Positive electrode active material for secondary battery, method of preparing the same, and secondary battery including the positive electrode active material

A method of preparing a positive electrode active material for a secondary battery comprising:

• preparing a first transition metal-containing solution including a nickel raw material, a cobalt raw material, and a manganese raw material and a second transition metal-containing solution including a nickel raw material, a cobalt raw material, and a manganese raw material in a concentration different from that of the first transition metal-containing solution;
• preparing a reaction solution, in which nickel manganese cobalt-based composite metal hydroxide particles are formed, by adding an ammonium cation-containing complexing agent and a basic compound as well as the second transition metal-containing solution to the first transition metal-containing solution to allow a mixing ratio of the first transition metal-containing solution to the second transition metal-containing solution to be gradually changed from 100 vol %:0 vol % to 0 vol %:100 vol % and performing a co-precipitation reaction in a pH range of 11 to 13;
• growing the nickel manganese cobalt-based composite metal hydroxide particles by adding an ammonium cation-containing complexing agent and a basic compound to the reaction solution until a pH of the reaction solution reaches 8 or more to less than 11;
• and
• mixing the grown nickel manganese cobalt-based composite metal hydroxide particles with a lithium-containing raw material and performing a heat treatment.