Li₂S–LiI Solid Solutions with Ionic Conductive Domains for Enhanced All-Solid-State Li/S Batteries

Lithium sulfur (Li/S) batteries are promising next-generation battery candidates owing to their high energy densities. In particular, the fast solid-state S/Li2S redox reactions are crucial to increase the energy density and extend the cycle life of such batteries. However, the poor electronic and ionic conductivities of S and Li2S result in a low reversible capacity. Therefore, an electrode design is required to achieve high-energy-density Li/S batteries. In this study, we investigated the charge–discharge mechanism of a solid solution of Li2S and LiI (Li2S–LiI) in all-solid-state batteries showing excellent electrochemical properties, including cycling performance. We found that a high reversible capacity was achieved despite the high conversion of Li2S into S because the ionic conductivity of the positive electrode was maintained during charging and discharging, and this was a result of the formation of an ionic conductive structure comprising LiI-rich domains. Crucially, essentially fully solid phase S/Li2S reactions in all-solid-state batteries were attained by fully eliminating the sulfide solid electrolyte from the positive electrode. These findings enable the design of S- and Li2S-based positive electrodes for solid phase redox reactions for use in high-energy-density Li/S batteries.