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

High Resolution Image Download MS PowerPoint Slide 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 Na 0.44 MnO 2 electrodes with the above electrolytes verified that a stable electrode | electrolyte interface was formed on the HC and Na 0.44 MnO 2 electrodes in [Na(G5)][FSA]/HFE.