Method for manufacturing a separator and a separator manufactured using the same.

Abstract

One aspect of the present invention provides a method for manufacturing a separator and a separator manufactured using the same, comprising the steps of (a) processing a composition containing a polyolefin and a porosity-forming agent to obtain a base sheet, (b) stretching the base sheet in the longitudinal (MD) and transverse (TD) directions and removing the porosity-forming agent to obtain a porous film, and (c) stretching the porous film in the transverse (TD) direction and heat-setting it.

Description

Technical Field

[0005] ,

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[0006]

[0001] The present invention relates to a method for manufacturing a separator and a separator manufactured using the same.

Background Art

[0002] Lithium secondary batteries are widely used as power sources for various electrical products that require miniaturization and weight reduction, such as smartphones, notebook computers, and tablet PCs. As the application fields expand to include medium and large-sized batteries for smart grids and electric vehicles, there is a demand for the development of lithium secondary batteries with large capacity, long life, and high stability.

[0003] Recently, in order to increase the capacity of lithium secondary batteries, the thinning of separators has emerged as a technical issue. However, such thinning is accompanied by a decrease in mechanical strength, so it is necessary to achieve both in a well-balanced manner. For this reason, separators applying resins having a higher weight average molecular weight than general-purpose polyethylene, such as ultra-high molecular weight polyethylene, have been continuously developed.

[0004] Ultra-high molecular weight polyethylene (UHMWPE) generally refers to polyethylene having a weight average molecular weight of 1,000,000 or more. According to Japanese Patent Laid-Open No. 60-242035 and the like, it is known that ultra-high molecular weight polyethylene is superior to general-purpose polyethylenes such as high-density polyethylene and low-density polyethylene in terms of mechanical strength, abrasion resistance, uniformity, self-lubricity, chemical resistance, and the like.

[0005] However, ultra-high molecular weight polyethylene has low fluidity even in a completely melted state due to its high molecular weight, so it is difficult to process, and there is a problem that the heat resistance decreases, such as an increase in the high-temperature (150 ° C) heat shrinkage rate of the manufactured separator.

[0006] Furthermore, in order to improve the heat resistance of the separator, a separator in which ceramic particles are coated on the surface of a porous support, so-called ceramic-coated separator, has been proposed. However, the aforementioned ceramic-coated separator still has considerable technical challenges related to permeability. Specifically, while coating the surface of the porous support with a heat-resistant layer containing ceramic particles improves the heat resistance of the separator, the heat-resistant layer clogs the pores formed in the porous support, reducing the permeability of the separator. As a result, the ion transport pathway between the positive and negative electrodes decreases, and consequently, the charging and discharging performance of the battery is significantly reduced.

[0007] Attempts have been made to improve the adhesion to the electrodes and extend the battery life by further forming an adhesive layer having adhesive force to the electrodes on the surface of the porous support and / or the surface of the heat-resistant layer.

[0008] Separators coated with functional layers, such as heat-resistant layers and adhesive layers, are generally manufactured by applying and drying a composition for forming the functional layer on one or both sides of a porous support while the porous support is moved at a constant speed. However, if the porous support contains or is made of ultra-high molecular weight polyethylene, there is a problem that the heat applied to the separator during drying for the formation of the functional layer further increases the thermal shrinkage rate of the porous support. [Overview of the project] [Problems that the invention aims to solve]

[0009] The present invention aims to solve the problems of the prior art described above, and the object of the present invention is to provide a method for manufacturing a separator that can achieve and improve a good balance of processability, heat resistance, and perforation strength, and a separator manufactured using the same. [Means for solving the problem]

[0010] One aspect of the present invention provides a method for manufacturing a separator that includes the steps of (a) processing a composition containing a polyolefin and a porosity-forming agent to obtain a base sheet, (b) stretching the base sheet in the longitudinal (MD) and transverse (TD) directions and removing the porosity-forming agent to obtain a porous film, and (c) stretching the porous film in the transverse (TD) direction and heat-setting it, and satisfying the following conditions (1) to (3). (1) 7 ≤ Mb ≤ 8, (2) 0 <Mb / Tb<1、(3)0<Mb / (Tb×Tc)<0.5 In (1) to (3) above, Mb and Tb are the stretch ratios of the base sheet along the longitudinal (MD) and transverse (TD) directions in step (b), respectively, and Tc is the stretch ratio of the porous film along the transverse (TD) direction in step (c).

[0011] In one embodiment, the weight-average molecular weight (Mw) of the polyolefin may be between 1,000,000 and 4,000,000.

[0012] In one embodiment, the polyolefin may include one selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, ethylene vinyl acetate, ethylene butyl acrylate, ethylene ethyl acrylate, and two or more combinations or copolymers thereof.

[0013] In one embodiment, Tc may be 1.5 to 2.0. In one embodiment, the ratio of Mb to Tc (Mb / Tc) may be 3.0 to 5.0.

[0014] In one embodiment, the porosity-forming agent may be a paraffin oil having a kinematic viscosity of 50 to 100 cSt at 40°C.

[0015] In one embodiment, the thickness of the base sheet obtained in step (a) is 1,000 to 1,500 μm, the thickness of the porous film obtained in step (c) is 10 μm or less, and the puncture strength may be 500 gf or more.

[0016] In one embodiment, after step (c), the further step may include (d) applying a coating solution containing a binder and a solvent to at least one surface of the porous film, drying it, and forming a functional layer.

[0017] In one embodiment, the binder may be one selected from the group consisting of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene, 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, cyanoethylcellulose, hydroxyethylcellulose, cyanoethyl sucrose, pullulan, carboxymethylcellulose, polyvinyl alcohol, polyvinyl butyral, acrylonitrile-acrylic acid copolymer, ethylene-acrylic acid copolymer, styrene-butadiene copolymer, alkyl acrylate-acrylonitrile copolymer, polyethylene glycol, acrylic rubber, and two or more combinations thereof.

[0018] In one embodiment, the solvent may be 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, and two or more combinations thereof.

[0019] In one embodiment, the coating solution may further contain one inorganic particle 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 two or more combinations thereof.

[0020] Another aspect of the present invention provides a separator manufactured using the above-described separator manufacturing method and satisfying the following conditions (i) to (iii). (i) The thermal shrinkage rate in the longitudinal direction (MD) at 150°C is 5% or less, (ii) The thermal shrinkage rate in the transverse direction (TD) at 150°C is 5% or less, and (iii) The ratio of the thermal shrinkage rate in the transverse direction (TD) to the thermal shrinkage rate in the longitudinal direction (MD) at 150°C is 1.1 or less. [Effects of the Invention]

[0021] A method for manufacturing a separator according to one aspect of the present invention includes the steps of (a) processing a composition containing a polyolefin and a porosity-forming agent to obtain a base sheet, (b) stretching the base sheet in the longitudinal (MD) and transverse (TD) directions and removing the porosity-forming agent to obtain a porous film, and (c) stretching the porous film in the transverse (TD) direction and heat-fixing it. By satisfying the following conditions (1) to (3), processability, heat resistance, and perforation strength can be balanced and improved. (1) 7 ≤ Mb ≤ 8, (2) 0 <Mb / Tb<1、(3)0<Mb / (Tb×Tc)<0.5 In (1) to (3) above, Mb and Tb are the stretch ratios of the base sheet along the longitudinal (MD) and transverse (TD) directions in step (b), respectively, and Tc is the stretch ratio of the porous film along the transverse (TD) direction in step (c).

[0022] The effects of the present invention are not limited to the above effects, and should be understood to include all effects inferable from the configuration of the invention described in the detailed description or claims of the present invention.

Mode for Carrying Out the Invention

[0023] Hereinafter, the present invention will be described. However, the present invention can be implemented in various different forms, and therefore is not limited to the embodiments described herein.

[0024] Throughout the specification, when a part is "connected" to another part, it includes not only the case where it is "directly connected", but also the case where it is "indirectly connected" with another member interposed therebetween. Further, when a part is described as "including" any component, it means that other components can be further provided without excluding other components, unless otherwise specified.

[0025] One aspect of the present invention provides a method for manufacturing a separator, including: (a) processing a composition containing a polyolefin and a pore-forming agent to obtain a base sheet; (b) stretching the base sheet in the machine direction (MD) and the transverse direction (TD), and removing the pore-forming agent to obtain a porous membrane; and (c) stretching the porous membrane in the transverse direction (TD) and thermally fixing it, satisfying the following conditions (1) to (3). (1) 7 ≦ Mb ≦ 8, (2) 0 < Mb / Tb < 1, (3) 0 < Mb / (Tb × Tc) < 0.5 In the above (1) to (3), Mb and Tb are the stretching ratios of the base sheet along the machine direction (MD) and the transverse direction (TD) in the step (b), respectively, and Tc is the stretching ratio of the porous membrane along the transverse direction (TD) in the step (c).

[0026] In step (a) above, a composition containing a polyolefin and a porosity-forming agent can be extruded and discharged through a T-die to obtain a base sheet. The weight-average molecular weight (Mw) of the polyolefin may be 1,000,000 to 4,000,000, preferably 1,500,000 to 3,000,000, and more preferably 1,500,000 to 2,500,000, and the molecular weight distribution (Mw / Mn) may be 3 to 7. If the weight-average molecular weight of the polyolefin is less than 1,000,000, the mechanical strength of the separator may decrease, and if it is greater than 4,000,000, the thermal shrinkage rate may increase excessively, and the heat resistance may decrease. Furthermore, if the molecular weight distribution of the polyolefin is less than 3, the dispersibility with the porosity-forming agent may decrease, and the uniformity of the manufactured separator may decrease, and if it is greater than 7, the mechanical properties of the separator may decrease. The terms "weight-average molecular weight" and "molecular weight distribution" as used herein may refer to values ​​measured by gel permeation chromatography (GPC) using polystyrene as a standard sample, according to the methods described in the literature (e.g., Macromolecules, Vol. 34, No. 19, pp. 6812-6820 (2001)).

[0027] The polyolefin may be selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, ethylene vinyl acetate, ethylene butyl acrylate, ethylene ethyl acrylate, and combinations or copolymers of two or more of these, preferably polyethylene and / or polypropylene, more preferably polyethylene, but is not limited thereto.

[0028] The porosity-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, naphthenic oil, and two or more combinations thereof. Preferably, it may be paraffin oil, and more preferably, it may be paraffin oil having a kinematic viscosity of 50 to 100 cSt at 40°C, but is not limited thereto. The composition may contain 10 to 50% by weight of the polyolefin, preferably 20 to 40% by weight, and 50 to 90% by weight of the porosity-forming agent, preferably 60 to 80% by weight.

[0029] The thickness of the base sheet obtained in step (a) above may be 1,000 to 1,500 μm, preferably 1,200 to 1,500 μm. When the composition contains an ultra-high molecular weight polyolefin, for example, ultra-high molecular weight polyethylene with a weight-average molecular weight of 2,000,000 or more, it is difficult to adjust the thickness of the base sheet to less than 1,000 μm by extrusion or casting, and when the thickness of the base sheet is greater than 1,500 μm, it is difficult to thin the separator by subsequent stretching.

[0030] In step (b) above, the base sheet is stretched in the longitudinal (MD) and transverse (TD) directions, and the pore-forming agent is removed to obtain a porous film. The stretching can be performed by sequential biaxial stretching, in which the base sheet is stretched in the longitudinal (MD) direction at a predetermined stretching ratio, and then stretched in the transverse (TD) direction, or by simultaneous biaxial stretching, in which the base sheet is stretched simultaneously in the longitudinal (MD) direction and the transverse (TD) direction. As used herein, the term "stretching ratio" means the ratio of the length in the longitudinal (MD) direction or transverse (TD) direction after stretching to the length in the longitudinal (MD) direction or transverse (TD) direction of the base sheet before stretching, and can be interpreted as having the same meaning as stretching ratio, elongation rate, etc.

[0031] The longitudinal (MD) stretching of the base sheet can be performed using a roll stretcher. The roll stretcher is a device that stretches the base sheet along the transport direction of the base sheet in a process line, and the direction of stretching the base sheet by the roll stretcher can be defined as the longitudinal direction (MD, mechanical direction). The roll stretcher includes a plurality of rolls installed along the transport direction of the base sheet, and can stretch the base sheet along the longitudinal direction at a predetermined magnification such that the rolls at the rear end rotate faster than the rolls at the front end.

[0032] The lateral (TD) stretching of the base sheet can be performed using a tenter stretcher. The tenter stretcher is a device that stretches the base sheet perpendicular to the transport direction of the base sheet in a process line, and can stretch the base sheet along the lateral direction by a predetermined magnification while fixing both lateral ends of the base sheet with predetermined members such as chucks and clips, and separating the members in the lateral direction.

[0033] The porosity-forming agent can be selectively extracted and removed from the base sheet by applying a predetermined extraction solvent to the base sheet. The porosity-forming agent can also be extracted and removed by immersing the base sheet in an impregnation tank containing a solution of the extraction solvent for a predetermined time.

[0034] After extraction, the content of the pore-forming agent remaining on the surface and / or inside the porous membrane may be 1% by weight or less. The extraction solvent may be, for example, methyl ethyl ketone, hexane, dichloromethane, etc., but is not limited to these. The time required for extraction and removal of the pore-forming agent can be determined by the thickness and porosity of the base sheet and / or the porous membrane, but if the thickness and porosity of the porous membrane are 1 to 10 μm and 30 to 70% by volume, respectively, it may be 10 minutes or less, preferably 5 minutes or less.

[0035] Furthermore, the porous membrane from which the pore-forming agent has been extracted and removed can be heated to remove any remaining extraction solvent from the porous membrane. Some of the extraction solvent applied in step (b) may remain on the surface and / or inside the porous membrane. Since the remaining extraction solvent can worsen the physical properties of subsequent steps and the separator produced thereby, the porous membrane can be appropriately heated to a temperature above the boiling point of the extraction solvent to remove any remaining extraction solvent from the porous membrane.

[0036] In step (c) above, the porous film can be stretched in the lateral direction (TD) and heat-fixed to obtain a separator.

[0037] The aforementioned heat setting refers to the process of stretching the porous membrane in the lateral direction (TD), then relaxing and fixing it, and then applying heat to forcibly fix the porous membrane that is about to shrink, thereby removing residual stress. A higher heat setting temperature is advantageous in reducing the thermal shrinkage rate of the separator, but if the temperature is too high, the porous membrane may partially melt, the formed pores may become blocked, and the air permeability may decrease.

[0038] The heat-setting temperature is preferably selected within a range in which 10 to 30% by weight of the crystalline portion of the porous film melts. When the heat-setting temperature is selected within this range, it is possible to prevent problems such as insufficient rearrangement of the polyolefin molecules within the porous film resulting in no residual stress relief effect, and partial melting causing blockage of pores and a decrease in air permeability. For example, the heat-setting temperature may be 120 to 140°C, preferably 123 to 135°C, and the heat-setting time may be 5 seconds to 1 minute.

[0039] In connection with the stretching carried out in steps (b) and (c) above, the method for manufacturing the separator can satisfy the following conditions (1) to (3). (1) 7 ≤ Mb ≤ 8, (2) 0 <Mb / Tb<1、(3)0<Mb / (Tb×Tc)<0.5 In (1) to (3) above, Mb and Tb are the stretch ratios of the base sheet along the longitudinal (MD) and transverse (TD) directions in step (b), respectively, and Tc is the stretch ratio of the porous film along the transverse (TD) direction in step (c).

[0040] The condition in (1) above relates to the stretch ratio (Mb) along the longitudinal direction (MD) of the base sheet in step (b), and Mb may be 7 to 8, preferably 7 to 7.5. If Mb is less than 7, the structural properties required of the porous membrane, such as pore size, porosity, and air permeability, may decrease, and if it is greater than 8, the longitudinal (MD) thermal shrinkage rate of the separator may increase, and the heat resistance may decrease. In particular, when adjusting Mb within the specified range, it can be understood that the base sheet shrinks relatively and substantially in the transverse direction (TD) while stretched in the longitudinal direction (MD), and such shrinkage can reduce the residual stress accumulated in the base sheet and / or the porous membrane by subsequent transverse (TD) stretching, thereby reducing the thermal shrinkage rate of the separator and contributing to improved heat resistance. Furthermore, if the heat resistance of the porous film is improved, the resistance to heat applied to the porous film during the coating and drying process for forming a functional layer on its surface can be improved, thereby preventing the separator from being arbitrarily deformed and / or altered.

[0041] The condition in (2) above relates to the ratio of the stretch ratio along the longitudinal direction (MD) of the base sheet to the stretch ratio along the transverse direction (TD) of the base sheet in step (b), and the ratio may be greater than 0 and less than 1, preferably 0.5 to 0.95, and more preferably 0.7 to 0.9. Here, Tb may be 8.0 to 10.0, preferably 8.5 to 9.5. Tb is the stretch ratio along the transverse direction (TD) of the biaxial stretching performed in step (b), and by controlling Tb to be less than Mb, it is possible to reduce the residual stress accumulated in the base sheet by the transverse (TD) stretching between biaxial stretching, thereby reducing the thermal shrinkage rate of the separator and contributing to improved heat resistance. Furthermore, if Tb is less than 0.5, the structural properties required of the porous film, such as pore size, porosity, pore orientation, and air permeability, may decrease.

[0042] The condition in (3) above relates to the ratio of the stretch ratio along the longitudinal direction (MD) of the base sheet in step (b) above to the total stretch ratio along the transverse direction (TD) throughout the entire process of manufacturing the separator, wherein the ratio may be greater than 0 and less than 0.5, preferably 0.3 or more, and more preferably 0.4 or more. Here, Tc may be 1.5 to 2.0, preferably 1.7 to 1.9. When Mb / (Tb×Tc) is 0.5 or more, the thermal shrinkage rates of the separator in both the longitudinal direction (MD) and transverse direction (TD) increase, and in particular, the thermal shrinkage rate in the transverse direction (TD) increases sharply, which may reduce the heat resistance.

[0043] In step (c), the ratio (Mb / Tc) of the stretch ratio (Tc) along the transverse direction (TD) of the porous film to the stretch ratio (Mb) along the longitudinal direction (MD) of the base sheet in step (b) is 3.0 to 5.0, preferably 3.5 to 4.8, and more preferably 3.5 to 4.5, but is not limited to these values.

[0044] The thickness of the porous film obtained in step (c) above may be 10 μm or less, preferably 1 to 8 μm, and more preferably 2 to 7 μm. If the thickness of the porous film is greater than 10 μm, it is difficult to thin the separator and miniaturize and integrate the battery containing it, and if it is less than 1 μm, the mechanical strength of the separator, in particular the perforation strength, may decrease. The perforation strength of the porous film obtained in step (c) above may be 500 gf or more, preferably 500 to 1,000 gf, and more preferably 510 to 800 gf.

[0045] The separator obtained in step (c) above can be used directly during battery assembly, and if necessary, a functional layer described later can be formed on its surface before application.

[0046] The process may further include (d) applying a coating solution containing a binder and a solvent to at least one surface of the porous film and drying it to form a functional layer. Specifically, in step (d), a coating solution containing a binder and a solvent is applied to at least one surface of the porous film and dried to remove the solvent and other liquid residue contained in the coating solution in one step, thereby forming a functional layer. The functional layer may be formed on one surface of the porous film, or on both surfaces as necessary. The functional layers formed on both surfaces of the porous film may have the same thickness, composition and effect, or at least one of these may differ as needed.

[0047] The binder may be one selected from the group consisting of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene, 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, cyanoethylcellulose, hydroxyethylcellulose, cyanoethylsucrose, pullulan, carboxymethylcellulose, polyvinyl alcohol, polyvinyl butyral, acrylonitrile-acrylic acid copolymer, ethylene-acrylic acid copolymer, styrene-butadiene copolymer, alkyl acrylate-acrylonitrile copolymer, polyethylene glycol, acrylic rubber, and two or more combinations thereof. Preferably, it may be carboxymethylcellulose and alkyl acrylate-acrylonitrile copolymer, but is not limited to these. For example, in the functional layer, the carboxymethylcellulose and the alkyl acrylate-acrylonitrile copolymer may be present in a weight ratio of 1:0.5 to 1.5, respectively.

[0048] The coating solution may further contain one inorganic particle 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 two or more combinations thereof, wherein the inorganic particle can contribute to improving the heat resistance of the functional layer and the separator. The inorganic particle is preferably Al2O3 and / or AlO(OH), but is not limited to these.

[0049] The content of the inorganic particles in the functional layer may be 50 to 95% by weight. If the content of the inorganic particles in the functional layer is less than 50% by weight, the required level of heat resistance cannot be provided, and if it is greater than 95% by weight, the dispersibility of the inorganic particles may decrease, or the coating workability and processability may decrease.

[0050] The inorganic particles tend to bind together and aggregate due to electrostatic attraction. Such aggregation of inorganic particles can impede the uniformity of physical properties on the surface of the separator. Therefore, the coating liquid may further contain additives to improve the dispersibility of the inorganic particles, such as dispersants and surfactants. In particular, the coating liquid may contain sodium hexametaphosphate ((NaPO3)6) as a dispersant, and the content of sodium hexametaphosphate in the functional layer may be 0.01 to 1% by weight, preferably 0.01 to 0.5% by weight. The sodium hexametaphosphate is adsorbed on the edges of the inorganic particles, especially plate-like inorganic particles, weakening the (-) charge at the edges, effectively preventing the aggregation of the inorganic particles, and improving the storage stability of the coating liquid for forming the functional layer and the dispersibility of the inorganic particles in the functional layer.

[0051] To obtain a water-soluble coating solution, the solvent may be 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, and two or more combinations thereof. Preferably, it may be a mixture of ethanol and water, more preferably a mixture of ethanol and water in a weight ratio of 1:10 to 50, but it is not limited to these.

[0052] The coating liquid may contain 30 to 100 parts by weight of inorganic particles and 1 to 20 parts by weight of binder per 100 parts by weight of solvent, and is preferably water-soluble (aqueous or water-based). The solid content in the coating liquid can be adjusted to 20 to 50% by weight, preferably 30 to 40% by weight, taking into consideration the balance between processability, workability, and heat resistance.

[0053] The thickness of the functional layer may be 1 to 10 μm. If the thickness of the functional layer is less than 1 μm, the required level of adhesion and heat resistance cannot be provided, and if it is greater than 10 μm, the separator becomes thicker, which can hinder the increase in capacity, miniaturization, and integration of the battery or device.

[0054] Furthermore, before applying the coating liquid to both sides of the porous film in step (d) above, the porous film may be plasma-treated in the presence of a mixed gas containing sulfur dioxide (SO2) and oxygen (O2).

[0055] The plasma treatment makes the surface of the porous film and / or the surface of the internal pores hydrophilic, thereby improving the bonding strength between both surfaces of the porous film and the coating liquid, and thereby significantly improving the durability of the separator, in particular its long-term durability and heat resistance.

[0056] Conventionally, a wet process has been mainly used to hydrophilize the surface of porous membranes by immersing them in sulfuric acid or the like for a certain period of time to sulfonate them. However, in this case, the wet process is performed separately from the plasma treatment, either before or after it, which makes the process complex and generates a large amount of process waste.

[0057] Therefore, since the process gas used during the plasma treatment includes not only conventional air, oxygen, and / or inert gases, but also a certain amount of sulfur dioxide gas, functional groups such as -SO3 can be generated on the surface and the surface of the internal pores of the porous film by a single dry process called plasma treatment, without wet processes such as immersing the porous film in sulfuric acid, thereby maximizing the hydrophilicity and ionic conductivity of the porous film through sulfurization. This simplifies conventional complex processes and is advantageous from an environmental standpoint.

[0058] The mixed gas, which is the process gas used during the plasma treatment, may contain 50-90 volume% sulfur dioxide and 10-50 volume% oxygen, preferably 60-80 volume% sulfur dioxide and 20-40 volume% oxygen, and more preferably 70-80 volume% sulfur dioxide and 20-30 volume% oxygen. If the sulfur dioxide content in the mixed gas is less than 50 volume%, the required level of hydrophilicity for the porous film cannot be achieved, and if it is greater than 90 volume%, the process becomes unstable.

[0059] The plasma treatment can be performed for 0.5 to 90 minutes, preferably 0.5 to 20 minutes. If the plasma treatment is performed for less than 0.5 minutes, the porous film may not be hydrophilized and sulfonated to the required level, and if it is performed for longer than 90 minutes, the degree of hydrophilization and sulfonation may converge to a certain level, which may reduce process efficiency.

[0060] A separator manufactured using the above-described method for manufacturing a separator can satisfy the following conditions (i) to (iii). (i) The thermal shrinkage rate in the longitudinal direction (MD) at 150°C is 5% or less, preferably 1.0 to 4.5%, more preferably 1.9 to 4.0%, (ii) The thermal shrinkage rate in the transverse direction (TD) at 150°C is 5% or less, preferably 1.0 to 4.8%, more preferably 1.5 to 4.0%, and (iii) The ratio of the thermal shrinkage rate in the transverse direction (TD) to the thermal shrinkage rate in the longitudinal direction (MD) at 150°C is 1.1 or less, preferably 0.5 to 1.0, more preferably 0.6 to 0.9.

[0061] The following describes in detail embodiments of the present invention. Example 1 20 parts by weight of ultra-high molecular weight polyethylene (UHMWPE) with a weight-average molecular weight of 2,000,000 and a molecular weight distribution (Mw / Mn) of 5, and 80 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C were mixed and fed into a twin-screw extruder (inner diameter 58 mm, L / D = 56). The mixture was extruded from the twin-screw extruder through a T-die with a width of 630 mm at a screw rotation speed of 40 rpm and a temperature of 200°C, and then passed through a casting roll at a temperature of 17°C to produce a base sheet with a thickness of 1,450 μm.

[0062] The base sheet was stretched 8.0 times in the longitudinal direction (MD) using a roll stretcher at 105°C (MDO step), then stretched 9.5 times in the transverse direction (TD) using a tenter stretcher at 125°C (TDO1 step), then impregnated in a dichloromethane leaching tank at 25°C to extract and remove paraffin oil for 1 minute, and dried at 50°C for 5 minutes to produce a porous membrane. The porous membrane was then stretched 1.7 times in the transverse direction (TD) at 134°C (TDO2 step), and then heat-set in an 18% relaxed state to produce a first separator with a thickness of 7 μm.

[0063] Carboxymethylcellulose salt (CMC), acrylic-acrylonitrile copolymer latex, alumina (aluminum oxide, Al2O3), dispersant ((NaPO3)6), surfactant, water, and ethanol were mixed in the proportions shown in Table 1 below, and then dispersed in a ball mill to produce a water-dispersible ceramic slurry.

[0064] [Table 1]

[0065] The water-dispersible ceramic slurry was applied to one surface of the first separator, then gravure coated with a 110-mesh coating roll, and dried in a hot air oven at 80°C for 1 hour to produce a second separator containing a functional layer (heat-resistant layer) with a thickness of 3 μm.

[0066] Example 2 The first separator and the second separator were manufactured in the same manner as in Example 1, except that the base sheet was stretched 7.5 times in the longitudinal direction (MD) during the MDO process.

[0067] Example 3 The first separator and the second separator were manufactured in the same manner as in Example 1, except that the base sheet was stretched 7.0 times in the longitudinal direction (MD) during the MDO process.

[0068] Example 4 The first separator and the second separator were manufactured in the same manner as in Example 1, except that the base sheet was stretched 9.0 times in the transverse direction (TD) in the TDO1 step and the porous film was stretched 1.9 times in the transverse direction (TD) in the TDO2 step.

[0069] Example 5 The first separator and the second separator were manufactured in the same manner as in Example 4, except that the base sheet was stretched 7.5 times in the longitudinal direction (MD) during the MDO process.

[0070] Example 6 The first separator and the second separator were manufactured in the same manner as in Example 4, except that the base sheet was stretched 7.0 times in the longitudinal direction (MD) during the MDO process.

[0071] Example 7 The first separator and the second separator were manufactured in the same manner as in Example 1, except that the base sheet was stretched 8.5 times in the transverse direction (TD) in the TDO1 step and the porous film was stretched 2.0 times in the transverse direction (TD) in the TDO2 step.

[0072] Example 8 The first separator and the second separator were manufactured in the same manner as in Example 7, except that the base sheet was stretched 7.5 times in the longitudinal direction (MD) during the MDO process.

[0073] Example 9 The first separator and the second separator were manufactured in the same manner as in Example 7, except that the base sheet was stretched 7.0 times in the longitudinal direction (MD) during the MDO process.

[0074] Comparative Example 1 The first separator and the second separator were manufactured in the same manner as in Example 4, except that the base sheet was stretched 9.0 times in the longitudinal direction (MD) during the MDO process.

[0075] Comparative Example 2 The first separator and the second separator were manufactured in the same manner as in Comparative Example 1, except that the base sheet was stretched 8.5 times in the transverse direction (TD) during the TDO1 step.

[0076] Comparative Example 3 The first separator and the second separator were manufactured in the same manner as in Comparative Example 1, except that the base sheet was stretched 8.0 times in the transverse direction (TD) during the TDO1 step.

[0077] Comparative Example 4 The first separator and the second separator were manufactured in the same manner as in Comparative Example 1, except that the porous membrane was stretched 2.0 times in the transverse direction (TD) during the TDO2 step.

[0078] Comparative Example 5 The first separator and the second separator were manufactured in the same manner as in Comparative Example 1, except that the porous membrane was stretched 1.7 times in the transverse direction (TD) during the TDO2 step.

[0079] Comparative Example 6 The first separator and the second separator were manufactured in the same manner as in Comparative Example 1, except that the porous membrane was stretched 1.5 times in the transverse direction (TD) during the TDO2 step.

[0080] Comparative Example 7 The first separator and the second separator were manufactured in the same manner as in Comparative Example 1, except that the porous membrane was stretched 1.8 times in the transverse direction (TD) during the TDO2 step.

[0081] Comparative Example 8 The first separator and the second separator were manufactured in the same manner as in Comparative Example 7, except that the base sheet was stretched 8.5 times in the longitudinal direction (MD) during the MDO process.

[0082] Comparative Example 9 The first separator and the second separator were manufactured in the same manner as in Comparative Example 7, except that the base sheet was stretched 8.3 times in the longitudinal direction (MD) during the MDO process.

[0083] Comparative Example 10 20 parts by weight of ultra-high molecular weight polyethylene (UHMWPE) with a weight-average molecular weight of 2,000,000 and a molecular weight distribution (Mw / Mn) of 5, and 80 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C were mixed and fed into a twin-screw extruder (inner diameter 58 mm, L / D = 56). The mixture was extruded from the twin-screw extruder through a T-die with a width of 630 mm at a screw rotation speed of 40 rpm and a temperature of 200°C, and then passed through a casting roll at a temperature of 17°C to produce a base sheet with a thickness of 1,500 μm, except that the first and second separators were produced in the same manner as in Example 1.

[0084] Comparative Example 11 20 parts by weight of ultra-high molecular weight polyethylene (UHMWPE) with a weight-average molecular weight of 2,000,000 and a molecular weight distribution (Mw / Mn) of 5, and 80 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C were mixed and fed into a twin-screw extruder (inner diameter 58 mm, L / D = 56). The mixture was extruded from the twin-screw extruder through a T-die with a width of 630 mm at a screw rotation speed of 40 rpm and a temperature of 200°C, and then passed through a casting roll at a temperature of 17°C to produce a base sheet with a thickness of 1,700 μm, except that the first and second separators were manufactured in the same manner as in Example 1.

[0085] The main control variables for the separator manufacturing method according to the above examples and comparative examples are shown in Table 2 below. In Table 2 below, Mb and Tb are the stretch ratios of the base sheet along the longitudinal direction (MD) and transverse direction (TD), respectively, and Tc is the stretch ratio of the porous film along the transverse direction (TD).

[0086] [Table 2]

[0087] Experimental example The perforation strength of the first separator and the thermal shrinkage rate of the second separator in the above-mentioned examples and comparative examples were measured by the following method. Unless otherwise specified, measurements were taken at room temperature (25°C), and the results are shown in Table 3 below.

[0088] - Penetration strength (gf): Using a penetration strength measuring device, force was applied with a stick to a 100mm x 50mm first separator test specimen, and the force applied until the specimen was penetrated was measured.

[0089] - Heat shrinkage rate (%): A 200 x 200 mm second separator test specimen was placed between A4 sheets of paper and left in an oven at 150°C for 1 hour. After cooling to room temperature, the length of the specimen's shrinkage in the longitudinal (MD) and transverse (TD) directions was measured, and the heat shrinkage rate was calculated using the following formula. Thermal shrinkage rate (%) = {(I3-I4) / I3} × 100 In the above formula, I3 is the longitudinal or transverse length of the second separator specimen before shrinkage, and I4 is the longitudinal or transverse length of the second separator specimen after shrinkage.

[0090] [Table 3]

[0091] The foregoing description of the present invention is for illustrative purposes only, and a person with ordinary skill in the art to which the present invention pertains will understand that it can be easily modified into other specific forms without altering the technical idea or essential features of the present invention. Accordingly, the embodiments described above are illustrative in all respects and should not be construed as limiting. For example, each component described in a single form can also be implemented in a distributed form, and similarly, components described in a distributed form can also be implemented in a combined form.

[0092] The scope of the present invention is defined by the claims set forth below, and all changes or modifications derived from the meaning and scope of those claims, as well as from equivalent concepts, should be interpreted as being within the scope of the present invention.

Claims

1. (a) A step of processing a composition containing a polyolefin and a pore-forming agent to obtain a base sheet, (b) A step of stretching the base sheet in the longitudinal direction (MD) and transverse direction (TD), removing the pore-forming agent, to obtain a porous film, (c) The process includes stretching the porous film in the lateral direction (TD) and heat-fixing it, A method for manufacturing a separator that satisfies the following conditions (1) to (3). (1) 7 ≤ Mb ≤ 8 (2) 0<Mb / Tb<1 (3) 0<Mb / (Tb×Tc)<0.5 In (1) to (3) above, Mb and Tb are the stretch ratios of the base sheet along the longitudinal direction (MD) and transverse direction (TD) in step (b), respectively, and Tc is the stretch ratio of the porous film along the transverse direction (TD) in step (c).

2. The method for producing a separator according to claim 1, wherein the weight-average molecular weight (Mw) of the polyolefin is 1,000,000 to 4,000,000.

3. The method for producing a separator according to claim 2, wherein the polyolefin comprises polyethylene, polypropylene, polybutylene, polymethylpentene, ethylene vinyl acetate, ethylene butyl acrylate, ethylene ethyl acrylate, and one selected from the group consisting of two or more combinations or copolymers thereof.

4. The method for manufacturing a separator according to claim 1, wherein the Tc is 1.5 to 2.

0.

5. The method for manufacturing a separator according to claim 1, wherein the ratio of Mb to Tc (Mb / Tc) is 3.0 to 5.

0.

6. The method for producing a separator according to claim 1, wherein the pore-forming agent is a paraffin oil having a kinematic viscosity of 50 to 100 cSt at 40°C.

7. The thickness of the base sheet obtained in step (a) above is 1,000 to 1,500 μm. The method for manufacturing a separator according to claim 1, wherein the thickness of the porous membrane obtained in step (c) is 10 μm or less, and the perforation strength is 500 gf or more.

8. After step (c), (d) A method for manufacturing a separator according to claim 1, further comprising the steps of (d) applying a coating solution containing a binder and a solvent to at least one surface of the porous membrane, drying it, and forming a functional layer.

9. The method for producing a separator according to claim 8, wherein the binder is one selected from the group consisting of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene, 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, cyanoethylcellulose, hydroxyethylcellulose, cyanoethyl sucrose, pullulan, carboxymethylcellulose, polyvinyl alcohol, polyvinyl butyral, acrylonitrile-acrylic acid copolymer, ethylene-acrylic acid copolymer, styrene-butadiene copolymer, alkyl acrylate-acrylonitrile copolymer, polyethylene glycol, acrylic rubber, and two or more combinations thereof.

10. The method for producing a separator according to claim 8, wherein the solvent is 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, and two or more combinations thereof.

11. The coating solution contains SiO ２ , AlO(OH), Mg(OH) ２ , Al(OH) ３ , TiO ２ , BaTiO ３ , Li ２ O, LiF, LiOH, Li ３ N, BaO, Na ２ O, Li ２ CO ３ , CaCO ３ , LiAlO ２ , Al ２ O ３ , SiO, SnO, SnO ２ , PbO ２ , ZnO, P ２ O ５ , CuO, MoO, V ２ O ５ , B ２ O ３ , Si ３ N ４ , CeO ２ , Mn ３ O ４ , Sn ２ P ２ O ７ , Sn ２ B ２ O ５ , Sn ２ BPO ６ The method for manufacturing a separator according to claim 8, further comprising one inorganic particle selected from the group consisting of SiO ２ , AlO(OH), Mg(OH) ２ , Al(OH) ３ , TiO ２ , BaTiO ３ , Li ２ O, LiF, LiOH, Li ３ N, BaO, Na ２ O, Li ２ CO ３ , CaCO ３ , LiAlO ２ , Al ２ O ３ , SiO, SnO, SnO ２ , PbO ２ , ZnO, P ２ O ５ , CuO, MoO, V ２ O ５ , B ２ O ３ , Si ３ N ４ , CeO ２ , Mn ３ O ４ , Sn ２ P ２ O ７ , Sn ２ B ２ O ５ , Sn ２ BPO ６ and combinations of two or more thereof.

12. Manufactured using the separator manufacturing method described in any one of claims 8 to 11, A separator that satisfies the following conditions (i) to (iii). (i) The longitudinal (MD) heat shrinkage rate at 150°C is 5% or less. (ii) Transverse (TD) heat shrinkage rate at 150°C is 5% or less. (iii) The ratio of the transverse (TD) thermal shrinkage rate to the longitudinal (MD) thermal shrinkage rate at 150°C is 1.1 or less.

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