Electrode surface engineering with electrolyte additives, improving reversibility of magnesium metal anode batteries

Rechargeable magnesium batteries (RMBs) are promising energy storage systems because of magnesium's high volumetric capacity and abundance. However, RMBs' advancement has been hindered by the lack of suitable electrolytes that combine high oxidative stability and electrode compatibility. In this study, we investigate the effects of three mechanistically different additives, namely, boron trifluoride diethyl etherate (BF₃·O(C₂H₅)₂), iodine (I₂), and boron triiodide (BI₃), in an Mg(TFSI)₂/DME:DG electrolyte on magnesium plating and stripping kinetics, voltage hysteresis, coulombic efficiency, and the composition and thickness of the electrode interphase. BI₃ demonstrates the greatest beneficial effect among these additives by forming a thin (<5 nm) interphase, rich in iodide and boron species, that enhances the coulombic efficiency (from <60 to 80-90%), reduces voltage hysteresis (from 2000 mV to 250-350 mV), and improves Mg plating/stripping kinetics. Electrochemical tests against Mo₆S₈ cathodes demonstrate that substrate choice also affects performance: Mo₆S₈ supported on stainless steel yields stable cycling with full capacity retention, whereas titanium exhibits poor adhesion of the cathode composite material. Compared to state-of-the-art Mg[B(HFIP)₄]₂ electrolytes, Mg(TFSI)₂/DME:DG + BI₃ offers a simpler, more scalable, and practical route with engineered interphase chemistry, though further studies are needed to match the outstanding performance of Mg[B(HFIP)₄]₂ and to fully understand the Mg(TFSI)₂/DME:DG + BI₃ bulk electrolyte properties. This work provides key insights for designing improved-performance RMBs via innovative interphase engineering and electrolyte formulation.