The electrochemical properties of the mechanically alloyed Mg35Ti10-xCrxNi55 (x=5, 7, 9) electrode alloys

The mechanically alloyed (MA) Mg35Ti10-xCrxNi55 (x=5, 7, 9) electrode alloys were investigated for their electrochemical performance. It was found that when the Cr content was low, with the increase of Cr, the maximum discharge capacity decreased gradually, while the cycling capacity retention was improved significantly. X-ray photoelectron spectroscopy (XPS) surface analysis revealed that the Cr content in alloys effectively depressed the oxidation of metallic elements behind the passivation layer, which was built up during corrosion at the surface. Further XPS and Auger electron spectroscopy (AES) studies revealed that on the external side of the surface of Mg35Ti5Cr5Ni55 a multi-component (Cr2O3)k(TiO2)l(NiO) m(Mg(OH)2)n composite passivation layer was formed, which was much thinner (nearly 150 nm) and more compact than the layer formed on the surface of Mg35Ti10Ni55. The corrosion current density Icorr of the alloys decreased also with the augmentation of Cr content. However, Cr in the alloy was found to cause an increase of the reaction resistance at the surface and at the same time a reduction in the surface reaction activty. For the optimum performance of the alloy, the amount of Cr substitution was suggested to be around 5 at.%. At this level of Cr substitution, Icorr is still too high for practical usage. The formation a more compact, less pervious to OH- and electrochemically less active surface passivation layer due to the introduction of Cr is believed to be the main cause for the improvement of the cycling stability, the reduction of exchange current density and the deterioration of surface reaction activity. © 2003 Elsevier B.V. All rights reserved.