Glyme based electrolytes are one of the promising candidates towards the development of sodium ion batteries since they enable sodium ions to intercalate with conventional graphite electrodes. However, the solvation and dynamics of sodium ions in glyme-based solvents are not fully characterized even though they are important to understand the electrochemical properties of these electrolytes. Here, we investigated the structure and dynamics of the ion pairs formed by sodium thiocyanate in different glyme solvents (mono- to tetra-glyme) using linear (FTIR) and multi-dimensional (2DIR) infrared spectroscopic techniques. The FTIR data shows that the majority of thiocyanate ions are found as contact ion pairs with the sodium ions irrespective of the glyme. The measured conductivity values of these systems are consistent with the FTIR results. The 2DIR studies of these systems reveal the two dynamical components for frequency-frequency correlation function (FFCF) of the thiocyanate ion. While the fast dynamical component has a time scale lower than 10 ps time scales and becomes slower for longer glyme lengths, the slow dynamical component does not change with the glyme length and it has a characteristic time of ~30 ps. The anisotropy measurements of the electrolytes reveal a fast motion which has the same characteristic time as the FFCF and is assigned to the thermal rotation motion of the thiocyanate ion in a cone. Modeling of the anisotropy shows that the cone angle becomes smaller as the length of the glyme increases, indicating that larger glymes produce more open coordination structures around the sodium ion. The results provide a new structural information of the coordinated sodium ion in glyme which has interesting implications for understating their susceptibility to the formation of ion pairs.

Structure and dynamics of ion pairs in glymes

255th National Meeting (2018)

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