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(Ni_{1–<i>x</i>–<i>y</i>}Mn_<i>x</i>Co_<i>y</i>)­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.