Disclosed is an electrolyte for a secondary battery comprising an electrolyte salt and an electrolyte solvent, the electrolyte comprising both a lactam-based compound and a sulfinyl group-containing compound. Also, disclosed is an electrode having a solid electrolyte interface (SEI) film partially or totally formed on a surface thereof, the SEI film being formed by electrical reduction of the above compounds. Further, a secondary battery comprising the electrolyte and/or the electrode is disclosed.