Improving Rapid Lithium Storage Capability of FeF3 by In-Situ Etching during Dehydration

H2O, with its mild etching ability, was frequently used to regulate the morphology and crystal structure of substances, thereby enhancing their properties. Inspired by this, we utilized the crystal H2O in FeF3·3H2O to perform an etching process with an aim to improve the conductivity of FeF3, which could enhance its capacity level and rate capabilities. Herein, this research reported that this in-situ etching was conducted during the dehydration of FeF3·3H2O; meanwhile, the surface of the FeF3 grains was roughened, and traces of iron oxyfluoride were produced on it. The conductivity of the FeF3 grains was significantly enhanced, attributed to the semiconducting property of iron oxyfluoride. Moreover, the rough surface increased the effective contact area between the grains and the electrolyte, thereby increasing the exchange flux of Li+. With the increased quantity of iron oxyfluoride compounds, the charge transfer resistance of the electrode was reduced from 61.87 to 37.2 Ω, and the initial discharge capacity increased from 83.5 to 108.1 mAh g–1 at 2C. This finding provided strategies and methods for the surface modification of compound materials. Especially for halogen metal compounds with high theoretical capacity energy density, their chemical performance can be improved by in-situ defect engineering in the process of dehydration of their hydrates.