Another challenge is the development of electrolytes that can effectively utilize an Mg metal anode. Extensive effort has been put in to develop electrolytes capable of reversibly depositing/stripping Mg. Most work has focused on complex electrolytes that allow fast and reversible Mg deposition/stripping. Such electrolytes were first proposed by Aurbach's group in 2000, [8] and subsequently they developed the all phenyl complex showing high anodic stability. [9] Later, a non-nucleophilic electrolyte, Mg-HMDS, was developed by Muldoon and co-workers, and Fichtner and co-workers for Mg/S batteries. [10,11] To eliminate the organic Grignard species in the electrolyte, an all inorganic electrolyte, magnesium aluminum chloride complex, was reported. [12][13][14][15] These complex electrolytes have enabled the usage of Mg-cathode half-cells to evaluate various cathode materials due to their capability of facile Mg deposition/stripping; [8,10,[16][17][18] however, their complicated synthesis procedure, incompatibility with oxide cathodes, sensitivity to air and moisture, low ionic conductivity, and high cost have rendered them less attractive for practical applications than conventional organic electrolytes based on simple salts (salts containing anions of PF 6 − , BF 4 − , TFSI − , etc.). Unfortunately, most of these simple salt electrolytes are not compatible with Mg metal because poorly or nonconductive surface layers are normally formed on the Mg surface. In the electrolytes based on common aprotic organic solvents (such as AN, PC, etc.), the surface film is dominated by the decomposition of solvent due to their weak reduction stability, and the formed layer can block the deposition/stripping of Mg. [19] Even for solvents that are stable with Mg metal (such as glyme), the decomposition of salts or the reaction of trace moisture with Mg can still form a surface film covering the Mg surface, leading to a large overpotential for Mg deposition/stripping. [20][21][22][23] Therefore, the formation of a poorly or even nonconductive surface layer on Mg metal in common simple salt organic electrolytes is inevitable due to Mg's strong reducing ability, which remains a challenge for the application of a Mg metal anode in these simple salt electrolytes. Herein, we propose for the first time a facile approach to solve this problem by tuning the composition of the surface layer and forming an Mg ion conductive layer. By adding a small concentration of iodine (<50 × 10 −3 m) into the electrolyte, an insoluble magnesium iodide layer is formed on the surface of the Mg metal, which acts as a solid electrolyte Electrolytes that are able to reversibly deposit/strip Mg are crucial for rechargeable Mg batteries. The most studied complex electrolytes based on Lewis acid-base chemistry are expensive, difficult to be synthesized, and show limited anodic stability. Conventional electrolytes using simple salts such as Mg(TFSI) 2 can be readily synthesized, but Mg deposition/stripping in these simple salt electrolytes is accompanied by a large...