K3V2(PO4)2F3 as a robust cathode for potassium-ion batteries

Potassium-ion batteries have emerged as promising candidates for low-cost and sustainable energy storage systems. The development of potassium-ion batteries is relatively slow due to the large size of potassium ions, rendering great difficulty in designing appropriate host materials. Herein, a K3V2(PO4)2F3 cathode is inherited from Na3V2(PO4)2F3 analog. The crystallographic structure and phase transformations are unveiled through in-situ X-ray diffraction, which shows only minor volume change of 6.2% during potassium ions insertion/extraction. Nearly two potassium ions could be provided by the electrode, delivering a capacity of over 100 mA h g−1 with a high average potential of ~3.7 V vs. K+/K. An energy density of around 400 W h kg−1 together with a respectable rate capability have been obtained. Coupling with a graphite anode, a 3.4 V-Class battery has been demonstrated, making potassium-ion batteries promising contenders to sodium ion batteries in large-scale energy storage. This discovery also sheds insights into the quest for potential electrodes from the analogs in Li/Na-ion batteries.