Disclosed is an electrolyte for a secondary battery, comprising an eutectic mixture consisting of: (a) am amide group-containing compound with at least one EDG introduced into the N-position thereof; and (b) an ionizable lithium salt. Also, provided are a secondary battery comprising such an electrolyte, and a method of adjusting an electrochemical stability window of an eutectic mixture consisting of an amide group-containing compound and a lithium salt by regulating electron donating properties of at least one substituent group introduced into the N-position of the amide group-containing compound.