Characterizing the MnO2 dissolution process via EQCM for rechargeable aqueous batteries

Abstract

Aqueous rechargeable batteries are attractive for grid-scale energy storage due to their intrinsic safety and low cost capability. Among potential cathode materials, manganese dioxide (MnO₂) has drawn significant attention because of its abundance and high theoretical capacity enabled by proton-coupled redox chemistry. Despite these advantages, MnO₂-based systems often suffer from limited cycling stability caused by dissolution-related capacity losses, the origins of which remain poorly understood. In this work, electrochemical quartz crystal microbalance (EQCM) measurements are employed to directly monitor the mass evolution of MnO₂ electrodes during deposition and dissolution in acidic to mildly acidic aqueous electrolytes. By correlating mass change with charge transfer in real time, electrochemical and chemical contributions to MnO₂ dissolution are distinguished and quantified. The results show that MnO₂ discharge proceeds through electrochemical proton-coupled dissolution to Mn²⁺, accompanied by non-faradaic chemical dissolution that leads to irreversible mass loss. The relative contributions of these processes are strongly dependent on electrolyte pH. Highly acidic electrolytes promote complete electrochemical dissolution but accelerate chemical dissolution, whereas higher pH electrolytes suppress chemical loss at the expense of incomplete MnO₂ dissolution. By balancing these competing effects, a mildly acidic electrolyte is identified that enables reversible MnO₂ cycling with coulombic efficiencies approaching 99%. These findings provide quantitative insight into MnO₂ dissolution chemistry and establish practical electrolyte design guidelines for improving the reversibility of Mn-based aqueous battery systems.