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

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 2 for Li+ and Na+, where n of Na+ is larger than that of Li+ at any given concentration. The results imply the existence of anionic magnesium solvates of varying sizes. The identity of the Mg2+ charge-carrying species is complex due to the presence of bridging AGG solvates in solution. It is likely that there is a combination of single Mg2+ solvate species and larger complexes containing two or more cations. In comparison, the primary Li+ and Na+ charge-carrying species are likely [Li(TFSI)(2)](-) and [Na(TESI)(3)](2-) in the concentration range successfully implemented in IL-based electrolyte batteries. These solvates result in Mg2+ cations that are mobile in the IL-based electrolytes as demonstrated by the reversible magnesiation/demagnesiation in V2O5 aerogel electrodes.