In situ investigation of the volume change and pulverization of hydride materials for Ni-MH batteries by concomitant generated force and acoustic emission measurements

Abstract

A new experimental set-up constituted of an electrochemical cell connected to a compression load cell and acoustic emission (AE) equipment was used for monitoring the force and the cracking of metal hydride electrodes generated by their volume expansion/contraction during their cycling. Two materials were studied: a commercial LaNi5-based alloy and an amorphous MgNi alloy prepared by ball milling. During the charge of the LaNi5-based electrode, a concordance was observed between the increase of the force generated by its volume expansion and the increase of the AE activity associated with the particle cracking. This correlation was not observed during the charge of the MgNi electrode, which tends to confirm that the main origin of the cracking of the MgNi particles is not their volume expansion but is rather due to the mechanical action of the H2 bubbles produced at the end of the charge step. In addition, on the basis of the generated/relaxed force rates, the volume expansion/contraction of the electrode appeared more progressive with the MgNi alloy than with the LaNi5-based alloy. This could result from the lack of abrupt α–β phase transition region in the MgNi alloy due to its amorphous structure.