Polyolefin microporous membrane, method for manufacturing thereof, and separator comprising the microporous membrane

Abstract

The present disclosure relates to a polyolefin microporous membrane, a method for manufacturing the same, and a separation membrane comprising the microporous membrane. According to one embodiment, a polyolefin microporous membrane is provided comprising 60% to 80% by weight of polypropylene having a viscosity-average molecular weight of 1×106 g / mol to 3×106 g / mol and 20% to 40% by weight of polyethylene having a weight-average molecular weight of 1×105 g / mol to 10×105 g / mol, a puncture strength of 0.25 N / ㎛ or more, a gas permeability of 1.0×10-5 Darcy or more, a porosity of 30% to 70%, an average pore size of 20 nm to 40 nm, a shutdown temperature of 150 ℃ or less, and a meltdown temperature of 180 ℃ or more.

Description

Technology Field

[0001] The present disclosure relates to a polyolefin microporous membrane, a method for manufacturing the same, and a separation membrane comprising the microporous membrane. Background Technology

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

[0003] Recently, as secondary batteries are becoming larger and higher in capacity for application in electric vehicles and ESS (Energy storage systems), ensuring battery safety has become an increasingly important factor. For example, when a battery is exposed to a high-temperature environment or operates, the separator may shrink, causing an internal short circuit, and there is a risk of fire due to the internal short circuit.

[0004] Current polyolefin microporous membranes for secondary battery separators are equipped with a shutdown function to ensure the safety of the battery. The shutdown function is a function that significantly increases the resistance of the separator by melting the polyolefin and blocking the pores. Even if a malfunction occurs in the battery and the temperature rises, safety is ensured because the internal resistance of the battery increases due to this function, effectively preventing current from flowing.

[0005] When a battery is exposed to higher temperatures, the separator may melt down, causing an internal short circuit between the positive and negative electrodes, which significantly increases the risk of fire. In particular, high-capacity and large-sized secondary batteries generate heat at an even faster rate, increasing the risk of meltdown before the shutdown function can operate properly.

[0006] Therefore, it is necessary to develop polyolefin microporous membranes with excellent heat resistance to withstand rapid temperature increases in batteries. In addition, along with heat resistance, high mechanical strength is required to enhance safety during the battery manufacturing process and operation, while high permeability is required to improve capacity and output. Prior art literature

[0007] Republic of Korea Published Patent Application No. 10-2011-0101202 The problem to be solved

[0008] According to one aspect of the present disclosure, a polyolefin microporous membrane with improved heat resistance at high temperatures, a method for manufacturing the same, and a separation membrane comprising said microporous membrane can be provided.

[0009] According to another aspect of the present disclosure, a polyolefin microporous membrane with improved mechanical strength and permeability, a method for manufacturing the same, and a separation membrane comprising said microporous membrane can be provided.

[0010] The polyolefin microporous membranes and separators of the present disclosure can be widely applied in green technology fields such as electric vehicles, battery charging stations, and other applications utilizing batteries, such as solar power generation and wind power generation. Furthermore, the polyolefin microporous membranes and separators of the present disclosure can be used in eco-friendly electric vehicles, hybrid vehicles, etc., to prevent climate change by suppressing air pollution and greenhouse gas emissions. means of solving the problem

[0011] The present disclosure describes a viscosity-average molecular weight of 1×10 6 g / mol to 3×10 6 60% to 80% by weight of polypropylene with a g / mol and a weight-average molecular weight of 1X10 5 g / mol to 10×10 5It contains 20% to 40% by weight of polyethylene with a g / mol content, has a puncture strength of 0.25 N / ㎛ or higher, and a gas permeability of 1.0×10⁻⁶ -5 The present invention provides a polyolefin microporous membrane having a Darcy or higher, a porosity of 30% to 70%, an average pore size of 20 nm to 40 nm, a shutdown temperature of 150 °C or lower, and a meltdown temperature of 180 °C or higher.

[0012] In one embodiment, the polypropylene may have a melting temperature of 160°C or higher.

[0013] In one embodiment, the polyethylene may have a melting temperature of 133°C or higher.

[0014] In one embodiment, the thickness of the microporous membrane may be 3 μm to 30 μm.

[0015] In one embodiment, the shutdown temperature may be 149°C or lower.

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

[0017] Additionally, the present disclosure comprises: (a) a step of preparing a melt by melt-kneading a mixture containing a polyolefin resin and a diluent through an extruder; (b) a step of extruding the melt to form a sheet; (c) a step of sequentially biaxially stretching the sheet in the longitudinal and transverse directions to form a film; (d) a step of extracting the diluent from the stretched film and drying it; and (e) a step of heat-treating the dried film. In this case, the polyolefin resin in step (a) has a viscosity-average molecular weight of 1×10⁻⁶ 6 g / mol to 3×10 6 60% to 80% by weight of polypropylene with a g / mol and a weight-average molecular weight of 1X10 5 g / mol to 10×10 5It contains 20% to 40% by weight of polyethylene with a g / mol content.

[0018] In one embodiment, step (d) may involve extracting a diluent from a stretched film and drying the film by shrinking it by 5% or less in the longitudinal direction and 10% or less in the transverse direction.

[0019] In one embodiment, step (e) may involve heat-treating the dried film at a temperature range from the melting temperature (Tm) of the polyethylene to a temperature 8°C higher than the melting temperature (Tm + 8°C).

[0020] In addition, the present disclosure provides a separation membrane comprising a polyolefin microporous membrane as described above. Effects of the invention

[0021] The polyolefin microporous membrane according to the present disclosure can secure significantly improved heat resistance at high temperatures.

[0022] In addition, the polyolefin microporous membrane according to the present disclosure has pores that become blocked at low temperatures and can maintain the shape of a separator at high temperatures.

[0023] In addition, the polyolefin microporous membrane according to the present disclosure may have a shutdown temperature of 150°C or lower and a meltdown temperature of 180°C or higher.

[0024] In addition, the polyolefin microporous membrane according to the present disclosure can have excellent mechanical strength and gas permeability.

[0025] In addition, the present disclosure can provide a battery having excellent high-temperature thermal stability by including a polyolefin microporous membrane according to one embodiment. Specific details for implementing the invention

[0026] The embodiments described in this specification may be modified in various different forms, and the technology according to one embodiment is not limited to the embodiments described below. Furthermore, the embodiment of one example is provided to more fully explain the present disclosure to those skilled in the art.

[0027] Additionally, the singular form used in the specification and the appended claims may be intended to include the plural form unless specifically indicated otherwise in the context.

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

[0029] Furthermore, throughout the specification, the term "comprising" a component means that, unless specifically stated otherwise, it does not exclude other components but rather may include additional components.

[0030] The present disclosure describes a viscosity-average molecular weight of 1×10 6 g / mol to 3×10 6 60% to 80% by weight of polypropylene with a g / mol and a weight-average molecular weight of 1X10 5 g / mol to 10×10 5 It contains 20% to 40% by weight of polyethylene with a g / mol content, has a puncture strength of 0.25 N / ㎛ or higher, and a gas permeability of 1.0×10⁻⁶ -5The present invention provides a polyolefin microporous membrane having a Darcy or higher, a porosity of 30% to 70%, an average pore size of 20 nm to 40 nm, a shutdown temperature of 150 °C or lower, and a meltdown temperature of 180 °C or higher.

[0031] Recently, as secondary batteries have become larger and more high-capacity, there is a demand to meet superior battery performance and safety standards. To this end, separators for secondary batteries are required to possess both higher levels of heat resistance and permeability. To this end, polypropylene, which has a high melting point, is used as a material for secondary battery separators; however, polypropylene microporous membranes manufactured by dry processes are not suitable for use in the secondary battery field due to low pore strength and excessively large and non-uniform pore sizes.

[0032] As a result of repeated research by the inventors, it was confirmed that it is possible to manufacture a polyolefin microporous membrane with excellent pore strength and permeability, while significantly improving heat resistance at high temperatures, by simultaneously achieving the pore strength, gas permeability, porosity, average pore size, shutdown temperature, and meltdown temperature within the aforementioned range.

[0033] A secondary battery according to one embodiment can secure both excellent battery performance and safety by including a polyolefin microporous membrane that simultaneously satisfies the physical properties described above. In particular, the present disclosure can provide a secondary battery comprising a microporous membrane in which pores are clogged at a temperature of 150°C or lower and the shape of the separator can be maintained even at a temperature of 180°C or higher. That is, the polyolefin microporous membrane of the present disclosure is suitable for application in high-output / high-capacity batteries as it simultaneously satisfies the physical properties described above.

[0034] The above polyolefin microporous membrane may be manufactured by a wet method including a sequential biaxial stretching process without departing from the scope of the present disclosure.

[0035] As one embodiment, a polyolefin microporous membrane satisfying the physical properties described above simultaneously can be manufactured by including a specific polyethylene resin and a specific polypropylene resin in a specific compositional ratio. For example, a microporous membrane according to one embodiment has a viscosity-average molecular weight of 1×10 6 g / mol to 3×10 6 60% to 80% by weight of polypropylene with a g / mol and a weight-average molecular weight of 1×10 5 g / mol to 10×10 5 It may contain 20% to 40% by weight of polyethylene with a g / mol. In addition, in one embodiment, the polypropylene may have a melting temperature of 160°C or higher, 162°C or higher, 170°C or lower, 165°C or lower, or a value between these values. For example, the polypropylene may have a melting temperature of 160°C to 170°C or 162°C to 165°C. In addition, in one embodiment, the polyethylene may have a melting temperature of 133°C or higher, 133°C to 140°C or 133°C to 135°C.

[0036] In one embodiment, the polyolefin microporous membrane can be manufactured by including the raw material resin used in a specific composition ratio and performing an extraction / drying process under specific conditions. For example, the polyolefin microporous membrane can be manufactured by extruding and sequentially biaxially stretching a mixture of a diluent dissolved in a polyolefin resin containing polyethylene resin and polypropylene resin in a specific composition ratio to form a film, extracting the diluent from the film, and drying the film at a specific shrinkage rate. For example, the diluent can be extracted from the stretched film, and the film can be dried by shrinking it by 5% or less in the longitudinal direction and 10% or less in the transverse direction. Accordingly, the polyolefin microporous membrane can satisfy all the aforementioned physical properties, possess excellent mechanical strength and permeability, and simultaneously have significantly improved heat resistance at high temperatures.

[0037] As one embodiment, the polyolefin microporous membrane can be manufactured by including the raw material resin used in a specific composition ratio and performing a heat treatment process at a specific temperature. For example, in the mixed polyolefin microporous membrane of polypropylene and polyolefin, the heat treatment process can be performed in a temperature range from the melting temperature (Tm) of polyethylene among the components to a temperature 8°C higher than the melting temperature (Tm + 8°C).

[0038] When the above conditions are combined, the polyolefin microporous membrane of the present disclosure satisfies all the aforementioned physical properties, thereby possessing excellent mechanical strength and permeability, while simultaneously having significantly improved heat resistance at high temperatures. That is, the present disclosure has a puncture strength of 0.25 N / µm or more and a gas permeability of 1.0 × 10⁻⁶ -5 A polyolefin microporous membrane can be provided that is at least Darcy, has a porosity of 30% to 70%, an average pore size of 20 nm to 40 nm, a shutdown temperature of 150 °C or lower, and a meltdown temperature of 180 °C or higher.

[0039] The above-mentioned polyolefin microporous membrane will be described in more detail below.

[0040] In one embodiment, the polypropylene included in the polyolefin microporous membrane has a viscosity-average molecular weight of 1×10 6 g / mol or more, 1.2×10 6 g / mol or more, 1.3×10 6 g / mol or more, 3×10 6 g / mol or less, 2.5×10 6 g / mol or less, 2.2×10 6 g / mol or less, 1.8×10 6 It may be g / mol or less, or a value between the above values. For example, the viscosity-average molecular weight of the polypropylene is 1×10⁻⁶ 6 g / mol to 3×10 6 g / mol, 1.2×10 6 g / mol to 2.5×10 6 g / mol, 1.3×10 6 g / mol to 2.2×10 6 g / mol or 1.3×10 6 g / mol to 1.8×10 6 It can be g / mol.

[0041] In one embodiment, the content of polypropylene included in the polyolefin microporous membrane may be 60 wt% or more, 65 wt% or more, 70 wt% or more, 80 wt% or less, 75 wt% or less, or a value between the above values, based on the total amount of polypropylene and polyethylene. For example, the content of polypropylene may be 60 wt% to 80 wt%, 65 wt% to 75 wt%, or 70 wt% to 75 wt%. According to one embodiment, if the content of polypropylene satisfies the above range, it may have excellent puncture strength and gas permeability, and may have a shutdown temperature of 150 ℃ or less and a meltdown temperature of 180 ℃ or more.

[0042] In one embodiment, the polypropylene may have a melting temperature of 160°C or higher, 162°C or higher, 170°C or lower, 165°C or lower, or a value between these values. For example, the melting temperature of the polypropylene may be 160°C to 170°C or 162°C to 165°C. According to one embodiment, if the melting temperature of the polypropylene satisfies the above range, it may have excellent puncture strength and gas permeability, a shutdown temperature of 150°C or lower, and a meltdown temperature of 180°C or higher.

[0043] In one embodiment, the polyethylene included in the polyolefin microporous membrane has a weight-average molecular weight of 1×10 5 g / mol or more, 3×10 5 g / mol or more, 10×10 5g / mol or less, 9×10 5 It may be g / mol or less, or a value between the above values. For example, the weight-average molecular weight of the polyethylene is 1×10⁻⁶ 5 g / mol to 10×10 5 g / mol, 3×10 5 g / mol to 10×10 5 g / mol or 3×10 5 g / mol to 9×10 5 It can be g / mol.

[0044] In one embodiment, the content of polyethylene included in the polyolefin microporous membrane may be 20 wt% or more, 25 wt% or more, 40 wt% or less, 35 wt% or less, or a value between the above values, based on the total amount of polypropylene and polyethylene. For example, the content of polyethylene may be 20 wt% to 40 wt% or 25 wt% to 35 wt%. According to one embodiment, if the content of polyethylene satisfies the above range, it may have excellent puncture strength and gas permeability, and may have a shutdown temperature of 150 ℃ or less and a meltdown temperature of 180 ℃ or more.

[0045] In one embodiment, the polyethylene may have a melting temperature of 133°C or higher, 133°C to 140°C, or 133°C to 135°C. According to one embodiment, if the melting temperature of the polyethylene satisfies the above range, it may have excellent puncture strength and gas permeability, a shutdown temperature of 150°C or lower, and a meltdown temperature of 180°C or higher.

[0046] In one embodiment, the polyolefin microporous membrane may have a puncture strength of 0.25 N / ㎛ or more, 0.30 N / ㎛ or more, 1.0 N / ㎛ or less, 0.8 N / ㎛ or less, 0.5 N / ㎛ or less, or a value between the above values. For example, the puncture strength may be 0.25 N / ㎛ to 1.0 N / ㎛, 0.30 N / ㎛ to 0.8 N / ㎛, or 0.30 N / ㎛ to 0.5 N / ㎛. By satisfying the puncture strength within the above-described range, excellent resistance to external stresses generated during battery manufacturing and dendrites generated during battery charging and discharging is achieved, thereby ensuring battery safety.

[0047] In one embodiment, the polyolefin microporous membrane has a gas permeability of 1.0 × 10⁻⁶ -5 Darcy, above, 1.15×10 -5 Darcy or higher, 5.0×10 -5 Darcy and below, 3.0×10 -5 It may be less than or equal to Darcy, or a value between the above figures. For example, the gas permeability is 1.0 × 10⁻⁶. -5 Darcy to 5.0×10 -5 Darcy or 1.15×10 -5 Darcy to 3.0×10 -5 It may be Darcy. By satisfying the gas permeability within the aforementioned range, it can have excellent ion conductivity, and the charge / discharge characteristics of the battery can be improved due to the low internal resistance of the battery.

[0048] In one embodiment, the polyolefin microporous membrane may have a porosity of 30% or more, 35% or more, 37% or more, 70% or less, 60% or less, 50% or less, or a value between the above values. For example, the porosity may be 30% to 70%, 35% to 60%, or 37% to 50%.

[0049] In one embodiment, the polyolefin microporous membrane may have an average pore size of 20 nm or more, 24 nm or more, 40 nm or less, 35 nm or less, or a value between the above values. For example, the average pore size may be 20 nm to 40 nm or 24 nm to 35 nm.

[0050] In one embodiment, the polyolefin microporous membrane may have a shutdown temperature of 150°C or lower, 149.5°C or lower, 140°C to 150°C, 145°C to 149.5°C, 145°C to 149°C, or 145°C to 148.5°C. By satisfying the shutdown temperature within the above-described range, excellent safety can be ensured even if a malfunction occurs in the battery to which it is applied and a temperature rise occurs.

[0051] In one embodiment, the polyolefin microporous membrane may have a meltdown temperature of 180°C or higher, or between 180°C and 190°C. By satisfying the meltdown temperature within the above-described range, even if the temperature of the battery to which it is applied rises rapidly, internal short circuits caused by the rupture of the microporous membrane can be suppressed, thereby ensuring excellent safety.

[0052] In one embodiment, the thickness of the polyolefin microporous membrane may be 3 μm or more, 5 μm or more, 30 μm or less, 20 μm or less, 15 μm or less, or a value between these values. For example, the thickness of the microporous membrane may be 3 μm to 30 μm, 3 μm to 20 μm, or 5 μm to 15 μm. The microporous membrane of the present disclosure can simultaneously achieve excellent puncture strength, gas permeability, porosity, average pore size, and heat resistance even under the thickness ranges described above, and in particular, can have a shutdown temperature of 150 °C or less and a meltdown temperature of 180 °C or more. Accordingly, according to one embodiment, thinning of the separator for a secondary battery is possible, making it suitable for application in high-capacity / high-output batteries.

[0053] In one embodiment, the polyolefin microporous membrane may be manufactured by a wet method including a sequential biaxial stretching process, thereby providing a polyolefin microporous membrane that simultaneously satisfies the physical properties described above. Accordingly, the polyolefin microporous membrane satisfies all the physical properties described above, possessing excellent mechanical strength and permeability, while simultaneously having significantly improved heat resistance at high temperatures.

[0054] Specifically, the polyolefin microporous membrane is manufactured by extruding and sequentially biaxially stretching a mixture of polyolefin resin containing polyethylene resin and polypropylene resin in a specific compositional ratio to form a film, and extracting the diluent from the film. It can be manufactured by a wet method including a conventional sequential biaxial stretching process known to those skilled in the art, and is not limited to cases where a microporous membrane having the physical properties described above can be manufactured.

[0055] In one embodiment, a diluent may be extracted from a sequentially biaxially stretched film and the film may be dried by shrinking it by 5% or less in the longitudinal direction and 10% or less in the transverse direction, but may be performed in other ways without departing from the scope of the present disclosure.

[0056] As one embodiment, the dried film obtained by the extraction / drying process may be heat-treated at a temperature range from the melting temperature (Tm) of the polyethylene to a temperature 8°C higher than the melting temperature (Tm + 8°C), but may be performed in other ways without departing from the scope of the present disclosure.

[0057] Hereinafter, a method for manufacturing a polyolefin microporous membrane of the present disclosure will be described.

[0058] A method for manufacturing a polyolefin microporous membrane according to one embodiment comprises: (a) a step of preparing a melt by melt-kneading a mixture containing a polyolefin resin and a diluent through an extruder; (b) a step of extruding the melt to form a sheet; (c) a step of sequentially biaxially stretching the sheet in the longitudinal and transverse directions to form a film; (d) a step of extracting the diluent from the stretched film and drying it; and (e) a step of heat-treating the dried film; wherein the polyolefin resin in step (a) has a viscosity-average molecular weight of 1×10 6 g / mol to 3×10 6 60% to 80% by weight of polypropylene with a g / mol and a weight-average molecular weight of 1×10 5 g / mol to 10×10 5 It may contain 20% to 40% by weight of polyethylene with a g / mol.

[0059] Each manufacturing step is described below.

[0060] First, step (a) is a step of preparing a melt by melt-kneading a mixture containing a polyolefin resin and a diluent through an extruder, wherein the mixture may contain the polyolefin resin and the diluent in a weight ratio of 10 to 60: 90 to 40 for pore formation, or in a weight ratio of 20 to 40: 80 to 60, but is not particularly limited thereto as long as it does not deviate from the scope of the present disclosure.

[0061] In step (a) above, the polyolefin resin has a viscosity-average molecular weight of 1×10 6 g / mol to 3×10 6 60% to 80% by weight of polypropylene with a g / mol and a weight-average molecular weight of 1×10 5 g / mol to 10×10 5It may contain 20% to 40% by weight of polyethylene with a g / mol. For example, it may contain 65% to 75% by weight of the polypropylene and 25% to 35% by weight of the polyethylene. According to one embodiment, if the polyolefin resin satisfies the above composition, the puncture strength is 0.25 N / μm or higher, and the gas permeability is 1.0×10⁻⁶ -5 It is possible to manufacture a polyolefin microporous membrane having a Darcy or higher, a porosity of 30% to 70%, an average pore size of 20 nm to 40 nm, a shutdown temperature of 150 °C or lower, and a meltdown temperature of 180 °C or higher. A secondary battery containing a microporous membrane satisfying all of these properties can secure both excellent battery performance and safety.

[0062] As the viscosity-average molecular weight (or weight-average molecular weight) and melting temperature of the above-mentioned polypropylene and polyethylene are the same as those described above, a detailed explanation is omitted.

[0063] In one embodiment, the diluent can be used without limitation as long as it is any organic compound that forms a single phase with the polyolefin resin at the extrusion temperature. For example, the above 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 embodiment of the above diluent may be paraffin oil having a kinetic viscosity of 20 cSt to 200 cSt at 40°C, but may be substituted with other types of diluents without departing from the scope of the present disclosure.

[0064] In addition, the above mixture may further include one or more of general additives for enhancing specific functions, such as oxidation stabilizers, UV stabilizers, and antistatic agents, within a range where the properties of the microporous membrane are not significantly degraded.

[0065] Next, step (b) is a step of extruding the molten material to form a sheet, which can be performed without limitation by methods known to those skilled in the art. For example, the molten material may be extruded through a T-die and formed into a sheet by casting or calendering while cooling to a temperature of 10°C to 80°C, but may be performed in other ways without departing from the scope of the present disclosure.

[0066] Next, step (c) is a step of forming the sheet into a film by sequentially biaxially stretching it in the longitudinal and transverse directions, wherein the stretching ratios in the longitudinal and transverse directions may each be independently 4 times or more, 6 times or more, 10 times or less, 15 times or less, or values ​​between the above figures. For example, the stretching ratios in the longitudinal and transverse directions may be 4 times to 15 times or 6 times to 10 times. As the stretching ratios for the longitudinal and transverse directions satisfy the above-described ranges, a polyolefin microporous membrane having the physical properties as described above can be manufactured.

[0067] (c) The stretching in step (c) is performed using a sequential stretching method of a roll method or a tenter method, and can be performed at a temperature ranging from 80°C lower than the melting temperature of polypropylene to the melting temperature of polypropylene. When stretching is performed at the temperature range described above, the fluidity of the polypropylene resin can be secured to enable effective stretching. Specifically, stretching can be performed uniformly over the entire sheet without fracture due to stretching, allowing for stable stretching; accordingly, a high-quality microporous membrane can be manufactured with properties such as uniform gas permeability and puncture strength across the entire membrane. For example, the stretching can be performed at 100°C to 170°C or 110°C to 160°C, but is not limited thereto.

[0068] Next, step (d) is a step of extracting and drying the diluent from the stretched film, The process is carried out by extracting the diluent inside the film using an organic solvent and drying the organic solvent from the film in which the diluent has been replaced with the organic solvent. The organic solvent is not particularly limited and can be used as long as it can extract the diluent. For example, methyl ethyl ketone, methylene chloride, hexane, etc., may be used as the organic solvent due to their high extraction efficiency and rapid drying, but the process may be carried out in other ways without departing from the scope of the present disclosure.

[0069] In one embodiment, step (d) may involve extracting a diluent from a stretched film and shrinking and drying the film by 5% or less in the longitudinal direction and 10% or less in the transverse direction. When a diluent is extracted from a film made of a polyolefin resin satisfying the composition described above and the film is shrunk and dried under the conditions described above, a microporous membrane can be provided that simultaneously satisfies the perforation strength, gas permeability, porosity, average pore size, shutdown temperature, and meltdown temperature as described above. A secondary battery comprising such a microporous membrane can secure both excellent battery performance and safety.

[0070] In one embodiment, the longitudinal shrinkage rate during the extraction / drying process may be 5% or less, 4% or less, 3.5% or less, or 1% or more, for example, 1% to 5%, 1% to 4%, or 1% to 3.5%. The transverse shrinkage rate during the extraction / drying process may be 10% or less, 9% or less, 8.5% or less, or 1% or more, for example, 1% to 10%, 1% to 9%, or 1% to 8.5%. At this time, the longitudinal (or transverse) shrinkage rate may be calculated by the following Equation 1.

[0071] [Mathematical Formula 1]

[0072] Longitudinal (or transverse) shrinkage rate (%) = [{Longitudinal (or transverse) length before extraction - Longitudinal (or transverse) length after drying} / Longitudinal (or transverse) length before extraction] × 100

[0073] Longitudinal or transverse shrinkage during the above extraction / drying process can be controlled by the tension applied to the film during the drying process. If the tension is high, shrinkage occurs less, and if the tension is low, shrinkage may occur more. The tension applied to the film during the drying process can be determined differently depending on the thickness of the film, and its magnitude can be appropriately changed as long as it does not depart from the scope of the present disclosure.

[0074] In one embodiment, step (d) may be performed at a temperature of 40°C or lower, but may be performed at other temperatures without departing from the scope of the present disclosure.

[0075] (e) Step is a heat treatment step of the dried film, and heat is applied while holding the film in a roll or tenter-type device so that no dimensional changes occur in the longitudinal and transverse directions, thereby removing residual stress within the film and improving the heat resistance of the final polyolefin microporous membrane.

[0076] In one embodiment, step (e) may involve heat-treating the dried film in a temperature range from the melting temperature (Tm) of the polyethylene to a temperature 8°C higher than the melting temperature (Tm + 8°C). When a film made of a polyolefin resin satisfying the composition described above is heat-treated under the temperature conditions described above, a microporous membrane can be provided that simultaneously satisfies the puncture strength, gas permeability, porosity, average pore size, shutdown temperature, and meltdown temperature as described above. A secondary battery comprising such a microporous membrane can secure both excellent battery performance and safety.

[0077] In one embodiment, the heat treatment of step (e) may be performed at a temperature in the range of 133 ℃ to 148 ℃, or at a temperature in the range of 133 ℃ to 145 ℃, or at a temperature in the range of 134 ℃ to 142 ℃. As the heat treatment is performed at the temperatures in the above-described ranges, a polyolefin microporous membrane satisfying all the physical properties described above can be produced.

[0078] In one embodiment, the heat treatment of step (e) may be heat-stretched and / or heat-relaxed to heat-set. That is, the heat treatment can be performed in various ways by adjusting the tension during the heat treatment. The heat treatment may be repeated 1 to 3 times, but is not necessarily limited thereto.

[0079] The above step (e) may include, for example, a heat stretching process for stretching in the longitudinal or transverse direction, a heat setting process for fixing the length / width in the longitudinal and transverse directions and applying heat, and a heat relaxing process for relaxing (shrinking) in the longitudinal or transverse direction. For example, the heat relaxing process may relax to 80% to 99% or 90% to 99% of the transverse width prior to the heat relaxing process, and the heat stretching process may stretch to 120% to 160% or 140% to 160% of the transverse width prior to the heat stretching process, but may be performed in other ways without departing from the scope of the present disclosure.

[0080] The present disclosure provides a separator comprising a polyolefin microporous membrane as described above, wherein the separator may be a separator used in any known energy storage device and is not particularly limited, but a non-limiting example may be a separator used in a lithium secondary battery.

[0081] In the following, embodiments of the present disclosure are further described with reference to specific experimental examples. The embodiments and comparative examples included in the experimental examples are merely illustrative of the present disclosure and are not intended to limit the appended claims. It is obvious to those skilled in the art that various changes and modifications to the embodiments are possible within the scope and spirit of the present disclosure, and that such variations and modifications fall within the scope of the appended claims.

[0082] [Methods for Measuring Physical Properties]

[0083] 1. Viscosity-average molecular weight (g / mol)

[0084] Viscosity-average molecular weight (Mv) was calculated by measuring the intrinsic viscosity (η) using the Polymer Char Crystex QC model and using the Margolies-equation in Equation 2 below. The intrinsic viscosity was measured using 1,2,4-trichlorobenzene (TCB) as the solvent.

[0085] [Mathematical Formula 2]

[0086] Mv = 5.37 × 10 4 × [η] 1.49

[0087] 2. Weight-average molecular weight (g / mol)

[0088] The weight-average molecular weight (Mw) was measured using a high-temperature Gel Permeation Chromatography (GPC) from Agilent Technologies. PLgel Guard and PLgel Olexis were used as the GPC columns, 1,2,4-trichlorobenzene (TCB) was used as the solvent, and polystyrene was used as the standard sample; the analysis was performed at 140 °C.

[0089] 3. Melting temperature (°C)

[0090] The melting temperature was determined by the peak position on the curve measured by differential scanning calorimetry (DSC). Using the Thaermal Analysis System DSC 3+ model, a differential scanning calorimeter (DSC) from Mettler Toledo, the temperature range from 25 ℃ to 200 ℃ was scanned under a nitrogen atmosphere at a heating rate of 10 ℃ / min. The amount of sample measured was 5 mg.

[0091] 4. Thickness of the microporous membrane (㎛)

[0092] The thickness of the microporous membrane is measured using a contact-type thickness gauge with a thickness precision of 0.1 μm. It is measured using a TESA Mu-Hite Electronic Height Gauge from TESA with a measurement pressure of 0.63 N.

[0093] 5. Piercing strength (N / ㎛)

[0094] The puncture strength is measured by attaching a pin tip with a diameter of 1.0 mm and a radius of curvature of 0.5 mm to INSTRON’s UTM (Universal Test Machine) 3345 and pressing the microporous membrane at a speed of 120 mm / min. At this time, the puncture strength is calculated by dividing the load (N) at which the microporous membrane breaks by the thickness (㎛) of the microporous membrane.

[0095] 6. Gas Permeability (Darcy)

[0096] Gas permeability is measured using a porometer (PMI’s CFP-1500-AEL). Generally, gas permeability is expressed in the Gurley number, but the Gurley number does not correct for the influence of thickness, making it difficult to determine the relative permeability according to the pore structure. To solve this, the gas permeability of the present disclosure is measured using the Darcy permeability constant calculated from the following Equation 3. Nitrogen is used as the gas, and the average value of the Darcy permeability constant measured in the 100 to 200 psi range is calculated.

[0097] [Mathematical Formula 3]

[0098] Darcy transmittance constant (C) = (8 F·T·V) / (πD 2 (P 2 -1))

[0099] F = flow velocity

[0100] T = Sample thickness

[0101] V = Gas viscosity (0.185 for N2)

[0102] D = Sample diameter

[0103] P = pressure

[0104] 7. Porosity (%)

[0105] The porosity of the microporous membrane was calculated using the following mathematical formula 4. Specifically, a sample with a width of A cm, a length of B cm, and a thickness of T cm was prepared, and its mass was measured to calculate the porosity through the ratio of the mass of the resin and the mass of the microporous membrane of the same volume.

[0106] [Mathematical Formula 4]

[0107] Porosity (%) = 100×{1-M / (A×B×T×ρ)}

[0108] In the above mathematical formula 4, M is the mass (g) of the microporous membrane, and ρ is the density (g / cm³) of the polyolefin resin forming the microporous membrane.

[0109] 8. Average pore size (nm)

[0110] The average pore size was measured using a pore meter (PMI CFP-1500-AEL) in accordance with ASTM F316-03. The average pore size was measured using the half-dry method, and Galwick solution (surface tension: 15.9 dyne / cm) provided by PMI was used to measure the pore size.

[0111] 9. Shutdown temperature and meltdown temperature

[0112] The shutdown and meltdown temperatures of the microporous membrane were measured using a self-made electrical resistance measuring cell. The electrical resistance measuring cell was fabricated as follows: A microporous membrane prepared with dimensions of 2.5 cm × 2.5 cm was placed between two glass cells, and the joints of the glass cells were firmly secured with clips to prevent movement. Next, an electrolyte solution containing 1 M hexafluorophosphate (LiPF6) dissolved in propylene carbonate (PC) solvent was injected into each glass cell with care to prevent the formation of bubbles. Aluminum electrodes were placed in each glass cell to fabricate the final electrical resistance measuring cell. The fabricated electrical resistance measuring cell was placed in an oil bath preheated to 120 °C, and the two electrodes were connected to an impedance analyzer (ZIVE SP2 from ZIVE Lab). The resistance value of the cell was measured using an alternating current of 1 kHz (5 mV) while increasing the temperature of the oil bath at a rate of 2 °C. The cell resistance value was plotted against temperature to measure the shutdown temperature and the meltdown temperature. The shutdown temperature was defined as the temperature at which the resistance value became 1000 Ω or higher, and the meltdown temperature was defined as the temperature at which the resistance value became 1000 Ω or lower again after the shutdown temperature.

[0113] <Example 1>

[0114] The weight-average molecular weight is 6.0 × 10⁻⁶ 5 25 wt% of polyethylene resin having a g / mol and a melting temperature of 134 °C and a viscosity-average molecular weight of 1.5 × 10 6 A polyolefin resin was prepared consisting of 75% by weight of a polypropylene resin having a g / mol and a melting temperature of 164°C. Next, a mixture containing the polyolefin resin and paraffin oil having 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 melt.

[0115] The above molten material was continuously extruded through a T-die, and a sheet with a width of 300 mm and an average thickness of 1200 μm was manufactured using a casting roll set to 30°C.

[0116] The above sheet was longitudinally stretched by a roll method to a length of 6 times at a stretching temperature of 115°C, and then guided to a tenter to transversely stretch to a length of 6 times at a stretching temperature of 140°C.

[0117] Paraffin oil was extracted from a film that had been stretched in the longitudinal and transverse directions at 25°C using methylene chloride. The film from which paraffin oil was extracted was dried at 60°C while adjusting the tension in the opposite direction of the shrinkage stress occurring during the drying process after extraction so that it would shrink by 2% in the longitudinal direction and 6% in the transverse direction compared to the film before extraction.

[0118] A polyolefin microporous membrane with a thickness of 10.5 μm was prepared by heat-setting the dried film at 140 ℃ for about 20 seconds using a tenter-type heat-setting device. The properties of the finally prepared microporous membrane are listed in Table 1 below.

[0119] <Example 2>

[0120] A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that a polyolefin resin composed of 20 wt% polyethylene resin and 80 wt% polypropylene resin was used instead of the polyolefin resin of Example 1, and the results are listed in Table 1.

[0121] <Example 3>

[0122] A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that a polyolefin resin composed of 40 wt% polyethylene resin and 60 wt% polypropylene resin was used instead of the polyolefin resin of Example 1, and the results are listed in Table 1.

[0123] <Example 4>

[0124] Instead of the polyethylene resin of Example 1, a weight-average molecular weight of 3.0 × 10 5 A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that a polyethylene resin with a g / mol and a melting temperature of 134 ℃ was used, and the results are listed in Table 1.

[0125] <Example 5>

[0126] Instead of the polyethylene resin of Example 1, a weight-average molecular weight of 10×10 5 A polyethylene resin having a g / mol and a melting temperature of 134 °C was used, and instead of the polypropylene resin of Example 1, a viscosity-average molecular weight of 2.0 × 10⁻⁶ 6 A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that a polypropylene resin with a g / mol and a melting temperature of 164 ℃ was used, and the results are listed in Table 1.

[0127] <Example 6>

[0128] A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that the paraffin oil-extracted film was dried to shrink by 3% in the longitudinal direction and 8% in the transverse direction compared to the film before extraction, and the results are listed in Table 1.

[0129] <Example 7>

[0130] A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that the paraffin oil-extracted film was dried to shrink by 5% in the longitudinal direction and 10% in the transverse direction compared to the film before extraction, and the results are listed in Table 1.

[0131] <Example 8>

[0132] A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that the heat setting temperature was 142 ℃, and the results are listed in Table 1.

[0133] <Example 9>

[0134] A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that the heat setting temperature was 136 ℃, and the results are listed in Table 1.

[0135] <Comparative Example 1>

[0136] A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that a polyolefin resin composed of 10 wt% polyethylene resin and 90 wt% polypropylene resin was used instead of the polyolefin resin of Example 1, and the results are listed in Table 2.

[0137] <Comparative Example 2>

[0138] A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that a polyolefin resin composed of 18 wt% polyethylene resin and 82 wt% polypropylene resin was used instead of the polyolefin resin of Example 1, and the results are listed in Table 2.

[0139] <Comparative Example 3>

[0140] A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that a polyolefin resin composed of 45 wt% polyethylene resin and 55 wt% polypropylene resin was used instead of the polyolefin resin of Example 1, and the results are listed in Table 2.

[0141] <Comparative Example 4>

[0142] Instead of the polyethylene resin of Example 1, a weight-average molecular weight of 1.1×10 6A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that a polyethylene resin with a g / mol and a melting temperature of 134 ℃ was used, and the paraffin oil extracted film was dried to shrink by 4% in the longitudinal direction and 9% in the transverse direction compared to the film before extraction, and the results are listed in Table 2.

[0143] <Comparative Example 5>

[0144] Instead of the polyethylene resin of Example 1, a weight-average molecular weight of 6.0×10 5 A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that a polyethylene resin with a g / mol and a melting temperature of 132 ℃ was used and the heat setting temperature was set to 138 ℃, and the results are listed in Table 2.

[0145] <Comparative Example 6>

[0146] Instead of the polypropylene resin of Example 1, a viscosity-average molecular weight of 1.5×10 6 A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that a polypropylene resin with a g / mol and a melting temperature of 158 ℃ was used, and the results are listed in Table 2.

[0147] <Comparative Example 7>

[0148] A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that the paraffin oil-extracted film was dried to shrink by 7% in the longitudinal direction and 14% in the transverse direction compared to the film before extraction, and the results are listed in Table 2.

[0149] <Comparative Example 8>

[0150] A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that the paraffin oil-extracted film was dried to be stretched 5% in the longitudinal direction and shrunk 8% in the transverse direction compared to the film before extraction, and the heat setting temperature was 136 ℃, and the results are listed in Table 2.

[0151] <Comparative Example 9>

[0152] A polyolefin microporous membrane was prepared in the same manner as in Example 1, except that the heat setting temperature was set to 145 ℃, and the results are listed in Table 2.

[0153]

[0154]

[0155] Referring to Table 1 above, it can be seen that the microporous membranes of Examples 1 to 9 not only have perforation strength, gas permeability, porosity, and average pore size that can secure the performance of the battery, but also have shutdown temperature and meltdown temperature that can secure the thermal safety of the battery.

[0156] On the other hand, the microporous membranes of Comparative Example 1 and Comparative Example 2 used more than 80 wt% of polypropylene resin, resulting in shutdown temperatures of 158.4°C and 155.9°C, respectively. Accordingly, the battery to which these are applied has the disadvantage that thermal stability is significantly reduced as the pores close at high temperatures.

[0157] As a result of using less than 60 weight% of polypropylene resin in the microporous membrane of Comparative Example 3, it failed to satisfy the range of pore strength and gas permeability intended to be achieved by the present disclosure. Accordingly, a battery to which this is applied has the disadvantage that it cannot achieve performance as a battery.

[0158] The microporous membrane of Comparative Example 4 has an average molecular weight of 1.0 × 10⁻⁶ 6As a result of using polyethylene resin in an amount exceeding g / mol, the range of gas permeability and shutdown temperature intended to be achieved by the present disclosure was not satisfied. Accordingly, a battery to which this is applied cannot achieve performance as a battery and has the disadvantage of significantly reduced thermal safety.

[0159] As a result of using a polyethylene resin with a melting temperature of less than 133°C for the microporous membrane of Comparative Example 5, it failed to satisfy the range of gas permeability intended to be achieved by the present disclosure. Accordingly, the battery to which it is applied has the disadvantage that it cannot achieve performance as a battery.

[0160] As a result of using a polypropylene resin with a melting temperature of less than 160°C for the microporous membrane of Comparative Example 6, the meltdown temperature range intended to be achieved by the present disclosure was not satisfied. Accordingly, the battery to which this is applied has the disadvantage that the thermal safety is significantly reduced due to the internal short circuit between the negative and positive electrodes caused by the rupture of the separator.

[0161] The microporous membrane of Comparative Example 7 had a longitudinal shrinkage of more than 5% and a transverse shrinkage of more than 10%, and as a result of performing the extraction / drying process, it failed to satisfy the range of gas permeability and porosity intended to be achieved by the present disclosure. Accordingly, the battery to which it is applied has the disadvantage that it cannot achieve performance as a battery.

[0162] As a result of performing the extraction / drying process by stretching the microporous membrane of Comparative Example 8 without shrinking longitudinally by 5% or less, it failed to satisfy the range of average pore size and shutdown temperature intended to be achieved by the present disclosure. Accordingly, a battery to which this is applied cannot achieve performance as a battery and has the disadvantage of significantly reduced thermal stability.

[0163] As a result of performing a heat-setting process on the microporous membrane of Comparative Example 9 at a temperature 11°C higher than the melting temperature of the polyethylene resin, it failed to satisfy the range of gas permeability and porosity intended to be achieved by the present disclosure. Accordingly, the battery to which it is applied has the disadvantage that it cannot achieve performance as a battery.

[0164] The above description is merely an example of applying the principles of the present disclosure, and other configurations may be included without departing from the scope of the present disclosure.

Claims

Claim 1 Viscosity-average molecular weight is 1×10 6 g / mol to 3×10 6 60% to 80% by weight of polypropylene with a g / mol and a weight-average molecular weight of 1×10 5 g / mol to 10×10 5 It contains 20% to 40% by weight of polyethylene with a g / mol content, has a puncture strength of 0.25 N / ㎛ or higher, and a gas permeability of 1.0×10⁻⁶ -5 A polyolefin microporous membrane having a Darcy or higher, a porosity of 30% to 70%, an average pore size of 20 nm to 40 nm, a shutdown temperature of 150 °C or lower, and a meltdown temperature of 180 °C or higher. Claim 2 In claim 1, the polypropylene is a polyolefin microporous membrane having a melting temperature of 160°C or higher. Claim 3 In claim 1, the polyethylene is a polyolefin microporous membrane having a melting temperature of 133°C or higher. Claim 4 In claim 1, a polyolefin microporous membrane having a thickness of 3 μm to 30 μm. Claim 5 A polyolefin microporous membrane according to claim 1, wherein the shutdown temperature is 149 ℃ or lower. Claim 6 A polyolefin microporous membrane according to claim 1, manufactured by a wet method including a sequential biaxial stretching process. Claim 7 (a) a step of preparing a melt by melt-kneading a mixture containing a polyolefin resin and a diluent through an extruder; (b) a step of extruding the melt to form a sheet; (c) a step of sequentially biaxially stretching the sheet in the longitudinal and transverse directions to form a film; (d) a step of extracting the diluent from the stretched film, shrinking the film by 5% or less in the longitudinal direction and 10% or less in the transverse direction, and drying it; and (e) a step of heat-treating the dried film; wherein the polyolefin resin in step (a) has a viscosity-average molecular weight of 1×10 6 g / mol to 3×10 6 60% to 80% by weight of polypropylene having g / mol and a melting temperature of 160°C or higher, and a weight-average molecular weight of 1×10 5 g / mol to 10×10 5 A method for manufacturing a polyolefin microporous membrane, comprising 20% ​​to 40% by weight of polyethylene having a g / mol and a melting temperature of 133°C or higher, wherein step (e) involves heat treatment in a temperature range from the melting temperature (Tm) of the polyethylene to a temperature 8°C higher than the melting temperature (Tm + 8°C). Claim 8 delete Claim 9 delete Claim 10 A separation membrane comprising a polyolefin microporous membrane according to any one of claims 1 to 6.

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