(Invited) Structure-Function Relationships in Electrolytes for Reversible Magnesium Batteries

Wednesday, 4 October 2017: 08:20
Chesapeake K (Gaylord National Resort and Convention Center)
B. M. Bartlett (University of Michigan)
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 in part due to the paucity of suitable electrolytes. The most widely published reports on magnesium electrolytes are composed of air- and moisture sensitive Grignard reagents, which decompose to form electron-insulating layers on either platinum or glassy carbon electrodes. Recent work in our group has unfurled structure-activity relationships in a series of salts formed from ROMgCl (R = phenolate or t-butoxide) and AlCl3 in THF solvent. A transmetalation reaction gives rise to the cationic species [Mg2(μ-Cl)3(thf)3]+ and a series of chloro- and alkoxy-aluminate anions in equilibrium at room temperature. We find that adding fluorinated substituents maximizes the anodic stability, and adding steric bulk maximizes solution conductivity. Accordingly, our most promising candidate is the combination of 1.2 M (CF3)2(CH)3COMgCl with 0.2 M AlCl3, which shows anodic stability on to 3.2 V vs. Mg2+/0 on a platinum working electrode with a solution conductivity of 3.5 mS/cm, both of which are competitive with electrolytes used in lithium-ion chemistry. Not surprisingly, this electrolyte shows lowest Tafel slope for dendrite-free magnesium metal deposition, ~ 80 mV/dec. Moreover, it supports reversible cycling of Chevrel phase Mo3S4 vs. Mg metal in 2016-type coin cells.