Metavalently bonded tellurides: the essence of improved thermoelectric performance in elemental Te.

Elemental Te is important for semiconductor applications including thermoelectric energy conversion. Introducing dopants such as As, Sb, and Bi has been proven critical for improving its thermoelectric performance. However, the remarkably low solubility of these elements in Te raises questions about the mechanism with which these dopants can improve the thermoelectric properties. Indeed, these dopants overwhelmingly form precipitates rather than dissolve in the Te lattice. To distinguish the role of doping and precipitation on the properties, we have developed a correlative method to locally determine the structure-property relationship for an individual matrix or precipitate. We reveal that the conspicuous enhancement of electrical conductivity and power factor of bulk Te stems from the dopant-induced metavalently bonded telluride precipitates. These precipitates form electrically beneficial interfaces with the Te matrix. A quantum-mechanical-derived map uncovers more candidates for advancing Te thermoelectrics. This unconventional doping scenario adds another recipe to the design options for thermoelectrics and opens interesting pathways for microstructure design.

Phase diagrams and superconductivity of ternary Ca-Al-H compounds under high pressure.

The search for high-temperature superconductors in hydrides under high pressure has always been a research hotspot. Hydrogen-based superconductors offer an avenue to achieve the long-sought goal of superconductivity at room temperature. Here we systematically explored the high-pressure phase diagram, electronic properties, lattice dynamics and superconductivity of the ternary Ca-Al-H system using ab initio methods. At 80 GPa, CaAlH5 transforms from Cmcm to P21/m phase. Both of Cmcm-CaAlH5 and Pnnm-CaAl2H8 are semiconductors. At 200 GPa, P4/mmm-CaAlH7 and a metastable compound Immm-Ca2AlH12 were found. Furthermore, P4/mmm-CaAlH7 shows obvious softening of the high frequency vibration modes, which improves the strength of electron-phonon coupling. Therefore, a superconducting transition temperature Tc of 71 K is generated in P4/mmm-CaAlH7 at 50 GPa. In addition, the thermodynamic metastable Immm-Ca2AlH12 exhibits a superconducting transition temperature of 118 K at 250 GPa. These results are very useful for the experimental searching of new high-Tc superconductors in ternary hydrides. Our work may provide an opportunity to search for high Tc superconductors at lower pressure.

Low Thermal Conductivity and Optimized Thermoelectric Properties of p-Type Te-Sb2Se3: Synergistic Effect of Doping and Defect Engineering.

Elemental tellurium exhibits a promising thermoelectric performance, largely due to the optimization of the carrier concentration stemming from effective chemical doping. In this study, we demonstrate a novel approach to realize the collaborative manipulation of the electrical and thermal transport properties in the Te system via introduction of Sb2Se3. A series of p-type Te1-x(Sb2Se3)x (0 ≤ x ≤ 0.2) samples were fabricated through the melting method followed by spark plasma sintering. Electrically, antimony as a successful dopant enables a remarkable improvement of the carrier concentration, from ∼1018 to ∼1019 cm-3, thus resulting in a desired power factor across the entire temperature range. Thermally, utilization of defects engineering containing point defects, grain boundaries, dislocations, and secondary phase precipitates effectively reduces the lattice thermal conductivity. The coexistence of multi-frequency phonon scattering centers derived from the addition of Sb2Se3 leads to a minimum lattice thermal conductivity of 0.5 W m-1 K-1, approaching the amorphous limit. As a result, Te0.95(Sb2Se3)0.05 shows the highest figure of merit ZT ∼1 at 600 K, comparable to that of the toxic Te(As) thermoelectrics. This work not only points out that synergistic effects of both doping and defect engineering play a vital role in decoupling the thermoelectric parameters in the Te1-x(Sb2Se3)x system but also gives a referential strategy for a higher thermoelectric performance in other Te-based materials.

Interconnected Ultrasmall V2O3 and Li4Ti5O12 Particles Construct Robust Interfaces for Long-Cycling Anodes of Lithium-Ion Batteries.

Designing composite structures of active materials is critical for high-performance lithium-ion batteries, as it determines the reversibility of lithium-ion insertion and extraction of the electrodes. The V2O3 anode has a high specific capacity but presents poor cycling stability due to a large volume change. Herein, a novel C@V2O3-Li4Ti5O12 composite with ultrastable cycling stability is constructed. In this composite structure, the interconnected ultrasmall V2O3 and Li4Ti5O12 nanoparticles (5-10 nm) construct robust interfaces in the carbon matrix. The Li4Ti5O12 nanoparticles with excellent cycling stability and a minor volume change act as fixtures that effectively restrict the volume change of V2O3 nanoparticles and improve the cycling stability of the C@V2O3-Li4Ti5O12 composite. The C@V2O3-Li4Ti5O12 composite maintains no degradation during 500 cycles under a current density of 100 mA g-1. The results demonstrate that constructing a highly stable interface between the active nanoparticles with smaller and larger volume changes is of great significance to suppress their pulverization and achieve high reversibility. This work contributes to a new strategy to design the structure of long-cycling anode materials for highly stable lithium-ion batteries.