Exploration of bifunctional Vanadium-based Metal-Organic framework with double active centers for Potassium-ion batteries.

Exploration of suitable electrode hosts with large open channels that can reversibly accommodate K with large radius have been extensively investigated. Nevertheless, the reported inorganic counterparts were inevitably restricted by the difficulty of large K diffusion capability in crystal structure and the huge volume change. Herein, we report a bifunctional vanadium-based metal-organic framework (MIL-47) with double active centers and larger lamellar spacing that could serve as both cathode and anode material, respectively by controlling the redox potential range in potassium-ion batteries. The results suggest that the stable K-storage mechanism is the reversible rearrangement of the conjugated carboxyl groups of organic terephthalic acid into enolate and the reversible redox activity of V ions, with the specific capacity of 272 mAh g (0.01-1.5 V) and 50 mAh g (1.5-3.8 V) at the current density of 10 mA g for MIL-47 anode and MIL-47 cathode, respectively. The unsaturated functional group of MIL-47 and the intermediate bridged V atom not only provide multi-dimensional channels for electron and ion transport but also stabilize its crystal structure. Additionally, a symmetric full-cell in potassium-ion batteries based on MIL-47 was constructed successfully by avoiding the utilization of K metal with safety concerns. Our results provide new insight into structure design for next-generation large-scale energy storage applications.