Highly active electrolytes for rechargeable Mg batteries based on a [Mg2(μ-Cl)2]2+ cation complex in dimethoxyethane

Abstract

A novel [Mg₂(μ-Cl)₂]²⁺ cation complex, which is highly active for reversible Mg electrodeposition, was identified for the first time in this work. This complex was found to be present in electrolytes formulated in dimethoxyethane (DME) through dehalodimerization of non-nucleophilic MgCl₂ by reacting with either Mg salts (such as Mg(TFSI)₂, TFSI = bis(trifluoromethane)sulfonylimide) or Lewis acid salts (such as AlEtCl₂ or AlCl₃). The molecular structure of the cation complex was characterized by single crystal X-ray diffraction, Raman spectroscopy and NMR. The electrolyte synthesis process was studied and rational approaches for formulating highly active electrolytes were proposed. Through control of the anions, electrolytes with an efficiency close to 100%, a wide electrochemical window (up to 3.5 V) and a high ionic conductivity (>6 mS cm⁻¹) were obtained. The understanding of electrolyte synthesis in DME developed in this work could bring significant opportunities for the rational formulation of electrolytes of the general formula [Mg₂(μ-Cl)₂][anion]_x for practical Mg batteries.