Organic liquid electrolytes in Li-S batteries: actualities and perspectives

To meet future application requirement for large-scale energy storage systems, as a promising candidate, lithium–sulfur battery has attracted extensive attention and has been thoroughly explored due to its high theoretical cathodic capacity (1, 675 mAh g−1) and the low cost of sulfur. Its practical application is hindered by the known challenges, including inferior cycle stability and low charging efficiency etc. despite extensive studies in last decades. Most researchers have focused on developing novel host materials for sulfur cathode, protecting lithium metal anode by artificial SEI layers, and structure optimization of Li-S battery. The primary issue is the high solubility of lithium sulfide intermediates (Li2Sn, 3≤n≤8) in liquid organic electrolytes, which results in a “polysulfide shuttle effect” and rapid capacity fading. Compared with solid-state electrolytes, up to now, traditional liquid state electrolytes are still the most favorable choice for the commercialization of Li-S batteries. Each component of liquid electrolyte, such as solvents, lithium salts and additives, is related to dissolution of polysulfide intermediates and formation of SEI film on lithium metal anode. Motivated by strong demand of the state-of-art electrolyte systems for high performance Li-S batteries, researchers have tried to explore more novel electrolyte systems. In this review, the recent progress of liquid electrolyte systems are summarized and discussed. In addition, we also review some new concepts and the reaction mechanisms of the electrolytes in Li-S batteries.