Sang-Chul Lee

Origin and hysteresis of lithium compositional spatiodynamics within battery primary particles

Watching batteries fail Rechargeable batteries lose capacity in part because of physical changes in the electrodes caused by electrochemical cycling. Lim et al. track the reaction dynamics of an electrode material, LiFePO4, by measuring the relative concentrations of Fe(II) and Fe(III) in it by means of high-resolution x-ray absorption spectrometry (see the Perspective by Schougaard). The exchange current density is then mapped for Li+ insertion and removal. At fast cycling rates, solid solutions form as Li+ is removed and inserted. However, at slow cycling rates, nanoscale phase separation occurs within battery particles, which eventually shortens battery life. Science, this issue p. 566; see also p. 543 X-ray microscopy shows the nanoscale evolution of the composition and reaction rate inside a Li-ion battery during cycling. The kinetics and uniformity of ion insertion reactions at the solid-liquid interface govern the rate capability and lifetime, respectively, of electrochemical devices such as Li-ion batteries. Using an operando x-ray microscopy platform that maps the dynamics of the Li composition and insertion rate in LixFePO4, we found that nanoscale spatial variations in rate and in composition control the lithiation pathway at the subparticle length scale. Specifically, spatial variations in the insertion rate constant lead to the formation of nonuniform domains, and the composition dependence of the rate constant amplifies nonuniformities during delithiation but suppresses them during lithiation, and moreover stabilizes the solid solution during lithiation. This coupling of lithium composition and surface reaction rates controls the kinetics and uniformity during electrochemical ion insertion.

Thermal conductivity anisotropy and grain structure in Ge2Sb2Te5 films

Although lateral thermal conduction in Ge2Sb2Te5 (GST) films can influence the performance of phase change memory (PCM), there are no data available for the in-plane thermal conductivity. This work measures both the in-plane and the out-of-plane thermal conductivities for the amorphous, face-centered-cubic, and hexagonal-close-packed phases of GST using two independent techniques. For crystalline GST, we report anisotropy favoring out-of-plane conduction by up to 54%, which varies with annealing time. Scaling arguments indicate that the anisotropy may be due to the thermal resistance of amorphous regions near grain boundaries. This explanation is consistent with transmission electron microscopy images showing columnar grains and amorphous phase at grain boundaries.

Chemical and Phase Evolution of Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production

Amorphous MoSx is a highly active, earth-abundant catalyst for the electrochemical hydrogen evolution reaction. Previous studies have revealed that this material initially has a composition of MoS3, but after electrochemical activation, the surface is reduced to form an active phase resembling MoS2 in composition and chemical state. However, structural changes in the MoSx catalyst and the mechanism of the activation process remain poorly understood. In this study, we employ transmission electron microscopy (TEM) to image amorphous MoSx catalysts activated under two hydrogen-rich conditions: ex situ in an electrochemical cell and in situ in an environmental TEM. For the first time, we directly observe the formation of crystalline domains in the MoSx catalyst after both activation procedures as well as spatially localized changes in the chemical state detected via electron energy loss spectroscopy. Using density functional theory calculations, we investigate the mechanisms for this phase transformation and find that the presence of hydrogen is critical for enabling the restructuring process. Our results suggest that the surface of the amorphous MoSx catalyst is dynamic: while the initial catalyst activation forms the primary active surface of amorphous MoS2, continued transformation to the crystalline phase during electrochemical operation could contribute to catalyst deactivation. These results have important implications for the application of this highly active electrocatalyst for sustainable H2 generation.

Effects of Particle Size, Electronic Connectivity, and Incoherent Nanoscale Domains on the Sequence of Lithiation in LiFePO4 Porous Electrodes

High-resolution X-ray microscopy is used to investigate the sequence of lithiation in LiFePO4 porous electrodes. For electrodes with homogeneous interparticle electronic connectivity via the carbon black network, the smaller particles lithiate first. For electrodes with heterogeneous connectivity, the better-connected particles preferentially lithiate. Correlative electron and X-ray microscopy also reveal the presence of incoherent nanodomains that lithiate as if they are separate particles.

Equilibrium oxygen storage capacity of ultrathin CeO2-δ depends non-monotonically on large biaxial strain

Elastic strain is being increasingly employed to enhance the catalytic properties of mixed ion–electron conducting oxides. However, its effect on oxygen storage capacity is not well established. Here, we fabricate ultrathin, coherently strained films of CeO2-δ between 5.6% biaxial compression and 2.1% tension. In situ ambient pressure X-ray photoelectron spectroscopy reveals up to a fourfold enhancement in equilibrium oxygen storage capacity under both compression and tension. This non-monotonic variation with strain departs from the conventional wisdom based on a chemical expansion dominated behaviour. Through depth profiling, film thickness variations and a coupled photoemission–thermodynamic analysis of space-charge effects, we show that the enhanced reducibility is not dominated by interfacial effects. On the basis of ab initio calculations of oxygen vacancy formation incorporating defect interactions and vibrational contributions, we suggest that the non-monotonicity arises from the tetragonal distortion under large biaxial strain. These results may guide the rational engineering of multilayer and core–shell oxide nanomaterials.

Effect of plate thickness on particle deposition velocity onto a face-up flat plate situated parallel to an airflow

The effect of plate thickness on particle deposition velocity onto a face-up flat plate in a parallel airflow was examined both numerically and experimentally. Plate thickness was varied as 0.05, 0.925, 2.3, and 6.35 mm, by considering flat plates of negligible thickness, 450 mm wafers, 5 in photomasks, and 6 in EUVL photomasks, respectively. Statistical Lagrangian particle tracking (SLPT) model with the use of commercial codes was employed. The SLPT model was validated by comparing the numerically obtained particle deposition velocities with either the theoretically predicted or the experimentally determined particle deposition velocities, for the flat plates of various thicknesses. Then, the effect of plate thickness on particle deposition velocity onto the face-up flat plate in a parallel airflow was investigated by employing the SLPT model. It was found that the effect of plate thickness should be taken into account, when the particle deposition velocity onto a 5 in photomask (2.3 mm thick) or a 6 in ...

Software plagiarism detection via the static API call frequency birthmark

In this paper, we propose a system for detecting software plagiarism using a birthmark. The birthmark is representative features of a program, which can be used to identify the program. We use a set of frequency of APIs used in a program as its birthmark. The proposed system consists of three components. First, it extracts the frequency of APIs employed in a program. Next, it generates the program birthmark using a set of frequency of APIs and weights to APIs to extract unique features of the program. Finally, it decides the plagiarism based on the cosine similarity between the birthmarks. Through extensive experiments, it was found that the proposed system can provide 97.2% of precision and 95.7% of recall in plagiarism detection.

On exploiting content and citations together to compute similarity of scientific papers

In computing the similarity of scientific papers, previous text-based and link-based similarity measures look at only a single side of the content and citations. In this paper, we propose a novel approach called SimCC that effectively combines the content and citation information to accurately compute the similarity of scientific papers. Unlike previous approaches, SimCC effectively represents both authority and context of a scientific paper simultaneously in computing similarities. Also, we propose SimCC+A to consider recently-published papers. The effectiveness of our proposed method is demonstrated via extensive experiments on a real-world dataset of scientific papers, with more than 100% improvement in accuracy compared with previous methods.

Inert matrix fuel - A new challenge for material technology in the nuclear fuel cycle

An innovative nuclear fuel concept for the utilization as energy resources and for the incineration of excess Pu arisings as well as for an effective transmutation of minor actinides (MAs; Am, Np and Cm) is discussed from the aspect of material technology. Stabilized cubic phase ZrO2 and other potential candidate materials for the Inert Matrix are compared in terms of the material properties and other behaviors such as the behavior against irradiation with the relevant information currently available. Strategies for the use of the Inert Matrix Fuel concept in various countries are discussed and compared for their options in nuclear fuel cycle technology.

Electrode Lithiation: Effects of Particle Size, Electronic Connectivity, and Incoherent Nanoscale Domains on the Sequence of Lithiation in LiFePO4 Porous Electrodes (Adv. Mater. 42/2015)

On page 6591, W. Chueh and co-workers use high-resolution X-ray microscopy to study the sequence of lithiation in LiFePO4 battery electrodes and reveal that local electronic connectivity limits the rate capability. For electrodes with homogeneous interparticle electronic connectivity via the carbon black network, the smaller particles lithiate first. For electrodes with heterogeneous connectivity, the better-connected particles preferentially lithiate.