A cross-linked structure-containing separation membrane for lithium secondary batteries, a method for manufacturing the same, and a lithium secondary battery equipped with the separation membrane.

Abstract

The present invention relates to a separator containing a crosslinked structure for a lithium secondary battery, the separator comprising: a polyolefin porous support having a crosslinked structure in which polymer chains are directly connected to each other; an inorganic composite porous layer located on at least one side of the polyolefin porous support and including an inorganic filler and a first binder polymer; and a porous adhesive layer located on the inorganic composite porous layer and including a second binder polymer; a method for manufacturing the separator; and a lithium secondary battery including the separator. The separator containing a crosslinked structure for a lithium secondary battery according to the present invention has improved high-temperature safety and excellent adhesion to an electrode. The method for manufacturing the separator containing a crosslinked structure for a lithium secondary battery according to the present invention can simplify the process by adding a photoinitiator to a coating solution for forming a porous adhesive layer, without requiring additional equipment for photocrosslinking the polyolefin porous support.

Description

Technical Field

[0001] This application claims priority based on Korean Patent Application No. 10-2021-0059581 filed on May 7, 2021.

[0002] The present invention relates to a cross-linked structure-containing separator for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the separator.

Background Art

[0003] In recent years, the interest in energy storage technology has been increasing. The application fields are expanding to mobile phones, camcorders, notebook computers, and even the energy of electric vehicles, and the demand for higher energy density of batteries used as power sources for such electronic devices is increasing. Lithium secondary batteries are the batteries that can best meet such demands, and research on 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 them, the separator is required to have insulation for separating the positive electrode and the negative electrode and electrically insulating them, and high ionic conductivity for enhancing the permeability of lithium ions based on high porosity.

[0005] Although polyolefin separators are widely used as such separators, in the case of a polyethylene (PE) separator, which is a typical polyolefin separator, since the melting point (Tm) is low, if the temperature of the battery rises above the melting point of polyethylene in a battery misuse environment, a melt-down phenomenon may occur, leading to the risk of ignition and explosion. Due to its material properties and manufacturing process characteristics, the separator shows significant heat shrinkage behavior in situations such as high temperature, which may lead to safety problems such as internal short circuits.

[0006] Therefore, there is an urgent need for a separator that can ensure safety at high temperatures.

Summary of the Invention

Problems to be Solved by the Invention

[0007] The problem to be solved by the present invention is to provide a crosslinked structure-containing separator for a lithium secondary battery with improved high-temperature safety and a lithium secondary battery equipped with the same.

[0008] Another problem to be solved by the present invention is to provide a simplified method for manufacturing a crosslinked structure-containing separator for a lithium secondary battery with improved high-temperature safety.

Means for Solving the Problems

[0009] To solve the above problems, according to one aspect of the present invention, a crosslinked structure-containing separator for a lithium secondary battery according to the following embodiments is provided.

[0010] The first embodiment is a crosslinked structure-containing polyolefin porous support having a crosslinked structure in which polymer chains are directly connected to each other, an inorganic composite void layer located on at least one surface of the crosslinked structure-containing polyolefin porous support and containing an inorganic filler and a first binder polymer, and a porous adhesive layer located on the inorganic composite void layer and containing a second binder polymer, and relates to a crosslinked structure-containing separator for a lithium secondary battery.

[0011] According to the second embodiment, in the first embodiment, the thermal shrinkage rates in the machine direction (MD) and the transverse direction (TD) measured after leaving the crosslinked structure-containing separator for a lithium secondary battery at 150 °C for 30 minutes can be 20% or less, respectively.

[0012] According to the third embodiment, in the first or second embodiment, the weight ratio of the inorganic filler to the first binder polymer can be 95:5 to 99.9:0.1.

[0013] According to the fourth example of implementation, in any one of the first to third examples of implementation, The first binder polymer may include an acrylic polymer, polyacrylic acid, styrene-butadiene rubber, polyvinyl alcohol, or two or more of these.

[0014] According to the fifth example of implementation, in any one of the first to fourth examples of implementation, 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 of these.

[0015] According to the sixth example, in any one of the first to fifth examples, The porous adhesive layer may have a pattern comprising one or more adhesive portions containing the second binder polymer and one or more plain (non-coating) regions where the adhesive portions are not formed.

[0016] According to the 7th example, in any one of the 1st to 6th examples, The meltdown temperature of the cross-linked structure-containing separation membrane for lithium secondary batteries may be 160°C or higher.

[0017] According to the eighth example of implementation, in any one of the seventh examples of implementation, The shutdown temperature of the cross-linked structure-containing separation membrane for lithium secondary batteries may be 145°C or lower.

[0018] To solve the above problems, according to one aspect of the present invention, a method for manufacturing a crosslinked structure-containing separation membrane for lithium secondary batteries is provided as shown in the following embodiment.

[0019] The ninth example of implementation is: (S1) A step of producing an inorganic composite void layer forming slurry containing an inorganic filler, a first binder polymer, and a dispersion medium, (S2) The step of forming an inorganic composite void layer by coating and drying the slurry for forming the inorganic composite void layer on at least one surface of a polyolefin porous support, (S3) A step of coating the upper surface of the inorganic composite void layer with a coating solution for forming a porous adhesive layer, which includes a second binder polymer, a solvent for the second binder polymer, and a photoinitiator. (S4) The step of immersing the result of step (S3) in a coagulation solution containing a non-solvent for the second binder polymer, and then drying it to form a porous adhesive layer, The present invention relates to a method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries, comprising the step of (S5) irradiating the result of step (S4) with ultraviolet light.

[0020] According to the 10th example, in the 9th example, The content of the photoinitiator may be 0.015 to 0.3 parts by weight per 100 parts by weight of the polyolefin porous support.

[0021] According to the 11th embodiment, in the 9th embodiment or the 10th embodiment, The weight ratio of the inorganic filler to the first binder polymer may be 95:5 to 99.9:0.1.

[0022] According to the 12th example, in any one of the 9th to 11th examples, The first binder polymer may include an acrylic polymer, polyacrylic acid, styrene-butadiene rubber, polyvinyl alcohol, or two or more of these.

[0023] According to the 13th example, in any one of the 9th to 12th examples, The aforementioned dispersion medium is an aqueous dispersion medium.

[0024] According to the 14th example, in any one of the 9th to 13th examples, 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 of these.

[0025] According to the 15th example, in any one of the 9th to 14th examples, The solvent for the second binder polymer 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; amides such as N-methylpyrrolidone and N,N-dimethylformamide; or two or more of these.

[0026] According to the 16th example, in any one of the 9th to 15th examples, The aforementioned photoinitiator may include a type II photoinitiator.

[0027] According to the 17th example, in any one of the 9th to 16th examples, The photoinitiator may include thioxanthone (TX), a thioxanthone derivative, benzophenone (BPO), a benzophenone derivative, or two or more of these.

[0028] According to the 18th example, in any one of the 9th to 17th examples, The aforementioned ultraviolet irradiation light intensity is 10 to 2000 mJ / cm². 2 It is possible.

[0029] To solve the above problems, according to one aspect of the present invention, a lithium secondary battery of the following embodiment is provided.

[0030] The 19th concrete example is: A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator membrane interposed between the positive electrode and the negative electrode, The present invention relates to a lithium secondary battery, wherein the separation membrane is a separation membrane containing a cross-linked structure for lithium secondary batteries according to any one of the first to eighth embodiments. [Effects of the Invention]

[0031] A cross-linked structure-containing separation membrane for lithium secondary batteries according to one aspect of the present invention has improved high-temperature safety and excellent adhesion to electrodes.

[0032] A method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to one aspect of the present invention simplifies the process by adding a photoinitiator to a coating liquid for forming a porous adhesive layer, thereby eliminating the need for additional equipment to photocrosslink a polyolefin porous support.

[0033] The following drawings accompanying this specification illustrate preferred embodiments of the invention and, together with the detailed description of the invention, serve to further illustrate the technical idea of ​​the invention. Therefore, the invention should not be construed as being limited solely to what is shown in the drawings. [Brief explanation of the drawing]

[0034] [Figure 1] This figure schematically shows a crosslinked structure-containing separation membrane for lithium secondary batteries according to one embodiment of the present invention. [Figure 2] This figure shows the meltdown temperature in the mechanical direction, measured by thermomechanical analysis (TMA) of the separation membranes produced in Example 1, Comparative Example 1, and Comparative Example 2. [Modes for carrying out the invention]

[0035] Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings. Prior to this, terms and words used in this specification and in the claims shall not be interpreted in their usual and dictionary sense, but rather in a sense and concept corresponding to the technical idea of ​​the present invention, in accordance with the principle that the inventor himself may appropriately define the concept of terms in order to best describe the invention.

[0036] Therefore, the embodiments described herein and the configurations shown in the drawings represent only one of the most preferred embodiments of the present invention and do not represent the entire technical concept of the invention. It should be understood that there are various equivalents and modifications that can be substituted for these at the time of filing this application.

[0037] In this specification, terms such as "first," "second," etc., are used to distinguish one component from another, and each component is not limited by these terms.

[0038] A separation membrane containing a crosslinked structure for lithium secondary batteries according to one embodiment of the present invention is A crosslinked structure-containing polyolefin porous support having a crosslinked structure in which polymer chains are directly linked to each other, An inorganic composite void layer comprising an inorganic filler and a first binder polymer, located on at least one surface of the crosslinked structure-containing polyolefin porous support, The material comprises a porous adhesive layer located on the inorganic composite void layer and containing a second binder polymer.

[0039] Figure 1 is a schematic diagram showing a crosslinked structure-containing separation membrane for lithium secondary batteries according to one embodiment of the present invention.

[0040] Referring to Figure 1, the crosslinked structure-containing separation membrane 1 for lithium secondary batteries according to one embodiment of the present invention includes a crosslinked structure-containing polyolefin porous support 10.

[0041] In this specification, "a crosslinked structure in which polymer chains are directly linked to each other" means a state in which polymer chains substantially made of polyolefins, more preferably polymer chains made solely of polyolefins, become reactive upon the addition of a photoinitiator, and the polymer chains directly crosslink with each other. Therefore, a crosslinking reaction that occurs between crosslinking agents due to the addition of an additional crosslinking agent does not constitute a "crosslinked structure in which polymer chains are directly linked to each other" as defined in this invention. Furthermore, a crosslinking reaction that occurs between an additional crosslinking agent and a polymer chain does not constitute a "crosslinked structure in which polymer chains are directly linked to each other" as defined in this invention, even if the polymer chain is substantially made of polyolefins or made solely of polyolefins.

[0042] While photoinitiators may be crosslinked with each other or with polymer chains, such crosslinked structures have a lower reaction enthalpy than crosslinked structures between polymer chains within a porous polyolefin support, and may decompose during battery charging and discharging, potentially causing side reactions. In one embodiment of the present invention, the porous polyolefin support containing the crosslinked structure may not include a crosslinked structure in which the photoinitiator and polymer chains are directly linked, but may only include a crosslinked structure in which polymer chains are directly linked to each other.

[0043] In one embodiment of the present invention, the crosslinked structure-containing polyolefin porous support includes only crosslinked structures in which polymer chains are directly linked to each other, and does not include crosslinked structures in which a photoinitiator and polymer chains are directly linked.

[0044] A separation membrane containing a crosslinked structure for lithium secondary batteries according to one embodiment of the present invention can have improved heat resistance by comprising a crosslinked structure-containing polyolefin porous support 10 having a crosslinked structure in which polymer chains are directly linked to each other.

[0045] In one embodiment of the present invention, the polyolefin porous support may be a porous film.

[0046] In one embodiment of the present invention, the polyolefin may include polyethylene; polypropylene; polybutylene; polypentene; polyhexene; polyoctene; copolymers of two or more ethylene, propylene, butene, pentene, 4-methylpentene, hexene, and octene; or mixtures thereof.

[0047] Non-limiting examples of the polyethylene include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE). When the polyethylene is high-density polyethylene with high crystallinity and a high melting point, it is easy to increase the modulus while maintaining the desired level of heat resistance.

[0048] 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, a separation film with excellent strength and heat resistance can be obtained while ensuring the uniformity and film-forming processability of the polyolefin porous support.

[0049] The aforementioned weight-average molecular weight can be measured using gel permeation chromatography (GPC: Gel Permeation Chromatography, PL GPC220, manufactured by Agilent Technologies) under the following conditions.

[0050] - Column: PL Olexis (Polymer Laboratories) - Solvent: TCB (trichlorobenzene) -Flow rate: 1.0ml / min -Sample concentration: 1.0 mg / ml -Injection volume: 200μl - Column temperature: 160℃ -Detector: Agilent high-temperature radioisotope detector - Standard: Polystyrene (corrected with a cubic function)

[0051] In one embodiment of the present invention, the degree of crosslinking of the crosslinked structure-containing polyolefin porous support may be 10% to 80%, 30% to 55%, 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 degrees, it is easy to increase the modulus while maintaining the desired level of heat resistance. For example, when the degree of crosslinking of the crosslinked structure-containing polyolefin porous support is 20% or more, the meltdown temperature of the crosslinked structure-containing polyolefin porous support is likely to be 170°C or higher.

[0052] In this case, the degree of crosslinking is calculated by immersing the crosslinked polyolefin porous support in a xylene solution at 135°C and boiling it for 12 hours, according to ASTM D2765, measuring the remaining weight, and then calculating the degree of crosslinking as a percentage of the remaining weight relative to the initial weight.

[0053] In one embodiment of the present invention, the crosslinked structure-containing polyolefin porous support may have 0.01 to 0.6 or 0.02 to 0.5 double bonds per 1000 carbon atoms in the polyolefin chain when 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 temperature and / or high voltage can be minimized.

[0054] In one embodiment of the present invention, the number of double bonds present in the polyolefin chain excluding the ends of the crosslinked structure-containing polyolefin porous support may be 0.005 to 0.59 per 1000 carbon atoms. "Double bonds present in the polyolefin chain excluding the ends" refers to double bonds present throughout the polyolefin chain excluding the ends of the polyolefin chain. Here, "ends" refers to the positions of carbon atoms connected to the ends on both sides of the polyolefin chain.

[0055] 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 separation membrane being easily damaged during battery use can be prevented, and energy density can be easily ensured.

[0056] Referring to Figure 1, a crosslinked structure-containing separation membrane 1 for lithium secondary batteries according to one embodiment of the present invention comprises an inorganic composite void layer 20 on at least one surface of the crosslinked structure-containing polyolefin porous support 10. Specifically, the inorganic composite void layer 20 may be formed on one or both surfaces of the crosslinked structure-containing polyolefin porous support 10. The inorganic composite void layer 20 comprises an inorganic filler and a first binder polymer.

[0057] The inorganic composite void layer 20 includes inorganic fillers and a first binder polymer that adheres the inorganic fillers to each other (i.e., the first binder polymer connects and fixes the inorganic fillers to each other) so that the inorganic fillers remain bonded to each other. The first binder polymer maintains the bonded state between the inorganic fillers and the crosslinked structure-containing polyolefin porous support 10. The inorganic composite void layer 20 prevents the crosslinked structure-containing polyolefin porous support 10 from exhibiting excessive thermal shrinkage behavior at high temperatures due to the inorganic fillers, thereby improving the safety of the separation membrane. For example, the thermal shrinkage rates of the separation membrane in the mechanical direction (MD) and transverse direction (TD), measured after being left at 150°C for 30 minutes, may be 20% or less, 2% to 15%, 2% to 10%, or 2% to 5%, respectively.

[0058] In this specification, "machine direction (MD)" refers to the direction of travel when the separation membrane is continuously produced, and is the longitudinal direction of the separation membrane; and "transverse direction (TD)" refers to the transverse direction of the machine direction, that is, the direction perpendicular to the direction of travel when the separation membrane is continuously produced, and is perpendicular to the longitudinal direction of the separation membrane.

[0059] The inorganic filler is not particularly limited as long as it is electrochemically stable. That is, the inorganic filler usable in the present invention is suitable for any lithium secondary battery operating voltage range (e.g., Li / Li + The voltage is not particularly limited as long as oxidation and / or reduction reactions do not occur at a voltage of 0-5V (reference). In particular, when inorganic particles with a high dielectric constant are used as inorganic fillers, it is possible to improve the ionic conductivity of the electrolyte by contributing to an increase in the degree of dissociation of electrolyte salts in the liquid electrolyte, such as lithium salts.

[0060] For the reasons stated above, 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-restrictive examples of inorganic fillers with a dielectric constant of 5 or more include BaTiO3, Pb(Zr,Ti)O3(PZT), and Pb 1-x Lax Zr 1-y Ti y O3 (PLZT, 0 < x < 1, 0 < y < 1), Pb(Mg 1 / 3 Nb 2 / 3 )O3 - PbTiO3 (PMN - PT), hafnium (HfO2), SrTiO3, SnO2, CeO2, MgO, Mg(OH)2, NiO, CaO, ZnO, ZrO2, SiO2, Y2O3, Al2O3, AlOOH, Al(OH)3, SiC, TiO2, or a mixture thereof, etc. can be mentioned.

[0061] Also, in another embodiment of the present invention, as the inorganic filler, an inorganic filler having lithium ion transfer ability, that is, an inorganic filler that contains lithium element but does not store lithium and has a function of moving lithium ions can be used. Non - limiting examples of inorganic fillers having lithium ion transfer ability include lithium phosphate (Li3PO4), lithium titanium phosphate (Li x Ti y (PO4)3, 0 < x < 2, 0 < y < 3), lithium aluminum titanium phosphate (Li x Al y Ti z (PO4)3, 0 < x < 2, 0 < y < 1, 0 < z < 3), (LiAlTiP) x O y - based glass (0 < x < 4, 0 < y < 13), lithium lanthanum titanate (Li x La y TiO3, 0 < x < 2, 0 < y < 3), Li 3.25 Ge 0.25 P 0.75 S4 and other lithium germanium thiophosphates (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 Siy S z 、 0 < x < 3, 0 < y < 2, 0 < z < 4), P2S5-based glasses such as LiI-Li2S-P2S5 (Li x P y S z 、 0 < x < 3, 0 < y < 3, 0 < z < 7), or mixtures thereof, etc. may be mentioned.

[0062] 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-described range, it is easy to form an inorganic composite void layer 20 having a uniform thickness and appropriate porosity, the dispersibility of the inorganic filler is good, and a desired energy density can be achieved.

[0063] At this time, the average particle size of the inorganic filler means D 50 particle size, and "D 50 particle size" means the particle size at the 50% point of the particle number cumulative distribution according to the particle size. The particle size can be measured using the laser diffraction method. Specifically, after dispersing the measurement target powder in a dispersion medium, it is introduced into a commercially available laser diffraction particle size measuring device (for example, Microtrac S3500), and 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 it becomes 50% of the particle number cumulative distribution according to the particle size in the measuring device, D 50 particle size can be measured.

[0064] The first binder polymer has a glass transition temperature (glass transition temperature, T gThe glass transition temperature of the first binder polymer can be -200 to 200°C. If the glass transition temperature of the first binder polymer satisfies the above range, the mechanical properties of the final inorganic composite void layer 20, such as flexibility and elasticity, can be improved. The first binder polymer may have ion conductivity. If the first binder polymer has ion conductivity, the performance of the battery can be further improved. The dielectric constant of the first binder polymer may be 1.0 to 100 (measurement frequency = 1 kHz) or 10 to 100. If the dielectric constant of the first binder polymer satisfies the above range, the degree of dissociation of the salt in the electrolyte can be improved.

[0065] In one embodiment of the present invention, the first binder polymer may be a binder polymer with excellent heat resistance. When the first binder polymer has excellent heat resistance, the heat resistance properties of the inorganic composite void layer can be further improved. For example, the thermal shrinkage rates of the separation membrane in the mechanical direction (MD) and transverse direction (TD), measured after being left at 150°C for 30 minutes, may be 20% or less, 2% to 15%, 2% to 10%, 2% to 5%, 0% to 5%, or 0% to 2%, respectively.

[0066] In one embodiment of the present invention, the first binder polymer may include an acrylic polymer, polyacrylic acid, styrene-butadiene rubber, polyvinyl alcohol, or two or more of these.

[0067] Specifically, the acrylic polymer may include an acrylic homopolymer obtained by polymerizing only acrylic monomers, a copolymer of an acrylic monomer and another monomer, and may include ethylhexyl acrylate-methyl methacrylate copolymer, polymethyl methacrylate, polyethylhexyl acrylate, polybutyl acrylate, polyacrylonitrile, butyl acrylate-methyl methacrylate copolymer, or two or more of these.

[0068] In one embodiment of the present invention, the first binder polymer may be in particulate form.

[0069] 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 separation membrane increases, improving the thermal safety of the separation membrane at high temperatures. For example, the thermal shrinkage rates of the separation membrane in the mechanical direction (MD) and transverse direction (TD), measured after being left at 150°C for 30 minutes, may be 20% or less, 2% to 15%, 2% to 10%, 2% to 5%, 0% to 5%, or 0% to 2%, respectively. Furthermore, sufficient adhesion between the inorganic fillers can be ensured while also ensuring sufficient space between them.

[0070] In one embodiment of the present invention, the inorganic composite void layer 20 may further contain additives such as dispersants and / or thickeners. In one embodiment of the present invention, the additives may include polyvinylpyrrolidone (PVP), hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), ethylhydroxyethylcellulose (EHEC), methylcellulose (MC), carboxymethylcellulose (CMC), hydroxyalkylmethylcellulose, cyanoethylene polyvinyl alcohol, or two or more of these.

[0071] In one embodiment of the present invention, the inorganic composite void layer 20 may have a structure in which the inorganic fillers are filled and in contact with each other and bound together by the binder polymer, thereby forming interstitial volumes between the inorganic fillers, and these interstitial volumes between the inorganic fillers become empty spaces that form pores.

[0072] In one embodiment of the present invention, the average pore diameter of the inorganic composite void layer 20 may be 0.001 μm to 10 μm. The average pore diameter of the inorganic composite void layer 20 can be measured by capillary flow porometry. Capillary flow porometry is a method that measures the diameter of the smallest pore in the thickness direction. Therefore, in order to measure the average pore diameter of only the inorganic composite void layer 20 by capillary flow porometry, the inorganic composite void layer 20 must be separated from the cross-linked structure-containing polyolefin porous support 10, and the separated inorganic composite void layer 20 must be wrapped in a nonwoven fabric capable of supporting it when measured. In this case, the pore size of the nonwoven fabric must be much larger than the pore size of the inorganic composite void layer 20.

[0073] In one embodiment of the present invention, the porosity of the inorganic composite void layer 20 may be 5% to 95%, 10% to 95%, 20% to 90%, or 30% to 80%. The porosity corresponds to the value obtained by subtracting the volume calculated from the weight and density of each component of the inorganic composite void layer 20 from the volume calculated from the thickness, width, and length of the inorganic composite void layer 20.

[0074] The porosity of the inorganic composite void layer 20 can be measured by the BET6 method using a scanning electron microscope (SEM) image, a mercury porosimeter, or a porosimetry analyzer (Bell Japan, Belsorp-II mini) with nitrogen gas adsorption flow.

[0075] In one embodiment of the present invention, the thickness of the inorganic composite void layer 20 can be 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 void layer 20 satisfies the above range, the cell strength of the battery can be easily increased while maintaining excellent adhesion to the electrodes.

[0076] Referring to Figure 1, the crosslinked structure-containing separation membrane 1 for lithium secondary batteries comprises a porous adhesive layer 30 on the inorganic composite void layer 20. The porous adhesive layer 30 contains a second binder polymer.

[0077] When the cross-linked structure-containing separation membrane 1 for lithium secondary batteries includes an inorganic composite void layer 20, the inclusion of inorganic fillers ensures thermal safety at high temperatures, but there is a risk of reduced adhesion to the electrodes. In particular, in one embodiment of the present invention, when the weight ratio of inorganic fillers to the first binder polymer is 95:5 to 99.9:0.1, the high content of inorganic fillers further improves thermal safety at high temperatures, but the reduced content of the first binder polymer may cause problems with the adhesion between the separation membrane 1 and the electrodes.

[0078] The porous adhesive layer 30 contains a second binder polymer, thereby ensuring adhesion between the separation membrane 1, which comprises the inorganic composite void layer 20, and the electrode. Furthermore, the porous adhesive layer 30 has pores, which prevents the resistance of the separation membrane from increasing.

[0079] In one embodiment of the present invention, the porous adhesive layer 30 can minimize the phenomenon of increased resistance of the separation membrane because the second binder polymer does not penetrate the surface and / or interior of the crosslinked structure-containing polyolefin porous support 10.

[0080] The second binder polymer may be a binder polymer commonly used for forming adhesive layers. The second binder polymer has a glass transition temperature (T gThe glass transition temperature of the second binder polymer can 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 final adhesive layer can be improved. The second binder polymer may have ion conductivity. When a binder polymer with ion conductivity is used as the second binder polymer, the performance of the battery can be further improved. The dielectric constant of the second binder polymer may be 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 the salt in the electrolyte can be improved.

[0081] 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 of these.

[0082] In one embodiment of the present invention, the porous adhesive layer 30 may have a pattern comprising one or more adhesive portions containing the second binder polymer and one or more plain portions where the adhesive portions are not formed. The pattern may be dot-shaped, stripe-shaped, diagonal, wave-shaped, triangular, square, or semicircular. When the porous adhesive layer 30 has a pattern, the resistance of the separation membrane is improved, and the electrolyte can be impregnated through the plain portions where the porous adhesive layer 30 is not formed, thereby improving the electrolyte impregnation of the separation membrane.

[0083] In one embodiment of the present invention, the thickness of the porous adhesive layer 30 may be 0.5 μm to 1.5 μm, 0.6 μm to 1.2 μm, or 0.6 μm to 1.0 μm. When the thickness of the porous adhesive layer 30 is within the above range, the adhesion to the electrode is excellent, and as a result, the cell strength of the battery can be increased. Furthermore, it is advantageous in terms of the battery's cycle characteristics and resistance characteristics.

[0084] A separation membrane containing a crosslinked structure for lithium secondary batteries according to one embodiment of the present invention is equipped with a crosslinked structure-containing polyolefin porous support having a crosslinked structure in which polymer chains are directly linked, thereby providing excellent high-temperature safety. For example, the meltdown temperature of the separation membrane containing the crosslinked structure for lithium secondary batteries can be increased compared to the meltdown temperature of a conventional separation membrane for lithium secondary batteries before crosslinking. For example, the meltdown temperature of the separation membrane may be 160°C or higher, 170°C or higher, or 180°C to 230°C.

[0085] In this specification, "separation membrane for lithium secondary battery before crosslinking" refers to a separation membrane comprising a non-crosslinked, crosslinked polyolefin porous support, an inorganic composite void layer located on at least one surface of the crosslinked polyolefin porous support and containing an inorganic filler and the first binder polymer, and a porous adhesive layer located on the inorganic composite void layer and containing a second binder polymer.

[0086] The meltdown temperature can be measured by thermomechanical analysis (TMA). For example, after taking samples in the mechanical and transverse directions, a sample measuring 4.8 mm wide x 8 mm long is placed in a TMA instrument (TA Instruments, Q400) and a tension of 0.01 N is applied. The temperature is then changed from 30°C to 220°C at a heating rate of 5°C / min, and the temperature at which the length rapidly increases and the sample breaks is measured to determine the meltdown temperature.

[0087] A separation membrane containing a crosslinked structure for lithium secondary batteries according to one embodiment of the present invention exhibits a smaller increase in shutdown temperature and a smaller rate of change compared to conventional separation membranes for lithium secondary batteries before crosslinking. In the separation membrane containing a crosslinked structure for lithium secondary batteries according to one embodiment of the present invention, the meltdown temperature of the separation membrane increases compared to before crosslinking, but the shutdown temperature does not increase significantly. Therefore, overcharge safety due to the shutdown temperature is ensured, and the high-temperature safety of the separation membrane is greatly improved.

[0088] In one embodiment of the present invention, the crosslinked structure-containing separation membrane for lithium secondary batteries may have a shutdown temperature of 145°C or less, 140°C or less, or 133°C to 140°C. When the crosslinked structure-containing separation membrane for lithium secondary batteries 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, pressurized process of battery assembly can be easily prevented.

[0089] The shutdown temperature can be determined by measuring the time (in 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 a Wang Ren type air permeability measuring device, and the temperature at which the air permeability of the separation membrane rapidly increases is defined as the shutdown temperature.

[0090] A cross-linked structure-containing separation membrane for lithium secondary batteries according to one embodiment of the present invention provides excellent thermal safety at high temperatures by comprising an inorganic composite void layer containing inorganic fillers. For example, the thermal shrinkage rates in the mechanical direction (MD) and transverse direction (TD), measured after being left at 150°C for 30 minutes, may be 20% or less, 2% to 15%, or 2% to 10%, respectively. Furthermore, the presence of a porous adhesive layer provides excellent adhesion to electrodes.

[0091] A separation membrane containing a crosslinked structure for lithium secondary batteries according to one embodiment of the present invention comprises a crosslinked structure-containing polyolefin porous support having a crosslinked structure in which polymer chains within the polyolefin porous support are directly linked, thereby allowing the porous structure of the polyolefin porous support to be substantially maintained in the same state as before crosslinking even after crosslinking.

[0092] A separation membrane containing a crosslinked structure for lithium secondary batteries according to one embodiment of the present invention exhibits less deterioration in permeability, basis weight, tensile strength, tensile elongation, puncture strength, and electrical resistance compared to the separation membrane for lithium secondary batteries before crosslinking, and the rate of change is also small.

[0093] A separation membrane containing a crosslinked structure for lithium secondary batteries 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 a separation membrane for lithium secondary batteries before crosslinking.

[0094] The rate of change in air permeability can be calculated using the following formula.

[0095] Percentage change (%) = [(Percentage of the cross-linked separation membrane for lithium secondary batteries after cross-linking) - (Percentage of the separation membrane for lithium secondary batteries before cross-linking)] / (Percentage of the separation membrane for lithium secondary batteries before cross-linking) × 100

[0096] Throughout this specification, "crosslinked structure-containing separation membrane for lithium secondary battery after crosslinking" refers to a separation membrane comprising a crosslinked structure-containing polyolefin porous support, an inorganic composite void layer located on at least one surface of the crosslinked structure-containing polyolefin porous support and containing an inorganic filler and a first binder polymer, and a porous adhesive layer located on the upper surface of the inorganic composite void layer and containing a second binder polymer.

[0097] The aforementioned air permeability (Gurley) can be measured by the ASTM D726-94 method. The Gurley used here is the resistance to airflow and is measured by a Gurley densometer. The air permeability values ​​described herein are based on 100 ml of air flowing through a 1 in a porous support under a pressure of 12.2 in H2O. 2 This is expressed as the time (in seconds) it takes for air to pass through the cross-section, i.e., the permeability time.

[0098] A separation membrane containing a crosslinked structure for lithium secondary batteries 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 a separation membrane for lithium secondary batteries before crosslinking.

[0099] The rate of change in basis weight can be calculated using the following formula.

[0100] Percentage change in basis weight (%) = [(Basis weight of the cross-linked separation membrane for lithium secondary batteries after cross-linking) - (Basis weight of the separation membrane for lithium secondary batteries before cross-linking)] / (Basis weight of the separation membrane for lithium secondary batteries before cross-linking) × 100

[0101] The basis weight (g / m 2 ) prepare a sample with dimensions of 1m in both length and width, measure its weight, and show the result.

[0102] A separation membrane containing a crosslinked structure for lithium secondary batteries according to one embodiment of the present invention may have a change in tensile strength in the mechanical 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 a separation membrane for lithium secondary batteries before crosslinking.

[0103] The rate of change in tensile strength can be calculated using the following formula.

[0104] Percentage change in tensile strength in the mechanical direction (%) = [(Tensile strength in the mechanical direction of the separation membrane for lithium secondary batteries before crosslinking) - (Tensile strength in the mechanical direction of the crosslinked structure-containing separation membrane for lithium secondary batteries after crosslinking)] / (Tensile strength in the mechanical direction of the separation membrane for lithium secondary batteries before crosslinking) × 100

[0105] Percentage change in tensile strength in the lateral direction (%) = [(Tensile strength in the lateral direction of the separation membrane for lithium secondary batteries before crosslinking) - (Tensile strength in the lateral direction of the crosslinked structure-containing separation membrane for lithium secondary batteries after crosslinking)] / (Tensile strength in the lateral direction of the separation membrane for lithium secondary batteries before crosslinking) × 100

[0106] The aforementioned tensile strength may be the strength at which the specimen breaks when it is pulled in the mechanical direction and transverse direction at a speed of 50 mm / min using Universal Testing Systems (Instron® 3345), in accordance with ASTM D882.

[0107] A separation membrane containing a crosslinked structure for lithium secondary batteries 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 mechanical direction and lateral direction compared to a separation membrane for lithium secondary batteries before crosslinking.

[0108] The rate of change in tensile elongation can be calculated using the following formula.

[0109] Percentage change in tensile elongation in the mechanical direction (%) = [(Tensile elongation in the mechanical direction of the separation membrane for lithium secondary batteries before crosslinking) - (Tensile elongation in the mechanical direction of the crosslinked structure-containing separation membrane for lithium secondary batteries after crosslinking)] / (Tensile elongation in the mechanical direction of the separation membrane for lithium secondary batteries before crosslinking) × 100

[0110] Percentage change in lateral tensile elongation (%) = [(Tensile elongation in the lateral direction of the separation membrane for lithium secondary batteries before crosslinking) - (Tensile elongation in the lateral direction of the crosslinked structure-containing separation membrane for lithium secondary batteries after crosslinking)] / (Tensile elongation in the lateral direction of the separation membrane for lithium secondary batteries before crosslinking) × 100

[0111] The tensile elongation can be calculated by measuring the maximum length of elongation until the specimen breaks, when the specimen is pulled in the mechanical direction and transverse direction at a speed of 50 mm / min using Universal Testing Systems (Instron® 3345), in accordance with ASTM D882, and using the following formula.

[0112] Tensile elongation in the mechanical direction (%) = (Length of specimen in the mechanical direction immediately before fracture - Length of specimen in the mechanical direction before stretching) / (Length of specimen in the mechanical direction before stretching) × 100

[0113] Transverse tensile elongation (%) = (Transverse length of specimen immediately before fracture - Transverse length of specimen before stretching) / (Transverse length of specimen before stretching) × 100

[0114] A separation membrane containing a crosslinked structure for lithium secondary batteries 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 a separation membrane for lithium secondary batteries before crosslinking.

[0115] The rate of change in puncture strength can be calculated using the following formula.

[0116] Percentage change in puncture strength (%) = [(Puncture strength of the separation membrane for lithium secondary batteries before crosslinking) - (Puncture strength of the crosslinked structure-containing separation membrane for lithium secondary batteries after crosslinking)] / (Puncture strength of the separation membrane for lithium secondary batteries before crosslinking) × 100

[0117] The aforementioned puncture strength can be measured in accordance with ASTM D2582. Specifically, the puncture strength can be measured according to ASTM D2582 after setting a 1 mm round tip to operate at a speed of 120 mm / min.

[0118] A separation membrane containing a crosslinked structure for lithium secondary batteries 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 a separation membrane for lithium secondary batteries before crosslinking.

[0119] The rate of change of electrical resistance can be calculated using the following formula.

[0120] Percentage change in electrical resistance (%) = [(Electrical resistance of the crosslinked structure-containing separation membrane for lithium secondary batteries after crosslinking) - (Electrical resistance of the separation membrane for lithium secondary batteries before crosslinking)] / (Electrical resistance of the separation membrane for lithium secondary batteries before crosslinking) × 100

[0121] The electrical resistance can be determined by measuring the resistance of the separation membrane using an impedance measurement method after leaving a coin cell containing the separation membrane sample at room temperature for one day.

[0122] A cross-linked structure-containing separation membrane for lithium secondary batteries according to one embodiment of the present invention may be manufactured by the following method, but is not limited thereto.

[0123] A method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to one embodiment of the present invention is: (S1) A step of producing an inorganic composite void layer forming slurry containing an inorganic filler, a first binder polymer, and a dispersion medium, (S2) The step of forming an inorganic composite void layer by coating and drying the slurry for forming the inorganic composite void layer on at least one surface of a polyolefin porous support, (S3) A step of coating the upper surface of the inorganic composite void layer with a coating solution for forming a porous adhesive layer, which includes a second binder polymer, a solvent for the second binder polymer, and a photoinitiator. (S4) The step of immersing the result of step (S3) in a coagulation solution containing a non-solvent for the second binder polymer, and then drying it to form a porous adhesive layer, (S5) The step of irradiating the result of step (S4) with ultraviolet light.

[0124] The following describes a method for manufacturing a cross-linked structure-containing separation membrane for lithium secondary batteries according to one embodiment of the present invention, focusing on its main components.

[0125] The photoinitiator directly photocrosslinks polymer chains within a porous polyolefin support. The photoinitiator alone can crosslink the porous polyolefin support without the need for other components such as crosslinking agents, co-initiators, or synergistic agents. Through light absorption alone, hydrogen atoms are removed from the photoinitiator via hydrogen abstraction, causing it to become a reactive compound. This photoinitiator then forms radicals on the polymer chains within the porous polyolefin support, making the chains reactive and causing them to directly link to each other through photocrosslinking. For example, since hydrogen abstraction by the photoinitiator is possible in small amounts of double bond or branched structures present within the polyolefin, radicals can be formed through hydrogen abstraction by light absorption alone, while hydrogen atoms are removed from the polyolefin chains.

[0126] In the method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to one embodiment of the present invention, radicals can be generated in the polymer chains within the polyolefin porous support by using the photoinitiator, thereby enabling the formation of a crosslinked structure in which polymer chains are directly linked to each other.

[0127] A method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to one embodiment of the present invention involves first producing an inorganic composite void layer forming slurry containing an inorganic filler, a first binder polymer, and a dispersion medium (S1).

[0128] For details regarding the inorganic fillers mentioned above, please refer to the information provided above.

[0129] The dispersion medium may act as a solvent that dissolves the first binder polymer, depending on the type of first binder polymer, or it may act as a dispersion medium that disperses the first binder polymer without dissolving it. The dispersion medium may be one that has a similar solubility index to the first binder polymer to be used and a low boiling point. In this case, uniform mixing and subsequent removal of the dispersion medium become easier.

[0130] 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 the formation of the inorganic composite void layer, and does not require additional explosion-proof equipment, thus making it easier to form the inorganic composite void layer.

[0131] For details regarding the first binder polymer, please refer to the information provided above.

[0132] In one embodiment of the present invention, the first binder polymer may be insoluble in the solvent for the second binder polymer and the non-solvent for the second binder polymer described later. In this case, even if a coating liquid described later is applied to form a porous adhesive layer after forming an inorganic composite void layer, the first binder polymer is not dissolved, making it easy to prevent the phenomenon of the first binder polymer dissolved in the solvent for the second binder polymer or the non-solvent for the second binder polymer blocking the pores.

[0133] 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 particulate form.

[0134] The slurry for forming the inorganic composite void layer can be manufactured by dissolving or dispersing the first binder polymer in the dispersion medium, then adding and dispersing the inorganic filler. The inorganic filler may be added in a state that has been crushed in advance to have a predetermined average particle size, or the inorganic filler may be added to the slurry in which the first binder polymer has been dissolved or dispersed, and then the inorganic filler may be crushed and dispersed using a ball mill or the like while controlling the particle size to have a predetermined average particle size. In this case, the crushing may be carried out for 1 to 20 hours, and the average particle size of the crushed inorganic filler is as described above. Conventional methods can be used as the crushing method, and the ball mill method may be used.

[0135] In one embodiment of the present invention, the slurry for forming an inorganic composite void layer may further contain additives such as a dispersant and / or a thickener. In one embodiment of the present invention, the additives may include polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), ethylhydroxyethylcellulose (EHEC), methylcellulose (MC), carboxymethylcellulose (CMC), polyacrylic acid, hydroxyalkylmethylcellulose, cyanoethylene polyvinyl alcohol, or two or more of these.

[0136] In one embodiment of the present invention, the solid content of the slurry for forming the inorganic composite void layer may be 5% to 60% by weight, or 30% to 50% by weight. When the solid content of the slurry for forming the inorganic composite void layer 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.

[0137] Subsequently, the slurry for forming the inorganic composite void layer is coated onto at least one surface of the polyolefin porous support and dried to form the inorganic composite void layer (S2).

[0138] The aforementioned polyolefin porous support can be manufactured by forming pores using conventional methods known in the industry, such as a wet process using a solvent, diluent, or pore-forming agent, or a dry process using a stretching method, in order to ensure excellent permeability and porosity from the polyolefin material described above.

[0139] In one embodiment of the present invention, the polyolefin porous support may have 0.01 to 0.5 or 0.01 to 0.3 double bonds per 1000 carbon atoms in the polyolefin chain when measured by H-NMR. When the polyolefin porous support has the above-mentioned number of double bonds, the polyolefin porous support can be effectively crosslinked, and the generation of excessive radicals and the occurrence of side reactions can be minimized. For example, side reactions can be prevented by crosslinking only between polymer chains without crosslinking between photoinitiators or between photoinitiators and polymer chains.

[0140] Furthermore, it is possible to prevent the phenomenon in which the separation membrane shrinks due to the rapid crosslinking reaction caused by the excessive generation of radicals. This prevents a decrease in the air permeability of the polyolefin porous support after crosslinking.

[0141] Furthermore, it is possible to prevent excessive main chain scission of the polyolefin, which would reduce the mechanical strength of the porous polyolefin support.

[0142] Since crosslinking between polyolefin chains occurs throughout the entire polyolefin chain excluding the terminal portions, the number of double bonds present in the polyolefin chain excluding the terminals can affect the crosslinking of the polyolefin porous support. In one embodiment of the present invention, the polyolefin porous support may have 0.005 to 0.49 double bonds per 1000 carbon atoms in the polyolefin chain excluding the terminals when measured by H-NMR. For details on "double bonds present in the polyolefin chain excluding the terminals," please refer to the above explanation.

[0143] In one embodiment of the present invention, the number of double bonds present in the polyolefin chain can be adjusted by adjusting the type and purity of the catalyst used in the synthesis of the polyolefin, the addition of a binder, and so on.

[0144] In one embodiment of the present invention, the polyolefin porous support has a BET specific surface area of ​​10 m². 2 / g~27m 2 / g, 13m 2 / g~25m 2 / g, or 15m 2 / g~23m 2 It may be / g. When the BET specific surface area of ​​the polyolefin porous support satisfies the above range, the surface area of ​​the polyolefin porous support increases, and the crosslinking efficiency of the polyolefin porous support can be increased even when a small amount of photoinitiator is used.

[0145] The BET specific surface area of ​​the aforementioned polyolefin porous support can be measured by the BET method. Specifically, the BET specific surface area of ​​inorganic particles can be calculated from the amount of nitrogen gas adsorbed under liquid nitrogen temperature (77K) using Belsorp-mini II from BEL Japan.

[0146] In one embodiment of the present invention, the polyolefin porous support may further contain an antioxidant. The antioxidant can regulate the crosslinking reaction between polymer chains by controlling radicals formed on the polyolefin chains. The antioxidant either oxidizes the polymer chains instead of them to prevent oxidation, or it absorbs the generated radicals to regulate the crosslinking reaction between polymer chains. This can affect the shutdown temperature and mechanical strength of the final separation membrane.

[0147] 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 satisfies the above range, it is possible to sufficiently control the radicals generated in excess of the antioxidant, easily prevent the occurrence of side reactions, and easily prevent the phenomenon of the surface of the polyolefin porous support becoming non-uniform.

[0148] Such antioxidants can be broadly classified into two types: radical scavengers, which react with radicals generated on polyolefins to stabilize them, and peroxide decomposers, which decompose peroxides generated by radicals into stable molecular forms. The radical scavengers stabilize radicals by abstracting hydrogen and become radicals themselves, but can remain in a stable form through resonance effects or electron rearrangement. The peroxide decomposers can exhibit superior effects when used in combination with radical scavengers.

[0149] In one embodiment of the present invention, the antioxidant may include a first antioxidant which is a radical scavenger and a second antioxidant which is a peroxide decomposer. Since the first antioxidant and the second antioxidant have different operating mechanisms, by including both the first antioxidant which is a radical scavenger and the second antioxidant which is a peroxide decomposer in the antioxidant, the synergistic effect of these antioxidants makes it easier to suppress the generation of unwanted radicals.

[0150] The content of the first antioxidant and the content of the second antioxidant may be the same or different.

[0151] In one embodiment of the present invention, the first antioxidant may include a phenolic antioxidant, an amine-based antioxidant, or a mixture thereof.

[0152] The aforementioned phenolic antioxidants, 2,6-di-t-butyl-4-methylphenol, 4,4'-thiobis(2-t-butyl-5-methylphenol), 2,2'-thiodiethylbis-[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], 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) It may contain [xyphenyl)-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 of these.

[0153] 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 satisfies the above range, the problem of side reactions occurring due to excessive radical generation can be easily prevented.

[0154] In one embodiment of the present invention, the second antioxidant may include a phosphorus-based antioxidant, a sulfur-based antioxidant, or a mixture thereof.

[0155] The phosphorus-based antioxidant decomposes peroxides to produce alcohols, which are converted into phosphates. The phosphorus-based antioxidants 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- This may contain a 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 of these.

[0156] 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 of these.

[0157] 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 satisfies the above range, the problem of side reactions occurring due to excessive radical generation can be easily prevented.

[0158] In one embodiment of the present invention, when the antioxidant simultaneously contains a first antioxidant which is a radical scavenger and a second antioxidant which is 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.

[0159] Non-limiting examples of methods for coating the inorganic composite void layer forming slurry onto the polyolefin porous support include dip coating, die coating, roll coating, comma coating, microgravure coating, doctor blade coating, reverse roll coating, and direct roll coating.

[0160] The slurry for forming the inorganic composite void layer can be dried by the drying method used in the production of separation membranes. For example, the coated slurry may be dried with air for 10 seconds to 30 minutes, 30 seconds to 20 minutes, or 3 minutes to 10 minutes. Drying within these time ranges has the effect of removing the solvent from the remaining second binder polymer without hindering productivity.

[0161] Subsequently, a coating solution for forming a porous adhesive layer, containing a second binder polymer, a solvent for the second binder polymer, and a photoinitiator, is coated onto the upper surface of the inorganic composite void layer (S3).

[0162] For details regarding the second binder polymer, please refer to the information provided above.

[0163] The solvent for the second binder polymer may be one that dissolves the second binder polymer in an amount of 5% or more by weight, 15% or more by weight, or 25% or more by weight at 25°C.

[0164] The solvent for the second binder polymer may be a non-solvent for the first binder polymer. For example, the solvent for the second binder polymer may be one that dissolves the first binder polymer at 25°C in less than 5% by weight.

[0165] In one embodiment of the present invention, the solvent for the second binder polymer 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; amides such as N-methylpyrrolidone and N,N-dimethylformamide; or two or more of these.

[0166] In one embodiment of the present invention, the second binder polymer may be included in an amount of 3% to 30% by weight, or 5% to 25% by weight, based on 100% by weight of the coating liquid for forming the porous adhesive layer.

[0167] By including the photoinitiator in the coating liquid for forming a porous adhesive layer, when the coating liquid for forming a porous adhesive layer is coated onto the upper surface of the inorganic composite void layer, the photoinitiator is introduced to the surface of the polyolefin porous support and a porous adhesive layer can be formed at the same time.

[0168] During the coating process using the coating solution for forming a porous adhesive layer, the polyolefin porous support is wetted with the solvent for the second binder polymer. At this time, the photoinitiator contained in the coating solution for forming a porous adhesive layer is introduced to the surface of the polyolefin porous support, and the polyolefin porous support can be photocrosslinked by the photoinitiator present on the surface of the polyolefin porous support when irradiated with ultraviolet light.

[0169] As a result, the method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to one embodiment of the present invention simplifies the process by eliminating the need for additional equipment for directly applying a photoinitiator to the polyolefin porous support in order to photocrosslink the polyolefin porous support, such as equipment for directly coating and drying a solution containing a photoinitiator on the polyolefin porous support, and enabling the photocrosslinking of the polyolefin porous support using a porous adhesive layer formation step.

[0170] The method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to one embodiment of the present invention does not require other components such as monomers to form radicals outside the photoinitiator in order to directly crosslink polymer chains within the polyolefin porous support. Therefore, even when the photoinitiator is added to the coating liquid for forming a porous adhesive layer, other components do not interfere with the photoinitiator reaching the surface of the polyolefin porous support, and the photoinitiator can be sufficiently introduced to the surface of the polyolefin porous support.

[0171] Furthermore, since polyolefin porous supports themselves and inorganic fillers generally have a high ultraviolet blocking effect, if ultraviolet light is irradiated after forming an inorganic composite void layer and a porous adhesive layer containing inorganic fillers, the amount of ultraviolet light irradiated to the polyolefin porous support may decrease. However, in the present invention, crosslinking is possible even with a small amount of ultraviolet light irradiated, so even if ultraviolet light is irradiated after the inorganic composite void layer and porous adhesive layer have been formed, the polymer chains within the polyolefin porous support can be crosslinked and directly linked to each other.

[0172] In one embodiment of the present invention, the photoinitiator may be a type II photoinitiator.

[0173] 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 of these.

[0174] In one embodiment of the present invention, the photoinitiator may include 2-isopropylthioxanthone, thioxanthone, or a mixture thereof. 2-isopropylthioxanthone or thioxanthone can be photocrosslinked even at long wavelengths with high transmittance. As a result, the polyolefin porous support can be easily crosslinked even when ultraviolet light is irradiated after the inorganic material-forming void layer and the porous adhesive layer have been formed.

[0175] The content of the photoinitiator present on the surface of the polyolefin porous support can affect the crosslinking of the polyolefin porous support. In one embodiment of the present invention, the content of the photoinitiator may be 0.015 to 0.3 parts by weight, 0.03 to 0.3 parts by weight, 0.03 to 0.09 parts by weight, or 0.06 to 0.07 parts by weight per 100 parts by weight of the polyolefin porous support. When the content of the photoinitiator satisfies the above range, the polyolefin porous support can be crosslinked while simultaneously preventing excessive crosslinking reactions. Even when the photoinitiator is coated with the above-mentioned 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 lower than conventionally used).

[0176] 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 filling the total pore volume of the polyolefin porous support. For example, assuming that the total pore volume of the polyolefin porous support is 100% filled with the solvent for the second binder polymer, and that there is no solvent for the second binder polymer on the surface of the polyolefin porous support, the weight of the solvent for the second binder polymer contained within the total pore volume of the polyolefin porous support can be determined from the density of the solvent for the second binder polymer, 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 for the second binder polymer.

[0177] In one embodiment of the present invention, the content of the photoinitiator in the photocrosslinking composition is 0.015 to 0.3 parts by weight per 100 parts by weight of the polyolefin porous support, and may be 0.01 to 0.5 parts by weight, 0.02 to 0.45 parts by weight, or 0.25 to 0.4 parts by weight based on 100 parts by weight of the solvent for the second binder polymer. When the content of the photoinitiator satisfies the above range based on the solvent for the second binder polymer, the polyolefin porous support can be crosslinked, and at the same time, the occurrence of side reactions due to excessive radical generation can be more easily prevented.

[0178] Furthermore, in one embodiment of the present invention, the content of the photoinitiator in the photocrosslinking composition is 0.015 to 0.3 parts by weight per 100 parts by weight of the polyolefin porous support, and is 0.01 mg / m² based on the specific surface area of ​​the polyolefin porous support. 2 ~1.0 mg / m² 2 , 0.03 mg / m² 2 ~0.8 mg / m² 2 , or 0.06 mg / m² 2 ~0.7 mg / m² 2 This is possible. When the content of the photoinitiator satisfies the above-mentioned range, the polyolefin porous support can be crosslinked, and at the same time, the occurrence of side reactions due to excessive radical generation can be more easily prevented.

[0179] The photoinitiator content, based on the specific surface area of ​​the polyolefin porous support, can be measured by NMR analysis.

[0180] Non-limiting examples of methods for coating the porous adhesive layer-forming coating liquid onto the upper surface of the inorganic composite void layer include dip coating, die coating, roll coating, comma coating, microgravure coating, doctor blade coating, reverse roll coating, Mayer bar coating, and direct roll coating.

[0181] In one embodiment of the present invention, a pattern can be formed on the porous adhesive layer that is finally manufactured by pattern coating the porous adhesive layer with the porous adhesive layer coating liquid on the upper surface of the inorganic composite void layer.

[0182] Subsequently, the result of step (S3) is immersed in a coagulation solution containing a non-solvent for the second binder polymer, and then dried to form a porous adhesive layer (S4). This causes the second binder polymer to solidify while a phase separation phenomenon is induced in the coated porous adhesive layer-forming coating solution. After that, the coagulation solution is removed by washing with water and the material is dried.

[0183] The drying can be carried out using methods known in the industry and can be done in batch or continuous mode using an oven or heated chamber within a temperature range that takes into account the vapor pressure of the solvent relative to the second binder polymer used. The drying is intended to remove most of the solvent from the second binder polymer present in the porous adhesive layer forming coating liquid and should preferably be carried out as quickly as possible considering productivity, for example, in a time of 1 minute or less or 30 seconds or less.

[0184] The coagulation solution may consist solely of a non-solvent for the second binder polymer, or it may contain a mixed solvent of a non-solvent for the second binder polymer and the solvent for the second binder polymer as described above. When a mixed solvent of a non-solvent for the second binder polymer and the solvent for the second binder polymer is used, the content of the non-solvent for the second binder polymer may be 50% by weight or more per 100% by weight of the coagulation solution, in order to form a good porous structure and improve productivity.

[0185] During the solidification process of the second binder polymer, it condenses, thereby preventing the second binder polymer from penetrating the surface and / or interior of the polyolefin porous support, and thus preventing the phenomenon of increased resistance of the separation membrane. Furthermore, the resistance of the separation membrane can be improved by making the adhesive layer containing the second binder polymer porous.

[0186] The non-solvent for the second binder polymer may have a solubility of less than 5% by weight for the second binder polymer at 25°C.

[0187] The non-solvent for the second binder polymer may also be a non-solvent for the first binder polymer. For example, the non-solvent for the second binder polymer may have a solubility of less than 5% by weight for the first binder polymer at 25°C.

[0188] 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 of these.

[0189] In one embodiment of the present invention, the immersion may be performed for 3 seconds to 1 minute. When the immersion time falls within the above range, phase separation occurs appropriately, ensuring adhesion between the inorganic composite void layer and the porous adhesive layer, and preventing the adhesive layer from detaching.

[0190] In one embodiment of the present invention, the drying may be carried out by a normal drying method used during the production of separation membranes. For example, it may be dried with air for 10 seconds to 30 minutes, 30 seconds to 20 minutes, or 3 minutes to 10 minutes. When drying is carried out within the above time ranges, it has the effect of removing residual dispersion medium without hindering productivity.

[0191] Subsequently, the product obtained in step (S4) is irradiated with ultraviolet light (S5).

[0192] When exposed to ultraviolet light, the polymer chains within the porous polyolefin support are crosslinked, resulting in a porous polyolefin support containing a crosslinked structure.

[0193] Ultraviolet irradiation can be performed using an ultraviolet crosslinking apparatus, with the irradiation time and intensity of the ultraviolet light appropriately adjusted, taking into account conditions such as the content ratio of the photoinitiator. For example, the irradiation time and intensity of the ultraviolet light can be set to conditions that ensure sufficient crosslinking of the polymer chains within the porous polyolefin support to achieve the desired heat resistance, while preventing damage to the separation membrane from the heat generated by the ultraviolet lamp. Furthermore, the ultraviolet lamp used in the ultraviolet crosslinking apparatus can be appropriately selected from high-pressure mercury lamps, metal lamps, gallium lamps, etc., depending on the photoinitiator used, and the emission wavelength and capacity of the ultraviolet lamp can be appropriately selected according to the process.

[0194] The method for manufacturing a crosslinked structure-containing separation membrane for lithium secondary batteries according to one embodiment of the present invention allows for photocrosslinking of polymer chains within a polyolefin porous support even with an irradiation dose of ultraviolet light that is significantly smaller than the amount of light used in general photocrosslinking, thus increasing the applicability of the crosslinked structure-containing separation membrane for lithium secondary batteries to the mass production process. For example, the irradiation dose of ultraviolet light is 10 to 2000 mJ / cm². 2 50-1000 mJ / cm² 2 , or 150-500 mJ / cm² 2 It is possible.

[0195] In one embodiment of the present invention, the amount of ultraviolet light irradiated can be measured using a portable light intensity meter H-type UV bulb and UV power pack manufactured by Miltec. When measuring the light intensity using a Miltec H-type UV bulb, three types of wavelength values ​​are obtained for each wavelength: UVA, UVB, and UVC. The ultraviolet light of the present invention corresponds to UVA.

[0196] In the present invention, the method for measuring the amount of ultraviolet light irradiation involves passing a UV power pack on a conveyor under a light source under the same conditions as the sample, and the ultraviolet light intensity value shown on the UV power pack at this time is referred to as the "amount of ultraviolet light irradiation".

[0197] A method for manufacturing a crosslinked structure-containing separation membrane for lithium secondary batteries according to one embodiment of the present invention allows for the formation of the porous adhesive layer in various forms by forming the inorganic composite void layer and the porous adhesive layer through separate steps. For example, the porous adhesive layer can be easily formed in a patterned form.

[0198] A lithium secondary battery can be manufactured by interposing the aforementioned cross-linked structure-containing separation membrane for lithium secondary batteries between the positive electrode and the negative electrode.

[0199] The lithium secondary battery may have various shapes, such as cylindrical, prismatic, or pouch-type.

[0200] 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.

[0201] The electrodes to be applied with the crosslinked structure-containing separation membrane for lithium secondary batteries of the present invention are not particularly limited and can be manufactured in a form in which an electrode active material layer, comprising an electrode active material, a conductive material, and a binder, is bonded to a current collector by conventional methods well known in the industry.

[0202] Non-limiting examples of positive electrode active materials among the electrode active materials include layered compounds such as lithium cobalt composite oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; chemical formula Li 1+x Mn 2-x Lithium manganese oxides such as O4 (x=0~0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O5, LiV3O4, V2O5, Cu2V2O7; chemical formula LiNi 1-x M x Ni-site type lithium nickel oxide represented as O2 (M=Co, Mn, Al, Cu, Fe, Mg, B or Ga, x=0.01~0.3); chemical formula LiMn 1-x M x Lithium manganese composite oxides represented as O2 (M=Co, Ni, Fe, Cr, Zn, or Ta, x=0.01~0.1) or Li2Mn3MO5 (M=Fe, Co, Ni, Cu, or Zn); LiMn2O4 in which part of the Li in the chemical formula is substituted with alkaline earth metal ions; disulfide compounds; Fe2(MoO4)3, etc. are examples, but are not limited to these.

[0203] Non-limiting examples of the negative electrode active material among the electrode active materials mentioned above include conventional negative electrode active materials used in the negative electrodes of lithium secondary batteries, and in particular lithium adsorbent materials such as lithium metal or lithium alloy, carbon, petroleum coke, activated carbon, graphite or other carbon compounds may be used.

[0204] Non-limiting examples of positive electrode current collectors include foil made from aluminum, nickel, or combinations thereof, and non-limiting examples of negative electrode current collectors include foil made from copper, gold, nickel, or copper alloys, or combinations thereof.

[0205] In one embodiment of the present invention, the conductive material used in the negative electrode and the positive electrode can each be added independently, usually in an amount of 1% to 30% by weight based on the total weight of the active material layer. Such conductive materials are not particularly limited as long as they do not induce chemical changes in the battery and are conductive, and examples of such materials that can be used include graphite such as natural graphite or artificial graphite; carbon blacks such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; 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.

[0206] In one embodiment of the present invention, the binders used in the negative electrode and the positive electrode are components that independently assist in the bonding of the active material to a conductive material and to the current collector, and can usually be added in an amount of 1% to 30% by weight based on the total weight of the active material layer. Examples of such binders include polyvinylidene fluoride (PVdF), polyacrylic acid (PAA), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers.

[0207] In one embodiment of the present invention, the lithium secondary battery includes an electrolyte, which may include an organic solvent and a lithium salt. Furthermore, an organic solid electrolyte or an inorganic solid electrolyte may be used as the electrolyte.

[0208] Examples of the aforementioned organic solvents 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, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0209] The lithium salt is a substance that is readily soluble in the organic solvent, for example, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenylborate, imides, etc. may be used.

[0210] Furthermore, to improve charge-discharge characteristics, flame retardancy, etc., the electrolyte may be supplemented with, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. In some cases, halogen-containing solvents such as carbon tetrachloride and trifluoroethylene may be further added to impart nonflammability, and carbon dioxide may be further added to improve high-temperature storage characteristics.

[0211] Examples of organic solid electrolytes that can be used include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyagitation lysine, polyester sulfides, polyvinyl alcohol, polyvinylidene fluoride, and polymers containing ionic dissociation groups.

[0212] Examples of inorganic solid electrolytes that can be used include lithium nitrides, halides, and sulfates such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH, and Li3PO4-Li2S-SiS2.

[0213] The injection of the electrolyte can be carried out at an appropriate stage in the battery manufacturing process, depending on the manufacturing process and required physical properties of the final product. That is, it can be done before battery assembly or at the final stage of battery assembly.

[0214] In one embodiment of the present invention, in addition to the conventional winding process, the process of applying the cross-linked structure-containing separation membrane for lithium secondary batteries to the battery can also include lamination (stack) and folding processes between the separation membrane and the electrodes.

[0215] In one embodiment of the present invention, the crosslinked structure-containing separation membrane for lithium secondary batteries is interposed between the positive and negative electrodes of the lithium secondary battery, and can be interposed between adjacent cells or electrodes when a plurality of cells or electrodes are assembled to form an electrode assembly. The electrode assembly can have a variety of structures, such as a simple stack type, a jelly roll type, a stack folding type, or a lamination stack type.

[0216] The present invention will be described in detail below with reference to examples to aid in understanding the present invention. However, the examples of 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 give a more complete explanation of the present invention to a person of average knowledge in the industry.

[0217] Example 1 For 1H-NMR measurements, a 6 μm thick polyethylene porous film (manufactured by Toray Industries, Inc., weight-average molecular weight: 600,000, porosity: 45%) was prepared as a polyolefin porous support. This film contained 0.2 double bonds per 1,000 carbon atoms in the polymer chain, and included 3,000 ppm of Irganox 1010 and 2,000 ppm of Irgafos 168 as antioxidants.

[0218] 600nm D 50 Al2O3 powder with particle size and 250 nm D 50 An inorganic filler was prepared by mixing γ-AlOOH powder with a specific particle size in a weight ratio of 9:1. An acrylic emulsion (CSB-130, manufactured by Toyo Ink Co., Ltd.) was prepared as the first binder polymer, and sodium carboxymethylcellulose (CMC-Na) (SG-L02, manufactured by GL Chem Co., Ltd.) was prepared as a dispersant.

[0219] After adding the prepared inorganic filler, first binder polymer, and dispersant to water in a weight ratio of 97:2:1, the inorganic filler was crushed and dispersed to produce a slurry for forming an inorganic composite void layer.

[0220] The inorganic composite void layer was formed by coating both sides of a polyolefin porous support with the slurry for forming the inorganic composite void layer and drying it.

[0221] A coating solution for forming a porous adhesive layer was prepared by adding polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) (LBG, Arkema) as a second binder polymer to the solvent N-methyl-2-pyrrolidone (NMP), and adding 2-isopropylthioxanthone (Sigma Aldrich) as a photoinitiator at a ratio of 0.1 parts by weight per 100 parts by weight of N-methyl-2-pyrrolidone.

[0222] After coating the porous adhesive layer-forming coating liquid onto the inorganic composite void layer, the photoinitiator content was adjusted to 0.07 parts by weight per 100 parts by weight of the polyolefin porous support, and the coating liquid was solidified by sequentially immersing it in a solidification solution and a water washing tank. The solidification solution and the water washing tank contained water, which is a non-solvent. After the coating liquid solidified, the solvent and non-solvent remaining in the coating liquid were dried simultaneously.

[0223] Subsequently, the resulting material was subjected to an integrated luminous intensity of 500 mJ / cm² using a high-pressure mercury lamp (Lichtzen LH-250 / 800-A). 2 By irradiating the polyolefin porous support with ultraviolet light to achieve a crosslinked structure, a crosslinked structure-containing separation membrane for lithium secondary batteries was obtained.

[0224] Comparative Example 1 A polyethylene porous film (manufactured by Toray Industries, Inc., weight-average molecular weight: 600,000, porosity: 45%) with a thickness of 6 μm and a double bond count of 0.2 per 1000 carbon atoms in the polymer chain at the time of H-NMR measurement, was used as a separation membrane for lithium secondary batteries without any further treatment.

[0225] Comparative Example 2 A separation membrane was prepared in the same manner as in Example 1, except that 2-isopropylthioxanthone (manufactured by Sigma Aldrich) was not added as a photoinitiator to the coating liquid for forming a porous adhesive layer, and ultraviolet light was not irradiated.

[0226] Evaluation Example 1: Evaluation of the physical properties of a separation membrane Table 1 shows the results of measuring the air permeability, basis weight, tensile strength in the mechanical and transverse directions, tensile elongation in the mechanical and transverse directions, puncture strength, thermal shrinkage rate in the mechanical and transverse directions (measured after being left at 150°C for 30 minutes), adhesion to electrodes, electrical resistance, and meltdown temperature for the separation membranes produced in Example 1, Comparative Example 1, and Comparative Example 2.

[0227] (1) Evaluation of breathability Air permeability (Gurley) was measured according to the ASTM D726-94 method. The Gurley used here is the resistance to airflow and is measured by a Gurley densometer. The air permeability values ​​described here are based on 100 ml of air flowing through a separation membrane at a pressure of 12.2 in H2O. 2 This is expressed as the time (in seconds) it takes for air to pass through the cross-section, i.e., the permeability time.

[0228] (2) Evaluation of basis weight Basis weight (g / m 2 For this evaluation, a sample was prepared with a separation membrane that was 1m in both length and width, and its weight was measured for assessment.

[0229] (3) Tensile strength in the mechanical direction and transverse direction A test piece measuring 150mm x 15mm was prepared.

[0230] In accordance with ASTM D882, the strength at which the specimen fractured when it was pulled in the mechanical direction (MD) and transverse direction (TD) at a speed of 50 mm / min using Universal Testing Systems (Instron® 3345) was defined as the tensile strength in the mechanical direction (MD) and transverse direction (TD).

[0231] (4) Tensile elongation in the machine direction and transverse direction A test piece measuring 150mm x 15mm was prepared.

[0232] In accordance with ASTM D882, the specimen was pulled in the mechanical and transverse directions at a speed of 50 mm / min using Universal Testing Systems (Instron® 3345). The maximum length of elongation until the specimen broke was measured, and the tensile elongation was calculated using the following formula.

[0233] Tensile elongation in the mechanical direction (%) = (Length of specimen in the mechanical direction immediately before fracture - Length of specimen in the mechanical direction before stretching) / (Length of specimen in the mechanical direction before stretching) × 100

[0234] Transverse tensile elongation (%) = (Transverse length of specimen immediately before fracture - Transverse length of specimen before stretching) / (Transverse length of specimen before stretching) × 100

[0235] (5) Puncture strength A sample measuring 50mm x 50mm was prepared.

[0236] After setting a 1mm round tip to operate at a speed of 120mm / min according to ASTM D2582, the puncture strength was measured.

[0237] (6) Evaluation of thermal shrinkage rate at 150°C The thermal shrinkage rate was calculated by cutting the separation membrane into 50mm (length) x 50mm (width) test pieces, placing them in an oven heated to 150°C for 30 minutes, retrieving the test pieces, and measuring the change in length in the mechanical and lateral directions.

[0238] Thermal shrinkage rate at 150°C (%) = {(Dimensions before shrinkage - Dimensions after shrinkage) / Dimensions before shrinkage} × 100

[0239] (7) Adhesion to the electrode The separation membranes produced in Example 1, Comparative Example 1, and Comparative Example 2 were cut to a size of 25 mm x 100 mm, and two of each were prepared.

[0240] The two prepared separation membranes were placed on top of each other with their porous adhesive layers facing each other, sandwiched between 100 μm PET films, and then laminated by passing them through a 100°C roll laminator. During this process, the laminator was heated at a speed of 0.3 m / min for 30 seconds, and the pressure was 2 kgf / cm². 2 That was the case.

[0241] After attaching the ends of the two attached separation membranes to a UTM device (LLOYD Instruments, LF Plus), force was applied in both directions at a measurement speed of 300 mm / min to measure the force required to separate the attached separation membranes.

[0242] (8) Evaluation of electrical resistance Electrical resistance was measured by fabricating a coin cell using a separator membrane, leaving the coin cell at room temperature for one day, and then measuring the resistance of the separator membrane using the impedance measurement method. The coin cell was fabricated as follows.

[0243] Manufacturing of negative electrodes A negative electrode slurry was prepared by mixing artificial graphite as the negative electrode active material, Denka Black (carbon black) as the conductive material, and polyvinylidene fluoride (PVdF) as the binder in a weight ratio of 75:5:20, and then adding N-methylpyrrolidone (NMP) as the solvent.

[0244] The aforementioned negative electrode slurry was given a flow rate of 3.8 mAh / cm³. 2 The negative electrode was prepared by coating and drying the copper current collector with the specified loading amount.

[0245] Manufacturing of positive electrodes A positive electrode active material slurry was prepared by adding LiCoO2 as the positive electrode active material, Denka Black as the conductive material, and polyvinylidene fluoride (PVdF) as the binder in a weight ratio of 85:5:10 to the solvent N-methylpyrrolidone (NMP). The positive electrode active material slurry was coated onto a sheet-shaped aluminum current collector and dried, resulting in a final positive electrode loading amount of 3.3 mAh / cm². 2 The positive electrode active material layer was formed in such a manner.

[0246] Manufacture of Coin Cell Between the negative electrode and the positive electrode manufactured as described above, each of the separator membranes of the above-described Examples and Comparative Examples was interposed, and a non-aqueous electrolyte (1M LiPF6, ethylene carbonate (EC) / propylene carbonate (PC) / diethyl carbonate (DEC)) (volume ratio = 3:3:4) was injected to manufacture a coin cell.

[0247] (9) Evaluation of Meltdown Temperature The meltdown temperature was measured by a thermomechanical analysis method (TMA) after samples in the machine direction (MD) and transverse direction (TD) of the separator membrane were respectively collected. Specifically, a sample with a width of 4.8 mm × a length of 8 mm was placed in a TMA apparatus (manufactured by TA Instruments, Q400), and while applying a tension of 0.01 N, the temperature was changed from 30°C to 220°C at a heating rate of 5°C / min. As the temperature increased, the length of the sample changed, and the temperature at which the length rapidly increased and the sample broke in each of the machine direction and the transverse direction was measured.

[0248]

Table 1

[0249] From Table 1, it can be confirmed that the separator membrane manufactured in Example 1 has excellent heat shrinkage rates in the machine direction and the transverse direction measured after being left at 150°C for 30 minutes and the adhesive force with the electrode, and excellent heat resistance.

[0250] On the other hand, the separator membrane manufactured in Comparative Example 1 has inferior heat shrinkage rates in the machine direction and the transverse direction measured after being left at 150°C for 30 minutes and the adhesive force with the electrode, and it can be confirmed that the meltdown temperature is at a significantly lower level compared to Example ①.

[0251] For the separator membrane manufactured in Comparative Example 2, although the heat shrinkage rates in the machine direction and the transverse direction measured after being left at 150°C for 30 minutes and the adhesive force with the electrode are excellent, it can be confirmed that the meltdown temperature is at a significantly lower level compared to Example 1.

[0252] The melt-down temperatures measured in the machine direction for the separation membranes produced in Example 1, Comparative Example 1, and Comparative Example 2 are shown in FIG. 2. The portion where a rapid dimension change occurs corresponds to the melt-down temperature.

[0253] As shown in FIG. 2, it can be confirmed that the separation membranes produced in Comparative Example 1 and Comparative Example 2 have a melt-down temperature of about 150°C.

[0254] On the other hand, it can be confirmed that the separation membrane produced in Example 1 has a melt-down temperature of about 220°C or higher.

Claims

1. A crosslinked structure-containing polyolefin porous support having a crosslinked structure in which polymer chains are directly linked to each other, An inorganic composite void layer comprising an inorganic filler and a first binder polymer, located on at least one surface of the cross-linked structure-containing polyolefin porous support, It comprises a porous adhesive layer located on the inorganic composite void layer and containing a second binder polymer, The crosslinked structure, in which the polymer chains are directly linked, is formed by introducing a photoinitiator containing thioxanthone, a thioxanthone derivative, or two or more of these onto the surface of a porous polyolefin support, wherein the photoinitiator is added to a coating liquid that forms a porous adhesive layer. The aforementioned polyolefin porous support is a crosslinked structure-containing separation membrane for lithium secondary batteries, which does not contain a crosslinked structure in which a photoinitiator and polymer chains are directly linked.

2. The separation membrane containing a cross-linked structure for lithium secondary batteries according to claim 1, wherein the thermal shrinkage rates in the mechanical direction and the transverse direction, measured after leaving the separation membrane containing the cross-linked structure for lithium secondary batteries at 150°C for 30 minutes, are each 20% or less.

3. The separation membrane containing a crosslinked structure for a lithium secondary battery according to claim 1, wherein the weight ratio of the inorganic filler to the first binder polymer is 95:5 to 99.9:0.

1.

4. The separation membrane containing a crosslinked structure for lithium secondary batteries according to claim 1, wherein the first binder polymer comprises an acrylic polymer, polyacrylic acid, styrene-butadiene rubber, polyvinyl alcohol, or two or more of these.

5. The separation membrane containing a crosslinked structure for lithium secondary batteries according to claim 1, wherein the second binder polymer comprises polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene, polyvinylidene fluoride-tetrafluoroethylene, polyvinylidene fluoride-trifluoroethylene, polymethyl methacrylate, polyethylhexyl acrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, ethylhexyl acrylate-methyl methacrylate copolymer, ethylene vinyl acetate copolymer, polyethylene oxide, polyarylate, or two or more of these.

6. The separation membrane containing a crosslinked structure for a lithium secondary battery according to claim 1, wherein the porous adhesive layer has a pattern comprising one or more adhesive portions containing the second binder polymer and one or more plain portions where the adhesive portions are not formed.

7. The separation membrane containing a cross-linked structure for lithium secondary batteries according to claim 1, wherein the meltdown temperature of the separation membrane containing the cross-linked structure for lithium secondary batteries is 160°C or higher.

8. The separation membrane containing a crosslinked structure for a lithium secondary battery according to claim 1, wherein the shutdown temperature of the separation membrane containing the crosslinked structure for a lithium secondary battery is 145°C or lower.

9. (S1) A step of producing an inorganic composite void layer forming slurry containing an inorganic filler, a first binder polymer, and a dispersion medium, (S2) The step of forming an inorganic composite void layer by coating and drying the slurry for forming the inorganic composite void layer on at least one surface of a polyolefin porous support, (S3) A step of coating the upper surface of the inorganic composite void layer with a coating liquid for forming a porous adhesive layer, which includes a second binder polymer, a solvent for the second binder polymer, and a photoinitiator. (S4) The process involves immersing the product obtained in step (S3) in a coagulation solution containing a non-solvent for the second binder polymer, and then drying it to form a porous adhesive layer. A method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries, comprising the step of (S5) irradiating the result of step (S4) with ultraviolet light.

10. The method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to claim 9, wherein the content of the photoinitiator is 0.015 to 0.3 parts by weight per 100 parts by weight of the polyolefin porous support.

11. A method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to claim 9, wherein the weight ratio of the inorganic filler to the first binder polymer is 95:5 to 99.9:0.

1.

12. The method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to claim 9, wherein the first binder polymer comprises an acrylic polymer, polyacrylic acid, styrene-butadiene rubber, polyvinyl alcohol, or two or more of these.

13. The method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to claim 9, wherein the dispersion medium is an aqueous dispersion medium.

14. A method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to claim 9, wherein the second binder polymer comprises polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene, polyvinylidene fluoride-tetrafluoroethylene, polyvinylidene fluoride-trifluoroethylene, polymethyl methacrylate, polyethylhexyl acrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, ethylhexyl acrylate-methyl methacrylate copolymer, ethylene vinyl acetate copolymer, polyethylene oxide, polyarylate, or two or more of these.

15. A method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to claim 9, wherein the solvent for the second binder polymer comprises acetone, tetrahydrofuran, methylene chloride, chloroform, trimethyl phosphate, triethyl phosphate, methyl ethyl ketone, toluene, hexane, cyclohexane, dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, or two or more of these.

16. The method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to claim 9, wherein the photoinitiator includes a type II photoinitiator.

17. The method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to claim 9, wherein the photoinitiator comprises a thioxanthone, a thioxanthone derivative, a benzophenone, a benzophenone derivative, or two or more of these.

18. The amount of ultraviolet light irradiated is 10 to 2000 mJ / cm². ２ The method for producing a crosslinked structure-containing separation membrane for lithium secondary batteries according to claim 9.

19. A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator membrane interposed between the positive electrode and the negative electrode, A lithium secondary battery wherein the separation membrane is a crosslinked structure-containing separation membrane for lithium secondary batteries according to any one of claims 1 to 8.

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