The electronic and electrochemical properties of the TiFe-based alloys

Ti(Fe,M)-based alloys (M=Ni, Mo, Cr, Co) were mechanically alloyed (MA) under an argon atmosphere to synthesize nanostructured materials. XRD analysis showed that, after 25 h of milling, the starting mixture of the elements had decomposed into an amorphous phase. Following the annealing in high purity argon at 750 degreesC for 0.5 h, XRD confirmed the formation of CsCl-type structures with crystallite sizes of about 30 nm. These materials were used as negative electrodes for a Ni-MHx, battery. The alloying elements of the 3d transition metals, Ni, Mo, Cr and Co, were substituted for iron atoms, and the structural, electronic and electrochemical properties were studied. With increasing nickel content in TiFe1-xNix, the material showed an increase in discharge capacity which passed through a maximum for x=3/4. In the nanocrystalline TiNi0.6Fe0.1Mo0.1Cr0.1Co0.1 powder, a discharge capacity of up to 135 mA h g(-1) (at 40 mA g(-1) discharge current) was measured. For this composition the capacity degraded much more slowly with cycling in comparison to other studied compositions. The electronic structure was studied by the tight-binding version of the linear muffin-tin method in the atomic sphere approximation (TB-LMTO ASA). In the TiFe1-xNix alloys, increasing the content of the Ni impurities extended the valence bands and increased the density of states at the Fermi level. Similar effects were observed for the TiNi1/2Fe1/8Mo1/8Cr1/8Co1/8 system. Mechanical alloying proved to be a suitable procedure for obtaining TiFe-based alloy electrodes for Ni-MHx batteries. (C) 2002 Elsevier Science B.V. All rights reserved.