Evaluation of (CF₃SO₂)₂N⁻(TFSI) Based Electrolyte Solutions for Mg Batteries

Abstract

MgTFSI2 is the only ether-soluble "simple" magnesium salt. The poor electrochemical performance of Mg electrodes in its solutions hinders its practicality as a viable electrolyte for Mg batteries. MgTFSI2/DME solutions were demonstrated to dissolve large quantities of MgCl2 and produce electrolyte solutions with superior performance, though the electrochemical performance, mainly in terms of reversibility, of MgTFSI2/MgCl2 (DME) solutions cannot yet compete with that of organometallic based electrolyte solutions. We believe that the solutions' purity level governs the overall electrochemical performance, especially in solutions where a strong reductant (i.e Grignard reagent) is not present to act as an impurity scavenger. In this work, we alter the performance of the MgTFSI2/MgCl2 (DME) solutions through chemical and electrochemical conditioning and demonstrate the effect on the solutions' electrochemical characteristics. We demonstrate relatively high reversible behavior of Mg deposition/dissolution with crystalline uniformity of the Mg deposits, complemented by a fully reversible intercalation/de-intercalation process of Mg ions into Mo6S8 cathodes. We also investigated LiTFSI/MgCl2 solutions which exhibited even higher reversibility than MgTFSI2/MgCl2 (DME) solutions, which we attribute to the higher purity level available for the LiTFSI salt.