Disclosed is a secondary battery including a cathode, an anode, a separator, and an electrolyte, wherein the electrolyte includes a ternary eutectic mixture prepared by adding (c) a carbonate-based compound to a eutectic mixture containing (a) an amide group-containing compound and (b) an ionizable lithium salt, and the carbonate-based compound is included in an amount of less than 50 parts by weight based on 100 parts by weight of the electrolyte. The use of the disclosed ternary eutectic mixture having flame resistance, chemical stability, high conductivity, and a broad electrochemical window, as the electrolyte material, improves both the thermal stability and quality of the battery.