Investigation of the structural and electrochemical properties of superstoichiometric Ti-Zr-V-Mn-Cr-Ni hydrogen storage alloys

In this paper, the structural and electrochemical properties of the superstoichiometric (Ti_0.8Zr_0.2)(V_0.533Mn_0.107Cr_{0.1 6}Ni_0.2)_x (x = 2, 3, 4, 5, 6) hydrogen storage alloys have been studied systematically. It is found by X-ray diffraction and energy dispersive spectra analysis that all these alloys mainly consist of two phases, a C14 Laves phase with hexagonal structure and a V-based solid solution phase with body-centered cubic structure. The lattice parameters and thus the cell volumes of the two phases all decrease when x is increased. The electrochemical measurements indicate that the maximum discharge capacity, the discharge equilibrium potential, the high rate dischargeability, the cyclic stability, the exchange current density I₀, and the limiting current density I_L of the alloys all increase with increasing x from 2 to 5. When x reaches 6, the discharge equilibrium potential, the high rate dischargeability, and the cyclic stability are still increasing proportionately, while the maximum discharge capacity, the exchange current density I₀, and the limiting current density I_L all decrease. Furthermore, the alloy electrodes are activated with more difficulty for the alloys with higher stoichiometry x. Consequently, we believe that the superstoichiometry is an effective way to improve the overall electrochemical properties of the Ti-based Laves-phase hydrogen storage alloys used for the negative electrodes of the Ni-MH secondary batteries.