In neutral electrolytes, proton insertion is thermodynamically favored regardless of polymorph (Fig ). In basic electrolytes, however, competition exists between the insertion of protons and most non-protonic cations. Even when [H + ] = 10 -14 M, only a few ions are strongly favored to insert over protons, namely Na + & K + in 𝛼-MnO 2 , and Li + & Zn 2+ in λ-MnO 2 . The insertion voltages of other ions in 𝛼 and λ-MnO 2 are close to or lower than that of H + , and in the polymorphs with small tunnels (β & R, Figs. ), H + is always favored. This last finding is consistent with the wellknown proton insertion mechanism of alkaline primary batteries with highly concentrated KOH electrolytes. 𝛾-MnO 2 , the electrode material in these batteries, has [1×1] and [2×1] tunnels identical to the β and R polymorphs.