Water driven phase transitions in Prussian White cathode materials

Abstract Prussian White (PW, Na2Fe[Fe(CN)6]∙zH2O) is a promising cathode material for use in sodium-ion batteries (SIBs) for large-scale energy storage applications which demand long cycling life-times. For use in non-aqueous battery applications PW must not contain any water, and yet its removal induces a large volume change destabilizing the structure and reducing cycling life. Indeed, the material undergoes multiple phase transitions depending upon both the sodium and water content. Due to the extensive use of X ray diffraction, the mechanism behind the observed phase transitions is poorly understood. Here, we show a neutron diffraction study exploring the influence of water on the structure of PW. For the first time, two temperature dependent and composition independent structures were observed near room temperature which differ in the FeN6 and FeC6 octahedral tilting configurations. The fundamental nature of this distortion, which drives the large volume changes, was determined to be connected to water ordering in the framework. Further, using distortion mode analysis during the dehydration process, it was determined that water does not modify the nature of the distortions present. Rather, water was found to modulate the magnitude of pre-existing structural distortions. These results provide a robust fundamental understanding to the chemical driving forces impacting the nature and magnitude of structural distortions in PBAs. The insight provides guidance for designing and quantifying tilt-engineering in PBAs ultimately enabling new materials with enhanced long-term electrochemical performance in battery applications.