Anion Dependence on Lithium Redox in Imidazolium Ionic Liquds Composed of Cyclic Amides
Authors: Matsumoto Hajime , Tsuzuki Seiji , Kubota Keigo 
Published: February 2020

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.
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