The Effect of Oxygen Vacancies on the Diffusion Characteristics of Zn(II) Ions in the Perovskite SrTiO3 Layer: A Computational Study.

A highly polar perovskite SrTiO3 (STO) layer is considered as one of the promising artificial protective layers for the Zn metal anode of aqueous zinc-ion batteries (AZIBs). Although it has been reported that oxygen vacancies tend to promote Zn(II) ion migration in the STO layer and thereby effectively suppress Zn dendrite growth, there is still a lack of a basic understanding of the quantitative effects of oxygen vacancies on the diffusion characteristics of Zn(II) ions. In this regard, we comprehensively studied the structural features of charge imbalances caused by oxygen vacancies and how these charge imbalances affect the diffusion dynamics of Zn(II) ions by utilizing density functional theory and molecular dynamics simulations. It was found that the charge imbalances are typically localized close to vacancy sites and those Ti atoms that are closest to them, whereas differential charge densities close to Sr atoms are essentially non-existent. We also demonstrated that there is virtually no difference in structural stability between the different locations of oxygen vacancies by analyzing the electronic total energies of STO crystals with the different vacancy locations. As a result, although the structural aspects of charge distribution strongly rely on the relative vacancy locations within the STO crystal, Zn(II) diffusion characteristics stay almost consistent with changing vacancy locations. No preference for vacancy locations causes isotropic Zn(II) ion transport inside the STO layer, which subsequently inhibits the formation of Zn dendrites. Due to the promoted dynamics of Zn(II) ions induced by charge imbalance near the oxygen vacancies, the Zn(II) ion diffusivity in the STO layer monotonously increases with the increasing vacancy concentration ranging from 0% to 16%. However, the growth rate of Zn(II) ion diffusivity tends to slow down at relatively high vacancy concentrations as the imbalance points become saturated across the entire STO domain. The atomic-level understanding of the characteristics of Zn(II) ion diffusion demonstrated in this study is expected to contribute to developing new long-life anode systems for AZIBs.

Recent Advances in Transition Metal Dichalcogenide Cathode Materials for Aqueous Rechargeable Multivalent Metal-Ion Batteries.

The generation of renewable energy is a promising solution to counter the rapid increase in energy consumption. Nevertheless, the availability of renewable resources (e.g., wind, solar, and tidal) is non-continuous and temporary in nature, posing new demands for the production of next-generation large-scale energy storage devices. Because of their low cost, highly abundant raw materials, high safety, and environmental friendliness, aqueous rechargeable multivalent metal-ion batteries (AMMIBs) have recently garnered immense attention. However, several challenges hamper the development of AMMIBs, including their narrow electrochemical stability, poor ion diffusion kinetics, and electrode instability. Transition metal dichalcogenides (TMDs) have been extensively investigated for applications in energy storage devices because of their distinct chemical and physical properties. The wide interlayer distance of layered TMDs is an appealing property for ion diffusion and intercalation. This review focuses on the most recent advances in TMDs as cathode materials for aqueous rechargeable batteries based on multivalent charge carriers (Zn2+, Mg2+, and Al3+). Through this review, the key aspects of TMD materials for high-performance AMMIBs are highlighted. Furthermore, additional suggestions and strategies for the development of improved TMDs are discussed to inspire new research directions.

Design of highly efficient phosphor-converted white light-emitting diodes with color rendering indices (R1 - R15) ≥ 95 for artificial lighting.

Phosphor-converted white light-emitting diodes (pc-WLEDs) are excellent energy-efficient light sources for artificial lighting applications. One goal of artificial lighting is to make objects/images look natural - as they look under the sunlight. The ability of a light source to accurately render the natural color of an object is gauged by the parameter - color rendering index (CRI). A conventional pc-WLED has an average CRI ~ 80, which is very low for accurate color reproduction. To utilize the pc-WLEDs for artificial lighting applications, all the CRI points (R1 - R15) should be above 95. However, there is a trade-off between CRI and luminous efficacy (LER), and it is challenging to increase both CRI and LER. Herein we propose a novel LED package (PKG) design to achieve CRI points ≥95 and efficiency ~100 lm/W by introducing two blue LEDs and a UV LED in combination with green and red phosphors. The silicone encapsulant, the current through the LEDs, and the green/red phosphor ratio were optimized for achieving high CRI and LER. Our re-designed LED PKG will find applications in stadium lighting as well as for ultra-high-definition television production where high CRI points are required for the artificial light source.

Effect of alkyl branches on the thermal stability of quaternary ammonium cations in organic electrolytes for electrochemical double layer capacitors.

Quaternary ammoniums are cations having widespread use in organic electrolytes for high performance electrochemical double layer capacitors (EDLCs) due to their various advantages such as high electrochemical stability and inexpensive production cost. However, the decomposition of quaternary ammoniums via Hofmann elimination hinders their applications for EDLCs operating at elevated temperatures. This study systematically investigates the reactivity of four different quaternary ammoniums (tetraethyl-, triethylmethyl-, diethyldimethyl-, and trimethylethyl-ammonium) in EDLC by utilizing density functional theory calculations and Brownian dynamics simulations complemented with molecular dynamics simulations. It is found that ammonium stability reduces upon increasing the number of ethyl branches that have a stronger positive charge than the methyl groups. However, the contribution of the entropy change to the reaction free energy makes trimethylethylammonium less stable than diethyldimethylammonium at room temperature although the former has less ethyl branches than the latter. Trimethylethylammonium becomes the most stable at a high temperature of 488 K above which the activation free energy becomes effectively negligible and thus the number of reactive sites determines the overall stability. The fundamental understanding of the ammonium decompositions through Hofmann elimination demonstrated in this study is expected to contribute to developing new long-life organic electrolyte systems for high-temperature applications.

Collective degrees of freedom involved in absorption and desorption of surfactant molecules in spherical non-ionic micelles.

Dynamics of absorption and desorption of a surfactant monomer into and out of a spherical non-ionic micelle is investigated by coarse-grained molecular dynamics (MD) simulations. It is shown that these processes involve a complex interplay between the micellar structure and the monomer configuration. A quantitative model for collective dynamics of these degrees of freedom is developed. This is accomplished by reconstructing a multi-dimensional free energy landscape of the surfactant-micelle system using constrained MD simulations in which the distance between the micellar and monomer centers of mass is held constant. Results of this analysis are verified by direct (unconstrained) MD simulations of surfactant absorption in the micelle. It is demonstrated that the system dynamics is likely to deviate from the minimum energy path on the energy landscape. These deviations create an energy barrier for the monomer absorption and increase an existing barrier for the monomer desorption. A reduced Fokker-Planck equation is proposed to model these effects.

Molecular transport through surfactant-covered oil-water interfaces: role of physical properties of solutes and surfactants in creating energy barriers for transport.

Mechanisms of molecular transport across oil-water interfaces covered by nonionic surfactants are investigated using coarse-grained molecular dynamics simulations. Resistance of the surfactant monolayer to the solute transport is shown to be controlled by dense regions in the monolayer. The dense regions are formed on both sides of the dividing surface and the barrier to the solute transport is created by those of them experiencing unfavorable interactions with the solute. Resistance to the transport of a hydrophobic (hydrophilic) solute increases with the excess density of the head (tail) group region of the monolayer, which in turn increases with the length of the surfactant head (tail) group. Barriers for solute transport through surfactant monolayers are also influenced by the solute size. However, the extent of this influence is determined by the monolayer thickness and the solute structure and composition. For example, it is shown that resistance offered by thin monolayers to transport of linear oligomers is relatively insensitive to the solute length. The barrier sensitivity to the length of these solutes increases with the monolayer thickness. In addition to the static barriers, the solute transport is shown to be affected by dynamic barriers due to a nonadiabatic coupling of the monolayer surface with the solute position and configuration. This coupling leads to deviations of the system dynamics from the minimum energy path. The deviations are most significant in the neighborhood of the static energy barrier, which effectively leads to an increase of the barrier for the solute transport.