JP2021168287A - Power storage device separator - Google Patents

Abstract

To provide a microporous membrane with improved quality (reduction of the amount of unmelted material).SOLUTION: A microporous membrane includes a polyethylene resin, and a resin having a melting point of 140°C to 330°C unlike the polyethylene resin, and in the body viscoelasticity measurement, both the storage elastic modulus (E') and the loss elastic modulus (E") are 10 Pa to 10,000,000 Pa in the range of 150°C to 300°C.SELECTED DRAWING: Figure 1

Description

The present invention relates to a microporous membrane containing two or more kinds of resins, a separator having such a microporous membrane (for example, a separator for a power storage device), and the like.

Microporous membranes include microfiltration membranes, fuel cell separators, condenser separators, functional membrane base materials for filling pores with functional materials to develop new functions, and separators for power storage devices or theirs. It is widely used as a constituent material.

Lithium-ion secondary batteries (LIBs) are widely used in notebook personal computers, mobile phones, digital cameras, and the like. Among these, for example, a polyolefin microporous membrane is known as a separator for LIB or a constituent material thereof. Patent Document 1 proposes a method for producing a polyolefin microporous membrane for the purpose of high strength, high specific surface area, high pore volume, and the like. In Patent Document 1, a polyolefin microporous membrane is produced by melt-kneading a polyolefin resin having a weight average molecular weight of 500,000 or more and liquid paraffin, and extracting the liquid paraffin from the obtained resin composition. .. In Patent Document 2, a polyolefin resin such as polyethylene and a resin other than polyolefin (for example, polyamide) are kneaded in advance with an extruder to be pelletized, and then the pellets and liquid paraffin are mixed and extruded. Further, a method for producing a microporous film by extracting liquid paraffin is described.

Japanese Unexamined Patent Publication No. 2002-088189 JP-A-2002-226639

In recent years, there is a further demand for quality improvement (reduction of the amount of unmelted matter) in separators used in power storage devices. By providing a microporous membrane with improved quality, it is possible to provide a separator with improved quality, and by providing a separator with improved quality, safety (safety in a nail piercing test). ), And it is expected that a power storage device with excellent capacity maintenance characteristics (cycle characteristics) when charging and discharging are repeated can be realized.
In contrast to the above situation, the microporous membranes described in Patent Documents 1 and 2 have room for improvement from the viewpoint of improving quality.

In view of the above circumstances, an object of the present invention is to provide a microporous membrane with improved quality (reduction of the amount of unmelted material). Further, according to one embodiment of the present invention, the microporous membrane capable of improving the safety of the power storage device (safety in a nail stick test) and the cycle characteristics, and a separator having such a microporous membrane (for a power storage device). A separator) and a power storage device provided with such a microporous membrane are also disclosed.

The present inventors have conducted studies to solve the above problems, and have found that the above problems can be solved by using a separator for a power storage device having the following configuration, and have completed the present invention. Some aspects of the present invention are illustrated below.
[1] A microporous membrane containing a polyethylene resin and a resin having a melting point of 140 ° C. to 330 ° C., unlike the polyethylene resin.
A microporous membrane in which both the storage elastic modulus (E') and the loss elastic modulus (E ") are 10 Pa to 10,000,000 Pa in the range of 150 ° C. to 300 ° C. in the solid viscoelasticity measurement.
[2] The microporous membrane according to [1], wherein the resin having a melting point of 140 ° C. to 330 ° C. is dispersed in the polyethylene resin in a 2D spectroscopic mapping measurement.
[3] The microporous film according to [2], wherein at least one particle size of the resin having a melting point of 140 ° C. to 330 ° C. is 5.1 μm to 10 μm in the 2D spectroscopic mapping measurement.
[4] The microporous film according to [2] or [3], wherein the resin having a melting point of 140 ° C. to 330 ° C. has an average particle size of 5.1 μm to 10 μm in the 2D spectroscopic mapping measurement.
[5] Any of [1] to [4], wherein the resin having a melting point of 140 ° C. to 330 ° C. contains at least one selected from the group consisting of a polyolefin resin, a benzene ring-containing resin, and a heteroatom-containing resin. The microporous film according to item 1.
[6] The microporous membrane according to [5], wherein the polyolefin resin excludes homopolypropylene.
[7] The microporous film according to [5], wherein the benzene ring-containing resin is at least one selected from the group consisting of polyethylene terephthalate, polyetheretherketone, and polyphenylene ether.
[8] The microporous membrane according to [5], wherein the heteroatom-containing resin is at least one selected from the group consisting of polyamide, polytetrafluoroethylene, and polyketone.
[9] The microporous membrane according to any one of [1] to [8], wherein the polyethylene resin contains ultra-high molecular weight polyethylene.
[10] The mass ratio of the resin having a melting point of 140 ° C. to 330 ° C. and the polyethylene resin (the resin having a melting point of 140 ° C. to 330 ° C./the polyethylene resin) is 0.01 / 0.99 to 0.90. The microporous film according to any one of [1] to [9], which is /0.10.
[11] A separator for a power storage device provided with the microporous membrane according to any one of [1] to [10].
[12] A lithium ion secondary battery comprising the microporous membrane according to any one of [1] to [10].
[13] A power storage device provided with the microporous membrane according to any one of [1] to [10].

According to the present invention, it is possible to provide a microporous membrane with improved quality (reduction of the amount of unmelted material). Further, according to the present invention, by providing such a microporous membrane, it is possible to provide a separator with improved quality. By providing such a separator, it is expected that a power storage device having excellent safety (safety in a nail piercing test) and excellent cycle characteristics can be realized.

FIG. 1 is an image showing an example of the measurement result of the 2D spectroscopic mapping according to the present embodiment.

Hereinafter, embodiments of the present invention (referred to as “the present embodiment”) will be described, but the present invention is not limited to the present embodiment. The present invention can be modified in various ways without departing from the gist thereof. In the present specification, "~" means that the numerical values at both ends thereof are included as the upper limit value and the lower limit value unless otherwise specified. Further, in the present specification, the upper limit value and the lower limit value of the numerical range can be arbitrarily combined.

<Microporous membrane>
The microporous membrane (hereinafter, may be simply referred to as “microporous membrane”) according to the present embodiment is different from the polyethylene resin (hereinafter, may be referred to as “first resin”) and the first resin thereof. Moreover, a resin having a melting point of 140 to 330 ° C. (hereinafter, may be referred to as “second resin”) is included. In the solid viscoelasticity measurement, the microporous membrane has both a storage elastic modulus (E') and a loss elastic modulus (E ") of 10 to 10,000,000 Pa in the range of 150 to 300 ° C.

The microporous membrane can be used as a microfiltration membrane, a fuel cell separator, a condenser separator, a power storage device separator, an electrolysis membrane, or a constituent material thereof.
When a microporous membrane is used as a separator for a power storage device or a constituent material thereof, particularly a separator for LIB or a constituent material thereof, the microporous membrane itself may be used as the separator, and at least one side of the microporous membrane. A material provided with another layer or another film may be used as the separator. As the microporous membrane used for the separator for a power storage device, a microporous membrane having low electron conductivity, ionic conductivity, high resistance to an organic solvent, and a fine pore size is preferable.

As described in the Example column, the microporous membrane corresponding to the Examples has a reduced amount of unmelted matter (aggregate amount, that is, gel content) as compared with the microporous membrane corresponding to the Comparative Example. That is, the quality is improved.

In order to achieve such an improvement in quality, it is preferable that the second resin is preferably dispersed in the first resin. That is, it is preferable that the second resin is dispersed in the first resin in a granular manner. It is difficult to obtain the above-mentioned suitable dispersion by simply mixing the first resin and the second resin. However, a first component containing the first resin (eg, powder of the first resin) and a second component containing the second resin (eg, pellets of the second resin) are prepared, and they are used in an extruder. Based on the feeding method, the above-mentioned suitable dispersion tends to be obtained. Such a method will be described later.

Regarding the temperature-E'graph and the temperature-E'graph of the microporous membrane according to the present embodiment, the line may be interrupted when the temperature exceeds 200 ° C. The reason is that the temperature of the microporous membrane is raised. At the same time, after the melting point of the resin component (for example, nylon 12) that can be contained in the microporous film is exceeded, the viscosity of the resin component that exceeds the melting point decreases, leading to breakage. However, the temperature at which the microporous film breaks. Does not generally correlate with the melting point of the resin component, the size of the domain dispersed in polyethylene (PE), the resin component that can be contained in the microporous membrane (for example, heteroatom-containing resin, etc., the molecular weight of the second resin, etc. ) Can be adjusted.
Even if the storage elastic modulus (E') and the loss elastic modulus (E ") are not measured up to 300 ° C., these tend to decrease at 150 ° C. or higher. If it is ,000,000 Pa or less, there is a high possibility that the above requirement "10 to 10,000,000 Pa in the range of 150 to 300 ° C." is satisfied.

In this embodiment, the significance of paying attention to the storage elastic modulus (E') and the loss elastic modulus (E ") is as follows.
The storage elastic modulus (E') represents the rigidity of the material during dynamic behavior, that is, the hardness of the material. The lower the storage modulus (E'), the lower the rigidity of the material. When the storage elastic modulus (E') is 10 to 10,000,000 Pa in the range of 150 to 300 ° C., the microporous membrane has a desired rigidity in the above temperature range.
Further, the loss elastic modulus (E ″) represents the disappearance energy of the material during the dynamic behavior, that is, the viscosity of the material. The lower the value of the loss elastic modulus (E ″), the lower the viscosity. When the loss elastic modulus (E ″) is 10 to 10,000,000 Pa in the range of 150 to 300 ° C., the microporous membrane has a desired viscosity in the above temperature range.
As described above, in the solid viscoelasticity measurement, the microporous membrane having both the storage elastic modulus (E') and the loss elastic modulus (E ") of 10 to 10,000,000 Pa in the range of 150 to 300 ° C. In the range of 150 to 300 ° C., both desired rigidity and desired viscosity are achieved.

In this respect, it is not desired to be bound by the theory, but in the solid viscoelasticity measurement, both the storage elastic modulus (E') and the loss elastic modulus (E ") are 10 Pa or more in the range of 150 to 300 ° C. Therefore, it is considered that when the microporous film in the power storage device melts due to the temperature rise due to the internal short circuit of the power storage device, the melted resin (microporous film) invades the pores of the electrode and exhibits an anchor effect. Then, since the molten resin stays in place in a state where it has penetrated into the pores of the electrode, it is considered that an increase in the short-circuit area can be suppressed.
Similarly, although not bound by theory, in solid viscoelasticity measurements, both the storage modulus (E') and the loss modulus (E ") are in the range of 150-300 ° C., 10,000, When it is 000 Pa or less, the molten resin (microporous film) has appropriate viscoelasticity, so that the fluidity of the molten resin does not increase too much, and the exposed or short-circuited area of the electrode due to the outflow of the resin It is considered that the increase can be suppressed.
Therefore, even if it is a high energy density power storage device, for example, a large LIB, more specifically, an in-vehicle LIB, such a power storage device includes a separator having a microporous membrane according to the present embodiment. Therefore, it becomes easy to prevent thermal runaway at the time of internal short circuit.

From the viewpoint of facilitating thermal runaway during an internal short circuit, both the storage elastic modulus (E') and the loss elastic modulus (E ") are in the range of 150 to 300 ° C., preferably 20 Pa or more, more preferably 30 Pa or more. Yes, preferably 9,950,000 Pa or less, more preferably 9,900,000 Pa or less.
The storage elastic modulus (E') may be 50 Pa or more, or 100 Pa or more. The loss elastic modulus (E ") may be 30 to 700,000 Pa or 50 to 600,000 Pa.

The obtained values may differ between the case where the solid viscoelasticity of the microporous membrane is measured and the case where the solid viscoelasticity of the resin raw material of the microporous membrane is measured. Further, even a method capable of measuring the solid viscoelasticity of a resin raw material is not preferable for measuring the solid viscoelasticity of a microporous membrane obtained from the resin raw material. In the present embodiment, the storage elastic modulus (E') and the loss elastic modulus (E ") are measured by a method for a microporous membrane (that is, the method described in Examples), respectively.
The significance of controlling the storage elastic modulus (E') and the loss elastic modulus (E ") in the range of 150 to 300 ° C. for the microporous membrane is when an internal short circuit of the power storage device occurs. The point is that the viscoelasticity of the resin (microporous membrane) constituting the separator in the power storage device can be easily controlled within a specific range. In the present embodiment, the microporous membrane is used instead of the resin raw material. As a target, since the parameters related to viscoelasticity are controlled within a specific range, when an internal short circuit occurs in the power storage device, the separator in the power storage device can easily obtain the desired viscoelasticity, and as a result, heat is generated during the internal short circuit. It becomes easier to prevent runaway.

(1st component)
The first component contains a first resin (polyethylene resin). The first resin has a viscosity average molecular weight (Mv) of 100,000 to 9,700,000 and a dispersity (Mw / Mn) expressed as a ratio of a weight average molecular weight (Mw) to a number average molecular weight (Mn). Is preferably 3 to 18. Such a polyethylene resin is also referred to as ultra high molecular weight polyethylene (UHMWPE).

The viscosity average molecular weight (Mv) of the first resin is more preferably 120,000 to 9,000,000, still more preferably 200,000 to 8,500,000. Further, the dispersity (Mw / Mn) is not limited to the above 3 to 7, and is preferably 3 to 18, more preferably 4 to 14, and extremely preferably 4 to 13.

Here, the first component is preferably a powder containing 2 to 100% by mass of the above-mentioned first resin based on the total mass of the first component from the viewpoint of facilitating the reduction of the amount of unmelted material. From the viewpoint of ensuring the strength of the obtained microporous membrane, the content of the first resin in the first component is more preferably 4% by mass or more.

The first resin may contain not only a single type but also a plurality of types of UHMWPE. From the viewpoint of high strength of the microporous film, UHMWPE is preferably poly (ethylene and / or propylene-co-α-olefin), and more preferably poly (ethylene-co-propylene) or poly (ethylene-co). -Butene), and at least one selected from the group consisting of poly (ethylene-co-propylene-co-butene). From the same viewpoint, UHMWPE preferably contains a structural unit derived from ethylene in an amount of 98.5 mol% or more and 100 mol% or less, and more preferably 0.0 mol of a structural unit derived from an α-olefin other than ethylene. It is contained in an amount of more than% and 1.5 mol% or less.

Further, the first resin may contain a polyethylene resin other than UHMWPE. Examples of polyethylene resins other than UHMWPE include low-density polyolefin (LDPE) such as linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), high-pressure low-density polyethylene, or a mixture thereof.

The preferable "powder" as the first component is that the number average particle size (Nd ₅₀ ) is 80 μm to 180 μm, the volume average particle size (Nd ₅₀ ) is 120 μm to 220 μm, and the number particle size distribution (Nd _80). / Nd ₂₀ ) is 1.1 to 4.2, preferably 1.2 to 4.1, and the volume particle size distribution (Vd ₈₀ / Vd ₂₀ ) is 1.1 to 3.3. It is preferably 1.15 to 3.2, and the crystallinity size is 10.0 nm to 30.0 nm, preferably 11.0 nm to 28.0 nm, and more preferably 12.0 nm to 23.0 nm. At least one selected from the group consisting of 30% to 99% crystallinity, preferably 32% to 98%, more preferably 38% to 97.5%, and containing polyethylene. Those that meet the conditions. These number average particle size (Nd ₅₀ ), volume average particle size (Vd ₅₀ ), number particle size distribution (Nd ₈₀ / Nd ₂₀ ), volume particle size distribution (Vd ₈₀ / Vd ₂₀ ), crystallinity size, crystallinity The degree and the like can be measured by a known method.

For example, for the number average particle size (Nd ₅₀ ), the volume average particle size (Vd ₅₀ ), the number particle size distribution (Nd ₈₀ / Nd ₂₀ ), and the volume particle size distribution (Vd ₈₀ / Vd ₂₀ ), the flow manufactured by Micromerics. Formula Image analysis It can be obtained by measurement using a particle size / shape measuring device, Particle Insight. Further, for example, the crystallite size and the crystallinity can be obtained by XRD measurement using an X-ray diffractometer Ultima-IV manufactured by Rigaku Corporation.

The first component may contain a resin other than the first resin as long as it does not interfere with the exertion of the effects of the present invention. Examples of the resin other than the first resin include resins that can be added as the second resin described later.
In addition, the first component may contain additives other than the resin as long as the effects of the present invention are not impaired. Examples of the additive include a dehydration condensation catalyst, metal soaps such as calcium stearate or zinc stearate, an ultraviolet absorber, a light stabilizer, an antistatic agent, an antifogging agent, and a coloring pigment.

(Second component)
The second component contains a second resin (resin having a melting point of 140 ° C. to 330 ° C.) different from the first resin described above. The second resin is not particularly limited, but for example, a polyolefin resin; a polyamide resin such as nylon 6, nylon 66, nylon 11, nylon 6-10, nylon 6-12, nylon 6-66, and aramid resin; a polyimide resin. Polyester resin such as polyethylene terephthalate (PET) and polybutene terephthalate (PBT); Fluororesin such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene; Copolymer of ethylene and vinyl alcohol (for example, manufactured by Kuraray Co., Ltd.) Ever melting point: 157 ° C to 190 ° C), polysulfone, polyethersulfone, polyketone, polyetheretherketone (PEEK) and the like can be mentioned. These resins can be used singularly or in plurals.

By using such a second resin, it becomes easy to improve the heat resistance of the obtained microporous membrane. Then, when such a second resin is suitably dispersed in the above-mentioned first resin, the second resin is wrapped by the first resin, that is, the second resin is taken into the inside of the microporous membrane. Become a shape. As a result, the exposure of the second resin on the surface of the microporous membrane can be reduced, so that even if the microporous membrane and the electrode are brought into contact with each other, the contact area between the electrode and the second resin can be reduced. Therefore, the possibility that the redox decomposition of the second resin is caused by the electrode can be reduced.

The second resin is preferably a polyolefin resin. The polyolefin resin is not particularly limited, but for example, homopolypropylene (PP) can be excluded, and / or a resin containing a repeating unit of propylene as a constituent component can be excluded. These polyolefin resins can be used singularly or in plurals.

The second resin is preferably a benzene ring-containing resin. The benzene ring-containing resin is preferably at least one selected from the group consisting of, for example, polyethylene terephthalate, polyetheretherketone, and polyphenylene ether. These benzene ring-containing resins can be used alone or in plurals.

Further, the second resin is preferably a heteroatom-containing resin. The heteroatom-containing resin is preferably at least one selected from the group consisting of, for example, polyamide (PA), polytetrafluoroethylene (PTFE), and polyketone. These heteroatom-containing resins can be used singularly or in plurals.

When the second resin is composed of a plurality of resins, the melting point of the second resin does not mean the melting point of each of the resins constituting the second resin, but the melting point of the second resin as a whole. Therefore, even if a resin having a melting point outside the range of 140 ° C. to 330 ° C. is contained, if the melting point as a whole falls within the range of 140 ° C. to 330 ° C., it corresponds to the second resin. The overall composition of the first resin and the second resin may be different. Therefore, for example, the resin constituting the second resin may contain the first resin (polyethylene resin). Even if the melting point of the polyethylene resin is less than 140 ° C, if it is configured so that the melting point as a whole falls within the range of 140 ° C to 330 ° C by combining with other resins, it corresponds to the second resin. do.

As such a second resin, the viscosity average molecular weight (Mv) is 10,000 to 300,000, and the dispersity (Mw / Mn) of the weight average molecular weight (Mw) with respect to the number average molecular weight (Mn) is 3 to 3. It is preferably 18.

Here, the second component is preferably a pellet containing 2% by mass to 100% by mass of the second resin based on the total mass of the second component. From the viewpoint of ensuring both film strength and developing fuse characteristics, the second resin in the second component is more preferably 2.1% by mass or more, still more preferably 2.3% by mass or more.

The second component may contain additives other than the resin as long as it does not interfere with the exertion of the effects of the present invention. Examples of the additive include a dehydration condensation catalyst, metal soaps such as calcium stearate or zinc stearate, an ultraviolet absorber, a light stabilizer, an antistatic agent, an antifogging agent, and a coloring pigment.

The second component, which is a pellet, can be obtained, for example, by drying the polymerized powder of the second resin, extruding it into a strand shape by an extruder, cooling it with water, and then cutting it into a pellet shape. The viscosity average molecular weight (Mv) of the raw material powder is preferably 30,000 to 3,500,000, and the dispersity (Mw / Mn) is preferably 3 to 15.
The viscosity average molecular weight (Mv) when the second component is molded into pellets is preferably 35,000 to 3,300,000, more preferably 30,000 to 3,200,000. .. The degree of dispersion is preferably 3 to 15, more preferably 4 to 14, and even more preferably 4 to 13.

Here, the "pellet" preferable as the second component _{is larger than the number average particle size (Nd 50} ) and the volume average particle size (Vd ₅₀ ) of the "powder" preferable as the first component, and has one side. It is a resin product having a maximum length of 10 mm or less and 1 mm or more. The shape of the pellet is not particularly limited, and may be, for example, a spherical shape, an elliptical spherical shape, or a pillar shape. It is obtained by melt-extruding the raw material with an extruder, arranging it into a strand shape while cooling it with water or air, and continuously cutting it. The size and shape can be adjusted by strand formation or cutting method.

By melt-extruding the above-mentioned "powder" into the above-mentioned "pellets" in this way, the swelling rate due to the plasticizer can be significantly delayed (as described later, the second component, which is a pellet, can be used as a pellet. Does not swell substantially). Inside the extruder, it is important not to inhibit the swelling of the first component. Further, in the melting step after the swelling step, from the viewpoint of uniform melting, the “pellet” preferable as the second component is larger than the crystallite size of the “powder” preferable as the first component and is in the above range. Those having a crystallite size.
Further, the "pellet" preferable as the second component means a substance having a crystallinity in the above range, which is smaller than the crystallinity of the "powder" preferable as the first component. In the method for producing pellets, the crystallite size, crystallinity, pellet size, and shape of the pellets, such as the temperature of the extruded resin in the form of strands, the cooling temperature at the time of cutting, or the pulling speed of the strands from extrusion (melt microstretching), etc. Can be adjusted. These number average particle diameters (Nd ₅₀ ), volume average particle diameters (Vd ₅₀ ), crystallite size, crystallinity, and the like can be measured by known methods.

For example, the number average particle size (Nd ₅₀ ), the volume average particle size (Vd ₅₀ ), the crystallinity size, and the crystallinity can be measured by the same method as that described in the item of the first component. ..
The size of the pellet can be obtained, for example, by measuring the length of one side with a calibrated caliper.

By forming the second component in the form of pellets, it becomes easy to remarkably improve the uniformity of swelling of the plasticizer in the twin-screw extruder when mixed with the first component (for example, the powder of the first resin). It is considered that this is because the swelling rate of the second component can be significantly slowed down as compared with, for example, such a powder of the second component by forming the second component in the form of pellets. Therefore, the second component does not excessively inhibit the swelling of the first component (for example, the powder of the first resin), and the pellet itself basically does not swell, and is suitable for subsequent melting and kneading. It will be used. As a result, even in the process of melt kneading, the second component can be easily and uniformly dispersed in the first component to the molecular level.

As described above, the second component, which is a pellet, has a significantly slower swelling rate than the swelling rate of the powder, and therefore does not substantially swell. That is why it is possible to prevent the swelling of the first component (for example, the powder of the first resin) from being excessively inhibited by the second component.

(2D spectroscopic mapping measurement)
FIG. 1 is an image showing a 2D spectroscopic mapping measurement result of the microporous membrane according to the present embodiment. In FIG. 1, an enlarged image of a portion surrounded by a square by a white line is shown, and the portion surrounded by a circle by a white line corresponds to a specific expansion / contraction vibration absorption band. In the 2D spectroscopic mapping measurement, it is preferable that the second resin is granularly dispersed in the first resin (polyethylene resin). "The second resin is dispersed in the first resin in a granular manner" means that the second resin is observed in the first resin in a sea-island shape or in a dot shape in the image obtained by the 2D spectroscopic mapping measurement. .. In the observation area, the area of the first resin may be larger than the area of the second resin. The observation area and the observation magnification can be appropriately changed so that the overall image of the second resin can be preferably confirmed. Specific examples of 2D spectroscopic mapping measurement are as described in Examples.

Further, in the 2D spectroscopic mapping measurement, the particle size of at least one of the second resin (resin having a melting point of 140 ° C. to 330 ° C.) is preferably 5.1 to 10 μm, and is preferably 5.2 to 9.8 μm. Is more preferable. The particle size referred to here may be the major axis of the second resin which is granular, the minor axis, or the average of the major axis and the minor axis.
The observation area can be changed as appropriate. A second resin having a particle size of at least 5.1 to 10 μm may be present in any observation area. When the second resin is preferably dispersed in the first resin, the second resin has a particle size of at least 5.1 to 10 μm.

Further, in the 2D spectroscopic mapping measurement, the average particle size of the second resin (resin having a melting point of 140 ° C. to 330 ° C.) is preferably 5.1 to 10 μm, and preferably 5.2 to 9.8 μm. More preferred. When the second resin is more preferably dispersed in the first resin, the average particle size of the second component becomes a value in the range of 5.1 to 10 μm. The method for calculating the average particle size of the second resin is as described in Examples.

As described above, by preferably dispersing the second component in the first component, the amount of unmelted material in the obtained microporous membrane can be reduced, that is, the quality can be improved. By providing the microporous membrane with improved quality, it is possible to provide a separator with improved quality. By providing such a separator, it is expected that a power storage device having excellent safety (safety in a nail piercing test) and excellent cycle characteristics can be realized.

(Mass of 1st resin and 2nd resin)
The mass ratio (second resin / first resin) of the second resin (resin having a melting point of 140 to 330 ° C.) and the first resin (polyethylene resin) is 0.01 / 0.99 to 0.90 / 0. It is preferably 10.
Conventionally, the second resin as described above is insoluble and thermodynamically incompatible because the compatibility parameters are different from those of the first resin, but can be physically dispersed in the first resin. rice field. Since such a dispersed alloy molded product can achieve both the characteristics of the first and second resins, the size of the domain forming the alloy is important for the expression of its performance. The present inventors have endeavored to construct a dispersion by limiting the size of the second resin. In reality, it is difficult for the second resin to be suitably dispersed in the first resin simply by mixing the first resin and the second resin. However, a first component containing the first resin (for example, powder of the first resin) and a second component containing the second resin (for example, pellets of the second resin) are prepared, and these are described later. By subjecting to a special extrusion step as in steps (1) and / or (2), the above-mentioned suitable dispersion tends to be obtained. Then, according to such a method, it becomes easier to disperse the second resin in the first resin more preferably in the above range.
From the viewpoint of preferably dispersing the second resin, the above mass ratio (second resin / first resin) is more preferably 0.02 / 0.98 to 0.88 / 0.12.

(Various characteristics of microporous membrane)
The characteristics of the microporous membrane will be described below.
These characteristics are obtained when the microporous membrane as the separator for the power storage device is a flat membrane, but when the separator for the power storage device is in the form of a laminated membrane, the layers other than the microporous membrane are removed from the laminated membrane. Can be measured.

The porosity of the microporous membrane is preferably 20% or more, more preferably 30% or more, still more preferably 32% or more or 35% or more. When the microporous membrane has a porosity of 20% or more, when the microporous membrane is used as a separator for LIB or a constituent material thereof, the followability to the rapid movement of lithium (Li) ions tends to be further improved. be. On the other hand, the porosity of the microporous membrane is preferably 90% or less, more preferably 80% or less, still more preferably 50% or less. When the porosity of the microporous membrane is 90% or less, the membrane strength tends to be further improved and self-discharge tends to be further suppressed. The porosity of the microporous membrane is measured by the method described in the Examples.

The air permeability of the microporous membrane is preferably 1 second or longer, more preferably 50 seconds or longer, ^{still more preferably 55 seconds or longer, still more preferably 100 seconds or longer, per 100 cm 3.} When the air permeability of the microporous membrane is 1 second or more, the balance between the film thickness, the porosity and the average pore size tends to be further improved. The air permeability of the microporous membrane is preferably 400 seconds or less, more preferably 300 seconds or less. When the air permeability of the microporous membrane is 400 seconds or less, the ion permeability tends to be further improved. The air permeability of the microporous membrane can be adjusted by adjusting the stretching ratio, stretching temperature, and the like. Such air permeability is measured by the method described in Examples.

The film thickness of the microporous membrane is preferably 1.0 μm or more, more preferably 2.0 μm or more, still more preferably 3.0 μm or more, or 4.0 μm or more. When the film thickness of the microporous film is 1.0 μm or more, the film strength tends to be further improved. The film thickness of the microporous membrane is preferably 500 μm or less, more preferably 100 μm or less, still more preferably 80 μm or less, 22 μm or less, or 19 μm or less. When the film thickness of the microporous membrane is 500 μm or less, the ion permeability tends to be further improved. The film thickness of the microporous membrane can be adjusted by adjusting the stretching ratio, stretching temperature, and the like. Such film thickness is measured by the method described in Examples.

In particular, when a microporous membrane is used as the separator for LIB or its constituent material, the film thickness of the microporous membrane is preferably 25 μm or less, more preferably 22 μm or less or 20 μm or less, and further preferably 18 μm or less. In this case, when the film thickness of the microporous membrane is 25 μm or less, the permeability tends to be further improved. In this case, the lower limit of the film thickness of the microporous membrane may be 1.0 μm or more, 3.0 μm or more, 4.0 μm or more, or 5.0 μm or more.

The puncture strength of the microporous membrane is preferably 200 gf / 20 μm or more, more preferably 300 gf / 20 μm or more, preferably 2000 gf / 20 μm or less, and more preferably 1000 gf / 20 μm or less. It is preferable that the puncture strength is 200 gf / 20 μm or more from the viewpoint of suppressing the film rupture due to the active material or the like that has fallen off when the battery is wound. It is also preferable from the viewpoint of suppressing the concern of short circuit due to expansion and contraction of the electrode due to charging and discharging. On the other hand, the value of 2000 gf / 20 μm or less is preferable from the viewpoint of reducing width shrinkage due to relaxation of orientation during heating. The puncture strength of the microporous membrane can be adjusted by adjusting the stretching ratio, stretching temperature, and the like. Such puncture strength is measured by the method described in Examples.

<Manufacturing method of microporous membrane>
The manufacturing method of the microporous membrane is as follows:
(1) A step of supplying a first component containing a first resin (polyethylene resin), a second component containing a second resin (resin having a melting point of 140 to 330 ° C.), and a plasticizer to a twin-screw extruder;
(2) A step of melt-kneading the first component, the second component, and the plasticizer with a twin-screw extruder to produce a resin composition; and (3) Extracting the plasticizer from the resin composition and microporous The process of manufacturing the film;
including.
Hereinafter, each step will be described in order.

[Step (1)]
In the step (1), the first component containing the first resin, the second component containing the second resin, and the plasticizer are supplied to the twin-screw extruder.
As described above, an example of the first component is a powder of the first resin, and an example of the second component is a pellet prepared from the powder of the second resin.

(Plasticizer)
The plasticizer can be a known material as long as it is liquid at a temperature of 20 ° C. to 70 ° C. and has excellent dispersibility of the first resin or the second resin. The plasticizer used in the step (1) is preferably a non-volatile solvent capable of forming a uniform solution at a temperature equal to or higher than the melting point of the first resin or the second resin in consideration of subsequent extraction. Specific examples of the non-volatile solvent include hydrocarbons such as liquid paraffin, paraffin wax, decane, and decalin; esters such as dioctyl phthalate and dibutyl phthalate; higher alcohols such as oleyl alcohol and stearyl alcohol. And so on. Among them, liquid paraffin is preferable because it has high compatibility with polyethylene, the interface peeling between the resin and the plasticizer is unlikely to occur even when the melt-kneaded product is stretched, and uniform stretching tends to be easily carried out.

(Supply to twin-screw extruder)
The first component and the second component can be supplied to, for example, a twin-screw extruder at the same time. To supply them at the same time, for example, a method of dry-blending the first component and the second component with a super mixer or the like and supplying them to a twin-screw extruder; the first component and the second component are used to prepare a mixed slurry described later. However, there is a method of supplying this to a twin-screw extruder.

Further, the first component and the second component can be individually supplied to, for example, a twin-screw extruder. To supply them individually, for example, a method of supplying the first component and the second component from individual feeders to the twin-screw extruder; a mixed slurry described later is prepared from the first component and the plasticizer, and this is used. A method of supplying the twin-screw extruder separately from the two components; the second component is melted by another extruder (second extruder), and this is separated from the first component by the above-mentioned twin-screw extruder (first extruder). The method of supplying to the extruder) can be mentioned. When the first component and the second component are individually supplied to the twin-screw extruder, the first component may be supplied first, or the second component may be supplied first.

Further, the plasticizer can be supplied to the twin-screw extruder together with the first component, for example. After the plasticizer is supplied to the twin-screw extruder together with the first component, the plasticizer may be additionally supplied from the same or different feeders.
This type of twin-screw extruder generally has an upper feed port arranged on the upstream side and a middle feed port located on the downstream side of the feed port and arranged in the middle of the melt kneading area. ing. Therefore, after the mixed slurry is supplied from the upper feed port of the twin-screw extruder, the plasticizer can be additionally supplied from the middle feed port of the twin-screw extruder. As a result, it becomes easy to adjust the ratio of the amount of liquid paraffin to the resin composition extruded from the twin-screw extruder to a desired ratio, and it becomes easy to adjust the temperature of the resin composition to a desired range. Of course, the first component or the second component can also be supplied from the middle feed port.

(Mixed slurry)
Here, step (1) uses a continuous mixer under the conditions of a temperature of 20 ° C. to 70 ° C., ^{a shear rate of 100 to 400,000 seconds-1} and a residence time of 1.0 seconds to 60 seconds. A step of producing a mixed slurry with a first component and a plasticizer and supplying the mixed slurry and the second component to a twin-screw extruder can be included.
Further, the step (1) is a step of producing a mixed slurry with the first component, the second component, and a plasticizer using a continuous mixer under the above conditions, and supplying the mixed slurry to the twin-screw extruder. It can also be included.

The lower limit of the set temperature of the continuous mixer is preferably 25 ° C. or higher, more preferably 30 ° C. or higher from the viewpoint of maximally swelling the first component, and the upper limit thereof is the dissolution of the first resin during mixing. From the viewpoint of suppressing and obtaining a slurry, the temperature is preferably 68 ° C. or lower, more preferably 67 ° C. or lower, 66 ° C. or lower, or 65 ° C. or lower.
Shear rate of the continuous mixer, uniformly contacted with a plasticizer of the first component, from the viewpoint of obtaining a dispersion, a 100～400,000 sec ^-1, preferably 120～398,000 sec ^-1, more It is preferably 1,000 to 100,000 seconds- ¹ .
The residence time of the continuous mixer is 1.0 to 60 seconds, preferably 2.0 to 58 seconds, and more preferably 2.0 seconds to 56 seconds from the viewpoint of ensuring the dispersion of the first resin in the plasticizer. Seconds.

The content of the first component (for example, the powder of the first resin) in the mixed slurry preferably exceeds 0% by mass from the viewpoint of the strength of the obtained microporous membrane based on the total mass of the mixed slurry. , More preferably 1% by mass or more, still more preferably 2% by mass or more or 4% by mass or more. Further, this content is preferably 50% by mass or less, more preferably 40% by mass or less, still more preferably 30% by mass or less or 20% by mass or less from the viewpoint of suppressing the generation of unmelted matter in the first resin. Is.

The content of the second component (for example, pellets of the second resin) in the mixed slurry is preferably 0% by mass from the viewpoint of the strength of the obtained microporous membrane based on the total mass of the mixed slurry. It is more preferably 1% by mass or more, still more preferably 2% by mass or more or 4% by mass or more. Further, this content is preferably 50% by mass or less, more preferably 40% by mass or less, still more preferably 30% by mass or less or 20% by mass or less from the viewpoint of suppressing the generation of unmelted matter in the second resin. Is.

The mixed slurry is preferably supplied to the twin-screw extruder at a temperature of 25 ° C to 80 ° C. From the viewpoint of ensuring the entanglement of the polymer chains to the extent that the molecular weight of the first resin does not decrease while ensuring the appropriate viscosity of the mixed slurry, it is preferable to adjust the supply temperature within the range of 25 ° C. to 80 ° C. .. From the same viewpoint, the supply temperature is more preferably 30 ° C. to 76 ° C., still more preferably 30 ° C. to 70 ° C.

(Additive)
The above mixed slurry contains known additives such as dehydration condensation catalyst, metal soaps such as calcium stearate or zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, coloring pigments and the like. It's fine.

[Step (2)]
In the step (2), the first component, the second component, and the plasticizer are melt-kneaded by a twin-screw extruder to produce a resin composition. In the present embodiment, in the above step (1), the uniformity of swelling of the plastic agent with respect to the first component is ensured. Therefore, in the step (2), as long as it does not deviate from the gist of the present invention. , Types of twin-screw extruder, supply of each raw material to twin-screw extruder, extrusion time, extrusion speed, shear rate, shearing force, and the like are not limited.

[Step (3)]
In the step (3), a plasticizer is extracted from the resin composition produced in the above step (2) to produce a microporous film.
In particular, the step (3) according to the present embodiment includes a sheet processing step (4), a stretching step (5), an extraction step (6), and a heat treatment step (7).

(Sheet processing process (4))
In the step (4), the resin composition obtained in the above step (2) is extruded into a sheet, cooled and solidified, and processed into a sheet-shaped molded product. The resin composition may contain a plasticizer or other additives and the like.

The ratio of the resin (the total amount of the first resin, the second resin, and other resins if contained) in the sheet-shaped molded product is preferably based on the mass of the sheet-shaped molded product from the viewpoint of sheet moldability. Is 10 to 80% by mass, more preferably 20 to 60% by mass, and most preferably 30 to 50% by mass.

As a method for producing a sheet-shaped molded product, for example, the resin composition obtained in the above step (2) is extruded into a sheet shape via a T-die or the like and brought into contact with a heat conductor to crystallize the resin component. Examples thereof include a method of cooling to a temperature lower than the crystallization temperature and solidifying. Examples of the heat conductor used for cooling solidification include metals, water, air, and plasticizers. Among these, it is preferable to use a metal roll because of its high heat conduction efficiency. Further, when the extruded resin composition is brought into contact with a metal roll, sandwiching it between the rolls further enhances the efficiency of heat conduction, and the sheet is oriented to increase the film strength and the surface smoothness of the sheet. It is preferable because it tends to be used. When the resin composition is extruded from the T die into a sheet, the die lip interval is preferably 200 μm or more and 3,000 μm or less, and more preferably 500 μm or more and 2,500 μm or less. When the die lip interval is 200 μm or more, the shavings and the like are reduced, the influence on the film quality such as streaks or defects is small, and the risk of film breakage or the like can be reduced in the subsequent stretching step. On the other hand, when the die lip interval is 3,000 μm or less, the cooling rate is high, cooling unevenness can be prevented, and the thickness stability of the sheet can be maintained.
Further, the extruded sheet-shaped molded product may be rolled during the step (4).

[Stretching step (5)]
In the step (5), the sheet-shaped molded product obtained in the above step (4) is biaxially stretched at a surface magnification of 20 times or more and 200 times or less to form a stretched product.

As the stretching treatment, biaxial stretching is preferable to uniaxial stretching from the viewpoint that the film thickness distribution and the air permeability distribution in the width direction can be reduced. By simultaneously stretching the sheet in the biaxial direction, the number of times the sheet-shaped molded product repeats cooling and heating in the film forming process is reduced, and the distribution in the width direction is improved. Examples of the biaxial stretching method include simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple stretching. From the viewpoint of improving the puncture strength and the uniformity of stretching, simultaneous biaxial stretching is preferable, and from the viewpoint of ease of controlling the plane orientation, successive biaxial stretching is preferable.

In the present specification, the simultaneous biaxial stretching is a stretching in which MD (mechanical direction of continuous molding of microporous membrane) and TD (direction of crossing MD of microporous membrane at an angle of 90 °) are simultaneously stretched. The method refers to a method, and the draw ratio in each direction may be different. Sequential biaxial stretching refers to a stretching method in which MD and TD are stretched independently, and when MD or TD is stretched, the other direction is fixed in an unconstrained state or a fixed length. It is assumed that it is in the state of being.

The draw ratio is preferably in the range of 20 times or more and 200 times or less in terms of surface magnification, more preferably in the range of 25 times or more and 170 times or less, and further preferably 30 times or more and 150 times or less. The stretching ratio in each axial direction is preferably in the range of 2 times or more and 15 times or less for MD and 2 times or more and 15 times or less for TD, 3 times or more and 12 times or less for MD, and 3 times or more and 12 times or less for TD. It is more preferable that the range is 5 times or more and 10 times or less for MD, and 5 times or more and 10 times or less for TD. When the total area magnification is 20 times or more, sufficient strength tends to be imparted to the obtained microporous film, while when the total area magnification is 200 times or less, the film breakage in the step (5) is prevented. High productivity tends to be obtained.

The stretching temperature is preferably 90 to 150 ° C., more preferably 100 to 140 ° C., still more preferably 110 to 130 ° C. from the viewpoint of meltability and film forming property of the first resin and the second resin.

[Extraction step (6)]
In the step (6), the plasticizer is extracted from the drawn product obtained in the above step (5) to form a porous body. Examples of the method for extracting the plasticizer include a method in which a stretched product is immersed in an extraction solvent to extract the plasticizer and then dried. The extraction method may be either a batch method or a continuous method. In order to suppress the shrinkage of the porous body, it is preferable to restrain the end portion of the sheet-shaped molded body during a series of steps of dipping and drying. Further, the residual amount of the plasticizer in the porous body is preferably less than 1% by mass with respect to the mass of the entire porous membrane.
After the step (6), the plasticizer may be recovered and reused by an operation such as distillation.

As the extraction solvent, a solvent that is poor with respect to the first resin and the second resin, is a good solvent with respect to the plasticizer, and has a boiling point lower than that of the first resin and the second resin is used. Is preferable. Examples of such an extraction solvent include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; non-chlorine type such as hydrofluoroether and hydrofluorocarbon. Hydrocarbon solvents; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; ketones such as acetone and methyl ethyl ketone can be mentioned. These extraction solvents may be recovered and reused by an operation such as distillation.

[Heat treatment step (7)]
In the step (7), the porous body is heat-treated at a temperature equal to or lower than the melting point of the porous body obtained in the above step (6), and then the porous body is stretched to produce a microporous membrane.

The porous body is heat-treated for the purpose of heat fixation from the viewpoint of suppressing shrinkage. The heat treatment method includes a stretching operation performed at a predetermined atmosphere, a predetermined temperature, and a predetermined stretching ratio for the purpose of adjusting physical properties, and / or a predetermined atmosphere, a predetermined temperature, for the purpose of reducing stretching stress. And the mitigation operation performed at a predetermined mitigation rate. A relaxation operation may be performed after the stretching operation. These heat treatments can be performed using a tenter or a roll stretching machine.

From the viewpoint of increasing the strength and porosity of the microporous membrane, the stretching operation preferably stretches the MD and / or TD of the membrane 1.1 times or more, and more preferably 1.2 times or more. conduct.
The relaxation operation is a reduction operation of the membrane to MD and / or TD. The relaxation rate is a value obtained by dividing the size of the film after the relaxation operation by the size of the film before the relaxation operation. When both MD and TD are relaxed, it is a value obtained by multiplying the relaxation rate of MD and the relaxation rate of TD. The relaxation rate is preferably 1.0 or less, more preferably 0.97 or less, still more preferably 0.95 or less. The relaxation rate is preferably 0.5 or more from the viewpoint of film quality. The relaxation operation may be performed in both the MD and TD directions or only in one of the MD and TD.

The temperature of the heat treatment including the stretching or relaxation operation is preferably in the range of 100 to 170 ° C. from the viewpoint of the melting points of the first resin and the second resin (hereinafter, also referred to as “Tm”). When the temperature of the stretching and relaxation operations is in the above range, it is preferable from the viewpoint of the balance between the reduction of the heat shrinkage rate and the porosity. The lower limit of the heat treatment temperature is more preferably 110 ° C. or higher, further preferably 120 ° C. or higher, still more preferably 125 ° C. or higher, and the upper limit thereof is more preferably 160 ° C. or lower, further preferably 150 ° C. or lower, still higher. It is preferably 140 ° C. or lower.

The microporous membrane may be subjected to post-treatment such as hydrophilization treatment with a surfactant or the like, cross-linking treatment with ionizing radiation or the like during or after the step (7). The order of the steps (5), (6), and (7) described above may be rearranged, or these steps may be performed at the same time, but from the viewpoint of film forming property, biaxial stretching may be performed. It is preferable to carry out these steps in the order of steps (5), (6), and (7) using a machine.
From the viewpoint of handleability and storage stability, the obtained microporous membrane can be wound by a winder to form a roll or cut by a slitter.

<Separator for power storage device, separator for LIB>
The microporous membrane can be used in a power storage device, for example, as a separator for LIB or a constituent material thereof. According to this, it becomes easy to suppress the redox deterioration of the separator, and it becomes easy to construct a dense and uniform porous structure.

Separators include flat membranes (eg, formed of one microporous membrane), laminated membranes (eg, laminates of multiple microporous membranes, laminates of microporous membranes and other membranes), coating membranes (eg, laminates of other membranes). , When a functional substance is coated on at least one side of the microporous membrane).

<LIB>
LIB uses a lithium transition metal oxide such as lithium cobalt oxide and lithium cobalt composite oxide as a positive electrode, a carbon material such as graphite and graphite as a negative electrode, and an organic solvent containing a lithium salt such _{as LiPF 6 as an electrolytic solution.} It is a storage battery. When charging / discharging the LIB, the ionized Li reciprocates between the electrodes. Further, since it is necessary for the ionized Li to move between the electrodes at a relatively high speed while suppressing the contact between the electrodes, a separator is arranged between the electrodes.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. However, the present invention is not limited to the following examples as long as the gist of the present invention is not exceeded. The physical properties in the examples were measured by the following methods.

<Melting point of second resin (° C)>
The melting point of the second resin was measured using a differential scanning calorimetry (DSC) measuring device "DSC-60" (manufactured by Shimadzu Corporation). First, the second resin was punched into a circle having a diameter of 5 mm, and several sheets were superposed to obtain 3 mg, which was used as a measurement sample. This sample was laid on an aluminum open sample pan having a diameter of 5 mm, a clamping cover was placed on the pan, and the sample was fixed in the aluminum pan by a sample sealer. Under a nitrogen atmosphere, the temperature is raised from 30 ° C to 200 ° C at a heating rate of 10 ° C / min (first temperature rise), held at 200 ° C for 5 minutes, and then lowered from 200 ° C to 30 ° C at a temperature lowering rate of 10 ° C / min. The temperature has dropped. Subsequently, after holding at 30 ° C. for 5 minutes, the temperature was raised again from 30 ° C. to 200 ° C. at a heating rate of 10 ° C./min (second temperature rise). In the melting endothermic curve of the second temperature rise, the maximum temperature was defined as the melting point of the second resin. When there are a plurality of maximum values, the temperature at which the maximum value of the largest melting endothermic curve is obtained is adopted as the melting point of the second resin.

<Measurement of solid viscoelasticity of microporous membrane>
The dynamic viscoelasticity of the separator can be measured using a dynamic viscoelasticity measuring device, and the storage elastic modulus (E') and the loss elastic modulus (E'') can be calculated.
For E'and E' _{, if} no fracture (rapid decrease in elastic modulus) of the sample is observed at 150 ° C to 300 ° C, it is calculated from the average value of 150 ° C to 300 ° C and is 150. When the sample was broken at ° C. to 300 ° C., it was calculated from the average value from 150 ° C. to the temperature at the breaking point.

・ Storage elastic modulus (E')
The measurement conditions for the storage elastic modulus (E') are defined by the following configurations (i) to (iv).
(I) Dynamic viscoelasticity measurement under the following conditions:
-Measuring device used: RSA-G2 (manufactured by TA Instruments)
-Sample film thickness: range of 5 μm to 50 μm-Measurement temperature range: -50 to 300 ° C
・ Temperature rise rate: 10 ° C / min
・ Measurement frequency: 1Hz
-Transformation mode: Sine wave tension mode (Linear tension)
・ Initial value of static tensile load: 0.5N
-Initial (at 25 ° C) gap distance: 25 mm
-Auto strike adjustment: Embedded (range: amplitude value 0.05 to 25%, sine wave load 0.02 to 5N)
Do it with.
(Ii) The above-mentioned static tensile load refers to an intermediate value between the maximum stress and the minimum stress in each periodic motion. The sine wave load refers to a vibration stress centered on the static tensile load.
(Iii) The above-mentioned sinusoidal tension mode refers to measuring the above-mentioned vibration stress while performing periodic motion with a fixed amplitude of 0.2%. In the above sinusoidal tension mode, the above vibration stress is measured by varying the gap distance and the above static tensile load so that the difference between the above static tensile load and the above sinusoidal load is within 20%. do. Then, when the sine wave load becomes 0.02 N or less, the amplitude value is amplified so that the sine wave load is within 5 N and the increase amount of the amplitude value is within 25%. The above vibration stress is measured.
(Iv) The relationship between the obtained sinusoidal load and the amplitude value, and the following equation:
σ ^* = σ ₀・ Exp [i (ωt + δ)],
ε ^* = ε ₀・ Exp (iωt),
σ ^* = E ^*・ ε ^*
E ^* = E'+ iE''
(In the formula, σ ^* : vibration stress, ε ^* : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between vibration stress and strain, E ^* : complex elastic coefficient, E ': Storage elastic coefficient, E'': Loss elastic coefficient Vibration stress: Sine wave load / initial cross-sectional area Static tensile load: Minimum point of vibration stress in each cycle (minimum point of distance between gaps in each cycle) Load Sine wave load: Difference between measured vibration stress and static tensile load)
The storage elastic modulus E'was calculated from.

・ Elastic modulus (E'')
The measurement conditions for the loss elastic modulus (E ″) are defined by the following configurations (i) to (iv).
(I) Dynamic viscoelasticity measurement under the following conditions:
-Measuring device used: RSA-G2 (manufactured by TA Instruments)
-Sample film thickness: range of 5 μm to 50 μm-Measurement temperature range: -50 to 300 ° C
・ Temperature rise rate: 10 ° C / min
・ Measurement frequency: 1Hz
-Transformation mode: Sine wave tension mode (Linear tension)
・ Initial value of static tensile load: 0.5N
-Initial (at 25 ° C) gap distance: 25 mm
-Auto strike adjustment: Embedded (range: amplitude value 0.05 to 25%, sine wave load 0.02 to 5N)
Do it with.
(Ii) The above-mentioned static tensile load refers to an intermediate value between the maximum stress and the minimum stress in each periodic motion. The sine wave load refers to a vibration stress centered on the static tensile load.
(Iii) The above-mentioned sinusoidal tension mode refers to measuring the above-mentioned vibration stress while performing periodic motion with a fixed amplitude of 0.2%. In the above sinusoidal tension mode, the above vibration stress is measured by varying the gap distance and the above static tensile load so that the difference between the above static tensile load and the above sinusoidal load is within 20%. do. Then, when the sine wave load becomes 0.02 N or less, the amplitude value is amplified so that the sine wave load is within 5 N and the amount of increase in the amplitude value is within 25%. Measure the vibration stress.
(Iv) The obtained sine wave load and amplitude value, and the following equation:
σ ^* = σ ₀・ Exp [i (ωt + δ)],
ε ^* = ε ₀・ Exp (iωt),
σ ^* = E ^*・ ε ^*
E ^* = E'+ iE''
(In the formula, σ ^* : vibration stress, ε ^* : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between vibration stress and strain, E ^* : complex elastic coefficient, E ': Storage elastic coefficient, E'': Loss elastic coefficient Vibration stress: Sine wave load / initial cross-sectional area Static tensile load: Minimum point of vibration stress in each cycle (minimum point of distance between gaps in each cycle) Load Sine wave load: Difference between measured vibration stress and static tensile load)
The loss elastic modulus E'' was calculated from.

<2D spectroscopic mapping measurement>
-Sample preparation The microporous membrane was cut into MD5 mm x TD5 mm and used for measurement.

-Measurement method Using a tip-enhanced Raman scattering microscope (TERSsense) manufactured by nanophoton, vibration absorption bands other than PE in the sample were detected and 2D mapping was performed. Specifically, as a vibration absorption band other than PE, for example, in the case of nylon 12 (Example 1), a stretchable vibration absorption band near ^{1650 cm -1 was used.}

-Method for calculating the average particle size of the second resin Image data obtained by 2D mapping was image-processed to obtain an area corresponding to a circle, and the diameter was calculated as the average particle size.

<Film thickness (μm)>
The measurement was carried out in an atmosphere of room temperature of 23 ° C. and humidity of 40% using a micro thickener (type KBM manufactured by Toyo Seiki Co., Ltd.). The measurement was performed by applying a load of 44 gf using a terminal having a terminal diameter of 5 mmφ.

<Porosity (%)>
A 10 cm × 10 cm square sample was cut from a microporous membrane, its volume (cm ³ ) and mass (g) were determined, and the pore ratio was calculated from ^{them and their density (g / cm 3) using the following equation.}
Porosity (%) = (volume-mass / density) / volume x 100

<Air permeability (seconds / 100 cm ³ )>
Air permeability of polyolefin microporous membrane in an atmosphere of temperature 23 ° C and humidity 40% using a Garley type air permeability meter manufactured by Toyo Seiki Co., Ltd., GB2 (trademark) in accordance with JIS P-8117. The resistance was measured and used as the air permeability.

<Puncture strength (gf)>
A microporous membrane was fixed with a sample holder having an opening diameter of 11.3 mm using a handy compression tester KES-G5 (trademark) manufactured by Katou Tech. Next, the central portion of the fixed microporous membrane is subjected to a puncture test at a radius of curvature of the needle tip of 0.5 mm, a puncture speed of 2 mm / sec, a temperature of 23 ° C., and a humidity of 40% to maximize puncture. Raw puncture strength (gf) was obtained as a load.

<Amount of unmelted material in separator (pieces / 1000m ² )>
The amount of unmelted material in the separator has an area of 100 μm in length × 100 μm in width or more when the separator obtained through the film forming steps of Examples and Comparative Examples is observed with a transmission optical microscope, and light is transmitted. Quantified by areas that do not. The number of resin agglomerates per ^{1000 m 2 of} separator area was measured by observation with a transmission optical microscope.

<Nail piercing evaluation (%)>
The positive electrode, the negative electrode, and the non-aqueous electrolytic solution were prepared by the following procedures a to c.
a. Preparation of positive electrode Nickel, manganese, cobalt composite oxide (NMC) (Ni: Mn: Co = 1: 1: 1 (element ratio), density 4.70 g / cm ³ ) was 90.4% by mass as the positive electrode active material. As a conductive auxiliary material, 1.6% by mass of graphite powder (KS6) (density 2.26 g / cm ³ , number average particle diameter 6.5 μm) and acetylene black powder (AB) (density 1.95 g / cm ³ , number). An average particle size of 48 nm) was mixed at a ratio of 3.8% by mass, and polyvinylidene fluoride (PVDF) (density 1.75 g / cm ³ ) as a binder was mixed at a ratio of 4.2% by mass, and these were mixed with N-methylpyrrolidone (NMP). ) To prepare a slurry. This slurry is applied to one side of a 20 μm-thick aluminum foil serving as a positive electrode current collector using a die coater, dried at 130 ° C. for 3 minutes, and then compression-molded using a roll press to obtain a positive electrode. Made. The amount of the positive electrode active material applied at this time was 109 g / m ² .

b. Graphite powder A (density 2.23 g / cm ^3, number average particle diameter of 12.7 [mu] m) as prepared negative active material of the negative electrode of 87.6 wt%, and graphite powder B (density 2.27 g / cm ^3, the number average particle 9.7% by mass (diameter 6.5 μm), 1.4% by mass (solid content equivalent) of ammonium salt of carboxymethyl cellulose as a binder (solid content concentration 1.83% by mass aqueous solution), and 1.7% by mass of diene rubber-based latex. % (Solid content equivalent) (solid content concentration 40% by mass aqueous solution) was dispersed in purified water to prepare a slurry. This slurry was applied to one side of a copper foil having a thickness of 12 μm to be a negative electrode current collector with a die coater, dried at 120 ° C. for 3 minutes, and then compression-molded with a roll press to prepare a negative electrode. The amount of the negative electrode active material applied at this time was 52 g / m ² .

c. _{Preparation of non-aqueous electrolyte solution A non-aqueous electrolyte solution is prepared by dissolving LiPF 6} as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) so as to have a concentration of 1.0 mol / L. Prepared.

d. Battery production Using the positive electrode, negative electrode, non-aqueous electrolytic solution, and separator (separator of Example or separator of Comparative Example) obtained in the above a to c, a current value of 1 A (0.3 C) and a termination battery voltage A laminated secondary battery having a size of 100 mm × 60 mm and a capacity of 3 Ah was prepared by charging with a constant current constant voltage (CCCV) for 3 hours under the condition of 4.2 V.
e. Nail piercing evaluation A laminated secondary battery was placed on an iron plate in an explosion-proof booth where temperature control was possible. The temperature inside the explosion-proof booth was set to 40 ° C. in the central portion of the laminated secondary battery, and an iron nail having a diameter of 3.0 mm was penetrated at a speed of 2 mm / sec, and the nail was maintained in a penetrated state. The temperature of the thermocouple installed inside the nail so that the temperature inside the laminated battery could be measured after the nail penetrated was measured, and the presence or absence of ignition was evaluated.
The evaluation was repeated using a laminated secondary battery newly prepared by the same method, and the number of samples that did not ignite (no ignition) was calculated as a% value by the following formula.
Evaluation result (%) = (100 x number of samples that did not ignite / total number of samples)

<Cycle test (%)>
Using the separators obtained in Examples and Comparative Examples, and using the simple battery obtained in step d above, the cycle characteristics were evaluated by the following procedure.
(1) Pretreatment The simple battery is charged with a constant current of 1 / 3C to a voltage of 4.2V, then charged with a constant voltage of 4.2V for 8 hours, and then charged with a current of 1 / 3C to 3.0V. The voltage was discharged to the final voltage of. Next, after a constant current charge to a voltage of 4.2 V with a current value of 1 C, a constant voltage charge of 4.2 V was performed for 3 hours, and then discharged to a final voltage of 3.0 V with a current of 1 C. Finally, a constant current charge of 4.2 V was performed with a current value of 1 C, and then a constant voltage charge of 4.2 V was performed for 3 hours. Note that 1C represents a current value that discharges the reference capacity of the battery in one hour.
(2) Cycle test The pretreated battery is discharged to a discharge end voltage of 3 V with a discharge current of 1 C under the condition of a temperature of 25 ° C., and then charged to a charge end voltage of 4.2 V with a charge current of 1 C. went. Charging and discharging were repeated with this as one cycle. Then, the capacity retention rate after 300 cycles with respect to the initial capacity (capacity in the first cycle) was calculated as a% value by the following formula.
Evaluation result (%) = (holding capacity after 100 x 300 cycles / initial capacity)

[Example 1]
Powder (first component) containing the first resin (weight average molecular weight and number average molecular weight are difficult to measure, UHMWPE with viscosity average molecular weight of 1 million) and second resin (nylon 12 at melting point 178 ° C. 12: Aldrich sales product number 181161, labeled as "PA12" in the table) (second component; columnar body with a diameter of 5 to 6 mm and a height of 3 to 2 mm) was placed in a supermixer at the mass ratio shown in Table 1. The first component and the second component were supplied to a twin-screw extruder and melt-mixed. Then, while melt-mixing, liquid paraffin (kinematic viscosity at 37.78 ° C. 7.59 × 10 ^-5 m ² / s) is extruded by an injection nozzle with a manifold (T die) having a die lip interval of 1500 μm. It was supplied to the machine, further kneaded, and the resin composition was extruded. At this time, the extrusion middle stage portion (middle stage feed port of the twin-screw extruder) so that the ratio of the amount of liquid paraffin to the resin composition extruded from the twin-screw extruder is 70% by mass and the temperature of the resin composition is 220 ° C. ) Further injected liquid paraffin. Subsequently, the extruded resin composition was extruded onto a cooling roll controlled to a surface temperature of 25 ° C. and cast to obtain a sheet-shaped molded product.
Next, it was guided to a simultaneous biaxial tenter stretching machine and biaxially stretched to obtain a stretched product. The stretching conditions were a stretched surface magnification of 50 to 180 times, and the porosity, air permeability, thickness, piercing strength, etc. were adjusted by adjusting the stretching temperature, the heating air volume, the magnification, and the like. In Example 1, the biaxial stretching temperature was 125 ° C.
Next, the stretched product was immersed in dichloromethane, and liquid paraffin was extracted from the stretched product to form a porous body.
Next, it is guided to a TD tenter to heat-fix the porous body, heat-fixed (HS) at 128 ° C., and then 0.5 times the relaxation operation in the TD direction (that is, the HS relaxation rate is 0.5 times). ) Was performed. Then, the above-mentioned evaluation was performed on the obtained microporous membrane. The evaluation results are shown in Table 1.

[Examples 2 to 10 and Comparative Examples 1 to 4]
A microporous membrane was prepared by the same method as in Example 1 except that the raw material composition was changed as shown in Table 1. The evaluation results of the obtained film are shown in Table 1. Similar to Example 1, the resin corresponding to the first component was used as a powder, and the resin corresponding to the second component was used as a pellet.

In addition, "EVOH" of Example 2 represents an ethylene-vinyl alcohol copolymer (Kuraray Eval (trademark) standard grade "G156B"). However, the commercially available products that can be used are not limited to this, and other grades manufactured by the above companies or ethylene-vinyl alcohol copolymers other than those manufactured by the above companies can also be used.

Moreover, "PE-PA" of Examples 3 and 5 represents a polyethylene-polyamide copolymer. Here, LP91 manufactured by ARKEMA was used as "PE-PA". However, the commercially available products that can be used are not limited to this, and other grades manufactured by the above companies or polyethylene-polyamide copolymers other than those manufactured by the above companies can also be used.

Further, the “nanocrystal structure controlled elastomer” of Example 7 represents a Toughmer (registered trademark) brand “PN-2070” manufactured by Mitsui Chemicals, Inc. However, the commercially available products that can be used are not limited to this, and other brands manufactured by the above-mentioned companies or nanocrystal structure-controlled elastomers other than those manufactured by the above-mentioned companies can also be used.

Further, "PA12 & PE-PA" of Example 10 represents a mixture of nylon 12 and a polyethylene-polyamide copolymer at a ratio of 10: 1 (10: 1 = PA12: PE-PA). Here, as "PA12 & PE-PA", nylon 12 having a melting point of 178 ° C.: sold by Aldrich (product number; 181161) and LP91 manufactured by ARKEMA were used. However, the commercially available products that can be used are not limited to this. Coalescence can also be used.

Further, "PA copolymer" of Comparative Example 1 represents alamine (registered trademark) grade "CM8000" available from Toray Industries, Inc., and "PEEK" of Comparative Example 2 is polyetheretherketone (Toray Plastics). Represents TPS (registered trademark) standard grade "NC") available from Seiko Co., Ltd.

From the results in Table 1, the microporous membranes of Examples 1 to 10 have a reduced amount of unmelted matter (aggregate amount, that is, gel content) as compared with Comparative Examples 1 to 4, that is, the quality of the microporous membranes is reduced. It was confirmed that improvements were being made. Further, from the results in Table 1, in Examples 1 to 10, it is possible to realize a power storage device having excellent safety (safety in a nail piercing test) and excellent cycle characteristics as compared with Comparative Examples 1 to 4. It was also confirmed that there was.

Claims (13)

A microporous membrane containing a polyethylene resin and a resin having a melting point of 140 ° C. to 330 ° C. unlike the polyethylene resin.
A microporous membrane in which both the storage elastic modulus (E') and the loss elastic modulus (E ") are 10 Pa to 10,000,000 Pa in the range of 150 ° C. to 300 ° C. in the solid viscoelasticity measurement. The microporous membrane according to claim 1, wherein the resin having a melting point of 140 ° C. to 330 ° C. is dispersed in the polyethylene resin in a 2D spectroscopic mapping measurement. The microporous membrane according to claim 2, wherein at least one particle size of the resin having a melting point of 140 ° C. to 330 ° C. is 5.1 μm to 10 μm in the 2D spectroscopic mapping measurement. The microporous membrane according to claim 2 or 3, wherein in the 2D spectroscopic mapping measurement, the average particle size of the resin having a melting point of 140 ° C. to 330 ° C. is 5.1 μm to 10 μm. The invention according to any one of claims 1 to 4, wherein the resin having a melting point of 140 ° C. to 330 ° C. contains at least one selected from the group consisting of a polyolefin resin, a benzene ring-containing resin, and a heteroatom-containing resin. Microporous film. The microporous membrane according to claim 5, wherein the polyolefin resin excludes homopolypropylene. The microporous film according to claim 5, wherein the benzene ring-containing resin is at least one selected from the group consisting of polyethylene terephthalate, polyetheretherketone, and polyphenylene ether. The microporous membrane according to claim 5, wherein the heteroatom-containing resin is at least one selected from the group consisting of polyamide, polytetrafluoroethylene, and polyketone. The microporous membrane according to any one of claims 1 to 8, wherein the polyethylene resin contains ultra-high molecular weight polyethylene. The mass ratio of the resin having a melting point of 140 ° C. to 330 ° C. and the polyethylene resin (the resin having a melting point of 140 ° C. to 330 ° C./the polyethylene resin) is 0.01 / 0.99 to 0.90 / 0. 10. The microporous film according to any one of claims 1 to 9. A separator for a power storage device comprising the microporous membrane according to any one of claims 1 to 10. A lithium ion secondary battery comprising the microporous membrane according to any one of claims 1 to 10. A power storage device comprising the microporous membrane according to any one of claims 1 to 10.

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