KFeRT exhibits two pairs of redox peaks at 3.50/3.74 and 2.88/3.16 V, corresponding to the redox reactions of C-coordinated low-spin (LS) Fe and N-coordinated high-spin (HS) Fe, respectively, suggesting a two-electron transfer process. Noted that an increase in the intercalation voltages by ~0.2 V is noticeable in KFeRT (3.50 and 2.88 V) compared with the NaFeFe-PBA counterparts in literature. As will be discussed in the next section, it is K + intercalation, instead of Na + intercalation, that is responsible for the voltage increase, even though the amount of Na + in the electrolyte is several magnitudes higher than the amount of K + in KFeRT, and such a difference is caused by the difference anion vacancy contents in the two samples. Interestingly, KFe60 exhibits an extra pair of peaks at 2.73 and 3.02 V. The intercalation voltage (2.73 V) agrees with the voltage at which HS Fe is reduced in NaFeFe-PBA cathodes, which suggests that an extra process of Na + intercalation may have occurred.