Typical thermoelectric materials: Progress and prospects

Currently, global efforts to address climate change and mitigate global warming are primarily focused on clean and renewable energy technologies. However, the significant loss of energy in the form of heat and the associated issues within conventional energy pathways highlight the critical need to explore new methods for energy production, conversion, and storage. The low efficiency of traditional energy sources is not only due to mechanical and thermodynamic limitations but also the substantial energy loss during conversion processes [1], [2], [3]. For instance, thermal-based power generation accounts for over 50% of global electricity production, yet its operational efficiency is only 30%-40%. Waste heat generated from power plants, industrial processes, smokestacks, geothermal sources, and engine exhaust systems is a major contributor to this energy loss, with more than half of global energy resources being wasted as heat each year. Effectively recovering and utilizing this waste heat could significantly enhance energy efficiency, reduce greenhouse gas emissions, and promote sustainable development. While there are various methods for recovering waste heat, thermoelectric (TE) devices stand out due to their unique ability to directly convert waste heat into valuable electrical energy [4]. As a technology that transforms heat into electricity, thermoelectric materials and devices show great promise in waste heat recovery and are expected to play a key role in improving energy efficiency in the future.

Despite significant progress, the practical application of TE materials is still limited by their low conversion efficiency, which is determined by the dimensionless figure of merit ZT. The ZT value is expressed as ZT=, where S is the Seebeck coefficient, σ is the electrical conductivity, and T is the absolute temperature. The power factor PF= characterizes the material’s ability to convert energy. Thermal conductivity κ is the sum of the lattice thermal conductivity and the electronic thermal conductivity , i.e., κ=+. The relationship between and σ can be described by the Wiedemann-Franz law [5], =LσT, where L is the Lorenz number. The key to enhancing TE efficiency lies in developing materials with high ZT values. Ideal materials should exhibit “phonon-glass electron-crystal” properties, characterized by low thermal conductivity, high electrical conductivity, and a large Seebeck coefficient [6]. However, there is a complex interdependence between S, σ and , which remains a bottleneck in current research. For example, increasing electrical conductivity often reduces the Seebeck coefficient, as these two parameters are coupled through the carrier concentration. Additionally, methods aimed at enhancing phonon scattering to reduce lattice thermal conductivity can simultaneously lower carrier mobility, negatively impacting electrical conductivity. In recent years, researchers have adopted various external strategies to improve ZT values, such as electronic band structure engineering [7], [8], [9], Nano-microstructures controlling [10], [11], [12], and carrier concentration optimization [13], [14], [15]. While these approaches have shown success, the most straightforward and effective route is still to identify materials that intrinsically possess low and high PF. Such materials, by design, avoid the trade-offs associated with external optimization techniques, making them the most practical solution for achieving high-efficiency thermoelectric materials.

The selection of thermoelectric materials varies significantly across different operating temperatures. Many excellent reviews have systematically summarized this field in earlier studies [18], [19], [20]. Thermoelectric materials are typically categorized into low-temperature, near-room-temperature, mid-temperature, and high-temperature materials based on the temperature range corresponding to their peak ZT values. Low-temperature thermoelectric materials typically have a narrow working temperature range and are primarily used in solid-state cooling applications. The bismuth telluride (BiTe_x) system is a typical low-temperature thermoelectric material [21], with a working temperature generally below 300 K. Near-room temperature thermoelectric materials have a working temperature range from 300 K to 500 K, including (Bi, Sb)₂(Te, Se)₃-based materials [22], Mg₃Sb₂-based compounds [23], and MgAgSb-based compounds [24]. These materials have not only been commercialized but also demonstrate excellent thermoelectric conversion efficiency within this temperature range, making them key research subjects in the field. As for the medium-temperature range (500 K–900 K), research primarily focuses on materials with a spinel structure, PbTe, SnSe, and other IV-VI compound semiconductors [25], [26], [27], [28]. These materials exhibit promising thermoelectric performance in this temperature range and are expected to play a significant role in future medium-temperature thermoelectric applications. As the temperature increases, materials such as SiGe alloys [29], Zintl compounds [30], [31], [32], and half-Heusler alloys [33], [34], [35] exhibit excellent thermoelectric performance, making them suitable for high-temperature environments. These materials typically operate above 900 K and are classified as high-temperature thermoelectric materials. In recent years, with the continuous advancement of thermoelectric materials research, the focus has gradually expanded to various new material systems. Fig. 1 illustrates the research distribution of several thermoelectric materials in recent literature, reflecting the research intensity and development trends of different material systems. Additionally, with a deeper understanding of the material properties and the factors influencing thermoelectric performance, significant progress has been made in material design and engineering optimization. The discovery and development of these novel materials signal ongoing innovation and breakthroughs in the field of thermoelectrics.

Starting from the basic theory of thermoelectric materials, this paper reviews the research progress of several types of classical materials with good thermoelectric potential, including full-Heusler alloys, half-Heusler alloys, perovskites, Zintl compounds, fluorite and antifluorite compounds. The current research focus is on optimizing the performance of existing materials or developing novel high-efficiency thermoelectric materials to achieve improved thermoelectric conversion efficiency. Therefore, we analyze the exceptional thermoelectric performance of these materials, focusing on intrinsic properties such as low lattice thermal conductivity or high power factor, and explore various approaches to enhance thermoelectric performance in known materials. By addressing aspects such as electronic structure, electrical and thermal transport, and thermoelectric performance modulation, this paper aims to provide valuable insights for further research in this field.