Na⁺ Solvation and Association in Na(SO₃CF₃)-Dimethoxyethane Electrolytes by Large-Angle X-Ray Scattering and DFT...

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.