Role of Coordination Structure of Magnesium Ions on Charge and Discharge Behavior of Magnesium Alloy Electrode

Abstract

Mechanism of magnesium ion alloying reaction into bismuth electrode in magnesium bis(trifluoromethanesulfonyl)amide (Mg(TFSA)2)/acetonitrile (AN) and Mg(TFSA)2/2-methyltetrahydrofuran (2-MeTHF) electrolyte was examined by a combination of operando soft X-ray absorption spectroscopy (XAS), Raman spectroscopy, and density functional theory (DFT) calculations. In 0.5 M Mg(TFSA)2/AN, the magnesium ions alloying reaction occurred, whereas the alloying reaction did not occur in 0.5 M Mg(TFSA)2/2-MeTHF. Raman spectroscopy showed that less than 15% of [TFSA]− coordinates with magnesium ions in 0.5 M Mg(TFSA)2/AN, while more than 90% of [TFSA]− coordinates with magnesium ions in Mg(TFSA)2/2-MeTHF. Using operando XAS measurements, we observed that electronic and local structure of magnesium ion changed similarly upon cathodic polarization in both electrolytes. These results indicate that the difference of the behavior of alloy formation should be affected by the difference of coordinate structure of [TFSA]− in both electrolytes. Our DFT calculation results indicates [TFSA]− coordinated to magnesium ions undergoes reduction decomposition more easily than [TFSA]− uncoordinated to magnesium ions. In 0.5 M Mg(TFSA)2/2-MeTHF, the [TFSA]− coordinating to magnesium ions undergoes reduction decomposition, which inhibits the alloying reaction into the bismuth electrode. On the other hand, in 0.5 M Mg(TFSA)2/AN, the [TFSA]− reduction decomposition occurs relatively slowly because of the weak coordination between [TFSA]− and magnesium ions, which allows the magnesium ions alloying into the bismuth electrode in the electrolyte.