LiNiO2–Li2MnO3–Li2SO4 Amorphous-Based Positive Electrode Active Materials for All-Solid-State Lithium-Ion Batteries

All-solid-state lithium secondary batteries are attractive owing to their high safety and energy density. Developing active materials for the positive electrode is important for enhancing the energy density. Generally, Co-based active materials, including LiCoO2 and Li(Ni1–x–yMnxCoy)O2, are widely used in positive electrodes. However, recent cost trends of these samples require Co-free materials. Furthermore, the formation of an active material/solid electrolyte interface can cause issues in the application of oxide active materials in all-solid-state batteries with sulfide electrolytes. In this study, we developed LiNiO2–Li2MnO3–Li2SO4 amorphous-based active materials comprising nanocrystals distributed in an amorphous matrix for positive electrodes. These active materials were prepared using a mechanochemical treatment and subsequent heat treatment, and the material composition and sintering temperature were optimized for improving the charge–discharge characteristics of all-solid-state batteries. All-solid-state batteries using the 60LiNiO2·20Li2MnO3·20Li2SO4 (mol %) electrode obtained by heat treatment at 300 °C exhibit the highest initial discharge capacity of 186 mA h g–1 and reversible cycle performance, because the addition of Li2SO4 increases the ductility and ionic conductivity of the active material. This study can guide the future development of Co-free positive electrode active materials for all-solid-state batteries with high energy densities.