Low‐Cost K4Fe(CN)6 as a High‐Voltage Cathode for Potassium‐Ion Batteries

: Potassium-ion batteries (KIBs) are of interest for large-scale electrical energy storage, owing to the abundance of K resources and potential high energy density. Low-cost cathodes with high performance are crucial for KIBs. Herein, K4 Fe(CN)6 is shown to be a low-cost and high-voltage cathode for KIBs. It can deliver a high voltage of approximately 3.6 V and a discharge capacity of 65.5 mAh g-1 with a lifespan of 400 cycles of discharge and charge. This is attributed to the strong σ bonds between C atoms and Fe and to the reduced particle size and good contact with conductive carbon brought about by ball milling, which benefit both the K+ ion and the electronic conduction. The [Fe(CN)6 ]3-/4- redox couple is found to be responsible for charge compensation upon reversible extraction/insertion of K+ from/into K4 Fe(CN)6 . The high voltage and stability of K4 Fe(CN)6 will make it a promising low-cost cathode for KIBs and encourage more investigations into high-performance cathode materials.