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

• Published in: Journal of physical chemistry. C 2016-10, Vol.120 (41), p.23339-23350
• Main Authors: Terada, Shoshi, Susa, Hiroko, Tsuzuki, Seiji, Mandai, Toshihiko, Ueno, Kazuhide, Umebayashi, Yasuhiro, Dokko, Kaoru, Watanabe, Masayoshi
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b06804

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 G5)]+ cation. Raman spectroscopy revealed that the complex structure of [Na­(glyme)]+ is maintained in the HFE solution, and free (uncoordinated) glymes 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.