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

There is provided a positive electrode active material with which a lithium ion secondary battery with both high battery capacity and high thermal stability achieved at a high level is obtainable. [0002] A positive electrode active material for a lithium ion secondary battery, containing a lithium-nickel-manganese composite oxide configured by secondary particles with a plurality of aggregated primary particles, in which the lithium-nickel-manganese composite oxide has a hexagonal layered structure and contains lithium (Li), nickel (Ni), manganese (Mn), an element M (M), and titanium (Ti), 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, 0.03 ≤ 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.