Influence of Pd addition on the electrochemical performance of Mg-Ni-Ti-Al-based metal hydride for Ni-MH batteries

Amorphous Mg0.9Ti0.1NiAl0.05 and Mg0.9Ti0.1NiAl0.05Pd0.1 alloys were prepared by high energy ball milling and evaluated as metal hydride electrodes for Ni-MH batteries. The Mg0.9Ti0.1NiAl0.05Pd0.1 alloy showed a much higher cycle life with a capacity retention of 72% after 100 cycles (C100th = 288 mAh g−1) compared to 26% for the Pd-free alloy (C100th = 117 mAh g−1). This was mainly attributed to the improvement of the alloy oxidation resistance in KOH electrolyte with Pd addition, as confirmed by cyclic voltammetry experiments and X-ray diffraction analyses on cycled electrodes. In addition, in situ acoustic emission (AE) measurements revealed that the energy of the AE signals related to the particle cracking is lower for the Mg0.9Ti0.1NiAl0.05Pd0.1 electrode, suggesting that the cracks are smaller in size than with the Pd-free alloy. The Mg0.9Ti0.1NiAl0.05Pd0.1 electrode also displayed a higher discharge rate capability than the Mg0.9Ti0.1NiAl0.05 electrode. On the basis of their respective electrochemical pressure-composition isotherm, it was shown that the presence of Pd in the alloy decreases the thermodynamic stability of the metal hydride. Through a comparative analysis of discharge polarization curves, it was also shown that Pd addition decreases substantially the H-diffusion resistance in the alloy whereas its positive effect on the charge-transfer resistance is limited.