In situ monitoring of the volume change and cracking of a MgTi hydride electrode

Metastable MgTi–10 wt% Pd alloy was synthesized by high-energy ball milling and evaluated as metal hydride electrode for Ni–MH batteries. In situ acoustic emission and generated/relaxed force measurements were performed to monitor the particle cracking and volume expansion/contraction of the electrode occurring during electrochemical charge–discharge cycling. On the basis on these measurements, it was shown that the electrochemical hydrogenation of the MgTi alloy occurs first by an irreversible hydrogenation (corresponding to a charge capacity of 500 mA h g−1) followed by a reversible hydrogenation (resulting in a discharge capacity of 370 mA h g−1). This second step induces MgTi particle cracking and electrode collapse. During the subsequent cycles, a more reversible volume expansion/contraction is observed and the particle cracking becomes progressively less intensive and originates from the hydrogen evolution reaction rather than to the hydrogen absorption reaction.