Highly Reversible Iron Fluoride Conversion Cathodes Enabled by Deep‐Eutectic Solvent Method and Heterostructure Design

Abstract Conversion‐reaction metal fluoride cathodes have the advantages of low cost, environmental friendliness, and high energy density. However, their tailored synthesis and structure design under safe, facile, and scalable conditions have always been a puzzle, which limits the popularization of conversion‐type cathodes in lithium metal batteries. To address these challenges, here an iron fluoride heterostructure with FeF2 and FeF3 binary phases is proposed and prepared by deep eutectic solvent method under the assistance of cobalt cation as an oxidization promoter. The compact heterogeneous contact between tetragonal FeF2 and hexagonal tungsten bronze FeF3 phases enables the promotion and stabilization of interface charge transfer and topotactic conversion reaction. The narrower band gap of FeF2, higher theoretical capacity and thermodynamic potential of FeF3 synergistically contribute to the balance between cycling reversibility and capacity. The heterostructure cathode with an optimized molar ratio of FeF2/FeF3 can realize a highly reversible capacity of ≈520 mAh g−1 in the first tens of cycles and 305 mAh g−1 after 200 cycles. The typical two‐stage reaction plateaus are well preserved even under the high current density of 1000 mA g−1. The green synthesis strategy and heterostructure fluoride design open a new horizon for the development of high‐energy‐density metal fluoride batteries.