This leads to an incomplete K + extraction in the initial deintercalation (Figure ) and more importantly, the deintercalated K + did not fully intercalate back in the framework due to the high K + migration energy barrier and the preference of Na + at certain intercalation sites (Figure and Table ). As a result, after the K-dominated intercalation process at the higher voltage range, a Na-dominated intercalation process occurs (Figure ), giving rise to a plateau split at the low voltage range (Figures and) and a low capacity, even though most of the sites in the PBA framework favors K + intercalation over Na + intercalation. In contrast, in the case of an anion vacancy-containing KFeFe-PBA framework (KFeVC, Figure ), K + diffuses faster because of the lower migration energy barrier and the higher diffusion coefficient (Figures and). This leads to a complete K + extraction in the initial deintercalation and the fully reversible K + insertion in the following intercalation (Figure ). As a result, the two steps of the intercalation process are both dominated by K + , which enables a high capacity and both the intercalation voltages being higher than those of NaFeFe-PBA cathodes in NIBs (Figures and). Therefore, designing defective structures is a crucial approach to regulate the cation intercalation mechanism in a hybrid ion battery system and thus an effective method to enhance the performance of hybrid ion batteries. This approach will be invaluable for hybrid systems in which severely sluggish ion diffusion kinetics take place, such as Mg 2+ and Ca 2+ as charge carriers.