Interfacial Behavior of Magnesium Ions at Electrode/Electrolyte Interface during Magnesium Deposition Reaction

Abstract

Introduction Magnesium has large theoretical volumetric capacity and high negative, also much cheaper than metal lithium on price and simple process for handle. Due to the higher safety compared with lithium batteries. rechargeable magnesium batteries with magnesium as the anode has become a hot spot of the study. Electrolyte for magnesium rechargeable batteries has been developed by Aurbach and co-worker [1,2], in which Grinard reagents are contained. The anodic stability and chemical stability of the Grignard reagents-based electrolyte is not satisfied with practical application. [3]. For the development of rechargeable magnesium batteries, it is necessary to elucidate the reaction mechanism at electrolyte/magnesium metal interface. The key factor that predominates the reaction mechanism is considered to be the coordination structures of magnesium ions in the bulk electrolyte solution and at the negative electrode surface. In this study, we examined the mechanism of magnesium deposition/dissolution reactions in the magnesium bis(trifluoromethanesulfonyl)amide Mg(TFSA)2/triglyme electrolyte, and compared them with those in Mg(AlCl2EtBu)2/tetrahydrofuran (THF) and Mg(TFSA)2/2-methyltetrahydrofuran (2-MeTHF)). The bulk structures of the electrolytes were examined by Raman spectroscopy and the theoretical calculation. And then, we applied operando soft X-ray absorption spectroscopy(XAS) to examine the electronic and local structure of magnesium ions at negative electrode/electrolyte interface during magnesium deposition. Experiments Electrochemical properties of the electrolyte were investigated by cyclic voltammetry with the three-electrode cell. The deposition after electrochemical measurement was characterized by XRD and SEM. The operando XAS spectra for Mg K-edge of the electrolytes were measured by the partial fluorescence yield method at the SPring-8 synchrotron radiation facility (BL27SU) in Hyogo, Japan [4]. Results and discussion In cyclic voltammetry, the currents attributed to magnesium deposition/dissolution reactions occurred in Mg(TFSA)2/triglyme and Mg(AlCl2EtBu)2/THF were observed. The currents were not observed in Mg(TFSA)2/2-MeTHF. 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. We conclude that controlling the interaction between Mg2+ and [TFSA]- and suppressing reduction decomposition of [TFSA]- is crucial to achieve successful magnesium deposition.