Phase structure and electrochemical characteristics of CaNi4.7Mn0.3 hydrogen storage alloy by mechanical alloying

In this paper, we study systematically the effect of ball/powder weight ratio on the morphological, structural, and electrochemical properties of CaNi4.7Mn0.3 powder alloy by mechanical milling. The CaNi4.7Mn0.3 alloy powder is elaborated for an optimal milling time of 40 h with 8:1 and 12:1 ball/powder weight ratios. The X-ray diffraction (XRD) characterization shows that the CaNi4.7Mn0.3 alloy powder is characterized by a nanocrystalline/amorphous crystallographic state and exhibits two major Ni and CaNi3 phases irrespective of the ball/powder weight ratio. The CaNi4.7Mn0.3 electrode activates rapidly in the first cycle regardless of the ball/powder weight ratio, and the best value of maximum discharge capacity is obtained for an 8:1 ratio (125 mAh g(-1)). The values of diffusion coefficient/mean grain size squared ratio D-H/a(2), Nernst's potential E-0, and exchange current density I-0 at the first activation cycle are the best for 8:1 ball/powder weight ratio. After activation, the discharge capacity decreases exponentially regardless of the ball/powder weight ratio. Indeed, the capacity loss and the degradation rate after 50th cycle are about 71%, 10.71 cycle(-1) and 53%, 2.95 cycle(-1) for 8:1 and 12:1, respectively. The evolution of the D-H/a(2) ratio, E-0, and I-0 during the cycling is in good agreement with that of the discharge capacity. The highest values of the diffusion coefficient D-H, Nernst's potential E-0, and exchange current density I-0 after 50th cycle are observed for 8:1 ball/powder weight ratio.