Releasing Free Anions by High Donor Number Cosolvent in Noncorrosive Electrolytes of Commercially Available Magnesium Salts

Abstract

Passivation of the magnesium (Mg) anode in the chloridefree electrolytes using commercially available Mg salts is a critical issue for rechargeable Mg batteries. Herein, a high donor number cosolvent of 1methylimidazolium (MeIm) is introduced into Mg(TFSI) 2 -and Mg-(HMDS) 2 -based electrolytes to address the passivation problem and realize highly reversible Mg plating/stripping. Theoretical calculations and experimental characterization results reveal that the strong coordination ability of MeIm with Mg 2+ can weaken the anion−cation interactions and promote the formation of free anions that have higher reduction stability, thus significantly suppressing anion-derived passivation layer formation. By adding MeIm cosolvent into Mg(TFSI) 2 -based electrolyte, the average Coulombic efficiency of the Mg//Cu cell is increased from less than 20% to over 90%, and the Mg//Mg cell can stably cycle for over 800 h with a low overpotential. In the MeIm-regulated Mg(HMDS) 2 -based electrolyte, the solvation structure change, featured by an effective separation of Mg 2+ and HMDS − , greatly increases the ionic conductivity by more than 30 times. This solvation structure regulation strategy for noncorrosive electrolytes of commercially available Mg salts has a great potential for application in future rechargeable Mg metal batteries.