Impact of Synthesis Conditions in Na-Rich Prussian Blue Analogues

Sodium-rich iron hexacyanoferrates were prepared by coprecipitation, hydrothermal route, and under reflux, with or without dehydration. They were obtained with different structures described in cubic, orthorhombic, or rhombohedral symmetry, with variable compositions in sodium, water, and cationic vacancies and with a variety of morphologies. This series of sodium-rich Prussian blue analogues allowed addressing the relationship between synthesis conditions, composition, structure, morphology, and electrochemical properties in Na-ion batteries. A new orthorhombic phase with the Na_1.8Fe₂(CN)₆·0.7H₂O composition synthesized by an hydrothermal route at 140 °C is reported for the first time, whereas a phase of Na₂Fe₂(CN)₆·2H₂O composition obtained under reflux, previously described with a monoclinic structure, shows in fact a rhombohedral structure.