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

Abstract

In this work, various amounts of (La,Mg)5Ni19 phase is successfully produced in a A2B7-type La0.75Mg0.25Ni3.5 alloy by zone heating the as-cast alloy at the peritectic reaction temperature of the (La,Mg)5Ni19 phase (1203 K) for different durations. The formation process, electrochemical effects and functioning mechanism of the (La,Mg)5Ni19 phase are studied. The as-cast alloy contains (La,Mg)2Ni7 and LaNi5 main phases, and (La,Mg)5Ni19 minor phase. During zone heating, the peritectic reaction between the LaNi5 solid phase and the melted (La,Mg)2Ni7 liquid phase occurs, forming (La,Mg)5Ni19 phase. Thus the (La,Mg)5Ni19 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)5Ni19 phase abundance, contributing to fast hydrogen desorption and large current dischargeability. In addition, the (La,Mg)5Ni19 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)5Ni19 phase) to 80.4% (46.2 wt% (La,Mg)5Ni19 phase).