When spinel-type lithium cobaltate is applied as cathode active material for a lithium ion battery, a spinel-type crystal phase is unstable and is easy to be dislocated to a layered rock-salt structure, which makes it easy to impair battery properties. Thus, manganese is partially substituted for cobalt in spinel-type lithium cobaltate, to achieve stabilization of the spinel-type crystal phase. Specifically, cathode active material is used in a lithium ion battery, the cathode active material including: a composite oxide of lithium and transition metal, wherein the transition metal consists of cobalt as a main constituent, and manganese, and the composite oxide has a spinel-type crystal phase that is formed of lithium, cobalt, manganese, and oxygen.
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