Suppressing Surface Degradation in Na-Rich Prussian Blue Cathodes via Liquid-Phase Dehydration

Na-rich Prussian Blue (PB) is an attractive sodium-ion battery cathode owing to its high capacity and low cost, yet approximately 10 wt.% of crystal water in its framework induces electrolyte side reactions and Fe dissolution. Conventional thermal dehydration has been used to remove crystal water, but it consistently results in capacity fading and poor air stability. Here, the primary cause of degradation is experimentally demonstrated to be surface oxidation driven by Fe-O bond formation during heat treatment. Guided by this insight, a liquid-phase, low-temperature dehydration strategy based on nitrogen bubbling is introduced, which simultaneously eliminates crystal water and stabilizes the surface. The approach can be seamlessly integrated into electrode fabrication to minimize air exposure and improve cycling stability. Overall, this work clarifies the mechanistic role of surface oxidation in moisture-sensitive PB cathodes and highlights the critical importance of surface chemistry control in achieving stable electrochemical performance.