Positive Electrode Active Material For Secondary Battery And Lithium Secondary Battery Including The Same

A positive electrode active material for a secondary battery which includes a nickel-based lithium composite transition metal oxide including nickel (Ni), wherein the lithium composite transition metal oxide satisfies Equation 1 and Equation 2 below 80 nm≤crystallite size≤150 nm [Equation 1] Δsize(|crystallite size−crystallite size|)≤20 [Equation 2]