Degradation mechanism of layered MnO2 cathodes in Zn/ZnSO4/MnO2 rechargeable cells

Layered-type MnO2 (δ- or naturally occurring birnessite-related MnO2) electrodes that suffer capacity degradation during extended cycling in Zn/ZnSO4/MnO2 rechargeable cells are investigated. When the composite cathodes consisting of MnO2 powder, carbon additive and Teflon binder are galvanostatically cycled in the potential range of 1.0–1.9 V (vs. Zn/Zn2+) where a two-step, two-electron charge/discharge reaction occurs, the cathodes lose their capacities within a few cycles. Such an abrupt capacity loss is found to be caused partly by the formation of basic zinc sulfates (BZS, ZnSO4·3Zn(OH)2·nH2O) on the cathode surface, and also by the Mn losses due to the irreversible nature of the cathodic cell reaction: Mn2+ ions, once produced during the discharge step, are not fully restored to MnO2 during the charging period. An addition of 0.1–0.5 M MnSO4 to 2 M ZnSO4 electrolyte, however, greatly alleviates these failure modes. With this addition, the Mn losses become insignificant as a result of facilitation in the charging reaction and BZS formation is discouraged. Carbon additives loaded in the composite MnO2 cathodes also critically affect the cathode cyclabilities by controlling the rate of charging reaction: the cathodes loaded with acetylene blacks display superior cyclabilities to those containing furnace blacks. From one observation whereby the acetylene blacks possess a lesser amount of surface oxygenic species than the furnace blacks and the other whereby the charging reaction more readily occurs in the former cathodes, it is proposed that the charging (deposition) reaction is significantly hindered by the presence of surface oxygenic species on carbon additives. Electron micrographs of cycled MnO2 cathodes reveal that larger and porous MnO2 deposits are well-grown on the acetylene-black-loaded cathodes whereas only irregular-shaped smaller deposits are formed on the furnace-black-loaded cathodes.