Electrochemical and crystallographic characterization of Co-free hydrogen storage alloys for use in nickel–metal hydride batteries

Electrochemical and crystallographic characterization of Co-free hydrogen storage alloys containing Si and/or Fe was carried out. In the charge–discharge cycle test, the maximum discharge capacity of a Co-free MmNi_4.3Mn_0.4Al_0.3 (Mm=misch metal) alloy electrode was decreased by the partial substitution of Si and/or Fe for Ni, but the cycle performance was improved. The MmNi_3.6Mn_0.4Al_0.3Si_0.1Fe_0.6 electrode showed the most excellent cycle durability in this study. It was found by SEM, EPMA and XRD that Fe and Si contained in these alloys more or less suppressed the dissolution of Al and the lattice expansion with hydrogen absorption, leading to the long cycle life. This effect was strengthened by the simultaneous partial substitution of Si and Fe for Ni in the Co-free alloy.