Interplay Between Structure and Conductivity in Imidazolium-Based Ionic Liquids As Electrolytes for Magnesium Batteries

The search for new electrolytes is one of the most challenging aspects in the development of secondary batteries based on Li and Mg ions [1].

The state of the art for Mg electrolytes is mainly constituted by organo-Mg compounds; in general, they show good Mg deposition and dissolution performance, high Coulombic efficiency and in some cases a wide electrochemical stability window [2-3]. The main drawback of this class of materials is associated with their dissolution in ethereal-based solvents characterized by a high vapor pressure and flammability. Nevertheless, the use of high-boiling solvents is known to lead to: (a) a decrease in the electrolyte conductivity; and (b) a decrease in the electrode reaction rate, which limits their practical applicability [2].

Ionic liquids (ILs) are salts that are molten at room temperature [4]. ILs are also characterized by: (a) negligible vapor pressure; (b) high thermal stability; and (c) low flammability.

Only a few examples of the applicability of ILs as electrolytes for secondary Mg batteries are reported in the literature [5]. Several issues regarding the conductivity mechanisms, formation of the solid-electrolyte interface (SEI) and long-term performance of these systems are still open questions.

From a fundamental point of view, the interplay between the ILs nanostructure and the conductivity is a crucial point to clarify the effect of the IL matrix on the migration mechanism of Mg²⁺ ions.

In this report, a comparison between two Mg electrolytes based on Ethyl-methylimidazolium tetrafluoroborate (EMImBF₄) and Ethyl-methylimidazolium chloride/AlCl₃ (EMImCl/(AlCl₃)_1.5) ILs is proposed. Both ionic liquids are doped with different amounts of δ-MgCl₂, to achieve a sufficient conductivity of Mg²⁺ ions.

The correlation between structure, thermal properties and conduction mechanism of the resulting EMImBF₄/(δ-MgCl₂)_x and [EMImCl/(AlCl₃)_1.5]/(δ-MgCl₂)_x is investigated by a variety of techniques: (a) FT-MIR and FIR at different temperatures; (b) differential scanning calorimetry (DSC); and (c) broadband electrical spectroscopy (BES).

Figure 1 shows a plot of σ_DC of EMImBF₄ vs. 1/T. The observed trend of σ_DC is strongly correlated to the thermal features detected by DSC. A decrease in σ_DC of six orders of magnitude is evidenced at temperatures higher than the crystallization temperature (T_c). This phenomenon is investigated as a function of: (a) Mg concentration; and (b) the different anions in the ILs.

BES measurements were undertaken to elucidate the electrical response of the electrolytes in terms of dielectric and polarization phenomena. At T < T_m (T_m = melting temperature) three dielectric relaxations are present, associated with the rotational motions of EMIm⁺ cations; at T > T_m, three and four interdomain polarization events are detected respectively for EMImBF₄ and [EMImCl/(AlCl₃)_1.5]. These polarization events are associated with the presence of cation and anion nanocluster aggregates with different permittivities.

References

[1] M. Armand, F. Endres, D. R. MacFarlane, H. Ohno, and B. Scrosati, Nature Materials, 8, 621 (2009).

[2] N. Amir, Y. Vestfrid, O. Chusid, Y. Gofer, and D. Aurbach, Journal of Power Sources, 174, 1234 (2007).

[3] C. Liebenow, Z. Yang, and P. Lobitz, Electrochemistry Communications, 2, 641 (2000).

[4] H. Ohno, Electrochemical Aspects of Ionic Liquids, Wiley, Hoboken (2005).

[5] Y. NuLi, J. Yang, P. Wang, Applied Surface Science, 252, 8086 (2006).