Electrolyte solutions design for lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries promise high energy density for next-generation energy storage systems, yet many challenges remain. Li-S batteries follow a conversion chemistry, which radically differs from intercalation-based lithium-ion batteries. Recently, it has become clear that the chemistry of electrolyte solutions and their ability to stabilize polysulfide Li2Sx species formed by sulfur reduction have a critical effect on energy density and cycling performance. This review evaluates the key role of solution properties and polysulfide solvation. Factors that determine the solvation are discussed, including the solvent, salt, concentration, and interaction with Li-polysulfide species. Three fundamental types of electrolyte solution-moderately (conventional), sparingly, and highly solvating-are presented along with a multi-dimensional analysis of solution chemistry, polysulfide solubility, sulfur reaction pathway, Li2S deposition, and solution quantity. The stability of lithium metal anodes with these solutions is discussed with respect to side reactions, protective surface film formation, and dendritic Li deposition. Emphasis is placed on options to reduce the electrolyte solution/sulfur ratio and prolong battery cycle life. The advantages and disadvantages of the three systems are compared in accordance with the multifaceted requirements. In conclusion, we offer our perspective for future development of Li-S batteries.