Title of invention: electrolyte for lithium secondary battery Technical field [One] Mutual citation with related applications [2] This application claims the benefit of priority based on Korean Patent Application No. 10-2018-0095549 filed on August 16, 2018, and all contents disclosed in the documents of the Korean patent application are included as part of this specification. [3] Technical field [4] The present invention relates to an electrolyte for a lithium secondary battery, and more particularly, to an electrolyte for a lithium secondary battery having improved ionic conductivity and safety while having excellent capacity characteristics by using a high-concentration lithium salt. Background [5] Lithium secondary batteries are not only portable power sources for mobile phones, notebook computers, digital cameras and camcorders, but also power tools, electric bicycles, hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (plug-in). HEV, PHEV) and other medium and large power supplies are rapidly expanding their application. With the expansion of such application fields and the increase in demand, the external shape and size of the battery are variously changed. In order to meet these demands, the battery components must stably implement battery performance under conditions of high current flow. [6] Lithium secondary batteries are manufactured by using a material capable of intercalating and deintercalating lithium ions as a negative electrode and a positive electrode, selectively including a separator between the two electrodes, and placing an electrolyte between the positive electrodes. Electricity is generated or consumed by redox reactions resulting from the insertion and desorption of ions. [7] Meanwhile, with the recent expansion of application fields, the utilization and importance of lithium secondary batteries are gradually increasing. In particular, in an environment requiring high output such as HEV/PHEV, improvement of output characteristics of the battery is required. The output characteristic of a battery is a measure of how much current can be passed at a given voltage. In general, when the current increases, the output that can be obtained from the battery initially increases and then decreases after reaching the maximum value. . This is related to the polarization phenomenon, because the battery voltage decreases when the current increases above a specific value, and the capacity obtainable in a given voltage range decreases. Since such a polarization phenomenon is related to the diffusion rate of lithium ions and the internal resistance of the battery, it is necessary to improve the diffusion rate of lithium ions and electrical conductivity characteristics in order to improve the output characteristics of the battery. [8] Recently, as one method for improving the output characteristics of a battery, an electrolyte containing a high concentration of lithium salt is used to increase the lithium ion yield (Li+ transference number) and the degree of dissociation of lithium ions to improve the output characteristics of the battery. A plan is being devised. [9] When a high-concentration lithium salt is used, not only the output characteristics of the battery are improved, but also high temperature safety can be improved by reducing the amount of a free solvent that does not bind lithium ions. Specifically, when the battery is charged, lithium may be desorbed from the positive electrode active material, so that it is structurally unstable. When exposed to high temperature conditions, oxygen radicals may be generated as the structure collapses. Oxygen radicals generated at this time have very high reactivity, and may react with a free solvent that is not bound to lithium ions to cause an exothermic reaction. In this case, if a high concentration of lithium salt is used in the electrolyte, the amount of free solvent is reduced and the exothermic reaction is suppressed, thereby increasing the high temperature safety of the battery. [10] However, when a high-concentration lithium salt is used, the viscosity of the electrolyte increases. At this time, since the mobility of ions in the electrolyte is inversely proportional to the viscosity of the electrolyte according to Stokes' law, there may be a problem that the ionic conductivity of the electrolyte is degraded. [11] Therefore, it is time to need an electrolyte for a lithium secondary battery capable of improving the output characteristics, capacity characteristics, and high-temperature safety of the battery using a high concentration of lithium salt while maintaining the ionic conductivity of the battery at a certain level or higher. [12] (Patent Document 1) Korean Patent Publication No. 10-2016-0040127 Detailed description of the invention Technical challenge [13] The present invention is to solve the above problems, and to provide an electrolyte for a lithium secondary battery capable of improving high temperature safety while improving output characteristics and capacity characteristics of a lithium secondary battery by using a high concentration of lithium salt. [14] Means of solving the task [15] In one aspect, the present invention is an oligomer mixture comprising a lithium salt having a concentration of 1.6 M to 5 M, a first oligomer comprising a unit represented by the following formula (1), and a second oligomer comprising a unit represented by the following formula (2) And it provides an electrolyte for a lithium secondary battery comprising an organic solvent. [16] [Formula 1] [17] [18] In Formula 1, R a , R b , R c and R d are each independently a fluorine element or an alkyl group having 1 to 3 carbon atoms unsubstituted or substituted with a fluorine element, and p is an integer of 1 to 50. [19] [Formula 2] [20] [21] In Formula 2, R g , R h , R i and R j are each independently a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms, and s is an integer of 1 to 50. [22] Meanwhile, the concentration of the lithium salt may be 1.6 M to 4 M. [23] Alternatively, the concentration of the lithium salt may be 1.6 M to 3 M. [24] The first oligomer may be an oligomer represented by the following Formula 1a. [25] [Formula 1a] [26] [27] In Formula 1a, R a , R b , R c and R d are each independently a fluorine element or an alkyl group having 1 to 3 carbon atoms unsubstituted or substituted with a fluorine element, and R e is an aliphatic hydrocarbon group or an aromatic hydrocarbon Note, wherein R f is an alkylene group having 1 to 5 carbon atoms substituted or unsubstituted with a fluorine element, and R'is hydrogen or an alkyl group having 1 to 3 carbon atoms, and o is an integer of 1 to 3, and p is It is an integer of 1-50, and q is an integer of 1-15. [28] The first oligomer may be an oligomer represented by Formula 1b below. [29] [Formula 1b] [30] [31] In Formula 1b, R a , R b , R c and R d are each independently a fluorine element or an alkyl group having 1 to 3 carbon atoms unsubstituted or substituted with a fluorine element, and R e is an aliphatic hydrocarbon group or an aromatic A hydrocarbon group, wherein R f is an alkylene group having 1 to 5 carbon atoms substituted or unsubstituted with a fluorine element, the o'is an integer of 1 to 2, the o" is an integer of 1 to 3, and p is 1 It is an integer of to 50, and q is an integer of 1 to 15. [32] Meanwhile, the second oligomer may be an oligomer represented by the following Formula 2a. [33] [Formula 2a] [34] [35] In Formula 2a, R g , R h , R i and R j are each independently a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms, and R k is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, and R" is hydrogen Or an alkyl group having 1 to 3 carbon atoms, r is an integer of 1 to 3, s is an integer of 1 to 50, t is an integer of 1 to 20, and u is an integer of 1 to 10. [36] The second oligomer may be an oligomer represented by Formula 2b below. [37] [Formula 2b] [38] [39] In Formula 2b, R g , R h , R i and R j are each independently a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms, and R k is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, and r′ is 1 Is an integer of 2 to 2, r" is an integer of 1 to 3, s is an integer of 1 to 50, t is an integer of 1 to 20, and u is an integer of 1 to 10. [40] In another aspect, the electrolyte for a lithium secondary battery may further include a halogenated benzene compound represented by Formula 3 below, and the halogenated benzene compound represented by Formula 3 is 0.01 parts by weight or more relative to 100 parts by weight of the electrolyte for a lithium secondary battery. It may be included in less than parts by weight. [41] [Formula 3] [42] [43] In Formula 3, X is at least one halogen element selected from the group consisting of F, Cl, and Br, and n is an integer of 1 to 3. [44] In one aspect, the electrolyte for a lithium secondary battery may be a liquid electrolyte. [45] When the electrolyte for a lithium secondary battery is a liquid electrolyte, the oligomer mixture may be included in an amount of 0.01 parts by weight to 1 part by weight based on 100 parts by weight of the electrolyte for a lithium secondary battery. [46] When the electrolyte for a lithium secondary battery is a liquid electrolyte, the first oligomer and the second oligomer may be mixed in a weight ratio of (10 to 50): (50 to 90). [47] In another aspect, the electrolyte for a lithium secondary battery may be a gel polymer electrolyte. [48] When the electrolyte for a lithium secondary battery is a gel polymer electrolyte, the oligomer mixture may be included in an amount of 2 to 10 parts by weight based on 100 parts by weight of the electrolyte for a lithium secondary battery. [49] When the electrolyte for a lithium secondary battery is a gel polymer electrolyte, the first oligomer and the second oligomer may be mixed in a weight ratio of (1 to 50): (50 to 99). [50] When the electrolyte for a lithium secondary battery is a gel polymer electrolyte, a polymerization initiator may be further included. [51] Effects of the Invention [52] The electrolyte for a lithium secondary battery according to the present invention uses a high-concentration lithium salt, so that the ionic conductivity is high and the surface tension can be adjusted to a certain level or less, and the initial capacity of the battery and the high temperature safety of the battery can be improved. Best mode for carrying out the invention [53] Hereinafter, the present invention will be described in more detail. [54] The terms or words used in the specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventor may appropriately define the concept of terms in order to describe his own invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of ​​the present invention based on the principle that there is. [55] The terms used in the present specification are only used to describe exemplary embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. [56] In the present specification, terms such as "comprise", "include" or "have" are intended to designate the presence of implemented features, numbers, steps, components, or a combination thereof, but one or more other features or It is to be understood that the possibility of the presence or addition of numbers, steps, components, or combinations thereof is not preliminarily excluded. [57] In the present specification, the weight average molecular weight may mean a value converted to standard polystyrene measured by gel permeation chromatography (GPC), and unless otherwise specified, the molecular weight will mean the weight average molecular weight. I can. For example, in the present invention, measurements are made using Agilent's 1200 series under GPC conditions, and the used column may be an Agilent's PL mixed B column, and THF may be used as a solvent. [58] [59] Electrolyte for lithium secondary battery [60] The electrolyte for a lithium secondary battery according to the present invention includes a lithium salt having a concentration of 1.6 M to 5 M; It may be an electrolyte for a lithium secondary battery including an oligomer mixture including a first oligomer including a unit represented by Formula 1 and a second oligomer including a unit represented by Formula 2, and an organic solvent. [61] [62] The lithium salt may be included in a concentration of 1.6M to 5M, preferably 1.6M to 4M, and more preferably 1.6M to 3M in the electrolyte for a lithium secondary battery. When the lithium salt is contained within the above concentration range, when the lithium salt is contained within the above range, lithium ions are sufficiently supplied, and the lithium ion yield (Li+ transference number) and the degree of dissociation of lithium ions are improved, thereby improving the output characteristics of the battery. Can be. [63] In addition, when a high-concentration lithium salt is used, high-temperature safety can be improved by reducing the amount of a free solvent that does not bind to lithium ions. Specifically, when the battery is charged, lithium may be desorbed from the positive electrode active material and thus is structurally unstable. When exposed to high temperature conditions, oxygen radicals may be generated as the structure collapses. Since the oxygen radical generated at this time has very high reactivity, it may react with a free solvent to cause an exothermic reaction. In this case, if a high concentration of lithium salt is used, a large amount of lithium ions react with the free solvent, thereby reducing the amount of the free solvent, thereby suppressing the exothermic reaction, thereby increasing the high temperature safety of the battery. [64] Specifically, any compound capable of providing lithium ions used in a lithium secondary battery may be used without particular limitation. Specifically, the lithium salt, Li cation + a, and the anion include F - , Cl - , Br - , I - , NO 3 - , N (CN) 2 - , BF 4 - , ClO 4 - , AlO 4 - , AlCl 4 - , PF 6 - , SbF 6 - , AsF 6 - , BF 2 C 2 O 4 - , BC 4 O 8 - , (CF 3 ) 2 PF 4 - , (CF 3 ) 3 PF 3 - , (CF 3 ) 4 PF 2 - , (CF 3 ) 5 PF - , (CF 3 ) 6 P - , CF 3 SO 3 - , C 4 F 9 SO 3 - , CF 3 CF 2 SO 3 - , (CF 3 SO 2 ) 2 N - , (FSO 2 ) 2 N - , CF 3 CF 2 (CF 3 ) 2 CO - , (CF 3 SO 2 ) 2 CH - , CF 3 (CF 2 ) 7 SO 3 - , CF 3 CO 2 - , CH 3 CO 2 - , SCN - , and (CF 3 CF 2 SO 2 ) 2 N - according to one or need to be selected from the group consisting of can be used as a mixture of two or more. [65] [66] The electrolyte for a lithium secondary battery includes an oligomer mixture including a first oligomer including a unit represented by Formula 1 below and a second oligomer including a unit represented by Formula 2 below. [67] First, a first oligomer including a unit represented by the following formula (1) will be described. [68] [Formula 1] [69] [70] In Formula 1, R a , R b , R c and R d are each independently a fluorine element or an alkyl group having 1 to 3 carbon atoms unsubstituted or substituted with a fluorine element, and p is an integer of 1 to 50. The p may be preferably an integer of 1 to 45, more preferably an integer of 1 to 40. [71] [72] Since the first oligomer containing the unit represented by Formula 1 contains an ethylene group substituted with a fluorine element having low reactivity with lithium ions, it can control side reactions of lithium ions and decomposition reactions of lithium salts. Therefore, it is possible to suppress side reactions that occur when a high concentration of lithium salt is used. In addition, since the first oligomer contains a fluorine element having excellent flame retardancy, when an electrolyte containing the first oligomer is used, heat generation and ignition of the lithium secondary battery can be suppressed, thereby improving high temperature safety. [73] [74] Specifically, the first oligomer may be an oligomer represented by the following Formula 1a. [75] [Formula 1a] [76] [77] In Formula 1a, R a , R b , R c and R d are each independently a fluorine element or an alkyl group having 1 to 3 carbon atoms unsubstituted or substituted with a fluorine element, and R e is an aliphatic hydrocarbon group or an aromatic hydrocarbon Note, wherein R f is an alkylene group having 1 to 5 carbon atoms substituted or unsubstituted with a fluorine element, and R'is hydrogen or an alkyl group having 1 to 3 carbon atoms, and o is an integer of 1 to 3, and p is It is an integer of 1-50, and q is an integer of 1-15. At this time, p may be preferably an integer of 1 to 45, more preferably an integer of 1 to 40. [78] In the oligomer represented by Formula 1a, the aliphatic hydrocarbon group includes an alicyclic hydrocarbon group or a linear hydrocarbon group. [79] The alicyclic hydrocarbon group is a substituted or unsubstituted cycloalkylene group having 4 to 20 carbon atoms; A substituted or unsubstituted C4-20 cycloalkylene group containing an isocyanate group (NCO); A substituted or unsubstituted C4 to C20 cycloalkenylene group; And it may include at least one or more selected from the group consisting of a substituted or unsubstituted heterocycloalkylene group having 2 to 20 carbon atoms. [80] The linear hydrocarbon group is a substituted or unsubstituted alkylene group having 1 to 20 carbon atoms; A substituted or unsubstituted C1-C20 alkylene group containing an isocyanate group (NCO); A substituted or unsubstituted C 1 to C 20 alkoxyl group; A substituted or unsubstituted alkenylene group having 2 to 20 carbon atoms; And at least one selected from the group consisting of a substituted or unsubstituted alkynylene group having 2 to 20 carbon atoms. [81] In addition, in the oligomer represented by Formula 1a, the aromatic hydrocarbon group may include a substituted or unsubstituted arylene group having 6 to 20 carbon atoms; Or a substituted or unsubstituted C2 to C20 heteroarylene group may be included. [82] For a specific example, the oligomer represented by Formula 1a may be an oligomer represented by Formula 1a-1 below. [83] [Formula 1a-1] [84] [85] In Formula 1a-1, p is an integer of 1 to 50, and q is an integer of 1 to 15. The p may be preferably an integer of 1 to 45, more preferably an integer of 1 to 40. [86] Alternatively, the first oligomer may be an oligomer represented by Formula 1b below. [87] [Formula 1b] [88] [89] In Formula 1b, R a , R b , R c and R d are each independently a fluorine element or an alkyl group having 1 to 3 carbon atoms unsubstituted or substituted with a fluorine element, and R e is an aliphatic hydrocarbon group or an aromatic A hydrocarbon group, wherein R f is an alkylene group having 1 to 5 carbon atoms substituted or unsubstituted with a fluorine element, the o'is an integer of 1 to 2, the o" is an integer of 1 to 3, and p is 1 It is an integer of to 50, and q is an integer of 1 to 15. The p may be an integer of preferably 1 to 45, more preferably an integer of 1 to 40. [90] [91] For a specific example, the oligomer represented by Formula 1b may be an oligomer represented by Formula 1b-1 below. [92] [Formula 1b-1] [93] [94] In Formula 1b-1, p is an integer of 1 to 50, and q is an integer of 1 to 15. [95] At this time, p is preferably an integer of 1 to 45, more preferably an integer of 1 to 40. [96] [97] When the first oligomer represented by Formula 1a and Formula 1b is included as an electrolyte for a lithium secondary battery, it contains a unit represented by Formula 1, thus maintaining the properties and forming crosslinks at both ends of itself Since it contains an acrylate-based functional group that can be a hydrophilic moiety and a fluorine-substituted ethylene group that is a hydrophobic moiety, it is further given a role of a surfactant in the battery, and the surface resistance with the electrode interface can be lowered. Accordingly, when the first oligomer represented by Formulas 1a and 1b is used, the wettability effect of the lithium secondary battery may be improved. [98] However, when only the first oligomer represented by Chemical Formulas 1a and 1b is used in the electrolyte alone, there is a problem that it is difficult to control the reactivity of the acrylate group. In particular, when applied to a gel polymer electrolyte, the pre-gelation phenomenon between the first oligomers has already been expressed in the wetting process step prior to the process of curing the first oligomer, so that the gel polymer electrolyte cannot be uniformly formed. Can occur. [99] Therefore, in order to solve such a problem, a second oligomer containing a unit represented by Formula 2 is used together, and the reactivity of the acrylate group included in the first oligomer represented by Formula 1a and Formula 1b is adjusted. You can solve the same problem. [100] Specifically, in the case of the second oligomer, a carbonate group is included, and since the carbonate group contains oxygen (O), the free electron density is high. Accordingly, the degree of radical reaction of the acrylate group can be controlled. [101] [102] At this time, the weight average molecular weight (MW) of the first oligomer represented by Chemical Formulas 1a and 1b may be adjusted by the number of repeating units, from about 500 g/mol to 200,000 g/mol, specifically 1,000 g/mol To 150,000 g/mol, more specifically 2,000 g/mol to 100,000 g/mol. When the weight average molecular weight of the first oligomer is within the above range, the affinity with the organic solvent is high, so that dispersion can be made well, the surface tension is reduced to a certain level or less to improve the wettability of the electrolyte, and the lithium salt decomposition reaction It is possible to suppress and prevent lithium ions from causing side reactions. [103] [104] Next, a second oligomer including the unit represented by Chemical Formula 2 will be described. [105] [Formula 2] [106] [107] In Formula 2, R g , R h , R i and R j are each independently a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms, and s is an integer of 1 to 50. [108] The s may be preferably an integer of 1 to 45, more preferably an integer of 1 to 40. [109] Meanwhile, the second oligomer may be an oligomer represented by the following Formula 2a. [110] [Formula 2a] [111] [112] In Formula 2a, R g , R h , R i and R j are each independently a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms, and R k is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, and R" is hydrogen Or an alkyl group having 1 to 3 carbon atoms, r is an integer of 1 to 3, s is an integer of 1 to 50, t is an integer of 1 to 20, and u is an integer of 1 to 10. [113] The s may be preferably an integer of 1 to 45, more preferably an integer of 1 to 40. [114] In the oligomer represented by Formula 2a, the aliphatic hydrocarbon group includes an alicyclic hydrocarbon group or a linear hydrocarbon group. [115] The alicyclic hydrocarbon group is a substituted or unsubstituted cycloalkylene group having 4 to 20 carbon atoms; A substituted or unsubstituted C4-20 cycloalkylene group containing an isocyanate group (NCO); A substituted or unsubstituted C4 to C20 cycloalkenylene group; And it may include at least one or more selected from the group consisting of a substituted or unsubstituted heterocycloalkylene group having 2 to 20 carbon atoms. [116] The linear hydrocarbon group is a substituted or unsubstituted alkylene group having 1 to 20 carbon atoms; A substituted or unsubstituted C1-C20 alkylene group containing an isocyanate group (NCO); A substituted or unsubstituted C 1 to C 20 alkoxyl group; A substituted or unsubstituted alkenylene group having 2 to 20 carbon atoms; And at least one selected from the group consisting of a substituted or unsubstituted alkynylene group having 2 to 20 carbon atoms. [117] In addition, in the oligomer represented by Formula 2a, the aromatic hydrocarbon group may include a substituted or unsubstituted arylene group having 6 to 20 carbon atoms; Or a substituted or unsubstituted C2 to C20 heteroarylene group may be included. [118] For a specific example, the oligomer represented by Formula 2a may be an oligomer represented by Formula 2a-1 below. [119] [Formula 2a-1] [120] [121] The s is an integer of 1 to 50, t is an integer of 1 to 20, and u is an integer of 1 to 10. The s may be preferably an integer of 1 to 45, more preferably an integer of 1 to 40. [122] [123] Alternatively, the second oligomer may be an oligomer represented by the following Formula 2b. [124] [Formula 2b] [125] [126] In Formula 2b, R g , R h , R i and R j are each independently a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms, and R k is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, and r′ is 1 Is an integer of 2 to 2, r" is an integer of 1 to 3, s is an integer of 1 to 50, t is an integer of 1 to 20, and u is an integer of 1 to 10. [127] In the oligomer represented by Formula 2b, the aliphatic hydrocarbon group includes an alicyclic hydrocarbon group or a linear hydrocarbon group. [128] The alicyclic hydrocarbon group is a substituted or unsubstituted cycloalkylene group having 4 to 20 carbon atoms; A substituted or unsubstituted C4-20 cycloalkylene group containing an isocyanate group (NCO); A substituted or unsubstituted C4 to C20 cycloalkenylene group; And it may include at least one or more selected from the group consisting of a substituted or unsubstituted heterocycloalkylene group having 2 to 20 carbon atoms. [129] The linear hydrocarbon group is a substituted or unsubstituted alkylene group having 1 to 20 carbon atoms; A substituted or unsubstituted C1-C20 alkylene group containing an isocyanate group (NCO); A substituted or unsubstituted C 1 to C 20 alkoxyl group; A substituted or unsubstituted alkenylene group having 2 to 20 carbon atoms; And at least one selected from the group consisting of a substituted or unsubstituted alkynylene group having 2 to 20 carbon atoms. [130] In addition, in the oligomer represented by Formula 2b, the aromatic hydrocarbon group may include a substituted or unsubstituted arylene group having 6 to 20 carbon atoms; Or a substituted or unsubstituted C2 to C20 heteroarylene group may be included. [131] [132] For a specific example, the oligomer represented by Formula 2b may be an oligomer represented by Formula 2b-1 below. [133] [Formula 2b-1] [134] [135] The s is an integer of 1 to 50, t is an integer of 1 to 20, and u is an integer of 1 to 10. The s may be preferably an integer of 1 to 45, more preferably an integer of 1 to 40. [136] [137] The second oligomer including the unit represented by Chemical Formula 2 contains a carbonate group, and the carbonate group has high affinity with an organic solvent and has excellent ability to dissociate lithium salts, and thus lithium ion solvation ) Can be improved. [138] However, when only the second oligomer is used in the electrolyte alone, there is a problem in that the surface tension and viscosity are high, so that the wetting of the electrolyte may be relatively deteriorated. [139] In this case, when the first oligomer containing the fluorine element is used together, the first oligomer acts as a surfactant, thereby reducing the surface tension of the electrolyte. Therefore, when the first oligomer and the second oligomer are used together, the degree of dissociation of the lithium salt of the electrolyte is improved to improve the output characteristics of the battery and at the same time, the wettability of the electrolyte is maintained at a certain level or higher, so that the interface between the electrolyte and the electrode is also You can keep it excellent. [140] [141] At this time, the weight average molecular weight (MW) of the second oligomer represented by Chemical Formulas 2a and 2b may be adjusted by the number of repeating units, from about 500 g/mol to 200,000 g/mol, specifically 1,000 g/mol To 150,000 g/mol, more specifically 2,000 g/mol to 100,000 g/mol. When the weight average molecular weight of the second oligomer is within the above range, the second oligomer is dissolved at a certain level in the electrolyte, so that dispersibility may be maintained. [142] [143] Meanwhile, the electrolyte for a lithium secondary battery may further include a halogenated benzene compound represented by Formula 3 below in order to further improve charge/discharge cycle characteristics of the battery and improve flame retardancy of the electrolyte. [144] [Formula 3] [145] [146] In Formula 3, X is at least one halogen element selected from the group consisting of F, Cl, and Br, and n is an integer of 1 to 3. [147] In this case, the halogenated benzene compound represented by Formula 3 may be included in an amount of 0.01 parts by weight or more and less than 50 parts by weight, more preferably 2 parts by weight to 40 parts by weight, based on 100 parts by weight of the electrolyte for the lithium secondary battery. [148] Specific examples of the halogenated benzene compound represented by Chemical Formula 3 include fluorobenzene, difluorobenzene, trichlorobenzene, chlorobenzene, dichlorobenzene, trichlorobenzene, bromobenzene, dibromobenzene, chlorofluorobenzene, Bromofluorobenzene, and the like, and one or more of them may be used in combination. [149] [150] Specifically, the electrolyte for a lithium secondary battery may be a liquid electrolyte or a gel polymer electrolyte. At this time, the content of the oligomer mixture and the mixing ratio between the first and second oligomers also vary according to the type of electrolyte for the lithium secondary battery. [151] For example, when the electrolyte for a lithium secondary battery is a liquid electrolyte, the oligomer mixture is 0.01 parts by weight to 1 part by weight, preferably 0.025 parts by weight to 0.75 parts by weight, more preferably based on 100 parts by weight of the lithium secondary battery electrolyte. May be included in 0.05 parts by weight to 0.75 parts by weight. When the oligomer mixture is included within the above range, an electrolyte having excellent wetting and a high degree of dissociation of a lithium salt may be provided. [152] On the other hand, when the electrolyte for a lithium secondary battery is a liquid electrolyte, the first oligomer and the second oligomer are (10 to 50): (50 to 90) weight ratio, preferably (10 to 45): (55 to 90), More preferably, it is mixed in a weight ratio of (10 to 40): (60 to 90). When the first and second oligomers are mixed in the above range, the surface tension of the electrolyte is maintained at a certain level, so that the wettability of the electrolyte may be controlled. [153] [154] For another example, when the electrolyte for a lithium secondary battery is a gel polymer electrolyte, the oligomer mixture is 2 to 10 parts by weight, preferably 2 to 9 parts by weight, based on 100 parts by weight of the electrolyte for a lithium secondary battery, More preferably, it may be included in 2 parts by weight to 8 parts by weight. When the oligomer mixture is included within the above range, the oligomers may stably form a polymer network structure, and the wetting of the electrolyte may be maintained above a certain level. On the other hand, the mobility of lithium ions is improved, and even if the viscosity is slightly increased by using a high-concentration lithium salt, the ionic conductivity of lithium ions can be maintained above a certain level. [155] On the other hand, when the electrolyte for a lithium secondary battery is a gel polymer electrolyte, the first oligomer and the second oligomer are (1 to 50): (50 to 99) weight ratio, preferably (1 to 45): (55 to 99) , More preferably, they are mixed in a weight ratio of (1 to 40): (60 to 99). When the first and second oligomers are mixed in the above range, the surface tension of the electrolyte is maintained at a certain level, so that the wettability of the electrolyte may be controlled. [156] On the other hand, when the electrolyte for a lithium secondary battery is a gel polymer electrolyte, a polymerization initiator is further included. Depending on the type of polymerization reaction, thermal polymerization or photopolymerization may be used, and the polymerization initiator may be a conventional polymerization initiator known in the art. For example, it may be at least one selected from the group consisting of an azo compound, a peroxide compound, or a mixture thereof. [157] Specifically, the polymerization initiator, benzoyl peroxide (benzoyl peroxide), acetyl peroxide (acetyl peroxide), dilauryl peroxide (dilauryl peroxide), di-tert-butyl peroxide (di-tert-butyl peroxide), t -Organic peroxides such as t-butyl peroxy-2-ethyl-hexanoate, cumyl hydroperoxide and hydrogen peroxide, or hydroperoxidation Logistics and 2,2'-azobis (2-cyanobutane), dimethyl 2,2'-azobis (2-methylpropionate), 2,2'-azobis (methylbutyronitrile), 2, 2'-Azobis (isobutyronitrile) (AIBN; 2,2'-Azobis (iso-butyronitrile)) and 2,2'-azobisdimethyl-valeronitrile (AMVN; 2,2'-Azobisdimethyl-Valeronitrile ), but at least one azo compound selected from the group consisting of, but is not limited thereto. [158] The polymerization initiator may be included in an amount of 0.1% to 5% by weight, preferably 0.1% to 4% by weight, more preferably 0.1% to 3% by weight based on the total weight of the oligomer mixture. When the polymerization initiator is included within the above range, the remaining unreacted polymerization initiator may be minimized, and gelation may be performed at a certain level or higher. [159] Meanwhile, when the electrolyte for a lithium secondary battery according to the present invention is a gel polymer electrolyte, the gel polymer electrolyte may be formed through a curing process. At this time, in order to cure with a gel polymer electrolyte, it may be cured by an electron beam (E-BEAM), gamma ray, room temperature or high temperature aging process. According to an embodiment of the present invention, it may be performed through thermal curing. In this case, the curing time may take about 2 minutes to 48 hours, and the thermal curing temperature may be 40°C to 100°C, specifically 40°C to 80°C. [160] [161] As the organic solvent, those commonly used in an electrolyte for a lithium secondary battery may be used without limitation. For example, an ether compound, an ester compound, an amide compound, a linear carbonate compound, or a cyclic carbonate compound may be used alone or in combination of two or more. [162] Specific examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, and fluoroethylene carbonate (FEC). In addition, specific examples of the linear carbonate compound include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethylmethyl carbonate (EMC), methylpropyl carbonate, and ethylpropyl carbonate. Any one selected from or a mixture of two or more of them may be representatively used, but is not limited thereto. [163] In particular, among the carbonate-based organic solvents, cyclic carbonates such as ethylene carbonate and propylene carbonate, which are known to dissociate lithium salts in the electrolyte well due to high dielectric constant as an organic solvent of high viscosity, may be used. In addition to these cyclic carbonates, dimethyl carbonate and diethyl An electrolyte having a high electrical conductivity can be prepared by mixing and using a low viscosity, low dielectric constant linear carbonate such as carbonate in an appropriate ratio. [164] In addition, as the ether compound, any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methylpropyl ether, and ethylpropyl ether, or a mixture of two or more of them may be used. It is not limited. [165] And the ester compound includes linear esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate; And any one selected from the group consisting of cyclic esters such as γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone, or a mixture of two or more thereof may be used. However, it is not limited thereto. [166] [167] Meanwhile, the electrolyte for a lithium secondary battery according to the present invention may further include an additive. As specific examples of additives, vinylene carbonate (VC), propane sultone (PS), polyphenylene sulfide, succinnitrile (SN), propene sultone (PRS), vinyl ethylene carbonate, lithium difluoro (oxalato ) One or more compounds selected from the group consisting of borate (LiODFB), ethylene sulfate, adiponitrile, and lithium bis(oxalato) borate may be used as an additive. When the compounds listed above are used together as an additive, a stable film can be formed on the anode and the cathode at the same time. At this time, by the film formed on the negative electrode, it is possible to suppress decomposition of the electrolyte even under high temperature and high pressure conditions, as well as suppressing the elution of the transition metal contained in the positive electrode by the film formed on the positive electrode. , High pressure characteristics and stability can be improved. [168] [169] Next, a lithium secondary battery according to the present invention will be described. [170] A lithium secondary battery according to an embodiment of the present invention includes at least one positive electrode, at least one negative electrode, a separator selectively disposed between the positive electrode and the negative electrode, and an electrolyte for the lithium secondary battery. In this case, since the electrolyte for the lithium secondary battery is the same as the above-described contents, a detailed description will be omitted. [171] [172] The positive electrode may be prepared by coating a positive electrode active material slurry including a positive electrode active material, an electrode binder, an electrode conductive material, and a solvent on the positive electrode current collector. [173] The positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes to the battery, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon on the surface of aluminum or stainless steel. , Nickel, titanium, silver, or the like may be used. [174] The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium, and specifically, may include at least one metal such as cobalt, manganese, nickel or aluminum, and a lithium composite metal oxide containing lithium. have. More specifically, the lithium composite metal oxide is a lithium-manganese-based oxide (eg, LiMnO 2 , LiMn 2 O 4, etc.), a lithium-cobalt-based oxide (eg, LiCoO 2, etc.), a lithium-nickel-based oxide (E.g., LiNiO 2 ), lithium-nickel-manganese oxide (e.g., LiNi 1-Y1 Mn Y1 O 2 (here, 0