In the 𝛼 polymorph, solvation shells have a slight stabilizing effect for smaller ions that preferentially occupy the 8h sites when water is not present. This can be attributed to two effects: a more stable coordination environment formed in part by the oxygen atoms of water molecules, and hydrogen bonding between the water molecule and the MnO 2 framework oxygen anions. The insertion of water molecules into the [2×2] tunnel creates new quasi-tetrahedral sites that small ions like Li + , Mg 2+ , Zn 2+ and Al 3+ occupy instead of the 8h sites (Fig. on the left). Because small, highly charged atoms also tend to have strong solvation energies, 98 it's highly probable that these ions will insert into the [2×2] tunnels with water molecules, particularly at low capacities. Na + , K + , and Ca 2+ , however, which are too large for the newly formed tetrahedral sites, don't exhibit a preference for inserting with water. In fact, Na + and K + exhibit a large destabilization with coinserted water because the extra water molecules displace these ions from the center of the 2a and 2b sites respectively.