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 (~ 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.