Modulating Solvation Structure through Steric Hindrance: A Diluent Strategy for Low-Temperature Sodium Metal...

The ability of sodium metal batteries (SMBs) to operate under low temperatures is highly desirable; however, achieving stable performance remains challenging due to limitations in cation-solvent interactions and sluggish desolvation kinetics. Salt precipitation or electrolyte solidification severely impedes ion transport, leading to low Coulombic efficiency for Na plating/stripping, rampant dendrite growth, fragile solid electrolyte interphase (SEI) formation, and severe capacity decay. Localized high-concentration electrolytes (LHCEs) offer a promising solution but often rely on costly and volatile fluorinated ether diluents. Herein, we introduce cyclopentyl methyl ether (CPME) as a cost-effective diluent to formulate a NaPF6-diethylene glycol dimethyl ether (G2)/CPME LHCE. Combining theoretical and experimental analyses, we demonstrate that the bulky cyclopentyl group in CPME introduces steric hindrance, which kinetically excludes CPME from the primary Na+ solvation sheath while weakening Na+-G2 coordination. This facilitates PF6- entry into the inner solvation sheath, forming an anion-enriched structure that promotes a gradient NaF-rich SEI. As a result, Na‖Na symmetric cells operate stably for over 3600 h at 25 °C, and Na‖Na3V2(PO4)3 full cells retain 90.1% of their initial capacity after 1000 cycles at 1 C. At -30 °C, they deliver 90.9% capacity retention after 500 cycles at 1 C, compared to 72.7% for the NaPF6-G2 electrolyte. This work establishes CPME as a cost-effective diluent for enabling low-temperature-tolerant SMBs.