Separators for power storage devices

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

Technical Field

[0001] The present invention relates to a microporous membrane containing two or more resins, a separator (for example, a separator for a power storage device) including such a microporous membrane, and the like.

Background Art

[0002] Microporous membranes are widely used as precision filtration membranes, separators for fuel cells, separators for capacitors, base materials for functional membranes for expressing new functions by filling functional materials into pores, separators for power storage devices, or constituent materials thereof.

[0003] Lithium ion secondary batteries (LIBs) are widely used in notebook personal computers, mobile phones, digital cameras, and the like. Among these, as a separator for LIBs or a constituent material thereof, for example, a polyolefin microporous membrane is known. 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 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 obtained resin composition to produce a polyolefin microporous membrane. Patent Document 2 describes a method for producing a microporous membrane by previously kneading a polyolefin resin such as polyethylene and a resin other than polyolefin (for example, polyamide) with an extruder to form pellets, then mixing the pellets and liquid paraffin, extruding them, and further extracting the liquid paraffin.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] In recent years, for separators used in energy storage devices, further improvement in quality (reduction in the amount of unmelted matter) has been demanded. By providing a microporous membrane with improved quality, a separator with improved quality can be provided, and by providing a separator with improved quality, an energy storage device excellent in safety (safety in a nail penetration test) and in capacity retention characteristics (cycle characteristics) when charge and discharge are repeated can be expected to be realized. In view of the above situation, there was room for improvement in the microporous membranes described in Patent Documents 1 and 2 from the viewpoint of improving quality.

[0006] In view of the above circumstances, an object of the present invention is to provide a microporous membrane in which quality improvement (reduction in the amount of unmelted matter) is achieved. Further, according to an embodiment of the present invention, there are also disclosed the microporous membrane capable of improving the safety (safety in a nail penetration test) and cycle characteristics of an energy storage device, a separator (separator for an energy storage device) including such a microporous membrane, and an energy storage device including such a microporous membrane.

[0007] The present inventors conducted studies to solve the above problems, and found that the above problems can be solved by using a separator for an energy storage device having the following configuration, and completed the present invention. Some aspects of the present invention are exemplified below. [1] A microporous membrane containing a polyethylene resin and a resin having a melting point of 140°C to 330°C, which is different from the polyethylene resin, In solid viscoelasticity measurement, a microporous membrane in which both the storage elastic modulus (E') and the loss elastic modulus (E") are in the range of 10 Pa to 10,000,000 Pa at 150°C to 300°C. [2] The microporous membrane according to [1], wherein in 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] 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 [2]. [4] 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 microporous membrane according to [2] or [3]. [5] The resin having a melting point of 140°C to 330°C includes at least one selected from the group consisting of a polyolefin resin, a benzene ring-containing resin, and a heteroatom-containing resin, the microporous membrane according to any one of [1] to [4]. [6] The polyolefin resin excludes homopolypropylene, the microporous membrane according to [5]. [7] 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 [5]. [8] The heteroatom-containing resin is at least one selected from the group consisting of polyamide, polytetrafluoroethylene, and polyketone, the microporous membrane according to [5]. [9] The polyethylene resin includes ultra-high molecular weight polyethylene, the microporous membrane according to any one of [1] to [8].

[10] The 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, the microporous membrane according to any one of [1] to [9].

[11] A separator for a power 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] A power storage device comprising the microporous membrane according to any one of [1] to

[10] .

Advantages of the Invention

[0008] According to the present invention, it is possible to provide a microporous membrane with improved quality (reduction in the amount of unmelted matter). 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 excellent in safety (safety in a nail penetration test) and cycle characteristics can be realized.

Brief Description of the Drawings

[0009]

Figure 1

Embodiments for Carrying Out the Invention

[0010] Hereinafter, embodiments of the present invention (referred to as "the present embodiments") will be described, but the present invention is not limited to only the present embodiments. The present invention can be variously modified without departing from its gist. In this specification, "~" means including the numerical values at both ends as the upper limit value and the lower limit value unless otherwise specified. Further, in this specification, the upper limit value and the lower limit value of the numerical range can be arbitrarily combined.

[0011] <Microporous Membrane> The microporous membrane according to the present embodiment (hereinafter sometimes simply referred to as "microporous membrane") includes a polyethylene resin (hereinafter sometimes referred to as "first resin") and a resin different from the first resin and having a melting point of 140 to 330°C (hereinafter sometimes referred to as "second resin"). And, in the solid viscoelastic measurement, the microporous membrane has both a storage elastic modulus (E') and a loss elastic modulus (E") in the range of 150 to 300°C and 10 to 10,000,000 Pa.

[0012] The microporous membrane can be used as a microfiltration membrane, a separator for a fuel cell, a separator for a capacitor, a separator for a power storage device, an electrolysis membrane, or a constituent material thereof. When a separator for a power storage device or a constituent material thereof, particularly when a microporous membrane is used as a separator for a LIB or a constituent material thereof, the microporous membrane itself may be used as the separator, or a structure in which another layer or another film is provided on at least one side of the microporous membrane may be used as the separator. As the microporous membrane used for the separator of the power storage device, those having low electronic conductivity, having ionic conductivity, having high resistance to organic solvents, and having fine pore diameters are preferable.

[0013] As described in the Examples section, the microporous membrane corresponding to the Examples has a reduced amount of unmelted matter (amount of aggregates, i.e., gel content) compared to the microporous membrane corresponding to the Comparative Examples, that is, the quality is improved.

[0014] In order to achieve such quality improvement, 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 form. The above-mentioned preferable dispersion is difficult to obtain simply by mixing the first resin and the second resin. However, based on a method in which 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 each prepared and then introduced into an extruder, the above-mentioned preferable dispersion tends to be obtained. Such a method will be described later.

[0015] Regarding the temperature-E' graph and the temperature-E'' graph of the microporous membrane according to the present embodiment, the line may be interrupted at around temperatures exceeding 200°C. The reason is that as the temperature of the microporous membrane rises, after exceeding the melting point of the resin component (for example, nylon 12) that the microporous membrane can contain, the viscosity of the resin component exceeding the melting point decreases, leading to breakage. However, the breaking temperature does not necessarily correlate with the melting point of the resin component and can be adjusted by the size of the domain dispersed in polyethylene (PE), the resin component that the microporous membrane can contain (for example, a heteroatom-containing resin, etc.), the molecular weight of the second resin, etc. Incidentally, even if the storage elastic modulus (E') and the loss elastic modulus (E") are not measured up to 300°C, they often tend to decrease at 150°C or higher. Therefore, if they are 10,000,000 Pa or less at 150°C, there is a high possibility of satisfying the above requirement of "10 to 10,000,000 Pa in the range of 150 to 300°C".

[0016] In this embodiment, the significance of focusing on 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 value of the storage elastic modulus (E'), the lower the rigidity of the material. When the storage elastic modulus (E') is in the range of 10 to 10,000,000 Pa in the range of 150 to 300°C, the microporous membrane has the desired rigidity in the above temperature range. Also, the loss elastic modulus (E") represents the dissipated energy of the material during 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 in the range of 10 to 10,000,000 Pa in the range of 150 to 300°C, the microporous membrane has the desired viscosity in the above temperature range. As described above, in solid viscoelasticity measurement, a microporous membrane in which both the storage elastic modulus (E') and the loss elastic modulus (E") are in the range of 10 to 10,000,000 Pa in the range of 150 to 300°C achieves both the desired rigidity and the desired viscosity in the range of 150 to 300°C.

[0017] In this regard, although not wishing to be bound by theory, in solid viscoelasticity measurement, when 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, when the temperature of the power storage device rises due to an internal short circuit and the microporous membrane in the power storage device melts, it is considered that the melted resin (microporous membrane) penetrates into the pores of the electrode and exhibits an anchor effect. And since the melted resin stays in place when it penetrates into the pores of the electrode, it is considered that an increase in the short-circuit area can be suppressed. Similarly, although not wishing to be bound by theory, in solid viscoelasticity measurements, when both the storage modulus (E') and the loss modulus (E") are 10,000,000 Pa or less in the range of 150 to 300 °C, the molten resin (microporous membrane) has appropriate viscoelasticity, so that the fluidity of the molten resin does not increase too much, and it is considered that the exposure of the electrodes or the increase in the short-circuit area due to the outflow of the resin can be suppressed. Therefore, even for a high energy density energy storage device, for example, a large LIB, and more specifically, an in-vehicle LIB, such an energy storage device is provided with a separator including the microporous membrane according to the present embodiment, making it easier to prevent thermal runaway during internal short circuit.

[0018] From the viewpoint of making it easier to prevent thermal runaway during internal short circuit, both the storage modulus (E') and the loss modulus (E") are 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 in the range of 150 to 300 °C. The storage modulus (E') may be 50 Pa or more, or may be 100 Pa or more. Also, the loss modulus (E") may be in the range of 30 to 700,000 Pa, or may be in the range of 50 to 600,000 Pa.

[0019] When measuring the solid viscoelasticity of the microporous membrane and when measuring the solid viscoelasticity of the resin raw material of the microporous membrane, the obtained values may be different. Also, among the methods capable of measuring the solid viscoelasticity of the resin raw material, there are methods that are not preferable for measuring the solid viscoelasticity of the microporous membrane obtained from the resin raw material. In the present embodiment, the storage modulus (E') and the loss modulus (E") are each measured by a method targeting the microporous membrane (that is, the method described in the examples). Regarding a microporous membrane, in the range of 150 to 300 °C, the significance of controlling the storage modulus (E') and the loss modulus (E") within a specific range is that when an internal short circuit occurs in a power storage device, it becomes easier to control the viscoelasticity of the resin (microporous membrane) constituting the separator in the power storage device within a specific range. In the present embodiment, since the target is the microporous membrane rather than the resin raw material, and 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, it becomes easier to prevent thermal runaway during the internal short circuit.

[0020] (First component) The first component contains 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) represented 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 referred to as ultra-high molecular weight polyethylene (UHMWPE).

[0021] 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. Also, its 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.

[0022] Here, from the viewpoint of easily reducing the amount of unmelted matter, the first component is preferably a powder containing 2 to 100% by mass of the above first resin based on the total mass of the first component. 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.

[0023] The first resin may include not only a single type but also a plurality of types of UHMWPE. From the perspective of the high strength of the microporous membrane, 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 perspective, UHMWPE preferably contains structural units derived from ethylene in an amount of 98.5 mol% or more and 100 mol% or less, and more preferably contains structural units derived from α-olefins other than ethylene in an amount of more than 0.0 mol% and 1.5 mol% or less.

[0024] In addition, the first resin may contain a polyethylene resin other than UHMWPE. Examples of the polyethylene resin other than UHMWPE include low-density polyolefins (LDPE) such as linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), high-pressure process low-density polyethylene, or mixtures thereof.

[0025] The preferable "powder" as the first component means 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, the number particle size distribution (Nd 80 / Nd 20 ) is 1.1 to 4.2, preferably 1.2 to 4.1, the volume particle size distribution (Vd 80 / Vd 20 ) is 1.1 to 3.3, preferably 1.15 to 3.2, the crystallite size is 10.0 nm to 30.0 nm, preferably 11.0 nm to 28.0 nm, more preferably 12.0 nm to 23.0 nm, the crystallinity is 30% to 99%, preferably 32% to 98%, more preferably 38% to 97.5%, and it contains polyethylene. It refers to those satisfying at least one condition selected from the group consisting of these. These number average particle sizes (Nd 50 ), volume average particle sizes (Vd 50 ), number particle size distributions (Nd 80 / Nd 20) Volume particle size distribution (Vd 80 / Vd 20 ), crystallite size, crystallinity, etc. can be measured by known methods.

[0026] For example, 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 and shape measuring device Particle Insight manufactured by Micromeritics. Also, for example, crystallite size and crystallinity can be obtained by XRD measurement using an X-ray diffractometer Ultima-IV manufactured by Rigaku Corporation.

[0027] Note that the first component may contain a resin other than the first resin as long as it does not inhibit 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. Also, the first component may contain additives other than the resin as long as it does not inhibit the exertion of the effects of the present invention. Examples of the additives include dehydration condensation catalysts, metal soaps such as calcium stearate or zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, coloring pigments, etc.

[0028] (Second component) The second component contains a second resin (a resin having a melting point of 140°C to 330°C) different from the above-mentioned first resin. The second resin is not particularly limited, and 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-based resins; polyester-based resins such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT); fluorine-based 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, polyetheretherketone (PEEK), and the like. These resins can be used singly or in plural.

[0029] By using such a second resin, it becomes easier to improve the heat resistance of the obtained microporous membrane. And 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 incorporated into the microporous membrane. Thereby, 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, the contact area between the electrode and the second resin can be reduced. Therefore, the possibility of the redox decomposition of the second resin being caused by the electrode can be reduced.

[0030] The second resin is preferably a polyolefin resin. The polyolefin resin is not particularly limited, and for example, homopolypropylene (PP) can be excluded, and / or resins containing a repeating unit of propylene as a constituent component can be excluded. These polyolefin resins can be used singly or in plural.

[0031] Further, the second resin is preferably a benzene ring-containing resin. As the benzene ring-containing resin, for example, at least one selected from the group consisting of polyethylene terephthalate, polyether ether ketone, and polyphenylene ether is preferable. These benzene ring-containing resins can be used singly or in plural.

[0032] Also, the second resin is preferably a heteroatom-containing resin. As the heteroatom-containing resin, for example, at least one selected from the group consisting of polyamide (PA), polytetrafluoroethylene (PTFE), and polyketone is preferable. These heteroatom-containing resins can be used singly or in plural.

[0033] When the second resin is composed of a plurality of resins, the melting point of the second resin means the melting point of the second resin as a whole, not the melting point of each resin constituting the second resin. Therefore, even if a resin having a melting point outside the range of 140°C to 330°C is included, if the melting point as a whole falls within the range of 140°C to 330°C, it corresponds to the second resin. The first resin and the second resin only need to have different overall compositions. For this reason, for example, the first resin (polyethylene resin) may be included in the resin constituting the second resin. Even if the melting point of the polyethylene resin is less than 140°C, if it is configured to fall within the range of 140°C to 330°C as a whole by combination with other resins, it corresponds to the second resin.

[0034] Such a second resin preferably 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) with respect to the number average molecular weight (Mn) of 3 to 18.

[0035] 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 film strength and expressing fuse characteristics, the second resin in the second component is more preferably 2.1% by mass or more, and still more preferably 2.3% by mass or more.

[0036] The second component may contain additives other than the resin as long as it does not inhibit the exertion of the effects of the present invention. Examples of the additives include dehydration condensation catalysts, metal soaps such as calcium stearate or zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, coloring pigments, and the like.

[0037] The second component in the form of pellets can be obtained, for example, by drying the powder of the polymerized second resin and then extruding it into strands by an extruder, followed by water cooling and cutting into pellets. The viscosity average molecular weight (Mv) of the powder as the raw material is preferably 30,000 to 3,500,000, and the dispersity (Mw / Mn) is preferably 3 to 15. Also, the viscosity average molecular weight (Mv) when the second component is formed into pellets is preferably 35,000 to 3,300,000, more preferably 30,000 to 3,200,000. And the dispersity is preferably 3 to 15, more preferably 4 to 14, and still more preferably 4 to 13.

[0038] Here, the preferred "pellets" as the second component are larger than the number average particle diameter (Nd 50 ) and the volume average particle diameter (Vd 50 ) of the preferred "powder" as the first component, and are resinous substances with 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, ellipsoidal, or cylindrical. They can be obtained by melt-extruding the raw material with an extruder, arranging it in strands while cooling it with water or air, and continuously cutting it. The size and shape can be adjusted by the strand formation and cutting method.

[0039] In this way, by melt-extruding the above-mentioned "powder" and adjusting it to the above-mentioned "pellet", the swelling rate due to the plasticizer can be significantly slowed down (as described later, the second component in the form of a pellet does not substantially swell). Inside the extruder, it is important not to inhibit the swelling of the first component. Also, in the melting process after the swelling process, from the perspective of uniform melting, the "pellet" which is preferable as the second component means one having a crystallite size larger than that of the "powder" which is preferable as the first component and having a crystallite size within the above range. Furthermore, the "pellet" which is preferable as the second component means one having a crystallinity smaller than that of the "powder" which is preferable as the first component and having a crystallinity within the above range. In the method for manufacturing pellets, the crystallite size, crystallinity, size, and shape of the pellets can be adjusted, such as the temperature of the extruded resin in the form of strands, the cooling temperature during cutting, or the pulling rate (melt micro-stretching) of the strands from extrusion. These number average particle diameters (Nd 50 ), volume average particle diameters (Vd 50 ), crystallite size, crystallinity, etc. can be measured by known methods.

[0040] For example, these number average particle diameters (Nd 50 ), volume average particle diameters (Vd 50 ), crystallite size, and crystallinity can be measured by the same methods as those described in the items of the first component. Also, the size of the pellet can be obtained, for example, by measuring the length of one side with a calibrated caliper.

[0041] By forming the second component into pellets, it becomes easier to significantly improve the uniformity of the swelling of the plasticizer in a twin-screw extruder when mixed with the first component (e.g., powder of the first resin). This is presumably because by forming the second component into pellets, its swelling rate can be made significantly slower compared to, for example, the powder of such a 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 basically do not swell and are suitably used in subsequent melt-kneading. As a result, even in the process of melt-kneading, it becomes easier to disperse the second component into the first component uniformly up to the molecular level.

[0042] As described above, the second component in the form of pellets has a swelling rate significantly slower than that of the powder and thus does not substantially swell. Therefore, it is possible to prevent the swelling of the first component (e.g., powder of the first resin) from being excessively inhibited by the second component.

[0043] (2D Spectroscopic Mapping Measurement) FIG. 1 is an image showing the 2D spectroscopic mapping measurement results of the microporous membrane according to the present embodiment. In FIG. 1, an enlarged image of the portion surrounded by a square with a white line is shown, and the portion 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 a granular form in the first resin (polyethylene resin). "The second resin is dispersed in a granular form in the first resin" means that in the image obtained by 2D spectroscopic mapping measurement, the second resin is observed in an island-like or dot-like form in the first resin. 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 suitably confirmed. Specific examples of the 2D spectroscopic mapping measurement are as described in the examples.

[0044] In addition, in the 2D spectroscopic mapping measurement, it is preferable that at least one particle size of the second resin (resin having a melting point of 140°C to 330°C) is 5.1 to 10 μm, and more preferably 5.2 to 9.8 μm. The particle size referred to here may be the major axis of the granular second resin, the minor axis, or the average of the major axis and the minor axis. The observation area can be changed as appropriate. In any observation area, it is sufficient that the second resin having a particle size of at least 5.1 to 10 μm is 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.

[0045] In addition, in the 2D spectroscopic mapping measurement, it is preferable that the average particle size of the second resin (resin having a melting point of 140°C to 330°C) is 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 calculation method of the average particle size of the second resin is as described in the examples.

[0046] As described above, by suitably dispersing the second component in the first component, the resulting microporous membrane can reduce the amount of unmelted matter, that is, improve the quality. By providing a microporous membrane with improved quality, a separator with improved quality can be provided. And by providing such a separator, it is expected that a power storage device excellent in safety (safety in the nail penetration test) and cycle characteristics can be realized.

[0047] (The mass of each of the first resin and the 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, since the second resin as described above has different solubility parameters from the first resin, it is non-molten and thermodynamically incompatible, but it could be physically dispersed in the first resin. Since such a dispersed alloy molded article can have both the characteristics of the first and second resins, the size of the domains forming the alloy is important for the manifestation of its performance. The inventors of the present invention have tried to construct a dispersion by limiting the size of the second resin. In practice, simply mixing the first resin and the second resin makes it difficult for the second resin to be preferably dispersed in the first resin. However, by preparing 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) respectively, and subjecting them to a special extrusion process as in the steps (1) and / or (2) described later, the above-mentioned preferable dispersion tends to be obtained. And according to such a method, it becomes easier to disperse the second resin in the first resin more preferably within 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.

[0048] (Various characteristics of the microporous membrane) The characteristics of the microporous membrane will be described below. These characteristics are measured when the microporous membrane as a separator for a power storage device is a flat membrane. When the separator for a power storage device is in the form of a laminated membrane, it can be measured after removing the layers other than the microporous membrane from the laminated membrane.

[0049] 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 porosity of the microporous membrane is 20% or more, the followability for the rapid movement of lithium (Li) ions tends to be further improved when the microporous membrane is 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, 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.

[0050] The air permeability of the microporous membrane is 100 cm 3 per second, preferably 1 second or more, more preferably 50 seconds or more, still more preferably 55 seconds or more, even more preferably 100 seconds or more. When the air permeability of the microporous membrane is 1 second or more, the balance of the membrane thickness, porosity, and average pore diameter tends to be further improved. Also, 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 draw ratio, draw temperature, etc. Such air permeability is measured by the method described in the examples.

[0051] The membrane 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 membrane thickness of the microporous membrane is 1.0 μm or more, the membrane strength tends to be further improved. Also, the membrane 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 membrane thickness of the microporous membrane is 500 μm or less, the ion permeability tends to be further improved. The membrane thickness of the microporous membrane can be adjusted by adjusting the draw ratio, draw temperature, etc. Such membrane thickness is measured by the method described in the examples.

[0052] In particular, when using a microporous membrane as a separator for a LIB or a constituent material thereof, 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 still more 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.

[0053] 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. The fact that the puncture strength is 200 gf / 20 μm or more is preferable from the viewpoint of suppressing film breakage due to fallen active material or the like during battery winding. It is also preferable from the viewpoint of suppressing the risk of short circuit due to the expansion and contraction of the electrode accompanying charge and discharge. On the other hand, setting it to 2000 gf / 20 μm or less is preferable from the viewpoint of reducing the width contraction due to the relaxation of orientation during heating. The puncture strength of the microporous membrane can be adjusted by adjusting the draw ratio, draw temperature, etc. Such puncture strength is measured by the method described in the examples.

[0054] <Manufacturing method of microporous membrane> The manufacturing method of the microporous membrane includes the following steps: (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 in a twin-screw extruder to produce a resin composition; and (3) A step of extracting the plasticizer from the resin composition to produce a microporous membrane; are included. Hereinafter, each step will be described in order.

[0055] [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, examples of the first component include the powder of the first resin, and examples of the second component include pellets made from the powder of the second resin.

[0056] (Plasticizer) The plasticizer is liquid at a temperature of 20°C to 70°C, and can be a known material as long as it has excellent dispersibility in the first resin or the second resin. Considering subsequent extraction, the plasticizer used in step (1) is preferably a non-volatile solvent that can form a homogeneous solution at a temperature equal to or higher than the melting point of the first resin or the second resin. 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; and higher alcohols such as oleyl alcohol and stearyl alcohol. Among them, liquid paraffin is preferred because it has high compatibility with polyethylene, and it is difficult for interfacial peeling between the resin and the plasticizer to occur even when the melt-kneaded product is stretched, and uniform stretching tends to be easily carried out.

[0057] (Supply to the twin-screw extruder) The first component and the second component can be simultaneously supplied to, for example, a twin-screw extruder. To supply them simultaneously, for example, there are methods of supplying a dry blend of the first component and the second component prepared by a super mixer or the like to the twin-screw extruder; methods of preparing a mixed slurry described later with the first component and the second component and supplying this to the twin-screw extruder.

[0058] Also, the first component and the second component can be individually supplied to, for example, a twin-screw extruder. To supply them individually, for example, there are methods of supplying the first component and the second component to the twin-screw extruder from individual feeders; methods of preparing a mixed slurry described later with the first component and the plasticizer and supplying this separately from the second component to the twin-screw extruder; methods of melting the second component by another extruder (the second extruder) and supplying this separately from the first component to the above-mentioned twin-screw extruder (the first extruder). 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.

[0059] Also, the plasticizer can be supplied to a twin-screw extruder together with the first component, for example. After supplying the plasticizer to the twin-screw extruder together with the first component, additional plasticizer may be supplied 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 and in the middle of the melt-kneading area. Therefore, after supplying the mixed slurry from the upper feed port of the twin-screw extruder, additional plasticizer can also be supplied from the middle feed port at the tip of the twin-screw extruder. This makes it easier to adjust the ratio of the amount of liquid paraffin in 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.

[0060] (Mixed slurry) Here, step (1) can include a step of producing a mixed slurry from the first component and the plasticizer using 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 second to 60 seconds, and supplying the mixed slurry and the second component to a twin-screw extruder. Also, step (1) can 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 conditions and supplying the mixed slurry to a twin-screw extruder.

[0061] 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 swelling the first component to the maximum extent. The upper limit is preferably 68°C or lower, more preferably 67°C or lower, 66°C or lower, or 65°C or lower, from the viewpoint of suppressing the dissolution of the first resin during mixing to obtain a slurry. The shear rate of the continuous mixer is 100 to 400,000 seconds -1 from the viewpoint of uniformly bringing the first component into contact with the plasticizer to obtain a dispersion, and is preferably 120 to 398,000 seconds -1 and more preferably 1,000 to 100,000 seconds-1 It is as follows. The residence time of the continuous mixer is 1.0 to 60 seconds, preferably 2.0 to 58 seconds, more preferably 2.0 to 56 seconds, from the viewpoint of ensuring the dispersion of the first resin in the plasticizer.

[0062] The content of the first component (for example, the powder of the first resin) in the mixed slurry is preferably more than 0% by mass, more preferably 1% by mass or more, still more preferably 2% by mass or more or 4% by mass or more, from the viewpoint of the strength of the obtained microporous membrane, based on the total mass of the mixed slurry. Also, 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.

[0063] Also, the content of the second component (for example, the pellets of the second resin) in the mixed slurry is preferably more than 0% by mass, more preferably 1% by mass or more, still more preferably 2% by mass or more or 4% by mass or more, from the viewpoint of the strength of the obtained microporous membrane, based on the total mass of the mixed slurry. Also, 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.

[0064] The mixed slurry is preferably supplied to a twin-screw extruder at a temperature of 25°C to 80°C. From the viewpoint of ensuring the entanglement of polymer chains to such an extent that the decrease in the molecular weight of the first resin does not occur while ensuring an 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.

[0065] (Additive) The above-mentioned mixed slurry may contain known additives, such as dehydration condensation catalysts, metal soaps such as calcium stearate or zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, coloring pigments, and the like.

[0066] [Step (2)] In step (2), the first component, the second component, and the plasticizer are melt-kneaded using a twin-screw extruder to produce a resin composition. In the present embodiment, in step (1) above, since the uniformity of the swelling of the plasticizer with respect to the first component is ensured, in step (2), as long as it is within the scope not departing from the gist of the present invention, the type of the twin-screw extruder, the supply of each raw material to the twin-screw extruder, the extrusion time, the extrusion speed, the shear rate, the shear force, and other conditions are not limited.

[0067] [Step (3)] Step (3) extracts the plasticizer from the resin composition produced in step (2) above to produce a microporous membrane. In particular, 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).

[0068] (Sheet Processing Step (4)) In step (4), the resin composition obtained in step (2) above is extruded into a sheet shape and cooled and solidified to be processed into a sheet-shaped molded body. The resin composition may contain a plasticizer or other additives.

[0069] The proportion of the resin (total amount of the first resin, the second resin, and other resins if included) in the sheet-shaped molded body 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-shaped molded body from the viewpoint of sheet formability.

[0070] As a method for manufacturing a sheet-shaped molded body, for example, the resin composition obtained in the above step (2) is extruded into a sheet through a T-die or the like, brought into contact with a heat conductor, and cooled to a temperature lower than the crystallization temperature of the resin component to be solidified. Examples of the heat conductor used for cooling and solidification include metals, water, air, and plasticizers. Among these, it is preferable to use a metal roll because of its high heat conduction efficiency. When the extruded resin composition is brought into contact with a metal roll, sandwiching it between the rolls is also preferable because it further increases the heat conduction efficiency, the sheet becomes oriented and the film strength increases, and the surface smoothness of the sheet tends to improve. The die lip interval when extruding the resin composition into a sheet from the T-die 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, meandering and the like are reduced, the influence on the film quality such as streaks or defects is small, and the risk of film breakage and the like in the subsequent stretching process can be reduced. On the other hand, when the die lip interval is 3,000 μm or less, the cooling rate is fast, cooling unevenness can be prevented, and the thickness stability of the sheet can be maintained. Also, during step (4), the extruded sheet-shaped molded body may be rolled.

[0071] [Stretching step (5)] In step (5), the sheet-shaped molded body 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.

[0072] From the viewpoint of being able to reduce the film thickness distribution and air permeability distribution in the width direction, biaxial stretching is preferable to uniaxial stretching as the stretching treatment. By stretching the sheet simultaneously in the biaxial direction, the number of times of repeating cooling and heating during the film forming process of the sheet-shaped molded body is reduced, and the distribution in the width direction is improved. Examples of the biaxial stretching method include methods such as simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple stretching. From the viewpoints of improving the puncture strength and the uniformity of stretching, simultaneous biaxial stretching is preferable, and from the viewpoint of ease of controlling the surface orientation, sequential biaxial stretching is preferable.

[0073] In this specification, simultaneous biaxial stretching refers to a stretching method in which stretching in the MD (machine direction of continuous forming of the microporous membrane) and stretching in the TD (direction perpendicular to the MD of the microporous membrane at an angle of 90°) are simultaneously performed, and the stretching ratios in each direction may be different. Sequential biaxial stretching refers to a stretching method in which stretching in the MD and TD is independently performed. When stretching is performed in the MD or TD, the other direction is in an unconstrained state or a state fixed at a constant length.

[0074] The stretching ratio is preferably in the range of 20 times or more and 200 times or less in terms of the area ratio, more preferably in the range of 25 times or more and 170 times or less, and still more 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 in the MD and 2 times or more and 15 times or less in the TD, more preferably in the range of 3 times or more and 12 times or less in the MD and 3 times or more and 12 times or less in the TD, and still more preferably in the range of 5 times or more and 10 times or less in the MD and 5 times or more and 10 times or less in the TD. When the total area ratio is 20 times or more, the resulting microporous membrane tends to have sufficient strength. On the other hand, when the total area ratio is 200 times or less, membrane breakage in step (5) can be prevented, and high productivity tends to be obtained.

[0075] From the viewpoints of the meltability and film-forming properties of the first resin and the second resin, the stretching temperature is preferably 90 to 150°C, more preferably 100 to 140°C, and still more preferably 110 to 130°C.

[0076] [Extraction step (6)] In step (6), a plasticizer is extracted from the stretched product obtained in step (5) above to form a porous body. Examples of the method for extracting the plasticizer include a method in which the stretched product is immersed in an extraction solvent to extract the plasticizer and then dried. The extraction method may be either a batch type or a continuous type. In order to suppress the shrinkage of the porous body, it is preferable to restrain the ends of the sheet-shaped molded body during a series of steps of immersion and drying. Further, the remaining amount of the plasticizer in the porous body is preferably less than 1% by mass based on the mass of the entire porous membrane. Note that after step (6), the plasticizer may be recovered and reused by operations such as distillation.

[0077] As the extraction solvent, it is preferable to use a solvent 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-chlorinated halogenated solvents such as hydrofluoroether and hydrofluorocarbon; 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 operations such as distillation and reused.

[0078] [Heat treatment step (7)] In step (7), after heat-treating the porous body obtained in step (6) above at a temperature below the melting point of the porous body, the porous body is stretched to produce a microporous membrane.

[0079] The porous body is heat-treated for the purpose of heat setting from the viewpoint of suppressing shrinkage. Examples of the heat treatment method include 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 relaxation operation performed at a predetermined atmosphere, a predetermined temperature, and a predetermined relaxation rate for the purpose of reducing stretching stress. The relaxation operation may be performed after the stretching operation. These heat treatments can be performed using a tenter or a roll stretching machine.

[0080] From the viewpoint of increasing the strength and porosity of the microporous membrane, the stretching operation is preferably performed at 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 on the MD and / or TD of the film. The relaxation rate is the value obtained by dividing the dimension of the film after the relaxation operation by the dimension of the film before the relaxation operation. When both MD and TD are relaxed, it is the value obtained by multiplying the relaxation rate of MD by the relaxation rate of TD. The relaxation rate is preferably 1.0 or less, more preferably 0.97 or less, and 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 directions of MD and TD or only in one of MD and TD.

[0081] The temperature of the heat treatment including stretching or relaxation operations is preferably in 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. When the temperatures of the stretching and relaxation operations are within the above range, it is preferable from the viewpoint of reducing the heat shrinkage rate and balancing the porosity. The lower limit of the heat treatment temperature is more preferably 110°C or higher, still more preferably 120°C or higher, and even more preferably 125°C or higher, and the upper limit thereof is more preferably 160°C or lower, still more preferably 150°C or lower, and even more preferably 140°C or lower.

[0082] After step (7) or after step (7), post-treatments such as hydrophilic treatment with a surfactant or the like and crosslinking treatment with ionizing radiation or the like may be performed on the microporous membrane. Although the order of steps (5), (6), and (7) described above may be rearranged or these steps may be performed simultaneously, from the viewpoint of film-forming properties, it is preferable to perform these steps in the order of steps (5), (6), and (7) using a biaxial stretching machine. The obtained microporous membrane can be wound by a winder to form a roll or cut by a slitter from the viewpoints of handleability and storage stability.

[0083] <Separator for energy storage device, separator for LIB> The microporous membrane can be used in an energy storage device, for example, as a separator for LIB or a constituent material thereof. According to this, it becomes easier to suppress the redox degradation of the separator, and it also becomes easier to construct a dense and uniform porous structure.

[0084] The separator may be in the form of a flat film (for example, formed of a single microporous membrane), a laminated film (for example, a laminate of a plurality of microporous membranes, a laminate of a microporous membrane and another membrane), a coated film (for example, when a functional substance is coated on at least one side of a microporous membrane), and the like.

[0085] <lib> The LIB uses, as the positive electrode, a lithium transition metal oxide such as lithium cobalt oxide or lithium cobalt composite oxide, as the negative electrode, a carbon material such as graphite or carbon black, and as the electrolyte, an organic solvent containing a lithium salt such as LiPF 6 and is a storage battery. During charging and discharging of the LIB, ionized Li reciprocates between the electrodes. Also, since ionized Li needs to move between the electrodes relatively quickly while suppressing contact between the electrodes, a separator is disposed between the electrodes.

Examples

[0086] Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited only 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.

[0087] <Melting point (°C) of the second resin> Using a differential scanning calorimetry (DSC) measurement device “DSC-60” (manufactured by Shimadzu Corporation), the melting point of the second resin was measured. First, the second resin was punched out into a circle with a diameter of 5 mm, and several pieces were stacked to make 3 mg, which was used as a measurement sample. This sample was placed in an aluminum open sample pan with a diameter of 5 mm, a clamping cover was placed on it, and it was fixed in the aluminum pan by a sample sealer. Under a nitrogen atmosphere, the temperature was raised from 30°C to 200°C at a rate of 10°C / min (first temperature rise), held at 200°C for 5 minutes, and then the temperature was lowered from 200°C to 30°C at a 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 rate of 10°C / min (second temperature rise). In the melting endotherm curve of the second temperature rise, the temperature at the maximum was taken as the melting point of the second resin. When there were multiple maximum values, the temperature at the maximum of the largest melting endotherm curve was adopted as the melting point of the second resin.

[0088] <Measurement of solid viscoelasticity of the microporous membrane> The dynamic viscoelasticity of the separator is measured using a dynamic viscoelasticity measuring device, and the storage modulus (E') and loss modulus (E'') can be calculated. Note that for E' 、 and E'', when no breakage (sudden decrease in modulus) of the sample is observed at 150°C to 300°C, they are calculated from the average value at 150°C to 300°C. When breakage of the sample is observed at 150°C to 300°C, they are calculated from the average value from 150°C to the temperature at the break point.

[0089] · Storage modulus (E') The measurement conditions for the storage modulus (E') are defined by the following configurations (i) to (iv). (i) The dynamic viscoelasticity measurement is carried out under the following conditions: · Measuring device used: RSA-G2 (manufactured by TA Instruments) · Sample film thickness: in the range of 5 μm to 50 μm · Measurement temperature range: -50 to 300°C · Heating rate: 10°C / min · Measurement frequency: 1 Hz · Deformation mode: sine wave tension mode (Linear tension) · Initial value of static tensile load: 0.5 N · Initial (at 25°C) gap distance: 25 mm · Auto strain adjustment: Enabled (range: amplitude value 0.05 to 25%, sine wave load 0.02 to 5 N) is performed. (ii) The above-mentioned static tensile load refers to the intermediate value between the maximum stress and the minimum stress in each periodic motion. Also, the above-mentioned sine wave load refers to the oscillating stress centered on the above-mentioned static tensile load. (iii) The above-mentioned sine wave tension mode refers to measuring the above-mentioned vibration stress while performing periodic motion with a fixed amplitude of 0.2%. In the above-mentioned sine wave tension mode, the vibration stress is measured by varying the gap distance and the above-mentioned static tensile load so that the difference between the above-mentioned static tensile load and the above-mentioned sine wave load is within 20%. Then, when the above-mentioned sine wave load becomes 0.02 N or less, the above-mentioned amplitude value is amplified so that the above-mentioned sine wave load is within 5 N and the increase amount of the above-mentioned amplitude value is within 25%, and the above-mentioned vibration stress is measured. (iv) The relationship between the obtained sine wave load and the amplitude value, and the following formula: σ * =σ 0 ·Exp[i(ωt + δ)], ε * =ε 0 ·Exp(iωt), σ * =E * ·ε * E * =E’ + iE’’ (where σ * : vibration stress, ε * : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between vibration stress and strain, E * : complex elastic modulus, E’: storage elastic modulus, E’’: loss elastic modulus Vibration stress: sine wave load / initial cross-sectional area Static tensile load: load at the minimum point of the vibration stress in each period (the minimum point of the gap distance in each period) Sine wave load: difference between the measured vibration stress and the static tensile load) The storage elastic modulus E’ was calculated from

[0090] · Loss elastic modulus (E’’) The measurement conditions for the loss elastic modulus (E’’) are defined by the following configurations (i) to (iv). (i) Dynamic viscoelastic measurement under the following conditions: · Measuring device used: RSA-G2 (manufactured by TA Instruments) · Sample film thickness: in the range of 5 μm to 50 μm · Measurement temperature range: -50 to 300 °C · Heating rate: 10 °C / min · Measurement frequency: 1 Hz · Deformation mode: Sine wave tension mode (Linear tension) · Initial value of static tensile load: 0.5 N · Initial (at 25 °C) gap distance: 25 mm · Auto strain adjustment: Enabled (range: amplitude value 0.05 - 25%, sine wave load 0.02 - 5 N) It is carried out as follows. (ii) The above-mentioned static tensile load refers to the intermediate value between the maximum stress and the minimum stress in each periodic motion. Also, the above-mentioned sine wave load refers to the oscillating stress centered on the above-mentioned static tensile load. (iii) The above-mentioned sine wave tension mode refers to measuring the above-mentioned oscillating stress while performing periodic motion with a fixed amplitude of 0.2%. In the above-mentioned sine wave tension mode, the gap distance and the above-mentioned static tensile load are varied so that the difference between the above-mentioned static tensile load and the above-mentioned sine wave load is within 20% to measure the above-mentioned oscillating stress. And when the above-mentioned sine wave load becomes 0.02 N or less, the above-mentioned amplitude value is amplified so that the above-mentioned sine wave load is within 5 N and the increase amount of the amplitude value is within 25% to measure the above-mentioned oscillating stress. (iv) The obtained sine wave load and amplitude value, and the following formula: σ * =σ 0 ·Exp[i(ωt + δ)], ε * =ε 0 ·Exp(iωt), σ * =E * ·ε * E * =E’ + iE’’ (where σ * : oscillating stress, ε * : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between oscillating stress and strain, E * : complex elastic modulus, E’: storage elastic modulus, E’’: loss elastic modulus Oscillating stress: Sine wave load / Initial cross-sectional area Static tensile load: The load at the minimum point of the vibration stress in each cycle (the minimum point of the gap distance in each cycle) Sine wave load: The difference between the measured vibration stress and the static tensile load) The loss elastic modulus E’’ was calculated from

[0091] <2D spectroscopic mapping measurement> · Sample preparation The microporous membrane was cut into MD5 mm × TD5 mm and used for measurement.

[0092] · Measurement method Using a tip-enhanced Raman scattering microscope (TERSsense) manufactured by nano photon, vibration absorption bands other than PE in the sample were detected and 2D mapping was performed. Specifically, as the vibration absorption band other than PE, for example, in the case of nylon 12 (Example 1), the stretching vibration absorption band around 1650 cm -1 was used.

[0093] · Method for calculating the average particle diameter of the second resin The image data obtained from the 2D mapping was image-processed to obtain the area equivalent to a circle, and its diameter was calculated as the average particle diameter.

[0094] <Film thickness (μm)> Measurement was performed using a micrometer thickness gauge (Type KBM manufactured by Toyo Seiki) under an atmosphere of room temperature 23°C and humidity 40%. A terminal with a terminal diameter of 5 mmφ was used, and measurement was performed by applying a load of 44 gf.

[0095] <Porosity (%)> A 10 cm × 10 cm square sample was cut from the microporous membrane, and its volume (cm 3 ) and mass (g) were determined, and the porosity was calculated using the following formula from these and the density (g / cm 3 ). Porosity (%) = (Volume - Mass / Density) / Volume × 100

[0096] <Air permeability (seconds / 100 cm 3 )> In accordance with JIS P-8117, the air permeability of the polyolefin microporous membrane was measured as the air permeability resistance under an atmosphere of 23°C and 40% humidity using a Gurley air permeability meter, G-B2 (trademark), manufactured by Toyo Seiki Co., Ltd.

[0097] <Puncture strength (gf)> Using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech Co., Ltd., the microporous membrane was fixed with a sample holder having a diameter of 11.3 mm at the opening. Next, the central part of the fixed microporous membrane was subjected to a puncture test at a needle tip curvature radius of 0.5 mm and a puncture speed of 2 mm / sec under an atmosphere of 23°C and 40% humidity to obtain the raw puncture strength (gf) as the maximum puncture load.

[0098] <Amount of unmelted matter in separator (pieces / 1000 m 2 )> The amount of unmelted matter in the separator was quantified by the area having a length of 100 μm × width of 100 μm or more and not allowing light to pass through when the separator obtained through the film-forming processes of the examples and comparative examples was observed with a transmission optical microscope. The number of resin aggregates per 1000 m of the separator area was measured by observation with a transmission optical microscope. 2 per unit area was measured.

[0099] <Stab evaluation (%)> The positive electrode, negative electrode, and non-aqueous electrolyte were prepared according to the following procedures a to c. a. Preparation of positive electrode As the positive electrode active material, 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, graphite powder (KS6) (density 2.26 g / cm 3 , number average particle diameter 6.5 μm) was 1.6% by mass, and acetylene black powder (AB) (density 1.95 g / cm 3 , number average particle diameter 48 nm) was 3.8% by mass, and polyvinylidene fluoride (PVDF) (density 1.75 g / cm 3 was mixed at a ratio of 4.2% by mass, and these were dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This slurry was applied to one side of an aluminum foil with a thickness of 20 μm serving as the positive electrode current collector using a die coater, dried at 130 °C for 3 minutes, and then compression molded using a roll press to fabricate a positive electrode. The coating amount of the positive electrode active material at this time was 109 g / m 2 was obtained.

[0100] b. Fabrication 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), 1.4% by mass (in terms of solid content) of ammonium carboxymethyl cellulose as a binder (aqueous solution with a solid content concentration of 1.83% by mass), and 1.7% by mass (in terms of solid content) of diene rubber latex (aqueous solution with a solid content concentration of 40% by mass) were dispersed in purified water to prepare a slurry. This slurry was applied to one side of a copper foil with a thickness of 12 μm serving as the negative electrode current collector using a die coater, dried at 120 °C for 3 minutes, and then compression molded using a roll press to fabricate a negative electrode. The coating amount of the negative electrode active material at this time was 52 g / m 2 was obtained.

[0101] 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.

[0102] d. Battery fabrication Using the positive electrode, negative electrode, non-aqueous electrolyte, and separator (the separator of the example or the separator of the comparative example) obtained in the above a~c, a laminated secondary battery with a size of 100 mm × 60 mm and a capacity of 3 Ah was fabricated by constant current constant voltage (CCCV) charging at a current value of 1 A (0.3C) and a final battery voltage of 4.2 V for 3 hours. e. Nail penetration evaluation The laminated secondary battery was placed still on an iron plate inside an explosion-proof booth with temperature control. At the center of the laminated secondary battery, the temperature inside the explosion-proof booth was set to 40°C, and an iron nail with a diameter of 3.0 mm was penetrated at a speed of 2 mm / sec, and the nail was maintained in the 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. Using the laminated secondary battery newly produced by the same method, the evaluation was repeated, and the number of samples that did not reach ignition (no ignition) was calculated as a % value by the following formula. Evaluation result (%) = (100 × number of samples that did not reach ignition / total number of samples)

[0103] <Cycle test (%)> Using the separators obtained in the examples and comparative examples respectively, and using the simple battery obtained in the above procedure d, the cycle characteristics were evaluated by the following procedure. (1) Pretreatment The above simple battery was charged at a constant current to a voltage of 4.2 V at a current value of 1 / 3 C, then charged at a constant voltage of 4.2 V for 8 hours, and then discharged at a current of 1 / 3 C to a cut-off voltage of 3.0 V. Next, it was charged at a constant current to a voltage of 4.2 V at a current value of 1 C, then charged at a constant voltage of 4.2 V for 3 hours, and further discharged at a current of 1 C to a cut-off voltage of 3.0 V. Finally, it was charged at a constant current to 4.2 V at a current value of 1 C, and then charged at a constant voltage of 4.2 V for 3 hours. Note that 1 C represents the current value for discharging the reference capacity of the battery in 1 hour. (2) Cycle test The battery that had undergone the above pretreatment was discharged at a discharge current of 1 C to a discharge cut-off voltage of 3 V under the condition of a temperature of 25°C, and then charged at a charge current of 1 C to a charge cut-off voltage of 4.2 V. This was repeated as one cycle for charge and discharge. Then, the capacity retention rate after 300 cycles with respect to the initial capacity (the capacity in the first cycle) was calculated as a % value by the following formula. Evaluation result (%) = (100 × retained capacity after 300 cycles / initial capacity)

[0104] [Example 1] A powder (first component) containing a first resin (UHMWPE with a viscosity-average molecular weight of 1 million, where the weight-average molecular weight and number-average molecular weight are difficult to measure) and a pellet (second component; a cylinder with a diameter of 5 - 6 mm and a height of 3 - 2 mm) containing a second resin (nylon 12 with a melting point of 178 °C; product number sold by Aldrich; 181161, denoted as "PA12" in the table) were dry-blended in a super mixer at the mass ratios described in Table 1, and the first component and the second component were fed into a twin-screw extruder and melt-mixed. Then, while melt-mixing, liquid paraffin (kinematic viscosity at 37.78 °C of 7.59×10 -5 m 2 / s) was fed through an injection nozzle into a twin-screw extruder equipped with a manifold (T-die) with a die lip interval of 1500 μm, and further kneading was performed to extrude the resin composition. At this time, liquid paraffin was further injected from the middle extrusion section (the middle feed port of the twin-screw extruder) so that the ratio of the amount of liquid paraffin in 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 controlled to a surface temperature of 25 °C and cast to obtain a sheet-shaped molded body. Next, it was led to a simultaneous biaxial tenter stretching machine and biaxially stretched to obtain a stretched product. The stretching conditions were a stretching surface magnification of 50 - 180 times, and the porosity, air permeability, thickness, puncture strength, etc. were adjusted by adjusting the stretching temperature, heating air volume, magnification, etc. In Example 1, the biaxial stretching temperature was 125 °C. Next, the stretched product was immersed in dichloromethane to extract liquid paraffin from the stretched product to form a porous body. Next, in order to perform heat fixation of the porous body, it was led to a TD tenter and heat fixation (HS) was performed at 128 °C, and then a relaxation operation 0.5 times in the TD direction (that is, the HS relaxation rate was 0.5 times) was performed. Thereafter, the above evaluations were performed on the obtained microporous membrane. The evaluation results are shown in Table 1.

[0105] [Examples 2 - 10, and Comparative Examples 1 - 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. 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 pellets.

[0106] Note that "EVOH" in Example 2 represents an ethylene-vinyl alcohol copolymer (Kuraray's Evalar (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 company or ethylene-vinyl alcohol copolymers other than those manufactured by the above company can also be used.

[0107] Also, "PE-PA" in 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 company or polyethylene-polyamide copolymers other than those manufactured by the above company can also be used.

[0108] Also, the "nanocrystal structure-controlled elastomer" in Example 7 represents 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 company or nanocrystal structure-controlled elastomers other than those manufactured by the above company can also be used.

[0109] Also, "PA12&PE-PA" in Example 10 represents a mixture of nylon 12 and polyethylene-polyamide copolymer at a ratio of 10:1 (10:1 = PA12:PE-PA). Here, nylon 12 with a melting point of 178°C sold by Aldrich (product number; 181161) and LP91 manufactured by ARKEMA were used as "PA12&PE-PA". However, the commercially available products that can be used are not limited to this, and other grades manufactured by the above company or nylon 12 other than those manufactured by the above company, and also other grades manufactured by the above company or polyethylene-polyamide copolymers other than those manufactured by the above company can also be used.

[0110] In addition, the "PA copolymer" in Comparative Example 1 represents Aramine (registered trademark) grade "CM8000" available from Toray Industries, Inc., and the "PEEK" in Comparative Example 2 represents polyetheretherketone (TPS (registered trademark) standard grade "NC" available from Toray Plastics (K.K.)).

[0111]

Table 1-1

[0112]

Table 1-2

[0113] From the results in Table 1, it was confirmed that the microporous membranes of Examples 1 to 10 had a reduced amount of unmelted matter (amount of aggregates, i.e., gel content) compared to Comparative Examples 1 to 4, that is, the quality was improved. Also, from the results in Table 1, it was confirmed that in Examples 1 to 10, it was possible to realize a power storage device that was excellent in safety (safety in the nail penetration test) and cycle characteristics compared to Comparative Examples 1 to 4.< / lib>

Claims

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, 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 polyethylene resin in a granular form; 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.

2. 2. The microporous membrane of 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.

3. The microporous membrane according to claim 1 or 2, wherein the resin having a melting point of 140°C to 330°C comprises at least one selected from the group consisting of polyolefin resins, benzene ring-containing resins, and heteroatom-containing resins.

4. The microporous membrane of claim 3 , wherein the polyolefin resin is other than homopolypropylene.

5. 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.

6. 4. The microporous membrane of claim 3, wherein the heteroatom-containing resin is at least one selected from the group consisting of polyamide, polytetrafluoroethylene, and polyketone.

7. The microporous membrane of any one of claims 1 to 6, wherein the polyethylene resin comprises ultra-high molecular weight polyethylene.

8. The microporous membrane according to any one of claims 1 to 7, 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.

9. A separator for an electricity storage device comprising the microporous membrane according to any one of claims 1 to 8.

10. A lithium ion secondary battery comprising the microporous membrane according to any one of claims 1 to 8.

11. An electricity storage device comprising the microporous membrane according to any one of claims 1 to 8.

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