A critical roadblock toward practical Mg-based energy storage technologies is the lack of reversible electrolytes that are safe and electrochemically stable. Ionic liquids (ILs) are possible candidates for the electrolytes because in addition to being endowed with high thermal and electrochemical stability, they exhibit negligible vapor pressure and are non-flammable. High-performance electrolytes based on 1-ethyl-3-methylimidazolium halide (EMImX, X = Cl or I) doped with AlX₃ and highly amorphous δ-MgX₂ salt have recently been developed for application in secondary magnesium batteries [1,2]. Using a magnesium anode in conjunction with the chloride-based electrolytes the coin cells are cyclically discharged at a high rate (35 mA/g) exhibiting an initial capacity of 80 mAh/g and a steady-state voltage of 2.3 V [1]. This development seems very promising, however the key to the improvement of these new materials is the structural identification as it directly determines the properties and behavior of the electrolyte.

This work presents the coupling between experimental and computational vibrational frequency investigations that characterizes a family of electrolytes with the general formula [EMImX/(AlX₃)_m]/(δ-MgX₂)_n. Experimental Raman and Fourier Transform Far Infrared (FT-FIR) spectra were measured for the electrolyte systems at various Mg salt concentrations. The spectra reveals new peaks indicative of the formation of new species and structural changes over the pristine IL. Hence, molecular-level investigations are necessary to assign the spectra and understand the ionic structure of these new materials. High-level density functional theory (DFT) calculations implementing the B3LYP functional were undertaken to determine the local structure and vibrational modes of the EMIm⁺ cation interacting with AlX₄⁻, Al₂X₇⁻, and MgX₂ (X = Cl or I).

The analysis of the calculated and experimental vibrational spectra suggests that the pristine IL, i.e., EMImX/(AlX₃)_m, contains both AlX₄⁻ and Al₂X₇⁻ anionic species. Addition of the buffering δ-MgX₂ salt to the pristine IL increases the ratio of the AlX₄⁻ to Al₂X₇⁻ anions in both chloride- and iodide-based electrolytes as shown in Figure 1. The experimental spectra reveal differences in the structure of the dissolved salt between the two halide-based electrolytes. The presence of vibrational peaks associated with the vibration of (MgCl₂)_n>1 species in the chloride-based electrolytes reveals that the dissolved salt can be found in the dimeric and polymeric forms. However, the corresponding vibrations are absent in the experimental spectra of the iodide-based material, indicating that the magnesium salt interacts with the IL in the form of a monomer rather than either a dimer or trimer (in contrast to the chloride-based).

References

[1]     F. Bertasi, C. Hettige, F. Sepehr, X. Bogle, G. Pagot, K. Vezzu, E. Negro, S.J. Paddison, S.G. Greenbaum, M. Vittadello, V. Di Noto, Chemsuschem 8 (2015) 3069-3076.

[2]     F. Sepehr, F. Bertasi, S.J. Paddison, V. Di Noto, ECS Meeting Abstracts MA2015-01 (2015) 2046.