Unveiling capacity limitations of MnO2 in rechargeable Zn chemistry

Abstract

Aqueous Zn–MnO₂ batteries with mildly acidic electrolytes deliver 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 mA h g⁻¹ MnO₂, 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-sharing MnO₂ octahedra, which, unlike the edge-sharing 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.