Direct crystallization of deep eutectic solvent into solid-state electrolyte for magnesium metal batteries

Solid-state magnesium rechargeable battery is a promising post-Li battery technology due to the high abundance of Mg element, the competitive volumetric capacity and enhanced safety of Mg metal anodes. However, current solid electrolyte formulas failed to realize considerably reversible Mg anodes caused by the passive layer formed on their surface. Herein, a novel Mg²⁺-conducting solid-state electrolyte (MCE) is proposed by directly crystalizing deep eutectic solvents composing of Mg(TFSI)₂ and urea at room temperature. Experimental and simulation results indicate the formation of anion-rich groups in urea-Mg²⁺ coordination structures, facilitating rapid Mg²⁺ migration in the MCE. When applied in the Mg//Mg symmetric cells, the new electrolyte derives a macroporous and fluoride-rich organic/inorganic hybrid interface layer on the Mg metal surface. The unique interfacial structure endows the interface layer with Mg²⁺ diffusion capability for reversible Mg metal stripping/plating. The practical feasibility of MCE is finally demonstrated by cycling in Mg//V₂O₅ full cells with competitive electrochemical performance. This work illustrates a new concept of crystalizing deep eutectic solvents into solid-state electrolytes at room temperature for multivalent metal batteries.