Supercritical-fluid synthesis of FeF₂ and CoF₂ Li-ion conversion materials

The synthesis of the Li-ion conversion candidates, FeF₂ and CoF₂, obtained from the single source organometallic precursors [Fe(tta)₃] (tta = C₈H₄F₃O ₂S), and [Co(hfac)₂·2H₂O] (hfac = C ₅H₁F₆O₂), respectively, via a novel supercritical fluid (SCF) method is presented. The nature of the synthesis led to highly-crystalline FeF₂ and CoF₂ powders requiring no additional thermal treatment. The as-obtained powders were investigated for use as potential positive Li-ion conversion electrodes by means of chronopotentiometric measurements. The FeF₂ cells displayed high initial capacities following electrochemical conversion (up to ∼1100 mA h g^-1 at a potential of 1.0 V vs. Li/Li⁺), with appreciable cyclic behaviour over 25 discharge-charge cycles. The deposition of a ∼5 nm layer of amorphous carbon onto the surface of the active material following SCF treatment, likely facilitated adequate electron transport through an otherwise poorly conducting FeF₂ phase. Similarly, CoF₂ cells displayed high initial capacities (up to ∼650 mA h g^-1 at a potential of 1.2 V vs. Li/Li⁺), although significant capacity fading ensued in the subsequent cycles. Ex situ XRD measurements confirmed a poor reversibility in the conversion sequence for CoF₂, with a complete loss of CoF₂ crystallinity and the sole presence of a crystalline LiF phase following charging.