A new phase diagram of AB(CD)1/2 system, where A, B, C, and D represent Li2MnO3, LiMO2 (M=Co, Al, Cr, etc.), LiMnO2, and LiM′O2 (M′=Ni, Cu, Fe, etc.), respectively, was developed to express all compositions of solid solutions with layered manganese oxides. The phase diagram satisfied the theoretical manganese oxidation state of +4 over the whole triangle plane of the diagram and was expressed with a new design equation of Li [ M x ( L i 1 / 3 M n 2 / 3 ) y ( M ′ 1 / 2 M n 1 / 2 ) 1 − x − y ] O 2 . Solid solutions whose compositions were randomly chosen from the phase diagram were experimentally prepared using a direct synthetic method and were subjected to the examination of the structural and electrochemical properties. All the synthesized solid solutions had layered structure with maintaining +4 and showed one-step monotonous discharge curve shape without structural transformation during charge/discharge processes.