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

Area of Science:

• Electrochemistry
• Materials Science
• Physical Chemistry

Background:

• Understanding ion solvation and local structure is crucial for optimizing electrolyte performance in sodium-based batteries.
• Sodium triflate (NaOTf) in dimethoxyethane (DME) is a promising electrolyte system, but its structural evolution with concentration requires detailed investigation.

Purpose of the Study:

• To systematically investigate the concentration-dependent local structure and ion solvation behavior in NaOTf/DME electrolytes.
• To elucidate the speciation of sodium ions and the formation of ion pairs/aggregates at varying salt concentrations.

Main Methods:

• Integration of large-angle X-ray scattering (LAXS) with atomic modeling.
• Complementary use of infrared (IR) spectroscopy and density functional theory (DFT) calculations.
• Analysis of radial distribution functions and coordination numbers across a range of NaOTf concentrations (0-0.2).

Main Results:

• Na+ ions induce DME to adopt a trans-gauche-trans (TGT) conformation, coordinating via a bidentate chelation mode.
• The number of coordinating DME molecules decreases, while associated triflate anions increase with rising NaOTf concentration.
• Two distinct Na+-triflate contact ion pair configurations (bidentate and monodentate) were identified, with their fractions changing significantly with concentration.
• Speciation analysis revealed a shift from solvated Na+ to neutral and anionic ion pairs/aggregates as NaOTf concentration increased.

Conclusions:

• The study provides a quantitative structural basis for the nonlinear ionic conductivity observed in glyme-based sodium electrolytes.
• The transition from solvent-separated ions to contact ion pairs and higher-order aggregates is a key factor governing electrolyte behavior.