JP2021021066A - Polyolefin microporous film, laminate, and non-aqueous electrolyte secondary battery using the same - Google Patents

Abstract

To provide a polyolefin microporous film for use as separator of non-aqueous electrolyte secondary battery such as lithium ion secondary battery, excellent in low resistance characteristics, capable of improving capacity retention rate under rapid charge/discharge conditions.SOLUTION: A polyolefin microporous film has 870 to 2800 fibrils/μm3, in a 2.7 μm square three-dimensional image created from each of cross sectional images obtained in FIB-SEM measurement of the microporous film.SELECTED DRAWING: None

Description

The present invention relates to a polyolefin-based microporous membrane, a laminate, and a non-aqueous electrolyte secondary battery using the same.

Thermoplastic resin microporous membranes are widely used as substance separation membranes, selective permeation membranes, isolation membranes and the like. Specific applications of microporous membranes include, for example, separators for non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, and polymer batteries, and separators for electric double layer capacitors. Various filters such as back-penetration filtration membranes, ultrafiltration membranes, precision filtration membranes, moisture-permeable waterproof clothing, medical materials, supports for fuel cells, etc.

In particular, a polyethylene microporous membrane is widely used as a separator for a lithium ion secondary battery. As its features, in addition to being excellent in mechanical strength that greatly contributes to battery safety and productivity, it also has ion permeability through the electrolytic solution that has penetrated into the micropores while ensuring electrical insulation, and the outside of the battery / At the time of an abnormal reaction inside, it has a pore closing function that suppresses an excessive temperature rise by automatically blocking the permeation of ions at about 120 to 150 ° C.

In addition, the use of lithium-ion secondary batteries for automobiles and home appliances is expanding, and the need for rapid charging and discharging is increasing in each application from the viewpoint of convenience. When the lithium ion secondary battery is rapidly charged and discharged, heat is generated due to the electrodes, separators, and other members that make up the lithium ion secondary battery, and the resistance that exists at the interface between the members, and the battery member. There was a problem that the thermal deterioration of the battery was promoted. In addition, when rapid charging / discharging is performed on a lithium ion secondary battery, the charging / discharging time is shortened, so that the time for lithium ions to penetrate into the fine parts of the electrode is short, and the number of ions that lithium ions are effectively taken in and out of. There is a problem that the battery capacity is reduced.

As an effort for a separator to improve the long-term reliability of a lithium ion secondary battery, a technique (Patent Document 1) for improving long-term compression resistance in a minute region of a film has been proposed, and specific stretching conditions have been proposed. By setting the parameters obtained by FIB-SEM image analysis for the coat layer of the polyolefin-based microporous film to a specific range, a technique for improving the battery capacity (rate characteristics) under rapid charge / discharge conditions (Patent Document 2). , A technique for reducing charging resistance (Patent Document 3) and the like have been proposed.

Japanese Unexamined Patent Publication No. 2009-242631 JP-A-2016-121327 JP-A-2018-181649

In Patent Documents 1 and 2, the puncture strength, porosity, heat shrinkage, film thickness retention rate due to puncture creep, etc. are adjusted by adjusting the raw material composition and manufacturing conditions, and the separator for a lithium ion secondary battery is used. Proposals have been made to improve long-term reliability when applied, but the internal structure of the microporous film has not been fully considered, and under low resistance characteristics that reduce the electrical resistance inside the battery and rapid charge / discharge conditions. In some cases, the capacity retention rate was insufficiently improved. Further, Patent Document 3 proposes to reduce the charging resistance by setting the fractal dimension of the insulating porous layer obtained by coating to a specific range, but the microporous film itself as the substrate to be coated is used. The structure of the film and the internal structure of the microporous film are not fully considered, and when considering the resistance of the base material and the coating layer as a whole, the strength and the capacity retention rate under rapid charge / discharge conditions are improved. In some cases, the compatibility between the two was insufficient.

Therefore, in the present invention, by eliminating the above-mentioned drawbacks and setting the fibril structure inside the polyolefin-based microporous film within a specific range, it can be applied as a separator for a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery. It is an object of the present invention to provide a polyolefin-based microporous film having excellent strength and improving the capacity retention rate during rapid charging and discharging.

The present invention for solving the above problems has the following configurations.
(1) Polyolefin having 870 fibrils / μm ³ or more and 2800 / μm ³ or less in a 2.7 μm square three-dimensional image created from each cross-sectional image obtained by FIB-SEM measurement of a microporous film. System microporous film.
(2) The polyolefin-based microporous membrane according to (1), which has an average fibril diameter of 25 nm or more and 60 nm or less.
(3) The polyolefin-based microporous membrane according to (1) or (2), wherein the number of fibril crossings is 1350 / μm ³ or more and 4400 / μm ³ or less.
(4) The polyolefin-based microporous membrane according to any one of (1) to (3), wherein the piercing strength is 180 gf or more and 700 gf or less.
(5) The polyolefin-based microporous membrane according to any one of (1) to (4), which has a thickness of 3 μm or more and 14 μm or less.
(6) The polyolefin-based microporous membrane according to any one of (1) to (5), wherein the porosity is 35% or more and 50% or less.
(7) The method according to any one of (1) to (6), wherein the maximum shrinkage stress temperature in the TD direction by the thermomechanical analyzer (TMA) is 143 ° C. or higher and the maximum shrinkage stress is 1.3 MPa or lower. Polyolefin-based microporous membrane.
(8) A laminate obtained by further laminating a heat-resistant resin layer on the polyolefin-based microporous membrane according to any one of (1) to (7).
(9) A non-aqueous electrolyte secondary battery comprising the polyolefin-based microporous membrane according to any one of (1) to (7) or the laminate according to (6).

The polyolefin-based microporous film according to the embodiment of the present invention has excellent strength when applied as a separator for a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery, and has a capacity retention rate during rapid charging and discharging. Has the effect of improving.

Hereinafter, the polyolefin-based microporous membrane according to the embodiment of the present invention will be described in detail. In this specification, when the numerical range is described as "A to B", it means the range of A or more and B or less.

The polyolefin-based microporous membrane according to the embodiment of the present invention has a number of fibrils of 870 / μm ³ in a 2.7 μm square three-dimensional image created from each cross-sectional image obtained by FIB-SEM measurement of the microporous membrane. It is important that the number is 2800 lines / μm ³ or less.

The number of fibrils can be used as one of the indexes representing the internal structure of the polyolefin-based microporous membrane.

The polyolefin-based microporous film in the embodiment of the present invention contains a polyolefin-based resin as a main component, and the main component is a polyolefin-based resin when the total mass of the polyolefin-based microporous film is 100% by mass. Means that it contains more than 50% by mass and 100% by mass or more. Here, examples of the polyolefin resin in the embodiment of the present invention include various polyethylene resins and various polypropylene resins, and the polyethylene resin in the embodiment of the present invention is the total mass of the polymer of the polyethylene resin. Is 100% by mass, it means a polymer in which the total amount of polyethylene-derived components exceeds 50% by mass and is 100% by mass or less.

In the present specification, the polyolefin-based microporous membrane may be simply referred to as "microporous membrane".

Further, the polypropylene-based resin in the embodiment of the present invention has a total of propylene-derived components exceeding 50% by mass and 100% by mass or less when the total mass of the polymer of the polypropylene-based resin is 100% by mass. Means a polymer of aspects.

In the present specification, the polyolefin-based microporous membrane may be simply referred to as "microporous membrane".

The polyethylene-based resin in the embodiment of the present invention is a homopolymer composed of ethylene only, or propylene, 1-butene, 1-pentene, 1-hexene, 3-methyl-1-butene, 3-methyl-1-pentene, 3-Ethyl-1-pentene, 4-methyl-1-pentene, 4-methyl-1-hexene, 4,4-dimethyl-1-hexene, 4,4-dimethyl-1-pentene, 4-ethyl-1 Chain olefins (α-olefins) such as −hexene, 3-ethyl-1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene, etc. Examples thereof include copolymers obtained by copolymerizing with.

Examples of the polypropylene-based resin in the embodiment of the present invention include a homopolymer composed of propylene only, or various polypropylene-based resins such as an ethylene-propylene copolymer, an ethylene-propylene-butene copolymer, and a propylene-butene copolymer. Be done.

Further, the polyolefin-based resin in the embodiment of the present invention may be either a single product or a mixture of two or more different polyolefin-based resins.

Among these various polyolefin resins, polyethylene is particularly preferable from the viewpoint of excellent pore closing performance. The melting point (softening point) of polyethylene is preferably 70 to 150 ° C. from the viewpoint of pore closing performance of the microporous membrane.

Hereinafter, a polyethylene resin will be described in detail as an example of the polyolefin resin used in the embodiment of the present invention. The types of polyethylene resin used in the embodiment of the present invention, medium density polyethylene in the range of high density polyethylene, such as density exceeds 0.94 g / cm ^3, density of 0.93~0.94g / cm ³ Examples thereof include low-density polyethylene having a density lower than 0.93 g / cm ^{3 and} linear low-density polyethylene. From the viewpoint of controlling the internal structure of the polyolefin-based microporous film described later to a desired range, polyolefin-based When the total mass of the microporous film is 100% by mass, it is preferable that the microporous film contains 80% by mass or more of ultra-high molecular weight polyethylene.

Ultra high molecular weight polyethylene used in the embodiment of the present invention has a weight average molecular weight of 1.0 × 10 ⁶ or more, preferably 1.0 × 10 ⁷ or less. When the weight-average molecular weight of 1.0 × 10 ⁶ or more, the relaxation time is fine fibrils suppressing an increase in the stretching temperature and the heat treatment temperature is not too short to melt, the pore number of the microporous membrane will be reduced Can be prevented. By weight average molecular weight is used 1.0 × 10 ⁶ or more ultra-high molecular weight polyethylene, increased entanglement of molecular chains, for uniformly stress the polyethylene resin layer in the stretching process is loaded, polyolefin described later It is possible to control the internal structure of the microporous membrane within a desired range. Therefore, the weight average molecular weight of the ultrahigh-molecular-weight polyethylene is preferably 1.0 × 10 ⁶ or more, more preferably 1.5 × 10 ⁶ or more, more preferably 2.0 × 10 ⁶ or more, and most preferably 3 is .0 × 10 ⁶ or more. The weight as the upper limit of the average molecular weight, preferably 8.0 × 10 ⁶ or less, more preferably 6.0 × 10 ⁶ or less, more preferably 5.0 × 10 ⁶ or less, and most preferably 4.0 × 10 It is ⁶ or less.

The molecular weight distribution (weight average molecular weight (Mw) / number average molecular weight (Mn)) of the ultra-high molecular weight polyethylene is preferably in the range of 3.0 to 100. The narrower the molecular weight distribution, the more unified the system is and the more uniform micropores can be obtained. Therefore, the narrower the molecular weight distribution is, the more preferable, but the narrower the distribution, the lower the molding processability. Therefore, the lower limit of the molecular weight distribution is preferably 4.0 or more, more preferably 5.0 or more, and further preferably 6.0 or more. As the molecular weight distribution increases, the low molecular weight components increase, which tends to reduce the strength and melt / fuse fine fibrils during stretching / heat fixation. Therefore, the upper limit is preferably 80 or less, more preferably 50 or less, still more preferable. Is 20 or less, most preferably 10 or less. Within the above range, good molding processability can be obtained and uniform micropores can be obtained because the system is unified.

High-density polyethylene used in the embodiment of the present invention, the weight-average molecular weight (Mw) of 1.0 × 10 ⁴ 1.0 × 10 ⁶ or less, 1.0 × 10 ⁵ 1.0 more preferably × 10 ⁶ or less, further preferably 5.0 × 10 ⁵ or more 9.0 × 10 ⁵ or less. By applying high-density polyethylene having a weight average molecular weight within the above range to the polyolefin-based microporous membrane of the embodiment of the present invention, pressure fluctuation of the resin in the extruder is less likely to occur, and uneven thickness of the polyolefin microporous membrane is reduced. In some cases, the quality of the microporous membrane can be improved.

The polyolefin-based microporous membrane of the present invention is made of ultra-high molecular weight polyethylene when the total mass of the polyolefin-based microporous membrane is 100% by mass from the viewpoint of controlling the internal structure of the polyolefin-based microporous membrane within a desired range. The content is preferably 80% by mass or more, more preferably 90% by mass or more, and further preferably 95% by mass or more. Further, from the viewpoint of controlling the internal structure of the polyolefin-based microporous film within a desired range, when the total mass of the polyolefin-based microporous film is 100% by mass, the ultra-high molecular weight polyethylene is 100% by mass. It may be present, but from the viewpoint of stabilizing the resin pressure in the extruder and improving the quality of the microporous film such as reducing the thickness unevenness of the polyolefin microporous film, it contains 20% by mass or less of high-density polyethylene. May be good.

In addition, the polyolefin-based microporous membrane according to the embodiment of the present invention is filled with an antioxidant, a heat stabilizer, an antistatic agent, an ultraviolet absorber, a blocking inhibitor, and the like, as long as the effects of the present invention are not impaired. Various additives such as materials may be contained. In particular, it is preferable to add an antioxidant for the purpose of suppressing oxidative deterioration due to the thermal history of the polyolefin resin.

Examples of the antioxidant include 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris (3,5-tris). It is preferable to use one or more selected from di-t-butyl-4-hydroxybenzyl) benzene, tetrakis [methylene-3 (3,5-di-t-butyl-4-hydroxyphenyl) propionate] methane and the like.

The FIB-SEM measurement in the embodiment of the present invention is an operation of scraping a cross section of a microporous membrane at regular intervals with an integrated ion beam (FIB) (FIB cutting), and an SEM (scanning electron microscope) image of the scraped surface. It refers to a method of measuring continuous images at regular intervals in the depth direction by repeating the operation of shooting. As a sample preparation method for FIB-SEM measurement, the microporous film according to the embodiment of the present invention is impregnated with an electronically dyed resin, and the pores are embedded, and then the cross section of the film is formed. Examples thereof include a method of preparing a section of a film cross section using a microtome so as to be an initial observation surface. Further, as a method for measuring FIB-SEM, there is a method in which a sample (section of a cross section of a film) prepared for measurement is cut by 10 nm in the depth direction and SEM images in the depth direction are sequentially taken. .. As a method of specifying the position information of each SEM image, a marking containing a metal component is made on a part of the observation screen of the microporous film to be FIB-cut, and each image is based on the marked position. A method of identifying the position correlation is used. The image area is preferably about 3 μm square to 10 μm square, and when the observation surface is inclined, the scale may be converted in consideration of the inclination.

Further, as a method of creating a three-dimensional image from each cross-sectional image, for example, using image processing software such as "ExFact (registered trademark) Analysis for Fiber" manufactured by Nippon Visual Science Co., Ltd., among the polyolefin-based microporous membranes, The part of the electronically dyed resin (that is, the part corresponding to the pores of the microporous film) and the part of the resin that constitutes the polyolefin-based microporous film (constituent resin) that are embedded and treated as bright areas. After performing binarization processing on the part), three-dimensional stereoscopic imaging is performed based on the information of the binarization processing, and image processing such as "ExFact (registered trademark) Analysis for Fiber" manufactured by Nippon Visual Science Co., Ltd. Examples thereof include a method of creating a three-dimensional image of the constituent resin portion inside the polyolefin-based microporous film by performing a thinning process of the resin portion constituting the polyolefin-based microporous film using software. Regarding the size of the three-dimensional image to be created, from the viewpoint of analysis time and reproducibility of analysis parameters, in the present invention, a cube surrounded by a side having a length of 2.7 μm is used.

The number of fibrils of the present invention is such that the constituent resin portion obtained by the three-dimensional image is thinned by using image processing software such as "ExFact (registered trademark) Analysis for Fiber" manufactured by Nippon Visual Science Co., Ltd. After obtaining the number of thin lines after the line of the resin part is divided at the intersecting and branched parts, the value is converted into the number per 1 μm ³ .

The polyolefin-based microporous film according to the embodiment of the present invention has a number of fibrils of 870 / μm ³ in a 2.7 μm square three-dimensional image created from each cross-sectional image obtained by FIB-SEM measurement of the microporous film. It is important that the number is 2800 lines / μm ³ or less.

In the polyolefin-based microporous membrane according to the embodiment of the present invention, cleavage of a higher-order structure occurs in the stretching step of the production method example described later. In the present invention, the fact that the number of fibrils is 870 / μm ³ or more indicates that the number of fibrous resin portions after cleavage is sufficient, and the formation of pores in the polyolefin-based microporous film is formed. It shows that the lithium ions can move smoothly when the process progresses sufficiently and the lithium ion secondary battery is used as a separator.

The polyolefin-based microporous film according to the embodiment of the present invention has a number of fibrils of 870 / μm ³ in a 2.7 μm square three-dimensional image created from each cross-sectional image obtained by FIB-SEM measurement of the microporous film. By doing so, it is possible to reduce the electrical resistance of the separator, suppress thermal deterioration of the battery, and improve the capacity retention rate during rapid charging and discharging. From the viewpoint of reducing electrical resistance, suppressing thermal deterioration, and improving the capacity retention rate during rapid charging and discharging, the number of fibrils is preferably 1000 / μm ³ or more, more preferably 1250 / μm ³ or more, and 1400. / Μm ³ or more is particularly preferable. On the other hand, if the number of fibrils is too large, the number of paths through which lithium ions pass may decrease and the resistance may increase. Therefore, 2800 lines / μm ³ or less is important.

In the present invention, in a 2.7 μm square three-dimensional image created from each cross-sectional image obtained by FIB-SEM measurement of a microporous film, the number of fibrils is 870 lines / μm ^{3 and} 2800 lines / μm ³ or less. 80% by mass or more of the resin constituting the polyolefin-based microporous film is ultra-high molecular weight polyethylene, the surface magnification of wet stretching is 60 times or more, and the resin concentration at the time of production is less than 30% by mass. The method can be mentioned. In the embodiment of the present invention, 80% by mass or more of the resin constituting the polyolefin-based microporous film is ultra-high molecular weight polyethylene, and the resin concentration at the time of production is less than 30% by mass, so that the cast sheet before stretching is used. It was found that the spherulite growth of polyethylene can be suppressed and the cast sheet structure can be made uniform. Furthermore, by combining with wet stretching with a wet surface magnification of 60 times or more, it is possible to significantly reduce the places where the pores are insufficiently formed and uniformly form the pores of the polyolefin-based microporous film. It becomes. Further, by uniformly forming the pores of the polyolefin-based microporous film, it is possible to increase the number of fibrils by reducing the locations where the pores are insufficiently formed.

Next, the average diameter of fibrils and the number of fibril crossings will be described. The average diameter of fibrils and the number of crossing fibrils can be used as one of the indexes representing the internal structure of the polyolefin-based microporous membrane.

The polyolefin-based microporous film of the present invention reduces the electrical resistance inside the battery when used as a separator for a lithium ion secondary battery, and improves the capacity retention rate during rapid charging and discharging. -It is preferable that the average diameter of the fibrils is 25 nm or more and 60 nm or less, which is obtained by analyzing a 2.7 μm square three-dimensional image created from each cross-sectional image obtained by SEM measurement. Here, the average diameter of fibrils is 2.7 μm square obtained by using image processing software such as “ExFact (registered trademark) Analysis for Fiber” manufactured by Nippon Visual Science Co., Ltd. in the same manner as the evaluation method of the number of fibrils. Refers to the average value of all fibril diameters contained in a three-dimensional image. By setting the fibril diameter within a specific range, lithium ions can move smoothly when used with a separator of a lithium ion secondary battery.

The average diameter of the fibrils in the embodiment of the present invention is more preferably 55 nm or less, further preferably 45 nm or less, and further preferably 35 nm or less, from the viewpoint of reducing electrical resistance and further enhancing the effect of improving the capacity retention rate during rapid charging / discharging. Is particularly preferable. On the other hand, from the viewpoint of improving mechanical properties such as strength, the average diameter of fibrils is preferably 25 nm or more.

In the embodiment of the present invention, as a method of setting the average diameter of the fibrils to 25 nm or more and 60 nm or less, a method of setting the wet stretching speed to a low specific range and uniformizing the formation of pores during stretching, or a method having two or more steps Examples thereof include a method of applying stepwise stretching to gradually promote the formation of pores.

The polyolefin-based microporous film according to the embodiment of the present invention has a fibril crossing number of 1350 / μm ³ or more and 4400 / μm ³ or less, which causes electrical resistance when used as a separator for a lithium ion secondary battery. It is preferable from the viewpoint of reducing the capacity and improving the capacity retention rate during rapid charging / discharging. Here, the number of fibril crossings is the same as the evaluation method for the number of fibrils, and the constituent resin portion of the microporous film is defined by using image processing software such as "ExFact (registered trademark) Analysis for Fiber" manufactured by Nippon Visual Science Co., Ltd. This is the number of points where the constituent resin parts intersect each other when the line is divided, and the number of points per 1 μm ³ after obtaining the number of points where the constituent resin parts intersect in a 2,7 μm square three-dimensional image. The value converted to.

In the embodiment of the present invention, the large number of fibril crossings indicates that a fine network structure is formed in the microporous membrane, and the formation of pores in the polyolefin-based microporous membrane is sufficiently advanced. However, it is shown that lithium ions can move smoothly when used as a separator for a lithium ion secondary battery.

In the embodiment of the present invention, the number of fibril crossings is more preferably 1700 / μm ³ or more, more preferably 2200 / μm, from the viewpoint of reducing electrical resistance and further enhancing the effect of improving the capacity retention rate during rapid charging / discharging. ³ or more is more preferable, and 3000 pieces / μm ³ or more is particularly preferable. On the other hand, from the viewpoint of improving mechanical properties such as strength, the number of fibril crossings is more preferably 4200 / μm ³ or less.

In the embodiment of the present invention, as a method of setting the number of fibril crossings to 1350 / μm ³ or more and 4400 / μm ³ or less, a large relaxation rate is set in the heat treatment step after wet stretching, and shrinkage in the plane direction is performed. Examples thereof include a method of advancing and making the hole path in the thickness direction uniform in the linear direction.

The polyolefin-based microporous membrane according to the embodiment of the present invention preferably has a piercing strength of 180 gf or more and 700 gf or less from the viewpoint of improving the impact resistance of the battery when used as a separator for a lithium ion secondary battery. .. From the viewpoint of further enhancing the impact resistance of the battery when used as a separator for a lithium ion secondary battery, the puncture strength is more preferably 250 gf or more, further preferably 350 gf or more, and particularly preferably 500 gf or more. Further, from the viewpoint of impact resistance of the battery, the higher the strength of the polyolefin-based microporous film is, the more preferable it is, but from the viewpoint of improving the balance with other physical characteristics such as the heat shrinkage rate, 700 gf or less is preferable. The piercing strength in the present invention is the piercing strength when the thickness is converted to 10 μm.

In the embodiment of the present invention, as a method of setting the puncture strength of the polyolefin-based microporous membrane to 180 gf or more and 700 gf or less, 70% by mass or more of the constituent resin of the polyolefin-based microporous membrane is made of ultra-high molecular weight polyethylene and wet. Examples thereof include a method in which the surface magnification of stretching is 60 times or more and the porosity of the polyolefin-based microporous film is 35% or more and 55% or less.

When the polyolefin-based microporous membrane according to the embodiment of the present invention has a thickness of 3 μm or more and 14 μm or less, the distance between the electrodes can be reduced when used as a separator for a lithium ion battery, and the battery member can be used. Since it is possible to increase the number of layers, it is preferable from the viewpoint of increasing the capacity of the battery. As a method for setting the thickness to 3 μm or more and 14 μm or less, a wet stretching method is adopted, and a method of increasing the stretching ratio and the line speed at the time of manufacturing is used. From the viewpoint of increasing the capacity of the battery, the thickness of the polyolefin-based microporous membrane is more preferably 12 μm or less, further preferably 10 μm or less, and particularly preferably 7 μm or less.

The polyolefin-based microporous membrane according to the embodiment of the present invention has a maximum shrinkage stress temperature of 143 ° C. or higher in the TD direction and a maximum shrinkage stress of 1.3 MPa or lower by a thermomechanical analyzer (TMA). It is preferable from the viewpoint of the safety of the secondary battery. When the lithium ion secondary battery becomes hot due to rapid charging and discharging, the contraction stress of the polyolefin-based microporous membrane contained in the lithium ion secondary battery increases, causing deformation in the TD direction that is not particularly wound. It will be easier. Deformation of the polyolefin-based microporous film in the TD direction causes insufficient insulation inside the lithium-ion secondary battery, which may cause thermal runaway and ignition of the lithium-ion battery. Therefore, the safety of the lithium-ion secondary battery From the viewpoint of enhancing the properties, in the embodiment of the present invention, it is preferable that the maximum contraction stress temperature in the TD direction by the thermomechanical analyzer (TMA) is high and the maximum contraction stress is low. However, from the viewpoint of the balance of the path structure, fibril structure, strength, etc. of the polyolefin-based microporous membrane, the maximum shrinkage stress temperature in the TD direction by the thermomechanical analyzer (TMA) is preferably 150 ° C. or less, and the maximum shrinkage stress is A range of 0.6 MPa or more is preferable.

In the embodiment of the present invention, as a method of setting the maximum shrinkage stress temperature in the TD direction to 143 ° C. or higher and the maximum shrinkage stress to 1.3 MPa or lower by a thermomechanical analyzer (TMA), ultra-high molecular weight polyethylene is used as a main component. In this configuration, a method of strengthening the relaxation of strain of the polyolefin-based microporous film by setting the heat fixing temperature to 130 ° C. or higher and the relaxation rate to 15% or higher can be mentioned.

The polyolefin-based microporous membrane according to the embodiment of the present invention has pores from the viewpoint of setting the fibril structure in a specific range, improving puncture strength, safety when used as a separator for a lithium ion secondary battery, and the like. The rate is preferably 35% or more and 50% or less.

In the embodiment of the present invention, examples of the method of setting the pore ratio to 35% or more and 50% or less include a method of adjusting various manufacturing conditions such as a draw ratio, a draw temperature, a heat treatment temperature, and a heat treatment time.

The polyolefin-based microporous membrane according to the embodiment of the present invention may be further laminated with a heat-resistant resin layer to form a laminated body from the viewpoint of improving heat resistance when mounted on a lithium ion battery. As the heat-resistant resin layer, a resin that is insoluble in the electrolytic shadow of the battery, such as various fluororesins, acrylic resins, and aromatic polyamide diameter resins, and that is electrically stable within the battery usage conditions is preferably used. .. Further, the heat-resistant resin layer may contain an organic powder, an inorganic powder, or a mixture thereof as a filler from the viewpoint of further improving the heat resistance. For example, the organic powder includes a fluorine-based resin, a melamine-based resin, and the like. Aromatic polyamide-based resins and the like are inorganic powders such as metal oxides, metal nitrides, metal carbides, metal hydroxides, carbonates, sulfates, and more specifically alumina, silica, titanium dioxide, and hydroxide. Aluminum, calcium carbonate and the like can be used.

Next, an example of a method for producing a polyolefin-based microporous membrane according to an embodiment of the present invention will be described below, but the present invention is not construed as being limited to such an example.

The method for producing a polyolefin-based microporous membrane according to the embodiment of the present invention preferably has the following steps (a) to (e).
(A) Step of melt-kneading a polymer material containing one or more kinds of polyolefin resins and, if necessary, a solvent to prepare a polyolefin resin solution (b) Extruding the solution and molding it into a sheet. Step of cooling and solidifying (c) Step of stretching the obtained sheet by a roll method or a tenter method (d) Then, a step of extracting a plasticizer from the obtained stretched film and drying the film (e) Heat treatment / re-stretching Process to do

Hereinafter, each step will be described.
(A) Step of preparing a polyolefin-based resin solution The polyolefin-based resin used according to the embodiment of the present invention is heated and dissolved in a plasticizer to prepare a polyolefin-based resin solution. The plasticizer is not particularly limited as long as it is a solvent capable of sufficiently dissolving the polyolefin resin, but the solvent is preferably a liquid at room temperature in order to enable stretching at a relatively high magnification.

Solvents include aliphatic, cyclic aliphatic or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin, mineral oil distillates having corresponding boiling points, and dibutylphthalates. Examples thereof include phthalates that are liquid at room temperature, such as dioctyl phthalates. In order to obtain a gel-like sheet having a stable liquid solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin.

The ratio of the solvent is preferably 400 parts by mass or more and 900 parts by mass or less with respect to 100 parts by mass of the total mass of the polyethylene resin from the viewpoint of facilitating control of the number of fibrils in a specific range.

In the melt-kneaded state, it is miscible with the polyolefin resin, but at room temperature, a solid solvent may be mixed with the liquid solvent. Examples of such a solid solvent include stearyl alcohol, ceryl alcohol, paraffin wax and the like. However, if only a solid solvent is used, uneven stretching may occur.

The viscosity of the liquid solvent is preferably 20 to 200 cSt at 40 ° C. When the viscosity at 40 ° C. is 20 cSt or more, the sheet obtained by extruding the polyolefin resin solution from the die is unlikely to become non-uniform. On the other hand, if the viscosity at 40 ° C. is 200 cSt or less, the liquid solvent can be easily removed. The viscosity of the liquid solvent is the viscosity measured at 40 ° C. using an Ubbelohde viscometer.

(B) Formation of Extruded Product and Formation of Gel-like Sheet The uniform melt-kneading method of the polyolefin-based resin solution is not particularly limited, but when it is desired to prepare a high-concentration polyolefin-based resin solution, it is carried out in a twin-screw extruder. Is preferable. If necessary, metal soaps such as calcium stearate, ultraviolet absorbers, light stabilizers, antistatic agents and other known additives are also added as long as the effects of the present invention are not impaired without impairing the film-forming property. You may. In particular, it is preferable to add an antioxidant in order to prevent oxidation of the polyolefin resin.

In the extruder, the polyolefin resin solution is uniformly mixed at a temperature at which the polyolefin resin is completely melted. The melt-kneading temperature varies depending on the polyolefin resin used, but is preferably (melting point of the polyolefin resin + 10 ° C.) to (melting point of the polyolefin resin + 120 ° C.). More preferably, it is (melting point of polyolefin resin + 20 ° C.) to (melting point of polyolefin resin + 100 ° C.).

Here, the melting point means a value measured by DSC (Differential scanning calorimetry) based on JIS K7121 (1987). For example, when the polyolefin-based resin is a polyethylene-based resin, the melt-kneading temperature of the polyethylene-based resin is preferably in the range of 140 to 250 ° C. More preferably, it is 160 to 230 ° C, and most preferably 170 to 200 ° C. Specifically, since the polyethylene resin has a melting point of about 130 to 140 ° C., the melt-kneading temperature is preferably 140 to 250 ° C., most preferably 180 to 230 ° C.

From the viewpoint of suppressing deterioration of the polyolefin resin, it is preferable that the melt-kneading temperature is low, but if the temperature is lower than the above temperature, unmelted material is generated in the extruded product extruded from the die, and the film breaks or the like in the subsequent stretching step. May cause. On the other hand, if the temperature is higher than the above temperature, the thermal decomposition of the polyolefin-based resin becomes severe, and the physical properties of the obtained polyolefin-based microporous film, for example, the strength and the porosity may deteriorate. In addition, the decomposed product precipitates on a cooling roll, a roll in the stretching process, or the like and adheres to the sheet, which leads to deterioration of the appearance. Therefore, it is preferable that the melt-kneading temperature is within the above range.

Next, a gel-like sheet is obtained by cooling the obtained extruded product, and the microphase of the polyolefin resin separated by the solvent can be immobilized by cooling. It is preferable to cool the gel sheet to 10 to 50 ° C. in the cooling step. This is because the final cooling temperature is set to be equal to or lower than the crystallization end temperature, and by making the higher-order structure finer, uniform stretching can be easily performed in the subsequent stretching. Therefore, cooling is preferably performed at a rate of 30 ° C./min or higher at least up to the gelation temperature or lower.

Generally, when the cooling rate is slow, relatively large crystals are formed, so that the higher-order structure of the gel-like sheet becomes coarse, and the gel structure forming the gel structure also becomes large. On the other hand, when the cooling rate is high, small and uniform crystals are formed, so that the higher-order structure of the gel-like sheet becomes dense, which leads to uniform stretching and reduction of unopened portions.

Examples of the cooling method include a method of directly contacting with cold air, cooling water, and other cooling media, a method of contacting with a roll cooled with a refrigerant, a method of using a casting drum, and the like.

Although the case where the polyolefin-based microporous membrane is a single layer has been described so far, the polyolefin-based microporous membrane according to the embodiment of the present invention is not limited to a single layer, and may be a laminated body. The number of layers is not particularly limited, and may be two layers or three or more layers.

As a method of forming a polyolefin-based microporous membrane as a laminate, for example, desired resins are prepared as required, and these resins are separately supplied to an extruder and melted at a desired temperature to form a polymer tube or a die. Examples thereof include a method of forming a laminated body by merging the layers inside and extruding from a slit-shaped die with a desired thickness of each layer.

(C) Stretching Step The obtained gel-like (including laminated sheet) sheet is stretched. The stretching method used includes uniaxial stretching in the sheet transport direction (MD direction) by a roll stretching machine, uniaxial stretching in the sheet width direction (TD direction) by a tenter, a roll stretching machine and a tenter, or a combination of a tenter and a tenter. Examples thereof include sequential biaxial stretching by the above and simultaneous biaxial stretching by the simultaneous biaxial tenter.

The draw ratio varies depending on the thickness of the gel-like sheet from the viewpoint of uniformity of the thickness of the film, but it is preferable to stretch 7 times or more in any direction. Further, from the viewpoint of setting the number of fibrils in a desired range, the surface magnification is preferably 60 times or more, more preferably 80 times or more, and particularly preferably 100 times or more. Further, from the viewpoint of suppressing tearing during the production of the polyolefin-based microporous film, the surface magnification is preferably 150 times or less.

From the viewpoint of improving the stretching uniformity in the stretching step, the preferred forms of the stretching ratio and the raw material composition are ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of 1 million or more, and the total mass of all polyolefin resins contained in the gel sheet. When the content is 100% by mass, the composition contains 80% by mass or more by stretching from a wet gel sheet with a surface magnification of 60 times or more, and more preferably 10 × 10 times or more by wet stretching. is there. A more preferable form contains ultra-high molecular weight polyethylene having a weight average molecular weight (Mw) of 2 million or more, which is 80% by mass or more when the total mass of the total polyolefin resin contained in the gel sheet is 100% by mass. As a configuration, it is stretched from a wet gel sheet at a surface magnification of 60 times or more, and most preferably it is stretched wet at a surface magnification of 10 × 10 times or more.

The stretching temperature is preferably in the range of (melting point of the gel-like sheet + 10 ° C. or lower) to (crystal dispersion temperature of polyolefin resin Tcd) to (melting point of the gel-like sheet + 5 ° C.). Specifically, since the polyethylene composition has a crystal dispersion temperature of about 90 to 110 ° C., the stretching temperature is preferably 100 to 130 ° C., more preferably 115 to 125 ° C., still more preferably 117. It is 5 to 125 ° C. The crystal dispersion temperature Tcd is obtained from the temperature characteristics of dynamic viscoelasticity measured according to ASTM D 4065 (2012).

If the stretching temperature is less than 90 ° C., the formation of pores is insufficient due to low temperature stretching, it is difficult to obtain uniformity in the thickness of the film, and the pore ratio is also low. If the stretching temperature is higher than 130 ° C., the sheet may be melted and the pores may be easily closed.

Due to the above stretching, the higher-order structure of the gel sheet is cleaved, the crystal phase becomes finer, and a large number of fibrils are formed. Fibrils form a three-dimensionally irregularly connected network structure. Since the mechanical strength is improved by stretching and pores are formed, the polyolefin-based microporous membrane according to the embodiment of the present invention is suitable for a battery separator.

Further, by stretching before removing the plasticizer, the polyolefin-based resin is in a state of being sufficiently plasticized and softened, so that the higher-order structure is smoothly cleaved and the crystal phase is uniformly refined. Can be done. In addition, since the higher-order structure is easily cleaved by stretching before removing the plasticizer, strain during stretching is less likely to remain, and the heat shrinkage rate should be lower than when stretching after removing the plasticizer. Can be done.

(D) Plasticizer extraction (cleaning) / drying step Next, the plasticizer (solvent) remaining in the gel-like sheet is removed using a cleaning solvent. Since the polyolefin-based resin phase and the solvent phase are separated, a polyolefin-based microporous membrane can be obtained by removing the solvent.

Examples of the cleaning solvent include saturated hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, ethers such as diethyl ether and dioxane, ketones such as methyl ethyl ketone, and ethane trifluoride. Chain fluorocarbon and the like can be mentioned.

These cleaning solvents have low surface tension (eg, 24 mN / m or less at 25 ° C.). By using a cleaning solvent with a low surface tension, the reticulated structure that forms microporous is suppressed by the surface tension of the gas-liquid interface during drying after cleaning, and polyolefin-based microporous with excellent porosity and permeability. A film is obtained. These cleaning solvents are appropriately selected according to the plasticizer and used alone or in combination.

Examples of the cleaning method include a method of immersing the gel-like sheet in a cleaning solvent for extraction, a method of showering the gel-like sheet with the cleaning solvent, a method using a combination thereof, and the like. The amount of the cleaning solvent used varies depending on the cleaning method, but is generally preferably 300 parts by mass or more with respect to 100 parts by mass of the gel sheet.

The washing temperature may be 15 to 30 ° C., and if necessary, heat to 80 ° C. or lower. At this time, from the viewpoint of enhancing the cleaning effect of the cleaning solvent, from the viewpoint of preventing the physical properties of the obtained reolefin-based microporous film (for example, the physical properties in the TD direction and / or the MD direction) from becoming non-uniform, the polyolefin-based microporous film. From the viewpoint of improving the mechanical and electrical characteristics of the gel sheet, the longer the gel sheet is immersed in the cleaning solvent, the better.

The above-mentioned washing is preferably carried out until the residual solvent in the gel-like sheet after washing, that is, the polyolefin-based microporous membrane becomes less than 1% by mass.

Then, in the drying step, the solvent in the polyolefin-based microporous membrane is dried and removed. The drying method is not particularly limited, and a method using a metal heating roll, a method using hot air, or the like can be selected. The drying temperature is preferably 40 to 100 ° C, more preferably 40 to 80 ° C. If the drying is insufficient, the porosity of the polyolefin-based microporous membrane may decrease in the subsequent heat treatment, and the permeability may deteriorate.

(E) Heat Treatment / Re-stretching Step The dried polyolefin-based microporous membrane may be stretched (re-stretched) at least in the uniaxial direction. The re-stretching can be performed by the tenter method or the like in the same manner as the above-mentioned stretching while heating the polyolefin-based microporous membrane. The re-stretching may be uniaxial stretching or biaxial stretching. In the case of multi-stage stretching, simultaneous biaxial or sequential stretching is performed in combination.

The re-stretching temperature is preferably equal to or lower than the melting point of the polyolefin resin, and more preferably within the range of (Tcd-20 ° C. of the polyolefin resin composition) to the melting point of the polyolefin resin. Specifically, in the case of a polyethylene-based resin, the re-stretching temperature is preferably 70 to 135 ° C., more preferably 110 to 135 ° C., further preferably 125 to 135 ° C., and even more preferably 130 to 135 ° C.

In the case of uniaxial stretching, the re-stretching ratio is preferably 1.0 to 2.0 times, particularly preferably 1.1 to 1.6 times, and more preferably 1.2 to 1.4 times in the TD direction. .. When biaxial stretching is performed, it is preferable to stretch 1.01 to 2.0 times in the MD direction and the TD direction, respectively. The re-stretching magnification may be different in the MD direction and the TD direction. By re-stretching within the above range, the porosity and permeability can be increased, the reaggregation of fibrils due to shrinkage can be suppressed, and the porosity of the polyolefin-based microporous membrane can be uniformly formed.

From the viewpoint of heat shrinkage and wrinkles and sagging, the relaxation rate from the maximum re-stretching ratio is preferably 20% or less, more preferably 10% or less, still more preferably 5% or less. When the relaxation rate is 20% or less, a uniform fibril structure can be obtained.

(F) Other Steps Further, depending on other uses, the polyolefin-based microporous membrane can be hydrophilized. The hydrophilization treatment can be performed by monomer grafting, surfactant treatment, corona discharge, or the like. The monomer graft is preferably carried out after the cross-linking treatment.

The method for measuring the characteristics and the method for evaluating the effect in the embodiment of the present invention are as follows. However, the embodiments of the present invention are not limited to these examples.

(1) Weight average molecular weight (Mw)
The weight average molecular weight of the polyethylene resin was determined by the gel permeation chromatography (GPC) method under the following conditions.
-Measuring device: GPC-150C manufactured by WATERS CORPORATION
-Column: SHODEX UT806M manufactured by Showa Denko KK
-Column temperature: 135 ° C
-Solvent (mobile phase): O-dichlorobenzene-Solvent flow rate: 1.0 mL / min-Sample concentration: 0.1 wt% (Dissolution condition: 135 ° C / 1H)
-Injection amount: 500 μL
-Detector: WATERS CORPORATION differential refractometer (RI detector)
-Calibration curve: Prepared from a calibration curve obtained using a monodisperse polystyrene standard sample using a predetermined conversion constant.

(2) Thickness After cutting an evaluation sample of a polyolefin-based microporous membrane to a size of 95 mm × 95 mm, marks were added so that two 40 mm square lattices were arranged vertically and horizontally, for a total of four. When filling in the marks, make sure that the apex of the center where the four grids overlap overlaps with the center position of the evaluation sample, and that the sides of the vertical and horizontal grids are parallel to the sides of the evaluation sample. went. The thickness of a total of 9 points corresponding to the apex of a total of 4 grids was measured with a contact thickness meter (Lightmatic manufactured by Mitutoyo Co., Ltd.), and the average value of the thicknesses of the 9 points was obtained.

(3) Pore ratio A sample for evaluation is cut out from a polyolefin-based microporous membrane in a size of 5 cm square, and its volume (cm ³ ) and mass (g) are determined, and from them and the resin density (g / cm ³ ), It was calculated using the following equation. The above measurement was performed at 5 points at different random sampling points in the same polyolefin-based microporous membrane, and the average value of the porosity at the 5 points was obtained. The resin density was determined by JIS K6922-2-2010 after the polyolefin-based microporous membrane was heated and melted to form a non-porous sheet.
Pore ratio = [(volume-mass / resin density) / volume] x 100

(4) Puncture strength Using a piercing meter manufactured by MARUBISHI, a needle with a spherical tip (radius of curvature R: 0.5 mm) and a diameter of 1 mm has a thickness of T1 (μm) and a polyolefin-based microporous film of 2 mm / sec. The measured value obtained by measuring the maximum load when piercing at a speed was defined as the piercing strength L1 (gf).
The piercing strength L1 (gf) is converted into the maximum load when the thickness is 10 μm by the formula: L2 (gf) = L1 (gf) / T1 (μm) × 10 μm, and the piercing strength L2 (gf) converted to a thickness of 10 μm. And said.
The above measurement was performed at 3 points at different random sampling points in the same polyolefin-based microporous membrane, and the average values of the piercing strength L1 (gf) and the piercing strength L2 (gf) converted to a thickness of 10 μm at the three points were taken respectively. The average value of the piercing strength L2 (gf) converted to a thickness of 10 μm was determined and listed in the table as “piercing strength (converted to 10 μm)”.

(5) FIB-SEM
Continuous images by FIB-SEM were measured under the following conditions.
-Sample preparation: After embedding a polyolefin-based microporous membrane with an epoxy-based resin, it was subjected to electron staining with OsO ₄ and used for measurement.
-Observation device: Helios G4 made by FEI
・ Observation condition: Acceleration voltage 1kV
・ Sample tilt: 52 °
-Pixel size: Image horizontal direction: 5.4 nm, image vertical direction: 6.8 nm (after tilt correction)
・ Slice interval in FIB: 10 nm
-Image alignment method: Pt was deposited on the top of the film for marking, and the position between each image was confirmed.
-Inclination correction: Since the FIB-SEM observation is performed from an angle of 52 °, the SEM image is observed contracted in the vertical direction. Therefore, in order to obtain an image observed from the front in the vertical direction of the image, 1.27. It is necessary to double (= / sin 52 °), and a three-dimensional image described later was prepared using the image after tilt correction.
-Measurement size: FIB processing was sequentially performed on 5 μm × 5 μm of the film cross section, slicing was performed until it became 4 μm in the depth direction, and a volume integral (400 captured images) of 5 μm × 5 μm × 4 μm was measured.

(6) Creation of 3D image Regarding the FIB-SEM image obtained in (5), among the microporous membranes, using the image processing software of "ExFact (registered trademark) Analysis for Fiber" manufactured by Nippon Visual Science Co., Ltd. After binarizing the electron-dyed resin portion (that is, the portion corresponding to the pores of the microporous membrane) and the resin portion constituting the microporous membrane, which are embedded, the binary value is applied. A three-dimensional stereoscopic image was created based on the information of the conversion process. After that, the image processing software of "ExFact (registered trademark) Analysis for Poros / Particles" manufactured by Nippon Visual Science Co., Ltd. is used to thin the resin portion constituting the microporous membrane to form a tertiary resin portion inside the microporous membrane. I created the original image. Regarding the size of the created 3D image, from the viewpoint of analysis time and reproducibility of analysis parameters, it was surrounded by a side with a length of 2.7 μm at the center of the FIB-SEM measurement size of 5 μm × 5 μm × 4 μm. It was made into a cube.

(7) Constituent resin portion obtained by thinning the three-dimensional image obtained in (6) using the image processing software of "ExFact (registered trademark) Analysis for Fiber" manufactured by Nippon Visual Science Co., Ltd. The number of fibrils in the present invention was taken as the value converted into the number of lines per 1 μm ³ after obtaining the number of thin lines after dividing the lines in the intersecting and branching portions.

(8) Mean free path of fibril The average value of all fibril diameters contained in the 2.7 μm square three-dimensional image obtained in (7) is an image of “ExFact® Analysis for Fiber” manufactured by Nippon Visual Science Co., Ltd. Obtained from the processing software and used as the mean diameter of fibrils.

(9) In the number of fibril crossings (7), when the line of the constituent resin portion of the microporous film is divided and processed by using the image processing software of "ExFact (registered trademark) Analysis for Fiber" manufactured by Nippon Visual Science Co., Ltd. After determining the number of intersections of the constituent resin portions, the value converted into the number per 1 μm ³ was taken as the number of fibril intersections in the present invention.

(10) Capacity retention rate under rapid charge / discharge conditions In order to evaluate the capacity retention rate under rapid charge / discharge conditions when a lithium ion secondary battery is configured, a non-aqueous electrolyte solution consisting of a positive electrode, a negative electrode, a separator, and an electrolyte is used. A charge / discharge test was carried out by incorporating a polyolefin-based microporous film as a separator in the secondary battery.

NMC532 (Lithium Nickel Manganese Cobalt Composite Oxide (Li _1.05 Ni _0.50 Mn _0.29 Co _0.21) at 9.5 mg / cm ² on an aluminum foil with a width of 38 mm, a length of 33 mm and a thickness of 20 μm. O ₂ )) is laminated on a cathode, and natural graphite with a density of 1.45 g / cm ³ is laminated on a copper foil having a width of 40 mm, a length of 35 mm, and a thickness of 10 μm at a unit area mass of 5.5 mg / cm ^2. Was used. The positive electrode and the negative electrode were dried and used in a vacuum oven at 120 ° C.

As the separator, a polyolefin-based microporous membrane having a length of 50 mm and a width of 50 mm was dried in a vacuum oven at room temperature and used. The electrolytic solution is a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate (30/35/35, volume ratio) in which vinylene carbonate (VC) and LiPF ₆ are dissolved, and the VC concentration is 0.5% by mass, LiPF _6. A solution of concentration: 1 mol / L was prepared.

A lithium ion secondary battery was produced by stacking a positive electrode, a separator, and a negative electrode, arranging the obtained laminate in a laminate pouch, injecting an electrolytic solution into the laminate pouch, and vacuum-sealing the laminate pouch. ..

The prepared lithium ion secondary battery was charged for 10 to 15% at a temperature of 35 ° C. and 0.1 C for the first time, and left at 35 ° C. for one night (12 hours or more) to degas. Next, CC-CV charging with a temperature of 35 ° C., a voltage range of 2.75 to 4.2 V, and a charging current value of 0.1 C (constant current constant voltage charging ((termination current condition 0.02 C)), discharge current value 0 .1C CC discharge (constant current discharge) was performed. Next, CC-CV charging (constant current constant voltage charging (constant current constant voltage charging)) with a temperature of 35 ° C., a voltage range of 2.75 to 4.2 V, and a charging current value of 0.2 C. (Termination current condition 0.05C)), the time when CC discharge (constant current discharge) with a discharge current value of 0.2C was performed for 3 cycles was defined as the initial stage of the non-aqueous electrolyte secondary battery.

Next, at 15 ° C. after CC-CV charging (constant current constant voltage charging ((termination current condition 0.05C)) with a temperature of 35 ° C., a voltage range of 2.75 to 4.2V, and a charging current value of 0.2C. A 0.2C CC discharge (constant current discharge) was performed, and the discharge capacity at that time was set to 0.2C capacity. Next, the temperature was 35 ° C., the voltage range was 2.75 to 4.2V, and the charging current value was 0. After CC-CV charging (constant current constant voltage charging ((termination current condition 0.05C)) at 5C, rate test at 10C (180mA, 14.4mA / cm ² ) of non-aqueous electrolyte secondary battery at 15 ° C. From this result, the ratio of 10C capacity to 0.2C capacity {(10C capacity / 0.2C capacity) × 100} (%) was defined as the capacity retention rate (%) under the rapid charge / discharge condition. % Or more was considered to have good characteristics.

(11) Maximum contraction stress in the TD direction by a thermomechanical analyzer (TMA) A polyolefin-based microporous membrane was cut into a rectangle of 3 mm in the MD direction and 15 mm in the TD direction to prepare a sample for evaluation. Using "TMA7100" manufactured by Hitachi High-Technology, the evaluation sample is fixed to the chuck so that the distance between the chucks (TD direction) is 10 mm, and the speed is 5 ° C / min from 30 ° C to 200 ° C in the constant length mode. The temperature was raised in. The temperature and shrinkage force when the temperature is raised to 200 ° C. are measured at 1-second intervals, and the value obtained by dividing the largest shrinkage force (gf) by the cross-sectional area of the evaluation sample is TD by a thermomechanical analyzer (TMA). The maximum shrinkage stress (MPa) in the direction was used.

(12) Maximum contraction stress temperature in the TD direction by the thermomechanical analyzer (TMA) In (11), the temperature indicating the maximum contraction stress in the TD direction by the thermomechanical analyzer (TMA) is determined by the thermomechanical analyzer (TMA). The maximum contraction stress temperature (° C.) in the TD direction was used.

(13) Safety Evaluation The lithium-ion secondary battery manufactured in the same manner as in (10) was charged with a constant current of 0.2 C to a voltage of 4.2 V, and then charged with a constant voltage of 4.2 V. After that, it was discharged to a final voltage of 3.0 V with a current of 1 C. Next, after constant current charging to 4.2 V with a current value of 0.2 C, constant voltage charging of 4.2 V was carried out. Then, the charged battery was put into an oven, the temperature was raised from room temperature at 5 ° C./min, and the battery was left at 150 ° C. for 60 minutes to evaluate according to the following criteria.
A: No ignition or smoke is seen after 60 minutes.
B: After reaching 150 ° C., ignition or smoke was observed over 30 minutes and within 60 minutes.
C: After reaching 150 ° C., ignition or smoke was observed over 10 minutes and within 30 minutes.
D: Ignition or smoke was observed within 10 minutes after reaching 150 ° C.

(Example 1)
The weight average molecular weight as the raw material (Mw) ultra high molecular weight polyethylene and the weight average molecular weight of 10 × 10 ⁵ (Mw) was used high density polyethylene 5 × 10 ^5. 80 parts by mass of liquid paraffin is added to 16 parts by mass of ultra-high molecular weight polyethylene and 4 parts by mass of high-density polyethylene, and 0.5 parts by mass of 2,6-di-t-butyl- based on the total mass of polyethylene-based resin. P-cresol and 0.7 parts by mass of tetrakis [methylene-3- (3,5-di-t-butyl-4-hydroxyphenyl) -propionate] methane were added as an antioxidant and mixed, and a polyethylene resin solution was added. Was prepared. When the mass of the total polyethylene resin was 100% by mass, the ratio of the ultrahigh molecular weight polyethylene was 80% by mass. Since liquid paraffin and antioxidants are almost removed in the manufacturing process, in the present invention, the ratio of ultra-high molecular weight polyethylene when the mass of the total polyethylene-based resin is 100% by mass is determined in the polyolefin-based microporous film. The ultra-high molecular weight ratio was used.
The obtained polyethylene-based resin solution was put into a twin-screw extruder, kneaded at 180 ° C., supplied to a T-die, and the extruded product was cooled with a cooling roll controlled at 15 ° C. to form a gel-like sheet.
The obtained gel-like sheet is longitudinally stretched 8 times at 118 ° C. in the longitudinal direction (MD direction) with a roll stretching machine (described as longitudinal stretching (MD1) in the table), cooled, and then a tenter stretching machine. In the width direction (TD direction), the sheet was laterally stretched 8 times at 118 ° C. (described as transverse stretching (TD) in the table), the sheet width was fixed as it was in the tenter stretching machine, and the sheet was held at a temperature of 115 ° C. for 10 seconds. The surface magnification, which is the product of the magnification of longitudinal stretching (MD1) and the magnification of transverse stretching (TD), was 64 times.
Next, the stretched gel-like sheet was immersed in a methylene chloride bath in a washing tank, and after removing liquid paraffin, it was dried to obtain a polyolefin-based microporous membrane. Finally, using an oven, heat fixation was carried out at a temperature of 130 ° C. for 10 minutes in a state of being relaxed by shrinking by 5% in the width direction to obtain a polyolefin-based microporous membrane.

(Example 2)
Regarding stretching in the longitudinal direction (MD direction) with a roll stretching machine, the first step (MD1) is quadrupled at 118 ° C (described as longitudinal stretching (MD1) in the table), and the second step (MD2) is 2 at 118 ° C. A polyolefin-based microporous film was obtained in the same manner as in Example 1 except that the film was stretched in two steps (described as longitudinal stretching (MD2) in the table).

(Examples 3 to 5, 8 to 19)
A polyolefin-based microporous membrane was obtained in the same manner as in Example 2 except that the raw material composition and production conditions were as shown in the table.

(Examples 6 and 7)
A polyolefin-based microporous membrane was obtained in the same manner as in Example 1 except that the raw material composition and production conditions were as shown in the table.

(Comparative Examples 1 to 4)
A polyolefin-based microporous membrane was obtained in the same manner as in Example 1 except that the raw material composition and production conditions were as shown in the table.

The polyolefin-based microporous film of the present invention has excellent low resistance characteristics when applied as a separator for non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries, and has a capacity retention rate under rapid charge / discharge conditions. Therefore, it is suitably used as a separator for a non-aqueous electrolyte secondary battery that can be charged and discharged rapidly.

Claims (9)

In a 2.7 μm square three-dimensional image created from each cross-sectional image obtained by FIB-SEM measurement of the microporous film, the number of fibrils is 870 lines / μm ³ or more and 2800 lines / μm ³ or less. film. The polyolefin-based microporous membrane according to claim 1, wherein the fibril average diameter is 25 nm or more and 60 nm or less. The polyolefin-based microporous membrane according to claim 1 or 2, wherein the number of fibril crossings is 1350 / μm ³ or more and 4400 / μm ³ or less. The polyolefin-based microporous membrane according to any one of claims 1 to 3, wherein the piercing strength is 180 gf or more and 700 gf or less. The polyolefin-based microporous membrane according to any one of claims 1 to 4, wherein the thickness is 3 μm or more and 14 μm or less. The polyolefin-based microporous membrane according to any one of claims 1 to 5, wherein the porosity is 35% or more and 50% or less. The polyolefin-based fine according to any one of claims 1 to 6, wherein the maximum shrinkage stress temperature in the TD direction by a thermomechanical analyzer (TMA) is 143 ° C. or higher and the maximum shrinkage stress is 1.3 MPa or lower. Porous membrane. A laminate obtained by further laminating a heat-resistant resin layer on the polyolefin-based microporous membrane according to any one of claims 1 to 7. A non-aqueous electrolyte secondary battery comprising the polyolefin-based microporous membrane according to any one of claims 1 to 7 or the laminate according to claim 6.