Metallic Hydrides II: Materials for Electrochemical Storage
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Metallic Hydrides II: Materials for Electrochemical Storage J.-M. Joubert, M. Latroche, and A. Percheron-Guégan Abstract For a century, nickel-cadmium (Ni-Cd) batteries have been widely used as electrochemical energy-storage cells. However, due to the rapid development of portable electronic devices and the increasing search for cleaner electric vehicles, new generations of batteries have been investigated during the last few decades. Among them, nickel metal hydride (Ni-MH) batteries, with their larger capacities and improved environmental compatibility, have shown their ability to replace Ni-Cd cells. The negative electrodes of Ni-MH batteries are made of reversibly hydride-forming intermetallic compounds. In this article, the crystallographic and thermodynamic properties of these compounds will be reviewed. Their hydrogen-absorption properties, their electrochemical performance, and the solutions that have been found to achieve reliable cycle life will be presented. The industrial market for Ni-MH batteries will also be discussed in comparison with other battery systems. Keywords: aging, alloys, energy-storage materials, hydrogen storage, intermetallic compounds, metallic hydrides.
Introduction The exceptional hydrogen-storage properties of LaNi5, which is able to store up to 6.7 hydrogen atoms per formula unit (H/f.u.), were discovered in 1970.1 Numerous applications were found for this compound and its derivatives, ranging from hydrogen-gas storage to thermal systems such as heat pumps or hydrogen gettering (for a review, see Reference 2). In 1973, Ewe et al. reported the use of this alloy as the negative electrode in an alkali medium,3 and the first patents for battery applications were filed a few years later.4 However, the high equilibrium pressure and poor cycle life of LaNi5 make it unusable in practical batteries.5 In this article, the structural and thermodynamic properties of LaNi5-derived, so-called AB5 metallic hydrides will be reviewed, and we will show how the equilibrium pressure of the hydride may be adapted to the electrochemical application. Then, the principal causes for degradation will be presented, 694
together with solutions adopted to overcome this problem. Other hydride-forming compounds like AB2 Laves phases will be briefly described. Finally, the performance of nickel metal hydride (Ni-MH) batteries will be discussed in comparison with other systems.
Structural and Thermodynamic Properties of AB5 Intermetallic Compounds and Hydrides Structure of AB5 and Related Substituted and Nonstoichiometric Compounds AB5 intermetallic compounds used for electrochemical applications crystallize in the hexagonal structure described for LaNi5 (P6/mmm space group, CaCu5 structure type) with A(1a) in (0, 0, 0), B(2c) in (1/3, 2/3, 0), and B(3g) in (1/2, 0, 1/2). Both the La and Ni atoms can be substituted with other elements while maintaining the hexagonal LaNi5 parent structure.
Lanthanum is easily substituted with other 4f elements in the whole range of concentration leading to a complete solid so
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