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 .