To the best of our knowledge, no direct evidence has been provided in literature with regard to the correlation between the [Fe(CN)6] 4-anion vacancy in K-PBAs and K + /Na + intercalation mechanism/K-PBA cathode performance in hybrid NIB cells, impeding the full potential of such an interesting yet promising battery system. Herein, we report the synthesis of KFeFe-PBAs with different [Fe(CN)6] 4-anion vacancy contents by adjusting reaction temperature and the applications of KFeFe-PBAs as cathode materials in hybrid NIBs. A comparative study on the electrochemical performance, cation intercalation mechanism, and cation diffusion kinetics of the cathodes is carried out to understand how anion vacancy content regulates K + /Na + intercalation mechanism. Our results demonstrate that a higher vacancy content enhances reversible K + intercalation, which in turn suppresses Na + intercalation in the KFeFe-PBA cathodes, realizing the increase in intercalation voltage throughout the discharge process and enhancing cycling stability over a long term. The KFeFe-PBA cathode with 20% [Fe(CN)6] 4- vacancy delivered an initial discharge capacity of 129 mAh g -1 at 25 mA g -1 and retained 87% and 77% capacity after 100 and 300 cycles, respectively. The cathode exhibited high intercalation voltages at 3.61 and 2.97 V in the hybrid NIB cells, and the voltages were kept throughout the cycles.