Separator and production method therefor

Abstract

One aspect of the present invention provides a separator and a production method therefor, the separator comprising: a porous support having a hydrophobic region comprising a polyolefin, and a hydrophilic region comprising a hydrophilic polymer dispersed in the hydrophobic region; and a heat-resistant layer formed on at least one surface of the porous support and comprising inorganic particles and a binder, wherein Do / Di derived according to the following method is 1.0-1.25. The steps of: adding 2 ul of propylene carbonate dropwise to one surface of the heat-resistant layer, and then leaving same to stand for 20 seconds; dividing a region in which the propylene carbonate is radially diffused along the surface of the heat-resistant layer into an initial diffuse region and a late diffuse region on the basis of brightness; and calculating the ratio Do / Di by measuring the maximum diameter (Di) of the initial diffuse region and the maximum diameter (Do) of the late diffuse region, wherein the brightness of the initial diffuse region is lower than the brightness of the late diffuse region.

Description

Separator and method of manufacturing the same

[0001] The present invention relates to a separator and a method for manufacturing the same, and more specifically, to a separator for a secondary battery with improved impregnation properties with an electrolyte and a method for manufacturing the same.

[0002] Rechargeable batteries are widely used as power sources for various electrical products requiring miniaturization and lightweight design, such as smartphones, laptops, and tablet PCs. As their application fields expand to include medium and large-sized batteries for smart grids and electric vehicles, there is a demand for the development of rechargeable batteries with high capacity, long lifespan, and high stability.

[0003] As a means to achieve the above objective, research and development is actively being conducted on separators with formed micropores that separate the anode and cathode to prevent internal short circuits and facilitate ion movement during the charging and discharging process; specifically, on microporous separators using polyolefins such as polyethylene, which are advantageous for pore formation via thermally induced phase separation, are economical, and facilitate the fulfillment of the physical properties required for separators.

[0004] Conventional polyolefin-based separators, which are widely used, suffer from poor heat resistance and mechanical strength. To address this, a technology has been proposed to coat the surface of the separator with a heat-resistant layer containing ceramic particles. However, this heat-resistant layer leaves significant technical challenges regarding air permeability and conductivity (resistance), which are critical factors affecting the performance of the separator. Specifically, while forming a heat-resistant layer containing ceramic particles on the surface of a porous substrate improves the heat resistance of the separator, the ceramic particles in the layer block the pores formed in the porous substrate, thereby reducing the separator's air permeability. Consequently, the ion transport pathway between the anode and cathode is significantly reduced, resulting in a substantial decline in the charging and discharging performance of the secondary battery. Furthermore, as the heat-resistant layer is continuously exposed to the electrolyte inside the battery, the ceramic particles may partially and continuously detach from the porous substrate; in this case, the heat resistance of the separator may also gradually decrease.

[0005] Meanwhile, the secondary battery is structured such that an electrolyte is impregnated into an electrode assembly in which a porous separator is interposed between a positive electrode and a negative electrode, each having an active material coated on an electrode current collector.

[0006] Secondary batteries are manufactured by producing an electrode assembly having a structure in which positive and negative electrodes are alternately stacked and a separator is interposed between the positive and negative electrodes, inserting the electrode assembly into a battery case consisting of a can or pouch of a specific size and shape, and finally injecting an electrolyte. At this time, the electrolyte permeates between the positive, negative, and separator via capillary force. However, due to the characteristics of the materials, the positive, negative, and separator are hydrophobic, whereas the electrolyte is hydrophilic; therefore, improving the impregnation or wettability of the electrolyte to the electrodes and separator requires considerable time and stringent process conditions. Consequently, there are limitations in achieving and improving a balance between the impregnation of the electrolyte and the productivity of the process.

[0007] To improve the impregnation of such electrolytes, methods such as injecting the electrolyte at high temperatures or injecting it under pressurized or reduced pressure are used; however, these methods present problems, such as the deformation of the existing electrode assembly and electrolyte due to heat, which can lead to internal short circuits. Furthermore, since the above process is performed after the electrode assembly is placed in the battery case along with the electrolyte, the impregnation of the electrolyte into the electrode assembly may be uneven. In particular, in the case of jelly-roll type electrode assemblies, uneven impregnation occurs between the center and the outer part of the winding, which leads to a shortened battery life.

[0008] It has been suggested that the impregnation of the electrolyte can be improved through a porous membrane or porous support containing polyolefins (polyethylene, polypropylene, etc.) and ethylene vinyl acetate, but there is a problem in that the pore structure of the membrane or support becomes non-uniform with the addition of ethylene vinyl acetate, a hydrophilic polymer, and the air permeability, mechanical properties, and heat resistance are all reduced. Furthermore, in order to resolve the trade-off between the hydrophilicity and other properties of the membrane or support, it is difficult to control the process and additional processes are required, which leads to a decrease in productivity.

[0009] In addition, it has been suggested that the impregnation of the electrolyte can be improved by coating at least one surface of a porous support with an aqueous slurry containing an inorganic material, a binder, and water, and by controlling the particle size of the inorganic material and the type of binder. However, the impregnation of the electrolyte is merely measured and evaluated as the diffusion area of ​​the electrolyte along the plane direction (MD, TD) of the separator, and there is a problem in that the diffusion and impregnation of the electrolyte along the plane direction and thickness direction of the separator are not smoothly carried out at the interface between the porous support, which is essentially hydrophobic, and the coating layer, which is hydrophilic.

[0010] The present invention aims to solve the problems of the aforementioned prior art. The objective of the present invention is to provide a separator capable of achieving a balanced combination of mechanical properties, heat resistance, impregnation into an electrolyte, and process productivity, as well as a method for manufacturing the same.

[0011] One aspect of the present invention provides a separation membrane comprising: a porous support having a hydrophobic region comprising a polyolefin and a hydrophilic region comprising a hydrophilic polymer dispersed in the hydrophobic region; and a heat-resistant layer formed on at least one surface of the porous support and comprising inorganic particles and a binder, wherein the separation membrane has a Do / Di ratio of 1.0 to 1.25 derived by a method comprising the steps of: dropping 2 μl of propylene carbonate onto one surface of the heat-resistant layer and leaving it for 20 seconds; dividing the region where the propylene carbonate diffuses radially along the surface of the heat-resistant layer into an initial diffusion region and a later diffusion region based on brightness; and measuring the maximum diameter (Di) of the initial diffusion region and the maximum diameter (Do) of the later diffusion region to calculate the ratio Do / Di. Here, the brightness of the initial diffusion region is lower than the brightness of the later diffusion region.

[0012] In one embodiment, the polyolefin may include one selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, and combinations or copolymers of two or more of these.

[0013] In one embodiment, the polyolefin may include polyethylene having a viscosity average molecular weight (Mv) of 200,000 to 3,000,000 g / mol.

[0014] In one embodiment, the content of the hydrophilic region in the porous support may be 0.1 to 30 weight%.

[0015] In one embodiment, the hydrophilic polymer may include one selected from the group consisting of ethylene vinyl acetate, ethylene vinyl alcohol, polyvinyl alcohol, polyacrylic acid, polyoxyethylene-polyoxypropylene block copolymer, polyethylene glycol, polyethylene oxide, polyvinylpyrrolidone, polyvinyl acetal, polyvinyl butyral, cellulose derivative, polyurethane, polyamide, polyester, glycerol, ethylene-based copolymer including hydrophilic monomer, and combinations of two or more of these.

[0016] In one embodiment, X derived according to a method comprising: a step of forming a laminate by laminating 30 or more of the porous supports; a step of dropping 2 µl of propylene carbonate onto one side of the laminate and leaving it for 5 minutes; a step of checking the wetting state of the propylene carbonate while peeling off the porous supports one by one from the other side of the laminate; and a step of determining the number (X) of the porous supports in the laminate in which the wetting state of the propylene carbonate is confirmed; may be 20 or more.

[0017] In one embodiment, the porous support may satisfy at least one of the following conditions (i) to (vi).

[0018] (i) Thickness 1~20㎛; (ii) MD tensile strength 2,500~3,500kgf / cm² 2 ; (iii) TD tensile strength 2,000~3,000 kgf / cm 2 (iv) MD tensile elongation 50~150%; (v) TD tensile elongation 50~150%; (vi) puncture strength 450~600gf.

[0019] In one embodiment, the inorganic particle may include one selected from the group consisting of SiO2, AlO(OH), Mg(OH)2, Al(OH)3, TiO2, BaTiO3, Li2O, LiF, LiOH, Li3N, BaO, Na2O, Li2CO3, CaCO3, LiAlO2, Al2O3, SiO, SnO, SnO2, PbO2, ZnO, P2O5, CuO, MoO, V2O5, B2O3, Si3N4, CeO2, Mn3O4, Sn2P2O7, Sn2B2O5, Sn2BPO6, and combinations of two or more of these.

[0020] In one embodiment, the thickness of the heat-resistant layer is 1 to 10 μm, and the content of the inorganic particles in the heat-resistant layer may be 50 to 99 weight percent.

[0021] In one embodiment, the binder may include a hydrophilic binder, a hydrophobic binder, or a combination thereof.

[0022] In one embodiment, the binder may include a hydrophilic binder and a hydrophobic binder, and the hydrophobic binder may be a copolymer including a hydrophilic unit and a hydrophobic unit.

[0023] In one embodiment, the content of the hydrophobic binder in the binder may be 25 to 80 weight percent.

[0024] In one embodiment, the hydrophilic unit may include one selected from the group consisting of acrylic acid, methacrylic acid, acrylamide, hydroxyethyl methacrylate, hydroxyethyl acrylate, vinyl alcohol, poly(ethylene glycol) methacrylate, N,N-dimethylacrylamide, sodium acrylate, sodium methacrylate, vinyl sulfone, propylene glycol methacrylate, 2-hydroxypropyl methacrylate, 2-methacryloyloxyethylphosphorylcholine, vinylpyrrolidone, itaconic acid, N-vinylcaprolactam, N,N-dimethylaminoethyl methacrylate, N-isopropylacrylamide, polyacrylic acid, polyvinyl alcohol, polyvinylpyrrolidone, and combinations of two or more of these.

[0025] In one embodiment, the hydrophobic unit comprises styrene, polystyrene, methyl methacrylate, butyl acrylate, hexyl acrylate, octyl acrylate, isobornyl acrylate, dodecyl methacrylate, isopropyl acrylate, lauryl methacrylate, tetrahydrofuran acrylate, ethyl methacrylate, trimethylsilyl methacrylate, 2-ethylhexyl methacrylate, methoxytrimethylsilyl methacrylate, isobornyl acrylate, cyclohexyl methacrylate, benzyl methacrylate, propyl methacrylate, phenyl methacrylate, isotrimethoxysilyl methacrylate, triethoxysilylpropyl acrylate, methoxysilylpropyl methacrylate, phenyltrimethoxysilane, hexamethyldisiloxane, pentamethyldisiloxane, vinyltrimethoxysilane, vinyltriethoxysilane, trimethoxysilylethyl methacrylate, It may include polydimethylsiloxane methacrylate and one selected from the group consisting of a combination of two or more of these.

[0026] In one embodiment, the RGB values ​​of the initial diffusion region may be (80~100, 80~100, 80~100), and the RGB values ​​of the later diffusion region may be (100~120, 100~120, 100~120).

[0027] In one embodiment, the absolute value of the difference between the RGB values ​​of the initial diffusion region and the later diffusion region may be (1~20, 1~20, 1~20).

[0028] Another aspect of the present invention provides a method for manufacturing a separation membrane comprising: (a) feeding a composition comprising a polyolefin, a hydrophilic polymer, and a pore-forming agent into an extruder comprising a screw and forming a base sheet; (b) stretching the base sheet and then extracting the pore-forming agent to produce a base film; (c) heat-setting the base film to produce a porous support; and (d) applying a slurry comprising inorganic particles, a binder, and a solvent to at least one surface of the porous support and then drying to form a heat-resistant layer.

[0029] In one embodiment, in step (a), the aspect ratio (L / D) of the screw may be 58 to 62, and the rotational speed of the screw may be 40 to 200 rpm.

[0030] In one embodiment, in step (b), the stretching may be performed at a temperature of 100 to 130°C along the MD and TD of the base sheet at an area ratio of 10 to 400 times.

[0031] In one embodiment, in step (c), the heat setting may be performed at a temperature of 120 to 135°C while the base film is relaxed or stretched along at least one of MD and TD by an area ratio of 0.8 to 3 times.

[0032] A separator according to one aspect of the present invention comprises: a porous support having a hydrophobic region comprising a polyolefin and a hydrophilic region comprising a hydrophilic polymer dispersed in the hydrophobic region; and a heat-resistant layer formed on at least one surface of the porous support and comprising inorganic particles and a binder; wherein, by adjusting the Do / Di derived according to a method comprising the steps of: dropping 2 µl of propylene carbonate onto one surface of the heat-resistant layer and leaving it for 20 seconds; dividing the region where the propylene carbonate has radially diffused along the surface of the heat-resistant layer into an initial diffusion region and a later diffusion region based on brightness; and measuring the maximum diameter (Di) of the initial diffusion region and the maximum diameter (Do) of the later diffusion region to calculate the ratio Do / Di to a range of 1.0 to 1.25, the mechanical properties, heat resistance, impregnation into an electrolyte, and process productivity can be achieved in a balanced manner.

[0033] The effects of the present invention are not limited to the effects described above, and should be understood to include all effects that can be inferred from the configuration of the invention described in the detailed description or claims of the present invention.

[0034] FIG. 1 shows a method for evaluating the impregnation of a separator with respect to an electrolyte according to one embodiment of the present invention.

[0035] FIG. 2 shows a method for evaluating the impregnation of a porous support with respect to an electrolyte according to one embodiment of the present invention.

[0036] Figure 3 shows the results of the evaluation of the impregnation of the separator membrane with respect to the electrolyte according to the embodiments and comparative examples of the present invention.

[0037] The present invention will be described below with reference to the attached drawings. However, the present invention may be implemented in various different forms and is therefore not limited to the embodiments described herein. Furthermore, in order to clearly explain the present invention in the drawings, parts unrelated to the explanation have been omitted, and similar parts throughout the specification have been given similar reference numerals.

[0038] Throughout the specification, when it is stated that a part is "connected" to another part, this includes not only cases where they are "directly connected," but also cases where they are "indirectly connected" with other members interposed between them. Furthermore, when it is stated that a part "includes" a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but rather allows for the inclusion of additional components.

[0039] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings.

[0040] Separator

[0041] A separator according to one aspect of the present invention may comprise: a porous support having a hydrophobic region comprising a polyolefin and a hydrophilic region comprising a hydrophilic polymer dispersed in the hydrophobic region; and a heat-resistant layer formed on at least one surface of the porous support and comprising inorganic particles and a binder.

[0042] Conventional porous separator membranes or porous supports composed solely of polyolefins are essentially hydrophobic, but by melting and mixing a certain amount of hydrophilic polymer with the polyolefin, a certain amount of hydrophilicity can be imparted to the porous support.

[0043] In the porous support, the hydrophobic region and the hydrophilic region may each constitute a continuous phase and a discontinuous phase. In the porous support, the hydrophilic region may be uniformly dispersed within a matrix composed of the hydrophobic region to impart substantially uniform hydrophilicity to the entire area along the face and / or thickness direction of the porous support, and accordingly, the impregnation of the porous support with respect to the electrolyte may be improved.

[0044] The term "matrix" as used in this specification refers to a component that constitutes a continuous phase in a porous support comprising two or more components. That is, in the porous support, the hydrophobic region containing the polyolefin exists as a continuous phase, and the hydrophobic region containing the hydrophilic polymer may exist as a discontinuous phase dispersed therein.

[0045] The above polyolefin may include one selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, and combinations or copolymers of two or more of these, and preferably may include polyethylene and / or polypropylene, but is not limited thereto.

[0046] The above polyolefin may include polyethylene having a viscosity average molecular weight (Mv) of 200,000 to 3,000,000 g / mol. The polyethylene may be one selected from the group consisting of ultra-high molecular weight polyethylene (UHMWPE, Mv: 1,000,000 to 3,000,000 g / mol), high molecular weight polyethylene (HMWPE, Mv: 100,000 to 1,000,000 g / mol), high density polyethylene (HDPE, Mv: 100,000 to 1,000,000 g / mol), low density polyethylene (LDPE, Mv: 10,000 to 100,000 g / mol), homogeneous linear and linear low density polyethylene (LLDPE), and combinations of two or more of these. For example, the polyethylene may be ultra-high molecular weight polyethylene having a viscosity-average molecular weight (Mv) of 1,000,000 to 2,000,000 g / mol, preferably 1,000,000 to 1,500,000 g / mol. Here, the viscosity-average molecular weight (Mv) is an average molecular weight calculated based on the viscosity of the polymer solution and can be experimentally determined using the Mark-Houwink equation.

[0047] If the viscosity-average molecular weight of the above polyethylene exceeds 3,000,000 g / mol, the viscosity increases and processability may decrease, and if it is less than 200,000 g / mol, the viscosity becomes excessively low, and the dispersibility with pore-forming agents, antioxidants, etc. used when manufacturing a porous support is extremely reduced, and in some cases, phase separation or layer separation may occur.

[0048] The content of the hydrophilic region in the above porous support may be 0.1 to 30 wt%, preferably 0.1 to 10 wt%, more preferably 1 to 7.5 wt%, and advantageously 2 to 6 wt%. If the content of the hydrophilic region is 0.1 wt%, the required level of electrolyte impregnation cannot be achieved, and if it exceeds 30 wt%, while the electrolyte impregnation may be further improved, the mechanical properties and heat resistance of the porous support that can be achieved through the polyolefin may be reduced. In addition, if the content of the hydrophilic region exceeds 30 wt%, the dispersibility of the hydrophilic polymer is reduced, and the number of surface defects having a size of 2 mm or more and a different brightness from the surroundings on the surface of the porous support increases, which may degrade the appearance quality, and the resistance may change rapidly in areas and / or regions where the hydrophilic polymer is randomly aggregated on the surface and / or inside the porous support, which may adversely affect the electrochemical characteristics of the battery.

[0049] The above hydrophilic polymer may be selected from the group consisting of ethylene vinyl acetate, ethylene vinyl alcohol, polyvinyl alcohol, polyacrylic acid, polyoxyethylene-polyoxypropylene block copolymer, polyethylene glycol, polyethylene oxide, polyvinylpyrrolidone, polyvinyl acetal, polyvinyl butyral, cellulose derivative, polyurethane, polyamide, polyester, glycerol, ethylene-based copolymers containing hydrophilic monomers, and combinations of two or more of these, preferably ethylene vinyl acetate, more preferably ethylene vinyl acetate having a vinyl acetate content of 15 to 30 weight%, but is not limited thereto. If the vinyl acetate content in the ethylene vinyl acetate is less than 15 weight%, the mechanical properties and hydrophilicity of the porous support may be reduced, and if it exceeds 30 weight%, processability and the dispersibility of the hydrophilic polymer accordingly may be reduced. Meanwhile, as the above-mentioned hydrophilic polymer, in addition to those mentioned above, various types of polymers having hydrophilic functional groups such as amine groups, amide groups, hydroxyl groups, carboxylic acid groups, etc., in the main chain and / or side chain may also be applied. When the above hydrophilic polymer is an ethylene-based copolymer comprising a hydrophilic monomer, the hydrophilic monomer may be, for example, acrylic acid, maleic anhydride, styrene maleic anhydride, acrylamide, methacrylamide, glycidyl methacrylate, vinyl alcohol, vinyl acetic acid, methacrylic acid, ethylene glycol dimethacrylate, propylene glycol methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, 2-sulfoethyl methacrylate, 3-sulfopropyl methacrylate, glutaric acid, fumaric acid, itaconic acid, acrylonitrile, ethyl acrylate, metabenzoic acid, benzoic acid derivatives, acetylene dicarboxylic acid, aryl sulfonic acid, bisphenol-A dimethacrylate, N-vinylpyrrolidone, N-isopropylacrylamide, oxo-olefin derivatives, etc., but is not limited thereto. no.

[0050] FIG. 1 illustrates a method for evaluating the impregnation of a separator with respect to an electrolyte according to an embodiment of the present invention. Referring to FIG. 1, the separator may have a Do / Di ratio of 1.0 to 1.25, preferably 1.0 to 1.24, more preferably 1.0 to 1.22, and advantageously 1.0 to 1.20, derived according to a method comprising the steps of: dropping 2 μl of propylene carbonate onto one surface of the heat-resistant layer and leaving it for 20 seconds; dividing the region where propylene carbonate has radially diffused along the surface of the heat-resistant layer into an initial diffusion region and a later diffusion region based on brightness; and measuring the maximum diameter (Di) of the initial diffusion region and the maximum diameter (Do) of the later diffusion region to calculate the ratio Do / Di. Here, the brightness of the initial diffusion region is lower than the brightness of the later diffusion region.

[0051] The above propylene carbonate is one of the electrolytes used in secondary batteries, for example, lithium secondary batteries, and instead of the above propylene carbonate, one selected from the group consisting of ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and combinations of two or more of these may be used, and the above propylene carbonate may also be mixed with these and used.

[0052] The propylene carbonate dropped onto the surface of the heat-resistant layer spreads radially along the surface of the heat-resistant layer, that is, in a circular or elliptical shape, and the planar dimensions of the area where the propylene carbonate has spread can be determined by the length of the propylene carbonate spread in the MD and TD directions of the heat-resistant layer.

[0053] Generally, in the case of a separator in which a heat-resistant layer containing inorganic particles and a binder is formed on at least one surface of a porous support, there is a problem in that the diffusion and impregnation of the electrolyte along the surface and thickness directions of the separator do not occur smoothly at the interface between the essentially hydrophobic porous support and the hydrophilic coating layer. That is, since the heat-resistant layer has a certain hydrophilicity, the impregnation and diffusion of the electrolyte can occur smoothly in the heat-resistant layer, whereas since the porous support is hydrophobic, the impregnation and diffusion of the electrolyte may be significantly reduced compared to the heat-resistant layer.

[0054] The electrolyte (propylene carbonate) dropwise applied to one surface of the heat-resistant layer can diffuse not only in the plane direction of the heat-resistant layer but also in the thickness direction toward the interface between the heat-resistant layer and the porous support. The electrolyte that reaches the interface can diffuse in the plane direction and the thickness direction of the porous support; however, since the hydrophilicity of the separator changes discontinuously at the interface, the diffusion of the electrolyte is also necessarily discontinuous.

[0055] This discontinuous diffusion of the electrolyte can be clearly observed on the surface of the heat-resistant layer. In the initial diffusion region, the electrolyte (propylene carbonate) diffuses along the thickness direction of the separator not only to the heat-resistant layer but also to at least a part of the porous support, so the brightness is observed to be low. In contrast, in the later diffusion region, the electrolyte (propylene carbonate) diffuses along the thickness direction of the separator to the heat-resistant layer, and then (1) does not diffuse into the porous support but diffuses along the plane direction of the heat-resistant layer, or (2) even if it diffuses into at least a part of the porous support, the diffusion depth of the electrolyte along the thickness direction tends to be significantly shallower than in the initial diffusion region, so the brightness is observed to be high.

[0056] The brightness of the initial diffusion region may be lower than the brightness of the subsequent diffusion region, and the boundary between the initial diffusion region and the subsequent diffusion region may be specified as a point and / or region where the brightness of the initial diffusion region changes discontinuously during the process of electrolyte diffusion, and the maximum diameter (Di) of the initial diffusion region may be determined based on this boundary.

[0057] In the plane-direction diffusion region of the above electrolyte, a predetermined color spectrum, for example, a black and white color spectrum, is observed. By setting a reference value where the brightness changes discontinuously in this black and white color spectrum as follows, the boundaries of the above initial diffusion region and the above later diffusion region can be specified. (1) Points of abrupt change in pixel distribution can be identified through brightness histogram analysis. The brightness histogram is a graph visualizing the brightness distribution of the surface of the above heat-resistant layer. Points where the number of pixels changes abruptly at a specific brightness value are likely to indicate discontinuity in brightness, and these points can reflect areas where a difference in brightness occurs due to the discontinuous diffusion of the above electrolyte. For example, in a brightness histogram where brightness is distributed from 0 to 255, points where the frequency of pixels increases or decreases abruptly at specific brightness (e.g., 120, 180) can be set as boundaries. (2) Areas of abrupt change in brightness can be identified by utilizing an edge detection algorithm. Algorithms such as the Sobel filter and Canny boundary detection are useful for accurately extracting the diffusion boundary of the electrolyte by calculating the gradient of brightness change. For example, when applying the Sobel filter, a point where the rate of change in brightness is 50 or more can be set as the boundary. (3) A threshold value can be set by calculating the difference in average brightness between the electrolyte penetration area and the non-penetration area. For example, if the average brightness of the initial diffusion area is 100 and the average brightness of the later diffusion area is 150, the image can be binarized by setting the midpoint of these values, 125, as the reference value to effectively distinguish the initial diffusion area and the later diffusion area. (4) The boundary can also be identified through the RGB (red, green, blue) values ​​or RGB color coordinates of the initial diffusion area and the later diffusion area.For example, the RGB values ​​of the initial diffusion region may be (80~100, 80~100, 80~100), and the RGB values ​​of the later diffusion region may be (100~120, 100~120, 100~120). Additionally, the difference between them may be (1~20, 1~20, 1~20), and in this case, the initial diffusion region and the later diffusion region can be distinguished by a virtual closed line connecting points where the RGB values ​​in the diffusion region are (100, 100, 100).

[0058] If the Do / Di value derived according to the above method exceeds 1.25, the impregnation and diffusion of the electrolyte in the heat-resistant layer and the porous support may become uneven, which may have an adverse effect on the electrochemical characteristics of the battery.

[0059] FIG. 2 illustrates a method for evaluating the impregnation of a porous support with respect to an electrolyte according to the present invention. Referring to FIG. 2, the porous support may be formed by a method comprising the steps of: laminating 30 or more sheets of the porous support to form a laminate; dropping 2 μl of propylene carbonate onto one surface of the laminate and leaving it for 5 minutes; peeling off the porous support one sheet at a time from the other surface of the laminate while checking the wetting state of the propylene carbonate; and determining the number (X) of the porous support in the laminate in which the wetting state of the propylene carbonate is confirmed. The value of X derived according to this method may be 20 or more, preferably 25 or more, more preferably 35 or more, and advantageously 40 or more. If the value of X derived according to the method is less than 20, the impregnation of the electrolyte in the thickness direction is reduced, and through this, it can be estimated that the pore size, pore structure, etc., in the thickness direction and / or plane direction of a single porous support becomes non-uniform.

[0060] The above propylene carbonate is one of the electrolytes used in secondary batteries, for example, lithium secondary batteries, and instead of the above propylene carbonate, one selected from the group consisting of ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and combinations of two or more of these may be used, and the above propylene carbonate may also be mixed with these and used.

[0061] Generally, the evaluation of electrolyte impregnation in battery separators is considered one of the critical factors determining battery performance and stability. Conventional evaluation methods primarily involved measuring the area of ​​electrolyte impregnation in the separator along the Transverse Direction (TD) and Machine Direction (MD). This method is useful for evaluating the planar impregnation of the separator and helps verify the physical structure and uniformity of the separator during the manufacturing process.

[0062] However, evaluation methods that consider only the TD and MD directions have the following limitations. Measurements along the TD and MD directions reflect only the two-dimensional impregnation of the separator. In actual batteries, the electrolyte moves three-dimensionally through the separator, and the movement of the electrolyte in the thickness direction (so-called Z direction) significantly affects battery performance. Therefore, without considering impregnation in the thickness direction, it is difficult to accurately predict performance in actual operating environments. The thickness-direction impregnation of the separator is determined by the electrolyte penetration rate and the pore structure within the separator. If the pore structure is non-uniform in the thickness direction of the separator or if the electrolyte is not properly impregnated, electrochemical reactions within the battery may occur inefficiently. This can lead to shortened battery life, increased internal resistance, and heat generation problems. However, it is difficult to detect these problems in advance using only measurements in the TD and MD directions. Furthermore, defects or non-uniformities in the thickness direction that may occur during the separator manufacturing process are not sufficiently detected by TD and MD direction evaluations. For example, if pore size gradually changes in the thickness direction or electrolyte penetration is inhibited in a specific layer, these issues can significantly impact actual battery performance. However, it is difficult to detect these issues using existing evaluation methods.

[0063] To address these limitations, a method for evaluating electrolyte impregnation along the thickness direction is required. Evaluation methods along the thickness direction are important for the following reasons: Thickness-direction evaluation allows for the identification of the three-dimensional impregnation characteristics of the separator. This enables the simulation of electrolyte movement in an environment similar to actual battery operating conditions, thereby allowing for a more precise evaluation of the separator's performance and stability. The thickness-direction evaluation method can identify thickness-direction non-uniformity that may occur during the manufacturing process and provide data for improvement; this contributes to enhancing separator quality and optimizing the overall performance of the battery. Furthermore, thickness-direction evaluation allows for an in-depth analysis of the interaction between the separator's pore structure and the electrolyte. Such analysis can provide fundamental data for the development of new materials and process optimization.

[0064] In other words, existing measurement and evaluation methods following the TD and MD directions in evaluating the electrolyte impregnation of battery separators have two-dimensional limitations, and by introducing a thickness direction evaluation method, the three-dimensional characteristics of the separator can be identified more accurately. This is an essential approach for improving the performance and stability of batteries and is expected to play an important role in the development of next-generation battery technology. Accordingly, the inventors have discovered that, through the evaluation method described below, parameter (X) can be characterized as an indicator of the design range of the separator suitable for improving electrolyte impregnation.

[0065] The above X is not specifically limited but can be controlled, for example, by the properties and composition of the raw material (polyolefin, pore-forming agent, etc.), the specifications of the screw included in the extruder during extrusion (diameter, length, aspect ratio, etc.), the rotational speed (rpm), the temperature during casting, the thickness of the base sheet, the stretching temperature according to the MD and TD directions during stretching, the stretching ratio, the solvent used for the extraction and removal of the pore-forming agent, the time required therefrom, the temperature during heat setting, the stretching ratio, etc. Accordingly, it can be determined by the size, structure, shape, connectivity of the pores formed in the separator, the uniformity and gradient along the plane direction and / or thickness direction, and the surface properties of the separator (roughness, smoothness), etc. For example, if at least one of the content of the hydrophilic polymer in the separator, the aspect ratio (L / D) of the screw included in the extruder, and the temperature during casting increases within a predetermined range, the diffusion resistance of the electrolyte along the thickness direction at the interlayer interface of the laminate tends to decrease.

[0066] The porous support may satisfy at least one of the following conditions (i) to (vi): (i) thickness of 1 to 20 μm, preferably 1 to 15 μm, more preferably 1 to 10 μm; (ii) MD tensile strength of 2,500 to 3,500 kgf / cm² 2 , preferably, 2,800~3,500 kgf / cm² 2 , more preferably, 2,900~3,500 kgf / cm² 2 ; (iii) TD tensile strength 2,000~3,000 kgf / cm 2 , preferably, 2,300~3,000 kgf / cm² 2 , more preferably, 2,400~3,000 kgf / cm² 2; (iv) MD tensile elongation 50~150%, preferably 50~120%, more preferably 70~100%; (v) TD tensile elongation 50~150%, preferably 70~150%, more preferably 90~120%; (vi) puncture strength 450~600gf, preferably 480~600gf, more preferably 490~580gf.

[0067] The heat-resistant layer is formed on at least one surface of the porous support and may include inorganic particles and a binder.

[0068] The above inorganic particles may include one selected from the group consisting of SiO2, AlO(OH), Mg(OH)2, Al(OH)3, TiO2, BaTiO3, Li2O, LiF, LiOH, Li3N, BaO, Na2O, Li2CO3, CaCO3, LiAlO2, Al2O3, SiO, SnO, SnO2, PbO2, ZnO, P2O5, CuO, MoO, V2O5, B2O3, Si3N4, CeO2, Mn3O4, Sn2P2O7, Sn2B2O5, Sn2BPO6, and combinations of two or more of these, and preferably may include boehmite (AlO(OH)), but is not limited thereto.

[0069] The content of the inorganic particles in the heat-resistant layer may be 50 to 99 weight percent. If the content of the inorganic particles in the heat-resistant layer is less than 50 weight percent, the required level of heat resistance cannot be provided, and if it exceeds 99 weight percent, the dispersibility of the inorganic particles may be reduced, or workability and processability may be reduced.

[0070] The average particle size (D50) of the inorganic particles may be larger than the average size of the pores contained in the porous support. If the average particle size of the inorganic particles is less than or equal to the average size of the pores contained in the porous support, the inorganic particles may penetrate into the interior of the pores of the porous support and close the pores, thereby significantly reducing the air permeability and ion conductivity of the separation membrane. The average particle size of the inorganic particles may be 100 to 1,000 nm, preferably 200 to 800 nm, more preferably 400 to 800 nm, but is not limited thereto.

[0071] The thickness of the heat-resistant layer may be 1 to 10 μm. If the thickness of the heat-resistant layer is less than 1 μm, it may not be possible to provide the required level of heat resistance, and if it exceeds 10 μm, the separator may become thick, which may hinder the miniaturization and integration of the battery.

[0072] The binder may melt and fuse in the heat-resistant layer to not only bind the inorganic particles together but also bind the inorganic particles to the porous support. The binder may include a hydrophilic binder, a hydrophobic binder, or a combination thereof, and preferably may include a hydrophilic binder and a hydrophobic binder. The hydrophobic binder may be a copolymer comprising a hydrophilic unit and a hydrophobic unit. As used herein, the term "unit" refers to a monomer, oligomer, or polymer constituting the copolymer.

[0073] The above hydrophilic binder may be, for example, one selected from the group consisting of polymethyl methacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, ethylene vinyl acetate, polyimide, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, hydroxyethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, polyvinyl alcohol, polyvinyl butyral, acrylonitrile-acrylic acid copolymer, ethylene-acrylic acid copolymer, styrene-butadiene copolymer, alkyl acrylate-acrylonitrile copolymer, polyethylene glycol, acrylic rubber, and combinations of two or more of these, but is not limited thereto.

[0074] Among the hydrophobic binders, the hydrophilic unit ensures that the inorganic particles and the binder are uniformly dispersed in the slurry for forming the heat-resistant layer, and the hydrophobic unit can effectively suppress the absorption of moisture from the surrounding atmosphere by the heat-resistant layer in an undried and / or dried state.

[0075] The above hydrophilic unit may include one selected from the group consisting of acrylic acid, methacrylic acid, acrylamide, hydroxyethyl methacrylate, hydroxyethyl acrylate, vinyl alcohol, poly(ethylene glycol) methacrylate, N,N-dimethylacrylamide, sodium acrylate, sodium methacrylate, vinyl sulfone, propylene glycol methacrylate, 2-hydroxypropyl methacrylate, 2-methacryloyloxyethylphosphorylcholine, vinylpyrrolidone, itaconic acid, N-vinylcaprolactam, N,N-dimethylaminoethyl methacrylate, N-isopropylacrylamide, polyacrylic acid, polyvinyl alcohol, polyvinylpyrrolidone, and combinations of two or more of these.

[0076] The above hydrophobic unit is styrene, polystyrene, methyl methacrylate, butyl acrylate, hexyl acrylate, octyl acrylate, isobornyl acrylate, dodecyl methacrylate, isopropyl acrylate, lauryl methacrylate, tetrahydrofuran acrylate, ethyl methacrylate, trimethylsilyl methacrylate, 2-ethylhexyl methacrylate, methoxytrimethylsilyl methacrylate, isobornyl acrylate, cyclohexyl methacrylate, benzyl methacrylate, propyl methacrylate, phenyl methacrylate, isotrimethoxysilyl methacrylate, triethoxysilylpropyl acrylate, methoxysilylpropyl methacrylate, phenyltrimethoxysilane, hexamethyldisiloxane, pentamethyldisiloxane, vinyltrimethoxysilane, vinyltriethoxysilane, trimethoxysilylethyl methacrylate, polydimethylsiloxane It may include methacrylate and one selected from the group consisting of a combination of two or more of these.

[0077] The above hydrophobic unit may be an aromatic monomer, an oligomer or polymer containing it. When the above hydrophobic unit comprises at least one aromatic monomer, the hydrophobic binder is, for example, a styrene-acrylic acid copolymer, a phenyl methacrylate-hydroxyethyl methacrylate copolymer, a benzyl methacrylate-acrylic acid copolymer, a styrene-hydroxyethyl acrylate copolymer, a phenyl methacrylate-acrylamide copolymer, a benzyl acrylate-acrylic acid copolymer, a styrene-N,N-dimethylacrylamide copolymer, a phenyl methacrylate-sodium methacrylate copolymer, a benzyl methacrylate-hydroxyethyl methacrylate copolymer, a styrene-methacrylic acid copolymer, a phenyl methacrylate-sodium acrylate copolymer, a benzyl acrylate-N,N-dimethylacrylamide copolymer, a styrene-poly(ethylene glycol) methacrylate copolymer, and a phenyl It may be one selected from the group consisting of acrylate-hydroxyethyl methacrylate copolymer, benzyl methacrylate-sodium acrylate copolymer, styrene-acrylamide copolymer, phenyl methacrylate-hydroxypropyl methacrylate copolymer, benzyl acrylate-methacrylic acid copolymer, styrene-hydroxyethyl methacrylate copolymer, phenyl acrylate-acrylic acid copolymer, and combinations of two or more of these, and preferably may be a styrene-acrylic acid copolymer, but is not limited thereto.

[0078] In addition, the hydrophobic unit may be a silicon-based monomer, an oligomer or polymer containing the same. When the above hydrophobic unit comprises at least one silicone-based monomer, the hydrophobic binder is a polydimethylsiloxane-acrylic acid copolymer, a trimethoxysilylpropyl methacrylate-hydroxyethyl acrylate copolymer, a methoxysilylpropyl methacrylate-acrylamide copolymer, a polydimethylsiloxane-methacrylic acid copolymer, a pentamethyldisiloxane-acrylic acid copolymer, a triethoxysilylpropyl acrylate-hydroxyethyl methacrylate copolymer, a methoxysilylpropyl acrylate-acrylic acid copolymer, a hexamethyldisiloxane-acrylamide copolymer, a polydimethylsiloxane-methacrylamide copolymer, a trimethoxysilylpropyl acrylate-acrylic acid copolymer, a vinyltrimethoxysilane-hydroxyethyl acrylate copolymer, a hexamethylsiloxane-acrylamide copolymer, It may be one selected from the group consisting of pentamethyldisiloxane-hydroxyethyl methacrylate copolymer, dimethylsiloxane-methacrylic acid copolymer, vinyltriethoxysilane-acrylic acid copolymer, polydimethylsiloxane-acrylamide copolymer, trimethoxysilylethyl methacrylate-acrylic acid copolymer, pentamethyldisiloxane-acrylic acid copolymer, dimethoxysilylpropyl methacrylate-hydroxyethyl acrylate copolymer, triethoxysilylethyl acrylate-methacrylamide copolymer, and combinations of two or more of these, and preferably may be polydimethylsiloxane-acrylic acid copolymer, but is not limited thereto.

[0079] The content of the hydrophobic binder in the above binder may be 25-80 wt%, 30-70 wt%, 40-70 wt%, 50-70 wt%, or 60-70 wt%. If the content of the hydrophobic binder in the above binder is less than 25 wt%, it is difficult to suppress moisture absorption by the separator, and consequently, various physical properties such as a decrease in the binding strength of the heat-resistant layer, an increase in resistance, and a decrease in dielectric breakdown voltage may be caused. If the content of the hydrophobic binder in the above binder exceeds 80 wt%, the content of the hydrophilic binder is relatively reduced, which may result in a decrease in the dispersibility of the inorganic particles or a decrease in workability and processability, causing uncoated areas to occur, and such uncoated areas may have an adverse effect on the heat resistance of the separator.

[0080] Method for manufacturing a separation membrane

[0081] A method for manufacturing a separator according to another aspect of the present invention may comprise: (a) a step of feeding a composition comprising a polyolefin, a hydrophilic polymer, and a pore-forming agent into an extruder comprising a screw and forming a base sheet; (b) a step of stretching the base sheet and then extracting the pore-forming agent to produce a base film; (c) a step of heat-setting the base film to produce a porous support; and (d) a step of applying a slurry comprising inorganic particles, a binder, and a solvent to at least one surface of the porous support and then drying to form a heat-resistant layer.

[0082] That is, a separator having the properties as described above can be manufactured by a method comprising: (a) a step of feeding a composition comprising a polyolefin, a hydrophilic polymer, and a pore-forming agent into an extruder including a screw and forming a base sheet; (b) a step of stretching the base sheet and then extracting the pore-forming agent to produce a base film; (c) a step of heat-setting the base film to produce a porous support; and (d) a step of applying a slurry comprising inorganic particles, a binder, and a solvent to at least one surface of the porous support and then drying to form a heat-resistant layer.

[0083] In step (a) above, a composition comprising 20 to 40 weight% of the polyolefin, 0.1 to 5 weight% of the hydrophilic polymer, and the remainder of a pore-forming agent may be fed into an extruder including the screw to form a base sheet. The types, physical properties, and effects of the polyolefin and the hydrophilic polymer are as described above. The input amounts of the polyolefin and the hydrophilic polymer may be adjusted so that the content of the hydrophilic region containing the hydrophilic polymer in the separation membrane is 0.1 to 30 weight%.

[0084] The above pore-forming agent may be one selected from the group consisting of paraffin oil, paraffin wax, mineral oil, solid paraffin, soybean oil, rapeseed oil, palm oil, coconut oil, di-2-ethylhexyl phthalate, dibutyl phthalate, diisononyl phthalate, diisodecyl phthalate, bis(2-propylheptyl)phthalate, naphthene oil, and combinations of two or more of these, preferably may be paraffin oil, and more preferably may be paraffin oil having a kinematic viscosity of 50 to 100 cSt at 40°C, but is not limited thereto.

[0085] In step (a) above, the aspect ratio (L / D) of the screw may be 58 to 62. The aspect ratio (Length-to-Diameter Ratio, L / D) of the screw is defined as the length of the screw divided by its diameter, and this can significantly affect the performance of the extruder and the quality of the final product. The aspect ratio can directly affect the residence time of the material within the extruder, mixing and melting efficiency, and the uniformity of the extruded product.

[0086] When the aspect ratio is high, the material remains inside the screw for a longer period, increasing the time it is subjected to thermal and shear stress. This allows the material to be sufficiently mixed and melted, resulting in improved uniformity of the extruder. Particularly when processing high-viscosity or composite materials, a high aspect ratio is advantageous for obtaining high-quality extruders as it ensures uniform mixing and temperature distribution. Conversely, a low aspect ratio results in a shorter residence time, which may hinder sufficient mixing and melting; this can degrade the uniformity of the extruder and lead to quality issues. Furthermore, the aspect ratio can also affect the design and energy efficiency of the screw. Screws with a high aspect ratio provide longer processing paths to enhance mixing and melting efficiency, but this can increase the power and thermal energy required to drive the extruder. On the other hand, a low aspect ratio consumes relatively less energy but has the disadvantage of making it difficult to produce uniform extruders. Therefore, it is important to design the aspect ratio appropriately, which can be optimized according to the requirements of the processed material and the final product.

[0087] If the aspect ratio of the screw is less than 58, the residence time of the material within the screw is short, which may result in incomplete mixing and melting processes and reduced uniformity of the extruder. Consequently, the mechanical properties, electrolyte impregnation, and electrochemical characteristics of the porous support and the separator containing it may deteriorate, which may be more pronounced in high-viscosity materials such as ultra-high molecular weight polyethylene. Additionally, the short residence time may cause non-uniformity in the temperature distribution of the material, leading to any defects in the separator. If the aspect ratio of the screw exceeds 62, the material may remain inside the extruder for an excessively long time, which may result in excessive heat and / or shear forces being applied to the material. This may cause thermal decomposition or thermal damage to the material, and consequently, the physical and chemical properties of the extruder may be compromised. Furthermore, it may accelerate wear on the screw and barrel, increasing equipment maintenance costs and energy consumption, which may lead to reduced productivity.

[0088] In step (a) above, the rotational speed of the screw may be 40 to 200 rpm, preferably 60 to 150 rpm, more preferably 60 to 100 rpm. Depending on the rotational speed of the screw, the mixing, melting, and pore formation of the material may vary, and thereby the performance and quality of the porous support and the separation membrane containing it may be determined.

[0089] If the rotational speed of the screw is less than 40 rpm, the mixing and melting of the material are not sufficiently achieved, and the pore structure of the porous support may be formed unevenly. This can degrade electrochemical performance and, in particular, have a negative impact on the electrolyte impregnation and ion transport efficiency inside the secondary battery. On the other hand, if the rotational speed of the screw exceeds 200 rpm, the material may be subjected to excessive shear stress, which may cause thermal damage or thermal decomposition. Consequently, the mechanical strength of the porous support may be weakened, and high-temperature stability may be reduced.

[0090] In step (b) above, a base film may be obtained by stretching the base sheet and then extracting the pore-forming agent, or by stretching the base sheet after extracting the pore-forming agent. The stretching may be performed by known methods such as uniaxial stretching or biaxial stretching (sequential or simultaneous).

[0091] In the case of sequential or simultaneous biaxial stretching, the stretching ratio may be 4 to 20 times in the transverse direction (MD) and the longitudinal direction (TD), respectively, and the corresponding surface ratio may be 10 to 400 times. In particular, in step (b) above, the stretching may be performed at a temperature of 100 to 130°C. The stretching temperature of the base sheet may have a significant effect on both the physical properties and productivity of the porous support.

[0092] From the perspective of the physical properties of the porous support, the stretching temperature can have a direct effect on the mechanical strength and thermal stability of the porous support. If the stretching temperature is below 100°C, the polyolefin molecular chains may not have sufficient fluidity, and cracks or defects may occur during stretching. These defects can weaken the mechanical strength of the porous support and impair the uniformity of the pore structure during electrolyte penetration, thereby negatively affecting the performance of the battery. Conversely, if the stretching temperature exceeds 130°C, the polyolefin material may soften excessively, resulting in the formation of an uneven pore structure. This not only reduces electrochemical stability but also lowers the heat resistance of the porous support, which can compromise the safety of the battery in high-temperature environments.

[0093] Meanwhile, setting an appropriate stretching temperature is necessary to increase process efficiency. If the stretching temperature is below 100°C, the elongation of the material is limited, making it difficult to achieve a high stretching ratio and potentially reducing the production speed of the porous support. On the other hand, if the stretching temperature exceeds 130°C, excessive heat can damage the polyolefin material or cause overheating issues with equipment such as screws, which can increase maintenance costs and compromise process stability.

[0094] In step (c) above, the heat setting may be performed at a temperature of 120 to 135°C while the base film is relaxed or stretched along at least one of MD and TD by an area ratio of 0.8 to 3 times.

[0095] Heat setting is a process of fixing a film and applying heat to forcibly hold the film that is about to shrink, thereby removing residual stress. While a high heat setting temperature is advantageous for reducing the shrinkage rate, if the temperature is excessively high, the pores formed by partial melting of the film may close, potentially reducing transmittance. It is preferable to select a heat setting temperature within a range where 10 to 30 weight percent of the crystalline portion of the base film melts. If the heat setting temperature is selected within this range, it is possible to prevent the problem of insufficient rearrangement of polyolefin molecules within the base film resulting in no effect of removing residual stress from the film, and the problem of reduced transmittance caused by the closure of pores due to partial melting. For example, the heat setting temperature may be 120 to 135°C, preferably 130 to 135°C, and the heat setting time may be 5 seconds to 10 minutes, preferably 10 seconds to 1 minute.

[0096] Meanwhile, in step (c) above, the physical properties and productivity of the porous support can be simultaneously optimized by relaxing or stretching the base film along at least one of MD and TD at an area ratio of 0.8 to 3 times during heat setting. The relaxation or stretching ratio in the heat setting step may affect the pore structure and mechanical strength of the porous support. If the area ratio is less than 0.8 times, the pores of the porous support are not sufficiently stabilized, which may result in reduced electrolyte impregnation and ion transport efficiency. Conversely, if the area ratio exceeds 3 times, excessive stress is applied to the porous support, which may cause the pore structure to be destroyed or the mechanical strength to be weakened.

[0097] In step (d) above, a heat-resistant layer can be formed by applying a slurry containing inorganic particles, a binder, and a solvent to at least one surface of the porous support and then drying it.

[0098] The above solvent may be an aqueous solvent, and the above aqueous solvent may be, for example, one selected from the group consisting of methanol, ethanol, propanol, butanol, methoxyethanol, ethoxyethanol, lactone, acetonitrile, n-methyl-2-pyrrolidone (NMP), formic acid, nitromethane, acetic acid, dimethyl sulfoxide, water (distilled water), and combinations of two or more of these, and preferably may be a mixed solvent including water and ethanol, but is not limited thereto. The type, properties, content, effects, etc. of the above inorganic particles and the above binder are as described above.

[0099] The application of the above slurry may be carried out by one selected from the group consisting of a roll coater, a bar coater, a spray coater, a die coater, a comma coater, and a combination of two or more of these, and preferably by a roll coater and / or a bar coater, but is not limited thereto.

[0100] The content of the solid component including the inorganic particles and the binder in the above slurry may be 10 to 50 weight%, preferably 15 to 45 weight%, and more preferably 20 to 40 weight%. If the content of the solid component in the above slurry is less than 10 weight%, uncoated areas may occur, and such uncoated areas may adversely affect the heat resistance of the separator membrane, and if it exceeds 50 weight%, the dispersibility of the slurry may be reduced, or workability and processability may be reduced.

[0101] The inorganic particles tend to bind and aggregate due to electrostatic attraction. Such aggregation of inorganic particles can impair the uniformity of physical properties on the surface of the separation membrane. Accordingly, the slurry may further include additives to improve the dispersibility of the inorganic particles, such as dispersants and surfactants. In particular, the slurry may include sodium hexametaphosphate ((NaPO3)6) as a dispersant, and the content of sodium hexametaphosphate in the heat-resistant layer may be 0.01 to 1 weight%, preferably 0.01 to 0.5 weight%. The sodium hexametaphosphate is adsorbed onto the edges of the inorganic particles, particularly plate-shaped inorganic particles, and weakens the negative charge on the edges, thereby effectively preventing the aggregation of the inorganic particles. This can improve the storage stability of the slurry for forming the heat-resistant layer and the dispersibility of the inorganic particles within the heat-resistant layer.

[0102] The above drying means a process of applying heat to the slurry applied to at least one surface of the porous support to remove the aqueous solvent and other liquid residues contained in the slurry.

[0103] The heat-resistant layer may be formed on one side of the porous support, and if necessary, may be formed on both sides. The heat-resistant layers formed on both sides of the porous support may have the same thickness, composition, and effect, and in some cases, at least one of these may differ.

[0104] Meanwhile, before applying the slurry to at least one surface of the porous support in step (d) above, the porous support may be plasma treated in the presence of a mixed gas containing sulfur dioxide (SO2) and oxygen (O2). Through the plasma treatment, the surface of the porous support and / or the surface of the internal pores may be hydrophilized to improve the bonding strength between both sides of the porous support and the slurry, thereby significantly improving the durability of the separation membrane, particularly its long-term durability and heat resistance.

[0105] Conventionally, a wet process in which the surface of a porous support is immersed in sulfuric acid or the like for a certain period of time to sulfonate it has been mainly used to hydrophilize the surface of the porous support. However, in this case, the wet process is performed separately from the plasma treatment, such as preceding or succeeding the plasma treatment, which makes the process complex and causes a problem of generating a large amount of process waste liquid.

[0106] In this regard, the process gas used in the above plasma treatment includes not only conventional air, oxygen, and / or inert gas but also a certain amount of sulfur dioxide gas. Therefore, functional groups such as -SO3 are generated on the surface of the porous support and the surface of the internal pores through a single dry process called plasma treatment without a wet process such as immersing the porous support in sulfuric acid, i.e., sulfonation, thereby maximizing the hydrophilicity and ion conductivity of the porous support, simplifying the complex conventional process, and is also advantageous in terms of the environment.

[0107] The mixed gas, which is the process gas used in the above plasma treatment, may contain 50 to 90 volume% sulfur dioxide and 10 to 50 volume% oxygen, preferably 60 to 80 volume% sulfur dioxide and 20 to 40 volume% oxygen, more preferably 70 to 80 volume% sulfur dioxide and 20 to 30 volume% oxygen. If the content of sulfur dioxide in the mixed gas is less than 50 volume%, the required level of hydrophilicity for the porous support cannot be achieved, and if it exceeds 90 volume%, the process may become unstable.

[0108] The above plasma treatment can be performed for 0.5 to 90 minutes, preferably 0.5 to 20 minutes. If the above plasma treatment is performed for less than 0.5 minutes, the porous support cannot be hydrophilized and sulfonated to the required level, and if it is performed for more than 90 minutes, the degree of hydrophilization and sulfonation may converge to a certain level, which may reduce process efficiency.

[0109] Hereinafter, embodiments of the present invention will be described in detail.

[0110] Example 1

[0111] 30 parts by weight of a main resin, in which polyethylene (PE) with a viscosity average molecular weight (Mv) of 1,000,000 and ethylene vinyl acetate (EVA) with a vinyl acetate content of 28% by weight were mixed in a weight ratio of 99:1, and 70 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C were mixed and fed into a twin extruder (inner diameter 58 mm, L / D=60). Under conditions of a screw rotation speed of 80 rpm and 200°C, the mixture was extruded from the twin extruder through a T-die with a width of 300 mm and then passed through a casting roll at a temperature of 40°C to produce a base sheet with a thickness of 800 μm.

[0112] A base film was prepared by stretching the above base sheet eight times in the longitudinal direction (MD) in a roll stretcher at 110°C and nine times in the transverse direction (TD) in a tenter stretcher at 125°C. The above base film was immersed in a dichloromethane leaching tank at 25°C to extract and remove paraffin oil for 1 minute, and then dried at 50°C for 5 minutes. The above film was heat-set at 135°C while relaxed by 15% in the transverse direction (TD) to produce a porous support having a thickness of 9 μm and an air permeability of 150 s / 100 cc.

[0113] A slurry was prepared by mixing 60g of acryl-acrylonitrile copolymer, 180g of styrene-acrylic acid copolymer, 750g of boehmite (AlO(OH)6), 1.5g of dispersant ((NaPO3)6), 1,500g of water, and 86g of ethanol, and then dispersing the mixture using a ball mill. The slurry was applied to one surface of the porous support using a bar coater, and then dried in a hot air oven at a temperature of 80°C for 1 hour to produce a separator containing a heat-resistant layer with a thickness of 3㎛.

[0114] Examples 2 to 9

[0115] A separator membrane was manufactured in the same manner as in Example 1 above, except that at least one process variable among the PE:EVA weight ratio, extrusion, casting, stretching, and heat setting was changed. The process variables applied to Examples 1 to 9 are shown in Table 1 below.

[0116]

[0117]

[0118] Comparative Examples 1 to 13

[0119] A separator membrane was manufactured in the same manner as in Example 1, except that at least one process variable among the PE:EVA weight ratio, extrusion, casting, stretching, and heat setting conditions was changed. The process variables applied to Examples 1 to 9 are shown in Table 2 below.

[0120]

[0121]

[0122] Experimental Example

[0123] The physical properties of the separator membranes prepared in the examples and comparative examples were measured and evaluated according to the following method, and the results are shown in Table 3, Table 4, and Figure 3 below.

[0124] - Thickness direction impregnation (X): Fifty separator specimens were laminated to form a laminate, and 2 μl of propylene carbonate was applied to one side of the laminate and left for 5 minutes. While peeling off the separator specimens one by one from the other side of the laminate, the wetting state of the propylene carbonate was visually checked based on the difference in brightness, and the number of separator specimens (X) in which the wetting state of the propylene carbonate was confirmed among the laminates was determined.

[0125] -Thickness (㎛): The thickness of the membrane specimen was measured using a micro-thickness gauge.

[0126] - Tensile strength (kgf / cm²) 2 Using a tensile strength tester, the stress applied until fracture occurred in the machine direction (MD) and transverse direction (TD) was measured on a separator specimen with dimensions of 20mm*200mm.

[0127] - Tensile elongation (%): Using a tensile strength tester, the ratio of the elongation of a 20mm x 200mm membrane specimen until fracture occurred in the machine direction (MD) and transverse direction (TD) was measured.

[0128] - Perforation strength (gf): Using a perforation strength meter, force was applied with a stick to a porous film specimen of size 100mm*50mm until the sample was pierced, and the force applied was measured.

[0129] - Maximum diameter of the initial diffusion region (Di, mm), maximum diameter of the later diffusion region (Do, mm), Do / Di: 2 μl of propylene carbonate was dropped onto one side of the heat-resistant layer of the separator and left for 20 seconds. The region where propylene carbonate diffused radially along the surface of the heat-resistant layer was divided into an initial diffusion region and a later diffusion region based on brightness. Specifically, a virtual closed line was drawn by connecting points where the RGB (red, green, blue) values ​​were (100, 100, 100) within the region where propylene carbonate diffused. The inner region of the closed line was designated as the initial diffusion region, and the outer region as the later diffusion region. Based on this, Di, Do, and Di / Do were measured and calculated.

[0130]

[0131]

[0132]

[0133]

[0134] The foregoing description of the present invention is for illustrative purposes only, and those skilled in the art will understand that other specific forms can be easily modified without altering the technical spirit or essential features of the present invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive. For example, each component described as a single unit may be implemented in a distributed manner, and components described as distributed may likewise be implemented in a combined form.

[0135] The scope of the present invention is defined by the claims set forth below, and all modifications or variations derived from the meaning and scope of the claims and equivalent concepts thereof should be interpreted as being included within the scope of the present invention.

[0136] <Explanation of Symbols>

[0137] S: Porous support (one membrane sheet or one sheet)

[0138] E: Electrolyte

[0139] CL: Heat-resistant layer

Claims

1. A porous support having a hydrophobic region comprising a polyolefin and a hydrophilic region comprising a hydrophilic polymer dispersed in the hydrophobic region; and A separation membrane comprising: a heat-resistant layer formed on at least one surface of the above-mentioned porous support and comprising inorganic particles and a binder; Separator with a Do / Di of 1.0–1.25 derived according to the following method: A step of dropping 2 µl of propylene carbonate onto one surface of the heat-resistant layer and leaving it for 20 seconds; A step of dividing the region in which propylene carbonate has radially diffused along the surface of the heat-resistant layer into an initial diffusion region and a later diffusion region based on brightness; and A step of calculating the ratio Do / Di by measuring the maximum diameter (Di) of the initial diffusion region and the maximum diameter (Do) of the later diffusion region, Here, the brightness of the initial diffusion region is lower than the brightness of the later diffusion region.

2. In Paragraph 1, The above polyolefin comprises one selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, and combinations or copolymers of two or more of these. Separator.

3. In Paragraph 2, The above polyolefin comprises polyethylene having a viscosity average molecular weight (Mv) of 200,000 to 3,000,000 g / mol, Separator.

4. In Paragraph 1, The content of the hydrophilic region in the above porous support is 0.1 to 30 weight%, Separator.

5. In Paragraph 1, The above hydrophilic polymer comprises one selected from the group consisting of ethylene vinyl acetate, ethylene vinyl alcohol, polyvinyl alcohol, polyacrylic acid, polyoxyethylene-polyoxypropylene block copolymer, polyethylene glycol, polyethylene oxide, polyvinylpyrrolidone, polyvinyl acetal, polyvinyl butyral, cellulose derivative, polyurethane, polyamide, polyester, glycerol, ethylene-based copolymer comprising a hydrophilic monomer, and combinations of two or more of these. Separator.

6. In Paragraph 1, A step of forming a laminate by laminating 30 or more of the above porous supports; A step of dropping 2 µl of propylene carbonate onto one surface of the laminate and leaving it for 5 minutes; A step of checking the wetting state of propylene carbonate while peeling off the porous support one sheet at a time from the other side of the laminate; and A method comprising the step of determining the number (X) of the porous support in which the wetting state of the propylene carbonate among the laminates is confirmed; wherein X derived according to the method is 20 or more, Separator.

7. In Paragraph 1, The above porous support satisfies at least one of the following conditions (i) to (vi), Separator: (i) Thickness 1~20㎛; (ii) MD tensile strength 2,500~3,500 kgf / cm² 2 ; (iii) TD tensile strength 2,000~3,000 kgf / cm² 2 ; (iv) MD tensile elongation 50~150%; (v) TD tensile elongation 50~150%; (vi) Perforation strength 450~600gf.

8. In Paragraph 1, The above inorganic particles comprise one selected from the group consisting of SiO2, AlO(OH), Mg(OH)2, Al(OH)3, TiO2, BaTiO3, Li2O, LiF, LiOH, Li3N, BaO, Na2O, Li2CO3, CaCO3, LiAlO2, Al2O3, SiO, SnO, SnO2, PbO2, ZnO, P2O5, CuO, MoO, V2O5, B2O3, Si3N4, CeO2, Mn3O4, Sn2P2O7, Sn2B2O5, Sn2BPO6, and combinations of two or more of these. Separator.

9. In Paragraph 1, The thickness of the above heat-resistant layer is 1 to 10 μm, and The content of the inorganic particles in the heat-resistant layer is 50 to 99 weight percent, Separator.

10. In Paragraph 1, The above binder comprises a hydrophilic binder, a hydrophobic binder, or a combination thereof. Separator.

11. In Paragraph 1, The above binder includes a hydrophilic binder and a hydrophobic binder, and The above hydrophobic binder is a copolymer comprising hydrophilic units and hydrophobic units, Separator.

12. In Paragraph 11, The content of the hydrophobic binder among the above binders is 25 to 80 weight percent, Separator.

13. In Paragraph 11, The above hydrophilic unit comprises one selected from the group consisting of acrylic acid, methacrylic acid, acrylamide, hydroxyethyl methacrylate, hydroxyethyl acrylate, vinyl alcohol, poly(ethylene glycol) methacrylate, N,N-dimethylacrylamide, sodium acrylate, sodium methacrylate, vinyl sulfone, propylene glycol methacrylate, 2-hydroxypropyl methacrylate, 2-methacryloyloxyethylphosphorylcholine, vinylpyrrolidone, itaconic acid, N-vinylcaprolactam, N,N-dimethylaminoethyl methacrylate, N-isopropylacrylamide, polyacrylic acid, polyvinyl alcohol, polyvinylpyrrolidone, and combinations of two or more of these. Separator.

14. In Paragraph 11, The above hydrophobic unit is styrene, polystyrene, methyl methacrylate, butyl acrylate, hexyl acrylate, octyl acrylate, isobornyl acrylate, dodecyl methacrylate, isopropyl acrylate, lauryl methacrylate, tetrahydrofuran acrylate, ethyl methacrylate, trimethylsilyl methacrylate, 2-ethylhexyl methacrylate, methoxytrimethylsilyl methacrylate, isobornyl acrylate, cyclohexyl methacrylate, benzyl methacrylate, propyl methacrylate, phenyl methacrylate, isotrimethoxysilyl methacrylate, triethoxysilylpropyl acrylate, methoxysilylpropyl methacrylate, phenyltrimethoxysilane, hexamethyldisiloxane, pentamethyldisiloxane, vinyltrimethoxysilane, vinyltriethoxysilane, trimethoxysilylethyl methacrylate, polydimethylsiloxane Serving with methacrylate and one selected from the group consisting of combinations of two or more of these, Separator.

15. In Paragraph 1, The RGB values ​​of the above initial diffusion region are (80~100, 80~100, 80~100), and The RGB values ​​of the above late diffusion region are (100~120, 100~120, 100~120), Separator.

16. In Paragraph 15, The absolute value of the difference between the RGB values ​​of the above initial diffusion region and the above late diffusion region is (1~20, 1~20, 1~20), Separator.

17. A method for manufacturing a separation membrane according to any one of claims 1 to 16, (a) a step of feeding a composition comprising a polyolefin, a hydrophilic polymer, and a pore-forming agent into an extruder comprising a screw and forming a base sheet; (b) a step of manufacturing a base film by stretching the base sheet and then extracting the pore-forming agent; (c) a step of manufacturing a porous support by heat-setting the base film; and (d) a step of forming a heat-resistant layer by applying a slurry containing inorganic particles, a binder, and a solvent to at least one surface of the porous support and then drying; comprising Method for manufacturing a separator.

18. In Paragraph 17, In step (a) above, The aspect ratio (L / D) of the screw is 58 to 62, and the rotational speed of the screw is 40 to 200 rpm. Method for manufacturing a separator.

19. In Paragraph 17, In step (b) above, The above stretching is performed at a temperature of 100 to 130°C along the MD and TD of the base sheet at an area ratio of 10 to 400 times. Method for manufacturing a separator.

20. In Paragraph 17, In step (c) above, The above heat setting is performed at a temperature of 120 to 135°C while the base film is relaxed or stretched along at least one of MD and TD by an area ratio of 0.8 to 3 times. Method for manufacturing a separator.

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