The behaviour of highly over-stoichiometric LaNi5 Mn2 alloy as negative electrode for Ni/MH batteries

Key electrochemical properties of highly over-stoichiometric LaNi₅Mn₂ alloy with CaCu₅-type structure have been measured and compared with those of stoichiometric LaNi₄Mn alloy. LaNi₅Mn₂ is obtained by mechanical alloying of two-phase La(Ni,Mn)₅+(Ni,Mn) alloy previously produced by induction melting. The as-milled alloy was thermally annealed at 450°C for one hour to crystallise milling-induced amorphous phase. As derived from DSC and XRD measurements, further annealing at 550°C produces segregation of minor NiMn-phase (21 wt%), which leaves stoichiometric LaNi₄Mn alloy (79 wt%) as a major phase. Electrochemical cycle-life and potential equilibrium measurements for both LaNi₅Mn₂ and LaNi₄Mn alloys show that the over-stoichiometric alloy has much lower discharge capacity than the stoichiometric one (135 mAh/g and 260 mAh/g, respectively). Conversely, the over-stoichiometric alloy exhibits much better cycle-life than the stoichiometric one (5% and 25% decay capacity after 55 cycles, respectively). These results demonstrate that stoichiometry is an effective parameter for tuning the discharge capacity and cycle-life of LaNi_5+x -type electrodes to the performances required by a particular application.