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 % to 75 atm % of total transition metals, and wherein the lithium composite transition metal oxide powder undergoes a 3% or less change in lithium-oxygen (Li—O) interlayer spacing (i.e., LiOslab thickness) in a state-of-charge (SOC) range of 58% to 86%.