Na3V2O2(PO4)2F3-2 as a stable positive electrode for potassium-ion batteries

Reversible potassium ion insertion into Na3V2O2x(PO4)2F3-2x, which is regarded as a solid solution between Na3V3+2(PO4)2F3 and Na3V4+2O2(PO4)2F is demonstrated for the first time. V3+/V4+-mixed Na3V2O2x (PO4)2F3-2x-multiwalled carbon nanotube (MWCNT) composite was prepared by a conventional hydrothermal reaction. Na3V2O2x(PO4)2F3-2x-MWCNT composite exhibiting an average electrode potential of 3.7 V in a potassium cell due to the cumulative inductive effect of phosphate fluoride oxide. The prepared composite delivers an initial discharge capacity of 118 mAh g−1 at 0.1C (13 mA g−1) rate with good cycle stability and high-rate capability performances. The electrochemical measurements, energy dispersive X-ray analysis, ex-situ, and operando X-ray diffractions provide new insights into reversible phase transformations during the subsequent cycles after an electrochemical K/Na exchange process at the initial cycle.