Positive Electrode Active Material For Lithium Secondary Battery And Lithium Secondary Battery

Provided is a lithium secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode includes, as a positive electrode active material, a lithium composite transition metal oxide powder having a layered structure and a nickel content accounting for 50 atm % or more of total transition metals, and wherein the layered structure of the positive electrode active material is phase-transformed into a spinel structure at a temperature of 300° C. or more in a fully charged state.