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.