In-Depth Understanding of Dehydration and Rehydration Behaviors of Prussian Blue Analogs in Sodium-Ion Batteries

Prussian blue analogs (PBAs) are considered one of the most promising cathode materials for sodium-ion batteries. Nevertheless, the high crystal water content in PBAs has been identified as the principal constraint on their commercial applications. To address the contradiction of PBAs being challenging to dehydrate at low temperatures while being susceptible to decomposition at high temperatures, we conduct a systematic investigation of the macroscopic and microscopic structural characteristics of pilot-synthesized PBAs during dehydration, elaborating on a series of thermodynamic and kinetic behaviors associated with this process. Based on the X-ray absorption fine structure analysis (XAFS) and theoretical calculations, we propose three modes of temperature-induced charge transfer during dehydration and innovatively introduce the fractal dimension to assess the effect of the dehydration level. In addition, we investigate the rehydration behavior of PBAs, quantify the impact of water absorbed on the phase transition, and finally identify an operation procedure suitable for industrial production. It has been demonstrated that dehydration of PBAs under the optimal heat treatment process can effectively activate the redox reaction of low-spin Fe and improve the flatulence of pouch cells, which exhibit a capacity retention of 77.9% after 1800 cycles at 1 C, indicating significant application potential.