Solvation effects. Ions in aqueous electrolytes are surrounded by large solvation shells of hydrogen-bonded water molecules. Because of their small radii and high polarity, water molecules that directly coordinate solvated ions can be tightly bound, particularly to smaller ions with higher charges. This strong coordination makes it possible for ions to insert into MnO 2 crystal structures with partial solvation shells. However, of the 6 most common MnO 2 polymorphs, only the 𝛼 and 𝛿 phases possess channels that are large enough to accommodate water molecules. These polymorphs are typically synthesized with water molecules inside their channels, but whether water intercalates dynamically during redox reactions remains a subject of debate. To investigate the effects that water can have on Mn oxide electrochemistry, we calculate the voltages and corresponding capacities of ions inserting into MnO 2 with partial solvation shells (Fig. ). In all cases, solvation shells were initialized with the oxygens of the water molecule residing approximately 2 Å away from the inserted ion and oriented according to the results of previous neutron diffraction experiments. Like in Fig. , higher voltages correspond to more favorable insertion events.