Provided is a lithium secondary battery with enhanced energy density including a cathode using a cathode active material containing lithium mixed transition metal oxide absorbing and discharging lithium ions, wherein a final discharge voltage is in the range of about 1.5 V to about 2.75 V. A final discharge voltage can be reduced to 1.5 V to 2.75 V from 3.0 V by using various lithium mixed transition metal oxides as a cathode active material, or by using the various lithium mixed transition metal oxides with the LCO-based cathode material mixed. Compared to typical LCO-based cathode materials of which capacity and energy density do not change even if a final discharge voltage is decreased, a cathode material of the present invention results in further improving capacity by 10-20% as the final discharge voltage is decreased.