Shoutao Zhang

Phase Diagram and High-Temperature Superconductivity of Compressed Selenium Hydrides

Recent discovery of high-temperature superconductivity (Tc = 190 K) in sulfur hydrides at megabar pressures breaks the traditional belief on the Tc limit of 40 K for conventional superconductors, and opens up the doors in searching new high-temperature superconductors in compounds made up of light elements. Selenium is a sister and isoelectronic element of sulfur, with a larger atomic core and a weaker electronegativity. Whether selenium hydrides share similar high-temperature superconductivity remains elusive, but it is a subject of considerable interest. First-principles swarm structure predictions are performed in an effort to seek for energetically stable and metallic selenium hydrides at high pressures. We find the phase diagram of selenium hydrides is rather different from its sulfur analogy, which is indicated by the emergence of new phases and the change of relative stabilities. Three stable and metallic species with stoichiometries of HSe2, HSe and H3Se are identified above ~120 GPa and they all exhibit superconductive behaviors, of which the hydrogen-rich HSe and H3Se phases show high Tc in the range of 40–110 K. Our simulations established the high-temperature superconductive nature of selenium hydrides and provided useful route for experimental verification.

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Stable and metallic two-dimensional TaC2 as an anode material for lithium-ion battery

Relative to advanced cathode materials, anode materials have become one of the key factors to hamper the performance improvement of lithium-ion batteries (LIBs). Recently, two-dimensional (2D) transition metal carbides (e.g. MXenes) have drawn great attention due to their high Li storage ability. However, metal-rich 2D transition metal carbides as anodes usually need surface functionalization, leading to a decrease in the rate performances. Here, we propose that increasing the carbon composition in 2D TaxCy is beneficial for not only eliminating surface functionalization but also greatly improving battery performance. First-principles swarm structural searches were used to explore structures and stabilities of 2D TaxCy (x = 1 and y = 1–4, or x = 2 and y = 1). Besides reproducing the reported 2D TaC, TaC2 and Ta2C are found to be stable, and have high thermal stabilities. Metallic TaC2 and Ta2C provide good electronic conductivity. Intriguingly, carbon-rich TaC2 contains carbon dimers exposed on the surface, enabling it to directly adsorb Li atoms. After adsorption of two-layer Li atoms, its structural integrity is well preserved. The resultant specific capacity, diffusion energy barrier, and open-circuit-voltage (OCV) of TaC2 are much better than those of commercial graphite, f-Ti3C2 or the Ti2C monolayer. Compared with TaC2, TaC, and Ta2C as anode materials, the overall performance of carbon-rich TaxCy is better. Our work provides a useful strategy for designing new-type 2D transition metal materials for LIBs.

Understanding the role of lithium sulfide clusters in lithium–sulfur batteries

Lithium sulfide (Li2S) as an electrode material not only has high capacity but also overcomes many problems caused by pure sulfur electrodes. In particular, the battery performance of nanoscale (Li2S)n clusters is much better than that of bulk sized Li2S. However, the structures, stability, and properties of (Li2S)n clusters, which are very important to improve the performance of Li–S batteries, are still unexplored. Herein, the most stable structures of (Li2S)n (n = 1–10) are reliably determined using the advanced swarm-intelligence structure prediction method. The (Li2S)n (n ≥ 4) clusters exhibit intriguing cage-like structures, which are favorable for eliminating dangling bonds and enhancing structural stability. Compared to the Li2S monomer, each sulfur atom in the clusters is coordinated with more lithium atoms, thus lengthening the Li–S bond length and decreasing the Li–S bond activation energy. Notably, the adsorption energy gradually increases on the considered anchoring materials (AMs) as the cluster size increases. Moreover, B-doped graphene is a good AM in comparison with graphene or N-doped graphene. The predicted characteristic peaks of infrared, Raman, and electronic absorption spectra provide useful information for in situ experimental investigation. Our work represents a significant step towards understanding (Li2S)n clusters and improving the performance of Li–S batteries.

ATLAS: A real-space finite-difference implementation of orbital-free density functional theory

Abstract Orbital-free density functional theory (OF-DFT) is a promising method for large-scale quantum mechanics simulation as it provides a good balance of accuracy and computational cost. Its applicability to large-scale simulations has been aided by progress in constructing kinetic energy functionals and local pseudopotentials. However, the widespread adoption of OF-DFT requires further improvement in its efficiency and robustly implemented software. Here we develop a real-space finite-difference (FD) method for the numerical solution of OF-DFT in periodic systems. Instead of the traditional self-consistent method, a powerful scheme for energy minimization is introduced to solve the Euler–Lagrange equation. Our approach engages both the real-space finite-difference method and a direct energy-minimization scheme for the OF-DFT calculations. The method is coded into the ATLAS software package and benchmarked using periodic systems of solid Mg, Al, and Al 3 Mg. The test results show that our implementation can achieve high accuracy, efficiency, and numerical stability for large-scale simulations.

Silicon Framework-Based Lithium Silicides at High Pressures.

The bandgap and optical properties of diamond silicon (Si) are not suitable for many advanced applications such as thin-film photovoltaic devices and light-emitting diodes. Thus, finding new Si allotropes with better bandgap and optical properties is desirable. Recently, a Si allotrope with a desirable bandgap of ∼1.3 eV was obtained by leaching Na from NaSi6 that was synthesized under high pressure [Nat. Mater. 2015, 14, 169], paving the way to finding new Si allotropes. Li is isoelectronic with Na, with a smaller atomic core and comparable electronegativity. It is unknown whether Li silicides share similar properties, but it is of considerable interest. Here, a swarm intelligence-based structural prediction is used in combination with first-principles calculations to investigate the chemical reactions between Si and Li at high pressures, where seven new compositions (LiSi4, LiSi3, LiSi2, Li2Si3, Li2Si, Li3Si, and Li4Si) become stable above 8.4 GPa. The Si-Si bonding patterns in these compounds evolve with increasing Li content sequentially from frameworks to layers, linear chains, and eventually isolated Si ions. Nearest-neighbor Si atoms, in Cmmm-structured LiSi4, form covalent open channels hosting one-dimensional Li atom chains, which have similar structural features to NaSi6. The analysis of integrated crystal orbital Hamilton populations reveals that the Si-Si interactions are mainly responsible for the structural stability. Moreover, this structure is dynamically stable even at ambient pressure. Our results are also important for understanding the structures and electronic properties of Li-Si binary compounds at high pressures.

Ten-fold coordinated polymorph and metallization of TiO2 under high pressure

Titanium dioxide (TiO2) has a wide range of industrial applications (e.g., in photocatalysis and solar cells). Pressure causes profound structural and electronic changes in TiO2, leading to the fundamental modification of its physical properties. We report the metallization of TiO2 at high pressures through first-principles swarm structure searching calculations. Metallization accompanies the stabilization of a body-centered tetragonal CaC2-type structure (space group I4/mmm), which is more stable than the Fe2P-type structure above 690 GPa. This phase adopts a ten-fold coordinated structure consisting of face-sharing TiO10 dodecahedrons, the highest coordination number among all TiO2 phases known so far. In contrast to the nine-fold Fe2P-type structure, the higher coordination and denser polyhedral packing makes the CaC2-type structure energetically favorable. Our work enables an opportunity to understand the structure and electronic properties of TiO2 at high pressures.

Phase diagram, stability and electronic properties of an Fe–P system under high pressure: a first principles study

Fe–P binary compounds have attracted much attention, particularly under high pressure, since they are the constituents of the Earths core. However, most studies focus on the single stoichiometry of Fe–P binary compounds at high pressure, and their whole phase diagram and relative stabilities have been unexplored thus far. Herein, first principles swarm structure predictions are performed to find stable structures of Fe–P compounds with various FexPy (x = 1–4 and y = 1, or x = 1 and y = 2) compositions. Then, their phase diagram and relative stabilities are reliably determined based on predicted structures. Specifically, the FeP, Fe2P and Fe4P compounds are found to be stable in the pressure range of 0–400 GPa. The Fe3P compound decomposes into Fe2P and Fe4P above 214 GPa. FeP2 becomes unstable above 82 GPa. Notably, two new phases (i.e. C2/c-structured Fe4P and Cmcm-structured Fe3P) are found to be more stable than the previously reported phases. In addition, the XRD pattern of the predicted Cmcm-structured Fe3P matches the experimental patterns, and we are awaiting future experimental confirmation. Electronic band calculations show that the Fe–P binary compounds are metallic, with a pronounced Fe 3d component crossing the Fermi level. Cmcm-structured Fe3P is ferromagnetic. Our study not only provides useful information for the further study of Fe–P binary compounds but also for the determination of the Earths core components.

First-principle optimal local pseudopotentials construction via optimized effective potential method

The local pseudopotential (LPP) is an important component of orbital-free density functional theory, a promising large-scale simulation method that can maintain information on a materials electron state. The LPP is usually extracted from solid-state density functional theory calculations, thereby it is difficult to assess its transferability to cases involving very different chemical environments. Here, we reveal a fundamental relation between the first-principles norm-conserving pseudopotential (NCPP) and the LPP. On the basis of this relationship, we demonstrate that the LPP can be constructed optimally from the NCPP for a large number of elements using the optimized effective potential method. Specially, our method provides a unified scheme for constructing and assessing the LPP within the framework of first-principles pseudopotentials. Our practice reveals that the existence of a valid LPP with high transferability may strongly depend on the element.

TiC3 Monolayer with High Specific Capacity for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have attracted considerable attention due to the intrinsic safety and high abundance of sodium. However, the lack of high-performance anode materials becomes a main obstacle for the development of SIBs. Here, we identify an ideal anode material, a metallic TiC3 monolayer with not only remarkably high storage capacity of 1278 mA h g-1 but also low barrier energy and open-circuit voltage, through first-principles swarm-intelligence structure calculations. TiC3 still keeps metallic after adsorbing two-layer Na atoms, ensuring good electrical conductivity during the battery cycle. Besides, high melting point and superior dynamical stability are in favor of practical application. Its excellent performance can be mainly attributed to the presence of an unusual n-biphenyl unit in the TiC3 monolayer. High cohesive energy, originating from multibonding coexistence (e.g., covalent, ionic, and metal bonds) in the TiC3 monolayer, provides strong feasibility for experimental synthesis. In comparison with TiC3, functionalized TiC3 with oxygen shows a higher storage capacity; meanwhile, it keeps nearly the same barrier energy. This is in sharp contrast with metal-rich MXenes. These intriguing properties make the TiC3 monolayer a promising anode material for SIBs.