It has been speculated that using K-PBAs as cathodes for NIBs is an attractive approach because compared with Na + , the bigger K + can better fit the large interstitial voids in the PBA framework, enabling higher structural stability. In a hybrid battery system containing two cation species, where K + residues in a K-PBA cathode and Na + is present in the electrolyte, cation intercalation mechanism has been elusive.