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.