How to achieve long-term electrochemical cycling stability with hydride-forming electrode materials

The long-term electrochemical cycling stability of AB5-type compounds in alkaline media can be improved either by lowering the specific surface area of the applied hydride-forming powder or by lowering the oxidation rate constant of the intermetallic compound. Both parameters are shown to be intrinsic material properties. Reducing the specific surface area can be accomplished by making use of non-stoichiometric compounds in which the A-type atoms in the crystal lattice are partly replaced by dumbell pairs of B-type atoms. Electrochemically stable non-stoichiometric compounds are characterized by the absence of a clear discrete α-to-β phase transition upon hydridization, through which particle size reduction of the powder during electrochemical activation remains limited. Compounds with a composition close to or above the as-denoted “critical composition” meet this requirement. On the other hand, lowering the oxidation rate constant can successfully be achieved by replacing part of the A-type atoms by other lanthanides and/or part of the B-type atoms by other transition metals within the AB5 stoichiometry. This leads to so-called multicomponent compounds. Although the hydride-formation/decomposition reaction mechanism for this type of compound is shown to be accompanied by a pronounced discrete phase transition, a relatively low oxidation rate constant ensures a good overall electrochemical cycling stability.