Phase transformation and electrochemical properties of La0.70Mg0.30Ni3.3 super-stacking metal hydride alloy

• Rising annealing temperature increases La3MgNi14 mass via phase transformations.
• LaNi5 and LaMgNi4 phases can be eliminated at lower temperature.
• La3MgNi14 phase benefits discharge capacity.
• La4MgNi19 phase enhances high rate dischargeability (HRD).

In this study, phase transformation and electrochemical properties of super-stacking La0.70Mg0.30Ni3.3 alloys obtained by annealing the induction melting samples were systematically studied. XRD and Rietveld refinement results show that the as-cast alloy consists of LaNi5, La4MgNi19, La3MgNi14, La2MgNi9 and LaMgNi4 phases, and the gradual rising annealing temperature effectively increases La3MgNi14 phase abundance via phase transformations. At 1123 K, LaNi5 and LaMgNi4 are eliminated, transforming to super-stacking phases. Further increase the temperature to 1173 K, partial La4MgNi19 and La2MgNi9 phases transform to La3MgNi14 phase and La4MgNi19 phase is depleted completely at 1223 K, resulting in La3MgNi14 as the main phase and La2MgNi9 as the surplus phase. Electrochemical results show that increasing abundance of La3MgNi14 phase makes a significant improvement on the discharge capacity of the alloy electrode, enhancing from 370 to 401 mAh g−1. And ascribing to the formation of La4MgNi19 phase, the high rate dischargeability (HRD1800) of the alloy electrode is ameliorated by 10%. Based on the phase transformation, purposeful controlling the alloy phase composition by annealing could efficiently improve the electrochemical properties of the super-stacking metal hydride alloys.