Phonon Scattering Engineering via Yb Doping in SnSe ₂ for Substantially Lowered Thermal Conductivity and Enhanced Thermoelectric Performance

SnSe 2 is a promising thermoelectric (TE) material with intrinsic n‐type characteristics and a high theoretical ZT value of 2.95 along the a ‐axis. However, its densely packed crystal lattice in the plane perpendicular to the c ‐axis leads to weak phonon scattering, limiting improvements through conventional defect or nanostructure‐based strategies. In this study, the rare‐earth element Yb is introduced into tin‐rich SnSe 2 , predominantly segregating at grain boundaries and enhancing phonon scattering, while a small fraction incorporates into the lattice and modifies the electronic structure, simultaneously tuning both electrical and thermal transport behaviors. Yb incorporation enhances multiple phonon scattering mechanisms, significantly reducing lattice thermal conductivity, reaching a minimum of ~0.48 W·m −1 ·K −1 . Meanwhile, it modulates the electronic structure by introducing impurity states, altering band alignment, and enhancing band degeneracy, collectively increasing the density‐of‐states (DOS) effective mass and Seebeck coefficient, contributing to a maximum power factor of 436.47 μW·m −1 ·K −2 at 773 K. As a result, the Yb‐doped SnSe 2 sample with 1.0 wt% achieves a peak ZT max of ~0.53 at 773 K along the direction parallel to the pressing direction, representing an ~95.3% enhancement over the undoped sample. This study presents a synergistic and effective strategy for optimizing SnSe 2 ‐based TE materials via rare‐earth doping, paving the way for next‐generation high‐performance TE devices.