A mixture of ionic liquids, 1,3-dimethylimidazolium 2-methoxy(2-ethoxy(2-ethoxy(2-ethoxy)))ethylphosphite ([DMIm][MPEGP]) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), which forms an ionic liquid crystal with a superior ionic conductivity, evolves a smectic structure through ion-ion interactions as a function of LiTFSI concentration. Nuclear magnetic resonance (NMR) spectroscopy and pulsed-field-gradient (PFG) NMR examinations showed that the morphology of the structures and the strength of ion-ion interactions are closely related to the ratio of Li+/[MPEGP]-. The results revealed structures composed of Li+/[MPEGP]-/TFSI- (≈3:4:1) mainly by the Coulombic interactions between Li+ cations and phosphite head groups in [MPEGP]- anions. NMR diffraction on PFG echo profiles revealed a cluster size of -2 μm, inversely proportional to the number of mobile [MPEGP]- anions, and an ion diffusion on the cluster surfaces that is 1000 times faster than that in the bulk liquid. These observations suggest that the superior ionic conductivity in the mixture is mainly due to the fast ion transport on the cluster surfaces of smectic ionic liquid crystals. The variations of diffusion ratios between mobile Li+, TFSI-, [DMIm]+, and [MPEGP]- ions indicate that these mobile ions remaining in the voids of structures preserve the bulk ionic liquid properties.
Bibliographical noteFunding Information:
This work was financially supported by a grant from the Fundamental R&D Program for Core Technology of Materials; Industrial Strategic Technology Development Program funded by the Ministry of Knowledge Economy, Republic of Korea; and partially supported by Materials Architecturing Research Center at the Korea Institute of Science and Technology (KIST). This work was partially supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2018R1D1A1B07050522). Synchrotron X-ray diffraction examinations were performed at the Pohang Light Source, South Korea, and NMR experiments were performed at EMSL, a DOE Office of Science user facility sponsored by the DOE BER located at PNNL.
© 2019 American Chemical Society.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- General Energy
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films