Stable Solid Electrolyte Interphase In Situ Formed on Magnesium-Metal Anode by using a Perfluorinated Alkoxide-Based All-Magnesium Salt Electrolyte.

Passivation of the Mg anode surface in conventional electrolytes constitutes a critical issue for practical Mg batteries. In this work, a perfluorinated tert-butoxide magnesium salt, Mg(pftb) 2 , is codissolved with MgCl 2 in tetrahydrofuran (THF) to form an all-magnesium salt electrolyte. Raman spectroscopy and density function theory calculation confirm that [Mg 2 Cl 3 ·6THF] + [Mg(pftb) 3 ] - is the main electrochemically active species of the electrolyte. The proper lowest unoccupied molecular orbital energy level of the [Mg(pftb) 3 ] - anion enables in situ formation of a stable solid electrolyte interphase (SEI) on Mg anodes. A detailed analysis of the SEI reveals that its stability originates from a dual-layered organic/inorganic hybrid structure. Mg//Cu and Mg//Mg cells using the electrolyte achieve a high Coulombic efficiency of 99.7% over 3000 cycles, and low overpotentials over ultralong-cycle lives of 8100, 3000, and 1500 h at current densities of 0.5, 1.0, and 2.0 mA cm -2 , respectively. The robust SEI layer, once formed on a Mg electrode, is also shown highly effective in suppressing side-reactions in a TFSI - -containing electrolyte. A high Coulombic efficiency of 99.5% over 800 cycles is also demonstrated for a Mg//Mo 6 S 8 full cell, showing great promise of the SEI forming electrolyte in future Mg batteries.