Polyolefin microporous membrane, battery separator, secondary battery, liquid filter, and filter unit

Abstract

Provided is a polyolefin microporous membrane which, when used as a battery separator, can impart high output characteristics and high safety, and which, when used as a liquid filter, can eliminate the tradeoff between small pore diameter and high permeability. When the arithmetic average roughness Sa (nm) of one surface and the other surface of the microporous membrane is set as Sa1 and the arithmetic average roughness Sa of the surface having a large arithmetic average roughness is set as Sa2, formula 1 and formula 2 are satisfied. When the intermediate value of the pore diameter based on the surface area determined by a water squeezing type porosity analyzer is set as Ds and the intermediate value of the pore diameter based on the pore volume is set as Dv, formula 3 is satisfied, and the puncture strength converted into the basis weight is 700 mN / (g / m < 2 >) or more. Formula 1: 10 < = (Sa1+Sa2) / 2 < = 80 Formula 2: Sa1 / Sa2 < = 1.4 Formula 3: 1.0 < = Dv / Ds < = 1.2

Description

TECHNICAL FIELD

[0001] The present application relates to a separation membrane used in separation, selective permeation, and the like of a substance, and a polyolefin microporous membrane widely used as a separator material for an electrochemical reaction device such as an alkaline battery, a lithium secondary battery, a fuel cell, a capacitor, and the like. In addition, the present application also relates to a battery separator, a secondary battery, a filter for liquids, and a filtration unit. BACKGROUND

[0002] The polyolefin microporous membrane is mainly used as a filter, a separator for a fuel cell, a separator for a capacitor. It is particularly suitable for use as a separator for a non-aqueous electrolyte secondary battery such as a lithium ion battery, which is widely used in notebook personal computers, mobile phones, and the like. The reasons can be listed as follows: the polyolefin microporous membrane has excellent mechanical strength, shutdown temperature, and ion permeability.

[0003] In addition, the polyolefin microporous membrane, due to its uniform fine pore structure and excellent solvent resistance and chemical resistance characteristics, is also widely used in various filter applications such as water treatment membranes, ultrafiltration membranes, precision filtration membranes, and waterproof clothing.

[0004] The lithium ion secondary battery, in various applications, has an increasing demand for miniaturization and high output power from the aspect of convenience, and in conjunction therewith, a separator is required to be thin and low in resistance. The strength of the separator is reduced due to the thinning and low resistance, and thus short circuiting caused by electrodes and foreign matter (reduction in resistance to foreign matter) and breakage of the separator when the battery is impacted (reduction in impact resistance) are easily caused, and the safety of the battery is reduced. Therefore, it is necessary to design the resin that constitutes the microporous membrane to have higher strength than in the past.

[0005] In addition, in a lithium ion secondary battery of small size and high output power, lithium deposition (dendrite) is easily caused, and thus the demand for the resistance to dendrite of the separator is increasing. The dendrite of the lithium ion secondary battery refers to needle-shaped crystals generated near the interface between the negative electrode and the separator along with charging and discharging, and if it grows to penetrate the separator, it can cause short circuiting, and thus in a battery in which dendrite is easily caused, a separator having a small pore diameter in which dendrite is difficult to penetrate is required. On the other hand, if the pore diameter of the microporous membrane is reduced, the permeability is sometimes reduced, and the resistance is deteriorated.

[0006] Furthermore, the separator also needs to have a function of ensuring safety when the battery abnormally heats. The above shutdown refers to a function in which the polyolefin microporous membrane melts to block the pores and block the current when the battery abnormally heats, and in particular, in a battery designed for high output, the risk of abnormal heating is high, and thus it is preferable to perform shutdown at a lower temperature. On the other hand, in order to reduce the shutdown temperature of the polyolefin microporous membrane, it is generally designed to reduce the crystalline orientation and crystallinity of the polyolefin, and thus the strength and the permeability are sometimes reduced.

[0007] With respect to the filter for liquid use, for example, in the case of removal of foreign matter for a semiconductor manufacturing process, it is required to efficiently capture smaller-sized foreign matter. On the other hand, from the viewpoint of the processing capacity, deterioration of the permeability is disadvantageous.

[0008] As described above, with the downsizing and high output power of lithium ion secondary batteries, it is required to balance the low resistance and high safety, which are opposite properties, at a higher level. For example, Patent Literature 1 discloses a laminated porous polyolefin film in which the safety and output characteristics are excellent when used as a separator by setting the relationship between the puncture strength and the porosity and the relationship between the melting point and the shutdown temperature within a prescribed range. In addition, Patent Literature 2 describes a polyolefin microporous film in which the cycle characteristics and the output characteristics are excellent and the safety can be ensured by specifying the lithium ion diffusion coefficient in the film thickness direction and the shutdown temperature. In addition, Patent Literature 3 describes a porous polyolefin film having excellent output characteristics, safety, and process transportability in which the porosity and the puncture strength, and the tensile modulus in the length direction are specified.

[0009] Prior Art Documents

[0010] Patent Literature

[0011] Patent Literature 1: Japanese Patent Application Publication No. 2019-143142

[0012] Patent Literature 2: Japanese Patent Application Publication No. 2020-084084

[0013] Patent Literature 3: Japanese Patent Application Publication No. 2021-038379 SUMMARY

[0014] PROBLEMS TO BE SOLVED BY THE INVENTION

[0015] The microporous film described in Patent Literature 1 is designed by laminating a layer (shutdown layer) having a low melting point to improve the shutdown characteristics, but when the microporous film is thinned, the weight per unit area of the shutdown layer is insufficient, and sometimes even if the pores are closed by melting, the safety cannot be ensured. In addition, for the microporous film described in Patent Literature 2, the macroporosity and the strength reduction due to the blending of inorganic particles can make it impossible to ensure the high level of safety required in recent years. In addition, the microporous film described in Patent Literature 3 has a high dry stretching ratio and is high in strength, but generally in the case of a high dry stretching ratio, not only the microporous film macroporosity but also the shutdown temperature rise becomes a problem.

[0016] As described above, the object of the present invention is to provide a polyolefin microporous membrane that, when used as a battery separator, can impart high output power characteristics and high safety, thus contributing to the miniaturization and high output power of batteries, and when used as a liquid filter, can eliminate the trade-off between small pore size and high permeability.

[0017] Problem-solving methods

[0018] To solve the above problems and achieve the objective, the present invention has the following structure. It should be noted that, in the following description, the numerical range indicated by "~" refers to the range including the values ​​before and after "~" as both the lower and upper limits.

[0019] [1] A polyolefin microporous membrane, wherein, regarding the arithmetic mean roughness Sa (nm) of one surface and another surface of the microporous membrane, when Sa of the side with larger arithmetic mean roughness is set as Sa1 and Sa of the side with smaller arithmetic mean roughness is set as Sa2, Equations 1 and 2 are satisfied; when the median value of the pore diameter based on the surface area determined by a water-dip porosity analyzer is set as Ds and the median value of the pore diameter based on the pore volume is set as Dv, Equation 3 is satisfied, and the puncture strength converted to unit area weight is 700 mN / (g / m²). 2 )above.

[0020] Equation 1: 10 ≤ (Sa1 + Sa2) / 2 ≤ 80

[0021] Equation 2: Sa1 / Sa2 ≤ 1.4

[0022] Equation 3: 1.0 ≤ Dv / Ds ≤ 1.2

[0023] [2] According to the polyolefin microporous membrane described in [1], in the three-dimensional structure of the microporous membrane with a diameter of 4 μm obtained by focused ion beam scanning electron microscopy (FIB-SEM), it has a density of 1.0 × 10⁻⁶. 8 nm 3 The resin portion of the above volume is less than 10.

[0024] [3] A polyolefin microporous membrane, in which the three-dimensional structure of the microporous membrane, measured by focused ion beam scanning electron microscopy (FIB-SEM), has a density of 1.0 × 10⁻⁶ m². 8 nm 3 The resin portion of the above volume is less than 10.

[0025] [4] According to the polyolefin microporous membrane described in [1] or [3], in the 4 μm square three-dimensional structure of the microporous membrane obtained by FIB-SEM measurement, the average volume of the resin portion is 1.3 × 10⁻⁶. 7 nm3 the following.

[0026] [5] The polyolefin microporous membrane according to any one of [1] to [4] has an average pore size of 35 nm or less as measured by a pore size analyzer.

[0027] [6] The polyolefin microporous membrane according to any one of [1] to [5] has a shut-off temperature of 140°C or below.

[0028] [7] The standard deviation of the orientation parameter values ​​of a total of 12 points measured by Raman spectroscopy at 15° intervals relative to any direction within the microporous membrane surface of the polyolefin microporous membrane according to any one of [1] to [6] is 0.30 or less.

[0029] [8] The polyolefin microporous membrane according to any one of [1] to [7] has a melting point of 134°C or less as determined by differential scanning calorimetry (DSC).

[0030] [9] The polyolefin microporous membrane according to any one of [1] to [8] satisfies the following formula 4 when the molecular weight differential distribution curve determined by gel permeation chromatography (GPC) is set as M1, the largest molecular weight with a height of 20% relative to the maximum value in the range of molecular weight from 10,000 to 10,000,000, and the smallest molecular weight is set as M2.

[0031] Formula 4: M1 / M2 ≥ 500

[0032]

[10] The polyolefin microporous membrane according to any one of [1] to [9] has, on the molecular weight differential distribution curve determined by gel permeation chromatography (GPC), the area ratio of molecular weight components with a molecular weight of 50,000 or less is 10% or more relative to the peak area of ​​all molecular weight components, and the area ratio of molecular weight components with a molecular weight of 1 million or more is 10% or more.

[0033]

[11] A battery separator, wherein a polyolefin microporous membrane as described in any one of [1] to

[10] is used.

[0034]

[12] A secondary battery in which the battery separator described in

[11] is used.

[0035]

[13] A polyolefin microporous membrane, wherein the standard deviation of orientation parameter values ​​measured at 12 points at 15° intervals relative to any direction within the microporous membrane surface using Raman spectroscopy is less than 0.3, and the puncture strength per unit area weight is 700 mN / (g / m²). 2 )above.

[0036]

[14] A liquid filter wherein a polyolefin microporous membrane as described in any one of [1] to

[10] is used.

[0037]

[15] A filtration unit in which the liquid filter described in

[14] is used.

[0038] Invention Effects

[0039] According to the present invention, a polyolefin microporous membrane can be provided, which, when used as a battery separator, can impart high output power characteristics and high safety, contributing to battery miniaturization and high output power. Furthermore, due to its excellent filtration accuracy and permeability when used as a liquid filter, it is suitable for use as a high-precision filter in semiconductor processes and other applications requiring the removal of minute foreign matter. Detailed Implementation

[0040] The embodiments of the present invention will now be described. It should be noted that the present invention is not limited to the embodiments described below.

[0041] [Polyolefin microporous membrane]

[0042] As one embodiment of the polyolefin microporous membrane according to the present invention, a polyolefin microporous membrane is provided. Regarding the arithmetic mean roughness Sa (nm) of one surface and another surface of the microporous membrane, when Sa is set as Sa1 for the side with larger arithmetic mean roughness and Sa2 for the side with smaller arithmetic mean roughness, equations 1 and 2 are satisfied. When the median value of the pore diameter based on the surface area determined by a water-dip porosity analyzer is set as Ds and the median value of the pore diameter based on the pore volume is set as Dv, equation 3 is satisfied, and the puncture strength converted to unit area weight is 700 mN / (g / m²). 2 )above.

[0043] Equation 1: 10 ≤ (Sa1 + Sa2) / 2 ≤ 80

[0044] Equation 2: Sa1 / Sa2 ≤ 1.4

[0045] Equation 3: 1.0 ≤ Dv / Ds ≤ 1.2

[0046] As one embodiment of the polyolefin microporous membrane according to the present invention, a polyolefin microporous membrane is provided. Regarding the arithmetic mean roughness Sa (nm) of one surface and the other surface of the microporous membrane, when Sa is set as Sa1 for the side with a larger arithmetic mean roughness and Sa2 for the side with a smaller arithmetic mean roughness, the average value obtained by (Sa1+Sa2) / 2, i.e., the average value of the arithmetic mean roughness of the surface and the back surface, is 10 nm or more and 80 nm or less. Preferably, it is 10 nm or more and 70 nm or less, more preferably 10 nm or more and 60 nm or less, further preferably 10 nm or more and 50 nm or less, and particularly preferably 10 nm or more and 45 nm or less. By setting (Sa1+Sa2) / 2 to the above range, the microporous membrane becomes smooth macroscopically and has a uniform and fine pore structure microscopically, thus enabling the fabrication of a membrane with excellent shut-off performance, dendrite resistance, and low resistance. Furthermore, when used as a liquid filter, a polyolefin microporous membrane with excellent filtration accuracy and permeability can be fabricated. The arithmetic mean roughness Sa can be specifically measured using the method described in the examples. As a method for setting the value of (Sa1+Sa2) / 2 to the above range, examples include controlling the molecular weight distribution of the microporous membrane, or setting the raw material composition and membrane forming conditions to the ranges described later.

[0047] As one embodiment of the polyolefin microporous membrane according to the present invention, a polyolefin microporous membrane with a Sa1 / Sa2 value of 1.4 or less can be cited. Preferably, it is 1.35 or less, more preferably 1.3 or less, further preferably 1.2 or less, and particularly preferably 1.1 or less. By setting Sa1 / Sa2 to the above range, the structural difference between the surface and back of the microporous membrane is reduced, which can reduce the membrane resistance. In addition, when used as a liquid filter, the permeability can be improved. According to the above, it can be seen that there is no particular limitation on the lower limit of Sa1 / Sa2, but considering the productivity of the microporous membrane, it is, for example, 1.001 or more. As a method to set the Sa1 / Sa2 value to the above range, examples include controlling the molecular weight distribution of the microporous membrane, or keeping the raw material composition and membrane forming conditions within the range described below.

[0048] As one embodiment of the polyolefin microporous membrane according to the present invention, a polyolefin microporous membrane is provided in which, when the median value of the pore diameter based on the surface area determined by a water-extruded porosity analyzer is set as Ds, and the median value of the pore diameter based on the pore volume is set as Dv, the Dv / Ds ratio is 1.0 or more and 1.2 or less. Preferably, it is 1.0 or more and 1.15 or less, more preferably 1.0 or more and 1.1 or less, and even more preferably 1.0 or more and 1.05 or less. By setting Dv / Ds to the above range, coarse pores are reduced, thereby forming a microporous membrane with excellent closure performance and dendrite resistance. In addition, when used as a liquid filter, the filtration accuracy can be improved. As a method for setting Dv / Ds to the above range, methods include controlling the molecular weight distribution of the microporous membrane, or setting the raw material composition and membrane forming conditions within the ranges described below.

[0049] As one embodiment of the polyolefin microporous membrane involved in the present invention, an example is a puncture strength of 700 mN / (g / m²) calculated based on unit area weight. 2 Polyolefin microporous membranes with a strength of 800 mN / (g / m³) or higher. Preferably, the strength is 800 mN / (g / m³). 2 ) or above, more preferably 900 mN / (g / m 2 The value is above 1000 mN / (g / m²), and more preferably 1000 mN / (g / m²). 2 The above range indicates that by setting the puncture strength, calculated per unit area weight, within this range, a diaphragm with excellent safety can be manufactured. Furthermore, when used as a liquid filter, it can suppress deformation of the pore structure due to pressure during filtration, improving filtration accuracy. Based on the above, there is no particular upper limit to the puncture strength calculated per unit area weight, but considering thermal shrinkage, a value of 3000 mN / (g / m²) is appropriate. 2 The above. In order to set the value of puncture intensity converted by unit area weight to the above range, it is preferable to set the raw material composition and membrane preparation conditions of the microporous membrane to the range described below.

[0050] The average pore size of the polyolefin microporous membrane according to embodiments of the present invention is preferably 35 nm or less, more preferably 32 nm or less, further preferably 28 nm or less, and particularly preferably 25 nm or less. By setting the average pore size within the above range, a microporous membrane with excellent strength and dendrite resistance can be produced. Furthermore, there is no particular limitation on the lower limit of the average pore size, but considering the permeability of the microporous membrane, it is preferably 14 nm or more, more preferably 18 nm or more.

[0051] The shut-off temperature of the polyolefin microporous membrane according to embodiments of the present invention is preferably below 140°C, more preferably below 139°C, further preferably below 138°C, and particularly preferably below 137°C. By setting the shut-off temperature within the above range, excellent low-temperature shut-off performance is achieved, thereby improving the safety of the membrane. Considering the above aspects, there is no particular limitation on the lower limit of the shut-off temperature, but considering both the permeability and strength of the microporous membrane, it is preferably above 125°C.

[0052] As one embodiment of the polyolefin microporous membrane according to the present invention, an example is a polyolefin microporous membrane in which the standard deviation of the orientation parameter values ​​measured at 12 points at 15° intervals relative to any direction within the microporous membrane surface using Raman spectroscopy is less than 0.30. The orientation parameter of the polyolefin microporous membrane based on Raman spectroscopy can be calculated by the following formula.

[0053] I=I 1130 / I 1063

[0054] I: Orientation parameter

[0055] I 1130 1130cm -1 Peak intensity at

[0056] I 1063 1060cm -1 Peak intensity at

[0057] The standard deviation of the orientation parameter is preferably 0.25 or less, more preferably 0.20 or less, and particularly preferably 0.15 or less. By setting the standard deviation of the orientation parameter within the above range, the fibrils (resin portion) constituting the microporous membrane are uniformly oriented within the membrane surface, thus achieving a uniform pore structure. When used as a battery separator, a microporous membrane with excellent shut-off properties, dendrite resistance, and permeability can be produced. Furthermore, when used as a liquid filter, a polyolefin microporous membrane with excellent filtration accuracy and permeability can be produced. Considering the above aspects, the lower limit of the standard deviation of the orientation parameter is not particularly limited, but from the perspective of balancing productivity, 0.01 or more is preferred. To set the standard deviation of the orientation parameter within the above range, it is preferable that the raw material composition and membrane-forming conditions of the microporous membrane are within the range described later.

[0058] The melting point of the polyolefin microporous membrane according to embodiments of the present invention, as determined by DSC, is preferably 134°C or lower, more preferably 133°C or lower, and even more preferably 132°C or lower. By setting the melting point within the above range, excellent shut-off characteristics are achieved, and the pore structure of the microporous membrane can be made uniform and fine. Considering both the permeability and strength of the microporous membrane, the lower limit of the melting point is preferably 125°C or higher, more preferably 127°C or higher, and even more preferably 129°C or higher. Methods for setting the melting point of the polyolefin microporous membrane within the above range include controlling the molecular weight distribution of the microporous membrane and ensuring the raw material composition is within the range described later.

[0059] In the polyolefin microporous membrane of the present invention, when the largest molecular weight (M1) in the range of 10,000 to 10,000,000 with a height of 20% relative to the maximum value on the molecular weight differential distribution curve determined by GPC is set, and the smallest molecular weight (M2) is set, the M1 / M2 ratio is preferably 500 or higher. More preferably, M1 / M2 is 600 or higher, further preferably 700 or higher, and particularly preferably 800 or higher and 1000 or lower. By setting M1 / M2 within the above range, it is easy to balance the closing characteristics and strength of the polyolefin microporous membrane, easy to adjust the standard deviation of the orientation parameters within the polyolefin microporous membrane surface to a preferred range, and easy to achieve a 1.3 × 10⁻⁶ m² three-dimensional structure in the 4 μm square microporous membrane obtained by FIB-SEM measurement. 7 nm 3 The amount of resin portion of the above volume is adjusted to a preferred range. In order to set the M1 / M2 of the polyolefin microporous membrane to the above range, it is preferable to make the raw material composition and mixing conditions of the microporous membrane within the range described later.

[0060] The polyolefin microporous membrane according to embodiments of the present invention preferably has an area ratio of 50,000 or less for molecular weight differential distribution curves determined by gel permeation chromatography (GPC) as described later, relative to the peak area of ​​all molecular weight components, with a molecular weight of 1,000,000 or more being 10% or more. More preferably, the area ratio of molecular weight of 50,000 or less is 15% or more, and even more preferably 20% or more. Furthermore, the area ratio of molecular weight of 50,000 or less is preferably 35% or less, more preferably 30% or less, and even more preferably 25% or less. The area ratio of molecular weight of 1,000,000 or more is more preferably 15% or more, and even more preferably 20% or more. Furthermore, the area ratio of molecular weight of 1,000,000 or more is preferably 35% or less, more preferably 30% or less, and even more preferably 25% or less. By setting the amount of molecular weight of 50,000 or less and molecular weight of 1,000,000 or more in the polyolefin microporous membrane to the above range, it is easy to balance the closure characteristics and strength of the polyolefin microporous membrane, and it is easy to adjust the standard deviation of the orientation parameters within the polyolefin microporous membrane surface to a preferred range.

[0061] As one embodiment of the polyolefin microporous membrane according to the present invention, an example is a microporous membrane with a 4 μm square three-dimensional structure obtained by FIB-SEM measurement, having a density of 1.0 × 10⁻⁶. 8 nm 3 The resin portion of the above volume is a polyolefin microporous membrane with 10 or fewer particles. More preferably, it has 8 or fewer particles, even more preferably 5 or fewer particles, and particularly preferably 1 or fewer particles. This is achieved by having a resin portion of 1.0 × 10⁻⁶ particles. 8 nm 3 The amount of resin portion in the above-mentioned volume is set within the range described above. When used as a battery separator, ion conductivity is improved, and due to its uniform structure, it exhibits excellent shut-off performance and dendrite resistance. Furthermore, when used as a liquid filter, it can form a polyolefin microporous membrane with excellent filtration accuracy and permeability. Regarding the 1.0 × 10⁻⁶ ppm in a 4 μm square three-dimensional structure as determined by FIB-SEM... 8 nm 3 The method for determining the quantity of resin portions of the above volume can specifically be performed using the method described in the examples. To obtain a resin portion having a volume of 1.0 × 10⁻⁶... 8 nm 3 The amount of resin portion of the above volume is set within the above range, and preferably the raw material composition and film-forming conditions of the microporous membrane are within the range described below.

[0062] The polyolefin microporous membrane according to embodiments of the present invention preferably has an average volume of 1.3 × 10⁻⁶ resin portions observed in the 4 μm square three-dimensional structure of the microporous membrane as measured by FIB-SEM. 7 nm3 Hereinafter, 1.2 × 10 is more preferred. 7 nm 3 Hereinafter, 1.0 × 10 is further preferred. 7 nm 3 The following describes how, by setting the average volume of each resin portion within the aforementioned range, ion conductivity is improved when used as a battery separator. Furthermore, when used as a liquid filter, a polyolefin microporous membrane with excellent filtration accuracy and permeability can be formed. Considering the above aspects, there is no particular limitation on the lower limit of the average volume of the resin portion, but considering both the permeability and thermal shrinkage rate of the microporous membrane, a volume of 1.0 × 10⁻⁶ is preferred. 6 nm or higher. In order to set the average volume of the resin portion within the above range, it is preferable that the raw material composition and film-forming conditions of the microporous membrane are within the range described below.

[0063] The polyolefin microporous membrane involved in the embodiments of the present invention preferably has an air permeability of 30 seconds / 100cm based on its thickness. 3 Below / μm, more preferably 20 seconds / 100cm 3 Below / μm, further preferably 15 seconds / 100cm 3 Below / μm. There is no specific lower limit for the thickness-to-permeability conversion, but considering the balance with membrane strength, 1 second / 100cm is preferred. 3 / μm or more. By setting the thickness-to-permeability to the above range, a microporous membrane with excellent charge-discharge characteristics can be formed when the microporous membrane is used as a battery separator. The thickness-to-permeability can be set to the above range by adjusting the proportion of raw materials, stretching ratio, heat-setting conditions, etc. during the manufacturing process.

[0064] The thickness of the polyolefin microporous membrane according to embodiments of the present invention can be appropriately adjusted according to the application, preferably 20 μm or less, more preferably 15 μm or less, and even more preferably 12 μm or less. Furthermore, it is preferably 2 μm or more, more preferably 5 μm or more, and particularly preferably 7 μm or more. By setting the thickness of the polyolefin microporous membrane to the above range, when used as a battery separator, both safety and high battery capacity can be achieved. The thickness can be set to the above range by appropriately adjusting film-forming conditions such as extrusion conditions.

[0065] The porosity of the polyolefin microporous membrane according to embodiments of the present invention is preferably 30% or more, more preferably 35% or more, and even more preferably 40% or more. While no specific upper limit is set for the porosity, it is preferably 80% or less from the viewpoint of suppressing a decrease in membrane strength. By setting the porosity to the above range, the microporous membrane exhibits excellent output characteristics when used as a separator for secondary batteries. The porosity can be set to the above range by adjusting the raw material formulation, stretching ratio, heat-fixing conditions, etc., during the manufacturing process.

[0066] The specific structure of the polyolefin microporous membrane in this invention will be described below, but it is not necessary to be limited to the embodiments described below.

[0067] The polyolefin microporous membrane according to embodiments of the present invention preferably has polyethylene resin as its main component. Furthermore, the main component referred to here is the component with the highest content, expressed as a percentage by mass, among the components constituting the polyolefin microporous membrane. The proportion of the polyethylene resin component in the polyolefin microporous membrane is preferably 80% by mass or more, more preferably 90% by mass or more, further preferably 96% by mass or more, and particularly preferably 99% by mass or more. By ensuring that its content in the polyolefin microporous membrane is within the above range, the microporous membrane exhibits excellent film-forming properties and uniformity, while also achieving an excellent balance of performance characteristics as a battery separator, such as membrane strength and permeability. Here, the polyolefin microporous membrane may contain two or more types of polyethylene resin; in this case, the total amount of polyethylene resin is considered as the amount of polyethylene resin component constituting the polyolefin microporous membrane.

[0068] The polyolefin microporous membrane according to embodiments of the present invention can use various polyethylene resins, including ultra-high molecular weight polyethylene, high-density polyethylene, medium-density polyethylene, branched low-density polyethylene, and linear low-density polyethylene. Furthermore, the polyethylene resin can be an ethylene homopolymer or a copolymer of ethylene and other α-olefins. Examples of α-olefins include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. Here, the polyethylene resin contains more than 50 mol% ethylene relative to all raw material monomer components.

[0069] The polyolefin microporous membrane of the embodiments of the present invention preferably contains ultra-high molecular weight polyethylene (hereinafter, described as resin A) of the above-mentioned polyethylene, and more preferably contains resin A and high-density polyethylene (hereinafter, described as resin B).

[0070] The weight-average molecular weight (Mw) of the ultra-high molecular weight polyethylene used as resin A is preferably 800,000 or more, more preferably 900,000 or more, even more preferably 1,000,000 or more, even more preferably 2,000,000 or more, and particularly preferably 2,400,000 or more. Furthermore, the weight-average molecular weight (Mw) is preferably 4,000,000 or less, more preferably 3,500,000 or less. By setting the weight-average molecular weight of resin A within the above range, it is easy to improve the strength of the polyolefin microporous membrane, or to reduce the arithmetic mean roughness (Sa) by making the pore structure uniform and fine, or to reduce the average volume of the resin portion.

[0071] The melting point of resin A is preferably below 135°C, more preferably below 134°C, and particularly preferably below 130°C. Furthermore, it is preferably above 125°C, more preferably above 127°C. By setting the melting point of resin A within the above range, the pore structure of the polyolefin microporous membrane becomes more uniform and finer, and the shut-off properties also improve. Additionally, the structural difference between the two surfaces of the polyolefin microporous membrane can be reduced, and the Sa1 / Sa2 value can be adjusted to an appropriate range. The relationship between the melting point of resin A and the structural difference between the two surfaces of the polyolefin microporous membrane is not yet clear, but it is speculated that by controlling the melting point of resin A, i.e., the crystallization behavior of the resin during cooling and solidification, the structural difference caused by the difference in cooling rates between the surface and the back surface is reduced in the process of forming the gel-like sheet described later. When using a polyolefin resin with a weight-average molecular weight (Mw) of 2.51 million to 4 million and a melting point of 130°C or lower as resin A, it is easy to make the pore structure of the polyolefin microporous membrane uniform and fine. In addition, it can reduce the structural difference between the two surfaces of the polyolefin microporous membrane and adjust the value of Sa1 / Sa2 to an appropriate range.

[0072] It should be noted that the melting point of resin A can be set to the aforementioned range by controlling the molecular structure (monomer types) of resin A and adjusting the number and length of side chains. For example, in the case of polyethylene resins, a method can be given by copolymerizing ethylene with other α-olefins and controlling the copolymerization rate and the types of α-olefins used.

[0073] The content of resin A in the polyolefin microporous membrane is preferably 30% by mass or more, more preferably 50% by mass or more, and even more preferably 60% by mass or more. Furthermore, it is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 80% by mass or less. By setting the content of resin A in the polyolefin microporous membrane within the above range, it is easy to improve membrane strength and to refine the pore structure.

[0074] High-density polyethylene (density: 0.940 g / m³) is used as resin B. 3 Above and 0.970 g / m 3The weight-average molecular weight (Mw) of the following resins is preferably 10,000 or more, more preferably 20,000 or more, and even more preferably 50,000 or more. Furthermore, the weight-average molecular weight (Mw) is preferably 200,000 or less, more preferably 150,000 or less, and even more preferably 100,000 or less. By setting the weight-average molecular weight of resin B within the above range, a structure in which the fibrils are uniformly oriented within the film surface can be obtained after stretching, making it easier to... V / D S The value and having 1.0 × 10 8 nm 3 The amount of resin portion of the above volume is adjusted to a preferred range.

[0075] The melting point of resin B is more preferably 128°C or higher, and even more preferably 130°C or higher. Furthermore, it is preferably 135°C or lower, and more preferably 134°C or lower. By setting the melting point of resin B within the above range, not only is the pore structure refined, but the pore-closing properties are also excellent.

[0076] The heat of fusion (ΔH) of resin B, as determined by differential scanning calorimetry (DSC), is preferably 200 J / g or more, more preferably 210 J / g or more, and even more preferably 220 J / g or more. By setting the heat of fusion (ΔH) of resin B within the above range, the rise in shut-off temperature can be suppressed and the film strength can be improved. It should be noted that, considering the above aspects, there is no specific upper limit set for the heat of fusion (ΔH) of resin B, but from the perspective of film-forming properties, it is preferably 280 J / g or less.

[0077] The content of resin B in the polyolefin microporous membrane is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 20% by mass or more. Furthermore, it is preferably 70% by mass or less, more preferably 50% by mass or less, and even more preferably 40% by mass or less. By setting the content of resin B in the polyolefin microporous membrane within the above range, it is easy to obtain a membrane with a content of 1.0 × 10⁻⁶. 8 nm 3 The amount of resin portion of the above volume is adjusted to a preferred range, and excessive crystallization orientation during stretching is suppressed, resulting in good shut-off properties.

[0078] The polyolefin microporous membranes described in the embodiments of the present invention may contain resins other than polyethylene resins. For example, from the perspective of improving the heat resistance of the microporous membrane, polypropylene resins are preferably added. Besides homopolymer polypropylene, block copolymers and random copolymers may also be used as polypropylene resins. Block copolymers and random copolymers may contain copolymer components copolymerized with α-olefins other than propylene. Examples of α-olefins include ethylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, and octene. Here, polypropylene resin refers to resins containing more than 50 mol% of propylene relative to all raw material monomer components.

[0079] The amount of polypropylene resin added is preferably 20% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, relative to the total mass of the polyolefin microporous membrane. By setting it within the above range, a polyolefin microporous membrane with excellent productivity, quality, and strength can be formed.

[0080] The polyolefin microporous membrane may, as needed, contain resin components other than polyethylene and polypropylene resins. Additionally, without impairing the effects of the invention, it may contain various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, anti-blocking agents, fillers, crystallizing nucleating agents, and crystallization delaying agents.

[0081] [Manufacturing method of polyolefin microporous membrane]

[0082] Next, a method for manufacturing a polyolefin microporous membrane according to an embodiment of the present invention will be shown. Examples of methods for manufacturing polyolefin microporous membranes include dry membrane fabrication methods and wet membrane fabrication methods. From the perspective of controlling the structure and properties of the membrane, a wet membrane fabrication method is preferred as the method for manufacturing the polyolefin microporous membrane in this embodiment.

[0083] The following describes a method for manufacturing wet-process polyolefin microporous membranes. It should be noted that the following description is one example of a manufacturing method and is not limited to this method.

[0084] As a method for manufacturing polyolefin microporous membrane in an embodiment of the present invention, it is preferable to include the following steps (1) to (5) in sequence, more preferably the following step (6), and may further include the following step (7) after step (6) or instead of step (6).

[0085] (1) The process of preparing a polyolefin resin composition by melt mixing the above-mentioned polyolefin resin and film-forming solvent; (2) The process of extruding and cooling the above-mentioned polyolefin resin composition to form a gel-like sheet; (3) The first stretching process of preheating and stretching the above-mentioned gel-like sheet; (4) The process of removing the film-forming solvent from the stretched gel-like sheet; (5) The process of drying the sheet after removing the solvent for film formation; (6) A second stretching process, which involves preheating and stretching the dried sheet; (7) The process of heat-treating the dried sheet.

[0086] (1) Preparation process of polyolefin resin composition

[0087] A polyolefin resin composition is prepared by heating and dissolving a polyolefin resin in a plasticizer (a film-forming solvent). As a plasticizer, there are no particular limitations on any solvent capable of uniformly dispersing the polyolefin resin; however, for higher stretching ratios, a liquid solvent at room temperature is preferred. Examples of suitable solvents include aliphatic, cyclic aliphatic, or aromatic hydrocarbons such as nonane, decane, decahydronaphthalene, p-xylene, undecane, dodecane, and liquid paraffin, as well as mineral oil fractions with boiling points corresponding to these, and phthalate esters such as dibutyl phthalate and dioctyl phthalate, which are liquid at room temperature. To obtain a gel-like sheet with a stable liquid solvent content, a non-volatile liquid solvent such as liquid paraffin is preferred.

[0088] The mixing ratio of polyolefin resin and plasticizer, relative to the total mass of the polyolefin resin composition, preferably sets the content of polyolefin resin to 10-50% by mass. By setting the content of polyolefin resin within the above range, the dispersion of polyolefin resin and plasticizer becomes better, and the resulting microporous membrane exhibits excellent strength, permeability, and heat resistance. Furthermore, when molded into sheets, the amount of expansion and contraction at the die outlet becomes appropriate, and the sheet's formability and film-forming properties also improve.

[0089] From the perspective of obtaining a uniform compound state, the melt mixing of polyolefin resin and plasticizer is preferably carried out in a twin-screw extruder.

[0090] The resin temperature during mixing is preferably 150°C or higher, more preferably 160°C or higher, even more preferably 180°C or higher, and preferably 250°C or lower, more preferably 240°C or lower, even more preferably 230°C or lower. By setting the temperature of the polyolefin resin composition during mixing within the above range, the reduction in strength caused by resin deterioration can be prevented, and the polyolefin resin and plasticizer can be uniformly melt-mixed.

[0091] Furthermore, when compounding using a twin-screw extruder, the ratio Q / Ns, calculated from the extruded mass Q (kg / hr) to the screw speed Ns (rpm), is preferably 0.01 or higher, more preferably 0.05 or higher, even more preferably 0.1 or higher, and particularly preferably 0.3 or higher. This prevents strength reduction due to resin deterioration during compounding. Additionally, the upper limit is preferably 5.0 or lower, more preferably 3.0 or lower, and even more preferably 2.0 or lower. This allows for sufficient shearing of the polyolefin resin composition, resulting in a uniform dispersion.

[0092] (2) Formation process of gel-like sheet

[0093] Molten polyolefin resin composition is fed from an extruder to a die and extruded into a sheet. The extrusion method can be either T-die extrusion or blow extrusion. Alternatively, multiple polyolefin resin compositions, either identical or with different compositions, can be fed from multiple extruders to a multi-manifold composite T-die, stacked into layers, and extruded into a laminated sheet. The preferred extrusion temperature is 140–250°C.

[0094] The shear rate applied when the melt is extruded from the die is 150 seconds. -1 The above, more preferably 200 seconds -1 The above is further preferred to be 250 seconds. -1 The above, especially preferred, is 300 seconds. -1 The above describes how setting the shear rate within the aforementioned range allows for control of the sheet surface orientation, reduces the structural difference between the surface and back sides during microporous membrane fabrication, and facilitates the miniaturization of the pore structure. Considering membrane quality, the upper limit for the shear rate is 1000 seconds. -1 Hereinafter, 800 seconds is preferred. -1 The shear rate, in extrusion using a T-die, can be calculated using the following formula.

[0095] γ=6Q / (Wt 2 )

[0096] γ: Shear rate ( / sec)

[0097] Q: Discharge volume (cm) 3 / sec)

[0098] W: Width of the T-shaped die outlet (cm)

[0099] t: Slit gap at the outlet of the T-shaped die head (cm)

[0100] The resin composition, melt-extruded into a sheet, is cooled and solidified into a gel-like sheet. In the cooling process, it is preferable to cool to 10-50°C before the first stretching process described later. This is because it is preferable to keep the final cooling temperature below the crystallization completion temperature, thereby refining the higher-order structure and facilitating uniform stretching in the subsequent stretching. Furthermore, the cooling rate at this time is preferably 50°C / min or higher, more preferably 100°C / min or higher, and even more preferably 150°C / min or higher. Generally, a slow cooling rate results in larger crystals, thus coarser higher-order structures in the gel-like sheet, and a larger gel structure. Conversely, a fast cooling rate results in smaller crystals, thus denser higher-order structures in the gel-like sheet, enabling uniform stretching. This, in turn, facilitates improved strength of the polyolefin microporous membrane and easier refinement of the pore structure.

[0101] (3) First stretching process

[0102] Next, the obtained gel-like sheet is stretched at least along a uniaxial direction, but it is preferable to preheat the gel-like sheet before stretching. The preheating temperature is preferably set to 90~130°C, more preferably 105°C or higher, even more preferably 110°C or higher, and even more preferably 120°C or lower, and even more preferably 118°C or lower. By setting the preheating temperature to the above conditions, a polyolefin microporous membrane with a uniform and fine pore structure can be obtained by uniform stretching during the stretching process.

[0103] The preheated gel-like sheet is preferably stretched at a specified ratio using a tenter frame, roller stretching, blow-blowing, or a combination thereof. Stretching can be uniaxial or biaxial, but biaxial stretching is preferred. In the case of biaxial stretching, it can be any of simultaneous biaxial stretching, sequential biaxial stretching, and multi-stage stretching (e.g., a combination of simultaneous and sequential biaxial stretching), with simultaneous biaxial stretching being preferred. By performing simultaneous biaxial stretching, it is easy to control the arithmetic mean roughness (Sa) of the polyolefin microporous membrane surface, the standard deviation of the orientation parameters within the membrane surface, and the surface roughness of the polyolefin microporous membrane with a value of 1.0 × 10⁻⁶. 8 nm 3 The resin portion of the above volume is adjusted to the preferred range.

[0104] The stretching ratio (area stretching ratio) in this process is preferably 16 times or more, more preferably 25 times or more. Furthermore, the stretching ratio is preferably 4 times or more, more preferably 5 times or more, in either the mechanical length direction (MD direction) or the mechanical width direction (TD direction). The stretching ratios in the MD and TD directions can be the same or different; by setting the area stretching ratio within the above range, mechanical strength and permeability can be improved. Additionally, the area stretching ratio in this process is preferably 100 times or less, more preferably 64 times or less, thereby enabling the production of polyolefin microporous membranes with excellent membrane strength while preventing membrane breakage.

[0105] The stretching temperature in this process is preferably 90°C or higher, more preferably 105°C or higher, and even more preferably 110°C or higher. Furthermore, it is preferably 130°C or lower, more preferably 120°C or lower, and even more preferably 118°C or lower. Through the stretching described above, cracks occur between the polyethylene sheets, and the polyethylene resin phase becomes micronized, forming numerous fibrils. These fibrils form a three-dimensional, irregularly interconnected network structure.

[0106] The stretching speed in the MD direction of this process is preferably 300% / min or more, more preferably 700% / min or more, further preferably 1000% / min or more, particularly preferably 1200% / min or more, and most preferably 1500% / min or more. It is also preferably 5000% / min or less. By setting the stretching speed in the MD direction within the above range, it is easy to achieve a fine and uniform pore structure. Furthermore, the stretching speed in this process is obtained by dividing the stretching rate from the start of stretching to the end of stretching by the required time, assuming the stretching rate of the microporous membrane before stretching is 0% and the stretching rate for each stretching ratio is 100%.

[0107] The stretching speed in the TD direction of this process is preferably 300% or more, more preferably 700% / min or more, further preferably 1000% / min or more, particularly preferably 1200% / min or more, and most preferably 1500% / min or more. Furthermore, it is preferably 5000% / min or less. By setting the stretching speed in the TD direction to the above range, it is easy to achieve a fine and uniform pore structure.

[0108] (4) Solvent removal process for film formation

[0109] The solvent used for film formation is removed (cleaned) using a cleaning solvent. The polyolefin resin phase and the solvent phase undergo phase separation. Therefore, if the solvent for film formation is removed, a porous membrane composed of fibrils forming a fine three-dimensional network structure and having irregularly interconnected pores (voids) in three dimensions can be obtained. The cleaning solvent and the method for removing the solvent using the cleaning solvent are known, and therefore description is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Unexamined Patent Application Publication No. 2002-256099 can be used.

[0110] (5) Drying process

[0111] The polyolefin microporous membrane, after the solvent for film formation has been removed, is dried by heating or air drying. The drying temperature is preferably below 100°C, more preferably below 95°C. The total mass of the polyolefin microporous membrane is considered as 100 parts by mass (dried mass), and drying is preferably carried out until the residual cleaning solvent is below 5 parts by mass, more preferably until the residual cleaning solvent is below 3 parts by mass.

[0112] (6) Second stretching process

[0113] Next, the dried microporous membrane is stretched at least along a uniaxial direction at a specified area stretching ratio. This stretching of the dried film (second stretching) is also called dry stretching. Stretching can be uniaxial or biaxial, preferably biaxial. In the case of biaxial stretching, it can be either synchronous or sequential stretching, preferably sequential stretching. In the case of sequential stretching, it is preferable to stretch along the MD direction followed by continuous stretching along the TD direction.

[0114] The area stretching ratio of dry stretching is preferably 1.2 times or more, more preferably 1.5 times or more, further preferably 2.0 times or more, particularly preferably 2.5 times or more, and most preferably 3.0 times or more. By setting the area stretching ratio of dry stretching within the above range, the structure is made more uniform, and the value of Sa1 / Sa2 is easily adjusted to an appropriate range, resulting in a structure with a diameter of 1.0 × 10⁻⁶. 8 nm 3 The amount of resin portion in the above volume is controlled within a preferred range. Furthermore, by setting the area stretching ratio of the dry stretching to 16 times or less, film-forming properties are stable. Additionally, the area stretching ratio described in this process represents the product of the dry stretching ratios in the MD direction and the TD direction.

[0115] The stretching ratio in the MD direction of the dry stretching is preferably 1.2 times or more, more preferably 1.4 times or more, even more preferably 1.6 times or more, particularly preferably 1.7 times or more, and more preferably 4 times or less. By setting the stretching ratio in the MD direction of the dry stretching to the above range, the strength and permeability of the polyolefin microporous membrane can be easily controlled within the desired range, and the structure can be made uniform.

[0116] The stretching ratio in the TD direction of dry stretching is preferably 1.2 times or more, more preferably 1.4 times or more, even more preferably 1.6 times or more, particularly preferably 1.7 times or more, and more preferably 4 times or less. By setting the stretching ratio in the TD direction of dry stretching to the above range, the strength and permeability of the polyolefin microporous membrane can be easily controlled within the desired range, and the structure can be made uniform.

[0117] The stretching ratio in the MD direction of this process is set to S. MD Set the stretch ratio in the TD direction to S. TD At that time, |S MD -S TD | Preferably 0.5 or less, more preferably 0.3 or less, and particularly preferably 0.1 or less. By |S MD -S TD By setting the parameters within the above range, a structure in which the fibrils of a polyolefin microporous membrane are uniformly oriented in-plane can be obtained. This facilitates the determination of the standard deviation of the orientation parameters and the arithmetic mean roughness (Sa) within the membrane surface, achieving a value of 1.0 × 10⁻⁶. 8 nm 3 The quantity of resin portion of the above volume is adjusted to the desired range. It should be noted that the stretch ratio in this process refers to the stretch ratio of the microporous membrane to be supplied to the next process, based on the microporous membrane to be processed in this process.

[0118] The stretching temperature in the MD direction during this process is preferably 60°C or higher, more preferably 80°C or higher. Furthermore, it is preferably 130°C or lower, more preferably 120°C or lower. By setting the stretching temperature in the MD direction within the above range, the resulting polyolefin microporous membrane exhibits excellent permeability and strength, and can be stretched uniformly.

[0119] The stretching temperature in the TD direction during this process is preferably 80°C or higher, more preferably 100°C or higher. Furthermore, it is preferably 145°C or lower, more preferably 135°C or lower. By setting the stretching temperature in the TD direction within the above range, the resulting polyolefin microporous membrane exhibits excellent permeability and strength, and can be stretched uniformly.

[0120] (7) Heat treatment process

[0121] Additionally, after or instead of step (6), the dried polyolefin microporous membrane can be heat-treated. Heat treatment stabilizes the crystals and homogenizes the lamellar crystals. Heat treatment methods include heat-fixing and / or heat-relaxation. Heat-fixing refers to heat treatment that maintains the membrane's dimensions while heating. Heat-relaxation refers to heat treatment that causes the membrane to shrink thermally in the MD and TD directions during heating. Preferably, heat-fixing is performed using a tenter frame or rollers. The relaxation rate in the relaxation treatment is obtained by dividing the size of the relaxed membrane by the size of the membrane before relaxation. The relaxation rate in both the MD and TD directions is preferably 1.0 or less, more preferably 0.98 or less, and even more preferably 0.96 or less. Furthermore, considering the planarity of the microporous membrane, a relaxation rate of 0.80 or more is preferred, more preferably 0.90 or more. For polyolefin resins, the heat treatment temperature is preferably in the range of 100 to 140°C.

[0122] The polyolefin microporous membrane obtained above can be used in various applications such as filters, separators for secondary batteries, separators for fuel cells, and separators for capacitors.

[0123] [Battery separators and secondary batteries]

[0124] As one aspect of the present invention, examples include battery separators using the polyolefin microporous membrane of the present invention and secondary batteries using the battery separator. Because the battery separator of the present invention exhibits excellent safety and output characteristics, it is particularly preferred for use in secondary batteries requiring high energy density, high capacity, and high output power.

[0125] [Liquid filters and filter units]

[0126] As one aspect of the present invention, examples include liquid filters using the polyolefin microporous membrane of the present invention and filter units using the liquid filter. When used as a liquid filter, it exhibits excellent filtration accuracy and high permeability; therefore, it is preferably used as a liquid filter for semiconductor photoresists requiring high-precision filtration. The polyolefin microporous membrane of the present invention can be used as a liquid filter for sheet-like, tubular, or pleated filter units; from the perspective of increasing the filtration area, a pleated filter unit is preferred. When assembling into a pleated filter unit, it is preferable to laminate a resin raw material mesh and a reinforcing membrane composed of porous bodies onto at least one surface of the polyolefin microporous membrane of the present invention. After being bonded to the reinforcing membrane, valley folds are formed to create a pleated shape, which is then assembled into the filter unit for use.

[0127] Example

[0128] The present invention will be further described in detail through embodiments, but the implementation of the present invention is not limited to these embodiments. It should be noted that, unless otherwise specified, the evaluations in this application are conducted in an environment with a temperature of 23°C and a humidity of 65%. The evaluation methods and analysis methods used in the embodiments are described below.

[0129] [Measurement Method]

[0130] [thickness]

[0131] Using a contact thickness gauge, namely the "Light Machick" (registered trademark) VL-50 (10.5mm) manufactured by Mitsutoyo Co., Ltd. The thickness of a polyolefin microporous membrane is measured at any 5 points within a 50mm × 50mm range using an ultra-hard spherical probe with a measuring load of 0.01N. The average value is taken as the thickness (μm).

[0132] Porosity

[0133] A 50mm × 50mm square sample was cut from a polyolefin microporous membrane, and its volume (cm³) was measured. 3 ) and mass (g). From these values ​​and membrane density (g / cm³) 3 The porosity of the polyolefin microporous membrane is calculated using the following formula. It should be noted that the membrane density is assumed to be 0.99 g / cm³. 3 The calculation is performed based on a constant value. In this determination, samples were cut from any three locations on the polyolefin microporous membrane, and the average value of the measured porosity was calculated.

[0134] Formula: Porosity (%) = [(volume - mass / membrane density) / volume] × 100.

[0135] [Breathability]

[0136] For polyolefin microporous membranes, according to JIS P-8117:2009, the air permeability (seconds / 100cm²) was measured using a Wang Yan-type air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) at 25°C in an atmosphere. 3 In addition, the thickness (μm) of the polyolefin microporous membrane, as measured by the above method, is used to calculate the thickness-to-permeability.

[0137] [Puncture intensity converted to weight per unit area]

[0138] For puncture strength, except that the test speed was set to 2 mm / s, it was measured according to JIS Z 1707 (2019). A force gauge (Imada DS2-20N, Inc.) was used to measure the maximum load (mN) when a needle with a spherical tip (radius of curvature R: 0.5 mm) and a diameter of 1.0 mm punctured a polyolefin microporous membrane. The value obtained by the following formula was used as the puncture strength converted to weight per unit area (mN / (g / m²)). 2 )).

[0139] Formula: Puncture intensity converted to unit area weight (mN / (g / m²)) 2 = Maximum load (mN) / Weight per unit area of ​​polyolefin microporous membrane (g / m³) 2 )

[0140] It should be noted that, for the unit area weight of polyolefin microporous membrane, a 50mm×50mm square sample is cut from the polyolefin microporous membrane, and the mass (g) at room temperature (25℃) is measured and calculated using the following formula.

[0141] Formula: Weight per unit area (g / m²) 2 = mass (g) / (50 (mm) × 50 (mm)) × 10 6

[0142] Gel permeation chromatography (GPC)

[0143] The weight-average molecular weight (Mw) of polyolefin resins and the molecular weight distribution of polyolefin microporous membranes were determined by GPC under the conditions shown below. On the differential molecular weight distribution curve obtained by GPC, the area ratio of each molecular weight component was determined by the ratio of the area of ​​each molecular weight region to the peak area of ​​all molecular weight components.

[0144] • Sample preparation: Add 5 mL of the test solvent to 5 mg of sample, heat and stir at 160~170℃ for 60 minutes, and then filter the resulting solution through a metal filter (pore size 0.5 μm).

[0145] • Measuring apparatus: High-temperature GPC apparatus, Toso HLC-8321GPC / HT

[0146] • Protective pillar: Showa Denko Co., Ltd. "Shodex" (registered trademark) HT-G

[0147] • Pillars: 2 pieces of "Shodex" (registered trademark) UT806M manufactured by Showa Denko Co., Ltd.

[0148] • Column temperature: 145℃

[0149] • Solvent (mobile phase): 1,2,4-trichlorobenzene (made by Fujifilm and Koichi Pharmaceutical, with the addition of 0.1 wt% BHT)

[0150] Solvent flow rate: 1.0 ml / min

[0151] Injection volume: 0.300 mL

[0152] • Detector: Differential Refractive Index Detector (RI Detector)

[0153] • Standard samples: Monodisperse polystyrene manufactured by Tokyo Soy Corporation, and bibenzyl polystyrene manufactured by Tokyo Chemical Industry Co., Ltd.

[0154] Then, convert the obtained Mw to polyethylene (PE). The conversion formula is as follows.

[0155] Mw (PE conversion) = Mw (PS conversion measurement) × 0.468.

[0156] Differential Scanning Calorimetry (DSC)

[0157] The melting point, heat of fusion, and melting point of polyolefin resins and polyolefin microporous membranes were determined using DSC. A 6.0 mg sample was sealed in an aluminum dish. Using a Parking Elmer PYRIS Diamond DSC, the temperature was increased from 30 °C to 230 °C at a rate of 10 °C / min (first heating), held at 230 °C for 5 minutes, cooled at a rate of 10 °C / min, and then increased again from 30 °C to 230 °C at a rate of 10 °C / min (second heating). The heat of fusion and melting point were calculated from the melting peak obtained by plotting a baseline between 60 °C and 200 °C on the temperature distribution curve of the heat endothermic measured during the second heating in the above DSC determination. It should be noted that the temperature at which the maximum heat endothermic value is displayed is taken as the melting point, and the heat of fusion is calculated from the area of ​​the melting peak.

[0158] [Arithmetic mean roughness Sa]

[0159] Surface roughness was measured using a Hitachi High-Tech VS-1540 scanning white interference microscope. VS-Measure (version 10.4.4) was used for measurement, and VS-Viewer (version 10.0.3) for analysis. A 50mm × 50mm square piece of polyolefin microporous membrane was cut and adhered to a circular frame with an inner diameter of 4cm in a wrinkle-free and loose manner, serving as the measurement sample. Measurements were performed under the conditions shown below, and after data processing, the arithmetic mean roughness Sa was calculated according to ISO 25178. Measurements were taken from two surfaces of each sample; the surface with the larger Sa value was designated Sa1, and the surface with the smaller Sa value was designated Sa2. Additionally, measurements were taken at four points on each surface, and the average value was calculated.

[0160] <Measurement Conditions>

[0161] Objective lens: 50x

[0162] • Lens tube: 0.5x

[0163] Wavelength filter: 530white

[0164] • Camera: High resolution

[0165] • Measurement mode: Wave

[0166] • Measuring equipment: piezoelectric

[0167] • Field of view size: 1024×1024 (pixels)

[0168] • Effective pixel count: 50%

[0169] • Cutoff frequency: None

[0170] <Data processing conditions>

[0171] • Surface correction (approximate surface shape): 4 times

[0172] • Interpolation: Full interpolation

[0173] • Filter: Smoothed 9×9 (pixel).

[0174] Standard deviation of orientation parameters

[0175] The orientation parameters of the polyolefin microporous membrane were determined and calculated using Raman spectroscopy. A laser beam was incident perpendicularly to the normal direction of the membrane surface (XY plane), and polarized light was generated using a polarizer. The sample was rotated, and any direction within the membrane surface was taken as 0°. Raman spectra were obtained at 12 points (0°~165°) every 15° intervals for each direction. For the obtained Raman spectra, the orientation parameters were measured at 800 cm⁻¹. -1 and 1350cm-1 Draw a straight line between these two points, use it as a baseline for correction, and then adjust the distance from 1045 to 1075 cm. -1 The maximum strength between them is taken as 1060cm. -1 The peak intensity (I1060) is between 1115 and 1145 cm⁻¹. -1 The maximum strength between them is 1130cm. -1 The peak intensity (I1130) is 1130 cm⁻¹. -1 With 1060cm -1 The peak intensity ratio (I1130 / I1060) was used as the orientation parameter. Any direction within the film surface was taken as 0°, and 12 points (0°~165°) were measured every 15°. The standard deviation was calculated from the obtained values.

[0176] • Apparatus: Micro Raman spectroscopy system (Renishaw "inVia")

[0177] ·180° backscattering position

[0178] • Spectral length 250mm

[0179] • 3000 diffraction gratings / mm

[0180] • Excitation laser 532nm

[0181] • 50x objective lens (NA=0.75)

[0182] • Spot size (spatial resolution) 5μm.

[0183] [Median pore size based on surface area (Ds) and median pore size based on pore volume (Dv)]

[0184] A rectangular test piece weighing 0.20 ± 0.02 g was cut from any position on the polyolefin microporous membrane, and its area and weight were measured. The volume of the polyolefin microporous membrane was calculated from the area of ​​the test piece and the membrane thickness determined using the above method. The cut sample was wound onto a sample core in purified water without introducing air bubbles and placed in the measuring chamber of a water-extruded porosity analyzer (model: WIP-3K-A-1) manufactured by POROUSMATERIALS, INC. Gas was introduced into the purified water in the chamber and the pressure was increased in stages until the maximum pressure reached 13.7 MPaG. The relationship between pressure and pore size, surface area, and pore volume is shown in the following formula. In the water-extruded porosity analyzer, the surface area was calculated by assuming the pore shape to be cylindrical. Regarding the pore size distribution obtained based on the relationship between the pore size calculated from the pressure in each pressurization stage and the surface area (cumulative value) of the pores into which pure water was injected in each pressurization stage, when the surface area (cumulative value) at the maximum pressure of 13.7 MPaG is considered as 100%, the pore size at the point where the surface area (cumulative value) reaches 50% is taken as the median value of the pore size based on surface area. For the median value of the pore size based on pore volume, the calculation method for the median value of the pore size based on surface area is applied by replacing the surface area portion with the pore volume and calculating it in the same way.

[0185] D=-4γcos θ / P

[0186] Dv = -4γcos θ / Pv 50

[0187] Ds = -4γcosθ / Ps 50

[0188] ΔS=4×ΔV / D

[0189] D: Aperture

[0190] Dv: Median value of pore diameter based on pore volume

[0191] Ds: Median value of pore size based on surface area

[0192] γ: Surface tension of water (set to 72.75 mN / m)

[0193] θ: Contact angle between polyethylene and water (set to 110°)

[0194] Pv 50 : When the cumulative orifice volume (indentation) at the pressure of 13.7 MPaG is considered as 100%, the pressure at the point when the cumulative orifice volume is 50% is considered as 100%.

[0195] Ps 50 Taking the cumulative surface area at 13.7 MPaG as 100%, the pressure at 50% cumulative surface area is...

[0196] ΔV: The volume of pure water injected during a single-stage pressurization (pore volume)

[0197] ΔS: The surface area of ​​the fine pores pressed in during a single-stage pressurization.

[0198] [Off Temperature]

[0199] While heating the polyolefin microporous membrane at a heating rate of 5°C / min, the air permeability resistance was measured using an air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T). The air permeability resistance was determined to reach a detection limit of 99,999 seconds / 100cm. 3 The temperature at which the air is turned off is taken as the shut-off temperature (°C).

[0200] The measuring cell is made of aluminum blocks and has a thermocouple structure formed directly below the polyolefin microporous membrane. The sample is cut into 5cm×5cm squares, and the surrounding area is fixed with an O-ring while the temperature is raised for measurement.

[0201] [Average Aperture]

[0202] The average pore size of the polyolefin microporous membrane was determined using a permeabilizer (PMI, CFP-1500A). The impregnation liquid for the polyolefin microporous membrane was GALWICK (surface tension: 15.9 dynes / cm), and measurements were performed in the order of dry-up and wet-up. The average pore size (nm) was measured based on ASTM E1294-89 (1999) (semi-dry method), and the pore size was calculated from the pressure (kPa) at the point where the curve representing half the slope of the pressure-flow curve in the dry-up measurement intersects with the curve in the wet-up measurement. The conversion between pressure and pore size was performed using the following mathematical formula.

[0203] d=C·γ / P

[0204] (In the above formula, "d (nm)" is the average pore size of the microporous membrane, "γ (dynes / cm)" is the surface tension of the impregnating liquid, "P (KPa)" is the pressure, and "C" is a constant, set to 2860).

[0205] [Methods for acquiring FIB-SEM (Focused Ion Beam Scanning Electron Microscopy) images of microporous membranes]

[0206] Continuous images were determined by FIB-SEM under the following conditions.

[0207] • Sample preparation: After embedding the polyolefin microporous membrane with epoxy resin, OsO4 was used for electron staining for determination.

[0208] • Observation device: Helios G4 manufactured by Thermo Fisher Scientific

[0209] • Observation conditions: accelerating voltage 1kV

[0210] • Sample tilt: 52°

[0211] • Pixel size: Image horizontal: 4.5 nm, Image vertical: 5.7 nm (after tilt correction)

[0212] • Slice spacing in FIB: 10nm

[0213] • To confirm and align the positions of the images, marks with Pt deposited on the side of the membrane are made.

[0214] • Tilt Correction: FIB-SEM observation is performed at a 52° angle, so the SEM image is observed to shrink in the vertical direction. Therefore, in order to form an image viewed from the front in the vertical direction of the image, it is necessary to set it to 1.27 times (= / sin52°). The tilt-corrected image is used for the 3D image production described later.

[0215] • Size determination: FIB processing was performed on the area larger than 4μm×4μm on the membrane cross section in sequence, and slices were made until the depth reached more than 4μm, and images of the area that could ensure a volume of 4μm×4μm×4μm (401 images were taken) were obtained.

[0216] Methods for creating 3D images

[0217] For the FIB-SEM images acquired using the above methods, three-dimensional images were created using the Thermo Fisher Scientific image processing and analysis software Avizo through the following processing.

[0218] • When reading the image into the software, tilt correction is performed at a ratio of 1.27 times the image's vertical axis.

[0219] • After aligning the image based on the Pt markers stacked on the side of the membrane, the image is cropped to form a volume of 4μm×4μm×4μm.

[0220] • In order to implement the image resolution described later, the voxels need to be isotropic, so the voxel size is resampled to 5nm×5nm×5nm.

[0221] • After binarizing the portion of the embedded resin in the microporous membrane that is electronically stained (i.e., the portion equivalent to the pores of the microporous membrane) and the resin portion constituting the microporous membrane, a three-dimensional image (three-dimensional data) is generated based on the binarized information, and the porosity of the three-dimensional whole is calculated.

[0222] (It should be noted that the porosity is calculated using the Volome Fraction function. As an internal software behavior, it counts the objects, i.e., the binarized pores and the number of voxels assigned, and divides the result by the total number of voxels in the resolution region.)

[0223] [with 1.0×10 8 nm 3 [Number of resin portions of the above volume, average volume of resin portions]

[0224] For the 3D data produced by the above method, image analysis was performed using the Thermo Fisher Scientific image processing and analysis software Avizo, through the following processing steps.

[0225] • The embedded resin portion (pore portion) and the resin portion constituting the microporous membrane (membrane portion) that have undergone electronic staining and binarization are segmented by a Watershed process. At this time, the segmentation parameters (makerextent) are set as 1 for the pore portion and 2 for the membrane portion.

[0226] • PNM (Pore Network Modeling) analysis was performed on the segmented resin portions to calculate the volume of each segment. The volumes of all resin portions were then averaged to calculate the average volume of the resin portions. Additionally, an analytical filter was applied to the obtained data to create a stereo image displaying only membrane portions with volumes above a specified threshold, and the number of these portions was counted. The threshold was set to 1.0 × 10⁻⁶. 8 nm 3 .

[0227] [Cyclic testing method and sample removal after cyclic testing]

[0228] Cyclic tests were conducted using the methods described below, and evaluation samples were removed from the batteries after the tests. Furthermore, the operations described in the sections concerning (cathode fabrication) to (battery fabrication) and (removal of the microporous membrane from the battery after the cyclic test) were performed in a dry chamber with a dew point below -30°C.

[0229] (The production of the positive electrode)

[0230] A slurry was prepared by mixing 94 parts by mass of nickel-cobalt-manganese composite oxide (NMC, Ni:Co:Mn = 6:2:2 (elemental ratio)) as the positive electrode active material, 3 parts by mass of carbon black as the conductive material, and 3 parts by mass of polyvinylidene fluoride (PVDF) as the resin binder, and dispersing them in N-methylpyrrolidone (NMP). This slurry was then uniformly coated onto both sides of an aluminum foil with a thickness of 20 μm forming the positive electrode current collector and dried. The foil was then compressed using a roller press. At this point, the coating weight of the positive electrode mixture on one side was set to 170 g / m². 2 The density is 3.0 g / cm³. 3 It is made in this way.

[0231] (Making the negative electrode)

[0232] 97 parts by weight of artificial graphite (as the negative electrode active material), 1 part by weight of carboxymethyl cellulose (as the resin binder), and 2 parts by weight of styrene-butadiene copolymer latex were mixed in a specific ratio and dispersed in purified water to prepare a slurry. This slurry was uniformly coated onto both sides of a 10 μm thick copper foil forming the negative electrode current collector and dried. Then, it was compressed and shaped using a roller press. At this point, the coating weight of the negative electrode mixture on one side was set to 100 g / m². 2 The density is 1.5 g / cm³ 3 It is made in this way.

[0233] (Non-aqueous electrolyte)

[0234] LiPF6 was added as a solute to a mixed solvent of ethylene carbonate: ethyl methyl carbonate: dimethyl carbonate = 3:3:4 (volume ratio) to a concentration of 1.0 mol / L. Then, 1 part by mass of ethylene carbonate was added to 100 parts by mass of this solution to prepare a non-aqueous electrolyte.

[0235] (Battery manufacturing)

[0236] Using the above-described positive electrode, the microporous membrane of this embodiment, and the above-described negative electrode, a flat, wound electrode body is fabricated. This flat, wound electrode body is clamped with an aluminum laminate, leaving a portion of the opening sealed. After drying it in a vacuum oven at 80°C for 6 hours, the above-described non-aqueous electrolyte is injected, and the electrode is sealed using a vacuum sealing machine to produce a laminated secondary battery with dimensions of 32mm × 32mm and a capacity of 300mAh.

[0237] (Pre-charge and discharge process)

[0238] The battery prepared above was charged at a constant current of 60mA until the battery voltage reached 4.2V. Then, it was charged at a constant voltage of 4.2V until the current reached 15mA. After resting for 30 minutes, it was discharged at a constant current of 60mA until the battery voltage reached 2.5V. After resting for 30 minutes, the above charge-discharge cycle was performed three times to prepare a battery for cycle testing.

[0239] (Cyclic test)

[0240] Using the aforementioned laminated secondary battery, charge it at a constant current of 900mA (3C) to a battery voltage of 4.2V, then charge it at a constant voltage of 4.2V until the current reaches 15mA (0.05C), pause for 30 minutes, and then discharge it at a constant current of 300mA (1C) to a battery voltage of 2.5V, pause for 30 minutes. Repeat this charge-discharge cycle 800 times.

[0241] (The microporous membrane was removed from the battery after the cycle test.)

[0242] The battery cell that underwent the above-mentioned cycle test was unsealed, and only the microporous membrane was removed. The removed microporous membrane was immersed in 50 mL of dimethyl carbonate for 5 minutes, then rinsed with 50 mL of fresh dimethyl carbonate and thoroughly air-dried to obtain the evaluation sample.

[0243] [Converted resistance of a 10μm thickness polyolefin microporous membrane] (Ω·cm) 2 / 10μm)

[0244] As samples for testing, 12 circular samples with a diameter of 19 mm were cut and vacuum-dried together with components of a CR2032 button battery (top cover, bottom cover, gasket (PP), spacer (cylindrical, 15.5 mm in diameter, 1.0 mm thick), and corrugated gasket) purchased from Hosen Co., Ltd. to remove moisture. In a drying chamber with an outdoor temperature below -35°C, three samples for testing and a PP gasket were placed sequentially on the bottom inner side of the bottom cover of the aforementioned button battery component. 0.15 mL of electrolyte (manufactured by Kishida Chemical Co.) containing dissolved LiPF6 was added to a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC / EMC = 4:6 [volume ratio]) at a concentration of 1 M. After placing the spacer on a polyolefin microporous membrane, the sample was allowed to stand for 10 minutes under a pressure of approximately -50 kPa to immerse it in the electrolyte. Then, a wave-shaped gasket and a top cover are placed on the battery, and it is sealed with a coin battery crimper (manufactured by Hosen Co., Ltd.) to obtain a sample battery.

[0245] The obtained sample batteries were placed in a high-temperature constant temperature bath at 25°C and allowed to stand for 2 hours. Then, the battery resistance (the real value when the imaginary axis value is 0) was measured using an impedance analyzer (manufactured by HIOKI Electric Co., Ltd.) at an amplitude of 20mV and a frequency of 200kHz. The above measurement was also performed on batteries separately manufactured with the number of polyolefin microporous membranes set to 4 or 5. The battery resistance was plotted against the number of polyolefin microporous membranes placed in the battery and approximated linearly. The slope (resistance per polyolefin microporous membrane) Z obtained was used to calculate the equivalent resistance of the polyolefin microporous membrane with a thickness of 10μm based on the following formula.

[0246] 10μm thickness converted resistance (Ω·cm) 2 ( / 10μm) = Z × area of ​​spacer × 10 / thickness of polyolefin microporous membrane

[0247] It should be noted that the area of ​​the spacer is set to 2.01 cm². 2 .

[0248] The 10μm thickness of the polyolefin microporous membrane is converted into resistance as an indicator of its output characteristics when used as a diaphragm.

[0249] ○: Less than 1.0 Ω·cm 2 / 10μm

[0250] ×:1.0 Ω·cm 2 / 10μm or larger.

[0251] [Safety of polyolefin microporous membranes]

[0252] The safety of polyolefin microporous membranes used as separators for secondary batteries is evaluated according to the following standards. Standards A and B are considered acceptable.

[0253] A: All three items below are 0.

[0254] B: In the following three items, there are 1 to 3 △ and 0 ×.

[0255] C: Other than A and B above (among the following 3 items, × means more than one)

[0256] Project 1

[0257] 〇: The puncture strength, converted to unit area weight, is above 900mN.

[0258] △: The puncture strength, converted to unit area weight, is 700mN or more but less than 900mN.

[0259] ×: The puncture strength converted to unit area weight is less than 700mN.

[0260] Project 2

[0261] 〇: Average pore size is below 30nm

[0262] △: Average pore size exceeding 30nm and below 35nm

[0263] ×: Average pore size exceeds 35nm

[0264] Project 3

[0265] 〇: The shut-off temperature is below 137℃

[0266] △: Shutdown temperature exceeds 137℃ but is below 140℃

[0267] ×: The shutdown temperature exceeds 140℃.

[0268] [Example 1]

[0269] In polyolefin feedstocks, using 70% by mass Mw results in 1.2 × 10⁻⁶. 6 Ultra-high molecular weight polyethylene with a melting point of 134℃ was used as resin A, and 30% by mass of Mw was used, with a yield of 6.0 × 10⁻⁶. 4 High-density polyethylene with a melting point of 132℃ and a ΔH of 220 J / g was used as resin B. Liquid paraffin (75% by mass) was added to 25% by mass of the above polyolefin raw material. Based on the mass of ultra-high molecular weight polyethylene, 0.5 parts by mass of 2,6-di-tert-butyl-p-cresol and 0.7 parts by mass of tetra[methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]methane were added as antioxidants and mixed to prepare a polyolefin resin composition. The obtained polyolefin resin composition was fed into a twin-screw extruder and kneaded at 180℃ to prepare a polyolefin solution. The obtained polyolefin solution was fed to a T-die set to 200℃ and melt-extruded under conditions of a shear rate of 350 sec⁻¹ at the die discharge point and a Q / Ns of 1.8. The solution was cooled using a casting drum controlled at 35℃ to form a gel-like sheet. For the gel-like sheet, a synchronous biaxial tenter frame was used to perform a first stretch at 118°C with a stretch ratio of 5 times in both the MD and TD directions and a stretching speed of 1000% / min in both the MD and TD directions. The wet-stretched gel-like sheet was then immersed in a dichloromethane bath to remove liquid paraffin and air-dried. Next, as a second stretch, the microporous membrane was stretched along the MD direction with a stretch ratio of 1.7 times on rollers set to 100°C using a roller stretcher. Then, it was stretched along the TD direction with a stretch ratio of 1.7 times using a tenter frame at 130°C to obtain a polyolefin microporous membrane. The membrane-forming conditions of the polyolefin microporous membrane are recorded in Table 1, and the evaluation results of the obtained polyolefin microporous membrane are recorded in Table 2.

[0270] [Examples 2-4, Comparative Examples 1-8]

[0271] Except for changes to the raw materials and film-forming conditions as described in the table, the procedure was carried out in the same manner as in Example 1.

[0272] The examples and comparative examples are shown in Tables 1-5. Table 5 shows the various analytical results after cycling tests on the polyolefin microporous membranes of Example 1 and Comparative Example 8 using the methods described above.

[0273]

Claims

1. A polyolefin microporous membrane, with respect to arithmetic average roughness Sa, units of nm, of one surface and the other surface of the microporous membrane, when the Sa of the one surface having a larger arithmetic average roughness is set as Sa1, the Sa of the one surface having a smaller arithmetic average roughness is set as Sa2, formula 1 and formula 2 are satisfied, when the median value of the pore diameter based on the surface area determined by a water extrusion porosimetry analyzer is set as Ds, the median value of the pore diameter based on the pore volume is set as Dv, formula 3 is satisfied, and the puncture strength converted to the weight per unit area is 700 mN / (g / m2) or more. 2 ) Formula 1: 10 ≤ (Sa1 + Sa2) / 2 ≤ 80 Formula 2: Sa1 / Sa2 ≤ 1.4 Formula 3: 1.0 ≤ Dv / Ds ≤ 1.

2.

2. The polyolefin microporous membrane according to claim 1, having a three-dimensional structure of 4 pm square determined by focused ion beam scanning electron microscopy (FIB-SEM) measurement of the microporous membrane, wherein the volume of the resin portion is 1.0 x 10 8 nm 3 The resin portion of the above volume is 10 or less.

3. A polyolefin microporous membrane having 1.0 x 10 8 nm 3 The resin portion of the above volume is 10 or less.

4. The polyolefin microporous membrane according to claim 1 or 3, wherein the average volume of the resin portion in the 4 μm square three-dimensional structure of the microporous membrane, as determined by FIB-SEM, is 1.3 × 10⁻⁶. 7 nm 3 the following.

5. The polyolefin microporous membrane according to claim 1 or 3, wherein the average pore diameter is 35 nm or less as measured by a pore size analyzer.

6. The polyolefin microporous membrane according to claim 1 or 3, wherein the shutdown temperature is 140°C or less.

7. The polyolefin microporous membrane according to claim 1 or 3, wherein the standard deviation of the orientation parameter values of a total of 12 points measured at intervals of 15° with respect to an arbitrary direction in the plane of the microporous membrane by Raman spectroscopy is 0.30 or less.

8. The polyolefin microporous membrane according to claim 1 or 3, wherein the melting point as measured by differential scanning calorimetry (DSC) is 134°C or less.

9. The polyolefin microporous membrane according to claim 1 or 3, wherein, in a molecular weight differential distribution curve as measured by gel permeation chromatography (GPC), the maximum molecular weight in the range of 100,000 to 10 million in which the height with respect to the maximum value is 20% is set as Ml, and the minimum molecular weight is set as M2, and Formula 4 shown below is satisfied: Formula 4: Ml / M2 ≥ 500.

10. The polyolefin microporous membrane according to claim 1 or 3, wherein, in a molecular weight differential distribution curve as measured by gel permeation chromatography (GPC), the area ratio of the molecular weight of 500,000 or less is 10% or more with respect to the peak area of the entire molecular weight component, and the area ratio of the molecular weight of 10 million or more is 10% or more.

11. A battery separator, wherein the polyolefin microporous membrane according to claim 1 or 3 is used.

12. A secondary battery, wherein the battery separator according to claim 11 is used.

14. A filter for a liquid, wherein the polyolefin microporous membrane according to claim 1 or 3 is used.

13. A polyolefin microporous membrane, a standard deviation of orientation parameter values measured by Raman spectroscopy at a total of 12 points at intervals of 15° with respect to an arbitrary direction in-plane of the microporous membrane is 0.3 or less, and a puncture strength converted into a weight per unit area is 700 mN / (g / m 2 ) or more.

15. A filtration unit, wherein the filter for a liquid according to claim 14 is used. ​

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