Interestingly, the three thermodynamically stable compounds in Figure all contain singly valent cations. However, Figure indicated that multivalent ion insertion could be favorable relative to proton redox in certain polymorphs given a low H + concentration (e.g. Zn 2+ in λ-MnO 2 , Al 3+ in 𝛿-MnO 2 ). The Pourbaix diagrams in Figure reveal precipitated (hydr)oxides to be a more stable configuration for multivalent cations in water at high pH. Phases like ZnO, Ca(OH) 2 and Mg(OH) 2 are thermodynamically preferred relative to their corresponding ion-inserted manganese oxides. As a result, developing a multivalent ion shuttle battery that can access the full 2 electron capacity of Mn oxide may require significant engineering of the electrolyte to limit formation of these precipitated phases.