Abstract
Intermetallic alloys of general formula AB5 (A, rare-earth or transition metal; B, transition metal) are able to store large amounts of hydrogen to form intermetallic hydrides. The absorption/desorption reaction is reversible in a large domain of temperature and pressure and therefore these compounds have been developed for electrochemical storage. These materials lead to large capacity electrodes in NiOOH/Ni(OH)2|KOH|AB5/AB5Hx-type batteries. It has been shown that partial substitutions on either the A or/and B sites allow one to adjust the thermodynamic properties of these hydrides. The LaNi5 compound is able to store more than 6H mol−1 at room temperature. However its equilibrium pressure is above the atmospheric one which is not suitable for electrochemical application. By substitution on the Ni sites by elements such as Al, Mn or Co, it is possible to decrease the plateau pressure and to improve the cycle life without significant loss of storage properties. Moreover, a multi-component compound with nominal composition LaNi3.55Mn0.4Al0.3Co0.75 exhibits a very good resistance to corrosion. It is noteworthy that substitution by the Mn, Al or Co elements alone does not lead to such an improvement. In order to understand the role played by each element, a study has been performed by in-situ neutron diffraction studies. This technique allows a bulk analysis of the working electrode in a geometry very close to that of commercial batteries. It gives crystallographic information (phases involved in the reaction, proportions, cell-volume evolutions) as well as kinetic information (system evolution depending on charge rates). We will present the results obtained on LaNi5−xMx (M = Al, Mn, Co) and LaNi3.55Mn0.4Al0.3Co0.75 electrodes in different absorption/desorption processes. Crystallographic, electrochemical and kinetic behaviours of these electrodes will be discussed in relation to the nature of the substituent.
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