Determining Factor on the Polarization Behavior of Magnesium Deposition for Magnesium Battery Anode.

To clarify the origin of the polarization of magnesium deposition/dissolution reactions, we combined electrochemical measurement, soft X-ray absorption spectroscopy ( SXAS), Raman, and density functional theory (DFT) techniques to three different electrolytes: magnesium bis(trifluoromethanesulfonyl)amide (Mg(TFSA))/triglyme, magnesium borohydride (Mg(BH))/tetrahydrofuran (THF), and Mg(TFSA)/2-methyltetrahydrofuran (2-MeTHF). Cyclic voltammetry revealed that magnesium deposition/dissolution reactions occur in Mg(TFSA)/triglyme and Mg(BH)/THF, while the reactions do not occur in Mg(TFSA)/2-MeTHF. Raman spectroscopy shows that the [TFSA] in the Mg(TFSA)/triglyme electrolyte largely does not coordinate to the magnesium ions, while all of the [TFSA] in Mg(TFSA)/2-MeTHF and [BH] in Mg(BH)/THF coordinate to the magnesium ions. In SXAS measurements, the intermediate, such as the Mg ion, was not observed at potentials above the magnesium deposition potential, and the local structure distortion around the magnesium ions increases in all of the electrolytes at the magnesium electrode|electrolyte interface during the cathodic polarization. Our DFT calculation and X-ray photoelectron spectroscopy results indicate that the [TFSA], strongly bound to the magnesium ion in the Mg(TFSA)/2-MeTHF electrolyte, undergoes reduction decomposition easily, instead of deposition of magnesium metal, which makes the electrolyte inactive electrochemically. In the Mg(BH)/THF electrolyte, because the [BH] coordinated to the magnesium ions is stable even under the potential of the magnesium deposition, the magnesium deposition is not inhibited by the decomposition of [BH]. Conversely, because [TFSA] is weakly bound to the magnesium ion in Mg(TFSA)/triglyme, the reduction decomposition occurs relatively slowly, which allows the magnesium deposition in the electrolyte.