Unveiling capacity limitations of MnO 2 in rechargeable Zn chemistry

Abstract

MnO 2 dissolution during first discharge in neutral Zn–MnO 2 batteries originates preferentially from Mn–Mn edge-sharing coordination. Aqueous Zn–MnO 2 batteries with mildly acidic electrolytes deliver attractive experimental capacities, however the underlying mechanisms remain elusive, particularly regarding the interactions of Zn 2+ and H + with MnO 2 , as well as the formation of Mn 2+ and Zn 4 SO 4 (OH) 6 · x H 2 O (ZSH). Although these products are compatible with a two-electron dissolution mechanism, the observed first-discharge capacity is limited to approximately 300 mA h g −1 MnO 2 , close to that of a one-electron reaction. To address this contradiction, commonly used α-MnO 2 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. MnO 2 dissolution into Mn 2+ 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-sharing MnO 2 octahedra, which, unlike the edge-sharing ones, are hindered from undergoing disproportion, limiting the MnO 2 dissolution and explaining the reduced capacity. This comprehensive mechanistic understanding opens new pathways for the selection of the most appropriate MnO 2 phases and the optimization of electrodes to improve the performance of aqueous Zn–MnO 2 battery systems.