Bulky Dimethyl Phosphate Anion-Based Ionic Liquid Enables Reversible Magnesium Electrodeposition

Rechargeable magnesium batteries have wide application prospects due to their abundant resources and high theoretical specific capacity. However, the Mg plating/stripping reversibility in conventional electrolytes is still not satisfactory because of the Mg2+/electron-impermeable passivation layer formation on the Mg-metal anode. In this work, 1-ethyl-3-methylimidazolium dimethyl phosphate (EMIM(CH3O)2PO2) has been proposed as an effective cosolvent to regulate the electrolyte solvation structures and ameliorate Mg electrochemistry reversibility. Through nuclear magnetic resonance spectroscopy, Raman spectroscopy, mass spectrometry, and molecular dynamics analyses, EMIM(CH3O)2PO2 has been verified to play a role in boosting the dissociation of Mg(CF3SO3)2 in diglyme (G2) solvent, transforming the electrochemically inactive [Mg(G2)2(CF3SO3)]+ ion pair species into a fresh [Mg(G2)[(CH3O)2PO2]]+ counterpart. Investigations on Mg electrodeposits reveal an isotropic granular-welding plate morphology with a thin and nonpassivating interlayer. As expected, electrolytes with EMIM(CH3O)2PO2 cosolvents display the best Mg plating/stripping properties, including a low Mg plating overpotential (0.80 V), high Coulombic efficiency (71.3%), and wide electrochemical window (3.2 V), among all ionic liquid-based electrolytes.