Boron-based electrolyte solutions with wide electrochemical windows for rechargeable magnesium batteries

Abstract

Recently, we have developed a boron based electrolyte system with outstanding electrochemical performance, formed through the reaction of tri(3,5-dimethylphenyl)borane (Mes₃B) and PhMgCl in THF, for rechargeable magnesium batteries. In this paper, the main components and equilibria of the unique boron based electrolyte solutions are identified by NMR, single-crystal XRD, etc. The results prove that the solutions contain various magnesium species, such as Mg₂Cl₃⁺, MgCl⁺, Ph₂Mg and the tetracoordinated boron anion [Mes₃BPh]⁻. Fluorescence spectra and Raman spectroscopy analyses indicate that the high anodic stability (ca. 3.5 V vs. Mg reference electrode (RE)) of the Mes₃B–(PhMgCl)₂ electrolyte is attributed to the non-covalent interactions between the anion [Mes₃BPh]⁻ and Ph₂Mg. Furthermore, the air sensitivity of the boron based electrolyte and its electrochemical stability on the different current collectors are studied. Finally, the reversible electrochemical process of Mg intercalation into a Mo₆S₈ cathode confirms that the boron based electrolyte could be practically used in rechargeable Mg battery systems.