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

The present invention relates to a positive electrode active material for a lithium ion secondary battery, including: lithium-nickel-manganese composite oxide particles having a layered hexagonal crystal system, wherein, the lithium-nickel-manganese composite oxide is represented by the following general formula: Li1+tNiaMnbMcMgdO2+α (1)in which an element M in the general formula (1) is at least one element selected from Co, Ti, W, B, Mo, V, Nb, Ca, Al, Cr, Zr, and Ta; -0.05≤t≤0.2, 0.50≤a≤0.95, 0.03≤b≤0.40, 0.02≤c≤0.40, 0.0005≤d≤0.05, a+b+c+d=1.0, and 0≤α≤0.5; wherein the lithium-nickel-manganese composite oxide particles contains secondary particles formed by aggregation of primary particles of the lithium-nickel-manganese composite oxide particles, and wherein lattice constants a and c of the lithium-nickel-manganese composite oxide as determined by an X-ray diffraction method are 2.8640Å≤a≤2.8750Å and 14.195Å≤c≤14.225Å.