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 nm≤crystallite size nm [Equation Δ size(|crystallite size−crystallite size|)≤ [Equation