Da Wang

A sulfur host based on titanium monoxide@carbon hollow spheres for advanced lithium–sulfur batteries

Lithium–sulfur batteries show advantages for next-generation electrical energy storage due to their high energy density and cost effectiveness. Enhancing the conductivity of the sulfur cathode and moderating the dissolution of lithium polysulfides are two key factors for the success of lithium–sulfur batteries. Here we report a sulfur host that overcomes both obstacles at once. With inherent metallic conductivity and strong adsorption capability for lithium-polysulfides, titanium monoxide@carbon hollow nanospheres can not only generate sufficient electrical contact to the insulating sulfur for high capacity, but also effectively confine lithium-polysulfides for prolonged cycle life. Additionally, the designed composite cathode further maximizes the lithium-polysulfide restriction capability by using the polar shells to prevent their outward diffusion, which avoids the need for chemically bonding all lithium-polysulfides on the surfaces of polar particles.

First-Principles Study of Phosphorene and Graphene Heterostructure as Anode Materials for Rechargeable Li Batteries

There is a great desire to develop the high-efficient anodes materials for Li batteries, which require not only large capacity but also high stability and mobility. In this work, the phosphorene/graphene heterostructure (P/G) was carefully explored based on first-principles calculations. The binding energy of Li on the pristine phosphorene is relatively weak (within 1.9 eV), whereas the phosphorene/graphene heterostructure (P/G) can greatly improve the binding energy (2.6 eV) without affecting the high mobility of Li within the layers. The electronic structures show that the large Li adsorption energy and fast diffusion ability of the P/G origin from the interfacial synergy effect. Interestingly, the P/G also displays ultrahigh stiffness (Cac = 350 N/m, Czz = 464 N/m), which can effectively avoid the distortion of the pristine phosphorene after the insertion of lithium. Thus, P/G can greatly enhance the cycle life of the battery. Owing to the high capacity, good conductivity, excellent Li mobility, and ultrahigh stiffness, P/G is a very promising anode material in Li-ion batteries (LIBs).

Pristine and defect-containing phosphorene as promising anode materials for rechargeable Li batteries

The pristine and defect-containing phosphorene as promising anode materials for Li-ion batteries (LIBs) have been systematically investigated by first-principles calculations. The results suggest that the binding energies of Li adsorption on the different sites vary within a narrow range, and the binding between Li atom and pristine phosphorene is relatively weak. Interestingly, the defect can greatly improve the performance of the phosphorene in terms of both binding energy and diffusion of Li on phosphorene. The binding energy of Li around the vacancy created is increased by about 1 eV compared to that of the perfect phosphorene. More importantly, Li atoms could diffuse between two adjacent grooves with an energy barrier of 0.13 eV, which opens a novel channel for Li diffusion. This would dramatically improve the fast charge/discharge capability. These interesting properties indicate that the defect-containing PP has great potential to be a good electrode material in LIBs.

β-MnO2 as a cathode material for lithium ion batteries from first principles calculations

The search for excellent cathodes for lithium batteries is the main topic in order to meet the requirements of low cost, high safety, and high capacity in many real applications. β-MnO2, as a potential candidate, has attracted great attention because of its high stability and potential high capacity among all the phases. Because of the complexity of β-MnO2, some fundamental questions at the atomic level during the charge-discharge process, remain unclear. The lithiation process of β-MnO2 has been systematically examined by first-principles calculations along with cluster expansion techniques. Five stable configurations during the lithium intercalation process are firstly determined, and the electrochemical voltages are from 3.47 to 2.77 eV, indicating the strongly correlated effects of the β-MnO2-LiMnO2 system. During the lithiation process, the changes in the lattice parameters are not symmetric. The analysis of electronic structures shows that Mn ions are in the mixed valence states of Mn(3+) and Mn(4+) during the lithiation process, which results in Jahn-Teller distortion in Mn(3+)O6 octahedra. Such results uncover the intrinsic origin of the asymmetric deformation during the charge-discharge process, resulting in the irreversible capacity fading during cycling. From the analysis of the thermal reduction of delithiated LixMnO2, the formation of oxygen is thermodynamically infeasible in the whole extraction process. Our results indicate that β-MnO2 has great potential as a cathode material for high capacity Li-ion batteries.

Diverse and tunable electronic structures of single-layer metal phosphorus trichalcogenides for photocatalytic water splitting

The family of bulk metal phosphorus trichalcogenides (APX3, A = M(II), M(I)(0.5)M(III)(0.5); X = S, Se; M(I), M(II), and M(III) represent Group-I, Group-II, and Group-III metals, respectively) has attracted great attentions because such materials not only own magnetic and ferroelectric properties, but also exhibit excellent properties in hydrogen storage and lithium battery because of the layered structures. Many layered materials have been exfoliated into two-dimensional (2D) materials, and they show distinct electronic properties compared with their bulks. Here we present a systematical study of single-layer metal phosphorus trichalcogenides by density functional theory calculations. The results show that the single layer metal phosphorus trichalcogenides have very low formation energies, which indicates that the exfoliation of single layer APX3 should not be difficult. The family of single layer metal phosphorus trichalcogenides exhibits a large range of band gaps from 1.77 to 3.94 eV, and the electronic structures are greatly affected by the metal or the chalcogenide atoms. The calculated band edges of metal phosphorus trichalcogenides further reveal that single-layer ZnPSe3, CdPSe3, Ag0.5Sc0.5PSe3, and Ag0.5In0.5PX3 (X = S and Se) have both suitable band gaps for visible-light driving and sufficient over-potentials for water splitting. More fascinatingly, single-layer Ag0.5Sc0.5PSe3 is a direct band gap semiconductor, and the calculated optical absorption further convinces that such materials own outstanding properties for light absorption. Such results demonstrate that the single layer metal phosphorus trichalcogenides own high stability, versatile electronic properties, and high optical absorption, thus such materials have great chances to be high efficient photocatalysts for water-splitting.

An ab initio study of TiS3: a promising electrode material for rechargeable Li and Na ion batteries

Titanium trisulfide (TiS3) was recently reported to be highly promising as an electrode material for Li-ion batteries, due to its multielectron processes with high theoretical capacity. However, theoretical work on the performance and mechanism of Li adsorption in bulk and monolayer TiS3 is still lacking. The constraint of lithium resource also requires replacement by an abundant material such as Na. Using first principles calculations based on density functional theory, this study extensively investigates the electronic structure, adsorption and diffusion properties, capacity and plateaus of Li and Na atoms in bulk and monolayer TiS3. The results reveal that as the thickness of the TiS3 material decreased to a monolayer, a transition from an indirect band gap to a direct band gap was induced. Both the difference in charge density and the Bader charge analysis show that a significant charge transfer occurs from a Li or Na adatom to its neighboring sulfur atoms. Additionally, in bulk and monolayer TiS3, both Li and Na show two diffusion pathways with a low diffusion barrier, and one pathway can be further enhanced as the TiS3 changes from bulk to monolayer. Moreover, monolayer TiS3 shows a lower energy barrier for Na atoms, and there is also no problem associated with volume expansion in bulk TiS3. At high Li/Na concentrations, the Li/Na atoms can also diffuse easily, and one diffusion pathway is viable in bulk TiS3, which is effective for direct diffusion. All these properties are promising for the development of Li and Na batteries based on bulk and monolayer TiS3.

Single-layer Group-IVB nitride halides as promising photocatalysts

In order to construct efficient solar-driven devices, many potential materials have been explored in search of desirable photocatalyts for water splitting. Layered structure nitride halides have received significant attention from different fields because of their unusual electronic properties. In this work, we have systematically studied the electronic structures and potential photocatalytic properties of single-layer Group-IVB nitride halides (MNX, M = Ti, Zr, Hf; X = Cl, Br, I) in different forms using first-principles calculations. The results show that the single-layer nitride halides have very low formation energies, which indicates that the isolation of these single-layer MNX materials should not be difficult. The calculated band structures reveal that all of the single-layer MNX are semiconductors, while each of them shows a distinct type of electronic properties. Among these semiconducting nitride halides, ten members of the single-layer MNX family are feasible photocatalysts for splitting water. Interestingly, single-layer α-ZrNX (X = Cl, Br, I) and α-HfNI are direct band gap semiconductors with desirable band gaps (2.23–2.83 eV), and the calculated optical adsorption spectra further confirm their excellent light absorption in visible light region. Finally, the electronic properties and optical absorption in visible light region of single-layer MNX can be easily tuned through hybridisation or doping between them because of the similarity of the MNXs. Their high stability, versatile electronic properties, and high optical absorption make single-layer Group-IVB nitride halides promising candidates for application in photocatalytic water splitting.

Robust vanadium pentoxide electrodes for sodium and calcium ion batteries: thermodynamic and diffusion mechanical insights

It has long been a critical challenge to find suitable electrodes for rechargeable Na/Ca-ion batteries (NIBs/CIBs) with superior electrochemical performance. Vanadium pentoxides offer the prospect of serving as cathodes in the development of high-capacity NIBs and CIBs. Here the concentration-dependent electrochemical characteristics of Na- and Ca-ions with α- and δ-V2O5 are examined using density functional theory with Hubbard U corrections. Multiple low energy configurations, stemming from the different ionic concentrations, are identified to evaluate the stability of α- and δ-V2O5 upon Na/Ca intercalation. It is computationally predicted that the α phase is more stable than the δ phase during both Na and Ca intercalation processes. Additionally, the energy barriers for Ca diffusion in α-V2O5 at high concentration are higher than that in δ-V2O5 (0.975–1.825 eV compared to 0.735–1.385 eV), which suggests that cycling V2O5 exclusively in the δ phase may improve performance. More importantly, lower surface-to-bulk diffusion barriers of 0.498 and 0.846 eV are found for Na- and Ca-ion insertion at the (010) surface, which account for the improved electrochemical properties found in nanostructured V2O5 compared to their bulk counterparts. Our results provide crucial insights into the thermodynamic and electrochemical response of V2O5 to Na/Ca-ion intercalation, thus contributing to the design of high capacity NIBs/CIBs.

First-Principles Study of Novel Two-Dimensional (C4H9NH3)2PbX4 Perovskites for Solar Cell Absorbers

Low-dimensional perovskites (A2BX4), in which the A cations are replaced by different organic cations, may be used for photovoltaic applications. In this contribution, we systematically study the two-dimensional (2D) (C4H9NH3)2PbX4 (X═Cl, Br and I) hybrid perovskites by density functional theory (DFT). A clear structures-properties relationship, with the photophysical characteristics directly related to the dimensionality and material compositions, was established. The strong s-p antibonding couplings in both bulk and monolayer (C4H9NH3)2PbI4 lead to low effective masses for both holes (mh*) and electrons (me*). However, mh* increases in proportion to the decreasing inorganic layer thickness, which eventually leads to a slightly shifted band edge emission found in 2D perovskites. Notably, the 2D (C4H9NH3)2PbX4 perovskites exhibit strong optical transitions in the visible light spectrum, and the optical absorption tunings can be achieved by varying the compositions and the layer thicknesses. Such work paves an important way to uncover the structures-properties relationship in 2D perovskites.

Iced photochemical reduction to synthesize atomically dispersed metals by suppressing nanocrystal growth

Photochemical solution-phase reactions have been widely applied for the syntheses of nanocrystals. In particular, tuning of the nucleation and growth of solids has been a major area of focus. Here we demonstrate a facile approach to generate atomically dispersed platinum via photochemical reduction of frozen chloroplatinic acid solution using ultraviolet light. Using this iced-photochemical reduction, the aggregation of atoms is prevented, and single atoms are successfully stabilized. The platinum atoms are deposited on various substrates, including mesoporous carbon, graphene, carbon nanotubes, titanium dioxide nanoparticles, and zinc oxide nanowires. The atomically dispersed platinum on mesoporous carbon exhibits efficient catalytic activity for the electrochemical hydrogen evolution reaction, with an overpotential of only 65 mV at a current density of 100 mA cm−2 and long-time durability (>10 h), superior to state-of-the-art platinum/carbon. This iced-photochemical reduction may be extended to other single atoms, for example gold and silver, as demonstrated in this study.Photochemical synthesis is a popular approach to fabricate metallic nanoparticles, however stabilizing individually-dispersed atoms by this method remains challenging. Here, the authors freeze their precursor solution prior to UV irradiation to obtain atomically-dispersed platinum catalysts with high electrocatalytic performance.