Designer Ionic Liquids for Reversible Electrochemical Deposition/Dissolution of Magnesium

Chelating ionic liquids (ILs), in which polyether chains are pendent from the organic pyrrolidinium cation of the ILs (PEGylated ILs), were prepared that facilitate reversible electrochemical deposition/dissolution of Mg from a Mg(BH₄)₂ source. Mg electrodeposition processes in two specific PEGylated-ILs were compared against that in the widely studied N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid (BMPyrTFSI). The two chelating IL systems (one with a pendent polyether chain with three ether oxygens, MPEG₃PyrTFSI, and the other with a seven-ether chain, MPEG₇PyrTFSI) showed substantial improvement over BMPyrTFSI for Mg electrodeposition/dissolution. The best overall electrochemical performance was in MPEG₇PyrTFSI. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were used to characterize galvanostatically deposited Mg, revealing production of pure, dendrite-free Mg deposits. Reversible Mg electrodeposition was achieved with high Coulombic efficiency (CE) of 90% and high current density (ca. 2 mA/cm² for the stripping peak). Raman spectroscopy was used to characterize Mg²⁺ speciation in the PEGylated ILs and BMPyrTFSI containing Mg(BH₄)₂ by study of Raman modes of the coordinated and free states of borohydride, TFSI^-, and polyether COC groups. Quantitative analysis revealed that the polyether chains can displace both TFSI^- and BH₄^- from the coordination sphere of Mg²⁺. Comparison of the different IL electrolytes suggested that these displacement reactions may play a role in enabling Mg deposition/dissolution with high CE and current density in these PEGylated IL media. These results represent the first demonstration of reversible electrochemical deposition/dissolution of Mg in an ionic liquid specifically designed with this task in mind.