Degradation Mechanism of Conversion-Type Iron Trifluoride: Toward Improvement of Cycle Performance

Conversion-type iron trifluoride (FeF₃) has attracted considerable attention as a positive electrode material for lithium secondary batteries due to its high energy density and low cost. However, the conversion process through which FeF₃ operates leads it to suffer from capacity degradation upon repeated cycling. To improve the cycle performance, in this study we investigated the degradation mechanism of conversion-type FeF₃ electrode material. Bulk analyses of FeF₃ upon cycling reveal incomplete oxidation to Fe³⁺ concomitant with the aggregation of LiF at the charged state. In addition, surface analyses of FeF₃ reveal that a film covered the electrode surface after 10 cycles, which leads to a remarkable increase in resistance. We show that the choice of the electrolyte formulation is crucial in preventing the formation of the film on the electrode surface; thus, FeF₃ shows better performance in an electrolyte comprising LiBF₄ solute in cyclic carbonate solvents than in chain carbonate-containing LiPF₆ as the electrolyte. This study underpins that a careful selection of solvent, rather than solute, is significantly essential to improve the cycle performance of the FeF₃ electrode.