Development of AB 2-type Zr–Ti–Mn–V–Ni–Fe hydride electrodes for Ni–MH secondary batteries

Abstract

A series of multicomponent Zr 0.5Ti 0.5Mn 0.4V 0.6Ni 1− x Fe x ( x=0.00, 0.15, 0.30, 0.45 and 0.60) alloys are prepared and their crystal structure and P– C– T curves are examined. The electrochemical properties of these alloys such as discharge capacity, cycling performance and rate capability are also investigated. Zr 0.5Ti 0.5Mn 0.4V 0.6Ni 1− x Fe x alloys ( x=0.00, 0.15, 0.30, 0.45 and 0.60) have the C14 hexagonal Laves phase structure. Their hydrogen storage capacities do not show significant differences. The discharge capacity just after activation decreases with the increase in the amount of the substituted Fe but the cycling performance is improved. The discharge capacity after activation of the alloy with x=0.00 is about 240 mAh/g at the current density 60 mA/g. Zr 0.5Ti 0.5Mn 0.4V 0.6Ni 0.85Fe 0.15 is the best composition with a relatively large discharge capacity and a good cycling performance. The increase in the discharge capacity of Zr 0.5Ti 0.5Mn 0.4V 0.6Ni 0.85Fe 0.15 with the increase in the current density (from 60 to 125 mA/g) is considered to result from the self-discharge property of the electrode. During activation Ni-rich regions form on the electrode surface, which may act as active sites for the electrochemical reaction. The formation of more minute cracks in the large particles of the alloys with higher Fe content is considered to result from the more severe destruction of the crystal structure due to more dissolution of zirconium and iron into the solution.