Amorphous FeVO4 as a promising anode material for potassium-ion batteries

Potassium ion batteries (KIBs) have attracted an increasing attention as a potential low-cost energy storage system. Currently, the reported anode materials for KIBs are mainly limited to carbonaceous materials, whereas high capacity metal oxides are scarcely investigated. Herein, we report amorphous FeVO4 as a promising anode material for KIBs. This material demonstrates a higher specific potassium storage capacity (similar to 350 mA h g(-1) at 100 mA g(-1)) and more favorable cycling performance than its crystalline counterparts. We identify that the charge storage in amorphous FeVO4 is achieved by both surface capacitive effects and diffusion-limited ion insertion/extraction processes. Both Fe and V are electrochemically active during charge storage, but instead of metallic iron and vanadium, nanosized crystalline oxides are generated at the end of potassiation/depotassiation, which is fundamentally different from the conventional conversion reactions. Finally, we demonstrate that with a simple ball-milling approach, the obtained amorphous FeVO4/carbon composite exhibits fast rate capability (180 mA h g(-1) at 2 A g(-1)) and stable cycling performance for 2000 cycles with a Coulombic efficiency of 99.8%. The present study may open up new opportunities for exploring novel amorphous materials as alternative electrode materials for KIBs.