Mixed-anion thermoelectrics: Advanced tuning of electron and phonon transport

Thermoelectric energy conversion is a promising renewable technology to generate electricity by recovering waste heat. Great progress has been made in energy conversion efficiency of thermoelectric materials, but further performance enhancement has been expected by developing new material design rules. Recently, “mixed-anion” materials, which consist of two or more anionic species in a single phase, have attracted much attention as a next-generation high-performance thermoelectric material. They form unique crystal structures and coordination not observed in single-anion systems and have demonstrated, for example, extremely low lattice thermal conductivity and also specific electronic structure enabling high thermoelectric performance. This paper provides a comprehensive review of the recent advances in mixed-anion thermoelectric materials and the mixed-anion effect on electron and phonon transport. We first provide an overview of the historical approach of multiple-anion substitution onto single-anion compounds and discuss the substantial impacts of multiple anion substitutions across different material systems. Then, we summarize the characteristics of crystal structures and physical properties, as well as the recent advances in thermoelectric properties for the mixed-anion compounds that naturally contain multiple anions. In the end, we point out the currently unsolved challenges and future prospects toward the development of mixed-anion thermoelectrics. Mixed-anion materials have a large degree of freedom regarding the choice of the constituent anion combinations, which provides a wide search space for new materials with further outstanding thermoelectric performance. Going forward, we expect that the mixed-anion strategy offers great potential for finding new classes of high-performance thermoelectric materials.