Understanding Solution Chemistry and Electrochemistry of Mg-Electrolytes for Rechargeable Mg Batteries

Mg batteries with the inherent merits of Mg anode have attracted increasing attention as a promising high energy density battery technology. However, the lack of high performance Mg²⁺ electrolyte has been a primary technic hurdle to the development of practical Mg²⁺ batteries.^1,2 The current popular methods in preparing Mg²⁺ electrolytes involve the use of nucleophilic Mg²⁺ sources, Grignard reagents or analogues (RMgCl, R= alkyl, aryl and amido etc.) and dialkyl (diaryl) magnesium (MgR₂). But handling these reactive Mg²⁺ chemicals is not synthetically feasible, and their presence essentially limits oxidation stability and compatibility of the resulting electrolytes with electrophiles (such as high capacity sulfur cathode). Recently we reported a very simple approach to produce high performance Mg²⁺ conductive electrolytes, termed as “mono-Cl abstraction”, employing cheap and non-nucleophilic MgCl₂ as the Mg²⁺ source and various Al Lewis acids (AlCl₃, AlEtCl₂ and AlPh₃).³ In this presentation, we will discuss solution chemistry of the “mono-Cl abstraction” approach, which is fundamentally distinct from the reported methods using nucleophilic Mg²⁺ sources. For example,  “mono-Cl abstraction” only involves a single Cl^- transmetalation between MgCl₂ and Lewis acid but both Cl^- and R^- transmetalation reactions take place between RMgCl and Lewis acid (see Scheme 1). In addition, electrochemistry of the MgCl₂/Al Lewis acid electrolytes will be interpreted based on experimental data and compared to electrolytes reported by others. In meeting with desired electrolytes properties for practical rechargeable Mg-batteries, the presented discussions of electrolyte chemistry in together with new results from our group at PNNL are attempted to provide designing rationales for next generation of Mg²⁺electrolytes.

            (1)        Yoo, H. D.; Shterenberg, I.; Gofer, Y.; Gershinsky, G.; Pour, N.; Aurbach, D. Energy Environ. Sci. 2013, 6, 2265.

            (2)        Muldoon, J.; Bucur, C. B.; Oliver, A. G.; Sugimoto, T.; Matsui, M.; Kim, H. S.; Allred, G. D.; Zajicek, J.; Kotani, Y. Energy Environ. Sci. 2012, 5, 5941.

            (3)        Liu, T.; Li, G.; Shao, Y.; Liu, J. 224th ECS meeting, San Francisco (CA), October, 2013.

       Acknowledgment

We thank Pacific Northwest National Laboratory for support of this research through the Lab Directed Research and Development Award. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.