JP5601681B2 - Polyolefin microporous membrane containing inorganic particles and separator for non-aqueous electrolyte battery - Google Patents

Description

  The present invention relates to an inorganic particle-containing polyolefin microporous membrane and a separator for a non-aqueous electrolyte battery.

  Microporous membranes have various pore diameters, pore shapes, and numbers of pores, and are used in a wide range of fields because of their characteristics that can be manifested by their unique structures. For example, separation membranes used for water treatment and concentration utilizing the sieving effect due to the difference in pore diameter, adsorption sheets used for water absorption, oil absorption, deodorizing materials utilizing large surface area and porous space due to microporosity, difference in molecular size Moisture permeable waterproof sheet using the feature that allows air and water vapor to pass through but not water, multi-functionalized by filling various materials in the porous space, polymer electrolyte membrane and humidifying membrane useful for fuel cells, etc. Furthermore, they are used as liquid crystal materials and battery materials.

In recent years, development of nonaqueous electrolyte batteries centering on lithium ion batteries has been actively conducted. Usually, in a nonaqueous electrolyte battery, a microporous membrane (separator) is provided between positive and negative electrodes. Such a separator has a function of preventing direct contact between the positive and negative electrodes and allowing ions to permeate through the electrolytic solution held in the micropores. As a constituent material of the separator, a polyolefin material is mainly used from the viewpoint of electrochemical stability and electrolytic solution resistance.
As a new application for non-aqueous electrolyte batteries, the development of in-vehicle applications is rapidly expanding. For in-vehicle applications, characteristics such as long life, high output, high capacity, and high voltage are required, and lithium ion secondary batteries are regarded as the most promising because of the characteristics of the batteries. In particular, there is a great demand for higher capacity, and constituent materials and battery structures are diversified every day, and separators in the battery are required to have safety in various states. From the viewpoint of long life, a separator in which inorganic particles are mixed has been studied for the purpose of improving liquid retention.

  In response to such a demand for safety, for example, Patent Document 1 includes an island-shaped structure containing a polyolefin resin and fine particles, and a fibril connecting the island-shaped structures, and the fibril is a polyolefin. A polyolefin microporous membrane containing a resin and arranged substantially in one direction has been proposed. Patent Document 2 proposes a microporous membrane comprising a polyolefin resin and inorganic particles, wherein the microporous membrane has a puncture strength and a film thickness retention rate in piercing creep adjusted to a specific range. ing.

JP 2008-218085 A International Publication No. 2008-035674 Pamphlet

However, the separator made of a polyolefin microporous film containing inorganic particles described in Patent Documents 1 and 2 has room for improvement in heat resistance in a state where both ends are constrained (for example, a separator in a prismatic battery). It is what you have.
In view of the above circumstances, the present invention provides a polyolefin microporous membrane containing inorganic particles, which has good heat resistance and exhibits good safety when used as a separator for a non-aqueous electrolyte battery. It is an object to provide a microporous membrane and a separator for a non-aqueous electrolyte battery including the same.

The present inventors paid attention to the melting behavior of the microporous membrane as a means for improving the heat resistance of the polyolefin microporous membrane containing inorganic particles.
That is, by including inorganic particles in the polyolefin microporous film, the mechanical strength and the heat resistance when the surface is fixed are seen, but the heat resistance when the sides are fixed is not necessarily sufficient.
As a result of intensive studies on the above problems, the present inventors have determined that the number of melting peaks observed by the first DSC in the polyolefin microporous membrane containing inorganic particles is P1N, and then the second time after cooling. When the number of melting peaks observed by DSC is P2N,
P1N> P2N
It is found that the inorganic particle-containing polyolefin microporous membrane to be excellent in heat resistance in a state where the sides are fixed, and further, it is found that safety as a separator for a nonaqueous electrolyte battery can be improved, and the present invention is completed. It came to.

That is, the present invention is as follows.
[1]
A microporous film containing a polyolefin resin and inorganic particles,
The content ratio of the inorganic particles is 10% by mass to 80% by mass with respect to the total weight of the microporous membrane,
In the DSC of the microporous membrane, there are two or more melting peak numbers (P1N) in the first measurement, and the melting peak number (P2N) in the second measurement is the melting peak number (P1N) in the first measurement. ) rather than less than,
An inorganic particle-containing polyolefin microporous membrane having a retention time of 240 seconds or longer in heat resistance evaluation by TMA .
[2]
The polyolefin resin is a microporous film containing polyethylene as a main component, and the DSC of the microporous film has two or more melting peak numbers (P1N) in the first measurement, and the melting peak temperature is 140 ° C. The inorganic microparticle-containing polyolefin microporous membrane according to the above [1], wherein the inorganic particle-containing polyolefin microporous membrane is present at a temperature below 140 ° C.
[3]
A separator for a non-aqueous electrolyte battery comprising the inorganic particle-containing polyolefin microporous membrane according to [1] or [2].

  The inorganic microparticle-containing polyolefin microporous membrane of the present invention exhibits excellent heat resistance when the sides are fixed, and can ensure good safety when used as a separator for a non-aqueous electrolyte battery.

The evaluation result of TMA in Example 1 is shown. The evaluation result of TMA in Example 2 is shown. The evaluation result of TMA in Example 3 is shown. The evaluation result of TMA in Example 4 is shown. The evaluation result of TMA in Example 5 is shown. The evaluation result of TMA in Example 6 is shown. The evaluation result of TMA in Example 7 is shown. The evaluation result of TMA in Example 8 is shown. The evaluation result of TMA in Comparative Example 1 is shown. The evaluation result of TMA in Comparative Example 2 is shown. The evaluation result of TMA in Comparative Example 3 is shown. The evaluation result of TMA in Comparative Example 4 is shown. The evaluation result of TMA in Comparative Example 5 is shown. The evaluation result of TMA in Comparative Example 6 is shown. The evaluation result of DSC in Example 1 is shown. The evaluation result of DSC in the comparative example 1 is shown.

  Hereinafter, a mode for carrying out the present invention (hereinafter abbreviated as “the present embodiment”) will be described in detail. The present invention is not limited to the following embodiment, and can be implemented with various modifications within the scope of the gist.

  The inorganic particle-containing polyolefin microporous membrane of the present embodiment is a microporous membrane containing a polyolefin resin and inorganic particles, and the content ratio of the inorganic particles is 10% by mass or more based on the total weight of the microporous membrane. 80% by mass or less, and in DSC of the microporous membrane, the number of melting peaks (P1N) in the first measurement is 2 or more, and the number of melting peaks (P2N) in the second measurement is the first measurement. (P1N> P2N) less than the number of melting peaks in (P1N). In the present embodiment, “DSC” refers to differential scanning calorimetry.

  The first DSC and the second DSC have the following meanings. In the first DSC, the crystal state (melting behavior) of the measurement piece can be measured. That is, when the microporous membrane is measured, the crystalline state of the polyolefin resin in the microporous membrane can be measured. Subsequently, the sample is cooled under a certain condition, and in the second DSC which is subsequently performed while the sample is unopened, it is possible to measure the crystalline state reflecting the characteristics of the polyolefin raw material constituting the microporous film. That is, in the first measurement, the crystal in the microporous film is once melted by heating to a temperature equal to or higher than the melting point. Subsequently, after the orientation is relaxed, the polyolefin resin forming the microporous film is recrystallized by cooling. In the second measurement, the recrystallized crystal state (the inherent melting point of the polyolefin raw material itself) can be measured.

  When the number of peaks observed in the first DSC is P1N and the number of peaks observed in the second DSC is P2N, P1N> P2N means that one kind of polyolefin resin has a plurality of crystalline states. And suggests that it constitutes a microporous membrane.

  When P1N> P2N, the heat resistance when the side of the microporous membrane is fixed is excellent. The microporous membrane (P1N> P2N) in which a single polyolefin has a plurality of crystal states is composed of one kind having a single melting point or a plurality of polyolefins having a single melting point (P1N = P2N, and P1N ≧ 1) Excellent heat resistance not found in microporous membranes. The reason is not clear, but when a single polyolefin has a plurality of crystal states, it is estimated that the plurality of crystal states are connected by the same molecular chain, and a finer structure is formed, thereby forming inorganic particles. This is thought to be due to the improved adhesion of the polyolefin resin. When exposed to a high temperature, the conventional polyolefin microporous membrane containing inorganic particles may be broken due to the interfacial delamination between the polyolefin and the inorganic particles due to the shrinkage of the mobility of the molecular chains. Even when the microporous membrane of this embodiment is exposed to a high temperature, the adhesion between the polyolefin and the inorganic particles is high. Is done.

  The means for adjusting P1N> P2N in the DSC of the microporous membrane can be achieved by controlling a plurality of influencing factors and setting the molding conditions within a specific range. For example, in the case of melt film formation using a polyolefin resin, inorganic particles, and a third substance (plasticizer extracted and removed in a subsequent process) as raw materials, (1) a melt is extruded from a T-die into a sheet and cooled by a roll The method of melt drawing at a specific magnification (control of the draw ratio), (2) the method of controlling the cooling rate of the melt, (3) the plasticizer ratio and the polyolefin ratio in the constituent materials Examples include a method of making the range, (4) a method of controlling the state of the polymer chain by controlling the ratio (Q / N) of the extrusion amount (Q) and the screw rotation speed (N) at the time of melt kneading, etc. Can be mentioned.

  The polyolefin resin is a microporous film containing polyethylene as a main component, and the DSC of the microporous film has two or more melting peak numbers (P1N) in the first measurement, and the melting peak temperature is It is preferred that one or more exist below 140 ° C and above 140 ° C. When P1N and melting peak temperature satisfy the above conditions, safety tends to be further improved. This is presumably because, at high temperatures, rapid stress relaxation by a polyethylene crystal component of less than 140 ° C. and strong adhesion with inorganic particles by a polyethylene crystal component of 140 ° C. or more can be achieved at the same time.

  The separator of the present embodiment is formed from a resin composition containing a polyolefin resin and inorganic particles. The polyolefin resin used in the present embodiment is not particularly limited. For example, a monomer such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene is polymerized. Examples include polymers obtained (homopolymers, copolymers, multistage polymers, etc.). These polymers can be used alone or in combination of two or more.

Examples of the polyolefin resin include low density polyethylene (density 0.910 to less than 0.930 g / cm ³ ), linear low density polyethylene (density 0.910 to less than 0.940 g / cm ³ ), and medium density. Polyethylene (density 0.930 to less than 0.942 g / cm ³ ), high density polyethylene (density 0.942 g / cm ³ or more), ultrahigh molecular weight polyethylene (density 0.910 to less than 0.970 g / cm ³ ), iso Examples include tactic polypropylene, atactic polypropylene, polybutene, and ethylene propylene rubber. These can be used alone or in combination of two or more.

  The polyolefin resin preferably contains polyethylene as a main component from the viewpoints of heat resistance, durability, stretchability, and the like, and particularly preferably contains high-density polyethylene. Here, “main component” means that the proportion of polyethylene in the polyolefin resin is preferably 30% by mass or more, more preferably 50% by mass or more, and further preferably 70% by mass or more. The upper limit of the polyethylene content is preferably 100% by mass or less, more preferably 80% by mass or less.

  In addition, the resin composition includes, as necessary, antioxidants such as phenols, phosphoruss, and sulfurs; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers, light stabilizers, and antistatic agents. Various known additives such as an agent, an antifogging agent, and a coloring pigment may be mixed.

  Viscosity average molecular weight of the polyolefin resin (measured according to the measurement method in Examples described later. In addition, when a plurality of types of polyolefin resins are used, it means a value measured for each polyolefin resin.) Is preferably 50,000 or more, more preferably 100,000 or more, and the upper limit is preferably 10 million or less, more preferably 3 million or less, and even more preferably 1 million or less. Setting the viscosity average molecular weight to 50,000 or more is from the viewpoint of maintaining high melt tension during melt molding and ensuring good moldability, or from the viewpoint of increasing the strength of the microporous film by imparting sufficient entanglement. preferable. On the other hand, setting the viscosity average molecular weight to 10 million or less is preferable from the viewpoint of achieving uniform melt-kneading and improving sheet formability, particularly thickness formability. Setting the viscosity average molecular weight to 1 million or less is preferable from the viewpoint of improving the thickness moldability.

  The inorganic particles are not particularly limited, and examples thereof include oxide ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, zinc oxide, iron oxide, silicon nitride, titanium nitride, boron nitride, and the like. Nitride ceramics, silicon carbide, calcium carbonate, aluminum sulfate, barium sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amicite, bentonite , Ceramics such as asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, silica sand, and glass fiber. These can be used alone or in combination of two or more. Among these, silica, alumina, titanium, and magnesia are more preferable from the viewpoint of electrochemical stability.

  The average primary particle size of the inorganic particles is preferably 1 nm or more, more preferably 6 nm or more, still more preferably 10 nm or more, and the upper limit is preferably 100 nm or less, more preferably 80 nm or less, still more preferably 60 nm or less. It is. When the average primary particle size of the inorganic particles is 100 nm or less, even when stretched or the like, there is a tendency that peeling between the polyolefin and the inorganic particles does not easily occur, the generation of macrovoids is reduced, and the pore size distribution is good. It is preferable from the viewpoint of controlling. Here, it is preferable that peeling between polyolefin and inorganic particles hardly occurs from the viewpoint of increasing the hardness of the fibril itself constituting the microporous film. On the other hand, setting the average primary particle size of the inorganic particles to 1 nm or more is preferable from the viewpoint of ensuring the dispersibility of the inorganic particles and increasing the hardness of the fibril itself constituting the microporous film.

  The proportion of the inorganic particles in the microporous membrane (the proportion of the inorganic particles with respect to the total weight of the microporous membrane) is 10% by mass or more, preferably 20% by mass or more, and the upper limit is 80% by mass or less. , Preferably it is 60 mass% or less. Setting the ratio to 10% by mass or more is preferable from the viewpoints of improving heat resistance, permeability, and strength when the sides are fixed. On the other hand, setting the ratio to 80% by mass or less is preferable from the viewpoint of improving heat resistance when the sides are fixed, and also preferable from the viewpoint of uniformly dispersing inorganic particles.

Although a plurality of methods for producing a microporous membrane containing a polyolefin resin and inorganic particles, and in particular, a polyolefin microporous membrane containing inorganic particles suitable as a separator, are disclosed in the following (A) and (B) 2 It is roughly divided into species.
(A): Melt film formation using polyolefin and inorganic particles as raw materials.
(B): Melt film formation using polyolefin, inorganic particles, and a third substance (substance extracted and removed in a later step) as raw materials.

Further, the above (A) and (B) are further subdivided as follows by further combining stretching.
(A) -a: A sheet is formed after melt-kneading polyolefin and inorganic particles. Porous by stretching.
(B) -a: After melt-kneading polyolefin, inorganic particles, and the third substance, a sheet is formed. It is made porous by extracting the third substance (all or part of it).
(B) -b: After kneading and kneading polyolefin, inorganic particles, and the third substance, forming a sheet. It is made porous by extracting the third substance (all or part of it). Next, the pore structure is controlled by stretching.
(B) -c: Polyolefin, inorganic particles, and a third substance are melt-kneaded and then sheeted. Controls the morphology of polyolefin, inorganic particles and third material by stretching. Next, the third substance is extracted (all or part of it remains) to make it porous.

  As a manufacturing method of the separator of the present embodiment, the method (B) is preferable from the viewpoint of controlling P1N> P2N and from the viewpoint of increasing the strength.

As a more specific manufacturing method of the inorganic particle-containing polyolefin microporous membrane in the present embodiment, for example, a manufacturing method including the following steps (1) to (4) can be used.
(1) A polyolefin resin composition containing a polyolefin resin and inorganic particles and a plasticizer in a predetermined ratio, a predetermined Q / N ratio (Q: extrusion amount [kg / hr], N: screw rotation speed [rpm]) Melting and kneading with,
(2) After the kneading step, the kneaded product is formed into a sheet shape, melt-drawn at a predetermined draw ratio, cooled and processed into a sheet-like molded body,
(3) After the forming step, the sheet-like formed body is biaxially stretched at a surface magnification of 20 to 200 times to form a stretched product,
(4) A porous body forming step of forming a porous body by extracting a plasticizer from the stretched product.

  The plasticizer used in the step (1) is preferably a non-volatile solvent capable of forming a uniform solution at a temperature equal to or higher than the melting point of the polyolefin resin when mixed with the polyolefin resin. Moreover, it is preferable that it is a liquid at normal temperature.

  Examples of the plasticizer include hydrocarbons such as liquid paraffin and paraffin wax; esters such as diethyl hexyl phthalate and dibutyl phthalate; higher alcohols such as oleyl alcohol and stearyl alcohol.

  The proportion of the plasticizer in the kneaded product is preferably more than 60% by mass, more preferably 65% by mass or more, and the upper limit is preferably 80% by mass or less. It is preferable from the viewpoint of forming a plurality of crystal structures that the ratio exceeds 60 mass% and is 80 mass% or less. Furthermore, it is preferable to make the said ratio 80 mass% or less from a viewpoint of ensuring sheet | seat moldability.

  The melt kneading is preferably performed using a twin screw. The Q / N ratio (Q: extrusion amount [kg / hr], N: screw rotation speed [rpm]) is preferably 0.3 or more, and more preferably 0.5 or more. As an upper limit, Preferably it is 1.5 or less, More preferably, it is 1.2 or less. When the Q / N ratio is 0.3 or more, the polymer molecular chain is not broken and the polymer can be uniformly dissolved in the plasticizer, so that a higher-strength microporous film tends to be obtained. Yes, when it is 1.5 or less, it becomes possible to unravel the polymer molecular chains and give sufficient shearing force to highly disperse the inorganic particles. It tends to be easy to obtain a crystal structure.

  The temperature of the melt-kneading part is preferably less than 200 ° C. from the viewpoint of dispersibility of the inorganic particles. The lower limit of the temperature is equal to or higher than the melting point of the polyolefin from the viewpoint of uniformly dissolving the polyolefin resin in the plasticizer.

  It is preferable to set the set temperature in the step of forming the melt into a sheet from the melt kneading step onwards higher than the set temperature of the extruder. The upper limit of the set temperature is preferably 300 ° C. or less and more preferably 260 ° C. or less from the viewpoint of thermal degradation of the polyolefin resin. For example, when manufacturing a sheet-like molded body continuously from an extruder, after the melt-kneading step, the step of forming into a sheet shape, that is, the path from the extruder outlet to the T die, and the set temperature of the T die are extruded. When the temperature is set higher than the set temperature of the process, it is preferable because the inorganic particles finely dispersed in the melt-kneading process can be formed into a sheet shape without re-aggregation. In particular, when inorganic particles having a small particle diameter are used, the effect of suppressing aggregation is large.

  The step (2) is, for example, a step of extruding the kneaded material into a sheet shape via a T-die or the like, sandwiching between rolls to create a thickness-controlled sheet, and contacting the heat conductor to cool and solidify the sheet. It is. As the heat conductor, metal, water, air, plasticizer itself, or the like can be used. In the idle running part from the T-die to the sheet shape and sandwiched between the rolls, the crystalline state can be controlled by cooling after being melt-stretched at a predetermined draw ratio. When the width of the T die is A and the width of the sheet sandwiched between rolls is B, the roll speed is controlled in the range of 0.98> B / A> 0.75, so that a single polyolefin resin is obtained. Therefore, a microporous film having a plurality of crystal states (P1N> P2N) is easily obtained.

  As the stretching method in the step (3), for example, any method such as simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple-time stretching may be used alone or in combination, but the piercing strength is increased. From the viewpoint of uniform film thickness, simultaneous biaxial stretching is preferred. Here, the simultaneous biaxial stretching means stretching in the length direction of the film (machine direction (MD), hereinafter sometimes referred to as “MD direction”) and the width direction of the film (direction perpendicular to the machine direction (TD). In the following, it may be described as “TD direction”)), and the deformation rate in each direction may be different. Sequential biaxial stretching is a method in which stretching in the MD direction or TD direction is performed independently. When stretching is performed in the MD direction or TD direction, the other direction is in an unconstrained state or a constant direction. It is in a state of being fixed to the length. The draw ratio is preferably in the range of 20 times to less than 200 times in terms of the total area ratio, more preferably 20 times to 100 times, and even more preferably 25 times to 50 times. When the total area magnification is 20 times or more, it tends to be high strength, and when it is less than 200 times, thermal shrinkage tends to be reduced.

  The stretching ratio in each axial direction is preferably 4 to 10 times in the MD direction, preferably 4 to 10 times in the TD direction, 5 to 8 times in the MD direction, and 5 to 8 times in the TD direction. The range of is more preferable. If the draw ratio is 4 times or more in the MD direction and 4 times or more in the TD direction, products having small film thickness unevenness tend to be obtained in both the MD direction and the TD direction. There is a tendency to be excellent in roundness.

  The stretching temperature is preferably −50 ° C. or higher and lower than the melting point of polyolefin, more preferably −30 ° C. or higher and −2 ° C. or lower, and more preferably −15 ° C. or higher and −3 ° C. or lower. When the stretching temperature is within the above range, a microporous membrane having higher strength and higher withstand voltage tends to be obtained.

  Examples of the step (4) include a method of immersing the stretched product in a solvent for the plasticizer. The residual amount of plasticizer in the microporous membrane after extraction is preferably less than 1% by mass. Further, the amount of the inorganic particles extracted by this step is preferably 1% by mass or less, more preferably substantially 0% by mass, based on the blending amount in the microporous membrane.

  Stretching can be performed before or after extraction of the plasticizer, but it is preferable to further stretch after extraction of the plasticizer from the viewpoint of controlling the pore diameter. Furthermore, it is preferable to add a heat treatment step such as heat fixation and heat relaxation subsequent to the stretching step from the viewpoint of further suppressing the shrinkage of the microporous membrane.

  In addition, the microporous membrane may be subjected to post-treatment such as a hydrophilic treatment with a surfactant or the like, or a cross-linking treatment with ionizing radiation or the like as long as the effects of the present invention are not impaired.

  The film thickness of the polyolefin microporous membrane in the present embodiment is preferably 2 μm or more, more preferably 5 μm or more, and the upper limit is preferably less than 100 μm, more preferably less than 60 μm, and still more preferably less than 40 μm. Setting the film thickness to 2 μm or more is preferable from the viewpoint of improving the mechanical strength and the withstanding voltage. On the other hand, when the film thickness is less than 100 μm, the occupied volume of the polyolefin microporous film is reduced, which tends to be advantageous in terms of increasing the battery capacity. In addition, the film thickness of a polyolefin microporous film is measured according to the measuring method in the Example mentioned later.

  The film thickness can be adjusted by adjusting the sheet thickness in the step (2), the stretching ratio in the step (3), the stretching temperature, and the like.

  The porosity of the polyolefin microporous membrane in the present embodiment is preferably 45% or more, more preferably 50% or more, and the upper limit is preferably less than 80%, more preferably 75% or less. Setting the porosity to 45% or more is preferable from the viewpoint of securing good output characteristics. On the other hand, it is preferable to make it less than 80% from the viewpoint of securing the puncture strength and the viewpoint of securing the withstand voltage. The porosity of the polyolefin microporous membrane is measured according to the measurement method in the examples described later.

  The porosity can be adjusted by adjusting the ratio of polyolefin resin / inorganic particles / plasticizer in the step (1), the stretching temperature in the step (3), and the stretching ratio.

  The air permeability of the polyolefin microporous membrane in the present embodiment is preferably 10 seconds or more, more preferably 40 seconds or more, and the upper limit is preferably 500 seconds or less, more preferably 300 seconds or less, and still more preferably. 200 seconds or less. Setting the air permeability to 10 seconds or more is preferable from the viewpoint of suppressing self-discharge of the battery. On the other hand, setting it to 500 seconds or less is suitable from the viewpoint of obtaining good charge / discharge characteristics. The air permeability of the polyolefin microporous membrane is measured according to the measurement method in the examples described later.

  The air permeability can be adjusted by adjusting the ratio of polyolefin resin / inorganic particles / plasticizer in the step (1), the stretching temperature in the step (3), and the stretching ratio.

  The polyolefin microporous membrane in the present embodiment exhibits good heat resistance characteristics, and can be particularly suitably used as a separator for non-aqueous electrolyte batteries.

  The non-aqueous electrolyte battery in the present embodiment includes a separator for a non-aqueous electrolyte battery including the above-described polyolefin microporous membrane, a positive electrode plate, a negative electrode plate, a non-aqueous electrolyte (a non-aqueous solvent and a solution thereof). A metal salt). Specifically, for example, a positive electrode plate containing a transition metal oxide capable of occluding and releasing lithium ions and a negative electrode plate capable of occluding and releasing lithium ions and the like are wound with a separator interposed therebetween. Or it is laminated | stacked, the nonaqueous electrolyte solution is hold | maintained and it accommodates in the container.

  The positive electrode plate will be described below. As the positive electrode active material, for example, lithium composite metal oxide such as lithium nickelate, lithium manganate or lithium cobaltate, lithium composite metal phosphate such as lithium iron phosphate, or the like can be used. The positive electrode active material is kneaded with a conductive agent and a binder, coated and dried as a positive electrode paste on a positive electrode current collector such as an aluminum foil, rolled to a predetermined thickness, and then cut into a predetermined dimension to form a positive electrode plate. Here, as the conductive agent, a metal powder that is stable under a positive electrode potential, for example, carbon black such as acetylene black or a graphite material can be used. As the binder, a material that is stable under a positive electrode potential, such as polyvinylidene fluoride, modified acrylic rubber, or polytetrafluoroethylene, can be used.

  The negative electrode plate will be described below. As the negative electrode active material, a material capable of occluding lithium can be used. Specifically, for example, at least one selected from the group consisting of graphite, silicide, titanium alloy material, and the like can be used. Moreover, as a negative electrode active material of a nonaqueous electrolyte secondary battery, a metal, a metal fiber, a carbon material, an oxide, a nitride, a silicon compound, a tin compound, or various alloy materials can be used, for example. In particular, silicon (Si) or tin (Sn), or a silicon compound or tin compound such as an alloy, a compound, or a solid solution is preferable because the capacity density of the battery tends to increase. Examples of the carbon material include various natural graphite, coke, graphitized carbon, carbon fiber, spherical carbon, various artificial graphite, and amorphous carbon. 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, coated and dried as a negative electrode paste on a negative electrode current collector such as a copper foil, rolled to a predetermined thickness, and then cut to a predetermined dimension to form a negative electrode plate. Here, as the binder, a material that is stable under a negative electrode potential, such as PVDF or a styrene-butadiene rubber copolymer, can be used.

The nonaqueous electrolytic solution 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. As the non-aqueous solvent, a cyclic carbonate, a chain carbonate, a cyclic carboxylic acid ester or the like is used. Examples of the lithium salt include 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 , and lower aliphatic carboxylic acid. Examples include lithium acid, LiCl, LiBr, LiI, borates, imide salts, and the like.
In addition, about the measuring method of the various parameters mentioned above, unless there is particular notice, it measures according to the measuring method in the Example mentioned later.

  Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist. In addition, the physical property in an Example was measured with the following method.

(1) Viscosity average molecular weight (Mv)
In order to prevent deterioration of the sample, 2,6-di-t-butyl-4-methylphenol is dissolved in decahydronaphthalene to a concentration of 0.1% by mass, and this (hereinafter abbreviated as DHN) is used as a sample solvent. Using.
The sample was dissolved in DHN at 150 ° C. to a concentration of 0.1% by mass to prepare a sample solution. 10 mL of the prepared sample solution was collected, and the number of seconds passing through the marked line (t) at 135 ° C. was measured with a Canon Fenceke viscometer (SO100). When inorganic particles contained in the sample, a solution in which the sample was dissolved in DHN was filtered to remove the inorganic particles. When inorganic particles can be dissolved and removed, a sample in which inorganic particles are dissolved and removed in advance may be used. Further, after heating the DHN to 0.99 ° C., and 10mL taken, the number of seconds passing between marked lines viscometer (t _B) was measured in the same manner. The resulting passage seconds t, was calculated intrinsic viscosity [eta] by the following conversion equation using the t _B.
[Η] = ((1.651 t / t _B −0.651) ^0.5 −1) /0.0834
Mv was calculated from the obtained [η] by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67

(2) Average primary particle size The average primary particle size of the inorganic particles was measured with a scanning electron microscope. Specifically, a 10 μm × 10 μm field-of-view image magnified by a scanning electron microscope (SEM) is directly or after being printed on a photo from a negative, is read into an image analyzer, and is calculated from the circle equivalent diameter of each particle The number average value of the diameters of the circles having the same area was defined as the average primary particle diameter of the inorganic particles. However, if the staining boundary is unclear when inputting from the photograph to the image analysis apparatus, the photograph was traced and input to the image analysis apparatus using this figure.

(3) Film thickness (μm)
The measurement was performed at a room temperature of 23 ° C. using a micro thickness measuring instrument manufactured by Toyo Seiki, KBM (trademark).

(4) Porosity (%)
A 10 cm × 10 cm square sample was cut from the polyolefin microporous membrane, its volume (cm ³ ) and mass (g) were determined, and calculated from these and density (g / cm ³ ) using the following equation.
Porosity (%) = (volume-mass / density of mixed composition) / volume × 100
In addition, the value calculated | required by calculating from the density and mixing ratio of each used polyolefin resin and an inorganic particle was used for the density of a mixed composition.

(5) Air permeability (sec)
Based on JIS P-8117, it measured with the Gurley type air permeability meter by the Toyo Seiki Co., Ltd. and G-B2 (trademark).

(6) Puncture strength (N)
Using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech, a polyolefin microporous membrane was fixed with a sample holder having a diameter of 11.3 mm at the opening. Next, the puncture strength (N) is obtained as the maximum puncture load by conducting a puncture test on the fixed microporous membrane at a needle radius of curvature of 0.5 mm and a puncture speed of 2 mm / sec in a 25 ° C. atmosphere. )

(7) DSC (Differential Scanning Calorimetry)
It measured using DSC60 by Shimadzu Corporation. A sample was punched into a circle with a diameter of 5 mm, and several sheets were stacked to give 3 mg, which was used as a measurement sample. This was spread on an aluminum open sample pan having a diameter of 5 mm, and a clamping cover was placed thereon and fixed in the aluminum pan with a sample sealer.
First measurement Under a nitrogen atmosphere, the temperature was increased from 30 ° C. to 200 ° C. at a rate of temperature increase of 10 ° C./min to obtain a melting endotherm curve. The number of peaks of the obtained melting endotherm curve was P1N, and the peak top temperature was the melting point (° C.).
Subsequently, the sample was held at 200 ° C. for 5 minutes, cooled from 200 ° C. to 30 ° C. at 10 ° C./min, and held at 30 ° C. for 5 minutes.
Second Measurement Following the first measurement, measurement was again performed from 30 ° C. to 200 ° C. at a rate of temperature increase of 10 ° C./min to obtain a melting endotherm curve. The number of peaks of the obtained melting endotherm curve was P2N, and the peak top temperature was the melting point (° C.).

(8) MD heat shrinkage The separator was cut to 100 mm in the MD direction and 100 mm in the TD direction, and left in an oven at 150 ° C. for 1 hour. At this time, the sample was sandwiched between two sheets of paper so that the hot air would not directly hit the sample. After taking out the sample from the oven and cooling, the length (mm) was measured, and the MD thermal shrinkage was calculated by the following formula.
MD thermal shrinkage (%) = (100−MD length after heating) / 100 × 100

(9) Tensile strength Based on JIS K7127, it measured about MD and TD sample (shape; width 10mm x length 100mm) using the Shimadzu Corporation tensile tester, Autograph AG-A type (trademark). Moreover, the sample used the thing which stuck the cellophane tape (the Nitto Denko Packaging System Co., Ltd. make, brand name: N.29) to the single side | surface of the both ends (each 25mm) of the sample between chuck | zippers 50mm. Furthermore, in order to prevent sample slipping during the test, 1 mm-thick fluororubber was affixed inside the chuck of the tensile tester. The tensile strength (kg / cm ² ) was determined by dividing the strength at break by the cross-sectional area of the sample before the test. The measurement was performed at a temperature of 23 ± 2 ° C., a chuck pressure of 0.30 MPa, and a tensile speed of 200 mm / min.

(10) Evaluation of heat resistance (TMA (Thermomechanical Analysis): Thermomechanical Analysis)
Measurement was performed using Shimadzu Corporation TMA50 (trademark). A sample cut to about 15 mm in the MD direction and 3 mm in the TD direction was fixed to the chuck so that the distance between the chucks (MD direction) was 10 mm, and set on a dedicated probe. The initial load was 0.0098 N (1.0 gf), and the probe was heated from 30 ° C. to 10 ° C./min in the constant length mode to 150 ° C., and then held at 150 ° C. for 10 minutes. The time until the shrinkage stress falls below 0.0098 N (1.0 gf) after reaching 150 ° C. was defined as the holding time. The time when the shrinkage stress was less than 0.0098N (1.0 gf) was regarded as breakage, and the holding time was used as an index for heat resistance evaluation.

(11) Battery evaluation (safety evaluation)
a. Battery Preparation (a-1) Positive Electrode Plate Preparation 92.2% by weight of lithium cobalt composite oxide LiCoO ₂ as an active material, 2.3% by weight of flake graphite and acetylene black as a conductive agent, and polyvinylidene fluoride as a binder A slurry was prepared by dispersing 3.2% by weight of (PVDF) in N-methylpyrrolidone (NMP). This slurry was applied to one side of a 20 μm thick aluminum foil serving as a positive electrode current collector with a die coater, dried at 130 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material application amount of the positive electrode was 250 g / m ² , and the active material bulk density was 3.00 g / cm ³ . This was cut into a width of about 40 mm to form a strip.
(A-2) Preparation of negative electrode plate 96.9% by weight of artificial graphite as an active material, 1.4% by weight of ammonium salt of carboxymethyl cellulose and 1.7% by weight of styrene-butadiene copolymer latex as dispersed in purified water. A slurry was prepared. This slurry was applied to one side of a 12 μm thick copper foil serving as a negative electrode current collector with a die coater, dried at 120 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material application amount of the negative electrode was set to 106 g / m ² , and the active material bulk density was set to 1.35 g / cm ³ . This was cut into a width of about 40 mm to form a strip.
(A-3) Preparation of non-aqueous electrolyte As a non-aqueous electrolyte, a mixed solvent of ethylene carbonate: ethyl methyl carbonate: dimethyl carbonate = 1: 1: 1 (volume ratio) and LiPF ₆ as a solute at a concentration of 1.0 mol / It was prepared by dissolving to liter.
(A-4) Winding / Assembly The electrode plate laminate is obtained by winding the microporous membrane separator, the strip-like positive electrode, and the strip-like negative electrode in the order of the strip-like negative electrode, the separator, the strip-like positive electrode, and the separator in a spiral manner. Was made.
After pressing this electrode plate laminate into a flat plate, it is housed in an aluminum container, the aluminum lead led out from the positive electrode current collector is used as the container wall, and the nickel lead derived from the negative electrode current collector is used as the container lid terminal part. Connected. Further, the non-aqueous electrolyte described above was poured into this container and sealed. The prismatic lithium ion battery thus manufactured was designed to have a vertical (thickness) of 6.3 mm, a horizontal of 30 mm, and a height of 48 mm, and a nominal discharge capacity of 620 mAh.

b. Capacity measurement (mAh)
The lithium ion battery assembled as described above was charged with a constant current and constant voltage (CCCV) for 6 hours under the conditions of a current value of 310 mA (0.5 C) and a final battery voltage of 4.2 V. At this time, the current value immediately before the end of charging was almost zero. Then, it was left (aged) for 1 week in an atmosphere at 25 ° C.
Next, it is charged with a constant current constant voltage (CCCV) for 3 hours under the conditions of a current value of 620 mA (1.0 C) and a termination battery voltage of 4.2 V, and discharged to a battery voltage of 3.0 V at a constant current value (CC) of 620 mA. I went through the cycle. The discharge capacity at this time was defined as the initial discharge capacity. A battery having an initial discharge capacity within ± 10 mA was used for safety evaluation.

c. Safety evaluation (nail penetration test)
For a battery charged with a constant current and constant voltage (CCCV) for 3 hours under the conditions of a current value of 620 mA (1.0 C) and a final battery voltage of 4.2 V, an iron round nail having a diameter of 2.7 mm is placed at 20 ° C. Under the environment, it penetrated at a speed of 5 mm / sec, and the heat generation state at that time was observed. The temperature after 1 second and the temperature after 60 seconds on the surface of the battery in the vicinity of the battery penetration location were measured. The case where the temperature after 1 second was less than 100 ° C was regarded as acceptable, and the case where the temperature was 100 ° C or higher was regarded as unacceptable (x). Furthermore, in the case of passing, the case where the battery surface temperature after 60 seconds was less than 100 ° C. was judged as suitable (◎), and the case where it was 100 ° C. or more was judged as suitable (◯).

[Example 1]
13.4 parts by mass of high-density polyethylene “SH800” (made by Asahi Kasei Chemicals Co., Ltd.) having a viscosity average molecular weight (Mv) of 270,000 and 5 ultra-high molecular weight polyethylene “UH850” (made by Asahi Kasei Chemicals Co., Ltd.) having an Mv of 2 million 12.8 parts by mass of silica “RX200” (manufactured by Nippon Aerosil Co., Ltd.) having an average primary particle size of 12 nm and liquid paraffin “Smoyl P-350P” as a plasticizer (Matsumura Oil Research Co., Ltd.) 15.4 parts by mass) and 0.1 part by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant Was premixed with a super mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. Further, the liquid paraffin was side-fed to a twin screw extruder cylinder so that the liquid paraffin content ratio in the total mixture (100 parts by mass) to be melt kneaded and extruded was 68 parts by mass. The melt-kneading conditions were performed with a screw rotation speed of 50 rpm and an extrusion rate of 30 kg / h. The Q / N ratio at this time was 0.6. The set temperature was 160 ° C. for the kneading section and 230 ° C. for the T die. Subsequently, the melt-kneaded product was extruded into a sheet form from a T-die and cooled with a cooling roll controlled at a surface temperature of 70 ° C. to obtain a sheet-shaped polyolefin resin composition having a thickness of 1800 μm. At this time, the speed of the cooling roll was adjusted so that the ratio of the width A [mm] of the T die to the sheet width B [mm] after cooling, that is, B / A was 0.8. Next, it was continuously led to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the longitudinal direction and 6.4 times in the transverse direction. The stretching setting temperature at this time was 122 ° C. Next, it was led to a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried. Further, after being led to a transverse tenter and stretched by 1.84 times in the transverse direction, the final exit is set to a relaxation rate of 10% so as to be 1.65 times (relaxation rate = (1.84-1.65) /1.84× 100 = 10%), and the inorganic-containing polyolefin microporous film was wound. The set temperature of the transverse stretch portion was 137 ° C., and the set temperature of the relaxation portion was 144 ° C. The film forming conditions are shown in Table 1, and the characteristics of the microporous film are shown in Table 2. Moreover, the evaluation result of TMA is shown in FIG. 1, and the evaluation result of DSC is shown in FIG. Further, the battery evaluation results are shown in Table 2. The obtained microporous membrane was calcined at 600 ° C. for 30 minutes, and the amount of silica was calculated from the remaining weight. As a result, it was 39.8% by mass, and the blended silica remained almost unextracted. Other Examples and Comparative Examples also showed values of less than ± 1% compared with the silica content calculated from the raw material composition ratio.

[Example 2] to [Example 8]
The film forming conditions were changed as shown in Table 1, and a microporous film was obtained by forming a film in the same manner as in Example 1 except for the conditions described in Table 1.
In Example 4, 13.0 parts by mass of high-density polyethylene “SH800” having an Mv of 270,000, and 5.5 parts by mass of ultrahigh molecular weight polyethylene “UH850” having an Mv of 2 million are used. Silica “RX200” (manufactured by Nippon Aerosil Co., Ltd.) ) Was changed to 18.5 parts by mass, and a film was produced in the same manner as in Example 1 except that the conditions described in Table 1 were obtained. Thus, a microporous membrane was obtained.
In Example 6, 10.8 parts by mass of high-density polyethylene “SH800” of Mv 270,000, ultra high molecular weight polyethylene “UH850” of Mv 2 million in 4.6 parts by mass, random polypropylene “B221WA” (Prime Polymer Co., Ltd.) The product was changed to 3.8 parts by mass, and a film was formed in the same manner as in Example 1 except for the conditions described in Table 1, thereby obtaining a microporous membrane.
In Example 7, Mv 270,000 high density polyethylene “SH800” is 23.0 parts by mass, Mv 2 million ultra high molecular weight polyethylene “UH850” is 5.8 parts by mass, silica “RX200” (manufactured by Nippon Aerosil Co., Ltd.) ) Was changed to 3.2 parts by mass, and a microporous membrane was obtained by forming a membrane in the same manner as in Example 1 except for the conditions described in Table 1.
In Example 8, Mv 2 million ultra high molecular weight polyethylene “UH850” was changed to 6.4 parts by mass and silica “RX200” (manufactured by Nippon Aerosil Co., Ltd.) was changed to 25.6 parts by mass. A microporous membrane was obtained by forming the membrane in the same manner as in Example 1 except for the above conditions.
Table 2 shows the characteristics of the microporous membrane. TMA evaluation results are shown in FIGS. Further, the battery evaluation results are shown in Table 2.

[Comparative Example 1]
16.8 parts by mass of high-density polyethylene “SH800” (manufactured by Asahi Kasei Chemicals Co., Ltd.) having a viscosity average molecular weight (Mv) of 270,000, and ultrahigh molecular weight polyethylene “UH850” (manufactured by Asahi Kasei Chemicals Co., Ltd.) having an Mv of 2 million are 7 .16 parts by mass of silica “RX200” (manufactured by Nippon Aerosil Co., Ltd.) having an average primary particle size of 12 nm and liquid paraffin “Smoyl P-350P” (made by Matsumura Oil Research Co., Ltd.) as a plasticizer 19.2 parts by mass and 0.1% by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant is super Premixed with a mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. Further, the liquid paraffin was side-fed to the twin screw extruder cylinder so that the liquid paraffin content ratio in the total mixture (100 parts by mass) to be melt kneaded and extruded was 60 parts by mass. The melt-kneading conditions were as follows: screw rotation speed 80 rpm, extrusion rate 18 kg / h. The Q / N ratio at this time was 0.23. The set temperature was 200 ° C. for the kneading part and 220 ° C. for the T die. Subsequently, the melt-kneaded product was extruded into a sheet form from a T-die, and cooled with a cooling roll controlled to a surface temperature of 40 ° C. to obtain a sheet-shaped polyolefin resin composition having a thickness of 1800 μm. At this time, the speed of the cooling roll was adjusted so that the ratio of the width A [mm] of the T die to the sheet width B [mm] after cooling, that is, B / A was 0.74. Next, it was continuously led to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the longitudinal direction and 6.4 times in the transverse direction. The stretching setting temperature at this time was 122 ° C. Next, it was led to a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried. Further, after being led to a transverse tenter and stretched by 1.84 times in the transverse direction, the final exit is set to a relaxation rate of 10% so as to be 1.65 times (relaxation rate = (1.84-1.65) /1.84× 100 = 10%), and the inorganic-containing polyolefin microporous film was wound. The set temperature of the transverse stretch portion was 137 ° C., and the set temperature of the relaxation portion was 139 ° C. Table 3 shows the film forming conditions, and Table 4 shows the characteristics of the microporous film. FIG. 9 shows the TMA evaluation results, and FIG. 16 shows the DSC evaluation results. Further, the battery evaluation results are shown in Table 4.

[Comparative Example 2] to [Comparative Example 6]
The film forming conditions were changed as shown in Table 3, and a microporous film was obtained by forming a film in the same manner as in Comparative Example 1 except for the conditions described in Table 3.
In Comparative Example 3, high-density polyethylene “SH800” with Mv of 270,000 was changed to 36.0 parts by mass and ultrahigh molecular weight polyethylene “UH850” with Mv of 2 million was changed to 9.0 parts by mass, and silica “RX200” (Nippon Aerosil Co., Ltd.) The microporous membrane was obtained by carrying out film formation in the same manner as in the conditions described in Table 3 and Comparative Example 1 except that the product ()) was not used.
In Comparative Example 4, 13.4 parts by mass of high-density polyethylene “SH800” having an Mv of 270,000, 5.8 parts by mass of ultrahigh molecular weight polyethylene “UH850” having an Mv of 2 million, and random polypropylene “B221WA” (Prime Polymer Co., Ltd.) The product was changed to 4.8 parts by mass, and a film was formed in the same manner as in Comparative Example 1 except for the conditions described in Table 3, to obtain a microporous membrane.
In Comparative Example 5, high-density polyethylene “SH800” with Mv 270,000 was changed to 24.3 parts by mass, and ultrahigh molecular weight polyethylene “UH850” with Mv 2 million was changed to 6.1 parts by mass, and silica “RX200” (Nippon Aerosil Co., Ltd.) )) Was changed to 1.6 parts by mass, and a film was produced in the same manner as in Comparative Example 1 except that the conditions described in Table 3 were obtained. Thus, a microporous membrane was obtained.
In Comparative Example 6, Mv 2 million ultra high molecular weight polyethylene “UH850” was changed to 5.6 parts by mass, silica “RX200” (manufactured by Nippon Aerosil Co., Ltd.) was changed to 34.4 parts by mass, and Table 3 A microporous membrane was obtained by forming the membrane in the same manner as in Comparative Example 1 except for the described conditions.
Table 4 shows the characteristics of the microporous membrane. TMA evaluation results are shown in FIGS. Further, the battery evaluation results are shown in Table 4.

  As is clear from the results in Table 2, the microporous membranes of Examples 1 to 8 in which the number of peaks obtained by DSC is P1N> P2N is 240 seconds or more in the heat resistance evaluation in TMA, whereas the comparative example It was 90 seconds or less for the microporous membranes 1 to 6, and the microporous membrane of the present embodiment clearly showed good heat resistance. Furthermore, when the microporous membranes of Examples 1 to 8 were used as battery separators for nonaqueous electrolyte solutions, it was revealed that good results were shown in safety evaluation.

  The microporous membrane of the present invention exhibits good heat resistance characteristics, and when used as a separator for a non-aqueous electrolyte battery, good safety can be ensured.

Claims (3)

A microporous film containing a polyolefin resin and inorganic particles,
The content ratio of the inorganic particles is 10% by mass to 80% by mass with respect to the total weight of the microporous membrane,
In the DSC of the microporous membrane, there are two or more melting peak numbers (P1N) in the first measurement, and the melting peak number (P2N) in the second measurement is the melting peak number (P1N) in the first measurement. ) rather than less than,
An inorganic particle-containing polyolefin microporous membrane having a retention time of 240 seconds or longer in heat resistance evaluation by TMA .   The polyolefin resin is a microporous film containing polyethylene as a main component, and the DSC of the microporous film has two or more melting peak numbers (P1N) in the first measurement, and the melting peak temperature is 140 ° C. The inorganic microparticle-containing polyolefin microporous membrane according to claim 1, wherein the inorganic particle-containing polyolefin microporous membrane is present at a temperature below 140 ° C.   A separator for a non-aqueous electrolyte battery comprising the inorganic particle-containing polyolefin microporous membrane according to claim 1 or 2.