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

Magnesium is a metal that has been considered to hold significant promise as an alternative to lithium-based battery systems. It has a higher specific volumetric capacity than lithium, which could allow it to be used in high energy-density batteries, and it is highly abundant in the earth’s crust.¹ Current electrolytes for rechargeable magnesium cells are based on Mg organohaloaluminate complex solutions.¹ Ionic liquids (ILs) have been investigated as alternative electrolyte components for magnesium-based batteries, but always in the presence of a Grignard or Lewis acid-base type salt.^2-4 Although electrolytes containing non-nucleophilic salts with ILs for the reversible deposition and dissolution of magnesium have been previously studied, the results could not be reproduced.^2,3 However, there are significant advantages in terms of safety if the nucleophilic salt could be replaced by one like magnesium bis(trifluoro-methanesulfonyl)imide. From the fundamental point of view, there seem to be few, if any, studies on the ionic interactions of ILs with magnesium cations.

ILs have received significant attention as an alternative to conventional electrolytes particularly in lithium battery applications due to their many favorable properties, but most notably their low volatility and flammability.⁵ One of the drawbacks of IL-based electrolytes is their low conductivity and high viscosity as compared to conventional organic liquid-based electrolytes. It has been reported that the conductivities of salt-in-IL electrolytes are lower than those of the ionic liquids themselves, which is a result of the increased viscosity of the system due to interactions between the metal cations and the IL anions.^6,7

The Raman shifts of the TFSI expansion-contraction mode in N-butyl-N-methylpyrrolidinium bis(trifluoro-methanesulfonyl)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(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/de-magnesiation in V₂O₅ aerogel electrodes.

 

References

 

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