SURFACE AND BULK PROPERTIES OF LANI5-XSIX ALLOYS FROM THE VIEWPOINT OF BATTERY APPLICATIONS

Because of their rather low hydrogen storage capacity, LaNi5-xSix alloys have not been studied well up to now and not considered for battery applications. We have found, however, that the alloy LaNi4.5Si0.5 shows fast activation and an incredibly high stability upon electrochemical cycling (-0.007 m A h g-1 cycle-1) with a fairly high electrochemical capacity (245 m A h g-1). X-ray diffraction, pressure-composition isotherms and electrochemical measurements have been carried out on LaNi4.7Si0.3 and LaNi4.5Si0.5. By means of photoelectron spectroscopy we have analysed the composition and thickness of surface layers formed on LaNi4.5Si0.5 powder electrodes after various pretreatments. Electrochemical cycling in 6 M KOH electrolyte forms a sublayer rich in metallic nickel. Contrary to earlier suggestions, no passivating silicon surface oxide film could be found on cycled LaNi4.5Si0.5 electrodes. We attribute the fast activation of these alloys to the very thin oxide layer and to metallic nickel just under the surface, and we attribute the high corrosion resistance to the low volume expansion upon hydriding and the compact surface layer which is therefore formed.