Effects of mechanical grinding on the hydrogen storage and electrochemical properties 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 easily activated under hydrogen atmosphere and the time for a 90% maximum hydrogen uptake was found to be close to that of the unmilled powder. The first hydrogenation cycle of the milled powder shows a strong slope indicating the presence of several hydrides. The hydrogenated ground powder was stabilized with a carbon monoxide surface treatment that markedly slows down the desorption kinetics. From the ball milled and hydrogenated powder, the formation of the intermediate phase LaNi5Hx with x approximate to 3 is observed, 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 activated 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 intermediate 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 cycles is large (24%). (C) 1999 Published by Elsevier Science S.A. All rights reserved.