This work demonstrates the crucial role of [Fe(CN)6] 4-anion vacancy in determining the Na/K intercalation processes in KFeFe-PBAs and their electrochemical performance as the cathode materials in hybrid NIBs. The KFeRT cathode with a higher content of anion vacancy outperforms the KFe60 cathode with a lower content of anion vacancy, by delivering an initial discharge capacity of ~130 mAh g -1 at 25 mA g -1 and retaining 86% and 77% capacity after 100 and 300 cycles, respectively, as well as 63 mAh g -1 at 500 mA g -1 . Electrochemical measurements and DFT calculations reveal that the presence of anion vacancy further promotes K + intercalation over Na + intercalation, reduces K + migration energy barrier, and enhances ion diffusion kinetics, which results in the K + dominated intercalation at both high and low plateaus while suppressing Na + intercalation. The calculations also reveal that K-PBA framework with different anion vacancy levels has different preference of K/Na intercalation, leading to different cation intercalation behavior. K-PBAs are an extremely interesting and promising class of cathode materials in Na cells, owing to the preferential insertion of K + over Na + in the K-PBA framework and the resulting increase in the insertion voltage. Taking full advantage of this kind of hybrid ion batteries requires the understanding of K/Na kinetics and how it plays with varying the structures of K-PBAs. This work highlights compositional design, including anion vacancy, is the key to enhance the electrochemical performance of hybrid ion batteries.