Abstract
Ionic liquids have many potential applications because they have non-flammability, high thermal stability, high ionic conductivity, and so on. Among various types of ionic liquids, quaternary-phosphonium-cation-based ionic liquids (P-ILs) have better physicochemical properties such as ionic conductivity, potential window compared with corresponding quaternary-ammonium-cation-based ionic liquids (N-ILs). Therefore, we have considered that P-ILs are better suitable for applications in electrochemical devices such as electrolytes for lithium-ion batteries than N-ILs [1].Recently, we spectroscopically demonstrated that such higher ionic conductivity for P-ILs can be explained by weaker intermolecular interaction between anion and cation [2]. This provided us a consideration that molecular interactions are controlling factors in translational motion of ions. For deeper discussion, we focused on the ether-functionalized P-ILs displayed in the Figure 1 because ether structure in phosphonium cations enhanced ionic conductivity and investigated by pulsed field gradient NMR (PFG-NMR), Raman, and terahertz spectroscopies.Based on the PFG-NMR, we calculated transport number of each ion specie and found that transport number of P111(1O1) cation was slightly higher than that for P1113 cation. In Raman spectroscopy, we obtained enthalpy change in conformational change of anion (ΔH) and demonstrated that ΔH for P111(1O1)cation was somewhat lower than that for P1113 cation. Terahertz spectroscopy indicated the weaker intermolecular interaction of P111(1O1) cation with anion than that of P1113 cation. Despite these differences in P111X cations, such ether group effects were hardly observed in P222X cations, which would be due to the different symmetry of cation and steric hindrance.
Published Version
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