Cycle‐life and charge/discharge curves of pure and physically Bi2O3‐doped pyrolusite (β‐MnO2) were studied. Crystalline intermediate products of the first three semi‐cycles give information about the effects of Bi2O3. A mechanism is postulated. In the first two semi cycles pyrolusite (β‐MnO2) is converted to the layered δ‐MnO2 with β‐Mn(OH)2 in the fully discharged state. In the course of the following discharge hausmannite Mn3O4, which is electrochemically inert, is formed. No hausmannite is found in presence of Bi2O3.
Pieces of information on mechanistical aspects can be gathered from the initial discharge process. During the initial discharge of β‐MnO2, δ‐MnO2 is formed as an intermediate product. In the undoped case this species can be found around MnO1.75. In presence of Bi2O3, however, δ‐MnO2 is existent nearly down to MnO1.25 over the whole discharge region.
The reaction path diverges at the [MnIII(OH)4(H2O)2]− ion, dissoluted during the cycling process: in presence of Bi2O3 the reaction is controlled kinetically by a quick disproportionation of the [MnIII(OH)4(H2O)2]− ions resulting in δ‐MnO2 and β‐Mn(OH)2. In absence of Bi2O3, thermodynamical control occurs, and [MnIII(OH)4(H2O)2]− reacts with [MnII(OH)4(H2O)2]2‐ forming the electrochemical non‐reactive hausmannite Mn3O4.