The preparation and characterization of iron fluorides polymorphs FeF3·0.33H2O and β-FeF3∙3H2O as cathode materials for lithium-ion batteries

Two kinds of iron fluorides, FeF3·0.33H2O and β-FeF3∙3H2O, have been synthesized by different chemical routes using Fe(NO3)3·9H2O and NH4F as precursors and investigated as cathode materials for secondary lithium batteries. β-FeF3·3H2O was obtained via a mechano-chemical process and FeF3·0.33H2O was obtained by a precipitation reaction from ethanol solution. To further improve their electrochemical properties, a ball milling process with carbon has been used to form composites. The fluorides were characterized by X-ray diffraction (XRD), thermal analysis, FTIR spectroscopy, galvanostatic charge/discharge and cyclic voltammetry measurements. The results showed that the as-prepared β-FeF3·3H2O/C exhibited better rate performance and cycling stability. It exhibited an initial discharge capacity of 199.6 mA h g−1 at a current density of 20 mA g−1 in a voltage range of 1.8–4.5 V and still reserved a discharge capacity of about 146.5 mA h g−1 after 10 cycles. Only a capacity loss of 15 mA h g−1 and 7 mA h g−1 was observed when the current density was at 40 mA g−1 and 80 mA g−1, respectively.