Structural and chemical analyses of the new ternary La5MgNi24 phase synthesized by Spark Plasma Sintering and used as negative electrode material for Ni-MH batteries

In the present work, starting from the nominal composition La0.85Mg0.15Ni3.8, samples have been synthesized by SPS (Spark Plasma Sintering) technique at different temperature from 810 °C to 900 °C. The crystallographic structures as well as the phase compositions have been studied by X-ray diffraction (XRD) and Electron-Probe Micro Analysis (EPMA). A new ternary La5MgNi24 phase with stacking structure (space group R-3m) has been identified and its structure determined by XRD analysis. This (1:4) phase forms at higher temperature than the (5:19) and (2:7) ones. In the present work, the phases (2:7); (5:19); (1:4) and (1:5) coexist for all samples. The substitution of La by Mg only occurs in the layer corresponding to the Laves phase for all the stacking structure phases. The substitution rate (La/Mg ratio in the Laves layer) is equal to half and does not change with the SPS temperature treatment. The hydrogen storage capacity and the electrochemical capacity are not too much influenced by the SPS temperature. In contrast, the cycling stability shows better resistance to corrosion for the samples containing larger amount of (5:19) phase.

► La0.85Mg0.15Ni3.8 samples have been synthesized by SPS at different temperatures.
► A new ternary La5MgNi24 phase with stacking structure has been identified.
► Relationship with H2 storage capacity, electrochemical one and phase fraction is discussed.