Effects of Composition in La/Ni-Based Intermetallic Compounds Used as Negative Electrodes in Ni-MH Batteries

Abstract

The choice of AB_n (A=Y, rare earth, B=transition metal, 1≤n≤5) intermetallic compounds as hydrogen storage materials comes from their ability to react with hydrogen gas to form intermetallic hydrides. From the point view of the electrochemical storage application, LaNi₅ is the most promising negative electrode material for rechargeable Ni-MH batteries because of the rapid and reversible storage of large quantities of hydrogen [1]. However the practical application of LaNi₅ is limited both from a thermodynamic and an electrochemical point of view: the hydride formation pressure is too high (~ 2.3 bars) and the capacity loss of such an electrode upon cycling is too large for practical application (50% loss of capacity upon 100 cycles). Such a decline has been commonly ascribed to corrosion of the alloy leading to La(OH)₃ and nickel particles. It is nevertheless possible to overcome such limitations by changing the composition of the parent LaNi₅ alloy. Partial substitutions on the lanthanum and the nickel sites have been proved to be very successful in adapting the thermodynamic properties of the hydrides and improving their life time upon cycling and the best compromise has been found for the three substituted compound [2]. Recently, it has also been discovered that cycling stability can also be significantly improved by leaving the stoichiometric AB5 composition and making use of over stoichiometric AB_x compounds, with x>5 [3]. However, it remains that in spite of such effects of the composition on the long-term electrochemical behaviour, little is known about its origin and the specific effect of each substituent. The fundamental investigation of the single substituted alloys is therefore of great interest to get a better insight into the relationships between the chemical composition and the electrode properties.