demonstrate that KFeRT and KFe60 exhibited the same cubic-to-monoclinic phase transition and structural change during discharge (cation intercalation), accompanying the reduction of Fe, and the phase transition was reversible for both samples. It is worth mentioning that Na Kedge XAS results (Figure ) further supported the cation intercalation mechanism concluded from the EDS results, i.e., Na content remaining low throughout the discharge-charge cycle of KFeRT (Figure ) but showing reversible change from states 3 to 5 for the case of KFe60 (Figure ). Therefore, the results solidifies the determining role of [Fe(CN)6] 4-anion vacancy in regulating cation intercalation and the resulting NIB cell performance. To further understand the role of anion vacancies, we carried out computational evaluations on Na/K intercalation energy and migration energy via the DFT calculations and experimental measurements on cation diffusion kinetics via the GITT measurement. The calculations were performed on the KFeVF and KFeVC models, as previously mentioned in Figures and, to represent KFe60 and KFeRT, respectively. Since the major difference of cation intercalation between the two samples occurs during the low-voltage intercalation (Figures and), energies were calculated at the state of 75% body-centered sites being occupied by K + (75% potassiation), meaning that in a 2×2×2 supercell, 24 of 32 sites are occupied by K + and 8 sites are available for cation intercalation (Figure ). We first calculated the intercalation energies of Na + and K + at the 8 sites (denoted as site x, x = 1-8, Table ). The KFeVC model has lower intercalation energies for K + than Na + in all 8 sites, while in the KFeVF model, 2 sites (site 4 and site 6) have lower intercalation energies for Na + than K + . This suggests that the presence of anion vacancy can regulate the cation intercalation in the term of thermodynamics way. It has been reported that PBA framework, when no intercalation site is occupied, has a strong selectivity towards large-sized alkali ions, and as a result K + intercalation is more energetically favorable than Na + intercalation. However, our results showed that in an anion vacancyfree model, some sites prefer Na + occupancy with 75% sites occupied by K + , as the large size of K + could possibly inhibit further K + intercalation. Also, we are mindful that K + intercalation process requires the migration of K + , and therefore we calculated the migration energy of K + from one body-centered site to the adjacent site along a migration path next to an anion vacancy in the KFeVC model (Figure ) and the equivalent site in the KFeVF model (Figure ). The calculations showed that a migration energy barrier of 1.07 eV is needed for the migration path to be energetically favorable in the KFeVF model (Figure ), whereas a lower migration energy barrier, 0.81 eV, is needed in the KFeVC model. The calculation results agree well with previous calculations of K + diffusion in non-potassiated and fully potassiated K-PBA cathodes, and are also reasonably consistent with our experimental observations. Figures and display the GITT curves and calculated diffusion coefficients during the discharging processes of the samples. In general, KFeRT shows smaller over potentials than KFe60, particularly at the two discharge plateaus. The diffusion coefficient of KFeRT is in the range of 10 -11 ~ 10 -12 cm 2 s -1 , being 1-2 magnitudes higher than that of KFe60 (10 -12 ~ 10 -14 cm 2 s -1 ).