Polyolefin microporous film

Abstract

To provide a polyolefin microporous film which suppresses contamination of foreign matter and coating omission and can achieve high film quality while achieving both a thin film and film strength, and has high detection accuracy of foreign matter in a defect inspection machine, and a separator for an electrochemical device including the same.SOLUTION: A polyolefin microporous film contains a polyolefin resin as a main component, satisfies at least one of the following characteristics (1) and (2): (1) a basis weight of 4.0 g / m2 or more; and (2) a film thickness of 6 μm or more, and has a light transmittance at a wavelength of 660 nm of the polyolefin microporous film of 10% or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a microporous polyolefin membrane and the like. [Background technology]

[0002] Polyolefin microporous membranes (hereinafter sometimes simply referred to as "PO microporous membranes") are widely used for separating various substances, or as permselective separation membranes, separators, etc. Applications include, for example, microfiltration membranes; separators for batteries such as lithium ion batteries and fuel cells; separators for capacitors; and base materials for functional membranes in which functional materials are filled into the pores to create new functions. In particular, PO microporous membranes are preferably used as separators for lithium ion batteries (LIBs), which are widely used in mobile phones, smartphones, wearable devices, notebook personal computers (PCs), tablet PCs, digital cameras, etc.

[0003] Conventionally, microporous PO membranes used as LIB separators have been required to have dimensional stability at temperatures below the melting point of PO or against external stress, shutdown performance near the melting point, and membrane rupture resistance at higher temperatures. Furthermore, various microporous PO membranes and their manufacturing methods have been proposed from the perspective of the relationship between the properties of LIB separators and the characteristics of LIBs.

[0004] For example, Patent Document 1 describes a microporous PO membrane having a porous layer with a thickness of 16 μm or less, in which the single-point total pore volume of pores with diameters of 98 nm or less, as measured by a nitrogen gas adsorption test, is 25% to 85% of the total pore volume, from the viewpoint of excellent results in a half-cell initial overcharge test.

[0005] Patent Document 2 describes a porous polyolefin film that has a porosity of 50% or more, a pin puncture strength of 3.7 N or more at a film thickness of 10 μm, and a tensile modulus in the machine direction (MD) of 980 MPa or more, from the viewpoints of output characteristics, safety, and process transportability when used as a battery separator.

[0006] In Patent Document 3, in order to provide a microporous membrane with an excellent balance of physical properties including thin film, uniform pore size, tensile strength, and high permeability, and thereby ensure the output characteristics of a secondary battery, the proportion of the total value of the pore size distribution of pores that are 0.9 Dp to 1.1 Dp, the maximum pore size, the MD tensile strength, and the ratio of the MD tensile strength to the TD tensile strength are studied, where Dp (nm) is the pore size showing the maximum peak in a special pore size distribution curve of the membrane in a dry state and a wet state.

[0007] Patent Document 4 investigates the basis weight, film thickness, and light transmittance at a specific wavelength of a microporous membrane containing polyethylene as a main component, from the viewpoint of enabling stable detection of defects such as scratches and pinholes when the membrane is made thinner or has a high porosity. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent Publication No. 2021-123614 [Patent Document 2] Japanese Patent Publication No. 2021-038379 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-120786 [Patent Document 4] International Publication No. 2018 / 164057 Summary of the Invention [Problem to be solved by the invention]

[0009] In recent years, electrochemical devices such as LIBs have become significantly smaller and thinner, while there is also a demand for LIBs with higher capacities than conventional products. To this end, electrochemical device manufacturers are working to make separators for electrochemical devices thinner, but excessive thinning can impair the safety, strength, and other performance aspects of the electrochemical device, so optimizing the film thickness is important.

[0010] Furthermore, if metal or other foreign matter gets mixed into conventional separators for electrochemical devices, problems include short circuits caused by the foreign matter, wrinkles in the PO microporous membrane used as the conventional separator substrate, and coating defects caused by polymer particles when coating the PO microporous membrane with a functional layer, etc. With the increasing demand for higher quality PO microporous membranes in recent years, stricter control is required than ever before to prevent short circuits and coating defects.

[0011] On the other hand, conventional separators for power storage devices that have a film thickness and / or basis weight above a certain value have low light transmittance and the accuracy of detecting foreign matter using a defect inspection machine is low, making it difficult to achieve both an appropriate thickness and / or basis weight and high quality.

[0012] In view of the above circumstances, the invention of the present disclosure aims to provide a polyolefin microporous membrane that can achieve high membrane quality by suppressing the inclusion of foreign matter and coating voids while achieving optimal membrane thickness and membrane strength, and that can detect foreign matter with high accuracy using a defect inspection machine, as well as a separator for an electrochemical device and an electrochemical device using the same. [Means for solving the problem]

[0013] The above problems can be solved by the following technical means. <1> A polyolefin microporous membrane containing a polyolefin resin as a main component, which has the following properties (1) and (2): (1) Basis weight: 4.0 g / m 2 That's it; (2) the film thickness is 6 μm or more; At least one of the following is satisfied, and The polyolefin microporous film has a light transmittance of 10% or more at a wavelength of 660 nm. <2> The polyolefin microporous membrane was stained with ruthenium, embedded in a room-temperature curing epoxy resin, and then a smooth cross section parallel to the TD was prepared using a broad ion beam (BIB). At least one scanning electron microscope (SEM) image of the smooth cross section was taken at 7000x magnification, and pore size analysis was performed using the local thickness method. The pore size distribution D TD Item 2. The polyolefin microporous membrane according to item 1, wherein the total proportion of pores having a pore size of 180 nm or less is 90% or less of the total. <3> When the pore diameters of five SEM images were analyzed, the average pore diameter d TD(N=5) Item 3. The polyolefin microporous membrane according to Item 2, wherein the standard deviation of the average particle diameter is 0.1 nm or more and 1.0 nm or less. <4> The polyolefin microporous membrane has a puncture strength converted into basis weight of 80 gf / (g / m 2 ) or more 150gf / (g / m 2 4. The polyolefin microporous membrane according to any one of items 1 to 3, wherein: <5> 5. The microporous polyolefin membrane according to any one of items 1 to 4, wherein the microporous polyolefin membrane has a viscosity average molecular weight (Mv) of 800,000 or more. <6> 6. The polyolefin microporous membrane according to any one of items 1 to 5, wherein the molecular weight distribution (Mw / Mn) of the polyolefin microporous membrane, expressed as the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn), is 5 or more and 10 or less. <7> 7. The polyolefin microporous membrane according to any one of items 1 to 6, wherein the polyolefin microporous membrane has a porosity measured at a temperature of 30°C of 30% or more and 65% or less. <8> 8. The microporous polyolefin membrane according to any one of items 1 to 7, wherein the thickness of the microporous polyolefin membrane is 6 μm or more and 12 μm or less. [Effects of the Invention]

[0014] According to the present invention, it is possible to achieve high membrane quality by suppressing inclusion of foreign matter and coating voids while achieving both the desired thinness and membrane strength of the polyolefin microporous membrane, and it is also possible to improve the accuracy of detecting foreign matter using a defect inspection machine, thereby providing a separator for electrochemical devices and an electrochemical device including the polyolefin microporous membrane. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, a detailed description will be given of an embodiment of the present invention (hereinafter, sometimes abbreviated as "the present embodiment"); however, the present invention is not limited to this embodiment, and various modifications are possible without departing from the spirit of the present invention. In this specification, the flow direction of the membrane during membrane production is defined as MD, and the direction intersecting MD at 90 degrees in the membrane plane is defined as TD. Furthermore, when a membrane or resin contains a specific component as a main component, it means that the content of the specific component is 50% by mass or more based on the mass of the membrane or resin.

[0016] The polyolefin microporous membrane (PO microporous membrane) according to this embodiment contains polyolefin (PO) as a main component, and is characterized by the relationship between basis weight and / or membrane thickness and light transmittance as shown below.

[0017] If desired, the pore size, pore size distribution, molecular weight, pin puncture strength, pin puncture strength per unit area, porosity, etc. of the PO microporous membrane can be specified as shown below, and an inorganic coating layer and / or adhesive layer can be formed on the surface. The properties described in this embodiment can be utilized independently or in any combination. Unless otherwise specified, the methods for measuring the physical properties of the PO microporous membrane are described in detail in the Examples section.

[0018] <Relationship between basis weight and / or film thickness and light transmittance> The microporous PO membrane of this embodiment has the following properties (1) and (2): (1) Basis weight: 4.0 g / m 2 That's it; (2) the film thickness is 6 μm or more; At least one of the following is satisfied, and The polyolefin microporous film has a light transmittance of 10% or more at a wavelength of 660 nm.

[0019] 4.0 g / m as shown in (1) above 2 The above basis weight is important for the PO microporous membrane of this embodiment in terms of membrane strength and optical properties, and can be utilized to ensure good safety in nail penetration tests and the like of electrochemical devices that include the PO microporous membrane as an electrochemical device separator.

[0020] The thickness of 6 μm or more as described in (2) above is important for the PO microporous membrane of this embodiment in terms of membrane strength and optical properties, and can be utilized to ensure good safety in nail penetration tests and the like of electrochemical devices containing the PO microporous membrane as a separator for electrochemical devices.

[0021] Generally, in foreign matter detection methods, an optical detection device is used to pass the membrane to be inspected between a light source and a light receiving unit, and abnormal areas are detected by comparing them with normal areas.Therefore, the lower the light transmittance of the membrane, the smaller the relative ratio and the more difficult it is to find abnormal areas.On the other hand, the thicker the membrane and the higher the basis weight, the greater the light loss path, and the lower the light transmittance of the membrane.

[0022] The PO microporous membrane of this embodiment, assuming that at least one of the above properties (1) and (2) is satisfied in the general trade-off relationship between the aforementioned thinning / reducing basis weight and the accuracy of foreign matter inspection, has a light transmittance of 10% or more at a wavelength of 660 nm. This improves the light transmittance of the PO microporous membrane with optimized thickness and / or basis weight, thereby improving the accuracy of foreign matter detection by a defect inspection machine and ultimately achieving high membrane quality by suppressing the inclusion of foreign matter and coating voids.

[0023] Although not wishing to be bound by theory in the foreign matter detection method, because the wavelength of light used in the inspection machine is in the visible light range (i.e., about 380 nm to about 780 nm), the light transmittance at a wavelength of 660 nm can be regarded as the amount of light that travels directly to the light receiving unit, with the amount of light emitted from the light source being 100%. Therefore, in this embodiment, assuming that at least one of the above properties (1) and (2) is satisfied, the light transmittance of a microporous polypropylene membrane at a wavelength of 660 nm was found to be 10% or more. From this perspective, the light transmittance of a microporous polypropylene membrane at a wavelength of 660 nm is preferably more than 10% and not more than 40%, more preferably 12% or more and less than 40%, even more preferably 14% or more and less than 40%, still more preferably 16% or more and less than 40%, and particularly preferably 18% or more and less than 40%.

[0024] The basis weight of the PO microporous membrane is preferably 4.0 g / m from the viewpoints of improving the accuracy of detecting foreign matter using the defect inspection machine described above, reducing the porosity, and ensuring the safety of electrochemical devices. 2 More than 7.9g / m 2 or less, more preferably 4.1 g / m 2 More than 7.0g / m 2 More preferably, 4.3 g / m or less 2 More than 6.1g / m 2 or less, even more preferably 4.4 g / m 2 More than 6.0g / m 2 Below 4.6 g / m, particularly preferably 2 More than 5.8g / m 2 The following is the result.

[0025] The microporous PO membrane preferably has a thickness of 6 μm or more and 12 μm or less from the viewpoints of improving the accuracy of detecting foreign matter using the defect inspection machine described above, and suppressing the bulkiness of the electricity storage device and increasing its capacity.

[0026] The microporous polypropylene membrane of this embodiment preferably has its basis weight, film thickness, and light transmittance at a wavelength of 660 nm all adjusted as described above to improve the accuracy of foreign matter detection by a defect inspection machine, to prevent foreign matter contamination and coating voids, and to achieve high membrane quality.

[0027] The physical means for satisfying at least one of the above properties (1) and (2) and adjusting the light transmittance at a wavelength of 660 nm as described above are as follows.

[0028] The causes of light loss include absorption, reflection, and scattering of light, and the scattering of light is represented by the following formula: α=πd / λ {where α is the size parameter, λ is the wavelength of the scattered wave, d is the diameter of the scattering particle, and π is the circular constant} In this case, it can be broadly divided into Rayleigh scattering where α<<1 and Mie scattering where α≒1.

[0029] Rayleigh scattering intensity σ s is the following formula:

number

[0030] In this embodiment, although it is not desired to be bound by theory, a microporous membrane with a maximum pore size of about 500 nm that can be used as a separator for an electrochemical device generates Rayleigh scattering, and therefore, the Rayleigh scattering intensity σ s It has been found that, when the particles in the formula are approximated as pores, the influence of scattering can be minimized by making the micropores in the membrane as small as possible. Therefore, physical means for satisfying at least one of the above properties (1) and (2) and adjusting the light transmittance at a wavelength of 660 nm as described above include, for example, controlling the pore size and pore size distribution of the microporous PO membrane, and more specifically, having membrane properties that can be obtained by the pore size analysis method described below.

[0031] As a manufacturing method for satisfying at least one of the above properties (1) and (2) and adjusting the light transmittance at a wavelength of 660 nm as described above, for example, at least one selected from the following (i) to (iv) can be mentioned: (i) Selection of the resin to be contained or the raw material resin, more specifically, selection of a polyolefin resin having a viscosity average molecular weight (Mv) of 800,000 to 1,500,000, a molecular weight distribution (Mw / Mn) expressed as the ratio of the mass average molecular weight (Mw) to the number average molecular weight (Mn) of 5 to 10, and / or a single resin composition (e.g., polyethylene homopolymer), and adjusting the total content (PC) of all resins contained in the PO resin composition to fall within the range of 20% to 25%; (ii) In the method for producing a microporous PO membrane, the cooling rate during casting is controlled to 1°C / sec to 12°C / sec. Means for controlling the cooling rate during casting include, for example, adjusting the sheet thickness to within the range of 1200 μm to 3000 μm, adjusting the resin temperature at the die outlet to within the range of 180°C to 215°C, using a cooling roll whose surface temperature is controlled to 50°C to 100°C to adjust the casting temperature, and adjusting the casting speed to within the range of 1 m / min to 12 m / min. (iii) simultaneous biaxial stretching in the production method of PO microporous membrane; (iv) The total stretching ratio in the method for producing a microporous PO membrane is adjusted to within the range of 70 to 120 times. Among the above-mentioned production methods, at least one selected from the group consisting of methods (i), (iii) and (iv) is preferred.

[0032] The preferred configuration or physical properties, preferred components, and production method of the microporous polypropylene membrane according to this embodiment are described below.

[0033] <Characteristics obtained by membrane pore size analysis> The microporous PO membrane according to the present embodiment is stained with ruthenium, embedded in a room-temperature curing epoxy resin, and then a smooth cross section parallel to the TD is prepared using a broad ion beam (BIB). At least one image of the smooth cross section is taken at 7000x magnification using a scanning electron microscope (SEM), and pore size analysis is performed using the local thickness method. The pore size distribution D TD In the present invention, the total proportion of pores having a diameter of 180 nm or less is preferably 90% or less of the total.

[0034] The above pore size analysis is detailed in the Examples section.

[0035] Without wishing to be bound by theory, the microporous PO membrane of this embodiment has a pore size distribution D TD In this case, if the total proportion of pore sizes of 180 nm or less is 90% or less of the total, light will be transmitted directly without being scattered or diffused, which is thought to increase light transmittance and make the film less susceptible to the effects of Rayleigh scattering. This will improve the accuracy of foreign matter detection by defect inspection equipment, and ultimately suppress the inclusion of foreign matter and coating voids, achieving high film quality.

[0036] In order to further improve the accuracy of detecting foreign matter using a defect inspection machine and to prevent foreign matter from entering or coating voids, five SEM images of a smooth cross section parallel to the TD prepared as described above were measured and analyzed for pore size by the local thickness method. The average pore size d TD(N=5) The standard deviation of the above is preferably 0.1 nm or more and 1.0 nm or less, more preferably 0.2 nm or more and less than 0.9 nm, even more preferably 0.2 nm or more and 0.8 nm or less, still more preferably 0.2 nm or more and 0.7 nm or less, and particularly preferably 0.2 nm or more and 0.5 nm or less.

[0037] The microporous PO membrane according to the present embodiment is stained with ruthenium, embedded in a room-temperature curing epoxy resin, and then a smooth cross section parallel to the TD is prepared by BIB. At least one SEM image of the smooth cross section is taken at 7000x magnification, and the pore size distribution D TDIt is preferable that the standard deviation of D is 35 nm or more and 55 nm or less. TD If the standard deviation of pore size distribution D is ≦55 nm, it tends to contribute to further improvement of the accuracy of detecting foreign matter in defect inspection equipment and to suppression of foreign matter contamination and coating voids. TD The standard deviation of is more preferably 40 nm or more and 50 nm or less, and even more preferably more than 40 nm and 46 nm or less.

[0038] The microporous PO membrane of this embodiment has an average pore diameter d TD However, it is preferable that the thickness is 100 nm or more and 160 nm or less. TD If the average pore diameter d is less than 160 nm, it tends to contribute to further improving the accuracy of detecting foreign matter in defect inspection machines and suppressing the inclusion of foreign matter and coating voids. TD is more preferably 105 nm or more and 150 nm or less, even more preferably 109 nm or more and less than 140 nm, and particularly preferably 110 nm or more and 125 nm or less.

[0039] The microporous PO membrane of this embodiment is designed to have a maximum pore size PS of 1 / 2 mm when at least one SEM image of a smooth cross section parallel to the TD prepared as described above is analyzed for pore size, in order to further improve the accuracy of detecting foreign matter using a defect inspection machine and to prevent foreign matter from being mixed in or missing from the coating. TD(max) However, it is preferable that the thickness is 250 nm or more and 400 nm or less.

[0040] At least one SEM image of the smooth cross section parallel to TD can be obtained, for example, by capturing at least one image of the smooth cross section parallel to TD in at least one field of view at 7000x magnification. Five SEM images of the smooth cross section parallel to TD can be obtained, for example, by capturing five images of the smooth cross section parallel to TD in different fields of view at 7000x magnification.

[0041] The microporous PO membrane according to the present embodiment was stained with ruthenium and embedded in room-temperature curing epoxy resin to further improve the accuracy of detecting foreign matter in a defect inspection machine and to prevent the inclusion of foreign matter or coating voids. Then, a smooth cross section parallel to the MD was prepared using BIB. Five SEM images of the smooth cross section were taken at 7000x magnification and measured. The average pore diameter d MD(N=5) The standard deviation of is preferably 0.2 nm or more and 0.9 nm or less, and more preferably 0.2 nm or more and less than 0.8 nm.

[0042] The microporous PO membrane according to the present embodiment is stained with ruthenium, embedded in a room-temperature curing epoxy resin, and then a smooth cross section parallel to the MD is prepared by BIB. When at least one SEM image of the smooth cross section taken at 7000x magnification is analyzed for pore size, the pore size distribution D MD It is preferable that the standard deviation of D is 38 nm or more and 45 nm or less. MD If the standard deviation of pore size distribution D is ≦45 nm, it tends to contribute to further improvement of the accuracy of detecting foreign matter in defect inspection equipment and to suppression of foreign matter contamination and coating voids. MD The standard deviation is more preferably 39 nm or more and 44 nm or less.

[0043] The microporous PO membrane of this embodiment has an average pore diameter d of 1.0 mm or less when at least one SEM image of a smooth cross section parallel to the MD prepared as described above is analyzed for pore size, from the viewpoints of further improving the accuracy of detecting foreign matter in a defect inspection machine and suppressing the inclusion of foreign matter and coating voids. MD However, it is preferably 100 nm or more and 155 nm or less, more preferably 102 nm or more and 149 nm or less, even more preferably 105 nm or more and 140 nm or less, and particularly preferably 107 nm or more and 125 nm or less.

[0044] At least one SEM image of the smooth cross section parallel to the MD can be obtained, for example, by capturing at least one image of the smooth cross section parallel to the MD in at least one field of view at 7000x magnification. Five SEM images of the smooth cross section parallel to the MD can be obtained, for example, by capturing five images of the smooth cross section parallel to the MD in separate fields of view at 7000x magnification.

[0045] The microporous PO membrane according to the present embodiment was stained with ruthenium and embedded in room-temperature curing epoxy resin to further improve the accuracy of detecting foreign matter in a defect inspection machine and to prevent the inclusion of foreign matter or coating voids. Smooth cross sections parallel to the MD and TD were then prepared using BIB. SEM images of the smooth cross sections were taken at 7000x magnification, and at least one image was analyzed for pore size by the local thickness method. The ratio of the average pore diameters of the cross sections prepared in both directions, i.e., the average pore diameter d MD and average pore diameter d TD The ratio (d MD / d TD ) is preferably 0.85 or more and 1.25 or less.

[0046] The means for adjusting the properties of a microporous PO membrane obtained by pore size analysis as described above is not limited, and may be, for example, the same as any of the manufacturing means (i) to (iv) for adjusting the light transmittance at a wavelength of 660 nm as described above while satisfying at least one of the properties (1) and (2). Among the means (i) to (iv), the simultaneous biaxial stretching method (iii) is preferred as a means for adjusting the properties obtained by pore size analysis.

[0047] In addition, the average pore diameter d MD and average pore diameter d TD The ratio (d MD / d TD ) can be adjusted to fall within the above range by not only the above (i) to (iii) but also by controlling the MD / TD stretch ratio in the production method for the microporous PO membrane.

[0048] <Membrane strength> PO microporous membrane basis weight (g / m 2) When converted to (hereinafter referred to as the areal density-converted puncture strength), the puncture strength is 80 gf / (g / m 2 ) or more and 150 gf / (g / m 2 ) or less, which is preferable. The PO microporous membrane having an areal density-converted puncture strength of 80 gf / (g / m 2 ) or more has an optimized pore diameter and pore size distribution, and tends to have good light transmittance. Also, the PO microporous membrane having an areal density-converted puncture strength of 80 gf / (g / m 2 ) to 150 gf / (g / m 2 ) tends to have improved burst membrane resistance and good safety. From this tendency, the areal density-converted puncture strength of the PO microporous membrane is more preferably 83 gf / (g / m 2 ) or more and 140 gf / (g / m 2 ) or less, and even more preferably 85 gf / (g / m 2 ) or more and 135 gf / (g / m 2 ) or less.

[0049] The puncture strength of the PO microporous membrane not converted to the areal density (hereinafter simply referred to as the puncture strength) is preferably 450 gf or more and 1000 gf or less, more preferably 550 gf or more and 1000 gf or less, and even more preferably 550 gf or more and 900 gf or less, from the viewpoint of improving the burst membrane resistance, similar to the areal density-converted puncture strength.

[0050] Note that the units "gf" and "N" are interchangeable according to the formula: 1 gf ≒ 0.0098 N. As means for controlling the areal density-converted puncture strength or the puncture strength of the PO microporous membrane within the above numerical ranges, for example, in the manufacturing process of the PO microporous membrane, adjustment of the molecular weight of the PO raw material or the contained resin, adjustment of the stretching surface magnification and / or stretching temperature, etc. can be mentioned. Among them, adjustment of the biaxial stretching temperature is preferable.

[0051] <Molecular weight of the PO microporous membrane> The viscosity-average molecular weight (Mv) of the PO microporous membrane according to this embodiment is preferably 800,000 or more. By setting the Mv of the PO microporous membrane itself to 800,000 or more, it tends to contribute to further improvement in the detection accuracy of foreign matters by the defect inspection machine and suppression of foreign matter contamination and coating omission. Also, an Mv of 800,000 or more for the PO microporous membrane is preferable from the viewpoints of reducing the pore diameter and increasing the permeability. From such viewpoints, the Mv of the PO microporous membrane is more preferably 850,000 or more, and still more preferably 900,000 or more. On the other hand, the Mv of the PO microporous membrane is preferably 2,000,000 or less, and more preferably 1,500,000 or less, from the viewpoint of suppressing thermal shrinkage.

[0052] The molecular weight distribution (Mw / Mn), expressed as the ratio of the mass average molecular weight (Mw) to the number average molecular weight (Mn) of the PO microporous membrane according to this embodiment, is preferably 5 or more and 10 or less. The PO microporous membrane within the numerical range of 5 ≦ Mw / Mn ≦ 10 is likely to achieve a uniform pore diameter and tends to contribute to further improvement in the detection accuracy of foreign matters by the defect inspection machine and suppression of foreign matter contamination and coating omission. Also, the PO microporous membrane within the numerical range of 5 ≦ Mw / Mn ≦ 10 is preferable from the viewpoints of reducing the pore diameter and increasing the permeability. From such viewpoints, the molecular weight distribution (Mw / Mn) of the PO microporous membrane is more preferably 6 or more and less than 10, and still more preferably 7 or more and 9 or less.

[0053] In this specification, the molecular weight of the PO microporous membrane is obtained by measuring the PO microporous membrane itself. The molecular weight of the PO microporous membrane can be adjusted to the above range, for example, by controlling the molecular weight or composition of the contained resin or raw material resin, or by adjusting the draw surface magnification and / or draw temperature in the manufacturing process of the PO microporous membrane. Among these, adjustment of the draw temperature is preferable, and adjustment of the biaxial draw temperature is more preferable.

[0054] <Air permeability of the PO microporous membrane> The air permeability of the PO microporous membrane is preferably 30 s / 100 cm 3 or more and 195 s / 100 cm 3More preferably, it is 50 s / 100 cm or less 3 180 s / 100 cm or more 3 More preferably, it is 70 s / 100 cm or less 3 170 s / 100 cm or more 3 More preferably, it is 78 s / 100 cm or less 3 160 s / 100 cm or more 3 It is less than the above values. The air permeability of the PO microporous membrane can be controlled within the above numerical range, for example, in the method for producing the PO microporous membrane, by adjusting the HS stretching ratio, HS relaxation ratio, HS relaxation temperature, etc.

[0055] <Porosity of the PO microporous membrane> The porosity of the PO microporous membrane measured at 30 °C is preferably 30% or more, more preferably 35% or more, and still more preferably 40% or more. A porosity of 30% or more is suitable from the viewpoint of ensuring good pore diameter, pore size distribution, ion permeability, and device output characteristics. Also, a porosity of 30% or more is preferable from the viewpoint of adjusting the light transmittance at a wavelength of 660 nm while satisfying at least one of the above characteristics (1) and (2). The upper limit of the porosity is preferably 65% or less, more preferably 60% or less. A porosity of 65% or less is preferable from the viewpoints of film strength and withstand voltage.

[0056] <Flux pore diameter> The Flux pore diameter of the PO microporous membrane according to this embodiment is preferably 30 nm or more and 70 nm or less. The PO microporous membrane within the range of 30 nm ≤ Flux pore diameter ≤ 70 nm has a tendency to improve the cycle performance when it is mounted in an electrochemical device as a separator for an electrochemical device without foreign substances entering, and also has a tendency to suppress Rayleigh scattering of light and increase light transmittance.

[0057] <Thermal shrinkage rate of the PO microporous membrane> With the expansion of the scope of use of electrochemical devices, in order to ensure device safety in a high-temperature environment, for example, in an oven test, it is preferable to control the thermal shrinkage rate of the PO microporous membrane used as a separator at a high temperature (for example, near the melting point of PO or near the melting point of the PO microporous membrane). Also, it is preferable to control the thermal shrinkage rate of the separator for electrochemical devices to prevent contact between electrodes in the power storage device. From such a perspective, the thermal shrinkage rate of the PO microporous membrane at 120 °C is preferably 33% or less in the MD direction and preferably 21% or less in the TD direction. The lower limit value of the thermal shrinkage rate of the PO microporous membrane at 120 °C may be, for example, -5% or more, -2% or more, -1% or more, or 0% or more in both the MD and TD directions.

[0058] <Maximum shrinkage stress of the TMA of the PO microporous membrane> In the thermomechanical analysis (TMA) of the PO microporous membrane, the maximum shrinkage stress is preferably 7.0 gf or less in the MD direction and preferably 9.7 gf or less in the TD direction. With the expansion of the scope of use of electrochemical devices, in order to ensure device safety in a high-temperature environment, the TMA of the PO microporous membrane used as a separator is considered important. From such a perspective, a PO microporous membrane having a maximum shrinkage stress of 7.0 gf or less in the MD direction and / or 9.7 gf or less in the TD direction, when incorporated into an electrochemical device as a separator, tends to improve the high-temperature safety of the device.

[0059] From the perspective of further improving the safety of the electrochemical device in a high-temperature environment, the maximum shrinkage stress in the TMA of the PO microporous membrane is more preferably 6.5 gf or less and even more preferably 6.1 gf or less in the MD direction, and more preferably 9.0 gf or less and even more preferably 8.5 gf or less in the TD direction. The lower limit value of the maximum shrinkage stress of the TMA is preferably 1.0 gf or more, more preferably 1.5 gf or more, and even more preferably 2.0 gf or more in both the MD and TD directions, from the perspectives of the productivity of the electrochemical device and the adhesion between the electrode and the separator.

[0060] <Optical Properties of PO Microporous Membrane> In the ultraviolet-visible absorption measurement of the PO microporous membrane, the absorbance A measured has the following relationship at wavelengths other than 600 nm: 6 A.U. ≤ A @500nm ≤ 15 A.U.; and / or 30 A.U. ≤ A @700nm ≤ 50 A.U.; It is preferably to have the above relationship. Although it is not desired to be restricted by theory, it is considered that the pore diameter of the microporous membrane is smaller than the wavelength of light, so it is difficult for light to be scattered and difficult to look white, so it is easily observed as transparent or translucent, and thus it is easier to adjust the film thickness and / or basis weight and the light transmittance at a wavelength of 660 nm as described above.

[0061] The optical properties of the PO microporous membrane can be adjusted as described above, for example, by the selection of the contained PO or raw material PO, more specifically, the use of relatively high molecular weight polyethylene (PE), and / or the non-use of polypropylene (PP), and the implementation of simultaneous biaxial stretching or the adjustment of the biaxial stretching temperature in the film-forming process.

[0062] <Shutdown Temperature and Film-Breaking Temperature (Melt-Down Temperature)> From the viewpoints of maintaining the performance of the electrochemical device in a high-temperature environment and ensuring safety during abnormal heat generation of the electrochemical device, the shutdown temperature of the PO microporous membrane is preferably 151 °C or lower, more preferably 150 °C or lower, 149 °C or lower, 148 °C or lower, 147 °C or lower, 146 °C or lower, 145 °C or lower, 144 °C or lower, 143 °C or lower, 142 °C or lower, or 141 °C or lower. Also, the lower limit value of the shutdown temperature may be, for example, 110 °C or higher, 120 °C or higher, 130 °C or higher, or 140 °C or higher in relation to the blockage of the membrane pores.

[0063] The membrane rupture temperature (melt-down temperature) of the PO microporous membrane according to this embodiment is preferably 150 °C or higher, 155 °C or higher, or 160 °C or higher, more preferably 170 °C or higher, 180 °C or higher, 190 °C or higher, or exceeding 200 °C, from the viewpoints of the stability and safety of the electrochemical device with respect to temperature. The upper limit value of the membrane rupture temperature of the PO microporous membrane is not limited, but can be, for example, 240 °C or lower, less than 240 °C, 235 °C or lower, or 230 °C or lower, depending on the type of the contained PO or raw material PO, the types of components other than PO, the mixing ratio of PO and other components, etc.

[0064] <Components contained in the PO microporous membrane> The PO microporous membrane according to this embodiment is formed from a resin composition containing a polyolefin resin. If desired, the resin composition may further contain inorganic particles, resins other than polyolefin, etc. The total content ratio (PC) of all the resins contained in the PO resin composition is preferably within the range of 20% to 25% from the viewpoints of adjusting the film thickness and / or basis weight of the PO microporous membrane and the light transmittance as described above, and the film strength.

[0065] The amount of the polyolefin resin (PO resin) contained in the PO microporous membrane is 50% by mass or more as the main component based on the mass of the PO microporous membrane, preferably 60% by mass or more, preferably 70% by mass or more, preferably 80% by mass or more, and may be 90% by mass or more and 100% by mass or less.

[0066] The polyolefin resin used in this embodiment is not particularly limited, and examples thereof include polymers obtained by polymerizing monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene (for example, homopolymers, copolymers, multistage polymers, etc.). These polymers can be used alone or in combination of two or more, and from the viewpoint of adjusting the film thickness and / or basis weight of the PO microporous membrane and the light transmittance as described above, a single type of polyolefin resin is preferred.

[0067] From the viewpoint of exhibiting shutdown properties, the total proportion of the PE raw material in the PO raw material is preferably 50% by mass to 100% by mass, and more preferably 80% by mass to 100% by mass.

[0068] From the viewpoint of adjusting the film thickness and / or basis weight and light transmittance of the PO microporous membrane as described above, and from the viewpoint of achieving a smaller pore size and higher transmittance, the PO raw material preferably has a single resin composition, more preferably consists of polyethylene (PE), and further preferably consists of PE alone, and may be, for example, a polyethylene homopolymer.

[0069] From the viewpoint of adjusting the film thickness and / or basis weight and light transmittance of the PO microporous membrane as described above, and from the viewpoint of achieving a smaller pore size and higher transmittance, the PO microporous membrane preferably has a single resin composition, more preferably consists of polyethylene (PE), and further preferably consists of PE alone, and may be, for example, a polyethylene homopolymer.

[0070] The viscosity-average molecular weight of the PO resin is preferably 500,000 or more and 2,000,000 or less. As the molecular weight of the PO resin itself increases, the film strength tends to be more easily exhibited and safety also tends to improve. From this viewpoint, the Mv of the PO resin is more preferably 600,000 or more, and even more preferably 700,000 or more. On the other hand, adjusting the viscosity-average molecular weight of the PO resin to 2,000,000 or less is preferable from the viewpoint of suppressing thermal shrinkage.

[0071] The polyethylene (PE) contained in the PO microporous membrane preferably has a viscosity average molecular weight (Mv) of 600,000 or more and 2,000,000 or less, more preferably 800,000 or more and 1,000,000 or less, and a weight average molecular weight (Mw) of 5.0 × 10 5 ~5.0×10 6and / or the polydispersity (Mw / Mn) is preferably in the range of 7.0 to 8.8. When the molecular weight of PE is within the above numerical range, not only can the thickness and / or basis weight and light transmittance of the PO microporous membrane be easily adjusted as described above, but also small pore size and high transmittance can be easily achieved. From the same viewpoint, the Mv of the PE raw material is preferably 600,000 to 2,000,000, more preferably 800,000 to 1,000,000, and Mw is preferably 5.0 × 10 5 ~5.0×10 6 and / or the polydispersity (Mw / Mn) is preferably in the range of 7.0 to 8.8. Among these, Mv is preferably in the range of 800,000 to 1,000,000 and Mw is preferably in the range of 5.0×10 5 ~5.0×10 6 and a polydispersity (Mw / Mn) of 7.0 or more and 8.8 or less are particularly preferred.

[0072] The Z-average molecular weight (Mz) of the polyethylene (PE) contained in the PO microporous membrane is 1.0 × 10 6 ~5.0×10 7 When the Mz of the PE is within the above range, it is not only easy to adjust the thickness and / or basis weight and light transmittance of the PO microporous membrane as described above, but also easy to achieve small pore size and high transmittance. From the same viewpoint, the Mz of the PE raw material is also preferably 1.0 × 10 6 ~5.0×10 7 It is preferable that the temperature is in the range of

[0073] It is preferable that the polyolefin resin does not contain polypropylene (PP) from the viewpoint of further improving the accuracy of detecting foreign matter using a defect inspection machine, from the viewpoint of suppressing the inclusion of foreign matter and coating voids, and from the viewpoint of adjusting the film thickness and / or basis weight and light transmittance as described above.

[0074] Furthermore, examples of polyolefin resins include low-density polyethylene (density 0.910 g / cm 3 More than 0.930g / cm 3less than 0.910 g / cm 3 More than 0.940g / cm 3 less than 0.930 g / cm 3 More than 0.942g / cm 3 less than 0.942 g / cm 3 or more), ultra-high molecular weight polyethylene (density 0.910 g / cm 3 More than 0.970g / cm 3 These may be used alone or in combination of two or more.

[0075] The resin composition may be mixed with various known additives, such as inorganic particles, phenolic, phosphorus-based, or sulfur-based antioxidants; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments, as needed.

[0076] <Method for producing polyolefin microporous membrane> The method for producing the microporous PO membrane according to this embodiment is not particularly limited, but may be, for example, A mixing step (a) of mixing a resin composition containing a polyolefin resin and, if desired, various additives; an extrusion step (b) of melt-kneading and extruding the mixture obtained in the step (a); a sheet forming step (c) of forming the extrudate obtained in step (b) into a sheet; a primary stretching step (d) of stretching the sheet-like molded product obtained in step (c) at least once in at least one axial direction; an extraction step (e) of extracting the pore-forming material from the primary stretched membrane obtained in step (d); and a heat setting step (f) of heat setting (HS) the extracted film obtained in step (e) at a predetermined temperature.

[0077] The above-described method for producing a PO microporous membrane can provide a PO microporous membrane that can achieve both improved cycle characteristics and suppressed short-circuit defects when used as a separator for an electrochemical device. Note that the method for producing a PO microporous membrane of this embodiment is not limited to the above-described method, and various modifications are possible without departing from the spirit and scope of the invention.

[0078] [Mixing step (a)] The mixing step (a) is a step of mixing a resin composition containing a polyolefin resin and, if desired, various additives. In the mixing step (a), other components may be mixed with the resin composition, if necessary.

[0079] In the mixing step (a), from the viewpoint of the means for adjusting the film thickness and / or basis weight and the optical properties as described above, and from the viewpoint of pore size control, it is preferable to select and use a PO raw material, and it is more preferable to select and use a PE raw material.

[0080] The pore-forming material may be any material as long as it is distinguished from the PO resin and inorganic particle materials, and may be, for example, a plasticizer. Examples of the plasticizer include non-volatile solvents capable of forming a homogeneous solution at temperatures equal to or higher than the melting point of the PO resin, such as hydrocarbons such as liquid paraffin (LP) and paraffin wax, esters such as dioctyl phthalate and dibutyl phthalate, and higher alcohols such as oleyl alcohol and stearyl alcohol.

[0081] The content of the plasticizer in the resin composition is preferably 60% by mass to 90% by mass, more preferably 70% by mass to 80% by mass. By adjusting the plasticizer content to 60% by mass or more, the melt viscosity of the resin composition decreases, and melt fracture is suppressed, which tends to improve film formability during extrusion. On the other hand, by adjusting the plasticizer content to 90% by mass or less, elongation of the raw sheet during the film formation process can sometimes be suppressed.

[0082] (Optional additives) In step (a), the resin composition containing PO may contain any additive. The additives are not particularly limited, but examples include polymers other than polyolefin resins; antioxidants such as phenolic compounds, phosphorus-based compounds, and sulfur-based compounds; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers; light stabilizers; antistatic agents; antifogging agents; and coloring pigments. The total amount of these additives added is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less, per 100 parts by mass of the polyolefin resin.

[0083] The mixing method in step (a) is not particularly limited, but examples thereof include a method in which some or all of the raw materials are premixed as needed using a Henschel mixer, ribbon blender, tumbler blender, etc. Among these, a method in which mixing is carried out using a Henschel mixer is preferred.

[0084] [Extrusion step (b)] In the extrusion step (b), the resin composition obtained in the step (a) is melt-kneaded and extruded. In the extrusion step (b), other components may be mixed with the resin composition, if necessary. From the viewpoint of adjusting the film thickness and / or basis weight and optical properties of the microporous PO membrane as described above, and from the viewpoint of pore size control, the extrudate is preferably prepared by extruding a single type of PE raw material.

[0085] The melt-kneading method in step (b) is not particularly limited, but examples include melt-kneading all raw materials, including the mixture mixed in step (a), using a screw extruder such as a single-screw extruder or a twin-screw extruder; a kneader; a mixer; etc. Among these, melt-kneading is preferably performed using a twin-screw extruder with a screw. Furthermore, when melt-kneading, it is preferable to add the plasticizer in two or more batches. Furthermore, when adding additives in multiple batches, it is preferable to adjust the amount added in the first batch to 80% by weight or less of the total amount added, from the viewpoint of suppressing aggregation of the components and uniformly dispersing them. This is preferable from the viewpoint of suppressing heat generation by shutting down over a large area and improving cell safety.

[0086] When a pore-forming agent is used in step (b), the temperature of the melt-kneading zone is preferably less than 200°C from the viewpoint of uniformly kneading the resin composition. The lower limit of the temperature of the melt-kneading zone is equal to or higher than the melting point of the polyolefin from the viewpoint of uniformly dissolving the polyolefin resin in the plasticizer.

[0087] In the present embodiment, although not particularly limited, during kneading, it is preferable to mix the raw material PO with an antioxidant at a predetermined concentration, then replace the atmosphere around the mixture with a nitrogen atmosphere, and perform melt-kneading while maintaining the nitrogen atmosphere.

[0088] In step (b), the kneaded mixture obtained through the above kneading is extruded using an extruder such as a T-die or a circular die. This may be single-layer extrusion or multi-layer extrusion. The extrusion conditions are not particularly limited, and known methods can be used, for example. Furthermore, from the viewpoint of controlling the thickness of the resulting microporous polypropylene membrane, it is preferable to control the die lip clearance, etc.

[0089] [Sheet forming process (c)] The sheet-forming step (c) is a step of forming the extrudate obtained in the extrusion step (b) into a sheet, and is also called a casting step. The sheet-shaped product obtained in the sheet-forming step (c) may be a single layer or a laminate. The sheet-forming method is not particularly limited, but examples thereof include a method of solidifying the extrudate by compression and cooling.

[0090] The compression cooling method is not particularly limited, and examples thereof include a method in which the extrudate is directly brought into contact with a cooling medium such as cold air or cooling water, and a method in which the extrudate is brought into contact with a metal roll, a press, etc. cooled with a refrigerant. Of these, the method in which the extrudate is brought into contact with a metal roll, a press, etc. cooled with a refrigerant is preferred from the viewpoints of adjusting the film thickness and / or basis weight and optical properties of the PO microporous membrane as described above, and of controlling the pore size.

[0091] From the viewpoints of controlling the film thickness and / or basis weight and the light transmittance at a wavelength of 660 nm of the microporous PO film, and of achieving a film with a small pore size and high transmittance, the cooling rate during casting is preferably controlled within the range of 1°C / sec to 12°C / sec. The cooling rate during casting, assuming that the extrudate from the die is the original web, is determined by the following two equations: Cooling rate during casting [℃ / sec.] = (resin temperature [℃] when the extrudate from the die first contacts the roll - resin temperature [℃] when the extrudate changes roll contact surface, is embraced by the next roll, and is released from that roll) ÷ residence time of the raw roll [sec.] Raw material residence time [sec.] = Length on roll [m] x 60 [sec.] ÷ Casting speed [m / min] The resin temperature during casting is measured using a non-contact infrared thermograph on the surface not touching the roll. Controlling the cooling rate during casting within the range of 1°C / sec to 12°C / sec means, but is not limited to, avoiding excessive cooling compared to conventional compression cooling methods and performing moderate cooling. If the cooling rate is too high, when the molten kneaded material is solidified in the sheet molding process, a smaller phase separation structure will form toward the surface of the cooling roll, easily resulting in a non-uniform pore structure in the film thickness direction. To obtain a uniform pore structure, it is desirable to slow the cooling rate to 12°C / sec or less. Furthermore, to cool and solidify on the roll, it is desirable to control the cooling rate to 1°C / sec or more.

[0092] Examples of ways to optimize the cooling rate during casting include adjusting the sheet thickness to within the range of 1200 μm to 3000 μm; adjusting the resin temperature at the die exit to within the range of 180°C to 215°C; using a cooling roll with a surface temperature controlled to 50°C to 100°C to adjust the casting temperature; and adjusting the casting speed to within the range of 1 m / min to 12 m / min. Adjusting the sheet thickness to within the range of 3000 μm or less is desirable to ensure that the resin is cooled and solidified by the time the extrudate changes contact surface with the roll, is embraced by the next roll, and then leaves that roll. Furthermore, controlling the sheet thickness to 1200 μm or greater is desirable to form a uniform film.

[0093] From the viewpoint of the film thickness explained in (2) above, it is also preferable to control the cast clearance etc. in the sheet forming step (c). The thickness of the obtained sheet-like molded product is preferably 1,000 μm or more and 3,300 μm or less, and more preferably 1,200 μm or more and 3,100 μm or less, depending on the thickness after stretching in step (d), for example.

[0094] [Primary stretching process (d)] The primary stretching step (d) is a step of stretching the sheet-like molded product obtained in the sheet molding step (c) at least once in at least one direction. This stretching step (a stretching step performed before the subsequent extraction step (e)) is referred to as "primary stretching," and the film obtained by primary stretching is referred to as "primarily stretched film." In the primary stretching, the sheet-like molded product can be stretched in at least one direction, and may be performed in both MD and TD, or in only one of MD or TD.

[0095] The stretching method for the primary stretching is not particularly limited, and examples thereof include uniaxial stretching using a roll stretching machine, TD uniaxial stretching using a tenter, sequential biaxial stretching using a roll stretching machine and a tenter or a combination of multiple tenters, and simultaneous biaxial stretching using a simultaneous biaxial tenter or inflation molding. Among these, simultaneous biaxial stretching is preferred from the viewpoint of uniform stretching of the film in relation to means for controlling the film thickness and / or basis weight and the light transmittance at a wavelength of 660 nm of the microporous PO film.

[0096] The MD and / or TD stretching ratio in the first stretching is preferably 5 times or more, more preferably 6 times or more. When the MD and / or TD stretching ratio in the first stretching is 5 times or more, the strength of the resulting PO microporous membrane tends to be further improved. Furthermore, the MD and / or TD stretching ratio in the first stretching is preferably 9 times or less, more preferably 8 times or less or 7 times or less. When the MD and / or TD stretching ratio in the first stretching is 9 times or less, breakage during stretching tends to be further suppressed. When biaxial stretching is performed, either sequential stretching or simultaneous biaxial stretching may be used. The stretching ratio in each axial direction is preferably 5 times or more and 9 times or less, more preferably 6 times or more and 8 times or less, or 6 times or more and 7 times or less, and a wet stretching ratio (wet stretching ratio) is even more preferred. It is also preferable to optimize the MD / TD stretching ratio ratio depending on the pore size or pore size distribution characteristics of the resulting PO microporous membrane.

[0097] The primary stretching temperature can be selected with reference to the composition and concentration of the raw material resins contained in the PO resin composition. The MD and / or TD stretching temperature is preferably within the range of 100°C to 135°C, more preferably 110°C to 130°C, even more preferably 115°C to 125°C, and still more preferably 116°C to 122°C. The stretching temperature in both MD and TD is preferably 100°C or higher from the viewpoint of preventing breakage, and is preferably 135°C or lower from the viewpoint of increasing film strength. The stretching temperature is preferably a wet stretching temperature (wet stretching temperature), and more preferably a biaxial stretching temperature.

[0098] [Extraction step (e)] The extraction step (e) is a step of extracting the pore-forming material from the primarily stretched membrane obtained in the primary stretching step (d) to obtain an extracted membrane. Examples of methods for removing the pore-forming material include immersing the primarily stretched membrane in an extraction solvent to extract the pore-forming material, followed by thorough drying. The method for extracting the pore-forming material may be either a batch method or a continuous method. Furthermore, it is preferable that the amount of the pore-forming material, particularly the plasticizer, remaining in the porous membrane is less than 1% by mass.

[0099] The extraction solvent used to extract the pore-forming material is preferably a poor solvent for the polyolefin resin and a good solvent for the pore-forming material or plasticizer, and has a boiling point lower than the melting point of the polyolefin resin. Such extraction solvents are not particularly limited, but include, for example, hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; non-chlorine-based 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 and reused by operations such as distillation.

[0100] [Heat setting process (f)] The heat-setting step (f) is a step of heat-setting the extracted film obtained in the extraction step (e) at a predetermined temperature. The heat treatment method at this time is not particularly limited, but examples thereof include a heat-setting method in which a tenter or a roll stretching machine is used to perform a stretching and relaxation operation.

[0101] The stretching operation in the heat-setting step (f) is an operation to stretch the PO microporous membrane in at least one of MD and TD, and may be performed in both MD and TD, or in only one of MD or TD. From the viewpoint of providing a PO microporous membrane that can achieve both improved cycle characteristics and suppressed short-circuit defects, it is preferable to heat-set at least in TD.

[0102] The MD and TD stretching ratios in the heat setting step (f) are preferably 1.70 to 2.40, more preferably 1.80 to 2.30. The MD and TD stretching ratios in step (f) are preferably 1.70 or more from the viewpoint of highly orienting the film, and are preferably 2.40 or less from the viewpoint of achieving both film thickness and film strength and suppressing film breakage. From the same viewpoint, the heat setting is preferably performed by dry uniaxial stretching.

[0103] The MD and / or TD stretching temperature during heat setting is preferably 128°C or higher from the viewpoint of adjusting the shutdown temperature and / or meltdown temperature, more preferably 129°C or higher from the viewpoint of the output characteristics of the electrochemical device, and even more preferably in the range of 130°C to 134°C from the viewpoint of preventing short-circuit defects and safety.

[0104] The relaxation operation in the heat-setting step (f) is an operation of shrinking the PO microporous membrane in at least one of MD and TD, and may be performed in both MD and TD, or only in MD or TD. The relaxation ratio in the heat-setting step (f) is preferably 0.95 or less, more preferably 0.93 or less. A relaxation ratio of 0.95 or less in step (f) tends to suppress thermal shrinkage. From the viewpoint of increasing the relaxation temperature, the relaxation ratio is preferably 0.50 or more, more preferably 0.70 or more. Here, the "relaxation ratio" refers to the value obtained by dividing the membrane dimensions after the relaxation operation by the membrane dimensions before the relaxation operation. When both MD and TD are relaxed, the relaxation ratio refers to the value obtained by multiplying the MD relaxation ratio by the TD relaxation ratio. Relaxation ratio = (membrane size after relaxation (m)) / (membrane size before relaxation (m))

[0105] The relaxation temperature in this relaxation operation is preferably 128°C or higher from the viewpoint of the relationship between the pore size and optical properties of the resulting microporous polypropylene membrane, more preferably 131°C or higher from the viewpoint of the cycle characteristics of the electrochemical device, and even more preferably in the range of 131°C to 134°C from the viewpoints of preventing short-circuit defects and safety.

[0106] The order of the steps (a) to (f) can be changed as desired as long as the effects of the present invention are not impaired. From the viewpoints of controlling the film thickness and / or basis weight and the light transmittance at a wavelength of 660 nm of the PO microporous membrane and improving film strength, the total stretch ratio of the PO microporous membrane after the steps (a) to (f) is preferably 70 to 120, more preferably 80 to 120, and even more preferably 90 to 120. Here, the "total stretch ratio" refers to the value obtained by multiplying the MD and / or TD stretch ratio in the primary stretching step (d) by the stretch ratio and / or relaxation ratio in the heat setting step.

[0107] [Other steps] The method for producing a PO microporous membrane of this embodiment may include steps other than the above steps (a) to (f). The other steps may include, but are not limited to, a lamination step of superposing multiple monolayer PO microporous membranes to obtain a laminated PO microporous membrane, in addition to the heat setting step. Alternatively, the method for producing a PO microporous membrane may include a peeling step of peeling the laminate obtained by coextrusion after steps (a) to (f) to obtain two or more monolayer membranes. The method for producing a PO microporous membrane of this embodiment may also include a surface treatment step of subjecting the surface of the PO microporous membrane to surface treatment such as electron beam irradiation, plasma irradiation, surfactant application, or chemical modification. Furthermore, an inorganic particle material may be applied to one or both surfaces of the PO microporous membrane to provide an inorganic material layer, or an adhesive layer containing a thermoplastic resin may be provided on the surface of the PO microporous membrane.

[0108] <Separators for electrochemical devices> The polyolefin microporous membrane according to this embodiment can be used as a separator for electrochemical devices such as lithium ion secondary batteries. When incorporated into a lithium ion secondary battery, the polyolefin microporous membrane can suppress thermal runaway in the lithium ion secondary battery.

[0109] <Electrochemical devices> An electrochemical device containing a wound or laminated body formed by winding or laminating the microporous polypropylene membrane according to this embodiment is also an aspect of the present invention. Examples of electrochemical devices include nonaqueous electrolyte batteries, nonaqueous electrolyte batteries, nonaqueous lithium-ion secondary batteries, nonaqueous gel secondary batteries, nonaqueous solid secondary batteries, lithium-ion capacitors, and electric double-layer capacitors.

[0110] The nonaqueous electrolyte battery according to this embodiment includes a separator for a nonaqueous electrolyte battery including the above-described microporous PO membrane, a positive electrode plate, a negative electrode plate, and a nonaqueous electrolyte solution (including a nonaqueous solvent and a metal salt dissolved therein). Specifically, for example, a positive electrode plate including a transition metal oxide capable of absorbing and desorbing lithium ions and a negative electrode plate also capable of absorbing and desorbing lithium ions are wound or stacked so as to face each other with the separator for a nonaqueous electrolyte battery interposed therebetween, and the nonaqueous electrolyte solution is held in a container.

[0111] The positive electrode plate will be described below. Examples of the positive electrode active material include lithium composite metal oxides such as lithium nickel oxide, lithium manganate, and lithium cobalt oxide, and lithium composite metal phosphates such as lithium iron phosphate. The positive electrode active material is mixed with a conductive agent and a binder, and the resulting paste is applied to a positive electrode current collector such as aluminum foil and dried. The paste is then rolled to a predetermined thickness and cut to a predetermined size to form a positive electrode plate. The conductive agent can be a metal powder that is stable under the positive electrode potential, such as carbon black or graphite, such as acetylene black. The binder can be a material that is stable under the positive electrode potential, such as polyvinylidene fluoride, modified acrylic rubber, or polytetrafluoroethylene.

[0112] The negative electrode plate will be described below. A material capable of absorbing lithium can be used as the negative electrode active material. Specifically, at least one material selected from the group consisting of graphite, silicide, and titanium alloy materials can be used. Furthermore, the negative electrode active material of a nonaqueous electrolyte secondary battery can be, for example, a metal, a metal fiber, a carbon material, an oxide, a nitride, a silicon compound, a tin compound, or various alloy materials. Silicon compounds or tin compounds, such as simple substances, alloys, compounds, and solid solutions of silicon (Si) or tin (Sn), are particularly preferred because they tend to increase the capacity density of the battery.

[0113] Examples of carbon materials include various natural graphites, coke, partially graphitized carbon, carbon fiber, spherical carbon, various artificial graphites, and amorphous carbon.

[0114] As the negative electrode active material, one of the above materials may be used alone, or two or more may be used in combination. The negative electrode active material is mixed with a binder, and the paste is applied to a negative electrode current collector such as copper foil, dried, rolled to a predetermined thickness, and then cut to a predetermined size to form a negative electrode plate. Here, the binder may be a material that is stable under the negative electrode potential, such as PVDF or a styrene-butadiene rubber copolymer.

[0115] The nonaqueous electrolyte is described below. Nonaqueous electrolytes generally contain a nonaqueous solvent and a metal salt, such as a lithium salt, a sodium salt, or a calcium salt, dissolved therein. Examples of nonaqueous solvents that can be used include cyclic carbonates, chain carbonates, and cyclic carboxylic acid esters. Examples of lithium salts include LiPF6, LiClO4, LiBF4, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, Li(CF3SO2)2, LiAsF6, lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, borates, and imide salts.

[0116] The above-mentioned various parameters are measured according to the measurement methods in the examples described below, unless otherwise specified. [Example]

[0117] Next, the present embodiment will be described in more detail with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples as long as it does not deviate from the gist of the present invention. The physical properties in the examples were measured by the following methods. Unless otherwise specified, measurements were taken at room temperature of 23°C and humidity of 40%.

[0118] (1)Molecular weight (1a) High-temperature GPC of membrane: Measurement of weight-average molecular weight (Mw) and number-average molecular weight (Mn) (GPC-relative method) Sample preparation The polyolefin raw material was weighed, and the eluent 1,2,4-trichlorobenzene (TCB) was added to a concentration of 1 mg / ml. Using a high-temperature dissolver, the mixture was left standing at 160°C for 30 minutes, then shaken at 160°C for 1 hour, and visual inspection confirmed that the sample had completely dissolved. While still at 160°C, the sample solution was filtered through a 0.5 μm filter, and the filtrate was used as a GPC measurement sample. GPC measurement The GPC apparatus used was a Waters ALC / GPC-150-C-plus model (trademark), with two 30 cm columns of GMH6-HT (trademark) and two 30 cm columns of GMH6-HTL (trademark) manufactured by Tosoh Corporation connected in series. GPC measurements were performed at 140°C with a sample concentration of 0.05 wt% using orthodichlorobenzene as the mobile phase solvent. A calibration curve was prepared using commercially available monodisperse polystyrene with a known molecular weight as a standard substance, and the molecular weight distribution data of each sample converted to polystyrene was multiplied by 0.43 (Q factor of polyethylene / Q factor of polystyrene = 17.7 / 41.3) to obtain the molecular weight distribution data converted to polyethylene. From this, the weight average molecular weight (Mw) and number average molecular weight (Mn) of each sample were calculated, and the molecular weight distribution (Mw / Mn) was also obtained.

[0119] (1b) Viscosity average molecular weight (Mv) The intrinsic viscosity [η] (dl / g) in decalin solvent at 135°C was determined according to ASTM-D4020. For the microporous PO membrane and the polyethylene raw material, Mv was calculated using the following formula: [η]=6.77×10 -4 Mv 0.67 For polypropylene raw materials, Mv was calculated using the following formula: [η]=1.10×10 -4 Mv 0.80

[0120] (2) Film thickness (μm) The thickness of the microporous PO membrane was measured at room temperature of 23±2°C using a micro thickness measuring instrument, KBM (trademark), manufactured by Toyo Seiki Co., Ltd.

[0121] (3) Porosity (%) A 10cm x 10cm square sample was cut from the PO microporous membrane, and its volume (cm 3 ) and mass (g), and then calculate the density (g / cm 3 ) the porosity was calculated using the following formula: Porosity (%) = (volume - mass / density) / volume x 100

[0122] (4) Air permeability (sec / 100cm 3 ) The air resistance in accordance with JIS P-8117 was taken as the air permeability. The air permeability of the PO microporous membrane was measured in accordance with JIS P-8117 using a Gurley air permeability meter, G-B2 (trademark), manufactured by Toyo Seiki Co., Ltd., in an atmosphere at a temperature of 23°C and a humidity of 40%, and the air permeability was determined as the air resistance.

[0123] (5) Puncture strength (gf) and area weight conversion puncture strength (gf / (g / m 2 )) Using a Kato Tech handy compression tester KES-G5 (trademark), a PO microporous membrane was fixed in a sample holder with an opening diameter of 11.3 mm. Next, a puncture test was performed on the center of the fixed microporous membrane using a needle with a tip diameter of 1.0 mm and a curvature radius of 0.5 mm at a puncture speed of 2 mm / sec under an atmosphere of 23°C and 40% humidity. The raw puncture strength (gf) was determined as the maximum puncture load, and the value converted to basis weight (gf / (g / m 2 )) was also calculated.

[0124] (6) Flux pore diameter (μm) The flux pore size was calculated from the permeation flow rate (Flux) measurement of the PO microporous membrane. It is known that the fluid inside the capillary follows Knudsen flow when the mean free path of the fluid is larger than the pore size of the capillary, and follows Poiseuille flow when it is smaller. Therefore, we assume that the air flow in measuring the air permeability of a porous membrane follows Knudsen flow, and that the water flow in measuring the water permeability of a porous membrane follows Poiseuille flow. In this case, the average pore diameter d (μm) of the porous membrane and the tortuosity τa (dimensionless) is the air permeation rate constant R gas (m 3 / (m 2 ·sec·Pa)), water permeation rate constant R liq (m 3 / (m 2 sec Pa), air molecular velocity ν (m / sec), water viscosity η (Pa sec), standard pressure P s (=101325 Pa), porosity ε (%), and film thickness L (μm) were calculated using the following formula: d=2ν×(R liq / R gas )×(16η / 3Ps)×10 6 τ a =(d×(ε / 100)×ν / (3L×P s ×R gas )) 1 / 2 where R gas was calculated from the air permeability (sec) using the following formula: R gas =0.0001 / (air permeability × (6.424 × 10 -4 )×(0.01276×101325)) Also, R liq is the permeability (cm 3 / (cm 2 sec Pa) using the following formula: Ta. R liq =water permeability / 100 The water permeability was determined as follows: A porous membrane that had been immersed in ethanol was placed in a stainless steel liquid permeation cell with a diameter of 41 mm, and after washing the ethanol off the membrane with water, water was passed through the membrane at a differential pressure of approximately 50,000 Pa. The amount of water permeated (cm 3 ) the amount of water permeable per unit time, unit pressure and unit area was calculated, and this was taken as the permeability. Also, ν is the gas constant R (= 8.314), absolute temperature T (K), pi (π), and the average molecular weight of air M (= 2.896 × 10 -2 kg / mol) using the following formula: ν=((8R×T) / (π×M)) 1 / 2

[0125] (7) Heat shrinkage rate at 120°C (%) The PO microporous membrane was cut into a 100 mm length in the MD direction and a 100 mm length in the TD direction and placed in an oven at a predetermined temperature (120°C or 150°C) for 1 hour. The sample was sandwiched between two sheets of paper to prevent direct exposure to hot air. The paper used was Fuji Xerox Interfield V-Paper monochrome copy / printer paper (basis weight 64 g / m). 2 After the sample was removed from the oven and cooled, the length (mm) was measured and the thermal shrinkage was calculated using the following formula. Measurements were taken in both the MD and TD directions. Heat shrinkage rate (%) = {(100 - length after heating) / 100} x 100

[0126] (8) Maximum contraction stress of TMA (gf) The thermal shrinkage of the sample was measured using a Shimadzu TMA50 (trademark). To measure the value in the MD (TD) direction, a sample cut to a width of 3 mm in the TD (TD) direction was fixed to a chuck with a 10 mm inter-chuck distance and set on a dedicated probe. The sample was heated from 30°C to 200°C at a heating rate of 10°C / min under an initial load of 1.0 g and in constant length measurement mode. The load (gf) generated at this time was measured, and the maximum value was taken as the MD (or TD) maximum thermal shrinkage stress (gf).

[0127] (9) Cross-sectional SEM observation and pore size analysis Cross-sectional SEM image of a resin-embedded sample The microporous PO membrane was stained with ruthenium and embedded in room-temperature curing epoxy resin. Smooth cross sections parallel to the MD and TD were then prepared using a broad ion beam (BIB). These cross sections were then observed under a scanning electron microscope (SEM) at 7000x magnification to obtain SEM images. From the smooth cross section, multiple SEM images can be obtained from multiple fields of view, or one SEM image can be obtained from one field of view. Specifically, the SEM image acquisition position can be determined as follows: SEM field of view, when the entire field of view is captured at 7000x: (1)-1: Obtain an image containing the full thickness; (1)-2: A field of view excluding 3% of the total thickness from both outermost surfaces is cut out and used for image analysis as described below. If the SEM field of view does not fit the entire field of view at 7000x magnification: (2)-1: Obtain an image of the entire screen centered on the center of the membrane; (2)-2: From the obtained image, a field of view excluding 3% thickness from both ends is cut out and used for the image analysis described below. When acquiring multiple SEM images, capture them at 50 μm intervals along the longitudinal direction of the smooth cross section.

[0128] Image analysis and pore size analysis The obtained SEM images were subjected to median filtering under a radius of 2.0 pixels and then binarized using the Otsu method under threshold conditions to calculate the area ratio of black and / or white areas. The binarized images were then subjected to pore size analysis using the local thickness method to calculate the pore size distribution, average pore size, maximum pore size, and other parameters. The local thickness method can be performed using the thickness analysis function of "BoneJ," a plugin for the image processing software "Image J." The size of the largest circle that fits within a given area can be defined as the spatial size. Therefore, even if there are no independent structures or spaces, spatial size can be defined and the pore size distribution, average pore size, maximum pore size, and other parameters can be calculated.

[0129] Pore ​​size analysis results When the pore size was analyzed by the local thickness method for at least one SEM image of each smooth cross section parallel to the MD and TD prepared by BIB as described above, the average pore size d MD and d TD , pore size distribution D MD and D TD , pore size distribution D MD Standard deviation and pore size distribution D TD Standard deviation of the maximum pore size PS MD(max) and P.S. TD(max) etc. can be calculated.

[0130] When five SEM images of smooth cross sections parallel to MD and TD prepared by BIB as described above were analyzed for pore size by the Local Thickness method, at least one of the pore size distributions D TD The total proportion of pore diameters of 180 nm or less in at least one pore size distribution D MD The total percentage of pores with diameters of 180 nm or less, and the average pore diameter d of five sheets MD(N=5) and the average pore diameter d of five sheets TD(N=5) The standard deviation etc. can be calculated.

[0131] (10) Measurement of optical properties A 5cm x 2.5cm sample was cut from the center of the TD of the microporous PO membrane, and from three randomly selected locations in the MD. Using a JASCO V-630 UV-Vis-NIR spectrophotometer, the sample was measured from 390 to 850nm, with a scanning interval of 1nm and a scanning speed of 400nm / min. The transmittance (%T) at 660nm was calculated from the measured transmittance (%T) in 1nm increments.

[0132] (11) Defect detection accuracy The inspection area of ​​the PO microporous membrane was approximately 1000 to 1300 mm in width and approximately 5000 to 7000 mm in length, and the number of defects was counted using an optical foreign matter inspection device. The defect detection accuracy was evaluated according to the following criteria. Note that the size and density of the defects were not taken into consideration, and only the number of dark defects such as unmelted material, carbides, and foreign matter was counted. A: It can count over 2,000 defects. B: Can count more than 1,000 defects. C: Can count more than 500 defects. D: Not countable ×: Uncountable and also rated D or E in the nail penetration test described below

[0133] (12) Nail penetration test a. Preparation of the positive electrode A slurry was prepared by dispersing lithium-cobalt composite oxide (LiCoO2) as the positive electrode active material, graphite, and acetylene black as conductive materials in polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP) as binders. This slurry was applied to a 15 μm thick aluminum foil positive electrode current collector using a die coater, dried at 130 °C for 3 minutes, and then compression molded using a roll press. The resulting molded body was slit to a width of 57.0 mm to obtain a positive electrode.

[0134] b. Preparation of negative electrode A slurry was prepared by dispersing artificial graphite as the negative electrode active material and carboxymethylcellulose ammonium salt and styrene-butadiene copolymer latex as the binder in purified water. This slurry was applied to a copper foil negative electrode current collector using a die coater, dried at 120°C for 3 minutes, and then compression molded using a roll press. The resulting molded body was slit to a width of 58.5 mm to obtain a negative electrode.

[0135] c. Preparation of non-aqueous electrolyte A non-aqueous electrolyte solution was prepared by dissolving LiPF6 as a solute in a mixed solvent of ethylene carbonate: dimethyl carbonate: ethyl methyl carbonate = 1:1:2 (volume ratio) to a concentration of 1 mol / L.

[0136] d. Battery assembly After winding the positive electrode, the PO porous membrane obtained in the Examples or Comparative Examples, and the negative electrode, a wound electrode assembly was prepared by a conventional method and pressed using a press to fit into an outer can. The number of windings was adjusted depending on the thickness and springback of the PO microporous membrane. The outermost periphery of the resulting wound electrode assembly was fixed by attaching insulating tape. The negative electrode lead was welded to the battery can, and the positive electrode lead was welded to the safety valve, and the wound electrode assembly was inserted into the battery can. 5 g of nonaqueous electrolyte was then injected into the battery can, and the lid was crimped to the battery can via a gasket, yielding a prismatic secondary battery measuring 42.0 mm in width, 63.0 mm in height, and 10.5 mm in thickness. This prismatic secondary battery was charged at a current of 0.2 C (0.2 times the 1-hour rate (1 C) of the rated electrical capacity) in an atmosphere of 25°C up to a battery voltage of 4.2 V. After reaching this voltage, the current was reduced to maintain the battery voltage at 4.2 V. This method allowed for a total of 3 hours of charging. Subsequently, the battery was discharged at a current of 0.2C until the battery voltage reached 3.0V.

[0137] e. Nail penetration safety test The battery assembled in step d above and selected for evaluation was placed on a steel plate in a temperature-controlled explosion-proof booth. A 3.0 mm diameter iron nail was prepared and a thermocouple was installed inside the nail. In the explosion-proof booth, at 30°C and under pressure of 3 MPa, the iron nail was driven through the center of the battery at a speed of 2 mm / sec, and the nail was maintained in the penetrated state. The battery was observed from the start of nail penetration until after the nail had penetrated, and safety was evaluated according to the following criteria. A: Nothing will happen. B: Smoke. C: Cell swelling and deformation are observed. D: Ignite. ×: Explosion occurs and the defect detection accuracy is rated D

[0138] [Example 1] PE1 was charged into a Henschel mixer according to the polyethylene (PE) type shown in Table 1 and the PE type and raw material composition ratio shown in Table 2, and an appropriate amount of pentaerythrityl tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] was added as an antioxidant and premixed. The resulting mixture was fed to the feed port of a twin-screw co-rotating screw extruder using a feeder. Liquid paraffin (LP) was added to the twin-screw extruder cylinder via side feed in two batches so that the total amount of liquid paraffin (LP) in the melt-kneaded, extruded mixture (100.1 parts by mass) was 76.0 parts by mass.

[0139] The extrusion and casting processes were carried out under the conditions shown in Table 2 to obtain a sheet-like molded product having a thickness of 1800 μm.

[0140] The obtained sheet-like molded product was introduced into a simultaneous biaxial stretching machine to obtain a primarily stretched membrane (primary stretching step). The set stretching conditions were as shown in Table 2. The obtained primarily stretched membrane was then introduced into a methylene chloride tank and thoroughly immersed to extract and remove the liquid paraffin, which is a plasticizer, and then the methylene chloride was dried and removed to obtain an extracted membrane.

[0141] The extracted membrane was then introduced into a TD uniaxial tenter for heat setting. In the heat setting process, stretching was performed under the TD stretching temperature and TD stretching ratio conditions shown in Table 2, followed by relaxation under the relaxation temperature and relaxation ratio conditions after stretching. The various properties of the resulting microporous PO membrane were evaluated by the above-mentioned methods. The membrane production conditions are shown in Table 2, and the results are shown in Table 4.

[0142] [Examples 2 to 6 and Comparative Examples 1 to 8] A microporous PO membrane was obtained in the same manner as in Example 1, except that the resin raw material types, raw material composition ratios, and membrane formation conditions were set as shown in Tables 2 and 3, respectively. The properties of the obtained microporous PO membrane were evaluated by the above-mentioned methods. The results are shown in Tables 4 and 5.

[0143] [Table 1]

[0144]

Table 2

[0145]

Table 3

[0146]

Table 4

[0147]

Table 5

Claims

1. A polyolefin microporous membrane containing a polyolefin resin as a main component, which has the following properties (1) and (2): (1) Basis weight 4.0 g / m ２ That's it; (2) The film thickness is 6 μm or more; At least one of the following is satisfied, and The polyolefin microporous film has a light transmittance of 10% or more at a wavelength of 660 nm.

2. The polyolefin microporous membrane was stained with ruthenium and embedded in a room-temperature curing epoxy resin. A smooth cross section parallel to the TD was then prepared using a broad ion beam (BIB). At least one scanning electron microscope (SEM) image of the smooth cross section was taken at 7000x magnification, and pore size analysis was performed using the local thickness method. The pore size distribution D ＴＤ The polyolefin microporous membrane according to claim 1, wherein the total proportion of pores having a pore size of 180 nm or less is 90% or less of the total.

3. When the pore diameter of five SEM images was analyzed, the average pore diameter d ＴＤ（Ｎ＝５） The polyolefin microporous membrane according to claim 2, wherein the standard deviation of the average particle diameter is 0.1 nm or more and 1.0 nm or less.

4. The polyolefin microporous membrane has a puncture strength converted into basis weight of 80 gf / (g / m ２ ) or more 150gf / (g / m ２ 4. The polyolefin microporous membrane according to claim 2 or 3, wherein the molecular weight is 1000 or less.

5. The polyolefin microporous membrane according to claim 1 or 2, wherein the polyolefin microporous membrane has a viscosity average molecular weight (Mv) of 800,000 or more.

6. 3. The polyolefin microporous membrane according to claim 1, wherein the polyolefin microporous membrane has a molecular weight distribution (Mw / Mn) expressed as the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of 5 or more and 10 or less.

7. 3. The polyolefin microporous membrane according to claim 1, wherein the polyolefin microporous membrane has a porosity measured at a temperature of 30°C of 30% or more and 65% or less.

8. The polyolefin microporous membrane according to claim 1 or 2, wherein the membrane thickness is 6 μm or more and 12 μm or less.

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