Complex Nature of Ionic Coordination in Magnesium Ionic Liquid-Based Electrolytes: Solvates with Mobile Mg2+ Cations

Abstract

The Raman shifts of the TFSI− expansion-contraction mode in N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid (IL) electrolytes were analyzed to compare the ionic coordination of magnesium with lithium and sodium. In the Mg2+-IL electrolytes, the TFSI– anions are found in three different potential energy environments, while only two populations of TFSI– are evident in the Na+- and Li+-IL electrolytes. For Mg2+, the high frequency peak component is associated with a TFSI– that is in a bidentate coordination with a single metal cation and can therefore be considered a contact ion pair (CIP) solvate. The mid frequency component is attributed primarily to bridging aggregate (AGG) TFSI– solvate or a weakly bound monodentate CIP TFSI–. The low frequency peak is well-known to be associated with “free” TFSI– anions. The average number of TFSI– per Mg2+ cation (n) is 3 to 4. In comparison, the value of n is 4 at very low concentrations and decreases with increasing salt mole fraction to...