<p>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 <i>in situ</i>
spectroelectrochemical methodology to unambiguously demonstrate that the reversible
proton-coupled MnO<sub>2</sub>-to-Mn<sup>2+</sup> conversion is the main charge
storage mechanism occurring at MnO<sub>2</sub> cathodes over a range of slightly
acidic Al<sup>3+</sup>-based aqueous electrolytes. In Zn/MnO<sub>2</sub> assemblies,
this mechanism is associated with high gravimetric capacity and discharge
potentials, up to 560 mAh·g<sup>-1</sup> and 1.76 V respectively, attractive
efficiencies (<i>CE</i> > 98.5 % and <i>EE</i> > 80%) and excellent cyclability (>
750 cycles at 10 A·g<sup>-1</sup>). Finally, we conducted a critical analysis of the
data previously published on MnO<sub>x</sub> cathodes in Al<sup>3+</sup>-based
aqueous electrolytes to conclude on a universal charge storage mechanism, <i>i.e.</i>, the reversible
electrodissolution/electrodeposition of MnO<sub>2</sub>.<i></i></p>
The Role of Al3+-Based Aqueous Electrolytes in the Charge Storage Mechanism of MnOx Cathodes