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

Potassium vanadium fluorides of K₅V₃F₁₄, K₃VF₆, and KVF₄ have been investigated as potential positive electrode materials for potassium-ion batteries. K₅V₃F₁₄ and KVF₄ exhibit low electrochemical activity with an ∼50 mAh g^-1 initial capacity. By contrast, K₃VF₆ 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 V⁴⁺ to V⁵⁺ (from K₂V⁴⁺F₆ to K₁V⁵⁺F₆) is mainly irreversible and V^3+/2+ (from K₃V³⁺F₆ to K₄V²⁺F₆) is almost inactive. K₃VF₆ displays partially irreversible structural changes during potassium extraction/insertion, especially when more than 1 mol of potassium ions are extracted from K₃VF₆. These results suggest that structural stability after potassium extraction is a key factor in the design of high-capacity fluoride materials.