Effects of mechanical grinding on the hydrogen storage and electrochemical properties of LaNi5

Abstract

LaNi 5 powders were mechanically ground under argon atmosphere. After 2 h of milling, a limit particle size of 1.5 μm 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 LaNi 5 H x with x ≈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 LaNi 5 powders are intensively used as the negative electrode of reversible metal hydrogen batteries. The unmilled but activated LaNi 5 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%).