The microstructures and electrochemical performances of La _0.6Gd_0.2Mg_0.2Ni_3.0Co _0.5-xAl_x (x=0-0.5) hydrogen storage alloys as negative electrodes for nickel/metal hydride secondary batteries

La_0.6Gd_0.2Mg_0.2Ni_3.0Co _0.5-xAl_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 microstructures 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) Ni₃ phase and LaNi₅ phase. The abundance of (La, Mg)₂Ni₇ phase decreases while the abundance of LaNi₅ phase and (La, Mg)Ni₃ 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;bsupesup ^-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.