Synergistic Microstructure and Composition Engineering via Na2S Enables High-Performance Porous PbTe Thermoelectrics with Ultrahigh Device Power Density.

Thermoelectric (TE) materials, capable of directly converting heat into electricity, offer a promising route for sustainable energy recovery. However, practical deployment is limited by the difficulty in simultaneously optimizing electrical and thermal transport properties. In this study, a synergistic microstructure-composition co-design strategy for enhancing the performance of PbTe-based TEs via Na2S-assisted solid-state synthesis is presented. The thermal decomposition of Na2S not only introduces hierarchical porosity but also facilitates initial Na doping, enabling the concurrent optimization of phonon scattering, carrier concentration, and band convergence. The optimized composition, Pb0.97Na0.03Te-1.0%Na2S, exhibits refined grains, dispersed Na2Te nanoprecipitates, and a high density of dislocations, leading to ultralow lattice thermal conductivity (≈0.50 W m-1 K-1 at 750 K) while preserving excellent electrical transport. A peak TE figure of merit zT≈2.2 at 823 K and a high average zT ≈1.9 across 623-823 K are achieved. To validate the device-level applicability, single-leg TE modules are fabricated, achieving a high conversion efficiency of 13.4% at ΔT = 395 K, which is among the best reported for a PbTe-based system. Furthermore, a unicouple module integrated with n-type skutterudite reaches a record power density of 2.2 W cm-2 at ΔT = 375 K. This study highlights a scalable pathway for advancing mid-temperature TE materials and devices through structural and compositional engineering.