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

Realizing practical lithium–sulfur batteries with high energy density requires lean electrolyte design. However, under low electrolyte/sulfur (E/S) ratios, highly concentrated lithium polysulfides in the electrolyte phase limit cycling and capacity. Here, we report that a small amount of Lewis acidic calcium cation in the electrolyte addresses the problems of lean electrolyte lithium–sulfur batteries. Because of its Lewis acidity, Ca2+ readily converts lithium polysulfides into CaS and S8, preventing electrolyte jamming, polysulfide shuttle and Li corrosion. The in situ-formed CaS catalyzes the reduction reaction of lithium polysulfides. Ca2+ rejuvenates via 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 energy density of 493 Wh kg−1 (E/S of 2.4 μL mg−1) based on the total mass of the cell excluding external packaging, with 70% capacity retention at 220 cycle under 1 mA cm−2 discharge, and 346 Wh kg−1 (2.9 μL mg−1) with 77% capacity retention at 360 cycle under 1.0 C 2 mA cm−2 discharge. 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 lithium–sulfur batteries.