On the decrepitation mechanism of MgNi and LaNi5-based electrodes studied by in situ acoustic emission

In situ monitoring of the pulverization of amorphous MgNi and crystalline LaNi5-based alloys has been studied during their hydrogen charge by combining acoustic emission and electrochemical measurements. In both alloys, two classes of acoustic signals with specific temporal and energetic characteristics were detected during their charge: a 131 class related to the particle cracking and a P2 class due to the release of H-2 bubbles. By comparing the P1 activity on both materials as a function of the charge input, it was shown that the pulverization phenomenon becomes significant at a much lower charge input for the LaNi5-based electrode (similar to 5-25 mAh g(-1)) than for the MgNi electrode (similar to 365 mAh g(-1)), reflecting the fact that the mechanism responsible of their decrepitation is not similar. Indeed, it was demonstrated that the cracking of the amorphous and porous MgNi material is mainly induced by the hydrogen evolution reaction whereas for the crystalline and denser LaNi5-based material, the alpha-beta lattice expansion is responsible of its decrepitation. It was also shown that the particle size and the charge current density have a major impact on the MgNi decrepitation. The correlation between the MgNi particle cracking and the discharge capacity decay with cycling was established. (C) 2011 Elsevier B.V. All rights reserved,