Passivation Layer of Magnesium-Based Negative Electrodes

Abstract

Rechargeable magnesium batteries are getting more interests as a potential beyond Li-ion chemistry in the battery field. Of course a lot of the challenges of the rechargeable magnesium batteries are remaining for cathodes, anodes and electrolytes. In the case of the anode active materials, since formation of the passivation layer at the surface of the magnesium metal is the most critical problem for the reversible deposition and dissolution process, halide based electroltye solutions are preferred. On the other hand, the halide-based electrolyte sotluion always has voltage limitation due to the corrosion of the current collector. As a consequence, it is very challenging to develop a high voltage magnesium batteries. Previously we reported that the magnesium metal can be easily passivated in the Mg(TFSA)2 BuMe-triglyme solution using XPS and in situ FTIR. It is mainly due to the electrochemical reduction of the TFSA anion to form the passivation layer resulting in the high overpotential. However, a magnesium-based intermetallic compound: Mg3Bi2 showed good reversibility in the Mg(TFSA)2 BuMe-tryglyme solution with very low overpotential. Even though the decomposition of the TFSA anion itself was still observed by XPS, the surface layer formed on the Mg3Bi2 could not be enough stable to work as the passivation layer. In the present study, we synthesized various magnesium-based intermetallic compounds such as Mg3Bi2, Ma3Sb2 and Mg2Sn via solid-state process and investigated the stability of the passivation layer formed at the surface of the intermetallic compounds. In the ambient atmosphere, Mg3Bi2 and Mg2Sn were easily decomposed to form Bi and Sn respectively. On the other hand, the Mg3Sb2 was extremely stable even with water. These results well matches with the electrochemical performances of the each intermetallic compounds. The obtaind results suggest that the formation of the alloy or intermetallic compounds de-stabilizes the surface of the magnesium. We think the destabilization of the magnesium improves the performance of the anode active materials.