On the Mechanisms of Reversible Magnesium Deposition Processes

Magnesium deposition processes from ethereal solutions of Grignard salts of the RMgX type (R = alkyl, aryl groups and X = halide; Cl, Br), and complexes of the Mg(AX_4-nR_n′R_′n″)₂ type (A = Al, B, X = halide; R,R′ = alkyl or aryl groups and n′ + n″ = n) were investigated. These complexes can be considered as interaction products between R′RMg bases and AX_{3 n}R_n′R′_n″ Lewis acids. The use of such complexes in ether solvents enables solutions of high anodic stability to be obtained, which can be suitable for rechargeable Mg battery systems. In situ scanning tunneling microscopy, scanning electron microscopy in conjunction with element analysis by dispersive X-ray, electrochemical quartz crystal microbalance, and impedance spectroscopy were used. Mg deposition in all the solutions studied initially form a porous deposit that becomes compact and crystalline as the process continues. It was found that the morphology of Mg deposition is strongly dependent on the solution's composition. This is because these processes are accompanied by adsorption processes. The specific adsorbed species in each solution probably influence the nucleation processes and thus affect the final morphology of Mg deposition in each solution. There is a clear correlation between the morphology of these processes and the cycling efficiency of Mg anodes measured in each solution. The results thus obtained are important for R&D of rechargeable Mg battery systems.