Rechargeable magnesium batteries have attracted considerable interest recently. In magnesium borohydride electrolyte, it has been reported that the use of LiBH4 and NaBH4 additives improve several desirable electrochemical properties, such as coulombic efficiency and current density. The primary goal of this study was to investigate the effects of LiBH4 and NaBH4 additives on the energetics of ion formation in Mg(BH4)(2) electrolyte. Density functional theory was used to elucidate the liquid structure and most stable species of LiBH4 and NaBH4 salts in two solvents, tetrahydrofuran and monoglyme. Calculations of cation dimer complexes, those containing two Li+ or Na+, indicate that dimers may form even at low concentrations of salt in tetrahydrofuran. The simultaneous formation of cationic and anionic complexes, such as Li+ and Li(BH4)(2)(-) with solvent ligands, is a favorable pathway for generating ions. This co-formation lowers the free energy of ion formation in general, which explains the synergetic effect when Li or Na is used as an additive to Mg-based electrolyte. The additives unlock additional reaction pathways that enhance the concentration of magnesium cations by orders of magnitude. (C) 2018 The Electrochemical Society.