Interconvertible and rejuvenated Lewis acidic electrolyte additivefor lean electrolyte lithium sulfur batteries

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.