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 <i>N</i>-butyl-<i>N</i>-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 (<i>n</i>) is 3 to 4. In comparison, the value of <i>n</i> is 4 at very low concentrations and decreases with increasing salt mole fraction to 2 for Li⁺ and Na⁺, where <i>n</i> 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(TFSI)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