The maximum ion capacity of a given polymorph was taken to be the highest degree of intercalation in which the original MnO 2 polymorph structure is maintained. Beyond this maximum capacity, continued ion insertion causes dramatic structural rearrangement, which can lead to irreversible capacity degradation. Examples of such rearrangements include broken Mn-O bonds (considered in this work to be at distances greater than 3 Å) and Mn atom migration to non-octahedral sites. Because we exclude such distorted phases, the maximum ion capacities we calculate for each polymorph differ from previous work. In Fig. , voltages are plotted against x in A x MnO 2 for direct comparison of maximum ion capacity.