MnO2/Mn2+ chemistry: Charging protocol and electrolyte regulation

Aqueous rechargeable Zn-MnO2 batteries based on the dissolution/deposition mechanism of MnO2/Mn2+ are gaining increasing attention due to their high capacity and structural simplicity. One of the major concerns is the Mn2+/Mn3+ side reaction, which hampers the coulombic efficiency (CE) due to Mn3+(aq) disproportionation. However, factors affecting Mn3+ formation have not been systematically investigated. In this study, we utilized in situ optical microscopy and Scanning Electron Microscopy (SEM) to evaluate the formation of Mn3+ by observing its disproportionation product: the randomly deposited MnO2. We found that an excessively high charging voltage and a low electrolyte pH (pH<4.2) were shown to adversely accelerate Mn3+ formation. Most reports on the Mn2+/MnO2 cathode indicate a coulombic efficiency of only 80 % on carbon felt (thickness: 2.5 mm) at 2 mAh/cm2 due to the inherently low electrical conductivity of MnO2. Here with the optimized charging protocol and the utilization of the anode-friendly, methanesulfonic acid (MSA)-containing electrolyte, we achieved a CE of nearly 100 % for up to 200 cycles at 2 mAh/cm2. This work gives guidelines on the electrolyte design and charging protocol optimization towards high-performance MnO2/Mn2+ cathodes.