JP2022142698A - Separator for power storage device and power storage device - Google Patents

Abstract

To provide a separator for a power storage device that suppresses clogging and has excellent thermal stability.SOLUTION: A separator for a power storage device includes a separator substrate having a microporous layer (A) mainly composed of polypropylene and a microporous layer (B) mainly composed of polypropylene. The area-average long pore diameter in the ND-MD cross section of the microporous layer (A) is 1.1 to 5.0 times the area-average long pore diameter in the ND-MD cross section of the microporous layer (B). The microporous layer (A) constitutes the outermost layer on at least one side of the separator substrate.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present disclosure relates to separators for power storage devices and the like.

Microporous membranes, especially polyolefin-based microporous membranes, are used in many technical fields such as microfiltration membranes, battery separators, condenser separators, and fuel cell materials. It is used as a battery separator. Lithium ion batteries are used in various applications such as small electronic devices such as mobile phones and notebook personal computers, and electric vehicles including hybrid vehicles and plug-in hybrid vehicles.

In recent years, there has been a demand for lithium-ion batteries with high energy capacity, high energy density, and high output characteristics.In line with this, there is a demand for separators that are thin films, have excellent battery performance, and are excellent in battery reliability and safety. is rising.

For example, U.S. Pat. No. 5,300,003 discloses multilayer microporous thin films or membranes that can exhibit improved properties, including improved dielectric breakdown and strength, compared to conventional single-layer or tri-layer microporous membranes of the same thickness. is described. A preferred multilayer microporous membrane comprises a microlayer and one or more laminated barriers.

Cited Document 2 discloses a separator for an electric storage device having a microporous film containing polyolefin as a main component. It describes a power storage device separator, wherein the microporous membrane has a melt flow rate (MFR) of 0.9 g/10 min or less when measured at 16 kg and a temperature of 230°C.

WO2018/089748 WO2020/196120

There is a problem that deposits generated due to cycle deterioration cause clogging of the separator and shorten the cycle life. In addition, with the increase in size of batteries, there is a demand for separators that are excellent in air permeability and dimensional stability even after being exposed to high temperatures.

Accordingly, an object of the present disclosure is to provide an electricity storage device separator that suppresses clogging and has excellent thermal stability.

Examples of embodiments of the present disclosure are listed in items [1] to [10] below.
[1]
Having a separator substrate having a microporous layer (A) containing polypropylene as a main component and a microporous layer (B) containing polypropylene as a main component,
The area-average long pore diameter in the ND-MD cross section of the microporous layer (A) is 1.1 to 5.0 times the area-average long pore diameter in the ND-MD cross section of the microporous layer (B). can be,
A separator for an electric storage device, wherein the microporous layer (A) constitutes the outermost layer on at least one side of the separator substrate.
[2]
The power storage device separator according to item 1, wherein the pores of the microporous layer (A) have an area-average long pore diameter in the ND-MD cross section of 100 nm or more and 600 nm or less.
[3]
3. The power storage device separator according to item 1 or 2, wherein the separator base material has a heat 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.
[4]
4. The power storage device separator according to any one of items 1 to 3, wherein the microporous layer (A) constitutes the outermost layers on both sides of the separator base material.
[5]
The electricity storage device according to any one of items 1 to 4, wherein the separator substrate includes a structure in which a microporous layer (A), a microporous layer (B), and a microporous layer (A) are laminated in this order. separator for
[6]
6. The power storage device separator according to any one of items 1 to 5, wherein the separator base material further has a microporous layer (C) containing polyolefin as a main component.
[7]
The area-average long pore diameter in the ND-MD cross section of the microporous layer (C) is 0.2 to 0.9 times the area-average long pore diameter in the ND-MD cross section of the microporous layer (B). 7. The power storage device separator according to item 6.
[8]
The electricity storage according to any one of items 1 to 7, wherein the separator base material includes a structure in which a microporous layer (A), a microporous layer (B), and a microporous layer (C) are laminated in this order. Device separator.
[9]
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 8.
[10]
The electricity storage device according to item 9, wherein the microporous layer (A) is arranged to face the negative electrode side.

According to the present disclosure, a power storage device separator that suppresses clogging and has excellent thermal stability is provided.

《Separators for power storage devices》
The power storage device separator of the present disclosure has a separator base material having a microporous layer (A) containing polypropylene as a main component and a microporous layer (B) containing polypropylene as a main component. The separator substrate is further coated on the microporous layer (A) and/or the microporous layer (B) (also referred to as a “surface layer”, “coating layer”, etc., hereinafter simply referred to as “coating layer”). .). In the present specification, the term “microporous layer” means each microporous layer that constitutes the base material of the separator, and the term “separator base material” means the base material of the separator excluding any coating layer. However, the “separator” means the entire separator including any coating layer.

<Microporous layer (A)>
The power storage device separator of the present disclosure has a microporous layer (A). The power storage device separator may have only one microporous layer (A), or may have two or more layers. At least one layer of the microporous layer (A) constitutes the outermost layer on at least one side of the separator substrate. When the power storage device separator has two or more microporous layers (A), the microporous layers (A) may form the outermost layers on both sides of the separator substrate. The microporous layer (A) is mainly composed of polypropylene, which allows good battery performance to be maintained even after storage at high temperature (130° C.). In the present specification, "mainly comprising 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 content of polypropylene in the microporous layer (A) is 50% by mass or more, preferably 55% by mass or more, 60% by mass or more, from the viewpoint of wettability of the separator, thinning, and shutdown characteristics. , 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more. The upper limit of the content of polypropylene in the microporous layer (A) is not limited, but is, 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 100% by mass.

<Material of microporous layer (A)>
The microporous layer (A) is mainly composed of polypropylene. The polypropylene of the microporous layer (A) may be the same material as the polypropylene of the microporous layer (B) and the microporous layer (C), which will be described later, or a different polypropylene in chemical structure, more specifically, The polypropylenes may differ in at least one of the monomer composition, stereoregularity, molecular weight, crystal structure, and the like.

Stereoregularity of polypropylene includes, but is not limited to, atactic, isotactic, or syndiotactic homopolymers. Polypropylenes according to the present disclosure are preferably isotactic or syndiotactic highly crystalline homopolymers.

The polypropylene of the microporous layer (A) is preferably a homopolymer, and may be a copolymer, such as a block polymer, copolymerized with a small amount of comonomer other than propylene, such as an α-olefin comonomer. The amount of propylene structures contained as repeating units in 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 a comonomer other than the propylene structure contained in the polypropylene is not limited, but is, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. can be Polypropylene can be used individually by 1 type or in mixture of 2 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, etc., and the pore size of the microporous layer is increased to avoid clogging. It is preferably 1,500,000 or less from the viewpoint of Mw of polypropylene as polypropylene is more preferably 500,000 or more and 1,300,000 or less, still more preferably 600,000 or more and 1,100,000 or less, still more preferably 700,000 or more, 1,050,000 or less, 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 7 or less, more preferably 6.5. less than or equal to 6, less than or equal to 5.5, or less than or equal to 5. The smaller the Mw/Mn value of polypropylene, the smaller the melt tension of the resulting microporous layer. Therefore, it is preferable that the Mw/Mn value of polypropylene is 7 or less in order to control the melt tension of the microporous layer (A) to 30 mN or less. Also, Mw/Mn may be preferably 1 or more, for example 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. When Mw/Mn is 1 or more, moderate entanglement of molecules is maintained, and stability during film formation may be improved. The weight-average molecular weight, number-average molecular weight, and Mw/Mn of the polyolefin of the present disclosure are polystyrene-equivalent molecular weights obtained by GPC (gel permeation chromatography) measurement.

The density of the polypropylene of the microporous layer (A) 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. good. Density of polypropylene 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, 0.98 g/cm 3 or less, It may be 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 polyolefin is related to the crystallinity of polypropylene, and a polypropylene density of 0.85 g/cm ³ or more improves the productivity of the microporous layer, which is particularly advantageous in dry processes.

The microporous layer (A) may contain other resins as long as the main component is polypropylene. Other resins include, for example, polyolefins other than polypropylene (also referred to as "other polyolefins") and copolymers of polystyrene and polyolefins. A polyolefin is a polymer 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 having a carbon-carbon double bond, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and the like. Polyolefins are, for example, homopolymers, copolymers, multistage polymers, etc., preferably homopolymers. Specifically, polyethylene is preferable as the polyolefin from the viewpoint of shutdown characteristics and the like. Preferred copolymers of polystyrene and polyolefin include styrene-(ethylene-propylene)-styrene copolymer (SEPS), styrene-(ethylene-butene)-styrene copolymer, and styrene-ethylene-styrene copolymer. can be raised. Particularly preferred is styrene-(ethylene-propylene)-styrene copolymer (SEPS).

The weight average molecular weight (Mw) of other polyolefins is preferably 300,000 or more from the viewpoint of the strength of the microporous layer, etc. 1,500,000 or less is preferred. Mw of the polyolefin is more preferably 500,000 or more and 1,300,000 or less, still more preferably 600,000 or more and 1,100,000 or less, still more preferably 700,000 or more and 1,000 ,000 or less, particularly preferably 800,000 or more and 960,000 or less.

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 other polyolefins is preferably 7 or less, more preferably 6.5 or less, 6 or less. 5 or less, or 5 or less. As the value of Mw/Mn becomes smaller, the entanglement between molecules becomes less, so the melt tension of the obtained microporous layer tends to become smaller. Therefore, it is preferable that the Mw/Mn value of the polyolefin of the microporous layer (A) is 7 or less because the melt tension of the microporous layer (A) can be controlled to be low and the thickness of the microporous layer (A) can be made thinner. Also, Mw/Mn of the polyolefin of the microporous layer (A) may be preferably 1 or more, for example 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. When Mw/Mn is 1 or more, moderate entanglement of molecules is maintained, and stability during film formation may be improved.

<Melt flow rate (MFR) of microporous layer (A)>
The upper limit of the melt flow rate (MFR) (MFR of a single layer) of the microporous layer (A) is preferably 4.0 g/10 minutes or less from the viewpoint of obtaining a higher-strength microporous layer (A). It may be 3.0 g/10 min or less, 2.0 g/10 min or less, 1.5 g/10 min or less, or 1.1 g/10 min or less. The lower limit of the MFR (single layer MFR) of the microporous layer (A) is not limited from the viewpoint of the moldability of the microporous layer (A). It may be 10 minutes or more, 0.4 g/10 minutes or more, 0.45 g/10 minutes or more, or 0.5 g/10 minutes 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.

That the MFR of the microporous layer (A) is 4.0 g/10 min or less means that the polyolefin contained in the microporous layer (A) has a somewhat high molecular weight. When the polyolefin has a high molecular weight, the number of tie molecules that bind crystalline particles together increases, so there is a tendency to obtain a high-strength microporous layer (A). When the MFR of the microporous layer (A) is 0.3 g/10 minutes or more, the melt tension of the microporous layer (A) does not become too low, and a high-strength and thin microporous layer can be obtained more easily.

From the viewpoint of obtaining a high-strength microporous layer (A), the MFR of the polypropylene of the microporous layer (A) is 0.3 to 4.0 when measured under conditions of a load of 2.16 kg and a temperature of 230°C. 0 g/10 min is preferred. From the viewpoint of obtaining a higher-strength microporous layer, the upper limit of MFR of polypropylene is, for example, 3.0 g/10 min or less, 2.0 g/10 min or less, 1.5 g/10 min or less, or 1.1 g /10 minutes or less. The lower limit of the MFR of polypropylene is not limited, but from the viewpoint of moldability of the microporous layer (A), for example, 0.3 g/10 min or more, 0.35 g/10 min or more, 0.4 g/10 min or more. , or 0.45 g/10 minutes or more.

<Pentad fraction of microporous layer (A)>
The lower limit of the pentad fraction of polypropylene in the microporous layer (A) is preferably 94.0% or more, for example, 95.0% or more, 96.0%, from the viewpoint of obtaining a microporous layer with low air permeability. 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 polypropylene is not limited, but may be 99.9% or less, 99.8% or less, or 99.5% or less. The pentad fraction of polypropylene is measured by ¹³ C-NMR (nuclear magnetic resonance spectroscopy).

A polypropylene having a pentad fraction of 94.0% or more indicates that the polypropylene has high crystallinity. In the separator obtained by the stretch opening method, especially the dry method, the pores are opened by stretching the amorphous portion between the crystalline layers. Since the air permeability can be kept low, it is possible to increase the output of the battery.

<Area average long pore diameter of microporous layer (A)>
The area-average long pore diameter in the ND-MD cross section of the microporous layer (A) (hereinafter also simply referred to as "area-average long pore diameter") is the area-average long pore diameter of the microporous layer (B) described later, 1. It is 1 time or more and 5.0 times or less. In the specification of the present application, "ND" indicates the thickness direction of the microporous layer, and "MD" indicates the film formation direction of the microporous layer. For example, the MD direction of a separator having a microporous layer is the longitudinal direction if it is a roll. A "long pore diameter" means a pore diameter in the MD direction. Further, when there are two or more microporous layers (A) and / or microporous layers (B), the microporous layer (A) and the microporous layer (B ) are compared.

That the area-average long pore diameter of the microporous layer (A) is 1.1 times or more that of the microporous layer (B) means that the microporous layer (A), which is the outermost layer of the separator substrate, is the microporous layer It means a microporous layer with a pore size larger than that of (B). When the area-average long pore diameter of the microporous layer (A) is 1.1 times or more that of the microporous layer (B), both clogging reduction and short-circuit suppression can be achieved. More specifically, the microporous layer (A), which is the outermost layer, has a large pore diameter, so that clogging of the separator due to deposits in battery evaluation (cycle test) can be reduced, and the microporous layer located in the inner layer It is considered that the layer (B) having a small pore diameter can suppress short circuits caused by contamination by foreign matter. Further, it is considered that the strength of the separator can be sufficiently ensured by setting the area-average long pore diameter of the microporous layer (A) to be 5 times or less that of the microporous layer (B). The area-average long pore diameter of the microporous layer (A) is preferably 1.2 to 3 times that of the microporous layer (B).

The area-average long pore size of the microporous layer (A) in the ND-MD cross section is preferably 100 nm or more and 600 nm or less. When the area-average long pore diameter of the microporous layer (A) is 100 nm or more, clogging of the separator due to deposits in the electric storage device can be more effectively reduced. Strength can be further improved. In a preferred embodiment, it is more important to reduce clogging due to deposits in the electricity storage device, and for that purpose, it is more important that the microporous layer (A) has an area-average long pore diameter of 100 nm or more. . The area-average long pore size of the microporous layer (A) in the ND-MD cross section is more preferably 120 nm or more and 550 nm or less, still more preferably 130 nm or more and 500 nm or less, and even more preferably 140 nm or more and 450 nm or less.

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 image analysis of a range of 20 μm in the MD direction×3 μm in the ND direction from the obtained image. Detailed conditions are shown in Examples. When measuring the average pore size from the cross-sectional SEM image, the number average pore size and the area average pore size can be calculated. It is difficult to obtain a sufficient correlation with the physical properties of Therefore, in the specification of the present application, the area-average pore size is used as the average pore size so that the correlation with the physical properties of the separator can be obtained.

<Porosity of microporous layer (A)>
The porosity of the microporous layer (A) is preferably 20% or more from the viewpoint of avoiding clogging in the electricity storage device and obtaining good air permeability of the separator, and 70% from the viewpoint of maintaining the strength of the separator. The following are preferable. The porosity of the microporous layer (A) is more preferably 25% or more and 65% or less, still more preferably 30% or more and 60% or less, and particularly preferably 35% or more and 55% or less.

<Thickness of microporous layer (A)>
The thickness of the microporous layer (A) is preferably 10 μm or less, for example, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm or less, or 4 μm or less, from the viewpoint of increasing the energy density of the electric storage device. you can The lower limit of the thickness of the microporous layer (A) is 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 and the like.

<Additive for microporous layer (A)>
The microporous layer (A) containing polypropylene as a main component may further contain additives such as elastomers, crystal nucleating agents, antioxidants, fillers, etc., in addition to polypropylene, if necessary. The amount of the additive is not particularly limited. It may be 10% by mass or less or 7% by mass or less.

<Microporous layer (B)>
The power storage device separator of the present disclosure has a microporous layer (B). The power storage device separator may have only one microporous layer (B), or may have two or more layers. The microporous layer (B) also contains polypropylene as a main component, which allows good battery performance to be maintained even after storage at high temperature (130° C.). In the present specification, "mainly comprising polypropylene" means that the microporous layer (B) contains 50% by mass or more of polypropylene based on the total mass of the microporous layer (B). The lower limit of the content of polypropylene in the microporous layer (B) is preferably 55% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass, or % by mass or more, 90% by mass or more, or 95% by mass or more. The upper limit of the polypropylene content in the microporous layer (B) is not limited, but is, 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 100% by mass.

<Material of microporous layer (B)>
The microporous layer (B) is mainly composed of polypropylene. The polypropylene of the microporous layer (B) may be the same material as the polypropylene of the microporous layer (A) and the microporous layer (C) to be described later, or may be a different polypropylene in chemical structure, more specifically, The polypropylenes may differ in at least one of the monomer composition, stereoregularity, molecular weight, crystal structure, and the like.

The stereoregularity of the polypropylene of the microporous layer (B) is not limited, but examples thereof include atactic, isotactic, and syndiotactic homopolymers. Polypropylenes according to the present disclosure are preferably isotactic or syndiotactic highly crystalline homopolymers.

The polypropylene of the microporous layer (B) is preferably a homopolymer, and may be a copolymer, such as a block polymer, copolymerized with a small amount of comonomer other than propylene, such as an α-olefin comonomer. The amount of propylene structures contained as repeating units in 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 a comonomer other than the propylene structure contained in the polypropylene is not limited, but is, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. can be Polypropylene can be used individually by 1 type or in mixture of 2 or more types.

The weight average molecular weight (Mw) of the polypropylene of the microporous layer (B) is preferably 300,000 or more from the viewpoint of the strength of the microporous layer, etc., and the pore size of the microporous layer is increased to avoid clogging. It is preferably 1,500,000 or less from the viewpoint of Mw of polypropylene as polypropylene is more preferably 500,000 or more and 1,300,000 or less, still more preferably 600,000 or more and 1,100,000 or less, still more preferably 700,000 or more, 1,050,000 or less, 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 (B) by the number average molecular weight (Mn) is preferably 7 or less, more preferably 6.5. less than or equal to 6, less than or equal to 5.5, or less than or equal to 5. The smaller the Mw/Mn value of polypropylene, the smaller the melt tension of the resulting microporous layer. Therefore, it is preferable that the Mw/Mn value of polypropylene is 7 or less in order to control the melt tension of the microporous layer (B) to 30 mN or less. Also, Mw/Mn may be preferably 1 or more, for example 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. When Mw/Mn is 1 or more, appropriate molecular entanglement is maintained, and stability during film formation may be improved. The weight-average molecular weight, number-average molecular weight, and Mw/Mn of the polyolefin of the present disclosure are polystyrene-equivalent molecular weights obtained by GPC (gel permeation chromatography) measurement.

The density of polypropylene in 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. good. Density of polypropylene 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, 0.98 g/cm 3 or less, It may be 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 polyolefin is related to the crystallinity of polypropylene, and a polypropylene density of 0.85 g/cm ³ or more improves the productivity of the microporous layer, which is particularly advantageous in dry processes.

The microporous layer (B) may contain other resins as long as the main component is polypropylene. Other resins include, for example, polyolefins other than polypropylene (also referred to as “other polyolefins”). A polyolefin is a polymer 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 having a carbon-carbon double bond, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and the like. Polyolefins are, for example, homopolymers, copolymers, multistage polymers, etc., preferably homopolymers. Specifically, polyethylene is preferable as the polyolefin from the viewpoint of shutdown characteristics and the like.

The weight average molecular weight (Mw) of other polyolefins is preferably 300,000 or more from the viewpoint of the strength of the microporous layer, etc. 1,500,000 or less is preferred. Mw of the polyolefin is more preferably 500,000 or more and 1,300,000 or less, still more preferably 600,000 or more and 1,100,000 or less, still more preferably 700,000 or more and 1,000 ,000 or less, particularly preferably 800,000 or more and 960,000 or less.

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 other polyolefins is preferably 7 or less, more preferably 6.5 or less, 6 or less. 5 or less, or 5 or less. As the value of Mw/Mn becomes smaller, the entanglement between molecules becomes less, so the melt tension of the obtained microporous layer tends to become smaller. Therefore, it is preferable that the Mw/Mn value of the polyolefin of the microporous layer (B) is 7 or less, because the melt tension of the microporous layer (B) can be controlled to be low and the thickness can be made thinner. Also, Mw/Mn of the polyolefin of the microporous layer (B) may be preferably 1 or more, for example 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. When Mw/Mn is 1 or more, moderate entanglement of molecules is maintained, and stability during film formation may be improved.

<Melt flow rate (MFR) of microporous layer (B)>
The upper limit of the melt flow rate (MFR) (MFR of a single layer) of the microporous layer (B) is preferably 1.5 g/10 minutes or less from the viewpoint of obtaining a higher-strength microporous layer (B). It may be 1.4 g/10 min or less, 1.3 g/10 min or less, 1.2 g/10 min or less, or 1.1 g/10 min or less. The lower limit of the MFR (single layer MFR) of the microporous layer (B) is not limited from the viewpoint of the moldability of the microporous layer (B), but is, for example, 0.2 g/10 minutes or more, 0.25 g/ It may be 10 minutes or more, 0.3 g/10 minutes or more, 0.35 g/10 minutes or more, or 0.4 g/10 minutes 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.

The fact that the MFR of the microporous layer (B) is 1.5 g/10 minutes or less means that the polyolefin contained in the microporous layer (B) has a somewhat high molecular weight. When the polyolefin has a high molecular weight, the number of tie molecules that bind crystalline particles together increases, so there is a tendency to obtain a high-strength microporous layer (B). When the MFR of the microporous layer (B) is 0.2 g/10 minutes or more, the melt tension of the microporous layer (B) does not become too low, and a high-strength and thin microporous layer can be obtained more easily.

From the viewpoint of obtaining a high-strength microporous layer (B), the MFR of the polypropylene of the microporous layer (B) is 0.2 to 1.0 when measured under conditions of a load of 2.16 kg and a temperature of 230°C. 5 g/10 minutes is preferred. From the viewpoint of obtaining a higher-strength microporous layer, the upper limit of the MFR of polypropylene is, for example, 1.4 g/10 min or less, 1.3 g/10 min or less, 1.2 g/10 min or less, or 1.1 g /10 minutes or less. The lower limit of the MFR of polypropylene is not limited, but from the viewpoint of moldability of the microporous layer (B), for example, 0.25 g/10 min or more, 0.3 g/10 min or more, 0.35 g/10 min or more. , or 0.4 g/10 minutes or more.
The MFR of the microporous layer (B) is preferably lower than the MFR of the microporous layer (A). By making the MFR of the microporous layer (B) lower than the MFR of the microporous layer (A), the pore size of the microporous layer (A) of the obtained separator is controlled to be larger than the pore size of the microporous layer (B). can be done.

<Pentad fraction of microporous layer (B)>
The lower limit of the pentad fraction of polypropylene in the microporous layer (B) is preferably 94.0% or more, for example, 95.0% or more, 96.0%, from the viewpoint of obtaining a microporous layer with low air permeability. 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 polypropylene is not limited, but may be 99.9% or less, 99.8% or less, or 99.5% or less. The pentad fraction of polypropylene is measured by ¹³ C-NMR (nuclear magnetic resonance spectroscopy).

A polypropylene having a pentad fraction of 94.0% or more indicates that the polypropylene has high crystallinity. In the separator obtained by the stretch opening method, especially the dry method, the pores are opened by stretching the amorphous portion between the crystalline layers. Since the air permeability can be kept low, it is possible to increase the output of the battery.

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

The area-average long pore diameter of the ND-MD cross section of the microporous layer (B) is preferably 20 nm or more and 500 nm or less, more preferably 30 nm or more and 450 nm or less, still more preferably 40 nm or more and 400 nm or less, still more preferably 50 nm or more and 350 nm or less. is. When the area-average long pore diameter of the microporous layer (B) is within this range, clogging of the separator can be more effectively reduced and short circuits can be suppressed.

<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 electricity storage device and obtaining good air permeability of the separator, and 70% from the viewpoint of maintaining the strength of the separator. The following are preferable. The porosity of the microporous layer (B) is more preferably 25% or more and 65% or less, still more preferably 30% or more and 60% or less, and particularly preferably 35% or more and 55% or less.

<Thickness of microporous layer (B)>
The thickness of the microporous layer (B) according to the present disclosure is preferably 10 μm or less, for example, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm or less, or It may be 4 μm or less. The lower limit of the thickness of the microporous layer (B) is preferably 1 µm or more, for example, 2 µm or more, 3 µm or more, or 3.5 µm or more, from the viewpoint of strength and the like.

<Additive for microporous layer (B)>
The microporous layer (B) containing polypropylene as a main component may further contain additives such as elastomers, crystal nucleating agents, antioxidants, fillers, etc., in addition to polypropylene, if necessary. The amount of the additive is not particularly limited, but is, 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, based on the total mass of the microporous layer (B). It may be 7% by mass or less or 5% by mass or less.

<Microporous layer (C)>
The power storage device separator of the present disclosure may further have a microporous layer (C) in addition to the microporous layer (A) and the microporous layer (B). In this case, the power storage device separator may have only one microporous layer (C), or may have two or more layers. The microporous layer (C) also contains polypropylene as a main component, which allows good battery performance to be maintained even after storage at high temperature (130° C.). In the present specification, "mainly comprising polypropylene" means that the microporous layer (C) contains 50% by mass or more of polypropylene based on the total mass of the microporous layer (C). The lower limit of the content of polypropylene in the microporous layer (C) is preferably 55% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass, or % by mass or more, 90% by mass or more, or 95% by mass or more. The upper limit of the content of polypropylene in the microporous layer (C) is not limited, but is, 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 100% by mass.

<Material of microporous layer (C)>
The microporous layer (C) is mainly composed of polypropylene. The polypropylene of the microporous layer (C) may be the same material as the polypropylene of the microporous layer (A) and the microporous layer (B), or may be different in chemical structure, more specifically in monomer composition. , stereoregularity, molecular weight, and crystal structure may be different.

The stereoregularity of the polypropylene of the microporous layer (C) is not limited, but examples thereof include atactic, isotactic, and syndiotactic homopolymers. Polypropylenes according to the present disclosure are preferably isotactic or syndiotactic highly crystalline homopolymers.

The polypropylene of the microporous layer (C) is preferably a homopolymer, and may be a copolymer, such as a block polymer, copolymerized with a small amount of comonomer other than propylene, such as an α-olefin comonomer. The amount of propylene structures contained as repeating units in 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 a comonomer other than the propylene structure contained in the polypropylene is not limited, but is, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. can be Polypropylene can be used individually by 1 type or in mixture of 2 or more types.

The weight average molecular weight (Mw) of the polypropylene of the microporous layer (C) is preferably 300,000 or more from the viewpoint of the strength of the microporous layer, etc., and the pore diameter of the microporous layer is increased to avoid clogging. It is preferably 1,500,000 or less from the viewpoint of Mw of polypropylene as polypropylene is more preferably 500,000 or more and 1,300,000 or less, still more preferably 600,000 or more and 1,100,000 or less, still more preferably 700,000 or more, 1,050,000 or less, 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 (C) by the number average molecular weight (Mn) is preferably 7 or less, more preferably 6.5. less than or equal to 6, less than or equal to 5.5, or less than or equal to 5. The smaller the Mw/Mn value of polypropylene, the smaller the melt tension of the resulting microporous layer. Therefore, it is preferable that the Mw/Mn value of polypropylene is 7 or less in order to control the melt tension of the microporous layer (C) to 30 mN or less. Also, Mw/Mn may be preferably 1 or more, for example 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. When Mw/Mn is 1 or more, moderate entanglement of molecules is maintained, and stability during film formation may be improved. The weight-average molecular weight, number-average molecular weight, and Mw/Mn of the polyolefin of the present disclosure are polystyrene-equivalent molecular weights obtained by GPC (gel permeation chromatography) measurement.

Density of the polypropylene of the microporous layer (C) 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. good. Density of polypropylene 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, 0.98 g/cm 3 or less, It may be 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 polyolefin is related to the crystallinity of polypropylene, and a polypropylene density of 0.85 g/cm ³ or more improves the productivity of the microporous layer, which is particularly advantageous in dry processes.

The microporous layer (C) may contain other resins as long as the main component is polypropylene. Other resins include, for example, polyolefins other than polypropylene (also referred to as “other polyolefins”). A polyolefin is a polymer 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 having a carbon-carbon double bond, such as ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and the like. Polyolefins are, for example, homopolymers, copolymers, multistage polymers, etc., preferably homopolymers. Specifically, polyethylene is preferable as the polyolefin from the viewpoint of shutdown characteristics and the like.

The weight average molecular weight (Mw) of other polyolefins is preferably 300,000 or more from the viewpoint of the strength of the microporous layer, etc. 1,500,000 or less is preferred. Mw of the polyolefin is more preferably 500,000 or more and 1,300,000 or less, still more preferably 600,000 or more and 1,100,000 or less, still more preferably 700,000 or more and 1,000 ,000 or less, particularly preferably 800,000 or more and 960,000 or less.

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 other polyolefins is preferably 7 or less, more preferably 6.5 or less, 6 or less. 5 or less, or 5 or less. As the value of Mw/Mn becomes smaller, the entanglement between molecules becomes less, so the melt tension of the obtained microporous layer tends to become smaller. Therefore, it is preferable that the polyolefin Mw/Mn value of the microporous layer (C) is 7 or less because the melt tension of the microporous layer (C) can be controlled to be low and the thickness can be made thinner. Also, Mw/Mn of the polyolefin of the microporous layer (C) may be preferably 1 or more, for example 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. When Mw/Mn is 1 or more, moderate entanglement of molecules is maintained, and stability during film formation may be improved.

<Melt flow rate (MFR) of microporous layer (C)>
The upper limit of the melt flow rate (MFR) (MFR of a single layer) of the microporous layer (C) is preferably 1.5 g/10 minutes or less from the viewpoint of obtaining a higher-strength microporous layer (C). It may be 1.4 g/10 min or less, 1.3 g/10 min or less, 1.2 g/10 min or less, or 1.1 g/10 min or less. The lower limit of the MFR (single layer MFR) of the microporous layer (C) is not limited from the viewpoint of the moldability of the microporous layer (C). It may be 10 minutes or more, 0.3 g/10 minutes or more, 0.35 g/10 minutes or more, or 0.4 g/10 minutes or more. The MFR of the microporous layer (C) is measured under conditions of a load of 2.16 kg and a temperature of 230°C.

That the MFR of the microporous layer (C) is 1.5 g/10 minutes or less means that the polyolefin contained in the microporous layer (C) has a somewhat high molecular weight. When the polyolefin has a high molecular weight, the number of tie molecules that bind crystalline particles together increases, so there is a tendency to obtain a high-strength microporous layer (C). When the MFR of the microporous layer (C) is 0.2 g/10 minutes or more, the melt tension of the microporous layer (C) does not become too low, and a high-strength and thin microporous layer can be obtained more easily.

From the viewpoint of obtaining a high-strength microporous layer (C), the MFR of the polypropylene constituting the microporous layer (C) is 0.2 to 0.2 when measured under conditions of a load of 2.16 kg and a temperature of 230 ° C. It is preferably 1.5 g/10 minutes. From the viewpoint of obtaining a higher-strength microporous layer, the upper limit of the MFR of polypropylene is, for example, 1.4 g/10 min or less, 1.3 g/10 min or less, 1.2 g/10 min or less, or 1.1 g /10 minutes or less. The lower limit of the MFR of polypropylene is not limited, but from the viewpoint of the moldability of the microporous layer (C), it is, for example, 0.25 g/10 min or more, 0.3 g/10 min or more, 0.35 g/10 min or more. , or 0.4 g/10 minutes or more.

<Pentad fraction of microporous layer (C)>
The lower limit of the pentad fraction of polypropylene in the microporous layer (C) is preferably 94.0% or more, for example, 95.0% or more, 96.0%, from the viewpoint of obtaining a microporous layer with low air permeability. 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 polypropylene is not limited, but may be 99.9% or less, 99.8% or less, or 99.5% or less. The pentad fraction of polypropylene is measured by ¹³ C-NMR (nuclear magnetic resonance spectroscopy).

A polypropylene having a pentad fraction of 94.0% or more indicates that the polypropylene has high crystallinity. In the separator obtained by the stretch opening method, especially the dry method, the pores are opened by stretching the amorphous portion between the crystalline layers. Since the air permeability can be kept low, it is possible to increase the output of the battery.

<Area average long pore diameter of microporous layer (C)>
The area-average long pore diameter in the ND-MD cross section of the microporous layer (C) (hereinafter also simply referred to as "area-average long pore diameter") is preferably smaller than the area-average long pore diameter of the microporous layer (B). . Specifically, the area-average long pore diameter of the microporous layer (C) is preferably 0.2 times or more and 0.9 times or less, more preferably 0.5 times the area-average long pore diameter of the microporous layer (B). It is more than 0.9 times and less than 0.9 times. This makes it possible to more effectively reduce clogging of the separator and suppress short circuits. In addition, when the microporous layer (C) and / or the microporous layer (B) has two or more layers, the microporous layer (C) and the microporous layer (B ) are compared.

The area-average long pore size of the microporous layer (C) in the ND-MD cross section is preferably 5 nm or more and 450 nm or less, more preferably 10 nm or more and 400 nm or less, still more preferably 40 nm or more and 350 nm or less, still more preferably 50 nm or more and 300 nm or less. is. When the area-average long pore size of the microporous layer (C) is within this range, clogging of the separator can be more effectively reduced and short circuits can be suppressed.

<Porosity of microporous layer (C)>
The porosity of the microporous layer (C) is preferably 20% or more from the viewpoint of avoiding clogging in the electrical storage device and obtaining good air permeability of the separator, and 70% from the viewpoint of maintaining the strength of the separator. The following are preferable. The porosity of the microporous layer (C) is more preferably 25% or more and 65% or less, still more preferably 30% or more and 60% or less, and particularly preferably 35% or more and 55% or less.

<Thickness of microporous layer (C)>
The thickness of the microporous layer (C) according to the present disclosure is preferably 10 μm or less, for example, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm or less, or It may be 4 μm or less. The lower limit of the thickness of the microporous layer (C) is 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 and the like.

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

<Layer structure of separator base material>
A base material of a separator for an electricity storage device (also simply referred to as a "separator base material" in the specification of the present application) has at least one microporous layer (A) and one microporous layer (B). The separator base material may have a multi-layer structure of three or more layers having two or more microporous layers (A) and/or microporous layers (B). Examples thereof 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). Moreover, the separator base material may 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 the above-described microporous layer (C), 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). A symmetrical laminated structure is preferable from the viewpoints of ease of manufacture, suppression of curling of the separator, and the like.

When the separator base material has a microporous layer (C), the separator base material preferably has a three-layer structure of microporous layer (A)/microporous layer (B)/microporous layer (C). When the separator base material has such a laminated structure, it is possible to more effectively reduce separator clogging and suppress short circuits.

<Thickness of separator base material>
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 electric storage device. you can The lower limit of the thickness of the separator base material is preferably 6 μm or more from the viewpoint of strength, and may be, for example, 7 μm or more, 8 μm or more, 9 μm or more, or 10 μm or more.

<Air Permeability (Air Permeability) of Separator Base Material>
The upper limit of the air permeability of the separator base material is preferably 290 sec/100 cm ³ or less, for example, 280 sec/100 cm ³ or less, 270 sec/100 cm ³ or less, when the thickness of the separator base material is converted to 16 μm. , 260 sec/100 cm ³ or less, or 250 sec/100 cm ³ or less. The lower limit of the air permeability of the separator base material is not limited, but when the thickness of the separator base material is converted to 16 μm, it is, for example, 50 seconds/100 cm ³ or more, 60 seconds/100 cm ³ or more, or 70 seconds/100 cm ³ . or more.

<Porosity of separator base material>
The porosity of the separator base material 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 70% or less from the viewpoint of maintaining the strength of the separator. is preferred. The porosity of the separator substrate is more preferably 25% or more and 65% or less, still more preferably 30% or more and 60% or less, and particularly preferably 35% or more and 55% or less.

<Puncture strength of separator base material>
The lower limit of the puncture strength of the separator base is preferably 230 gf or more, more preferably 240 gf or more, 250 gf or more, 260 gf or more, or 280 gf or more, and even more preferably 280 gf or more, when the thickness of the separator base is converted to 16 μm. is 300 gf or more, particularly preferably 320 gf or more. The upper limit of the puncture strength of the separator base material is not limited, but when the thickness of the separator base material is converted to 16 μm, it is preferably 550 gf or less, for example, 500 gf or less, or 480 gf or less.

<Thermal shrinkage rate of the separator base material>
The separator base material preferably has a heat shrinkage rate of −1.0% or more and 3.0% or less in the width direction (TD direction) after heat treatment at 150° C. for 1 hour. That is, the separator base material has very little heat shrinkage in the width direction even at high temperatures. A short circuit at high temperatures can be effectively suppressed when the thermal shrinkage is 3.0% or less. The reason why the thermal shrinkage rate is -1.0% or more is that when the thermal shrinkage rate is measured, the base material swells slightly in the width direction, and the thermal shrinkage rate may be a negative value smaller than 0%. is. The heat shrinkage may be 0% or more, or greater than 0%. As a method for producing a separator substrate having a heat shrinkage rate of -1.0% or more and 3.0% or less, for example, a method of producing by a dry method of uniaxial stretching can be mentioned. In wet separators, heat shrinkage in the width direction is generally very large. It is easy to obtain a separator base material with a content of 0% or less.

<<Method for producing separator for power storage device>>
A method for producing a separator for an electricity storage device includes a melt extrusion step of obtaining a resin film by melt-extrusion of a resin composition containing polypropylene as a main component (hereinafter also referred to as a "polypropylene-based resin composition"), and the resulting resin film. includes a pore-forming step of opening a hole to make it porous. Methods for producing a microporous layer are broadly classified into a dry method that does not use a solvent in the pore forming process and a wet method that uses a solvent.

As a dry method, a polypropylene resin composition is melt-kneaded and extruded, followed by heat treatment and stretching to separate the polypropylene crystal interface, and a polypropylene resin composition and an inorganic filler are melt-kneaded and formed into a film. After stretching, the interface between the polypropylene and the inorganic filler is separated by stretching.

As the wet method, the polypropylene resin composition and the pore-forming material are melt-kneaded to form a film, and if necessary, the film is stretched and then the pore-forming material is extracted. and a method in which the polypropylene is coagulated by immersing it in a poor solvent for polypropylene and the solvent is removed at the same time.

A single-screw extruder and a twin-screw extruder can be used for melt-kneading the polypropylene-based resin composition, and in addition to these, for example, a kneader, a laboplastomill, a kneading roll, and a Banbury mixer are used. can also

The polypropylene-based resin composition may optionally contain resins other than polypropylene, additives, and the like depending on the manufacturing method of the microporous layer or the desired physical properties of the microporous layer. Additives include, for example, pore-forming agents, fluorine-based flow modifiers, waxes, crystal nucleating agents, antioxidants, metal soaps such as aliphatic carboxylic acid metal salts, ultraviolet absorbers, light stabilizers, and antistatic agents. agents, antifogging agents, and coloring pigments. Pore formers include plasticizers, inorganic fillers, or 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-based ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitrides such as silicon nitride, titanium nitride, and boron nitride; Physical ceramics; 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, silica sand, and other ceramics; and glass fiber.

As a method for producing the separator base material, a dry lamellar crystal pore-opening process in which polypropylene crystal interfaces are exfoliated by heat treatment and stretching is preferred. Here, as a method for producing a separator substrate having a microporous layer (A) and a microporous layer (B), it is preferable to use at least one of the following methods (a) and (b):
(i) A method for producing a separator substrate by co-extrusion film formation, wherein the microporous layer (A) and the microporous layer (B) are co-extruded and subjected to annealing, cold drawing, hot drawing, and heat relaxation steps; and (ii) The microporous layer (A) and the microporous layer (B) are separately extruded into films, laminated together, and then subjected to annealing, cold stretching, hot stretching, and thermal relaxation steps. Separator by lamination. A method for manufacturing a substrate.

In the case of further having a microporous layer (C), the following methods can be mentioned as well:
(i) A separator formed by co-extrusion, in which the microporous layer (A), the microporous layer (B), and the microporous layer (C) are co-extruded and subjected to annealing, cold stretching, hot stretching, and thermal relaxation steps. a method for manufacturing a substrate;
(ii) At least one of the microporous layer (A), the microporous layer (B), and the microporous layer (C) is separately extruded, laminated, and then annealed, cold stretched, and hot stretched. , a method for producing a separator base material by lamination, which is subjected to a thermal relaxation process.

As a method of adjusting the absolute values and ratios of the area-average long pore diameters in the ND-MD cross section of the microporous layer (A) and the microporous layer (B), and optionally the microporous layer (C), within the preferred range of the present disclosure. Examples include changing the molecular weight of polypropylene in each layer and adding additives. The present inventors have found that by using polypropylene with a higher molecular weight than the other layers for one microporous layer, it is possible to control the pore size to be smaller than the other layers. Further, the inventors have found that by adding an additive having a specific structure represented by a styrene-olefin copolymer to the microporous layer, it is possible to control the pore size to be larger than that of other layers. Furthermore, the inventors have found that by strictly controlling the pore size of each layer, it is possible to achieve both battery performance, heat resistance, and short-circuit suppression.

Of the co-extrusion process (i) and the lamination process (ii), the co-extrusion process (i) is preferred from the viewpoint of manufacturing costs and the like. In the co-extrusion process (i), the microporous layers (A) to (C) are extruded and film-formed at a temperature as low as possible, and can be effectively quenched by blowing low-temperature air. preferable. It is preferable to rapidly cool with air after film formation, and the temperature of the blown air is preferably 20° C. or less, more preferably 15° C. or less. By blowing cold air controlled to such a low temperature, the resin after film formation is quenched and oriented uniformly in the MD direction.

The method for manufacturing the separator base material may include an annealing step after the extrusion film formation. By performing the annealing step, the crystal structure of the microporous layers (A) to (C) tends to grow and the openness tends to improve. By applying annealing at a specific temperature for a long time, it tends to be possible to obtain a good area-average long pore diameter for all of the microporous layers (A) to (C). The reason for this is thought to be that crystals grow without disturbing the crystal structure and high porosity can be obtained. In the annealing step, the temperature range is preferably 115° C. or higher and 160° C. or lower, preferably 20 minutes or longer, and more preferably 60 minutes or longer. It is preferable from the viewpoint of preventing clogging.

The method for manufacturing the separator substrate may include a stretching step after the annealing step, during the hole forming step, or before or after the hole forming step. As the stretching treatment, either uniaxial stretching or biaxial stretching can be used. Uniaxial stretching is preferable, although not limited, from the viewpoint of manufacturing costs when using a dry method. Biaxial stretching is preferable from the viewpoint of improving the strength and the like of the resulting separator base material. Examples of biaxial stretching include methods such as simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, and multiple times stretching. Simultaneous biaxial stretching is preferable from the viewpoints of improvement in puncture strength, uniformity of stretching, and shutdown properties. From the viewpoint of ease of control of plane orientation, sequential biaxial stretching is preferred. When the sheet-like molded body is stretched in the biaxial direction at a high magnification, the molecules are oriented in the plane direction, so there is a tendency to obtain a separator substrate that is difficult to tear and has high puncture strength.

In order to suppress heat shrinkage of the separator base material, a heat treatment process may be performed for the purpose of heat setting after the stretching process or after the hole forming process. The heat treatment step includes a stretching operation performed at a predetermined temperature atmosphere and a predetermined stretching rate for the purpose of adjusting physical properties, and / or a relaxation operation performed at a predetermined temperature atmosphere and a predetermined relaxation rate for the purpose of reducing stretching stress. may contain. A relaxation operation may be performed after the stretching operation. These heat treatment steps can be performed using a tenter or a roll stretching machine.

The obtained separator base material itself can be used as a separator for an electric storage device as it is. Optionally, one or both sides of the separator substrate may be provided with additional layers of coating layers.

《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 preferably has a positive electrode and a negative electrode, and the power storage device separator is preferably laminated between the positive electrode and the negative electrode. The microporous layer (A) constituting the outermost layer of the separator base material is preferably arranged facing the negative electrode side. Clogging of the separator in the electricity storage device is mostly caused by deposits on the surface of the negative electrode. Clogging can be effectively reduced.

Electricity storage devices include, but are not limited to, lithium secondary batteries, lithium ion secondary batteries, sodium secondary batteries, sodium ion secondary batteries, magnesium secondary batteries, magnesium ion secondary batteries, calcium secondary batteries, calcium Ion secondary battery, aluminum secondary battery, aluminum ion secondary battery, nickel hydrogen battery, nickel cadmium battery, electric double layer capacitor, lithium ion capacitor, redox flow battery, lithium sulfur battery, lithium air battery, zinc air battery, etc. is mentioned. Among these, lithium secondary batteries, lithium ion secondary batteries, nickel-hydrogen batteries, or lithium ion capacitors are preferred, and lithium ion secondary batteries are more preferred, from the viewpoint of practicality.

In the electric storage device, for example, the positive electrode and the negative electrode are laminated with the separator described above interposed therebetween, and are wound as necessary to form a laminated electrode body or a wound electrode body, which is then wrapped as an outer body. The positive and negative electrodes are connected to the positive and negative terminals of the exterior body via a lead body or the like, and further, 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 is exteriorized. It can be produced by sealing the exterior body after injecting it into the body.

《Measurement and evaluation method》
[Measurement of melt flow rate (MFR)]
The melt flow rate (MFR) of the microporous layer (A) was measured according to JIS K 7210 under conditions of a temperature of 230°C and a load of 2.16 kg (unit: g/10 minutes). The MFR of polypropylene was measured according to JIS K 7210 under conditions of a temperature of 230°C and a load of 2.16 kg. The melt flow rate (MFR) of polyethylene was measured according to JIS K 7210 under the conditions of a temperature of 190°C and a load of 2.16 kg.

[Measurement of Mw and Mn by GPC (gel permeation chromatography)]
Using Agilent PL-GPC220, standard polystyrene was measured under the following conditions to create a calibration curve. The chromatograph was also measured under the same conditions for the sample polymer, and based on the calibration curve, the polystyrene-equivalent weight average molecular weight (Mw), number average molecular weight (Mn), and weight average molecular weight (Mw) of the polymer were calculated according to the following conditions. was calculated by dividing by the number average molecular weight (Mn) (Mw/Mn).
Column: TSKgel GMHHR-H (20) HT (7.8 mm I.D. × 30 cm) 2 Mobile phase: 1,2,4-trichlorobenzene Detector: RI
Column temperature: 160°C
Sample concentration: 1 mg/ml
Calibration curve: Polystyrene

[Measurement of melt tension]
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: 2 mm/min ・Take-up speed: 60 m/min ・Temperature: 230°C

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

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

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

[Air permeability (sec/100 cm ³ )]
Using a Gurley air permeability meter conforming to JIS P-8117, the air permeability resistance (sec/100 cm ³ ) of the separator base material is measured, divided by the thickness and then multiplied by 16 to obtain a value converted to a thickness of 16 μm. Air permeability was calculated.

[Air permeability after high temperature treatment (sec/100 cm ³ )]
A sample obtained by cutting the separator substrate into a square of 100 mm in MD/TD was placed in a hot air dryer (manufactured by Yamato Scientific Co., Ltd., DF1032) heated to 140° C. and heat-treated in the atmosphere at normal pressure. After 30 minutes, the sample was removed from the hot air dryer, allowed to cool at room temperature for 10 minutes, and then measured using a Gurley air permeability meter conforming to JIS P-8117 to determine the air resistance of the separator substrate (seconds/100 cm ³ ) was measured, divided by the thickness, and then multiplied by 16 to calculate the air permeability after high-temperature treatment (converted to a thickness of 16 μm).

[TD heat shrinkage (%)]
A sample obtained by cutting the separator substrate into a square of 50 mm in MD/TD was placed in a hot air dryer (manufactured by Yamato Scientific Co., Ltd., DF1032) heated to 150° C. and heat-treated in the atmosphere at normal pressure. After 1 hour, the sample was taken out from the hot air dryer, allowed to cool at room temperature for 10 minutes, and then measured for dimensional shrinkage. The sample was placed on copy paper or the like so as not to adhere to the inner wall of the dryer and to prevent the samples from being fused together.
Thermal shrinkage rate (%): (Dimensions before heating (mm) - Dimensions after heating (mm)) / (Dimensions before heating (mm)) x 100

[Evaluation of Area Average Long Pore Diameter]
The area-average long pore diameter was measured by image analysis in cross-sectional SEM observation. As a pretreatment, the separator was dyed with ruthenium, and a cross-sectional sample was prepared by freeze fracture. The cross section is the MD-ND plane. After fixing the cross-sectional sample to the SEM sample table for cross-sectional observation with a conductive adhesive (carbon-based) and drying, an osmium coater (HPC-30W, manufactured by Vacuum Device Co., Ltd.) is used for conductive treatment, and the applied voltage adjustment knob is applied. Osmium coating was performed under the conditions of a setting of 4.5 and a discharge time of 0.5 seconds, and a microscopic sample was obtained. Next, using a scanning electron microscope (Hitachi High-Technologies S-4800), arbitrary three points on the surface of the microporous membrane were observed under the conditions of an acceleration voltage of 1 kV, a detection signal of LA10, a working distance of 5 mm, and a magnification of 5000 times.

The observation image was subjected to binarization processing using the Otsu method using image processing software ImageJ to separate the resin portion and the pore portion, and the average length of the pore portion was calculated. At this time, micropores existing between the photographing range and outside the photographing range and holes with a hole area of 0.001 μm ² or less were excluded from the measurement targets. The average diameter was calculated by area average from the area of each hole.

[Observation of cycle capacity retention rate and clogging]
As an electrolytic solution, an electrolytic solution containing 1 mol/L of LiPF ₆ as a lithium salt in a mixture of ethylene carbonate and ethyl methyl carbonate at a volume ratio of 1:2 was used.

Lithium-nickel-manganese-cobalt mixed oxide (LiNi _0.5 Co _0.2 Mn _0.3 O ₂ ) as a positive electrode active material, and carbon black powder (manufactured by Timcal, trade name: SuperP Li) as a conductive aid. and PVDF as a binder at a mass ratio of mixed oxide: conductive aid: binder = 100: 3.5: 3, and applied to both sides of an aluminum foil as a positive electrode current collector having a thickness of 15 μm. After drying, it was pressed with a roll press to prepare a double-sided coated positive electrode.

Graphite powder with a particle size of 22 μm (D50) as a negative electrode active material (manufactured by Hitachi Chemical Co., Ltd., trade name: MAG), a binder (manufactured by Nippon Zeon Co., Ltd., trade name: BM400B), and a thickener carboxymethyl cellulose (manufactured by Daicel) , trade name: #2200) were mixed at a mass ratio of graphite powder: binder: thickener = 100: 1.5: 1.1, and one side of a copper foil as a negative electrode current collector having a thickness of 10 µm, The solvent was removed by drying, and then the coated copper foil was pressed with a roll press to prepare a single-sided coated negative electrode and a double-sided coated negative electrode, respectively.

The resulting positive electrode and negative electrode are placed on the opposite surfaces of the respective active materials, while sandwiching the separator prepared below, one-side coated negative electrode/double-sided coated positive electrode/double-sided coated negative electrode/double-sided coated positive electrode/single-sided coated negative electrode were laminated in this order. The microporous layer (A) was arranged so as to face the negative electrode. Next, the obtained laminate was inserted into a bag (battery outer packaging) made of a laminate film in which both sides of an aluminum foil (thickness 40 μm) were coated with a resin layer, with the positive and negative terminals projecting therefrom. After that, the electrolytic solution prepared as described above was poured into a 0.8 mL bag, and the bag was vacuum-sealed to prepare a sheet-like lithium ion secondary battery.

The resulting sheet-shaped lithium ion secondary battery was housed in a constant temperature bath (manufactured by Futaba Kagaku Co., Ltd., trade name: PLM-73S) set at 25 ° C., and charged and discharged by a charging/discharging device (manufactured by Aska Denshi Co., Ltd., trade name: ACD-01) and allowed to stand for 16 hours. Then, the battery was charged at a constant current of 0.05 C and charged at a constant voltage of 4.35 V for 2 hours after the voltage reached 4.35 V, and then charged at a constant current of 0.2 C to 3.0 V. The battery was initially charged and discharged by repeating the charge-discharge cycle three times. Note that 1C indicates a current value when discharging the entire capacity of the battery in 1 hour.

After the initial charge and discharge, the battery was housed in a constant temperature bath set at 25 ° C., and the battery was charged at a constant current of 1 C. After the voltage reached 4.35 V, the battery was charged at a constant voltage of 4.35 V. After charging for 1 hour, the battery was subjected to a cycle test by repeating 100 charge-discharge cycles of discharging to 3.0 V at a constant current of 1C.

The value (percentage) obtained by dividing the discharge capacity (mAh) at the 100th cycle by the discharge capacity (mAh) at the 1st cycle was taken as the cycle capacity retention rate. After 100 cycles, the sheet-shaped lithium ion secondary battery was dismantled in an argon atmosphere, the separator was taken out, and after washing by immersion three times using ethyl methyl carbonate, the negative electrode side surface of the separator was examined with a microscope. The presence or absence of clogging on the surface of the separator was confirmed by observing in the range of 1 mm square. When the pores on the surface of the separator were 50% or more filled with sediments, it was determined that clogging was present, and when the pores on the surface of the separator were not 50% or more filled with deposits, it was determined that there was no clogging. Table 1 shows the presence or absence of clogging.

<<Example 1>>
[Preparation of microporous layer]
As the resin for the microporous layer (A), 95% by weight of a high molecular weight polypropylene resin (“PP1” in the table, MFR (230° C.) = 1.0 g/10 min, density = 0.91 g/cm ³ ), 5% by weight of an ethylene-butene random copolymer elastomer (“C2C4” in the table) was dry-blended to obtain a resin material. The resulting resin material 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. Further, as the resin for the microporous layer (B), a high molecular weight polypropylene resin (“PP1” in the table, MFR (230° C.) = 1.0 g/10 min, density = 0.91 g/cm ³ ) was used at 2.5 It was melted in an inch extruder and supplied to the inner layer of the two-kind three-layer coextrusion T-die using a gear pump. The temperature of the T-die was set to 220° C., and after the molten polymer was extruded from the T-die, it was wound on a roll while cooling the extruded resin with blown air, thereby forming a precursor of an A/B/A layer structure with a thickness of about 17 μm. Got a body sheet. Here, the lip width of the T-die in the TD direction was set to 500 mm, the distance between the lips of the T-die (lip clearance) was set to 2.4 mm, and the discharge was performed at a discharge rate of 6 kg/h.

Next, the obtained precursor was put into a dryer and annealed at 120° C. for 20 minutes. After that, the annealed precursor was subjected to 20% cold stretching at room temperature, and the stretched film was put into an oven at 125°C without shrinking, and subjected to hot stretching to 140%, and then to 15%. By relaxing, a separator substrate having a three-layer structure consisting of A/B/A layers was obtained. Table 1 shows the structure, physical properties, and battery performance evaluation results of the obtained separator base material.

<<Examples 2 to 4, Comparative Examples 1 to 3>>
A microporous membrane was obtained in the same manner as in Example 1 except that the raw materials were changed as shown in Table 1, and the obtained separator was evaluated. In the table, "PP2" indicates polypropylene resin (MFR (230°C) = 0.5 g/10 min, density = 0.91 g/cm ³ ). "SEPS" in the table indicates a block copolymer of styrene-ethylenepropylene-styrene. "PE" in the table indicates polyethylene (MFR (230°C) = 0.4 g/10 minutes).

<<Example 5>>
Instead of the coextrusion T die of two kinds and three layers, a coextrusion T die of two kinds and two layers was installed, and the raw materials as shown in Table 2 were used to form a film under the same conditions as in Example 1. A thick A/B layer structure precursor sheet was obtained.

Next, the obtained precursor was put into a dryer and annealed at 120° C. for 20 minutes. After that, the annealed precursor was subjected to 20% cold stretching at room temperature, and the stretched film was put into an oven at 125°C without shrinking, and subjected to hot stretching to 140%, and then to 15%. By relaxing, a separator substrate having a two-layer structure consisting of A/B layers was obtained. Table 1 shows the structure, physical properties, and battery performance evaluation results of the obtained separator base material.

<<Example 6>>
Instead of the coextrusion T die of two kinds and three layers, a coextrusion T die of three kinds and three layers was installed, and the raw materials as shown in Table 2 were used to form a film under the same conditions as in Example 1, and the thickness was about 17 μm. A precursor sheet having an A/B/C layer structure was obtained.

Next, the obtained precursor was put into a dryer and annealed at 120° C. for 20 minutes. After that, the annealed precursor was subjected to 20% cold stretching at room temperature, and the stretched film was put into an oven at 125°C without shrinking, and subjected to hot stretching to 140%, and then to 15%. By relaxing, a separator substrate having a three-layer structure consisting of A/B/C layers was obtained. Table 1 shows the structure, physical properties, and battery performance evaluation results of the obtained separator base material.

From Table 1, by strictly controlling the composition of each layer and strictly controlling the film formation conditions, the area average pore diameter of the microporous layer (A) arranged as the outermost layer was It can be seen that it is possible to make it larger than the area average pore size of It is also found that such a separator configuration suppresses clogging and is excellent in maintaining performance at high temperatures.

The power storage device separator of the present disclosure can be suitably used as a separator for power storage devices such as lithium ion secondary batteries.

Claims (10)

Having a separator substrate having a microporous layer (A) containing polypropylene as a main component and a microporous layer (B) containing polypropylene as a main component,
The area-average long pore diameter in the ND-MD cross section of the microporous layer (A) is 1.1 to 5.0 times the area-average long pore diameter in the ND-MD cross section of the microporous layer (B). can be,
A separator for an electricity storage device, wherein the microporous layer (A) constitutes the outermost layer on at least one side of the separator substrate. 2. The electricity storage device separator according to claim 1, wherein the pores of said microporous layer (A) have an area-average long pore diameter in the ND-MD cross section of 100 nm or more and 600 nm or less. 3. The power storage device separator according to claim 1, wherein said separator base material has a widthwise heat shrinkage of -1.0% or more and 3.0% or less after heat treatment at 150° C. for 1 hour. The power storage device separator according to any one of claims 1 to 3, wherein the microporous layer (A) constitutes the outermost layers on both sides of the separator substrate. The electricity storage according to any one of claims 1 to 4, wherein the separator base material includes a structure in which a microporous layer (A), a microporous layer (B), and a microporous layer (A) are laminated in this order. Device separator. The power storage device separator according to any one of claims 1 to 5, wherein the separator substrate further has a microporous layer (C) containing polyolefin as a main component. The area-average long pore diameter in the ND-MD cross section of the microporous layer (C) is 0.2 to 0.9 times the area-average long pore diameter in the ND-MD cross section of the microporous layer (B). 7. The power storage device separator according to claim 6. The separator substrate according to any one of claims 1 to 7, comprising a structure in which a microporous layer (A), a microporous layer (B) and a microporous layer (C) are laminated in this order. Separator for power storage device. An electricity storage device comprising a positive electrode, a negative electrode, and the separator for an electricity storage device according to any one of claims 1 to 8. The electricity storage device according to claim 9, wherein the microporous layer (A) is arranged facing the negative electrode side.