Jong-Pil Jegal

Solid-state microwave irradiation synthesis of high quality graphene nanosheets under hydrogen containing atmosphere

High quality graphene nanosheets were fabricated within 1 min by solid-state microwave irradiation of a mixture of graphite oxide and graphene nanosheets under a hydrogen atmosphere. The graphene nanosheets in the mixture acted as an effective microwave susceptor under microwave irradiation synthesis, and could provide sufficiently rapid heating for the effective exfoliation of graphite oxide. A hydrogen containing atmosphere played important roles in improving the quality of the graphene nanosheets by increasing the level of reduction and preventing the formation of defects in the graphene nanosheets. The graphene nanosheets thus obtained exhibited a specific surface area of 586 m2 g−1 and an outstanding carbon/oxygen ratio of 18.5.

A lithium iron phosphate/nitrogen-doped reduced graphene oxide nanocomposite as a cathode material for high-power lithium ion batteries

A LiFePO4/nitrogen-doped reduced graphene oxide nanocomposite was synthesized using a solution-based method followed by heat treatment. The nitrogen-doped reduced graphene oxide surrounding the LiFePO4 nanoparticles facilitates the transfer of electrons throughout the electrodes, which significantly reduces the internal resistance of the electrodes, resulting in high utilization of LiFePO4. Electrodes fabricated with the LiFePO4/nitrogen-doped reduced graphene nanocomposite show high discharge capacities and voltages at high rates including sub-zero temperature conditions, even at commercially acceptable loading levels.

Three-Dimensional Layer-by-Layer Anode Structure Based on Co3O4 Nanoplates Strongly Tied by Capillary-like Multiwall Carbon Nanotubes for Use in High-Performance Lithium-Ion Batteries

A layer-by-layer (LBL) structure composed of Co3O4 nanoplates and capillary-like three-dimensional (3D) multiwall carbon nanotube (MWCNT) nets was developed as an anode with simultaneous high-rate and long-term cycling performance in a lithium-ion battery. As the current density was increased to 50 A g(-1), the LBL structure exhibited excellent long-term cycling and rate performance. Thus, the Co3O4 nanoplates were in good electrical contact with the capillary-like 3D MWCNT nets under mechanically severe strain during long-term, high-rate cyclic operation.

Synthesis of LiMn0.75Fe0.25PO4/C microspheres using a microwave-assisted process with a complexing agent for high-rate lithium ion batteries

LiMn0.75Fe0.25PO4/C microspheres were synthesized using a microwave-assisted process with a complexing agent. In this process, it was found that the various states of the complexing agent for different pH values of the precursor solution have significant effects on the obtained micro-spherical morphology. Furthermore, the concentration of antisite defects in the samples was also found to be affected by the pH of the precursor. The prepared secondary spheres have a high tap density of 1.3 g cm−3 and deliver a reversible capacity of 163 mA h g−1 at a 0.05 C-rate. Furthermore, remarkable rate capability is obtained, with 57% capacity retention at a 60 C-rate, as well as excellent cyclability, with 99.3% capacity retention after 100 cycles at 1 C-rate.

In situ fabrication of lithium titanium oxide by microwave-assisted alkalization for high-rate lithium-ion batteries

A phase-pure Li4Ti5O12/reduced graphene oxide nanocomposite was prepared using a simple one-pot synthesis of the Li–Ti–O precursor and subsequent heat treatment. The prepared nanocomposite delivers a reversible capacity of 168 mA h g−1 at 1 C-rate and a remarkable rate capability with 59% capacity retention at 50 C-rate.

Phase Transition Method To Form Group 6A Nanoparticles on Carbonaceous Templates

Considerable effort has been made to develop unique methods of preparing and characterizing nanoparticles and nanocomposites in order to exploit the true potential of nanotechnology. We used a facile, versatile phase-transition method for forming Group 6A nanoparticles on carbonaceous templates to produce homogeneous 5-10 nm diameter Group 6A nanoparticles on carbon nanotubes (CNTs) and reduced graphene oxide (RGO), to obtain nanocomposites. The method involved melting and recrystallizing mixtures of elemental sulfur and either CNTs or RGO on carbonaceous templates. The surface tension and hydrophilicity of the molten Group 6A species surfaces and the oxygen functional groups on the carbonaceous template surfaces were considered in depth to provide important guidelines for forming Group 6A nanoparticles on carbonaceous templates. The surface tension of the molten Group 6A species should be intrinsically low, leading to effective wetting on the carbonaceous template. In addition, the molten Group 6A species hydrophilic surfaces were essential for enabling hydrophilic-hydrophilic interaction for selective wetting at the oxygen functional groups on the carbonaceous template, leading to the heterogeneous nucleation of nanoparticles. Furthermore, the size and morphology (isolated vs layer-like) of the Group 6A nanoparticles were tuned by adjusting the oxidation state of the carbonaceous template. We investigated the potential application of the nanocomposites prepared using this method to cathode materials in lithium-sulfur secondary batteries.