Amorphous manganese oxides were prepared by oxidation of Mn(II) precursors in aqueous solutions at room temperature. The oxidation procedure offers a number of advantages including a good control of chemical composition and morphology. X-ray and electron diffraction of as-synthesized materials reveal a complete lack of long-range order. Synthesized amorphous manganese dioxide with a low sodium content (0.09 Na/Mn) exhibits a specific capacity of 350, 290, and 250 mAh/g at discharge/charge rates of C/100, C/50, and C/5, respectively. The performance indicates not only extremely high lithium intercalation capacities, but also excellent rate capabilities. Electrochemical behavior during discharge/charge cycling suggests occurrence of local structural relaxations in the amorphous material; however, no trend toward a spinel-like behavior is observed. It is found that the presence of a large amount of sodium in the amorphous material (0.27 Na/Mn) lowers the lithium intercalation capacity but greatly improves capacity retention upon cycling. The sodium ions, which reside stably in the amorphous host as an electrochemically inactive species, may help stabilize the local structure thus leading to excellent cycling performance. (C) 2002 The Electrochemical Society.