Electrochemical and surface properties of iron-containing AB(5)-type alloys

LaNi5-type metal hydride electrodes can attain a long cycle life in rechargeable batteries by partial substitution of Ni by expensive Co. We have investigated the surface and electrochemical properties of (La,Mm)Ni-5-x(Fe,Al,Mn,Cu)(x)-type alloys with regard to battery applications. We found that partial substitution of nickel by iron did not increase the durability of LaNi5. The initial capacity of LaNi4.5Fe0.5 was 320 mA h g(-1), while after 200 charge-discharge cycles it decreased to 130 mA h g(-1). With the combined substitution of iron and aluminium the durability increased. A more dramatic improvement to the stable alloys was attained for mischmetal- based alloys (e.g. MmNi(3.6)Fe(0.7)Al(0.3)Mn(0.4)). Deep discharges and cycling at 40 degrees C caused only a minor capacity decrease for these alloys. Surface analysis using X-ray photoelectron spectroscopy (XPS) of LaNi4Fe and LaNi4.2Fe0.5Al0.3 showed significant changes in the surface composition of electrochemically cycled alloys. The surface changed from being lanthanum oxide rich to nickel rich. No aluminium or iron enrichment was found in the cycled alloys. The content of metallic nickel was lower than observed previously on the surface of LaNi1-xSix alloys. LaNi4.2Fe0.5Al0.3 with good cyclic stability also showed a higher metallic Ni content in the subsurface layer. For all analysed samples, lanthanum was oxidized almost throughout the entire sputtering depth, whereas nickel became metallic at a depth of 0 to 60 Angstrom and iron at a depth of 50 to 400 Angstrom.