[0001]This application claims priority from Korean Patent Application No. 2019-0072177, filed on June 18, 2019, the 10 disclosure of which is incorporated by reference herein. Technical Field [0002] 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 15 for a lithium secondary battery, which may suppress a self- discharge phenomenon of the lithium secondary battery by suppressing dissolution of transition metal in a positive electrode, and a lithium secondary battery including the same. BACKGROUND ART 20 [0003] There is a need to develop technology for efficiently storing and utilizing electrical energy as personal IT devices and computer networks are developed with the development of information society and the accompanying dependency of society as a whole on the electrical energy is 25 increased. 1 [0004] A technology based on secondary batteries is the most suitable technology for various applications, wherein, since a secondary battery may be miniaturized, it is applicable to a personal IT device, and it is also applicable to a large 5 device such as a power storage device. [0005] Among these secondary battery technologies, lithium ion batteries, which are battery systems having the theoretically highest energy density, are in the spotlight. [0006] The lithium ion battery is largely composed of a 10 positive electrode formed of a transition metal oxide containing lithium, a negative electrode capable of storing lithium, an electrolyte that becomes a medium for transferring lithium ions, and a separator, and, among them, a significant amount of research on the electrolyte has been 15 conducted while the electrolyte is known as a component that greatly affects stability and safety of the battery. [0007] The electrolyte causes a reduction decomposition reaction on a negative electrode interface during an activation process of the battery to form a solid electrolyte 20 interphase (SEI). The SEI suppresses additional decomposition of an electrolyte solution and may transmit lithium ions. [0008] Transition metal ions may be dissolved from a positive electrode active material by a decomposition product 25 of a lithium salt included in the electrolyte under high- 2 temperature conditions, and the dissolved transition metal ions may be re-deposited to the positive electrode to increase resistance of the positive electrode. Also, the dissolved transition metal ions may be electrodeposited on 5 the negative electrode interface thorough the electrolyte to cause a self-discharge phenomenon of the negative electrode, and may decompose the SEI formed on a surface of the negative electrode to reduce passivation ability of the SEI. [0009] Thus, there is an urgent need for research into an 10 electrolyte which includes a component capable of scavenging the decomposition product of the lithium salt. [0010] Prior Art Document: International Patent Publication No. 2009-157261 DISCLOSURE OF THE INVENTION 15 TECHNICAL PROBLEM [0011] An aspect of the present invention provides an electrolyte for a lithium secondary battery, which may suppress an electrolyte decomposition reaction and may minimize generation of a lithium salt decomposition product 20 and a transition metal ion dissolution phenomenon in a positive electrode even when the lithium secondary battery is operated under high-temperature conditions, and a lithium secondary battery including the same. TECHNICAL SOLUTION 25 [0012] According to an aspect of the present invention, 3 there is provided an electrolyte for a lithium secondary battery which includes: a lithium salt, an organic solvent, and a compound represented by Formula 1. [0013] [Formula 1] [0014] In Formula 1, [0015] R1 and R2 are each independently an alkyl group having 1 to 3 carbon atoms. [0016] According to another aspect of the present invention, 10 there is provided a lithium secondary battery including a positive electrode, a negative electrode, and the electrolyte for a lithium secondary battery. ADVANTAGEOUS EFFECTS [0017] Since an electrolyte for a lithium secondary battery 15 according to the present invention may not only suppress an additional electrolyte decomposition reaction and minimize generation of a decomposition product of a lithium salt even when the lithium secondary battery is stored or repeatedly charged and discharged at high temperatures but may also 20 minimize dissolution of transition metal ions in a positive electrode, a lithium secondary battery having improved high-temperature life characteristics and resistance characteristics may be prepared. 4 BRIEF DESCRIPTION OF THE DRAWINGS [0018] The following drawings attached to the specification illustrate preferred examples of the present invention by example, and serve to enable technical concepts of the 5 present invention to be further understood together with detailed description of the invention given below, and therefore the present invention should not be interpreted only with matters in such drawings. [0019] FIG. 1 is a graph illustrating the results of 10 evaluation of resistance increase rates (%) after high- temperature (60°C) storage according to Experimental Example 1; [0020] FIG. 2 is a graph illustrating the results of evaluation of capacity retentions (%) after high-temperature 15 (45°C) charge and discharge according to Experimental Example 2; [0021] FIG. 3 is a graph illustrating the results of evaluation of resistance increase rates (%) after high-temperature (45°C) charge and discharge according to 20 Experimental Example 3; and [0022] FIG. 4 is a graph illustrating the results of evaluation of resistance increase rates (%) and capacity retentions (%) after high-temperature (45°C) charge and discharge according to Experimental Example 4. 25 MODE FOR CARRYING OUT THE INVENTION 5 [0023] Hereinafter, the present invention will be described in more detail. [0024] It will be understood that words or terms used in the specification and claims shall not be interpreted as the 5 meaning defined in commonly used dictionaries, and it will be further understood that the words or terms should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the technical idea of the invention, based on the principle that an 10 inventor may properly define the meaning of the words or terms to best explain the invention. [0025] The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the present invention. In the 15 specification, the terms of a singular form may comprise plural forms unless referred to the contrary. [0026] It will be further understood that the terms “include,” “comprise,” or "have" when used in this specification, specify the presence of stated features, 20 numbers, steps, elements, or combinations thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, elements, or combinations thereof. [0027] 25 [0028] An electrolyte for a lithium secondary battery 6 according to the present invention includes: a lithium salt, an organic solvent, and an additive including a compound represented by Formula 1 below. [0029] [Formula 1] 5 [0030] In Formula 1, [0031] R1 and R2 are each independently an alkyl group having 1 to 3 carbon atoms. 10 [0032] (1) Lithium Salt [0033] First, a lithium salt will be described. [0034] The lithium salt is used as a medium for transferring ions in a lithium secondary battery, wherein it is desirable that the lithium salt is included in a concentration of 0.1 M 15 to 3 M, preferably 0.8 M to 2.5 M, and more preferably 1 M to 1.5 M in the electrolyte for a lithium secondary battery. In a case in which the lithium salt is included within the above range, an increase in resistance in the battery may be prevented by preventing decomposition of a solid electrolyte 20 interphase (SEI) formed on an electrode interface when the battery is operated at a high voltage while minimizing a by-product generated by the dissolution of the lithium salt in 7 the electrolyte. [0035] For example, the lithium salt may include at least one compound selected from the group consisting of LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiN(C2F5SO2)2, LiN(CF3SO2)2, 5 CF3SO3Li, LiC(CF3SO2)3, LiC4BO8, LiTFSI, LiFSI, and LiClO4. [0036] Particularly, among the lithium salts, LiPF6 is widely used because it has relatively higher ionic conductivity than other lithium salts. However, in a case in which an organic solvent included in an electrolyte is 10 decomposed at high temperature, PF6-, as an anion, may be decomposed at high temperature or by-products, such as HF and PF5, may be generated due to moisture included in the electrolyte. The by-products, such as HF and PF5, may be a cause of destructing the SEI on a surface of an electron-rich 15 negative electrode as described above or dissolving transition metal ions from a positive electrode. [0037] Thus, in order to suppress a side reaction due to the by-products, the present invention aims at providing the compound represented by Formula 1, as an additive for forming 20 an SEI, which may scavenge the Lewis acid by-products and may simultaneously suppress the side reaction of the Lewis acid by-products at high temperatures by being reduced on the surface of the negative electrode. 25 [0038] (2) Organic Solvent 8 [0039] Next, the organic solvent will be described. [0040] Various organic solvents typically used in a lithium electrolyte may be used as the organic solvent without limitation. For example, the organic solvent may include a 5 cyclic carbonate-based organic solvent, a linear carbonate- based organic solvent, or a mixed organic solvent thereof. [0041] The cyclic carbonate-based organic solvent is an organic solvent which may well dissociate the lithium salt in the electrolyte due to high permittivity as a highly viscous 10 organic solvent, wherein specific examples of the cyclic carbonate-based organic solvent may be at least one organic solvent selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 15 2,3-pentylene carbonate, and vinylene carbonate, and, among them, the cyclic carbonate-based organic solvent may include at least one of ethylene carbonate and propylene carbonate (PC). [0042] Also, the linear carbonate-based organic solvent is 20 an organic solvent having low viscosity and low permittivity, wherein typical examples of the linear carbonate-based organic solvent may be at least one organic solvent selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl 25 carbonate (EMC), methylpropyl carbonate, and ethylpropyl 9 carbonate, and the linear carbonate-based organic solvent may specifically include ethyl methyl carbonate (EMC). [0043] It is desirable that an electrolyte having high electrical conductivity may be prepared if a linear carbonate 5 organic solvent is mixed with a cyclic carbonate organic solvent in an appropriate ratio and used as the organic solvent. Specifically, the cyclic carbonate organic solvent and the linear carbonate organic solvent may be used by being mixed in a volume ratio of 1:9 to 5:5, for example, 2:8 to 10 3:7. [0044] Furthermore, the organic solvent may further include an ester (acetates and propionates) organic solvent having low melting point and high stability at high temperature, for example, a linear ester-based organic solvent and/or a cyclic 15 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. [0045] Typical examples of the linear ester-based organic 20 solvent may be 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. [0046] Also, the cyclic ester-based organic solvent may 25 include at least one organic solvent selected from the group 10 consisting of y-butyrolactone, y-valerolactone, y-caprolactone, o-valerolactone, and s-caprolactone. [0047] If necessary, the organic solvent may be further mixed with an amide compound or a nitrile compound and used. 5 [0048] (3) Additive: Compound Represented By Formula 1 [0049] Next, the electrolyte for a lithium secondary battery of the present invention includes a compound represented by the following Formula 1 as an additive. 10 [0050] [Formula 1] [0051] In Formula 1, [0052] R1 and R2 are each independently an alkyl group having 1 to 3 carbon atoms. 15 [0053] An electrolyte for a lithium secondary battery generally includes a lithium salt such as LiPF6, and the lithium salt generates a Lewis acid by-product, such as PF5 or HF, when the lithium salt is decomposed in the electrolyte 20 under high-temperature conditions. The Lewis acid by-product may dissolve transition metal ions in a positive electrode active material, and the dissolved transition metal ions may 11 be re-deposited on a surface of the positive electrode to increase resistance of the positive electrode. The transition metal ions may be electrodeposited on a negative electrode interface thorough the electrolyte and may react 5 with components constituting a solid electrolyte interphase (SEI) on the electrode interface or an organic solvent to cause a decomposition reaction, and thus, the transition metal ions may reduce passivation ability of the SEI. [0054] Thus, it is necessary to scavenge the Lewis acid 10 compound, as a high-temperature decomposition product of the lithium salt, in order to address problems, such as an increase in resistance in the battery and a degradation of battery life characteristics, due to the generation of the decomposition reaction product by suppressing the 15 decomposition reaction of the SEI under high-temperature conditions. [0055] Thus, in the present invention, the compound represented by Formula 1 corresponding to a Lewis base, which may react with the Lewis acid, was used as the electrolyte 20 additive. Since the compound represented by Formula 1 contains an electron-rich sulfur (S) element in a ring, it acts as the Lewis base. That is, since it provides electrons to the Lewis acid decomposition product such as HF or PF5, it performs a Lewis acid-base reaction with the Lewis acid 25 decomposition product instead of the components constituting 12 the SEI formed on the surface of the negative electrode or a transition metal oxide in the positive electrode. [0056] Also, since the compound represented by Formula 1 contains a double bond in its molecular structure, it has 5 excellent reducibility on the surface of the negative electrode during initial charge of the lithium secondary battery. Thus, the compound represented by Formula 1 may preferentially be reductively decomposed to improve the components of the SEI so that the SEI formed on the surface 10 of the negative electrode may suppress the side reaction of the Lewis acid by-product derived from the lithium salt. [0057] Specifically, the compound represented by Formula 1 may include at least one selected from the group consisting of compounds represented by Formulae 1A to 1C below. [0059] [Formula 1B] 13 15 [0058] [Formula 1A] [0060] [Formula 1C] 5 [0061] The compound represented by Formula 1 may be included in an amount of 0.1 part by weight to 10 parts by weight, preferably 0.1 part by weight to 5 parts by weight, and more preferably 0.1 part by weight to 3 parts by weight based on 10 100 parts by weight of the electrolyte for a lithium secondary battery. In a case in which the compound represented by Formula 1 is included in an amount within the above range, an effective SEI may not only be formed on the surface of the negative electrode, but the Lewis acid by- 15 products derived from the lithium salt, for example, HF and PF5, may be effectively scavenged. However, in a case in which the compound represented by Formula 1 is included in an amount greater than the above range, a decomposition reaction 14 may excessively occur to increase initial resistance of the lithium secondary battery, and, in a case in which the compound represented by Formula 1 is included in an amount less than the above range, an effect as an additive may be 5 insignificant. [0062] (4) Other Additives [0063] The electrolyte for a lithium secondary battery of the present invention may additionally further include other 10 additives which may form a stable film on the surfaces of the negative electrode and the positive electrode while not significantly increasing the initial resistance in addition to the effect from the compound represented by Formula 1, or which may act as a complementary agent for suppressing the 15 decomposition of the solvent in the electrolyte for a lithium secondary battery and improving mobility of lithium ions. [0064] These other additives are not particularly limited as long as these are additives capable of forming a stable film on the surfaces of the positive electrode and the negative 20 electrode. As a representative example, the other additive may include at least one selected from the group consisting of a halogen-substituted or unsubstituted carbonate-based compound, a vinyl silane-based compound, a phosphate-based compound, a phosphite-based compound, a sulfite-based 25 compound, a sulfone-based compound, a sulfate-based compound, 15 a sultone-based compound, a halogen-substituted carbonate- based compound, a halogen-substituted benzene-based compound, a nitrile-based compound, a borate-based compound, and a lithium salt-based compound. 5 [0065] Specifically, the other additive may include at least one compound selected from the group consisting of a vinyl silane-based compound, a phosphate-based compound, a sulfate-based compound, a sultone-based compound, a halogen-substituted benzene-based compound, and a borate-based 10 compound. [0066] The halogen-substituted or unsubstituted carbonate-based compound may include vinylene carbonate (VC) or fluoroethylene carbonate (FEC). [0067] The vinyl silane-based compound may improve 15 durability of the battery by forming a stable film through electrochemical reduction on the surface of the negative electrode. Specifically, tetravinylsilane (TVS) may be included as the vinyl silane-based compound. [0068] The phosphate-based or phosphite-based compound is a 20 component for assisting the formation of the SEI by being electrochemically decomposed on the surfaces of the positive electrode and the negative electrode, wherein an effect of improving long-term cycle life characteristics of the secondary battery may be achieved by the phosphate-based or 25 phosphite-based compound. Representative examples thereof 16 may be at least one compound selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate (LiDFP), tetramethyl trimethylsilyl phosphate (TMSPa), trimethylsilyl phosphite (TMSPi), 5 tris(2,2,2-trifluoroethyl) phosphate (TFEPa), and tris(trifluoroethyl) phosphite (TFEPi). [0069] The sulfite-based compound may include at least one compound selected from the group consisting of ethylene sulfite, methylethylene sulfite, ethylethylene sulfite, 4,5- 10 dimethylethylene sulfite, 4,5-diethylethylene sulfite, propylene sulfite, 4,5-dimethylpropylene sulfite, 4,5- diethylpropylene sulfite, 4,6-dimethylpropylene sulfite, 4,6- diethylpropylene sulfite, and 1,3-butylene glycol sulfite. [0070] The sulfone-based compound may include at least one 15 compound selected from the group consisting of divinyl sulfone, dimethyl sulfone, diethyl sulfone, methylethyl sulfone, and methylvinyl sulfone. [0071] The sulfate-based compound may include ethylene sulfate (Esa), trimethylene sulfate (TMS), and methyl 20 trimethylene sulfate (MTMS). [0072] The sultone-based compound may include at least one compound selected from the group consisting of 1,3-propane sultone (PS), 1,4-butane sultone, ethane sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene 25 sultone. 17 [0073] The halogen-substituted benzene-based compound may include fluorobenzene (FB). [0074] Also, the nitrile-based compound may include at least one compound selected from the group consisting of 5 succinonitrile (SN), adiponitrile (Adn), acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentane carbonitrile, cyclohexane carbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, 10 phenylacetonitrile, 2-fluorophenylacetonitrile, and 4- fluorophenylacetonitrile. [0075] The borate-based compound may include lithium oxalyldifluoroborate (LiODFB), lithium bis(oxalato)borate (LiB(C2O4)2; LiBOB), or lithium tetrafluoroborate (LiBF4). 15 [0076] The lithium salt-based compound is a compound different from the lithium salt included in the electrolyte, wherein the lithium salt-based compound may be LiPO2F2. [0077] The compounds listed as the other additives may be included alone or as a mixture of two or more thereof, and 20 may be included in an amount of 1 part by weight to 40 parts by weight based on 100 parts by weight of the electrolyte for a lithium secondary battery, particularly 1 part by weight to 30 parts by weight based on 100 parts by weight of the electrolyte for a lithium secondary battery, and more 25 particularly 1 part by weight to 20 parts by weight based on 18 100 parts by weight of the electrolyte for a lithium secondary battery. [0078] If the amount of the other additives is greater than the above range, a side reaction in the electrolyte may occur 5 excessively during charge and discharge of the battery, and, since an excessive decomposition reaction may occur at high temperatures, the initial resistance of the lithium secondary battery may be increased, or the resistance may be continuously increased during charge and discharge of the 10 lithium secondary battery to degrade discharge capacity and life characteristics of the battery. [0079] [0080] Next, a lithium secondary battery according to the 15 present invention will be described. [0081] The lithium secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, and the electrolyte for a lithium secondary battery, and may optionally further include 20 a separator which may be disposed between the positive electrode and the negative electrode. In this case, since the electrolyte for a lithium secondary battery is the same as described above, a detailed description thereof will be omitted. 25 19 [0082] (1) Positive Electrode [0083] The positive electrode may be prepared by coating a positive electrode collector with a positive electrode active material slurry including a positive electrode active 5 material, a binder for an electrode, a conductive agent for an electrode, and a solvent. [0084] The positive electrode collector is not particularly limited so long as it has conductivity without causing adverse chemical changes in the battery, and, for example, 10 stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel that is surface-treated with one of carbon, nickel, titanium, silver, or the like may be used. In this case, the positive electrode collector may have fine surface roughness to improve bonding strength with the 15 positive electrode active material, and the positive electrode collector may be used in various shapes such as a film, a sheet, a foil, a net, a porous body, a foam body, a non-woven fabric body, and the like. [0085] The positive electrode active material is a compound 20 capable of reversibly intercalating and deintercalating lithium, wherein the positive electrode active material may specifically include a lithium composite metal oxide including lithium and at least one metal such as cobalt, manganese, nickel, or aluminum. Specifically, the lithium 25 composite metal oxide may include lithium-manganese-based 20 oxide (e.g., LiMnO2, LiMn2O4, etc.), lithium-cobalt-based oxide (e.g., LiCoO2, etc.), lithium-nickel-based oxide (e.g., LiNiO2, etc.), lithium iron-phosphate-based positive electrode material (e.g., LiFePO4), lithium-nickel-manganese- 5 based oxide (e.g., LiNi1-Y1MnY1O2 (where 0