Fe-based potassium Prussian blue analogues (K-PBAs) are traditionally used as K-ion battery cathodes. Interestingly, K-PBAs are appealing cathodes for Na-ion batteries (NIBs), due to the increased cation intercalation voltage compared to Na-PBAs. In such a hybrid NIB cell, where Na+ is in the electrolyte and K+ 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 [Fe(CN)6]4- anion vacancy in K-PBAs on regulating K+/Na+ intercalation mechanism in hybrid NIB cells, by comparing two K-PBA cathodes with different vacancy contents. Experimental and computational results demonstrate that introducing a level of anion vacancy can maximize the number of K+ intercalation sites and enhance K+ diffusion in the PBA framework. This facilitates K+ intercalation and suppresses Na+ intercalation, resulting in a K+-dominated and high-discharge-voltage ion storage process in the hybrid NIB cell. The K-PBA cathode with 20% anion vacancy delivers 127 mAh g-1 at 50 mA g-1 and 63 mAh g-1 at 500 mA g-1, as well as retains 87% and 77% capacity after 100 and 300 cycles, respectively. It completely outperforms the counterpart with 7% anion vacancy, which exhibits increased Na+ intercalation but overall deteriorated ion storage. Our results show the promise of hybrid battery systems and the crucial role of vacancy regulation in designing electrode materials for these systems.
Replacing EIS results with GITT results.