Papers by Author: Ki Won Kim

Abstract: Ni-3at.%W alloy tapes for YBCO coated conductor were fabricated by powder metallurgy process including powder compaction, cold isostatic pressing(CIP), cold rolling and annealing for recrystallization. The Ni-3at.%W tapes annealed in an atmosphere of 96%Ar and 4%H2 mixing gas were characterized by X-ray pole figures, orientation distribution function(ODF) and optical microscopy(OM). The Ni-W alloy tapes were heat-treated at 700~1000°C for the development of cube texture {001}<100>. The texture analysis indicated that as fabricated tapes have a sharp cube texture after annealing at 900 and 1000°C. Also, thoroughly recrystallization takes place for the annealed tapes at the both temperatures and the average grain size of the two tapes is about the same, which is about 35㎛.

Abstract: Si electrode with different amount of polymer binder was fabricated by ball milling. With increasing of polymer content, not only porosity but also the first charge capacity of Si electrode was increased. The Si electrode enhanced electrical conductivity showed high first capacity of 3644mAh/g and improved cycle performance. Si electrode having good electrochemical property could be fabricated by controlling amount of polymer.

Abstract: The effect of lithium salts such as LiPF6, LiBF4, LiCF3SO3 and LiN(CF3SO2)2 (LiTFSI) in tetra(ethylene glycol) dimethyl ether (TEGDME) electrolyte on the ionic conductivity, interfacial resistance and discharge properties of Li/pyrite cell at room temperature was studied. The electrolytes had good ionic conductivity at room temperature in the range 0.61 to 1.86 × 10-3 S/
. The discharge capacities of Li/pyrite cells with 1M LiPF6 and LiBF4 in TEGDME were lower compared to those of the other two non-HF containing salts. The best cycle performance was exhibited by LiTFSI in TEGDME electrolyte, with a discharge capacity of 438 mAhg-1 after 20 cycles, which is ~49% of FeS2 theoretical capacity (894 mAhg-1). The good performance of LiTFSI-TEGDME electrolyte resulted mainly from its low interfacial resistance in Li/FeS2 cells, which showed a decreasing trend with cycling.

Abstract: The nickel sulfide (Ni3S2) thin film could be prepared from Ni/S double layer, which was deposited on nickel foil using evaporation and sputtering. The nickel sulfide electrode was discharged and charged between 0.6V and 2.6V versus Li/Li+ at room temperature. The nickel sulfide film had the first discharge capacity of 270mAh/g, and two plateaus at 1.3V and 1.8V.

Abstract: Si film electrodes for Li micro-film batteries were fabricated on a Cu substrate and a Ci/Cu film. In the structural properties, FE-SEM observation demonstrated difference in surface morphologies of Si films with different under layers. Surface of the Si film deposited consists of clusters and average size of the cluster was 165 nm for Si/Cu film and 80 nm for Si/C/Cu film. Si film has amorphous phase in spite of different under layers; Cu substrate and C/Cu film. In the electrochemical properties, the carbon-inserted film showed a good cycleability compared with Si/Cu electrode. It is believed that the insertion of carbon film as a buffer film absorbed the stress generated during charge-discharge process and improved cycle performance of Si anode electrode.

Abstract: Iron, sulfur and transition metal powders were used as the starting materials to prepare iron disulfide (FeS2) cathode material at room temperature by high energy mechanical alloying. Modified FeS2 were also prepared by incorporation of transition metals like Co and Ni. Li/FeS2 cells with the prepared iron disulfides as cathodes were studied for discharge properties at room temperature using the 0.5M LiTFSI in tetra(ethylene glycol) dimethyl ether (TEGDME). The first discharge capacities of Li/composite FeS2 cell with 5 wt.% Co and 3 wt.% Ni were 571 and 844 mAh/g, respectively, compared to 389 mAh/g for the cell without any additive. The enhanced properties resulted from the better electronic conductivity of the material containing the metallic additive. The initial capacity and cyclic performance were improved when nickel and cobalt were added to prepare the modified iron disulfide.

Abstract: In this study, we investigated ionic conductivities of the electrolytes and cycle performances of Li/S cells using the electrolyte. (PEO)10LiCF3SO3 composite polymer electrolyte(CPE) containing carbon powders and Brij dispersant was prepared by ball milling for 12hr. The 5wt% carbon powders having high surface area (~ 80 m2/g) was added into the (PEO)10LiCF3SO3 electrolyte. To get a well-dispersed structure, Brij dispersant was also added into the (PEO)10LiCF3SO3-5wt%Carbon electrolyte. Li/CPEs/50wt%S cells showed initial discharge capacities of between 1,250 and 1,413 mAh/g-sulfur with current density of 100 mA/g-sulfur at 80 °C. These results led us to conclude that the dispersants added into the CPE improved the initial discharge capacities and cycle performances.

Abstract: We investigated on the additive effect of carbon nanotube in the sulfur electrode on the first discharge curve and cycling property of lithium/sulfur cell. The sulfur electrode with carbon nanotube had two discharge plateau potentials and the first discharge capacity about 1200 mAh/g sulfur. The addition carbon nanotube into the sulfur electrode did not affect the first discharge behavior, but improved the cycling property of lithium/sulfur cell. The optimum content of carbon nanotube was 6 wt% of sulfur electrode.

Abstract: Various physical, chemical and mechanical methods, such as inert gas condensation, chemical vapor condensation, sol-gel, pulsed wire evaporation, evaporation technique, and mechanical alloying have been used to synthesize nanoparticles. Among them, chemical vapor condensation(CVC) represents the benefit for its applicability to almost materials because a wide range of precursors are available for large-scale production with a non-agglomerated state. In this work, iron nanoparticles and nanowires have synthesized by chemical vapor condensation(CVC) process, using iron pentacarbonyl(Fe(CO)5) as precursor. The effects of processing parameters on the morphology, microstructure and size of iron nanoparticles and nanowires were studied. Iron nanoparticles and nanowires having various diameters were obtained by controlling the inflow of metallic organic precursor. Both nanoparticles and nanowires were crystallized. Characterization of obtained nanoparticles and nanowires were investigated by using a field emission scanning electron microscopy, transmission microscopy and X-ray diffraction.

Abstract: Gallium phosphide nanowires were successfully synthesized by the catalytic chemical vapor deposition (CVD) method using MgO powder-impregnated nickel oxide as catalyst and gallium phosphide and gallium powders as GaP source. The synthesis of GaP nanowires were carried out at 900°C for 30min under argon ambient and directly vaporized Ga and GaP powder. The diameter of GaP nanowires is about 25~70nm and the length is up to several tens of micrometers. The GaP NWs was core-shell structure, which consists of the GaP core and the Ga oxide outer layers. The GaP nanowires have a single-crystalline zinc blend structured crystals with the [111] growth direction. Nanowires larger than around 50nm in diameter exhibited twinning faults, which appears in the TEM images as discrete dark lines and alternating wire contrast. We demonstrate that MgO powder-impregnated nickel oxide catalyst exhibited a large catalytic effect on the growth of high-purity and -quantity gallium phosphide(GaP).