Cross-Citation with Related Application(s) [2] This application claims the benefit of priority based on Korean Patent Application No. 2019-0006405 dated January 17, 2019 and Korean Patent Application No. 2020-0005939 dated January 16, 2020, and all The content is incorporated as part of this specification. [3] [4] technical field [5] The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, to an electrolyte for a lithium secondary battery having improved battery performance at high voltage or high temperature. background [6] As personal IT devices and computer networks are developed due to the development of the information society, and the overall society's dependence on electric energy increases accordingly, technology development for efficiently storing and utilizing electric energy is required. [7] Among the technologies developed for this purpose, the most suitable technology for various uses is a secondary battery-based technology. In the case of a secondary battery, it is possible to miniaturize it to the extent that it can be applied to personal IT devices and the like, and since it can be applied to electric vehicles and power storage devices, interest in this is emerging. Among these secondary batteries, a lithium ion battery, which is a battery system with high energy density, is in the spotlight and is currently being applied to various devices. [8] In the case of lithium ion battery systems, unlike the early days when lithium metal was directly applied to the system, a transition metal oxide material containing lithium was used as a cathode material, and a carbon-based material such as graphite and an alloy-based material such as silicon were used as an anode material. It is implemented as a system in which lithium metal is not directly used inside the battery, such as applying a material as an anode. [9] In the case of such a lithium ion battery, it is largely composed of a positive electrode composed of a transition metal oxide containing lithium, a negative electrode capable of storing lithium, an electrolyte as a medium for transferring lithium ions, and a separator. As it is known as a component that has a great influence on stability and safety, many studies are being conducted on it. [10] In the case of an electrolyte for a lithium ion battery, it is composed of a lithium salt, an organic solvent dissolving the same, and a functional additive, and proper selection of these components is important in order to improve the electrochemical properties of the battery. Representative lithium salts currently used include LiPF 6 , LiBF 4 , LiFSI (lithium fluorosulfonyl imide, LiN(SO 2 F) 2 ), LiTFSI (lithium (bis)trifluoromethanesulfonyl imide, LiN(SO 2 CF 3 ) 2 ) or LiBOB ( lithium bis(oxalate) borate, LiB(C 2 O 4 ) 2 ), etc. are used, and in the case of an organic solvent, a carbonate-based organic solvent, an ester-based organic solvent, or an ether-based organic solvent is used. [11] In the case of such a lithium ion battery, an increase in resistance and a decrease in capacity during charging/discharging or storage at high temperatures are suggested as major problems in performance degradation, and one of the causes of these problems is degradation of electrolyte at high temperature. It is a side reaction that occurs due to decomposition of salts at high temperature, among others. When the by-product of these salts decomposes the film formed on the surface of the anode and the cathode after activation, there is a problem of lowering the passivation ability of the film, thereby causing additional decomposition of the electrolyte and accompanying self-discharge. there is [12] Among the electrode materials of lithium ion batteries, especially in the case of negative electrodes, graphite-based negative electrodes are mostly used. In the case of graphite, its operating potential is 0.3 V (vs. Li/Li+) or less. It is lower than the stability window, and the electrolyte currently used is reduced and decomposed. This reductive decomposition product forms a solid electrolyte interphase (SEI) film that transmits lithium ions but inhibits further decomposition of the electrolyte. [13] However, when the SEI film does not have sufficient passivation ability to suppress the further decomposition of the electrolyte, the electrolyte is additionally decomposed during storage and the charged graphite is self-discharged, and consequently the potential of the entire battery is lowered. Therefore, in order to maintain the passivation ability of SEI at high temperature , HF, PF 5, etc. , which are decomposition products such as LiPF 6 , which are representative lithium salts generated due to heat/moisture, etc. , are removed to suppress damage to the SEI film, or There is an urgent need to propose and introduce an additive that can further form an additional stable film on the SEI film formed in . [14] Prior art literature [15] Japanese Laid-Open Patent Publication No. 2003-217655 DETAILED DESCRIPTION OF THE INVENTION technical challenge [16] The present invention is to solve the above problems, and the electrolyte for a lithium secondary battery having improved high-temperature characteristics of a lithium secondary battery by suppressing side reactions caused by by-products generated when lithium salts are decomposed at high voltage or high temperature, and including the same The invention relates to a lithium secondary battery. means of solving the problem [17] According to one embodiment, the present invention is an additive comprising a compound represented by the following formula (1); an oligomer including a unit represented by the following formula (2) and including an acrylate group at the terminal; lithium salt; and an organic solvent; provides an electrolyte for a lithium secondary battery comprising. [18] [Formula 1] [19] [20] In Formula 1, R 1 is an alkyl group having 1 to 5 carbon atoms in which a halogen element is substituted or unsubstituted, an alkoxy group having 1 to 5 carbon atoms in which a halogen element is substituted or unsubstituted, an alkyl group having 1 to 3 carbon atoms is substituted or An unsubstituted phenyl group and an alkyl group having 1 to 5 carbon atoms are selected from the group consisting of a substituted or unsubstituted amine group. [21] [Formula 2] [22] [23] In Formula 2, Ra, Rb, Rc, and Rd are each independently a fluorine element or an alkyl group having 1 to 3 carbon atoms substituted or unsubstituted with an elemental fluorine, and p is an integer from 1 to 50. [24] [25] According to another embodiment, the present invention provides a positive electrode; cathode; And it provides a lithium secondary battery comprising the electrolyte for a lithium secondary battery of the present invention. Effects of the Invention [26] Since the electrolyte for a lithium secondary battery according to the present invention contains specific oligomers and additives, it has excellent high-temperature performance and can minimize deterioration of battery performance even when the voltage is increased. Best mode for carrying out the invention [27] Hereinafter, the present invention will be described in more detail. [28] The terms or words used in the present specification and claims should not be construed as being limited to their ordinary or dictionary meanings, and the inventor may properly define the concept of the term in order to best describe his invention. It should be interpreted as meaning and concept consistent with the technical idea of ​​the present invention based on the principle that there is. [29] The terminology used herein is used to describe exemplary embodiments only, and is not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly dictates otherwise. [30] In the present specification, terms such as "comprise", "comprising" or "having" are intended to designate the presence of an embodied feature, number, step, element, or a combination thereof, but one or more other features or It should be understood that the existence or addition of numbers, steps, elements, or combinations thereof is not precluded in advance. [31] 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 may mean a weight average molecular weight. can For example, in the present invention, measurement is performed using Agilent's 1200 series under GPC conditions, and the column used at this time may be Agilent's PL mixed B column, and the solvent may be THF. [32] [33] Electrolyte for lithium secondary battery [34] The electrolyte for a lithium secondary battery according to the present invention includes an additive comprising a compound represented by Formula 1; an oligomer including a unit represented by Formula 2 and including an acrylate group at the terminal; lithium salt; and organic solvents. [35] [36] Hereinafter, each component of the electrolyte for a lithium secondary battery of the present invention will be described in more detail. [37] [38] (1) additives [39] First, the additive will be described. The additive includes a compound represented by the following Chemical Formula 1, and other additives may be further added depending on the type of electrode used or the use of the battery. [40] [Formula 1] [41] [42] In Formula 1, R 1 is an alkyl group having 1 to 5 carbon atoms in which a halogen element is substituted or unsubstituted, an alkoxy group having 1 to 5 carbon atoms in which a halogen element is substituted or unsubstituted, or an alkyl group having 1 to 3 carbon atoms is substituted or An unsubstituted phenyl group and an alkyl group having 1 to 5 carbon atoms may be selected from the group consisting of a substituted or unsubstituted amine group. [43] When the driving voltage of a lithium secondary battery increases or the battery is exposed to high temperatures, transition metals are eluted from the positive electrode, and an unstable SEI (Solid Electrolyte Interphase) film is formed on the surface of the negative electrode. There is a problem that this cannot be suppressed. [44] In the case of the present invention, in order to solve the above problems, a compound represented by Formula 1 is further formed on the positive/negative interface to prevent the electrolyte decomposition reaction and the SEI membrane from being decomposed. was added to the electrolyte and used. The compound represented by Formula 1 may form a Solid Electrolyte Interphase (SEI) film on the positive electrode interface, thereby preventing transition metal ions eluted from the positive electrode active material from adhering to the negative electrode, thereby improving the deterioration of battery performance. In addition, by forming a passive layer on the interface between the anode and the cathode, it is possible to suppress the further decomposition reaction of the electrolyte. [45] As a specific example, the compound represented by Chemical Formula 1 may be at least one selected from the group consisting of compounds represented by the following Chemical Formulas 1A to 1E. [46] [Formula 1A] [47] [48] In Formula 1A, n is an integer of 0 to 4. [49] [50] [Formula 1B] [51] [52] In Formula 1B, m is an integer of 0 to 4, wherein X, X' and X" are each independently one of hydrogen or a halogen element, and at least one is a halogen element. [53] [54] [Formula 1C] [55] [56] In Formula 1C, k is an integer of 0 to 4, Y, Y' and Y" are each independently one of hydrogen or a halogen element, and at least one is a halogen element. [57] [58] [Formula 1D] [59] [60] In Formula 1D, s is an integer of 0 to 2. [61] [62] [Formula 1E] [63] [64] In Formula 1E, R 2 and R 3 are each independently hydrogen or an alkyl group having 1 to 5 carbon atoms. [65] [66] More specifically, the compound represented by Chemical Formula 1A may be a compound represented by the following Chemical Formulas 1A-1 to 1A-3. [67] [Formula 1A-1] [68] [69] [70] [Formula 1A-2] [71] [72] [73] [Formula 1A-3] [74] [75] [76] In addition, the compound represented by Formula 1B may be a compound represented by Formula 1B-1 below. [77] [Formula 1B-1] [78] [79] [80] In addition, the compound represented by Formula 1C may be at least one selected from the group consisting of compounds represented by the following Formulas 1C-1 to 1C-5. [81] [Formula 1C-1] [82] [83] [84] [Formula 1C-2] [85] [86] [87] [Formula 1C-3] [88] [89] [90] [Formula 1C-4] [91] [92] [93] [Formula 1C-5] [94] [95] [96] In addition, the compound represented by Formula 1D may be a compound represented by Formula 1D-1 below. [97] [Formula 1D-1] [98] [99] [100] In addition, the compound represented by Formula 1E may be a compound represented by Formula 1E-1 below. [101] [Formula 1E-1] [102] [103] [104] On the other hand, the compound represented by Formula 1 is 0.1 parts by weight to 5 parts by weight, preferably 0.1 parts by weight to 3 parts by weight, more preferably 0.1 parts by weight to 1 part by weight based on 100 parts by weight of the electrolyte for a lithium secondary battery. may be included. When the compound represented by Formula 1 is included within the above range, it is possible to effectively remove lithium salt byproducts such as PF 5 while controlling the increase in internal resistance . [105] [106] On the other hand, the additive according to the present invention, in addition to the above-described components, according to the use of the battery, the internal configuration of the battery, etc., in order to give the effect of reducing the resistance in the battery, other additives that can implement these properties known in the art, etc. may contain more. The other additives include, for example, vinylene carbonate (Vinylene Carbonate, VC), vinyl ethylene carbonate (VEC), propane sultone (PS), succinonitrile (SN), Adiponitrile (AdN), ethylene sulfate (ESa), propene sultone (PRS), fluoroethylene carbonate (FEC), lithium difluorophosphate (LiPO 2 F 2 ), lithium difluoro (oxalate) borate (LiODFB), lithium bis- (oxalato) borate (LiBOB), 3-trimethoxysila Other additives such as nyl-propyl-N-aniline (3-trimethoxysilanyl-propyl-N-aniline, TMSPa), tris(trimethylsilyl)phosphate ((Tris(trimethylsilyl) Phosphite), TMSPi) may be used. [107] [108] (2) oligomers [109] Next, an oligomer including a unit represented by the following formula (2) and including an acrylate at the terminal will be described. [110] [Formula 2] [111] [112] In Formula 2, Ra, Rb, Rc, and Rd are each independently a fluorine element or an alkyl group having 1 to 3 carbon atoms substituted or unsubstituted with an elemental fluorine, and p is an integer from 1 to 50. [113] Since the oligomer including the unit represented by Formula 2 and including an acrylate group at the terminal contains an ethylene group substituted with a fluorine element having low reactivity with lithium ions, a side reaction of lithium ions and a lithium salt (salt) It is possible to control the decomposition reaction and the like, and it is possible to suppress side reactions occurring when a high concentration of lithium salt is used. In addition, since the oligomer contains elemental fluorine having excellent flame retardancy, when an electrolyte including the oligomer is used, heat generation and ignition of the lithium secondary battery may be suppressed, thereby improving high-temperature safety. [114] On the other hand, since the oligomer includes a unit containing a fluorine element having hydrophobicity and at the same time a hydrophilic acrylate group at the terminal, it serves as a surfactant to lower the surface resistance with the electrode interface, and a lithium secondary battery of the wetting effect (wetting) can be improved. [115] Specifically, the oligomer may be an oligomer represented by the following Chemical Formula 2A. [116] [Formula 2A] [117] [118] In Formula 2A, [119] Wherein 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 an element fluorine, [120] Wherein R e is an aliphatic hydrocarbon group or an aromatic hydrocarbon group, [121] Wherein R f is an alkylene group having 1 to 5 carbon atoms unsubstituted or substituted with a fluorine element, [122] Wherein R' is hydrogen or an alkyl group having 1 to 3 carbon atoms, [123] Wherein o is an integer from 1 to 3, [124] Wherein p is an integer from 1 to 50, [125] Wherein q is an integer of 1 to 15. [126] In this case, p may be an integer of preferably 1 to 45, more preferably an integer of 1 to 40. [127] In the oligomer represented by Formula 2A, 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-C20 cycloalkylene group containing an isocyanate group (NCO); a substituted or unsubstituted C4-C20 cycloalkenylene group; And it may include at least one 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 C 1 to C 20 alkylene group; A substituted or unsubstituted C1-C20 alkylene group containing an isocyanate group (NCO); A substituted or unsubstituted C1-C20 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 2A, the aromatic hydrocarbon group may be a substituted or unsubstituted C 6 to C 20 arylene group; Or it may include a substituted or unsubstituted heteroarylene group having 2 to 20 carbon atoms. [131] As a specific example, the oligomer represented by Formula 2A may be an oligomer represented by Formula 2A-1 below. [132] [Formula 2A-1] [133] [134] In Formula 2A-1, p' is an integer from 1 to 50, and q is an integer from 1 to 15. The p' may be an integer of preferably 1 to 45, more preferably an integer of 1 to 40. [135] [136] Alternatively, the oligomer may be an oligomer represented by the following Chemical Formula 2B. [137] [Formula 2B] [138] [139] In Formula 2B, 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 an element fluorine, and R e ' is an aliphatic a hydrocarbon group or an aromatic hydrocarbon group, wherein R f ' is an alkylene group having 1 to 5 carbon atoms substituted or unsubstituted with a fluorine element, wherein r is an integer of 1 to 2, and r' is an integer of 1 to 3, Wherein p' is an integer from 1 to 50, and q' is an integer from 1 to 15. The p may be an integer of preferably 1 to 45, more preferably an integer of 1 to 40. [140] [141] As a specific example, the oligomer represented by Formula 2B may be an oligomer represented by Formula 2B-1 below. [142] [Formula 2B-1] [143] [144] In Formula 2B-1, p is an integer from 1 to 50, and q is an integer from 1 to 15. In this case, p is preferably an integer of 1 to 45, more preferably an integer of 1 to 40. [145] Meanwhile, the weight average molecular weight (MW) of the oligomer may be controlled by the number of repeating units, and may be about 500 to 200,000, specifically 1,000 to 150,000, and more specifically 2,000 to 100,000. When the weight average molecular weight of the oligomer is within the above range, it has a high affinity with an organic solvent and can be well dispersed, can improve the wettability of the electrolyte by lowering the surface tension to a certain level or less, and suppress the decomposition reaction of the lithium salt and lithium ions can be prevented from causing side reactions. [146] In this case, the oligomer may be included in an amount of 0.1 parts by weight to 5 parts by weight, preferably 0.1 parts by weight to 3 parts by weight, more preferably 0.1 parts by weight to 1 part by weight based on 100 parts by weight of the lithium secondary electrolyte. When the oligomer is included within the above range, it is possible to minimize the interfacial resistance in the battery by acting as a surfactant while maintaining the mobility and ionic conductivity of lithium ions above a certain level to suppress side reactions. [147] [148] (3) lithium salt [149] The lithium salt may be included in the electrolyte for a lithium secondary battery in a molar concentration of 1M to 3M, preferably 1M to 2M, more preferably 1M to 1.5M. When the lithium salt is included within the molar concentration range, lithium ions may be sufficiently supplied, so that a lithium ion yield (Li+ transference number) and a degree of dissociation of lithium ions may be improved, thereby improving output characteristics of the battery. [150] Typically, the lithium salt is LiPF 6 , LiBF 4 , LiSbF 6 , LiAsF 6 , LiClO 4 , LiN(C 2 F 5 SO 2 ) 2 , LiN(CF 3 SO 2 ) 2 , CF 3 SO 3 Li, LiC(CF) 3 SO 2 ) 3 , LiC 4 BO 8 , LiTFSI, LiFSI and LiClO 4 may include at least one compound selected from the group consisting of, preferably LiPF 6 and/or LiBF 4 , but is not limited thereto. [151] Among lithium salts, LiPF 6 and/or LiBF 4 are generally used because of their high ionic conductivity. However, when the organic solvent is decomposed at a high temperature, the decomposition product of the organic solvent reacts with PF 6 − , which is an anion of the lithium salt, and a Lewis acid byproduct such as PF 5 may be generated. In the case of a Lewis acid by-product, it promotes a spontaneous decomposition reaction of the organic solvent and causes a side reaction that collapses the SEI film formed on the electrode interface. If the side reaction is not suppressed, the resistance in the battery may rapidly increase, and the capacity characteristics of the battery may be deteriorated. [152] More specifically, when LiPF 6 is used as the lithium salt , PF 6 − , which is an anion, loses electrons from the negative electrode side, and PF 5 may be generated. At this time, the following chemical reaction may proceed in a chain. [153] [154] [155] When the chain reaction proceeds, decomposition of an organic solvent or a side reaction with the SEI film may occur due to other by-products including generated HF, so that the performance of the battery may be continuously deteriorated. Therefore, in the case of the present invention, in order to solve the above problems, an oligomer including an additive including a compound represented by Chemical Formula 1 and a unit represented by Chemical Formula 2 and an acrylate group at the terminal is used as a lithium secondary compound. It is used in addition to the electrolyte for batteries. [156] [157] (4) organic solvents [158] In the nonaqueous electrolyte for a lithium secondary battery according to the present specification, the organic solvent may include a cyclic carbonate-based organic solvent, a linear carbonate-based organic solvent, or a mixed organic solvent thereof. [159] The cyclic carbonate-based organic solvent is a high-viscosity organic solvent that has a high dielectric constant and can well dissociate lithium salts in the electrolyte, and specific examples thereof include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene. At least one organic solvent selected from the group consisting of carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and vinylene carbonate, among them, ethylene carbonate may include. [160] In addition, the linear carbonate-based organic solvent is an organic solvent having a low viscosity and a low dielectric constant, and representative examples thereof include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethylmethyl carbonate ( EMC), at least one organic solvent selected from the group consisting of methylpropyl carbonate and ethylpropyl carbonate may be used, and specifically, ethylmethyl carbonate (EMC) may be included. [161] In addition, the organic solvent may further include a linear ester-based organic solvent and/or a cyclic ester-based organic solvent in the cyclic carbonate-based organic solvent and/or the linear carbonate-based organic solvent to prepare an electrolyte solution having high ionic conductivity. may [162] Specific examples of the linear ester-based organic solvent include at least one organic solvent selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate and butyl propionate. can be heard [163] In addition, the cyclic ester-based organic solvent includes at least one organic solvent selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone and ε-caprolactone. can [164] Meanwhile, as the organic solvent, an organic solvent commonly used in an electrolyte for a lithium secondary battery may be added without limitation, if necessary. For example, at least one organic solvent of an ether-based organic solvent, an amide-based organic solvent, and a nitrile-based organic solvent may be further included. [165] [166] lithium secondary battery [167] Next, a lithium secondary battery according to the present invention will be described. A lithium secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, and the electrolyte for the lithium secondary battery, and optionally further includes a separator. Meanwhile, since the electrolyte for a lithium secondary battery is the same as described above, a detailed description thereof will be omitted. [168] [169] (1) anode [170] The positive electrode may be prepared by coating a positive electrode active material slurry including a positive electrode active material, a binder, a conductive material, and a solvent on a positive electrode current collector. [171] The positive electrode may be prepared by coating a positive electrode active material slurry including a positive electrode active material, a binder, a conductive material, and a solvent on a positive electrode current collector. [172] The positive electrode current collector is not particularly limited as long as it has conductivity without causing a chemical change in 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 surface-treated may be used. [173] The positive active material is a compound capable of reversible intercalation and deintercalation of lithium, and specifically, may include a lithium composite metal oxide including lithium and one or more metals such as cobalt, manganese, nickel or aluminum. have. More specifically, the lithium composite metal oxide is a lithium-manganese oxide (eg, LiMnO 2 , LiMn 2 O 4 , etc.), a lithium-cobalt-based oxide (eg, LiCoO 2 , etc.), lithium-nickel-based oxide (eg, LiNiO 2 , etc.), lithium-nickel-manganese oxide (eg, LiNi 1-Y1 Mn Y1 O 2 (here, 0