  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.
