Potassium Vanadium Fluorides as Positive Electrode Materials for K-ion Batteries

High Resolution Image Download MS PowerPoint Slide Potassium vanadium fluorides of K 5 V 3 F 14, K 3 VF 6, and KVF 4 have been investigated as potential positive electrode materials for potassium-ion batteries. K 5 V 3 F 14 and KVF 4 exhibit low electrochemical activity with an ∼50 mAh g –1 initial capacity. By contrast, K 3 VF 6 exhibits a promising electrochemical performance, displaying a voltage plateau at 3.7 V and a large initial capacity of 95 mA h g –1 . A reversible V 4+/3+ one-electron process is evidenced by X-ray absorption spectroscopy, whereas oxidation from V 4+ to V 5+ (from K 2 V 4+ F 6 to K 1 V 5+ F 6 ) is mainly irreversible and V 3+/2+ (from K 3 V 3+ F 6 to K 4 V 2+ F 6 ) is almost inactive. K 3 VF 6 displays partially irreversible structural changes during potassium extraction/insertion, especially when more than 1 mol of potassium ions are extracted from K 3 VF 6 . These results suggest that structural stability after potassium extraction is a key factor in the design of high-capacity fluoride materials.