The study of reversible magnesium deposition by in situ scanning tunneling microscopy

Magnesium can be reversibly deposited 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_3-nR_n′R′_n″)₂ type (A = Al, B; X = halide; R,R′ = alkyl or aryl groups and n′ + n″ = n). These complexes can be considered as products of the interaction between R′RMg bases and AX_3-nR_n′R′_n″ Lewis acids. The use of such complexes in ether solvents enables us to obtain solutions of high anodic stability, which can be suitable for rechargeable Mg battery systems. Mg electrodes in both Grignard salts and complex ethereal solutions developed very high impedance under open-circuit conditions. However, it appears that this high impedance is not due to stable passivation phenomena, but rather to some adsorption processes. Hence, it was possible to probe in situ the morphology of Mg deposition and dissolution processes by scanning tunneling microscopy. In this paper, we report on the study of Mg deposition in tetrahydrofuran solutions of BuMgCl (Bu = butyl group) and Mg(AlCl)₂BuEt (Et = ethyl group). The latter solution was found to be useful and suitable for rechargeable magnesium battery systems.