Dehydration Conditions and Ultrafast Rehydration of Prussian White: Phase Transition Dynamics and Implications for Sodium-Ion Batteries.

Prussian White (PW) is a strategic cathode material for sodium-ion batteries, offering a high theoretical capacity and voltage. However, the crystalline structure and the electrochemical performance of PW strongly depend on the hydration level, which is difficult to control, leading to discrepancies in the results and interpretations presented in the literature. This work aims to provide a deeper insight into the dehydration process of PW materials and a better understanding of the impact of their fast rehydration, upon exposure to moisture, on their characterization. For this purpose, a Na_1.87Mn[Fe(CN)₆]_0.99·1.99H₂O sample was synthesized by a coprecipitation method and subsequently dehydrated to remove water. After thorough characterization, our findings show that drying parameters, such as temperature and pressure, strongly influence the post-drying result. Moreover, the dehydrated samples rehydrate within minutes of exposure to air, which may explain some discrepancies observed in the literature and highlights the necessity to work under fully air-tight conditions.