Effects of mechanical grinding on the hydrogen storage and electrochemicalproperties of LaNi5

LaNi5 powders were mechanically ground under argon atmosphere. After 2 h of milling, a limit particle size of 1.5 mu m was obtained. This powder was e asily activated under hydrogen atmosphere and the time for a 90% maximum hy drogen uptake was found to be close to that of the unmilled powder. The fir st hydrogenation cycle of the milled powder shows a strong slope indicating the presence of several hydrides. The hydrogenated ground powder was stabi lized with a carbon monoxide surface treatment that markedly slows down the desorption kinetics. From the ball milled and hydrogenated powder, the for mation of the intermediate phase LaNi5Hx with x approximate to 3 is observe d, contrarily to what is found with hydrogenated but unground powders, such a different behavior was questioned in terms of electrochemical conditions , as milled or unmilled LaNi5 powders are intensively used as the negative electrode of reversible metal hydrogen batteries. The unmilled but activate d LaNi5 electrode exhibits the lowest discharge capacity but the best cycle life time (loss of only 5% of the discharge capacity after 50 cycles). The powder milled for only 1 h before activation does not present any intermed iate hydride and shows a better discharge capacity. The best performance is obtained for the powder containing the intermediate phase with a discharge capacity of 307 mA h g(-1). Nevertheless, the loss in capacity after 50 cy cles is large (24%). (C) 1999 Published by Elsevier Science S.A. All rights reserved.