Ionic liquids (IL) have a great potential of applications in electrochemical devices such as electrolytes for fuel cells. In particular, quaternary phosphonium cation-based ionic liquids (P-ILs) is much suitable due to lower viscosity and better ionic conductivity compared with corresponding quaternary ammonium cation-based ionic liquids (A-ILs). Over the past decade, fundamental properties of P-ILs have been investigated by viscometry, calorimetry, spectroscopy, calculation, and so on. However, little is known about molecular mechanism for such better potential of P-ILs. The present study aimed to reveal the relationship between molecular structure and macroscopic material properties of P-ILs in terms of alkyl chain asymmetry.

The P-ILs used in this study were prepared by quadrification of trialkylphosphine followed by ion exchange with bis(trifluoromethanesulfonyl)imide (TFSA) anions. We named P-ILs based on the number of alkyl chain length. For example, P2225 means triethyl(penta)phosphonium cation. In this study, we used P2225-TFSA, P2228-TFSA, and P222(12)-TFSA. We conducted temperature-variable broadband dielectric spectroscopy between 20 Hz and 100 MHz. We also carried out Raman spectroscopy (laser excitation at 532 nm) and THz spectroscopy.

The obtained complex dielectric spectra were dominated by (i) electrode polarization, (ii) ionic conduction, and (iii) reorientational relaxation, depending on the frequency and temperature. The spectra at the frequency regions between ionic conduction and reorientational relaxation were well-fitted with a Havriliak-Negami dielectric relaxation function, superimposed with a conductivity term. Thus, we determined both ionic conductivity (σ_dc) and relaxation time for structural relaxation (τ_HN) of ionic liquids. Although the σ_dc for all of the P-ILs used in this study exhibited similar Vogel-Fulcher-Tamman-type temperature dependence, the σ_dc slightly decreased with increasing the asymmetric alkyl chain length. Such temperature dependence was also observed in τ_HN, meaning that translational dynamics partly coupled with segmental dynamics. This result is consistent with that internal motion of the alkyl chains should depend on the chain length. Raman and THz spectroscopy provided an information on molecular structure (such as conformation) and inter- and intra-molecular interaction between anions and cations. In the presentation, we will describe the relationship of charge carrier transportation with ion conformations and intermolecular structures.