The crosslinked structure contains a polyolefin porous support, a crosslinked structure-containing separator for a lithium secondary battery containing the same, and a lithium secondary battery including the separator

Abstract

The present invention relates to a crosslinked structure-containing polyolefin porous support having a crosslinked structure in which polymer chains are directly linked to each other, and in which a first peak is detected at a g value of 2.010 to 2.030 when irradiated with ultraviolet light of 500 W and measured by electron spin resonance, a crosslinked structure-containing separator for lithium secondary batteries containing the same, and a lithium secondary battery comprising the separator. The crosslinked structure-containing polyolefin porous support according to the present invention has excellent heat resistance.

Description

Technical Field

[0001] This application claims priority based on Korean Patent Application No. 10-2021-0059583, filed on May 7, 2021.

[0002] The present invention relates to a crosslinked structure-containing polyolefin porous support, a crosslinked structure-containing separator for a lithium secondary battery including the same, and a lithium secondary battery including the separator.

Background Art

[0003] In recent years, interest in energy storage technology has been increasing. The application fields are expanding to include mobile phones, camcorders, notebook computers, and even the energy of electric vehicles, and there is an increasing demand for higher energy density of batteries used as power sources for such electronic devices. 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 can lead to safety problems such as internal short circuits.

[0006] Therefore, the demand for a separator that can ensure safety at high temperatures remains high.

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 cross-linked structure-containing polyolefin porous support with improved high-temperature safety.

[0008] Another problem to be solved by the present invention is to provide a cross-linked structure-containing separator for a lithium secondary battery including the cross-linked structure-containing polyolefin porous support, and a lithium secondary battery including the separator.

Means for Solving the Problems

[0009] In order to solve the above problems, according to one aspect of the present invention, a cross-linked structure-containing polyolefin porous support of the following embodiments is provided.

[0010] The first embodiment is It has a cross-linked structure in which polymer chains are directly connected to each other, When irradiated with ultraviolet rays of 500 W and measured by electron spin resonance (ESR) method, a first peak is detected at a g value of 2.010 to 2.030, and relates to a cross-linked structure-containing polyolefin porous support.

[0011] According to the second embodiment, in the first embodiment, When irradiated with ultraviolet rays of 500 W and measured by electron spin resonance method, a second peak may be further detected at a g value of 1.990 to 2.009.

[0012] According to the third embodiment, in the second embodiment, When irradiated with ultraviolet rays of 500 W and measured by electron spin resonance method, the ratio of the area of the first peak to the area of the second peak may be 10% to 200%.

[0013] According to the fourth embodiment, in any one of the first to third embodiments, In a frequency-loss storage modulus curve where the horizontal axis is the frequency (rad / s) converted to a logarithmic scale and the vertical axis is the storage modulus (G’, storage modulus) (A) and loss modulus (G’’, loss modulus) (B) both converted to logarithmic scales, In the range where the frequency is 1 rad / s or less, the ratio (A / B) of the storage modulus (G’) (A) to the loss modulus (G’’) (B) of the crosslinked structure-containing polyolefin porous support may be 2 or more.

[0014] According to a fifth embodiment, in any one of the first to fourth embodiments, In a frequency-loss storage modulus curve where the horizontal axis is the frequency (rad / s) converted to a logarithmic scale and the vertical axis is the storage modulus (G’, storage modulus) (A) and loss modulus (G’’, loss modulus) (B) both converted to logarithmic scales, where the frequency is 10 -1 ~1 rad / s, the slope of the storage modulus (G’) (A) curve with respect to the frequency of the crosslinked structure-containing polyolefin porous support may be 0.05 to 0.4.

[0015] According to a sixth embodiment, in the fourth or fifth embodiment, the value of the storage modulus may be 1.0×10 5 ~1.0×10 7 Pa.

[0016] According to a seventh embodiment, in the fourth or fifth embodiment, the value of the loss modulus may be 3.0×10 5 Pa or less.

[0017] To solve the above problems, according to one aspect of the present invention, a crosslinked structure-containing separator for a lithium secondary battery of the following embodiments is provided.

[0018] An eighth embodiment is Relates to a crosslinked structure-containing separator for a lithium secondary battery including a crosslinked structure-containing polyolefin porous support according to any one of the first to seventh embodiments.

[0019] According to the ninth embodiment, in the eighth embodiment, The crosslinked structure-containing separator for a lithium secondary battery is located on at least one surface of the crosslinked structure-containing polyolefin porous support, and may further include an inorganic composite void layer containing an inorganic filler and a binder polymer.

[0020] According to the tenth embodiment, in the ninth embodiment, The crosslinked structure-containing separator for a lithium secondary battery is located on at least one surface of the crosslinked structure-containing polyolefin porous support, and includes an inorganic composite void layer 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 may further include the same.

[0021] According to the eleventh embodiment, in any one of the eighth to tenth embodiments, The melt-down temperature of the crosslinked structure-containing separator for a lithium secondary battery may be 160 °C or higher.

[0022] According to the twelfth embodiment, in any one of the eighth to eleventh embodiments, The shut-down temperature of the crosslinked structure-containing separator for a lithium secondary battery may be 145 °C or lower.

[0023] To solve the above problems, according to one aspect of the present invention, a lithium secondary battery of the following embodiment is provided.

[0024] The thirteenth embodiment is It includes a positive electrode, a negative electrode, and a separator for a lithium secondary battery interposed between the positive electrode and the negative electrode, Relates to a lithium secondary battery in which the separator for a lithium secondary battery is a crosslinked structure-containing separator for a lithium secondary battery according to any one of the eighth to twelfth embodiments.

Effect of the Invention

[0025] The crosslinked structure-containing polyolefin porous support according to one aspect of the present invention has excellent heat resistance.

[0026] The crosslinked structure-containing separator for a lithium secondary battery including the crosslinked structure-containing polyolefin porous support according to one aspect of the present invention has excellent heat resistance by including the crosslinked structure-containing polyolefin porous support having excellent heat resistance.

[0027] The following drawings attached to this specification illustrate desirable embodiments of the present invention and serve to further understand the technical idea of the present invention together with the detailed description of the invention. Therefore, the present invention should not be construed as being limited only to the matters described in the drawings.

Brief Description of the Drawings

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Embodiments for Carrying Out the Invention

[0029] Hereinafter, preferred embodiments of the present invention will be described in detail. Prior to this, the terms and words used in this specification and the claims are not to be construed as being limited to their ordinary and dictionary meanings. The inventor himself interprets them in accordance with the meaning and concept corresponding to the technical idea of the present invention in accordance with the principle that he can appropriately define the concept of the terms in order to explain the invention in the best way.

[0030] Therefore, it should be understood that the embodiments described in this specification and the configurations shown in the drawings are only the most preferred embodiments of the present invention and do not represent all of the technical ideas of the present invention. Therefore, there can be various equivalents and modifications that can replace them at the time of this application.

[0031] In this specification, terms such as "first" and "second" are used to distinguish one component from another component, and each component is not limited by these terms.

[0032] The crosslinked structure-containing polyolefin porous support according to an embodiment of the present invention is It has a crosslinked structure in which polymer chains are directly connected to each other, When irradiated with ultraviolet rays of 500 W and measured by the electron spin resonance method, a first peak is detected at a g value of 2.010 to 2.030.

[0033] As used herein, the "crosslinked structure in which polymer chains are directly linked to each other" means a state in which polymer chains substantially composed of polyolefin, more preferably polymer chains composed only of polyolefin, become reactive by the addition of a type II photoinitiator, and the polymer chains form a crosslinked bond directly with each other. Therefore, the crosslinking reaction that occurs between crosslinking agents by the addition of an additional crosslinking agent does not correspond to the "crosslinked structure in which polymer chains are directly linked to each other" referred to in the present invention. Further, the crosslinking reaction that occurs between an additional crosslinking agent and a polymer chain does not correspond to the "crosslinked structure in which polymer chains are directly linked to each other" referred to in the present invention, even if the polymer chain is substantially composed of polyolefin or composed only of polyolefin.

[0034] In addition, type II photoinitiators can be crosslinked with each other or type II photoinitiators and polymer chains can be crosslinked. However, since such a crosslinked structure has a lower reaction enthalpy than the crosslinked structure between polymer chains in the polyolefin porous support, it may be decomposed during charging and discharging of the battery to cause side reactions. The crosslinked structure-containing polyolefin porous support may not include a crosslinked structure in which a type II photoinitiator and a polymer chain are directly linked, and may include only a crosslinked structure in which polymer chains are directly linked to each other.

[0035] In one embodiment of the present invention, the crosslinked structure-containing polyolefin porous support includes only a crosslinked structure in which polymer chains are directly linked to each other, and does not include a crosslinked structure in which a type II photoinitiator and a polymer chain are directly linked.

[0036] In one embodiment of the present invention, the degree of crosslinking of the crosslinked structure-containing polyolefin porous support can be 10% to 45%, 15% to 40%, or 20% to 35%. When the crosslinked structure-containing polyolefin porous support satisfies the above-mentioned range of the degree of crosslinking, it is easy to increase the modulus while having the target level of heat resistance. For example, when the degree of crosslinking of the crosslinked structure-containing polyolefin porous support is 20% or more, the melt-down temperature of the separation membrane including the crosslinked structure-containing polyolefin porous support tends to be 170 °C or higher.

[0037] At this time, the degree of crosslinking is calculated as the percentage of the residual weight to the initial weight by measuring the residual weight after immersing the polyolefin porous support containing a crosslinked structure in a xylene solution at 135 °C and boiling for 12 hours according to ASTM D2765.

[0038] In one embodiment of the present invention, in the polyolefin porous support containing a crosslinked structure, double bonds can be generated in the polyolefin chains by a crosslinking reaction using a type II photoinitiator.

[0039] In one embodiment of the present invention, in the polyolefin porous support containing a crosslinked structure, the number of double bonds present in the polyolefin chains during H-NMR measurement can be 0.01 to 0.6, or 0.02 to 0.5 per 1000 carbon atoms. When the polyolefin porous support containing a crosslinked structure has the double bonds in the above-mentioned number, the portion where side reactions occur can be minimized.

[0040] In one embodiment of the present invention, the number of double bonds present in the polyolefin chains excluding the ends of the polyolefin porous support containing a crosslinked structure can be 0.005 to 0.59 per 1000 carbon atoms. In the present specification, the "double bonds present in the polyolefin chains excluding the ends" refers to the double bonds present in the entire polyolefin chains excluding the ends of the polyolefin chains. Here, the "ends" mean the positions of the carbon atoms respectively connected to both ends of the polyolefin chains.

[0041] In one embodiment of the present invention, the polyolefin porous support containing a crosslinked structure can be a porous film.

[0042] In one embodiment of the present invention, the polyolefin may include polyethylene; polypropylene; polybutylene; polypentene; polyhexene; polyoctene; a copolymer of two or more of ethylene, propylene, butene, pentene, 4-methylpentene, hexene, and octene; or a mixture thereof.

[0043] Non-limiting examples of the polyethylene include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), and the like. When the polyethylene is high-density polyethylene with high crystallinity and a high melting point of the resin, it is easy to increase the modulus while having the desired level of heat resistance.

[0044] 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-described range, a separation membrane excellent in strength and heat resistance can be finally obtained while ensuring the uniformity and film-forming processability of the polyolefin porous support containing a crosslinked structure.

[0045] The weight average molecular weight can be measured under the following conditions using gel permeation chromatography (GPC: Gel Permeation Chromatography, PL GPC220, manufactured by Agilent Technologies).

[0046] - Column: PL Olexis (Polymer Laboratories) - Solvent: TCB (trichlorobenzene) - Flow rate: 1.0 ml / min - Sample concentration: 1.0 mg / ml - Injection volume: 200 μl - Column temperature: 160 °C - Detector: High-temperature RI detector manufactured by Agilent - Standard: Polystyrene (corrected by a cubic function)

[0047] 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-described range, it is possible to prevent the problem that the separation membrane is easily damaged during the use of the battery, and it is possible to easily ensure the energy density.

[0048] When the crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention is irradiated with 500 W of ultraviolet light and measured by electron spin resonance method, a first peak is detected at a g value of 2.010 to 2.030. The "first peak" means that the crosslinked structure-containing polyolefin porous support forms radicals by light absorption. That is, as a result of irradiation with ultraviolet light, it means that radicals are formed in the polymer chains in the crosslinked structure-containing polyolefin porous support.

[0049] In one embodiment of the present invention, when irradiated with 500 W of ultraviolet light and measured by electron spin resonance method, in addition to the first peak, a second peak may be further detected at a g value of 1.990 to 2.009. The "second peak" means the presence of a single electron or an electron showing single electron-like behavior by the ultraviolet light irradiated during the process of measuring the spectrum by the electron spin resonance method. The second peak may appear symmetrically with a peak convex upward and a peak convex downward.

[0050] In one embodiment of the present invention, the crosslinked structure-containing polyolefin porous support may contain a photoinitiator, specifically a type II photoinitiator. The type II photoinitiator crosslinks between polymer chains and may remain in the porous support. When the type II photoinitiator remains in the porous support, the second peak may be detected when irradiated with 500 W of ultraviolet light.

[0051] The greater the increase in the area of the first peak relative to the area of the second peak, the smoother the formation of radicals from the polymer chains in the crosslinked structure-containing polyolefin porous support.

[0052] In one embodiment of the present invention, when irradiated with 500 W of ultraviolet light and measured by electron spin resonance method, the ratio of the area of the first peak to the area of the second peak can be 10% to 200%, or 10% to 180%.

[0053] The ratio of the area of the first peak to the area of the second peak can be the ratio of the area of the first peak to the area of the second peak when 30 mg to 40 mg of the crosslinked structure-containing polyolefin porous support is introduced into an electron spin resonance measuring device.

[0054] The crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention can improve heat resistance by having a crosslinked structure in which polymer chains are directly connected to each other.

[0055] The crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention can substantially maintain the pore structure of the polyolefin porous support even after crosslinking as it was before crosslinking.

[0056] In a frequency-loss storage modulus curve in which the horizontal axis is the frequency (rad / s) converted to a logarithmic scale and the vertical axis is the storage modulus (G’, A) and loss modulus (G’’, B) converted to a logarithmic scale, In the range where the frequency is 1 rad / s or less, the ratio (A / B) of the storage modulus (G’) (A) to the loss modulus (G’’) (B) of the crosslinked structure-containing polyolefin porous support can be 2 or more.

[0057] The present inventors have intensively studied to find a crosslinked structure-containing polyolefin porous support that can ensure safety even at high temperatures. As a means to solve this, while reducing the viscosity of the crosslinked structure-containing polyolefin porous support at high temperatures, the elasticity of the crosslinked structure-containing polyolefin porous support was increased to consequently improve high-temperature safety.

[0058] Specifically, when reducing the viscosity of the crosslinked structure-containing polyolefin porous support at high temperature and increasing its elasticity, the strength of the crosslinked structure-containing polyolefin porous support is maintained at high temperature, and since the fluidity of the crosslinked structure-containing polyolefin porous support itself does not occur, short circuit between the positive electrode and the negative electrode can be prevented, and as a result, the safety of the crosslinked structure-containing polyolefin porous support is improved.

[0059] In a specific embodiment of the present invention, in a frequency-loss storage elastic modulus curve in which the horizontal axis is the frequency (rad / s) converted to a logarithmic scale and the vertical axis is the storage elastic modulus (G’) (A) and loss elastic modulus (G’’) (B) of the crosslinked structure-containing polyolefin porous support converted to a logarithmic scale, when the value of G’ is larger than G’’, specifically when it is 2 or more, a crosslinked structure-containing polyolefin porous support with improved safety can be provided.

[0060] When the value of G’’ is larger than the value of G’, the viscosity becomes larger than the elasticity of the crosslinked structure-containing polyolefin porous support, and there is a problem that the pores of the crosslinked structure-containing polyolefin porous support are quickly blocked at high temperature.

[0061] On the other hand, when the value of G’ relative to G’’ is less than 2, the fluidity of the crosslinked structure-containing polyolefin porous support occurs, and the separation function by the separation membrane including the crosslinked structure-containing polyolefin porous support is lost, which is not appropriate.

[0062] In the present invention, the storage elastic modulus (G’, storage modulus) means the ability of a substance to store energy and can be expressed as in Equation 1.

[0063] [Equation 1] G’ = (stress / strain) cosδ

[0064] In the present invention, the storage elastic modulus can be measured using dynamic mechanical analysis.

[0065] In the present invention, the storage elastic modulus is measured using dynamic viscoelasticity measurement in a temperature sweep test at a temperature range of 180 to 220 °C and a frequency of 1 rad / s.

[0066] In a specific embodiment of the present invention, the value of the storage elastic modulus is 1.0×10 5 ~1.0×10 7 Pa, 1.2×10 5 ~5.0×10 6 Pa, 1.5×10 5 ~2.0×10 6 Pa, 1.7×10 5 ~1.0×10 6 Pa, or 1.9×10 5 ~3.8×10 5 Pa. When the value of the storage elastic modulus is within the above numerical range, it is advantageous in that the strength of the crosslinked structure-containing polyolefin porous support at high temperatures can be maintained.

[0067] In the present invention, the loss elastic modulus (G'', loss modulus) means the ability of a substance to lose energy due to deformation and can be expressed as in Equation 2.

[0068] [Equation 2] G'' = (stress / strain)sinδ

[0069] In the present invention, the loss elastic modulus can be measured using dynamic viscoelasticity measurement.

[0070] In the present invention, the loss elastic modulus is measured using dynamic viscoelasticity measurement in a temperature sweep test at a temperature range of 180 to 220 °C and a frequency of 1 rad / s.

[0071] In a specific embodiment of the present invention, the value of the loss elastic modulus is 3.0×10 5 Pa or less, 1.0×10 4 ~3.0×10 5 Pa, 2.0×104 ~1.5×10 5 Pa, 5.0×10 4 ~1.2×10 5 Pa, or 7.0×10 4 ~1.1×10 5 Pa. When the value of the loss elastic modulus is within the above numerical range, it is advantageous in that the fluidity of the crosslinked structure-containing polyolefin porous support does not occur.

[0072] In the present invention, the meaning of the ratio (A / B) of the storage elastic modulus (G') (A) to the loss elastic modulus (G'') (B) of the crosslinked structure-containing polyolefin porous support is a relative measure of the elastic contribution to the viscous contribution.

[0073] Specifically, when the ratio is 1 or more, it exhibits characteristics similar to a solid, and when the ratio is 1 or less, it exhibits characteristics similar to a liquid.

[0074] On the other hand, in the casting and stretching processes, the A / B is used as a relative measure for determining the uniformity and fluidity of the extruded sheet.

[0075] Therefore, in the manufacturing process of the crosslinked structure-containing polyolefin porous support, for the smooth progress of the process, it is desirable that the value of A / B is low. In the final crosslinked structure-containing polyolefin porous support, it is desirable that the ratio of A / B is high, and in particular, in the present invention, it is desirable that the ratio is 2 or more.

[0076] In a specific embodiment of the present invention, in the range of a vibration frequency of 1 rad / s or less, the ratio (A / B) of the storage elastic modulus (G') (A) to the loss elastic modulus (G'') (B) of the crosslinked structure-containing polyolefin porous support can be 2 or more, 2 to 7, 2 to 5, 2.1 to 4.7, or 2.18 to 4.68. When the ratio of A / B is within the above range, it is advantageous in that the separation membrane including the crosslinked structure-containing polyolefin porous support can maintain the separation function while maintaining the strength of the crosslinked structure-containing polyolefin porous support at high temperature.

[0077] In one embodiment of the present invention, in a frequency-loss storage modulus curve where the horizontal axis is the frequency (rad / s) converted to a logarithmic scale and the vertical axis is the storage modulus (G’) (A) and loss modulus (G’’) (B) converted to a logarithmic scale, when the frequency is in the range of 10 -1 ~1 rad / s, the slope of the storage modulus (G’) (A) curve with respect to the frequency of the crosslinked structure-containing polyolefin porous support can be 0.05 to 0.4.

[0078] At this time, for the storage modulus and loss modulus, please refer to the above description.

[0079] In a specific embodiment of the present invention, when the frequency of the crosslinked structure-containing polyolefin porous support is in the range of 10 -1 ~1 rad / s, the slope of the storage modulus (G’) (A) curve with respect to the frequency can be 0.05 to 0.4, 0.07 to 0.35, 0.1 to 0.3, 0.12 to 0.28, or 0.133 to 0.267. When the ratio of A / B is within the above range, it is advantageous in that the separation membrane containing the crosslinked structure-containing polyolefin porous support can maintain the isolation function at high temperatures.

[0080] The crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention can be used as a crosslinked structure-containing separation membrane for a lithium secondary battery.

[0081] The crosslinked structure-containing separation membrane for a lithium secondary battery according to one embodiment of the present invention may include the crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention.

[0082] In one embodiment of the present invention, when the crosslinked structure-containing separation membrane for a lithium secondary battery includes the crosslinked structure-containing polyolefin porous support having the above-mentioned number of double bonds, the problem of deterioration of battery performance at high temperatures and / or high voltages can be easily prevented.

[0083] The separator containing a crosslinked structure for a lithium secondary battery according to an embodiment of the present invention may include a polyolefin porous support containing a crosslinked structure according to an embodiment of the present invention.

[0084] The separator containing a crosslinked structure for a lithium secondary battery according to another embodiment of the present invention may further include an inorganic composite void layer located on at least one surface of the polyolefin porous support containing a crosslinked structure and containing an inorganic filler and a binder polymer. This is shown in FIG. 1.

[0085] Referring to FIG. 1, the separator 1 containing a crosslinked structure for a lithium secondary battery according to an embodiment of the present invention may include a polyolefin porous support 10 containing a crosslinked structure and an inorganic composite void layer 20 located on at least one surface of the polyolefin porous support 10 containing a crosslinked structure and containing an inorganic filler and a binder polymer.

[0086] The inorganic composite void layer 20 may be formed on one or both surfaces of the polyolefin porous support 10 containing a crosslinked structure. The inorganic composite void layer 20 includes an inorganic filler and a binder polymer that adheres these inorganic fillers to each other (i.e., the binder polymer connects and fixes between the inorganic fillers) so that they can maintain the state of binding to each other, and the binder polymer can maintain the state of binding between the inorganic filler and the polyolefin porous support 10 containing a crosslinked structure. The inorganic composite void layer 20 can prevent the polyolefin porous support 10 containing a crosslinked structure from showing extremely large heat shrinkage behavior at high temperatures, and can improve the safety of the separator. For example, the heat shrinkage rates of the separator in the machine direction (MD) and the transverse direction (TD) measured after standing at 120° C. for 30 minutes may be 20% or less, 2% to 15%, or 2% to 10%, respectively.

[0087] In this specification, the "Machine direction (MD)" refers to the advancing direction when the separation membrane is continuously produced, which is the longitudinal direction of the separation membrane, and the "Transverse direction (TD)" refers to the transverse direction of the machine direction, that is, the direction perpendicular to the advancing direction when the separation membrane is continuously produced, which is the direction perpendicular to the longitudinal direction of the separation membrane.

[0088] The inorganic filler is not particularly limited as long as it is electrochemically stable. That is, the inorganic filler that can be used in the present invention is not particularly limited as long as oxidation and / or reduction reactions do not occur within the operating voltage range of the applied electrochemical element (for example, 0 to 5V based on Li / Li + standard). In particular, when using inorganic particles with a high dielectric constant as the inorganic filler, it can contribute to an increase in the dissociation degree of electrolyte salts in the liquid electrolyte, such as lithium salts, and improve the ionic conductivity of the electrolyte.

[0089] For the reasons described 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-limiting examples of inorganic fillers having a dielectric constant of 5 or more include BaTiO3, Pb(Zr,Ti)O3 (PZT), Pb 1-x La x Zr 1-y Ti y O3 (PLZT, 0 < x < 1, 0 < y < 1), Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3 (PMN-PT), hafnia (HfO2), SrTiO3, SnO2, CeO2, MgO, Mg(OH)2, NiO, CaO, ZnO, ZrO2, SiO2, Y2O3, Al2O3, AlOOH, Al(OH)3, SiC, TiO2, or a mixture thereof, and the like.

[0090] In another embodiment of the present invention, as the inorganic filler, an inorganic filler having lithium ion conduction ability, that is, an inorganic filler that does not store lithium but has a function of moving lithium ions and contains a lithium element can be used. Non-limiting examples of inorganic fillers having lithium ion conduction ability include lithium phosphate (Li3PO4), lithium titanium phosphate (Li x Ti y (PO4)3, 0 < x < 2, 0 < y < 3), lithium aluminum titanium phosphate (Li x Al y Ti z (PO4)3, 0 < x < 2, 0 < y < 1, 0 < z < 3), (LiAlTiP) x O y -based glasses such as 14Li2O - 9Al2O3 - 38TiO2 - 39P2O5 (0 < x < 4, 0 < y < 13), lithium lanthanum titanate (Li x La y TiO3, 0 < x < 2, 0 < y < 3), lithium germanium thiophosphate such as Li 3.25 Ge 0.25 P 0.75 S4 (Li x Ge y P z S w , 0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5), lithium nitride such as Li3N (Li x N y , 0 < x < 4, 0 < y < 2), SiS2 - based glasses such as Li3PO4 - Li2S - SiS2 (Li x Si y S z , 0 < x < 3, 0 < y < 2, 0 < z < 4), P2S5 - based glasses such as LiI - Li2S - P2S5 (Li x P y S z , 0 < x < 3, 0 < y < 3, 0 < z < 7), or mixtures thereof, and the like.

[0091] 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 the inorganic composite void layer 20 having a uniform thickness and an appropriate porosity, the dispersibility of the inorganic filler is good, and a desired energy density can be achieved.

[0092] 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 cumulative particle number distribution according to the particle size. The particle size can be measured using the laser diffraction method. Specifically, after dispersing the powder to be measured in a dispersion medium, it is introduced into a commercially available laser diffraction particle size measuring device (for example, Microtrac S3500). When the particles pass through the laser beam, the difference in the diffraction pattern according to the particle size is measured to calculate the particle size distribution. By calculating the particle diameter at the point where the cumulative particle number distribution according to the particle size in the measuring device reaches 50%, D 50 particle size can be measured.

[0093] The binder polymer can have a glass transition temperature (glass transition temperature, T g ) of -200 to 200°C. When the glass transition temperature of the binder polymer satisfies the above-described range, the mechanical properties such as flexibility and elasticity of the finally formed inorganic composite void layer 20 can be improved. The binder polymer can have an ion conduction ability. When the binder polymer has an ion conduction ability, the performance of the battery can be further improved. The binder polymer can have a dielectric constant of 1.0 to 100 (measurement frequency = 1 kHz) or 10 to 100. When the dielectric constant of the binder polymer satisfies the above-described range, the degree of dissociation of the salt in the electrolyte can be improved.

[0094] In one embodiment of the present invention, the binder polymer may include poly(vinylidene fluoride - hexafluoropropylene), poly(vinylidene fluoride - chlorotrifluoroethylene), poly(vinylidene fluoride - tetrafluoroethylene), poly(vinylidene fluoride - trichloroethylene), acrylic copolymers, styrene - butadiene copolymers, polyacrylic acid, polymethyl methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl alcohol, polyvinyl acetate, ethylene - vinyl acetate copolymer, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, or two or more thereof.

[0095] The acrylic copolymer may include, but is not limited to, ethyl acrylate - acrylic acid - N,N - dimethylacrylamide copolymer, ethyl acrylate - acrylic acid - 2 - (dimethylamino)ethyl acrylate copolymer, ethyl acrylate - acrylic acid - N,N - diethylacrylamide copolymer, ethyl acrylate - acrylic acid - 2 - (diethylamino)ethyl acrylate copolymer, or two or more thereof.

[0096] In one embodiment of the present invention, the weight ratio of the inorganic filler to the binder polymer is determined in consideration of the thickness, pore diameter, and porosity of the finally produced inorganic composite void layer 20, and may be 50:50 to 99.9:0.1, or 60:40 to 99.5:0.5. When the weight ratio of the inorganic filler to the binder polymer is within the above - described range, sufficient empty spaces formed between the inorganic fillers can be ensured, and the pore diameter and porosity of the inorganic composite void layer 20 can be easily ensured. Also, the adhesive force between the inorganic fillers can be easily ensured.

[0097] In one embodiment of the present invention, the inorganic composite void layer 20 may further include additives such as a dispersant and / or a thickener. In one embodiment of the present invention, the additives may include polyvinylpyrrolidone (PVP), hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), ethylhydroxyethyl cellulose (EHEC), methyl cellulose (MC), carboxymethyl cellulose (CMC), hydroxyalkylmethyl cellulose, cyanoethylated polyvinyl alcohol, or two or more of these.

[0098] In one embodiment of the present invention, the inorganic composite void layer 20 has a structure in which the inorganic fillers are filled and in contact with each other, and are bound to each other by the binder polymer, whereby an interstitial volume is formed between the inorganic fillers, and the interstitial volume between the inorganic fillers may become empty space to form pores.

[0099] In another embodiment of the present invention, the inorganic composite void layer 20 includes a plurality of nodes including the inorganic filler and a binder polymer covering at least a part of the surface of the inorganic filler, and one or more filaments formed in a thread shape from the binder polymer of the nodes. The filaments extend from the nodes and include node connection portions that connect other nodes, and the node connection portions may have a structure in which a plurality of filaments derived from the binder polymer cross each other to form a three-dimensional network structure.

[0100] In one embodiment of the present invention, the average pore diameter of the inorganic composite void layer 20 can be 0.001 μm to 10 μm. The average pore diameter of the inorganic composite void layer 20 can be measured by a capillary flow porometry method. Capillary flow porometry is a method of measuring the diameter of the smallest pores 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 crosslinked structure-containing polyolefin porous support 10 and measured in a state where the separated inorganic composite void layer 20 is wrapped with a non-woven fabric that can support it. At this time, the pore size of the non-woven fabric must be much larger than the pore size of the inorganic composite void layer 20.

[0101] In one embodiment of the present invention, the porosity of the inorganic composite void layer 20 can be 5% to 95%, 10% to 95%, 20% to 90%, or 30% to 80%. The porosity corresponds to a value obtained by subtracting the volume converted from the weight and density of each component of the inorganic composite void layer 20 from the volume calculated from the thickness, horizontal length, and vertical length of the inorganic composite void layer 20.

[0102] The porosity of the inorganic composite void layer 20 can be measured by the BET six-point method using a scanning electron microscope (SEM) image, a mercury porosimeter, or a porosimetry analyzer (Bell Japan, Belsorp-II mini) by nitrogen gas adsorption flow.

[0103] 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-described range, the cell strength of the battery can be easily increased while the adhesion to the electrode is excellent.

[0104] The crosslinked structure-containing separator for a lithium secondary battery according to another embodiment of the present invention is located on at least one surface of the crosslinked structure-containing polyolefin porous support, and may further include an inorganic composite void layer containing an inorganic filler and a first binder polymer, and a porous adhesive layer containing a second binder polymer located on the inorganic composite void layer. This is shown in FIG. 2.

[0105] Referring to FIG. 2, the crosslinked structure-containing separator 1' for a lithium secondary battery according to an embodiment of the present invention includes a crosslinked structure-containing polyolefin porous support 10' having a crosslinked structure in which polymer chains are directly connected to each other, and an inorganic composite void layer 20' located on at least one surface of the crosslinked structure-containing polyolefin porous support 10' and containing an inorganic filler and a first binder polymer, and a porous adhesive layer 30' located on the inorganic composite void layer 20' and containing a second binder polymer.

[0106] 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' includes an inorganic filler and a first binder polymer that adheres these inorganic fillers to each other (i.e., the first binder polymer connects and fixes between the inorganic fillers) so that the inorganic fillers can maintain a bound state with each other. The first binder polymer can maintain a bound state between the inorganic filler and the crosslinked structure-containing polyolefin porous support 10'. The inorganic composite void layer 20' can prevent the crosslinked structure-containing polyolefin porous support 10' from showing extremely large heat shrinkage behavior at high temperatures, and can improve the safety of the separator. For example, the heat shrinkage rates of the separator in the machine direction (MD) and the transverse direction (TD) measured after leaving it at 150 °C for 30 minutes may be 20% or less, 2% - 15%, or 2% - 10% respectively.

[0107] For the inorganic filler, refer to the above-described content.

[0108] The first binder polymer has a glass transition temperature (T g) can be -200 to 200°C. When the glass transition temperature of the first binder polymer satisfies the above-described range, the mechanical properties such as flexibility and elasticity of the finally formed inorganic composite void layer 20' can be improved. The first binder polymer can have ion conduction ability. When the first binder polymer has ion conduction ability, the performance of the battery can be further improved. The first binder polymer can have a dielectric constant of 1.0 to 100 (measurement frequency = 1 kHz) or 10 to 100. When the dielectric constant of the first binder polymer satisfies the above-described range, the dissociation degree of the salt in the electrolyte can be improved.

[0109] In one embodiment of the present invention, the first binder polymer can be a binder polymer having excellent heat resistance. When the first binder polymer has excellent heat resistance, the heat resistance characteristics of the inorganic composite void layer can be further improved. For example, the heat shrinkage rates of the separator in the machine direction (MD) and the transverse direction (TD) measured after leaving it at 150°C for 30 minutes can be 20% or less, 2% to 15%, 2% to 10%, 2% to 5%, 0% to 5%, or 0% to 2%, respectively. In one embodiment of the present invention, the first binder polymer can include an acrylic polymer, polyacrylic acid, styrene butadiene rubber, carboxymethyl cellulose, polyvinyl alcohol, or two or more thereof.

[0110] Specifically, the acrylic polymer can include an acrylic homopolymer obtained by polymerizing only acrylic monomers, and can also include a copolymer of acrylic monomers and other monomers. For example, the acrylic polymer can include an ethylhexyl acrylate-methyl methacrylate copolymer, polymethyl methacrylate, polyethylhexyl acrylate, polybutyl acrylate, polyacrylonitrile, a butyl acrylate-methyl methacrylate copolymer, or two or more thereof.

[0111] In one embodiment of the present invention, the first binder polymer can be in particulate form.

[0112] In one embodiment of the present invention, the weight ratio of the inorganic filler to the first binder polymer can 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-mentioned range, the content of the inorganic filler distributed per unit area of the separation membrane increases, and the thermal safety of the separation membrane at high temperatures can be improved. For example, the thermal shrinkage rates of the separation membrane in the machine direction (MD) and the transverse direction (TD) measured after leaving it at 150 °C for 30 minutes can be 20% or less, 2% to 15%, 2% to 10%, 2% to 5%, 0% to 5%, or 0% to 2%, respectively. Hereinafter, the characteristics of the inorganic composite void layer 20' different from the above-mentioned inorganic composite void layer 20 will be described.

[0113] In one embodiment of the present invention, in the inorganic composite void layer 20', the inorganic fillers are filled and in contact with each other, and are bound to each other by the first binder polymer, whereby an interstitial volume is formed between the inorganic fillers, and the interstitial volume between the inorganic fillers can have a structure that becomes an empty space to form pores.

[0114] By including a second binder polymer, the porous adhesive layer 30' can ensure the adhesive force between the separation membrane provided with the inorganic composite void layer 20' and the electrode. In addition, pores are formed in the porous adhesive layer 30', and it is possible to prevent the resistance of the separation membrane from increasing.

[0115] In one embodiment of the present invention, since the second binder polymer does not penetrate the surface and / or the inside of the crosslinked structure-containing polyolefin porous support 10', the phenomenon of the resistance of the separation membrane increasing can be minimized in the porous adhesive layer 30'.

[0116] The second binder polymer can be a binder polymer commonly used for forming an adhesive layer. The second binder polymer has a glass transition temperature (T g) can be -200 to 200 °C. When the glass transition temperature of the second binder polymer satisfies the above-described range, mechanical properties such as the flexibility and elasticity of the finally formed adhesive layer can be improved. The second binder polymer may have ion-conducting ability. When a binder polymer having ion-conducting ability is used as the second binder polymer, the performance of the battery can be further improved. The second binder polymer may have a dielectric constant of 1.0 to 100 (measurement frequency = 1 kHz) or 10 to 100. When the dielectric constant of the second binder polymer satisfies the above-described range, the degree of dissociation of the salt in the electrolyte can be improved.

[0117] 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 may include two or more of these.

[0118] In one embodiment of the present invention, the porous adhesive layer 30' may have a pattern including one or more adhesive portions containing the second binder polymer and one or more non-coating regions where the adhesive portions are not formed. The pattern may be a dot type, stripe type, diagonal type, wave type, triangle, square, or semi-circular. When the porous adhesive layer has a pattern, the resistance of the separator is improved, and the electrolyte can be impregnated through the non-coated region where the porous adhesive layer is not formed, so that the electrolyte impregnation property of the separator can be improved.

[0119] In one embodiment of the present invention, the thickness of the porous adhesive layer 30' can be 0.5 μm to 1.5 μm, 0.6 μm to 1.2 μm, or 0.6 μm to 1.0 μm. When the thickness of the porous adhesive layer is within the above-described range, the adhesive force with the electrode is excellent, and as a result, the cell strength of the battery can be increased. Also, it is advantageous in terms of the cycle characteristics and resistance characteristics of the battery.

[0120] The separator containing a crosslinked structure for a lithium secondary battery according to one embodiment of the present invention includes a crosslinked structure-containing polyolefin porous support according to one embodiment of the present invention in which polymer chains are directly connected to each other, and thus has excellent high-temperature safety. For example, the melt-down temperature of the separator containing a crosslinked structure for a lithium secondary battery can be increased compared to a separator including a non-crosslinked polyolefin porous support. For example, the melt-down temperature of the separator can be 160°C or higher, 170°C or higher, or 180°C to 230°C.

[0121] As used herein, the "separator including a non-crosslinked polyolefin porous support" refers to a separator composed of a non-crosslinked polyolefin porous support without a crosslinked structure; a separator including a non-crosslinked polyolefin porous support without a crosslinked structure and an inorganic composite void layer located on at least one surface of the non-crosslinked polyolefin porous support without a crosslinked structure and containing an inorganic filler and a binder polymer; or a separator including a non-crosslinked polyolefin porous support without a crosslinked structure, an inorganic composite void layer located on at least one surface of the non-crosslinked polyolefin porous support without a crosslinked structure 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.

[0122] The melting temperature can be measured by a Thermomechanical Analysis (TMA) method. For example, after samples in the machine direction and the transverse direction are respectively taken, a sample with a width of 4.8 mm and a length of 8 mm is placed in a TMA device (Q400 manufactured by TA Instruments), and while applying a tension of 0.01 N, the temperature is 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 can be measured as the melting temperature.

[0123] The crosslinked structure-containing separator for a lithium secondary battery according to an embodiment of the present invention has a small increase in the shutdown temperature and a small change rate compared to a separator including an uncrosslinked polyolefin porous support. The crosslinked structure-containing separator for a lithium secondary battery according to an embodiment of the present invention has an increased melting temperature of the separator while the shutdown temperature does not increase much compared to a separator including an uncrosslinked polyolefin porous support. Therefore, overcharge safety due to the shutdown temperature can be ensured, and at the same time, the high-temperature safety of the separator is greatly improved.

[0124] In one embodiment of the present invention, the crosslinked structure-containing separator for a lithium secondary battery may have a shutdown temperature of 145 °C or less, 140 °C or less, or 133 °C to 140 °C. When the crosslinked structure-containing separator for a lithium secondary battery has the above-described shutdown temperature, overcharge safety can be ensured, and at the same time, the problem that the pores of the crosslinked structure-containing polyolefin porous support are damaged and the resistance increases during the high-temperature and pressurization processes during battery assembly can be easily prevented.

[0125] The shutdown temperature can be obtained by measuring the time (seconds) required for 100 ml of air to pass through the separator at a constant pressure of 0.05 MPa while increasing the temperature by 5 °C per minute using Wang's air permeability measuring device, and taking the temperature at which the air permeability of the separator rapidly increases as the shutdown temperature.

[0126] The crosslinked structure-containing separator for a lithium secondary battery according to an embodiment of the present invention has its air permeability, basis weight, tensile strength, tensile elongation, puncture strength, electrical resistance, etc., without significantly deteriorating compared to those of the separator including the polyolefin porous support before crosslinking, and the rate of change is also small.

[0127] The crosslinked structure-containing separator for a lithium secondary battery according to an embodiment of the present invention may have a rate of change in air permeability of 10% or less, 0% to 10%, 0% to 5%, or 0% to 3% compared to the separator for a lithium secondary battery before crosslinking.

[0128] The rate of change in air permeability can be calculated by the following formula.

[0129] Rate of change in air permeability (%) = [(Air permeability of the crosslinked structure-containing separator for a lithium secondary battery after crosslinking) - (Air permeability of the separator for a lithium secondary battery before crosslinking)] / (Air permeability of the separator for a lithium secondary battery before crosslinking) × 100

[0130] Throughout this specification, the "crosslinked structure-containing separator for a lithium secondary battery after crosslinking" refers to a separator composed of a crosslinked structure-containing polyolefin porous support; a separator including a crosslinked structure-containing polyolefin porous support and 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 binder polymer; or a separator including 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.

[0131] The air permeability (Gurley) can be measured by the ASTM D726-94 method. The Gurley used here is the resistance to the flow of air and is measured by a Gurley densometer. The value of the air permeability described here is the time (seconds) required for 100 ml of air to pass through the cross-section of the sample porous support under a pressure of 12.2 in H2O, that is, the air permeability time, with a sample porous support of 1 in 2 is shown.

[0132] The crosslinked structure-containing separator for a lithium secondary battery according to an embodiment of the present invention may have a change rate of basis weight of 5% or less, or 0% to 5% compared with the separator for a lithium secondary battery before crosslinking.

[0133] The change rate of basis weight can be calculated by the following formula.

[0134] Change rate of basis weight (%) = [(basis weight of the crosslinked structure-containing separator for a lithium secondary battery after crosslinking) - (basis weight of the separator for a lithium secondary battery before crosslinking)] / (basis weight of the separator for a lithium secondary battery before crosslinking) × 100

[0135] The basis weight (g / m 2 ) is shown by preparing a sample with a length and width of 1 m each and measuring its weight.

[0136] The crosslinked structure-containing separator for a lithium secondary battery according to an embodiment of the present invention may have a change rate of tensile strength in the machine direction and the transverse direction of 20% or less, 0% to 20%, 0% to 10%, 0% to 9%, 0% to 8%, or 0% to 7.53% compared with the separator for a lithium secondary battery before crosslinking.

[0137] The change rate of tensile strength can be calculated by the following formula.

[0138] Change rate of tensile strength in the machine direction (%) = [(tensile strength in the machine direction of the separator for a lithium secondary battery before crosslinking) - (tensile strength in the machine direction of the crosslinked structure-containing separator for a lithium secondary battery after crosslinking)] / (tensile strength in the machine direction of the separator for a lithium secondary battery before crosslinking) × 100

[0139] Change rate of tensile strength in the transverse direction (%) = [(Tensile strength in the transverse direction of the separator for lithium secondary batteries before cross-linking) - (Tensile strength in the transverse direction of the cross-linked structure-containing separator for lithium secondary batteries after cross-linking)] / (Tensile strength in the transverse direction of the separator for lithium secondary batteries before cross-linking) × 100

[0140] The tensile strength may be the strength at the time when the specimen breaks when the specimen is pulled in the machine direction and the transverse direction at a speed of 50 mm / min using Universal Testing Systems (Instron (registered trademark) 3345) in accordance with ASTM D882.

[0141] The cross-linked structure-containing separator for lithium secondary batteries according to an embodiment of the present invention may have a change rate of tensile elongation in the machine direction and the transverse direction of 20% or less, or 0% to 20%, compared with the separator for lithium secondary batteries before cross-linking.

[0142] The change rate of tensile elongation can be calculated by the following formula.

[0143] Change rate of tensile elongation in the machine direction (%) = [(Tensile elongation in the machine direction of the separator for lithium secondary batteries before cross-linking) - (Tensile elongation in the machine direction of the cross-linked structure-containing separator for lithium secondary batteries after cross-linking)] / (Tensile elongation in the machine direction of the separator for lithium secondary batteries before cross-linking) × 100

[0144] Change rate of tensile elongation in the transverse direction (%) = [(Tensile elongation in the transverse direction of the separator for lithium secondary batteries before cross-linking) - (Tensile elongation in the transverse direction of the cross-linked structure-containing separator for lithium secondary batteries after cross-linking)] / (Tensile elongation in the transverse direction of the separator for lithium secondary batteries before cross-linking) × 100

[0145] The tensile elongation is measured in accordance with ASTM D882 by using a Universal Testing Systems (Instron (registered trademark) 3345) to stretch the test piece at a speed of 50 mm / min in the machine direction and the transverse direction respectively, and measuring the maximum length of elongation until the test piece breaks, and can be calculated by the following formula.

[0146] Tensile elongation in the machine direction (%) = (Length of the test piece in the machine direction immediately before break - Length of the test piece in the machine direction before stretching) / (Length of the test piece in the machine direction before stretching) × 100

[0147] Tensile elongation in the transverse direction (%) = (Length of the test piece in the transverse direction immediately before break - Length of the test piece in the transverse direction before stretching) / (Length of the test piece in the transverse direction before stretching) × 100

[0148] The crosslinked structure-containing separator for lithium secondary batteries according to an embodiment of the present invention may have a change rate of puncture strength of 10% or less, 0.5% to 10%, 1% to 9%, or 1.18% to 8.71% compared to the separator for lithium secondary batteries before crosslinking.

[0149] The change rate of puncture strength can be calculated by the following formula.

[0150] Change rate of puncture strength (%) = [(Puncture strength of the separator for lithium secondary batteries before crosslinking) - (Puncture strength of the crosslinked structure-containing separator for lithium secondary batteries after crosslinking)] / (Puncture strength of the separator for lithium secondary batteries before crosslinking) × 100

[0151] The puncture strength can be measured in accordance with ASTM D2582. Specifically, after setting a 1 mm round tip to operate at a speed of 120 mm / min, the puncture strength can be measured in accordance with ASTM D2582.

[0152] The crosslinked structure-containing separator for lithium secondary batteries according to an embodiment of the present invention may have a change rate of electrical resistance of 15% or less, 2% to 10%, or 2% to 5% compared to the separator for lithium secondary batteries before crosslinking.

[0153] The change rate of electrical resistance can be calculated by the following formula.

[0154] Change rate of electrical resistance (%) = [(Electrical resistance of the crosslinked structure-containing separator for lithium secondary batteries after crosslinking) - (Electrical resistance of the separator for lithium secondary batteries before crosslinking)] / (Electrical resistance of the separator for lithium secondary batteries before crosslinking) × 100

[0155] The electrical resistance can be obtained by measuring the resistance of the separator by an impedance measurement method after leaving a coin cell manufactured including a separator sample at room temperature for one day.

[0156] The crosslinked structure-containing polyolefin porous support according to the present invention can be manufactured by the following method, but is not limited thereto.

[0157] A method for manufacturing a crosslinked structure-containing polyolefin porous support according to an embodiment of the present invention is preparing a polyolefin porous support containing a type II photoinitiator, and irradiating the polyolefin porous support with ultraviolet rays (UV).

[0158] Hereinafter, a method for manufacturing a crosslinked structure-containing polyolefin porous support according to an embodiment of the present invention will be described centering on the main part.

[0159] First, a polyolefin porous support containing a type II photoinitiator is prepared.

[0160] The type II photoinitiator directly photo-crosslinks the polymer chains in the polyolefin porous support. In the present invention, the type II photoinitiator is introduced onto the surface of the polyolefin porous support so that the polyolefin porous support can be crosslinked upon irradiation with ultraviolet rays. Here, the "surface of the polyolefin porous support" refers to the surface of the polymer chains of several to several tens of nm constituting the polyolefin porous support.

[0161] Conventionally, type I photoinitiators used to photocrosslink polyolefin porous supports generally use a crosslinking agent together. After absorbing light, the type I photoinitiator undergoes unimolecular bond cleavage to become a reactive compound species, and the photoinitiator or crosslinking agent binds to the polymer chains in the polyolefin porous support, resulting in photocrosslinking.

[0162] On the other hand, the type II photoinitiator can crosslink the polyolefin porous support with only the photoinitiator, without a crosslinking agent or other components such as a co-initiator or synergist. Only by light absorption, while the hydrogen atom in the type II photoinitiator is removed by hydrogen abstraction, the type II photoinitiator becomes a reactive compound. Such a type II photoinitiator forms radicals in the polymer chains in the polyolefin porous support to make the polymer chains reactive, and the polymer chains are directly linked to each other for photocrosslinking. For example, since the hydrogen abstraction reaction by the type II photoinitiator is possible in a small amount of double bond structure or branched structure present in the polyolefin, radicals can be formed while hydrogen atoms are abstracted from the polyolefin chains by the hydrogen abstraction reaction only by light absorption.

[0163] The manufacturing method of the crosslinked structure-containing polyolefin porous support according to an embodiment of the present invention can generate radicals in the polymer chains in the polyolefin porous support by using the type II photoinitiator, so that a crosslinked structure in which the polymer chains are directly connected to each other can be formed.

[0164] Also, by using the type II photoinitiator, crosslinking can be achieved with a smaller amount of light than when using the type I photoinitiator or other crosslinking agents, which is more advantageous in terms of mass production.

[0165] After forming radicals in the polymer chains, the type II photoinitiator is stabilized and no longer generates radicals from the polymer chains in the polyolefin porous support. As a result, the finally manufactured crosslinked structure-containing polyolefin porous support also does not have activated radicals.

[0166] However, when ultraviolet light is irradiated during the process of measuring the spectrum by the electron spin resonance method, such ultraviolet light reactivates the type II photoinitiator, and the activated type II photoinitiator forms radicals again from the polymer chains in the polyolefin porous support. As a result, a first peak is detected at a g-value of 2.010 to 2.030.

[0167] In one embodiment of the present invention, the type II photoinitiator may include thioxanthone (TX), a thioxanthone derivative, benzophenone (BPO), a benzophenone derivative, or two or more of these.

[0168] The thioxanthone derivative may include, for example, 2-isopropyl thioxanthone (2-Isopropyl thioxanthone: ITX), 2-chlorothioxanthone, 2-dodecylthioxanthone, 2,4-diethylthioxanthone, 2,4-dimethylthioxanthone, 1-methoxycarbonylthioxanthone, 2-ethoxycarbonylthioxanthone, 3-(2-methoxyethoxycarbonyl)-thioxanthone, 4-butoxycarbonyl-thioxanthone, 3-butoxycarbonyl-7-methylthioxanthone, 1-cyano-3-chlorothioxanthone, 1-ethoxycarbonyl-3-chlorothioxanthone, 1-ethoxycarbonyl-3-ethoxythioxanthone, 1-ethoxycarbonyl-3-aminothioxanthone, 1-ethoxycarbonyl-3-phenylsulfurylthioxanthone, 3,4-di[2-(2-methoxyethoxy)ethoxycarbonyl]thioxanthone, 1-ethoxycarbonyl-3-(1-methyl-1-morpholino-ethyl)-thioxanthone, 2-methyl-6-dimethoxymethyl-thioxanthone, 2-methyl-6-(1,1-dimethoxy-benzyl)-thioxanthone, 2-morpholinomethylthioxanthone, 2-methyl-6-morpholinomethyl-thioxanthone, N-allylthioxanthone-3,4-dicarboximide, N-octylthioxanthone-3,4-dicarboximide, N-(1,1,3,3-tetramethylbutyl)-thioxanthone-3,4-dicarboximide, 1-phenoxythioxanthone, 6-ethoxycarbonyl-2-methoxythioxanthone, 6-ethoxycarbonyl-2-methylthioxanthone, thioxanthone-2-polyethylene glycol ester, 2-hydroxy-3-(3,4-dimethyl-9-oxo-9H-thioxanthone-2-yloxy)-N,N,N-trimethyl-1-propanaminium chloride, etc., but is not limited thereto.

[0169] The benzophenone derivative may include, for example, 4-phenylbenzophenone, 4-methoxybenzophenone, 4,4'-dimethoxy-benzophenone, 4,4'-dimethylbenzophenone, 4,4'-dichlorobenzophenone, 4,4'-dimethylaminobenzophenone, 4,4'-diethylaminobenzophenone, 4-methylbenzophenone, 2,4,6-trimethylbenzophenone, 4-(4-methylthiophenyl)-benzophenone, 3,3'-dimethyl-4-methoxy-benzophenone, methyl 2-benzoylbenzoate, 4-(2-hydroxyethylthio)-benzophenone, 4-(4-tolylthio)benzophenone, 4-benzoyl-N,N,N-trimethylbenzenemethanaminium chloride, 2-hydroxy-3-(4-benzoylphenoxy)-N,N,N-trimethyl-propanaminium chloride monohydrate, 4-hydroxybenzophenone, 4-(13-acryloyl-1,4,7,10,13-pentaoxatridecyl)-benzophenone, 4-benzoyl-N,N-dimethyl-N-[2-(1-oxo-2-propenyl)oxy]ethyl-benzenemethanaminium chloride, etc., but is not limited thereto.

[0170] In particular, 2-isopropylthioxanthone or thioxanthone has even better reactivity to ultraviolet rays. Therefore, when the type II photoinitiator contains 2-isopropylthioxanthone, thioxanthone, or a mixture thereof, the amount of light, for example, 500 mJ / cm 2 is sufficient to enable photocrosslinking of the polyolefin porous support even at this level, which is more advantageous in terms of mass production. As a result, when irradiated with 500 W of ultraviolet rays and measured by electron spin resonance method, the ratio of the area of the first peak to the area of the second peak becomes larger than when containing benzophenone, etc.

[0171] In addition, when the Type II photoinitiator contains 2-isopropylthioxanthone (ITX), since the melting point of ITX is as low as about 70°C to 80°C, when the photocrosslinking temperature is adjusted to 80°C to 100°C, the ITX on the surface of the polyolefin porous support melts while the mobility of ITX into the polyolefin porous support occurs, and the crosslinking efficiency can be increased. Finally, physical property changes of the crosslinked structure-containing polyolefin porous support to be manufactured can be easily prevented.

[0172] In one embodiment of the present invention, the content of the Type II photoinitiator can be 0.015 parts by weight to 0.36 parts by weight, 0.015 parts by weight to 0.09 parts by weight, 0.03 parts by weight to 0.07 parts by weight, or 0.036 parts by weight to 0.073 parts by weight based on 100 parts by weight of the polyolefin porous support. When the content of the Type II photoinitiator satisfies the above-described range, crosslinking occurs only between the polymer chains in which radicals are formed, and the Type II photoinitiator is not crosslinked with the polymer chains. Thus, the problem of side reactions caused by excessive radicals due to the crosslinking of the Type II photoinitiator and the polymer chains can be more easily prevented. When the Type II photoinitiators are crosslinked with each other or the Type II photoinitiator and the polymer chains are crosslinked with each other, such a crosslinked structure has a lower reaction enthalpy than the crosslinked structure between the polymer chains in the polyolefin porous support, and thus may be decomposed to cause side reactions. In addition, when the Type II photoinitiator and the polymer chains are crosslinked, there is a possibility of degrading properties such as the shutdown temperature by lowering the melting temperature of the polyolefin chains.

[0173] Also, when the content of the Type II photoinitiator satisfies the above-described range, radicals are formed only between the polymer chains where radicals are formed, so that crosslinking occurs. Therefore, it is possible to easily prevent the crosslinked structure-containing polyolefin porous support from shrinking due to a rapid crosslinking reaction or the occurrence of main chain scission of the polyolefin, etc., which would lower other physical properties such as the mechanical strength of the crosslinked structure-containing polyolefin porous support. Even when the Type II photoinitiator is contained within the above-described range, the polyolefin porous support can be crosslinked by irradiation with ultraviolet light having a light amount that can ensure mass production productivity (i.e., a smaller light amount than before).

[0174] The content of the Type II photoinitiator with respect to 100 parts by weight of the polyolefin porous support can be determined by measuring the content of the Type II photoinitiator filled in 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 a solvent described later and there is no solvent present on the surface of the polyolefin porous support, the weight of the solvent contained in the total pore volume of the polyolefin porous support is determined from the density of the solvent, and the content of the Type II photoinitiator with respect to 100 parts by weight of the polyolefin porous support can be determined from the content of the Type II photoinitiator contained in the solvent.

[0175] The polyolefin porous support can be produced by forming pores using a normal method known in the art, for example, a wet method using a solvent, a diluent, or a pore former, or a dry method using a stretching method, in order to ensure excellent air permeability and porosity from the above-described polyolefin substance.

[0176] In one embodiment of the present invention, when measured by H-NMR, the number of double bonds present in the polyolefin chain of the polyolefin porous support may be 0.01 to 0.5, 0.01 to 0.3, or 0.01 to 0.2 per 1000 carbon atoms. When the polyolefin porous support has the double bonds in the above-mentioned number, the radicals formed by the hydrogen abstraction reaction by the type II photoinitiator from the double bond structure present in the polyolefin chain can be regulated, the polyolefin porous support can be effectively crosslinked, and the generation of excessive radicals and the occurrence of side reactions can be minimized.

[0177] Among the double bonds present in the polyolefin chain, the double bonds present in the polyolefin chain excluding the terminals can affect the crosslinking between the polymer chains. In one embodiment of the present invention, the number of double bonds present in the polyolefin chain excluding the terminals of the crosslinked structure-containing polyolefin porous support may be 0.005 to 0.49 per 1000 carbon atoms.

[0178] In one embodiment of the present invention, the number of double bonds present in the polyolefin chain can be adjusted by adjusting the type, purity, addition of a linking agent, etc. of the catalyst during the synthesis of the polyolefin.

[0179] In one embodiment of the present invention, the polyolefin porous support has a BET specific surface area of 10 m 2 / g to 27 m 2 / g, 13 m 2 / g to 25 m 2 / g, or 15 m 2 / g to 23 m 2 / g. When the BET specific surface area of the polyolefin porous support satisfies the above-mentioned 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 type II photoinitiator is used.

[0180] The BET specific surface area of the polyolefin porous support can be measured by the BET method. Specifically, the BET specific surface area of the inorganic particles can be calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77K) using Belsorp-mini II manufactured by BEL Japan, Inc.

[0181] 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 the radicals formed in the polyolefin chain. The antioxidant can prevent the oxidation of the polymer chain by oxidizing in place of the polymer chain or can absorb the generated radicals to regulate the crosslinking reaction between polymer chains.

[0182] In one embodiment of the present invention, the content of the antioxidant may be 500 ppm to 20000 ppm, 1000 ppm to 15000 ppm, or 2000 ppm to 13000 ppm based on the content of the polyolefin porous support. When the content of the antioxidant satisfies the above-described range, not only can the problem of side reactions caused by the antioxidant's ability to sufficiently control the excessively generated radicals be easily prevented, but also the phenomenon of the surface of the polyolefin porous support becoming non-uniform can be easily prevented.

[0183] Such antioxidants can be broadly classified into radical scavengers that react with the radicals generated in the polyolefin to stabilize the polyolefin and peroxide decomposers that decompose the peroxides generated by the radicals into molecules in a stable form. The radical scavenger can extract hydrogen to stabilize the radical and become a radical itself, but can remain in a stable form through resonance effects or electron rearrangement. The peroxide decomposer can exhibit a more excellent effect when used in combination with the radical scavenger.

[0184] In one embodiment of the present invention, the antioxidant may include a first antioxidant that is a radical scavenger and a second antioxidant that is a peroxide decomposer. Since the operating mechanisms of the first antioxidant and the second antioxidant are different, by simultaneously including the first antioxidant that is a radical scavenger and the second antioxidant that is a peroxide decomposer in the antioxidant, the synergistic effect of these antioxidants can more easily suppress the generation of unnecessary radicals.

[0185] The content of the first antioxidant and the content of the second antioxidant may be the same or different.

[0186] In one embodiment of the present invention, the first antioxidant may include a phenolic antioxidant, an amine antioxidant, or a mixture thereof.

[0187] The phenolic antioxidants include 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], triethylene glycol-bis-[3-(3-t-butyl-4-hydroxy-5-methylphenyl)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 may include two or more of these.

[0188] 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-described range, the problem of side reactions caused by excessive generation of radicals can be easily prevented.

[0189] In one embodiment of the present invention, the second antioxidant may include a phosphorus-based antioxidant, a sulfur-based antioxidant, or a mixture thereof.

[0190] The phosphorus-based antioxidant decomposes peroxides to generate alcohols and changes to phosphates. The phosphorus-based antioxidant may 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, bis(2,4-di-t-butyl-6-methylphenyl)-ethyl-phosphite, bis(2,6-di-t-butyl-4-methylphenyl)pentaerythritol diphosphite, bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite, bis(2,4-dicumylphenyl)pentaerythritol diphosphite, distearyl pentaerythritol diphosphite, tris(2,4-di-t-butylphenyl) phosphite, or two or more of these.

[0191] The sulfur-based antioxidant may include 3,3'-thiobis-1,1'-dodecyl ester, dimethyl 3,3'-thiodipropionate, dioctadecyl 3,3'-thiodipropionate, 2,2-bis{[3-(dodecylthio)-1-oxopropoxy]methyl}propane-1,3-diyl bis[3-(dodecylthio)propionate], or two or more of these.

[0192] 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-described range, the problem of side reactions occurring due to excessive generation of radicals can be easily prevented.

[0193] In one embodiment of the present invention, when the antioxidant simultaneously contains a first antioxidant that is a radical scavenger and a second antioxidant that 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.

[0194] In one embodiment of the present invention, the step of preparing the polyolefin porous support containing the type II photoinitiator may include the step of adding the type II photoinitiator to an extruder when extruding a polyolefin composition for forming the polyolefin porous support to prepare the polyolefin porous support.

[0195] In another embodiment of the present invention, the step of preparing the polyolefin porous support may include the step of coating and drying a photocrosslinking composition containing the type II photoinitiator and a solvent on the outside of the polyolefin porous support.

[0196] In this specification, the "step of coating and drying on the outside" includes not only the case of coating and drying the photocrosslinking composition on the surface of the polyolefin porous support, but also the case of coating and drying the photocrosslinking composition on the surface of another layer after another layer is formed on the polyolefin porous support.

[0197] In one embodiment of the present invention, before coating the photoinitiator solution on the polyolefin porous support, the polyolefin porous support can be subjected to corona discharge treatment. The corona discharge treatment can be performed by applying a high-frequency and high-voltage output generated by a predetermined drive circuit unit between a predetermined discharge electrode and a processing roll provided in a corona discharge treatment machine. Through the corona discharge treatment, the surface of the polyolefin porous support is modified, and the wettability of the polyolefin porous support with respect to the photocrosslinking composition can be further improved. Thereby, even when the same content of type II photoinitiator is contained, the crosslinking of the polyolefin porous support can be performed more efficiently. The corona discharge treatment can be performed by an atmospheric pressure plasma method.

[0198] In one embodiment of the present invention, the solvent may be 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; chlorine-based 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.

[0199] In one embodiment of the present invention, the content of the type II photoinitiator in the photocrosslinking composition is 0.015 to 0.36 parts by weight with respect to 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.

[0200] When the content of the type II photoinitiator satisfies the above-described range, the polyolefin porous support can be crosslinked, and at the same time, side reactions caused by excessive generation of radicals can be more easily prevented.

[0201] Also, in one embodiment of the present invention, the content of the type II photoinitiator in the photo-crosslinking composition is 0.015 to 0.36 parts by weight based on 100 parts by weight of the polyolefin porous support, and based on the specific surface area of the polyolefin porous support, it is 0.01 mg / m 2 ~1.0 mg / m 2 、0.03 mg / m 2 ~0.8 mg / m 2 、or 0.06 mg / m 2 ~0.7 mg / m 2 It can be. When the content of the type II photoinitiator satisfies the above-described range, the polyolefin porous support can be crosslinked, and at the same time, side reactions caused by excessive generation of radicals can be more easily prevented.

[0202] The content of the type II photoinitiator based on the specific surface area of the polyolefin porous support can be measured through NMR analysis.

[0203] In one embodiment of the present invention, the photo-crosslinking composition can be a photoinitiator solution containing the type II photoinitiator and the solvent.

[0204] Non-limiting examples of the method for coating the photoinitiator solution on the polyolefin porous support include dip coating method, die coating method, roll coating method, comma coating method, microgravure coating method, doctor blade coating method, reverse roll coating method, Mayer bar coating method, direct roll coating method, etc.

[0205] After coating the photoinitiator solution onto the polyolefin porous support, the drying step can be carried out batchwise or continuously using an oven or a heating chamber within a temperature range taking into account the vapor pressure of the solvent used, which is a method known in the art. The drying substantially removes the solvent present in the photoinitiator solution and is desirably carried out as quickly as possible considering productivity and the like, and can be carried out, for example, in a time of 1 minute or less or 30 seconds or less.

[0206] In still other embodiments of the present invention, the photocrosslinking composition can be a slurry for forming an inorganic composite void layer containing an inorganic filler, a binder polymer, the type II photoinitiator, and the solvent.

[0207] When the photocrosslinking composition is the slurry for forming an inorganic composite void layer, the type II photoinitiator is introduced onto the surface of the polyolefin porous support while the photocrosslinking composition is coated on the polyolefin porous support, and when irradiated with ultraviolet rays, the polyolefin porous support is crosslinked and an inorganic composite void layer can be formed on at least one surface of the polyolefin porous support. Thereby, a crosslinked structure-containing separator for a lithium secondary battery can be produced, which includes a polyolefin porous support containing a crosslinked structure and 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 binder polymer.

[0208] When using a slurry for forming an inorganic composite void layer as the photocrosslinking composition, additional equipment for directly applying the type II photoinitiator to the polyolefin porous support, such as equipment for directly coating and drying a solution containing the type II photoinitiator on the polyolefin porous support, is not required, and the polyolefin porous support can be crosslinked using the inorganic composite void layer forming step.

[0209] In addition, since the slurry for forming the inorganic composite void layer does not require other monomers or the like in addition to the type II photoinitiator to directly crosslink the polymer chains in the polyolefin porous support, even if the type II photoinitiator is included in the slurry for forming the inorganic composite void layer together with the inorganic filler and the binder polymer, monomers or the like do not interfere with the type II photoinitiator reaching the surface of the polyolefin porous support, and the type II photoinitiator can be sufficiently introduced onto the surface of the polyolefin porous support.

[0210] Generally, since the polyolefin porous support itself and the inorganic filler have a high ultraviolet blocking effect, if ultraviolet light is irradiated after forming the inorganic composite void layer containing the inorganic filler, the amount of irradiated light of the ultraviolet light reaching the polyolefin porous support may decrease. However, in the present invention, even if ultraviolet light is irradiated after the inorganic composite void layer is formed, the polymer chains in the polyolefin porous support can be directly crosslinked with each other.

[0211] The solvent may serve as a solvent for dissolving the binder polymer according to the type of the binder polymer, or may serve as a dispersion medium for dispersing the binder polymer without dissolving it. Further, the solvent can dissolve the type II photoinitiator. As the solvent, one having a solubility index similar to that of the binder polymer to be used and a low boiling point can be used. In this case, uniform mixing and subsequent solvent removal become easy. For non-limiting examples of such solvents, refer to the description of the solvents described above.

[0212] For the inorganic filler and the binder polymer, refer to the above-described content.

[0213] The binder polymer may be dissolved in the solvent according to the type of the binder polymer, or may be dispersed without being dissolved in the solvent.

[0214] In one embodiment of the present invention, when the composition for photocrosslinking is the slurry for forming the inorganic composite void layer, the type II photoinitiator may include 2-isopropylthioxanthone, thioxanthone, or a mixture thereof. 2-Isopropylthioxanthone or thioxanthone enables photocrosslinking even at a long wavelength with high transmittance. Thereby, even if the type II photoinitiator is included in the slurry for forming the inorganic composite void layer containing an inorganic filler, a binder polymer, etc., the polyolefin porous support can be easily crosslinked.

[0215] The slurry for forming the inorganic composite void layer can be produced by dissolving or dispersing the binder polymer in the solvent and then adding the inorganic filler and dispersing it. The inorganic filler may be added in a state where it has been crushed in advance to have a predetermined average particle size, or after adding the inorganic filler to the slurry in which the binder polymer is dissolved or dispersed, the inorganic filler may be crushed and dispersed while controlling it to have a predetermined average particle size using a ball mill method or the like. At this time, the crushing can be performed for 1 to 20 hours, and the average particle size of the crushed inorganic filler is as described above. As the crushing method, a normal method can be used, and the ball mill method can be used.

[0216] In one embodiment of the present invention, the solid content of the slurry for forming the inorganic composite void layer may be 5 wt% to 60 wt%, or 30 wt% to 50 wt%. When the solid content of the slurry for forming the inorganic composite void layer is within the above-described range, coating uniformity can be easily ensured, and it can be easily prevented that the slurry flows and unevenness occurs or that a large amount of energy is required for drying the slurry.

[0217] In one embodiment of the present invention, when the composition for photocrosslinking is the slurry for forming the inorganic composite void layer, after coating the composition for photocrosslinking on the polyolefin porous support, a phase separation process can be performed. The phase separation can be performed by a humid phase separation or an immersion phase separation method.

[0218] Hereinafter, the humidification phase separation among the phase separations will be described.

[0219] First, the humidification phase separation can be carried out under the conditions of a temperature range of 15°C to 70°C or 20°C to 50°C, and a relative humidity range of 15% to 80% or 30% to 50%. While the slurry for forming the inorganic composite void layer undergoes a drying process, it acquires phase transition characteristics due to the vapor-induced phase separation phenomenon known in the art.

[0220] For the humidification phase separation, a non-solvent for the binder polymer can be introduced in a gaseous state. The non-solvent for the binder polymer is not particularly limited as long as it has partial compatibility with the solvent without dissolving the binder polymer. For example, those with a solubility of less than 5% by weight of the binder polymer under the condition of 25°C can be used. For example, the non-solvent for the binder polymer can be water, methanol, ethanol, isopropanol, butanol, butanediol, ethylene glycol, propylene glycol, tripropylene glycol, or two or more of these.

[0221] Regarding the immersion phase separation among the phase separations, it is as follows.

[0222] After coating the slurry for forming the inorganic composite void layer on the outside of the polyolefin porous support, it is immersed in a coagulating liquid containing a non-solvent for the binder polymer for a predetermined time. Thereby, while inducing a phase separation phenomenon in the coated inorganic composite void layer slurry, the binder polymer is solidified. In this step, an inorganic composite void layer that is porous is formed. Then, the coagulating liquid is removed by washing with water and dried. For the drying, a method known in the art can be used, and it can be carried out batchwise or continuously using an oven or a heating chamber within a temperature range considering the vapor pressure of the solvent used. The drying is to remove almost all of the solvent present in the slurry, and it is desirable to be carried out as quickly as possible considering productivity and the like, and it can be carried out, for example, in a time of 1 minute or less or 30 seconds or less.

[0223] As the coagulating liquid, only a non-solvent for the binder polymer can be used, or a mixed solvent of a non-solvent for the binder polymer and a solvent as described above can be used. When using a mixed solvent of a non-solvent for the binder polymer and a solvent, from the viewpoint of forming a good porous structure and improving productivity, the content of the non-solvent for the binder polymer can be 50% by weight or more based on 100% by weight of the coagulating liquid.

[0224] In another embodiment of the present invention, the step of coating and drying the photocrosslinking composition containing the type II photoinitiator and the solvent on the outside of the polyolefin porous support is coating and drying a slurry for forming an inorganic composite void layer containing an inorganic filler, a first binder polymer, and a dispersion medium on at least one surface of the polyolefin porous support to form an inorganic composite void layer; coating and drying a coating liquid for forming a porous adhesive layer containing a second binder polymer, the type II photoinitiator, and the solvent on the upper surface of the inorganic composite void layer, may be included.

[0225] Thus, a crosslinked structure-containing separator for a lithium secondary battery can be manufactured, which includes 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 containing a second binder polymer.

[0226] For the above-mentioned inorganic filler, please refer to the content described above.

[0227] The dispersion medium may serve as a solvent for dissolving the first binder polymer according to the type of the first binder polymer, or may serve as a dispersion medium for dispersing the first binder polymer without dissolving it. The dispersion medium having a solubility index similar to that of the first binder polymer to be used and a low boiling point can be used. In this case, uniform mixing and subsequent removal of the dispersion medium become easier.

[0228] In one embodiment of the present invention, the dispersion medium can be an aqueous dispersion medium. When the dispersion medium is an aqueous dispersion medium, it is environmentally friendly, does not require excessive heat during the drying process after forming the inorganic composite void layer, and does not require additional explosion-proof equipment, so that the inorganic composite void layer can be formed more easily.

[0229] In one embodiment of the present invention, the first binder polymer may be insoluble in the solvent and a non-solvent for the second binder polymer described below. In this case, even when the coating liquid described below is applied to form the porous adhesive layer after forming the inorganic composite void layer, since the first binder polymer is not dissolved, the phenomenon that the first binder polymer dissolved in the solvent and / or the non-solvent for the second binder polymer blocks the pores can be easily prevented.

[0230] In one embodiment of the present invention, the first binder polymer can be an aqueous binder polymer. At this time, the first binder polymer can be dissolved in an aqueous solvent or dispersed by an aqueous dispersion medium. When the first binder polymer is dispersed by an aqueous dispersion medium, the first binder polymer can be in a particulate form.

[0231] For the slurry for forming the inorganic composite void layer, refer to the above-described content.

[0232] The slurry for forming the inorganic composite void layer can be dried by a drying method during the production of a normal separation membrane. For example, the drying of the coated slurry can be performed by air for 10 seconds to 30 minutes, 30 seconds to 20 minutes, or 3 minutes to 10 minutes. When dried within the above time range, it has the effect of removing residual solvents while not inhibiting productivity.

[0233] For the second binder polymer, refer to the above-described content.

[0234] The solvent can be one that dissolves 5 wt% or more, 15 wt% or more, or 25 wt% or more of the second binder polymer at 25°C.

[0235] The solvent can be a non-solvent for the first binder polymer. For example, the solvent can be one that dissolves less than 5 wt% of the first binder polymer at 25°C.

[0236] For the type of the solvent, refer to the above-described content.

[0237] In one embodiment of the present invention, the second binder polymer can be contained in an amount of 3 wt% to 30 wt%, or 5 wt% to 25 wt% based on 100 wt% of the coating liquid for forming the porous adhesive layer.

[0238] When the type II photoinitiator is included in the coating liquid for forming a porous adhesive layer, when the coating liquid for forming a porous adhesive layer is coated on the upper surface of the inorganic composite void layer, the type II photoinitiator can be introduced onto the surface of the polyolefin porous support and at the same time a porous adhesive layer can be formed.

[0239] In the process of coating the coating liquid for forming a porous adhesive layer, the polyolefin porous support gets wet with the solvent. At this time, the type II photoinitiator contained in the coating liquid for forming a porous adhesive layer is introduced onto the surface of the polyolefin porous support, and the polyolefin porous support can be photocrosslinked by the type II photoinitiator present on the surface of the polyolefin porous support during ultraviolet irradiation.

[0240] Accordingly, the manufacturing method of the crosslinked structure-containing separator for a lithium secondary battery according to an embodiment of the present invention does not require additional equipment for directly applying a type II 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 the type II photoinitiator on the polyolefin porous support. In terms of being able to photocrosslink the polyolefin porous support using the porous adhesive layer forming step, the process can be simplified.

[0241] The manufacturing method of the crosslinked structure-containing separator for a lithium secondary battery according to an embodiment of the present invention does not require other components such as monomers for forming radicals in addition to the type II photoinitiator in order to directly crosslink the polymer chains in the polyolefin porous support. Therefore, even when the type II photoinitiator is added to the coating liquid for forming a porous adhesive layer, other components do not interfere with the type II photoinitiator reaching the surface of the polyolefin porous support, and the type II photoinitiator can be sufficiently introduced onto the surface of the polyolefin porous support.

[0242] In general, since the polyolefin porous support itself and the inorganic filler have a high ultraviolet blocking effect, when ultraviolet rays are irradiated after forming the inorganic composite void layer and the porous adhesive layer, the irradiation light amount of the ultraviolet rays reaching the polyolefin porous support may decrease. However, in the present invention, since crosslinking is possible even with a small irradiation light amount of ultraviolet rays, even when ultraviolet rays are irradiated after the inorganic composite void layer and the porous adhesive layer are formed, the polymer chains in the polyolefin porous support can be crosslinked and directly connected.

[0243] In one embodiment of the present invention, the coating liquid for forming the porous coating layer may contain 2-isopropylthioxanthone, thioxanthone, or a mixture thereof as the type II photoinitiator. 2-Isopropylthioxanthone or thioxanthone enables photocrosslinking even at a long wavelength with high transmittance. Thereby, even when ultraviolet rays are irradiated after the inorganic formation void layer and the porous adhesive layer are formed, the polyolefin porous support can be easily crosslinked.

[0244] In one embodiment of the present invention, by pattern coating the coating liquid for forming the porous adhesive layer on the upper surface of the inorganic composite void layer, a pattern can be formed on the finally produced porous adhesive layer.

[0245] In one embodiment of the present invention, after the coating liquid for forming the porous adhesive layer is coated on the upper surface of the inorganic composite void layer, a phase separation process can be performed. The phase separation can be performed by an immersion phase separation method.

[0246] After coating the upper surface of the inorganic composite void layer with the coating liquid for forming the porous adhesive layer, it is immersed in a coagulating liquid containing a non-solvent for the second binder polymer for a predetermined time. Thereby, the second binder polymer is solidified while inducing a phase separation phenomenon in the coated coating liquid for forming the porous adhesive layer. A porous adhesive layer is formed in this step. Then, the coagulating liquid is removed by washing with water and dried. For the drying, a method known in the art can be used, and it can be carried out batchwise or continuously using an oven or a heating chamber within a temperature range considering the vapor pressure of the solvent used. The drying is to remove almost all of the solvent present in the coating liquid for forming the porous adhesive layer, and it is desirable to be carried out as quickly as possible considering productivity and the like, and it can be carried out, for example, in a time of 1 minute or less or 30 seconds or less.

[0247] As the coagulating liquid, only a non-solvent for the second binder polymer can be used, or a mixed solvent of a non-solvent for the second binder polymer and a solvent as described above can be used. When using a mixed solvent of a non-solvent for the second binder polymer and a solvent, from the viewpoint of forming a good porous structure and improving productivity, the content of the non-solvent for the second binder polymer can be 50% by weight or more based on 100% by weight of the coagulating liquid.

[0248] In the process of solidifying the second binder polymer, the second binder polymer condenses, thereby preventing the second binder polymer from penetrating into the surface and / or inside of the polyolefin porous support, and preventing the phenomenon of an increase in the resistance of the separation membrane. Further, the resistance of the separation membrane can be improved by making the adhesive layer containing the second binder polymer porous.

[0249] The non-solvent for the second binder polymer can have a solubility in the second binder polymer of less than 5% by weight at 25°C.

[0250] 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 in the first binder polymer of less than 5% by weight at 25°C.

[0251] In one embodiment of the present invention, the non-solvent for the second binder polymer may include water, methanol, ethanol, propyl alcohol, butyl alcohol, butanediol, ethylene glycol, propylene glycol, tripropylene glycol, or two or more thereof.

[0252] In one embodiment of the present invention, the immersion can be performed for 3 seconds to 1 minute. When the immersion time satisfies the above-described range, phase separation occurs appropriately and the adhesive force between the inorganic composite void layer and the porous adhesive layer is ensured, and the detachment of the adhesive layer can be prevented.

[0253] In one embodiment of the present invention, the coating liquid for forming the porous adhesive layer can be dried by a drying method during the production of a normal separation membrane. For example, it can be dried by air for 10 seconds to 30 minutes, 30 seconds to 20 minutes, or 3 minutes to 10 minutes. When dried within the above time range, it has the effect of removing the residual dispersion medium while not inhibiting productivity.

[0254] By such a manufacturing method, since the inorganic composite void layer and the porous adhesive layer are formed through separate steps, the porous adhesive layer can be formed in various forms. For example, the porous adhesive layer can be easily formed in a pattern form.

[0255] Thereafter, the polyolefin porous support is irradiated with ultraviolet rays. By irradiating with ultraviolet rays, the polymer chains in the polyolefin porous support are crosslinked to obtain a polyolefin porous support containing a crosslinked structure.

[0256] The ultraviolet irradiation can be carried out by appropriately adjusting the irradiation time and irradiation light amount of ultraviolet rays using an ultraviolet crosslinking device in consideration of conditions such as the content ratio of the type II photoinitiator. For example, the irradiation time and irradiation light amount of the ultraviolet rays can be set under conditions such that the polymer chains in the polyolefin porous support are sufficiently crosslinked to ensure the desired heat resistance and the separation membrane is not damaged by the heat generated by the ultraviolet lamp. Further, the ultraviolet lamp used in the ultraviolet crosslinking device can be appropriately selected and used from a high-pressure mercury lamp, a metal lamp, a gallium lamp, etc. according to the type II photoinitiator to be used, and the emission wavelength and capacity of the ultraviolet lamp can be appropriately selected according to the process.

[0257] The method for producing a crosslinked structure-containing polyolefin porous support according to an embodiment of the present invention can photocrosslink the polymer chains in the polyolefin porous support even with an extremely small irradiation light amount of ultraviolet rays compared to the light amount used in general photocrosslinking, so that the applicability to a mass production process can be enhanced. For example, the irradiation light amount of the ultraviolet rays can be 10 to 2000 mJ / cm 2 , 50 to 1000 mJ / cm 2 , or 150 to 500 mJ / cm 2 .

[0258] In one embodiment of the present invention, the irradiation light amount of the ultraviolet rays can be measured using a portable light amount measuring device H type UV bulb and UV power pack manufactured by Miltec. When measuring the light amount using the H type UV bulb manufactured by Miltec, three types of wavelength values of UVA, UVB, and UVC can be obtained for each wavelength, and the ultraviolet rays of the present invention correspond to UVA.

[0259] In the present invention, the method for measuring the irradiation light amount of the ultraviolet rays is to pass the UV power pack under the light source on the conveyor under the same conditions as the sample, and at this time, the ultraviolet light amount numerical value indicated on the UV power pack is referred to as the "irradiation light amount of the ultraviolet rays".

[0260] A lithium secondary battery can be manufactured by interposing a crosslinked structure-containing separator for a lithium secondary battery according to an embodiment of the present invention between a positive electrode and a negative electrode.

[0261] The lithium secondary battery can have various shapes such as cylindrical, rectangular, or pouch type.

[0262] The lithium secondary battery can include a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, a lithium ion polymer secondary battery, or the like.

[0263] The electrode applied together with the crosslinked structure-containing separator for a lithium secondary battery of the present invention is not particularly limited, and an electrode active material layer containing an electrode active material, a conductive material, and a binder can be produced in a form adhered to a current collector by a conventional method well known in the art.

[0264] Non-limiting examples of the positive electrode active material among the electrode active materials include layered compounds such as lithium cobalt composite oxide (LiCoO2), lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; chemical formula Li 1+x Mn 2-x O4 (x = 0 to 0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O5, LiV3O4, V2O5, Cu2V2O7; chemical formula LiNi 1-x M x O2 (M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, x = 0.01 to 0.3) Ni-site type lithium nickel oxide represented by; chemical formula LiMn 1-x M x O2 (M = Co, Ni, Fe, Cr, Zn or Ta, x = 0.01 to 0.1) or lithium manganese composite oxide represented by Li2Mn3MO5 (M = Fe, Co, Ni, Cu or Zn); LiMn2O4 in which part of Li in the chemical formula is substituted with an alkaline earth metal ion; disulfide compounds; Fe2(MoO4)3, etc., but is not limited thereto.

[0265] Among the electrode active materials, non-limiting examples of the negative electrode active material include ordinary negative electrode active materials conventionally used for the negative electrode of a lithium secondary battery. In particular, lithium metal or lithium alloy, carbon, petroleum coke, activated carbon, graphite, or other carbonaceous materials such as lithium adsorbing materials can be used.

[0266] Non-limiting examples of the positive electrode current collector include foils made of aluminum, nickel, or combinations thereof. Non-limiting examples of the negative electrode current collector include foils made of copper, gold, nickel, or copper alloys, or combinations thereof.

[0267] In one embodiment of the present invention, the conductive materials used in the negative electrode and the positive electrode can each independently be added usually in an amount of 1 wt% to 30 wt% based on the total weight of the active material layer. Such conductive materials are not particularly limited as long as they have conductivity without inducing a chemical change in the battery. For example, graphite such as natural graphite and artificial graphite; carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon 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 can be used.

[0268] In one embodiment of the present invention, the binders used in the negative electrode and the positive electrode are each independently components that assist in binding the active material, the conductive material, etc., and binding to the current collector, and are usually added in an amount of 1 wt% to 30 wt% based on the total weight of the active material layer. Examples of such binders include polyvinylidene fluoride (PVdF), polyacrylic acid (PAA), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, various copolymers, and the like.

[0269] In one embodiment of the present invention, the lithium secondary battery contains an electrolytic solution, and the electrolytic solution may contain an organic solvent and a lithium salt. Further, as the electrolytic solution, an organic solid electrolyte, an inorganic solid electrolyte, or the like may be used.

[0270] Examples of the organic solvent 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, triphosphate ester, trimethoxymethane, dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl propionate, ethyl propionate, and the like.

[0271] The lithium salt is a substance that is easily dissolved in the organic solvent, and examples thereof include 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, imide, etc. can be used.

[0272] Also, for the purpose of improving charge-discharge characteristics, flame retardancy, etc., for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivative, sulfur, quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. can be added to the electrolyte. In some cases, for imparting non-flammability, a halogen-containing solvent such as carbon tetrachloride and vinylidene fluoride may be further included, and for improving high-temperature storage characteristics, carbon dioxide gas may be further included.

[0273] Examples of the organic solid electrolyte include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate esters, polyagitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, polymers containing ionic dissociation groups, etc. can be used.

[0274] Examples of the inorganic solid electrolyte include nitrides, halides, sulfates of Li such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH, Li3PO4-Li2S-SiS2, etc. can be used.

[0275] The injection of the electrolyte can be carried out at an appropriate stage in the battery manufacturing process according to the manufacturing process of the final product and the required physical properties. That is, it can be carried out before battery assembly or at the final stage of battery assembly.

[0276] In one embodiment of the present invention, as a step of applying the crosslinked structure-containing separator for a lithium secondary battery to a battery, in addition to the normal winding process, a lamination and folding process of the separator and the electrode can be applied.

[0277] In one embodiment of the present invention, the crosslinked structure-containing separator for a lithium secondary battery is interposed between the positive electrode and the negative electrode of the lithium secondary battery, and when a plurality of cells or electrodes are assembled to form an electrode assembly, it can be interposed between adjacent cells or electrodes. The electrode assembly can have various structures such as a simple stack type, jelly roll type, stack folding type, lamination stack type, etc.

[0278] Hereinafter, in order to assist in the understanding of the present invention, examples will be given and described in detail. However, the examples according to the present invention can be deformed into many other forms, and the scope of the present invention should not be construed as being limited to the examples described below. The examples of the present invention are provided to more fully explain the present invention to those with average knowledge in the industry.

[0279] Example 1 A polyethylene porous film (manufactured by Senior, weight average molecular weight: 600,000, porosity: 50%) with a thickness of 9 μm, in which the number of double bonds present in the polymer chain is 0.2 per 1000 carbon atoms, and Irganox 1010 is added at 3000 ppm and Irgafos 168 is added at 2000 ppm as antioxidants, was prepared as a polyolefin porous support.

[0280] 2-Isopropylthioxanthone (manufactured by Sigma Aldrich) was prepared as a photoinitiator.

[0281] As a UV light source, a high-pressure mercury lamp (Lichtzen high-pressure mercury lamp, LH-250 / 800-A) was prepared.

[0282] The photoinitiator was dissolved in an acetone solvent, and a photocrosslinking composition containing 0.1 part by weight of the photoinitiator based on 100 parts by weight of acetone was prepared.

[0283] The produced polyolefin porous support was immersed in the photocrosslinking composition and then taken out. While cutting the coating liquid using a bar so that no photocrosslinking composition remained on the surface of the porous support, it was dried so that the content of the photoinitiator became 0.073 part by weight with respect to 100 parts by weight of the polyolefin porous support.

[0284] Next, on the upper surface of the polyolefin porous support coated with the photocrosslinking composition, UV was irradiated so that the integrated light amount, that is, the irradiation light amount of UV, became 500 mJ / cm 2 . At this time, the irradiation intensity of UV was set to 80% of the UV light source.

[0285] Thereby, a crosslinked structure-containing polyolefin porous support having a crosslinked structure in which polymer chains are directly connected to each other was obtained.

[0286] Example 2 After preparing a photocrosslinking composition by adding 2-isopropylthioxanthone as a photoinitiator so as to be 0.05 part by weight based on 100 parts by weight of acetone, the produced polyolefin porous support was immersed in the photocrosslinking composition and then taken out. While cutting the coating liquid using a bar so that no photocrosslinking composition remained on the surface of the porous support, a crosslinked structure-containing polyolefin porous support was obtained in the same manner as in Example 1, except that the content of the photoinitiator was made 0.036 part by weight with respect to 100 parts by weight of the polyolefin porous support.

[0287] Example 3 As a photoinitiator, benzophenone (manufactured by Sigma Aldrich) was added to 0.1 part by weight based on 100 parts by weight of acetone to prepare a photo-crosslinking composition. Then, the produced polyolefin porous support was immersed in the photo-crosslinking composition and taken out. While cutting the coating liquid using a bar so that no photo-crosslinking composition remained on the surface of the porous support, a crosslinked-structure-containing polyolefin porous support was obtained in the same manner as in Example 1, except that the content of the photoinitiator was adjusted to 0.073 part by weight based on 100 parts by weight of the polyolefin porous support.

[0288] Comparative Example 1 No treatment was performed on a polyethylene porous film (manufactured by Senior, weight average molecular weight: 600,000, porosity: 50%) as a polyolefin porous support with a thickness of 9 μm in which the number of double bonds present in the polymer chain was 0.2 per 1000 carbon atoms during 1H-NMR measurement.

[0289] Comparative Example 2 A polyolefin porous support was obtained in the same manner as in Example 1, except that phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Irgacure 819) was used as the photoinitiator instead of 2-isopropylthioxanthone.

[0290] Comparative Example 3 Instead of 2-isopropylthioxanthone as the photoinitiator, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Irgacure 819) was added so that it was 0.1 part by weight based on 100 parts by weight of acetone, and tris(2-acryloyloxyethyl)isocyanurate (TEICTA, manufactured by Sigma Aldrich) was added so that it was 0.3 part by weight based on 100 parts by weight of acetone to prepare a photocrosslinking composition. After that, the produced polyolefin porous support was immersed in the photocrosslinking composition and then taken out. While cutting the coating liquid using a bar so that no photocrosslinking composition remained on the surface of the porous support, a polyolefin porous support was obtained in the same manner as in Example 1, except that the content of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide was 0.073 part by weight and tris(2-acryloyloxyethyl)isocyanurate was 0.219 part by weight with respect to 100 parts by weight of the polyolefin porous support.

[0291] Evaluation Example 1: Evaluation of Electron Spin Resonance (ESR) Spectrum Peaks of Polyolefin Porous Supports For the polyolefin porous supports produced in Examples 1 to 3 and Comparative Examples 1 to 3, the electron spin resonance spectra when not irradiated with ultraviolet light and when irradiated with 500 W of ultraviolet light are shown in FIGS. 3 to 8, respectively.

[0292] In FIGS. 3 to 8, the electron spin resonance spectrum when 30 mg of the polyolefin porous support was irradiated with 500 W of ultraviolet light is indicated by A, and the case where the separation membrane was not irradiated with ultraviolet light is indicated by (B).

[0293] In addition, the electron spin resonance spectra of the crosslinked structure-containing polyolefin porous supports produced in Examples 1 to 3 are shown in Table 1 below.

[0294] The electron spin resonance spectrum when irradiated with 500 W of ultraviolet light was measured under the following conditions.

[0295] Measuring device: JEOL ESR device JES-FA100 / Frequency 9215 MHz / Output 0.998 mW / Sweep time 30 seconds

[0296]

Table 1

[0297] From FIGS. 3 to 5, in the case of the crosslinked structure-containing polyolefin porous support produced in Examples 1 to 3, when irradiated with 500 W of ultraviolet light, it can be confirmed that a first peak is detected at a g value of 2.010 to 2.030. Also, it can be confirmed that a second peak is detected at a g value of 1.990 to 2.009. The ratio of the area of the first peak obtained by integrating the area of the first peak to the area of the second peak obtained by integrating the area of the second peak was 10% to 200%.

[0298] From these, it can be confirmed that in the crosslinked structure-containing polyolefin porous support produced in Examples 1 to 3, radicals were formed from the polymer chains in the polyolefin porous support.

[0299] On the other hand, as shown in FIG. 6, no peak occurred in the polyolefin porous support of Comparative Example 1.

[0300] As can be confirmed from FIGS. 7 and 8, in the polyolefin porous support produced in Comparative Examples 2 and 3, when irradiated with 500 W of ultraviolet light, a peak was detected at a g value of 1.990 to 2.009, but no peak was detected at a g value of 2.010 to 2.030.

[0301] From these, it can be confirmed that in the polyolefin porous support produced in Comparative Examples 1 to 3, no radicals were formed from the polymer chains in the polyolefin porous support.

[0302] Evaluation Example 2: Evaluation of the physical properties of the polyolefin porous support The results of measuring the air permeability, porosity, rate of change in basis weight, electrical resistance, degree of crosslinking, and melt-down temperature of the polyolefin porous supports produced in Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 2.

[0303] (1) Evaluation of air permeability The air permeability (Gurley) was measured by the ASTM D726-94 method. The Gurley used here is the resistance to the flow of air and is measured by a Gurley densitometer. The value of the air permeability described here is shown as the time (seconds), i.e., the air passage time, required for 100 ml of air to pass through a cross-section of 1 in2 of the polyolefin porous support under a pressure of 12.2 in H2O.

[0304] (2) Evaluation of porosity The porosity was measured by calculating the ratio of the weight of the polyolefin porous support to the weight when the polyolefin porous support occupies 100% in the volume obtained by measuring the horizontal length / vertical length / thickness of the polyolefin porous support and determining the volume.

[0305] Porosity (%) = 100 × (1 - weight of the sample of the polyolefin porous support / (horizontal length (50 mm) × vertical length (50 mm) × thickness of the sample of the polyolefin porous support × density of the separation membrane))

[0306] (3) Evaluation of the rate of change in basis weight Basis weight (g / m 2 ) was evaluated by preparing a sample with a length and width of 1 m each of the polyolefin porous support and measuring its weight.

[0307] The rate of change in basis weight can be calculated by the following formula.

[0308] Rate of change in basis weight (%) = [(basis weight of the polyolefin porous support after crosslinking) - (basis weight of the polyolefin porous support before crosslinking)] / (basis weight of the polyolefin porous support before crosslinking) × 100

[0309] Here, in the case of Comparative Example 1, since crosslinking did not occur, the rate of change in basis weight was 0%.

[0310] (4) Evaluation of electrical resistance For the evaluation of electrical resistance, coin cells were fabricated using the polyolefin porous supports produced in Examples 1 to 3 and Comparative Examples 1 to 3 as separator membranes. After leaving the coin cells at room temperature for 1 day, the resistance of the polyolefin porous supports was measured by the impedance measurement method. The coin cells were fabricated as follows.

[0311] Fabrication of the negative electrode Artificial graphite as the negative electrode active material, Denka black (carbon black) as the conductive material, and polyvinylidene fluoride (PVdF) as the binder were mixed at a weight ratio of 75:5:20, and N-methylpyrrolidone (NMP) as the solvent was added to produce a negative electrode slurry.

[0312] The negative electrode slurry was coated on a copper current collector at a loading amount of 3.8 mAh / cm 2 and dried to prepare a negative electrode.

[0313] Fabrication of the positive electrode LiCoO2 as the positive electrode active material, Denka black as the conductive material, and polyvinylidene fluoride (PVdF) as the binder were added to N-methylpyrrolidone (NMP) as the solvent at a weight ratio of 85:5:10 to prepare a positive electrode active material slurry. The positive electrode active material slurry was coated on a sheet-like aluminum current collector and dried to form a positive electrode active material layer such that the final positive electrode loading amount was 3.3 mAh / cm 2 was achieved.

[0314] Fabrication of the coin cell Between the negative electrode and the positive electrode fabricated as described above, the polyolefin porous supports produced in the above-described Examples and Comparative Examples were interposed as separator membranes, and a non-aqueous electrolyte (1 M LiPF6, ethylene carbonate (EC) / propylene carbonate (PC) / diethyl carbonate (DEC)) (volume ratio = 3:3:4) was injected to fabricate a coin cell.

[0315] (5) Evaluation of crosslinking degree The crosslinking degree was calculated as the percentage of the residual weight to the initial weight after immersing the polyolefin porous supports produced in Examples 1 to 3 and Comparative Examples 1 to 3 in a xylene solution at 135 °C according to ASTM D2765, boiling for 12 hours, and then measuring the residual weight.

[0316] (6) Evaluation of the melt-down temperature The melt-down temperature was measured by a thermomechanical analysis method (TMA) after sampling a sample in the machine direction of the polyolefin porous support. Specifically, a sample with a width of 4.8 mm and a length of 8 mm was placed in a TMA apparatus (Q400 manufactured by TA Instruments), and the temperature was changed from 30 °C to 220 °C at a heating rate of 5 °C / min while applying a tensile force of 0.01 N. As the temperature increased, the length of the sample changed, and the temperature at which the length increased rapidly and the sample broke in the machine direction was measured and defined as the melt-down temperature.

[0317]

Table 2

[0318] As can be seen from Table 2, in the case of the crosslinked structure-containing polyolefin porous supports produced in Examples 1 to 3, the melt-down temperature was 160 °C or higher. This is because a type II photoinitiator such as 2-isopropylthioxanthone and benzophenone was used, enabling direct photocrosslinking of the polymer chains in the polyolefin porous support.

[0319] On the other hand, in the case of Comparative Example 1, direct photocrosslinking between the polymer chains in the polyolefin porous support did not occur, and the melt-down temperature did not even reach 150 °C.

[0320] In the case of the polyolefin porous supports produced in Comparative Examples 2 and 3, the melt-down temperature did not reach 160 °C. This is because a type II photoinitiator was not used, resulting in little direct photocrosslinking of the polymer chains in the polyolefin porous support.

[0321] Evaluation Example 3: Measurement of Storage Elastic Modulus and Loss Elastic Modulus of Polyolefin Porous Support Table 3 shows the results of measuring the storage elastic modulus and loss elastic modulus of the polyolefin porous supports produced in Examples 1 and 2 and Comparative Example 1.

[0322] Using the polyolefin porous supports produced in Examples 1 and 2 and Comparative Example 1, circular parallel-plate samples with a diameter of 25 mm and a thickness of 1 mm were prepared. With the prepared samples, a frequency sweep test was performed at a temperature of 190 °C using a rheological property measuring device (ARES-G2, TA Instruments). For the polyolefin constituting the polyolefin porous support, 1) the storage elastic modulus (G’, storage modulus) at a frequency of 1 rad / s, 2) the loss elastic modulus (G’’, loss modulus) at 1 rad / s, and 3) the storage elastic modulus (G’, storage modulus) gradient from a frequency of 10 -1 ~1 rad / s were measured respectively. The results are shown in Table 3 below. From the measured G’ and G’’ of the polyolefin constituting the polyolefin porous support, the fluidity of the polyolefin porous support composed of such a polyolefin can be confirmed.

[0323]

Table 3

[0324] Referring to Table 3, in the range where the frequency of the polyolefin porous support is 1 rad / s or less, the ratio (A / B) of the storage elastic modulus (G’) (A) to the loss elastic modulus (G’’) (B) is 2 or more, or the gradient of the storage elastic modulus (G’) (A) curve with respect to the frequency satisfies 0.05 to 0.4. The crosslinked structure-containing polyolefin porous supports produced in Examples 1 and 2 show a significantly higher coating temperature than the polyolefin porous support of Comparative Example 1, indicating that the thermal stability has been greatly improved.

Claims

**Claim 1** A crosslinked structure-containing polyolefin porous support, having a crosslinked structure in which polymer chains are directly connected to each other, when irradiated with ultraviolet light of 500 W and measured by electron spin resonance method, a first peak is detected at a g value of 2.010 to 2.030, wherein the polyolefin is a crosslinked structure-containing polyolefin porous support containing polyethylene. **Claim 2** The crosslinked structure-containing polyolefin porous support according to claim 1, wherein when irradiated with ultraviolet light of 500 W and measured by electron spin resonance method, a second peak is further detected at a g value of 1.990 to 2.

009. **Claim 3** The crosslinked structure-containing polyolefin porous support according to claim 2, wherein when irradiated with ultraviolet light of 500 W and measured by electron spin resonance method, the ratio of the area of the first peak to the area of the second peak is 10% to 200%. **Claim 4** In a frequency-loss storage modulus curve where the horizontal axis is the frequency (rad / s) converted to a logarithmic scale and the vertical axis is the storage modulus (G', storage modulus) (A) and loss modulus (G'', loss modulus) (B) converted to a logarithmic scale, the crosslinked structure-containing polyolefin porous support according to claim 1, wherein in the range where the frequency is 1 rad / s or less, the ratio (A / B) of the storage modulus (G') (A) to the loss modulus (G'') (B) of the crosslinked structure-containing polyolefin porous support is 2 or more. **Claim 5** In a frequency-loss storage modulus curve where the horizontal axis is the frequency (rad / s) converted to a logarithmic scale and the vertical axis is the storage modulus (G') (A) and loss modulus (G'') (B) converted to a logarithmic scale, The vibration frequency is 10 －１ to 1 rad / s, and the slope of the storage elastic modulus (G') (A) curve of the crosslinked structure-containing polyolefin porous support with respect to the vibration frequency is 0.05 to 0.

4. The crosslinked structure-containing polyolefin porous support according to claim 1. **Claim 6** The value of the storage elastic modulus is 1.0 × 10 ５ to 1.0 × 10 ７ Pa, the crosslinked structure-containing polyolefin porous support according to claim 4 or 5. **Claim 7** The value of the loss elastic modulus is 3.0×10 ５ Pa or less, the crosslinked structure-containing polyolefin porous support according to claim 4 or 5. **Claim 8** A crosslinked structure-containing separator for a lithium secondary battery, comprising the crosslinked structure-containing polyolefin porous support according to claim 1. **Claim 9** The crosslinked structure-containing separator for a lithium secondary battery according to claim 8, wherein the crosslinked structure-containing separator for a lithium secondary battery is located on at least one surface of the crosslinked structure-containing polyolefin porous support and further comprises an inorganic composite void layer containing an inorganic filler and a binder polymer. **Claim 10** The crosslinked structure-containing separator for a lithium secondary battery, is located on at least one surface of the crosslinked structure-containing polyolefin porous support, and an inorganic composite void layer containing an inorganic filler and a first binder polymer, A porous adhesive layer that is located on the inorganic composite void layer and contains a second binder polymer; the crosslinked structure-containing separator membrane for a lithium secondary battery according to claim 8.

11. The crosslinked structure-containing separator membrane for a lithium secondary battery according to claim 8, wherein the melt-down temperature of the crosslinked structure-containing separator membrane for a lithium secondary battery is 160°C or higher.

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

13. Including a positive electrode, a negative electrode, and a separator membrane for a lithium secondary battery interposed between the positive electrode and the negative electrode. A lithium secondary battery, wherein the separator membrane for a lithium secondary battery is the crosslinked structure-containing separator membrane for a lithium secondary battery according to claim 8.

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