Polyolefin-based microporous membrane, laminate, and non-aqueous electrolyte secondary battery using the same

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

[Technical Field]

[0001] This invention relates to a polyolefin-based microporous membrane, a laminate, and a non-aqueous electrolyte secondary battery using the same. [Background technology]

[0002] Thermoplastic resin microporous membranes are widely used as separation membranes, selective permeable membranes, and isolation membranes for substances. Specific applications of microporous membranes include separators for non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries, and polymer batteries; separators for electric double-layer capacitors; various filters such as reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes; breathable waterproof clothing; medical materials; and fuel cell supports.

[0003] In particular, polyethylene microporous membranes are widely used as separators for lithium-ion secondary batteries. Their characteristics include excellent mechanical strength, which greatly contributes to battery safety and productivity, as well as electrical insulation while simultaneously providing ion permeability through the electrolyte that permeates the micropores. Furthermore, they possess a pore-closing function that automatically blocks ion permeation at around 120-150°C in the event of abnormal reactions outside or inside the battery, thereby suppressing excessive temperature rises.

[0004] Furthermore, lithium-ion secondary batteries are increasingly being used in automotive and home appliance applications, and in each application, there is a growing need for rapid charging and discharging from a convenience standpoint. However, rapid charging and discharging of lithium-ion secondary batteries has presented challenges, such as the problem of heat generation due to resistance present in the electrodes, separators, and other components that make up the lithium-ion secondary battery, as well as at the interfaces between these components, accelerating the thermal degradation of the battery components. Additionally, when rapid charging and discharging of lithium-ion secondary batteries, the shorter charging and discharging time means that lithium ions have less time to penetrate into the fine parts of the electrodes, reducing the number of ions that can be effectively moved in and out, and thus lowering the battery capacity.

[0005] As an effort to improve the long-term reliability of lithium-ion secondary batteries, technologies have been proposed to improve the long-term compression resistance in the minute regions of the membrane (Patent Document 1), and technologies to improve the battery capacity (rate characteristics) under rapid charge / discharge conditions by specific stretching conditions (Patent Document 2), and a technology to reduce the charging resistance by setting parameters obtained by FIB-SEM image analysis of the coating layer of a polyolefin-based microporous membrane within a specific range (Patent Document 3), etc.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] In Patent Documents 1 and 2, by adjusting the raw material composition and manufacturing conditions, the puncture strength, porosity, heat shrinkage rate, film thickness retention rate due to puncture creep, etc. are adjusted, and proposals are made to improve the long-term reliability when applied to the separator of a lithium-ion secondary battery. However, the internal structure of the microporous membrane has not been sufficiently considered, and there are cases where the low-resistance characteristics for reducing the internal electrical resistance of the battery and the improvement of the capacity retention rate under rapid charge / discharge conditions are insufficient. Also, in Patent Document 3, a proposal is made to reduce the charging resistance by setting the fractal dimension of the insulating porous layer obtained by coating within a specific range. However, the structure of the microporous membrane itself as the coated substrate and further the internal structure of the microporous membrane have not been sufficiently considered, and there are cases where the compatibility between strength and the improvement of the capacity retention rate under rapid charge / discharge conditions is insufficient when considering the resistance of the entire substrate and coating layer.

[0008] Therefore, the present invention aims to overcome the above-mentioned drawbacks and provide a polyolefin-based microporous membrane that exhibits superior strength and improves capacity retention during rapid charging and discharging when applied as a separator in non-aqueous electrolyte secondary batteries, including lithium-ion secondary batteries, by specifying the fibril structure inside the polyolefin-based microporous membrane to a particular range. [Means for solving the problem]

[0009] The present invention, which solves the above problems, has the following configuration. (1) In a 2.7 μm square three-dimensional image created from cross-sectional images obtained by FIB-SEM measurement of a microporous membrane, the number of fibrils was 870 / μm 3 More than 2800 lines / μm 3 The following are polyolefin-based microporous membranes. (2) A polyolefin-based microporous membrane as described in (1), wherein the average fibril diameter is 25 nm or more and 60 nm or less. (3) Fibril crossing count is 1350 / μm 3 More than 4400 pieces / μm 3 The following is a polyolefin-based microporous membrane as described in (1) or (2). (4) A polyolefin-based microporous membrane according to any one of (1) to (3), having a puncture strength of 180 gf or more and 700 gf or less. (5) A polyolefin-based microporous membrane according to any one of (1) to (4), having a thickness of 3 μm or more and 14 μm or less. (6) A polyolefin-based microporous membrane according to any one of (1) to (5), having a porosity of 35% or more and 50% or less. (7) A polyolefin-based microporous membrane according to any one of (1) to (6), wherein the maximum shrinkage stress temperature in the TD direction determined by a thermomechanical analyzer (TMA) is 143°C or higher, and the maximum shrinkage stress is 1.3 MPa or lower. (8) A laminate comprising a polyolefin-based microporous film described in any one of (1) to (7), with a heat-resistant resin layer further laminated on top. (9) A non-aqueous electrolyte secondary battery comprising a polyolefin-based microporous membrane according to any one of (1) to (7), or a laminate according to (6). [Effects of the Invention]

[0010] The polyolefin-based microporous membrane according to the embodiment of the present invention exhibits excellent strength and improves capacity retention during rapid charging and discharging when applied as a separator in non-aqueous electrolyte secondary batteries, including lithium-ion secondary batteries. [Modes for carrying out the invention]

[0011] The following describes in detail the polyolefin-based microporous membrane according to embodiments of the present invention. In this specification, when a numerical range is described as "A to B", it refers to a range of A or greater and B or less.

[0012] The polyolefin-based microporous membrane according to the embodiment of the present invention has a fibril count of 870 fibers / μm in a 2.7 μm square three-dimensional image created from cross-sectional images obtained by FIB-SEM measurement of the microporous membrane. 3 More than 2800 lines / μm 3 It is important that the following conditions are met.

[0013] The number of fibrils can be used as one indicator to represent the internal structure of polyolefin-based microporous membranes.

[0014] The polyolefin microporous membrane in the embodiments of the present invention mainly consists of a polyolefin resin, where "main component" means that when the total mass of the polyolefin microporous membrane is 100% by mass, it contains more than 50% by mass and up to 100% by mass of polyolefin resin. Here, examples of polyolefin resins in the embodiments of the present invention include various polyethylene resins and various polypropylene resins, where the polyethylene resin in the embodiments of the present invention means a polymer in which, when the total mass of the polyethylene polymer is 100% by mass, the total amount of ethylene-derived components is more than 50% by mass and up to 100% by mass.

[0015] In this specification, polyolefin-based microporous membranes may be simply referred to as "microporous membranes."

[0016] Furthermore, in the embodiments of the present invention, the polypropylene resin refers to a polymer in which, when the total mass of the polypropylene resin polymer is taken as 100% by mass, the total amount of propylene-derived components exceeds 50% by mass and is 100% by mass or less.

[0017] In this specification, polyolefin-based microporous membranes may be simply referred to as "microporous membranes."

[0018] The polyethylene resins in embodiments of the present invention include homopolymers consisting solely of ethylene, or copolymers obtained by copolymerizing 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-hexene, 3-ethyl-1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene, and other chain-like olefins (α-olefins).

[0019] The polypropylene resins in embodiments of the present invention include homopolymers consisting solely of propylene, or various polypropylene resins such as ethylene-propylene copolymers, ethylene-propylene-butene copolymers, and propylene-butene copolymers.

[0020] Furthermore, the polyolefin resin in the embodiments of the present invention may be a single substance or a mixture of two or more different polyolefin resins.

[0021] Among these various polyolefin resins, polyethylene is particularly preferred from the viewpoint of excellent pore blocking performance. The melting point (softening point) of polyethylene is preferably 70 to 150 °C from the viewpoint of the pore blocking performance of the microporous membrane.

[0022] Hereinafter, a polyethylene resin will be described in detail as an example of the polyolefin resin used in the embodiments of the present invention. As the types of polyethylene resins used in the embodiments of the present invention, high-density polyethylene having a density exceeding 0.94 g / cm 3 medium-density polyethylene in the range of 0.93 to 0.94 g / cm 3 low-density polyethylene having a density lower than 0.93 g / cm 3 linear low-density polyethylene, etc. can be mentioned. From the viewpoint of controlling the internal structure of the polyolefin-based microporous membrane described later within a desired range, when the total mass of the polyolefin-based microporous membrane is 100% by mass, a configuration containing 80% by mass or more of ultra-high molecular weight polyethylene is preferable. [[ID=XI]]

[0023] The ultra-high molecular weight polyethylene used in the embodiments of the present invention preferably has a weight average molecular weight of 1.0 × 10 6 or more and 8.0 × 10 7 or less. If the weight average molecular weight is 1.0 × 10 6 or more, the relaxation time will not become too short, suppressing the increase in the stretching temperature and heat treatment temperature, preventing the melting of fine fibrils, and reducing the number of pores in the microporous membrane. By using ultra-high molecular weight polyethylene having a weight average molecular weight of 1.0 × 10 6 or more, the entanglement of molecular chains increases, and stress is uniformly applied to the polyethylene resin layer in the stretching process, so that the internal structure of the polyolefin-based microporous membrane described later can be controlled within a desired range. Therefore, the weight average molecular weight of the ultra-high molecular weight polyethylene is preferably 1.0 × 10 6 or more, more preferably 6 1.5 × 10 6 or more, still more preferably 6 2.0 × 10 6 or more, and most preferably 6 3.0 × 10 6 or more. Also, as the upper limit of the weight average molecular weight, preferably 6 8.0 × 10 6The following is more preferable: 6.0 × 10 6 More preferably 5.0 × 10 6 The following is most preferably 4.0 × 10 6 The following applies:

[0024] The molecular weight distribution (weight-average molecular weight (Mw) / number-average molecular weight (Mn)) of ultra-high molecular weight polyethylene is preferably in the range of 3.0 to 100. A narrower molecular weight distribution is preferable because it leads to a more unified system and easier acquisition of uniform micropores. However, a narrower distribution reduces moldability. Therefore, the lower limit of the molecular weight distribution is preferably 4.0 or higher, more preferably 5.0 or higher, and even more preferably 6.0 or higher. As the molecular weight distribution increases, the amount of low molecular weight components increases, leading to a decrease in strength and making it easier for fine fibrils to melt and fuse during stretching and heat fixing. Therefore, the upper limit is preferably 80 or lower, more preferably 50 or lower, even more preferably 20 or lower, and most preferably 10 or lower. By setting the molecular weight within the above range, good moldability can be obtained, and uniform micropores can be obtained because the system is unified.

[0025] The high-density polyethylene used in the embodiments of the present invention has a weight-average molecular weight (Mw) of 1.0 × 10⁻⁶. 4 The above 1.0 × 10 6 Preferably, it is 1.0 × 10 5 The above 1.0 × 10 6 It is more preferable that the following conditions apply: 5.0 × 10 5 The above 9.0 x 10 5 The following is even more preferable: 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 fluctuations of the resin in the extruder become less likely to occur, and the quality of the microporous membrane, such as reducing thickness unevenness of the polyolefin microporous membrane, can be improved.

[0026] In the polyolefin-based microporous membrane of the present invention, from the viewpoint of controlling the internal structure of the polyolefin-based microporous membrane to a desired range, the content of ultra-high molecular weight polyethylene is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, when the total mass of the polyolefin-based microporous membrane is 100% by mass. Furthermore, from the viewpoint of controlling the internal structure of the polyolefin-based microporous membrane to a desired range, the structure may consist of 100% by mass of ultra-high molecular weight polyethylene when the total mass of the polyolefin-based microporous membrane is 100% by mass. However, from the viewpoint of stabilizing the resin pressure in the extruder and improving the quality of the microporous membrane, such as reducing thickness unevenness of the polyolefin-based microporous membrane, it may also contain 20% by mass or less of high-density polyethylene.

[0027] Furthermore, the polyolefin-based microporous membrane according to the embodiments of the present invention may contain various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, and even blocking inhibitors and fillers, to the extent that they do not impair the effects of the present invention. In particular, it is preferable to add antioxidants in order to suppress oxidative degradation of the polyolefin resin due to its thermal history.

[0028] As an antioxidant, it is preferable to use one or more selected from, for example, 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4), 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane, etc.

[0029] In the embodiments of the present invention, FIB-SEM measurement refers to a method of measuring continuous images at regular intervals in the depth direction by repeatedly performing the operation of scraping the cross-section of a microporous membrane at regular intervals using an integrated ion beam (FIB) (FIB cutting), and taking SEM (scanning electron microscope) images of the scraped surface. A sample preparation method for FIB-SEM measurement includes impregnating the microporous membrane according to the embodiments of the present invention with an electron-stained resin, embedding the voids, and then preparing sections of the film cross-section using a microtome so that the film cross-section becomes the initial observation surface. A FIB-SEM measurement method includes sequentially taking SEM images in the depth direction while cutting the prepared sample (section of the film cross-section) in 10 nm increments in the depth direction. A method for identifying the positional information of each SEM image includes marking a portion of the observation screen of the microporous membrane being FIB-cut with a metallic component, and then identifying the correlation between the positions of each image based on the marked positions. The image area is preferably between 3 μm square and 10 μm square. If the observation surface is inclined, the scale may be adjusted to account for the inclination.

[0030] Furthermore, as a method for creating a three-dimensional image from each cross-sectional image, for example, an image processing software such as "ExFact® Analysis for Fiber" manufactured by Visual Science Japan Inc. is used to perform binarization on the embedded, electron-stained resin portion (i.e., the portion corresponding to the pores of the microporous membrane) and the resin portion constituting the polyolefin microporous membrane (constituent resin portion) that are observed as bright areas in the polyolefin microporous membrane. Then, a three-dimensional stereoscopic image is created based on the information from the binarization process, and the resin portion constituting the polyolefin microporous membrane is thinned using image processing software such as "ExFact® Analysis for Fiber" manufactured by Visual Science Japan Inc. to create a three-dimensional image of the constituent resin portion inside the polyolefin microporous membrane. Regarding the size of the three-dimensional image to be created, from the viewpoint of analysis time and reproducibility of analysis parameters, in this invention it is set to a cube enclosed by sides with a length of 2.7 μm.

[0031] The number of fibrils in this invention is determined by first performing a thinning process on the constituent resin portion obtained from a three-dimensional image using image processing software such as "ExFact® Analysis for Fiber" manufactured by Visual Science Japan, and then dividing the lines of the constituent resin portion at the points where they intersect and branch to determine the number of thin lines, and then determining the number of 1 μm 3 This value is calculated by converting it to the number of items per unit.

[0032] The polyolefin-based microporous membrane according to the embodiment of the present invention has a fibril count of 870 fibers / μm in a 2.7 μm square three-dimensional image created from cross-sectional images obtained by FIB-SEM measurement of the microporous membrane. 3 More than 2800 lines / μm 3 The following is important:

[0033] In the polyolefin-based microporous membrane according to the embodiment of the present invention, cleavage of the higher-order structure occurs during the stretching step of the manufacturing method example described later. In the present invention, the number of fibrils is 870 / μm 3 The above indicates that there are a sufficient number of fibrous resin fibers after cleavage, and that the formation of pores in the polyolefin microporous membrane proceeds sufficiently, allowing lithium ions to move smoothly when used as a separator in a lithium-ion secondary battery.

[0034] The polyolefin-based microporous membrane according to the embodiment of the present invention has a fibril count of 870 fibers / μm in a 2.7 μm square three-dimensional image created from each cross-sectional image obtained by FIB-SEM measurement of the microporous membrane. 3 By doing so, the electrical resistance of the separator is reduced, which suppresses thermal degradation of the battery and improves the capacity retention rate during rapid charging and discharging. From the perspective of reducing electrical resistance and suppressing thermal degradation and improving the capacity retention rate during rapid charging and discharging, the number of fibrils is 1000 / μm. 3 The above is preferable, with 1250 lines / μm 3 The above is more preferable, with 1400 lines / μm. 3The above is particularly preferable. On the other hand, if the number of fibrils is too high, the number of paths through which lithium ions can pass decreases, which can increase resistance, so 2800 fibrils / μm is preferable. 3 The following is important.

[0035] In this invention, in a 2.7 μm square three-dimensional image created from cross-sectional images obtained by FIB-SEM measurement of a microporous membrane, the number of fibrils is 870 / μm. 3 2800 lines / μm 3 One method involves using ultra-high molecular weight polyethylene for 80% or more of the resin constituting the polyolefin microporous membrane, setting the surface ratio of the wet stretching to 60 times or more, and further setting the resin concentration during manufacturing to less than 30% by mass. In an embodiment of the present invention, it was found that by using ultra-high molecular weight polyethylene for 80% or more of the resin constituting the polyolefin microporous membrane and setting the resin concentration during manufacturing to less than 30% by mass, spherulite growth of polyethylene in the cast sheet before stretching can be suppressed, and the cast sheet structure can be made uniform. Furthermore, by combining this with wet stretching with a wet surface ratio of 60 times or more, it is possible to significantly reduce areas where pore formation is insufficient and to uniformly form the pores of the polyolefin microporous membrane. In addition, by uniformly forming the pores of the polyolefin microporous membrane and reducing areas where pore formation is insufficient, it becomes possible to increase the number of fibrils.

[0036] Next, we will explain the average fibril diameter and the number of fibril crossovers. The average fibril diameter and the number of fibril crossovers can be used as indicators to represent the internal structure of polyolefin-based microporous membranes.

[0037] The polyolefin-based microporous membrane of the present invention, when used as a separator in a lithium-ion secondary battery, preferably has an average fibril diameter of 25 nm to 60 nm, determined by analyzing a 2.7 μm square three-dimensional image created from cross-sectional images obtained by FIB-SEM measurement of the microporous membrane, from the viewpoint of reducing the electrical resistance inside the battery and improving the capacity retention rate during rapid charging and discharging. Here, the average fibril diameter refers to the average value of the total fibril diameter contained in the 2.7 μm square three-dimensional image, determined using image processing software such as "ExFact(registered trademark) Analysis for Fiber" manufactured by Visual Science Japan, in the same manner as the method for evaluating the number of fibrils. By setting the fibril diameter within a specific range, lithium ions can move smoothly when used as a separator in a lithium-ion secondary battery.

[0038] In the embodiments of the present invention, the average fibril diameter is more preferably 55 nm or less, even more preferably 45 nm or less, and particularly preferably 35 nm or less, from the viewpoint of reducing electrical resistance and further enhancing the effect of improving capacity retention during rapid charging and discharging. On the other hand, from the viewpoint of improving mechanical properties such as strength, the average fibril diameter is preferably 25 nm or more.

[0039] In embodiments of the present invention, methods for setting the average fibril diameter to 25 nm or more and 60 nm or less include setting the wet stretching speed to a specific low range to homogenize the formation of voids during stretching, and applying multi-stage stretching of two or more stages to gradually promote the formation of voids.

[0040] The polyolefin-based microporous membrane according to the embodiment of the present invention has a fibril crossing number of 1350 cells / μm. 3 More than 4400 pieces / μm 3The following is preferable from the viewpoint of reducing electrical resistance and improving capacity retention during rapid charging and discharging when used as a separator in lithium-ion secondary batteries. Here, the fibril crossing number is the number of points where the constituent resin parts intersect when dividing the lines of the constituent resin parts of a microporous membrane using image processing software such as "ExFact(registered trademark) Analysis for Fiber" manufactured by Visual Science Japan, in the same way as the evaluation method for the number of fibrils. After determining the number of intersection points in a 2.7 μm square three-dimensional image, 1 μm 3 This value is calculated by converting it to the number of items per unit.

[0041] In embodiments of the present invention, a high number of fibril crossings indicates the formation of a fine network structure in the microporous membrane, demonstrating that the formation of pores in the polyolefin-based microporous membrane is sufficiently advanced, allowing lithium ions to move smoothly when used as a separator in a lithium-ion secondary battery.

[0042] In the embodiments of the present invention, the number of fibril crossings is set to 1700 / μm, from the viewpoint of reducing electrical resistance and further enhancing the effect of improving capacity retention during rapid charging and discharging. 3 The above is more preferable, with 2200 particles / μm 3 The above is even more preferable, with 3000 particles / μm 3 The above is particularly preferable. On the other hand, from the viewpoint of improving mechanical properties such as strength, the fibril crossing count should be 4200 / μm. 3 The following are preferable.

[0043] In an embodiment of the present invention, the number of fibril crossings is 1350 / μm 3 More than 4400 pieces / μm 3 Methods to achieve this include setting a high relaxation rate in the heat treatment process after wet stretching to promote shrinkage in the planar direction and to homogenize the pore paths in the thickness direction in a linear direction.

[0044] The polyolefin microporous membrane according to the embodiments of the present invention preferably has a puncture 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 in a lithium-ion secondary battery. From the viewpoint of further improving the impact resistance of the battery when used as a separator in a lithium-ion secondary battery, a puncture strength of 250 gf or more is more preferable, 350 gf or more is even more preferable, and 500 gf or more is particularly preferable. Furthermore, from the viewpoint of the impact resistance of the battery, a higher strength of the polyolefin microporous membrane is preferable, but from the viewpoint of achieving a good balance with other physical properties such as thermal shrinkage rate, 700 gf or less is preferable. Note that the puncture strength in the present invention is the puncture strength when the thickness is converted to 10 μm.

[0045] In embodiments of the present invention, a method for achieving a puncture strength of a polyolefin-based microporous membrane of 180 gf or more and 700 gf or less includes using ultra-high molecular weight polyethylene for 70% or more of the constituent resin of the polyolefin-based microporous membrane, setting the surface ratio of wet stretching to 60 times or more, and setting the porosity of the polyolefin-based microporous membrane to 35% or more and 55% or less.

[0046] The polyolefin microporous membrane according to the embodiment of the present invention is preferable to have a thickness of 3 μm to 14 μm when used as a separator in a lithium-ion battery, as this allows for a thinner distance between electrodes and an increase in the number of stacked battery components, thus enabling a higher battery capacity. Methods for achieving a thickness of 3 μm to 14 μm include employing a wet stretching method and increasing the stretching ratio and the line speed during manufacturing. From the viewpoint of increasing battery capacity, the thickness of the polyolefin microporous membrane is more preferably 12 μm or less, even more preferably 10 μm or less, and particularly preferably 7 μm or less.

[0047] In the embodiments of the present invention, it is preferable from the viewpoint of safety for lithium-ion secondary batteries that the polyolefin-based microporous membrane has a maximum shrinkage stress temperature in the TD direction of 143°C or higher and a maximum shrinkage stress of 1.3 MPa or lower, as measured by a thermomechanical analyzer (TMA). When lithium-ion secondary batteries reach high temperatures due to rapid charging and discharging, the shrinkage stress of the polyolefin-based microporous membrane contained in the lithium-ion secondary battery increases, and deformation in the TD direction, especially in the unwound portion, becomes more likely. Since deformation of the polyolefin-based microporous membrane in the TD direction can lead to insufficient insulation within the lithium-ion secondary battery, potentially causing thermal runaway and ignition, from the viewpoint of enhancing the safety of lithium-ion secondary batteries, an embodiment of the present invention is preferable in which the maximum shrinkage stress temperature in the TD direction measured by a thermomechanical analyzer (TMA) is high and the maximum shrinkage stress is low. However, from the viewpoint of balancing the path structure, fibril structure, and strength of the polyolefin-based microporous membrane, it is preferable that the maximum shrinkage stress temperature in the TD direction measured by a thermomechanical analyzer (TMA) is 150°C or lower and the maximum shrinkage stress is in the range of 0.6 MPa or higher.

[0048] In embodiments of the present invention, a method for achieving a maximum shrinkage stress temperature of 143°C or higher and a maximum shrinkage stress of 1.3 MPa or lower in the TD direction, as measured by a thermomechanical analyzer (TMA), is a method that enhances the relaxation of strain in a polyolefin-based microporous membrane by using a configuration mainly composed of ultra-high molecular weight polyethylene, setting the heat-fixing temperature to 130°C or higher and the relaxation rate to 15% or higher.

[0049] In the embodiments of the present invention, the polyolefin-based microporous membrane preferably has a porosity of 35% to 50% from the viewpoint of specifying the fibril structure within a particular range and improving puncture strength and safety when used as a separator in lithium-ion secondary batteries.

[0050] In embodiments of the present invention, methods for achieving a porosity of 35% or more and 50% or less include adjusting various manufacturing conditions such as the stretching ratio, stretching temperature, heat treatment temperature, and heat treatment time.

[0051] The polyolefin microporous membrane according to the embodiment of the present invention may be further laminated with a heat-resistant resin layer to improve heat resistance when mounted on a lithium-ion battery, thereby forming a laminate. Preferably, the heat-resistant resin layer is a resin that is insoluble in the electrolytic resin of the battery and electrically stable within the range of battery operating conditions, such as various fluororesins, acrylic resins, and aromatic polyamide resins. Furthermore, the heat-resistant resin layer may contain organic powders, inorganic powders, or mixtures thereof as fillers to further improve heat resistance. For example, organic powders may include fluororesins, melamine resins, and aromatic polyamide resins, while inorganic powders may include metal oxides, metal nitrides, metal carbides, metal hydroxides, carbonates, sulfates, and more specifically, alumina, silica, titanium dioxide, aluminum hydroxide, and calcium carbonate.

[0052] 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 limited to such examples.

[0053] The method for producing a polyolefin-based microporous membrane according to embodiments of the present invention preferably comprises the following steps (a) to (e). (a) A step of melt-kneading a polymer material containing one or more types of polyolefin resins and a solvent as necessary to prepare a polyolefin resin solution. (b) A process of extruding the molten material, forming it into a sheet, and cooling and solidifying it. (c) A process of stretching the obtained sheet using a roll method or a tenter method. (d) The process of extracting a plasticizer from the stretched film and drying the film. (e) Process of heat treatment / re-stretching

[0054] The following describes each step. (a) Preparation of polyolefin resin solution A polyolefin resin solution is prepared by heating and dissolving the polyolefin resin used in the embodiments of the present invention in a plasticizer. The plasticizer is not particularly limited as long as it is a solvent that can sufficiently dissolve the polyolefin resin, but it is preferable that the solvent is liquid at room temperature in order to enable relatively high-magnification stretching.

[0055] Examples of solvents include aliphatic, cyclic aliphatic, or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, as well as mineral oil fractions with corresponding boiling points, and phthalate esters that are liquid at room temperature, such as dibutyl phthalate and dioctyl phthalate. To obtain a gel-like sheet with a stable liquid solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin.

[0056] Regarding the solvent ratio, from the viewpoint of easily controlling the number of fibrils within a specific range, it is preferable to use 400 parts by mass or more and 900 parts by mass or less of solvent per 100 parts by mass of the total mass of polyethylene resin.

[0057] In a molten and kneaded state, a solvent that is miscible with polyolefin resin but is solid at room temperature may be mixed with the liquid solvent. Examples of such solid solvents include stearyl alcohol, ceryl alcohol, and paraffin wax. However, using only a solid solvent may result in uneven stretching.

[0058] The viscosity of the liquid solvent is preferably 20 to 200 cSt at 40°C. A viscosity of 20 cSt or higher at 40°C reduces the likelihood of the sheet extruded from the die containing the polyolefin resin solution becoming non-uniform. On the other hand, a viscosity of 200 cSt or lower at 40°C facilitates the removal of the liquid solvent. The viscosity of the liquid solvent is measured at 40°C using an Ubbelohde viscometer.

[0059] (b) Formation of extruded material and formation of gel sheet The method for uniformly melting and kneading a polyolefin resin solution is not particularly limited, but when preparing a high-concentration polyolefin resin solution, it is preferable to do so in a twin-screw extruder. If necessary, known additives such as metal soaps like calcium stearate, ultraviolet absorbers, light stabilizers, and antistatic agents may be added within a range that does not impair film-forming properties or impair the effects of the present invention. In particular, it is preferable to add an antioxidant to prevent oxidation of the polyolefin resin.

[0060] In the extruder, the polyolefin resin solution is uniformly mixed at a temperature at which the polyolefin resin completely melts. The melt-mixing temperature varies depending on the polyolefin resin used, but it is preferably between (melting point of polyolefin resin + 10°C) and (melting point of polyolefin resin + 120°C). More preferably, it is between (melting point of polyolefin resin + 20°C) and (melting point of polyolefin resin + 100°C).

[0061] Here, the melting point refers to the value measured by DSC (Differential scanning calorimetry) based on JIS K7121 (1987). For example, if the polyolefin resin is a polyethylene resin, the melting and mixing temperature of the polyethylene 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 polyethylene resin has a melting point of about 130 to 140°C, the melting and mixing temperature is preferably 140 to 250°C, and most preferably 180 to 230°C.

[0062] From the viewpoint of suppressing the degradation of polyolefin resins, a lower melt-mixing temperature is preferable. However, if the temperature is lower than the above-mentioned temperature, unmelted material may be generated in the extruded product from the die, which may cause film rupture or other problems in the subsequent stretching process. Conversely, if the temperature is higher than the above-mentioned temperature, the thermal decomposition of the polyolefin resin becomes more severe, which may worsen the physical properties of the resulting polyolefin microporous film, such as strength and porosity. In addition, decomposition products may precipitate on cooling rolls or rolls in the stretching process and adhere to the sheet, leading to a deterioration in appearance. Therefore, it is preferable to perform the melt-mixing within the above-mentioned range.

[0063] Next, a gel-like sheet is obtained by cooling the resulting extruded material. Cooling allows for the immobilization of the microphase of the polyolefin resin separated by the solvent. It is preferable to cool the gel-like sheet to 10-50°C during the cooling process. This is to ensure that the final cooling temperature is below the crystallization completion temperature, and by refining the higher-order structure, uniform stretching becomes easier during subsequent stretching. Therefore, it is preferable to cool at a rate of 30°C / min or more until the temperature is at least below the gelation temperature.

[0064] Generally, a slow cooling rate leads to the formation of relatively large crystals, resulting in a coarser higher-order structure of the gel-like sheet and a larger gel structure. Conversely, a fast cooling rate leads to the formation of small, uniform crystals, resulting in a denser higher-order structure of the gel-like sheet, which in turn leads to uniform stretching and a reduction in unopened areas.

[0065] Cooling methods include direct contact with cold air, cooling water, or other cooling media; contact with rolls cooled by a refrigerant; and the use of casting drums, etc.

[0066] While we have described the case of a single-layer polyolefin microporous membrane so far, the polyolefin microporous membrane according to the embodiments of the present invention is not limited to a single layer, but may be a laminate. There is no particular limit to the number of layers; it may be a two-layer laminate or a laminate of three or more layers.

[0067] Methods for forming a laminate of polyolefin-based microporous membranes include, for example, preparing the desired resins as needed, supplying these resins separately to an extruder and melting them at the desired temperature, combining them in a polymer tube or die, and then extruding them from a slit-shaped die to the desired thickness of each layer to form the laminate.

[0068] (c) Stretching process The resulting gel-like sheet (including laminated sheets) is stretched. The stretching methods used include uniaxial stretching in the sheet transport direction (MD direction) using a roll stretcher, uniaxial stretching in the sheet width direction (TD direction) using a tenter, sequential biaxial stretching using a combination of a roll stretcher and a tenter, or two tenters, and simultaneous biaxial stretching using a simultaneous biaxial tenter.

[0069] The stretching ratio varies depending on the thickness of the gel-like sheet, from the viewpoint of uniformity of film thickness, but it is preferable to stretch it to 7 times or more in any direction. Furthermore, from the viewpoint of setting the number of fibrils within a desired range, the surface ratio is preferably 60 times or more, more preferably 80 times or more, and particularly preferably 100 times or more. In addition, from the viewpoint of suppressing tearing during the manufacturing of polyolefin-based microporous films, the surface ratio is preferably 150 times or less.

[0070] From the viewpoint of improving stretch uniformity in the stretching process, the preferred stretching ratio and raw material composition is to have a composition in which ultra-high molecular weight polyethylene with a weight-average molecular weight (Mw) of 1 million or more is contained in an amount of 80% by mass or more when the total mass of all polyolefin resins contained in the gel-like sheet is set to 100% by mass, and to stretch from a wet gel-like sheet at a face ratio of 60 times or more, and more preferably at a wet stretch of 10 × 10 times or more. An even more preferred configuration is to have an ultra-high molecular weight polyethylene with a weight-average molecular weight (Mw) of 2 million or more is contained in an amount of 80% by mass or more when the total mass of all polyolefin resins contained in the gel-like sheet is set to 100% by mass, and to stretch from a wet gel-like sheet at a face ratio of 60 times or more, and most preferably at a wet stretch of 10 × 10 times or more.

[0071] The stretching temperature is preferably 10°C or less above the melting point of the gel-like sheet, and more preferably in the range of (crystal dispersion temperature Tcd of the polyolefin resin) to (melting point of the gel-like sheet + 5°C). Specifically, in the case of polyethylene compositions, since the crystal dispersion temperature is about 90 to 110°C, the stretching temperature is preferably 100 to 130°C, more preferably 115 to 125°C, and even more preferably 117.5 to 125°C. The crystal dispersion temperature Tcd is determined from the temperature characteristics of the dynamic viscoelasticity measured according to ASTM D 4065 (2012).

[0072] If the stretching temperature is below 90°C, the formation of pores will be insufficient due to low-temperature stretching, making it difficult to obtain uniformity in film thickness and resulting in a low porosity. If the stretching temperature is above 130°C, the sheet may melt, and pore blockage may occur.

[0073] The stretching described above causes cleavage of the higher-order structure of the gel sheet, refinement of the crystalline phase, and formation of numerous fibrils. The fibrils form a network structure that is irregularly linked in three dimensions. Since stretching improves mechanical strength and creates pores, the polyolefin-based microporous film according to the embodiment of the present invention is suitable for use as a battery separator.

[0074] Furthermore, by stretching the polyolefin resin before removing the plasticizer, the resin is sufficiently plasticized and softened, which allows for smoother cleavage of the higher-order structure and uniform refinement of the crystalline phase. In addition, because the higher-order structure is easily cleaved by stretching before removing the plasticizer, less strain remains after stretching, and the thermal shrinkage rate can be lower compared to stretching after removing the plasticizer.

[0075] (d) Plasticizer extraction (washing) and drying process Next, the plasticizer (solvent) remaining in the gel-like sheet is removed using a washing solvent. Since the polyolefin resin phase and the solvent phase are separated, a polyolefin microporous film is obtained by removing the solvent.

[0076] Examples of cleaning solvents 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 chain-like fluorocarbons such as trifluorinated ethane.

[0077] These cleaning solvents have low surface tension (e.g., 24 mN / m or less at 25°C). By using cleaning solvents with low surface tension, the shrinkage of the microporous network structure is suppressed by the surface tension of the gas-liquid interface during drying after cleaning, resulting in a polyolefin-based microporous film with excellent porosity and permeability. These cleaning solvents are appropriately selected depending on the plasticizer and used individually or in mixtures.

[0078] Cleaning methods include immersing the gel sheet in a cleaning solvent for extraction, showering the gel sheet with the cleaning solvent, or a combination of these methods. The amount of cleaning solvent used varies depending on the cleaning method, but generally, it is preferable to use 300 parts by mass or more per 100 parts by mass of the gel sheet.

[0079] The washing temperature should be 15-30°C, and may be heated to 80°C or below if necessary. At this time, from the viewpoint of enhancing the washing effect of the washing solvent, ensuring that the physical properties of the resulting polyolefin microporous membrane (e.g., physical properties in the TD direction and / or MD direction) are not non-uniform, and improving the mechanical and electrical properties of the polyolefin microporous membrane, the longer the time the gel sheet is immersed in the washing solvent, the better.

[0080] The cleaning described above is preferably carried out until the residual solvent in the gel-like sheet, i.e., the polyolefin-based microporous membrane, is less than 1% by mass.

[0081] Subsequently, the solvent in the polyolefin microporous film is dried and removed in a drying process. There are no particular limitations on the drying method, and methods such as using a metal heating roll or using hot air can be selected. The drying temperature is preferably 40 to 100°C, and more preferably 40 to 80°C. If drying is insufficient, the porosity of the polyolefin microporous film may decrease during subsequent heat treatment, resulting in poor permeability.

[0082] (e) Heat treatment / re-stretching process The dried polyolefin microporous membrane may be stretched (re-stretched) in at least one axial direction. Re-stretching can be performed by heating the polyolefin microporous membrane and using the Tenter method or the like, similar to the stretching described above. Re-stretching may be uniaxial or biaxial. In the case of multi-stage stretching, it can be performed by combining simultaneous biaxial or sequential stretching.

[0083] The re-drawing temperature is preferably below the melting point of the polyolefin resin, and more preferably within the range of (Tcd of the polyolefin resin composition - 20°C) to the melting point of the polyolefin resin. Specifically, in the case of polyethylene resins, the re-drawing temperature is preferably 70 to 135°C, more preferably 110 to 135°C, even more preferably 125 to 135°C, and even more preferably 130 to 135°C.

[0084] For uniaxial stretching, the re-stretching ratio is preferably 1.01 to 2.0 times, and particularly preferably 1.1 to 1.6 times in the TD direction, with 1.2 to 1.4 times being more preferable. When biaxial stretching is performed, it is preferable to stretch by 1.01 to 2.0 times in both the MD and TD directions. Note that the re-stretching ratio may differ between the MD and TD directions. By re-stretching within the above range, porosity and permeability are increased, and re-aggregation of fibrils due to shrinkage can be suppressed, enabling the uniform formation of pores in the polyolefin microporous membrane.

[0085] From the viewpoint of thermal shrinkage rate and wrinkles and sagging, the relaxation rate from the maximum re-stretching ratio is preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less. When the relaxation rate is 20% or less, a uniform fibril structure can be obtained.

[0086] (f) Other processes Furthermore, depending on the application, the polyolefin microporous membrane can be subjected to hydrophilic treatment. Hydrophilic treatment can be carried out by monomer grafting, surfactant treatment, corona discharge, etc. It is preferable to perform monomer grafting after crosslinking treatment. [Examples]

[0087] The method for measuring the characteristics and evaluating the effects in the embodiments of the present invention is as follows. However, the embodiments of the present invention are not limited to these embodiments. Examples 1-7, 13, 15-17, and 19 should be interpreted as Reference Examples 1-7, 13, 15-17, and 19.

[0088] (1) Weight average molecular weight (Mw) The weight-average molecular weight of polyethylene resins was determined by gel permeation chromatography (GPC) under the following conditions. • Measuring device: GPC-150C manufactured by WATERS CORPORATION • Column: SHODEX UT806M manufactured by Showa Denko Corporation • Column temperature: 135℃ • Solvent (mobile phase): O-dichlorobenzene • Solvent flow rate: 1.0 mL / min • Sample concentration: 0.1 wt% (Dissolution conditions: 135°C / 1H) • Injection volume: 500 μL • Detector: Differential refractometer (RI detector) manufactured by WATERS CORPORATION • Calibration curve: Created from a calibration curve obtained using monodisperse polystyrene standard samples, using a predetermined conversion constant.

[0089] (2) Thickness After cutting a polyolefin microporous membrane into a 95mm x 95mm size evaluation sample, marks were made so that four 40mm square grids were arranged in a row, two vertically and two horizontally. When making the marks, the central vertex where the four grids overlapped was aligned with the center of the evaluation sample, and the edges of the vertical and horizontal grids were made parallel to the edges of the evaluation sample. The thickness was measured at a total of nine points corresponding to the vertices of the four interconnected grids using a contact thickness gauge (Mitutoyo Lightmatic), and the average thickness of the nine points was calculated.

[0090] (3) Porosity A sample for evaluation was cut from a polyolefin microporous membrane to a size of 5 cm square, and its volume (cm³) was measured. 3 Calculate the weight and mass (g), and then compare them with the resin density (g / cm³). 3 The following formula was used for the calculation. The above measurements were performed at five randomly selected locations within the same polyolefin microporous membrane, and the average value of the porosity at the five locations was determined. The resin density was determined according to JIS K6922-2-2010 after the polyolefin microporous membrane was heated and melted to form a non-porous sheet. Porosity = [(Volume - Mass / Resin Density) / Volume] × 100

[0091] (4) Puncture strength Using a puncture meter manufactured by MARUBISHI, the maximum load measured when puncturing a polyolefin microporous membrane with a thickness T1 (μm) at a speed of 2 mm / second with a 1 mm diameter needle with a spherical tip (radius of curvature R: 0.5 mm) was defined as the puncture strength L1 (gf). The puncture strength L1(gf) was converted to the maximum load when the thickness is 10 μm using the formula: L2(gf) = L1(gf) / T1(μm) × 10 μm, and this was defined as the puncture strength L2(gf) for a thickness of 10 μm. The above measurements were performed at three different, randomly selected locations within the same polyolefin microporous membrane. The average values ​​of the puncture strength L1(gf) and the puncture strength L2(gf) converted to a thickness of 10 μm were calculated for each of the three locations, and the average value of the puncture strength L2(gf) converted to a thickness of 10 μm was recorded in the table as "Puncture Strength (converted to 10 μm)".

[0092] (5) FIB-SEM Sequential images were measured using FIB-SEM under the following conditions. • Sample preparation: Polyolefin microporous membranes were embedded in epoxy resin, then electron-stained with OsO4, and subjected to measurement. • Observation equipment: FEI Helios G4 Observation conditions: Acceleration voltage 1kV • Sample tilt: 52° • Pixel size: Image horizontal: 5.4nm, Image vertical: 6.8nm (after tilt correction) • Slicing interval in FIB: 10nm • Image alignment method: Markings were made by depositing Pt on the top of the film, and the position of each image was confirmed. • Tilt correction: Since FIB-SEM observation is performed from a 52° angle, the SEM image is observed compressed vertically. Therefore, to obtain an image that appears as if it were observed from the front in the vertical direction, it is necessary to multiply by 1.27 (= / sin52°). The three-dimensional image described later was created using the tilt-corrected image. • Measurement size: FIB processing was sequentially performed on a 5μm x 5μm section of the film cross-section, and the film was sliced ​​in the depth direction until it was 4μm. Measurements were then taken on a volume of 5μm x 5μm x 4μm (400 captured images).

[0093] (6) Creation of three-dimensional images The FIB-SEM images obtained in (5) were binarized using the image processing software "ExFact(registered trademark) Analysis for Fiber" from Visual Science Japan Inc. to separate the embedded, electron-stained resin portion (i.e., the portion corresponding to the pores of the microporous membrane) from the resin portion constituting the microporous membrane. Three-dimensional images were then created based on the binarized information. Subsequently, the resin portion constituting the microporous membrane was thinned using the image processing software "ExFact(registered trademark) Analysis for Porous / Particles" from Visual Science Japan Inc. to create a three-dimensional image of the constituent resin portion inside the microporous membrane. The size of the three-dimensional image to be created was set to a cube enclosed by a central side of 2.7 μm length within the 5 μm × 5 μm × 4 μm FIB-SEM measurement size, from the viewpoint of analysis time and reproducibility of analysis parameters.

[0094] (7) Number of fibrils (6) Using the image processing software "ExFact(registered trademark) Analysis for Fiber" manufactured by Visual Science Japan, the lines of the constituent resin that have undergone thinning processing are divided at the points where they intersect and branch, and the number of thin lines after this process is determined, and then 1 μm 3 The value converted to the number of fibers per unit was defined as the number of fibrils in this invention.

[0095] (8) Average diameter of fibrils The average value of all fibril diameters contained in the 2.7 μm square three-dimensional image obtained in (7) was calculated using the image processing software "ExFact(registered trademark) Analysis for Fiber" manufactured by Visual Science Japan, Inc., and was defined as the average fibril diameter.

[0096] (9) Number of fibril crossings (7) In the image processing software "ExFact(registered trademark) Analysis for Fiber" manufactured by Visual Science Japan, the number of points where the constituent resin parts intersect when dividing the lines of the constituent resin parts of the microporous membrane is determined, and then 1 μm 3The value converted to the number of elements per unit was defined as the fibril crossing number in this invention.

[0097] (10) Capacity retention rate under rapid charge / discharge conditions To evaluate the capacity retention rate under rapid charge-discharge conditions when using a lithium-ion secondary battery configuration, a non-aqueous electrolyte secondary battery consisting of a positive electrode, negative electrode, separator, and electrolyte was equipped with a polyolefin-based microporous membrane as the separator, and charge-discharge tests were conducted.

[0098] A base weight of 9.5 mg / cm² is applied to an aluminum foil measuring 38 mm wide x 33 mm long x 20 μm thick. 2 NMC532 (Lithium Nickel Manganese Cobalt Composite Oxide (Li 1.05 Ni 0.50 Mn 0.29 Co 0.21 A cathode made of laminated O2)) and a copper foil measuring 40 mm wide x 35 mm long x 10 μm thick with a density of 1.45 g / cm³ 3 Natural graphite with a unit area mass of 5.5 mg / cm² 2 A laminated anode was used. The positive and negative electrodes were dried in a vacuum oven at 120°C before use.

[0099] The separator used was a polyolefin microporous membrane measuring 50 mm in length and 50 mm in width, dried in a vacuum oven at room temperature. The electrolyte was prepared by dissolving vinylene carbonate (VC) and LiPF6 in a mixture of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate (30 / 35 / 35, volume ratio), resulting in a solution with a VC concentration of 0.5% by mass and a LiPF6 concentration of 1 mol / L.

[0100] A lithium-ion secondary battery was fabricated by stacking a positive electrode, a separator, and a negative electrode, placing the resulting laminate in a laminate pouch, pouring an electrolyte solution into the laminate pouch, and then vacuum sealing the laminate pouch.

[0101] The fabricated lithium-ion secondary battery was initially charged to 10-15% at 35°C and 0.1C, then left overnight (12 hours or more) at 35°C to allow degassing. Next, CC-CV charging (constant current constant voltage charging (termination current condition 0.02C)) was performed at 35°C, voltage range 2.75-4.2V, and charging current value 0.1C, followed by CC discharge (constant current discharge) at a discharge current value of 0.1C. Then, CC-CV charging (constant current constant voltage charging (termination current condition 0.05C)) was performed at 35°C, voltage range 2.75-4.2V, and charging current value 0.2C, followed by CC discharge (constant current discharge) at a discharge current value of 0.2C. The initial state of the non-aqueous electrolyte secondary battery was defined as having completed three cycles of this process.

[0102] Next, CC-CV charging (constant current constant voltage charging (termination current condition 0.05C)) was performed at a temperature of 35°C, voltage range 2.75~4.2V, and charging current value 0.2C, followed by CC discharge (constant current discharge) at 15°C at 0.2C, and the discharge capacity at that time was taken as the 0.2C capacity. Next, CC-CV charging (constant current constant voltage charging (termination current condition 0.05C)) was performed at a temperature of 35°C, voltage range 2.75~4.2V, and charging current value 0.5C, followed by 10C (180mA, 14.4mA / cm²) of a non-aqueous electrolyte secondary battery at 15°C. 2 A rate test was conducted under the following conditions. Based on these results, the ratio of the 10C capacity to the 0.2C capacity {(10C capacity / 0.2C capacity) × 100} (%) was defined as the capacity retention rate (%) under rapid charge and discharge conditions. A rate of 55% or higher was considered to indicate good performance.

[0103] (11) Maximum contraction stress in the TD direction determined by thermomechanical analysis (TMA) A polyolefin microporous membrane was cut into a rectangle measuring 3 mm in the MD direction and 15 mm in the TD direction to prepare an evaluation sample. Using a Hitachi High-Technologies Corporation "TMA7100," the evaluation sample was fixed to the chuck so that the distance between the chucks (TD direction) was 10 mm, and the temperature was increased from 30°C to 200°C at a rate of 5°C / min in constant length mode. The temperature and shrinkage force at the point of heating to 200°C were 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 was defined as the maximum shrinkage stress (MPa) in the TD direction as measured by a thermomechanical analyzer (TMA).

[0104] (12) Maximum contraction stress temperature in the TD direction determined by thermomechanical analysis (TMA) In (11), the temperature at which the maximum shrinkage stress in the TD direction is observed by a thermomechanical analyzer (TMA) is defined as the maximum shrinkage stress temperature in the TD direction (°C) as determined by a thermomechanical analyzer (TMA).

[0105] (13) Safety evaluation For lithium-ion secondary batteries prepared in the same manner as in (10), constant current charging was performed at a current of 0.2C up to a voltage of 4.2V, followed by constant voltage charging at 4.2V, and then discharged at a current of 1C up to a cutoff voltage of 3.0V. Next, constant current charging was performed at a current of 0.2C up to 4.2V, followed by constant voltage charging at 4.2V. After that, the charged batteries were placed in an oven, heated from room temperature at 5°C / min, left at 150°C for 60 minutes, and evaluated according to the following criteria. A: No fire or smoke is observed after 60 minutes. B: After reaching 150℃, ignition or smoke was observed between 30 and 60 minutes. C: After reaching 150℃, ignition or smoke was observed between 10 and 30 minutes. D: Ignition or smoke was observed within 10 minutes after reaching 150℃.

[0106] (Example 1) The raw material has a weight-average molecular weight (Mw) of 10 × 10 5 Ultra-high molecular weight polyethylene and weight-average molecular weight (Mw) of 5 × 10 5High-density polyethylene was used. 16 parts by mass of ultra-high molecular weight polyethylene and 4 parts by mass of high-density polyethylene were added to 80 parts by mass of liquid paraffin. Furthermore, 0.5 parts by mass of 2,6-di-t-butyl-p-cresol and 0.7 parts by mass of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate]methane were added as antioxidants based on the total mass of the polyethylene resin, and the mixture was prepared to create a polyethylene resin solution. The ratio of ultra-high molecular weight polyethylene when the total mass of the polyethylene resin is taken as 100% by mass was 80% by mass. Since the liquid paraffin and antioxidants are almost completely removed during the manufacturing process, in this invention, the ratio of ultra-high molecular weight polyethylene when the total mass of the polyethylene resin is taken as the ultra-high molecular weight ratio in the polyolefin microporous membrane. The obtained polyethylene resin solution was fed into a twin-screw extruder, kneaded at 180°C, supplied to a T-die, and the extruded material was cooled on a cooling roll controlled to 15°C to form a gel-like sheet. The resulting gel-like sheet was stretched longitudinally (MD direction) by 8 times at 118°C using a roll stretcher (indicated as longitudinal stretching (MD1) in the table), cooled, and then stretched transversely (TD direction) by 8 times at 118°C using a tenter stretcher (indicated as transverse stretching (TD) in the table). The sheet width was then fixed in the tenter stretcher and held at a temperature of 115°C for 10 seconds. The surface magnification, which is the product of the magnification ratio of longitudinal stretching (MD1) and transverse stretching (TD), was 64 times. Next, the stretched gel-like sheet was immersed in a methylene chloride bath in a washing tank to remove the liquid paraffin, and then dried to obtain a polyolefin-based microporous membrane. Finally, using an oven, the sheet was reduced by 5% in the width direction and, in a relaxed state, heat-fixed at 130°C for 10 minutes to obtain a polyolefin-based microporous membrane.

[0107] (Example 2) A polyolefin microporous film was obtained in the same manner as in Example 1, except that the stretching in the longitudinal direction (MD direction) using a roll stretcher was performed in two stages: the first stage (MD1) at 118°C to 4 times the material (indicated as longitudinal stretching (MD1) in the table) and the second stage (MD2) at 118°C to 2 times the material (indicated as longitudinal stretching (MD2) in the table).

[0108] (Examples 3-5, 8-19) A polyolefin-based microporous membrane was obtained in the same manner as in Example 2, except that the raw material composition and manufacturing conditions were as shown in the table.

[0109] (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 manufacturing conditions were as shown in the table.

[0110] (Comparative Examples 1-4) A polyolefin-based microporous membrane was obtained in the same manner as in Example 1, except that the raw material composition and manufacturing conditions were as shown in the table.

[0111] [Table 1]

[0112] [Table 2]

[0113] [Table 3]

[0114] [Table 4]

[0115] [Table 5] [Industrial applicability]

[0116] The polyolefin-based microporous membrane of the present invention is suitable for use as a separator in non-aqueous electrolyte secondary batteries, including lithium-ion secondary batteries, because it exhibits excellent low-resistance characteristics and can improve capacity retention under rapid charge-discharge conditions.

Claims

1. The polyolefin resin used as a raw material is made of ultra-high molecular weight polyethylene, and in a 2.7 μm square three-dimensional image created from cross-sectional images obtained by FIB-SEM measurement of a microporous membrane, the number of fibrils is 870 / μm. ３ More than 2800 lines / μm ３ A polyolefin-based microporous membrane having the following characteristics: a thickness of 3 μm or more and 14 μm or less; a maximum shrinkage stress temperature in the TD direction measured by a thermomechanical analyzer (TMA) of 143°C or higher; and a maximum shrinkage stress of 1.3 MPa or less.

2. The polyolefin-based microporous membrane according to claim 1, wherein the average fibril diameter is 25 nm or more and 60 nm or less.

3. Fibril crossing count: 1350 / μm ３ More than 4400 pieces / μm ３ The polyolefin-based microporous membrane according to claim 1 or 2, which is as follows:

4. A polyolefin-based microporous membrane according to any one of claims 1 to 3, wherein the puncture strength is 180 gf or more and 700 gf or less.

5. A polyolefin-based microporous membrane according to any one of claims 1 to 4, wherein the porosity is 35% or more and 50% or less.

6. A laminate comprising a polyolefin-based microporous film according to any one of claims 1 to 5, further laminated with a heat-resistant resin layer.

7. A non-aqueous electrolyte secondary battery comprising a polyolefin-based microporous membrane according to any one of claims 1 to 5, or a laminate according to claim 6.

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