Unveiling Capacity Limitations of MnO2 in Rechargeable Zn Chemistry

Aqueous Zn–MnO2 batteries with mildly acidic electrolytes show attractive experimental capacities, however the underlying mechanisms remain elusive, particularly regarding the interactions of Zn2+ and H+ with MnO2, as well as the formation of Mn2+ and Zn4SO4(OH)6·xH2O (ZSH). Although these products are compatible with a two-electron dissolution mechanism, the observed first-discharge capacity is limited to approximately 300 mAh g-1MnO2, close to that of a one-electron reaction. To address this contradiction, commonly used α-MnO2 nanowires were chosen as cathode material and investigated by a systematic multimodal and multiscale approach under operando or ex-situ conditions to analyze the processes that occur during the first discharge. MnO2 dissolution into Mn2+ and ZSH precipitation were confirmed, and the formation of a disordered phase at the nanowire surface with the accumulation of Mn(III) was detected. An in-depth analysis indicates that such Mn(III) species correspond to protonated corner-shared MnO2 octahedra, which, unlike the edge-shared ones, are hindered from undergoing disproportion, limiting the MnO2 dissolution and explaining the reduced capacity. This comprehensive mechanistic understanding opens new pathways for the selection of the most appropriate MnO2 phases and the optimization of electrodes to improve the performance of aqueous Zn–MnO2 battery systems.