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 strength, capable of improving capacity retention rate under rapid charge / discharge conditions.SOLUTION: A polyolefin microporous film has 35 passes / μm2 or more in thickness direction and a peak value in tortuosity factor distribution of passes in thickness direction of 1.30 or more and 1.80 or less, 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] Efforts to improve the long-term reliability of lithium-ion secondary batteries include a technology that improves the long-term compressive resistance in minute regions of the film (Patent Document 1), a technology that improves the battery capacity (rate characteristics) under rapid charge-discharge conditions by specific stretching conditions (Patent Document 2), and a technology that reduces charging resistance by setting parameters obtained by FIB-SEM image analysis of the coating layer of a polyolefin-based microporous film within a specific range (Patent Document 3). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2009-242631 [Patent Document 2] Japanese Patent Publication No. 2016-121327 [Patent Document 3] Japanese Patent Publication No. 2018-181649 [Overview of the project] [Problems that the invention aims to solve]

[0007] Patent documents 1 and 2 propose adjusting puncture strength, porosity, thermal shrinkage rate, and film thickness retention rate due to puncture creep by adjusting the raw material composition and manufacturing conditions, thereby improving the long-term reliability when applied to separators in lithium-ion secondary batteries. However, the internal structure in the thickness direction of the microporous film is not sufficiently considered, and there were cases where it was insufficient to achieve both strength and improved capacity retention rate under rapid charge and discharge conditions. Furthermore, patent document 3 proposes reducing charging resistance by setting the fractal dimension of the insulating porous layer obtained by coating to a specific range. However, the structure of the microporous film itself that serves as the substrate to be coated, and furthermore, the internal structure in the thickness direction of the microporous film, are not sufficiently considered, and there were cases where it was insufficient to achieve both strength and improved capacity retention rate under rapid charge and discharge conditions when considering the overall resistance of the substrate and coating layer.

[0008] Therefore, the present invention eliminates the above drawbacks, and by setting the pore structure inside the polyolefin microporous membrane within a specific range, it is excellent in strength when applied as a separator for non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries, and aims to provide a polyolefin microporous membrane capable of improving the capacity retention rate under rapid charge and discharge conditions.

Means for Solving the Problems

[0009] The present invention for solving the above problems has the following configuration. (1) In a 2.7 μm square three-dimensional image created from each cross-sectional image obtained by FIB-SEM measurement of the microporous membrane, the number of paths in the thickness direction is 35 paths / μm 2 or more, and the peak value of the tortuosity distribution of the paths in the thickness direction is 1.30 or more and 1.80 or less, a polyolefin microporous membrane. (2) The polyolefin microporous membrane according to (1), wherein the frequency of the peak value of the tortuosity distribution of the paths in the thickness direction is 6% or more and 30% or less. (3) The polyolefin microporous membrane according to (1) or (2), wherein the half-value width of the tortuosity distribution of the paths in the thickness direction is 0.06 or more and 0.25 or less. (4) The polyolefin microporous membrane according to any one of (1) to (3), wherein the puncture strength is 180 gf or more and 700 gf or less. (5) The polyolefin microporous membrane according to any one of (1) to (4), wherein the thickness is 3 μm or more and 14 μm or less. (6) The polyolefin microporous membrane according to any one of (1) to (5), wherein the porosity is 35% or more and 50% or less. (7) The polyolefin microporous membrane according to any one of (1) to (6), wherein the maximum shrinkage stress temperature in the TD direction by a thermomechanical analyzer (TMA) is 143 °C or more, and the maximum shrinkage stress is 1.3 MPa or less. (8) A laminate in which a heat-resistant resin layer is further laminated on the polyolefin microporous membrane according to any one of (1) to (7). (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 (8). [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 under rapid charge-discharge conditions 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 thickness-direction pass count of 35 lines / 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. 2 Therefore, it is important that the peak value of the curvature distribution of the path in the thickness direction is between 1.30 and 1.80.

[0013] The number of passes in the thickness direction and the peak values ​​of the curve ratio distribution of passes in the thickness direction can each be used as indicators representing the pore structure inside a polyolefin-based microporous membrane.

[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 the 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." Furthermore, in the embodiments of the present invention, the polypropylene-based resin refers to a polymer in which, when the total mass of the polypropylene-based 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.

[0016] 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).

[0017] 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.

[0018] In addition, the polyolefin resin in the embodiment of the present invention may be either a single substance or a mixture of two or more different polyolefin resins.

[0019] Among these various polyolefin resins, a polyethylene resin is particularly preferable from the viewpoint of excellent pore blocking performance. The melting point (softening point) of the polyethylene resin is preferably 70 to 150 °C from the viewpoint of the pore blocking performance of the microporous membrane.

[0020] Hereinafter, a polyethylene resin will be described in detail as an example of the polyolefin resin used in the embodiment of the present invention. Examples of the type of polyethylene resin used in the embodiment of the present invention include high-density polyethylene having a density exceeding 0.94 g / cm 3 medium-density polyethylene having a density 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, ultra-high molecular weight polyethylene having a specific molecular weight, and the like. From the viewpoint of controlling the internal pore 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.

[0021] The ultra-high molecular weight polyethylene used in the embodiment of the present invention preferably has a weight average molecular weight of 1.0×10 6 or more and 1.0×10 7 or less. If the weight average molecular weight is 1.0×10 6 or more, the relaxation time does not become too short, the increase in the stretching temperature and the heat treatment temperature is suppressed, and the fine fibrils are melted, preventing the reduction of 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 various structures in the thickness direction 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 6The above is more comfortable 1.5 × 10 6 More preferably 2.0 × 10 6 In summary, the most preferred is 3.0 × 10 6 That concludes the explanation. Furthermore, the upper limit for the weight-average molecular weight is preferably 8.0 × 10⁻⁶. 6 The 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:

[0022] 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, but 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 most 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, 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.

[0023] 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 particularly preferable: By applying high-density polyethylene with 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, and the quality can be improved.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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 Porous / Particles" 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 microporous membrane. Then, a three-dimensional stereoscopic image is created based on the information from the binarization process, and a three-dimensional image of the pores inside the microporous membrane is created by performing a thinning process on the pore portions of the microporous membrane using image processing software such as "ExFact® Analysis for Porous / Particles" manufactured by Visual Science Japan Inc. 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.

[0028] In this application, the number of passes in the thickness direction is determined by detecting the shortest path among all paths (paths) consisting of voids in the microporous membrane, from the starting surface to the ending surface, of a three-dimensional image cube enclosed by sides of length 2.7 μm, created by the method described above. The number of passes in the thickness direction from the starting surface with an area of ​​2.7 μm × 2.7 μm to the ending surface with an area of ​​2.7 μm × 2.7 μm within the three-dimensional image cube is counted, and 1 μm 2 The value converted to the number of lines per unit is the number of passes in the thickness direction (lines / μm) in the embodiment of the present invention. 2 )

[0029] The polyolefin-based microporous membrane according to the embodiment of the present invention has a thickness-direction pass count of 35 lines / 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. 2By doing so, when used as a separator in a lithium-ion secondary battery, the number of paths for lithium ions to move increases, allowing for smoother lithium ion movement. This reduces the electrical resistance of the separator and improves the capacity retention rate during rapid charging and discharging of the battery. From the perspective of reducing electrical resistance and improving the capacity retention rate during rapid charging and discharging, the number of paths in the thickness direction is 40 lines / μm. 2 The above is preferable, with 55 lines / μm 2 The above is more preferable: 65 lines / μm 2 The above is even more preferable, with 80 lines / μm 2 The above is particularly preferable. Furthermore, from the viewpoint of reducing electrical resistance, a higher number of passes in the thickness direction in the 2.7 μm square three-dimensional image created from each cross-sectional image obtained by FIB-SEM measurement of the microporous film is preferable. However, from the viewpoint of maintaining ease of handling during processing, the number of passes in the thickness direction should be 250 / μm. 2 The following are preferred, and more preferably, 200 lines / μm 2 The following applies:

[0030] In the polyolefin-based microporous membrane according to the embodiment of the present invention, it is important that the peak value of the curvature distribution of paths in the thickness direction in a 2.7 μm square three-dimensional image created from each cross-sectional image obtained by FIB-SEM measurement of the microporous membrane is between 1.30 and 1.80. Here, the peak value of the curvature distribution of paths in the thickness direction refers to the curvature that shows the highest frequency in the graph of the curvature frequency distribution obtained by smoothing a frequency distribution of curvature for all paths in the thickness direction of the target of analysis, after calculating the curvature of each path in the thickness direction obtained by the method described above using image processing software such as "ExFact(registered trademark) Analysis for Porous / Particles". Note that a larger peak value of curvature indicates that the paths in the thickness direction of the microporous membrane are more detours, while a smaller value indicates that the paths in the thickness direction of the microporous membrane have paths that are closer to straight lines. Furthermore, in this application, if two or more peak values ​​for the curvature ratio of the path in the thickness direction are observed, the peak value with the highest frequency is adopted. If two or more peaks with the same frequency are observed, the peak value that results in the lowest curvature ratio is adopted.

[0031] In the polyolefin-based microporous membrane according to the embodiment of the present invention, by setting the peak value of the curvature distribution of the paths in the thickness direction to 1.30 or more and 1.80 or less in a 2.7 μm square three-dimensional image created from each cross-sectional image obtained by FIB-SEM measurement of the microporous membrane, when used as a separator in a lithium-ion secondary battery, the path through which lithium ions move becomes closer to a straight line, allowing lithium ions to move smoothly. This reduces the electrical resistance of the separator and improves the capacity retention rate during rapid charging and discharging of the battery. From the viewpoint of reducing electrical resistance and improving the capacity retention rate during rapid charging and discharging, the peak value of the curvature distribution of the paths in the thickness direction is preferably 1.75 or less, more preferably 1.70 or less, even more preferably 1.65 or less, and particularly preferably 1.60 or less. Furthermore, from the viewpoint of reducing electrical resistance, it is preferable that the peak value of the curvature distribution of the path in the thickness direction in the 2.7 μm square three-dimensional image created from each cross-sectional image obtained by FIB-SEM measurement of the microporous membrane be as low as possible. However, if the peak value of the curvature distribution becomes too low, when used as a separator in a lithium-ion battery, dendritic lithium metallic deposits called dendrites are more likely to be formed on the electrode surface, which may cause short circuits between the positive and negative electrodes. Therefore, it is preferable that the peak value of the curvature distribution be 1.30 or higher.

[0032] In an embodiment of 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 membrane, the number of passes in the thickness direction is 35 / μm. 2As described above, a method for achieving a peak value of the curvature distribution of paths in the thickness direction of 1.30 or more and 1.80 or less is to use ultra-high molecular weight polyethylene for 80% or more of the constituent resin of the polyolefin-based microporous membrane, to use a wet surface magnification of 60 times or more, and to use a resin concentration of less than 30% by mass during manufacturing. In the embodiment of the present invention, it has been found that by using ultra-high molecular weight polyethylene for 80% or more of the constituent resin of the polyolefin-based microporous membrane and to use a resin concentration of less than 30% by mass during manufacturing, 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 magnification of 60 times or more, the areas with insufficient porosity can be significantly reduced, and uniform porosity of the polyolefin-based microporous membrane can be achieved. Furthermore, by achieving uniform porosity in the polyolefin-based microporous membrane, the number of insufficiently porosity-filled areas is reduced, thereby increasing the number of paths in the thickness direction. Additionally, by reducing insufficiently porosity-filled areas, bypassing paths in the thickness direction is suppressed, making it possible to control the peak value of the curve ratio distribution to a specific range with low values.

[0033] In the embodiment of the present invention, the polyolefin-based microporous membrane is preferable when used as a separator for lithium-ion secondary batteries, from the viewpoint of reducing electrical resistance and improving capacity retention during rapid charging and discharging, such that the frequency of peak values ​​in the curvature distribution of paths in the thickness direction is 6% or more and 30% or less. Here, the frequency of peak values ​​in the curvature distribution refers to the frequency value of the peak value in the curvature frequency distribution described above, and the larger the value, the greater the proportion of paths with peak curvature values, that is, the more uniform the curvature of each path in the thickness direction.

[0034] In embodiments of the present invention, the frequency of peak values ​​in the curvature distribution of the paths in the thickness direction is more preferably 7% or more, even more preferably 9% or more, and particularly preferably 12% or more, from the viewpoint of further enhancing the effects of reducing electrical resistance and improving capacity retention during rapid charging and discharging. On the other hand, from the viewpoint of improving mechanical properties such as strength, the frequency of peak values ​​in the curvature distribution of the paths in the thickness direction is preferably 30% or less.

[0035] In embodiments of the present invention, methods for setting the frequency of peak values ​​in the curvature distribution of paths in the thickness direction to 6% or more and 30% or less include setting the wet stretching speed to a specific low range to homogenize the opening properties during stretching, and applying multi-stage stretching of two or more stages to gradually advance the opening and increase the number of paths with the same curvature.

[0036] In the embodiments of the present invention, the polyolefin-based microporous membrane is preferable when used as a separator in a lithium-ion secondary battery, from the viewpoint of reducing electrical resistance and improving capacity retention during rapid charging and discharging, if the full width at half maximum of the curvature distribution of the paths in the thickness direction is 0.06 or more and 0.25 or less. Here, the full width at half maximum of the curvature distribution refers to the arithmetic full width at half maximum of the graph of the curvature frequency distribution described above, and the smaller the value, the sharper the curvature distribution, indicating that the curvature of the paths in the thickness direction is uniform.

[0037] In embodiments of the present invention, the width at half maximum of the curvature distribution of the paths in the thickness direction is more preferably 0.20 or less, even more preferably 0.15 or less, and particularly preferably 0.11 or less, from the viewpoint of further enhancing the effects of reducing electrical resistance and improving capacity retention during rapid charging and discharging. On the other hand, from the viewpoint of improving mechanical properties such as strength, the width at half maximum of the curvature distribution of the paths in the thickness direction is more preferably 0.06 or more, and even more preferably 0.07 or more.

[0038] In embodiments of the present invention, a method for making the width at half maximum of the curvature distribution of the paths in the thickness direction 0.06 or more and 0.25 or less includes setting a large relaxation rate in the heat treatment process after wet stretching to promote shrinkage in the plane direction and homogenizing the hole paths in the thickness direction in the linear direction.

[0039] 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.

[0040] 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.

[0041] The polyolefin-based 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 for a lithium-ion secondary battery. From the viewpoint of further improving the impact resistance of the battery when used as a separator for 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-based 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 more is preferable. Note that the puncture strength in the embodiments of the present invention is the puncture strength when the thickness is converted to 10 μm.

[0042] 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.

[0043] 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.

[0044] 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 various structures of the paths in the thickness direction, and improving puncture strength and safety when used as a separator in lithium-ion secondary batteries.

[0045] 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.

[0046] The polyolefin-based microporous membrane according to the embodiment of the present invention may be a laminate in which a heat-resistant resin layer is further laminated, from the viewpoint of improving heat resistance when mounted on a lithium-ion battery.

[0047] Preferably, the heat-resistant resin layer is made of a resin that is insoluble in the battery electrolyte 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.

[0048] 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.

[0049] 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 process of melt-kneading a polymer material containing one or more 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

[0050] 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.

[0051] 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.

[0052] Regarding the solvent ratio, from the viewpoint of easily controlling the structure of the paths in the thickness direction to 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.

[0053] 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.

[0054] 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.

[0055] (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.

[0056] 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).

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] (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.

[0065] 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 achieving a desired range for various path structures in the thickness direction, a surface ratio of 60 times or more is preferable, more preferably 80 times or more, and particularly preferable 100 times or more. In addition, from the viewpoint of suppressing tearing during the manufacturing of polyolefin-based microporous films, a surface ratio of 150 times or less is preferable.

[0066] From the viewpoint of improving stretch uniformity in the stretching process, the preferred form of 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 form is to have a composition in which 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.

[0067] 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).

[0068] If the stretching temperature is below 90°C, the pores will not open sufficiently due to low-temperature stretching, making it difficult to obtain uniform 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.

[0069] 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.

[0070] 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.

[0071] (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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] (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.

[0079] 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.

[0080] 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 uniform porosity formation in the polyolefin microporous membrane.

[0081] 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.

[0082] (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]

[0083] 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.

[0084] (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.

[0085] (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 locations 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.

[0086] (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

[0087] (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 10 μm film thickness were calculated for each of the three locations, and the average value of the puncture strength L2(gf) converted to a 10 μm film thickness was recorded in the table as "Puncture Strength (10 μm equivalent)".

[0088] (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).

[0089] (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 Porous / Particles" from Visual Science Japan Inc. The embedded, electron-stained resin portion (i.e., the portion corresponding to the pores of the microporous membrane) and the resin portion constituting the microporous membrane were then binarized. Based on the binarized information, a three-dimensional image was created. Subsequently, the pore portions of the microporous membrane were 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 pore structure 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.

[0090] (7) Number of passes in the thickness direction (6) Using the image processing software "ExFact(registered trademark) Analysis for Porous / Particles" manufactured by Visual Science Japan, the shortest path was detected from all paths consisting of the pores of the polyolefin microporous membrane, from the starting surface to the ending surface in the thickness direction, and was counted as one path. In the cube of the three-dimensional image enclosed by sides of 2.7 μm length, all paths in the thickness direction from the starting surface with an area of ​​2.7 μm × 2.7 μm to the ending surface with an area of ​​2.7 μm × 2.7 μm were counted, and 1 μm 2 The value converted to the number of strokes per unit was used as the number of passes in the thickness direction in this invention.

[0091] (8) Peak value of the curvature distribution of the path in the thickness direction For each path in the thickness direction obtained in (7), the curvature ratio was calculated using image processing software such as "ExFact(registered trademark) Analysis for Porous / Particles". Then, a frequency distribution of curvature ratios was created for all paths in the thickness direction of the analysis target, and a graph of the curvature ratio frequency distribution was created after smoothing. For the obtained graph, the curvature ratio showing the highest frequency was taken as the peak value of the curvature ratio distribution of the paths in the thickness direction. If two or more peak values ​​of curvature ratios were observed in the paths in the thickness direction, the peak value with the highest frequency was adopted. If two or more peaks with the highest frequency were observed at the same frequency, the peak value with the lowest curvature ratio was adopted.

[0092] (9) Frequency of peak values ​​in the curve ratio distribution of paths in the thickness direction This value represents the frequency at the peak of the curve ratio distribution obtained in (8).

[0093] (10) Width at half maximum of the curvature distribution of the path in the thickness direction The graph of the curve frequency distribution obtained in the same manner as in (8) was fitted with a function using numerical analysis software, and the arithmetic full width at half maximum was obtained.

[0094] (11) 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.

[0095] 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 / cm²2 A laminated anode was used. The positive and negative electrodes were dried in a vacuum oven at 120°C before use.

[0096] 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.

[0097] 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.

[0098] 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. Subsequently, CC-CV charging (constant current constant voltage charging (termination current condition 0.05C)) at 35°C, voltage range 2.75-4.2V, and charging current value 0.2C was performed, 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.

[0099] 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 of 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 of 0.5C, followed by 10C (180mA, 14.4mA / cm²) of a non-aqueous electrolyte secondary battery at 15°C. 2A 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.

[0100] (12) 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).

[0101] (13) Maximum contraction stress temperature in the TD direction determined by thermomechanical analysis (TMA) In (12), 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).

[0102] (14) Safety evaluation For lithium-ion secondary batteries prepared in the same manner as in (11), 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℃.

[0103] (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 5 High-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 in width by 5% (indicated as the relaxation rate in the table) and then heat-set at 130°C for 10 minutes to obtain a polyolefin-based microporous membrane. In the table, the percentage of reduction in width during heat-set is referred to as the relaxation rate, and the time spent on heat-set is referred to as the heat-set time.

[0104] (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).

[0105] (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.

[0106] (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.

[0107] (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.

[0108] [Table 1]

[0109] [Table 2]

[0110] [Table 3]

[0111] [Table 4]

[0112] [Table 5] [Industrial applicability]

[0113] The polyolefin-based microporous membrane according to the embodiment 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 strength 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 passes in the thickness direction is 35 / μm. ２ The above describes a polyolefin-based microporous membrane in which the peak value of the path curvature distribution in the thickness direction is 1.30 or more and 1.80 or less, the thickness is 3 μm or more and 14 μm or less, and the maximum shrinkage stress temperature in the TD direction measured by thermomechanical analysis (TMA) is 143°C or higher and the maximum shrinkage stress is 1.3 MPa or lower.

2. The polyolefin-based microporous membrane according to claim 1, wherein the frequency of peak values ​​in the distribution of curvature ratios of paths in the thickness direction is 6% or more and 30% or less.

3. A polyolefin-based microporous membrane according to claim 1 or claim 2, wherein the width at half maximum of the path curvature distribution in the thickness direction is 0.06 or more and 0.25 or less.

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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