(Invited) Structural Aspect on Ion Solvation in Ionic Liquid-Based Electrolyte Solutions for Electrochemical Devices

Abstract

In development of ionic liquid (IL)-based electrolytes for electrochemical devices such as Li ion batteries and electric double-layer and Li ion hybrid electrochemical capacitors, one of important issues is to reduce the viscosity. Bis(fluorosulfonyl)amide (FSA)-based IL is well known as one of low viscous ILs. FSA anion has a similar molecular structure to bis(trifluoromethanesulfonyl)amide (TFSA) anion; CF3 groups of TFSA are only substituted by F atoms, however, the FSA-based ILs show a significantly lower viscosity than the TFSA-based ILs.[1] In general, addition of Li salt to solvent IL leads to an increase in viscosity. Interestingly, the viscosity increase of FSA-based ILs with adding LiFSA salt is appreciably small relative to the corresponding TFSA system,[2] resulting in excellent performance of electrochemical devices with FSA-based ILs. Ion-ion interaction, particularly, “ion solvation” in the bulk plays a key role for understanding not just fundamental dynamic properties of electrolytes but electrode/electrolyte interface structure. In this presentation, we report ion-ion interactions (or liquid structures) in neat FSA- and TFSA-based ILs by using Raman spectroscopy, high energy X-ray total scattering (HEXTS), and theoretical calculations. Solvation structure of Li ion in both ILs are also reported and will be discussed their difference and similarity at the molecular level. From radial distribution functions, G(r)s, obtained by HEXTS and molecular dynamics (MD) simulations,[3] we found that static structure (distance and orientation) of the nearest neighbor cation - anion interaction is almost the same in both [C2mIm][FSA] and [C2mIm][TFSA] (C2mIm: 1-ethyl-3-methylimidazolium cation). On the other hand, in the long-range structure, the peak intensity or oscillation in the G(r) is appreciably weaker for [C2mIm][FSA] than for [C2mIm][TFSA]. This implies that the extent of the long-range ordering or structuredness is rather low in FSA system relative to TFSA system. Raman spectra with the aid of DFT calculations revealed that solvation structure of Li ion in [C2mIm][FSA] is largely different from that in [C2mIm][TFSA]. The Li ion is solvated with two TFSA anions acting as a bi-dentate ligand in [C2mIm][TFSA] system to form [Li(TFSA)2]– complex. In [C2mIm][FSA] system, solvation number of FSA anions bound to Li ion was estimated to be »3, indicating the formation of [Li(FSA)3]2 – complex. By quantitative analysis on HEXTS and MD data, it was found that mono- and bi-dentate FSA anions coexist in the [Li(FSA)3]2 – complex, i.e., [Li(mono-FSA)2(bi-FSA)1]2–.[4] [1] H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, M. Kono, J. Power Sources, 160 (2006) 1308. [2] S. Tsuzuki, K. Hayamizu, S. Seki, J. Phys. Chem. B, 114 (2010) 16329-16336. [3] K. Fujii, S. Seki, K. Ohara, Y. Kameda, H. Doi, S. Saito, Y. Umebayashi, J. Solution Chem., 43 (2014) 1655-1668. [4] K. Fujii, H. Hamano, H. Doi, X. Song, S. Tsuzuki, H. Hayamizu, S. Seki, Y. Kameda, K. Dokko, M. Watanabe, Y. Umebayashi, J. Phys. Chem. C, 117 (2013) 19314-19324.