Nickel-based positive electrode active material, method of preparing the same, and lithium battery using the nickel-based positive electrode active material

A positive electrode active material for a secondary lithium battery, a method of preparing the same, and a lithium battery using the nickel-based positive electrode active material, wherein said positive electrode active material comprises a lithium nickel complex oxide material having a surface portion and a core portion, in which lithium ions are distributed in a concentration gradient decreasing from the surface portion to the core portion of the lithium nickel complex oxide material.