A comparison between the electrochemical behavior of reversible magnesium and lithium electrodes

This paper describes briefly the difference between reversible lithium and magnesium electrodes. In the case of lithium, the active metal is always co vered by surface films. Li dissolution-deposition is reversible only when t he surface films contain elastomers and are flexible. Hence, they can accom modate the morphological changes of the electrode during the electrochemica l processes without breaking down. In an ideal situation, lithium is deposi ted beneath the surface films, while being constantly protected in a way th at prevents reactions between freshly deposited lithium and solution specie s. In contrast to lithium, magnesium electrodes are reversible only in solu tions where surface film free conditions exist. Mg does not react with ethe rs, and thus, in ethereal solutions of Grignard reagents (RMgX, where R = a lkyl, aryl, X = halide) and complexes of the following type: Mg(AlX4-nRn 'R ' (n "))(2), R and R ' = alkyl groups, X = halide, A = Al, 0 < n < 4 and n ' + n " = n, magnesium electrodes behave reversibly. However, it should be noted that the above stoichiometry of the Mg salts does not reflect the tr ue structure of the active ions in solutions. Mg deposition does not occur via electron transfer to simply solvated Mg2+ ions. The behavior of Mg elec trodes in these solutions is discussed in light of studies by EQCM, EIS, FT IR, XPS, STM and standard electrochemical techniques. (C) 2001 Elsevier Sci ence B.V. All rights reserved.