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

A positive electrode active material for lithium ion secondary batteries containing secondary particles in which a plurality of primary particles are mutually agglomerated contains: a lithium transition metal complex oxide having a layered rock salt-type structure; and crystallized lithium phosphate. In a crystal structure analysis of the lithium transition metal complex oxide having the layered rock salt-type structure by neutron diffraction, a ratio by which an element other than lithium occupies a lithium site (3b site) is 3% or greater and 8% or less.