High temperature behavior of NaxFe[Fe(CN)6] Prussian blue analogues probed by in-situ Mössbauer spectroscopy

Abstract Prussian blue (PB), Na x Fe[Fe(CN) 6 ], is an intriguing cathode material for low-cost sodium ion batteries. Due to the aqueous processing routes employed in its synthesis, the complete removal of water from the material is an essential step before the cathode can be incorporated into non-aqueous battery systems. The challenge is that water plays a critical role in stabilizing the open framework structure and its removal induces phase transitions that ultimately leads to structural collapse. This temperature dependent investigation uses in-situ X-ray diffraction and 57 Fe Mössbauer spectroscopy to investigate how the structural changes induced by dehydration of cubic ( x = 0.6) and monoclinic ( x = 1.8) PB impacts the local electron dynamics surrounding the two distinct redox active Fe sites. With XRD, phase transitions were observed with temperature in both materials, ultimately resulting in formation of the dehydrated rhombohedral structure and the resulting structural collapse around 240 °C. Mössbauer spectroscopy revealed that the cubic x = 0.6 sample retains its structure until dehydration, but that the monoclinic x = 1.8 sample undergoes significant structural changes as it transitions to rhombohedral and cubic phases. Both materials oxidized during the Mössbauer measurements around 240 °C following structural collapse.