Method of Preparing Positive Electrode Active Material Precursor and Positive Electrode Active Material Precursor

A method of preparing a positive electrode active material precursor includes: providing a transition metal-containing solution including nickel, cobalt, and manganese; and introducing the transition metal-containing solution into a reactor, adding a basic aqueous solution and an ammonium cation-containing complex-forming agent, and performing a co-precipitation reaction to prepare a transition metal hydroxide in the form of a secondary particle formed by agglomerating primary particles. The co-precipitation reaction is performed under conditions satisfying Expression 1 described in the specification, and a positive electrode active material precursor whose crystalline grain has a controlled aspect ratio. A positive electrode active material prepared using the positive electrode active material precursor, a positive electrode for a lithium secondary battery, which includes the positive electrode active material, and a lithium secondary battery are also provided.