Magnesium metal is an ideal anode active material because of its high theoretical capacity > 3500 mAh·cm^-3and dendrite-free morphology.  However the surface of the magnesium metal anode is easily passivated in most of the ionic electrolyte solution, as a consequence, very limited choices of the electrolyte solutions are available for the reversible magnesium deposition/dissolution. 

  Magnesium organohaloalminates, typically synthesized by mixing Grignard reagents and alminate-based Lewis acids, are well known as magnesium electrolytes having excellent reversibility and forming highly crystalline magnesium.  Despite of its good electrochemical properties, the Mg organohaloaluminate has a lot of difficulties to be an electrolyte solution of a practical battery system. 

  Recently ionic electrolyte solutions containing ionic magnesium salt: Mg(TFSI)₂ were proposed as the alternative to the Mg organohaloalminates [2, 3].  Compered with the Mg organohaloalminate-based system, the Mg(TFSI)₂based electrolyte solutions show poor reversibility and high overpotential.

  In the present study, we conducted surface analyses of the electrodeposited magnesium electrodes in various electrolyte solutions: Mg organohaloalminates, Mg(TFSI)₂in triglyme and other solvents, to understand the key property of the magnesium deposition/dissolution process. 

 2.     Experimental

  In this study, we used two electrolyte solutions.  An Mg organohaloalminate solution was prepared by mixing with EtMgCl in THF and Et₂AlCl resulting to form the electrolyte containing 0.25 mol dm^-3 of magnesium.  A 0.5 M of Mg(TFSI)₂in triethylene glycol dimethyl ether (G3) was also prepared as the comparison.

  The electrodeposition test was carried out using a three-electrode cell.  The working electrode was platinum sheet, and the magnesium metal was employed as the quasi-reference electrode and the counter electrode.  The electrodes deposited Mg were rinsed with THF or DME, and observed the surface morphology by SEM.

  The surface analysis was performed for Mg metal electrodes immersed in the electrolyte solutions for 24 hours.  The Mg metal electrodes were rinsed with solvents and the surface analyses were performed using XPS. 

 3.     Results and Discussion

  Fig. 1 shows the SEM images of electrodeposited Mg on the Pt electrodes.  At a glance the surface morphologies of the electrodeposited Mg were significantly dependent upon the electrolyte solution.  In the case of the Mg organohaloalminate solution, the surface morphology is very smooth, and the formation of hexagonal-particles is clearly observed in the high magnification SEM images, whereas the magnesium metal deposited in the Mg(TFSI)₂solution showed spherical particles .  Moreover, the XRD pattern detected low crystalline of electrodeposited Mg from the TFSI system.

  Fig. 2 shows the XPS C 1s spectra of immersed Mg in the electrolyte solutions.  The XPS spectra of the Mg metal immersed in the Mg organohaloalminate solution had a peak assigned the C-C bonds from the surface to bulk.  However, the case of the Mg metal immersed in the Mg(TFSI)₂system, a peak corresponding to the C-O-C bonds and a peak assigned C-F bonds were detected.  It clearly suggests that the TFSI anion is easily reduced at the surface of the Mg metal resulting in the formation of the surface film.  Hence we assume that the surface film hinders the smooth deposition and dissolution of the Mg. Also we think that the reduction stability of the anion is crucial for Mg deposition/dissolution process.   

 4.     Reference

[1] D. Aurbach. Z. Lu, A. Schechter, Y. Gofer, R. Turgeman, Y. Cohen, M. Moshkovich, and E. Levi, Nature, 407, 724-727 (2000)

[2] T. Fukutsuka, K. Asaka, A. Inoo, R. Yasui, K. Miyazaki, T. Abe, K. Nishio, and Y. Uchimoto, Chem. Lett., 43, 1788-1790 (2014)

[3] H. Senoh, H. Sakaebe, H. Sano, M. Yao, K. Kuratani, N. Takeuchi, and T. Kiyobayashi, J. Electrochem. Soc., 161, A1315 (2014)