Method for manufacturing a separator containing a cross-linked structure for a lithium secondary battery, a separator containing a cross-linked structure for a lithium secondary battery manufactured by the method, and a lithium secondary battery including the same

Abstract

In the present invention, the content of the photoinitiator is 0.01 mg / m based on the specific surface area of ​​the polyolefin porous support. 2 ~1.0mg / m 2 The photoinitiator is added so that the BET specific surface area is 10 m 2 / g~27m 2 The present invention relates to a method for producing a separator containing a crosslinked structure for a lithium secondary battery, the method including the steps of preparing a polyolefin porous support having a specific surface area of ​​100 nm to 150 nm and irradiating the support with ultraviolet light, and a separator containing a crosslinked structure for a lithium secondary battery produced by the method, and a lithium secondary battery including the separator. By adjusting the BET specific surface area of ​​the polyolefin porous support, the polyolefin porous support can be crosslinked with only a small amount of a photoinitiator.

Description

[Technical Field]

[0001] This application claims priority from Korean Patent Application No. 10-2021-0059587, filed on May 7, 2021.

[0002] The present invention relates to a method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery, a separator containing a crosslinked structure for a lithium secondary battery manufactured by the method, and a lithium secondary battery including the same. [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] Polyolefin separators are widely used as such separators, but polyethylene (PE) separators, a typical polyolefin separator, have a low melting point (Tm). Therefore, if the battery temperature rises above the melting point of polyethylene during battery misuse, a meltdown phenomenon may occur, leading to fire and explosion. Furthermore, due to the characteristics of the material and manufacturing process, separators exhibit severe thermal shrinkage in high temperature situations, which can lead to safety issues such as internal short circuits.

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

[0007] The present invention provides a method for preparing a separator having a crosslinked structure for a lithium secondary battery, which can crosslink a polyolefin porous support with only a small amount of photoinitiator.

[0008] Another object of the present invention is to provide a separator containing a crosslinked structure for a lithium secondary battery, which has improved safety at high temperatures, and a lithium secondary battery including the separator. [Means for solving the problem]

[0009] 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.

[0010] The first embodiment is providing a polyolefin porous support containing a photoinitiator; and irradiating the polyolefin porous support with ultraviolet light, The polyolefin porous support is BET specific surface area is 10m 2 / g~27m 2 / g, The content of the photoinitiator is 0.01 mg / m based on the specific surface area of ​​the polyolefin porous support. 2 ~1.0mg / m 2 The present invention relates to a method for producing a separator containing a crosslinked structure for a lithium secondary battery.

[0011] According to the second embodiment, in the first embodiment, providing a polyolefin porous support containing the photoinitiator, feeding a polyolefin and a liquid phase diluent to an extruder; extruding a polyolefin composition from the extruder; passing the extruded polyolefin composition through a die and a chill roll to form and stretch the composition into a sheet; extracting the liquid diluent from the stretched sheet to produce a pre-porous support; The pre-porous support was heat-set to a BET specific surface area of ​​10 m 2 / g~27m 2 / g of the polyolefin porous support; and coating the photocrosslinking composition containing the photoinitiator and a solvent on the outside of the polyolefin porous support and drying the composition.

[0012] According to the third embodiment, in the second embodiment, The liquid diluent may have a kinematic viscosity at 40°C of 25 cSt to 100 cSt.

[0013] According to the fourth embodiment, in the second or third embodiment, The temperature of the cooling roll may be 30°C to 65°C.

[0014] According to the fifth embodiment, in any one of the second to fourth embodiments, The heat setting temperature may be 125°C to 132°C.

[0015] According to the sixth embodiment, in any one of the second to fifth embodiments, The photocrosslinking composition may be a photoinitiator solution containing the photoinitiator and the solvent.

[0016] According to the seventh embodiment, in any one of the second to fifth embodiments, 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.

[0017] According to the eighth embodiment, in any one of the second to fifth embodiments, 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.

[0018] According to the ninth embodiment, in any one of the first to eighth embodiments, The photoinitiator may include a Type II photoinitiator.

[0019] According to the tenth embodiment, in any one of the first to ninth embodiments, The photoinitiator may include thioxanthone (TX), a thioxanthone derivative, benzophenone (BPO), a benzophenone derivative, or two or more thereof.

[0020] According to an eleventh embodiment, in any one of the first to tenth embodiments, The irradiation amount of the ultraviolet light is 10 to 2000 mJ / cm 2 It could be.

[0021] 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.

[0022] The twelfth embodiment is: The present invention relates to a separator having a crosslinked structure for a lithium secondary battery, which is manufactured according to any one of the first to eleventh embodiments.

[0023] The thirteenth embodiment is The porous support includes a crosslinked structure-containing polyolefin having a crosslinked structure in which polymer chains are directly linked to each other, The crosslinked structure-containing polyolefin porous support has a BET specific surface area of ​​10 m 2 / g~27m 2 / g.

[0024] According to the fourteenth embodiment, in the thirteenth embodiment, The crosslinked structure-containing polyolefin porous support may have an average pore diameter of 30 nm to 80 nm.

[0025] According to the fifteenth embodiment, in the thirteenth or fourteenth embodiment, The crosslinked structure-containing polyolefin porous support may have a porosity of 45% to 70%.

[0026] According to the 16th embodiment, in any one of the 13th to 15th embodiments, The degree of crosslinking of the crosslinked structure-containing polyolefin porous support may be 10% to 45%.

[0027] According to the seventeenth embodiment, in any one of the thirteenth to sixteenth embodiments, The porous support 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.

[0028] According to the 18th embodiment, in any one of the 13th to 16th embodiments, The separation membrane 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; The porous adhesive layer may further include a porous adhesive layer positioned on the inorganic composite porous layer and including a second binder polymer.

[0029] According to the 19th embodiment, in any one of the 13th to 18th embodiments, The cross-linked structure-containing separator for a lithium secondary battery may have a meltdown temperature of 160° C. or higher.

[0030] According to the 20th embodiment, in any one of the 13th to 19th embodiments, The separator for a lithium secondary battery having a crosslinked structure may have a shutdown temperature of 145° C. or less.

[0031] 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.

[0032] The 21st embodiment is The lithium secondary battery includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, The separator for a lithium secondary battery may be a separator containing a crosslinked structure for a lithium secondary battery according to any one of the thirteenth to twentieth embodiments. [Effects of the Invention]

[0033] According to one aspect of the present invention, a method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery is provided. 2 / g~27m 2 Since the crosslinking efficiency is high using a polyolefin porous support having a crosslinking rate of 1000 kJ / g, the polyolefin porous support can be crosslinked with only a small amount of photoinitiator.

[0034] The method for manufacturing a separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention can crosslink a polyolefin porous support with only a small amount of photoinitiator, thereby minimizing side reactions.

[0035] The separator for a lithium secondary battery according to one embodiment of the present invention has excellent heat resistance by including a porous support body having a crosslinked structure in which polymer chains are directly linked to each other.

[0036] 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]

[0037] [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

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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 with ultraviolet light, The polyolefin porous support has a BET specific surface area of ​​10 m 2 / g~27m 2 / g, and the content of the photoinitiator is 0.01 mg / m based on the specific surface area of ​​the polyolefin porous support. 2 ~1.0mg / m 2 is.

[0042] 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.

[0043] 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.

[0044] The photoinitiator directly photocrosslinks the polymer chains within the polyolefin porous support.

[0045] The photoinitiator alone can crosslink the polyolefin porous substrate without the need for other components such as a crosslinker, coinitiator, or synergist. Upon light absorption alone, hydrogen atoms within the photoinitiator are removed through a hydrogen abstraction reaction, turning the photoinitiator into a reactive compound. This photoinitiator then forms radicals on the polymer chains within the polyolefin porous substrate, making the polymer chains reactive and directly linking them to form photocrosslinks. 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 or branched structures within the polyolefin to form radicals on the polymer chains.

[0046] 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.

[0047] Since the photoinitiator plays a role in forming radicals on the polymer chains in the polyolefin porous support, the content of the photoinitiator is very important in the formation of radicals.

[0048] If an excessive amount of photoinitiator is used to increase the crosslinking efficiency of the polyolefin porous support, the photoinitiator may cause a side reaction. That is, the photoinitiator aims to crosslink polyolefin chains by using the polyolefin as a hydrogen donor (H donor). However, as the photoinitiator content increases, the photoinitiator may react with the polymer chains or with other photoinitiators, resulting in the formation of undesired reaction products. In particular, photoinitiators that react with polyolefin chains are undesirable because they lower the melting temperature of the polyolefin chains, degrading the properties of the separator.

[0049] Therefore, it is necessary to minimize the content of the photoinitiator, but reducing the content of the photoinitiator makes it difficult to crosslink the polyolefin porous support.

[0050] The present inventors have discovered that by adjusting the BET specific surface area of ​​the polyolefin porous support to a specific level, the crosslinking efficiency of the polyolefin porous support can be increased while minimizing the content of the photoinitiator, and have completed the present invention.

[0051] As the surface area of ​​the polyolefin porous support increases, the area where crosslinking occurs between polymer chains increases, and the crosslinking efficiency of the polyolefin porous support increases.

[0052] The polyolefin porous support has a BET specific surface area of ​​10 m 2 / g~27m 2When the BET specific surface area of ​​the polyolefin porous support satisfies the above range, the surface area of ​​the polyolefin porous support into which the photoinitiator is introduced increases, and the crosslinking efficiency of the polyolefin porous support can be increased even when a small amount of photoinitiator is used.

[0053] The polyolefin porous support has a BET specific surface area of ​​10 m 2 If the amount is less than 1 / g, the surface area of ​​the polyolefin porous support into which the photoinitiator is introduced is reduced, making it difficult to ensure crosslinking efficiency with only a small amount of photoinitiator.

[0054] The polyolefin porous support has a BET specific surface area of ​​27 m 2 If it exceeds 1 / g, the photoinitiator penetrates into the pores of the polyolefin porous support, making it difficult to crosslink the surfaces of the polyolefin chains in the polyolefin porous support, and therefore crosslinking of the polyolefin porous support does not proceed smoothly.

[0055] In one embodiment of the present invention, the polyolefin porous support has a BET specific surface area of ​​13 m 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 crosslinking efficiency of the polyolefin porous support can be increased.

[0056] 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 polyolefin porous support can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-MAX G manufactured by MicrotracBEL.

[0057] In one embodiment of the present invention, the average pore diameter of the polyolefin porous support may be 30 nm to 80 nm, 30 nm to 75 nm, or 30 nm to 70 nm. When the average pore diameter of the polyolefin porous support satisfies the above range, the BET specific surface area of ​​the polyolefin porous support is 10 m 2 / g~27m 2 / g.

[0058] As used herein, "average pore size" refers to the arithmetic mean value of a large number of pore sizes. The average pore size of the polyolefin porous support or the crosslinked polyolefin porous support described below can be measured by the BET 6-point method using a mercury porosimeter, capillary flow porometry, or a porosimetry analyzer (Bell Japan, Belsorp-II mini) via nitrogen gas adsorption flow. Capillary flow porometry provides the most accurate measurement results because it requires the lowest pressure to apply to the separation membrane specimen.

[0059] In one embodiment of the present invention, the porosity of the polyolefin porous support may be 45% to 70%, 45% to 65%, or 45% to 60%. When the porosity of the polyolefin porous support satisfies the above range, sufficient lithium ion migration paths can be secured while achieving high crosslink density with only a small amount of photocrosslinking initiator added.

[0060] The porosity of the polyolefin porous support can be calculated based on the average thickness and weight measured by taking a separator sample of a certain area and taking into account the density of the polyolefin.

[0061] In one embodiment of the present invention, the polyolefin porous support may have a number of double bonds present in the polyolefin chain per 1,000 carbon atoms as measured by H-NMR of 0.01 to 0.5, or 0.01 to 0.3. When the polyolefin porous support has the above-mentioned number of double bonds, it is possible to control radicals formed by a hydrogen abstraction reaction by a photoinitiator from the double bond structure present in the polyolefin chain, thereby effectively crosslinking the polyolefin porous support and minimizing side reactions caused by excessive generation of radicals.

[0062] 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 0.49 per 1,000 carbon atoms. In this specification, "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.

[0063] 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.

[0064] In one embodiment of the present invention, the polyolefin porous support may be a porous film.

[0065] 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.

[0066] In another embodiment of the present invention, the step of preparing a polyolefin porous support containing a photoinitiator includes the steps of feeding a polyolefin and a liquid phase diluent to an extruder, extruding a polyolefin composition from the extruder, passing the extruded polyolefin composition through a die and a chill roll to form and stretch the extruded polyolefin composition into a sheet, extracting the liquid phase diluent from the stretched sheet to prepare a pre-porous support, and heat-setting the pre-porous support to prepare a polyolefin porous support having a BET specific surface area of ​​10 m 2 / g~27m 2 / g of the polyolefin porous support; and coating a photocrosslinking composition containing the photoinitiator and a solvent on the outside of the polyolefin porous support and drying the composition.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] The weight average molecular weight can be measured using gel permeation chromatography (GPC, PL GPC220, manufactured by Agilent Technologies) under the following conditions.

[0071] -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)

[0072] In one embodiment of the present invention, the liquid diluent may have a kinematic viscosity at 40°C of 25 cSt to 100 cSt, 30 cSt to 95 cSt, or 35 cSt to 90 cSt. When the liquid diluent has a kinematic viscosity at 40°C in the above range, the BET specific surface area of ​​the liquid diluent is 10 m 2 / g~27m 2 It is easy to produce a polyolefin porous support having an average pore diameter of 30 nm to 80 nm.

[0073] In this specification, the term "kinematic viscosity" refers to viscosity taking gravity into consideration and corresponds to the value obtained by dividing absolute viscosity by melt density. Here, absolute viscosity refers to viscosity ignoring gravity and refers to the inherent viscous resistance of an object itself.

[0074] The kinematic viscosity may be measured with a Brookfield rotational viscometer.

[0075] In one embodiment of the present invention, the liquid diluent may include paraffin; wax; soybean oil; phthalate esters such as dibutyl phthalate, dihexyl phthalate, and dioctyl phthalate; aromatic ethers such as diphenyl ether and benzyl ether; fatty acids having 10 to 20 carbon atoms such as palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid; fatty acid alcohols having 10 to 20 carbon atoms such as palmitic alcohol, stearic alcohol, and oleic alcohol; saturated and unsaturated fatty acids having a fatty acid group with 4 to 26 carbon atoms, such as palmitic acid mono-, di-, or triester, stearic acid mono-, di-, or triester, oleic acid mono-, di-, or triester, and linoleic acid mono-, di-, or triester, or fatty acid esters in which one or more fatty acids in which the double bond of an unsaturated fatty acid is substituted with epoxy are ester-bonded to an alcohol having 1 to 10 carbon atoms and 1 to 8 hydroxy groups; or two or more of these.

[0076] The weight ratio of the polyolefin to the liquid diluent may be 50:50 to 20:80, or 40:60 to 30:70. When the weight ratio of the polyolefin to the liquid diluent satisfies the above range, the polyolefin porous support finally produced can have an appropriate level of porosity and average pore size, the pores can be interconnected to improve permeability, an increase in extrusion load can be prevented, and a viscosity that is easy to process can be ensured. Furthermore, problems such as breakage and thickness variation during stretching, which are caused by the polyolefin being extruded in a gel form without being thermodynamically mixed with the liquid diluent, can be prevented. Furthermore, a decrease in the strength of the finally produced polyolefin porous support can be easily prevented.

[0077] In addition to the polyolefin and diluent, an antioxidant may be added to the extruder. The antioxidant controls the radicals formed on the polyolefin chains, thereby regulating the cross-linking reaction between polymer chains. The antioxidant either oxidizes the polymer chains to prevent their oxidation, or absorbs the generated radicals to regulate the cross-linking reaction between polymer chains. This can affect the shutdown temperature and mechanical strength of the final separator.

[0078] 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, thereby easily preventing side reactions and easily preventing the surface of the polyolefin porous support from becoming uneven.

[0079] 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.

[0080] 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.

[0081] The content of the first antioxidant and the content of the second antioxidant may be the same or different.

[0082] In one embodiment of the present invention, the first antioxidant may comprise a phenolic antioxidant, an amine antioxidant, or a mixture thereof.

[0083] 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.

[0084] 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.

[0085] In one embodiment of the present invention, the second antioxidant may include a phosphorus-based antioxidant, a sulfur-based antioxidant, or a mixture thereof.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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. The extrusion may be carried out at a temperature of 160°C to 240°C for several minutes using a single-screw or twin-screw extruder. For uniform reactive extrusion, an extruder with a screw L / D (length / diameter) of 30 or more may be used.

[0090] The extruded polyolefin composition is cooled while passing through a die and a cooling roll, during which phase separation between the polyolefin and the liquid diluent may occur. The time for phase separation between the polyolefin and the liquid diluent may affect the BET specific surface area, average pore size, pore pattern, etc. of the final polyolefin porous support.

[0091] In one embodiment of the present invention, the temperature of the cooling roll may be 30°C to 65°C, or 35°C to 60°C. Generally, the temperature of the cooling roll is 20°C to 25°C. In the method for producing a separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention, the temperature of the cooling roll is increased to 30°C to 65°C, allowing sufficient time for phase separation between the polyolefin and the liquid diluent. As a result, the BET specific surface area of ​​the finally produced polyolefin porous support is 10 m 2 / g~27m 2 / g. In addition, it is easy to produce a polyolefin porous support having an average pore diameter of 30 nm to 80 nm.

[0092] In one embodiment of the present invention, the stretching may be performed sequentially or simultaneously using a roll or tenter. The stretching ratio may be 3x or more, or 5x to 10x, in both the longitudinal and transverse directions, with a total stretching ratio of 20x to 80x. When the stretching ratio is within the above range, problems such as insufficient orientation in one direction, resulting in imbalance of physical properties between the machine direction and the transverse direction, and reduced tensile strength and puncture strength, can be easily prevented. In addition, problems such as understretching, resulting in no pore formation, or breakage during stretching, resulting in increased shrinkage of the final film, can be easily prevented.

[0093] In this specification, the term "machine direction (MD)" refers to the direction in which a separation membrane travels when continuously produced or the longitudinal direction of the separation membrane in the direction in which the produced separation membrane is wound up, and the term "transverse direction (TD)" refers to the transverse direction of the machine direction, i.e., the direction perpendicular to the direction in which a separation membrane travels when continuously produced or the direction perpendicular to the longitudinal direction of the separation membrane in the direction in which the produced separation membrane is wound up.

[0094] The stretching temperature may vary depending on the melting point of the polyolefin used and the concentration and type of liquid diluent, and may be selected within a temperature range in which 30% to 80% by weight of the crystalline portion of the polyolefin in the sheet melts. When the stretching temperature is within the above range, the sheet does not become soft, which can easily prevent breakage or incomplete stretching during stretching. Furthermore, it can easily prevent uneven thickness due to partial overstretching or reduced polyolefin orientation, which can reduce physical properties. Meanwhile, the degree of melting of the crystalline portion as a function of temperature can be measured by DSC (differential scanning calorimetry) of the sheet.

[0095] In the step of extracting the liquid diluent from the stretched sheet, the liquid diluent may be extracted using an organic solvent. Specifically, the liquid diluent may be extracted from the stretched sheet using an organic solvent, followed by drying. The organic solvent is not particularly limited as long as it can extract the liquid diluent. For example, methyl ethyl ketone, methylene chloride, hexane, etc. may be used.

[0096] The extraction method may be any common solvent extraction method, such as immersion, solvent spray, or ultrasonic, either singly or in combination. After the extraction process, the content of the liquid diluent remaining may be 1 wt% or less based on 100 wt% of the polyolefin porous support. In this case, deterioration of the physical properties of the polyolefin porous support, resulting in a decrease in permeability, can be easily prevented.

[0097] The content of the residual liquid diluent is affected by the extraction temperature and extraction time, which may be below 40°C, taking into account the increased solubility of the diluent in the organic solvent and the safety issue of boiling the organic solvent.

[0098] The extraction time varies depending on the thickness of the polyolefin porous support to be produced, but may be 2 to 4 minutes in the case of a polyolefin porous support having a thickness of 10 μm to 30 μm.

[0099] In the heat setting, the preliminary porous support is fixed and heat is applied to forcibly fix the polyolefin porous support that is finally manufactured so as to prevent shrinkage, thereby removing residual stress.

[0100] In one embodiment of the present invention, the heat setting temperature may be 125° C. to 132° C., or 125° C. to 130° C. When the heat setting temperature is within the above range, it is possible to easily prevent the pore size of the finally produced polyolefin porous support from excessively increasing.

[0101] In one embodiment of the present invention, the content of the photoinitiator is 0.01 mg / m based on the specific surface area of ​​the polyolefin porous support. 2 ~1.0mg / m 2 When the content of the photoinitiator satisfies the above range, the polyolefin porous support can be effectively crosslinked and side reactions caused by excessive radical generation can be prevented. For example, side reactions can be prevented by preventing excessive radical generation, which would result in crosslinking between photoinitiators or between the photoinitiator and polymer chains, and by allowing crosslinking only between polymer chains. Even when the photoinitiator is coated in the above content, the polyolefin porous support can be crosslinked by irradiation with ultraviolet light at a light intensity that ensures mass production productivity (i.e., a light intensity that is lower than conventional light intensity).

[0102] In addition, excessive generation of radicals can be prevented, which can cause rapid crosslinking reaction and shrinkage of the separator, thereby preventing a decrease in the air permeability of the polyolefin porous support after crosslinking.

[0103] In addition, it is possible to prevent excessive main chain scission of the polyolefin from occurring, which would otherwise reduce the mechanical strength of the polyolefin porous support.

[0104] The content of the photoinitiator is 0.01 mg / m based on the specific surface area of ​​the polyolefin porous support. 2 If the temperature is less than this, radicals are not formed to an extent that allows the polyolefin porous support to be sufficiently crosslinked, and the crosslinking of the polyolefin porous support does not proceed smoothly.

[0105] The content of the photoinitiator is 1.0 mg / m based on the specific surface area of ​​the polyolefin porous support. 2If the temperature exceeds this range, the polyolefin porous support is crosslinked, but excessive radicals are generated, causing side reactions. For example, photoinitiators may crosslink with each other or with the polyolefin chain. Furthermore, the crosslinking reaction occurs rapidly upon UV irradiation, causing the separator to shrink and the polyolefin main chain to scission, resulting in a decrease in mechanical strength.

[0106] In one embodiment of the present invention, the content of the photoinitiator is 0.03 mg / m based on the specific surface area of ​​the polyolefin porous support. 2 ~0.8mg / m 2 , or 0.06 mg / m 2 ~0.7mg / m 2 When the content of the photoinitiator satisfies the above range, the polyolefin porous support can be effectively crosslinked and side reactions caused by excessive generation of radicals can be more easily prevented. The content of the photoinitiator based on the specific surface area of ​​the polyolefin porous support can be measured by NMR analysis.

[0107] In one embodiment of the present invention, the photoinitiator may include a Type II photoinitiator.

[0108] 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.

[0109] 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-phenoxythioxanthone, 6-ethoxycarbonyl-2-methoxythioxanthone, 6-ethoxycarbonyl-2-methylthioxanthone, thioxanthone-2-polyethylene glycol ester, 2-hydroxy-3-(3,4-dimethyl-9-oxo-9H-thioxanthone-2-yloxy)-N,N,N-trimethyl-1-propanaminium chloride, and the like.

[0110] 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.

[0111] 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 a photocrosslinking initiator such as benzophenone is used. 2 Since photocrosslinking of the polyolefin porous support is possible even at this level, it is more advantageous in terms of mass production.

[0112] 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 mobility of the ITX into the pores of the polyolefin porous support, thereby increasing crosslinking efficiency and easily preventing changes in the physical properties of the final separator.

[0113] 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.

[0114] 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.

[0115] In the present specification, the "step of coating on the outside and drying" includes not only the case where a photo-crosslinking composition is coated on the surface of a polyolefin porous support and then dried, but also the case where another layer is formed on a polyolefin porous support and then a photo-crosslinking composition is coated on the surface of the other layer and then dried.

[0116] In one embodiment of the present invention, the polyolefin porous support may be corona discharge treated before the photoinitiator solution 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 photocrosslinking composition. This allows for more efficient crosslinking of the polyolefin porous support even when the same amount of photoinitiator is used. The corona discharge treatment may be performed using atmospheric pressure plasma.

[0117] 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.

[0118] In one embodiment of the present invention, the content of the photoinitiator in the photocrosslinking composition is 0.01 mg / m based on the specific surface area of ​​the polyolefin porous support. 2 ~1.0mg / m 2 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.

[0119] When 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.

[0120] In addition, in one embodiment of the present invention, the content of the photoinitiator in the photocrosslinking composition is 0.01 mg / m based on the specific surface area of ​​the polyolefin porous support. 2 ~1.0mg / m 2and may be 0.015 to 0.36 parts by weight, 0.015 to 0.09 parts by weight, or 0.015 to 0.07 parts by weight, relative to 100 parts by weight of the polyolefin porous support. When the content of the photoinitiator 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.

[0121] 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.

[0122] In one embodiment of the present invention, the photocrosslinking composition may be a photoinitiator solution containing the photoinitiator and the solvent.

[0123] 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.

[0124] 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 in a batch or continuous manner using an oven or a 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 photocrosslinking initiator solution, and is preferably performed as quickly as possible considering productivity, for example, within 1 minute or 30 seconds.

[0125] 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.

[0126] 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.

[0127] 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.

[0128] 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.

[0129] Generally, since the polyolefin porous support itself and the inorganic filler have a high ultraviolet blocking effect, if an inorganic composite porous layer containing an inorganic filler is formed and then irradiated with ultraviolet light, the amount of ultraviolet light reaching the polyolefin porous support may be reduced. However, in the present invention, even if ultraviolet light is irradiated after the inorganic composite porous layer is formed, the polymer chains within the polyolefin porous support can be crosslinked and directly linked.

[0130] In one embodiment of the present invention, when the photo-crosslinking composition is a slurry for forming the 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 the polyolefin porous support to be easily crosslinked by irradiating ultraviolet light after the inorganic composite porous layer is formed.

[0131] 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.

[0132] 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.

[0133] 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.

[0134] In another embodiment of the present invention, the inorganic filler may be an inorganic filler having lithium ion transport ability, i.e., an inorganic filler that contains lithium element but does not store lithium but has the function of transporting lithium ions. Non-limiting examples of inorganic fillers having lithium ion transport ability include lithium phosphate (LiPO), lithium titanium phosphate (Li x Ti y (PO4)3,0 <x<2、0<y<3)、リチウムアルミニウムチタンホスフェート(Li x Al y Ti z (PO4)3,0 <x<2、0<y<1、0<z<3)、14Li2O-9Al2O3-38TiO2-39P2O5などの(LiAlTiP) x O y Glass (0 <x<4、0<y<13)、リチウムランタンチタネート(Li x La y TiO3, 0 <x<2、0<y<3)、Li 3.25 Ge 0.25 P 0.75 Lithium germanium thiophosphate (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 glass such as Li3PO4-Li2S-SiS2 (Li x Si y S z , (0 < x < 3, 0 < y < 2, 0 < z < 4), P2S5-based glass such as LiI-Li2S-P2S5 (Li x P y S z , (0 < x < 3, 0 < y < 3, 0 < z < 7), or mixtures thereof, etc. may be mentioned.

[0135] In one embodiment of the present invention, the average particle size of the inorganic filler can be 0.01 μm to 1.5 μm. When the average particle size of the inorganic filler satisfies the above-mentioned range, it is easy to form an inorganic composite void layer having a uniform thickness and appropriate porosity, the dispersibility of the inorganic filler is good, and a desired energy density can be achieved. The average particle size of the inorganic filler means the particle size D

[0136] At this time, the average particle size of the inorganic filler means the particle size D 50 particle size, and "D 50 particle size" means the particle size at the 50% point of the particle number cumulative distribution according to the particle size. The particle size can be measured using the laser diffraction method. Specifically, after dispersing the powder to be measured in a dispersion medium, it is introduced into a commercially available laser diffraction particle size measuring device (for example, Microtrac S3500). When the particles pass through the laser beam, the difference in the diffraction pattern according to the particle size is measured to calculate the particle size distribution. By calculating the particle diameter at the point where the particle number cumulative distribution according to the particle size in the measuring device reaches 50%, D 50 particle size can be measured.

[0137] The binder polymer may be dissolved in the solvent according to the type of the binder polymer, or may be dispersed without being dissolved in the solvent.

[0138] The binder polymer has a glass transition temperature (T g ) may be -200 to 200°C. When the glass transition temperature of the binder polymer satisfies the above range, the mechanical properties such as flexibility and elasticity of the finally formed inorganic composite porous layer 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.

[0139] 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.

[0140] 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.

[0141] 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 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 can be formed between the inorganic fillers, making it easy to ensure the pore size and porosity of the inorganic composite porous layer. In addition, the adhesive strength between the inorganic fillers can be easily ensured.

[0142] 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.

[0143] In one embodiment of the present invention, the inorganic composite porous layer forming slurry 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.

[0144] 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.

[0145] 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.

[0146] Among the phase separations, the humidification phase separation will be described below.

[0147] 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.

[0148] 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.

[0149] Among the phase separations, the immersion phase separation will be described below.

[0150] 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.

[0151] The coagulation liquid may be a non-solvent for the binder polymer alone or a mixture of a non-solvent and the above-mentioned solvent. When a mixture of a non-solvent and a solvent is used, the content of the non-solvent 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.

[0152] 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.

[0153] For the inorganic filler, please refer to the above content.

[0154] 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.

[0155] 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.

[0156] 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.

[0157] 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.

[0158] In one embodiment of the present invention, the first binder polymer may be a binder polymer having 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.

[0159] 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.

[0160] 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.

[0161] 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.

[0162] For the inorganic composite porous layer forming slurry, please refer to the above description.

[0163] When the inorganic composite porous layer-forming slurry contains an inorganic filler, a first binder polymer, and a dispersion medium, 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 with 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.

[0164] 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.

[0165] 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.

[0166] The solvent contained in the coating liquid for forming the porous adhesive layer 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.

[0167] The solvent contained in the coating liquid for forming the porous adhesive layer may be a non-solvent for the first binder polymer. For example, the solvent may dissolve the first binder polymer to a concentration of less than 5 wt % at 25° C. For non-limiting examples of such solvents, see the description of the solvent above.

[0168] 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.

[0169] 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.

[0170] 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.

[0171] 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.

[0172] 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.

[0173] In addition, since the polyolefin porous support itself and the inorganic filler generally have a high ultraviolet blocking effect, if an inorganic composite porous layer containing an inorganic filler is formed and then irradiated with ultraviolet light, the amount of ultraviolet light reaching the polyolefin porous support may be reduced. However, in the present invention, crosslinking is possible even with a small amount of ultraviolet light irradiation, so even if ultraviolet light is irradiated after the inorganic composite porous layer and porous adhesive layer are formed, the polymer chains in the polyolefin porous support can be crosslinked and directly connected to each other.

[0174] 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.

[0175] 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.

[0176] 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.

[0177] After the porous adhesive layer-forming coating liquid is coated on the inorganic composite porous layer, it is 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, and the substrate is dried. 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.

[0178] 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.

[0179] As the second binder polymer solidifies, it condenses, preventing it from penetrating into the surface and / or interior of the polyolefin porous support, thereby preventing an increase in the resistance of the separator. In addition, the adhesive layer containing the second binder polymer becomes porous, improving the resistance of the separator.

[0180] 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.

[0181] 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.

[0182] 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.

[0183] 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.

[0184] 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.

[0185] 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.

[0186] Thereafter, the polyolefin porous support is irradiated with ultraviolet light, which crosslinks the polymer chains in the polyolefin porous support, thereby obtaining a polyolefin porous support having a crosslinked structure.

[0187] 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.

[0188] 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.

[0189] 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.

[0190] 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."

[0191] In the method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery according to an embodiment of the present invention, a separator having a BET specific surface area of ​​10 to 27 m 2 / g, based on the specific surface area of ​​the polyolefin porous support, 2 ~1.0mg / m 2 The photoinitiator alone can crosslink the polyolefin porous support, thereby minimizing side reactions.

[0192] As described above, the separator having a crosslinked structure for a lithium secondary battery finally manufactured by the method for manufacturing a separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention has excellent heat resistance by including a crosslinked structure-containing polyolefin porous support having a crosslinked structure in which polymer chains are directly connected to each other.

[0193] According to one embodiment of the present invention, a separator containing a crosslinked structure for a lithium secondary battery includes a porous support having a crosslinked structure in which polymer chains are directly connected to each other, and the porous support has a BET specific surface area of ​​10 m 2 / g~27m 2 / g.

[0194] 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.

[0195] Although photoinitiators may be crosslinked with each other or crosslinked photoinitiators 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 decompose and cause side reactions during charge and discharge of the battery. In one embodiment of the present invention, the crosslinked structure-containing polyolefin porous support may not include a crosslinked structure in which a photoinitiator is directly linked to a polymer chain, but may include only a crosslinked structure in which polymer chains are directly linked to each other.

[0196] 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.

[0197] The separator for a lithium secondary battery according to one embodiment of the present invention may have improved heat resistance by including a porous support body made of polyolefin having a crosslinked structure, in which polymer chains are directly connected to each other.

[0198] The crosslinked structure-containing polyolefin porous support has a BET specific surface area of ​​10 m 2 / g~27m 2 / g.

[0199] In the separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention, there is almost no difference between the BET specific surface area of ​​the polyolefin porous support before crosslinking and the BET specific surface area of ​​the polyolefin porous support having a crosslinked structure after crosslinking.

[0200] As a result, when the separator contains the crosslinked polyolefin porous support, the basic physical properties of the separator other than the meltdown temperature are not affected by crosslinking. For example, even after crosslinking, the pores in the crosslinked polyolefin porous support are formed to an extent that allows easy movement of lithium ions, so the resistance of the separator is not affected.

[0201] The crosslinked structure-containing polyolefin porous support has a BET specific surface area of ​​27 m 2If the solubility exceeds 1 / g, it is difficult to ensure rapid charging and output characteristics due to high resistance after injection of the electrolyte.

[0202] In one embodiment of the present invention, the crosslinked structure-containing polyolefin porous support has a BET specific surface area of ​​13 m 2 / g~25m 2 / g, or 15m 2 / g~23m 2 / g.

[0203] The BET specific surface area of ​​the crosslinked structure-containing polyolefin porous support can be measured by the BET method. Specifically, the BET specific surface area of ​​the crosslinked structure-containing polyolefin porous support can be calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77 K) using a BELSORP-MAX G manufactured by MicrotracBEL.

[0204] In one embodiment of the present invention, the average pore diameter of the crosslinked structure-containing polyolefin porous support may be 30 nm to 80 nm, 30 nm to 75 nm, or 30 nm to 70 nm. When the average pore diameter of the crosslinked structure-containing polyolefin porous support satisfies the above-mentioned range, the BET specific surface area of ​​the crosslinked structure-containing polyolefin porous support is 10 m 2 / g~27m 2 / g.

[0205] In one embodiment of the present invention, the porosity of the crosslinked structure-containing polyolefin porous support may be 45% to 70%, 45% to 65%, or 45% to 60%. When the porosity of the crosslinked structure-containing polyolefin porous support satisfies the above range, sufficient paths for lithium ion migration can be secured.

[0206] The porosity of the crosslinked polyolefin porous support can be measured by a mercury porosimeter, capillary flow porometry, or a porosimetry analyzer (Bell Japan, Belsorp-II mini) using a BET 6-point method with nitrogen gas adsorption flow. Capillary flow porometry provides the most accurate measurement results because it requires the lowest pressure to apply to the membrane specimen.

[0207] 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 at high temperatures of 150°C or higher 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 higher, the meltdown temperature of the crosslinked structure-containing polyolefin porous support is easy to reach 170°C or higher.

[0208] 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.

[0209] In one embodiment of the present invention, the crosslinked structure-containing polyolefin porous support may have a polyolefin chain with 0.01 to 0.6 or 0.02 to 0.5 double bonds per 1000 carbon atoms as measured by H-NMR. When the crosslinked structure-containing polyolefin porous support has the above-mentioned number of double bonds, the problem of battery performance degradation at high temperatures and / or high voltages can be minimized.

[0210] In one embodiment of the present invention, the number of double bonds present in the polyolefin chain excluding the terminal end of the crosslinked structure-containing polyolefin porous support may be 0.005 to 0.59 per 1000 carbon atoms.

[0211] In one embodiment of the present invention, the thickness of the crosslinked structure-containing polyolefin porous support may be 3 μm to 16 μm, or 5 μm to 12 μm. When the thickness of the crosslinked structure-containing polyolefin porous support is within the above range, the problem of the separator being easily damaged during use of the battery can be prevented and the energy density can be easily ensured.

[0212] A separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention may include a porous support body made of polyolefin having a crosslinked structure in which polymer chains are directly connected to each other.

[0213] 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.

[0214] 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.

[0215] 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.

[0216] For the inorganic filler and the binder polymer, please refer to the above description.

[0217] 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.

[0218] 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.

[0219] 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. Capillary flow porometry is a method for measuring the diameter of the smallest pore in the thickness direction. Therefore, in order 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 the separated inorganic composite porous layer 20 must be 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.

[0220] 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.

[0221] The porosity of the inorganic composite porous layer 20 can be measured by a scanning electron microscope (SEM) image, a mercury porosimeter, or a porosimetry analyzer (Bell Japan, Belsorp-II mini) using a BET 6-point method with nitrogen gas adsorption flow.

[0222] 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.

[0223] 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.

[0224] 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.

[0225] 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.

[0226] For the inorganic filler, the first binder polymer, and the second binder polymer, please refer to the above description.

[0227] The following describes the characteristics of the inorganic composite porous layer 20' that differ from the inorganic composite porous layer 20 described above.

[0228] 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.

[0229] The porous adhesive layer contains a second binder polymer to ensure adhesion between the separator having the inorganic composite porous layer and the electrode, and the porous adhesive layer has pores formed therein to prevent the separator from becoming too resistive.

[0230] In one embodiment of the present invention, the porous adhesive layer 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.

[0231] In one embodiment of the present invention, the porous adhesive layer 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 pattern. When the porous adhesive layer 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.

[0232] In one embodiment of the present invention, the thickness of the porous adhesive layer 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 is within the above range, the adhesive strength with the electrode is excellent, which increases the cell strength of the battery. This is also advantageous in terms of the cycle characteristics and resistance characteristics of the battery.

[0233] The separator for a lithium secondary battery according to one embodiment of the present invention has excellent heat resistance due to the inclusion of a porous support body having a crosslinked structure in which polymer chains are directly linked to each other. For example, the meltdown temperature of the separator for a lithium secondary battery having a crosslinked structure can be increased compared to the meltdown temperature of a conventional separator for a lithium secondary battery before crosslinking. For example, the meltdown temperature of the separator can be 180°C or higher, 195°C or higher, 210°C or higher, or 180°C to 230°C.

[0234] In this specification, "a separator for a lithium secondary battery before crosslinking" refers to a separator made of a polyolefin porous support that is not crosslinked and does not contain a crosslinked structure; a separator comprising a polyolefin porous support that is not crosslinked and does not contain a crosslinked structure, and an inorganic composite porous layer that is located on at least one surface of the polyolefin porous support that is not crosslinked and contains an inorganic filler and a binder polymer; or a separator comprising a polyolefin porous support that is not crosslinked and does not contain a crosslinked structure, an inorganic composite porous layer that is located on at least one surface of the polyolefin porous support that is not crosslinked and contains an inorganic filler and the first binder polymer, and a porous adhesive layer that is located on the inorganic composite porous layer and contains a second binder polymer.

[0235] 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.

[0236] The separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention has a smaller increase in shutdown temperature and a smaller rate of change than a conventional separator for a lithium secondary battery before crosslinking. The separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention has a higher meltdown temperature but a smaller increase in shutdown temperature compared to a separator before crosslinking, thereby ensuring overcharge safety due to the shutdown temperature and significantly improving the high-temperature safety of the separator.

[0237] In one embodiment of the present invention, the separator containing a crosslinked structure for a lithium secondary battery may have a shutdown temperature of 145° C. or less, 140° C. or less, or 133° C. to 140° C. When the separator containing a crosslinked structure for a lithium secondary battery has the above-mentioned shutdown temperature, overcharge safety can be ensured and the problem of increased resistance due to damage to the pores of the crosslinked structure-containing polyolefin porous support during the high-temperature and pressure process during battery assembly can be easily prevented.

[0238] The shutdown temperature can be determined by measuring the time (seconds) it takes for 100 ml of air to pass through the separation membrane at a constant pressure of 0.05 MPa when the temperature is raised by 5°C per minute using an Oken air permeability measuring device, and determining the temperature at which the air permeability of the separation membrane increases rapidly.

[0239] A separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention includes a polyolefin porous support having a crosslinked structure in which polymer chains within the polyolefin porous support are directly linked, thereby allowing the pore structure of the polyolefin porous support to be substantially maintained even after crosslinking.

[0240] The separator for a lithium secondary battery having a crosslinked structure according to one embodiment of the present invention does not significantly deteriorate in terms of air permeability, basis weight, tensile strength, tensile elongation, puncture strength, electrical resistance, etc., compared to the separator for a lithium secondary battery before crosslinking, and the rate of change is also small.

[0241] 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.

[0242] The rate of change in air permeability can be calculated using the following formula:

[0243] 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

[0244] 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.

[0245] 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. 2 The time (seconds) required for the air to pass through the cross section is shown as the ventilation time.

[0246] 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.

[0247] The rate of change in basis weight can be calculated using the following formula:

[0248] 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

[0249] The basis weight (g / m 2 ) is measured by preparing a sample that is 1m long and 1m wide, and measuring its weight.

[0250] The separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention may have a change in tensile strength in the machine direction and transverse direction of 20% or less, 0% to 20%, 0% to 10%, 0% to 9%, 0% to 8%, or 0% to 7.53% compared to the separator for a lithium secondary battery before crosslinking.

[0251] The rate of change in tensile strength can be calculated using the following formula:

[0252] Change rate of tensile strength in the machine direction (%)=[(tensile strength in the machine direction of the separator for lithium secondary batteries before crosslinking)−(tensile strength in the machine direction of the separator for lithium secondary batteries containing a crosslinked structure after crosslinking)] / (tensile strength in the machine direction of the separator for lithium secondary batteries before crosslinking)×100

[0253] Change rate of tensile strength in the transverse direction (%)=[(tensile strength in the transverse direction of separator for lithium secondary battery before crosslinking)−(tensile strength in the transverse direction of separator containing a crosslinked structure for lithium secondary battery after crosslinking)] / (tensile strength in the transverse direction of separator for lithium secondary battery before crosslinking)×100

[0254] The tensile strength may be the strength at which the specimen breaks when the specimen is pulled in the machine direction and the transverse direction at a rate of 50 mm / min using Universal Testing Systems (Instron (registered trademark) 3345) in accordance with ASTM D882.

[0255] The separator having a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention may have a change rate of 20% or less, or 0% to 20%, in tensile elongation in the machine direction and transverse direction compared to the separator for a lithium secondary battery before crosslinking.

[0256] The rate of change in tensile elongation can be calculated using the following formula:

[0257] Change rate of tensile elongation in the machine direction (%) = [(tensile elongation in the machine direction of the separator for lithium secondary batteries before crosslinking) - (tensile elongation in the machine direction of the separator for lithium secondary batteries containing a crosslinked structure after crosslinking)] / (tensile elongation in the machine direction of the separator for lithium secondary batteries before crosslinking) × 100

[0258] Change rate of tensile elongation in the transverse direction (%) = [(tensile elongation in the transverse direction of separator for lithium secondary battery before crosslinking) - (tensile elongation in the transverse direction of separator containing crosslinked structure for lithium secondary battery after crosslinking)] / (tensile elongation in the transverse direction of separator for lithium secondary battery before crosslinking) × 100

[0259] The tensile elongation was measured in accordance with ASTM D882 by measuring the maximum length of the specimen stretched in both the machine direction and the transverse direction at a rate of 50 mm / min using Universal Testing Systems (Instron® 3345) until the specimen broke, and calculated using the following formula:

[0260] Tensile elongation in the machine direction (%) = (length in the machine direction of the specimen just before breakage - length in the machine direction of the specimen before stretching) / (length in the machine direction of the specimen before stretching) × 100

[0261] Tensile elongation in the transverse direction (%) = (transverse length of specimen just before breakage - transverse length of specimen before stretching) / (transverse length of specimen before stretching) × 100

[0262] The separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention may have a change in puncture strength of 10% or less, 0.5% to 10%, 1% to 9%, or 1.18% to 8.71% compared to the separator for a lithium secondary battery before crosslinking.

[0263] The rate of change in puncture strength can be calculated using the following formula:

[0264] Change in puncture strength (%)=[(puncture strength of separator for lithium secondary battery before crosslinking)−(puncture strength of separator for lithium secondary battery containing crosslinked structure after crosslinking)] / (puncture strength of separator for lithium secondary battery before crosslinking)×100

[0265] The puncture strength can be measured in accordance with ASTM D2582. Specifically, after setting a 1 mm round tip to operate at a speed of 120 mm / min, the puncture strength can be measured in accordance with ASTM D2582.

[0266] 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.

[0267] The rate of change in electrical resistance can be calculated using the following formula:

[0268] 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

[0269] 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.

[0270] 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.

[0271] The lithium secondary battery may be in various shapes such as a cylindrical shape, a prismatic shape, or a pouch shape.

[0272] 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.

[0273] 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.

[0274] 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-xM x Ni-site lithium nickel oxide represented by O2 (M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, x = 0.01 to 0.3); chemical formula: LiMn 1-x M x Examples of suitable lithium manganese composite oxides include, but are not limited to, lithium manganese composite oxides represented by Li2Mn3MO5 (M=Fe, Co, Ni, Fe, Cr, Zn, or Ta, x=0.01 to 0.1) or Li2Mn3MO5 (M=Fe, Co, Ni, Cu, or Zn); LiMn2O4, in which part of the Li in the chemical formula is replaced with an alkaline earth metal ion; disulfide compounds; and Fe2(MoO4)3.

[0275] 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.

[0276] 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.

[0277] 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.

[0278] 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.

[0279] 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.

[0280] 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.

[0281] The lithium salt is a substance that is easily dissolved in the organic solvent, such as LiCl, LiBr, LiI, LiClO4, LiBF4, LiB10 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.

[0282] 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.

[0283] 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.

[0284] 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.

[0285] 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.

[0286] 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.

[0287] 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.

[0288] 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.

[0289] Example 1 Production of polyolefin porous supports 30 parts by weight of an ethylene vinyl polymer (weight average molecular weight: 600,000, manufactured by Daehan Petrochemicals, grade: VH035) with a polymer chain containing 0.33 double bonds per 1000 carbon atoms as measured by H-NMR, 70 parts by weight of liquid paraffin oil (manufactured by Far East Petrochemicals, LP350, kinematic viscosity at 40°C: 68 cSt), 2000 ppm of Irganox 1010 as a primary antioxidant, and 2000 ppm of Irgafos 168 as a secondary antioxidant were placed in a twin-screw extruder with an L / D ratio of 60, mixed, and extruded.

[0290] This was formed into a sheet through a T-die, cooled by passing through a cooling roll maintained at 35°C, and then biaxially stretched using a tenter-type sequential stretching machine that stretched in the machine direction (MD) and then in the transverse direction (TD).

[0291] The liquid paraffin oil was extracted from the resulting stretched sheet with methylene chloride, and the sheet was heat-set at 126°C to produce a polyolefin porous support.

[0292] As a result of measurement using BELSORP-MAX G (manufactured by MicrotracBEL), the BET specific surface area of ​​the polyolefin porous support was 15.9 m 2 The average pore size of the polyolefin porous support measured with BPT-101-A (PMI) was 58 nm, and the porosity of the polyolefin porous support calculated using the true density of polyolefin after measuring the thickness and weight of a specimen of a certain area (5 cm x 5 cm) was 42%.

[0293] Manufacturing of separators containing cross-linked structures for lithium secondary batteries As a photoinitiator, 2-isopropylthioxanthone (manufactured by Sigma Aldrich) was prepared.

[0294] A high-pressure mercury lamp (Lichtzen high-pressure mercury lamp, LH-250 / 800-A) was prepared as the UV light source.

[0295] The photoinitiator was dissolved in an acetone solvent to prepare a photocrosslinking composition.

[0296] The content of the photoinitiator on one surface of the polyolefin porous support is 0.3 mg / m based on the specific surface area of ​​the polyolefin porous support. 2 The photocrosslinking composition was coated onto the substrate so that the coated substrate was coated with the photocrosslinking composition and dried at room temperature (25° C.) for 1 minute.

[0297] 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 ultraviolet light was irradiated so that the intensity of the ultraviolet light was 80% of that of the ultraviolet light source.

[0298] As a result, a separator containing a crosslinked structure for a lithium secondary battery was obtained, which included a polyolefin porous support having a crosslinked structure in which polymer chains are directly linked to each other.

[0299] Example 2 Production of polyolefin porous supports A polyolefin porous support was produced in the same manner as in Example 1, except that the oil used was changed to liquid paraffin oil (LP150, manufactured by Kyokuto Petrochemical Co., Ltd., kinematic viscosity at 40°C: 29.5 cSt) and the temperature of the cooling roll was maintained at 45°C.

[0300] As a result of measurement using BELSORP-MAX G (manufactured by MicrotracBEL), the BET specific surface area of ​​the polyolefin porous support was 21.9 m 2 The average pore size of the polyolefin porous support measured with BPT-101-A (PMI) was 44 nm, and the porosity of the polyolefin porous support calculated using the true density of polyolefin after measuring the thickness and weight of a specimen of a certain area (5 cm x 5 cm) was 45%.

[0301] Manufacturing of separators containing cross-linked structures for lithium secondary batteries The polyolefin porous support prepared as described above was used, and the photoinitiator was applied to one surface of the polyolefin porous support in a concentration of 0.2 mg / m based on the specific surface area of ​​the polyolefin porous support. 2 A separator having a crosslinked structure for a lithium secondary battery was prepared in the same manner as in Example 1, except that the photocrosslinking composition was coated so that the crosslinked structure was 0.015 μm.

[0302] Example 3 Production of polyolefin porous supports A polyolefin porous support was produced in the same manner as in Example 2, except that the heat setting temperature was changed to 125°C.

[0303] As a result of measurement using BELSORP-MAX G (manufactured by MicrotracBEL), the BET specific surface area of ​​the polyolefin porous support was 26.3 m 2 The average pore size of the polyolefin porous support measured with BPT-101-A (PMI) was 32 nm, and the porosity of the polyolefin porous support calculated using the true density of polyolefin after measuring the thickness and weight of a specimen of a certain area (5 cm x 5 cm) was 45%.

[0304] Manufacturing of separators containing cross-linked structures for lithium secondary batteries The polyolefin porous support prepared in Example 3 was used, and the photoinitiator was applied to one surface of the polyolefin porous support in an amount of 0.15 mg / m based on the specific surface area of ​​the polyolefin porous support. 2 A separator having a crosslinked structure for a lithium secondary battery was prepared in the same manner as in Example 1, except that the photocrosslinking composition was coated so that the crosslinked structure was 0.015 μm.

[0305] Comparative Example 1 Measurement using BELSORP-MAX G (MicrotracBEL) revealed that the BET specific surface area was 33.5 m 2 A polyolefin porous support having an average pore size of 21 nm measured with BPT-101-A (manufactured by PMI Co., Ltd.) and a porosity of 32% calculated using the true density of polyolefin after measuring the thickness and weight of a test piece of a certain area (5 cm x 5 cm) was used, and the content of the photoinitiator was 1.0 mg / m based on the specific surface area of ​​the polyolefin porous support on one side of the polyolefin porous support. 2 A separator was prepared in the same manner as in Example 1, except that the photocrosslinking composition was coated so that the thickness of the separator was 1 / 2 mm.

[0306] Comparative Example 2 Measurement using BELSORP-MAX G (MicrotracBEL) revealed that the BET specific surface area was 39.1 m 2A polyolefin porous support having an average pore size of 20 nm measured with BPT-101-A (manufactured by PMI Co., Ltd.) and a porosity of 43%, calculated using the true density of polyolefin after measuring the thickness and weight of a test piece of a certain area (5 cm x 5 cm), was used, and the content of the photoinitiator was 1.0 mg / m based on the specific surface area of ​​the polyolefin porous support on one side of the polyolefin porous support. 2 A separator was prepared in the same manner as in Example 1, except that the photocrosslinking composition was coated so that the thickness of the separator was 1 / 2 mm.

[0307] Comparative Example 3 Measurement using BELSORP-MAX G (MicrotracBEL) revealed that the BET specific surface area was 5.1 m 2 A polyolefin porous support having an average pore size of 820 nm as measured by BPT-101-A (manufactured by PMI Co., Ltd.) and a porosity of 47%, calculated using the true density of polyolefin after measuring the thickness and weight of a test piece of a certain area (5 cm x 5 cm), was used. The content of the photoinitiator was 1.0 mg / m based on the specific surface area of ​​the polyolefin porous support on one side of the polyolefin porous support. 2 A separator was prepared in the same manner as in Example 1, except that the photocrosslinking composition was coated so that the thickness of the separator was 1 / 2 mm.

[0308] Comparative Example 4 The polyolefin porous support prepared in Example 3 was used, and the content of the photoinitiator was 2.0 mg / m based on the specific surface area of ​​the polyolefin porous support. 2 A separation membrane was produced in the same manner as in Example 1, except that the coating was performed so that the thickness of the membrane was 100 μm.

[0309] Evaluation Example 1: Evaluation of the physical properties of a cross-linked polyolefin porous support The BET specific surface area, average pore diameter, and porosity of the crosslinked structure-containing polyolefin porous supports produced in Examples 1 to 3 and Comparative Examples 1 to 4 were measured and are shown in Table 1 below.

[0310] The BET specific surface area was measured using a BSLSORP-MAX G manufactured by MicrotracBEL.

[0311] The average pore diameter was measured using a PMI BPT-101-A.

[0312] The porosity was calculated using the true density of the polyolefin after measuring the thickness and weight of a test piece of a certain area (5 cm x 5 cm).

[0313] [Table 1]

[0314] Evaluation example 2: Evaluation of the physical properties of a separator containing a cross-linked structure for lithium secondary batteries The crosslinking degree, tensile strength, meltdown temperature, and shutdown temperature of the separators containing a crosslinked structure for lithium secondary batteries prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were measured and are shown in Table 2 below.

[0315] Evaluation of cross-linking degree The crosslinking degree was calculated as a percentage of the residual weight relative to the initial weight after immersing each of the crosslinked structure-containing polyolefin porous supports prepared in Examples 1 to 3 and Comparative Examples 1 to 4 in a xylene solution at 135°C and boiling for 12 hours according to ASTM D2765.

[0316] Evaluation of tensile strength in machine and cross directions A test piece measuring 100 mm x 15 mm was prepared.

[0317] According to ASTM D882, the specimen was pulled in the machine direction (MD) and 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 and the transverse direction.

[0318] Meltdown temperature assessment The meltdown temperature was measured by thermomechanical analysis (TMA) after taking samples from the separator in the machine direction (MD) and transverse direction (TD). Specifically, a 4.8 mm wide x 8 mm long sample was placed in a TMA device (TA Instruments, Q400) and heated from 30°C to 220°C at a heating rate of 5°C / min under a tension of 0.01 N. As the temperature increased, the length of the sample changed, and the temperature at which the length increased rapidly and the sample broke in both the machine direction and transverse direction was measured and defined as the meltdown temperature.

[0319] Shutdown temperature evaluation 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.

[0320] [Table 2]

[0321] From Table 2, it can be seen that the separators prepared in Examples 1 to 4 have a specific BET specific surface area, and therefore the polyolefin porous support is crosslinked, even when a small amount of photoinitiator is used. As a result, the shutdown temperature does not increase, but the meltdown temperature increases.

[0322] On the other hand, in the separation membranes produced in Comparative Examples 1 to 3, the BET specific surface area of ​​the polyolefin porous support was 10 m 2 / g~27m 2 / g, 1.0 mg / m 2 It can be seen that the following photoinitiators did not result in crosslinking of the polyolefin porous support.

[0323] The separation membrane produced in Comparative Example 4 had a molecular weight of 1.0 mg / m 2It can be seen that although crosslinking of the polyolefin porous support was possible when an excessive amount of photoinitiator exceeding 1000 ppm was used, excessive radicals were formed, resulting in a decrease in the mechanical strength of the separator.

Claims

1. providing a polyolefin porous support containing a photoinitiator; and irradiating the polyolefin porous support with ultraviolet light, The polyolefin porous support is BET specific surface area is 10m ２ / g~27m ２ / g, The content of the photoinitiator is 0.01 mg / m based on the specific surface area of ​​the polyolefin porous support. ２ ~1.0 mg / m ２ The present invention provides a method for producing a separator containing a crosslinked structure for a lithium secondary battery.

2. providing a polyolefin porous support containing the photoinitiator, feeding a polyolefin and a liquid phase diluent to an extruder; extruding a polyolefin composition from the extruder; passing the extruded polyolefin composition through a die and a chill roll to form and stretch the composition into a sheet; extracting the liquid diluent from the stretched sheet to produce a pre-porous support; The pre-porous support was heat-set to a BET specific surface area of ​​10 m ２ / g~27m ２ / g of the polyolefin porous support; 2. The method of claim 1, further comprising: coating a photo-crosslinking composition containing the photoinitiator and the solvent on the outer surface of the polyolefin porous support, and drying the composition.

3. 3. The method for producing a separator containing a crosslinked structure for a lithium secondary battery according to claim 2, wherein the liquid diluent has a kinematic viscosity at 40° C. of 25 cSt to 100 cSt.

4. The method for producing a separator containing a crosslinked structure for a lithium secondary battery according to claim 2, wherein the temperature of the cooling roll is 30°C to 65°C.

5. The method for producing a separator containing a crosslinked structure for a lithium secondary battery according to claim 2, wherein the heat setting temperature is 125°C to 132°C.

6. The method for producing a separator containing a crosslinked structure for a lithium secondary battery according to claim 2 , wherein the composition for photocrosslinking is a photoinitiator solution containing the photoinitiator and the solvent.

7. 3. The method for producing a separator containing a crosslinked structure for a lithium secondary battery according to claim 2, 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.

8. 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; 3. The method of claim 2, 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.

9. The method for manufacturing a separator containing a crosslinked structure for a lithium secondary battery according to claim 1 , wherein the photoinitiator comprises a type II photoinitiator.

10. 2. The method of claim 1, wherein the photoinitiator comprises thioxanthone, a thioxanthone derivative, benzophenone, a benzophenone derivative, or two or more thereof.

11. The amount of ultraviolet light irradiation is 10 to 2000 mJ / cm ２ The method for producing a separator containing a crosslinked structure for a lithium secondary battery according to claim 1,

12. The porous support includes a crosslinked structure-containing polyolefin having a crosslinked structure in which polymer chains are directly linked to each other, The crosslinked structure-containing polyolefin porous support has a BET specific surface area of ​​10 m ２ / g~27m ２ / g.

13. The crosslinked structure-containing separator for a lithium secondary battery according to claim 12, wherein the crosslinked structure-containing polyolefin porous support has an average pore size of 30 nm to 80 nm.

14. 13. The separator for a lithium secondary battery according to claim 12, wherein the porosity of the crosslinked polyolefin porous support is 45% to 70%.

15. The separator for a lithium secondary battery according to claim 12, wherein the degree of crosslinking of the crosslinked structure-containing polyolefin porous support is 10% to 45%.

16. The separator for a lithium secondary battery according to claim 12, 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.

17. The separation membrane 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; The separator for a lithium secondary battery according to claim 12, further comprising: a porous adhesive layer positioned on the inorganic composite porous layer and including a second binder polymer.

18. The separator for a lithium secondary battery having a crosslinked structure according to claim 12, wherein the meltdown temperature of the separator for a lithium secondary battery having a crosslinked structure is 160°C or higher.

19. The separator containing a crosslinked structure for a lithium secondary battery according to claim 12, wherein the separator containing a crosslinked structure for a lithium secondary battery has a shutdown temperature of 145° C. or less.

20. The lithium secondary battery includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, A lithium secondary battery, wherein the separator for a lithium secondary battery is the crosslinked structure-containing separator for a lithium secondary battery according to any one of claims 12 to 19.

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