The hydrogenation characteristics of Ti-Zr-V-Mn-Ni C14 type Laves phase alloys for metal hydride electrodes

In order to develop high discharge capacity negative electrode materials of Ni-MH batteries, Ti-Zr-V-Mn-Ni AB(2) type Laves phase alloys have been developed. In Ti-Zr-V-Mn-Ni alloys with single C14 Laves phase alloys, gaseous hydrogen storage capacities are 1.44-1.71 wt.% H-2 galloy g(-1) at 50 degrees C and discharge capacities are 363-464 mAh g(-1) at a current density of 50 mA g(-1) at 30 degrees C in proportion to the Ni concentration. However, the cycle life of these alloys were found to be poor. In order to examine the reason for the initial abrupt degradation of Ti-Zr-V-Mn-Ni alloys, surface morphologies, surface concentration profiles and the polarization resistance of the electrodes were investigated. It was found that a Ti-oxide film formed on the degraded electrode surface and the polarization resistance of the degraded electrode increased greatly. These results indicate that the degradation of Ti-Zr-V-Mn-Ni alloys is caused by the Ti-oxide film, which induces an increase of the polarization resistance, including the resistance of the hydrogen diffusion through the Ti-oxide film and the resistance of electron conduction between electrodes and external circuits as well as between alloy particles.