The Role of Al 3+ ‐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 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 .