Cationic lithium polysulfides in lithium–sulfur batteries

Lithium polysulfides (LiPSs) serve as vital intermediates that participate in every working electrochemical process of lithium–sulfur (Li–S) batteries. Revealing the existing form of LiPSs in electrolyte is essential for understanding the reaction mechanism and rationally designing high-performance Li–S batteries. Herein, we pioneeringly demonstrate that LiPSs are weakly dissociated in electrolyte but exhibit a strong tendency to bond extra lithium ions and form cationic LiPSs (e.g., Li3S6+). Kinetic evaluations further reveal that cationic LiPSs are more sluggish in cathode reactions while more aggressive to react with Li metal anodes. Accordingly, decreasing the salt concentration is demonstrated to inhibit the formation of cationic LiPSs and is manifested effective to improve the performance of practical high-energy-density Li–S pouch cells. This work reveals the existence of cationic LiPSs in electrolyte and affords directional guidance on improving the electrochemical performance of Li–S batteries.