While several highly metastable ion-inserted solids exist between Mn 3+ and Mn 4+ , metastability is less pronounced at lower oxidation states. Among Mn 2+ solids, layered 𝛿-Mn(OH) 2 is the most stable, while the next lowest energy phase, λ-Mn(OH) 2 , is close to 100 meV less stable than 𝛿-Mn(OH) 2 and the β and 𝛼 hydroxides are less stable by more than 150 meV. This larger difference in stability compared to the Mn 3+ and Mn 4+ solids rationalizes why layered phases often form at reducing voltages regardless of the polymorph present initially. Lastly, we note that debate exists over whether bixbyite 𝛼-Mn 2 O 3 is thermodynamically stable with respect to MnOOH. 𝛼-Mn 2 O 3 appears in the original Pourbaix diagram, but no MnOOH phases were considered. Recent DFT calculations have predicted MnOOH to be more stable, which agrees qualitatively with experimental observations that 𝛼-Mn 2 O 3 formation requires elevated temperatures and that MnOOH forms when 𝛼-Mn 2 O 3 and Mn 3 O 4 are aged in aerated water. However, solution-phase potential-pH measurements of MnOOH phases have reported free energies slightly less stable than that of 𝛼-Mn 2 O 3 . To the best of our knowledge, no hightemperature heat capacity measurements of the Mn oxyhydroxides have been reported to resolve this debate. Therefore, in our analysis, we use the experimental values that have been reported thus far, which our calculations reproduce (Table ).