Reducing Mg Anode Overpotential via Ion Conductive Surface Layer Formation by Iodine Additive

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 overpotential due to the formation of a surface layer on the Mg metal with a low Mg ion conductivity. Here the overpotential for Mg deposition/stripping in a simple salt, Mg(TFSI)(2)-1,2-dimethoxyethane (DME), electrolyte is significantly reduced by adding a small concentration of iodine (<= 50 x 10(-3) M) as an additive. Mechanism studies demonstrate that an Mg ion conductive solid MgI2 layer is formed on the surface of the Mg metal and acts as a solid electrolyte interface. With the Mg(TFSI)(2)-DME-I-2 electrolyte, a very small voltage hysteresis is achieved in an Mg-S full cell.