Opening the Bandgap of Metallic Half-Heuslers via the Introduction of d-d Orbital Interactions.

Half-Heusler compounds with semiconducting behavior have been developed as high-performance thermoelectric materials for power generation. Many half-Heusler compounds also exhibit metallic behavior without a bandgap and thus inferior thermoelectric performance. Here, taking metallic half-Heusler MgNiSb as an example, a bandgap opening strategy is proposed by introducing the d-d orbital interactions, which enables the opening of the bandgap and the improvement of the thermoelectric performance. The width of the bandgap can be engineered by tuning the strength of the d-d orbital interactions. The conduction type and the carrier density can also be modulated in the Mg1- x Tix NiSb system. Both improved n-type and p-type thermoelectric properties are realized, which are much higher than that of the metallic MgNiSb. The proposed bandgap opening strategy can be employed to design and develop new half-Heusler semiconductors for functional and energy applications.

High-Performance Stretchable Thermoelectric Generator for Self-Powered Wearable Electronics.

Wearable thermoelectric generators (TEGs), which can convert human body heat to electricity, provide a promising solution for self-powered wearable electronics. However, their power densities still need to be improved aiming at broad practical applications. Here, a stretchable TEG that achieves comfortable wearability and outstanding output performance simultaneously is reported. When worn on the forehead at an ambient temperature of 15 °C, the stretchable TEG exhibits excellent power densities with a maximum value of 13.8 µW cm-2 under the breezeless condition, and even as high as 71.8 µW cm-2 at an air speed of 2 m s-1 , being one of the highest values for wearable TEGs. Furthermore, this study demonstrates that this stretchable TEG can effectively power a commercial light-emitting diode and stably drive an electrocardiogram module in real-time without the assistance of any additional power supply. These results highlight the great potential of these stretchable TEGs for power generation applications.

An Elastic Cross-Linked Binder for Silicon Anodes in Lithium-Ion Batteries with a High Mass Loading.

Due to the urgent demand for lithium-ion batteries (LIBs) with a high energy density, silicon (Si) possessing an ultrahigh capacity has aroused wide attention. However, its practical application is seriously hindered by enormous volume changes of the Si anode during cycling. Developing novel binders suitable for the Si anode has proven to be an effective strategy to improve its electrochemical performance. Herein, we constructed a three-dimensional network binder, in which the polyacrylic acid (PAA) long chains are cross-linked with one kind of amino acid, lysine (Lys). The abundant polar groups in PAA/Lys enable it to tightly adhere to the Si particles via hydrogen bonds, and the cross-linked structure prevents irreversible slipping of the PAA chains upon volume variation of the particles. The Si used was obtained from a sustainable route by recycling photovoltaic waste silicon. With high elasticity and strong adhesion, the PAA/Lys binder can effectively keep the structural integrity of the Si electrode and improve its electrochemical performance. The Si electrode using the PAA/Lys binder exhibits a good cycling stability (1008 mAh g-1 at 2 A g-1 after 250 cycles). Even with a high mass loading of 3.03 mg cm-2, the Si anode can remain stable for 100 cycles at a high fixed areal capacity of 3.03 mAh cm-2. This work gives a practical method to make stable Si electrodes using sustainable Si source and environmentally friendly amino acid-based binders.

Achieving metal-like malleability and ductility in Ag2Te1-x S x inorganic thermoelectric semiconductors with high mobility.

Inorganic semiconductor Ag2Te1-x S x has been recently found to exhibit unexpected plastic deformation with compressive strain up to 30%. However, the origin of the abnormal plasticity and how to simultaneously achieve superb ductility and high mobility are still elusive. Here, we demonstrate that crystalline/amorphous Ag2Te1-x S x (x = 0.3, 0.4, and 0.5) composites can exhibit excellent compressive strain up to 70% if the monoclinic Ag2Te phase, which commonly exists in the matrix, is eliminated. Significantly, an ultra-high tensile elongation reaching 107.3% was found in Ag2Te0.7S0.3, which is the highest one yet reported in the system and even surpasses those achieved in some metals and high-entropy alloys. Moreover, high mobility of above 1000 cm2 V-1 s-1 at room temperature and good thermoelectric performance are simultaneously maintained. A modified Ashby plot with ductility factor versus carrier mobility is thereby proposed to highlight the potential of solid materials for applications in flexible/wearable electronics.

High-Power-Density Wearable Thermoelectric Generators for Human Body Heat Harvesting.

Wearable thermoelectrics has attracted significant interest in recent years. Among them, rigid-structure thermoelectric generators (TEGs) were seldomly employed for wearable applications, although those exhibit significant advantages of high device output performance and impact resistance. Here, we report a type of rigid wearable TEGs (w-TEGs) without ceramic substrates made using a simple cutting-and-bonding method. Owing to the small contact area, the w-TEGs comprising 48-n/p-pairs can be well attached to the human body. The lack of ceramic substrates leaves more space in the height direction, which benefits the wearability in practical applications and high power density. We demonstrated that increasing the height of w-TEGs from 1.38 to 3.14 mm significantly improves the power density by a factor of 10. As a result, the maximum power densities of 7.9 µW cm-2 and 43.6 µW cm-2 for the w-TEGs were realized under the breezeless condition and a wind speed for normal walking, respectively. This work provides a feasible design solution for rigid-structure free-substrate w-TEGs with very high power density, which will speed up the research of wearable thermoelectrics.

Demonstration of valley anisotropy utilized to enhance the thermoelectric power factor.

Valley anisotropy is a favorable electronic structure feature that could be utilized for good thermoelectric performance. Here, taking advantage of the single anisotropic Fermi pocket in p-type Mg3Sb2, a feasible strategy utilizing the valley anisotropy to enhance the thermoelectric power factor is demonstrated by synergistic studies on both single crystals and textured polycrystalline samples. Compared to the heavy-band direction, a higher carrier mobility by a factor of 3 is observed along the light-band direction, while the Seebeck coefficient remains similar. Together with lower lattice thermal conductivity, an increased room-temperature zT by a factor of 3.6 is found. Moreover, the first-principles calculations of 66 isostructural Zintl phase compounds are conducted and 9 of them are screened out displaying a pz-orbital-dominated valence band, similar to Mg3Sb2. In this work, we experimentally demonstrate that valley anisotropy is an effective strategy for the enhancement of thermoelectric performance in materials with anisotropic Fermi pockets.

Medium Entropy-Enabled High Performance Cubic GeTe Thermoelectrics.

The configurational entropy is an emerging descriptor in the functional materials genome. In thermoelectric materials, the configurational entropy helps tune the delicate trade-off between carrier mobility and lattice thermal conductivity, as well as the structural phase transition, if any. Taking GeTe as an example, low-entropy GeTe generally have high carrier mobility and distinguished zT > 2, but the rhombohedral-cubic phase transition restricts the applications. In contrast, despite cubic structure and ultralow lattice thermal conductivity, the degraded carrier mobility leads to a low zT in high-entropy GeTe. Herein, medium-entropy alloying is implemented to suppress the phase transition and achieve the cubic GeTe with ultralow lattice thermal conductivity yet decent carrier mobility. In addition, co-alloying of (Mn, Pb, Sb, Cd) facilitates multivalence bands convergence and band flattening, thereby yielding good Seebeck coefficients and compensating for decreased carrier mobility. For the first time, a state-of-the-art zT of 2.1 at 873 K and average zT ave of 1.3 between 300 and 873 K are attained in cubic phased Ge0.63Mn0.15Pb0.1Sb0.06Cd0.06Te. Moreover, a record-high Vickers hardness of 270 is attained. These results not only promote GeTe materials for practical applications, but also present a breakthrough in the burgeoning field of entropy engineering.

Mo-Fe/NbFeSb Thermoelectric Junctions: Anti-Thermal Aging Interface and Low Contact Resistivity.

In recent years, high-performance half-Heusler compounds have been developed as promising thermoelectric materials for power generation. Aiming at practical device applications, one key step is to seek suitable metal electrodes so that low interfacial resistivity is guaranteed under long-term thermal aging. In the previous work, the fresh Mo/Nb0.8Ti0.2FeSb junction was found exhibiting low contact resistivity below 1 µΩ cm2; however, it increased by tens of times under long-term thermal aging, mainly originating from the formation of the high-resistivity FeSb2 phase and the appearance of cracks. Here, the Mo-Fe electrodes are employed to build the junctions with Nb0.8Ti0.2FeSb. The interfacial behavior and contact resistance in these junctions were investigated both before and after the thermal aging. Interestingly, no obvious formation of FeSb2 phase and cracks were observed. As a result, the contact resistivity was below ∼1 µΩ cm2 after 15 days' thermal aging, indicating better connection reliability and lower contact resistivity compared to the Mo/Nb0.8Ti0.2FeSb junction. These findings highlight the applicability of Mo-Fe electrodes and pave the way for NbFeSb-based half-Heusler thermoelectric materials for device applications.

Stable cycling of Prussian blue/Zn battery in a nonflammable aqueous/organic hybrid electrolyte.

Although rechargeable aqueous batteries are attracting increasing attention in recent years due to high safety, low cost, high power density and environmental friendliness, the aqueous batteries suffer from limited cycle life due to a narrow electrochemical window of the aqueous electrolytes, severe side reaction and instability of electrode materials in aqueous electrolytes. In this work, we propose a hybrid aqueous electrolyte with a mixed solvent of water and acetonitrile (ACN), which exhibits a wide electrochemical window, high ionic conductivity, and nonflammability. An aqueous battery with an iron hexacyanoferrate (FeHCF) cathode, Zn anode and H2O/ACN hybrid electrolyte shows a high capacity of 69.1 mA h g-1 at 10C (89.5% relative to that at 1C) and an extremely long cycle life with 51.4% capacity retention after 19 000 cycles at 10C. The excellent cycling performance of the aqueous FeHCF/Zn batteries can be attributed to the reduced water activity and extended electrochemical window because of the strong hydrogen-bonding interaction between ACN and H2O. Besides, the large particle size and good crystallization of FeHCF can inhibit its dissolution in the aqueous electrolyte which further improves cycling performance. This work will shed light on the design of safe aqueous batteries for applications in large-scale energy storage.

Tuning Optimum Temperature Range of Bi2 Te3 -Based Thermoelectric Materials by Defect Engineering.

Bi2 Te3 -based solid solutions, which have been widely used as thermoelectric (TE) materials for the room temperature TE refrigeration, are also the potential candidates for the power generators with medium and low-temperature heat sources. Therefore, depending on the applications, Bi2 Te3 -based materials are expected to exhibit excellent TE properties in different temperature ranges. Manipulating the point defects in Bi2 Te3 -based materials is an effective and important method to realize this purpose. In this review, we focus on how to optimize the TE properties of Bi2 Te3 -based TE materials in different temperature ranges by defect engineering. Our calculation results of two-band model revel that tuning the carrier concentration and band gap, which is easily realized by defects engineering, can obtain better TE properties at different temperatures. Then, the typical paradigms about optimizing the TE properties at different temperatures for n-type and p-type Bi2 Te3 -based ZM ingots and polycrystals are discussed in the perspective of defects engineering. This review can provide the guidance to improve the TE properties of Bi2 Te3 -based materials at different temperatures by defects engineering.

Trace fluorinated-carbon-nanotube-induced lithium dendrite elimination for high-performance lithium-oxygen cells.

Lithium metal has attracted considerable attention due to its ultrahigh theoretical capacity. Nevertheless, issues such as dendritic Li formation and instability of the Li metal/electrolyte interface still restrain its practical applications. In this work, we design a Li composite anode with fluorinated carbon nanotubes (FCNT) fabricated by a simple melting-soaking method. It was found that trace amounts of added FCNT (only 1.6 wt%) lead to a significant chemical/electrochemical stability of metallic Li. The obtained Li/FCNT composite electrode (LFCNT) exhibits much better stability in open air and electrolyte than bare Li. The LFCNT enables uniform plating/stripping of metallic Li, preventing the dendrite formation during repeated cycling. In situ optical microscopy observations confirm dendrite-free Li deposition with the mechanism clarified by density functional theory calculations. Compared with bare Li, the LFCNT shows a considerable improvement in rate capability, voltage hysteresis and cycle performance, sustaining stable cycling at a high current density of 3 mA cm-2 or a capacity up to 5 mA h cm-2. Li-O2 cells with a LFCNT anode exhibit a long life of 135 cycles at a capacity of 1000 mA h g-1, which is six-fold than that with the bare Li anode.

Revealing the Intrinsic Electronic Structure of 3D Half-Heusler Thermoelectric Materials by Angle-Resolved Photoemission Spectroscopy.

Accurate determination of the intrinsic electronic structure of thermoelectric materials is a prerequisite for utilizing an electronic band engineering strategy to improve their thermoelectric performance. Herein, with high-resolution angle-resolved photoemission spectroscopy (ARPES), the intrinsic electronic structure of the 3D half-Heusler thermoelectric material ZrNiSn is revealed. An unexpectedly large intrinsic bandgap is directly observed by ARPES and is further confirmed by electrical and optical measurements and first-principles calculations. Moreover, a large anisotropic conduction band with an anisotropic factor of 6 is identified by ARPES and attributed to be one of the most important reasons leading to the high thermoelectric performance of ZrNiSn. These successful findings rely on the grown high-quality single crystals, which have fewer Ni interstitial defects and negligible in-gap states on the electronic structure. This work demonstrates a realistic paradigm to investigate the electronic structure of 3D solid materials by using ARPES and provides new insights into the intrinsic electronic structure of the half-Heusler system benefiting further optimization of thermoelectric performance.

High-Performance Mg3Sb2-x Bi x Thermoelectrics: Progress and Perspective.

Since the first successful implementation of n-type doping, low-cost Mg3Sb2-x Bi x alloys have been rapidly developed as excellent thermoelectric materials in recent years. An average figure of merit zT above unity over the temperature range 300-700 K makes this new system become a promising alternative to the commercially used n-type Bi2Te3-x Se x alloys for either refrigeration or low-grade heat power generation near room temperature. In this review, with the structure-property-application relationship as the mainline, we first discuss how the crystallographic, electronic, and phononic structures lay the foundation of the high thermoelectric performance. Then, optimization strategies, including the physical aspects of band engineering with Sb/Bi alloying and carrier scattering mechanism with grain boundary modification and the chemical aspects of Mg defects and aliovalent doping, are extensively reviewed. Mainstream directions targeting the improvement of zT near room temperature are outlined. Finally, device applications and related engineering issues are discussed. We hope this review could help to promote the understanding and future developments of low-cost Mg3Sb2-x Bi x alloys for practical thermoelectric applications.

Liquid-Phase Hot Deformation to Enhance Thermoelectric Performance of n-type Bismuth-Telluride-Based Solid Solutions.

Bismuth-telluride-based solid solutions are the best commercial thermoelectric materials near room temperature. For their n-type polycrystalline compounds, the maximum figures of merit (zTs) are often less than 1.0 due to the degraded carrier mobility resulting from the loss of texture. Herein, a liquid-phase hot deformation procedure, during which the Bi2(Te,Se)3 ingots are directly hot deformed with the extrusion of liquid eutectic phase, is performed to enhance the thermoelectric performance of n-type Bi2(Te,Se)3 alloys. The deformation-induced dynamic recrystallization is remarkably suppressed due to the reduction of nucleation sites and the release of deformation stress by liquid phase, contributing to a weakened carrier scattering and enhanced carrier mobility. The liquid eutectic phase also facilitates the rotation of grains and enhanced (000l) texture, further improving carrier mobility. In addition, the dense dislocations and lattice distortion introduced into the matrix reduce the lattice thermal conductivity. As a result, a high zT value of 1.1 at 400 K is obtained, about 75% increment over the normal one-step hot deformed alloys. This work not only demonstrates a simple and efficient technique for achieving superior n-type Bi2Te3-based materials, but also elucidates the important role of liquid eutectic phase in hot deformation.

Evolution of the Intrinsic Point Defects in Bismuth Telluride-Based Thermoelectric Materials.

In polycrystalline bismuth telluride-based thermoelectric materials, mechanical-deformation-induced donor-like effects can introduce a high concentration of electrons to change the thermoelectric properties through the evolution of intrinsic point defects. However, the evolution law of these point defects during sample preparation remains elusive. Herein, we systematically investigate the evolution of intrinsic point defects in n-type Bi2Te3-based materials from the perspective of thermodynamics and kinetics, in combination with positron annihilation measurement. It is found that not only the mechanical deformation but also the sintering temperature is vital to the donor-like effect. The mechanical deformation can promote the formation of cation vacancies and facilitate the donor-like effect, and the sintering process can provide excess energy for Bi antisite atoms to surmount the diffusion potential barrier. This work provides us a better understanding of the evolution law of intrinsic point defects in Bi2Te3-based alloys and guides us to control the carrier concentration by manipulating intrinsic point defects.

Dendrite-Free Fluorinated Graphene/Lithium Anodes Enabling in Situ LiF Formation for High-Performance Lithium-Oxygen Cells.

Metallic Li is considered as the ultimate choice of negative electrodes for Li batteries because of its largest theoretical specific capacity. However, formidable issues such as poor safety and cyclability caused by lithium dendrite growth and tremendous interfacial side reactions have strictly hindered its practical applications. Here, we report a fluorinated graphene (FG)-modified Li negative electrode (LFG) for high-performance lithium-oxygen (Li-O2) cells. The results show that only 3 wt % FG introduction leads to a significant enhancement on rate capability and cycling life of Li electrodes. Compared with the half cells with bare Li, the cells with LFG exhibit much more stable voltage profiles even at a large areal capacity up to 5 mA h cm-2 or a large current density up to 5 mA cm-2. Li-O2 cells with the LFG anode show a longer cycle life than the cell with the pristine lithium anode. It was found that a LiF-rich layer could be in situ built upon cycling when FG was used, which ensures uniform Li stripping/plating and effectively suppresses Li dendrite growth. Density functional theory calculations confirm the possibility of conversion from FG to graphene and LiF after Li intercalation into LFG during cycling. In situ optical microscopy observation vividly exhibits the obvious inhibition effect of FG for Li dendrite growth.

Low Contact Resistivity and Interfacial Behavior of p-Type NbFeSb/Mo Thermoelectric Junction.

Half-Heusler compounds are a class of promising thermoelectric (TE) materials for power generation. However, the large contact resistivity at the interface between TE legs and metal electrode of the TE device seriously hinders the full play of the material performance. Here we report an Ohmic contact for the junction of p-type Nb0.8Ti0.2FeSb and Mo electrode with a low contact resistivity of <1 µΩ cm2 due to the matching of work functions between Nb0.8Ti0.2FeSb and FeMo interlayer. The interface carrier transport is dominated by the field emission and consequently a strong tunneling electric current is obtained due to the high doping level and relatively low dielectric constant of p-type Nb0.8Ti0.2FeSb semiconductor. The interface microstructure analysis indicates that there is a FeMo alloy interlayer with a thickness of 5 µm and a mixing layer of Nb0.8Ti0.2FeSb and Nb3Ti with a thickness of 25 µm. After a long time heat treatment at 1073 K, the FeMo alloy transforms into a FeSb2 layer, while the mixing layer is occupied totally by Nb3Ti. Due to the relatively high electrical resistivity for FeSb2 phase, the increasing content of Nb3Ti and the crack at both sides of Nb3Ti interlayer, the contact resistivity rises up to 18.4 µΩ cm2 after 32 days' aging. These results demonstrate that the applicability of low contact resistivity NbFeSb/Mo junction in high performance TE devices.

Nonflammable quasi-solid-state electrolyte for stable lithium-metal batteries.

Rechargeable lithium batteries with high-voltage/capacity cathodes are regarded as promising high-energy-density energy-storage systems. Nevertheless, these systems are restricted by some critical challenges, such as flammable electrolyte, lithium dendrite formation and rapid capacity fade at high voltage and elevated temperature. In this work, we report a quasi-solid-state composite electrolyte (QCE) prepared by in situ polymerization reactions. The electrolyte consists of polymer matrix, inorganic filler, nonflammable plasticizers and Li salt, and shows a good thermal stability, a moderate ionic conductivity of 2.8 × 10-4 S cm-1 at 25 °C, and a wide electrochemical window up to 6.7 V. The batteries with the QCE show good electrochemical performance when coupled with lithium metal anode and LiCoO2 or LiNi0.8Mn0.1Co0.1O2 cathodes. Pouch-type batteries with the QCE also exhibit stable cycling, and can tolerate abuse testes such as folding, cutting and nail penetration. The in situ formed fluorides and phosphides from the plasticizers stabilize the interfaces between the QCE and electrodes, which enables stable cycling of Li metal batteries.

Approaching the minimum lattice thermal conductivity of p-type SnTe thermoelectric materials by Sb and Mg alloying.

SnTe, as the nontoxic analogue to high-performance PbTe thermoelectric material, has captured the worldwide interest recently. Many triumphant instances focus on the strategies of band convergence, resonant doping, and nano-precipitates phonon scattering. Herein, the p-type SnTe-based materials Sn0.85-xSb0.15MgxTe (x = 0-0.10) are fabricated and a combined effect of Sb and Mg is investigated. Sb alloying tunes the hole carrier concentration of SnTe and decreases the lattice thermal conductivity. Mg alloying leads to a nearly hundredfold rise of disorder parameter due to the large mass and strain fluctuations, and as a consequence the lattice thermal conductivity decreases further down to ∼0.64 W m-1 K-1 at 773 K, close to the theoretical minimum of the lattice thermal conductivity (∼0.50 W m-1 K-1) of SnTe. In conjunction with the enhancement of the Seebeck coefficient caused by band convergence due to Mg alloying, the maximum zTmax reaches ∼1.02 and the device zTdevice of ∼0.50 at 773 K for Sn0.79Sb0.15Mg0.06Te, suggesting this SnTe-based composition has a promising potential in intermediate temperature thermoelectric applications.

Unexpected Low-Temperature Performance of Li-O2 Cells with Inhibited Side Reactions.

In this work, we found that the side reactions of both the Li anode and cathode with the electrolyte can be obviously alleviated at low temperature. This favorable merit enables long cycle life of the Li-O2 cells at low temperature. At 0 °C, the cells can sustain stable cycling of 279 and 1025 cycles at 400 mA g-1 with limited capacities of 1000 and 500 mA h g-1, respectively. Even at -20 °C, the cell can be stably cycled for 83 cycles at 200 mA g-1 with a limited capacity of 500 mA h g-1.