In Situ Reaction Induced Core-Shell Structure to Ultralow κlat and High Thermoelectric Performance of SnTe

Synergistically optimized electron and phonon transport in high-performance copper sulfides thermoelectric materials via one-pot modulation

Optimizing thermoelectric conversion efficiency requires the compromise of electrical and thermal properties of materials, which are hard to simultaneously improve due to the strong coupling of carrier and phonon transport. Herein, a one-pot approach realizing simultaneous second phase and Cu vacancies modulation is proposed, which is effective in synergistically optimizing thermoelectric performance in copper sulfides. Multiple lattice defects, including nanoprecipitates, dislocations, and nanopores are produced by adding a refined ratio of Sn and Se. Phonon transport is significantly suppressed by multiple mechanisms. An ultralow lattice thermal conductivity is therefore obtained. Furthermore, extra Se is added in the copper sulfide for optimizing electrical transport properties by inducing generating Cu vacancies. Ultimately, an excellent figure of merit of ~1.6 at 873 K is realized in the Cu1.992SSe0.016(Cu2SnSe4)0.004 bulk sample. The simple strategy of inducing compositional and structural modulation for improving thermoelectric parameters promotes low-cost high-performance copper sulfides as alternatives in thermoelectric applications.

Toward Ultrahigh Thermoelectric Performance of Cu2 SnS3 -Based Materials by Analog Alloying

Cu₂ SnS₃ is a promising thermoelectric candidate for power generation at medium temperature due to its low-cost and environmental-benign features. However, the high electrical resistivity due to low hole concentration severely restricts its final thermoelectric performance. Here, analog alloying with CuInSe₂ is first adopted to optimize the electrical resistivity by promoting the formation of Sn vacancies and the precipitation of In, and optimize lattice thermal conductivity through the formation of stacking faults and nanotwins. Such analog alloying enables a greatly enhanced power factor of 8.03 µW cm^-1 K^-2 and a largely reduced lattice thermal conductivity of 0.38 W m^-1  K^-1 for Cu₂ SnS₃ - 9 mol.% CuInSe₂ . Eventually, a peak ZT as high as 1.14 at 773 K is achieved for Cu₂ SnS₃ - 9 mol.% CuInSe₂ , which is one of the highest ZT among the researches on Cu₂ SnS₃ -based thermoelectric materials. The work implies analog alloying with CuInSe₂ is a very effective route to unleash superior thermoelectric performance of Cu₂ SnS₃ .

Bottom-Up Synthesis of SnTe-Based Thermoelectric Composites

There is a need for the development of lead-free thermoelectric materials for medium-/high-temperature applications. Here, we report a thiol-free tin telluride (SnTe) precursor that can be thermally decomposed to produce SnTe crystals with sizes ranging from tens to several hundreds of nanometers. We further engineer SnTe-Cu₂SnTe₃ nanocomposites with a homogeneous phase distribution by decomposing the liquid SnTe precursor containing a dispersion of Cu_1.5Te colloidal nanoparticles. The presence of Cu within the SnTe and the segregated semimetallic Cu₂SnTe₃ phase effectively improves the electrical conductivity of SnTe while simultaneously reducing the lattice thermal conductivity without compromising the Seebeck coefficient. Overall, power factors up to 3.63 mW m^-1 K^-2 and thermoelectric figures of merit up to 1.04 are obtained at 823 K, which represent a 167% enhancement compared with pristine SnTe.

Achieving High Thermoelectric Performance of Eco-Friendly SnTe-Based Materials by Selective Alloying and Defect Modulation

Recently, rock-salt lead-free chalcogenide SnTe-based thermoelectric (TE) materials have been considered an alternative to PbTe because of the nontoxic properties of Sn as compared to Pb. However, high carrier concentration that originated from intrinsic Sn vacancies and relatively high thermal conductivity of pristine SnTe lead to poor TE efficiency, which makes room for improving its TE properties. In this study, we present that the Na incorporation into the SnTe matrix is helpful for modifying the electronic band structure, optimization of carrier concentration, introducing dislocations, and kink planes; benefiting from these synergistic effects obviates the disadvantages of SnTe and makes a significant improvement in TE performance. We reveal that Na favorably impacts the structure of electronic bands by valence, conduction band engineering, leading to a nice enhancement in the Seebeck coefficient, which exhibits the highest power factor value of 37.93 μWcm^-1 K^-2 at 898 K, representing the best result for the SnTe material system. Moreover, a broader phonon spectrum is introduced by new phonon-scattering centers, scattered by dislocations and kink planes which suppressed lattice thermal conductivity to 0.57 Wm^-1 K^-1 at 898 K, which is much lower than that of pristine SnTe. Ultimately, a maximum ZT of 1.26 at 898 K is achieved in the Sn_1.03Te + 3% Na sample, which is 97% higher than that of the pristine SnTe, suggesting that SnTe-based materials are a robust candidate for TE applications specifically, an ideal alternative of lead chalcogenides for TE power generation at high temperatures.

Improving thermoelectric performance by constructing a SnTe/ZnO core–shell structure

SnTe is becoming a new research focus as an intermediate temperature thermoelectric material for its environment-friendly property. Herein, the SnTe/ZnO core–shell structure prepared by a facile hydrothermal method is firstly constructed to enhance the thermoelectric performance. The characterization results demonstrate that ZnO nanosheets are coated on the surface of SnTe particles by in situ synthesis and converted into ZnO nano-dots by spark plasma sintering. The energy barriers built by the SnTe/ZnO core–shell structure improve the Seebeck coefficient effectively. Additionally, the increased density of interfaces induced by ZnO can effectively scatter low/medium frequency phonons, reducing the lattice thermal conductivity in the low/medium temperature region. Further, the point defects caused by Cu₂Te-alloying strengthen the scattering of high frequency phonons. The lattice thermal conductivity reaches 0.48 W m⁻¹ K⁻¹, which is close to the amorphous limit of pristine SnTe. As a result, a peak ZT value of 0.94 is achieved at 823 K for SnTe(Cu₂Te)_0.06–1.5% ZnO, benefiting from the synergistic optimization of thermal and electrical properties. This provides a new idea for exploring an optimization strategy of thermoelectric performance.

Energy filtering effect introduced by the SnTe/ZnO core–shell structure in SnTe-based TE materials increases the ZT by approximately 50%.

Enhanced Thermoelectric Performance of SnTe-Based Materials via Interface Engineering

Interface engineering has been regarded as an effective strategy to improve thermoelectric (TE) performance by modulating electrical transport and enhancing phonon scattering. Herein, we develop a new interface engineering strategy in SnTe-based TE materials. We first use a one-step solvothermal method to synthesize SnTe powders decorated by Sb₂Te₃ nanoplates. After subsequent spark plasma sintering, we found that an ion-exchange reaction between the Sb₂Te₃ and SnTe matrixes happens to result in Sb doping and the formation of SnSb nanoparticles and the recrystallization of the nanograined SnTe at the grain boundaries of the SnTe matrix. Benefitting from this unique engineering, a significantly reduced lattice thermal conductivity of ∼0.64 W m^-1 K^-1 and a high zT of ∼1.08 (∼100% enhanced) at 873 K are achieved in SnTe-Sb_0.06. Such improved TE properties are attributed to the optimized carrier concentration and valence band convergence due to the Sb doping and enhanced phonon scattering by interface engineering at the grain boundaries. This work has demonstrated a facile and effective method to realize high-TE-performance SnTe via interface engineering.

Defect engineering in thermoelectric materials: what have we learned?

Thermoelectric energy conversion is an all solid-state technology that relies on exceptional semiconductor materials that are generally optimized through sophisticated strategies involving the engineering of defects in their structure. In this review, we summarize the recent advances of defect engineering to improve the thermoelectric (TE) performance and mechanical properties of inorganic materials. First, we introduce the various types of defects categorized by dimensionality, i.e. point defects (vacancies, interstitials, and antisites), dislocations, planar defects (twin boundaries, stacking faults and grain boundaries), and volume defects (precipitation and voids). Next, we discuss the advanced methods for characterizing defects in TE materials. Subsequently, we elaborate on the influences of defect engineering on the electrical and thermal transport properties as well as mechanical performance of TE materials. In the end, we discuss the outlook for the future development of defect engineering to further advance the TE field.

Optimized Electronic Bands and Ultralow Lattice Thermal Conductivity in Ag and Y Codoped SnTe

As a lead-free thermoelectric material, SnTe is inhibited by its inherent high carrier concentration and high thermal conductivity. This work describes the synergistic effect on the modulation of band structure and microstructural defects of SnTe by Ag and Y codoping, which gives rise to band convergence and multiple microstructural defects (secondary phases, dislocations, and boundaries) in the matrix and endows Sn_0.94Ag_0.09Y_0.05Te with an increased power factor of ∼2485 μW m^-1 K^-2, an extremely low lattice thermal conductivity of ∼0.61 W m^-1 K^-1, and a peak zT as high as ∼1.2 at 873 K. This work reveals that the combination of Ag and Y could play a role in the synergistic optimization of electronic and phonon transport properties of SnTe by modifying the band structure and microstructures, providing guidance for enhancing the thermoelectric performance of the relevant materials.

Inorganic Thermoelectric Fibers: A Review of Materials, Fabrication Methods, and Applications

Thermoelectric technology can directly harvest the waste heat into electricity, which is a promising field of green and sustainable energy. In this aspect, flexible thermoelectrics (FTE) such as wearable fabrics, smart biosensing, and biomedical electronics offer a variety of applications. Since the nanofibers are one of the important constructions of FTE, inorganic thermoelectric fibers are focused on here due to their excellent thermoelectric performance and acceptable flexibility. Additionally, measurement and microstructure characterizations for various thermoelectric fibers (Bi-Sb-Te, Ag₂Te, PbTe, SnSe and NaCo₂O₄) made by different fabrication methods, such as electrospinning, two-step anodization process, solution-phase deposition method, focused ion beam, and self-heated 3ω method, are detailed. This review further illustrates that some techniques, such as thermal drawing method, result in high performance of fiber-based thermoelectric properties, which can emerge in wearable devices and smart electronics in the near future.