The microstructures and electrochemical performances of La_(0.6)Gd_(0.2)Mg_(0.2)Ni_(3.0)Co_(0.5-x)Al_x (x=0-0.5) hydrogen storage alloys as negative electrodes for nickel/metal hydride secondary batteries

摘要

La_(0.6)Gd_(0.2)Mg_(0.2)Ni_(3.0)Co_(0.5-x)Al_x (x = 0-0.5) hydrogen storage alloys were prepared by induction melting followed by annealing treatment at 1173 K for 8 h. The effects of substitution Al for Co on the micro-structures and electrochemical performances were studied systematically. The structure analyses show that all alloys consist of multiphase structures such as (La, Mg)_2Ni_7 phase, (La, Mg) Ni_3 phase and LaNi_5 phase. The abundance of (La, Mg)_2Ni_7 phase decreases while the abundance of LaNis phase and (La, Mg) Ni_3 phase increases directly as the Al content increasing. The electrochemical tests show that the maximum discharge capacity of alloy electrodes are almost unchanged when x ≤ 0.2 while the cyclic stability of the alloy electrode are improved significantly after proper amount of Al substitution for Co. The alloy electrode with x = 0.1 exhibits the better balance between discharge capacity and cycling life than any others. Moreover, at the discharge current density of 900 mA g~(-1), the high rate dischargeability (HRD) of the alloy electrodes decreases with increasing Al substitution and the relative analyses reveal that the charge transfer on alloy surface is more important than the hydrogen diffusion in alloy bulk for the kinetic properties of the alloy electrodes.