Designer Ionic Liquids for Reversible Electrochemical Deposition/Dissolution of Magnesium

Abstract

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/cm(2) for the stripping peak). Raman spectroscopy was used to characterize Mg(2+) 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 Mg(2+). 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.