To provide a lithium ion secondary battery that contains a Li₂MnO₃-LiMO₂ solid solution as a positive electrode active material and has a high electrode density, and a method for manufacturing a positive electrode active material for the lithium ion secondary battery.

A lithium ion battery contains a solid solution of Li₂MnO₃ and LiMO₂, where M represents at least one of Co, Mn, and Ni, as a positive electrode active material so that a positive electrode has an electrode density of 2.9 g/cc or higher. The solid solution is obtained by mixing and then burning a mixture of a precursor of the solid solution and a lithium compound, or the solid solution, and a molybdate.