Anion Dependence on Lithium Redox in Imidazolium Ionic Liquds Composed of Cyclic Amides

Ionic liquids (ILs) composed of perfluoroanion such as BF₄⁻ and Tf₂N⁻ ([(CF₃SO₂)₂N]⁻) have attracted attention as less flammable and less volatile electrolyte for a lithium secondary battery. Especially, Tf₂N^- forms ILs not only 1-ethyl-3-methylimidazolium (C₂mim⁺), but also various aliphatic quaternary ammonium (AQA⁺). The AQA-Tf₂N exhibited large apparent electrochemical windows enough wide to use a solvent free electrolyte for 4 V-class battery applications with the use of lithium metal anode. [1] The cathodic stability of AQA-Tf₂N such as N-methyl-N-butylpyrrolidinium indeed enough negative to perform lithium redox in the ILs, however, another amide ILs composed of f₂N^- (bis(fluoromethylsulfonyl)amide) also exhibited very good lithium redox on metal electrodes even in the [C₂mim][f₂N]. [2] Considering that the addition of lithium salts in [C₂mim][Tf₂N] extended their cathodic limit enough negative potential [3], the existence of a lithium salt also one of the important factor to exhibit good lithium redox in C₂mim-ILs, however, no good lithium redox was observed in the [C₂mim][Tf₂N]. The [C₂mim][f₂N] could be used as a 4V class battery with the use of not only the lithium metal anode but also conventional carbon anodes. [4] The most striking point of such f₂N^--based ILs was the rate performance was quite high among various ILs composed of perfluoroanions. [5]

 We have recently much focused on one of asymmetric amide such as (fluoromethylsulfonyl)(trifluoromethylsulfonyl)amide (fTfN^-) because the anions possesses superior ability to reduce melting point of their salts. Not only tetraethylammonium but also 5-azoniaspiro[4.4.]nonane, which exhibited much high melting point with the Tf₂N^- and the f₂N^-, forms ILs with fTfN^-. Furthermore, these ILs could be used as a full cell application with the use of carbon anode like f₂N^-. [6,7] These results strongly suggest that the existence of FSO₂- group in amide anions have an important substituent in the application of ILs as lithium battery electrolytes. One of the possible reasons for such good performance even in C₂mim⁺-ILs might be due to the existence of good SEI film on an electrode, which protects further decomposition of cathodically unstable C₂mim⁺.

  In this presentation, we would like to show C₂mim-ILs composed of one of the cyclic amides (cTf₂N) also exhibited very good lithium redox on a metal electrode. This seems important because the good lithium redox can be achieved in C₂mim-ILs without FSO₂- group. To discuss the effect of such anion dependence on the lithium redox in C₂mim-ILs, EQCM measurements were performed. The results indicate that a certain surface film on the metal electrode formed in both the [C₂mim][f₂N] and the [C₂mim][cTf₂N]. However the mass change observed in the former ILs was much larger than the latter. From these results, FSO₂^- group in amide anion might be act as a surface modification reagents like a carbonate solvent molecule. On the other hand, the results for cyclic amide without FSO₂- group indicate that the anion structure is quite important to achieve good lithium cyclability.  All these results imply that C₂mim-ILs also attractive candidate for a 4 V - class lithium battery electrolyte if we choose appropriate anions.

[1] H. Sakaebe and H. Matsumoto., Electrochem. Commun., 5,594 (2003).

[2] H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, M. Kono, J. Power Sources, 160(2), 1308 (2006).

[3] H. Matsumoto, H. Kageyama, Y. Miyazaki, Electrochemisty,  71(12), 1058 (2003).

[4] M. Ishikawa, T. Sugimoto, M. Kikuta, E. Ishiko, M. Kono, J. Power Sources, 162(1), 658 (2006).

[5] H. Matsumoto, H. Sakaebe, K. Tatsumi, ECS Transaction, 16(35), 59 (2009).

[6] H. Matsumoto, N. Terasawa, T. Umecky, S. Tsuzuki, H. Sakaebe, K. Asaka, K. Tatsumi, Chem. Lett., 37, 1020 (2008).

[7] H. Matsumoto, N. Terasawa, H. Sakaebe, S. Tsuzuki, WO 2009/136608 A1.