Electronic-phononic decoupling and Fermi-level tuning enable high thermoelectric performance in Ag₈SnSe₆

Liquid-like superionic conductors, with highly mobile ions in a rigid framework, offer intrinsically low lattice thermal conductivity without compromising electronic transport. Argyrodite-type Ag₈SnSe₆ exhibits a melt-like Ag sublattice that drives lattice thermal conductivity (κ_L) below 0.2 watts per meter per kelvin, yet its low carrier concentration limits the power factor. Here, interstitial Ag atoms raise the Fermi level into the conduction band, substantially increasing the electron concentration. Simultaneously, the formation of a secondary Ag₂Se phase generates lattice distortions that enhance phonon scattering. A pronounced mismatch between electronic (~200 nanometers) and phononic (~0.22 nanometers) mean free paths decouples charge and heat transport, enabling concurrent suppression of κ_L and retention of high electrical conductivity. This coupled electronic-phononic modulation yields a record ZT of 0.72 at ambient temperature and a peak ZT of 1.1 at 735 kelvins, with an average ZT_avg of 0.72 over 320 to 735 kelvins. A unicouple device achieves 6.3% efficiency under a 357-kelvin gradient, highlighting a practical strategy for high-performance midtemperature thermoelectrics.