Positive electrode active material for rechargeable lithium-ion battery

A positive electrode active material is a layered lithium manganese compound represented by a general formula Li1-xMO2, where x is a lithium-deficient quantity and larger than 1/5, and M is manganese or metals of two or more kinds containing manganese as a main component. The metals are preferably 3d-transition metals. The positive electrode active material has a high capacity and is excellent in structure stability. A rechargeable lithium-ion battery uses a positive electrode material containing the positive electrode active material and is excellent in cyclic stability.