Determination of Average Coulombic Efficiency for Rechargeable Magnesium Metal Anodes in Prospective Electrolyte Solutions

The design of electrolyte solutions that permit reversible and efficient Mg metal electrodeposition is one of the most important tasks in the development of rechargeable Mg batteries. Several types of electrolyte solutions for Mg metal anodes have been developed and explored over the last two decades. These investigations have contributed to a better understanding of the Mg deposition and stripping processes. However, the Coulombic efficiency (CE) for reversible electrodeposition reported for these various systems and their performance in comparison to one another remained unclear. We used rigorous electrochemical methods to accurately quantify the average CE of the major electrolyte solutions considered for secondary Mg metal batteries. We demonstrated how changes in the experiential protocols influence CE measurements, resulting in inconsistent reports. Even though exceptional efficiency has been reported for a variety of systems, we discovered that the only candidate that currently meets the 99% CE benchmark during a prolonged cycling procedure is the dichloro-complex, which is a first-generation Grignard-based electrolyte solution. Second- and third-generation Grignard-free and chloride-free solutions showed reasonable CE only when the deposition currents densities were lowered. This comprehensive and systematic investigation will help to create a more accurate treasure map for potential electrolyte solutions for rechargeable Mg metal anodes.