Nanostructured and layered lithium manganese oxide powders and methods of producing same. The powders are represented by the chemical formula, LixMn1-yMyO2, where 0.5x1.33, 0y0.5 and have an average primary particle diameter from 5 nm to 300 nm, preferably between 5 and 100 nm, and M is at least one cation dopant. The powders can be formed into active cathode materials in Li-ion and Li rechargeable batteries.