Surface Analysis of Magnesium Metal Anode
Authors: Matsui Masaki , Kuwata Hiroko , Masuda Takuya , Imanishi Nobuyuki 
Published: February 2020

Introduction

Rechargeable magnesium battery is expected as a candidate of post lithium-ion battery1. One of the biggest challenges in the rechargeable magnesium battery is reversible deposition/dissolution process of the magnesium metal anode, because the magnesium deposition process does not occur in the most of the conventional ionic electrolyte solution, while organohaloalminate-based electrolytes show highly reversible magnesium deposition/dissolution2. Recently several research group reported that Mg(TFSI)2 in ether-based solvents show reversible magnesium deposition/dissolution3, 4. However since the electrochemical behaviors of the Mg(TFSI)2-based ionic electrolyte solutions are very different from the organohaloalminate-based electrolytes; the ionic electrolytes show high overpotential and low coulombic efficiency, we think the electrodeposition process of the magnesium metal is not that simple as lithium, sodium or any other metal. Here we conducted in situFTIR spectroscopy and Lab-HAXPES to confirm the formation process of so-called SEI layer and passivation layer on magnesium metal in various electrolyte solutions.

Experimental

A three-electrode in situ FTIR cell with internal reflection geometry was build upon a diamond ATR disc of DuraSamplIR (Smiths Detection). Platinum thin film electrode was fabricated on the diamond-window as the working electrode. A magnesium quasi-reference electrode and a magnesium counter electrode were employed for the electrochemical measurement. The FTIR spectra were taken by single beam mode and subtractively normalized interfacial FTIR spectra (SNIFTIRS) were calculated. The SNIFTIRS have positive peaks and negative peaks corresponding to the decreased species and the increased species during the electrochemical measurement respectively. The Lab-HAXPES measurement was performed using Cr-Ka radiation.

Results

Figure 1 shows a comparison of in situ FTIR spectra for an organohaloalminate-based electrolyte (0.25M EtMgCl : EtAlCl2 = 1 : 2 by mol in THF) and an ionic electrolyte solution(0.5 M Mg(TFSI)2 in BuMe-triglyme) during a two-cycles of CV measurement. The cathodic scan was terminated at the electrode potential slightly above the magnesium deposition. In the case of the organohaloalminate-based electrolyte, reversible spectra changes corresponding to the adsorption state changes of THF were observed for two cycles, as shown in Figure 1 (a). It suggests that no irreversible electrochemical reaction takes place at the surface of the electrode. In other words, no SEI layer was formed in the organohaloalminate-based electrolyte. On the other hand, irreversible spectra changes were observed in the case of Mg(TFSI)2 in BuMe-triglyme. The irreversible reaction should be corresponding to the cathodic decomposition of the TFSI anion. The XPS analysis proved that a surface layer containing some fluorinated organic species were formed at the surface of the Mg negative electrode in the case of Mg(TFSI)2-based electrolyte. The high overpotential and poor reversibility of the Mg(TFSI)2 system could be due to the formation of the surface layer.

References

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Figure 1