Microstructure and Charge–Discharge Mechanism of a Li₃CuS₂ Positive Electrode Material for All-Solid-State Lithium-Ion Batteries

To develop all-solid-state lithium batteries, high-capacity positive electrode materials are necessary. An antifluorite-type material, Li2S, exhibits a high theoretical capacity. However, Li2S cannot be used as a positive electrode for the all-solid-state cell because of its insulating behavior. To provide electronic and ionic conduction, recently, antifluorite-type Li3CuS2 has been developed by activation of Li2S by Cu substitution. Li3CuS2 is a favorable candidate for positive electrodes as sulfide-based all-solid-state cells with Li3CuS2 exhibit high charge–discharge performance. However, structural changes and redox species during the charge–discharge cycle have not been understood yet. To clarify the charge–discharge mechanism of Li3CuS2, in this study, we examined the microstructural changes in a Li3CuS2–Li3PS4 positive electrode composite during charge and discharge by transmission electron microscopy (TEM). The hollow-cone dark-field imaging technique was employed to evaluate the crystallite size distribution. The result shows that the crystallite size of Li3CuS2 reversibly decreases and increases in the charging and discharging states, respectively. The electron diffraction pattern shows that LiCuS2 was formed during charging, which is attributed to Li+ extraction from Li3CuS2. In the discharging state, the crystallite size increased and Li3CuS2 was reproduced. The TEM results suggest that the reversible structural changes (Li3CuS2 ⇆ LiCuS2 + 2Li+ + 2e–) would contribute to high charge–discharge characteristics.