Significant degradation in electrochemical performance of a low crystalline Prussian Blue, with almost complete loss of capacity over 10 voltammetry cycles, was identified as being caused by K + becoming trapped within the partially intercalated Prussian blue structure. The degradation is caused by an increase in structural disorder on cycling, which can be observed from the perspectives of framework Fe and also the K + species. Specifically, non-conductive ferrihydrite forms on repeated cycling, which results in a loss of electrochemical activity, and K + becomes trapped within the remaining thermodynamically stable Prussian blue within the cathode. Potentially this process is exacerbated by the poor electrical conductivity of these small crystallite containing samples that aggregate together when forming composite electrodes.