Spatially Informed Qualitative Design of Weakly Solvating Electrolytes for Fast-Charging Sodium-Ion Batteries

The fast-charging performance of hard carbon (HC) anodes with low sodiation plateau potentials critically depends on Na+ transport within the electrolyte and across the electrode–electrolyte interface. Anion-derived solid electrolyte interphases (SEIs) are known to reduce interfacial resistance and facilitate rapid ion transport. Introducing steric hindrance in solvents can weaken cation–solvent coordination and promote the formation of anion-derived SEIs. However, a universal strategy for comparing solvent steric effects remains elusive. Herein, Molar volume and Sterimol parameters are employed to qualitatively assess steric effects in alkyl mononitriles, a class of solvents capable of supporting high-ionic-conductivity electrolytes. Using this approach, trimethylacetonitrile (TMAN) is identified as a sterically hindered solvent with weak coordinating ability. TMAN-based electrolytes reduce Na+ desolvation energy and enhance anion participation in the primary solvation shell, yielding compact, low-impedance SEIs on HC anodes. Consequently, a simple electrolyte of 1 M sodium bis(fluorosulfonyl)imide in TMAN enables HC fast charging at 3C without sodium plating, while Ah-level HC||NaNi1/3Fe1/3Mn1/3O2 pouch cells retain 93.1% of their capacity after 3000 cycles at 1C, and 91.7% after 1300 cycles at 3C charging rate. This study established a generalizable solvent screening strategy, laying the foundation for the efficient development of electrolytes.