Sun-il Mho

Identification of Two Luminescence Sites of Sr2SiO4:Eu2+ and (Sr,Ba)2SiO4:Eu2+ Phosphors

Europium activated strontium orthosilicate Sr2SiO4:Eu phosphors were prepared by a solid state reaction method using flux. Two emissions bands of blueish-green at 490 nm (SI) and of yellow at 560 nm (SII) were observed. The two emission bands are assigned to the 4f-5d transition of Eu ions in two different cation sites in α’-Sr2SiO4 orthorhombic lattices. The relative intensities of two emission bands and the efficient yellow-emission strongly depend on the fluxes used for the preparation process and the Eu concentration in the Sr2SiO4:Eu phosphors. The emission intensity of Eu was also increased especially for the SII site, by incorporating Ba into the lattice (BaxSr2-xSiO4).

Characteristics of Conducting Polymer-Coated Nanosized LiFePO4 Cathode in the Li+ Batteries

Both nano-sized and micron-sized LiFePO4 crystalline particles with rhombus structure were synthesized by a hydrothermal method for the cathode materials. Coating conductive polymers, such as polypyrrole (PPy) and poly-3,4-ethylenedioxythiophene (PEDOT), on the surfaces of LiFePO4 particles improved the battery capacities by increasing the ionic and electric conductivities of the cathode materials. The nano-sized LiFePO4 coated with PEDOT shows the largest specific capacity (167 mAhg for initial to 158 mAhg after 50 cycles), compared to the cases coated with PPy (165 mAhg to 127 mAhg) and with carbon (152 mAhg to 147 mAhg). Compared to the nano-sized particles, the micron-sized particles have much smaller surface-tovolume ratios, which resulted in increased diffusion distance requirements for lithium ions and, also, reduced accessibility of lithium ions.

Impedance Monitoring of the Anodic Dissolution of PANi during the V2O5 Deposition Step for the PANi/V2O5 Composite Film Preparation

Vanadium pentoxide / polyaniline (V2O5/PANi) composite films were prepared by a two-step electrochemical method for the cathode material application in lithium batteries. As a first step the PANi film was potentiodynamically grown in an acid solution containing aniline monomer, and secondly vanadium oxide was oxidatively deposited on the polyaniline film in a temperature controlled VOSO4 solution. The increased anodic current efficiency in the hot VOSO4 solution results in high contents of V2O5 in the composite films. The in-situ electrochemical impedance spectra for the second step of V2O5-deposition at the PANi-film electrodes were measured. The impedance increases as the oxidative deposition of V2O5 proceeds. Polyaniline film is also oxidized during the second V2O5-deposition step in the high temperature acidic solutions, which result in the polyaniline being overoxidized to a less conductive form.