Potassium nickel hexacyanoferrate as cathode for high voltage and ultralong life potassium-ion batteries

Potassium-ion batteries (KIBs) are considered as one of the most promising energy storage devices owing to the close redox potential to lithium, rich abundance and low cost potassium resource, promoting the exploration on cathode materials for K-ions storage. This paper reports a detailed experimental study on potassium nickel hexacyanoferrate, K_1.81Ni[Fe(CN)₆]_0.97·0.086H₂O (KNHCF), cathode for KIBs. KNHCF synthesized possesses few [Fe(CN)₆]^4- vacancies and coordinated water molecules, and high specific surface area along with developed microporous/mesoporous structures, which collectively induce highly reversible electrochemical intercalation process and fast kinetics. The resultant KNHCF cathode demonstrates reversible capacity of 57.0 mAh·g⁻¹, high operating voltage of 3.74 V, outstanding voltage stability with no decay, good cycling stability (87.34%) after 1000 cycles and long-term lifetime over 8000 cycles with capacity fading rate of 0.0052% per cycle. A solid solution mechanism with zero-strain, where one-electron shuttle relied on C-coordinated Fe^Ⅱ/Fe^Ⅲ couple, plays an important role for K-ions intercalation/deintercalation. A 3.34 V full cell assembled with graphite anode delivers a high energy density of 142.6 Wh·kg⁻¹ with the cpacity retention of 88.5% after 100 cycles, and ultra-long life-span over 5000 cycles can be achieved.