Molecular Design of Nitrile Electrolytes Enabling Lithiated Silicon–Sulfur Batteries with Quasi‐Solid‐State Sulfur Reaction

The development of lithium-sulfur (Li-S) batteries is hindered by the polysulfide dissolving, cross-over and the inherent lithium metal anode instability. We herein instead describe a lithiated silicon-sulfur (LiSi-S) battery enabled by molecular engineering of highly solvating nitrile electrolytes toward weakly solvating to fundamentally decouple the reactions of the two electrodes and eliminate their cross-talk. Specifically, by controlled fluorination of the ethoxy-nitrile base solvent, the charge distribution on the solvent is manipulated which suppresses the solvation for polysulfides promoting a quasi-solid-state sulfur reaction (QSSSR) mechanism. The promoted anion participation in Li⁺ solvation, along with the fluoroethylene carbonate additive, further stabilizes the interphases at both sulfur cathode and LiSi anode mitigating the mechanical degradations. The QSSSR-based LiSi-S cell shows a high capacity of 1499.0 mA h g_sulfur ^-1 at 0.1C, and achieves a high capacity retention of 90.2% over 100 cycles at 0.2C with an average Coulombic efficiency of 99.9%. This work highlights the essence of molecular engineering for manipulating the primary reactions and interphasial behaviors at both electrodes toward high performance sulfur batteries.