Mechanistic understanding of a bifunctional carbonate additive for enhanced performance in lithium-sulfur battery

Lithium-sulfur (Li-S) batteries stand promising for next-generation energy storage systems due to their high specific capacity and cost-effectiveness. However, their commercialization is hindered by sluggish sulfur reduction reaction (SRR) kinetics and polysulfide migration. To address these challenges, we introduce bis(4-nitrophenyl) carbonate (BNC) as a bifunctional electrolyte additive. At an optimal concentration, BNC leverages its polar nature to anchor soluble polysulfides while simultaneously modifying the Li⁺ solvation structure at the molecular level, enhancing SRR kinetics. This dual functionality is confirmed through molecular dynamics simulations and electrochemical analyses. In situ electrochemical impedance spectroscopy (EIS) further shows that optimal BNC concentration reduces activation energy for polysulfides formation by 40.6%. Operando spectroscopic techniques, including Raman and X-ray absorption spectroscopy (XAS), demonstrate BNC's dual effect, with a focus on the middle-chain polysulfides conversion, supported by detailed polysulfide quantification. X-ray fluorescence (XRF) mapping reveals decreased sulfur deposition on lithium, indicating the effectiveness of shuttle suppression. These effects contribute to outstanding cycling performance under practical conditions, achieving 650.93 mAh ${\text{g}}_{\text{sulfur}}^{-1}$ and coulombic efficiency of 93% over 200 cycles at a C-rate of C/2. This work not only offers valuable insights into the use of unconventional carbonate-based additives but also provides a blueprint for advancing Li-S battery designs through targeted solvation structure modifications.