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

PROBLEM TO BE SOLVED: To provide a positive electrode active material for lithium ion secondary battery that is easily synthesized in a short time, and compatibly attain initial discharge capacity and repetition characteristics of a lithium ion secondary battery.SOLUTION: A method of manufacturing a positive electrode active material for lithium ion secondary battery includes a process (I-1) of obtaining an aqueous composition (1) by dissolving in an aqueous solvent one or more kinds selected out of a lithium compound, a silicon compound, an iron compound, a magnesium compound, and a manganese compound, and a phosphate compound when (x) in composition formula (A) of LiM(SiO)(PO)(where M is one or more kinds of element selected out of Fe, Mg and Mn, and 0≤x≤1) satisfies 0