Electrolytes for Mg-based energy storage devices can be formed from non-nucleophilic Mg.sup.2+ sources to provide outstanding electrochemical performance and improved electrophilic susceptibility compared to electrolytes employing nucleophilic sources. The instant 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). Synthesis of the Mg.sup.2+ electrolytes utilizes inexpensive and safe magnesium dihalides as non-nucleophilic Mg.sup.2+ sources in combination with Lewis acids, MR.sub.aX.sub.3-a (for 3.gtoreq.a.gtoreq.1). Furthermore, addition of free-halide-anion donors can improve the coulombic efficiency of Mg electrolytes from nucleophilic or non-nucleophilic Mg.sup.2+ sources.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
 This invention was made with Government support under Contract DE-AC0576RLO1830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.