The Role of Al3+‐Based Aqueous Electrolytes in the Charge Storage Mechanism of MnOx Cathodes

Abstract 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 MnO 2 ‐to‐Mn 2+ conversion is the main charge storage mechanism occurring at MnO 2 cathodes for a range of slightly acidic Al 3+ ‐based aqueous electrolytes, with the Al 3+ hexaaquo complex playing the key role of proton donor. In Zn/MnO 2 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 Al 3+ ‐based aqueous electrolytes is conducted to conclude on a universal charge storage mechanism, i.e., the reversible electrodissolution/electrodeposition of MnO 2 .