High Thermoelectric Performance in SnTeâAgSbTe2 Alloys from

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High Thermoelectric Performance in SnTe-AgSbTe2 Alloys from Lattice Softening, Giant Phonon-Vacancy Scattering, and Valence Band Convergence Gangjian Tan, Shiqiang Hao, Riley Hanus, Xiaomi Zhang, Shashwat Anand, Trevor Paul Bailey, Alexander J. E. Rettie, Xianli Su, Ctirad Uher, Vinayak P. Dravid, G. Jeffrey Snyder, Chris Wolverton, and Mercouri G. Kanatzidis ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00137 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 13, 2018

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ACS Energy Letters

High Thermoelectric Performance in SnTe-AgSbTe2 Alloys from Lattice Softening, Giant PhononVacancy Scattering, and Valence Band Convergence Gangjian Tan,1 Shiqiang Hao,2 Riley C. Hanus,2 Xiaomi Zhang,2 Shashwat Anand,2 Trevor P. Bailey,3 Alexander J. E. Rettie,4 Xianli Su,1,5 Ctirad Uher,3 Vinayak P. Dravid,2 G. Jeffrey Snyder,2 Chris Wolverton,2 Mercouri G. Kanatzidis*,1,4 1

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States

2

Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States 3

Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, United States

4

Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

5

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China

1

ACS Paragon Plus Environment

ACS Energy Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract We report on the underlying mechanism that enables the SnTe-AgSbTe2 system to exhibit superior thermoelectric figure of merit (ZT) than its parent compound SnTe. We show that AgSbTe2 alloying has a profound impact on the band structure of SnTe by converging the energies of its light and heavy valence bands, leading to significantly enhanced Seebeck coefficients. We have also unraveled a significant connection between alloying and defect stability in this system, wherein the Sn vacancy concentration increases significantly when Ag and Sb are alloyed on the Sn site. The increased Sn vacancy concentration dramatically reduces the lattice thermal conductivity through both lattice softening and phonon-vacancy scattering to ~0.4 Wm-1K-1 at 800 K. Consequently, a ZT value of 1.2 at 800 K for AgSn5SbTe7 can be achieved by doping I on Te sites. This represents a 300% improvement over pristine SnTe, outperforming many reported SnTebased thermoelectric materials.

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ACS Energy Letters

Graphical Abstract

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Thermoelectric materials can realize the direct conversion from heat to electricity without any hazardous emissions, and vice versa. Therefore, thermoelectricity has long been considered as a possible means of enhancing energy efficiency and utilization while helping to mitigate environmental issues.1-10 The key to bringing thermoelectric technology into large-scale deployment is to improve the thermoelectric conversion efficiency which depend on the dimensionless figure of merit ZT=S2σT/κ=S2σT/(κel+κlat). In the expression of ZT, S is the Seebeck coefficient, σ represents the electrical conductivity, T denotes the absolute temperature in Kelvin, and κ is the total thermal conductivity that consists of two main contributions from charge carriers (κel) and the lattice (κlat). Except for κlat which is tied to phonon transport, all transport parameters are strongly interrelated through the carrier concentration.5,7 The ZTmax can be reached by improving the electronic band structure (resonance states, band convergence, etc.)11-13 and/or minimizing κlat through the introduction of defects at different scales to strongly scatter phonons9,14-18. A less studied effect, which is highlighted here, is that defects also have the capacity to soften a material’s lattice which, in addition to modifying scattering processes, reduces phonon velocity. Indeed, the primary material descriptor for predicting a material’s thermal conductivity is the stiffness of its lattice (bulk modulus, speed of sound, Debye temperature, etc.)19,20, which if controlled through defect engineering, can provide another avenue to improve ZTmax . Spectacular progress has been made in the past decade on PbTe-based compounds with the best ZT readily exceeding 2, especially for the p-type compositions.15,21,22 However, due to the perceived toxicity of Pb in PbTe, researchers have been exploring Pb-free substitutes, among which SnTe holds the greatest promise because it shares many similarities with PbTe on crystal and electronic structures.17,23-36 Nonetheless, SnTe itself historically has not been regarded as a good thermoelectric material (ZT 2 by multi-functional alloying. J. Mater. 2016, 2, 141-149. Tan, G.; Zhao, L.-D.; Shi, F.; Doak, J. W.; Lo, S.-H.; Sun, H.; Wolverton, C.; Dravid, V. P.; Uher, C.; Kanatzidis, M. G. High Thermoelectric Performance of pType SnTe via a Synergistic Band Engineering and Nanostructuring Approach. J. Am. Chem. Soc. 2014, 136, 7006-7017. Tan, G.; Shi, F.; Doak, J. W.; Sun, H.; Zhao, L.-D.; Wang, P.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Extraordinary role of Hg in enhancing the thermoelectric performance of p-type SnTe. Energy Environ. Sci. 2015, 8, 267277. Zhang, Q.; Liao, B.; Lan, Y.; Lukas, K.; Liu, W.; Esfarjani, K.; Opeil, C.; Broido, D.; Chen, G.; Ren, Z. High thermoelectric performance by resonant dopant indium in nanostructured SnTe. Proc. Natl. Acad. Sci. USA 2013, 110, 1326113266. Tan, G.; Zeier, W. G.; Shi, F.; Wang, P.; Snyder, G. J.; Dravid, V. P.; Kanatzidis, M. G. High Thermoelectric Performance SnTe–In2Te3 Solid Solutions Enabled by Resonant Levels and Strong Vacancy Phonon Scattering. Chem. Mater. 2015, 27, 7801-7811. Zhou, M.; Gibbs, Z. M.; Wang, H.; Han, Y.; Xin, C.; Li, L.; Snyder, G. J. 16


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Optimization of thermoelectric efficiency in SnTe: the case for the light band. Phys. Chem. Chem. Phys. 2014, 16, 20741-20748. Tan, G.; Shi, F.; Hao, S.; Chi, H.; Bailey, T. P.; Zhao, L.-D.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Valence Band Modification and High Thermoelectric Performance in SnTe Heavily Alloyed with MnTe. J. Am. Chem. Soc. 2015, 137, 11507-11516. Tan, G.; Shi, F.; Hao, S.; Chi, H.; Zhao, L.-D.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Codoping in SnTe: Enhancement of Thermoelectric Performance through Synergy of Resonance Levels and Band Convergence. J. Am. Chem. Soc. 2015, 137, 5100-5112. Tan, G.; Shi, F.; Sun, H.; Zhao, L.-D.; Uher, C.; Dravid, V. P.; Kanatzidis, M. G. SnTe-AgBiTe2 as an efficient thermoelectric material with low thermal conductivity. J. Mater. Chem. A 2014, 2, 20849-20854. He, J.; Tan, X.; Xu, J.; Liu, G.-Q.; Shao, H.; Fu, Y.; Wang, X.; Liu, Z.; Xu, J.; Jiang, H.; et al. Valence band engineering and thermoelectric performance optimization in SnTe by Mn-alloying via a zone-melting method. J. Mater. Chem. A 2015, 3, 19974-19979. Banik, A.; Shenoy, U. S.; Saha, S.; Waghmare, U. V.; Biswas, K. High Power Factor and Enhanced Thermoelectric Performance of SnTe-AgInTe2: Synergistic Effect of Resonance Level and Valence Band Convergence. J. Am. Chem. Soc. 2016, 138, 13068-13075. Li, W.; Zheng, L.; Ge, B.; Lin, S.; Zhang, X.; Chen, Z.; Chang, Y.; Pei, Y. Promoting SnTe as an Eco-Friendly Solution for p-PbTe Thermoelectric via Band Convergence and Interstitial Defects. Adv. Mater. 2017, 29, 1605887. Tan, X. J.; Shao, H. Z.; He, J.; Liu, G. Q.; Xu, J. T.; Jiang, J.; Jiang, H. C. Band engineering and improved thermoelectric performance in M-doped SnTe (M = Mg, Mn, Cd, and Hg). Phys. Chem. Chem. Phys. 2016, 18, 7141-7147. Zhao, L.-D.; Lo, S.-H.; Zhang, Y.; Sun, H.; Tan, G.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals. Nature 2014, 508, 373-377. Tan, G.; Hao, S.; Zhao, J.; Wolverton, C.; Kanatzidis, M. G. High Thermoelectric Performance in Electron-Doped AgBi3S5 with Ultralow Thermal Conductivity. J. Am. Chem. Soc. 2017, 139, 6467-6473. Brebrick, R. F. Deviations from stoichiometry and electrical properties in SnTe. J. Phys. Chem. Solids 1963, 24, 27-36. Vedeneev, V. P.; Krivoruchko, S. P.; Sabo, E. P. Tin telluride based thermoelectrical alloys. Semiconductors 1998, 32, 241-244. SINGH, D. J. Thermopower of SnTe from Boltzmann transport calculations. Funct. Mater. Lett. 2010, 03, 223-226. 17



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Influence on the Thermoelectric Properties of the AgSbTe2-SnTe Quaternary System. MRS Proceed. 2011, 886, 508-515. Androulakis, J.; Pcionek, R.; Quarez, E.; Do, J.-H.; Kong, H.; Palchik, O.; Uher, C.; D'Angelo, J. J.; Short, J.; Hogan, T.; et al. Coexistence of Large Thermopower and Degenerate Doping in the Nanostructured Material Ag0.85SnSb1.15Te3. Chem. Mater. 2006, 18, 4719-4721. Yang, S. H.; Zhu, T. J.; Sun, T.; He, J.; Zhang, S. N.; Zhao, X. B. Nanostructures in high-performance (GeTe)x(AgSbTe2)100−x thermoelectric materials. Nanotechnology 2008, 19, 245707. Placheova, S. K. Thermoelectric figure of merit of the system (GeTe)1−x(AgSbTe2)x. Phys. Status Solid. A 1984, 83, 349-355. Falkenbach, O.; Schmitz, A.; Dankwort, T.; Koch, G.; Kienle, L.; Mueller, E.; Schlecht, S. Tin Telluride-Based Nanocomposites of the Type AgSnmBiTe2+m (BTST-m) as Effective Lead-Free Thermoelectric Materials. Chem. Mater. 2015, 27, 7296-7305. Quarez, E.; Hsu, K.-F.; Pcionek, R.; Frangis, N.; Polychroniadis, E. K.; Kanatzidis, M. G. Nanostructuring, Compositional Fluctuations, and Atomic Ordering in the Thermoelectric Materials AgPbmSbTe2+m: The Myth of Solid Solutions. J. Am. Chem. Soc. 2005, 127, 9177-9190. Zhou, M.; Li, J.-F.; Kita, T. Nanostructured AgPbmSbTem+2 System Bulk Materials with Enhanced Thermoelectric Performance. J. Am. Chem. Soc. 2008, 130, 45274532. Ke, X.; Chen, C.; Yang, J.; Wu, L.; Zhou, J.; Li, Q.; Zhu, Y.; Kent, P. R. C. Microstructure and a Nucleation Mechanism for Nanoprecipitates in PbTeAgSbTe2. Phys. Rev. Lett. 2009, 103, 145502. Putley, E. H. Galvano- and thermo-magnetic coefficients for a multi-band conductor. J. Phys. C Solid State Phys. 1975, 8, 1837. Jovovic, V.; Heremans, J. P. Measurements of the energy band gap and valence band structure of AgSbTe2. Phys. Rev. B 2008, 77, 245204. Cahill, D. G.; Watson, S. K.; Pohl, R. O. Lower limit to the thermal conductivity of disordered crystals. Phys. Rev. B 1992, 46, 6131-6140. Callaway, J. Model for Lattice Thermal Conductivity at Low Temperatures. Phys. Rev. 1959, 113, 1046-1051. Kaviany, M. Heat Transfer Physics; Cambridge University Press: Cambridge, U.K.; 2008. Three degenerate phonon modes with a speed of sound equal to the average speed of sound. Damon, D. H. Thermal Conductivity of SnTe between 100° and 500°K. J. Appl. Phys. 1966, 37, 3181-3190. 19



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Figure Captions Figure 1. (a) Room temperature lattice parameters of SnTe as a function of AgSbTe2 content; (b) conventional transmission electron microscopy image, high resolution TEM image (upper right inset), as well as selective area diffraction pattern (lower right inset), (c) backscattered electron image (BSE) and (d) corresponding elemental maps obtained from energy dispersive X-ray spectroscopy for the sample AgSn5SbTe7. There is no evident phase separation at either the micro- or nano scale, which suggests a solid solution behavior between SnTe and AgSbTe2. Figure 2. Temperature dependent (a) electrical conductivity and (b) Seebeck coefficient for AgSnmSbTe2+m; (c) Room temperature Seebeck coefficient as a function of carrier concentration for AgSnmSbTe2+m, in comparison with the theoretical Pisarenko plot of SnTe as indicated by the blue line25; (d) Temperature-variant normalized Hall coefficients of SnTe,28 AgSn25SbTe27 and AgSn15SbTe17. Figure 3. Electronic band structures for (a) SnTe and (b) AgSn25SbTe27. The band gap (Eg) and energy separation between the light valence band at L point and heavy valence band at Σ point (∆Ev) are labeled to guide eyes. Figure 4. (a) Total and (b) lattice thermal conductivities as a function of temperature for AgSnmSbTe2+m. Figure 5. (a) Lattice thermal conductivity modeling under Debye approximation. Data from Damon67 of a SnTe single crystal was modeled along with the polycrystalline SnTe sample and m=5 sample, AgSn5SbTe7 of this study. The grey shaded region shows the reduction in κlat from SnTe to AgSn5SbTe7 solely due to lattice softening because of decreased sound velocity which is measured using ultrasound. The further reduction from gray to blue (blue shaded region) is due to increased [VSn ] and phonon-vacancy scattering. (b) ZT values as a function of temperature for AgSnmSbTe2+m; Comparison of (c) temperature dependent ZT and (d) 400-800 K average ZT (ZTave) in this study with previous studies28,29,33.

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Table 1. Room temperature physical properties, including mass density (ρ), Hall carrier concentrations (Np) and carrier mobilities (µ) for AgSnmSbTe2+m. Sample

ρ (g/cm3)

Np (1020 cm-3)

µ (cm2V-1s-1)

SnTe

6.12

2.6

187

m=50

6.17

5.4

62

m=25

6.14

7.8

35

m=20

6.27

8.3

32

m=15

6.13

m=10

6.26

9.1 -

26 -

m=5

6.11

14.5

10

Table 2. Parameters for κlat modeling. The phonon-phonon scattering parameter, A, and microstructural scattering parameter d were fit, while everything else was set to experimental values. A (107 m3K-1s-2) Sample

d (µ µm)

[VSn] (1020 cm-3)

υs (m/s)

Debye

BvK

Debye

BvK

single crystal

2.5

4.3

2

5

0.8

2151

m=0 polycrystal

2.5

4.3

2

5

1.3

2151

m=5 polycrystal

2.5

4.3

2

5

7.3

1862

22


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Figure 1

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Figure 2

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Figure 3

25


ACS Energy Letters

-1

5

SnTe m=50 m=25 m=20 m=15 m=10 m=5

AgSnmSbTe2+m

2.5

m=20 m=15 m=10 m=5

SnTe m=50 m=25

-1

-1

6

(b) 3.0 -1

7

AgSnmSbTe2+m

κlat (Wm K )

(a) 8 κ (Wm K )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.0 1.5

4

1.0

3 2

0.5

1

0.0

-1

κmin=0.4 Wm K

-1

300 400 500 600 700 800

300 400 500 600 700 800

T (K)

T (K)

Figure 4

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Figure 5

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High Thermoelectric Performance in SnTeâAgSbTe2 Alloys from

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High Thermoelectric Performance in SnTeâAgSbTe2 Alloys from