METHODS

1. MMC/G4 Synthesis. Electrolyte synthesis and preparation was conducted in an argon-filled glovebox. (HNEt3)(CB11H12) was purchased from Katchem (Czech Republic) and used as received. Mg powder (99.5%, 325 mesh), anhydrous 1,2-dimethoxyethane (DME), and tetraglyme were purchased from Sigma-Aldrich. DME was distilled from Na metal and stored over activated 3 Å molecular sieves. Tetraglyme was distilled from Na metal and stirred in the presence of freshly prepared Mg shavings at 100 °C for 15 h, where it was stored until use. [Mg(DME)3](CB11H12)2 has been reported previously27,53 but was prepared per a modified procedure described below.

Synthesis of [Mg(DME)3](CB11H12)2. (HNEt3)[CB11H12] (5 g, 20.5 mmol) and Mg powder (1.32 g, 54.3 mmol) were weighed in a 125mL flat bottom flask, and anhydrous DME (60 mL) was added. The dark gray suspension was vigorously stirred until H2 evolution ceased and the suspension turned light gray, typically taking about 3 h at room temperature when stirring is vigorous enough. The suspension was cooled down externally until the original dark gray color was recovered. Unreacted Mg powder was filtered off using a fine frit, and the filtrate was immediately heated up to 60 °C for a minimum of 1 h to induce crystallization. The solid was collected by filtration, washed with warm (50 °C) anhydrous DME (10 mL + 2 mL), and dried under vacuum. This resulted in 5.66 g (95% yield) of white microcrystalline powder. NMR characterization was found to match the one reported previously.27 

General Procedure for the Preparation of 0.45 M MMC/G4 Electrolyte Solution. G4 (3.192 μL) was added to [Mg(DME)3]-(CB11H12)2(927 mg, 1.6 mmol), and the mixture was stirred until dissolution. The clear solution was further stirred under vacuum for 30 min to remove DME solvent.

2. Preparation of Standard Coin-Cells. Mg―Mg symmetrical cells were constructed using standard CR-2032 coin-cells.Each coin-cell utilized 15 mm diameter Mg foil pieces, which were punched from a 100 μm thick Mg foil stock supply purchased from ESPI Metals (3N purity). Each punched Mg foil piece was polished to remove surface oxides using common sandpaper prior to use. A glass-fiber separator was utilized to separate the two Mg foil pieces and soaked in 100 μL of electrolyte for each test. Coin-cells were sealed using a standard coin cell crimper (Xiamen Tmaxcn Inc.) and dismantled for analysis using a standard Hoshen manual coin-cell decrimper.