(669c) Understanding the Electrode/Electrolyte Interphase in Magnesium-Ion Electrolytes with Simple Mg Salts

Researchers are highly interested in Magnesium-ion (Mg-ion) electrochemistry owing to its high natural ambulance, high volumetric capacity/energy density and non-dendritic platting/stripping of Mg metal anodes. Commercialization of Mg-ion rechargeable batteries is hampered due to the lack of practical electrolytes and cathode materials. The Solid Electrolyte Interphase (SEI) layer formed during the deposition/stripping of Mg metal is highly detrimental towards the diffusion of Mg ions into the anode. A thorough understanding of the composition of these layers and the effect of the electrolyte properties on the electrochemical performance is deeply lacking. The preliminary comparative study on the SEI formed in Magnesium(II) Bis(trifluoromethanesulfonyl)imide (Mg(TFSI)₂) in tetraglyme (G4) and Magnesium Monocarborane (Mg(CB₁₁H₁₂)₂) in G4 was performed. Reversible deposition/stripping was observed in both electrolytes with water content less than 5 ppm. Mg(TFSI)₂ displayed high overpotential of deposition and stripping and high potential hysteresis. Whereas; Mg(CB₁₁H₁₂)₂ displayed considerably lower overpotential and hysteresis under the same conditions. X-ray Photoelectron Spectroscopy (XPS) measurements conducted on Mg deposited on Copper (Cu) surface indicated side products from the Mg(TFSI)₂ electrolyte but not for the Mg(CB₁₁H₁₂)₂ electrolyte. SEM, and XPS analysis were also conducted on Mg metal surface after chronopotentiometry experiments to get a thorough understanding of Mg metal as an anode for the proposed electrolytes. The surface characterization results indicated Mg(TFSI)₂ has unstable anion prone to cathodic decomposition leading to the formation of SEI composed by magnesium oxide, magnesium sulfide and magnesium fluoride that severely impedes efficient deposition and stripping of Mg.