Hydrothermal Synthesis and Characterization of K_xMnO₂·yH₂O

We report here the direct synthesis of a hexagonal form of manganese dioxide using mild hydrothermal methods. The reaction of potassium or sodium permanganate in water at 170 °C leads directly to potassium or sodium manganese dioxide, Alk ≈0.25 MnO 2 ·0.6H 2 O, with a R 3̄ m rhombohedral structure like Li x (H 2 O)TiS 2 and a 7 Å repeat distance indicative of a monolayer of water between the manganese dioxide layers. This manganese oxide reacts readily and reversibly with lithium ions.