Phase structure, crystallography and electrochemical properties of Laves phase compounds Ti0.8Zr0.2V1.6Mn0.8−xMxNi0.6 (M=Fe,Al,Cr,Co)

The effects of partial substitution of Mn by M (M=Fe,Al,Cr,Co) on the phase structures, crystallography and electrochemical properties of Laves phase compounds Ti0.8Zr0.2V1.6Mn0.8−xMxNi0.6 (M=Fe,Al,Cr,Co) have been studied. It is found by X-ray powder diffraction and energy dispersive X-ray spectrometer analysis that most alloys are composed of a AB2-type hexagonal structure C14 Laves phase matrix and a BCC structure TiNi-based secondary phase alloy and some of them contain another BCC phase. The optical photomicrograph shows that the C14 Lave phase forms a continuous matrix and the TiNi-based secondary phase intersperses among it in the form of dendritic structures. With the substitution of Mn by Fe and Cr, the dendrites are larger, while Co and Al substitutions make the dendrites smaller. Electrochemical studies indicate that Cr substitution improves the cyclic durability of the alloys greatly despite the decrease of discharge capacity, which can be attributed to the suppression of surface segregation of Ti, Zr and V elements by the presence of Cr, which suppresses the oxidation and the dissolution of V and the oxidation of Ti and Zr greatly as evidenced by the XPS analysis. Other element substitutions such as the substitution with Fe, Al and Co do not have significant effect on the alloy electrode performance.