Na ⁺ Solvation and Association in Na(SO ₃ CF ₃ )–Dimethoxyethane Electrolytes by Large-Angle X-Ray Scattering and DFT Calculations

The concentration-dependent local structure and ion solvation behavior in sodium triflate NaOTf/dimethoxyethane (DME) electrolytes were systematically investigated by integrating large-angle X-ray scattering (LAXS) with atomic modeling, IR, and density functional theory (DFT) calculations across a range of salt concentrations ( x (NaOTf) = 0, 0.05, 0.1, and 0.2). LAXS data-driven all-atomic modeling reveals that Na + ions preferentially induce DME to adopt the trans–gauche–trans (TGT) conformation, with each DME molecule coordinating to Na + in a bidentate chelation mode via both oxygen atoms. Quantitative analysis of center-of-mass radial distribution functions shows that the average number of DME molecules coordinating to Na + decreases from 2.2 at x (NaOTf) = 0.05 to 1.1 at x (NaOTf) = 0.2, while the average number of associated OTf – anions per Na + increases from 0.9 to 2.3 over the same concentration range. Pair distribution functions combined with DFT-optimized cluster geometries indicate that Na + maintains a total coordination number of 5.4–5.8, with concentration-dependent contributions from the oxygen atoms in both DME and OTf – . The Na + -OTf – contact ion pairs exhibit two distinct configurations: bidentate (CIPB) with a Na–S distance of 2.94 Å and monodentate (CIPM) with a Na–S distance of 3.69 Å. The fraction of CIPB decreases from 56% at x (NaOTf) = 0.05 to 28% at x (NaOTf) = 0.2, while the fraction of CIPM increases correspondingly from 44% to 72%. Speciation analysis based on coordination number distributions reveals that at x (NaOTf) = 0.05, 49% of Na + exists as [Na-3DME] +, 44% as neutral [Na-1OTf-2DME] 0, and 7% as anionic [Na-2OTf-1DME] −; at x (NaOTf) = 0.2, the dominant species becomes [Na-2OTf-1DME] − (35%), followed by [Na-3OTf] 2– (29%), with only 20% remaining as neutral [Na-1OTf-2DME] 0 . This systematic evolution from solvent-separated ions to contact ion pairs and higher-order ionic aggregates with increasing salt concentration provides a quantitative structural basis for understanding the nonlinear ionic conductivity behavior in glyme-based sodium electrolytes.