Insertion of Magnesium into Antimony Layers on Gold Electrodes:Kinetic Behaviour

Abstract

AbstractMagnesium based secondary batteries are regarded as a viable alternative to the immensely popular Li‐ion systems. One of the largest challenges is the selection of a Mg anode material since the insertion/extraction processes are kinetically slow because of the large ionic radius and high charge density of Mg2+. In an attempt to bridge the gap between insertion measurements in 3D composite electrode materials and that in an idealized pure model system, we studied the insertion and diffusion of Mg in a thin, massive layer of Sb deposited on Au by using PITT, CV, and potential step experiments. Sb has been suggested as an insertion material because magnesium can form intermetallic compound with it. The layered crystal structure of Sb leads should facilitate formation of such an intermetallic phase. Mg insertion from a MACC/tetraglyme electrolyte into Sb starts 300 mV positive of the onset potential of Mg deposition as shown by cyclic voltammetry. The molar ratio of Mg to Sb agrees well with the stoichiometry of Mg3Sb2 alloy (Zintl‐phase). The diffusion coefficient of Mg‐insertion into Sb – layers and the charge transfer rate have been estimated by the above techniques. Such diffusion coefficients, albeit still somewhat “apparent”, are much more closely related to the true diffusion coefficient in the metal or alloy. The solid‐state diffusion coefficient of Mg into the Sb layers is in the range of 4–8×10−14 cm2 s−1. A very high Tafel slope of 370 mV/dec was found in potential step experiments. Mg insertion was further investigated by XPS measurements. Besides Mg and Sb, Al and Cl signals were also detected, particularly at the outer parts of the layer.