Petrus Henricus Laurentius Notten

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.

Low-Pressure Chemical Vapor Deposition of LiCoO2 Thin Films: A Systematic Investigation of the Deposition Parameters

The feasibility of volatile precursor low-pressure chemical vapor deposition (LPCVD) for the production of LiCoO2 cathodes for all solid-state microbatteries was examined. To test this feasibility, and gain insight into the deposition behavior, the influence of the deposition parameters on the properties of LPCVD grown thin-film LiCoO2 cathodes was systematically investigated. The deposition temperature, concentration of the various reactants, and duration of thin-film growth were varied. The resulting LiCoO2 layers were subjected to X-ray diffraction, inductively coupled plasma-atomic emission spectrometry, Rutherford backscattering spectroscopy, and electrochemical analyses. Stoichiometry of the films could be controlled by varying the precursor flows. Samples deposited at high temperatures with the optimum stoichiometry showed a high crystallinity and a high electrochemical activity; a storage capacity corresponding to a reversible Li-content around the theoretical value of 0.55 per Co was reached, and a good cycling stability was obtained when using this electrode in combination with a solid-state electrolyte.

Electronic Network Modeling of Rechargeable Batteries I. The Nickel and Cadmium Electrodes

This work introduces electronic network modeling as a new way of simulating battery behavior. Mathematical descriptions are given for the Ni and Cd electrode charge/discharge reactions. These mathematical descriptions are subsequently introduced in the form of electronic components, combined in an electronic network. The advantage of this way of modeling is that it provides a compact electronic model of a battery, which can be used in any electronic-circuit simulation, providing a tool to develop new battery-management systems. In addition, due to the discretization of the electrochemical reactions into separate electronic components, local currents and overpotentials occurring in the system can be studied. This information can be used to investigate the origin of certain battery behavior.

Thermodynamic properties of non-stoichiometric LaNix−1CuH systems

Abstract A series of non-stoichiometric LaNi x−1 Cu intermetallic compounds with x = 5.0 to 6.0 has been prepared and characterized by X-ray diffraction and shown to be single phase. The thermodynamics of hydrogen absorption and desorption by these compounds have been determined from their equilibrium hydrogen pressures and from the calorimetric measurement of their heats of reaction with H 2 (g). For x > 5.4 the system appears to be above its critical composition for the co-existence of the dilute and hydride phases, in agreement with earlier results obtained from in situ X-ray diffraction. As x increases from 5.0 to 6.0, the hydrogen pressures corresponding to the plateaux, or else to the “plateau-like” regions, increase and the hydrogen capacities decrease. For the plateaux and plateau-like regions the calorimetrically measured enthalpies of reaction are found to increase in magnitude with x , whereas the corresponding entropy changes are nearly independent of x , and hence the enthalpy changes cause the observed increase of p H 2 . The hysteresis decreases with increasing x .