Polypropylene microporous membrane, method for manufacturing the same, and separator containing the microporous membrane

Abstract

To provide a method for manufacturing a polypropylene microporous membrane that exhibits excellent permeability and permeability, as well as significantly improved heat resistance at high temperatures, a polypropylene microporous membrane, and a separator containing the microporous membrane. [Solution] A method for manufacturing a polypropylene microporous membrane is a) a viscosity-average molecular weight of 1 × 10 6 g / mol ~ 3 × 10 6 The process includes: (b) producing a molten product by melting and kneading a mixture containing a polypropylene resin in g / mol and a diluent using an extruder; (c) forming a sheet by extruding the molten product; (d) forming a film by sequentially biaxially stretching the sheet in the longitudinal and transverse directions; (e) extracting the diluent from the stretched film and drying it; and (e) heat-stretching the dried film at 130°C to 135°C, and then heat-treating the heat-stretched film by heat-setting and heat-relaxing it at 155°C to 165°C.

Description

[Technical Field]

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

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

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

[0004] High molecular weight polyethylene, used as the main material for separators in secondary batteries, has a low melting point of around 140°C, limiting its heat resistance. To overcome this limitation, polypropylene, which has a higher melting point, is used as the material for separators in secondary batteries.

[0005] However, conventional polypropylene microporous membranes are manufactured by a dry process. Due to the characteristics of such a dry process, the longitudinal / transverse stretching ratio of the dry polypropylene microporous membrane is low, or it is mainly stretched only in one direction, and the puncture strength is relatively low. Therefore, it can only be complemented by increasing the thickness. In addition, the pores of the microporous membrane obtained by the dry process have a huge linear size, and there is a characteristic that the uniformity of the pore size is low. Such characteristics have a negative effect on the performance of the battery. Therefore, the dry polypropylene microporous membrane is not suitable for use in the field of secondary batteries.

[0006] To solve this problem, Korean Patent Publication No. 10-2011-0101202 discloses a polypropylene microporous membrane manufactured by a wet process and having excellent heat resistance and strength. However, this microporous membrane has a significantly low puncture strength of less than 0.05 N / μm and its heat shrinkage stability is not significantly improved compared to the conventional one. Therefore, it is inferior in the safety of the battery in a high-temperature environment such as a hot-box evaluation, and thus has a drawback of not being suitable for application to high-capacity and large-sized batteries.

[0007] Therefore, from the viewpoints of battery performance and safety, there is a demand for the development of a polypropylene microporous membrane with high puncture strength and permeability, significantly improved heat resistance at a higher temperature, and particularly excellent battery safety even in a 140°C hot-box evaluation which is an evaluation index for high-temperature safety.

Summary of the Invention

Problems to be Solved by the Invention

[0008] In order to solve the above problems, an object of the present disclosure is to provide a polypropylene microporous membrane excellent in puncture strength and permeability, with significantly improved heat resistance at high temperatures, a method for manufacturing the same, and a separator including the microporous membrane.

[0009] In particular, an object is to provide a polypropylene microporous membrane capable of achieving excellent battery safety even in a 140°C hot-box evaluation after the battery is assembled.

[0010] The polypropylene microporous membrane of the present disclosure can be widely applied to green technology fields such as electric vehicles, battery charging stands, and solar power generation and wind power generation that use batteries. In addition, the polypropylene microporous membrane of the present disclosure can be used in eco-friendly electric vehicles, hybrid vehicles, etc. that suppress air pollution and greenhouse gas emissions and prevent climate change.

Means for Solving the Problems

[0011] As one means for achieving the above problems, the present disclosure includes polypropylene having a viscosity average molecular weight of 1×10 6 g / mol to 3×10 6 g / mol, the thickness of the microporous membrane is 3 μm to 30 μm, the puncture strength is 0.20 N / μm or more, the gas permeability is 1.0×10 -5 Darcy or more, the porosity is 25% to 60%, the average pore size is 25 nm to 50 nm, and the lateral shrinkage rate measured after standing at 150 °C for 1 hour is 20% or less. A polypropylene microporous membrane is provided. The porosity means the ratio composed of empty space or pores in the volume of the microporous membrane. The polypropylene resin may be homopolypropylene. As an example, the homopolypropylene resin may have a high crystallinity of 40% or more, 50% or more, 60% or more, or 70% or more. As an example, the high-crystallinity homopolypropylene resin may have a melting point of 160 to 170 °C.

[0012] In one aspect, the polypropylene microporous membrane may have a melt fracture temperature of 165 °C or higher.

[0013] In one aspect, the polypropylene microporous membrane may have a lateral shrinkage rate of 15% or less.

[0014] In one embodiment, the polypropylene microporous film may be manufactured by a wet process including a sequential biaxial stretching step.

[0015] Furthermore, as another means to achieve the above-mentioned objectives, this disclosure provides (a) a viscosity-average molecular weight of 1 × 10 6 g / mol ~ 3 × 10 6 The present invention provides a method for producing a polypropylene microporous film, comprising the steps of: (b) producing a molten product by melting and kneading a mixture containing a polypropylene resin in g / mol and a diluent using an extruder; (c) forming the molten product into a sheet by extruding the product; (d) forming a film by sequentially biaxially stretching the sheet in the longitudinal and transverse directions; (e) extracting the diluent from the stretched film and drying it; and (e) heat-stretching the dried film at a temperature at which 2% to 5% of the polypropylene resin crystals melt, and then heat-treating the heat-stretched film by heat-fixing and heat-relaxing it at a temperature at which 25% to 60% of the polypropylene resin crystals melt.

[0016] In one embodiment, the thermal stretching in step (e) may be carried out at a temperature of 125°C to 135°C.

[0017] In one embodiment, the thermal setting and thermal relaxation in step (e) may be carried out at a temperature of 155°C to 165°C.

[0018] In another aspect of this disclosure, a polypropylene microporous membrane manufactured by the above manufacturing method is provided.

[0019] In other embodiments of this disclosure, the viscosity-average molecular weight is 1 × 10 6 g / mol ~ 3 × 10 6 It contains polypropylene in g / mol, has a microporous membrane thickness of 3 μm to 30 μm, a puncture strength of 0.20 N / μm or higher, and a gas permeability of 1.0 × 10⁻⁶ -5 The present invention provides a microporous membrane for use as a separator in secondary batteries, which is Darcy or higher, has a porosity of 25% to 60%, an average pore size of 25 nm to 50 nm, and a lateral shrinkage rate of 20% or less measured after being left at 150°C for 1 hour.

[0020] In one aspect, the microporous membrane for the separator of the secondary battery may have a melting rupture temperature of 165° C. or higher.

[0021] In another aspect of the present disclosure, there is provided a secondary battery including a positive electrode, a negative electrode, and the separator described in the above aspect.

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

Advantages of the Invention

[0023] The polypropylene microporous membrane according to the present disclosure has excellent puncture strength and permeability, and can ensure significantly improved heat resistance at high temperatures.

[0024] Also, the polypropylene microporous membrane according to the present disclosure has a puncture strength of 0.20 N / μm or more and a gas permeability of 1.0×10 -5 Darcy or more.

[0025] Also, the polypropylene microporous membrane according to the present disclosure can have a lateral shrinkage rate of 20% or less measured after being left at 150° C. for 1 hour.

[0026] Also, by including the polypropylene microporous membrane according to one aspect, the present disclosure can ensure excellent battery performance and provide a battery having excellent safety in which smoking or ignition does not occur in a high-temperature environment. Specifically, the present disclosure can provide a battery having excellent safety in which smoking or ignition of the battery does not occur in a hot box evaluation at a high temperature of 140° C.

Brief Description of the Drawings

[0027] [Figure 1] It is a diagram showing a frame for measuring the melting rupture temperature of the microporous membrane manufactured in one embodiment. [Figure 2]This figure shows a configuration in which a microporous membrane, manufactured in one embodiment, is fixed with tape to a frame for measuring the melt-break temperature of the microporous membrane. [Modes for carrying out the invention]

[0028] The embodiments described herein can be modified into various other forms, and the technology relating to one aspect is not limited to the embodiments described below. Furthermore, embodiments of one aspect are provided to give a more complete explanation of this disclosure to a person with average skill in the art.

[0029] Furthermore, the singular form used in the specification and the attached claims is intended to include plural forms unless otherwise indicated in the context.

[0030] Furthermore, the numerical ranges used herein include lower and upper limits, all values ​​within those limits, increments logically derived from the form and width of the defined range, all double-limited values, and all possible combinations of upper and lower limits of numerical ranges limited in different forms. Unless otherwise defined herein, values ​​outside the numerical range that may occur due to experimental error or rounding up are also included in the defined numerical range.

[0031] Furthermore, throughout the specification, "including" a certain component means, unless otherwise stated, that it may include other components rather than excluding them.

[0032] This disclosure relates to a viscosity-average molecular weight of 1 × 10 6 g / mol ~ 3 × 10 6 It contains polypropylene in g / mol, has a microporous membrane thickness of 3 μm to 30 μm, a puncture strength of 0.20 N / μm or higher, and a gas permeability of 1.0 × 10⁻⁶ -5 The present invention provides a polypropylene microporous membrane that is Darcy or better, has a porosity of 25% to 60%, an average pore size of 25 nm to 50 nm, and a lateral shrinkage rate of 20% or less measured after being left at 150°C for 1 hour.

[0033] In recent years, secondary batteries have been required to meet higher battery performance and safety standards as they have become larger and have higher capacities. To this end, separators for secondary batteries are required to have both higher levels of heat resistance and permeability. For this reason, polypropylene, which has a high melting point, is used as the material for separators for secondary batteries. However, polypropylene microporous membranes manufactured by dry processes are unsuitable for use in secondary batteries due to their low puncture strength and excessively large and non-uniform pore sizes. Furthermore, polypropylene microporous membranes manufactured by wet processes also have low puncture strength and their thermal shrinkage stability has not improved significantly compared to conventional materials. Therefore, they have the disadvantage of being unsuitable for high-capacity and large-scale batteries because they are less safe for use in high-temperature environments such as hot box evaluations.

[0034] As a result of extensive research by the inventors, a puncture strength of 0.20 N / μm or higher, and 1.0 × 10⁻⁶ -5 We have confirmed that it is possible to manufacture a polypropylene microporous membrane that exhibits excellent puncture strength and permeability, as well as significantly improved heat resistance at high temperatures, by achieving gas permeability greater than Darcy, a porosity of 25% to 60%, an average pore size of 25 nm to 50 nm, and a lateral shrinkage rate of 20% or less, as measured after being left at 150°C for 1 hour.

[0035] A secondary battery according to one embodiment can ensure both excellent battery performance and safety at high temperatures by including a polypropylene microporous membrane that satisfies both of the above-mentioned physical properties. In particular, this disclosure can provide a battery with excellent thermal safety at high temperatures, such as one that does not smoke or catch fire during hot box evaluation at a high temperature of 140°C. The manufacturing and evaluation method of the battery for the above evaluation is as follows: A positive electrode using NCM622 (Ni:Co:Mn=6:2:2) as the active material and a negative electrode using graphite carbon as the active material are wound together with the microporous membrane of this disclosure and placed in an aluminum pouch to manufacture a battery. Next, an electrolyte in which 1M lithium hexafluorophosphate (LiPF6) is dissolved in a solution containing ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume ratio of 3:5:2 is injected into the inside of the battery and sealed to produce a 2Ah capacity battery. Next, the battery is subjected to aging and degassing, fully charged to 4.2V, and then heated in an oven at a rate of 5°C / min. After reaching 140°C, it is left for 30 minutes to check whether smoke or fire occurs in the battery. In other words, the polypropylene microporous film of this disclosure satisfies both of the above physical properties and is therefore suitably applicable to high-power / high-capacity batteries.

[0036] In one embodiment, a polypropylene microporous membrane satisfying both of the above-mentioned physical properties can be manufactured by using a polypropylene resin having a viscosity-average molecular weight within a specific range, or by heat treatment at a specific temperature after extraction of a diluent.

[0037] In one embodiment, the polypropylene resin contained in the polypropylene microporous membrane has a viscosity-average molecular weight of 1 × 10⁻⁶ 6 g / mol ~ 3 × 10 6 The value can be g / mol, specifically 1.2 × 10⁻⁶. 6 g / mol ~ 2.5 × 10 6 g / mol, more specifically 1.3 × 10⁻⁶ 6 g / mol ~ 2.2 × 10 6 It can be expressed as g / mol.

[0038] In one embodiment, heat treatment at a specific temperature after diluent extraction may involve heat-stretching the film obtained after diluent extraction at a first temperature, and then heat-fixing and heat-relaxing the heat-stretched film at a second temperature higher than the first temperature.

[0039] As a result, the polypropylene microporous membrane can have excellent puncture strength and permeability, as well as significantly improved heat resistance at high temperatures.

[0040] The polypropylene microporous membrane will be described in more detail below.

[0041] In one embodiment, the thickness of the polypropylene microporous film is 3 μm to 30 μm, more specifically 5 μm to 20 μm, and more specifically 5 μm to 15 μm. The microporous film of this disclosure can achieve excellent perforation strength, gas permeability, and thermal shrinkage rate even with the thickness in the above range, and in particular, it has been found to exhibit excellent battery safety in hot box evaluation using the evaluation method of the embodiment described below. Furthermore, it has excellent resistance to external stress generated during battery manufacturing and to temperature rise and dendrite formation that occur during battery charging and discharging, and because the internal resistance of the battery is low, the battery's charging and discharging performance can be improved.

[0042] In one embodiment, the polypropylene microporous film can have a puncture strength of 0.20 N / μm or more even with a thickness within the above range. Specifically, the puncture strength may be 0.25 N / μm or more, or 0.30 N / μm or more, and there is no particular upper limit, but for example, it may be 1.0 N / μm or less. In one specific embodiment, the puncture strength may be 0.20 N / μm to 1.0 N / μm, 0.25 N / μm to 1.0 N / μm, or 0.30 N / μm to 1.0 N / μm, but is not limited to these. By satisfying the above range of puncture strength, excellent resistance to external stress generated during battery manufacturing and to dendrites generated during battery charging and discharging can be achieved, ensuring battery safety. Furthermore, it is possible to thin the separator for secondary batteries, making it suitably applicable to high-capacity / high-output batteries.

[0043] In one embodiment, the polypropylene microporous membrane is 1.0 × 10 -5 It can have a gas permeability of 1.3 × 10⁻¹⁰ or higher, specifically, the gas permeability is 1.3 × 10⁻¹⁰. -5 Darcy or higher, 1.5×10 -5 Darcy or higher, 3.0×10 -5 Darcy or lower, or 5.0 x 10 -5 It may be less than or equal to Darcy. In one specific embodiment, the gas permeability is 1.0 × 10⁻⁶. -5 Darcy ~ 5.0 x 10 -5 Darcy, or 1.3 × 10 -5 Darcy~3.0×10 -5 It may be Darcy, but is not limited to it. By satisfying the gas permeability within the above range, it is possible to have excellent ionic conductivity, and the low internal resistance of the battery can improve the charge and discharge characteristics of the battery.

[0044] In one embodiment, the polypropylene microporous film may have a porosity of 25% to 60% from the viewpoint of mechanical strength and ionic conductivity, specifically 30% to 50% or 35% to 39%, but is not limited thereto.

[0045] In one embodiment, the polypropylene microporous film may have an average pore size of 25 nm to 50 nm, more specifically 25 nm to 40 nm, and more specifically 25 nm to 35 nm. In this case, the average pore size may be measured using the half-dry method. In one embodiment, the average pore size can be measured using a porometer according to ASTM F316-03.

[0046] In one embodiment, the polypropylene microporous membrane may have a lateral shrinkage rate of 20% or less, 15% or less, or 13% or less, measured after being left at 150°C for 1 hour, and the lower limit is not particularly limited, but may be, for example, 0.1%, 0.5%, or 1%. In a specific embodiment, the shrinkage rate may be 0.1% to 20%, 0.5% to 15%, or 1% to 13%, but is not limited thereto.

[0047] In one embodiment, the polypropylene microporous film may have a melt-rupture temperature of 165°C or higher, or 170°C or higher, and there is no particular upper limit, but it may be, for example, 250°C. In a specific embodiment, the melt-rupture temperature may be 165°C to 250°C or 170°C to 250°C, but is not limited thereto.

[0048] In one embodiment, the polypropylene microporous film may be manufactured by a wet process including a sequential biaxial stretching step, thereby providing a polypropylene microporous film that satisfies all of the above-mentioned physical properties.

[0049] Specifically, the polypropylene microporous membrane is manufactured by extruding and sequentially biaxially stretching a mixture of polypropylene resin in which diluent is dissolved to form a film, and then extracting diluent from the film. It may be manufactured by a wet process that includes a normal sequential biaxial stretching step known to those skilled in the art, and is not limited as long as a microporous membrane having the above-mentioned physical properties can be manufactured.

[0050] In one embodiment, a polypropylene microporous membrane satisfying both of the above-mentioned physical properties can be manufactured by using a polypropylene resin having a viscosity-average molecular weight within a specific range, or by heat treatment at a specific temperature after extraction of a diluent. As a result, the polypropylene microporous membrane can have excellent puncture strength and permeability, as well as significantly improved heat resistance at high temperatures. By including a polypropylene microporous membrane satisfying both of the above-mentioned physical properties, this disclosure can provide a battery that ensures excellent battery performance while also having excellent thermal safety at high temperatures, such as not producing smoke or igniting during hot box evaluation at a high temperature of 140°C.

[0051] In one embodiment, the polypropylene resin contained in the polypropylene microporous membrane has a viscosity-average molecular weight of 1 × 10⁻⁶ 6 g / mol ~ 3 × 10 6 The value can be g / mol, specifically 1.2 × 10⁻⁶. 6 g / mol ~ 2.5 × 10 6 g / mol, more specifically 1.3 × 10⁻⁶ 6 g / mol ~ 2.2 × 10 6 It can be expressed as g / mol.

[0052] In one embodiment, heat treatment at a specific temperature after diluent extraction may involve heat-stretching the film obtained after diluent extraction at a first temperature, and then heat-fixing and heat-relaxing the heat-stretched film at a second temperature higher than the first temperature.

[0053] The method for manufacturing the polypropylene microporous membrane described herein will be explained below.

[0054] A method for producing a polypropylene microporous membrane according to one embodiment includes (a) a viscosity-average molecular weight of 1 × 10 6 g / mol ~ 3 × 10 6The process may include the steps of: (b) producing a molten product by melting and kneading a mixture containing a polypropylene resin in g / mol and a diluent using an extruder; (c) forming the molten product into a sheet by extruding it; (d) forming a film by sequentially biaxially stretching the sheet in the longitudinal and transverse directions; (e) extracting the diluent from the stretched film and drying it; and (e) heat-stretching the dried film at a temperature at which 2% to 5% of the polypropylene resin crystals melt, and then heat-treating the heat-stretched film by heat-setting and heat-relaxing it at a temperature at which 25% to 60% of the polypropylene resin crystals melt.

[0055] The following describes each manufacturing step.

[0056] First, step (a) is a step of producing a molten product by melting and kneading a mixture containing polypropylene resin and diluent using an extruder, wherein the mixture contains polypropylene resin and diluent in a weight ratio of 10-60:90-40 for pore formation, specifically in a weight ratio of 20-40:80-60, but is not particularly limited as long as the objectives of this disclosure are achieved. When the weight ratio within the above range is satisfied, the molten product has sufficient fluidity, a uniform sheet can be easily formed in the subsequent steps, and sufficient orientation is achieved during the stretching process, ensuring mechanical strength, thus preventing problems such as breakage during the stretching process.

[0057] The aforementioned polypropylene resin has a viscosity-average molecular weight of 1 × 10 6 g / mol ~ 3 × 10 6 g / mol, preferably 1.2 × 10⁻⁶ 6 g / mol ~ 2.5 × 10 6 g / mol, more preferably 1.3 × 10⁻⁶ 6 g / mol ~ 2.2 × 10 6The material may be polypropylene with a viscosity-average molecular weight of g / mol, or it may contain polypropylene having a viscosity-average molecular weight within the range described above. Having a viscosity-average molecular weight within the range described above makes it possible to achieve the physical properties intended by this disclosure. A battery to which a microporous film satisfying the above physical properties is applied has low initial resistance and excellent battery performance, and can achieve excellent thermal safety in hot box evaluation.

[0058] In one embodiment, the polypropylene resin may have a melting temperature of 160°C or higher, specifically 160°C to 170°C, but is not necessarily limited thereto. The melting temperature of the polypropylene resin can be determined by DSC.

[0059] In one embodiment, the diluent can be any organic compound that forms a single phase with the polypropylene resin at the extrusion temperature, without any limitations. For example, the diluent may be one or more combinations selected from the group consisting of aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, paraffin oil, and paraffin wax; phthalic acid esters such as dibutyl phthalate and dioctyl phthalate; C10-C20 fatty acids such as palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid; and C10-C20 fatty alcohols such as cetyl alcohol, stearyl alcohol, and oleyl alcohol. One specific example of the aforementioned diluent is, but is not limited to, paraffin oil having a kinetic viscosity of 20 cSt to 200 cSt at 40°C. Alternatively, other diluents that have the same or similar kinetic viscosity at an appropriate extrusion temperature may be used. For example, diluents with a kinetic viscosity of 1 cSt to 1000 cSt or 10 cSt to 500 cSt may be used.

[0060] Furthermore, the mixture may further contain one or more common additives for improving specific functions, such as oxidation stabilizers, UV stabilizers, and antistatic agents, to the extent that the properties of the microporous membrane do not significantly deteriorate.

[0061] (b) Step is a step of extruding the molten material into a sheet, which may be carried out in a manner known to those skilled in the art, and may be, for example, by extruding the molten material with a T-die and forming it into a sheet by casting or calendering while cooling to a temperature of 10°C to 80°C.

[0062] (c) The longitudinal and transverse stretch ratios in step (c) may be independently 4 times or more, 6 times or more, 10 times or less, or 15 times or less, for example, 4 times to 15 times or 6 times to 10 times. By satisfying the above ranges for longitudinal and transverse stretch ratios, a polypropylene microporous film having the physical properties targeted in this disclosure can be manufactured.

[0063] (c) The stretching in step (c) is carried out by a sequential stretching method using a roll or tenter, and may be performed at a temperature in the range of 80°C below the melting temperature of polypropylene to the melting temperature of polypropylene. When stretching is performed at the above-mentioned temperature range, the fluidity of the polypropylene resin can be ensured for effective stretching. Specifically, the sheet is stretched uniformly throughout and no breakage occurs due to stretching, so the stretching can be performed stably. As a result, a high-quality microporous membrane can be manufactured with uniform physical properties such as gas permeability and puncture strength throughout 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 to these.

[0064] Step (d) is to extract the diluent from the stretched film and dry it, which is done by extracting the diluent from the film using an organic solvent and drying the organic solvent with the film in which the diluent has been replaced by the organic solvent. The organic solvent is not particularly limited as long as it can extract the diluent. Specifically, methyl ethyl ketone, methylene chloride, hexane, etc. may be used as the organic solvent because they have high extraction efficiency and dry quickly.

[0065] (d) Step is preferably carried out at a high temperature to increase the solubility of the diluent and the organic solvent, but may be carried out at a temperature of 40°C or lower from the viewpoint of safety due to the boiling of the organic solvent.

[0066] (e) Step (e) involves heat-stretching the dried film using a roll-type or tenter-type apparatus at a temperature at which 2% to 5% of the polypropylene resin crystals melt, and then heat-treating the heat-stretched film by heat-setting and heat-relaxing it at a temperature at which 25% to 60% of the polypropylene resin crystals melt. By heat-treating at the above temperatures, the physical properties intended in this disclosure can be achieved. Microporous films having such physical properties are suitably applied to high-power / high-capacity batteries. In detail, batteries to which a microporous film satisfying the above physical properties is applied have low initial resistance and excellent battery performance, and can achieve excellent thermal safety in hot-box evaluation.

[0067] In one embodiment, the temperature at which the crystals of the polypropylene resin melt to a certain level may vary depending on the viscosity-average molecular weight and crystallinity of the polypropylene used. In this disclosure, since polypropylene having a viscosity-average molecular weight within the above range is used as the polypropylene resin for producing a polypropylene microporous film, the thermal stretching in step (e) may be carried out at a temperature of 125°C to 135°C, specifically at a temperature of 127°C to 133°C. Furthermore, the thermal setting and thermal relaxation in step (e) may be carried out at a temperature of 155°C to 165°C, specifically at a temperature of 157°C to 165°C.

[0068] The thermal stretching in step (e) above may be, for example, stretching in the longitudinal or transverse direction, and from the viewpoint of thermal shrinkage rate, it may be stretching in the transverse direction, and may be stretching to 120% to 160% or 130% to 150% of the transverse width before thermal stretching.

[0069] The thermal relaxation in step (e) above is to relax (shrink) in the longitudinal or transverse direction, and from the viewpoint of the thermal shrinkage rate, it may be to relax in the transverse direction, and may be to relax to 80% to 99% or 90% to 99% of the transverse width before thermal relaxation.

[0070] This disclosure provides a separator comprising a polypropylene microporous membrane as described above, which may be a separator used in any known energy storage device, and is not particularly limited, but a non-limiting example is a separator used in a lithium secondary battery.

[0071] Examples and experimental cases are described below with specific illustrations. However, the examples and experimental cases described below are merely illustrative and the technology described herein is not limited thereto.

[0072] [Physical property measurement method] 1.Viscosity average molecular weight (g / mol) The viscosity-average molecular weight (Mv) of polypropylene was calculated by measuring the intrinsic viscosity (η) at 165°C according to ISO 16152:2022 using Polymer Char's Crystex QC model, and then calculating it using the Margolies equation shown in Mathematical Equation 1 below. The intrinsic viscosity was measured using 1,2,4-trichlorobenzene (TCB) as the solvent.

[0073] [Mathematical formula 1] Mv = 5.37 x 10 4 x[η] 1.49

[0074] 2. Thickness of microporous membrane (μm) The thickness of the microporous membrane was measured using a contact-type thickness measuring instrument with a thickness accuracy of 0.1 μm. The measurement was performed using a TESA Mu-Hite Electronic Height Gauge from TESA Corporation, with a measurement pressure of 0.63 N.

[0075] 3.Punching strength (N / μm) Penetration strength was measured using an INSTRON UTM (Universal Test Machine) 3345, with a pin tip having a diameter of 1.0 mm and a radius of curvature of 0.5 mm attached, and by pressing a microporous membrane at a speed of 120 mm / min. The penetration strength was calculated by dividing the load (N) at which the microporous membrane ruptured by the thickness of the microporous membrane (μm).

[0076] 4. Gas permeability (Darcy) Gas permeability was measured using a porometer (CFP-1500-AEL from Porous Materials Inc. (PMI)). Generally, gas permeability is expressed in terms of the Gurley number, but the Gurley number does not correct for the effect of thickness, making it difficult to understand the relative permeability due to the porosity structure. To solve this problem, the gas permeability in this disclosure is measured using the Darcy permeability constant calculated by the following mathematical formula 2. Nitrogen was used as the gas, and the average value of the Darcy permeability constant measured in the 100-200 psi range was calculated.

[0077] [Mathematical formula 2] Darcy transmission constant (C)=(8F·T·V) / (πD 2 (P 2 -1)) F=Flow rate (cc / cm) T = Sample thickness (mm) V = Viscosity of the gas (0.185 cP at 20℃ for N2) D = Diameter of the sample (mm) P = pressure (psi)

[0078] 5. Lateral contraction rate (%) A 15cm x 15cm microporous membrane, with its length and width indicated on a 10cm length, was placed in a temperature-stabilized oven (DKN612, Yamato Scientific Co., Ltd.) at 150°C for 1 hour. The change in length was then measured, and the lateral shrinkage rate was calculated using the method shown in mathematical formula 3 below.

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

[0080] 6. Average pore size (nm) The average pore size was measured according to ASTM F316-03 using a porometer (CFP-1500-AEL from PMI). The average pore size was measured using the half-dry method (ASTM F316-03), and Galwick solution (surface tension: 15.9 dyne / cm) provided by PMI was used to measure pore size.

[0081] 7. Melt-break temperature The melt-break temperature of the microporous membrane was measured by fixing a microporous membrane 12 (5cm × 5cm) as shown in Figure 2 to a frame 10 (outer dimensions: 7.5cm × 7.5cm, inner dimensions: 2.5cm × 2.5cm) as shown in Figure 1 using polyimide tape 14, and then leaving it in a convection oven maintained at a set temperature for 10 minutes. After that, the presence or absence of rupture of the microporous membrane was observed and measured. The highest temperature at which the microporous membrane did not rupture after 10 minutes was defined as the melt-break temperature.

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

[0083] If the initial resistance is 50 mΩ or less, the discharge test using 2C-rate will confirm that the capacitance is maintained at 20% or more, and the initial resistance value will serve as an evaluation index for the output characteristics.

[0084] 9. Hotbox Evaluation The hot box evaluation was performed using a battery assembled with a polypropylene microporous membrane as the separator. Specifically, a positive electrode using NCM622 (Ni:Co:Mn=6:2:2) as the active material and a negative electrode using graphite carbon as the active material were wound together with the manufactured microporous membrane and placed in an aluminum pouch. An electrolyte solution containing ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a volume ratio of 3:5:2, with 1M lithium hexafluorophosphate (LiPF6) dissolved in it was then injected and the pouch was sealed to assemble a 2Ah capacity battery. After aging and degassing the assembled battery, it was fully charged to 4.2V and heated in an oven at 5°C / min. After reaching 140°C, the battery was left for 30 minutes and the changes in the battery were measured.

[0085] After being left at 140°C for 30 minutes, the test was judged as "Fail" if smoke or fire occurred in the battery, and "Pass" if there was no change in battery voltage / current and no smoke or fire occurred.

[0086] 10. Thermal property analysis of the film The phenomenon of polypropylene resin crystals melting due to temperature was analyzed using a Discovery DSC250 differential scanning calorimeter (DSC) from TA Instrument Co., Ltd., under conditions of a sample weight of 5 mg and a scan speed of 10°C / min.

[0087] The temperature at which n% of polypropylene resin crystals melt (T n The heat capacity was measured by a method that calculates the temperature at the point where n% of the total heat capacity (Heat of fusion) is represented. In this disclosure, T2, T5, and T5 are the temperatures at which 2%, 5%, 25%, and 60% of the crystals of the film introduced into the heat fixing device melt, respectively.25 , and T 60 The values ​​were measured and recorded in Table 1.

[0088] 11.Porosity (%) The porosity was calculated using the following mathematical formula. Specifically, a sample with dimensions of A cm (e.g., 30 cm) in width, B cm (e.g., 20 cm) in height, and T cm (e.g., 0.0003 to 0.002 cm) was prepared, its mass was measured, and the porosity was calculated from the ratio of the mass of the resin to the mass of the microporous membrane for the same volume.

[0089] Porosity (%)=100×{1-M / (A×B×T×ρ)} In the above mathematical formula, M is the mass (g) of the microporous membrane, and ρ is the density (g / cm³) of the polypropylene resin forming the microporous membrane. 3 )

[0090] <Example 1> Viscosity-average molecular weight is 1.5 × 10 6 A mixture containing polypropylene resin with a melting point of 165°C and paraffin oil with a kinematic viscosity of 80 cSt at 40°C in a weight ratio of 30:70 was prepared by melt-kneading using a twin-screw extruder. The melting point was measured using a Discovery DSC250 differential scanning calorimeter (DSC) provided by TA Instruments, under conditions of a sample weight of 5 mg and a scan speed of 10°C / min.

[0091] The molten material was continuously extruded using a T-die, and a sheet with a width of 300 mm and an average thickness of 1100 μm was produced using a casting roll set to 30°C. The sheet was then stretched longitudinally using a roll method at a stretching temperature of 115°C to a six-fold increase, and subsequently guided to a tenter for transverse stretching at a stretching temperature of 140°C to a six-fold increase.

[0092] Using methylene chloride, paraffin oil was extracted from the film after it had been stretched in both the longitudinal and transverse directions at 25°C, and the film from which the paraffin oil had been extracted was dried at 60°C.

[0093] The dried film was heat-stretched laterally to 145% using a tenter-type heat-setting device at 130°C, the temperature at which 3% of the polypropylene resin crystals melt. Next, it was heat-set for 10 seconds at 160°C, the temperature at which 35% of the polypropylene resin crystals melt, and then heat-relaxed (shrinked) to 93% of its width before the heat relaxation process, thereby producing a microporous film with a thickness of 12.8 μm.

[0094] The physical properties of the ultimately manufactured microporous membrane and the performance of the battery to which it was applied are recorded in Table 2 below.

[0095] <Examples 2-3 and Comparative Examples 1-6> Except for changing the viscosity-average molecular weight (PP Mv), longitudinal / transverse stretching ratio, and heat treatment temperature of the polypropylene resin used to the conditions listed in Table 1, a polypropylene microporous film was manufactured in the same manner as in Example 1. The physical properties of the finally manufactured microporous film and the performance of the battery to which it was applied are recorded in Table 2 below.

[0096] [Table 1]

[0097] [Table 2]

[0098] Referring to Tables 1 and 2 above, the microporous membranes of the examples satisfy all the physical properties targeted by this disclosure, and the batteries to which they are applied have an initial resistance of 50 mΩ or less and pass hot box evaluation, achieving excellent battery performance and safety in high-temperature environments.

[0099] The microporous membrane of Comparative Example 1 is 1.0 × 10⁻⁶ 6 When manufactured using polypropylene resin with a viscosity-average molecular weight of less than g / mol, less entanglement between chains occurred, resulting in film breakage during the heat stretching step of the heat treatment process.

[0100] The microporous membrane of Comparative Example 2 is 3.0 × 10⁻⁶ 6 When manufactured using polypropylene resin with a viscosity-average molecular weight exceeding g / mol, the mixing of the resin and diluent is uneven, resulting in the production of sheets with uneven thickness and the disadvantage of uneven product quality.

[0101] In Comparative Examples 3 and 4, the microporous films underwent a heat treatment step where the thermal stretching was performed at a temperature higher than the temperature at which 5% of the film crystals melted (T5) before being introduced into the heat-fixing apparatus. As a result, the gas permeability, porosity, and average pore size were 1.0 × 10⁻⁶, respectively. -5 Because it is less than Darcy, less than 25%, and less than 24nm, batteries to which it is applied have the same thermal safety as those described in this disclosure, but have the disadvantage of having an initial resistance of 55mΩ or more, resulting in inferior battery performance.

[0102] The microporous film of Comparative Example 5 undergoes thermal setting and thermal relaxation in the heat treatment step, and is introduced into the heat setting device at a temperature at which 25% of the crystals of the film melt (T 25 When the experiment was conducted at a lower temperature, the physical properties such as gas permeability were met, but the lateral shrinkage rate was found to be 25.1%, resulting in a significant decrease in the safety of batteries using this method in high-temperature environments.

[0103] The microporous film of Comparative Example 6 undergoes thermal setting and thermal relaxation in the heat treatment step, and is introduced into the heat setting device at a temperature at which 60% of the crystals of the film melt (T 60 The results obtained at temperatures higher than 0.2 × 10⁻⁶ showed gas permeability, porosity, and average pore size to be 0.2 × 10⁻⁶, respectively. -5 Because it is less than Darcy, less than 16%, and less than 19nm, batteries to which it is applied have the same thermal safety as those described in this disclosure, but have the disadvantage of having an initial resistance of 65mΩ and inferior battery performance.

[0104] As described above, this disclosure has been explained through specific matters and limited embodiments, which are provided only for a more general understanding of the disclosure, and the disclosure is not limited to the embodiments described above. A person with ordinary skill in the art to which this disclosure belongs can make various modifications and variations from such descriptions.

Claims

1. Viscosity-average molecular weight is 1 × 10 ６ g / mol ~ 3 × 10 ６ It contains polypropylene in g / mol, has a microporous membrane thickness of 3 μm to 30 μm, a puncture strength of 0.20 N / μm or more, and a gas permeability of 1.0 × 10⁻⁶. －５ A polypropylene microporous membrane that is Darcy or higher, has a porosity of 25% to 60%, an average pore size of 25 nm to 50 nm, and a lateral shrinkage rate of 20% or less measured after being left at 150°C for 1 hour.

2. The polypropylene microporous membrane according to claim 1, wherein the melting and rupture temperature is 165°C or higher.

3. The polypropylene microporous membrane according to claim 1, wherein the lateral shrinkage rate is 15% or less.

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

5. (a) Viscosity-average molecular weight is 1 × 10 ６ g / mol ~ 3 × 10 ６ A step of producing a molten product by melt-kneading a mixture containing polypropylene resin and diluent in g / mol using an extruder, (b) The step of extruding the molten material to form it into a sheet, (c) A step of forming the sheet into a film by successively biaxially stretching it in the longitudinal and transverse directions, (d) A step of extracting diluent from the stretched film and drying it, (e) A method for producing a polypropylene microporous film, comprising the steps of (e) heat-stretching a dried film at a temperature in which 2% to 5% of the polypropylene resin crystals melt, and heat-treating the heat-stretched film by heat-setting and heat-relaxing it at a temperature in which 25% to 60% of the polypropylene resin crystals melt.

6. The method for producing a polypropylene microporous film according to claim 5, wherein the thermal stretching in step (e) is carried out at a temperature of 125°C to 135°C.

7. The method for producing a polypropylene microporous film according to claim 5, wherein the thermal setting and thermal relaxation in step (e) are carried out at a temperature of 155°C to 165°C.

8. A separator comprising a polypropylene microporous membrane according to any one of claims 1 to 4.

9. Viscosity-average molecular weight is 1 × 10 ６ g / mol ~ 3 × 10 ６ It contains polypropylene in g / mol, has a microporous membrane thickness of 3 μm to 30 μm, a puncture strength of 0.20 N / μm or more, and a gas permeability of 1.0 × 10⁻⁶. －５ A microporous membrane for use as a separator in secondary batteries, having a Darcy grade or higher, a porosity of 25% to 60%, an average pore size of 25 nm to 50 nm, and a lateral shrinkage rate of 20% or less measured after being left at 150°C for 1 hour.

10. A microporous membrane for a separator in a secondary battery according to claim 9, wherein the melting and rupture temperature is 165°C or higher.

11. A polypropylene microporous membrane manufactured by the manufacturing method described in any one of claims 5 to 7.

12. A secondary battery comprising a positive electrode, a negative electrode, and the separator described in claim 8.

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