An investigation on electrochemical performances of as-cast and annealed La_0.8Mg_0.2Ni_3.3Co_0.2Si_x (x = 0–0.2) alloy electrodes for Ni/MH battery application

Abstract

The La-Mg-Ni-based A₂B₇-type La_0.8Mg_0.2Ni_3.3Co_0.2Si_x (x=0–0.2) electrode alloys were prepared by casting and annealing. The influences of the additional silicon and the annealing treatment on the structure and electrochemical performances of the alloys were investigated systemically. Both of the analyses of XRD and SEM reveal that the as-cast and annealed alloys are of a multiphase structure, involving two main phases (La, Mg)₂Ni₇ and LaNi₅ as well as one minor phase LaNi₃. The addition of Si and annealing treatment bring on an evident change in the phase abundances and cell parameters of (La, Mg)₂Ni₇ and LaNi₅ phase for the alloy without altering its phase structure. The phase abundances decrease from 74.3% (x=0) to 57.8% (x=0.2) for the (La, Mg)₂Ni₇ phase, and those of LaNi5 phase increase from 20.2% (x=0) to 37.3% (x=0.2). As for the electrochemical measurements, adding Si and performing annealing treatment have engendered obvious impacts. The cycle stability of the alloys is improved dramatically, being enhanced from 80.3% to 93.7% for the as-annealed (950 °C) alloys with Si content increasing from 0 to 0.2. However, the discharge capacity is reduced by adding Si, from 399.4 to 345.3 mA·h/g as the Si content increases from 0 to 0.2. Furthermore, such addition makes the electrochemical kinetic properties of the alloy electrodes first increase and then decrease. Also, it is found that the overall electrochemical properties of the alloys first augment and then fall with the annealing temperature rising.

Keywords

A₂B₇-type electrode alloy / adding Si / annealing treatment / structure / electrochemical performances

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