Solvation of Sodium Salts in Glymes

Sodium ion batteries are promising candidates to replace high cost lithium ion batteries due to the high natural abundance of sodium. However, conventional graphite anodes cannot be used in sodium ion batteries due to difficulties in intercalating sodium ions into graphite. Previous studies have shown that ether based electrolytes, such as glymes, are promising candidates since they do not present the problem of intercalation of sodium ions into graphite. Interestingly, the solvation of sodium ions in glymes has not been fully characterized. Here, we studied the solvation of sodium ions in glymes of different lengths, from two to four units, via two dimensional infrared spectroscopy, 2D-IR, and FTIR spectroscopy. Our studies show that the solvent molecules and counter ion play a significant role in the solvation of the sodium ion. For example, the sodium ion is mainly associated to its thiocyanate counter ion through the sulfur atom irrespective of the glyme length. Moreover, the ratio between free ions and ion pairs changes with the type of glyme. In addition, 2D-IR spectroscopic studies reveal the presence of two distinct motions in the molecular environment of the thiocyanate ion with the picosecond time scales that increase their characteristic times with the length of the glyme. In contrast, studies on sodium bis(trifluoromethane)sulfonimide (TFSI) in different glymes show a contrasting picture in which most of the counter ions do not form ion pairs with the sulfonate groups of the anion. Moreover, the TFSI ions do not seem to observe any changes in their dynamics in glymes of different lengths. Our results indicate that the molecular solvation of the sodium ion is strongly influenced by the effectiveness of the glyme to solvate the cation. However, the delocalization of the charge in the counter ion seems to be key for explaining the tendency of ion pair formation in glyme-based sodium ion electrolytes.