Positive Electrode Active Material Precursor for Secondary Battery, Positive Electrode Active Material, Preparation Methods Thereof, and Lithium Secondary Battery Including the Positive Electrode Active Material

A method of preparing a positive electrode active material precursor for a secondary battery includes continuously adding a nickel (Ni), cobalt (Co), and manganese (Mn) transition metal cation-containing solution, an alkaline solution, and an ammonium ion-containing solution to a reactor, and forming a positive electrode active material precursor, in which nickel (Ni) and cobalt (Co) are in non-oxidized hydroxide forms and manganese (Mn) is in an oxidized form, by co-precipitation while a gas is not added or an oxygen-containing gas is continuously added to the reactor. A positive electrode active material precursor for a secondary battery is also provided which includes nickel (Ni), cobalt (Co), and manganese (Mn), wherein the nickel (Ni) and the cobalt (Co) are in non-oxidized hydroxide forms, and the manganese (Mn) is in an oxidized form.