Young Jun Yun

Polyethylenimine-Mediated Electrostatic Assembly of MnO2 Nanorods on Graphene Oxides for Use as Anodes in Lithium-Ion Batteries.

In recent years, the development of electrochemically active materials with excellent lithium storage capacity has attracted tremendous attention for application in high-performance lithium-ion batteries. MnO2-based composite materials have been recognized as one of promising candidates owing to their high theoretical capacity and cost-effectiveness. In this study, a previously unrecognized chemical method is proposed to induce intra-stacked assembly from MnO2 nanorods and graphene oxide (GO), which is incorporated as an electrically conductive medium and a structural template, through polyethylenimine (PEI)-derived electrostatic modulation between both constituent materials. It is revealed that PEI, a cationic polyelectrolyte, is capable of effectively forming hierarchical, two-dimensional MnO2-RGO composites, enabling highly reversible capacities of 880, 770, 630, and 460 mA·h/g at current densities of 0.1, 1, 3, and 5 A/g, respectively. The role of PEI in electrostatically assembled composite materials is clarified through electrochemical impedance spectroscopy-based comparative analysis.

Thermally stable white-emitting single composition Na(Sr,Ba)PO 4 :Eu 2+ , Mn 2+ phosphor for near-ultraviolet-pumped light-emitting diodes

Eu(2+) and Mn(2+) codoped, white-emitting Na(Sr,Ba)PO(4) phosphors are prepared, and their emission properties, especially for thermal stability, are thoroughly investigated. The thermal quenching and Eu(2+)/Mn(2+) energy transfer efficiency are totally different in the ratio of alkaline earth metals in host composition, NaBaPO(4), Na(Sr(0.5)Ba(0.5))PO(4), and NaSrPO(4), respectively. Furthermore, by using near-ultraviolet light-emitting diodes (LEDs) and the corresponding Na(Sr(0.5)Ba(0.5))PO(4):Eu(2+), Mn(2+) phosphor as light converters, we demonstrate a bright and thermally stable white-emitting LED. The resultant LED exhibits a warm white light [~4900 K, CIE coordinates of (0.33, 0.31)] with excellent thermal and hydrolytic stabilities comparable to those of commercially available ones, Y(3)Al(5)O(12):Ce(3+) and BaMg(2)Al(16)O(27):Eu(2+). The proposed composition, with its efficient energy transfer, could enable Eu(2+) and Mn(2+) codoped Na(Sr,Ba)PO(4) to be a promising single component phosphor for cost-effective white-emitting LEDs.

Superior Lithium Storage Performance using Sequentially Stacked MnO2/Reduced Graphene Oxide Composite Electrodes

Hybrid nanostructures based on graphene and metal oxides hold great potential for use in high-performance electrode materials for next-generation lithium-ion batteries. Herein, a new strategy to fabricate sequentially stacked α-MnO2 /reduced graphene oxide composites driven by surface-charge-induced mutual electrostatic interactions is proposed. The resultant composite anode exhibits an excellent reversible charge/discharge capacity as high as 1100 mA h g(-1) without any traceable capacity fading, even after 100 cycles, which leads to a high rate capability electrode performance for lithium ion batteries. Thus, the proposed synthetic procedures guarantee a synergistic effect of multidimensional nanoscale media between one (metal oxide nanowire) and two dimensions (graphene sheet) for superior energy-storage electrodes.

Color tuning and thermal quenching of different sensitizer ion, Mn 2+ or Ce 3+ , doped Ba 2 Mg(BO 3 ) 2 :Eu 2+ phosphor

Ba2Mg(BO3)2:Eu2+ phosphors incorporated with two different sensitizers, Mn2+ or Ce3+, were prepared and their emission properties, especially for color purity and thermal stability, were investigated thoroughly. The overall emission property induced by the Eu2+ ion and the resultant thermal behavior were strongly dependent on the type of codoped sensitizer ions, Mn2+ or Ce3+. Intense red emission peaking at 620 nm was obtained upon 370 nm excitation of the Mn2+-sensitized phosphor and the resultant light-emitting diode lamps using the given phosphor exhibited a more reddish emission with chromaticity coordinates of (0.602, 0.340). Thus, we can meet the purpose of illuminating elements by designing the proper chemical composition of the Eu2+-activated Ba2Mg(BO3)2 phosphor using different sensitizers: a more reddish emitting Mn2+-sensitized one for backlight units and a bright yellow-emitting Ce3+-sensitized one for solid-state lightings.

Morphology Effect on Enhanced Li

The electrochemical performance of Li(Mn, M)PO4 (M = Co2+/3+, Ni2+/3+) was investigated with regard to the particle morphology. Within a controlled chemical composition, Li(Mn0.92Co0.04Ni0.04)PO4, the resultant cathode exhibited somewhat spherical-shaped nanocrystalline particles and enhanced Li+-ion storage, which was even better than the undoped LiMnPO4, up to 16% in discharge capacity at 0.05 C. The outstanding electrochemical performance is attributed to the well-dispersed spherical-shaped particle morphology, which allows the fast Li+-ion migration during the electrochemical lithiation/delithiation process, especially at high current density.