Unveiling the Dual Functionality of a Bifunctional Carbonate Additive in Practical Li-S Batteries through Multimodal Operando Characterizations

Lithium-sulfur (Li–S) batteries promise ultra-high energy densities but suffer from sluggish sulfur reduction reaction (SRR) kinetics and polysulfide shuttling. Here, we demonstrate that bis(4-nitrophenyl) carbonate (BNC), a previously overlooked carbonate additive, exhibits concentration-dependent bifunctionality in practical Li–S cells (4 mg cm -2 sulfur loading, E/S = 8 µL mg -1 ). At an optimized concentration of 0.01 M, BNC simultaneously modifies the Li + solvation environment and anchors soluble polysulfides, thereby enhancing SRR kinetics and suppressing shuttle effects. We validate these dual mechanisms through an integrated suite of molecular simulations and operando techniques. Ab initio molecular dynamics (AIMD) and 7 Li NMR spectroscopy reveal that BNC induces a redistribution of solvent molecules in the Li + solvation shell, weakening DME coordination and facilitating desolvation for faster reaction kinetics. Operando Raman spectroscopy indicates that middle- and long-chain polysulfide peaks diminish earlier in BNC-containing cells, while operando X-ray absorption spectroscopy (XAS) captures the formation and transformation of short-chain polysulfides (Li 2 S x , x<4), culminating in enhanced Li 2 S formation. Quantitative deconvolution using MCR-ALS confirms a more complete 16-electron conversion under BNC regulation. Electrochemical studies further corroborate these findings. Randles–Sevcik analysis and potentiostatic Li 2 S nucleation tests show a 46% increase in Li-ion diffusion and a 30 mAh g s -1 enhancement in Li 2 S nucleation capacity. In situ EIS reveals that BNC lowers the SRR activation energy by 40.6%, particularly at the most kinetically hindered stages. Galvanostatic cycling achieves 650.93 mAh g s -1 with 93% coulombic efficiency over 200 cycles at C/2. Meanwhile, XRF mapping and shuttle current measurements confirm reduced sulfur deposition on the Li anode, directly linking BNC’s function to suppressed parasitic reactions. This work not only reveals BNC’s unique ability to simultaneously optimize solvation structure and immobilize polysulfides but also sets a blueprint for additive design in high-loading, lean-electrolyte Li–S systems, advancing their viability for commercial applications. Figure 1