Diagnosing the Role of Mn Additives in Aqueous Zn/MnO 2 Batteries

ABSTRACT Manganese dioxides are pervasive as cathode materials for aqueous zinc‐ion batteries; however, their energy storage mechanism has proven to be complex. Increasing evidence suggests that Mn dissolution contributes to cathode capacity via a reversible dissolution‐redeposition process, in addition to capacity from Zn 2+ /H + (de)intercalation. Understanding the role played by Mn 2+ in the electrolyte, particularly when introduced as an electrolyte additive, is important for diagnosing cathode degradation. Using operando X‐ray fluorescence (XRF) spectroscopy, we track Mn 2+ in operating cells to gain insights into the complexity of the MnO 2 cathode. While some suppression of cathode dissolution by the additive cannot be excluded, our measurements suggest that Mn 2+ electrolyte additives improve cycling performance by supplying extra redox‐active Mn during charge, rather than solely by suppressing Mn dissolution, thereby compensating for cathode degradation. Benefiting from operando XRF, we propose that a potential contributor to capacity fading is the incomplete dissolution of the MnO 2 cathode and incomplete redeposition of Mn 2+ from the electrolyte, and that this fading does not appear to be mitigated by using Mn 2+ additives. These findings underscore the necessity of directly tracking Mn 2+ evolution, and that improving reversibility requires addressing both the dissolution of MnO 2 and the redeposition of Mn 2+ .