Remarkable electrochemical and ion-transport characteristics of magnesium-fluorinated alkoxyaluminate–diglyme electrolytes for magnesium batteries

Abstract

Magnesium batteries (MBs) are fascinating options for large-scale energy-storage devices because the properties of magnesium metal anodes are more advantageous than those of their lithium or sodium counterparts. Moreover, the recent rapid progress in electrolyte materials will surely benefit MB research and development. Magnesium salts incorporating fluorinated alkoxyborate or alkoxyaluminate anions are an emerging class of potential MB electrolyte materials owing to their outstanding characteristics, such as favorable anodic stability and excellent compatibility with magnesium metal. Despite the growing number of reports on such electrolytes, the optimal electrolyte compositions remain unclear and, hence, must be investigated for practical application to MBs. Therefore, we comprehensively compared the bulk physicochemical properties, electrochemical characteristics, and ion-transport behaviors of ethereal solutions of magnesium-fluorinated alkoxyborate and alkoxyaluminate. By systematically characterizing the composition–property relationships, we found that the magnesium-fluorinated alkoxyaluminate Mg[Al(HFIP)₄]₂ (HFIP = hexafluoro-iso-propoxyl group) and diglyme (G2) salt–solvent combination exhibited outstanding electrochemical activity. The optimal electrolyte allowed highly stable and efficient magnesium deposition/dissolution cycling for over 250 cycles with a coulombic efficiency of 99.4% under an exceptionally low polarization <±60 mV despite the absence of strong Lewis acidic agents. As evidenced by the Vogel–Tammann–Fulcher fitting analysis, impedance spectroscopy results, and ab initio molecular dynamics calculations, the well-balanced solvation/desolvation of G2 toward Mg²⁺ ions and the remarkable ion-transport and interfacial characteristics and sufficient reduction stability of the [Al(HFIP)₄]⁻ were the reasons for the outstanding electrochemical performance.