The Study of MgTFSI2/Ether Solutions for Rechargeable Magnesium Batteries
Authors: Shterenberg Ivgeni , Salama Michael , Gofer Yosef , Nitoker Neta , Major Dan Thomas , Aurbach Doron 
Published: February 2020

Rechargeable Mg batteries are the focus of extensive work, throughout the world1-3. The high abundance of magnesium in the earth's crust and the chance to gain high volumetric energy density and prolonged cycle-life with non-aqueous rechargeable Mg batteries, make them very attractive. Over the years, we see an interesting and impressive evolution of the electrolyte solutions in which Mg anodes are reversible and their electrochemical windows exceed 3V.

In this presentation we focus on the challenge of electrolyte solutions for rechargeable Mg batteries. In general, such solutions should be based on ether solvents. Highly interesting are poly-ethers, which are not volatile and may be easily purified. The first generations of electrolyte solutions for rechargeable Mg batteries were complexes which included electrolyte moieties with organo-metallic bonds (with elements such as aluminum or boron).  Such complex electrolyte solutions cannot be compatible with high capacity/high voltage cathode materials based on transition metal oxides. In turn, conventional electrolyte solutions comprising Mg salts, in which transition metal oxides may intercalate reversibly with Mg ions, are irrelevant for reversible Mg anodes due to complicated passivation phenomena. Electrolyte solutions based on “simple” salts are assumed to be the most practical candidates for rechargeable Mg battery systems if Mg anodes can behave reversibly in them. By simple we mean Mg salts, with non-organometallic components, similar to the ones regularly used in lithium ion battery systems with anions such as TFSI-, ClO4
-, PF6
- etc.  Due to reactivity issues, only ethers are considered as viable solvents for secondary Mg-metal based batteries, as they were shown to be inert with Mg metal and do not form passivation layers. Until now, Mg(N(SO2CF3)2)2 (denoted as MgTFSI2) is the only ethers-soluble “simple” salt reported that forms ethereal solutions which allow reversible behavior of Mg anodes4
,
5.  MgTFSI2 is an attractive salt for Mg batteries electrolyte solutions, endowed with a high thermal and oxidation stability .It also forms highly ionic conductive solutions with ether solvents. Magnesium can be reversibly deposited, to some extent, from MgTFSI2/glyme solutions, indicating that the MgTFSI2 salt is stable with Mg metal anodes. Although its long-term electrochemical cycling performance is rather poor, it can be significantly enhanced with addition of chlorides and through conditioning process5.

A highly interesting ether solvent for Mg batteries is dimethoxyethane - DME.  We encounter very interesting phenomenon with DME based solutions. At certain salt concentrations MgTFSI2/DME mixtures may form two immiscible solution phases with DME. This phenomenon is extremely unique and, as far as we know, had not been investigated deeply.  This phenomenon resembles miscibility-gap to a great extent. We studied this phenomenon using a wide variety of analytical tools, such as SCXRD SSNMR, and Raman coupled with DFT calculations. In this study we were able to determine the solutions structures of the different MgTFSI2/DME phases and theorize a plausible explanation for the two-phase phenomenon. Knowing the arrangement of the solvent/anion/cation in the solution matrix can help explains the properties of these electrolyte solutions and produces a viable model for their electrochemical behavior. We believe that our study can help to improve the relevance of MgTFSI2/poly-ether solutions for practical rechargeable Mg batteries.

References

1              Liao, C. et al. The unexpected discovery of the Mg(HMDS)2/MgCl2 complex as a magnesium electrolyte for rechargeable magnesium batteries. Journal of Materials Chemistry A
3, 6082-6087(2015).

2              Tutusaus, O. et al. An Efficient Halogen-Free Electrolyte for Use in Rechargeable Magnesium Batteries. Angewandte Chemie International Edition
54, 7900-7904(2015).

3              Pour, N., Gofer, Y., Major, D. T. & Aurbach, D. Structural Analysis of Electrolyte Solutions for Rechargeable Mg Batteries by Stereoscopic Means and DFT Calculations. Journal of the American Chemical Society
133,6270-6278(2011).

4              Ha, S.-Y. et al. Magnesium(II) Bis(trifluoromethane sulfonyl) Imide-Based Electrolytes with Wide Electrochemical Windows for Rechargeable Magnesium Batteries. ACS applied materials & interfaces
6, 4063-4073(2014).

5              Shterenberg, I. et al. Evaluation of (CF3SO2)2N− (TFSI) Based Electrolyte Solutions for Mg Batteries. Journal of the Electrochemical Society
162, A7118-A7128(2015).