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LaNi 5 and related substituted alloys belong to the most interesting electrode materials for rechargeable Ni/MH batteries, because large quantities of hydrogen can be rapidly and reversibly stored in these compounds. However, the storage capacity decreases during repeated charge/discharge cycles. This degradation process is closely related to corrosion of the alloy in the aqueous KOH electrolyte and is enhanced by decrepitation (fissuring of the powder particles). The latter phenomenon is the consequence of volume changes resulting from the hydrogen solid solution/hydride phase transformation upon cycling. 1 The exact mechanisms governing the initiation and propagation of alloy corrosion depend on the material's chemical composition and microstructure, on the nature of the electrolyte, and the cycling conditions and are controlled through a complex interplay between various chemical reaction and matter/charge-transport steps at an atomic scale. Accurate prediction of the lifetime behavior of Ni/MH batteries is hindered by a lack of knowledge about corrosion scale structure and chemistry, as well as about its time evolution during corrosion. Predictive models for the specific corrosion mechanisms and their interplay, together with a fundamental understanding of the processes, can be obtained only by systematic experimental analysis of the corrosion products. In the present work, a substituted AB 5 alloy with minimal decrepitation upon cycling was chosen to study corrosion mechanisms and kinetics. The alloy was cycled in a model cell for different time and temperature conditions. Corrosion products were identified by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In the past, several similar studies were made by surface analysis techniques with excellent depth resolution but poor lateral resolution. In this study, TEM offers the advantage to provide both high lateral and spatial resolution and yields insight about the corrosion scale features at the atomic level. Corrosion models are developed from the corrosion scale microstructure and time evolution.Experimental Description of alloy, electrodes, and cells.-The material studied in the present work was a mechanically ground industrial alloy annealed for 6 h at 1100ЊC with an average particle diameter of 40 m.Its crystal structure was hexagonal (CaCu 5 type) with lattice parameters a ϭ 0.4988 nm and c ϭ 0.4055 nm. The alloy was composed of rare earth metals (La, Ce, Pr, and Nd), nickel, cobalt, manganese, and aluminum (exact chemical composition given in Table I). The alloy was almost homogeneous, only a few rare earth precipitates being present (bright contrast in the SEM image of Fig. 1). Lanthanum was highly concentrated in these precipitates, probably due its larger atomic radius compared to the other Mm elements in presence.For forming the negative MH electrodes, a slurry of metal powder, carbon, and binder was pasted into a foam nickel substrate, dried, and compressed (details are given in Ref. 2). MH n...

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The degradation mechanism of MmNi,,,Co,,,Mn,4A1,, hydrogen storage alloy in nickel-metal hydride (Ni-MH) sealed batteries and effects of corrosion on cell performance during cycling are investigated. A general equation for the corrosion of AB, alloy is derived from analyses of corrosion products in cycled electrodes and measurements of negative discharge reserve. Water consumption due to alloy corrosion explains the increase of impedance during cycling while internal pressure evolution is directly linked to the loss of charge reserve. The parameters controlling the corrosion rate of AB, alloys are also examined. It is shown that the pulverization mode of the alloy particles rather than the surface reactivity to electrolyte explains the differences in cycle-life performance of alloys of various compositions. InfrocluctionAmong different electrochemical systems, Ni-MH batteries are one of the most promising energy sources for cordless appliances and electric vehicles (EVs) because they are nonpolluting and have higher energy-storage capabilities than Ni-Cd batteries. The use of sealed Ni-MR batteries in ENs or mobile phones requires good stability of the impedance during the whole cycle life as well as a limited increase of internal pressure at the end of charge. These two characteristics can be achieved by using hydrogen storage alloys with low corrosion rates.The first purpose of this study was to measure corrosion rates of AB, alloys during cycling and evaluate the consequences of this corrosion on sealed Ni-MR battery performance. The second purpose was to determine the main parameters responsible for AB, corrosion and particularly to separate the effects of the reactivity of the surface and the decrepitation phenomenon (particle pulverization), which continuously generates new surfaces. ExperimentalPreparation of alloy powders, electrodes, and cells.-Alloy ingots of composition MmNi,55Coai,Mn,4A1,, were prepared by melting Mm (Ce 50%, La 30%, Nd 15%, Pr 5%), Ni, Co, Mn, and Al in an induction furnace. After annealing, the ingots were mechanically ground into a powder of 35 jim in average diameter. A slurry containing metal hydride powder, a carbon black powder of high specific surface area, and styrene butadiene rubber was pasted into a foam nickel substrate. A carbon coating was then applied on the MR electrodes in order to improve their oxygen recombination ability. Positive electrodes were prepared by filling a nickel foam substrate with an active material consisting of Ni(OH), and cobalt compounds as conductive agents. Negative and positive electrodes as well as a polyamide separator were spirally wound to form cylindrical sealed cells (4/5 A size). Average capacity of the cells was 1.6 Ah at 1 C. The electrolyte was 8.7 M KOH-0.5 M NaOH-0.7 M LiOH.Electrical evaluation of the cells.-Cycle life was evaluated at room temperature using the following operating conditions: charge at 1 C for 1.2 h; discharge at I C to a cutoff potential of 0.9 V. Cells were periodically sampled for internal impedance, corrosion...