LAUSR

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 MgCl2 in tetrahydrofuran (THF) to form an all‐magnesium salt electrolyte. Raman spectroscopy and density function theory calculation confirm that [Mg2Cl3·6THF]+[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//Mo6S8 full cell, showing great promise of the SEI forming electrolyte in future Mg batteries.