Provided are a cathode active material with excellent high-temperature life that suppresses the elution of metal elements from the cathode active metal and the generation of different phase therein and exhibits a high potential, as well as a high-voltage lithium ion battery that achieves excellent high-temperature life by applying such a cathode active material. The cathode active material for a lithium ion battery is represented by a compositional formula LiaNixMnyMzO4-δFδ (where M is one or more elements selected from the group consisting of Ti, Ge, Mg, Co, Fe, Cu, and Al, and a, x, y, z, and δ satisfy the following formulas: 1≦a<1.08, 0.4≦x<0.5, 0
