Dissociation and Diffusion of Glyme-Sodium Bis(trifluoromethanesulfonyl)amide Complexes in Hydrofluoroether-Based Electrolytes for Sodium Batteries

Physicochemical properties and battery performance of [Na-(glyme)][TFSA] complexes (TFSA: bis(trifluoromethanesulfonyl)amide) dissolved in a hydrofluoroether (HFE) were investigated. Glyme (tetraglyme (G4) or pentaglyme (G5)) coordinates to Na+ to form a 1:1 complex [Na(G4 or GS)](+) cation. Raman spectroscopy revealed that the complex structure of [Na(glyme)](+) is maintained in the HFE solution, and free (uncoordinated) glyrnes are not liberated on adding HFE. HFE molecules are scarcely involved in the first solvation shell of Na+ because of their low electron-pair-donating ability. Raman spectra of the [TFSA](-) anion suggests that the attractive interaction between the complex [Na(glyme)](+) cation and [TFSA](-) anion is enhanced on adding HFE. The population of contact ion-pair (CIP) and/or aggregate (AGG) is smaller for the G5 system than for the G4 one, and the [Na(G5)][TFSA]/HFE has higher ionic conductivity. The self-diffusion coefficients of the [Na(glyme)]+ complex and [TFSA](-) were measured by pulsed field gradient (PFG) NMR, and the dissociativity of [Na(glyme)] [TFSA] was assessed. The dissociativity of the G5 system is greater than that of the G4 one, and the dissociativity can be correlated with the attractive interaction between [Na(glyme)](+) and [TFSA](-), as evaluated by ab initio calculations. The dissociativity of the complexes gave significant effects on the battery performance.