Jeongyeon Lee

Scalable Synthesis of Few-Layer MoS2 Incorporated into Hierarchical Porous Carbon Nanosheets for High-Performance Li- and Na-Ion Battery Anodes

It is still a challenging task to develop a facile and scalable process to synthesize porous hybrid materials with high electrochemical performance. Herein, a scalable strategy is developed for the synthesis of few-layer MoS2 incorporated into hierarchical porous carbon (MHPC) nanosheet composites as anode materials for both Li- (LIB) and Na-ion battery (SIB). An inexpensive oleylamine (OA) is introduced to not only serve as a hinder the stacking of MoS2 nanosheets but also to provide a conductive carbon, allowing large scale production. In addition, a SiO2 template is adopted to direct the growth of both carbon and MoS2 nanosheets, resulting in the formation of hierarchical porous structures with interconnected networks. Due to these unique features, the as-obtained MHPC shows substantial reversible capacity and very long cycling performance when used as an anode material for LIBs and SIBs, even at high current density. Indeed, this material delivers reversible capacities of 732 and 280 mA h g(-1) after 300 cycles at 1 A g(-1) in LIBs and SIBs, respectively. The results suggest that these MHPC composites also have tremendous potential for applications in other fields.

Sulfur-loaded monodisperse carbon nanocapsules anchored on graphene nanosheets as cathodes for high performance lithium–sulfur batteries

There is a growing demand to enhance the electrical conductivity of the cathode and to restrain the fast capacity decay during a charge–discharge process in lithium–sulfur (Li–S) batteries. This can be accomplished by developing novel methods for the synthesis of nanostructured materials that can act as effective cathode hosts. In this study, monodisperse carbon nanocapsules with a diameter of ∼20 nm anchored on a graphene nanosheet (MCNC/G) were prepared by a facile strategy, which involved mixing of iron-oleate and graphene, heat treatment, and finally, acid etching of iron oxide nanoparticles. This simple synthesis method could be suitable for mass production. We loaded MCNC/G with sulfur by a melting process, and tested the performance of the resulting MCNC/G–sulfur (MCNC/G–S) composites as the cathode material. As a result, the MCNC/G–S electrode infiltrated with 60 wt% sulfur delivers a high and stable reversible capacity of 525 mA h g−1 after 100 cycles at a 0.5 C-rate with good capacity retention and excellent rate capability (630.5 mA h g−1 at a high current density of 1C). The improved electrochemical performance could be attributed to the monodisperse carbon nanocapsules in the MCNC/G composite, which lead to small volume expansion and physical confinement of sulfur due to the void spaces inside the carbon nanocapsules during the charge–discharge process. Thus, these uniquely structured monodisperse carbon nanocapsules anchored on graphene nanosheets can be promising candidates for other energy storage applications.

Scalable Synthesis of Honeycomb-like Ordered Mesoporous Carbon Nanosheets and Their Application in Lithium–Sulfur Batteries

There is a growing need to improve the electrical conductivity of the cathode and to suppress the rapid capacity decay during cycling in lithium-sulfur (Li-S) batteries. This can be achieved by developing facile methods for the synthesis of novel nanostructured carbon materials that can function as effective cathode hosts. In this Article, we report the scalable synthesis of ordered mesoporous carbon nanosheets (OMCNS) via the etching of self-assembled iron oxide/carbon hybrid nanosheets (IO-C NS), which serve as an advanced sulfur host for Li-S batteries. The obtained two-dimensional (2D) nanosheets have close-packed uniform cubic mesopores of ∼20 nm side length, and the gap between the pores is ∼4 nm, which resembles the honeycomb structure consisting of an ordered array of hexagonal pores. We loaded OMCNS with sulfur by a simple melting infusion process and evaluated the performance of the resulting OMCNS-sulfur composites as the cathode material. As a result, the sulfur-loaded OMCNS hybrid (OMCNS-S) electrode infiltrated with 70 wt % sulfur delivers a high and stable reversible capacity of 505.7 mA h g-1 after 500 cycles at 0.5 C-rate with excellent capacity retention (a decay of 0.081% per cycle) and excellent rate capability (580.6 mA h g-1 at a high current density of 2 C). The improved electrochemical properties could be attributed to the fact that the uniform cubic mesopores offer sufficient space for the volume expansion of sulfur inside them and therefore trap the polysulfides during the charging-discharging process. Therefore, these unique structured carbon nanosheets can be promising candidates for other energy-storage applications.

Transformational Leadership and Inter-Team Collaboration Exploring the Mediating Role of Teamwork Quality and Moderating Role of Team Size

We develop and test a model of the effects of transformational leadership beyond the boundaries of a team to the level of collaboration with other teams. We also test team size as a boundary condition in this relationship. In a study using a crisscross design to control common method bias, using a sample of 388 teams from a leading, global electronics company, the results from structural equation modeling (SEM) reveal that teams with transformational leaders are more likely to have higher internal teamwork quality (TWQ) and receive high inter-team collaboration ratings from other teams. Results further support the expectation that the association between transformational leadership and inter-team collaboration is mediated by TWQ and moderated by team size. The importance of transformational leadership for team-level research, as well as its hidden benefits beyond internal team boundaries, is discussed.

Multi-Heteroatom-Doped Hollow Carbon Attached on Graphene Using LiFePO4 Nanoparticles as Hard Templates for High-Performance Lithium–Sulfur Batteries

P, O, and N heteroatom-doped hollow carbon on graphene (PONHC/G) from nanosized LiFePO4 (LFP) as a hard template is shown to be a very efficient sulfur host for lithium-sulfur (Li-S) batteries. The PONHC/G made from LFP nanoparticles as hard materials provides sufficient voids with various pore sizes for sulfur storage, and doping of the carbon structures with various heteroatoms minimized dissolution/diffusion of the polysulfides. The obtained PONHC/G can store sulfur and mitigate diffusion of the dissolved polysulfide owing to the well-organized host structure and the strong chemical affinity for polysulfides because of the polarization effect of the heteroatom dopants. As a cathode, S@PONHC/G shows excellent cycle stability and rate capability, as confirmed by polysulfide adsorption analysis. Therefore, PONHC/G may show high potential as a sulfur scaffold in the commercialization of Li-S batteries through additional modification and optimization of these host materials.

Allowable Compressive Stress of Pre-Tensioned Members with Tee or Inverted Tee Sections at Transfer

In a previous research performed by the authors, the allowable compressive stress coefficient (K) in pretensioned members with rectangular section at transfer was proposed based on strength design theory. In this study, a subsequent research of an enormous analysis was performed to determine the K factor for Tee and inverted Tee section members, considering the effect of section height (h), section type, amount of tendons (), and eccentricity ratio (e/h). Based on the analysis results, the allowable compressive stress coefficients (K) for Tee and inverted Tee section members at transfer were derived, which limit the maximum allowable stresses as 80% and 70% of the compressive strengths at the time of release for Tee section and inverted Tee section, respectively. And these were larger than the allowable stresses specified in domestic and other international codes. In order to verify the proposed equations, they were compared to the test results available in literature and other codes, which showed that the allowable stresses in domestic and international codes are unconservative for the cases with low eccentricity ratios while conservative for those with high eccentricity ratios. The proposed equations, however, estimate the allowable stresses of the Tee and inverted Tee section members reasonably close to test results.