Tuning the Valence of MnO x Cathodes Reveals the Universality of the Proton‐Coupled Electrodissolution/Electrodeposition Mechanism in Rechargeable Aqueous Zn Batteries

Abstract The charge storage mechanism of manganese oxides (MnO x ) in rechargeable mild aqueous Zn batteries is still a matter of debate, with no consensus on mechanism and charge carriers. In this work, this issue is addressed by relying on a quantitative and comparative electrochemical study of low‐valence crystalline MnO x cathodes in a range of aqueous electrolytes, with a primary focus on spinel Mn 3 O 4 . The results demonstrate that the oxidation state of Mn in MnO x structures as well as their degree of crystallinity have little impact on the electrochemical reactivity, all MnO x being fully converted into soluble Mn 2+ upon discharge. It is further revealed that this electrodissolution mechanism is affected by Mn 3 O 4 dismutation in the most acidic electrolytes. Accordingly, the true thermodynamics and kinetics of Mn 3 O 4 are only accessible at mildly acidic to neutral pHs. In addition, the study confirms the phase transformation of Mn 3 O 4 to Mn 5 O 8 upon galvanostatic oxidation, and demonstrates that this transformation only proceeds in Mn 2+ ‐free electrolytes, since otherwise amorphous MnO 2 electrodeposits on top of Mn 3 O 4 . Finally, it is concluded that regardless of the starting MnO x material, after the first discharge or a first charge/discharge cycle, subsequent charge/discharge cycles rely on a reversible proton‐coupled electrodeposition of amorphous MnO 2 .