Charge Transport and Structural Dynamics in Ionic Liquids

Charge transport and structural dynamics in a variety of glass-forming ionic liquids (ILs) are investigated over wide frequency, temperature, and pressure ranges by means of broadband dielectric spectroscopy (BDS), pulsed field gradient nuclear magnetic resonance (PFG NMR), frequency-dependent calorimetry, differential scanning calorimetry and rheology [1-3]. The dielectric spectra are dominated – on the low-frequency side – by electrode polarization effects while, for higher frequencies, charge transport in a disordered matrix is the underlying physical mechanism. While the absolute values of dc conductivity, and viscosity vary over more than 11 decades with temperature and upon systematic structural variation of the ILs, quantitative agreement is found between the characteristic diffusion rates and the structural alpha-relaxation rates. Using a combination of Einstein, Einstein-Smoluchowski, Maxwell and Langevin relations, the observed universal scaling of charge transport in ionic liquids is traced back to the dominant role of Brownian dynamics. A novel approach is applied to extract diffusion coefficients from BDS spectra in quantitative agreement with PFG NMR values but in a much broader range. It becomes possible to separately determine the effective number density and the mobilities of the charge carriers as well as the type of their thermal activation from the measured dielectric/conductivity spectra. It is shown that the observed Vogel-Fulcher-Tammann (VFT) dependence of the dc conductivity can be traced back to a similar temperature dependence of the mobility while for the number density an Arrhenius-type thermal activation is found.

Comparing low molecular weight and polymerized ILs having the same cations and anions, a huge shift is found in the glass transition temperature from -70 ° C to + 55 ° C for the polymerized system. As a result the dc conductivity is decreased by 7 orders of magnitude. While the dc conductivity and fluidity exhibit practically identical temperature dependence for the non-polymerized IL, a significant decoupling of ionic conduction from structural dynamics is observed for the polymerized IL. In addition, the dc conductivity of the polymerized IL exceeds that of its molecular counterpart by four orders of magnitude at their respective calorimetric glass transition temperatures. This is attributed to the unusually high mobility of the anions especially at lower temperatures when the structural dynamics is significantly slowed down. A simple physical explanation of the possible origin of the remarkable decoupling of ionic conductivity from structural dynamics will be discussed.

Ionic liquids are promising materials for the development of safe electrolytes in modern electrochemical energy devices such as batteries, super-capacitors, fuel cells and dye-sensitized solar cells. However, progress in the use of ionic liquids in these applications is still hindered by limited understanding of the mechanism of charge transport as well as the interplay between molecular structure and dynamics in ionic liquids. In this talk, new insights into the dominant mechanisms of charge transport and structural dynamics obtained from studies involving broadband dielectric spectroscopy, pulsed field gradient nuclear magnetic resonance, dynamic mechanical spectroscopy, and frequency-dependent calorimetry will be discussed. We will also discuss some immediate technological benefits of these findings.

References

1)       Sangoro, J. R. and Kremer, F. Acc. Chem. Res., 2012, 45, 525-532.

2)       Sangoro et al. Soft Matter, 2011, 7, 1678-1681.

3)       Sangoro et al. Phys. Chem. Chem. Phys., 2009, 11, 913.