Effect of microstructure, composition and non-stoichiometry on electrochemical properties of low-Co rare-earth nickel hydrogen storage alloys

A series of low-Co stoichiometric and non-stoichiometric rare-earth nickel hydrogen storage alloys were examined for the possible application as negative electrodes in NiMH batteries. Partial substitution of Co by one transitional metal such as Cu, Fe, Cr or Zn did not effectively improve the electrochemical cycling stability, while a combined replacement of Co by Cr, Cu and Si was very effective in improving the cycling durability though the initial capacity was slightly low as compared with the parent alloy. In this work, the low-Co non-stoichiometric alloys did not appear superior in the capacity, rate capability and the charge-discharge cycling stability. The discharge capacity seems to be determined by the alloying components and compositional homogeneity. The cycling durability was dominated by the microstructure of alloys. The microstructure and chemical composition of alloys were examined and analyzed by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The relationship between the microstructure and electrochemical properties of alloys was discussed. (C) 1998 Elsevier Science S.A. All rights reserved.