Lead-free SnTe-based compounds as advanced thermoelectrics

Highlights

• Recent progress for SnTe-based thermoelectric materials is summarized.

• Basic structural chemistry and physical properties are discussed in detail.

• Future prospects for further improving ZT of SnTe are outlined.

Abstract

Thermoelectric materials are able to realize the direct energy conversion between heat and electricity and have great potential in power generation and Peltier cooling. SnTe, an old but intriguing mid-temperature thermoelectric material, has attracted much attention in the past half-decade because of its non-toxic chemical elements as well as distinctive two-valence-bands (L and Σ bands) electronic structure. However, pristine SnTe shows inferior thermoelectric performance largely due to its intrinsically large population of holes as charge carriers, big energy difference between L and Σ bands and high lattice thermal conductivity. In recent years, numerous efforts have been carried out to optimize the transport coefficients of SnTe, endowing the peak thermoelectric figure of merit ZT above 1.8. In this review, the fundamental properties of SnTe are first outlined, and the optimization strategies are subsequently summarized for both electronic and phonon transport. Finally, future possible directions to further enhance the thermoelectric performance of SnTe are put forward at the end of this article. We believe that this review will spark people’s inspiration in exploring other IV-VI thermoelectric compounds.

Introduction

Thermoelectric (TE) materials can be used to convert temperature gradient into electric power, and present an advanced technology for energy harvesting [[1], [2], [3], [4]]. There has been a long history of Radioisotope Thermoelectric Generators (RTGs) powering the spacecraft launched by NASA since 1930s [5]. TEs are now actively considered for a wide variety of new applications, such as the conversion of automobile exhaust heat into electricity [[6], [7], [8], [9], [10]]. The effectiveness of a TE material is evaluated by the dimensionless figure of merit ZT = S²σT/κ = S²σT/(κ_lat+κ_ele), where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity that comprises two major components: the electrical contribution κ_ele = LσT (L is the Lorenz number) and the lattice contribution κ_lat [11,12]. Usually, the term S²σ is called as power factor which reflects the electrical properties of TE materials. An ideal TE material requires high σ like in metals, large S like in insulators, and low κ like in glasses [[13], [14], [15]]. The strong interdependence of these physical parameters makes it quite challenging to increase ZT of a given material in a large freedom [12,16,17].

Thanks to the development of new concepts relating to size effects and band structure engineering, we have witnessed the rapid advance of TE materials and the explosive growth of TE community over the past three decades [10,[18], [19], [20], [21], [22], [23], [24], [25], [26]]. The best ZT, which had staggered around unity for half a century before 1990s, was steadily improved to ∼1.5 in 2000s and is further promoted to well above 2 as recently demonstrated in PbTe system [[27], [28], [29], [30]]. However, due to the concern on the perceived toxicity of Pb, there has been a growing interest of developing high performance lead-free TE materials to replace PbTe [[31], [32], [33]]. The typical examples include skutterudites, half-Heusler alloys, tin or copper chalcogenides, magnesium silicide, etc. [29,[34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47]] Among these materials, SnTe holds the greatest promise because it resembles PbTe in many aspects [32,48,49].

The exploration of SnTe as a potential TE material can be traced back to 1960s when its anomalous electron transport behavior (two valence bands character) was identified [50,51]. Some explorative doping studies were also performed with the aim to optimize the TE performance of SnTe [[52], [53], [54], [55], [56], [57], [58]]. However, the ZT (far below unity) of SnTe was still much lower than that of PbTe at that time because of its very low Seebeck coefficient and over high thermal conductivity originating from the intrinsically large volume of tin vacancies [32,59]. Therefore, for a long time, the progress of SnTe had a slow pace until very recently when the band structure engineering [29,48,[60], [61], [62], [63], [64], [65], [66], [67]] and nanostructuring stratagems [29,62,[68], [69], [70], [71], [72]] were applied to this old but intriguing system. As a consequence, a competitive maximum ZT of 1.4–1.8 at 900 K has been achieved in optimized SnTe, making it a robust material for middle temperature TE power generation when lead-free materials are required [[73], [74], [75], [76], [77], [78], [79]].

Several nice review articles have been published recently highlighting the rapid progress of SnTe-based thermoelectric materials [31,59,[80], [81], [82]]. We hereby aim at providing a comprehensive and timely overview on the historical researches and recent developments of SnTe TE materials in an attempt to understand its basic structural chemistry and physical properties, and to guide its further development. Unlike the pervious articles, we expound electronic structure feature of SnTe in the way of describing the connection between PbTe and SnTe, and discuss the influence on hole concentration dependent Seebeck coefficient. Moreover, the intrinsic lattice thermal conductivity has also been analyzed from the perspective of lattice dynamics and resonance bonds by comparing with InSb. Generally, we organize this review as follows. First, we summarize the crystal, electronic structures and lattice dynamics of SnTe. Then, the effective technologies that successfully enhanced the TE performance of SnTe are discussed in detail. Future possible directions for further enhancing ZT of SnTe are proposed at the end of this review.