Positive electrode active material for lithium ion secondary battery and lithium ion secondary battery

A positive electrode active material for a lithium ion secondary battery, the positive electrode active material comprising a lithium-nickel composite oxide having a hexagonal layered structure and configured by particles including at least either single primary particles or secondary particles with a plurality of aggregated primary particles,

• wherein metal elements constituting the lithium-nickel composite oxide include lithium (Li), nickel (Ni), and optionally at least one element M (M) selected from the group consisting of Co, Mn, Al, V, Mg, Mo, Ca, Cr, Zr, Ti, Nb, Na, K, W, Fe, Zn, B, Si, P, and Ta,
• a mole number ratio of the metal elements is represented as Li:Ni:M=a:b:c (provided that, 0.95≤a≤1.10, 0.30≤b≤1.00, 0.00≤c≤0.70, and b+c=1), and
• the particles included in the positive electrode active material have a cross section having an area proportion of a crystal face having a maximum area to the cross section of 80% or more when one crystal face is defined as a region having a crystal misorientation of 15° or less.