the microporous layer (X) does not become too high, making it easier to produce a high-strength, high-power, thin-film separator.

[0034] When the polyolefin (A) is polypropylene, the upper limit of the MFR measured under a load of 2.16 kg at a temperature of 230° C. is preferably 0.9 g / 10 min or less, 0.90 g / 10 min or less, 0.85 g / 10 min or less, 0.8 g / 10 min or less, 0.7 g / 10 min or less, 0.6 g / 10 min or less, or 0.55 g / 10 min or less, from the viewpoint of obtaining a separator with high strength. The lower limit of the MFR of the polypropylene is preferably 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, 0.45 g / 10 min or more, or 0.5 g / 10 min or more, from the viewpoint of moldability of the microporous layer (X).

[0035] <Molecular weight of microporous layer (X)> The lower limit of the weight-average molecular weight (Mw) of the microporous layer (X) is preferably 500,000 or more, more preferably 700,000 or more, from the viewpoints of the strength of the microporous layer (X) and the MFR of the microporous layer (X). The upper limit of the Mw of the microporous layer (X) is preferably 1,500,000 or less, more preferably 1,100,000 or less, from the viewpoints of increasing the pore size of the microporous layer (X) to suppress clogging and obtain high output.

[0036] The upper limit of the value (Mw / Mn) obtained by dividing Mw of the microporous layer (X) by the number average molecular weight (Mn) is preferably 6 or less, 5.5 or less, or 5 or less, from the viewpoints of the strength of the microporous layer (X), controlling the melt tension of the microporous layer (X) to be thin, and increasing the pore size of the microporous layer (X) to suppress clogging and obtain high output. The lower limit of Mw / Mn of the microporous layer (X) is preferably 1 or more, 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more, from the viewpoint of the stability of the microporous layer (X).

[0037] <Pentad fraction> When the polyolefin (A) is polypropylene, in this embodiment, 13 From the viewpoint of obtaining a microporous layer (X) with low air permeability, the lower limit of the pentad fraction of the polypropylene measured by C-NMR (nuclear magnetic resonance) is preferably 94.0% or more, 95.0% or more, 96.0% or more, 96.5% or more, 97.0% or more, 97.5% or more, 98.0% or more, 98.5% or more, or 99.0% or more. The upper limit of the pentad fraction of the polypropylene may be, for example, 99.9% or less, 99.8% or less, or 99.5% or less.

[0038] When the pentad fraction of the polypropylene is 94.0% or higher, the polypropylene has high crystallinity. In separators obtained by the stretching method, particularly the dry method, pores are opened by stretching the amorphous portions between multiple crystalline portions. Therefore, high crystallinity of the polypropylene improves pore opening, increasing the porosity and the average long pore diameter of the pores present in the microporous layer (X) as determined by MD-TD surface observation or ND-MD cross-sectional observation using a scanning electron microscope (SEM). This is preferable because it suppresses clogging and allows for low air permeability. Using high-molecular-weight polypropylene increases strength, but the increased entanglement of molecular chains reduces pore opening, making it difficult to achieve low air permeability. This is thought to be one of the major reasons why high-molecular-weight polypropylene is difficult to use in the dry method. In contrast, using polypropylene with a well-controlled structure, such as polypropylene with a high pentad fraction, makes it possible to achieve high pore opening even when using high-molecular-weight polypropylene. More preferably, by using polypropylene with such a controlled structure and applying controlled production conditions to the production process, it becomes possible to obtain higher pore opening even when using high molecular weight polypropylene.

[0039] <Average pore diameter and maximum pore diameter of the microporous layer (X)> In the separator for an electricity storage device of this embodiment, when the MD-TD surface is observed with a scanning electron microscope (SEM), the average long pore diameter of the pores present in the microporous layer (X) is preferably 100 nm or more and / or 400 nm or less. In the present disclosure, the machine direction (MD) refers to the film-forming direction of the microporous layer, the width direction (TD) refers to the direction perpendicular to the film-forming direction of the microporous layer, and the normal direction (ND) refers to the thickness direction of the microporous layer (i.e., the direction perpendicular to MD and TD). The MD of a separator having a microporous layer is the longitudinal direction if it is a roll. Setting the average long pore diameter of the pores present on the MD-TD surface within this range tends to contribute to suppressing clogging in electricity storage devices such as lithium-ion secondary batteries and adjusting the air permeability of the separator.

[0040] In MD-TD surface observation by SEM, the lower limit of the average pore diameter of the pores present in the microporous layer (X) is preferably 100 nm or more, more preferably 130 nm or more, and even more preferably 140 nm or more, from the viewpoint of suppressing clogging in the electricity storage device. In MD-TD surface observation by SEM, the upper limit of the average pore diameter of the pores present in the microporous layer (X) is preferably 400 nm or less, more preferably 350 nm or less, and even more preferably 300 nm or less, from the viewpoint of suppressing short circuits in the electricity storage device.

[0041] In ND-MD cross-section observation by SEM, the lower limit of the average pore diameter of the pores present in the microporous layer (X) is preferably 100 nm or more, more preferably 130 nm or more, and even more preferably 140 nm or more, from the viewpoint of suppressing clogging in the electricity storage device. In ND-MD cross-section observation by SEM, the upper limit of the average pore diameter of the pores present in the microporous layer (X) is preferably 400 nm or less, more preferably 350 nm or less, and even more preferably 300 nm or less, from the viewpoint of suppressing short circuits in the electricity storage device.

[0042] In MD-TD surface observation or ND-MD cross-section observation by SEM, the lower limit of the maximum pore diameter of the pores present in the microporous layer (X) is preferably 100 nm or more, more preferably 220 nm or more, and even more preferably 230 nm or more, from the viewpoint of suppressing clogging in the electricity storage device. In MD-TD surface observation or ND-MD cross-section observation by SEM, the upper limit of the maximum pore diameter of the pores present in the microporous layer (X) is preferably 400 nm or less, more preferably 375 nm or less, even more preferably 360 nm or less, and particularly preferably 350 nm or less, from the viewpoint of suppressing short circuits in the electricity storage device.

[0043] In the present disclosure, the average pore diameter is the area average value of the major diameters calculated based on the area of ​​each pore present in the microporous layer when the microporous layer is observed by MD-TD surface or ND-MD cross section by SEM. The maximum pore diameter is the largest of the major diameters of each pore present in the microporous layer when the microporous layer is observed by MD-TD surface or ND-MD cross section by SEM. The average pore diameter and maximum pore diameter can be measured by SEM (scanning electron microscope) observation of the separator's MD-TD surface (when determining the average pore diameter of pores present on the MD-TD surface) or ND-MD cross section (when determining the average pore diameter of pores present on the ND-MD cross section) and image analysis of the obtained SEM image in a 4 μm × 4 μm range. Detailed conditions are shown in the Examples.

[0044] In MD-TD surface observation or ND-MD cross-sectional observation of the microporous layer by SEM, when the average pore diameter and maximum pore diameter of the microporous layer are large, the air permeability of the separator decreases, and clogging in the electricity storage device is reduced, which is expected to improve the life of the lithium-ion battery. Furthermore, if the average pore diameter and maximum pore diameter of the microporous layer (X) are too large, the strength and insulating properties of the separator including the microporous layer (X) may decrease, raising concerns about the safety of the electricity storage device.

[0045] <Achieving both MFR and average pore diameter in the microporous layer (X)> In the separator for an electricity storage device of this embodiment, the microporous layer (X) preferably has an MFR of 0.90 g / 10 min or less, and an average pore diameter of 100 nm or more on the MD-TD surface or ND-MD cross section when observed by SEM on the surface or cross section. Conventionally, in separators having a multilayer structure, particularly multilayer separators obtained by a dry process, no plasticizer was used during production, making it very difficult to achieve an MFR of 0.90 g / 10 min or less and an average pore diameter of 100 nm or more on the MD-TD surface or ND-MD cross section. When the MFR is as low as 0.90 g / 10 min or less, i.e., when the molecular weight of the polyolefin is high, pores tend to be difficult to open during stretching, and even when pores are opened, the average pore diameter tends to be small. Furthermore, even in conventional techniques in which the air permeability is adjusted to about 200 sec / 100 ml as described above, a small average pore diameter in the separator tends to make it difficult to suppress clogging in the electricity storage device. Furthermore, in a multilayer separator, the thickness of each layer must be particularly thin. For example, in a three-layer separator with a thickness of 18 μm, if the thickness ratio of each layer is 1:1:1, the thickness of each layer must be 6 μm. It has been even more difficult to achieve a large pore diameter in such a thin film while using a high-molecular-weight polyolefin. In this embodiment, although not limited thereto, by applying precisely controlled film formation and stretching conditions as exemplified in the section "Method for producing a separator for an electricity storage device" described later and / or using polypropylene with a controlled pentad fraction as the polyolefin, the average pore diameter in MD-TD surface observation or ND-MD cross-sectional observation of the microporous layer (X) by SEM can be controlled within the above range even in a microporous layer (X) mainly composed of a low-MFR (i.e., high-molecular-weight) polyolefin.

[0046] <Porosity of the microporous layer (X)> The lower limit of the porosity of the microporous layer (X) according to this embodiment is preferably 20% or more, more preferably 25% or more, even more preferably 30% or more, still more preferably 35% or more, and particularly preferably 40% or more, from the viewpoint of suppressing clogging in the electricity storage device and controlling the air permeability of the separator. The upper limit of the porosity of the microporous layer (X) is preferably 70% or less, more preferably 65% ​​or less, even more preferably 60% or less, and particularly preferably 55% or less, from the viewpoint of maintaining the strength of the separator. The porosity is measured by the method described in the Examples.

[0047] <Thickness of the microporous layer (X)> The upper limit of the thickness of the microporous layer (X) according to this embodiment is preferably 10 μm or less, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm or less, or 4 μm or less, from the viewpoint of increasing the energy density of the electricity storage device, etc. The lower limit of the thickness of the microporous layer (X) is preferably 1 μm or more, 2 μm or more, 3 μm or more, or 3.5 μm or more, from the viewpoint of strength, etc.

[0048] <Additives> In this embodiment, the microporous layer (X) containing polyolefin (A) as a main component may further contain, in addition to polyolefin (A), additives such as elastomers, flow modifiers (e.g., fluorine-based flow modifiers), waxes, crystal nucleating agents, antioxidants, metal soaps such as metal salts of aliphatic carboxylic acids, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, color pigments, and fillers, as needed.

[0049] <Microporous layer (Y)> The separator for an electricity storage device of this embodiment may have a microporous layer (Y) containing polyolefin (B) as a main component. Here, the separator for an electricity storage device of this embodiment may have only one microporous layer (Y), or two or more microporous layers (Y). In this embodiment, "containing polyolefin (B) as a main component" means that the microporous layer (Y) contains 50 mass% or more of polyolefin (B) based on the total mass of the microporous layer (Y). The lower limit of the content of polyolefin (B) in the microporous layer (Y) is preferably 55 mass% or more, 60 mass% or more, 70 mass% or more, 80 mass% or more, 90 mass% or more, or 95 mass% or more, from the viewpoints of the wettability of the separator to the electrolyte, thinning of the film, shutdown properties, etc. The upper limit of the content of polyolefin (B) in the microporous layer (Y) may be, for example, 60% by mass or less, 70% by mass or less, 80% by mass or less, 90% by mass or less, 95% by mass or less, 98% by mass or less, or 99% by mass or less, or may be 100% by mass.

[0050] <Polyolefin (B)> The polyolefin (B) in this embodiment is a polymer that contains a monomer having a carbon-carbon double bond as a repeating unit and has a molecular structure (more specifically, chemical composition, molecular weight, crystalline structure, etc.) that is different from that of the polyolefin (A) in this embodiment. Monomers that constitute the polyolefin (B) are not limited, but include monomers having 1 to 10 carbon atoms and a carbon-carbon double bond, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. The polyolefin (B) is, for example, a homopolymer, copolymer, or multistage polymer, and is preferably a homopolymer.

[0051] Specifically, the polyolefin (B) is preferably polyethylene, polypropylene, a copolymer of ethylene and propylene, or a mixture thereof, from the viewpoint of shutdown characteristics, etc. Polyolefin (B) is more preferably polyethylene.

[0052] When polyethylene is used as the polyolefin (B), the lower limit of the MFR of the polyethylene measured under a load of 2.16 kg at a temperature of 190°C is preferably 0.1 g / 10 min or more, more preferably 0.15 g / 10 min or more, even more preferably 0.18 g / 10 min or more, and particularly preferably 0.2 g / 10 min or more, from the viewpoint of good pore opening property and suppression of clogging. The upper limit of the MFR of the polyethylene is preferably 2.0 g / 10 min or less, more preferably 1.0 g / 10 min or less, even more preferably 0.8 g / 10 min or less, and particularly preferably 0.5 g / 10 min or less, from the viewpoint of separator strength.

[0053] <Average pore diameter of the microporous layer (Y)> In this embodiment, in ND-MD cross-section observation by SEM, the lower limit of the average pore diameter of the pores present in the microporous layer (Y) is preferably 100 nm or more, more preferably 150 nm or more, from the viewpoints of preventing clogging and achieving low air permeability of the separator. In ND-MD cross-section observation by SEM, the upper limit of the average pore diameter of the pores present in the microporous layer (Y) is preferably 2000 nm or less, more preferably 1000 nm or less, from the viewpoint of preventing short circuits in electricity storage devices such as lithium ion batteries.

[0054] <Ratio of average pore diameters of microporous layer (X) and microporous layer (Y)> In this embodiment, when ND-MD cross sections of the microporous layer (X) and the microporous layer (Y) are observed by SEM, the average pore diameter of the pores present in the microporous layer (Y) is preferably 1.2 to 10 times the average pore diameter of the pores present in the microporous layer (X). This means that the average pore diameter of the pores present in the ND-MD cross section of the microporous layer (Y) is larger than the average pore diameter of the pores present in the ND-MD cross section of the microporous layer (X) by a certain range. When the ND-MD cross section is observed by SEM, the separator is provided with a microporous layer (Y) having an average pore diameter that is 1.2 times or more larger than the average pore diameter of the pores present in the microporous layer (X), thereby making it possible to effectively suppress clogging. From the viewpoint of suppressing short circuits in the power storage device, when the ND-MD cross section is observed by SEM, the average pore diameter of the pores present in the microporous layer (Y) is preferably 10 times or less the average pore diameter of the pores present in the microporous layer (X). In ND-MD cross section observation by SEM, the ratio of the average long pore diameter of the pores present in the microporous layer (Y) to the average long pore diameter of the pores present in the microporous layer (X) is more preferably 1.4 to 8 times.

[0055] <Thickness of the microporous layer (Y)> The upper limit of the thickness of the microporous layer (Y) according to this embodiment is preferably 10 μm or less, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm or less, or 4 μm or less, from the viewpoint of increasing the energy density of the electricity storage device, etc. The lower limit of the thickness of the microporous layer (Y) is preferably 1 μm or more, 2 μm or more, 3 μm or more, or 3.5 μm or more, from the viewpoint of strength, etc.

[0056] <Multilayer structure> The separator for an electricity storage device may be a single layer of the microporous layer (X) primarily composed of the above-described polyolefin (A), or may have a multilayer structure in which the microporous layer (X) is laminated with a microporous layer (Y) primarily composed of a polyolefin (B). The additional microporous layer (Y) primarily composed of a polyolefin (B) may, in one embodiment, be a polyethylene microporous layer having the properties described above for the microporous layer (X) (e.g., one or more of a specific melt flow rate (MFR) and a molecular weight distribution (Mw / Mn)). In another embodiment, the microporous layer (Y) may be a polyethylene microporous layer lacking these properties. Furthermore, the microporous layer (Y) may be a polypropylene microporous layer. Because polyethylene has a melting point suitable for melt shutdown, it is preferable that the microporous layer (Y) be primarily composed of polyethylene from the viewpoint of shutdown properties.

[0057] The multilayer structure refers to a structure of two or more layers including a microporous layer (X) primarily composed of the polyolefin (A) of this embodiment. In one aspect, a multilayer structure in which three or more microporous layers are laminated is preferred. The multilayer structure more preferably has at least two microporous layers (X) primarily composed of the polyolefin (A) of this embodiment and at least one microporous layer (Y) primarily composed of the polyolefin (B). The multilayer structure even more preferably has at least two microporous layers (X) primarily composed of polypropylene (PP microporous layers) and at least one microporous layer (Y) primarily composed of polyethylene (PE microporous layer).

[0058] The advantages of this embodiment can be achieved regardless of the order in which the layers are laminated in the multilayer structure, but a three-layer structure in which layers are laminated in the order of PP microporous layer / PE microporous layer / PP microporous layer is particularly preferred. The three-layer structure of PP microporous layer / PE microporous layer / PP microporous layer allows the PE microporous layer to provide good shutdown properties while the PP microporous layer maintains good mechanical strength. The PP microporous layer is preferably a microporous layer (X) having the specific melt flow rate (MFR) and other properties described above.

[0059] <Separator thickness> The upper limit of the thickness of the separator for an electricity storage device according to this embodiment is preferably 25 μm or less, 22 μm or less, 20 μm or less, 18 μm or less, 16 μm or less, 14 μm or less, or 12 μm or less, from the viewpoint of increasing the energy density of the electricity storage device, etc. The lower limit of the thickness of the separator for an electricity storage device according to this embodiment is preferably 6 μm or more, 7 μm or more, 8 μm or more, 9 μm or more, 10 μm or more, or 11 μm or more, from the viewpoint of strength, etc.

[0060] <Porosity of separator> The lower limit of the porosity of the separator for an electricity storage device is preferably 20% or more, more preferably 25% or more, even more preferably 30% or more, and particularly preferably 35% or more, from the viewpoint of suppressing clogging in the electricity storage device and controlling the air permeability of the separator. The upper limit of the porosity of the separator for an electricity storage device is preferably 70% or less, more preferably 65% ​​or less, even more preferably 60% or less, and particularly preferably 55% or less, from the viewpoint of maintaining the strength of the separator. The porosity is measured by the method described in the Examples.

[0061] <Separator air permeability (air resistance)> The upper limit of the air permeability of the separator for an electricity storage device is preferably 300 seconds / 100 cm when converted into a thickness of 14 μm. 3 More preferably, it is 290 seconds / 100 cm or less. 3 Below, 280 seconds / 100cm 3 Below, 270 seconds / 100cm 3 or less, or 250 seconds / 100cm 3 More preferably, it is 180 seconds / 100 cm or less. 3 Below, 170 seconds / 100cm 3 Below, 160 seconds / 100cm 3 Below, 150 seconds / 100cm 3 or less, or 140 seconds / 100cm 3 The lower limit of the separator's air permeability is, for example, 50 seconds / 100 cm when the separator thickness is converted to 14 μm. 3 Over 60 seconds / 100cm 3Over 70 seconds / 100cm 3 Over, 100 seconds / 100cm 3 Over, 110 seconds / 100cm 3 or more, or 120 seconds / 100cm 3 That's all.

[0062] <Piercing strength> The lower limit of the puncture strength of the separator for an electricity storage device, when converted to a thickness of 14 μm, is preferably 230 gf or more, 240 gf or more, 250 gf or more, 260 gf or more, 280 gf or more, or 300 gf or more, and more preferably 310 gf or more, 320 gf or more, 330 gf or more, 340 gf or more, 350 gf or more, or 360 gf or more. The upper limit of the puncture strength of the separator for an electricity storage device, when converted to a thickness of 14 μm, is preferably 550 gf or less, 500 gf or less, or 480 gf or less. In a particularly preferred embodiment, the thickness of the separator for an electricity storage device is 8 μm or more and 18 μm or less, and the puncture strength is 230 gf or more.

[0063] <Balance of thickness, breathability and puncture resistance> As described above, the separator for an electricity storage device of this embodiment uses a microporous layer mainly composed of a polyolefin having a specific melt tension and melt flow rate (MFR), thereby making it possible to obtain a separator for an electricity storage device that is thin yet has low air permeability and high strength. For example, the separator for an electricity storage device of this embodiment includes a multilayer structure of a microporous layer, and the thickness of the multilayer structure is 18 μm or less, and the air permeability when the thickness of the multilayer structure is converted to 14 μm is 300 sec / 100 cm. 3 It is more preferable that the thickness of the multilayer structure is 18 μm or less, the thickness of the microporous layer of the present embodiment included in the multilayer structure (thickness as a single layer) is 6 μm or less, and the air permeability of the multilayer structure is 300 sec / 100 cm when converted to a thickness of 14 μm. 3 It is even more preferable that the thickness is not more than 100 mm and the puncture strength is 300 gf or more.

[0064] <Separator tensile test characteristics> In a tensile test of the electricity storage device separator according to this embodiment, it is preferable that the ratio (SMD / STD) of the tensile strength in the machine direction (SMD) to the tensile strength in the width direction (STD) satisfies the following relationship. SMD / STD>5 A separator with an SMD / STD of more than 5 can be used as a high-strength separator. As a high-strength separator, SMD / STD≧7 is more preferable, SMD / STD≧10 is even more preferable, SMD / STD≧12 is even more preferable, and SMD / STD≧14 is particularly preferable. The tensile test of the separator is performed by the method described in the Examples section.

[0065] <Heat shrinkage rate> The separator for an electricity storage device according to this embodiment preferably has a heat shrinkage of 1% or less in TD and 4% or less in MD after heat treatment at 105°C for 1 hour, and a heat shrinkage of 1% or less in TD and 10% or less in MD after heat treatment at 120°C for 1 hour. The heat shrinkage after heat treatment at 105°C is an indicator of the dimensional stability of the separator for an electricity storage device during use of the electricity storage device, particularly when the electricity storage device is placed in a harsh high-temperature environment or in an abnormal heat generation state. The heat shrinkage after heat treatment at 120°C is an indicator of the dimensional stability of the separator for an electricity storage device during use of the electricity storage device, particularly under conditions in which the electricity storage device is in an abnormal state. The separator for an electricity storage device according to this embodiment has heat shrinkages in TD and MD controlled within specific ranges after heat treatment at 105°C for 1 hour and after heat treatment at 120°C for 1 hour, thereby ensuring dimensional stability at high temperatures inside an electricity storage device such as a battery, thereby suppressing abnormal states in the electricity storage device and ensuring good device characteristics. The method for measuring the heat shrinkage rate of the present disclosure will be described in detail in the Examples.

[0066] The TD heat shrinkage rate after heat treatment at 105°C for 1 hour is 1% or less, preferably 0.9% or less, and more preferably 0.8% or less, from the viewpoint of dimensional stability inside the electricity storage device. While a lower TD heat shrinkage rate is desirable, from the viewpoint of ease of manufacturing the electricity storage device separator, it may be, for example, -1% or more, -0.5% or more, or 0% or more. Furthermore, the MD heat shrinkage rate after heat treatment at 105°C for 1 hour is 4% or less, preferably 3.8% or less, more preferably 3.5% or less, even more preferably 3.2% or less, particularly preferably 3% or less, and most preferably 2.7% or less, from the viewpoint of dimensional stability inside the electricity storage device. Although a lower MD heat shrinkage rate is desirable, from the viewpoint of ease of manufacturing the electricity storage device separator, it may be, for example, 0% or more, 0.5% or more, or 1% or more.

[0067] The TD heat shrinkage rate after heat treatment at 120°C for 1 hour is 1% or less, preferably 0.9% or less, and more preferably 0.8% or less, from the viewpoint of dimensional stability inside the electricity storage device. While a lower TD heat shrinkage rate is desirable, from the viewpoint of ease of manufacturing the electricity storage device separator, it may be, for example, -1% or more, -0.5% or more, or 0% or more. Furthermore, the MD heat shrinkage rate after heat treatment at 120°C for 1 hour is 10% or less, preferably 9.0% or less, more preferably 8.5% or less, even more preferably 8.0% or less, particularly preferably 7.5% or less, and most preferably 7.0% or less, from the viewpoint of dimensional stability inside the electricity storage device. Although a lower MD heat shrinkage rate is desirable, from the viewpoint of ease of manufacturing the electricity storage device separator, it may be, for example, 0% or more, 0.5% or more, or 1% or more.

[0068] <Balance of air permeability, puncture resistance and heat shrinkage rate> The separator for an electric storage device of this embodiment uses a microporous layer (X) primarily composed of a polyolefin (A) having a specific melt flow rate (MFR), and exhibits low air permeability, high strength, and low heat shrinkage. Generally, low MFR, i.e., high molecular weight, polyolefins exhibit significant intertwining between polymers, resulting in insufficient pore opening, making it difficult to control the air permeability low. Therefore, conventional separators have either been unable to use polyolefins with sufficiently high molecular weights, resulting in insufficient pin puncture strength, or, when high molecular weight polyolefins are used, the pore opening is insufficient, making it difficult to achieve low air permeability and large pore size. Thus, conventionally, low air permeability and high strength (especially high pin puncture strength) have been incompatible. Furthermore, conventional dry separators tend to have high MD heat shrinkage, making it extremely difficult to obtain a separator that simultaneously achieves high strength, low air permeability, and low heat shrinkage. The separator for an electricity storage device of this embodiment can have low air permeability, high pin puncture strength, and low heat shrinkage by using a specific polyolefin (A) in the microporous layer (X). Such a separator for an electricity storage device can be produced, for example, by applying precisely controlled film formation and stretching conditions as exemplified in the section "Method for producing a separator for an electricity storage device" below, and / or by using polypropylene with a controlled pentad fraction as the polyolefin (A), although this is not limited thereto.

[0069] <<Method for manufacturing separator for electricity storage device>> The method for producing the microporous layer of this embodiment generally includes a melt-extrusion step in which a resin composition containing the above-described polyolefin (hereinafter referred to as a polyolefin resin composition) is melt-extruded to obtain a resin film, and a pore-forming step in which the obtained resin film is perforated to make it porous, and optionally includes an annealing step, a stretching step, a heat-relaxing step, etc. The method for producing the microporous layer is broadly divided into a dry method in which no solvent is used in the pore-forming step, and a wet method in which a solvent is used. Examples of the melt-extrusion method include a T-die method and an inflation method.

[0070] In one embodiment, the polyolefin resin composition is a composition comprising the components of the microporous layer (X), and in another embodiment, a composition comprising the components of the microporous layer (Y). The polyolefin resin composition may optionally contain resins other than polyolefins, additives, etc., depending on the method for producing the microporous layer or the desired physical properties of the microporous layer. Examples of additives include pore-forming materials, fluorine-based flow modifiers, elastomers, waxes, crystal nucleating materials, antioxidants, metal soaps such as metal salts of aliphatic carboxylic acids, UV absorbers, light stabilizers, antistatic agents, antifogging agents, and color pigments. Examples of pore-forming materials include plasticizers, inorganic fillers, and combinations thereof.

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

[0072] Examples of inorganic fillers include oxide ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fiber.

[0073] The temperature of the extruder during melt-kneading is preferably at least 20°C higher than the melting point of the polyolefin resin composition and not higher than 110°C higher than the melting point of the polyolefin resin composition. When the temperature is above the lower limit, the resulting polyolefin resin film has a uniform thickness, making it less likely to break during extrusion. When the temperature is below the upper limit, the polyolefin contained in the polyolefin resin composition is highly oriented, resulting in a favorable lamellar structure in the polyolefin. This results in low air permeability for the resulting separator for an electrical storage device, resulting in low resistance for an electrical storage device such as a lithium-ion secondary battery. When a polyolefin resin film has multiple layers formed using different polyolefin resin compositions, the melting point refers to the melting point of the polyolefin resin composition with the lower melting point. In one embodiment, the melting point is evaluated as the peak-top temperature of the largest endothermic peak observed when the temperature is increased at a heating rate of 10°C / min using a DSC (differential scanning calorimeter).

[0074] The draw ratio when the polyolefin resin composition is extruded into a film from an extruder is preferably 50 to 400, more preferably 75 to 350, and particularly preferably 100 to 300. The draw ratio refers to the value obtained by dividing the lip clearance of a T-die or inflation die by the thickness of the polyolefin resin film extruded from the die lip. By adjusting the draw ratio within the above range, the molecular orientation of the polyolefin is improved, lamellae can be formed well, and the air permeability of the separator can be controlled low. At the same time, the film formation stability of the film is not reduced, and the thickness and width precision of the resulting polyolefin resin film can be improved.

[0075] The polyolefin resin composition extruded from the die may be cooled with air. When the polyolefin resin extruded from the die is sufficiently cooled with air, the polyolefin resin crystallizes to form lamellae, resulting in a separator with a microporous layer having large average and maximum pore diameters, high porosity, and low air permeability. The air flow rate is preferably 300 L / min or more and 1000 L / min or less for a die width of 500 mm. Setting the air flow rate above the lower limit above is preferable because the polyolefin resin composition extruded from the die is sufficiently cooled by the air, causing the polyolefin to crystallize to form lamellae, resulting in a microporous layer with low air permeability. Setting the air flow rate below the upper limit above is preferable from the viewpoint of improving the film formation stability and the thickness and width accuracy of the resulting polyolefin resin sheet. The air temperature is preferably 0°C or more and 40°C or less from the viewpoints of film formation stability, thickness accuracy, and width accuracy.

[0076] Methods for producing a power storage device separator having a multilayer structure including the microporous layer of this embodiment and multiple laminated microporous layers include, but are not limited to, coextrusion and lamination. In the coextrusion method, the resin compositions of each layer are coextruded and laminated into two or more layers to produce a raw film. The resulting raw film of two or more layers can be stretched and perforated to produce a microporous layer. It is easier to obtain a high-strength microporous layer by producing a raw film of a laminate of two or more layers and then perforating it, rather than by producing each single microporous layer individually. In the lamination method, each layer is produced separately and then bonded together to adhere the layers to each other. Examples of lamination methods include dry lamination using an adhesive or the like, and thermal lamination in which multiple layers are bonded by heating. However, thermal lamination is preferred from the viewpoint of further improving the air permeability and strength of the resulting power storage device separator.

[0077] Next, the resulting polyolefin resin film is subjected to an annealing step in which the film is heat-treated. This annealing step grows the lamellae formed in the polyolefin resin film during the extrusion step. This treatment improves the film's openability and reduces the air permeability of the resulting separator for an electricity storage device.

[0078] The annealing temperature of the polyolefin resin film is preferably set to a temperature 40°C lower than the melting point of the polyolefin resin film or higher and 1°C lower than the melting point of the polyolefin resin film or lower. However, when the polyolefin resin film has a laminate structure, the annealing temperature is preferably limited to a temperature 40°C lower than the melting point of the resin film with the lowest melting point or higher and 1°C lower than the melting point of the resin film with the lowest melting point or lower. The annealing temperature refers to the temperature of the atmosphere inside the annealing device. When the annealing temperature is above the lower limit, the lamellae grow well and are easily opened during the film stretching process. When the annealing temperature is below the upper limit, collapse of the lamellar structure due to relaxation of the orientation of the polyolefin in the polyolefin resin film is suppressed.

[0079] The annealing time of the polyolefin resin film is preferably 10 minutes or more from the viewpoint of favorably growing lamellae and favorably opening holes in the polyolefin resin film in the stretching step, and in one embodiment, the annealing time may be 180 minutes or less.

[0080] The annealing of the polyolefin resin film may be carried out while the polyolefin resin film is running, or may be carried out while the polyolefin resin film is wound up in a roll.

[0081] Next, a stretching step is carried out in which the annealed polyolefin resin film is stretched to form holes. This stretching step preferably includes a first stretching step and a second stretching step subsequent to the first stretching step.

[0082] In the first stretching step, the lamellae formed in the polyolefin resin film are separated from each other, causing minute cracks in the amorphous parts between the lamellae, and these cracks act as starting points to form numerous micropores. In the first stretching step, uniaxial stretching in the MD is performed.

[0083] In the first stretching step, the lower limit of the temperature of the polyolefin resin film is preferably −20° C. or higher, more preferably 0° C. or higher. In the first stretching step, the upper limit of the temperature of the polyolefin resin film is preferably 110° C. or lower, more preferably 80° C. or lower. When the temperature is equal to or higher than the lower limit, breakage of the polyolefin resin film during stretching is suppressed, and when the temperature is equal to or lower than the upper limit, cracks are effectively generated in the amorphous portions between the lamellae, suppressing necking of the film.

[0084] In the first stretching step, the lower limit of the stretching ratio of the polyolefin resin film is preferably 1.02 times or more, more preferably 1.06 times or more. In the first stretching step, the upper limit of the stretching ratio of the polyolefin resin film is preferably 1.5 times or less, more preferably 1.4 times or less. When the stretching ratio is equal to or greater than the lower limit, micropores are likely to be formed in the amorphous portions between the lamellae. When the stretching ratio is equal to or less than the upper limit, excessive micropores are not formed, and the pore size does not become too small, thereby reducing the air permeability. In the present disclosure, the stretching ratio of the polyolefin resin film refers to the value obtained by dividing the length of the polyolefin resin film after stretching by the length of the polyolefin resin film before stretching.

[0085] The stretching speed in the first stretching step of the polyolefin resin film is preferably 10 to 1000% / min, more preferably 50 to 600% / min. When the stretching speed is equal to or higher than the lower limit, micropores are likely to be uniformly formed in the amorphous portions between the lamellae, and when the stretching speed is equal to or lower than the upper limit, breakage of the polyolefin resin film can be suppressed.

[0086] In the present disclosure, the stretching speed of a polyolefin resin film refers to the rate of change in dimension of the polyolefin resin film in the stretching direction per unit time.

[0087] The method for stretching the polyolefin-based resin film in the first stretching step is not particularly limited as long as it can uniaxially stretch the polyolefin-based resin film, and examples include a method in which the polyolefin-based resin film is stretched at a predetermined temperature using a uniaxial stretching device.

[0088] Next, the polyolefin resin film after uniaxial stretching in the first stretching step is preferably subjected to a second stretching step in which the ambient temperature inside the apparatus is higher than the ambient temperature during uniaxial stretching in the first stretching step and is not higher than a temperature 1°C to 60°C lower than the melting point of the polyolefin resin film (or the resin film with the lowest melting point in the case of a multilayer structure). In the second stretching step, the polyolefin resin film is preferably uniaxially stretched only in the machine direction. In this way, by stretching the polyolefin resin film at an ambient temperature higher than the ambient temperature inside the apparatus in the first stretching step, the numerous micropores formed in the polyolefin resin film in the first stretching step can be grown. If the temperature is above the above lower limit, the micropores formed in the polyolefin resin film in the first stretching step are likely to grow, and the air permeability of the resulting separator for an electricity storage device can be reduced. When the temperature is equal to or lower than the upper limit, the micropores formed in the polyolefin resin film in the first stretching step are less likely to be blocked, and the air permeability of the resulting electricity storage device separator can be reduced.

[0089] In the second stretching step, the stretching ratio of the polyolefin resin film is preferably 1.5 to 3 times, more preferably 1.8 to 2.5 times. If the stretching ratio is above the lower limit, the micropores formed in the polyolefin resin film during the first stretching step tend to grow, thereby reducing the air permeability of the resulting separator for a storage battery device. If the stretching ratio is below the upper limit, the micropores formed in the polyolefin resin film during the first stretching step tend not to close, thereby reducing the air permeability of the resulting separator for a storage battery device.

[0090] In the second stretching step, the stretching speed of the polyolefin resin film is preferably 60% / min or less, or 30% / min or less, from the viewpoint of uniformly widening the micropores formed in the first stretching step, and may be, for example, 2% / min or more, or 3% / min or more, from the viewpoint of process efficiency.

[0091] The method for stretching the polyolefin-based resin film in the second stretching step is not particularly limited as long as it can uniaxially stretch the polyolefin-based resin film, and examples include a method of uniaxially stretching the polyolefin-based resin film at a predetermined temperature using a uniaxial stretching device.

[0092] The polyolefin resin film uniaxially stretched in the second stretching step is subjected to a heat-relaxing step in which the film is heated to relieve residual stress. Residual stress may be generated in the polyolefin resin film due to the stretching in the second stretching step. The heat-relaxing step is performed to relieve the residual stress and suppress thermal shrinkage of the resulting polyolefin resin microporous layer due to heating other than that in the heat-relaxing step, thereby improving the safety of the resulting power storage device separator.

[0093] As described above, in order to improve the dimensional stability of the polyolefin resin microporous layer during heating, it is necessary to relax the residual stress in the polyolefin resin film. To achieve this, the ambient temperature in the heat-relaxing device in the heat-relaxing step is preferably 40°C lower or higher, or 20°C lower or higher, than the melting point of the polyolefin resin film (the resin film with the lowest melting point in the case of a multilayer structure). Furthermore, from the viewpoint of suppressing blockage of the micropores formed in the stretching step, the temperature is preferably 1°C or higher, or 4°C or higher, than the melting point of the polyolefin resin film (the resin film with the lowest melting point in the case of a multilayer structure).

[0094] The lower limit of the heat shrinkage rate of the polyolefin resin film in the heat-relaxing step is preferably 25% or more, more preferably 30% or more. The upper limit of the heat shrinkage rate of the polyolefin resin film in the heat-relaxing step is preferably 60% or less, more preferably 50% or less. The heat shrinkage rate of the polyolefin resin film in the heat-relaxing step refers to the value obtained by dividing the shrinkage length of the polyolefin resin film in the stretching direction in the heat-relaxing step by the length of the polyolefin resin film in the stretching direction after the second stretching step and multiplying the result by 100. When the heat shrinkage rate is equal to or greater than the lower limit, the residual stress in the polyolefin resin film is sufficiently relaxed, the resulting polyolefin resin microporous layer has good dimensional stability during heating, and the safety of electricity storage devices such as lithium-ion secondary batteries at high temperatures is good. Furthermore, when the shrinkage rate is equal to or less than the upper limit, sagging of the polyolefin resin film is unlikely to occur, and poor winding or deterioration of uniformity onto a roll is suppressed.

[0095] In the second stretching step and the heat-relaxing step, it is preferable to adjust the stretching speed and the conveying speed so that they do not become excessively high, from the viewpoint of sufficiently relaxing the stress and reducing the thermal shrinkage rate.

[0096] After the heat-relaxing step, it is preferable to apply heat-relaxing again at a temperature equal to or higher than the temperature in the heat-relaxing step and 20° C. higher or lower than the temperature in the heat-relaxing step. By applying this step, it is possible to obtain a separator for an electricity storage device having better heat shrinkage properties.

[0097] <Energy storage device> The electricity storage device of this embodiment includes the electricity storage device separator of this embodiment. The electricity storage device of this embodiment has a positive electrode and a negative electrode. The electricity storage device separator is preferably laminated between the positive electrode and the negative electrode, or is located on the outside of the positive electrode or negative electrode within the battery exterior, or wraps the electrode. If desired, lead bodies may be connected to the positive electrode and the negative electrode, respectively, so that the positive electrode and the negative electrode can be connected to external equipment, etc.

[0098] Examples of the power storage device include, but are not limited to, lithium secondary batteries, lithium ion secondary batteries, sodium secondary batteries, sodium ion secondary batteries, magnesium secondary batteries, magnesium ion secondary batteries, calcium secondary batteries, calcium ion secondary batteries, aluminum secondary batteries, aluminum ion secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries, electric double layer capacitors, lithium ion capacitors, redox flow batteries, lithium-sulfur batteries, lithium-air batteries, and zinc-air batteries. Among these, from the viewpoint of compatibility with the separator according to this embodiment, lithium secondary batteries, lithium ion secondary batteries, nickel-metal hydride batteries, and lithium ion capacitors are preferred, and lithium ion secondary batteries are more preferred.

[0099] The electricity storage device can be produced, for example, by the following method: The positive and negative electrodes are stacked with the separator described above interposed therebetween and wound as necessary to form a stacked electrode body or a wound electrode body. The stacked or wound electrode body is then loaded into an exterior housing. At this time, lead bodies are connected to the positive and negative electrodes within the exterior housing, and the ends of the lead bodies can be arranged so that they extend outside the exterior housing. When there are multiple positive and negative electrodes, tabs of the same electrode may be joined by welding or other means to form a single lead body that extends outside the exterior housing. The tabs of the same electrode may be formed from exposed portions of the current collectors, or may be formed by welding metal pieces to exposed portions of the current collectors. The positive and negative electrodes are connected to the positive and negative electrode terminals of the exterior housing via lead bodies or other means. At this time, portions of the lead body and portions of the exterior housing may be joined by heat fusion or other means. A nonaqueous electrolyte solution containing a nonaqueous solvent such as a chain and / or cyclic carbonate and an electrolyte such as a lithium salt is then injected into the exterior housing. The exterior housing is then sealed to produce an electricity storage device. [Example]

[0100] EXAMPLES The present invention will be specifically explained below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0101] <<Measurement and Evaluation Methods>> [Melt flow rate (MFR) measurement] The melt flow rate (MFR) of the microporous layer was measured in accordance with JIS K 7210 at a temperature of 230°C and a load of 2.16 kg (unit: g / 10 min). The MFR of polypropylene was measured in accordance with JIS K 7210 at a temperature of 230°C and a load of 2.16 kg. The melt flow rate (MFR) of polyethylene was measured in accordance with JIS K 7210 at a temperature of 190°C and a load of 2.16 kg. The MFR of elastomers was measured in accordance with JIS K 7210 at a temperature of 230°C and a load of 2.16 kg.

[0102] [Measurement of Mw and Mn by GPC (Gel Permeation Chromatography)] A calibration curve was created by measuring standard polystyrene under the following conditions using an Agilent PL-GPC220 from Agilent Technologies, Inc. Chromatography was also performed on the sample polymer under the same conditions, and based on the calibration curve, the weight-average molecular weight (Mw), number-average molecular weight (Mn), and the value obtained by dividing the weight-average molecular weight (Mw) by the number-average molecular weight (Mn) (Mw / Mn) of the polymer were calculated under the following conditions.

[0103] Column: TSKgel GMHHR-H(20) HT (7.8mm I.D. x 30cm) x 2 Mobile phase: 1,2,4-trichlorobenzene Detector: RI Column temperature: 160℃ Sample concentration: 1mg / ml Calibration curve: Polystyrene

[0104] [Melt tension measurement] The melt tension (mN) of the microporous layer was measured under the following conditions using a Capillograph manufactured by Toyo Seiki Seisaku-sho, Ltd. Capillary: diameter 1.0 mm, length 20 mm Cylinder extrusion speed: 2mm / min Take-up speed: 60m / min ·Temperature: 230℃

[0105] [Measurement of pentad fraction] The pentad fraction of polypropylene was assigned based on the description in the Polymer Analysis Handbook (edited by the Japan Society for Analytical Chemistry). 13 Calculations were made from the C-NMR spectrum using the peak height method. 13 The C-NMR spectrum was measured using a JEOL-ECZ500 manufactured by JEOL Ltd., by dissolving polypropylene in o-dichlorobenzene-d, at a measurement temperature of 145°C and 25,000 cumulative cycles.

[0106] [Thickness (μm) measurement] The thickness (μm) of the separator was measured at room temperature of 23±2°C using a Digimatic Indicator IDC112 manufactured by Mitutoyo Corporation.

[0107] [Porosity (%) measurement] A 10cm x 10cm square sample was cut from the separator and its volume (cm 3 ) and mass (g), and then calculate the density (g / cm 3 ) and the porosity was calculated.

[0108] [Air resistance (sec / 100cm) 3 ) Measurement The separator's air permeability resistance (sec / 100cm) was measured using a Gurley air permeability meter conforming to JIS P-8117. 3 ) was measured, divided by the thickness in μm, and then multiplied by 14 to calculate the air resistance (air permeability) converted into a 14 μm thickness.

[0109] [Measurement of puncture strength] A needle with a hemispherical tip and a radius of 0.5 mm was prepared, and a separator was sandwiched between two plates with openings of 11 mm diameter (dia.), and the needle, separator, and plates were set in place. A puncture test was performed using an Imada MX2-50N instrument under the following conditions: a needle tip curvature radius of 0.5 mm, an opening diameter of 11 mm in the separator holding plate, and a puncture speed of 25 mm / min. The needle and separator were brought into contact, and the maximum puncture load (i.e., puncture strength (gf)) was measured. The puncture strength obtained was divided by the thickness and then multiplied by 14 to calculate the puncture strength equivalent to a thickness of 14 μm.

[0110] [Tensile test] A tensile test was performed using a tensile testing machine (Shimadzu Corporation, Autograph AG-A type), and the strength at the time of sample breakage was divided by the cross-sectional area of ​​the sample before the test to obtain the tensile breaking strength (kg / cm 2 The measurement conditions were temperature: 23±2°C, sample shape: width 10 mm × length 100 mm, chuck distance: 50 mm, and tensile speed: 200 mm / min, and the tensile strength in the MD (SMD) and the tensile strength in the TD (STD) were measured and the SMD / STD ratio was calculated.

[0111] [Measurement of average and maximum long pore diameters] The average and maximum pore diameters were measured by image analysis of the pores present on the MD-TD surface or ND-MD cross section using an SEM. As a pretreatment, the separator was stained with ruthenium. The obtained sample was fixed to the SEM sample stage with a conductive adhesive (carbon-based), dried, and then subjected to a conductive treatment using an osmium coater (HPC-30W, manufactured by Vacuum Device Co., Ltd.). Osmium coating was then performed with the applied voltage adjustment knob set to 4.5 and a discharge time of 0.5 seconds. The specimen was then used for microscopic examination.

[0112] The above-mentioned specimen was observed with a scanning electron microscope (SEM) at an accelerating voltage of 1.0 kV, and SEM images were obtained. The obtained surface or cross-sectional SEM images were taken within a range of 4 μm × 4 μm. of The diameter of each hole was measured, and if necessary, binarization processing was performed to separate the resin part from the hole part, and the maximum diameter and average diameter of the hole were calculated. At this time, micropores existing across the 4 μm × 4 μm range and the part outside the range, and micropores with an area of ​​0.001 n m 2 The following pores were excluded from the measurement range. The average pore diameter was obtained by calculating the area-weighted average value of the pore diameters. The largest pore diameter was used as the maximum pore diameter. Tables 1 to 6 show the maximum pore diameters or average pore diameters measured by SEM observation of the MD-TD surface.

[0113] [Measurement of heat shrinkage rate] The separator was cut into 50 mm squares in both the MD and TD directions, and the resulting samples were placed in hot air dryers (DF1032, manufactured by Yamato Scientific Co., Ltd.) heated to 105°C and 120°C (atmospheric pressure, in the atmosphere). After 1 hour and 2 hours, the samples were removed from the hot air dryer and the thermal shrinkage was determined. The samples were placed on copy paper before being placed in the hot air dryer to prevent them from adhering to the inner walls of the dryer and to prevent the samples from fusing together. Heat shrinkage rate (%): (dimension before heating (mm) - dimension after heating (mm)) / (dimension before heating (mm)) x 100

[0114] [Fabrication of sheet-type lithium-ion secondary batteries] The electrolyte used was a mixture of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:2, containing 1 mol / L of LiPF6 as a lithium salt.

[0115] The positive electrode active material was lithium nickel manganese cobalt composite oxide (LiNi 0.5 Co 0.2 Mn 0.3 O2), carbon black powder (manufactured by Timcal Corporation, product name: SuperP Li) as a conductive additive, and PVDF as a binder were mixed in a mass ratio of composite oxide:conductive additive:binder = 100:3.5:3. The resulting mixture was dispersed in a solvent (N-methylpyrrolidone) to form a dispersion. The dispersion was applied to both sides of a 15 μm-thick aluminum foil serving as a positive electrode current collector, and after the solvent was dried and removed, the foil was pressed with a roll press to produce a double-sided coated positive electrode.

[0116] Graphite powder (Hitachi Chemical Co., Ltd., product name: MAG) with a particle diameter of 22 μm (D50) was used as the negative electrode active material, a binder (Zeon Corporation, product name: BM400B), and a carboxymethyl cellulose thickener (Daicel Corporation, product name: #2200) were mixed in a mass ratio of graphite powder:binder:thickener = 100:1.5:1.1. The resulting mixture was dispersed in a solvent (water) to prepare an aqueous dispersion. The aqueous dispersion was applied to one side of a 10 μm-thick copper foil serving as a negative electrode current collector to prepare a single-sided coated body. Separately from the single-sided coated body, the aqueous dispersion was applied to both sides of a 10 μm-thick copper foil serving as a negative electrode current collector to prepare a double-sided coated body. The solvent (water) was dried and removed from the single-sided coated and double-sided coated bodies, and then the coated copper foil was pressed with a roll press to prepare single-sided coated negative electrodes and double-sided coated negative electrodes, respectively.

[0117] The resulting positive and negative electrodes were stacked in the following order: single-sided coated negative electrode / double-sided coated positive electrode / double-sided coated negative electrode / double-sided coated positive electrode / single-sided coated negative electrode, with the separator prepared below sandwiched between the opposing surfaces of the active materials. The resulting stack was then inserted into a bag (battery exterior) made of a laminate film in which both sides of aluminum foil (40 μm thick) were coated with a resin layer. Terminals of the inserted electrodes protruded from the battery exterior. 0.8 mL of the electrolyte prepared as described above was then poured into the bag, and the bag was vacuum-sealed to produce a sheet-shaped lithium-ion secondary battery.

[0118] [Battery performance evaluation and clogging observation] The resulting sheet-shaped lithium-ion secondary battery was placed in a thermostatic chamber (Futaba Scientific Co., Ltd., product name: PLM-73S) set at 25°C, connected to a charge / discharge device (Asuka Electronics Co., Ltd., product name: ACD-01), and left to stand for 16 hours. The battery was then charged at a constant current of 0.05 C, and after the voltage reached 4.35 V, it was charged at a constant voltage of 4.35 V for 2 hours, and then discharged at a constant current of 0.2 C to 3.0 V. This charge / discharge cycle was repeated three times to perform initial charging and discharging of the battery. 1 C refers to the current value when the battery's full capacity is discharged in 1 hour.

[0119] After the initial charge and discharge, the battery was placed in a thermostatic chamber set at 50°C and charged at a constant current of 1 C. After the voltage reached 4.35 V, the battery was charged at a constant voltage of 4.35 V for 1 hour, and then discharged at a constant current of 1 C to 3.0 V. This charge and discharge cycle was repeated 100 times. The battery cycle test involved repeating the above charge and discharge cycle 100 times.

[0120] The cycle capacity retention rate was determined as a percentage by dividing the discharge capacity (mAh) at the 100th cycle by the discharge capacity (mAh) at the 1st cycle. After 100 cycles, the sheet-shaped lithium-ion secondary battery was disassembled under an argon atmosphere, and the separator was removed. The first immersion cleaning was performed by immersing the separator in an ethyl methyl carbonate-containing bath for 30 seconds and then removing it. A total of three immersion cleanings were performed. The ethyl methyl carbonate in the bath was replaced during the second and third immersion cleanings. The negative electrode side surface of the separator was then observed in a 1 mm square area using an optical microscope at magnifications of 100 to 1000 times to check for clogging on the separator surface. Tables 1 to 4 show the presence or absence of clogging.

[0121] [Evaluation of short circuits in lithium-ion secondary batteries] The resulting sheet-shaped lithium-ion secondary battery was placed in a thermostatic chamber (Futaba Scientific Co., Ltd., product name: PLM-73S) set at 25°C, connected to a charge / discharge device (Asuka Electronics Co., Ltd., product name: ACD-01), and left to stand for 16 hours. The battery was then charged at a constant current of 0.05 C, and after the voltage reached 4.35 V, it was charged at a constant voltage of 4.35 V for 2 hours, and then discharged at a constant current of 0.2 C to 3.0 V. This charge / discharge cycle was repeated three times to perform initial charging and discharging of the battery. 1 C refers to the current value when the battery's full capacity is discharged in 1 hour.

[0122] After the initial charge and discharge, the battery was charged to 4.0 V at a constant current of 0.2 C, and then placed in a thermostatic chamber. The temperature of the thermostatic chamber was increased from 25°C by 5°C every 10 minutes, and the temperature at which the battery voltage dropped to 0.5 V or less was confirmed and used as the short-circuit temperature, which serves as an indicator of battery safety.

[0123] Example 1 [Preparation of single-layer separator (microporous layer (X))] High molecular weight polypropylene resin (PP, MFR (230°C) = 0.51 g / 10 min, density = 0.91 g / cm 3The polymer (Mw / Mn = 5.2, pentad fraction = 99.3%) was melted in a 2.5-inch extruder and fed to a T-die with a die width of 500 mm and a die lip clearance of 3.3 mm using a gear pump. The T-die temperature was set to 210°C. The molten polymer was extruded from the T-die. The extruded resin was then sufficiently cooled by blowing in 12°C air at a rate of 500 L / min and wound onto a roll. The wound raw film had a thickness of 15 μm and a draw ratio of 220. The raw film was then annealed at 130°C for 20 minutes. The annealed raw film was cold-stretched to 8% at room temperature, then hot-stretched to 185% at 116°C, and heat-relaxed to 45% at 126°C to form micropores, yielding a separator consisting of a microporous layer (X) with a single PP layer structure. After the above-mentioned stretching and perforation, the air permeability, puncture strength, porosity, average long pore diameter, and maximum long pore diameter of the obtained separator were measured. The results are shown in Table 1.

[0124] Examples 2 to 11 Separators comprising a microporous layer (X) were obtained in the same manner as in Example 1, except that the raw materials and production conditions were changed as shown in Table 1, and the obtained separators were evaluated.

[0125] Comparative Examples 1 to 11 Separators comprising a microporous layer (X) were produced in the same manner as in Example 1, except that the raw materials and production conditions were changed as shown in Table 2, and the separators were evaluated.

[0126] Example 12 [Preparation of three-layer separator (microporous layer (X) / microporous layer (Y) / microporous layer (X)] High molecular weight polypropylene resin (PP, MFR (230°C) = 0.51 g / 10 min, density = 0.91 g / cm 3The polymer was melted in a 2.5-inch extruder and fed to a T-die using a gear pump. The temperature of the T-die was set to 230°C. The molten polymer was extruded from the T-die. The extruded resin was then cooled by blowing air, and the PP raw film (X'), a precursor of the microporous layer (X), was taken up on a roll. Similarly, polyethylene resin (PE, MFR (190°C) = 0.38 g / 10 min, density = 0.96 g / cm 3 ) was melted in a 2.5-inch extruder and fed to a T-die using a gear pump. The temperature of the T-die was set to 220°C. The molten polymer was extruded from the T-die. The extruded resin was then cooled by blowing air, and the PE raw film (Y'), which is a precursor of the microporous layer (Y), was taken up on a roll. The PP raw film (X') and PE raw film (Y') wound onto rolls each had a thickness of 5 μm. The PP raw film (X') and PE raw film (Y') were then bonded together to form the PP raw film (X') / PE raw film (Y') / PP raw film (X'), resulting in a raw film with a PP / PE / PP three-layer structure. The three-layer raw film was then annealed at 130°C for 20 minutes. The annealed raw film was cold stretched to 11% at room temperature, then hot stretched to 158% at 125°C, and relaxed to 113% at 125°C to form micropores, resulting in a PP / PE / PP three-layer separator consisting of microporous layer (X) / microporous layer (Y) / microporous layer (X).

[0127] After the stretching and perforation, the air resistance and puncture strength of the resulting three-layer separator were measured, as well as the MFR and melt tension of the microporous layer (X). The minimum thickness of the PP raw film (X') was evaluated by increasing the take-up speed while maintaining a constant discharge rate after the PP raw film (X') was produced. The resulting PP raw film with the minimum thickness was laminated with the 5 μm-thick PE raw film (Y') and then perforated under the annealing and stretching conditions described above (annealing at 125°C for 20 minutes, cold stretching to 11% at room temperature, hot stretching to 158% at 125°C, and relaxation to 113% at 125°C). A three-layer separator was then fabricated, and the thickness of the separator was confirmed. The results are shown in Table 3.

[0128] Examples 13 to 16 Separators having a three-layer structure were obtained in the same manner as in Example 12, except that the raw materials and production conditions were changed as shown in Table 3, and the obtained separators were evaluated.

[0129] Comparative Examples 12 to 18 Separators having a three-layer structure were produced in the same manner as in Example 12, except that the raw materials and production conditions were changed as shown in Table 4, and the resulting separators were evaluated.

[0130] Example 17 [Preparation of microporous layer] High molecular weight polypropylene (PP, MFR (230°C) = 0.51 g / 10 min, density = 0.91 g / cm 3 The polymer (Mw=900,000, Mw / Mn=5.2, pentad fraction=99.3%) was melted in a 2.5-inch extruder and fed to a T-die using a gear pump. The T-die temperature was set to 230°C. The molten polymer was extruded from the T-die. The extruded resin was then rapidly cooled with 800 L / min of blown air cooled to 10°C using a chiller, while being wound around a roll at a speed of 25 m / min to obtain a precursor (X') (precursor of the microporous layer (X)) with a thickness of approximately 6 μm. The lip width of the T-die TD was set to 500 mm, the distance between the lips of the T-die (lip clearance) was set to 2.4 mm, and the extrusion rate was 3.8 kg / h. Similarly, polyethylene (PE, MFR (190°C) = 0.38 g / 10 min, density = 0.96 g / cm) was used as polyolefin (B). 3 ) was melted in a 2.5-inch extruder and fed to a T-die using a gear pump. The temperature of the T-die was set to 210°C. The molten polymer was extruded from the T-die. The extruded resin was then rapidly cooled by blowing in 800 L / min of air cooled to 10°C using a chiller, while being wound around a roll at a roll speed of 25 m / min to obtain a precursor (Y') (precursor of the microporous layer (Y)) with a thickness of approximately 6 μm. Here, the TD lip width of the T-die was set to 500 mm, the distance between the lips of the T-die (lip clearance) was set to 2.4 mm, and the extrusion rate was 3.8 kg / h.

[0131] The resulting precursor (X') and precursor (Y') were then thermocompressed at 120°C and 4 m / min using a thermocompression laminator to form precursor (X') / precursor (Y') / precursor (X'), yielding a three-layer precursor (Z). The resulting precursor (Z) was then placed in a dryer and annealed at 120°C for 1 hour. The annealed precursor (Z) was then cold stretched 8% at room temperature, placed in a 116°C oven without shrinking the stretched film, and hot stretched to 185%, followed by relaxation at 124°C by 25% and then at 128°C by 20% to obtain a separator with a three-layer structure consisting of microporous layer (X) / microporous layer (Y) / microporous layer (X). The structure, physical properties, and battery performance evaluation results of the resulting separator are shown in Table 5.

[0132] Examples 18 to 21, Comparative Examples 19 and 20 A microporous layer was obtained in the same manner as in Example 17, except that the raw materials were changed as shown in Tables 5 and 6, and the resulting separator was evaluated.

[0133] Example 22 A microporous layer was obtained in the same manner as in Example 17, except that in the heat-relaxing step, the first-stage heat-relaxation was 15% at 124°C and the second-stage heat-relaxation was 10% at 128°C, and the microporous layer and a separator obtained using the microporous layer were evaluated.

[0134] Example 23 A microporous layer was obtained in the same manner as in Example 17, except that in the heat-relaxing step, the first-stage heat-relaxing was performed at 126°C by 20% and the second-stage heat-relaxing was not performed, and the microporous layer and a separator obtained using the microporous layer were evaluated.

[0135] Comparative Example 21 The polyolefin raw materials were changed as shown in Table 6, and film formation was performed by a coextrusion process rather than a lamination process. Specifically, the polypropylene shown in Table 6 was melted at 220°C in a 2.5-inch extruder, and the polyethylene described in Example 17 was melted at 200°C. The molten polypropylene and molten polyethylene were then fed into a coextrusion T-die (220°C) at a discharge rate ratio of 1:1:1 to form a three-layer structure of polypropylene / polyethylene / polypropylene. The molten polymer was extruded from the T-die at 11.4 kg / h. The extruded resin was then rapidly cooled to 20°C by blowing in 800 L / min of air using a chiller, while being wound around a roll at a roll speed of 25 m / min, yielding a precursor (Z) (precursor of the microporous layer (Z)) with a three-layer structure approximately 18 μm thick. The resulting precursor (Z) was placed in a dryer and annealed at 120°C for 1 hour. The annealed precursor (Z) was then cold-stretched by 8% at room temperature, and the stretched film was placed in a 125°C oven without shrinkage and hot-stretched to 150%, followed by relaxation by 25% to obtain a separator having a three-layer structure consisting of microporous layer (X) / microporous layer (Y) / microporous layer (X). The structure, physical properties, and battery performance evaluation results of the obtained separator are shown in Table 6.

[0136] [Table 1-1]

[0137] [Table 1-2]

[0138] [Table 1-3]

[0139] [Table 2-1]

[0140] [Table 2-2]

[0141] [Table 3]

[0142] [Table 4]

[0143] [Table 5]

[0144] [Table 6] [Industrial Applicability]

[0145] The separator for an electricity storage device of this embodiment has high strength and can be made thin, and can be suitably used as a separator for an electricity storage device, such as a lithium ion secondary battery.

Claims

1. a microporous layer (X) whose main component is polyolefin (A), the microporous layer (X) has a melt flow rate (MFR) of 0.90 g / 10 min or less under a load of 2.16 kg and at a temperature of 230°C; In MD-TD surface observation or ND-MD cross-sectional observation of the microporous layer (X) with a scanning electron microscope (SEM), the average long pore diameter of pores present in the microporous layer (X) is 100 nm or more, and the maximum long pore diameter of pores present in the microporous layer (X) is 100 nm or more and 400 nm or less, The polyolefin (A) comprises polypropylene, and The pentad fraction of the polypropylene is 98.5% or more. Separators for power storage devices.

2. The melt tension of the microporous layer (X) measured at a temperature of 230°C is 16 mN or more. The separator for an electricity storage device according to claim 1 .

3. the melt tension of the microporous layer (X) measured at a temperature of 230°C is 16 mN or more and 40 mN or less; The separator for an electricity storage device according to claim 1 .

4. the ratio (SMD / STD) of the tensile strength in the machine direction (SMD) to the tensile strength in the width direction (STD) of the separator for an electricity storage device is SMD / STD>5; The separator for an electricity storage device according to any one of claims 1 to 3.

5. the separator for an electricity storage device has a heat shrinkage rate of 1% or less in the TD and 4% or less in the MD after heat treatment at 105°C for 1 hour, The separator for a storage battery device according to any one of claims 1 to 4, wherein the separator for a storage battery device has a thermal shrinkage rate of 1% or less in the TD and 10% or less in the MD after heat treatment at 120°C for 1 hour.

6. The air permeability when the thickness of the separator for the electricity storage device is converted to 14 μm is 250 sec / 100 cm ３ Below is the The separator for an electricity storage device according to any one of claims 1 to 5.

7. 7. The separator for an electricity storage device according to claim 1, which has a thickness of 8 μm or more and 18 μm or less, and a puncture strength of 230 gf or more when converted to a thickness of 14 μm.

8. The ratio of the polypropylene to the polyolefin (A) is 50 to 100% by mass. The separator for an electricity storage device according to any one of claims 1 to 7.

9. The weight average molecular weight (Mw) of the microporous layer (X) is 500,000 or more and 1,500,000 or less. The separator for an electricity storage device according to any one of claims 1 to 8.

10. the value (Mw / Mn) obtained by dividing Mw by number average molecular weight (Mn) of the microporous layer (X) is 6 or less; The separator for an electricity storage device according to any one of claims 1 to 9.

11. The separator for an electricity storage device according to any one of claims 1 to 10, further comprising a microporous layer (Y) containing polyolefin (B) as a main component.

12. The separator for an electricity storage device according to claim 11, wherein the polyolefin (B) is polyethylene.

13. An electricity storage device comprising the electricity storage device separator according to any one of claims 1 to 12.

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