Li(+) Local Structure in Hydrofluoroether Diluted Li-Glyme Solvate Ionic Liquid

Area of Science:

• Electrochemistry
• Materials Science
• Computational Chemistry

Background:

• Hydrofluoroethers (HFEs) are used as diluents in lithium battery electrolytes to tune ionic conductivity and viscosity.
• Understanding the local structure of lithium ions (Li+) is crucial for optimizing electrolyte performance.

Purpose of the Study:

• To clarify the Li+ local structure in 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) diluted [Li(G4)][TFSA] solvate ionic liquid.
• To investigate the coordination behavior of HFEs and tetraglyme (G4) with Li+ and bis(trifluoromethanesulfonyl)amide (TFSA) anions.

Main Methods:

• Raman spectroscopy combined with Density Functional Theory (DFT) calculations.
• X-ray total scattering experiments (HEXTS) supported by Molecular Dynamics (MD) simulations.
• Development of new intermolecular force field parameters for HFEs and glymes.

Main Results:

• HFEs do not directly coordinate to Li+.
• Both solvent-shared ion pairs (SSIP) and contact ion pairs (CIP) exist in neat and HFE-diluted electrolytes.
• Two types of CIPs (monodentate and bidentate) were identified, with monodentate CIPs decreasing upon HFE dilution.
• X-ray scattering revealed a new peak attributed to correlations between [Li(G4)][TFSA] ion pairs.
• MD simulations indicated dynamic coordination of G4's terminal oxygen atoms to Li+.

Conclusions:

• HFE dilution alters the ion pairing in [Li(G4)][TFSA] electrolytes by reducing monodentate CIPs.
• The dynamic coordination of G4 to Li+ is a key feature in both neat and HFE-diluted systems.
• This study provides fundamental insights into the structural evolution of lithium battery electrolytes with HFE additives.