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, operando soft X-ray absorption spectroscopy (operando SXAS), Raman, and density functional theory (DFT) techniques to three different electrolytes: magnesium bis(trifluoromethanesulfonyl)amide (Mg(TFSA)₂)/triglyme, magnesium borohydride (Mg(BH₄)₂)/tetrahydrofuran (THF), and Mg(TFSA)₂/2-methyltetrahydrofuran (2-MeTHF). Cyclic voltammetry revealed that magnesium deposition/dissolution reactions occur in Mg(TFSA)₂/triglyme and Mg(BH₄)₂/THF, while the reactions do not occur in Mg(TFSA)₂/2-MeTHF. Raman spectroscopy shows that the [TFSA]^- in the Mg(TFSA)₂/triglyme electrolyte largely does not coordinate to the magnesium ions, while all of the [TFSA]^- in Mg(TFSA)₂/2-MeTHF and [BH₄]^- in Mg(BH₄)₂/THF coordinate to the magnesium ions. In operando 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)₂/2-MeTHF electrolyte, undergoes reduction decomposition easily, instead of deposition of magnesium metal, which makes the electrolyte inactive electrochemically. In the Mg(BH₄)₂/THF electrolyte, because the [BH₄]^- 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 [BH₄]^-. Conversely, because [TFSA]^- is weakly bound to the magnesium ion in Mg(TFSA)₂/triglyme, the reduction decomposition occurs relatively slowly, which allows the magnesium deposition in the electrolyte.