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

The microstructural and electrochemical properties of La2.3−xCaxMg0.7Ni9 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.