Polyolefin microporous membrane, method for manufacturing the same, and separator including microporous membrane

Abstract

To provide a polyolefin microporous membrane having improved heat resistance at a high temperature, a method for manufacturing the same, and a separator including the microporous membrane.SOLUTION: In an embodiment, a polyolefin microporous membrane includes: 60 wt.% to 80 wt.% of a polypropylene having a viscosity average molecular weight of 1×106 g / mol to 3×106 g / mol and 20 wt.% to 40 wt.% of a polyethylene having a weight average molecular weight of 1×105 g / mol to 10×105 g / mol, wherein the polyolefin microporous membrane has a puncture strength of 0.25 N / μm or more, a gas permeability of 1.0×10-5Darcy or more, 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.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a polyolefin microporous membrane, a method for producing the same, and a separator including the microporous membrane.

Background Art

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

[0003] In recent years, secondary batteries have been increased in capacity and size for application to electric vehicles, ESS (Energy storage system), etc., and ensuring the safety of the battery has become an even more important factor. For example, when the battery is exposed to or operated in a high-temperature environment, the separator may shrink and cause 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 have a shutdown function as a function for ensuring battery safety. The shutdown function is a function that significantly increases the resistance of the separator by melting the polyolefin and closing the pores. Even when an abnormality occurs in the battery and the temperature rises, the internal resistance of the battery increases due to this function, so that substantially no current flows and safety is ensured.

[0005] When the battery is further exposed to high temperatures, the separator breaks down (melt-down), causing an internal short circuit between the positive and negative electrodes, so the possibility of fire becomes very high. In particular, secondary batteries with increased capacity and size have a higher heat generation rate, so there is also a higher risk of melt-down before the shutdown function fully operates.

[0006] Therefore, there is a need to develop a polyolefin microporous membrane with excellent heat resistance that can prepare for a rapid temperature rise of the battery. Also, in order to improve the safety during the manufacturing process and use of the battery along with heat resistance, high mechanical strength is required, and high permeability is required for improving capacity and output.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] According to one aspect of the present disclosure, there is provided a polyolefin microporous membrane with improved heat resistance at high temperatures, a method for manufacturing the same, and a separator including the microporous membrane.

[0009] According to another aspect of the present disclosure, there is provided a polyolefin microporous membrane with improved mechanical strength and permeability, a method for manufacturing the same, and a separator including the microporous membrane.

[0010] The polyolefin microporous membrane and separator of the present disclosure are widely applicable to fields of green technologies such as electric vehicles, battery charging stations, and other solar power generation and wind power generation that use batteries. Also, the polyolefin microporous membrane and separator of the present disclosure can be used in eco-friendly electric vehicles, hybrid vehicles, etc., which suppress air pollution and greenhouse gas emissions and prevent climate change.

Means for Solving the Problems

[0011] This disclosure relates to a polyolefin microporous membrane having a viscosity-average molecular weight of 1×10 6 g / mol to 3×10 660% to 80% by weight of polypropylene with a density of g / mol, and 20% to 40% by weight of polyethylene with a weight average molecular weight of 1×10 5 g / mol to 10×10 5 g / mol, having a puncture strength of 0.25 N / μm or more and 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°C or less, and a melt-down temperature of 180°C or more, provides a polyolefin microporous membrane.

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

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

[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 less.

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

[0017] Further, the present disclosure includes: (a) a step of melt-kneading a mixture containing a polyolefin resin and a diluent by an extruder to produce a melt; (b) a step of extruding the melt to form it into a sheet; (c) a step of sequentially biaxially stretching the sheet in the longitudinal and transverse directions to form it into a film; (d) a step of extracting and drying the diluent from the stretched film; and (e) a step of heat-treating the dried film. At this time, the polyolefin resin in step (a) has a viscosity average molecular weight of 1×10 6 g / mol to 3×10 6 g / mol, 60% to 80% by weight of polypropylene, and a weight average molecular weight of 1×105 g / mol to 10×10 5 It contains 20% to 40% by weight of polyethylene with a density of 5 g / mol.

[0018] In one embodiment, the step (d) may be to extract the diluent from the stretched film and shrink and dry the film by 5% or less in the longitudinal direction and 10% or less in the transverse direction.

[0019] In one embodiment, the step (e) may be to heat-treat 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).

[0020] The present disclosure also provides a separator including the above-described polyolefin microporous membrane.

Advantages of the Invention

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

[0022] Also, the polyolefin microporous membrane according to the present disclosure can have pores blocked at low temperatures and maintain the form of the separator at high temperatures.

[0023] Also, the polyolefin microporous membrane according to the present disclosure can have a shutdown temperature of 150°C or lower and a melt-down temperature of 180°C or higher.

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

[0025] The present disclosure can also provide a battery having excellent high-temperature thermal safety by including the polyolefin microporous membrane according to one embodiment.

Embodiments for Carrying Out the Invention

[0026] The embodiments described in this specification can be modified into various other forms, and the technology according to one embodiment is not limited to the embodiments described below. Also, the embodiments of one embodiment are provided to more fully explain the present disclosure to those with average knowledge in the relevant technical field.

[0027] Also, the singular forms used in the specification and the appended claims are intended to include the plural forms as well, unless the context clearly indicates otherwise.

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

[0029] Furthermore, throughout the specification, when a component is described as "including", it means that it can further include other components, rather than excluding other components, unless there is a contrary description.

[0030] The present disclosure provides a polyolefin microporous membrane comprising 60 wt% to 80 wt% of polypropylene having a viscosity-average molecular weight of 1×10 6 g / mol to 3×10 6 g / mol, and 20 wt% to 40 wt% of polyethylene having a weight-average molecular weight of 1×10 5 g / mol to 10×10 5 g / mol, having a puncture strength of 0.25 N / μm 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°C or less, and a melt-down temperature of 180°C or more.

[0031] In recent years, with the increase in capacity and size of secondary batteries, it has been required to satisfy more excellent battery performance and safety. For this purpose, separators for secondary batteries are required to have both higher levels of heat resistance and permeability. For this reason, polypropylene with a high melting temperature is used as a material for separators for secondary batteries. However, polypropylene microporous membranes manufactured by a dry process are not suitable for use in the field of secondary batteries due to their low puncture strength and excessively large and non-uniform pore sizes.

[0032] As a result of the inventors' repeated research, by realizing both the puncture strength, gas permeability, porosity, average pore size, shutdown temperature, and melt-down temperature within the above-mentioned ranges, it was confirmed that a polyolefin microporous membrane excellent in puncture strength and permeability and significantly improved in heat resistance at high temperatures can be manufactured.

[0033] A secondary battery according to one embodiment can ensure both excellent battery performance and safety by including a polyolefin microporous membrane that satisfies the physical properties as described above. In particular, the present disclosure can provide a secondary battery including a microporous membrane in which pores are blocked at a temperature of 150°C or lower and the form 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 can be suitably applied to high-output / high-capacity batteries by satisfying the above-mentioned physical properties.

[0034] The polyolefin microporous membrane may be manufactured by a wet method including a sequential biaxial stretching process as long as it does not deviate from the scope of the present disclosure.

[0035] As one embodiment, a polyolefin microporous membrane that satisfies the physical properties as described above may be manufactured by including a specific polyethylene resin and a specific polypropylene resin in a specific composition ratio. For example, the microporous membrane according to one embodiment contains 60% to 80% by weight of polypropylene having a viscosity average molecular weight of 1×10 6 g / mol to 3×10 6 g / mol, and a weight average molecular weight of 1×10 5g / mol~10×10 5 It may contain 20% to 40% by weight of polyethylene having a range of g / mol. Further, in the above-described embodiment, the melting temperature of the polypropylene may be 160°C or higher, 162°C or higher, 170°C or lower, 165°C or lower, or a value between the above numerical values. For example, the melting temperature of the polypropylene may be 160°C to 170°C or 162°C to 165°C. Further, in one embodiment, the melting temperature of the polyethylene may be 133°C or higher, 133°C to 140°C, or 133°C to 135°C.

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

[0037] As one embodiment, the polyolefin microporous membrane may be manufactured by including a 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 polyethylene, the heat treatment process may be performed in a temperature range of the melting temperature (Tm) of polyethylene to a temperature (Tm + 8°C) 8°C higher than the melting temperature among its components.

[0038] When the above conditions are combined with each other, the polyolefin microporous membrane of the present disclosure can satisfy all of the above physical properties, and thus has excellent mechanical strength and permeability, and can have significantly improved heat resistance at high temperatures. That is, the present disclosure provides a polyolefin microporous membrane having a puncture strength of 0.25 N / μm 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° C. or less, and a melt-down temperature of 180° C. or more.

[0039] Hereinafter, the polyolefin microporous membrane will be described in more detail.

[0040] In one embodiment, the polypropylene contained 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 g / mol or less, or a value between the above numerical values. For example, the viscosity-average molecular weight of the polypropylene may be 1×10 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 g / mol.

[0041] In one embodiment, the content of polypropylene contained 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 numerical values based on the total amount of polypropylene and polyethylene. For example, the content of the polypropylene may be 60 wt% to 80 wt%, 65 wt% to 75 wt%, or 70 wt% to 75 wt%. According to one embodiment, when the content of polypropylene satisfies the above range, it has excellent puncture strength and gas permeability, and can have a shutdown temperature of 150°C or lower and a melt-down temperature of 180°C or higher.

[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 the above numerical 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, when the melting temperature of polypropylene satisfies the above range, it has excellent puncture strength and gas permeability, and can have a shutdown temperature of 150°C or lower and a melt-down temperature of 180°C or higher.

[0043] In one embodiment, the polyethylene contained 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 5 g / mol or less, 9×10 5 g / mol or less, or a value between the above numerical values. For example, the weight average molecular weight of the polyethylene may be 1×10 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 g / mol.

[0044] In one embodiment, the content of polyethylene in the polyolefin microporous membrane may be 20% by weight or more, 25% by weight or more, 40% by weight or less, 35% by weight or less, or a value between the above numerical values based on the total amount of polypropylene and polyethylene. For example, the content of the polyethylene may be 20% by weight to 40% by weight or 25% by weight to 35% by weight. According to one embodiment, when the content of polyethylene satisfies the above range, it has excellent puncture strength and gas permeability, and can have a shutdown temperature of 150 °C or lower and a melt-down temperature of 180 °C or higher.

[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, when the melting temperature of the polyethylene satisfies the above range, it has excellent puncture strength and gas permeability, and can have a shutdown temperature of 150 °C or lower and a melt-down temperature of 180 °C or higher.

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

[0047] In one embodiment, the polyolefin microporous membrane has a gas permeability of 1.0×10 -5 Darcy or more, 1.15×10 -5 Darcy or more, 5.0×10 -5 Darcy or less, 3.0×10 -5 Darcy or less, or a value between the above numerical values. For example, the gas permeability may be 1.0×10 -5 Darcy to 5.0×10 -5Darcy or 1.15×10 -5 Darcy to 3.0×10 -5 Darcy may be sufficient. By satisfying the gas permeability within the above range, excellent ionic conductivity can be achieved, and due to the low internal resistance of the battery, the charge and discharge characteristics of the battery can be improved.

[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 numerical values. For example, the porosity may be 30% - 70%, 35% - 60%, or 37% - 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 numerical values. For example, the average pore size may be 20 nm - 40 nm or 24 nm - 35 nm.

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

[0051] In one embodiment, the polyolefin microporous membrane may have a melting - down temperature of 180°C or more, or 180°C - 190°C. By satisfying the melting - down temperature within the above range, even if the temperature of the battery to which it is applied rises rapidly, an internal short - circuit due to the membrane breakage of the microporous membrane can be suppressed, and excellent safety can be ensured.

[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 the above numerical 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. Even under the thickness within the above range, the microporous membrane of the present disclosure can achieve excellent puncture strength, gas permeability, porosity, average pore size, and heat resistance. In particular, it can have a shutdown temperature of 150 °C or lower and a melt-down temperature of 180 °C or higher. Thereby, according to one embodiment, it is possible to thin the separator for secondary batteries, and it is suitably applicable to 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 satisfies the above physical properties. Thereby, the polyolefin microporous membrane satisfies all of the above physical properties, has excellent mechanical strength and permeability, and can have significantly improved heat resistance at high temperatures.

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

[0055] As one embodiment, the diluent may be extracted from the sequentially biaxially stretched film, and the film may be shrunk by 5% or less in the longitudinal direction and 10% or less in the transverse direction and dried, but it may be performed by other methods as long as it does not deviate from the scope of the present disclosure.

[0056] As one embodiment, the dried film obtained by the extraction / drying step may be heat-treated 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), but may be performed by other methods as long as it does not deviate from the scope of the present disclosure.

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

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

[0059] Hereinafter, each manufacturing step will be described.

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

[0061] In step (a), the polyolefin resin has a viscosity average molecular weight of 1×10 6 g / mol to 3×10 660% to 80% by weight of polypropylene, which is g / mol, and a weight average molecular weight of 1×10 5 g / mol to 10×10 5 g / mol may contain 20% to 40% by weight of polyethylene. 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, when the polyolefin resin satisfies the above composition, the puncture strength is 0.25 N / μm or more, the gas permeability is 1.0×10 -5 Darcy or more, the porosity is 30% to 70%, the average pore size is 20 nm to 40 nm, the shutdown temperature is 150°C or less, and the melt-down temperature is 180°C or more, a polyolefin microporous membrane can be produced. A secondary battery including a microporous membrane that satisfies all of such physical properties can ensure both excellent battery performance and safety.

[0062] Since the viscosity average molecular weight (or weight average molecular weight) and the melting temperature of the polypropylene and polyethylene are as described above, specific descriptions are omitted.

[0063] In one embodiment, as the diluent, any organic compound that forms a single phase with the polyolefin resin at the extrusion temperature can be used without limitation. For example, the diluent may be an aliphatic or cyclic hydrocarbon such as nonane, decane, decalin, paraffin oil, paraffin wax, a phthalic acid ester such as dibutyl phthalate, dioctyl phthalate, a C10-C20 fatty acid such as palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, and one or a combination of two or more selected from the group consisting of C10-C20 fatty alcohols such as cetyl alcohol, stearyl alcohol, oleyl alcohol. As a specific example of the diluent, paraffin oil having a kinematic viscosity at 40°C of 20 cSt to 200 cSt can be mentioned, but it may be replaced with other types of diluents as long as the scope of the present disclosure is not deviated from.

[0064] Further, the mixture may further contain any one or more of ordinary additives for improving specific functions, such as an antioxidant, a UV stabilizer, an antistatic agent, within a range where the characteristics of the microporous membrane are not significantly deteriorated.

[0065] Next, step (b) is a step of extruding the melt to form a sheet, which may be carried out without being limited by a method known to those skilled in the art. As an example, the melt may be extruded by a T-die and formed into a sheet by a casting or calendaring method while cooling to a temperature of 10°C to 80°C, but may be carried out by other methods as long as it does not deviate from the scope of the present disclosure.

[0066] Next, step (c) is a step of sequentially biaxially stretching the sheet in the longitudinal and transverse directions to form a film. The stretching ratios in the longitudinal and transverse directions may each independently be 4 times or more, 6 times or more, 10 times or less, 15 times or less, or a value between the above numerical values. For example, the stretching ratios in the longitudinal and transverse directions may be 4 to 15 times or 6 to 10 times. By satisfying the above ranges for the stretching ratios in the longitudinal and transverse directions, a polyolefin microporous membrane having the above-described physical properties can be manufactured.

[0067] The stretching in step (c) is carried out by a sequential stretching method of a roll method or a tenter method, and may be carried out at a temperature in the range of 80°C lower than the melting temperature of polypropylene to the melting temperature of polypropylene. When stretching is carried out at a temperature in the above range, the fluidity of the polypropylene resin for ensuring effective stretching can be ensured. Specifically, stretching is uniformly carried out over the entire sheet, and since breakage due to stretching does not occur, stretching can be carried out stably. Thereby, a high-quality microporous membrane can be manufactured in which physical properties such as uniform gas permeability and puncture strength are realized over the entire membrane. As an example, the stretching may be carried out 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, which is performed by extracting the diluent in the film using an organic solvent and drying the organic solvent in the film in which the diluent has been replaced with the organic solvent. The organic solvent can be used without particular limitation as long as it can extract the diluent. For example, as the organic solvent, methyl ethyl ketone, methylene chloride, hexane, etc. may be used from the viewpoints of high extraction efficiency and fast drying, but other methods may be employed as long as they do not deviate from the scope of the present disclosure.

[0069] As one embodiment, step (d) may be to extract the diluent from the stretched film and shrink and dry the film to 5% or less in the longitudinal direction and 10% or less in the transverse direction. When the diluent is extracted from a film made of a polyolefin resin satisfying the above composition and the film is shrink-dried under the above conditions, a microporous membrane that satisfies both the above-mentioned puncture strength, gas permeability, porosity, average pore size, shutdown temperature, and melt-down temperature can be provided. A secondary battery including such a microporous membrane can ensure both excellent battery performance and safety.

[0070] As one embodiment, the shrinkage rate in the longitudinal direction in the extraction / drying step may be 5% or less, 4% or less, 3.5% or less, and 1% or more, for example, 1% - 5%, 1% - 4%, or 1% - 3.5%. The shrinkage rate in the transverse direction in the extraction / drying step may be 10% or less, 9% or less, 8.5% or less, and 1% or more, for example, 1% - 10%, 1% - 9%, or 1% - 8.5%. At this time, the shrinkage rate in the longitudinal direction (or transverse direction) can be calculated by the following Mathematical Formula 1.

[0071] [Mathematical Formula 1] Shrinkage rate in the longitudinal direction (or transverse direction) (%) = [{Length in the longitudinal direction (or transverse direction) before extraction - Length in the longitudinal direction (or transverse direction) after drying} / Length in the longitudinal direction (or transverse direction) before extraction] × 100

[0072] The longitudinal or transverse shrinkage in the extraction / drying process may be adjusted by the tension applied to the film in the drying process. When the tension is increased, less shrinkage occurs, and when the tension is decreased, more shrinkage occurs. The tension applied to the film in the drying process is determined differently according to the thickness of the film, and its magnitude can be appropriately changed as long as it does not deviate from the scope of the present disclosure.

[0073] As one embodiment, step (d) may be performed at a temperature of 40°C or lower, but may be performed at other temperatures as long as it does not deviate from the scope of the present disclosure.

[0074] Step (e) is a step of heat-treating the dried film. Using a roll method or tenter method device, heat is applied while being forcibly grasped so that dimensional changes do not occur in the longitudinal and transverse directions, and the residual stress in the film is removed, and the heat resistance of the finally produced polyolefin microporous membrane can be improved.

[0075] As one embodiment, the step (e) may be to heat-treat 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 above composition is heat-treated at the temperature under the above conditions, a microporous membrane that satisfies both the above-mentioned punching strength, gas permeability, porosity, average pore size, shutdown temperature, and melt-down temperature can be provided. A secondary battery including such a microporous membrane can ensure both excellent battery performance and safety.

[0076] In one embodiment, the heat treatment in step (e) may be performed at a temperature in the range of 133°C to 148°C, and may be performed at a temperature in the range of 133°C to 145°C, or 134°C to 142°C. By performing the heat treatment at a temperature within the above range, a polyolefin microporous membrane that satisfies all of the above physical properties can be manufactured.

[0077] In one embodiment, the heat treatment in step (e) may be heat stretching and / or heat relaxation for heat fixation. That is, the tension may be adjusted during the heat treatment, and the heat treatment may be performed in various ways. The heat treatment may be repeated 1 to 3 times, but is not necessarily limited thereto.

[0078] The step (e) may include, for example, any one or more of a heat stretching step of stretching in the longitudinal or transverse direction, a heat fixation step of applying heat while fixing the length / width in the longitudinal and transverse directions, and a heat relaxation step of relaxing (contracting) in the longitudinal or transverse direction. As an example, the heat relaxation step may be to relax to 80% - 99% or 90% - 99% of the transverse width before the heat relaxation step, and the heat stretching step may be to stretch to 120% - 160% or 140% - 160% of the transverse width before the heat stretching step, but may be performed by other methods without departing from the scope of the present disclosure.

[0079] The present disclosure provides a separator including the polyolefin microporous membrane as described above. The separator may be a separator used in all known energy storage devices, and is not particularly limited. As a non-limiting example, a separator used in a lithium secondary battery may be mentioned.

[0080] Hereinafter, with reference to specific experimental examples, the embodiments of the present disclosure will be further described. The examples and comparative examples included in the experimental examples are merely illustrative of the present disclosure and do not limit the scope of the appended claims. It is obvious to those skilled in the art that various changes and modifications of the examples are possible within the scope and technical idea of the present disclosure, and it goes without saying that such variations and modifications belong to the scope of the appended claims.

[0081] [Physical Property Measurement Method] 1. Viscosity-average molecular weight (g / mol) The viscosity-average molecular weight (Mv) was calculated from the intrinsic viscosity (η) measured using the Crystex QC model of Polymer char according to the Margolies-equation of the following Mathematical formula 2. The intrinsic viscosity was measured using 1,2,4-trichlorobenzene (TCB) as the solvent (the temperature during measurement was 165 °C).

[0082] [Mathematical formula 2] Mv = 5.37x10 4 x [η] 1.49

[0083] 2. Weight-average molecular weight (g / mol) The weight-average molecular weight (Mw) was measured using high-temperature GPC (Gel Permeation Chromatography) of Agilent Technologies. Using PLgel Guard and PLgel Olexis as GPC columns, 1,2,4-trichlorobenzene (TCB) as the solvent, and polystyrene as the standard sample, the analysis was performed at 140 °C.

[0084] 3. Melting temperature (°C) The melting temperature was determined by the peak position in the curve measured with a differential scanning calorimeter (DSC). Using the Thermal Analysis System DSC 3+ model, a differential scanning calorimeter (DSC) of Mettler Toledo, in a nitrogen atmosphere, the temperature range from 25 °C to 200 °C was scanned at a heating rate of 10 °C / min. The amount of the measurement sample at this time was 5 mg.

[0085] 4. Thickness (μm) of the microporous membrane The thickness of the microporous membrane was measured using a contact-type thickness measuring instrument with a precision of 0.1 μm for thickness. Using the TESA Mu-Hite Electronic Height Gauge of TESA, the measurement pressure was set to 0.63 N for measurement.

[0086] 5. Punching strength (N / μm) The puncture strength was measured by attaching a pin tip with a diameter of 1.0 mm and a radius of curvature of 0.5 mm to an INSTRON UTM (Universal Test Machine) 3345 and pressing the microporous membrane at a speed of 120 mm / min. At this time, the puncture strength was calculated by dividing the load (N) when the microporous membrane was broken by the thickness (μm) of the microporous membrane.

[0087] 6. Gas Permeability (Darcy) The gas permeability was measured using a porometer (CFP-1500-AEL manufactured by PMI). Generally, the gas permeability is represented by the Gurley number. However, since the influence of the thickness is not corrected in the Gurley number, it is difficult to understand the relative permeability due to the void structure. To solve this problem, the gas permeability of the present disclosure is measured using the Darcy permeability constant calculated from the following Mathematical Formula 3. Nitrogen was used as the gas, and the average value of the Darcy permeability constant measured in the 100 - 200 psi region was calculated.

[0088] [Mathematical Formula 3] Darcy Permeability Constant (C) = (8F·T·V) / (πD 2 (P 2 -1)) F = Flow Rate T = Thickness of the Sample V = Viscosity of the Gas (0.185 for N2) D = Diameter of the Sample P = Pressure

[0089] 7. Porosity (%) The porosity of the microporous membrane was calculated by 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. The porosity was calculated from the ratio of the mass of the resin of the same volume and the mass of the microporous membrane.

[0090] [Mathematical Formula 4] Porosity (%) = 100 × {1 - M / (A × B × T × ρ)} In the above Mathematical Formula 4, M is the mass (g) of the microporous membrane, and ρ is the density (g / cm 3) is as follows.

[0091] 8. Average pore size (nm) The average pore size was measured using a porometer (CFP-1500-AEL manufactured by PMI) in accordance with ASTM F316-03. The average pore size was measured by the half-dry method, and Galwick liquid (surface tension: 15.9 dyne / cm) provided by PMI was used to measure the pore size.

[0092] 9. Shutdown temperature and melt-down temperature An electrically resistive measurement cell fabricated by ourselves was used to measure the shutdown temperature and melt-down temperature of the microporous membrane. The electrically resistive measurement cell was fabricated in the following manner: A microporous membrane prepared with a size of 2.5 cm × 2.5 cm was positioned between two glass cells, and then firmly fixed with clips so that the bonding site of the glass cells did not move. Next, an electrolytic solution in which 1 M of hexafluorophosphate (LiPF6) was dissolved in a propylene carbonate (PC) solvent was injected into each glass cell while taking care not to generate bubbles. Aluminum electrodes were positioned in each glass cell, and finally, the electrically resistive measurement cell was fabricated. The fabricated electrically resistive measurement cell was placed in an oil bath preheated to 120°C, and the two electrodes were connected to an impedance analyzer (ZIVE SP2 manufactured by ZIVE Lab). While raising the temperature of the oil bath at a rate of 2°C / min, the resistance value of the cell was measured using an alternating current (5 mV) of 1 kHz. The cell resistance value was plotted against the temperature, and the shutdown temperature and melt-down temperature were measured. The shutdown temperature was defined as the temperature at which the resistance value became 1000 Ω or more, and the melt-down temperature was defined as the temperature at which it became 1000 Ω or less after the shutdown temperature.

[0093] <Example 1> The weight-average molecular weight is 6.0×10 5 g / mol and the melting temperature is 134°C, 25% by weight of a polyethylene resin, and the viscosity-average molecular weight is 1.5×10 6A polyolefin resin composed of 75% by weight of polypropylene resin with a melting temperature of 164 °C at g / mol was prepared. Next, a mixture containing the polyolefin resin and 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 melt.

[0094] The melt was continuously extruded through a T-die, and a sheet with a width of 300 mm and an average thickness of 1200 μm was produced using a casting roll set at 30 °C.

[0095] The sheet was longitudinally stretched by a roll method at a stretching temperature of 115 °C to a factor of 6, and then continuously induced into a tenter and transversely stretched at a stretching temperature of 140 °C to a factor of 6.

[0096] Paraffin oil was extracted from the film that had been stretched in the longitudinal and transverse directions at 25 °C using methylene chloride. After extraction, the film from which paraffin oil had been extracted was dried at 60 °C while adjusting the tension in the opposite direction of the shrinkage stress generated during the drying process so that it shrank by 2% in the longitudinal direction and 6% in the transverse direction compared to the film before extraction.

[0097] The dried film was heat-fixed at 140 °C for about 20 seconds using a tenter-type heat-fixing device to produce a polyolefin microporous membrane with a thickness of 10.5 μm. The physical properties of the finally produced microporous membrane were recorded in Table 1 below.

[0098] <Example 2> A polyolefin microporous membrane was produced in the same manner as in Example 1, except that a polyolefin resin composed of 20% by weight of polyethylene resin and 80% by weight of polypropylene resin was used instead of the polyolefin resin in Example 1, and the results were recorded in Table 1.

[0099] <Example 3> A polyolefin microporous membrane was produced in the same manner as in Example 1, except that a polyolefin resin composed of 40% by weight of a polyethylene resin and 60% by weight of a polypropylene resin was used instead of the polyolefin resin of Example 1, and the results were recorded in Table 1.

[0100] <Example 4> A polyolefin microporous membrane was produced in the same manner as in Example 1, except that a polyethylene resin having a weight average molecular weight of 3.0×10 5 g / mol and a melting temperature of 134°C was used instead of the polyethylene resin of Example 1, and the results were recorded in Table 1.

[0101] <Example 5> A polyethylene resin having a weight average molecular weight of 10×10 5 g / mol and a melting temperature of 134°C was used instead of the polyethylene resin of Example 1, and a polypropylene resin having a viscosity average molecular weight of 2.0×10 6 g / mol and a melting temperature of 164°C was used instead of the polypropylene resin of Example 1. A polyolefin microporous membrane was produced in the same manner as in Example 1, and the results were recorded in Table 1.

[0102] <Example 6> A polyolefin microporous membrane was produced in the same manner as in Example 1, except that the film from which paraffin oil had been extracted was dried so that it shrank by 3% in the longitudinal direction and 8% in the transverse direction compared to the film before extraction, and the results were recorded in Table 1.

[0103] <Example 7> A polyolefin microporous membrane was produced in the same manner as in Example 1, except that the film from which paraffin oil had been extracted was dried so that it shrank by 5% in the longitudinal direction and 10% in the transverse direction compared to the film before extraction, and the results were recorded in Table 1.

[0104] <Example 8> A polyolefin microporous membrane was produced in the same manner as in Example 1, except that the heat setting temperature was 142°C, and the results were recorded in Table 1.

[0105] <Example 9> A polyolefin microporous membrane was produced in the same manner as in Example 1, except that the heat-fixing temperature was set at 136°C, and the results were recorded in Table 1.

[0106] <Comparative Example 1> A polyolefin microporous membrane was produced in the same manner as in Example 1, except that a polyolefin resin composed of 10% by weight of polyethylene resin and 90% by weight of polypropylene resin was used instead of the polyolefin resin in Example 1, and the results were recorded in Table 2.

[0107] <Comparative Example 2> A polyolefin microporous membrane was produced in the same manner as in Example 1, except that a polyolefin resin composed of 18% by weight of polyethylene resin and 82% by weight of polypropylene resin was used instead of the polyolefin resin in Example 1, and the results were recorded in Table 2.

[0108] <Comparative Example 3> A polyolefin microporous membrane was produced in the same manner as in Example 1, except that a polyolefin resin composed of 45% by weight of polyethylene resin and 55% by weight of polypropylene resin was used instead of the polyolefin resin in Example 1, and the results were recorded in Table 2.

[0109] <Comparative Example 4> <Instead of the polyethylene resin in Example 1, a polyethylene resin having a weight-average molecular weight of 1.1×10 6 g / mol and a melting temperature of 134°C was used, and the film from which paraffin oil was extracted was dried so that it shrank by 4% in the longitudinal direction and 9% in the transverse direction compared to the film before extraction. A polyolefin microporous membrane was produced in the same manner as in Example 1, and the results were recorded in Table 2.

[0110] <Comparative Example 5> <Instead of the polyethylene resin in Example 1, a polyethylene resin having a weight-average molecular weight of 6.0×10 5A polyolefin microporous membrane was produced in the same manner as in Example 1, except that a polyethylene resin having a melt temperature of 132°C and a viscosity average molecular weight of [value] g / mol was used, and the heat setting temperature was 138°C. The results were recorded in Table 2.

[0111] <Comparative Example 6> Instead of the polypropylene resin of Example 1, a polypropylene resin having a melt temperature of 158°C and a viscosity average molecular weight of 1.5×10 6 g / mol was used. A polyolefin microporous membrane was produced in the same manner as in Example 1, except for this point. The results were recorded in Table 2.

[0112] <Comparative Example 7> A polyolefin microporous membrane was produced in the same manner as in Example 1, except that the film from which paraffin oil was extracted was dried so that it shrank by 7% in the longitudinal direction and 14% in the transverse direction compared to the film before extraction. The results were recorded in Table 2.

[0113] <Comparative Example 8> A polyolefin microporous membrane was produced in the same manner as in Example 1, except that the film from which paraffin oil was extracted was dried so that it was stretched by 5% in the longitudinal direction and shrank by 8% in the transverse direction compared to the film before extraction, and the heat setting temperature was 136°C. The results were recorded in Table 2.

[0114] <Comparative Example 9> A polyolefin microporous membrane was produced in the same manner as in Example 1, except that the heat setting temperature was 145°C. The results were recorded in Table 2.

[0115]

Table 1

[0116]

Table 2

[0117] Referring to Table 1 above, it can be seen that the microporous membranes of Examples 1 to 9 not only have the punching strength, gas permeability, porosity, and average pore size that can ensure the performance of the battery, but also have the shutdown temperature and melting temperature that can ensure the thermal safety of the battery.

[0118] In contrast, as a result of using more than 80% by weight of polypropylene resin in the microporous membranes of Comparative Example 1 and Comparative Example 2, the shutdown temperatures were 158.4 °C and 155.9 °C, respectively. As a result, the batteries to which these were applied had the drawback that the pores closed at high temperatures and the thermal safety was significantly reduced.

[0119] As a result of using less than 60% by weight of polypropylene resin in the microporous membrane of Comparative Example 3, the range of punching strength and gas permeability to be achieved by the present disclosure was not satisfied. As a result, the battery to which this was applied had the drawback that it was impossible to achieve the performance as a battery.

[0120] As a result of using polyethylene resin with an average weight molecular weight exceeding 1.0×10 6 g / mol in the microporous membrane of Comparative Example 4, the range of gas permeability and shutdown temperature to be achieved by the present disclosure was not satisfied. As a result, the battery to which this was applied had the drawbacks that it was impossible to achieve the performance as a battery and the thermal safety was significantly reduced.

[0121] As a result of using polyethylene resin with a melting temperature of less than 133 °C in the microporous membrane of Comparative Example 5, the range of gas permeability to be achieved by the present disclosure was not satisfied. As a result, the battery to which this was applied had the drawback that it was impossible to achieve the performance as a battery.

[0122] As a result of using polypropylene resin with a melting temperature of less than 160 °C in the microporous membrane of Comparative Example 6, the range of melting temperature to be achieved by the present disclosure was not satisfied. As a result, the battery to which this was applied had the drawback that internal short circuit between the negative electrode and the positive electrode occurred due to membrane breakage of the separator, and the thermal safety was significantly reduced.

[0123] As a result of performing the extraction / drying process on the microporous membrane of Comparative Example 7, the shrinkage in the longitudinal direction exceeded 5% and the shrinkage in the lateral direction exceeded 10%, failing to meet the ranges of gas permeability and porosity that the present disclosure aims to achieve. Accordingly, the battery to which this was applied had the drawback that it was impossible to realize the performance as a battery.

[0124] As a result of performing the extraction / drying process on the microporous membrane of Comparative Example 8 by stretching instead of shrinking in the longitudinal direction by 5% or less, the average pore size and the range of shutdown temperature that the present disclosure aims to achieve were not satisfied. Accordingly, the battery to which this was applied had the drawback that it was impossible to realize the performance as a battery and the thermal safety was significantly reduced.

[0125] As a result of performing the heat fixation process on the microporous membrane of Comparative Example 9 at a temperature 11 °C higher than the melting temperature of the polyethylene resin, the ranges of gas permeability and porosity that the present disclosure aims to achieve were not satisfied. Accordingly, the battery to which this was applied had the drawback that it was impossible to realize the performance as a battery.

[0126] The content described above is merely an exemplification to which the principle of the present disclosure is applied, and other configurations may be further included without departing from the scope of the present disclosure.

Claims

1. The viscosity-average molecular weight is 1 × 10 ６ g / mol to 3 × 10 ６ g / mol of 60% to 80% by weight of polypropylene, and the weight-average molecular weight is 1 × 10 ５ g / mol to 10 × 10 ５ g / mol of 20% to 40% by weight of polyethylene, having a puncture strength of 0.25 N / μm 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°C or less, and a melt-down temperature of 180°C or more, a polyolefin microporous membrane.

2. The polyolefin microporous membrane according to claim 1, wherein the polypropylene has a melting temperature of 160°C or higher.

3. The polyolefin microporous membrane according to claim 1, wherein the polyethylene has a melting temperature of 133°C or higher.

4. The polyolefin microporous membrane according to claim 1, wherein the thickness of the microporous membrane is 3 μm to 30 μm.

5. The polyolefin microporous membrane according to claim 1, wherein the shutdown temperature is 149°C or lower.

6. The polyolefin microporous membrane according to claim 1, which is produced by a wet method including a sequential biaxial stretching step.

7. (a) A step of melt-kneading a mixture containing a polyolefin resin and a diluent by an extruder to produce a melt; (b) A step of extruding the melt and molding it into 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 and drying the diluent from the stretched film; (e) A step of heat-treating the dried film, and including The polyolefin resin in the step (a) has a viscosity-average molecular weight of 1 × 10 ６ g / mol to 3 × 10 ６ g / mol, 60% to 80% by weight of polypropylene, and a weight-average molecular weight of 1 × 10 ５ g / mol to 10 × 10 ５ g / mol, 20% to 40% by weight of polyethylene, and is a method for producing a polyolefin microporous membrane.

8. The method for producing a polyolefin microporous membrane according to claim 7, wherein the step (d) is to extract the diluent from the stretched film and shrink the film by 5% or less in the longitudinal direction and 10% or less in the transverse direction and then dry it.

9. The method for producing a polyolefin microporous membrane according to claim 7, wherein the step (e) is to heat-treat 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).

10. A separator including the polyolefin microporous membrane according to any one of claims 1 to 6.

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