Towards safe lithium-sulfur batteries from liquid-state electrolyte to solid-state electrolyte

Lithium-sulfur (Li−S) battery has been considered as one of the most promising future batteries owing to the high theoretical energy density (2600 W·h·kg⁻¹) and the usage of the inexpensive active materials (elemental sulfur). The recent progress in fundamental research and engineering of the Li−S battery, involved in electrode, electrolyte, membrane, binder, and current collector, has greatly promoted the performance of Li−S batteries from the laboratory level to the approaching practical level. However, the safety concerns still deserve attention in the following application stage. This review focuses on the development of the electrolyte for Li−S batteries from liquid state to solid state. Some problems and the corresponding solutions are emphasized, such as the soluble lithium polysulfides migration, ionic conductivity of electrolyte, the interface contact between electrolyte and electrode, and the reaction kinetics. Moreover, future perspectives of the safe and high-performance Li−S batteries are also introduced.