Preparation, Electrochemical Properties and Capacity Degradation Mechanism of Gd₂Co₇-type Superlattice Structure La_0.65Gd_0.10Mg_0.25Ni_3.50Metal Hydride Alloy

Abstract

Single Gd 2 Co 7 -type phase La 0.65 Gd 0.10 Mg 0.25 Ni 3.50 alloy is prepared with induction melting followed by annealing treatment. Rietveld refinements show that the as-cast alloy consists of CaCu 5 -type phase, Gd 2 Co 7 -type phase and Ce 5 Co 19 -type phase. At temperature of 1123-1248 K, the CaCu 5 -type and Ce 5 Co 19 -type phases transform into Gd 2 Co 7 -type phase via peritectic reactions. As annealed at 1248 K for 36 h, the CaCu 5 -type and Ce 5 Co 19 -type phases totally transform into Gd 2 Co 7 -type phase and the alloy shows a single phase structure. The increasing abundance of Gd 2 Co 7 -type phase makes a significant improvement on the discharge capacity and the Gd 2 Co 7 -type single phase alloy shows a superior discharge capacity of 386 mAh g −1 . Moreover, the Gd 2 Co 7 -type single phase alloy exhibits preferable capacity retention, which is up to 88.4% at the 160th cycles. The pulverization of the single phase alloy mainly occurs at earlier stage and the average particle size is not significantly reduced after 50 charge/discharge cycles. It is found that the oxidation of Mg mainly occurs at an early stage, while the oxidation of La happens in the whole process of charge/discharge cycling. Moreover, the oxidation of La gets aggravated after 100 cycles as the hydroxides of La is observed growing from nanoscale (<100 nm) needles (100th cycles) to micrometer-size (>2 μm) rods (150th cycles).

Self-Discharge Characteristic of La0.60Nd0.15Mg0.25Ni3.50 Alloy with Single-Phase Ce2Ni7-Type Super-Stacking Structure

“…In this stage, the CaCu 5 and Ce 5 Co 19 phases form the Gd 2 Co 7 type phase via a peritectic reaction. 21 As annealed at 1248 K for 24 h, the alloy (#3) shows a single-phase Gd 2 Co 7 -type structure and its Rietveld refinement pattern are shown in Figure 2c. In the temperature interval of 1173-1248 K, the CaCu 5 and Ce 5 Co 19 phases transform into Gd 2 Co 7 -type phase and the transformation totally accomplishes at 1248 K. When the annealing temperature increases to 1273 K (#4), a new Ce 2 Ni 7 -type phase appears.…”

“…After storage of 7 d, for the as-cast alloy, there are many materials with flake and needle-shape on the surface of the alloy particles, which can be Mg(OH) 2 and La(OH) 3 . 20,21 For the Gd 2 Co 7type alloy, the hexagonal Mg(OH) 2 and needle-shaped La(OH) 3 still appear, but the amount gets smaller. As for the Ce 2 Ni 7 -type alloy electrode, the amount of the hydroxide get even smaller and cannot cover over the surface of the alloy particles.…”