The Role of Al³⁺ -Based Aqueous Electrolytes in the Charge Storage Mechanism of MnO_x Cathodes

Rechargeable aqueous aluminium batteries are the subject of growing interest, however, the charge storage mechanisms at manganese oxide-based cathodes remain poorly understood. In essense, every study proposes a different mechanism. Here, an in situ spectroelectrochemical methodology is used to unambiguously demonstrate that reversible proton-coupled MnO₂ -to-Mn²⁺ conversion is the main charge storage mechanism occurring at MnO₂ cathodes for a range of slightly acidic Al³⁺ -based aqueous electrolytes, with the Al³⁺ hexaaquo complex playing the key role of proton donor. In Zn/MnO₂ assemblies, this mechanism is associated with high gravimetric capacities and discharge potentials, up to 560 mAh g^-1 and 1.65 V respectively, attractive efficiencies (CE > 99.5% and EE > 82%) and excellent cyclability (almost 100% capacity retention over 1 400 cycles at 2 A g^-1 ). Finally, a critical analysis of the data previously published on MnO_x cathodes in Al³⁺ -based aqueous electrolytes is conducted to conclude on a universal charge storage mechanism, i.e., the reversible electrodissolution/electrodeposition of MnO₂ .