K1.5VOPO4F0.5: a novel high-power and high-voltage cathode for rechargeable K-ion batteries

High-performance cathode materials for K-ion batteries require large K+ diffusion pathways and numerous K+ sites in the stable crystal structure. Herein, we investigate K1.5VOPO4F0.5 composed of three-dimensionally interconnected [V2O10F] bi-octahedra and [PO4] tetrahedra as a promising cathode material for K-ion batteries using a combined computational and experimental approach. First-principles calculation results reveal that K+ ions can be facilely diffused along large two-dimensional pathways in the K1.5VOPO4F0.5 structure and that numerous K+ ions can be reversibly de/intercalated in the available voltage range, demonstrating the potential of K1.5VOPO4F0.5 to deliver outstanding electrochemical performance in a K-ion battery system. At a current rate of C/20 (1C = 116 mA g−1), the specific capacity of K1.5VOPO4F0.5 is retained up to ∼116 mA h g−1 with a high average operation voltage of ∼3.8 V (vs. K+/K), corresponding to reversible de/intercalation of ∼1 mol K+ in the structure. Even at 5C, K1.5VOPO4F0.5 delivers ∼79% of the capacity measured at C/20, indicating its excellent power-capability despite the large ionic size of K+. Moreover, K1.5VOPO4F0.5 delivers excellent cycle performance, with ∼86% retention of the initial capacity after 300 cycles at 1C and a high coulombic efficiency of over 99%. Combined studies using ex situ XANES analyses and first-principles calculation indicate occurrence of V4+/V5+ redox reaction of K1.5VOPO4F0.5 for its high operation voltage. We believe that our findings will provide highly useful guidance for the discovery of high-performance electrode materials for rechargeable batteries.