Electrochemical Charge and Discharge of Magnesium Anode in Triglyme-Based Solution

Magnesium rechargeable batteries have paid attention for the next generation battery, because magnesium metal has a higher volumetric specific capacity and higher melting point than these of lithium metal.  Electrolyte for magnesium rechargeable batteries has been developed by Aurbach and co-workers [1, 2], in which Grignard reagents (RMgX, where R is an alkyl or aryl group, and X is Cl or Br) are contained.  The anodic stability (about 1.7 V vs. Mg²⁺ / Mg) and chemical stability of the Grignard reagents-based electrolyte is not satisfied with practical application [3].  Recently, Muldoon and co-workers have developed magnesium electrolytes contain [Mg₂(μ-Cl)₃-6THF] [4].  The anodic stability in this electrolyte is drastically improved.   However, it is still desired to improve the anodic and chemical stability.  The development of new electrolyte system for magnesium rechargeable batteries is required.  In this study, we investigate magnesium deposition and dissolution in triglyme-based electrolyte having high chemical stability.

Electrochemical properties of the electrolyte were investigated by cyclic voltammetry with the three-electrode cell.  Platinum plate and magnesium rod were used as working electrode and counter electrode, respectively.  A silver wire was used as a reference electrode.  Magnesium trifuluoromethanesulfonyl-imide (Mg(TFSI)₂) / triglyme was used as the electrolyte.  The deposition after electrochemical measurement was characterized by XRD and SEM.  XRD measurements were carried out at the beam line BL02B2 at SPring-8 (Japan).  Magnesium K-edge XAFS measurements were also carried out for the triglyme-based electrolyte and the Grignard reagents at the beam line BL27SU at Spring-8 (Japan).

Figure 1(a) shows cyclic voltammogram of the platinum working electrode in Mg(TFSI)₂ / triglyme  electrolyte.  The cathodic and anodic peaks correspond to magnesium deposition and dissolution, respectively.  The anodic stability in this electrolyte is higher than 3.5 V vs. Mg²⁺ / Mg. This value is higher than that of the potential window in Grignard reagents / THF and Mg₂(μ-Cl)₃-6THF system.  The deposited product was characterized by XRD and SEM (Fig. 1(b)). The diffraction pattern of the product is fully indexed to the P6₃/mmc space group, which is similar to the space group of magnesium metal. The particles with a thickness of approximately 5 mm were deposited on a platinum-working electrode. These results validate that the Mg(TFSI)₂ / triglyme system can practically be used in magnesium rechargeable batteries coupled with high-voltage cathode materials.

ACKNOWLEDGMENTS

 This work was partially supported by Core Research for Evolutional Science and Technology (CREST) program of Japan Science and Technology Agency (JST) in Japan.

References

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

[2]D. Aurbach, Y. Gofer, A. Schechter, O. Chusid, H. Gizbar, Y. Cohen, M. Moshkovich & R. Turgeman, J. Power Sources, 97-98, 269 (2001).

[3]Z. Lu, A. Schechter, M. Moshkovich & D. Aurbach, J. Electroanal. Chem., 466, 203 (1999).

[4] J. Muldoon, C. B. Bucur, A. G. Oliver, T. Sugimoto, M. Matsui, H. S. Kim, G. D. Allred, J. Zajicek & Y. Kotani, Energy Environ. Sci., 5, 5941-5950 (2012).