Polyolefin microporous membrane

Abstract

A polyolefin microporous membrane is provided, wherein the polyolefin microporous membrane is stained with ruthenium, embedded in room temperature curing epoxy resin, and a smooth cross-section parallel to the TD is prepared using a broad ion beam (BIB). When pore size analysis is performed using the local thickness method on scanning electron microscopy (SEM) images of five points at 50 μm intervals at 7000x magnification within this smooth cross-section, the average pore size d is determined. TD(N=5) The standard deviation is above 0.1 and below 1.0.

Description

Technical Field

[0001] This invention relates to polyolefin microporous membranes, etc. Background Technology

[0002] Polyolefin microporous membranes (sometimes abbreviated as "PO microporous membranes") are used to separate various substances or are widely used as selective permeation separation membranes, separators, etc. Examples of their applications include precision filtration membranes; separators for lithium-ion batteries, fuel cells, etc.; separators for capacitors; and master materials for functional membranes that exhibit new functions by filling the pores with functional materials. Among these applications, PO microporous membranes are particularly suitable as separators for lithium-ion batteries (LIBs), which are widely used in mobile phones, smartphones, wearable devices, laptop computers (PCs), tablet PCs, and digital cameras.

[0003] Previously, PO microporous membranes, intended for use as separators in LIBs, sought to possess the following properties: dimensional stability at temperatures below the melting point of PO or under external stress; shut-off properties near the melting point; and, further, rupture-resistant membrane properties at high temperatures. Furthermore, based on the correlation between the properties of LIB separators and the characteristics of LIBs, various PO microporous membranes and their manufacturing methods have been proposed.

[0004] For example, Patent Document 1 describes a PO microporous membrane having a porous layer. From the viewpoint of excellent initial overcharge test results of half-cells, the membrane thickness of the porous layer is less than 16 μm, and the total micropore volume by one-point method with a micropore diameter of less than 98 nm as determined by nitrogen adsorption test is more than 25% and less than 85% of the total pore volume.

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

[0006] In Patent Document 3, in order to provide a microporous membrane with excellent physical property balance of thin film, uniform micropore size, tensile strength and high permeability, and thus ensure the output characteristics of secondary battery, the proportion of the total value of the micropore diameter distribution of pores with a maximum peak in the special micropore diameter distribution curves of the membrane in dry and wet states, the maximum pore size, MD tensile strength, and the ratio of MD tensile strength to TD tensile strength were studied.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2021-123614

[0010] Patent Document 2: Japanese Patent Application Publication No. 2021-038379

[0011] Patent Document 3: Japanese Patent Application Publication No. 2015-120786 Summary of the Invention

[0012] The problem the invention aims to solve

[0013] In recent years, electrochemical devices such as LIBs have seen significant miniaturization and thinning. On the other hand, there is a demand for increased LIB capacity compared to existing products. Therefore, electrochemical device manufacturers are promoting the increase of nickel (Ni) content in the cathode and the thinning of separators for electrochemical devices. However, thinning can sometimes lead to safety issues such as reduced shock resistance and short circuits caused by foreign objects.

[0014] In particular, existing LIBs suffer from the problem of deposits (dendritices) from the electrodes, thus requiring improvements in cycling characteristics by suppressing dendrites.

[0015] Furthermore, to achieve high output in electrochemical devices, it is necessary to improve the ion permeability of the separator while ensuring thin-film properties and absolute strength, thus requiring high porosity. However, even in electrochemical devices using separators with high porosity, short-circuit failures still easily occur.

[0016] In view of the above, the object of the present invention is to provide a polyolefin microporous membrane that can improve the cycle characteristics of an electrochemical device and suppress short-circuit defects, as well as a separator and an electrochemical device using the same.

[0017] Solution for solving the problem

[0018] The aforementioned problems can be solved using the following technical means.

[0019] <1>

[0020] A polyolefin microporous membrane, wherein,

[0021] The polyolefin microporous membrane was stained with ruthenium and embedded in room-temperature curing epoxy resin. A smooth cross-section parallel to the TD was fabricated using a broad ion beam (BIB). The average pore size d was determined by scanning electron microscopy (SEM) images of five points at 7000x magnification and 50 μm intervals within this smooth cross-section using the local thickness method. TD(N=5) The standard deviation is above 0.1 nm and below 1.0 nm.

[0022] <2>

[0023] According to the polyolefin microporous membrane described in Project 1, the thickness of the polyolefin microporous membrane is more than 6 μm and less than 16 μm.

[0024] <3>

[0025] According to the polyolefin microporous membrane described in Project 1 or 2, when performing pore size analysis on at least one point of the SEM image, the pore size distribution D TD The standard deviation is above 35nm and below 55nm.

[0026] <4>

[0027] According to any one of Projects 1 to 3, the polyolefin microporous membrane, wherein the average pore size d when performing pore size analysis at least one point on the SEM image. TD It is between 100nm and 160nm.

[0028] <5>

[0029] According to any one of items 1 to 4, the polyolefin microporous membrane has a per-unit area weight converted puncture strength of 80 gf / (g / m²). 2 ) or above and 150gf / (g / m 2 )the following.

[0030] <6>

[0031] The polyolefin microporous membrane according to any one of items 1 to 5, wherein the viscosity-average molecular weight (Mv) of the polyolefin microporous membrane is 800,000 or more and 1,500,000 or less.

[0032] <7>

[0033] The polyolefin microporous membrane according to any one of items 1 to 6, wherein the molecular weight distribution (Mw / Mn) of the polyolefin microporous membrane, expressed as the ratio of mass-average molecular weight (Mw) to number-average molecular weight (Mn), is 5 or more and 10 or less.

[0034] <8>

[0035] The polyolefin microporous membrane according to any one of items 1 to 7, wherein the puncture strength of the polyolefin microporous membrane is above 450 gf and below 1000 gf.

[0036] <9>

[0037] The polyolefin microporous membrane according to any one of items 1 to 8, wherein the air permeability of the polyolefin microporous membrane is 30s / 100cm. 3 Above and 195s / 100cm 3 the following.

[0038] <10>

[0039] According to any one of items 1 to 9, the amount of powder falling off the polyolefin microporous membrane when it is fed from the feeder with a tension of 10N and conveyed for only 200m on a rubber with a dynamic friction coefficient of 0.5 to SUS304 is more than 0.001mg / cm and less than 0.04mg / cm.

[0040] The effects of the invention

[0041] According to the present invention, the cycle characteristics of electrochemical devices comprising polyolefin microporous membranes or separators for electrochemical devices can be improved, and short-circuit defects can be suppressed. Detailed Implementation

[0042] The following describes in detail the method for implementing the present invention (hereinafter sometimes simply referred to as "this embodiment"), but the present invention is not limited thereto, and various modifications can be made without departing from its spirit. It should be noted that in this specification, the flow direction of the membrane during film formation is defined as MD, and the direction intersecting MD at a 90-degree angle within the membrane plane is defined as TD. Furthermore, "the membrane or resin contains a specific component as a main component" means that, based on the mass of the membrane or resin, the content of the specific component is 50% by mass or more.

[0043] The polyolefin microporous membrane (PO microporous membrane) of this embodiment contains polyolefin (PO) as the main component, and the relationship between the membrane thickness and the properties obtained by pore size analysis has the following characteristics.

[0044] As desired, the membrane thickness, molecular weight, puncture strength, puncture strength converted from unit area weight, and air permeability of the PO microporous membrane can also be determined as follows, and an inorganic coating layer or adhesive layer can also be formed on its surface. The various properties described in this embodiment can be used independently or in arbitrary combinations. It should be noted that, unless otherwise specified, the methods for determining the physical properties of the PO microporous membrane are detailed in the examples.

[0045] <Relationship between membrane thickness and membrane properties obtained using pore size analysis>

[0046] In this embodiment, the PO microporous membrane was stained with ruthenium and embedded in room-temperature curing epoxy resin. A smooth cross-section parallel to the TD was fabricated using a broad ion beam (BIB). When pore size analysis was performed using the local thickness method on scanning electron microscopy (SEM) images of five points at 7000x magnification with 50 μm intervals within this smooth cross-section, the average pore size d was determined. TD(N=5) The standard deviation is above 0.1 nm and below 1.0 nm.

[0047] The aperture analysis described above is detailed in the project of the embodiment. It should be noted that the SEM images of the above 5 points can be obtained, for example, by taking 5 images at 50 μm intervals along the length direction in the above smooth cross section at different field of view magnifications of 7000x.

[0048] The PO microporous membrane involved in this embodiment also has an average pore size d. TD(N=5) With a standard deviation of 0.1 nm or more and 1.0 nm or less, when used as a separator in an electrochemical device, it not only makes the current more uniform and suppresses dendrites to improve the cycle characteristics of the electrochemical device, but also increases the membrane strength, thereby improving the rupture resistance and voltage withstand capability, thus improving both cycle characteristics and suppressing short-circuit failure.

[0049] The average pore size d of the PO microporous membrane in this embodiment can be clearly defined. TD(N=5) The smaller the standard deviation, the better. From the viewpoint of balancing improved cycle characteristics and suppression of short-circuit defects, it is preferred to be greater than 0.1 nm and less than 1.0 nm, more preferably greater than 0.2 nm and less than 0.9 nm, further preferably greater than 0.2 nm and less than 0.8 nm, even more preferably greater than 0.2 nm and less than 0.7 nm, and particularly preferably greater than 0.2 nm and less than 0.5 nm.

[0050] For the PO microporous membrane of this embodiment, after ruthenium staining and embedding with room-temperature curing epoxy resin, a smooth cross-section parallel to TD is fabricated using BIB. When performing pore size analysis on SEM images of at least one point obtained by photographing this smooth cross-section at 7000x magnification, the pore size distribution D... TD The standard deviation is preferably above 35nm and below 55nm. If 35nm ≤ D is satisfied... TD If the standard deviation is ≤55nm, there is a tendency to suppress short-circuit defects while further improving cycle characteristics and withstand voltage. Based on this tendency, the aperture distribution D TD The standard deviation is more preferably 38nm or more and 53nm or less, further preferably 40nm or more and 50nm or less, even more preferably more than 40nm and 49nm or less, even more preferably more than 40nm and less than 49nm, and particularly preferably more than 40nm and 46nm or less.

[0051] In this embodiment, the PO microporous membrane preferably has an average pore size d when at least one point of pore size analysis is performed on the SEM image of a smooth cross-section parallel to TD prepared as described above. TD It is between 100nm and 160nm. If 100nm ≤ d TD For wavelengths ≤160nm, there is a tendency to simultaneously improve cycle performance, suppress short-circuit defects, and further enhance the film's voltage withstand capability. Not adhering to theoretical constraints, it is assumed that within 100nm ≤ d... TD Within the ≤160nm range, the pore structure of PO microporous membranes is small and uniform. Therefore, the branches of the fibril backbone increase, the interface between the air and the separator backbone becomes denser, and surface discharge is suppressed. From this perspective, the average pore size d... TD More preferably, the nm diameter is 105nm or more and 150nm or less; even more preferably, the nm diameter is 108nm or more and 140nm or less; even more preferably, the nm diameter is 109nm or more and less than 140nm; even more preferably, the nm diameter is 110nm or more and 130nm or less; and particularly preferably, the nm diameter is 110nm or more and 125nm or less.

[0052] For the PO microporous membrane of this embodiment, from the viewpoint of balancing improved circulation characteristics and suppression of short-circuit defects, the maximum pore size PS is determined by performing at least one point pore size analysis on the SEM image of the smooth cross-section parallel to TD prepared as described above. TD(max) Preferably, the wavelength is above 250nm and below 400nm.

[0053] It should be noted that the SEM image of at least one point of the smooth cross section parallel to the TD can be obtained, for example, by taking a picture of at least one point of the smooth cross section parallel to the TD with at least one field of view at 7000x magnification.

[0054] For the PO microporous membrane involved in this embodiment, from the viewpoint of balancing improved circulation characteristics and suppression of short-circuit defects, after ruthenium staining and embedding with room-temperature curing epoxy resin, a smooth cross-section parallel to the microstructure was fabricated using BIB. When performing pore size analysis on SEM images of five points at 7000x magnification with 50μm intervals in this smooth cross-section using the local thickness method, the average pore size d was determined. MD(N=5) The standard deviation is preferably 0.2 nm or more and 0.9 nm or less, more preferably 0.2 nm or more and less than 0.8 nm.

[0055] For the PO microporous membrane of this embodiment, after ruthenium staining and embedding with room-temperature curing epoxy resin, a smooth cross-section parallel to the microstructure was fabricated using BIB. When performing pore size analysis on SEM images of at least one point obtained by photographing this smooth cross-section at 7000x magnification, the pore size distribution D... MDThe standard deviation is preferably above 38 nm and below 45 nm, satisfying 38 nm ≤ D. MD When the standard deviation is ≤45nm, there is a tendency to easily balance improved cycle characteristics and suppression of short-circuit defects. Based on this tendency, the aperture distribution D... MD The standard deviation is preferably above 39nm and below 44nm.

[0056] For the PO microporous membrane of this embodiment, from the viewpoint of balancing improved circulation characteristics and suppression of short-circuit defects, the average pore size d is determined by performing pore size analysis at least at one point on the SEM image of the smooth cross-section parallel to the MD prepared as described above. MD Preferably, the 100nm or more and 155nm or less is preferred, more preferably 102nm or more and 149nm or less is preferred, even more preferably 105nm or more and 140nm or less is preferred, even more preferably 106nm or more and 130nm or less is preferred, and particularly preferably 107nm or more and 125nm or less is preferred.

[0057] It should be noted that a SEM image of at least one point of a smooth cross section parallel to the MD can be obtained, for example, by taking a picture of at least one point of the smooth cross section parallel to the MD with at least one field of view at 7000x magnification.

[0058] For the PO microporous membrane of this embodiment, from the viewpoint of balancing improved circulation characteristics and suppression of short-circuit defects, after ruthenium staining and embedding with room-temperature curing epoxy resin, smooth cross-sections parallel to MD and TD are fabricated using BIB. When performing pore size analysis on SEM images of at least one point obtained by taking pictures of these smooth cross-sections at 7000x magnification using the local thickness method, the ratio of the average pore size of the cross-sections fabricated along the two directions, i.e., the average pore size d, is determined. MD With average aperture d TD The ratio (d) MD / d TD Preferably, the value is 0.85 or higher and 1.25 or lower, more preferably 0.90 or higher and 1.20 or lower, and even more preferably 0.91 or higher and 1.08 or lower.

[0059] As a means of adjusting both the film thickness and the properties of the film obtained by pore size analysis as described above, at least one of the following (i) to (v) can be cited as an example.

[0060] (i) The selection of the resin or raw material resin, more specifically, may include: selecting a polyolefin resin with a viscosity-average molecular weight (Mv) of 800,000 to 1,500,000, a molecular weight distribution (Mw / Mn) of 5 to 10 expressed as the ratio of mass-average molecular weight (Mw) to number-average molecular weight (Mn), and / or having a single resin composition (e.g., polyethylene homopolymer); and adjusting the total content (PC) of all resins contained in the PO resin composition to a range of 20% to 25%, etc.

[0061] (ii) In the manufacturing method of PO microporous membrane, the mixing index in the extrusion process is controlled.

[0062] The compounding index is expressed by the following formula:

[0063] The mixing index (rpm·min.) = screw speed (rpm) × resin residence time (min.). More specifically, it is preferable to adjust the mixing index to the range of 800 rpm·min. to 2,200 rpm·min. When the mixing index is 800 rpm·min. or higher, the resin is mixed to a degree sufficient to maintain the pore size uniformity of the membrane and to suppress unmelted resin, thereby eliminating or reducing defects. When the mixing index is 2,200 rpm·min. or lower, the molecular weight of the resin is maintained or increased, the membrane strength is improved, and the amount of powder falling off is reduced, which can eliminate or reduce defects during production.

[0064] (iii) In the manufacturing method of PO microporous membrane, the cooling rate during casting is controlled to be 1℃ / sec.~12℃ / sec., and the means of controlling the cooling rate during casting can be, for example, adjusting the film thickness to be in the range of 1200μm~3000μm, adjusting the resin temperature at the mold exit to be in the range of 180℃~215℃, adjusting the temperature of the roller that the resin first contacts (hereinafter referred to as "casting temperature" or "temperature of the roller that first receives the extruded preform"), using a cooling roller with a surface temperature controlled to be 50℃~100℃ in order to adjust the casting temperature, and adjusting the casting speed to be in the range of 1m / min.~12m / min., etc.

[0065] (iv) Simultaneous biaxial stretching can be listed as a method for manufacturing PO microporous membranes.

[0066] (v) In the manufacturing method of PO microporous membrane, the control of the stretching ratio can be cited, for example, adjusting the stretching ratio in the stretching process before extraction of the pore-forming material to the range of 35 to 60 times, adjusting the transverse (TD) stretching ratio in the stretching process after extraction to the range of 1.4 to 2.4 times, and adjusting the total stretching ratio to the range of 70 to 120 times, etc.

[0067] In addition, as a method for increasing the average pore size d of PO microporous membranes MD With average aperture d TD The ratio (d) MD / d TD The means of adjusting to the above numerical range can be not only exemplified by (i) to (iv) above, but also by controlling the MD / TD stretching ratio in the manufacturing method of PO microporous membranes.

[0068] The preferred structure or properties, preferred constituent elements, and manufacturing method of the PO microporous membrane of this embodiment will be described below.

[0069] <Film thickness>

[0070] The thickness of the PO microporous membrane is preferably 6 μm or more and 16 μm or less. A membrane thickness of 6 μm or more and 16 μm or less is important from the viewpoint of uniform pore size for the PO microporous membrane of this embodiment.

[0071] The thinner the PO microporous membrane, the more significant its membrane strength, current uniformity, rupture resistance, and voltage withstand capability. Therefore, from the viewpoint of balancing improved cycle characteristics and suppression of short-circuit defects, the membrane thickness of the PO microporous membrane is more preferably less than 16 μm, further preferably less than 15 μm, even more preferably less than 13 μm, even more preferably less than 12 μm, and particularly preferably less than 10 μm. Maintaining insulation between electrodes and satisfying the above-described average pore size d based on pore size analysis are also important considerations. TD(N=5) From the perspective of the standard deviation range, the thickness of PO microporous membrane is preferably 6 μm or more, and can also exceed 6 μm.

[0072] As a means of controlling the thickness of the PO microporous membrane within the above-mentioned numerical range, examples include not only (i) to (v) mentioned above, but also the control of the blank thickness, heat setting (HS) temperature, etc. in the PO microporous membrane manufacturing method described later.

[0073] <Membrane Strength>

[0074] Conversion of PO microporous membrane to weight per unit area (g / m²) 2 The preferred puncture strength (hereinafter referred to as the puncture strength converted to unit area weight) is 80 gf / (g / m²). 2 ) or above and 150gf / (g / m 2 Below ) . Has 80gf / (g / m 2 )~150gf / (g / m 2 PO microporous membranes with a puncture strength per unit area weight tend to exhibit improved rupture resistance. Based on this tendency, a more preferable puncture strength per unit area weight for PO microporous membranes is 83 gf / (g / m²). 2)Above and 140 gf / (g / m 2 )Below, more preferably 85 gf / (g / m 2 )Above and 135 gf / (g / m 2 )Below.

[0075] From the viewpoint of similarly improving the bursting membrane resistance as the puncture strength converted to the weight per unit area, the puncture strength of the PO microporous membrane not converted to the weight per unit area (hereinafter simply referred to as the puncture strength) is preferably 450 gf or more and 1000 gf or less, more preferably 550 gf or more and 1000 gf or less, and further preferably 550 gf or more and 900 gf or less.

[0076] It should be noted that the units "gf" and "N" can be interchanged according to the formula: 1 gf ≈ 0.0098 N. As a means of controlling the puncture strength converted to the weight per unit area or the puncture strength of the PO microporous membrane within the above numerical range, for example, adjusting the molecular weight of the PO raw material or the resin contained, adjusting the draw ratio of the stretching surface and / or the stretching temperature, etc. in the manufacturing process of the PO microporous membrane can be cited.

[0077] <Molecular weight of PO microporous membrane>

[0078] The viscosity-average molecular weight (Mv) of the PO microporous membrane of the present embodiment is preferably 800,000 or more. By making the Mv of the PO microporous membrane itself 800,000 or more, there is a tendency to easily achieve a uniform pore size. According to such a tendency, the Mv of the PO microporous membrane is more preferably 850,000 or more, and further preferably 900,000 or more. On the other hand, from the viewpoint of suppressing thermal shrinkage, the Mv of the PO microporous membrane is preferably 2,000,000 or less, more preferably 1,500,000 or less.

[0079] The molecular weight distribution (Mw / Mn) of the PO microporous membrane of the present embodiment, expressed as the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn), is preferably 5 or more and 10 or less. The PO microporous membrane within the numerical range of 5 ≤ Mw / Mn ≤ 10 has a tendency to easily achieve a uniform pore size. According to such a tendency, the molecular weight distribution (Mw / Mn) of the PO microporous membrane is more preferably 6 or more and less than 10, and further preferably 7 or more and 9 or less.

[0080] In this specification, the molecular weight of the PO microporous membrane is obtained by measuring the PO microporous membrane itself. The molecular weight of the PO microporous membrane can be adjusted to the above range, for example, by controlling the molecular weight or composition of the resin or raw material resin contained.

[0081] <Air permeability of PO microporous membrane>

[0082] From the viewpoint of ensuring ion permeability and ensuring the output characteristics of the electrochemical device, the air permeability of the PO microporous membrane is preferably 30 s / 100 cm 3 or more and 195 s / 100 cm 3 or less, more preferably 50 s / 100 cm 3 or more and 180 s / 100 cm 3 or less, still more preferably 70 s / 100 cm 3 or more and 170 s / 100 cm 3 or less, particularly preferably 78 s / 100 cm 3 or more and 160 s / 100 cm 3 or less. The air permeability of the PO microporous membrane can be controlled within the above numerical range, for example, by adjusting the HS draw ratio, HS relaxation ratio, HS relaxation temperature, etc. in the manufacturing method of the PO microporous membrane.

[0083] <Porosity of the PO microporous membrane>

[0084] The porosity of the PO microporous membrane is preferably 25% or more, more preferably 30% or more, still more preferably 35% or more, and particularly preferably 40% or more. From the viewpoint of ensuring good output characteristics, a porosity of 25% or more is preferred. As the upper limit of the porosity, it is preferably less than 70%, more preferably 65% or less, and still more preferably 60% or less. From the viewpoints of film strength and withstand voltage, a porosity of less than 70% is preferred.

[0085] <Flux pore size>

[0086] The Flux pore size of the PO microporous membrane of this embodiment is preferably 30 nm or more and 70 nm or less. The PO microporous membrane within the range of 30 nm ≤ Flux pore size ≤ 70 nm has no foreign matter entering, and when it is installed as a separator for an electrochemical device in the electrochemical device, there is a tendency to improve the cycle performance, and there is also a tendency to suppress light scattering and increase the light transmittance.

[0087] <Thermal shrinkage rate of the PO microporous membrane>

[0088] As the scope of utilization of electrochemical devices expands, in a high-temperature environment, such as in an oven test, in order to ensure device safety, it is preferable to control the thermal shrinkage rate of the PO microporous membrane used as a separator at high temperatures (such as near the melting point of PO, or near the melting point of the PO microporous membrane, etc.). In addition, it is also preferable to control the thermal shrinkage rate of the separator for electrochemical devices to prevent contact between electrodes in the energy storage device. From this perspective, the thermal shrinkage rate of the PO microporous membrane at 120 °C is preferably 33% or less in the MD direction and preferably 21% or less in the TD direction. The lower limit value of the thermal shrinkage rate of the PO microporous membrane at 120 °C can be, for example, -5% or more, -2% or more, -1% or more, or 0% or more in both the MD and TD directions.

[0089] <Maximum shrinkage stress of the TMA of the PO microporous membrane>

[0090] In the thermomechanical analysis (TMA) of the PO microporous membrane, the maximum shrinkage stress is preferably 7.0 gf or less in the MD direction and preferably 9.7 gf or less in the TD direction. As the scope of utilization of electrochemical devices expands, in order to ensure device safety in a high-temperature environment, the TMA of the PO microporous membrane used as a separator is considered important. From this perspective, when a PO microporous membrane having a maximum shrinkage stress of 7.0 gf or less in the MD direction and / or 9.7 gf or less in the TD direction is assembled as a separator into an electrochemical device, there is a tendency to improve the high-temperature safety of the device.

[0091] From the perspective of further improving the safety of electrochemical devices in a high-temperature environment, the maximum shrinkage stress of the TMA of the PO microporous membrane is more preferably 6.5 gf or less in the MD direction, further preferably 6.1 gf or less, and then more preferably 9.0 gf or less in the TD direction, further preferably 8.5 gf or less. From the perspective of the productivity of electrochemical devices and the adhesion between the electrodes and the separator, the lower limit value of the maximum shrinkage stress of the TMA is preferably 1.0 gf or more, more preferably 1.5 gf or more, and further preferably 2.0 gf or more in both the MD and TD directions.

[0092] <Closure temperature and film rupture temperature (melting temperature)>

[0093] From the perspective of maintaining the performance of electrochemical devices in a high-temperature environment and ensuring safety when the electrochemical device abnormally generates heat, the closure temperature of the PO microporous membrane is preferably 151 °C or less, more preferably 150 °C or less, 149 °C or less, 148 °C or less, 147 °C or less, 146 °C or less, 145 °C or less, 144 °C or less, 143 °C or less, 142 °C or less, or 141 °C or less. In addition, the lower limit value of the closure temperature is associated with the occlusion of the membrane pores and can be, for example, 110 °C or more, 120 °C or more, 130 °C or more, or 140 °C or more.

[0094] From the viewpoints of the stability and safety of the electrochemical device with respect to temperature, the film breakage temperature (melting temperature) of the PO microporous membrane of the present embodiment is preferably 150°C or higher, 155°C or higher, or 160°C or higher, and more preferably 170°C or higher, 180°C or higher, 190°C or higher, or exceeding 200°C. The upper limit value of the film breakage temperature of the PO microporous membrane is not limited, and depending on the type of PO or raw material PO contained, the type of components other than PO, the mixing ratio of PO and other components, etc., it can be, for example, 240°C or lower, less than 240°C, 235°C or lower, or 230°C or lower.

[0095] <Powder dropping amount of PO microporous membrane>

[0096] The powder dropping amount when the PO microporous membrane is sent out from the feeder at a tension of 10 N and conveyed on a rubber with a dynamic friction coefficient of 0.5 with SUS304 for only 200 m is preferably 0.001 mg / cm or more and 0.04 mg / cm or less. The generation of low molecular weight components of the PO microporous membrane in the range of 0.001 mg / cm to 0.04 mg / cm is small, and if it is included in the electrochemical device, the cycle characteristics can be improved. The powder dropping amount can be measured in the powder dropping test. The powder dropping test is described in detail in the examples.

[0097] As a means of controlling the powder dropping amount within the above numerical range, for example, kneading under mild conditions in the manufacturing method of the PO microporous membrane can be cited. More specifically, as shown in (ii) above, it is preferable to adjust the kneading index to 2,200 rpm·min or less, and more preferably to adjust it within the range of 800 rpm·min to 2,200 rpm·min. When the kneading index is 2,200 rpm·min or less, the molecular weight of the resin is maintained or increased, the strength of the membrane is also improved, the generation of low molecular weight components becomes less, and thus the powder dropping amount is also reduced, and the disadvantages during production can be eliminated or reduced. When the kneading index is 800 rpm·min or more, the unmolten resin can be suppressed and the disadvantages can be eliminated or reduced.

[0098] <Components contained in PO microporous membrane>

[0099] The PO microporous membrane of the present embodiment is formed of a resin composition containing a polyolefin resin. If necessary, the resin composition can further contain inorganic particles, resins other than polyolefin, etc. From the viewpoints of adjusting both the film thickness of the PO microporous membrane and the characteristics obtained by the pore size analysis method, and the viewpoint of film strength, the total content ratio (PC) of all the resins contained in the PO resin composition is preferably within the range of 20% to 25%.

[0100] Based on the mass of the PO microporous membrane, 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 can be 90% by mass or more and 100% by mass or less.

[0101] The polyolefin resin used in this embodiment 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. One type of these polymers can be used alone, or two or more can be used in combination. Furthermore, from the viewpoint of adjusting the membrane thickness of the PO microporous membrane as described above and considering the properties obtained using pore size analysis, a single type of polyolefin resin is preferred.

[0102] From the viewpoint of exhibiting shut-off characteristics, the total proportion of PE raw materials in PO raw materials is preferably 50% to 100% by mass, and more preferably 80% to 100% by mass.

[0103] From the perspective of adjusting the membrane thickness of the PO microporous membrane as described above and using the characteristics obtained by pore size analysis, the PO raw material preferably has a single resin composition, more preferably is composed of polyethylene (PE), and even more preferably is composed of only PE, for example, it can be a polyethylene homopolymer.

[0104] From the perspective of adjusting the membrane thickness of the PO microporous membrane as described above and the characteristics obtained by pore size analysis, the PO microporous membrane preferably has a single resin composition, more preferably is composed of polyethylene (PE), and even more preferably is composed of only PE, for example, it can be a polyethylene homopolymer.

[0105] The viscosity-average molecular weight (Mv) of the PO resin is preferably 500,000 or more and 2,000,000 or less. As the molecular weight of the PO resin increases, the film strength tends to be more readily apparent, and the safety also improves. From this perspective, the Mv of the PO resin is more preferably 600,000 or more, and even more preferably 700,000 or more. On the other hand, from the viewpoint of suppressing heat shrinkage, it is preferable to adjust the viscosity-average molecular weight of the PO resin to 2,000,000 or less.

[0106] The viscosity-average molecular weight (Mv) of the polyethylene (PE) contained in the PO microporous membrane is preferably 600,000 or more and 2,000,000 or less, more preferably 800,000 or more and 1,000,000 or less, and the weight-average molecular weight (Mw) is preferably 5.0 × 10⁻⁶. 5 ~5.0×10 6Within the range of [value missing], and / or the polydispersity (Mw / Mn) is preferably 7.0 or higher and 8.8 or lower. If the molecular weight of PE is within the above-mentioned range, it is easy to adjust both the membrane thickness of the PO microporous membrane and the properties obtained by pore size analysis as described above. From the same point of view, for PE raw materials, Mv is preferably 600,000 or higher and 2,000,000 or lower, more preferably 800,000 or higher and 1,000,000 or lower, and Mw is preferably 5.0 × 10⁻⁶. 5 ~5.0×10 6 Within the specified range, and / or the polydispersity (Mw / Mn) is preferably 7.0 or higher and 8.8 or lower. Particularly preferred are Mv of 800,000 or higher and 1,000,000 or lower, and Mw of 5.0 × 10⁻⁶. 5 ~5.0×10 6 Within the range of [specific range], and PE resins or PE raw materials with polydispersity (Mw / Mn of 7.0 or higher and 8.8 or lower).

[0107] The Z-average molecular weight (Mz) of the polyethylene (PE) contained in the PO microporous membrane is preferably 1.0 × 10⁻⁶. 6 ~5.0×10 7 Within the aforementioned range, if the Mz of PE is within this range, it is easy to adjust both the film thickness of the PO microporous membrane and the properties obtained using pore size analysis, as described above. From the same perspective, for PE raw materials, Mz is also preferably within 1.0 × 10⁻⁶. 6 ~5.0×10 7 Within the range.

[0108] From the perspective of adjusting the properties obtained by pore size analysis, polyolefin resins are preferably free of polypropylene (PP).

[0109] In addition, examples of polyolefin resins include low-density polyethylene (density 0.910 g / cm³). 3 Above and less than 0.930 g / cm 3 ), linear low-density polyethylene (density 0.910 g / cm³) 3 Above and below 0.940 g / cm 3 Medium-density polyethylene (density 0.930 g / cm³) 3 Above and below 0.942 g / cm³ 3 High-density polyethylene (density 0.942 g / cm³) 3 (above), ultra-high molecular weight polyethylene (density 0.910 g / cm³) 3 Above and less than 0.970 g / cm 3 They can be used individually or in combination of two or more.

[0110] In the above resin composition, inorganic particles, phenolic, phosphorus or sulfur antioxidants, metallic soaps such as calcium stearate and zinc stearate, and various known additives such as ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments can be mixed as needed.

[0111] <Methods for Manufacturing Polyolefin Microporous Membranes>

[0112] The method for manufacturing the PO microporous membrane described in this embodiment is not particularly limited, and methods including the following steps can be listed as examples:

[0113] The mixing step (a) involves mixing a resin composition containing a polyolefin resin with various additives to be added as desired.

[0114] Extrusion step (b) involves melting and kneading the mixture obtained in step (a) and then extruding it.

[0115] Sheet forming process (c) shapes the extrudate obtained in process (b) into sheets;

[0116] In one stretching process (d), the sheet-shaped material obtained in process (c) is stretched at least once along a single axis.

[0117] Extraction step (e) involves extracting the pore-forming material from the primary stretched membrane obtained in step (d); and

[0118] The heat setting process (f) involves heat-setting the extracted membrane obtained in process (e) at a specified temperature (HS).

[0119] The above-described method for manufacturing PO microporous membranes provides a PO microporous membrane that can simultaneously improve cycle characteristics and suppress short-circuit defects when used as a separator in electrochemical devices. It should be noted that the method for manufacturing the PO microporous membrane in this embodiment is not limited to the above-described method, and various modifications can be made without departing from its essence.

[0120] [Mixed Process (a)]

[0121] The mixing step (a) is a step in which the resin composition containing the polyolefin resin is mixed with various additives as desired. It should be noted that, in the mixing step (a), other components may be mixed with the resin composition as needed.

[0122] In the mixing process (a), for the means (i) used to control the properties and film thickness obtained by pore size analysis and the Flux pore size, it is preferable to use PO raw material as described above, and more preferably to use PE raw material.

[0123] The pore-forming material can be any material as long as it differs from the PO resin and inorganic particles; for example, it can be a plasticizer. As a plasticizer, non-volatile solvents capable of forming a homogeneous solution above the melting point of PO resin can be used, such as hydrocarbons like liquid paraffin (LP) and alkane waxes; esters like dioctyl phthalate and dibutyl phthalate; and higher alcohols like oleyl alcohol and stearyl alcohol.

[0124] The plasticizer content in the resin composition is preferably 60% to 90% by mass, more preferably 70% to 80% by mass. By adjusting the plasticizer content to 60% by mass or more, the film-forming properties during extrusion tend to improve due to the reduction in melt viscosity of the resin composition and the suppression of melt cracking. On the other hand, by adjusting the plasticizer content to 90% by mass or less, it is sometimes possible to suppress the preform elongation during the film-forming process.

[0125] (Optional additives)

[0126] In step (a), the resin composition containing PO may contain optional additives. There are no particular limitations on the additives, 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; and coloring pigments. The total amount of these additives added relative to 100 parts by weight of the polyolefin resin is preferably 20 parts by weight or less, more preferably 10 parts by weight or less, and even more preferably 5 parts by weight or less.

[0127] The mixing method in step (a) is not particularly limited, and examples include pre-mixing part or all of the raw materials as needed using a Henschel mixer, ribbon mixer, drum mixer, etc. Among these, the method of mixing using a Henschel mixer is preferred.

[0128] [Extrusion process (b)]

[0129] Extrusion step (b) is a process in which the resin composition obtained in step (a) is melt-blended and extruded. It should be noted that other components may be mixed with the resin composition in extrusion step (b) as needed. Related to the aforementioned means (i) for controlling the properties and film thickness of PO microporous membranes obtained by pore size analysis, the extrudate is preferably formed from a single type of PE raw material.

[0130] The melt mixing method in step (b) is not particularly limited, and examples include melting and mixing all raw materials, including the mixture obtained in step (a), using a screw extruder such as a single-screw extruder or a twin-screw extruder; a kneader; or a mixer. Melt mixing is preferably performed using a twin-screw extruder with the screws. Furthermore, when performing melt mixing, the plasticizer is preferably added in two or more stages. Moreover, when adding the additive in multiple stages, from the viewpoint of suppressing the aggregation of the contained components and ensuring uniform dispersion, it is preferable to adjust the amount added in the first stage to be 95% by weight or less of the total added amount. This is preferable from the viewpoint of suppressing heat release and improving the safety of the battery cell by closing off a large area.

[0131] In connection with the aforementioned means (ii) used to control the properties, film thickness, and powder fallout of PO microporous membranes obtained by pore size analysis, in the extrusion process (b), it is preferable to adjust the mixing index, expressed as the product of screw speed (rpm) and resin residence time (min.), to a range of 800 rpm·min. to 2,200 rpm·min. When the mixing index is 800 rpm·min. or higher, the resin composition is thoroughly mixed, the pore size uniformity of the resulting membrane is maintained, and unmelted resin can be suppressed, thus eliminating or reducing defects. When the mixing index is 2,200 rpm·min. or lower, the molecular weight of the resin is maintained or increased, the strength of the resulting membrane is improved, and the powder fallout is reduced, thus eliminating or reducing defects during production.

[0132] When using a pore-forming agent in step (b), from the viewpoint of uniformly mixing the resin composition, the temperature of the melt mixing section is preferably less than 200°C. From the viewpoint of uniformly dissolving the polyolefin resin in the plasticizer, the lower limit of the temperature of the melt mixing section is above the melting point of the polyolefin.

[0133] In this embodiment, there are no particular limitations on the mixing process. Preferably, after mixing the antioxidant into the PO of the raw material at a specified concentration, the mixture is surrounded by a nitrogen atmosphere, and the melt mixing is carried out while maintaining the nitrogen atmosphere.

[0134] In step (b), the compound obtained through the above-described mixing process is extruded using an extruder such as a T-die or a ring die. This can be either single-layer extrusion or multi-layer extrusion. There are no particular limitations on the extrusion conditions; for example, known methods can be used. Furthermore, from the viewpoint of the film thickness of the resulting PO microporous membrane as described above, it is preferable to control the die lip gap, etc.

[0135] [Sheet forming process (c)]

[0136] Sheet forming process (c) is the process of forming the extrudate obtained in extrusion process (b) into a sheet shape, also known as a casting process. The sheet shape obtained by sheet forming process (c) can be a single layer or a stack. There are no particular limitations on the sheet forming method, and methods such as solidifying the extrudate by compression and cooling can be cited as examples.

[0137] There are no particular limitations on the compression cooling method, and examples include methods that allow the extrudate to directly contact cooling media such as cold air or cooling water; and methods that allow the extrudate to contact metal rollers or presses cooled by refrigerant. Among these, from the viewpoints of adjusting the film thickness and utilizing the characteristics obtained by aperture analysis as described above, the method of contacting the extrudate with metal rollers or presses cooled by refrigerant is preferred.

[0138] In connection with the aforementioned means (iii) for controlling the properties and film thickness of the PO microporous membrane obtained by pore size analysis, it is preferable to control the cooling rate during casting within the range of 1°C / sec. to 12°C / sec.

[0139] Regarding the cooling rate during casting, if the extruded material from the die is considered as the blank, it can be calculated using the following two formulas:

[0140] Cooling rate during casting [°C / sec.] = (Resin temperature [°C] when the extrudate initially contacts the rollers from the die - Resin temperature [°C] when the extrudate's roller contact surface changes and it leaves the next roller after being picked up by the next roller) ÷ Preform residence time [sec.]

[0141] The dwell time of the blank [sec.] = the length on the roller [m] × 60 [sec.] ÷ the casting speed [m / min].

[0142] Here, the resin temperature during casting is measured using a non-contact infrared thermal imaging device on the surface not in contact with the roller. There is no particular limitation on controlling the cooling rate during casting within the range of 1°C / sec. to 12°C / sec.; it simply means moderate cooling to avoid excessive cooling compared to conventional compression cooling methods. If the cooling rate is too high, when the molten compound solidifies in the sheet forming process, smaller phase separation structures will form closer to the surface of the cooling roller, easily leading to uneven pore structures in the film thickness direction. To obtain a uniform pore structure, it is preferable to slow down the cooling rate and control it below 12°C / sec. Furthermore, to ensure cooling and solidification on the roller, it is preferable to control the cooling rate above 1°C / sec.

[0143] As a means of optimizing the cooling rate during casting as described above, examples include: adjusting the film thickness to a range of 1200 μm to 3000 μm; adjusting the resin temperature at the die exit to a range of 180°C to 215°C; using a cooling roller with a surface temperature controlled at 50°C to 100°C to adjust the casting temperature; and adjusting the casting speed to a range of 1 m / min to 12 m / min. When the film thickness is adjusted to a range of 3000 μm or less, the roller contact surface of the extruded material changes and is received by the next roller, and the resin is reliably cooled and cured before leaving the roller, which is therefore preferable. In addition, in order to form a uniform film thickness, it is preferable to control the film thickness to 1200 μm or more.

[0144] In connection with the aforementioned means (iii) for controlling the properties and film thickness of PO microporous membranes obtained by pore size analysis, the temperature of the roller initially receiving the extruded preform can be adjusted to a range preferably 41°C to 100°C, more preferably 45°C to 98°C, further preferably 50°C to 97°C, and particularly preferably 55°C to 95°C.

[0145] Furthermore, from the viewpoint of film thickness described above, it is preferable to control the casting gap and the like in the sheet forming process (c). The thickness of the resulting sheet-shaped body, for example, based on the thickness after stretching in process (d), is preferably 1,000 μm or more and 3,300 μm or less, more preferably 1,200 μm or more and 3,100 μm or less.

[0146] [Single stretching process (d)]

[0147] A single stretching process (d) is a process in which the sheet formed in the sheet forming process (c) is stretched at least once, at least along a uniaxial direction. This stretching process (the stretching process performed before the subsequent extraction process (e)) is called "single stretching," and the film obtained by single stretching is called "single stretch film." In single stretching, the sheet formed can be stretched at least along one direction, which can be both the MD and TD directions, or only one of the MD or TD directions.

[0148] As for the stretching method for a single stretching step, there is no particular limitation, and examples include, for instance, uniaxial stretching based on a roll stretching machine; TD uniaxial stretching based on a tenter frame; successive biaxial stretching based on a combination of a roll stretching machine and a tenter frame or multiple tenter frames; and simultaneous biaxial stretching based on a simultaneous biaxial tenter frame or blow molding. Among these, related to the aforementioned means (iv) for controlling the characteristics and film thickness obtained by pore size analysis of PO microporous membranes, from the viewpoint of uniform membrane stretching, simultaneous biaxial stretching is preferred.

[0149] The stretching ratio of the MD and / or TD in a single stretch is preferably 5 times or more, more preferably 6 times or more. By making the stretching ratio of the MD and / or TD in a single stretch 5 times or more, there is a tendency to further improve the strength of the obtained PO microporous membrane. In addition, the stretching ratio of the MD and / or TD in a single stretch is preferably 9 times or less, more preferably 8 times or less or 7 times or less. By making the stretching ratio of the MD and / or TD in a single stretch 9 times or less, there is a tendency to further suppress fracture during stretching. When performing biaxial stretching, it can be sequential stretching or simultaneous biaxial stretching, and the stretching ratio in each axis direction is preferably 5 times or more and 9 times or less, more preferably 6 times or more and 8 times or less, or 6 times or more and 7 times or less, and even more preferably a wet stretching ratio. In addition, it is also preferable to adjust the stretching ratio according to the average pore size d of the obtained PO microporous membrane. MD With average aperture d TD The ratio (d) MD / d TD ), optimize the MD / TD stretch ratio.

[0150] The primary stretching temperature can be selected with reference to the composition and concentration of the raw resins contained in the PO resin composition. The stretching temperature of MD and / or TD is preferably in the range of 100°C to 135°C, more preferably 110°C to 130°C, further preferably 115°C to 125°C, and even more preferably in the range of 116°C to 122°C. For either MD or TD, from the viewpoint of suppressing breakage, the stretching temperature is preferably 100°C or higher, and from the viewpoint of improving film strength, the stretching temperature is preferably 135°C or lower. The stretching temperature is preferably the wet stretching temperature.

[0151] When a stretching process (d) is performed before the extraction process (e), in connection with the aforementioned means (v) used to control the properties and membrane thickness of the PO microporous membrane obtained by pore size analysis, it is preferable to adjust the stretching ratio in the stretching process (d) before extracting the porous material from the membrane to a range of 35 to 60 times.

[0152] [Extraction process (e)]

[0153] Extraction step (e) is a step of obtaining an extracted membrane by extracting the pore-forming material from the primary stretched membrane obtained in the primary stretching step (d). Methods for removing the pore-forming material include, for example, immersing the primary stretched membrane in an extraction solvent to extract the pore-forming material and then thoroughly drying it. The method for extracting the pore-forming material can be either batch or continuous. Furthermore, the residual amount of pore-forming material, especially plasticizer, in the porous membrane is preferably less than 1% by mass.

[0154] The extraction solvent used when extracting pore-forming materials is preferably a solvent that is a poor solvent for polyolefin resins but a good solvent for pore-forming materials or plasticizers, and has a boiling point lower than the melting point of polyolefin resins. There are no particular limitations on such extraction solvents, and examples include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as dichloromethane 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. It should be noted that these extraction solvents can be recovered and reused through operations such as distillation.

[0155] [Heat setting process (f)]

[0156] The heat setting process (f) is a process in which the extracted membrane obtained in the extraction process (e) is heat-set at a specified temperature. There are no particular limitations on the heat treatment method used at this stage; examples include heat setting methods that utilize a tenter frame or a roll stretching machine for stretching and relaxation operations.

[0157] The stretching operation in the heat setting process (f) is an operation of stretching the PO microporous membrane along at least one of the MD and TD directions. It can be performed in both MD and TD directions, or only in one of the MD or TD directions. From the viewpoint of providing a PO microporous membrane that can simultaneously improve cycle characteristics and suppress short-circuit defects, it is preferable to perform heat setting at least in the TD direction.

[0158] In the heat setting process (f), the stretching ratios of MD and TD are preferably 1.70 times or more and 2.40 times or less, more preferably 1.80 times or more and 2.30 times or less. From the viewpoint of achieving high film orientation, the stretching ratios of MD and TD in process (f) are preferably 1.70 times or more, and from the viewpoint of balancing film thickness and film strength while suppressing film breakage, they are preferably 2.40 times or less. From the same viewpoint, heat setting is preferably performed by dry uniaxial stretching.

[0159] In the case of a stretching process (hereinafter also referred to as "secondary stretching process") after the extraction process (e), for example, in the heat setting process (f), in connection with the above-mentioned means (v) for controlling the characteristics and film thickness of the PO microporous membrane obtained by pore size analysis, it is preferable to adjust the TD stretching ratio in the secondary stretching process after extracting the pore-forming material from the membrane to a range of 1.4 to 2.4 times.

[0160] The MD and / or TD stretching temperature during heat setting is preferably 128°C or higher from the viewpoint of adjusting the shut-off temperature and / or melting temperature, more preferably 129°C or higher from the viewpoint of the output characteristics of the electrochemical device, and further preferably in the range of 130°C to 134°C from the viewpoint of suppressing short-circuit failure and safety.

[0161] The relaxation operation in the heat setting process (f) is an operation that shrinks the PO microporous membrane along at least one of the MD and TD directions. This can be performed along both MD and TD directions, or only along one of the MD or TD directions. The relaxation ratio in the heat setting process (f) is preferably 0.95 times or less, more preferably 0.93 times or less. By keeping the relaxation ratio in process (f) below 0.95, there is a tendency to suppress heat shrinkage. Furthermore, from the viewpoint of increasing the relaxation temperature, the relaxation ratio is preferably 0.50 times or more, more preferably 0.70 times or more. Here, "relaxation ratio" refers to the value obtained by dividing the size of the membrane after the relaxation operation by the size of the membrane before the relaxation operation. When relaxing both MD and TD, it is the value obtained by multiplying the relaxation ratio of MD by the relaxation ratio of TD.

[0162] Relaxation ratio = (Membrane size after relaxation (m)) / (Membrane size before relaxation (m))

[0163] From the viewpoint of the pore size of the obtained PO microporous membrane, the relaxation temperature in this relaxation operation is preferably 128°C or higher, more preferably 131°C or higher from the viewpoint of the cycle characteristics of the electrochemical device, and further preferably in the range of 131°C to 134°C from the viewpoint of suppressing short-circuit defects and safety.

[0164] The order of the above steps (a) to (f) can be arbitrarily changed as long as it does not impair the effect of the present invention. After the above steps (a) to (f), the total stretching ratio of the PO microporous membrane is related to the above-mentioned means (v) used to control the characteristics and membrane thickness obtained by pore size analysis. From the viewpoint of improving membrane strength, it is preferably 70 times or more and 120 times or less, more preferably 80 times or more and 120 times or less, and even more preferably 90 times or more and 120 times or less. Here, "total stretching ratio" refers to the value obtained by multiplying the stretching ratio of MD and / or TD in the first stretching step (d) with the stretching ratio and / or relaxation ratio in the heat setting step.

[0165] [Other processes]

[0166] The method for manufacturing a PO microporous membrane according to this embodiment may include other steps besides steps (a) to (f) described above. These other steps are not particularly limited; for example, based on the heat-setting step described above, a lamination step in which multiple sheets of a PO microporous membrane, which is a monolayer, are stacked together to obtain the PO microporous membrane as a laminate. Alternatively, the method for manufacturing a PO microporous membrane may include a peeling step after steps (a) to (f) of a laminate obtained by co-extrusion to obtain two or more monolayers. Furthermore, the method for manufacturing a PO microporous membrane according to this embodiment may include a surface treatment step of performing surface treatments on the surface of the PO microporous membrane, such as electron beam irradiation, plasma irradiation, surfactant coating, or chemical modification. Furthermore, inorganic particulate material may be coated onto one or both sides of the PO microporous membrane, an inorganic material layer may be provided, or an adhesive layer containing thermoplastic resin may be provided on the surface of the PO microporous membrane.

[0167] <Separators for Electrochemical Devices>

[0168] The polyolefin microporous membrane described in this embodiment can be used as a separator in electrochemical devices such as lithium-ion secondary batteries. By assembling the polyolefin microporous membrane into a lithium-ion secondary battery, it can suppress thermal runaway.

[0169] <Electrochemical Device>

[0170] An electrochemical device that houses multiple wound or stacked PO microporous membranes as described in this embodiment is also an aspect of the present invention. Examples of electrochemical devices include, for instance, non-aqueous electrolyte batteries, non-aqueous electrolyte cells, non-aqueous lithium-ion secondary batteries, non-aqueous gel secondary batteries, non-aqueous solid-state secondary batteries, lithium-ion capacitors, and double-layer capacitors.

[0171] The non-aqueous electrolyte battery of this embodiment includes: a separator for a non-aqueous electrolyte battery containing the above-described 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 absorbing and releasing lithium ions and a negative electrode plate capable of absorbing and releasing lithium ions are wound or stacked opposite each other across the separator for a non-aqueous electrolyte battery, thereby retaining the non-aqueous electrolyte and storing it in a container.

[0172] The following describes the positive electrode plate. As the positive electrode active material, lithium composite metal oxides such as lithium nickel oxide, lithium manganese oxide, or lithium cobalt oxide, and lithium composite metal phosphates such as lithium iron phosphate can be used. The positive electrode active material is mixed with a conductive agent and a binder, coated as a positive electrode paste onto a positive electrode current collector such as aluminum foil, and dried. After being rolled to a specified thickness, it is cut into specified dimensions to form the positive electrode plate. Here, as the conductive agent, a metal powder stable at the positive electrode potential, such as carbon black or graphite materials like acetylene black, can be used. Furthermore, as the binder, a material stable at the positive electrode potential, such as polyvinylidene fluoride, modified acrylic rubber, or polytetrafluoroethylene, can be used.

[0173] The following describes the negative electrode plate. As the negative electrode active material, a material capable of absorbing lithium can be used. Specifically, at least one material selected from the group consisting of graphite, silicides, and titanium alloys can be used. Furthermore, as the negative electrode active material for non-aqueous electrolyte secondary batteries, materials such as 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) compounds, such as elemental silicon or alloys, compounds, and solid solutions, tend to increase the capacity density of the battery and are therefore preferred.

[0174] Examples of carbon materials include various natural graphite, coke, carbon produced during graphitization, carbon fibers, spherical carbon, various artificial graphite, and amorphous carbon.

[0175] As the negative electrode active material, one of the above materials can be used alone, or two or more can be used in combination. The negative electrode active material is mixed with a binder, coated onto a negative electrode current collector such as copper foil as a negative electrode paste, and dried. After being calendered to a specified thickness, it is cut into specified dimensions to form a negative electrode plate. Here, as the binder, a material stable at the negative electrode potential can be used, such as PVDF or styrene-butadiene rubber copolymer.

[0176] The following section describes non-aqueous electrolytes. Non-aqueous electrolytes typically contain a non-aqueous solvent and dissolved metal salts such as lithium, sodium, and calcium salts. Cyclic carbonates, chain carbonates, and cyclic carboxylic acid esters can be used as non-aqueous solvents. Examples of lithium salts include LiPF6, LiClO4, LiBF4, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, Li(CF3SO2)2, LiAsF6, lower aliphatic lithium carboxylate salts, LiCl, LiBr, LiI, borates, and imide salts.

[0177] It should be noted that, unless otherwise specified, the measurement methods for the above parameters shall be performed in accordance with the measurement methods in the embodiments described later.

[0178] Example

[0179] Next, examples and comparative examples are given to illustrate this embodiment in more detail. However, this embodiment is not limited to the following examples as long as it does not deviate from its spirit. It should be noted that the physical properties in the examples were measured using the following methods. Unless otherwise specified, the measurements were taken at room temperature of 23°C and humidity of 40%.

[0180] (1) Molecular weight

[0181] (1a) High-temperature GPC of membranes: determination of weight-average molecular weight (Mw) and number-average molecular weight (Mn) (GPC-relative method)

[0182] • Sample preparation

[0183] Weigh the polyolefin raw material and add it to the eluent 1,2,4-trichlorobenzene (TCB) at a concentration of 1 mg / ml. Using a high-temperature dissolver, let it stand at 160°C for 30 minutes, then shake it at 160°C for 1 hour, visually confirming that the sample has completely dissolved. While maintaining 160°C, filter the sample solution through a 0.5 μm filter, and use the filtrate as the sample for GPC determination.

[0184] GPC measurement

[0185] As the GPC apparatus, a Waters ALC / GPC-150-C-plus (trademark) model was used, with two 30cm columns of GMH6-HT (trademark) and two 30cm columns of GMH6-HTL (trademark) manufactured by Tosoh Corporation connected in series. o-Dichlorobenzene was used as the mobile phase solvent, and GPC determination was performed at 140°C with a sample concentration of 0.05wt%.

[0186] It should be noted that commercially available monodisperse polystyrene with known molecular weight was used as the standard material to prepare the standard curve. The calculated molecular weight distribution data of polystyrene for each sample was multiplied by 0.43 (Q factor of polyethylene / Q factor of polystyrene = 17.7 / 41.3) to obtain the molecular weight distribution data of polyethylene. Therefore, by calculating the weight-average molecular weight (Mw) and number-average molecular weight (Mn) of each sample, the molecular weight distribution (Mw / Mn) was also obtained.

[0187] (1b) Viscosity-average molecular weight (Mv)

[0188] Based on ASTM-D4020, determine the intrinsic viscosity [η] (dl / g) of decahydronaphthalene solvent at 135°C. For PO microporous membranes and polyethylene raw materials, calculate Mv using the following formula.

[0189] [η] = 6.77 × 10 -4 Mv 0.67

[0190] For polypropylene raw materials, Mv can be calculated using the following formula.

[0191] [η]=1.10×10 -4 Mv 0.80

[0192] (2) Film thickness (μm)

[0193] The thickness of the PO microporous membrane was measured at room temperature (23±2℃) using a miniature thickness gauge manufactured by Toyo Seiki Co., Ltd. (KBM trademark).

[0194] (3) Porosity (%)

[0195] A 10cm × 10cm square sample was cut from the PO microporous membrane. Its volume (cm³) was calculated. 3 ) and mass (g), based on them and density (g / cm³) 3 The porosity is calculated using the following formula.

[0196] Porosity (%) = (Volume - Mass / Density) / Volume × 100

[0197] (4) Breathability (sec / 100cm) 3 )

[0198] The air permeability will be determined based on the air resistance rating of JIS P-8117.

[0199] In the determination of the air permeability of PO microporous membrane, in accordance with JIS P-8117, the air permeability resistance of PO microporous membrane was measured using a Gurley air permeability meter G-B2 (trademark) manufactured by Toyo Seiki Co., Ltd., under an atmosphere of 23°C and 40% humidity, and the air permeability was used as the measure.

[0200] (5) Conversion of puncture intensity (gf) and weight per unit area to puncture intensity (gf / (g / m²)) 2 ))

[0201] Using a KATOTECH KES-G5 handheld compression tester (trademark), a PO microporous membrane was fixed with a sample holder having an opening diameter of 11.3 mm. Next, a puncture test was performed on the central portion 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 / sec, in an atmosphere of 23°C and 40% humidity. The raw puncture strength (gf), representing the maximum puncture load, was determined from this. The value converted to weight per unit area (gf / (g / m²)) was also calculated. 2 )).

[0202] (6) Flux pore size (μm)

[0203] The fluid pore size is calculated based on the permeation velocity (flux) of the PO microporous membrane. It is known that fluid within a capillary follows Knudsen flow when the mean free path of the fluid is larger than the capillary pore size, and Poiseuille flow when it is smaller. Therefore, it is assumed that air flow in the permeability measurement of the porous membrane follows Knudsen flow, and water flow in the water permeability measurement follows Poiseuille flow.

[0204] At this point, the average pore size d (μm) and curvature τ of the porous membrane... a (Dimensionless) Derived from the air permeability constant R gas (m 3 / (m 2 ·sec·Pa), the water permeability constant R liq (m 3 / (m 2 (·sec·Pa)), air molecular velocity ν (m / sec), water viscosity η (Pa·sec), standard pressure P s The porosity ε (%) and film thickness L (μm) are calculated using the following formulas. (=101325Pa)

[0205] d=2ν×(R liq / R gas )×(16η / 3Ps)×10 6

[0206] τ a =(d×(ε / 100)×ν / (3L×P s ×R gas )) 1 / 2

[0207] Among them, R gas The air permeability (sec) is calculated using the following formula.

[0208] R gas =0.0001 / (breathability × (6.424 × 10) -4 )×(0.01276×101325))

[0209] Additionally, R liq Permeability (cm) 3 / (cm 2 The value of ·sec·Pa) is obtained using the following formula.

[0210] R liq =Permeability / 100

[0211] It should be noted that the water permeability was calculated as follows: A porous membrane pre-impregnated with ethanol was installed on a 41mm diameter stainless steel permeation tank. After washing the membrane with ethanol and water, water was permeated through it under a differential pressure of approximately 50,000 Pa. The water permeation rate (cm³) after 120 seconds was calculated. 3 ), calculate the water permeability per unit time, unit pressure, and unit area, and use it as the permeability.

[0212] Additionally, ν is determined based on the gas constant R (=8.314), absolute temperature T (K), pi π, and the average molecular weight of air M (=2.896×10⁻⁶). -2 (kg / mol) can be calculated using the following formula.

[0213] ν=((8R×T) / (π×M)) 1 / 2

[0214] (7) Thermal shrinkage rate (%) at 120℃

[0215] The PO microporous membrane was cut into 100mm pieces along the MD direction and 100mm along the TD direction, and then placed in an oven at a specified temperature (120℃ or 150℃) for 1 hour. During this time, the sample was sandwiched between two sheets of paper, with hot air not directly blowing onto it. The paper used was Fuji Xerox Interfield Co., Ltd. V-Paper monochrome copier / printer paper (64g / m²). 2 After removing the sample from the oven and allowing it to cool, measure its length (mm) and calculate the thermal shrinkage rate using the following formula. Measurements were performed in both the MD and TD directions.

[0216] Heat shrinkage rate (%) = {(100 - length after heating) / 100} × 100

[0217] (8) Maximum shrinkage stress (gf) of TMA

[0218] The thermal shrinkage of the sample was measured using a Shimadzu TMA50 (trademark). When measuring the value in the MD (TD) direction, a sample cut 3 mm wide along the TD (MD) direction was fixed in a fixture with a 10 mm gap between clamps and mounted on a dedicated probe. The initial load was set to 1.0 g, and the measurement was performed in constant length mode. The sample was heated from 30 °C to 200 °C at a heating rate of 10 °C / min, and the resulting load (gf) was measured. The maximum value was taken as the maximum thermal shrinkage stress (gf) in MD (or TD).

[0219] (9) Cross-sectional SEM observation and aperture analysis

[0220] Cross-sectional SEM images of resin-embedded samples

[0221] After staining the PO microporous membrane with ruthenium and embedding it in room-temperature curing epoxy resin, smooth cross-sections parallel to the MD and TD layers were fabricated using a broad ion beam (BIB) microscope. These smooth cross-sections were then observed using a scanning electron microscope (SEM) at 7000x magnification to obtain SEM images. Multiple SEM images can be obtained from the smooth cross-section in multiple fields of view, or a single SEM image can be obtained from a single field of view. Specifically, the location for obtaining the SEM images can be determined as follows.

[0222] • When SEM magnification at 7000x can capture the entire field of view:

[0223] (1)-1: Obtain an image that incorporates the entire thickness;

[0224] (1)-2: The field of view is cut out from the two outermost surfaces, removing 3% of the total thickness, for image analysis described later.

[0225] • When the field of view of SEM cannot be fully captured at 7000x magnification:

[0226] (2)-1: Using the center of the membrane as the center, obtain the overall image of the picture;

[0227] (2)-2: In the obtained image, the field of view with 3% thickness removed from both ends is cut out for image analysis described later.

[0228] • When multiple SEM images are obtained, SEM images are taken at 50μm intervals along the length direction of the smooth cross section.

[0229] (3)-1: As a specific example, the SEM image of N=5 is sampled as follows.

[0230] A 3cm square test piece was collected from the center of the PO microporous membrane.

[0231] A cross-section is fabricated on the TD (or MD) along the longitudinal direction of the PO microporous membrane using BIB processing.

[0232] Five SEM images were taken at 50 μm intervals on the TD (or MD) at any position on the cross-section of the PO microporous membrane for analysis.

[0233] Image analysis and aperture analysis

[0234] The obtained SEM image was processed using a median filter at a radius of 2.0 pixels, and then binarized using the Otsu method at a threshold to calculate the area ratio of the black and / or white regions. Next, the binarized image was analyzed using the local thickness method to calculate the pore size distribution, average pore size, and maximum pore size. The local thickness method can be performed using the "Thickness Analysis" plugin of the image processing software "Image J," which defines the size of the largest circle entering that location as the spatial dimension. Therefore, even without an independent structure / space, the spatial dimension can be defined to calculate the pore size distribution, average pore size, and maximum pore size.

[0235] Results of aperture analysis

[0236] For the SEM images taken from smooth cross sections parallel to MD and TD produced by BIB as described above, when performing pore size analysis at least at one point using the local thickness method, the average pore size d can be calculated. MD and d TD Aperture distribution D MD and D TD Aperture distribution D MD Standard deviation and aperture distribution D TD Standard deviation, maximum aperture PS MD(max) and PS TD(max) wait.

[0237] For the SEM images taken from smooth cross sections parallel to MD and TD produced by BIB as described above, the average pore size d can be calculated by performing pore size analysis at 5 points using the local thickness method. MD(N=5) Standard deviation and average aperture d TD(N=5) Standard deviation, etc.

[0238] (10) Dust shedding test

[0239] A 20cm wide rubber sheet with a dynamic friction coefficient of 0.5 to SUS304 is circularly attached to a roller with a diameter of 5cm and a wrap angle θ of 20°. A microporous membrane is then fed from a feeder with a tension of 10N, allowing the membrane to move on the rubber sheet under the following conditions, thereby conducting a powder shedding test. It should be noted that the mass of the 20cm wide circular rubber sheet is measured beforehand.

[0240] (condition)

[0241] Measurement length: 200m (i.e., the microporous membrane is fed from the feeder with a tension of 10N and travels only 200m on the rubber sheet).

[0242] Velocity of the microporous membrane: 30 m / min.

[0243] The mass difference of the rubber sheet before and after the dust drop test is measured, or the mass of the powder adhering to the rubber sheet after the dust drop test is measured, and the dust drop amount (mg / cm) is calculated from this.

[0244] (11) Withstand voltage test

[0245] Using the PO microporous membrane of the test object as the thin film sample, the thin film sample and aluminum foil were cut into 100mm × 50mm pieces. The sample was placed on the aluminum foil, and then a 5mm diameter aluminum plate was placed on the sample. Using a withstand voltage tester (device name: TOS9201) with a load of 45gf on the 5mm diameter electrode, the current type was set to AC (60Hz), the starting voltage was set to 0kV, and the voltage rise rate was set to 0.1kV / s. The voltage was applied to the electrode and gradually increased, and the voltage when a current of 0.2mA or more flowed was measured. This measurement was performed at at least 20 different positions within the same thin film sample. At this time, the setting and measurement of the measurement positions were performed without in-plane deviation at a distance of more than 15mm from the measurement position. The voltage value of the current of 0.2mA or more flowing through the thin film was converted into a value per 1μm film thickness as the withstand voltage value of the thin film sample. The average value of the 20 measured values ​​was calculated and evaluated according to the following criteria. The measurements were conducted in a dry chamber at room temperature of 18°C ​​and dew point of -40°C.

[0246] A: Withstand voltage is above 1.25kV / μm.

[0247] B: Withstand voltage value is above 1.00kV / μm and less than 1.25kV / μm.

[0248] C: Withstand voltage value is less than 1.00kV / μm.

[0249] (12) Battery Evaluation

[0250] Make the battery using the following steps a-1 to a-5.

[0251] a-1. Production of the positive electrode

[0252] According to the nickel, manganese, and cobalt composite oxide (NMC) used as the positive electrode active material (Ni:Mn:Co=1:1:1 (elemental ratio), density is 4.70 g / cm³. 3 90.4% by mass of graphite powder (KS6) as a conductive additive (density 2.26 g / cm³) 3 1.6% by mass of acetylene black powder (AB) (with a number-average particle size of 6.5 μm) and acetylene black powder (AB) (with a density of 1.95 g / cm³). 3The composition includes 3.8% by mass of polyvinylidene fluoride (PVDF) as a binder (with a density of 1.75 g / cm³), and a number-average particle size of 48 nm. 3 4.2% by mass were mixed and dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This slurry was then coated onto one side of a 20 μm thick aluminum foil, which would become the positive electrode current collector, using a die coater. After drying at 130°C for 3 minutes, the foil was compressed using a roller press to form the positive electrode. The coating weight of the positive electrode active material at this point was 109 g / m². 2 .

[0253] a-2. Negative electrode fabrication

[0254] Graphite powder A (density 2.23 g / cm³) is used as the negative electrode active material. 3 The number-average particle size was 12.7 μm (87.6% by mass) and graphite powder B (density 2.27 g / cm³) was also present. 3 A slurry was prepared by dispersing 9.7% by mass of a mixture of carboxymethyl cellulose (with a number average particle size of 6.5 μm), 1.4% by mass of ammonium salt of carboxymethyl cellulose (converted from solids) (solids concentration of 1.83% by mass aqueous solution), and 1.7% by mass of diene rubber latex (converted from solids) (solids concentration of 40% by mass aqueous solution) in purified water. The slurry was then coated onto one side of a 12 μm thick copper foil, which would serve as the negative electrode current collector, using a die coater. After drying at 120°C for 3 minutes, the foil was compressed using a roller press to form the negative electrode. The coating amount of the negative electrode active material was 52 g / m². 2 .

[0255] a-3. Preparation of non-aqueous electrolytes

[0256] A non-aqueous electrolyte was prepared by dissolving LiPF6 as a solute in a mixed solvent of ethylene carbonate and methyl ethyl carbonate at a concentration of 1.0 mol / L in a ratio of 1:2 (volume ratio).

[0257] a-4. Battery Manufacturing

[0258] 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 examples or comparative examples as separators, a laminated secondary battery with dimensions of 100mm × 60mm and a capacity of 3000mAh was fabricated under constant current constant voltage (CCCV) charging for 3 hours at a current value of 1A (0.3C) and a terminal battery voltage of 4.2V.

[0259] a-5. Volume Measurement (mAh)

[0260] The laminated secondary battery assembled using the above procedure was subjected to constant current constant voltage (CCCV) charging for 6 hours at a current of 1500mA (0.5C) and a terminal battery voltage of 4.2V. At this point, the current value was approximately 0 just before the charging was completed. Subsequently, the battery was left to mature (cured) at 25°C for one week.

[0261] The following cycle was then performed: Constant current constant voltage (CCCV) charging was conducted for 3 hours at a current of 3000mA (1.0C) and a final battery voltage of 4.2V, followed by discharging at a fixed current (CC) of 3000mA until the battery voltage reached 3.0V. The discharge capacity at this point was recorded as the initial discharge capacity X. Batteries with an initial discharge capacity X of 3000±10mAh or less were used for battery evaluation.

[0262] b. Output test (25℃)

[0263] For the laminated secondary battery assembled as described above and selected for evaluation, the 1C discharge capacity and 5C discharge capacity were measured under isothermal conditions at 25°C up to the discharge termination voltage of 3V. The 5C capacity / 1C capacity was taken as the output characteristic value. It should be noted that the output characteristic value was evaluated according to the following criteria.

[0264] A: The output characteristic value is above 0.90.

[0265] B: Output characteristic value is above 0.80 and less than 0.90.

[0266] C: Output characteristic value is less than 0.80.

[0267] c. Cyclic test

[0268] Using the battery assembled as described above and selected for evaluation, a total of 100 charge-discharge cycles were performed under the following conditions: (i) constant current and constant voltage charging at a current of 0.5C and an upper limit voltage of 4.2V for a total of 8 hours; (ii) a 10-minute rest; (iii) constant current discharging at a current of 0.5C and a termination voltage of 3.0V; and (iv) a 10-minute rest. All the above charge-discharge processes were performed separately at 25°C. Subsequently, the capacity retention rate (%) was calculated by multiplying the ratio of the discharge capacity of the 100th cycle to the initial battery capacity X (mAh) by 100. It should be noted that the capacity retention rate was evaluated according to the following criteria.

[0269] A: Capacity retention rate (%) is above 90%.

[0270] B: Capacity retention rate (%) is above 80% and less than 90%.

[0271] C: Capacity maintenance rate (%) is less than 80%.

[0272] (13) Nail puncture test

[0273] a. Production of the positive electrode

[0274] A slurry was prepared by dispersing lithium cobalt composite oxide (LiCoO2) as the positive electrode active material, and graphite and acetylene black as conductive materials in polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP) as binders. The slurry was then coated onto a 15 μm thick aluminum foil, which served as the positive electrode current collector, using a die coater. After drying at 130°C for 3 minutes, the foil was compressed using a roller press. The resulting molded body was cut into 57.0 mm wide pieces to obtain the positive electrode.

[0275] b. Negative electrode fabrication

[0276] Artificial graphite, used as the negative electrode active material, and ammonium salt of carboxymethyl cellulose and styrene-butadiene copolymer latex, used as a binder, were dispersed in purified water to prepare a slurry. The slurry was coated onto copper foil, which served as the negative electrode current collector, using a die coater. After drying at 120°C for 3 minutes, the foil was compressed and shaped using a roller press. The resulting shaped body was cut into 58.5 mm wide pieces to obtain the negative electrode.

[0277] c. Preparation of non-aqueous electrolytes

[0278] A non-aqueous electrolyte was prepared by dissolving LiPF6 as a solute in a mixed solvent of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a ratio of 1:1:2 (volume ratio).

[0279] d. Battery assembly

[0280] After winding the positive electrode, the PO porous membrane obtained in the examples or comparative examples, and the negative electrode, a wound electrode body was fabricated using conventional methods and pressed using a press to be placed into an outer packaging can. It should be noted that the number of windings is adjusted according to the thickness and resilience of the PO microporous membrane. The outermost periphery of the wound electrode body was fixed by attaching insulating tape. The negative electrode lead was soldered to the battery can, and the positive electrode lead was soldered to the safety valve. The wound electrode body was then inserted into the battery can. Then, 5g of non-aqueous electrolyte was injected into the battery can, and the cap was riveted to the battery can through a gasket, thereby obtaining a square secondary battery with a width of 42.0mm, a height of 63.0mm, and a thickness of 10.5mm. For this square secondary battery, under an atmosphere of 25°C, it was charged to a battery voltage of 4.2V at a current value of 0.2C (0.2 times the 1-hour rate (1C) of the rated capacity), and then the current value was gradually reduced while maintaining 4.2V. This method was used for a total charging time of 3 hours. Next, discharge the battery at a current of 0.2C until the battery voltage reaches 3.0V.

[0281] e. Nail puncture safety test

[0282] The battery assembled in section d above and selected for evaluation is placed on an iron plate in a temperature-controlled explosion-proof chamber. A 3.0 mm diameter iron nail is prepared, with a thermocouple installed inside. Under an environment of 30°C and 3 MPa pressure within the explosion-proof chamber, the iron nail is driven through the center of the battery at a speed of 2 mm / sec, maintaining the penetration. The battery is observed from the start of the nail penetration until it is complete, and its safety is evaluated according to the following criteria.

[0283] A: Nothing happened.

[0284] B: Smoke.

[0285] C: Battery swelling and deformation were observed.

[0286] D: On fire.

[0287] E: Explosion.

[0288] [Example 1]

[0289] According to the polyethylene (PE) types shown in Table 1 and the PE types and raw material composition ratios shown in Table 2, PE1 was fed into a Henschel mixer, and pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] as an antioxidant was added in appropriate amounts and premixed. The resulting mixture was fed into the feed port of a twin-screw extruder via a feeder. In addition, liquid paraffin (LP) was added to the barrel of the twin-screw extruder in two stages via side feeding, with the proportion of LP in the total mixture (100.1 parts by mass) being 76.0 parts by mass.

[0290] Extrusion and casting processes were performed under the conditions shown in Table 2 to obtain sheet-like molded products with a thickness of 1800 μm.

[0291] The obtained sheet-like material is fed into a simultaneous biaxial stretching machine to obtain a single-stretch film (single-stretch process). The stretching conditions are set as shown in Table 2. Next, the obtained single-stretch film is fed into a dichloromethane bath for thorough impregnation. After removing the liquid paraffin as a plasticizer, it is dried to remove the dichloromethane, resulting in an extracted film.

[0292] Next, for heat setting, the extracted membrane was introduced into a TD uniaxial tenter frame. During the heat setting process, as shown in Table 2, stretching was performed under TD stretching temperature and TD stretching ratio conditions, followed by relaxation under relaxation temperature and relaxation ratio conditions. Various properties of the obtained PO microporous membrane were evaluated using the above method. The membrane preparation conditions are shown in Table 2, and the results are shown in Table 4.

[0293] [Examples 2-20 and Comparative Examples 1-13]

[0294] The resin raw material types, raw material composition ratios, and film-forming conditions were set as shown in Table 2 or Table 3, and PO microporous membranes were obtained in the same manner as in Example 1. The various properties of the obtained PO microporous membranes were evaluated using the above method. The results are shown in Table 4 or Table 5.

[0295] [Table 1]

[0296]

[0297] [Table 2-1]

[0298]

[0299] [Table 2-2]

[0300]

[0301] [Table 3-1]

[0302]

[0303] [Table 3-2]

[0304]

[0305] [Table 4-1]

[0306]

[0307] [Table 4-2]

[0308]

[0309] [Table 5-1]

[0310]

[0311] [Table 5-2]

[0312]

Claims

1. A polyolefin microporous membrane, wherein, The polyolefin microporous membrane was stained with ruthenium and embedded in room-temperature curing epoxy resin. A smooth cross-section parallel to the TD was fabricated using a wide ion beam (BIB). When pore size analysis was performed using the local thickness method on five scanning electron microscope (SEM) images of this smooth cross-section taken at 7000x magnification with 50 μm intervals, the average pore size d was determined. TD(N=5) The standard deviation is above 0.1 nm and below 1.0 nm.

2. The polyolefin microporous membrane according to claim 1, wherein, The thickness of the polyolefin microporous membrane is greater than 6 μm and less than 16 μm.

3. The polyolefin microporous membrane according to claim 1, wherein, When performing aperture analysis on the SEM image at least at one point, the aperture distribution D TD The standard deviation is above 35nm and below 55nm.

4. The polyolefin microporous membrane according to claim 1 or 2, wherein, The average pore size d when performing pore size analysis at least one point on the SEM image TD It is between 100nm and 160nm.

5. The polyolefin microporous membrane according to claim 1 or 2, wherein, The puncture strength per unit area weight of the polyolefin microporous membrane is 80 gf / (g / m²). 2 ) or above and 150gf / (g / m 2 )the following.

6. The polyolefin microporous membrane according to claim 1 or 2, wherein, The viscosity-average molecular weight (Mv) of the polyolefin microporous membrane is above 800,000 and below 1,500,000.

7. The polyolefin microporous membrane according to claim 1 or 2, wherein, The molecular weight distribution of the polyolefin microporous membrane, expressed as the ratio of mass-average molecular weight Mw to number-average molecular weight Mn, is Mw / Mn, which is greater than 5 and less than 10.

8. The polyolefin microporous membrane according to claim 1 or 2, wherein, The puncture strength of the polyolefin microporous membrane is above 450 gf and below 1000 gf.

9. The polyolefin microporous membrane according to claim 1 or 2, wherein, The air permeability of the polyolefin microporous membrane is 30s / 100cm. 3 Above and 195s / 100cm 3 the following.

10. The polyolefin microporous membrane according to claim 1 or 2, wherein, When the polyolefin microporous membrane is fed from the feeder with a tension of 10N and conveyed for only 200m on rubber with a dynamic friction coefficient of 0.5 to SUS304, the amount of powder falling off is more than 0.001mg / cm and less than 0.04mg / cm.

Images