Direct Current Treatment Tuning Crystallinity Leading to High-Performance p-Type Sb2Te3 Flexible Thin Films.

With the development of wearable electronics, inorganic flexible thin films (f-TFs) with high thermoelectric performance have attracted increasing research interest. To further enhance the thermoelectric performance of p-type inorganic Sb2Te3-based f-TFs, we employed direct current treatment to tune the crystallinity by rationally tuning the direct current treatment time. Correspondingly, a high electrical conductivity of >845 S cm-1 and a moderate Seebeck coefficient of >110 µV K-1 within the entire measurement temperature range have been simultaneously achieved. Consequently, a high power factor of 12.84 µW cm-1 K-2 at 423 K has been realized in the as-prepared p-type Sb2Te3 f-TF treated by a direct current of 5 A for 4 min. A flexible thermoelectric device has been further assembled to demonstrate the power-generating capacity. This study indicates that the direct current treatment is an effective method to improve the thermoelectric performance of Sb2Te3 f-TFs.

Manganese and Cobalt-Free Ultrahigh-Ni-Rich Single-Crystal Cathode for High-Performance Lithium Batteries.

Current commercial nickel (Ni)-rich Mn, Co, and Al-containing cathodes are employed in high-energy-density lithium (Li) batteries all around the globe. The presence of Mn/Co in them brings out several problems, such as high toxicity, high cost, severe transition-metal dissolution, and quick surface degradation. Herein, a Mn/Co-free ultrahigh-Ni-rich single-crystal LiNi0.94Fe0.05Cu0.01O2 (SCNFCu) cathode with acceptable electrochemical performance is benchmarked against a Mn/Co-containing cathode. Despite having a slightly lower discharge capacity, the SCNFCu cathode retaining 77% of its capacity across 600 deep cycles in full-cell outperforms comparable to a high-Ni single-crystal LiNi0.9Mn0.05Co0.05O2 (SCNMC; 66%) cathode. It is shown that the stabilizing ions Fe/Cu in the SCNFCu cathode reduce structural disintegration, undesirable side reactions with the electrolyte, transition-metal dissolution, and active Li loss. This discovery provides a new extent for cathode material development for next-generation high-energy, Mn/Co-free Li batteries due to the compositional tuning flexibility and quick scalability of SCNFCu, which is comparable to the SCNMC cathode.

N doped FeP nanospheres decorated carbon matrix as an efficient electrocatalyst for durable lithium-sulfur batteries.

Rational design and synthesis of multifunctional electrocatalysts with high electrochemical activity and low cost are significantly important for new-generation lithium-sulfur (Li-S) batteries. Herein, N-doped FeP nanospheres decorated N doped carbon matrix is successfully synthesized by facile one-pot pyrolysis and in-situ phosphorization technique to mitigate the conversion kinetics and suppress the shuttle effect. The large specific surface area with mesopores can incorporate up to 81.5% sulfur, with the conductive carbon and nitrogen co-matrix providing Li+/e- passage and fastening the redox kinetics. The remarkable adsorption properties and the electrocatalytic activity through physical confinement and chemical immobilization is thoroughly verified. Consequently, the FeP/CN@S deliver a high reversible capacity of 1183 mAh g-1 at 0.1C compared to Co/P/CN@S (961 mAh g-1); whereas, at 1C, a negligible decay rate of 0.04% is observed for 1000 cycles, possessing outstanding cycling stability and rate capability. Hence, the cost-effective in-situ phosphorization strategy to synthesize FeP/CN@S as an efficient nanoreactor is constructive to be applied in Li-S batteries.

Optimized Thermoelectric Properties of Sulfide Compound Bi2SeS2 by Iodine Doping.

The Te-free compound Bi2SeS2 is considered as a potential thermoelectric material with less environmentally hazardous composition. Herein, the effect of iodine (I) substitution on its thermoelectric transport properties was studied. The electrical conductivity was enhanced due to the increased carrier concentration caused by the carrier provided defect Ise. Thus, an enhanced power factor over 690 µWm−1K−2 was obtained at 300 K by combining a moderate Seebeck coefficient above 150 µV/K due to its large effective mass, which indicated iodine was an effective n-type dopant for Bi2SeS2. Furthermore, a large drop in the lattice thermal conductivity was observed due to the enhanced phonon scattering caused by nanoprecipitates, which resulted in a low total thermal conductivity (<0.95 Wm−1K−1) for all doped samples. Consequently, a maximum ZT value of 0.56 was achieved at 773 K for a Bi2Se1−xIxS2 (x = 1.1%) sample, a nearly threefold improvement compared to the undoped sample.

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 Sn1.03Te + 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.

Novel Thermal Diffusion Temperature Engineering Leading to High Thermoelectric Performance in Bi2 Te3 -Based Flexible Thin-Films.

Flexible Bi2 Te3 -based thermoelectric devices can function as power generators for powering wearable electronics or chip-sensors for internet-of-things. However, the unsatisfied performance of n-type Bi2 Te3 flexible thin films significantly limits their wide application. In this study, a novel thermal diffusion method is employed to fabricate n-type Te-embedded Bi2 Te3 flexible thin films on flexible polyimide substrates, where Te embeddings can be achieved by tuning the thermal diffusion temperature and correspondingly result in an energy filtering effect at the Bi2 Te3 /Te interfaces. The energy filtering effect can lead to a high Seebeck coefficient ≈160 µV K-1 as well as high carrier mobility of ≈200 cm2 V-1 s-1 at room-temperature. Consequently, an ultrahigh room-temperature power factor of 14.65 µW cm-1 K-2 can be observed in the Te-embedded Bi2 Te3 flexible thin films prepared at the diffusion temperature of 623 K. A thermoelectric sensor is also assembled through integrating the n-type Bi2 Te3 flexible thin films with p-type Sb2 Te3 counterparts, which can fast reflect finger-touch status and demonstrate the applicability of as-prepared Te-embedded Bi2 Te3 flexible thin films. This study indicates that the thermal diffusion method is an effective way to fabricate high-performance and applicable flexible Te-embedded Bi2 Te3 -based thin films.

Homo-composition and hetero-structure nanocomposite Pnma Bi2SeS2 - Pnnm Bi2SeS2 with high thermoelectric performance.

Nanocomposite engineering decouples the transport of phonons and electrons. This usually involves the in-situ formation or ex-situ addition of nanoparticles to a material matrix with hetero-composition and hetero-structure (heC-heS) interfaces or hetero-composition and homo-structure (heC-hoS) interfaces. Herein, a quasi homo-composition and hetero-structure (hoC-heS) nanocomposite consisting of Pnma Bi2SeS2 - Pnnm Bi2SeS2 is obtained through a Br dopant-induced phase transition, providing a coherent interface between the Pnma matrix and Pnnm second phase due to the slight structural difference between the two phases. This hoC-heS nanocomposite demonstrates a significant reduction in lattice thermal conductivity (~0.40 W m-1 K-1) and an enhanced power factor (7.39 µW cm-1 K-2). Consequently, a record high figure-of-merit ZTmax = 1.12 (at 773 K) and a high average figure-of-merit ZTave = 0.72 (in the range of 323-773 K) are achieved. This work provides a general strategy for synergistically tuning electrical and thermal transport properties by designing hoC-heS nanocomposites through a dopant-induced phase transition.

Boosting Thermoelectric Performance of Cu2SnSe3 via Comprehensive Band Structure Regulation and Intensified Phonon Scattering by Multidimensional Defects.

As an eco-friendly thermoelectric material, Cu2SnSe3 has recently drawn much attention. However, its high electrical resistivity ρ and low thermopower S prohibit its thermoelectric performance. Herein, we show that a widened band gap and the increased density of states are achieved via S alloying, resulting in 1.6 times enhancement of S (from 170 to 277 µV/K). Moreover, doping In at the Sn site can cause a 19-fold decrease of ρ and a 2.2 times enhancement of S (at room temperature) due to both multivalence bands' participation in electrical transport and the further enhancement of the density of states effective mass, which allows a sharp increase in the power factor. As a result, PF = 9.3 µW cm-1 K-2 was achieved at ∼800 K for the Cu2Sn0.82In0.18Se2.7S0.3 sample. Besides, as large as 44% reduction of lattice thermal conductivity is obtained via intensified phonon scattering by In-doping-induced formation of multidimensional defects, such as Sn vacancies, dislocations, twin boundaries, and CuInSe2 nanoprecipitates. Consequently, a record high figure of merit of ZT = 1.51 at 858 K is acquired for Cu2Sn0.82In0.18Se2.7S0.3, which is 4.7-fold larger than that of pristine Cu2SnSe3.

Improving the thermoelectric performance of Cu2SnSe3via regulating micro- and electronic structures.

As a p-type thermoelectric material, Cu2SnSe3 (CSS) has recently drawn much attention, with its constituents being abundant and free of toxic elements. However, the low electrical conductivity σ and thermopower S of CSS prohibit its thermoelectric performance. Here, we show that through mechanical milling, a 14 times increase in σ, around a 2-fold rise in S and a 40% reduction in the lattice thermal conductivity κL (at 300 K) can be achieved, amazingly. Microstructural analysis combined with first-principles calculations reveal that the increased σ originates from the generated Sn vacancies , Se dangling bonds and the reconstructed Cu-Sn-terminated acceptor-like surface states; while the enhanced S comes mainly from the enhanced density of states effective mass caused by the Sn vacancies. In addition, the generated Sn vacancies and the in situ formed SnO2 nanoparticles give rise to strong phonon scattering, leading to the reduced κL. As a result, a maximum ZTm = 0.9 at 848 K is obtained for the CSS specimen milled for 2 h, which is ∼3 times larger than that of CSS milled for 0.5 h.

Design of Domain Structure and Realization of Ultralow Thermal Conductivity for Record-High Thermoelectric Performance in Chalcopyrite.

Chalcopyrite compound CuGaTe2 is the focus of much research interest due to its high power factor. However, its high intrinsic lattice thermal conductivity seriously impedes the promotion of its thermoelectric performance. Here, it is shown that through alloying of isoelectronic elements In and Ag in CuGaTe2 , a quinary alloy compound system Cu1- x Agx Ga0.4 In0.6 Te2 (0 ≤ x ≤ 0.4) with complex nanosized strain domain structure is prepared. Due to strong phonon scattering mainly by this domain structure, thermal conductivity (at 300 K) drops from 6.1 W m-1 K-1 for the host compound to 1.5 W m-1 K-1 for the sample with x = 0.4. As a result, the optimized chalcopyrite sample Cu0.7 Ag0.3 Ga0.4 In0.6 Te2 presents an outstanding performance, with record-high figure of merit (ZT) reaching 1.64 (at 873 K) and average ZT reaching 0.73 (between ≈300 and 873 K), which are ≈37 and ≈35% larger than the corresponding values for pristine CuGaTe2 , respectively, demonstrating that such domain structure arising from isoelectronic multielement alloying in chalcopyrite compound can effectively suppress its thermal conductivity and elevate its thermoelectric performance remarkably.