Low-Cost K₄ Fe(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.