Separator for power storage device and power storage device

Abstract

The present disclosure provides a separator for a power storage device, which is excellent in terms of at least one of suppression of dendrite short-circuiting, low resistance and high piercing strength. This separator for a power storage device comprises a separator substrate having: one or more microporous layers (A) comprising mainly polypropylene; and one or more microporous layers (B) which comprise mainly polypropylene and which are layered on at least one of the microporous layers (A). The melt tension MtA of the microporous layer (A) at 240ºC is 10-40 mN. The ratio of the melt tension MtA of the microporous layer (A) at 240ºC and the melt tension MtB of the microporous layer (B) at 240ºC (MtA / MtB) is 1.05-4.0. The area-average long pore diameter of the microporous layer (A) is 50-500 nm.

Description

[Technical Field]

[0001] The present disclosure 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 secondary batteries and lithium ion secondary batteries. Lithium ion batteries are used in a variety of applications, including small electronic devices such as mobile phones and notebook personal computers, as well as electric vehicles, including hybrid vehicles and plug-in hybrid vehicles.

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

[0004] For example, US Patent No. 5,999, 167 describes a multilayer microporous thin film or membrane that can improve properties including dielectric breakdown and strength. A preferred multilayer microporous membrane comprises a microlayer and one or more stacked barriers.

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

[0006] [Patent Document 1] International Publication No. 2018 / 089748 [Patent Document 2] International Publication No. 2020 / 196120 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present disclosure is to provide a separator for an electricity storage device that excels in at least one of suppressing dendritic short circuits, low resistance, and high puncture strength. [Means for solving the problem]

[0008] Examples of embodiments of the present disclosure are listed in the following items [1] to

[15] . [1] A separator for an electricity storage device, comprising a separator substrate having one or more microporous layers (A) mainly composed of polypropylene, and one or more microporous layers (B) mainly composed of polypropylene laminated on at least one of the microporous layers (A), The melt tension Mt of the microporous layer (A) at 240°C A is 10mN or more and 40mN or less, The melt tension Mt of the microporous layer (A) at 240°C A and the melt tension Mt of the microporous layer (B) at 240°C. B Compared to Mt A / Mt B is between 1.05 and 4.0, The separator for an electricity storage device, wherein the microporous layer (A) has an area average pore diameter of 50 nm or more and 500 nm or less. [2] The melt tension Mt of the microporous layer (A) at 240°C A and the melt tension Mt of the microporous layer (B) at 240°C. B Compared to Mt A / Mt B 2. The separator for an electricity storage device according to item 1, wherein the σ is 1.1 or more and 2.2 or less. [3] The MD tensile strength of the separator for the electricity storage device is 2000 kgf / cm 2 More than 2800kgf / cm 23. The separator for an electricity storage device according to item 1 or 2, wherein: [4] 4. The separator for an electricity storage device according to any one of items 1 to 3, wherein the separator substrate has a thermal shrinkage rate in the width direction after heat treatment at 150° C. for 1 hour of -1.0% or more and 3.0% or less. [5] 5. The separator for an electricity storage device according to any one of items 1 to 4, wherein at least one of the microporous layers (A) and at least one of the microporous layers (B) are adjacent to each other. [6] 6. The separator for an electricity storage device according to any one of items 1 to 5, wherein the separator substrate has the microporous layer (A) on each of the outermost layers on both sides. [7] 7. The separator for an electricity storage device according to any one of items 1 to 6, wherein the microporous layer (A) has a thickness of 5 μm or less, and the microporous layer (B) has a thickness of 5 μm or less. [8] 8. The separator for a power storage device according to any one of items 1 to 7, wherein the porosity of the separator for a power storage device is 40% or more and 60% or less. [9] 9. The separator for a power storage device according to any one of items 1 to 8, wherein the separator for a power storage device has an air permeability of 10 sec / 100 cc or more and less than 220 sec / 100 cc.

[10] 10. The separator for an electricity storage device according to any one of items 1 to 9, wherein the microporous layer (B) has an area average pore diameter of 50 nm or more and 240 nm or less.

[11] 11. The separator for a power storage device according to any one of items 1 to 10, wherein the separator for a power storage device has an MD / TD tensile strength ratio of 16 or more and 30 or less.

[12] 12. An electricity storage device comprising: a positive electrode; a negative electrode; and the electricity storage device separator according to any one of items 1 to 11, disposed between the positive electrode and the negative electrode.

[13] Item 13. The power storage device according to item 12, wherein the positive electrode contains lithium iron phosphate as a positive electrode active material.

[14] A method for producing a separator for an electricity storage device, comprising a separator substrate having one or more microporous layers (A) mainly composed of polypropylene and one or more microporous layers (B) mainly composed of polypropylene, the method comprising: (i) a melt-extrusion step of co-extruding the microporous layer (A) and the microporous layer (B), or (ii) separately extruding the microporous layer (A) and the microporous layer (B) and then laminating them together to obtain a precursor sheet of the separator substrate; annealing the obtained precursor sheet at a temperature ranging from 135°C to 160°C; cold stretching the precursor sheet in MD at a stretching ratio of 5% to 20%; hot stretching and heat relaxing the precursor sheet; A method for producing a separator for an electricity storage device, comprising:

[15] Item 15. The method according to item 14, wherein the hot stretching and heat relaxing are performed by stretching the precursor sheet to 140% or more and 220% or less, with the MD dimension of the precursor sheet before stretching being 100%, and then relaxing the precursor sheet in the MD by 10% or more and 50% or less. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to provide a separator for an electricity storage device that excels in at least one of suppression of dendritic short circuits, low resistance, and high puncture strength. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram defining ΔR and Δδ for evaluating the membrane resistance value of a microporous membrane. DETAILED DESCRIPTION OF THE INVENTION

[0011] <Separator for power storage device> The separator for an electric storage device of the present disclosure has a separator substrate having one or more microporous layers (A) primarily made of polypropylene and one or more microporous layers (B) primarily made of polypropylene laminated to at least one of the microporous layers (A). The separator substrate may further have a coating layer (also called a "surface layer," "coating layer," etc.; hereinafter simply referred to as a "coating layer") on the microporous layer (A) and / or the microporous layer (B). In the present disclosure, the term "microporous layer" refers to each microporous layer constituting the separator substrate, the term "separator substrate" refers to the separator substrate excluding any coating layers, and the term "separator" refers to the entire separator including any coating layers.

[0012] <Microporous layer (A)> The separator for an electricity storage device of the present disclosure has a microporous layer (A). The separator for an electricity storage device may have only one microporous layer (A), or two or more microporous layers (A). The microporous layer (A) is primarily composed of polypropylene, which allows the battery to maintain good performance even after storage at high temperatures (e.g., 130°C). In the present disclosure, "primarily composed of polypropylene" means that polypropylene accounts for the largest mass % based on the total mass of the microporous layer (A). The lower limit of the polypropylene content in the microporous layer (A) is preferably 50 mass % or more, more preferably 60 mass % or more, and even more preferably 75 mass % or more, 80 mass % or more, 90 mass % or more, 95 mass % or more, 97 mass % or more, or 99 mass % or more, from the viewpoints of the wettability, thinning, shutdown properties, etc. of the separator. The upper limit of the polypropylene content in the microporous layer (A) is not limited, but 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 even 100% by mass. When the microporous layer (A) is used as the outermost layer, the polypropylene content is preferably 97% or more. This prevents contamination of the manufacturing equipment due to bleed-out of impurities and production problems due to poor peeling between separators, allowing the separator substrate to be annealed at a higher temperature. For example, the separator substrate can be annealed at 135°C or higher, which tends to result in a separator substrate with high permeability and high porosity, and a separator with low resistance is easily obtained.

[0013] <Material for the microporous layer (A)> The microporous layer (A) is primarily composed of polypropylene. The polypropylene of the microporous layer (A) may be the same material as the polypropylene of the microporous layer (B) described below, or may be a polypropylene with a different chemical structure, more specifically, a polypropylene differing in at least one of monomer composition, stereoregularity, molecular weight, and crystalline structure. The stereoregularity of the polypropylene is not limited, but examples include atactic, isotactic, and syndiotactic homopolymers. The polypropylene according to the present disclosure is preferably a highly crystalline isotactic or syndiotactic homopolymer.

[0014] The polypropylene of the microporous layer (A) is preferably a homopolymer, but may also be a copolymer, such as a block polymer, in which a small amount of a comonomer other than propylene, such as an α-olefin comonomer, is copolymerized. The amount of propylene structures contained as repeating units in the polypropylene is not limited, but may be, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The amount of repeating units derived from comonomers other than the propylene structure contained in the polypropylene is not limited, but may be, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. The polypropylene may be used alone or in combination of two or more types.

[0015] The weight-average molecular weight (Mw) of the polypropylene of the microporous layer (A) is preferably 300,000 or more from the viewpoints of the strength of the microporous layer and suppressing dendrites, and is preferably 1,300,000 or less from the viewpoints of ensuring good film-forming properties and productivity. The Mw of the polypropylene is more preferably 500,000 or more and 1,200,000 or less, even more preferably 650,000 or more and 1,100,000 or less, still more preferably 750,000 or more and 1,000,000 or less, and particularly preferably 800,000 or more and 1,000,000 or less.

[0016] The upper limit of the value (Mw / Mn) obtained by dividing the weight-average molecular weight (Mw) of the polypropylene in the microporous layer (A) by the number-average molecular weight (Mn) is preferably 20 or less, more preferably 18 or less, 16 or less, 14 or less, or 12 or less. Setting Mw / Mn to 20 or less tends to ensure good film-formability and productivity. Furthermore, Mw / Mn is preferably 3 or more, more preferably 4 or more, 4.5 or more, or 5.0 or more. As the Mw / Mn value of polypropylene increases, the melt tension of the resulting microporous layer tends to increase, and increasing the melt tension of the microporous layer (A) is also preferable for dendrite suppression. Therefore, a polypropylene Mw / Mn value of 3 or more is preferable for controlling the melt tension of the microporous layer (A) at a high level. The weight-average molecular weight, number-average molecular weight, and Mw / Mn of the polyolefins of the present disclosure are polystyrene-equivalent molecular weights obtained by GPC (gel permeation chromatography) measurement.

[0017] The density of the polypropylene of the microporous layer (A) is preferably 0.85 g / cm 3 For example, 0.88 g / cm 3 More than 0.89g / cm 3 or more, or 0.90 g / cm 3 The density of polypropylene is preferably 1.1 g / cm or more. 3 For example, 1.0 g / cm 3 Below, 0.98g / cm 3 Below, 0.97g / cm 3 Below, 0.96g / cm 3 Below, 0.95g / cm 3 Below, 0.94g / cm 3 Below, 0.93g / cm 3 or less than 0.92 g / cm 3 The density of polyolefins is related to the crystallinity of polypropylene, and the density of polypropylene can be 0.85 g / cm or less. 3 By setting the above, the productivity of the microporous layer is improved, which is particularly advantageous in the dry method.

[0018] The microporous layer (A) may contain other resins as long as it is mainly composed of polypropylene. Examples of other resins include polyolefins other than polypropylene (also referred to as "other polyolefins") and copolymers of polystyrene and polyolefin. Polyolefins are polymers containing a monomer having a carbon-carbon double bond as a repeating unit. Monomers constituting polyolefins other than polypropylene include, but are not limited to, monomers having 2 or 4 to 10 carbon atoms and a carbon-carbon double bond, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Polyolefins are, for example, homopolymers, copolymers, or multi-stage polymers, and polyethylene may also be contained. Preferred examples of copolymers of polystyrene and polyolefins include styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer, and styrene-ethylene-styrene copolymer. Styrene-(ethylene-propylene)-styrene copolymer (SEPS) is particularly preferred.

[0019] <Melt flow rate (MFR) of microporous layer (A)> The upper limit of the melt flow rate (MFR) of the microporous layer (A) (MFR of a single layer) is preferably 4.0 g / 10 min or less, from the viewpoint of obtaining a microporous layer (A) with higher strength and suppressing dendrite short-circuiting, and may be, for example, 3.0 g / 10 min or less, 2.0 g / 10 min or less, 1.5 g / 10 min or less, 1.1 g / 10 min or less, or 0.5 g / 10 min or less. The lower limit of the MFR of the microporous layer (A) (MFR of a single layer) is not limited from the viewpoint of formability of the microporous layer (A), and may be, for example, 0.2 g / 10 min or more, 0.25 g / 10 min or more, 0.3 g / 10 min or more, or 0.35 g / 10 min or more. The MFR of the microporous layer (A) is measured under conditions of a load of 2.16 kg and a temperature of 230°C. A microporous layer (A) having an MFR of 4.0 g / 10 min or less means that the molecular weight of the polyolefin contained in the microporous layer (A) is relatively high. A high molecular weight polyolefin increases the number of tie molecules that bond crystalline phases together, which tends to result in a microporous layer (A) with high strength and also makes it possible to impart a sufficiently high melt tension, making it easier to achieve a pore structure that suppresses dendritic short circuits. A microporous layer (A) having an MFR of 0.2 g / 10 min or more prevents the melt tension of the microporous layer (A) from becoming too high, ensuring good film-forming properties and productivity. To obtain a microporous layer (A) with high strength and high melt tension, the MFR of the polypropylene of the microporous layer (A) is preferably 0.2 to 4.0 g / 10 min when measured under conditions of a load of 2.16 kg and a temperature of 230°C. The upper limit of the MFR of the polypropylene may be, for example, 4.0 g / 10 min or less, 3.0 g / 10 min or less, 2.0 g / 10 min or less, 1.5 g / 10 min or less, 1.1 g / 10 min or less, or 0.5 g / 10 min or less, from the viewpoint of obtaining a microporous layer with higher strength. The lower limit of the MFR of the polypropylene is not limited, but may be, for example, 0.2 g / 10 min or more, 0.25 g / 10 min or more, 0.3 g / 10 min or more, or 0.35 g / 10 min or more, from the viewpoint of the formability of the microporous layer (A).

[0020] <Pentad fraction of microporous layer (A)> From the viewpoint of obtaining a microporous layer with low air permeability, the lower limit of the pentad fraction of the polypropylene in the microporous layer (A) is preferably 94.0% or more, for example, 95.0% or more, 96.0% or more, 96.5% or more, 97.0% or more, 97.5% or more, 98.0% or more, 98.5% or more, or 99.0% or more. The upper limit of the pentad fraction of the polypropylene is not limited, but may be 99.9% or less, 99.8% or less, or 99.5% or less. The pentad fraction of the polypropylene is 13 Measured by C-NMR (nuclear magnetic resonance).

[0021] A polypropylene with a pentad fraction of 94.0% or higher indicates high crystallinity. Separators obtained by the stretch-perforation method, particularly the dry method, have pores formed by stretching the amorphous portions between crystalline portions. Therefore, if the polypropylene has high crystallinity, the pores become good and the air permeability can be kept low, enabling the battery to have high output.

[0022] <Melt tension of microporous layer (A)> Melt tension Mt of microporous layer (A) at 240°C A is 10 mN or more and 40 mN or less. A The lower limit of the melt tension Mt is preferably 13 mN or more, more preferably 16 mN or more, even more preferably 20 mN or more, and particularly preferably 23 mN or more, from the viewpoint of achieving a sufficiently fine pore structure, small pore diameter, and exhibiting good dendrite suppression performance. A From the viewpoint of good film-forming properties and productivity, the upper limit is preferably 40 mN or less, more preferably 37 mN or less, even more preferably 34 mN or less, and most preferably 32 mN or less.

[0023] <Area-average pore diameter of the microporous layer (A)> The area-average pore diameter (hereinafter simply referred to as "area-average pore diameter") of the microporous layer (A) in the ND-MD cross section is 50 nm to 500 nm, and preferably the area-average pore diameter of the microporous layer (A) is smaller than the area-average pore diameter of the microporous layer (B), i.e., 50 nm to 500 nm. In the present disclosure, "ND" refers to the thickness direction of the microporous layer, and "MD" refers to the deposition direction of the microporous layer. For example, the MD of a separator having a microporous layer is the longitudinal direction if it is a roll. "Long pore diameter" refers to the pore diameter in the MD. When there are two or more microporous layers (A) and / or microporous layers (B), the area-average pore diameters of the microporous layer (A) and the microporous layer (B) are compared based on the average area-average pore diameter of each layer. The lower limit of the area-average pore diameter of the microporous layer (A) is preferably 50 nm or more, more preferably 80 nm or more, even more preferably 100 nm or more, even more preferably 130 nm or more, and particularly preferably 150 nm or more, from the viewpoint of ensuring good output in the power storage device, i.e., low membrane resistance. It is believed that a larger pore diameter shortens the Li-ion conductive path, making it easier to achieve low membrane resistance. Furthermore, the upper limit of the area-average pore diameter of the microporous layer (A) is preferably 500 nm or less, preferably 400 nm or less, more preferably 300 nm or less, even more preferably 250 nm or less, even more preferably 220 nm or less, and particularly preferably 350 nm or less, from the viewpoint of good pinning or suppressing dendritic short-circuiting. A smaller pore diameter increases the amount of molecules (tie molecules) connecting lamellar crystals, which tends to result in a separator with high pinning.

[0024] The area-average pore diameter can be measured by performing cross-sectional SEM observation of the MD-ND cross section of the separator and analyzing the obtained image in a range of MD 20 μm × ND 3 μm. Detailed conditions are shown in the Examples. When measuring the average pore diameter from a cross-sectional SEM image, the number-average pore diameter and the area-average pore diameter can be calculated, but in this disclosure, the area-average pore diameter is used as the average pore diameter to better correlate with the physical properties of the separator.

[0025] <Porosity of the microporous layer (A)> The porosity of the microporous layer (A) is preferably 40% or more from the viewpoints of avoiding clogging in the electricity storage device and obtaining low separator membrane resistance, and is preferably 70% or less from the viewpoints of maintaining separator strength and high puncture resistance. From the viewpoint of low membrane resistance, the porosity of the microporous layer (A) is more preferably 45% to 65%, even more preferably 47% to 60%, and particularly preferably 50% to 57%.

[0026] <Thickness of the microporous layer (A)> The thickness of the microporous layer (A) may be preferably 10 μm or less, for example, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm or less, or 4 μm or less, from the viewpoint of increasing the energy density of the electricity storage device, etc. The lower limit of the thickness of the microporous layer (A) may be preferably 1 μm or more, for example, 2 μm or more, 3 μm or more, or 3.5 μm or more, from the viewpoint of strength, etc.

[0027] <Additives for the microporous layer (A)> The microporous layer (A) containing polypropylene as a main component may further contain, in addition to polypropylene, additives such as elastomers, crystal nucleating agents, antioxidants, fillers, etc. The amount of the additives is not particularly limited, and may be, for example, 0.01% by mass or more, 0.1% by mass or more, or 1% by mass or more, and 20% by mass or less, 10% by mass or less, or 7% by mass or less, based on the total mass of the microporous layer (A).

[0028] <Microporous layer (B)> The separator for an electricity storage device of the present disclosure has a microporous layer (B). The separator for an electricity storage device may have only one microporous layer (B) or two or more microporous layers (B). At least one of the microporous layers (B) preferably constitutes the outermost layer on at least one side of the separator substrate. When the separator for an electricity storage device has two or more microporous layers (A), the microporous layer (B) may constitute the outermost layer on both sides of the separator substrate. The use of polypropylene with low melt tension makes it easier to obtain a large pore size, which is expected to improve cycle performance. The microporous layer (B) also contains polypropylene as a main component, which allows the battery to maintain good battery performance even after storage at high temperatures (e.g., 130°C). In the present disclosure, "containing polypropylene as a main component" means that polypropylene accounts for the largest mass % based on the total mass of the microporous layer (B). The lower limit of the polypropylene content in the microporous layer (B) is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 75% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, 97% by mass or more, or 99% by mass or more, from the viewpoints of separator wettability, thinning, and shutdown characteristics. The upper limit of the polypropylene content in the microporous layer (A) is not limited, but 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 even 100% by mass. When the microporous layer (B) is used as the outermost layer, the polypropylene content is preferably 97% or more. This prevents contamination of production equipment due to bleed-out of impurities and production problems due to poor peeling between separators, allowing the separator substrate to be annealed at a higher temperature. For example, the separator substrate can be annealed at 135° C. or higher, which tends to result in a separator substrate with high permeability and high porosity, and is likely to result in a separator with low resistance.

[0029] <Material for the microporous layer (B)> The microporous layer (B) is primarily composed of polypropylene. The polypropylene of the microporous layer (B) may be the same material as the polypropylene of the microporous layer (A), or may be a polypropylene with a different chemical structure, more specifically, a polypropylene differing in at least one of the following: monomer composition, stereoregularity, molecular weight, and crystalline structure. The stereoregularity of the polypropylene of the microporous layer (B) is not limited, and examples thereof include atactic, isotactic, or syndiotactic homopolymers. The polypropylene according to the present disclosure is preferably a highly crystalline isotactic or syndiotactic homopolymer.

[0030] The polypropylene of the microporous layer (B) is preferably a homopolymer, but may also be a copolymer, such as a block polymer, in which a small amount of a comonomer other than propylene, such as an α-olefin comonomer, is copolymerized. The amount of propylene structures contained as repeating units in the polypropylene is not limited, but may be, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The amount of repeating units derived from comonomers other than the propylene structure contained in the polypropylene is not limited, but may be, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. The polypropylene can be used alone or in combination of two or more types.

[0031] The weight-average molecular weight (Mw) of the polypropylene of the microporous layer (B) is preferably 250,000 or more from the viewpoint of the strength of the microporous layer, and is preferably 1,000,000 or less from the viewpoint of increasing the pore size of the microporous layer and exhibiting good dendrite suppression performance. The Mw of the polypropylene is more preferably 400,000 or more and 950,000 or less, even more preferably 550,000 or more and 900,000 or less, still more preferably 600,000 or more and 900,000 or less, and particularly preferably 700,000 or more and 900,000 or less.

[0032] The upper limit of the value (Mw / Mn) obtained by dividing the weight-average molecular weight (Mw) by the number-average molecular weight (Mn) of the polypropylene in the microporous layer (B) is preferably 7 or less, more preferably 6.5 or less, 6 or less, 5.5 or less, or 5 or less. The smaller the Mw / Mn value of the polypropylene, the lower the melt tension of the resulting microporous layer tends to be. Therefore, it is preferable that the Mw / Mn value of the polypropylene is 7 or less in order to control the melt tension of the microporous layer (B) to a low level. Furthermore, Mw / Mn is preferably 1 or more, for example, 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. An Mw / Mn of 1 or more may maintain appropriate molecular entanglement and improve stability during film formation. The weight-average molecular weight, number-average molecular weight, and Mw / Mn of the polyolefins of the present disclosure are polystyrene-equivalent molecular weights obtained by GPC (gel permeation chromatography) measurement.

[0033] The density of the polypropylene of the microporous layer (B) is preferably 0.85 g / cm 3 For example, 0.88 g / cm 3 More than 0.89g / cm 3 or more, or 0.90 g / cm 3 The density of polypropylene is preferably 1.1 g / cm or more. 3 For example, 1.0 g / cm 3 Below, 0.98g / cm 3 Below, 0.97g / cm 3 Below, 0.96g / cm 3 Below, 0.95g / cm 3 Below, 0.94g / cm 3 Below, 0.93g / cm 3 or less than 0.92 g / cm 3 The density of polyolefins is related to the crystallinity of polypropylene, and the density of polypropylene can be 0.85 g / cm or less. 3 By setting the above, the productivity of the microporous layer is improved, which is particularly advantageous in the dry method.

[0034] The microporous layer (B) may contain other resins as long as it is mainly composed of polypropylene. Examples of other resins include polyolefins other than polypropylene (also referred to as "other polyolefins"). Polyolefins are polymers containing a monomer having a carbon-carbon double bond as a repeating unit. Monomers constituting polyolefins other than polypropylene are not limited, but include monomers having 2 or 4 to 10 carbon atoms and a carbon-carbon double bond, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene.

[0035] The microporous layer (B) may contain a thermoplastic elastomer other than polypropylene. Examples of thermoplastic elastomers include, but are not limited to, polypropylene, polyolefins other than polypropylene (also referred to as "other polyolefins"), and copolymers of polystyrene and polyolefin. Examples of polypropylene include low-crystalline polypropylenes having low stereoregularity regions. Polyolefins are polymers containing a monomer having a carbon-carbon double bond as a repeating unit. Examples of monomers constituting polyolefins other than polypropylene include, but are not limited to, monomers having 2 or 4 to 10 carbon atoms and a carbon-carbon double bond, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Examples of polyolefins include homopolymers, copolymers, and multi-stage polymers. For example, polyethylene may be contained. Preferred examples of copolymers of polystyrene and polyolefin include styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer (SEBS), hydrogenated styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), styrene-ethylene-styrene copolymer, olefin crystalline-(ethylene-butene)-olefin crystalline copolymer (CEBC), hydrogenated olefin crystalline-(ethylene-butene)-olefin crystalline copolymer (hydrogenated CEBC), styrene-(ethylene-butene)-olefin crystalline copolymer (SEBC), hydrogenated styrene-(ethylene-butene)-olefin crystalline copolymer (hydrogenated SEBC), etc. Particularly preferred are hydrogenated styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), hydrogenated styrene-(ethylene-butene)-olefin crystalline copolymer (hydrogenated SEBC), and styrene-(ethylene-propylene)-styrene copolymer (SEPS).

[0036] The thermoplastic elastomer contained in the microporous layer (B) is preferably an elastomer incompatible with polypropylene from the viewpoints of pore opening and increasing the pore size. The elastomer incompatible with polypropylene is not particularly limited, but preferred examples include copolymers of polyethylene and other polyolefins and copolymers of polystyrene and polyolefins. Preferred examples of the copolymers of polystyrene and polyolefins include styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer (SEBS), hydrogenated styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), styrene-ethylene-styrene copolymer, olefin crystalline-(ethylene-butene)-olefin crystalline copolymer (CEBC), hydrogenated olefin crystalline-(ethylene-butene)-olefin crystalline copolymer (hydrogenated CEBC), styrene-(ethylene-butene)-olefin crystalline copolymer (SEBC), and hydrogenated styrene-(ethylene-butene)-olefin crystalline copolymer (hydrogenated SEBC). Particularly preferred are hydrogenated styrene-(ethylene-butene)-styrene copolymer (hydrogenated SEBS), hydrogenated styrene-(ethylene-butene)-olefin crystalline copolymer (hydrogenated SEBC), and styrene-(ethylene-propylene)-styrene copolymer (SEPS).

[0037] <Melt flow rate (MFR) of microporous layer (B)> The upper limit of the melt flow rate (MFR) of the microporous layer (B) (MFR of a single layer) is preferably 8.0 g / 10 min or less, from the viewpoint of obtaining a microporous layer (B) with higher strength, and may be, for example, 6.0 g / 10 min or less, 4.0 g / 10 min or less, 3.0 g / 10 min or less, 2.0 g / 10 min or less, or 1.1 g / 10 min or less. The lower limit of the MFR of the microporous layer (B) (MFR of a single layer) is not limited, from the viewpoint of exhibiting a good dendrite suppression effect, and may be, for example, 0.3 g / 10 min or more, 0.35 g / 10 min or more, 0.4 g / 10 min or more, 0.45 g / 10 min or more, or 0.5 g / 10 min or more. The MFR of the microporous layer (B) is measured under conditions of a load of 2.16 kg and a temperature of 230°C. A microporous layer (B) having an MFR of 8.0 g / 10 min or less means that the molecular weight of the polyolefin contained in the microporous layer (B) is relatively high. A high molecular weight polyolefin results in more tie molecules bonding crystalline substances together, which tends to result in a microporous layer (B) with high strength. A microporous layer (B) having an MFR of 0.3 g / 10 min or more prevents the melt tension of the microporous layer (B) from becoming too high, making it easier to obtain a separator that exhibits good dendrite suppression effects.

[0038] The MFR of the polypropylene of the microporous layer (B) is preferably 8.0 g / 10 min or less, and may be, for example, 6.0 g / 10 min or less, 4.0 g / 10 min or less, 3.0 g / 10 min or less, 2.0 g / 10 min or less, or 1.1 g / 10 min or less, from the viewpoint of obtaining a microporous layer (B) with higher strength. The lower limit of the MFR of the microporous layer (B) (single-layer MFR) is not limited, from the viewpoint of exhibiting a good dendrite-inhibiting effect, and may be, for example, 0.3 g / 10 min or more, 0.35 g / 10 min or more, 0.4 g / 10 min or more, 0.45 g / 10 min or more, or 0.5 g / 10 min or more.

[0039] The MFR of the microporous layer (B) is preferably higher than the MFR of the microporous layer (A). By making the MFR of the microporous layer (B) higher than the MFR of the microporous layer (A), the pore size of the microporous layer (B) in the obtained separator can be controlled to be larger than the pore size of the microporous layer (A).

[0040] <Pentad fraction of microporous layer (B)> From the viewpoint of obtaining a microporous layer with low air permeability, the lower limit of the pentad fraction of the polypropylene in the microporous layer (B) is preferably 94.0% or more, for example, 95.0% or more, 96.0% or more, 96.5% or more, 97.0% or more, 97.5% or more, 98.0% or more, 98.5% or more, or 99.0% or more. The upper limit of the pentad fraction of the polypropylene is not limited, but may be 99.9% or less, 99.8% or less, or 99.5% or less. The pentad fraction of the polypropylene is 13 Measured by C-NMR (nuclear magnetic resonance).

[0041] A polypropylene with a pentad fraction of 94.0% or higher indicates high crystallinity. Separators obtained by the stretch-perforation method, particularly the dry method, have pores formed by stretching the amorphous portions between crystalline portions. Therefore, if the polypropylene has high crystallinity, the pores become good and the air permeability can be kept low, enabling the battery to have high output.

[0042] <Melt tension of microporous layer (B)> Melt tension Mt of microporous layer (B) at 240°C B The melt tension Mt is preferably 4 mN or more and 30 mN or less. B From the viewpoint of good film-forming properties and productivity of the separator substrate having microporous layer A and microporous layer B, the lower limit of melt tension Mt is preferably 4 mN or more, more preferably 7 mN or more, even more preferably 10 mN or more, particularly preferably 13 mN or more, and most preferably 16 mN or more. B The upper limit of the pressure is preferably 30 mN or less, more preferably 28 mN or less, even more preferably 26 mN or less, and most preferably 24 mN or less, from the viewpoint of achieving a large pore diameter and exhibiting good dendrite suppression performance.

[0043] <Area-average pore diameter of the microporous layer (B)> The area-average pore diameter (hereinafter also simply referred to as "area-average pore diameter") of the microporous layer (B) in the ND-MD cross section is preferably larger than the area-average pore diameter of the microporous layer (A). For details about the relationship with the area-average pore diameter of the microporous layer (A), see the section "Area-average pore diameter of the microporous layer (A)."

[0044] The area-average pore diameter of the microporous layer (B) in the ND-MD cross section is preferably 50 nm or more and 600 nm or less. The lower limit of the area-average pore diameter of the microporous layer (B) is more preferably 120 nm or more, even more preferably 140 nm or more, and even more preferably 160 nm or more. The upper limit of the area-average pore diameter of the microporous layer (B), which can be arbitrarily combined with these lower limits, is more preferably 500 nm or less, even more preferably 400 nm or less, and even more preferably 350 nm or less, 300 nm or less, 250 nm or less, or 240 nm or less. When the area-average pore diameter of the microporous layer (B) is within this range, dendritic short-circuiting can be more effectively suppressed.

[0045] <Porosity of microporous layer (B)> The porosity of the microporous layer (B) is preferably 40% or more from the viewpoint of avoiding clogging in the electricity storage device and obtaining a good low membrane resistance of the separator, and is preferably 70% or less from the viewpoint of maintaining the strength of the separator. The porosity of the microporous layer (A) is more preferably 45% to 65%, even more preferably 47% to 60%, and particularly preferably 50% to 57%.

[0046] <Thickness of the microporous layer (B)> The thickness of the microporous layer (B) according to the present disclosure may be preferably 10 μm or less, for example, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm or less, or 4 μm or less, from the viewpoint of increasing the energy density of the electricity storage device, etc. The lower limit of the thickness of the microporous layer (B) may be preferably 1 μm or more, for example, 2 μm or more, 3 μm or more, or 3.5 μm or more, from the viewpoint of strength, etc.

[0047] <Additives for the microporous layer (B)> The microporous layer (B) containing polypropylene as a main component may further contain, in addition to polypropylene, additives such as elastomers, crystal nucleating agents, antioxidants, fillers, etc. The amount of the additives is not particularly limited, and may be, for example, 0.01% by mass or more, 0.1% by mass or more, or 1% by mass or more, and 10% by mass or less, 7% by mass or less, or 5% by mass or less, based on the total mass of the microporous layer (B).

[0048] <Relationship between microporous layer (A) and microporous layer (B)> Melt tension Mt of microporous layer (A) at 240°C A and the melt tension Mt of the microporous layer (B) at 240°C. B Compared to Mt A / Mt B is 1.05 or more and 4.0 or less. A / Mt B By making Mt 1.05 or more, the pore size of the microporous layer (A) of the obtained separator can be controlled to be sufficiently small and the pore size of the microporous layer (B) can be controlled to be sufficiently large. Furthermore, dendrite growth is suppressed at the lamination interface of the discontinuous pore structure, thereby effectively suppressing dendrite short-circuiting in the electricity storage device. A / Mt B By setting the Mt to 4.0 or less, the crystal orientation during film formation is promoted, resulting in a highly oriented crystal state, making it possible to obtain a separator with good pore opening and air permeability. High orientation also leads to high MD tensile strength. Good pore opening also has the effect of suppressing dendrite formation in battery devices and suppressing dendrite short circuits. A / Mt B The lower limit value is more preferably 1.1 or more, 1.15 or more, 1.2 or more, or 1.25 or more, and the upper limit value that can be arbitrarily combined with these lower limit values ​​is preferably 3.5 or less, 3.3 or less, 3.0 or less, 2.5 or less, 2.4 or less, 2.3 or less, or 2.2 or less.

[0049] MFR of microporous layer (B) B ) and MFR of the microporous layer (A) (MFR A ) ratio, MFR B / MFR A The MFR is preferably 1.02 or more and 10.0 or less. B / MFR A By setting the MFR to 1.02 or more, the pore size of the microporous layer (A) of the obtained separator can be controlled to be sufficiently small and the pore size of the microporous layer (B) can be controlled to be sufficiently large, thereby effectively suppressing dendritic short-circuiting in the electricity storage device.B / MFR A By making the MFR equal to or less than 10.0, it is possible to obtain a separator having stable film-forming properties, productivity, good pore opening properties, and good air permeability. B / MFR A is preferably 1.05 or more and 6.0 or less, more preferably 1.1 or more and 5.0 or less, particularly preferably 1.1 or more and 4.0 or less, and most preferably 1.1 or more and 3.0 or less.

[0050] Weight average molecular weight Mw of polypropylene of microporous layer (A) A and the weight average molecular weight Mw of the polypropylene of the microporous layer (B). B Ratio of Mw A / Mw B Mw is preferably 1.02 or more and 2.0 or less. A / Mw B By setting the melt tension ratio Mt between the microporous layer (A) and the microporous layer (B) to 1.02 or more, A / Mt B As a result, it is possible to effectively suppress dendrite formation in the power storage device. A / Mt B By making the Mw 2.0 or less, it is possible to obtain a separator having stable film-forming properties, productivity, good pore opening properties, and good air permeability. A / Mw B is preferably 1.02 or more and 1.8 or less, more preferably 1.03 or more and 1.6 or less, and most preferably 1.05 or more and 1.4 or less.

[0051] The smaller of the area-average pore diameters of the microporous layer (A) and the microporous layer (B) is 170 nm or more and 800 nm or less. The lower limit of the smaller value is preferably 170 nm or more, more preferably 200 nm or more, even more preferably 230 nm or more, still more preferably 260 nm or more, and particularly preferably 280 nm or more, from the viewpoint of ensuring good output in the power storage device, i.e., low membrane resistance. The upper limit of the smaller value is preferably 800 nm or less, preferably 600 nm or less, more preferably 500 nm or less, even more preferably 450 nm or less, still more preferably 400 nm or less, and particularly preferably 350 nm or less, from the viewpoint of good puncture strength or suppression of dendrite short-circuiting. The area-average pore diameter of the microporous layer (A) is preferably smaller than the area-average pore diameter of the microporous layer (B). The larger of the area-average pore diameter of the microporous layer (A) and the area-average pore diameter of the microporous layer (B) is preferably 200 nm or more and 600 nm or less, more preferably 210 nm or more and 500 nm or less, and even more preferably 220 nm or more and 400 nm or less, from the viewpoint of more effectively suppressing dendrite short-circuiting. B and the area average pore diameter L of the microporous layer (A). A The ratio of L B / L A is preferably 1.1 or more and 6.0 or less. B / L A By setting L to 1.1 or more, the pore size of the microporous layer (A) can be made sufficiently small and the pore size of the microporous layer (B) can be made sufficiently large, thereby effectively suppressing dendritic short-circuiting in the electricity storage device. B / L A By making L 6 or less, it is possible to obtain a separator with good pore opening and air permeability. B / L A is preferably 1.1 or more and 4.0 or less, more preferably 1.15 or more and 3.0 or less, particularly preferably 1.2 or more and 2.5 or less, and most preferably 1.3 or more and 2.0 or less.

[0052] <Layer structure of separator substrate> The substrate of the separator for an electricity storage device (also simply referred to as "separator substrate" in the present disclosure) has at least one microporous layer (A) and one microporous layer (B). The separator substrate may have a multilayer structure of three or more layers, including two or more microporous layers (A) and / or two or more microporous layers (B). Examples of such a multilayer structure include a two-layer structure of microporous layer (A) / microporous layer (B) and a three-layer structure of microporous layer (A) / microporous layer (B) / microporous layer (A). The separator substrate may also have layers other than the microporous layer (A) and the microporous layer (B). Examples of layers other than the microporous layer (A) and the microporous layer (B) include a microporous layer mainly composed of a polyolefin other than (A) and (B), a layer containing an inorganic substance, and a layer containing a heat-resistant resin. For example, the separator substrate may have a multilayer structure of four or more layers, such as microporous layer (A) / microporous layer (B) / microporous layer (C) / microporous layer (A). From the viewpoints of ease of production and suppression of curling of the separator, a symmetrical laminate structure is preferred.

[0053] In terms of the layer structure of the substrate of the separator for an electricity storage device, from the viewpoint of effectively suppressing dendritic short-circuiting, it is preferable that at least one microporous layer (A) and at least one microporous layer (B) are adjacent to each other. "Adjacent" means that the microporous layer (A) and the microporous layer (B) are directly laminated. When the separator substrate has a multilayer structure of three or more layers, it is preferable that each microporous layer (A) is adjacent to each microporous layer (B). For example, when the separator substrate has a three-layer structure of microporous layer (A) / microporous layer (B) / microporous layer (A), it is preferable that the two outermost microporous layers (A) are adjacent to the inner microporous layer (B). It is more preferable that the microporous layer (A) constitutes the outermost layer on at least one side of the separator substrate, and particularly preferable that it constitutes the outermost layer on both sides of the separator substrate. When the microporous layer (A) constitutes the outermost layer, dendritic short-circuiting tends to be easily suppressed. Alternatively, it is also preferable that the microporous layer (B) constitutes the outermost layer on at least one side of the separator substrate, or may constitute the outermost layer on both sides of the separator substrate. When the microporous layer (B) constitutes the outermost layer, the membrane resistance tends to be further reduced and clogging by dendrites tends to be prevented, thereby improving cycle performance.

[0054] <Separator substrate thickness> The upper limit of the thickness of the separator substrate is preferably 25 μm or less, for example, 22 μm or less, 20 μm or less, 18 μm or less, 16 μm or less, 14 μm or less, or 12 μm or less, from the viewpoint of increasing the energy density of the power storage device, etc. The lower limit of the thickness of the separator substrate is preferably 6 μm or more, for example, 7 μm or more, 8 μm or more, 9 μm or more, or 10 μm or more, from the viewpoint of strength, etc.

[0055] <Air permeability (air resistance) of separator substrate> The upper limit of the air permeability of the separator substrate is preferably 220 sec / 100 cm 3 less than 200 seconds / 100 cm 3 For example, 190 seconds / 100 cm 3 Below, 185 seconds / 100cm 3 Below, 180 seconds / 100cm 3 or less, or 175 seconds / 100cm 3 It is believed that a low air permeability tends to increase the connectivity of the pore structure, which results in less obstruction to the movement of Li ions, thereby achieving low membrane resistance. The lower limit of the air permeability of the separator substrate is preferably 10 sec / 100 cm. 3 or more, and is not limited to, for example, 50 seconds / 100 cm 3 Over 60 seconds / 100cm 3 or more, or 70 seconds / 100cm 3 If the air permeability is high, the basis weight increases and a dense membrane structure is formed, which tends to result in a separator with high puncture strength.

[0056] <Tensile strength of separator substrate> The separator for an electricity storage device preferably has a MD tensile strength of 2000 kgf / cm 2 More preferably, 2200 kgf / cm 2 More preferably, 2400 kgf / cm 2 MD tensile strength is 2000kgf / cm2 When the MD tensile strength is 2000 kgf / cm or more, the separator is strongly oriented, which increases the MD strength, making it easier to achieve high puncture strength and facilitating the winding operation of the battery during the battery production process. 2 When the MD tensile strength is 2000 kgf / cm or more, the thermal shrinkage rate of the TD can be reduced, and the resistance to short circuits caused by thermal shrinkage of the TD when the battery is wound can be increased. 2 If the MD tensile strength is 3200 kgf / cm or more, the crystals during film formation tend to be highly oriented in the MD, which promotes efficient pore opening by stretching, resulting in low film resistance. 2 or less, and more preferably 3000 kgf / cm 2 More preferably, 2800 kgf / cm or less 2 MD tensile strength is 3200kgf / cm or less. 2 When the MD tensile strength is 3200 kgf / cm or less, cracks are less likely to occur in the MD, the separator is more likely to have high puncture strength, and the separator is less likely to tear in the MD when handled in the battery production process. 2 When the thickness is equal to or less than this, collapse of the pores during stretching can be suppressed, and the membrane resistance is reduced.

[0057] The MD / TD tensile strength ratio is preferably 16 or more and 30 or less, more preferably 18 or more and 28 or less, and even more preferably 20 or more and 26 or less. When the MD / TD tensile strength ratio is 16 or more, the TD heat shrinkage rate can be reduced, thereby increasing resistance to short circuits due to TD heat shrinkage during battery winding. When the MD / TD tensile strength ratio is 30 or less, cracks are less likely to occur in the MD, and high puncture strength is likely to be achieved, making it less likely that the separator will tear in the longitudinal direction (MD) during handling in the battery production process.

[0058] <Porosity of separator substrate> The porosity of the separator substrate is preferably 40% or more from the viewpoint of avoiding clogging in the power storage device and obtaining a good low membrane resistance of the separator, and is preferably 70% or less from the viewpoint of maintaining the separator strength. The lower limit of the porosity of the separator substrate is more preferably 45% or more, even more preferably 47% or more, and particularly preferably 50% or more. The upper limit of the porosity of the separator substrate, which can be arbitrarily combined with these lower limit values, is more preferably 65% ​​or less, even more preferably 60% or less, and particularly preferably 57% or less.

[0059] <Puncture strength of separator substrate> The lower limit of the puncture strength of the separator substrate, when converted to a thickness of 14 μm, is preferably 240 gf or more, more preferably 260 gf or more, 280 gf or more, 290 gf or more, or 300 gf or more, and particularly preferably 320 gf or more. The upper limit of the puncture strength of the separator substrate is not limited, but may be preferably 600 gf or less, for example 580 gf or less, or 550 gf or less, when converted to a thickness of 14 μm.

[0060] <Thermal shrinkage rate of separator substrate> The separator substrate preferably has a thermal shrinkage in the transverse direction (TD) of -1.0% or more and 3.0% or less after heat treatment at 150°C for 1 hour. This means that the separator substrate has very little thermal shrinkage in the TD even at high temperatures. A thermal shrinkage of 3.0% or less can effectively prevent short circuits at high temperatures. The reason for the thermal shrinkage of -1.0% or more is that when measuring the thermal shrinkage, the substrate expands in the TD, which can result in a negative value of less than 0%. The thermal shrinkage may be 0% or more, or greater than 0%. A method for producing a separator substrate having a thermal shrinkage of -1.0% or more and 3.0% or less includes, for example, a separator production method using uniaxial MD stretching, preferably a dry uniaxial stretching method. In separator manufacturing methods that involve biaxial stretching in MD and TD, such as wet separators, the thermal shrinkage in TD is generally very large, whereas in dry separators that involve uniaxial stretching, it is easy to obtain a separator substrate with a thermal shrinkage rate of -1.0% or more and 3.0% or less.

[0061] <<Method for manufacturing separator for electricity storage device>> The method for producing a separator for an electricity storage device includes a melt-extrusion step in which a resin composition containing polypropylene as a main component (hereinafter also referred to as a "polypropylene-based resin composition") is melt-extruded to obtain a resin sheet (precursor sheet), and a pore-forming step in which the obtained precursor sheet is perforated to make it porous. The method for producing a 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.

[0062] Examples of dry methods include a method in which a polypropylene-based resin composition is melt-kneaded and extruded, and then the polypropylene crystal interface is peeled off by heat treatment and stretching; and a method in which a polypropylene-based resin composition and an inorganic filler are melt-kneaded and formed into a film, and then the interface between the polypropylene and the inorganic filler is peeled off by stretching.

[0063] Examples of the wet method include a method in which a polypropylene resin composition and a pore-forming material are melt-kneaded to form a film, which is stretched as necessary, and then the pore-forming material is extracted; and a method in which a polypropylene resin composition is dissolved, and then the film is immersed in a poor solvent for polypropylene to solidify the polypropylene and simultaneously remove the solvent.

[0064] A single-screw extruder or a twin-screw extruder can be used for melt-kneading the polypropylene-based resin composition. In addition to these, for example, a kneader, a Laboplastomill, a kneading roll, a Banbury mixer, etc. can also be used.

[0065] The polypropylene resin composition may optionally contain resins other than polypropylene, additives, etc., depending on the method for producing the microporous layer or the desired physical properties of the microporous layer. Examples of additives include pore-forming materials, fluorine-based flow modifiers, waxes, crystal nucleating materials, antioxidants, metal soaps such as metal salts of aliphatic carboxylic acids, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments. Examples of pore-forming materials include plasticizers, inorganic fillers, and combinations thereof.

[0066] 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.

[0067] 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.

[0068] A preferred method for producing a separator substrate is a dry lamellar crystal opening process in which the polypropylene crystal interface is peeled off by heat treatment and stretching. A preferred method for producing a separator substrate having a microporous layer (A) and a microporous layer (B) is a melt extrusion process using at least one of the following methods (i) and (ii): (i) co-extrusion of the microporous layer (A) and the microporous layer (B) to obtain a precursor sheet; or (ii) The microporous layer (A) and the microporous layer (B) are extrusion-formed separately and then bonded together to obtain a precursor sheet. The resulting precursor sheet is then subjected to annealing, cold stretching, hot stretching and heat relaxation steps.

[0069] Of the coextrusion process (i) and lamination process (ii), the coextrusion process (i) is preferred from the viewpoint of production costs, etc. In the coextrusion process (i), the extrusion film formation conditions for the microporous layers (A) and (B) are preferably such that the resin is discharged at as low a temperature as possible and effectively quenched by blowing low-temperature air. After film formation, quenching with air is preferred, and the temperature of the blown air is preferably 20°C or lower, more preferably 15°C or lower. By blowing cold air controlled at such a low temperature, the resin after film formation is uniformly oriented in the MD.

[0070] In both the coextrusion process (i) and the lamination process (ii), an annealing step tends to grow the crystal structure of the microporous layers (A) and (B) and improve pore openness. Annealing at a specific temperature for a predetermined time tends to enable both the microporous layers (A) and (B) to have a good area-average long pore diameter, high porosity, and low air permeability. This is thought to be because the crystals grow without disrupting the crystal structure, resulting in high pore openness. The temperature range for the annealing step is preferably 135°C or higher and 160°C or lower, preferably 140°C or higher, and more preferably 150°C or higher. The duration of the annealing step is preferably 20 minutes or longer, more preferably 60 minutes or longer. This facilitates high orientation, resulting in high MD tensile strength and high pin puncture strength, and promoting efficient stretching and pore opening. As a result, low resistance and high pin puncture strength tend to be compatible. In particular, in the case of a separator substrate produced using the coextrusion process (i), which is composed of laminated resins with different melt tensions, it is difficult to form all layers of the separator substrate under optimal temperature and cooling conditions. Failure to form the film at the optimal temperature can inhibit crystal orientation, resulting in low MD tensile strength, inefficient perforation, and high film resistance. In such cases, it is effective to anneal the film at the high annealing temperature described above and then hot stretch and heat-relax at a high hot stretch ratio, as described below.

[0071] The method for producing a separator substrate may include a stretching step after the annealing step. Either uniaxial or biaxial stretching can be used as the stretching treatment. While not limited thereto, uniaxial stretching is preferred from the viewpoints of production costs when using a dry method and reducing TD thermal shrinkage. The MD stretch ratio of cold stretching ((dimension after stretching - dimension before stretching) / dimension before stretching x 100 (%)) is preferably 5% to 20%, more preferably 8% to less than 20%, and even more preferably 8% to 15%, which tends to produce a separator with a relatively large area-average long pore diameter. The cold stretching temperature is preferably 10°C to 50°C, more preferably 20°C to 30°C, and may be performed at room temperature (23±2°C) from the viewpoint of production costs. Uniaxial stretching is preferred from the viewpoints of improving both the strength and low resistance of the resulting separator substrate, production costs, and reducing TD thermal shrinkage.

[0072] To suppress thermal shrinkage of the separator substrate, a heat treatment step for heat setting may be performed after the stretching step or the hole-forming step. The heat treatment step may include a hot stretching operation performed at a predetermined temperature and a predetermined stretch ratio to adjust physical properties, and / or a heat-relaxing operation performed at a predetermined temperature and a predetermined relaxation ratio to reduce shrinkage stress imparted during film formation and stretching. A heat-relaxing operation may be performed after the hot stretching operation. In the hot stretching and heat-relaxing, the MD dimension before stretching is stretched preferably to 140% to 220%, more preferably to 160% to 200%, based on 100%. Thereafter, the MD dimension is relaxed preferably by 10% to 50%, more preferably by 20% to 40%. These heat treatment steps can be performed using a tenter or roll stretching machine. The temperature in the heat treatment step is preferably 120°C to 160°C, more preferably 130°C to 150°C.

[0073] The obtained separator substrate can be used as it is as a separator for an electricity storage device. Optionally, one or both sides of the separator substrate may be provided with an additional layer such as a coating layer, and if necessary, may be subjected to a surface treatment such as a corona treatment.

[0074] <Energy storage device> The power storage device of the present disclosure includes the power storage device separator of the present disclosure. The power storage device of the present disclosure has a positive electrode and a negative electrode, and the power storage device separator of the present disclosure is preferably disposed between the positive electrode and the negative electrode. Because dendrites generated in the power storage device grow to pass through the pores in the separator surface and have a high penetration force into the positive electrode surface, disposing a microporous layer (A) with a relatively high melt tension and small pore size on the surface of the separator can effectively suppress the growth of dendrites into the separator, which tends to effectively suppress dendrite short-circuiting.

[0075] Examples of the power storage device include, but are not limited to, lithium secondary batteries (including all-solid-state lithium batteries, lithium-sulfur batteries, and lithium-air batteries), lithium ion secondary batteries, sodium secondary batteries, sodium ion secondary batteries, magnesium secondary batteries, magnesium ion secondary batteries, calcium secondary batteries, calcium ion secondary batteries, aluminum secondary batteries, aluminum ion secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries, electric double layer capacitors, lithium ion capacitors, redox flow batteries, and zinc-air batteries. Among these, from the viewpoints of high energy density, low cost, and durability, lithium secondary batteries, lithium ion secondary batteries, and lithium ion capacitors are preferred, and lithium ion secondary batteries are more preferred.

[0076] The electricity storage device can be produced, for example, by stacking a positive electrode and a negative electrode with the separator described above interposed therebetween and winding them as necessary to form a stacked electrode body or a wound electrode body, which is then loaded into an outer casing, connecting the positive and negative electrodes to the positive and negative electrode terminals of the outer casing via lead bodies or the like, and further injecting a non-aqueous electrolyte solution containing a non-aqueous solvent such as a chain or cyclic carbonate and an electrolyte such as a lithium salt into the outer casing, and then sealing the outer casing.

[0077] The present electricity storage device is more preferably a lithium ion secondary battery, and preferred embodiments of the lithium ion secondary battery will now be described.

[0078] The positive electrode is not particularly limited as long as it functions as a positive electrode of a lithium ion secondary battery, and known positive electrodes can be used. The positive electrode preferably contains, as a positive electrode active material, one or more materials selected from the group consisting of materials capable of absorbing and releasing lithium ions. From the viewpoint of battery capacity and safety, the positive electrode is preferably made of lithium cobalt oxides typified by LiCoO2, spinel lithium manganese oxides typified by Li2Mn2O4, or Li2Mn 1.5 Ni 0.5 Examples of suitable lithium-containing composite metal oxides include spinel-based lithium nickel manganese oxides such as LiCoO4, lithium nickel oxides such as LiNiO2, lithium-containing composite metal oxides such as LiMO2 (where M represents two or more elements selected from the group consisting of Ni, Mn, Co, AI, and Mg), and lithium iron phosphate compounds such as LiFePO4. Among these, from the viewpoint of high safety and long-term stability, more preferred are lithium cobalt oxides such as LiCoO2, lithium nickel oxides such as LiNiO2, lithium-containing composite metal oxides such as LiMO2 (where M represents two or more elements selected from the group consisting of Ni, Mn, Co, AI, and Mg), and lithium iron phosphate compounds such as LiFePO4, with the lithium iron phosphate compound such as LiFePO4 being particularly preferred.

[0079] The negative electrode is not particularly limited as long as it functions as a negative electrode for a lithium-ion secondary battery, and may be any known material. The negative electrode preferably contains, as the negative electrode active material, one or more materials selected from the group consisting of materials capable of absorbing and releasing lithium ions and metallic lithium. That is, the negative electrode preferably contains, as the negative electrode active material, one or more materials selected from the group consisting of metallic lithium, carbon materials, materials containing elements capable of forming an alloy with lithium, and lithium-containing compounds. Examples of such materials include metallic lithium, as well as carbon materials such as hard carbon, soft carbon, artificial graphite, natural graphite, graphite, pyrolytic carbon, coke, glassy carbon, baked bodies of organic polymer compounds, mesocarbon microbeads, carbon fiber, activated carbon, graphite, carbon colloids, and carbon black. [Example]

[0080] <<Measurement and Evaluation Methods>> [Melt flow rate (MFR) measurement] The melt flow rate (MFR) of the microporous layer (A) was measured (unit: g / 10 min) at a temperature of 230°C and a load of 2.16 kg in accordance with JIS K 7210. The MFR of polypropylene was measured at a temperature of 230°C and a load of 2.16 kg in accordance with JIS K 7210. The melt flow rate (MFR) of polyethylene was measured at a temperature of 230°C and a load of 2.16 kg in accordance with JIS K 7210.

[0081] [Measurement of Mw and Mn by GPC (Gel Permeation Chromatography)] A calibration curve was created using an Agilent PL-GPC220 by measuring standard polystyrene under the following conditions. 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 MWD (Mw / Mn), calculated as the weight average molecular weight (Mw) divided by the number average molecular weight (Mn), were calculated using the following conditions. Column: TSKgel GMHHR-H(20) HT (7.8mm I.D. x 30 cm) x 2 Mobile phase: 1,2,4-trichlorobenzene Detector: RI Column temperature: 160℃ Sample concentration: 1mg / ml Calibration curve: Polystyrene

[0082] [Melt tension measurement] The melt tension (mN) of the microporous membrane was measured under the following conditions using a Capillograph manufactured by Toyo Seiki Seisakusho. Capillary: diameter 1.0 mm, length 20 mm Cylinder extrusion speed: 2mm / min Take-up speed: 60m / min ·Temperature: 230℃

[0083] [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 by dissolving polypropylene pellets in o-dichlorobenzene-d at a measurement temperature of 145°C and 25,000 cumulative cycles.

[0084] [Thickness (μm) measurement] The thickness (μm) of the separator substrate was measured using a Mitutoyo Digimatic Indicator IDC112 at room temperature of 23±2° C. The thickness of each microporous layer was calculated from cross-sectional SEM image data obtained by the method for evaluating the area average pore diameter described below.

[0085] [Porosity (%) measurement] A sample measuring 10 cm x 10 cm was cut from the separator or microporous layer, and its volume (cm 3 ) and mass (g), and then calculate the density (g / cm 3 ) the porosity was calculated using the following formula: Porosity (%) = (volume - mass / density) / volume x 100

[0086] [Air permeability (sec / 100cm) 3 )] The air permeability resistance (sec / 100cm) of the separator substrate was measured using a Gurley air permeability meter conforming to JIS P-8117. 3 ) was measured.

[0087] [Air permeability after high temperature treatment (sec / 100cm 3 )] The separator substrate was cut into 100 mm squares in both the MD and TD directions, and the resulting samples were placed in a hot air dryer (Yamato Scientific Co., Ltd., DF1032) and heat-treated at 140°C for 30 minutes in the atmosphere at normal pressure. The samples were removed from the hot air dryer and allowed to cool at 25°C for 10 minutes. The air permeability resistance (sec / 100cm) of the separator substrate was then measured using a Gurley air permeability tester conforming to JIS P-8117. 3 ) was measured.

[0088] [TD heat shrinkage rate (%)] The separator substrate was cut into 50 mm squares in both the MD and TD directions, and the resulting samples were placed on copy paper in a hot air dryer (Yamato Scientific, DF1032) and heat-treated at 150°C for 1 hour in air at normal pressure. The sample was then removed from the hot air dryer and allowed to cool at 25°C for 10 minutes, after which the dimensional shrinkage was determined. Heat shrinkage rate (%): (dimension before heating (mm) - dimension after heating (mm)) / (dimension before heating (mm)) x 100

[0089] [Separator MD tensile strength] The tensile strength of the separator was measured using a tensile tester (Minebea Co., Ltd. TG-1kN type) by setting the sample length before the test to 35 mm and pulling the sample at a speed of 100 mm / min. The tensile strength was determined as the strength (tensile load value) when the sample yielded, or if it broke (fractured) before yielding, the strength (tensile load value) at the time of breaking divided by the cross-sectional area of ​​the test piece. The tensile strength (kgf / cm) in the MD of the separator was 2 ) was measured.

[0090] [Piercing 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 Co., Ltd. "MX2-50N" 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 (gf) was calculated by dividing the puncture strength (gf) by the separator thickness (μm) and multiplying by 14 μm to obtain the puncture strength (gf) when the separator substrate thickness was converted to 14 μm.

[0091] [Area average long pore diameter (nm)] The area-average pore diameter was measured by image analysis of cross-sectional SEM observations. As a pretreatment, the separator was stained with ruthenium and freeze-fractured to prepare cross-sectional samples. The cross-section was MD-ND. The cross-sectional sample was fixed to a SEM sample stage for cross-sectional observation using a conductive carbon adhesive and dried. An osmium coater (HPC-30W, Vacuum Device Co., Ltd.) was used to apply an osmium coating to the sample, with the applied voltage adjustment knob set to 4.5 and a discharge time of 0.5 seconds. This resulted in a microscopic specimen. Next, a scanning electron microscope (Hitachi High-Technologies S-4800) was used to observe three random points on the microporous membrane surface at an accelerating voltage of 1 kV, a detection signal of LA10, a working distance of 5 mm, and a magnification of 5000x.

[0092] The observed images were binarized using the image processing software ImageJ using the Otsu method, and the resin part and the pore part were separated, and the average long diameter of the pores was calculated. At this time, the area of ​​the pores that existed both within the photographed range and outside the photographed range was 0.001 nm 2 The following pores were excluded from the measurement: The average diameter was calculated by averaging the area of ​​each pore.

[0093] [Battery performance (dendrite short circuit evaluation)] 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.

[0094] The positive electrode active material was lithium nickel manganese cobalt mixed oxide (LiNi 0.5 Co 0.2 Mn 0.3 O2), carbon black powder (manufactured by Timcal, product name: SuperP Li) as a conductive additive, and PVDF as a binder were mixed in a mass ratio of mixed oxide:conductive additive:binder = 100:3.5:3. This mixture was applied to both sides of an aluminum foil serving as a positive electrode current collector with a thickness of 15 μm, dried, and then pressed with a roll press to prepare a double-sided coated positive electrode.

[0095] Graphite powder (manufactured by 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 (manufactured by Zeon Corporation, product name: BM400B), and carboxymethyl cellulose (manufactured by Daicel Corporation, product name: #2200) were used as a thickener in a mass ratio of graphite powder:binder:thickener = 100:1.5:1.1, and the mixture was applied to one side and both sides of copper foil as a 10 μm-thick negative electrode current collector. The solvent was then dried and removed, and the coated copper foil was then pressed with a roll press to produce a single-sided coated negative electrode and a double-sided coated negative electrode, respectively.

[0096] 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 microporous layer (A) was positioned facing the negative electrode. The resulting laminate 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, with the positive and negative electrode terminals protruding. 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.

[0097] 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.

[0098] After the initial charge and discharge, the battery was placed in a thermostatic chamber set at 10°C. The battery was charged at a constant current of 1 C. After the voltage reached 4.50 V, it was charged at a constant voltage of 4.50 V for one hour, then left to stand in an open-circuit state for one hour, and then discharged at a constant current of 1 C to 3.0 V. This charge-discharge cycle was repeated 10 times, and the presence or absence of dendritic short-circuiting was confirmed using 10 sheet-type lithium-ion secondary batteries for each cycle. The pass rate (%) was calculated from the number of sheet-type lithium-ion secondary batteries that did not develop a short circuit. The presence or absence of dendritic short-circuiting was determined when the voltage dropped by 0.3 V or more while left to stand in an open-circuit state after charging, or when the battery could not be charged to 4.5 V within two hours during constant-current charging at 1 C.

[0099] [Membrane resistance measurement] The membrane resistance (measured ionic resistance) of a microporous membrane immersed in an electrolyte is crucial for battery manufacturing due to its influence on electrical performance. The membrane resistance is a more comprehensive measure of permeability than the Gurley value, since it is measured in an actual electrolyte for practical battery applications. The membrane resistance of a microporous membrane is essentially the ionic resistance of the electrolyte infiltrated into its pores. Typically, a microporous membrane immersed in an electrolyte has an electrical resistance approximately 6 to 7 times that of the equivalent volume of electrolyte displaced. It is a function of the membrane's porosity (pore volume), tortuosity, electrolyte resistance, membrane thickness, and the degree to which the electrolyte wets the membrane's pores. The membrane resistance is characterized by cutting small pieces of the microporous membrane from the final material and then placing them between two blocking electrodes. The microporous membrane is fully saturated with a battery electrolyte consisting of 1.0 M LiPF6 salt in EC / EMC solvent at a volume ratio of 3:7. The resistance value R (Ω) of the separator is measured using a four-probe AC ​​impedance technique. To reduce measurement errors at the electrode / separator interface, multiple measurements are required by adding more separator layers. Then, based on the measurements of the multiple layers, the ionic resistance value Rs (Ω) of the separator saturated with electrolyte is calculated using the following equation (1): Rs=ρsl / A (1) ρs is the ionic resistivity of the separation membrane Ω-cm, and A is the area of ​​the electrode cm 2 where l is the thickness of the separation membrane in cm. The ρs / A ratio is the calculated slope of the change in the resistance of a microporous membrane given by the following equation (2): Slope = ρs / A = ΔR / Δδ (2) ΔR and Δδ are defined in Figure 1. The slope calculation in Figure 1 is used to estimate the membrane resistance of microporous membranes using the multilayer measurement technique.

[0100] The ionic resistance of microporous membranes is measured using a four-probe AC ​​impedance technique. The cell used to measure resistance has two leads, one for sensing current and the other for sensing voltage, from the top and bottom probes of the cell. The electrolyte used for all resistance measurements is 1.0 M LiPF6 salt in a 3:7 volume ratio of EC:EMC solvent. A microporous membrane sample is placed on the bottom electrode. The microporous membrane should completely cover the bottom electrode and be completely wetted with the electrolyte. A second electrode is then gently placed on top of the bottom electrode and the impedance value is measured. The impedance value is measured using a potentiostat-based impedance meter. To reduce measurement error, begin adding more microporous layers and measure the cumulative resistance value. It is possible to measure the resistance of the electrolyte alone by adding a Teflon® spacer with a recessed center placed on the bottom probe. The electrolyte is then added to fill the center of the recess, and the top probe is then placed on the spacer.

[0101] Example 1 [Preparation of microporous layer] The resin for the microporous layer (A) was a high-molecular-weight polypropylene resin (MFR (230°C) = 0.25 g / 10 min, density = 0.91 g / cm 3 ) was melted in a 2.5-inch extruder and fed to both outer layers of a two-kind, three-layer coextrusion T-die using a gear pump. Furthermore, a high-molecular-weight polypropylene resin (MFR (230°C) = 0.44 g / 10 min, density = 0.91 g / cm) was used as the resin for the microporous layer (B). 3 ) was melted in a 2.5-inch extruder and fed to the inner layer of the two-component, three-layer coextrusion T-die using a gear pump. The T-die temperature was set to 240°C, and the molten polymer was extruded from the T-die. After extrusion, the extruded resin was cooled by blown air while being wound up on a roll, yielding a precursor sheet with a B / A / B layer structure approximately 14 μm thick. The T-die's TD lip width was set to 500 mm, the distance between the T-die lips (lip clearance) was set to 2.4 mm, and extrusion was performed at a rate of 6 kg / h.

[0102] The resulting precursor was then placed in a dryer and annealed at 150°C for 180 minutes. The annealed precursor was then cold stretched 11% in the MD at room temperature. The stretched membrane was then placed in a 140°C oven without shrinkage and hot stretched to 180% in the MD, with the pre-stretched dimensions at 100%. This was followed by relaxation in the MD by 30% to obtain a separator substrate with a three-layer structure consisting of B / A / B layers. The structure, physical properties, and battery performance evaluation results of the resulting separator substrate are shown in Table 1.

[0103] Examples 2 to 19 and Comparative Examples 1 to 4 Microporous membranes were obtained in the same manner as in Example 1, except that the raw materials and lamination order were changed as shown in Tables 1 and 2, and the annealing temperature, cold draw ratio, and hot stretching and heat-relaxing conditions were adjusted, and the resulting separators were evaluated.

[0104] In Example 6, a high molecular weight polypropylene resin (MFR (230°C) = 0.51 g / 10 min, density = 0.91 g / cm) was used as the resin for the microporous layer (B). 3 A resin material was obtained by dry blending 92% by weight of styrene-ethylene propylene-styrene block copolymer (referred to as "SEPS" in the table) with 8% by weight of styrene-ethylene propylene-styrene block copolymer (referred to as "SEPS" in the table). The obtained resin material was melted in a 2.5-inch extruder and fed to the inner layer of a two-kind, three-layer co-extrusion T-die using a gear pump. Furthermore, a high-molecular-weight polypropylene resin (MFR (230°C) = 0.44 g / 10 min, density = 0.91 g / cm) was used as the resin for the microporous layer (A). 3The polymer mixture was melted in a 2.5-inch extruder and fed to both outer layers of the two-component, three-layer coextrusion T-die using a gear pump. The T-die temperature was set to 240°C, and the molten polymer was extruded from the T-die. The extruded resin was cooled by blown air while being wound around a roll, yielding a precursor sheet with an A / B / A layer structure approximately 14 μm thick. The T-die's TD lip width was set to 500 mm, the T-die lip distance (lip clearance) was set to 2.4 mm, and extrusion was performed at a discharge rate of 6 kg / h. Next, the sheet was annealed, cold-stretched, hot-stretched, and heat-relaxed in the same manner as in Example 1 to obtain a separator substrate with a three-layer structure consisting of A / B / A layers.

[0105] In Comparative Example 3, turbulence occurred at the interface of the A layer / B layer / A layer during coextrusion film formation, and therefore a good precursor sheet was not obtained. In Comparative Example 4, a polyethylene resin (MFR (230°C) = 1.5 g / 10 min, density = 0.96 g / cm) was used as the resin for the microporous layer (B). 3 ) 100% by weight was melted in a 2.5-inch extruder and fed to a single-layer T-die using a gear pump. The T-die temperature was set to 210°C, and the molten polymer was extruded from the T-die. After extruding, the extruded resin was cooled by blown air while being wound up on a roll to obtain a precursor sheet of microporous layer (B) approximately 5 μm thick. Next, a lamination process was performed at 120°C to obtain a precursor sheet with a three-layer structure of microporous layer (A) / microporous layer (B) / microporous layer (A). The obtained precursor was then placed in a dryer and annealed at 120°C for 180 minutes. The annealed precursor was then cold stretched by 10% in each MD direction at room temperature, and the stretched film was placed in an oven at 125°C without shrinking. The pre-stretched dimensions were taken as 100%, and the film was hot stretched to 180% in each MD direction. Subsequently, the film was relaxed by 30% in each MD direction to obtain a separator substrate with a three-layer structure consisting of A / B / A layers.

[0106] [Table 1]

[0107] [Table 2] [Industrial Applicability]

[0108] The separator for an electricity storage device according to the present disclosure can be suitably used as a separator for an electricity storage device, such as a lithium ion secondary battery.

Claims

1. A separator for an electricity storage device, comprising a separator substrate having one or more microporous layers (A) mainly composed of polypropylene, and one or more microporous layers (B) mainly composed of polypropylene laminated on at least one of the microporous layers (A), The melt tension Mt of the microporous layer (A) at 240°C Ａ is 10 mN or more and 40 mN or less, The melt tension Mt of the microporous layer (A) at 240°C Ａ and the melt tension Mt of the microporous layer (B) at 240°C. Ｂ Relative to Mt Ａ / Mt Ｂ is equal to or greater than 1.05 and equal to or less than 4.0, The separator for an electricity storage device, wherein the microporous layer (A) has an area average pore diameter of 50 nm or more and 500 nm or less.

2. The melt tension Mt of the microporous layer (A) at 240°C Ａ and the melt tension Mt of the microporous layer (B) at 240°C. Ｂ Relative to Mt Ａ / Mt Ｂ The separator for an electricity storage device according to claim 1 , wherein the σ is 1.1 or more and 2.2 or less.

3. The separator for an electricity storage device has an MD tensile strength of 2000 kgf / cm ２ More than 2800kgf / cm ２ The separator for an electricity storage device according to claim 1 or 2, wherein:

4. 3. The separator for an electricity storage device according to claim 1, wherein the separator substrate has a thermal shrinkage rate in the width direction after heat treatment at 150° C. for 1 hour of -1.0% or more and 3.0% or less.

5. The separator for an electricity storage device according to claim 1 or 2, wherein at least one of the microporous layers (A) and at least one of the microporous layers (B) are adjacent to each other.

6. The separator for an electricity storage device according to claim 1 or 2, wherein the separator substrate has the microporous layer (A) as an outermost layer on both sides.

7. The separator for an electricity storage device according to claim 1 or 2, wherein the microporous layer (A) has a thickness of 5 μm or less, and the microporous layer (B) has a thickness of 5 μm or less.

8. The separator for a power storage device according to claim 1 or 2, wherein the porosity of the separator for a power storage device is 40% or more and 60% or less.

9. 3. The separator for an electricity storage device according to claim 1, wherein the separator for an electricity storage device has an air permeability of 10 seconds / 100 cc or more and less than 220 seconds / 100 cc.

10. The separator for an electricity storage device according to claim 1 or 2, wherein the microporous layer (B) has an area average pore diameter of 50 nm or more and 240 nm or less.

11. 3. The separator for an electricity storage device according to claim 1, wherein the separator for an electricity storage device has an MD / TD tensile strength ratio of 16 or more and 30 or less.

12. An electricity storage device comprising: a positive electrode; a negative electrode; and the electricity storage device separator according to claim 1 or 2 disposed between the positive electrode and the negative electrode.

13. The power storage device according to claim 12 , wherein the positive electrode contains lithium iron phosphate as a positive electrode active material.

14. A method for producing a separator for an electricity storage device, comprising a separator substrate having one or more microporous layers (A) mainly composed of polypropylene and one or more microporous layers (B) mainly composed of polypropylene, the method comprising: (i) a melt-extrusion step of co-extruding the microporous layer (A) and the microporous layer (B), or (ii) separately extruding the microporous layer (A) and the microporous layer (B) and then laminating them together to obtain a precursor sheet of the separator substrate; annealing the obtained precursor sheet at a temperature ranging from 135°C to 160°C; cold stretching the precursor sheet in MD at a stretching ratio of 5% to 20%; hot stretching and heat relaxing the precursor sheet; A method for producing a separator for an electricity storage device, comprising:

15. The method according to claim 14 , wherein the hot stretching and heat relaxing are performed by stretching the precursor sheet to 140% or more and 220% or less, with the MD dimension of the precursor sheet before stretching being 100%, and then relaxing the precursor sheet in the MD by 10% or more and 50% or less.

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