Positive Electrode Active Material, and Positive Electrode and Lithium Secondary Battery Including Same

A positive electrode active material, a positive electrode including the positive electrode active material, and a lithium secondary battery including the same are disclosed herein. In some embodiments, the positive electrode active material includes a lithium transition metal oxide containing nickel in an amount of 60 mol% or greater based on a total number of moles of transition metals in the lithium transition metal oxide, and in the form of a secondary particle which is an aggregate of primary particles. The positive active material satisfies Equation (1) : -0.021x + 4.0 ≤ y ≤ -0.021x + 5.5, wherein x is a crystal grain size (nm) of the positive electrode active material, and y is a crystal grain aspect ratio of the positive electrode active material.