A study on the improvement of the cyclic durability by Cr substitution in V–Ti alloy and surface modification by the ball-milling process

Abstract

The Ni–MH battery has been developed as a potential power source with a high energy density and excellent performance for mobile electrical appliances and hydride electric vehicles (HEVs). Research and development are, however, still essential for further improvement of the energy density and cycling life. Recently, vanadium-based solid solution hydrogen storage alloys have been considered as promising candidates for negative electrode materials because of their higher reversible hydrogen storage capacities. However, for the practical use of V-based alloy as a negative electrode, it is necessary to improve both the electrocatalytic activity and cyclic durability in alkaline solution. As a novel method for changing the surface properties of an alloy electrode to obtain a higher electro-catalytic activity, we have employed the ball-milling process with Ni powder without deterioration of the alloy bulk properties. In order to overcome the poor cyclic durability of V–Ti alloy, elemental Cr, which forms a protective oxide layer against corrosion in alkaline solution, was partially substituted for V. Through systematic experimentation, a V 0.68Ti 0.20Cr 0.12 alloy electrode ball-milled with Ni powder was found to have an advanced discharge capacity of 420 mAh/g and a good cycle life (80% after 200 cycles). For further promotion of the surface catalytic activity, instead of normal Ni powder (spherical Ni powder), filamentary Ni with a larger specific surface was used as a new surface modifier. Under optimum ball-milling conditions (7 wt% filamentary Ni for 25 min), V 0.68Ti 0.20Cr 0.12 alloy was effectively coated with Ni particles and showed an increased discharge capacity of 460 mAh/g.