Polyolefin microporous membrane

Abstract

Provided is a polyolefin microporous membrane containing polyethylene. The membrane has a porosity of 45% or more, and the all-trans amount of the polyethylene measured by Raman scattering spectroscopy is 1.30 or more.

Description

Polyolefin microporous membrane

[0001] This invention relates to polyolefin microporous membranes and the like.

[0002] Polyolefin microporous membranes (hereinafter sometimes simply abbreviated as "PO microporous membranes") are widely used for the separation of various substances, or as selective permeable separation membranes, isolation materials, etc. Their applications include, for example, microfiltration membranes; separators for batteries such as lithium-ion batteries and fuel cells; separators for capacitors; and base materials for functional membranes that allow functional materials to be filled into the pores to create new functions. Among these, PO microporous membranes are particularly suitable as separators for lithium-ion batteries (LIBs), which are widely used in mobile phones, smartphones, wearable devices, notebook personal computers (PCs), tablet PCs, digital cameras, and the like.

[0003] Conventionally, PO microporous membranes, used as separators for LIBs, have been required to possess dimensional stability at temperatures below the melting point of PO or under external stress, shutdown performance near the melting point, and even greater resistance to film rupture at high temperatures. Furthermore, various PO microporous membranes and their manufacturing methods have been proposed from the perspective of balancing the thermal stability of LIB separators with other properties.

[0004] For example, Patent Document 1 describes a polyolefin microporous membrane containing polyethylene, wherein the melting peak of the Nonreversing Heat Flow, measured by a temperature-modulated DSC method at a heating rate of 1°C / min, is in the range of 141.0°C to 150.0°C.

[0005] Patent document 2 describes a polyolefin microporous film having a film thickness of 12 μm or less, a Raman orientation parameter value in the mechanical direction of 14 or more, and a bubble point pore diameter of 25 nm or less.

[0006] Patent Document 3 describes a porous polyolefin film characterized by having a porosity of 50% or more, a puncture strength of 3.7 N or more based on a film thickness of 10 μm, and a longitudinal tensile modulus (MD tensile strength) of 980 MPa or more.

[0007] International Publication No. 2020 / 256138, Japanese Patent Publication No. 2020-095950, Japanese Patent Publication No. 2021-038379

[0008] Lithium-ion batteries used in smartphones, wearable devices, and other devices require fast charging performance. Achieving fast charging performance necessitates low battery resistance and high capacity retention at high charging rates, and one way to achieve this is by increasing the porosity of the separator. However, batteries using highly porosity separators are prone to voltage withstand failures, posing safety concerns.

[0009] Patent documents 1 to 3 discuss the safety of polyolefin microporous membranes, but there was room for improvement in achieving both high porosity and safety.

[0010] In view of the above circumstances, this disclosure aims to provide a polyolefin microporous membrane that is highly porosity yet excellent in safety, as well as a film roll and a lithium-ion battery using the same.

[0011] Examples of embodiments of the present invention are listed below: (1) A polyolefin microporous membrane containing polyethylene, wherein the porosity is 45% or more, and the all-trans amount of the polyethylene measured by Raman scattering spectroscopy is 1.30 or more. (2) The polyolefin microporous membrane according to item (1), wherein the sum of the crystalline phase ratio and the intermediate phase ratio of the polyethylene measured by Raman scattering spectroscopy is 0.75 or more. (3) The polyolefin microporous membrane according to item (1) or (2), wherein the branching amount of the polyethylene measured by NMR spectroscopy is 0.05 mol% or less. (4) The polyolefin microporous membrane according to any one of items (1) to (3), wherein the melting point at the second heating stage measured by DSC is 135°C or higher. (5) A polyolefin microporous membrane according to any of items (1) to (4), wherein the weight-average molecular weight (Mw) measured by high-temperature gel permeation chromatography (GPC) is 600,000 or more. (6) A polyolefin microporous membrane according to item (5), wherein the ratio of the weight-average molecular weight (Mw / Mn) measured by high-temperature GPC to the number-average molecular weight (Mn) measured by high-temperature GPC is 3.0 or more and 10 or less. (7) A polyolefin microporous membrane according to any of items (1) to (6), wherein the ratio of MD tensile strength to TD tensile strength (MD tensile strength / TD tensile strength) is 0.8 or more and 1.25 or less. (8) A puncture strength equivalent to a base weight of 120 gf / (g / m ２ ) A polyolefin microporous membrane as described in any of items (1) to (7), which is above (9) Air permeability resistance of 150 seconds / 100 cm ３The following polyolefin microporous membranes as described in any of items (1) to (8): (10) The polyolefin microporous membrane as described in any of items (1) to (9), wherein the average thermal shrinkage rate of MD and TD at 120°C is 30.0% or less. (11) A film roll formed by winding the polyolefin microporous membrane as described in any of items (1) to (10). (12) A separator for lithium-ion secondary batteries having an inorganic particle-containing layer with a thickness of 5 μm or less on at least one surface of the polyolefin microporous membrane as described in any of items (1) to (10). (13) A lithium-ion secondary battery comprising the separator for lithium-ion secondary batteries as described in item (12).

[0012] The polyolefin microporous membrane of this disclosure has high capacity and excellent safety when used as a separator for lithium-ion secondary batteries, and therefore can be suitably used as a separator for lithium-ion secondary batteries.

[0013] The following describes in detail embodiments for carrying out the present invention (hereinafter sometimes abbreviated as "this disclosure"), but the present invention is not limited thereto, and various modifications are possible without departing from its essence. In this specification, the flow direction of the film during film formation is defined as MD, and the direction (width direction) that intersects MD at a 90-degree angle in the film plane is defined as TD.

[0014] The polyolefin microporous membrane of this disclosure (in one embodiment, a PO microporous membrane) is a polyolefin microporous membrane containing polyethylene (PE), having a porosity of 45% or more, an all-trans amount of polyethylene measured by Raman scattering spectroscopy of 1.30 or more, and having the following characteristics.

[0015] If desired, the PO microporous film may also be specified in terms of film thickness, average pore size, thermal shrinkage rate, porosity, air permeability resistance, puncture strength, and basis weight puncture strength, as shown below, and an inorganic particle-containing layer or adhesive layer may be formed on its surface. The various properties described in this disclosure can be used independently or in any combination. Unless otherwise specified, the methods for measuring the physical properties of the PO microporous film are described in detail in the Examples section.

[0016] <Characteristics measured by Raman scattering spectroscopy> The polyolefin microporous film of this disclosure has an all-trans amount of polyethylene measured by Raman scattering spectroscopy within the range of 1.30 or more. When the all-trans amount of polyethylene measured by Raman scattering spectroscopy is within the above range, it is possible to manufacture a polyolefin microporous film that has high porosity while also exhibiting high strength and high dielectric strength.

[0017] In this disclosure, the all-trans amount of polyethylene, as measured by Raman scattering spectroscopy, is an index representing the proportion of polyethylene molecular chains where the C-C bond conformation is in the trans type. The all-trans amount of polyethylene originates from the trans-type C-C symmetric vibration of 1130 cm⁻¹. －１ The peak intensity is the peak intensity independent of the conformation (1298 cm). －１ , 1305cm －１ ) can be calculated by normalizing, and in detail, it is measured by the method described in the examples. In this disclosure, the crystalline part of polyethylene refers to the orthorhombic crystal, which is the stable structure of polyethylene. In this disclosure, the intermediate phase of polyethylene is a third phase that exists at the interface between the crystalline and amorphous parts of polyethylene, and refers to a partially ordered amorphous structure. In this disclosure, the amorphous part of polyethylene refers to the same structure as the molten state of polyethylene.

[0018] While this disclosure does not wish to be bound by theory, the inventors speculate that polyolefin microporous films with an all-trans amount of polyethylene of 1.30 or higher, as measured by Raman scattering spectroscopy, exhibit high strength and high dielectric strength despite their high porosity. The polyolefin microporous films of this disclosure have low resistance and high rate characteristics due to their high porosity, and because the all-trans amount of polyethylene measured by Raman scattering spectroscopy is a high value of 1.30 or higher, they form fibrils composed of highly oriented molecular chains, resulting in higher strength, fewer defects, and better safety features such as high dielectric strength compared to conventional products. In particular, it is important to increase the degree of orientation of the intermediate phase, and it has been found that the all-trans amount, which is the degree of orientation of the intermediate phase and amorphous region as well as the crystalline region, is important for achieving high strength and high dielectric strength.

[0019] The polyolefin microporous membrane of this disclosure, from the viewpoint of achieving both high strength and high dielectric strength while maintaining high porosity, preferably has an all-trans content of polyethylene of 1.30 or more, more preferably 1.40 or more, more preferably 1.50 or more, and even more preferably 1.70 or more, as measured by Raman scattering spectroscopy. The upper limit of the all-trans content of polyethylene is not particularly limited, but from the viewpoint of feasibility, it is preferable that the all-trans content of polyethylene is 2.90 or less as measured by Raman scattering spectroscopy.

[0020] From the viewpoint that increasing the proportion of the intermediate phase reduces defects and contributes to high dielectric strength, it is preferable that the sum of the crystalline phase ratio and the intermediate phase ratio of polyethylene, as measured by Raman scattering spectroscopy, be 0.75 or higher, more preferably 0.80 or higher, and particularly preferably 0.85 or higher. There is no particular upper limit to the sum of the crystalline phase ratio and the intermediate phase ratio of polyethylene, but from the viewpoint of feasibility, it is preferable that the sum of the crystalline phase ratio and the intermediate phase ratio of polyethylene, as measured by Raman scattering spectroscopy, be 0.99 or lower.

[0021] From the viewpoint that increasing the proportion of the crystalline phase reduces defects and contributes to high voltage resistance, the crystalline phase proportion of polyethylene measured by Raman scattering spectroscopy is preferably 0.50 or more, more preferably 0.60 or more, and particularly preferably 0.65 or more. In addition, the upper limit of the crystalline phase proportion of polyethylene measured by Raman scattering spectroscopy is not particularly limited, but from the viewpoint of feasibility, it is preferably 0.90 or less.

[0022] The crystalline phase proportion of polyethylene, the intermediate phase proportion of polyethylene, and the amorphous phase proportion of polyethylene are the CH of the crystalline phase, the intermediate phase, and the amorphous phase measured by peak Raman scattering spectroscopy ２ scissoring vibration derived from 1416 cm －１ , 1440 cm －１ and 1460 cm －１ It is obtained by calculating the proportion of each phase from the peak intensity ratio of the Raman peaks. Specifically, the crystalline phase proportion of polyethylene, the intermediate phase proportion of polyethylene, and the amorphous phase proportion of polyethylene are measured by the method described in the examples.

[0023] The all-trans content of polyethylene measured by Raman scattering spectroscopy, as well as the crystalline phase proportion of polyethylene and the intermediate phase proportion of polyethylene, can be adjusted within the numerical ranges explained above, for example, in the manufacturing process of a microporous polyolefin membrane, by the use of a single-component polyethylene raw material, the physical properties of the polyethylene used (e.g., melting point and molecular weight), stretching conditions (e.g., stretching ratio), and thermal relaxation conditions (e.g., relaxation rate, relaxation temperature, relaxation rate and cooling rate).

[0024] <Weight-average molecular weight of polyolefin microporous membrane> The weight-average molecular weight (Mw) of the polyolefin microporous membrane, measured by high-temperature gel permeation chromatography (GPC), is preferably 600,000 to 1,200,000, more preferably 700,000 to 1,000,000, and particularly preferably 800,000 to 950,000. It is preferable that the weight-average molecular weight (Mw) of the polyolefin microporous membrane is within the predetermined range because the ultra-high molecular weight leads to increased entanglement and high orientation of the molecular chains, increasing the all-trans content of polyethylene and resulting in high dielectric strength. The weight-average molecular weight (Mw) of the polyolefin microporous membrane is measured by the method described in the examples.

[0025] <Molecular weight distribution (Mw / Mn) of polyolefin microporous membrane> The ratio of the weight-average molecular weight (Mw) of the polyolefin microporous membrane, measured by high-temperature GPC, to the number-average molecular weight (Mn) measured by high-temperature GPC (molecular weight distribution: Mw / Mn) is preferably 3.0 or more and 10 or less, more preferably 4.0 or more and 8.0 or less, and particularly preferably 4.5 or more and 6.0 or less. When the molecular weight distribution (Mw / Mn) of the polyolefin microporous membrane is within the predetermined range, the molecular chain lengths become uniform and highly oriented, the all-trans amount of polyethylene increases and high dielectric strength is achieved, which is therefore preferable. The number-average molecular weight (Mn) of the polyolefin microporous membrane is measured by the method described in the examples.

[0026] The polyolefin microporous membrane of this disclosure preferably has a melting point of 135.0°C or higher, more preferably 135.5°C or higher, and particularly preferably 136.0°C or higher, as measured by DSC during the second heating stage. The polyolefin microporous membrane of this disclosure preferably has a melting point of 150.0°C or lower, as measured by DSC during the second heating stage. It is preferable that the melting point of the polyolefin microporous membrane of this disclosure is within a predetermined range, as this results in high orientation and fewer defects due to less branching of the polyethylene molecular chains, an increase in the all-trans content of polyethylene, and a high dielectric strength. The melting point of the polyolefin microporous membrane of the second heating stage, as measured by DSC, can be adjusted to within the numerical range described above by using a single-component polyethylene raw material and the physical properties of the polyethylene used (e.g., melting point and molecular weight). The melting point of the polyolefin microporous membrane of the polyolefin microporous membrane of the second heating stage, as measured by DSC, is measured by the method described in the examples.

[0027] <MD Tensile Strength and TD Tensile Strength> In this disclosure, the tensile strength in the longitudinal direction of the polyolefin microporous membrane is defined as the MD tensile strength, and the tensile strength in the width direction of the polyolefin microporous membrane is defined as the TD tensile strength. From the viewpoint of high dielectric strength and enhanced safety, the MD tensile strength and TD tensile strength of the polyolefin microporous membrane in this disclosure are preferably 2000 kgf / cm². ２ The pressure is above (approximately 196 MPa or higher), more preferably 2500 kgf / cm². ２ More preferably, 3000 kgf / cm² ２ In particular, 3500 kgf / cm² is preferred. ２ That concludes the explanation. From the viewpoint of suppressing thermal shrinkage, the MD tensile strength and TD tensile strength of the polyolefin microporous membrane of this disclosure are preferably 5000 kgf / cm. ２ More preferably, 4500 kgf / cm² ２ The following is particularly preferable: 4000 kgf / cm² ２The following applies. In order to achieve the above-described MD tensile strength and TD tensile strength of the polyolefin microporous membrane of this disclosure, it is preferable to set the raw material composition of the polyolefin microporous membrane within the range described below, and to set the stretching conditions during film formation within the range described below. The MD tensile strength and TD tensile strength of the polyolefin microporous membrane of this disclosure are measured by the method described in the examples.

[0028] <Ratio of MD tensile strength to TD tensile strength (MD tensile strength / TD tensile strength)> The polyolefin microporous film of this disclosure preferably has a ratio of MD tensile strength to TD tensile strength: MD tensile strength / TD tensile strength of 0.8 to 1.25, more preferably 0.85 to 1.17, and particularly preferably 0.9 to 1.11. When the MD tensile strength / TD tensile strength is within a predetermined range, the in-plane orientation of the polyolefin microporous film can be increased isotropically by stretching in both the MD and TD directions, resulting in a high dielectric strength, which is preferable.

[0029] <Viscosity-average molecular weight of PO microporous membrane> The viscosity-average molecular weight (Mv) of the PO microporous membrane according to this disclosure is preferably 400,000 or more and 2,000,000 or less. A higher molecular weight of the PO microporous membrane itself tends to result in increased membrane strength and improved safety in impact tests and oven tests. From this viewpoint, the Mv of the PO microporous membrane is more preferably 500,000 or more, even more preferably 600,000 or more, and most preferably 700,000 or more. On the other hand, adjusting the viscosity-average molecular weight of the PO microporous membrane to 2,000,000 or less is preferable from the viewpoint of suppressing thermal shrinkage by the heat-fixing (HS) process. From a similar viewpoint, the upper limit of the Mv of the PO microporous membrane is more preferably 1,500,000 or less.

[0030] In this specification, the viscosity-average molecular weight of a PO microporous membrane is obtained by measuring the Mv of the PO microporous membrane itself. The viscosity-average molecular weight of a PO microporous membrane can be adjusted to the above range, for example, by changing the composition ratio of raw material polymers with different molecular weights.

[0031] <Thermal Shrinkage Rate> With the expanding range of applications for electrochemical devices, it is considered important to control the thermal shrinkage rate of PO microporous films used as separators at high temperatures (e.g., near the melting point of PO, or near the melting point of the PO microporous film) in order to ensure device safety in high-temperature environments, such as oven tests, as well as in impact tests. PO microporous films in which PE crystals are highly oriented and whose thermal shrinkage rate is within a certain range are suitable from the viewpoint of ensuring good performance in impact tests and oven tests. Furthermore, it is also preferable to control the thermal shrinkage rate of separators for electrochemical devices to prevent contact between electrodes in energy storage devices.

[0032] More specifically, to improve safety in impact tests and oven tests, the upper limit of the thermal shrinkage rate of the PO microporous membrane at 120°C is preferably 35.0% or less, more preferably 30.0% or less, even more preferably 25.0% or less, and particularly preferably 20.0% or less for MD and TD. The lower limit of the thermal shrinkage rate of the PO microporous membrane at 120°C may be, for example, -5.0% or more, -2.0% or more, -1.0% or more, or 0.0% or more for MD and TD.

[0033] Means for controlling the thermal shrinkage rate of the PO microporous membrane at 120°C to within the above numerical range include, for example, adjusting the Mv of the PE raw material, and adjusting the TD stretching and / or relaxation ratio and temperature during the manufacturing process of the PO microporous membrane.

[0034] <Average Thermal Shrinkage Rate of MD and TD at 120°C> From the viewpoint of safety at high temperatures, the average thermal shrinkage rate of MD and TD at 120°C of the polyolefin microporous membrane of this disclosure is preferably 30.0% or less, more preferably 25.0% or less, and particularly preferably 20.0% or less. Means for controlling the average value of the thermal shrinkage rate of the PO microporous membrane at 120°C to be within the above numerical range include, for example, adjusting the Mv of the PE raw material, and adjusting the TD stretching and / or relaxation rate and temperature during heat setting (HS) in the manufacturing process of the PO microporous membrane. The thermal shrinkage rates of MD and TD of the polyolefin microporous membrane of this disclosure at 120°C are measured by the method described in the examples.

[0035] <Puncture Strength Based on Basis Weight> Basis weight (g / m²) of PO microporous membrane ２ The puncture strength obtained by dividing by (hereinafter referred to as the basis weight equivalent puncture strength) is 120 gf / (g / m²). ２ ) or more (approximately 1.177N / (g / m ２ It is preferable that the puncture strength of the PO microporous membrane is 120 gf / (g / m²). ２ When the value is above 140 gf / (g / m²), the safety of the impact test tends to be good. Based on this trend, the basis weight equivalent puncture strength of PO microporous membrane is 140 gf / (g / m²). ２ It is more preferable that it be 170 gf / (g / m³) or more, ２ It is even more preferable that it be ) or more. The upper limit of the puncture strength converted to basis weight should be, from the viewpoint of suppressing thermal shrinkage, for example, 250 gf / (g / m). ２ ) More preferably 200 gf / (g / m³) ２ ) may be less than or equal to the specified range. It is preferable that the basis weight equivalent puncture strength of the polyolefin microporous film be within a predetermined range, as this leads to a higher orientation of the molecular chains and increased safety. In order to make the basis weight equivalent puncture strength of the PO microporous film within the above range, it is preferable to set the stretching ratio during film formation of the polyolefin microporous film to the range described later. The basis weight equivalent puncture strength of the polyolefin microporous film of this disclosure is measured by the method described in the examples.

[0036] The puncture strength of the PO microporous membrane, which has not been converted to basis weight (hereinafter simply referred to as puncture strength), has a lower limit of preferably 200 gf or more (approximately 1.961 N or more), more preferably 250 gf or more, and even more preferably 300 gf or more. A puncture strength of 200 gf or more is preferable from the viewpoint of safety when an electrochemical device is subjected to impact. Furthermore, the upper limit of the puncture strength of the PO microporous membrane is preferably 680 gf or less from the viewpoint of orientation relaxation during heating of the membrane and the membrane stretching process. The puncture strength of the polyolefin microporous membrane of this disclosure is measured by the method described in the examples.

[0037] Means for controlling the basis weight-equivalent puncture strength or puncture strength of a PO microporous membrane within the above numerical range include, for example, using homo-PE raw material, adjusting the PE raw material Mv, and adjusting the stretching surface ratio and / or stretching temperature in the manufacturing process of the PO microporous membrane.

[0038] <Film Thickness> The film thickness of the polyolefin microporous film of this disclosure is preferably 2.0 μm or more, more preferably 4.0 μm or more, and the upper limit of the film thickness is preferably 15 μm or less, more preferably 10 μm or less. A film thickness of 2.0 μm or more is preferable from the viewpoint of improving mechanical strength and dielectric strength. On the other hand, a film thickness of 15 μm or less is preferable from the viewpoint of ensuring good output characteristics, and a film thickness of 15 μm or less tends to be advantageous in terms of increasing the capacity of the battery because the occupied volume of the polyolefin microporous film is reduced. The film thickness of the polyolefin microporous film of this disclosure is measured by the method described in the examples.

[0039] The thickness of the polyolefin microporous film can be adjusted by adjusting the sheet thickness in step (b), the stretching ratio and stretching temperature in step (c), and by combining these factors.

[0040] <Air Permeability Resistance> The air permeability resistance of a PO microporous membrane is determined from the viewpoint of ensuring membrane permeability and contributing to low resistance and high rate characteristics, at 100 cm² of air. ３The per unit is preferably 150 seconds or less, more preferably 100 seconds or less, even more preferably 80 seconds or less, and particularly preferably 60 seconds or less. The lower limit of air permeability is set at 100 cm² of air, from the viewpoint of balancing film thickness, porosity, and pore size. ３ The interval is preferably 15 seconds or more, more preferably 20 seconds or more, and even more preferably 30 seconds or more. The air permeability resistance of the PO microporous membrane can be controlled within the above numerical range by adjusting, for example, the HS stretching ratio, HS relaxation ratio, HS relaxation temperature, etc., in the method for manufacturing the PO microporous membrane described later. The air permeability resistance of the polyolefin microporous membrane of this disclosure is measured by the method described in the examples.

[0041] <Porrosivity> The porosity of the PO microporous membrane of this disclosure is 45% or more, preferably 50% or more, and more preferably 55% or more. A porosity of 45% or more is important from the viewpoint of ensuring low resistance and high capacity retention at high rates. The upper limit of the porosity is preferably 80% or less, more preferably 75% or less, and even more preferably 70% or less. A porosity of 80% or less is preferred from the viewpoint of ensuring the puncture strength and dielectric strength described above. The porosity of the polyolefin microporous membrane of this disclosure is measured by the method described in the examples.

[0042] <Components of the PO Microporous Film> The PO microporous film according to this disclosure is formed from a resin composition containing a polyolefin resin.Optionally, the PO resin composition may further contain inorganic particles, resins other than polyolefins, etc.From the viewpoint of the strength and film thickness of the PO microporous film, the total content (PC) of all resins contained in the PO resin composition is preferably 10% by mass or more and 34% by mass or less.

[0043] The amount of polyolefin resin (PO resin) contained in the PO microporous membrane is 50% by mass or more, preferably 60% by mass or more, preferably 70% by mass or more, preferably 80% by mass or more, and may be 90% by mass or more and 100% by mass or less, based on the mass of the PO microporous membrane.

[0044] The polyolefin resin used in this disclosure is not particularly limited, and examples include polymers (e.g., homopolymers, copolymers, multi-stage polymers, etc.) obtained by polymerizing monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. These polymers can be used individually or in combination of two or more. From the viewpoint of adjusting the properties obtained by the all-trans amount of polyethylene measured by Raman scattering spectroscopy as described above, a single type of polyolefin resin is preferred as the polyolefin resin.

[0045] From the viewpoint of exhibiting shutdown characteristics, the total proportion of PE raw materials in PO raw materials is preferably 50% by mass or more and 100% by mass or less, and more preferably 80% by mass or more and 100% by mass or less.

[0046] From the viewpoint of adjusting the properties obtained by the all-trans amount of polyethylene measured by Raman scattering spectroscopy as described above, the PO raw material preferably contains a single resin composition, more preferably consists of polyethylene (PE), and even more preferably consists of PE alone. For example, it may be a polyethylene homopolymer.

[0047] From the viewpoint of adjusting the properties obtained by the all-trans amount of polyethylene measured by Raman scattering spectroscopy as described above, the PO microporous membrane preferably has a single resin composition, more preferably consists of polyethylene (PE), and even more preferably consists of PE alone. For example, it may be a polyethylene homopolymer.

[0048] From the viewpoint of improving safety in impact tests and oven tests, it is preferable that the viscosity-average molecular weight (Mv) of the PO resin is 300,000 or more and 2,000,000 or less, as this increases the molecular weight of the PO resin itself, making it easier to achieve film strength. From this viewpoint, the Mv of the PO resin is more preferably 500,000 or more, and even more preferably 700,000 or more. On the other hand, adjusting the viscosity-average molecular weight of the PO resin to 2,000,000 or less is preferable from the viewpoint of suppressing thermal shrinkage.

[0049] The polyethylene (PE) contained in the PO microporous membrane preferably has a viscosity-average molecular weight (Mv) of 300,000 to 2,000,000, more preferably 500,000 to 1,200,000, and even more preferably 800,000 to 1,000,000. When the viscosity-average molecular weight of PE is within the above range, the entanglement of PE molecular chains increases, resulting in high tensile stress and easier orientation. This makes it easier to adjust the properties obtained by the all-trans amount of polyethylene measured by Raman scattering spectroscopy as described above. The viscosity-average molecular weight of polyethylene is measured by the method described in the examples.

[0050] Furthermore, as a polyolefin resin, for example, low-density polyethylene (density 0.910 g / cm³) is used. ３ 0.930g / cm or more ３ Less than ), linear low-density polyethylene (density 0.910 g / cm³) ３ 0.940g / cm or more ３ (less than), medium-density polyethylene (density 0.930 g / cm³) ３ 0.942g / cm or more ３ (less than), high-density polyethylene (density 0.942 g / cm³) ３ (The above), ultra-high molecular weight polyethylene (density 0.910 g / cm³) ３ 0.970g / cm or more ３Examples include polyethylene (less than 350 lb), isotactic polypropylene, atactic polypropylene, polybutene, ethylene propylene rubber, etc. These can be used individually or in combination of two or more. In particular, using only polyethylene, only polypropylene, or a mixture of polyethylene and polypropylene is preferable from the viewpoint of obtaining a uniform film.

[0051] Furthermore, from the perspective of safety in electrochemical devices that utilize a PO microporous membrane as a separator, polyolefin resin is suitable at 0.930 g / cm³. ３ 0.942g / cm or more ３ It is preferable that the PO microporous membrane is a medium-density polyethylene (PE) having a density of less than 1 million, for example, any PE other than high-density polyethylene (HDPE) may be used. Furthermore, from the viewpoint of improving the safety of electrochemical devices even when the PO microporous membrane is a thin film, medium-density polyethylene with a viscosity-average molecular weight of less than 1 million, and a viscosity-average molecular weight of 1 million to 2 million and a density of 0.930 g / cm³ are preferred. ３ 0.942g / cm or more ３ It is preferable that at least one type selected from ultra-high molecular weight polyethylene of less than 50% by mass be included, more preferably 60% by mass or more, and most preferably 70% by mass or more, based on the mass of the PO microporous membrane.

[0052] The branching amount of polyethylene in the polyolefin microporous membrane of this disclosure, as measured by NMR spectroscopy, is preferably 0.05 mol% or less, more preferably 0.04 mol% or less, and particularly preferably 0.03 mol% or less. A branching amount of polyethylene within this predetermined range is preferable because it results in highly oriented polyethylene molecular chains with fewer defects, an increase in the all-trans content of polyethylene, and thus a higher dielectric strength. The branching amount of polyethylene is measured by the method described in the examples.

[0053] The resin composition may optionally contain inorganic particles, antioxidants such as phenolic, phosphorus-based, or sulfur-based agents; metal soaps such as calcium stearate or zinc stearate; and various known additives such as ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments.

[0054] [Method for Manufacturing Polyolefin Microporous Films] The method for manufacturing PO microporous films according to this disclosure is not particularly limited, but examples include: a mixing step (a) of mixing a resin composition containing a polyolefin resin and various additives as desired; an extrusion step (b) of melt-kneading the mixture obtained in step (a) and extruding it; a sheet molding step (c) of forming the extruded product obtained in step (b) into a sheet; a primary stretching step (d) of stretching the sheet-like molded product obtained in step (c) at least once in at least one axial direction; an extraction step (e) of extracting pore-forming material from the primary stretched film obtained in step (d); and a dry re-stretching / thermal relaxation step (f) of re-stretching / thermal relaxation the extracted film obtained in step (e) at a predetermined temperature.

[0055] The above-described method for manufacturing a PO microporous membrane makes it possible to provide a PO microporous membrane that can achieve a high degree of safety in impact tests and oven tests when used as a separator for lithium-ion secondary batteries and other electrochemical devices. In particular, the method of stretching to MD and TD in the primary stretching step (d), followed by the extraction step (e), and then re-stretching / heat-relaxing to MD and / or TD in the dry re-stretching / heat-relaxation step (f) tends to make it easier to adjust the pore size and thermal shrinkage rate of the resulting PO microporous membrane to within the numerical range described above. It should be noted that the method for manufacturing a PO microporous membrane described herein is not limited to the above-described method, and various modifications are possible without departing from the gist of the method.

[0056] [Mixing step (a)] Mixing step (a) is a step of mixing a resin composition containing a polyolefin resin and various additives as desired. In addition, other components may be mixed with the resin composition in mixing step (a) as needed.

[0057] The pore-forming material can be any material as long as it is distinguishable from the PO resin and inorganic particle material, and can be, for example, a plasticizer. As the plasticizer, a non-volatile solvent that can form a homogeneous solution above the melting point of the PO resin may be used, such as hydrocarbons such as liquid paraffin (LP) and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; or higher alcohols such as oleyl alcohol and stearyl alcohol.

[0058] The plasticizer content in the resin composition is preferably 66% by mass or more and 90% by mass or less, and more preferably 70% by mass or more and 80% by mass or less. By adjusting the plasticizer content to 66% by mass or more, the melt viscosity of the resin composition decreases, melt fracture is suppressed, and film-forming properties during extrusion tend to improve. On the other hand, by adjusting the plasticizer content to 90% by mass or less, it may be possible to suppress the elongation of the raw material during the film-forming process.

[0059] (Optional Additives) In step (a), the resin composition containing PO may contain any additives. The additives are not particularly limited, but examples include polymers other than polyolefin resins; antioxidants such as phenolic compounds, phosphorus compounds, and sulfur compounds; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers; light stabilizers; antistatic agents; antifogging agents; coloring pigments, etc. The total amount of these additives added is preferably 5 parts by mass or less, more preferably 3 parts by mass or less, and even more preferably 1 part by mass or less, per 100 parts by mass of polyolefin resin.

[0060] The mixing method in step (a) is not particularly limited, but for example, one method is to pre-mix some or all of the raw materials using a Henschel mixer, ribbon blender, tumbler blender, etc., as needed. Among these, the method of mixing using a Henschel mixer is preferred.

[0061] [Extrusion Process (b)] Extrusion process (b) is a process in which the resin composition obtained in process (a) is melted, kneaded, and extruded. In addition, other components may be mixed with the resin composition in extrusion process (b) as needed.

[0062] The method of melt-kneading in step (b) is not particularly limited, but for example, all raw materials, including the mixture mixed in step (a), can be melt-kneaded using a screw extruder such as a single-screw extruder or twin-screw extruder; a kneader; a mixer, etc. Among these, it is preferable to perform the melt-kneading using a screw with a twin-screw extruder. Furthermore, when performing melt-kneading, it is preferable to add the plasticizer in two or more stages, and when adding the additive in multiple stages, it is preferable to adjust the amount added in the first stage so that it is 90% by weight or less of the total amount added, from the viewpoint of suppressing aggregation of contained components and dispersing them uniformly. This is preferable from the viewpoint of suppressing heat generation by shutting down a large area and improving the safety of the cell.

[0063] When a pore-forming agent is used in step (b), the temperature of the melt-mixing section is preferably less than 200°C from the viewpoint of uniformly mixing the resin composition. The lower limit of the temperature of the melt-mixing section is above the melting point of the polyolefin from the viewpoint of uniformly dissolving the polyolefin resin into the plasticizer.

[0064] In this disclosure, during the kneading process, although not particularly limited, it is preferable to first mix an antioxidant with the raw material PO at a predetermined concentration, then replace the surrounding atmosphere of the mixture with a nitrogen atmosphere, and perform melt kneading while maintaining the nitrogen atmosphere.

[0065] In step (b), the kneaded material obtained after the kneading process is extruded by an extruder such as a T-die or an annular die. At this time, it may be single-layer extrusion or laminated extrusion. The conditions during extrusion are not particularly limited, and known methods can be used, for example. Furthermore, from the viewpoint of the thickness of the resulting PO microporous film, it is preferable to control the (die) lip clearance and the like.

[0066] [Sheet forming process (c)] The sheet forming process (c) is a process of forming the extruded material obtained in the extrusion process (b) into a sheet. The sheet-like molded product obtained in the sheet forming process (c) may be a single layer or a laminate. The method of sheet forming is not particularly limited, but one example is a method of solidifying the extruded material by compression and cooling.

[0067] The compression cooling method is not particularly limited, but examples include a method of directly contacting the extruded material with a cooling medium such as cold air or cooling water; and a method of contacting the extruded material with a metal roll or press machine cooled with a refrigerant. Among these, the method of contacting the extruded material with a metal roll or press machine cooled with a refrigerant is preferred because it allows for easy control of the film thickness.

[0068] In the process of forming the molten material into a sheet after the melt-kneading step (b), it is preferable to set the temperature higher than the set temperature of the extruder. From the viewpoint of thermal degradation of the polyolefin resin, the upper limit of the set temperature is preferably 300°C or less, and more preferably 260°C or less. For example, when continuously producing sheet-shaped molded articles from an extruder, it is preferable that the temperature of the process of forming the molten material into a sheet after the melt-kneading step, i.e., the path from the extruder outlet to the T-die, and the T-die, are set higher than the set temperature of the extrusion step, because it is possible to form the molten material into a sheet without separation of the resin composition and the pore-forming material. Furthermore, from the viewpoint of the film thickness of the resulting PO microporous film, it is preferable to control the cast clearance and the like.

[0069] The thickness of the resulting sheet-like molded article can be between 0.5 mm and 3.0 mm, depending on, for example, the thickness after stretching in step (d).

[0070] [Primary stretching step (d)] The primary stretching step (d) is a step in which the sheet-like molded product obtained in the sheet forming step (c) is stretched at least once in at least one axial direction. This stretching step (a stretching step performed before the next extraction step (e)) will be called "primary stretching" or "wet stretching," and the film obtained by primary stretching will be called "primary stretched film." In primary stretching, the sheet-like molded product can be stretched in at least one direction, and may be performed in both MD and TD directions, or in only one of MD or TD directions.

[0071] The primary stretching method is not particularly limited, but can be either uniaxial stretching or biaxial stretching. From the viewpoint of adjusting the properties obtained by measuring the all-trans amount of polyethylene by Raman scattering spectroscopy as described above, biaxial stretching is preferred. In this case, either simultaneous biaxial stretching or sequential biaxial stretching can be used as the biaxial stretching method. Examples include MD uniaxial stretching using a roll stretcher, MD uniaxial stretching using a tenter having a stretching mechanism in the flow direction; TD uniaxial stretching using a tenter having a stretching mechanism in the width direction; sequential biaxial stretching using a combination of a roll stretcher and a tenter (hereinafter referred to as roll sequential stretching), or sequential biaxial stretching using a combination of one or more tenters (hereinafter referred to as tenter sequential biaxial stretching); simultaneous biaxial stretching using a simultaneous biaxial tenter (hereinafter referred to as simultaneous biaxial stretching), or simultaneous biaxial stretching by inflation molding; and so on.

[0072] From the viewpoint of adjusting the properties obtained by measuring the all-trans amount of polyethylene by Raman scattering spectroscopy as described above, biaxial stretching in both MD and TD directions is preferred as the primary stretching method for PO microporous membranes, sequential biaxial stretching with a tenter or simultaneous biaxial stretching is more preferred, and simultaneous biaxial stretching is most preferred. Sequential biaxial stretching with a tenter or simultaneous biaxial stretching is preferred because the membrane is gripped in the width direction when the PO microporous membrane is formed, thereby suppressing neck-in, improving the effective area ratio, and resulting in highly oriented polyethylene molecular chains. Neck-in is shrinkage that occurs in the width direction when stretching a PO microporous membrane, and tends to occur easily when stretching a PO microporous membrane with a roll stretcher.

[0073] The stretching ratio of the MD and / or TD in the primary stretching is preferably 7 times or more, more preferably 8 times or more, and particularly preferably 9 times or more. By having a stretching ratio of 7 times or more for the MD and / or TD in the primary stretching, the properties obtained by the all-trans amount of polyethylene measured by Raman scattering spectroscopy can be adjusted as described above, and the strength and dielectric strength of the resulting PO microporous film tend to be further improved. Alternatively, the stretching ratio of the MD and / or TD in the primary stretching is 12 times or less, more preferably 11 times or less, and even more preferably 10 times or less. By having a stretching ratio of 12 times or less for the MD and TD in the primary stretching, breakage during stretching tends to be further suppressed. If the total stretching ratio of the primary stretching is 60 times or more and 140 times or less, the properties obtained by the all-trans amount of polyethylene measured by Raman scattering spectroscopy can be adjusted as described above, and the strength of the resulting PO microporous film tend to be further improved. The primary total stretch ratio refers to the stretch ratios of MD and TD respectively in the case of uniaxial stretching, and the stretch ratio obtained by multiplying the stretch ratios of MD and TD respectively in the case of biaxial stretching.

[0074] The primary drawing temperature can be selected by referring to the raw material resin composition and concentration contained in the PO resin composition. The primary drawing temperature for MD and / or TD is preferably in the range of 100°C to 135°C, and more preferably in the range of 110°C to 130°C. For both MD and TD, the primary drawing temperature is preferably 100°C or higher from the viewpoint of suppressing fracture, and preferably 135°C or lower from the viewpoint of increasing strength and dielectric strength.

[0075] The primary stretching speed can be selected by referring to the raw material resin composition and concentration contained in the PO resin composition. The primary stretching speed for MD and / or TD is preferably in the range of 15% / second to 30% / second, and more preferably in the range of 18% / second to 25% / second. For both MD and TD, the primary stretching speed is preferably 15% / second or higher from the viewpoint of increasing strength and dielectric strength, and preferably 30% / second or lower from the viewpoint of suppressing fracture.

[0076] In this disclosure, prior to the primary stretching step (d) described above, the unstretched sheet may be pre-stretched to MD (pre-stretching). When pre-stretching is performed, it is preferable to perform pre-stretching to MD before simultaneous biaxial stretching in the primary stretching step, from the viewpoint of adjusting the properties obtained by the all-trans amount of polyethylene measured by Raman scattering spectroscopy as described above. The stretching ratio of the pre-stretching is not particularly limited, but is usually 1.00 times or more and 2.0 times or less, and preferably 1.5 times or more and 1.9 times or less. By having the pre-stretching ratio within a predetermined range, the properties obtained by the all-trans amount of polyethylene measured by Raman scattering spectroscopy can be adjusted as described above.

[0077] The stretching temperature during preliminary stretching is not particularly limited, but can usually be appropriately set within the range of 100°C to 130°C, and is preferably 110°C to 120°C. From the viewpoint of suppressing fracture, the preliminary stretching temperature is preferably 100°C or higher, and from the viewpoint of increasing strength and dielectric strength, it is preferably 130°C or lower.

[0078] The stretching speed in the case of preliminary stretching is not particularly limited, but is preferably in the range of 5% / second to 20% / second, and more preferably in the range of 8% / second to 15% / second. From the viewpoint of increasing strength and dielectric strength, the preliminary stretching speed is preferably 5% / second or more, and from the viewpoint of suppressing fracture, it is preferably 20% / second or less.

[0079] [Extraction step (e)] Extraction step (e) is a step of extracting pore-forming material from the primary stretched film obtained in the primary stretching step (d) to obtain an extracted film. Methods for removing the pore-forming material include, for example, immersing the primary stretched film in an extraction solvent to extract the pore-forming material and then drying it thoroughly. The method for extracting the pore-forming material may be either batch or continuous. Furthermore, it is preferable that the residual amount of pore-forming material, especially plasticizer, in the porous film be less than 1% by mass.

[0080] When extracting pore-forming materials, it is preferable to use an extraction solvent that is a poor solvent for polyolefin resins and a good solvent for pore-forming materials or plasticizers, and whose boiling point is lower than the melting point of the polyolefin resin. Such extraction solvents are not particularly limited, but examples include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; non-chlorinated halogenated solvents such as hydrofluoroethers and hydrofluorocarbons; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; and ketones such as acetone and methyl ethyl ketone. These extraction solvents may be recovered and reused by operations such as distillation.

[0081] [Dry re-stretching / thermal relaxation step (f)] The dry re-stretching / thermal relaxation step (f) is a step in which the extracted film obtained in the extraction step (e) is re-stretched / thermally relaxed at a predetermined temperature. The method of re-stretching / thermal relaxation at this time is not particularly limited, but one example is a method in which re-stretching (hereinafter also referred to as dry re-stretching) and relaxation operations are performed using a tenter or a roll stretcher. By performing stretching and relaxation operations in the dry re-stretching / thermal relaxation step (f), the polyolefin microporous film of this disclosure tends to have high porosity.

[0082] The stretching operation in the dry re-stretching / thermal relaxation process (f) is an operation to re-stretch the PO microporous membrane in at least one of the MD and TD directions. From the viewpoint of adjusting the properties obtained by the all-trans amount of polyethylene measured by Raman scattering spectroscopy as described above, the stretching operation in the dry re-stretching / thermal relaxation process (f) of the PO microporous membrane is preferably tenter sequential biaxial stretching, simultaneous biaxial stretching, and uniaxial stretching to the TD direction, with simultaneous biaxial stretching being more preferable.

[0083] In the dry re-stretching / thermal relaxation process (f), the stretching ratios of MD and TD are preferably 1.4 times or more and 3.0 times or less, and more preferably 2.0 times or more and 2.5 times or less. The properties obtained by the all-trans amount of polyethylene measured by Raman scattering spectroscopy can be adjusted as described above, and from the viewpoint of highly oriented polyethylene molecular chains to exhibit film strength, the stretching ratios of MD and TD in the thermal setting process (f) are preferably 1.4 times or more, and from the viewpoint of suppressing breakage, 3.0 times or less is preferred.

[0084] The stretching temperature in the dry re-stretching / thermal relaxation process (f) is preferably in the range of 130°C to 140°C, more preferably in the range of 132°C to 139°C, and even more preferably in the range of 134°C to 138°C. For both MD and TD, this stretching temperature is preferably 130°C or higher from the viewpoint of suppressing fracture, and preferably 140°C or lower from the viewpoint of improving strength and dielectric strength.

[0085] The stretching speed in the dry re-stretching / thermal relaxation process (f) is preferably in the range of 3% / second to 10% / second, and more preferably in the range of 5% / second to 8% / second. For both MD and TD, this stretching speed is preferably 3% / second or higher from the viewpoint of improving strength and dielectric strength, and preferably 10% / second or lower from the viewpoint of suppressing fracture.

[0086] The relaxation operation in the dry re-stretching / thermal relaxation process (f) is an operation to shrink the PO microporous membrane in at least one of the MD and TD directions, and may be performed in both directions of MD and TD, or in only one of MD or TD. From the viewpoint of adjusting the properties obtained by the all-trans amount of polyethylene measured by Raman scattering spectroscopy as described above, the relaxation operation during the dry re-stretching / thermal relaxation process (f) of the PO microporous membrane is preferably tenter sequential biaxial stretching, simultaneous biaxial stretching, and uniaxial stretching toward TD, with simultaneous biaxial stretching being more preferable.

[0087] From the viewpoint of achieving high porosity, the relaxation rates of MD and TD in the dry re-stretching / thermal relaxation process (f) are preferably 0% to 15%, more preferably 2% to 10%, and particularly preferably 4% to 8%. Here, "relaxation rate" is the value obtained by subtracting the dimensions of the film after the relaxation operation from the dimensions of the film before the relaxation operation and dividing the result by the dimensions of the film before the relaxation operation. Relaxation rate (%) = (Dimensions of the film before relaxation operation (m) - Dimensions of the film after relaxation operation (m)) / (Dimensions of the film before relaxation operation (m)) × 100

[0088] The relaxation temperature in this relaxation operation is preferably 120°C or higher, and preferably less than or equal to the stretching temperature (°C) of MD and TD in the dry re-stretching / thermal relaxation process (f). From the viewpoint of suppressing the thermal shrinkage rate at 120°C, the relaxation temperature is preferably 120°C or higher. From the viewpoint of rapidly cooling and fixing the structure formed by the stretching of MD and TD in the dry re-stretching / thermal relaxation process (f), it is preferable that the relaxation temperature is less than or equal to the stretching temperature (°C) of MD and TD in the dry re-stretching / thermal relaxation process (f).

[0089] In this relaxation operation, the relaxation rates for MD and TD are preferably in the range of 1.0% / second to 2.0% / second, more preferably in the range of 1.1% / second to 1.8% / second, and particularly preferably in the range of 1.2% / second to 1.7% / second, from the viewpoint of suppressing excessive orientation relaxation. Here, "relaxation rate" is the value obtained by dividing the relaxation rate by the relaxation time, where relaxation rate (% / second) = relaxation rate (%) ÷ relaxation time (seconds). Here, "relaxation time" is the time from the stretching of MD and TD in the dry re-stretching / thermal relaxation process (f) until the polyolefin microporous film is transported out of the tenter. The relaxation time is usually in the range of 3 seconds to 10 seconds.

[0090] The relaxation cooling rates for MD and TD in this relaxation operation are preferably in the range of 10°C / sec to 40°C / sec, and more preferably in the range of 20°C / sec to 30°C / sec, from the viewpoint of fixing the orientation of the intermediate phase of the polyethylene molecular chain formed by stretching by rapid cooling. Here, "relaxation cooling rate (°C / sec)" is the rate at which the film is cooled from the maximum temperature in the dry re-stretching / thermal relaxation process (f) to room temperature (25°C), and is the value obtained by dividing (maximum temperature (°C) - 25°C) by the relaxation time. Note that 25°C is the room temperature after the cooling operation. Here, "maximum temperature" is the highest value between the stretching temperature of MD or TD during the last re-stretching operation performed in the dry re-stretching / thermal relaxation process (f) and the relaxation temperature of MD or TD during the subsequent relaxation operation. To increase the relaxation cooling rate, methods such as performing the dry re-stretching / thermal relaxation process at a low temperature, or shortening the time from the start of the relaxation process until the polyolefin microporous membrane is removed from the tenter (relaxation time), can be used in appropriate combinations. Furthermore, methods such as blowing air onto both or one side of the polyolefin microporous membrane removed from the tenter after the thermal relaxation process may be used to increase the effective cooling rate.

[0091] In this embodiment, the final total stretch ratios for MD and TD in the stretching operations performed in the primary stretching step (d) and the dry re-stretching / heat relaxation step (f) are preferably 10 times or more and 30 times or less, and more preferably 15 times or more and 25 times or less. Here, "total stretch ratios for MD and TD" refers to the value obtained by multiplying the stretch ratios for MD and TD in the primary stretching step (d) by the stretch ratios for MD and TD in the dry re-stretching / heat relaxation step (f). Furthermore, the final total stretch ratio (hereinafter referred to as the total stretch ratio) of the stretching operations performed in the primary stretching step (d) and the dry re-stretching / thermal relaxation step (f) of this embodiment is preferably 120 times or more and 700 times or less, more preferably 150 times or more and 650 times or less, even more preferably 200 times or more and 600 times or less, particularly preferably 300 times or more and 550 times or less, and most preferably 400 times or more and 500 times or less. By having the total stretch ratios of MD and TD respectively within the above ranges, the characteristics obtained by the all-trans amount of polyethylene measured by Raman scattering spectroscopy can be adjusted as described above. Here, "total stretch ratio" refers to the value obtained by multiplying the MD total stretch ratio and the TD total stretch ratio.

[0092] The order of steps (a) to (f) described above can be changed as desired, as long as it does not impair the effects of the present invention.

[0093] [Other Steps] The method for manufacturing a PO microporous film according to this disclosure may include other steps besides steps (a) to (f) described above. Other steps are not particularly limited, but for example, in addition to the heat setting step described above, a lamination step may be used to obtain a laminated PO microporous film by stacking multiple single-layer PO microporous films. Alternatively, the method for manufacturing a PO microporous film may include a peeling step to obtain two or more single-layer films by peeling off the laminate obtained by co-extrusion after steps (a) to (f). Furthermore, the method for manufacturing a PO microporous film according to this disclosure may include a surface treatment step in which the surface of the PO microporous film is subjected to surface treatment such as electron beam irradiation, plasma irradiation, application of a surfactant, or chemical modification. Moreover, an inorganic particle material may be coated onto one or both sides of the PO microporous film to obtain a PO microporous film having an inorganic particle-containing layer.

[0094] <Formation of an inorganic particle-containing layer> From the viewpoint of safety, dimensional stability, and heat resistance, an inorganic particle-containing layer can be provided on the surface of the PO microporous film. The inorganic particle-containing layer is a layer containing inorganic components such as inorganic particles, and may optionally contain a binder resin to bind the inorganic particles together, a dispersant to disperse the inorganic particles in a solvent, etc.

[0095] As a method for forming an inorganic particle-containing layer, for example, a method can be used in which a coating solution (in one embodiment, an inorganic particle-containing slurry) containing inorganic particles, a binder resin, and a solvent is applied to at least one surface of a polyolefin microporous membrane, and then dried to form an inorganic particle-containing layer. As for the method of removing the solvent from the coating film after applying the inorganic particle-containing slurry, there are no particular limitations as long as it does not adversely affect the polyolefin microporous membrane. Examples include drying the polyolefin microporous membrane at a temperature below its melting point while fixing it in place, or drying it under reduced pressure at a low temperature.

[0096] Examples of materials for the inorganic particles contained in the inorganic particle-containing layer include oxide ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, magnesium sulfate, aluminum sulfate, barium sulfate, aluminum hydroxide, aluminum hydroxide oxide, potassium titanate, talc, kaolinite, decite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amethyst, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fibers. The inorganic particles may be used individually or in combination. Examples of binder resins include conjugated diene polymers, acrylic polymers, polyvinyl alcohol resins, and fluoropolymer resins. The binder resin may be in the form of latex and may contain water or an aqueous solvent. The solvent for the coating solution containing inorganic particles and binder resin is preferably one that can uniformly and stably disperse the inorganic particles and binder resin, such as N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, water, ethanol, toluene, hot xylene, methylene chloride, and hexane. The dispersant is one that adsorbs onto the surface of the inorganic particles in the slurry and stabilizes the inorganic particles by electrostatic repulsion, such as polycarboxylates, sulfonates, polyoxyethers, and surfactants.

[0097] The thickness of the inorganic particle-containing layer is preferably 0.5 μm or more, more preferably 1.0 μm or more, from the viewpoint of suppressing thermal shrinkage, and is 5.0 μm or less, more preferably 3.0 μm or less, from the viewpoint of ensuring good output characteristics.

[0098] The solid content in 100% by mass of the inorganic particle-containing slurry is preferably 1.0% by mass or more and 50% by mass or less, from the viewpoint of being able to adjust the thickness of the inorganic particle-containing layer to a predetermined range.

[0099] There are no particular limitations on the method for coating an inorganic particle-containing slurry onto a polyolefin microporous film, as long as it can achieve the desired thickness of the inorganic particle-containing layer. Examples include gravure coater, small-diameter gravure coater, reverse roll coater, transfer roll coater, kiss coater, dip coater, knife coater, air doctor coater, blade coater, rod coater, squeeze coater, cast coater, die coater, screen printing, spray coating, spray coater coating, and inkjet coating.

[0100] <Formation of Adhesive Layer> To prevent deformation or bulging due to gas generation in laminate-type batteries, which have recently been increasingly adopted in automotive batteries to increase energy density, an adhesive layer containing a thermoplastic resin can be provided on the surface of the PO microporous membrane. The thermoplastic resin contained in the adhesive layer is not particularly limited and includes, for example, polyolefins such as polyethylene or polypropylene; fluororesins such as polyvinylidene fluoride and polytetrafluoroethylene; fluororubbers such as vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, and ethylene-tetrafluoroethylene copolymer; styrene-butadiene copolymer and its hydride, acrylonitrile-butadiene copolymer and its hydride, acrylonitrile-butadiene-styrene Examples include rubbers such as styrene copolymers and their hydrogenates, (meth)acrylic acid ester copolymers, styrene-acrylic acid ester copolymers, acrylonitrile-acrylic acid ester copolymers, ethylene propylene rubber, polyvinyl alcohol, and polyvinyl acetate; cellulose derivatives such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and carboxymethyl cellulose; and resins with a melting point and / or glass transition temperature of 180°C or higher, such as polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, and polyester.

[0101] Furthermore, after the dry re-stretching / thermal relaxation process (f), the lamination process, or the surface treatment process, the master roll around which the PO microporous film is wound can be subjected to an aging treatment under predetermined temperature conditions before the rewinding operation of the master roll. This tends to make it easier to obtain a PO microporous film with higher thermal stability than the PO microporous film before rewinding. In the above case, the temperature during the aging treatment of the master roll is not particularly limited, but is preferably 35°C or higher, more preferably 45°C or higher, and even more preferably 60°C or higher. Also, from the viewpoint of maintaining the permeability of the PO microporous film, the temperature during the aging treatment of the master roll is preferably 120°C or lower. The time required for the aging treatment is not particularly limited, but is preferably 24 hours or more because the above effects are more likely to manifest.

[0102] <Separator for Electrochemical Devices> The polyolefin microporous membrane according to this disclosure can be used as a separator for electrochemical devices such as lithium-ion secondary batteries. By being incorporated into a lithium-ion secondary battery, the polyolefin microporous membrane can suppress thermal runaway of the lithium-ion secondary battery.

[0103] <Electrochemical Devices> An electrochemical device that houses a wound or laminated body made of a PO microporous film according to this disclosure, either wound or laminated, is also an embodiment of the present invention. Examples of electrochemical devices include film rolls, non-aqueous electrolyte batteries, non-aqueous lithium-ion secondary batteries, non-aqueous gel secondary batteries, non-aqueous solid secondary batteries, lithium-ion capacitors, electric double-layer capacitors, and the like.

[0104] The non-aqueous lithium-ion secondary battery according to this disclosure comprises a lithium-ion secondary battery separator containing the above-mentioned PO microporous membrane, a positive electrode plate, a negative electrode plate, and a non-aqueous electrolyte (containing a non-aqueous solvent and a metal salt dissolved therein). Specifically, for example, a positive electrode plate containing a transition metal oxide capable of intercepting and releasing lithium ions, etc., and a negative electrode plate capable of intercepting and releasing lithium ions, etc., are wound or laminated facing each other via a non-aqueous electrolyte battery separator, and the non-aqueous electrolyte is retained and housed in a container. In one embodiment, the lithium-ion secondary battery separator of this disclosure has an inorganic particle-containing layer with a thickness of 5.0 μm or less on at least one surface of the polyolefin microporous membrane of this disclosure. In one embodiment, the lithium-ion secondary battery of this disclosure includes the lithium-ion secondary battery separator of this disclosure.

[0105] The positive electrode plate is described below. As the positive electrode active material, for example, lithium composite metal oxides such as lithium nickelate, lithium manganeseate, or lithium cobaltate, or lithium composite metal phosphates such as lithium iron phosphate can be used. The positive electrode active material is kneaded with a conductive agent and a binder, and the positive electrode paste is applied to a positive electrode current collector such as aluminum foil and dried, then rolled to a predetermined thickness and cut to a predetermined size to form a positive electrode plate. Here, as the conductive agent, a metal powder that is stable under positive electrode potential, for example, carbon black such as acetylene black or graphite material can be used. As the binder, a material that is stable under positive electrode potential, for example, polyvinylidene fluoride, modified acrylic rubber, or polytetrafluoroethylene can be used.

[0106] The negative electrode plate is described below. As the negative electrode active material, a material capable of intercalating lithium can be used. Specifically, for example, at least one material selected from the group consisting of graphite, silicide, and titanium alloy materials can be used. Furthermore, as the negative electrode active material for a non-aqueous electrolyte secondary battery, for example, metals, metal fibers, carbon materials, oxides, nitrides, silicon compounds, tin compounds, or various alloy materials can be used. In particular, silicon (Si) or tin (Sn) elements or alloys, compounds, solid solutions, or other silicon or tin compounds are preferred because they tend to increase the capacity density of the battery.

[0107] Examples of carbon materials include various types of natural graphite, coke, carbon in the process of graphitization, carbon fibers, spheroidal carbon, various types of artificial graphite, and amorphous carbon.

[0108] As the negative electrode active material, one of the above materials may be used alone, or two or more may be used in combination. The negative electrode active material is kneaded with a binder, and the negative electrode paste is applied to a negative electrode current collector such as copper foil and dried, then rolled to a predetermined thickness and cut to a predetermined size to form a negative electrode plate. Here, as the binder, a material that is stable under negative electrode potential, such as PVDF or styrene-butadiene rubber copolymer, can be used.

[0109] Non-aqueous electrolytes are described below. Generally, non-aqueous electrolytes contain a non-aqueous solvent and metal salts such as lithium salts, sodium salts, and calcium salts dissolved in it. Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, and cyclic carboxylic acid esters. Examples of lithium salts include LiPF4. ６ LiClO ４ LiBF ４ LiAlCl ４ LiSbF ６ , LiSCN, LiCF ３ SO ３ LiCF ３ CO ２ , Li(CF ３ SO ２ ) ２ LiAsF ６Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, and imide salts.

[0110] Unless otherwise specified, the various parameters mentioned above are measured in accordance with the measurement methods described in the examples below.

[0111] Next, this embodiment will be described in more detail with reference to examples and comparative examples, but this embodiment is not limited to the following examples unless it exceeds the gist of the embodiment. The physical properties in the examples were measured by the following methods. Unless otherwise specified, measurements were taken in an environment of 23°C and 40% humidity.

[0112] (1) Molecular weight (1a) High-temperature GPC of polyolefin microporous membrane: Measurement of weight-average molecular weight (Mw) and number-average molecular weight (Mn) (GPC-relative method) ・Preparation of sample The polyolefin raw material was weighed and 1,2,4-trichlorobenzene (TCB) eluent was added to a concentration of 0.2 mg / mL. Using a high-temperature dissolver, the mixture was heated at 150°C for 1.0 hour by standing, then shaken for 2.5 hours, and heated again by standing for 0.5 hours, and it was visually confirmed that the entire sample had dissolved. The sample solution was pressurized with a syringe and filtered through a 1.0 μm filter, and the filtrate was used as the GPC measurement sample.

[0113] ・GPC Measurement An Agilent PL-GPC220™ GPC instrument manufactured by Agilent Inc. was used, and two columns manufactured by Tosoh Corporation, TSKgel guardcolumn HHR(S)HT2 and TSKgel GMHHRmainasuH(S)HT2 (7.8 mm I.D. kakeru 30 cm), were connected in series. Trichlorobenzene (TCB), 0.05% and 4,4'-thiobis(6-t-butyl-3-methyl)phenol-containing eluent was used. The sample was weighed into a high-temperature filter to a sample concentration of approximately 0.2 mg / mL, 5 mL of eluent was added, and it was dissolved at 150°C. GPC measurement was performed using an RI detector at a flow rate of 1 mL / min and a measurement temperature of 160°C. Furthermore, the calibration curve, which was created using commercially available monodisperse polystyrene with a known molecular weight as a standard substance, was converted to polyethylene-equivalent molecular weight using the concept of a general-purpose calibration curve. The coefficient was KPS = 1.21 × 10⁻⁶. ―４ , αPS=0.707, KPE=5.50×10 ―４ A value of αPE = 0.704 was used. This allowed for the calculation of the weight-average molecular weight (Mw) and number-average molecular weight (Mn) for each sample. The molecular weight distribution (Mw / Mn) was then obtained by dividing the weight-average molecular weight (Mw) by the number-average molecular weight (Mn).

[0114] (1b) Viscosity-average molecular weight (Mv) Based on ASTM-D4020, the intrinsic viscosity [η] (dl / g) at 135°C in decalin solvent was determined. For polyethylene raw materials, Mv was calculated using the following formula: [η] = 6.77 × 10⁻⁶ －４ Mv ０．６７ For polypropylene raw materials, Mv was calculated using the following formula: [η] = 1.10 × 10 －４ Mv ０．８０

[0115] (2) Film thickness (μm) The film thickness of the PO microporous film or porous layer was measured at room temperature of 23±2℃ using a microthickness gauge, KBM (trademark), manufactured by Toyo Seiki Co., Ltd.

[0116] (3) Porosity (%) A 10 cm x 10 cm square sample is cut from the PO microporous membrane, and its volume (cm³) ３ Calculate the PO density (g / cm³) and mass (g), and then compare them with the PO density (g / cm³).３ The porosity was calculated using the following formula. However, the PO density was 0.95 g / cm³. ３ The following formula was used: Porosity (%) = {(Volume of PO microporous membrane - Mass of PO microporous membrane) / PO density} / Volume of PO microporous membrane × 100

[0117] (4) Air permeability resistance (sec / 100cm ３ The air permeability resistance of the PO microporous membrane was measured in accordance with JIS P-8117, using a Gurley-type air permeability meter, G-B2 (trademark), manufactured by Toyo Seiki Co., Ltd., under conditions of 23°C and 40% humidity.

[0118] (5) Puncture strength (gf) and area weight conversion puncture strength (gf / (g / m ２ Using a Kato Tech KES-G5 (trademark) handy compression tester, the PO microporous membrane was fixed in a sample holder with an opening diameter of 11.3 mm. Next, a puncture test was performed on the center of the fixed microporous membrane using a needle with a tip diameter of 1.0 mm and a radius of curvature of 0.5 mm at a puncture speed of 2 mm / second in an atmosphere of 23°C and 40% humidity. The raw puncture strength (gf) was determined as the maximum puncture load and divided by the basis weight of the PO microporous membrane (gf / (g / m)). ２ )) was also calculated.

[0119] (6) Thermal shrinkage rate (%) of MD and TD at 120°C A PO microporous membrane was cut to 100 mm in the MD direction and 100 mm in the TD direction, and left standing in an oven at a predetermined temperature (120°C) for 1 hour. At this time, the sample was sandwiched between two sheets of paper so that the hot air did not directly hit the sample. The paper used was Fuji Xerox Interfield V-Paper monochrome copier / printer paper (basis weight 64 g / m²). ２ The following was used: After removing the samples from the oven and letting them cool, the length (mm) was measured and the thermal shrinkage rate was calculated using the following formula. Measurements were taken for both MD and TD. Thermal shrinkage rate (%) = {(100 - length after heating) / 100} × 100 The average thermal shrinkage rate (%) of MD and TD at 120°C was calculated as (thermal shrinkage rate of MD at 120°C + thermal shrinkage rate of TD at 120°C) ÷ 2.

[0120] (7) The total amount of all-trans of polyethylene, the crystalline phase ratio of polyethylene, and the intermediate phase ratio of polyethylene, as measured by Raman scattering spectroscopy. For the polyolefin microporous membranes prepared in the examples, a sample was prepared by cutting the polyolefin microporous membrane into a 3 cm square with scissors so that the Raman linear polarization direction and MD were parallel. The following conditions were set for the sample: - Apparatus: Renishaw InVia Reflex - Laser wavelength: 532 nm - Laser power: 5% - Objective lens: 20x - Exposure time: 5 s - Number of integrations: 4 - Polarization measurement: Linear polarization measurement was performed to obtain the Raman spectroscopic spectrum of the polyolefin microporous membrane.

[0121] The acquired Raman spectral spectra were fitted using a Gaussian function, and peak separation was performed. The Raman shifts were 1130 cm⁻¹ for each spectrum. －１ , 1298cm －１ , 1305cm －１ , 1416cm －１ , 1440cm －１ , and 1460cm －１ Peak intensity I １１３０ , I １２９８ , I １３０５ , I １４１６ , I １４４０ , I １４６０ By applying the following equations I, II, and III, the total all-trans amount of polyethylene, the crystalline phase ratio of polyethylene, the intermediate phase ratio of polyethylene, and the crystalline phase ratio of polyethylene were calculated. All-trans amount of polyethylene = I １１３０ / [0.80 × (I １２９８ +I １３０５ ) ] ... Equation I The sum of the crystalline phase ratio and the intermediate phase ratio of polyethylene = (I １４１６ +I １４４０ ) / (I １４１６ +I １４４０ +I １４６０ ) ... Equation II: Crystalline phase ratio of polyethylene = I １４１６ / (I １４１６ +I １４４０ +I １４６０ )...Formula III

[0122] (8) Amount of branches of polyethylene measured by NMR spectroscopy (mol%) 70 mg of a microporous polyolefin membrane was cut out to obtain a sample. After folding the sample to a height of 4.5 cm or less and placing it in a sample tube, o-dichlorobenzene-d4 was added to the sample tube, and the sample was dissolved at 130°C for about 3 to 4 hours. After allowing to stand overnight at room temperature, the following conditions are applied:  <13C-NMR measurement conditions>  • Apparatus: Bruckner Avance NEO 600  • Observed nucleus: 13C  • Pulse program: zgpg30  • Pulse wait time: 5 sec  • Number of scans: 8064 • Measurement temperature: 130°C  • Chemical shift reference: 29.9 ppm (-CH ２ -)  • Sample concentration: 10 wt / vol%  • Sample tube: 5 mmφ  • BF: 1 Hz (topspin)  13C-NMR measurement was performed under the above conditions.

[0123] In the obtained 13C-NMR spectrum, the sum of the integrated values of the branch signal derived from C3 monomer (19.9 ppm), the branch signal derived from C4 monomer (11.1 ppm), and the branch signal derived from C6 monomer (23.3 ppm) was compared with the half of the integrated value of the main signal derived from the main backbone of polyethylene (PE) molecular chain (29.9 ppm, integration range: 27.5-31.9 ppm). The amount of branches (mol%) of polyethylene in the microporous polyolefin membrane was calculated from the intensity ratio. The integrated value of branch signals was normalized such that the integrated value of the main signal was 10000.

[0124] (9) Melting point (°C) during the second heating step measured by DSC A suitable amount of polyolefin microporous membrane sample was placed in an aluminum dish and dried in a hot air dryer at 130°C for 30 minutes. Approximately 5 mg of the dried membrane was packed into an aluminum container for measurement, and DSC and DDSC curves under a nitrogen atmosphere were obtained using a DSC measuring device (TA Instruments, DSC Q2000). The measurement conditions were as follows: (1st heating program) Started at 40°C and heated at a rate of 50°C per minute. Maintained at 200°C for 5 minutes after reaching it. (2nd cooling program) Defrosted from 200°C at a rate of 20°C per minute. Maintained at -50°C for 5 minutes after reaching it. (3rd heating program) Heated from -50°C at a rate of 20°C per minute to 200°C. DSC and DDSC data were acquired during this 3rd heating step. Temperature and calorific calibration were performed using indium as the standard material at a heating rate of 1.0 °C / min. Sample container: Hitachi High-Tech Science AI open-type sample container and crimp cover. Sampling method: Folded film samples were punched out to 4.5 mm in diameter, placed in 5-7 mg sample containers, and crimped.

[0125] (10) Dielectric Strength Test Using the PO microporous film to be measured as a film sample, the film sample and aluminum foil were cut to 100 mm x 50 mm. The sample was placed on the aluminum foil, and a 5 mm diameter aluminum plate was placed on top of the sample. Using a 5 mm diameter electrode with a load of 45 gf, a dielectric strength tester (device name: TOS9201) was used, and the voltage was applied to the electrode with the current type set to AC (60 Hz), the starting voltage set to 0 kV, and the boosting rate set to 0.1 kV / s. The voltage was gradually increased, and the voltage when a current of 0.2 mA or more flowed was measured. This measurement was taken at at least 20 different locations within the plane of the same film sample. At this time, the setting of the measurement locations and the measurement were performed at locations at least 15 mm away from the measurement locations, ensuring no bias within the plane. The voltage value at which a current of 0.2 mA or more flowed was converted to a voltage withstand value per 1 μm of film thickness, and the average of 20 measured values ​​was calculated and evaluated according to the following criteria. Measurements were taken in a dry room at room temperature of 18°C ​​and a dew point of -40°C. S: Voltage withstand value of 0.12 kV / μm or more. A: Voltage withstand value of 0.10 kV / μm or more and less than 0.12 kV / μm. B: Voltage withstand value of 0.08 kV / μm or more and less than 0.10 kV / μm. C: Voltage withstand value less than 0.08 kV / μm.

[0126] (11) TD tensile strength and MD tensile strength (kgf / cm ２ ), and the ratio of MD tensile strength to TD tensile strength (MD tensile strength / TD tensile strength) The MD and TD tensile strengths were measured using strip-shaped test specimens with a width of 10 mm, according to the method compliant with ASTM D882. The test specimens were fixed using double-sided tape (NW-15, manufactured by Nichiban Co., Ltd.), and the TD tensile strength and MD tensile strength were determined by measuring under conditions of a grip distance of 20 mm and a test speed of 100 mm / min. The MD tensile strength / TD tensile strength ratio was obtained by dividing the value of MD tensile strength by the value of TD tensile strength.

[0127] (12) Battery Evaluation A battery was fabricated according to the following procedures a-1 to a-5. a-1. Fabrication of the positive electrode Nickel, manganese, cobalt composite oxide (NMC) (Ni:Mn:Co = 1:1:1 (elemental ratio), density 4.70 g / cm³) was used as the positive electrode active material. ３90.4% by mass of , graphite powder (KS6) (density 2.26 g / cm ３ , number average particle diameter 6.5 μm) in 1.6% by mass and acetylene black powder (AB) (density 1.95 g / cm ３ , number average particle diameter 48 nm) in 3.8% by mass, and polyvinylidene fluoride (PVDF) (density 1.75 g / cm ３ ) in 4.2% by mass, these were mixed and dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This slurry was applied onto one side of a 20 μm-thick aluminum foil serving as a positive electrode current collector using a die coater, dried at 130° C. for 3 minutes, and then compression-molded using a roll press to produce a positive electrode. The coating amount of the positive electrode active material at this time was 109 g / m ２ .

[0128] a-2. Preparation of negative electrode As the negative electrode active material, graphite powder A (density 2.23 g / cm ３ , number average particle diameter 12.7 μm) in 87.6% by mass and graphite powder B (density 2.27 g / cm ３ , number average particle diameter 6.5 μm) in 9.7% by mass, and 1.4% by mass (in terms of solid content) of ammonium salt of carboxymethyl cellulose (aqueous solution with 1.83% by mass solid content concentration) and 1.7% by mass (in terms of solid content) of diene rubber latex (aqueous solution with 40% by mass solid content concentration) as binders were dispersed in purified water to prepare a slurry. This slurry was applied onto one side of a 12 μm-thick copper foil serving as a negative electrode current collector using a die coater, dried at 120° C. for 3 minutes, and then compression-molded using a roll press to produce a negative electrode. The coating amount of the negative electrode active material at this time was 52 g / m ２ .

[0129] a-3. Preparation of non-aqueous electrolyte A non-aqueous electrolyte was prepared by dissolving LiPF ６ as a solute in a mixed solvent of ethylene carbonate:ethyl methyl carbonate = 1:2 (volume ratio) to a concentration of 1.0 mol / L.

[0130] a-4. Battery Fabrication Using the positive electrode, negative electrode, and non-aqueous electrolyte obtained in a-1 to a-3 above, as well as the microporous membrane obtained in the example or comparative example, a laminate-type secondary battery with a size of 100 mm x 60 mm and a capacity of 3000 mAh was fabricated by charging it under constant current and constant voltage (CCCV) conditions for 3 hours at a current of 1 A (0.3 C) and a terminal battery voltage of 4.2 V.

[0131] a-5. Capacity Measurement (mAh) The laminated secondary battery assembled as described above was charged under constant current constant voltage (CCCV) conditions for 6 hours at a current of 1500 mA (0.5 C) and a terminal battery voltage of 4.2 V. At this time, the current value was approximately 0 just before the end of charging. After that, the battery was left (aged) for one week in a 25°C atmosphere.

[0132] Next, a cycle was performed in which the battery was charged under constant current and constant voltage (CCCV) conditions for 3 hours at a current of 3000 mA (1.0 C) and a terminal battery voltage of 4.2 V, and then discharged to a battery voltage of 3.0 V at a constant current (CC) of 3000 mA. The discharge capacity at this time was defined as the initial discharge capacity X. Batteries with an initial discharge capacity X of 3000 ± 10 mAh or less were used for battery evaluation.

[0133] b. Cycle Test Using the batteries assembled and selected for evaluation as described above, a total of 100 charge-discharge cycles were performed under the following cycle conditions: (i) constant current constant voltage charging at a current of 20C and an upper voltage limit of 4.2V for a total of 8 hours, (ii) a 10-minute pause, (iii) constant current discharge at a current of 20C and a cutoff voltage of 3.0V, and (iv) a 10-minute pause. All of the above charge-discharge processes were performed in an atmosphere of 20°C. After that, the capacity retention rate (%) was calculated by multiplying the ratio of the discharge capacity after the 100th cycle to the initial battery capacity X (mAh) by 100. The capacity retention rate was evaluated according to the following criteria: S: Capacity retention rate (%) at 20°C is 50% or more. A: Capacity retention rate (%) at 20°C is 30% or more and less than 50%. B: Capacity retention rate (%) at 20°C is 20% or more and less than 30%. C: Capacity retention rate (%) at 20°C is less than 20%.

[0134] (13) Nail piercing test a. Preparation of the positive electrode Lithium cobalt composite oxide LiCoO ２Furthermore, graphite and acetylene black were dispersed as conductive materials in polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP) as binders to prepare a slurry. This slurry was applied to a 15 μm thick aluminum foil, which would serve as the positive electrode current collector, using a die coater, dried at 130°C for 3 minutes, and then compressed and molded using a roll press. The resulting molded body was slit to a width of 57.0 mm to obtain the positive electrode.

[0135] b. Preparation of the negative electrode A slurry was prepared by dispersing artificial graphite as the negative electrode active material and carboxymethylcellulose ammonium salt and styrene-butadiene copolymer latex as the binder in purified water. This slurry was applied to copper foil, which would serve as the negative electrode current collector, using a die coater, dried at 120°C for 3 minutes, and then compressed and molded using a roll press. The resulting molded body was slit to a width of 58.5 mm to obtain the negative electrode.

[0136] c. Preparation of non-aqueous electrolyte: A mixed solvent of ethylene carbonate:dimethyl carbonate:ethyl methyl carbonate = 1:1:2 (volume ratio) is used, with LiPF as the solute. ６ A non-aqueous electrolyte was prepared by dissolving the substance to a concentration of 1 mol / L.

[0137] d. After winding the battery assembly positive electrode, the PO porous film obtained in the example or comparative example, and the negative electrode, a wound electrode body was fabricated by a conventional method and pressed with a press machine so that it would fit into the outer casing. The number of turns was adjusted according to the thickness of the PO microporous film and the degree of springback. The outermost end of the obtained wound electrode body was fixed by attaching insulating tape. The negative electrode lead was welded to the battery casing and the positive electrode lead was welded to the safety valve, and the wound electrode body was inserted into the battery casing. Then, 5 g of non-aqueous electrolyte was injected into the battery casing, and the lid was crimped onto the battery casing via a gasket to obtain a rectangular secondary battery with a width of 42.0 mm, a height of 63.0 mm, and a thickness of 10.5 mm. This rectangular rechargeable battery was charged to a voltage of 4.2V at a current of 0.2C (0.2 times the hourly rate (1C) of the rated electrical capacity) in an atmosphere of 25°C for a total of 3 hours. After reaching 4.2V, the current was gradually reduced to maintain that voltage. Subsequently, it was discharged to a voltage of 3.0V at a current of 0.2C.

[0138] e. Nail Penetration Test The batteries assembled and selected for evaluation in d. above were placed on a steel plate inside a temperature-controlled explosion-proof booth. A 3.0 mm diameter steel nail was prepared, and a thermocouple was installed inside the nail. Under a 30°C, 3 MPa pressurized environment inside the explosion-proof booth, the steel nail was driven through the center of the battery at a speed of 2 mm / sec, and the nail was kept in the penetrating position. The battery was observed from the start of nail penetration until after the nail had penetrated, and its safety was evaluated according to the following criteria: S: Nothing happens. A: Smoke is emitted. B: Cell swelling or deformation is observed. C: Ignition or explosion occurs.

[0139] [Example 1] As shown in Table 2, PE1 with a viscosity-average molecular weight of 900,000 was used as the polyethylene (PE) species, and 0.1 parts by mass of pentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] was added as an antioxidant and pre-mixed. The resulting mixture was supplied to the feed port of a twin-screw extruder using a feeder. Liquid paraffin (LP) was added to the twin-screw extruder cylinder from the side feed in two stages so that the amount ratio of liquid paraffin as a plasticizer in the total mixture (100.1 parts by mass) to be melt-kneaded and extruded was 76.0 parts by mass. The set temperatures were 160°C for the kneading section and 200°C for the T-die. Subsequently, the molten mixture was extruded from the T-die into a sheet, cooled with a cooling roll controlled to a surface temperature of 70°C, and a sheet-like molded product with a thickness of 2.8 mm was obtained.

[0140] The obtained sheet-like molded material was guided to a simultaneous biaxial stretching machine to obtain a primary stretched film (primary stretching step). The set stretching conditions were an MD and TD stretching ratio of 9.6 times, an MD and TD stretching temperature of 113°C, and an MD and TD stretching speed of 20% / second. Next, the obtained primary stretched film was guided to a methylene chloride bath and thoroughly immersed to extract and remove the liquid paraffin, which is a plasticizer, and then the methylene chloride was dried off to obtain an extracted film. The total stretching ratio during the primary stretching step was 92.2 times.

[0141] Next, the extracted film was guided to a simultaneous biaxial stretcher for heat fixation. For the heat fixation process, after stretching under the conditions of MD and TD stretching temperature of 136°C, MD and TD stretching ratio of 2.4 times, and MD and TD stretching speed of 6.0% / second, relaxation was performed under the conditions of MD and TD relaxation temperature of 130°C, MD and TD relaxation rate of 5.0%, MD and TD relaxation speed of 1.3% / second, and MD and TD relaxation cooling speed of 28°C / second. Furthermore, cooling was accelerated by blowing air from both sides of the microporous film immediately after it was unloaded from the tenter after heat relaxation. Various properties of the obtained PO microporous film were evaluated using the above method. The film formation conditions are shown in Table 2, and the results are shown in Table 4. The total stretching ratio was 479.1 times. The resin types used in the examples and comparative examples are shown in Table 1.

[0142] [Examples 2-13 and Comparative Examples 1-12] PO microporous films were obtained in the same manner as in Example 1, except that the resin raw material type, raw material composition ratio, and film formation conditions were set as shown in Tables 2 and 3, respectively. In Examples 4, 5, and 11, air was blown from both sides of the microporous film immediately after it was unloaded from the tenter following thermal relaxation, similar to Example 1. In Examples 3 and 9, air was blown only from the bottom side of the microporous film immediately after it was unloaded from the tenter following thermal relaxation. In the other examples and comparative examples, air was not blown, and the relaxation cooling rate was controlled by controlling the temperature and relaxation time of the dry re-stretching / thermal relaxation process, respectively. Various properties of the obtained PO microporous films were evaluated using the above method. The results are shown in Tables 4 and 5. In Example 7, before simultaneous biaxial stretching, a sheet-like molded product was pre-stretched in a roll stretcher under the conditions of a stretching ratio of 1.8 times, a stretching temperature of 117°C, and a stretching speed of 10% / second. In the table, the terms "simultaneous," "TD," "MD," "sequential tenter," and "sequential roll" in the wet stretching method and dry re-stretching / thermal relaxation method entries indicate that the stretching and relaxation operations were performed using a simultaneous twin-screw stretcher, a TD unscrew tenter, an MD unscrew roll stretcher, a sequential tenter twin-screw stretcher, and a sequential roll twin-screw stretcher, respectively, in the primary stretching process and the dry re-stretching / thermal relaxation process. In the dry re-stretching / thermal relaxation process, the same stretcher was used for both the re-stretching and relaxation operations. For example, "simultaneous" means that a simultaneous twin-screw stretcher was used for both the re-stretching and relaxation operations.

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Claims

1. A polyolefin microporous membrane containing polyethylene, wherein the porosity is 45% or more, and the all-trans amount of the polyethylene, as measured by Raman scattering spectroscopy, is 1.30 or more.

2. The polyolefin microporous membrane according to claim 1, wherein the sum of the crystalline phase ratio and the intermediate phase ratio of polyethylene, as measured by the Raman scattering spectroscopy, is 0.75 or more.

3. The polyolefin microporous membrane according to claim 1 or 2, wherein the branching amount of the polyethylene, as measured by NMR spectroscopy, is 0.05 mol% or less.

4. The polyolefin microporous membrane according to claim 1 or 2, wherein the melting point at the second heating stage, as measured by DSC, is 135°C or higher.

5. The polyolefin microporous membrane according to claim 1 or 2, wherein the weight-average molecular weight (Mw) measured by high-temperature gel permeation chromatography (GPC) is 600,000 or more.

6. Number average molecular weight measured by high-temperature GPC: The polyolefin microporous membrane according to claim 5, wherein the ratio of the weight average molecular weight measured by high-temperature GPC to Mn (Mw / Mn) is 3.0 or more and 10 or less.

7. The polyolefin microporous membrane according to claim 1 or 2, wherein the ratio of MD tensile strength to TD tensile strength: MD tensile strength / TD tensile strength is 0.8 or more and 1.25 or less.

8. Puncture strength equivalent to base weight is 120 gf / (g / m). ２ The polyolefin microporous membrane according to claim 1 or 2, wherein the above conditions are met.

9. Air permeability resistance is 150 seconds / 100 cm ３ The polyolefin microporous membrane according to claim 1 or 2, which is as follows:

10. The polyolefin microporous membrane according to claim 1 or 2, wherein the average thermal shrinkage rate of MD and TD at 120°C is 30.0% or less.

11. A film roll formed by winding a polyolefin microporous film according to claim 1 or 2.

12. A separator for a lithium-ion secondary battery having an inorganic particle-containing layer with a thickness of 5 μm or less on at least one surface of the polyolefin microporous film according to claim 1 or 2.

13. A lithium-ion secondary battery comprising the lithium-ion secondary battery separator described in claim 12.

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