Entropy and Enthalpy Co‐Tuning Strategy in Cu 2‐y Ag y (In 0.06 Sn 0.94 )Se 2 S System toward Excellent Thermoelectric Performance

With the reduction in size, nanoparticles (NPs) exhibit unique chemical and physical properties due to surface effects and size‐dependent electronic bands. However, a significant challenge that limits their practical application is the tendency for grain growth at elevated temperatures. Herein, an entropic and enthalpic‐co‐stabilization strategy is employed in a series of high‐entropy Cu 2‐y Ag y (In 0.06 Sn 0.94 )Se 2 S materials. Remarkably, the grain size of these materials remains at 70 nm even after repeated tests up to 873 K. This stability has enabled to achieve an ultra‐low thermal conductivity ( κ tot ) of 0.22 W m −1 K −1 , a record‐high peak zT value of 1.76, and a high average zT value of 0.54 for nanostructured Cu 1.73 Ag 0.27 (In 0.06 Sn 0.94 )Se 2 S. Additionally, a high thermoelectric conversion efficiency of 4.5% for the segmented single‐leg module and 3.8% for the 2‐pair module at Δ T ≈ 460 K are achieved with the Cu 2 SnSe 3 ‐based material, marking its debut in thermoelectric power generation. These findings provide a paradigm to stabilize grain size and improve the thermoelectric performance of nano‐sized high‐entropy materials (HEMs), presenting a promising avenue for substantial advancements across various applications.