Unveiling the origin of the capacity fade in MnO$_{2}$ zinc-ion battery cathodes through an analysis of the Mn vacancy formation
Currently explored rechargeable aqueous zinc-ion battery (RAZIB) cathode materials, such as $α$-MnO$_{2}$, suffer from severe capacity fade when cycling at rates appropriate for grid-scale operation. Mn dissolution has been previously identified as the cause of $α$-MnO$_{2}$ cathode degradation during RAZIB cycling, with conflicting evidence being found in support of the proposed Jahn-Teller effect-assisted charge disproportionation reaction as the mechanism behind Mn dissolution. In order to unveil the Mn dissolution mechanism in MnO$_{2}$ cathode cells under RAZIB operation conditions, the energetic feasibility for Mn vacancy formation was probed in both charged (MnO$_{2}$) and discharged (ZnMn$_{2}$O$_{4}$) phases of $α$ and $λ$ polymorphs of MnO$_{2}$ using density functional theory. The formation of a Mn vacancy, and consequently the dissolution of Mn as Mn$^{2+}_{(aq)}$, was found to be thermodynamically feasible for the $α$-ZnMn$_{2}$O$_{4}$ phase due to the energetically unfavourable Zn bent coordination formed during the Zn$^{2+}$ intercalation process, indicating that Mn dissolution is promoted by an unstable Zn coordination environment. The theoretical calculations were then corroborated by operando $^{1}$H nuclear magnetic resonance experiments which captured the Mn dissolution occurring throughout the RAZIB discharge, with subsequent electrochemical deposition of the Mn atoms on the electrode during charge. The combined computational and experimental analysis reveals the critical role of defect energetics and coordination environment in driving active material dissolution, and consequently capacity fade, with the proposed mechanism also relevant for understanding cathode degradation in other intercalating ion battery chemistries.