G.L. Lu

The electrochemical properties of LaxMg3-xNi9 (x=1.0-2.0) hydrogen storage alloys

Abstract LaxMg3−xNi9 (x=1.0–2.0) alloys were prepared by a powder sintering method and the influence of partial substitution of Mg by La on the crystal structure and electrochemical performance of the alloys was investigated. The ternary alloys with different La/Mg ratio are related to the hexagonal PuNi3-type structure. The increase of La/Mg ratio in the alloys leads to a linear increase of the unit cell volume. The La-rich alloys (x=1.8–2.0) show promising electrode properties including a large discharge capacity (∼400 mAh/g), easy activation and good high-rate dischargeability, although the cycling stability needs further improvement.

Influence of the material processing on the electrochemical properties of cobalt-free Ml(NiMnAlFe)5 alloy

Abstract Co-free MlNi 4.1 Mn 0.35 Al 0.3 Fe 0.25 (Ml: La-rich mischmetal) alloy samples were prepared in three different ways. The original as-cast alloy was prepared by vacuum levitation melting in a cold crucible. One-third of the as-cast ingot was annealed at 1273 K for 10 h, and another one-third was remelted and rapidly solidified by melt-spinning. The influence of the alloy preparation methods on their microstructure and electrochemical properties was studied. XRD and SEM results revealed that all of the three differently prepared alloys were of the single CaCu 5 -type structure phase, but their microstructure and electrochemical properties were changed markedly. The as-cast alloy had a typical dendrite structure with noticeable composition segregation and rather poor cycling endurance. While the annealed and melt-spun alloy were of an equiaxed structure and a very fine cellular structure, respectively, and had a more homogeneous composition and dramatically improved cycling endurance, although their activation rate and high-rate dischargeability were lowered somewhat. In this study, the melt-spun Co-free alloy showed the best cycling stability ( S 500 =69.2%) and a reasonably high capacity (305 mA h g −1 ) and 1C rate dischargeability (85.6%), which is attributed to its lower degree of pulverization and more uniform composition.

Effect of Ti on the structure and electrochemical performance of Zr-based AB2 alloys for nickel-metal rechargeable batteries

Abstract The effects of titanium on the crystalline structure and electrochemical properties of Zr-based AB 2 electrode alloys were investigated. SEM, XRD and Rietveld analysis were used to analyze the microstructures and phase composition and abundance of the alloys. It was found that titanium could significantly change the phase abundance of Zr 1− x Ti x (NiVMnCr) 2.1 ( x =0, 0.1, 0.3) alloys. The C14 Laves phase increases and the C15 Laves phase and non-Laves phase decrease as the amount of titanium additive increases. Titanium also causes the cell parameters to contract. The more titanium added to the alloys, the more the cell volume of the alloys contracts. The microstructures of the three alloys, with or without titanium addition, were observed by scanning electron microscopy (SEM) and were shown to be dendrites. This shows that titanium has no effect on the microstructure of the alloys. Electrochemical analysis showed that titanium additive electrodes have a higher discharge capacity, but no obvious effect on the activation process. It is believed that the main reason for the increase in capacity of titanium additive electrodes is the increase in the amount of Laves phases (C14+C15 phases). Titanium additive improved the charging/discharging cycle life of Zr 1− x Ti x (NiVMnCr) 2.1 ( x =0, 0.1, 0.3) alloys and reduced the average capacity attenuation rate during charging/discharging cycling due to the excellent corrosion resistance of titanium.

Influences of annealing heat treatment on phase structure and electrochemical properties of Zr(MnVNi)2 hydrogen storage alloys

Abstract The influences of annealing treatment on the phase structure and the electrochemical properties of Zr(Mn 0.25 V 0.20 Ni 0.55 ) 2 and Zr(Mn 0.05 V 0.40 Ni 0.55 ) 2 hydrogen storage alloys were investigated by means of XRD analysis and electrochemical tests. As-cast alloys were comprised of Laves phases (i.e., C15 and C14) dispersed Zr–Ni phases (Zr 9 Ni 11 and ZrNi) and exhibited good overall electrochemical properties. When the as-cast alloys were annealed, the Zr–Ni phases and part of C14 phase decomposed completely with increasing annealing time. The thermodynamically stable phase structure of the annealed alloys was a mixture of C15 and C14 Laves phase. The decomposition of Zr–Ni phases and part of C14 phase resulted in a remarkable decrease in capacity and activation property as compared to those of the as-cast alloy. A synergistic effect due to the presence of Zr–Ni phases in the Laves phases is inferred. The deterioration in the overall electrochemical properties is attributed to the decomposition of the Zr–Ni phases.

The effect of solidification rate on the microstructure and electrochemical properties of Co-free Ml(NiMnAlFe)5 alloys

Abstract In this work, Fe-containing Co-free Ml(NiMnAlFe) 5 alloy samples were prepared by both arc-melting and melt-spinning processes at different solidification rates, and tested for electrochemical performance. The electrochemical tests indicated that higher solidification rates led to a better cycling stability, but lower discharge capacity and high-rate dischargeability. The X-ray diffraction results indicated that the as-cast alloy and the melt-spun alloys were both of the CaCu 5 -type structure single phase, but the lattice constants of the alloys were varying. The scanning electron microscope and EPMA results indicated that the microstructure of melt-spun alloy formed at the relative lower solidification rate (5 m / s ) was of very fine dendritic structure and that for the alloy formed at a relative higher solidification rate (15 m / s ) was of cellular structure, and both had a more homogeneous composition than the as-cast alloy, which had a typical dendrite structure with noticeable composition segregation. The Co-free MlNi 4.1 Mn 0.35 Al 0.3 Fe 0.25 prepared by melt-spinning at 15 m / s rate is a promising candidate for low-cost electrode alloy.

Study on structure and electrochemical performance of melt-spun non-stoichiometry alloys Ml(NiCoMnTi)5+X

Abstract The structure, secondary phases and electrochemical properties of over-stoichiometry alloys Ml(NiCoMnTi)5+X prepared by melt-spinning method were investigated. The microstructure of alloys with different X was a fine columnar structure as solidified at the high cooling rate of about 10 6 K s −1 . XRD analysis revealed that over-stoichiometry alloys would segregate TiNi3 secondary phase as the X⩾0.4 and segregated Ni as the X⩾0.7. XRD analysis also indicated that the cell parameters were changed and the cell volume contracted with the X increased from 0 to 1.0. Electrochemical testing showed that the capacity of alloys Ml(NiCoMnTi)5+X were closely related to the X, and the capacity of alloys was decreased quickly from 310 to 155 mA h g −1 as the X increased from 0 to 1.0. But all of the alloys with different stoichiometry X were activated within 5 charging/discharging cycles. Cycle life of the alloy was greatly prolonged as the X increased. The capacity decay rate kept very small as the X reached above 0.7, which was ascribed to the small cell expansion and pulverization in charging/discharging process due to their small capacity. High rate dischargeability of alloys was obviously improved as the X increased due to the secondary phase, both Ni and/or TiNi3, segregated in the grain boundary to help to conduct the hydrogen atoms. Among the alloys Ml(NiCoMnTi)5+X investigated, Ml(NiCoMnTi) 5.1 (X=0.1) had the best comprehensive electrochemical properties with high discharge capacity and longer cycle life and suitable for practical use in Ni/MH batteries.

The phase structure and electrochemical properties of the melt-spun alloy Zr0.7Ti0.3Mn0.4V0.4Ni1.2

Abstract The crystal structure, phase abundance, microstructure and electrochemical properties of the AB 2 -type Laves phase hydrogen storage alloy Zr 0.7 Ti 0.3 Mn 0.4 V 0.4 Ni 1.2 were studied, prepared both by arc-melting and melt-spinning. The XRD patterns have revealed that two Laves phases, C14 and C15 are formed in the as-cast alloy. In the melt-spun alloy, besides the C14 and C15 phase in very fine crystal grains, there appears a new crystalline C14 phase with very fine crystallites, which is named the nanocrystallite C14 in this paper. SEM and EDS analyses have indicated that the microstructure of melt-spun alloy is a very fine dendritic structure (10×3 μm), while the as-cast alloy is a much coarser dendritic structure (300×50 μm) with noticeable composition segregation. Electrochemical tests have indicated that the melt-spun alloy has a higher discharge capacity of 385 mA h g −1 in comparison with 371 mA h g −1 for the as-cast alloy and the melt-spun alloy has very good cycling stability. After 500 cycles, 80.7% of the initial discharge capacity is retained, much higher than 63.1% of the as-cast alloy. It is thus inferred that the electrochemical capacity of the nanocrystallite C14 phase is higher than that of the conventional C14 phase. Nevertheless, the activation property and the high-rate dischargeability of the melt-spun alloy were both found to decrease noticeably, most probably due to the higher resistance against decrepitation and cracking of the alloy with a more refined grain structure.

Micro-crystalline C14 Laves phase in melt-spun AB2 type Zr-based alloy

Abstract The single phase of micro-crystalline C14 Laves of Zr 0.7 Ti 0.3 (NiVMnCr) 2.1 alloy was successfully prepared by melt-spinning processing with a certain cooling rate. The investigation with XRD Rietveld analysis and TEM showed that micro-crystalline C14 Laves phase had a unique microstructure, the grain was composed of staked c -axis textured thin plates which contained a lot of crystallites with the mean size of about 3.1×3.1×1 nm 3 . The electrochemical measurement showed that it had longer cycle life, more activation cycles and lower discharge capacity than those of conventional mixture C14+C15 Laves phase.

Effect of Cr and Co additives on microstructure and electrochemical performance of Zr(NiVMn)2M0.1 alloys

Abstract Zr-based Laves alloys of Zr(NiVMn) 2 M 0.1 (M=Cr or Co) were prepared by an arc melting process. The effect of the Cr or Co additive on the microstructure and electrochemical performance of the Zr(NiVMn) 2 M 0.1 Laves phase hydride electrode alloys was investigated. Microstructure observation showed that the morphology of the Zr(NiVMn) 2 Cr 0.1 alloy was dendritic, while the morphology of the Zr(NiVMn) 2 Co 0.1 alloy was of equiaxial grains. The XRD results showed that both alloys consisted of the predominant C14 and C15 Laves phases, and the minor Zr 7 Ni 10 , Zr 9 Ni 11 non-Laves phases. There is no big difference in the amount of Laves phase in these two alloys as they were prepared under the same solidification conditions. The effect of the Cr or Co additive on phase composition and phase abundance of the obtained Zr(NiVMn) 2 M 0.1 alloys was quite similar. The electrochemical properties of the Zr(NiVMn) 2 M 0.1 alloys were dependent on the microstructures obtained. The electrode prepared from the alloy with equiaxial grain showed less activation cycles, higher discharge capacity and poorer cycle stability than those prepared from the alloy with dendrite structure. The microstructure-dependent electrochemical properties are also discussed.

Contribution of rare-earths to activation property of Zr-based hydride electrodes

Abstract The effect of rare earth RE alloying (RE=Ce, Ce-rich mischmetal Mm or La-rich mischmetal Ml) on the crystalline characteristics and electrochemical performances of AB2 type (ZrRE)(MnVCrNi)2 hydride electrodes was investigated. It is found that RE-alloyed electrodes are activated at the first charge/discharge cycle in comparison with 15 cycles needed to activate RE-free alloys. The electrochemical capacity of the RE modified alloy series studied is also found to increase by about 15%. The RE modification effect on activation has been discussed on the basis of electrochemical impedance spectroscopy (EIS) measurement and XRD analysis.