Flower-like K1+δVOPO4F crystallite with a layered framework structure as a robust cathode for potassium-ion batteries

Potassium-ion batteries (KIBs) have attracted more and more attention due to the abundant potassium resources and hence low cost of raw materials. Nevertheless, the performances of KIBs are still unsatisfactory for practical applications, mainly due to the lack of suitable cathode materials. In this work, we report for the first time a novel polyanionic compound K1+delta VOPO4F as a potassium ion storage host. Thanks to its flexible while stable layered [VOPO4F]infinity framework, as a KIB cathode material, it can provide a reversible capacity of about 101 mAh g(-1) at a rate of 0.2 C with a 87% capacity retention after 250 cycles. The corresponding energy density of 363 Wh kg(-1) is among the highest for K-ion storage. The working mechanism of the V4+/V5+ redox reaction is deter-mined by in-situ X-ray diffraction (XRD) and ex situ X-ray photoelectron spectra (XPS) analyses. Notably, it de-livers a stable average capacity of 75 mAh g(-1) for 700 cycles at 1 C with a capacity retention rate of 76%. This work presents that K1+delta VOPO4F is a promising K+ storage cathode material with merits of high energy density and long cyclability.