Synthesis by Electrolysis of Iron-Based Fluoride as Cathode Materials for Lithium Ion Batteries

The hydrated iron fluoride (Fe₃F₈·2H₂O) with mixed valence cations is successfully synthesized through a rapid electrolytic synthesis route for the first time using low-concentration HF solution as fluorine source and cheap carbon steel as iron source. By controlling the value of current density, submicron structured hydrated iron fluoride with different grain sizes is obtained. The thermal behavior of Fe₃F₈·2H₂O under air atmosphere is studied. The product cooling to room temperature after heat treatment is FeF_2.2(OH)_0.8·0.33H₂O, which is determined by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Fourier transform infrared spectrometer (FT-IR), and thermogravimetry/differential scanning calorimetry (TG/DSC). The evaluation of the electrochemical performance of FeF_2.2(OH)_0.8·0.33H₂O as a cathode for lithium batteries shows that it has an initial discharge capacity as high as 580 mAh g⁻¹ in a wide voltage range of 1.0–4.5 V at a current density of 20 mA g⁻¹, but the cycle performance is not very satisfactory, only 170 mAh g⁻¹ after 50 cycles.