Tailoring Electrode–Electrolyte Interface Using an Electron-Deficient Borate-Based Additive in MgTFSI2-MgCl2/DME Electrolyte for Rechargeable Magnesium Batteries

Rechargeable magnesium metal batteries need an electrolyte that forms a stable and ionically conductive solid electrolyte interphase (SEI) on the anodes. Here, we used molecular dynamic simulation, density functional theory calculation, and X-ray photoelectron spectroscopy analysis to investigate the solvation structures and SEI compositions in electrolytes consisting of dual-salts, magnesium bis(trifluoromethanesulfonyl)imide (MgTFSI₂), and MgCl₂, with different additives in 1,2-dimethoxyethane (DME) solvent. We found that the formed [Mg₃(μ-Cl)₄(DME)_mTFSI₂] (m = 3, 5) inner-shell solvation clusters in MgTFSI₂-MgCl₂/DME electrolyte could easily decompose and form a MgO- and MgF₂-rich SEI. Such electron-rich inorganic species in the SEI, especially MgF₂, turned out to be detrimental for Mg plating/stripping. To reduce the MgF₂ and MgO contents in SEI, we introduce an electron-deficient tri(2,2,2-trifluoroethyl) borate (TFEB) additive in the electrolyte. Mg//Mg cells using the MgTFSI₂-MgCl₂/DME-TFEB electrolyte could cycle stably for over 400 h with a small polarization voltage of ~150 mV. Even with the presence of 800 ppm H₂O, the electrolyte with TFEB additive could still preserve its good electrochemical performance. The optimized electrolyte also enabled stable cycling and high-rate capability for Mg//Mo₆S₈ and Mg//CuS full cells, showing great potential for future applications.