Enhanced cycling stability and high rate dischargeability of A2B7-type La–Mg–Ni-based alloys by in-situ formed (La,Mg)5Ni19 superlattice phase

In this work, various amounts of (La,Mg)(5)Ni-19 phase is successfully produced in a A(2)B(7)-type La0.75Mg0.25Ni3.5 alloy by zone heating the as-cast alloy at the peritectic reaction temperature of the (La,Mg)(5)Ni-19 phase (1203 K) for different durations. The formation process, electrochemical effects and functioning mechanism of the (La,Mg)(5)Ni-19 phase are studied. The as-cast alloy contains (La,Mg)(2)Ni-7 and LaNi5 main phases, and (La,Mg)(5)Ni-19 minor phase. During zone heating, the peritectic reaction between the LaNi5 solid phase and the melted (La,Mg)(2)Ni-7 liquid phase occurs, forming (La,Mg)(5)Ni-19 phase. Thus the (La,Mg)(5)Ni-19 phase abundance increases from 9.8 wt% (as-cast) to 46.2 wt% (heated for 24 h). The hydrogen desorption plateau pressure of the alloys increases with increasing (La,Mg)(5)Ni-19 phase abundance, contributing to fast hydrogen desorption and large current dischargeability. In addition, the (La,Mg)(5)Ni-19 phase network in the alloy matrix has a good structural stability against repeated hydrogen absorption/desorption, keeping the alloy from serious lattice destruction and crystal defects. Moreover, the discrete expansion/contraction between the LaNi5 phase and the superlattice phases during cycling decreases with the consumption of the LaNi5 phase, which relives the alloys' pulverization, and thus enhancing the oxidation resistance. Thereby, the cycling stability of the alloy electrodes of the 150th cycle increases from 65.4% (9.8 wt% (La,Mg)(5)Ni-19 phase) to 80.4% (46.2 wt% (La,Mg)(5)Ni-19 phase). (C) 2018 Elsevier B.V. All rights reserved.