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||NaNi_1/3Fe_1/3Mn_1/3O₂ 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.