All-solid-state batteries have attracted great attention because of their high safety owing to the use of non-flammable inorganic solid electrolytes. Solid electrolytes with an extremely high ionic conductivity have already been discovered, and therefore the discovery of new and efficient electrode materials is key for the construction of high energy density all-solid-state batteries. In the electrode layers for conventional all-solid-state batteries, mixtures composed of active material, solid electrolyte and carbon conductive additive powders are often applied to secure lithium ionic and electronic conduction pathways. Moreover, most of all-solid-state batteries reported so far comprise a thin electrode layer with a small amount of active materials and a thick electrolyte layer as the separator. Therefore, the energy density of all-solid-state batteries at the present stage is considerably lower than that of typical lithium ion batteries. To enhance the energy density of all-solid-state batteries, high capacity electrode materials should be developed and the active material content should be increased in the composite electrode layers.

With consideration to the demands and features described above, we have successfully developed a novel Li₂Ru_0.8S_0.2O_3.2 (80Li₂RuO₃·20Li₂SO₄ in mol%) positive electrode material for all-solid-state batteries. Mechanochemical treatment with Li₂SO₄ imparts ionic conductivity and favourable ductility to the Li₂RuO₃ active material. Because of the favourable formability and high electronic and ionic conductivities, we achieved the fabrication of all-solid-state cells, where the positive electrode layer was composed of the active material without any conductive additives. The all-solid-state cell using a monolithic Li₂Ru_0.8S_0.2O_3.2 positive electrode functioned as a secondary battery showing a high reversible capacity of about 270 mAh g^-1. In this study, the detail charge-discharge mechanism of the Li₂Ru_0.8S_0.2O_3.2 positive electrode was investigated by XRD, TEM, XPS, and XAFS measurements.