All-solid-state rechargeable Li or Na batteries attract much attention because of their high safety, long cycle life, versatile geometry and high energy density. Sulfide solid electrolytes based on the system Li2S-P2S5 have several advantages of high conductivity, wide electrochemical window, and appropriate mechanical properties suitable for forming close solid-solid interfaces with electrode active materials. Dense sulfide electrolyte pellets with about 90% relative density are prepared by only mold-pressing at room temperature without any sintering at high temperatures. To increase energy density of the batteries, the use of positive electrode active materials with high capacity and the increase of their content in a positive electrode layer are desired. Formation of favorable and large contact areas between electrode and electrolyte is achieved by electrolyte coating on electrode particles via gas-phase or liquid-phase techniques. Sulfide amorphous electrolyte coating on LiCoO2 particles by pulsed laser deposition is effective in increasing reversible capacity of all-solid-state batteries. Closely-attached electrolyte-LiCoO2 interfaces with large contact area are achieved by electrolyte coating. The coating reduces the volume of electrolyte content in an electrode layer, and energy density of solid-state batteries is thus increased. Preparation of nanocomposite electrodes with solid electrolytes and nano-carbons by high-energy ball milling is useful for sulfur or Li2S active materials. Conductivity enhancement of Li2S by the combination of LiI contributes to the improvement of utilization of Li2S active material in all-solid state batteries. Amorphous transition metal sulfides such as amorphous TiS3 are attractive as a mixed conductor with large capacity, and all-solid-state Li and Na batteries with high energy density are achieved.
Ref. A. Hayashi, A. Sakuda and M. Tatsumisago, Front. Energy Res., 4, 25 (2016).