Using experimental values where available (calibration set, Table ) and filling in the gaps with our DFT-computed energies, we construct Pourbaix diagrams for 1 molal aqueous solutions of Li + , Na + , K + , Mg 2+ , Ca 2+ , Zn 2+ and Al 3+ . The energies of ionic species are taken from reference data. We find that of the 100+ A x MnO 2 compounds we calculated, only three (𝛼-K 0.125 MnO 2 , 𝛼-Na 0.125 MnO 2 , and λ-Li 0.5 MnO 2 ) are thermodynamically stable at any value of pH and voltage (Fig ). All other ion-polymorph pairs are found to be metastable with respect to β-MnO 2 , Mn 2 O 3 , Mn 3 O 4 , and 𝛿-Mn(OH) 2 . The stability windows of these three compounds exist predominately at high pH, in agreement with the finding that proton redox dominates at lower pH (Fig ). The inserted ions in these compounds are also the same ones found previously to be strongly favored over proton insertion at pH 14 (Fig. ). The conditions chosen for this Pourbaix analysis are reflective of those experienced by Mn oxide electrodes in electrochemical devices. Previous work has shown, however, that a greater number of ion-inserted phases can be stabilized with supersaturated solutions that have even greater ion concentrations. While only three ion-inserted polymorphs are thermodynamically stable under electrochemical conditions, several others are highly metastable. Figure shows the free energies of five of these low-lying metastable phases (𝛿-Ca 0.25 MnO 2 , 𝛿-K 0.25 MnO 2 , λ-Mg 0.5 MnO 2 , 𝛿-Na 0.25 MnO 2 , and λ-Zn 0.5 MnO 2 ) as a function of applied voltage. For example, at pH 14 and 950 mV vs. RHE, 𝛿-Ca 0.25 MnO 2 and 𝛿-K 0.25 MnO 2 are within 35 meV of the ground state, while λ-Mg 0.5 MnO 2 and 𝛿-Na 0.25 MnO 2 are within 65 meV. Such low driving forces to form the ground state phases implies that these metastable ion-inserted polymorphs can be trapped kinetically over a wider range of conditions than our Pourbaix analysis would suggest. Therefore, to gain a comprehensive understanding of MnO x electrode behavior, possible reaction pathways involving metastable phases and the associated driving forces to form more stable phases must be evaluated. The energies tabulated in Table enable this type of analysis for many different ions in the Mn oxide system. In general, the small energy differences between many different Mn oxide phases suggests that electrochemical reaction pathways in this class of materials depends heavily on kinetic barriers. Estimation of these barriers with, for example, MD simulations or with the recently developed surface-stochastic walking method, will be critical to gain a complete understanding of device performance and degradation modes in A x MnO 2 electrodes.