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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To initiate wider discussion about promising research directions, this paper highlights a number of challenges in the development of rechargeable Mg batteries, especially those related to the slow solid-state Mg diffusion in common hosts. With a focus on the intercalation mechanism, we compare for the first time different strategies proposed in the literature for developing Mg battery cathodes, like the use of (i) nanoscale cathode materials; (ii) hybrid intercalation compounds containing bound water or other additional anion groups that can presumably screen the charge of the inserted cations, (iii) cluster-containing compounds with efficient attainment of local electroneutrality. This comparative analysis shows that cathodes whose function is based on a combination of the two first strategies, e.g., V2O5 gels and their hybrids, can exhibit relatively high voltage and capacity upon Mg insertion, but their kinetics is insufficiently fast. A proper intercalation mechanism for such materials is still unknown, but their relatively slow cation transport seems to be intrinsic: The paradox is that the high capacity testifies Mg insertion into crystal sites with incomplete charge screening. In contrast, the high rate capability and exclusively stable cycling of cathodes based on Chevrel phases (Mo6-cluster- containing compounds) are appropriate for Mg battery design, but they offer low energy density because of the low voltage. On the basis of the knowledge of the intercalation mechanism in these phases, we believe that the future search for cathode materials in rechargeable Mg batteries should be focused on new cluster-containing intercalation compounds with higher capacity and working potential.

This paper compares the electroanalytical behavior of lithiated graphite, LiCoO2, LiNiO2, and Li,Mn2O4 spinel ekctrodes. Slow scan rate cyclic voltammetry (SSCV), potentiostatic intermittent titration (PITT), and electrochemical impedance spectroscopy (EIS) were applied in order to study the potentiodynamic behavior, the variation of the solidstate diffusion coefficient, and the impedance of these electrodes. In addition, X-ray diffractometry and Fourier transform infrared (FTIR) spectroscopy were used in order to follow structural and surface chemical changes of these electrodes upon cycling. It was found that all four types of electrodes behave very similarly. Their SSCV are characterized by narrow peaks which may reflect phase transition between intercalation stages, and the potential-dependent Li chemical diffusion coefficient is a function with sharp minima in the vicinity of the CV peak potentials, in which the differential electrode capacity is maximal. The impedance spectra of these electrodes reflect an overall process of various steps in series. These include Li ion migration through surface films, charge transfer which depends strongly on the potential, solid-state diffusion and, finally, accumulation of the intercalants in their sites in the bulk of the active mass, which appears as a strongly potential-dependent, low-frequency capacitive element. It is demonstrated that the above electroanalytical response, which can be considered as the electrochemical fingerprint of these electrodes, may serve as a good in situ tool for the study of capacity fading mechanisms.