Computational Insights into Mg-Cl Complex Electrolytes for Rechargeable Magnesium Batteries

DFT calculations were conducted to provide unprecedented thermodynamic insights on tetrahydrofuran (THF) solvation, isomerization, and complexation of possible Mg−Cl coordination species for popular Mg−Cl electrolytes for magnesium batteries. Computational results using the M06-2x functional with the 6-31+G(d) basis set indicate that trigonal bipyramidal e,e-cis-tbp-MgCl₂(THF)₃ dichloride species and octahedral [MgCl(THF)₅]⁺ monochloride species are the dominant mononuclear species. These two can combine to form the dinuclear species [(μ-Cl)₃Mg₂(THF)₆]⁺ with a free energy −6.30 kcal/mol, which is calculated to be the dominant Mg−Cl species in solution. Two mono-cation species, [(μ-Cl)₃Mg₂(THF)₆]⁺ and [MgCl(THF)₅]⁺, have comparable LUMO energies, thus both of them can act as active species for Mg deposition. However, the significant dominance of the dinuclear species in the electrolyte indicates that it is the primary species involved in reversible Mg deposition.