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

Abstract

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

Description

Technical Field

[0001] This invention relates to polyolefin microporous membranes, battery separators, and lithium-ion secondary batteries. Background Technology

[0002] Microporous membranes are used in various fields, including filters such as filtration membranes and dialysis membranes, battery separators, and electrolytic capacitor separators. Among them, microporous membranes made of polyolefin resin materials have excellent chemical resistance, insulation, and mechanical strength, and possess shut-down properties, making them widely used as separators for secondary batteries. Moreover, to prevent fires or smoke when batteries are subjected to external impacts, safety tests, such as impact resistance tests, are required, necessitating increased mechanical strength for polyolefin microporous membranes.

[0003] From the perspective of battery impact resistance, Patent Document 1 proposes a polyolefin microporous membrane that satisfies the following: tensile strength (MPa) and elongation (%) in the MD and TD directions are [(tensile strength in the MD direction × elongation in the MD direction / 100)]. 2 + (Tensile strength in the TD direction × Elongation in the TD direction / 100) 2 ] 1 / 2 ≥300, tensile strength in the MD and TD directions is above 196MPa, maximum pore size measured by a perm pore meter is below 60nm, average flow pore size measured by a perm pore meter is below 40nm, and porosity is above 40%.

[0004] Patent document 2 proposes a polyolefin microporous membrane whose gas permeability resistance change rate after heating and compressing for 5 minutes at 90°C and 5.0MPa is less than 50%, and the membrane thickness change rate after heating and compressing for 5 minutes at 90°C and 5.0MPa is less than 10% when the membrane thickness of the polyolefin microporous membrane before heating and compression is taken as 100%.

[0005] Patent document 3 discloses a polyolefin microporous membrane, characterized in that the membrane thickness is less than 10 μm, the tensile strength in the length direction is more than 270 MPa and less than 350 MPa, the tensile strength in the width direction is more than 220 MPa and less than 280 MPa, the elongation in the length direction is more than 100% and less than 150%, and the elongation in the width direction is more than 100% and less than 180%.

[0006] Patent document 4 proposes a microporous membrane made of polyolefin with an average tensile strength of 150 MPa or more, an average tensile elongation of 185% or more, an average solid thermal shrinkage rate of less than 10.0% calculated based on the solid thermal shrinkage rate measured after heating at 105°C for 8 hours, and a thermal shrinkage rate of less than 14% in the TD direction at the shut-off temperature as determined by thermomechanical analysis.

[0007] Patent document 5 proposes a polyolefin microporous membrane in which the total degree of orientation obtained by measuring 360° using Raman spectroscopy at a 15° scale is 70 to 90, and the proportion of amorphous component (α135) at 135°C, as determined by pulsed NMR solid echo method, is 35% or more.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: International Publication No. 2018 / 180714

[0011] Patent Document 2: International Publication No. 2015 / 194504

[0012] Patent Document 3: Japanese Patent Application Publication No. 2020-164791

[0013] Patent Document 4: Japanese Patent Application Publication No. 2020-164858

[0014] Patent Document 5: Japanese Patent Application Publication No. 2022-48093 Summary of the Invention

[0015] In recent years, the demand for lithium-ion rechargeable batteries has been increasing for portable tools such as power tools. Due to their widespread portability, power tools must be used in various unpredictable environments, including at heights and in high temperatures, thus requiring increasingly stringent safety standards, particularly impact resistance. To improve battery impact resistance, high mechanical strength of the separator is necessary; however, generally, increased mechanical strength leads to a decrease in elongation. Under impact accompanied by large deformation, a low elongation of the separator can cause rupture, resulting in battery fire and smoke. Conversely, increased mechanical elongation reduces mechanical strength, making the separator more susceptible to rupture under strong impacts. Therefore, the mechanical properties of existing separators are insufficient for future lithium-ion rechargeable battery applications.

[0016] In view of the above circumstances, the present invention aims to provide a polyolefin microporous membrane with excellent impact resistance.

[0017] Methods for solving problems

[0018] To address the aforementioned issues, the present invention has the following features.

[0019] [I] A polyolefin microporous membrane having a tensile strength of 185 MPa or more in the longitudinal direction (MD) and an elongation of MD of 145% or more.

[0020] [II] The polyolefin microporous membrane described in [I] has a tensile strength in the width direction (TD) of 200 MPa or more and a tensile elongation of TD of 100% or more.

[0021] [III] For polyolefin microporous membranes as described in [I] or [II], the Raman orientation parameter R value is 1.55 or less, and the total value of the Raman orientation parameter in all in-plane directions is 72 or less, as determined by the following method.

[0022] [Methods for determining Raman orientation parameters]

[0023] Device:

[0024] The measuring apparatus used was a micro Raman spectroscopy system manufactured by Renishaw, inVia.

[0025] • 180° backscatter configuration • 250mm spectral length • 3000 diffraction gratings / mm • 532nm excitation laser

[0026] A 50x objective lens with NA=0.75 has a spot size, i.e., a spatial resolution of 5μm.

[0027] Polarized light conditions:

[0028] A laser beam is incident perpendicularly to the normal direction of the film surface (XY plane). Polarized light is generated using a polarizer. The sample is rotated, and the maximum depth (MD) is set to 0°. Raman spectra are obtained in 24 directions at intervals from 15° to 345°.

[0029] Peak intensity calculation:

[0030] For the obtained Raman spectrum, at 1020 cm⁻¹ -1 Above and 1160cm -1 The baseline for the following regions was obtained through a linear approximation, and the 1060cm value was calculated for each region. -1 and 1130cm -1 The maximum values ​​of the Raman bands are taken as the peak intensities I1130 and I1160.

[0031] Orientation degree:

[0032] 1130cm -1 and 1060cm -1 The peak intensity ratio, I1130 / I1060, is used as the orientation degree.

[0033] R value:

[0034] Set MD to 0°, and calculate the difference between the maximum and minimum values ​​of the orientation degrees in 24 directions at 15° intervals up to 345° as the orientation degree R.

[0035] The sum of the Raman orientation parameters in all in-plane directions:

[0036] Set MD to 0° and measure 24 directions from 15° intervals to 345°. Calculate the total orientation degree of each direction in 15°×n as the total value of the Raman orientation parameter in each direction in the plane, where 1≤n≤24 and n is an integer.

[0037] [IV]. The polyolefin microporous membrane described in any of [I] to [III] has a weight-average molecular weight (Mw) of 5.0 × 10⁻⁶ as determined by gel permeation chromatography (GPC). 5 ~1.0×10 6 .

[0038] [V]. The polyolefin microporous membrane described in any of [I] to [IV] has a number-average molecular weight (Mn) of 1.0 × 10⁻⁶ as determined by gel permeation chromatography (GPC). 5 ~3.0×10 5 The peak molecular weight Mp is 2.5 × 10⁻⁶. 5 ~4.5×10 5 .

[0039] [VI]. The polyolefin microporous membrane described in any one of [I] to [V] has a porous layer on at least one side of the polyolefin microporous membrane.

[0040] [VII]. A battery separator comprising the polyolefin microporous membrane described in any one of [I] to [VI].

[0041] [VIII]. The battery separator described in [VII] is for use in power tools.

[0042] [XI]. A lithium-ion secondary battery comprising a polyolefin microporous membrane as described in any one of [I] to [VI].

[0043] According to the present invention, a polyolefin microporous membrane with excellent impact resistance can be provided. In particular, the polyolefin microporous membrane of the present invention is suitable for use as a separator in batteries. Furthermore, the polyolefin microporous membrane of the present invention is well suited for use in lithium-ion secondary batteries. Detailed Implementation

[0044] Hereinafter, this embodiment of the present invention will be described. However, the present invention is not limited to the embodiments described below.

[0045] The polyolefin microporous membrane of the present invention has a tensile strength of 185 MPa or more in the longitudinal direction (MD) and an elongation at the MD of 145% or more. More preferably, the tensile strength at the MD is 220 MPa or more, and even more preferably 240 MPa or more. When the tensile strength at the MD is below 185 MPa, the impact resistance is poor. When the tensile strength at the MD is within the above-mentioned preferred range, the polyolefin microporous membrane exhibits excellent impact resistance when used as a battery separator. The tensile strength at the MD can be adjusted within a specified range by combining film-forming conditions such as the molecular weight of the polyolefin, the mixing ratio, and the stretching temperature during the manufacturing process. There is no particular upper limit, but from the viewpoint of film-forming properties, an upper limit of 400 MPa or less can be cited.

[0046] The tensile elongation of the polyolefin microporous membrane (MD) of the present invention is more preferably 150% or more, and even more preferably 155% or more. When the tensile strength of the MD is below 145%, its impact resistance is poor. When the tensile elongation of the MD is within the above-mentioned preferred range, the polyolefin microporous membrane exhibits excellent impact resistance when used as a battery separator. The tensile elongation of the MD can be adjusted within a specified range by combining film-forming conditions such as the molecular weight of the polyolefin, the mixing ratio, and the stretching temperature during the manufacturing process. While there is no particular upper limit, from the viewpoint of film-forming properties, values ​​of 300% or less can be cited.

[0047] The polyolefin microporous membrane of the present invention preferably has a tensile strength in the width direction (TD) of 200 MPa or more. More preferably, it has a tensile strength of 225 MPa or more, and even more preferably, it has a tensile strength of 250 MPa or more. When the tensile strength of TD is lower than 200 MPa, the impact resistance is poor. When the tensile strength of TD is within the above-mentioned preferred range, the polyolefin microporous membrane exhibits excellent impact resistance when used as a battery separator. The tensile strength of TD can be adjusted by combining film-forming conditions such as the molecular weight of the polyolefin, the mixing ratio, and the stretching temperature during the manufacturing process, and is within a specified range. There is no particular upper limit, but from the viewpoint of film-forming properties, 400 MPa or less can be cited as an example.

[0048] The tensile elongation of the polyolefin microporous membrane TD of the present invention is preferably 100% or more. More preferably, it is 105% or more, and even more preferably 110% or more. When the tensile strength of TD is below 100%, its impact resistance is poor. When the tensile elongation of TD is within the above-mentioned preferred range, the polyolefin microporous membrane exhibits excellent impact resistance when used as a battery separator. The tensile elongation of TD can be adjusted within a specified range by combining film-forming conditions such as the molecular weight of the polyolefin, the mixing ratio, and the stretching temperature during the manufacturing process. While there is no particular upper limit, from the viewpoint of film-forming properties, values ​​of 300% or less can be cited.

[0049] The Raman orientation degrees of the polyolefin microporous membrane of the present invention, measured by the method described later, are preferably 2.0 to 4.0 for both MD and TD. More preferably 2.5 to 3.5. Furthermore, the orientation ratio of MD to TD is preferably 0.8 to 1.2, more preferably 0.9 to 1.1.

[0050] Furthermore, the R value of the Raman orientation parameter 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 values ​​of 0 or more can be listed. If the Raman orientation degree and R value are within the above range, it indicates that the molecules are not oriented in a specific direction, and therefore there are no areas with weak local mechanical strength. If the molecules are strongly oriented in a specific direction, the elongation at break in that direction is low, or the membrane is prone to cracking due to external impact. Therefore, by keeping the Raman orientation degree and R value within the above range, a polyolefin microporous membrane with excellent impact resistance can be formed. Additionally, the total degree of orientation in each in-plane direction in the method described later is preferably 72 or less, more preferably 70 or less, and even more preferably 68 or less. By setting the total degree of orientation to the above range, the low elongation at break due to high orientation of molecular chains in the in-plane direction, or the tendency to crack due to external impact, can be suppressed. From the viewpoint of maintaining tensile strength, the lower limit of the total degree of orientation is preferably 55 or more. The Raman orientation value can be adjusted by combining the molecular weight, blending ratio, stretching temperature, and other film-forming conditions of the polyolefin during the manufacturing process, so that the above Raman orientation value is within the specified range.

[0051] From the viewpoint of easily controlling tensile strength, elongation, and molecular orientation, the polyolefin constituting the polyolefin microporous membrane of the present invention preferably has a weight-average molecular weight (Mw) of 5.0 × 10⁻⁶, as determined by gel permeation chromatography (GPC). 5 ~1.0×10 6 The range is more preferably 5.6 × 10 5 ~7.5×10 5 Within the range. Particularly preferred are those containing a weight-average molecular weight (Mw) of 5.0 × 10⁻⁶. 5 Above and 1.0×10 6 The following polyethylene.

[0052] Furthermore, the polyolefin microporous membrane of the present invention preferably has a number-average molecular weight (Mn) of 1.0 × 10⁻⁶. 5 ~3.0×10 5 The peak molecular weight (Mp) is 2.5 × 10⁻⁶. 5 ~4.5×10 5By using polyolefins within the aforementioned range to constitute the polyolefin microporous membrane, it is easy to achieve high tensile strength and elongation, and to form a uniform orientation in the in-plane direction. The weight-average molecular weight, number-average molecular weight, and peak molecular weight of the polyolefin resin composition constituting the polyolefin microporous membrane can be determined by GPC under the conditions described later.

[0053] The polyolefin microporous membrane of the present invention preferably has a thickness of 3 μm or more and 30 μm or less, and more preferably 8 μm or more and 20 μm or less.

[0054] The preferred unit area mass of the polyolefin microporous membrane of the present invention is 5 g / m³. 2 Above 20g / m 2 The preferred value is 6g / m 2 Above 15g / m 2 the following.

[0055] The porosity of the polyolefin microporous membrane of the present invention is preferably 30% to 70%, more preferably 40% to 60%. By setting the membrane thickness, mass per unit area, and porosity within the above ranges, good ion permeability can be obtained without compromising impact resistance.

[0056] The preferred gas permeability resistance of the polyolefin microporous membrane of the present invention, calculated at 12 μm, is 300 sec / 100 cm. 2 The following is more preferably 250 sec / 100 cm 2 The following is not a specific limitation on the lower limit, but from the viewpoint of preventing overcurrent, 50 sec / 100 cm is preferred. 2 If the air permeability resistance is within the above range, good ion permeability can be obtained.

[0057] The puncture strength of the polyolefin microporous membrane of the present invention is preferably 5.0 N or more, more preferably 6.0 N or more. There is no particular limitation on the upper limit, but 10.0 N or less is more preferred. Good mechanical properties can be obtained within the above range.

[0058] (Manufacturing method of polyolefin microporous membrane)

[0059] The polyolefin microporous membrane of the present invention can be a single-layer microporous membrane or a multilayer microporous membrane composed of multiple layers. The polyolefin resin composition in the single layer is described below.

[0060] (1) Polyolefin resin composition

[0061] The polyolefin resin composition may contain polyethylene. From the viewpoint of the uniform orientation of the polyolefin microporous membrane, copolymers of ethylene are preferred.

[0062] (Polyethylene)

[0063] The weight-average molecular weight (Mw) of polyethylene is preferably 5.0 × 10⁻⁶. 5 The above. Polyethylene can also be a copolymer containing small amounts of other α-olefin copolymers besides ethylene. Preferred α-olefin copolymers besides ethylene include propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, and styrene. When the α-olefin copolymer is considered as 100 mol%, the content of other α-olefins besides ethylene is preferably 5 mol% or less.

[0064] From the viewpoint of easily controlling the strength and tensile properties of microporous membranes, the weight-average molecular weight (Mw) of polyethylene is preferably 5.0 × 10⁻⁶. 5 Above and less than 1.2 × 10 6 More preferably 6.0×10 5 Above and 1.0×10 6 The following is a further preferred value: 5.6 × 10⁻⁶ 5 ~7.5×10 5 The range of values ​​is specified. Furthermore, from the viewpoint of uniform orientation and air permeability resistance, the melting point of polyethylene is preferably 128°C or higher and less than 133°C, more preferably 130°C or higher and less than 132°C.

[0065] The polyolefin resin composition may contain other resin components besides polyethylene as needed. As other resin components, it may further contain, for example, a resin that imparts heat resistance. Furthermore, without impairing the effects of the present invention, it may contain various additives such as antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, anti-blocking agents or fillers, crystallization nucleating agents, and crystallization delay agents.

[0066] In this embodiment, a porous membrane may also be formed by laminating a porous layer on at least one side of the polyolefin microporous membrane. The porous layer is not particularly limited; for example, a porous layer made of resin may be laminated. The resin used herein is not particularly limited; known resins can be used, such as acrylic resins, polyvinylidene fluoride resins, polyamide-imide resins, polyamide resins, aromatic polyamide resins, and polyimide resins. The porous layer may also contain inorganic particles. The inorganic particles are not particularly limited; known materials can be used, such as alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, and silicon.

[0067] (2) Manufacturing method of polyolefin microporous membrane

[0068] The method for manufacturing the polyolefin microporous membrane of the present invention preferably includes the following steps.

[0069] (a) Preparation of the solution,

[0070] (b) Formation of gel-like sheets,

[0071] (c) First stretch,

[0072] (d) Second stretch,

[0073] (e) Removal and drying of plasticizers,

[0074] (f) Third stretch, and

[0075] (g) Heat treatment.

[0076] Each process is described in detail.

[0077] (a) Preparation of solution

[0078] A plasticizer is added to a polyolefin resin composition in a biaxial extruder, and the mixture is melt-blended to prepare a solution. The polyolefin resin composition preferably contains 10% by mass to 30% by weight relative to the total resin solution. By maintaining the concentration of the polyolefin resin composition within this range, melt fracture and necking at the die exit can be prevented during extrusion of the polyolefin solution, resulting in good formability and appearance of the extruded article. The extruded article is obtained by conveying the solution from the extruder to the die and extruding it into a sheet. The extrusion method can be either flat-die extrusion or blow molding. The die clearance is preferably 0.1 mm to 5 mm. The extrusion temperature is preferably 140°C or higher and less than 240°C, and the extrusion speed is preferably 0.2 to 15 m / min.

[0079] (b) Formation of gel sheets

[0080] A gel sheet is formed by cooling the resulting extruded article. Cooling methods include contact with a refrigerant such as cold air or cooling water, or contact with a cooling roller, but contact with a roller cooled by a refrigerant is preferred. Cooling is performed at least to the gelation temperature, preferably at a rate of 50°C / minute or higher. Cooling is preferably performed to below 25°C. When the cooling rate is within the above range, the crystallinity remains within an appropriate range, resulting in a gel sheet suitable for stretching.

[0081] (c) First stretch

[0082] Next, the gel sheet is stretched. After preheating, the gel sheet is preferably stretched at a specified ratio using a stretching method, roller stretching, blow molding, or a combination thereof. Stretching can 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 on the MD and TD can be the same or different, but the stretching ratios on both the MD and TD are preferably 3 times or more.

[0083] The first stretching temperature is preferably 115°C to 130°C, more preferably 120°C to 125°C. Furthermore, the stretching ratio MD is preferably 6 times or more, more preferably 6.3 times or more, even more preferably 6.7 times or more, and preferably 8 times or less. When the stretching temperature is below the above range or the stretching ratio exceeds the above range, the orientation on the MD is too strong, which may sometimes cause a decrease in the MD tensile elongation and TD tensile strength. Conversely, when the stretching temperature exceeds the above range or the stretching ratio is below the above range, the orientation on the MD is too weak, and sometimes sufficient MD tensile strength cannot be obtained. By setting the MD stretching temperature and stretching ratio to the above range, the molecular chains can be precisely oriented on the MD, resulting in improved MD and TD tensile strength. Therefore, when used as a battery separator, it exhibits excellent impact resistance.

[0084] (d) Second stretching

[0085] The preheating and stretching temperature for the second stretching is preferably between 120°C and 130°C, and the stretching ratio is preferably between 8.0 and 10 times that of the TD. When the preheating and stretching temperature is below the above range or the stretching ratio exceeds the above range, the orientation on the TD is too strong, which may sometimes cause a decrease in the tensile strength of the MD and the tensile elongation of the TD. In addition, when the preheating and stretching temperature exceeds the above range or the stretching ratio is below the above range, the orientation on the TD is too weak, and sometimes sufficient TD strength cannot be obtained. By setting the preheating and stretching temperature and the stretching ratio on the TD to the above range, the molecular chains can be properly oriented on the TD, resulting in improved tensile strength of both the MD and the TD. Therefore, when used as a separator for batteries, it exhibits excellent impact resistance.

[0086] (e) Removal of plasticizers

[0087] Next, a cleaning solvent is used to remove the plasticizer contained in the gel sheet and then it is dried. The cleaning solvent and the method for removing the plasticizer using the cleaning solvent can be any known method. For example, the method disclosed in Japanese Patent No. 2132327 or Japanese Patent Application Publication No. 2002-256099 can be used. After removing the plasticizer, drying is preferably performed by heating or air drying. Any method capable of removing the cleaning solvent, including conventional methods such as heating and air drying (moving air), can be used.

[0088] (f) Third stretch

[0089] After preheating the dried sheet, it is stretched in at least one direction (dry stretching) to obtain a polyolefin microporous membrane. The second stretching is preferably performed while heating using a stretching method or similar technique. The final stretching ratio of 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, if the stretching is too high, the tensile strength (MD) and elongation (TD) decrease, so a ratio of 1.5 times or less is preferred.

[0090] (g) Heat treatment

[0091] After the second stretching, heat treatment is preferably performed while the membrane is held in a clamped position and at a fixed width. The heat treatment temperature is preferably between 115.0°C and 135.0°C. By setting the heat treatment temperature within this range, the thermal shrinkage rate of the polyolefin microporous membrane can be suppressed. During heat treatment, a thermal relaxation process can also be performed. When performing the thermal relaxation process, the relaxation rate is preferably between 4.5% and 30% when the length to be thermally relaxed is set to 100%. By setting the relaxation rate within this range, the tensile elongation at the tensile depth (TD) can be improved. If the relaxation rate is below 4.5%, the tensile elongation at the tensile depth (MD) and the tensile strength (TD) become lower; if it exceeds 30%, wrinkles and vibrations of the membrane may occur during the conveying process after relaxation.

[0092] Example

[0093] The present invention will now be described in more detail through examples. However, the present invention is not limited to these examples.

[0094] [Determination Method]

[0095] (1) Film thickness

[0096] The film thickness at five points (top left, top right, center, bottom left, and bottom right) within a 95mm × 95mm area of ​​the polyolefin microporous membrane was measured using a contact thickness gauge (Mitutoyo Litematic, 0.01N contact pressure, using a 10.5mm φ probe), and the average value was taken as the film thickness (μm). Alternatively, if the sample size cannot be 95mm × 95mm, it can be cut out of any size, and the five points (top left, top right, center, bottom left, and bottom right) can be measured.

[0097] (2) Mass per unit area

[0098] Prepare polyolefin microporous membranes cut into 5cm squares. Weigh each membrane using a precision balance (5 significant figures (0.0000g)). Divide each weight by 25cm.2 The calculation can be performed by dividing the measured weight by the area of ​​the sample if the sample size cannot be 5cm×5cm.

[0099] (3) Porosity

[0100] Cut a polyolefin microporous membrane into pieces measuring 95mm × 95mm and calculate its volume (cm²). 3 ) and weight (g), based on these and membrane density (g / cm³) 3 The porosity (%) is calculated using the following formula.

[0101] Formula: Porosity = (Volume - Weight / Membrane Density) / Volume) × 100

[0102] The membrane density is taken as 0.99 g / cm³. 3 In addition, the film thickness measured in (1) above is used in the volume calculation.

[0103] (4) Regarding air permeability resistance

[0104] For polyolefin microporous membranes, the air permeability (sec / 100cm) was measured using an air permeability meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T) according to JIS P-8117:2009. 3 ).

[0105] In addition, the 12μm equivalent air permeability resistance is calculated using the following formula (1).

[0106] 12μm converted air permeability resistance = air permeability resistance (sec / 100cm) 3 )×12 / film thickness (μm)・・Form (1).

[0107] (5) Puncture strength

[0108] Using a needle with a diameter of 1 mm (0.5 mmR at the tip), pierce a unit area of ​​mass W (g / m²) at a speed of 2 mm / s. 2 The maximum load value S(N) of a polyolefin microporous membrane was measured. The equivalent puncture strength of a 5 μm membrane was calculated using the following formula.

[0109] Formula: Puncture strength converted from mass per unit area = S(N) / W(g / m²) 2 ).

[0110] (6) Tensile strength and elongation

[0111] For the tensile strength (MPa) corresponding to each direction, the following conditions were determined using an Instron 5543 tensile testing machine manufactured by Instron, according to ASTM D882.

[0112] • Sample shape: Rectangle with dimensions of 100mm in length and 10mm in width;

[0113] • Measurement directions: MD (length direction), TD (width direction);

[0114] • Chalcolide distance: 20mm;

[0115] • Stretching speed: 100mm / min;

[0116] • Handles: Instron 2702-018 Jaw Faces for Flats (rubber-coated, 50×38mm);

[0117] • Load unit: 500N;

[0118] • Chuck pressure: 0.50 MPa;

[0119] • Temperature: 23℃

[0120] Tensile strength (MPa) is calculated by dividing the strength at which the specimen breaks by the cross-sectional area of ​​the specimen before the test.

[0121] Regarding tensile strength, the average value obtained from measurements at 5 points on the specimen in each direction was calculated.

[0122] (7) Raman orientation parameters

[0123] [Apparatus]

[0124] • The measuring apparatus used was the inVia micro Raman spectroscopy system (manufactured by Renishaw).

[0125] • 180° backscatter configuration • 250mm spectral length • 3000 diffraction gratings / mm • 532nm excitation laser

[0126] A 50x objective lens with NA=0.75 has a spot size, i.e., a spatial resolution of 5μm.

[0127] Polarized light conditions:

[0128] A laser beam is incident perpendicularly to the normal direction of the film surface (XY plane). Polarized light is generated using a polarizer. The sample is rotated, and the maximum depth (MD) is set to 0°. Raman spectra are obtained in 24 directions at intervals from 15° to 345°.

[0129] Peak intensity calculation:

[0130] For the obtained Raman spectrum, at 1020 cm⁻¹ -1 Above and 1160cm -1 The baseline for the following regions was obtained through a linear approximation, and the 1060cm value was calculated for each region.-1 and 1130cm -1 The maximum values ​​of the Raman bands are taken as the peak intensities I1130 and I1160.

[0131] Orientation degree:

[0132] 1130cm -1 and 1060cm -1 The peak intensity ratio, I1130 / I1060, is used as the orientation degree. The MD orientation degree / TD orientation degree is calculated based on the 0° orientation degree (MD orientation degree) and the 90° orientation degree (TD orientation degree). Furthermore, setting MD to 0°, the difference between the maximum and minimum values ​​of the orientation degree in 24 directions at 15° intervals up to 345° is calculated as the orientation degree R. Then, setting MD to 0°, measurements are performed in 24 directions at 15° intervals up to 345°, and the total of the orientation degrees in each direction (15° × n) is calculated as the total value of the Raman orientation parameter in all in-plane directions, where 1 ≤ n ≤ 24, and n is an integer.

[0133] (8) Weight-average molecular weight (Mw), number-average molecular weight (Mn), peak molecular weight (Mp)

[0134] The weight-average molecular weight (Mw), number-average molecular weight (Mn), and peak molecular weight (Mp) of polyolefin resins and polyolefin microporous membranes were determined by gel permeation chromatography (GPC) under the following conditions. Determination Conditions

[0135] Measurement apparatus: Agilent PL-GPC220 high-temperature GPC apparatus

[0136] • Columns: 2 Agilent PL1110-6200 (20μm MIXED-A)

[0137] • Column temperature: 160℃

[0138] Solvent (mobile phase): 1,2,4-trichlorobenzene

[0139] Solvent flow rate: 1.0 mL / min

[0140] • Sample concentration: 0.1% by weight (Dissolution conditions: 160℃ / 3.5H)

[0141] • Injection volume: 500μL

[0142] • Detector: Agilent differential refractive index detector (RI detector)

[0143] • Viscometer: Viscometer manufactured by Agilent

[0144] • Standard curve: Generally prepared using the pass curve method with monodisperse polystyrene standard samples.

[0145] (9) Melting point

[0146] The melting point of the polyolefin resin was determined using a scanning differential calorimeter (manufactured by Parking Elmer, Pyris Diamondsc). The polyolefin resin was placed in a sample holder and heated from 30°C to 230°C until completely melted. It was then held at 230°C for 3 minutes and decreased to 30°C at a rate of 10°C / min. This was considered the first heating cycle, and the same measurement was repeated. The melting point (Tm) of the polyolefin resin was determined based on the endothermic peak observed during the second heating cycle. For polyolefin resins, peaks with a heat of fusion greater than 2.0 J / g were considered endothermic peaks.

[0147] (10) Impact resistance test

[0148] Fabricate a cylindrical battery according to the following steps and conduct an impact test.

[0149] (The production of the positive electrode)

[0150] A slurry was prepared by dispersing 92.2% by mass of lithium cobalt composite oxide (LiCoO2) as the active material, 2.3% by mass each of flake graphite and acetylene black as conductive agents, and 3.2% by mass of polyvinylidene fluoride (PVDF) as a binder in N-methylpyrrolidone (NMP). The slurry was then coated onto one side of a 20 μm thick aluminum foil, which would serve as the positive electrode current collector, resulting in an active material coating weight of 250 g / m². 2 The bulk density of the active substance is 3.00 g / cm³. 3 Then, it is dried at 130°C for 3 minutes, compressed and shaped using a roller press, and cut into strips with a width of about 57mm.

[0151] (Making the negative electrode)

[0152] A slurry was prepared by dispersing 96.9% by mass of artificial graphite (as the active material), 1.4% by mass of ammonium salt of carboxymethyl cellulose (as the binder), and 1.7% by mass of styrene-butadiene copolymer latex in purified water. This slurry was then coated onto one side of a 12 μm thick copper foil, which would serve as the negative electrode current collector, resulting in an active material coating of 106 g / m². 2 The bulk density of the active substance is 1.55 g / cm³. 3 The high filling density is then dried at 120°C for 3 minutes, compressed and shaped using a roller press, and cut into strips approximately 58 mm wide.

[0153] (Preparation of non-aqueous electrolyte)

[0154] LiPF6, as a solute, was dissolved in a mixed solvent of ethylene carbonate and methyl ethyl carbonate in a volume ratio of 1 / 2.0 mol / L.

[0155] <Septum>

[0156] The diaphragms described in the examples and comparative examples were cut into strips of 60 mm.

[0157] <Battery Assembly>

[0158] An electrode plate laminate is fabricated by sequentially overlapping a strip negative electrode, a separator, a strip positive electrode, and a separator, and then repeatedly winding them in a spiral shape on a magnetic substrate (MD) with a winding tension of 2.45 N. This electrode plate laminate is then placed in a stainless steel container with an outer diameter of 18 mm and a height of 65 mm. An aluminum tab extending from the positive current collector is welded to the terminal portion of the container cap, and a nickel tab extending from the negative current collector is welded to the container wall. The container is then dried under vacuum at 80°C for 12 hours. Finally, the aforementioned non-aqueous electrolyte is injected into the container in an argon chamber, and the container is sealed.

[0159] (Impact resistance test)

[0160] First, the assembled batteries are charged with a constant current of 500mA until the battery voltage reaches 4.20V. Then, they are charged at constant voltage until the current value is below 10mA, resulting in a fully charged battery. Next, the fully charged cylindrical batteries are arranged with their long sides horizontal. A 9.1kg rod with a diameter of 15.8mm is dropped from a height of 61cm onto the flat center surface of the batteries, thus impacting each battery.

[0161] The heat generated by the impact on the battery was measured, and the impact resistance was evaluated as follows.

[0162] Fever less than 90℃: A

[0163] Fever above 90℃ but below 100℃: B

[0164] Fever above 100℃: C

[0165] In addition, for samples where MD and TD are unclear in the above measurements, the tensile elongation in the in-plane direction is measured, and the direction with the largest value is taken as MD, and the direction orthogonal to MD is taken as TD.

[0166] [Example 1]

[0167] (1) Preparation of polyolefin solution

[0168] The weight-average molecular weight (Mw) is 6.5 × 10⁻⁶. 5A polyolefin solution was prepared by melt-blending polyethylene with a melting point of 131.5℃ with liquid paraffin using a twin-screw extruder to achieve a resin concentration of 27% by mass.

[0169] (2) Formation of gel sheets

[0170] A polyolefin solution is fed from a twin-screw extruder into a T-die and extruded. The extruded molded part is pulled and cooled by cooling rollers with the temperature adjusted to 25°C to form a gel-like sheet.

[0171] (3) First stretch

[0172] The gel-like sheets were stretched to 6.7 times their original size on MD at 120°C using a roller stretching machine.

[0173] (4) Second stretching, removal of film-forming solvent, and drying

[0174] The first stretched sheet was stretched 8.3 times on a TD (Dichloromethane) surface at a preheating temperature of 125℃ and a stretching temperature of 124℃ using a tenter frame. The stretched sheet was then immersed in a dichloromethane bath to remove the liquid paraffin, and then dried to obtain a dried microporous membrane.

[0175] (5) Third stretching and heat treatment

[0176] After preheating at 131°C, the membrane was stretched to 1.54 times its original size on a TD using a tenter frame, and then relaxed by 5.8% on the TD. While holding it on the tenter frame, it was heat-set at 131°C to obtain a polyolefin microporous membrane. The conditions for (1) to (5) above are shown in Table 1.

[0177] [Examples 2-8, Comparative Examples 1-2]

[0178] Except for applying the stretching and heat-setting conditions as described in the table, stretching was performed in the same manner as in Example 1 to obtain a polyolefin microporous membrane.

[0179] [Comparative Example 3]

[0180] (1) Preparation of polyolefin solution

[0181] The weight-average molecular weight (Mw) is 2.0 × 10⁻⁶. 6 30% by mass of polyethylene with a melting point of 133.0℃ and a weight-average molecular weight (Mw) of 3.0 × 10⁻⁶ 5 A polyolefin solution was prepared by melt-blending 70% by mass of polyethylene with a melting point of 136.0℃ with liquid paraffin using a twin-screw extruder to achieve a resin concentration of 28.5% by mass.

[0182] (2) Formation of gel sheets

[0183] A polyolefin solution is fed from a twin-screw extruder into a T-die and extruded. The extruded molded part is pulled and cooled simultaneously by cooling rollers adjusted to a temperature of 25°C to form a gel-like sheet.

[0184] (3) First stretch

[0185] The gel-like sheets were stretched to 7.5 times their original length along the MD at 125°C using a roller stretching machine.

[0186] (4) Second stretching, removal of film-forming solvent, and drying

[0187] The first stretched sheet was stretched to 8.9 times its original length along the TD line at a preheating temperature of 126°C and a stretching temperature of 127°C using a tenter frame. The stretched sheet was then immersed in a dichloromethane bath to remove the liquid paraffin and dried to obtain a dried microporous membrane.

[0188] (5) Third stretching and heat treatment

[0189] After preheating at 131.0℃, the membrane is stretched to 1.57 times its original size along the TD using a tenter frame, and then relaxed by 5.7% along the TD. While holding it on the tenter frame, it is heat-set at 131.0℃ to obtain a polyolefin microporous membrane.

[0190] [Comparative Examples 4-5]

[0191] Except for applying the stretching and heat-setting conditions as described in the table, stretching was performed in the same manner as in Comparative Example 3 to obtain a polyolefin microporous membrane.

[0192] [Comparative Example 6]

[0193] The weight-average molecular weight (Mw) is 1.5 × 10⁻⁶. 6 Polyethylene with a melting point of 136.0℃ (60% by mass) and a weight-average molecular weight (Mw) of 1.0 × 10⁻⁶ 5 40% by mass of polyethylene with a melting point of 132.0°C was melt-blended with liquid paraffin in a twin-screw extruder at a resin concentration of 28.0% by mass. The first stretching temperature was 120°C and the stretching ratio was 6.7 times. The second stretching temperature was 125°C and the stretching temperature was 125°C and the stretching ratio was 7.5 times. The third stretching temperature was 132°C and the maximum stretching ratio was 1.67 times. The relaxation rate was 8.5%. Except for these, stretching was carried out in the same manner as in Comparative Example 3 to obtain a polyolefin microporous membrane.

[0194] [Comparative Example 7]

[0195] Except that the stretching and heat setting conditions were set as described in the table, stretching was performed in the same manner as in Comparative Example 6 to obtain a polyolefin microporous membrane.

[0196] [result]

[0197] The physical property test results of the obtained polyolefin microporous membrane are shown in Table 2. Compared with the comparative example, the polyolefin microporous membrane obtained in the examples has a higher MD tensile elongation, and the batteries obtained by using it as a battery separator exhibit excellent battery safety, as represented by impact tests.

[0198]

[0199] Industry availability

[0200] The polyolefin microporous membrane of the present invention, when used as a battery separator, provides a polyolefin microporous membrane that can maintain a safe state even when the battery is subjected to external impact. Battery separators using the polyolefin microporous membrane of the present invention can utilize their properties for use in lithium-ion secondary batteries and are suitable for use in power tools.

Claims

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

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

3. The polyolefin microporous membrane according to claim 1 or 2, having an R value of a Raman orientation parameter of 1.55 or less and a total value of the in-plane directions of the Raman orientation parameter of 72 or less, as measured by the following method. Method for measuring Raman orientation parameter: Apparatus: • The measuring apparatus uses a micro-Raman spectroscopy system inVia manufactured by Renishaw Co. • 180° backscattering configuration, spectral length 250 mm, diffraction grating 3000 lines / mm, excitation laser 532 nm • 50x objective lens with N.A. = 0.75, spot size (spatial resolution) 5 μm Polarized light conditions: Perpendicularly incident laser light from the normal direction of the film surface (XY plane), polarized light is formed using a polarizer, the measurement sample is rotated, MD is set to 0°, and the Raman spectrum of each direction is obtained for 24 directions at intervals of 15° up to 345°. Calculation of peak intensity: For the obtained Raman spectrum, the maximum value of the Raman band at 1020 cm -1 above and 1160 cm -1 The maximum value of the Raman band at 1060 cm -1 and 1130 cm -1 was obtained as the peak intensity I1130, I1160, respectively, by linear approximation of the baseline in the following regions: Degree of orientation: the peak intensity ratio of 1130 cm -1 and 1060 cm -1 , i.e. I1130 / I1060, as the degree of orientation, R value: The difference between the maximum value and the minimum value of the degree of orientation in 24 directions at intervals of 15° up to 345° is taken as the degree of orientation R, with MD set to 0°. Total value of the in-plane directions of the Raman orientation parameter: MD is set to 0°, and the measurement is performed for 24 directions at intervals of 15° up to 345°, and the sum of the orientation degree of each direction of 15° x n is calculated as the sum of the in-plane direction of the Raman orientation parameter, wherein, 1 ≤ n ≤ 24, n is an integer.

4. The polyolefin microporous membrane according to claim 1 or 2, having a weight average molecular weight Mw of 5.0 x 10 5 1.0 x 10 6 .

5. The polyolefin microporous membrane according to claim 1 or 2, having a number average molecular weight Mn of 1.0 x 104 to 3.0 x 104, a peak molecular weight Mp of 2.5 x 104 to 4.5 x 104, and a weight average molecular weight Mw of 2.0 x 104 to 6.0 x 104, as determined by gel permeation chromatography (GPC). 5 5 5 5 .​​​ 6. The polyolefin microporous membrane according to claim 1 or 2, having a porous layer on at least one side of the polyolefin microporous membrane.

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

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

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

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