Effect of Rare Earth Elements on the Structure and Electrochemical Properties of La0.63R0.2Mg0.17Ni3.1Co0.3 Al0.1 Alloy Electrodes

Hydrogen storage alloys La0.63R0.2Mg0.17Ni3.1Co0.3Al0.1 (R = La, Ce, Pr, Nd, Y, Sm, Gd) based on A2B7 type were prepared by induction melting method. The alloys were annealed at 1173K during a week in a sealed stainless steel tube. The structure and the electrochemical properties of the annealed alloys have been studied systematically by XRD, EPMA and electrochemical studies. The alloys structure consists mainly of Ce2Ni7-type (SG: P63/mmc) La2Ni7 phase as well as minor Gd2Co7-type (SG: R-3m) phase, LaNi5 (CaCu5-type, SG:P6/mmm) phase. It is resulted that La0.65Y0.2Mg0.15Ni3.1Co0.3Al0.1 alloy exhibited the maximum electrochemical discharge capacity of 381.2 mAh·g-1. The best cycling stability was obtained with the La0.65Gd0.2Mg0.15Ni3.1Co0.3Al0.1 based alloy. This stability measured as the capacity retention rate at the 100th cycle (S100) was the highest for this sample (92.7%). The variation of the high rate discharge ability with the alloy composition. Its displayed a wave-like change. Firstly it increased from 24.5% (R = La) to 78.4% (R = Ce), then decreased to 14.4 % (R = Sm), and increased again to 63.8% with R = Gd.