Evaluation of (CF₃SO₂)₂N- (TFSI) based electrolyte solutions for Mg batteries

MgTFSI₂ is the only ether-soluble "simple" magnesium salt. The poor electrochemical performance ofMg 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.