Influence of the material processing on the electrochemical properties of cobalt-free Ml(NiMnAlFe)5 alloy

Co-free MlNi(4.1)Mn(0.35)Al(0.3)Fe(0.25) (Ml: La-rich mischmetal) alloy samples were prepared in three different ways. The original as-cast alloy was prepared by vacuum levitation melting in a cold crucible. One-third of the as-cast ingot was annealed at 1273 K for 10 h, and another one-third was remelted and rapidly solidified by melt-spinning. The influence of the alloy preparation methods on their microstructure and electrochemical properties was studied. XRD and SEM results revealed that all of the three differently prepared alloys were of the single CaCu5-type structure phase, but their microstructure and electrochemical properties were changed markedly. The as-cast alloy had a typical dendrite structure with noticeable composition segregation and rather poor cycling endurance. While the annealed and melt-spun alloy were of an equiaxed structure and a very fine cellular structure, respectively, and had a more homogeneous composition and dramatically improved cycling endurance, although their activation rate and high-rate dischargeability were lowered somewhat. In this study, the melt-spun Co-free alloy showed the best cycling stability (S-500 = 69.2%) and a reasonably high capacity (305 mA h g(-1)) and 1C rate dischargeability (85.6%), which is attributed to its lower degree of pulverization and more uniform composition. (C) 2002 Elsevier Science B.V. All rights reserved.