Despite the highly touted goals of moving beyond Li-ion batteries for electrochemical energy storage, lithium-based technology remains king of the hill. Moving to magnesium, which is similar in size, but carries out two-electron chemistry, is hampered for several reasons—largely due to the paucity of suitable electrolytes. First, the most widely published reports on magnesium electrolytes are composed of air- and moisture sensitive Grignard reagents, and they are limited to flammable and volatile ethereal solvents such as tetrahydrofuran (THF). In addition, although the anodic stability of an electrolyte in a particular solvent is dictated by the highest occupied molecular orbital (HOMO) of the most oxidizable species, many combinations that are electrochemically inert on platinum as the working electrode fail to show the same stability on common non-noble metal current collectors such as stainless steel. This decrease in the usable potential window is attributed to corrosion reactions occurring on the electrode, and is thought to arise due to the high chloride content of these common electrolytes. We have unfurled structure-activity relationships in a series of electrolytes composed of 0.5 M RPhOMgCl and 0.25 M AlX3 (where RPhO- representes a substituted phenolate ligand and X is a halide). We can tune the anodic stability over 600 mV by adding electron withdrawing groups to the para position. Moreover, increasing the steric bulk in the ortho positions gives rise to high solution conductivity. We find striking correlations between the ionization energies of [(RPhO)AlX3]–, one of the transmetallation products, and the experimentally observed anodic stability in solution. Finally, moving from AlCl3 to AlBr3 results in a drastic reduction in corrosion when magnesium deposition/stripping is carried out on a stainless steel current collector.

Structure-function relationships in electrolytes for reversible magnesium batteries

251st National Meeting (2016)

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