Positive Electrode Active Material for Secondary Battery and Lithium Secondary Battery Including the Same

A positive electrode active material, and a positive electrode and a lithium secondary battery including the same are disclosed herein. In some embodiments, a positive electrode active material includes a lithium composite transition metal oxide containing nickel, cobalt, and manganese and having a nickel content for 60 mol % or more, based on metals (M) excluding lithium, and is in the form of single particles having an average particle diameter (D) of 1 to 10 μm, wherein a 100-region extending from the surface toward the center of a single particle has crystal structures of a Fd3M and a Fm3space group, and a phase ratio is 0.2 to 0.7, which is a ratio of a first portion of a maximum straight length of the 100-region occupied by the crystal structure of the Fd3M space group to a second portion occupied by the crystal structure of the Fm3space group.