WO2024019069A1

WO2024019069A1 - Polyolefin microporous membrane, separator for batteries, and battery

Info

Publication number: WO2024019069A1
Application number: PCT/JP2023/026352
Authority: WO
Inventors: 寛也岡本; 和久光畑; 颯也熊谷
Original assignee: 東レ株式会社
Priority date: 2022-07-20
Filing date: 2023-07-19
Publication date: 2024-01-25

Abstract

The present invention addresses the problem of providing a separator which achieves a good balance between ion permeability and withstand voltage properties at high levels by adequately controlling the pore diameters of a polyolefin microporous membrane. The present invention provides a polyolefin microporous membrane which has the features (1) and (2) described below. (1) The mean flow pore diameter as determined by a porometer is 32 nm or less. (2) With respect to the pore diameter distribution as determined by a porometer, the skewness is -1.0 to 1.5.

Description

Polyolefin microporous membrane, battery separator and battery

The present invention relates to a polyolefin microporous membrane, a battery separator, and a battery.

Microporous membranes are used in various fields such as filters such as filtration membranes and dialysis membranes, separators for batteries and separators for electrolytic capacitors. Among these, in recent years, it has attracted attention as a battery separator used in lithium ion batteries, which are small, lightweight, and have high energy density.

Battery separators are required to have various properties such as electrical insulation, chemical stability, mechanical strength, and ion permeability from the viewpoint of battery safety and output characteristics. In particular, lithium-ion batteries have a high energy density, so if a short circuit occurs, the battery temperature will rise rapidly, leading to the risk of fire or explosion. Therefore, separators used in lithium ion batteries are required to have a shutdown (SD) function that melts the separator to close the pores and stop the battery reaction when the temperature rises excessively. As a separator for lithium ion batteries, microporous polyolefin membranes are particularly preferably used from the viewpoint of chemical stability and shutdown characteristics.

In recent years, in order to increase the capacity and output of lithium ion batteries, separators are required to be thinner and have higher ion permeability. As the separator becomes thinner, the distance between the electrodes becomes shorter and short circuits are more likely to occur. Therefore, the separator is required to have higher voltage resistance (dielectric breakdown field strength). In addition to this, in recent years there has been a demand for improvements in the mechanical strength of separators and reductions in shutdown temperatures in order to improve battery safety.

For example, Patent Document 1 describes that a separator with excellent voltage resistance can be produced by controlling the viscosity average molecular weight of the polyolefin constituting a microporous polyolefin membrane.

Additionally, Patent Document 2 describes that by containing polypropylene as a raw material and making it a laminated separator with a polyethylene layer, a separator with excellent permeability can be produced despite having small pores.

JP2022-051238A JP2022-059158A

The ion permeability, voltage resistance, mechanical strength, and low-temperature shutdown characteristics required of lithium-ion battery separators are often in a trade-off relationship, and it is extremely difficult to achieve high levels of both.

For example, Patent Document 1 describes that by increasing the viscosity average molecular weight of the polyolefin constituting a microporous polyolefin membrane, the pore diameter can be made smaller and more uniform, and a separator with excellent voltage resistance can be produced. There is. However, such a separator with a small pore size has a high air permeation resistance and tends to have insufficient ion permeability. Furthermore, there is no quantitative description of the uniformity of pore diameters, and the relationship with voltage resistance is not clear.

Patent Document 2 uses the ratio of distribution intensity near the pore diameter that shows the maximum peak in the pore size distribution curve as a measure of pore diameter uniformity, but does not describe the relationship between the ratio and withstand voltage property. do not have. Since dielectric breakdown is likely to occur in the large pore diameter region of the separator, the evaluation index of pore diameter uniformity described in Patent Document 2 is insufficient to guarantee the voltage resistance of the separator. In addition, the separator described in Patent Document 2 has a laminated structure of a layer containing polypropylene and a layer consisting only of polyethylene to achieve both small pore size and high permeability. The structure is not uniform and is considered to be disadvantageous in terms of voltage resistance.

In view of these circumstances, the present invention aims to provide a separator that achieves both ion permeability and voltage resistance at a high level by appropriately controlling the pore diameter of a microporous polyolefin membrane. .

In order to solve the above problems, the present inventors conducted intensive studies and found that by controlling the average flow diameter and skewness in the pore size distribution curve of a microporous polyolefin membrane within a predetermined range, high ion permeability and high resistance can be achieved. It was discovered that it is possible to achieve both voltage characteristics, and the present invention was achieved. The configuration of the present invention is shown below.
(1) A polyolefin microporous membrane having the following characteristics A and B.
A. The average flow diameter measured with a porometer is 32 nm or less.
B. The skewness in the pore size distribution curve measured with a porometer is -1.0 or more and 1.5 or less.
(2) The polyolefin microporous membrane described in (1) above, which has the characteristics C and D below.
C. In the differential molecular weight distribution curve of the polyolefin constituting the polyolefin microporous membrane measured by GPC, the molecular weight at the maximum peak is 4.0 × 10 ⁵ or more.
D. In the differential molecular weight distribution curve of the polyolefin constituting the polyolefin microporous membrane measured by GPC, the area ratio of the molecular weight of 3.0 x 10 ⁴ or less is 15% or more.
(3) The microporous polyolefin membrane according to (1) or (2) above, which has a puncture strength in terms of basis weight of 80 gf/(g/m ² ) or more.
(4) The microporous polyolefin membrane according to any one of (1) to (3) above, which has a shutdown temperature of 138° C. or lower.
(5) A battery separator comprising the microporous polyolefin membrane according to any one of (1) to (4) above.
(6) A battery comprising the battery separator described in (5) above.

The microporous polyolefin membrane of the present invention has both high ion permeability and high voltage resistance, and can provide a high level of output characteristics and short circuit resistance in a battery incorporated as a battery separator.

The present invention relates to a microporous polyolefin membrane with defined mean flow diameter and skewness in a pore size distribution curve.

The configuration of the present invention will be described in detail below based on an example of an embodiment, but the present invention is not limited thereto, and various modifications can be made without departing from the gist thereof.

<Average flow diameter>
The average flow diameter of the polyolefin microporous membrane in the present invention was determined by POROUS MATERIALS, INC. under the measurement conditions described below. Calculated using the dry sample ventilation curve (DryCurve) and wet sample ventilation curve (WetCurve) obtained using a Palm porometer (Model: CFP-1500A) made by Co., Ltd. or a Palm porometer with an equivalent measurement function. represents a value.

The polyolefin microporous membrane of the present invention has an average flow diameter of 32 nm or less. The thickness is more preferably 31 nm or less, and even more preferably 30 nm or less. By setting the average flow diameter to 32 nm or less, the polyolefin microporous membrane has excellent voltage resistance. The lower limit of the average flow diameter is substantially 10 nm or more. By setting the average flow diameter to 10 nm or more, the polyolefin microporous membrane has excellent ion permeability. Setting the average flow diameter within the above range can be achieved by appropriately adjusting the molecular weight distribution of the polyolefin constituting the microporous polyolefin membrane and the film forming conditions. For example, when the molecular weight of the polyolefin constituting the polyolefin microporous membrane is increased, or when the concentration of the polyolefin resin in the polyolefin solution is increased, the number of crosslinking points of molecular chains increases during melt-kneading of the polyolefin solution, which tends to reduce the average flow diameter. In order to increase the molecular weight of the polyolefin constituting the polyolefin microporous membrane, it is possible to increase the molecular weight during melt-kneading by using a polyolefin raw material with a high molecular weight or by increasing the ratio of screw rotation speed to extrusion amount in a twin-screw extruder. Examples of methods include suppressing molecular weight deterioration due to shearing.

<Skewness in pore size distribution curve>
The skewness in the pore size distribution curve of the microporous polyolefin membrane of the present invention was determined by POROUS MATERIALS, INC. under the measurement conditions described below. The values are measured using a palm porometer (model: CFP-1500A) manufactured by Co., Ltd. or a palm porometer with an equivalent measurement function.

The polyolefin microporous membrane of the present invention has a skewness of −1.0 or more and 1.5 or less in a pore size distribution curve described below. More preferably -0.8 or more and 1.0 or less, still more preferably -0.5 or more and 0.5 or less. Skewness in a pore size distribution curve represents the degree of bias in the distribution; a high value means that there is a large distribution of pore diameters larger than the average flow diameter, and a low value means that there are many distributions with pore diameters smaller than the average flow diameter. It means that. In addition, the skewness is strongly influenced by the pore diameter, which has a large difference from the average flow diameter. Therefore, if the skewness in the pore size distribution curve is too low, the ion permeability of the polyolefin microporous membrane will be low due to the presence of regions with insufficient openings, and if it is too high, the ion permeability of the polyolefin microporous membrane will be low when a voltage is applied. Since the acceleration of the charges is promoted, the voltage resistance of the polyolefin microporous membrane becomes low. In particular, dielectric breakdown of a microporous polyolefin film is a local phenomenon, and if there is even a partial portion with large pores, it is thought that the voltage resistance of the microporous polyolefin film will be significantly reduced. Although it is not necessarily clear why some areas have large pores, the pore-opening process in microporous polyolefin membranes proceeds through the destruction of spherulites due to yielding during stretching, followed by the opening of micropores through stretching. Therefore, it is estimated that if there is structural unevenness or stretching unevenness at the time of crystallization, large pores created in the step of spherulite destruction at the initial stage of stretching will remain, and the distribution will tend to be biased towards the large pore diameter side.

When the skewness in the pore size distribution curve is within the above range, the polyolefin microporous membrane has an excellent balance between air permeability resistance and voltage resistance. Setting the skewness in the pore size distribution curve within the above range can be achieved by appropriately adjusting the molecular weight distribution of the polyolefin constituting the microporous polyolefin membrane and the film forming conditions. For example, increasing the molecular weight of the polyolefin constituting a microporous polyolefin membrane, or increasing the concentration of polyolefin resin in the polyolefin solution, increases the number of crosslinking points in molecular chains during melt-kneading of the polyolefin solution, which reduces structural unevenness and stretching unevenness. , the absolute value of skewness becomes smaller. Further, by narrowing the molecular weight distribution of the polyolefin constituting the microporous polyolefin membrane, structural unevenness and stretching unevenness are similarly reduced, and the absolute value of the skewness is reduced. Increasing the stretching ratio is also effective as a means for eliminating stretching unevenness.

<Peak molecular weight in differential molecular weight distribution curve>
The polyolefin microporous membrane of the present invention has a differential molecular weight distribution curve of the polyolefin constituting the polyolefin microporous membrane obtained by GPC, in which the value dw/dlog (M) obtained by differentiating the concentration fraction w by the logarithm of the molecular weight is the maximum. The molecular weight (hereinafter referred to as the molecular weight at the maximum peak) is preferably 4.0×10 ⁵ or more, more preferably 5.0×10 ⁵ or more. The upper limit is preferably 1.0×10 ⁶ or less from the viewpoint of shutdown temperature and the like. If the molecular weight at the maximum peak is within the above range, the high molecular weight component will form the main skeleton of the polyolefin that makes up the microporous polyolefin membrane, and the number of crosslinking points in the molecular chain will increase, which will prevent the formation of unopened pores and large pores. suppressed. Therefore, the absolute value of skewness tends to become small. Further, when the stretching ratio is increased, the pore size is promoted to be smaller, and in addition, the puncture strength can be increased.

<Area ratio with a molecular weight of 30,000 or less in the differential molecular weight distribution curve>
In the microporous polyolefin membrane of the present invention, it is preferable that the area ratio of the molecular weight of 30,000 or less in the entire differential molecular weight distribution curve of the polyolefin constituting the microporous polyolefin membrane measured by GPC is 15% or more. Moreover, it is preferably 25% or less, more preferably 20% or less. When the area ratio of the molecular weight of 30,000 or less is within the above range, the shutdown temperature can be reduced while maintaining the crosslinking point density and strength of the skeleton structure of the polyolefin microporous membrane at a high level.

The maximum peak molecular weight in the differential molecular weight distribution curve and the area ratio with a molecular weight of 30,000 or less can be controlled by the molecular weight of the raw material used for the microporous polyolefin membrane and the melt-kneading conditions. For example, the molecular weight at the maximum peak in the differential molecular weight distribution curve can be increased by using a high molecular weight raw material and a low molecular weight raw material and setting the ratio of the low molecular weight raw materials to a certain level or more. In addition, the molecular weight at the maximum peak in the differential molecular weight distribution curve can be increased by increasing the ratio of the extrusion amount to the screw rotation speed in a twin-screw extruder to suppress molecular weight deterioration due to shearing during melt-kneading. Can be done.

<Piercing strength converted to basis weight>
The polyolefin microporous membrane of the present invention preferably has a value obtained by dividing the maximum load by the basis weight in a puncture test described below (hereinafter referred to as basis weight equivalent puncture strength) of 80 gf/(g/m ² ) or more. Note that in the description of the puncture test described below, the model number of the device used is specified, but this does not preclude the use of a measuring device with equivalent functionality. When the puncture strength in terms of basis weight is within the above range, membrane rupture due to electrode protrusions or foreign matter is prevented when the battery is rolled up, and short-circuit resistance is increased. The upper limit of the puncture strength in terms of fabric weight is not particularly limited, but is preferably 130 gf/(g/m ² ) or less from the viewpoint of suppressing a rise in shutdown temperature. Setting the puncture strength within the above range is possible by adjusting the molecular weight of the polyolefin constituting the microporous polyolefin membrane and film forming conditions. For example, by increasing the molecular weight of the polyolefin constituting the microporous polyolefin membrane, the entanglement between molecular chains becomes stronger and the puncture strength can be controlled to be higher. In addition, by increasing the stretching ratio, oriented crystallization of molecular chains is promoted, and the puncture strength can be controlled in a direction that increases.

<Shutdown temperature>
The polyolefin microporous membrane of the present invention preferably has a shutdown temperature of 138°C or lower. If the shutdown temperature is within the above range, even if the battery temperature suddenly rises, the separator will melt before ignition and stop the battery reaction by blocking the flow of ions, which will improve safety in the event of an abnormality. It can be made into The lower limit of the shutdown temperature is preferably 120° C. or higher from the viewpoint of the operating temperature range of the battery. Setting the shutdown temperature within the above range is possible by adding low molecular weight components or adjusting film forming conditions. For example, in the differential molecular weight distribution curve of the polyolefin constituting the microporous polyolefin membrane, if the area ratio of the low molecular weight region is increased, the shutdown temperature can be reduced. The low molecular weight range is, for example, a range where the molecular weight is 30,000 or less. Alternatively, oriented crystallization can be suppressed and the shutdown temperature can be lowered by decreasing the stretching ratio or increasing the stretching temperature, but this reduces the puncture strength. Therefore, achieving both desired puncture strength and shutdown temperature is achieved by controlling the molecular weight distribution of the polyolefin constituting the microporous polyolefin membrane and adjusting film forming conditions suitable for the molecular weight distribution.

<Dielectric breakdown voltage>
This is a characteristic measured by a withstand voltage test described later, and the minimum voltage at which a short circuit occurs when a voltage is applied to a microporous polyolefin membrane is called a dielectric breakdown voltage. Note that in the description of the withstand voltage test described later, the model number of the device used is specified, but this does not preclude the use of a measuring device with equivalent functionality. Since the dielectric breakdown voltage is proportional to the film thickness, it is necessary to increase the dielectric breakdown voltage per film thickness in order to obtain a high dielectric breakdown voltage even if the film is made thinner. The dielectric breakdown voltage per film thickness is preferably 0.20 kV/μm or more. Although the upper limit is not particularly determined, it is preferably 0.50 kV/μm or less from the viewpoint of suppressing an increase in air permeability resistance. By setting the dielectric breakdown voltage within the above range, the separator provided with the polyolefin microporous membrane of the present invention has suitable voltage resistance when used in a battery.

<Air permeability resistance>
This is a property measured by the test described below, and represents the difficulty of air passing through a microporous polyolefin membrane. When the air permeability resistance is low, the lithium ion permeability of the polyolefin microporous membrane is improved. Although the model number of the device used is specified in the description of the test described below, this does not preclude the use of a measuring device with equivalent functionality. The air permeability resistance is proportional to the film thickness, and the air permeation resistance per film thickness is preferably in the range of 10 seconds/100 cc/μm to 20 seconds/100 cc/μm. By setting it within the above range, the ion permeability and membrane strength of the polyolefin microporous membrane become suitable.

<Production method of polyolefin microporous membrane>
The polyolefin microporous membrane of the present invention is preferably a single layer from the viewpoint of structural uniformity, but may be a multilayer microporous membrane consisting of a plurality of layers. The number of laminated layers is not particularly limited, and may be two layers or three or more layers.

In the present invention, the direction parallel to the film forming direction of the polyolefin microporous membrane is referred to as the film forming direction, longitudinal direction, or MD (Machine Direction), and the direction perpendicular to the film forming direction within the surface of the polyolefin microporous membrane is referred to as the film forming direction. is called the width direction or TD (Transverse Direction).

The manufacturing method for manufacturing the polyolefin microporous membrane of the present invention preferably includes the following steps (1) to (6).
(1) A step of melt-kneading a polyolefin resin with a solvent to prepare a polyolefin solution. (2) A step of extruding the polyolefin solution and then cooling it to form a gel sheet. (3) A step of stretching the gel sheet in uniaxial or biaxial directions. Step (4) Step of removing the solvent from the stretched film and drying it. (5) Step of re-stretching the dried film in uniaxial or biaxial directions and heat-treating it. (6) After winding the heat-treated film around a core, Steps of performing aging treatment according to the following. Each step will be explained below.

(1) Preparation of polyolefin solution The polyolefin microporous membrane is a microporous membrane containing one or more types of polyolefin resins, and may contain two or more different types of polyolefin resins. In addition, from the viewpoint of achieving both mechanical strength and shutdown characteristics, it is particularly preferable to contain two or more types of polyolefin resins.

The polyolefin resin is preferably a homopolymer of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, etc., and a homopolymer of ethylene (polyethylene) is particularly preferred. Polyethylene may be a homopolymer of ethylene or a copolymer containing ethylene and other α-olefins.

The polyolefin resin used in the present invention must contain 40% by mass or more of an ultra-high molecular weight polyolefin resin (UHPO) having a weight average molecular weight (Mw) of 1.0×10 ⁶ or more when the entire polyolefin resin is 100% by mass. is preferred. The content of the ultra-high molecular weight polyolefin resin is more preferably 45% by mass or more, and even more preferably 50% by mass or more. When the content ratio of ultra-high molecular weight polyolefin resin (UHPO) with Mw of 1.0 x 10 ⁶ or more is within the above range, the network of molecular chains during melt kneading becomes strong and the number of crosslinking points increases, resulting in formation of large pores. The average flow diameter becomes smaller, and the skewness in the pore size distribution becomes smaller. In addition, the stretching stress increases, making it easier to obtain a high-strength microporous polyolefin membrane. In addition, it is preferable to also contain high-density polyolefin (HDPO) having an Mw of 3.0×10 ⁵ or less. The content of the high-density polyolefin resin having a Mw of 3.0×10 ⁵ or less is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more. By containing a high-density polyolefin resin with Mw in the above range, the melting point and shutdown temperature of the entire microporous polyolefin membrane can be lowered without significantly affecting the skeleton structure of the microporous polyolefin membrane.

The microporous polyolefin membrane may contain resin components other than polyethylene resin and polypropylene resin as necessary. In addition, various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, fillers, crystal nucleating agents, and crystallization retarders may be added to the extent that they do not impair the effects of the present invention. May be contained.

The polyolefin solution is prepared by heating and dissolving the polyolefin resin in a plasticizer. The plasticizer is not particularly limited as long as it is a solvent that can sufficiently dissolve the polyolefin resin, but in order to enable stretching at a high magnification, it is preferable to use a solvent that is liquid at room temperature. As the solvent, aliphatic, cycloaliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin, mineral oil fractions with boiling points corresponding to these, and dibutyl phthalate, Examples include phthalic acid esters that are liquid at room temperature, such as dioctyl phthalate. In order to obtain a gel-like sheet with a stable liquid solvent content, it is particularly preferable to use a nonvolatile liquid solvent such as liquid paraffin. The concentration of the polyolefin resin in the polyolefin solution when the polyolefin resin is dissolved in the plasticizer is preferably 20 to 35 parts by mass, more preferably 23 to 30 parts by mass, based on the plasticizer. As the concentration of the polyolefin resin in the polyolefin solution increases, the pore size becomes smaller, and the skewness in the pore size distribution becomes smaller, making it possible to obtain a microporous polyolefin membrane with an excellent balance between voltage resistance and permeability. On the other hand, if the concentration of the polyolefin resin in the polyolefin solution becomes too high, the fluidity of the polyolefin solution decreases, causing uneven kneading, and the appearance of the polyolefin microporous membrane deteriorates. By setting the polyolefin resin concentration in the polyolefin solution within the above range, a microporous polyolefin membrane with an excellent balance of voltage resistance, permeability, and appearance can be obtained.

(2) Formation of extrudate and gel sheet In the extruder, the polyolefin solution is uniformly mixed at a temperature at which the polyolefin resin completely melts. The resin temperature during melt-kneading (abbreviated as melt-kneading temperature) needs to be set differently depending on the polyolefin resin used, but it ranges from (melting point of polyolefin resin + 10°C) to (melting point of polyolefin resin + 120°C). It is preferable to do so. More preferably, the temperature range is from (melting point of polyolefin resin +20°C) to (melting point of polyolefin resin +100°C). Here, the melting point refers to a value measured by DSC based on JIS K7121:2012 (the same applies hereinafter). Specifically, for example, since the polyethylene composition has a melting point of about 120 to 140°C, the melt-kneading temperature is preferably 130 to 260°C, more preferably 160 to 230°C.

By setting the melt-kneading temperature within the above range, it is possible to suppress the generation of unmelted substances in the extrudate, which may cause membrane rupture or the like in the subsequent stretching step. In addition, thermal decomposition of the polyolefin is suppressed, and a decrease in strength of the resulting microporous polyolefin membrane can be suppressed. Furthermore, it is preferable because deterioration of the appearance due to decomposition products of the extrudate depositing on cooling rolls, rolls during the stretching process, etc. and adhering to the sheet is suppressed.

Next, a gel-like sheet is obtained by cooling the obtained extrudate. Cooling allows the solvent-separated polyolefin microphase to be immobilized. In the cooling step, the gel sheet is preferably cooled to 10 to 50°C. Cooling is preferably carried out at a rate of 30° C./min or more until the temperature reaches 100° C. or less. If the cooling rate is less than 30° C./min, the crystallinity will increase and it will be difficult to form a gel-like sheet suitable for stretching. Generally, when the cooling rate is slow, relatively large crystals are formed, so that the higher-order structure of the gel-like sheet becomes coarse and the gel structure forming it also becomes large. On the other hand, when the cooling rate is high, relatively small crystals are formed, so that the higher-order structure of the gel-like sheet becomes dense, which leads to uniform stretching and increased toughness of the film.

Cooling methods include direct contact with cold air, cooling water, or other cooling medium, contact with a roll cooled with a refrigerant, and use of a casting drum.

In addition to the above-mentioned single layer, the microporous polyolefin membrane of the present invention may be a laminate in which gel-like sheets each containing a desired resin are laminated to the extent that the effects of the present invention are not impaired. To make a laminate from a polyolefin microporous membrane, for example, desired resins are prepared as necessary, these resins are separately fed to an extruder, mixed with a solvent, melted at a desired temperature, and the polymer is made into a laminate. There is a method of forming a laminate by merging them inside a tube or die and extruding them from a slit-shaped die to the desired thickness of each layer.

(3) Stretching process Next, the obtained gel-like sheet is stretched. Stretching methods used include MD uniaxial stretching using a roll stretching machine, TD uniaxial stretching using a tenter, sequential biaxial stretching using a combination of a roll stretching machine and a tenter, or a combination of a tenter and a tenter, and simultaneous biaxial stretching using a simultaneous biaxial tenter. can be mentioned. Although the stretching ratio varies depending on the thickness of the gel-like sheet from the viewpoint of uniformity of film thickness, it is preferable to stretch the gel sheet to 5 times or more in any direction. The area magnification is preferably 25 times or more, more preferably 35 times or more, and even more preferably 45 times or more. By setting the area magnification within the above range, it is possible to prevent the uniformity of the film from being impaired due to insufficient stretching, and to obtain a polyolefin microporous film that is excellent in terms of voltage resistance and strength. Further, the area magnification is preferably 150 times or less. By setting the area magnification to 150 times or less, it is possible to prevent tearing during the production of the microporous polyolefin membrane and reduce productivity, and also to prevent the melting point and shutdown of the microporous polyolefin membrane due to excessive orientation. Prevents temperature from rising.

The stretching temperature is preferably below the melting point of the gel-like sheet +10°C, and more preferably within the range of (crystal dispersion temperature T _cd of the polyolefin resin) to (melting point of the gel-like sheet +5°C). Specifically, for example, a polyethylene composition has a crystal dispersion temperature of about 90 to 100°C, so the stretching temperature is preferably 90 to 130°C, more preferably 90 to 125°C. The crystal dispersion temperature T _cd is determined from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D4065 (1995). By setting the stretching temperature to 90° C. or higher, the openings become sufficient, uniformity of the film thickness is obtained, and the porosity becomes suitable. By setting the stretching temperature to 130° C. or lower, clogging of pores due to melting of the sheet is suppressed, and ion permeability becomes suitable.

(4) Washing/Drying Step Next, the solvent remaining in the stretched gel-like sheet is removed using a washing solvent. Since the polyolefin phase and the solvent phase are separated, a microporous polyolefin membrane is obtained by removing the solvent. Examples of cleaning solvents include saturated hydrocarbons such as pentane, hexane, and heptane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; ethers such as diethyl ether and dioxane; ketones such as methyl ethyl ketone; and trifluoroethane. Examples include chain fluorocarbons. These cleaning solvents have low surface tension, so the network structure that forms micropores suppresses shrinkage due to the surface tension of the air-liquid interface during drying after cleaning, and polyolefins with suitable porosity and ion permeability are used. A microporous membrane is obtained. These cleaning solvents are appropriately selected depending on the plasticizer and used alone or in combination.

Cleaning can be carried out by immersing the gel-like sheet in a cleaning solvent and extracting it, by showering the gel-like sheet with a cleaning solvent, or by a combination of these methods. The amount of cleaning solvent used varies depending on the cleaning method, but is generally preferably 300 parts by mass or more per 100 parts by mass of the gel sheet. The washing temperature may be 15 to 30°C, and if necessary, the temperature may be heated to 80°C or lower. At this time, the viewpoints of improving the cleaning effect of the solvent, preventing the physical properties of the obtained microporous polyolefin membrane from becoming uneven in TD and/or MD, and the mechanical properties of the microporous polyolefin membrane And from the viewpoint of improving electrical properties, the longer the time the gel-like sheet is immersed in the cleaning solvent, the better. The above-mentioned cleaning is preferably carried out until the residual solvent in the gel sheet, that is, the polyolefin microporous membrane after cleaning, becomes less than 1% by mass.

Thereafter, in a drying step, the solvent in the polyolefin microporous membrane is dried and removed. The drying method is not particularly limited, and a method using a metal heating roll, a method using hot air, etc. can be selected. The drying temperature is preferably 40 to 100°C, more preferably 40 to 80°C. Sufficient drying can suppress a decrease in porosity and deterioration in ion permeability of the polyolefin microporous membrane in the subsequent heat treatment step.

(5) Re-stretching/heat treatment step The dried microporous polyolefin membrane may be stretched (re-stretched) in at least one direction. Re-stretching can be carried out by the tenter method or the like in the same manner as the above-mentioned stretching while heating the polyolefin microporous membrane. The re-stretching may be uniaxial or biaxial stretching.

The re-stretching temperature is preferably below the melting point of the polyolefin composition, more preferably within the range of (T _cd -20°C) to the melting point. Specifically, the temperature is preferably 70 to 135°C, more preferably 110 to 132°C. Most preferably it is 120-130°C.

In the case of uniaxial stretching, the re-stretching ratio is preferably 1.01 to 2.0 times, particularly TD is preferably 1.1 to 1.8 times, and more preferably 1.2 to 1.6 times. In the case of biaxial stretching, the MD and TD are preferably 1.01 to 1.6 times each. Note that the re-stretching ratio may be different between MD and TD. By stretching within the above-mentioned range, the porosity and average flow diameter can be increased, and the ion permeability can be increased. By stretching at a magnification of 2.0 or less, the orientation becomes appropriate and the melting point and shutdown temperature of the microporous polyolefin membrane become suitable. From the viewpoint of heat shrinkage rate and wrinkles and sagging, the relaxation rate from the maximum re-stretching ratio is preferably 0.95 or less, more preferably 0.9 or less.

(6) Winding/Aging Step The wound body obtained by winding up the polyolefin microporous membrane may be subjected to an aging treatment in a constant temperature warehouse. By aging, residual stress in the microporous polyolefin membrane is relaxed, and shrinkage around room temperature that occurs during storage or transportation of the microporous polyolefin membrane package is reduced. It is desirable that the aging be performed at a temperature lower than the crystal dispersion temperature of the polyolefin. The aging treatment temperature is preferably 40°C to 80°C, more preferably 45°C to 75°C, even more preferably 50°C to 70°C. By setting the aging treatment temperature to 40° C. or higher, shrinkage due to heat from outside air is suppressed during storage or transportation in a high-temperature environment. Setting the temperature to 80° C. or lower is preferable because it prevents the product from shrinking excessively and decreasing the width-cutting yield of the product since it is close to the crystal dispersion temperature.

<Battery separator and secondary battery>
The polyolefin microporous membrane of the present invention has both high ion permeability and high voltage resistance by controlling the average flow rate diameter and skewness in the pore size distribution curve within a predetermined range. A battery separator comprising the above is preferable because it has both high levels of ion permeability and voltage resistance.

Furthermore, a battery equipped with the battery separator of the present invention is preferable because it has both safety and output characteristics.

The method for measuring each physical property of the polyolefin microporous membrane used in the examples will be explained below. Note that for measurements where the number of measurement samples is not listed, only one sample is measured.

<Film thickness>
A test piece was cut into a 95 mm square piece from an arbitrary position of the polyolefin microporous membrane. Using a contact film thickness meter (manufactured by Mitutoyo, "Lightmatic" (registered trademark), VL-50B (carbide spherical measuring tip φ10.5 mm)), measure each of five arbitrary points on the test piece. The thickness of the sample was measured at a measurement pressure of 0.01N. The average value of the thicknesses at these five points was defined as the thickness of the polyolefin microporous membrane. The measurement environment was within the range of 23±2°C.

<Porosity>
A test piece was cut into a 95 mm square piece from an arbitrary position of the polyolefin microporous membrane. The porosity was determined from the volume (cm ³ ) and mass (g) of the test piece using the following formula.
Porosity = (1 - mass of test piece / (polyolefin resin density x volume of test piece)) x 100
The polyolefin resin density was 0.99 g/cm ³ .

<Piercing strength converted to basis weight>
A microporous polyolefin membrane with a basis weight of W1 (g/m ² ) fixed on a sample holder was pierced at a speed of 2 (mm/sec) with a needle with a diameter of 1 mm and a spherical tip using Kato Tech's KES-G5. The maximum load La at that time was measured under the following conditions. This measurement is sometimes abbreviated as a puncture test.
・Sample holder opening diameter: 11.3mm
・Curvature radius of needle tip: 0.5mm
・Piercing speed: 2mm/sec
・Ambient temperature: 23℃
From the measured value La of the maximum load and the basis weight W1 of the sample, the basis weight equivalent puncture strength Lb (gf/(g/m ² )) was obtained based on the following formula.
Lb=La/W1
<Skewness in average flow diameter and pore size distribution curve>
The average flow rate diameter and pore size distribution curve of the polyolefin microporous membrane were measured by the following method. First, a sample in a dry state (hereinafter also simply referred to as "dry sample") and a sample in a wet state whose pores are filled with a measurement liquid Galwick (perfluoropolyether) whose surface tension is known (hereinafter simply referred to as " (also referred to as "wet sample"), POROUS MATERIALS, INC. The relationship between air pressure and air flow rate was measured using a Palm porometer (model: CFP-1500A) manufactured by CFP Corporation, and a ventilation curve (DryCurve) for a dry sample and a ventilation curve (WetCurve) for a wet sample were obtained.

A wet sample whose pores are filled with the measuring liquid exhibits properties similar to those of a capillary tube filled with liquid. When a wet sample is set in a porometer and the air pressure is gradually increased, the air pressure overcomes the surface tension of the liquid to be measured in the pores, starting with the pores with the largest diameter, and the liquid to be measured is forced out of the pores. As the air flow rate gradually increases accordingly, the sample eventually becomes dry. Therefore, the pore diameter can be calculated by measuring the pressure at which liquid is forced out of the pore. Here, assuming that the shape of the pore is cylindrical, the conditions for air at pressure P to enter the pore of diameter D are as follows: where the surface tension of the measurement liquid is γ, and the contact angle of the measurement liquid is θ It is expressed by the Washburn equation shown in Equation 1 below.
PD=4γcosθ...(Formula 1)
The average flow diameter "d (μm)" was determined using the following formula.
d=C・γ/p
In the formula, "γ (dynes/cm)" is the surface tension of the liquid, "p (Pa)" is the pressure at the point where the WetCurve intersects the curve showing the slope of 1/2 of the DryCurve, and "C" is a constant. The Galwick surface tension used in the measurement of this example is γ=15.6 (dynes/cm), and the pressure constant C used in the measurement is 2860.

In addition, in a palm porometer, when the pressure at each measurement point is P _j , the air flow rate of a wet sample at pressure P _j is F _wet,j , and the air flow rate of a dry sample is F _dry,j , the cumulative filter flow rate is (CFF: Cumulative Filter Flow, unit: %) and pore size distribution (PSF: PoreSizeFrequency, unit: %) are each calculated by the following formulas.
CFF _j = [(F _{wet, j} /F _{dry, j} )×100]...(Formula 2)
PSF _j = (CFF) _j+1 - (CFF) _j ... (Formula 3)
By combining the above (Formula 1) to (Formula 3), it is possible to obtain a pore size distribution curve showing the relationship between the pore diameter Dj and the pore size distribution PSFj based on the pressure change of the air flow rate in the dry state and the wet state. can.

The skewness S in the pore size distribution curve was determined by the following formula. First, the pore size distribution PSF _sj was obtained by normalizing the total value of the pore size distribution PSF _j to 1 using (Equation 4), and the average pore diameter was obtained from the pore size distribution curve using (Equation 5), and was set as D _avg . Furthermore, the skewness S was determined using (Equation 6).
PSF _sj =PSF _j /ΣPSF _j ... (Formula 4)
D _avg = Σ(D _j ×PSF _sj )...(Formula 5)
S=Σ((D _j −D _avg ) ³ ×PSF _sj ) (Equation 6).

<Weight average molecular weight, maximum peak molecular weight in differential molecular weight distribution curve, and area ratio with molecular weight of 30,000 or less>
The weight average molecular weights of the polyolefin resin and the polyolefin constituting the microporous polyolefin membrane were determined by gel permeation chromatography (GPC) under the following measurement conditions. Moreover, the differential molecular weight distribution curve of the polyolefin constituting the polyolefin microporous membrane was calculated by the following procedure.

(A) Detection intensity (elution curve) with respect to elution time was calculated from the differential refractive index detector (RI detector) of GPC, and the elution time was converted into molecular weight. Here, the baseline of the elution curve was set as the starting point at the retention time at the peak rise, the end point at the retention time at the peak end, and the peak detection interval was 0.017 minutes.

(B) The intensity area was determined when the entire area ratio of the elution curve was taken as 100%, and the concentration fraction of each molecular weight was determined. An integrated molecular weight curve was obtained by sequentially integrating the concentration fractions and plotting the logarithm of the molecular weight (log (M)) on the horizontal axis and the integrated value of the concentration fraction w on the vertical axis.

(C) Calculate the differential value of the curve with respect to the logarithm of each molecular weight, and the horizontal axis is the logarithm of the molecular weight log(M), and the vertical axis is the value dw/dlog(M) obtained by differentiating the concentration fraction with the logarithm of the molecular weight. A differential molecular weight distribution curve was obtained by plotting. In the obtained differential molecular weight distribution curve, the molecular weight at which the value dw/dlog (M) obtained by differentiating the concentration fraction by the logarithm of the molecular weight was maximized was determined. Furthermore, the area percentage of the molecular weight of 30,000 or less was calculated when the entire area percentage of the obtained differential molecular weight distribution curve was taken as 100%.
Measurement conditions/measuring device: Agilent high temperature GPC device PL-GPC220
・Column: Agilent PL1110-6200 (20 μm MIXED-A) x 2 ・Column temperature: 160°C
・Solvent (mobile phase): 1,2,4-trichlorobenzene ・Solvent flow rate: 1.0 mL/min ・Sample concentration: 0.1% by mass (dissolution conditions: 160°C/3.5 hours)
・Injection volume: 500μL
・Detector: Agilent differential refractive index detector (RI detector)
- Viscometer: Agilent viscosity detector - Calibration curve: Created by the universal calibration curve method using a monodisperse polystyrene standard sample.

<Shutdown temperature>
The shutdown temperature of the polyolefin microporous membrane was measured by the following method. The polyolefin microporous membrane is exposed to an atmosphere at 30°C, and the air permeation resistance is measured while increasing the temperature at a rate of 5°C/min. The temperature at which the air permeability resistance of the polyolefin microporous membrane reached 100,000 seconds/100 cc was defined as the shutdown temperature. The air resistance was measured using an air resistance meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) in accordance with JIS P8117:2009.

<Air permeability resistance>
A test piece was cut into a 5 cm square piece from an arbitrary position of the microporous polyolefin membrane. Using the digital Oken type air resistance tester EGO-1T manufactured by Asahi Seiko Co., Ltd., fix the test piece so that there are no wrinkles in the measurement area, and measure the air resistance in accordance with JIS P8117:2009. The degree was measured.

<Dielectric breakdown voltage>
The dielectric breakdown voltage of the polyolefin microporous membrane was evaluated using the following method. A polyolefin microporous membrane cut into a circle with a diameter of 60 mm was placed on a square aluminum plate with sides of 150 mm, and a brass cylindrical electrode with a diameter of 50 mm, a height of 30 mm, and a weight of 500 g was placed on top of the membrane. An industrial TOS9201 dielectric breakdown property tester was connected. A voltage was applied at a boost rate of 0.1 kV/sec, and the voltage value at which dielectric breakdown occurred was defined as the dielectric breakdown voltage. The above operation was repeated 10 times by replacing the sample, and the average value of the obtained dielectric breakdown voltage values was determined. In addition, the average value of the obtained dielectric breakdown voltage values was converted into a value per film thickness, and was defined as the thickness-converted dielectric breakdown voltage (dielectric breakdown voltage/film thickness) of the microporous polyolefin film.

[Example 1]
21 parts by mass of a polyolefin resin containing 70% by mass of ultra-high molecular weight polyethylene with a Mw of 1.5×10 ⁶ and a melting point of 136°C and 30% by mass of high-density polyethylene with a Mw of 1.0×10 ⁵ and a melting point of 132°C. The mixture was charged into a screw extruder, and 79 parts by mass of liquid paraffin [35 cSt (40° C.)] was added from the side feeder of the twin screw extruder, and the mixture was melted and kneaded in the twin screw extruder to prepare a polyolefin solution. The obtained polyolefin solution was supplied from a twin-screw extruder to a T-die and extruded into a sheet-like molded product. The extruded molded product was taken up with a cooling roll to form a gel-like sheet. The obtained gel-like sheet was longitudinally stretched by a roll method at a stretching temperature of 109.0° C. so that the stretching temperature was 6.1 times. Subsequently, it was introduced into a tenter and transversely stretched at a stretching temperature of 127.0° C. and a stretching ratio of 7.6 times. The membrane after stretching was washed in a methylene chloride washing tank to remove liquid paraffin. The washed membrane was dried, re-stretched in a tenter at 130.8° C. at a stretching ratio of 1.6 times, and then thermally relaxed at a relaxation rate of 0.94%. The wound body was placed in a thermostatic chamber at 60° C. for 24 hours for aging treatment, and a microporous polyolefin membrane was obtained.

The results are shown in Table 1. In addition, in the table, "ultra high molecular weight polyethylene" is referred to as "UHPE", "high density polyethylene" is referred to as "HDPE", "polypropylene" is referred to as "PP", and "average flow diameter measured with a porometer" is referred to as "average flow diameter". ”, “Distortion in the pore size distribution curve measured with a porometer”, “Distortion in the pore size distribution curve”, “Molecular weight at the maximum peak in the differential molecular weight distribution curve of the polyolefin constituting the polyolefin microporous membrane measured by GPC” " is the "maximum peak molecular weight," and "area ratio with a molecular weight of 30,000 or less in the differential molecular weight distribution curve of the polyolefin constituting the polyolefin microporous membrane measured by GPC" is "area ratio with a molecular weight of 30,000 or less." Each is abbreviated.

[Example 2]
In polyethylene resin, the mixing ratio of ultra-high molecular weight polyethylene, the mixing ratio of low molecular weight polyethylene, the mixing ratio of polyolefin resin and liquid paraffin, the magnification and temperature of longitudinal stretching, the magnification and temperature of transverse stretching, and the magnification and temperature of re-transverse stretching. The polyolefin was produced in the same manner as in Example 1, except that the conditions were changed to those listed in Table 1, and the rotational speed and discharge rate of the twin-screw extruder were adjusted so that the final film thickness became the values listed in Table 1. A microporous membrane was obtained.

[Example 3]
20 parts by mass of a polyolefin resin containing 90% by mass of ultra-high molecular weight polyethylene with a Mw of 1.5×10 ⁶ and a melting point of 136° C. and 10% by mass of high-density polyethylene with a Mw of 1.0×10 ⁵ and a melting point of 132°C. The mixture was charged into a screw extruder, 80 parts by mass of liquid paraffin [35 cSt (40° C.)] was added from the side feeder of the twin screw extruder, and the mixture was melted and kneaded in the twin screw extruder to prepare a polyolefin solution. The obtained polyolefin solution was supplied from a twin-screw extruder to a T-die and extruded into a sheet-like molded product. The extruded molded product was taken up with a cooling roll to form a gel-like sheet. The obtained gel-like sheet was introduced into a tenter, and longitudinally stretched at a stretching temperature of 115.0°C to a stretching ratio of 8.0 times, and transversely stretched at a stretching temperature of 115.0°C to a stretching ratio of 8.0 times. was carried out. The membrane after stretching was washed in a methylene chloride washing tank to remove liquid paraffin. The washed membrane was dried and heat-treated at 125.0°C in a constant temperature bath to obtain a microporous polyolefin membrane.

[Comparative example 1]
Using a polyethylene resin composition consisting of 100% by mass of ultra-high molecular weight polyethylene with a Mw of 7.0×10 ⁵ and a melting point of 136° C., the mixing ratio of polyolefin resin and liquid paraffin, the longitudinal stretching ratio and temperature, and the transverse stretching ratio・Other than changing the temperature, the magnification and temperature of re-transverse stretching to the conditions listed in Table 1, and adjusting the rotation speed and discharge amount of the twin-screw extruder so that the final film thickness became the values listed in Table 1. A microporous polyolefin membrane was obtained in the same manner as in Example 1.

[Comparative example 2]
A polyethylene resin composition consisting of 30% by mass of ultra-high molecular weight polyethylene with a Mw of 2.0×10 ⁶ and a melting point of 133°C, and 70% by mass of high-density polyethylene with a Mw of 5.0×10 ⁵ and a melting point of 135°C. The mixing ratio of polyolefin resin and liquid paraffin, the magnification and temperature of longitudinal stretching, the magnification and temperature of transverse stretching, and the magnification and temperature of re-transverse stretching were changed to the conditions listed in Table 1, and the final film thickness was as shown in Table 1. A microporous polyolefin membrane was obtained in the same manner as in Example 1, except that the rotational speed and discharge amount of the twin-screw extruder were adjusted to the values described in Example 1.

[Comparative example 3]
A polyethylene resin composition consisting of 30% by mass of ultra-high molecular weight polyethylene with a Mw of 2.0×10 ⁶ and a melting point of 133°C, and 70% by mass of high-density polyethylene with a Mw of 5.0×10 ⁵ and a melting point of 135°C. The mixing ratio of polyolefin resin and liquid paraffin, the magnification and temperature of longitudinal stretching, the magnification and temperature of transverse stretching, and the magnification and temperature of re-transverse stretching were changed to the conditions listed in Table 1, and the final film thickness was as shown in Table 1. A microporous polyolefin membrane was obtained in the same manner as in Example 1, except that the rotational speed and discharge amount of the twin-screw extruder were adjusted to the values described in Example 1.

[Comparative example 4]
(Preparation of first polyolefin solution)
28.5 parts by mass of a first polyolefin resin consisting of 30% by mass of ultra-high molecular weight polyethylene with an Mw of 2.0×10 ⁶ and 70% by mass of high-density polyethylene with an Mw of 5.0×10 ⁵ was placed in a twin-screw extruder. Then, 71.5 parts by mass of liquid paraffin [35 cSt (40° C.)] was supplied from the side feeder of the twin-screw press and melt-kneaded to prepare a first polyolefin solution.

(Preparation of second polyolefin solution)
30 parts by mass of a second polyolefin resin consisting of 50% by mass of ultra-high molecular weight polyethylene with an Mw of 5.0×10 ⁵ and 50% by mass of polypropylene with an Mw of 1.6×10 ⁶ were charged into a twin-screw extruder. 70 parts by mass of liquid paraffin [35 cSt (40° C.)] was supplied from the side feeder of the axial extruder and melt-kneaded to prepare a second polyolefin solution.

(extrusion)
The first and second polyolefin solutions are supplied from each twin-screw extruder to the three-layer die, and the layer thickness ratio of the first layer/second layer/first layer is 35/30/35. I pushed it out like that. The extruded product was cooled while being taken up by a cooling roll whose temperature was controlled at 30° C. to form an unstretched gel-like three-layer sheet.

(First stretching, removal of film-forming solvent, drying)
The obtained unstretched gel-like three-layer sheet was subjected to simultaneous biaxial stretching of 5 times in MD and 5 times in TD using a tenter device set at a temperature of 113.0° C. as a first stretch. Heat setting was performed at 0°C to obtain a biaxially stretched sheet. The obtained biaxially stretched sheet was washed with methylene chloride to extract and remove residual liquid paraffin, and then dried. The dried biaxially stretched sheet was heated to 111.0°C for second stretching using a tenter-type stretching machine, and then laterally stretched again to a width 1.4 times the width at the entrance of the stretching machine, and then fixed. Heat treatment was performed at a temperature of 110.0° C. and a relaxation rate of 0.93 to obtain a polyolefin microporous membrane with a thickness of 9.5 μm. The blending ratio of each component, manufacturing conditions, physical properties, etc. of the obtained microporous polyolefin membrane are listed in Table 1.

[Comparative example 5]
Except that the mixing ratio of polyolefin resin and liquid paraffin was changed to the conditions listed in Table 1, and the rotation speed and discharge rate of the twin-screw extruder were adjusted so that the final film thickness was the value listed in Table 1. A microporous polyolefin membrane was obtained in the same manner as in Example 3.

(evaluation)
As shown in Table 1, the polyolefin microporous membranes of Examples 1 to 3 had small and large pores with an average flow diameter of 26.5 to 30.4 nm and a skewness of 0.3 to 1.1 in the pore size distribution curve. This resulted in a microporous polyolefin membrane with a suppressed pore size distribution on the pore diameter side, and was able to achieve both high voltage resistance and air permeation resistance.

On the other hand, in the polyolefin microporous membrane of Comparative Example 1, the average flow diameter was 40.1 nm, and in the polyolefin microporous membranes of Comparative Examples 2 to 5, the skewness in the pore size distribution curve was 1.6 to 3.3. The distribution on the large pore diameter side became larger. As a result, voltage resistance decreased or air permeation resistance increased.

The battery separator according to the embodiment of the present invention is applicable to nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, lithium ion secondary batteries, lithium polymer secondary batteries, lithium-sulfur batteries, etc. It can be suitably used as a separator for batteries such as secondary batteries. In particular, it is preferable to use it as a separator for lithium ion secondary batteries.

Claims

A microporous polyolefin membrane having the following characteristics (1) to (2).
(1) The average flow diameter measured with a porometer is 32 nm or less.
(2) The skewness in the pore size distribution curve measured with a porometer is -1.0 or more and 1.5 or less.
The polyolefin microporous membrane according to claim 1, having the following characteristics (3) to (4).
(3) In the differential molecular weight distribution curve of the polyolefin constituting the polyolefin microporous membrane measured by GPC, the molecular weight at the maximum peak is 4.0×10 5 or more.
(4) In the differential molecular weight distribution curve of the polyolefin constituting the polyolefin microporous membrane measured by GPC, the area ratio of the molecular weight of 3.0×10 4 or less is 15% or more.
The microporous polyolefin membrane according to claim 2, which has a puncture strength in terms of basis weight of 80 gf/(g/m 2 ) or more.
The microporous polyolefin membrane according to claim 3, having a shutdown temperature of 138°C or less.
A battery separator comprising the polyolefin microporous membrane according to any one of claims 1 to 4.
A battery comprising the battery separator according to claim 5.