Operando Studies of Antiperovskite Lithium Battery Cathode Material (Li₂Fe)SO

Spectroscopic and X-ray diffraction operando techniques were used to investigate polycrystalline antiperovskite (Li2Fe)SO as cathode materials in a Li-battery setup. During Li removal, several intermediate, relatively stable phases exist. At low charging, Fe is oxidized from +2 to +3, but at higher charging, S2– is also partly oxidized to elemental sulfur, suggesting a cathode bifunctionality, and both redox processes seem reversible. On cycling (Li2Fe)SO in a battery, spectroscopy data suggest that a part of the Fe atoms irreversibly vacate the high-symmetry positions in the crystal lattice, in line with the broadening of X-ray diffraction peaks. Instead, new, relatively broad reflections appear in the X-ray patterns that might be explained by a crystallographic superstructure, corresponding to a doubling of the cubic unit cell axis, but the peak broadness indicates a lowering in crystallographic symmetry. Using a standard electrolyte and a moderate charging rate of C/10 results in typical capacity loss per cycle, but by using an electrolyte with low sulfur solubility, the (Li2Fe)SO cathode is stabilized, and charge densities of more than 200 mAh g–1 at a 1C charging rate are obtained. Additionally, a Li-deficient precursor (Li0.8Fe)SO served as a cathode material in a Na battery, providing presumably reversible Na intercalation and removal.