Shuqin Yang

Phase transformation and cycling characteristics of a Ce2Ni7-type single-phase La0.78Mg0.22Ni3.45 metal hydride alloy

A Ce2Ni7-type single-phase La0.78Mg0.22Ni3.45 alloy has been prepared by zoning annealing of the as-cast sample. It is found that at temperatures below 890 °C, non-super-stacking CaCu5- and MgCu4Sn-type phases disappear and super-stacking Ce5Co19-, Gd2Co7- and Ce2Ni7-type phases remain. The Ce5Co19-type phase can totally transform into the Ce2Ni7-type phase via a peritectic reaction at temperatures of 890–900 °C. At temperatures of 900–950 °C, the Gd2Co7-type phase melts and decomposes into the Ce5Co19-type phase, and the newly formed Ce5Co19-type phase subsequently reacts to form the Ce2Ni7-type phase. The Ce2Ni7-type single phase remains stable even at higher temperatures of 950–975 °C. The single-phase alloy shows a superior discharge capacity, close to 394 mA h g−1, and high electrochemical cycling stability, which can achieve 413 cycles as its discharge capacity reduces to 60% of the maximum value. We found that the capacity attenuation of the single-phase alloy is mainly due to the loss of active material at the alloy surface caused by oxidization of La and Mg, and the pulverization of the alloy is not severe with 100 charge/discharge cycles. The crystal structure of the single-phase alloy can be preserved well. Oxidation of La occurs prior to that of Mg. La hydroxide grows from nano-structured needles to larger-scaled rods then to unformed lamellar hydroxide, whereas the precipitation of Mg forms as irregular lamellae inlaid with hexagonal flakes.

Enhanced electrochemical properties of LaFeO3 with Ni modification for MH–Ni batteries

In this work, we synthesized LaFeO3–xwt%Ni (x = 0, 5, 10, 15) composites via a solid-state reaction method by adding Ni to the reactants, La2O3 and Fe2O3. Field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS) results revealed that Ni powders evenly dispersed among the LaFeO3 particles and apparently reduced their aggregation, which imparted the composites with a loose structure. Moreover, the Ni formed a conductive network, thus improving the conductivity of the composites. The maximum discharge capacity of the LaFeO3 electrodes remarkably increased from 266.8 mAh·g–1 (x = 0) to 339.7 mAh·g–1 (x = 10). In particular, the high-rate dischargeability of the LaFeO3–10wt%Ni electrode at a discharge current density of 1500 mA·g−1 reached 54.6%, which was approximately 1.5 times higher than that of the pure LaFeO3. Such a Ni-modified loose structure not only increased the charge transfer rate on the surface of the LaFeO3 particles but also enhanced the hydrogen diffusion rate in the bulk LaFeO3.

Effects of Annealing Temperature on Microstructure and Electrochemical Properties of Perovskite-type Oxide LaFeO 3 as Negative Electrode for Metal Hydride/Nickel(MH/Ni) Batteries

We reported the effects of annealing temperatures on microstructure and electrochemical properties of perovskite-type oxide LaFeO3 prepared by stearic acid combustion method. X-Ray diffraction(XRD) patterns show that the annealed LaFeO3 powder has orthorhombic structure. Scanning electron microscopy(SEM) and transmission electron microscopy(TEM) images show the presence of homogeneously dispersed, less aggregated, and small crystals(30―40 nm) at annealing temperatures of 500 and 600 °C. However, as the annealing temperature was increased to 700 and 800 °C, the crystals began to combine with each other and grew into further larger crystals(90―100 nm). The electrochemical performance of the annealed oxides was measured at 60 °C using chronopotentiometry, potentiodynamic polarization, and cyclic voltammetry. As the annealing temperature increased, the discharge capacity and anti-corrosion ability of the oxide electrode first increased and then decreased, reaching the optimum values at 600 °C, with a maximum discharge capacity of 563 mA·h/g. The better electrochemical performance of LaFeO3 annealed at 600 °C could be ascribed to their smaller and more homogeneous crystals.