Physicochemical and Electrochemical Properties of Glyme-Na Salt Molten Complex/Hydrofluoroether Mixtures
Authors: Terada Shoshi , Susa Hiroko , Dokko Kaoru , Watanabe Masayoshi 
Published: February 2020

1. Introduction:

 Our group has been investigating a specific highly concentrated electrolyte that behaves like ionic liquids, an equimolar mixture of certain Li salt, e.g. Li[TFSA] (lithium bis(trifluoromethanesulfonyl)amide: LiN(SO2CF3)2 ) and glymes (Gn: (CH3O(CH2CH2O)
n
CH3)). Li+ ion forms complex cation with glyme in 1:1 ratio when the chain length of glyme are n = 3 or 4 (G3 or G4), and nearly all glymes in the equimolar mixture coordinate to Li+ ions. Therefore the solvate cation [Li(glyme)]+ is the only cation in the system. Consequently, the mixture [Li(Glyme)][TFSA] behaves as a ionic liquid consists of [Li(glyme)]+ and a counter anion [TFSA]−, having low volatility, low flammability, high thermal stability, and electrochemical stability. Such mixture are classified as solvate ionic liquids (SILs).1,2) The glyme-Li salt SILs can be applied to various Li secondary batteries as a thermally stable electrolyte.2,3) In a similar manner, the sodium SILs can be prepared by simply mixing equimolar amount of Na[TFSA] and glymes, and applied to sodium secondary battery.4)The sodium secondary battery is one of the most promising low cost next generation battery owing to the natural abundance of sodium resource. To realize such battery, investigating reversible and stable anode is inevitable. In this work, the physicochemical properties and sodium metal deposition/dissolution behavior of glyme-Na SILs mixed with a hydrofluoroether (HFE: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether) are explored.

2. Experimental: The Na[TFSA] or Na[FSA] (sodium bis(flurorosulfonyl)amide: NaN(SO2F)2), G5, and HFE are mixed at 1:1:4 molar ratio under Ar atmosphere glove box and used as the electrolyte. The deposition and dissolution of Na metal was tested by 2032 type coin cell at 30 °C with Al foil as working electrode and Na metal as counter electrode. The current density was set at 100 μA cm−2with deposition for 1 hour and then reversed the current until the cell reaches 2 V. The cell was disassembled after 50th dissolution and decomposition product on Al electrode was observed.

3. Result and discussion:

 Figure 1 shows the coulombic efficiencies of Na metal dissolution/deposition and optical images of Al electrode after 50 cycles with electrolyte composed of TFSA or FSA anion. A clear difference in coulombic efficiency was observed with electrolyte composed of TFSA anion showing poor coulombic efficiency, lower than 1 % throughout the cycling. The optical image of Al electrode after 50 cycles of Na metal deposition and dissolution in TFSA electrolyte shows the electrode fully covered with decomposition product of Na metal and electrolyte. This indicates the irreversible reaction such as decomposition of electrolyte by the deposited Na metal taking place, resulting in poor coulombic efficiency. On the other hand, the FSA electrolyte shows high coulombic efficiency of over 90 % after 10 cycles, indicating the reversible deposition and dissolution of sodium metal. The Al electrode after 50 cycles still shows metallic appearance of Al foil and small amount of decomposition products were observed. The analysis of decomposition products and surface layer of sodium metal are underway and will be reported.

4. References

1) T. Mandai et al., Phys. Chem. Chem. Phys., 16, 8761 (2014).

2) K. Yoshida et al., J. Am. Chem. Soc., 113, 13121 (2011).

3) K. Dokko et al., J. Electrochem. Soc., 160, A1304 (2013).

4) S. Terada et al., Phys.Chem.Chem. Phys., 16, 11737 (2014).

Figure 1