Electrolytes with Solvating Inner Sheath Engineering for Practical Na–S Batteries

Abstract Sodium–sulfur (Na–S) batteries with durable Na‐metal stability, shuttle‐free cyclability, and long lifespan are promising to large‐scale energy storages. However, meeting these stringent requirements poses huge challenges with the existing electrolytes. Herein, a localized saturated electrolyte (LSE) is proposed with 2‐methyltetrahydrofuran (MeTHF) as an inner sheath solvent, which represents a new category of electrolyte for Na–S system. Unlike the traditional high concentration electrolytes, the LSE is realized with a low salt‐to‐solvent ratio and low diluent‐to‐solvent ratio, which pushes the limit of localized high concentration electrolyte (LHCE). The appropriate molecular structure and solvation ability of MeTHF regulate a saturated inner sheath, which features a reinforced coordination of Na⁺ to anions, enlarged Na⁺‐solvent distance, and weakened anion‐diluent interaction. Such electrolyte configuration is found to be the key to build a sustainable interphase and a quasi‐solid–solid sulfur redox process, making a dendrite‐inhibited and shuttle‐free Na–S battery possible. With this electrolyte, pouch cells with decent cycling performance under rather demanding conditions are demonstrated.