Oxidative Electrochemical Coupling Between Bi and Mn in Rechargeable Alkaline MnO2 Cathodes

The Bi-modified MnO2 cathode is an enabling technology for rechargeable alkaline Zn batteries, which are under intense study for low-cost, sustainable, non-flammable grid-scale energy storage. Although the effect of Bi on MnO2 rechargeability has been known for decades, its mechanism has remained a longstanding challenge in the field. Here, we detail an oxidative electrochemical coupling between Bi and Mn in electrodes of commercially relevant areal capacity (12 mAh/cm2). Specifically, operando X-ray absorption spectroscopy and differential capacity analysis reveal that electrochemical capacity associated with Bi oxidation is shifted up into a voltage range where only Mn oxidation is expected. This oxidative coupling occurs precisely when failure by Mn3O4 formation is known to occur in the absence of Bi, and coincides with the potential range at which the β'-MnOOH intermediate exists during charge. The stabilization of β'-MnOOH by Bi-Mn coupling reveals its critical role in rechargeability, as this intermediate is the primary material source of Mn3O4 in unmodified electrodes. Only a small fraction of the total Bi capacity, corresponding to approximately 2 wt % Bi2O3, participates in this coupling, and it is demonstrated that the overall extent of rechargeability is controlled by the diffusion of dissolved Bi ions through the microporous structure of the MnO2 active material. Consistent with this transport-limited scenario, structurally incorporating Bi3+ directly into the MnO2 lattice is shown to be counterproductive, as it pillars the structure and impedes the electrochemical conversion it was intended to assist.