Polyethylene microporous membrane, method for producing the same, and separator containing the microporous membrane

Abstract

The present invention provides a method for manufacturing a polyethylene microporous membrane with significantly high gas permeability and porosity, as well as significantly improved heat resistance at high temperatures, a polyethylene microporous membrane, a separator containing the microporous membrane, and an electrochemical element. [Solution] A method for producing a polyethylene microporous membrane, comprising the steps of (a) producing a molten product by melting and kneading a mixture containing polyethylene resin and diluent using an extruder; (b) extruding the molten product to form a sheet; (c) stretching the sheet by four times or more in the longitudinal direction; (d) extracting diluent from the sheet stretched in the longitudinal direction and drying it; (e) stretching the dried sheet by four times or more in the transverse direction to form a film; and (f) heat-treating the transversely stretched film.

Description

[Technical Field]

[0001] This disclosure relates to a polyethylene microporous membrane, a method for producing the same, and a separator containing the microporous membrane. In one embodiment, the disclosure relates to a polyethylene microporous membrane having significantly high gas permeability and porosity, as well as improved heat resistance, a method for producing the same, and a separator containing the microporous membrane. [Background technology]

[0002] Polyethylene microporous membranes are used in a variety of fields, including separation filters, separators for secondary batteries, separators for fuel cells, and separators for supercapacitors. In particular, they are widely used as separators for secondary batteries due to their excellent electrical insulation and ion permeability.

[0003] In recent years, secondary batteries have become larger and more powerful for applications in electric vehicles, ESS (Energy Storage Systems), and other applications, making battery safety an even more crucial factor. For example, when batteries are exposed to or operated in high-temperature environments, the separator may contract, causing an internal short circuit, which could lead to a fire. Therefore, there is a need to develop a heat-resistant polyethylene microporous membrane that can withstand the temperature rise of batteries. Along with heat resistance, high mechanical strength is required to improve safety during the battery manufacturing process and use, and high permeability and high porosity are required to improve capacity and output.

[0004] In particular, given the recent trend towards higher capacity and larger size batteries, there is a strong need to develop separators that possess both higher levels of heat resistance and permeability. However, since high permeability is inversely related to excellent heat resistance and / or strength, conventional separators that achieve high permeability, while superior in capacity and output characteristics, suffer from the drawback of reduced thermal safety.

[0005] As a method to solve the above-mentioned problems, Korean Patent Publication No. 10-2012-0032539 discloses a polyolefin microporous membrane with a large pore diameter, excellent electrical properties, and excellent strength and low thermal shrinkage. However, such a microporous membrane does not satisfy the physical properties required for application to higher capacity and higher output batteries, as it satisfies the requirement of a thermal shrinkage rate of 20% or less in the width direction at 130°C, a porosity of 40-50%, and a gas permeability of only 100-200 sec / 100 ml.

[0006] Therefore, there is a need to develop polyethylene microporous membranes that have significantly high permeability and porosity, as well as significantly improved heat resistance at high temperatures. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] To solve the above-mentioned problems, this disclosure aims to provide a polyethylene microporous membrane with significantly high gas permeability and porosity, as well as significantly improved heat resistance at high temperatures, a method for producing the same, and a separator containing the microporous membrane.

[0008] The separators disclosed herein are widely applicable to green technology fields such as electric vehicles, battery charging stations, and solar and wind power generation that utilize batteries. Furthermore, the separators disclosed herein can be used in environmentally friendly electric vehicles and hybrid vehicles to reduce air pollution and greenhouse gas emissions and prevent climate change. [Means for solving the problem]

[0009] As one means of achieving the above-mentioned objectives, this disclosure provides a polyethylene microporous membrane having a thickness of 3 μm to 30 μm, a perforation strength of 0.15 N / μm or more, a lateral shrinkage rate of 5% or less measured after being left at 121°C for 1 hour, and a PS index of 110 or more represented by the following formula 1.

[0010] [Formula 1] PS index = [Gas permeability × Porosity] ÷ [Lateral shrinkage rate at 121°C]

[0011] In the above Formula 1, the unit of gas permeability is “×10 -5 Darcy”, the unit of porosity is “%”, and the unit of lateral shrinkage rate at 121°C is “%”.

[0012] In one aspect, the polyethylene microporous membrane may have a gas permeability of 10.0×10 -5 Darcy or more.

[0013] In one aspect, the polyethylene microporous membrane may have a porosity of 55% to 70%, specifically 60% to 70%.

[0014] In one aspect, the polyethylene microporous membrane may have a PS index of 220 or more, specifically 400 or more.

[0015] In one aspect, the polyethylene microporous membrane may contain polyethylene having a weight average molecular weight of 1×10 5 g / mol to 10×10 5 g / mol.

[0016] In one aspect, the polyethylene microporous membrane may be manufactured by a wet method including a sequential biaxial stretching process.

[0017] Also, as another means for achieving the above-described problems, the present disclosure provides a method for manufacturing a polyethylene microporous membrane, including: (a) a step of melt-kneading a mixture containing a polyethylene resin and a diluent by an extruder to produce a melt; (b) a step of extruding the melt and forming it into a sheet; (c) a step of stretching the sheet 4 times or more in the longitudinal direction; (d) a step of extracting and drying the diluent from the sheet stretched in the longitudinal direction; (e) a step of stretching the dried sheet 4 times or more in the transverse direction to form a film; and (f) a step of heat-treating the film stretched in the transverse direction.

[0018] In one aspect, the step (f) includes a heat relaxation process of fixing the longitudinal length and contracting the transverse width, and the heat relaxation process may be relaxed to 80% to 95% of the transverse width before the heat relaxation process.

[0019] In another aspect of the present disclosure, a separator including the polyethylene microporous membrane is provided.

[0020] In another aspect of the present disclosure, an electrochemical device including the separator is provided.

[0021] In one aspect, the electrochemical device may be a secondary battery including the separator between a positive electrode and a negative electrode.

[0022] Also, as another means for achieving the above-described problems, the present disclosure provides a separator including the above-described polyethylene microporous membrane.

Advantages of the Invention

[0023] The polyethylene microporous membrane according to the present disclosure has a high gas permeability and porosity, and can ensure significantly improved heat resistance at high temperatures.

[0024] Furthermore, the polyethylene microporous membrane according to this disclosure has a perforation strength of 0.15 N / μm or more, a lateral shrinkage rate of 5% or less measured after being left at 121°C for 1 hour, and can have remarkably high gas permeability and porosity.

[0025] Therefore, this disclosure can provide a battery with improved output characteristics and thermal safety at high temperatures by including the aforementioned polyethylene microporous membrane. [Modes for carrying out the invention]

[0026] The embodiments described herein can be modified into various other forms, and the technology relating to one aspect is not limited to the embodiments described below. Furthermore, embodiments of one aspect are provided to give a more complete explanation of this disclosure to a person with average skill in the art.

[0027] Furthermore, the singular form used in the specification and the attached claims is intended to include plural forms unless otherwise indicated in the context.

[0028] Furthermore, the numerical ranges used herein include lower and upper limits, all values ​​within those limits, increments logically derived from the form and width of the defined range, all double-limited values, and all possible combinations of upper and lower limits of numerical ranges limited in different forms. Unless otherwise defined herein, values ​​outside the numerical range that may occur due to experimental error or rounding up are also included in the defined numerical range.

[0029] Furthermore, throughout the specification, "including" a certain component means, unless otherwise stated, that it may include other components rather than excluding them.

[0030] This disclosure provides a polyethylene microporous membrane having a thickness of 3 μm to 30 μm, a perforation strength of 0.15 N / μm or more, a lateral shrinkage rate of 5% or less measured after being left at 121°C for 1 hour, and a PS index of 110 or more represented by the following formula 1.

[0031] [Formula 1] PS index = [gas permeability (×10 -5 [Darcy) × Porosity (%) ÷ [121°C Lateral Shrinkage Rate (%)]

[0032] In recent years, as secondary batteries have become larger and more powerful, they are required to meet the demands for superior output characteristics and thermal safety. Therefore, separators for secondary batteries need to possess both higher levels of heat resistance and permeability. However, conventionally developed separators have limitations in achieving both high permeability and heat resistance above a certain level due to the inherent incompatibility between these two factors.

[0033] As a result of extensive research by the inventors, it has been confirmed that a polyethylene microporous membrane can be manufactured that satisfies the following conditions: a lateral shrinkage rate of 5% or less measured after being left at 121°C for 1 hour, and a PS index of 110 or higher according to Equation 1, resulting in significantly high gas permeability and porosity, as well as significantly improved heat resistance at high temperatures.

[0034] A secondary battery according to one embodiment can ensure both excellent output characteristics and thermal safety by including a polyethylene microporous membrane that satisfies both of the above-mentioned physical properties. In particular, this disclosure can provide a battery with excellent high-temperature thermal safety, such that no smoke or fire occurs in a hot-box evaluation at a high temperature of 125°C. The manufacturing and evaluation method of the battery for the above evaluation is as follows: A positive electrode using NCM622 (Ni:Co:Mn=6:2:2) as the active material and a negative electrode using graphite carbon as the active material are wound together with the microporous membrane of this disclosure and placed in an aluminum pouch to manufacture a battery. Next, an electrolyte in which 1M lithium hexafluorophosphate (LiPF6) is dissolved in a solution containing ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume ratio of 3:5:2 is injected into the inside of the battery and sealed to produce a 2Ah capacity battery. Next, after performing aging and degassing operations on the battery, it is fully charged to 4.2V, heated in an oven at 5°C / min, and left for 30 minutes after reaching 125°C to check whether or not smoke or fire occurs in the battery. In other words, the polyethylene microporous membrane of this disclosure satisfies both of the above physical properties and is suitable for use in high-power / high-capacity batteries.

[0035] The above-described properties of the polyethylene microporous membrane of this disclosure can be achieved, but are not limited to, performing a diluent extraction step or a lateral heat treatment step under specific conditions before the lateral stretching step in the manufacture of the polyethylene microporous membrane.

[0036] The PS index according to Equation 1 increases as gas permeability and porosity increase and lateral thermal contraction decreases, and may be 110 or higher. Specifically, it may be 220 or higher, more specifically 400 or higher, and even more specifically 500 or higher. A higher PS index is preferable, and there is no particular upper limit, but it may be, for example, 2000 or 1500. In a specific embodiment, the PS index may be 110-2000, 220-2000, 400-1500, or 500-1500, but is not limited thereto. When the PS index is within the above range, it has high gas permeability and porosity and excellent heat resistance, making it suitable for high-power / high-capacity batteries.

[0037] In one embodiment, the polyethylene microporous membrane has a PS index within the range described above, and the lateral shrinkage rate measured after being left at 121°C for 1 hour may be 5% or less, 3% or less, or 2.5% or less, with no particular lower limit, but may be, for example, 0.1%, 0.5%, or 1%. In a specific embodiment, the shrinkage rate may be 0.1% to 5%, 0.5% to 5%, 1% to 3%, or 1% to 2.5%, but is not limited thereto.

[0038] In one embodiment, the thickness of the polyethylene microporous membrane is 3 μm to 30 μm, more specifically 5 μm to 20 μm, and more specifically 5 μm to 15 μm. Even with the thickness in the above range, the microporous membrane of this disclosure can achieve excellent levels of puncture strength, gas permeability, and thermal shrinkage. As a result, it has excellent resistance to external stress generated during battery manufacturing and to temperature rise and dendrite formation during battery charging and discharging, and because the internal resistance of the battery is low, the battery's charging and discharging performance can be improved.

[0039] In one aspect, even if the polyethylene microporous membrane has a thickness within the above-mentioned range, it can have a puncture strength of 0.15 N / μm or more. Specifically, the puncture strength may be 0.17 N / μm or more, 0.26 N / μm or more, and the upper limit thereof is not particularly limited, but for example, it may be 1.0 N / μm or less. In a specific aspect, the puncture strength may be 0.15 N / μm to 1.0 N / μm, or 0.17 N / μm to 1.0 N / μm, or 0.26 N / μm to 1.0 N / μm, but is not limited thereto. By satisfying the puncture strength within the above-mentioned range, it is excellent in resistance to external stress generated during battery manufacturing and dendrites generated during charging and discharging of the battery, and can ensure battery safety. In addition, the separator for secondary batteries can be made thinner and is preferably applicable to high-capacity / high-output batteries.

[0040] In one aspect, the polyethylene microporous membrane can have a gas permeability of 10.0×10 -5 Darcy or more. Specifically, the gas permeability may be 15.0×10 -5 Darcy or more, more specifically 20.0×10 -5 Darcy or more, and even more specifically 25.0×10 -5 Darcy or more. The upper limit of the gas permeability is not particularly limited, but for example, it may be 50.0×10 -5 Darcy or less, 40.0×10 -5 Darcy or less, or 35.0×10 -5 Darcy or less. In a specific aspect, the gas permeability is 10.0×10 -5 Darcy to 50.0×10 -5 Darcy, 15.0×10 -5 Darcy to 50.0×10 -5 Darcy, 20.0×10 -5 Darcy to 40.0×10 -5 Darcy, or 25.0×10 -5 Darcy to 35.0×10 -5It may be Darcy, but is not limited to it. By satisfying the gas permeability within the above range, it is possible to have excellent ionic conductivity, and the output characteristics of the battery can be significantly improved due to the low internal resistance of the battery.

[0041] In one embodiment, the polyethylene microporous membrane may have a porosity of 55% to 70%, specifically 60% to 70%. When the porosity is within the above range, the output characteristics of the battery can be significantly improved due to the low internal resistance of the battery.

[0042] In one embodiment, the polyethylene microporous membrane has a weight-average molecular weight of 1 × 10 5 g / mol ~ 10 × 10 5 It contains polyethylene in g / mol, specifically 3 × 10 5 g / mol ~ 8 × 10 5 It may contain polyethylene in a concentration of g / mol, but is not necessarily limited to this.

[0043] In one embodiment, the polyethylene microporous membrane may be manufactured by a wet process including a sequential biaxial stretching step, and by adopting a specific process sequence or changing the conditions of a specific heat treatment step, a polyethylene microporous membrane that satisfies both of the above physical properties can be provided.

[0044] In one embodiment, the polyethylene microporous membrane can be manufactured by stretching in one direction, then extracting the diluent and stretching in the other direction; by sequentially biaxially stretching the film and then heat-treating it under specific conditions; or by a combination thereof.

[0045] Specifically, the polyethylene microporous membrane may be manufactured by performing a step of extracting diluent after longitudinal stretching and before transverse stretching in the manufacturing of the polyethylene microporous membrane, but is not necessarily limited to this as long as a microporous membrane having the above-described physical properties can be manufactured.

[0046] The method for manufacturing the polyethylene microporous membrane described herein will be explained below.

[0047] A method for producing a polyethylene microporous membrane according to one embodiment may include: (a) a step of producing a molten product by melting and kneading a mixture containing polyethylene resin and diluent using an extruder; (b) a step of extruding the molten product to form a sheet; (c) a step of stretching the sheet longitudinally by four times or more; (d) a step of extracting diluent from the longitudinally stretched sheet and drying it; (e) a step of stretching the dried sheet transversely by four times or more to form a film; and (f) a step of heat treating the film.

[0048] Unlike conventional methods that involve stretching in the longitudinal and transverse directions followed by a diluent extraction step, this disclosure involves extracting the diluent from a sheet that has been stretched in the longitudinal direction, followed by transverse stretching. This allows the pores to be further and significantly expanded by stretching, making it possible to produce a polyethylene microporous membrane with remarkably high porosity and gas permeability, as well as high heat resistance.

[0049] The following describes each manufacturing step.

[0050] First, step (a) is a step of producing a molten product by melting and kneading a mixture containing polyethylene resin and diluent using an extruder, wherein the mixture contains polyethylene resin and diluent in a weight ratio of 10-60:90-40 for pore formation, specifically in a weight ratio of 20-40:80-60, but is not particularly limited as long as the objectives of this disclosure are achieved. When the weight ratio within the above range is satisfied, the molten product has sufficient fluidity, a uniform sheet can be easily formed in the subsequent steps, and sufficient orientation is achieved during the stretching process, ensuring mechanical strength, thus preventing problems such as breakage during the stretching process.

[0051] The polyethylene resin may be high-density polyethylene or contain high-density polyethylene, from the viewpoint of strength, extrusion and kneadability, stretchability, and heat resistance of the final microporous membrane.

[0052] In one embodiment, the polyethylene resin has a weight-average molecular weight of 1 × 10 5 g / mol ~ 10 × 10 5 It may be g / mol, specifically 3 × 10 5 g / mol ~ 8 × 10 5 The expression may be in g / mol, but is not necessarily limited to this.

[0053] In one embodiment, the polyethylene resin may have a melting temperature of 130°C or higher, specifically 130°C to 140°C, but is not necessarily limited thereto. The melting temperature of the polyethylene resin can be measured by DSC.

[0054] In one embodiment, the diluent can be any organic compound that forms a single phase with the polyethylene resin at the extrusion temperature, without any limitations. For example, the diluent may be one or more combinations selected from the group consisting of aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, paraffin oil, and paraffin wax; phthalic acid esters such as dibutyl phthalate and dioctyl phthalate; C10-C20 fatty acids such as palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid; and C10-C20 fatty alcohols such as cetyl alcohol, stearyl alcohol, and oleyl alcohol. One specific example of the aforementioned diluent is, but is not limited to, paraffin oil having a kinetic viscosity of 20 cSt to 200 cSt at 40°C.

[0055] Furthermore, the mixture may further contain one or more common additives for improving specific functions, such as oxidation stabilizers, UV stabilizers, and antistatic agents, to the extent that the properties of the microporous membrane do not significantly deteriorate.

[0056] (b) Step is a step of extruding the molten material into a sheet, which may be carried out in a manner known to those skilled in the art, and may be, for example, by extruding the molten material with a T-die and forming it into a sheet by casting or calendering while cooling to a temperature of 10°C to 80°C.

[0057] (c) The longitudinal stretch ratio in step (c) may be specifically 4 times or more, 4 to 15 times, or more specifically 6 to 10 times, and by satisfying the above range for the longitudinal stretch ratio, a polyethylene microporous membrane having the physical properties targeted in this disclosure can be manufactured.

[0058] (c) The stretching in step (c) may be carried out by a roll method or a tenter method, and may be performed at a temperature in the range of 60°C below the melting temperature of polyethylene to the melting temperature of polyethylene. When stretching is performed at the above-mentioned temperature range, the fluidity of the polyethylene resin can be ensured for effective stretching. Specifically, the sheet is stretched uniformly throughout and no breakage occurs due to stretching, so the stretching can be performed stably. As a result, a high-quality microporous membrane can be manufactured in which physical properties such as gas permeability, perforation strength, and porosity are achieved uniformly throughout the membrane. As an example, the stretching may be carried out at 80°C to 140°C or 90°C to 125°C, but is not limited thereto.

[0059] Step (d) is a step of extracting diluent from a sheet stretched in the longitudinal direction and drying it, which is carried out by extracting diluent from the film using an organic solvent and drying the organic solvent with the film in which the diluent has been replaced by the organic solvent. The organic solvent can be used without particular limitation as long as it can extract the diluent. Specifically, methyl ethyl ketone, methylene chloride, hexane, etc. may be used as the organic solvent because they have high extraction efficiency and dry quickly.

[0060] (d) Step is preferably carried out at a high temperature to increase the solubility of the diluent and the organic solvent, but may be carried out at a temperature of 40°C or lower from the viewpoint of safety due to the boiling of the organic solvent.

[0061] (e) Step is to stretch the dried sheet transversely by four times or more to form a film. The stretch ratio, stretching method, and stretching temperature for transverse stretching are similar to those for longitudinal stretching, so a detailed explanation is omitted. For example, stretching may be carried out at 80°C to 140°C or 90°C to 125°C, but is not limited to these.

[0062] (f) Step is a step of heat-treating the transversely stretched film, which may be carried out using a roll-type or tenter-type apparatus, and may be carried out at a temperature of 120°C to 140°C, for example.

[0063] In one embodiment, step (f) may include a thermal relaxation step that fixes the longitudinal length of the transversely stretched film and relaxes (shrinks) the transverse width. By including the thermal relaxation step described above, a polyolefin microporous film having the properties targeted in this disclosure can be produced.

[0064] Specifically, a polyethylene microporous membrane according to one embodiment, manufactured including the above-mentioned thermal relaxation process, has a high porosity (or gas permeability) and significantly improved heat resistance. In particular, a battery to which the above-mentioned microporous membrane is applied has low initial resistance and can achieve excellent thermal safety in hot box evaluations.

[0065] The aforementioned thermal relaxation step may involve relaxing the material to 80% to 100% of its lateral width before the thermal relaxation step.

[0066] The thermal relaxation step may be performed to relax the temperature to 80% to 95% or 90% to 95% in order to more effectively realize the physical properties for which this disclosure is intended.

[0067] In one embodiment, step (f) may include a heat stretching step of fixing the longitudinal length and stretching the transverse width of the transversely stretched film, a heat setting step of fixing the longitudinal and transverse lengths / widths and applying heat, and the heat relaxation step described above. Not limited to, the heat stretching step may be stretching to 120% to 160% or 120% to 140% of the transverse width before the heat stretching step.

[0068] When a film stretched in the order of longitudinal stretching, extraction, and transverse stretching is heat-treated under the above conditions, contrary to the conventional wisdom that it is difficult to achieve both high permeability and heat resistance above a certain level due to the incompatible relationship between high permeability and heat resistance, it is possible to produce a polyethylene microporous membrane with remarkably high gas permeability and porosity, as well as significantly improved heat resistance at high temperatures. Microporous membranes with such properties are suitably applicable to high-power / high-capacity batteries.

[0069] This disclosure provides a separator comprising a polyethylene microporous membrane as described above, the separator may be any separator used in any known energy storage device, and is not particularly limited, but a non-limiting example is a separator used in a lithium secondary battery.

[0070] Examples and experimental cases are described below with specific illustrations. However, the examples and experimental cases described below are merely illustrative and the technology described herein is not limited thereto.

[0071] [Physical property measurement method] 1. Weight average molecular weight (g / mol) The weight-average molecular weight (Mw) was measured using Agilent Technologies' high-temperature GPC (Gel Permeation Chromatography) model name PL220. PLgel Guard and PLgel Olexis were used as GPC columns, 1,2,4-trichlorobenzene (TCB) as the solvent, and polystyrene as the standard sample, and the analysis was performed at 140°C.

[0072] 2. Thickness of microporous membrane (μm) The thickness of the microporous membrane was measured using a contact-type thickness measuring instrument with a thickness accuracy of 0.1 μm. The measurement was performed using a TESA μ-Hite Electronic Height Gauge from TESA Corporation, with a measurement pressure of 0.63 N.

[0073] 3.Punching strength (N / μm) Penetration strength was measured 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 attached, and by pressing a microporous membrane at a speed of 120 mm / min. The penetration strength was calculated by dividing the load (N) at which the microporous membrane ruptured by the thickness of the microporous membrane (μm).

[0074] 4. Gas permeability (Darcy) Gas permeability was measured using a porometer (CFP-1500-AEL from PMI). Generally, gas permeability is expressed in terms of the Gurley number, but the Gurley number does not correct for the effect of thickness, making it difficult to understand the relative permeability due to the porosity structure. To solve this problem, the gas permeability in this disclosure is measured using the Darcy permeability constant calculated by the following mathematical formula 1. Nitrogen was used as the gas, and the average value of the Darcy permeability constant measured in the 100-200 psi range was calculated.

[0075] [Mathematical formula 1] Darcy transmission constant (C)=(8F·T·V) / (πD 2 (P 2 -1)) F=Flow rate (cc / min) T = Sample thickness (mm) V = viscosity of gas (0.185 cP for N2) D = Diameter of the sample (mm) P = pressure (psi)

[0076] 5.Porosity (%) The porosity of the microporous membrane was calculated using the following mathematical formula 2. Specifically, a sample with dimensions A cm (width), B cm (length), and T cm (thickness) was prepared, its mass was measured, and the porosity was calculated from the ratio of the mass of the resin to the mass of the microporous membrane for the same volume.

[0077] [Mathematical formula 2] Porosity (%)=100×{1-M / (A×B×T×ρ)} In the above mathematical formula 2, M is the mass (g) of the microporous membrane, and ρ is the density (g / cm³) of the polyethylene resin forming the microporous membrane. 3 )

[0078] 6. Lateral contraction rate (%) A 15cm x 15cm microporous membrane, with its length and width indicated on a 10cm length, was placed in a temperature-stabilized oven (Yamato Scientific Co., Ltd. DKN612) at 121°C for 1 hour. The change in length was then measured, and the lateral shrinkage rate was calculated using the method shown in mathematical formula 3 below.

[0079] [Mathematical formula 3] Lateral shrinkage rate (%) = {(Lateral length before heating - Lateral length after heating) / Lateral length before heating} × 100

[0080] 7.PS Index (Performance & Safety Index) The PS index of a microporous membrane is an index used to evaluate both output performance and safety, and was calculated using the following formula 1.

[0081] [Formula 1] PS index = [gas permeability (×10 -5 [Darcy) × Porosity (%) ÷ [121°C Lateral Shrinkage Rate (%)]

[0082] 8. Initial Resistance Test The initial resistance test was performed using a battery assembled with a polyethylene microporous membrane as the separator. Specifically, a positive electrode using NCM622 (Ni:Co:Mn=6:2:2) as the active material and a negative electrode using graphite carbon as the active material were wound together with the manufactured microporous membrane and placed in an aluminum pouch. An electrolyte solution containing ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume ratio of 3:5:2, with 1M lithium hexafluorophosphate (LiPF6) dissolved in it was then injected and the pouch was sealed to assemble a 2Ah capacity battery. After aging and degassing the assembled battery, it was fully charged to 4.2V and the initial resistance (mΩ) was measured.

[0083] If the initial resistance is 30 mΩ or less, the discharge test using 3C-rate will confirm that the capacitance is maintained at 50% or more, and the initial resistance value will serve as an evaluation index for the output characteristics.

[0084] 9. Hotbox Evaluation The hot box evaluation was performed using a battery assembled with a polyethylene microporous membrane as the separator. Specifically, a positive electrode using NCM622 (Ni:Co:Mn=6:2:2) as the active material and a negative electrode using graphite carbon as the active material were wound together with the manufactured microporous membrane and placed in an aluminum pouch. An electrolyte solution containing ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume ratio of 3:5:2, with 1M lithium hexafluorophosphate (LiPF6) dissolved in it, was then injected and the pouch was sealed to assemble a 2Ah capacity battery. After aging and degassing the assembled battery, it was fully charged to 4.2V and heated in an oven at 5°C / min. After reaching 125°C, the battery was left for 30 minutes and the changes were measured.

[0085] After being left at 125°C for 30 minutes, the test was judged as "Fail" if smoke or fire occurred in the battery, and "Pass" if there was no change in battery voltage / current and no smoke or fire occurred.

[0086] <Example 1> Weight-average molecular weight is 5.0 × 10⁻⁶ 5 High-density polyethylene resin (0.95 g / cm³) has a melting point of 135°C and is expressed in g / mol. 3 A mixture containing paraffin oil with a kinematic viscosity of 80 cSt at 40°C in a weight ratio of 30:70 was melt-kneaded using a twin-screw extruder to produce a molten product. The melting temperature was measured using a Discovery DSC250 differential scanning calorimeter (DSC) provided by TA Instruments, under conditions of a sample weight of 5 mg and a scan speed of 10°C / min.

[0087] The molten material was continuously extruded using a T-die, and a sheet with a width of 300 mm and an average thickness of 1100 μm was manufactured using a casting roll set to 30°C.

[0088] The aforementioned sheet was stretched longitudinally using a roll method to increase its size sevenfold at a stretching temperature of 95°C. Paraffin oil was extracted from the sheet after longitudinal stretching was completed using methylene chloride at 25°C. The sheet from which the paraffin oil had been extracted was dried at 60°C. The dried sheet was then stretched transversely using a tenter method to increase its size eightfold at a stretching temperature of 122°C.

[0089] A microporous film with a thickness of 11.5 μm was manufactured by heat-treating a film that had been stretched transversely using a tenter-type heat-setting device at 130°C, while keeping the length in the longitudinal direction fixed. In detail, the transverse heat treatment involved heat-stretching the film to 135% transversely, then heat-setting it for 10 seconds, and finally heat-relaxing (shrinking) it to 93% of its width before the heat relaxation process.

[0090] The physical properties of the ultimately manufactured microporous membrane and the performance of the battery to which it was applied are recorded in Table 1 below.

[0091] <Example 2> A polyethylene microporous membrane was manufactured in the same manner as in Example 1, except that the heat treatment was performed while both the vertical and horizontal directions were fixed. The results are recorded in Table 1.

[0092] <Comparative Example 1> Weight-average molecular weight is 3.0 × 10⁻⁶ 5 A polyethylene microporous membrane was manufactured in the same manner as in Example 1, except that a high-density polyethylene resin with a density of g / mol was used, the material was sequentially stretched eight times in both the longitudinal and transverse directions before the extraction step, and heat treatment was performed at 130.5°C. The results are recorded in Table 1.

[0093] <Comparative Example 2> A polyethylene microporous membrane was manufactured in the same manner as in Example 1, except that the extraction step was performed after sequentially stretching the material eight times in both the longitudinal and transverse directions, and that after heat treatment at 132°C, the material was heat-stretched to 155% in the transverse direction, then heat-set for 10 seconds to allow it to heat-relax (shrink) to 97% of its width before the heat relaxation step. The results are recorded in Table 1.

[0094] [Table 1]

[0095] Referring to Table 1 above, the microporous membranes of Example 1 and Example 2 showed a lateral shrinkage rate of 5% or less after being left at 121°C for 1 hour, and a PS index of 110 or higher. As a result, the batteries to which these membranes were applied had an initial resistance of 30 mΩ or less and passed hot box evaluation, achieving both excellent output characteristics and thermal safety.

[0096] In particular, Example 1, in which the length in the vertical direction was fixed while heat treatment was performed in the horizontal direction, exhibits even better gas permeability, porosity, and thermal shrinkage rate than Example 2, in which heat treatment was performed while both the vertical and horizontal directions were fixed, and it can be seen that it has a PS index approximately five times higher than Example 2.

[0097] In Comparative Example 1, the microporous membrane underwent a diluent extraction step after the conventional stretching steps in the longitudinal and transverse directions. As a result, the transverse shrinkage rate measured after being left at 121°C for 1 hour was 5% or less, but the PS index was less than 110. Therefore, while batteries using this membrane have excellent thermal safety, they suffer from the disadvantage of significantly reduced output characteristics.

[0098] In Comparative Example 2, the microporous membrane underwent a diluent extraction step after the conventional stretching steps in the longitudinal and transverse directions. As a result, after being left at 121°C for 1 hour, the transverse shrinkage rate measured was more than 5%, and the PS index was less than 110. Therefore, batteries using this membrane have the disadvantage of being inferior in both thermal safety and output characteristics.

[0099] As described above, this disclosure has been explained through specific matters and limited embodiments, which are provided only for a more general understanding of the disclosure, and the disclosure is not limited to the embodiments described above. A person with ordinary skill in the art to which this disclosure belongs can make various modifications and variations from such descriptions.

Claims

1. A polyethylene microporous membrane having a thickness of 3 μm to 30 μm, a puncture strength of 0.15 N / μm or more, a lateral shrinkage rate of 5% or less measured after being left at 121°C for 1 hour, and a PS index of 110 or more as expressed by the following formula 1. [Formula 1] PS index = [gas permeability (x 10)] －５ [Darcy) × Porosity (%)] ÷ [121°C Lateral Shrinkage Rate (%)]

2. Gas permeability is 10.0 × 10 －５ A polyethylene microporous membrane according to claim 1, wherein the darcy level is greater than or equal to darcy.

3. A polyethylene microporous membrane according to claim 1, wherein the porosity is 55% to 70%.

4. A polyethylene microporous membrane according to claim 1, wherein the porosity is 60% to 70%.

5. The polyethylene microporous membrane according to claim 1, wherein the PS index is 220 or higher.

6. The polyethylene microporous membrane according to claim 1, wherein the PS index is 400 or more.

7. Weight-average molecular weight is 1 × 10 ５ g / mol~10×10 ５ A polyethylene microporous membrane according to claim 1, comprising polyethylene in a concentration of g / mol.

8. A polyethylene microporous membrane according to claim 1, manufactured by a wet process including a sequential biaxial stretching step.

9. (a) A step of producing a molten product by melting and kneading a mixture containing polyethylene resin and diluent using an extruder, (b) The step of extruding the molten material to form it into a sheet, (c) A step of stretching the sheet vertically by more than four times, (d) A step of extracting diluent from a sheet stretched in the longitudinal direction and drying it, (e) A step of stretching the dried sheet transversely by four times or more to form a film, A method for producing a polyethylene microporous membrane, comprising the step of (f) heat-treating the transversely stretched film.

10. The method for producing a polyethylene microporous membrane according to claim 9, wherein step (f) includes a thermal relaxation step of fixing the length in the vertical direction and shrinking the width in the horizontal direction, the thermal relaxation step of relaxing to 80% to 95% of the width in the horizontal direction before the thermal relaxation step.

11. A separator containing a polyethylene microporous membrane as described in any one of items 1 through 8 of the invoice.

12. An electrochemical element comprising the separator described in claim 11.

13. The electrochemical element according to claim 12, which is a secondary battery including the separator between the positive electrode and the negative electrode.

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