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 including 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, a ratio of a (003) diffraction peak intensity Ito a (104) diffraction peak intensity I(I/I) is 2.0 or more, and a degree of circularity is 0.93 or more and 1.00 or less.