In 𝛼-MnO 2 (Fig. ), large cations like Na + , K + and Ca 2+ are favored to insert at low capacity. These large cations occupy the 4-coordinated 2a (Na + & Ca 2+ ) and 8-coordinated 2b (K + ) Wyckoff sites at the center of the polymorph's [2×2] tunnels. For K + and Na + , the ionic bonds formed with the surrounding oxygens are strong enough that removing these cations from their sites entirely requires applied voltages beyond the oxidative stability limit of water in alkaline electrolytes. This finding agrees well with experiment, as 𝛼-MnO 2 is typically synthesized with small amounts of intercalated K + , Na + or Ba 2+ , and extraction of these ions is achieved with acid treatment. However, the large sizes of Na + , K + , and Ca 2+ severely limit the maximum capacities that can be achieved, with the sites filling completely at < 200 mAh/g. Higher capacities can be achieved with smaller ions (e.g. Mg 2+ , Zn 2+ , Al 3+ ), which preferentially fill the 8h sites. However, beyond Mn 3.875+ , proton insertion is thermodynamically favored relative to these other ions. Notably, because the large [2×2] tunnels limit the number of available host sites, we find that the full 2-electron capacity of MnO 2 can only be achieved with protons in the 𝛼 polymorph.