In 𝛿-MnO 2 , the stacked layers of Mn octahedra expand and slide upon insertion of water molecules. This rearrangement, along with the presence of extra oxygen atoms in the interlayer gap, disrupts the regular bonding between inserted ion and Mn oxide framework. Therefore, partial solvation shells have a slight destabilizing effect, with ion insertion voltages being slightly lower with water than without. This finding suggests that co-insertion with water in the 𝛿 polymorph is primarily a kinetic effect, governed by high barriers to ion desolvation rather than by strong interactions with the Mn oxide structure. Previous experiments have also shown a relationship between temperature-driven water loss in 𝛿-MnO 2 and the hydration energies of inserted ions, demonstrating that interlayer water is tightly-bound. Large ions like Na + , K + , and Ca 2+ reside in the same plane as the co-inserted water molecules whereas smaller ions sit in between the water layer and the layer of Mn octahedra. In both cases, 𝛿-MnO 2 undergoes a c-axis expansion from ~4 Å to ~7 Å upon ion insertion, consistent with experimental data on the birnessite family of structures. While it is well known that highly charged ions like Zn 2+ and Mg 2+ can insert with multiple water layers, leading to c-axis expansions as large as 10 Å, the energetic effect of water co-insertion should be well-captured by calculations containing a single water layer. The ability of these layers to expand is what gives 𝛿-MnO 2 a higher maximum ion capacity than the 𝛼 polymorph.