JP2024075699A - Separators for power storage devices - Google Patents

Abstract

The problem to be solved by the present invention is to provide a microporous membrane with improved quality (reduced amount of unmelted material).
[Solution] A microporous membrane is provided that contains a polyethylene resin and a resin different from the polyethylene resin and having a melting point of 140°C to 330°C, and that in solid viscoelasticity measurement, the microporous membrane has a storage modulus (E') and a loss modulus (E") both of 10 Pa to 10,000,000 Pa in the range of 150°C to 300°C.
[Selected Figure] Figure 1

Description

The present invention relates to a microporous membrane containing two or more types of resins, and a separator (e.g., a separator for an electricity storage device) that includes such a microporous membrane.

Microporous membranes are widely used as precision filtration membranes, fuel cell separators, condenser separators, base materials for functional membranes in which functional materials are filled into the pores to provide new functions, and separators for power storage devices, or as constituent materials for these.

Lithium ion secondary batteries (LIBs) are widely used in notebook personal computers, mobile phones, digital cameras, and the like. Among these, polyolefin microporous membranes are known as separators for LIBs or their constituent materials. Patent Document 1 proposes a method for producing a polyolefin microporous membrane with the aim of achieving high strength, a large specific surface area, and a large pore volume. In Patent Document 1, a polyolefin resin having a weight average molecular weight of 500,000 or more and liquid paraffin are melt-kneaded, and the liquid paraffin is extracted from the resulting resin composition to produce a polyolefin microporous membrane. Patent Document 2 describes a method for producing a microporous membrane by kneading a polyolefin resin such as polyethylene and a resin other than polyolefin (e.g., polyamide) in an extruder in advance to form pellets, mixing the pellets with liquid paraffin, extruding, and further extracting the liquid paraffin.

JP 2002-088189 A JP 2002-226639 A

In recent years, separators used in electricity storage devices are increasingly required to have improved quality (reduced amount of unmelted material). By providing a microporous membrane with improved quality, it is possible to provide a separator with improved quality, and it is expected that the use of a separator with improved quality will enable the realization of an electricity storage device that is excellent in safety (safety in a nail penetration test) and in capacity retention characteristics (cycle characteristics) during repeated charging and discharging.
In view of the above situation, the microporous membranes described in Patent Documents 1 and 2 have room for improvement in terms of quality.

In view of the above circumstances, the present invention aims to provide a microporous membrane with improved quality (reduced amount of unmelted material). In addition, one embodiment of the present invention discloses a microporous membrane that can improve the safety (safety in a nail penetration test) and cycle characteristics of an electricity storage device, a separator (separator for electricity storage device) that includes such a microporous membrane, and an electricity storage device that includes such a microporous membrane.

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 an electricity storage device having the following configuration, thereby completing the present invention. Some aspects of the present invention are exemplified below.
[1] A microporous membrane comprising a polyethylene resin and a resin having a melting point of 140°C to 330°C different from the polyethylene resin,
A microporous film having a storage modulus (E') and a loss modulus (E") both of 10 Pa to 10,000,000 Pa in the range of 150°C to 300°C in solid viscoelastic measurements.
[2] The microporous membrane according to [1], wherein, in a 2D spectroscopic mapping measurement, the resin having a melting point of 140°C to 330°C is dispersed in the form of particles in the polyethylene resin.
[3] The microporous membrane according to [2], wherein in the 2D spectroscopic mapping measurement, 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.
[4] The microporous membrane according to [2] or [3], wherein, in the 2D spectroscopic mapping measurement, the resin having a melting point of 140°C to 330°C has an average particle size of 5.1 μm to 10 μm.
[5] The microporous membrane according to any one of [1] to [4], wherein the resin having a melting point of 140°C to 330°C includes at least one selected from the group consisting of polyolefin resins, benzene ring-containing resins, and heteroatom-containing resins.
[6] The microporous membrane according to [5], wherein the polyolefin resin is other than homopolypropylene.
[7] The microporous membrane according to [5], wherein the benzene ring-containing resin is at least one selected from the group consisting of polyethylene terephthalate, polyether ether ketone, 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 microporous membrane according to any one of [1] to [9], wherein a mass ratio of the resin having a melting point of 140°C to 330°C to 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.
[11] A separator for an electricity storage device, comprising 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] An electricity storage device comprising 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 (reduced amount of unmelted material). Furthermore, according to the present invention, by providing such a microporous membrane, it is possible to provide a separator with improved quality. It is expected that by providing such a separator, it will be possible to realize an electricity storage device with excellent safety (safety in a nail penetration test) and excellent cycle characteristics.

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

The following describes an embodiment of the present invention (referred to as the "present embodiment"); however, the present invention is not limited to this embodiment. Various modifications of the present invention are possible without departing from the gist of the present invention. In this specification, unless otherwise specified, "to" means that the numerical values at both ends are included as the upper and lower limits. In addition, in this specification, the upper and lower limits of a numerical range can be combined in any way.

<Microporous membrane>
The microporous membrane according to this embodiment (hereinafter sometimes simply referred to as "microporous membrane") contains a polyethylene resin (hereinafter sometimes referred to as "first resin") and a resin (hereinafter sometimes referred to as "second resin") different from the first resin and having a melting point of 140 to 330°C. In solid viscoelasticity measurement, the microporous membrane has a storage modulus (E') and a loss 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 precision filtration membrane, a fuel cell separator, a capacitor separator, a power storage device separator, an electrolysis membrane, or a constituent material thereof, and the like.
When a microporous membrane is used as a separator for an electricity storage device or a constituent material thereof, particularly as a separator for an LIB or a constituent material thereof, the microporous membrane itself may be used as the separator, or a microporous membrane having another layer or another membrane provided on at least one side thereof may be used as the separator. The microporous membrane used for the separator for an electricity storage device preferably has low electronic conductivity, ionic conductivity, high resistance to organic solvents, and fine pores.

As described in the Examples section, the microporous membranes of the Examples have a reduced amount of unmelted material (amount of aggregates, i.e., gel content) compared to the microporous membranes of the Comparative Examples, which means that the quality is improved.

To achieve such improved quality, it is preferable that the second resin is suitably dispersed in the first resin. In other words, it is preferable that the second resin is dispersed in the first resin in a granular form. The above-mentioned suitable dispersion is difficult to obtain by simply mixing the first resin and the second resin. However, the above-mentioned suitable dispersion tends to be obtained by a method in which a first component containing the first resin (e.g., powder of the first resin) and a second component containing the second resin (e.g., pellets of the second resin) are separately prepared and then fed into an extruder. Such a method will be described later.

In the temperature-E' graph and temperature-E" graph of the microporous membrane according to this embodiment, the lines may be broken when the temperature exceeds 200°C. This is because as the temperature of the microporous membrane increases, the melting point of the resin component that can be contained in the microporous membrane (e.g., nylon 12) is exceeded, and the viscosity of the resin component that has exceeded the melting point decreases, leading to rupture. However, the rupture temperature does not necessarily correlate with the melting point of the resin component and can be adjusted by the size of the domains dispersed in polyethylene (PE) and the resin components that can be contained in the microporous membrane (e.g., a heteroatom-containing resin, the molecular weight of the second resin, etc.).
Incidentally, even if the storage modulus (E') and loss modulus (E") are not measured up to 300°C, they often tend to decrease at temperatures of 150°C or higher. Therefore, if they are 10,000,000 Pa or less at 150°C, there is a high possibility that the above requirement of "10 to 10,000,000 Pa in the range of 150 to 300°C" is met.

In the present embodiment, the significance of focusing on the storage modulus (E') and the loss modulus (E'') is as follows.
The storage modulus (E') represents the stiffness of a material during dynamic behavior, i.e., the hardness of the material. The lower the value of the storage modulus (E'), the lower the stiffness of the material. A storage modulus (E') of 10 to 10,000,000 Pa in the range of 150 to 300°C allows the microporous membrane to have the desired stiffness in the above temperature range.
Additionally, the loss modulus (E") represents the energy dissipated by the material during dynamic behavior, i.e., the viscosity of the material. The lower the loss modulus (E") value, the lower the viscosity. A loss modulus (E") of 10 to 10,000,000 Pa in the range of 150 to 300°C allows the microporous membrane to have the desired viscosity in the above temperature range.
As described above, a microporous membrane having a storage modulus (E') and loss modulus (E") both of 10 to 10,000,000 Pa in the range of 150 to 300°C in solid viscoelasticity measurements combines the desired rigidity and viscosity in that range of 150 to 300°C.

Without wishing to be bound by theory in this regard, it is believed that when the storage modulus (E') and loss modulus (E") are both 10 Pa or more in the range of 150 to 300°C in solid viscoelasticity measurements, when the microporous membrane in the electricity storage device melts due to a temperature rise caused by an internal short circuit in the electricity storage device, the molten resin (microporous membrane) penetrates into the pores of the electrode to exert an anchor effect. And, because the molten resin remains in place in the state of penetrating the pores of the electrode, an increase in the short-circuit area can be suppressed.
Similarly, without wishing to be bound by theory, it is believed that when the storage modulus (E') and loss modulus (E") are both 10,000,000 Pa or less in the range of 150 to 300°C in solid viscoelasticity measurements, the molten resin (microporous membrane) has appropriate viscoelasticity, so that the fluidity of the molten resin does not increase too much, and exposure of the electrodes or an increase in the short-circuit area due to outflow of the resin can be suppressed.
Therefore, even if the power storage device has a high energy density, for example, a large LIB, more specifically, an in-vehicle LIB, the device can easily prevent thermal runaway during an internal short circuit by including a separator having the microporous membrane according to the present embodiment.

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

When the solid viscoelasticity of a microporous membrane is measured, the value obtained may differ from that when the solid viscoelasticity of a resin raw material of the microporous membrane is measured. Furthermore, even if a method can measure the solid viscoelasticity of a resin raw material, there may be a method that is not suitable for measuring the solid viscoelasticity of a microporous membrane obtained from that resin raw material. In this embodiment, the storage modulus (E') and loss modulus (E") are each measured by a method for a microporous membrane (i.e., the method described in the Examples).
The significance of controlling the storage modulus (E') and loss modulus (E") of a microporous membrane within specific ranges at 150 to 300°C is that when an internal short circuit occurs in the electricity storage device, the viscoelasticity of the resin (microporous membrane) constituting the separator in the electricity storage device can be easily controlled within a specific range. In this embodiment, since parameters related to viscoelasticity are controlled within specific ranges not for the resin raw material but for the microporous membrane, when an internal short circuit occurs in the electricity storage device, the separator in the electricity storage device can easily obtain the desired viscoelasticity, which in turn makes it easier to prevent thermal runaway during an internal short circuit.

(First Component)
The first component includes a first resin (polyethylene resin). The first resin preferably has a viscosity average molecular weight (Mv) of 100,000 to 9,700,000 and a dispersity (Mw/Mn) expressed as the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of 3 to 18. Such a polyethylene resin is also called 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, and even more preferably 200,000 to 8,500,000. In addition, the dispersity (Mw/Mn) is not limited to the above range of 3 to 7, but is also preferably 3 to 18, more preferably 4 to 14, and extremely preferably 4 to 13.

The first component is preferably a powder containing 2 to 100% by mass of the first resin, based on the total mass of the first component, from the viewpoint of facilitating a reduction in the amount of unmelted material. From the viewpoint of ensuring the strength of the resulting microporous film, 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 multiple types of UHMWPE. From the viewpoint of high strength of the microporous membrane, the UHMWPE is preferably poly(ethylene and/or propylene-co-α-olefin), more preferably at least one selected from the group consisting of poly(ethylene-co-propylene), poly(ethylene-co-butene), and poly(ethylene-co-propylene-co-butene). From the same viewpoint, the UHMWPE preferably contains 98.5 mol% or more and 100 mol% or less of structural units derived from ethylene, and more preferably contains more than 0.0 mol% and 1.5 mol% or less of structural units derived from α-olefins other than ethylene.

The first resin may also contain a polyethylene resin other than UHMWPE. Examples of polyethylene resins other than UHMWPE include low-density polyolefins (LDPE) such as linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), high-pressure low-density polyethylene, or mixtures thereof.

The "powder" preferred as the first component satisfies at least one condition selected from the group consisting of a number average particle size ( _Nd50 ) of 80 μm to 180 μm, a volume average particle size ( _Nd50 ) of 120 μm to 220 μm, a number particle size distribution ( _Nd80 / _Nd20 ) of 1.1 to 4.2, preferably 1.2 to 4.1, a volume particle size distribution ( _Vd80 / _Vd20 ) of 1.1 to 3.3, preferably 1.15 to 3.2, a crystallite size of 10.0 nm to 30.0 nm, preferably 11.0 nm to 28.0 nm, more preferably 12.0 nm to 23.0 nm, a crystallinity of 30% to 99%, preferably 32% to 98%, more preferably 38% to 97.5%, and containing polyethylene. The 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 ), crystallite size, crystallinity, etc. can be measured by known techniques.

For example, the number average particle size (Nd ₅₀ ), volume average particle size (Vd ₅₀ ), number particle size distribution (Nd ₈₀ /Nd ₂₀ ), and volume particle size distribution (Vd ₈₀ /Vd ₂₀ ) can be obtained by measurement using a flow-type image analysis particle size/shape measurement device Particle Insight manufactured by Micromeritics. Also, for example, the crystallite size and 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 the effect of the present invention is not impaired. Examples of the resin other than the first resin include a resin that can be added as the second resin described below.
The first component may contain additives other than the resin, as long as the additives do not impair the effect of the present invention. Examples of the additives 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 color pigment.

(Second Component)
The second component includes a second resin (a resin having a melting point of 140°C to 330°C) different from the first resin. The second resin is not particularly limited, but examples thereof include polyolefin resins; polyamide resins such as nylon 6, nylon 66, nylon 11, nylon 6-10, nylon 6-12, nylon 6-66, and aramid resins; polyimide resins; polyester resins such as polyethylene terephthalate (PET) and polybutene terephthalate (PBT); fluorine resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene; copolymers of ethylene and vinyl alcohol (for example, EVAL manufactured by Kuraray Co., Ltd., melting point: 157°C to 190°C), polysulfone, polyethersulfone, polyketone, and polyetheretherketone (PEEK). These resins can be used singly or in a plurality of amounts.

By using such a second resin, it becomes easier to improve the heat resistance of the obtained microporous membrane. Furthermore, by suitably dispersing such a second resin in the above-mentioned first resin, the second resin is enveloped by the first resin, that is, the second resin is incorporated inside the microporous membrane. This reduces the exposure of the second resin on the surface of the microporous membrane, so that even if the microporous membrane is brought into contact with an electrode, the contact area between the electrode and the second resin can be reduced. Therefore, it is possible to reduce the possibility that the electrode will cause oxidation-reduction decomposition of the second resin.

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

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, polyether ether ketone, and polyphenylene ether. These benzene ring-containing resins can be used singly or in combination.

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

When the second resin is composed of multiple resins, the melting point of the second resin does not refer to the melting point of each resin that constitutes the second resin, but rather to the melting point of the second resin as a whole. Therefore, even if the second resin contains a resin whose melting point is outside the range of 140°C to 330°C, it is considered to be the second resin as long as the overall melting point is within the range of 140°C to 330°C. The first resin and the second resin only need to have different overall compositions. For this reason, for example, the resin that constitutes the second resin may contain the first resin (polyethylene resin). Even if the melting point of the polyethylene resin is less than 140°C, it is considered to be the second resin as long as it is combined with other resins so that the overall melting point is within the range of 140°C to 330°C.

It is preferable that such a second resin has a viscosity average molecular weight (Mv) of 10,000 to 300,000 and a dispersity (Mw/Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of 3 to 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 achieving both ensuring the film strength and expressing the fuse characteristics, the second resin in the second component is more preferably 2.1% by mass or more, and even more preferably 2.3% by mass or more.

The second component may contain additives other than the resin, as long as they do not impede the effect of the present invention. Examples of additives include dehydration condensation catalysts, metal soaps such as calcium stearate or zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, color pigments, etc.

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

Here, the "pellets" preferred as the second component are resinous objects having a number average particle size ( _Nd50 ) and a volume average particle size ( _Vd50 ) larger than those of the "powder" preferred as the first component, and a maximum side length of 10 mm or less and 1 mm or more. The shape of the pellets is not particularly limited, and may be, for example, spherical, elliptical, or cylindrical. The pellets are obtained by melt-extruding the raw material using an extruder, arranging the resultant into a strand shape while cooling with water or air, and continuously cutting the resultant. The size and shape can be adjusted by the strand formation or cutting method.

In this way, by melt-extruding the above-mentioned "powder" to prepare the above-mentioned "pellets", the swelling rate caused by the plasticizer can be significantly delayed (as described later, the second component, which is a pellet, does not substantially swell). Inside the extruder, it is important not to inhibit the swelling of the first component. Furthermore, in the melting step after the swelling step, from the viewpoint of uniform melting, the "pellets" preferred as the second component are those having a crystallite size larger than that of the "powder" preferred as the first component and within the above-mentioned range.
Furthermore, the "pellets" preferred as the second component are those having a crystallinity lower than that of the "powder" preferred as the first component and within the above range. In the method of producing pellets, the crystallite size, crystallinity, size and shape of the pellets can be adjusted by the temperature of the extruded resin into strands, the cooling temperature during cutting, or the pulling speed of the strands from the extrusion (melt micro-stretching). The number average particle size ( _Nd50 ), volume average particle size ( _Vd50 ), crystallite size, crystallinity, etc. can be measured by known methods.

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

By forming the second component into pellets, it becomes easier to significantly improve the uniformity of swelling of the plasticizer in the twin-screw extruder when mixed with the first component (e.g., powder of the first resin). This is thought to be because by forming the second component into pellets, the swelling rate can be significantly slowed down compared to, for example, the powder of the second component. Therefore, the second component does not excessively inhibit the swelling of the first component (e.g., powder of the first resin), and such pellets themselves do not basically swell, and are preferably used in the subsequent melt-kneading. This makes it easier to easily and uniformly disperse the second component in the first component down to the molecular level even in the melt-kneading process.

As mentioned above, the swelling rate of the second component, which is a pellet, is significantly slower than that of the powder, and therefore does not substantially swell. This makes it possible to prevent the swelling of the first component (e.g., the powder of the first resin) from being excessively inhibited by the second component.

(2D Spectroscopic Mapping Measurement)
FIG. 1 is an image showing the results of 2D spectroscopic mapping of the microporous membrane according to the present embodiment. In FIG. 1, an enlarged image of a part surrounded by a square with a white line is shown, and the part surrounded by a circle with a white line corresponds to a specific stretching vibration absorption band. In the 2D spectroscopic mapping measurement, it is preferable that the second resin is dispersed in the first resin (polyethylene resin) in a granular form. "The second resin is dispersed in the first resin in a granular form" means that the second resin is observed in the first resin in an island-like or dot-like form 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 entire image of the second resin can be suitably confirmed. Specific examples of the 2D spectroscopic mapping measurement are as described in the examples.

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

In addition, in the 2D spectroscopic mapping measurement, the average particle size of the second resin (a resin with a melting point of 140°C to 330°C) is preferably 5.1 to 10 μm, and more preferably 5.2 to 9.8 μm. By more suitably dispersing the second resin in the first resin, the average particle size of the second component becomes a value within the range of 5.1 to 10 μm. The method for calculating the average particle size of the second resin is as described in the examples.

As described above, by suitably dispersing the second component in the first component, the amount of unmelted material in the resulting microporous film is reduced, i.e., the quality is improved. By providing a microporous film with improved quality, a separator with improved quality can be provided. It is expected that by providing such a separator, it will be possible to realize an electricity storage device with excellent safety (safety in a nail penetration test) and excellent cycle characteristics.

(Mass of First Resin and Second Resin)
The mass ratio (second resin/first resin) of the second resin (resin having a melting point of 140 to 330° C.) to the first resin (polyethylene resin) is preferably 0.01/0.99 to 0.90/0.10.
Conventionally, the second resin as described above is non-melted and thermodynamically incompatible with the first resin because it has a different compatibility parameter from the first resin, but it can be physically dispersed in the first resin. Since such a dispersion-type alloy molded product can achieve both the properties 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 to suitably disperse the second resin in the first resin by simply mixing the first resin and the second resin. However, by preparing a first component containing the first resin (e.g., powder of the first resin) and a second component containing the second resin (e.g., pellets of the second resin), respectively, and subjecting them to a special extrusion process such as the steps (1) and/or (2) described later, the above-mentioned suitable dispersion tends to be obtained. And, according to such a method, it becomes easier to disperse the second resin in the first resin more suitably within the above range.
From the viewpoint of suitably 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 properties of the microporous membrane are described below.
These properties are measured when the microporous membrane used as the electricity storage device separator is a flat membrane. When the electricity storage device separator is in the form of a laminated membrane, the properties can be measured after removing layers other than the microporous membrane from the laminated membrane.

The porosity of the microporous membrane is preferably 20% or more, more preferably 30% or more, and even more preferably 32% or more or 35% or more. When the microporous membrane has a porosity of 20% or more, the membrane tends to have better compliance with the rapid movement of lithium (Li) ions when used as a separator for LIB or a constituent material thereof. On the other hand, the porosity of the microporous membrane is preferably 90% or less, more preferably 80% or less, and even more preferably 50% or less. When the microporous membrane has a porosity of 90% or less, the membrane strength tends to be improved and self-discharge tends to be 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 more, more preferably 50 seconds or more, even more preferably 55 seconds or more, and even more preferably 100 seconds or more per ¹⁰⁰ cm3. When the air permeability of the microporous membrane is 1 second or more, the balance between the membrane thickness, the porosity, and the average pore size tends to be further improved. In addition, 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, the stretching temperature, etc. Such air permeability is measured by the method described in the examples.

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

In particular, when a microporous membrane is used as a separator for LIB or a constituent material thereof, the membrane thickness of the microporous membrane is preferably 25 μm or less, more preferably 22 μm or less or 20 μm or less, and even more preferably 18 μm or less. In this case, by having the membrane thickness of the microporous membrane be 25 μm or less, the permeability tends to be further improved. In this case, the lower limit of the membrane 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, and preferably 2000 gf/20 μm or less, more preferably 1000 gf/20 μm or less. A puncture strength of 200 gf/20 μm or more is preferable from the viewpoint of suppressing rupture of the membrane due to active material that falls off when the battery is wound. It is also preferable from the viewpoint of suppressing concerns about short circuits due to expansion and contraction of the electrodes accompanying charging and discharging. On the other hand, a puncture strength of 2000 gf/20 μm or less is preferable from the viewpoint of reducing width contraction due to orientation relaxation during heating. The puncture strength of the microporous membrane can be adjusted by adjusting the stretching ratio, stretching temperature, etc. Such puncture strength is measured by the method described in the examples.

<Method for producing microporous membrane>
The method for producing a microporous membrane comprises the following steps:
(1) A step of supplying a first component including a first resin (polyethylene resin), a second component including a second resin (a resin having a melting point of 140 to 330° C.), and a plasticizer to a twin-screw extruder;
(2) melt-kneading the first component, the second component, and the plasticizer in a twin-screw extruder to produce a resin composition; and (3) extracting the plasticizer from the resin composition to produce a microporous membrane.
including.
Each step will be described in order below.

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

(Plasticizer)
The plasticizer may 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. In consideration of the subsequent extraction, the plasticizer used in step (1) is preferably a non-volatile solvent capable of forming a homogeneous solution at or above the melting point of the first resin or the second resin. Specific examples of non-volatile solvents include, for example, hydrocarbons such as liquid paraffin, paraffin wax, decane, and decalin; esters such as dioctyl phthalate and dibutyl phthalate; and higher alcohols such as oleyl alcohol and stearyl alcohol. Among them, liquid paraffin is preferred because it has high compatibility with polyethylene, and even when the molten kneaded product is stretched, interfacial peeling between the resin and the plasticizer is unlikely to occur, making it easier to perform uniform stretching.

(Feeding to twin screw extruder)
The first component and the second component can be simultaneously supplied to the twin-screw extruder, for example. Examples of the simultaneous supply include a method of dry-blending the first component and the second component using a super mixer or the like and supplying the resultant mixture to the twin-screw extruder, and a method of preparing a mixed slurry from the first component and the second component, which will be described later, and supplying the resulting mixture to the twin-screw extruder.

The first component and the second component can also be supplied separately to the twin-screw extruder, for example. Examples of the method for supplying the first component and the second component separately include a method of supplying the first component and the second component from separate feeders to the twin-screw extruder; a method of preparing a mixed slurry from the first component and a plasticizer, which will be described later, and supplying this to the twin-screw extruder separately from the second component; and a method of melting the second component using a separate extruder (second extruder) and supplying this to the twin-screw extruder (first extruder) separately from the first component. When the first component and the second component are supplied separately to the twin-screw extruder, the first component may be supplied first, or the second component may be supplied first.

The plasticizer may be fed, for example, to the twin-screw extruder together with the first component. After the plasticizer is fed to the twin-screw extruder together with the first component, additional plasticizer may be fed from the same or a different feeder.
This type of twin-screw extruder generally has an upper feed port arranged on the upstream side and a middle feed port arranged downstream of the feed port in the middle of the melt kneading area. Therefore, after the mixed slurry is supplied from the upper feed port of the twin-screw extruder, a plasticizer can be additionally supplied from the middle feed port of the twin-screw extruder. This makes it easier to adjust the liquid paraffin content of the resin composition extruded from the twin-screw extruder to a desired ratio, and also makes it easier 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, the step (1) can include a step of producing a mixed slurry from the first component and a plasticizer using a continuous mixer under conditions of a temperature of 20° C. to 70° C., a shear rate of 100 to 400,000 sec ^- 1, and a residence time of 1.0 to 60 seconds, and supplying the mixed slurry and the second component to a twin-screw extruder.
Step (1) may also include a step of producing a mixed slurry from the first component, the second component, and the plasticizer using a continuous mixer under the above-mentioned conditions, and supplying the mixed slurry to a twin-screw extruder.

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

The content of the first component (e.g., powder of the first resin) in the mixed slurry is preferably more than 0% by mass, more preferably 1% by mass or more, even more preferably 2% by mass or more or 4% by mass or more, based on the total mass of the mixed slurry, from the viewpoint of the strength of the resulting microporous film. In addition, from the viewpoint of suppressing the generation of unmelted matter in the first resin, this content is preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 30% by mass or less or 20% by mass or less.

The content of the second component (e.g., pellets of the second resin) in the mixed slurry is preferably more than 0% by mass, more preferably 1% by mass or more, and even more preferably 2% by mass or more or 4% by mass or more, based on the total mass of the mixed slurry, from the viewpoint of the strength of the resulting microporous film. Also, from the viewpoint of suppressing the generation of unmelted matter in the second resin, this content is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less or 20% by mass or less.

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 appropriate viscosity of the mixed slurry while ensuring entanglement of the polymer chains to an extent that does not cause a decrease in the molecular weight of the first resin, 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, and even more preferably 30°C to 70°C.

(Additive)
The mixed slurry may contain known additives such as 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.

[Step (2)]
In step (2), the first component, the second component, and the plasticizer are melt-kneaded in a twin-screw extruder to produce a resin composition. In this embodiment, since uniformity of swelling of the plasticizer with respect to the first component is ensured in the above step (1), in step (2), conditions such as the type of twin-screw extruder, supply of each raw material to the twin-screw extruder, extrusion time, extrusion speed, shear speed, shear force, etc. are not limited as long as they do not deviate from the gist of the present invention.

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

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

From the viewpoint of sheet moldability, the proportion of resin in the sheet-form molding (the total amount of the first resin, the second resin, and other resins, if included) is preferably 10 to 80% by mass, more preferably 20 to 60% by mass, and most preferably 30 to 50% by mass, based on the mass of the sheet-form molding.

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 through a T-die or the like, and then brought into contact with a heat conductor to cool to a temperature lower than the crystallization temperature of the resin component to solidify. Examples of heat conductors used for cooling and solidifying include metal, water, air, and plasticizers. Among these, it is preferable to use a metal roll because of its high thermal conductivity. In addition, when the extruded resin composition is brought into contact with a metal roll, it is also preferable to sandwich it between the rolls, because this further increases the efficiency of thermal conduction, and the sheet is oriented to increase the film strength and improve the surface smoothness of the sheet. When the resin composition is extruded into a sheet from a T-die, 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 amount of scum is reduced, and the impact on the film quality such as streaks or defects is small, and the risk of film breakage in the subsequent stretching process can be reduced. On the other hand, when the die lip distance is 3,000 μm or less, the cooling rate is high, which makes it possible to prevent uneven cooling and to maintain the thickness stability of the sheet.
In addition, during the step (4), the extruded sheet-like molding may be rolled.

[Stretching step (5)]
In step (5), the sheet-like molded article obtained in step (4) is biaxially stretched at an areal stretching ratio of 20 to 200 times to form a stretched product.

As a stretching process, biaxial stretching is preferred over uniaxial stretching from the viewpoint of reducing the film thickness distribution and air permeability distribution in the width direction. By simultaneously stretching the sheet in two axial directions, the number of times the sheet-like molded body is cooled and heated repeatedly during the film-making process is reduced, improving the distribution in the width direction. Examples of biaxial stretching methods include simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple stretching. From the viewpoint of improving puncture strength and uniformity of stretching, simultaneous biaxial stretching is preferred, and from the viewpoint of ease of control of surface orientation, sequential biaxial stretching is preferred.

In this specification, simultaneous biaxial stretching refers to a stretching method in which stretching in MD (machine direction of continuous microporous membrane molding) and stretching in TD (direction crossing the MD of the microporous membrane at an angle of 90°) are performed simultaneously, and the stretching ratios in each direction may be different. Sequential biaxial stretching refers to a stretching method in which stretching in MD and TD is performed independently, and while stretching is performed in MD or TD, the other direction is in an unconstrained state or fixed at a fixed length.

The stretching ratio is preferably in the range of 20 to 200 times in area ratio, more preferably in the range of 25 to 170 times, and even more preferably in the range of 30 to 150 times. The stretching ratio in each axial direction is preferably in the range of 2 to 15 times in MD and 2 to 15 times in TD, more preferably in the range of 3 to 12 times in MD and 3 to 12 times in TD, and even more preferably in the range of 5 to 10 times in MD and 5 to 10 times in TD. If the total area ratio is 20 times or more, the obtained microporous membrane tends to have sufficient strength, while if the total area ratio is 200 times or less, membrane breakage in step (5) is prevented and high productivity tends to be obtained.

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

[Extraction step (6)]
In step (6), the plasticizer is extracted from the stretched product obtained in step (5) to form a porous body. For example, the method of extracting the plasticizer includes a method of immersing the stretched product in an extraction solvent to extract the plasticizer and drying the product. 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 of the sheet-shaped molded body during a series of steps of immersion and drying. In addition, it is preferable that the amount of plasticizer remaining in the porous body is less than 1 mass% with respect to the mass of the entire porous film.
After the step (6), the plasticizer may be recovered by an operation such as distillation and reused.

As the extraction solvent, it is preferable to use one that is a poor solvent for the first resin and the second resin, a good solvent for the plasticizer, and has a boiling point lower than the melting points of the first resin and the second resin. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; non-chlorine halogenated solvents such as hydrofluoroethers and hydrofluorocarbons; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; and ketones such as acetone and methyl ethyl ketone. These extraction solvents may be recovered by distillation or other operations and reused.

[Heat treatment step (7)]
In step (7), the porous body obtained in step (6) above is heat-treated at a temperature not exceeding the melting point of the porous body, and then the porous body is stretched to produce a microporous membrane.

The porous body is heat-treated for the purpose of heat setting in order to suppress shrinkage. Methods of heat treatment include a stretching operation carried out in a specified atmosphere, at a specified temperature, and at a specified stretch ratio for the purpose of adjusting physical properties, and/or a relaxation operation carried out in a specified atmosphere, at a specified temperature, and at a specified relaxation ratio for the purpose of reducing stretching stress. A relaxation operation may be carried out after the stretching operation. These heat treatments can be carried out using a tenter or roll stretching machine.

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

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

During or after step (7), the microporous membrane may be subjected to post-treatment such as hydrophilization treatment with a surfactant, etc., crosslinking treatment with ionizing radiation, etc. The order of steps (5), (6), and (7) described above may be rearranged or these steps may be performed simultaneously, but from the viewpoint of film formability, it is preferable to perform these steps in the order of steps (5), (6), and (7) using a biaxial stretching machine.
From the viewpoints of handling and storage stability, the obtained microporous membrane can be wound up by a winder to form a roll, or cut by a slitter.

<Separators for power storage devices, separators for LIBs>
The microporous membrane can be used in an electricity storage device, for example, as a separator for an LIB or a constituent material thereof, which makes it easier to suppress oxidation-reduction deterioration of the separator and to construct a dense and uniform porous structure.

The separator may be in the form of a flat membrane (e.g., formed from a single microporous membrane), a laminated membrane (e.g., a laminate of multiple microporous membranes, or a laminate of a microporous membrane and another membrane), or a coated membrane (e.g., a microporous membrane with at least one side coated with a functional substance).

<LIB>
LIB is a storage battery that uses lithium transition metal oxides such as lithium cobalt oxide and lithium cobalt composite oxide as the positive electrode, carbon materials such as graphite and graphite as the negative electrode, and an organic solvent containing lithium salts such as _LiPF6 as the electrolyte. When LIB is charged and discharged, ionized Li moves back and forth between the electrodes. In addition, since it is necessary for ionized Li to move between the electrodes at a relatively high speed while suppressing contact between the electrodes, a separator is placed between the electrodes.

The present invention will be explained in more detail below 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 calorimetric (DSC) measuring device "DSC-60" (manufactured by Shimadzu Corporation). First, the second resin was punched into a circle with a diameter of 5 mm, and several sheets were stacked together to make 3 mg, which was used as a measurement sample. This sample was laid on an aluminum open sample pan with a diameter of 5 mm, a clamping cover was placed on it, and the sample was fixed in the aluminum pan with a sample sealer. In a nitrogen atmosphere, the temperature was raised from 30 ° C. to 200 ° C. at a heating rate of 10 ° C. / min (first heating), held at 200 ° C. for 5 minutes, and then cooled from 200 ° C. to 30 ° C. at a heating rate of 10 ° C. / min. 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 heating). The temperature at which the melting endothermic curve of the second heating had a maximum was taken as the melting point of the second resin. When there were multiple maximum values, the temperature at which the largest maximum value of the melting endothermic curve was taken as the melting point of the second resin.

<Solid viscoelasticity measurement of microporous membrane>
The dynamic viscoelasticity of the separator can be measured using a dynamic viscoelasticity measuring device, and the storage modulus (E') and loss modulus (E'') can be calculated.
Regarding E' _and E'', if no breakage (sudden drop in elastic modulus) of the sample was observed between 150°C and 300°C, they were calculated from the average value between 150°C and 300°C, whereas if breakage of the sample was observed between 150°C and 300°C, they were calculated from the average value from 150°C to the temperature at the breakage point.

Storage modulus (E')
The conditions for measuring the storage modulus (E') are specified in the following configurations (i) to (iv).
(i) Dynamic viscoelasticity measurement was performed under the following conditions:
Measurement device used: RSA-G2 (manufactured by TA Instruments)
・Sample film thickness: 5μm to 50μm range ・Measurement temperature range: -50 to 300℃
Heating rate: 10°C/min
Measurement frequency: 1Hz
Deformation mode: Sine wave tension mode (Linear tension)
Initial value of static tensile load: 0.5N
Initial gap distance (at 25°C): 25 mm
· Auto strain adjustment: Enabled (range: amplitude value 0.05 to 25%, sine wave load 0.02 to 5N)
It is done by.
(ii) The static tensile load refers to the intermediate value between the maximum stress and the minimum stress in each periodic motion, and the sinusoidal load refers to a vibration stress centered on the static tensile load.
(iii) The sinusoidal tensile mode refers to measuring the vibration stress while performing periodic motion with a fixed amplitude of 0.2%. In the sinusoidal tensile mode, the gap distance and the static tensile load are varied so that the difference between the static tensile load and the sinusoidal load is within 20% to measure the vibration stress. When the sinusoidal load becomes 0.02 N or less, the amplitude value is amplified so that the sinusoidal load is within 5 N and the increase in the amplitude value is within 25%, and the vibration stress is measured.
(iv) The relationship between the obtained sinusoidal load and the amplitude value, and the following formula:
σ ^* = _σ0 ·Exp[i(ωt+δ)],
ε ^* = ε ₀ Exp(iωt),
σ ^* =E ^* ·ε ^*
E ^* =E'+iE''
(In the formula, σ ^* : oscillatory stress, ε ^* : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between oscillatory stress and strain, E ^* : complex elastic modulus, E': storage elastic modulus, E'': loss elastic modulus. Oscillatory stress: sinusoidal load/initial cross-sectional area. Static tensile load: load at the minimum point of oscillatory stress in each cycle (minimum point of gap distance in each cycle). Sine wave load: difference between measured oscillatory stress and static tensile load.)
The storage modulus E' was calculated from the above.

Loss modulus (E'')
The measurement conditions for the loss modulus (E'') are specified in the following configurations (i) to (iv).
(i) Dynamic viscoelasticity measurement was performed under the following conditions:
Measurement device used: RSA-G2 (manufactured by TA Instruments)
・Sample film thickness: 5μm to 50μm range ・Measurement temperature range: -50 to 300℃
Heating rate: 10°C/min
Measurement frequency: 1Hz
Deformation mode: Sine wave tension mode (Linear tension)
Initial value of static tensile load: 0.5N
Initial gap distance (at 25°C): 25mm
· Auto strain adjustment: Enabled (range: amplitude value 0.05 to 25%, sine wave load 0.02 to 5N)
It is done by.
(ii) The static tensile load refers to the intermediate value between the maximum stress and the minimum stress in each periodic motion, and the sinusoidal load refers to a vibration stress centered on the static tensile load.
(iii) The sinusoidal tensile mode refers to measuring the vibration stress while performing periodic motion with a fixed amplitude of 0.2%. In the sinusoidal tensile mode, the gap distance and the static tensile load are varied so that the difference between the static tensile load and the sinusoidal load is within 20% to measure the vibration stress. When the sinusoidal load becomes 0.02 N or less, the amplitude is amplified so that the sinusoidal load is within 5 N and the increase in the amplitude is within 25%, and the vibration stress is measured.
(iv) The obtained sinusoidal load and amplitude value, and the following formula:
σ ^* = _σ0 ·Exp[i(ωt+δ)],
ε ^* = ε ₀ Exp(iωt),
σ ^* =E ^* ·ε ^*
E ^* =E'+iE''
(In the formula, σ ^* : oscillatory stress, ε ^* : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between oscillatory stress and strain, E ^* : complex elastic modulus, E': storage elastic modulus, E'': loss elastic modulus. Oscillatory stress: sinusoidal load/initial cross-sectional area. Static tensile load: load at the minimum point of oscillatory stress in each cycle (minimum point of gap distance in each cycle). Sine wave load: difference between measured oscillatory stress and static tensile load.)
The loss modulus E'' was calculated from

<2D Spectroscopic Mapping Measurement>
- Preparation of Sample The microporous membrane was cut into a size of MD 5 mm x TD 5 mm and used for measurement.

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

- Method for calculating average particle size of second resin Image data obtained by 2D mapping was subjected to image processing to determine the area equivalent to a circle, and the diameter was calculated as the average particle size.

<Film thickness (μm)>
The thickness was measured using a micro thickness gauge (Type KBM, manufactured by Toyo Seiki Seisakusho) at room temperature of 23° C. and humidity of 40%. A terminal with a terminal diameter of 5 mm was used and a load of 44 gf was applied.

<Porosity (%)>
A 10 cm x 10 cm square sample was cut from the microporous membrane, and its volume (cm ³ ) and mass (g) were determined. From these and the density (g/cm ³ ), the porosity was calculated using the following formula.
Porosity (%)=(volume−mass/density)/volume×100

<Air permeability (sec/100 cm ³ )>
In accordance with JIS P-8117, the air resistance of the polyolefin microporous membrane was measured in an atmosphere at a temperature of 23° C. and a humidity of 40% using a Gurley air permeability meter, G-B2 (trademark) manufactured by Toyo Seiki Co., Ltd., and this was taken as the air permeability.

<Puncture strength (gf)>
Using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech, the microporous membrane was fixed with a sample holder having an opening diameter of 11.3 mm. Next, a puncture test was performed on the center of the fixed microporous membrane in an atmosphere of a temperature of 23° C. and a humidity of 40% with a needle tip curvature radius of 0.5 mm and a puncture speed of 2 mm/sec, to obtain raw puncture strength (gf) as the maximum puncture load.

<Amount of unmelted material in separator (pieces/1000 m ² )>
The amount of unmelted material in the separator is quantified by a region having an area of 100 μm length × 100 μm width or more and through which light does not pass, when the separator obtained through the film-forming process of the examples and comparative examples is observed with a transmission optical microscope. The number of resin aggregates per 1000 ^m2 of separator area was measured by observation with a transmission optical microscope.

<Nail penetration rating (%)>
A positive electrode, a negative electrode, and a non-aqueous electrolyte solution were prepared according to the following procedures a to c.
A positive electrode was prepared by mixing 90.4 mass% of nickel, manganese, cobalt composite oxide (NMC) (Ni:Mn:Co=1:1:1 (element ratio), density 4.70 g/cm ³ ) as a positive electrode active material, 1.6 mass% of graphite powder (KS6) (density 2.26 g/cm ³ , number average particle size 6.5 μm) ^as a conductive additive, and 3.8 mass% of acetylene black powder (AB) (density 1.95 g/cm ³ , number average particle size 48 nm) as a binder in a ratio of 4.2 mass%, and dispersing these in N-methylpyrrolidone (NMP) to prepare a slurry. The slurry was applied to one side of an aluminum foil having a thickness of 20 μm 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 prepare a positive electrode. The amount of the positive electrode active material applied was 109 g/m ² .

b. Preparation of negative electrode As the negative electrode active material, 87.6% by mass of graphite powder A (density 2.23 g / cm ³ , number average particle diameter 12.7 μm), 9.7% by mass of graphite powder B (density 2.27 g / cm ³ , number average particle diameter 6.5 μm), and 1.4% by mass (solid content equivalent) of ammonium salt of carboxymethylcellulose (solid content concentration 1.83 mass% aqueous solution) and 1.7% by mass (solid content equivalent) of diene rubber latex (solid content concentration 40 mass% aqueous solution) were 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 machine to prepare a negative electrode. The amount of negative electrode active material applied at this time was 52 g / m ² .

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

d. Battery Fabrication A laminated secondary battery having a size of 100 mm x 60 mm and a capacity of 3 Ah was fabricated by using the positive electrode, negative electrode, and nonaqueous electrolyte obtained in the above a to c, as well as a separator (the separator of the Example or the separator of the Comparative Example) and charging at a current value of 1 A (0.3 C) and a final battery voltage of 4.2 V for 3 hours at a constant current constant voltage (CCCV).
e. Nail penetration evaluation The laminated secondary battery was placed on an iron plate in a temperature-controllable explosion-proof booth. The temperature in the explosion-proof booth was set to 40°C, and an iron nail with a diameter of 3.0 mm was penetrated into the center of the laminated secondary battery at a speed of 2 mm/sec, and the nail was maintained in the penetrated state. The temperature of a thermocouple installed inside the nail so that the temperature inside the laminated battery can be measured after the nail penetrated was measured, and the presence or absence of ignition was evaluated.
The evaluation was repeated using newly prepared laminated secondary batteries in the same manner, and the number of samples that did not ignite (no ignition) was calculated as a percentage using the following formula.
Evaluation result (%) = (100 x number of samples that did not ignite / total number of samples)

<Cycle test (%)>
The separators obtained in the examples and comparative examples were used, and the simplified batteries obtained in the above procedure d were used to evaluate cycle characteristics in the following manner.
(1) Pretreatment The above-mentioned simple battery was charged at a constant current of 1/3C to a voltage of 4.2V, then charged at a constant voltage of 4.2V for 8 hours, and then discharged at a current of 1/3C to an end voltage of 3.0V. Next, the battery was charged at a constant current of 1C to a voltage of 4.2V, then charged at a constant voltage of 4.2V for 3 hours, and then discharged at a current of 1C to an end voltage of 3.0V. Finally, the battery was charged at a constant current of 1C to 4.2V, and then charged at a constant voltage of 4.2V for 3 hours. Here, 1C represents the current value at which the battery's reference capacity is discharged in 1 hour.
(2) Cycle Test The above pretreated battery was discharged at a discharge current of 1 C to a discharge end voltage of 3 V under a temperature condition of 25° C., and then charged at a charge current of 1 C to a charge end voltage of 4.2 V. This constituted one cycle, and charging and discharging were repeated. The capacity retention rate after 300 cycles relative to the initial capacity (capacity in the first cycle) was calculated in percentage by the following formula.
Evaluation result (%)=(100×retention capacity after 300 cycles/initial capacity)

[Example 1]
A powder (first component) containing a first resin (weight average molecular weight and number average molecular weight are difficult to measure, viscosity average molecular weight is 1 million UHMWPE) and a pellet (second component; cylindrical body with diameter 5-6 mm, height 3-2 mm) containing a second resin (nylon 12 with melting point 178°C: Aldrich product number; 181161, shown as "PA12" in the table) were dry blended in a super mixer in the mass ratio shown in Table 1, and the first component and the second component were fed to a twin-screw extruder and melt-mixed. Then, while melt-mixing, liquid paraffin (dynamic viscosity at 37.78°C is 7.59 x ^{10-5 m2} /s) was fed to a twin-screw extruder equipped with a manifold (T die) with a die lip spacing of 1500 μm by an injection nozzle, and further kneaded to extrude a resin composition. At this time, liquid paraffin was further injected from the middle stage of the extrusion (middle stage feed port of the twin-screw extruder) so that the ratio of liquid paraffin to the resin composition extruded from the twin-screw extruder was 70% by mass and the temperature of the resin composition was 220° C. Subsequently, the extruded resin composition was extruded onto a cooling roll whose surface temperature was controlled to 25° C., and cast to obtain a sheet-like molded product.
Next, the film was introduced into a simultaneous biaxial tenter stretching machine and biaxially stretched to obtain a stretched product. The stretching conditions were a stretching areal ratio of 50 to 180 times, and the porosity, air permeability, thickness, and puncture strength were adjusted by adjusting the stretching temperature, heating air volume, and ratio, etc. In Example 1, the biaxial stretching temperature was 125°C.
Next, the stretched product was immersed in dichloromethane to extract the liquid paraffin from the stretched product, thereby forming a porous body.
Next, the porous body was introduced into a TD tenter to perform heat setting, and heat setting (HS) was performed at 128°C, and then a relaxation operation of 0.5 times in the TD direction (i.e., the HS relaxation rate was 0.5 times).Then, the obtained microporous membrane was evaluated as described above.The evaluation results are shown in Table 1.

[Examples 2 to 10 and Comparative Examples 1 to 4]
A microporous membrane was produced in the same manner as in Example 1, except that the raw material composition was changed as shown in Table 1. The evaluation results of the obtained membrane are shown in Table 1. As in 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 pellets.

In Example 2, "EVOH" refers to an ethylene-vinyl alcohol copolymer (EVAL (trademark) standard grade "G156B" manufactured by Kuraray). However, 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 manufactured by companies other than the above companies can also be used.

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

The "nanocrystalline structure-controlled elastomer" in Example 7 refers to the "PN-2070" brand of Toughmer (registered trademark) manufactured by Mitsui Chemicals, Inc. However, usable commercial products are not limited to this, and other brands manufactured by the above company or nanocrystalline structure-controlled elastomers manufactured by companies other than the above company can also be used.

In Example 10, "PA12 & PE-PA" refers to a mixture of nylon 12 and polyethylene-polyamide copolymer in a ratio of 10:1 (10:1 = PA12:PE-PA). Here, "PA12 & PE-PA" used is nylon 12 with a melting point of 178°C (product number: 181161) sold by Aldrich and LP91 manufactured by ARKEMA. However, usable commercial products are not limited to these, and other grades of nylon 12 manufactured by the above companies or other grades of nylon 12 manufactured by the above companies or other grades of polyethylene-polyamide copolymers manufactured by the above companies can also be used.

In addition, "PA copolymer" in Comparative Example 1 represents Alamin® grade "CM8000" available from Toray Industries, Inc., and "PEEK" in Comparative Example 2 represents polyetheretherketone (TPS® standard grade "NC" available from Toray Plastics Precision Co., Ltd.).

The results in Table 1 confirm that the microporous membranes of Examples 1 to 10 have a reduced amount of unmelted material (amount of aggregates, i.e., gel content) compared to Comparative Examples 1 to 4, which means that the quality has been improved. The results in Table 1 also confirm that, compared to Comparative Examples 1 to 4, Examples 1 to 10 can realize an electricity storage device that is superior in safety (safety in a nail penetration test) and cycle characteristics.

Claims (11)

A microporous membrane comprising a polyethylene resin and a resin having a melting point of 140°C to 330°C different from the polyethylene resin,
In solid viscoelasticity measurements, the storage modulus (E') is 1,000,000 Pa to 10,000,000 Pa in the range of 150°C to 300°C, and the loss modulus (E") is 10 Pa to 580,000 Pa in the range of 150°C to 300°C;
In a 2D spectroscopic mapping measurement, the resin having a melting point of 140°C to 330°C is dispersed in the form of particles in the polyethylene resin, and in the 2D spectroscopic mapping measurement, the resin having a melting point of 140°C to 330°C has an average particle size of 6 μm to 10 μm. The microporous membrane according to claim 1, wherein in the 2D spectroscopic mapping measurement, 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. The microporous membrane according to claim 1 or 2, wherein the resin having a melting point of 140°C to 330°C includes at least one selected from the group consisting of polyolefin resins, benzene ring-containing resins, and heteroatom-containing resins. The microporous membrane according to claim 3, wherein the polyolefin resin is other than homopolypropylene. The microporous membrane according to claim 3, wherein the benzene ring-containing resin is at least one selected from the group consisting of polyethylene terephthalate, polyether ether ketone, and polyphenylene ether. The microporous membrane according to claim 3, 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 6, wherein the polyethylene resin contains ultra-high molecular weight polyethylene. The microporous membrane according to any one of claims 1 to 7, wherein the mass ratio of the resin having a melting point of 140°C to 330°C to the polyethylene resin (resin having a melting point of 140°C to 330°C/polyethylene resin) is 0.01/0.99 to 0.90/0.10. A separator for an electricity storage device comprising the microporous membrane according to any one of claims 1 to 8. A lithium ion secondary battery comprising the microporous membrane according to any one of claims 1 to 8. An electricity storage device comprising the microporous membrane according to any one of claims 1 to 8.