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 single primary particles or by single primary particles and secondary particles with a plurality of aggregated primary particles,

• wherein a number proportion of the single primary particles to all of the particles is 30% or more,
• 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),
• a ratio of a (003) diffraction peak intensity I(003) to a (104) diffraction peak intensity I(104) (I(003)/I(104)) is 2.0 or more, the (003) diffraction peak intensity and the (104) diffraction peak intensity determined from X-ray diffraction measured using a flat plate sample holder of a Bragg Brentano optical system using a Cu-kα ray as an X-ray source, and
• a degree of circularity is 0.93 or more and 1.00 or less, the degree of circularity determined with a flow type image analysis method using a wet flow type particle size/shape analyzer.