Modulating Solvation Structure through Steric Hindrance: A Diluent Strategy for Low-Temperature Sodium Metal Batteries with Robust Inorganic Interphase

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 NaPF 6 −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 PF 6 − 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‖Na 3 V 2 (PO 4 ) 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 NaPF 6 −G2 electrolyte. This work establishes CPME as a cost-effective diluent for enabling low-temperature-tolerant SMBs.