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

Rechargeable aqueous aluminium batteries are the subject of growing interest, but the charge storage mechanisms at manganese oxide-based cathodes remain poorly understood with as many mechanisms as studies. Here, we use an original in situ spectroelectrochemical methodology to unambiguously demonstrate that the reversible proton-coupled MnO₂-to-Mn²⁺ conversion is the main charge storage mechanism occurring at MnO₂ cathodes over a range of slightly acidic Al³⁺-based aqueous electrolytes. In Zn/MnO₂ assemblies, this mechanism is associated with high gravimetric capacity and discharge potentials, up to 560 mAh·g^-1 and 1.76 V respectively, attractive efficiencies (CE > 98.5 % and EE > 80%) and excellent cyclability (> 750 cycles at 10 A·g^-1). Finally, we conducted a critical analysis of the data previously published on MnO_x cathodes in Al³⁺-based aqueous electrolytes to conclude on a universal charge storage mechanism, i.e., the reversible electrodissolution/electrodeposition of MnO₂.