The development of rechargeable Mg batteries, driven by the desire to overcome the limiting specific energy density and high cost of Li ion batteries, is currently frustrated by the lack of stable, functional electrolytes compatible with high voltage cathodes.  Successful electrolytes have typically employed chloride complexes as the Mg cation delivering species in order to avoid the formation of Mg cation blocking films on the anode surface.  However, such electrolytes exhibit questionable compatibility with the high voltage cathodes and current collector materials needed to attain large energy density and low cost in a complete magnesium battery system. Motivated by this problem, novel “conventional” electrolytes resembling those used in Li-ion batteries have been proposed, including those based on the weakly coordinating bis(trifluoromethylsulfonyl)imide (TFSI) and monocarborane (CB₁₁H₁₂^-) anions.  The electrochemical behavior and stability of such electrolytes is not well understood, and their study is necessary in order to guide magnesium battery development efforts.    

In this presentation we describe the results of in operando studies focused on evaluating the relationship between interfacial speciation and electrochemical behavior from the perspective of the efficiency of magnesium electrodeposition and dissolution in novel electrolyte chemistries. The techniques utilized by these studies include Raman spectroscopy and X-ray absorption spectroscopy, conducted in such a way as to provide interfacial or near-interfacial information concerning the interactions of the Mg cation with the solvent and anion species in the presence of the electrified interface. We demonstrate that these in situ experiments provide a level of insight that is not attained by bulk or otherwise ex situ electrolyte characterization techniques.  In particular, we find that interfacial changes in MgTFSI₂/glyme electrolyte speciation promote instability in the TFSI anion contributing to the poor reversibility of the Mg metal deposition process.  We further demonstrate that the addition of chloride species to the TFSI-based electrolyte modifies these interfacial processes, suppressing the TFSI decomposition pathway.  We will also discuss how the nature of the specific ether solvent employed influences these findings.  We believe that this information provides important insight into the design and implementation of electrolytes for future magnesium batteries.

This work is supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science. Sandia is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. DOE’s NNSA under contract DE-AC04-94AL85000.