Unveiling anomalous lattice shrinkage induced by Pi-backbonding in Prussian blue analogues

Abstract

Transition-metal (TM)-based Prussian blue and its analogues (TM-PBAs) have attracted considerable attention as cathode materials owing to their versatile ion storage capability with tunable working voltages. TM-PBAs with different crystal structures, morphologies, and TM combinations can exhibit excellent electrochemical properties because of their unique and robust host frameworks with well-defined <100> ionic diffusion channels. Nonetheless, there is still a lack of understanding regarding the performance dependence of TM-PBAs on structural changes during charging/discharging processes. In this study, in situ X-ray diffraction and X-ray absorption fine structure analyses elucidate the TM-dependent structural changes in a series of TM-PBAs during the charging and discharging processes. During the discharging process, the lattice volume of Fe-PBA increased while those of Ni- and Cu-PBAs decreased. This discrepancy is attributed to the extent of size reduction of the cyanometallate complex ([Fe(CN)6]) via pi-backbonding from Fe to C due to redox flips of the low-spin Fe3+/2+ ion. This study presents a comprehensive understanding of how TM selection affects capacity acquisition and phase transition in TM-PBAs, a promising class of cathode materials.