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

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