Properties of mechanically alloyed Mg―Ni―Ti ternary hydrogen storage alloys for Ni-MH batteries

摘要

MgNiTi_x, Mg_(1-x)Ti_xNi and MgNi_(1-x)Ti_x (with x varying from 0 to 0.5) alloys have been prepared by high energy ball milling and tested as hydrogen storage electrodes. The initial discharge capacities of the Mg―Ni―Ti ternary alloys are inferior to the MgNi electrode capacity. However, an exception is observed with MgNi_(0.95)Ti_(0.05), which has an initial discharge capacity of 575 mAh/g compared to 522 mAh/g for the MgNi electrode. The Mg-Ni-Ti ternary alloys show improved cycle life compared to Mg-Ni binary alloys with the same Mg/Ni atomic ratio. The best cycle life is observed with Mg_(0.5)Ti_(0.5)Ni electrode which retains 75% of initial capacity after 10 cycles in comparison to 39% for MgNi electrodes, in addition to improved high-rate dischargeability (HRD). According to the XPS analysis, the cycle life improvement of the Mg_(0.5)Ti_(0.5)Ni electrode can be related to the formation of TiO_2 which limits Mg(OH)_2 formation. The anodic polarization curve of Mg_(0.5)Ti_(0.5)Ni electrode shows that the current related to the active/passive transition is much less important and that the passive region is more extended than for the MgNi electrode but the corrosion of the electrode is still significant. This suggests that the cycle life improvement would be also associated with a decrease of the particle pulverization upon cycling.