Tuning the valence of MnOx cathodes reveals the universality of the proton-coupled electrodissolution/electrodeposition mechanism in rechargeable aqueous Zn batteries

The charge storage mechanism of manganese oxides (MnOx) in rechargeable mild aqueous Zn batteries is still a matter of debate, with no consensus on mechanism and charge carriers. In this work, we address this issue by relying on a quantitative and comparative electrochemical study of low-valence crystalline MnOx cathodes in a range of aqueous electrolytes, with a primary focus on spinel Mn3O4. Our results unambiguously demonstrate that the oxidation state of Mn in MnOx structures as well as their degree of crystallinity have little impact on the electrochemical reactivity, all MnOx being fully converted into soluble Mn2+ upon discharge. We further reveal that this electrodissolution mechanism is affected by Mn3O4 dismutation in the most acidic electrolytes. Accordingly, the true thermodynamics and kinetics of Mn3O4 are only accessible at mildly acidic to neutral pH values. In addition, we confirm the phase transformation of Mn3O4 to Mn5O8 upon galvanostatic oxidation, and demonstrate that this transformation only proceeds in Mn2+-free electrolytes, since otherwise amorphous MnO2 electrodeposits on top of Mn3O4. Finally, we conclude that regardless of the starting MnOx material, after the first discharge or a first charge/discharge cycle, subsequent charge/discharge cycles rely exclusively on a reversible proton-coupled electrodeposition of amorphous MnO2.