Solid-Electrolyte-Free O3-Li_xTiS₂ Cathode for High-Energy-Density All-Solid-State Lithium-Metal Batteries

Composites made of high-capacity and high-potential LiNixMnyCo1–x–yO2 (NMC) lamellar transition-metal oxides and S-based ionic conductors are primarily used as positive electrodes in all-solid-state batteries (ASSBs). However, NMC coatings are necessary to prevent the chemical reactivity of oxygen and sulfur at the expense of some penalty in capacity and in ionic conduction. To overcome these problems, the current trend is to move to S-based positive electrodes, which exhibit higher electronic and ionic conductivities and no S–O reactivity. Herein we succeed in preparing highly divided O3-LixTiS2 powders by ball milling, which can be used as a positive electrode free of the electrochemically dead solid electrolyte (SE) in ASSBs that shows energy densities of more than 350 Wh/kg and stable cycling under low pressures down to 1 bar. These enhanced performances were rationalized by 7Li NMR measurements that revealed a decrease in the site-to-site activation energy barrier with increasing ball-milling time. This finding, based on diffusion considerations associated with particle size reduction, can be generalized to other 3d metal sulfides and provides insights on the benefits of using a SE-free and C-free design relying on highly divided Li-containing sulfides to boost energy densities in ASSBs.