The thermodynamic properties of magnesium make it a natural choice for use as an anode material in rechargeable batteries, because it may provide a considerably higher energy density than the commonly used lead-acid and nickel-cadmium systems. Moreover, in contrast to lead and cadmium, magnesium is inexpensive, environmentally friendly and safe to handle. But the development of Mg batteries has been hindered by two problems. First, owing to the chemical activity of Mg, only solutions that neither donate nor accept protons are suitable as electrolytes; but most of these solutions allow the growth of passivating surface films, which inhibit any electrochemical reaction. Second, the choice of cathode materials has been limited by the difficulty of intercalating Mg ions in many hosts. Following previous studies of the electrochemistry of Mg electrodes in various non-aqueous solutions, and of a variety of intercalation electrodes, we have now developed rechargeable Mg battery systems that show promise for applications. The systems comprise electrolyte solutions based on Mg organohaloaluminate salts, and Mg(x)Mo3S4 cathodes, into which Mg ions can be intercalated reversibly, and with relatively fast kinetics. We expect that further improvements in the energy density will make these batteries a viable alternative to existing systems.

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This work is a comparative study of the electrochemical windows and the basic processes on gold electrodes in LiClO 4 , NaClO 4 , and KClO 4 solutions in propylene carbonate ͑PC͒. The analytical tools included cyclic voltammetry, electrochemical quartz crystal microbalance, surface-sensitive Fourier transform infrared spectroscopy ͑ex situ, external reflectance mode͒, and X-ray photoelectron spectroscopy. The apparent electrochemical windows of these systems are anodically limited at potentials above 1.3 V ͑vs. Ag pseudoreference electrode corresponding to 4.3 vs. Li/Li ϩ ͒ due to solvent oxidation. The apparent cathodic side is limited due to the reversible bulk active metal deposition occurring at approximately Ϫ3 and ϽϪ2.7 V vs. Ag pseudoreference electrode for Li and Na, respectively. In the case of the potassium salt solution, the electrochemical window is limited by a pronounced cathodic process below Ϫ2 V ͑vs. Ag reference electrode͒, which is attributed to irreversible reduction of solution species. Irreversible potassium deposition occurs at potentials below Ϫ2.5 V. This process cannot be separated from the reduction processes of the solution starting below Ϫ2 V. The study revealed that irreversible trace O 2 , trace H 2 O, and PC reduction form passivating surface films on these electrodes. These films act as a solid electrolyte interphase, i.e., they allow transport of the alkali metal ions through them. The study also found that the major constituent in the surface films is the PC reduction product CH 3 CH͑OCO 2 M͒CH 2 OCO 2 M. In general, the surface films formed on the noble metal electrodes in the Li and K salt solutions are more stable than those formed in the Na salt solutions, because the sodium oxides, hydroxide, and carbonates thus formed are more soluble in PC than the corresponding Li and K compounds.The use of polar aprotic electrolyte solutions in modern electrochemistry considerably extends the applicability of electrochemical techniques to a wide variety of substances ͑electrode materials, reagents, compounds, and electrolytes͒ because of two important factors:1. Aprotic organic solvents dissolve a wide variety of organic species.2. The electrochemical windows of polar aprotic solutions are usually much wider than those of protic solutions. Apparent stability higher than 4.5 V can be obtained with organic electrolyte solutions based on solvents such as alkyl carbonates.This paper addresses this second aspect, namely, the intrinsic electrochemical reactions of representative polar aprotic solutions, which apparently show wide electrochemical windows ͑Ͼ4 V at steady state͒ on noble metal electrodes.Much information has been accumulated over the last decades about the anodic and cathodic stability of polar aprotic solutions with both active and nonactive metal electrodes. 1,2 Special attention was paid to lithium salt solutions because of their importance to the field of lithium batteries. Steady-state voltammograms of commonly used lithium salts such as LiClO 4 , LiAsF 6 , LiPF 6 , LiBF 4 ...