Joonhyeon Kang

Effective wrapping of graphene on individual Li4Ti5O12 grains for high-rate Li-ion batteries

An effective way of synthesizing graphene-wrapped Li4Ti5O12 particles was developed by solid-state reaction between graphene oxide-wrapped P25 (TiO2) and Li2CO3. Compared to the previously reported graphene/Li4Ti5O12 composites, prior wrapping of TiO2 with subsequent chemical lithiation led to more effectively confined Li4Ti5O12. The Li4Ti5O12 tightly bound by graphene exhibited a remarkable specific capacity of 147 mA h g−1 at a rate of 10 C (1 C = 175 mA g−1) after 100 cycles. This rate capability is one of the highest values among reported Li4Ti5O12 with 150 ± 50 nm grains. The improved rate capability was attributed to the enhanced electronic conductivity of each Li4Ti5O12 grain via uniform graphene wrapping, with single-grain growth during annealing from the initial ∼25 nm TiO2 nanoparticles enclosed by outer graphene sheets. Graphene-eliminated Li4Ti5O12 by thermal decomposition was also directly compared to the graphene-coated sample, to clarify the role of graphene with nearly equivalent particle size/morphology distributions.

Facile preparation of reduced graphene oxide-based gas barrier films for organic photovoltaic devices

Reduced graphene oxide-based films were prepared to assess their effects as gas barriers on the stability of organic photovoltaic (OPV) devices. The direct spin-casting of a graphene oxide suspension onto an aluminum electrode was performed to encapsulate the associated OPV device with a reduced graphene oxide film. The lifetime of the OPV device after the reduction process was found to be increased by a factor of 50. The gas barrier properties of a graphene oxide layer are closely related to its surface roughness and dispersibility. Furthermore, these gas barrier properties can be enhanced by controlling the thermal reduction conditions. The thermal reduction of a graphene oxide film at a low heating rate results in a low water vapor permeability, only 0.1% of that of an as-prepared polyethylene naphthalate film. These results indicate that the dispersibility, surface roughness, and reduction conditions of a graphene oxide film significantly influence its gas barrier performance. Further investigations of the reduction of graphene oxide films are expected to enable further improvements in performance.

Reduced graphene oxide/carbon double-coated 3-D porous ZnO aggregates as high-performance Li-ion anode materials

The reduced graphene oxide (RGO)/carbon double-coated 3-D porous ZnO aggregates (RGO/C/ZnO) have been successfully synthesized as anode materials for Li-ion batteries with excellent cyclability and rate capability. The mesoporous ZnO aggregates prepared by a simple solvothermal method are sequentially modified through distinct carbon-based double coating. These novel architectures take unique advantages of mesopores acting as space to accommodate volume expansion during cycling, while the conformal carbon layer on each nanoparticle buffering volume changes, and conductive RGO sheets connect the aggregates to each other. Consequently, the RGO/C/ZnO exhibits superior electrochemical performance, including remarkably prolonged cycle life and excellent rate capability. Such improved performance of RGO/C/ZnO may be attributed to synergistic effects of both the 3-D porous nanostructures and RGO/C double coating.

Development of carbon-based cathodes for Li-air batteries: Present and future

Rechargeable lithium-air (Li-air) batteries are regarded as one of the most fascinating energy storage devices for use in the future electric vehicles, since Li-air batteries provide ten-times-higher theoretical capacities than those from current Li-ion batteries. Nonetheless, Li-air batteries have not yet been implemented to the market because of several major drawbacks such as low capacity, poor cycle life, and low round-trip efficiency. These battery performances are highly dependent on the design of air cathodes, thus much effort has been devoted to the development of high performance cathode. Among various materials, carbonaceous materials have been widely studied as the basis of air cathodes especially for non-aqueous Li-O2 cells due to their high electric conductivity, low cost, and ease of fabrication. This review summarizes the history, scientific background, and perspectives of Liair batteries, particularly from the viewpoint of carbon-based air cathodes.Graphical abstract

Evaluation of graphene-wrapped LiFePO4 as novel cathode materials for Li-ion batteries

Graphene-wrapped LiFePO4 (LiFePO4/G) is introduced as a cathode material for Li-ion batteries with an excellent rate capability. A straightforward solid-state reaction between graphene oxide-wrapped FePO4 and a lithium precursor resulted in highly conducting LiFePO4/G composites, which feature ∼70 nm-sized LiFePO4 crystallites with robust connection to the external graphene network. This unique morphology enables all LiFePO4 particles to be readily accessed by electrons during battery operation, leading to a remarkably enhanced rate capability. The in situ electrochemical impedance spectra were studied in detail throughout charge and discharge processes, by which enhanced electronic conductance and thereby reduced charge transfer resistance was confirmed as the origin of the superior performance in the novel LiFePO4/G.

OP22.02: The value of the ultrasonographic score in predicting successful induction of labor

Conclusions: It might be expected that risk of T21 would be the major factor in deciding whether to have an amniocentesis. Although we have shown that risk is indeed a factor that women use to make that decision, our data support our experience that the risk itself is of relatively little importance and other issues are frequently the deciding factor. The issues to resolve are what are the most important factors in the decision making process and what would women chose if not divided into low and high risk but just given their figure?

Optimum Morphology of Mixed-Olivine Mesocrystals for a Li-Ion Battery

In this present work, we report on the synthesis of micron-sized LiMn0.8Fe0.2PO4 (LMFP) mesocrystals via a solvothermal method with varying pH and precursor ratios. The morphologies of resultant LMFP secondary particles are classified into two major classes, flakes and ellipsoids, both of which are featured by the mesocrystalline aggregates where the primary particles constituting LMFP secondary particles are crystallographically aligned. Assessment of the battery performance reveals that the flake-shaped LMFP mesocrystals exhibit a specific capacity and rate capability superior to those of other mesocrystals. The origin of the enhanced electrochemical performance is investigated in terms of primary particle size, pore structure, antisite-defect concentration, and secondary particle shape. It is shown that the shape of the secondary particle has just as much of a significant effect on the battery performance as the crystallite size and antisite defects do. We believe that this work provides a rule of design for electrochemically favorable meso/nanostructures, which is of great potential for improving battery performance by tuning the morphology of particles on multilength scales.