Glyme-Sodium Bis(fluorosulfonyl)amide Complex Electrolytes for Sodium Ion Batteries

The physicochemical and electrochemical properties of an equimolar complex of pentaglyme (G5) and sodium bis(fluorosulfonyl)amide (NaFSA), [Na(G5)][FSA], mixed with a hydrofluoroether (HFE) were investigated. Ab initio calculations and Raman spectroscopy showed that the coordination structure of [Na(G5)][FSA] was similar to that of [Na(G5)][TFSA] (TFSA: bis(trifluoromethanesulfonyl)amide). The ligand G5 remained coordinated to Na+ and was not liberated from the cationic [Na(G5)](+) complex even in the presence of HFE. The charge transport property was greater in [Na(G5)][FSA]/HFE than in [Na(G5)][TFSA]/HFE. A prominent difference was found in the Na metal deposition/dissolution behavior. Highly reversible Na deposition/dissolution with a Coulombic efficiency (95%) could be achieved in [Na(G5)][FSA]/HFE; however, the reversibility in [Na(G5)][TFSA]/HFE was very low. X-ray photoelectron spectroscopy (XPS) of the deposited Na metal in each electrolyte revealed that a thin and compact layer of electrolyte decomposition products was formed on the Na deposits in [Na(G5)][FSA]/HFE. The FSA-derived thin layer can effectively inhibit the further decomposition of the electrolyte. By contrast, a thick electrolyte decomposition product found for [Na(G5)][TFSA]/HFE suggested continuous decomposition of the electrolyte during Na deposition-dissolution. Highly stable charge and discharge of a hard carbon (HC) electrode was accomplished in [Na(G5)][FSA]/HFE, with high Coulombic efficiency over 99.9% and negligible capacity decrease over 300 cycles. Electrochemical impedance measurements of a symmetrical cell for Na, HC, and Na0.44MnO2 electrodes with the above electrolytes verified that a stable electrode vertical bar electrolyte interface was formed on the HC and Na0.44MnO2 electrodes in [Na(G5)][FSA]/HFE.