Improved Electrochemical Cycle Stability of La–Mg–Ni-Based A<sub>2</sub>B<sub>7</sub>-Type Alloys by Melt Spinning and Substituting La with Pr

Abstract

The poor electrochemical cycle stability of RE–Mg–Ni system A2B7-type electrode alloys have limited their practical application as the negative electrode materials of Ni–MH battery. In order to improve the electrochemical cycle stability of the La–Mg–Ni system A2B7-type electrode alloys, the partial substitution of Pr for La has been performed. The La0.75−xPrxMg0.25Ni3.2Co0.2Al0.1 (x = 0–0.4) electrode alloys were fabricated by casting and melt-spinning. The microstructures and electrochemical cycle stability of the as-cast and spun alloys were investigated by XRD, SEM and HRTEM. The results show that the as-cast and spun alloys have a multiphase structure, consisting of two main phases (La, Mg)2Ni7 and LaNi5 as well as a residual phase LaNi2. The substitution of Pr for La brings on a notable grain refinement of the as-cast alloys instead of altering the phase structure of the alloys. The electrochemical measurement indicates that the cycle stability of the alloy remarkably grows with increasing both Pr content and spinning rate. The substitution of Pr for La and the melt spinning significantly ameliorate electrochemical cycle stability of the alloys. The capacity retaining rate (S100) of the as-spun (15 m/s) alloys at 100th charging/discharging cycle is enhanced from 72.38% to 90.33% by increasing Pr content from 0 to 0.4. And that of the Pr0.3 alloy is increased from 78.11% to 92.32% by growing spinning rate from 0 (as-cast was defined as the spinning rate of 0 m/s) to 20 m/s.