Effects of annealing temperature on structure and the electrochemical properties of La0.7Mg0.3Ni2.45Co0.75Mn0.1Al0.2 hydrogen storage alloy

In our endeavor to improve the cyclic stability of the La-Mg-Ni-Co type alloys, the as-cast and annealed La0.7Mg0.3Ni2.45Co0.75Mn0.1Al0.2 hydrogen storage alloys were prepared and the effects of annealing at 1173, 1273 and 1373 K for 8 h on the crystal structures and the overall electrochemical properties of the alloys are reported in this paper. XRD and Rietveld analyses have revealed that the major phases of all the alloys are the (La,Mg)Ni3 phase and the LaNi5 phase. Both the lattice parameters and cell volumes of the (La,Mg)Ni3 phase and the LaNi5 phase increase with increasing annealing temperature, but the phase abundance of these two phases varies differently. It is found that the discharge capacity and the cycle life of the alloys are improved after annealing treatment owing to the variable phase abundance and composition homogenization, respectively. However, high rate dischargeability (HRD), electrochemical impedance spectra (EIS), linear polarization, anode polarization and potential-step studies indicate that the electrochemical kinetics of the alloy electrodes is deteriorated after annealing treatment. This phenomenon can be ascribed to that the composition homogenization and the suppression of the pulverization due to decreasing of the crystal lattice strain and defect after annealing treatment, which increases the charge-transfer resistance of the alloy electrodes and also reduces the rate of the diffusion of hydrogen from the interior of the bulk to the surface.