Crosslinked separator for lithium secondary battery comprising crosslinked polyolefin and method for manufacturing the same

Abstract

The present invention relates to a crosslinked separator for lithium secondary batteries and a method for manufacturing the same, the crosslinked separator comprising a crosslinked polyolefin porous substrate, the crosslinked polyolefin porous substrate comprising a plurality of fibrils and pores formed by the plurality of fibrils crossing each other, and wherein polyolefin chains constituting the fibrils are directly crosslinked to each other, and the crosslinked separator has an excellent thermal safety without sacrificing other physical properties, with a change in tensile strength in the machine direction of 20% or less when compared to an uncrosslinked separator comprising an uncrosslinked polyolefin porous substrate.

Description

Technical Field

[0001] The present disclosure relates to a cross-linked separator for a lithium secondary battery comprising a cross-linked polyolefin and a method for manufacturing the same.

[0002] This application claims priority from Korean Patent Application No. 10-2019-0142909 filed in Korea on November 8, 2019, the disclosure of which is incorporated herein by reference. Background Art

[0003] Energy storage technology has recently received increasing attention. As its application has expanded to include energy for mobile phones, camcorders, laptop computers, and even electric vehicles, research and development efforts in electrochemical devices have become increasingly focused. Against this backdrop, electrochemical devices have garnered the most attention. Among these electrochemical devices, the development of rechargeable secondary batteries has been a key focus.

[0004] Among commercially available secondary batteries, lithium secondary batteries developed in the early 1990s have a higher operating voltage and significantly higher energy density than conventional batteries such as Ni-MH, Ni-Cd, and sulfate-lead batteries using aqueous electrolytes and have therefore attracted attention.

[0005] Such a lithium secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. In particular, the separator needs to have insulating properties for separating and electrically insulating the positive and negative electrodes from each other and high ion conductivity based on high porosity for increasing lithium ion permeability.

[0006] Generally, the separator can be obtained by mixing polyolefin with a diluent, performing extrusion and stretching to form a film, and forming pores by extracting the diluent using a solvent or the like.

[0007] On the other hand, in order to apply lithium secondary batteries to electric vehicles (EVs), safety and cost of lithium secondary batteries need to be significantly improved.

[0008] In the case of a typical polyolefin separator, i.e., a polyethylene (PE) separator, it has a low melting point (Tm), and when the battery is used abnormally and the battery temperature rises above the melting point of polyethylene to produce a melting phenomenon, it may cause fire and explosion. As a method of enhancing the safety of the separator, attempts have been made to use a PE / PP / PE three-layer separator obtained by blending polypropylene (PP) having a relatively higher melting point than polyethylene instead of a polyethylene single-layer separator. This PE / PP / PE three-layer separator has the advantage that it shows an increased melting temperature compared to a polyethylene single-layer separator, but has the limitation that it requires a more complicated manufacturing process compared to a wet single-layer polyethylene separator. In addition, as another method of enhancing the safety of the separator, attempts have been made to form a cross-linked coating on polyethylene using a cross-linking agent. However, due to the generation of by-products, this method shows low processing efficiency and results in the formation of foreign matter on the separator. Summary of the Invention

[0009] Technical issues

[0010] The present disclosure is designed to solve the problems in the related art, and thus, the present disclosure is directed to providing a cross-linked separator for a lithium secondary battery that includes a cross-linked polyolefin and has improved high-temperature safety.

[0011] The present disclosure also provides a simplified method for manufacturing a cross-linked separator for a lithium secondary battery including a cross-linked polyolefin.

[0012] Technical Solution

[0013] In one aspect of the present disclosure, according to a first embodiment, there is provided a cross-linked separator for a lithium secondary battery, comprising a cross-linked polyolefin porous substrate, wherein the cross-linked polyolefin porous substrate comprises a plurality of fibrils and pores formed by entanglement of the fibrils with each other, wherein the polyolefin chains forming the fibrils are directly cross-linked with each other; and compared with an uncross-linked separator comprising the polyolefin porous substrate before cross-linking, the cross-linked separator has a tensile strength change in the machine direction of less than 20%.

[0014] According to a second embodiment, there is provided a cross-linked separator for a lithium secondary battery as defined in the first embodiment, wherein the cross-linked separator may have a tensile strength in a machine direction that varies by 0 to 20% compared to the non-cross-linked separator.

[0015] According to a third embodiment, there is provided a cross-linked separator for a lithium secondary battery as defined in the first or second embodiment, which may show a puncture strength change of 10% or less compared to the non-cross-linked separator.

[0016] According to a fourth embodiment, there is provided a cross-linked separator for a lithium secondary battery as defined in any one of the first to third embodiments, further comprising a porous coating layer formed on at least one surface of the cross-linked polyolefin porous substrate.

[0017] wherein the uncrosslinked separator may further comprise a porous coating layer formed on at least one surface of the polyolefin porous substrate before crosslinking, and

[0018] The porous coating layer may include a binder polymer and inorganic particles, and may have a gap volume formed by the inorganic particles that are substantially in contact with each other, wherein the gap volume means a space defined by the inorganic particles that are substantially in contact with each other in a close-packed or densely packed structure of the inorganic particles, and the gap volume between the inorganic particles becomes an empty space forming pores of the porous coating layer.

[0019] According to a fifth embodiment, there is provided a cross-linked separator for a lithium secondary battery as defined in any one of the first to fourth embodiments, which may show an air permeability change of 10% or less compared to the non-cross-linked separator.

[0020] According to a sixth embodiment, there is provided a cross-linked separator for a lithium secondary battery as defined in any one of the first to fifth embodiments, which may show a weight change per unit area of ​​5% or less compared to the non-cross-linked separator.

[0021] According to a seventh embodiment, there is provided a cross-linked separator for a lithium secondary battery as defined in any one of the first to sixth embodiments, which may show a resistance change of 15% or less compared to the non-cross-linked separator.

[0022] According to an eighth embodiment, there is provided a cross-linked separator for a lithium secondary battery as defined in any one of the first to seventh embodiments, wherein the cross-linked polyolefin porous substrate may have a cross-linking degree of 10% to 80%.

[0023] In another aspect of the present disclosure, according to a ninth embodiment, there is provided a method for manufacturing the cross-linked separator for a lithium secondary battery as defined in the first embodiment, the method comprising the following steps:

[0024] applying a Type 2 photoinitiator composition comprising a Type 2 photoinitiator and a solvent for the Type 2 photoinitiator to a polyolefin porous substrate; and

[0025] irradiating ultraviolet rays onto the polyolefin porous substrate coated with the type 2 photoinitiator composition,

[0026] Wherein, based on 100 parts by weight of the solvent for the type 2 photoinitiator, the content of the type 2 photoinitiator is 0.05 to 0.3 parts by weight.

[0027] According to a tenth embodiment, there is provided a method for manufacturing a cross-linked separator for a lithium secondary battery as defined in the ninth embodiment, wherein the type 2 photoinitiator composition may be a porous coating layer-forming composition further comprising inorganic particles and a binder polymer.

[0028] According to an eleventh embodiment, a method for manufacturing a cross-linked separator for a lithium secondary battery as defined in the ninth or tenth embodiment is provided, wherein the type 2 photoinitiator may include thioxanthone (TX), a thioxanthone derivative, benzophenone (BPO), a benzophenone derivative or a mixture of two or more thereof.

[0029] According to a twelfth embodiment, there is provided a method for manufacturing a cross-linked separator for a lithium secondary battery as defined in the eleventh embodiment, wherein the type 2 photoinitiator may include 2-isopropylthioxanthone (ITX), thioxanthone (TX) or a mixture thereof.

[0030] According to a thirteenth embodiment, there is provided a method for manufacturing a cross-linked separator for a lithium secondary battery as defined in any one of the ninth to twelfth embodiments, wherein the ultraviolet rays can be 10 to 1000 mJ / cm 2 The irradiation light dose is irradiated.

[0031] In another aspect of the present disclosure, according to the fourteenth embodiment, a lithium secondary battery is provided, comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the separator is a cross-linked separator for a lithium secondary battery as defined in any one of the first to eighth embodiments.

[0032] Beneficial effects

[0033] The crosslinked separator for lithium secondary batteries according to the present disclosure has excellent heat resistance without adversely affecting other physical properties of the polyolefin porous substrate due to crosslinking formed by direct crosslinking of polyolefin chains forming fibrils of the polyolefin porous substrate.

[0034] In particular, the cross-linked separator for a lithium secondary battery according to the present disclosure comprises a cross-linked polyolefin porous substrate comprising a plurality of polyolefin fibrils; and pores formed by the entanglement of the polyolefin fibrils, wherein the polyolefin chains forming the fibrils are directly cross-linked with each other. Therefore, even when the final cross-linked separator does not include a separate surface coating for improving heat resistance, it can provide improved mechanical strength reduction and ensure heat resistance.

[0035] Additionally, the method of manufacturing a cross-linked separator for a lithium secondary battery according to the present disclosure uses a significantly small amount of a Type 2 photoinitiator to perform cross-linking of the polyolefin chains without adversely affecting other physical properties of the polyolefin porous substrate.

[0036] In the method for manufacturing a cross-linked separator for a lithium secondary battery according to the present disclosure, the polyolefin chains can be cross-linked using a significantly lower UV light dose than that used for conventional UV cross-linking. Therefore, the cross-linked separator for a lithium secondary battery comprising a cross-linked polyolefin porous substrate exhibits high applicability to large-scale manufacturing processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The accompanying drawings illustrate preferred embodiments of the present disclosure together with the aforementioned disclosure, to provide a further understanding of the technical features of the present disclosure. Therefore, the present disclosure should not be construed as being limited to the accompanying drawings.

[0038] Figure 1 is a graph showing the thermal shrinkage rate of the cross-linked separator produced in Example 5, the thermal shrinkage rate of the substrate obtained after removing the porous coating layer from the cross-linked separator, and the thermal shrinkage rate of the substrate after washing with acetone. DETAILED DESCRIPTION

[0039] Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Before the description, it should be understood that the terms used in this specification and the appended claims should not be interpreted as limited to the general meaning and dictionary meaning, but should be interpreted based on the meaning and concept corresponding to the technical aspects of the present disclosure based on the principle of allowing the inventor to appropriately define the terms for the best interpretation.

[0040] In one aspect of the present disclosure, there is provided a cross-linked separator for a lithium secondary battery, comprising a cross-linked polyolefin porous substrate, wherein the cross-linked polyolefin porous substrate comprises a plurality of fibrils and pores formed by entanglement of the fibrils with each other, wherein the polyolefin chains forming the fibrils are directly cross-linked with each other; and compared with an uncross-linked separator comprising the polyolefin porous substrate before cross-linking, the cross-linked separator has a tensile strength change in the machine direction of less than 20%.

[0041] As used herein, "fibrils" refer to fibrils formed by longitudinally stretching and orienting the polymer chains forming the polyolefin porous substrate during the manufacture of the porous substrate, thereby increasing the bonding force between adjacent molecular chains and allowing the chains to aggregate in the longitudinal direction. As a result, the cross-linked polyolefin porous substrate according to the present disclosure has a layered structure comprising a plurality of fibrils aligned parallel to the substrate surface.

[0042] Herein, the expression "directly crosslinked (direct crosslinking)" refers to a state in which, after fibrils substantially comprising polyolefin (preferably, fibrils consisting solely of polyolefin) are rendered reactive by the addition of a Type 2 photoinitiator, polyolefin chains forming the fibrils are directly crosslinked. Therefore, crosslinking occurring between a separately introduced crosslinking agent does not correspond to "direct crosslinking." Furthermore, crosslinking occurring between a separately introduced crosslinking agent and the polyolefin chains does not correspond to "direct crosslinking" as defined in the present disclosure, even when the fibrils substantially comprise or consist solely of polyolefin.

[0043] The term "direct crosslinking" refers to crosslinking achieved by a Type 2 photoinitiator.

[0044] It is generally known that photoinitiators are divided into type 1 photoinitiators and type 2 photoinitiators.

[0045] The Type 1 photoinitiator undergoes unimolecular bond cleavage upon absorption of light and is then converted into a reactive species. The Type 1 photoinitiator does not require any other chemical species to perform its function. It is known that when crosslinking polyolefin (e.g., polyethylene) chains is performed using the Type 1 photoinitiator and a curing agent, the initiator or curing agent combines with free radicals generated by the polyethylene chains, resulting in crosslinking.

[0046] In contrast, the Type 2 photoinitiators are known to undergo a bimolecular reaction and, upon absorption of light, react with another molecule (eg, a co-initiator or synergist) to form a reactive compound.

[0047] However, even when using the Type 2 photoinitiator according to the present disclosure, the Type 2 photoinitiator utilizes light absorption to abstract hydrogen atoms without the aid of other co-initiators or synergists, thereby removing hydrogen atoms and simultaneously forming free radicals that are converted into reactive compounds, thereby converting the polyolefin itself into a reactive polyolefin. Therefore, according to one embodiment of the present disclosure, a crosslinked polyolefin porous substrate can be provided, obtained by directly crosslinking the polyolefin chains of fibrils formed from polyolefin compounds.

[0048] According to one embodiment of the present disclosure, the degree of crosslinking of the polyolefin in the crosslinked polyolefin porous substrate may be 10% to 80%, or 30% to 55%. Herein, the degree of crosslinking is calculated according to ASTM D2765 by immersing the crosslinked polyolefin porous substrate in a xylene solution at 135°C, boiling the solution for 12 hours, weighing the residue, and calculating the weight percentage of the residue based on the initial weight. When the crosslinked polyolefin porous substrate according to the present disclosure has a degree of crosslinking within the above-defined range, a crosslinked separator having a desired level of melting temperature, improved thermal shrinkage, and increased modulus can be provided.

[0049] According to one embodiment of the present disclosure, the cross-linked polyolefin porous substrate may be formed of a polyolefin porous film, a polyolefin porous non-woven fabric, or a combination thereof.

[0050] According to one embodiment of the present disclosure, the polyolefin may be polyethylene; polypropylene; polybutene; polypentene; polyhexene; polyoctene; a copolymer of at least two of ethylene, propylene, butene, pentene, 4-methylpentene, hexene and octene; or a mixture thereof.

[0051] Specifically, the polyethylene includes low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), etc. Among them, high-density polyethylene having high crystallinity and high resin melting point is most preferred.

[0052] According to one embodiment of the present disclosure, the polyolefin may have a weight average molecular weight of 200,000 to 1,500,000, 220,000 to 1,000,000, or 250,000 to 800,000. According to the present disclosure, by using a high molecular weight polyolefin having a weight average molecular weight within the range defined above as a starting material for manufacturing a cross-linked separator for a lithium secondary battery, a separator having excellent strength and heat resistance can be obtained while ensuring the uniformity and film processability of the separator.

[0053] The crosslinked separator for lithium secondary batteries according to the present disclosure has excellent mechanical strength even after crosslinking. According to the present disclosure, the crosslinked separator has a tensile strength change in the machine direction of 20% or less compared to an uncrosslinked separator comprising a polyolefin porous substrate before crosslinking.

[0054] Herein, the change in tensile strength of the cross-linked separator in the machine direction compared to the non-cross-linked separator can be calculated according to the following formula.

[0055] Change in tensile strength in the machine direction (%) = [(tensile strength in the machine direction of the uncrosslinked membrane comprising the polyolefin porous substrate before crosslinking) - (tensile strength in the machine direction of the crosslinked membrane comprising the polyolefin porous substrate after crosslinking)] / (tensile strength in the machine direction of the uncrosslinked membrane comprising the polyolefin porous substrate before crosslinking) × 100

[0056] According to one embodiment of the present disclosure, the tensile strength in the machine direction can be determined according to ASTM D 882. Specifically, the tensile strength in the machine direction can be obtained by measuring the strength at the time point when a sample having a size of 100 mm x 15 mm is stretched in the machine direction at a rate of 50 mm / min using a universal testing machine (UTM) (available from Instron, model: 3345) and breaking the test piece.

[0057] According to one embodiment of the present disclosure, the cross-linked separator may have a tensile strength change in the machine direction of 0-20%, 0-10%, 0-9%, 0-8% or 0-7.53% compared to the non-cross-linked separator.

[0058] According to one embodiment of the present disclosure, the cross-linked separator may show a puncture strength change of 10% or less, 0.5% to 10%, 1% to 9%, or 1.18% to 8.71% compared to the non-cross-linked separator.

[0059] Herein, the change in puncture strength of the cross-linked separator compared to the non-cross-linked separator can be calculated according to the following formula.

[0060] Puncture strength change (%) = [(puncture strength of the uncrosslinked membrane containing the polyolefin porous substrate before crosslinking) - (puncture strength of the crosslinked membrane containing the polyolefin porous substrate after crosslinking)] / (puncture strength of the uncrosslinked membrane containing the polyolefin porous substrate before crosslinking) × 100

[0061] According to one embodiment of the present disclosure, the puncture strength can be determined according to ASTM D 2582. Specifically, after setting a circular tip with a diameter of 1 mm to run at a speed of 120 mm / min, the puncture strength can be determined according to ASTM D 2582.

[0062] According to the present disclosure, the polyolefin chains of the fibrils contained in the polyolefin porous substrate are directly crosslinked. As a result, the polyolefin porous substrate can maintain the same pore structure as before crosslinking after crosslinking, and the air permeability and weight per unit area of ​​the crosslinked separator do not increase significantly compared to the separator before crosslinking, and show little change.

[0063] According to one embodiment of the present disclosure, the cross-linked separator may exhibit an air permeability change of 10% or less, 0-10%, 0-5% or 0-3%, and a weight per unit area change of 5% or less or 0-5% compared to the uncross-linked separator.

[0064] According to one embodiment of the present disclosure, when the cross-linked separator shows a change in air permeability and a change in weight per unit area within the range defined above after cross-linking compared to the uncross-linked separator before cross-linking, physical properties such as thermal safety can be improved without causing changes in separator performance.

[0065] Herein, the change in air permeability and weight per unit area of ​​the cross-linked separator before and after cross-linking can be calculated according to the following formula.

[0066] Air permeability change (%) = [(air permeability of the cross-linked membrane containing the polyolefin porous substrate after cross-linking) - (air permeability of the uncross-linked membrane containing the polyolefin porous substrate before cross-linking)] / (air permeability of the uncross-linked membrane containing the polyolefin porous substrate before cross-linking) × 100

[0067] Weight change per unit area (%) = [(weight per unit area of ​​the cross-linked membrane including the polyolefin porous substrate after cross-linking) - (weight per unit area of ​​the uncross-linked membrane including the polyolefin porous substrate before cross-linking)] / (weight per unit area of ​​the uncross-linked membrane including the polyolefin porous substrate before cross-linking) × 100

[0068] Gurley air permeability can be determined according to ASTM D726-94. As used herein, Gurley refers to the resistance to air flow and is determined by a Gurley air permeability meter. The air permeability values ​​described herein are determined by the flow of 100 ml of air at a pressure of 12.2 inches of H2O through an area of ​​1 inch. 2 The time (seconds) required to measure the cross section of the sample substrate is expressed as the air permeation time.

[0069] Weight per unit area (g / m 2 ) refers to the weight of a sample with a width of 1 m and a length of 1 m.

[0070] According to one embodiment of the present disclosure, the cross-linked separator may show a resistance (Ω) change of 15% or less, 2% to 10% or 2% to 5% compared to the uncross-linked separator. When the cross-linked separator shows a resistance change within the range defined above, it may show low resistance to prevent degradation of battery performance. The resistance can be determined by placing a coin cell manufactured using a separator sample at room temperature for 1 day and measuring the resistance of the separator by impedance analysis.

[0071] Herein, the resistance change of the cross-linked separator after cross-linking compared to the non-cross-linked separator before cross-linking can be calculated according to the following formula.

[0072] Resistance change (%) = [(resistance of the cross-linked separator containing the polyolefin porous substrate after cross-linking) - (resistance of the uncross-linked separator containing the polyolefin porous substrate before cross-linking)] / (resistance of the uncross-linked separator containing the polyolefin porous substrate before cross-linking) × 100

[0073] According to one embodiment of the present disclosure, the cross-linked separator may further include a porous coating layer formed on at least one surface of the cross-linked polyolefin porous substrate.

[0074] wherein the uncrosslinked separator may further comprise a porous coating layer formed on at least one surface of the polyolefin porous substrate before crosslinking, and

[0075] The porous coating layer may include a binder polymer and inorganic particles, and have a gap volume formed by the inorganic particles that are substantially in contact with each other, wherein the gap volume means a space defined by the inorganic particles that are substantially in contact with each other in a close-packed or densely packed structure of the inorganic particles, and the gap volume between the inorganic particles becomes an empty space forming pores of the porous coating layer.

[0076] The porous coating layer can have a microporous structure due to the interstitial volume between the inorganic particles, and the interstitial volume can serve as a spacer, utilizing which the porous coating layer can maintain its physical shape. In addition, the inorganic particles are generally characterized in that they do not undergo changes in physical properties even at high temperatures above 200°C. Therefore, the porous coating layer can provide the cross-linked separator for lithium secondary batteries with excellent heat resistance, such as improved thermal shrinkage.

[0077] According to one embodiment of the present disclosure, the porous coating layer may have a thickness of 1 to 50 μm, 2 to 30 μm, or 2 to 20 μm.

[0078] According to one embodiment of the present disclosure, the weight ratio of the inorganic particles to the binder polymer in the porous coating layer can be determined in consideration of the thickness, pore size and porosity of the porous coating layer finally manufactured, and can be 50:50 to 99.9:0.1 or 60:40 to 99.5:0.5. When the weight ratio of the inorganic particles to the binder polymer satisfies the range defined above, the problem of the pore size and porosity of the resulting coating layer being reduced due to an excessive increase in the content of the binder polymer and a reduction in the empty space formed between the inorganic particles can be prevented. The problem of deterioration of the mechanical properties of the resulting coating layer caused by an excessive reduction in the content of the binder polymer and deterioration in the adhesion between the inorganic particles can also be solved.

[0079] According to one embodiment of the present disclosure, there is no particular limitation on the size of the inorganic particles of the porous coating. However, in order to form a coating with uniform thickness and provide suitable porosity, the inorganic particles may preferably have a size of 0.001 to 10 μm, 0.01 to 10 μm, or 0.05 to 5 μm, or 0.1 to 2 μm. When the size of the inorganic particles meets the range defined above, the inorganic particles maintain dispersibility so as to control the physical properties of the diaphragm and avoid an increase in the thickness of the porous coating, thereby providing improved mechanical properties. In addition, the risk of internal short circuits caused by excessive pore size during charge / discharge of the battery can be reduced.

[0080] There is no particular limitation on the inorganic particles as long as they are electrochemically stable. In other words, there is no particular limitation on the inorganic particles as long as they do not cause oxidation and / or reduction within the operating voltage range of the applicable electrochemical device (e.g., 0 to 5 V based on Li / Li + ). In particular, when inorganic particles with ion transport ability are used, the ionic conductivity in the electrochemical device can be improved, thereby contributing to the improvement of the performance of the electrochemical device. In addition, when inorganic particles with a high dielectric constant are used, the ionic conductivity of the electrolyte can be improved by increasing the degree of dissociation of electrolyte salts such as lithium salts in the liquid electrolyte.

[0081] For the above reasons, the inorganic particles may include inorganic particles with a dielectric constant of 5 or more or 10 or more, inorganic particles with lithium ion transport ability, or a mixture thereof. Non-limiting examples of inorganic particles with a dielectric constant of 5 or more may include any one selected from the group consisting of: BaTiO3, Pb(Zr,Ti)O3 (PZT), Pb 1-x La x Zr 1-y Ti y O3 (PLZT, where 0 < x < 1, 0 < y < 1), Pb(Mg 1 / 3 Nb 2 / 3 )O3 - PbTiO3 (PMN - PT), hafnium dioxide (HfO2), SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, Y2O3, Al2O3, γ - AlOOH, SiC, TiO2. In addition, when high dielectric constant inorganic particles are used in combination with inorganic particles with ion transport ability, a synergistic effect can be obtained.

[0082] Non-limiting examples of the inorganic particles with lithium ion transport ability include lithium phosphate (Li3PO4), lithium titanium phosphate (Li x Ti y (PO4)3, 0 < x < 2, 0 < y < 3), lithium aluminum titanium phosphate (Li x Al y Ti z (PO4)3, 0 < x < 2, 0 < y < 1, 0 < z < 3), (LiAlTiP) x O y -like glass (1 < x < 4, 0 < y < 13) such as 14LiOH - 9Al2O3 - 38TiO2 - 39P2O5, lithium lanthanum titanate (Li x La y TiO3, 0 < x < 2, 0 < y < 3), lithium germanium thiophosphate (Li x Gey P z S w ,0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5) such as Li 3.25 Ge 0.25 P 0.75 S4, lithium nitride (Li x N y ,0 < x < 4, 0 < y < 2) such as Li3N, SiS2-based glass (Li x Si y S z ,0 < x < 3, 0 < y < 2, 0 < z < 4) such as Li3PO4-Li2S-SiS2 and P2S5-based glass (Li x P y S z ,0 < x < 3, 0 < y < 3, 0 < z < 7) such as LiI-Li2S-P2S5 or a mixture thereof.

[0083] The binder polymer contained in the porous coating may include polyvinylidene fluoride, polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-chlorotrifluoroethylene, polyvinylidene fluoride-co-trifluoroethylene, polymethyl methacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, poly(ethylene oxide), cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, acrylonitrile-butadiene-styrene copolymer, polyimide, or a mixture of two or more of them.

[0084] According to one embodiment of the present disclosure, the porous coating may further include a dispersant or a dispersible binder polymer. The function of the dispersant is to improve the dispersibility of the inorganic particles. In addition to the function of improving dispersibility, the dispersant also has the function of acting as an adhesive binder polymer and can thus be referred to as a dispersible binder polymer. Non-limiting examples of the dispersant include polymers such as acrylic copolymers; cyanoethyl polyvinyl alcohol; phenolic compounds, including baicalin, luteolin, taxifolin, myricetin, quercetin, rutin, catechin, epigallocatechin gallate, butein, piceatannol, and tannic acid; pyrogallol; amylose; amylopectin; xanthan gum; and fatty acid compounds; or a mixture of two or more of them.

[0085] The acrylic copolymer may be a copolymer containing functional groups selected from OH, COOH, CN, amine, and amide groups or two or more of them.

[0086] Specific examples of such acrylic copolymers may include, but are not limited to, ethyl acrylate-acrylic acid-N,N-dimethylacrylamide copolymer, ethyl acrylate-acrylic acid-2-(dimethylamino)ethyl acrylate copolymer, ethyl acrylate-acrylic acid-N,N-diethylacrylamide copolymer, ethyl acrylate-acrylic acid-2-(diethylamino)ethyl acrylate copolymer, or two or more thereof.

[0087] The pore size and porosity of the porous coating are primarily determined by the size of the inorganic particles. For example, when inorganic particles with a particle size of 1 μm or less are used, the resulting pores also have a size of 1 μm or less. This porous structure is then filled with an electrolyte that is subsequently injected, and the function of the electrolyte is to transport ions. Therefore, pore size and porosity are important factors affecting the ionic conductivity of the porous inorganic coating.

[0088] The porous coating according to one embodiment of the present disclosure may have a pore size of 0.001 to 10 μm or 0.001 to 1 μm. In addition, the porous coating may have a porosity of 5% to 95%, 10% to 95%, 20% to 90%, or 30% to 80%. The pore size can be measured by capillary flow porometry. The porosity corresponds to the value obtained by subtracting the volume of the porous inorganic coating calculated from the thickness, width, and length of the porous inorganic coating from the volume of the coating derived from the weight and density of the components of the coating.

[0089] When the porous coating layer has a pore size and / or porosity within the above-defined range, the cross-linked separator according to one embodiment of the present disclosure may prevent a short circuit from occurring in an abnormal state and may simultaneously provide suitable resistance characteristics and air permeability.

[0090] In another aspect of the present disclosure, a method for manufacturing a cross-linked separator for a lithium secondary battery is provided, comprising the following steps:

[0091] applying a Type 2 photoinitiator composition comprising a Type 2 photoinitiator and a solvent for the Type 2 photoinitiator to a polyolefin porous substrate (step S1); and

[0092] Irradiating the polyolefin porous substrate coated with the type 2 photoinitiator composition with ultraviolet light (step S2),

[0093] Wherein, based on 100 parts by weight of the solvent for the type 2 photoinitiator, the content of the type 2 photoinitiator is 0.05 to 0.3 parts by weight.

[0094] First, a Type 2 photoinitiator composition including a Type 2 photoinitiator and a solvent for the Type 2 photoinitiator is applied to a polyolefin porous substrate (step S1 ).

[0095] According to one embodiment of the present disclosure, the type 2 photoinitiator may include thioxanthone (TX), a thioxanthone derivative, benzophenone (BPO), a benzophenone derivative, or a mixture of two or more thereof.

[0096] Specific examples of the thioxanthone derivatives include, but are not limited to, 2-isopropylthioxanthone, 2-chlorothioxanthone, 2-dodecylthioxanthone, 2,4-diethylthioxanthone, 2,4-dimethylthioxanthone, 1-methoxycarbonylthioxanthone, 2-ethoxycarbonylthioxanthone, 3-(2-methoxyethoxycarbonyl)-thioxanthone, 4-butoxycarbonyl-thioxanthone, 3-butoxycarbonyl-7-methylthioxanthone, 1-cyano-3-chlorothioxanthone, 1-ethoxycarbonyl-3-chlorothioxanthone, 1-ethoxycarbonyl-3-ethoxythioxanthone, 1-ethoxycarbonyl-3-aminothioxanthone, 1-ethoxycarbonyl-3-phenylsulfonylthioxanthone, 3,4-bis[2-(2-methoxyethoxy)ethoxycarbonyl]thioxanthone, 1-ethoxycarbonyl-3-(1-methylthioxanthone)-1,2-dimethylthioxanthone, 1-methyl-2-thioxanthone, 1-methyl-3- ... thioxanthone, 2-methyl-6-(1,1-dimethoxy-benzyl)-thioxanthone, 2-morpholinomethylthioxanthone, 2-methyl-6-morpholinomethylthioxanthone, N-allylthioxanthone-3,4-dicarboximide, N-octylthioxanthone-3,4-dicarboximide, N-(1,1,3,3-tetramethylbutyl)-thioxanthone-3,4-dicarboximide, 1-phenoxythioxanthone, 6-ethoxycarbonyl-2-methoxythioxanthone, 6-ethoxycarbonyl-2-methylthioxanthone, thioxanthone-2-polyethylene glycol ester, 2-hydroxy-3-(3,4-dimethyl-9-oxo-9H-thioxanthone-2-yloxy)-N,N,N-trimethyl-1-propylammonium chloride, and the like.

[0097] According to the present disclosure, specific examples of the benzophenone derivatives may include, but are not limited to, 4-phenylbenzophenone, 4-methoxybenzophenone, 4,4'-dimethoxybenzophenone, 4,4'-dimethylbenzophenone, 4,4'-dichlorobenzophenone, 4,4'-dimethylaminobenzophenone, 4,4'-diethylaminobenzophenone, 4-methylbenzophenone, 2,4,6-trimethylbenzophenone, 4-(4-methylthiophenyl)-benzophenone, 3,3'-dimethyl-4-methoxybenzophenone, methyl-2-benzoylphenylbenzene, formic acid ester, 4-(2-hydroxyethylthio)-benzophenone, 4-(4-tolylthio)benzophenone, 4-benzoyl-N,N,N-trimethylbenzyl ammonium chloride, 2-hydroxy-3-(4-benzoylphenoxy)-N,N,N-trimethylpropylammonium chloride monohydrate, 4-hydroxybenzophenone, 4-(13-acryloyl-1,4,7,10,13-pentaoxatridecanyl)-benzophenone, 4-benzoyl-N,N-dimethyl-N-[2-(1-oxo-2-propenyl)oxy]ethyl-benzyl ammonium chloride, and the like.

[0098] According to the present disclosure, when the Type 2 photoinitiator is used, there is an advantage in that crosslinking can be accomplished with a lower light dose compared to crosslinking using Type 1 photoinitiators and / or crosslinkers.

[0099] According to one embodiment of the present disclosure, the type 2 photoinitiator may be 2-isopropylthioxanthone, thioxanthone or a mixture thereof. When the type 2 photoinitiator is 2-isopropylthioxanthone, thioxanthone or a mixture thereof, a lower light dose, such as 500 mJ / cm, can be used for crosslinking compared to using any other photoinitiator such as benzophenone. 2 Perform cross-linking.

[0100] According to one embodiment of the present disclosure, a composition comprising a Type 2 photoinitiator can be prepared by dissolving the Type 2 photoinitiator in a solvent. The solvent can be acetone, isopropyl alcohol (IPA), N-methylpyrrolidone (NMP), methanol, or a mixture of two or more thereof.

[0101] The Type 2 photoinitiator is present in an amount of 0.05 to 0.3 parts by weight based on 100 parts by weight of the solvent for the Type 2 photoinitiator. When the content of the Type 2 photoinitiator is excessively higher than the above-defined range, rapid crosslinking occurs during UV irradiation, causing the separator to shrink and the polyolefin backbone to sever, resulting in reduced mechanical strength. Furthermore, when the content of the Type 2 photoinitiator is less than the above-defined range, crosslinking does not proceed smoothly even under UV irradiation.

[0102] According to one embodiment of the present disclosure, the Type 2 photoinitiator may be present in an amount of 0.05 to 0.2 parts by weight or 0.05 to 0.1 parts by weight based on 100 parts by weight of the solvent for the Type 2 photoinitiator.

[0103] With regard to the polyolefin porous substrate that can be used in the present disclosure, reference is made to the above description.

[0104] According to one embodiment of the present disclosure, the Type 2 photoinitiator composition can be applied to the polyolefin porous substrate by immersing the polyolefin porous substrate in the Type 2 photoinitiator composition, or by applying the Type 2 photoinitiator composition to at least one surface of the polyolefin porous substrate by spraying, etc. However, the scope of the present disclosure is not limited thereto. For example, the Type 2 photoinitiator composition can be applied to the polyolefin porous substrate for 0.1 seconds to 5 minutes, but the scope of the present disclosure is not limited thereto. The polyolefin porous substrate coated with the Type 2 photoinitiator composition can then be dried. For example, the drying can be performed at room temperature for 30 seconds to 10 minutes.

[0105] Thereafter, ultraviolet rays are irradiated onto the polyolefin porous substrate coated with the type 2 photoinitiator composition (step S2).

[0106] UV irradiation can be performed using a UV curing system, while appropriately controlling the UV irradiation time and dose by taking into account other conditions, such as the weight ratio of the photoinitiator. For example, the UV irradiation time and dose can be set so that the polyolefin fibrils are sufficiently crosslinked to provide a polyolefin porous substrate having a melting point of approximately 160°C or above, or 170°C or above, while also preventing the polyolefin porous substrate from being damaged by the heat generated by the UV lamp. Furthermore, the UV lamp used in the UV curing system can be appropriately selected from a high-pressure mercury lamp, a metal lamp, a gallium lamp, and the like, depending on the photoinitiator used for crosslinking. The emission wavelength and dose can also be appropriately selected based on the overall process.

[0107] According to one embodiment of the present disclosure, ultraviolet rays may be irradiated to the polyolefin porous substrate coated with the type 2 photoinitiator composition, wherein the UV light dose may be 10 to 1000 mJ / cm 2 50~1000mJ / cm 2 or 150-500 mJ / cm 2 .

[0108] According to one embodiment of the present disclosure, the "UV light dose" can be determined by using a portable light dose meter called an H-type UV bulb and a UV power puck available from Miltec. When the light dose is measured by using the H-type UV bulb available from Miltec, three types of wavelength values, UVA, UVB, and UVC, are provided according to the wavelength, and the ultraviolet light used herein corresponds to UVA.

[0109] According to the present disclosure, a method for determining “UV light dose” includes passing a UV power puck through a conveyor in the presence of a light source under the same conditions as the sample, and the UV light dose value displayed in the UV power puck is defined as “UV light dose”.

[0110] According to one embodiment of the present disclosure, the type 2 photoinitiator composition may be a porous coating-forming composition further comprising inorganic particles and a binder polymer. Therefore, the method for manufacturing a cross-linked separator may include the following steps: preparing a polyolefin porous substrate (step P1); preparing a porous coating-forming composition, the porous coating-forming composition comprising a type 2 photoinitiator, a solvent for the type 2 photoinitiator, inorganic particles, and a binder polymer (step P2); applying the porous coating-forming composition to at least one surface of the polyolefin porous substrate (step P3); and irradiating ultraviolet light to the porous coating formed on at least one surface of the polyolefin porous substrate (step P4).

[0111] First, a polyolefin porous substrate is prepared (step P1). With regard to the polyolefin porous substrate, reference is made to the above description.

[0112] Next, a porous coating layer-forming composition is prepared, the composition comprising a type 2 photoinitiator, a solvent for the type 2 photoinitiator, inorganic particles, and a binder polymer (step P2 ).

[0113] With regard to the inorganic particles, the binder polymer, and the type 2 photoinitiator, reference is made to the above description.

[0114] The solvent for the type 2 photoinitiator is a solvent capable of dissolving the type 2 photoinitiator and may be used as a solvent for dissolving the binder polymer or as a dispersion medium for not dissolving but dispersing the binder polymer, depending on the specific type of the binder polymer.

[0115] According to one embodiment of the present disclosure, the solvent for the Type 2 photoinitiator is an organic solvent, and any organic solvent may be used without particular limitation as long as it can uniformly disperse the inorganic particles, the binder polymer, and the Type 2 photoinitiator.

[0116] Specific examples of the organic solvent may include: cyclic aliphatic hydrocarbons such as cyclopentane and cyclohexane; aromatic hydrocarbons such as toluene, xylene and ethylbenzene; ketones such as acetone, ethyl methyl ketone, diisopropyl ketone, cyclohexanone, methylcyclohexane and ethylcyclohexane; chlorinated aliphatic hydrocarbons such as dichloromethane, chloroform and carbon tetrachloride; esters such as ethyl acetate, butyl acetate, γ-butyrolactone and ε-caprolactone; acrylonitrile such as acetonitrile and propionitrile; ethers such as tetrahydrofuran and ethylene glycol diethyl ether; alcohols such as methanol, ethanol, isopropyl alcohol, ethylene glycol and ethylene glycol monomethyl ether; and amides such as N-methylpyrrolidone and N,N-dimethylformamide. According to one embodiment of the present disclosure, the solvent may include acetone in view of the advantages during the drying process.

[0117] Such solvents can be used alone or in combination. In particular, solvents with low boiling points and high volatility are preferred because they can be removed in a short time at low temperatures. Particularly preferred solvents can include acetone, toluene, cyclohexanone, cyclopentane, tetrahydrofuran, cyclohexane, xylene, N-methylpyrrolidone, or a mixed solvent containing two or more of these.

[0118] Preferably, the ratio of the inorganic particles to the binder polymer in the porous coating layer-forming composition is the same as described above with respect to the porous coating layer.

[0119] Herein, based on 100 parts by weight of a solvent such as acetone or N-methylpyrrolidone for preparing a porous coating, the type 2 photoinitiator is used in an amount of 0.05 to 0.3 parts by weight, 0.05 to 0.2 parts by weight or 0.05 to 0.1 parts by weight. When the type 2 photoinitiator is used within the range defined above, the direct crosslinking of the polyolefin can be achieved by irradiating ultraviolet light with a light dose (i.e., a lower light dose than in the past) that can ensure mass production productivity, and a crosslinked separator for a lithium secondary battery comprising such a crosslinked polyolefin substrate can have improved heat resistance. In addition, crosslinking can be appropriately performed without including the problem of membrane shrinkage caused by rapid crosslinking during UV irradiation and reduced mechanical strength caused by severing the polyolefin main chain.

[0120] Then, a porous coating layer-forming composition is applied to at least one surface of the polyolefin porous substrate (step P3).

[0121] According to one embodiment of the present disclosure, there is no particular limitation on the method for forming a porous coating layer by applying the porous coating layer-forming composition onto at least one surface of the polyolefin porous substrate, and specific examples of the method may include dip coating, die coating, roll coating, comma coating, blade coating, reverse roll coating, direct roll coating, and the like.

[0122] Phase separation may be performed to form a pore structure having higher quality in the porous coating layer, and may be performed by steam-induced phase separation or immersion phase separation.

[0123] Hereinafter, steam-induced phase separation will be explained in more detail.

[0124] First, steam-induced phase separation can be performed at a temperature of 15 to 70° C. or 20 to 50° C. and a relative humidity of 15% to 80% or 30% to 50%. After the porous coating layer-forming composition is applied to at least one surface of the polyolefin porous substrate, the porous coating layer undergoes phase transition properties due to the steam-induced phase separation phenomenon while drying, and an interstitial volume structure is formed in the porous coating layer.

[0125] To induce steam phase separation, a gaseous non-solvent may be introduced. The non-solvent is not particularly limited, as long as it does not dissolve the binder polymer and is partially miscible with the solvent. For example, the non-solvent may be water, methanol, ethanol, isopropanol, butanol, or two or more thereof.

[0126] When a gaseous non-solvent is introduced and added, there are advantages in that phase separation can be performed by using a small amount of the non-solvent, and the porous coating layer-forming composition can be easily dried.

[0127] Here, the gaseous non-solvent may be added at a temperature of 15 to 70° C. When the temperature is lower than 15° C., the non-solvent hardly maintains its gaseous state, and the porous coating layer-forming composition dries at a low rate, resulting in reduced productivity. When the temperature is higher than 70° C., the solvent and the non-solvent dry at an excessively high rate, making it difficult to sufficiently separate the phases.

[0128] In addition, during the phase separation, the non-solvent may be added so that the vapor pressure of the non-solvent is 15% to 80% or 30% to 50% based on its saturated vapor pressure, and then the phase separation may be sequentially performed. When the vapor pressure of the non-solvent is less than 15% based on its saturated vapor pressure, the amount of the non-solvent is too small to sufficiently perform the phase separation. When the vapor pressure of the non-solvent is greater than 80% based on its saturated vapor pressure, excessive phase separation occurs, making it difficult to form a uniform coating.

[0129] In order to perform phase separation by adding a gaseous non-solvent, it is advantageous that the solvent has a low boiling point and is therefore easily evaporated. In other words, when the solvent evaporates and the temperature is lowered, solvent exchange can easily occur while the gaseous non-solvent condenses. According to one embodiment, when a gaseous non-solvent is added, the solvent may have a boiling point of 30 to 80°C. In addition, the solvent of the porous coating layer-forming composition to which the gaseous non-solvent is added may be acetone, methyl ethyl ketone, or a mixture thereof.

[0130] Hereinafter, the impregnation phase separation will be explained in more detail.

[0131] In order to perform impregnation phase separation, the type 2 photoinitiator and the binder polymer are dissolved in a solvent, and inorganic particles are introduced therein and mixed therewith to prepare a porous coating composition. Then, after applying the porous coating composition on at least one surface of the polyolefin porous substrate, the coated substrate is immersed in a curing solution comprising a suitable non-solvent for a predetermined time. In this way, phase separation occurs in the porous coating composition, and at the same time the binder polymer is cured. In this process, the coating comprising the binder polymer and inorganic particles is converted into a porous layer. Thereafter, the resulting product is washed with water to remove the curing solution and then dried to integrally form a porous coating on the polyolefin porous substrate. According to one embodiment of the present disclosure, a porous coating composition (comprising a type 2 photoinitiator, a solvent for a type 2 photoinitiator, a binder polymer and inorganic particles) may preferably include a binder polymer having a concentration of 3 to 10% by weight based on 100% by weight of the composition.

[0132] The solvent used to dissolve the binder polymer may be a solvent capable of dissolving the binder polymer to a concentration of 5% by weight or more, preferably 15% by weight or more, and more preferably 25% by weight or more at 25°C. Non-limiting examples of the solvent include polar amide solvents such as N-methylpyrrolidone, dimethylacetamide, and dimethylformamide; acetone; cyclopentanone; methyl acetate; γ-butyrolactone; trimethyl phosphate; triethyl phosphate; and dimethylethoxymethane. If the solubility of the binder polymer in the solvent is below the above-defined range, there is a problem that phase separation may proceed excessively.

[0133] The non-solvent may be a substance having a solubility of the binder polymer of less than 5 wt % at 25° C. The non-solvent may include at least one selected from water, methanol, ethanol, propanol, butanol, butanediol, ethylene glycol, propylene glycol, and tripropylene glycol.

[0134] The non-solvent may be used alone or as a mixed solvent with the above-mentioned solvents as the solidifying solution. When a mixed solvent of the non-solvent and the above-mentioned solvent is used, the content of the non-solvent is 95% by weight or more based on 100% by weight of the solidifying solution in order to form a high-quality porous structure and improve productivity.

[0135] On the other hand, according to one embodiment of the present disclosure, the binder polymer can be cured by preparing two or more curing solutions and sequentially immersing the diaphragm coated with the porous coating forming composition in each curing solution for a predetermined time. Herein, multiple curing solutions can be prepared in such a way that the concentration of the non-solvent can be sequentially increased compared to the previous step. The concentration of the non-solvent in at least the second or subsequent curing solution can be higher than the concentration of the non-solvent in the first curing solution. For example, the concentration of the non-solvent in the first curing solution can be 95% by weight, and the concentration of the non-solvent in the subsequent curing solution can be controlled to be higher than 95% by weight.

[0136] Since the separator is immersed in a curing solution containing an excess of a non-solvent, the solvent in the coating layer is exchanged with the curing solution, and the proportion of the non-solvent in the coating layer gradually increases. Therefore, when curing is performed in multiple steps by preparing multiple curing solutions, it is preferable to gradually increase the proportion of the non-solvent in the curing solution. On the other hand, when the non-solvent content of the first curing solution is 100%, the curing solutions after the first curing solution contain only the non-solvent.

[0137] According to one embodiment of the present disclosure, the curing solution can be maintained at a temperature of 5°C or higher and lower than 20°C. At temperatures below the range defined above, condensation of the non-solvent does not occur undesirably. At temperatures above the range defined above, phase separation occurs rapidly, so that the coating may not have a dense structure. Therefore, the desired porous coating cannot be formed, and the separator has a structure in which the binder polymer is too dense in local areas, which is not preferred in terms of resistance characteristics and adhesion. On the other hand, when multiple curing steps are performed by preparing multiple curing solutions as described above, the temperature of the first curing solution can be set to 5°C or higher and lower than 20°C, and then the temperature of the second or subsequent curing solution can be sequentially increased until the drying step is performed. At least, the second or subsequent curing solution can be prepared to have a temperature higher than that of the first curing solution. However, it is preferred to control the temperature of the second or subsequent curing solution to a temperature below 40°C. At temperatures above the range defined above, evaporation of the non-solvent does not occur undesirably. At temperatures below the range defined above, thermal shock occurs when introduced into the drying furnace, resulting in the risk of changes in the width of the substrate.

[0138] On the other hand, according to one embodiment of the present disclosure, the immersion time can be controlled to be less than 1 minute. When the immersion time is greater than 1 minute, excessive phase separation occurs, resulting in reduced adhesion between the polyolefin porous substrate and the porous coating layer and separation of the coating layer. On the other hand, when multiple curing steps are performed by preparing multiple curing solutions as described above, the immersion time in the first curing solution can be controlled to be 3 to 25 seconds.

[0139] According to one embodiment of the present disclosure, after the phase separation step, a drying step may be performed. The drying step may be performed using known methods and may be performed in an intermittent or continuous mode using an oven or a heating chamber within a temperature range that takes into account the vapor pressure of the solvent used herein. The drying step is to substantially remove the solvent present in the composition and, taking into account productivity, is preferably performed as quickly as possible. For example, the drying step may be performed for less than 1 minute or less than 30 seconds.

[0140] The porous coating layer may be formed on both surfaces of the polyolefin porous substrate, or selectively formed on only one surface of the polyolefin porous substrate.

[0141] Then, ultraviolet rays are irradiated to the porous coating layer formed on at least one surface of the polyolefin porous substrate (step P4).

[0142] Before irradiating the separator with ultraviolet light, a porous coating layer-forming composition containing a Type 2 photoinitiator is applied, thereby distributing the Type 2 photoinitiator on the fibril surfaces of the polyolefin porous substrate. Subsequently, ultraviolet light is irradiated, and the Type 2 photoinitiator present on the porous substrate surface directly crosslinks polyolefin chains on the fibril surfaces of the polyolefin porous substrate.

[0143] According to one embodiment of the present disclosure, ultraviolet rays may be irradiated to the porous coating layer-forming composition coated on at least one surface of the porous polyolefin substrate, wherein the UV light dose may be 10 to 1000 mJ / cm 2 50~1000mJ / cm 2 or 150-500 mJ / cm 2 .

[0144] According to one embodiment of the present disclosure, when 2-isopropylthioxanthone (ITX) is used as a Type 2 photoinitiator, ITX has a low melting point of approximately 70-80°C. Therefore, when the UV curing temperature is controlled at 80-100°C, the ITX on the polyolefin porous substrate may melt, and the photoinitiator may migrate into the substrate, thereby improving the curing efficiency and preventing changes in the physical properties of the cross-linked separator.

[0145] The cross-linked separator for a lithium secondary battery according to the present disclosure may be interposed between a positive electrode and a negative electrode to provide an electrochemical device.

[0146] The electrochemical device includes any device that performs an electrochemical reaction, and specific examples thereof include all types of primary batteries, secondary batteries, fuel cells, solar cells or capacitors such as supercapacitor devices. In particular, among secondary batteries, lithium secondary batteries are preferred, including lithium metal secondary batteries, lithium ion secondary batteries, lithium polymer secondary batteries or lithium ion polymer batteries.

[0147] The electrode used in combination with the separator according to the present disclosure is not particularly limited and can be obtained by bonding an electrode active material to an electrode current collector by a method generally known in the art.

[0148] Among the electrode active materials, non-limiting examples of positive electrode active materials include conventional positive electrode active materials that can be used for positive electrodes for conventional electrochemical devices. In particular, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a lithium composite oxide containing a combination thereof is preferably used.

[0149] Non-limiting examples of negative electrode active materials include conventional negative electrode active materials that can be used for negative electrodes of conventional electrochemical devices. In particular, lithium intercalation materials such as lithium metal or lithium alloys, carbon, petroleum coke, activated carbon, graphite or other carbonaceous materials are preferably used. Non-limiting examples of positive electrode current collectors include foils made of aluminum, nickel or a combination thereof. Non-limiting examples of negative electrode current collectors include foils made of copper, gold, nickel, copper alloys or a combination thereof.

[0150] According to one embodiment of the present disclosure, the electrolyte that can be used in the electrochemical device according to the present disclosure is an electrolyte having A + B - A salt of the structure + Contains alkali metal cations such as Li + 、Na + , K + or a combination thereof, and B - Contains anions such as PF6 - 、BF4 - 、Cl - Br - , I - 、ClO4 - 、AsF6 - 、CH3CO2 - CF3SO3 - 、N(CF3SO2)2 -, C(CF2SO2)3- or a combination thereof, wherein the salt is dissolved or dissociated in an organic solvent, and the organic solvent comprises propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), gamma-butyrolactone (gamma-butyrolactone) or a combination thereof. However, the present disclosure is not limited thereto.

[0151] The injection of the electrolyte can be carried out at an appropriate step during the process of manufacturing the battery, depending on the manufacturing process of the final product and the properties required of the final product. In other words, the injection of the electrolyte can be carried out before the battery is assembled or in the last step of the battery assembly.

[0152] According to one embodiment of the present disclosure, the cross-linked separator for a lithium secondary battery may be applied to a battery by laminating, stacking, and folding the cross-linked separator and an electrode, in addition to winding by a conventional method.

[0153] According to one embodiment of the present disclosure, the cross-linked separator for a lithium secondary battery may be interposed between the positive electrode and the negative electrode. When an electrode assembly is formed by assembling a plurality of batteries or electrodes, the cross-linked separator may be interposed between adjacent batteries or electrodes. The electrode assembly may have various structures, such as a simple stack type, a jelly roll type, a stacked folding type, a laminated stack type, and the like.

[0154] The following will be described in more detail with reference to the embodiments so that the present disclosure can be readily understood. However, the following embodiments may be embodied in many different forms, and the scope of the present invention should not be construed as being limited to the exemplary embodiments set forth herein. On the contrary, these exemplary embodiments are provided to provide a thorough and comprehensive overview of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0155] Example 1

[0156] A polyethylene porous film having a thickness of 9 μm (available from Toray Industries, Inc., porosity: 45%) was prepared as a polyolefin porous substrate.

[0157] As a type 2 photoinitiator, 2-isopropylthioxanthone (ITX, commercially available from Sigma-Aldrich) was prepared.

[0158] As a UV light source, a high-pressure mercury lamp (a high-temperature mercury lamp available from Lichtzen, LH-250 / 800-A) was prepared.

[0159] The Type 2 photoinitiator was dissolved in acetone as a solvent to prepare a Type 2 photoinitiator composition including 0.05 parts by weight of the Type 2 photoinitiator based on 100 parts by weight of acetone.

[0160] The polyolefin porous substrate was immersed in the Type 2 photoinitiator composition for 30 seconds, taken out therefrom, and then dried at room temperature (25° C.) for 1 minute.

[0161] Then, the UV light was applied at a cumulative dose (i.e. 500 mJ / cm 2 The UV irradiation light dose of 200 nm was irradiated to the upper surface of the polyolefin porous substrate coated with the Type 2 photoinitiator composition, wherein the UV irradiation intensity was set to 80% of the UV light source and the line speed was set to 10 m / min.

[0162] In this manner, a crosslinked separator for a lithium secondary battery is obtained, comprising a crosslinked polyolefin porous substrate comprising a plurality of fibrils and pores formed by entanglement of the fibrils, wherein polyethylene chains forming the fibrils are directly crosslinked with each other.

[0163] Example 2

[0164] A cross-linked separator for a lithium secondary battery was obtained in the same manner as in Example 1, except that thioxanthone (available from TCI) was used instead of 2-isopropylthioxanthone (available from Sigma-Aldrich).

[0165] Example 3

[0166] A cross-linked separator for a lithium secondary battery was obtained in the same manner as in Example 1, except that 0.1 parts by weight of benzophenone (available from Sigma-Aldrich) based on 100 parts by weight of acetone was used instead of 2-isopropylthioxanthone (available from Sigma-Aldrich), and ultraviolet light was applied at 500 mJ / cm 2 The cumulative light dose is irradiated onto the polyethylene porous substrate.

[0167] Example 4

[0168] A cross-linked separator for a lithium secondary battery was obtained in the same manner as in Example 1, except that 0.1 parts by weight of 4-hydroxybenzophenone (available from Sigma-Aldrich) was used instead of 2-isopropylthioxanthone (available from Sigma-Aldrich) based on 100 parts by weight of acetone, and ultraviolet light was applied at 500 mJ / cm 2 The cumulative light dose is irradiated onto the polyethylene porous substrate.

[0169] Example 5

[0170] A polyethylene porous film having a thickness of 9 μm (available from Toray Industries, Inc., porosity: 45%) was prepared as a polyolefin porous substrate.

[0171] As inorganic particles, D was mixed at a weight ratio of 9:1. 50 Al2O3 powder with a particle size of 500nm and D 50 γ-AlOOH powder with a particle size of 250 nm. As used herein, “particle size D 50 " means the particle size at the 50% point in the cumulative distribution of the number of particles depending on the particle size. 50 It can be determined by using a laser diffraction method. Specifically, the powder to be analyzed is dispersed in a dispersion medium and introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500) to measure the difference in diffraction patterns depending on the particle size when the particles pass through the laser beam. The particle size distribution can then be calculated. D is then determined by calculating the particle size at the 50% point in the cumulative distribution of the number of particles depending on the particle size in the analyzer. 50 As a binder polymer, polyvinylidene fluoride (PVDF) was prepared.

[0172] As a type 2 photoinitiator, thioxanthone (commercially available from Sigma-Aldrich) was prepared.

[0173] As a UV light source, a high-pressure mercury lamp (a high-temperature mercury lamp available from Lichtzen, LH-250 / 800-A) was prepared.

[0174] The binder polymer was added to acetone as a solvent and dissolved therein at 50°C for about 4 hours. Inorganic particles were added to the resulting solution in a weight ratio of the binder polymer to the total inorganic particles of 1:4. Then, cyanoethyl polyvinyl alcohol was added thereto as a dispersant to 2% by weight based on the total inorganic particle content, and 0.05 parts by weight of a type 2 photoinitiator based on 100 parts by weight of acetone was added. Thereafter, the inorganic particles were pulverized and dispersed for 12 hours using a ball mill to prepare a composition for forming a porous coating layer. Herein, the ratio of solvent to solid content was 4:1.

[0175] The porous coating layer-forming composition was applied to both surfaces of a polyethylene porous substrate having a size of 6 cm x 15 cm by dip coating at 23° C. and a relative humidity of 42% to a total coating amount of 13.5 g / m 2 , and then the coated polyethylene porous substrate was dried at 23°C for 1 minute.

[0176] Then, ultraviolet rays were irradiated at 500 mJ / cm by using a high-pressure mercury lamp (a high-temperature mercury lamp available from Lichtzen, LH-250 / 800-A). 2The porous coating layers formed on both surfaces of the polyolefin porous substrate were irradiated with a cumulative light dose of 100 nm, wherein the UV irradiation intensity was set to 80% of the UV light source and the line speed was set to 10 m / min. In this manner, a crosslinked separator for a lithium secondary battery was obtained, comprising a crosslinked polyolefin porous substrate and porous coating layers formed on both surfaces of the crosslinked polyolefin porous substrate.

[0177] The porous coating layer includes interstitial volumes formed by inorganic particles that are substantially in contact with each other, wherein the interstitial volumes mean spaces defined by the inorganic particles that are substantially in contact with each other in a close-packed or densely packed structure of the inorganic particles, and the interstitial volumes between the inorganic particles become empty spaces that form pores of the porous coating layer.

[0178] Example 6

[0179] A polyethylene porous film having a thickness of 9 μm (available from Toray Industries, Inc., porosity: 45%) was prepared as a polyolefin porous substrate.

[0180] As inorganic particles, D was mixed at a weight ratio of 9:1. 50 Al2O3 powder with a particle size of 500nm and D 50 γ-AlOOH powder with a particle size of 250 nm and polyvinylidene fluoride (PVDF) as a binder polymer were prepared.

[0181] As a type 2 photoinitiator, 2-isopropylthioxanthone (commercially available from Sigma-Aldrich) was prepared.

[0182] As a UV light source, a high-pressure mercury lamp (a high-temperature mercury lamp available from Lichtzen, LH-250 / 800-A) was prepared.

[0183] The binder polymer was added to acetone as a solvent and dissolved therein at 50°C for about 4 hours. Inorganic particles were added to the resulting solution in a manner such that the weight ratio of the binder polymer to the total inorganic particles was 1:4. Then, cyanoethyl polyvinyl alcohol was added thereto as a dispersant to 2% by weight based on the total inorganic particle content, and 0.1 parts by weight of a type 2 photoinitiator based on 100 parts by weight of acetone was added. Thereafter, the inorganic particles were pulverized and dispersed for 12 hours using a ball mill to prepare a composition for forming a porous coating layer. Herein, the ratio of solvent to solid content was 4:1.

[0184] The porous coating layer-forming composition was applied to both surfaces of a polyethylene porous film having a size of 6 cm x 15 cm by dip coating at 23° C. under a relative humidity of 42% to a total coating amount of 13.5 g / m 2, the coated polyethylene porous substrate is then sequentially immersed in a first curing bath and a second curing bath to cure the porous coating layer-forming composition. The first curing bath contains a curing solution containing NMP as a solvent and water as a non-solvent mixed in a weight ratio of 5:95, the temperature of the curing solution is controlled at 15°C, and the immersion time is 10 seconds. The second curing bath contains a curing solution consisting solely of water as a non-solvent, the temperature of the curing solution is controlled at 23°C, and the immersion time is 30 seconds. After the porous coating layer is cured, it is removed from the curing solution, and the residual solvent and non-solvent on the porous coating layer are dried simultaneously.

[0185] Then, ultraviolet rays were irradiated at 500 mJ / cm by using a high-pressure mercury lamp (a high-temperature mercury lamp available from Lichtzen, LH-250 / 800-A). 2 The porous coating layers formed on both surfaces of the polyolefin porous substrate were irradiated with a cumulative light dose of 100 nm. In this manner, a cross-linked separator for a lithium secondary battery was obtained, comprising a cross-linked polyolefin porous substrate and porous coating layers formed on both surfaces of the cross-linked polyolefin porous substrate.

[0186] Example 7

[0187] A cross-linked separator for a lithium secondary battery was obtained in the same manner as in Example 1, except that a Type 2 photoinitiator composition including 0.1 parts by weight of the Type 2 photoinitiator based on 100 parts by weight of acetone was used.

[0188] Example 8

[0189] A cross-linked separator for a lithium secondary battery was obtained in the same manner as in Example 1, except that a Type 2 photoinitiator composition including 0.15 parts by weight of the Type 2 photoinitiator based on 100 parts by weight of acetone was used.

[0190] Example 9

[0191] A cross-linked separator for a lithium secondary battery was obtained in the same manner as in Example 1, except that a Type 2 photoinitiator composition including 0.2 parts by weight of the Type 2 photoinitiator based on 100 parts by weight of acetone was used.

[0192] Example 10

[0193] A cross-linked separator for a lithium secondary battery was obtained in the same manner as in Example 1, except that a Type 2 photoinitiator composition including 0.3 parts by weight of the Type 2 photoinitiator based on 100 parts by weight of acetone was used.

[0194] Comparative Example 1

[0195] A polyethylene porous film having a thickness of 9 μm (available from Toray Industries, Inc., porosity: 45%) without any treatment was used as a separator for a lithium secondary battery.

[0196] Comparative Example 2

[0197] As a photocurable composition, a photocurable composition was prepared by preparing diphenyl (2,4,6-trimethylbenzoyl) -phosphine oxide (TPO) (available from Sigma-Aldrich) and tris (2-acryloyloxyethyl) isocyanurate (TEICTA, available from Sigma-Aldrich) and dissolving them in acetone in an amount of 0.3 parts by weight based on 100 parts by weight of acetone.

[0198] As the polyolefin porous substrate, a polyethylene porous film having a thickness of 9 μm (available from Toray Industries, Inc., porosity: 45%) was prepared.

[0199] The polyethylene porous substrate was immersed in the photocurable composition for 30 seconds, taken out therefrom, and then dried at room temperature (25° C.) for 1 minute.

[0200] Then, ultraviolet rays were irradiated at 1500 mJ / cm by using a high-pressure mercury lamp (a high-temperature mercury lamp available from Lichtzen, LH-250 / 800-A). 2 The cumulative light dose of irradiation was directed to the upper surface of the polyolefin porous substrate coated with the photocurable composition. In this manner, a crosslinked separator for a lithium secondary battery comprising a crosslinked polyolefin porous membrane was obtained.

[0201] Comparative Example 3

[0202] A cross-linked separator for a lithium secondary battery comprising a cross-linked polyolefin porous film was obtained in the same manner as in Comparative Example 2, except that phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Irgacure 819) was used instead of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) (available from Sigma-Aldrich) in the components for the photocurable composition.

[0203] Comparative Example 4

[0204] A cross-linked separator for a lithium secondary battery including a cross-linked polyolefin porous film was obtained in the same manner as in Comparative Example 2, except that among the components for the photocurable composition, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Irgacure 819) was used in an amount of 0.3 parts by weight based on 100 parts by weight of acetone and tris(2-acryloyloxyethyl)isocyanurate (TEICTA, available from Sigma-Aldrich) was used in an amount of 0.6 parts by weight based on 100 parts by weight of acetone.

[0205] Comparative Example 5

[0206] A cross-linked separator for a lithium secondary battery including a cross-linked polyolefin porous film was obtained in the same manner as in Comparative Example 2, except that phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Irgacure 819) was used alone in an amount of 0.3 parts by weight based on 100 parts by weight of acetone in the components for the photocurable composition.

[0207] Comparative Example 6

[0208] A cross-linked separator for a lithium secondary battery comprising a cross-linked polyolefin porous film was obtained in the same manner as in Example 1, except that a photocurable composition comprising 0.3 parts by weight of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Irgacure 819) based on 100 parts by weight of acetone, 0.6 parts by weight of tris(2-acryloyloxyethyl)isocyanurate (TEICTA, available from Sigma-Aldrich) based on 100 parts by weight of acetone, and 0.1 parts by weight of thioxanthone (available from TCI) based on 100 parts by weight of acetone was used instead of the Type 2 photoinitiator composition.

[0209] Comparative Example 7

[0210] An electron beam from an electron beam (E-beam) (EB TECH Co., Ltd.) was irradiated at a dose of 200 kGy to a polyethylene porous film having a thickness of 9 μm (available from Toray Industries, Inc., porosity: 45%) to obtain a crosslinked separator for a lithium secondary battery comprising a crosslinked polyolefin porous film. Herein, the 200 kGy electron beam corresponds to a dose at which crosslinking can occur.

[0211] Comparative Example 8

[0212] A separator for a lithium secondary battery was obtained in the same manner as in Example 5, except that thioxanthone was not added to the porous coating layer-forming composition of Example 5 and UV irradiation was not performed.

[0213] Comparative Example 9

[0214] A separator for a lithium secondary battery was obtained in the same manner as in Example 6, except that 2-isopropylthioxanthone was not added to the porous coating layer-forming composition of Example 6 and UV irradiation was not performed.

[0215] Comparative Example 10

[0216] A separator for a lithium secondary battery was obtained in the same manner as in Example 5, except that 0.3 parts by weight of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Irgacure 819) based on 100 parts by weight of acetone and 0.6 parts by weight of tris(2-acryloyloxyethyl)isocyanurate (TEICTA) (available from Sigma-Aldrich) based on 100 parts by weight of acetone were used instead of 0.05 parts by weight of thioxanthone in the porous coating layer-forming composition of Example 5, and the reaction mixture was heated at 1500 mJ / cm 2 The cumulative light dose is irradiated with ultraviolet light.

[0217] Comparative Example 11

[0218] A separator for a lithium secondary battery was obtained in the same manner as in Example 1, except that 0.5 parts by weight of thioxanthone (available from TCI) based on 100 parts by weight of acetone was used instead of 2-isopropylthioxanthone (available from Sigma-Aldrich).

[0219] Comparative Example 12

[0220] A separator for a lithium secondary battery was obtained in the same manner as in Example 1, except that 0.5 parts by weight of 2-isopropylthioxanthone was used based on 100 parts by weight of acetone.

[0221] Test Example 1: Performance evaluation of a separator comprising only a porous substrate

[0222] The separators produced in Examples 1 to 4, Comparative Examples 1 to 7, and Comparative Example 11 were each measured for air permeability, porosity, basis weight, degree of crosslinking, meltdown temperature, tensile strength (MD / TD), change in tensile strength in the machine direction (MD), puncture strength, change in puncture strength, high-temperature shrinkage at 120°C (MD / TD), and electrical resistance. The results are shown in Table 1 below.

[0223] (1) Evaluation of air permeability

[0224] Gurley is measured according to ASTM D726-94. Gurley as used herein refers to the resistance of a membrane to air flow and is measured using a Gurley densometer. The air permeability value defined herein is determined by 100 ml of air passing through 1 inch of a membrane at a pressure of 12.2 inches of H2O. 2 The time (seconds) required to cross the diaphragm cross section is expressed as the ventilation time.

[0225] (2) Evaluation of porosity

[0226] The porosity is determined by measuring the width / length / thickness of the separator to calculate the volume, measuring the weight, and then calculating the ratio of the weight relative to the volume if 100% was occupied by the separator.

[0227] Porosity (%) = 100 x (1-membrane sample weight / (membrane sample width (50 mm) x membrane sample length (50 mm) x thickness x membrane density))

[0228] (3) Evaluation of weight per unit area

[0229] The weight per unit area (g / m2) was evaluated by preparing a sample with a size of 1 m x 1 m in width x length and measuring the weight of the sample. 2 ).

[0230] (4) Evaluation of cross-linking degree

[0231] The degree of crosslinking was evaluated according to ASTM D2765 by immersing a polyolefin porous substrate in a xylene solution at 135° C., boiling the substrate for 12 hours, measuring the weight of the residue, and calculating the weight percentage of the residue based on the initial weight.

[0232] (5) Evaluation of melting temperature

[0233] The fusing temperature is determined by collecting each sample in the machine direction (MD) and the transverse direction (TD) and analyzing each sample by thermomechanical analysis (TMA). In particular, a sample with a width x length dimension of 4.5 mm x 8 mm is introduced into a TMA instrument (TA Instruments, Q400) and heated from a temperature of 30° C. to 220° C. at a heating rate of 5° C. / minute while applying a tension of 0.01 N. As the temperature rises, the sample shows a change in length. The temperature of the sample when it breaks after a rapid increase in length is then measured. Herein, the fusing temperature is measured along the machine direction (MD).

[0234] (6) Evaluation of tensile strength in the machine direction and transverse direction and changes in tensile strength in the machine direction

[0235] Prepare a test piece with a size of 100 mm x 15 mm.

[0236] According to ASTM D882, by using a universal testing machine ( 3345) The test piece was stretched at a rate of 50 mm / min in each of the machine direction and the transverse direction, and the strength at the breaking point was defined as the tensile strength in each of the machine direction and the transverse direction.

[0237] In addition, the change in tensile strength in the machine direction was calculated according to the following formula: In the following formula, the “uncrosslinked separator comprising the polyolefin porous substrate before crosslinking” corresponds to Comparative Example 1.

[0238] Change in tensile strength in the machine direction (%) = [(tensile strength in the machine direction of the uncrosslinked separator comprising the polyolefin porous substrate before crosslinking) - (tensile strength in the machine direction of the crosslinked separator comprising the polyolefin porous substrate after crosslinking)] / (tensile strength in the machine direction of the uncrosslinked separator comprising the polyolefin porous substrate before crosslinking) × 100

[0239] (7) Evaluation of puncture strength and puncture strength changes

[0240] Prepare a test piece with a size of 50 mm x 50 mm.

[0241] According to ASTM D2582, a round tip having a diameter of 1 mm was run at a rate of 120 mm / min, and the puncture strength was determined.

[0242] In addition, the change in puncture strength was calculated according to the following formula: In the following formula, “the non-crosslinked separator including the polyolefin porous substrate before crosslinking” corresponds to Comparative Example 1.

[0243] Puncture strength change (%) = [(puncture strength of the uncrosslinked membrane containing the polyolefin porous substrate before crosslinking) - (puncture strength of the crosslinked membrane containing the polyolefin porous substrate after crosslinking)] / (puncture strength of the uncrosslinked membrane containing the polyolefin porous substrate before crosslinking) × 100

[0244] (8) Evaluation of high temperature shrinkage at 120°C

[0245] Each of the separators of Examples 1 to 4, Comparative Examples 1 to 7, and Comparative Example 11 was cut into a size of 50 mm (length) x 50 mm (width) to prepare a test piece. The test piece was placed in an oven heated to 120° C. for 30 minutes. The test piece was recovered and the length change in the machine direction and the transverse direction was measured to calculate the high-temperature shrinkage rate:

[0246] High-temperature shrinkage at 120°C (%) = [(size before shrinkage - size after shrinkage) / size before shrinkage] x 100

[0247] (9) Evaluation of resistance

[0248] Coin cells were fabricated using the separators of Examples 1 to 4, Comparative Examples 1 to 7, and Comparative Example 11. The coin cells were allowed to stand at room temperature for 1 day, and then the resistance of the separators was measured using impedance analysis to determine the resistance. The coin cells were fabricated as follows.

[0249] Anode manufacturing

[0250] First, artificial graphite as a negative electrode active material, Danka black (carbon black) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed in a weight ratio of 75:5:20, and N-methylpyrrolidone as a solvent was added to the resulting mixture to prepare a negative electrode slurry.

[0251] The negative electrode slurry was heated at 3.8 mAh / cm 2 The negative electrode was obtained by coating the negative electrode on a copper current collector and then drying the negative electrode.

[0252] Cathode manufacturing

[0253] First, LiCoO2 as a positive electrode active material, Danka black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed in a weight ratio of 85:5:10, and the resulting mixture was added to N-methylpyrrolidone (NMP) as a solvent to prepare a positive electrode active material slurry. The positive electrode active material slurry was coated on a sheet-like aluminum current collector and then dried to form a positive electrode active material layer, reaching 3.3 mAh / cm 2 The final cathode loading.

[0254] Coin cell battery manufacturing

[0255] Each separator of the embodiment and comparative example was inserted between the negative electrode and the positive electrode obtained as described above, and a non-aqueous electrolyte (1M LiPF6, ethylene carbonate (EC) / propylene carbonate (PC) / diethyl carbonate (DEC), volume ratio = 3:3:4) was injected therein to obtain a coin battery.

[0256] [Table 1]

[0257]

[0258] Referring to Table 1, it can be seen that compared with the uncross-linked diaphragm according to Comparative Example 1, the cross-linked diaphragms manufactured in Examples 1 to 4 have almost or no difference in thickness, air permeability, porosity, weight per unit area, tensile strength, puncture strength and resistance, while having significantly improved cross-linking degree, melting temperature, high temperature shrinkage rate at 120°C and safety.

[0259] On the other hand, compared with Examples 1 to 4, the cross-linked separators prepared in Comparative Examples 2 to 4 were only 2 The curing was achieved only when irradiated with a high light dose of 100 nm and showed significant degradation in cross-linking degree, high temperature shrinkage at 120°C, and electrical resistance.

[0260] Even when using Type 1 initiator alone, Comparative Example 5 completed crosslinking. However, it can be seen that Comparative Example 5 has a low degree of crosslinking compared to Examples 1 to 4, and exhibits significant deterioration in high-temperature shrinkage at 120°C and resistance.

[0261] When using the Type 1 initiator, curing agent, and Type 2 initiator, Comparative Example 6 completed curing with a light dose at a similar level to that in Examples 1 to 4. However, it can be seen that Comparative Example 6 showed deterioration in air permeability and electrical resistance compared to Examples 1 to 4.

[0262] It can be seen that Comparative Example 7 showed a lower degree of crosslinking and significantly deteriorated tensile strength when irradiated with electron beams compared to Examples 1 to 4.

[0263] It was confirmed that Comparative Example 11 resulted in significant deterioration in the tensile strength of the cross-linked separator because the content of the type 2 photoinitiator was greater than 0.3 parts by weight based on 100 parts by weight of the solvent.

[0264] Test Example 2: Performance evaluation of a separator including a porous coating layer

[0265] Each of the separators produced in Examples 5 and 6 and Comparative Examples 8 to 10 was measured for air permeability, meltdown temperature, tensile strength, change in tensile strength in the machine direction, puncture strength, and change in puncture strength. The results are shown in Table 2 below.

[0266] Regarding the methods of determining the air permeability, the melting temperature, the puncture strength, and the change in puncture strength, refer to the description in Test Example 1 above.

[0267] In Example 5, regarding the method of determining tensile strength and tensile strength change in the machine direction, reference is made to the description above in Test Example 1. Herein, it should be understood that “an uncrosslinked separator comprising a polyolefin porous substrate before crosslinking” corresponds to Comparative Example 8.

[0268] In Example 6, regarding the method of determining tensile strength and tensile strength change in the machine direction, reference is made to the description above in Test Example 1. Herein, it should be understood that “an uncrosslinked separator comprising a polyolefin porous substrate before crosslinking” corresponds to Comparative Example 9.

[0269] [Table 2]

[0270]

[0271] Referring to Table 2, the cross-linked separators manufactured in Examples 5 and 6 showed significantly improved melting temperatures compared to the uncross-linked separators according to Comparative Examples 8 and 9 in which no photoinitiator was used and no cross-linking was performed, and also showed significantly improved melting temperatures compared to Comparative Example 10 in which a photocuring agent was used.

[0272] In addition, in the case of Comparative Example 10, it can be seen that the melting temperature is further deteriorated compared with Comparative Example 4. The above results are considered to be derived from the low UV transmittance of the porous coating layer and insufficient cross-linking of the porous substrate.

[0273] Test Example 3: Determination of the cause of improvement in melting temperature

[0274] The thermal shrinkage of the cross-linked separator produced in Example 5 ("curve 1"), the thermal shrinkage of a substrate obtained by removing the porous coating layer from the cross-linked separator produced in Example 5 using Scotch tape ("curve 2"), and the thermal shrinkage of the substrate from which the porous coating layer was removed after being thoroughly washed with acetone ("curve 3") were determined. The results are shown in FIG. Figure 1 middle.

[0275] from Figure 1 It can be seen from the figure that the substrate obtained after removing the porous coating layer does not cause a significant change in the thermal shrinkage. Therefore, it can be inferred that the heat resistance of the cross-linked separator is derived from the cross-linked polyolefin porous substrate.

[0276] Test Example 4: Determination of Changes in Mechanical Strength of Separators Depending on Type 2 Photoinitiator Content

[0277] Each of the separators produced in Examples 1 and 7 to 10 and Comparative Examples 1 and 12 was evaluated in terms of air permeability, porosity, weight per unit area, degree of crosslinking, melting temperature, tensile strength, change in tensile strength in the machine direction, puncture strength, and change in puncture strength. The results are shown in Table 3 below.

[0278] Regarding the methods for determining the air permeability, porosity, weight per unit area, degree of crosslinking, melting temperature, tensile strength, tensile strength change in the machine direction, puncture strength and puncture strength change, refer to the description in Test Example 1 above.

[0279] [Table 3]

[0280]

[0281] Referring to Table 3, it can be seen that the tensile strength in the machine direction of the membranes manufactured in Examples 1 and 7 to 10 and containing 0.05 to 0.3 parts by weight of the type 2 photoinitiator based on 100 parts by weight of the solvent is reduced to less than 20% compared to the uncrosslinked membrane manufactured in Comparative Example 1.

[0282] It can also be seen that the separators manufactured in Examples 1 and 7 to 9 containing 0.05 to 0.2 parts by weight of the type 2 photoinitiator based on 100 parts by weight of the solvent showed a decrease in puncture strength of less than 10% compared to the uncrosslinked separator manufactured in Comparative Example 1 without crosslinking.

[0283] In contrast, it can be seen that the diaphragm manufactured in Comparative Example 12 and containing more than 0.3 parts by weight of a Type 2 photoinitiator based on 100 parts by weight of a solvent has a tensile strength reduction of more than 20% in the machine direction and shows a puncture strength reduction of more than 10% compared to the uncrosslinked diaphragm manufactured in Comparative Example 1 that is not crosslinked.

Claims

1. A cross-linked separator for a lithium secondary battery, comprising a cross-linked polyolefin porous substrate, the cross-linked polyolefin porous substrate comprising a plurality of fibrils and pores between the fibrils entangled with each other, the plurality of fibrils comprising polyolefin chains directly cross-linked with each other; and The cross-linked separator has a tensile strength in the machine direction that varies by 0 to 10% compared to a non-cross-linked separator comprising a polyolefin porous substrate before cross-linking. The cross-linked polyolefin porous substrate has a cross-linking degree of 30% to 55%, The tensile strength change is calculated as follows: in: x is the change in tensile strength in the machine direction in %, y is the tensile strength of the uncrosslinked separator in the machine direction, z is the tensile strength of the cross-linked separator in the machine direction. 2 . The cross-linked separator for a lithium secondary battery according to claim 1 , wherein the cross-linked separator exhibits a puncture strength change of 10% or less compared to the non-cross-linked separator.

3. The cross-linked separator for lithium secondary batteries according to claim 1, further comprising a porous coating layer provided on at least one surface of the cross-linked polyolefin porous substrate, wherein the uncrosslinked separator further comprises a porous coating layer disposed on at least one surface of the polyolefin porous substrate before crosslinking, and The porous coating layer comprises a binder polymer and inorganic particles, and has a gap volume between the inorganic particles in contact with each other, wherein the gap volume is a space defined by the inorganic particles in contact with each other in a stacked structure of the inorganic particles, and the gap volume between the inorganic particles corresponds to the pores of the porous coating layer. 4 . The cross-linked separator for a lithium secondary battery according to claim 1 , wherein the cross-linked separator exhibits an air permeability change of 10% or less compared to the non-cross-linked separator. 5 . The cross-linked separator for a lithium secondary battery according to claim 1 , wherein the cross-linked separator exhibits a weight change per unit area of ​​5% or less compared to the non-cross-linked separator. 6 . The cross-linked separator for a lithium secondary battery according to claim 1 , wherein the cross-linked separator exhibits a resistance change of 15% or less compared to the non-cross-linked separator.

7. A method for manufacturing the cross-linked separator for a lithium secondary battery according to claim 1, the method comprising the following steps: applying a Type 2 photoinitiator composition comprising a Type 2 photoinitiator and a solvent for the Type 2 photoinitiator to a polyolefin porous substrate; and irradiating ultraviolet rays onto the polyolefin porous substrate coated with the type 2 photoinitiator composition, Wherein, based on 100 parts by weight of the solvent for the type 2 photoinitiator, the content of the type 2 photoinitiator is 0.05 to 0.3 parts by weight, The ultraviolet light is 10 to 1000 mJ / cm 2 The irradiation light dose is irradiated. 8 . The method for producing a cross-linked separator for a lithium secondary battery according to claim 7 , wherein the type 2 photoinitiator composition is a porous coating layer-forming composition further comprising inorganic particles and a binder polymer. 9 . The method for manufacturing a cross-linked separator for a lithium secondary battery according to claim 7 , wherein the type 2 photoinitiator comprises thioxanthone (TX), a thioxanthone derivative, benzophenone (BPO), a benzophenone derivative, or a mixture of two or more thereof. 10 . The method for manufacturing a cross-linked separator for a lithium secondary battery according to claim 9 , wherein the type 2 photoinitiator comprises 2-isopropylthioxanthone (ITX), thioxanthone (TX) or a mixture thereof. 11 . A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the separator is the cross-linked separator for a lithium secondary battery according to claim 1 .

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