A micron-size carbon-free K3V2O2(PO4)2F cathode with high-rate performance for potassium-ion batteries

(Fluoro)phosphate-based polyanionic materials have been considered as promising cathode candidates for sodium/potassium-ion batteries (SIBs/PIBs) due to their excellent structure stability and high reaction potential. However, compared with the rapid Na+ storage of (fluoro)phosphates, the potassium analogues still exhibit poor rate capability for K+ storage. Herein, we demonstrate a micron-size fluorophosphate-based polyanionic cathode K3V4+2O2(PO4)2F (KVOPF) without needing extra carbon coating for realizing high-capacity and high-rate K+ storage. This KVOPF cathode is reported for the first time, which can deliver 105 mAh g−1 at 100 mA g−1 and 62 mAh g−1 at 2000 mA g−1, and the rate capability is comparable with its analogue Na3V2O2(PO4)2F for Na+ storage. The energy storage mechanism of KVOPF is revealed by in situ XRD: it undergoes a solid solution process followed by two continuous phase transitions during charging process, and the discharging process is opposite, exhibiting its high reversibility. In addition, a full cell was constructed by coupling with hard carbon anode. This work provides a promising cathode candidate for constructing high power-high energy PIBs.