Electrochemical stability of ZnMn2O4: Understanding Zn-ion rechargeable battery capacity and degradation

We present a refined Mn-Zn-H$_2$O Pourbaix diagram with the emphasis on parameters relevant for the Zn/MnO$_2$ rechargeable cells. It maps out boundaries of electrochemical stability for MnO$_2$, ZnMn$_2$O$_4$, ZnMn$_3$O$_7$, and MnOOH. The diagram helps to rationalize experimental observation on processes and phases occurring during charge/discharge, including the position of charge/discharge redox peaks and capacity fade observed in rechargeable aqueous Zn-ion batteries for stationary storage. The proposed Pourbaix diagram is validated by observing the pH-dependent transformation of electrolytic manganese dioxide to hetaerolite and chalcophanite during discharge and charge, respectively. Our results can guide the selection of operating conditions (the potential range and pH) for existing aqueous Zn/MnO$_2$ rechargeable cells to maximise their longevity. In addition, the relation between electrochemical stability boundaries and operating conditions can be used as an additional design criterion in exploration of future cathode materials for aqueous rechargeable batteries.