An electrolyte for a lithium secondary battery, the electrolyte including a lithium salt, a non-aqueous organic solvent, and a polar additive based on a substituted hetero-bicyclic compound. Oxidation of the electrolyte is prevented by formation of a polar thin film on a surface portion of the positive electrode, which facilitates transfer of lithium ions. The lithium secondary batteries using the electrolyte have excellent high temperature life characteristics and high temperature conservation characteristics.