Factors influencing fast ion transport in glyme-based electrolytes for rechargeable lithium–air batteries

Abstract

To elucidate the determination factors affecting Li-ion transport in glyme-based electrolytes, six kinds of 1.0 M tetraglyme (G4) electrolytes were prepared containing a Li salt (LiSO₃CF₃, LiN(SO₂CF₃)₂, or LiN(SO₂F)₂) or different concentrations (0.5, 2.0, or 2.7 M) of LiN(SO₂CF₃)₂. In addition to conventional bulk parameters such as ionic conductivity (σ), viscosity (η), and density, self-diffusion coefficients of Li⁺, anions, and G4 were measured by pulsed-gradient spin-echo nuclear magnetic resonance method. Interaction energies (ΔE) were determined by density functional theory calculations based on the supermolecule method for Li⁺–anion (salt dissociation) and G4–Li⁺ (Li⁺ solvation) interactions. The ΔE values corresponded to ion diffusion radii formed by solvation and/or ion pairs. The order of dissociation energies ΔE was LiSO₃CF₃ > LiN(SO₂CF₃)₂ > LiN(SO₂F)₂, which agreed well with the dissociation degree of these salts in the electrolytes. From the obtained knowledge, we also demonstrated that increasing the mobility and number of carrier ions are effective ways to enhance σ of glyme-based electrolytes by using 1,2-dimethoxyethane with lower η and similar dielectric constant to those of G4.