Fig. 2: The coordination interactions between Na⁺ and different solvent molecules in the EMTMSA-based and carbonate-based electrolytes by Raman spectroscopy and NMR techniques.

Deconvolution of Na⁺–FSI⁻ Raman peaks for the carbonate-based (a) and EMTMSA-based (b) electrolytes with peak fits for different cluster types, including SSIP, CIP, and AGG. ¹⁷O NMR spectra of the carbonate-based (c) and EMTMSA-based (d) electrolytes before and after incorporating 0.1 M and 1 M NaFSI into the solvent mixtures. The corresponding functional groups in different molecules and chemical shifts are denoted. Schematic figures of the representative Na⁺ solvation structures in the carbonate-based (e) and EMTMSA-based (f) electrolytes. ¹H VT-NMR spectra of the PC/EMC (g) and EMTMSA/PC/EMC (h) solvent mixtures at 25 °C, −20 °C, and −50 °C. Peak broadening is observed for the carbonate mixtures upon cooling, corresponding to significantly decreased molecular rotation. In contrast, the peaks remain almost unchanged for the EMTMSA-carbonate mixture. Source data are provided as a Source data file.