POSITIVE ELECTRODE ACTIVE MATERIAL FOR LITHIUM ION SECONDARY BATTERY, METHOD FOR PRODUCING THE SAME, AND LITHIUM ION SECONDARY BATTERY

A positive electrode active material for a lithium ion secondary battery, in which the lithium-nickel-manganese composite oxide has a hexagonal layered structure, a mole number ratio of elements is represented as Li:Ni:Mn:M:Ti=a:(1-x-y-z):x:y:z, provided that 0.97≤a≤1.25, 0.035≤x≤0.15, 0≤y≤0.15, and 0.01≤z≤0.05, a ratio of a total amount of peak intensities of most intense peaks of a titanium compound to a (003) diffraction peak intensity that is the most intense peak of the hexagonal layered structure is 0.2 or less, a crystallite diameter at (003) plane is 80 nm or more and less than 160 nm, and a specific surface area is 0.7 m/g or more and 4.0 m/g or less.