In the β and R polymorphs (Fig. ), H + insertion is favored, even at pH 14. The smaller [1×1] tunnels of the rutile β structure prevent ions larger than Zn 2+ (e.g. Na + , Ca 2+ , K + ) from inserting while the [2×1] tunnels of R-MnO 2 are just large enough to accommodate Na + at low concentrations. The voltages and maximum capacities of both Zn 2+ and Mg 2+ insertion into the 2b octahedral sites of β-MnO 2 are low, however. Insertion beyond Mn 3.75+ leads to broken Mn-O bonds and structural distortions (Fig ). The slightly larger tunnels of the R polymorph are a more suitable host for these ions. Like the 𝛼 polymorph, the full 2-electron capacity of β-MnO 2 is only accessible via proton insertion. While the R polymorph could, in theory, achieve 2-electron redox with Mg 2+ and Zn 2+ ions, H + will insert preferentially, if available.