Interplay Between Vibrational Modes and Relaxations in Electrolytes for Secondary Magnesium Batteries Based on Haloaluminate Ionic Liquids

The search for electrolytes with enhanced performance is a crucial point for the development of efficient secondary Mg batteries.¹ Early work on Mg deposition and stripping from Grignard-based non-aqueous solution were proposed in 1990 by Gregory et al.,² while the first example of polymer electrolytes comprising PEG400 and δ-MgCl₂ was proposed in 1998.³ In the following years, electrolytes based on EthylMgBr and PEO,⁴ Grignard,^5,6 and other organo-Mg⁷compounds were explored.

Among all proposed materials, Grignard and organo-Mg compounds, suffer from several drawbacks associated with their chemical stability in ethereal-based solvents, which are characterized by a high vapor pressure and flammability. On the contrary, Ionic Liquids (ILs) seem a very appealing class of materials for applications in Mg secondary batteries due to their: a) very low volatility, high thermal stability and non-flammability; b) high ion density and high conductivity; c) wide electrochemical stability window; and d) easiness of synthesis by evaluating the best cation and the anion for each application.⁸

There are still several aspects that are not fully understood regarding the use of ILs in electrochemical devices. Indeed, the conductivity mechanisms, the formation of the solid-electrolyte interface (SEI) and long-term performance of these systems are still open questions. Nevertheless, from a fundamental point of view, understanding the interplay between the ILs relaxations and the charge carrier migration is crucial in order to clarify the effect of the ILs matrix on the conductivity mechanism of Mg²⁺ions.

Here we present a family of electrolytes based on 1-ethyl-3-methylimidazolium iodide, aluminium iodide, and δ-MgI₂⁸ with general formula [EMImI/(AlI₃)_m]/(δ-MgI₂)_n. The short-range structural features and the interactions in the electrolytes are elucidated by coupling Raman and Infrared (both in the medium and in the far infrared) spectroscopy with DFT calculations.

The detailed electrical response of the [EMImI/(AlI₃)_m]/(δ-MgI₂)_n materials in terms of polarization and relaxation events at temperatures higher and lower than the melting of EMImI/(AlI₃)_m are investigated by using Broadband Electrical Spectroscopy (BES). The results allow us to correlate the dielectric relaxation of the imidazolium cations with the overall long-range charge migrations, thus elucidating the interplay existing between conductivity and nanostructure of this new class of IL.

References

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