Electrochemical Properties\nand Microstructural Analysis\nof the Li<sub>2</sub>FeSO Cathode Material

Recent research has focused on the oxy-sulfide Li<sub>2</sub>FeSO\nwith an antiperovskite structure as a cathode because of its high\ntheoretical capacity (455 mAh g<sup>–1</sup>). An all-solid-state\nbattery with a Li<sub>2</sub>FeSO cathode achieved a high discharge\ncapacity of approximately 270 mAh g<sup>–1</sup> at a high\ncathode loading ratio (90 wt %). However, the reason for the superior\nbattery performance in the all-solid-state battery is unclear. In\nthis study, Li<sub>2</sub>FeSO was synthesized through mechanical\nmilling, and the charge compensation mechanism during the battery\nreaction and the cross-sectional microstructure were analyzed. Fe\noxidation in the low-voltage region and S oxidation in the high-voltage\nregion proceeded during charging. On the other hand, S reduction in\nthe high-voltage region and Fe reduction in the low-voltage region\noccurred during discharge. Li<sub>2</sub>FeSO in all-solid-state batteries\ncan use sulfur redox reactions to enable the high capacity of 270\nmAh g<sup>–1</sup>. A dense body was observed even at a high\nLi<sub>2</sub>FeSO active material loading ratio (90 wt %) by scanning\nelectron microscopy (SEM). The elastic modulus, Mayer hardness, and\nyield point were 24.9, 0.46, and 0.82 GPa, respectively, similar to\nthose of sulfide solid electrolytes. These excellent mechanical properties,\nsuch as low modulus and high formability due to the presence of sulfur,\ncontribute to superior battery performance, even at a high Li<sub>2</sub>FeSO active material loading ratio (90 wt %).