Amide-type ionic liquids (ILs) have been investigated as the electrolyte for secondary batteries because they have such favorable properties as non-flammability, low-volatility, and the electrochemical stability.^1,2 However, the ILs have been reported to decompose cathodically at the potentials more positive than the deposition potential of Li and form solid-electrolyte interphase (SEI) on the electrode.³ Although the formation of the SEI is favorable for lithium secondary batteries, the SEI may hinder the electrode reactions of the active materials when the ILs are used as the electrolytes for redox flow batteries. In the present study, the formation potentials of the SEI in two amide-type ILs, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing metal salts were investigated in the presence of a redox species.

BMPBr was prepared by the reaction of 1-methylpyrrolidine and 1-bromobutane in acetonitrile. BMPFSA was obtained by the interaction of BMPBr with LiFSA in distilled water. BMPFSA was dried under vacuum at 80 °C for 48 hours before use. BMPTFSA and the metal salts were used as supplied. The water content in BMPFSA and BMPTFSA was less than 10 ppm. The electrochemical measurement was carried out with a three-electrode cell in an argon-filled globe box. A Pt disk and wire were used as a working and counter electrode, respectively. Ag wire immersed in BMPFSA or BMPTFSA containing 0.10 M AgCF₃SO₃ was used as a reference electrode, which is denoted by Ag|Ag(I).

The anodic peak potentials in the cyclic voltammograms in the ILs containing the redox species were almost constant after holding the potentials more positive than the cathodic decomposition potentials. On the other hand, the anodic peak potentials changed by holding the potential at the negative potential region in the presence of the metal species, suggesting that the potential of the SEI formation can be determined more sensitively by monitoring the redox reaction.

References

[1] M. Yamagata, Y. Katayama, and T. Miura, J. Electrochem. Soc., 153, E5 (2006).

[2] K. Matsumoto, T. Hosokawa, T. Nohira, R. Hagiwara, A. Fukunaga, K. Numata, E. Itani, S. Sakai, K. Nitta, and S. Inazawa, J. Power Sources, 265, 36 (2014).

[3] R. Furuya, N. Tachikawa, K. Yoshii, Y. Katayama, and T. Miura, J. Electrochem. Soc., 162, H634 (2015).