JP2022051521A - Separator for power storage device - Google Patents

Abstract

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

Description

The present invention relates to a separator for a power storage device and the like.

Microporous membranes, especially polyolefin microporous membranes, are used in many technical fields such as precision filtration membranes, battery separators, condenser separators, fuel cell materials, etc., and are particularly storage devices typified by lithium-ion batteries. It is used as a separator. Lithium-ion batteries are applied to various applications such as small electronic devices such as mobile phones and notebook personal computers, as well as hybrid vehicles and electric vehicles including plug-in hybrid vehicles.

In recent years, lithium ion batteries having high energy capacity, high energy density, and high output characteristics have been demanded, and along with this, there is an increasing demand for thin film and high strength (for example, high piercing strength) separators.

Patent Document 1 describes polypropylene resin (X) of 5 to 30% by weight and polypropylene resin (Y) of 95 to 70% by weight from the viewpoint of the rigidity of the microporous film and the reduction of the product defect rate at a certain thickness. Describes the polypropylene resin composition for a microporous film containing. The polypropylene resin (X) described in Patent Document 1 has a specific melt flow rate (MFR), a molecular weight distribution (Mw / Mn), and a long-chain branched structure. The polypropylene resin (Y) described in Patent Document 1 has a specific MFR and excludes the polypropylene resin (X).

Patent Document 2 describes the following three layers (a) to (c) from the viewpoint of an improved balance of electrochemical stability of a microporous membrane, high meltdown temperature, high electrolyte affinity, and low water retention. ) Described:
(A) A first layer containing 40.0 to 85.0% by weight of isotactic polypropylene having a weight average molecular weight (Mw) of 6.0 × ¹⁰⁵ or more.
(B) A second layer containing polyolefin,
(C) A third layer containing 40.0 to 85.0% by weight of isotactic polypropylene having a Mw of 6.0 × ¹⁰⁵ or more.

Patent Document 3 includes a first microporous film containing a first resin composition having a melting point _TmA and a second resin composition containing a second resin composition having a melting point _TmB lower than the melting point _TmA . Described is a laminated microporous film comprising a microporous film and having an extensional viscosity of 18,000 to 40,000 Pa · s and a shear viscosity of 5,000 to 10,000 Pa · s.

In Patent Document 4, the component weight having a molecular weight of 50,000 or less is 25 to 60% by weight, the component weight having a molecular weight of 700,000 or more is 19 to 30% by weight, the weight average molecular weight is 350,000 to 500,000, and the melt tension is 1. A propylene-based resin micropore film made of a propylene-based resin weighing 1 to 3.2 g is described.

In Patent Document 5, the ultimate viscosity [η] is 1 dl / g or more and less than 7 dl / g, the mesopentad fraction is in the range of 94.0 to 99.5%, and the elution integral amount up to 100 ° C. at the time of temperature rise. Is 10% or less, the melting point is 153 to 167 ° C., the peak top temperature of the maximum peak exists in 105 to 130 ° C. in the elution temperature-elution amount curve, and the half price range of the peak is 7.0 ° C. or less. A polypropylene resin composition for forming a microporous film, which comprises a propylene homopolymer as an essential component, is described.

Patent Document 6 describes a polyolefin microporous membrane containing a polypropylene-based resin having a meltdown temperature of 195 ° C. or higher and 230 ° C. or lower, and the weight average molecular weight of the polypropylene-based resin is 500,000 or more. It is described that it may be 800,000 or less, and that the molecular weight distribution may be 7.5 or more and 16 or less.

Patent Document 7 comprises laminating a first microporous film made of a first resin composition and a second microporous film made of a second resin composition, and the first resin composition. And the laminated microporous film in which the MFR of the second resin composition is 1.0 g / 10 minutes or less and the molecular weight distribution (Mw / Mn) of the resin contained in the first resin composition is 10 or more. Is described.

Japanese Unexamined Patent Publication No. 2018-030992 Special Table 2013-517152 Gazette Japanese Unexamined Patent Publication No. 2016-022676 Japanese Unexamined Patent Publication No. 2012-09286 International Publication No. 2010/07784 International Publication No. 2017/138512 Japanese Unexamined Patent Publication No. 2016-22679

It is known that a high-strength microporous membrane can be provided by using a high molecular weight polyolefin for the microporous membrane.

However, when a high molecular weight polyolefin is used for the production of the microporous layer, the film forming property and the pore opening property at the time of manufacturing become insufficient, the pore diameter of the microporous film becomes small, and the air permeability (that is, the air permeability resistance). ) Deteriorates and the viscosity at the time of melting tends to be high, which makes it difficult to produce a separator of a thin film (for example, a thickness of 20 μm or less).

Therefore, an object of the present invention is to provide a thin film separator for a power storage device, which has high strength and suppresses clogging due to long-term use.
Further, the present invention provides a separator for a power storage device capable of high output and thinning while having high strength (particularly high puncture strength) and high dimensional stability at high temperature, which contributes to high safety of the power storage device. The purpose is to provide.

As a result of diligent studies to solve the above problems, the present inventors use a microporous layer containing a polyolefin having a specific melt flow rate (MFR) as a main component and having a specific average long pore size. However, they have found that it is advantageous in solving the above-mentioned problems, and have completed the present invention. Examples of embodiments of the present invention are listed below.
[1] Contains a microporous layer (X) whose main component is polyolefin (A),
The microporous layer (X) has a load of 2.16 kg and a melt flow rate (MFR) of 0.90 g / 10 minutes or less at a temperature of 230 ° C., and the microporous layer (X) is measured by a scanning electron microscope (SEM). ), The average major pore diameter of the pores existing in the microporous layer (X) is 100 nm or more in the MD-TD surface observation or the ND-MD cross-sectional observation.
Separator for power storage device.
[2] In the MD-TD surface observation or ND-MD cross-sectional observation of the microporous layer (X) by the SEM, the maximum long pore diameter of the pores existing in the microporous layer (X) is 100 nm or more and 400 nm or less. ,
The separator for a power storage device according to the first aspect.
[3] The melt tension of the microporous layer (X) when measured at a temperature of 230 ° C. is 16 mN or more.
The separator for a power storage device according to the above aspect 1 or 2.
[4] The melt tension of the microporous layer (X) when measured at a temperature of 230 ° C. is 16 mN or more and 40 mN or less.
The separator for a power storage device according to the above aspect 1 or 2.
[5] The ratio (SMD / STD) of the tensile strength (SMD) in the mechanical direction and the tensile strength (STD) in the width direction of the separator for the power storage device is SMD / STD> 5.
The separator for a power storage device according to any one of the above aspects 1 to 4.
[6] The heat shrinkage of the separator for a power storage device after heat treatment at 105 ° C. for 1 hour is 1% or less in TD and 4% or less in MD.
The separator for a power storage device according to any one of the above aspects 1 to 5, wherein the heat shrinkage rate of the separator for a power storage device after heat treatment at 120 ° C. for 1 hour is 1% or less in TD and 10% or less in MD.
[7] The air permeability when the thickness of the separator for a power storage device is converted to 14 μm is 250 seconds / 100 cm ³ or less.
The separator for a power storage device according to any one of the above aspects 1 to 6.
[8] The separator for a power storage device according to any one of the above aspects 1 to 7, wherein the thickness is 8 μm or more and 18 μm or less, and the puncture strength when the thickness is converted to 14 μm is 230 gf or more.
[9] The polyolefin (A) contains polypropylene.
The separator for a power storage device according to any one of the above aspects 1 to 8.
[10] The ratio of the polypropylene to the polyolefin (A) is 50 to 100% by mass.
The separator for a power storage device according to the above aspect 9.
[11] The polypropylene has a pentad fraction of 95.0% or more.
The separator for a power storage device according to the above aspect 9 or 10.
[12] The weight average molecular weight (Mw) of the microporous layer (X) is 500,000 or more and 1,500,000 or less.
The separator for a power storage device according to any one of the above aspects 1 to 11.
[13] The value (Mw / Mn) obtained by dividing Mw of the microporous layer (X) by the number average molecular weight (Mn) is 6 or less.
The separator for a power storage device according to any one of the above aspects 1 to 12.
[14] The separator for a power storage device according to any one of the above aspects 1 to 13, further comprising a microporous layer (Y) containing polyolefin (B) as a main component.
[15] The separator for a power storage device according to the above aspect 14, wherein the polyolefin (B) is polyethylene.
[16] A power storage device comprising the separator for the power storage device according to any one of the above aspects 1 to 15.

According to the present invention, it is possible to provide a separator for a power storage device having high strength and capable of suppressing clogging in a power storage device such as a lithium ion secondary battery. Further, according to the present invention, the power storage capable of high output and thinning while having high strength (particularly high puncture strength) and high dimensional stability at high temperature, which contributes to high safety of the power storage device. A separator for a device can be provided. It should be noted that the above description shall not be deemed to disclose all the embodiments of the present invention and all the advantages relating to the present invention. Further embodiments of the present invention and their advantages will become apparent with reference to the following description.

Hereinafter, the present invention will be described in detail for the purpose of exemplifying an embodiment of the present invention (hereinafter referred to as “the present embodiment”), but the present invention is not limited to the present embodiment. The weight average molecular weight (Mw), number average molecular weight (Mn), and Mw / Mn of the polyolefin of the present embodiment are polystyrene-equivalent molecular weights obtained by GPC (gel permeation chromatography) measurement.

<< Separator for power storage device >>
<Microporous layer>
The separator for a power storage device of the present embodiment includes a microporous layer containing polyolefin as a main component. In the present embodiment, the microporous layer refers to a single microporous layer constituting a separator for a power storage device, and two or more layers may be laminated and used in multiple layers. In one embodiment, the separator for a power storage device has a microporous layer (X) containing polyolefin (A) as a main component. If desired, the separator for a power storage device may further include a microporous layer (Y) containing polyolefin (B) as a main component.

Even when the separator is composed of a multi-layered microporous layer, the adhesive tape is attached to the end of the separator and pulled to easily peel off each microporous layer for recovery, and the melt tension and melt flow rate (MFR) described later are described. ), Molecular weight, long pore size, porosity, thickness and other physical properties can be measured.

<Microporous layer (X)>
The separator for a power storage device of this embodiment has a microporous layer (X). The separator for a power storage device may have only one microporous layer (X) or may have two or more layers. In the present embodiment, the term "polyolefin (A) as a main component" means that the polyolefin (A) is contained in an amount of 50% by mass or more based on the total mass of the microporous layer (X). The lower limit of the content of the polyolefin (A) in the microporous layer (X) is 50% by mass or more, preferably 55% by mass, from the viewpoint of wettability of the separator with respect to the electrolytic solution, thinning, and shutdown characteristics. As mentioned above, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more. The upper limit of the content of the polyolefin (A) in the microporous layer (X) 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. , 99% by mass or less, or 100% by mass.

<Polyolefin (A)>
The polyolefin (A) in the present embodiment is a polymer containing a monomer having a carbon-carbon double bond as a repeating unit. The monomer constituting the polyolefin (A) is not limited, but is a monomer having a carbon-carbon double bond and having 1 or more and less than 10 carbon atoms, for example, ethylene, propylene, 1-butene, 4-methyl-1-pentene. , 1-hexene, 1-octene and the like. The polyolefin (A) is, for example, a homopolymer, a copolymer, a multistage polymer, or the like, and is preferably a homopolymer.

As the polyolefin (A), specifically, polyethylene, polypropylene, a copolymer of ethylene and propylene, and a mixture thereof are preferable from the viewpoint of shutdown characteristics and the like. The polyolefin (A) is more preferably polypropylene. From the same viewpoint, the lower limit of the ratio of polypropylene to the polyolefin (A) which is the main component of the microporous layer (X) is preferably 50% by mass or more, more preferably 60% by mass or more, still more preferably 70% by mass or more. Is. The lower limit of the ratio of polypropylene to the polyolefin (A), which is the main component of the microporous layer (X), is preferably 100% by mass or less.

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

The polypropylene that can be used as the polyolefin (A) is preferably a homopolymer, and may be a copolymer obtained by copolymerizing a comonomer other than propylene, for example, an α-olefin comonomer, for example, a block polymer. The lower limit of the amount of the propylene structure contained as a repeating unit of polypropylene may be, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. Polypropylene may contain repeating units other than the propylene structure. In that case, the upper limit of the amount of the repeating unit (excluding the propylene structure) derived from the comonomer is, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. good. Polypropylene can be used alone or in admixture of two or more.

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

The upper limit of the value (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) of the polyolefin (A) by the number average molecular weight (Mn) is preferably 7 or less, 6.5 or less, 6 or less, 5.5 or less, Or 5 or less. The smaller the value of Mw / Mn, the less the entanglement between the molecules, and the smaller the melt tension of the obtained microporous layer (X) tends to be. Therefore, when the value of Mw / Mn of the polyolefin (A) is 7 or less, the melt tension of the microporous layer (X) can be controlled to be low, and the microporous layer (X) can be stably produced with a thinner film. preferable. The lower limit of Mw / Mn of the polyolefin (A) is preferably 1 or more, 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. When Mw / Mn is 1 or more, appropriate molecular entanglement is maintained, and the stability during film formation tends to be good.

Most of the conventional separators have a thickness of 20 μm to 40 μm, and it has been difficult to produce a thin-film separator, especially in a separator having a multi-layer structure. It is considered that this is because when a high molecular weight polyolefin is used, the melt tension at the time of film formation becomes high, and it is difficult to stably form a microporous layer of a thin film. Controlling the molecular weight of the polyolefin (A) and the molecular weight distribution represented by Mw / Mn within the numerical ranges described above is advantageous from the viewpoint of producing a thinner film separator.

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

When the polyolefin (A) is polypropylene, the upper limit of the value (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) of polypropylene by the number average molecular weight (Mn) is preferably 7 or less, 6.5 or less, 6 Hereinafter, it is 5.5 or less, or 5 or less. The smaller the Mw / Mn value of polypropylene, the smaller the melt tension of the obtained microporous layer (X) tends to be. Therefore, it is preferable that the value of Mw / Mn of polypropylene is 7 or less in order to control the melt tension of the microporous layer (X) to 30 mN or less. The Mw / Mn of polypropylene is preferably 1 or more, 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. When Mw / Mn of polypropylene is 1 or more, appropriate molecular entanglement is maintained, and stability during film formation tends to be good.

The lower limit of the density of the polyolefin (A) is preferably 0.85 g / cm ³ or more, 0.88 g / cm ³ or more, 0.89 g / cm ³ or more, or 0.90 g / cm ³ or more. The upper limit of the density of the polyolefin (A) is preferably 1.1 g / cm ³ or less, 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. Hereinafter, it is 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 (A) is related to the crystallinity of the polyolefin (A), and by setting the density of the polyolefin (A) to 0.85 g / cm ³ or more, the pore-opening property of the microporous layer (X) is improved. In particular, it is advantageous in a method of making a resin raw fabric porous by a dry method (hereinafter referred to as a dry method).

<Melting tension of microporous layer (X)>
The upper limit of the melt tension (melt tension of a single layer) of the microporous layer (X) when measured at a temperature of 230 ° C. is preferably 40 mN or less, more preferably 40 mN or less, from the viewpoint of moldability of the microporous layer (X). It is 38 mN or less, more preferably 35 mN or less, and most preferably 30 mN or less. The lower limit of the melt tension (melt tension of a single layer) of the microporous layer (X) is preferably 16 mN or more, more preferably 17 mN or more, still more preferably 20 mN or more from the viewpoint of the strength of the microporous layer (X). Most preferably, it is 24 mN or more.

<Melt flow rate (MFR)>
The upper limit of the melt flow rate (MFR) (that is, single layer MFR) of the microporous layer (X) when measured at a load of 2.16 kg and a temperature of 230 ° C. is 0.9 g from the viewpoint of obtaining a high-strength separator. / 10 minutes or less, for example 0.90 g / 10 minutes or less, 0.85 g / 10 minutes or less, 0.7 g / 10 minutes or less, 0.65 g / 10 minutes or less, 0.6 g / 10 minutes or less, or 0 It may be .55 g / 10 minutes or less. The lower limit of the MFR (single layer MFR) of the microporous layer (X) is, for example, 0.15 g / 10 minutes or more, 0.2 g / 10 minutes or more, from the viewpoint of moldability of the microporous layer (X). 0.25 g / 10 minutes or more, 0.3 g / 10 minutes or more, 0.35 g / 10 minutes or more, 0.38 g / 10 minutes or more, or 0.4 g / 10 minutes or more, 0.45 g / 10 minutes or more, 0 .50 g / 10 minutes or more, 0.55 g / 10 minutes or more, 0.60 g / 10 minutes or more, 0.65 g / 10 minutes or more, or 0.70 g / 10 minutes or more.

Due to the high molecular weight of the polyolefin (A) contained in the microporous layer (X), the MFR of the microporous layer (X) tends to be 0.9 g / 10 minutes or less. Since the molecular weight of the polyolefin is high, the number of Thai molecules that bond the crystals to each other increases, so that a high-strength microporous layer tends to be obtained. In one embodiment, the molecular weight distribution represented by Mw / Mn of the polyolefin (A) is 7 or less and the MFR is 0.9 g / 10 minutes or less, particularly 0.6 g / 10 minutes or less, which means that the thin film has high strength. It is advantageous from the viewpoint of obtaining a separator for a power storage device. In one embodiment, when the MFR of the microporous layer (X) is 0.15 g / 10 minutes or more, the melt tension of the microporous layer (X) does not become too high, and the separator has high strength, high output, and a thin film. Is easier to obtain.

When the polyolefin (A) is polypropylene, the upper limit of MFR when measured at a load of 2.16 kg and a temperature of 230 ° C. is preferably 0.9 g / 10 minutes or less, 0. 90 g / 10 minutes or less, 0.85 g / 10 minutes or less, 0.8 g / 10 minutes or less, 0.7 g / 10 minutes or less, 0.6 g / 10 minutes or less, or 0.55 g / 10 minutes or less. The lower limit of the MFR of polypropylene is preferably 0.2 g / 10 minutes or more, 0.25 g / 10 minutes or more, 0.3 g / 10 minutes or more, 0. 35 g / 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.

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

The upper limit of the value (Mw / Mn) obtained by dividing Mw of the microporous layer (X) by the number average molecular weight (Mn) is to lower the melt tension of the microporous layer (X) from the viewpoint of the strength of the microporous layer (X). It is preferably 6 or less, 5.5 or less, or 5 or less from the viewpoint of controlling the thickness and increasing the pore size of the microporous layer (X) to suppress clogging and obtain high output. The lower limit of Mw / Mn of the microporous layer (X) is preferably 1 or more, 1.3 or more, 1.5 or more, 2.0 or more, or 2. 5 or more.

<Pentad fraction>
When the polyolefin (A) is polypropylene, in this embodiment, the lower limit of the pentad fraction of polypropylene measured by ¹³ C-NMR (nuclear magnetic resonance method) is the microporous layer (X) with low air permeability. From the viewpoint of obtaining 94.0% or more, 95.0% or more, 96.0% or more, 96.5% or more, 97.0% or more, 97.5% or more, 98.0% or more, 98. It is 5.5% or more, or 99.0% or more. The upper limit of the pentad fraction of polypropylene may be, for example, 99.9% or less, 99.8% or less, or 99.5% or less.

When the pentad fraction of polypropylene is 94.0% or more, the crystallinity of polypropylene is high. The separator obtained by the stretch-opening method, particularly the dry method, opens pores by stretching an amorphous portion between a plurality of crystalline portions. Therefore, when polypropylene has high crystallinity, the pore-opening property becomes high. It becomes good, and the porosity and the average long pore diameter of the pores existing in the microporous layer (X) can be increased in MD-TD surface observation or ND-MD cross-sectional observation with a scanning electron microscope (SEM), resulting in clogging. Is preferable because the air permeability can be controlled to be low. When high molecular weight polypropylene is used, the strength is high, but the entanglement between the molecular chains is large, so that the pore opening property is deteriorated, and as a result, it is difficult to obtain low air permeability. This is considered to be one of the major factors that make it difficult to use high molecular weight polypropylene in the dry method. On the other hand, by using polypropylene having a very well-controlled structure such as polypropylene having a high pentad fraction, it is possible to obtain high pore-opening property even if a high molecular weight polypropylene is used. More preferably, by using polypropylene having such a controlled structure and imparting controlled manufacturing conditions to the manufacturing process, it is possible to obtain higher porosity even when polypropylene having a high molecular weight is used. Will be.

<Average long hole diameter and maximum long hole diameter of microporous layer (X)>
In the separator for a power storage device of the present embodiment, in the case of MD-TD surface observation by a scanning electron microscope (SEM), the average long pore diameter of the pores existing in the microporous layer (X) is preferably 100 nm or more. And / or preferably 400 nm or less. In the present disclosure, the mechanical direction (MD) indicates the film forming direction of the microporous layer, and the width direction (TD) indicates the direction perpendicular to the film forming direction of the microporous layer, and the normal direction (ND). ) Indicates the thickness direction of the microporous layer (that is, the direction perpendicular to MD and TD). The MD of the separator having the microporous layer is in the longitudinal direction if it is a roll. Setting the average long hole diameter of the holes existing on the surface of the MD-TD to this range tends to contribute to the suppression of clogging in the power storage device such as a lithium ion secondary battery and the adjustment of the air permeability of the separator. It is in.

In the MD-TD surface observation by SEM, the lower limit of the average major pore diameter of the pores existing in the microporous layer (X) is preferably 100 nm or more, more preferably 130 nm or more, from the viewpoint of suppressing clogging in the power storage device. More preferably, it is 140 nm or more. In the MD-TD surface observation by SEM, the upper limit of the average long pore diameter of the pores existing in the microporous layer (X) is preferably 400 nm or less, more preferably 350 nm or less, and further, from the viewpoint of suppressing a short circuit in the power storage device. It is preferably 300 nm or less.

In the ND-MD cross-sectional observation by SEM, the lower limit of the average major pore diameter of the pores existing in the microporous layer (X) is preferably 100 nm or more, more preferably 130 nm or more, from the viewpoint of suppressing clogging in the power storage device. More preferably, it is 140 nm or more. In the ND-MD cross-sectional observation by SEM, the upper limit of the average long pore diameter of the pores existing in the microporous layer (X) is preferably 400 nm or less, more preferably 350 nm or less, and further, from the viewpoint of suppressing a short circuit in the power storage device. It is preferably 300 nm or less.

In MD-TD surface observation or ND-MD cross-section observation by SEM, the lower limit of the maximum long pore diameter of the pores existing in the microporous layer (X) is preferably 100 nm or more from the viewpoint of suppressing clogging in the power storage device. It is more preferably 220 nm or more, still more preferably 230 nm or more. In MD-TD surface observation or ND-MD cross-section observation by SEM, the upper limit of the maximum long pore diameter of the pores existing in the microporous layer (X) is preferably 400 nm or less from the viewpoint of suppressing short circuit in the power storage device. It is preferably 375 nm or less, more preferably 360 nm or less, and particularly preferably 350 nm or less.

In the present disclosure, the average long pore diameter is the area average value of the long diameter calculated based on the area of each pore existing in the microporous layer in the MD-TD surface observation or the ND-MD cross-sectional observation of the microporous layer by SEM. Is. The maximum long pore diameter is the largest of the major diameters of each pore existing in the microporous layer in MD-TD surface observation or ND-MD cross-sectional observation of the microporous layer by SEM. The average long hole diameter and the maximum long hole diameter are the MD-TD surface of the separator (when the average long hole diameter of the holes existing on the MD-TD surface is obtained) or the ND-MD cross section (the average long hole diameter of the holes existing in the ND-MD cross section). It can be measured by performing SEM (scanning electron microscope) observation of the obtained SEM image and performing image analysis in a range of 4 μm × 4 μm of the obtained SEM image. Detailed conditions are shown in Examples.

In the MD-TD surface observation or ND-MD cross-sectional observation of the microporous layer by SEM, if the average long pore diameter and the maximum long pore diameter of the microporous layer become large, the air permeability of the separator decreases and clogging in the power storage device occurs. It can be expected that the life of the lithium-ion battery will be improved by reducing the amount. Further, if the average long pore diameter and the maximum long pore diameter of the microporous layer (X) are too large, the strength and insulation of the separator including the microporous layer (X) may decrease, and the safety of the power storage device may decrease. ..

<Both MFR of microporous layer (X) and average long pore size>
In the separator for a power storage device of the present embodiment, the microporous layer (X) preferably has an MFR of 0.90 g / 10 minutes or less, and is MD-TD surface or ND in the case of surface or cross-sectional SEM observation. -The average long hole diameter in the MD cross section is 100 nm or more. Conventionally, a separator having a multi-layer structure, particularly a multi-layer separator obtained by a dry method, does not use a plasticizer at the time of manufacture, so that the MFR is lowered to 0.90 g / 10 minutes or less and MD-TD is used. It was very difficult to increase the average long hole diameter on the surface or ND-MD cross section to 100 nm or more. When the MFR was as low as 0.90 g / 10 minutes or less, that is, when the molecular weight of the polyolefin was high, it was difficult to open pores at the time of stretching and opening, and even if the pores were opened, the average long pore diameter tended to be small. Further, even in the conventional technique in which the air permeability is adjusted to about 200 seconds / 100 ml as described above, if the average long hole diameter of the separator is small, it tends to be difficult to suppress clogging in the power storage device. Further, the multilayer structure separator needs to make the film thickness of each layer particularly thin. For example, in the case of a separator having a three-layer structure with a thickness of 18 μm, when the thickness ratio of each layer is 1: 1: 1, the thickness of each layer needs to be 6 μm. With such a thin film, it has been more difficult to realize a large pore size while using a high molecular weight polyolefin. In the present embodiment, the application of precisely controlled film formation and stretching conditions as exemplified in the section "Method for manufacturing separators for power storage devices" described later, and / or pentad, is not limited to this. By using polypropylene with a controlled fraction as the polyolefin, even in the microporous layer (X) containing a low MFR (that is, high molecular weight) polyolefin as the main component, the MD-TD of the microporous layer (X) by SEM The average long hole diameter in surface observation or ND-MD cross-sectional observation can be controlled within the above range.

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

<Thickness of microporous layer (X)>
The upper limit of the thickness of the microporous layer (X) according to the present embodiment is preferably 10 μm or less, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4.5 μm from the viewpoint of increasing the energy density of the power storage device. Below, or 4 μm or less. The lower limit of the thickness of the microporous layer (X) is preferably 1 μm or more, 2 μm or more, 3 μm or more, or 3.5 m or more from the viewpoint of strength and the like.

<Additive>
In the present embodiment, the microporous layer (X) containing the polyolefin (A) as a main component is, in addition to the polyolefin (A), an elastomer, a flow modifier (for example, a fluorine-based flow modifier), waxes, and crystal nuclei. Additives such as agents, antioxidants, metal soaps such as aliphatic carboxylic acid metal salts, ultraviolet absorbers, light stabilizers, antistatic agents, antistatic agents, coloring pigments, fillers, etc. are further contained as needed. May be.

<Microporous layer (Y)>
The separator for a power storage device of the present embodiment may have a microporous layer (Y) containing polyolefin (B) as a main component. Here, the separator for a power storage device of the present embodiment may have only one microporous layer (Y), or may have two or more layers. In the present embodiment, the term "polyolefin (B) as a main component" means that the polyolefin (B) is contained in an amount of 50% by mass or more based on the total mass of the microporous layer (Y). The lower limit of the content of the polyolefin (B) in the microporous layer (Y) is preferably 55% by mass or more, 60% by mass or more, from the viewpoint of wettability of the separator with respect to the electrolytic solution, thinning, and shutdown characteristics. It is 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 the polyolefin (B) in the microporous layer (Y) 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, Alternatively, it may be 99% by mass or less, and may be 100% by mass.

<Polyolefin (B)>
The polyolefin (B) in the present embodiment contains a monomer having a carbon-carbon double bond as a repeating unit, and has a molecular structure (more specifically, a chemical composition, a molecular weight, and a crystal) with the polyolefin (A) of the present embodiment. It is a polymer with a different structure, etc.). The monomer constituting the polyolefin (B) is not limited, but is a monomer having a carbon-carbon double bond and having 1 to 10 carbon atoms, for example, ethylene, propylene, 1-butene, 4-methyl-1-pentene, and the like. Examples thereof include 1-hexene and 1-octene. The polyolefin (B) is, for example, a homopolymer, a copolymer, a multistage polymer, or the like, and is preferably a homopolymer.

As the polyolefin (B), specifically, polyethylene, polypropylene, a copolymer of ethylene and propylene, and a mixture thereof are preferable from the viewpoint of shutdown characteristics and the like. The polyolefin (B) is more preferably polyethylene.

When polyethylene is used as the polyolefin (B), the lower limit of the MFR of polyethylene measured at a load of 2.16 kg and a temperature of 190 ° C. is preferably 0.1 g / 10 from the viewpoint of good pore opening and suppression of clogging. Minutes or more, more preferably 0.15 g / 10 minutes or more, still more preferably 0.18 g / 10 minutes or more, and particularly preferably 0.2 g / 10 minutes or more. From the viewpoint of the strength of the separator, the upper limit of the MFR of polyethylene is preferably 2.0 g / 10 minutes or less, more preferably 1.0 g / 10 minutes or less, still more preferably 0.8 g / 10 minutes or less, and particularly preferably 0. .5 g / 10 minutes or less.

<Average long pore diameter of microporous layer (Y)>
In the present embodiment, in the case of ND-MD cross-sectional observation by SEM, the lower limit of the average long pore diameter of the pores existing in the microporous layer (Y) is preferable from the viewpoint of suppressing clogging and low air permeability of the separator. Is 100 nm or more, more preferably 150 nm or more. The upper limit of the average long hole diameter of the pores existing in the microporous layer (Y) in the ND-MD cross-sectional observation by SEM is preferably 2000 nm or less, more preferably 2000 nm or less from the viewpoint of suppressing a short circuit in a power storage device such as a lithium ion battery. It is 1000 nm or less.

<Ratio of average major pore diameter of microporous layer (X) to microporous layer (Y)>
In the present embodiment, in the case of ND-MD cross-sectional observation of the microporous layer (X) and the microporous layer (Y) by SEM, the average long pore diameter of the pores existing in the microporous layer (Y) is the microporous layer. It is preferable that the average long hole diameter of the holes present in (X) is 1.2 times or more and 10 times or less. This means that the average long hole diameter of the holes present in the ND-MD cross section of the microporous layer (Y) is specific to the average long hole diameter of the holes present in the ND-MD cross section of the microporous layer (X). Indicates that it is large in the range of. In ND-MD cross-sectional observation by SEM, it is effective that the separator includes a microporous layer (Y) having an average elongated pore diameter 1.2 times or more larger than the average elongated pore diameter of the pores existing in the microporous layer (X). It is possible to suppress clogging. From the viewpoint of suppressing a short circuit in the power storage device, the average long hole diameter of the pores existing in the microporous layer (Y) is the average length of the holes existing in the microporous layer (X) in the ND-MD cross-sectional observation by SEM. It is preferably 10 times or less the pore diameter. In the ND-MD cross-sectional observation by SEM, the ratio of the average long pore diameter of the pores present in the microporous layer (Y) to the average long pore diameter of the pores present in the microporous layer (X) is more preferably 1.4 times or more. It is 8 times or less.

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

<Multi-layer structure>
The separator for a power storage device may be a single layer of the microporous layer (X) containing the polyolefin (A) as the main component described above, or the microporous layer (X) and the polyolefin (B) as the main components. It may have a multilayer structure in which a microporous layer (Y) is laminated. In one embodiment, the further microporous layer (Y) containing the polyolefin (B) as a main component has the characteristics (eg, specific melt flow rate (MFR) and molecular weight distribution (eg, specific melt flow rate (MFR)) and molecular weight distribution described above with respect to the microporous layer (X). It may be a polyethylene microporous layer having one or more of Mw / Mn) and the like, and in one embodiment, it may be a polyethylene microporous layer having no of these characteristics. Further, the microporous layer (Y) may be a polypropylene microporous layer. Since polyethylene has a melting point suitable for melt shutdown, it is preferable that the microporous layer (Y) contains polyethylene as a main component from the viewpoint of shutdown characteristics.

The multilayer structure means a structure having two or more layers including a microporous layer (X) containing the polyolefin (A) as a main component of the present embodiment. In one aspect, a multilayer structure in which three or more microporous layers are laminated is preferable. The multilayer structure has at least two layers of the microporous layer (X) containing the polyolefin (A) as a main component and at least one layer of the microporous layer (Y) containing the polyolefin (B) as a main component. Is even more preferable. The multilayer structure consists of at least two layers of a microporous layer (X) (PP microporous layer) containing polypropylene as a main component and at least one layer of a microporous layer (Y) (PE microporous layer) containing polyethylene as a main component. It is even more preferable to have.

The multilayer structure can exhibit the advantages of the present embodiment in any order, but has a three-layer structure in which the PP microporous layer / PE microporous layer / PP microporous layer are laminated in this order. Is particularly preferable. By having a three-layer structure of PP microporous layer / PE microporous layer / PP microporous layer, the PP microporous layer maintains good mechanical strength while having good shutdown characteristics due to the PE microporous layer. Can be done. The PP microporous layer is preferably a microporous layer (X) having the specific melt flow rate (MFR) or the like described above.

<Thickness of separator>
The upper limit of the thickness of the separator for a power storage device according to the present embodiment is preferably 25 μm or less, 22 μm or less, 20 μm or less, 18 μm or less, 16 μm or less, 14 μm or less, or 12 μm from the viewpoint of increasing the energy density of the power storage device. It is as follows. The lower limit of the thickness of the separator for a power storage device according to the present embodiment is preferably 6 μm or more, 7 μm or more, 8 μm or more, 9 μm or more, 10 μm or more, or 11 μm or more from the viewpoint of strength and the like.

<Porosity of separator>
The lower limit of the porosity of the separator for a power storage device is preferably 20% or more, more preferably 25% or more, still more preferably 25% or more, from the viewpoint of suppressing clogging in the power storage device and controlling the air permeability of the separator. It is 30% or more, particularly preferably 35% or more. The upper limit of the pore ratio of the separator for a power storage device is preferably 70% or less, more preferably 65% or less, still more preferably 60% or less, and particularly preferably 55% or less from the viewpoint of maintaining the strength of the separator. Porosity is measured by the method described in the Examples.

<Air permeability of separator (air permeability resistance)>
The upper limit of the air permeability of the separator for a power storage device is preferably 300 seconds / 100 cm ³ or less, more preferably 290 seconds / 100 cm ³ or less, 280 seconds / 100 cm ³ or less when the thickness is converted to 14 μm. 270 seconds / 100 cm ³ or less, or 250 seconds / 100 cm ³ or less, more preferably 180 seconds / 100 cm ³ or less, 170 seconds / 100 cm ³ or less, 160 seconds / 100 cm ³ or less, 150 seconds / 100 cm ³ or less, or 140 seconds / 100 cm ³ or less. The lower limit of the air permeability of the separator is, for example, 50 seconds / 100 cm ³ or more, 60 seconds / 100 cm ³ or more, 70 seconds / 100 cm ³ or more, 100 seconds / 100 cm ³ or more, when the thickness of the separator is converted to 14 μm. It may be 110 seconds / 100 cm ³ or more, or 120 seconds / 100 cm ³ or more.

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

<Balance of thickness, air permeability and piercing strength>
As described above, the separator for a power storage device of the present embodiment is a thin film by using a microporous layer containing a polyolefin as a main component having a specific melt tension and melt flow rate (MFR). It is possible to obtain a separator for a power storage device having low air permeability and high strength. For example, the separator for a power storage device of the present embodiment includes a multi-layer structure of microporous layers, the thickness of the multi-layer structure is 18 μm or less, and the air permeability when the thickness of the multi-layer structure is converted to 14 μm is 300 seconds / second. It is more preferable that the thickness is 100 cm ³ or less and the puncture strength when the thickness is converted to 14 μm is 300 gf or more. The thickness of the multi-layer structure is 18 μm or less, the thickness of the microporous layer of the present embodiment included in the multi-layer structure (thickness as a single layer) is 6 μm or less, and the thickness of the multi-layer structure is converted into 14 μm. It is even more preferable that the air permeability is 300 seconds / 100 cm ³ or less and the puncture strength is 300 gf or more.

<Tensile test characteristics of separator>
In the tensile test of the separator for a power storage device according to the present embodiment, it is preferable that the ratio (SMD / STD) of the tensile strength (SMD) in the mechanical direction and the tensile strength (STD) in the width direction satisfies the following relationship.
SMD / STD> 5
A separator having an SMD / STD of more than 5 can be used as a high-strength separator. As the high-strength separator, SMD / STD ≧ 7, SMD / STD ≧ 10, SMD / STD ≧ 12, and SMD / STD ≧ 14 are particularly preferable. The tensile test of the separator is performed by the method described in the item of the example.

<Heat shrinkage rate>
The separator for a power storage device of the present embodiment has a heat shrinkage rate of 1% or less in TD and 4% or less in MD after heat treatment at 105 ° C. for 1 hour, and a heat shrinkage rate after heat treatment at 120 ° C. for 1 hour. It is preferably 1% or less in TD and 10% or less in MD. The heat shrinkage rate after the heat treatment at 105 ° C. is an index of the dimensional stability of the separator for the power storage device when the power storage device is used, particularly when it is placed in a harsh high temperature environment or when it is in an abnormal heat generation state. The heat shrinkage rate after the heat treatment at 120 ° C. is an index of the dimensional stability of the separator for the power storage device when the power storage device is used, especially under the condition that the power storage device is in an abnormal state. The separator for a power storage device of the present embodiment is a power storage device such as a battery because the heat shrinkage of TD and MD is controlled within a specific range after the heat treatment at 105 ° C. for 1 hour and the heat treatment at 120 ° C. for 1 hour. It is possible to ensure the dimensional stability at high temperature inside, suppress the abnormal state of the power storage device, and secure good device characteristics. Details of the method for measuring the heat shrinkage rate of the present disclosure will be described in Examples.

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

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

<Balance of air permeability, puncture strength and heat shrinkage>
The separator for a power storage device of the present embodiment uses a microporous layer (X) containing a polyolefin (A) having a specific melt flow rate (MFR) as a main component, and has low air permeability, high strength, and low heat shrinkage. The rate. In general, low MFR, that is, high molecular weight polyolefin has a large entanglement between polymers, so that a separator using the polymer does not have sufficient pore-opening property, and as a result, it becomes difficult to control the air permeability to a low level. Therefore, with the conventional separator, a polyolefin having a sufficiently high molecular weight cannot be used, and the piercing strength tends to be insufficient, or when a high molecular weight polyolefin is used, the porosity is poor. It became sufficient, and it was difficult to achieve low air permeability and large pore diameter. As described above, conventionally, low air permeability and high strength (particularly high puncture strength) have been in a contradictory relationship. Further, in the conventional dry type separator, the heat shrinkage rate of MD tends to be high, and it is very difficult to obtain a separator having both high strength, low air permeability, and low heat shrinkage rate. By using the specific polyolefin (A) for the microporous layer (X), the separator for a power storage device of the present embodiment can have both low air permeability, high puncture strength, and low heat shrinkage. Such a separator for a power storage device is not limited to this, and for example, the application of precisely controlled film formation and stretching conditions as exemplified in the section << Manufacturing method of a separator for a power storage device >> described later. And / or can be produced by using polypropylene having a controlled pentad fraction as the polyolefin (A).

<< Manufacturing method of separator for power storage device >>
As a method for producing a microporous layer of the present embodiment, generally, a melt extrusion step of melt-extruding a resin composition containing a polyolefin described above (hereinafter referred to as a polyolefin-based resin composition) to obtain a resin film, and obtaining It includes a pore forming step of opening a hole in the obtained resin film to make it porous, and optionally includes an annealing step, a stretching step, a heat relaxation step, and the like. The method for producing a microporous layer is roughly classified into a dry method that does not use a solvent in the pore forming step and a wet method that uses a solvent. Examples of the melt extrusion method include a T-die method and an inflation method.

The polyolefin-based resin composition is a composition composed of a constituent component of the microporous layer (X) in one embodiment, and a composition composed of a constituent component of the microporous layer (Y) in one embodiment. The polyolefin-based resin composition may optionally contain a resin other than the polyolefin, an additive, or the like, depending on the method for producing the microporous layer or the physical characteristics of the target microporous layer. Examples of the additive include pore-forming materials, fluorine-based flow modifiers, elastomers, waxes, crystal nuclei, antioxidants, metal soaps such as aliphatic carboxylic acid metal salts, ultraviolet absorbers, light stabilizers, and antistatic agents. Examples thereof include an inhibitor, an antifogging agent, and a coloring pigment. Examples of the pore-forming material include a plasticizer, an inorganic filler, or a combination thereof.

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

Examples of the inorganic filler include oxide-based ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, ittoria, zinc oxide, and iron oxide; nitrided silicon nitride, titanium nitride, boron nitride, etc. 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, kaolin, kaolin, and other ceramics; glass fibers can be mentioned.

The temperature of the extruder at the time of melt-kneading is preferably "a temperature 20 ° C. higher than the melting point of the polyolefin resin composition" and "a temperature 110 ° C. higher than the melting point of the polyolefin resin composition" or less. When the temperature is equal to or higher than the above lower limit, the thickness of the obtained polyolefin resin film becomes uniform, and the phenomenon that the film is cut during extrusion is unlikely to occur. When the temperature is not more than the above upper limit, the orientation of the polyolefin contained in the polyolefin-based resin composition is high and the lamellar structure is satisfactorily generated in the polyolefin, so that the air permeability of the obtained separator for a power storage device is low. , Lithium-ion secondary batteries and other energy storage devices have low resistance. When the polyolefin-based resin film has a plurality of layers formed by using different polyolefin-based resin compositions, the melting point refers to the melting point of the polyolefin-based resin composition having a lower melting point. The melting point is evaluated as the peak top temperature of the endothermic peak observed when the temperature is raised at a heating rate of 10 ° C./min using a DSC (Differential Scanning Calorimetry) in one embodiment. Will be done.

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

The polyolefin-based resin composition extruded from the die may be cooled with air.
When the polyolefin resin extruded from the die is sufficiently cooled with air, the polyolefin resin crystallizes to form lamella, so that the average long pore diameter and maximum long pore diameter are large, the porosity is high, and the microporous with low air permeability is high. A separator having a layer is obtained. The air volume is preferably 300 L / min or more and 1000 L / min or less with respect to a die width of 500 mm. When the air volume is equal to or higher than the above lower limit, the polyolefin-based resin composition extruded from the die is sufficiently cooled by air, so that the polyolefin crystallizes to form lamella, and the microporous layer tends to have low air permeability. It is preferable because there is. Further, it is preferable to set the air volume to the above upper limit or less from the viewpoint of improving the film formation stability of the film and the thickness accuracy and width accuracy of the obtained polyolefin resin sheet. The temperature of the air is preferably 0 ° C. or higher and 40 ° C. or lower from the viewpoint of film formation stability, thickness accuracy and width accuracy.

The method for manufacturing a separator for a power storage device including the microporous layer of the present embodiment and having a multilayer structure in which a plurality of microporous layers are laminated is not limited to these, and examples thereof include a coextrusion method and a laminating method. Can be mentioned. In the coextrusion method, the resin composition of each layer is coextruded and laminated into two or more layers to prepare a raw film. A microporous layer can be produced by stretching and opening two or more layers of the obtained raw fabric film. Rather than preparing each of the single microporous layers, it is better to prepare the original fabric of the film laminated in two or more layers and then open the pores to prepare the microporous layer with high strength. It is easy to obtain a porous layer. In the laminating method, the layers are brought into close contact with each other by forming the layers separately and then laminating them. Examples of the laminating method include a dry laminating method using an adhesive and a thermal laminating method in which a plurality of layers are bonded by heating. From the viewpoint of further improving the air permeability and strength of the obtained separator for a power storage device. Therefore, the thermal laminating method is preferable.

Next, an annealing step of heat-treating the obtained polyolefin-based resin film is performed. This annealing step grows the lamella produced on the polyolefin-based resin film in the extrusion step. By performing this treatment, the porosity of the film can be improved and the air permeability of the obtained separator for a power storage device can be reduced.

The annealing temperature of the polyolefin-based resin film is preferably set to "a temperature 40 ° C. lower than the melting point of the polyolefin-based resin film" or more and "a temperature 1 ° C. lower than the melting point of the polyolefin-based resin film" or less. .. However, when the polyolefin resin film has a laminated structure, the annealing temperature is "40 ° C lower than the melting point of the resin film having the lowest melting point" and "1 ° C lower than the melting point of the resin film having the lowest melting point". It is preferably limited to "temperature" or lower. The annealing temperature is the temperature of the atmosphere inside the annealing device. When the annealing temperature is equal to or higher than the above lower limit, the lamella grows satisfactorily and is easily opened in the film stretching step. Structural collapse is suppressed.

The annealing time of the polyolefin-based resin film is preferably 10 minutes or more from the viewpoint of satisfactorily growing the lamella and satisfactorily opening the pores of the polyolefin-based resin film in the stretching step. The annealing time may be 180 minutes or less in one embodiment.

The annealing of the polyolefin-based resin film may be performed while running the polyolefin-based resin film, or may be performed in a state where the polyolefin-based resin film is wound into a roll.

Next, a stretching step of stretching and opening the annealed polyolefin resin film is performed. This stretching step preferably includes a first stretching step and a second stretching step following the first stretching step.

In the first stretching step, the lamellas generated in the polyolefin-based resin film are separated from each other to generate fine cracks in the amorphous portion between the lamellas, and a large number of micropores are formed from these cracks as a starting point. .. In the first stretching step, MD is uniaxially stretched.

In the first stretching step, the lower limit of the temperature of the polyolefin resin film is preferably −20 ° C. or higher, more preferably 0 ° C. or higher. In the first stretching step, the upper limit of the temperature of the polyolefin resin film is preferably 110 ° C. or lower, more preferably 80 ° C. or lower. When the temperature is above the above lower limit, the breaking of the polyolefin resin film during stretching is suppressed, and when the temperature is below the above upper limit, cracks are satisfactorily generated in the amorphous portion between the lamellas, and the film. Neck-in is suppressed.

In the first stretching step, the lower limit of the draw ratio of the polyolefin resin film is preferably 1.02 times or more, more preferably 1.06 times or more. In the first stretching step, the upper limit of the draw ratio of the polyolefin resin film is preferably 1.5 times or less, more preferably 1.4 times or less. When the draw ratio is equal to or higher than the above lower limit, micropores are likely to be formed in the amorphous portion between the lamellae. When the draw ratio is not more than the above upper limit, the micropores are not excessively formed and the pore diameter is not too small, so that the air permeability can be lowered. In the present disclosure, the stretch ratio of the polyolefin-based resin film means a value obtained by dividing the length of the polyolefin-based resin film after stretching by the length of the polyolefin-based resin film before stretching.

The stretching speed in the first stretching step of the polyolefin resin film is preferably 10 to 1000% / min, more preferably 50 to 600% / min. When the stretching speed is at least the above lower limit, micropores are likely to be uniformly formed in the amorphous portion between the lamellae, and when it is at least the above upper limit, breakage of the polyolefin resin film can be suppressed.

In the present disclosure, the stretching speed of the polyolefin-based resin film means the rate of change in dimensions of the polyolefin-based resin film in the stretching direction per unit time.

The method for stretching the polyolefin-based resin film in the first stretching step is not particularly limited as long as the polyolefin-based resin film can be uniaxially stretched, and for example, the polyolefin-based resin film is stretched at a predetermined temperature using a uniaxial stretching device. Examples thereof include a method of stretching.

Next, for the polyolefin-based resin film after uniaxial stretching in the first stretching step, preferably, the atmospheric temperature inside the apparatus is higher than the atmospheric temperature at the time of uniaxial stretching in the first stretching step, and the polyolefin-based resin film (multilayer). In the case of the structure, it is preferable to carry out the second stretching step of stretching so that the temperature is 1 ° C. or higher and 60 ° C. or lower lower than the melting point of the resin film having the lowest melting point). Also in the second stretching step, the polyolefin-based resin film is preferably uniaxially stretched only in the mechanical direction. As described above, by performing the stretching treatment on the polyolefin-based resin film at an atmospheric temperature higher than the atmospheric temperature in the apparatus in the first stretching step, a large number of minute particles formed on the polyolefin-based resin film in the first stretching step. The holes can be grown. When the temperature is equal to or higher than the above lower limit, the micropores formed in the polyolefin-based resin film are likely to grow in the first stretching step, and the air permeability of the obtained separator for a power storage device can be lowered. When the temperature is not more than the above upper limit, the micropores formed in the polyolefin-based resin film in the first stretching step are less likely to be closed, and the air permeability of the obtained separator for a power storage device can be lowered.

In the second stretching step, the stretching ratio of the polyolefin-based resin film is preferably 1.5 times or more and 3 times or less, and more preferably 1.8 times or more and 2.5 times or less. When the draw ratio is at least the above lower limit, the micropores formed in the polyolefin-based resin film during the first stretching step are likely to grow, and the air permeability of the obtained separator for a power storage device can be lowered. When the draw ratio is not more than the above upper limit, the micropores formed in the polyolefin-based resin film in the first stretching step are less likely to be closed, and the air permeability of the obtained separator for a power storage device can be lowered.

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

The method for stretching the polyolefin-based resin film in the second stretching step is not particularly limited as long as the polyolefin-based resin film can be uniaxially stretched, and for example, a method for uniaxially stretching the polyolefin-based resin film at a predetermined temperature using a uniaxial stretching device is used. Can be mentioned.

The polyolefin-based resin film uniaxially stretched in the second stretching step is subjected to a heat relaxation step of heating to relieve residual stress. Residual stress may occur in the polyolefin-based resin film due to stretching in the second stretching step. In the heat relaxation step, the residual stress is relaxed, and the obtained polyolefin-based resin microporous layer is suppressed from being thermally shrunk by heating different from the heat relaxation step, so that the safety of the obtained separator for a power storage device is improved. It is done to improve.

As described above, in order to improve the dimensional stability of the microporous polyolefin resin film during heating, it is necessary to relax the residual stress of the polyolefin resin film. For that purpose, the ambient temperature in the apparatus in the heat relaxation step is 40 ° C. or higher, or 20 ° C. lower than the melting point of the polyolefin resin film (resin film having the lowest melting point in the case of a multilayer structure). It is preferably more than that. The temperature is 1 ° C. or higher, or 4 ° C., higher than the melting point of the polyolefin-based resin film (the resin film having the lowest melting point in the case of a multilayer structure) from the viewpoint of suppressing the clogging of the micropores formed in the stretching step. As mentioned above, it is preferable that it is low.

The lower limit of the heat shrinkage of the polyolefin resin film in the heat relaxation step is preferably 25% or more, more preferably 30% or more. The upper limit of the heat shrinkage rate of the polyolefin-based resin film in the heat relaxation step is preferably 60% or less, more preferably 50% or less. The heat shrinkage rate of the polyolefin resin film in the heat relaxation step is the shrinkage length of the polyolefin resin film in the stretching direction in the heat relaxation step, and the length of the polyolefin resin film in the stretching direction after the second stretching step. Divided by and multiplied by 100. When the heat shrinkage rate is equal to or higher than the above lower limit, the residual stress of the polyolefin-based resin film is sufficiently relaxed, the dimensional stability of the obtained polyolefin-based resin microporous layer during heating is good, and lithium ions at high temperatures are obtained. The safety of power storage devices such as secondary batteries is good. Further, when the shrinkage rate heat is not more than the above upper limit, the polyolefin-based resin film is less likely to sag, and poor winding of the roll or deterioration of uniformity is suppressed.

In the second stretching step and the heat relaxation step, it is preferable to adjust the stretching speed and the transport speed so that they do not become excessively large from the viewpoint of sufficiently relaxing the stress and reducing the heat shrinkage rate.

Further, after the heat relaxation step, the heat relaxation is applied again at a temperature equal to or higher than the temperature of the heat relaxation step and at a temperature 20 ° C. higher than or lower than the temperature of the heat relaxation step. Is preferable. By applying this step, it becomes possible to obtain a separator for a power storage device having better heat shrinkage physical properties.

《Power storage device》
The power storage device of the present embodiment includes the separator for the power storage device of the present embodiment. The power storage device of the present embodiment has a positive electrode and a negative electrode. The separator for a power storage device is preferably laminated between the positive electrode and the negative electrode, located outside the positive electrode or the negative electrode in the battery exterior, or encloses the electrode. If desired, the lead body may be connected to the positive electrode and the negative electrode, respectively, so that the positive electrode and the negative electrode can be connected to an external device or the like.

The power storage device is not limited, and is, for example, a lithium secondary battery, a lithium ion secondary battery, a sodium secondary battery, a sodium ion secondary battery, a magnesium secondary battery, a magnesium ion secondary battery, a calcium secondary battery, and a 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, lithium sulfur batteries, lithium air batteries, zinc air batteries, etc. Can be mentioned. Among these, a lithium secondary battery, a lithium ion secondary battery, a nickel hydrogen battery, or a lithium ion capacitor is preferable, and a lithium ion secondary battery is more preferable, from the viewpoint of compatibility with the separator according to the present embodiment. ..

The power storage device can be manufactured, for example, by the following method:
The positive electrode and the negative electrode are overlapped with each other via the separator described above and wound as needed to form a laminated electrode body or a wound electrode body. After that, the laminated electrode body or the wound electrode body is loaded into the exterior body. At that time, the positive and negative electrodes in the exterior body can be arranged so that the lead body is connected to each of the positive and negative electrodes and the end portion of the lead body is exposed to the outside of the exterior body. When there are a plurality of positive electrodes and a plurality of negative electrodes, tabs having the same electrode may be joined by welding or the like to be joined to one lead body and exposed to the outside of the exterior body. The tabs of the same pole can be composed of an exposed portion of the current collector, or can be configured by welding a metal piece to the exposed portion of the current collector. The positive and negative terminals and the positive and negative terminals of the exterior body are connected via a lead body or the like. At that time, a part of the lead body and a part of the exterior body may be joined by heat fusion or the like. Further, a non-aqueous electrolyte solution containing a non-aqueous solvent such as a chain and / or cyclic carbonate and an electrolyte such as a lithium salt is injected into the exterior body. After that, the exterior body can be sealed to manufacture a power storage device.

Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

<< Measurement and evaluation method >>
[Measurement of melt flow rate (MFR)]
The melt flow rate (MFR) of the microporous layer was measured according to JIS K 7210 under the 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 the 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. The MFR of the elastomer was measured according to JIS K 7210 under the conditions of a temperature of 230 ° C. and a load of 2.16 kg.

[Measurement of Mw and Mn by GPC (Gel Permeation Chromatography)]
Using the Agilent PL-GPC220 from Agilent Technologies, Inc., standard polystyrene was measured under the following conditions to create a calibration curve. For the polymer of the sample, the chromatograph was measured under the same conditions, and based on the calibration curve, the weight average molecular weight (Mw), the number average molecular weight (Mn), and the weight average molecular weight (Mw) of the polymer were numerically averaged under the following conditions. The value (Mw / Mn) divided by the molecular weight (Mn) was calculated.

Column: TSKgel GMHHR-H (20) HT (7.8 mm ID x 30 cm) 2 mobile phases: 1,2,4-trichlorobenzene detector: RI
Column temperature: 160 ° C
Sample concentration: 1 mg / ml
Calibration curve: Polystyrene

[Measurement of melt tension]
Using a capillograph manufactured by Toyo Seiki Seisakusho Co., Ltd., the melt tension (mN) of the microporous layer was measured under the following conditions.
・ Capillary: diameter 1.0 mm, length 20 mm
・ Cylinder extrusion speed: 2 mm / min ・ Pick-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 spectra assigned based on the description in the Polymer Analysis Handbook (edited by the Japan Society for Analytical Chemistry). ¹³ The measurement of the C-NMR spectrum was carried out using JEOL-ECZ500 manufactured by JEOL Ltd. under the conditions that polypropylene was dissolved in o-dichlorobenzene-d, the measurement temperature was 145 ° C, and the number of integrations was 25,000.

[Measurement of thickness (μm)]
The thickness (μm) of the separator was measured at room temperature of 23 ± 2 ° C. using the Digimatic Indicator IDC112 manufactured by Mitutoyo Co., Ltd.

[Measurement of porosity (%)]
A 10 cm × 10 cm square sample was cut from a separator, its volume (cm ³ ) and mass (g) were determined, and the porosity was calculated from them and the density (g / cm ³ ).

[Measurement of air permeability resistance (seconds / 100 cm ³ )]
Using a Garley type air permeability meter compliant with JIS P-8117, measure the air permeability resistance (seconds / 100 cm ³ ) of the separator, divide by the thickness in which the unit is μm, and then multiply by 14. The air permeability resistance (air permeability) converted to 14 μm was calculated.

[Measurement of puncture strength]
A needle having a hemispherical tip with a radius of 0.5 mm was prepared, a separator was sandwiched between two plates having an opening having a diameter (dia.) Of 11 mm, and the needle, separator and plate were set. A puncture test was performed using "MX2-50N" manufactured by Imada Co., Ltd. under the conditions of a radius of curvature of the needle tip of 0.5 mm, an opening diameter of the separator holding plate of 11 mm, and a piercing speed of 25 mm / min. The needle and separator were brought into contact and the maximum piercing load (ie, piercing strength (gf)) was measured. By dividing the obtained piercing strength by the thickness and then multiplying by 14, the piercing strength converted to a thickness of 14 μm was calculated.

[Tensile test]
A tensile test was performed using a tensile tester (Shimadzu Corporation Autograph AG-A type), and the strength at break of the sample was divided by the cross-sectional area of the sample before the test to obtain the tensile strength at break (kg / cm ² ). bottom. The measurement conditions were temperature; 23 ± 2 ° C., sample shape; width 10 mm × length 100 mm, distance between chucks; 50 mm, tensile speed; 200 mm / min, and the tensile strength SMD of MD and the tensile strength STD of TD were measured. The SMD / STD was calculated.

[Measurement of average long hole diameter and maximum long hole diameter]
The average long hole diameter and the maximum long hole diameter were measured by image analysis by SEM observation of holes existing on the MD-TD surface or the ND-MD cross section. As a pretreatment, the separator was stained with ruthenium. The obtained sample is fixed to an SEM sample table 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.), and then an applied voltage. Osmium coating was performed under the conditions of the adjustment knob setting of 4.5 and the discharge time of 0.5 seconds, and used as a microscopic sample.

The microscopic sample was observed with a scanning electron microscope (SEM) at an acceleration voltage of 1.0 kV, and an SEM image was obtained. The long hole diameter of each hole in the range of 4 μm × 4 μm of the obtained surface or cross-sectional SEM image is measured, and if necessary, binarization treatment for separating the resin part and the hole part is performed to obtain the maximum long hole diameter and the average length. The hole diameter was calculated. At this time, the micropores existing in the range of 4 μm × 4 μm and the portion outside the range, and the pores having an area of 0.001 μm ² or less were excluded from the measurement range. For the long hole diameter of the hole, the value of the average long hole diameter was obtained by calculating the average value of the area weighting of the holes. Of the major diameters of the holes, the largest one was adopted as the maximum major pore diameter. Tables 1 to 6 show the maximum long hole diameter or the average long hole diameter measured by SEM observation of the MD-TD surface.

[Measurement of heat shrinkage]
The sample obtained by cutting the separator into squares of 50 mm each for MD / TD was placed in a hot air dryer (manufactured by Yamato Scientific Co., Ltd., DF1032) heated to 105 ° C and 120 ° C, respectively (normal pressure, under the atmosphere). I put it in. After 1 hour and 2 hours, the sample was taken out from the hot air dryer and the heat shrinkage was determined. The sample was placed on a copy paper and then placed in a hot air dryer so as not to adhere to the inner wall of the dryer and to prevent the samples from fusing to each other.
Heat shrinkage rate (%): (Dimension before heating (mm) -Dimension after heating (mm)) / (Dimension before heating (mm)) x 100

[Manufacturing of sheet-shaped lithium-ion secondary battery]
As the 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 composite oxide (LiNi _0.5 Co _0.2 Mn _0.3 O ₂ ) as the positive electrode active material and carbon black powder (manufactured by Timcal Co., Ltd., trade name: SuperP Li) as the conductive auxiliary agent. ) And PVDF as a binder were mixed in a mass ratio of composite oxide: conductive auxiliary agent: binder = 100: 3.5: 3. The obtained mixture was dispersed in a solvent (N-methylpyrrolidone) to form a dispersion. A dispersion was applied to both sides of an aluminum foil as a positive electrode current collector having a thickness of 15 μm, the solvent was removed by drying, and then pressed with a roll press to prepare a double-sided coated positive electrode.

Graphite powder with a particle size of 22 μm (D50) (manufactured by Hitachi Kasei Co., Ltd., trade name: MAG), binder (manufactured by Nippon Zeon Co., Ltd., trade name: BM400B) as a negative electrode active material, and carboxymethyl cellulose (Dycel) as a thickener. Co., Ltd., trade name: # 2200) was mixed in a mass ratio of graphite powder: binder: thickener = 100: 1.5: 1.1. The obtained mixture was dispersed in a solvent (water) to prepare an aqueous dispersion. The aqueous dispersion was applied to one side of a copper foil as a negative electrode current collector having a thickness of 10 μm to prepare a single-sided coated body. Separately from the single-sided coated body, a water dispersion was applied to both sides of a copper foil as a negative electrode current collector having a thickness of 10 μm to prepare a double-sided coated body. The solvent (water) was dried and removed from the single-sided coating body and the double-sided coating body, and then the coated copper foil was pressed with a roll press to prepare a single-sided coating negative electrode and a double-sided coating negative electrode, respectively.

The obtained positive electrode and negative electrode are placed on the facing surfaces of the respective active materials with the separator prepared below sandwiched between them. Was laminated in the order of. Next, the obtained laminated body was inserted into a bag (battery exterior) formed of a laminated film in which both sides of an aluminum foil (thickness 40 μm) were coated with a resin layer. Terminals of the inserted electrodes are projected from the battery exterior. Then, 0.8 mL of the electrolytic solution prepared as described above was injected into the bag, and the bag was vacuum-sealed to prepare a sheet-shaped lithium ion secondary battery.

[Battery performance evaluation and observation of clogging]
The obtained sheet-shaped lithium-ion secondary battery is housed in a constant temperature bath (manufactured by Futaba Kagaku Co., Ltd., trade name: PLM-73S) set at 25 ° C., and charged / discharged (manufactured by Asuka Electronics Co., Ltd., trade name: ACD). It was connected to -01) and allowed to stand for 16 hours. Next, the battery is charged with a constant current of 0.05 C, and after the voltage reaches 4.35 V, it is charged with a constant voltage of 4.35 V for 2 hours, and then 3.0 V with a constant current of 0.2 C. The initial charge / discharge of the battery was performed by repeating the charge / discharge cycle of discharging to 3 times. Note that 1C indicates a current value when the entire capacity of the battery is discharged in 1 hour.

After the initial charge / discharge, the battery is housed in a constant temperature bath set at 50 ° C., the battery is charged with a constant current of 1C, and after the voltage reaches 4.35V, the constant voltage is 4.35V. After charging for 1 hour, the charge / discharge cycle of discharging to 3.0 V with a constant current of 1 C was repeated 100 times. The battery cycle test is a test in which the above charge / discharge cycle is repeated 100 times.

The value (percentage) obtained by dividing the discharge capacity (mAh) of the 100th cycle by the discharge capacity (mAh) of the first cycle was defined as the cycle capacity retention rate. Further, the sheet-shaped lithium ion secondary battery after 100 cycles was disassembled in an argon atmosphere, and the separator was taken out. The separator was immersed in an ethylmethyl carbonate-containing tank for 30 seconds and taken out to perform the first immersion washing. A total of 3 immersion washes were performed. In the second and third immersion washes, the ethylmethyl carbonate in the tank was replaced. Then, the surface on the negative electrode side of the separator was observed in a range of 1 mm square under the condition of a magnification of 100 to 1000 times with an optical microscope, and the presence or absence of clogging on the surface of the separator was confirmed. Tables 1 to 4 show the presence or absence of clogging.

[Short-circuit evaluation in lithium-ion secondary battery]
The obtained sheet-shaped lithium-ion secondary battery is housed in a constant temperature bath (manufactured by Futaba Kagaku Co., Ltd., trade name: PLM-73S) set at 25 ° C., and charged / discharged (manufactured by Asuka Electronics Co., Ltd., trade name: ACD). It was connected to -01) and allowed to stand for 16 hours. Next, the battery is charged with a constant current of 0.05 C, and after the voltage reaches 4.35 V, it is charged with a constant voltage of 4.35 V for 2 hours, and then 3.0 V with a constant current of 0.2 C. The initial charge / discharge of the battery was performed by repeating the charge / discharge cycle of discharging to 3 times. Note that 1C indicates a current value when the entire capacity of the battery is discharged in 1 hour.

After the initial charge / discharge, the battery is charged to 4.0 V with a constant current of 0.2 C, the battery is housed in a constant temperature bath, the temperature of the constant temperature bath is raised from 25 ° C. to 5 ° C. every 10 minutes, and the battery voltage is increased. The temperature at which the voltage drops to 0.5 V or less was confirmed, and the temperature was set as the short-circuit generation temperature, which is an index of battery safety.

<< Example 1 >>
[Preparation of single-layer separator (microporous layer (X))]
2. High molecular weight polypropylene resin (PP, MFR (230 ° C.) = 0.51 g / 10 min, density = 0.91 g / cm ³ , Mw / Mn = 5.2, pentad fraction = 99.3%). It was melted in a 5-inch extruder and supplied to a T-die with a die width of 500 mm and a die lip clearance of 3.3 mm using a gear pump. The temperature of the T-die was set to 210 ° C. The molten polymer was ejected from the T-die. Then, the discharged resin was sufficiently cooled by the blown air at 12 ° C. with an air volume of 500 L / min and wound on a roll. The raw film wound on the roll had a thickness of 15 μm and had a draw ratio of 220. The raw film was then annealed at 130 ° C. for 20 minutes. The annealed raw film was cold-stretched to 8% at room temperature, then 185% hot-stretched at 116 ° C, and heat-relaxed 45% at 126 ° C to form microporous, resulting in a PP single-layer structure. A separator made of a microporous layer (X) was obtained. After the stretching and opening, the air permeability, puncture strength, pore ratio, average long hole diameter and maximum long hole diameter of the obtained separator were measured. The results are shown in Table 1.

<< Examples 2 to 11 >>
A separator composed of a microporous layer (X) was obtained according to the same method as in Example 1 except that the raw materials and production conditions were changed as shown in Table 1, and the obtained separator was evaluated.

<< Comparative Examples 1 to 11 >>
A separator composed of a microporous layer (X) was prepared according to the same method as in Example 1 except that the raw materials and production conditions were changed as shown in Table 2, and the separator was evaluated.

<< Example 12 >>
[Preparation of three-layer separator (microporous layer (X) / microporous layer (Y) / microporous layer (X)]
2. High molecular weight polypropylene resin (PP, MFR (230 ° C.) = 0.51 g / 10 minutes, density = 0.91 g / cm ³ , Mw / Mn = 5.2, pentad fraction = 99.3%). It was melted in a 5-inch extruder and supplied to the T-die using a gear pump. The temperature of the T-die was set to 230 ° C. The molten polymer was ejected from the T-die. Then, the discharged resin was cooled by the blown air, and the PP raw film (X'), which is a precursor of the microporous layer (X), was wound on a roll.
Similarly, polyethylene resin (PE, MFR (190 ° C.) = 0.38 g / 10 minutes, density = 0.96 g / cm ³ ) is melted in a 2.5 inch extruder and transferred to a T-die using a gear pump. And supplied. The temperature of the T-die was set to 220 ° C. The molten polymer was ejected from the T-die. Then, the discharged resin was cooled by the blown air, and the PE raw film (Y'), which is a precursor of the microporous layer (Y), was wound on a roll.
The PP raw fabric film (X') and the PE raw fabric film (Y') wound on the roll each have a thickness of 5 μm, followed by the PP raw fabric film (X') and the PE raw fabric film (Y'). Y') is bound so as to be a PP raw film (X') / PE raw film (Y') / PP raw film (X'), and has a three-layer structure of PP / PE / PP. An original film was obtained. Then, the original film having a three-layer structure was annealed at 130 ° C. for 20 minutes. The annealed raw film was cold-stretched to 11% at room temperature, then hot-stretched to 158% at 125 ° C., and relaxed to 113% at 125 ° C. to form microporous layers (microporous layer). A separator having a PP / PE / PP three-layer structure composed of X) / microporous layer (Y) / microporous layer (X) was obtained.

After the stretching and opening, the air permeability resistance and the piercing strength of the obtained three-layer structure separator were measured, and the MFR and melt tension of the microporous layer (X) were measured. The lower limit thickness of the PP raw fabric film (X') was evaluated by increasing the take-up speed while keeping the discharge amount constant after the PP raw fabric film (X') was manufactured. Further, the obtained PP raw film having the lower limit thickness was laminated with the PE raw film (Y') having a thickness of 5 μm, and then the annealing and stretching conditions (annealed at 125 ° C. for 20 minutes and up to 11% at room temperature) were performed. A separator having a three-layer structure was produced by cold stretching, hot stretching to 158% at 125 ° C., and relaxing to 113% at 125 ° C.) to prepare a separator having a three-layer structure, and the thickness of the separator was confirmed. The results are shown in Table 3.

<< Examples 13 to 16 >>
Separator having a three-layer structure was obtained according to the same method as in Example 12 except that the raw materials and production conditions were changed as shown in Table 3, and the obtained separator was evaluated.

<< Comparative Examples 12-18 >>
Separators having a three-layer structure were prepared according to the same method as in Example 12 except that the raw materials and production conditions were changed as shown in Table 4, and the obtained separators were evaluated.

<< Example 17 >>
[Preparation of microporous layer]
High molecular weight polypropylene (PP, MFR (230 ° C.) = 0.51 g / 10 minutes, density = 0.91 g / cm ³ , Mw = 900,000, Mw / Mn = 5.2, pentad fraction = 99.3%) Was melted in a 2.5 inch extruder and supplied to the T-die using a gear pump. The temperature of the T-die was set to 230 ° C. The molten polymer was ejected from the T-die. Then, while quenching the discharged resin with 800 L / min of blown air cooled to 10 ° C. by a chiller, the precursor (X') (microporous layer) having a thickness of about 6 μm was wound around the roll at a roll speed of 25 m / min. A precursor of X)) was obtained. Here, the lip width of the TD of the T-die was set to 500 mm, the distance between the lips of the T-die (lip clearance) was set to 2.4 mm, and the discharge was performed at a discharge rate of 3.8 kg / h.
Similarly, as the polyolefin (B), polyethylene (PE, MFR (190 ° C.) = 0.38 g / 10 minutes, density = 0.96 g / cm ³ ) is melted in a 2.5 inch extruder and T-die. It was supplied using a gear pump. The temperature of the T-die was set to 210 ° C. The molten polymer was ejected from the T-die. Then, while quenching the discharged resin with 800 L / min of blown air cooled to 10 ° C. by a chiller, the precursor (Y') (microporous layer) having a thickness of about 6 μm was wound around the roll at a roll speed of 25 m / min. Y) precursor) was obtained. Here, the lip width of the TD of the T-die was set to 500 mm, the distance between the lips of the T-die (lip clearance) was set to 2.4 mm, and the discharge was performed at a discharge rate of 3.8 kg / h.

Next, a thermocompression bonding laminator was used to convert the obtained precursor (X') and precursor (Y') into a precursor (X') / precursor (Y') / precursor (X'). The precursor (Z) having a three-layer structure was obtained by thermocompression bonding at 120 ° C. and 4 m / min. Next, the obtained precursor (Z) was put into a dryer and subjected to annealing treatment at 120 ° C. for 1 hour. Then, the annealed precursor (Z) was cold-stretched at room temperature by 8%, placed in an oven at 116 ° C. without shrinking the stretched membrane, hot-stretched to 185%, and then 124. By relaxing 25% at ° C. and further relaxing 20% at 128 ° C., a separator having a three-layer structure consisting of a microporous layer (X) / microporous layer (Y) / microporous layer (X) was obtained. .. Table 5 shows the structure, physical properties, and battery performance evaluation results of the obtained separator.

<< Examples 18-21, Comparative Example 19, Comparative Example 20 >>
A microporous layer was obtained according to the same method as in Example 17 except that the raw materials were changed as shown in Tables 5 and 6, and the obtained separator was evaluated.

<< Example 22 >>
In the heat relaxation step, the microporous layer was relaxed according to the same method as in Example 17 except that the heat relaxation in the first step was relaxed by 15% at 124 ° C. and the heat relaxation in the second step was relaxed by 10% at 128 ° C. Was obtained, and the microporous layer and the separator obtained using the same were evaluated.

<< Example 23 >>
In the heat relaxation step, a microporous layer was obtained according to the same method as in Example 17 except that the heat relaxation in the first step was relaxed by 20% at 126 ° C. and the heat relaxation in the second step was not performed. The layers and the separators obtained with them were evaluated.

<< Comparative Example 21 >>
As shown in Table 6, the raw material of the polyolefin was changed, and the film was formed by the coextrusion process instead of the laminating process. Specifically, the polypropylene shown in Table 6 was melted at 220 ° C. using a 2.5 inch extruder, and the polyethylene according to Example 17 was melted at 200 ° C., respectively. Next, the polypropylene molten resin and the polyethylene molten resin were supplied to a coextrusion T-die (220 ° C.) at a discharge rate ratio of 1: 1: 1 so as to have a three-layer structure of polypropylene / polyethylene / polypropylene, and the polymer was melted. Was discharged from the T die at 11.4 kg / h. Then, while quenching the discharged resin with 800 L / min of blown air cooled to 20 ° C. by a chiller, the precursor (Z) (fine) having a three-layer structure having a thickness of about 18 μm is wound around the roll at a roll speed of 25 m / min. A precursor of the porous layer (Z)) was obtained. The obtained precursor (Z) was put into a dryer and subjected to annealing treatment at 120 ° C. for 1 hour. Then, the annealed precursor (Z) was cold-stretched at room temperature by 8%, placed in an oven at 125 ° C. without shrinking the stretched membrane, hot-stretched to 150%, and then hot-stretched. By relaxing by 25%, a separator having a three-layer structure composed of a microporous layer (X) / microporous layer (Y) / microporous layer (X) was obtained. Table 6 shows the structure, physical properties, and battery performance evaluation results of the obtained separator.

The separator for a power storage device of the present embodiment has high strength and can be made into a thin film, and can be suitably used as a separator for a power storage device, for example, a lithium ion secondary battery or the like.

The microscopic sample was observed with a scanning electron microscope (SEM) at an acceleration voltage of 1.0 kV, and an SEM image was obtained. The long hole diameter of each hole in the range of 4 μm × 4 μm of the obtained surface or cross-sectional SEM image is measured, and if necessary, binarization treatment for separating the resin part and the hole part is performed, and the maximum long hole diameter and the average long hole diameter are performed. Was calculated. At this time, the micropores existing in the range of 4 μm × 4 μm and the portion outside the range, and the pores having an area of 0.001 nm ² or less were excluded from the measurement range. For the long hole diameter of the hole, the value of the average long hole diameter was obtained by calculating the average value of the area weighting of the holes. Of the major diameters of the holes, the largest one was adopted as the maximum major pore diameter. Tables 1 to 6 show the maximum long hole diameter or the average long hole diameter measured by SEM observation of the MD-TD surface.

Claims (16)

It contains a microporous layer (X) whose main component is polyolefin (A), and contains.
The microporous layer (X) has a load of 2.16 kg and a melt flow rate (MFR) of 0.90 g / 10 minutes or less at a temperature of 230 ° C., and the microporous layer (X) is measured by a scanning electron microscope (SEM). ), The average major pore diameter of the pores existing in the microporous layer (X) is 100 nm or more in the MD-TD surface observation or the ND-MD cross-sectional observation.
Separator for power storage device. In the MD-TD surface observation or ND-MD cross-sectional observation of the microporous layer (X) by the SEM, the maximum long pore diameter of the pores existing in the microporous layer (X) is 100 nm or more and 400 nm or less.
The separator for a power storage device according to claim 1. The melt tension of the microporous layer (X) when measured at a temperature of 230 ° C. is 16 mN or more.
The separator for a power storage device according to claim 1 or 2. The melt tension of the microporous layer (X) when measured at a temperature of 230 ° C. is 16 mN or more and 40 mN or less.
The separator for a power storage device according to claim 1 or 2. The ratio (SMD / STD) of the tensile strength (SMD) in the mechanical direction and the tensile strength (STD) in the width direction of the separator for a power storage device is SMD / STD> 5.
The separator for a power storage device according to any one of claims 1 to 4. The heat shrinkage of the separator for a power storage device after heat treatment at 105 ° C. for 1 hour is 1% or less in TD and 4% or less in MD.
The separator for a power storage device according to any one of claims 1 to 5, wherein the heat shrinkage rate of the separator for a power storage device after heat treatment at 120 ° C. for 1 hour is 1% or less in TD and 10% or less in MD. The air permeability when the thickness of the separator for a power storage device is converted to 14 μm is 250 seconds / 100 cm ³ or less.
The separator for a power storage device according to any one of claims 1 to 6. The separator for a power storage device according to any one of claims 1 to 7, wherein the thickness is 8 μm or more and 18 μm or less, and the puncture strength when the thickness is converted to 14 μm is 230 gf or more. The polyolefin (A) contains polypropylene.
The separator for a power storage device according to any one of claims 1 to 8. The ratio of the polypropylene to the polyolefin (A) is 50 to 100% by mass.
The separator for a power storage device according to claim 9. The polypropylene has a pentad fraction of 95.0% or more.
The separator for a power storage device according to claim 9 or 10. The weight average molecular weight (Mw) of the microporous layer (X) is 500,000 or more and 1,500,000 or less.
The separator for a power storage device according to any one of claims 1 to 11. The value (Mw / Mn) obtained by dividing Mw of the microporous layer (X) by the number average molecular weight (Mn) is 6 or less.
The separator for a power storage device according to any one of claims 1 to 12. The separator for a power storage device according to any one of claims 1 to 13, further comprising a microporous layer (Y) containing polyolefin (B) as a main component. The separator for a power storage device according to claim 14, wherein the polyolefin (B) is polyethylene. A power storage device comprising the separator for the power storage device according to any one of claims 1 to 15.