Composition for forming gel polymer electrolyte, gel polymer electrolyte, and lithium secondary battery comprising gel polymer electrolyte

Abstract

The present invention provides a composition for forming a gel polymer electrolyte, comprising: a lithium salt; an organic solvent; and a crosslinking compound represented by chemical formula 1. [chemical formula 1] In chemical formula 1, L1, L2, L3, and L4 are each independently selected from a direct bond or an alkylene group of 1 to 3 carbon atoms; L5 and L6 may each be an alkylene group of 1 to 20 carbon atoms in which an ether group is substituted for at least one methylene group and fluorine is substituted for at least one hydrogen; and R1 and R2 are each independently selected from hydrogen, an alkyl group of 1 to 3 carbon atoms, or a halogen.

Description

Composition for forming a gel polymer electrolyte, gel polymer electrolyte, and lithium secondary battery including the gel polymer electrolyte Cross-citation with related applications This application claims the benefit of priority to Korean Patent Application No. 10-2023-0124966, filed September 19, 2023, the entire contents of which are incorporated herein by reference. Technology field The present invention relates to a composition for forming a gel polymer electrolyte, a gel polymer electrolyte, and a lithium secondary battery including the gel polymer electrolyte. Demand for high-energy-density lithium secondary batteries is rapidly increasing due to technological advancements in mobile electronic devices, electric vehicles (EVs), and grid-scale energy storage systems (ESS). In general, a lithium secondary battery can be composed of a cathode made of a transition metal oxide containing lithium, a cathode that can store lithium, an electrolyte that serves as a medium for transferring lithium ions, and materials such as a separator. The electrolyte used in the aforementioned lithium secondary batteries typically uses a liquid electrolyte with high ionic conductivity and excellent electrochemical properties. However, the liquid electrolyte is highly flammable, highly reactive with electrode materials, and can cause side reactions. Furthermore, it exhibits low stability, such as leakage, which can lead to fires and explosions under abnormal operating conditions. Accordingly, in order to improve the stability of lithium-ion batteries, research is being conducted to develop more stable electrolytes instead of liquid electrolytes, and for example, ionic liquid electrolytes, solid electrolytes, and gel polymer electrolytes are being proposed. The above-mentioned ionic liquid electrolyte has excellent thermal and oxidation stability, but has the disadvantages of being unstable on the negative electrode surface, having low wettability for polyolefin-based separators due to high viscosity, and being high in cost. On the other hand, the above-mentioned solid electrolyte improves the stability of the battery by excluding flammable organic solvents, but has the problem of reduced ionic conductivity around the electrolyte and a shortened lifespan due to increased interfacial resistance between the solid electrolyte and the electrode. Meanwhile, the gel polymer electrolyte is an electrolyte system that encapsulates and impregnates a liquid electrolyte within a polymer structure, and has the advantages of high structural, thermal, and mechanical stability over time, securing relatively high ionic conductivity, and reasonable manufacturing cost. At this time, the gel polymer electrolyte can be classified into a physically cross-linked gel polymer electrolyte that forms a polymer structure through physical bonding of polymers, and a chemically cross-linked gel polymer electrolyte that forms a polymer structure through a chemical reaction of a reactive oligomer or a cross-linking agent. Since the bonding strength of the physically cross-linked gel polymer electrolyte may weaken over time, the chemically cross-linked gel polymer electrolyte that forms a polymer structure through chemical bonding has higher structural, thermal, and mechanical stability. However, the chemical cross-linked gel polymer electrolyte contains a large amount of liquid electrolyte inside, and still has problems such as leakage, ignition, and explosion. Therefore, there is an urgent need to develop a gel polymer electrolyte that can improve all performances, including stability and lifespan, of lithium secondary batteries. One object of the present invention is to provide a composition for forming a gel polymer electrolyte, which has excellent oxidation stability and flame retardancy and is effective in suppressing the heat generation of an electrode, thereby simultaneously improving the safety and life performance of a lithium secondary battery. In addition, another object of the present invention is to provide a gel polymer electrolyte having excellent oxidation stability and flame retardancy and being effective in suppressing the heat generation of an electrode, thereby simultaneously improving the safety and life performance of a lithium secondary battery. In addition, another object of the present invention is to provide a lithium secondary battery including the aforementioned gel polymer electrolyte. [1] The present invention provides a composition for forming a gel polymer electrolyte, comprising: a lithium salt; an organic solvent; and a crosslinking compound represented by the following chemical formula 1. [Chemical Formula 1] In the above chemical formula 1, L1, L2, L3, and L4 are each independently selected from a direct bond or an alkylene group having 1 to 3 carbon atoms, L5 and L6 are each independently an alkylene group having 1 to 20 carbon atoms in which at least one methylene group may be substituted with an ether group and at least one hydrogen is substituted with fluorine, and R1 and R2 are each independently selected from hydrogen, an alkyl group having 1 to 3 carbon atoms, or halogen. [2] The present invention provides a composition for forming a gel polymer electrolyte, further comprising a polymerization initiator, in the above [1]. [3] The present invention provides a composition for forming a gel polymer electrolyte, wherein the molecular weight of the crosslinking compound is 700 g / mol to 2,000 g / mol, in at least one of the above [1] and [2]. [4] The present invention provides a composition for forming a gel polymer electrolyte, wherein in at least one of the above [1] to [3], the crosslinking compound is included in an amount of 1% to 30% by weight based on the total weight of the composition for forming a gel polymer electrolyte. [5] The present invention provides a composition for forming a gel polymer electrolyte, wherein in at least one of the above [1] to [4], in the above chemical formula 1, L1, L2, L3, and L4 are independently selected from direct bonds or methylene groups. [6] The present invention provides a composition for forming a gel polymer electrolyte, wherein in at least one of the above [1] to [5], in the above chemical formula 1, L1, L2, L3, and L4 are each a methylene group. [7] The present invention provides a composition for forming a gel polymer electrolyte, wherein in at least one of the above [1] to [6], L5 and L6 are each independently represented by the following chemical formula 2. [Chemical Formula 2] *-[(CH2) n -(CF2) m -(CH2) p -(O) q -(CH2) r -(CF2) s -(CH2) t ] u -* In the above chemical formula 2, n, m, p, r, s, and t are each integers greater than or equal to 0, q is 0 or 1, m+s is an integer greater than or equal to 1, u is an integer greater than or equal to 1, and the product of (n+m+p+q+s+t) and u is an integer from 1 to 20, provided that when q is 1, n+m+p is an integer greater than or equal to 1, and r+s+t is an integer greater than or equal to 1. [8] The present invention provides a composition for forming a gel polymer electrolyte, wherein the crosslinking compound represented by the chemical formula 1 in at least one of the above [1] to [7] comprises a crosslinking compound represented by the following chemical formula 1-1. [Chemical Formula 1-1] [9] The present invention is characterized in that in at least one of the above [1] to [8], the lithium salt is LiCl, LiBr, LiI, LiBF4, LiClO4, LiAlO4, LiAlCl4, LiPF6, LiSbF6, LiAsF6, LiB 10 Cl 10 , LiBOB (LiB(C2O4)2), LiCF3SO3, LiFSI (LiN(SO2F)2), LiCH3SO3, LiCF3CO2, LiCH3CO2, and LiBETI (LiN(SO2CF2CF3)2) are provided.

[0010] The present invention provides a composition for forming a gel polymer electrolyte, wherein in at least one of the above [1] to [9], the organic solvent comprises at least one selected from the group consisting of a carbonate solvent, an ester solvent, an ether solvent, a ketone solvent, an aromatic hydrocarbon solvent, a phosphate solvent, a sulfone solvent, a nitrile solvent, and an ionic liquid.

[0011] The present invention provides a composition for forming a gel polymer electrolyte, wherein the composition further comprises an additive in at least one of the above [1] to

[0010] , wherein the additive comprises at least one selected from the group consisting of a sultone compound, a halogen-substituted carbonate compound, a nitrile compound, a cyclic carbonate compound, a sulfite compound, a sulfone compound, a sulfate compound, a phosphate or phosphite compound, a borate compound, and a lithium salt compound.

[0012] The present invention provides a composition for forming a gel polymer electrolyte, wherein in at least one of the above [1] to

[0011] , the additive comprises a halogen-substituted carbonate-based compound, and the halogen-substituted carbonate-based compound is fluoroethylene carbonate.

[0013] The present invention provides a gel polymer electrolyte comprising a polymer matrix formed by a crosslinking reaction of a lithium salt; an organic solvent; and a crosslinking compound represented by the following chemical formula 1. [Chemical Formula 1] In the above chemical formula 1, L1, L2, L3, and L4 are each independently selected from a direct bond or an alkylene group having 1 to 3 carbon atoms, L5 and L6 are each independently an alkylene group having 1 to 10 carbon atoms in which at least one methylene group may be substituted with an ether group and at least one hydrogen is substituted with fluorine, and R1 and R2 are each independently selected from hydrogen, an alkyl group having 1 to 3 carbon atoms, or halogen.

[0014] The present invention provides a lithium secondary battery comprising: a positive electrode; a negative electrode opposite to the positive electrode; a separator interposed between the positive electrode and the negative electrode; and a gel polymer electrolyte according to

[0013] . The composition for forming a gel polymer electrolyte according to the present invention is characterized by including a crosslinking compound of a specific chemical formula, and since the crosslinking compound includes a fluorine group and a spiro cyclic diphosphate group within the compound, the gel polymer electrolyte formed by the crosslinking reaction of the crosslinking compound can have excellent oxidation stability and flame retardancy, and has an excellent effect of suppressing the heat generation of an electrode. Accordingly, the composition for forming a gel polymer electrolyte or a lithium secondary battery including the gel polymer electrolyte can prevent leakage of an organic solvent and short-circuiting of the battery, and can also have excellent safety and life performance. Figure 1 is a schematic diagram illustrating a synthesis process of a crosslinked compound according to Manufacturing Example 1. Figure 2 shows the FT-IR spectrum results of fluorinated ether monoacrylate (PFE-mono AC) and spirocyclic pentaerythritol diphosphate perfluorinated ether acrylate (SPDFA) manufactured through steps 2 and 3 of Manufacturing Example 1. Figure 3 is a diagram of spirocyclic pentaerythritol bisphosphorate diphosphoryl chloride (SPDPC) manufactured through step 1 of manufacturing example 1. 1 This is the result of the H NMR spectrum. Figure 4 is a schematic diagram of spirocyclic pentaerythritol diphosphate perfluorinated ether acrylate (SPDFA) manufactured through step 3 of manufacturing example 1. 1 This is the result of the H NMR spectrum. Figure 5 shows the FT-IR spectrum analysis results of Examples 1 and 2. Figure 6 shows the results of measuring ionic conductivity according to temperature of the gel polymer electrolyte of Example 2 and the liquid electrolyte of Comparative Example 1. Figure 7 is a graph showing the current value measured according to the applied voltage (3.0 to 6.0 V) using linear sweep voltammetry (LSV) of the liquid electrolyte of Comparative Example 1 and the gel polymer electrolytes of Comparative Examples 4, 6, and 8. Figure 8 shows charge / discharge curves of the 1st, 10th, 30th, 50th, 100th, 200th, and 300th cycles of the lithium secondary battery of Example 3. Figure 9 is a graph showing the change in discharge capacity according to the cycle of the lithium secondary battery of Example 3. Figure 10 is a photograph showing the results of the combustion test of the liquid electrolyte of Comparative Example 1 before (left) and after (right) the combustion test. Figure 11 is a photograph showing the results of a combustion test of the gel polymer electrolyte of Example 2 before (left) and after (right) the combustion test. This is a photograph showing the results before (left) and after (right) storage of a polyethylene separator not impregnated with a gel polymer electrolyte according to the present invention of FIG. 12 when stored at 140°C. This is a photograph showing the results before (left) and after (right) storage of a polyethylene separator impregnated with a gel polymer electrolyte according to the present invention of FIG. 13 when stored at 140°C. Figure 14 is a graph showing the battery voltage measured over time when the lithium secondary batteries of Example 3 and Comparative Example 2 were stored in a hot box at 140°C. Figure 15 is a graph showing the amount of heat generated according to the temperature of the negative electrode in the lithium secondary batteries of Example 3 and Comparative Example 2. Figure 16 is a graph showing the amount of heat generated according to the temperature of the positive electrode in the lithium secondary batteries of Example 3 and Comparative Example 2. Terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as meanings and concepts that conform to the technical spirit of the present invention, based on the principle that the inventor can appropriately define the concept of the term to explain his or her own invention in the best possible manner. It should be understood that the terms “include,” “comprising,” or “having” used in this specification are intended to specify the presence of a feature, number, step, component, or combination thereof, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof. Meanwhile, before explaining the present invention, unless otherwise specified, “*” in the present invention means a connected portion (bonding site) between the terminals of the same or different atoms or chemical formulas. In addition, in the description of "carbon atoms a to b" in the present specification, "a" and "b" refer to the number of carbon atoms included in a specific functional group. That is, the functional group may include "a" to "b" carbon atoms. For example, "an alkyl group having 1 to 5 carbon atoms" refers to an alkyl group including 1 to 5 carbon atoms, i.e., CH3-, CH3CH2-, CH3CH2CH2-, (CH3)2CH-, CH3CH2CH2CH2-, (CH3)2CHCH2-, CH3CH2CH2CH2CH2-, (CH3)2CHCH2CH2CH2-, (CH3)2CHCH2CH2CH2-, etc. Additionally, all alkyl groups in this specification may be substituted or unsubstituted. The above "substitution" means, unless otherwise defined, that at least one hydrogen bonded to carbon is replaced by an element other than hydrogen, for example, an alkyl group having 1 to 20 carbon atoms, an alkenyl group having 2 to 20 carbon atoms, an alkynyl group having 2 to 20 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, a cycloalkyl group having 3 to 12 carbon atoms, a cycloalkenyl group having 3 to 12 carbon atoms, a cycloalkynyl group having 3 to 12 carbon atoms, a heterocycloalkyl group having 3 to 12 carbon atoms, a heterocycloalkenyl group having 3 to 12 carbon atoms, a heterocycloalkynyl group having 2 to 12 carbon atoms, an aryloxy group having 6 to 12 carbon atoms, a halogen atom, a fluoroalkyl group having 1 to 20 carbon atoms, a nitro group, an aryl group having 6 to 20 carbon atoms, an aryl group having 2 to 20 carbon atoms, an alkyl ... It means substituted with a heteroaryl group having 20 carbon atoms, a haloaryl group having 6 to 20 carbon atoms, etc. Hereinafter, the present invention will be described in more detail. Composition for forming gel polymer electrolyte The present invention provides a composition for forming a gel polymer electrolyte. Specifically, the composition for forming the gel polymer electrolyte is characterized by including a lithium salt; an organic solvent; and a crosslinking compound represented by the following chemical formula 1. [Chemical Formula 1] In the above chemical formula 1, L1, L2, L3, and L4 are each independently selected from a direct bond or an alkylene group having 1 to 3 carbon atoms, L5 and L6 are each independently an alkylene group having 1 to 20 carbon atoms in which at least one methylene group may be substituted with an ether group and at least one hydrogen is substituted with fluorine, and R1 and R2 are each independently selected from hydrogen, an alkyl group having 1 to 3 carbon atoms, or halogen. 1) Lithium salt As the lithium salt used in the present invention, various lithium salts commonly used in electrolytes for lithium secondary batteries can be used without limitation. For example, the lithium salt may be Li as a cation. + , and the anion is F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , AlO4 - , AlCl4 - , PF6 - , SbF6 - , AsF6 - , B 10 Cl 10 - , BF2C2O4 - , BC4O8 - , PF4C2O4 - , PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , C4F9SO3 - , CF3CF2SO3 - , (FSO2)2N - , CF3CF2(CF3)2CO- , (CF3SO2)2CH - , CH3SO3 - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - It may include at least one selected from the group consisting of . Specifically, the lithium salt is LiCl, LiBr, LiI, LiBF4, LiClO4, LiAlO4, LiAlCl4, LiPF6, LiSbF6, LiAsF6, LiB 10 Cl 10 , LiBOB (LiB(C2O4)2), LiCF3SO3, LiFSI (LiN(SO2F)2), LiCH3SO3, LiCF3CO2, LiCH3CO2, and LiBETI (LiN(SO2CF2CF3)2). Specifically, the lithium salt may include at least one selected from the group consisting of LiBF4, LiClO4, LiPF6, LiBOB (LiB(C2O4)2), LiCF3SO3, LiTFSI (LiN(SO2CF3)2), LiFSI ((LiN(SO2F)2), and LiBETI (LiN(SO2CF2CF3)2). The lithium salt may be included in the non-aqueous electrolyte at a concentration of 0.5 M to 5 M, specifically at a concentration of 0.8 M to 4 M, and more specifically at a concentration of 0.8 M to 2.0 M. When the concentration of the lithium salt satisfies the above range, the lithium ion yield (Li + The output characteristics of the battery can be improved by improving the transference number and the degree of dissociation of lithium ions. 2) Organic solvent The organic solvent mentioned above is a non-aqueous solvent commonly used in lithium secondary batteries, and is not particularly limited as long as decomposition due to oxidation reactions, etc. during the charging and discharging process of the secondary battery can be minimized. The organic solvent may include at least one selected from the group consisting of carbonate solvents, ester solvents, ether solvents, ketone solvents, aromatic hydrocarbon solvents, phosphate solvents, sulfone solvents, nitrile solvents, and ionic liquids. Specifically, the organic solvent may include at least one selected from the group consisting of carbonate solvents and ester solvents, and more specifically, a carbonate solvent. When the organic solvent includes a carbonate solvent, the carbonate solvent may include a cyclic carbonate solvent, a linear carbonate solvent, or a mixture thereof. The above cyclic carbonate solvent is a high-viscosity organic solvent having a high dielectric constant and thus capable of dissociating a lithium salt in an electrolyte well, and specifically may include at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, fluoroethylene carbonate, and vinylene carbonate, and more specifically may include ethylene carbonate. In addition, the linear carbonate solvent is an organic solvent having low viscosity and low dielectric constant, and may specifically include at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethylmethyl carbonate (EMC), methylpropyl carbonate, and ethylpropyl carbonate, and more specifically, may include ethylmethyl carbonate (EMC). The above organic solvent may be a mixture of a cyclic carbonate solvent and a linear carbonate solvent. In this case, the cyclic carbonate solvent and the linear carbonate solvent may be mixed in a volume ratio of 10:90 to 40:60, specifically, a volume ratio of 15:85 to 35:65. When the mixing ratio of the cyclic carbonate solvent and the linear carbonate solvent satisfies the above range, high dielectric constant and low viscosity characteristics can be simultaneously satisfied, and excellent ionic conductivity characteristics can be implemented. Meanwhile, the ester solvent may be a linear ester solvent or a cyclic ester solvent. The linear ester solvent may specifically include at least one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate. In addition, the cyclic ester solvent may specifically include at least one selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone. As the above ether solvent, any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, 1,3-dioxolane (DOL), and 2,2-bis(trifluoromethyl)-1,3-dioxolane (TFDOL) or a mixture of two or more thereof may be used, but is not limited thereto. The above nitrile solvent may be at least one selected from the group consisting of acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentane carbonitrile, cyclohexane carbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile, but is not limited thereto. The ionic liquid may include at least one selected from the group consisting of diethylmethylammonium trifluoromethanesulfonate, dimethylpropylammonium trifluoromethanesulfonate, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide, N-methyl-N-propylpiperidium bis(trifluoromethanesulfonyl)imide, N-butyl-N-methyl pyrrolidium bis(trifluoromethanesulfonyl)imide, and methylpropylpiperidiumtrifluoro methanesulfonyl imide. Cyclohexanone can be used as the above ketone solvent. Specific examples of the above aromatic hydrocarbon solvents include benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, fluorotoluene, 2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4-trifluorotoluene, 2,3,5-trifluorotoluene, chlorotoluene, 2,3-dichlorotoluene, 2,4-dichlorotoluene, 2,5-dichlorotoluene, 2,3,4-trichlorotoluene, 2,3,5-trichlorotoluene, iodotoluene, 2,3-diiodotoluene, 2,4-diiodotoluene, 2,5-diiodotoluene, 2,3,4-triiodotoluene, 2,3,5-triiodotoluene, xylene, and combinations thereof. The above sulfone solvent may be at least one selected from the group consisting of dimethyl sulfone, diethyl sulfone, ethyl methyl sulfone, dipropyl sulfone, dibutyl sulfone, diphenyl sulfone, and dimethyl sulfoxide. The above phosphate solvent may be at least one selected from the group consisting of trimethyl phosphate, triethyl phosphate, tris(2-chloroethyl) phosphate, tris(2,2,2-trifluoroethyl) phosphate, tripropyl phosphate, triisopropyl phosphate, tributyl phosphate, trihexyl phosphate, triphenyl phosphate, tritolyl phosphate, methyl ethylene phosphate, and ethyl ethylene phosphate. 3) Cross-linked compound represented by chemical formula 1 The above crosslinked compound is characterized by being represented by the following chemical formula 1. [Chemical Formula 1] In the above chemical formula 1, L1, L2, L3, and L4 are each independently selected from a direct bond or an alkylene group having 1 to 3 carbon atoms, L5 and L6 are each independently an alkylene group having 1 to 20 carbon atoms in which at least one methylene group may be substituted with an ether group and at least one hydrogen is substituted with fluorine, and R1 and R2 are each independently selected from hydrogen, an alkyl group having 1 to 3 carbon atoms, or halogen. The crosslinking compound included in the composition for forming the gel polymer electrolyte is characterized in that acrylate groups are bonded to both terminals of a spiro cyclic diphosphate matrix, and a fluorine-substituted linker group (L5 and L6) exists between the spiro cyclic diphosphate matrix and the acrylate group, and the polymer matrix formed by the crosslinking reaction of the crosslinking compound can have excellent oxidation stability and flame retardancy, so that the thermal safety, prevention of ignition or explosion of a lithium secondary battery to which the gel polymer electrolyte is applied can be ensured, and excellent life performance can be achieved. Specifically, the spiro cyclic diphosphate matrix can form radicals when vaporized upon heating, and the radicals formed at this time can scavenge OH radicals and H radicals that cause ignition of the battery, so it is effective in preventing ignition and improving flame retardancy. However, the spiro cyclic diphosphate has poor oxidation stability, and in particular, stability is a problem at high voltage, and there is a problem that it easily opens. This problem of reduced oxidation stability of the spiro cyclic diphosphate, along with the problem of the aforementioned flame retardant effect not being expressed, causes a reduction in the life performance of the lithium secondary battery. To solve these problems, the crosslinking compound of the present invention is characterized by the presence of fluorine-substituted linker groups (L5 and L6) between the spiro cyclic diphosphate parent and the acrylate group. Since the decomposition and ring-opening of the spiro cyclic diphosphate can be prevented to an excellent level through the increase in electronegativity due to the fluorine-substituted linker group, the crosslinking compound or the polymer matrix formed through the crosslinking reaction thereof can exhibit excellent oxidation stability and flame retardancy, and accordingly, a lithium secondary battery including the composition for forming a gel polymer electrolyte or a gel polymer electrolyte prepared therefrom can exhibit both excellent safety and life performance. In the above chemical formula 1, L1, L2, L3, and L4 may be independently selected from a direct bond or an alkylene group having 1 to 3 carbon atoms, and specifically, may be independently selected from a direct bond or a methylene group (-CH2-), and more specifically, may be a methylene group in terms of improving the structural stability of the spiro cyclic diphosphate. In the above chemical formula 1, L5 and L6 are alkylene groups having 1 to 20 carbon atoms in which at least one hydrogen is replaced with fluorine. The L5 and L6 can function as a linker group that binds the spiro cyclic diphosphate and acrylate in the above chemical formula 1. In addition, the L5 and L6 are alkylene groups having 1 to 20 carbon atoms in which at least one hydrogen is replaced with fluorine and have excellent electronegativity, thereby preventing ring opening or decomposition of the adjacent spiro cyclic diphosphate and improving oxidation stability. The above L5 and L6 may be independently an alkylene group having 1 to 20 carbon atoms in which at least one hydrogen is substituted with fluorine, specifically an alkylene group having 1 to 10 carbon atoms in which at least one hydrogen is substituted with fluorine, more specifically an alkylene group having 5 to 10 carbon atoms in which at least one hydrogen is substituted with fluorine, independently of each other. Within the above range, the structural stability of the crosslinked compound is improved, and the effect of preventing ring opening or decomposition of the above-described spiro cyclic diphosphate can be further improved. In addition, the above L5 and L6 may independently have one or more methylene groups (or difluoromethylene groups) substituted with an ether group (-O-). Specifically, the above L5 and L6 may independently have one or more methylene groups substituted or not substituted with an ether group. The above L5 and L6 may be independently an alkylene group having 1 to 20 carbon atoms in which at least one hydrogen is substituted with fluorine without any special limitation. For example, the above L5 and L6 may each be independently represented by the following chemical formula 2. [Chemical Formula 2] *-[(CH2) n -(CF2) m -(CH2) p -(O) q -(CH2) r -(CF2) s -(CH2) t ] u-* In the above chemical formula 2, n, m, p, r, s, and t are each integers greater than or equal to 0, q is 0 or 1, m+s is an integer greater than or equal to 1, u is an integer greater than or equal to 1, and the product of (n+m+p+q+s+t) and u is an integer from 1 to 20, provided that when q is 1, n+m+p is an integer greater than or equal to 1, and r+s+t is an integer greater than or equal to 1. In the above chemical formula 2, when u is 2 or more, n, m, p, r, s and t of each unit may be the same or different. In the above chemical formula 2, the product of (n+m+p+q+s+t) and u may be an integer from 1 to 20, specifically an integer from 1 to 10, and more specifically an integer from 5 to 10. Specifically, the crosslinked compound represented by the above chemical formula 1 may include a crosslinked compound represented by the following chemical formula 1-1. [Chemical Formula 1-1] The molecular weight of the above crosslinked compound may be 700 g / mol to 2,000 g / mol, specifically 850 g / mol to 1,200 g / mol. The above molecular weight can be measured using a terminal group quantification method that quantitatively analyzes the functional group at the end of the molecular chain to obtain the molecular weight, a comprehensive method using physical properties such as osmotic pressure, vapor pressure depression, boiling point elevation, and freezing point depression (membrane osmosis, vapor pressure osmosis, etc.), a light scattering method that uses light scattering, an ultracentrifugation method that measures the molecular weight by analyzing the sedimentation velocity or concentration distribution after centrifuging a polymer solution, a viscometry method that uses the viscosity of a polymer solution, and gel permeation chromatography (GPC) using high-performance liquid chromatography (HPLC). The cross-linking compound may be included in an amount of 1 wt% to 30 wt% based on the total weight of the composition for forming the gel polymer electrolyte. Specifically, the content of the cross-linking compound may be 1 wt% or more, 2 wt% or more, 3 wt% or more, 4 wt% or more, 5 wt% or more, 6 wt% or more, 7 wt% or more, or 8 wt% or more based on the total weight of the composition for forming the gel polymer electrolyte. The content of the cross-linking compound may be 30 wt% or less, 25 wt% or less, 20 wt% or less, 15 wt% or less, 12 wt% or less, or 11 wt% or less based on the total weight of the composition for forming the gel polymer electrolyte. More specifically, the content of the cross-linking compound may be 3 wt% to 30 wt%, 5 wt% to 30 wt%, 5 wt% to 25 wt%, or 8 wt% to 15 wt% based on the total weight of the composition for forming the gel polymer electrolyte. When within the above range, the oxidation stability and flame retardancy performance of the composition for forming the gel polymer electrolyte or the gel polymer electrolyte described above are exhibited at an excellent level, while the increase in resistance and the decrease in ionic conductivity due to excessive addition of a crosslinking compound can be prevented, which is preferable. 4) Polymerization initiator The composition for forming the above gel polymer electrolyte may further include a polymerization initiator. As the above polymerization initiator, a general thermal polymerization initiator known in the art can be used. Specifically, the polymerization initiator is diisobutyl peroxide, t-amylperoxydicarbonate, di(4-tert-butylcyclohexyl)peroxydicarbonate, diethylhexyl peroxydicarbonate, dibutyl peroxydicarbonate, diisopropyl peroxydicarbonate, dicetyl peroxydicarbonate, dimyristyl peroxydicarbonate, tert-butyl peroxypivalate, dilauroyl peroxide, didecanoyl peroxide. peroxide), 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate, tert-amylperoxy-2-ethylhexanoate, dibenzoyl peroxide, tert-butylperoxy-2-ethylhexanoate, tert-butylperoxy ethyl acetate, tert-butylperoxy isobutylate, and 1,4-di(t-butylperoxycarbo)cyclohexane(1,It may include at least one selected from the group consisting of 4-di(tert-butylperoxy carbo)cyclohexane, and specifically, it may include t-butylperoxypivalate, which is a liquid thermal initiator with a low initiation temperature and generates little gas during the initiation reaction. The above polymerization initiator is easily soluble in organic solvents and can be decomposed by heat at 30°C to 80°C to form radicals. The crosslinking reaction of the crosslinking compound can be promoted by the radicals thus formed, and a polymer matrix (network) with secured mechanical strength and ionic conductivity can be formed. The polymerization initiator may be included in the composition for forming the gel polymer electrolyte in an amount of 0.01 to 20 parts by weight, specifically 0.1 to 10 parts by weight, based on 100 parts by weight of the crosslinking compound. When the polymerization initiator is included in the above range, the gel polymer conversion rate can be increased, thereby securing the gel polymer electrolyte properties, and the pre-gel reaction can be prevented, thereby improving the electrolyte wetting property for the electrode. 5) Additives In addition, the composition for forming a gel polymer electrolyte of the present invention may additionally include an additive capable of forming a more stable ion conductivity film on the electrode surface, if necessary, in order to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, and swelling improvement effects during high-temperature storage. The above additive may include at least one selected from the group consisting of sultone compounds, halogen-substituted carbonate compounds, nitrile compounds, cyclic carbonate compounds, sulfite compounds, sulfone compounds, sulfate compounds, phosphate or phosphite compounds, borate compounds, and lithium salt compounds. The above sultone-based compound may include at least one compound selected from the group consisting of 1,3-propane sultone (PS), 1,4-butane sultone, ethene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone, and may be included in an amount of 5 wt% or less based on the total weight of the precursor composition for a polymer electrolyte. When the content of the sultone-based compound in the precursor composition for a polymer electrolyte exceeds 5 wt%, an excessively thick film may be formed on the electrode surface, which may cause an increase in resistance and a deterioration in output, and an excessive amount of additives in the precursor composition for a polymer electrolyte may increase the resistance, thereby deteriorating the output characteristics. In addition, the halogen-substituted carbonate-based compound may include fluoroethylene carbonate (FEC), and may be included in an amount of 5 wt% or less based on the total weight of the precursor composition for a polymer electrolyte. If the content of the halogen-substituted carbonate-based compound in the precursor composition for a polymer electrolyte exceeds 5 wt%, cell swelling performance may deteriorate. In addition, the nitrile compound may include at least one compound selected from the group consisting of succinonitrile (SN), adiponitrile (Adn), acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentane carbonitrile, cyclohexane carbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile. The nitrile compound may be included in an amount of 8 wt% or less based on the total weight of the precursor composition for the polymer electrolyte. If the total weight of the nitrile compound in the precursor composition for the polymer electrolyte exceeds 8 wt%, the resistance may increase due to an increase in the film formed on the electrode surface, which may deteriorate battery performance. In addition, the cyclic carbonate compound may be vinylene carbonate (VC) or vinylethylene carbonate, and may be included in an amount of 5 wt% or less based on the total weight of the precursor composition for the polymer electrolyte. If the content of the cyclic carbonate compound in the precursor composition for the polymer electrolyte exceeds 5 wt%, the cell swelling inhibition performance may deteriorate. The above sulfite compound may include at least one compound selected from the group consisting of ethylene sulfite, methyl ethylene sulfite, ethyl ethylene sulfite, 4,5-dimethyl ethylene sulfite, 4,5-diethyl ethylene sulfite, propylene sulfite, 4,5-dimethyl propylene sulfite, 4,5-diethyl propylene sulfite, 4,6-dimethyl propylene sulfite, 4,6-diethyl propylene sulfite, and 1,3-butylene glycol sulfite, and may be included in an amount of 5 wt% or less based on the total weight of the precursor composition. The above sulfone compound may include at least one compound selected from the group consisting of divinyl sulfone, dimethyl sulfone, diethyl sulfone, methylethyl sulfone, and methylvinyl sulfone, and may be included in an amount of 5 wt% or less based on the total weight of the precursor composition. The above sulfate-based compound may be ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS), and may be included in an amount of 5 wt% or less based on the total weight of the precursor composition. The above phosphate-based or phosphite-based compound may include at least one compound selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tris(trimethylsilyl) phosphate (TMSPa), tris(trimethylsilyl) phosphite (TMSPi), tris(2,2,2-trifluoroethyl) phosphate (TFEPa), and tris(trifluoroethyl) phosphite (TFEPi), and may be included in an amount of 3 wt% or less based on the total weight of the precursor composition for a polymer electrolyte. The above borate compound may include tetraphenylborate, lithium oxalyldifluoroborate (LiODFB), or lithium bisoxalatoborate (LiB(C2O4)2, LiBOB), and may be included in an amount of 3 wt% or less based on the total weight of the precursor composition for the polymer electrolyte. The lithium salt-based compound is a compound different from the lithium salt included in the precursor composition, and may include LiPO2F2 or LiBF4, and may be included in an amount of 3 wt% or less based on the total weight of the precursor composition for the polymer electrolyte. Specifically, the additive may include a halogen-substituted carbonate compound, i.e., fluoroethylene carbonate (FEC). In addition, two or more of the above-mentioned other additives may be included as a mixture, and the total weight of the additives may be included in an amount of 20 wt% or less, specifically 0.01 wt% to 10 wt%, based on the total weight of the composition for forming a gel polymer electrolyte. gel polymer electrolyte In addition, the present invention provides a gel polymer electrolyte. The gel polymer electrolyte can be prepared from the aforementioned gel polymer electrolyte forming composition. More specifically, the gel polymer electrolyte can be formed by a crosslinking reaction of the aforementioned gel polymer electrolyte forming composition or a crosslinking reaction of a crosslinking compound included in the aforementioned gel polymer electrolyte forming composition. Specifically, the gel polymer electrolyte comprises a lithium salt; an organic solvent; and a polymer matrix formed by a crosslinking reaction of a crosslinking compound represented by the following chemical formula 1. [Chemical Formula 1] In the above chemical formula 1, L1, L2, L3, and L4 are each independently selected from a direct bond or an alkylene group having 1 to 3 carbon atoms, L5 and L6 are each independently an alkylene group having 1 to 10 carbon atoms in which at least one methylene group may be substituted with an ether group and at least one hydrogen is substituted with fluorine, and R1 and R2 are each independently selected from hydrogen, an alkyl group having 1 to 3 carbon atoms, or halogen. The above gel polymer electrolyte can have excellent oxidation stability and flame retardancy by including a polymer matrix formed by a crosslinked compound represented by the above chemical formula 1, so that a lithium secondary battery to which the gel polymer electrolyte is applied can have thermal safety, prevention of ignition or explosion, and excellent life performance. The specific description of the lithium salt, organic solvent, and crosslinked compound represented by chemical formula 1 is as described above. lithium secondary battery In addition, the present invention provides a lithium secondary battery including the aforementioned gel polymer electrolyte. Specifically, the present invention provides a lithium secondary battery comprising: a positive electrode; a negative electrode facing the positive electrode; a separator interposed between the positive electrode and the negative electrode; and the aforementioned gel polymer electrolyte. Alternatively, the present invention provides a lithium secondary battery comprising: a positive electrode; a negative electrode facing the positive electrode; a separator interposed between the positive electrode and the negative electrode; and a gel polymer electrolyte formed from the aforementioned gel polymer electrolyte forming composition. Specifically, the lithium secondary battery can be manufactured by manufacturing an electrode assembly by interposing a separator between the positive electrode and the negative electrode, then injecting the assembled electrode assembly and the gel polymer electrolyte forming composition of the present invention into a battery case, and performing a crosslinking reaction (thermal polymerization) of the crosslinking compound. The composition for forming the above gel polymer electrolyte and the above gel polymer electrolyte have been described above, and the positive electrode, negative electrode, and separator will be described below. 1) Bipolar The above positive electrode may include a positive electrode active material. The above lithium transition metal oxide is a compound capable of reversible intercalation and deintercalation of lithium, and includes lithium cobalt oxides (e.g., LiCoO2, etc.), lithium nickel oxides (e.g., LiNiO2, etc.), lithium manganese oxides (e.g., LiMnO2, LiMn2O4, etc.), lithium-nickel-manganese oxides (e.g., LiNi 1-Y Mn Y O2 (here, 0 <Y<1), LiMn 2-z Ni z O4 (where 0<Z<2), etc.), lithium-nickel-cobalt oxides (e.g., LiNi 1-Y1 Co Y1 O2 (here, 0 <Y1<1) 등), 리튬-망간-코발트계 산화물(예를 들면, LiCo 1-Y2 Mn Y2 O2 (here, 0 <Y2<1), LiMn 2-z1 Co z1 O4 (wherein, 0<Z1<2) etc.), lithium-nickel-manganese-cobalt oxides (e.g., Li(Ni p Co q Mn r1 )O2(wherein, 0<p<1, 0<q<1, 0<r1<1, p+q+r1=1) or Li(Ni p1 Co q1 Mn r2)O4 (wherein, 0<p1<2, 0<q1<2, 0<r2<2, p1+q1+r2=2) etc.), lithium-nickel-cobalt-manganese-transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r3 M S2 )O2 (wherein, M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg and Mo, and p2, q2, r3 and s2 are atomic fractions of independent elements, 0<p2<1, 0<q2<1, 0<r3<1, 0<s2<1, p2+q2+r3+s2=1) etc.), nickel-lithium transition metal composite oxide, lithium manganese-rich oxide, lithium iron phosphate (e.g., LiFePO4 etc.), and overlithiated layered oxide (OLO). The above high nickel lithium transition metal composite oxide may contain nickel in an amount of 50 mol% or more, specifically 60 mol% or more, based on the total mole number of transition metals included in the lithium transition metal composite oxide. Specifically, the high nickel lithium transition metal composite oxide may contain nickel as a transition metal; and at least one selected from manganese, cobalt, and aluminum, and may contain nickel in an amount of 50 mol% or more, specifically 60 mol% or more, and more specifically 60 mol% to 90 mol%, based on the total mole number of transition metals. Additionally, the lithium transition metal oxide may be a compound represented by the following chemical formula 5. [Chemical Formula 5] Li 1+x (Ni a Co b Mn c M d )O2 In the above chemical formula 5, M is at least one selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B and Mo, and 1+x, a, b, c and d are atomic fractions of independent elements, respectively, 0≤x≤0.2, 0.50≤a<1, 0 <b≤0.30, 0<c≤0.30, 0≤d≤0.1, a+b+c+d=1이다. Preferably, a, b, c, and d may be 0.60≤a≤0.95, 0.025≤b≤0.25, 0.025≤c≤0.25, and 0≤d≤0.05, respectively. In addition, a, b, c, and d may be 0.80≤a≤0.95, 0.025≤b≤0.15, 0.025≤c≤0.15, and 0≤d≤0.05, respectively. In addition, a, b, c, and d may be 0.85≤a≤0.90, 0.05≤b≤0.10, 0.05≤c≤0.10, and 0≤d≤0.03, respectively. The above lithium transition metal oxide may be a lithium manganese-rich oxide. The lithium manganese-rich oxide may contain Mn in an amount of 50 mol% or more among all metals excluding lithium, and the molar ratio of lithium to transition metal may exceed 1. Specifically, the lithium manganese-rich oxide may be a compound represented by the following chemical formula 6. [Chemical Formula 6] Li 1+s [Ni t Co u Mn v M 1 w ]O 2+z In the above chemical formula 6, 0.05≤s≤1, 0≤t≤0.5, 0≤u≤0.3, 0.5≤v<1.0, 0≤w≤0.2, 0≤z≤1. Preferably, 0.05≤s≤1.0, 0.1≤t≤0.5, 0≤u≤0.1, 0.5≤v<1.0, 0≤w≤0.2, 0≤z≤1, and more preferably, 0.10≤s≤0.50, 0.1≤t≤0.5, 0≤u≤0.1, 0.6≤v<1.0, 0≤w≤0.1, 0≤z≤0.50. M 1 It may be at least one selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B and Mo. More specifically, the lithium manganese-rich oxide may be a compound represented by the following chemical formula 6-1. [Chemical Formula 6-1] X Li2MnO3·(1-X)Li[Ni 1-y-z-w Mn y Co z M 1 w ]O2 In the above chemical formula 6-1, 0.1≤X≤0.5, 0.5≤y<1, 0≤z≤0.3, 0≤w≤0.2 may be satisfied, preferably 0.2≤X≤0.5, 0.5≤y<1, 0≤z≤0.1, 0≤w≤0.2, more preferably 0.3≤X≤0.5, 0.6≤y<1, 0≤z≤0.1, 0≤w≤0.2 may be satisfied. M 1 It may be at least one selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B and Mo. The positive electrode may include a positive electrode current collector; and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. In this case, the positive electrode active material layer may include the positive electrode active material. The positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. Specifically, the positive electrode current collector may include at least one selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, and an aluminum-cadmium alloy, preferably aluminum. The thickness of the above positive electrode collector may typically be 3 to 500 μm. The positive electrode current collector may also form fine irregularities on its surface to enhance the bonding strength of the negative electrode active material. For example, the positive electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous body, foam, or non-woven fabric. The positive electrode active material layer is disposed on at least one surface of the positive electrode current collector. Specifically, the positive electrode active material layer may be disposed on one or both surfaces of the positive electrode current collector. The above-mentioned positive electrode active material may be included in the positive electrode active material layer at 80 wt% to 99 wt%, preferably 92 wt% to 98.5 wt%, taking into account sufficient capacity of the positive electrode active material. The above-described positive electrode active material layer may further include a binder and / or a conductive material together with the above-described positive electrode active material. The above binder is a component that assists in the binding of the active material and the conductive material and the binding to the current collector, and specifically, may include at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, preferably polyvinylidene fluoride. The above binder may be included in the positive electrode active material layer at 1 wt% to 20 wt%, preferably 1.2 wt% to 10 wt%, in order to sufficiently secure binding force between components such as the positive electrode active material. The conductive material may be used to assist and improve conductivity in a secondary battery, and is not particularly limited as long as it has conductivity without causing chemical changes. Specifically, the positive electrode conductive material may include at least one selected from the group consisting of graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, paneth black, lamp black, thermal black, etc.; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives, and preferably, it may include carbon black in terms of improving conductivity. The above-mentioned conductive material may be included in the positive electrode active material layer at 1 wt% to 20 wt%, preferably 1.2 wt% to 10 wt%, in order to sufficiently secure electrical conductivity. The thickness of the positive electrode active material layer may be 30 µm to 400 µm, preferably 40 µm to 110 µm. The above positive electrode can be manufactured by coating a positive electrode slurry containing a positive electrode active material and optionally a binder, a conductive material, and a solvent for forming a positive electrode slurry on the positive electrode current collector, and then drying and rolling. The solvent for forming the positive electrode slurry may include an organic solvent such as NMP (N-methyl-2-pyrrolidone). The solid content of the positive electrode slurry may be 40 wt% to 90 wt%, specifically 50 wt% to 80 wt%. 2) Cathode The above cathode may be opposite to the above anode. The above negative electrode may include a negative electrode current collector; and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The above negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. Specifically, the negative electrode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., or an aluminum-cadmium alloy. The above negative electrode current collector may typically have a thickness of 3 to 500 μm. The above-described negative electrode current collector may have fine irregularities formed on its surface to enhance the bonding strength of the negative electrode active material. For example, the above-described negative electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous body, foam, or non-woven fabric. The negative electrode active material layer is disposed on at least one surface of the negative electrode current collector. Specifically, the negative electrode active material layer may be disposed on one or both surfaces of the negative electrode current collector. The above negative electrode active material layer may include a negative electrode active material. The above negative electrode active material is a material capable of reversibly inserting / de-inserting lithium ions, and may include at least one selected from the group consisting of carbon-based active materials, (semi)metal-based active materials, and lithium metal, and specifically may include at least one selected from among carbon-based active materials and (semi)metal-based active materials. The above carbon-based active material may include at least one selected from the group consisting of artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, graphene, and fibrous carbon, and preferably may include at least one selected from the group consisting of artificial graphite and natural graphite. The average particle diameter (D) of the above carbon-based active material 50) may be 10㎛ to 30㎛, preferably 15㎛ to 25㎛, in order to ensure structural stability during charging and discharging and reduce side reactions with the electrolyte. Specifically, the (semi-)metal-based active material may include at least one (semi-)metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, and Sn; an alloy of lithium with at least one (semi-)metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, and Sn; an oxide of at least one (semi-)metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, and Sn; lithium titanium oxide (LTO); lithium vanadium oxide; etc. More specifically, the (semi)metal-based active material may include a silicon-based active material. The above silicon-based active material is SiO x It may include a compound represented by (0≤x<2). In the case of SiO2, since it does not react with lithium ions and thus cannot store lithium, it is preferable that x is within the above range, and more preferably, the silicon-based active material may be SiO. The average particle diameter (D) of the above silicon-based active material 50 ) may be 1㎛ to 30㎛, preferably 2㎛ to 15㎛, in order to reduce side reactions with the electrolyte while ensuring structural stability during charging and discharging. The above negative active material may be included in the negative active material layer in an amount of 60 wt% to 99 wt%, preferably 75 wt% to 95 wt%. The above negative electrode active material layer may further include a binder and / or a conductive material together with the negative electrode active material. The above binder is used to improve the performance of the battery by improving the adhesion between the negative electrode active material layer and the negative electrode current collector, and may include, for example, at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, and materials in which hydrogens of these are substituted with Li, Na, Ca, etc., and may also include various copolymers thereof. May include. The above binder may be included in the negative electrode active material layer in an amount of 0.5 wt% to 10 wt%, preferably 1 wt% to 5 wt%. The conductive material is not particularly limited as long as it is conductive and does not cause a chemical change in the battery, and examples thereof include graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, paneth black, lamp black, thermal black, and the like; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. The above conductive material may be included in the negative electrode active material layer in an amount of 0.5 wt% to 10 wt%, preferably 1 wt% to 5 wt%. The thickness of the negative electrode active material layer may be 10 µm to 100 µm, preferably 50 µm to 80 µm. The above negative electrode can be manufactured by coating a negative electrode slurry including a negative electrode active material, a binder, a conductive material, and / or a solvent for forming a negative electrode slurry on at least one surface of a negative electrode current collector, and then drying and rolling. The solvent for forming the negative electrode slurry may include at least one selected from the group consisting of distilled water, NMP (N-methyl-2-pyrrolidone), ethanol, methanol, and isopropyl alcohol, specifically NMP, in order to facilitate dispersion of the negative electrode active material, binder, and / or conductive material, for example. The solid content of the negative electrode slurry may be 30 wt% to 80 wt%, specifically 40 wt% to 70 wt%. 3) Membrane In addition, as the separator, a conventional porous polymer film that has been conventionally used as a separator, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, may be used alone or in a laminated manner, or a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. may be used, but is not limited thereto. In addition, a coated separator containing a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and may optionally be used in a single-layer or multi-layer structure. There is no particular limitation on the external shape of the lithium secondary battery of the present invention, but it may be a cylindrical shape using a can, a square shape, a pouch shape, or a coin shape. Hereinafter, the present invention will be described in more detail through specific examples. However, the following examples are merely illustrative and serve to aid understanding of the present invention and do not limit its scope. It will be apparent to those skilled in the art that various modifications and variations are possible within the scope and technical spirit of this disclosure, and such modifications and variations are naturally within the scope of the appended claims. Examples and Comparative Examples Manufacturing Example 1: Preparation of a cross-linked compound: Synthesis of spirocyclic pentaerythritol diphosphate perfluorinated ether acrylate (SPDFA, chemical formula 1-1) (1) Step 1: Synthesis of spirocyclic pentaerythritol bisphosphorate diphosphoryl chloride (SPDPC) A schematic diagram for synthesizing spirocyclic pentaerythritol bisphosphorate diphosphoryl chloride (SPDPC) from pentaerythritol and phosphorus oxychloride is shown in Figure 1 (Step 2). First, pentaerythritol (6.81 g, 50 mmol) is placed in a double jacket, cooled to 0°C, and then phosphorus oxychloride (53.67 g, 350 mmol) is slowly added dropwise while stirring. After the dropping is complete, the mixed solution is raised to 80°C and stirred for 2 hours, then raised to 110°C and the reaction is carried out for 20 hours. The compound whose reaction was completed was filtered, washed with tetrahydrofuran and ethanol, and then vacuum dried at 80°C for 12 hours to obtain spirocyclic pentaerythritol bisphosphorate diphosphoryl chloride, and the chemical structure of the product was confirmed through NMR spectrum. [Chemical Structure Confirmation of Spirocyclic Pentaerythritol Bisphosphorate Diphosphoryl Chloride (NMR)] 1H NMR (DMSO), δ (ppm): 4.25-4.1 (dd, 4H) 31 P NMR (CDCl3), δ (ppm): -7.95 (2) Step 2: Synthesis of fluorinated ether monoacrylate (PFE-mono AC) A schematic diagram for synthesizing fluorinated ether monoacrylate (PFE-mono AC) from perfluoroether diol (PFE-OH) is shown in Figure 1 (Step 2). First, 10 g of 1H,1H,8H,8H-perfluoro-3,6-dioxaoctan-1,8-diol (PFE-OH) as reactants and 3.08 g of acryloyl chloride are dissolved in 56.6 ml of tetrahydrofuran (THF) and placed in a double jacket. At 0 °C, the reactants and 4.15 g of triethyl amine (TEA) catalyst are added to THF, and the reaction temperature is raised to 25 °C and the reaction is carried out for 12 hours. After vacuum drying the THF solvent from the generated solution, unreacted and by-products were completely removed through a column process, and the chemical structure of the obtained product was confirmed through an NMR spectrum. [Confirmation of the chemical structure of fluorinated ether monoacrylate (NMR)] 1 H NMR (CDCl3), δ (ppm): 6.54-6.5 (dd, 1H), 6.22-6.16 (dd, 1H), 6.05-5.98 (dd, 1H), 4.6-4.55 (t, 2H), 3.95-3.9 (q, 2H), 2.54-5.48 (t, 1H) (3) Step 3: Synthesis of spirocyclic pentaerythritol diphosphate perfluorinated ether acrylate (SPDFA) A schematic diagram for synthesizing spirocyclic pentaerythritol diphosphate perfluorinated ether acrylate (SPDFA) is shown in Fig. 1 (step 3). First, 10.0 g of spirocyclic pentaerythritol bisphosphorate diphosphoryl chloride (SPDPC) and 9.8 g of fluorinated ether monoacrylate (PFE-mono AC) prepared as reactants are dissolved in 192.6 ml of acetonitrile solvent and placed in a double jacket. After adding all of the reactants and 18.2 g of triethylamine catalyst at 0 °C, the reaction temperature is raised to 25 °C and the reaction is carried out for 48 hours (2 days). After vacuum drying the acetonitrile solvent in the resulting solution, unreacted and side-reacted substances are completely removed through a column process, and the chemical structure of the obtained product is shown in Fig. 2, FT-IR and Fig. 3. 1 This was confirmed through the H NMR spectrum results. The molecular weight of the spirocyclic pentaerythritol diphosphate perfluorinated ether acrylate (SPDFA) prepared above was 952.4 g / mol. Specifically, Fig. 2 is the FT-IR spectrum results of fluorinated ether monoacrylate (PFE-mono AC) and spirocyclic pentaerythritol diphosphate perfluorinated ether acrylate (SPDFA) manufactured through the above steps 2 and 3. In addition, Fig. 3 is the FT-IR spectrum results of spirocyclic pentaerythritol bisphosphorate diphosphoryl chloride (SPDPC) manufactured through the above step 1. 1 This is the H NMR spectrum result. In addition, Fig. 4 shows the spirocyclic pentaerythritol diphosphate perfluorinated ether acrylate (SPDFA) prepared through the above step 3. 1 This is the result of the H NMR spectrum. Example 1: Preparation of a composition for forming a gel polymer electrolyte Lithium hexafluorophosphate (LiPF6) having a concentration of 1.15 M was dissolved in a mixed solvent of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) (volume ratio 30 / 50 / 20), and then fluoroethylene carbonate (FEC) was added to prepare a liquid electrolyte. The FEC was added in an amount of 5 wt% in the liquid electrolyte. 9 g of the liquid electrolyte and 1 g of a spirocyclic pentaerythritol diphosphate perfluorinated ether acrylate (SPDFA) crosslinking compound were mixed, and 0.01 g of t-butylperoxypivalate, which is 1 part by weight per 100 parts by weight of the crosslinking compound, was added, and stirred to prepare a composition for forming a gel polymer electrolyte. Example 2: Preparation of gel polymer electrolyte The composition for forming a gel polymer electrolyte prepared in Example 1 was thermally crosslinked at 70°C for 1 hour to prepare a gel polymer electrolyte (SPDFA GPE) in which a polymer matrix was formed by the crosslinking compound. Example 3: Manufacturing of a lithium secondary battery Cathode active material (Li[Ni 0.6 Co 0.2 Mn 0.2 ]O2), a conductive agent (Super P), and a binder (PVdF) were added to N-methylpyrrolidone (NMP) solvent at a weight ratio of 95:3:2 to prepare a positive electrode slurry. The positive electrode slurry was applied to one surface of an aluminum positive electrode collector having a thickness of 15 μm, and dried and rolled to prepare a positive electrode. A negative electrode slurry was prepared by adding a negative electrode active material (graphite), a conductive agent (Super P), and a binder (PVdF) in a weight ratio of 90:3:7 to an N-methylpyrrolidone solvent. The negative electrode slurry was applied to one surface of a copper negative electrode collector having a thickness of 15 μm, and dried and rolled to prepare a negative electrode. An electrode assembly was manufactured by interposing a polyethylene porous separator between the positive and negative electrodes manufactured above, the electrode assembly was placed in a battery case, the composition for forming a gel polymer electrolyte of Example 1 was injected into the battery case, sealed, and then heat-treated at 70°C to manufacture a lithium secondary battery in which a gel polymer electrolyte was formed using the composition for forming a gel polymer electrolyte. Comparative Example 1: Preparation of liquid electrolyte A liquid electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1.15 M in a mixed solvent of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) (volume ratio 30 / 50 / 20), and then adding fluoroethylene carbonate (FEC). The fluoroethylene was added to the liquid electrolyte in an amount of 5 wt%. Comparative Example 2: Manufacturing of Lithium Secondary Battery A lithium secondary battery was manufactured in the same manner as in Example 3, except that the liquid electrolyte of Comparative Example 1 was used instead of the gel polymer electrolyte forming composition of Example 1 and that no separate heat treatment for forming the gel polymer electrolyte was performed. Comparative Example 3: Preparation of a composition for forming a gel polymer electrolyte A composition for forming a gel polymer electrolyte was prepared in the same manner as in Example 1, except that 1 g of a compound represented by the following chemical formula A was added to the composition for forming a gel polymer electrolyte instead of 1 g of a spirocyclic pentaerythritol diphosphate perfluorinated ether acrylate (SPDFA) cross-linking compound. [Chemical Formula A] (In the above chemical formula A, n1 is 3) Comparative Example 4: Preparation of gel polymer electrolyte The composition for forming a gel polymer electrolyte prepared in Comparative Example 3 was thermally crosslinked at 70°C for 1 hour to prepare a gel polymer electrolyte (SPDFA GPE) in which a polymer matrix was formed by the crosslinking compound. Comparative Example 5: Preparation of a composition for forming a gel polymer electrolyte A gel polymer electrolyte forming composition was prepared in the same manner as in Example 1, except that 1 g of a compound represented by the following chemical formula B was added to the gel polymer electrolyte forming composition instead of 1 g of a spirocyclic pentaerythritol diphosphate perfluorinated ether acrylate (SPDFA) cross-linking compound. [Chemical Formula B] (In the above chemical formula B, m1 is 3) Comparative Example 6: Preparation of gel polymer electrolyte The composition for forming a gel polymer electrolyte prepared in Comparative Example 5 was thermally crosslinked at 70°C for 1 hour to prepare a gel polymer electrolyte (SPDFA GPE) in which a polymer matrix was formed by the crosslinking compound. Comparative Example 7: Preparation of a composition for forming a gel polymer electrolyte A gel polymer electrolyte forming composition was prepared in the same manner as in Example 1, except that 1 g of a compound represented by the following chemical formula C was added to the gel polymer electrolyte forming composition instead of 1 g of a spirocyclic pentaerythritol diphosphate perfluorinated ether acrylate (SPDFA) crosslinking compound. [Chemical Formula C] Comparative Example 8: Preparation of gel polymer electrolyte The composition for forming a gel polymer electrolyte prepared in Comparative Example 7 was thermally crosslinked at 70°C for 1 hour to prepare a gel polymer electrolyte (SPDFA GPE) in which a polymer matrix was formed by the crosslinking compound. Experimental example Experimental Example 1: Confirmation of cross-linking reaction of gel polymer electrolyte To confirm whether the crosslinking reaction of the gel polymer electrolyte (SPDFA GPE) of Example 2 proceeded smoothly, FT-IR spectral analysis was performed. The results are shown in Fig. 5. C=C (1635 cm-) of SPDFA of Example 1 1 ) It is confirmed that the double bond peak disappears in the SPDFA GPE of Example 2, thereby confirming the completion of the crosslinking reaction and the formation of a gel polymer electrolyte. Experimental Example 2: Ionic Conductivity Measurement Using a CH device (CHI 600D) with the gel polymer electrolyte of Example 2 and the liquid electrolyte of Comparative Example 1, 10 to 10 were measured at an amplitude of 50 mV each. 6 The AC impedance between sandwich electrodes was measured over a frequency range of Hz, and the measurements were analyzed using a frequency response analyzer to determine the ionic conductivity as a function of temperature. The results are shown in Fig. 6. Referring to Figure 6, the gel polymer electrolyte of Example 2 has a density of 4 × 10 at room temperature. -3 S cm -1 It showed high ionic conductivity. Experimental Example 3: Electrochemical Stability Evaluation In order to compare the electrochemical stability of the gel polymer electrolyte of Example 2, the liquid electrolyte of Comparative Example 1, and the gel polymer electrolytes of Comparative Examples 4, 6, and 8, the current value according to the applied voltage was measured using linear sweep voltammetry (LSV) (scan speed 1 mV / s), and the results are shown in Fig. 7. Fig. 7 is a graph (oxidation scan) specifically showing values ​​according to the range of the x-axis of 3.0 to 6.0 V. Referring to FIG. 7, it can be seen that the gel polymer electrolyte of Example 2 exhibits improved electrochemical stability compared to the liquid electrolyte of Comparative Example 1, and the gel polymer electrolytes of Comparative Examples 4, 6, and 8 due to the characteristic structure of the crosslinked compound according to Chemical Formula 1. Experimental Example 4: Cycle Characteristics Evaluation The lithium secondary battery of Example 3 manufactured above was charged to 4.2 V at a constant current of 0.1 C at 25°C, and discharged to 2.5 V at a constant current of 0.1 C, which constituted one cycle, and 4 cycles of activation steps were performed. Afterwards, 300 cycles of charge and discharge were performed, with one cycle being charging to 4.2 V with a 0.2 C constant current at 25°C and discharging to 2.5 V with a 0.2 C constant current. Figure 8 shows charge / discharge curves of the 1st, 10th, 30th, 50th, 100th, 200th, and 300th cycles of the lithium secondary battery of Example 3. Figure 9 shows a graph showing the change in discharge capacity according to the cycle of the lithium secondary battery of Example 3. It was found that the lithium secondary battery of Example 3 using the gel polymer electrolyte of Example 3 performed charge and discharge cycles stably. Experimental Example 5: Flame Retardant Characteristics Evaluation (1) Experimental Example 5-1 To evaluate the flame retardant properties of the gel polymer electrolyte manufactured in Example 2 and the liquid electrolyte of Comparative Example 1, a combustion experiment was conducted using a flame torch. Figure 10 is a photograph showing the results before (left) and after (right) a combustion test of the liquid electrolyte of Comparative Example 1. Figure 11 is a photograph showing the results before (left) and after (right) a combustion test of the gel polymer electrolyte of Example 2. Referring to Fig. 10, the liquid electrolyte of Comparative Example 1 shows high flammability when applied with a flame torch. On the other hand, referring to FIG. 11, it can be seen that the gel polymer electrolyte of Example 2 exhibits excellent self-extinguishing properties and flame retardant properties that do not burn. (2) Experimental Example 5-2 To evaluate the flame retardancy properties of the gel polymer electrolyte manufactured in Example 2, the liquid electrolyte of Comparative Example 1, and the gel polymer electrolytes of Comparative Examples 4, 6, and 8, a combustion experiment was conducted using a flame torch. For the gel polymer electrolyte manufactured in Example 2, the liquid electrolyte of Comparative Example 1, and the gel polymer electrolytes of Comparative Examples 4, 6, and 8, a self-extinguishing time experiment was conducted to measure the time it takes for 1 g of each electrolyte to be ignited and for the sample to catch fire, thereby evaluating the flame retardancy. The results are shown in Table 1 below. Self-extinguishing time (sec) Example 20 Comparative Example 134.2 Comparative Example 42.5 Comparative Example 61.8 Comparative Example 86.1 Looking at Table 1 above, it can be seen that the gel polymer electrolyte for a lithium secondary battery manufactured in Example 2 of the present invention did not catch fire, whereas the liquid electrolyte of Comparative Example 1 and the gel polymer electrolytes of Comparative Examples 4, 6 and 8 showed inferior flame retardancy properties compared to the gel polymer electrolyte of Example 2, which showed complete non-flammability properties. Experimental Example 6: Evaluation of the degree of membrane shrinkage Two identical polyethylene membranes were prepared. One of two polyethylene separators was impregnated with the composition for forming a gel polymer electrolyte of Example 1, and then a cross-linking reaction was performed at 70°C for 1 hour to produce a polyethylene separator impregnated with a gel polymer electrolyte. In addition, no treatment was performed on the other polyethylene separator. A polyethylene separator that was not impregnated with a gel polymer electrolyte (i.e., not treated at all) was stored at 140°C for 30 minutes. Figure 12 shows a photograph of the polyethylene separator before storage at 140°C on the left, and a photograph of the polyethylene separator after storage at 140°C for 30 minutes on the right. Additionally, a polyethylene separator impregnated with a gel polymer electrolyte was stored at 140°C for 30 minutes. Figure 13 shows a photograph of a polyethylene separator before storage at 140°C on the left, and a photograph of a polyethylene separator after storage at 140°C for 30 minutes on the right. Referring to Fig. 12, it was found that the polyethylene separator without the gel polymer electrolyte according to the present invention exhibited significant shrinkage at high temperatures. Meanwhile, referring to Fig. 13, it can be confirmed that the polyethylene separator without the gel polymer electrolyte according to the present invention exhibited very low shrinkage when stored at 140°C and had high thermal stability at high temperatures. Experimental Example 7: Lithium Secondary Battery Short-Circuit Test The lithium secondary batteries of Example 3 and Comparative Example 2 were charged to 4.2 V at a constant current of 0.1 C at 25°C, stored in a hot box at 140°C, and the battery voltage over time was measured. The results are shown in Fig. 14. Referring to Fig. 14, the lithium secondary battery of Example 3, which applied the gel polymer electrolyte of Example 2, maintained the battery voltage stably even at high temperatures compared to the lithium secondary battery of Comparative Example 2, which used the liquid electrolyte of Comparative Example 1. Through these results, it can be seen that when the gel polymer electrolyte according to the present invention is applied, the thermal stability of the lithium secondary battery can be significantly improved by suppressing the micro-internal short-circuit phenomenon of the lithium secondary battery. Experimental Example 8: Measurement of electrode heat generation In the lithium secondary batteries of Example 3 and Comparative Example 2, the calorific value of the electrodes according to the electrolyte was measured using differential scanning calorimetry (DSC) to compare the calorific value of the negative electrode and the positive electrode. Specifically, the lithium secondary batteries manufactured in Example 3 and Comparative Example 2 above were charged to 4.2 V at 0.1 C constant current at 25°C, and discharged to 2.5 V at 0.1 C constant current, which was considered one cycle, and 4 cycles of activation steps were performed. Thereafter, the batteries were charged to 4.2 V at 0.2 C constant current at 25°C, discharged to 2.5 V at 0.2 C constant current, and charged and discharged once at 0.2 C constant current, and then fully charged again at 0.2 C constant current. After disassembling the lithium secondary batteries to recover the positive and negative electrode materials, the calorific value was measured through a DSC experiment. The DSC experiment was performed by heating from 0°C to 300°C at a rate of 10°C / min. The results of the DSC experiment for the cathode are shown in Fig. 15, and the results of the DSC experiment for the anode are shown in Fig. 16. Referring to FIGS. 15 and 16, it can be seen that the calorific value of both the negative electrode and the positive electrode of the lithium secondary battery of Example 3 is significantly reduced compared to that of Comparative Example 2. Through these results, it can be seen that the composition for forming a gel polymer electrolyte or the gel polymer electrolyte of the present invention can significantly improve the thermal safety of a lithium secondary battery.

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