POSITIVE ELECTRODE ACTIVE MATERIAL FOR LITHIUM-ION SECONDARY CELL, AND LITHIUM-ION SECONDARY CELL

A positive electrode active material that can achieve high thermal stability at low cost is provided.Provided is a positive electrode active material for a lithium ion secondary battery, the positive electrode active material containing a lithium-nickel-manganese composite oxide, in which metal elements constituting the lithium-nickel-manganese composite oxide include lithium (Li), nickel (Ni), manganese (Mn), cobalt (Co), titanium (Ti), niobium (Nb), and optionally zirconium (Zr), an amount of substance ratio of the elements is represented as Li : Ni : Mn : Co : Zr : Ti : Nb = a : b : c : d : e : f : g (provided that, 0.97 ≤ a ≤ 1.10, 0.80 ≤ b ≤ 0.88, 0.04 ≤ c ≤ 0.12, 0.04 ≤ d ≤ 0.10, 0 ≤ e ≤ 0.004, 0.003 < f ≤ 0.030, 0.001 < g ≤ 0.006, and b + c + d + e + f + g = 1), in the amount of substance ratio, (f + g) ≤ 0.030 and f > g are satisfied, and an amount of lithium to be eluted in water when the positive electrode active material is immersed in water is 0.20% by mass or less with respect to the entire positive electrode active material.