Microporous polyolefin film, method for manufacturing same, and secondary battery comprising same

Abstract

The present invention relates to a microporous polyolefin film and, more specifically, to a microporous polyolefin film, a method for manufacturing same, and a secondary battery comprising same, wherein the polyolefin comprises an alpha-olefin-derived unit and, with respect to the alpha-olefin-derived unit, the average pore size of the microporous film is 10 nm to 45 nm depending on the number of short chain branches (SCBs) per 1,000 carbon atoms in the polyolefin backbone.

Description

Polyolefin microporous membrane, method for producing the same, and secondary battery including the same

[0001] Cross-citation with related applications

[0002] This application claims the benefit of priority from Korean Patent Application No. 10-2024-0098445, filed July 25, 2024, the entire contents of which are incorporated herein by reference.

[0003] Technology field

[0004] The present invention relates to a polyolefin microporous film, a method for producing the same, and a secondary battery including the same.

[0005]

[0006] Lithium secondary batteries, which have the characteristics of high output, high capacity, light weight, and small size, are composed of four elements: a cathode, an anode, an electrolyte, and a separator. Among them, the separator electrically separates the anode and cathode, thereby increasing the stability of the battery.

[0007] Among these, the separator has a microporous structure that not only ensures smooth ion conductivity and excellent impregnation of the electrolyte, but also prevents electrical short-circuiting due to physical contact between the positive and negative electrodes, and performs a shutdown function in the event of an excessive temperature rise due to abnormal battery behavior. The shutdown function refers to the function that when a charged battery is short-circuited, the potential difference between the positive and negative electrodes rapidly narrows, causing an exothermic reaction, and the electrolyte decomposes to generate gases such as methane, hydrogen, and carbon dioxide, which may cause an explosion. In this case, the separator melts and blocks the porous pores, delaying the flow of current and stopping the battery reaction and exothermic reaction, thereby ensuring stability.

[0008] The required characteristics of the separator vary depending on the intended use and purpose. For general small devices, a polyolefin-based microporous membrane with a thickness of approximately 15 μm to 25 μm and pores of 50 nm to 1 μm is generally used. It requires characteristics such as insulation to prevent short circuits between electrodes, a thin thickness for high capacity, high tensile strength and puncture resistance to prevent deformation during battery assembly and rupture due to metal impurities, low shrinkage at high temperatures, and excellent electrochemical stability. In addition, a permeability of approximately 400 sec / 100 cc and a porosity of approximately 40% are required for smooth ion conduction and excellent impregnation of the electrolyte.

[0009] In comparison, in cases where high output characteristics such as HEV and PHEV are required, the spectroscopic value should be lower to improve output characteristics, and the meltdown temperature should be high, approximately 180℃ or higher, so that short circuits between electrodes can be prevented even when exposed to high temperatures due to abnormal battery behavior.

[0010] Methods for manufacturing microporous membranes are classified into dry and wet methods depending on whether or not a solvent is used. The dry method involves melting and extruding a crystalline polyolefin polymer to form a plate-shaped sheet, heat-treating it, and then stretching it at low and high temperatures to form pores. Because it does not use a solvent, the process is simple and highly productive. However, it is disadvantageous for producing products with wide widths, and the membrane thickness can easily be uneven. Furthermore, uniaxial stretching can cause directional dependence of mechanical strength. The wet method involves mixing a low-molecular-weight organic substance (pore-forming agent) such as liquid paraffin or solid wax with a polyolefin polymer, heating and melting it in an extruder. The sheet is then passed through a T-die and casting roll, stretched at a temperature near the crystal melting point, washed with a non-volatile solvent, and any remaining solvent is removed. The pore structure is then fixed through drying and heat treatment.

[0011] Among the components of lithium secondary batteries, the separator is a crucial technology, and its physical properties and quality are crucial factors in determining battery performance. In particular, physical properties such as tensile strength and puncture strength, and pore characteristics such as air permeability and porosity, exhibit conflicting characteristics. Therefore, it is crucial to tailor these properties to the battery's intended use. Various attempts have been made to address this issue, but to date, no commercially available solution has been sufficiently satisfactory.

[0012] To enhance the performance of microporous membranes, and furthermore, of secondary batteries containing microporous membranes, the pore size and size distribution must be appropriately controlled. This is because electrolyte impregnation and output characteristics vary depending on the pore characteristics of the microporous membrane.

[0013] In order to improve the porosity characteristics, a method of increasing the content of a porosity-forming agent in high-density polyethylene (HDPE) and lowering the porosity and air permeability while lowering the physical properties can be generally used. However, this method has the disadvantage of causing a pressure drop in the extruder during extrusion processing, which lowers the melt mixing characteristics, resulting in unmelted products and poor workability.

[0014] Prior art literature

[0015] Republic of Korea Patent No. 10-2260536

[0016]

[0017] The present invention, which aims to solve the problems described above, provides a polyolefin microporous membrane in which the average pore size of the microporous membrane is controlled by controlling the number of short chain branches (SCBs) of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain.

[0018] In addition, the present invention aims to provide a method for manufacturing the polyolefin microporous membrane.

[0019] In addition, the present invention aims to provide a secondary battery including the polyolefin microporous film.

[0020] The problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

[0021]

[0022] One embodiment of the present invention for solving the above-described problem provides a polyolefin microporous membrane, wherein the polyolefin includes an alpha-olefin derived unit, and the average pore size of the microporous membrane is 10 nm to 45 nm depending on the number of short chain branches (SCBs) of the alpha-olefin derived unit per 1,000 carbon atoms of the polyolefin main chain.

[0023] The above polyolefin microporous membrane can satisfy at least one of the following equations 1 and 2.

[0024] [Formula 1]

[0025] SCB / 1000C ≤ 4.0

[0026] [Formula 2]

[0027] -10 x + 30 ≤ y ≤ -10 x + 55

[0028] In the above formula 1, SCB / 1000C represents the number of short-chain branches of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain,

[0029] In the above equation 2, x represents SCB / 1000C, and y represents the average pore size of the microporous membrane.

[0030] The above alpha olefin may have 3 to 8 carbon atoms.

[0031] The above polyolefin may be an ethylene-alphaolefin copolymer or a mixture of an ethylene-alphaolefin copolymer and an ethylene homopolymer.

[0032] The above mixture may contain an ethylene-alphaolefin copolymer and an ethylene homopolymer in a weight ratio of 20:80 to 80:20.

[0033] The viscosity average molecular weight of the above polyolefin may be 200,000 g / mol to 5,000,000 g / mol.

[0034] The tensile strength of the above microporous membrane is 80 gf / (g / m 2 ) may be more than that.

[0035] The porosity of the above microporous membrane may be 30% to 70%.

[0036] The ionic conductivity of the above microporous membrane may be 0.50 mS / cm or more.

[0037] The above microporous membrane has a Gurley permeability of 30 s / 100cm. 3 200 s / 100cm 3 It could be.

[0038] The thickness of the above microporous membrane may be 3 ㎛ to 20 ㎛.

[0039] In addition, one embodiment of the present invention for solving the above-described problem provides a method for producing the above-described polyolefin microporous membrane, and specifically, provides a method for producing the above-described polyolefin microporous membrane, including the steps of: supplying raw materials including polyolefin and oil to an extruder, mixing them, extruding a melted product, and obtaining a sheet-shaped extruded product; and stretching the extruded product.

[0040] The above oil may be a paraffinic oil.

[0041] The weight ratio of the polyolefin resin blend supplied during the extrusion and the oil may be 10:90 to 40:60.

[0042] The step of stretching the extrudate may include a first stretching step of stretching the extrudate 2 to 6 times in the machine direction (MD) at 100°C to 125°C; and a second stretching step of stretching the extrudate stretched in the first stretching step 7 to 12.5 times in the transverse direction (TD) at 116°C to 135°C.

[0043] After the step of extending the above-mentioned discharge, a step of removing oil through extraction may be further included.

[0044] After the step of removing the oil, a step of heat-setting in the transverse direction (TD) at 1 to 1.8 times the transverse direction (TD) at 110 to 135°C may be further included.

[0045] In addition, one embodiment of the present invention for solving the above-described problem provides a secondary battery including the above-described polyolefin microporous film.

[0046] The above polyolefin microporous membrane may be a separator of the secondary battery.

[0047]

[0048] The microporous membrane according to the present invention contains a short-chain branch composed of alpha-olefin at a certain ratio, so that the average pore size of the microporous membrane can be controlled within the range of 10 nm to 45 nm, and a secondary battery including the microporous membrane has excellent battery stability, assembly properties, and output characteristics.

[0049] The effects of the present invention are not limited to the effects mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the description below.

[0050]

[0051] Figure 1 is a graph showing the average pore size of the microporous membrane according to SCB / 1000C of the polyolefin of the examples.

[0052] Figure 2 shows an equation for calculating ionic conductivity.

[0053]

[0054] The advantages and features of the present invention, and the methods for achieving them, will become clearer with reference to the embodiments described in detail below. However, the present invention is not limited to the embodiments disclosed below and may be implemented in various different forms. These embodiments are provided solely to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the scope of the present invention, and the present invention is defined solely by the scope of the claims.

[0055] Before proceeding, the meanings of terms used in this specification will be briefly explained. However, please note that the explanation of terms is intended to aid understanding of this specification and, unless explicitly stated to limit the invention, they are not intended to limit the technical spirit of the invention.

[0056] The terminology used herein is for the purpose of describing embodiments only and is not intended to limit the present invention. In this specification, the singular also includes the plural unless specifically stated otherwise. As used herein, the terms "comprises" and / or "comprising" do not exclude the presence or addition of one or more other components in addition to the mentioned components. Throughout the specification, the term "and / or" includes each and any and all combinations of one or more of the mentioned components. Although "primary", "secondary", etc. are used to describe various components, these components are not limited by these terms. These terms are only used to distinguish one component from another.

[0057] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in their common sense to those skilled in the art to which the present invention pertains. Furthermore, terms defined in commonly used dictionaries are not to be interpreted ideally or excessively unless explicitly and specifically defined otherwise.

[0058]

[0059] Hereinafter, the present invention will be described in more detail to help understand the present invention.

[0060] <Polyolefin microporous membrane>

[0061] According to one embodiment, the present invention provides a polyolefin microporous membrane, and more specifically, the polyolefin includes an alpha-olefin derived unit, and the average pore size of the microporous membrane can be provided in a range of 10 nm to 45 nm depending on the number of short chain branches (SCBs) of the alpha-olefin derived unit per 1,000 carbon atoms of the polyolefin main chain.

[0062] In one embodiment, the alpha-olefin may have 3 to 8 or 3 to 6 carbon atoms. For example, the alpha-olefin may be at least one selected from the group consisting of propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, and 1-octene. As a specific example, the alpha-olefin may be at least one selected from the group consisting of propylene, 1-butene, and 1-hexene.

[0063] Depending on the type of the alpha-olefin and the amount of the copolymerized alpha-olefin, the number of short chain branches (SCB) of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain varies. In the present invention, by controlling the number of short chain branches (SCB) of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain, the average diameter of the pores formed in the polyolefin microporous membrane can be controlled to 10 nm to 45 nm, 10 nm to 43 nm, 10 nm to 40 nm, 10 nm to 35 nm, 10 nm to 30 nm, or 15 nm to 40 nm. By having an average pore size within the above-mentioned range, electrolyte affinity, mechanical properties, and battery properties can be excellent.

[0064] In one embodiment, the polyolefin microporous membrane may satisfy at least one of the following equations 1 and 2.

[0065] [Formula 1]

[0066] SCB / 1000C ≤ 4.0

[0067] [Formula 2]

[0068] -10 x + 30 ≤ y ≤ -10 x + 55

[0069] In the above formula 1, SCB / 1000C represents the number of short-chain branches of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain,

[0070] In the above equation 2, x represents SCB / 1000C, and y represents the average pore size of the microporous membrane.

[0071] The above formula 1 may mean that the number of short-chain branches of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain is 4 or less, and for example, the SCB / 1000C may be 3.5 or less, 0 to 3.5, 0.2 to 0.2 to 3.5, 0.2 to 3.0, 0.5 to 2.5, or 0.5 to 2.0. By having the SCB / 1000C within the above-mentioned range, the average diameter of the pores formed in the polyolefin microporous membrane can be controlled. In this way, when the average diameter of the pores formed in the microporous membrane is controlled through SCB / 1000C of polyolefin, the pore size can be adjusted without changing other processing conditions simply by changing the type of polyolefin, and a microporous membrane with high ionic conductivity can be manufactured through a small pore size.

[0072] The above equation 2 is an equation representing the relationship between the number of short-chain branches of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain and the average pore size of the microporous membrane. However, the average pore size (y) value of the microporous membrane is 0 or a positive number. Specifically, the above equation 2 may mean a negative correlation in which the average pore size of the microporous membrane decreases as the content of short-chain branches increases. The above equation 2 is an equation empirically derived through the results of numerous experiments, and may mean that the average pore size, which affects the performance of the microporous membrane, can be controlled by controlling the amount of short-chain branches of the polyolefin used.

[0073] In one embodiment, the polyolefin microporous membrane can satisfy both Equations 1 and 2 below. Specifically, by satisfying Equation 2 while the number of short-chain branches of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain is 4 or less, a microporous membrane having excellent ionic conductivity can be obtained.

[0074] In one embodiment, the viscosity average molecular weight of the polyolefin may be 200,000 g / mol to 5,000,000 g / mol, 250,000 g / mol to 4,000,000 g / mol, or 250,000 g / mol to 3,000,000 g / mol. When a polyolefin having a viscosity average molecular weight within the above-mentioned range is included, a microporous membrane having excellent ionic conductivity can be obtained.

[0075] The above viscosity average molecular weight can be calculated from the intrinsic viscosity using the following formula according to ASTM D 4020.

[0076] Mw = 5.37 Х 10 4 Х [η] 1.49

[0077] (Mw stands for viscosity-average molecular weight (g / mol), and η stands for intrinsic viscosity (dl / g).)

[0078] In one embodiment, the melting point of the polyolefin may be 110°C to 140°C, 115°C to 135°C, or 125°C to 135°C. When a polyolefin having a melting point within the above-described range is included, a microporous membrane having excellent ionic conductivity can be obtained.

[0079] In one embodiment, the polyolefin may be an ethylene-alphaolefin copolymer or a mixture of an ethylene-alphaolefin copolymer and an ethylene homopolymer.

[0080] When an ethylene-alphaolefin copolymer is used alone as the polyolefin, the viscosity average molecular weight of the ethylene-alphaolefin copolymer may be 250,000 g / mol to 3,000,000 g / mol. As a specific example, when the alphaolefin is propylene, the viscosity average molecular weight may be 250,000 g / mol to 2,000,000 g / mol, when the alphaolefin is 1-butene, the viscosity average molecular weight may be 2,000,000 g / mol to 3,000,000 g / mol, and when the alphaolefin is 1-hexene, the viscosity average molecular weight may be 500,000 g / mol to 1,000,000 g / mol. When an ethylene-alpha-olefin copolymer having a viscosity-average molecular weight within the aforementioned range is used alone, the average pore size of the microporous membrane can be adjusted to a desired range by controlling SCB / 1000C, thereby further improving battery performance when the microporous membrane is used as a battery separator.

[0081] When a mixture of an ethylene-alphaolefin copolymer and an ethylene homopolymer is used as the polyolefin, the viscosity average molecular weight of the ethylene-alphaolefin copolymer may be 250,000 g / mol to 1,500,000 g / mol. As a specific example, when the alphaolefin is propylene, the viscosity average molecular weight may be 250,000 g / mol to 2,000,000 g / mol, when the alphaolefin is 1-butene, the viscosity average molecular weight may be 1,200,000 g / mol to 1,800,000 g / mol, and when the alphaolefin is 1-hexene, the viscosity average molecular weight may be 500,000 g / mol to 1,000,000 g / mol. Additionally, the viscosity average molecular weight of the ethylene homopolymer may be 250,000 g / mol to 2,000,000 g / mol.

[0082] When a mixture of an ethylene-alphaolefin copolymer and an ethylene homopolymer is used as the above polyolefin, the weight ratio of the ethylene-alphaolefin copolymer and the ethylene homopolymer may be 20:80 to 80:20, 25:75 to 75:25, or 60:40 to 40:60.

[0083] When a mixture of an ethylene-alphaolefin copolymer and an ethylene homopolymer within the aforementioned range is used, the average pore size of the microporous membrane can be adjusted to a desired range by controlling SCB / 1000C, thereby further improving battery performance when the microporous membrane is used as a battery separator.

[0084] In one embodiment, the microporous membrane may have a thickness of 3 μm to 20 μm, 5 μm to 17 μm, 8 μm to 15 μm, or 8 μm to 10 μm. The thickness may be measured according to ASTM D374. The polyolefin microporous membrane may have a constant thin thickness within the above-described range, and may improve the stability of a secondary battery including the polyolefin microporous membrane as a separator.

[0085] In one embodiment, the tensile strength of the microporous membrane for the basis weight is 80 gf / (g / m 2 ) or more, 80 gf / (g / m 2 ) to 150 gf / (g / m 2 ) or 82 gf / (g / m 2 ) to 140 gf / (g / m 2 ) may be. The polyolefin microporous membrane having a buckling strength for a basis weight within the aforementioned range can prevent the polyolefin microporous membrane from being broken by an external force during the secondary battery assembly process or by internal foreign matter after assembly.

[0086] In one embodiment, the porosity of the microporous membrane may be 30% to 70%, 32% to 70%, 35% to 60%, or 35% to 55%. By having a porosity within the aforementioned range, the buckling strength relative to the basis weight is excellent, and the battery characteristics can be improved.

[0087] In one embodiment, the microporous membrane may have an ionic conductivity of 0.5 mS / cm or more, 0.7 mS / cm or more, 0.7 mS / cm to 1 mS / cm, or 0.7 mS / cm to 0.8 mS / cm. By having an ionic conductivity within the above-described range, battery characteristics may be improved.

[0088] In one embodiment, the microporous membrane has a Gurley permeability of 30 s / 100 cm. 3 200 s / 100cm 3 , 50 s / 100cm 3 180 s / 100cm 3 or 100 s / 100cm 3 160 s / 100cm 3 It is possible to improve membrane permeability and battery characteristics by having a gullibility within the aforementioned range.

[0089]

[0090] <Method for manufacturing polyolefin microporous membrane>

[0091] According to one embodiment, a method for producing the above-described polyolefin microporous membrane is provided, comprising the steps of: supplying raw materials including polyolefin and oil to an extruder, mixing the raw materials, and extruding the mixed melt to obtain a sheet-shaped extruded product; and stretching the extruded product.

[0092] In one embodiment, the alpha-olefin may have 3 to 8 or 3 to 6 carbon atoms. For example, the alpha-olefin may be at least one selected from the group consisting of propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, and 1-octene. As a specific example, the alpha-olefin may be at least one selected from the group consisting of propylene, 1-butene, and 1-hexene.

[0093] Depending on the type of the alpha-olefin and the amount of the copolymerized alpha-olefin, the number of short chain branches (SCB) of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain varies. In the present invention, by controlling the number of short chain branches (SCB) of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain, the average diameter of the pores formed in the manufactured polyolefin microporous membrane can be controlled to 10 nm to 45 nm, 10 nm to 43 nm, 10 nm to 40 nm, 10 nm to 35 nm, 10 nm to 30 nm, or 15 nm to 40 nm. By having an average pore size within the above-mentioned range, electrolyte affinity, mechanical properties, and battery properties can be excellent.

[0094] In one embodiment, the manufactured polyolefin microporous membrane may satisfy at least one of the following equations 1 and 2.

[0095] [Formula 1]

[0096] SCB / 1000C ≤ 4.0

[0097] [Formula 2]

[0098] -10 x + 30 ≤ y ≤ -10 x + 55

[0099] In the above formula 1, SCB / 1000C represents the number of short-chain branches of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain,

[0100] In the above equation 2, x represents SCB / 1000C, and y represents the average pore size of the microporous membrane.

[0101] The above formula 1 may mean that the number of short-chain branches of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain is 4 or less, and for example, the SCB / 1000C may be 3.5 or less, 0 to 3.5, 0.2 to 0.2 to 3.5, 0.2 to 3.0, 0.5 to 2.5, or 0.5 to 2.0. By having the SCB / 1000C within the above-mentioned range, the average diameter of the pores formed in the polyolefin microporous membrane manufactured can be controlled. In this way, when the average diameter of the pores formed in the microporous membrane is controlled through SCB / 1000C of polyolefin, the pore size can be adjusted without changing other processing conditions simply by changing the type of polyolefin, and a microporous membrane with high ionic conductivity can be manufactured through a small pore size.

[0102] The above equation 2 is an equation that represents the relationship between the number of short-chain branches of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain and the average pore size of the microporous membrane. However, the average pore size (y) value of the microporous membrane is 0 or a positive number. Specifically, the above equation 2 may mean a negative correlation in which the average pore size of the microporous membrane decreases as the content of short-chain branches increases.

[0103] In one embodiment, the polyolefin microporous membrane manufactured above can satisfy both Equations 1 and 2 below. Specifically, by satisfying Equation 2 while the number of short-chain branches of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain is 4 or less, a microporous membrane having excellent ionic conductivity can be obtained.

[0104] In one embodiment, the viscosity average molecular weight of the polyolefin may be 200,000 g / mol to 5,000,000 g / mol, 250,000 g / mol to 4,000,000 g / mol, or 250,000 g / mol to 3,000,000 g / mol. When a polyolefin having a viscosity average molecular weight within the above-described range is included, a microporous membrane having excellent ionic conductivity can be obtained.

[0105] The above viscosity average molecular weight can be calculated from the intrinsic viscosity using the following formula according to ASTM D 4020.

[0106] Mw = 5.37 Х 10 4 Х [η] 1.49

[0107] (Mw stands for viscosity-average molecular weight (g / mol), and η stands for intrinsic viscosity (dl / g).)

[0108] In one embodiment, the melting point of the polyolefin may be 110°C to 140°C, 115°C to 135°C, or 125°C to 135°C. When a polyolefin having a melting point within the above-described range is included, a microporous membrane having excellent ionic conductivity can be obtained.

[0109] In one embodiment, the polyolefin may be an ethylene-alphaolefin copolymer or a mixture of an ethylene-alphaolefin copolymer and an ethylene homopolymer.

[0110] When an ethylene-alphaolefin copolymer is used alone as the polyolefin, the viscosity average molecular weight of the ethylene-alphaolefin copolymer may be 250,000 g / mol to 3,000,000 g / mol. As a specific example, when the alphaolefin is propylene, the viscosity average molecular weight may be 250,000 g / mol to 2,000,000 g / mol, when the alphaolefin is 1-butene, the viscosity average molecular weight may be 2,000,000 g / mol to 3,000,000 g / mol, and when the alphaolefin is 1-hexene, the viscosity average molecular weight may be 500,000 g / mol to 1,000,000 g / mol. When an ethylene-alpha-olefin copolymer having a viscosity-average molecular weight within the aforementioned range is used alone, the average pore size of the microporous membrane can be adjusted to a desired range by controlling SCB / 1000C, thereby further improving battery performance when the microporous membrane is used as a battery separator.

[0111] When a mixture of an ethylene-alphaolefin copolymer and an ethylene homopolymer is used as the polyolefin, the viscosity average molecular weight of the ethylene-alphaolefin copolymer may be 250,000 g / mol to 1,500,000 g / mol. As a specific example, when the alphaolefin is propylene, the viscosity average molecular weight may be 250,000 g / mol to 2,000,000 g / mol, when the alphaolefin is 1-butene, the viscosity average molecular weight may be 1,200,000 g / mol to 1,800,000 g / mol, and when the alphaolefin is 1-hexene, the viscosity average molecular weight may be 500,000 g / mol to 1,000,000 g / mol. Additionally, the viscosity average molecular weight of the ethylene homopolymer may be 250,000 g / mol to 2,000,000 g / mol.

[0112] When a mixture of an ethylene-alphaolefin copolymer and an ethylene homopolymer is used as the above polyolefin, the weight ratio of the ethylene-alphaolefin copolymer and the ethylene homopolymer may be 20:80 to 80:20, 25:75 to 75:25, or 60:40 to 40:60.

[0113] When a mixture of an ethylene-alphaolefin copolymer and an ethylene homopolymer within the aforementioned range is used, the average pore size of the microporous membrane can be adjusted to a desired range by controlling SCB / 1000C, thereby further improving battery performance when the microporous membrane is used as a battery separator.

[0114] In one embodiment, the raw material may further include at least one selected from the group consisting of antioxidants and neutralizing agents.

[0115] The above antioxidants are 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, 1,6-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionamido]hexane, 1,6-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionamido]propane, tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane, pentaerythritol-tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenol)propionate), It may be at least one selected from the group consisting of 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazinane-2,4,6-trione, bis(octadecyl)hydroxyamine, tris(2,4-di-tert-butylphenyl)-phosphite, bis(2,6-di-tert-butyl-4-methylphenyl)pentaerythritol-di-phosphite and bis(2,4-di-tert-butylphenyl)pentaerythritol-di-phosphite. As a specific example, the antioxidant may be a mixture of pentaerythritol-tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenol)propionate) and tris(2,4-di-tert-butylphenyl)-phosphite.

[0116] The neutralizing agent may be at least one selected from the group consisting of calcium stearate, zinc stearate, magnesium aluminum hydroxy carbonate, zinc oxide, and magnesium hydroxy stearate. As a specific example, the neutralizing agent may be calcium stearate.

[0117] In one embodiment, the oil supplied to the extruder when producing the melt may be paraffinic oil. The paraffinic oil may be any paraffinic hydrocarbon oil commonly used in the art without limitation.

[0118] In one embodiment, the weight ratio of the raw material and the oil supplied during the extrusion may be 10:90 to 40:60, 15:85 to 35:65, or 20:80 to 30:70. When the raw material and the oil are supplied to the extruder and mixed within the above-mentioned range, the mixing property and the extrusion processability are improved, and pores can be appropriately formed in the polyolefin microporous membrane produced.

[0119] In one embodiment, conditions such as the temperature at which the extrusion is performed can be adjusted to conditions that are advantageous for extrusion processing, such that the polyolefin resin is melted and has excellent compatibility with oil. The temperature at which the extrusion is performed may be, for example, 150°C to 250°C, 180°C to 250°C, or 200°C to 230°C.

[0120] In one embodiment, the step of stretching the extrudate may be performed through a biaxial stretching process. In the biaxial stretching process, a tenter frame process may be used to perform a stable biaxial stretching process on the extrudate in the machine direction (MD) and the transverse direction (TD). Through this, the extrudate can be stretched at a high elongation ratio to realize sufficient crystal orientation, and a polyolefin microporous membrane with a uniform thickness can be formed, thereby achieving the desired property improvement effect.

[0121] The step of stretching the above-mentioned discharged product may include a first stretching step of stretching the discharged product 2 to 6 times in the machine direction (MD) at 100°C to 125°C; and a second stretching step of stretching the discharged product stretched in the first stretching step 7 to 12.5 times in the transverse direction (TD) at 116°C to 135°C. During the stretching step, the polyolefin resin and oil undergo phase separation, thereby forming pores in the polyolefin microporous membrane that is ultimately produced.

[0122] In one embodiment, the oil removal process is a process for removing oil through extraction from a polyolefin microporous membrane manufactured through the extrusion and stretching process, and may include a step of dissolving oil present in the polyolefin microporous membrane in an organic solvent; and a step of removing the organic solvent in which the oil is dissolved by drying.

[0123] The organic solvent may be any conventional organic solvent used in the art to dissolve oil without limitation, and for example, the organic solvent may be methylene chloride.

[0124] In one embodiment, after the step of removing the oil, a step of heat-setting in the transverse direction (TD) at 110°C to 135°C for 1 to 1.8 times or 1 to 1.5 times may be further included. By performing the heat-setting step further in the aforementioned range, residual stress can be removed.

[0125] A microporous membrane manufactured through a method for manufacturing a polyolefin microporous membrane according to the present invention can have small, uniform pores formed therein.

[0126] In one embodiment, the manufactured microporous membrane may have a thickness of 3 μm to 20 μm, 5 μm to 17 μm, 8 μm to 15 μm, or 8 μm to 10 μm. The thickness may be measured according to ASTM D374. The polyolefin microporous membrane may have a constant thin thickness within the above-described range, and may improve the stability of a secondary battery including the polyolefin microporous membrane as a separator.

[0127] In one embodiment, the tensile strength of the manufactured microporous membrane is 80 gf / (g / m 2 ) or more, 80 gf / (g / m 2 ) to 150 gf / (g / m 2 ) or 82 gf / (g / m 2 ) to 140 gf / (g / m 2 ) may be. The polyolefin microporous membrane having a buckling strength for a basis weight within the aforementioned range can prevent the polyolefin microporous membrane from being broken by an external force during the secondary battery assembly process or by internal foreign matter after assembly.

[0128] In one embodiment, the porosity of the microporous membrane manufactured above may be 30% to 70%, 32% to 70%, 35% to 60%, or 35% to 55%. By having a porosity within the aforementioned range, the buckling strength relative to the basis weight is excellent, and the battery characteristics can be improved.

[0129] In one embodiment, the microporous membrane may have an ionic conductivity of 0.5 mS / cm or more, 0.7 mS / cm or more, 0.7 mS / cm to 1 mS / cm, or 0.7 mS / cm to 0.8 mS / cm. By having an ionic conductivity within the above-described range, battery characteristics may be improved.

[0130] In one embodiment, the manufactured microporous membrane has a Gurley permeability of 30 s / 100 cm. 3 200 s / 100cm 3 , 50 s / 100cm 3180 s / 100cm 3 or 100 s / 100cm 3 160 s / 100cm 3 It is possible to improve membrane permeability and battery characteristics by having a gullibility within the aforementioned range.

[0131]

[0132] Secondary battery

[0133] According to one embodiment, a secondary battery including the polyolefin microporous film described above is provided. The secondary battery may be, more specifically, a lithium ion secondary battery.

[0134] The form of the secondary battery may be various, for example, an electrolytic type, a laminated type, and a coiled type, and as a specific example, a separator including the polyolefin microporous film may be used as a separator of a coiled secondary battery.

[0135] The term "secondary battery" is commonly used in the industry and can refer to a power storage system that offers superior energy density by converting electrical energy into chemical energy and storing it. Compared to non-rechargeable primary batteries, secondary batteries are rechargeable and are widely used in IT devices such as smartphones, cellphones, laptops, and tablet PCs. Recently, interest in electric vehicles has grown to prevent environmental pollution, leading to the adoption of high-capacity secondary batteries in electric vehicles.

[0136] In the above secondary battery, the polyolefin microporous membrane according to the present invention may be included as a separator, and components other than the separator may include those commonly used in the art.

[0137] The four core materials of the aforementioned lithium-ion secondary battery components are the anode, cathode, separator, and electrolyte. The separator, among these, contains micropores that allow lithium ions to move smoothly between the two electrodes during the charging and discharging process. This also prevents physical contact (short circuit) between the anode and cathode, thereby preventing thermal runaway.

[0138] Lithium-ion secondary battery separators isolate the anode and cathode, prevent electrical shorts between the two electrodes, and allow the passage of electrolyte and ions. While the separator itself does not participate in the battery's electrochemical reactions, its physical properties, such as electrolyte impregnation, porosity, and thermal shrinkage, significantly impact the battery's performance and safety.

[0139] Among these, the electrolyte impregnation rate is a critical factor affecting the battery's lifespan and capacity. A higher electrolyte impregnation rate is advantageous for producing superior batteries. Furthermore, improving the electrolyte impregnation rate is a key factor in accelerating the overall battery production process.

[0140] In this regard, in the present invention, by using the polyolefin microporous film having the above-described excellent properties as a separator of the secondary battery, productivity can be improved and the performance of the secondary battery can be improved.

[0141]

[0142] Below, specific embodiments of the present invention are presented. However, the embodiments described below are intended solely to specifically illustrate or explain the present invention and should not be construed as limiting it. Furthermore, any details not described herein are technically feasible to those skilled in the art, and therefore, their description is omitted.

[0143]

[0144] [Examples and Comparative Examples]

[0145] Examples 1 to 15 and Comparative Examples 1 to 4

[0146] Different types of polyolefins and different mixing ratios of polyolefins were used, and the manufacturing conditions of the microporous membranes were all the same. The polyolefins shown in Table 1 below were used, and the physical properties were measured as follows.

[0147]

[0148] viscosity average molecular weight

[0149] The viscosity-average molecular weight (Mw) was calculated from the intrinsic viscosity [η] according to ASTM D 4020. In the case of polymers, viscosity can provide useful information in a dilute solution, and the value obtained by dividing the viscosity of the polymer by the viscosity of the solution and the concentration is called the specific viscosity, and the extrapolated value of the specific viscosity when the polymer concentration becomes 0 is defined as the intrinsic viscosity (IV). Since the IV value of linear polymers is mainly affected by the size of the polymer, it has a high correlation with the molecular weight, and in the case of ultra-high molecular weight polyethylene, the Margolies equation below is widely used.

[0150] Mw = 5.37 Х 10 4 Х [η] 1.49

[0151] (Mw stands for viscosity-average molecular weight (g / mol), and η stands for intrinsic viscosity (dl / g).)

[0152]

[0153] SCB / 1000C

[0154] SCB / 1000C was measured by nuclear magnetic resonance spectroscopy (NMR) using Bruker's Avance Neo NMR (600 MHz) instrument. The sample was dissolved in a solvent mixture and measured at high temperature (403 K) to calculate the amount of short-chain branches.

[0155]

[0156] melting point

[0157] Melting point (Tm) was measured using a Differential Scanning Calorimeter (DSC) according to ASTM E793. The sample was heated to 200°C, maintained isothermally for 10 minutes, then cooled to 30°C at a rate of 10°C / min to remove thermal history, maintained for another minute, and then increased to 200°C at a rate of 10°C / min, comparing the difference in heat input with that of a vacant dish to measure Tm.

[0158] Ethylene-alphaolefin copolymerPolyethylene homopolymerPolyolefinC1C2C3C4C5C6C7C8C9H1H2H3H4H5ViscosityAverage molecular weight (g / mol)250,000,800,000,800,1.5 million1,500,000,2 million2 million3 million250,000,600,1.2 million1,500,000,2 millionAlphaolefin typepropylene1-hexene1-hexenepropylene1-butene1-butene1-butenepropylene1-butene-----SCB / 1000C (units)2.54.62.52.81.31.10.21.53.300000Melting point(℃)129126128128132132133131128134134134134134

[0159] The polyolefins in Table 1 were evenly mixed using a Henschel mixer in the ratios corresponding to the examples and comparative examples shown in Tables 2 and 3, to prepare a polyolefin mixed powder. Thereafter, 2000 ppm of Irganox 1010 as an antioxidant and 600 ppm of Ca-stearate as a neutralizer were added to the prepared polyolefin mixed powder, and a mixture was prepared using a Henschel mixer. While the prepared mixture was supplied from a hopper through a quantitative feeder, paraffin oil (Kukdong Oil & Chemical, LP-350F) was injected into the front end of the extruder according to the polyolefin / oil ratios included in Tables 2 and 3. The sheet coming out of the extruder passed through a casting roll and was stretched in the MD (Machine direction) and TD (transverse direction) directions while controlling the thickness of the sheet to be constant under the conditions of Tables 2 and 3, thereby preparing a microporous membrane. The oil of the manufactured microporous membrane was removed during the process of passing through an extraction tank containing methylene chloride (MC), and the microporous membrane was dried and heat-set in the TD direction under the conditions shown in Tables 2 and 3 below to complete the final microporous membrane.

[0160] Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Example 10 Polyolefin Polyolefin type C1 C3 C4 C7 C9 C1 / H1 C1 / H1 C2 / H2 C2 / H2 C3 / H2 Polyolefin mixing ratio Exclusive Exclusive Exclusive Exclusive Exclusive 75 / 25 50 / 50 75 / 25 50 / 50 75 / 25 SCB / 1000 C2.5 2.5 2.8 0.2 3.3 1.9 1.3 3.5 2.3 1.9 Microporous membrane manufacturing conditions Polyolefin / oil ratio 30 / 70 30 / 70 30 / 70 30 / 70 30 / 70 30 / 70 30 / 70 30 / 70 30 / 70 Extruder temperature (℃)220220220220220220220220220220Discharge (kg / hr)3030303030303030303030MD Stretching Temperature (℃)110110110110110110110110110110110MD Stretching Ratio44444444444TD Stretching Temperature (℃)122122122122122122122122122122122122TD Stretching Ratio10101010101010101010101010TD Heat Setting Temperature (℃)120120120120120120120120120120120TD Heat Setting Multiplier 1.21.21.21.21.21.21.21.21.21.21.2

[0161] Example 11 Example 12 Example 13 Example 14 Example 15 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Polyolefin Polyolefin type C3 / H5 C4 / H2 C5 / H2 C5 / H2 C3 / H5 H3 H2 H5 C2 Polyolefin mixing ratio 25 / 75 50 / 50 25 / 75 50 / 50 50 / 50 Single Single Single Single SCB / 1000 C 0.6 1.4 0.3 0.7 1.3 0 0 0 4.6 Microporous membrane manufacturing conditions Polyolefin / oil ratio 30 / 70 30 / 70 30 / 70 30 / 70 30 / 70 30 / 70 30 / 70 30 / 70 Extruder temperature (℃)220220220220220220220220220Discharge rate(kg / hr)30303030303030303030MD stretching temperature (℃)110110110110110110110110110110MD stretching ratio4444444444TD stretching temperature (℃)122122122122122122122122122122TD stretching ratio10101010101010101010101010TD heat setting temperature (℃)120120120120120120120120120120TD heat setting ratio1.2 ...

[0162]

[0163] [Experimental Example]

[0164] The physical properties of the microporous membranes manufactured in Examples 1 to 15 and Comparative Examples 1 to 4 were measured as follows and are shown in Tables 4 and 5 below.

[0165]

[0166] thickness

[0167] The VL-50 device from Mitutoyo, Japan was used, and the thickness was measured according to ASTM D374.

[0168]

[0169] average pore size

[0170] The average pore size was obtained using a capillary flow porometer (Poretech Innova iCFP-500) using the bubble point measurement method according to ASTM F316-03.

[0171]

[0172] Pin Puncture

[0173] The buckling strength was measured at a speed of 10 mm / sec using a tip with a distal diameter of 1 mm in accordance with ASTM D-4833 using a KES-G5 device from Kato Tech, Japan.

[0174]

[0175] Gurley number

[0176] The polyolefin microporous membrane was cut to a size of 50 mm x 50 mm, and the air permeability was obtained by measuring the time it took for 100 ml of air to pass through it using a Gurley-type densometer.

[0177]

[0178] basic weight

[0179] A 50 mm x 50 mm sample was taken from the polyolefin microporous membrane, weighed, and the basis weight was calculated.

[0180]

[0181] Porosity (%)

[0182] Based on the basis of the basis weight of the polyolefin microporous membrane, it was calculated using the following formula.

[0183] Porosity (%) = [1 - ρ ÷ (bxd)] x 100

[0184] (ρ is the density of polyolefin, b is the basis weight of the microporous membrane, and d is the thickness of the microporous membrane.)

[0185]

[0186] Ion Conductivity (Electrochmical Impedance Spectroscopy)

[0187] An ion conductivity cell having SUS (D: 16 mm) poles on both sides of a polyolefin microporous membrane (D: 18 mm) was manufactured to measure AC impedance, and the ion conductivity (mS / cm) was calculated using the equation in Figure 2 below.

[0188]

[0189] Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Example 10 Thickness (㎛) 9.28.799.188.599.28.38.2 Average pore size (nm) 24151342221833152021 Strength (gf) 450480510700640540540410520550 Air permeability (Gurley, s / 100ml) 130121134110154120140112130110 Basis weight (g / m 2 )5.24.855.44.655.155.25.3Porosity (%)46423540364545483842Strength / Basis Weight (gf / (g / m) 2 )8710010213013910810682100104Ionic conductivity (mS / cm)0.820.70.720.60.981.10.651.250.830.82

[0190] Example 11 Example 12 Example 13 Example 14 Example 15 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Thickness (㎛) 9.6 8.8 10.18 89.18 8.8 9.5 8.8 Average pore size (nm) 3 3 3 5 3 8 3 1 2 8 4 7 4 7 5 0 1 8 Strength (gf) 6 6 0 5 2 0 5 6 0 5 3 0 5 2 0 5 7 0 5 7 0 7 7 0 3 3 0 Air permeability (Gurley, s / 100ml) 1 0 8 1 3 0 1 1 9 1 1 9 1 5 7 1 3 6 1 2 7 1 4 6 3 0 0 Weight (g / m) 2 )5.4554.85.25.45.455Porosity (%)444648424041403628Strength / Basis Weight (gf / (g / m) 2)12210411211010010610615466Ionic conductivity (mS / cm)0.720.681.080.710.790.510.560.550.35

[0191] As confirmed from Tables 4 and 5 above, in the case of Examples 1 to 15, it can be confirmed that the average pore size is controlled within the range according to the present invention according to SCB / 1000C, and accordingly, the physical properties such as porosity, air permeability, burr strength / basis weight ratio, and ionic conductivity were excellent.

[0192] On the other hand, it can be confirmed through Comparative Examples 1 to 3 that when only polyethylene homopolymer is used as the polyolefin, an average pore size larger than 45 nm and low ionic conductivity are exhibited. In addition, as seen in Example 8 and Comparative Example 4, it can be confirmed that when the SCB / 1000C content is too high, the pore size may actually slightly increase and the puncture resistance, air permeability, porosity, and ionic conductivity deteriorate. This indicates that there is an optimal value for the SCB / 1000C content of the polyolefin in terms of the performance of the microporous membrane used.

[0193] Through Fig. 1 showing the average pore size according to SCB / 1000C of the above examples, it can be confirmed that the average pore size decreases in proportion to SCB / 1000C according to Equation 2, and through the results of the microporous membrane analysis of the above examples, it can be confirmed that when SCB / 1000C is less than 4.5, as the average pore size decreases, the physical properties of the microporous membrane such as porosity, air permeability, puncture strength / basis weight ratio, and ionic conductivity can be improved. In addition, it can be seen that even if the processing conditions such as extrusion, stretching, and extraction of the membrane are the same, the average pore size can be successfully controlled depending on the content of short-chain branches of the polyolefin used in manufacturing the membrane.

[0194] While the embodiments of the present invention have been described above, those skilled in the art will appreciate that the present invention can be implemented in other specific forms without altering the technical concept or essential features thereof. Therefore, the embodiments described above should be understood to be illustrative in all respects, not restrictive.

Claims

1. In the polyolefin microporous membrane, The above polyolefin contains an alpha-olefin derived unit, A polyolefin microporous membrane having an average pore size of 10 nm to 45 nm depending on the number of short chain branches (SCBs) of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain.

2. In paragraph 1, A polyolefin microporous membrane satisfying at least one of the following equations 1 and 2: [Formula 1] SCB / 1000C ≤ 4.0 [Formula 2] -10 x + 30 ≤ y ≤ -10 x + 55 In the above formula 1, SCB / 1000C represents the number of short-chain branches of the alpha-olefin-derived unit per 1,000 carbon atoms of the polyolefin main chain, In the above equation 2, x represents SCB / 1000C, and y represents the average pore size of the microporous membrane.

3. In paragraph 1, The above alpha olefin is a polyolefin microporous membrane having 3 to 8 carbon atoms.

4. In paragraph 1, The above polyolefin is a polyolefin microporous membrane which is an ethylene-alpha olefin copolymer or a mixture of an ethylene-alpha olefin copolymer and an ethylene homopolymer in a weight ratio of 20:80 to 80:

20.

5. In paragraph 1, A polyolefin microporous membrane having a viscosity average molecular weight of 200,000 g / mol to 5,000,000 g / mol.

6. In paragraph 1, The tensile strength of the above microporous membrane is 80 gf / (g / m 2 ) polyolefin microporous membrane.

7. In paragraph 1, A polyolefin microporous membrane having a porosity of 30% to 70%.

8. In paragraph 1, A polyolefin microporous membrane having an ionic conductivity of 0.50 mS / cm or more.

9. In paragraph 1, The above microporous membrane has a Gurley permeability of 30 s / 100cm. 3 200 s / 100cm 3 Polyolefin microporous membrane.

10. A step of supplying raw materials and oil containing polyolefin to an extruder and extruding the mixed melt to obtain a sheet-shaped extruded product; and A method for producing a polyolefin microporous membrane according to claim 1, comprising a step of extending the above-mentioned discharge.

11. In paragraph 10, The step of extending the above discharge is: A first stretching step of stretching the above extruded material in the machine direction (MD) at a magnification of 2 to 6 times at 100 to 125°C; and A method for manufacturing a polyolefin microporous membrane, comprising a second stretching step of stretching the ejected material stretched in the first stretching step at a magnification of 7 to 12.5 times in the width direction (TD) at 116 to 135°C.

12. In paragraph 10, A method for manufacturing a polyolefin microporous membrane further comprising a step of removing oil through extraction after the step of extending the above-mentioned discharge.

13. In paragraph 10, A method for producing a polyolefin microporous membrane, further comprising a step of heat-setting at a temperature of 110°C to 135°C in the transverse direction (TD) at a magnification of 1 to 1.8 times after the step of removing the oil.

14. A secondary battery comprising a polyolefin microporous film according to Article 1.

15. In paragraph 14, The above polyolefin microporous membrane is a secondary battery separator.

Images