Lithium-ion battery was commercialized in 1991, and its market has grown tremendously for IT mobiles. Today, the demand for high energy density is rapidly increasing for automotive and stationary energy storage. In the history, lithium metal secondary battery has attracted great attention for digital media applications about thirty years ago. However, due to significant technical challenges such as dendrite formation during charge-discharge cycles, its development had faded and the interests shifted to lithium-ion batteries. From the view point of energy density, the interest is coming back and effort for the lithium metal and related alloys is intensifying.

              In this presentation, our recent work on the fabrication of all-solid-state batteries and sulfide solid electrolyte development will be reported. The sulfide solid-state batteries have the advantages of (1) small grain boundary resistance, (2) fast kinetics at Li/solid electrolyte interface, (3) stable cycle with a lithium metal anode, (4), lack of convection of the electrolyte material, and (5) non-volatile and incombustible electrolyte. [1-3] We have demonstrated charge-discharge operation of solid state Li-S batteries, and verified its low activation energy of the charge transfer process at the anode. Further improvements are still needed in the loading-level of active materials and cell performance.

              Besides the fabrication of the solid-state cells, our efforts on the materials development will be presented. In particular, some results on LiI substituted Li₃PS₄ solid electrolyte will be shown. This material is amorphous and exhibits >1mScm^-1at 25℃.[4] Finally, challenges and outlook of our research and development will be discussed.

1. S. Ito, et al., J. Power Sources 248 (2014) 943.
2. M. Agostini, et al., Solid State Ionics 244 (2013) 48.
3. T. Yamada, et al., J. Electrochem. Soc.(2015) A646.
4. Y. Aihara, et al., elsewhere