Haim Gizbar

Prototype systems for rechargeable magnesium batteries.

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 MgxMo 3S4 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.

Improved Electrolyte Solutions for Rechargeable Magnesium Batteries

Electrolyte solutions of magnesium organo-halo-aluminates in ethers are suitable for rechargeable magnesium batteries as they enable highly reversible electrodeposition for magnesium while they possess a wide electrochemical window (>2.2 V). Adding LiCI or tetrabutylammonium chloride to these solutions considerably improves their ionic conductivity, the kinetics of the Mg deposition-dissolution processes, and the intercalation behavior of Mg x MO 6 S 8 Chevrel cathodes. The dissolution of both salts in the electrolytic solutions involves acid-base reactions with complex species. Multinuclei nuclear magnetic resonance and Raman spectroscopy were used in conjunction with electrochemical techniques to study these systems. The nature of these reactions, their products, and the way they influence the various properties of these solutions, are discussed herein.

Kinetic and Thermodynamic Studies of Mg2 + and Li + Ion Insertion into the Mo6 S 8 Chevrel Phase

Slow-scan rate cyclic voltammetry (SSCV) and chronopotentiometry were used for a quantitative comparison of the thermodynamic and kinetic characteristics of Li + and Mg 2+ -ion insertion into the Mo 6 S 8 chevrel phase compound. The Li-insertion process consists mainly of three stages with the relative stoichiometries 1:2:1, corresponding to the formation of Li 1 Mo 6 S 8 , Li 3 Mo 6 S 8 , and Li 4 Mo 6 S 8 , respectively. The kinetics of the intercalation is relatively fast. Mg-ion insertion was found to have the stoichiometry 2:2, i.e., Mg 1 Mo 6 S 8 and Mg 2 Mo 6 S 8 are formed. The initial magnesiation and the final demagnesiation of the chevrel phase (Mo 6 S 8 ↔ Mg 1 Mo 6 S 8 ) reveal intrinsically slow kinetics, accompanied by a substantial decrease in the intercalation level. This probably results from a low ionic conductivity of the electrode bulk caused by both small concentration and low mobility of the Mg ion in this potential region, related to the sites that the Mg intercalants occupy in the Mg x Mo 6 S 8 phase. A moderate increase in temperature results in a drastic increase of ion mobility. In Mg(AlCl (4-n) R n ) 2 solution, the difference of the two sequential insertions of Mg ion into the chevrel phase was found to be 0.26 V, i.e., by 0.08 V lower than that for the insertion of Li ion.

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

Abstract This paper describes briefly the difference between reversible lithium and magnesium electrodes. In the case of lithium, the active metal is always covered by surface films. Li dissolution–deposition is reversible only when the surface films contain elastomers and are flexible. Hence, they can accommodate the morphological changes of the electrode during the electrochemical processes without breaking down. In an ideal situation, lithium is deposited beneath the surface films, while being constantly protected in a way that prevents reactions between freshly deposited lithium and solution species. In contrast to lithium, magnesium electrodes are reversible only in solutions where surface film free conditions exist. Mg does not react with ethers, and thus, in ethereal solutions of Grignard reagents (RMgX, where R=alkyl, aryl, X=halide) and complexes of the following type: Mg(AlX 4− n R n ′ R n ″ ′) 2 , R and R′=alkyl groups, X=halide, A=Al, 0 n n ′+ n ′′= n , magnesium electrodes behave reversibly. However, it should be noted that the above stoichiometry of the Mg salts does not reflect the true structure of the active ions in solutions. Mg deposition does not occur via electron transfer to simply solvated Mg 2+ ions. The behavior of Mg electrodes in these solutions is discussed in light of studies by EQCM, EIS, FTIR, XPS, STM and standard electrochemical techniques.