Crosslinked structure-containing separator for lithium secondary battery, manufacturing method therefor, and lithium secondary battery comprising separator

Abstract

To provide a crosslinked structure-containing separator for a lithium secondary battery including a crosslinked structure-containing polyolefin porous support having a crosslinked structure including polymer chains interconnected directly with one another and a photoinitiator having an oxidation potential value at least 0.02 V higher than the fully charged voltage of the lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the separator, where the lithium secondary battery including the separator has excellent high-temperature safety and can improve the problem of degradation of capacity after high-temperature storage.SOLUTION: Provided is a crosslinked structure-containing separator for a lithium secondary battery, including: a crosslinked structure-containing polyolefin porous support having a crosslinked structure including polymer chains interconnected directly with one another; and a photoinitiator having an oxidation potential value at least 0.02 V higher than the fully charged voltage of the lithium secondary battery.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application claims priority from Korean Patent Application No. 10-2021-0059582, filed on May 7, 2021.

[0002] The present invention relates to a separator containing a crosslinked structure for a lithium secondary battery, a method for producing the same, and a lithium secondary battery including the separator. [Background technology]

[0003] In recent years, interest in energy storage technology has been growing. As the range of applications for energy storage has expanded to include mobile phones, camcorders, laptops, and even electric vehicles, there has been a growing demand for higher energy density batteries used as power sources for such electronic devices. Lithium secondary batteries are the type of battery that best meets this demand, and research into them is currently being actively conducted.

[0004] Such a lithium secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator, among which the separator is required to have insulating properties to separate and electrically insulate the positive electrode and the negative electrode, and high ionic conductivity to increase the permeability of lithium ions due to high porosity.

[0005] The separator serves to electrically insulate the positive and negative electrodes, so they must remain electrically insulated even when the battery is subjected to abnormal conditions such as high temperatures. However, polyolefin separators, which are commonly used as separators, have a low melting point (Tm). If the battery temperature rises above the melting point of polyolefin during battery misuse, they may melt down, resulting in fire and explosion. Furthermore, due to the characteristics of the material and manufacturing process, separators can exhibit severe thermal shrinkage at high temperatures, leading to safety issues such as internal short circuits.

[0006] Therefore, there is a strong demand for a separation membrane that can ensure safety at high temperatures. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention provides a separator containing a crosslinked structure for a lithium secondary battery that is electrochemically stable and has excellent safety at high temperatures.

[0008] Another object of the present invention is to provide a method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery using a photoinitiator that is electrochemically stable and can effectively crosslink a polyolefin porous support.

[0009] Another object of the present invention is to provide a lithium secondary battery having a separator containing a crosslinked structure, which exhibits improved capacity reduction after high-temperature storage. [Means for solving the problem]

[0010] In order to solve the above problems, according to one aspect of the present invention, there is provided a separator having a crosslinked structure for a lithium secondary battery according to the following embodiment.

[0011] The first embodiment is A separator containing a crosslinked structure for a lithium secondary battery, a crosslinked structure-containing polyolefin porous support having a crosslinked structure in which polymer chains are directly linked to each other; and a photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more.

[0012] According to the second embodiment, in the first embodiment, The photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02V or more may have an oxidation potential value of 4.4V to 8V.

[0013] According to the third embodiment, in the first or second embodiment, The content of the photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more may be 0.015 to 0.36 parts by weight based on 100 parts by weight of the crosslinked structure-containing polyolefin porous support.

[0014] According to a fourth embodiment, in any one of the first to third embodiments, The separator for a lithium secondary battery having a crosslinked structure may further include an inorganic composite porous layer located on at least one surface of the crosslinked structure-containing polyolefin porous support and including an inorganic filler and a binder polymer.

[0015] According to a fifth embodiment, in any one of the first to third embodiments, The separator for a lithium secondary battery having a crosslinked structure is disposed on at least one surface of the crosslinked structure-containing polyolefin porous support, and the inorganic composite porous layer includes an inorganic filler and a first binder polymer; The porous adhesive layer may further include a porous adhesive layer positioned on the inorganic composite porous layer and including a second binder polymer.

[0016] According to the sixth embodiment, in any one of the first to fifth embodiments, The photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of the lithium secondary battery may include thioxanthone (TX), a thioxanthone derivative, benzophenone (BPO), a benzophenone derivative, or two or more thereof.

[0017] According to the seventh embodiment, in any one of the first to sixth embodiments, The cross-linked structure-containing separator for a lithium secondary battery may have a meltdown temperature of 160° C. or higher.

[0018] According to the eighth embodiment, in any one of the first to seventh embodiments, The separator for a lithium secondary battery having a crosslinked structure may have a shutdown temperature of 145° C. or less.

[0019] In order to solve the above problems, according to one aspect of the present invention, there is provided a method for manufacturing a separator having a crosslinked structure for a lithium secondary battery, according to the following embodiment.

[0020] The ninth embodiment is: A method for producing a separator containing a crosslinked structure for a lithium secondary battery, comprising: preparing a polyolefin porous support containing a photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more; and irradiating the polyolefin porous support with ultraviolet light.

[0021] According to the tenth embodiment, in the ninth embodiment, preparing a polyolefin porous support containing a photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more; The method may include coating an outer surface of the polyolefin porous support with a photoinitiator composition including a photoinitiator having an oxidation potential value higher than a full charge voltage of the lithium secondary battery by 0.02 V or more and a solvent, and drying the composition.

[0022] According to the eleventh embodiment, in the tenth embodiment, The photoinitiator composition may be a slurry for forming an inorganic composite porous layer, including an inorganic filler, a binder polymer, a photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more, and the solvent.

[0023] According to the twelfth embodiment, in the tenth embodiment, a step of coating a photoinitiator composition including a photoinitiator having an oxidation potential value higher than a full charge voltage of the lithium secondary battery by 0.02 V or more and a solvent on the outside of the polyolefin porous support and drying the composition; forming an inorganic composite porous layer by coating an inorganic composite porous layer-forming slurry containing an inorganic filler, a first binder polymer, and a dispersion medium on at least one surface of the polyolefin porous support and drying the coating; and coating a coating liquid for forming a porous adhesive layer, the coating liquid including a second binder polymer, a photoinitiator having an oxidation potential value higher than a full charge voltage of the lithium secondary battery by 0.02 V or more, and the solvent, on the upper surface of the inorganic composite porous layer, and drying the coating liquid.

[0024] According to the thirteenth embodiment, in any one of the ninth to twelfth embodiments, The photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02V or more may have an oxidation potential value of 4.4V to 8V.

[0025] According to the fourteenth embodiment, in any one of the ninth to thirteenth embodiments, The content of the photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more may be 0.015 to 0.36 parts by weight based on 100 parts by weight of the polyolefin porous support.

[0026] According to the fifteenth embodiment, in any one of the ninth to fourteenth embodiments, The photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of the lithium secondary battery may include thioxanthone (TX), a thioxanthone derivative, benzophenone (BPO), a benzophenone derivative, or two or more thereof.

[0027] According to the 16th embodiment, in any one of the 9th to 15th embodiments, The irradiation dose of the ultraviolet light is 10 to 2000 mJ / cm 2 It could be.

[0028] In order to solve the above problems, according to one aspect of the present invention, there is provided a lithium secondary battery according to the following embodiment.

[0029] The seventeenth embodiment is The lithium secondary battery includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, The present invention relates to a lithium secondary battery, wherein the separator for the lithium secondary battery is a separator for the lithium secondary battery containing a crosslinked structure according to any one of the first to eighth embodiments. [Effects of the Invention]

[0030] The separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention is electrochemically stable even when it contains a photoinitiator.

[0031] The separator for a lithium secondary battery according to one embodiment of the present invention has excellent heat resistance by including a porous support body having a crosslinked structure in which polymer chains are directly linked to each other.

[0032] In accordance with one aspect of the present invention, a method for manufacturing a separator having a crosslinked structure for a lithium secondary battery can effectively crosslink a polyolefin porous support using a photoinitiator having an oxidation potential value that is at least 0.02 V higher than the full charge voltage of the lithium secondary battery.

[0033] The separator for a lithium secondary battery having a crosslinked structure according to one embodiment of the present invention has a meltdown temperature of 160°C or higher, and therefore has excellent safety at high temperatures. A lithium secondary battery having such a separator having a crosslinked structure can improve the problem of capacity reduction after high-temperature storage even when a photoinitiator is included, and can exhibit capacity characteristics equivalent to or better than those of conventional non-crosslinked separators.

[0034] The following drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the detailed description of the invention, serve to further understand the technical concept of the present invention, so the present invention should not be interpreted as being limited to the matters described in the drawings. [Brief explanation of the drawings]

[0035] [Figure 1]1 is a schematic view illustrating a separator having a crosslinked structure for a lithium secondary battery according to an embodiment of the present invention. [Figure 2] 1 is a schematic view illustrating a separator having a cross-linked structure for a lithium secondary battery according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0036] Hereinafter, preferred embodiments of the present invention will be described in detail. Prior to this, the terms and phrases used in the specification and claims should not be construed as being limited to their ordinary and dictionary meanings, but should be construed as having meanings and concepts corresponding to the technical ideas of the present invention, in accordance with the principle that the inventors themselves can appropriately define the concepts of terms in order to best describe the invention.

[0037] Therefore, it should be understood that the embodiments described in this specification and the configurations shown in the drawings are merely the most desirable embodiments of the present invention and do not represent the entire technical idea of ​​the present invention, and that there may be various equivalents and modifications that can be substituted therefor at the time of this application.

[0038] In this specification, terms such as "first" and "second" are used to distinguish one component from another, and each component is not limited by these terms.

[0039] According to one embodiment of the present invention, a separator containing a crosslinked structure for a lithium secondary battery is a crosslinked structure-containing polyolefin porous support having a crosslinked structure in which polymer chains are directly linked to each other; The photoinitiator has an oxidation potential value that is 0.02 V or more higher than the fully charged voltage of the lithium secondary battery.

[0040] As used herein, the term "crosslinked structure in which polymer chains are directly linked" refers to a state in which polymer chains consisting essentially of polyolefin, more preferably polymer chains consisting only of polyolefin, become reactive with the addition of a photoinitiator, and the polymer chains are directly crosslinked with each other. Therefore, a crosslinking reaction between crosslinkers caused by the addition of an additional crosslinking agent does not fall under the "crosslinked structure in which polymer chains are directly linked" referred to in the present invention. Furthermore, a crosslinking reaction between an additional crosslinking agent and a polymer chain does not fall under the "crosslinked structure in which polymer chains are directly linked" referred to in the present invention, even if the polymer chains are substantially composed of polyolefin or consist only of polyolefin.

[0041] In the process of crosslinking between polymer chains, photoinitiators may be crosslinked with each other or the photoinitiator may be crosslinked with the polymer chains, but such crosslinked structures have a lower reaction enthalpy than crosslinked structures between polymer chains within the polyolefin porous support, and therefore may decompose and cause side reactions during charge and discharge of the battery. In one embodiment of the present invention, the crosslinked structure-containing polyolefin porous support may not include a crosslinked structure in which a photoinitiator is directly linked to a polymer chain, but may include only a crosslinked structure in which polymer chains are directly linked to each other.

[0042] In one embodiment of the present invention, the crosslinked structure-containing polyolefin porous support contains only crosslinked structures in which polymer chains are directly linked to each other, and does not contain crosslinked structures in which a photoinitiator is directly linked to a polymer chain.

[0043] The separator for a lithium secondary battery according to one embodiment of the present invention includes a porous support body having a crosslinked structure, in which polymer chains are directly connected to each other, thereby improving heat resistance.

[0044] In one embodiment of the present invention, the polyolefin porous support may be a porous film.

[0045] In one embodiment of the present invention, the polyolefin may include polyethylene; polypropylene; polybutylene; polypentene; polyhexene; polyoctene; a copolymer of two or more of ethylene, propylene, butene, pentene, 4-methylpentene, hexene, and octene; or a mixture thereof.

[0046] Non-limiting examples of the polyethylene include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), etc. When the polyethylene is a high-density polyethylene having a high crystallinity and a high melting point of the resin, it is easy to increase the modulus while maintaining a desired level of heat resistance.

[0047] In one embodiment of the present invention, the weight-average molecular weight of the polyolefin may be 200,000 to 1,500,000, 220,000 to 1,000,000, or 250,000 to 800,000. When the weight-average molecular weight of the polyolefin is within the above range, the uniformity of the polyolefin porous support and the membrane formation processability are ensured, and ultimately a separation membrane with excellent strength and heat resistance can be obtained.

[0048] The weight average molecular weight can be measured using gel permeation chromatography (GPC, PL GPC220, manufactured by Agilent Technologies) under the following conditions. -Column: PL Olexis (Polymer Laboratories) -Solvent: TCB (trichlorobenzene) -Flow rate: 1.0ml / min -Sample concentration: 1.0mg / ml -Injection volume: 200μl -Column temperature: 160℃ -Detector: Agilent high temperature RI detector -Standard: Polystyrene (corrected by a cubic function)

[0049] In one embodiment of the present invention, the degree of crosslinking of the crosslinked structure-containing polyolefin porous support may be 10% to 45%, 15% to 40%, or 20% to 35%. When the crosslinked structure-containing polyolefin porous support satisfies the above-mentioned range of crosslinking degree, the modulus is easily increased while maintaining a desired level of heat resistance. For example, when the crosslinked structure-containing polyolefin porous support has a degree of crosslinking of 20% or more, the meltdown temperature of the crosslinked structure-containing polyolefin porous support is likely to be 170°C or higher.

[0050] In this case, the degree of crosslinking is determined by immersing a crosslinked structure-containing polyolefin porous support in a xylene solution at 135°C and boiling it for 12 hours according to ASTM D2765, measuring the residual weight, and calculating the percentage of the residual weight relative to the initial weight.

[0051] In one embodiment of the present invention, the crosslinked structure-containing polyolefin porous support may have a polyolefin chain with 0.01 to 0.6 or 0.02 to 0.5 double bonds per 1000 carbon atoms as measured by H-NMR. When the crosslinked structure-containing polyolefin porous support has the above-mentioned number of double bonds, the problem of battery performance degradation at high temperatures and / or high voltages can be minimized.

[0052] In one embodiment of the present invention, the number of double bonds present in the polyolefin chain excluding the terminals of the crosslinked structure-containing polyolefin porous support may be 0.005 to 0.59 per 1,000 carbon atoms. "Double bonds present in the polyolefin chain excluding the terminals" refers to double bonds present throughout the polyolefin chain excluding the terminals of the polyolefin chain. Here, "terminals" refers to the positions of the carbon atoms connected to both ends of the polyolefin chain.

[0053] In one embodiment of the present invention, the thickness of the crosslinked structure-containing polyolefin porous support may be 3 μm to 16 μm, or 5 μm to 12 μm. When the thickness of the crosslinked structure-containing polyolefin porous support is within the above range, the problem of the separator being easily damaged during use of the battery can be prevented and the energy density can be easily ensured.

[0054] The photoinitiator has an oxidation potential value that is at least 0.02 V higher than the full charge voltage of the lithium secondary battery. Since the oxidation potential value of the photoinitiator is at least 0.02 V higher than the full charge voltage of the lithium secondary battery, it is possible to prevent the photoinitiator from being oxidized and causing a side reaction even after the lithium secondary battery is fully charged.

[0055] A separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention includes a photoinitiator that is electrochemically stable and has an oxidation potential value that is 0.02 V or more higher than the full charge voltage of the lithium secondary battery. Therefore, after a lithium secondary battery including the separator is fully charged, an oxidation reaction of the photoinitiator due to the electrochemical environment within the battery does not occur, thereby preventing a deterioration in battery performance.

[0056] In this specification, the "full charge voltage of a lithium secondary battery" refers to the voltage when the battery is 100% charged. In one embodiment of the present invention, the full charge voltage of the lithium secondary battery may be, for example, 4.2 V or more, or 4.2 V to 4.7 V.

[0057] If the oxidation potential value of the photoinitiator contained in the separator is lower than the full charge voltage of the lithium secondary battery, when the lithium secondary battery is fully charged, the photoinitiator is oxidized due to the electrochemical environment within the battery, causing a side reaction.

[0058] In addition, if the oxidation potential value of the photoinitiator contained in the separator is higher than the full charge voltage of the lithium secondary battery but is higher by less than 0.02 V, the base potential of the negative electrode is 0.02 V, so when the lithium secondary battery is fully charged, the photoinitiator will still be oxidized due to the electrochemical environment within the battery, causing a side reaction.

[0059] In one embodiment of the present invention, the oxidation potential of the photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of the lithium secondary battery may be 4.38 V to 8.0 V, 4.4 to 7.5 V, or 4.4 to 7.0 V. When the oxidation potential value of the photoinitiator satisfies the above range, the oxidation potential value of the photoinitiator significantly exceeds the full charge voltage of the lithium secondary battery, and it is possible to more easily prevent a lithium secondary battery including a separator containing the photoinitiator from deteriorating in battery performance after being fully charged.

[0060] In one embodiment of the present invention, the content of the photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of the lithium secondary battery may be 0.015 to 0.36 parts by weight, 0.015 to 0.09 parts by weight, or 0.015 to 0.07 parts by weight, relative to 100 parts by weight of the crosslinked structure-containing polyolefin porous support. When the content of the photoinitiator is within the above range, the occurrence of side reactions can be more easily prevented. In addition, an excessive increase in resistance can be easily prevented, and when the photoinitiator is dissolved in an electrolyte, an excessive increase in the viscosity of the electrolyte can be easily prevented.

[0061] The content of the photoinitiator having an oxidation potential higher by 0.02 V or more than the full charge voltage of the lithium secondary battery per 100 parts by weight of the polyolefin porous support can be determined by measuring the content of the photoinitiator having an oxidation potential higher by 0.02 V or more than the full charge voltage of the lithium secondary battery that is filled in the entire pore volume of the polyolefin porous support. For example, assuming that the entire pore volume of the polyolefin porous support is filled 100% with a solvent (described below) and that no solvent is present on the surface of the polyolefin porous support, the weight of the solvent contained in the entire pore volume of the polyolefin porous support can be determined from the density of the solvent, and the content of the photoinitiator (having an oxidation potential higher by 0.02 V or more than the full charge voltage of the lithium secondary battery) per 100 parts by weight of the polyolefin porous support can be determined from the content of the photoinitiator (having an oxidation potential higher by 0.02 V or more than the full charge voltage of the lithium secondary battery) contained in the solvent.

[0062] In one embodiment of the present invention, the photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of the lithium secondary battery may include thioxanthone (TX), a thioxanthone derivative, benzophenone (BPO), a benzophenone derivative, or two or more thereof.

[0063] Examples of the thioxanthone derivatives include 2-isopropyl thioxanthone (ITX), 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-phenylsulfurylthioxanthone, 3,4-di[2-(2-methoxyethoxy)ethoxycarbonyl]thioxanthone, 1-ethoxycarbonyl-3-(1-methyl-1-morpholino-ethyl)-thio Xanthone, 2-methyl-6-dimethoxymethyl-thioxanthone, 2-methyl-6-(1,1-dimethoxy-benzyl)-thioxanthone, 2-morpholinomethylthioxanthone, 2-methyl-6-morpholinomethyl-thioxanthone, N-allylthioxanthone-3,4-dicarboximide, N-octylthioxanthone-3,4-dicarboximide, N-(1,1,3,3-tetramethylbutyl)-thioxanthone-3,4- dicarboximide, 1-penoxythioxanthone, 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-propanaminium chloride, and the like.

[0064] Examples of the benzophenone derivatives include 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-benzoylbenzoate, 4-(2-hydroxyethylthio)benzophenone, and the like. The benzophenone may include, but is not limited to, 4-(4-tolylthio)benzophenone, 4-benzoyl-N,N,N-trimethylbenzenemethanaminium chloride, 2-hydroxy-3-(4-benzoylphenoxy)-N,N,N-trimethyl-propanaminium chloride monohydrate, 4-hydroxybenzophenone, 4-(13-acryloyl-1,4,7,10,13-pentaoxatridecyl)-benzophenone, 4-benzoyl-N,N-dimethyl-N-[2-(1-oxo-2-propenyl)oxy]ethyl-benzenemethanaminium chloride, and the like.

[0065] A separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention may include a porous support having a crosslinked structure in which polymer chains are directly connected to each other, and a photoinitiator having an oxidation potential value that is 0.02 V or more higher than the full charge voltage of the lithium secondary battery.

[0066] According to another embodiment of the present invention, a separator having a crosslinked structure for a lithium secondary battery may further include an inorganic composite porous layer, which is located on at least one surface of the crosslinked structure-containing polyolefin porous support and includes an inorganic filler and a binder polymer, as shown in FIG.

[0067] Referring to FIG. 1, a separator 1 having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention may include a photoinitiator (not shown) having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more, a crosslinked polyolefin porous support 10, and an inorganic composite porous layer 20 located on at least one surface of the crosslinked polyolefin porous support 10 and including an inorganic filler and a binder polymer.

[0068] The inorganic composite porous layer 20 may be formed on one or both sides of the crosslinked polyolefin porous support 10. The inorganic composite porous layer 20 includes an inorganic filler and a binder polymer that bonds the inorganic fillers together (i.e., the binder polymer connects and fixes the inorganic fillers) so that the inorganic fillers remain bound to each other. The binder polymer maintains the binding between the inorganic filler and the crosslinked polyolefin porous support 10. The inorganic filler in the inorganic composite porous layer 20 prevents the crosslinked polyolefin porous support 10 from exhibiting significant thermal shrinkage at high temperatures, thereby improving the safety of the separator. For example, the thermal shrinkage of the separator in the machine direction (MD) and transverse direction (TD) measured after leaving it at 120°C for 30 minutes may be 20% or less, 2% to 15%, or 2% to 10%, respectively.

[0069] In this specification, the term "machine direction (MD)" refers to the direction in which a separation membrane advances when it is continuously produced, and refers to the longitudinal direction of the separation membrane, and the term "transverse direction (TD)" refers to the direction transverse to the machine direction, i.e., the direction perpendicular to the direction in which a separation membrane advances when it is continuously produced, and refers to the direction perpendicular to the longitudinal direction of the separation membrane.

[0070] The inorganic filler is not particularly limited as long as it is electrochemically stable. That is, the inorganic filler that can be used in the present invention is selected from the group consisting of inorganic fillers that are suitable for use in the electrochemical device, and ... +The inorganic filler is not particularly limited as long as it does not undergo oxidation and / or reduction reactions at a voltage of 0 to 5 V relative to the reference voltage. In particular, when inorganic particles with a high dielectric constant are used as the inorganic filler, they can contribute to increasing the degree of dissociation of electrolyte salts, such as lithium salts, in the liquid electrolyte, thereby improving the ionic conductivity of the electrolyte solution.

[0071] For the above reasons, in one embodiment of the present invention, the inorganic filler may include a high dielectric constant inorganic filler having a dielectric constant of 5 or more, preferably 10 or more. Non-limiting examples of inorganic fillers having a dielectric constant of 5 or more include BaTiO3, Pb(Zr,Ti)O3 (PZT), Pb 1-x La x Zr 1-y Ti y O3(PLZT, 0 <x<1、0<y<1)、Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3 (PMN-PT), hafnia (HfO2), SrTiO3, SnO2, CeO2, MgO, Mg(OH)2, NiO, CaO, ZnO, ZrO2, SiO2, Y2O3, Al2O3, AlOOH, Al(OH)3, SiC, TiO2, or mixtures thereof.

[0072] In another embodiment of the present invention, the inorganic filler may be an inorganic filler having lithium ion transport ability, i.e., an inorganic filler that contains lithium element but does not store lithium but has the function of transporting lithium ions. Non-limiting examples of inorganic fillers having lithium ion transport ability include lithium phosphate (LiPO), lithium titanium phosphate (Li x Ti y (PO4)3,0 <x<2、0<y<3)、リチウムアルミニウムチタンホスフェート(Li x Al y Ti z (PO4)3,0 <x<2、0<y<1、0<z<3)、14Li2O-9Al2O3-38TiO2-39P2O5などのような(LiAlTiP) x O y Glass (0 <x<4、0<y<13)、リチウムランタンチタネート(Lix La y TiO3, where 0 < x < 2, 0 < y < 3), Li 3.25 Ge 0.25 P 0.75 lithium germanium thiophosphate such as Li x Ge y P z S w , where 0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5), lithium nitride such as Li3N x N y , where 0 < x < 4, 0 < y < 2), SiS2-based glass such as Li3PO4-Li2S-SiS2 x Si y S z , where 0 < x < 3, 0 < y < 2, 0 < z < 4), P2S5-based glass such as LiI-Li2S-P2S5 x P y S z , where 0 < x < 3, 0 < y < 3, 0 < z < 7), or mixtures thereof, etc. can be mentioned.

[0073] In one embodiment of the present invention, the average particle size of the inorganic filler can be 0.01 μm to 1.5 μm. When the average particle size of the inorganic filler satisfies the above-mentioned range, it is easy to form an inorganic composite void layer having a uniform thickness and appropriate porosity, the dispersibility of the inorganic filler is good, and a desired energy density can be achieved.

[0074] At this time, the average particle size of the inorganic filler means D 50 particle size, and "D 50 particle size" means the particle size at the 50% point of the particle number cumulative distribution according to the particle size. The particle size can be measured using the laser diffraction method. Specifically, after dispersing the measurement target powder in a dispersion medium, it is introduced into a commercially available laser diffraction particle size measuring device (for example, Microtrac S3500). When the particles pass through the laser beam, the difference in the diffraction pattern according to the particle size is measured to calculate the particle size distribution. By calculating the particle diameter at the point where the particle number cumulative distribution according to the particle size in the measuring device reaches 50%, D50 The particle size may be measured.

[0075] The binder polymer has a glass transition temperature (T g ) may be -200 to 200°C. When the glass transition temperature of the binder polymer satisfies the above range, the mechanical properties such as flexibility and elasticity of the finally formed inorganic composite porous layer may be improved. The binder polymer may have ion conductivity. When the binder polymer has ion conductivity, the battery performance may be further improved. The binder polymer may have a dielectric constant of 1.0 to 100 (measurement frequency = 1 kHz) or 10 to 100. When the dielectric constant of the binder polymer satisfies the above range, the degree of dissociation of salt in the electrolyte may be improved.

[0076] In one embodiment of the present invention, the binder polymer may include polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-chlorotrifluoroethylene, polyvinylidene fluoride-tetrafluoroethylene, polyvinylidene fluoride-trichloroethylene, an acrylic copolymer, a styrene-butadiene copolymer, polyacrylic acid, polymethyl methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl alcohol, polyvinyl acetate, an ethylene-vinyl acetate copolymer, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, or two or more thereof.

[0077] The acrylic copolymer may include, but is 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.

[0078] In one embodiment of the present invention, the weight ratio of the inorganic filler to the binder polymer is determined taking into consideration the thickness, pore size, and porosity of the inorganic composite porous layer 20 to be finally produced, and may be 50:50 to 99.9:0.1, or 60:40 to 99.5:0.5. When the weight ratio of the inorganic filler to the binder polymer is within the above range, sufficient void space is formed between the inorganic fillers, making it easy to ensure the pore size and porosity of the inorganic composite porous layer 20. In addition, the adhesive strength between the inorganic fillers can be easily ensured.

[0079] In one embodiment of the present invention, the inorganic composite porous layer 20 may further include additives such as a dispersant and / or a thickener, such as polyvinylpyrrolidone (PVP), hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), ethylhydroxyethyl cellulose (EHEC), methyl cellulose (MC), carboxymethyl cellulose (CMC), hydroxyalkyl methyl cellulose, cyanoethylene polyvinyl alcohol, or two or more thereof.

[0080] In one embodiment of the present invention, the inorganic composite porous layer 20 may have a structure in which the inorganic fillers are filled and bound together by the binder polymer while in contact with each other, thereby forming interstitial volumes between the inorganic fillers, and the interstitial volumes between the inorganic fillers become empty spaces to form pores.

[0081] In another embodiment of the present invention, the inorganic composite porous layer 20 includes a plurality of nodes including the inorganic filler and a binder polymer that coats at least a portion of the surface of the inorganic filler, and one or more filaments formed in a thread shape from the binder polymer of the nodes, the filaments having node-connecting portions that extend from the nodes and connect other nodes, and the node-connecting portions may have a structure in which a plurality of filaments derived from the binder polymer cross each other to form a three-dimensional network structure.

[0082] In one embodiment of the present invention, the inorganic composite porous layer 20 may have an average pore size of 0.001 μm to 10 μm. The average pore size of the inorganic composite porous layer 20 may be measured by capillary flow porometry, which measures the diameter of the smallest pore in the thickness direction. Therefore, to measure the average pore size of the inorganic composite porous layer 20 alone by capillary flow porometry, the inorganic composite porous layer 20 must be separated from the crosslinked polyolefin porous support 10 and wrapped in a supportable nonwoven fabric before measurement. In this case, the pore size of the nonwoven fabric must be much larger than the pore size of the inorganic composite porous layer 20.

[0083] In one embodiment of the present invention, the porosity of the inorganic composite porous layer 20 may be 5% to 95%, 10% to 95%, 20% to 90%, or 30% to 80%. The porosity corresponds to a value obtained by subtracting the volume calculated from the weight and density of each component of the inorganic composite porous layer 20 from the volume calculated from the thickness, width, and length of the inorganic composite porous layer 20.

[0084] The porosity of the inorganic composite porous layer 20 can be measured by a BET 6-point method using a scanning electron microscope (SEM) image, a mercury porosimeter, or a porosimetry analyzer (Bell Japan, Belsorp-II mini) with nitrogen gas adsorption flow.

[0085] In one embodiment of the present invention, the inorganic composite porous layer 20 may have a thickness of 1.5 μm to 5.0 μm on one surface of the crosslinked structure-containing polyolefin porous support 10. When the thickness of the inorganic composite porous layer 20 satisfies the above range, the adhesive strength with the electrode can be excellent and the cell strength of the battery can be easily increased.

[0086] According to another embodiment of the present invention, a separator having a crosslinked structure for a lithium secondary battery may further include an inorganic composite porous layer disposed on at least one surface of the crosslinked structure-containing polyolefin porous support and including an inorganic filler and a first binder polymer, and a porous adhesive layer disposed on the inorganic composite porous layer and including a second binder polymer, as shown in FIG.

[0087] Referring to FIG. 2, a separator 1′ having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention may include a photoinitiator (not shown) having an oxidation potential value 0.02 V or more higher than the full charge voltage of the lithium secondary battery; a polyolefin porous support 10′ having a crosslinked structure in which polymer chains are directly connected to each other; an inorganic composite porous layer 20′ disposed on at least one surface of the polyolefin porous support 10′ having a crosslinked structure and including an inorganic filler and a first binder polymer; and a porous adhesive layer 30′ disposed on the inorganic composite porous layer 20′ and including a second binder polymer.

[0088] The inorganic composite porous layer 20' may be formed on one or both sides of the crosslinked polyolefin porous support 10'. The inorganic composite porous layer 20' includes inorganic fillers and a first binder polymer that bonds the inorganic fillers together (i.e., the first binder polymer connects and fixes the inorganic fillers) so that the inorganic fillers remain bound to each other. The first binder polymer maintains the binding between the inorganic fillers and the crosslinked polyolefin porous support 10'. The inorganic filler in the inorganic composite porous layer 20' prevents the crosslinked polyolefin porous support 10' from exhibiting significant thermal shrinkage at high temperatures, thereby improving the safety of the separator. For example, the thermal shrinkage of the separator in the machine direction (MD) and transverse direction (TD) measured after leaving it at 150°C for 30 minutes may be 20% or less, 2% to 15%, or 2% to 10%, respectively.

[0089] For the inorganic filler, please refer to the above content.

[0090] The first binder polymer has a glass transition temperature (T g ) may be -200 to 200°C. When the glass transition temperature of the first binder polymer satisfies the above range, the mechanical properties such as flexibility and elasticity of the finally formed inorganic composite porous layer may be improved. The first binder polymer may have ion conductivity. When a binder polymer having ion conductivity is used as the first binder polymer, the performance of the battery may be further improved. The first binder polymer may have a dielectric constant of 1.0 to 100 (measurement frequency = 1 kHz) or 10 to 100. When the dielectric constant of the first binder polymer satisfies the above range, the degree of dissociation of salt in the electrolyte may be improved.

[0091] In one embodiment of the present invention, the first binder polymer may be a binder polymer with excellent heat resistance. When the first binder polymer has excellent heat resistance, the heat resistance of the inorganic composite porous layer may be further improved. For example, the separator may have a thermal shrinkage of 20% or less, 2% to 15%, 2% to 10%, 2% to 5%, 0% to 5%, or 0% to 2% in the machine direction (MD) and transverse direction (TD), respectively, measured after being left at 150°C for 30 minutes. In one embodiment of the present invention, the first binder polymer may include an acrylic polymer, polyacrylic acid, styrene-butadiene rubber, polyvinyl alcohol, or two or more thereof.

[0092] Specifically, the acrylic polymer may include an acrylic homopolymer obtained by polymerizing only an acrylic monomer, or a copolymer of an acrylic monomer with another monomer, such as an ethylhexyl acrylate-methyl methacrylate copolymer, polymethyl methacrylate, polyethylhexyl acrylate, polybutyl acrylate, polyacrylonitrile, or a butyl acrylate-methyl methacrylate copolymer, or two or more thereof.

[0093] In one embodiment of the present invention, the first binder polymer may be in the form of particles.

[0094] In one embodiment of the present invention, the weight ratio of the inorganic filler to the first binder polymer may be 95:5 to 99.9:0.1, 96:4 to 99.5:0.5, or 97:3 to 99:1. When the weight ratio of the inorganic filler to the first binder polymer is within the above range, the content of the inorganic filler distributed per unit area of ​​the separator increases, thereby improving the thermal stability of the separator at high temperatures. For example, the thermal shrinkage of the separator in the machine direction (MD) and transverse direction (TD) measured after leaving the separator at 150°C for 30 minutes may be 20% or less, 2% to 15%, 2% to 10%, 2% to 5%, 0% to 5%, or 0% to 2%, respectively. Furthermore, sufficient adhesive strength between the inorganic fillers can be ensured while sufficient void space can be formed between the inorganic fillers.

[0095] The following describes the characteristics of the inorganic composite porous layer 20' that differ from the inorganic composite porous layer 20 described above.

[0096] In one embodiment of the present invention, the inorganic composite porous layer 20' may have a structure in which the inorganic fillers are filled and in contact with each other and bound together by the first binder polymer, thereby forming interstitial volumes between the inorganic fillers, and the interstitial volumes between the inorganic fillers become empty spaces to form pores.

[0097] The porous adhesive layer 30' contains a second binder polymer, thereby ensuring adhesive strength between the separator including the inorganic composite porous layer 20' and the electrode. In addition, the porous adhesive layer 30' has pores formed therein, which prevents the separator from becoming too resistive.

[0098] In one embodiment of the present invention, the porous adhesive layer 30' can minimize the phenomenon of increased resistance of the separator because the second binder polymer does not penetrate into the surface and / or interior of the cross-linked structure-containing polyolefin porous support 10'.

[0099] The second binder polymer may be a binder polymer that is commonly used to form an adhesive layer. The second binder polymer may have a glass transition temperature (T g) may be -200°C to 200°C. When the glass transition temperature of the second binder polymer satisfies the above range, the mechanical properties such as flexibility and elasticity of the finally formed porous adhesive layer 30' may be improved. The second binder polymer may have ion conductivity. When a binder polymer having ion conductivity is used as the second binder polymer, the performance of the battery may be further improved. The second binder polymer may have a dielectric constant of 1.0 to 100 (measurement frequency = 1 kHz) or 10 to 100. When the dielectric constant of the second binder polymer satisfies the above range, the degree of dissociation of salt in the electrolyte may be improved.

[0100] In one embodiment of the present invention, the second binder polymer may include polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene, polyvinylidene fluoride-tetrafluoroethylene, polyvinylidene fluoride-trifluoroethylene, polymethyl methacrylate, polyethylhexyl acrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, ethylhexyl acrylate-methyl methacrylate copolymer, ethylene-vinyl acetate copolymer, polyethylene oxide, polyarylate, or two or more thereof.

[0101] In one embodiment of the present invention, the porous adhesive layer 30' may have a pattern including one or more adhesive regions containing the second binder polymer and one or more non-coating regions where the adhesive regions are not formed. The pattern may be a dotted, striped, diagonal, wavy, triangular, rectangular, or semicircular shape. When the porous adhesive layer 30' has a pattern, the resistance of the separator is improved, and the electrolyte can be impregnated through the non-coating regions where the porous adhesive layer 30' is not formed, thereby improving the electrolyte impregnation of the separator.

[0102] In one embodiment of the present invention, the thickness of the porous adhesive layer 30' may be 0.5 μm to 1.5 μm, 0.6 μm to 1.2 μm, or 0.6 μm to 1.0 μm. When the thickness of the porous adhesive layer 30' is within the above range, the adhesive strength with the electrode is excellent, thereby increasing the cell strength of the battery. This is also advantageous in terms of the cycle characteristics and resistance characteristics of the battery.

[0103] The separator for a lithium secondary battery according to one embodiment of the present invention has excellent high-temperature safety due to the inclusion of a porous support body having a crosslinked structure in which polymer chains are directly connected to each other. For example, the meltdown temperature of the separator for a lithium secondary battery having a crosslinked structure can be increased compared to the meltdown temperature of a conventional separator for a lithium secondary battery before crosslinking. For example, the meltdown temperature of the separator can be 160°C or higher, 170°C or higher, or 180 to 230°C.

[0104] In this specification, "a separator for a lithium secondary battery before crosslinking" refers to a separator made of a polyolefin porous support that is not crosslinked and does not contain a crosslinked structure; a separator comprising a polyolefin porous support that is not crosslinked and does not contain a crosslinked structure, and an inorganic composite porous layer that is located on at least one surface of the polyolefin porous support that is not crosslinked and contains an inorganic filler and a binder polymer; or a separator comprising a polyolefin porous support that is not crosslinked and does not contain a crosslinked structure, an inorganic composite porous layer that is located on at least one surface of the polyolefin porous support that is not crosslinked and contains an inorganic filler and the first binder polymer, and a porous adhesive layer that is located on the inorganic composite porous layer and contains a second binder polymer.

[0105] The meltdown temperature can be measured by thermomechanical analysis (TMA). For example, samples are taken in the machine direction and the transverse direction, and then a 4.8 mm wide x 8 mm long sample is placed in a TMA device (TA Instruments, Q400). Under a tension of 0.01 N, the temperature is increased from 30°C to 220°C at a rate of 5°C / min. The meltdown temperature is measured at the temperature at which the sample suddenly increases in length and breaks.

[0106] The separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention has a smaller increase in shutdown temperature and a smaller rate of change than a conventional separator for a lithium secondary battery before crosslinking. The separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention has a higher meltdown temperature but a smaller increase in shutdown temperature compared to a separator before crosslinking, thereby ensuring overcharge safety due to the shutdown temperature and significantly improving the high-temperature safety of the separator.

[0107] In one embodiment of the present invention, the separator containing a crosslinked structure for a lithium secondary battery may have a shutdown temperature of 145° C. or less, 140° C. or less, or 133° C. to 140° C. When the separator containing a crosslinked structure for a lithium secondary battery has the above-mentioned shutdown temperature, overcharge safety can be ensured and the problem of increased resistance due to damage to the pores of the crosslinked structure-containing polyolefin porous support during the high-temperature and pressure process during battery assembly can be easily prevented.

[0108] The shutdown temperature can be determined by measuring the time (seconds) it takes for 100 cc of air to pass through the separation membrane at a constant pressure of 0.05 MPa when the temperature is raised by 5°C per minute using an Oken air permeability measuring device, and determining the temperature at which the air permeability of the separation membrane increases rapidly.

[0109] A separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention includes a polyolefin porous support having a crosslinked structure in which polymer chains within the polyolefin porous support are directly linked, thereby allowing the pore structure of the polyolefin porous support to be substantially maintained even after crosslinking.

[0110] The separator for a lithium secondary battery having a crosslinked structure according to one embodiment of the present invention does not significantly deteriorate in terms of air permeability, basis weight, tensile strength, tensile elongation, puncture strength, electrical resistance, etc., compared to the separator for a lithium secondary battery before crosslinking, and the rate of change is also small.

[0111] The separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention may have a change in air permeability of 10% or less, 0% to 10%, 0% to 5%, or 0% to 3% compared to the separator for a lithium secondary battery before crosslinking.

[0112] The rate of change in air permeability can be calculated using the following formula: Rate of change in air permeability (%) = [(air permeability of separator containing crosslinked structure for lithium secondary battery after crosslinking) - (air permeability of separator for lithium secondary battery before crosslinking)] / (air permeability of separator for lithium secondary battery before crosslinking) × 100

[0113] Throughout this specification, the term "crosslinked structure-containing separator for a lithium secondary battery after crosslinking" refers to a separator made of a crosslinked structure-containing polyolefin porous support; a separator comprising a crosslinked structure-containing polyolefin porous support and an inorganic composite porous layer located on at least one surface of the crosslinked structure-containing polyolefin porous support and comprising an inorganic filler and a binder polymer; or a separator comprising a crosslinked structure-containing polyolefin porous support, an inorganic composite porous layer located on at least one surface of the crosslinked structure-containing polyolefin porous support and comprising an inorganic filler and a first binder polymer, and a porous adhesive layer located on the upper surface of the inorganic composite porous layer and comprising a second binder polymer.

[0114] The air permeability (Gurley) may be measured by ASTM D726-94 method. As used herein, Gurley is the resistance to air flow, as measured by a Gurley densometer. Air permeability values ​​described herein are calculated by measuring 100 cc of air at a pressure of 12.2 in. H2O through 1 in. of a sample porous support. 2 The time (seconds) required for the air to pass through the cross section is shown as the ventilation time.

[0115] The crosslinked structure-containing separator for a lithium secondary battery according to one embodiment of the present invention may have a basis weight change rate of 5% or less, or 0% to 5%, compared to the separator for a lithium secondary battery before crosslinking.

[0116] The rate of change in basis weight can be calculated using the following formula: Rate of change in basis weight (%)=[(basis weight of separator for lithium secondary battery containing crosslinked structure after crosslinking)−(basis weight of separator for lithium secondary battery before crosslinking)] / (basis weight of separator for lithium secondary battery before crosslinking)×100 The basis weight (g / m 2 ) is measured by preparing a sample that is 1m long and 1m wide, and measuring its weight.

[0117] The separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention may have a change in tensile strength in the machine direction and transverse direction of 20% or less, 0% to 20%, 0% to 10%, 0% to 9%, 0% to 8%, or 0% to 7.53% compared to the separator for a lithium secondary battery before crosslinking.

[0118] The rate of change in tensile strength can be calculated using the following formula: Change rate of tensile strength in the machine direction (%)=[(tensile strength in the machine direction of the separator for lithium secondary batteries before crosslinking)−(tensile strength in the machine direction of the separator for lithium secondary batteries containing a crosslinked structure after crosslinking)] / (tensile strength in the machine direction of the separator for lithium secondary batteries before crosslinking)×100 Change rate of tensile strength in the transverse direction (%)=[(tensile strength in the transverse direction of separator for lithium secondary battery before crosslinking)−(tensile strength in the transverse direction of separator containing a crosslinked structure for lithium secondary battery after crosslinking)] / (tensile strength in the transverse direction of separator for lithium secondary battery before crosslinking)×100

[0119] The tensile strength may be the strength at which the specimen breaks when the specimen is pulled in the machine direction and the transverse direction at a rate of 50 mm / min using Universal Testing Systems (Instron (registered trademark) 3345) in accordance with ASTM D882.

[0120] The separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention may have a change rate of 20% or less, or 0% to 20%, in tensile elongation in the machine direction and transverse direction compared to the separator for a lithium secondary battery before crosslinking.

[0121] The rate of change in tensile elongation can be calculated using the following formula: Change rate of tensile elongation in the machine direction (%) = [(tensile elongation in the machine direction of the separator for lithium secondary batteries before crosslinking) - (tensile elongation in the machine direction of the separator for lithium secondary batteries containing a crosslinked structure after crosslinking)] / (tensile elongation in the machine direction of the separator for lithium secondary batteries before crosslinking) × 100 Change rate of tensile elongation in the transverse direction (%) = [(tensile elongation in the transverse direction of separator for lithium secondary battery before crosslinking) - (tensile elongation in the transverse direction of separator containing crosslinked structure for lithium secondary battery after crosslinking)] / (tensile elongation in the transverse direction of separator for lithium secondary battery before crosslinking) × 100

[0122] The tensile elongation was measured in accordance with ASTM D882 by measuring the maximum length of the specimen stretched in both the machine direction and the transverse direction at a rate of 50 mm / min using Universal Testing Systems (Instron® 3345) until the specimen broke, and calculated using the following formula: Tensile elongation in the machine direction (%) = (length in the machine direction of the specimen just before breakage - length in the machine direction of the specimen before stretching) / (length in the machine direction of the specimen before stretching) × 100 Tensile elongation in the transverse direction (%) = (transverse length of specimen just before breakage - transverse length of specimen before stretching) / (transverse length of specimen before stretching) × 100

[0123] The separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention may have a change in puncture strength of 10% or less, 0.5% to 10%, 1% to 9%, or 1.18% to 8.71% compared to the separator for a lithium secondary battery before crosslinking.

[0124] The rate of change in puncture strength can be calculated using the following formula: Change in puncture strength (%)=[(puncture strength of separator for lithium secondary battery before crosslinking)−(puncture strength of separator for lithium secondary battery containing crosslinked structure after crosslinking)] / (puncture strength of separator for lithium secondary battery before crosslinking)×100

[0125] The puncture strength can be measured in accordance with ASTM D2582. Specifically, after setting a 1 mm round tip to operate at a speed of 120 mm / min, the puncture strength can be measured in accordance with ASTM D2582.

[0126] The separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention may have a change in electrical resistance of 15% or less, 2% to 10%, or 2% to 5% compared to the separator for a lithium secondary battery before crosslinking.

[0127] The rate of change in electrical resistance can be calculated using the following formula: Rate of change in electrical resistance (%) = [(electrical resistance of separator containing crosslinked structure for lithium secondary battery after crosslinking) - (electrical resistance of separator for lithium secondary battery before crosslinking)] / (electrical resistance of separator for lithium secondary battery before crosslinking) × 100

[0128] The electrical resistance can be determined by leaving a coin cell containing a separator sample at room temperature for one day and then measuring the resistance of the separator using an impedance measurement method.

[0129] The separator having a crosslinked structure for a lithium secondary battery according to an embodiment of the present invention may be prepared by the following method, but is not limited thereto.

[0130] According to one embodiment of the present invention, a method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery includes: preparing a polyolefin porous support containing a photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more; and irradiating the polyolefin porous support with ultraviolet light.

[0131] Hereinafter, a method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery according to an embodiment of the present invention will be described focusing on the main points.

[0132] First, a polyolefin porous support is prepared, containing a photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of a lithium secondary battery. The photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of the lithium secondary battery is introduced onto the surface of the polyolefin porous support, and the polyolefin porous support can be crosslinked upon irradiation with ultraviolet light. Here, the "surface of the polyolefin porous support" can include not only the surface of the outermost layer of the polyolefin porous support, but also the surfaces of the pores present inside the polyolefin porous support.

[0133] A photoinitiator having an oxidation potential at least 0.02 V higher than the full charge voltage of the lithium secondary battery directly photocrosslinks polymer chains in the polyolefin porous support. A photoinitiator having an oxidation potential at least 0.02 V higher than the full charge voltage of the lithium secondary battery can crosslink the polyolefin porous support alone, without the need for other components such as a crosslinker, coinitiator, or synergist. Upon light absorption alone, hydrogen atoms within the photoinitiator are removed through a hydrogen abstraction reaction, turning the photoinitiator into a reactive compound. This photoinitiator then forms radicals on the polymer chains in the polyolefin porous support, making the polymer chains reactive and resulting in photocrosslinking by directly linking the polymer chains to each other. For example, a small amount of double bond or branched structure present in the polyolefin can undergo a hydrogen abstraction reaction with a photoinitiator having an oxidation potential at least 0.02 V higher than the full charge voltage of the lithium secondary battery. Therefore, upon light absorption alone, hydrogen atoms can be abstracted from the double bond or branched structure in the polyolefin to form radicals on the polymer chain.

[0134] In a method for preparing a separator having a crosslinked structure for a lithium secondary battery according to an embodiment of the present invention, a photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more can be used to generate radicals in polymer chains in a polyolefin porous support, thereby forming a crosslinked structure in which polymer chains are directly linked to each other.

[0135] For the photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more, please refer to the above content.

[0136] When conventional photoinitiators are used to UV-crosslink a polyolefin porous support, some of the photoinitiator may not be completely removed after crosslinking of the polyolefin porous support, leaving the separator. This can lead to side reactions caused by oxidation / reduction in the electrochemical environment within the battery. Therefore, a process of removing the photoinitiator is required after crosslinking the polyolefin porous support.

[0137] A method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention uses an electrochemically stable photoinitiator having an oxidation potential at least 0.02 V higher than the full charge voltage of the lithium secondary battery. This prevents the photoinitiator from oxidizing and causing a side reaction even if it remains in the separator after photo-crosslinking the polyolefin porous support. This eliminates the need for a process for removing the photoinitiator after crosslinking the polyolefin porous support, further simplifying the process. In addition, the polymer chains in the polyolefin porous support can be directly photo-crosslinked, preventing the problem of battery performance degradation due to remaining photoinitiator.

[0138] In one embodiment of the present invention, the content of the photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of the lithium secondary battery may be 0.015 to 0.36 parts by weight, 0.015 to 0.09 parts by weight, or 0.015 to 0.07 parts by weight, relative to 100 parts by weight of the polyolefin porous support. When the content of the photoinitiator is within the above range, side reactions can be more easily prevented. For example, crosslinking occurs only between polymer chains, without crosslinking between photoinitiators or between the photoinitiator and the polymer chain. In addition, excessive radical generation leading to rapid crosslinking reactions, which could result in shrinkage of the separator, or excessive polyolefin main chain scission, which could result in a decrease in the mechanical strength of the polyolefin porous support, can be prevented.

[0139] The content of the photoinitiator having an oxidation potential higher by 0.02 V or more than the full charge voltage of a lithium secondary battery per 100 parts by weight of the polyolefin porous support can be determined by measuring the content of the photoinitiator having an oxidation potential higher by 0.02 V or more than the full charge voltage of a lithium secondary battery, filled in the entire pore volume of the polyolefin porous support. For example, assuming that the entire pore volume of the polyolefin porous support is filled 100% with a solvent (described below) and no solvent is present on the surface of the polyolefin porous support, the weight of the solvent contained in the entire pore volume of the polyolefin porous support can be determined from the density of the solvent, and the content of the photoinitiator having an oxidation potential higher by 0.02 V or more than the full charge voltage of a lithium secondary battery, contained in the solvent, can be determined.

[0140] In particular, when the photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more includes 2-isopropylthioxanthone, thioxanthone, or a mixture thereof, the photoinitiator can emit a smaller amount of light, for example, 500 mJ / cm , than when it includes benzophenone. 2 Since photocrosslinking of the polyolefin porous support is possible even at this level, it is more advantageous in terms of mass production.

[0141] Furthermore, when the photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of the lithium secondary battery includes 2-isopropylthioxanthone (ITX), the melting point of ITX is low, about 70°C to 80°C. Therefore, when the photocrosslinking temperature is adjusted to 80°C to 100°C, the ITX on the surface of the polyolefin porous support melts, causing mobility of the ITX into the polyolefin porous support, thereby increasing crosslinking efficiency and easily preventing changes in the physical properties of the final separator.

[0142] The polyolefin porous support may be prepared by forming pores from the polyolefin material using a conventional method known in the art, such as a wet method using a solvent, diluent, or pore-forming agent, or a dry method using a stretching method, in order to ensure excellent breathability and porosity.

[0143] In one embodiment of the present invention, the polyolefin porous support may have a polyolefin chain with 0.01 to 0.5 or 0.01 to 0.3 double bonds per 1000 carbon atoms as measured by H-NMR. When the polyolefin porous support has the above-mentioned number of double bonds, the polyolefin porous support can be effectively crosslinked and side reactions caused by excessive radical formation can be easily prevented.

[0144] In one embodiment of the present invention, the number of double bonds present in the polyolefin chain excluding the terminals of the polyolefin porous support may be 0.005 to 0.49 per 1,000 carbon atoms. "Double bonds present in the polyolefin chain excluding the terminals" refers to double bonds present throughout the polyolefin chain excluding the terminals of the polyolefin chain. Here, "terminals" refers to the positions of the carbon atoms connected to both ends of the polyolefin.

[0145] In one embodiment of the present invention, the number of double bonds present in the polyolefin chain may be adjusted by adjusting the type and purity of the catalyst, addition of a linking agent, etc. during polyolefin synthesis.

[0146] In one embodiment of the present invention, the polyolefin porous support has a BET specific surface area of ​​10 m 2 / g~27m 2 / g, 13m 2 / g~25m 2 / g, or 15m 2 / g~23m 2When the BET specific surface area of ​​the polyolefin porous support satisfies the above range, the surface area of ​​the polyolefin porous support increases, and the crosslinking efficiency of the polyolefin porous support can be increased even when a small amount of a photoinitiator having an oxidation potential value higher than the full charge voltage of a lithium secondary battery by 0.02 V or more is used.

[0147] The BET specific surface area of ​​the polyolefin porous support can be measured by the BET method. Specifically, the BET specific surface area of ​​the inorganic particles can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a Belsorp-mini II manufactured by BEL Japan.

[0148] In one embodiment of the present invention, the polyolefin porous support may further include an antioxidant. The antioxidant can control the crosslinking reaction between polymer chains by controlling the radicals formed in the polyolefin chains. The antioxidant either oxidizes the polymer chains to prevent oxidation of the polymer chains or absorbs the generated radicals to control the crosslinking reaction between the polymer chains. This can affect the shutdown temperature and mechanical strength of the final separator.

[0149] In one embodiment of the present invention, the content of the antioxidant may be 500 ppm to 20,000 ppm, 1,000 ppm to 15,000 ppm, or 2,000 ppm to 13,000 ppm based on the content of the polyolefin porous support. When the content of the antioxidant is within the above range, the antioxidant can sufficiently control excessively generated radicals, easily preventing side reactions and easily preventing the surface of the polyolefin porous support from becoming uneven. As a result, the finally manufactured separator has a tensile strength of 1,500 kgf / cm after being exposed to 180°C for 1 minute. 2 It's easy to become more than that.

[0150] These antioxidants can be broadly divided into radical scavengers, which react with radicals generated in polyolefins to stabilize the polyolefins, and peroxide decomposers, which decompose peroxides generated by the radicals into stable molecules. The radical scavengers abstract hydrogen to stabilize the radicals and become radicals themselves, but can remain in a stable form through a resonance effect or electron rearrangement. The peroxide decomposers can be more effective when used in combination with a radical scavenger.

[0151] In one embodiment of the present invention, the antioxidant may include a first antioxidant that is a radical scavenger and a second antioxidant that is a peroxide decomposer. Since the first antioxidant and the second antioxidant have different mechanisms of action, by simultaneously including the first antioxidant that is a radical scavenger and the second antioxidant that is a peroxide decomposer, the generation of unnecessary radicals can be more easily suppressed by the synergistic effect of these antioxidants.

[0152] The content of the first antioxidant and the content of the second antioxidant may be the same or different.

[0153] In one embodiment of the present invention, the first antioxidant may comprise a phenolic antioxidant, an amine antioxidant, or a mixture thereof.

[0154] The phenolic antioxidants include 2,6-di-t-butyl-4-methylphenol, 4,4'-thiobis(2-t-butyl-5-methylphenol), 2,2'-thiodiethyl bis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate], pentaerythritol tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate], 4,4'-thiobis(2-methyl-6-t-butylphenol), 2,2'-thiobis(6-t-butyl-4-methylphenol), octadecyl-[3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate], ... hydroxyphenyl)propionate], triethylene glycol-bis-[3-(3-t-butyl-4-hydroxy-5-methylphenol)propionate], thiodiethylene bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 6,6'-di-t-butyl-2,2'-thiodi-p-cresol, 1,3,5-tris(4-t-butyl-3-hydroxy-2,6-xylyl)methyl-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione, dioctadecyl 3,3'-thiodipropionate, or two or more thereof.

[0155] In one embodiment of the present invention, the content of the first antioxidant may be 500 ppm to 10,000 ppm, 1,000 ppm to 12,000 ppm, or 1,000 ppm to 10,000 ppm based on the content of the polyolefin porous support. When the content of the first antioxidant is within the above range, side reactions caused by excessive generation of radicals can be easily prevented.

[0156] In one embodiment of the present invention, the second antioxidant may include a phosphorus-based antioxidant, a sulfur-based antioxidant, or a mixture thereof.

[0157] The phosphorus-based antioxidant decomposes peroxide to generate alcohol, which is converted into phosphate. Examples of the phosphorus-based antioxidant include 3,9-bis(2,6-di-t-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane, bis(2,6-dicumylphenyl)pentaerythritol diphosphite, 2,2'-methylenebis(4,6-di-t-butylphenyl)2-ethylhexyl phosphite, and bis(2,4-di-t-butyl-6-methylphenyl)-ethyl- phosphite, bis(2,6-di-t-butyl-4-methylphenyl)pentaerythritol diphosphite, bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite, bis(2,4-dicumylphenyl)pentaerythritol diphosphite, distearyl pentaerythritol diphosphite, tris(2,4-di-t-butylphenyl)phosphite, or two or more thereof.

[0158] The sulfur-based antioxidant may include 3,3′-thiobis-1,1′-didodecyl ester, dimethyl 3,3′-thiodipropionate, dioctadecyl 3,3′-thiodipropionate, 2,2-bis{[3-(dodecylthio)-1-oxopropoxy]methyl}propane-1,3-diylbis[3-(dodecylthio)propionate], or two or more thereof.

[0159] In one embodiment of the present invention, the content of the second antioxidant may be 500 ppm to 10,000 ppm, 1,000 ppm to 12,000 ppm, or 1,000 ppm to 10,000 ppm based on the content of the polyolefin porous support. When the content of the second antioxidant is within the above range, side reactions caused by excessive generation of radicals can be easily prevented.

[0160] In one embodiment of the present invention, when the antioxidant comprises a first antioxidant as a radical scavenger and a second antioxidant as a peroxide decomposer, the content of the first antioxidant may be 500 ppm to 10,000 ppm based on the content of the polyolefin porous support, and the content of the second antioxidant may be 500 ppm to 10,000 ppm based on the content of the polyolefin porous support.

[0161] In one embodiment of the present invention, the step of preparing a polyolefin porous support including a photoinitiator having an oxidation potential value higher than the full charge voltage of a lithium secondary battery by 0.02 V or more may include adding a photoinitiator having an oxidation potential value higher than the full charge voltage of a lithium secondary battery to an extruder when extruding a polyolefin composition for forming the polyolefin porous support.

[0162] In another embodiment of the present invention, the step of preparing the polyolefin porous support may include coating a photoinitiator composition, including a photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more, and a solvent, on the outside of the polyolefin porous support, and drying the composition.

[0163] As used herein, the term "coating on the outside and drying" refers not only to a case where a photoinitiator composition is directly coated and dried on the surface of a polyolefin porous support, but also to a case where another layer is formed on the polyolefin porous support and then a photoinitiator composition is coated and dried on the surface of the other layer.

[0164] In one embodiment of the present invention, the polyolefin porous support may be corona discharge treated before the photoinitiator composition is coated on the polyolefin porous support. The corona discharge treatment may be performed by applying a high-frequency, high-voltage output generated by a predetermined driving circuit between a predetermined discharge electrode and a treatment roll in a corona discharge treatment machine. The corona discharge treatment modifies the surface of the polyolefin porous support, thereby further improving the wettability of the polyolefin porous support to the photoinitiator composition. As a result, crosslinking of the polyolefin porous support can be more efficiently performed even when a photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of a lithium secondary battery is used at the same content. The corona discharge treatment may be performed using atmospheric pressure plasma.

[0165] In one embodiment of the present invention, the 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 methylene chloride, chloroform, and carbon tetrachloride; esters such as ethyl acetate, butyl acetate, γ-butyrolactone, and ε-caprolactone; acyl nitriles such as acetonitrile and propionitrile; ethers such as tetrahydrofuran and ethylene glycol diethyl ether; alcohols such as methanol, ethanol, isopropanol, ethylene glycol, and ethylene glycol monomethyl ether; and amides such as N-methylpyrrolidone and N,N-dimethylformamide; or two or more of these.

[0166] In one embodiment of the present invention, the content of the photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of a lithium secondary battery in the photoinitiator composition is 0.015 to 0.36 parts by weight per 100 parts by weight of the polyolefin porous support, and may be 0.01 to 0.5 parts by weight, 0.02 to 0.45 parts by weight, or 0.25 to 0.4 parts by weight per 100 parts by weight of the solvent. When the content of the photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of a lithium secondary battery satisfies the above-mentioned range based on the solvent, the polyolefin porous support can be crosslinked, and side reactions due to excessive radical generation can be more easily prevented.

[0167] In one embodiment of the present invention, the content of the photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more in the photoinitiator composition is 0.015 to 0.36 parts by weight based on 100 parts by weight of the polyolefin porous support, and is 0.01 mg / m based on the specific surface area of ​​the polyolefin porous support. 2 ~1.0mg / m 2 , 0.03 mg / m 2 ~0.8mg / m 2 , or 0.06 mg / m 2 ~0.7mg / m 2 When the content of the photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more in the photoinitiator composition is within the above-mentioned range, the polyolefin porous support can be crosslinked, and side reactions due to excessive generation of radicals can be more easily prevented.

[0168] The content of the photoinitiator having an oxidation potential value higher than the full charge voltage of a lithium secondary battery by 0.02 V or more based on the specific surface area of ​​the polyolefin porous support can be measured by NMR analysis.

[0169] In one embodiment of the present invention, the photoinitiator composition may be a photoinitiator solution including a photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more and the solvent.

[0170] Non-limiting examples of the method for coating the photoinitiator solution on the polyolefin porous support include dip coating, die coating, roll coating, comma coating, microgravure coating, doctor blade coating, reverse roll coating, Mayer bar coating, and direct roll coating.

[0171] The drying step after coating the photoinitiator solution on the polyolefin porous support can be performed by a method known in the art, and can be performed batchwise or continuously using an oven or heated chamber at a temperature range that takes into account the vapor pressure of the solvent used. The drying removes most of the solvent present in the photoinitiator solution and is preferably performed as quickly as possible, taking productivity into consideration, for example, within 1 minute or 30 seconds.

[0172] In yet another embodiment of the present invention, the photoinitiator composition may be a slurry for forming an inorganic composite porous layer, including an inorganic filler, a binder polymer, a photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more, and the solvent.

[0173] When the photoinitiator composition is a slurry for forming the inorganic composite porous layer, the photoinitiator composition is coated onto a polyolefin porous support, and a photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of a lithium secondary battery is introduced onto the surface of the polyolefin porous support, and when irradiated with ultraviolet light, the polyolefin porous support can be crosslinked and an inorganic composite porous layer can be formed on at least one surface of the polyolefin porous support.

[0174] When a slurry for forming an inorganic composite porous layer is used as the photoinitiator composition, the polyolefin porous support can be photocrosslinked using the inorganic composite porous layer formation process without the need for additional equipment for directly applying a photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of the lithium secondary battery to the polyolefin porous support, for example, equipment for directly coating and drying a solution containing a photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of the lithium secondary battery onto the polyolefin porous support.

[0175] Furthermore, the inorganic composite porous layer-forming slurry directly crosslinks polymer chains within the polyolefin porous support, and therefore does not require other monomers in addition to the photoinitiator having an oxidation potential 0.02 V or more higher than the full charge voltage of the lithium secondary battery. Therefore, even if the photoinitiator having an oxidation potential 0.02 V or more higher than the full charge voltage of the lithium secondary battery is included in the inorganic composite porous layer-forming slurry along with the inorganic filler and binder polymer, the monomers do not prevent the photoinitiator having an oxidation potential 0.02 V or more higher than the full charge voltage of the lithium secondary battery from reaching the surface of the polyolefin porous support, and the photoinitiator having an oxidation potential 0.02 V or more higher than the full charge voltage of the lithium secondary battery can be sufficiently introduced onto the surface of the polyolefin porous support.

[0176] Generally, since the polyolefin porous support itself and the inorganic filler have a high UV blocking effect, if an inorganic composite porous layer containing an inorganic filler is formed and then irradiated with UV rays, the amount of UV rays reaching the polyolefin porous support may be reduced. However, in the method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention, even if UV rays are irradiated after the inorganic composite porous layer is formed, the polymer chains in the polyolefin porous support can be crosslinked and directly connected to each other.

[0177] In one embodiment of the present invention, when the photoinitiator composition is used as a slurry for forming the inorganic composite porous layer, the photoinitiator may include 2-isopropylthioxanthone, thioxanthone, or a mixture thereof, as a photoinitiator having an oxidation potential at least 0.02 V higher than the full charge voltage of the lithium secondary battery. 2-Isopropylthioxanthone or thioxanthone is capable of photocrosslinking even at long wavelengths with high transmittance. As a result, even when a slurry for forming the inorganic composite porous layer containing an inorganic filler, a binder polymer, etc., contains a photoinitiator having an oxidation potential at least 0.02 V higher than the full charge voltage of the lithium secondary battery, the polyolefin porous support can be easily crosslinked.

[0178] Depending on the type of binder polymer, the solvent may function as a solvent that dissolves the binder polymer, or as a dispersion medium that disperses the binder polymer without dissolving it. The solvent may also dissolve a photoinitiator having an oxidation potential that is 0.02 V or more higher than the full charge voltage of the lithium secondary battery. The solvent may have a solubility index similar to that of the binder polymer to be used and a low boiling point. This facilitates uniform mixing and subsequent solvent removal. For non-limiting examples of such solvents, see the description of the solvent above.

[0179] For the inorganic filler and the binder polymer, please refer to the above description.

[0180] The inorganic composite porous layer-forming slurry can be prepared by dissolving or dispersing the binder polymer in the solvent, then adding and dispersing the inorganic filler. The inorganic filler may be added in a pre-crushed state to have a predetermined average particle size, or the inorganic filler may be added to the slurry in which the binder polymer is dissolved or dispersed, and then crushed and dispersed using a ball mill or other method to have a predetermined average particle size. Crushing may be performed for 1 to 20 hours, and the average particle size of the crushed inorganic filler is as described above. Conventional crushing methods can be used, and a ball mill method may be used.

[0181] In one embodiment of the present invention, the solid content of the inorganic composite porous layer forming slurry may be 5 wt% to 60 wt%, or 30 wt% to 50 wt%. When the solid content of the inorganic composite porous layer forming slurry is within the above range, coating uniformity can be easily ensured and unevenness due to slurry flow or the need for a large amount of energy to dry the slurry can be easily prevented.

[0182] In an embodiment of the present invention, when the photoinitiator composition is a slurry for forming the inorganic composite porous layer, a phase separation process may be performed after coating the photoinitiator composition on the polyolefin porous support. The phase separation may be performed by humidification phase separation or immersion phase separation.

[0183] Among the phase separations, the humidification phase separation will be described below.

[0184] First, the humidification phase separation can be performed at a temperature range of 15°C to 70°C or a temperature range of 20°C to 50°C and a relative humidity range of 15% to 80% or a relative humidity range of 30% to 50%. As the inorganic composite porous layer forming slurry undergoes a drying process, it acquires phase transition characteristics due to a vapor-induced phase separation phenomenon known in the art.

[0185] For the humidification phase separation, a non-solvent for the binder polymer may be introduced in a gaseous state. The non-solvent for the binder polymer is not particularly limited as long as it does not dissolve the binder polymer and is partially compatible with the solvent. For example, a non-solvent in which the binder polymer has a solubility of less than 5 wt % at 25°C may be used. For example, the non-solvent for the binder polymer may be water, methanol, ethanol, isopropanol, butanol, butanediol, ethylene glycol, propylene glycol, tripropylene glycol, or two or more thereof.

[0186] Among the phase separations, the immersion phase separation will be described below.

[0187] The inorganic composite porous layer-forming slurry is coated onto the outer surface of the polyolefin porous support, which is then immersed in a coagulation solution containing a non-solvent for the binder polymer for a predetermined period of time. This induces phase separation in the coated inorganic composite porous layer slurry, solidifying the binder polymer. This process forms a porous inorganic composite porous layer. The coagulation solution is then removed by rinsing with water, and the substrate is then dried. The drying can be performed by any method known in the art, and can be carried out batchwise or continuously using an oven or heated chamber at a temperature range that takes into account the vapor pressure of the solvent used. The drying removes most of the solvent present in the slurry and is preferably carried out as quickly as possible, taking productivity into consideration, for example, within 1 minute or 30 seconds.

[0188] The coagulation liquid may be a non-solvent for the binder polymer alone or a mixture of the non-solvent for the binder polymer and the above-mentioned solvent. When a mixture of the non-solvent for the binder polymer and a solvent is used, the content of the non-solvent for the binder polymer may be 50 wt % or more relative to 100 wt % of the coagulation liquid in order to form a good porous structure and improve productivity.

[0189] In another embodiment of the present invention, the step of coating a photoinitiator composition including a photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more and a solvent on the outside of the polyolefin porous support and drying the photoinitiator composition comprises:

[0190] forming an inorganic composite porous layer by coating an inorganic composite porous layer-forming slurry containing an inorganic filler, a first binder polymer, and a dispersion medium on at least one surface of the polyolefin porous support and drying the coating;

[0191] and coating a coating liquid for forming a porous adhesive layer, the coating liquid including a second binder polymer, a photoinitiator having an oxidation potential value higher than a full charge voltage of the lithium secondary battery by 0.02 V or more, and the solvent, on the upper surface of the inorganic composite porous layer, and drying the coating liquid.

[0192] For the inorganic filler and the slurry for forming the inorganic composite porous layer, please refer to the above contents.

[0193] Depending on the type of first binder polymer, the dispersion medium may function as a solvent that dissolves the first binder polymer, or as a dispersion medium that disperses the first binder polymer without dissolving it. The dispersion medium may have a solubility index similar to that of the first binder polymer to be used and a low boiling point. In this case, uniform mixing and subsequent removal of the dispersion medium are facilitated.

[0194] In one embodiment of the present invention, the dispersion medium may be an aqueous dispersion medium. When the dispersion medium is an aqueous dispersion medium, it is environmentally friendly, does not require excessive heat during the drying process after forming the inorganic composite porous layer, and does not require additional explosion-proof equipment, making it easier to form the inorganic composite porous layer.

[0195] In one embodiment of the present invention, the first binder polymer may be insoluble in the solvent or a non-solvent for the second binder polymer (described later). In this case, even if a coating liquid (described later) is applied to form a porous adhesive layer after forming the inorganic composite porous layer, the first binder polymer does not dissolve, so it is possible to easily prevent the first binder polymer dissolved in the solvent or a non-solvent for the second binder polymer from blocking pores.

[0196] In one embodiment of the present invention, the first binder polymer may be an aqueous binder polymer. In this case, the first binder polymer may be dissolved in an aqueous solvent or dispersed in an aqueous dispersion medium. When the first binder polymer is dispersed in an aqueous dispersion medium, the first binder polymer may be in a particulate form.

[0197] The inorganic composite porous layer-forming slurry can be dried using a drying method typically used in the manufacture of separation membranes. For example, the coated slurry can be dried in air for 10 seconds to 30 minutes, 30 seconds to 20 minutes, or 3 minutes to 10 minutes. Drying within the above time ranges effectively removes residual solvent without impairing productivity.

[0198] For the second binder polymer, please refer to the above description.

[0199] The solvent may dissolve the second binder polymer at 25° C. in an amount of 5 wt % or more, 15 wt % or more, or 25 wt % or more.

[0200] The solvent may be a non-solvent for the first binder polymer, for example, the solvent may dissolve less than 5% by weight of the first binder polymer at 25°C.

[0201] For the type of the solvent, please refer to the above content.

[0202] In one embodiment of the present invention, the second binder polymer may be included in an amount of 3 to 30 wt %, or 5 to 25 wt %, based on 100 wt % of the coating liquid for forming the porous adhesive layer.

[0203] Since the coating liquid for forming a porous adhesive layer contains a photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of the lithium secondary battery, when the coating liquid for forming a porous adhesive layer is coated on the upper surface of the inorganic composite porous layer, the photoinitiator having an oxidation potential value 0.02 V or more higher than the full charge voltage of the lithium secondary battery is introduced onto the surface of the polyolefin porous support, and a porous adhesive layer can be formed at the same time.

[0204] During the coating process of the coating liquid for forming a porous adhesive layer, the polyolefin porous support is wetted with the solvent. At this time, a photoinitiator contained in the coating liquid for forming a porous adhesive layer and having an oxidation potential value higher than the full charge voltage of a lithium secondary battery by 0.02 V or more is introduced onto the surface of the polyolefin porous support, and the polyolefin porous support can be photo-crosslinked by the photoinitiator present on the surface of the polyolefin porous support upon irradiation with ultraviolet light.

[0205] Therefore, the method for manufacturing a separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention does not require additional equipment for directly applying a photoinitiator having an oxidation potential higher than the full charge voltage of a lithium secondary battery by 0.02 V or more to the polyolefin porous support in order to photo-crosslink the polyolefin porous support, e.g., equipment for directly coating the polyolefin porous support with a solution containing a photoinitiator having an oxidation potential higher than the full charge voltage of a lithium secondary battery by 0.02 V or more and drying the solution. This simplifies the process in that the polyolefin porous support can be photo-crosslinked using a porous adhesive layer formation process.

[0206] In a method for manufacturing a separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention, polymer chains in the polyolefin porous support are directly crosslinked, and therefore no other components, such as a monomer for forming radicals, are required in addition to the photoinitiator having an oxidation potential 0.02 V or more higher than the full charge voltage of the lithium secondary battery. Therefore, even when a photoinitiator having an oxidation potential 0.02 V or more higher than the full charge voltage of the lithium secondary battery is added to a coating solution for forming a porous adhesive layer, other components do not prevent the photoinitiator having an oxidation potential 0.02 V or more higher than the full charge voltage of the lithium secondary battery from reaching the surface of the polyolefin porous support, and the photoinitiator having an oxidation potential 0.02 V or more higher than the full charge voltage of the lithium secondary battery can be sufficiently introduced onto the surface of the polyolefin porous support.

[0207] In addition, since the polyolefin porous support itself and the inorganic filler generally have a high UV blocking effect, if an inorganic composite porous layer containing an inorganic filler is formed and then irradiated with UV rays, the amount of UV rays reaching the polyolefin porous support may be reduced. However, in the method for manufacturing a separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention, crosslinking can be achieved even with a small amount of UV irradiation, and therefore, even if UV rays are irradiated after the inorganic composite porous layer and the porous adhesive layer are formed, the polymer chains in the polyolefin porous support can be crosslinked and directly connected to each other.

[0208] In one embodiment of the present invention, the coating solution for forming the porous coating layer may include 2-isopropylthioxanthone, thioxanthone, or a mixture thereof as a photoinitiator having an oxidation potential value at least 0.02 V higher than the full charge voltage of the lithium secondary battery. 2-Isopropylthioxanthone or thioxanthone can be photocrosslinked even with long wavelengths that have high transmittance. This allows the polyolefin porous support to be easily crosslinked by irradiating ultraviolet light after the inorganic material-forming porous layer and porous adhesive layer are formed.

[0209] In one embodiment of the present invention, the coating liquid for forming a porous adhesive layer may be pattern-coated on the upper surface of the inorganic composite porous layer to form a pattern on the finally manufactured porous adhesive layer.

[0210] In one embodiment of the present invention, after the coating solution for forming a porous adhesive layer is coated on the upper surface of the inorganic composite porous layer, a phase separation process may be performed. The phase separation may be performed by a immersion phase separation method.

[0211] In the immersion phase separation method, the porous adhesive layer-forming coating liquid is coated on the upper surface of the inorganic composite porous layer and then immersed in a coagulation liquid containing a non-solvent for the second binder polymer for a predetermined period of time. This induces phase separation in the coated porous adhesive layer-forming coating liquid, solidifying the second binder polymer. This process forms a porous adhesive layer. The coagulation liquid is then removed by rinsing with water, followed by drying. The drying can be performed by a method known in the art and can be carried out batchwise or continuously using an oven or heated chamber at a temperature range that takes into account the vapor pressure of the solvent used. The drying removes most of the solvent present in the porous adhesive layer-forming coating liquid. It is desirable to perform the drying as quickly as possible, taking productivity into consideration, for example, within 1 minute or 30 seconds.

[0212] The solidification liquid may be a non-solvent for the second binder polymer alone, or a mixed solvent of the non-solvent for the second binder polymer and the above-mentioned solvent. When a mixed solvent of the non-solvent for the second binder polymer and a solvent is used, the content of the non-solvent for the second binder polymer may be 50 wt % or more relative to 100 wt % of the solidification liquid in order to form a good porous structure and improve productivity.

[0213] When immersion phase separation is performed, the second binder polymer condenses during the solidification process, preventing the second binder polymer from penetrating the surface and / or interior of the polyolefin porous support, thereby preventing an increase in the resistance of the separation membrane. In addition, the adhesive layer containing the second binder polymer becomes porous, thereby improving the resistance of the separation membrane.

[0214] The non-solvent for the second binder polymer may have a solubility for the second binder polymer of less than 5 wt % at 25°C.

[0215] The non-solvent for the second binder polymer may also be a non-solvent for the first binder polymer, e.g., the non-solvent for the second binder polymer may have a solubility for the first binder polymer of less than 5 wt % at 25°C.

[0216] In one embodiment of the present invention, the non-solvent for the second binder polymer may include water, methanol, ethanol, propyl alcohol, butyl alcohol, butanediol, ethylene glycol, propylene glycol, tripropylene glycol, or two or more thereof.

[0217] In one embodiment of the present invention, the immersion may be performed for 3 seconds to 1 minute. When the immersion time is within the above range, phase separation occurs appropriately, ensuring adhesion between the inorganic composite porous layer and the porous adhesive layer and preventing detachment of the adhesive layer.

[0218] In one embodiment of the present invention, the coating solution for forming a porous adhesive layer can be dried by a drying method typically used in the production of separation membranes. For example, it can be dried by air for 10 seconds to 30 minutes, 30 seconds to 20 minutes, or 3 minutes to 10 minutes. Drying within the above time ranges has the effect of removing residual dispersion medium without impairing productivity.

[0219] In the method for manufacturing a separator having a crosslinked structure for a lithium secondary battery according to an embodiment of the present invention, the inorganic composite porous layer and the porous adhesive layer are formed through separate steps, so that the porous adhesive layer can be formed in various shapes. For example, the porous adhesive layer can be easily formed in a pattern.

[0220] Thereafter, the polyolefin porous support is irradiated with ultraviolet light, which crosslinks the polymer chains in the polyolefin porous support, thereby obtaining a crosslinked structure-containing polyolefin porous support having a crosslinked structure in which the polymer chains are directly linked to each other.

[0221] The UV irradiation can be performed using an UV cross-linking device by appropriately adjusting the UV irradiation time and dose, taking into account factors such as the content of a photoinitiator having an oxidation potential at least 0.02 V higher than the full charge voltage of the lithium secondary battery. For example, the UV irradiation time and dose can be set to ensure sufficient cross-linking of the polymer chains in the polyolefin porous support to achieve the desired heat resistance and to prevent damage to the separator due to heat generated by the UV lamp. The UV lamp used in the UV cross-linking device can be appropriately selected from a high-pressure mercury lamp, a metal lamp, a gallium lamp, etc., depending on the photoinitiator having an oxidation potential at least 0.02 V higher than the full charge voltage of the lithium secondary battery, and the emission wavelength and capacity of the UV lamp can be appropriately selected depending on the process.

[0222] The method for producing a separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention can photocrosslink polymer chains in a polyolefin porous support with only a significantly smaller amount of UV irradiation light than that used in general photocrosslinking, thereby enhancing applicability to mass production of separators containing a crosslinked structure for a lithium secondary battery. For example, the UV irradiation light dose is 10 to 2000 mJ / cm. 2 , 30~1500mJ / cm 2 , 50-1000mJ / cm 2 , 150-500mJ / cm 2 , or 500-1500mJ / cm 2It could be.

[0223] In one embodiment of the present invention, the UV radiation intensity can be measured using a Miltec H-type UV bulb and UV power pack, a portable light meter. When measuring the radiation intensity using a Miltec H-type UV bulb, three wavelength values, UVA, UVB, and UVC, are obtained for each wavelength, and the UV radiation of the present invention corresponds to UVA.

[0224] In the present invention, the method for measuring the amount of ultraviolet light irradiation is to pass a UV power pack on a conveyor under a light source under the same conditions as the sample, and the numerical value of the amount of ultraviolet light displayed on the UV power pack at this time is referred to as the "amount of ultraviolet light irradiation."

[0225] The separator containing a cross-linked structure for a lithium secondary battery may be interposed between a positive electrode and a negative electrode to manufacture a lithium secondary battery.

[0226] The lithium secondary battery including the separator having a crosslinked structure for a lithium secondary battery according to an embodiment of the present invention does not lose capacity after high-temperature storage even though it contains a photoinitiator.

[0227] Generally, when a lithium secondary battery is provided with a separator containing a crosslinked structure for a lithium secondary battery, which contains a photoinitiator, side reactions occur due to the electrochemically unstable photoinitiator when the battery is stored at high temperatures, for example, at about 80°C to 90°C, resulting in a problem of a decrease in battery performance, for example, the voltage and capacity of the battery.

[0228] However, a lithium secondary battery including a separator having a crosslinked structure for a lithium secondary battery according to an embodiment of the present invention may exhibit substantially the same level of performance after high-temperature storage as a lithium secondary battery including a separator for a lithium secondary battery that does not include a crosslinked structure, even if the photoinitiator remains in the battery, because the photoinitiator is electrochemically stable under voltage conditions within the battery's operating range. Specifically, the lithium secondary battery may exhibit substantially the same level of capacity loss after high-temperature storage as a lithium secondary battery including a separator for a lithium secondary battery that does not include a crosslinked structure.

[0229] In the present invention, the term "lithium secondary battery separator without a crosslinked structure" refers to a separator comprising a polyolefin porous support in which polymer chains are not crosslinked. For example, the term "lithium secondary battery separator without a crosslinked structure" refers to a separator made of a polyolefin porous support in which polymer chains are not crosslinked; a separator comprising the polyolefin porous support in which the crosslinked structure is not crosslinked and an inorganic composite porous layer located on at least one surface of the polyolefin porous support in which the inorganic composite porous layer contains an inorganic filler and a binder polymer; or a separator comprising a polyolefin porous support in which the crosslinked structure is not crosslinked and an inorganic composite porous layer located on at least one surface of the polyolefin porous support in which the inorganic filler and the first binder polymer are present, and a porous adhesive layer located on the inorganic composite porous layer in which the second binder polymer is present.

[0230] The lithium secondary battery may be in various shapes such as a cylindrical shape, a prismatic shape, or a pouch shape.

[0231] The lithium secondary battery may include a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0232] The electrode to be used together with the separator containing a crosslinked structure for a lithium secondary battery of the present invention is not particularly limited, and may be prepared by a conventional method known in the art in the form of an electrode active material layer including an electrode active material, a conductive material, and a binder bound to a current collector.

[0233] Non-limiting examples of the positive electrode active material include layered compounds such as lithium cobalt complex oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; 1+x Mn 2-xLithium manganese oxides such as O4 (x=0~0.33), LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O5, LiV3O4, V2O5, and Cu2V2O7; chemical formula LiNi 1-x M x Ni-site lithium nickel oxide represented by O2 (M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, x = 0.01 to 0.3); chemical formula: LiMn 1-x M x Examples of suitable lithium manganese composite oxides include, but are not limited to, lithium manganese composite oxides represented by Li2Mn3MO5 (M=Fe, Co, Ni, Fe, Cr, Zn, or Ta, x=0.01 to 0.1) or Li2Mn3MO5 (M=Fe, Co, Ni, Cu, or Zn); LiMn2O4, in which part of the Li in the chemical formula is replaced with an alkaline earth metal ion; disulfide compounds; and Fe2(MoO4)3.

[0234] Non-limiting examples of the negative electrode active material among the electrode active materials include conventional negative electrode active materials used in the negative electrodes of lithium secondary batteries, particularly lithium metal or lithium alloys, and lithium adsorbent materials such as carbon, petroleum coke, activated carbon, graphite, or other carbons.

[0235] Non-limiting examples of positive electrode current collectors include foils made of aluminum, nickel, or a combination thereof, and non-limiting examples of negative electrode current collectors include foils made of copper, gold, nickel, or a copper alloy, or a combination thereof.

[0236] In one embodiment of the present invention, the conductive materials used in the negative electrode and the positive electrode may each be independently added in an amount of 1 wt % to 30 wt % based on the total weight of the active material layer. The conductive material is not particularly limited as long as it does not induce chemical changes in the battery and has conductivity. Examples of such conductive materials include graphite such as natural graphite and artificial graphite; carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0237] In one embodiment of the present invention, the binders used in the negative and positive electrodes are components that independently aid in binding between the active material and the conductive material, and between the active material and the current collector, and are typically added in an amount of 1 to 30 wt % based on the total weight of the active material layer. Examples of such binders include polyvinylidene fluoride (PVdF), polyacrylic acid (PAA), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers.

[0238] In an embodiment of the present invention, the lithium secondary battery includes an electrolyte solution, which may include an organic solvent and a lithium salt. The electrolyte solution may be an organic solid electrolyte or an inorganic solid electrolyte.

[0239] Examples of the organic solvent that can be used include aprotic organic solvents such as N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0240] The lithium salt is a substance that is easily dissolved in the organic solvent, such as LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, lithium chloroborane, lithium lower aliphatic carboxylates, lithium tetraphenylborate, imides, etc. may be used.

[0241] In addition, for the purpose of improving charge / discharge characteristics, flame retardancy, etc., the electrolyte solution may contain, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. In some cases, the electrolyte may further contain a halogen-containing solvent such as carbon tetrachloride or trifluoroethylene to impart non-flammability, and may further contain carbon dioxide to improve high-temperature storage properties.

[0242] Examples of the organic solid electrolyte that can be used include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyagitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and polymers containing ionic dissociative groups.

[0243] Examples of the inorganic solid electrolyte that can be used include nitrides, halides, and sulfates of Li, such as LiN, LiI, LiNI, LiN-LiI-LiOH, LiSiO, LiSiO-LiI-LiOH, LiSiS, LiSiO, LiSiO-LiI-LiOH, and LiPO-LiS-SiS.

[0244] The electrolyte injection may be performed at an appropriate stage in the battery manufacturing process depending on the manufacturing process and required properties of the final product, i.e., before battery assembly or at the final stage of battery assembly.

[0245] In one embodiment of the present invention, the process of applying the separator having a crosslinked structure for a lithium secondary battery to a battery may include lamination (stack) and folding of the separator and electrodes in addition to the conventional winding process.

[0246] In one embodiment of the present invention, the separator for a lithium secondary battery having a crosslinked structure is interposed between a positive electrode and a negative electrode of the lithium secondary battery, and may be interposed between adjacent cells or electrodes when a plurality of cells or electrodes are assembled to form an electrode assembly. The electrode assembly may have various structures, such as a simple stack type, a jelly roll type, a stack folding type, or a lamination stack type. [Example]

[0247] The present invention will be described in detail below with reference to examples to aid in understanding the present invention. However, the examples according to the present invention can be modified into many other forms, and the scope of the present invention should not be construed as being limited to the examples described below. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0248] Example 1 A 6.5 μm thick polyethylene porous film (Senior, weight average molecular weight: 600,000, porosity: 50%) was prepared as a polyolefin porous support. The number of double bonds present in the polymer chain during H-NMR measurement was 0.2 per 1000 carbon atoms, and the antioxidants contained Irganox 1010 at 3000 ppm and Irgafos 168 at 2000 ppm.

[0249] D at 600 nm 50 Al2O3 powder with a particle size of 250 nm and D 50 The inorganic filler was prepared by mixing γ-AlOOH powder with a particle size of 1.5 mm in a weight ratio of 9:1. An acrylic emulsion (CSB-130, manufactured by Toyo Ink) was prepared as the first binder polymer, and sodium carboxymethylcellulose (CMC-Na) (SG-L02, manufactured by GL Chem) was prepared as the dispersant.

[0250] The prepared inorganic filler, first binder polymer, and dispersant were added to water in a weight ratio of 97:2:1, and then the inorganic filler was crushed and dispersed to prepare a slurry for forming an inorganic composite porous layer.

[0251] The inorganic composite porous layer forming slurry was coated on both sides of a polyolefin porous support and dried to form an inorganic composite porous layer.

[0252] Polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) (LBG, manufactured by Arkema) as a second binder polymer was added to N-methyl-2-pyrrolidone (NMP) as a solvent, and 2-isopropylthioxanthone (manufactured by Sigma Aldrich, oxidation potential: 4.468 V) as a photoinitiator was added in an amount of 0.1 parts by weight per 100 parts by weight of N-methyl-2-pyrrolidone to prepare a coating solution for forming a porous adhesive layer.

[0253] After coating the coating solution for forming a porous adhesive layer on the inorganic composite porous layer, the content of the photoinitiator was adjusted to 0.07 parts by weight per 100 parts by weight of the polyolefin porous support, and the coating solution for forming a porous adhesive layer was solidified by sequentially immersing it in a coagulation bath and a water rinsing bath. The coagulation bath contained water as a non-solvent, and the water rinsing bath contained a rinse solution consisting solely of water as a non-solvent. After the coating solution for forming a porous adhesive layer solidified, the solvent and non-solvent remaining in the coating solution were simultaneously dried.

[0254] Thereafter, the resultant product was irradiated with a high-pressure mercury lamp (Lichtzen high-pressure mercury lamp, LH-250 / 800-A) at an integrated light dose of 500 mJ / cm 2 2 The polyolefin porous support was crosslinked by irradiating it with ultraviolet light so that the crosslinked structure-containing separator for a lithium secondary battery was obtained.

[0255] Example 2 A crosslinked structure-containing separator for lithium secondary batteries was obtained in the same manner as in Example 1, except that thioxanthone (oxidation potential: 4.591 V) (manufactured by TCI) was used instead of 2-isopropylthioxanthone (manufactured by Sigma-Aldrich).

[0256] Example 3 Benzophenone (oxidation potential: 5.383 V) (Sigma Aldrich) was used instead of 2-isopropylthioxanthone (Sigma Aldrich), and the UV irradiation dose was 1500 mJ / cm 2A separator having a crosslinked structure for a lithium secondary battery was obtained in the same manner as in Example 1, except that the irradiation was performed so that the crosslinked structure for a lithium secondary battery was

[0257] Example 4 A 6.5 μm thick polyethylene porous film (manufactured by Toray Industries, Inc., porosity: 45%) was prepared as a polyolefin porous support. The number of double bonds present in the polymer chain during H-NMR measurement was 0.2 per 1000 carbon atoms, and the film contained 3000 ppm of Irganox 1010 and 2000 ppm of Irgafos 168 as antioxidants.

[0258] A photoinitiator composition was prepared by dissolving 2-isopropylthioxanthone (Sigma Aldrich, oxidation potential: 4.468 V) as a photoinitiator in 0.05 parts by weight based on 100 parts by weight of acetone, and the resulting composition was applied to both sides of a 6 cm x 15 cm polyolefin porous support in a total coating amount of 13.5 g / m 2 The composition was cut using a bar so that the composition would not remain on the surface of the porous support, and the content of the photoinitiator was adjusted to be 0.03 parts by weight based on 100 parts by weight of the polyolefin porous support.

[0259] Next, the upper surface of the polyolefin porous support coated with the photoinitiator composition was exposed to an integrated light dose, i.e., UV irradiation dose of 500 mJ / cm 2 2 The separator containing a cross-linked structure for a lithium secondary battery was obtained by irradiating the separator with UV light so that the cross-linked structure for a lithium secondary battery was obtained.

[0260] Comparative Example 1 A separation membrane was obtained in the same manner as in Example 1, except that 2-isopropylthioxanthone (Sigma Aldrich) was not used.

[0261] Comparative Example 2 Instead of 2-isopropylthioxanthone (Sigma Aldrich), 2-carboxymethoxythioxanthone (CMTX) (oxidation potential: 4.378 V) (Alpha Chemistry) was used, and the UV irradiation dose was 1500 mJ / cm 2 A separator having a crosslinked structure for a lithium secondary battery was obtained in the same manner as in Example 1, except that the irradiation was performed so that the crosslinked structure for a lithium secondary battery was

[0262] Evaluation example 1: Evaluation of the physical properties of the separation membrane and results of high-temperature storage tests The air permeability, meltdown temperature, heat shrinkage rate after leaving at 150° C. for 30 minutes, and high-temperature storage test results of the separators produced in Examples 1 to 4 and Comparative Examples 1 and 2 are shown in Table 1 below.

[0263] (1) Evaluation of breathability Air permeability (Gurley) was measured by ASTM D726-94 method. As used herein, Gurley is the resistance to air flow, as measured by a Gurley densometer. Air permeability values ​​described herein are calculated by passing 100 cc of air through 1 in of a separator at a pressure of 12.2 in H2O. 2 The time (seconds) required for the air to pass through the cross section is shown as the ventilation time.

[0264] (2) Evaluation of meltdown temperature The meltdown temperature was measured by thermomechanical analysis (TMA) after taking samples from the separator in the machine direction (MD) and transverse direction (TD). Specifically, a 4.8 mm wide x 8 mm long sample was placed in a TMA device (TA Instruments, Q400) and heated from 30°C to 220°C at a heating rate of 5°C / min under a tension of 0.01 N. As the temperature increased, the length of the sample changed, and the temperature at which the length increased rapidly and the sample broke in both the machine direction and the transverse direction was measured.

[0265] (3) Evaluation of thermal shrinkage after leaving at 150°C for 30 minutes The heat shrinkage rate was calculated by cutting the separator into a size of 50 mm (length) x 50 mm (width) to prepare a test specimen, placing it in an oven heated to 150°C for 30 minutes, and then recovering the specimen and measuring the change in length in the machine direction and transverse direction. Heat shrinkage rate (%) after leaving at 150°C for 30 minutes = {(dimension before shrinkage - dimension after shrinkage) / dimension before shrinkage} x 100

[0266] (4) High temperature storage test Coin cells each including the separators for lithium secondary batteries having a crosslinked structure prepared in Examples 1 to 4 and Comparative Examples 1 and 2 were manufactured.

[0267] Anode manufacturing Artificial graphite as a negative electrode active material, 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 (NMP) as a solvent was added to prepare a negative electrode slurry. The negative electrode slurry was charged to 3.8 mAh / cm 2 The resulting mixture was coated on a copper current collector in a loading amount of 0.1g and dried to prepare a negative electrode.

[0268] Cathode manufacturing A cathode active material slurry was prepared by adding LiCoO2 as a cathode active material, Denka black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 85:5:10 to N-methylpyrrolidone (NMP) as a solvent. The cathode active material slurry was coated on a sheet-shaped aluminum current collector and dried, resulting in a final cathode loading of 3.3 mAh / cm. 2 The positive electrode active material layer was formed so as to have the following structure.

[0269] Coin Cell Manufacturing A separator according to each of the Examples and Comparative Examples was interposed between the negative electrode and the positive electrode prepared as described above, and a non-aqueous electrolyte (1M LiPF, ethylene carbonate (EC) / propylene carbonate (PC) / diethyl carbonate (DEC)) (volume ratio = 3:3:4) was poured into the separator to prepare a coin cell.

[0270] The manufactured coin cells were fully charged to 4.43 V and stored at about 85° C. for 8 hours, after which the voltage, remaining capacity, and recovered capacity were measured and shown in Table 1 below.

[0271] Here, the recovery capacity means the capacity when the coin cell is stored at 85° C. for 8 hours and then charged and discharged again.

[0272] [Table 1]

[0273] From Table 1, it can be seen that the separators produced in Examples 1 to 4 have a meltdown temperature of 160°C or higher, demonstrating excellent safety at high temperatures. It can also be seen that the voltage and capacity of the battery after high-temperature storage are almost the same as those of the separator of Comparative Example 1, which does not contain a crosslinked structure.

[0274] It can be seen that the separation membrane produced in Comparative Example 1 has poor safety at high temperatures.

[0275] The separator prepared in Comparative Example 2 had an increased meltdown temperature and was therefore highly safe at high temperatures. However, because the oxidation potential of the photoinitiator used was not more than 0.02 V higher than the full charge voltage of the lithium secondary battery, the voltage and capacity of the battery were significantly reduced after high-temperature storage.

Claims

1. A separator containing a crosslinked structure for a lithium secondary battery, a crosslinked structure-containing polyolefin porous support having a crosslinked structure in which polymer chains are directly linked to each other; a photoinitiator having an oxidation potential value higher than a full charge voltage of the lithium secondary battery by 0.02 V or more.

2. 2. The crosslinked structure-containing separator for a lithium secondary battery according to claim 1, wherein the photoinitiator has an oxidation potential value 0.02 V or more higher than the full charge voltage of the lithium secondary battery, and the oxidation potential value is 4.4 V to 8 V.

3. 2. The crosslinked structure-containing separator for a lithium secondary battery according to claim 1, wherein the amount of the photoinitiator having an oxidation potential value higher by 0.02 V or more than a full charge voltage of the lithium secondary battery is 0.015 to 0.36 parts by weight, based on 100 parts by weight of the crosslinked structure-containing polyolefin porous support.

4. 2. The separator for a lithium secondary battery according to claim 1, further comprising an inorganic composite porous layer located on at least one surface of the crosslinked structure-containing polyolefin porous support, the inorganic composite porous layer comprising an inorganic filler and a binder polymer.

5. The separator for a lithium secondary battery having a crosslinked structure is disposed on at least one surface of the crosslinked structure-containing polyolefin porous support, and the inorganic composite porous layer includes an inorganic filler and a first binder polymer; 10. The separator for a lithium secondary battery according to claim 1, further comprising: a porous adhesive layer positioned on the inorganic composite porous layer and including a second binder polymer.

6. 2. The crosslinked structure-containing separator for a lithium secondary battery according to claim 1, wherein the photoinitiator having an oxidation potential value higher by 0.02 V or more than a full charge voltage of the lithium secondary battery comprises thioxanthone, a thioxanthone derivative, benzophenone, a benzophenone derivative, or two or more thereof.

7. 2. The separator for a lithium secondary battery according to claim 1, wherein the separator for a lithium secondary battery having a crosslinked structure has a meltdown temperature of 160[deg.] C. or higher.

8. 2. The separator for a lithium secondary battery according to claim 1, wherein the separator for a lithium secondary battery having a crosslinked structure has a shutdown temperature of 145[deg.] C. or less.

9. A method for producing a separator containing a crosslinked structure for a lithium secondary battery, comprising: preparing a polyolefin porous support including a photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more; and irradiating the polyolefin porous support with ultraviolet light.

10. preparing a polyolefin porous support containing a photoinitiator having an oxidation potential value higher than the full charge voltage of the lithium secondary battery by 0.02 V or more; 10. The method for preparing a separator containing a crosslinked structure for a lithium secondary battery according to claim 9, comprising: coating a photoinitiator composition, including a photoinitiator having an oxidation potential value higher than a full charge voltage of the lithium secondary battery by 0.02 V or more and a solvent, on the outside of the polyolefin porous support, and drying the coating.

11. 11. The method for manufacturing a crosslinked structure-containing separator for a lithium secondary battery according to claim 10, wherein the photoinitiator composition is a slurry for forming an inorganic composite porous layer, the slurry comprising an inorganic filler, a binder polymer, a photoinitiator having an oxidation potential value higher than a full charge voltage of the lithium secondary battery by 0.02 V or more, and the solvent.

12. a step of coating a photoinitiator composition including a photoinitiator having an oxidation potential value higher than a full charge voltage of the lithium secondary battery by 0.02 V or more and a solvent on the outside of the polyolefin porous support and drying the composition; forming an inorganic composite porous layer by coating at least one surface of the polyolefin porous support with a slurry for forming the inorganic composite porous layer, the slurry including an inorganic filler, a first binder polymer, and a dispersion medium, and drying the slurry; 11. The method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery according to claim 10, further comprising: coating a coating liquid for forming a porous adhesive layer on an upper surface of the inorganic composite porous layer, the coating liquid including a second binder polymer, a photoinitiator having an oxidation potential value higher by 0.02 V or more than a full charge voltage of the lithium secondary battery, and the solvent, and drying the coating liquid.

13. 10. The method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery according to claim 9, wherein the oxidation potential of the photoinitiator having an oxidation potential value higher by 0.02 V or more than the full charge voltage of the lithium secondary battery is 4.4 V to 8 V.

14. 10. The method for manufacturing a crosslinked structure-containing separator for a lithium secondary battery according to claim 9, wherein the amount of the photoinitiator having an oxidation potential value higher than a full charge voltage of the lithium secondary battery by 0.02 V or more is 0.015 to 0.36 parts by weight, based on 100 parts by weight of the polyolefin porous support.

15. 10. The method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery according to claim 9, wherein the photoinitiator having an oxidation potential value higher by 0.02 V or more than a full charge voltage of the lithium secondary battery comprises thioxanthone, a thioxanthone derivative, benzophenone, a benzophenone derivative, or two or more thereof.

16. The irradiation dose of the ultraviolet light is 10 to 2000 mJ / cm ２ The method for producing a separator containing a crosslinked structure for a lithium secondary battery according to claim 9,

17. The lithium secondary battery includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, A lithium secondary battery, wherein the separator for a lithium secondary battery is the crosslinked structure-containing separator for a lithium secondary battery according to claim 1 .

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