Papers by Author: Jenq Gong Duh

Abstract: From the history of commercial anode materials, Sn/C-based material is the focus to enhance the cycling performance. A chemical solution method is used to synthesize the Sn compounds / graphite composite anodes. At the first part of this study, the cycle life was enhanced by adjusting various pH values. The multi-phase Sn compound containing Sn₆O₄(OH)₄, SnO₂, Sn₃(PO₄)₂ were deposited on slices of graphite of sample pH6. They were expected to provide a higher spectator to Sn ratio for improved cycleability. These phases could be reduced to metallic Sn, resulting in buffer matrix during 1^st cycle. Therefore, the sample pH6 exhibited the best of electrochemical performance. For the cell cycled between 0.001V and 1.5V, the 1^st charge capacity was 758 mAhg-1. Even after 50 cycles, the capacity remained higher than 460 mAhg-¹. Synthesizing Sn compounds / graphite composite anodes at pH6 with different initial Sn concentrations improved the cycling performance in the second part of this study. When the Sn concentration reached 0.12M, multi-phases of Sn compounds were deposited on the slices of graphite and the amount of Sn was the maximum. The agglomeration of Sn clusters was suppressed by buffering agent generated by the multi-phase of Sn compounds containing Sn₆O₄(OH)₄, SnO₂, Sn₃(PO₄)2_{. Therefore, the sample Sn0.12 M exhibited}the best cycling behavior among all. During cycling between 0.001 V and 1.5 V, the 1^st charge capacity was around 734 mAhg^-1. The capacity remained as high as 440 mAhg-1 even after 50 cycles.

Abstract: The effect of non-ferromagnetic Cr interlayer on the high-frequency ferromagnetic properties (HFFMPs) was investigated by use of FeCoTa/Cr/ FeCoTa triple layered films. In conventional thought, the metal interlayer gives rise to a high eddy current loss and therefore a deteriorated HFFMP. However, it is found that HFFMPs depend on the thickness of Cr interlayer. The HFFPMs are improved by Cr-interlayer with a thickness less than 12 nm (sample C1). Comparing with the Cr-interlayer-free FeCoHf single layered film (sample C0), the magnetic anisotropy field of C1 dramatically increases from 185 Oe for C0 to 558 Oe for C1. As a consequence, a high ferromagnetic resonance frequency over than 3 GHz is achieved for sample C1. When the thickness of Cr-interlayer is more than 120 nm (C2), the HFFMPs are reduced due to the increase of eddy current loss and magnetic decoupling between the ferromagnetic layers.

Abstract: Recently, combination of ductile carbonaceous materials with the metallic Sn has received a great deal of interest to be a novel anode material for lithium ion batteries, because of their higher capacity than the conventional graphite anodes and better cycleability than the pure Sn anodes. Electrochemical performance of the Sn/C composite anodes is influenced by the material system, particle size and size distribution of Sn as well as the amount of deposited Sn. This study revealed that a favorable Sn/C composite anode exhibited reduced size and uniformly distributed tin particles. The crystal structure, morphology and elemental distribution were analyzed by XRD patterns, SEM and EPMA, respectively. The carbothermal-reducted Sn/Mesophase graphite powder (MGP) composite anodes exhibited much higher capacity than the bare MGP, and the initial efficiency was also much higher than the metallic tin anode in literatures.

Abstract: CrAlSiN hard coatings were fabricated on the Si substrate from metallurgical Cr0.45Al0.45Si0.10 alloy target by reactive r.f. magnetron sputtering. The oxidation resistance of CrAlSiN coatings was investigated after annealing at temperatures between 900 and 1100°C for 1 hr in air. The phase identification and microstructure of CrAlSiN coatings after heat treatment were analyzed by X-ray diffraction (XRD). The hardness of CrAlSiN coating after heat treatment at 900oC for 1hr in air is slightly decreased from 30.2GPa to 28.3±1.3GPa, which was caused by the thin oxide formation on the surface of the film. The microstructure of CrAlSiN coating after heat treatment at 1000oC from 1 hr analyzed by TEM revealed two types of layer feature, including the nanocrystalline grain embedded in the Al-riched amorphous layer and reaction interface with relative high content of Si.

Abstract: Mn3O4 hausmannite, which is a normal spinel with the Mn2+ in the tetrahedral site and the Mn3+ in the tetrahedral site, is one of the most stable manganese oxides. Variation in the valence of Mn ions (2+, 3+ and 4+) contributes to several different structures of manganese oxides. The autoxidation of precipitated manganese hydroxide in an alkaline solution is a practical approach to synthesize hausmannite (Mn3O4) at low temperature. During the process, the particle size and morphology of derived products were totally different from the precursors even though nanometer-sized Mn(OH)2 crystals were fabricated at first. It was observed that the variation was resulted from the accumulation of produced Mn3O4 crystallites which departed from the original crystals. This study has not only discussed the influence of reactant concentrations on the particle size and morphology of derived powders, but also revealed the morphological transformation of crystals involved in autoxidation with the aid of electron micrographs

Abstract: Powdery hausmannite (Mn3O4) is of interest in many industrial and technological applications. It is widely used as reactive catalysts, raw material of humidity sensors, and the cathode oxides of Li-ion secondary batteries. In this study, sub-micron and nano-meter sized Mn3O4 powders are prepared by an efficient method at room temperature. Mn(OH)2 nanocrystalsare commonly precipitated at first and then oxidized in the alkaline solution containing excess OH- anions. However, conventionally prepared Mn3O4 powders by the above process are ill-crystallized. To enhance the crystallinity of fabricated powders, CO3 2- anions are introduced into the process. The modified autoxidation method is practical to fabricate low-cost and high grade powders of Mn3O4. Advantages of the modified method are confirmed by both the electron micrographs and XRD patterns of synthesized powders. It is revealed that particle size of the products is in the sub-micron meter range, and the particle morphology can be adjusted by altering the precipitation sequence.

Abstract: The newly developed LiNi0.6Co0.4-xMnxO2 (0.1 < x < 0.3) cathode materials were synthesized by calcining the mixture of NixCoyMn1-x-y(OH)2 and Li2CO3 at 900-940 oC for 15 hr in flowing O2 atmosphere. The NixCoyMn1-x-y(OH)2 precursor was obtained by the chemical co-precipitation method at the pH value controlled by the concentration of NaOH, NH4OH and transition metal sulfate solution. The X-ray diffraction patterns indicated the pure layered hexagonal structure LiNi0.6Co0.4-xMnxO2. The electrochemical behavior of LiNixCoyMn1-x-yO2 powder was examined by using test cells cycled within the voltage range 3-4.3 V at the 0.1C rate for the first cycle and then at the 0.2C rate afterwards. LiNixCoyMn1-x-yO2 cathode materials showed good initial discharge capacity (165-180 mAh/g) and cycling performance. The fading rate was less than 5 % after 20 cycling test. It is demonstrated that LiNixCoyMn1-x-yO2 electrode should exhibit great potential for the future application in lithium-ion battery cathode material.

Abstract: Surface modification on the electrode has a vital impact on lithium-ion batteries, and it is essential to probe the mechanism of the modified film on the surface of the electrode. In this study, a Li2O-2B2O3 film was coated on the surface of the cathode material by solution method. The cathode powders derived from co-precipitation method were calcined with various weight percent of the surface modified glass to form fine powder of single spinel phase with different particle size, size distribution and morphology. The thermogravimetry/differential thermal analysis was used to evaluate the appropriate heat treatment temperature. The structure was confirmed by the X-ray diffractometer along with the composition measured by the electron probe microanalyzer. From the field emission scanning electron microscope image and Laser Scattering measurements, the average particle size was in the range of 7-8µm. The electrochemical behavior of the cathode powder was examined by using two-electrode test cells consisted of a cathode, metallic lithium anode, and an electrolyte of 1M LiPF6. Cyclic charge/discharge testing of the coin cells, fabricated by both coated and un-coated cathode material, provided high discharge capacity. Furthermore, the coated cathode powder showed better cyclability than the un-coated one after the cyclic test. The introduction of the glass-coated cathode material revealed high discharge capacity and appreciably decreased the decay rate after cyclic test.

Abstract: LiCoO2 spinel is one of the most promising cathode materials for Li-ion batteries. However, the capacity fading is aggravated at high voltage, resulting from cathode degradation and electrolyte decomposition owing to over-charging. To improve structural stability, surface modification is an effective method. In this study, nano-crystallized ZnO was coated on the surface of commercial LiCoO2 powders via sol-gel method. The correlation among the amount of coated ZnO, microstructure of modified cathode and the cycling behavior of surface-treated LiCoO2 powders is discussed. Moreover, the effects of cycling for cathodes with as-derived powders on the phase and morphology are also considered. The surface morphology observed from the scanning electron microscope (SEM) images shows that nano-crystallized and spherical ZnO particles with an average size of about 20 nm have developed after coating. The size of ZnO nanocrystallites is related to the initial concentration of Zn2+ cations. In comparing the characteristics of bare and coated LiCoO2 powders, improvement in cyceability of the ZnO-coated cathode is explored. It is confirmed that Zn2+ ions diffuse into the surface region of LiCoO2 particles. To reveal the effects of Zone coating on enhancing the electrochemical properties of LiCoO2 cathode during charge and discharge, the morphological differences between the cathode material before and after cycling are discussed.