Elucidation of discharge–charge reaction mechanism of FeF₂ cathode aimed at efficient use of conversion reaction for lithium-ion batteries

Iron-based conversion-type materials, which are inexpensive and have low environmental impact, are promising as cathodes for large-scale Li-ion batteries. Among these materials, iron fluoride (FeF₂) is notable for its relatively high operating voltage and large reversible capacity. Here, the effect of the electrolyte type on the FeF₂ conversion reaction was examined, revealing that the cyclabilities of FeF₂ were improved by changing the electrolyte solvent from chain carbonate to cyclic coronate. During the discharge process, lithium carbonate was generated on the surface of the electrode with EC:PC electrolyte. On the other hand, lithium phosphate was produced on the surface electrodes regardless of which electrolyte was used. In addition, the amount of iron elution in EC:DMC was larger than that in EC:PC. The primary factor in the deterioration of the cycle was the elution of iron into electrolyte rather than the side reactants generated during the discharge–charge reaction. In addition, Fe³⁺ formed on the electrode surface by repeating the discharge–charge reaction, and this is the cause of the plateau appearing at 3.0 V during the discharge process of FeF₂ after a few cycles.