Electrochemical Properties and Microstructural Analysis of the Li2FeSO Cathode Material

Recent research has focused on the oxy-sulfide Li2FeSO with an antiperovskite structure as a cathode because of its high theoretical capacity (455 mAh g–1). An all-solid-state battery with a Li2FeSO cathode achieved a high discharge capacity of approximately 270 mAh g–1 at a high cathode loading ratio (90 wt %). However, the reason for the superior battery performance in the all-solid-state battery is unclear. In this study, Li2FeSO was synthesized through mechanical milling, and the charge compensation mechanism during the battery reaction and the cross-sectional microstructure were analyzed. Fe oxidation in the low-voltage region and S oxidation in the high-voltage region proceeded during charging. On the other hand, S reduction in the high-voltage region and Fe reduction in the low-voltage region occurred during discharge. Li2FeSO in all-solid-state batteries can use sulfur redox reactions to enable the high capacity of 270 mAh g–1. A dense body was observed even at a high Li2FeSO active material loading ratio (90 wt %) by scanning electron microscopy (SEM). The elastic modulus, Mayer hardness, and yield point were 24.9, 0.46, and 0.82 GPa, respectively, similar to those of sulfide solid electrolytes. These excellent mechanical properties, such as low modulus and high formability due to the presence of sulfur, contribute to superior battery performance, even at a high Li2FeSO active material loading ratio (90 wt %).