Anion Vacancy Regulated Sodium/Potassium Intercalation in Potassium Prussian Blue Analog Cathodes for Hybrid Sodium‐Ion Batteries

Abstract Fe‐based potassium Prussian blue analogs (K‐PBAs) are commonly used as K‐ion battery (KIB) cathodes. Interestingly, K‐PBAs are appealing cathodes for Na‐ion batteries (NIBs). In a hybrid NIB cell, where Na‐ion is in the electrolyte and K‐ion is in the PBA cathode, cation intercalation and electrochemical performance of the cathode can be significantly affected by [Fe(CN)6]⁴⁻ anion vacancy. This work studies the effect of anion vacancy in K‐PBAs on regulating K‐ion/Na‐ion intercalation mechanism in hybrid NIB cells, by comparing two K‐PBA cathodes with different vacancy contents. The results demonstrate that introducing a level of anion vacancy can maximize the number of K‐ion intercalation sites and enhance K‐ion diffusion in the PBA framework. This facilitates K‐ion intercalation and suppresses Na‐ion intercalation, resulting in a K‐ion‐dominated and high‐discharge‐voltage ion storage process in the hybrid NIB cell. The K‐PBA cathode with 20% anion vacancy delivers 128 mAh g⁻¹ at 25 mA g⁻¹ and 67 mAh g⁻¹ at 1000 mA g⁻¹, as well as retains 89% and 81% capacity after 100 and 300 cycles, respectively. It completely outperforms the counterpart with 7% anion vacancy, which exhibits increased Na‐ion intercalation but overall deteriorated ion storage.