Abstract: Disclosed is a lithium secondary battery comprising an electrode assembly composed of a cathode, an anode and a separator, and a lithium electrolyte, wherein the electrodes and/or electrolyte include a compound (“urethane compound”) and/or polymer (“poly urethane”) which are decomposed upon overcharge of the battery and contain a urethane group in a molecular structure thereof. The lithium secondary battery of the present invention is characterized by addition of a compound or polymer having a urethane group to electrodes and/or electrolyte, wherein the thus-added compound and/or polymer have substantially no adverse effects on general performance of the battery under normal operating conditions and are decomposed with high reliability upon overcharge of the battery, thereby offering time delay effects for securing battery safety, and the decomposition products thereof result in sharply increased internal resistance of the battery, thereby increasing battery safety.

Abstract: The present invention provides a non-aqueous electrolyte-based high power lithium secondary battery having a long-term service life and superior safety at both room temperature and high temperature, even after repeated high-current charging and discharging, wherein the battery comprises a mixture of a particular lithium manganese-metal composite oxide (A) having a spinel structure and a particular lithium nickel-manganese-cobalt composite oxide (B) having a layered structure, as a cathode active material.