Polyolefin-based microporous membrane, method for producing the same, and secondary battery containing the same

Abstract

The present invention provides a polyolefin-based microporous membrane with improved heat resistance, a method for producing the same, a separator containing the same, and a secondary battery containing the same. [Solution] The polyolefin-based microporous membrane contains polyolefin and has a gas permeability of 2.5 × 10⁻⁶ -5 The condition is Darcy or better, the transverse direction (TD) contraction rate at 120°C is 10% or less, and the BDV index calculated using formula 1 below is 15 or higher. JPEG2026047143000016.jpg22170 (P: Puncture strength of polyolefin microporous membrane (N / μm); M: Viscosity-average molecular weight of polyolefin (×10) 5 g / mol); D: Gas permeability of polyolefin-based microporous membrane (×10 -5 Darcy); d: Average pore size of polyolefin microporous membrane (nm); ε: Porosity of polyolefin microporous membrane)

Description

[Technical Field]

[0001] This disclosure relates to a polyolefin-based microporous membrane, a method for producing the same, and a secondary battery containing the same. [Background technology]

[0002] In recent years, interest in electrochemical elements and their energy storage technologies used in mobile phones, laptop computers, electric vehicles, and other devices has surged. In particular, research on separators, one of the main components that influence the characteristics of secondary batteries (which are electrochemical elements), is actively being conducted. Because separators are impregnated with electrolytes and function as pathways for ions, they significantly impact the physical properties of secondary batteries.

[0003] On the other hand, as separators for secondary batteries, polyolefin-based microporous membranes are used because they have ion permeability, excellent electrical insulation properties, and a pore-closing function that cuts off the current and suppresses excessive temperature rise when abnormal temperature rise occurs inside the battery.

[0004] However, when polyolefin-based microporous membranes are heated to temperatures above their melting point, they may shrink and rupture, so research is needed to improve their heat resistance. [Overview of the project] [Problems that the invention aims to solve]

[0005] One embodiment aims to provide a polyolefin-based microporous membrane, a separator containing the same, and a secondary battery containing the same. Another embodiment aims to provide a method for producing a polyolefin-based microporous membrane according to the above-described embodiment. [Means for solving the problem]

[0006] One embodiment is a polyolefin-based microporous membrane containing a polyolefin resin, having a gas permeability of 2.5 × 10⁻⁶ -5The present invention provides a polyolefin-based microporous membrane that is Darcy or better, has a transverse (TD) shrinkage rate of 10% or less at 120°C, and has a BDV index of 15 or higher according to the following formula 1.

[0007]

number

[0008] (In formula 1 above, P: porosity of polyolefin microporous membrane (N / μm); M: viscosity-average molecular weight of polyolefin (×10) 5 g / mol); D: Gas permeability of polyolefin-based microporous membrane (×10 -5 Darcy); d: Average pore size of polyolefin microporous membrane (nm); ε: Porosity of polyolefin microporous membrane)

[0009] In one embodiment, the polyolefin resin has a viscosity-average molecular weight (Mv) of 3 × 10 5 g / mol ~ 50 × 10 5 g / mol may be In one embodiment, the polyolefin-based microporous film may have a breakdown voltage (BDV) of 0.13 kV / μm or higher relative to its average thickness, and the breakdown voltage is measured in accordance with ASTM D149.

[0010] In one embodiment, the puncture strength of the polyolefin-based microporous membrane may be 0.4 N / μm or greater. In one embodiment, the porosity of the polyolefin-based microporous membrane is determined by the mass (M, g) measured after cutting the polyolefin-based microporous membrane into a rectangle with dimensions A cm × B cm (thickness: T, μm), and the density (ρ, g / cm³) of the polyolefin. 3 This was calculated using the following formula 2.

[0011] [Formula 2] Porosity={1-(M×10000) / (ABTρ)}

[0012] In one embodiment, the porosity of the polyolefin microporous membrane may be 0.2 or more. In one embodiment, the thickness of the polyolefin microporous membrane may be 3 μm to 20 μm.

[0013] In one embodiment, the average pore size of the polyolefin microporous membrane measured according to ASTM F316-03 may be 10 nm to 100 nm. In one embodiment, the polyolefin microporous membrane may have a shrinkage rate in the machine direction (MD) at 120°C of 10% or less.

[0014] Another embodiment is a method for producing a polyolefin microporous membrane, including the steps of kneading a polyolefin resin and a diluent to prepare a melt, forming the melt into a sheet shape, and extracting the diluent after stretching the formed sheet, wherein the gas permeability is 2.5×10 -5 Darcy or more, the shrinkage rate in the transverse direction (TD) at 120°C is 10% or less, and the BDV index of the following formula 1 is 15 or more.

[0015]

Number

[0016] (In the above formula 1, P: puncture strength of the polyolefin microporous membrane (N / μm); M: viscosity average molecular weight of the polyolefin (×10 5 g / mol); D: gas permeability of the polyolefin microporous membrane (×10 -5 Darcy); d: average pore size of the polyolefin microporous membrane (nm); ε: porosity of the polyolefin microporous membrane)

[0017] In one embodiment, the step of stretching the molded sheet may include stretching it 6 to 15 times in the longitudinal direction (MD) at a temperature of 60°C to 130°C.

[0018] In one embodiment, the step of stretching the molded sheet may further include stretching it 6 to 15 times in the transverse direction (TD) at a temperature of 80°C to 130°C. In one embodiment, the stretch ratio in the longitudinal (MD) and transverse (TD) directions (MD / TD) may be less than 1.0.

[0019] In one embodiment, the method for producing the polyolefin-based microporous membrane may further include a drying step after the step of extracting the diluent. In one embodiment, the step of heat-fixing the diluent at a temperature of 100°C to 150°C may be further included after the step of extracting the diluent. In one embodiment, the polyolefin-based microporous membrane may be a separator.

[0020] Another embodiment includes a polyolefin and has a gas permeability of 2.5 × 10 -5 The present invention provides a separator comprising a polyolefin-based microporous membrane that is Darcy or better, has a transverse (TD) shrinkage rate of 10% or less at 120°C, and has a BDV index of 15 or higher according to the following formula 1.

[0021]

number

[0022] (In formula 1 above, P: porosity of polyolefin microporous membrane (N / μm); M: viscosity-average molecular weight of polyolefin (×10) 5 g / mol); D: Gas permeability of polyolefin-based microporous membrane (×10 -5 Darcy); d: Average pore size of polyolefin microporous membrane (nm); ε: Porosity of polyolefin microporous membrane)

[0023] Another embodiment provides a secondary battery that includes a separator according to the aforementioned embodiment. [Effects of the Invention]

[0024] According to one embodiment, the polyolefin-based microporous membrane has a gas permeability of 2.5 × 10⁻⁶. -5 By simultaneously satisfying the conditions of being Darcy or higher, having a transverse (TD) shrinkage rate of 10% or less at 120°C, and having a BDV index of 15 or higher in the following formula 1, the strength, permeability, and insulation properties of the film are simultaneously improved. Therefore, when the polyolefin-based microporous film according to the above embodiment is used as a separator, the stability of the battery can be improved. [Brief explanation of the drawing]

[0025] [Figure 1] This is a schematic plan view of a secondary battery according to one embodiment. [Figure 2] This is a schematic cross-sectional view of a secondary battery according to one embodiment. [Figure 3] This graph shows the correlation between the BDV index and the dielectric breakdown voltage (kV / μm) value relative to the average thickness. [Modes for carrying out the invention]

[0026] The embodiments described herein may be modified into various different forms, and the technology according to one embodiment is not limited to the embodiments described later. Furthermore, throughout the specification, when a component is described as "comprising, including, containing," "providing," "containing," or "having," it means that it may further include other components, rather than excluding other components, and does not exclude elements, materials, or processes that are not additionally listed.

[0027] Numerical ranges as used herein include lower and upper limits, all values ​​within that range, increments logically derived from the form and width of the defined range, all limited values ​​among them, and all possible combinations of upper and lower limits of numerical ranges limited in different forms. For example, if the content of a composition is limited to 10% to 80% or 20% to 50%, the numerical ranges of 10% to 50% or 50% to 80% must also be interpreted as being described herein. Unless otherwise defined herein, values ​​outside the numerical range that may occur due to experimental error or rounding of values ​​are also included in the defined numerical range.

[0028] Hereafter, unless otherwise defined, “approximately” in this specification may be considered to be a value within 30%, 25%, 20%, 15%, 10%, 5%, 3%, 2%, 1%, or 0.5% of the explicitly stated value.

[0029] In this specification, “longitudinal direction (MD)” means the direction in which the product is manufactured. This is the axis that coincides with the direction of operation of the machine and indicates the primary direction in which the raw materials are primarily processed or formed. In this specification, the transverse direction (TD) means the direction perpendicular to the longitudinal direction (MD). This is the horizontal direction of the product and intersects the longitudinal direction at a right angle.

[0030] The present disclosure will be described in detail below with reference to the attached drawings. However, this is illustrative only, and the present disclosure is not limited to the specific embodiments described illustratively.

[0031] One embodiment provides a polyolefin-based resin that exhibits excellent film strength and permeability, as well as high insulating properties. Specifically, the polyolefin-based microporous film according to one embodiment comprises a polyolefin-based resin and has a gas permeability of 2.5 × 10⁻⁶. -5 The condition is Darcy or better, the transverse direction (TD) contraction rate at 120°C is 10% or less, and the BDV index calculated using formula 1 below is 15 or higher.

[0032]

number

[0033] P: Puncture strength (N / μm) of polyolefin-based microporous membranes M: Viscosity-average molecular weight of polyolefins (×10 5 g / mol) D: Gas permeability of polyolefin-based microporous membrane (×10 -5 Darcy) d: Average pore size (nm) of polyolefin-based microporous membranes ε: Porosity of polyolefin-based microporous membranes

[0034] In one embodiment, the polyolefin-based microporous membrane is not particularly limited in its means of simultaneously satisfying the above-mentioned limited gas permeability, lateral shrinkage rate, and BDV index, and can be satisfied, for example, by various variables in the composition and manufacturing method of the polyolefin-based microporous membrane.

[0035] In one embodiment, the gas permeability is 2.6 × 10⁻⁶. -5 Darcy or higher, 2.7×10 -5 Darcy or higher, 2.9×10 -5 Darcy or higher, or 3.0 x 10 -5 It may be Darcy or higher. In this case, there is no particular upper limit, but for example, 5.0 × 10 -5 Below Darcy, 4.5×10 -5 Below Darcy, 4.2×10 -5 Darcy and below, 4.0×10 -5 Below Darcy, 3.8×10 -5 Darcy or lower, or 3.5 x 10 -5 It may be less than or equal to Darcy. In one embodiment, the gas permeability can be measured using a void measuring instrument, the CFP-1500-AEL manufactured by PMI. In one embodiment, the gas permeability was calculated using the following Darcy permeability constant (C), and the average value of the Darcy permeability constant in the range of 100 psi to 200 psi was calculated.

[0036] [Formula 2] C = (8FTV) / πD 2 (P 2 -1)

[0037] C: Darcy transmission constant F:Flow rate (cc / min) T: Thickness of microporous membrane (mm) V: Viscosity of gas (N2) (0.185 cP) D: Diameter of the microporous membrane (mm) P: Pressure (psi)

[0038] In one embodiment, the transverse (TD) shrinkage rate at 120°C may be 9.5% or less, 9% or less, 8.5% or less, or 8% or less. In this case, the lower limit is not particularly limited, but for example, it may be 2% or more, 3% or more, 4% or more, or 4.5% or more. In one embodiment, the shrinkage rate can be calculated by cutting the microporous membrane into 15cm x 15cm pieces, marking them at 10cm intervals in the vertical (MD) and transverse (TD) directions, inserting A4 paper, placing it in an oven (Yamato Corporation, DKN612) that has been temperature-stabilized at 120°C, leaving it for 1 hour, measuring the change in the intervals, and calculating the transverse shrinkage rate using the following formula 4.

[0039] [Equation 4] Shrinkage rate = 100 × (initial interval - interval after leaving at 120°C) / initial interval

[0040] In one embodiment, the BDV index may be 15.2 or higher, 15.3 or higher, 15.5 or higher, 15.6 or higher, 16.0 or higher, or 19.0 or higher. In this case, the upper limit is not particularly limited, but for example, it may be 25.0 or lower, 22.0 or lower, 20.0 or lower, or 19.5 or lower. The BDV index according to one embodiment is a novel parameter provided for the first time in this application, and the inventors have confirmed that the BDV index has a correlation with the dielectric breakdown voltage (kV / μm) value with respect to the average thickness, that is, the dielectric breakdown voltage tends to increase as the BDV index increases. In particular, as a result of collecting experimental values ​​using multiple polyolefin-based microporous films, it was confirmed that when the BDV index is 15 or higher, a dielectric breakdown voltage of 0.13kV / μm or higher can be achieved (Figure 3). Therefore, the polyolefin-based microporous film according to one embodiment achieves a BDV index of 15 or higher, thereby achieving a dielectric breakdown voltage of 0.13 kV / μm or higher, and thus exhibiting excellent insulating properties.

[0041] In one embodiment, the polyolefin (PO) resin has a viscosity-average molecular weight (Mv) of 3 × 10 5 g / mol ~ 50 × 10 5 g / mol, 3 × 10⁻⁶ 5 g / mol ~ 40 × 10 5 g / mol, 3 × 10⁻⁶ 5 g / mol ~ 30 × 10 5 g / mol, 3 × 10⁻⁶ 5 g / mol ~ 20 × 10 5 g / mol, 5 × 10 5 g / mol ~ 20 × 10 5 g / mol, or 6 × 10⁻⁶ 5 g / mol ~ 20 × 10 5 It may be g / mol. In one embodiment, the viscosity-average molecular weight of the polyolefin resin is not necessarily limited, but is 3 × 10 5 If the viscosity-average molecular weight is lower than g / mol, there is a limit to how much the strength can be increased by increasing the stretch ratio during the production of the microporous film, and as a result, it may not be possible to achieve a BDV index of 15 or higher. Also, if the viscosity-average molecular weight of the polyolefin resin is 50 × 10 5Higher than g / mol, or 30 × 10 5 If the concentration is higher than g / mol, it becomes difficult to effectively ensure kneadability during extrusion, which can make it difficult to produce a uniform microporous membrane.

[0042] In one embodiment, the viscosity-average molecular weight of the polyolefin resin was calculated using the Margolies equation shown in Equation 1 below, after measuring the intrinsic viscosity (η) using the Crystex® model (solvent: TCB (trichlorobenzene)) manufactured by Polymer Char.

[0043] [Formula 1] Viscosity average molecular weight (Mv)=5.37×10 4 ×[η] 1.49

[0044] In one embodiment, the polyolefin resin may include, for example, polyethylene (or a polyethylene copolymer) or polypropylene (or a polypropylene copolymer). Alternatively, for example, the polyolefin resin may include a mixture of polyethylene and polypropylene having different viscosity-average molecular weights, and the polyethylene and polypropylene may include a first polyethylene and first polypropylene having relatively high molecular weights, and a second polyethylene and second polypropylene having relatively low molecular weights, respectively. In one embodiment, when the polyolefin resin includes both polyethylene and polypropylene, the weight ratio may be, for example, 10:90 to 90:10, 30:70 to 70:30, 40:60 to 60:40, 30:70 to 40:60, or 60:40 to 70:30. Alternatively, according to one embodiment, polyethylene alone may be used as the polyolefin resin.

[0045] In one embodiment, the melting temperature of the polyolefin resin (polyethylene, polypropylene, or a mixture thereof) is not particularly limited, but may be, for example, 100°C to 180°C, 110°C to 180°C, 100°C to 150°C, 110°C to 140°C, 120°C to 140°C, 120°C to 135°C, 120°C to 180°C, 130°C to 180°C, 130°C to 170°C, 140°C to 170°C, or 145°C to 165°C.

[0046] In one embodiment, the polyolefin-based microporous film may have a breakdown voltage (BDV) of 0.13kV / μm or higher, 0.135kV / μm or higher, 0.14kV / μm or higher, or 0.15kV / μm or higher relative to its average thickness. In this case, the upper limit is not particularly limited, but for example, it may be 0.3kV / μm or lower, 0.25kV / μm or lower, 0.2kV / μm or lower, 0.18kV / μm or lower, 0.17kV / μm or lower, or 0.16kV / μm or lower.

[0047] In one embodiment, the dielectric breakdown voltage is measured in accordance with ASTM D149. Specifically, after placing the microporous film between the electrodes of a dielectric strength tester (Croma, model 19052) in a dry room (dew point temperature: -60°C), the voltage (kV) is measured when the leakage current is 5 mA under the condition that the applied voltage is increased at 5 kV / 10 sec, and this value is calculated by dividing it by the average thickness. In this case, the average thickness can be obtained by stacking eight layers of microporous film, measuring the thickness at five arbitrary points in the TD direction using a thickness measuring instrument manufactured by Mitutoyo, dividing by 5 to derive the average thickness of the eight layers of microporous film, and then dividing again by 8 to derive the average thickness of a single microporous film.

[0048] In one embodiment, the puncture strength of the polyolefin-based microporous membrane may be 0.4 N / μm or more, 0.45 N / μm or more, 0.5 N / μm or more, 0.53 N / μm or more, 0.54 N / μm or more, or 0.55 N / μm or more. In this case, the upper limit is not particularly limited, but for example, it may be 1.0 N / μm or less, 0.8 N / μm or less, 0.7 N / μm or less, 0.65 N / μm or less, or 0.6 N / μm or less.

[0049] In one embodiment, the puncture strength may be a value measured using a pin tip with a diameter of 1.0 mm and a radius of curvature of 0.5 mm, at a speed of 120 mm / min using an INSTRON UTM (Universal Test Machine) 3345.

[0050] In one embodiment, the porosity of the polyolefin-based microporous membrane is determined by the mass (M, g) measured after cutting the polyolefin-based microporous membrane into a rectangle with dimensions A cm × B cm (thickness: T, μm), and the density (ρ, g / cm³) of the polyolefin. 3 This was calculated using the following formula 2.

[0051] [Formula 2] Porosity={1-(M×10000) / (ABTρ)}

[0052] In this case, the porosity may be, for example, 0.2 or more, 0.25 or more, 0.3 or more, 0.35 or more, or 0.4 or more. In this case, there is no particular upper limit, but for example, it may be 0.7 or less, 0.6 or less, 0.5 or less, or 0.45 or less.

[0053] In one embodiment, the thickness of the polyolefin-based microporous membrane may be, for example, 3 μm to 20 μm, 3 μm to 15 μm, 5 μm to 15 μm, or 6 μm to 12 μm. In one embodiment, the thickness can be measured using a TESA Micro-Hite Electronic Gauge manufactured by TESA, which is a contact-type thickness measuring instrument having an accuracy of 0.1 μm under a measurement pressure condition of 0.63 N. The polyolefin-based microporous membrane according to one embodiment can achieve high strength and thermal safety even at a thin thickness by combining the gas permeability, lateral shrinkage rate at 120°C, and BDV index of the polyolefin-based microporous membrane as described above.

[0054] In one embodiment, the average pore size of the polyolefin-based microporous membrane may be 10 nm to 100 nm, 10 nm to 80 nm, 20 nm to 80 nm, 20 nm to 60 nm, 30 nm to 50 nm, or 35 nm to 45 nm. In one embodiment, the average pore size can be measured using a pore analyzer, the CFP-1500-AEL, manufactured by PMI, in accordance with ASTM F316-03. Specifically, it can be measured by the half-dry method, and a Galwick solution (surface tension: 15.9 dyne / cm) manufactured by PMI can be used.

[0055] In one embodiment, the polyolefin-based microporous membrane may have a machine-direction (MD) shrinkage rate of 10% or less, 9% or less, 8.5% or less, 8% or less, or 7.5% or less at 120°C. In this case, the lower limit is not particularly limited, but for example, it may be 2% or more, 3% or more, 4% or more, or 5% or more. In one embodiment, the shrinkage rate can be calculated by cutting the microporous membrane into 15cm x 15cm pieces, marking them at 10cm intervals in the machine-direction (MD) and horizontal (TD) directions, inserting paper, placing it in an oven (Yamato Corporation, DKN612) that has been temperature-stabilized at 120°C, leaving it for 1 hour, measuring the change in the intervals, and calculating the shrinkage rate in the machine-direction using the following formula 4.

[0056] [Equation 4] Shrinkage rate = 100 × (initial interval - interval after leaving at 120°C) / initial interval

[0057] Another embodiment is a method for producing a polyolefin microporous membrane, comprising the steps of: preparing a molten material by kneading a polyolefin resin and a diluent; forming the molten material into a sheet; and extracting the diluent after stretching the formed sheet, wherein the gas permeability is 2.5 × 10⁻⁶. -5 The present invention provides a method for producing a polyolefin-based microporous membrane that is Darcy or higher, has a transverse (TD) shrinkage rate of 10% or less at 120°C, and has a BDV index of 15 or higher according to the formula 1.

[0058] In one embodiment, the polyolefin-based microporous membrane can be applied in the same manner as described above, so a redundant explanation will be omitted below. In one embodiment, the weight ratio of the polyolefin resin to the diluent may be, for example, 5:95 to 50:50, 10:90 to 50:50, 10:90 to 40:60, 10:90 to 35:65, or 15:85 to 40:60.

[0059] In one embodiment, the step of stretching the molded sheet may include stretching it 6 to 15 times in the longitudinal (MD) direction at a temperature of 60°C to 130°C. In this case, the temperature conditions may be, for example, 60°C to 120°C, 60°C to 110°C, 60°C to 100°C, 60°C to 95°C, 60°C to 90°C, 70°C to 100°C, 70°C to 95°C, 80°C to 100°C, or 80°C to 95°C. The stretch ratio may also be, for example, 6 to 13 times, 6 to 12 times, 6 to 10 times, 7 to 12 times, 7 to 10 times, 8 to 12 times, or 8 to 10 times.

[0060] In one embodiment, the longitudinal stretching temperature is not necessarily limited, but in one embodiment, it may be set to a temperature about 30°C or more lower than the melting temperature of the sheet manufactured in the step in which the extruded molten material is formed into a sheet. If the longitudinal stretching temperature is set to a temperature less than 30°C lower than the melting temperature of the manufactured sheet, the transverse shrinkage rate at 120°C will be 10% or less, and 2.5 × 10 -5 Achieving gas permeability higher than Darcy may become difficult.

[0061] In one embodiment, the melting temperature of the sheet produced in the step in which the extruded molten material is formed into a sheet can be determined from the peak position on the curve measured using a differential scanning calorimetry (DSC). Specifically, a Thermal analysis system DSC 3+ model differential scanning calorimetry manufactured by Mettler Toledo was used to scan the temperature range from 25°C to 200°C at a heating rate of 10°C / min under a nitrogen atmosphere. In this case, the amount of sample measured was set to 5 mg, and the melting temperature was defined as the temperature showing the maximum (peak) heat value. In one embodiment, the melting temperature of the sheet is not particularly limited, but may be, for example, 100°C to 150°C, 120°C to 140°C, 120°C to 130°C, or about 120°C.

[0062] In one embodiment, the step of stretching the molded sheet may further include a step of stretching it transversely (TD) by 6 to 15 times at a temperature of 80°C to 130°C after the longitudinal stretching. In this case, the temperature conditions may be, for example, 80°C to 130°C, 90°C to 130°C, 90°C to 120°C, or 100°C to 120°C. Furthermore, the stretch ratio may be, for example, 7 to 15 times, 7 to 13 times, 8 to 12 times, or 9 to 11 times.

[0063] In one embodiment, the transverse stretching may include two or more stretching sections, for example, a PH section, an ST section, and an HS section in that order. In this case, the PH (Pre-heating) section is a section in which heat is applied to a sheet that has been stretched longitudinally without stretching, the ST (Stretching) section is a section in which heat is applied to the sheet and stretched transversely, and the HS (Heat-setting) section is a section in which heat is applied to the stretched film without stretching / relaxation. In this case, the temperature of the ST section may be set to a temperature below the melting temperature of the sheet after longitudinal stretching. The temperature of the PH section may be selected to be lower than the temperature of the ST section, for example, 1°C to 5°C, 1°C to 3°C, or about 2°C lower. The temperature of the HS section may be selected to be higher than the temperature of the ST section, for example, 1°C to 5°C, 2°C to 4°C, or about 3°C ​​higher. Setting the above temperatures in each section is effective because it can make the pores of the microporous membrane smaller. Specifically, for example, the temperature of the PH section may be set within the range of 110°C to 130°C, 110°C to 125°C, or 115°C to 125°C; the temperature of the ST section may be set within the range of 115°C to 135°C, 115°C to 130°C, or 120°C to 130°C; and the temperature of the HS section may be set within the range of 120°C to 140°C, 120°C to 135°C, or 120°C to 130°C.

[0064] In one embodiment, the stretch ratio in the longitudinal (MD) and transverse (TD) directions (MD / TD) may be less than 1.0, 0.95 or less, 0.9 or less, or 0.89 or less. In this case, the lower limit is not particularly limited, but for example, it may be 0.5 or more, 0.6 or more, 0.7 or more, 0.8 or more, or 0.82 or more. In one embodiment, if the stretch ratio (MD / TD) exceeds the upper limit, it may become difficult to achieve a transverse shrinkage rate of 10% or less at 120°C.

[0065] In one embodiment, the step of preparing the molten material may specifically involve melting and kneading a polyolefin resin and a diluent through an extruder to prepare a thermodynamically single-phase molten material.

[0066] In one embodiment, the type of diluent is not particularly limited and may be any organic compound that forms a single phase with the polyolefin resin (or a mixture of the polyolefin resin and other types of resins) during extrusion. Non-limiting examples of the diluent include, for example, aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, liquid paraffin (or paraffin oil), paraffin wax; phthalates such as dibutyl phthalate and dioctyl phthalate; fatty acids having 10 to 20 carbon atoms such as palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid; and fatty acid alcohols having 10 to 20 carbon atoms such as palmitic acid alcohol, stearic acid alcohol, and oleic acid alcohol. These may be used individually or in combination of two or more. Specifically, among the diluents, liquid paraffin, which is an example of a low molecular weight organic substance having a molecular structure similar to that of a polyolefin, may be used.

[0067] In one embodiment, the step of drying may be further included after the step of extracting the diluent. In one embodiment, a heat treatment step may be further included after the step of extracting the diluent or after the drying step. The heat treatment step may include stretching, heat setting, and / or relaxation steps. In one embodiment, the heat treatment step or heat setting step may be carried out at a temperature of 100°C to 150°C. The temperature conditions may be, for example, 110°C to 150°C, 120°C to 150°C, or 130°C to 140°C. In this case, the temperature of the heat treatment or heat setting step may be selected to be 3°C to 20°C lower or 5°C to 10°C lower than the melting temperature of the extracted and / or dried film. If the temperature is higher than the above, 2.5 × 10 -5 Achieving a gas permeability of Darcy or higher is difficult, and at temperatures lower than the aforementioned temperature, it may be difficult to achieve a lateral shrinkage rate of 10% or less at 120°C. In this case, the melting temperature was measured by the method described above.

[0068] In one embodiment, if the heat treatment step includes a stretching / heat setting / relaxation step, the stretching step can stretch by 1.3 to 3 times, 1.3 to 2 times, or 1.5 to 2 times, and if this is not met, 2.5 × 10 -5 Achieving a gas permeability higher than Darcy may become difficult. Furthermore, in the relaxation step, relaxation can be 0.5 to 0.95 times, 0.7 to 0.95 times, 0.8 to 0.95 times, or 0.8 to 0.9 times. If these conditions are not met, it may become difficult to achieve a lateral shrinkage rate of 10% or less at 120°C. The polyolefin-based microporous membrane may be a separator, and more specifically, it may be a separator for secondary batteries.

[0069] Another embodiment includes a polyolefin and has a gas permeability of 2.5 × 10 -5 The present invention provides a separator comprising a polyolefin-based microporous membrane that is Darcy or higher, has a transverse (TD) shrinkage rate of 10% or less at 120°C, and has a BDV index of 15 or higher according to formula 1.

[0070] In one embodiment, the polyolefin-based microporous membrane included in the separator can be the same as described above, so a redundant explanation will be omitted below.

[0071] In one embodiment, the separator may specifically be a separator for a secondary battery. In one embodiment, the separator achieves good insulation properties by simultaneously satisfying the gas permeability, lateral shrinkage rate, and BDV index conditions. Therefore, even if the battery undergoes many charge-discharge cycles, it can effectively prevent side reactions caused by volume changes of the electrodes inside the battery and / or dendrite formation on the negative electrode. Furthermore, because the separator in one embodiment has a high dielectric breakdown voltage, it can effectively prevent localized damage to the separator even when an overvoltage is applied from the outside, such as during overcharging. Thus, it can suppress the phenomenon of leakage current generation due to separator damage.

[0072] Another embodiment provides a secondary battery that includes a separator according to the aforementioned embodiment. In one embodiment, the secondary battery may include a separator according to one embodiment, a positive electrode, a negative electrode facing the positive electrode, and an electrolyte.

[0073] In one embodiment, the initial resistance of the secondary battery may be 54 mΩ or less, or 53 mΩ or less, and in this case, the lower limit is not particularly limited, but for example, it may be 30 mΩ or more, 35 mΩ or more, 40 mΩ or more, or 50 mΩ or more. In one embodiment, the initial electrical resistance per unit thickness of the secondary battery may be about 6.6 mΩ / μm or less.

[0074] Figures 1 and 2 are schematic plan and cross-sectional views, respectively, of a secondary battery according to an exemplary embodiment. For example, Figure 2 is a cross-sectional view taken along the line I-I' in Figure 1.

[0075] Referring to Figures 1 and 2, the secondary battery may include a positive electrode 100 and a negative electrode 130 opposite to the positive electrode 100. The positive electrode 100 may include a positive electrode current collector 105 and a positive electrode active material layer 110 on the positive electrode current collector 105.

[0076] The positive electrode active material layer 110 may include a positive electrode active material, and optionally a positive electrode binder and a conductive material. The positive electrode 100 can be manufactured, for example, by mixing and stirring a positive electrode active material, a positive electrode binder, a conductive material, a dispersion medium, etc. to produce a positive electrode slurry, which is then applied to the positive electrode current collector 105, dried, and rolled.

[0077] The positive electrode current collector 105 may include, for example, stainless steel, nickel, aluminum, titanium, copper, or alloys thereof, and more preferably, aluminum or an aluminum alloy.

[0078] In one embodiment, the secondary battery may be a lithium secondary battery, for example, a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery. The following provides a general description of a secondary battery according to one embodiment.

[0079] [Positive electrode] The positive electrode may include a positive electrode current collector and a positive electrode mixture layer disposed on at least one side of the positive electrode current collector.

[0080] (Positive electrode current collector) The positive electrode current collector may include stainless steel, nickel, aluminum, titanium, or alloys thereof. The positive electrode current collector may also include aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver. The thickness of the positive electrode current collector is not limited to, but may be, for example, 10 μm to 50 μm.

[0081] (Positive electrode material) The positive electrode mixture layer may contain a positive electrode active material. The positive electrode active material may contain a compound capable of reversibly intercalating and deintercalating lithium ions.

[0082] According to exemplary embodiments, the positive electrode active material may include a lithium-nickel metal oxide. The lithium-nickel metal oxide may further include at least one of cobalt (Co), manganese (Mn), and aluminum (Al).

[0083] In some embodiments, the positive electrode active material or the lithium-nickel metal oxide may include a layered structure or crystalline structure represented by the following chemical formula 1.

[0084] [Chemical formula 1] Li x Ni a M b O 2+z

[0085] In chemical formula 1, 0.9 ≤ x ≤ 1.2, 0.6 ≤ a ≤ 0.99, 0.01 ≤ b ≤ 0.4, and -0.5 ≤ z ≤ 0.1 may also be true. As previously mentioned, M may include Co, Mn, and / or Al.

[0086] The chemical structure represented by Chemical Formula 1 indicates the bonding relationships contained within the layered or crystalline structure of the positive electrode active material and does not exclude other additional elements. For example, M may include Co and / or Mn, and Co and / or Mn, together with Ni, may be provided as the main active element of the positive electrode active material. Chemical Formula 1 is provided to represent the bonding relationships of the said main active element and should be understood to include the introduction and substitution of additional elements.

[0087] In one embodiment, auxiliary elements may be further included in addition to the main active element to enhance the chemical stability of the positive electrode active material or the layered / crystalline structure. These auxiliary elements may be mixed together with the layered / crystalline structure to form bonds, and in this case, they must be understood to be included within the range of the chemical structure represented by chemical formula 1.

[0088] The auxiliary element may include, for example, at least one of Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, or Zr. The auxiliary element may act as an auxiliary active element that contributes to the capacity / power activity of the positive electrode active material, together with Co or Mn, such as Al. For example, the positive electrode active material or the lithium-nickel metal oxide may include a layered structure or crystalline structure represented by the following chemical formula 1-1.

[0089] [Chemical formula 1-1] Li x Ni a M1 b1 M2 b2 O 2+z

[0090] In chemical formula 1-1, M1 may include Co, Mn, and / or Al. M2 may include the aforementioned auxiliary elements. In chemical formula 1-1, 0.9≦x≦1.2, 0.6≦a≦0.99, 0.01≦b1+b2≦0.4, and -0.5≦z≦0.1 may also be true.

[0091] The positive electrode active material may further contain coating elements or doping elements. For example, elements substantially identical or similar to the auxiliary elements described above may be used as coating elements or doping elements. For example, the elements described above may be used individually or in combination of two or more elements as coating elements or doping elements.

[0092] The coating element or doping element may be present on the surface of the lithium-nickel metal oxide particles or may penetrate through the surface of the lithium-nickel metal oxide particles and be contained within the bonding structure represented by chemical formula 1 or chemical formula 1-1.

[0093] The positive electrode active material may include nickel-cobalt-manganese (NCM) lithium oxide. In this case, an NCM lithium oxide with an increased nickel content may be used.

[0094] Ni may be provided as a transition metal related to the output and capacity of lithium secondary batteries. Therefore, as described above, by employing a high-Ni composition in the positive electrode active material, a high-capacity positive electrode and a high-capacity lithium secondary battery can be provided.

[0095] However, as the Ni content increases, the long-term storage stability and lifespan stability of the positive electrode or secondary battery relatively decrease, and side reactions with the electrolyte may also increase. However, according to exemplary embodiments, by including Co, electrical conductivity can be maintained, and lifespan stability and capacity maintenance characteristics can be improved via Mn.

[0096] The Ni content in the NCM-based lithium oxide (for example, the mole fraction of nickel relative to the total number of moles of nickel, cobalt, and manganese) may be 0.6 or higher, 0.7 or higher, or 0.8 or higher. In some embodiments, the Ni content may be 0.8 to 0.95, 0.82 to 0.95, 0.83 to 0.95, 0.84 to 0.95, 0.85 to 0.95, or 0.88 to 0.95.

[0097] In some embodiments, the positive electrode active material may include a lithium cobalt oxide-based active material, a lithium manganese oxide-based active material, a lithium nickel oxide-based active material, or a lithium iron phosphate (LFP)-based active material (e.g., LiFePO4).

[0098] In some embodiments, the positive electrode active material may include, for example, a Mn-rich active material having a chemical structure or crystalline structure represented by chemical formula 2, an LLO (Li-rich layered oxide) / OLO (Over Lithiated Oxide) active material, or a Co-less active material.

[0099] [Chemical formula 2] p[Li2MnO3]·(1 - p)[Li q JO2]

[0100] In Chemical Formula 2, 0 < p < 1, 0.9 ≦ q ≦ 1.2, and J may contain at least one element among Mn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, and B.

[0101] (Method for manufacturing the positive electrode) For example, the positive electrode active material can be mixed in a solvent to produce a positive electrode slurry. After coating the positive electrode slurry on a positive electrode current collector, it can be dried and rolled to produce a positive electrode mixture layer. The coating process may be performed by methods such as gravure coating, slot die coating, multilayer simultaneous die coating, imprinting, doctor blade coating, dip coating, bar coating, casting, etc., and is not limited thereto. The positive electrode mixture layer may further contain a binder and optionally may further contain a conductive material, a thickener, etc. At this time, the binder and the conductive material are as described above.

[0102] (Positive electrode solvent) Non - limiting examples of the solvent used for manufacturing the positive electrode mixture include N - methyl - 2 - pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N,N - dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, isobutyl isobutyrate, butyl butyrate, xylene, anisole, etc.

[0103] (Positive electrode binder) The binder may include a non-aqueous binder and / or an aqueous binder, or a rubber-based binder and / or a fluorine-based binder, for example, polyvinylidene fluoride (PVDF), vinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), polyacrylonitrile, polymethyl methacrylate, acrylonitrile butadiene rubber (NBR), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), etc. In one embodiment, a PVDF-based binder may be used as the positive electrode binder.

[0104] (Positive electrode conductive material) The conductive material may be added to enhance the conductivity and / or the mobility of lithium ions or electrons in the positive electrode mixture layer. For example, the conductive material may be a linear conductive material and / or a point conductive material, and may include, but is not limited to, carbon-based conductive materials such as graphite, carbon black, acetylene black, Ketjenblack, graphene, carbon nanotubes, VGCF (vapor-grown carbon fiber), carbon fibers, and carbon nanofibers, and / or metallic conductive materials such as tin, tin oxide, titanium oxide, LaSrCoO3, and LaSrMnO3.

[0105] (Positive electrode thickener / dispersant) If necessary, the cathode mixture may further contain a thickener and / or a dispersant. In one embodiment, the cathode mixture may contain a thickener such as carboxymethylcellulose (CMC).

[0106] [Negative electrode] The negative electrode may include a negative electrode current collector and a negative electrode mixture layer disposed on at least one side of the negative electrode current collector.

[0107] (Negative electrode current collector) Non-limiting examples of negative electrode current collectors include copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, and polymer substrates coated with conductive metals. The negative electrode current collector is not limited to these, but may be, for example, 10 to 50 μm thick.

[0108] (Negative electrode material) The negative electrode mixture layer may contain a negative electrode active material. The negative electrode active material may be a substance capable of adsorbing and desorbing lithium ions. For example, the negative electrode active material may be a carbon-based material such as crystalline carbon, amorphous carbon, carbon composites, or carbon fibers; lithium metal; lithium alloy; silicon (Si)-containing material or tin (Sn)-containing material.

[0109] Examples of amorphous carbon include hard carbon, soft carbon, coke, mesocarbon microbeads (MCMB), and mesophase pitch-based carbon fiber (MPCF). Examples of crystalline carbon include graphite-based carbons such as natural graphite, artificial graphite, graphitized coke, graphitized MCMB, and graphitized MPCF.

[0110] Examples of the lithium metal include pure lithium metal or lithium metal on which a protective layer for purposes such as inhibiting dendrite growth has been formed. In one embodiment, a lithium metal-containing layer deposited or coated on a negative electrode current collector may be used as the negative electrode active material layer. In another embodiment, a lithium thin film layer may be used as the negative electrode active material layer.

[0111] The elements contained in the lithium alloy include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium.

[0112] The silicon-containing substance can provide increased capacity characteristics. The silicon-containing substance may include Si, SiOx (0 < x < 2), metal-doped SiOx (0 < x < 2), silicon-carbon composites, etc. The metal may include lithium and / or magnesium, and the metal-doped SiOx (0 < x < 2) may include metal silicates.

[0113] (Method for manufacturing the negative electrode) For example, the negative electrode active material can be mixed in a solvent to produce a negative electrode slurry. After coating / vapor-depositing the negative electrode slurry on a negative electrode current collector, it can be dried and rolled to produce a negative electrode binder layer. The coating process may be performed by methods such as gravure coating, slot die coating, multilayer simultaneous die coating, imprinting, doctor blade coating, dip coating, bar coating, casting, etc., and is not limited thereto. The negative electrode binder layer may further include a binder, and optionally may further include a conductive material, a thickening agent, etc. In some embodiments, the negative electrode may include a negative electrode active material layer in the form of lithium metal formed by a vapor deposition / coating process.

[0114] (Negative electrode solvent) Non-limiting examples of the solvent for the negative electrode binder include water, pure water, deionized water, distilled water, ethanol, isopropanol, methanol, acetone, n-propanol, t-butanol, isobutyl isobutyrate, butyl butyrate, xylene, anisole, etc.

[0115] (Negative electrode binder / conductive material / thickening agent) As the binder, conductive material, and thickening agent, the aforementioned substances that can be used during the manufacture of the positive electrode may be used.

[0116] In some embodiments, the negative electrode binder may be a rubber-based binder such as a styrene-butadiene rubber (SBR)-based binder, a carboxymethylcellulose (CMC)-based binder, a polyacrylic acid-based binder, or a polyethylenedioxythiophene (poly(3,4-ethylenedioxythiophene, PEDOT)-based binder.

[0117] [Electrode assembly] According to exemplary embodiments, an electrode assembly can be formed by repeatedly arranging a positive electrode, a negative electrode, and a separator. In some embodiments, the electrode assembly may be of the winding, stacking, z-folding, or stack-folding type.

[0118] [Electrolyte] An electrode assembly is housed in a case along with an electrolyte, thus defining a lithium secondary battery. According to an exemplary embodiment, a non-aqueous electrolyte may be used as the electrolyte.

[0119] (Lithium salt / organic solvent) The non-aqueous electrolyte contains a lithium salt, which is the electrolyte, and an organic solvent, wherein the lithium salt is, for example, Li + X - Represented by the anion (X) of the lithium salt. - ) as F - Cl - , Br - , I - NO3 - , N(CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 -, CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - , and (CF3CF2SO2)2N - etc. can be mentioned.

[0120] The organic solvent may contain organic compounds that have sufficient solubility in the lithium salt and additives and that do not react within the battery. The organic solvent may include, for example, at least one of carbonate solvents, ester solvents, ether solvents, ketone solvents, alcohol solvents, and aprotic solvents.Examples of the aforementioned organic solvents include propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate, diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, vinylene carbonate, methyl acetate (MA), ethyl acetate (EA), n-propyl acetate (n-PA), 1,1-dimethylethyl acetate (DMEA), methyl propionate (MP), ethyl propionate (EP), fluoroethyl acetate (FEA), difluoroethyl acetate (DFEA), trifluoroethyl acetate (TFEA), and dibutyl ether (dibutyl Other substances that may be used include ether, tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether (DEGDME), dimethoxyethane, tetrahydrofuran (THF), 2-methyltetrahydrofuran, ethyl alcohol, isopropyl alcohol, dimethyl sulfoxide, acetonitrile, diethoxyethane, sulfolane, γ-butyrolactone, and propylene sulfite. These may be used individually or in combination of two or more.

[0121] (Additives) The non-aqueous electrolyte may further contain additives. These additives may include, for example, cyclic carbonate compounds, fluorine-substituted carbonate compounds, sultone compounds, cyclic sulfate compounds, cyclic sulfite compounds, phosphate compounds, and borate compounds.

[0122] The aforementioned cyclic carbonate compound may include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and the like. The fluorine-substituted carbonate compound may include fluoroethylene carbonate (FEC), etc.

[0123] The sultone compound may include 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, and the like.

[0124] The cyclic sulfate compound may include 1,2-ethylene sulfate, 1,2-propylene sulfate, and the like.

[0125] The aforementioned cyclic sulfite compound may include ethylene sulfite, butylene sulfite, and the like.

[0126] The phosphate compound may include lithium difluorobis-oxalato phosphate, lithium difluorophosphate, and the like. The borate compound may include lithium bis(oxalate)borate or the like.

[0127] [Solid electrolyte] In some embodiments, a solid electrolyte may be used instead of the non-aqueous electrolyte described above. In this case, the lithium secondary battery may be manufactured in the form of an all-solid-state battery. Also, a solid electrolyte layer may be disposed between the positive electrode and the negative electrode instead of the separator described above.

[0128] The solid electrolyte may include a sulfide-based electrolyte. As non-limiting examples, the sulfide-based electrolyte may be Li2S-P2S5, Li2S-P2S5-LiCl, Li2S-P2S5-LiBr, Li2S-P2S5-LiCl-LiBr, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-Z m S n (m and n are positive numbers, Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li p MO q (p and q are positive numbers, M is P, Si, Ge, B, Al, Ga, or In), Li 7-x PS 6-x Cl x (0≦x≦2), Li 7-x PS 6-x Br x (0≦x≦2), Li 7-x PS 6-x I x (0≦x≦2) and the like. These may be used alone or in combination of two or more.

[0129] In one embodiment, the solid electrolyte may include, for example, oxide-based amorphous solid electrolytes such as Li2O-B2O3-P2O5, Li2O-SiO2, Li2O-B2O3, and Li2O-B2O3-ZnO.

[0130] [Cell structure] For example, electrode tabs (positive electrode tab and negative electrode tab) may protrude from the positive electrode current collector and the negative electrode current collector, respectively, and extend to one side of the case. The electrode tabs may be fused together with the aforementioned side of the case and connected to electrode leads (positive electrode lead and negative electrode lead) that extend or are exposed outside the case. For example, pouch-type cases, rectangular cases, cylindrical cases, coin-type cases, etc., may be used.

[0131] The examples will be further described below with reference to specific experimental examples. The examples and comparative examples included in the experimental examples are merely illustrative of one embodiment and do not limit the scope of the attached claims. It will be obvious to those skilled in the art that various changes and modifications to the examples are possible within the scope of this disclosure and the technical concept, and that such variations and modifications naturally fall within the scope of the attached claims.

[0132] Test method 1.Viscosity average molecular weight (Mv, g / mol) The viscosity-average molecular weight of polyolefins (PO) was obtained by first measuring the intrinsic viscosity (η) using the Crystex® model (solvent: TCB (trichlorobenzene)) manufactured by Polymer Char, and then calculating it using the following Margolies equation.

[0133] [Formula 1] Viscosity average molecular weight (Mv)=5.37×10 4 ×[η] 1.49

[0134] 2. Thickness (μm) The thickness of the microporous membrane was measured using a TESA Micro-Hite Electronic Gauge, a contact-type thickness measuring instrument manufactured by TESA Corporation, with an accuracy of 0.1 μm, and the measurement pressure was set to 0.63 N.

[0135] 3.Punching strength (N / μm) The perforation strength of the microporous membrane was measured at a speed of 120 mm / min using an INSTRON UTM (Universal Test Machine) 3345, with a pin tip having a diameter of 1.0 mm and a radius of curvature of 0.5 mm.

[0136] 4. Gas permeability (×10 -5 Darcy) The gas permeability of the microporous membrane was measured using a void measuring instrument, the CFP-1500-AEL, manufactured by PMI. In the example, the gas permeability was calculated using the Darcy permeability constant (C) shown below, and the average value of the Darcy permeability constant in the range of 100 psi to 200 psi was calculated.

[0137] [Formula 2] C = (8FTV) / πD 2 (P 2 -1)

[0138] C: Darcy transmission constant F:Flow rate (cc / min) T: Thickness of microporous membrane (mm) V: Viscosity of gas (N2) (0.185 cP) D: Diameter of the microporous membrane (mm) P: Pressure (psi)

[0139] 5.Porosity The porosity of the microporous membrane was calculated by determining the space within the microporous membrane. Specifically, the mass (M, g) measured after cutting the microporous membrane sample into a rectangle of A cm × B cm (thickness: T, μm), and the density (ρ, g / cm³) of the polyolefin were used. 3 Using ), the calculation was performed using the following formula 2.

[0140] [Formula 2] Porosity={1-(M×10000) / (ABTρ)}

[0141] 6. Average pore size (nm) The average pore size of the microporous membrane was measured according to ASTM F316-03 using a PMI CFP-1500-AEL pore analyzer. The measurement was performed using the half-dry method, with PMI Galwick solution (surface tension: 15.9 dyne / cm) used.

[0142] 7. Shrinkage rates of MD and TD at 120°C for 1 hour (MD / TD, %) The microporous membrane was cut into 15cm x 15cm pieces, marked at 10cm points in both the longitudinal (MD) and transverse (TD) directions, and then paper was placed between the pieces. The pieces were then placed in an oven (Yamato, DKN612) with a temperature stabilized at 120°C for 1 hour, after which the change in spacing was measured. The shrinkage rates in the transverse and longitudinal directions were calculated using formula 4 below (initial spacing: 10cm).

[0143] [Equation 4] Shrinkage rate = 100 × (initial interval - interval after leaving at 120°C) / initial interval

[0144] 8.BDV Index The BDV index was calculated using the following formula 1.

[0145]

number

[0146] P: Puncture strength of microporous membrane (N / μm) M: Viscosity-average molecular weight of polyolefins (×10 5 g / mol) D: Gas permeability of microporous membrane (×10 -5 Darcy) d: Average pore size (nm) of microporous membranes ε: Porosity of a microporous membrane

[0147] 9. Dielectric breakdown voltage (BDV (Break-Down Voltage), kV / μm) The dielectric breakdown voltage of the microporous film was measured according to ASTM D149. The microporous film was placed between the electrodes of a dielectric strength tester (Croma, model 19052) in a dry room (dew point temperature: -60°C), and the voltage (kV) was evaluated when the leakage current was 5 mA, measured under the condition that the applied voltage was increased at 5 kV / 10 sec.

[0148] Furthermore, the average thickness (t, μm) of the entire microporous membrane was calculated by stacking eight layers of microporous membranes and then measuring the thickness at five arbitrary points in the TD direction using a thickness measuring instrument manufactured by Mitutoyo. The average thickness of the eight layers of microporous membranes was then divided by five to obtain the average thickness of the entire single microporous membrane, and this was then divided again by eight to obtain the average thickness of the entire single microporous membrane.

[0149] Subsequently, in order to compare the calculated dielectric breakdown voltage with respect to thickness, the value of BDV / t, which is the ratio of the calculated dielectric breakdown voltage (BDV) to the average thickness (t) of the entire microporous film, was determined.

[0150] 10. Hot-box evaluation The hot-box evaluation was performed using a battery assembled as follows, with a microporous membrane applied as a separator.

[0151] Cathode Manufacturing: A cathode mixture slurry was prepared by adding 92% by weight of lithium cobalt composite oxide (LiCoO2) as the cathode active material, 4% by weight of carbon black as the conductive material, and 4% by weight of polyvinylidene fluoride (PVdF) as the binder to N-methyl-2-pyrrolidone (NMP) as the solvent. The prepared slurry was coated onto a 30 μm thick aluminum (Al) thin film, dried at a temperature of 120°C, and then roll-pressed to produce a 140 μm thick cathode.

[0152] Negative electrode manufacturing: Graphite carbon, PVdF as a binder, and carbon black as a conductive material were prepared in concentrations of 96% by weight, 3% by weight, and 1% by weight, respectively, and added to the solvent NMP to produce a negative electrode mixture slurry. The prepared slurry was applied to a 20 μm thick copper (Cu) thin film, dried at 120°C, and then pressed with a roll press to produce a 150 μm thick negative electrode.

[0153] A pouch-type battery was assembled using a stacking method, with a separator manufactured between the positive and negative electrodes. An electrolyte solution consisting of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / dimethyl carbonate (DMC) = 3:5:2 (volume ratio) in which 1M lithium hexafluorophosphate (LiPF6) was dissolved was injected into each assembled battery to manufacture a lithium secondary battery. This resulted in the production of a pouch-type lithium secondary battery with a capacity of 2Ah.

[0154] After the assembled batteries underwent aging and degassing, they were fully charged to 4.2V and placed in an oven. The temperature was gradually increased in 5°C increments until it reached 130°C, at which point it was left for 30 minutes to measure the changes in the batteries. After 30 minutes at 130°C, if the battery emitted smoke or caught fire, it was judged as a Failure. If there was no change in the battery's voltage / current and no smoke or fire occurred, it was judged as a Pass.

[0155] 11. Overcharge evaluation The overcharge evaluation involved charging the battery manufactured during the test method 10.Hot-box evaluation in an insulated chamber with a current of 200mA to 4.2V, discharging it to 3V with a current of 200mA to complete one charge-discharge cycle, then charging it to 4.2V with a constant current of 1000mA, maintaining the voltage constant until the charging current fell below 30mA, and then boosting the voltage to 5.0V with 1000mA and maintaining the voltage for 2 hours while observing the battery's appearance and temperature changes to confirm safety against overcharging. If smoke or fire occurred, it was judged as a Fail; if no smoke or fire occurred, it was judged as a Pass.

[0156] 12. Initial Resistance Evaluation The initial resistance evaluation was performed using batteries manufactured during the Hot-box evaluation described in Test Method 10. When the initial resistance is 54 mΩ or less, the capacity is maintained at 50% or more in the 2C-rate discharge test; therefore, the initial resistance value serves as an evaluation index for the battery output characteristics.

[0157] <Example 1> Production of a microporous membrane Polyethylene (viscosity-average molecular weight: 6 × 10) 5 A thermodynamically single-phase molten material was prepared by melt-kneading 30% by weight of (g / mol) and 70% by weight of diluent via an extruder, and then the molten material was formed into a sheet using a cooling roll. Next, the sheet was stretched 9 times in the machine direction (MD) at 90°C, and then stretched 11 times in the transverse direction (TD) at 117°C, 120°C, and 124°C in PH (Pre-heating; section where only heat is applied without stretching) - ST (Stretching; section where heat is applied and stretching is performed) - HS (Heat-setting; section where only heat is applied to the stretched sheet without stretching / relaxation), respectively. After that, the diluent was extracted and the sheet was moved to a drying roll for drying. The dried film was stretched (1.55 times), heat-set, and relaxed (0.87 times) at 134°C to produce a microporous membrane with a thickness of 9.4 μm.

[0158] <Examples 2-4> A microporous membrane was manufactured using the same method as in Example 1, referring to Table 1 below.

[0159] <Comparative Example 1 to Comparative Example 7> A microporous membrane was manufactured using the same method as in Example 1, referring to Table 2 below.

[0160] [Table 1]

[0161] [Table 2]

[0162] The microporous membranes of the examples and comparative examples, and the batteries manufactured using them, were evaluated using the aforementioned test method, and the results are shown in Tables 3 to 6 below.

[0163] [Table 3]

[0164] [Table 4]

[0165] [Table 5]

[0166] [Table 6]

[0167] As can be seen from Tables 3 to 6 above, 2.5 × 10 -5 The secondary battery containing the polyolefin-based microporous membrane of the example, which simultaneously satisfies a gas permeability of 10% or more, a lateral (TD) shrinkage rate of 10% or less at 120°C, and a BDV index of 15 or more, exhibits a low initial resistance of 54 mΩ or less, excellent charge-discharge characteristics, and superior insulation characteristics and stability at high voltage and / or high temperature compared to the secondary battery containing the polyolefin-based microporous membrane of the comparative example.

[0168] The above description is merely an example of applying the principles of this disclosure, and other configurations may be included without departing from the scope of this disclosure. Although one embodiment has been described in detail with reference to examples and experimental examples, the scope of one embodiment is not limited to a specific example and should be interpreted in accordance with the appended claims. [Explanation of symbols]

[0169] 100: Positive electrode 105: Positive electrode current collector 107: Positive lead 110: Positive electrode active material layer 120: Negative electrode active material layer 125: Negative electrode current collector 127: Negative lead 130: Negative electrode 140: Separator 150: Electrode assembly 160: Case

Claims

1. It contains polyolefin resin and has a gas permeability of 2.5 × 10⁻⁶ －５ A polyolefin-based microporous membrane having a Darcy rating of 15 or higher, a transverse direction (TD) shrinkage rate of 10% or less at 120°C, and a BDV index of 15 or higher according to formula 1 below. [Math 1] P: Puncture strength (N / μm) of polyolefin-based microporous membranes M: Viscosity-average molecular weight of polyolefin (×10 ５ g / mol) D: Gas permeability of polyolefin-based microporous membrane (×10 －５ Darcy) d: Average pore size (nm) of polyolefin-based microporous membranes ε: Porosity of polyolefin-based microporous membranes

2. The aforementioned polyolefin resin has a viscosity-average molecular weight of 3 × 10 ５ g / mol~50×10 ５ A polyolefin-based microporous membrane according to claim 1, wherein the concentration is g / mol.

3. The polyolefin-based microporous film according to claim 1, wherein the breakdown voltage (Break-Down Voltage, BDV) with respect to the average thickness is 0.13 kV / μm or more, and the breakdown voltage is measured in accordance with ASTM D149.

4. The polyolefin-based microporous membrane according to claim 1, wherein the puncture strength of the polyolefin-based microporous membrane is 0.4 N / μm or more.

5. The porosity of the polyolefin-based microporous membrane is determined by the mass (M, g) measured after cutting the polyolefin-based microporous membrane into a rectangle with dimensions A cm × B cm (thickness: T, μm), and the density (ρ, g / cm³) of the polyolefin. ３ The polyolefin-based microporous membrane according to claim 1, calculated using the following formula 2. [Formula 2] Porosity = {1-(M×10000) / (ABTρ)}

6. The polyolefin-based microporous membrane according to claim 5, wherein the porosity of the polyolefin-based microporous membrane is 0.2 or more.

7. The polyolefin-based microporous membrane according to claim 1, wherein the thickness of the polyolefin-based microporous membrane is 3 μm to 20 μm.

8. The polyolefin-based microporous membrane according to claim 1, wherein the average pore size of the polyolefin-based microporous membrane, as measured in accordance with ASTM F316-03, is 10 nm to 100 nm.

9. The polyolefin-based microporous membrane according to claim 1, wherein the shrinkage rate in the machine direction (MD) at 120°C is 10% or less.

10. The steps include: preparing a molten material by mixing a polyolefin resin with a diluent, The steps include: forming the molten material into a sheet; A method for producing a polyolefin-based microporous membrane, comprising the step of stretching a molded sheet and then extracting a diluent, Gas permeability is 2.5 × 10 －５ A method for producing a polyolefin-based microporous membrane, wherein the film is Darcy or higher, the transverse direction (TD) shrinkage rate at 120°C is 10% or less, and the BDV index of formula 1 below is 15 or higher. [Math 2] P: Puncture strength (N / μm) of polyolefin-based microporous membranes M: Viscosity-average molecular weight of polyolefin (×10 ５ g / mol) D: Gas permeability of the polyolefin microporous membrane (×10 －５ Darcy) d: Average pore size (nm) of polyolefin-based microporous membranes ε: Porosity of polyolefin-based microporous membranes

11. The method for producing a polyolefin-based microporous membrane according to claim 10, wherein the step of stretching the molded sheet includes stretching it 6 to 15 times in the longitudinal direction (MD) at a temperature of 60°C to 130°C.

12. The method for producing a polyolefin-based microporous membrane according to claim 11, wherein the step of stretching the molded sheet further comprises stretching it 6 to 15 times in the transverse direction (TD) at a temperature of 80°C to 130°C.

13. A method for producing a polyolefin-based microporous film according to claim 12, wherein the stretching ratio in the longitudinal direction (MD) and transverse direction (TD) (MD / TD) is less than 1.

0.

14. A method for producing a polyolefin-based microporous membrane according to claim 10, further comprising a drying step after the step of extracting the diluent.

15. A method for producing a polyolefin-based microporous membrane according to claim 10, further comprising the step of heat-fixing at a temperature of 100°C to 150°C after the step of extracting the diluent.

16. The method for producing a polyolefin-based microporous membrane according to claim 10, wherein the polyolefin-based microporous membrane is a separator.

17. It contains polyolefin and has a gas permeability of 2.5 × 10⁻⁶. －５ A separator comprising a polyolefin-based microporous membrane that is Darcy or higher, has a transverse direction (TD) shrinkage rate of 10% or less at 120°C, and has a BDV index of 15 or higher according to the following formula 1. [Math 3] P: Puncture strength (N / μm) of polyolefin-based microporous membranes M: Viscosity-average molecular weight of polyolefin (×10 ５ g / mol) D: Gas permeability of polyolefin-based microporous membrane (×10 －５ Darcy) d: Average pore size (nm) of polyolefin-based microporous membranes ε: Porosity of polyolefin-based microporous membranes

18. A secondary battery comprising the separator described in claim 17.

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