Positive Electrode Active Material for Secondary Battery, Method of Preparing the Same, and Lithium Secondary Battery Including the Same

The present invention relates to a positive electrode active material for a secondary battery which includes a lithium composite transition metal oxide including nickel (Ni), cobalt (Co), and manganese (Mn), wherein the lithium composite transition metal oxide includes the nickel (Ni) in an amount of 65 mol % or more and the manganese (Mn) in an amount of 5 mol % or more based on a total amount of transition metals, and wherein the electrode positive active material is composed of a single particle, having a crystallite size of 180 nm or more.