Effect of Ca on the microstructural and electrochemical properties of La2.3−xCaxMg0.7Ni9 hydrogen storage alloys

The microstructural and electrochemical properties of La_2.3−xCa_xMg_0.7Ni₉ hydrogen storage alloys have been studied systematically. The microstructure examined by XRD, SEM and EDX shows that the alloys consist of multi-phases, which are (La, Mg)₂Ni₇ phase, LaMgNi₄ phase, (La, Mg)Ni₃ phase and LaNi₅ phase. It is can be found that Ca does not appear to segregate. This phenomenon is different from Mg. With increasing Ca content, the main phase varies from (La, Mg)₂Ni₇ phase (x = 0) to (La, Mg)Ni₃ phase (x = 0.3), LaNi₅ phase (x = 0.6, 0.8) and (La, Mg)Ni₃ phase (x = 1.0, 1.3). The maximum discharge capacities of the alloy electrodes increase from 244.6 mAh/g (x = 0) to 380 mAh/g (x = 1.0), and then decrease to 353.6 mAh/g (x = 1.3). The discharge capacities of the alloys are related to phase content. Cell volumes of LaNi₅ phase, (La, Mg)₂Ni₇ phase and (La, Mg)Ni₃ phase all decrease and the high rate dischargeability (HRD) is improved by adding Ca. The alloy electrodes also show relative good cycling stability up to 100 cycles.