Molecular structure and ultrafast dynamics of sodium thiocyanate ion pairs formed in glymes of different lengths
Glyme-based electrolytes are one of the promising candidates in the development of sodium ion batteries due to their compatibility with conventional graphite electrodes. Recent studies have shown that the chelation effect significantly affects the ion pair formation in these sodium–glyme based electrolytes. However, the solvation structure and dynamics of the sodium–glyme complex have yet to be fully characterized. In this paper, the structure and the motions of the sodium–glyme complex are investigated by using the thiocyanate ion as a reporter of the structure. To this end, steady state and time resolved infrared spectroscopy in conjunction with computational simulations and numerical modeling are used. Overall, the experiments show that the anion is mostly associated with the cation forming contact ion-pairs in all solutions. Time resolved vibrational anisotropy shows a bi-exponential dynamics which is in agreement with the reorientational dynamics of the thiocyanate ion describing its restricted and the overall rotations in the ion-pair-glyme complex. In addition, two dimensional infrared spectroscopy with parallel polarization reveals two dynamical processes for the anion with time scales that increase as a function of glyme length. The molecular motions giving rise to the observed vibrational dynamics are derived by comparing the results with a model describing the restricted rotational diffusion of an axially symmetric particle. The simulated anisotropy shows a good agreement with the experimental measurement. However, to obtain a good agreement of the simulated decorrelation time of the frequency–frequency correlation function (FFCF) with the experiment, a loose tethered oscillator with large stochastic fluctuations is needed. The large stochastic motions are described in terms of the dynamics of the glyme end chains given the observed correlation between the dynamics of the FFCF and the glyme length.