CdSe Quantum Dots Enable High Thermoelectric Performance in Solution-Processed Polycrystalline SnSe.

Here, a high peak ZT of ≈2.0 is reported in solution-processed polycrystalline Ge and Cd codoped SnSe. Microstructural characterization reveals that CdSe quantum dots are successfully introduced by solution process method. Ultraviolet photoelectron spectroscopy evinces that CdSe quantum dots enhance the density of states in the electronic structure of SnSe, which leads to a large Seebeck coefficient. It is found that Ge and Cd codoping simultaneously optimizes carrier concentration and improves electrical conductivity. The enhanced Seebeck coefficient and optimization of carrier concentration lead to marked increase in power factor. CdSe quantum dots combined with strong lattice strain give rise to strong phonon scattering, leading to an ultralow lattice thermal conductivity. Consequently, high thermoelectric performance is realized in solution-processed polycrystalline SnSe by designing quantum dot structures and introducing lattice strain. This work provides a new route for designing prospective thermoelectric materials by microstructural manipulation in solution chemistry.

All-Scale Hierarchical Structuring, Optimized Carrier Concentration, and Band Manipulation Lead to Ultra-High Thermoelectric Performance in Eco-Friendly MnTe.

MnTe emerges as an enormous potential for medium-temperature thermoelectric applications due to its lead-free nature, high content of Mn in the earth's crust, and superior mechanical properties. Here, it is demonstrate that multiple valence band convergence can be realized through Pb and Ag incorporations, producing large Seebeck coefficient. Furthermore, the carrier concentration can be obviously enhance by Pb and Ag codoping, contributing to significant enhancement of power factor. Moreover, microstructural characterizations reveal that PbTe nanorods can be introduced into MnTe matrix by alloying Pb. This can modify the microstructure into all-scale hierarchical architectures (including PbTe nanorods, enhances point-defect scattering, dense dislocations and stacking faults), strongly lowering lattice thermal conductivity to a record low value of 0.376 W m-1 K-1 in MnTe system. As a result, an ultra-high ZT of 1.5 can be achieved in MnTe thermoelectric through all-scale hierarchical structuring, optimized carrier concentration, and valence band convergence, outperforming most of MnTe-based thermoelectric materials.

Lattice Distortions and Multiple Valence Band Convergence Contributing to High Thermoelectric Performance in MnTe.

Here, a new route is proposed for the minimization of lattice thermal conductivity in MnTe through considerable increasing phonon scattering by introducing dense lattice distortions. Dense lattice distortions can be induced by Cu and Ag dopants possessing large differences in atom radius with host elements, which causes strong phonon scattering and results in extremely low lattice thermal conductivity. Density functional theory (DFT) calculations reveal that Cu and Ag codoping enables multiple valence band convergence and produces a high density of state values in the electronic structure of MnTe, contributing to the large Seebeck coefficient. Cu and Ag codoping not only optimizes the Seebeck coefficient but also substantially increases the carrier concentration and electrical conductivity, resulting in the significant enhancement of power factor. The maximum power factor reaches 11.36 µW cm-1 K-2 in Mn0.98 Cu0.04 Ag0.04 Te. Consequently, an outstanding ZT of 1.3 is achieved for Mn0.98 Cu0.04 Ag0.04 Te by these synergistic effects. This study provides guidelines for developing high-performance thermoelectric materials through the rational design of effective dopants.

Divacancy and resonance level enables high thermoelectric performance in n-type SnSe polycrystals.

N-type polycrystalline SnSe is considered as a highly promising candidates for thermoelectric applications due to facile processing, machinability, and scalability. However, existing efforts do not enable a peak ZT value exceeding 2.0 in n-type polycrystalline SnSe. Here, we realized a significant ZT enhancement by leveraging the synergistic effects of divacancy defect and introducing resonance level into the conduction band. The resonance level and increased density of states resulting from tungsten boost the Seebeck coefficient. The combination of the enhanced electrical conductivity (achieved by increasing carrier concentration through WCl6 doping and Se vacancies) and large Seebeck coefficient lead to a high power factor. Microstructural analyses reveal that the co-existence of divacancy defects (Se vacancies and Sn vacancies) and endotaxial W- and Cl-rich nanoprecipitates scatter phonons effectively, resulting in ultralow lattice conductivity. Ultimately, a record-high peak ZT of 2.2 at 773 K is achieved in n-type SnSe0.92 + 0.03WCl6.

Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in Ge1-x-yBixCayTe with Ultrafine Ferroelectric Domain Structure.

GeTe and its derivatives emerging as a promising lead-free thermoelectric candidate have received extensive attention. Here, a new route was proposed that the minimization of κL in GeTe through considerable enhancement of acoustic phonon scattering by introducing ultrafine ferroelectric domain structure. We found that Bi and Ca dopants induce strong atomic strain disturbance in the GeTe matrix because of large differences in atom radius with host elements, leading to the formation of ultrafine ferroelectric domain structure. Furthermore, large strain field and mass fluctuation induced by Bi and Ca codoping result in further reduced κL by effectively shortening the phonon relaxation time. The co-existence of ultrafine ferroelectric domain structure, large strain field, and mass fluctuation contribute to an ultralow lattice thermal conductivity of 0.48 W m-1 K-1 at 823 K. Bi and Ca codoping significantly enhances the Seebeck coefficient and power factor through reducing the energy offset between light and heavy valence bands of GeTe. The modified band structure boosts the power factor up to 47 µW cm-1 K-2 in Ge0.85Bi0.09Ca0.06Te. Ultimately, a high ZT of ∼2.2 can be attained. This work demonstrates a new design paradigm for developing high-performance thermoelectric materials.

Enhanced Density of States Facilitates High Thermoelectric Performance in Solution-Grown Ge- and In-Codoped SnSe Nanoplates.

SnSe single crystals have gained great interest due to their excellent thermoelectric performance. However, polycrystalline SnSe is greatly desired due to facile processing, machinability, and scale-up application. Here, we report an outstanding high average ZT of 0.88 as well as a high peak ZT of 1.92 in solution-processed SnSe nanoplates. Nanosized boundaries formed by nanoplates and lattice strain created by lattice dislocations and stacking faults effectively scatter heat-carrying phonons, resulting in an ultralow lattice thermal conductivity of 0.19 W m-1 K-1 at 873 K. Ultraviolet photoelectron spectroscopy reveals that Ge and In incorporation produces an enhanced density of states in the electronic structure of SnSe, resulting in a large Seebeck coefficient. Ge and In codoping not only optimizes the Seebeck coefficient but also substantially increases the carrier concentration and electrical conductivity, helping to maintain a high power factor over a wide temperature range. Benefiting from an enhanced power factor and markedly reduced lattice thermal conductivity, high average ZT and peak ZT are achieved in Ge- and In-codoped SnSe nanoplates. This work achieves an ultrahigh average ZT of 0.88 in polycrystalline SnSe by adopting nontoxic element doping, potentially expanding its usefulness for various thermoelectric generator applications.

Realizing High Thermoelectric Performance in p-Type SnSe Crystals via Convergence of Multiple Electronic Valence Bands.

SnSe crystals have gained considerable interest for their outstanding thermoelectric performance. Here, we achieve excellent thermoelectric properties in Sn0.99-xPbxZn0.01Se crystals via valence band convergence and point-defect engineering strategies. We demonstrate that Pb and Zn codoping converges the energy offset between multiple valence bands by significantly modifying the band structure, contributing to the enhancement of the Seebeck coefficient. The carrier concentration and electrical conductivity can be optimized, leading to an enhanced power factor. The dual-atom point-defect effect created by the substitution of Pb and Zn in the SnSe lattice introduces strong phonon scattering, significantly reducing the lattice thermal conductivity to as low as 0.284 W m-1 K-1. As a result, a maximum ZT value of 1.9 at 773 K is achieved in Sn0.93Pb0.06Zn0.01Se crystals along the bc-plane direction. This study highlights the crucial role of manipulating multiple electronic valence bands in further improving SnSe thermoelectrics.

Lattice Strain Leads to High Thermoelectric Performance in Polycrystalline SnSe.

Polycrystalline SnSe materials with ZT values comparable to those of SnSe crystals are greatly desired due to facile processing, machinability, and scale-up application. Here manipulating interatomic force by harnessing lattice strains was proposed for achieving significantly reduced lattice thermal conductivity in polycrystalline SnSe. Large static lattice strain created by lattice dislocations and stacking faults causes an effective shortening in phonon relaxation time, resulting in ultralow lattice thermal conductivity. A combination of band convergence and resonance levels induced by Ga incorporation contribute to a sharp increase of Seebeck coefficient and power factor. These lead to a high thermoelectric performance ZT ∼ 2.2, which is a record high ZT reported so far for solution-processed SnSe polycrystals. Besides the high peak ZT, a high average ZT of 0.72 and outstanding thermoelectric conversion efficiency of 12.4% were achieved by adopting nontoxic element doping, highlighting great potential for power generation application at intermediate temperatures. Engineering lattice strain to achieve ultralow lattice thermal conductivity with the aid of band convergence and resonance levels provides a great opportunity for designing prospective thermoelectrics.

Achieving Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in GeTe Alloys via Introducing Cu2Te Nanocrystals and Resonant Level Doping.

The binary compound of GeTe emerging as a potential medium-temperature thermoelectric material has drawn a great deal of attention. Here, we achieve ultralow lattice thermal conductivity and high thermoelectric performance in In and a heavy content of Cu codoped GeTe thermoelectrics. In dopants improve the density of state near the surface of Femi of GeTe by introducing resonant levels, producing a sharp increase of the Seebeck coefficient. In and Cu codoping not only optimizes carrier concentration but also substantially increases carrier mobility to a high value of 87 cm2 V-1 s-1 due to the diminution of Ge vacancies. The enhanced Seebeck coefficient coupled with dramatically enhanced carrier mobility results in significant enhancement of PF in Ge1.04-x-yInxCuyTe series. Moreover, we introduce Cu2Te nanocrystals' secondary phase into GeTe by alloying a heavy content of Cu. Cu2Te nanocrystals and a high density of dislocations cause strong phonon scattering, significantly diminishing lattice thermal conductivity. The lattice thermal conductivity reduced as low as 0.31 W m-1 K-1 at 823 K, which is not only lower than the amorphous limit of GeTe but also competitive with those of thermoelectric materials with strong lattice anharmonicity or complex crystal structures. Consequently, a high ZT of 2.0 was achieved for Ge0.9In0.015Cu0.125Te by decoupling electron and phonon transport of GeTe. This work highlights the importance of phonon engineering in advancing high-performance GeTe thermoelectrics.

Nano/micro lithium transitionmetal (Fe, Mn, Co and Ni) silicate cathode materials for lithium ion batteries.

Lithium transitionmetal (Fe, Mn, Co, Ni) silicate cathode materials are new promising substituting cathode materials for lithium ion batteries. They had caught the researchers' eyes in the past several years. Nowadays, there are growing interests for silicate cathode materials in the field of lithium ion batteries. Among the silicate cathode materials, Li2FeSiO4 is the most promising cathode materials because of its high structure stability, high reversible capacity, high electronic conductivity and the abundant resource of iron and silicon. Although Li2MnSiO4 and Li2CoSiO4 have much higher theoretic specific capacity than Li2FeSiO4, they all have inferior electrochemical behaviours due to different reasons. There are only calculation results about Li2NiSiO4 till now. This brief critical review firstly discussed some papers about the first-principle calculation of Li2MSiO4 (M=Fe, Mn, Co Ni), and then collects and discusses relevant papers and recent patents about the fabrication, structure, particle size and electrochemical performance of nano/micro Li2MSiO4 (M=Fe, Mn, Co Ni) and their composites. Finally, the future challenges of Li2FeSiO4 are also discussed.

Carbon nanomaterials used as conductive additives in lithium ion batteries.

As the vital part of lithium ion batteries, conductive additives play important roles in the electrochemical performance of lithium ion batteries. They construct a conductive percolation network to increase and keep the electronic conductivity of electrode, enabling it charge and discharge faster. In addition, conductive additives absorb and retain electrolyte, allowing an intimate contact between the lithium ions and active materials. Carbon nanomaterials are carbon black, Super P, acetylene black, carbon nanofibers, and carbon nanotubes, which all have superior properties such as low weight, high chemical inertia and high specific surface area. They are the ideal conductive additives for lithium ion batteries. This review will discuss some registered patents and relevant papers about the carbon nanomaterials that are used as conductive additives in cathode or anode to improve the electrochemical performance of lithium ion batteries.