Papers by Author: Hyo Jun Ahn

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: 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: 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: Lithium/sulfur battery has some problems such as low utilization of active material and poor cycle life owing to the dissolution of lithium polysulfide into electrolyte, aggregation of sulfur during charge-discharge process, and structural change of sulfur electrode. To overcome such problems, carbon nano tubes (CNTs) and graphitic nano fibers (GNFs) were added into the sulfur electrode. The addition of CNTs and GNFs having a network-like structure is expected to offer the structural stability and good electrical path of sulfur electrode. The morphology of fabricated sulfur electrode was observed by using scanning electron microscope (SEM), and the crystalline structure was characterized by using X-ray diffraction (XRD). The charge/discharge tests were conducted in the voltage range 3.2/1.5V (vs.Li) with a galvanostatic method.

Abstract: Iron disulfide (FeS2) is attractive as a positive electrode material in lithium batteries because of its low material cost, environmental non-toxicity, and high specific energy density. Furthermore, natural pyrite is a secondary product of the mining extraction of coal. For these reasons, natural and synthetic pyrites have been proposed as active cathode materials in secondary lithium batteries. We investigated the effect of various solvents on the electrochemical properties of lithium-FeS2 batteries. The specific discharge capacity of Li/FeS2 cells varied from 500 to 780mAh/g based on FeS2.

Abstract: The lithium ionic conductivity of Poly (acrylonitrile) (PAN) gel polymer electrolyte with PC/EC was found to be about 1.3 x 10-3S/cm at room temperature. The discharge curve of Li/ PAN (PC+EC)/S battery showed only one plateau region, which is different from that using PVdF(TEGDME) gel polymer electrolyte. Also, the first discharge capacity was 556mAh/g-sulfur in Li/S battery using PAN (PC+EC) gel electrolyte at room temperature.

Abstract: To find out the proper sodium ion conducting electrolyte at room temperature, we investigated the ac impedance measurement of PVdF gel polymer electrolyte and liquid tetraglyme(TEGDME) with various concentrations of sodium trifluoromethane sulfonate(NaCF3SO3). The concentration of NaCF3SO3 did not severely affect the ionic conductivity. The sodium ionic conductivity using TEGDME with NaCF3SO3 was about 3.3×10-4 S㎝-1 which was lower than that of the PVdF gel polymer electrolyte, 5.0×10-4 S㎝-1. From the viewpoint of ionic conductivity, PVdF gel polymer electrolyte was proper electrolyte for sodium battery.

Abstract: The sulfur electrodes were prepared from sulfur, carbon, and PEO as a binder. Poly(ethylene-oxide) with LiCF3SO3 was used as a solid polymer electrolyte for Li/S cell. Sulfur content of the sulfur electrode was 70wt%, and the carbon content was varied from 10wt% to 25wt%. The weight ratio of PEO and LiCF3SO3 in the polymer electrolyte was 9:1. The lithium/PEO/sulfur cell showed two plateau potential regions (2.4V, 2.1V) and high discharge capacity, i.e., 1068mAh/g(63.7% utilization of sulfur). The discharge capacity decreased drastically during charge-discharge cycling. The capacity fade depended on the composition of sulfur electrode regardless of similar initial discharge capacity. The sulfur electrode with high carbon content retained high capacity after repeated cycling. The optimum composition of 70wt% sulfur electrode was composed of 20wt% carbon and 10% PEO.