Evoking High-Donor-Number-Assisted and Organosulfur-Mediated Conversion in Lithium–Sulfur Batteries

The solution-mediated behavior of lithium–sulfur (Li–S) batteries presents a wide range of opportunity for evaluating and improving the performance at practical lean-electrolyte conditions. Here, we introduce methyl trifluoroacetate (CH3TFA) as an additive to the Li–S electrolyte to evaluate the joint effects of two distinct strategies: high donor number solvents/salts and organosulfur-mediated discharge. CH3TFA is shown to react with lithium polysulfides in situ to form lithium trifluoroacetate (LiTFA) and dimethyl polysulfides. We find that both the methyl group and trifluoroacetate anion considerably enhance Li–S discharge behavior over the course of cycling, though they have distinctly beneficial effects. The TFA anion impacts solution coordination behavior, improving polarization and discharge kinetics during cycling. Meanwhile, the derivatization to dimethyl polysulfides improves the solubility of intermediate species, enhancing overall utilization under lean-electrolyte conditions. CH3TFA thus represents a new class of additives for Li–S batteries, enabling an in situ systematic molecular engineering of intermediate species for improved performance.