Synergistic Microstructure and Composition Engineering via Na 2 S 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, Pb_0.97Na_0.03Te-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.