Robert E. F. Einerhand

On the nature of the electrochemical cycling stability of non-stoichiometric LaNi5-based hydride-forming compounds Part I. crystallography and electrochemistry

Single-phase, non-stoichiometric La(Ni/Cu)x compounds (5.0⩽x⩽6.0) have been prepared by annealing the solids at the appropriate temperatures within the homogeneity regions of the materials phase diagrams. The effects of both the non-stoichiometric composition and the chemical composition (Ni-to-Cu ratio) on the crystallographic and electrochemical properties have been investigated. A special substitutional mechanism is presented which can account for the crystallographic data. This mechanism involves the partial replacement of La atoms by dumbbell pairs of Ni atoms, whereas Cu atoms are argued to occupy preferentially the crystallographic positions surrounding these dumb-bells. Electrochemical parameters, such as the storage capacity, cycling stability and discharge efficiency, have been determined and are found to be strongly dependent on both the non-stoichiometric and the chemical composition of the compounds. Microscopic investigations of the electrochemically cycled electrodes revealed an unequivocal correlation between particle size reduction and cycling stability. A model is proposed which can account for the cycle life behaviour of these non-stoichiometric compounds. This model, in which the electrode surface area and the materials oxidation rate constant play an essential role, has been tested using electrodes with different surface areas.

On the nature of the electrochemical cycling stability of non-stoichiometric LaNi5-based hydride-forming compounds Part II. In situ x-ray diffractometry

Abstract In order to elucidate the origin of the mechanical stability of ternary non-stoichiometric compounds, the influence of hydrogen gas absorption-desorption on the crystallographic lattice expansion has been investigated in single-phase LaNi χ and LaNi χ-1 Cu compounds (5.0 ⩽ χ ⩽ 6.0) by means of in situ X-ray powder diffractometry. The extent of the hydrogen plateau region where the α and β phases coexist is strongly dependent on both the non-stoichiometric and the chemical composition of the compounds. The discrete α-β lattice expansion rather than the total lattice expansion is found to be essential for the mechanical stability of the hydride-forming powder particles. Materials which are characterized by a large discrete lattice expansion suffer from a considerable particle size reduction, whereas materials lacking such a pronounced discrete expansion are found to be mechanically much more stable. Transition between these types of materials is characterized by the proposed critical composition. The critical composition for the investigated ternary non-stoichiometric system has been determined to be close to LaNi 4.4 Cu.

HOW TO ACHIEVE LONG-TERM ELECTROCHEMICAL CYCLING STABILITY WITH HYDRIDE-FORMING ELECTRODE MATERIALS

Abstract The long-term electrochemical cycling stability of AB 5 -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 AB 5 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.