A facile approach using MgCl2 to formulate high performance Mg2+ electrolytes for rechargeable Mg batteries

Rechargeable Mg batteries have been regarded as a viable battery technology for grid scale energy storage and transportation applications. However, the limited performance of Mg2+ electrolytes has been a primary technical hurdle to develop high energy density rechargeable Mg batteries. In this study, MgCl2 is demonstrated as a non-nucleophilic and cheap Mg2+ source in combination with Al Lewis acids (AlCl3, AlPh3 and AlEtCl2) to formulate a series of Mg2+ electrolytes, representing the simplest method to prepare Mg2+ conductive electrolytes (no precursor synthesis, free of recrystallization and giving quantitative yield). These electrolytes are characterized by high oxidation stability (up to 3.4 V vs. Mg), improved electrophile compatibility and electrochemical reversibility (up to 100% coulombic efficiency). Three electrolyte systems (MgCl2-AlCl3, MgCl2-AlPh3, and MgCl2-AlEtCl2) were fully characterized by multinuclear NMR (H-1, Al-27{H-1} and Mg-25{H-1}) spectroscopies and electrochemical analysis. Single crystal X-ray diffraction and NMR studies consistently established molecular structures of the three electrolytes sharing a common Mg2+-dimer mono-cation, [(mu-Cl)(3)Mg-2(THF)(6)](+), along with an anion (AlCl4-, AlPh3Cl- and AlEtCl3- respectively). Clean and dendrite free Mg bulk plating and viable battery performance were validated through representative studies using the MgCl2-AlEtCl2 electrolyte. The reaction mechanism of MgCl2 and the Al Lewis acids in THF is discussed to highlight the formation of the electrochemically active [(mu-Cl)(3)Mg-2(THF)(6)](+) dimer mono-cation in these electrolytes and their improved performance compared to reported electrolytes using nucleophilic Mg2+ sources.