indicate that anion vacancy content can affect the cation intercalation mechanism in K-PBAs when they are used as cathodes in NIBs, and as a result, a higher vacancy content can enhance the NIB performance of the K-PBA cathode. How increasing anion vacancy content in K-PBAs regulates Na + /K + intercalation and thus improves NIB performance is not only exciting but also intriguing, because previous literature favored a low anion vacancy content to be a crucial prerequisite for PBA cathodes in NIBs. We are mindful that previous literature Na cells, but this makes our results even more exciting and intriguing because to the best of our knowledge the understanding of the dependence of Na + /K + intercalation on the [Fe(CN)6] 4- vacancy content in KFeFe-PBAs has never been reported. We then draw our attention to cation intercalation processes, as the CV and GCD results have clearly suggested the processes were different between the two samples. We carried out ex-situ EDS analysis on the KFeRT and KFe60 electrodes at various states cation (de)intercalation (Figures and) and examined the Na and K contents (normalized to Fe) at each state (Figures and). Noted that the key difference lies in the discharge/charge plateau split at 2.8 V (state 3)/3.05 V (state 5) for KFe60. In the case of KFeRT (Figure ), the Na and K contents increased almost linearly during discharge. However, K + was the dominating cation, as K/Fe ratio was 0.47 after the first discharge plateau (state 2) and further increased to 0.77 at the end of discharge (state 3). K/Fe ratio was consistently and significantly higher than Na/Fe ratio (0.12 at state 2 and 0.19 at state 3). The K-dominated intercalation seen here confirms the selectivity of the PBA framework towards K + rather than Na + and explains the higher intercalation voltages in this work compared with the voltages of the NaFeFe-PBA cathode in literature. A reverse change of the K/Fe and Na/Fe ratios was observed during the following charge (states 4 and 5). Also, the K/Fe and Na/Fe ratios were 0.75 and 0.14, respectively, after 300 cycles (Table ), being very close to those at state 3 shown in Figure . This proves that no significant Na + -K + exchange occurred during long-term cycling and more importantly, the reversible K-dominated intercalation in KFeRT was promoted by a high anion vacancy content. It is the first reported evidence of long-term reversible K + (de)intercalation reported for KFeFe-PBAs used as NIB cathodes, which is different from the previous results observed from KMnFe-PBAs. In the case of KFe60 (Figure ), K + dominated the intercalation before the plateau spitted at 2.8 V (state 3). But a sharp increase in Na/Fe ratio appeared after the split, and the ratio peaked at 0.32 at the end of discharge (state 4), whereas the K/Fe ratio remained nearly unchanged from states 3 to 4. This suggests that Na + was the dominating cation and responsible for the semi-plateau between 2.8 and 2.5 V (Figure ). Subsequently, a sharp decrease in Na/Fe ratio appeared during charge between states 4 and 5, while the K/Fe ratio kept nearly unchanged. Therefore, despite a higher initial K + content in KFe60 than KFeRT (1.91 vs. 1.49 per formula), the lower anion vacancy resulted in less K + (de)intercalation in KFe60 than KFeRT and an additional Na-dominated intercalation in KFe60. Combining the analysis from KFeRT, the results further confirm that anion vacancy content in KFeFe-PBAs can regulate Na + /K + intercalation, and a lower vacancy content limits K + intercalation. and starts promoting Na + intercalation, being different from previous work. In addition, we employed a series of characterization techniques to characterize the KFeRT and KFe60 electrodes at difference discharge and charge states, in order to examine whether various anion vacancy contents may vary the phase transition and/or structural change, which may in turn affect Na + /K + intercalation mechanism. The collective from the XRD (Figures and), Raman (Figures and), and XAS analysis (Figures )