JP2020145008A - Separator for lithium ion secondary battery - Google Patents

Abstract

To provide a high strength separator or the like which is applicable to an electric storage device represented by a lithium ion secondary battery (LIB).SOLUTION: Disclosed is a separator which includes: a microporous layer containing a polyolefin resin as a main component; and a functional layer on a microporous layer. The functional layer contains a water-insoluble organic fiber and an average length pore diameter of the microporous layer is 1 μ m or less.SELECTED DRAWING: None

Description

The present invention relates to a separator and the like applicable to a power storage device represented by a lithium ion secondary battery (LIB).

The microporous layer, particularly the microporous layer containing polyolefin resin as a main component, is used as a precision filter film, a battery separator, a capacitor separator, a fuel cell material, or a component thereof, and is particularly used for a secondary battery such as a LIB. It is widely used as a separator or a component thereof. In recent years, LIB has been used for small electronic devices such as mobile phones and notebook personal computers, while it has also been applied to electric vehicles including hybrid vehicles and plug-in hybrid vehicles.

LIBs used in electric vehicles are generally large in size and have a high energy capacity, so that higher safety is required to be ensured. Further, since a LIB used in an electric vehicle extracts a large amount of energy in a short time, it is required to have higher output characteristics than a conventional battery. Further, a battery having a high energy density is also required, and accordingly, a thin film of the separator is required. In applications for electric vehicles, LIBs tend to be used for a long period of time in harsher conditions than in applications for small electronic devices, so the required level of product safety or long-term reliability (life characteristics) is also high. There is. That is, in the future, LIBs used in electric vehicles will be strongly required to have both high energy output and high product safety. In particular, separators have high strength (for example, tensile strength), low shrinkage, and high heat resistance. Characteristics such as low electrical resistance are required.

For the above circumstances, for example, a separator containing a fine-diameter cellulose fiber is disclosed (see Patent Documents 1 to 3). On the other hand, all of the separators described in Patent Documents 1 to 3 have been studied from the viewpoint of adopting a laminated structure including a microporous layer (base material) containing a polyolefin resin as a main component and a specific functional layer. Not done.

Japanese Unexamined Patent Publication No. 2017-21001 International Publication No. 2015/063997 JP-A-2010-202987

The separators described in Patent Documents 1 to 3 still have room for study from the viewpoint of improving the strength, for example, the TD strength (particularly, the TD tensile strength). From the viewpoint of being applicable to a power storage device represented by LIB, the separator must also have good air permeation resistance (low air permeation resistance) when the strength is improved. It should be noted that such a problem is not limited to the power storage device used in the electric vehicle, but also exists in both the power storage device used in the large electronic device other than the electric vehicle and the power storage device used in the small electronic device.

Therefore, an object of the present invention is to provide a high-strength separator or the like applicable to a power storage device represented by LIB.

The present inventors adopt a laminated structure containing a microporous layer (base material) containing a polyolefin resin as a main component and having a relatively small average long pore size, and a functional layer containing water-insoluble organic fibers. We have found that the above problems can be solved by this, and completed the present invention. That is, the present invention is as follows.
[1]
A separator containing a microporous layer containing a polyolefin resin as a main component and a functional layer on the microporous layer.
The functional layer contains water-insoluble organic fibers and contains
A separator having an average long pore diameter of 1 μm or less in the microporous layer.
[2]
The separator according to [1], wherein the water-insoluble organic fiber contains cellulose nanofibers.
[3]
The separator according to [1] or [2], wherein the functional layer contains a resin binder.
[4]
The separator according to [3], wherein the resin binder contains a water-soluble polymer and / or a resin emulsion.
[5]
The separator according to [4], wherein the resin binder contains polyethylene oxide, which is the water-soluble polymer.
[6]
The separator according to any one of [3] to [5], wherein the resin binder has a breaking elongation of 100% or more.
[7]
The separator according to any one of [1] to [6], wherein the functional layer contains an inorganic filler.
[8]
The separator according to [7], wherein the inorganic filler has an average particle size of 0.01 to 10 μm.
[9]
The separator according to any one of [1] to [8], wherein the water-insoluble organic fiber has a fiber diameter of 5 to 2000 nm.
[10]
The separator according to any one of [1] to [9], wherein the breaking elongation of the functional layer is 2.0% or more.
[11]
The separator according to any one of [1] to [10], wherein the microporous layer has an average long pore diameter of 10 to 500 nm or less.
[12]
The separator according to any one of [1] to [11], which comprises the microporous layer porousd by a dry method.
[13]
The separator according to any one of [1] to [11], which comprises the microporous layer porousd by a wet method.
[14]
The microporous layer is
The polymer matrix containing the polyolefin resin and
Fibril extending from the polymer matrix in one direction of the microporous layer and containing the polyolefin resin,
The separator according to any one of [1] to [11], which comprises a porous body existing between a plurality of the fibrils.
[15]
The microporous layer is
The polymer matrix containing the polyolefin resin and
A fibril that extends three-dimensionally from the polymer matrix and contains the polyolefin resin,
The separator according to any one of [1] to [11], which comprises a porous body existing between a plurality of the fibrils.
[16]
A slurry for producing a functional layer that can be formed on a microporous layer containing a polyolefin resin as a main component.
A slurry containing water-insoluble organic fibers.
[17]
The slurry according to [16], wherein the water-insoluble organic fiber contains cellulose nanofibers.
[18]
The slurry further contains a resin binder and contains
The slurry according to [16] or [17], wherein the content ratio of the resin binder is 10 to 150% by mass with respect to the water-insoluble organic fiber.
[19]
The slurry according to [18], wherein the resin binder contains a water-soluble polymer and / or a resin emulsion.
[20]
The slurry according to [19], wherein the resin binder contains polyethylene oxide, which is the water-soluble polymer.
[21]
The slurry according to any one of [18] to [20], wherein the resin binder has a breaking elongation of 100% or more.
[22]
The slurry according to any one of [16] to [21], further comprising an inorganic filler.
[23]
The slurry according to [22], wherein the inorganic filler has an average particle size of 0.01 to 10 μm.
[24]
The slurry according to any one of [16] to [23], wherein the water-insoluble organic fiber has a fiber diameter of 5 to 2000 nm.
[25]
A functional layer containing water-insoluble organic fibers.
[26]
The functional layer according to [25], wherein the water-insoluble organic fiber contains cellulose nanofibers.
[27]
The functional layer according to [25] or [26], which comprises a resin binder.
[28]
The functional layer according to [27], wherein the resin binder contains a water-soluble polymer and / or a resin emulsion.
[29]
The functional layer according to [28], wherein the resin binder contains polyethylene oxide, which is the water-soluble polymer.
[30]
The functional layer according to any one of [27] to [29], wherein the resin binder has a breaking elongation of 100% or more.
[31]
The functional layer according to any one of [25] to [30], which contains an inorganic filler.
[32]
The functional layer according to [31], wherein the inorganic filler has an average particle size of 0.01 to 10 μm.
[33]
The functional layer according to any one of [25] to [32], wherein the water-insoluble organic fiber has a fiber diameter of 5 to 2000 nm.
[34]
The functional layer according to any one of [25] to [33], wherein the breaking elongation of the functional layer is 2.0% or more.

According to the present invention, it is possible to provide a high-strength separator or the like applicable to a power storage device represented by LIB.

Hereinafter, embodiments for carrying out the present invention (hereinafter, referred to as “the present embodiment”) will be described. However, the present invention is not limited to the following embodiments. The present embodiment can be appropriately modified and implemented within the scope of the gist of the present invention.
Unless otherwise specified, the expression "-" in the present specification means to include the numerical values at both ends as the upper limit value and the lower limit value.
Since the expressions "on the layer", "on the film" and "on the surface" in the present specification mean the positional relationship in the stacking direction with respect to the layer, the film or the surface, the positional relationship of each member is directly above the layer and the film. It is not limited to directly above the surface or directly above the surface. Therefore, for example, the expression "functional layer on the microporous layer" refers to an arbitrary layer (for example, an inorganic layer that does not correspond to either the microporous layer and the functional layer) between the microporous layer and the functional layer. Does not exclude aspects that include.

In the present specification, since a single layer or a plurality of layers can form a film, the layer and the film may be used interchangeably.
In the present specification, the abbreviation "MD" means the mechanical direction when the microporous membrane is continuously formed, and the abbreviation "TD" means the direction across the MD at an angle of 90 °.
The microporous film is a film composed of one or more polyolefin-based microporous layers, a microporous single-layer film composed of a resin layer other than the polyolefin-based microporous layer, and a composite microporous single having a polyolefin-based resin and a resin other than the polyolefin-based resin. Means at least one of layered membranes. Such a microporous membrane may also be referred to as a microporous layer.
The microporous laminated film is at least one of a multilayer film in which a plurality of polyolefin-based microporous layers are laminated, and a composite microporous film in which a polyolefin-based microporous film and a microporous film containing another resin are laminated. Means one. In the following, it may be simply referred to as a "laminate".

The polyolefin-based microporous membrane is formed from a polyolefin resin composition containing a polyolefin resin as a main component.
The polypropylene-based microporous film is formed from a polypropylene resin composition containing polypropylene as a main component.

In the present specification, the fact that the microporous layer contains a polyolefin resin as a main component means that the ratio of the polyolefin resin in the microporous layer is 50% by mass or more with respect to the total mass of the microporous layer. .. At this time, the polyolefin resin can form a polymer network in the microporous layer.
In the present specification, the fact that the polypropylene resin composition contains polypropylene as a main component means that the proportion of polypropylene in the polypropylene resin composition is 50% by mass or more with respect to the total mass of the polypropylene resin composition. To do.

A separator applicable to a power storage device represented by LIB (hereinafter, may be simply abbreviated as "separator") is arranged between a plurality of electrodes in the power storage device, and has ion permeability and, if necessary. A member having a shutdown characteristic accordingly. The separator contains a microporous membrane and / or a microporous laminated film as a base material, and has a functional layer on the base material.
The separator has a functional layer on at least one side of the substrate. Therefore, both a mode in which the functional layer is provided on one side of the base material and a mode in which the functional layer is provided on both sides of the base material are included in the present invention.
The separator may further include any layer (a layer that does not correspond to either a microporous layer or a functional layer), if desired. The separator may be provided with any such layer on, for example, the microporous layer, on the functional layer, or between the microporous layer and the functional layer.

[Separator for power storage device]
One aspect of the present invention is a separator for a power storage device. Such a separator includes a microporous layer containing a polyolefin resin as a main component and a functional layer on the microporous layer. The functional layer contains water-insoluble organic fibers, and the average long pore size of the microporous layer is 1 μm or less.

(Improved separator strength)
The water-insoluble organic fibers contained in the functional layer form a binding or entangled structure between the water-insoluble organic fibers, whereby the functional layer can be increased in strength, and by extension, a separator containing the functional layer. Can be increased in strength. In particular, it is presumed that cellulose nanofibers (also referred to as "fine cellulose fibers") having a large amount of hydroxyl groups on the surface have high strength due to the binding force derived from hydrogen bonds between the fibers. Further, it is presumed that by using a fiber having a high aspect ratio (fiber length / fiber diameter), the fibers are more entangled with each other and more fibers are bound to one fiber. , It is considered that the strength can be increased by this.

The average long pore diameter of the microporous layer is 1 μm or less. By adjusting the average long pore size of the microporous layer within the above range, it is possible to achieve good air permeation resistance, high tensile strength, and high safety. Then, based on the viewpoint of achieving these, it is also possible to appropriately set the lower limit of the average major pore diameter of the microporous layer. A separator different from the present invention, for example, a separator having a base material containing a non-woven fabric as a main component, cannot achieve such an extremely small average long pore diameter of 1 μm or less (for example, 1000 nm). The method for measuring the average long hole diameter will be described in the Example column.

Therefore, in the laminated structure including the above functional layer and the above microporous layer, the tough functional layer can prevent the microporous layer from being broken, whereby the tensile strength of the separator (for example, TD tensile strength) can be prevented. Can be improved. Further, by adopting such a laminated structure, the strength can be maintained even at a high temperature, so that the heat resistance of the separator can be improved. Since such a separator has good air permeation resistance, high tensile strength, and high safety, it is suitable not only for a power storage device used for a small electronic device but also for a power storage device used for a large electronic device such as an electric vehicle. Applicable.
Hereinafter, each member capable of forming the separator according to the present embodiment will be described.

(Microporous layer)
The microporous layer contains a polyolefin resin as a main component. The content of the polyolefin resin in the microporous layer is preferably 55 to 99.9% by mass or 55 to 98% by mass based on the total mass of the microporous layer from the viewpoint of ensuring good basic physical properties of the separator. is there. The content of the polyolefin resin in the microporous layer may be 60% by mass or more, 70% by mass or more, or 80% by mass or more.

The polyolefin resin is not particularly limited, but is, for example, a homopolymer, a copolymer or a multi-stage obtained by using ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and the like as monomers. Polymerized polymers and the like can be mentioned. These polyolefin resins may be used alone or in combination of two or more.

As the polyolefin resin, ultra-high molecular weight polyolefin is preferable from the viewpoint of improving the strength of the separator, the output performance of the power storage device, and the cycle performance. The viscosity average molecular weight of ultrahigh molecular weight polyolefins is generally 500,000 to 16,000,000, preferably 1,000,000 to 1,000,000, more preferably 2,000,000 to 5,000,000. is there.

As the type of polyolefin resin, polyethylene resin, polypropylene resin, copolymers of a plurality of monomers constituting these, and a mixture thereof are preferable from the viewpoint of ensuring good shutdown characteristics, and good shutdown characteristics, From the viewpoint of facilitating both the strength of the separator and the strength of the separator, polyethylene resin and polypropylene resin are more preferable, and one or more polypropylene resins are further preferable.

Specific examples of the polyethylene resin are not particularly limited, and examples thereof include low-density polyethylene, linear low-density polyethylene, medium-density polyethylene, high-density polyethylene, and ultra-high molecular weight polyethylene. In the present specification, the high-density polyethylene means polyethylene having a density of 0.942 to 0.970 g / cm ³ . The density of polyethylene means a value measured according to the D) density gradient tube method described in JIS K7112 (1999).

Specific examples of the polypropylene resin include, but are not limited to, low-density polypropylene, linear polypropylene, linear or branched-chain polypropylene, high-density polypropylene, ultra-high molecular weight polypropylene, and the like. The molecular weight of the polypropylene resin is determined according to the MFR. The polypropylene resin MFR (230 ° C., 2.16 kg load) used is preferably 2.2 g / g from the viewpoint of balancing the strength, porosity, air permeation resistance, and dimensional stability of the microporous membrane. It is less than 10 minutes, more preferably 1.1 g / 10 minutes or less, 0.9 g / 10 minutes or less, and even more preferably 0.6 g / 10 minutes or less. The lower limit of the MFR of the polypropylene resin can be, for example, 0.10 g / 10 minutes or more, 0.20 g / 10 minutes or more, and the like.

The microporous layer is a thermoplastic elastomer in addition to the polyolefin resin from the viewpoint of ensuring the safety of the separator, that is, from the viewpoint of balancing the strength and the pore ratio, and the air permeability resistance and the dimensional stability. , A copolymer having a branched chain, a copolymer having a conjugated double bond, and a copolymer having a crystalline region and an amorphous region in the molecule (hereinafter, referred to as "subcomponent (B)"). .) Can be included. The subcomponent (B) can improve the strength of the microporous layer. Such a sub-component (B) is a component different from the polyolefin resin.

The polyolefin resin composition for forming the microporous layer has, as the subcomponent (B), for example, polypropylene elastomer, ethylene / α-olefin copolymer, propylene / α-olefin copolymer, styrene olefin copolymer and the like. Can be included. Of these, polypropylene elastomers, ethylene / α-olefin copolymers, and propylene / α-olefin copolymers are more preferable, and ethylene / α-olefin copolymers are even more preferable, among others, from the viewpoint of improving the puncture strength of the separator. Ethylene / 1-butene copolymers are particularly preferred.

When the microporous membrane contains an auxiliary component (B) in addition to the polyolefin resin, the ratio of the total mass (T) to the auxiliary component (B) is the high strength of the microporous membrane, where T is the total amount of the polyolefin resin. (T) / (B) = 99/1 to 80/20 (mass) from the viewpoints of conversion, reduction of the heat shrinkage rate of the microporous membrane, and good pore-opening property of the microporous membrane. %), More preferably (T) / (B) = 98/2 to 82/18 (mass%), and even more preferably (T) / (B) = 97/3 to 85/15 (mass%). ..

The microporous layer preferably satisfies at least one of the following items (a) to (c):
(A) The average long pore diameter of the microporous layer is 10 to 500 nm;
(A) The pore diameter ratio a / b of the average long pore diameter a of the microporous layer and the pore diameter b orthogonal to the average long pore diameter a is 1.5 or more and 30 or less;
(C) The average long pore diameter of the microporous layer is arranged along one direction (for example, MD) of the microporous layer.
The method for measuring the average long hole diameter will be described in Examples.

Regarding the above item (a), by adjusting the average long pore diameter of the microporous layer within the above preferable range, it becomes easy to achieve good air permeation resistance, high tensile strength, and high safety. From the viewpoint of further facilitating the achievement of these, the average major hole diameter is more preferably 30 to 450 nm or 50 to 400 nm.
However, the average long pore size of the microporous layer is not limited to the above-mentioned preferable range, and may exceed, for example, 500 nm, but needs to be 1 μm or less (for example, 1000 nm). As described above, if a separator different from the present invention, for example, a separator having a base material containing a non-woven fabric as a main component, an extremely small average long pore size of 1 μm or less cannot be achieved, and by extension, the action and effect of the present invention. Cannot be obtained.

Regarding the above item (a), the pore diameter ratio (a) / (b) is preferably 1.5 or more and 30 or less, more preferably, from the viewpoint of ensuring the safety of the separator and the safety of the power storage device. Is 3 or more and 20 or less.

Although it is not desired that the above item (c) is bound by theory, it is presumed that it is derived from the high molecular weight component in the microporous layer. As shown in the above item (c), when the average long pore diameter is arranged along one direction (particularly MD) of the microporous layer, it is easy to secure good air permeation resistance and to develop high tensile strength. In addition, it becomes easy to reduce unsafe modes such as dendrite generation in the power storage device. For example, when the main component of the microporous layer is polypropylene resin, if the average long pore diameter is arranged along the MD of the microporous layer, other contained components improve the adhesiveness at the interface with the polypropylene resin. Therefore, it has the advantage of being easily decentralized.

Further, in a preferred embodiment of the present invention, the microporous layer is
(I) A polymer matrix containing a polyolefin resin and
(Ii) A fibril extending from the polymer matrix in one direction of the microporous layer and containing a polyolefin resin,
(Iii) Porousness existing between multiple fibrils,
Have.
The microporous layer specified by the above items (i) to (iii) is achieved by, for example, a dry method, for example, a dry stretching method, a dry lamella opening method, or the like.

Fibril means the smallest unit of a network structure containing a polyolefin resin. At least one hole is defined by fibril. That is, the fibril constitutes the outer edge of the hole, and in other words, the space surrounded by the fibril is formed as the hole. This fibril is formed by stretching a polymer chain of polyolefin at the time of stretching and opening. In the case of uniaxial stretching, lamella crystals are arranged in places other than fibrils (polymer matrix), which brings about high strength in one direction (for example, MD) and keeps shrinkage rate in the other direction (for example, TD) low. it can. The fibrils and polymer matrix can be confirmed, for example, from SEM photographs of the surface of the microporous layer during uniaxial stretching.

One direction of the microporous layer in which the fibrils extend is, for example, MD. When the fibril extends in one direction, a hole having a long axis along the one direction and a short axis perpendicular to the long axis is formed.
Here, "extending in one direction" means that 90% or more of fibril is included in the angle range of ± 20 degrees in the extending direction in the SEM photograph. That is, in the SEM photograph, it means that 90% or more of the long axes of the holes are included in the angle range of ± 20 degrees with each other.
As described above, since the microporous layer specified by the above items (i) to (iii) is achieved by, for example, a dry method, the microporous layer (that is, 90%) that does not meet the above definition in the SEM photograph. The microporous layer in which the above fibrils intersect each other beyond an angle range of ± 20 degrees in the extending direction) may be a microporous layer achieved by a wet method described later.

Further, in a preferred embodiment of the present invention, the microporous layer is
(I) A polymer matrix containing a polyolefin resin and
(Ii) A fibril that extends three-dimensionally from the polymer matrix and contains a polyolefin resin,
(Iii) Porousness existing between multiple fibrils,
Have.
The microporous layer specified by the above items (i) to (iii) is achieved by, for example, a wet method.

The three-dimensional extension of the fibrils means that the fibrils form a three-dimensional network structure. For example, when the above-mentioned dry method of uniaxial stretching is adopted, since the fibrils extend only in the stretching direction, a three-dimensional network structure is not formed. In the three-dimensional network structure, the fibril forms a structure in which the polymer matrix is crossed, connected or branched, and it can be confirmed from the SEM photograph that the fibril extends from the polymer matrix with a depth.
In the case of the microporous layer made porous by the above dry method, substantially elliptical pores along the long axis in one direction (for example, MD) are observed in the SEM photograph, but the microporous layer is made porous by the wet method. In the case of the microporous layer, holes having various shapes such as a circular shape, an elliptical shape, a square shape, and an amorphous shape are observed in the SEM photograph.

The porosity of the microporous layer is preferably 30-80%, more preferably 33-77%, even more preferably 37-73%. When the porosity is 30% or more, it becomes easy to secure ion permeability when the microporous layer is used for a power storage device application. Further, when the porosity is 80% or less, it becomes easy to maintain the strength of the microporous layer. The porosity can be adjusted by controlling the resin composition, the mixing ratio of the resin and the plasticizer, the orientation crystallization conditions, the stretching opening conditions, the heat fixing conditions, and the like.

The air permeation resistance of the separator containing the microporous layer is preferably 100 to 800 seconds / 100 ml, more preferably 120 to 750 seconds / 100 ml, still more preferably 150 to 700 seconds / 100 ml, in terms of film thickness of 14 μm. More preferably, it is 680 seconds / 100 ml or less. Adjusting the air permeation resistance to 100 seconds or more is preferable from the viewpoint of obtaining a separator containing a homogeneous microporous layer without defects. Further, adjusting the air permeation resistance to 600 seconds or less can contribute to ensuring sufficient ion permeability.
In the present specification, the mere "air permeation resistance" or "separator air permeation resistance" is obtained by dividing the measured value of the air permeability resistance of the separator by the film thickness of the separator and then multiplying by 14 μm. It is a value (air permeation resistance in terms of film thickness of 14 μm).

In the present embodiment, for example, the microporous layer containing polyethylene as a main component can be laminated on the microporous layer described above, and more specifically, the microporous layer of the polypropylene resin composition described above can be laminated. The layer (a) and the microporous layer (b) containing polyethylene as a main component can be laminated to form a microporous laminated film (laminated body).

Specific configurations of the laminate include, for example, the following configurations:
(I) Polypropylene layer (a) / Polyethylene layer (b)
(II) Polypropylene layer (a) / Polyethylene layer (b) / Polypropylene layer (a)
(III) Polyethylene layer (b) / Polypropylene layer (a) / Polyethylene layer (b)
(IV) Polyethylene layer (b) / polyethylene layer (b) / polypropylene layer (a)
(V) Polypropylene layer (a) / Polyethylene layer (b) / Polyethylene layer (b) / Polypropylene layer (a)
Above all, a structure in which polypropylene layers (a) and polyethylene layers (b) are alternately laminated is preferable, and from the viewpoint of improving productivity, two types of three layers having the same surface layer and different intermediate layers. The structure is more preferable, and the configuration of (II) above is particularly preferable.

As a method for producing the above-mentioned laminated body, for example, a coextrusion method or a laminating method in which each layer is separately extruded and then bonded to each other can be adopted. As the laminating method, either a dry laminating method using an adhesive or the like and a molten laminating method in which a plurality of layers are bonded together in a molten state can be adopted.

The content of polyethylene in the microporous layer (b) is preferably 50% by mass or more, preferably 60% by mass or more, preferably 70% by mass or more, and preferably 80% by mass or more. It is preferably 90% by mass or more, and preferably 95% by mass or more. The density of polyethylene in the microporous layer (b) is preferably 0.96 g / cm ³ or more. The upper limit thereof is not particularly limited, but may be 0.98 g / cm ³ or less, or 0.97 g / cm ³ or less. The MFR of the microporous layer (b) is preferably 0.6 g / 10 minutes or less, and the lower limit thereof is not particularly limited, but may be 0.05 g / 10 minutes or more, and 0.1 g. It may be 10 minutes or more, or 0.15 g / 10 minutes or more.

[Functional layer]
(Water-insoluble organic fiber)
The functional layer contains water-insoluble organic fibers. The water-insoluble organic fiber refers to a fine fiber formed into a fibrous form by refining a water-insoluble organic raw material. The term "water-insoluble" as used herein refers to the property that the functional layer can be dispersed in an aqueous system to the extent possible. Such water-insoluble organic fibers are derived from acid-modified polyolefin fibers, polyolefin fibers containing a water-soluble polymer, acrylic fibers, and cellulose nanofibers from the viewpoint of improving the strength of the separator and ion permeability. It is preferable to include at least one selected from the group consisting of. From the viewpoint of achieving both the strength of the separator and the heat resistance of the separator, it is particularly preferable to contain cellulose nanofibers. The cellulose nanofibers may be derived from wood or organic substances other than wood. Further, as cellulosic nanofibers, cellulosic fine fibers to which the surface is chemically treated and / or cellulose in which the hydroxyl group at the 6-position is oxidized by a TEMPO oxidation catalyst to become a carboxyl group (including acid type and salt type). Cellulosic fine fibers can also be used. In the former case, it can be used by subjecting various surface chemical treatments according to the purpose. For example, some or most of the hydroxyl groups present on the surface of fine cellulose fibers are esterified containing acetate, nitrate, and sulfate, alkyl ethers such as methyl ethers, and carboxymethyl ethers as representatives. An etherified product containing carboxy ether and cyanoethyl ether can be appropriately prepared and used. Further, in the latter, that is, in the preparation of the fine cellulose fiber in which the hydroxyl group at the 6-position is oxidized by the TEMPO oxidation catalyst, it is not always necessary to use a miniaturization device such as a high-pressure homogenizer that requires high energy. Dispersions can be obtained. For example, as described in the literature (A. Isogai et al., Biomacromolecules, 7, 1687-1691 (2006)), 2,2,6,6-tetramethylpiperidinooxy in an aqueous dispersion of natural cellulose. A catalyst called TEMPO such as radical and an alkyl halide coexist, an oxidizing agent such as hypochlorous acid is added thereto, and the reaction is allowed to proceed for a certain period of time. After purification treatment such as washing with water, a normal mixer is used. Cellulose nanofibers can be obtained extremely easily by subjecting the treatment.

Examples of the polyolefin fiber include polyethylene fiber and polypropylene fiber. In general, fibers commercially available under the name of polyolefin fibers or polyolefin nanofibers are also preferably used.
The water-insoluble organic fiber contained in the functional layer is not limited to any one of the above, and two or more may be contained. Therefore, for example, as the water-insoluble organic fiber, cellulose nanofibers and polyolefin nanofibers (for example, polyethylene nanofibers) may be used in combination. When two or more kinds of water-insoluble organic fibers are used in combination, the blending ratio thereof can be appropriately adjusted.
However, specific examples of water-insoluble organic fibers are not limited to the above.

The water-insoluble organic fiber can be produced by dispersing the organic raw material in water and then treating it with a device such as a high-pressure homogenizer, an ultra-high-pressure homogenizer, or a grinder, which applies a high shearing force. Any device other than these may be used as long as it is a device that performs miniaturization by a mechanism substantially similar to these devices. The solid content concentration of the organic raw material in the aqueous dispersion when the water-insoluble organic fiber is produced by these devices is not particularly limited. The organic raw material may be appropriately pretreated by an autoclave treatment, a beating treatment, an enzyme treatment or a combination thereof under impregnation in water at a temperature of 100 ° C. or higher. A known method can be used in producing water-insoluble organic fibers from organic raw materials.

The fiber diameter (average fiber diameter) of the water-insoluble organic fiber is not particularly limited, but is preferably 5 to 2000 nm, for example. From the viewpoint of maintaining ion permeability in the pores formed between the fibers, the fiber diameter is preferably 5 nm or more, and from the viewpoint of ensuring the binding force between the fibers and the strength derived from the pore diameter that is less likely to cause stress concentration. , 2000 nm or less is preferable. From the same viewpoint, the fiber diameter is more preferably 10 to 300 nm, further preferably 15 to 200 nm. The method for measuring the fiber diameter will be described in the Example column.

The blending ratio (mass%) of the water-insoluble organic fiber in the functional layer is not particularly limited, but is, for example, 20 to 100%, preferably 30 to 95%, and more preferably 30 to 90%. When the blending ratio is 20% or more, it is advantageous from the viewpoint of improving the strength by improving the elastic modulus of the functional layer. If the fiber diameter of the water-insoluble organic fiber is small, the elongation of the functional layer may be small. Therefore, from the viewpoint of improving the strength by improving the elongation of the functional layer, it is effective to reduce the blending ratio of the water-insoluble organic fiber and increase the blending amount of the resin binder.

The breaking elongation of the functional layer (TD breaking elongation at the time of breaking the functional layer in the separator) is not particularly limited, but is preferably 2% or more, and more preferably 2.0 to 150%, for example. .. When the elongation at break is 2.0 to 150%, the tensile strength of the separator tends to be improved. When the elongation at break is in the above range, the functional layer is unlikely to break before reaching the stress peak of the separator, and therefore it is easy to improve the strength. The breaking elongation of the functional layer can be controlled by adjusting the constituent materials of the functional layer and the compounding ratio of each constituent material from the viewpoint of improving the strength of the functional layer. The method for measuring the tensile strength will be described in the Example column.

Here, the functional layer preferably further contains a resin binder, an inorganic filler, and the like in addition to the water-insoluble organic fiber.

(Resin binder)
By including the resin binder in the functional layer, flexibility can be imparted to the functional layer, and as a result, the strength can be increased. Since the water-insoluble organic fiber has high binding property, the obtained functional layer may have low elongation and become brittle. On the other hand, by using the resin binder, the elongation of the functional layer is improved and it tends to be flexible, and as a result, it becomes easy to obtain a tough functional layer.

The resin binder preferably contains a water-soluble polymer and / or a resin emulsion from the viewpoint of good binding property with the water-insoluble organic fiber. In particular, it preferably contains a water-soluble polymer, and more preferably contains at least one of polyethylene oxide (PEO) and polyvinyl alcohol. Of these, PEO is more preferable because it has the characteristic of easily opening holes when compounded with water-insoluble organic fibers. From the viewpoint of improving the strength of the separator and the ion permeability, the molecular weight of PEO is preferably 100,000 to 10 million. The "water-soluble polymer" here refers to a polymer that dissolves 1 g or more in 100 g of water at 25 ° C.
When the resin binder contains a resin emulsion, the melting point of the resin binder (for example, when the resin binder contains a polyolefin polymer) or the minimum film forming temperature (for example, when the resin binder contains an acrylic copolymer) is determined. It is preferably 80 ° C. or lower. When the temperature is 80 ° C. or lower, the particles of the emulsion tend to fuse during drying, and the TD tensile strength of the functional layer tends to improve.
When the resin binder is a polyolefin polymer, its melting point, and when the resin binder is an acrylic copolymer, its minimum film forming temperature is a guideline for the drying temperature of the functional layer slurry. Therefore, in Examples 1 and 4 in which the resin binder is an acrylic copolymer and Comparative Example 1 in Examples described later, the item of the melting point of the acrylic copolymer is set to "Unknown" (see Table 2).

The blending ratio (mass%) of the resin binder is not particularly limited, but is preferably 10 to 200% with respect to the water-insoluble fiber, for example. The content ratio is preferably 10% or more from the viewpoint of improving the strength by improving the elongation of the functional layer, and preferably 200% or less from the viewpoint of improving the strength by improving the elastic modulus of the functional layer.

The elongation at break of the resin binder is not particularly limited, but is preferably 100% or more, for example. When the breaking elongation of the resin binder is 100% or more, the breaking elongation of the functional layer also tends to improve. The method for measuring the elongation at break will be described in the Example column.
The tensile strength of the resin binder is preferably 100 MPa or less. When the tensile strength of the resin binder is 100 MPa or less, the breaking strain of the functional layer is improved and the TD tensile strength tends to be improved. The method for measuring the tensile strength will be described in the Example column.

(Inorganic filler)
By containing the inorganic filler in the functional layer, the porosity of the functional layer can be improved and the ion permeability can be improved. Since the water-insoluble organic fibers have high binding properties, the void structure between the fibers tends to be crushed, and as a result, the ion permeability may deteriorate. The resin binder may also enter between the void structures between the fibers and further deteriorate the ion permeability. On the other hand, by adding the inorganic filler, the air-shelter structure is easily formed, and the ion permeability can be surely improved.

Examples of the inorganic filler are inorganic particles, and examples thereof include metal particles, metal oxide particles, metal nitride particles, and carbon-based particles. The inorganic filler is preferably at least one of alumina or boehmite from the viewpoint of safety and heat resistance in the battery. However, the inorganic filler is not limited to the above example, and may be fibrous instead of particulate.

The average particle size of the inorganic filler is not particularly limited, but is preferably 0.01 nm to 5 μm, preferably 0.05 nm to 4 μm, and preferably 0.8 nm to 3 μm, for example. The average particle size is preferably 0.01 nm or more from the viewpoint of avoiding pore clogging and improving ion permeability, and preferably 5 μm or less from the viewpoint of avoiding a decrease in strength due to stress concentration caused by an increase in the pore size. The method for measuring the average particle size of the inorganic filler will be described in the Example column.

The blending ratio (mass%) of the inorganic filler in the functional layer is not particularly limited, but is preferably 0 to 70%, preferably 0 to 50%, and 0 to 40%, for example. The blending ratio is preferably 70% or less from the viewpoint of avoiding a decrease in strength due to maintaining the binding force between the water-insoluble organic fibers and a decrease in strength due to avoiding stress concentration caused by an increase in the opening diameter.

[Slurry for functional layer]
The slurry (dispersion liquid) according to the present embodiment is used for producing the above-mentioned functional layer that can be formed on a microporous layer containing a polyolefin resin as a main component. Such dispersion contains water-insoluble organic fibers. The water-insoluble organic fibers are as described above.
Such a dispersion can be obtained by dispersing the above-mentioned water-insoluble organic fiber in water and kneading it if necessary. Further, the dispersion liquid may contain the above resin binder, the above inorganic filler and the like, if necessary, in addition to the above water-insoluble organic fiber. In order to obtain a slurry containing a resin binder and an inorganic filler, the resin binder and the inorganic filler may be dispersed together in water in which the water-insoluble organic fibers are dispersed. The inorganic filler may be pulverized and an aqueous dispersion thereof may be prepared, and the aqueous dispersion may be blended in water in which the water-insoluble organic fibers are dispersed.

The preferable composition of the water-insoluble organic fiber, the resin binder, and the inorganic filler contained in the dispersion liquid is as described above. For example, it is preferable that the water-insoluble organic fiber contains cellulose nanofibers. Further, the content ratio of the resin binder is preferably 10 to 150% by mass with respect to the water-insoluble organic fiber. Further, it is preferable that the resin binder contains a water-soluble polymer and / or a resin emulsion. Further, it is preferable that the resin binder contains PEO, which is a water-soluble polymer. It is preferable that the breaking elongation of the resin binder is 100% or more. Further, the average particle size of the inorganic filler is preferably 0.01 to 10 μm. Further, the fiber diameter of the water-insoluble organic fiber is preferably 5 to 2000 nm.

[Various configurations of separator]
The thickness of the separator is not particularly limited, but is, for example, preferably 1 to 60 μm, more preferably 5 to 40 μm, and further preferably 10 to 20 μm. When the film thickness is 1 μm or more, it becomes easy to improve the mechanical strength. Further, when the film thickness is 60 μm or less, the occupied volume of the separator in the power storage device is reduced, which tends to be advantageous in terms of increasing the capacity of the power storage device. The method for measuring the thickness of the separator will be described in the Example column.

Basis weight of the separator is not particularly limited, for example, is preferably 1.0 to 20 g / m ^2, more preferably 1.0~10g / m ^2, more preferably 1.0~6.0g / m ² .. A basis weight of 1.0 g / m ² or more is preferable because it is easy to handle in the process of assembling the power storage device. When the basis weight is 20 g / m ² or less, it becomes easy to thin the entire separator while maintaining a structural balance with the functional layer. The method for measuring the basis weight of the separator will be described in the Example column.

[Separator manufacturing method]
(Stretched hole)
The microporous layer according to this embodiment can be made into a film by using a coextrusion facility or a laminating facility. As an example, a resin composition having the same component can be laminated in two or more layers to prepare a raw film, and then the two or more layers of the laminated film can be stretched and opened to form a microporous laminated film. It is higher to prepare a microporous layer through a pore-opening step after laminating a single-layer film in two or more layers to prepare a raw film of the film, rather than producing a single-layer film having the same resin composition. It is easy to obtain a strong microporous layer.

In another embodiment, at least one microporous layer containing a polypropylene resin as a main component and at least one microporous layer containing a polyethylene resin as a main component are laminated to prepare a raw film, and then two layers. A microporous laminated film can be produced by stretching and opening the above laminated film. Rather than producing a single-layer film, it is better to laminate two or more layers of a single-layer film to produce the original fabric of the film and then to produce a microporous layer through a pore-opening step to obtain a high-strength microporous layer. Easy to get. From this point of view, in the case of a multilayer film in which a plurality of polyolefin-based microporous layers are laminated, it is preferable that three or more polyolefin-based microporous layers are laminated, and microporous containing a polypropylene resin as a main component. It is more preferable that at least two layers (PP microporous layer) and at least one microporous layer containing polyethylene resin as a main component (PE microporous layer) are laminated, and PP microporous layer / PE microporous layer / A three-layer film in which PP microporous layers are laminated in this order is more preferable.

Examples of the method for producing a multilayer film in which a plurality of polyolefin-based microporous layers are laminated or the method for producing a composite microporous film having a polyolefin-based microporous layer and another resin microporous layer include a coextrusion method or each layer. A method of bringing the layers into close contact with each other can be adopted by a laminating method or the like in which the layers are separately extruded and then bonded. As the laminating method, either a dry laminating method using an adhesive or the like and a thermal laminating method in which a plurality of layers are bonded by heating can be adopted.

The microporous layer is produced by a dry stretching method in which a film is directly stretched and oriented after melt-kneading in an extruder without using a solvent, and then an annealing step, a cold stretching step, and a heat stretching step are performed in this order. Is preferable. A method of stretching and orienting the molten resin from the extruder via a T-die, a circular die extrusion method, or the like can be used, and it is particularly preferable to use the circular die extrusion method because thinning is easy. In contrast to the wet method, the dry stretching method, in particular, the method of orienting the lamella crystals and forming the pores by interfacial peeling of the crystals, makes it easy to align the pores and is obtained by the method. The microporous layer is preferable because it can exhibit a low air permeation resistance with respect to the porosity.

The extension opening will be described in more detail. The single layer or laminated body of the raw film is subjected to a stretching treatment. As the stretching condition, uniaxial stretching (MD stretching) can be adopted. Regarding the stretching temperature, the molding characteristics of the microporous layer (a) containing polypropylene resin as a main component or the microporous layer (b) containing polyethylene resin as a main component, the mode of voids formed in each layer, and the like. It can be set as appropriate according to the situation. By such a stretching treatment, voids are provided in each of the microporous layers (a) and (b). Here, as a mechanism (method) for providing voids, for example, a pore-opening method at a lamella crystal interface can be mentioned.
In the perforation method at the crystal interface, for example, a crystalline resin such as polyethylene is melt-extruded at a high drawdown ratio to form a precursor film, and the precursor film is in a range of 5 to 50 ° C. lower than the crystal melting point of the crystalline resin. To form an annealed precursor film by annealing at the above temperature, the annealed precursor film is uniaxially cold-stretched 1.1 to 2 times at a temperature in the range of -20 to 70 ° C., and then 5 from the crystal melting point of the crystalline resin. Examples thereof include a method of obtaining a microporous layer (for example, providing voids in the film) by uniaxially stretching 1.5 to 5 times at a temperature in the range of about 50 ° C. lower.

The uniaxially stretched microporous layer can set the shrinkage rate of TD to be extremely low. The microporous layer according to this embodiment is preferably used alone or in combination with other layers as a separator for a power storage device.

(Manufacturing method of microporous layer)
The polyolefin resin composition containing the polyolefin resin described above and optionally the subcomponent (B) can be produced by a melt-kneading method of a single-screw or twin-screw extruder.

In order to efficiently finely disperse the sulfur atom-containing compound (A) in the high molecular weight polyolefin resin, it is preferable to adjust the shearing force and the temperature higher than the conventional levels. However, the higher the shearing force and temperature, the more likely it is that polymer decomposition will occur.Therefore, sufficient consideration should be given to the screw design or screw rotation speed in the extruder, the optimization of the temperature setting of each cylinder, and the reduction of air mixing. is necessary. In particular, since air is mixed into the extruder together with the resin pellets, the decomposition of the resin composition is accelerated, so that it is necessary to take measures such as melting and kneading the composition under nitrogen purge or nitrogen flow. The obtained resin composition can be used for producing a microporous membrane by a dry method or a wet method.

As a dry method, a polyolefin resin composition is melt-kneaded and extruded, and then a highly oriented film is directly formed from a T-die, or a highly oriented film is formed by a circular die extrusion method to prepare a raw film. After that, a method of peeling the lamella crystal interface of the polyolefin by thermal stretching through an annealing treatment of the raw film, micropore formation by cold stretching, and the like can be mentioned. In the circular die extrusion method, for example, a melt-kneaded product of a polypropylene resin composition is blown up from a circular die mainly to MD and wound up via a guide plate and a nip roll to obtain a highly crystalline MD-oriented raw fabric. Can be done.

As a wet method, a method in which a polyolefin resin composition and a pore-forming material are melt-kneaded to form a sheet, stretched as necessary, and then the pore-forming material is extracted from the sheet, after dissolution of the polyolefin resin composition. , A method of immersing the polyolefin in a poor solvent to coagulate the polyolefin and at the same time removing the solvent.

The polyolefin resin composition may contain a resin other than polyolefin, an arbitrary additive, or the like. Examples of additives include fluorine-based flow modifiers, waxes, crystal nuclei, metal soaps such as aliphatic carboxylic acid metal salts, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring agents. Examples include pigments.

The melt-kneading of the polyolefin resin composition can be performed by, for example, a kneader, a lab plast mill, a kneading roll, a Banbury mixer or the like, in addition to the single-screw and twin-screw extruder. Further, a direct compound method in which the resin composition is melt-kneaded in an extruder and then directly molded into a film can also be used. Examples of plasticizers that can be used 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.

The perforation step can be performed by a known dry method and / or a wet method. A stretching step may also be performed during or after the pore forming step or before or after the pore forming step. As the stretching treatment, both uniaxial stretching and biaxial stretching can be used, but at least MD stretching is preferable. When the film is stretched in one direction, the other direction is in an unconstrained state or a fixed length.

In order to suppress the shrinkage of the microporous layer, heat treatment may be performed for the purpose of heat fixation after stretching or pore formation. The heat treatment includes a stretching operation performed at a predetermined temperature atmosphere and a predetermined stretching rate for the purpose of adjusting physical properties, and / or relaxation performed at a predetermined temperature atmosphere and a predetermined relaxation rate for the purpose of reducing stretching stress. The operation can be mentioned. The relaxation operation may be performed after the stretching operation. These heat treatments can be performed using a tenter or a roll stretching machine.

As an example, a method for producing a microporous layer by a dry lamella opening method will be described. The dry lamellar pore opening method does not use a solvent such as water or an organic solvent, and cleaves the lamellar interface by stretching a non-porous precursor in which a plurality of lamellar structures are bonded via tie molecules. Is a method of forming.

The dry lamellar pore opening method is a non-porous precursor (highly oriented film) formed from a resin composition containing (a) a polyolefin resin, a sulfur atom-containing compound (A), and optionally a subcomponent (B). It is preferable to include a step of extruding (raw) and (a) a step of uniaxially stretching the extruded non-porous precursor. The microporous film obtained by the dry lamellar perforation method including the steps (a) and (b) can then be functionalized through a coating, dipping, impregnating step or the like.

Step (a) can be performed by a conventional extrusion method (single-screw or twin-screw extrusion method). The extruder can include a T-die or circular die with elongated holes. The uniaxial stretching of step (a) can be performed as described above. Longitudinal (MD) stretching can include both cold stretching and hot stretching.
From the viewpoint of suppressing the internal strain of the non-porous precursor, the non-porous precursor can be annealed during step (a), after step (b) or before stretching in step (a). Annealing is, for example, within the range between a temperature 50 ° C. lower than the melting point of the polypropylene resin as the main component and a temperature 10 ° C. lower than the melting point of the polypropylene resin or 50 than the melting point of the polypropylene resin as the main component. It can be carried out within a temperature range of ° C. lower and a temperature 15 ° C. lower than the melting point of the polypropylene resin.

(Manufacturing method of functional layer)
To prepare the functional layer, the above functional layer slurry (dispersion liquid) can be applied onto the microporous layer and, if necessary, dried. The surface of the microporous layer is treated by a known technique such as a corona discharge treatment machine, a plasma treatment machine, an ozone treatment machine, a flame treatment machine, etc. for the purpose of improving the affinity with the dispersion liquid. It is also possible. A known method can be used to apply the dispersion liquid onto the microporous layer. The drying time and drying temperature are not particularly limited.

[Power storage device]
The power storage device includes a positive electrode, a negative electrode, and the above-mentioned separator. In LIB, which is a typical example of a power storage device, for example, a positive electrode and a negative electrode are overlapped with each other via the above-mentioned separator and wound as necessary to form a laminated electrode body or a wound electrode body. Is loaded into the exterior body, and the positive and negative electrodes of the exterior body are connected via a lead body or the like, and further, non-aqueous electrolysis containing a non-aqueous solvent such as a chain or cyclic carbonate and an electrolyte such as a lithium salt. After injecting the liquid into the outer body, the outer body can be sealed and produced.

The present embodiment will be described with reference to examples. However, the present invention is not limited to the following examples. The evaluation methods for various raw materials and various characteristics are as follows.

[Average long pore size (nm) of microporous layer]
The average long pore size of the microporous layer was obtained by observing the surface of the microporous layer with an SEM and measuring the pore size of the obtained SEM image. Specifically, the surface of the microporous film was observed by SEM at a magnification of 30,000, the long pore size of the microporous film within the image range of 4.2 μm × 3 μm was measured, and the average was calculated to obtain the average long pore size. ..

[Fiber diameter (nm) of water-insoluble organic fiber]
The surface on which the fibers were applied and dried was randomly observed at three locations by SEM at a magnification equivalent to 10,000 to 100,000 times depending on the fiber diameter. With respect to the obtained SEM image, lines are drawn in the horizontal and vertical directions with respect to the screen, the fiber diameters of the fibers intersecting the lines are measured from the enlarged image, and the number of intersecting fibers and the fiber diameter of each fiber are measured. count. In this way, the number average fiber diameter was calculated using the measurement results of two vertical and horizontal series for one image. Furthermore, the number average fiber diameter was calculated for the other two extracted SEM images in the same manner, and the results for a total of three images were averaged to obtain the fiber diameter of the target sample.

[Elongation at break (%)]
The elongation at break of the functional layer of the separator at the time of film rupture was determined by using a tensile tester (TG-1 kN type manufactured by Minevea Co., Ltd.) to set the sample length before the test to 35 mm and pull at a speed of 100 mm / min. The elongation when the functional layer was cut (broken). The breaking elongation of the resin binder is determined by the same method as in which a sample is prepared by a hot press method or a casting method and the breaking elongation of the functional layer (the above-mentioned TD breaking elongation) is measured, and the resin binder is cut (broken). It was defined as the elongation when the film was formed. The elongation at break is calculated by the following formula.
Tensile elongation (%) = 100 × (L-L0) / L0
L0: Sample length before test L: Sample length at break

[Tensile strength (kgf / cm ² )]
The TD tensile strength of the separator is when the sample length before the test is 35 mm, the sample is pulled at a speed of 100 mm / min, and the sample is cut (broken) using a tensile tester (TG-1 kN type manufactured by Minevea Co., Ltd.). (The value obtained by dividing the tensile load value by the cross-sectional area of the test piece).

[Average particle size (μm) of inorganic filler (inorganic particles)]
The particle size distribution was measured using a laser particle size distribution measuring device (Microtrack MT3300EX manufactured by Nikkiso Co., Ltd.), and the particle size at which the cumulative frequency was 50% was defined as the average particle size.

[Separator thickness (μm)]
A 10 cm square sample was cut from the separator. The thickness of the sample was measured at room temperature of 23 ± 2 ° C. using a Digimatic Indicator IDC112 manufactured by Mitutoyo.

[Metsuke of separator (g / cm ² )]
The mass (g) of the above 10 cm square sample was measured and converted into the mass per cm ² .

[Air permeation resistance of separator (seconds / 100 ml)]
The air permeability resistance of the separator was measured with a Garley type air permeability meter conforming to JIS P-8117.

[Example 1]
(Preparation of microporous layer)
Polypropylene resin (MFR2.0, density 0.91), pore diameter 30 mm, L / D (L: distance from raw material supply port to discharge port of extruder (m), D: inner diameter of extruder (m). The same.) = 30, and the mixture was put into a single-screw extruder set at a temperature of 200 ° C. via a feeder and extruded from a T-die (200 ° C.) having a lip thickness of 2.5 mm installed at the tip of the extruder. Immediately after that, the molten resin was blown with cold air at 25 ° C. using an air knife and wound up with a cast roll set at 95 ° C. under the conditions of a draw ratio of 200 and a winding speed of 20 m / min to form a film.
The resulting film was annealed in a hot air circulation oven heated to 145 ° C. for 1 hour. Next, the annealed film was uniaxially stretched 1.2 times in the longitudinal direction at a temperature of 25 ° C. to obtain a cold stretched film. Next, the cold stretched film was uniaxially stretched 2.5 times in the longitudinal direction at a temperature of 140 ° C. and heat-fixed at 150 ° C. to obtain a uniaxially stretched film. Further, the uniaxially stretched film was uniaxially stretched 4.0 times in the transverse direction at a temperature of 145 ° C. and heat-fixed at 145 ° C. to obtain a microporous layer (C0). In addition, according to the above evaluation method, it was confirmed that the average long pore size of the obtained microporous layer (C0) was 1 μm or less.

(Preparation of separator C1)
1 kg of boehmite (C01 manufactured by Daimei Kagaku Co., Ltd.) and 9 kg of distilled water are mixed and atomized 5 times under an operating pressure of 200 MPa using Starburst (HJP-25090) manufactured by Sugino Machine Co., Ltd. as an atomizing device. This was carried out to obtain an aqueous dispersion B1 of boehmite (solid content concentration: 10 wt%). The average particle size of the aqueous dispersion B1 was 0.1 μm.
Further, as a water-insoluble organic fiber, 20.0 g of a 2.2% solid content solution of water-dispersed fine cellulose fibers (manufactured by Sugino Machine Limited, BMa, fiber diameter 20 to 50 nm) was prepared. Further, as a resin binder, 0.44 g of a 13 wt% solid content adjusting solution prepared by adding distilled water to an aqueous dispersion of an acrylic copolymer (A65S manufactured by Asahi Kasei Corporation) was prepared. Further, 5.0 g of a boehmite dispersion B1 was prepared as an inorganic filler. These water-insoluble organic fibers, resin binder, and inorganic filler were weighed in a resin container and treated with a defoaming kneader (Nippon Seiki NBK-1) at 1500 rpm for 5 minutes to obtain a dispersion. Table 1 shows the composition (mass%) of each raw material in the obtained dispersion. Hereinafter, the composition (mass%) in Table 1 is also understood as the composition (mass%) in the functional layer.

The obtained dispersion was coated on a microporous film (C0) on a dust-free paper laid on an aluminum plate using a bar coater (# 18), and then a dryer set at 90 ° C. It was dried in the room for 20 to 30 minutes. As a result, a separator (C1) having a composite layer (functional layer) containing an organic fiber, a resin binder, and an inorganic filler on the surface was obtained. From the mass change before and after coating, it was confirmed that the separator (C1) had a functional layer of 1.2 g / m ² . The TD tensile strength of the obtained separator (C1) is as high as 149 kgf / cm ² , the TD breaking elongation at the time when the functional layer is ruptured is as high as 4.8%, and the air permeation resistance is 446 seconds / 100 ml. Met.

[Example 2]
(Preparation of separator C2)
Linter pulp, which is a natural cellulose, is immersed in water to a concentration of 4% by weight, heat-treated in an autoclave at 130 ° C. for 4 hours, and the obtained swollen pulp is washed with water and swollen in a state impregnated with water. Pulp (aqueous dispersion) was obtained. An SDR14 type lab refiner (pressurized DISK type) manufactured by Aikawa Iron Works Co., Ltd. was used as a disc refiner device, and the aqueous dispersion was beaten for 20 minutes with a clearance of 1 mm between the discs. Subsequently, beating was carried out thoroughly under the condition that the clearance was reduced to a level close to zero, and a beating aqueous dispersion (solid content concentration: 1.60% by weight) was obtained. The obtained beating water dispersion was directly subjected to a micronization treatment 10 times under an operating pressure of 100 MPa using a high-pressure homogenizer (NS015H manufactured by Niro Soabi (Italy)), and the water dispersion M1 of fine cellulose fibers (M1). Solid content concentration: 1.60% by weight for both) was obtained.
Further, as a water-insoluble organic fiber, 20.0 g of a solid content 1.10% adjusting solution prepared by adding distilled water to an aqueous dispersion M1 of fine cellulose fibers was prepared. Further, as a resin binder, 0.42 g of a solid content 13 wt% adjusting solution prepared by adding distilled water to an aqueous dispersion of a polyolefin polymer (Sumitomo Seika Chemicals, Zyxen L) was prepared. These water-insoluble organic fibers and the resin binder were weighed in a resin container and treated with a defoaming kneader (Nippon Seiki NBK-1) at 1500 rpm for 5 minutes to obtain a dispersion liquid. Table 1 shows the composition (mass%) of each raw material in the obtained dispersion.

The obtained dispersion is applied onto the microporous layer (C0) on a dust-free paper laid on an aluminum plate using an applicator (4 mil), and then in a dryer set at 90 ° C. It was dried for 20 to 30 minutes. As a result, a separator (C2) having a composite layer (functional layer) containing an organic fiber and a resin binder on the surface was obtained. From the mass change before and after coating, it was confirmed that the separator (C2) had a functional layer of 1.1 g / m ² . The TD tensile strength of the obtained separator (C2) is as high as 157 kgf / cm ² , the TD breaking elongation at the time when the functional layer ruptures is as high as 6.0%, and the air permeation resistance is 498 seconds / 100 ml. Met.

[Example 3]
(Preparation of separator C3)
As a water-insoluble organic fiber, 20.0 g of a 1.54% solid content adjusting solution prepared by adding distilled water to an aqueous dispersion M1 of fine cellulose fibers was prepared. Further, as a resin binder, 0.94 g of a solid content 8 wt% adjusting solution prepared by adding distilled water to polyethylene oxide (manufactured by Meisei Chemicals, E30, molecular weight 400,000) was prepared. These water-insoluble organic fibers and the resin binder were weighed in a resin container and treated with a defoaming kneader (Nippon Seiki NBK-1) at 1500 rpm for 5 minutes to obtain a dispersion liquid. Table 1 shows the composition (mass%) of each raw material in the obtained dispersion.

The obtained dispersion is applied onto the microporous layer (C0) on a dust-free paper laid on an aluminum plate using an applicator (10 mil), and then in a dryer set at 60 ° C. It was dried for 20 to 30 minutes. As a result, a separator (C3) having a composite layer (functional layer) containing an organic fiber and a resin binder on the surface was obtained. From the change in mass change before and after coating, it was confirmed that the separator (C3) had a functional layer of 2.1 g / m ² . The TD tensile strength of the obtained separator (C3) is as high as 154 kgf / cm ² , the TD breaking elongation at the time when the functional layer ruptures is as high as 4.7%, and the air permeation resistance is 671 seconds / 100 ml. Met.

[Example 4]
(Preparation of separator C4)
As a water-insoluble organic fiber, 15.0 g of a 0.70% solid content adjusting solution prepared by adding distilled water to an aqueous dispersion M1 of fine cellulose fibers was prepared. Further, as a resin binder, 0.40 g of a 13 wt% solid content adjusting solution prepared by adding distilled water to an aqueous dispersion of an acrylic copolymer (A65S manufactured by Asahi Kasei Corporation) was prepared. Further, as an inorganic filler, distilled water was added to the dispersion B1 of boehmite to prepare 2.1 g of a 5% solid content adjusting solution. These water-insoluble organic fibers, resin binder, and inorganic filler were weighed in a resin container and treated with a defoaming kneader (Nippon Seiki NBK-1) at 1500 rpm for 5 minutes to obtain a dispersion. Table 1 shows the composition (mass%) of each raw material in the obtained dispersion.

The obtained dispersion is applied onto the microporous layer (C0) on a dust-free paper laid on an aluminum plate using an applicator (10 mil), and then in a dryer set at 90 ° C. It was dried for 20 to 30 minutes. As a result, a separator (C4) having a composite layer (functional layer) containing an organic fiber, a resin binder, and an inorganic filler on the surface was obtained. From the mass change before and after coating, it was confirmed that the separator (C4) had a functional layer of 2.9 g / m ² . The TD tensile strength of the obtained separator (C4) is as high as 166 kgf / cm ² , the TD breaking elongation at the time when the functional layer is ruptured is as high as 4.4%, and the air permeation resistance is 504 seconds / 100 ml. Met.

[Example 5]
(Preparation of separator C5)
70 g of boehmite (manufactured by Kawai lime, BMMW, average particle size 1.9 μm) and 630 g of distilled water are mixed, and a dispersant (manufactured by San Nopco, ED001, low molecular weight ammonium polycarboxylic acid type) is added to the boehmite. In addition to 0%, the mixture was stirred with a three-one motor for 3 minutes to obtain an aqueous dispersion B2 of boehmite (solid content concentration: 10 wt%). The average particle size of the aqueous dispersion B2 was 1.9 μm.
Further, as a water-insoluble organic fiber, 20.0 g of a 1.54% solid content adjusting solution prepared by adding distilled water to an aqueous dispersion M1 of fine cellulose fibers was prepared. Further, as a resin binder, 1.5 g of a 5 wt% solid content adjusting solution prepared by adding distilled water to polyethylene oxide (manufactured by Meisei Chemicals, E60, molecular weight 1 million) was prepared. Further, as an inorganic filler, 1.5 g of a dispersion B2 (solid content 10%) of boehmite was prepared. These water-insoluble organic fibers, resin binder, and inorganic filler were weighed in a resin container and treated with a defoaming kneader (Nippon Seiki NBK-1) at 1500 rpm for 5 minutes to obtain a dispersion liquid. Table 1 shows the composition (mass%) of each raw material in the obtained dispersion.

The obtained dispersion is applied onto the microporous layer (C0) on a dust-free paper laid on an aluminum plate using an applicator (10 mil), and then in a dryer set at 60 ° C. It was dried for 20 to 30 minutes. As a result, a separator (C5) having a composite layer (functional layer) containing an organic fiber, a resin binder, and an inorganic filler was obtained. From the mass change before and after coating, it was confirmed that the separator (C5) had a functional layer of 4.3 g / m ² . The TD tensile strength of the obtained separator (C5) is as high as 198 kgf / cm ² , the TD breaking elongation at the time when the functional layer ruptures is as high as 9.4%, and the air permeation resistance is 345 seconds / 100 ml. Met.

[Example 6]
(Preparation of separator C6)
70 g of alumina (Nippon Light Metal, LS-110F, average particle size 1.1 μm) and 630 g of distilled water are mixed, and a dispersant (San Nopco, E-D001, low molecular weight ammonium polycarboxylic acid type) is added to boehmite. In addition to 1.0%, zirconia beads Φ0.1, filling rate 64%, once under peripheral speed 10m / MIN using a bead mill (UAM-015) manufactured by Hiroshima Metal & Machinery as an atomizing device. Aluminization treatment was carried out to obtain an aqueous dispersion of alumina B3 (solid content concentration: 10 weight wt%). The average particle size of the aqueous dispersion B3 was 0.6 μm.
Further, as a water-insoluble organic fiber, 20.0 g of a 1.54% solid content adjusting solution prepared by adding distilled water to an aqueous dispersion M1 of fine cellulose fibers was prepared. Further, as a resin binder, 1.5 g of a 5 wt% solid content adjusting solution prepared by adding distilled water to polyethylene oxide (manufactured by Meisei Chemicals, E60, molecular weight 1 million) was prepared. Further, 1.5 g of an alumina dispersion B3 (solid content 10%) was prepared as an inorganic filler. These water-insoluble organic fibers, resin binder, and inorganic filler were weighed in a resin container and treated with a defoaming kneader (Nippon Seiki NBK-1) at 1500 rpm for 5 minutes to obtain a dispersion. Table 1 shows the composition (mass%) of each raw material in the obtained dispersion.

The obtained dispersion is applied onto the microporous layer (C0) on a dust-free paper laid on an aluminum plate using an applicator (10 mil), and then in a dryer set at 60 ° C. It was dried for 20 to 30 minutes. As a result, a separator (C6) having a composite layer (functional layer) containing an organic fiber, a resin binder, and an inorganic filler was obtained. From the mass change before and after coating, it was confirmed that the separator (C6) had a functional layer of 5.3 g / m ² . The TD tensile strength of the obtained separator (C6) is as high as 198 kgf / cm ² , the TD breaking elongation at the time when the functional layer ruptures is as high as 9.0%, and the air permeation resistance is 434 seconds / 100 ml. Met.

[Example 7]
(Preparation of separator C7)
0.3 kg of polyolefin binder fiber (Mitsui Chemicals, E600, solid content 33%) and 9.9 kg of distilled water are mixed and operated using Starburst (HJP-2509) manufactured by Sugino Machine Co., Ltd. as an atomizing device. The micronization treatment was performed 5 times under a pressure of 200 MPa to obtain an aqueous dispersion M2 (solid content concentration: 1.0 wt%) of polyolefin fibers.
Further, as the water-insoluble organic fiber, 20.5 g of a solid content 1.54% adjusting solution obtained by adding distilled water to the aqueous dispersion M1 of the fine cellulose fiber and the aqueous dispersion M2 of the polyolefin fiber (solid content 1.0). %) Was prepared as 16 g. Further, as a resin binder, 6.0 g of a solid content 2 wt% adjusting solution prepared by adding distilled water to polyethylene oxide (manufactured by Meisei Chemicals, E240, molecular weight 5 million) was prepared. These water-insoluble organic fibers and the resin binder were weighed in a resin container and treated with a defoaming kneader (Nippon Seiki NBK-1) at 1500 rpm for 5 minutes to obtain a dispersion liquid. Table 1 shows the composition (mass%) of each raw material in the obtained dispersion.

The obtained dispersion is applied onto the microporous layer (C0) on a dust-free paper laid on an aluminum plate using an applicator (10 mil), and then in a dryer set at 90 ° C. It was dried for 20 to 30 minutes. As a result, a separator (C7) having a composite layer (functional layer) containing an organic fiber and a resin binder on the surface was obtained. From the mass change before and after coating, it was confirmed that the separator (C7) had a functional layer of 1.9 g / m ² . The TD tensile strength of the obtained separator (C7) is as high as 149 kgf / cm ² , the TD breaking elongation at the time when the functional layer ruptures is as high as 4.5%, and the air permeation resistance is 369 seconds / 100 ml. Met.

[Comparative Example 1]
(Preparation of separator C8)
As an inorganic filler, 10 g of boehmite (made of Kawai lime, BMMW, average particle size of 1.9 μm) mixed with 10 g of distilled water was prepared. Further, as a resin binder, 4 g of an aqueous dispersion of an acrylic copolymer (Asahi Kasei, A65S, solid content 65 wt%) was prepared. These inorganic fillers and resin binders were weighed in a flask and stirred with a stirrer to obtain a dispersion liquid. Table 1 shows the composition (mass%) of each raw material in the obtained dispersion.

The obtained dispersion is coated on the microporous layer (C0) on a dust-free paper laid on an aluminum plate using a bar coater (# 4), and then in a dryer set at 90 ° C. It was dried for 20 to 30 minutes. As a result, a separator (C8) having a composite layer of a resin binder and an inorganic filler was obtained. From the mass change before and after coating, it was confirmed that the separator (C8) had a composite layer of 5.3 g / m ² . The TD tensile strength of the obtained separator (C8) was 104 kgf / cm ² , and the stress at the time when the composite layer broke was small, so the TD breaking elongation at this time was not detected. The air permeation resistance was 243 seconds / 100 ml.

[Comparative Example 2]
The performance of the microporous layer (C0) obtained in Example 1 was confirmed without treating with water-insoluble organic fibers. As a result, the TD tensile strength of the microporous layer (C0) was 130 kgf / cm ² , and the air permeation resistance was 225 seconds / 100 ml. The TD breaking elongation has not been detected.

Table 2 shows various configurations and evaluation results in Examples 1 to 7 and Comparative Examples 1 and 2. From the results in Table 2, a separator whose functional layer contains water-insoluble organic fibers and whose microporous layer has an average major pore diameter of 1 μm or less can improve the TD tensile strength and has good air permeability. Since the resistance (low air permeation resistance) is also secured, it can be seen that it can be suitably applied to a power storage device represented by LIB.

Claims (34)

A separator containing a microporous layer containing a polyolefin resin as a main component and a functional layer on the microporous layer.
The functional layer contains water-insoluble organic fibers and contains
A separator having an average long pore diameter of 1 μm or less in the microporous layer. The separator according to claim 1, wherein the water-insoluble organic fiber contains cellulose nanofibers. The separator according to claim 1 or 2, wherein the functional layer contains a resin binder. The separator according to claim 3, wherein the resin binder contains a water-soluble polymer and / or a resin emulsion. The separator according to claim 4, wherein the resin binder contains polyethylene oxide, which is the water-soluble polymer. The separator according to any one of claims 3 to 5, wherein the resin binder has a breaking elongation of 100% or more. The separator according to any one of claims 1 to 6, wherein the functional layer contains an inorganic filler. The separator according to claim 7, wherein the inorganic filler has an average particle size of 0.01 to 10 μm. The separator according to any one of claims 1 to 8, wherein the water-insoluble organic fiber has a fiber diameter of 5 to 2000 nm. The separator according to any one of claims 1 to 9, wherein the functional layer has a breaking elongation of 2.0% or more. The separator according to any one of claims 1 to 10, wherein the microporous layer has an average long pore diameter of 10 to 500 nm or less. The separator according to any one of claims 1 to 11, which comprises the microporous layer porousd by a dry method. The separator according to any one of claims 1 to 11, which comprises the microporous layer that has been porousized by a wet method. The microporous layer is
The polymer matrix containing the polyolefin resin and
Fibril extending from the polymer matrix in one direction of the microporous layer and containing the polyolefin resin,
The separator according to any one of claims 1 to 11, which comprises a porous body existing between a plurality of the fibrils. The microporous layer is
The polymer matrix containing the polyolefin resin and
A fibril that extends three-dimensionally from the polymer matrix and contains the polyolefin resin,
The separator according to any one of claims 1 to 11, which comprises a porous body existing between a plurality of the fibrils. A slurry for producing a functional layer that can be formed on a microporous layer containing a polyolefin resin as a main component.
A slurry containing water-insoluble organic fibers. The slurry according to claim 16, wherein the water-insoluble organic fiber contains cellulose nanofibers. The slurry further contains a resin binder and contains
The slurry according to claim 16 or 17, wherein the content ratio of the resin binder is 10 to 150% by mass with respect to the water-insoluble organic fiber. The slurry according to claim 18, wherein the resin binder contains a water-soluble polymer and / or a resin emulsion. The slurry according to claim 19, wherein the resin binder contains polyethylene oxide, which is the water-soluble polymer. The slurry according to any one of claims 18 to 20, wherein the resin binder has a breaking elongation of 100% or more. The slurry according to any one of claims 16 to 21, further comprising an inorganic filler. The slurry according to claim 22, wherein the inorganic filler has an average particle size of 0.01 to 10 μm. The slurry according to any one of claims 16 to 23, wherein the water-insoluble organic fiber has a fiber diameter of 5 to 2000 nm. A functional layer containing water-insoluble organic fibers. The functional layer according to claim 25, wherein the water-insoluble organic fiber contains cellulose nanofibers. The functional layer according to claim 25 or 26, which comprises a resin binder. The functional layer according to claim 27, wherein the resin binder contains a water-soluble polymer and / or a resin emulsion. 28. The functional layer according to claim 28, wherein the resin binder contains polyethylene oxide, which is the water-soluble polymer. The functional layer according to any one of claims 27 to 29, wherein the resin binder has a breaking elongation of 100% or more. The functional layer according to any one of claims 25 to 30, which comprises an inorganic filler. The functional layer according to claim 31, wherein the inorganic filler has an average particle size of 0.01 to 10 μm. The functional layer according to any one of claims 25 to 32, wherein the water-insoluble organic fiber has a fiber diameter of 5 to 2000 nm. The functional layer according to any one of claims 25 to 33, wherein the breaking elongation of the functional layer is 2.0% or more.