As will be discussed later, increasing vacancy content is crucial to enhance the NIB performance of KFeFe-PBAs. We next examined the electrochemical performance of KFeRT and KFe60 as cathode materials in Na half-cells, using 1 M NaClO4 in EC:PC (50:50% vol.) as the electrolyte in the voltage range of 2.3-4.3 V (vs. Na + /Na). Since the pristine samples were pre-intercalated with K + and the electrolyte only contained Na + , the electrochemical measurement began with a charge process to extract K + . Figure compares the typical CV curves after the initial charge process.