Positive Electrode Active Material for Lithium Secondary Battery and Lithium Secondary Battery

Provided is a lithium secondary battery which includes 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 lithium composite transition metal oxide powder has an amount of change in lithium-oxygen (Li—O) interlayer spacing (ΔT), as represented by Equation (1), of 0 or more: Δ Equation (1):