Lithium nickel manganese composite oxide, method for producing same, cathode and electricity storage device using same

Provided is a method for producing a 5V spinel structured lithium nickel manganese composite oxide which has a low nickel oxide content in a minor phase. A mixture including a pre-prepared spinel structured lithium nickel manganese composite oxide which has a low molar ratio of nickel to manganese (Ni/Mn=0.10 to 0.29), a lithium compound, and a nickel compound is fired to produce the spinel structured lithium nickel manganese composite oxide which has a high molar ratio of nickel to manganese (Ni/Mn= about 1/3).