Effect of annealing treatment on structure and electrochemical properties of La 0.67Mg 0.33Ni 2.5Co 0.5 alloy electrodes

Abstract

La 0.67Mg 0.33Ni 2.5Co 0.5 alloys were prepared by induction melting followed by different annealing treatments (1023, 1073, 1123 and 1173 K) for 24 h. Alloy structure and electrochemical properties of different annealed alloys have been studied systematically by X-ray diffraction (XRD), scanning electron microscope (SEM) and electrochemical experiments. Alloy structure analyses show that all of the alloys consisted of complex phases such as (La, Mg)(Ni, Co) 3 phase (PuNi 3-type, SG: R-3 m), (La, Mg) 2(Ni, Co) 7 phase (Ce 2Ni 7-type, SG: P6 3/mmc), LaMg(Ni, Co) 4 phase (MgCu 4Sn-type, SG: F-4 3 m) and La(Ni, Co) 5 phase (CaCu 5-type, SG:P6/mmm). One thousand one hundred and twenty-three kelvin benefited formation of (La, Mg)(Ni, Co) 3 phase best. Main phase in alloy became (La, Mg) 2(Ni, Co) 7 phase at 1173 K annealing treatment. Electrochemical experiments show that absorption/desorption plateau became flatter and wider after annealing treatment, that all of the alloy electrodes exhibited good activation characteristics, that annealing treatment improved discharge capacities of alloy electrodes from 315 mAh g −1 (as-cast) to 402.5 mAh g −1 (1173 K). At the same time, cyclic stability of alloy electrodes was also improved with rise of annealing temperature, especially for alloy electrode (S 70 = 92.9%) with main phase (La, Mg) 2(Ni, Co) 7 at 1173 K. As (La, Mg)(Ni, Co) 3 phase in alloys increased, high rate dischargeability characteristics were deteriorated. However, alloy electrode with main phase (La, Mg) 2(Ni, Co) 7 exhibited the best kinetic characteristics. All experiments imply that alloy electrode with main phase (La, Mg) 2(Ni, Co) 7 possessed excellent overall electrochemical properties.