Interconvertible and rejuvenated Lewis acidic electrolyte additive for lean electrolyte lithium sulfur batteries

Abstract Realizing practical lithium–sulfur batteries (LSBs) with high energy density requires a lean electrolyte design. However, under low electrolyte/sulfur (E/S) ratios, highly concentrated lithium polysulfides (LiPSs) in the electrolyte phase limit cycling and capacity. Here, we report that a small amount of Lewis acidic calcium cation (Ca2+) in the electrolyte addresses the problems of lean electrolyte LSB. Because of its strong Lewis acidity, Ca2+ readily converts LiPSs into CaS and S8, preventing electrolyte jamming, PS shuttle and Li corrosion. The in situ formed CaS catalyzes the reduction reaction of LiPSs. Ca2+ can be rejuvenated via the electrochemical oxidation of CaS during charging, enabling a sustainable interconversion between Ca2+ and CaS during cycling. Li-S pouch cells with Ca2+ additive delivered an impressive energy density of 493 Wh kg−1 with 70% capacity retention (E/S ratio of 2.4 μL mg-1) at 220 cycle, and 346 Wh kg-1 with 77% capacity retention (2.9 μL mg-1) at 360 cycle. The judicious integration of lithium-sulfur and calcium-sulfur chemistries offers a handy but effective approach to overcome the long-lasting trade-off between energy density and cycling stability in the development of LSBs.