MgTFSI 2 /MgCl 2  /DME Solution Structure Analysis

Abstract

Recently, MgTFSI 2 /MgCl 2 electrolyte solutions in dimethoxyethane (DME) have been shown to function as viable electrolyte solutions for secondary Mg batteries that can facilitate reversible magnesium deposition/dissolution 1 . MgCl 2 is a crucial component in these solutions. On its own, however, it is practically insoluble in DME. Therefore, the fact that it is readily dissolved in MgTFSI 2 /DME solution is remarkable. Addition of MgCl 2 greatly improves the electrochemical performance of MgTFSI 2 /DME electrolyte solutions. MgTFSI 2 /DME electrolyte show large overpotential for deposition (-0.6 V vs Mg) and dissolution (1.5V vs Mg). Furthermore this electrolyte exhibit poor magnesium deposition reversibility. Adding MgCl 2 to this electrolyte reduces the overpotentials for both deposition and dissolution and showed 98% columbic efficiency. Thus, identifying the species formed in MgTFSI 2 /MgCl 2 solutions are very intriguing. In this study, we identified numerus solution species that some of them were not previously identified. We believe that the newly discovered Mg x Cl y complexes play a crucial rule in the improved electrochemical performance of the MgTFSI 2 /DME electrolyte solutions. The complexes that were identified were Mg 3 Cl 4 2+ (figure 1) 2 , Mg 2 Cl 2 2+ .Furthermore we showed that the hexacoordinated nature of magnesium effectively dictate the solution structure. We showed that in THF based electrolyte Mg 2 Cl 3 + is the dominant solution specie and we didn’t see any evidence for its existence in DME based electrolytes(. We implemented a wide variety of analytical tools, including single crystal X-ray diffraction, multinuclear NMR, and Raman spectroscopy, to elucidate the structure of these solutions. Various solution species were determined, and a suitable reaction scheme is suggested(figure 1). We believe that this type of fundamental study is essential for designing new and improved electrolyte solutions for secondary magnesium batteries. Shterenberg, I.; Salama, M.; Yoo, H. D.; Gofer, Y.; Park, J.-B.; Sun, Y.-K.; Aurbach, D. Evaluation of (CF3SO2) 2N−(TFSI) based electrolyte solutions for Mg batteries. J. Electrochem. Soc. 2015 , 162 , A7118-A7128. Salama, M.; Shterenberg, I.; JW Shimon, L.; Keinan-Adamsky, K.; Afri, M.; Gofer, Y.; Aurbach, D. Structural Analysis of Magnesium-Chloride Complexes in Dimethoxyethane Solutions in the Context of Mg Batteries Research. The Journal of Physical Chemistry C 2017 . Figure 1