JP2024156561A - Separator for power storage device and power storage device - Google Patents

Abstract

An object of the present disclosure is to provide a separator for an electricity storage device that is thin and has both air permeability and puncture strength.
[Solution] A separator for an electricity storage device is provided which has a multilayer structure of a microporous layer (A) mainly composed of polypropylene and a microporous layer (B) mainly composed of polyethylene, at least one of the microporous layers (A) forming the outermost layer on at least one side of a separator substrate, the MFR of the microporous layer (A) being 0.90 g/10 min or less when measured under a load of 2.16 kg and at a temperature of 230°C, the thickness of the separator substrate being 15 μm or less, and the porosity of the separator substrate being 40% or more.
[Selection diagram] None

Description

This disclosure relates to separators for electricity storage devices.

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 hybrid automobiles and electric automobiles including plug-in hybrid automobiles.

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 increased demand for thin-film separators that offer excellent battery performance, reliability, and safety.

For example, U.S. Patent No. 5,399,933 describes multilayer microporous thin films or membranes that can improve properties including dielectric breakdown and strength. A preferred multilayer microporous membrane includes a microlayer and one or more stacked barriers.

Cited Document 2 describes a separator for an electricity storage device having a microporous membrane that is mainly composed of polyolefin 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.

In recent years, heterogeneous polyolefin multilayer separators that have a microporous layer mainly made of polypropylene and a microporous layer mainly made of polyethylene have been attracting attention. Heterogeneous polyolefin multilayer separators improve the safety of batteries by combining high-temperature durability due to the high-melting point polypropylene layer and shutdown performance during thermal runaway due to the low-melting point polyethylene layer.

The mainstream manufacturing method for heterogeneous polyolefin multilayer separators is the lamination process, which allows each microporous layer to be formed separately, which is thought to enable more precise temperature control and orientation during film formation.

Cited Document 3 describes a separator for an electricity storage device that has a microporous layer mainly made of polypropylene and a microporous layer mainly made of polyethylene.

International Publication No. 2018/089748 International Publication No. 2020/196120 International Publication No. 2022/59744

Electricity storage device separators manufactured using a lamination process are limited in how thin they can be made because each microporous layer is individually formed and then laminated. In addition, as the separator becomes thinner, its physical strength decreases, making it difficult to improve puncture strength while maintaining excellent air permeability. Therefore, the object of the present disclosure is to provide a thin electricity storage device separator that is both air permeable and has good puncture strength.

Examples of embodiments of the present disclosure are listed in the following items [1] to [10].
(1) A separator for an electricity storage device having a multilayer structure of a microporous layer (A) mainly composed of polypropylene and a microporous layer (B) mainly composed of polyethylene,
At least one of the microporous layers (A) constitutes an outermost layer on at least one surface of the separator substrate,
the melt flow rate (MFR) of the microporous layer (A) is 0.90 g/10 min or less when measured under a load of 2.16 kg and at a temperature of 230° C.;
The separator for an electricity storage device, wherein the separator substrate has a thickness of 15 μm or less, and a porosity of 40% or more.
(2) The separator for an electricity storage device according to item 1, wherein, in MD-ND cross-section observation of the microporous layer (A) with a scanning electron microscope (SEM), the area average long pore diameter of pores present in the microporous layer (A) is 150 nm or less.
(3) The separator for an electricity storage device according to item 1 or 2, wherein the separator has an air permeability of 200 sec/100 ^cm3 or less when converted to a thickness of 8 μm.
(4) The separator for an electricity storage device according to any one of items 1 to 3, wherein, in MD-ND cross-section observation of the microporous layer (A) with a scanning electron microscope (SEM), the area average long pore diameter of pores present in the microporous layer (A) is 100 nm or more.
(5) The separator for an electricity storage device according to any one of items 1 to 4, wherein the microporous layer (B) has a thickness of 5 μm or less.
(6) The separator for an electricity storage device according to any one of items 1 to 5, wherein the MFR of the microporous layer (A) is 0.3 g/10 min or more when measured under a load of 2.16 kg at a temperature of 230° C.
(7) The separator for an electricity storage device according to any one of items 1 to 6, wherein the separator substrate has a thickness of 9.5 μm or less.
(8) An electricity storage device comprising: a positive electrode; a negative electrode; and the separator for an electricity storage device according to any one of items 1 to 7, disposed between the positive electrode and the negative electrode.
(9) A method for producing a separator for an electricity storage device according to any one of items 1 to 7, comprising the following steps (1) to (3):
Step (1): The stretching ratio (cold stretching ratio) of the resin film in the first stretching step is 10% or more;
step (2): the stretching ratio (hot stretching ratio) of the resin film in the second stretching step is 160% or more; and step (3): the annealing temperature in the annealing step is 125° C. or more.
(10) The method for producing a separator for an electricity storage device according to item 9, wherein the microporous layer (A) and the microporous layer (B) are coextruded.

This disclosure provides a separator for an electricity storage device that is thin and has both air permeability and puncture strength.

<Separator for power storage device>
The separator for an electric storage device of the present disclosure has a separator substrate having a microporous layer (A) mainly composed of polypropylene and a microporous layer (B) mainly composed of polyethylene. The separator substrate may further have a coating layer (also called a "surface layer", a "coating layer", etc., hereinafter simply referred to as a "coating layer") on the microporous layer (A) and/or the microporous layer (B). In this specification, the term "microporous layer" refers to each microporous layer constituting the separator substrate, the term "separator substrate" refers to the substrate of the separator excluding any coating layer, and the term "separator" refers to the entire separator including any coating layer.

<Microporous layer (A)>
The separator for an electric storage device of the present disclosure has a microporous layer (A). The separator for an electric storage device may have only one microporous layer (A) or two or more layers. At least one of the microporous layers (A) constitutes the outermost layer on at least one side of the separator substrate. When the separator for an electric storage device has two or more microporous layers (A), the microporous layer (A) may constitute the outermost layer on both sides of the separator substrate. The microporous layer (A) is mainly composed of polypropylene, which allows the battery to maintain good performance even after storage at high temperatures (e.g., 130°C). In the present specification, "mainly composed of polypropylene" means that the microporous layer (A) contains 50% by mass or more of polypropylene based on the total mass of the microporous layer (A). The lower limit of the polypropylene content in the microporous layer (A) is 50% by mass or more, preferably 55% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more, from the viewpoints of the wettability, thinning, and shutdown characteristics of the separator, etc. The upper limit of the polypropylene content in the microporous layer (A) is not limited, and may be, for example, 60% by mass or less, 70% by mass or less, 80% by mass or less, 90% by mass or less, 95% by mass or less, 98% by mass or less, or 99% by mass or less, or may be 100% by mass.

<Material for Microporous Layer (A)>
The microporous layer (A) is mainly composed of polypropylene. The stereoregularity of the polypropylene is not limited, but may be, for example, atactic, isotactic, or syndiotactic homopolymer. The polypropylene according to the present disclosure is preferably an isotactic or syndiotactic high crystalline homopolymer.

The polypropylene of the microporous layer (A) is preferably a homopolymer, and may 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.

The weight average molecular weight (Mw) of the polypropylene of the microporous layer (A) is preferably 300,000 or more from the viewpoint of the strength of the microporous layer, and is preferably 1,300,000 or less from the viewpoint 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.

The upper limit of the value (Mw/Mn) obtained by dividing the weight average molecular weight (Mw) of the polypropylene of the microporous layer (A) by the number average molecular weight (Mn) is preferably 20 or less, more preferably 18 or less, 16 or less, 14 or less, or 12 or less. By setting Mw/Mn to 20 or less, good film-forming properties and productivity tend to be ensured. In addition, Mw/Mn is preferably 3 or more, more preferably 3.5 or more, and even more preferably 4 or more. The larger the Mw/Mn value of the polypropylene, the larger the melt tension of the obtained microporous layer tends to be, and it is preferable to increase the melt tension of the microporous layer (A) even in increasing the strength of the microporous layer (A). Therefore, it is preferable that the Mw/Mn value of the polypropylene is 3 or more in order to control the melt tension of the microporous layer (A) to a high level. The weight average molecular weight, number average molecular weight, and Mw/Mn of the polyolefins disclosed herein are polystyrene-equivalent molecular weights obtained by GPC (gel permeation chromatography) measurement.

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

As long as the microporous layer (A) is mainly composed of polypropylene, it may contain other resins. Examples of other resins include polyolefins other than polypropylene (also referred to as "other polyolefins") and copolymers of polystyrene and polyolefins. Polyolefins are polymers containing a monomer having a carbon-carbon double bond as a repeating unit. Monomers constituting polyolefins other than polypropylene include, but are not limited to, monomers having 2 or 4 to 10 carbon atoms and having 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 as an example, it is possible to contain polyethylene. Examples of copolymers of polystyrene and polyolefins include styrene-(ethylene-propylene)-styrene copolymers (SEPS), styrene-(ethylene-butene)-styrene copolymers, and styrene-ethylene-styrene copolymers. Styrene-(ethylene-propylene)-styrene copolymers (SEPS) are particularly preferred.

<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 0.90 g/10 min or less from the viewpoint of obtaining a microporous layer (A) with higher strength, and may be, for example, 0.6 g/10 min or less, 0.55 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 the formability and thin film 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, 0.35 g/10 min or more, or 0.5 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.

The MFR of the microporous layer (A) being 0.90 g/10 min or less means that the molecular weight of the polyolefin contained in the microporous layer (A) is relatively high. When the molecular weight of the polyolefin is high, there are more tie molecules that bond the crystalline substances together, so that a microporous layer (A) with high strength tends to be obtained, and a separator substrate with high strength tends to be obtained. In addition, when the molecular weight of the polyolefin is high, holes are less likely to open during the stretching process, so that the area average long pore diameter tends to be low.

By having the MFR of the microporous layer (A) be 0.2 g/10 min or more, the melt tension of the microporous layer (A) does not become too high, making it possible to ensure good film-forming properties and productivity.

From the viewpoint of obtaining a microporous layer (A) having high strength and high melt tension, the MFR of the polypropylene of the microporous layer (A) is preferably 0.2 to 0.9 g/10 min when measured under conditions of a load of 2.16 kg and a temperature of 230°C. From the viewpoint of obtaining a microporous layer having higher strength, the upper limit of the MFR of the polypropylene may be, for example, 0.9 g/10 min or less, 0.6 g/10 min or less, 0.55 g/10 min or less, or 0.5 g/10 min or less. The lower limit of the MFR of the polypropylene is not limited, but from the viewpoint of the moldability and thin film formability of the microporous layer (A), it may be, for example, 0.2 g/10 min or more, 0.25 g/10 min or more, 0.3 g/10 min or more, 0.35 g/10 min or more, or 0.5 g/10 min or more.

<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) may be 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 measured by ¹³ C-NMR (nuclear magnetic resonance).

A polypropylene with a pentad fraction of 94.0% or more indicates that the polypropylene has high crystallinity. In separators obtained by the stretching perforation method, particularly the dry method, the amorphous parts between the crystalline parts are stretched to create holes, so if the polypropylene has high crystallinity, the perforation is good and the air permeability can be kept low, enabling the battery to have high output.

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

<Area-average long pore diameter of microporous layer (A)>
The area average long pore diameter (hereinafter also simply referred to as "area average long pore diameter") in the MD-ND cross section of the microporous layer (A) is preferably 50 nm or more and 400 nm or less. In this specification, "ND" refers to the thickness direction of the microporous layer, and "MD" refers to the film formation 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" means the pore diameter in the MD. In addition, when there are two or more microporous layers (A) and/or microporous layers (B), the area average long pore diameters of the microporous layer (A) and the microporous layer (B) are compared based on the average area average long pore diameter value of each microporous 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, particularly preferably 120 nm or more, and most preferably 130 nm or more, from the viewpoint of maintaining excellent air permeability and ensuring good output in an electricity storage device. Also, the upper limit of the area average pore diameter of the microporous layer (A) is preferably 400 nm or less, more preferably 250 nm or less, even more preferably 200 nm or less, particularly preferably 170 nm or less, and most preferably 150 nm or less, from the viewpoint of obtaining a microporous layer (A) with higher strength.

In a multilayer separator having a multilayer structure of a microporous layer (A) mainly composed of polypropylene and a microporous layer (B) mainly composed of polyethylene, which is obtained by a dry co-extrusion process, it has been revealed that the strength of the microporous layer (A) and the separator substrate is increased by reducing the area average long pore diameter of the microporous layer (A). The mechanism is presumed to be as follows:
The area-average long pore diameter of the microporous layer (A) is reduced, and the number of pores per unit area is increased. Since each pore is formed by a tie molecule that bonds crystalline substances, the number of tie molecules per unit area is increased. Therefore, a microporous layer (A) and a separator substrate having high strength tend to be obtained. In addition, the area-average long pore diameter is reduced, and therefore the stress applied to the end of each hole during a puncture test is reduced, and therefore a microporous layer (A) and a separator substrate having high strength tend to be obtained.

The area-average long 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. Detailed conditions are shown in the Examples. When measuring the average pore diameter from the cross-sectional SEM image, the number-average pore diameter and area-average pore diameter can be calculated, but in this specification, the area-average pore diameter is used as the average pore diameter to obtain a better correlation with the physical properties of the separator.

<Porosity of microporous layer (A)>
The porosity of the microporous layer (A) is preferably 20% or more, more preferably 25% or more, even more preferably 30% or more, and particularly preferably 35% or more from the viewpoint of avoiding clogging in the electricity storage device and obtaining good air permeability of the separator. Also, the porosity of the microporous layer (A) is preferably 70% or less, more preferably 65% or less, and even more preferably 60% or less from the viewpoint of maintaining the strength of the separator.

<Achieving both MFR of the microporous layer (A) and porosity of the separator substrate>
In the separator for a power storage device of this embodiment, the microporous layer (A) has an MFR of 0.90 g/10 min or less, a film thickness of 15 μm or less, and a porosity of 40% or more. Conventionally, in a multilayer separator having a multilayer structure of a microporous layer mainly composed of polypropylene and a microporous layer mainly composed of polyethylene, obtained by a dry method and a coextrusion process, it was very difficult to adjust the porosity to 40% or more using a microporous layer (A) having an MFR of 0.90 g/10 min or less. When the MFR of the microporous layer (A) mainly composed of polypropylene is as low as 0.90 g/10 min or less, the melt viscosity of polypropylene is significantly higher than that of polyethylene. Therefore, when the resin composition is extruded into a film shape from the extruder during extrusion film formation, stress is concentrated in the high-viscosity polypropylene resin and is not sufficiently transmitted to the low-viscosity polyethylene resin. The microporous layer (B) mainly composed of polyethylene to which stress was not transmitted had a tendency to show a significant decrease in openness due to a decrease in molecular orientation, and the porosity of the microporous layer (B) and the separator substrate tended to decrease.

Furthermore, in a multi-layer separator, the thickness of each microporous layer must be particularly thin. For example, in a three-layer separator with a thickness of 13 μm, if the thickness ratio of each microporous layer is 1:1:1, the thickness of each microporous layer must be 4.3 μm. It was even more difficult to achieve high porosity in such a thin film while using a high molecular weight polyolefin. In this embodiment, although not limited thereto, by applying precisely controlled annealing and stretching conditions as exemplified in the section "Manufacturing method for separator for power storage device" described later, the porosity of the separator substrate can be controlled to the above range even when the MFR of the microporous layer (A) mainly composed of polypropylene is as low as 0.90 g/10 min or less.

<Thickness of microporous layer (A)>
The thickness of the microporous layer (A) may be preferably 6 μm or less, for example, 5.5 μm or less, 5 μm or less, 4.5 μm or less, 4 μm or less, 3.5 μm or less, or 3 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, 2.5 μm or more, or 3 μm or more, from the viewpoint of strength, etc.

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).

<Microporous layer (B)>
The separator for an electric storage device of the present disclosure has a microporous layer (B). The separator for an electric storage device may have only one layer of the microporous layer (B), or may have two or more layers. The microporous layer (B) is mainly composed of polyethylene, which allows the shutdown performance to be exhibited at a low temperature during thermal runaway of the battery. In the present specification, "mainly composed of polyethylene" means that the microporous layer (B) contains 50% by mass or more of polyethylene based on the total mass of the microporous layer (B). The lower limit of the content of polyethylene in the microporous layer (B) may be preferably 55% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more from the viewpoint of the wettability, thinning, etc. of the separator. The upper limit of the polyethylene content in the microporous layer (B) is not limited, and may be, for example, 60% by mass or less, 70% by mass or less, 80% by mass or less, 90% by mass or less, 95% by mass or less, 98% by mass or less, or 99% by mass or less, or may be 100% by mass.

<Material for Microporous Layer (B)>
The microporous layer (B) is mainly composed of polyethylene.
The polyethylene of the microporous layer (B) is preferably a homopolymer, and may be a copolymer, such as a block polymer, in which a small amount of a comonomer other than ethylene, such as an α-olefin comonomer, is copolymerized. The amount of ethylene structure contained as a repeating unit in the polyethylene is not limited, but may be, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The amount of repeating units derived from comonomers other than the ethylene structure contained in the polyethylene is not limited, but may be, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. The polyethylene may be used alone or in combination of two or more kinds.

The density of the polyethylene of the microporous layer (B) is preferably 0.85 g/cm ³ or more, for example, 0.88 g/cm ³ or more, 0.89 g/cm ³ or more, or 0.90 g/cm ³ or more. The density of the polyethylene is preferably 1.1 g/cm ³ or less, for example, 1.0 g/cm ³ or less, 0.98 g/cm ³ or less, 0.97 g/cm ³ or less, 0.96 g/cm ³ or less, 0.95 g/cm ³ or less, 0.94 g/cm ³ or less, 0.93 g/cm ³ or less, or 0.92 g/cm ³ or less. The density of the polyethylene is related to the crystallinity of the polyethylene, and by setting the density of the polyethylene to 0.85 g/cm ³ or more, the productivity of the microporous layer is improved, which is particularly advantageous in the dry method.

As long as the microporous layer (B) is mainly composed of polyethylene, it may contain other resins. Examples of other resins include polyolefins other than polyethylene (also referred to as "other polyolefins"). Polyolefins are polymers that contain monomers having carbon-carbon double bonds as repeating units. Monomers that constitute polyolefins other than polyethylene include, but are not limited to, monomers having 3 to 10 carbon atoms and having carbon-carbon double bonds, such as propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene.

<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 2.0 g/10 min or less, more preferably 1.0 g/10 min or less, even more preferably 0.8 g/10 min or less, and particularly preferably 0.5 g/10 min or less, from the viewpoint of obtaining a microporous layer (B) with higher strength. The lower limit of the MFR of the microporous layer (B) (MFR of a single layer) is preferably 0.1 g/10 min or more, more preferably 0.15 g/10 min or more, even more preferably 0.18 g/10 min or more, and particularly preferably 0.2 g/10 min or more, from the viewpoint of good pore opening and suppression of clogging. The MFR of the microporous layer (B) is measured under the conditions of a load of 2.16 kg and a temperature of 190°C.

From the viewpoint of obtaining a microporous layer (B) having a higher strength, the upper limit of the MFR of the polyethylene of the microporous layer (B) is preferably 2.0 g/10 min or less, more preferably 1.0 g/10 min or less, even more preferably 0.8 g/10 min or less, and particularly preferably 0.5 g/10 min or less. From the viewpoint of good pore opening and suppression of clogging, the lower limit of the MFR of the polyethylene of the microporous layer (B) is preferably 0.1 g/10 min or more, more preferably 0.15 g/10 min or more, even more preferably 0.18 g/10 min or more, and particularly preferably 0.2 g/10 min or more. The MFR of the polyethylene of the microporous layer (B) is measured under conditions of a load of 2.16 kg and a temperature of 190°C.

<Area-average long pore diameter of microporous layer (B)>
The area average long pore diameter of the microporous layer (B) in the MD-ND cross section is preferably 100 nm to 1000 nm, more preferably 200 nm to 900 nm, even more preferably 300 nm to 850 nm, and still more preferably 400 nm to 750 nm. When the area average long pore diameter of the microporous layer (B) is within this range, both air permeability and high strength can be achieved while maintaining a high porosity.

<Porosity of microporous layer (B)>
The porosity of the microporous layer (B) is preferably 20% or more from the viewpoint of avoiding clogging in the electric storage device and obtaining good air permeability of the separator, and is preferably 90% or less from the viewpoint of maintaining the strength of the separator. The porosity of the microporous layer (B) is more preferably 25% or more and 85% or less, further preferably 25% or more and 80% or less, and particularly preferably 30% or more and 80% or less. In this embodiment, although not limited thereto, by applying precisely controlled annealing and stretching conditions as exemplified in the section "Manufacturing method for separator for electric storage device" described later, even when the MFR of the microporous layer (A) mainly composed of polypropylene is as low as 0.90 g/10 min or less, the porosity of the microporous layer (B) can be controlled to the above range.

<Thickness of 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, 6 μm or less, 5 μm or less, 4.5 μm or less, 4 μm or less, or 3 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 microporous layer (B) may be preferably 1 μm or more, for example 1.5 μm or more, 2 μm or more, or 2.5 μm or more, from the viewpoint of strength, etc.

Additives for the Microporous Layer (B)
The microporous layer (B) containing polyethylene as a main component may further contain, in addition to polyethylene, additives such as elastomers, crystal nucleating agents, antioxidants, fillers, etc., as necessary. 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).

<Relationship between microporous layer (A) and microporous layer (B)>
<Layer structure of separator substrate>
The substrate of the separator for an electric storage device (also simply referred to as "separator substrate" in the present specification) 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 having two or more layers of at least one of the microporous layer (A) and/or the microporous layer (B). For example, a two-layer structure of the microporous layer (A)/microporous layer (B) and a three-layer structure of the microporous layer (A)/microporous layer (B)/microporous layer (A) may be mentioned. The separator substrate may also have a layer other than the microporous layer (A) and the microporous layer (B). For example, the layer other than the microporous layer (A) and the microporous layer (B) may be, for example, 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.

From the viewpoints of reducing the total thickness of the separator substrate and achieving both air permeability and puncture strength, the ratio of the thickness of the microporous layer (B) to the thickness of the microporous layer (A) is preferably within the range of 0.2 to 1.5, 0.35 to 1.25, or 0.5 to 1.0. When the microporous layer (A) or the microporous layer (B) is multilayered, this ratio is calculated in terms of the thickness per single layer.

As for the layer structure of the substrate of the separator for an electricity storage device, it is preferable that the microporous layer (A) constitutes the outermost layer on at least one side of the separator substrate, and constitutes the outermost layers on both sides of the separator substrate. When the microporous layer (A) constitutes the outermost layer, the strength tends to be improved.

<Thickness of separator substrate>
The upper limit of the thickness of the separator substrate is 15 μm or less, and preferably 14 μm or less, for example, 13 μm or less, 12 μm or less, 11 μm or less, 10 μm or less, or 9.5 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 5 μm or more, and may be, for example, 6 μm or more, 7 μm or more, 8 μm or more, or 9 μm or more, from the viewpoint of strength, etc.

<Air permeability of separator substrate>
The upper limit of the air permeability of the separator substrate, calculated as a thickness of 8 μm, is preferably 250 sec/100 cm3 or ^less , more preferably 200 sec/100 cm3 ^or less, and may be, for example, 180 sec/100 cm3 or ^less , 160 sec/100 cm3 ^or less, 150 sec/100 cm3 ^or less, or 140 sec/100 ^cm3 or less. The lower limit of the air permeability of the separator substrate is not limited, and may be, for example, 10 sec/100 cm3 ^or more, 20 sec/100 cm3 ^or more, or 30 sec/100 cm3 ^or more.
The air permeability of the separator substrate when the thickness of the separator substrate is converted to 8 μm (air permeability converted to 8 μm thickness) is calculated using the following formula.
(Air permeability converted to 8 μm thickness [sec/100 cm ³ ])=(Air permeability of separator substrate [sec/100 cm ³ ])/(Thickness of separator substrate [μm])×(8 [μm])

<Porosity of separator substrate>
The lower limit of the porosity of the separator substrate is 40% or more, preferably 42% or more, more preferably 44% or more, and particularly preferably 45% or more, from the viewpoint of avoiding clogging in the power storage device and obtaining good air permeability of the separator. The upper limit of the porosity of the separator substrate is preferably 70% or less, more preferably 65% or less, even more preferably 60% or less, and particularly preferably 55% or less, from the viewpoint of maintaining the strength of the separator.

<Puncture strength of separator substrate>
From the viewpoint of suppressing short circuits in the event of physical damage to the battery, the lower limit of the puncture strength of the separator substrate is preferably 145 gf or more (about 1.42 N or more), more preferably 150 gf or more, even more preferably 160 gf or more, and particularly preferably 170 gf or more, when the thickness of the separator substrate is converted to 8 μm. The upper limit of the puncture strength of the separator substrate is not limited, but may be preferably 500 gf or less, for example 480 gf or less, or 450 gf or less, when the thickness of the separator substrate is converted to 8 μm.
The puncture strength of the separator substrate when the thickness of the separator substrate is converted to 8 μm (8 μm film thickness converted puncture strength) is calculated using the following formula.
(8 μm film thickness equivalent puncture strength [gf])=(separator substrate puncture strength [gf])/(separator substrate thickness [μm])×(8 [μm])

<Thermal shrinkage rate of separator substrate>
The separator substrate preferably has a thermal shrinkage rate in the transverse direction (TD) of -1.0% or more and 3.0% or less after heat treatment at 150°C for 1 hour. That is, the separator substrate has very small thermal shrinkage in the TD even at high temperatures. When the thermal shrinkage rate is 3.0% or less, short circuits at high temperatures can be effectively suppressed. The reason why the thermal shrinkage rate is -1.0% or more is that when the thermal shrinkage rate is measured, the substrate expands in the TD, and the thermal shrinkage rate may become a negative value less than 0%. The thermal shrinkage rate may be 0% or more, or may be greater than 0%. As a method for producing a separator substrate having a thermal shrinkage rate of -1.0% or more and 3.0% or less, for example, a method for producing a separator by uniaxial stretching in the MD, preferably a method for producing a separator by a dry method of uniaxial stretching, can be mentioned. In a separator manufacturing method in which the separator is stretched in two axial directions, MD and TD, as typified by a wet separator, the thermal shrinkage in TD is generally very large, whereas in a dry separator in which the separator is uniaxially stretched, it is easy to obtain a separator substrate in which the thermal shrinkage rate is -1.0% or more and 3.0% or less.

<<Method for producing separator for power storage device>>
The method for producing a separator for an electricity storage device includes a melt extrusion step of melt extruding a resin composition mainly composed of polypropylene or polyethylene (hereinafter also referred to as a "polyolefin resin composition") to obtain a resin film (precursor film), and a pore forming step of opening holes in the obtained precursor film to make it porous. The method for producing a microporous layer is roughly divided into a dry method in which no solvent is used in the pore forming step, and a wet method in which a solvent is used.

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

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

A single screw extruder or a twin screw extruder can be used to melt-knead the polyolefin resin composition. In addition, other devices such as a kneader, a lab plast mill, a kneading roll, and a Banbury mixer can also be used.

The polyolefin resin composition may optionally contain resins other than polypropylene, resins other than polyethylene, and additives, etc., depending on the manufacturing method of the microporous layer or the physical properties of the desired microporous layer. Examples of additives include pore-forming materials, fluorine-based flow modifiers, wax, 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.

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

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.

A preferred method for producing a separator substrate is a dry lamellar crystal opening process in which the polyolefin crystal interface is peeled off by heat treatment and stretching. Here, the following methods (i) and (ii) are known for producing a separator substrate having a microporous layer (A) and a microporous layer (B):
(i) a method for producing a separator substrate by coextrusion, in which a microporous layer (A) and a microporous layer (B) are coextruded and then subjected to an annealing, cold stretching, hot stretching and heat relaxation process; and (ii) a method for producing a separator substrate by lamination, in which a microporous layer (A) and a microporous layer (B) are separately extruded and then bonded together by lamination, and then subjected to an annealing, cold stretching, hot stretching and heat relaxation process.

Of the above co-extrusion process (i) and lamination process (ii), the co-extrusion process (i) is used from the viewpoints of thin film formation properties and production costs.
The reasons for not using the lamination process (ii) are as follows:
In the multilayer separator, the thickness of each microporous layer must be particularly thin. For example, in the case of a three-layer separator having a thickness of 13 μm, when the thickness ratio of each microporous layer is 1:1:1, the thickness of each microporous layer must be 4.3 μm. In addition, for example, in the case of a three-layer separator having a thickness of 9 μm, when the thickness ratio of each microporous layer is 1:1:1, the thickness of each microporous layer must be 3 μm. In addition, for example, in the case of a three-layer separator having a thickness of 9.5 μm, when the thickness ratio of each microporous layer is 1:0.5:1, the thickness of the thinnest microporous layer must be 1.9 μm. In the lamination process (ii), each microporous layer must be extruded separately, but it is difficult to stably produce a resin film having a thickness of, for example, 4.3 μm, 3 μm, or 1.9 μm. In particular, when the microporous layer (A) containing polypropylene as a main component has an MFR of 0.90 g/10 min or less, i.e., when a high molecular weight polyolefin is used, it is extremely difficult to stably produce a resin film having a thickness of, for example, 4.3 μm, 3 μm, or 1.9 μm.
In addition, the lamination process requires a step of extruding each microporous layer separately and a step of bonding the layers together by lamination, and therefore requires more steps than a coextrusion process, resulting in increased production costs.

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

Methods for distinguishing between coextrusion and laminate products include, but are not limited to, the following two methods:
(a) When 40 arbitrary points of the laminated product are observed under the condition of 5000 times magnification by cross-sectional SEM observation, two or more peeled parts (same contrast parts as holes) with a length of 5000 nm or more are observed in the MD, whereas no peeled parts are observed at all in the coextruded product, so that the two products can be distinguished based on the criterion of the presence or absence of peeled parts by such a cross-sectional SEM observation method; and (b) A laminated product consisting of layers A and B is attached with adhesive tape (Nichiban Co., Ltd. Cellotape (registered trademark) No. 405) on the front and back of the product, and left in this state for 20 minutes in an environment of 23 ° C. and 50% relative humidity, and then the adhesive tape is peeled off at a peel angle of 180 °. When this operation is repeated 40 times for the laminated product, peeling of layers A and B tends to be observed after 10 times or more. In contrast, when the same operation is performed on a coextruded product consisting of layers A and B, peeling of layers A and B tends to be observed after one time or less. The two products can be distinguished based on the difference in peeling tendency caused by the peeling method.

The above-mentioned method for producing a separator substrate by the co-extrusion process (i) may include an annealing step after extrusion film formation. By carrying out the annealing step, the crystal structure of the microporous layers (A) and (B) tends to grow, and the pore opening tends to be improved. By applying annealing at a specific temperature for a specific time, it tends to be possible to obtain a good area average long pore diameter for both the microporous layers (A) and (B). This is thought to be because the crystals grow without disturbing the crystal structure, and high pore opening is obtained.

The lower limit of the annealing temperature is preferably 125°C or higher, more preferably 128°C or higher, and particularly preferably 130°C or higher. When the MFR of the polypropylene-based microporous layer (A) is as low as 0.90 g/10 min or lower, the molecular orientation of the polyethylene-based microporous layer (B) is reduced, which significantly reduces the openness, and the porosity of the microporous layer (B) and the separator substrate tends to decrease. This is a phenomenon specific to multilayer separators obtained by a dry method and a coextrusion process, which have a multilayer structure of a polypropylene-based microporous layer and a polyethylene-based microporous layer. When the annealing temperature is within this range, the openness is improved by the growth of the crystal structure, and the porosity can be improved. In addition, the upper limit of the annealing temperature is preferably 150°C or lower, more preferably 140°C or lower, and particularly preferably 135°C or lower. When the annealing temperature is within this range, the disturbance of the crystal structure caused by the melting of polyethylene is suppressed, and the openness is improved.

The lower limit of the annealing time is preferably 10 minutes or more, more preferably 15 minutes or more, 20 minutes or more, 25 minutes or more, or 30 minutes or more, and particularly preferably 60 minutes or more. When the annealing time is within this range, the openness is improved by the growth of the crystal structure, and the porosity can be improved. Furthermore, the upper limit of the annealing time is preferably 600 minutes or less, more preferably 300 minutes or less. When the annealing time is within this range, the openness is improved while maintaining a high production rate of the separator.

The annealing process may be performed while the resin film obtained by coextrusion is running, or may be performed while the resin film is wound up in a roll.

The method for producing the separator substrate may include a stretching step after the annealing step. Either uniaxial stretching or biaxial stretching can be used as the stretching process. Although not limited thereto, uniaxial stretching is preferred from the viewpoints of production costs when using a dry method and reducing thermal shrinkage in TD. Biaxial stretching is preferred from the viewpoints of improving the strength of the resulting separator substrate. Examples of biaxial stretching include simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple stretching.

The stretching process preferably includes a first stretching process (hereinafter also referred to as "cold stretching") and a second stretching process (hereinafter also referred to as "hot stretching") subsequent to the first stretching process.

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

In the first stretching step, the lower limit of the temperature of the resin film is preferably -20°C or higher, more preferably 0°C or higher. In the first stretching step, the upper limit of the temperature of the resin film is preferably 110°C or lower, more preferably 80°C or lower. When the temperature is equal to or higher than the above lower limit, breakage of the resin film during stretching is suppressed, and when the temperature is equal to or lower than the above upper limit, cracks are effectively generated in the amorphous parts between the lamellae, and neck-in of the film is suppressed.

The lower limit of the stretching ratio of the resin film in the first stretching step (hereinafter also referred to as "cold stretching ratio") is preferably 10% or more, more preferably 15% or more, even more preferably 20% or more, and particularly preferably 25% or more. When the MFR of the microporous layer (A) mainly composed of polypropylene is as low as 0.90 g/10 min or less, the molecular orientation of the microporous layer (B) mainly composed of polyethylene tends to be low, so that the formation of micropores is difficult to proceed in the amorphous part between the lamellae. This is a phenomenon specific to a multilayer separator obtained by a dry method and a coextrusion process, which has a multilayer structure of a microporous layer mainly composed of polypropylene and a microporous layer mainly composed of polyethylene. When the cold stretching ratio is equal to or higher than the above lower limit, micropores are likely to be formed in the amorphous part between the lamellae.
The upper limit of the cold stretching ratio is preferably 60% or less, more preferably 50% or less, and even more preferably 40% or less. When the cold stretching ratio is equal to or less than the upper limit, micropores are not excessively formed and the pore size is not too small, so that the air permeability can be reduced. In the present disclosure, the cold stretching ratio is represented by the following formula:
Cold stretch ratio (%)=((length of resin film after cold stretching/length of resin film before cold stretching)−1)×100

The stretching speed of the resin film in the first stretching step is preferably 10%/min or more, more preferably 50%/min or more, and preferably 1000%/min or less, more preferably 600%/min or less. When the stretching speed is equal to or higher than the lower limit, micropores are likely to be uniformly formed in the amorphous portion between the lamellae, and when the stretching speed is equal to or lower than the upper limit, breakage of the resin film can be suppressed. In this disclosure, the stretching speed of the resin film refers to the rate of change in the dimension of the resin film in the stretching direction per unit time.

The method for stretching the resin film in the first stretching step is not particularly limited as long as it can stretch the resin film uniaxially. For example, a method of stretching the resin film at a predetermined temperature using a uniaxial stretching device can be used.

Next, the resin film after the uniaxial stretching in the first stretching step is preferably subjected to a second stretching step (i.e., the first stretching step as cold stretching and the second stretching step as hot stretching) so that the ambient temperature inside the device is higher than the ambient temperature during uniaxial stretching in the first stretching step and is 1°C to 60°C lower than the melting point of the resin film (the resin film with the lowest melting point in the case of a multilayer structure) (in the first embodiment), or 1°C to 40°C lower than the melting point of the resin film (the resin film with the lowest melting point in the case of a multilayer structure) (i.e., the first stretching step as cold stretching and the second stretching step as hot stretching are performed). In the second stretching step, the resin film is also preferably uniaxially stretched only in the machine direction. In this way, by performing a stretching process on the resin film at an ambient temperature higher than the ambient temperature in the device in the first stretching step, it is possible to grow a large number of micropores formed in the resin film in the first stretching step. When the temperature is equal to or higher than the above lower limit, the micropores formed in the resin film in the first stretching step are likely to grow, and the air permeability of the resulting separator for a storage device can be reduced. If the temperature is below the upper limit, the micropores formed in the resin film in the first stretching step are less likely to be blocked, and the air permeability of the resulting separator for the electricity storage device can be reduced.

The lower limit of the stretching ratio of the resin film in the second stretching step (hereinafter also referred to as "hot stretching ratio") is preferably 160% or more, more preferably 170% or more, and even more preferably 180% or more. When the MFR of the microporous layer (A) mainly composed of polypropylene is as low as 0.90 g/10 min or less, the molecular orientation of the microporous layer (B) mainly composed of polyethylene is reduced, so that the openness is significantly reduced, and the porosity and air permeability of the microporous layer (B) and the separator substrate tend to be reduced. This is a phenomenon specific to a multilayer separator obtained by a dry method and a coextrusion process, which has a multilayer structure of a microporous layer mainly composed of polypropylene and a microporous layer mainly composed of polyethylene. When the hot stretching ratio is equal to or more than the above lower limit, the micropores formed in the resin film during cold stretching tend to grow easily, and the air permeability of the obtained separator for a storage battery device can be reduced. The upper limit of the stretching ratio of the resin film in the second stretching step is preferably 300% or less, more preferably 260% or less, and even more preferably 220% or less. When the hot stretching ratio is equal to or less than the above upper limit, micropores formed in the resin film during cold stretching are less likely to be blocked, and the air permeability of the resulting separator for a power storage device can be reduced. In the present disclosure, the hot stretching ratio is represented by the following formula:
Hot stretch ratio (%) = ((length of resin film after hot stretching/length of resin film before hot stretching) - 1) x (cold stretch ratio + 100)

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

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

After the stretching step (preferably after uniaxial stretching in the second stretching step), the resin film may be subjected to a heat-relaxing step in which residual stress is relaxed by heating. Residual stress may be generated in the resin film due to stretching in the second stretching step. The heat-relaxing step is performed to relax the residual stress and suppress thermal shrinkage of the resulting resin microporous layer due to heating separate from the heat-relaxing step, thereby improving the safety of the resulting separator for an electricity storage device. The heat-relaxing step can be performed using a tenter or a roll stretching machine.

As described above, in order to improve the dimensional stability of the resin microporous layer when it is heated, it is necessary to relax the residual stress in the resin film. To achieve this, it is preferable that the ambient temperature in the apparatus in the heat relaxation step is 40°C lower or higher, or 20°C lower or higher than the melting point of the resin film (the resin film with the lowest melting point in the case of a multi-layer structure). In addition, from the viewpoint of suppressing the blocking of the micropores formed in the stretching step, the above temperature is preferably 1°C or more, or 4°C or more lower than the melting point of the resin film (the resin film with the lowest melting point in the case of a multi-layer structure).

The lower limit of the heat shrinkage rate of the resin film in the heat relaxation step (hereinafter also referred to as "heat relaxation ratio") is preferably 25% or more, more preferably 30% or more. When the heat relaxation ratio is equal to or more than the above lower limit, the residual stress of the resin film is sufficiently relaxed, the dimensional stability of the obtained resin microporous layer during heating is good, and the safety of the lithium ion secondary battery or other electric storage device at high temperatures is good. In addition, the upper limit of the heat relaxation ratio is preferably 80% or less, more preferably 60% or less, and even more preferably 50% or less. When the heat relaxation ratio is equal to or less than the above upper limit, sagging is unlikely to occur in the resin film, and poor winding or deterioration of uniformity of the roll is suppressed. In the present disclosure, the heat relaxation ratio is represented by the following formula:
Heat relaxation ratio (%)=((length of resin film after heat relaxation process/length of resin film before heat relaxation process)−1)×(cold stretch ratio+hot stretch ratio+100)

In the heat stretching and heat relaxation processes, it is preferable to adjust the stretching speed and conveying speed so that they are not excessively high, in order to sufficiently relax the stress and reduce the heat relaxation ratio.

After the heat-relaxing step, it is preferable to apply heat relaxation again at a temperature equal to or higher than the temperature of the heat-relaxing step and 20°C higher or lower than the temperature of the heat-relaxing step. By applying this step, it is possible to obtain a separator for an electricity storage device with better heat shrink properties.

By adopting the manufacturing conditions of the present invention described above, it is possible to solve the problem of using high Mw PP resin when producing a multi-layer separator including a PE layer by co-extrusion, specifically, the problem that "the difference in melt viscosity between the PP layer and the PE layer becomes large, causing poor film formation of the PE layer and a significant decrease in porosity," and provide a separator that is thin and simultaneously satisfies both air permeability and puncture strength.

The obtained separator substrate can be used as it is as a separator for an electricity storage device. Optionally, a further layer such as a coating layer may be provided on one or both sides of the separator substrate, and a surface treatment such as a corona treatment may be applied as necessary.

<Electricity storage device>
The power storage device of the present disclosure includes the separator for the power storage device of the present disclosure. The power storage device of the present disclosure preferably has a positive electrode and a negative electrode, and the separator for the power storage device of the present disclosure is preferably disposed between the positive electrode and the negative electrode.

Examples of power storage devices 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-hydrogen 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.

The electricity storage device can be produced, for example, by stacking positive and negative electrodes with the separator described above between them and winding them as necessary to form a laminated electrode body or a wound electrode body, which is then loaded into an exterior body, connecting the positive and negative electrodes to the positive and negative electrode terminals of the exterior body via a lead body 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 exterior body, and then sealing the exterior body.

The present power storage device is more preferably a lithium ion secondary battery, and preferred embodiments of the lithium ion secondary battery are described here.

The positive electrode is not particularly limited as long as it acts as a positive electrode of a lithium ion secondary battery, and a known one can be used. The positive electrode preferably contains one or more selected from the group consisting of materials capable of absorbing and releasing lithium ions as a positive electrode active material. From the viewpoint of battery capacity and safety, the positive electrode is preferably a lithium cobalt oxide represented by LiCoO ₂ , a spinel lithium manganese oxide represented by Li ₂ Mn ₂ O ₄ , a spinel lithium nickel manganese oxide represented by Li ₂ Mn _1.5 Ni _0.5 O ₄ , a lithium nickel oxide represented by LiNiO ₂ , a lithium-containing composite metal oxide represented by LiMO ₂ (M represents two or more elements selected from the group consisting of Ni, Mn, Co, Al and Mg), and a lithium iron phosphate compound represented by LiFePO ₄ . Among these, from the viewpoint of high safety and long-term stability, more preferred are lithium cobalt oxides represented by _LiCoO2 , lithium nickel oxides represented by _LiNiO2 , lithium-containing composite metal oxides represented by _LiMO2 (M represents two or more elements selected from the group consisting of Ni, Mn, Co, Al, and Mg), and lithium iron phosphate compounds represented by _LiFePO4 , with the lithium iron phosphate compound represented by _LiFePO4 being particularly preferred.

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

Measurement and evaluation methods
[Measurement of Melt Flow Rate (MFR)]
The melt flow rate (MFR) of the microporous layer (A) was measured in accordance with JIS K 7210 under conditions of a temperature of 230°C and a load of 2.16 kg (unit: g/10 min). The melt flow rate (MFR) of the microporous layer (B) was measured in accordance with JIS K 7210 under conditions of a temperature of 190°C and a load of 2.16 kg. The MFR of the polypropylene was measured in accordance with JIS K 7210 under conditions of a temperature of 230°C and a load of 2.16 kg. The melt flow rate (MFR) of the polyethylene was measured in accordance with JIS K 7210 under conditions of a temperature of 190°C and a load of 2.16 kg.

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

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

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

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

[Air permeability (sec/100cm ³ )]
The air permeability (sec/100 cm ³ ) of the separator substrate was measured using a Gurley air permeability meter conforming to JIS P-8117. The air permeability (sec/100 cm ³ ) was expressed as a function of the separator thickness (μm). This was divided and multiplied by 8 μm to obtain the air permeability (sec/100 cm ³ ) when the thickness of the separator substrate was converted to 8 μm.

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

[Puncture strength]
A needle with a hemispherical tip with a radius of 0.5 mm was prepared, and the separator was sandwiched between two plates with an opening with a diameter (dia.) of 11 mm, and the needle, separator, and plates were set. A puncture test was performed using "MX2-50N" manufactured by Imada Co., Ltd. under the conditions of a needle tip curvature radius of 0.5 mm, an opening diameter of the separator holding plate of 11 mm, and a puncture speed of 25 mm/min. The needle and the separator were brought into contact, and the maximum puncture load (i.e., puncture strength (gf)) was measured. The puncture strength (gf) was divided by the separator thickness (μm) and multiplied by 8 μm to obtain the puncture strength (gf) when the thickness of the separator substrate was converted to 8 μm.

[Area average long pore diameter (nm)]
The area average long pore diameter was measured by image analysis of cross-sectional SEM observation. As a pretreatment, the separator was stained with ruthenium, and a cross-sectional sample was prepared by freeze fracturing. The cross section was taken along the MD-ND plane. The sample was fixed to a SEM sample stage for cross-sectional observation with a conductive adhesive (carbon-based), dried, and then subjected to a conductive treatment using an osmium coater (HPC-30W, manufactured by Vacuum Device Co., Ltd.) with the applied voltage adjustment knob set to setting 4. Then, osmium coating was performed under the conditions of 0.5 and discharge time of 0.5 seconds to prepare a microscopic specimen. Next, a scanning electron microscope (Hitachi High-Technologies Corporation, S-4800) was used to examine each micropore of the microporous membrane. Eight arbitrary points on the layer cross section were observed under conditions of an accelerating voltage of 1 kV, a detection signal LA of 10, a working distance of 5 mm, and a magnification of 30,000 times.

The observed image was trimmed using a function of OpenCV, an image analysis library, in a programming language Python environment so that only the cross section of one microporous layer was included, and the surface, outside, and other microporous layers were removed. The image was then binarized using the Otsu method to separate the resin part from the pores, and the average major axis of the pores was calculated. At this time, holes that existed between the photographed range and outside the photographed range and had a pore area of 0.001 ^nm2 or less were excluded from the measurement. The average diameter was calculated from the area of each hole by area averaging.

Example 1
[Preparation of microporous layer]
As the resin for the microporous layer (A), 100% by weight of polypropylene resin (MFR (230°C) = 0.30 g/10 min, density = 0.91 g/ ^cm3 ) was melted in a 2.5-inch extruder and fed to both outer layers of the two-kind, three-layer co-extrusion inflation die at a discharge rate of 6 kg/h using a gear pump. As the resin for the microporous layer (B), polyethylene resin (MFR (190°C) = 0.32 g/10 min, density = 0.96 g/ ^cm3 ) was melted in a 2.5-inch extruder and fed to the inner layer of the two-kind, three-layer co-extrusion inflation die at a discharge rate of 3 kg/h using a gear pump. The temperature of the inflation die was set to 240°C, and the molten polymer was extruded from the inflation die, and then the extruded resin was cooled by blown air while being wound on a roll to obtain a precursor film having an A/B/A layer structure with a thickness of about 14 μm. Here, the lip distance (lip clearance) of the inflation die was set to 1.8 mm. Extrusion was performed at a total extrusion rate of 9 kg/h.

Then, the obtained precursor film was placed in a dryer and annealed at 130°C for 180 minutes. The annealed precursor film was then cold stretched at room temperature to a cold stretch ratio of 30%, and the stretched film was placed in an oven at 120°C without shrinking and hot stretched to a hot stretch ratio of 180%, and then heat relaxed in an oven at 130°C to a heat relaxation ratio of 40%, to obtain a separator substrate having a three-layer structure consisting of A/B/A layers. The structure and physical properties of the obtained separator substrate are shown in Table 1.

Examples 2 to 8 and Comparative Examples 1 to 8
A microporous membrane was obtained in the same manner as in Example 1, except that the raw materials, extrusion amount conditions, precursor film thickness, temperature conditions in the annealing step, and cold stretch ratio and hot stretch ratio conditions in the stretching step were changed as shown in Table 1, and the resulting separator was evaluated.

From Table 1, it can be seen that by strictly controlling the composition of each layer and strictly controlling the annealing and stretching conditions, it is possible to control the porosity of the separator substrate even when the MFR of the microporous layer (A) mainly composed of polypropylene is as low as 0.90 g/10 min or less, and that such a separator configuration makes it possible to achieve both a thin film, air permeability, and pin puncture strength.
<Explanation of abbreviations in Table 1>
MFR: Melt flow rate PP: Polypropylene PE: Polyethylene Mw: Weight average molecular weight MwD: Molecular weight distribution (weight average molecular weight (Mw) divided by number average molecular weight (Mn) (Mw/Mn))

The separator for an electricity storage device disclosed herein can be suitably used as a separator for an electricity storage device, such as a lithium ion secondary battery.

Claims (10)

A separator for an electricity storage device having a multilayer structure of a microporous layer (A) mainly composed of polypropylene and a microporous layer (B) mainly composed of polyethylene,
At least one of the microporous layers (A) constitutes an outermost layer on at least one surface of the separator substrate,
the melt flow rate (MFR) of the microporous layer (A) is 0.90 g/10 min or less when measured under a load of 2.16 kg and at a temperature of 230° C.;
The separator for an electricity storage device, wherein the separator substrate has a thickness of 15 μm or less, and a porosity of 40% or more. The separator for an electricity storage device according to claim 1, wherein, in MD-ND cross-sectional observation of the microporous layer (A) using a scanning electron microscope (SEM), the area-average long pore diameter of the pores present in the microporous layer (A) is 150 nm or less. 3. The separator for an electricity storage device according to claim 1, wherein the separator has an air permeability of 200 sec/100 ^cm3 or less when converted to a thickness of 8 μm. The separator for an electric storage device according to claim 3, wherein the area average long pore diameter of the pores present in the microporous layer (A) is 100 nm or more when the microporous layer (A) is observed in an MD-ND cross section using a scanning electron microscope (SEM). The separator for an electricity storage device according to claim 1 or 2, wherein the thickness of the microporous layer (B) is 5 μm or less. The separator for an electricity storage device according to claim 1 or 2, wherein the MFR of the microporous layer (A) is 0.3 g/10 min or more when measured under a load of 2.16 kg and at a temperature of 230°C. The separator for an electric storage device according to claim 1 or 2, wherein the thickness of the separator substrate is 9.5 μm or less. 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. A method for producing the separator for an electricity storage device according to claim 1, comprising the following steps (1) to (3):
Step (1): The stretching ratio (cold stretching ratio) of the resin film in the first stretching step is 10% or more;
step (2): the stretching ratio (hot stretching ratio) of the resin film in the second stretching step is 160% or more; and step (3): the annealing temperature in the annealing step is 125° C. or more. The method for producing a separator for an electric storage device according to claim 9, wherein the microporous layer (A) and the microporous layer (B) are co-extruded.