Annealing effect on phase composition and electrochemical properties of the Co-free La2MgNi9 anode for Ni-metal hydride batteries

Present paper focuses on studies of the Co-free La2MgNi9 alloys as active materials of negative electrodes in nickel-metal hydride (NiMH) batteries. The effect of annealing treatment on the phase composition, microstructure, hydrogen absorption–desorption and electrochemical properties was investigated. The phase-structural composition, microstructures and morphologies of the phases were analyzed by X-ray diffraction and by scanning electron microscopy. Increase of the annealing temperature to 950 °C leads to a higher abundance of the La2MgNi9 and La3MgNi14 phases and an elimination of the present at lower temperatures LaNi5−x and LaMgNi4 intermetallics. The hydrogen absorption–desorption behaviors, the electrochemical performance and electrochemical cycling stability significantly improve after the annealing. For pasted electrodes, the annealed alloys had a discharge capacity of 350–360 mAh g−1 compared to 325 mAh g−1 for the as-cast sample. The discharge capacity of the annealed samples remained high, almost 50% after 300 cycles with 100% depth of discharge (DOD) in half-cell tests. Pellet electrodes prepared from the annealed alloy and carbonyl nickel powder showed a discharge capacity of 396 mAh g−1. In present work we also report the performance of a small prototype NiMH cell where the annealed alloy was used as the active material in the negative MH electrode and a sintered Ni electrode acted as the positive electrode. After 300 cycles at charge/discharge rates of 0.2 C the cell showed a very good cycling stability with its capacity remaining on the level of 87%.

► La2MgNi9 alloys were studied as negative electrodes of the metal hydride battery. ► Phase-structural composition was related to the annealing at 800, 900 and 950 °C. ► Abundance of the La2MgNi9 and La3MgNi14 reaches maximum after annealing at 950 °C. ► The annealing improves the electrochemical performance.