Despite the identified potential, unexpected complexities within the PB structure have made fundamental understanding of intercalation processes difficult. The PB structural framework is highly flexible, forming a cubic structure that can accommodate up to 25% [Fe II (CN) 6 ] 4- defects, with the subsequent under-coordinated Fe 3+ ions being bound to water, forming (H 2 O) 6 clusters (Scheme 1). In addition, the structure can accommodate significant amounts of uncoordinated occluded water. These structural variations influence the location of A site alkali cations within PB, which has ramifications in understanding the balance of charges during redox processes. In an ideal structure (Scheme 1a), reduction of Fe 3+/III sites is balanced by the intercalation of K + within the vacant channels of the structure, with the fully reduced Fe 2+ Fe II structure, known as Prussian white (PW) having a 1:1 K:Fe ratio. In the fully oxidized Fe 3+ Fe III structure, commonly referred to as Berlin Green (BG), K + is fully deintercalated. However, the influence of (H 2 O) 6 clusters, which are prevalent within the defective structure, has been debated, with some work concluding that these sites can inhibit ion insertion into the channels, while others deducing that K + sits within the (H 2 O) 6 cluster itself (Scheme 1b). The complications in understanding K + ion location have resulted in ambiguity when assigning the charge-balancing species during framework redox processes, with suggestions that in aqueous electrolytes H 3 O + or K + can act as counterions. Indeed, similar uncertainty is also observed for other cations that can be incorporated into the structure, such as Mg . Another challenge in utilizing PB for energy storage applications is its varied stability over repeated cycling. Highly crystalline materials, considered to have a limited number of defects, are reported to favor superior cycle-life and rate capability. The importance of [Fe(CN) 6 ] 4- defects in contributing to structural collapse is stated to be due to their inhibition of ion insertion and decrease in electronic conductivity. However, understanding the loss of capacity is again complicated by the differing process mechanisms that are suggested for charge balancing and K + intercalation.