Unveiling the origin of the capacity fade in MnO2 zinc-ion battery cathodes through an analysis of Mn vacancy formation

Currently explored rechargeable aqueous zinc-ion battery (RAZIB) cathode materials, such as α-MnO₂, suffer from severe capacity fade when charging and discharging at rates appropriate for grid-scale operation. Mn dissolution has been previously identified as the cause of α-MnO₂ 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 MnO₂ cathode cells under RAZIB operation conditions, the energetic feasibility for Mn vacancy formation was probed in both charged (MnO₂) and discharged (ZnMn₂O₄) phases of α and λ polymorphs of MnO₂ using density functional theory. The formation of a Mn vacancy, and consequently the dissolution of Mn as Mn²⁺_(aq), was found to be thermodynamically feasible for the α-ZnMn₂O₄ phase due to the energetically unfavourable Zn bent coordination formed during the Zn²⁺ intercalation process, indicating that Mn dissolution is promoted by an unstable Zn coordination environment. The theoretical calculations were then corroborated by operando¹H 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 environments 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. This article provides fundamental insight into the active material dissolution mechanism, offering the basis for the development of capacity fade mitigation strategies and discovery of novel cathode materials for prolonged stable battery operation.