Annealing effect on phase composition and electrochemical properties of the Co-free La(2)MgNi(9) anode for Ni-metal hydride batteries

Present paper focuses on studies of the Co-free La(2)MgNi(9) 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 degree C leads to a higher abundance of the La(2)MgNi(9) and La(3)MgNi(14) phases and an elimination of the present at lower temperatures LaNi(5-x) and LaMgNi(4) 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 mAhg(-1) compared to 325 mAhg(-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 degree C the cell showed a very good cycling stability with its capacity remaining on the level of 87%.