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

The hydrated iron fluoride (Fe 3 F 8 ·2H 2 O) 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 Fe 3 F 8 ·2H 2 O under air atmosphere is studied. The product cooling to room temperature after heat treatment is FeF 2.2 (OH) 0.8 ·0.33H 2 O, 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 ·0.33H 2 O 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. Graphical abstract