Facile synthesis of C-FeF₂ nanocomposites from CFx: influence of carbon precursor on reversible lithium storage

Transition metal fluorides are an important class of cathode materials for lithium batteries owing to their high specific energy and safety. However, metal fluorides are electrical insulators, exhibiting slow reaction kinetics with Li. Consequently, metal fluorides can show poor electrochemical performance. Instead, carbon-metal fluoride nanocomposites (CMNFCs) were suggested to enhance electrochemical activity. Chemical synthesis of CMNFCs poses particular challenges due to the poor chemical stability of metal fluorides. Recently, we reported a facile one-step method to synthesize carbon-FeF₂ nanocomposites by reacting fluorinated carbon (CFx) with iron pentacarbonyl (Fe(CO)₅) at 250 °C. The method resulted in C-FeF₂ nanocomposites with improved electrochemical properties. Here, we have synthesized four different C-FeF₂ nanocomposites by reacting four different CFx precursors made of petro-coke, carbon black, graphite, and carbon-fibers with Fe(CO)₅. Electrochemical performance of all four C-FeF₂ nanocomposites was evaluated at 25 °C and 40 °C. It is shown that the nature of CFx has a critical impact on the electrochemical performance of the corresponding C-FeF₂ nanocomposites. The C-FeF₂ nanocomposites were characterized by using various experimental techniques such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, resistivity measurement, and ⁵⁷Fe Mössbauer spectroscopy to shed light on the differences in electrochemical behaviour of different C-FeF₂ nanocomposites.