Microporous polyolefin membrane

Abstract

Provided is a microporous polyolefin film. When the polyolefin microporous film is dyed with ruthenium and embedded in a room-temperature-curable epoxy resin, a smooth cross-section parallel to TD is produced by a broad ion beam (BIB), and 7000-fold scanning electron microscope (SEM) images captured at five points located at an interval of 50 μm on the smooth cross-section are subjected to pore diameter analysis by the local thickness method, the standard deviation of the average pore diameter dTD(N=5) is 0.1-1.0.

Description

Polyolefin microporous membrane

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

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

[0003] Conventionally, in order to be used as a separator for LIB, a PO microporous membrane has been required to have dimensional stability against temperatures below the melting point of PO or against external stress, shutdown performance near the melting point, and membrane rupture resistance at higher temperatures. Also, from the viewpoint of the relationship between the properties of the LIB separator and the characteristics of the LIB, 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 with a membrane thickness of 16 μm or less, in which the single-point total pore volume of pores with a diameter of 98 nm or less, as measured by a nitrogen gas adsorption test, is 25% to 85% of the total pore volume, from the viewpoint of achieving excellent results in a half-cell initial overcharge test.

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

[0006] In Patent Document 3, when the pore size showing the maximum peak in a special pore size distribution curve of the membrane in a dry state and a wet state is defined as Dp (nm), the proportion of the total value of the pore size distribution of pores having a maximum Dp to 1.1 Dp, the maximum pore size, the MD tensile strength, and the ratio of the MD tensile strength to the TD tensile strength are investigated in order to provide a microporous membrane having an excellent balance of physical properties such as thinness, uniform pore size, tensile strength, and high permeability, thereby ensuring the output characteristics of a secondary battery.

[0007] JP 2021-123614 A JP 2021-038379 A JP 2015-120786 A

[0008] In recent years, electrochemical devices such as LIBs have become significantly smaller and thinner, while there is a demand for LIBs with higher capacities than conventional products. To achieve this, electrochemical device manufacturers are promoting efforts such as increasing the nickel (Ni) content of positive electrodes and thinning separators for electrochemical devices. However, thinning can lead to safety issues such as reduced impact resistance of electrochemical devices and short circuits caused by foreign objects.

[0009] In particular, conventional LIBs have had the problem of deposits (dendrites) from the electrodes, so there is a need to improve cycle characteristics by suppressing dendrites.

[0010] Furthermore, in order to increase the output of electrochemical devices, it is necessary to increase the ion permeability of separators for electrochemical devices and increase the porosity while ensuring thinness and absolute strength. However, electrochemical devices using separators for electrochemical devices with high porosity still have the problem of being prone to short-circuit defects.

[0011] In view of the above circumstances, an object of the present disclosure 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 for an electrochemical device and an electrochemical device using the same.

[0012] The above-mentioned problems can be solved by the following technical means: <1> A polyolefin microporous membrane, wherein the polyolefin microporous membrane is stained with ruthenium, embedded in a room-temperature curing epoxy resin, and a smooth cross section parallel to the TD is prepared using a broad ion beam (BIB). Five scanning electron microscope (SEM) images of the smooth cross section taken at 50 μm intervals at 7000x magnification are analyzed by a local thickness method to find an average pore diameter d ＴＤ（Ｎ＝５） <2> The polyolefin microporous membrane according to item 1, wherein the polyolefin microporous membrane has a thickness of 6 μm or more and 16 μm or less. <3> When the SEM image is subjected to at least one pore size analysis, the pore size distribution D ＴＤ <4> The polyolefin microporous membrane according to item 1 or 2, wherein the standard deviation of the average pore diameter d ＴＤ <5> The polyolefin microporous membrane according to any one of items 1 to 3, wherein the average particle diameter is 100 nm or more and 160 nm or less. ２ ) or more 150gf / (g / m ２ ) or less. <6> The polyolefin microporous membrane of any one of items 1 to 5, having a viscosity average molecular weight (Mv) of 800,000 or more and 1,500,000 or less. <7> The polyolefin microporous membrane of any one of items 1 to 6, having a molecular weight distribution (Mw / Mn) expressed as the ratio of the mass average molecular weight (Mw) to the number average molecular weight (Mn) of the polyolefin microporous membrane of 5 or more and 10 or less. <8> The polyolefin microporous membrane of any one of items 1 to 7, having a pin puncture strength of 450 gf or more and 1,000 gf or less. <9> The polyolefin microporous membrane of any one of items 1 to 7, having an air permeability of 30 s / 100 cm ３ More than 195s / 100cm ３<10> The polyolefin microporous membrane according to any one of items 1 to 9, wherein the amount of powder falling off when the polyolefin microporous membrane is unwound from a unwinder at a tension of 10 N and conveyed for 200 m on rubber having a dynamic friction coefficient of 0.5 with SUS304 is 0.001 mg / cm or more and 0.04 mg / cm or less.

[0013] According to the present invention, the cycle characteristics of an electrochemical device including a polyolefin microporous membrane or a separator for an electrochemical device can be improved and short-circuit defects can be suppressed.

[0014] Hereinafter, the embodiment for carrying out the present invention (hereinafter sometimes abbreviated as "present embodiment") will be described in detail, but the present invention is not limited thereto, and various modifications are possible within the scope of the gist thereof. In this specification, the flow direction of the membrane during membrane production is defined as MD, and the direction intersecting with MD at 90 degrees in the membrane plane is defined as TD. In addition, when a membrane or resin contains a specific component as a main component, it means that the content of the specific component is 50 mass% or more based on the mass of the membrane or resin.

[0015] The polyolefin microporous membrane (PO microporous membrane) according to this embodiment contains polyolefin (PO) as a main component, and is characterized by the relationship between membrane thickness and properties obtained by pore size analysis as shown below.

[0016] If desired, the PO microporous membrane can be specified in terms of thickness, molecular weight, pin puncture strength, basis weight equivalent pin puncture strength, air permeability, and the like, as shown below, and an inorganic coating layer or adhesive layer can be formed on its surface. The properties described in this embodiment can be utilized independently or in any combination. Unless otherwise specified, the methods for measuring the physical properties of the PO microporous membrane are described in detail in the Examples section.

[0017] <Relationship Between Film Thickness and Properties Obtained by Pore Size Analysis of Membrane> The PO microporous membrane according to this embodiment was stained with ruthenium, embedded in a room-temperature curing epoxy resin, and then a smooth cross section parallel to the TD was prepared using a broad ion beam (BIB). Five scanning electron microscope (SEM) images were taken at 50 μm intervals at 7000x magnification on the smooth cross section, and the pore size was analyzed by the local thickness method. The average pore size d ＴＤ（Ｎ＝５） The standard deviation of the thickness is 0.1 nm or more and 1.0 nm or less.

[0018] The pore size analysis is described in detail in the Examples section. The five SEM images can be obtained, for example, by capturing images of five different points at 50 μm intervals in the longitudinal direction of the smooth cross section at 7000x magnification.

[0019] The PO microporous membrane according to this embodiment has an average pore diameter d ＴＤ（Ｎ＝５） The standard deviation of 0.1 nm or more and 1.0 nm or less is such that when the separator is used for an electrochemical device, not only can the current become uniform and dendrites be suppressed to improve the cycle characteristics of the electrochemical device, but also the film strength can be increased, and the film rupture resistance and voltage resistance can be improved, thereby achieving both improved cycle characteristics and suppression of short-circuit defects.

[0020] The average pore diameter d of the PO microporous membrane according to this embodiment ＴＤ（Ｎ＝５） It is obvious that the smaller the standard deviation, the better. From the viewpoint of achieving both improved cycle characteristics and suppression of short-circuit defects, the standard deviation is preferably more than 0.1 nm and less than 1.0 nm, more preferably 0.2 nm or more and less than 0.9 nm, even more preferably 0.2 nm or more and 0.8 nm or less, still more preferably 0.2 nm or more and 0.7 nm or less, and particularly preferably 0.2 nm or more and 0.5 nm or less.

[0021] The PO microporous membrane according to this embodiment is stained with ruthenium, embedded in a room-temperature curing epoxy resin, and then a smooth cross section parallel to the TD is prepared using BIB. At least one SEM image of the smooth cross section is taken at 7000x magnification, and pore size analysis is performed on the image. The pore size distribution D ＴＤ It is preferable that the standard deviation of D is 35 nm or more and 55 nm or less.ＴＤ When the standard deviation of the pore size distribution D satisfies the condition of ≦55 nm, the cycle characteristics and the voltage resistance tend to be further improved while suppressing short-circuit defects. ＴＤ The standard deviation of is more preferably 38 nm or more and 53 nm or less, even more preferably 40 nm or more and 50 nm or less, still more preferably more than 40 nm and 49 nm or less, still more preferably more than 40 nm and less than 49 nm, and particularly preferably more than 40 nm and 46 nm or less.

[0022] The PO microporous membrane according to this embodiment has an average pore diameter d ＴＤ is preferably 100 nm or more and 160 nm or less. ＴＤ When the range of 100 nm≦d≦160 nm is satisfied, the voltage endurance of the film tends to be further improved while improving cycle characteristics and suppressing short circuit defects. ＴＤ In the range of ≦160 nm, the pore structure of the PO microporous membrane is small and uniform, resulting in more branches at the fibril trunk, which results in a dense interface between the air and the separator trunk, and is thought to suppress creeping discharge. ＴＤ is more preferably 105 nm or more and 150 nm or less, even more preferably 108 nm or more and 140 nm or less, still more preferably 109 nm or more and less than 140 nm, still more preferably 110 nm or more and 130 nm or less, and particularly preferably 110 nm or more and 125 nm or less.

[0023] From the viewpoint of both improving cycle characteristics and suppressing short-circuit defects, the PO microporous membrane according to this embodiment has a maximum pore size PS when at least one point pore size analysis is performed on an SEM image of a smooth cross section parallel to the TD prepared as described above. ＴＤ（ｍａｘ） However, it is preferable that the thickness is 250 nm or more and 400 nm or less.

[0024] The SEM image of at least one point on the smooth cross section parallel to the TD can be obtained, for example, by capturing an image of at least one point on the smooth cross section parallel to the TD in at least one field of view at 7000x magnification.

[0025] From the viewpoint of achieving both improved cycle characteristics and suppression of short-circuit defects, the PO microporous membrane according to this embodiment was stained with ruthenium, embedded in a room-temperature curing epoxy resin, and then a smooth cross section parallel to the MD was prepared using BIB. Five SEM images were taken at 50 μm intervals at 7000x magnification on the smooth cross section, and the pore size was analyzed by the local thickness method. The average pore size d ＭＤ（Ｎ＝５） The standard deviation of the thickness is preferably 0.2 nm or more and 0.9 nm or less, and more preferably 0.2 nm or more and less than 0.8 nm.

[0026] The PO microporous membrane according to this embodiment is stained with ruthenium, embedded in a room-temperature curing epoxy resin, and then a smooth cross section parallel to the MD is prepared using BIB. At least one SEM image of the smooth cross section is taken at 7000x magnification, and pore size analysis is performed on the image. The pore size distribution D ＭＤ It is preferable that the standard deviation of D is 38 nm or more and 45 nm or less. ＭＤ When the standard deviation of the pore size distribution D satisfies the condition of ≦45 nm, it tends to be easier to achieve both improvement in cycle characteristics and suppression of short circuit defects. ＭＤ The standard deviation is more preferably 39 nm or more and 44 nm or less.

[0027] From the viewpoint of both improving cycle characteristics and suppressing short-circuit defects, the PO microporous membrane according to this embodiment has an average pore diameter d ＭＤ However, it is preferably 100 nm or more and 155 nm or less, more preferably 102 nm or more and 149 nm or less, even more preferably 105 nm or more and 140 nm or less, still more preferably 106 nm or more and 130 nm or less, and particularly preferably 107 nm or more and 125 nm or less.

[0028] The SEM image of at least one point on the smooth cross section parallel to the MD can be obtained, for example, by imaging at least one point on the smooth cross section parallel to the MD in at least one field of view at 7000x magnification.

[0029] From the viewpoint of achieving both improved cycle characteristics and suppression of short-circuit defects, the PO microporous membrane according to this embodiment is stained with ruthenium, embedded in a room-temperature curing epoxy resin, and then smooth cross sections parallel to the MD and TD are prepared by BIB. At least one SEM image of the smooth cross sections is taken at 7000x magnification, and pore size analysis is performed by the local thickness method. The ratio of the average pore sizes of the cross sections prepared in both directions, i.e., the average pore size d ＭＤ and average pore diameter d ＴＤ The ratio (d ＭＤ / d ＴＤ ) is preferably 0.85 or more and 1.25 or less, more preferably 0.90 or more and 1.20 or less, and even more preferably 0.91 or more and 1.08 or less.

[0030] As a means for adjusting both the membrane thickness and the properties obtained by membrane pore size analysis as described above, for example, at least one selected from the following (i) to (v) can be mentioned.

[0031] (i) Selection of the resin to be contained or the raw material resin, more specifically, selection of a polyolefin resin having a viscosity average molecular weight (Mv) of 800,000 to 1,500,000, a molecular weight distribution (Mw / Mn) expressed as the ratio of the mass average molecular weight (Mw) to the number average molecular weight (Mn) of 5 to 10, and / or a single resin composition (e.g., polyethylene homopolymer), and adjusting the total content (PC) of all resins contained in the PO resin composition to fall within the range of 20% to 25%;

[0032] (ii) In the method for producing a PO microporous membrane, the kneading index during the extrusion process is controlled. The kneading index is expressed by the following formula: kneading index (rpm·min.) = screw rotation speed (rpm) × resin residence time (min.). More specifically, the kneading index is preferably adjusted within the range of 800 rpm·min. to 2,200 rpm·min. When the kneading index is 800 rpm·min. or higher, the resin is kneaded sufficiently to maintain the uniformity of the membrane pore size, and unmelted resin is suppressed, eliminating or reducing defects. When the kneading index is 2,200 rpm·min. or lower, the molecular weight of the resin is maintained or increased, membrane strength is improved, and the amount of powder shedding is reduced, eliminating or reducing defects during production.

[0033] (iii) In the method for producing a PO microporous membrane, the cooling rate during casting is controlled to 1°C / sec to 12°C / sec. Means for controlling the cooling rate during casting include, for example, adjusting the sheet thickness to within the range of 1,200 μm to 3,000 μm, adjusting the resin temperature at the die outlet to within the range of 180°C to 215°C, adjusting the temperature of the roll with which the resin first comes into contact (hereinafter referred to as the "casting temperature" or the "temperature of the roll that first embraces the extruded web"), using a cooling roll whose surface temperature is controlled to 50°C to 100°C to adjust the casting temperature, and adjusting the casting speed to within the range of 1 m / min to 12 m / min.

[0034] (iv) Simultaneous biaxial stretching in the production method of a PO microporous membrane.

[0035] (v) In the method for producing a PO microporous membrane, the stretch ratio can be controlled, for example, by adjusting the stretch ratio in the pre-extraction stretching step of the pore-forming material to within the range of 35 to 60 times, adjusting the transverse (TD) stretch ratio in the post-extraction stretching step to within the range of 1.4 to 2.4 times, and adjusting the total stretch ratio to within the range of 70 to 120 times.

[0036] In addition, the average pore diameter d of the PO microporous membrane ＭＤ and average pore diameter d ＴＤ The ratio (d ＭＤ / d ＴＤ) within the above range can be achieved not only by the above (i) to (iv) but also by controlling the MD / TD stretch ratio in the production method for the PO microporous membrane.

[0037] The preferred configuration or physical properties, preferred components, and production method of the microporous PO membrane according to this embodiment are described below.

[0038] <Film Thickness> The film thickness of the PO microporous film is preferably 6 μm or more and 16 μm or less. A film thickness of 6 μm or more and 16 μm or less is important for the PO microporous film of this embodiment from the viewpoint of uniform pore diameter.

[0039] The thinner the PO microporous membrane, the more pronounced its membrane strength, uniform current, membrane rupture resistance, and voltage resistance become. Therefore, from the viewpoint of achieving both improved cycle characteristics and suppression of short-circuit defects, the membrane thickness of the PO microporous membrane is more preferably less than 16 μm, even more preferably 15 μm or less, still more preferably 13 μm or less, even more preferably 12 μm or less, and particularly preferably 10 μm or less. The membrane thickness of the PO microporous membrane is such that it maintains insulation between electrodes and has an average pore diameter d ＴＤ（Ｎ＝５） From the viewpoint of satisfying the standard deviation range of the above, it is preferably 6 μm or more, and may exceed 6 μm.

[0040] Examples of means for controlling the thickness of the PO microporous membrane within the above range include not only the above (i) to (v) but also control of the thickness of the raw sheet and the heat setting (HS) temperature in the production method of the PO microporous membrane described below.

[0041] <Membrane strength> The basis weight (g / m ２ The puncture strength when converted to basis weight (hereinafter referred to as basis weight converted puncture strength) is 80 gf / (g / m ２ ) or more 150gf / (g / m ２ ) or less. ２ )~150gf / (g / m ２ PO microporous membranes having a puncture strength converted into basis weight of 83 gf / (g / m) tend to have improved membrane rupture resistance. ２ ) or more 140gf / (g / m２ ) or less, more preferably 85 gf / (g / m ２ ) or more 135gf / (g / m ２ ) below.

[0042] The pin puncture strength of the PO microporous membrane not converted into basis weight (hereinafter simply referred to as pin puncture strength) is preferably from 450 gf to 1,000 gf, more preferably from 550 gf to 1,000 gf, and even more preferably from 550 gf to 900 gf, from the viewpoint of improving membrane rupture resistance, similar to the pin puncture strength converted into basis weight.

[0043] The units "gf" and "N" are interchangeable according to the formula: 1 gf ≈ 0.0098 N. Methods for controlling the basis weight-equivalent pin puncture strength or pin puncture strength of the PO microporous membrane within the above-mentioned range include, for example, adjusting the molecular weight of the PO raw material or the resin contained therein, and adjusting the areal stretching ratio and / or stretching temperature in the production process of the PO microporous membrane.

[0044] <Molecular Weight of PO Microporous Membrane> The viscosity average molecular weight (Mv) of the PO microporous membrane according to this embodiment is preferably 800,000 or more. By making the Mv of the PO microporous membrane itself 800,000 or more, a uniform pore size tends to be more easily achieved. In view of this tendency, the Mv of the PO microporous membrane is more preferably 850,000 or more, and even more 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.

[0045] The molecular weight distribution (Mw / Mn), expressed as the ratio of the mass average molecular weight (Mw) to the number average molecular weight (Mn), of the PO microporous membrane according to this embodiment is preferably 5 or more and 10 or less. A PO microporous membrane within the range of 5≦Mw / Mn≦10 tends to easily achieve a uniform pore size. In view of this tendency, the molecular weight distribution (Mw / Mn) of the PO microporous membrane is more preferably 6 or more and less than 10, and even more preferably 7 or more and 9 or less.

[0046] In this specification, the molecular weight of the PO microporous membrane is obtained by measuring the molecular weight of the PO microporous membrane itself. The molecular weight of the PO microporous membrane can be adjusted to the above range, for example, by controlling the molecular weight or composition of the contained resin or raw material resin.

[0047] <Air permeability of PO microporous membrane> The air permeability of the PO microporous membrane is preferably 30 s / 100 cm from the viewpoint of ensuring ion permeability and ensuring the output characteristics of the electrochemical device. ３ More than 195s / 100cm ３ Less than 50 s / 100 cm ３ Over 180s / 100cm ３ More preferably, 70 s / 100 cm ３ Over 170s / 100cm ３ Particularly preferably 78 s / 100 cm ３ More than 160s / 100cm ３ The air permeability of the PO microporous membrane can be controlled within the above-mentioned range by, for example, adjusting the HS stretch ratio, HS relaxation ratio, HS relaxation temperature, etc. in the method for producing the PO microporous membrane.

[0048] <Porosity of PO microporous membrane> The porosity of the PO microporous membrane is preferably 25% or more, more preferably 30% or more, even more preferably 35% or more, and particularly preferably 40% or more. A porosity of 25% or more is suitable from the viewpoint of ensuring good output characteristics. The upper limit of the porosity is preferably less than 70%, more preferably 65% ​​or less, and even more preferably 60% or less. A porosity of less than 70% is preferred from the viewpoints of membrane strength and voltage resistance.

[0049] <Flux pore size> The PO microporous membrane according to this embodiment preferably has a flux pore size of 30 nm or more and 70 nm or less. A PO microporous membrane having a flux pore size within the range of 30 nm≦Flux pore size≦70 nm does not allow foreign matter to enter, and when mounted in an electrochemical device as a separator, tends to improve cycle performance and also tends to suppress light scattering and increase light transmittance.

[0050] <Heat shrinkage of PO microporous membrane> As the range of use of electrochemical devices expands, it is preferable to control the heat shrinkage of a PO microporous membrane used as a separator at high temperatures (e.g., near the melting point of PO or near the melting point of the PO microporous membrane) to ensure device safety in high-temperature environments, such as oven tests. It is also preferable to control the heat shrinkage of a separator for electrochemical devices to prevent contact between electrodes in an electricity storage device. From this perspective, the heat shrinkage of the PO microporous membrane at 120°C is preferably 33% or less in the MD and 21% or less in the TD. The lower limit of the heat shrinkage of the PO microporous membrane at 120°C may be, for example, −5% or more, −2% or more, −1% or more, or 0% or more in both the MD and TD.

[0051] <Maximum shrinkage stress of TMA of PO microporous membrane> In thermomechanical analysis (TMA) of a PO microporous membrane, the maximum shrinkage stress is preferably 7.0 gf or less in MD and preferably 9.7 gf or less in TD. As the range of use of electrochemical devices expands, TMA of PO microporous membranes used as separators is considered important for ensuring device safety in high-temperature environments. From this perspective, when a PO microporous membrane having a maximum shrinkage stress of 7.0 gf or less in MD and / or 9.7 gf or less in TD is incorporated into an electrochemical device as a separator, it tends to improve the high-temperature safety of the device.

[0052] From the viewpoint of further improving the safety of electrochemical devices in high-temperature environments, the maximum shrinkage stress of the PO microporous membrane in TMA is more preferably 6.5 gf or less, and even more preferably 6.1 gf or less, in MD, and more preferably 9.0 gf or less, and even more preferably 8.5 gf or less in TD. From the viewpoints of the productivity of electrochemical devices and the adhesion between electrodes and separators, the lower limit of the maximum shrinkage stress in TMA is preferably 1.0 gf or more, more preferably 1.5 gf or more, and even more preferably 2.0 gf or more, in both MD and TD.

[0053] <Shutdown temperature and membrane rupture temperature (meltdown temperature)> From the viewpoints of maintaining the performance of an electrochemical device in a high-temperature environment and ensuring safety in the event of abnormal heat generation in the electrochemical device, the shutdown temperature of the PO microporous membrane is preferably 151° C. or lower, and more preferably 150° C. or lower, 149° C. or lower, 148° C. or lower, 147° C. or lower, 146° C. or lower, 145° C. or lower, 144° C. or lower, 143° C. or lower, 142° C. or lower, or 141° C. or lower. The lower limit of the shutdown temperature, related to clogging of membrane pores, may be, for example, 110° C. or higher, 120° C. or higher, 130° C. or higher, or 140° C. or higher.

[0054] From the viewpoints of temperature stability and safety of electrochemical devices, the PO microporous membrane according to this embodiment has a membrane rupture temperature (meltdown temperature) of preferably at least 150° C., at least 155° C., or at least 160° C., and more preferably at least 170° C., at least 180° C., at least 190° C., or more than 200° C. The upper limit of the membrane rupture temperature of the PO microporous membrane is not limited, and can be, for example, at most 240° C., less than 240° C., at most 235° C., or at most 230° C., depending on the type of PO contained or starting PO, the types of components other than PO, the mixing ratio of PO to other components, etc.

[0055] <Amount of powder shedding from PO microporous membrane> When a PO microporous membrane is unwound from a unwinder at a tension of 10 N and conveyed for 200 m over rubber having a dynamic friction coefficient of 0.5 with SUS304, the amount of powder shedding is preferably 0.001 mg / cm or more and 0.04 mg / cm or less. PO microporous membranes with an amount of powder shedding in the range of 0.001 mg / cm to 0.04 mg / cm generate less low-molecular-weight components, and when incorporated into electrochemical devices, can improve cycle characteristics. The amount of powder shedding can be measured in a powder shedding test. The powder shedding test is described in detail in the Examples.

[0056] An example of a means for controlling the amount of powder falling within the above numerical range is kneading under mild conditions in the production method of a PO microporous membrane. More specifically, as described in (ii) above, it is preferable to adjust the kneading index to 2,200 rpm·min or less, and more preferably to 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 improved, and the generation of low-molecular-weight components is reduced, thereby reducing the amount of powder falling and eliminating or reducing defects during production. When the kneading index is 800 rpm·min or more, it is possible to suppress unmelted resin and eliminate or reduce defects.

[0057] <Components of PO Microporous Membrane> The PO microporous membrane of this embodiment is formed from a resin composition containing a polyolefin resin. If desired, the resin composition may further contain inorganic particles, a resin other than polyolefin, etc. The total content (PC) of all resins contained in the PO resin composition is preferably within the range of 20% to 25% from the viewpoints of adjusting both the membrane thickness and the properties obtained by pore size analysis of the PO microporous membrane as described above, and from the viewpoint of membrane strength.

[0058] 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, or preferably 80% by mass or more, and may be 90% by mass or more and 100% by mass or less, based on the mass of the PO microporous membrane, since it is contained as a main component.

[0059] The polyolefin resin used in this embodiment is not particularly limited and includes, for example, polymers (e.g., homopolymers, copolymers, multistage polymers, etc.) obtained by polymerizing monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. These polymers can be used alone or in combination of two or more. From the viewpoint of adjusting both the membrane thickness and the properties obtained by pore size analysis of the PO microporous membrane as described above, a single type of polyolefin resin is preferred.

[0060] From the viewpoint of exhibiting shutdown properties, the total proportion of the PE raw material in the PO raw material is preferably 50% by mass to 100% by mass, and more preferably 80% by mass to 100% by mass.

[0061] From the viewpoint of adjusting both the membrane thickness of the PO microporous membrane and the properties obtained by pore size analysis as described above, the PO raw material preferably has a single resin composition, more preferably consists of polyethylene (PE), and even more preferably consists of PE alone, and may be, for example, a polyethylene homopolymer.

[0062] From the viewpoint of adjusting both the membrane thickness and the properties obtained by pore size analysis as described above, the PO microporous membrane preferably has a single resin composition, more preferably consists of polyethylene (PE), and even more preferably consists of PE alone, and may be, for example, a polyethylene homopolymer.

[0063] The viscosity-average molecular weight of the PO resin is preferably 500,000 or more and 2,000,000 or less. As the molecular weight of the PO resin itself increases, the film tends to exhibit increased strength and safety. 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, adjusting the viscosity-average molecular weight of the PO resin to 2,000,000 or less is preferred from the viewpoint of suppressing thermal shrinkage.

[0064] The polyethylene (PE) contained in the PO microporous membrane preferably has a viscosity average molecular weight (Mv) of 600,000 or more and 2,000,000 or less, more preferably 800,000 or more and 1,000,000 or less, and a weight average molecular weight (Mw) of 5.0×10 ５ ~5.0 x 10 ６and / or the polydispersity (Mw / Mn) is preferably in the range of 7.0 to 8.8. When the molecular weight of PE is within the above numerical range, it becomes easier 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 viewpoint, the Mv of the PE raw material is also preferably 600,000 to 2,000,000, more preferably 800,000 to 1,000,000, and the Mw is preferably 5.0×10 ５ ~5.0 x 10 ６ and / or the polydispersity (Mw / Mn) is preferably in the range of 7.0 to 8.8. Among these, Mv is preferably in the range of 800,000 to 1,000,000 and Mw is preferably in the range of 5.0×10 ５ ~5.0 x 10 ６ and a polydispersity (Mw / Mn) of 7.0 or more and 8.8 or less are particularly preferred.

[0065] The Z-average molecular weight (Mz) of the polyethylene (PE) contained in the PO microporous membrane is 1.0 × 10 ６ ~5.0 x 10 ７ When the Mz of the PE is within the above range, it becomes easier to adjust both the thickness of the PO microporous membrane and the properties obtained by pore size analysis as described above. From the same viewpoint, the Mz of the PE raw material is also preferably 1.0 × 10 ６ ~5.0 x 10 ７ It is preferable that the temperature is in the range of

[0066] From the viewpoint of adjusting the properties obtained by pore size analysis, it is preferable that the polyolefin resin does not contain polypropylene (PP).

[0067] Furthermore, examples of polyolefin resins include low-density polyethylene (density 0.910 g / cm ３ 0.930g / cm or more ３ less than 0.910 g / cm ３ 0.940g / cm or more ３ less than 0.930 g / cm ３ 0.942g / cm or more ３less than 0.942 g / cm ３ or more), ultra-high molecular weight polyethylene (density 0.910 g / cm ３ 0.970g / cm or more ３ These may be used alone or in combination of two or more.

[0068] The resin composition may be mixed with various known additives, such as inorganic particles, phenolic, phosphorus, or sulfur-based antioxidants; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments, as needed.

[0069] <Method for producing polyolefin microporous membrane> The method for producing the PO microporous membrane of this embodiment is not particularly limited, and examples thereof include a method comprising: a mixing step (a) of mixing a resin composition containing a polyolefin resin and, if desired, various additives; an extrusion step (b) of melt-kneading and extruding the mixture obtained in step (a); a sheet-forming step (c) of molding the extrudate obtained in step (b) into a sheet; a primary stretching step (d) of stretching the sheet-shaped product obtained in step (c) at least once in at least one direction; an extraction step (e) of extracting a pore-forming material from the primarily stretched membrane obtained in step (d); and a heat-setting step (f) of heat-setting (HS) the extracted membrane obtained in step (e) at a predetermined temperature.

[0070] The above-described method for producing a PO microporous membrane can provide a PO microporous membrane that can achieve both improved cycle characteristics and suppressed short-circuit defects when used as a separator for an electrochemical device. Note that the method for producing a PO microporous membrane of this embodiment is not limited to the above-described method, and various modifications are possible without departing from the spirit of the invention.

[0071] [Mixing step (a)] The mixing step (a) is a step of mixing a resin composition containing a polyolefin resin and, if desired, various additives. In the mixing step (a), other components may be mixed with the resin composition as needed.

[0072] In the mixing step (a), it is preferable to select and use a PO raw material, and it is more preferable to select and use a PE raw material, in accordance with the characteristics obtained by pore size analysis, the means for controlling the film thickness (i), and the flux pore size as explained above.

[0073] The pore-forming material may be any material as long as it is distinguished from the PO resin and inorganic particle materials, and may be, for example, a plasticizer. Examples of the plasticizer include non-volatile solvents capable of forming a homogeneous solution at or above the melting point of the PO resin, such as hydrocarbons such as liquid paraffin (LP) and paraffin wax, esters such as dioctyl phthalate and dibutyl phthalate, and higher alcohols such as oleyl alcohol and stearyl alcohol.

[0074] The content of the plasticizer in the resin composition is preferably 60% by mass to 90% by mass, more preferably 70% by mass to 80% by mass. By adjusting the plasticizer content to 60% by mass or more, the melt viscosity of the resin composition decreases, melt fracture is suppressed, and film formability during extrusion tends to improve. On the other hand, adjusting the plasticizer content to 90% by mass or less may suppress elongation of the raw web during the film formation process.

[0075] (Optional Additives) In step (a), the resin composition containing PO may contain optional additives. The additives are not particularly limited, but examples thereof include polymers other than polyolefin resins; antioxidants such as phenolic compounds, phosphorus-based compounds, and sulfur-based compounds; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers; light stabilizers; antistatic agents; antifogging agents; and coloring pigments. The total amount of these additives added is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less, per 100 parts by mass of the polyolefin resin.

[0076] The mixing method in step (a) is not particularly limited, but examples thereof include a method in which some or all of the raw materials are premixed as necessary using a Henschel mixer, ribbon blender, tumbler blender, etc. Among these, a method in which mixing is performed using a Henschel mixer is preferred.

[0077] [Extrusion Step (b)] The extrusion step (b) is a step of melt-kneading and extruding the resin composition obtained in step (a). In the extrusion step (b), other components may be mixed with the resin composition, if necessary. In relation to the above-mentioned means (i) for controlling the properties and membrane thickness of the PO microporous membrane obtained by pore size analysis, the extrudate is preferably obtained by extruding a single type of PE raw material.

[0078] The melt-kneading method in step (b) is not particularly limited, but examples include a method in which all raw materials, including the mixture mixed in step (a), are melt-kneaded using a screw extruder such as a single-screw extruder or a twin-screw extruder; a kneader; a mixer; etc. Among these, melt-kneading is preferably performed using a screw in a twin-screw extruder. Furthermore, when melt-kneading, it is preferable to add the plasticizer in two or more batches. Furthermore, when adding the additive in multiple batches, it is preferable to adjust the amount added in the first batch to 95% by weight or less of the total amount added, from the viewpoint of suppressing aggregation of the contained components and uniformly dispersing them. This is preferable from the viewpoint of suppressing heat generation by shutting down over a large area and improving cell safety.

[0079] In relation to the above-mentioned means (ii) for controlling the properties, membrane thickness, amount of powder shedding, etc., of the PO microporous membrane obtained by pore size analysis, in the extrusion step (b), it is preferable to adjust the kneading index, expressed as the product of the screw rotation speed (rpm) and the resin residence time (min), within the range of 800 rpm·min to 2,200 rpm·min. When the kneading index is 800 rpm·min or higher, the resin composition is sufficiently kneaded, maintaining the pore size uniformity of the resulting membrane and suppressing unmelted resin, thereby eliminating or reducing defects. When the kneading 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, the amount of powder shedding is reduced, and defects during production are eliminated or reduced.

[0080] When a pore-forming agent is used in step (b), the temperature of the melt-kneading zone is preferably less than 200° C. from the viewpoint of uniformly kneading the resin composition. The lower limit of the temperature of the melt-kneading zone is equal to or higher than the melting point of the polyolefin from the viewpoint of uniformly dissolving the polyolefin resin in the plasticizer.

[0081] In the present embodiment, although not particularly limited, during kneading, it is preferable to mix the raw material PO with an antioxidant at a predetermined concentration, then replace the atmosphere around the mixture with a nitrogen atmosphere, and perform melt-kneading while maintaining the nitrogen atmosphere.

[0082] In step (b), the kneaded mixture obtained through the above kneading is extruded using an extruder such as a T-die or a circular die. This may be single-layer extrusion or multi-layer extrusion. The extrusion conditions are not particularly limited, and known methods can be used, for example. Furthermore, it is preferable to control the die lip clearance and the like from the viewpoint of the film thickness of the resulting PO microporous membrane described above.

[0083] [Sheet molding step (c)] The sheet molding step (c) is a step of molding the extrudate obtained in the extrusion step (b) into a sheet, and is also called a casting step. The sheet-shaped molded product obtained in the sheet molding step (c) may be a single layer or a laminate. The sheet molding method is not particularly limited, but examples thereof include a method of solidifying the extrudate by compression and cooling.

[0084] The compression cooling method is not particularly limited, but examples include a method in which the extrudate is brought into direct contact with a cooling medium such as cold air or cooling water, and a method in which the extrudate is brought into contact with a metal roll, a press, etc. cooled with a refrigerant. Among these, the method in which the extrudate is brought into contact with a metal roll, a press, etc. cooled with a refrigerant is preferred from the viewpoint of adjusting both the film thickness and the properties obtained by pore size analysis as described above.

[0085] In connection with the above-mentioned means (iii) for controlling the properties and membrane thickness of a 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. The cooling rate during casting, assuming that the extrudate from the die is the original web, is calculated using the following two equations: Cooling rate during casting [°C / sec] = (resin temperature when the extrudate from the die first contacts the roll [°C] - resin temperature when the extrudate's roll contact surface changes, it is embraced by the next roll, and it is released from that roll [°C]) ÷ residence time of original web [sec.] Residence time of original web [sec.] = length on roll [m] × 60 [sec.] ÷ casting speed [m / min]. Here, the resin temperature during casting is measured by non-contact infrared thermography on the surface not in contact with the roll. The cooling rate during casting is 1°C / sec to 12°C / sec. Controlling the cooling rate within this range means, but is not limited to, avoiding excessive cooling compared to conventional compression cooling methods and performing moderate cooling. If the cooling rate is excessively high, when the molten kneaded material is solidified in the sheet forming process, a smaller phase separation structure is formed toward the surface of the cooling roll, and a non-uniform pore structure is likely to be formed in the film thickness direction. To obtain a uniform pore structure, it is desirable to slow the cooling rate and control it to 12°C / sec or less. Furthermore, to cool and solidify on the roll, it is desirable to control the cooling rate to 1°C / sec or more.

[0086] Examples of means for optimizing the cooling rate during casting by controlling it as described above include adjusting the sheet thickness to within a range of 1200 μm to 3000 μm; adjusting the resin temperature at the die exit to within a range of 180°C to 215°C; using a chill roll whose surface temperature is controlled to 50°C to 100°C to adjust the casting temperature; and adjusting the casting speed to within a range of 1 m / min to 12 m / min. Adjusting the sheet thickness to within a range of 3000 μm or less is desirable in order to ensure that the resin is cooled and solidified by the time the roll contact surface of the extrudate changes, it is embraced by the next roll, and it is released from that roll. Furthermore, controlling the sheet thickness to 1200 μm or more is desirable in order to form a uniform film thickness.

[0087] In connection with the above-mentioned means (iii) for controlling the properties and membrane thickness of the microporous PO membrane obtained by pore size analysis, the temperature of the rolls that initially hold the extruded raw fabric can be adjusted within the range of preferably 41°C to 100°C, more preferably 45°C to 98°C, even more preferably 50°C to 97°C, and particularly preferably 55°C to 95°C.

[0088] From the viewpoint of the film thickness described above, it is also preferable to control the cast clearance etc. in the sheet forming step (c). The thickness of the obtained sheet-like molded product is preferably 1,000 μm or more and 3,300 μm or less, and more preferably 1,200 μm or more and 3,100 μm or less, depending on the thickness after stretching in step (d), for example.

[0089] [Primary Stretching Step (d)] The primary stretching step (d) is a step of stretching the sheet-like molded product obtained in the sheet molding step (c) at least once in at least one axial direction. This stretching step (a stretching step performed before the subsequent extraction step (e)) is referred to as "primary stretching," and the film obtained by primary stretching is referred to as "primary stretched film." In the primary stretching, the sheet-like molded product can be stretched in at least one direction, and may be performed in both MD and TD, or only in MD or TD.

[0090] The stretching method for the primary stretching is not particularly limited, and examples thereof include uniaxial stretching using a roll stretching machine; TD uniaxial stretching using a tenter; sequential biaxial stretching using a roll stretching machine and a tenter or a combination of multiple tenters; simultaneous biaxial stretching using a simultaneous biaxial tenter or inflation molding, etc. Among these, simultaneous biaxial stretching is preferred from the viewpoint of uniform stretching of the membrane in connection with the above-mentioned means (iv) for controlling the properties and membrane thickness obtained by pore size analysis of the PO microporous membrane.

[0091] The MD and / or TD stretching ratio in the primary stretching is preferably 5 times or more, more preferably 6 times or more. When the MD and / or TD stretching ratio in the primary stretching is 5 times or more, the strength of the resulting PO microporous membrane tends to be further improved. Furthermore, the MD and / or TD stretching ratio in the primary stretching is preferably 9 times or less, more preferably 8 times or less or 7 times or less. When the MD and / or TD stretching ratio in the primary stretching is 9 times or less, breakage during stretching tends to be further suppressed. When biaxial stretching is performed, either sequential stretching or simultaneous biaxial stretching may be used, and the stretching ratio in each axial direction is preferably 5 times or more and 9 times or less, more preferably 6 times or more and 8 times or less, or 6 times or more and 7 times or less, and a wet stretching ratio (wet stretching ratio) is even more preferable. Furthermore, the average pore diameter d of the resulting PO microporous membrane ＭＤ and average pore diameter d ＴＤ The ratio (d ＭＤ / d ＴＤ It is also preferable to optimize the MD / TD stretching ratio depending on the

[0092] The primary stretching temperature can be selected with reference to the composition and concentration of the raw material resins contained in the PO resin composition. The stretching temperature in the MD and / or TD is preferably within the range of 100°C to 135°C, more preferably 110°C to 130°C, even more preferably 115°C to 125°C, and still more preferably 116°C to 122°C. The stretching temperature in both the MD and TD is preferably 100°C or higher from the viewpoint of suppressing breakage, and is preferably 135°C or lower from the viewpoint of increasing film strength. The stretching temperature is preferably a wet stretching temperature (wet stretching temperature).

[0093] When the first stretching step (d) is carried out before the extraction step (e), in connection with the above-mentioned means (v) for controlling the properties obtained by pore size analysis of the PO microporous membrane and the membrane thickness, it is preferable to adjust the stretch ratio in the first stretching step (d) before extracting the pore-forming material from the membrane to within the range of 35 to 60 times.

[0094] [Extraction step (e)] The extraction step (e) is a step of extracting the pore-forming material from the first stretched membrane obtained in the first stretching step (d) to obtain an extracted membrane. For example, a method of removing the pore-forming material may be mentioned, such as immersing the first stretched membrane in an extraction solvent to extract the pore-forming material and then thoroughly drying it. The method of extracting the pore-forming material may be either a batch method or a continuous method. In addition, it is preferable that the remaining amount of the pore-forming material, particularly the plasticizer, in the porous membrane is less than 1 mass%.

[0095] The extraction solvent used to extract the pore-forming material is preferably a poor solvent for the polyolefin resin and a good solvent for the pore-forming material or plasticizer, and has a boiling point lower than the melting point of the polyolefin resin. Examples of such extraction solvents include, but are not limited to, hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; non-chlorinated halogenated solvents such as hydrofluoroethers and hydrofluorocarbons; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; and ketones such as acetone and methyl ethyl ketone. These extraction solvents may be recovered and reused by distillation or other procedures.

[0096] [Heat setting step (f)] The heat setting step (f) is a step of heat setting the extracted film obtained in the extraction step (e) at a predetermined temperature. The heat treatment method at this time is not particularly limited, but may be a heat setting method using a tenter or roll stretching machine to perform stretching and relaxation operations.

[0097] The stretching operation in the heat-setting step (f) is an operation of stretching the PO microporous membrane in at least one of MD and TD, and may be performed in both MD and TD, or in only one of MD or TD. From the viewpoint of providing a PO microporous membrane that can achieve both improved cycle characteristics and suppressed short-circuit defects, it is preferable to heat-set at least in TD.

[0098] The MD and TD stretching ratios in the heat setting step (f) 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. The MD and TD stretching ratios in step (f) are preferably 1.70 times or more from the viewpoint of highly orienting the film, and are preferably 2.40 times or less from the viewpoint of achieving both film thickness and film strength and suppressing film breakage. From the same viewpoint, the heat setting is preferably performed by dry uniaxial stretching.

[0099] When a stretching step (hereinafter also referred to as a "secondary stretching step") is performed after the extraction step (e), for example, during the heat setting step (f), in connection with the above-mentioned means (v) for controlling the properties and membrane thickness of the PO microporous membrane obtained by pore size analysis, it is preferable to adjust the TD stretch ratio in the secondary stretching step after the extraction of the pore-forming material from the membrane to within the range of 1.4 to 2.4 times.

[0100] The MD and / or TD stretching temperature during heat setting is preferably 128°C or higher from the viewpoint of adjusting the shutdown temperature and / or meltdown temperature, more preferably 129°C or higher from the viewpoint of the output characteristics of the electrochemical device, and even more preferably within the range of 130°C to 134°C from the viewpoint of suppressing short-circuit defects and safety.

[0101] The relaxation operation in the heat-setting step (f) is an operation of shrinking the PO microporous membrane in at least one of MD and TD, and may be performed in both MD and TD, or only in MD or TD. The relaxation ratio in the heat-setting step (f) is preferably 0.95 or less, more preferably 0.93 or less. A relaxation ratio of 0.95 or less in step (f) tends to suppress thermal shrinkage. From the viewpoint of increasing the relaxation temperature, the relaxation ratio is preferably 0.50 or more, more preferably 0.70 or more. Here, the "relaxation ratio" refers to the value obtained by dividing the membrane dimension after the relaxation operation by the membrane dimension before the relaxation operation. When both MD and TD are relaxed, the relaxation ratio refers to the value obtained by multiplying the relaxation ratio in MD by the relaxation ratio in TD. Relaxation ratio = (membrane dimension after relaxation operation (m)) / (membrane dimension before relaxation operation (m))

[0102] The relaxation temperature in this relaxation operation is preferably 128°C or higher from the viewpoint of the pore size of the resulting PO microporous membrane, more preferably 131°C or higher from the viewpoint of the cycle characteristics of the electrochemical device, and even more preferably within the range of 131°C to 134°C from the viewpoint of suppressing short-circuit defects and safety.

[0103] The order of the steps (a) to (f) can be changed as desired as long as the effects of the present invention are not impaired. After the steps (a) to (f), the total stretch ratio of the PO microporous membrane is preferably 70 to 120 times, more preferably 80 to 120 times, and even more preferably 90 to 120 times, from the viewpoint of improving membrane strength, in relation to the means (v) for controlling the properties obtained by pore size analysis and the membrane thickness. Here, the "total stretch ratio" refers to the value obtained by multiplying the MD and / or TD stretch ratio in the primary stretching step (d) by the stretch ratio and / or relaxation ratio in the heat setting step.

[0104] [Other Steps] The method for producing a PO microporous membrane of this embodiment may include steps other than the above steps (a) to (f). The other steps may include, but are not limited to, a lamination step of superposing multiple monolayer PO microporous membranes to obtain a laminated PO microporous membrane, in addition to the heat setting step. Alternatively, the method for producing a PO microporous membrane may include a peeling step of peeling the laminate obtained by coextrusion after steps (a) to (f) to obtain two or more monolayer membranes. The method for producing a PO microporous membrane of this embodiment may also include a surface treatment step of subjecting the surface of the PO microporous membrane to surface treatment such as electron beam irradiation, plasma irradiation, surfactant application, or chemical modification. Furthermore, an inorganic particle material may be applied to one or both surfaces of the PO microporous membrane to provide an inorganic material layer, or an adhesive layer containing a thermoplastic resin may be provided on the surface of the PO microporous membrane.

[0105] <Separator for electrochemical device> The polyolefin microporous membrane according to this embodiment can be used as a separator for electrochemical devices such as lithium ion secondary batteries. When incorporated into a lithium ion secondary battery, the polyolefin microporous membrane can suppress thermal runaway of the lithium ion secondary battery.

[0106] <Electrochemical Device> An electrochemical device containing a wound or laminated body formed by winding or laminating the PO microporous membrane according to this embodiment is also an aspect of the present invention. Examples of electrochemical devices include nonaqueous electrolyte batteries, nonaqueous electrolyte batteries, nonaqueous lithium ion secondary batteries, nonaqueous gel secondary batteries, nonaqueous solid secondary batteries, lithium ion capacitors, and electric double layer capacitors.

[0107] The nonaqueous electrolyte battery according to this embodiment includes a separator for a nonaqueous electrolyte battery including the above-described PO microporous membrane, a positive electrode plate, a negative electrode plate, and a nonaqueous electrolyte (including a nonaqueous solvent and a metal salt dissolved therein). Specifically, for example, a positive electrode plate including a transition metal oxide capable of absorbing and desorbing lithium ions and a negative electrode plate also capable of absorbing and desorbing lithium ions are wound or stacked so as to face each other with the separator for a nonaqueous electrolyte battery interposed therebetween, and the nonaqueous electrolyte is held in a container.

[0108] The positive electrode plate will be described below. Examples of the positive electrode active material include lithium composite metal oxides such as lithium nickel oxide, lithium manganate, and lithium cobalt oxide, and lithium composite metal phosphates such as lithium iron phosphate. The positive electrode active material is mixed with a conductive agent and a binder, and the resulting paste is applied to a positive electrode current collector such as aluminum foil and dried. The paste is then rolled to a predetermined thickness and cut to a predetermined size to form a positive electrode plate. The conductive agent can be a metal powder that is stable under the positive electrode potential, such as carbon black or graphite, such as acetylene black. The binder can be a material that is stable under the positive electrode potential, such as polyvinylidene fluoride, modified acrylic rubber, or polytetrafluoroethylene.

[0109] The negative electrode plate will be described below. A material capable of absorbing lithium can be used as the negative electrode active material. Specifically, at least one material selected from the group consisting of graphite, silicide, and titanium alloy materials can be used. Furthermore, the negative electrode active material of a nonaqueous electrolyte secondary battery can be, for example, a metal, a metal fiber, a carbon material, an oxide, a nitride, a silicon compound, a tin compound, or various alloy materials. Silicon compounds or tin compounds, such as silicon (Si) or tin (Sn) as simple substances, alloys, compounds, or solid solutions, are particularly preferred because they tend to increase the capacity density of the battery.

[0110] Examples of carbon materials include various natural graphites, coke, partially graphitized carbon, carbon fiber, spherical carbon, various artificial graphites, and amorphous carbon.

[0111] As the negative electrode active material, one of the above materials may be used alone, or two or more may be used in combination. The negative electrode active material is kneaded with a binder, and the resulting negative electrode paste is applied to a negative electrode current collector such as copper foil, dried, rolled to a predetermined thickness, and then cut to a predetermined size to form a negative electrode plate. Here, as the binder, a material that is stable under the negative electrode potential, such as PVDF or a styrene-butadiene rubber copolymer, can be used.

[0112] The non-aqueous electrolyte will be described below. The non-aqueous electrolyte generally contains a non-aqueous solvent and a metal salt, such as a lithium salt, a sodium salt, or a calcium salt, dissolved therein. The non-aqueous solvent may be a cyclic carbonate, a chain carbonate, or a cyclic carboxylic acid ester. The lithium salt may be, for example, LiPF ６ , LiClO ４ , LiBF ４ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , Li(CF ３ SO ２ ) ２ , LiAsF ６, lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, borates, imide salts, and the like.

[0113] The above-mentioned various parameters are measured according to the measurement methods in the examples described below, unless otherwise specified.

[0114] Next, the present embodiment will be described in more detail with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples as long as it does not deviate from the gist of the present invention. The physical properties in the examples were measured by the following methods. Unless otherwise specified, measurements were taken at room temperature of 23°C and humidity of 40%.

[0115] (1) Molecular Weight (1a) High-Temperature GPC of Membrane: Measurement of Weight-Average Molecular Weight (Mw) and Number-Average Molecular Weight (Mn) (GPC-Relative Method) Sample Preparation The polyolefin raw material was weighed, and the eluent 1,2,4-trichlorobenzene (TCB) was added so that the concentration became 1 mg / ml. Using a high-temperature dissolver, the mixture was left standing at 160°C for 30 minutes, and then shaken at 160°C for 1 hour, and it was visually confirmed that the sample had completely dissolved. While still at 160°C, the sample solution was filtered through a 0.5 μm filter, and the filtrate was used as a GPC measurement sample. - GPC measurement As a GPC apparatus, a Waters ALC / GPC-150-C-plus type (trademark) was used, and two 30 cm columns of GMH6-HT (trademark) and two 30 cm columns of GMH6-HTL (trademark) manufactured by Tosoh Corporation were connected in series. Orthodichlorobenzene was used as the mobile phase solvent, and GPC measurements were performed at a sample concentration of 0.05 wt% and 140 ° C. The calibration curve was created using commercially available monodisperse polystyrene with a known molecular weight as the standard substance, and the obtained molecular weight distribution data in terms of polystyrene of each sample was multiplied by 0.43 (Q factor of polyethylene / Q factor of polystyrene = 17.7 / 41.3) to obtain molecular weight distribution data in terms of polyethylene. As a result, the weight average molecular weight (Mw) and number average molecular weight (Mn) of each sample were calculated, and the molecular weight distribution (Mw / Mn) was also obtained.

[0116] (1b) Viscosity-average molecular weight (Mv) The intrinsic viscosity [η] (dl / g) in decalin solvent at 135°C was determined according to ASTM-D4020. For the PO microporous membrane and the polyethylene raw material, Mv was calculated using the following formula: [η] = 6.77 × 10 －４ Mv ０．６７ For polypropylene raw materials, Mv was calculated using the following formula: [η] = 1.10 × 10 －４ Mv ０．８０

[0117] (2) Film Thickness (μm) The film thickness of the PO microporous film was measured at room temperature of 23±2° C. using a micro thickness measuring instrument, KBM (trademark), manufactured by Toyo Seiki Co., Ltd.

[0118] (3) Porosity (%) A 10 cm x 10 cm square sample was cut out from the PO microporous membrane, and its volume (cm ３ ) and mass (g), and calculate the density (g / cm ３ ) and the porosity was calculated using the following formula: Porosity (%) = (volume - mass / density) / volume x 100

[0119] (4) Air permeability (sec / 100cm ３ The air permeability was determined as the air resistance in accordance with JIS P-8117. The air permeability of the PO microporous membrane was measured in accordance with JIS P-8117 using a Gurley air permeability meter, G-B2 (trademark), manufactured by Toyo Seiki Co., Ltd., in an atmosphere at a temperature of 23°C and a humidity of 40%, and the measured air resistance was used as the air permeability.

[0120] (5) Puncture strength (gf) and area weight conversion puncture strength (gf / (g / m ２ Using a Kato Tech handy compression tester KES-G5 (trademark), a PO microporous membrane was fixed in a sample holder with an opening diameter of 11.3 mm. Next, a puncture test was performed on the center of the fixed microporous membrane using a needle with a tip diameter of 1.0 mm and a curvature radius of 0.5 mm at a puncture speed of 2 mm / sec in an atmosphere of 23°C and 40% humidity to determine the raw puncture strength (gf) as the maximum puncture load, and the value converted to basis weight (gf / (g / m ２ ) was also calculated.

[0121] (6) Flux pore size (μm) Flux pore size was calculated from the permeation flow rate (Flux) measurement of the PO microporous membrane. It is known that the fluid inside the capillary follows Knudsen flow when the mean free path of the fluid is larger than the pore size of the capillary, and follows Poiseuille flow when it is smaller. Therefore, it is assumed that the air flow in measuring the air permeability of the porous membrane follows Knudsen flow, and the water flow in measuring the water permeability of the porous membrane follows Poiseuille flow. In this case, the mean pore size d (μm) and tortuosity τ of the porous membrane are ａ (dimensionless) is the air permeation rate constant R ｇａｓ (m ３ / (m ２ sec Pa), water permeation rate constant R ｌｉｑ (m ３ / (m ２ sec Pa), air molecular velocity ν (m / sec), water viscosity η (Pa sec), standard pressure P ｓ (=101325 Pa), porosity ε (%), and film thickness L (μm) were calculated using the following formula: d=2ν×(R ｌｉｑ / R ｇａｓ )×(16η / 3Ps)×10 ６ τ ａ =(d×(ε / 100)×ν / (3L×P ｓ ×R ｇａｓ )) １／２ Here, R ｇａｓ was calculated from the air permeability (sec) using the following formula: ｇａｓ = 0.0001 / (air permeability × (6.424 × 10 －４ ) × (0.01276 × 101325)) Also, R ｌｉｑ is the permeability (cm ３ / (cm ２ sec Pa) was calculated using the following formula: R ｌｉｑ = water permeability / 100 The water permeability was determined as follows: A porous membrane that had been immersed in ethanol was set in a stainless steel liquid permeation cell with a diameter of 41 mm, and after washing the ethanol off the membrane with water, water was passed through the membrane at a differential pressure of about 50,000 Pa. The water permeation rate (cm ３) was used to calculate the amount of water permeable per unit time, unit pressure, and unit area, and this was taken as the water permeability. ν is the gas constant R (= 8.314), absolute temperature T (K), pi (π), and the average molecular weight of air M (= 2.896 × 10 －２ The σ was calculated from the σ (kg / mol) using the following formula: ν = ((8R × T) / (π × M)) １／２

[0122] (7) Heat shrinkage rate (%) at 120°C A PO microporous membrane was cut into a piece of 100 mm in the MD direction and 100 mm in the TD direction, and left to stand in an oven at a predetermined temperature (120°C or 150°C) for 1 hour. The sample was sandwiched between two sheets of paper to prevent direct exposure to hot air. The paper used was Fuji Xerox Interfield V-Paper monochrome copy / printer paper (basis weight 64 g / m²). ２ After the sample was removed from the oven and cooled, the length (mm) was measured and the heat shrinkage was calculated using the following formula. Measurements were carried out in both the MD and TD directions. Heat shrinkage (%) = {(100 - length after heating) / 100} x 100

[0123] (8) Maximum Shrinkage Stress (gf) of TMA The thermal shrinkage of the sample was measured using a TMA50 (trademark) manufactured by Shimadzu Corporation. When measuring the value in the MD (TD) direction, a sample cut to a width of 3 mm in the TD (MD) direction was fixed to a chuck with a chuck distance of 10 mm and set on a dedicated probe. The sample was heated from 30°C to 200°C at a heating rate of 10°C / min under an initial load of 1.0 g and in constant length measurement mode, and the load (gf) generated at that time was measured, and the maximum value was taken as the MD (or TD) maximum thermal shrinkage stress (gf).

[0124] (9) Cross-sectional SEM Observation and Pore Size Analysis Cross-sectional SEM Images of Resin-Embedded Samples A PO microporous membrane was stained with ruthenium and embedded in a room-temperature curing epoxy resin. Smooth cross sections parallel to the MD and TD were then prepared using a broad ion beam (BIB). These smooth cross sections were then observed with a scanning electron microscope (SEM) and imaged at 7000x magnification to obtain SEM images. From the smooth cross section, multiple SEM images can be obtained from multiple fields of view, or one SEM image can be obtained from one field of view. Specifically, the acquisition position of the SEM image can be determined as follows: When the SEM field of view is 7000x and covers the entire field of view: (1)-1: An image including the entire thickness is obtained; (1)-2: A field of view excluding 3% of the total thickness from both outermost surfaces is cut out and used for the image analysis described below. When the SEM field of view is too large to fit the entire field of view at 7000x magnification: (2)-1: An image of the entire screen is obtained, centered on the central part of the membrane; (2)-2: A portion of the obtained image, excluding 3% of the thickness from both ends, is cut out and used for the image analysis described below. When multiple SEM images are obtained, SEM images are taken at 50 μm intervals in the longitudinal direction of the smooth cross section. (3)-1: As a specific example, N=5 SEM images were sampled as follows: A 3 cm square test piece was taken from the center of the PO microporous membrane, a cross section was prepared in the TD (or MD) as the longitudinal direction of the PO microporous membrane by BIB processing, and SEM images of five points were taken at 50 μm intervals in the TD (or MD) from any position on the PO microporous membrane cross section and used for analysis.

[0125] Image analysis and pore size analysis The obtained SEM images were subjected to median filter processing under a radius of 2.0 pixels, and then binarized under threshold conditions using the Otsu method to calculate the area ratio of black and / or white areas. The binarized images were then subjected to pore size analysis using the local thickness method to calculate the pore size distribution, average pore size, maximum pore size, etc. The local thickness method can be performed using the thickness analysis of "BoneJ," a plug-in for the image processing software "Image J." The size of the largest circle that fits into that location can be defined as the spatial size. Therefore, even if there is no independent structure / independent space, the spatial size can be defined and the pore size distribution, average pore size, maximum pore size, etc. can be calculated.

[0126] Results of pore size analysis When the SEM images of the smooth cross sections parallel to the MD and TD prepared by BIB as described above were analyzed by at least one point local thickness method, the average pore size d ＭＤ and d ＴＤ , pore size distribution D ＭＤ and D ＴＤ , pore size distribution D ＭＤ Standard deviation and pore size distribution D ＴＤ Standard deviation of pore size, maximum pore size PS ＭＤ（ｍａｘ） and P.S. ＴＤ（ｍａｘ） etc. can be calculated.

[0127] When the SEM images of the smooth cross sections parallel to the MD and TD prepared by BIB as described above were analyzed for pore size at five points by the local thickness method, the average pore diameter d ＭＤ（Ｎ＝５） and the standard deviation of the average pore diameter d ＴＤ（Ｎ＝５） The standard deviation etc. can be calculated.

[0128] (10) Powder Shedding Test A 20 cm wide rubber sheet having a dynamic friction coefficient of 0.5 with SUS304 was attached to a 5 cm diameter roll in a circular shape so that the embrace angle θ was 20°. A microporous membrane was unwound from a dispenser at a tension of 10 N and run over the rubber sheet under the following conditions to perform a powder shedding test. The mass of the 20 cm wide circular rubber sheet was measured in advance. (Conditions) Measurement length: 200 m (i.e., the microporous membrane was unwound from the dispenser at a tension of 10 N and run over the rubber sheet for 200 m) Microporous membrane speed: 30 m / min. The amount of powder shedding (mg / cm) was calculated by measuring the difference in mass of the rubber sheet before and after the powder shedding test, or by measuring the mass of powder attached to the rubber sheet after the powder shedding test.

[0129] (11) Voltage Withstanding Test The PO microporous membrane to be measured was used as a film sample, and the film sample and aluminum foil were cut into 100 mm x 50 mm pieces. The sample was placed on the aluminum foil, and then a 5 mm diameter aluminum plate was placed on the sample. Using a voltage withstanding tester (device name: TOS9201) under a load of 45 gf, a voltage was applied to a 5 mm diameter electrode at an AC current (60 Hz), a starting voltage of 0 kV, and a voltage rise rate of 0.1 kV / s. The voltage was gradually increased, and the voltage when a current of 0.2 mA or more flowed was measured. This measurement was performed at at least 20 different points within the same film sample's surface. The measurement points were set and measured at locations 15 mm or more away from each measurement point, without any in-plane bias. The voltage value at which a current of 0.2 mA or more flowed was converted per 1 μm of film thickness to be the withstand voltage value of the film sample, and the average of the measurements at 20 points was calculated and evaluated according to the following criteria. Measurements were carried out in a dry room at a room temperature of 18°C ​​and a dew point of -40°C. A: Withstand voltage value of 1.25 kV / μm or more. B: Withstand voltage value of 1.00 kV / μm or more but less than 1.25 kV / μm. C: Withstand voltage value less than 1.00 kV / μm.

[0130] (12) Battery Evaluation Batteries were fabricated according to the following steps a-1 to a-5. a-1. Fabrication of Positive Electrode Nickel, manganese, and cobalt composite oxide (NMC) (Ni:Mn:Co = 1:1:1 (element ratio), density 4.70 g / cm 3) was used as the positive electrode active material. ３ ) as a conductive additive, graphite powder (KS6) (density 2.26 g / cm ３ 1.6 mass% of acetylene black powder (AB) (density 1.95 g / cm ３ , number average particle diameter 48 nm) at 3.8 mass %, and polyvinylidene fluoride (PVDF) (density 1.75 g / cm ３ ) were mixed in a ratio of 4.2 mass %, and dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This slurry was applied to one side of a 20 μm thick aluminum foil serving as a positive electrode current collector using a die coater, dried at 130°C for 3 minutes, and then compression-molded using a roll press to prepare a positive electrode. The amount of positive electrode active material applied at this time was 109 g / m ２ It was.

[0131] a-2. Preparation of negative electrode Graphite powder A (density 2.23 g / cm 3 ) was used as the negative electrode active material. ３ 87.6 mass% of graphite powder B (density 2.27 g / cm ３ A slurry was prepared by dispersing 9.7% by mass of carboxymethylcellulose (number average particle diameter: 6.5 μm) and 1.4% by mass (solids content equivalent) of ammonium salt of carboxymethylcellulose (aqueous solution with a solids content of 1.83% by mass) and 1.7% by mass (solids content equivalent) of diene rubber latex (aqueous solution with a solids content of 40% by mass) as binders in purified water. This slurry was applied to one side of a 12 μm-thick copper foil serving as a negative electrode current collector using a die coater, dried at 120° C. for 3 minutes, and then compression-molded using a roll press to produce a negative electrode. The amount of negative electrode active material applied was 52 g / m ２ It was.

[0132] a-3. Preparation of non-aqueous electrolyte: LiPF as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1:2 (volume ratio) ６was dissolved in the solution to a concentration of 1.0 mol / L to prepare a non-aqueous electrolyte solution.

[0133] a-4. Battery Fabrication A laminate-type secondary battery having a size of 100 mm x 60 mm and a capacity of 3000 mAh was fabricated by using the positive electrode, negative electrode, and nonaqueous electrolyte solution obtained in a-1 to a-3 above, and a microporous membrane obtained in an Example or Comparative Example as a separator, and charging the battery at a constant current / constant voltage (CCCV) for 3 hours at a current value of 1 A (0.3 C) and an end-of-battery voltage of 4.2 V.

[0134] a-5. Capacity measurement (mAh) The laminated secondary battery assembled as described above was subjected to constant current constant voltage (CCCV) charging for 6 hours under conditions of a current value of 1500 mA (0.5 C) and a final battery voltage of 4.2 V. The current value immediately before the end of charging was nearly 0. Thereafter, the battery was left to stand (aging) in an atmosphere at 25°C for 1 week.

[0135] Next, the battery was cycled by charging at a constant current and constant voltage (CCCV) for 3 hours at a current of 3000 mA (1.0 C) and a final battery voltage of 4.2 V, followed by discharging at a constant current (CC) of 3000 mA to a battery voltage of 3.0 V. The discharge capacity at this time was defined as the initial discharge capacity X. Batteries with an initial discharge capacity X within 3000±10 mAh were used for battery evaluation.

[0136] b. Output Test (25°C) For the laminated secondary batteries assembled as described above and selected for evaluation, the 1C discharge capacity and 5C discharge capacity were measured at a constant temperature of 25°C up to a discharge end voltage of 3V, and the 5C capacity / 1C capacity was taken as the output characteristic value. The output characteristic value was evaluated according to the following criteria: A: Output characteristic value is 0.90 or more. B: Output characteristic value is 0.80 or more but less than 0.90. C: Output characteristic value is less than 0.80.

[0137] c. Cycle Test Using the battery assembled as described above and selected for evaluation, a total of 100 charge / discharge cycles were performed under the following cycle conditions: (i) constant-current / constant-voltage charging at a current of 0.5 C and an upper limit voltage of 4.2 V for a total of 8 hours, (ii) a 10-minute break, (iii) constant-current discharge at a current of 0.5 C and an end voltage of 3.0 V, and (iv) a 10-minute break. All of the charge / discharge cycles were performed in an atmosphere of 25°C. The capacity retention rate (%) was then calculated by multiplying the ratio of the discharge capacity at the 100th cycle to the initial battery capacity X (mAh) by 100. The capacity retention rate was evaluated according to the following criteria: A: Capacity retention rate (%) was 90% or more; B: Capacity retention rate (%) was 80% or more but less than 90%; C: Capacity retention rate (%) was less than 80%.

[0138] (13) Nail penetration test a. Preparation of positive electrode Lithium cobalt composite oxide (LiCoO) was used as the positive electrode active material. ２ The conductive material, graphite and acetylene black, were dispersed in a binder of polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP) to prepare a slurry. This slurry was applied to a 15 μm thick aluminum foil serving as a positive electrode current collector using a die coater, dried at 130°C for 3 minutes, and then compression molded using a roll press. The resulting molded body was slit to a width of 57.0 mm to obtain a positive electrode.

[0139] b. Preparation of negative electrode: Artificial graphite as the negative electrode active material, and carboxymethyl cellulose ammonium salt and styrene-butadiene copolymer latex as binders were dispersed in purified water to prepare a slurry. This slurry was applied to copper foil, which would serve as the negative electrode current collector, using a die coater, dried at 120°C for 3 minutes, and then compression molded using a roll press. The resulting molded body was slit to a width of 58.5 mm to obtain a negative electrode.

[0140] c. Preparation of non-aqueous electrolyte: A non-aqueous electrolyte solution was prepared by dissolving LiPF as a solute in a mixed solvent of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a volume ratio of 1:1:2. ６ was dissolved in the solution to a concentration of 1 mol / L to prepare a non-aqueous electrolyte solution.

[0141] d. Battery Assembly After winding the positive electrode, the PO porous membrane obtained in the Examples or Comparative Examples, and the negative electrode, a wound electrode body was prepared by a conventional method and pressed with a press so as to fit into an outer can. The number of windings was adjusted depending on the thickness of the PO microporous membrane and the degree of springback. The outermost peripheral end of the obtained wound electrode body was fixed by attaching insulating tape. The negative electrode lead was welded to the battery can, and the positive electrode lead was welded to the safety valve, and the wound electrode body was inserted into the battery can. Then, 5 g of nonaqueous electrolyte was injected into the battery can, and the lid was crimped to the battery can via a gasket, to obtain a prismatic secondary battery with a width of 42.0 mm, a height of 63.0 mm, and a thickness of 10.5 mm. This rectangular secondary battery was charged in an atmosphere of 25° C. at a current value of 0.2 C (a current 0.2 times the 1-hour rate (1 C) of the rated electrical capacity) up to a battery voltage of 4.2 V, and after reaching this voltage, the current value was started to be reduced so as to maintain 4.2 V. This method of charging was carried out for a total of 3 hours. Subsequently, the battery was discharged at a current value of 0.2 C down to a battery voltage of 3.0 V.

[0142] e. Nail penetration safety test The battery assembled in d. above and selected for evaluation was placed on an iron plate in a temperature-controllable explosion-proof booth. A 3.0 mm diameter iron nail was prepared and a thermocouple was installed inside the nail. In an environment of 30°C and 3 MPa pressure in the explosion-proof booth, the iron nail was penetrated into the center of the battery at a speed of 2 mm / sec, and the nail was maintained in the penetrated state. The battery was observed from the start of nail penetration until after the nail penetrated, and safety was evaluated according to the following criteria: A: Nothing happened. B: Smoke was emitted. C: Cell swelling and deformation were observed. D: Fire was generated. E: Explosion occurred.

[0143] Example 1 PE1 was charged into a Henschel mixer according to the polyethylene (PE) type shown in Table 1 and the PE type and raw material composition ratio shown in Table 2, and an appropriate amount of pentaerythrityl tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] was added as an antioxidant and premixed. The resulting mixture was fed to the feed port of a twin-screw co-rotating screw extruder using a feeder. Furthermore, the mixture was melt-kneaded, and liquid paraffin (LP) was added to the twin-screw extruder cylinder via side feed in two portions so that the amount of liquid paraffin (LP) in the total mixture (100.1 parts by mass) extruded was 76.0 parts by mass.

[0144] The extrusion and casting processes were carried out under the conditions shown in Table 2 to obtain a sheet-like molded product having a thickness of 1800 μm.

[0145] The obtained sheet-like molded product was introduced into a simultaneous biaxial stretching machine to obtain a primarily stretched membrane (primary stretching step). The set stretching conditions were as shown in Table 2. The obtained primarily stretched membrane was then introduced into a methylene chloride tank and thoroughly immersed to extract and remove the liquid paraffin, which is a plasticizer, and then the methylene chloride was dried and removed to obtain an extracted membrane.

[0146] The extracted membrane was then introduced into a TD uniaxial tenter for heat setting. In the heat setting step, a stretching operation was performed under the TD stretching temperature and TD stretching ratio conditions shown in Table 2, and a relaxation operation was performed after the stretching operation under the relaxation temperature and relaxation ratio conditions. The various properties of the resulting PO microporous membrane were evaluated by the above-mentioned methods. The membrane formation conditions are shown in Table 2, and the results are shown in Table 4.

[0147] Examples 2 to 20 and Comparative Examples 1 to 13 PO microporous membranes were obtained in the same manner as in Example 1, except that the resin raw material types, raw material composition ratios, and membrane formation conditions were set as shown in Tables 2 and 3, respectively. The various properties of the obtained PO microporous membranes were evaluated by the methods described above. The results are shown in Tables 4 and 5.

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Claims

1. A polyolefin microporous membrane, wherein the polyolefin microporous membrane is stained with ruthenium, embedded in a room-temperature curing epoxy resin, and a smooth cross section parallel to the TD is prepared using a broad ion beam (BIB). Five scanning electron microscope (SEM) images of the smooth cross section taken at 50 μm intervals at 7000x magnification are analyzed by the local thickness method to find an average pore diameter d ＴＤ（Ｎ＝５） A microporous polyolefin membrane, wherein the standard deviation of the surface roughness is 0.1 nm or more and 1.0 nm or less.

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

3. When the SEM image is analyzed for pore size at least at one point, the pore size distribution D ＴＤ The polyolefin microporous membrane according to claim 1, wherein the standard deviation of the average particle diameter (μm) is 35 nm or more and 55 nm or less.

4. The average pore diameter d when the SEM image is analyzed for pore diameter at least at one point ＴＤ The polyolefin microporous membrane according to claim 1 or 2, wherein the average particle diameter is 100 nm or more and 160 nm or less.

5. The polyolefin microporous membrane has a puncture strength calculated based on basis weight of 80 gf / (g / m ２ ) or more 150gf / (g / m ２ 3. The polyolefin microporous membrane according to claim 1 or 2, wherein the molecular weight is 1000 or less.

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

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

8. The microporous polyolefin membrane according to claim 1 or 2, wherein the microporous polyolefin membrane has a pin puncture strength of 450 gf or more and 1000 gf or less.

9. The polyolefin microporous membrane has an air permeability of 30 s / 100 cm ３ More than 195s / 100cm ３ The polyolefin microporous membrane according to claim 1 or 2, wherein:

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

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