The past two decades have witnessed the rapid growth of thermoelectric (TE) research. Novel concepts and paradigms are described here that have emerged, targeting superior TE materials and higher TE performance. These superior aspects include band convergence, "phonon-glass electron-crystal", multiscale phonon scattering, resonant states, anharmonicity, etc. Based on these concepts, some new TE materials with distinct features have been identified, including solids with high band degeneracy, with cages in which atoms rattle, with nanostructures at various length scales, etc. In addition, the performance of classical materials has been improved remarkably. However, the figure of merit zT of most TE materials is still lower than 2.0, generally around 1.0, due to interrelated TE properties. In order to realize an "overall zT > 2.0," it is imperative that the interrelated properties are decoupled more thoroughly, or new degrees of freedom are added to the overall optimization problem. The electrical and thermal transport must be synergistically optimized. Here, a detailed discussion about the commonly adopted strategies to optimize individual TE properties is presented. Then, four main compromises between the TE properties are elaborated from the point of view of the underlying mechanisms and decoupling strategies. Finally, some representative systems of synergistic optimization are also presented, which can serve as references for other TE materials. In conclusion, some of the newest ideas for the future are discussed.

Developing high‐performance thermoelectric materials is one of the crucial aspects for direct thermal‐to‐electric energy conversion. Herein, atomic scale point defect engineering is introduced as a new strategy to simultaneously optimize the electrical properties and lattice thermal conductivity of thermoelectric materials, and (Bi,Sb)2(Te,Se)3 thermoelectric solid solutions are selected as a paradigm to demonstrate the applicability of this new approach. Intrinsic point defects play an important role in enhancing the thermoelectric properties. Antisite defects and donor‐like effects are engineered in this system by tuning the formation energy of point defects and hot deformation. As a result, a record value of the figure of merit ZT of ≈1.2 at 445 K is obtained for n‐type polycrystalline Bi2Te2.3Se0.7 alloys, and a high ZT value of ≈1.3 at 380 K is achieved for p‐type polycrystalline Bi0.3Sb1.7Te3 alloys, both values being higher than those of commercial zone‐melted ingots. These results demonstrate the promise of point defect engineering as a new strategy to optimize thermoelectric properties.

Solid-state thermoelectric technology offers a promising solution for converting waste heat to useful electrical power. Both high operating temperature and high figure of merit zT are desirable for high-efficiency thermoelectric power generation. Here we report a high zT of ∼1.5 at 1,200 K for the p-type FeNbSb heavy-band half-Heusler alloys. High content of heavier Hf dopant simultaneously optimizes the electrical power factor and suppresses thermal conductivity. Both the enhanced point-defect and electron–phonon scatterings contribute to a significant reduction in the lattice thermal conductivity. An eight couple prototype thermoelectric module exhibits a high conversion efficiency of 6.2% and a high power density of 2.2 W cm−2 at a temperature difference of 655 K. These findings highlight the optimization strategy for heavy-band thermoelectric materials and demonstrate a realistic prospect of high-temperature thermoelectric modules based on half-Heusler alloys with low cost, excellent mechanical robustness and stability.

Microstructure manipulation plays an important role in enhancing physical and mechanical properties of materials. Here a high figure of merit zT of 1.2 at 357 K for n‐type bismuth‐telluride‐based thermoelectric (TE) materials through directly hot deforming the commercial zone melted (ZM) ingots is reported. The high TE performance is attributed to a synergistic combination of reduced lattice thermal conductivity and maintained high power factor. The lattice thermal conductivity is substantially decreased by broad wavelength phonon scattering via tuning multiscale microstructures, which includes microscale grain size reduction and texture loss, nanoscale distorted regions, and atomic scale lattice distotions and point defects. The high power factor of ZM ingots is maintained by the offset between weak donor‐like effect and texture loss during the hot deformation. The resulted high zT highlights the role of multiscale microstructures in improving Bi2Te3‐based materials and demonstrates the effective strategy in enhancing TE properties.

High performance p-type half-Heusler compounds FeNb1−xTixSb are developed via a band engineering approach and a record zT of 1.1 is achieved.

Electron and phonon transport characteristics determines the potential of thermoelectric materials for power generation or refrigeration. This work shows that, different from most of high performance thermoelectric materials with dominant acoustic phonon scattering, the promising ZrNiSn based half‐Heusler thermoelectric solid solutions exhibit an alloy scattering dominated charge transport. A low deformation potential and a low alloy scattering potential are found for the solid solutions, which is beneficial to maintain a relatively high electron mobility despite of the large effective mass, and can be intrinsic favorable features contributing to the noticeably high power factors of ZrNiSn based alloys. A quantitive description of the different phonon scattering mechanisms suggests that the point defect scattering is the most important mechanism that determines the phonon transport process of the solid solutions. The present results indicate that alloying can be an effective approach for such materials systems to enhance thermoelectric figure of merit ZT by reducing phonon thermal conductivity, while minimizing the deterioration of charge mobility due to the low alloy scatteirng potential.

Nanotubes of quasilayered bismuth telluride compound were prepared by hydrothermal synthesis. Nanotubes have diameters smaller than 100 nm and spiral tube-walls. The low-dimensional morphology and hollow structure enable bismuth telluride nanotubes to be a potential thermoelectric material with a high figure of merit due to the efficient phonon blocking effect. The experimental results show that the addition of nanotubes leads to a remarkable decrease in the thermal conductivity with the electrical conductivity much less affected and thus to an increase in the figure of merit of the Bi 2 Te 3-based material.

respectively the temperature of the hot side and cold side, Δ T = T H -T C and T avg are the temperature gradient and average between hot and cold sides. [ 4 ] The fi gure of merit zT is defi ned as zT = α 2 σT /( κ e + κ L ), where α , σ , T , κ e and κ L are respectively the Seebeck coeffi cient, the electrical conductivity, the absolute temperature, and the electrical and lattice components of total thermal conductivity κ . [ 2 ] High conversion effi ciency of 15%-20% is thought of as the "Holy Grail" for large scale application of TE technologies. [ 5 ] As depicted in Figure 1 , this conversion efficiency can be realized by using middle temperature (500-900 K) TE materials with very high zT or high temperature (>900 K) TE materials with compromised zT . Recently, much progress has been made in developing high effi ciency middle temperature TE materials: high zT 's of ≈1.5 at 750 K for n-type and ≈2.0 at for p-type have been achieved in PbTe-based alloys, [6][7][8][9] while n-type and p-type fi lled skutterudites have the maximum zT 's of ≈1.7 and ≈1.0, respectively, near 800 K. [ 10,11 ] Some other TE materials made of earth-abundant and non-toxic elements have also been identifi ed, such as single crystal SnSe ( zT ≈ 2.6 at 923 K), [ 12 ] silicides [ 13,14 ] and α -MgAgSb. [ 15 ] However, there remain tremendous challenges for most of these TE materials for large scale commercial application, mainly due to their relatively poor thermal stability and weak mechanical strength at high temperatures. In Half-Heusler (HH) compounds have gained ever-increasing popularity as promising high temperature thermoelectric materials. High fi gure of merit zT of ≈1.0 above 1000 K has recently been realized for both n-type and p-type HH compounds, demonstrating the realistic prospect of these high temperature compounds for high effi ciency power generation. Here, recent progress in advanced fabrication techniques and the intrinsic atomic disorders in HH compounds, which are linked to the understanding of the electrical transport, is discussed. Thermoelectric transport features of n-type ZrNiSn-based HH alloys are particularly emphasized, which is benefi cial to further improving thermoelectric performance and comprehensively understanding the underlying mechanisms in HH thermoelectric materials. The rational design and realization of new high performance p-type Fe(V,Nb)Sb-based HH compounds are also demonstrated. The outlook for future research directions of HH thermoelectric materials is also discussed.