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

Abstract

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 K_1.5VOPO₄F_0.5 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 K_1.5VOPO₄F_0.5 structure and that numerous K⁺ ions can be reversibly de/intercalated in the available voltage range, demonstrating the potential of K_1.5VOPO₄F_0.5 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 K_1.5VOPO₄F_0.5 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, K_1.5VOPO₄F_0.5 delivers ∼79% of the capacity measured at C/20, indicating its excellent power-capability despite the large ionic size of K⁺. Moreover, K_1.5VOPO₄F_0.5 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 K_1.5VOPO₄F_0.5 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.