Polyolefin microporous film, separator for batteries, and lithium ion secondary battery

Abstract

The purpose of the present invention is to provide a polyolefin microporous film that has high tensile strength and elongation in the machine direction and also has excellent impact resistance when used as a separator. The polyolefin microporous film has tensile strength in the machine direction (MD) of 185 MPa or more and a tensile elongation in the MD of 145% or more.

Description

Polyolefin microporous membrane, battery separator, and lithium ion secondary battery

[0001] The present invention relates to a polyolefin microporous membrane, a battery separator, and a lithium ion secondary battery.

[0002] Microporous membranes are used in various fields, such as filters such as filtration membranes and dialysis membranes, battery separators, and separators for electrolytic capacitors. Among these, microporous membranes made of polyolefin as a resin material are widely used as separators for secondary batteries because they have excellent chemical resistance, insulating properties, mechanical strength, and shutdown properties. Furthermore, batteries must pass safety tests, such as impact resistance tests, to prevent fire or smoke generation when subjected to external impact, and polyolefin microporous membranes are therefore required to have improved mechanical strength, etc.

[0003] From the viewpoint of impact resistance of the battery, Patent Document 1 specifies that the tensile strength (MPa) and tensile elongation (%) in the MD and TD directions are [(tensile strength in MD direction × tensile elongation in MD direction / 100)]. ２ + (tensile strength in TD direction × tensile elongation in TD direction / 100) ２ ] １／２ ≧300, has a tensile strength of 196 MPa or more in the MD and TD directions, a maximum pore size measured using a perm porometer of 60 nm or less, a mean flow pore size measured using a perm porometer of 40 nm or less, and a porosity of 40% or more.

[0004] Patent Document 2 proposes a polyolefin microporous membrane in which the rate of change in air resistance after heat-compression at a temperature of 90°C and a pressure of 5.0 MPa for 5 minutes is 50% or less, and the rate of change in membrane thickness after heat-compression at a temperature of 90°C and a pressure of 5.0 MPa for 5 minutes is 10% or less, with the membrane thickness of the polyolefin microporous membrane before heat-compression being 100%.

[0005] Patent Document 3 proposes a polyolefin microporous membrane having a membrane thickness of 10 μm or less, a tensile strength in the longitudinal direction of 270 MPa or more and 350 MPa or less, a tensile strength in the transverse direction of 220 MPa or more and 280 MPa or less, a tensile elongation in the longitudinal direction of 100% or more and 150% or less, and a tensile elongation in the transverse direction of 100% or more and 180% or less.

[0006] Patent Document 4 proposes a polyolefin microporous membrane having an average tensile strength of 150 MPa or more, an average tensile elongation of 185% or more, an average solid-state heat shrinkage of 10.0% or less calculated from the solid-state heat shrinkage measured after heating at 105°C for 8 hours, and a TD heat shrinkage of 14% or less at the shutdown temperature determined by thermomechanical analysis.

[0007] Patent Document 5 proposes a polyolefin microporous membrane having a total degree of orientation obtained by 360° measurement at 15° intervals by Raman spectroscopy of 70 to 90, and a proportion of amorphous components (α135) at 135°C of 35% or more as measured by the solid echo method of pulsed NMR.

[0008] International Publication No. 2018 / 180714 International Publication No. 2015 / 194504 JP 2020-164791 A JP 2020-164858 A JP 2022-48093 A

[0009] In recent years, there has been an increasing demand for portable tools, such as power tools, that utilize lithium-ion secondary batteries. Because power tools are highly portable, they are likely to be used at high altitudes, at high temperatures, and in a variety of other unpredictable environments. Therefore, the requirements for safety, particularly impact resistance, are becoming increasingly stringent. To improve the impact resistance of batteries, separator films must have high mechanical strength. However, increasing mechanical strength generally also reduces elongation. When subjected to impacts that involve large deformation, separator films with low elongation can rupture, leading to battery fires and smoke. Conversely, increasing mechanical elongation reduces mechanical strength, making them more susceptible to rupture under strong impacts. Therefore, the mechanical properties of conventional separators may be insufficient for future lithium-ion secondary battery separators.

[0010] In view of the above circumstances, an object of the present invention is to provide a polyolefin microporous membrane having excellent impact resistance.

[0011] To solve the above problems, the present invention has the following features. [I] A polyolefin microporous membrane having a tensile strength in the longitudinal direction (MD) of 185 MPa or more and a tensile elongation in the MD of 145% or more. [II] A polyolefin microporous membrane according to [I] having a tensile strength in the transverse direction (TD) of 200 MPa or more and a tensile elongation in the TD of 100% or more. [III] A polyolefin microporous membrane according to [I] or [II] having an R value of the Raman orientation parameter measured by the following method of 1.55 or less and a total value of the Raman orientation parameter in each in-plane direction of 72 or less. [Method for measuring Raman orientation parameter] [Apparatus] The measurement apparatus used is an inVia microscopic Raman spectroscopy system (manufactured by Renishaw).・180° backscattering configuration・Spectral length 250 mm・Diffraction grating 3000 lines / mm・Excitation laser 532 nm・50x objective lens (N.A. = 0.75)・Spot size (spatial resolution) 5 μm [Polarization conditions] The laser is perpendicularly incident from the normal direction of the film surface (XY plane) and polarized using a polarizer. The measurement sample is rotated, and Raman spectra are obtained in each direction for 24 directions up to 345° in 15° increments, with MD at 0°. [Calculation of peak intensity] The Raman spectrum obtained is calculated at 1020 cm －１ 1160cm or more －１ The baseline was obtained by linear approximation in the following region: 1060 cm －１ and 1130 cm －１ The maximum values ​​of the Raman bands are calculated as peak intensities I1130 and I1160, respectively. [Degree of orientation] 1130 cm －１ and 1060 cm －１The peak intensity ratio (I1130 / I1060) is taken as the degree of orientation. [R value] The difference between the largest and smallest values ​​of the degrees of orientation in 24 directions from 0° MD to 345° in 15° increments is calculated as the degree of orientation R. [Total value of Raman orientation parameters in each in-plane direction] The sum of the degrees of orientation in each direction (15° x n (1≦n≦24 (n is an integer))) when measuring 24 directions from 0° MD to 345° in 15° increments is calculated as the total value of the Raman orientation parameters in each in-plane direction. [IV] The weight average molecular weight (Mw) determined by gel permeation chromatography (GPC) measurement is 5.0×10 ５ ~1.0 x 10 ６ [V] The polyolefin microporous membrane according to any one of [I] to [III], wherein the number average molecular weight (Mn) measured by gel permeation chromatography (GPC) is 1.0 × 10 ５ ~3.0 x 10 ５ and the peak top molecular weight (Mp) is 2.5 × 10 ５ ~4.5 x 10 ５ [VI] The polyolefin microporous membrane according to any one of [I] to [V], which has a porous layer on at least one surface of the polyolefin microporous membrane. [VII] A battery separator comprising the polyolefin microporous membrane according to any one of [I] to [VI]. [VIII] The battery separator according to [VII], which is used in a power tool. [IX] A lithium ion secondary battery comprising the polyolefin microporous membrane according to any one of [I] to [VI].

[0012] The present invention provides a polyolefin microporous membrane with excellent impact resistance. In particular, the polyolefin microporous membrane of the present invention is suitable for use as a battery separator. Furthermore, the polyolefin microporous membrane of the present invention is suitable for use in lithium ion secondary batteries.

[0013] The present embodiment of the present invention will be described below, but the present invention is not limited to the embodiment described below.

[0014] The polyolefin microporous membrane of the present invention has a longitudinal (MD) tensile strength of 185 MPa or more and an MD tensile elongation of 145% or more. The MD tensile strength is more preferably 220 MPa or more, and even more preferably 240 MPa or more. An MD tensile strength below 185 MPa results in poor impact resistance. When the MD tensile strength is within the above preferred range, the polyolefin microporous membrane exhibits excellent impact resistance when used as a battery separator. The MD tensile strength can be adjusted to a predetermined range by combining membrane-forming conditions such as the molecular weight, blending ratio, and stretching temperature of the polyolefin in the production process. The upper limit is not particularly limited, but from the viewpoint of membrane formability, it can be set to 400 MPa or less.

[0015] The polyolefin microporous membrane of the present invention preferably has an MD tensile elongation of 150% or more, and even more preferably 155% or more. If the MD tensile strength is below 145%, the impact resistance is poor. When the MD tensile elongation is within the above preferred range, the polyolefin microporous membrane exhibits excellent impact resistance when used as a battery separator. The MD tensile elongation can be adjusted to a predetermined range by adjusting the membrane-forming conditions, such as the molecular weight, blending ratio, and stretching temperature of the polyolefin, in the production process. The upper limit is not particularly limited, but from the viewpoint of membrane formability, it can be set to 300% or less.

[0016] The polyolefin microporous membrane of the present invention preferably has a width direction (TD) tensile strength of 200 MPa or more. It is more preferably 225 MPa or more, and even more preferably 250 MPa or more. A TD tensile strength below 200 MPa results in poor impact resistance. When the TD tensile strength is within the above preferred range, the polyolefin microporous membrane exhibits excellent impact resistance when used as a battery separator. The TD tensile strength can be adjusted to a predetermined range by combining membrane-forming conditions such as the molecular weight, blending ratio, and stretching temperature of the polyolefin in the production process. The upper limit is not particularly limited, but from the viewpoint of membrane formability, it can be set to 400 MPa or less.

[0017] The polyolefin microporous membrane of the present invention preferably has a TD tensile elongation of 100% or more. It is more preferably 105% or more, and even more preferably 110% or more. A TD tensile strength below 100% results in poor impact resistance. When the TD tensile elongation is within the above preferred range, the polyolefin microporous membrane exhibits excellent impact resistance when used as a battery separator. The TD tensile elongation can be adjusted to a predetermined range by combining membrane-forming conditions such as the molecular weight, blending ratio, and stretching temperature of the polyolefin in the production process. The upper limit is not particularly limited, but from the viewpoint of membrane formability, it can be set to 300% or less.

[0018] The polyolefin microporous membrane of the present invention preferably has a Raman orientation degree in MD and TD, each measured by the method described below, of 2.0 to 4.0, more preferably 2.5 to 3.5. The ratio of the orientation degrees in MD and TD is preferably 0.8 to 1.2, more preferably 0.9 to 1.1.

[0019] The Raman orientation parameter R value is preferably 1.55 or less, more preferably 1.25 or less, and even more preferably 1.0 or less. The lower limit is not particularly limited, but can be set to 0 or more. When the Raman orientation degree and R value are within the above ranges, it indicates that the molecules are not oriented in a specific direction, and therefore there are no locally weak areas in mechanical strength. If the molecules are strongly oriented in a specific direction, the tensile elongation in that direction will be low and the film will be more susceptible to tearing due to external impact. Therefore, by setting the Raman orientation degree and R value within the above ranges, a polyolefin microporous film with excellent impact resistance can be obtained. Furthermore, the total degree of orientation in each in-plane direction in the method described below is preferably 72 or less, more preferably 70 or less, and even more preferably 68 or less. By setting the total degree of orientation within the above range, it is possible to prevent the molecular difference from being highly oriented in the in-plane direction, resulting in low tensile elongation and more susceptible to tearing due to external impact. The lower limit of the total degree of orientation is preferably 55 or more, from the viewpoint of maintaining tensile strength. The Raman orientation value can be adjusted within a predetermined range by adjusting the film-forming conditions, such as the molecular weight and compounding ratio of the polyolefin and the stretching temperature, in the production process.

[0020] The polyolefin constituting the polyolefin microporous membrane of the present invention has a weight average molecular weight (Mw) of 5.0 × 10 as determined by gel permeation chromatography (GPC) measurement, from the viewpoint of facilitating control of tensile strength / elongation and molecular orientation state. ５ ~1.0 x 10 ６ It is preferable that the range is 5.6×10 ５ ~7.5 x 10 ５ It is more preferable that the weight average molecular weight (Mw) is in the range of 5.0×10 ５ Above 1.0 x 10 ６ It is preferred that the polyethylene contains:

[0021] The polyolefin microporous membrane of the present invention has a number average molecular weight (Mn) of 1.0 × 10 ５ ~3.0 x 10 ５ and the peak top molecular weight (Mp) is 2.5 × 10 ５ ~4.5 x 10 ５Preferably, the polyolefin constituting the polyolefin microporous membrane is within the above range, which makes it easy to achieve high tensile strength and elongation and uniform in-plane orientation. The weight average molecular weight, number average molecular weight, and peak top molecular weight of the polyolefin resin composition constituting the polyolefin microporous membrane can be determined by GPC under the conditions described below.

[0022] The polyolefin microporous membrane of the present invention preferably has a membrane thickness of 3 μm or more and 30 μm or less, more preferably 8 μm or more and 20 μm or less.

[0023] The polyolefin microporous membrane of the present invention has a basis weight of 5 g / m ２ 20g / m or more ２ Preferably, the amount is 6 g / m or less. ２ 15g / m or more ２ More preferably, it is:

[0024] The polyolefin microporous membrane of the present invention preferably has a porosity of 30% to 70%, more preferably 40% to 60%. By setting the membrane thickness, basis weight, and porosity within the above ranges, good ion permeability can be obtained without impairing impact resistance.

[0025] The polyolefin microporous membrane of the present invention has a 12 μm equivalent air resistance of 300 sec / 100 cm ２ It is preferable that the speed is 250 sec / 100 cm or less. ２ The lower limit is not particularly limited, but from the viewpoint of preventing overcurrent, it is preferably 50 sec / 100 cm ２ When the air permeability is within the above range, good ion permeability can be obtained.

[0026] The polyolefin microporous membrane of the present invention preferably has a pin puncture strength of 5.0 N or more, more preferably 6.0 N or more. There is no particular upper limit, but it is more preferably 10.0 N or less. When the pin puncture strength is within the above range, good mechanical properties can be obtained.

[0027] (Method for producing polyolefin microporous membrane) The polyolefin microporous membrane of the present invention may be a single-layer microporous membrane or a multi-layer microporous membrane composed of multiple layers. The polyolefin resin composition in the form of a single layer will be described below.

[0028] (1) Polyolefin Resin Composition The polyolefin resin composition may contain polyethylene. From the viewpoint of uniform orientation of the polyolefin microporous film, it is preferably an ethylene copolymer.

[0029] (Polyethylene) The polyethylene has a weight average molecular weight (Mw) of 5.0 × 10 ５ or more. The polyethylene may be a copolymer containing a small amount of an α-olefin copolymer other than ethylene. Preferred α-olefin copolymers other than ethylene are propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, and styrene. The content of the α-olefin other than ethylene is preferably 5 mol % or less, assuming the α-olefin copolymer to be 100 mol %.

[0030] The polyethylene has a weight average molecular weight (Mw) of 5.0 × 10 in order to facilitate control of the strength and stretchability of the microporous membrane. ５ 1.2 x 10 ６ Preferably, it is less than 6.0 × 10 ５ Above 1.0 x 10 ６ More preferably, it is 5.6×10 or less. ５ ~7.5 x 10 ５ The melting point of the polyethylene is preferably 128°C or higher and lower than 133°C, more preferably 130°C or higher and lower than 132°C, from the viewpoints of uniform orientation and low air permeability.

[0031] The polyolefin resin composition may contain other resin components in addition to the polyethylene as needed. Examples of such other resin components include resins that further impart heat resistance. Furthermore, various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, antiblocking agents, fillers, crystal nucleating agents, and crystallization retarders may be added within the range that does not impair the effects of the present invention.

[0032] In this embodiment, a microporous membrane may be formed by laminating a porous layer on at least one side of the polyolefin microporous membrane.The porous layer is not particularly limited, but for example, a porous layer made of a resin may be laminated.The resin used here is not particularly limited, and known resins can be used, such as acrylic resin, polyvinylidene fluoride resin, polyamideimide resin, polyamide resin, aromatic polyamide resin, and polyimide resin.The porous layer may further contain inorganic particles, and the inorganic particles are not particularly limited, and known materials can be used, such as alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, and silicon.

[0033] (2) Method for producing a polyolefin microporous membrane The method for producing a polyolefin microporous membrane of the present invention preferably includes the following steps: (a) preparation of a solution, (b) forming a gel-like sheet, (c) first stretching, (d) second stretching, (e) removal of plasticizer and drying, (f) third stretching, and (g) heat treatment.

[0034] Each step will be described in detail.

[0035] (a) Preparation of Solution A plasticizer is added to a polyolefin resin composition in a twin-screw extruder, followed by melt-kneading to prepare a solution. The polyolefin resin composition preferably contains 10% by mass or more and 30% by weight or less of the total resin solution. By keeping the concentration of the polyolefin resin composition within the above range, melt fracture and necking at the die outlet can be prevented when the polyolefin solution is extruded, resulting in good moldability and appearance of the extrusion molded product. The solution is fed from the extruder to a die and extruded into a sheet to obtain an extrusion molded product. The extrusion method may be either a flat die method or an inflation method. A preferred example of the flat die gap is 0.1 mm or more and 5 mm or less. The extrusion temperature is preferably 140°C or more and less than 240°C, and the extrusion speed is preferably 0.2 to 15 m / min.

[0036] (b) Forming of a Gel-Like Sheet The resulting extrusion is cooled to form a gel-like sheet. Cooling methods include contacting the extrusion with a refrigerant such as cold air or cooling water, or contacting a cooling roll, but cooling by contacting a roll cooled with a refrigerant is preferred. Cooling is preferably carried out at a rate of 50°C / min or more to at least the gelation temperature. Cooling is preferably carried out to 25°C or below. When the cooling rate is within the above range, the crystallinity is maintained within an appropriate range, resulting in a gel-like sheet suitable for stretching.

[0037] (c) First Stretching Next, the gel-like sheet is stretched. After preheating, the gel-like sheet is preferably stretched at a predetermined ratio by a tenter method, a roll method, an inflation method, or a combination thereof. Stretching may be uniaxial or biaxial. The stretching ratio (area stretching ratio) is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. The stretching ratios in MD and TD may be the same or different, and a stretching ratio of 3 times or more is preferred in both MD and TD.

[0038] The first stretching temperature is preferably 115°C or higher and 130°C or lower, more preferably 120°C or higher and 125°C or lower. The stretching ratio in the MD is preferably 6 times or higher, more preferably 6.3 times or higher, even more preferably 6.7 times or higher, and preferably 8 times or lower. If the stretching temperature is lower than the above range or the stretching ratio exceeds the above range, the orientation in the MD may be too strong, resulting in a decrease in the MD tensile elongation and TD tensile strength. If the stretching temperature exceeds the above range or the stretching ratio is lower than the above range, the orientation in the MD may be too weak, making it impossible to obtain sufficient MD tensile strength. By setting the MD stretching temperature and stretching ratio within the above ranges, the molecular chains are moderately oriented in the MD, resulting in high tensile strength and elongation in the MD and TD. Therefore, when used as a battery separator, the impact resistance is excellent.

[0039] (d) Second Stretching The preheating and stretching temperatures for the second stretching are preferably 120°C or higher and 130°C or lower, and the stretch ratio is preferably 8.0 times or higher and 10 times or lower in the TD. If the preheating and stretching temperatures are lower than the above range or the stretch ratio exceeds the above range, the TD orientation may be too strong, resulting in a decrease in the MD tensile strength and TD tensile elongation. Furthermore, if the preheating and stretching temperatures exceed the above range or the stretch ratio is lower than the above range, the TD orientation may be too weak, making it impossible to obtain sufficient TD strength. By setting the TD preheating and stretching temperatures and stretch ratio within the above ranges, the molecular chains are moderately oriented in the TD, resulting in high tensile strength and elongation in the MD and TD. Therefore, when used as a battery separator, the impact resistance is excellent.

[0040] (e) Removal of Plasticizer Next, a cleaning solvent is used to remove the plasticizer contained in the gel-like sheet, followed by drying. Known cleaning solvents and methods for removing plasticizers using the same can be used. For example, the methods described in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used. After removing the plasticizer, it is preferable to dry the sheet by a heat drying method or an air drying method. Any method capable of removing the cleaning solvent may be used, including conventional methods such as heat drying and air drying (moving air).

[0041] (f) Third Stretching The dried sheet is preheated and then stretched in at least one direction (dry stretching) to obtain a polyolefin microporous membrane. The second stretching is preferably performed by a tenter method or the like while heating. The final stretching ratio in the second stretching is preferably 1.2 times or more, more preferably 1.25 times or more, and even more preferably 1.35 times or more. By setting the final stretching ratio within the above range, the tensile strength and elongation can be controlled within the desired range. However, since stretching at a high ratio reduces the MD tensile strength and TD tensile elongation, it is preferably 1.5 times or less.

[0042] (g) Heat Treatment After the second stretching, it is preferable to perform a heat treatment while holding the film with clips and keeping the width fixed. The heat treatment is preferably performed at 115.0°C or higher and 135.0°C or lower. By setting the heat treatment temperature within the above range, the heat shrinkage rate of the polyolefin microporous membrane can be reduced. A heat-relaxing treatment may be performed during the heat treatment. When performing a heat-relaxing treatment, the relaxation rate is preferably 4.5% or higher and 30% or lower, with the length immediately before being 100%. By setting the relaxation rate within the above range, the TD tensile elongation can be improved. If the relaxation rate is lower than 4.5%, the MD tensile elongation and TD tensile strength will be low, and if it exceeds 30%, the film may wrinkle or flutter during the transport process after relaxation.

[0043] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0044] [Measurement method] (1) Thickness The thickness of a polyolefin microporous membrane was measured at five points within a 95 mm × 95 mm area: the upper left, upper right, center, lower left, and lower right using a contact thickness meter (a Litematic manufactured by Mitutoyo Corporation, contact pressure 0.01 N, 10.5 mmφ probe), and the average value was taken as the thickness (μm). If the sample size cannot be made into 95 mm × 95 mm, a sample of any size may be cut out, and measurements may be made at the five points: the upper left, upper right, center, lower left, and lower right.

[0045] (2) Basis Weight: A polyolefin microporous membrane cut into 5 cm squares was prepared, and the weight of each piece was measured using a precision balance (5 significant digits (0.0000 g)).２ If the sample size cannot be made 5 cm x 5 cm, it is also possible to cut out a piece of any size and divide the measured weight by the area to calculate the weight.

[0046] (3) Porosity A polyolefin microporous membrane was cut into a size of 95 mm x 95 mm, and its volume (cm ３ ) and weight (g), and the film density (g / cm ３ ), the porosity (%) was calculated by the following formula: Porosity = ((volume - weight / membrane density) / volume) x 100, where membrane density is 0.99 g / cm ３ The film thickness measured in (1) above was used to calculate the volume.

[0047] (4) Air Resistance The air resistance (sec / 100 cm) of the polyolefin microporous membrane was measured using an air resistance meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) in accordance with JIS P-8117:2009. ３ The 12 μm equivalent air permeability drop was calculated using the following formula (1): 12 μm equivalent air permeability drop = air permeability drop (sec / 100 cm ３ )×12 / film thickness (μm) Formula (1).

[0048] (5) Puncture Strength: Using a needle with a diameter of 1 mm (tip 0.5 mmR), the test was performed at a speed of 2 mm / sec with a basis weight W (g / m ２ The maximum load S (N) when a polyolefin microporous membrane of 1000 g / cm2 was punctured was measured. The converted puncture strength for a membrane of 5 μm thickness was calculated using the following formula: Equation: Basis weight converted puncture strength=S(N) / W (g / m ２ ).

[0049] (6) Tensile Strength and Elongation The tensile strength (MPa) corresponding to each direction was measured using an Instron tensile tester, Instron 5543, in accordance with ASTM D882 under the following conditions. Sample shape: 100 mm x 10 mm rectangular measurement direction: MD (length direction), TD (width direction); Chuck distance: 20 mm; Tensile speed: 100 mm / min; Grip: Instron 2702-018 Jaw Faces for Flats (Rubber Coated, 50 x 38 mm); Load cell: 500 N; Chuck pressure: 0.50 MPa; Temperature: 23 ° C. The tensile strength (MPa) was determined by dividing the strength at sample break by the cross-sectional area of ​​the sample before the test. The tensile strength was measured at five points on each sample in each direction, and the average value was calculated.

[0050] (7) Raman Orientation Parameter [Apparatus] The measurement apparatus used was a microscopic Raman spectroscopy system inVia (manufactured by Renishaw). 180° backscattering configuration, spectroscopy length 250 mm, diffraction grating 3000 lines / mm, excitation laser 532 nm, 50x objective lens (N.A. = 0.75), spot size (spatial resolution) 5 μm [Polarization Conditions] The laser was incident perpendicularly from the normal direction to the film surface (XY plane), and polarized using a polarizer. The measurement sample was rotated, and Raman spectra were obtained in 24 directions up to 345° in 15° increments, with MD at 0°. [Peak Intensity Calculation] The Raman spectra obtained were measured at 1020 cm －１ 1160cm or more －１ The baseline was obtained by linear approximation in the following region: 1060 cm －１ and 1130 cm －１ The maximum values ​​of the Raman bands were determined as peak intensities I1130 and I1160, respectively. [Degree of orientation] 1130 cm －１ and 1060 cm －１The peak intensity ratio (I1130 / I1060) was taken as the degree of orientation, and the MD orientation degree / TD orientation degree was calculated from the degree of orientation at 0° (MD orientation degree) and the degree of orientation at 90° (TD orientation degree). Furthermore, the difference between the largest and smallest values ​​of the orientation degrees in 24 directions up to 345° in 15° increments, with MD at 0°, was calculated as the orientation degree R. Furthermore, the sum of the orientation degrees in each direction (15° × n (1≦n≦24 (n is an integer))) when measuring 24 directions up to 345° in 15° increments, with MD at 0°, was calculated as the sum of the Raman orientation parameters in each in-plane direction.

[0051] (8) Weight-average molecular weight (Mw), number-average molecular weight (Mn), and peak-top molecular weight (Mp) The weight-average molecular weight (Mw), number-average molecular weight (Mn), and peak-top molecular weight (Mp) of a polyolefin resin and a polyolefin microporous membrane were determined by gel permeation chromatography (GPC) under the following measurement conditions. Measurement conditions: Measurement apparatus: Agilent high-temperature GPC apparatus PL-GPC220; Column: Agilent PL1110-6200 (20 μm MIXED-A) × 2; Column temperature: 160°C; Solvent (mobile phase): 1,2,4-trichlorobenzene; Solvent flow rate: 1.0 mL / min; Sample concentration: 0.1 wt% (dissolution conditions: 160°C / 3.5H); Injection amount: 500 μL; Detector: Agilent differential refractive index detector (RI detector); Viscometer: Agilent viscosity detector; Calibration curve: Prepared by the universal calibration curve method using a monodisperse polystyrene standard sample.

[0052] (9) Melting Point The melting point of the polyolefin resin was determined using a differential scanning calorimeter (PYRIS DIAMOND DSC manufactured by PARKING ELMER). The polyolefin resin was placed in a sample holder, heated from 30 ° C. to 230 ° C. until completely melted, then held at 230 ° C. for 3 minutes and cooled to 30 ° C. at a rate of 10 ° C. / min. This was considered the first heating run, and the same measurement was repeated again. The melting point (Tm) of the polyolefin resin was determined from the endothermic peak during the second heating run. For polyolefin resins, a peak with a heat of fusion of 2.0 J / g or more was considered an endothermic peak.

[0053] (10) Impact Resistance Test Cylindrical batteries were prepared according to the following procedure, and an impact test was carried out. <Preparation of Positive Electrode> A slurry was prepared by dispersing 92.2 mass% of lithium-cobalt composite oxide LiCoO2 as the active material, 2.3 mass% each of flake graphite and acetylene black as conductive agents, and 3.2 mass% of polyvinylidene fluoride (PVDF) as a binder in N-methylpyrrolidone (NMP). This slurry was applied to a cylindrical battery with an active material coating amount of 250 g / m. ２ , active material bulk density 3.00 g / cm ３ The resulting mixture was applied to one side of a 20 μm thick aluminum foil, which would serve as a positive electrode current collector, using a die coater. The mixture was then dried at 130°C for 3 minutes, compression molded using a roll press, and cut into strips with a width of approximately 57 mm. <Preparation of Negative Electrode> 96.9% by mass of artificial graphite as the active material, 1.4% by mass of ammonium salt of carboxymethyl cellulose, and 1.7% by mass of styrene-butadiene copolymer latex as binders were dispersed in purified water to prepare a slurry. This slurry was applied to a substrate with an active material coating amount of 106 g / m. ２ , active material bulk density 1.55g / cm ３The resulting mixture was applied to one side of a 12 μm thick copper foil (which served as a negative electrode current collector) using a die coater at a high packing density of 100 μm. The mixture was then dried at 120°C for 3 minutes, compression-molded using a roll press, and cut into strips approximately 58 mm wide. <Preparation of Non-Aqueous Electrolyte> A non-aqueous electrolyte solution was prepared by dissolving LiPF6 as a solute in a 1 / 2 volume ratio ethylene carbonate / ethyl methyl carbonate mixed solvent to a concentration of 1.0 mol / L. <Separator> The separator described in the Examples and Comparative Examples was slit into a 60 mm strip. <Battery Assembly> An electrode plate laminate was produced by stacking the strip-shaped negative electrode, separator, strip-shaped positive electrode, and separator in this order and spirally winding the laminate multiple times in the MD at a winding tension of 2.45 N. This electrode plate stack was placed in a stainless steel container with an outer diameter of 18 mm and a height of 65 mm. An aluminum tab derived from the positive electrode current collector was welded to the container lid terminal, and a nickel tab derived from the negative electrode current collector was welded to the container wall. After drying for 12 hours at 80°C under vacuum, the container was filled with the above nonaqueous electrolyte in an argon box and sealed. <Impact Resistance Test> The assembled batteries were first charged at a constant current of 500 mA. After the battery voltage reached 4.20 V, they were charged at each constant voltage until the current value fell to 10 mA or less to obtain fully charged batteries. Next, the fully charged cylindrical batteries were placed with their long sides horizontal, and a 9.1 kg rod with a diameter of 15.8 mm was dropped from a height of 61 cm onto the flat center surface of each battery to impact each battery. The heat generated by the impact was measured, and impact resistance was evaluated as follows:

[0054] Heat generation was less than 90°C: A Heat generation was 90°C or more but less than 100°C: B Heat generation was 100°C or more: C In the above measurements, for samples whose MD and TD were unknown, the tensile elongation was measured in the in-plane direction, and the direction with the largest value was designated as MD, and the direction perpendicular to that was designated as TD.

[0055] [Example 1] (1) Preparation of polyolefin solution Weight average molecular weight (Mw) 6.5 × 10 ５Polyethylene with a melting point of 131.5°C was melt-kneaded with liquid paraffin in a twin-screw extruder to give a resin concentration of 27% by mass, to prepare a polyolefin solution. (2) Molding of Gel-Like Sheet The polyolefin solution was fed from the twin-screw extruder to a T-die and extruded. The extrudate was cooled while being taken up with a cooling roll controlled at 25°C to form a gel-like sheet. (3) First Stretching The gel-like sheet was stretched 6.7 times in the MD at 120°C using a roll stretcher. (4) Second Stretching, Removal of Membrane-Forming Solvent, and Drying The sheet after the first stretching was stretched 8.3 times in the TD at a preheating temperature of 125°C and a stretching temperature of 124°C using a tenter stretcher. After stretching, the sheet was immersed in a methylene chloride bath to remove the liquid paraffin, and then dried to obtain a dried microporous membrane. (5) Third Stretching and Heat Treatment The film was then preheated at 131°C and stretched 1.54 times in the TD using a tenter stretching machine, then relaxed 5.8% in the TD, and heat-set at 131°C while held in the tenter to obtain a polyolefin microporous membrane. The conditions for (1) to (5) above are shown in Table 1.

[0056] [Examples 2 to 8, Comparative Examples 1 and 2] Stretching was carried out in the same manner as in Example 1, except that the stretching and heat setting conditions were as shown in the table, to obtain a microporous polyolefin membrane.

[0057] Comparative Example 3 (1) Preparation of polyolefin solution ６ 30% by mass of polyethylene having a melting point of 133.0°C and a weight average molecular weight (Mw) of 3.0 × 10 ５A polyolefin solution was prepared by melt-kneading 70% by mass of polyethylene having a melting point of 136.0°C with liquid paraffin in a twin-screw extruder to a resin concentration of 28.5% by mass. (2) Molding of Gel-Like Sheet The polyolefin solution was fed from the twin-screw extruder to a T-die and extruded. The extrudate was cooled while being taken up with a cooling roll controlled at 25°C to form a gel-like sheet. (3) First Stretching The gel-like sheet was stretched 7.5 times in the MD at 125°C using a roll stretcher. (4) Second Stretching, Removal of Membrane-Forming Solvent, and Drying The sheet after the first stretching was stretched 8.9 times in the TD at a preheating temperature of 126°C and a stretching temperature of 127°C using a tenter stretcher. After stretching, the sheet was immersed in a methylene chloride bath to remove the liquid paraffin and then dried to obtain a dried microporous membrane. (5) Third Stretching and Heat Treatment Thereafter, the film was preheated at 131.0°C and stretched 1.57 times in the TD using a tenter stretching machine, then relaxed 5.7% in the TD, and heat-set at 131.0°C while held in the tenter to obtain a polyolefin microporous membrane.

[0058] Comparative Examples 4 and 5 Stretching was carried out in the same manner as in Comparative Example 3, except that the stretching and heat setting conditions were as shown in the table, to obtain a microporous polyolefin membrane.

[0059] [Comparative Example 6] Weight average molecular weight (Mw) 1.5 × 10 ６ 60% by mass of polyethylene having a melting point of 136.0°C and a weight average molecular weight (Mw) of 1.0 × 10 ５ and 40% by mass of polyethylene having a melting point of 132.0°C were melt-kneaded with liquid paraffin in a twin-screw extruder to give a resin concentration of 28.0% by mass, and stretched in the same manner as in Comparative Example 3 except that the stretching temperature in the first stretching was 120°C, the stretching ratio was 6.7 times, the preheating temperature in the second stretching was 125°C, the stretching temperature was 125°C, the stretching ratio was 7.5 times, and the stretching temperature in the third stretching was 132°C, the maximum stretching ratio was 1.67 times, and the relaxation rate was 8.5%, to give a polyolefin microporous membrane.

[0060] Comparative Example 7 Stretching was carried out in the same manner as in Comparative Example 6, except that the stretching and heat setting conditions were as shown in the table, to obtain a microporous polyolefin membrane.

[0061] [Results] The measurement results of the physical properties of the obtained polyolefin microporous membranes are shown in Table 2. The polyolefin microporous membranes obtained in the Examples had higher MD tensile strength and elongation than those of the Comparative Examples, and the batteries using them as battery separators had excellent battery safety as typified by impact tests.

[0062]

[0063]

[0064] When used as a battery separator, the polyolefin microporous membrane of the present invention can provide a polyolefin microporous membrane that can maintain a safe state even when the battery is subjected to an external impact. Taking advantage of its properties, the battery separator using the polyolefin microporous membrane of the present invention can be used in lithium ion secondary batteries and is suitable for use in power tools.

Claims

1. A microporous polyolefin membrane having a tensile strength in the longitudinal direction (MD) of 185 MPa or more and a tensile elongation in the MD of 145% or more.

2. The microporous polyolefin membrane according to claim 1, having a tensile strength in the transverse direction (TD) of 200 MPa or more and a tensile elongation in the TD of 100% or more.

3. The polyolefin microporous film according to claim 1 or 2, wherein the R value of the Raman orientation parameter measured by the following method is 1.55 or less, and the total value of the Raman orientation parameter in each in-plane direction is 72 or less. [Method of measuring Raman orientation parameter] [Apparatus] The measurement apparatus is a microscopic Raman spectroscopy system inVia (manufactured by Renishaw). 180° backscattering arrangement, spectral length 250 mm, diffraction grating 3000 lines / mm, excitation laser 532 nm, 50x objective lens (N.A. = 0.75), spot size (spatial resolution) 5 μm [Polarization conditions] The laser is perpendicularly incident from the normal direction of the film surface (XY plane), and polarized using a polarizer. The measurement sample is rotated, and Raman spectra are obtained for each direction in 24 directions up to 345° at 15° intervals with MD at 0°. [Calculation of peak intensity] The Raman spectrum obtained is measured at 1020 cm －１ 1160cm or more －１ The baseline was obtained by linear approximation in the following region: 1060 cm －１ and 1130 cm －１ The maximum values ​​of the Raman bands are calculated as peak intensities I1130 and I1160, respectively. [Degree of orientation] 1130 cm －１ and 1060 cm －１ The peak intensity ratio (I1130 / I1060) is taken as the degree of orientation. [R value] The difference between the largest and smallest values ​​of the degrees of orientation in 24 directions up to 345° in increments of 15°, with MD at 0°, is taken as the degree of orientation R. [Total value of Raman orientation parameters in each in-plane direction] The total value of the degrees of orientation in each direction (15°×n (1≦n≦24 (n is an integer))) when measuring 24 directions up to 345° in increments of 15°, with MD at 0°, is taken as the total value of the Raman orientation parameters in each in-plane direction.

4. The weight average molecular weight (Mw) measured by gel permeation chromatography (GPC) is 5.0 x 10 ５ ~1.0 x 10 ６ The microporous polyolefin membrane according to claim 1 or 2, 5. The number average molecular weight (Mn) measured by gel permeation chromatography (GPC) is 1.0 x 10 ５ ~3.0 x 10 ５ and the peak top molecular weight (Mp) is 2.5×10 ５ ~4.5 x 10 ５ The microporous polyolefin membrane according to claim 1 or 2, 6. The microporous polyolefin membrane according to claim 1 or 2, which has a porous layer on at least one surface of the microporous polyolefin membrane.

7. A battery separator comprising the microporous polyolefin membrane according to claim 1 or 2.

8. The battery separator according to claim 7, which is used in an electric power tool.

9. A lithium ion secondary battery comprising the microporous polyolefin membrane according to claim 1 or 2.

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