Characterization of the bismuth-modified manganese dioxide cathodes in rechargeable alkaline cells

Bismuth-modified manganese dioxide (BMD) cathodes are shown to exhibit good cycling characteristics with a theoretical two-electron capacity in rechargeable alkaline cells. With an aim to understand the discharge-charge mechanisms, the BMD cathodes are characterized by X-ray diffraction, scanning electron microscopy, and wet-chemical analysis at various levels of discharge and charge during the first two cycles and after various numbers of cycles. It is found that a well-ordered, crystalline birnessite MnO2 is formed at the end of first charge, irrespective of the initial form of the manganese oxide. The discharge-charge mechanism involves a reversible conversion of birnessite MnO2 to MnOOH to Mn(OH)(2) in the subsequent cycles. Wet-chemical analyses demonstrates for the first time that the discharge/ charge process in rechargeable alkaline cells involves a reversible dissolution/incorporation of K+ ions from/into the cathode lattice into/from the electrolyte. The incorporation of the K+ ions into the lattice appears to stabilize a well-ordered birnessite structure during charge. (C) 2002 The Electrochemical Society.