Deposition of ZnO on bismuth species towards a rechargeable Zn-based aqueous battery.

Zn aqueous batteries typically suffer from poor cycle life because water soluble zincate ions are formed during the oxidation of Zn. When Zn is oxidized, most of the Zn2+ ions detach from the current collector and become electrochemically inactive, leaving the battery non-rechargeable. Numerous reports demonstrate the use of Bi2O3 as an electrode additive to enhance electrochemical performance and they attribute this phenomenon to the improvement in electrical conductivity. However, conductivity does not have an effect on the intrinsic solubility of the zincate ion. We conduct a series of characterizations to provide a comprehensive mechanistic role of Bi2O3 in the Zn electrode. We find that upon oxidation, zincate ions are formed but they relax into ZnO on the surface of the bismuth species. This work proposes that the reason for the prolonged cycle life is due to the deposition of ZnO through relaxation and this prevents losing electrochemically active materials. This finding paves the way for further improving the cycle life and understanding the mechanism of the Zn based rechargeable aqueous batteries and possibly other conversion types of rechargeable batteries.