Rare Earth Stoichiometric Phase Structure And Electrochemical Performance Of Hydrogen Storage Electrode Alloys

In this paper, the MmNiyy-1.4-xAl0.5Mn0.5Cu0.4Cox(x=0~0.2; y=5.2~5.5) Co-low nonstoichiometric rare earth hydrogen storage alloys were studied. By means of XRD analysis, SEM/EDS investigation and electrochemical measurements, the effects of chemical composition, the composition and the preparation process such as annealing and rapidly quenching on the phase structures and electrochemical properties were studied systemically.For the as-cast and annealed alloys, the results show that the as-cast alloys have a typical coarse dendritic structure, containing the second phase besides the LaNi5 main phase. The second phase distributes along the grain boundary of the main phase and its amount increases with y increases. The as-cast+annealing alloy has nearly the same phase structure as that of the as-cast alloys. The electrochemical performance measurement shows that the as-cast alloys have fewer activation number, only 3 to 4 times is needed to reach the maximum discharge capacity, and the discharge capacity of the alloys decreases with the decrease of the y value. The cycle stability is improved with the increase of y and x value. The alloys annealed at 1000℃ for 10h have higher activation number, while the maximum discharge capacity and the cycle stability have no obvious change. Both the as-cast and annealed alloys have good high-rate discharge ability. Among all the alloy samples prepared, the one with x=0.2,y=5.3, and annealed at 1000℃ for 10h has the best general performance: its maximum discharge capacity Cmax=281.58mAh/g, high-rate discharge ability HRD600=87.5%, and capacity retention S100=88.15% after 100 cycles at 250 mA/g.The results of XRD analysis, SEM/EDS and matalograph observation revealed that melt-spun alloys with different cooling rates (10, 15, 20m/s) consist of single LaNi; phase with CaCus type crystal structure with fine and homogenous cellular and strip structure. The single phase structure is due to the rapid solidification process which can effectively restrain the occurrence of the second phase. Annealed at lower temperature, the alloy remains the single phase, and at higher temperature, it changed to two phases, ie, a second phase appears. The electrochemical performance measurement shows that the melt-spun alloys have the more activation number (usually 6 to 9 times), and the discharge capacity of the melt-spun alloys decreases with increasing the cooling rate. Compared to the as-cast alloys, the mult-spun alloys has lower discharge capacity, higher cycle stability, greatly high-rate discharge ability. After annealing, the activation property is largely improved (only needs 3 to 5 times). Alloys annealed at lower temperature have higher discharge capacity, higher capacity retention. While the ones annealed at higher temperature have relatively lower discharge capacity and capacity retention. The high-rate discharge ability has largely improved with increasing annealed temperature. Both the melt-spun andmelt-spun+annealed alloys have good high-rate discharge ability. Among all the melt-spun and melt-spun+annealed alloy samples prepared, the one with x=0.1,y=5.2, and annealed at 400℃ for qh has the best general performance: its activation number 5 times, the maximum discharge capacity Cmax=280.38mAh/g, high-rate discharge ability HRD600=90.76%, and capacity retention S100=93.5% after 100 cycles at 250 mA/g.It is seen therefore that nonstochiometric hydrogen storage alloys after melt-spun and low temperature annealing will have an improved cycle stability with decreased usage of cobalt metal.