Shiqiang Hao

Ultrahigh power factor and thermoelectric performance in hole-doped single-crystal SnSe

Heat conversion gets a power boost Thermoelectric materials convert waste heat into electricity, but often achieve high conversion efficiencies only at high temperatures. Zhao et al. tackle this problem by introducing small amounts of sodium to the thermoelectric SnSe (see the Perspective by Behnia). This boosts the power factor, allowing the material to generate more energy while maintaining good conversion efficiency. The effect holds across a wide temperature range, which is attractive for developing new applications. Science, this issue p. 141; see also p. 124 A thermoelectric derived by sodium doping of tin selenide has a high power factor and conversion efficiency over a wide temperature range. [Also see Perspective by Behnia] Thermoelectric technology, harvesting electric power directly from heat, is a promising environmentally friendly means of energy savings and power generation. The thermoelectric efficiency is determined by the device dimensionless figure of merit ZTdev, and optimizing this efficiency requires maximizing ZT values over a broad temperature range. Here, we report a record high ZTdev ∼1.34, with ZT ranging from 0.7 to 2.0 at 300 to 773 kelvin, realized in hole-doped tin selenide (SnSe) crystals. The exceptional performance arises from the ultrahigh power factor, which comes from a high electrical conductivity and a strongly enhanced Seebeck coefficient enabled by the contribution of multiple electronic valence bands present in SnSe. SnSe is a robust thermoelectric candidate for energy conversion applications in the low and moderate temperature range.

High thermoelectric performance via hierarchical compositionally alloyed nanostructures.

Previous efforts to enhance thermoelectric performance have primarily focused on reduction in lattice thermal conductivity caused by broad-based phonon scattering across multiple length scales. Herein, we demonstrate a design strategy which provides for simultaneous improvement of electrical and thermal properties of p-type PbSe and leads to ZT ~ 1.6 at 923 K, the highest ever reported for a tellurium-free chalcogenide. Our strategy goes beyond the recent ideas of reducing thermal conductivity by adding two key new theory-guided concepts in engineering, both electronic structure and band alignment across nanostructure-matrix interface. Utilizing density functional theory for calculations of valence band energy levels of nanoscale precipitates of CdS, CdSe, ZnS, and ZnSe, we infer favorable valence band alignments between PbSe and compositionally alloyed nanostructures of CdS1-xSex/ZnS1-xSex. Then by alloying Cd on the cation sublattice of PbSe, we tailor the electronic structure of its two valence bands (light hole L and heavy hole Σ) to move closer in energy, thereby enabling the enhancement of the Seebeck coefficients and the power factor.

Raising the Thermoelectric Performance of p-Type PbS with Endotaxial Nanostructuring and Valence-Band Offset Engineering Using CdS and ZnS

We have investigated in detail the effect of CdS and ZnS as second phases on the thermoelectric properties of p-type PbS. We report a ZT of ~1.3 at 923 K for 2.5 at.% Na-doped p-type PbS with endotaxially nanostructured 3.0 at.% CdS. We attribute the high ZT to the combination of broad-based phonon scattering on multiple length scales to reduce (lattice) thermal conductivity and favorable charge transport through coherent interfaces between the PbS matrix and metal sulfide nanophase precipitates, which maintains the requisite high carrier conductivity and the associated power factor. Similar to large ionically bonded metal sulfides (ZnS, CaS, and SrS), the covalently bonded CdS can also effectively reduce the lattice thermal conductivity in p-type PbS. The presence of ubiquitous nanostructuring was confirmed by transmission electron microscopy. Valence and conduction band energy levels of the NaCl-type metal sulfides, MS (M = Pb, Cd, Zn, Ca, and Sr) were calculated from density functional theory to gain insight into the band alignment between PbS and the second phases in these materials. The hole transport is controlled by band offset minimization through the alignment of valence bands between the host PbS and the embedded second phases, MS (M = Cd, Zn, Ca, and Sr). The smallest valence band offset of about 0.13 eV at 0 K was found between PbS and CdS which is diminished further by thermal band broadening at elevated temperature. This allows carrier transport between the endotaxially aligned components (i.e., matrix and nanostructure), thus minimizing significant deterioration of the hole mobility and power factor. We conclude the thermoelectric performance of the PbS system and, by extension, other systems can be enhanced by means of a closely coupled phonon-blocking/electron-transmitting approach through embedding endotaxially nanostructured second phases.

Valence Band Modification and High Thermoelectric Performance in SnTe Heavily Alloyed with MnTe

We demonstrate a high solubility limit of >9 mol% for MnTe alloying in SnTe. The electrical conductivity of SnTe decreases gradually while the Seebeck coefficient increases remarkably with increasing MnTe content, leading to enhanced power factors. The room-temperature Seebeck coefficients of Mn-doped SnTe are significantly higher than those predicted by theoretical Pisarenko plots for pure SnTe, indicating a modified band structure. The high-temperature Hall data of Sn1-xMnxTe show strong temperature dependence, suggestive of a two-valence-band conduction behavior. Moreover, the peak temperature of the Hall plot of Sn1-xMnxTe shifts toward lower temperature as MnTe content is increased, which is clear evidence of decreased energy separation (band convergence) between the two valence bands. The first-principles electronic structure calculations based on density functional theory also support this point. The higher doping fraction (>9%) of Mn in comparison with ∼3% for Cd and Hg in SnTe gives rise to a much better valence band convergence that is responsible for the observed highest Seebeck coefficient of ∼230 μV/K at 900 K. The high doping fraction of Mn in SnTe also creates stronger point defect scattering, which when combined with ubiquitous endotaxial MnTe nanostructures when the solubility of Mn is exceeded scatters a wide spectrum of phonons for a low lattice thermal conductivity of 0.9 W m(-1) K(-1) at 800 K. The synergistic role that Mn plays in regulating the electron and phonon transport of SnTe yields a high thermoelectric figure of merit of 1.3 at 900 K.

Codoping in SnTe: Enhancement of Thermoelectric Performance through Synergy of Resonance Levels and Band Convergence

We report a significant enhancement of the thermoelectric performance of p-type SnTe over a broad temperature plateau with a peak ZT value of ∼1.4 at 923 K through In/Cd codoping and a CdS nanostructuring approach. Indium and cadmium play different but complementary roles in modifying the valence band structure of SnTe. Specifically, In-doping introduces resonant levels inside the valence bands, leading to a considerably improved Seebeck coefficient at low temperature. Cd-doping, however, increases the Seebeck coefficient of SnTe remarkably in the mid- to high-temperature region via a convergence of the light and heavy hole bands and an enlargement of the band gap. Combining the two dopants in SnTe yields enhanced Seebeck coefficient and power factor over a wide temperature range due to the synergy of resonance levels and valence band convergence, as demonstrated by the Pisarenko plot and supported by first-principles band structure calculations. Moreover, these codoped samples can be hierarchically structured on all scales (atomic point defects by doping, nanoscale precipitations by CdS nanostructuring, and mesoscale grains by SPS treatment) to achieve highly effective phonon scattering leading to strongly reduced thermal conductivities. In addition to the high maximum ZT the resultant large average ZT of ∼0.8 between 300 and 923 K makes SnTe an attractive p-type material for high-temperature thermoelectric power generation.

Origin of the High Performance in GeTe-Based Thermoelectric Materials upon Bi2Te3 Doping

As a lead-free material, GeTe has drawn growing attention in thermoelectrics, and a figure of merit (ZT) close to unity was previously obtained via traditional doping/alloying, largely through hole carrier concentration tuning. In this report, we show that a remarkably high ZT of ∼1.9 can be achieved at 773 K in Ge0.87Pb0.13Te upon the introduction of 3 mol % Bi2Te3. Bismuth telluride promotes the solubility of PbTe in the GeTe matrix, thus leading to a significantly reduced thermal conductivity. At the same time, it enhances the thermopower by activating a much higher fraction of charge transport from the highly degenerate Σ valence band, as evidenced by density functional theory calculations. These mechanisms are incorporated and discussed in a three-band (L + Σ + C) model and are found to explain the experimental results well. Analysis of the detailed microstructure (including rhombohedral twin structures) in Ge0.87Pb0.13Te + 3 mol % Bi2Te3 was carried out using transmission electron microscopy and crystallographic group theory. The complex microstructure explains the reduced lattice thermal conductivity and electrical conductivity as well.

Non-equilibrium processing leads to record high thermoelectric figure of merit in PbTe-SrTe.

The broad-based implementation of thermoelectric materials in converting heat to electricity hinges on the achievement of high conversion efficiency. Here we demonstrate a thermoelectric figure of merit ZT of 2.5 at 923 K by the cumulative integration of several performance-enhancing concepts in a single material system. Using non-equilibrium processing we show that hole-doped samples of PbTe can be heavily alloyed with SrTe well beyond its thermodynamic solubility limit of <1 mol%. The much higher levels of Sr alloyed into the PbTe matrix widen the bandgap and create convergence of the two valence bands of PbTe, greatly boosting the power factors with maximal values over 30 μW cm−1 K−2. Exceeding the 5 mol% solubility limit leads to endotaxial SrTe nanostructures which produce extremely low lattice thermal conductivity of 0.5 W m−1 K−1 but preserve high hole mobilities because of the matrix/precipitate valence band alignment. The best composition is hole-doped PbTe–8%SrTe.

Free-standing nitrogen-doped graphene paper as electrodes for high-performance lithium/dissolved polysulfide batteries.

Free-standing N-doped graphene papers (NGP), generated by pyrolysis of polydiallyldimethylammonium chloride, were successfully used as binder-free electrodes for the state-of-the-art Li/polysulfide-catholyte batteries. They exhibited high specific capacities of approximately 1000 mA h g(-1) (based on S) after 100 cycles and coulombic efficiencies great than 98%, significantly better than undoped graphene paper (GP). These NGP were characterized with XRD, X-ray photoelectron spectroscopy, thermogravimetric analysis, AFM, electron microscopy, and Raman and impedance spectroscopy before and after cycling. Spectroscopic evidence suggested stronger binding of sulfide to NGP relative to GP, and modelling results from DFT calculation, substantiated with experimental data, indicated that pyrrolic and pyridinic N atoms interacted more strongly with Li polysulfides than quaternary N atoms. Thus, more favorable partition of polysulfides between the electrode and the electrolyte and the corresponding effect on the morphology of the passivation layer were the causes of the beneficial effect of N doping.

Concerted Rattling in CsAg5Te3 Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance

Thermoelectric (TE) materials convert heat energy directly into electricity, and introducing new materials with high conversion efficiency is a great challenge because of the rare combination of interdependent electrical and thermal transport properties required to be present in a single material. The TE efficiency is defined by the figure of merit ZT=(S(2) σ) T/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity, and T is the absolute temperature. A new p-type thermoelectric material, CsAg5 Te3 , is presented that exhibits ultralow lattice thermal conductivity (ca. 0.18 Wm(-1)  K(-1) ) and a high figure of merit of about 1.5 at 727 K. The lattice thermal conductivity is the lowest among state-of-the-art thermoelectrics; it is attributed to a previously unrecognized phonon scattering mechanism that involves the concerted rattling of a group of Ag ions that strongly raises the Grüneisen parameters of the material.

Lithium transport through lithium-ion battery cathode coatings

The surface coating of cathodes using insulator films has proven to be a promising method for high-voltage cathode stabilization in Li-ion batteries, but there is still substantial uncertainty about how these films function. More specifically, there is limited knowledge of lithium solubility and transport through the films, which is important for coating design and development. This study uses first-principles calculations based on density functional theory to examine the diffusivity of interstitial lithium in the crystals of α-AlF3, α-Al2O3, m-ZrO2, c-MgO, and α-quartz SiO2, which provide benchmark cases for further understanding of insulator coatings in general. In addition, we propose an ohmic electrolyte model to predict resistivities and overpotential contributions under battery operating conditions. For the crystalline materials considered we predict that Li+ diffuses quite slowly, with a migration barrier larger than 0.9 eV in all crystalline materials except α-quartz SiO2, which is predicted to have a migration barrier of 0.276 eV along 〈001〉. These results suggest that the stable crystalline forms of these insulator materials, except for oriented α-quartz SiO2, are not practical for conformal cathode coatings. Amorphous Al2O3 and AlF3 have higher Li+ diffusivities than their crystalline counterparts. Our predicted amorphous Al2O3 resistivity (1789 MΩ m) is close to the top of the range of the fitted resistivities extracted from previous experiments on nominal Al2O3 coatings (7.8 to 913 MΩ m) while our predicted amorphous AlF3 resistivity (114 MΩ m) is very close to the middle of the range. These comparisons support our framework for modeling and understanding the impact on overpotential of conformal coatings in terms of their fundamental thermodynamic and kinetic properties, and support that these materials can provide practical conformal coatings in their amorphous form.