Crosslinked polyolefin porous support, crosslinked separator for lithium secondary battery including the same, method for producing the same, and lithium secondary battery including the separator

Abstract

The present invention relates to a crosslinked structure-containing polyolefin porous support which has a crosslinked structure in which polymer chains are directly linked to each other, has a shutdown temperature of 145°C or lower, does not break even after being exposed to 180°C for 1 minute, and has a ratio of puncture strength after being exposed to 180°C for 1 minute to 50% to 100% of the puncture strength at 25°C; a crosslinked structure-containing separator for lithium secondary batteries which includes the same, a method for producing the same, and a lithium secondary battery which includes the separator.

Description

[Technical Field]

[0001] This application claims priority from Korean Patent Application No. 10-2021-0059584, filed on May 7, 2021.

[0002] The present invention relates to a crosslinked structure-containing polyolefin porous support, a crosslinked structure-containing separator for a lithium secondary battery including the same, 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 problem to be solved by the present invention is to provide a porous support of a crosslinked structure-containing polyolefin that has improved high temperature stability while maintaining a shutdown temperature.

[0008] Another object of the present invention is to provide a separator containing a crosslinked structure for a lithium secondary battery, which includes the crosslinked structure-containing polyolefin porous support, and a lithium secondary battery including the separator.

[0009] Another object of the present invention is to provide a simplified method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery, which has improved high-temperature safety while maintaining a shutdown temperature. [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 crosslinked structure-containing polyolefin porous support according to the following embodiment.

[0011] The first embodiment is It has a cross-linked structure in which polymer chains are directly linked to each other, The shutdown temperature is 145°C or less, It did not break even after being exposed to 180°C for 1 minute. The present invention relates to a crosslinked structure-containing polyolefin porous support, in which the ratio of the puncture strength after exposure to 180°C for 1 minute to the puncture strength at 25°C is 50% to 100%.

[0012] According to the second embodiment, in the first embodiment, The crosslinked structure-containing polyolefin porous support may have a puncture strength of 50 gf or more after being exposed to 180° C. for 1 minute.

[0013] According to the third embodiment, in the first or second embodiment, After being exposed to 180°C for 1 minute, the crosslinked structure-containing polyolefin porous support has a tensile strength of 500 kgf / cm in both the machine direction (MD) and the transverse direction (TD). 2 It could be more than that.

[0014] According to a fourth embodiment, in any one of the first to third embodiments, The degree of crosslinking of the crosslinked structure-containing polyolefin porous support may be 10% to 45%.

[0015] 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.

[0016] The fifth embodiment is The present invention relates to a separator containing a crosslinked structure for a lithium secondary battery, which includes a crosslinked structure-containing polyolefin porous support according to any one of the first to fourth embodiments.

[0017] According to the sixth embodiment, in the fifth embodiment, 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.

[0018] According to the seventh embodiment, in the fifth embodiment, 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.

[0019] According to the eighth embodiment, in any one of the fifth to seventh embodiments, The cross-linked structure-containing separator for a lithium secondary battery may have a meltdown temperature of 160° C. or higher.

[0020] According to the ninth embodiment, in the seventh or eighth embodiment, The weight ratio of the inorganic filler to the first binder polymer may be 95:5 to 99.9:0.1.

[0021] According to a tenth embodiment, in any one of the seventh to ninth embodiments, The first binder polymer may include an acrylic polymer, polyacrylic acid, styrene butadiene rubber, polyvinyl alcohol, or two or more thereof.

[0022] According to an eleventh embodiment, in any one of the seventh to tenth embodiments, 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.

[0023] 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.

[0024] The twelfth embodiment is: providing a polyolefin porous support containing a photoinitiator; and irradiating the polyolefin porous support containing the photoinitiator with ultraviolet light, The content of the photoinitiator is 0.015 to 0.36 parts by weight based on 100 parts by weight of the polyolefin porous support.

[0025] According to the thirteenth embodiment, in the twelfth embodiment, providing a polyolefin porous support containing the photoinitiator, The method may include coating a photocrosslinking composition containing the photoinitiator and a solvent on the outer surface of the polyolefin porous support and drying the coating.

[0026] According to the fourteenth embodiment, in the thirteenth embodiment, The photocrosslinking composition may be a slurry for forming an inorganic composite porous layer, which includes an inorganic filler, a binder polymer, the photoinitiator, and the solvent.

[0027] According to the fifteenth embodiment, in the thirteenth embodiment, a step of coating a photocrosslinking composition containing the photoinitiator and a solvent on the outer surface of the polyolefin porous support and drying the coating; 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, the photoinitiator, and the solvent, on the upper surface of the inorganic composite porous layer, and drying the coating liquid.

[0028] According to the 16th embodiment, in the 12th to 15th embodiments, The photoinitiator may include a Type II photoinitiator.

[0029] According to the seventeenth embodiment, in the twelfth to sixteenth embodiments, The photoinitiator may include thioxanthone (TX), a thioxanthone derivative, benzophenone (BPO), a benzophenone derivative, or two or more thereof.

[0030] According to the 18th embodiment, in the 12th to 17th embodiments, The irradiation amount of the ultraviolet light is 10 to 2000 mJ / cm 2 It could be. 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.

[0031] The 19th embodiment is: A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, The separator is a lithium secondary battery having a crosslinked structure according to any one of the fifth to eleventh embodiments. [Effects of the Invention]

[0032] The crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention has a low shutdown temperature and can simultaneously improve high temperature safety.

[0033] The separator for a lithium secondary battery including a crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention has a low shutdown temperature and improved high-temperature safety due to the inclusion of the crosslinked structure-containing polyolefin porous support.

[0034] According to one aspect of the present invention, a method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery can be simplified to manufacture a separator containing a crosslinked structure for a lithium secondary battery having a low shutdown temperature and improved high-temperature safety.

[0035] 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]

[0036] [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

[0037] 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.

[0038] Therefore, it should be understood that the embodiments described in this specification and the configurations shown in the drawings are merely the most preferred 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.

[0039] 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.

[0040] The crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention comprises: It has a cross-linked structure in which polymer chains are directly linked to each other, The shutdown temperature is 145°C or less, It did not break even after being exposed to 180°C for 1 minute. The ratio of the puncture strength after exposure to 180°C for 1 minute to the puncture strength at 25°C is 50% to 100%.

[0041] 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.

[0042] Although photoinitiators may be crosslinked with each other or with polymer chains, such crosslinked structures have a lower reaction enthalpy than crosslinked structures between polymer chains within the polyolefin porous support, and therefore may be decomposed and cause side reactions. 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.

[0043] 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.

[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, it is easy to increase the modulus 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 a separation membrane including the crosslinked structure-containing polyolefin porous support is easy to reach 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 the crosslinked polyolefin porous support, double bonds may be generated in the polyolefin chains due to the crosslinking reaction caused by the photoinitiator. Since side reactions occur at the double bonds, the number of double bonds may affect the performance of the crosslinked polyolefin porous support.

[0052] In one embodiment of the present invention, the crosslinked structure-containing polyolefin porous support may have a number of double bonds present in the polyolefin chain per 1000 carbon atoms as measured by H-NMR of 0.01 to 0.6, or 0.02 to 0.5. When the crosslinked structure-containing polyolefin porous support has the above-mentioned number of double bonds, it is possible to minimize the portion where side reactions occur.

[0053] 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.

[0054] 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, when it is used as a separator, the problem of the separator being easily damaged during use of the battery can be prevented and the energy density can be easily ensured.

[0055] The crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention has a crosslinked structure in which polymer chains are directly linked to each other, thereby improving heat resistance.

[0056] In the crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention, the pore structure of the polyolefin porous support can be substantially maintained even after crosslinking.

[0057] The crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention has a shutdown temperature of 145° C. or lower. It also does not break after being exposed to 180° C. for 1 minute, and the ratio of the puncture strength after being exposed to 180° C. for 1 minute to the puncture strength at 25° C. is 50% to 100%.

[0058] In one embodiment of the present invention, the crosslinked structure-containing polyolefin porous support may have a shutdown temperature of 140°C or less, 133°C to 140°C, or 135°C to 140°C.

[0059] The shutdown temperature can be determined by measuring the temperature at which the air permeability of a crosslinked polyolefin porous support suddenly increases when the support is heated after being fixed to an air permeability measuring device. For example, using an Oken air permeability measuring device (manufactured by Asahi Seiko, model: EG01-55-1MR), the temperature is raised by 5°C per minute, and the time (seconds) required for 100cc of air to pass through the support at a constant pressure of 0.05 MPa can be measured, and the temperature at which the air permeability of the support suddenly increases can be determined as the shutdown temperature.

[0060] The crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention has improved stability at high temperatures, and can maintain the shape of the polyolefin porous support even after being exposed to 180°C for 1 minute.

[0061] To check whether the crosslinked structure-containing polyolefin porous support breaks after being exposed to 180°C for 1 minute, a frame measuring 20 cm wide x 20 cm long is prepared, the crosslinked structure-containing polyolefin porous support is fixed using heat-resistant tape, and the frame is placed in a high-temperature oven set to 180°C and left for 1 minute, after which it is visually inspected for breakage.

[0062] Although conventional polyolefin porous supports have improved heat resistance and do not break even after being exposed to 180°C for 1 minute, the crosslinked structure-containing polyolefin porous supports are damaged during the heat treatment at 180°C, and the crosslinked structure-containing polyolefin porous supports do not have sufficient mechanical strength.

[0063] In the crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention, a crosslinking reaction between polymer chains occurs at a level that does not cause shrinkage of the polyolefin porous support or main chain scission of the polyolefin, and therefore the ratio of the puncture strength after exposure to 180°C for 1 minute to the puncture strength at 25°C is 50% to 100%. In other words, the puncture strength after exposure to 180°C for 1 minute does not show a significant decrease compared to the puncture strength at 25°C. Therefore, the support has sufficient mechanical strength even at high temperatures such as 180°C.

[0064] The puncture strength at 25°C can be measured in accordance with ASTM D2582. Specifically, the puncture strength can be measured in accordance with ASTM D2582 at 25°C after setting a 1 mm round tip to operate at a speed of 120 mm / min.

[0065] The puncture strength after exposure to 180°C for 1 minute can be measured in accordance with ASTM D2582. Specifically, the specimen can be exposed to 180°C for 1 minute, and a 1 mm round tip can be set to operate at a speed of 120 mm / min, and then the puncture strength of the specimen can be measured at 25°C in accordance with ASTM D2582.

[0066] In one embodiment of the present invention, the crosslinked structure-containing polyolefin porous support may have a ratio of 50% to 80%, 50% to 55%, 50% to 54.9%, or 51.8% to 54.9% of the puncture strength after exposure to 180°C for 1 minute to the puncture strength at 25°C. When the crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention has a ratio of puncture strength after exposure to 180°C for 1 minute to the puncture strength at 25°C within the above-mentioned range, it can have sufficient mechanical strength even at high temperatures such as 180°C.

[0067] In one embodiment of the present invention, the puncture strength of the crosslinked structure-containing polyolefin porous support after exposure to 180°C for 1 minute may be 50 gf or more, 100 gf or more, 100 gf to 700 gf, 150 gf to 500 gf, 200 gf to 300 gf, or 210 gf to 225 gf. When the puncture strength of the crosslinked structure-containing polyolefin porous support after exposure to 180°C for 1 minute is within the above-mentioned range, it can have sufficient mechanical strength even at high temperatures such as 180°C.

[0068] In one embodiment of the present invention, the tensile strength of the crosslinked structure-containing polyolefin porous support in the machine direction (MD) and transverse direction (TD) after exposure to 180°C for 1 minute is 500 kgf / cm 2 More than 700kgf / cm 2 More than 700kgf / cm 2 ~1500kgf / cm 2 , or 780 kgf / cm 2 ~1335kgf / cm 2 When the tensile strength of the crosslinked structure-containing polyolefin porous support after exposure to 180°C for 1 minute is within the above range, it can have sufficient mechanical strength even at a high temperature such as 180°C.

[0069] The tensile strength after exposure to 180°C for 1 minute can be measured by exposing a test specimen to 180°C for 1 minute, and then pulling the specimen at 25°C in both 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, and measuring the strength at which the test specimen breaks.

[0070] The cross-linked structure-containing polyolefin porous support according to one embodiment of the present invention can be used as a cross-linked structure-containing separator for a lithium secondary battery.

[0071] A separator having a crosslinked structure for a lithium secondary battery according to an embodiment of the present invention may include a porous support material having a crosslinked structure made of polyolefin according to an embodiment of the present invention.

[0072] In one embodiment of the present invention, when the separator having a crosslinked structure for a lithium secondary battery includes a porous support of a crosslinked polyolefin having the above-described number of double bonds, the problem of battery performance degradation at high temperatures and / or high voltages can be easily prevented.

[0073] A separator having a crosslinked structure for a lithium secondary battery according to an embodiment of the present invention may include a porous support material having a crosslinked structure made of polyolefin according to an embodiment of the present invention.

[0074] 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.

[0075] 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 crosslinked structure-containing polyolefin porous support 10 and an inorganic composite porous layer 20 located on at least one surface of the crosslinked structure-containing polyolefin porous support 10 and including an inorganic filler and a binder polymer.

[0076] 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.

[0077] 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.

[0078] 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 those that are within the operating voltage range (e.g., Li / Li) of the applied electrochemical element. + 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.

[0079] 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.

[0080] In another embodiment of the present invention, as the inorganic filler, an inorganic filler having lithium ion transfer ability, that is, an inorganic filler that does not store lithium but has a function of moving lithium ions and contains a lithium element, can be used. Non-limiting examples of inorganic fillers having lithium ion transfer ability include lithium phosphate (Li3PO4), lithium titanium phosphate (Li x Ti y (PO4)3, 0 < x < 2, 0 < y < 3), lithium aluminum titanium phosphate (Li x Al y Ti z (PO4)3, 0 < x < 2, 0 < y < 1, 0 < z < 3), (LiAlTiP) x O y -based glasses such as 14Li2O-9Al2O3-38TiO2-39P2O5 (0 < x < 4, 0 < y < 13), lithium lanthanum titanate (Li x La y TiO3, 0 < x < 2, 0 < y < 3), lithium germanium thiophosphate such as Li 3.25 Ge 0.25 P 0.75 S4 (Li x Ge y P z S w , 0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5), lithium nitride such as Li3N (Li x N y , 0 < x < 4, 0 < y < 2), SiS2-based glasses such as Li3PO4-Li2S-SiS2 (Li x Si y S z , 0 < x < 3, 0 < y < 2, 0 < z < 4), P2S5-based glasses such as LiI-Li2S-P2S5 (Li x P y S z , 0 < x < 3, 0 < y < 3, 0 < z < 7), or mixtures thereof, and the like. [[ID= forty-nine]]

[0081] [[ID= fifty-one]] In one embodiment of the present invention, the inorganic filler may have an average particle size of 0.01 μm to 1.5 μm. When the average particle size of the inorganic filler is within the above range, it is easy to form an inorganic composite porous layer 20 having a uniform thickness and appropriate porosity, and the inorganic filler has good dispersibility, thereby achieving a desired energy density.

[0082] In this case, the average particle diameter of the inorganic filler is D 50 "D" means particle size. 50 The particle size "means the particle size at 50% of the cumulative particle number distribution according to the particle size. The particle size can be measured using a laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium, and then introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac S3500). When the particles pass through a laser beam, the difference in the diffraction pattern according to the particle size is measured to calculate the particle size distribution. The particle diameter at 50% of the cumulative particle number distribution according to the particle size in the measuring device is calculated, and the D 50 The particle size may be measured.

[0083] 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 inorganic composite porous layer 20 finally formed can be improved. The binder polymer may have ion conductivity. When the binder polymer has ion conductivity, the battery performance can 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 can be improved.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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 crosslinked 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 crosslinked polyolefin porous support 10' 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.

[0096] 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.

[0097] For the inorganic filler, please refer to the above content.

[0098] 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.

[0099] 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 can 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.

[0100] In an 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.

[0101] 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.

[0102] In one embodiment of the present invention, the first binder polymer may be in the form of particles.

[0103] 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.

[0104] The following describes the characteristics of the inorganic composite porous layer 20' that differ from the inorganic composite porous layer 20 described above.

[0105] In one embodiment of the present invention, the inorganic composite porous layer may have a structure in which the inorganic fillers are filled and in contact with each other and bound to each other 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.

[0106] 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.

[0107] 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'.

[0108] 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 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 adhesive layer 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.

[0109] 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.

[0110] 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 is not formed, thereby improving the electrolyte impregnation of the separator.

[0111] 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.

[0112] The separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention includes a crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention, and therefore has a shutdown temperature of 145°C or less, does not break even after being exposed to 180°C for 1 minute, and has a puncture strength ratio of 50% to 100% after being exposed to 180°C for 1 minute relative to the puncture strength at 25°C.

[0113] The separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention has a shutdown temperature of 145°C or less, thereby ensuring safety in situations such as overcharging. Furthermore, it does not break even after being exposed to 180°C for 1 minute. That is, the separator's safety at high temperatures is improved, and it can maintain its shape even after being exposed to 180°C for 1 minute. As a result, the separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention can ensure safety in situations such as overcharging, and can also function as a separator that insulates the positive and negative electrodes even at high temperatures.

[0114] The shutdown temperature can be determined by measuring the temperature at which the air permeability of the separation membrane increases rapidly when the temperature is increased after the separation membrane is fixed to an air permeability measuring device. For example, 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 increased by 5°C per minute using an Oken air permeability measuring device (manufactured by Asahi Seiko, Model: EG01-55-1MR).

[0115] In one embodiment of the present invention, the separator having a crosslinked structure for a lithium secondary battery may have a shutdown temperature of 140°C or less, 133°C to 140°C, or 135°C to 140°C. When the separator's shutdown temperature satisfies this range, overcharge safety can be ensured and the problem of pore damage to the crosslinked polyolefin porous support during the high-temperature and pressure process during battery assembly can be easily prevented. For example, the crosslinked polyolefin porous support does not include a case in which a photoinitiator is directly linked to a polymer chain or a case in which photoinitiators are directly linked to each other, and therefore the problem of an excessive decrease in shutdown temperature due to a lower melting temperature of the polymer chain can be easily prevented.

[0116] To check whether the separation membrane breaks after being exposed to 180°C for 1 minute, a frame measuring 20 cm wide x 20 cm long is prepared, the separation membrane is fixed to the frame using heat-resistant tape, and the frame is placed in a high-temperature oven set to 180°C and left for 1 minute, after which the separation membrane is visually inspected for breakage.

[0117] Although conventional separators for lithium secondary batteries have improved heat resistance and do not break even after being exposed to 180°C for one minute, the separator is damaged during the heat treatment at 180°C and does not have sufficient strength to function properly.

[0118] In a separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention, a crosslinking reaction occurs between polymer chains at a level that does not cause separator shrinkage or polyolefin main chain scission, and therefore the ratio of puncture strength after exposure to 180°C for 1 minute to the puncture strength at 25°C is 50% to 100%. As a result, the separator has sufficient mechanical strength even after exposure to high temperatures such as 180°C, and can fully function as a separator to prevent short circuits between the positive and negative electrodes.

[0119] Please refer to the above for the method of measuring the puncture strength at 25°C and after exposure to 180°C for 1 minute.

[0120] In one embodiment of the present invention, the crosslinked structure-containing separator for a lithium secondary battery may have a ratio of puncture strength after exposure to 180°C for 1 minute to 50% to 80%, 50% to 55%, 50% to 54.9%, or 51.8% to 54.9% relative to the puncture strength at 25°C. When the crosslinked structure-containing separator for a lithium secondary battery has a ratio of puncture strength after exposure to 180°C for 1 minute to the puncture strength at 25°C within the above range, the separator has sufficient mechanical strength even after exposure to a high temperature such as 180°C, and can more easily perform its function as a separator to prevent short circuits between the positive and negative electrodes.

[0121] In one embodiment of the present invention, the separator containing a crosslinked structure for a lithium secondary battery may have a puncture strength of 50 gf or more, 100 gf or more, 100 gf to 700 gf, 150 gf to 500 gf, 200 gf to 300 gf, or 210 gf to 225 gf after exposure to 180°C for 1 minute. When the separator containing a crosslinked structure for a lithium secondary battery has a puncture strength within the above range after exposure to 180°C for 1 minute, the separator has sufficient mechanical strength even after exposure to high temperatures such as 180°C, and can more easily perform its function as a separator to prevent short circuits between the positive and negative electrodes.

[0122] In one embodiment of the present invention, the separator containing a crosslinked structure for a lithium secondary battery has a tensile strength of 500 kgf / cm in both the machine direction and the transverse direction after being exposed to 180° C. for 1 minute. 2 More than 700kgf / cm 2 More than 700kgf / cm 2 ~1500kgf / cm 2 , or 780 kgf / cm 2 ~1335kgf / cm 2 When the tensile strength of the separator having a crosslinked structure for a lithium secondary battery after exposure to 180°C for 1 minute is within the above range, the separator has sufficient mechanical strength even after exposure to a high temperature such as 180°C, and can more easily perform its function as a separator to prevent short circuits between the positive electrode and the negative electrode.

[0123] Please refer to the above for the method of measuring the tensile strength after exposure to 180°C for 1 minute.

[0124] The separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention may have a higher meltdown temperature than a separator including a non-crosslinked polyolefin porous support. In one embodiment of the present invention, the separator containing a crosslinked structure for a lithium secondary battery may have a meltdown temperature of 160°C or higher, 170°C or higher, or 180°C to 230°C. When the meltdown temperature of the separator containing a crosslinked structure for a lithium secondary battery satisfies the above range, no breakage occurs even after exposure to 180°C for 1 minute.

[0125] 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.

[0126] In this specification, "a separation membrane comprising a non-crosslinked polyolefin porous support" refers to a separation membrane made of a non-crosslinked polyolefin porous support that does not contain a crosslinked structure; a separation membrane comprising a non-crosslinked polyolefin porous support that does not contain a crosslinked structure and an inorganic composite porous layer that is located on at least one surface of the non-crosslinked polyolefin porous support and contains an inorganic filler and a binder polymer; or a separation membrane comprising a non-crosslinked polyolefin porous support that does not contain a crosslinked structure, an inorganic composite porous layer that is located on at least one surface of the non-crosslinked polyolefin porous support 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.

[0127] 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 at 25°C, tensile elongation at 25°C, puncture strength at 25°C, electrical resistance, etc. compared to the separator for a lithium secondary battery before crosslinking, and the rate of change is also small.

[0128] 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.

[0129] 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

[0130] 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.

[0131] The air permeability (Gurley) may be measured by ASTM D726-94 method. As used herein, Gurley is the resistance to air flow and is measured by a Gurley densometer. Air permeability values ​​described herein are measured by passing 100 ml of air through a sample porous support at a pressure of 12.2 in. H2O. 2The time (seconds) required for the air to pass through the cross section is shown as the ventilation time.

[0132] 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.

[0133] 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

[0134] The basis weight (g / m 2 ) is measured by preparing a sample that is 1m long and 1m wide, and measuring its weight.

[0135] 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 at 25°C 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.

[0136] The rate of change in tensile strength at 25°C can be calculated using the following formula: Change rate (%) of tensile strength in the machine direction at 25°C = [(tensile strength in the machine direction at 25°C of separator for lithium secondary battery before crosslinking) - (tensile strength in the machine direction at 25°C of separator for lithium secondary battery after crosslinking)] / (tensile strength in the machine direction at 25°C of separator for lithium secondary battery before crosslinking) × 100 Change rate (%) of tensile strength in the transverse direction at 25°C = [(tensile strength in the transverse direction at 25°C of separator for lithium secondary battery before crosslinking) - (tensile strength in the transverse direction at 25°C of separator for lithium secondary battery after crosslinking)] / (tensile strength in the transverse direction at 25°C of separator for lithium secondary battery before crosslinking) × 100

[0137] The tensile strength at 25°C may be the strength at which a specimen breaks when the specimen is pulled at 25°C in both 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.

[0138] 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 modulus at 25°C in the machine direction and transverse direction of 20% or less, or 0% to 20%, compared to the separator for a lithium secondary battery before crosslinking.

[0139] The rate of change in tensile elongation at 25°C can be calculated using the following formula: Change rate (%) in tensile elongation in the machine direction at 25°C = [(tensile elongation in the machine direction at 25°C of separator for lithium secondary battery before crosslinking) - (tensile elongation in the machine direction at 25°C of separator for lithium secondary battery after crosslinking)] / (tensile elongation in the machine direction at 25°C of separator for lithium secondary battery before crosslinking) × 100 Change rate (%) of tensile elongation in the transverse direction at 25°C = [(tensile elongation in the transverse direction at 25°C of separator for lithium secondary battery before crosslinking) - (tensile elongation in the transverse direction at 25°C of separator for lithium secondary battery after crosslinking)] / (tensile elongation in the transverse direction at 25°C of separator for lithium secondary battery before crosslinking) × 100

[0140] The tensile elongation was measured by measuring the maximum length of the specimen stretched until it broke when the specimen was stretched at a rate of 50 mm / min in both the machine direction and the transverse direction using Universal Testing Systems (Instron® 3345) at 25°C in accordance with ASTM D882, and calculated using the following formula: Tensile elongation in the machine direction at 25°C (%) = (length in the machine direction of the specimen immediately before break at 25°C - length in the machine direction of the specimen before stretching at 25°C) / (length in the machine direction of the specimen before stretching at 25°C) x 100 Tensile elongation in the transverse direction at 25°C (%) = (length of the specimen immediately before break at 25°C in the transverse direction at 25°C - length of the specimen before stretching at 25°C in the transverse direction) / (length of the specimen before stretching at 25°C in the transverse direction) × 100

[0141] 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 at 25°C 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.

[0142] The rate of change in puncture strength at 25°C can be calculated using the following formula: Percent change in puncture strength at 25°C (%) = [(puncture strength at 25°C of separator for lithium secondary battery before crosslinking) - (puncture strength at 25°C of separator for lithium secondary battery containing crosslinked structure after crosslinking)] / (puncture strength at 25°C of separator for lithium secondary battery before crosslinking) × 100

[0143] For the method of measuring the puncture strength, please refer to the above content.

[0144] 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.

[0145] 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

[0146] 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.

[0147] 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.

[0148] According to one embodiment of the present invention, a method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery includes: providing a polyolefin porous support containing a photoinitiator; and irradiating the polyolefin porous support containing the photoinitiator with ultraviolet light, The content of the photoinitiator is 0.015 to 0.36 parts by weight based on 100 parts by weight of the polyolefin porous support.

[0149] 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.

[0150] First, a polyolefin porous support containing a photoinitiator is prepared. The photoinitiator 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" may include not only the surface of the outermost layer of the polyolefin porous support, but also the surfaces of pores present inside the polyolefin porous support.

[0151] The photoinitiator directly photocrosslinks polymer chains within the polyolefin porous support. The photoinitiator can crosslink the polyolefin porous support using only the photoinitiator without 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 within the polyolefin porous support, making the polymer chains reactive and resulting in direct linkages between the polymer chains for photocrosslinking. For example, since the photoinitiator is capable of performing a hydrogen abstraction reaction on a small amount of double bond structures or branched structures present in the polyolefin, upon light absorption alone, hydrogen atoms can be abstracted from the double bond structures within the polyolefin to form radicals on the polymer chains.

[0152] In the method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery according to an embodiment of the present invention, the photoinitiator can 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.

[0153] In one embodiment of the present invention, the photoinitiator may include a Type II photoinitiator.

[0154] In one embodiment of the present invention, the photoinitiator may include thioxanthone (TX), a thioxanthone derivative, benzophenone (BPO), a benzophenone derivative, or two or more thereof.

[0155] 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.

[0156] 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.

[0157] In particular, when the photoinitiator includes 2-isopropylthioxanthone, thioxanthone, or a mixture thereof, a lower light dose, e.g., 500 mJ / cm, is required than when using a photoinitiator such as benzophenone. 2 Since photocrosslinking of the polyolefin porous support is possible even at this level, it is more advantageous in terms of mass production.

[0158] Furthermore, when the photoinitiator 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 the mobility of ITX into the substrate, thereby increasing the crosslinking efficiency and easily preventing changes in the physical properties of the final separator.

[0159] In one embodiment of the present invention, the content of the photoinitiator is 0.015 to 0.36 parts by weight based on 100 parts by weight of the polyolefin porous support. When the content of the photoinitiator satisfies the above range, the polyolefin porous support can be crosslinked while preventing excessive crosslinking, so that the final separator does not break even after being exposed to 180°C for 1 minute, has a shutdown temperature of 145°C or less, and exhibits a 50% to 100% ratio of the puncture strength after being exposed to 180°C for 1 minute to the puncture strength at 25°C. Even when the photoinitiator is coated in the above amount, the polyolefin porous support can be crosslinked by irradiation with UV light at a light intensity that ensures mass production (i.e., a light intensity that is lower than conventional light intensity).

[0160] The content of the photoinitiator per 100 parts by weight of the polyolefin porous support can be determined by measuring the content of the photoinitiator 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 per 100 parts by weight of the polyolefin porous support can be determined from the content of the photoinitiator contained in the solvent.

[0161] In the present invention, when the content of the photoinitiator satisfies the above-mentioned range, crosslinking occurs only between polymer chains on which radicals are formed, and the photoinitiator only serves to form radicals on the polymer chains but does not crosslink the polymer chains.

[0162] Furthermore, if an excessive amount of radicals are formed, the radicals may cause excessive scission of the main chain of the polyolefin, which may result in a decrease in the physical properties of the crosslinked structure-containing polyolefin porous support, such as mechanical strength.

[0163] In the present invention, when the content of the photoinitiator satisfies the above-mentioned range, the polyolefin porous support can be crosslinked, and at the same time, it is possible to prevent a decrease in the mechanical strength of the crosslinked structure-containing polyolefin porous support due to excessive scission of the main chain by radicals, etc. Therefore, the separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention does not break even after being exposed to 180°C for 1 minute, and the ratio of the puncture strength after being exposed to 180°C for 1 minute to the puncture strength at 25°C is 50% to 100% or more.

[0164] If the content of the photoinitiator is less than 0.015 parts by weight based on 100 parts by weight of the polyolefin porous support, the crosslinking reaction of the polyolefin porous support does not proceed smoothly even when irradiated with ultraviolet light.

[0165] If the content of the photoinitiator per 100 parts by weight of the polyolefin porous support exceeds 0.36 parts by weight, the separator may shrink due to a rapid crosslinking reaction or the polyolefin main chain may be cleaved, and the ratio of the puncture strength after exposure to 180°C for 1 minute to the puncture strength at 25°C may not meet the 50% to 100% requirement.

[0166] In one embodiment of the present invention, the content of the photoinitiator relative to 100 parts by weight of the polyolefin porous support may be 0.03 to 0.36 parts by weight, 0.015 to 0.09 parts by weight, 0.015 to 0.073 parts by weight, or 0.0365 to 0.07 parts by weight. When the content of the photoinitiator satisfies the above range, the polyolefin porous support can be crosslinked while preventing excessive crosslinking, and the ratio of the puncture strength after exposure to 180°C for 1 minute to the puncture strength at 25°C is 50% to 100%.

[0167] 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.

[0168] 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 1,000 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 the shutdown temperature is 145°C or less, preventing excessive crosslinking reactions. As a result, the ratio of the puncture strength after exposure to 180°C for 1 minute to the puncture strength at 25°C is 50% to 100%.

[0169] Since crosslinking between polyolefin chains occurs not at the ends of the polyolefin chains but throughout the entire polyolefin chain excluding the ends, the number of double bonds present in the polyolefin chain excluding the ends may further affect the generation of radicals. In one embodiment of the present invention, the number of double bonds present in the polyolefin chain excluding the ends of the crosslinked structure-containing polyolefin porous support may be 0.005 to 0.49 per 1,000 carbon atoms.

[0170] 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.

[0171] 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 2 When 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 with the use of a small amount of photoinitiator.

[0172] 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.

[0173] 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.

[0174] 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 1,000 kgf / cm in the machine direction after being exposed to 180°C for 1 minute. 2 It's easier than that.

[0175] 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.

[0176] 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.

[0177] The content of the first antioxidant and the content of the second antioxidant may be the same or different.

[0178] In one embodiment of the present invention, the first antioxidant may comprise a phenolic antioxidant, an amine antioxidant, or a mixture thereof.

[0179] 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.

[0180] 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.

[0181] In one embodiment of the present invention, the second antioxidant may include a phosphorus-based antioxidant, a sulfur-based antioxidant, or a mixture thereof.

[0182] 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.

[0183] 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.

[0184] 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.

[0185] In one embodiment of the present invention, when the antioxidant comprises both 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.

[0186] In one embodiment of the present invention, the step of preparing a polyolefin porous support containing a photoinitiator may include adding the photoinitiator to an extruder for extruding the polyolefin composition to prepare a polyolefin porous support.

[0187] In another embodiment of the present invention, the step of preparing the polyolefin porous support may include coating a photocrosslinking composition containing the photoinitiator and a solvent on the outside of the polyolefin porous support and drying the composition.

[0188] In the present specification, the "step of coating on the outside and drying" includes not only the case of coating and drying a photo-crosslinking composition on the surface of a polyolefin porous support, but also the case of forming another layer on a polyolefin porous support and then coating and drying a photo-crosslinking composition on the surface of the other layer.

[0189] In one embodiment of the present invention, the polyolefin porous support may be subjected to a corona discharge treatment before the photo-crosslinking 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 provided 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 photo-crosslinking composition. This allows for more efficient crosslinking of the polyolefin porous support using the same amount of photoinitiator. The corona discharge treatment may be performed using atmospheric pressure plasma.

[0190] 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.

[0191] In one embodiment of the present invention, the content of the photoinitiator in the photocrosslinking composition is 0.015 to 0.3 parts by weight based on 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 based on 100 parts by weight of the solvent. When the content of the photoinitiator satisfies the above-mentioned range based on the solvent, the polyolefin porous support can be crosslinked, and side reactions due to excessive generation of radicals can be more easily prevented.

[0192] In one embodiment of the present invention, the content of the photoinitiator in the photocrosslinking composition is 0.015 to 0.3 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 2When the content of the photoinitiator satisfies the above range, the polyolefin porous support can be crosslinked, and side reactions caused by excessive generation of radicals can be more easily prevented.

[0193] The content of the photoinitiator based on the specific surface area of ​​the polyolefin porous support can be measured by NMR analysis.

[0194] In one embodiment of the present invention, the photocrosslinking composition may be a photoinitiator solution containing the photoinitiator and the solvent.

[0195] 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.

[0196] 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.

[0197] In yet another embodiment of the present invention, the photocrosslinking composition may be a slurry for forming an inorganic composite porous layer, including an inorganic filler, a binder polymer, the photoinitiator, and the solvent.

[0198] When the photocrosslinking composition is a slurry for forming the inorganic composite porous layer, the photocrosslinking composition is coated onto a polyolefin porous support, and a photoinitiator 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.

[0199] When a slurry for forming an inorganic composite porous layer is used as the photocrosslinking composition, the polyolefin porous support can be photocrosslinked using the inorganic composite porous layer forming process without requiring additional equipment for directly applying the photoinitiator to the polyolefin porous support, such as equipment for directly coating and drying a solution containing the photoinitiator onto the polyolefin porous support.

[0200] Furthermore, the inorganic composite porous layer forming slurry does not require other monomers in addition to the photoinitiator to directly crosslink the polymer chains within the polyolefin porous support. Therefore, even if the photoinitiator is included in the inorganic composite porous layer forming slurry together with the inorganic filler and binder polymer, the monomers do not prevent the photoinitiator from reaching the surface of the polyolefin porous support, and the photoinitiator can be sufficiently introduced to the surface of the polyolefin porous support.

[0201] 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.

[0202] In one embodiment of the present invention, when the photo-crosslinking composition is a slurry for forming an inorganic composite porous layer, the photoinitiator may include 2-isopropylthioxanthone, thioxanthone, or a mixture thereof. 2-Isopropylthioxanthone or thioxanthone can be photo-crosslinked even at long wavelengths with high transmittance. This allows for easy crosslinking of the polyolefin porous support, even when a photoinitiator is included in a slurry for forming an inorganic composite porous layer containing an inorganic filler and a binder polymer.

[0203] 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 the photoinitiator. The solvent may have a solubility index similar to that of the binder polymer to be used and a low boiling point. In this case, uniform mixing and subsequent solvent removal are facilitated. For non-limiting examples of such solvents, see the description of the solvent above.

[0204] For the inorganic filler and the binder polymer, please refer to the above description.

[0205] 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.

[0206] 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.

[0207] In an embodiment of the present invention, when the photo-crosslinking composition is a slurry for forming the inorganic composite porous layer, the photo-crosslinking composition may be coated on the polyolefin porous support, followed by a phase separation process, which may be performed by humidification phase separation or immersion phase separation.

[0208] Among the phase separations, the humidification phase separation will be described below.

[0209] 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.

[0210] 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.

[0211] Among the phase separations, the immersion phase separation will be described below.

[0212] 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.

[0213] 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.

[0214] In another embodiment of the present invention, the step of coating the photocrosslinking composition containing the photoinitiator and the solvent on the outer surface of the polyolefin porous support and drying the same comprises: 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, the photoinitiator, and the solvent, on the upper surface of the inorganic composite porous layer, and drying the coating liquid.

[0215] For the inorganic filler and the slurry for forming the inorganic composite porous layer, please refer to the above contents.

[0216] 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.

[0217] 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.

[0218] In one embodiment of the present invention, the first binder polymer may be insoluble in the solvent and a non-solvent for the second binder polymer, which will be described later. In this case, even if a coating liquid, which will be described later, is applied to form a porous adhesive layer after forming the inorganic composite porous layer, the first binder polymer does not dissolve, which can easily prevent the first binder polymer dissolved in the solvent and / or a non-solvent for the second binder polymer from blocking pores.

[0219] 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.

[0220] 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.

[0221] For the second binder polymer, please refer to the above description.

[0222] 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.

[0223] 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.

[0224] For the type of the solvent, please refer to the above content.

[0225] 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.

[0226] Since the photoinitiator is contained in the coating liquid for forming a porous adhesive layer, when the coating liquid for forming a porous adhesive layer is coated on the upper surface of the inorganic composite porous layer, the photoinitiator is introduced onto the surface of the polyolefin porous support and simultaneously a porous adhesive layer can be formed.

[0227] 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, the photoinitiator contained in the coating liquid for forming a porous adhesive layer is introduced onto the surface of the polyolefin porous support, and upon irradiation with ultraviolet light, the polyolefin porous support can be photo-crosslinked by the photoinitiator present on the surface of the polyolefin porous support.

[0228] Therefore, the method for manufacturing a separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention can simplify the process in that the polyolefin porous support can be photo-crosslinked using a porous adhesive layer formation process without requiring additional equipment for directly applying a photoinitiator to the polyolefin porous support in order to photo-crosslink the polyolefin porous support, for example, equipment for directly coating a solution containing a photoinitiator onto the polyolefin porous support and drying the same.

[0229] The method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention does not require other components, such as a monomer for forming radicals, in addition to the photoinitiator to directly crosslink polymer chains in the polyolefin porous support. Therefore, even when the photoinitiator is added to the coating solution for forming the porous adhesive layer, other components do not prevent the photoinitiator from reaching the surface of the polyolefin porous support, and the photoinitiator can be sufficiently introduced onto the surface of the polyolefin porous support.

[0230] In addition, since the polyolefin porous support itself and the inorganic filler generally have a high UV blocking effect, if UV light is irradiated after the formation of the inorganic composite porous layer and the porous adhesive layer, the amount of UV light 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 light irradiation, and therefore, even if UV light is irradiated after the formation of the inorganic composite porous layer and the porous adhesive layer, the polymer chains in the polyolefin porous support can be crosslinked and directly connected to each other.

[0231] In one embodiment of the present invention, the coating solution for forming the porous coating layer may contain 2-isopropylthioxanthone, thioxanthone, or a mixture thereof as the photoinitiator. 2-Isopropylthioxanthone or thioxanthone can be photocrosslinked even at long wavelengths with high transmittance. This allows the polyolefin porous support to be easily crosslinked by irradiating it with ultraviolet light after the inorganic material-forming porous layer and porous adhesive layer are formed.

[0232] 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.

[0233] 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.

[0234] 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.

[0235] 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.

[0236] 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.

[0237] 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.

[0238] 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.

[0239] 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.

[0240] 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.

[0241] 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.

[0242] 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.

[0243] Then, the polyolefin porous support containing the photoinitiator 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.

[0244] The UV irradiation can be performed using a UV crosslinking device, with the UV irradiation time and dose appropriately adjusted taking into account factors such as the content ratio of the photoinitiator. For example, the UV irradiation time and dose can be set to a value that sufficiently crosslinks the polymer chains in the polyolefin porous support to ensure the desired heat resistance and that prevents damage to the separator due to heat generated by the UV lamp. The UV lamp used in the UV crosslinking device can be appropriately selected from a high-pressure mercury lamp, a metal lamp, a gallium lamp, etc., depending on the photoinitiator used, and the emission wavelength and capacity of the UV lamp can be appropriately selected depending on the process.

[0245] 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 , 50-1000mJ / cm 2 , or 150-500mJ / cm 2 It could be.

[0246] 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.

[0247] 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."

[0248] 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.

[0249] The lithium secondary battery may be in various shapes such as a cylindrical shape, a prismatic shape, or a pouch shape.

[0250] 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.

[0251] 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.

[0252] 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-x Lithium 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 type 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.

[0253] 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.

[0254] 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.

[0255] 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.

[0256] 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.

[0257] 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.

[0258] 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.

[0259] 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.

[0260] 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.

[0261] 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.

[0262] 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.

[0263] 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.

[0264] 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.

[0265] 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]

[0266] 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.

[0267] Example 1 As a photoinitiator, 2-isopropylthioxanthone (manufactured by Sigma Aldrich) was prepared.

[0268] As a polyolefin porous support, a 9.6 μm thick polyethylene porous film (Senior Corporation, weight average molecular weight: 600,000, porosity: 50%) was prepared. The number of double bonds present in the polymer chain during H-NMR measurement was 0.2 per 1000 carbon atoms.

[0269] A high-pressure mercury lamp (Lichtzen high-pressure mercury lamp, LH-250 / 800-A) was prepared as the UV light source.

[0270] The photoinitiator was dissolved in acetone to prepare a photocrosslinking composition containing 0.05 parts by weight of the photoinitiator based on 100 parts by weight of acetone.

[0271] The polyolefin porous support was immersed in the photocrosslinking composition for 30 seconds, then removed, and the coating solution was cut off using a bar to prevent any coating solution from remaining on the surface of the separator, while the content of the photoinitiator was adjusted to 0.0365 parts by weight based on 100 parts by weight of the polyolefin porous support, and the support was dried at room temperature (25°C) for 1 minute.

[0272] Next, the upper surface of the polyolefin porous support coated with the photocrosslinking composition was exposed to an integrated light dose, i.e., UV irradiation dose of 500 mJ / cm 2The cross-linked polyolefin porous support was obtained as a separator containing a cross-linked structure for lithium secondary batteries by irradiating it with UV light at a UV intensity of 80% of that of the UV light source and at a process line speed of 10 m / min.

[0273] Example 2 A crosslinked structure-containing polyolefin porous support was obtained as a crosslinked structure-containing separator for lithium secondary batteries in the same manner as in Example 1, except that thioxanthone (manufactured by TCI) was used instead of 2-isopropylthioxanthone.

[0274] Example 3 As a polyolefin porous support, a 6.5 μm thick polyethylene porous film (Senior Corporation, weight average molecular weight: 600,000, porosity: 50%) was prepared. The number of double bonds present in the polymer chain (excluding the terminals) during H-NMR measurement was 0.1 per 1000 carbon atoms.

[0275] The inorganic filler was prepared by mixing Al2O3 powder with a D50 particle size of 600 nm and γ-AlOOH powder with a D50 particle size of 250 nm in a weight ratio of 9:1. The first binder polymer was an acrylic emulsion (CSB-130, manufactured by Toyo Ink Co., Ltd.), and the dispersant was sodium carboxymethylcellulose (CMC-Na) (SG-L02, manufactured by GL Chem Co., Ltd.).

[0276] 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.

[0277] 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.

[0278] 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 0.1 parts by weight of 2-isopropylthioxanthone (manufactured by Sigma-Aldrich) as a photoinitiator was added per 100 parts by weight of N-methyl-2-pyrrolidone to prepare a coating solution for forming a porous adhesive layer.

[0279] 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.

[0280] 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.

[0281] Comparative Example 1 A 9.6 μm thick polyethylene porous film (Senior Corporation, weight average molecular weight: 600,000, porosity: 50%), which had a double bond count of 0.2 per 1,000 carbon atoms (excluding the terminals) during H-NMR measurement, was used as a separator for a lithium secondary battery without any further treatment.

[0282] Comparative Example 2 A 14.0 μm thick polypropylene porous film (Senior, porosity: 45%) was used as a separator for a lithium secondary battery without any treatment.

[0283] Comparative Example 3 A 16.3 μm thick imide polymer porous film (manufactured by Guhyun Co., Ltd., porosity: 45%) was used as a separator for a lithium secondary battery without any treatment.

[0284] Comparative Example 4 A photocrosslinking composition containing 1.0 part by weight of a photoinitiator based on 100 parts by weight of acetone was used, and the polyolefin porous support was immersed in the photocrosslinking composition for 30 seconds, then removed. The coating solution was then cut off using a bar to prevent any coating solution from remaining on the surface of the separator. A crosslinked structure-containing separator for a lithium secondary battery was prepared in the same manner as in Example 1, except that the content of the photoinitiator was adjusted to 0.7 parts by weight based on 100 parts by weight of the polyolefin porous support.

[0285] Comparative Example 5 A separation membrane was produced in the same manner as in Example 3, except that 2-isopropylthioxanthone was not used.

[0286] Evaluation example: Evaluation of the physical properties of separation membranes The separation membranes produced in Examples 1 to 3 and Comparative Examples 1 to 5 were measured for air permeability, porosity, electrical resistance, meltdown temperature, shutdown temperature, tensile strength at 25°C, puncture strength at 25°C, tensile strength after exposure to 180°C for 1 minute, and puncture strength after exposure to 180°C for 1 minute. The results are shown in Table 1.

[0287] (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. The air permeability values ​​described herein are calculated by measuring the resistance of 100 ml of air to the flow of 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.

[0288] (2) Evaluation of porosity The porosity was measured by measuring the width / length / thickness of the separator to determine the volume, measuring the weight, and calculating the ratio to the weight when the separator occupied 100% of the volume. Porosity (%) = 100 × (1 - weight of separation membrane sample / (width (50 mm) × length (50 mm) × thickness × density of separation membrane sample))

[0289] (3) Evaluation of electrical resistance Coin cells were fabricated using the separators prepared in Examples 1 to 3 and Comparative Examples 1 to 5, and the coin cells were left at room temperature for 1 day, after which the resistance of the separator was measured using an impedance measurement method. The coin cells were fabricated as follows.

[0290] 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.

[0291] 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.

[0292] Cathode manufacturing A positive electrode slurry was prepared by adding LiCoO2 as a positive electrode 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 positive electrode slurry was coated on a sheet-shaped aluminum current collector and dried, resulting in a final positive electrode loading of 3.3 mAh / cm. 2 The positive electrode active material layer was formed so as to have the following structure.

[0293] Coin Cell Manufacturing A separator of each of the above-mentioned 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 injected to prepare a coin cell.

[0294] (4) Evaluation of meltdown temperature The meltdown temperature was measured by thermomechanical analysis (TMA) after a sample was taken in the machine direction (MD) of the separator. Specifically, a 4.5 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 was measured.

[0295] (5) Evaluation of shutdown temperature After fixing the separation membrane to the air permeability measuring device, the air permeability was measured while raising the temperature by 5°C every minute. The air permeability was measured using an Oken air permeability measuring device (manufactured by Asahi Seiko, model: EG01-55-1MR) as the time (seconds) required for 100 ml of air to pass through the separation membrane at a constant pressure of 0.05 MPa. The temperature at which the air permeability of the separation membrane suddenly increased was defined as the shutdown temperature.

[0296] (6) Evaluation of tensile strength at 25°C in the machine direction and cross direction A test piece measuring 100 mm x 15 mm was prepared. According to ASTM D882, the specimen was pulled at 25°C in both the machine direction (MD) and the transverse direction (TD) at a rate of 50 mm / min using Universal Testing Systems (Instron® 3345). The strength at which the specimen broke was defined as the tensile strength in the machine direction (MD) and the transverse direction (TD).

[0297] (7) Puncture strength at 25°C A test piece measuring 50 mm x 50 mm was prepared. The puncture strength was measured at 25°C after setting a 1 mm round tip to operate at a speed of 120 mm / min according to ASTM D2582.

[0298] (8) Evaluation of tensile strength in the machine and transverse directions after exposure to 180°C for 1 minute A test piece measuring 100 mm x 15 mm was prepared. The specimen was exposed to 180°C for 1 minute, and then pulled in both the machine and transverse directions at a rate of 50 mm / min according to ASTM D882 at 25°C using Universal Testing Systems (Instron® 3345). The strength at which the specimen broke was defined as the tensile strength in the machine and transverse directions.

[0299] (9) Puncture strength after exposure to 180°C for 1 minute A test piece measuring 50 mm x 50 mm was prepared. The specimen was exposed to 180°C for 1 minute, and then the puncture strength of the specimen was measured at 25°C using a 1 mm round tip set to operate at a speed of 120 mm / min according to ASTM D2582.

[0300] (10) Evaluation of whether the separator breaks after being exposed to 180°C for 1 minute A frame measuring 20 cm wide x 20 cm long was prepared, and a separation membrane was fixed to it using heat-resistant tape. The frame was then placed in a high-temperature oven set to 180°C and left for 1 minute, after which the separation membrane was visually inspected for rupture.

[0301] [Table 1]

[0302] As can be seen from Table 1, the separators prepared in Examples 1 to 3 had a shutdown temperature of 145°C or less, did not break when exposed to 180°C for 1 minute, and had a puncture strength ratio of 50% or more after exposure to 180°C for 1 minute relative to the puncture strength at 25°C. In other words, it was confirmed that the separators prepared in Examples 1 to 3 maintained a reasonable level of mechanical strength even after exposure to 180°C for 1 minute.

[0303] On the other hand, the separation membrane produced in Comparative Example 1 had a shutdown temperature of 140°C or lower, but was broken when exposed to 180°C for 1 minute.

[0304] The separator produced in Comparative Example 2 had a shutdown temperature exceeding 140°C, and the separator broke when exposed to 180°C for 1 minute.

[0305] The separator prepared in Comparative Example 3 did not break when exposed to 180°C for 1 minute, but the shutdown temperature exceeded 140°C.

[0306] The separator prepared in Comparative Example 4 had a shutdown temperature of 145°C or less, and did not break when exposed to 180°C for 1 minute. However, the ratio of the puncture strength after exposure to 180°C for 1 minute to the puncture strength at 25°C was less than 50%. This indicates that the mechanical strength of the separator is significantly reduced when exposed to a high temperature of 180°C.

[0307] The separator prepared in Comparative Example 5 had a shutdown temperature of 145°C or less, and did not break when exposed to 180°C for 1 minute. However, the ratio of the puncture strength after exposure to 180°C for 1 minute to the puncture strength at 25°C was less than 50%. This indicates that the mechanical strength of the separator is significantly reduced when exposed to a high temperature of 180°C.

Claims

1. It has a cross-linked structure in which polymer chains are directly linked to each other, The shutdown temperature is 145°C or less, It did not break even after being exposed to 180°C for 1 minute. A separator containing a crosslinked structure for a lithium secondary battery, comprising a porous support of a polyolefin containing a crosslinked structure, the ratio of the puncture strength after exposure to 180°C for 1 minute to the puncture strength at 25°C being 50% to 100%.

2. 2. The separator for a lithium secondary battery according to claim 1, wherein the porous support body having a crosslinked structure has a puncture strength of 50 gf or more after being exposed to 180° C. for 1 minute.

3. After being exposed to 180°C for 1 minute, the crosslinked structure-containing polyolefin porous support has a tensile strength of 500 kgf / cm in both the machine direction (MD) and the transverse direction (TD). ２ The separator containing a crosslinked structure for a lithium secondary battery according to claim 1 .

4. 2. The separator for a lithium secondary battery according to claim 1, wherein the degree of crosslinking of the crosslinked structure-containing polyolefin porous support is 10% to 45%.

5. 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.

6. The separator containing a crosslinked structure for a lithium secondary battery is an inorganic composite porous layer located on at least one surface of the crosslinked structure-containing polyolefin porous support, the inorganic composite porous layer including 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.

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. 7. The separator for a lithium secondary battery according to claim 6, wherein a weight ratio of the inorganic filler to the first binder polymer is 95:5 to 99.9:0.

1.

9. 7. The separator for a lithium secondary battery according to claim 6, wherein the first binder polymer comprises an acrylic polymer, polyacrylic acid, styrene-butadiene rubber, polyvinyl alcohol, or two or more thereof.

10. 7. The cross-linked structure-containing separator for a lithium secondary battery according to claim 6, wherein the second binder polymer comprises 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.

11. providing a polyolefin porous support containing a photoinitiator; and irradiating the polyolefin porous support containing the photoinitiator with ultraviolet light, The content of the photoinitiator is 0.015 to 0.07 parts by weight based on 100 parts by weight of the polyolefin porous support; the photoinitiator comprises 2-isopropylthioxanthone, thioxanthone, or a mixture thereof; The method for producing a separator containing a crosslinked structure for a lithium secondary battery, wherein the ultraviolet irradiation dose is 150 to 500 mJ / cm 2 .

12. providing a polyolefin porous support containing the photoinitiator, 12. The method for preparing a separator having a crosslinked structure for a lithium secondary battery according to claim 11, comprising coating a photo-crosslinking composition containing the photoinitiator and a solvent on the outer surface of the polyolefin porous support, and drying the composition.

13. 13. The method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery according to claim 12, wherein the photocrosslinking composition is a slurry for forming an inorganic composite porous layer, the slurry comprising an inorganic filler, a binder polymer, the photoinitiator, and the solvent.

14. a step of coating a photocrosslinking composition containing the photoinitiator and a solvent on the outer surface of the polyolefin porous support and drying the coating; 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; 13. The method of claim 12, further comprising: coating a coating liquid for forming a porous adhesive layer, the coating liquid including a second binder polymer, the photoinitiator, and the solvent, on the upper surface of the inorganic composite porous layer, and drying the coating liquid.

15. A lithium secondary battery comprising 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 is the crosslinked structure-containing separator for lithium secondary batteries according to claim 1 .

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