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 Mg 2+ -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 Mg 2+, 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 Mg 2+ 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 Mg 2+ 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 Mg 2+ 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(TFSI) 3 ] 2– in the concentration range successfully implemented in IL-based electrolyte batteries. These solvates result in Mg 2+ cations that are mobile in the IL-based electrolytes as demonstrated by the reversible magnesiation/demagnesiation in V 2 O 5 aerogel electrodes.