Kyung Suk Yun

Electrochemical properties of PAN-based carbon fibers as anodes for rechargeable lithium ion batteries

Abstract Polyacrylonitrile(PAN)-based carbon fibers were tested as anodes for lithium ion rechargeable batteries. PAN-based fibers were first stabilized under tension in air at about 200°C (stabilization tension) and then carbonized in different gas environments (carbonization atmospheres) at heat treatment temperatures (HTT) between 700 and 1500°C. The carbon fiber electrodes were prepared at various conditions using the stabilized PAN fibers and then their electrochemical characteristics were investigated. The PAN-based carbon fiber prepared at an oxidative stabilization tension of ca. 10 MPa showed the highest discharge capacity in our experimental range. We found that the effective diffusion coefficient of lithium in the carbon fiber electrode was influenced by the carbonization environment employed. The electrochemical intercalation process depended on mass transfer of lithium into carbon layer which is rate-determining during the electrode charge–discharge process. The effect of HTT on discharge capacity varied depending on the combined effect of both the amount of intercalation sites available and the electric conductivity of the carbon fiber used.

Electrochemical properties and performance of poly(acrylonitrile)-based polymer electrolyte for Li/LiCoO2 cells

Abstract It has been demonstrated that LiCoO 2 is a very attractive cathode active material for lithium rechargeable cells. Poly(acrylonitrile) (PAN)-based polymer electrolyte is used for Li/LiCoO 2 cells. PAN-based polymer electrolyte shows ionic conductivity of the order 1 mS cm −1 at room temperature, irrespective of time evolution, and wide electrochemical stability up to 4.3 V (versus Li + /Li). An LiCoO 2 composite cathode containing 4 wt.% conductive material displays a good cycling performance. From a.c. impedance results, the interfacial resistance of Li/LiCoO 2 cells is dominated by the passive layer formed at the lithium/polymer electrolyte interface.

Corrosion behaviour of Zr1−xTixV0.6Ni1.2M0.2 (M=Ni, Cr, Mn) AB2-type metal hydride alloys in alkaline solution

Abstract An examination is made of the discharge and cycle life of Zr 0.5 Ti 0.5 V 0.6 Ni 1.4 alloys when a fraction (0.2 at.%) of the Ni-component is substituted by Cr or Mn. In addition, the Zr:Ti component ratios are varied to extend the cycle life of high capacity, Mn-substituted Zr 1− x Ti x V 0.6 Ni 1.2 Mn 0.2 ( x =0.0, 0.25, 0.5, 0.75) alloys. The metallurgical microstructure is observed by X-ray diffraction analysis, scanning electron microscopy, and energy dispersive X-ray analysis. Active–passive potentiodynamic behaviour, as well as charge–discharge cycle characteristics, is evaluated, and dissolved V-species in the electrolytic solution is analyzed by inductively coupled plasma spectroscopy. The corrosion behaviour of the V–Cr or the V–Mn phase in the alkaline electrolyte solution is found to determine the cycle life of an AB 2 alloy. Cr-substituted (Zr 0.5 Ti 0.5 Ni 1.2 Cr 0.2 ) alloy, containing a V–Cr phase, is estimated to involve a dissolution rate of 0.028 wt.% vanadium per cycle in an alkaline electrolytic solution, while Mn-substituted (Zr 0.5 Ti 0.5 V 0.6 Ni 1.2 Mn 0.2 ) alloy, containing a V–Mn phase, is estimated to have a dissolution rate of 0.138 wt.% vanadium per cycle. For Mn-substituted alloys, an optimum Zr:Ti ratio of 3:1, i.e., Zr 0.75 Ti 0.25 V 0.6 Ni 1.2 Mn 0.2 , is found to have the most stable cycle life. The improvement in cycle life caused by increasing the Zr content in the alloy is attributed to increase in the corrosion resistance of the alloy due to less formation of the corrosive V–Mn phase.

The effects of Cu-doping in V2O5 thin film cathode for microbattery

Copper-doped vanadium oxide (CuxV2O5) thin film cathode materials for a thin film microbattery have been prepared by DC reactive magnetron co-sputtering with O2/Ar ratio of 10/90 and compared with pure V2O5 thin film. The film structures have been characterized by x-ray diffraction analysis, transmission electron microscopy, Auger electron spectroscopy and X-ray photoelectron spectroscopy. X-ray diffraction and TEM studies show that the CuxV2O5 film was amorphous and phenomenal behavior of copper present in thin film with substrate has been explained by thermodynamical model. Copper doping helps to increase the thickness of the film more than 1 micrometer resulting increase of total capacity. Cycling behavior of the CuxV2O5/Lipon/Li configuration cell system was beyond 500 cycles with average capacity of 50 μAh/cm2-μm, which is higher than the pure V2O5 thin film system.

A MOSFET type sensor for oxygen sensing using LaF3 as a gate material

Abstract A MOSFET type sensor for O2 gas detection with a gate material of LaF3 film deposited on an ordinary n-channel MOSFET was fabricated. The LaF3 film of 800 A thickness and the Pt film of 500 A thickness were deposited, sequentially, by the e-beam evaporation method. The amount of drain current was decreased by about 130 μA with O2 gas and the response and regeneration times of this sensor were 1 and 17 min at 80 °C, respectively, when measured by the constant voltage method (VD = 3 V, VG = 0 V). In the constant voltage method, the higher the operating temperature, the shorter the response time. As oxygen gas was injected, the gate voltage of this sensor was shifted to 30 mV and its response time was 3.5 min with the constant current method (ID = 500 nA). When the oxygen concentration was 0.05% in nitrogen, the gate voltage ratio of the oxygen gas sensor approached 0.5.

Discharge characteristics of an Li/LiCoO2 cell with poly(acrylonitirile)-based polymer electrolyte

Abstract The electrochemical properties of poly(acrylonitrile) (PAN)-based polymer electrolyte and the discharge characteristics of an Li/LiCoO 2 cell were investigated. The PAN-based polymer electrolyte maintained an ionic conductivity of 1 mS/cm at room temperature, irrespective of storage time. Also, this electrolyte showed high electrochemical stability up to 4.3 V (versus Li + /Li). An Li/LiCoO 2 cell battery with this electrolyte has been fabricated and tested. This battery showed good cycling performance.