The present invention provides a lithium secondary battery comprising an electrode assembly and an electrolyte, wherein the electrode assembly includes a cathode, an anode, and a separator disposed between the cathode and the anode. The lithium secondary battery of the present invention is characterized in that the cathode includes, as a cathode active material, a lithium manganese composite oxide having a spinel structure, expressed by the following formula 1: LixMyMn<2-y>O<4-z>A (1) where, 0.9<=x<=1.2; 0