To understand the mechanism of magnesium deposition/dissolution reactions in the magnesium bis(trifluoromethanesulfonyl)amide (Mg(TFSA)2)/triglyme electrolyte, we examined the electronic and local structures of magnesium ions present in Mg(TFSA)2/triglyme during the magnesium deposition reaction using operando soft X-ray absorption spectroscopy (operando XAS, and compared them with those in Mg(AlCl2EtBu)2/tetrahydrofuran (THF) and Mg(TFSA)2/2-methyltetrahydrofuran (2-MeTHF). Cyclic voltammetry revealed that magnesium deposition/dissolution reactions occurred in Mg(TFSA)2/triglyme and Mg(AlCl2EtBu)2/THF, while the reactions did not occur in Mg(TFSA)2/2-MeTHF. Raman spectroscopy showed that a little [TFSA]- in Mg(TFSA)2/triglyme coordinates to the magnesium ions, while all of [TFSA]- in Mg(TFSA)2/2-MeTHF participates in the coordination of the magnesium ions. operando XAS measurements supported that valence of magnesium ions did not change and local structure distortion around magnesium ions increased in Mg(TFSA)2/triglyme and Mg(TFSA)2/2-MeTHF at the magnesium electrode | electrolyte interface during cathodic polarization. These results indicate that the difference of the behavior of magnesium deposition should be affected by the difference of coordinate structure of [TFSA]- in both electrolytes. Our DFT calculation results indicate that the [TFSA]- strongly bound to magnesium ion in Mg(TFSA)2/2-MeTHF electrolyte undergoes reduction decomposition easily instead deposition of magnesium metal, which makes the electrolyte inactive electrochemically. On the other hand, because the [TFSA]- weakly bound to magnesium ion in Mg(TFSA)2/triglyme, the reduction decomposition occurs relatively slowly, which allows magnesium deposition in the electrolyte.

Effect of anion coordination with magnesium ions for magnesium deposition and polarization behavior for magnesium battery anode

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.