JP2021116316A - Polyolefin microporous film - Google Patents

Abstract

To provide a polyolefin microporous film capable of securing cycle characteristics of an electrochemical device (excellent characteristics in a thermal cold circulation test) when used as a separator for the electrochemical device.SOLUTION: The sum of the MD maximum stress and TD maximum stress of the constant length thermomechanical analysis (TMA) is 0.10 N/mm2 or larger and smaller than 1.20 N/mm2, the average film thickness is 3.0 μm or larger and smaller than 18.0 μm, and polypropylene is contained in the range of 1.0 mass% to 20.0 mass%, and the film thickness conversion permeability is 1.0 s/(100 cm3 μm) or larger and 10.0 s/(100 cm3 μm) or smaller.SELECTED DRAWING: None

Description

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

Polyolefin microporous membranes are used in battery separators, condenser separators, fuel cell materials, microfiltration membranes, etc., and are particularly used as lithium ion secondary battery (LIB) separators or constituent materials thereof. The separator prevents direct contact between the positive and negative electrodes, and also allows ions to permeate through the electrolytic solution held in the microporous structure.

In recent years, LIB has been applied not only to small electronic devices such as mobile phones and notebook computers, but also to electric vehicles such as electric vehicles and small electric motorcycles. Since the size of the in-vehicle LIB and the capacity per single cell tend to be large in order to extend the cruising range, a battery having a large capacity per volume has been developed. Therefore, a polyolefin microporous membrane that can be used as a separator for LIB has been studied (Patent Documents 1 to 4).

In Patent Document 1, paying attention to the fact that the film is easily broken when a foreign substance is mixed into the microporous polyolefin film, it is studied to prevent the concentration of local current and thereby suppress the thermal runaway of the battery. According to Patent Document 1, the thickness of the polyolefin microporous membrane may be 1 μm or more and 100 μm or less, and the stress peak values in the width direction (TD) and the longitudinal direction (MD) in thermomechanical analysis (TMA) are 1.6 g, respectively. It is described that the following and can be 2.0 g or less, and the polypropylene content may be 1% to 40% by weight.

Patent Document 2 studies a polyolefin microporous membrane having dimensional stability in the temperature range of 100 ° C. to 120 ° C. and rapidly closing pores near the melting point of polyolefin. Patent Document 2 describes that the film thickness of the microporous polyolefin membrane can be 0.1 μm or more and 100 μm or less, and a mixture of polyethylene and polypropylene may be used as the polyolefin raw material from the viewpoint of heat resistance. ing.

Patent Document 3 describes polyethylene raw materials and plasticizers having a weight average molecular weight (Mw) of 1,000,000 or more, particularly in relation to separators for LIB used in notebook personal computers, mobile phones, digital cameras, and the like. It is described that the heat absorption ratio of the above is adjusted to produce a microporous film having an excellent balance between productivity, permeability and strength. However, the microporous membrane produced by the method described in Patent Document 3 has been poorly examined for TMA behavior.

Patent Document 4 studies a separator for a high output density lithium ion secondary battery, which can realize a lithium ion secondary battery in which self-discharge is suppressed and has excellent high rate characteristics, and also has good homogeneity. , And 1% by mass to 80% by mass is described as the blend amount of polypropylene in the polyolefin microporous film.

International Publication No. 2019/045077 International Publication No. 2018/179810 International Publication No. 2016/194962 International Publication No. 2009/136648

In recent years, attention has been paid to the installation of lithium ion secondary batteries (LIBs) in electric vehicles. Large-sized, high-capacity LiB used in electric vehicles needs to ensure safety in the event of abnormal heat generation, and has improved cycle characteristics due to high output when the electric vehicle starts and frequent charging and discharging. Is required.

Therefore, an object of the present invention is to provide a polyolefin microporous membrane capable of ensuring the cycle characteristics (good characteristics in a thermal-cooled circulation test) of an electrochemical device when used as a separator for an electrochemical device. ..

The inventors have found that the above problems can be solved by specifying the TMA behavior, the film thickness, the resin composition and the air permeability of the microporous polyolefin membrane, and have completed the present invention. An example of the aspect of the present invention is shown below.
[1]
In constant length thermomechanical analysis (TMA), the sum of the maximum stress in the longitudinal direction (MD) and the maximum stress in the width direction (TD) is 0.10 N / mm ² or more and less than 1.20 N / mm ^2.
The average film thickness is 3.0 μm or more and less than 18.0 μm.
It contains 1.0% by mass to 20.0% by mass of polypropylene and has a film thickness equivalent air permeability of 1.0 s / (100 cm ^3. μm) or more and 10.0 s / (100 cm ^3. μm) or less. Polyolefin microporous membrane.
[2]
The polyolefin microporous membrane according to item 1, wherein the average pore size is 0.030 μm or more and 0.075 μm or less.
[3]
MD tensile modulus, 3000 kgf / cm ² or more 8000kgf / cm ² or less, and TD tensile modulus is 3000 kgf / cm ² or more 5000 kgf / cm ² or less, the microporous polyolefin membrane of claim 1 or 2 ..
[4]
The polyolefin microporous film according to any one of Items 1 to 3, wherein the film thickness-equivalent puncture strength is 0.10 N / μm or more and 0.30 N / μm or less.
[5]
The polyolefin microporous membrane according to any one of Items 1 to 4, wherein the bending ratio is 1.0 or more and 2.0 or less.
[6]
The polyolefin microporous membrane according to any one of items 1 to 5, which comprises 50.0% by mass to 99.0% by mass of high-density polyethylene and / or ultra-high molecular weight polyethylene.
[7]
The polyolefin microporous membrane according to any one of Items 1 to 6, wherein the polyolefin microporous membrane has a viscosity average molecular weight of 300,000 or more and 700,000 or less.
[8]
A separator for an electrochemical device, which comprises the polyolefin microporous membrane according to any one of items 1 to 7.
[9]
An electrochemical device comprising the separator for an electrochemical device according to item 8.

According to the present invention, there is provided a polyolefin microporous membrane capable of ensuring the cycle characteristics of an electrochemical device (good characteristics in a thermal-cooled circulation test) when used as a separator for an electrochemical device.

Hereinafter, the embodiment of the present invention (hereinafter, referred to as “the present embodiment”) will be described in detail for the purpose of exemplifying, but the present invention is not limited to the present embodiment. In the present specification, the upper limit value and the lower limit value of each numerical range can be arbitrarily combined. Further, in the present specification, "~" means that the numerical values at both ends thereof are included as the upper limit value and the lower limit value unless otherwise specified. Further, in the present specification, the longitudinal direction (MD) is the mechanical direction for continuous molding of the microporous membrane, and the width direction (TD) is the direction across the MD of the microporous membrane at an angle of 90 °.

<Microporous membrane>
One aspect of the present invention is a polyolefin microporous membrane. The polyolefin microporous membrane preferably has low electron conductivity, ionic conductivity, high resistance to an organic solvent, and a fine pore size. In addition, the polyolefin microporous membrane can be used as a separator for an electrochemical device such as a secondary battery.

Examples of the polyolefin microporous film include a microporous film containing a polyolefin resin, a resin such as polyethylene terephthalate, polycycloolefin, polyethersulfone, polyamide, polyimide, polyimideamide, polyaramid, polycycloolefin, nylon, and polytetrafluoroethylene. Examples thereof include microporous films containing polyolefins, polyolefin-based fiber woven fabrics (woven fabrics), polyolefin-based fiber non-woven fabrics, paper, and aggregates of insulating substance particles. Among these, it has excellent coatability of the coating liquid in the coating process on the film, the film thickness of the separator is thinner than that of the conventional separator, the ratio of the active material in the electrochemical device is increased, and the capacity per volume is increased. From the viewpoint of increasing, a porous membrane containing a polyolefin resin is preferable.

The porous membrane containing a polyolefin resin is formed of polyolefin in an amount of 50% by mass or more and 100% by mass or less. From the viewpoint of shutdown characteristics when the polyolefin microporous membrane is used as a separator for an electrochemical device, the proportion of polyolefin in the polyolefin microporous membrane is preferably 60% by mass or more and 100% by mass or less, preferably 70% by mass. It is more preferably 100% by mass or less.

The polyolefin microporous membrane according to this embodiment is
The average film thickness is 3.0 μm or more and less than 18.0 μm.
It contains 1.0% by mass to 20.0% by mass of polypropylene based on its mass.
In the constant length thermomechanical analysis (TMA), the sum of the maximum MD stress and the maximum TD stress is 0.10 N / mm ² or more and less than 1.20 N / mm ² , and the film thickness-equivalent air permeability is 1. It is 0.0 s / (100 cm ^3. μm) or more and 10.0 s / (100 cm ^3. μm) or less.

The mechanism by which such a specific microporous membrane gives good thermal-cooled circulation test results is considered as follows. That is, when a thermal history from high temperature to low temperature is applied, a specific stress may be generated inside the microporous membrane, but by adjusting this stress value appropriately and blending polypropylene, it is fine. It is considered that the voltage can be maintained by suppressing the peeling of the interface between the porous membrane and the electrode.

When the average thickness of the polyolefin microporous film is 3.0 μm or more, the mechanical strength tends to be improved, and when it is less than 18.0 μm, it is mounted on an electrochemical device such as a lithium ion battery (LIB). At that time, there is a tendency to improve the device capacity by increasing the active material ratio in the device and increasing the capacity per volume. From the viewpoint of improving the mechanical strength and device capacity of the polyolefin microporous membrane, the average film thickness is preferably in the range of 5.0 μm or more and less than 14.0 μm.

The average film thickness of the polyolefin microporous film can be adjusted within the numerical range described above by controlling, for example, the die lip interval, the stretching ratio in the stretching step, and the like in the manufacturing process.

In the present embodiment, when the polyolefin microporous film contains 1.0% by mass to 20.0% by mass of polypropylene (PP) based on its mass, it has excellent thermal rupture resistance and tends to have a smaller pore diameter. be. From the viewpoint of further improving the thermal rupture resistance and the reduction in pore size, the PP content of the polyolefin microporous film is preferably 1.0% by mass to 15.0% by mass, preferably 1.0% by mass to 12%. More preferably, it is 0.0% by mass.

The viscosity average molecular weight (Mv) of PP is preferably 200,000 or more and 1,000,000 or less, more preferably 250,000 or more and 900,000 or less, and further preferably 300,000 or more and 800,000 or less. Although it is not desired to be bound by theory, when the Mv of PP is 200,000 or more, PP is uniformly dispersed in PE, for example, when used in combination with polyethylene (PE), and the heat resistance of PP is effective. Moreover, it is preferable because the viscosity of the microporous polyolefin film does not increase too much even when it reaches a high temperature close to 300 ° C. When the Mv of PP is 1,000,000 or less, it becomes easy to suppress the deterioration of the molecular weight of the polymer in the membrane, and it becomes easy to suppress the residual stress of the polyolefin microporous membrane.

The Mv of polypropylene can be calculated according to the following equation by measuring the ultimate viscosity [η] (dl / g) in a decalin solvent at 135 ° C. based on ASTM-D4020.
[Η] = 1.10 × 10 ^-4 Mv ^0.80

The polypropylene (PP) contained in the polyolefin microporous membrane is preferably a homopolymer from the viewpoint of appropriately increasing heat resistance and melt viscosity at high temperature. Specific examples of PP include isotactic polypropylene, syndiotactic polypropylene, atactic polypropylene and the like. Of these, isotactic polypropylene is preferable. The amount of isotactic polypropylene is preferably 90% by mass or more, more preferably 95% by mass or more, still more preferably 98% by mass or more, still more preferably 100% by mass or more, based on the total mass of PP of the entire polyolefin microporous membrane. Mass% (all).

Since the isotactic polypropylene occupies 90% by mass or more of the total PP, it is possible to suppress further melting of the microporous membrane due to the temperature rise at the time of short circuit. Further, since isotactic polypropylene has high crystallinity, phase separation from the plasticizer tends to proceed easily, and a film having good porosity and high permeability tends to be obtained. Therefore, it can have a favorable effect on the output or cycle characteristics of the electrochemical device. Furthermore, since the homo-PP has few amorphous parts, it is possible to suppress an increase in thermal shrinkage when heat below the melting point is applied or when the amorphous parts shrink due to residual stress, and the separator temperature at the initial stage of short circuit. It becomes easy to suppress the problem that the short-circuit area increases due to the shrinkage of the amorphous portion when the temperature reaches around 100 ° C.

30.0% by mass or more and 99.0% by mass or less of the polyolefin microporous film is a polyolefin other than homo-PP, for example, ethylene, 1-butene, 4-methyl-1-pentene, 1 Homopolymers, copolymers or multistage polymers obtained using −hexene, 1-octene and the like as monomers; ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1- It can be a copolymer or a multi-stage polymer obtained by using octene or the like as a monomer.

From the viewpoint of shutdown characteristics when the polyolefin microporous membrane is used as a separator for an electrochemical device, the polyolefin constituting the microporous membrane is a mixture of polypropylene and polyethylene (PE) or an ethylene-propylene copolymer. preferable. Specific examples of the ethylene-propylene copolymer include ethylene-propylene random copolymer and ethylene-propylene rubber.

In particular, PE is preferable from the viewpoint of processability and fuse characteristics because the melting point and the melt viscosity are in a suitable range. Specific examples of PE include low density polyethylene (LDPE), linear low density polyethylene (LLDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), high molecular weight polyethylene (HMWPE), and ultra high molecular weight polyethylene (HMWPE). UHMWPE) and the like. Above all, from the viewpoint of stopping the thermal runaway of the electrochemical device at an initial stage, the melting point of PE is preferably 125 ° C. or higher and 140 ° C. or lower, and more preferably 130 ° C. or higher and 140 ° C. or lower.

In the present specification, the ultra high molecular weight polyethylene (HMWPE) means polyethylene having a viscosity average molecular weight (Mv) of 100,000 or more. The Mv of polyethylene can be calculated by the following formula by measuring the ultimate viscosity [η] (dl / g) in a decalin solvent at 135 ° C. based on ASTM-D4020.
[Η] = 6.77 × 10 ^-4 Mv ^0.67
In general, the Mv of ultra high molecular weight polyethylene (UHMWPE) is 1,000,000 or more. Therefore, according to such a definition, the high molecular weight polyethylene (HMWPE) in the present specification includes UHMWPE by definition. Further, even if the polyethylene is called "ultra-high molecular weight polyethylene" based on a definition different from the definition, if the Mv is 100,000 or more, it may correspond to the high molecular weight polyethylene in the present embodiment. There is.

In the present specification, high density polyethylene (HDPE) refers to polyethylene having ^{a density of 0.942 g / cm 3 to} 0.970 g / cm ^3. The density of polyethylene means a value measured according to the D) density gradient tube method described in JIS K7112 (1999).

The polyolefin microporous film preferably contains HDPE and / or UHMWPE, and more preferably contains HDPE, from the viewpoint of raising the melting point of the film to improve the film forming property and improving the heat resistance. .. From the same viewpoint, the total content of HDPE and UHMWPE in the polyolefin microporous film is preferably 50.0% by mass to 99.0% by mass, preferably 70.0% by mass, based on the mass of the polyolefin microporous film. It is more preferably from mass% to 99.0% by mass, and even more preferably from 80.0% by mass to 99.0% by mass.

The PP content of the polyolefin microporous film and the content of polyolefins other than PP are adjusted by, for example, adjusting the type and blending ratio of the polyolefin resin raw material, the resin composition of the polyolefin resin composition, etc. in the method for producing the polyolefin microporous film. Can be optimized by.

From the viewpoint of the physical properties or raw material characteristics of the polyolefin microporous film, the polyolefin contained in the polyolefin microporous film has a ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) of 1.0 (molecular weight distribution: Mw / Mn) of 1.0. It is preferably 15.0 or more, more preferably 3.0 or more and 12.0 or less, and further preferably 5.0 or more and 9.0 or less.

The polyolefin microporous membrane can optionally contain any additive. As additives, for example, polymers other than polyolefins; inorganic fillers; antioxidants such as phenol-based, phosphorus-based, and sulfur-based; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers; light stabilizers; Antistatic agents; antifogging agents; coloring pigments and the like can be mentioned. The total amount of these additives added is preferably 20% by mass or less with respect to 100% by mass of the polyolefin from the viewpoint of improving shutdown performance and the like, more preferably 10% by mass or less, still more preferably 5% by mass. It is less than or equal to 0.01% by mass or more.

<Stress control>
In the constant length thermomechanical analysis (TMA) of a polyolefin microporous membrane, when the sum of the maximum MD stress and the maximum TD stress is 0.10 N / mm ² or more and less than 1.20 N / mm ² , a separator for an electrochemical device is used. At high temperatures, for example, PE melting point, 130 ° C to 140 ° C, or 130 ° C or higher and 136 ° C or lower, the shrinkage stress of the separator is suppressed, and the short circuit of the electrode due to shrinkage of the insulating layer at high temperature is caused. Can be prevented.

Further, when the fixed length method TMA microporous polyolefin membrane is 0.10N / mm ² ≦ MD maximum stress + TD maximum stress <1.20N / mm ^2, even in the heat-cold circulation test, the residual stress of the separator is reduced Since the stress applied to the expansion and contraction of each member of the electrochemical device due to a sudden temperature change is reduced, damage and rupture of the electrochemical device can be suppressed.

The TMA constant length method of the polyolefin microporous membrane can be carried out by setting the TMA apparatus to the fixed length mode, and more specifically, by the method described in Examples. The sum of the maximum MD stress and the maximum TD stress measured by the constant length method TMA of a polyolefin microporous membrane is 0. it is preferably ^{3N / mm 2 ~1.1N / mm 2} , and more preferably ^{0.5N / mm 2 ~1.0N / mm 2} .

In the TMA measurement of the polyolefin microporous membrane, the ratio of the MD maximum stress to the TD maximum stress (MD maximum stress / TD maximum stress) is preferably 0.75 or more and 1.50 or less, more preferably 0.80 or more and 1.45. Hereinafter, it is more preferably 0.85 or more and 1.40 or less, and even more preferably 0.90 or more and 1.35 or less. When the above ratio (maximum load of MD / maximum load of TD) is 0.75 or more and 1.5 or less, it becomes easy to suppress an increase in the short-circuit area due to the occurrence of cracks due to anisotropy.

From the viewpoint of suppressing the shrinkage stress at high temperature and the residual stress during the thermal cold circulation test, the maximum MD stress is preferably larger than the maximum TD stress. From the same point of view, the maximum MD stress is preferably 0.30 N / mm ^{2 to} 0.80 N / mm ² , more preferably 0.38 N / mm ^{2 to} 0.70 N / mm ² , and the maximum TD stress is. , Preferably 0.20 N / mm ^{2 to} 0.50 N / mm ² , and more preferably 0.31 N / mm ^{2 to} 0.48 N / mm ² .

The MD maximum stress, the TD maximum stress, and the sum thereof of the polyolefin microporous film measured by the constant length method TMA are, for example, not limited, but in the process of producing the polyolefin microporous film, (i) polyolefin. Do not use UHMWPE with a viscosity average molecular weight (Mv) of 1,000,000 or more as a raw material; (Iii) Stretching the molded product at a surface magnification of 40 times or more; (iv) Stretching the molded product at a temperature higher than the preheating temperature; (v) Simultaneously biaxially stretching the molded product, etc. Can be adjusted within the range described above.

In the present specification, "Tm2onset" refers to the rising temperature of the second temperature rising peak in the differential scanning calorimetry (DSC) measurement of the polyolefin microporous membrane, and the melting point (Tm) refers to the differential scanning calorimetry of the polyolefin microporous membrane. (DSC) Refers to the maximum temperature of the second temperature rise peak in the measurement. If there are a plurality of second temperature rise peaks, the peak with the largest endothermic area is selected from among them. DSC can be used to measure the melting point of polyolefins, more specifically by the methods described in the Examples.

In the present specification, "preheating" refers to the process of stretching a molded product in a wet state (sometimes referred to as "raw fabric") from the time the raw fabric enters the stretching machine furnace to the start of stretching. Performed, and "preheating coefficient (min · ° C.)" = preheating temperature x preheating time.

The viscosity average molecular weight (Mv) of the polyolefin microporous membrane is preferably 300,000 or more and 900,000 or less. When the Mv of the microporous membrane is in the range of 300,000 or more and 900,000 or less, it is easy to suppress an increase in the contraction stress of the membrane and secure an insulating function by maintaining the shape when the membrane is melted. From this point of view, the Mv of the microporous membrane is more preferably in the range of 350,000 to 850,000, still more preferably in the range of 400,000 to 850,000.

The Mv of the polyolefin microporous membrane can be calculated by the following formula by measuring the ultimate viscosity [η] (dl / g) in a decalin solvent at 135 ° C. based on ASTM-D4020.
[Η] = 6.77 × 10 ^-4 Mv ^0.67
In addition, even when PP is contained in the polyolefin microporous membrane, it is calculated by the above formula.

From the viewpoint of suppressing the stress applied to each member of the electrochemical device during the thermal cold circulation test or during the expansion and contraction of the electrochemical device after being mounted on the electrochemical device, MD is used in the tensile test of the polyolefin microporous film. tensile modulus, is preferably 3000 kgf / cm ² or more 8000kgf / cm ² or less, more preferably 3000 kgf / cm ² or more 7000kgf / cm ² or less, is 3000 kgf / cm ² or more 6500kgf / cm ² or less it is more preferably, and / or TD tensile modulus is preferably at 3000 kgf / cm ² or more 5000 kgf / cm ² or less, more preferably 3000 kgf / cm ² or more 4500kgf / cm ² or less, 3000 kgf / cm and more preferably ² or more 4000 kgf / cm ² or less.

From the viewpoint of further suppressing the shrinkage stress at high temperature and the residual stress during the thermal-cold circulation test, the MD tensile elastic modulus is preferably larger than the TD tensile elastic modulus in the tensile test of the polyolefin microporous membrane.

The MD tensile elastic modulus of the polyolefin microporous film is controlled, for example, by controlling the stretching temperature of the molded product of the polyolefin resin composition within the range of (Tc + 5 ° C.) or more (Tc + 15 ° C.) or less in the process of manufacturing the polyolefin microporous film. It can be adjusted within the numerical range described above by optimizing the crystallinity of the PE raw material used in combination with the PP raw material. As used herein, the term "Tc" refers to the crystallization temperature, which is the maximum temperature of the temperature drop peak in the differential scanning calorimetry (DSC) measurement of the microporous polyolefin. If there are a plurality of temperature drop peaks, the peak with the largest heat generation area is selected from among them. In the present embodiment, when the raw material is PE, Tc ≈ 118 ° C.

The TD tensile elastic modulus of the polyolefin microporous membrane is controlled, for example, by controlling the stretching temperature of the molded product of the polyolefin resin composition within the range of (Tc + 5 ° C.) or more (Tc + 15 ° C.) or less in the process of manufacturing the polyolefin microporous membrane. It can be adjusted within the numerical range described above by setting the relaxation rate in the heat fixing step to 15% or less.

The polyolefin microporous film suppresses shrinkage stress at high temperatures and prevents short circuits of electrodes due to shrinkage of the insulating layer at high temperatures when mounted on an electrochemical device. It is preferable to have a piercing strength, and the piercing strength per 1 μm of the film thickness (thickness-equivalent piercing strength) is preferably 0.10 N / μm or more and 0.30 N / μm or less, and 0.12 N / μm or more and 0. It is more preferably .30 N / μm or less, and further preferably 0.15 N / μm or more and 0.30 N / μm or less. In relation to the film thickness-equivalent puncture strength, the average film thickness of the polyolefin microporous film is preferably 3.0 μm or more and less than 18.0 μm, and preferably 5.0 μm or more and less than 14.0 μm.

The puncture strength and the film thickness-equivalent puncture strength of the polyolefin microporous film are determined in the process of producing the polyolefin microporous film including the above (i) to (v), as in the control of the maximum stress measured by the constant length method TMA. It can be adjusted within the range described above.

<Control of air permeability>
The air permeability per thickness 1μm microporous polyolefin membrane (thickness in terms of air permeability) is in 1.0s / (100cm ³ · μm) than 10.0s / (100cm ³ · μm) within the range As a result, the ion diffusivity of the membrane is increased, and the output of the electrochemical device can be improved by also increasing the Li ion diffusivity in an electrochemical device such as a lithium ion storage device.

^{The film thickness-equivalent air permeability of the polyolefin microporous membrane is 3.0 s / (100 cm 3.} μm) or more and 10.0 s / from the viewpoint of further improving the output of the electrochemical device by increasing the ion diffusivity of the membrane. preferably (100cm ³ · μm) or ^{less, 5.0s / (100cm 3 · μm} ) or 8.0s / (100cm ³ · μm) is more preferably less. In relation to the film thickness-equivalent air permeability, the average film thickness of the polyolefin microporous film is preferably 3.0 μm or more and less than 18.0 μm, and preferably 5.0 μm or more and less than 14.0 μm.

From the viewpoint of using the polyolefin microporous membrane as a separator for an electrochemical device, the air permeability of the polyolefin microporous membrane is preferably 30 s / 100 cm ³ or more, more preferably 40 s / 100 cm ³ or more, and further preferably 50 s / 100 cm. ^{It is 3} or more, and the air permeability is preferably 500 s / 100 cm ³ or less, more preferably 400 s / 100 cm ³ or less, still more preferably 300 s / 100 cm ³ or less, still more preferably 200 s / 100 cm ³ or less, and most preferably. It is 100 s / 100 cm ³ or less. When the air permeability is 30 s / 100 cm ³ or more, self-discharge of the separator can be suppressed. When the air permeability is 500 s / 100 cm ³ or less, the output of the electrochemical device can be guaranteed.

The air permeability of the polyolefin microporous membrane and the air permeability in terms of film thickness are not limited, for example, but in the process of manufacturing the polyolefin microporous membrane, the heat fixing temperature is set to (Tm-7 ° C.) or higher (Tm-). It can be adjusted within the numerical range described above by controlling the temperature within the range of 3 ° C. or lower, or by setting the thermal fixation ratio of the TD to 1.3 times or more, for example.

<Relationship between stress control and air permeability control>
According to the polyolefin microporous membranes described above, when used as a separator for an electrochemical device, it not only controls the stress applied to the device to improve safety and cycle characteristics, but also the air permeability of the separator. It was also found that the device output can be improved by optimizing.

<Control of hole diameter and curve ratio>
In the present embodiment, in addition to suppressing the shrinkage stress and the residual stress at high temperature of the polyolefin microporous membrane used as a separator for an electrochemical device, in order to improve the cycle characteristics and output of the electrochemical device, It has been found that it is preferable to control the pore size and / or the curvature ratio of the polyolefin microporous membrane.

By using a polyolefin microporous membrane having a pore diameter smaller than a specific level as a separator for an electrochemical device, it is possible to suppress deterioration of the electrochemical device due to a concentrated current flowing at one point and improve cycle characteristics. .. From this point of view, the average pore size of the polyolefin microporous membrane is preferably 0.030 μm or more and 0.075 μm or less, more preferably 0.030 μm or more and 0.072 μm or less, and 0.030 μm or more and 0. It is more preferably 070 μm or less. When the average pore diameter is 0.030 μm or more, the output of the electrochemical device tends to be improved.

The average pore size of the polyolefin microporous film is, for example, within the range of 30% by mass or more and less than 40% by mass of the polymer content (Pc) of the polyolefin resin composition to be extruded in the process of producing the polyolefin microporous film. Control, PP is contained in the polyolefin resin composition, the heat fixing step is performed under the condition of 5 ° C. <(heat fixing temperature-stretching temperature) <20 ° C., and the stretching strain rate during the heat fixing step is set to 5. By controlling the stretching temperature within the range of% to 10%, controlling the stretching temperature at the time of heat fixing within the range of Tc or more (Tc + 10 ° C.), or setting the total stretching ratio to 70 times or less, the above It can be adjusted within the numerical range described in.

The bending ratio of the polyolefin microporous film is preferably 1.0 or more and 2.0 or less, and more preferably 1.0 or more and 1.9 or less. The polyolefin microporous membrane designed so that the curvature ratio is within the above numerical range controls the ion diffusivity as a separator for an electrochemical device, particularly the lithium (Li) ion diffusivity, and adjusts the pore size of the separator. There is a tendency to enable higher output of devices without increasing the size. This tendency becomes remarkable when used in combination with the control of the average pore size of the polyolefin microporous membrane.

Regarding the bending rate of the polyolefin microporous film, for example, in the process of manufacturing the polyolefin microporous film, the surface magnification of biaxial stretching is controlled to 40 times or more, and in the heat fixing step, the relaxation rate is set to 15% or less and /. Alternatively, the heat fixing temperature can be adjusted within the numerical range described above by controlling the heat fixing temperature within the range of (Tm-7 ° C.) or higher (Tm-3 ° C.) or lower.

<Relationship between stress control and pore diameter / curve ratio control>
The polyolefin microporous membrane according to the present embodiment suppresses the thermal shrinkage of the separator for an electrochemical device and the electrochemical device by suppressing the shrinkage stress during heating after controlling the pore size and the bending ratio as described above. It has been found that it is possible to improve the cycle characteristics and increase the output of the above.

<Other properties>
The average pore size of the polyolefin microporous film is preferably 0.01 μm or more and 0.70 μm or less, more preferably 0.02 μm or more and 0.20 μm or less, still more preferably 0.03 μm or more and 0.15 μm or less, still more preferably 0. It is 04 μm or more and 0.10 μm or less, most preferably 0.05 μm or more and 0.08 μm or less. It is preferable that the average pore size is 0.01 μm or more because it has good ionic conductivity. It is preferable that the average pore size is 0.70 μm or less because it has good cycle characteristics. The average pore size can be adjusted by controlling the composition ratio of polyolefin, biaxial stretching temperature, stretching ratio, heat fixing temperature, stretching ratio during heat fixing, and relaxation rate during heat fixing, and by combining these. can.

The maximum pore size of the polyolefin microporous film is preferably 0.02 μm or more and 1.00 μm or less, more preferably 0.03 μm or more and 0.30 μm or less, still more preferably 0.04 μm or more and 0.20 μm or less, still more preferably 0. It is 05 μm or more and 0.15 μm or less, most preferably 0.06 μm or more and 0.10 μm or less. It is preferable that the maximum pore diameter is 0.02 μm or more because it has good ionic conductivity. When the maximum pore diameter is 1.00 μm or less, clogging or self-discharge due to by-products in the battery can be prevented.

The difference between the maximum pore size and the average pore size (maximum pore size-average pore size) of the polyolefin microporous film is preferably 0.001 μm or more and 0.3 μm or less, and more preferably 0.003 μm or more and 0.1 μm from the viewpoint of good cycle characteristics. Hereinafter, it is more preferably 0.005 μm or more and 0.05 μm or less, still more preferably 0.008 μm or more and 0.03 μm or less, and most preferably 0.01 μm or more and 0.02 μm or less.

The porosity of the polyolefin microporous membrane is preferably 25% or more and 95% or less, more preferably 30% or more and 65% or less, further preferably 35% or more and 55% or less, and most preferably 37% or more and 50% or less. The porosity of the polyolefin microporous membrane is preferably 25% or more from the viewpoint of improving ionic conductivity, and preferably 95% or less from the viewpoint of withstand voltage characteristics. The porosity of the polyolefin microporous film controls the mixing ratio of the polyolefin resin composition and the plasticizer, the biaxial stretching temperature, the stretching ratio, the heat fixing temperature, the stretching ratio during heat fixing, and the relaxation rate during heat fixing. , And these can be adjusted by combining them.

The microporous polyolefin membrane preferably has a shutdown temperature of 150 ° C. or lower as measured at a heating rate of 15 ° C./min. Since the shutdown temperature measured at a temperature rise rate of 15 ° C / min is 150 ° C or less, thermal runaway is suppressed by instantly increasing the internal resistance when the temperature rises suddenly when the electrochemical device is short-circuited. It will be easier. The shutdown temperature is preferably 150 ° C. or lower, more preferably 149 ° C. or lower, still more preferably 148 ° C. or lower, still more preferably 147 ° C. or lower, most preferably 146 ° C. or lower, and the shutdown temperature is preferably 135 ° C. or lower. As mentioned above, it is more preferably 137 ° C. or higher, and even more preferably 139 ° C. or higher. It can be inferred that when the shutdown temperature is 135 ° C. or higher, it becomes easier to prevent thermal runaway due to melting and outflow of the resin at a low temperature.

The shutdown temperature measured at a heating rate of 15 ° C./min includes, for example, selection of the polyolefin raw material to be used, specific energy during melt kneading, polymer concentration or kneading temperature during melt kneading, strain rate during stretching, and the like. By controlling various manufacturing conditions (for example, the film forming conditions shown in Table 1 below), the adjustment can be made as described above.

<Manufacturing method of microporous polyolefin membrane>
Another aspect of the present invention provides a method for producing a polyolefin microporous membrane. Examples of the method for producing the polyolefin microporous membrane include the following methods:
(1) A method in which a polyolefin resin composition and a pore-forming material are melt-kneaded to form a sheet, stretched as necessary, and then the pore-forming material is extracted to make the pore-forming material porous.
(2) A method of melt-kneading a polyolefin resin composition, extruding it at a high draw ratio, and then exfoliating the polyolefin crystal interface by heat treatment and stretching to make it porous;
(3) A method in which a polyolefin resin composition and an inorganic filler are melt-kneaded to form a sheet, and then the interface between the polyolefin and the inorganic filler is peeled off by stretching to make the porous material porous.
(4) Examples thereof include a method in which the polyolefin resin composition is dissolved and then immersed in a poor solvent for the polyolefin to coagulate the polyolefin and at the same time remove the solvent to make the polyolefin porous.

Hereinafter, as an example of a method for producing a polyolefin microporous film, the method of (1) melt-kneading the polyolefin resin composition and the pore-forming material to form a sheet and then extracting the pore-forming material will be described.

As described above, the polyolefin microporous membrane according to the present embodiment is made of both polypropylene (PP) and polyethylene (PE) in order to suppress shrinkage stress, air permeability, pore diameter, and curb ratio at high temperatures. It is preferable to include. Generally, in order to suppress the shrinkage stress of a polyolefin microporous membrane at a high temperature, it is necessary to reduce the strain stress applied when the separator is stretched. However, it has been found that under such general stretching conditions, the PE lamella does not cleave sufficiently and the pore size does not decrease. Therefore, the method (1) described above is performed in the stretching step in order to reduce the pore size while suppressing the shrinkage stress at a high temperature by keeping the strain stress during stretching small by the preheating conditions of the stretching step described later. The feature is that the pore diameter and curvature ratio are controlled by the preheating temperature, preheating coefficient, stretching ratio, stretching temperature, relaxation rate, relaxation temperature (hereinafter, also referred to as "heat fixing temperature") in the heat fixing (HS) process. It is possible to provide a polyolefin microporous membrane and a separator having a low shrinkage stress and a controlled pore structure. Further, in the present embodiment, it is preferable to use Tm2onset as an index of preheating, Tc as an index of stretching, and Tm as an index of heat fixation.

In the method for producing a polyolefin microporous film, first, the polyolefin resin composition and the pore-forming material are melt-kneaded. As a melt-kneading method, for example, a polyolefin resin and, if necessary, other additives are put into a resin kneading device such as an extruder, a kneader, a lab plast mill, a kneading roll, or a Banbury mixer to heat and melt the resin components. However, there is a method of introducing a pore-forming material at an arbitrary ratio and kneading.

Examples of the pore-forming material include a plasticizer, an inorganic material, or a combination thereof.

The plasticizer is not particularly limited, but it is preferable to use a non-volatile solvent capable of forming a uniform solution above the melting point of polyolefin. Specific examples of such a non-volatile solvent include hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; higher alcohols such as oleyl alcohol and stearyl alcohol. .. After extraction, these plasticizers may be recovered and reused by an operation such as distillation. Further, preferably, the polyolefin resin, other additives, and the plasticizer are pre-kneaded at a predetermined ratio using a Henschel mixer or the like before being put into the resin kneading apparatus. More preferably, in the pre-kneading, a part of the plasticizer to be used is added, and the remaining plasticizer is kneaded while being appropriately heated in a resin kneading device and side-fed. By using such a kneading method, the dispersibility of the plasticizer is enhanced, and when the sheet-shaped molded product of the melt-kneaded product of the resin composition and the plasticizer is stretched in a later step, the film is not broken and the magnification is high. It tends to be able to be stretched with.

Among the plasticizers, liquid paraffin has high compatibility with polyethylene or polypropylene when the polyolefin resin is polyethylene, and even if the melt-kneaded product is stretched, interfacial peeling between the resin and the plasticizer is unlikely to occur, and uniform stretching is possible. It is preferable because it tends to be easy to carry out.

The mass fraction of the polyolefin raw material in the composition containing the polyolefin raw material and the plasticizing agent is preferably 18% by mass or more and less than 35% by mass, more preferably 20% by mass or more and 33% by mass, based on the mass of the resin composition. Less than, more preferably 22% by mass or more and less than 31% by mass. If the mass fraction of the polyolefin raw material is less than 35% by mass, the energy during kneading does not rise too much, and deterioration of the molecular weight due to excessive entanglement of the polymers can be suppressed. Does not impair the characteristics. On the other hand, when the mass fraction of the polyolefin raw material is 18% by mass or more, sufficient energy can be given at the time of melt kneading, and the polymers are uniformly kneaded due to entanglement with each other. Even when the mixture of the above is stretched at a high magnification, the entanglement of the polyolefin molecular chains does not occur, a uniform and fine pore structure is easily formed, and the strength is easily increased.

From the viewpoint of controlling the thermal rupture property and / or the average pore size of the obtained polyolefin microporous film as described above, it is preferable to use PP as the polyolefin raw material. Further, from the viewpoint of heat-resistant film rupture and small pore size of the obtained microporous film, it is preferable to use a mixture of polyethylene (PE) and polypropylene (PP) as the polyolefin raw material. The proportion of PP used in the polyolefin resin composition is preferably 1.0% by mass or more and 20.0% by mass or less, more preferably 1.0% by mass or more and 15.0% by mass, assuming that the total amount of the polyolefin raw material is 100% by mass. % Or less, more preferably 1.0% by mass or more and 12.0% by mass or less, or 1.0% by mass or more and 10.0% by mass or less, still more preferably 2.0% by mass or more and 8.0% by mass or less. be. When the PP ratio is 1.0% by mass or more, it becomes difficult for the polyolefin microporous film to easily break when it reaches a high temperature of about 150 ° C., and it is difficult for minute pinholes to occur at the initial stage when the battery is short-circuited. Become. When the PP ratio is 20.0% by mass or less, the fluidity of the molten resin does not become too large when the temperature reaches a high temperature close to 300 ° C., and the electrode is exposed due to the outflow of the resin or excessive penetration into the electrode. It becomes easier to avoid thermal runaway.

When the polyolefin resin composition contains both PP and PE, the PE content is preferably 75% by mass or more, more preferably 85% by mass or more, still more preferably 90%, with the total amount of the polyolefin raw materials being 100% by mass. It can be 9% by mass or more, more preferably 93% by mass or more, and less than 99% by mass.

When the polyolefin resin composition contains both PP and PE, the Mv of PE is preferably 200,000 or more and 950,000 or less. When the Mv of PE is 200,000 or more when PE and PP are used in combination, the entanglement effect of polyolefin molecules can be sufficiently exhibited, and the strength of the microporous membrane can be ensured. If it is 950,000 or less, the difference in melt viscosity between the raw materials does not spread too much, and the polymer flows stably during sheet molding, so that a microporous film having a good appearance can be obtained. From this point of view, the Mv of PE is more preferably in the range of 250,000 to 900,000, still more preferably in the range of 300,000 to 850,000. Further, the crystallinity obtained from the differential scanning calorimetry (DSC) of PE is adjusted from the viewpoint of adjusting the MD and TD tensile elastic modulus and the melting point Tm of the obtained polyolefin microporous film within the numerical ranges described above. , 50% or more and less than 80%.

As the polyolefin raw material of the polyolefin resin composition, a plurality of PE resins may be mixed and used. When a plurality of PE resins are used in combination, the polyolefin resin composition preferably contains PE having an Mv of 100,000 or more and 300,000 or less and PE having an Mv of 500,000 or more and less than 1,000,000. By containing PE having an Mv of 100,000 or more and 300,000 or less, the polyolefin microporous film can suppress deterioration of the molecular weight of polyolefin without (a) excessive increase in viscosity during melt kneading, and is excessive during stretching. Residual stress does not remain, heat shrinkage tends to be small, (b) pores are easily closed when the temperature rises, and a good shutdown function tends to be obtained, and (c) viscosity when the microporous film is melted. It is presumed that when the electrochemical device is melted after a short circuit, it is likely to penetrate the electrode appropriately to exhibit the anchor effect, suppress heat shrinkage, and suppress an increase in the short circuit area. Since the polyolefin microporous membrane contains PE having an Mv of 500,000 or more and less than 1,000,000, (d) the stress increases during melt kneading, and the resin can be uniformly kneaded, and (e). Since the polyolefin microporous membrane develops entanglement between the polymers, it tends to have high strength, and when the polyolefin microporous membrane melts and reaches a high temperature of about 300 ° C., the viscosity does not decrease too much, and the resin becomes It is presumed that it will be easier to suppress thermal runaway because it will be easier to stay in place without flowing out.

The proportion of PE having an Mv of 100,000 or more and 300,000 or less used as a polyolefin raw material is preferably 25% by mass or more and 75% by mass or less, more preferably 30% by mass or more and 70% by mass, with the total amount of the polyolefin raw material being 100% by mass. Below, it is more preferably 35% by mass or more and 65% by mass or less, and even more preferably 40% by mass or more and 60% by mass or less. When the proportion of PE with Mv of 100,000 or more and 300,000 or less is 25% by mass or more, it has good shutdown characteristics, heat shrinkage suppression effect, and suppression of increase in short-circuit area due to having appropriate viscosity when reaching high temperature. It tends to be effective. When the proportion of polyethylene having an Mv of 100,000 or more and 300,000 or less is 75% by mass or less, the polymers tend to be entangled with each other during melt kneading, and the polyolefin microporous film becomes hot. When it reaches the level, the fluidity of the molten resin does not become too large, and there is a tendency that thermal runaway due to exposure of the electrodes due to the outflow of the resin can be avoided.

The proportion of PE having an Mv of 500,000 or more and less than 1 million used as a polyolefin raw material is preferably 25% by mass or more and 75% by mass or less, more preferably 30% by mass or more and 70% by mass, with the total amount of the polyolefin raw material being 100% by mass. Below, it is more preferably 35% by mass or more and 65% by mass or less, and even more preferably 40% by mass or more and 60% by mass or less. When the proportion of PE with Mv of 500,000 or more and less than 1 million is 25% by mass or more, the strength of the polyolefin microporous membrane is increased, and the effect of suppressing thermal runaway without resin flowing out when the temperature is reached is improved. It tends to be available. When the proportion of PE having an Mv of 500,000 or more and less than 1,000,000 is 70% by mass or less, excessive residual stress does not remain during stretching, heat shrinkage tends to be small, and holes are closed when the temperature rises. It is easy to obtain a good shutdown function, and at the same time, viscosity is generated when the polyolefin microporous film is melted. Therefore, when the battery is melted after a short circuit, it appropriately penetrates into the electrode to exhibit an anchor effect and heat. It is presumed that shrinkage can be suppressed and an increase in short-circuit area can be easily suppressed.

The polyolefin resin composition contains high-density polyethylene (HDPE) and / or ultra-high molecular weight polyethylene (UHMWPE) as a polyolefin raw material from the viewpoint of improving film-forming property by increasing the melting point of the obtained polyolefin microporous film and heat resistance. It is preferable, and it is more preferable to include HDPE. From the same viewpoint, the total content of HDPE and UHMWPE in the polyolefin resin composition is preferably 50.0% by mass to 95.0% by mass, preferably 70.0% by mass, with the total amount of the polyolefin raw materials as 100% by mass. It is more preferably from mass% to 95.0 mass%, and even more preferably from 80.0 mass% to 95.0 mass%.

On the other hand, the polyolefin resin composition preferably does not contain UHMWPE having an Mv of 1,000,000 or more as a polyolefin raw material from the viewpoint of controlling the shrinkage stress of the obtained polyolefin microporous film as described above. ..

Therefore, the presence or absence of UHMWPE in the polyolefin raw material can be determined according to the desired characteristics of the polyolefin microporous film, for example, the balance of melting point, film forming property, heat resistance, shrinkage stress and the like.

For low-density polyethylene (LDPE) as a polyolefin raw material, the LDPE content in the polyolefin resin composition is preferably 10% by mass or less, more preferably 8% by mass or less, and further preferably 8% by mass or less, with the total amount of the polyolefin raw material being 100% by mass. It is preferably 6% by mass or less or 5% by mass or less, more preferably 4% by mass or less (or less than 3% by mass, even less than 1% by mass), and most preferably the polyolefin resin composition contains LDPE. do not have. When the LDPE ratio is 10% by mass or less, the microporous polyolefin membrane easily breaks easily when it reaches a high temperature of about 150 ° C., and the fluidity of the molten resin when it reaches a high temperature of about 300 ° C. Does not become too large, and there is a tendency that thermal runaway due to exposure of the electrodes due to resin outflow can be avoided. For the same reason, low molecular weight polyethylene (LMWPE) having an Mv of less than 50,000 may be contained in the polyolefin resin composition as long as it does not significantly impair the exertion of the effects in the present invention, and the content thereof is, for example, Similar to the case of LDPE, and LMWPE having an Mv of less than 50,000 is preferably not included in the polyolefin resin composition.

From the viewpoint of the physical properties of the porous polyolefin resin film or the characteristics of the raw material, the polyolefin raw material has a ratio (molecular weight distribution: Mw / Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of 1.0 or more and 15.0 or less. It is preferably 3.0 or more and 12.0 or less, and more preferably 5.0 or more and 9.0 or less.

Any additive can be contained in the polyolefin resin composition. Additives include, for example, polymers other than polyolefin resins; inorganic fillers; antioxidants such as phenol-based, phosphorus-based, and sulfur-based; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers; light stabilizers. Antistatic agents; antifogging agents; coloring pigments and the like. The total amount of these additives added is preferably 20% by mass or less with respect to 100% by mass of the polyolefin resin from the viewpoint of improving shutdown performance and the like, more preferably 10% by mass or less, still more preferably 5% by mass. % Or less.

The polymer content (PC) in the polyolefin resin composition is 25, based on the mass of the polyolefin resin composition, from the viewpoint of adjusting the average pore size of the obtained polyolefin microporous film within the numerical range described above. It is preferably 3% by mass or more and less than 40% by mass, and preferably 30% by mass or more and 35% by mass or less. In the present specification, the PC calculation includes all the raw materials that finally constitute the film, regardless of whether or not they are polyolefins.

When melt-kneading the pore-forming material and the polyolefin raw material, the specific energy at the time of kneading the polyolefin raw material and the pore-forming material must be 0.10 kW · h / kg or more and 0.40 kW · h / kg or less. It is preferable, more preferably 0.12 kW · h / kg or more and 0.35 kW · h / kg or less, and even more preferably 0.14 kW · h / kg or more and 0.30 kW · h / kg or less. The specific energy is the value obtained by dividing the power P (kW) of the screw of the extruder applied during melt kneading of the pore-forming material and the polyolefin raw material by the extrusion amount Q (kg / h) per unit time of the pore-forming material and the polyolefin raw material. be. The power P (kW) of the screw of the extruder can be obtained from the following formula, where the torque applied to the screw during extrusion is T (Nm) and the screw rotation speed is N (rpm).
P = T × N / 9550
When the specific energy is 0.10 kW · h / kg or more, the entanglement of the polymers is promoted, and by uniformly kneading different polyolefin raw materials, a polyolefin microporous film having a uniform pore diameter and high strength can be obtained. Tend to be able to. Further, it is presumed that when the microporous polyolefin membrane is melted, a rapid decrease in viscosity can be suppressed due to entanglement between the polymers. When the specific energy is 0.40 kW · h / kg or less, molecular weight deterioration or oxidative deterioration due to cleavage or decomposition of the polymer due to excessive kneading is suppressed, and the polyolefin microporous membrane melts and reaches a high temperature. It is presumed that it becomes easier to suppress the decrease in viscosity of.

When melt-kneading the pore-forming material and the polyolefin raw material with an extruder, the temperature of the melt-kneading section (kneading temperature) is the specific energy at the time of melt-kneading, the film strength of the polyolefin microporous film, and the uniform pore size. From the viewpoint of properties, it is preferably 140 ° C. or higher and lower than 200 ° C., and more preferably 150 ° C. or higher and lower than 190 ° C.

When melt-kneading the pore-forming material and the polyolefin raw material with an extruder, the extrusion amount of the polyolefin raw material and the pore-forming material per unit time (that is, the discharge amount Q of the extruder Q: kg / hour) and the screw of the extruder The ratio (Q / N, unit: kg / (h · rpm)) to the number of revolutions N (rpm) is preferably from the viewpoint of the specific energy during melt kneading, the film strength of the polyolefin microporous film, and the uniformity of pore size. It is 2.2 or more and 7.8 or less, more preferably 2.5 or more and 7.5 or less, still more preferably 2.8 or more and 7.2 or less, and even more preferably 3.1 or more and 6.9 or less.

Next, the melt-kneaded product is formed into a sheet. As a method for producing a sheet-shaped molded product, for example, the melt-kneaded product is extruded into a sheet shape via a T-die or the like, brought into contact with a heat conductor, and cooled to a temperature sufficiently lower than the crystallization temperature of the resin component. There is a method of solidifying. Examples of the heat conductor used for cooling and solidification include metals, water, air, and plasticizers. Among these, it is preferable to use a metal roll because of its high heat conduction efficiency. Further, when the extruded kneaded product is brought into contact with a metal roll, sandwiching it between at least a pair of rolls further enhances the efficiency of heat conduction, and the sheet is oriented to increase the film strength and the surface smoothness of the sheet. Is also more preferable because it tends to improve. The die lip interval when extruding the melt-kneaded product from the T die into a sheet is preferably 200 μm or more and 3,000 μm or less, and more preferably 500 μm or more and 2,500 μm or less. When the die lip interval is 200 μm or more, the shavings and the like are reduced, the influence on the film quality such as streaks or defects is small, and the risk of film breakage or the like can be reduced in the subsequent stretching step. On the other hand, when the die lip interval is 3,000 μm or less, the cooling rate is high, cooling unevenness can be prevented, and the thickness stability of the sheet can be maintained.

Further, the sheet-shaped molded product may be rolled. Rolling can be carried out by, for example, a pressing method using a double belt press machine or the like. By rolling the sheet-shaped molded product, the orientation of the surface layer portion can be increased in particular. The rolled surface magnification is preferably more than 1 time and 3 times or less, and more preferably more than 1 time and 2 times or less. When the rolling ratio exceeds 1 times, the plane orientation tends to increase, and the film strength of the finally obtained porous film tends to increase. On the other hand, when the rolling ratio is 3 times or less, the orientation difference between the surface layer portion and the inside of the center is small, and a uniform porous structure tends to be formed in the thickness direction of the film.

(Stretching)
The stretching step in which the sheet-shaped molded product or the porous film is stretched may be performed before the step of extracting the pore-forming material from the sheet-shaped molded product (pore-forming step), or the pore-forming material is extracted from the sheet-shaped molded product. It may be performed on the perforated film. Further, the stretching step may be performed before and after the extraction of the pore-forming material from the sheet-shaped molded product.

Preheating is performed in the stretching step from the viewpoint of controlling the shrinkage stress of the obtained polyolefin microporous film, the maximum stress measured by the constant length method TMA, the puncture strength, the puncture strength in terms of film thickness, etc. as described above. Is preferable. From the same viewpoint, it is preferable to subject the wet sheet-shaped molded product to preheating, and the preheating temperature is preferably (Tm2onset-8 ° C.) or higher, more preferably 120 ° C. to 131 ° C., and / Or the preheating coefficient is preferably 200 or more and less than 300.

As the stretching treatment, either uniaxial stretching or biaxial stretching can be used, but biaxial stretching is performed from the viewpoint of controlling the stress and / or puncture strength of the obtained microporous membrane as described above. preferable. Further, from the viewpoint of heat shrinkage of the obtained porous membrane, it is preferable to carry out the stretching step at least twice.

When the sheet-shaped molded body is stretched at a high magnification in the biaxial direction, the molecules are oriented in the plane direction, the finally obtained porous film is hard to tear, and the sheet-like molded body has high puncture strength. Examples of the stretching method include simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple stretching. Simultaneous biaxial stretching is preferable from the viewpoint of hole diameter uniformity, stretching uniformity, and shutdown property.

In the present specification, the simultaneous biaxial stretching refers to a stretching method in which MD stretching and TD stretching are performed at the same time, and the stretching ratio in each direction may be different. Sequential biaxial stretching refers to a stretching method in which MD and TD are stretched independently, and when MD or TD is stretched, the other direction is fixed in an unconstrained state or a fixed length. It is assumed that it is in the state of being.

The draw ratio is preferably in the range of 28 times or more and less than 100 times, more preferably 32 times or more and 70 times or less, and 36 times or more and 50 times in terms of surface magnification for both uniaxial stretching and biaxial stretching. It is more preferably double or less. When the surface magnification is 28 times or more, the strength of the obtained polyolefin microporous film is increased and the pore size is not too small, so that the cycle characteristics are excellent. On the other hand, when the surface magnification is less than 100 times, the residual stress does not become too large, so that excessive heat shrinkage can be prevented, the fracture elongation can be prevented from decreasing, and the pore diameter becomes excessively large or the pore diameter is not large. Uniformization can be prevented. In the case of biaxial stretching, the stretching ratio adjusts the maximum stress measured by the constant length method TMA of the obtained polyolefin microporous film, the film thickness-equivalent puncture strength, and the bending ratio within the numerical ranges described above. From the viewpoint of It is more preferable that the content is within the range of 49 times or more. From the same viewpoint, the draw ratio in each axial direction is preferably in the range of 4 times or more and less than 10 times for MD, 4 times or more and less than 10 times for TD, 5 times or more and less than 9 times for MD, and TD. It is more preferably in the range of 5 times or more and less than 9 times, and further preferably in the range of 5.5 times or more and less than 8.5 times for MD and 5.5 times or more and less than 8.5 times for TD. ..

The temperature at the time of stretching the sheet-shaped molded product or the polyolefin microporous film is preferably higher than the preheating temperature from the viewpoint of controlling the stress and / or puncture strength of the microporous film as described above. And / or from the viewpoint of adjusting the MD and TD tensile elastic modulus of the microporous membrane within the numerical range described above, it is preferably within the range of (Tc + 5 ° C.) or more (Tc + 15 ° C.) or less. The specific stretching temperature is preferably more than 120 ° C., more preferably more than 122 ° C., more preferably 131 ° C. or lower, and more preferably 129 ° C. or lower. When the stretching temperature, particularly the temperature during biaxial stretching, exceeds 120 ° C., it is possible to suppress an increase in thermal shrinkage due to excessive residual stress. When the stretching temperature, particularly the temperature at the time of biaxial stretching, is 131 ° C. or less, sufficient strength can be given to the polyolefin microporous membrane, and the pore size distribution is prevented from being disturbed due to melting of the membrane surface, and the battery is charged and discharged. It is possible to guarantee the cycle performance when the above is repeated.

In order to suppress the heat shrinkage of the polyolefin microporous film, it is preferable to perform the heat fixing step by heat treatment after the stretching step or after forming the polyolefin microporous film.

The heat fixing method is performed at a predetermined temperature atmosphere and a predetermined stretching rate for the purpose of adjusting the physical properties, and / or at a predetermined temperature atmosphere and a predetermined relaxation rate for the purpose of reducing the stretching stress. Relaxation operation can be mentioned. The relaxation operation may be performed after the stretching operation. These heat fixings can be performed using a tenter or a roll stretching machine. When the heat fixing step is performed after the plasticizer is extracted, it is preferable that the relaxation operation at the time of heat fixing is performed in TD.

From the viewpoint of obtaining a polyolefin microporous film having higher strength and higher porosity, the magnification of the stretching operation during the heat fixing step is preferably 1.1 times or more, more preferably 1. times or more for the MD and / or TD of the film. It is 2 times or more, more preferably 1.4 times or more, preferably less than 2.3 times, and more preferably less than 2.0 times. When both MD and TD are stretched during heat fixing, the product of the stretch ratios of MD and TD is preferably less than 3.5 times, more preferably less than 3.0 times. When the draw ratio of MD and / or TD at the time of heat fixation is 1.1 times or more, the effects of high porosity and low heat shrinkage can be obtained, and when it is less than 2.3 times, it is excessive. It is possible to prevent an increase in pore diameter or a decrease in tensile elongation. When the product of the draw ratios of MD and TD during the heat treatment is less than 3.5 times, the increase in heat shrinkage can be suppressed.

When the stretching operation during the heat fixing step is simultaneous biaxial or sequential biaxial stretching of the sheet-shaped molded product or the porous film, the absolute value of the stretching strain rate is preferably 2% / s or more and less than 10% / s. , More preferably 3% / sec or more and 9% / s or less. Although it is not desired to be bound by the theory, when the stretching strain rate is 2% / s or more, the polymers are stretched in a state where the entanglement of the polymers in the sheet-shaped molded body is maintained, so that the polyolefin is microporous. It is presumed that the film has high strength and the pore size becomes uniform, and even if it melts and reaches a high temperature, the decrease in viscosity can be suppressed. When the stretching strain rate is less than 10% / s, the residual stress of the obtained polyolefin microporous film is reduced, and the heat shrinkage tends to be low, which is preferable.

The temperature in the stretching operation during the heat fixing step is preferably in the range of Tc or more (Tc + 10 ° C.) or less from the viewpoint of adjusting the average pore size of the polyolefin microporous film within the numerical range described above. From the viewpoint of suppressing TMA stress while maintaining the ion permeability of the membrane and maintaining the uniformity of pore size, from the viewpoint of adjusting the MD and TD tensile elastic modulus of the polyolefin microporous membrane within the numerical range described above, from the viewpoint of adjusting it within the numerical range described above. It is preferably in the range of (Tc + 5 ° C.) or higher (Tc + 15 ° C.) or lower. The specific stretching temperature can be, for example, in the range of 110 ° C. or higher and 140 ° C. or lower, or 123 ° C. or higher and 133 ° C. or lower.

The relaxation operation at the time of heat fixation is an operation of reducing the film to MD and / or TD. By performing the relaxation operation within a predetermined condition range, the stress decrease due to the temperature rise after melting can be moderated, and a polyolefin microporous film that does not easily break even at around 160 ° C. can be obtained.

The relaxation temperature (heat fixing temperature) during the heat fixing step is preferably 123 ° C. or higher and 133 ° C. or lower, and 129 ° C. or higher and 133 ° C. or lower, from the viewpoint of suppressing the TMA stress of the film and maintaining the uniformity of the pore size. Is more preferable.

The relationship between the temperature and the heat-fixing temperature in the stretching operation during the heat-fixing step is to adjust the air permeability, average pore size, and / or curvature of the obtained polyolefin microporous film within the numerical ranges described above. From the viewpoint, it is preferable to satisfy at least one of the following temperature conditions (a) and (b).
(A) 5 ° C <(heat fixation temperature-stretching temperature) <20 ° C
(A) (Tm-7 ° C) ≤ heat fixing temperature ≤ (Tm-3 ° C)

The relaxation rate is a value obtained by dividing the size of the film after the relaxation operation by the size of the film before the relaxation operation. When both MD and TD are relaxed, it is a value obtained by multiplying the relaxation rate of MD and the relaxation rate of TD. The relaxation rate is preferably 15% or less from the viewpoint of adjusting the TD tensile elastic modulus and / or the bending rate of the microporous polyolefin membrane within the numerical range described above. The relaxation rate is preferably more than 10% from the viewpoint of the heat shrinkage rate.

The mitigation operation may be performed on one or both of MD and TD. In the heat fixing step, the heat shrinkage of MD and / or TD of the obtained polyolefin microporous membrane can be controlled within an appropriate range by performing stretching and relaxation at the magnification and strain rate described above. ..

Further, from the viewpoint of adjusting the air permeability of the polyolefin microporous membrane within the numerical range explained above, the draw ratio of the polyolefin microporous membrane is 1.3 times when comparing before and after the heat fixing step. The above is preferable, and the TD stretching ratio is more preferably 1.3 times or more.

The sequence of all steps described above can be arbitrarily modified as long as the effects of the present invention are not compromised. After performing all the steps, the total draw ratio of the polyolefin microporous membrane is preferably 70 times or less, preferably 50 times or more and 70 times, from the viewpoint of adjusting the average pore size within the numerical range described above. It is more preferable that it is within the following range. In the present specification, the total draw ratio means a value obtained by multiplying the draw ratio of MD and / or TD in the draw step by the draw ratio and / or the relaxation ratio in the heat fixing step.

In addition, from the viewpoint of adjusting the film thickness-equivalent puncture strength of the polyolefin microporous film within the numerical range described above and improving the capacity of the electrochemical device on which the microporous film is mounted, it is finally determined. All steps are performed so that the average film thickness of the obtained polyolefin microporous film is preferably in the range of 3.0 μm or more and 18.0 μm or less, more preferably in the range of 5.0 μm or more and less than 14.0 μm. ..

<Separator for electrochemical device>
The microporous polyolefin membrane according to this embodiment can be used as a separator for an electrochemical device such as a lithium ion secondary battery. By incorporating the polyolefin microporous membrane into the lithium ion secondary battery, thermal runaway of the lithium ion secondary battery can be suppressed.

<Electrochemical device>
An electrochemical device in which a wound body or a laminated body formed by winding or laminating a plurality of polyolefin microporous membranes according to the present embodiment is also an aspect of the present invention. Examples of the electrochemical device include a non-aqueous electrolyte battery, a non-aqueous electrolyte battery, a non-aqueous lithium ion secondary battery, a non-aqueous gel secondary battery, a non-aqueous solid secondary battery, a lithium ion capacitor, an electric double layer capacitor, and the like. Can be mentioned.

The non-aqueous electrolyte battery according to the present embodiment includes the separator for a non-aqueous electrolyte battery containing the above-mentioned polyolefin microporous film, a positive electrode plate, a negative electrode plate, and a non-aqueous electrolyte solution (a non-aqueous solvent and a metal dissolved therein). Contains salt.). Specifically, for example, a positive electrode plate containing a transition metal oxide capable of occluding and releasing lithium ions and the like and a negative electrode plate capable of occluding and releasing lithium ions and the like are formed via a separator for a non-aqueous electrolyte battery. It is wound or laminated so as to face each other, holds a non-aqueous electrolyte solution, and is housed in a container.

The positive electrode plate will be described below. As the positive electrode active material, for example, a lithium composite metal oxide such as lithium nickelate, lithium manganate or lithium cobalt oxide, a 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, applied as a positive electrode paste to a positive electrode current collector such as aluminum foil, rolled to a predetermined thickness, and then cut to a predetermined size to obtain a positive electrode plate. Here, as the conductive agent, a metal powder stable under a positive electrode potential, for example, carbon black such as acetylene black or a graphite material can be used. Further, as the binder, a material stable under the positive electrode potential, for example, polyvinylidene fluoride, modified acrylic rubber, polytetrafluoroethylene, or the like 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 materials and the like can be used. Further, as the negative electrode active material of the non-aqueous electrolyte secondary battery, for example, metals, metal fibers, carbon materials, oxides, nitrides, silicon compounds, tin compounds, various alloy materials and the like can be used. In particular, a simple substance of silicon (Si) or tin (Sn) or a silicon compound or tin compound such as an alloy, compound, or solid solution is preferable because the capacity density of the battery tends to increase.

Examples of the carbon material include various natural graphites, cokes, developing carbons, carbon fibers, spherical carbons, various artificial graphites, and amorphous carbons.

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

The non-aqueous electrolyte solution will be described below. The non-aqueous electrolyte solution 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 ester, a chain carbonate ester, a cyclic carboxylic acid ester and the like are used. Examples of lithium salts 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 carboxylics. Examples thereof include lithium acid, LiCl, LiBr, LiI, borates, imide salts and the like.

Unless otherwise specified, the above-mentioned measurement methods for various parameters are measured according to the measurement methods in the examples described later.

Hereinafter, embodiments of the present invention will be specifically described with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples and Comparative Examples.

<TMA measurement (thermomechanical analysis)>
For the TMA measurement of the polyolefin microporous membrane, Shimadzu TMA50 (trademark) was used, and a tensile type was used as a dedicated probe. When measuring the value of MD (TD), a sample cut out to an MD (TD) of 15 mm and a width of 3.0 mm is fixed to the chuck so that the distance between the chucks is 10 mm, and is set in a dedicated probe. The initial load was 0.0049 N (0.5 gf), and the probe was heated from 30 ° C. to 250 ° C. at a rate of 10 ° C./min in the constant length mode. While reaching 250 ° C., the temperature and load were sampled at 1 second intervals to obtain the maximum load value.

<Tensile test>
The MD and TD tensile tests of the sample were performed using a tensile tester (Shimadzu Autograph AG-A type), and the tensile elastic modulus was (kgf / cm ² ) and the elongation (strain) of the sample was between 1 and 4%. It was obtained from the slope of the stress-strain straight line of. The measurement conditions are temperature: 23 ± 2 ° C., humidity: 40%, sample shape: width 10 mm × length 100 mm, distance between chucks: 50 mm, tensile speed: 200 mm / min.

<Viscosity average molecular weight (Mv)>
Based on ASTM-D4020, the ultimate viscosity [η] (dl / g) in a decalin solvent at 135 ° C. was determined.
The polyethylene and polyolefin microporous membranes were calculated by the following formula.
[Η] = 6.77 × 10 ^-4 Mv ^0.67
For polypropylene, Mv was calculated by the following formula.
[Η] = 1.10 × 10 ^-4 Mv ^0.80

<Gel Permeation Chromatography of Polyolefin Raw Materials (GPC)>
-Sample preparation The polyolefin raw material was weighed, and eluents 1,2,4-trichlorobenzene (TCB) were added so that the concentration became 1 mg / ml. After allowing to stand at 160 ° C. for 30 minutes using a high-temperature melter, the sample was shaken at 160 ° C. for 1 hour, and it was visually confirmed that all the samples had melted. The filtrate was filtered through a 0.5 μm filter at 160 ° C., and the filtrate was used as a GPC measurement sample.
-GPC measurement As a GPC device, PL-GPC220 (trademark) manufactured by Agilent was used, and two 30 cm columns of TSKgel GMHHR-H (20) HT (trademark) manufactured by Toso Co., Ltd. were used and adjusted as described above. 500 μl of the GPC measurement sample was injected into the measuring machine, and GPC measurement was performed at 160 ° C.
A calibration curve was prepared using commercially available monodisperse polystyrene having a known molecular weight as a standard substance, and polystyrene-equivalent molecular weight distribution data of each obtained sample was obtained. In the case of polyethylene, the polystyrene-equivalent molecular weight distribution data was obtained by multiplying the polystyrene-equivalent molecular weight distribution data by 0.43 (polyethylene Q factor / polystyrene Q factor = 17.7 / 41.3). In the case of polypropylene, polypropylene-equivalent molecular weight distribution data was obtained by multiplying by (polypropylene Q factor / polystyrene Q factor = 26.4 / 41.3). As a result, the weight average molecular weight (Mw) and the molecular weight distribution (Mw / Mn) of each sample were obtained.

<Measuring method of polypropylene content in microporous membrane>
The proportion of polypropylene contained in the polyolefin microporous film can be determined by infrared spectroscopy (IR) or Raman spectroscopy. For example, in order to calculate the ratio of polypropylene to polyethylene, ^{a peak of 1473 cm -1} derived from polyethylene and a peak of 1376 cm ^-1 derived from polypropylene were used as marker bands in the IR spectrum, and a sample having a known polypropylene content was prepared. The proportion of polypropylene can be calculated based on the calibration curve.

<DSC measurement (differential scanning calorimetry)>
The DSC was measured using a DSC60 manufactured by Shimadzu Corporation. A polyolefin resin or a microporous polyolefin membrane was inserted into an aluminum open sample pan having a diameter of 5 mm, a clamping cover was placed on the pan, and the film was fixed in the aluminum pan by a sample sealer. Under a nitrogen atmosphere, the temperature is raised from 30 ° C to 200 ° C at a heating rate of 10 ° C / min (first temperature rise), held at 200 ° C for 5 minutes, and then from 200 ° C to 30 ° C at a temperature lowering rate of 10 ° C / min. The temperature has dropped. Subsequently, after holding at 30 ° C. for 5 minutes, the temperature was raised again from 30 ° C. to 200 ° C. at a heating rate of 10 ° C./min (second temperature rise). If necessary, the maximum temperature in the crystallization heat generation curve during temperature reduction (crystallization temperature Tc of the resin sample), the peak rising temperature (Tm2onset) in the melting endothermic curve of the second temperature rise, and the maximum temperature (resin sample). Melting point Tm) and the like were observed. The crystallinity can be calculated by the following formula from the heat absorption amount ΔHm (J / g) obtained from the peak area at the time of the second temperature rise.
Crystallinity (%) = ΔHm / ΔH × 100
Here, ΔH is the amount of heat of fusion in a perfect crystal, and in the case of polyethylene, it can be calculated as ΔH = 293 J / g.

<Average pore size (μm) / curve ratio>
The average pore size and curving ratio can be measured by the gas-liquid method. Specifically, it is known that the fluid inside the capillary follows the Knudsen flow when the mean free path of the fluid is larger than the pore size of the capillary, and follows the Poiseuille flow when it is smaller. Therefore, it is assumed that the air flow in the permeability measurement of the microporous membrane follows the Knudsen flow and the water flow in the water permeability measurement follows the Poiseuille flow. In this case, the average pore diameter d (μm) and the curvature ratio τ _a (dimensionless) of the microporous film are the air permeation rate constant R _gas (m ³ / (m ² · sec · Pa)) and the 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), pore ratio ε It is obtained from (%) and the film thickness L (μm) by using the following formula.
d = 2ν × (R _liq / R _gas ) × (16η / 3P _s ) × 10 ⁶
τ _a = (d × (ε/100) × ν / (3L × P _s × R _gas )) ^1/2
Here, R _gas was obtained from the air permeability (seconds) using the following equation.
R _gas = 0.0001 / (air permeability × (6.424 × 10 ^-4 ) × (0.01276 × 101325))
R _liq was calculated from the water permeability (cm ³ / (cm ² · sec · Pa)) using the following equation.
R _liq = water permeability / 100
The water permeability was calculated as follows. A microporous membrane soaked in ethanol in advance is set in a stainless steel liquid-permeable cell having a diameter of 41 mm, the ethanol in the membrane is washed with water, and then water is permeated at a differential pressure of about 50,000 Pa for 120 seconds. From the water permeability (cm ³ ), the water permeation per unit time, unit pressure, and unit area was calculated, and this was taken as the water permeability.
Further, ν is a gas constant R (= 8.314 J / (K · mol)), an absolute temperature T (K), a pi, and an average molecular weight of air M (= 2.896 × 10 ⁻² kg / mol). Therefore, it was calculated using the following equation.
ν = ((8R × T) / (π × M)) ^1/2

<Average film thickness (μm)>
Using a micro thickener (Type KBM manufactured by Toyo Seiki), the film thickness of a 10 cm x 10 cm square sample was measured 9 times in an atmosphere of room temperature of 23 ° C and humidity of 40%, and the average value was calculated. did. If a 10 cm × 10 cm square sample cannot be secured, the film thickness of the sample is measured 5 times. In order to improve the measurement accuracy, when the average film thickness was less than 20 μm, the measurements were carried out by stacking the films until the total film thickness was 20 μm or more. The measurement was carried out by applying a load of 44 gf using a terminal having a terminal diameter of 5 mmφ.

<Porosity (%)>
A 10 cm × 10 cm square sample was cut from a microporous membrane, its volume (cm ³ ) and mass (g) were determined, and ^{the porosity was calculated from them and their density (g / cm 3} ) using the following equation.
Porosity (%) = (volume-mass / density) / volume x 100

<Air permeability (seconds / 100 cm ³ ) and film thickness equivalent ^{air permeability (seconds / 100 cm 3} / μm)>
Air permeability of polyolefin microporous membrane in an atmosphere of temperature 23 ° C and humidity 40% using a Garley type air permeability meter manufactured by Toyo Seiki Co., Ltd., GB2 (trademark) in accordance with JIS P-8117. The resistance was measured as the air permeability, and the film thickness-equivalent air permeability was calculated.

<Puncture strength (N) and film thickness equivalent puncture strength (N / μm)>
A microporous membrane was fixed with a sample holder having an opening diameter of 11.3 mm using a handy compression tester KES-G5 (trademark) manufactured by Katou Tech. Next, the central portion of the fixed microporous film was pierced at a maximum piercing by performing a piercing test at the tip of the needle with a radius of curvature of 0.5 mm, a piercing speed of 2 mm / sec, an atmosphere of a temperature of 23 ° C. and a humidity of 40%. The raw piercing strength (N) was obtained as a load, and the film thickness-equivalent piercing strength (N / μm) was further calculated.

<< Example 1 >>
<Manufacturing of microporous polyolefin membrane>
A microporous polyolefin membrane was prepared by the following procedure. The composition of the resin raw material is 15 parts by mass of homopolyethylene (Mw / Mn = 9.0, melting point Tm = 136.0 ° C., Tm2onset = 130.0 ° C.) having a viscosity average molecular weight of 300,000 as the first type of polyethylene (PE1). As the second type of polyethylene (PE2), 15 parts by mass of homopolyethylene (Mw / Mn = 11.0, melting point Tm = 136.0 ° C., Tm2onset = 130.0 ° C.) having a molecular weight average of 700,000, and polypropylene (PP). ) Was 2 parts by mass of isotactic polypropylene having a viscosity average molecular weight of 400,000. 0.3 parts by mass of tetrakis- (methylene-3- (3', 5'-di-t-butyl-4'-hydroxyphenyl) propionate) methane was mixed with the resin composition as an antioxidant. Each of the obtained mixtures was charged into a twin-screw extruder via a feeder. Further, liquid paraffin (kinematic viscosity at 37.78 ° C. 75.90 cSt) is injected into the extruder by side feed so that the total of the resin raw material + liquid paraffin is 100 parts by mass and the liquid paraffin is 68 parts by mass as a plasticizer. Then, the mixture was kneaded under the conditions that the kneading temperature was 160 ° C., the Q / N was 3.5 kg / (h · rpm), and the specific energy was 0.21 kWh / kg, and the mixture was extruded from a T die installed at the tip of the extruder. Immediately after extrusion, it was cooled and solidified with a cast roll cooled to 30 ° C. to form a sheet having a thickness of 1300 μm. This sheet was preheated under the conditions of a preheating temperature of 123 ° C. and a preheating coefficient of 217 minutes / ° C. Further, this sheet was stretched at a temperature of 124 ° C. in a simultaneous biaxial stretching machine at a stretching ratio (MD × TD) of 7 × 6.4 times and a surface magnification of 44.8 times. Next, the obtained stretched sheet was immersed in methylene chloride to extract and remove liquid paraffin. Then, the sheet was dried and stretched 1.5 times to TD under the conditions of a temperature of 123 ° C. and a strain rate of 7% / sec by a tenter stretching machine. Then, the stretched sheet was heat-treated to relax to TD under the conditions of a temperature of 130 ° C. and a relaxation rate of 12% to obtain a polyolefin microporous film.

<< Examples 2 to 10 and Comparative Examples 1 to 7 >>
Polyolefin microporous membranes of Examples 2 to 10 and Comparative Examples 1 to 7 were prepared under the film forming conditions shown in Table 1 according to the production method of Example 1. Regarding the raw material composition, the first type of polyethylene was indicated as PE1, the second type of polyethylene was indicated as PE2, and polypropylene was indicated as PP. It should be noted that the labeling of PE1, PE2 and PP is for convenience only, and the order of feeding the raw materials in the present invention is not limited to the order of PE1, PE2 and PP.

<< Example 11 >>
After producing a microporous film under the film forming conditions of Example 10, the film was thinned by the following procedure.
Using a feeding machine, the precursor sheets are fed out from each of the two rolls of the obtained single-layer microporous membrane precursor sheet, and the two sheets are set in one MD stretching machine in a stacked state to form a laminated porous film. Was formed and stretched 1.5 times in the longitudinal direction at a temperature of 126 ° C. Subsequently, the laminated porous film was set in a TD stretching machine, stretched 1.7 times in the TD direction at a temperature of 133 ° C., and heat-fixed. The thickness of the laminated porous membrane after heat fixing was 6.0 μm. Then, using a winder, the laminated porous membrane after heat fixing was wound as a roll.

A roll of the laminated porous membrane was set on a release slitter to separate the laminated porous membrane into two polyolefin microporous membranes. The physical characteristics of one microporous membrane after peeling are shown in Table 1.

<Making secondary batteries>
The positive electrode, the negative electrode, and the non-aqueous electrolytic solution were prepared by the following procedures a to c.
(A. Preparation of positive electrode)
As a positive electrode active material, nickel, manganese, cobalt composite oxide (NMC) (Ni: Mn: Co = 1: 1: 1 (element ratio), density 4.70 g / cm ³ ) is 90.4% by mass, a conductive auxiliary material. As graphite powder (KS6) (density 2.26 g / cm ³ , number average particle diameter 6.5 μm) is 1.6% by mass, and acetylene black powder (AB) (density 1.95 g / cm ³ , number average particle diameter). 48 nm) was mixed at a ratio of 3.8% by mass, and polyvinylidene fluoride (PVDF) (density 1.75 g / cm ³ ) as a binder was mixed at a ratio of 4.2% by mass, and these were mixed in N-methylpyrrolidone (NMP). A slurry was prepared by dispersing. This slurry is 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 obtain a positive electrode. Made. The amount of the positive electrode active material applied at this time was 109 g / m ² .

(B. Preparation of negative electrode)
As the negative electrode active material, graphite powder A (density 2.23 g / cm ³ , number average particle diameter 12.7 μm) was 87.6% by mass, and graphite powder B (density 2.27 g / cm ³ , number average particle diameter 6. 9.7% by mass (5 μm), 1.4% by mass (solid content equivalent) of ammonium salt of carboxymethyl cellulose as a binder (solid content concentration 1.83% by mass aqueous solution), and 1.7% by mass of diene rubber-based latex (solid). (Minute conversion) (solid content concentration 40% by mass aqueous solution) was dispersed in purified water to prepare a slurry. This slurry was applied to one side of a copper foil having a thickness of 12 μm 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 to prepare a negative electrode. The amount of the negative electrode active material applied at this time was 52 g / m ² .

(C. Preparation of non-aqueous electrolyte solution)
A non-aqueous electrolyte solution was prepared by dissolving _{LiPF 6} as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) so as to have a concentration of 1.0 mol / L.

(D. Battery production)
Using the positive electrode, the negative electrode, and the non-aqueous electrolyte solution obtained in the above a to c, and the separators obtained in Examples 1 to 10 and Comparative Examples 1 to 6, the current value was 1A (0.3C), and the termination was performed. A laminated secondary battery having a size of 100 mm × 60 mm and a capacity of 3 Ah was prepared by charging with a constant current constant voltage (CCCV) for 3 hours under the condition of a battery voltage of 4.2 V.

<Cycle test>
The cycle characteristics were evaluated by the following procedure.
(1) Pretreatment The simple battery is charged with a constant current of 1 / 3C to a voltage of 4.2V, then charged with a constant voltage of 4.2V for 8 hours, and then charged with a current of 1 / 3C to 3.0V. The voltage was discharged to the final voltage of. Next, after a constant current charge to a voltage of 4.2 V with a current value of 1 C, a constant voltage charge of 4.2 V was performed for 3 hours, and then discharged to a final voltage of 3.0 V with a current of 1 C. Finally, a constant current charge of 4.2 V was performed with a current value of 1 C, and then a constant voltage charge of 4.2 V was performed for 3 hours. Note that 1C represents a current value that discharges the reference capacity of the battery in one hour.

(2) Cycle test The pretreated battery is discharged to a discharge end voltage of 3 V with a discharge current of 1 C under the condition of a temperature of 25 ° C., and then charged to a charge end voltage of 4.2 V with a charge current of 1 C. went. Charging and discharging were repeated with this as one cycle. Then, using the capacity retention rate after 200 cycles with respect to the initial capacity (capacity in the first cycle), the cycle characteristics were evaluated according to the following criteria.

(3) Evaluation criteria for cycle characteristics A (Remarkably good): Capacity retention rate of 90% or more and 100% or less B (Good): Capacity retention rate of 85% or more and less than 90% C (Allowable): 80% or more and less than 85% Capacity retention rate D (defective): Capacity retention rate of less than 80%

<Oven test>
Using the above secondary battery, the charged secondary battery was heated from room temperature to 120 ° C. at 5 ° C./min and held in that state for 30 minutes. Then, the secondary battery was further heated to 150 ° C. at 30 ° C./min, the time until ignition was measured, and evaluated according to the following criteria. When the microporous polyolefin membrane broke during the test, the temperature was recorded as the oven rupture temperature.
A (Remarkably good): Those that did not ignite for 45 minutes or more at 150 ° C.
B (Good): Ignition occurred in 30 minutes or more and less than 45 minutes at 150 ° C.
C (allowable): Ignition occurred in 15 minutes or more and less than 30 minutes at 150 ° C.
D (defective): Those that ignited in less than 15 minutes at 150 ° C, or those that ignited before reaching 150 ° C.

<Heat-cooled circulation test>
Using the above secondary battery, the atmosphere of the secondary battery after charging was raised from room temperature to 75 ± 2 ° C., and kept in that state for 6 hours. After that, the temperature was lowered to −40 ± 2 ° C., and the state was maintained for 6 hours. The above ascending / descending temperature was set as one cycle, and the open circuit voltage (OCV) after 10 cycles was measured and evaluated according to the following criteria.
A (significantly good): OCV after 10 cycles is 97% or more of the voltage immediately after the test.
B (good): OCV after 10 cycles is 95% or more and less than 97% of the voltage immediately after the test.
C (allowable): OCV after 10 cycles is 90% or more and less than 95% of the voltage immediately after the test.
D (defective): OCV after 10 cycles is less than 90% of the voltage immediately after the test.

<Output test>
(1) 1C capacity measurement (mAh)
The above secondary battery is charged to a battery voltage of 4.2 V with a current value of 1.1 A (1.0 C) under an atmosphere of 25 ° C., and the current value is started to be throttled from 1.1 A so as to maintain 4.2 V. The battery was charged for a total of 3 hours. Next, the battery was discharged to a battery voltage of 2.0 V at a current value of 1.1 A (1.0 C) to obtain a 1 C discharge capacity.

(2) Output measurement (W / kg)
The battery whose initial capacity was measured in (1) above is charged to a battery voltage of 4.2 V with a current value of 1 C, and after reaching 4.2 V, the current value is started to be throttled for a total of 3 hours. Charge the battery to 100% SOC. After resting for 10 minutes, the current value of 0.3C is discharged to 50% SOC, and the rest is paused for 1 hour. After that, (a) discharge at 0.5C for 10 seconds, pause for 1 minute, charge for 10 seconds at 0.5C, pause for 1 minute, (b) discharge for 10 seconds at 1C, pause for 1 minute, charge for 10 seconds at 1C, charge for 1 minute. Pause, (c) 2C discharge for 10 seconds, 1 minute pause, 2C charge for 10 seconds, 1 minute pause, (d) 3C discharge for 10 seconds, 1 minute pause, 3C charge for 10 seconds, 1 minute pause, (e) ) Discharge for 10 seconds at 5C, pause for 1 minute, charge for 10 seconds at 5C, and pause for 1 minute.

The battery voltage after discharging for 10 seconds in each of the above (a) to (e) is measured, and each voltage is plotted against the current value. The current value at which the approximate straight line by the least squares method intersects the discharge lower limit voltage (V) was defined as (I), and the value divided by the battery mass (Wt) was defined as the output from the following equation.
Output (P) = (V × I) / Wt

The polyolefin microporous membranes obtained in Examples 1 to 11 and Comparative Examples 1 to 7 were evaluated according to the above evaluation method. Tables 1 to 3 show the film forming conditions and characteristics of the microporous membranes obtained in Examples 1 to 11 and Comparative Examples 1 to 7, and the evaluation results when they were incorporated into a secondary battery.

Claims (9)

In constant length thermomechanical analysis (TMA), the sum of the maximum stress in the longitudinal direction (MD) and the maximum stress in the width direction (TD) is 0.10 N / mm ² or more and less than 1.20 N / mm ^2.
The average film thickness is 3.0 μm or more and less than 18.0 μm.
It contains 1.0% by mass to 20.0% by mass of polypropylene and has a film thickness equivalent air permeability of 1.0 s / (100 cm ^3. μm) or more and 10.0 s / (100 cm ^3. μm) or less. Polyolefin microporous membrane. The polyolefin microporous membrane according to claim 1, wherein the average pore size is 0.030 μm or more and 0.075 μm or less. MD tensile modulus, 3000 kgf / cm ² or more 8000kgf / cm ² or less, and TD tensile modulus is 3000 kgf / cm ² or more 5000 kgf / cm ² or less, the microporous polyolefin according to claim 1 or 2 film. The polyolefin microporous film according to any one of claims 1 to 3, wherein the film thickness-equivalent puncture strength is 0.10 N / μm or more and 0.30 N / μm or less. The polyolefin microporous membrane according to any one of claims 1 to 4, wherein the bending ratio is 1.0 or more and 2.0 or less. The polyolefin microporous membrane according to any one of claims 1 to 5, which comprises 50.0% by mass to 99.0% by mass of high-density polyethylene and / or ultra-high molecular weight polyethylene. The polyolefin microporous membrane according to any one of claims 1 to 6, wherein the polyolefin microporous membrane has a viscosity average molecular weight of 300,000 or more and 700,000 or less. A separator for an electrochemical device, which comprises the polyolefin microporous membrane according to any one of claims 1 to 7. An electrochemical device comprising the separator for an electrochemical device according to claim 8.