POSITIVE ELECTRODE ACTIVE MATERIAL AND SECONDARY BATTERY

A positive electrode active material with high capacity and excellent charging and discharging cycle performance for a lithium-ion secondary battery is provided. The positive electrode active material contains lithium, cobalt, and oxygen, and the spin density attributed to a bivalent cobalt ion and a tetravalent cobalt ion is within a predetermined range. It is preferable that the positive electrode active material further contain magnesium. An appropriate magnesium concentration is represented as a concentration with respect to cobalt. It is also preferable that the positive electrode active material further contain fluorine.