Young-Kyu Han

Hierarchical Hollow Spheres of Fe2O3@Polyaniline for Lithium Ion Battery Anodes

Hierarchical hollow spheres of Fe2 O3 @polyaniline are fabricated by template-free synthesis of iron oxides followed by a post in- and exterior construction. A combination of large surface area with porous structure, fast ion/electron transport, and mechanical integrity renders this material attractive as a lithium-ion anode, showing superior rate capability and cycling performance.

Improved reversibility in lithium-oxygen battery: Understanding elementary reactions and surface charge engineering of metal alloy catalyst

Most Li-O2 batteries suffer from sluggish kinetics during oxygen evolution reactions (OERs). To overcome this drawback, we take the lesson from other catalysis researches that showed improved catalytic activities by employing metal alloy catalysts. Such research effort has led us to find Pt3Co nanoparticles as an effective OER catalyst in Li-O2 batteries. The superior catalytic activity was reflected in the substantially decreased overpotentials and improved cycling/rate performance compared to those of other catalysts. Density functional theory calculations suggested that the low OER overpotentials are associated with the reduced adsorption strength of LiO2 on the outermost Pt catalytic sites. Also, the alloy catalyst generates amorphous Li2O2 conformally coated around the catalyst and thus facilitates easier decomposition and higher reversibility. This investigation conveys an important message that understanding elementary reactions and surface charge engineering of air-catalysts are one of the most effective approaches in resolving the chronic sluggish charging kinetics in Li-O2 batteries.

Anisotropic Volume Expansion of Crystalline Silicon during Electrochemical Lithium Insertion: An Atomic Level Rationale

The volume expansion of silicon is the most important feature for electrochemical operations of high capacity Si anodes in lithium ion batteries. Recently, the unexpected anisotropic volume expansion of Si during lithiation has been experimentally observed, but its atomic-level origin is still unclear. By employing first-principles molecular dynamics simulations, herein, we report that the interfacial energy at the phase boundary of amorphous Li(x)Si/crystalline Si plays a very critical role in lithium diffusion and thus volume expansion. While the interface formation turns out to be favorable at x = 3.4 for all of the (100), (110), and (111) orientations, the interfacial energy for the (110) interface is the smallest, which is indeed linked to the preferential volume expansion along the <110> direction because the preferred (110) interface would promote lithiation behind the interface. Utilizing the structural characteristic of the Si(110) surface, local Li density at the (110) interface is especially high reaching Li(5.5)Si. Our atomic-level calculations enlighten the importance of the interfacial energy in the volume expansion of Si and offer an explanation for the previously unsolved perspective.

Structure of Au8: planar or nonplanar?

We have studied the structures and stabilities of Au6 and Au8 at the density-functional theory (DFT) and ab initio correlated levels of theory. For Au8, our ab initio calculations predict the lowest Au8 isomer to be planar, in line with the DFT calculations.

Enhanced Pseudocapacitance of Ionic Liquid/Cobalt Hydroxide Nanohybrids

Development of nanostructured materials with enhanced redox reaction capabilities is important for achieving high energy and power densities in energy storage systems. Here, we demonstrate that the nanohybridization of ionic liquids (ILs, 1-butyl-3-methylimidazolium tetrafluoroborate) and cobalt hydroxide (Co(OH)2) through ionothermal synthesis leads to a rapid and reversible redox reaction. The as-synthesized IL-Co(OH)2 has a favorable, tailored morphology with a large surface area of 400.4 m(2)/g and a mesopore size of 4.8 nm. In particular, the IL-Co(OH)2-based electrode exhibits improvement in electrochemical characteristics compared with bare Co(OH)2, showing a high specific capacitance of 859 F/g at 1 A/g, high-rate capability (∼95% retention at 30 A/g), and excellent cycling performance (∼96% retention over 1000 cycles). AC impedance analysis demonstrates that the introduction of ILs on Co(OH)2 facilitates ion transport and charge transfer: IL-Co(OH)2 shows a higher ion diffusion coefficient (1.06 × 10(-11) cm(2)/s) and lower charge transfer resistance (1.53 Ω) than those of bare Co(OH)2 (2.55 × 10(-12) cm(2)/s and 2.59 Ω). Our density functional theory (DFT) calculations reveal that the IL molecules, consisting of anion and cation groups, enable easier hydrogen desorption/adsorption process, that is, a more favorable redox reaction on the Co(OH)2 surface.

Spin–orbit effects on the transactinide p-block element monohydrides MH (M=element 113–118)

Spin–orbit effects on the bond lengths and dissociation energies of sixth- and seventh-row p-block element monohydrides MH(M=Tl–Rn and element 113–118) are evaluated using relativistic effective core potentials at the coupled-cluster level of theory. Spin–orbit effects play a dominant role in the determination of molecular properties for the seventh-row hydrides. Spin–orbit effects on the bond lengths and dissociation energies of seventh-row hydrides are qualitatively similar to, but substantially larger than those of the sixth-row homologs due to the enormous spin–orbit splitting of 7p orbitals. Spin–orbit interactions change the bond lengths of sixth- and seventh-row hydrides by −0.02∼+0.03 A and −0.21∼+0.21 A , respectively. Spin–orbit interactions usually elongate the bond lengths except for the molecules of the (p1/2)1-valence atoms, i.e., TlH and (113)H. The maximum elongation is predicted for (115)H, where the element 115(eka-bismuth) has the (7p3/2)1 configuration outside the inner (7p1/2)2 closed...

Atom-Level Understanding of the Sodiation Process in Silicon Anode Material.

Despite the exceptionally large capacities in Li ion batteries, Si has been considered inappropriate for applications in Na ion batteries. We report an atomic-level study on the applicability of a Si anode in Na ion batteries using ab initio molecular dynamics simulations. While crystalline Si is not suitable for alloying with Na atoms, amorphous Si can accommodate 0.76 Na atoms per Si atom, corresponding to a specific capacity of 725 mA h g(-1). Bader charge analyses reveal that the sodiation of an amorphous Si electrode continues until before the local Na-rich clusters containing neutral Na atoms are formed. The amorphous Na0.76Si phase undergoes a volume expansion of 114% and shows a Na diffusivity of 7 × 10(-10) cm(2) s(-1) at room temperature. Overall, the amorphous Si phase turns out quite attractive in performance compared to other alloy-type anode materials. This work suggests that amorphous Si might be a competitive candidate for Na ion battery anodes.

Remarkably efficient photocurrent generation based on a [60]fullerene-triosmium cluster/Zn-porphyrin/boron-dipyrrin triad SAM.

A new artificial photosynthetic triad array, a [60]fullerene-triosmium cluster/zinc-porphyrin/boron-dipyrrin complex (1, Os(3)C(60)/ZnP/Bodipy), has been prepared by decarbonylation of Os(3)(CO)(8)(CN(CH(2))(3)Si(OEt)(3))(mu(3)-eta(2):eta(2):eta(2)-C(60)) (6) with Me(3)NO/MeCN and subsequent reaction with the isocyanide ligand CNZnP/Bodipy (5) containing zinc porphyrin (ZnP) and boron dipyrrin (Bodipy) moieties. Triad 1 has been characterized by various spectroscopic methods (MS, NMR, IR, UV/Vis, photoluminescence, and transient absorption spectroscopy). The electrochemical properties of 1 in chlorobenzene (CB) have been examined by cyclic voltammetry; the general feature of the cyclic voltammogram of 1 is nine reversible one-electron redox couples, that is, the sum of those of 5 and 6. DFT has been applied to study the molecular and electronic structures of 1. On the basis of fluorescence-lifetime measurements and transient absorption spectroscopic data, 1 undergoes an efficient energy transfer from Bodipy to ZnP and a fast electron transfer from ZnP to C(60); the detailed kinetics involved in both events have been elucidated. The SAM of triad 1 (1/ITO; ITO=indium-tin oxide) has been prepared by immersion of an ITO electrode in a CB solution of 1 and diazabicyclo-octane (2:1 equiv), and characterized by UV/Vis absorption spectroscopy, water contact angle, X-ray photoelectron spectroscopy, and cyclic voltammetry. The photoelectrochemical properties of 1/ITO have been investigated by a standard three-electrode system in the presence of an ascorbic acid sacrificial electron donor. The quantum yield of the photoelectrochemical cell has been estimated to be 29 % based on the number of photons absorbed by the chromophores. Our triad 1 is unique when compared to previously reported photoinduced electron-transfer arrays, in that C(60) is linked by pi bonding with little perturbation of the C(60) electron delocalization.

Sodium ion diffusion in Al2O3: a distinct perspective compared with lithium ion diffusion.

Surface coating of active materials has been one of the most effective strategies to mitigate undesirable side reactions and thereby improve the overall battery performance. In this direction, aluminum oxide (Al2O3) is one of the most widely adopted coating materials due to its easy synthesis and low material cost. Nevertheless, the effect of Al2O3 coating on carrier ion diffusion has been investigated mainly for Li ion batteries, and the corresponding understanding for emerging Na ion batteries is currently missing. Using ab initio molecular dynamics calculations, herein, we first find that, unlike lithiation, sodiation of Al2O3 is thermodynamically unfavorable. Nonetheless, there can still exist a threshold in the Na ion content in Al2O3 before further diffusion into the adjacent active material, delivering a new insight that both thermodynamics and kinetics should be taken into account to describe ionic diffusion in any material media. Furthermore, Na ion diffusivity in NaxAl2O3 turns out to be much higher than Li ion diffusivity in LixAl2O3, a result opposite to the conventional stereotype based on the atomic radius consideration. While hopping between the O-rich trapping sites via an Na-O bond breaking/making process is identified as the main Na ion diffusion mechanism, the weaker Na-O bond strength than the Li-O counterpart turns out to be the origin of the superior diffusivity of Na ions.

Two-component calculations for the molecules containing superheavy elements: Spin–orbit effects for (117)H, (113)H, and (113)F

We have calculated bond lengths, harmonic vibrational frequencies, and dissociation energies for (117)H, (113)H, and (113)F using relativistic effective core potentials (RECPs) with one-electron spin–orbit operators at the two-component coupled-cluster levels of theory. It is shown that any reasonable theoretical descriptions of the electronic structures of molecules containing superheavy elements require consideration of relativistic interactions and electron correlations. Comparisons with available all-electron Dirac–Fock (DF) based results indicate that our two-component approaches are very promising tools in the calculations for the molecules containing superheavy elements. The spin–orbit effects calculated from one- and two-component RECPs are in good agreement with those from all-electron Douglas–Kroll and DF results, implying that the potential average scheme is useful for obtaining one-component RECPs even for superheavy elements. Spin–orbit and electron correlation effects are not additive for mo...