Byungwoo Park

Preparation and Exceptional Lithium Anodic Performance of Porous Carbon-Coated ZnO Quantum Dots Derived from a Metal–Organic Framework

Hierarchically porous carbon-coated ZnO quantum dots (QDs) (~3.5 nm) were synthesized by a one-step controlled pyrolysis of the metal-organic framework IRMOF-1. We have demonstrated a scalable and facile synthesis of carbon-coated ZnO QDs without agglomeration by structural reorganization. This unique microstructure exhibits outstanding electrochemical performance (capacity, cyclability, and rate capability) when evaluated as an anode material for lithium ion batteries.

LiCoO2 Cathode Material That Does Not Show a Phase Transition from Hexagonal to Monoclinic Phase

Structural instability of LiCoO 2 can be improved by sol-gel coating of Al 2 O 3 and subsequent heat-treatments. While Al 2 O 3 phase does not exist after heat-treatments, solid solution LiCo 1-x Al x O 2 that has discretely higher Al concentration was formed at the surface up to ∼500 A inside the particle. However, heat-treatment to 700°C results in the presence of the solid solution beyond ∼500 A. The different Al concentration at the surface significantly affects the structural stability of the materials during cycling, and those prepared at 400°C do not show a phase transition from hexagonal to monoclinic phase. Disappearance of such a phase transition improves capacity retention of the cathode. Moreover, cathodes prepared at 400 and 500°C show improved layered characteristics with cation order.

Suppression of Cobalt Dissolution from the LiCoO2 Cathodes with Various Metal-Oxide Coatings

ZrO 2 -coated LiCoO 2 showed negligible capacity loss up to 70 cycles at the cutoff voltage of 4.4 V, while bare LiCoO 2 exhibited ∼60% of its original capacity after only 30 cycles. The improved electrochemical behavior was caused by the suppression of cobalt dissolution by nanoscale metal-oxide coating. The amount of cobalt dissolution in the electrolyte from the charged LiCoO 2 held at 25 and 90°C, respectively, correlates well with the capacity retention, among coatings of various metal oxides. The trend of the open-circuit voltage reduction is again well correlated with both the cobalt dissolution and capacity retention.

Synthesis, Thermal, and Electrochemical Properties of AlPO4-Coated LiNi0.8Co0.1Mn0.1 O 2 Cathode Materials for a Li-Ion Cell

Although LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode material has a larger specific capacity than LiCoO 2 , their thermal instability has hindered their use in Li-ion cells. An AlPO 4 coating on the LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode, however, noticeably diminished the violent exothermic reaction of the cathode material with the electrolyte, without sacrificing the specific capacity of the bare LiNi 0.8 Co 0.1 Mn 0.1 O 2 (188 mAh/g at 4.3 V charge cut off). The results were consistent with the thermal abuse tests using Li-ion cells; the AlPO 4 -coated LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode did not exhibit thermal runaway with smoke and explosion, in contrast to the cell containing the bare cathode. In addition, the AlPO 4 -coated LiNi 0.8 Co 0.1 Mo 0.1 O 2 cathode exhibited a superior cycle-life performance compared to the bare LiNi 0.8 Co 0.1 Mn 0.1 O 2 .

Optical and electronic properties of post-annealed ZnO:Al thin films

This study examined the optical and electronic properties of post-annealed Al-doped ZnO (ZnO:Al) thin films. The lowest resistivity was observed after annealing a sputter-deposited ZnO:Al film at 350 °C. X-ray photoelectron spectroscopy revealed a ∼0.4 eV shift in the Fermi level when the carrier concentration was increased to 1.6×1020 cm−3 by Al doping and annealing. The optical band gap increased from 3.2 eV for insulating ZnO to 3.4 eV for conducting ZnO:Al, and was associated with conduction-band filling up to ∼0.4 eV in a renormalized band gap. Schematic band diagrams are shown for the ZnO and ZnO:Al films.

Ultrathin Zirconium Disulfide Nanodiscs

We present a colloidal route for the synthesis of ultrathin ZrS(2) (UT-ZrS(2)) nanodiscs that are ~1.6 nm thick and consist of approximately two unit cells of S-Zr-S. The lateral size of the discs can be tuned to 20, 35, or 60 nm while their thickness is kept constant. Under the appropriate conditions, these individual discs can self-assemble into face-to-face-stacked structures containing multiple discs. Because the S-Zr-S layers within individual discs are held together by weak van der Waals interactions, each UT-ZrS(2) disc provides spaces that can serve as host sites for intercalation. When we tested UT-ZrS(2) discs as anodic materials for Li(+) intercalation, they showed excellent nanoscale size effects, enhancing the discharge capacity by 230% and greatly improving the stability in comparison with bulk ZrS(2). The nanoscale size effect was especially prominent for their performance in fast charging/discharging cycles, where an 88% average recovery of reversible capacity was observed for UT-ZrS(2) discs with a lateral diameter of 20 nm. The nanoscale thickness and lateral size of UT-ZrS(2) discs are critical for fast and reliable intercalation cycling because those dimensions both increase the surface area and provide open edges that enhance the diffusion kinetics for guest molecules.

Mixture Behavior and Microwave Dielectric Properties in the Low-fired TiO2–CuO System

The mixture behavior and microwave dielectric properties of TiO2 doped with CuO sintered at around 900°C for 2 h were investigated using X-ray powder diffraction and a network analyzer. Low-fired TiO2 with 2% CuO had a quality factor of 14000, relative dielectric constant (er) of 98, and a temperature coefficient of resonant frequency (τf) of 374 ppm/°C. The microwave dielectric properties of low-fired, CuO doped TiO2 could be interpreted by observing the dielectric properties of CuO: high loss tangent (tan δ), low dielectric constant, and a negative temperature coefficient of resonant frequency. The microwave dielectric properties of low-fired, CuO doped TiO2 showed a dependence on the mixture formation of TiO2 and CuO. More importantly, the er of low-fired TiO2 with CuO could be predicted by the logarithmic mixing model. Therefore, the variation of the microwave dielectric properties was attributed to the mixture behavior of TiO2 and CuO.

Correlation between strain and dielectric properties in ZrTiO4 thin films

Single-phase paraelectric ZrTiO4 thin films were synthesized at various temperatures using direct-current magnetron reactive sputtering. The dielectric constants (e) and dielectric losses (tan δ) of as-deposited and annealed films were measured in the 100 kHz range using a Pt upper electrode and a phosphorous-doped Si (100) bottom electrode. Data showed that as the deposition temperature increased, the dielectric losses decreased, while the dielectric constants did not change much. Similar trends for dielectric losses were observed after annealing at 800 °C. These results of dielectric losses correlated well with strains in ZrTiO4 thin films, analyzed from x-ray diffraction peak widths at various scattering angles.

Comparison of Overcharge Behavior of AlPO4-Coated LiCoO2 and LiNi0.8Co0.1Mn0.1 O 2 Cathode Materials in Li-Ion Cells

The overcharge behavior of AlPO 4 -coated Li x Ni 0.8 Co 0.1 Mn 0.1 O 2 cathodes was compared to that of AlPO 4 -coated Li x CoO 2 cathodes in Li-ion cells. The former exhibited less heat generation than the latter as the charge voltage increased. Moreover, the coated Li x Ni 0.8 Co 0.1 Mn 0.1 O 2 cathode showed two distinct sets of exothermic peaks beginning at ∼75 and ∼200°C, respectively, while the coated Li x CoO 2 exhibited relatively continuously increasing exothermic peak beginning at ∼150°C. The results were consistent with the 12 V overcharge tests using Li-ion full cells. As the C rate increased from 1 to 3 C, the cell-surface temperature with the AlPO 4 -coated Li x Ni 0.8 Co 0.1 Mn 0.1 O 2 cathode did not exceed ∼125°C, and that of the coated Li x CoO 2 exceeded ∼170°C. However, the Li-ion cells containing either cathode with the AlPO 4 -nanoparticle coating did not exhibit thermal runaway, although short-circuits occurred at 12 V during a 3 C overcharging rate. It is evident that the AlPC 4 -coating layer on the powder drastically reduced the violent exothermic reaction between the electrolyte and cathode, controlling the overall safety of the Li-ion cells.

The effects of 100 nm-diameter Au nanoparticles on dye-sensitized solar cells

Gold nanoparticles of ∼100 nm in diameter were incorporated into TiO2 nanoparticles for dye-sensitized solar cells (DSSCs). At the optimum Au/TiO2 mass ratio of 0.05, the power-conversion efficiency of the DSSC improved to 3.3% from a value of 2.7% without Au, and this improvement was mainly attributed to the photocurrent density. The Au nanoparticles embedded in the nanoparticulate-TiO2 film strongly absorbed light due to the localized surface-plasmon resonance, and thereby promoted light absorption of the dye. In the DSSCs, the 100 nm-diameter Au nanoparticles generate field enhancement by surface-plasmon resonance rather than prolonged optical paths by light scattering.