Fluorine‐Free Cosolvent Chemistry Empowering Sodium‐Sulfurized Polyacrylonitrile Batteries

Abstract Localized high‐concentration electrolytes (LHCE) show great promise for room‐temperature sodium‐sulfur batteries. However, the majority of diluents in LHCE systems consist of fluorinated ethers, which are not only dense and expensive but also demonstrate poor reductive stability with sodium metal. Herein, a low‐density, non‐fluorinated ether electrolyte is presented that demonstrates localized high‐concentration behavior. This feature is driven by the weak solvating capabilities of 1,2‐dimethoxypropane (DMP) and the ultra‐weak solvating nature of cyclopentyl methyl ether (CPME). Impressively, the fluorine‐free CPME cosolvent acts as a diluent within the electrolyte. Therefore, the electrolyte achieves a tailored solvation structure characterized by anion‐rich species, which fosters the development of a resilient inorganic‐rich SEI with superior Na‐ion transport. Consequently, with a high sulfur‐content sulfurized polyacrylonitrile (SPAN, S content > 45% in SPAN) loading of 4.4 mg cm⁻ 2 (sulfur loading: 2 mg cm⁻ 2 ) and a low electrolyte‐to‐SPAN ratio of 9 µL mg⁻¹ (E/SPAN = 9), the Na‐SPAN cell demonstrates remarkable reversibility of 530 mA h g sulfur ⁻¹ after 200 cycles at C/5 rate. This performance surpasses the majority of state‐of‐the‐art Na‐SPAN ether‐based electrolyte systems reported to date. Hence, this work presents a novel approach for designing cost‐effective, high‐performance electrolytes for stable, practical Na‐SPAN batteries.